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High-performance CO oxidation catalysts engineered for COâ‚‚ Lasers

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High-performance CO oxidation catalysts engineered for COâ‚‚ Lasers
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Gardner, Steven Dwayne, 1961-
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vi, 172 leaves : ill. ; 29 cm.

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Alloys ( jstor )
Annealing ( jstor )
Binding energy ( jstor )
Catalysts ( jstor )
Chemicals ( jstor )
Lasers ( jstor )
Oxidation ( jstor )
Oxygen ( jstor )
Pretreatment ( jstor )
Tin oxides ( jstor )
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bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

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Thesis:
Thesis (Ph. D.)--University of Florida, 1990.
Bibliography:
Includes bibliographical references (leaves 162-171).
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Typescript.
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Vita.
Statement of Responsibility:
by Steven Dwayne Gardner.

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Copyright [name of dissertation author]. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
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HIGH-PERFORMANCE CO OXIDATION CATALYSTS
ENGINEERED FOR COz LASERS













By

STEVEN DWAYNE GARDNER


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


1990














ACKNOWLEDGMENTS


I wish to express my sincere thanks to Professor Gar

B. Hoflund whose instruction and guidance made this re-

search possible. We have shared countless discussions

through which his knowledge and optimism have served to

motivate me both personally and professionally. I would

also like to acknowledge the efforts of David R. Schryer,

Billy T. Upchurch, Erik J. Kielin and Jacqueline Schryer at

NASA Langley Research Center who provided the catalytic ac-

tivity measurements. As exemplified during several NASA-

sponsored workshops, their contributions have had an enor-

mous impact on the quality of this entire research project.

Support of this research under NASA grant NAG-I-794 is also

gratefully acknowledged. Finally, I wish to thank my wife,

Carmen, for her encouragement and forbearance during the

seemingly endless years of graduate study. Throughout the

exhausting hours of research, she has been a constant

reminder of what is most important in my life.














TABLE OF CONTENTS


pace

ACKNOWLEDGMENTS...................................... ii

ABSTRACT .............................................. iv

CHAPTERS

1 INTRODUCTION.................................... 1

COz Lasers in Remote Sensing Applications... 1
__,COz Lasers and Low-Temperature CO Oxidation
Catalysis ................................ 5

2 EVIDENCE OF ALLOY FORMATION DURING REDUCTION
OF PLATINIZED TIN OXIDE SURFACES............ 10

Introduction ................................. 10
Experimental................................ 11
Results and Discussion...................... 13
Summary ..................................... 23

3 SURFACE CHARACTERIZATION STUDY OF THE REDUC-
TION OF AN AIR-EXPOSED PtaSn ALLOY........... 24

Introduction................................. 24
Experimental................................ 25
Results and Discussion ...................... 26
Summary.................................... 37

_. 4 EFFECT OF PRETREATMENT ON A PLATINIZED TIN
OXIDE CATALYST USED FOR LOW-TEMPERATURE
CO OXIDATION ................................ 39

Introduction................................. 39
Experimental................................ 41
Results and Discussion...................... 42
Summary..................................... 61

5 CHARACTERIZATION STUDY OF SILICA-SUPPORTED
PLATINIZED TIN OXIDE CATALYSTS USED FOR
LOW-TEMPERATURE CO OXIDATION: EFFECT OF
PRETREATMENT TEMPERATURE.................... 63


iii









Introduction ................................ 63
Experimental .................................. 65
Results and Discussion................. ..... 66
Summary .................................... 80

6 CATALYTIC BEHAVIOR OF NOBLE METAL/REDUCIBLE
OXIDE MATERIALS FOR LOW-TEMPERATURE CO
OXIDATION: COMPARISON OF CATALYST
PERFORMANCE................................... 82

Introduction................................ 82
Catalyst Preparation........................ 85
Experimental ...... ........................ 87
Results and Discussion.... ......... ......... 88
Summary..................................... 105

7 COMPARISON OF THE PERFORMANCE CHARACTERISTICS
OF Pt/SnOx AND Au/MnOx CATALYSTS FOR LOW-
TEMPERATURE CO OXIDATION.................... 107

Introduction............................... 107
Experimental ................................ 108
Results and Discussion..................... 110
Summary ........................................ 121

8 CATALYTIC BEHAVIOR OF NOBLE METAL/REDUCIBLE
OXIDE MATERIALS FOR LOW-TEMPERATURE CO
OXIDATION: SURFACE CHARACTERIZATION OF
Au/MnOx .................. .................... 123

Introduction................................. 123
Experimental ................................ 126
Results and Discussion...................... 128
Summary..................................... 153

9 SUMMARY............................................ 154

APPENDIX DESCRIPTION OF THE ULTRAHIGH VACUUM SUR-
FACE ANALYSIS SYSTEM.................... 158

REFERENCES..... ........... ........................... 162

BIOGRAPHICAL SKETCH................................. 172














Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

HIGH-PERFORMANCE CO OXIDATION CATALYSTS
ENGINEERED FOR COz LASERS

By

STEVEN DWAYNE GARDNER

December, 1990

Chairman: Gar B. Hoflund
Major Department: Chemical Engineering

The low-temperature CO oxidation activity of numerous

materials has been evaluated in order to develop efficient

catalysts for use in COz lasers. The materials were

screened for activity in small, stoichiometric concentra-

tions of CO and Oz at temperatures near 55 C. An Au/MnOx

catalyst was synthesized which exhibited exceptional CO

oxidation activity while maintaining negligible performance

decay over a period of at least 70 days. The data suggest

that Au/MnOx has potential applications in air purification

and CO gas sensing as well.

Extensive surface characterization data from Pt/SnOx

and Au/MnOx catalysts are reported which relate surface

composition and chemical state information to corresponding

CO oxidation activity data. Ion__scattering spectroscopy

(ISS), Auger electron spectroscopy (AES), angle-resolved










Auger electron spectroscopy (ARAES) and X-ray photoelectron

spectroscopy (XPS) were utilized to observe the behavior of

these surfaces as a function of numerous pretreatments

which alter their catalytic activity. The results suggest

that Pt(OH)2 and Pt/Sn alloy formation may play a key role

in the CO oxidation mechanism on Pt/SnOx surfaces. A PtsSn

alloy was subsequently characterized before and after Hz

reduction to study its surface characteristics.

Surface characterization of Au/MnOx and MnOx was per-

formed in order to elucidate the CO oxidation mechanism.

The spectral data yield evidence that the enhanced CO

oxidation activity of Au/MnOx is related to Mn present

primarily as Mn3sO with substantial amounts of water or

hydroxyl groups. The spectra are consistent with very

small Au particles which may exist in an oxidized state.

The behavior of Au/MnOx and MnOx toward an inert pretreat-

ment suggests the possibility of a Au-MnOx interaction.














CHAPTER 1
INTRODUCTION


This research is a collection of studies whose main

objective is to develop low-temperature CO oxidation

catalysts which can be effectively utilized in COz lasers.

The studies themselves represent systematic efforts to syn-

thesize and evaluate new catalysts for this application and

to gain fundamental information about the low-temperature

CO oxidation mechanism. It will become apparent during the

course of the investigations, however, that the results

have potential applications beyond COz lasers.

In order to provide an understanding of the basis for

this research, this introductory section highlights impor-

tant aspects of COz laser operation and identifies the need

for low-temperature CO oxidation catalysts. This is fol-

lowed by a review which summarizes previous, related re-

search on COz lasers utilizing CO oxidation catalysis. Af-

terwards the individual studies themselves are presented.

The research concludes with a summary of the overall

results with recommendations for future research.

COz Lasers in Remote Sensing Applications

The advent of pulsed lasers has made it possible to

utilize pulsed laser energy in a radar fashion (laser radar










or lidar) with applications in altimetry, ranging, pollu-

tion detection, atmospheric chemistry, and weather monitor-

ing [1-51. Recently, the National Aeronautics and Space Ad-

ministration (NASA) has undertaken a project to develop a

laser atmospheric wind sounder (LAWS) which will incor-

porate a COz Doppler lidar [6]. The LAWS system will

operate from a satellite platform in earth orbit and

measure global wind velocities for a period of up to three

years. It is anticipated that the data obtained from LAWS

will enhance the understanding of atmospheric dynamics and

aid in weather prediction. The same techniques may be

employed to detect dangerous wind shear near airports. A

COz lidar was chosen because the technology is mature and

the corresponding COz laser radiation is safe to the eye.

However, as described below, there are significant problems

which need to be solved before COz lidars can be operated

in space or other remote locations.

i Carbon dioxide lasers are commonly operated in an

open-cycle mode. That is, fresh COz laser gas flows con-

tinuously through the laser from an external storage tank

and is subsequently exhausted to the environment. Unat-

tended operation for an extended period of time, therefore,

requires large, massive reservoirs of COz laser gas. Con-

sequently, this method of operation is undesirable for COz

lidars which will be deployed in remote locations such as










earth orbit. An additional complication arises for COz

lidars which must transmit signals long distances through

the atmosphere. The atmosphere contains a significant con-

centration of COz (approximately 300 parts per million) and

HzO both of which absorb COz laser radiation. As a result,

the COz lidar beam may experience considerable attenuation

as it propagates through the atmosphere [4,6]. A solution

is to use a CO2 laser gas which contains a COz isotope such

as C*'Oz. Relative to a common-isotope COz (C'60) laser

which operates at a wavelength of 10.6 micrometers, a laser

containing C1'Oz emits radiation at 9.1 micrometers which

interacts less with atmospheric constituents. However,

C'1Oz is much more expensive than COz and therefore open-

cycle laser configurations which use C'0Oz are not economi-

cal.

An alternative to open-cycle COz laser operation is

known as closed-cycle operation [7]. A COz laser operating

in a closed-cycle mode is initially charged with a COz gas

mixture and subsequently sealed. This configuration is ad-

vantageous because it eliminates the need for an external

CO2 storage tank and the COz gas mixture itself is con-

served. Unfortunately, there is a serious problem with COz

lasers which precludes closed-cycle operation. During the

lasing process, COz in the laser gas is dissociated into

stoichiometric amounts of CO and Oz. The gradual loss of










COz steadily decreases the power output of the laser. Per-

haps most critical, however, is the increase in Oz con-

centration which can lead to arcing and ultimately to laser

failure. An overall solution is to incorporate a CO oxida-

tion catalyst into the working confines of the COz laser to

recombine the CO and Oz into COz. Successful closed-cycle

operation is achieved by continuously recirculating the

laser gas across a suitable catalyst bed. The resulting

configuration is compact and portable and permits the COz

laser to be utilized in remote locations for an extended

period of time.

The requirements for an effective COz laser catalyst

are rather stringent [7,8]. An appropriate catalyst must

exhibit exceptional CO oxidation activity under the condi-

tions which correspond to steady-state COz laser operation.

That is, the catalyst must maintain activity at tempera-

tures near or below 50 C in a gas mixture which contains

small, stoichiometric concentrations of CO and 02 and as

much as 50% COz. In addition, the use of C18Oz requires

that the catalyst preserve the integrity of the isotopic

laser gas mixture. The catalyst should also be resistant

to dusting which could damage the delicate optics within

the laser.










COz Lasers and Low-Temperature CO Oxidation Catalysis

Few catalysts have been researched and targeted

specifically for use in COz lasers. Ordinarily, CO oxida-

tion catalysts are screened for activity using test gases

which consist of small concentrations of CO in air (excess

Oz). As a result, the performance of most CO oxidation

catalysts remains to be evaluated in the COz laser environ-

ment. Excess Oz facilitates CO oxidation and therefore

numerous catalysts perform well under these conditions even

at low temperatures (less than 100 OC) [9-171. However, CO

oxidation in stoichiometric concentrations of CO and Oz is

much more difficult. Consequently, under these conditions

only a limited number of catalysts are available which ex-

hibit significant low-temperature CO oxidation activity.

The following paragraphs summarize previous research

directed towards testing and evaluating low-temperature CO

catalysts in COz laser environments.

Hopcalite (a mixed-oxide catalyst consisting of ap-

proximately 60% MnOz and 40% CuO with trace amounts of

other oxides) has been tested in a sealed CO2 laser with

limited success [18]. Appreciable CO oxidation activity

was realized only when excess CO (approximately 0.5%) was

introduced into the COz laser gas mixture. This is due

largely to the fact that the CO oxidation rate over

hopcalite is approximately first-order with respect to CO










concentration [11,12,19]. Contrary to the COz laser's low

tolerance for Oz, a small excess in CO concentration is not

harmful to the COz laser performance. The hopcalite

catalyst was able to sustain laser performance for a period

of up to 105 pulses with no indication of failure. In or-

der to satisfy the requirements for LAWS, the laser must be

capable of at least 109 pulses [6]. Therefore, more exten-

sive testing is needed to verify the performance of hop-

calite. However, considering the activity decay which has

previously been observed for hopcalite catalysts [19], it

is doubtful that hopcalite will provide a sufficient

lifetime for the laser.

It has been shown that a heated Pt wire may be used to

recombine CO and Oz in atmospheres similar to those in

sealed COz lasers [20,211. Activity measurements between

700 and 1300 OC indicated significant recombination rates

which were independent of the CO and Oz partial pressures.

Nevertheless, this overall approach to CO and 02 recombina-

tion is undesirable in that it is fragile and requires ad-

ditional energy input to heat the Pt catalyst. In addi-

tion, the power dissipated in the laser may cause thermal

expansion of the laser components resulting in decreased

performance. It is much more advantageous to utilize a

catalyst which is effective near the steady-state










temperatures of the COz laser, particularly for space

applications where there are weight and power constraints.

Catalysts consisting of Pt and/or Pd supported on SnOx

(Pt/SnOx, Pd/SnOx, Pt-Pd/SnOx) have received considerable

attention with respect to CO oxidation in COz lasers

[7,8,22-26]. As a result, several interesting observations

have been made regarding their performance characteristics.

It has been demonstrated that optimum Pt/SnOx performance

results from Pt concentrations between 15 and 20 wt% (27].

Further enhancements in activity are realized by pretreat-

ing the Pt/SnOx surfaces with CO or Hz at elevated tempera-

tures (near 125 to 225 C) [26]. It has also been deter-

mined that relative to Pt/SnOx and Pd/SnOx alone, bimetal-

lic catalysts consisting of Pt-Pd/SnOx yield better perfor-

mance [27,281. The performance of a 2 wt% Pt/SnOx catalyst

has been verified during actual closed-cycle COz laser

operation. Utilizing an external catalyst bed containing

150 grams of 2 wt% Pt/SnOx at 100 C, the COz laser main-

tained 96% of its maximum power output for at least 28

hours with no indication of failure [29].

Although Pt/SnOx and Pd/SnOx appear to be promising

catalysts for the COz laser application, they are not yet

totally compatible with long-term laser operation in space.

Isotopic reaction studies [30] have indicated that about

15% of the total amount of Oz which participates in CO










oxidation over Pt/SnOx originates from the Pt/SnOx surface

itself. This is likely to be the case for Pd/SnOx and Pt-

Pd/SnOx as well. Since these SnOx-based surfaces or-

dinarily contain *Oz, the isotopic integrity of a C*"Oz

laser gas mixture would be destroyed. However, a procedure

has been developed to label Pt/SnOx with 1Osz yielding

Pt/Sn1XOx [30]. A preliminary test (lasting 17 days) has

indicated that C'O oxidation over the *'80-labelled

Pt/SnOx catalyst yields essentially 100% C"Oz within ex-

perimental error. Nevertheless, the tests need to be per-

formed over longer periods to ensure isotopic com-

patibility. The long-term performance characteristics of

Pt/SnOx, Pd/SnOx and Pt-Pd/SnOx represent another area for

concern. In the presence of stoichiometric CO and Oz near

ambient temperatures, all of these SnOx-based catalysts ex-

hibit significant decay in CO oxidation activity over ex-

tended time [27,28]. Consequently, their performance is

currently unacceptable in long-term COz laser applications

such as LAWS.

A study has recently evaluated the performance of a

sealed COz laser containing a Au catalyst [311. The inter-

nal wall of the laser's discharge tube was coated with a

thin, discontinuous Au film to recombine the products of

COz dissociation. The activity of the Au film appears to

result from the presence of atomic oxygen in the laser










discharge. However, the presence of a conductive Au film

in the laser amplification volume can create serious

problems under certain conditions. Care must be taken to

ensure that the Au film is sufficiently discontinuous (Au

islands) so as to avoid interference with the laser's

electrical requirements. In addition, the Au surface area

must be kept low to minimize stray reflection lasing which

reduces power output. Therefore, the disadvantages as-

sociated with coating such a large surface with Au under

conditions which minimize (but not eliminate) the drawbacks

noted above render this approach highly cost-ineffective

and inefficient.

Considering the previous research on CO oxidation

catalysis in COz lasers, much of the present research has

been directed toward SnOx-based materials such as Pt/SnOx.

Chapters 1 through 4 describe surface characterization

studies of Pt/SnOx and Pt-Sn alloy surfaces. These studies

were performed to learn more about the low-temperature CO

oxidation mechanism over Pt/SnOx catalysts. Chapter 5

presents results of activity screening experiments designed

to identify new low-temperature CO oxidation catalysts for

the CO2 laser application. Finally, Chapters 6 and 7

report activity and surface characterization data for a

Au/MnOx catalyst whose exceptional activity was identified

in the former screening experiments.














CHAPTER 2
EVIDENCE OF ALLOY FORMATION DURING REDUCTION
OF PLATINIZED TIN OXIDE SURFACES


Introduction

Platinized tin oxide surfaces (Pt/SnOz or Pt/SnOx)

have been proven to be efficient, low-temperature, CO

oxidation catalysts which makes them useful in COz laser

applications [7,8,231. However, relatively little is un-

derstood about the oxidation of CO over Pt supported on

reducible oxides. In order to understand the mechanism, it

is necessary to determine the surface composition, chemical

states of the surface species, and types of interactions

between the surface species. It also appears that surface

hydrogen has a large influence on the catalytic behavior of

these surfaces (26].

Alumina-supported Pt/Sn bimetallic catalysts are im-

portant in hydrocarbon reforming processes [32-34]. The

addition of Sn generally leads to prolonged catalyst

lifetime through an improved resistance toward coke forma-

tion. Furthermore, the reaction selectivity is enhanced

toward aromatization which results in an increase in the

reformate octane number.










It is important to understand the nature of the Pt/Sn

interaction in both the CO oxidation catalysts and the

reforming catalysts. This point has been controversial for

many years with regard to Pt/Sn reforming catalysts. Many

studies suggest that a Pt/Sn alloy is formed during reduc-

tion [35,36] while others suggest that tin is present as an

oxide [37-42]. Recent studies by Davis and coworkers

(43,44] are particularly convincing in demonstrating that

an alloy forms during reduction using X-ray photoelectron

spectroscopy (XPS) and in-situ X-ray diffraction. The dif-

fraction patterns indicate that the composition of the al-

loy is PtSn, but other alloys such as PtsSn may form at

different metal loadings.

The purpose of the present study is to investigate the

Pt/Sn interaction on a platinized tin oxide surface similar

to those used in CO oxidation. Auger electron spectroscopy

(AES), ion scattering spectroscopy (ISS), and XPS have been

used to examine the platinized tin oxide surface before,

during and after reduction by vacuum annealing and after

exposure of the reduced surface to oxygen at room tempera-

ture.

Experimental

A tin oxide film was prepared by thermal hydrolysis of

Sn4* from a solution containing 3 M SnCl4 and 1.5 M HCI

1451. The solution was sprayed onto a titanium foil which










had been heated and maintained at 500 C in air. The

resulting tin oxide film was about 5000 angstroms thick and

it contained significant amounts of hydrogen [46-50].

Platinum was deposited by impregnating the tin oxide film

with a saturated chloroplatinic acid solution. The film

was then rinsed with distilled water and calcined in air at

500 OC for one hour.

The sample was inserted into an ultrahigh vacuum (UHV)

system (base pressure of 10-s1 Torr) which is described in

detail in the Appendix. The UHV system contained a double-

pass cylindrical mirror analyzer (CMA) (Perkin-Elmer PHI

Model 25-270AR) used for AES, ISS and XPS. The AES experi-

ments were performed in the nonretarding mode using a 3-

keV, 10-pA primary beam with a 0.2-mm spot diameter. The

ISS spectra were collected in the nonretarding mode using a

1470-scattering angle and pulse counting detection [51]. A

1-keV, 100-nA 4He' primary beam was defocused over a 1-cm-

diameter area to minimize sputter damage. The XPS spectra

were collected in the retarding mode using a 25-eV pass

energy and Mg Ka excitation. The sample was heated

radiantly using a tungsten filament and its temperature was

measured with an optical pyrometer.










Results and Discussion

An AES spectrum and an ISS spectrum taken from the as-

prepared surface are shown in Figure 2-1. The AES spectrum

exhibits peaks due to Pt, Sn, O, C1 and C. The large Pt

feature at 64 eV indicates that the surface contains a high

concentration of Pt which is consistent with the ISS

spectrum shown in Figure 2-1(b). Several features appear

in the ISS spectrum. The Sn peak is a shoulder on the Pt

peak and the peak at 0.6 E/Eo is due to contamination

(probably Na). The secondary ions apparently desorb with a

threshold at 0.12 E/Eo. This suggests that charging is oc-

curring on this surface which shifts the elemental features

to higher E/Eo than predicted by the binary collision model

[52]. A very small O peak is at 0.49 E/Eo and the peak at

0.28 E/Eo may be due to C. A crude estimate of the elemen-

tal cross sections [53] and surface composition suggests

that the outermost layer of atoms contains about 60 at% Pt.

Next the sample was annealed under vacuum in incre-

ments of 50 C from 250 to 450 C for 30 minutes at each

temperature. The ISS spectra taken after annealing at 300

and 450 C are shown in Figures 2-2(a) and 2-2(b), respec-

tively. Several changes have occurred after annealing the

as-prepared sample at 300 OC. The Sn shoulder has become a

more prominent peak and its size has increased relative to

the Pt peak. Charging is no longer apparent on this






















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surface so the peak positions are approximately those pre-

dicted by the elastic binary scattering equation. Both the

contamination peak at 0.52 E/Eo and the O peak at 0.39 E/Eo

are reduced in intensity, and the C peak is no longer ap-

parent. Furthermore, the background intensity is sig-

nificantly decreased from that observed in Figure 2-1(b).

A possible explanation is discussed below.

The trends observed in annealing to 300 OC are con-

tinued by annealing to higher temperatures. Figure 2-2(b)

shows that the surface formed after annealing at 450 OC is

very different from the as-prepared surface. In comparing

Figures 2-2(a) and 2-2(b), the Sn peak is increased with

respect to the Pt and is even more well defined. The con-

taminant peak is no longer present and the O peak ap-

parently is further decreased in intensity. Also, the in-

elastically scattered ions composing the background are

greatly decreased in intensity. Several similarities are

observed between the ISS spectrum shown in Figure 2-2(b)

and the spectra taken from PtsSn surfaces in previous

studies (see Reference 54, Figure 5(b)]. Both surfaces

produce comparable inelastic backgrounds and display

similar Pt, Sn and O features with regard to peak shapes

and relative peak heights. The small inelastic background

is characteristic of spectra taken from metallic surfaces

where the electron mobility is sufficiently high to rapidly










neutralize inelastically scattered ions. The fact that the

ISS spectrum taken from a reduced platinized tin oxide sur-

face closely resembles an ISS spectrum taken from a Pt/Sn

alloy surface (rather than an ISS spectrum taken from a

nonreduced platinized tin oxide surface) supports the as-

sertion that a Pt/Sn alloy forms during reduction of the

platinized tin oxide surface.

The AES spectrum shown in Figure 2-3(a) and the Sn 3d

XPS spectrum shown in Figure 2-3(b) were also taken from

the sample after annealing at 450 OC. Comparing Figures 2-

l(a) and 2-3(a) shows that the Pt-to-Sn peak height ratio

essentially remains constant at a value of 1.12 during the

annealing. However, the Sn-to-O peak height ratio in-

creases from 1.10 to 1.37 during annealing by either migra-

tion of O below the near-surface region or desorption of 0

from the surface. These facts suggest an increased inter-

action between Sn and Pt in the near-surface region. The

Sn 3d XPS spectrum shown in Figure 2-3(b) has a dominant

peak at a binding energy of 486.4 eV. This feature is due

to Sn present as an oxide, but it is difficult to determine

the precise nature of this oxide from the Sn 3d features as

discussed previously [32]. The arrows in Figure 2-3(b) in-

dicate shoulders which are apparent at an approximate bind-

ing energy of 484.5 eV. This binding energy suggests that
































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Sn is also present in metallic or alloyed form. Only the

oxidic feature at 486.4 eV is present before annealing the

sample so tin oxide is reduced to metallic or alloyed Sn

during the vacuum annealing, which is a reductive process.

A study by Paffett and Windham (55] provides evidence that

the form of the Sn is alloyed rather than metallic. They

deposited metallic Sn on a Pt(lll) surface and showed that

an alloy forms during annealing at or above 150 C. The

450 OC annealing temperature used in the present study then

should result in alloy formation. Thus, the ISS, AES and

XPS results are all consistent with the assertion that a

Pt/Sn alloy is formed during the reduction of platinized

tin oxide.

Similar annealing treatments have been carried out on

tin oxide films which did not contain Pt [45,48,50,56]. In

these studies vacuum annealing to 450 C or above did not

result in reduction of the tin oxide to metallic Sn. Water

of hydration or hydroxyl groups are removed from the sur-

face by vacuum annealing and a portion of the SnOz is

reduced to SnO, but metallic Sn does not form. In fact, a

tin oxide surface which has been reduced by argon-ion sput-

tering is actually enriched in 0 by vacuum annealing

through migration of bulk O to the surface [56]. These ob-

servations demonstrate the important role which Pt plays in










reducing the Sn to metallic form which then alloys with the

Pt.

Next, the oxidative behavior of the surface was ex-

amined by exposing the sample to 10-7 Torr of oxygen for 1

hour at room temperature. The ISS and XPS spectra taken

from this surface are shown in Figures 2-4(a) and 2-4(b),

respectively. The expanded O feature in the ISS spectrum

indicates that the amount of 0 in the outermost surface

layer of atoms increases by about 30% during the oxygen ex-

posure. By comparison of Figures 2-2(b) and 2-4(a), it can

be seen that the oxygen exposure has increased the height

of the Sn peak with respect to the height of the Pt peak.

This behavior is characteristic of Pt/Sn alloys as

described in previous studies [54,571. The differences be-

tween the ISS spectra shown if Figures 2-2(b) and 2-4(a)

are quite similar to those obtained by exposing a Pt3Sn al-

loy surface to low-pressure oxygen at room temperature [see

Reference 57, Figures 5(a) and 5(b)]. Also, there is a

very slight reduction in the size of the shoulders on the

XPS Sn 3d peaks. The extent of this reduction is so small

that it is necessary to hold an expanded version of Figure

2-3(b) over Figure 2-4(b) in order to observe the decrease

in the size of the shoulders. The nonreactive behavior of

this surface toward oxygen as detected by XPS is also

characteristic of Pt/Sn alloy surfaces [57,58]. Using XPS,




















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Cheung [58] found that Sn alloyed with Pt exhibits greater

resistance toward oxidation than metallic Sn. It appears

that the Pt/Sn alloy bond causes the Sn to be less suscep-

tible to oxidation. Therefore, this finding also supports

the assertion that the shoulders observed in the XPS Sn 3d

spectra are due to alloyed Sn rather than elemental Sn, but

quantification of the rates of oxygen adsorption on a Pt/Sn

alloy and polycrystalline Sn has not yet been carried out.

It is interesting to compare the oxidative and reduc-

tive properties of platinized titania (Pt/TiOz) with those

of platinized tin oxide presented in this study since both

titania and tin oxide are reducible, conductive oxides. A

recent study by Hoflund and co-workers [59] of platinized

titania shows that the outermost layer always exhibits a

large ISS peak due to 0. Reduction of this surface results

in encapsulation of the Pt by TiO, and oxygen exposure

forms TiOz which migrates back off the Pt. This behavior

is quite different than that for the platinized tin oxide

surface for which the O lies underneath the Sn and Pt. It

is interesting to note that oxygen exposure of a polycrys-

talline Sn surface results in incorporation of 0 beneath an

outermost layer of Sn atoms [60]. One possible explanation

for the dramatically different behavior of platinized tin

oxide and platinized titania is that tin oxide is more

readily reduced to metal than titania in the presence of










Pt. Reduction of a platinized tin oxide surface by anneal-

ing under vacuum then results in the formation of a Pt/Sn

alloy rather than encapsulation of the Pt by reduced oxidic

support species as observed on platinized titania.

Summary

The Pt/Sn interaction at a platinized tin oxide

catalyst surface has been examined using ISS, AES and XPS

after reduction of the surface by vacuum annealing and ex-

posure of the reduced surface to oxygen. After reduction

XPS shows that metallic or alloyed Sn is formed, AES shows

an increase in the Sn-to-O peak height ratio and the ISS

spectrum becomes similar to that taken from a Pt/Sn alloy

surface. Furthermore, exposure to oxygen at room tempera-

ture induces changes similar to those found for a PtsSn al-

loy surface. These facts all support the assertion that a

Pt/Sn alloy is formed during reduction.














CHAPTER 3
SURFACE CHARACTERIZATION STUDY OF THE REDUCTION
OF AN AIR-EXPOSED PtaSn ALLOY


Introduction

Platinum/tin systems are important catalytic materials

for hydrocarbon reforming (33-40,61-66] and low-temperature

CO oxidation [7,8,22,23]. The presence of Sn alters the

catalytic behavior of Pt so it is important to understand

the nature of the Pt-Sn interaction. In the previous chap-

ter it was shown that a Pt/Sn alloy forms when a platinized

tin oxide film is reduced by annealing in vacuum. This

result suggests the importance of characterizing Pt/Sn al-

loy surfaces since these surfaces may be responsible for

the unique catalytic properties of Pt/Sn systems. Early

studies by Bouwman and co-workers (67-70] show that the

surface compositions of PtSn and PtaSn samples are readily

altered by ion sputtering, annealing in vacuum, exposure to

oxygen or exposure to hydrogen. More recent studies by

Hoflund and co-workers [54,57,71,72] have focused primarily

on the surface enrichment in Sn of PtsSn alloy surfaces

caused by annealing in vacuum or oxygen exposure. These

studies utilized angle-resolved Auger electron spectroscopy

(ARAES) [731, high-energy-resolution Auger electron










spectroscopy (HRAES) [74], scanning Auger microscopy (SAM),

ion scattering spectroscopy (ISS) and angle-resolved X-ray

photoelectron spectroscopy (ARXPS).

Relatively little work has been done on the reduction

of oxidized Pt/Sn alloy surfaces. The purpose of the

present study is to examine an air-exposed PtsSn alloy sur-

face before and after reducing the surface by annealing

hydrogen. Results from ISS, ARAES and XPS are presented.

Experimental

Details of the method used to prepare the PtsSn sample

appear in a related study [71]. The sample was stored in

argon until use. It was inserted into the UHV system (see

Appendix) and sputter-cleaned until all C and 0 contamina-

tion was removed. Then the sample was exposed to air and

reinserted into the vacuum system. After the oxidized

sample was characterized, it was moved into a preparation

chamber attached to the UHV system and reduced under 1 Torr

of hydrogen at 300 OC for 1 hour. The heating was carried

out using a special heating system [75] which did not ex-

pose any hot spots to the hydrogen which would cause dis-

sociation to atomic hydrogen. Thus, the effects of the

reduction are due only to exposure to molecular hydrogen.

The sample temperature was measured using an optical

pyrometer. After reduction the sample was moved back into










the UHV system without air exposure for further charac-

terization.

The ARAES, ISS and XPS experiments were performed

using a Perkin-Elmer PHI Model 25-270AR double-pass

cylindrical mirror analyzer (CMA) containing an internal

electron gun and movable aperture as the charged particle

energy analyzer. The ARAES experiments were performed in

the nonretarding mode using a 3-keV, 10-pA primary beam

from an external, glancing incidence electron gun. The

electron beam incidence angle was approximately 20 off the

alloy surface plane. Rotation of the 900-slotted aperture

allowed for selection of emission angles of 75 to obtain

more bulk-sensitive spectra and 200 to obtain more surface-

sensitive spectra. The ISS spectra were also collected in

the nonretarding mode using a 1470-scattering angle and

pulse counting detection [511. Sputter damage was mini-

mized through the use of a 1-keV, 100-nA 4He* primary beam

defocused over an area of about 1 cm2. Both survey and

high-resolution XPS spectra were collected with Mg Ka ex-

citation in the retarding mode using 50- and 25-eV pass

energies, respectively.

Results and Discussion

An ISS spectrum taken after sample cleaning and air

exposure is shown if Figure 3-1(a). This spectrum contains

a predominant peak due to Sn and very small peaks due to Pt





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and 0. The O peak is shifted from its appropriate E/Eo

value of 0.4 to 0.5, the background is fairly large and no

ions are detected below an E/Eo of 0.26. These facts indi-

cate that the surface is oxidic in nature and that charging

is occurring. This ISS spectrum is quite similar to that

taken from an oxygen-exposed Sn surface [601.

Annealing in hydrogen dramatically alters the composi-

tion of the outermost layer of surface atoms as seen in the

ISS spectrum of Figure 3-1(b). The Pt peak has grown in

size nearly to that of the Sn peak, the O peak is sig-

nificantly decreased in size, the 0 peak appears at its

predicted E/Eo value of 0.4 indicating that charging is no

longer a problem and the inelastic background is reduced

indicating that the outermost surface layer is more metal-

lic in nature. Figures 3-1(a) and 3-1(b) were collected

using the same instrument settings and are scaled ac-

curately with respect to each other. Thus, the Sn peak

height does not change significantly during the reduction.

The ISS cross section of O is much smaller than that of Pt

[53]. This suggests that the decrease in the number of 0

atoms may roughly correspond to the increase in the number

of Pt atoms in the outermost surface layer. It is probable

that the O atoms are removed from the surface layer through

water formation during the reduction. Although

speculative, it is possible that the Pt which migrates to










the surface fills the vacancies left by the 0. The chemi-

cal interaction between the Pt and hydrogen provides a

driving force for the Pt migration as discussed by Bouwman

and co-workers [671 much like the chemical interaction be-

tween the Sn and O provides a driving force for Sn migra-

tion to the surface during an oxygen exposure

[54,57,72,73]. These studies also demonstrate that anneal-

ing a Pt/Sn alloy surface in vacuum rather than hydrogen

enriches the near-surface region in Sn and not in Pt. A

hydrogen environment is necessary to enrich the near-

surface region in Pt.

Figure 3-2(a) shows a bulk-sensitive AES spectrum and

Figure 3-3(a) shows a surface-sensitive AES spectrum taken

from the cleaned and air-exposed sample. There are Sn, C

and 0 peaks appearing in these spectra but no peaks due to

Pt appear. This fact provides clear evidence that a fairly

thick (>30 angstroms) layer of tin oxide forms over the Pt-

rich region during the air exposure. The same conclusion

has been reached in a previous study by Hoflund and Asbury

[72] using angle-resolved XPS. The O/Sn peak-height ratios

in Figures 3-2(a) and 3-3(a) are 0.61 and 0.58, respec-

tively. These comparable values indicate that the composi-

tion of the tin oxide layer is fairly uniform with depth.

The ratios are also typical of those obtained from tin

oxide surfaces (50]. The C peak is probably due to






























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adsorption of hydrocarbons during the air exposure and al-

ways appears after exposing these samples to air.

Figures 3-2(b) and 3-3(b) show the bulk-sensitive and

surface-sensitive AES spectra, respectively, taken after

the reduction. Two changes due to the reduction are ob-

served by comparing the bulk-sensitive AES spectra shown in

Figures 3-2(a) and 3-2(b). The O/Sn ratio is substantially

reduced and Pt migrates to the near-surface region during

the reduction. Both of these facts are consistent with the

ISS spectrum shown in Figure 3-1(b) even though the bulk-

sensitive AES probes more deeply beneath the surface than

ISS. Comparing the surface-sensitive spectrum shown in

Figure 3-3(b) and the bulk-sensitive spectrum shown in

Figure 3-2(b), it is seen that more Pt and less O are con-

tained in the near-surface region than at greater depths.

This observation supports the assertion that Pt replaces O

which has been removed during the reduction. It is also

possible to qualitatively describe the depth composition

profiles of the Pt and 0 from these spectra. Initially,

the O had a fairly uniform concentration through the oxidic

region (of greater depth than that probed by the bulk-

sensitive AES). After the reduction the 0 concentration is

low at the surface and increases at greater depths. The Pt

is more concentrated at the surface and less so at greater

depths. It is seen that the increase in Pt and decrease in










O in the near-surface region is large by comparing the

surface-sensitive spectra shown in Figures 3-3(a) and 3-

3(b).

The XPS survey spectra taken before and after the

reduction are shown in Figures 3-4(a) and 3-4(b), respec-

tively. Peaks due to Sn, O, Pt, C, and Ta are observed in

the spectra. The Ta peak appears because a Ta strip was

used to hold the sample on the mounting block. As observed

by the presence of the large Pt 4f peaks in Figure 3-4(a),

XPS probes more deeply than bulk-sensitive AES. This Pt

signal originates from a Sn-depleted region which lies

beneath the tin oxide layer [57,72]. After the reduction

the Pt peaks are increased in size and the height of the O

Is peak is decreased. These results are consistent with

both the ISS and AES spectra even though XPS probes more

deeply than the other two techniques.

High-resolution XPS spectra of the Sn 3d, Pt 4f and 0

Is features before (A(a), B(a) and C(a), respectively) and

after reduction (A(b), B(b) and C(b), respectively) are

shown in Figure 3-5. The Sn 3d peaks shown in A(a) have a

binding energy of 486.4 eV which corresponds to that of

oxidic Sn (SnO, SnOz or Sn hydroxides) [32,501. However,

these peaks are broad and contain a shoulder at about 485.0

eV which is due either to alloyed or to metallic tin lying

beneath the oxide layer [72]. After the reduction most of
























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the Sn is in the form of alloyed or metallic Sn as

evidenced by the peak position of about 485.3 eV. This

value is larger than the Sn 3d binding energy of 484.6 eV

for metallic Sn. Since a detailed deconvolution of these

peaks has not been attempted in this study, a rigorous as-

signment of binding energies to species is not made. Fur-

ther work on this topic is in progress. A higher binding

energy shoulder at approximately 486.4 eV (shown as a

dotted line) is also present after the reduction. This is

consistent with the AES spectra shown in Figures 3-2(b) and

3-3(b), which show that most of the O lies beneath the sur-

face.

The Pt 4f peaks shown in Figure 3-5B(a) lie at a bind-

ing energy of 71.5 eV which is about 0.6 eV larger than

that of metallic Pt. The width and shapes of these peaks

suggest that only one chemical form of Pt contributes to

this spectrum. As discussed above, this Pt is probably an

Sn-depleted alloy, but the reason for the binding energy

difference from metallic Pt is not currently understood.

After reduction the Pt 4f binding energy is 71.7 eV.

Again, it appears that only one chemical form of Pt is

present. One possibility is that the Sn-depleted Pt alloy

has a binding energy of 71.5 eV and that this binding

energy increases to 71.7 eV as the Sn concentration in-

creases after reduction of the alloy. Two studies [55,58]










suggest that the Sn and Pt peaks obtained from an alloy are

shifted to higher binding energy. This and other pos-

sibilities are being investigated.

Figure 3-5C(a) shows the XPS O Is peak taken from the

air-exposed alloy surface and Figure 3-5C(b) shows the O Is

peak taken from the reduced alloy surface. The width and

shape of the peak in Figure 3-5C(a) indicates that two

forms of O are present on the air-exposed sample: an

oxidic form with a binding energy at 530.5 eV and hydroxyl

groups with a binding energy at about 531.5 eV. The reduc-

tion process removes the hydroxyl groups leaving only the

oxidic form beneath the surface.

Summary

An air-exposed, polycrystalline Pt3Sn surface has been

examined using ISS, ARAES and XPS before and after reduc-

tion by heating at 300 OC under 1 Torr of hydrogen for 1

hour. Before reduction the surface was covered with a

fairly thick (about 40 angstroms), uniform layer of tin

oxide and hydroxide. Beneath this layer was a Pt-rich

region. The ISS data shows that the outermost atomic layer

contains mostly Sn and O with only a very small amount of

Pt present.

Reduction results in loss of 0 and enrichment in Pt of

the near-surface region. The ISS data shows (1) that the

outermost atomic layer is strongly enriched in Pt through










migration of Pt to the surface, (2) that the outermost

atomic layer becomes more metallic and (3) that O is

removed from the outermost atomic layer during the reduc-

tion. The ARAES data suggest that the Pt migrates to the

surface from the Pt-rich region by moving through vacancies

left by the O which reacted with the hydrogen and desorbed,

thus strongly enriching the outermost layer in Pt. The XPS

data shows that most of the tin oxide is reduced to metal-

lic form and probably is present as alloyed Sn. The reduc-

tion results in loss of hydroxyl groups but a subsurface,

oxidic form remains under the reductive conditions used.

It is likely that reduction under the conditions used in

this study but for longer periods would result in complete

removal of the O leaving a Pt-rich alloy at the surface.














CHAPTER 4
EFFECT OF PRETREATMENT ON A PLATINIZED TIN OXIDE
CATALYST USED FOR LOW-TEMPERATURE CO OXIDATION


Introduction

It has been observed that the performance of

platinized tin oxide catalysts used for low-temperature CO

oxidation can be enhanced through reductive pretreatments

prior to reaction [26]. For the pretreatment and reaction

conditions used (see Figure 4-1), the unpretreated catalyst

exhibits the lowest long-term activity. Pretreatment in CO

at 100 OC greatly enhances the long-term activity, and a

further enhancement occurs following CO pretreatment at

125 C. Utilizing higher pretreatment temperatures up to

225 C yields no change in the catalytic activity compared

to the pretreatment at 125 OC. In order to understand how

platinized tin oxide catalysts function in converting CO

and Oz _to COz at low temperatures, it is necessary to

characterize the composition and chemical states of the

species present at these surfaces. This has been ac-

complished in this study using several surface charac-

terization techniques, including ion scattering spectros-

copy (ISS), X-ray photoelectron spectroscopy (XPS) and

Auger electron spectroscopy (AES) to examine the surface of













































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platinized tin oxide before and after pretreatment in CO as

a function of the pretreatment temperature. It is an-

ticipated that these studies will lead to a better under-

standing of this catalytic process and eventually to the

development of improved catalysts for this application.

Experimental

The platinized tin oxide catalyst was acquired from

Engelhard Industries in powdered form. It has an average

particle size of 1 ijm and a BET surface area of 6.9

m2/gram. For the characterization studies, the powder was

pressed into thin disks approximately 1 cm2 in diameter.

These were inserted directly into the ultrahigh vacuum

(UHV) system (see Appendix) for pretreatment and charac-

terization. Pretreatments were performed by moving the

sample into a preparation chamber (base pressure of 10-7

Torr), introducing CO at a pressure of 40 Torr and heating

the sample at the prescribed temperature for 1 hour. Then

the sample was allowed to cool while the preparation cham-

ber was pumped down to 10-7 Torr before moving the sample

into the main analytical chamber (base pressure of 10-1

Torr) for analysis by ISS, XPS and AES. The samples were

heated at 75, 100, 125 and 175 OC in the preparation cham-

ber using a custom sample heater [75], which did not expose

the reducing gas to hot spots that would have dissociated

the CO. Two different as-prepared samples were analyzed










without prereduction and the results were found to be

reproducible. A new sample was prepared and introduced

into the analysis chamber for each reduction temperature

studied.

The ISS, XPS and AES data were taken using a double-

pass cylindrical mirror analyzer (CMA) (Perkin-Elmer PHI

Model 25-270AR) as the charged-particle energy analyzer.

The ISS spectra were collected in the nonretarding mode

using a 1470-scattering angle and pulse counting detection

[51]. A 1-keV, 100-nA 4He* primary ion beam was defocused

over a 1-cm-diameter area and spectra were collected as

quickly as possible (typically 90 s) to minimize damage

from sputtering. The AES experiments were performed in the

nonretarding mode using a 3-keV, 10--A primary electron

beam with a 0.2-mm spot diameter. The XPS experiments were

performed in the retarding mode using Mg Ka excitation.

Survey XPS spectra were collected using a 50-eV pass energy

whereas a 25-eV pass energy was utilized for obtaining

elemental lineshape information.

Results and Discussion

An XPS survey spectrum taken from the as-received

Engelhard platinized tin oxide catalyst is shown in Figure

4-2. The peaks of significant size are due only to 0 and

Sn, and no peaks due to C or other typical contaminants ap-

pear. In fact, this spectrum is essentially identical to


























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an XPS spectrum obtained from a very clean tin oxide sur-

face. It is most interesting that the predominant Pt 4f

peaks are so small that they cannot be discerned readily in

this spectrum. A corresponding AES spectrum taken from the

same surface is shown in Figure 4-3. Again, the

predominant features are due to Sn and 0, and this spectrum

is similar to one obtained from a clean tin oxide surface.

However, a feature due to Pt appears at a binding energy

near 64 eV. It can be described as an edge rather than a

typically shaped AES peak. Similar Pt features have been

observed in a study of the electro-chemisorption of Pt on

tin oxide (76]. This feature is more prominent than the Pt

XPS peaks in Figure 4-2. In agreement with the XPS data,

no contaminant peaks are apparent in this AES spectrum. As

the catalyst is reduced at various temperatures, very small

changes are observed in the XPS and AES spectra correspond-

ing to Figures 4-2 and 4-3, but taken from the reduced sur-

faces. Therefore, these survey spectra are not shown.

Ion scattering spectroscopy is a particularly useful

technique for examining catalytic surfaces because it is

very highly surface sensitive (outermost one or two atomic

layers). The ISS spectra taken before (a) and after (b-e)

reduction are shown in Figure 4-4. The spectrum shown in

(a) consists of peaks due to O, Sn and Pt, and the high in-

elastic background is characteristic of ISS spectra taken

























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from nonmetallic surfaces. In such a case, inelastically

scattered ions are not efficiently neutralized since the

electron mobility at a non-metallic surface is so low that

these ions contribute to the background signal. A previous

study by Asbury and Hoflund [60] showed that O penetrates

beneath the surface during room-temperature oxygen exposure

of polycrystalline Sn. This suggests that the fairly large

O peak in (a) is due to O associated with the Pt and/or

perhaps with hydroxyl groups attached to Sn that may lie

above the surface [50].

The ISS spectra shown if Figure 4-4(b-e) were obtained

from samples reduced in 40 Torr of CO for 1 hour at 75,

100, 125 and 175 C respectively. All four spectra have

two characteristics in common. Firstly, the pretreatments

have resulted in negligible inelastic backgrounds, which is

indicative of the formation of surfaces with a metallic na-

ture. Secondly, the O peak is no longer discernible after

the reductions, which is also consistent with the observa-

tion that the surfaces appear to be metallic. The reduc-

tive pretreatment results in an increase in the Sn/Pt

ratio, and the extent of this increase is greater at higher

reduction temperatures. As shown in chapter 1, an increase

in the ISS Sn/Pt ratio during reduction of platinized tin

oxide surfaces has been found to be indicative of alloy

formation. All of the ISS spectra shown in Figure 4-4 were









taken using the same instrument settings, but the maximum

peak heights vary considerably. Although the variation is

not understood, it could be due to changes in ion

neutralization probability, surface morphological changes

or changes in the concentration of surface hydrogen, which

have been shown to alter the ISS signal strength [77,781.

The Sn 3d XPS spectra and Sn MNN AES spectra taken

before and after reduction are shown in Figures 4-5 and 4-

6, respectively. Before pretreatment (air-exposed sample)

the Sn 3ds/2 lineshape and peak position (486.4 eV) indi-

cate that Sn is present in the +2 or +4 oxidation states

most likely as SnO, Sn(OH)z, SnOz or Sn(OH)e and that

metallic Sn is absent. As discussed by Hoflund and co-

workers [32], it is not possible to distinguish between

these species based on the XPS Sn 3d peaks, but more

specific information can be gained about these species

using electron energy-loss spectroscopy (ELS) [48,561,

valence band XPS [48,50], electron-stimulated desorption

(ESD) [49,50] or secondary ion mass spectrometry (SIMS)

[46,47,79]. The metallic XPS Sn 3d5/z peak appears at an

energy of 484.6 eV [80]. When a small amount of metallic

Sn and a relatively large amount of tin oxides or

hydroxides are present, a slight broadening appears on the

low-binding-energy sides of the oxidic XPS Sn 3d features.

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Figure 4-5. Samples reduced at lower temperatures also

yield this metallic shoulder, and the amount of metallic Sn

produced generally increases as the annealing temperature

increases with the most being produced at 175 C. However,

the amounts of metallic Sn produced at 75 and 100 C ap-

pear similar. Similar observations can be made by con-

sidering the AES Sn MNN peaks shown in Figure 4-6. The

high-kinetic-energy oxidic peak lies at 430 eV, while the

high-kinetic-energy metallic peak lies at 423 eV. The

peaks shown in Figure 4-6(a) are characteristic of oxidic

Sn. When metallic Sn is present, the height of the split-

ting between the two primary peaks decreases. This is ob-

served in spectra (b)-(e) taken from the reduced samples.

In agreement with the XPS Sn 3d spectra, the extent of

metallic Sn formation is greater at elevated reduction tem-

peratures with the maximum amount of metallic Sn being

produced at 175 OC. Further reduction, either for pro-

longed periods or at higher temperatures than used in this

study, would undoubtedly result in the production of in-

creased amounts of metallic Sn.

The XPS O Is peaks are shown in Figure 4-7. These

were taken before (a) and after (b) reduction at 175 OC.

The peak shown in (a) exhibits a distinct asymmetry on the

high-binding-energy side due to the presence of hydroxyl

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approximately 531.8 eV. It is also possible that the very

small shoulder near 533.0 eV is due to adsorbed water.

This assignment is consistent with results obtained in the

study of hydrated polycrystalline tin oxide films by

Tarlov and Evans [81]. Pretreatment at 75 OC slightly

reduces the amounts of adsorbed water and hydroxyl groups

present while pretreatment at 100 C or above eliminates

the adsorbed water and further reduces the concentration of

hydroxyl groups. The 175 C pretreatment results in the

lowest surface hydroxyl group concentration. It is inter-

esting to note that these hydroxyl species are strongly

bonded to the surface and require annealing at 600 C in

vacuum for nearly complete removal [48,50,82].

The O content of the near-surface region is decreased

by the pretreatment process. The AES and XPS O/Sn atomic

ratios obtained at the various pretreatment temperatures

are listed in Table 4-1. A decrease in the O/Sn ratio is

caused by loss of adsorbed water, a decrease in hydroxyl

group concentration, reduction of tin oxides and hydroxides

to metallic Sn and reduction SnOz to SnO. However, the

relative importance of these factors cannot be assessed

completely from the types of data taken in this study. The

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ferent with respect to both magnitude and trend. The AES

data indicate that the near-surface region loses about 20%

of its 0 regardless of the reduction temperature. The XPS

O/Sn atomic ratios are considerably larger than the AES

values and decrease monotonically as the pretreatment tem-

perature increases. Either the XPS values differ from the

AES values due to inaccuracies in the tabulated cross sec-

tions or the variation is due to the fact that AES and XPS

probe different volumes of the near-surface region. If the

difference were due only to inaccurate cross sections, then

the XPS O/Sn ratios obtained from the reduced samples would

not show such a large variation with reduction temperature.

Thus, the variation is due to the fact that XPS probes more

deeply than AES, which is consistent with mean-free-path

arguments also. The kinetic energies of the XPS O Is and

Sn 3d electrons are 723 eV and approximately 770 eV,

respectively, while the kinetic energies of the AES O and

Sn electrons are near 510 eV and 430 eV, respectively. The

corresponding mean free paths (T) are about 6 angstroms for

XPS and 4 angstroms for AES [833. Since most of the XPS

and AES electrons originate within a depth of approximately

3(T), XPS probes about 18 angstroms beneath the surface

while AES probes at a depth of about 12 angstroms. Then,

the data in Table 4-1 indicate that the near-surface region










probed by AES contains less O than the region probed by XPS

for the untreated sample and samples reduced at 75-175 OC.

Reduction at any of the temperatures used causes the AES

O/Sn ratio to drop from 1.32 to about 1.06 whereas the XPS

O/Sn ratio decreases monotonically as the reduction tem-

perature increases. The essentially constant AES value

probably results from competing processes, that is, O leav-

ing the surface as CO2 during the reduction and O migrating

to the near-surface region from farther beneath the sur-

face. This is consistent with the trend of the XPS data

implying that subsurface reduction takes place to a greater

extent at higher temperatures. A similar phenomenon has

been observed previously for the reduction of a TiOz (001)

surface [84].

The XPS Pt 4f peaks obtained from the unpretreated

samples are shown in Figure 4-8(a), and the peak assign-

ments used in this study are listed in Table 4-2. Most of

these assignments were taken from a standard reference

[801, but the 72.3 eV feature has been assigned as Pt-O-Sn

in previous studies of platinized tin oxide surfaces

[32,561. The spectrum shown in Figure 4-8(a) indicates

that very little metallic Pt is present and that the Pt

species consist mostly of Pt-O-Sn, Pt(OH)z and Pt oxides.

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from a co-deposited platinum/tin oxide film after calcining

in air at 725 K for 1.5 hours [32].

The XPS Pt 4f spectra taken after pretreating at 75

and 100 C are shown in Figures 4-8(b) and 4-8(c), respec-

tively. These two spectra are quite similar in that very

little metallic Pt is present and the predominant species

consist of Pt-O-Sn and Pt(OH)z. The metallic Pt apparently

is in the form of small crystallites. A shoulder due to

PtOz may also be present in both spectra, but this shoulder

is smaller after the 100 C reduction. The relative

amounts of Pt-O-Sn and Pt(OH)z are similar in both spectra.

Pretreating at a temperature 25 C higher produces a sig-

nificant change in the Pt species present, as shown in

Figure 4-8(d). The predominant Pt species after the 125 C

reduction are Pt(OH)z and Pt-O-Sn in approximately equal

amounts. Also, a small amount of metallic Pt is present

after this pretreatment, and features due to Pt oxides do

not appear.

Reduction at 175 C produces a further and more

pronounced shift in the Pt chemical state toward Pt(OH)2 as

shown in Figure 4-8(e). The Pt-O-Sn feature is now a

shoulder on the Pt(OH)2 peak, and the amount of metallic Pt

present is decreased compared to the lower temperature

reductions. The spectrum shown in Figure 4-8(a), taken

from the unpretreated sample, exhibits a prominent feature









at 79.8 eV binding energy. Since the splitting between the

Pt 4f7/2 and Pt 4f5/2 peaks is approximately 3.35 eV, this

feature does not correspond to the Pt 4f5/z peak of any of

the species listed in Table 4-2. The 75 OC pretreatment

reduces the size of this feature, and it does not appear

after the 100 or 125 OC pretreatment. Therefore, it be-

haves like an oxidic feature and reappears in Figure 4-

8(e).

It is interesting to compare the spectral information

contained in Figure 4-8 with the kinetic information con-

tained in Figure 4-1. The nonpretreated catalyst exhibits

a low activity compare to any reduced catalyst. This

correlates with the facts that the Pt is predominantly

oxidic on the unpretreated catalyst and that significant

changes in the Pt chemical state occur during the reductive

pretreatments used in these studies. As the pretreatment

temperature increases, more of the Pt is converted into

Pt(OH)z. This fact suggests that the Pt(OH)z plays an im-

portant role in the conversion of CO and Oz into COz at

low temperatures. In this discussion a relationship is

being drawn between the long-term catalytic behavior (past

the first 500 minutes of reaction) and the chemical state

of the Pt after the pretreatment but before the onset of

reaction (time=0 in Figure 4-1). The initial catalytic be-

havior is quite complex [261, and changes may occur in the










Pt chemical state during this period. Consequently,

characterization studies are in progress in which the state

of the catalytic surface is being examined as the reaction

is run for various periods of time. These studies should

lead to an understanding of the chemical changes respon-

sible for the unusual, initial catalytic behavior and the

long-term decay in catalytic activity.

A small amount of metallic Pt forms during both the

125 and 175 OC pretreatment, and metallic Sn is also

present as stated above. Paffett and Windham [55] have

deposited layers of Sn on Pt(lll) and found that alloy for-

mation during annealing at or above 150 C is strongly sug-

gested by their data. Also, Fryberger and Semancik [85]

have found that Pd deposited on SnOz (110) alloys with the

Sn even at room temperature. It is anticipated that alloy

formation occurs in the Engelhard catalyst under the

pretreatment conditions used in this study. Since the

amount of Pt present on this surface is small, it is likely

that most of the metallic Sn is not alloyed while probably

all of the Pt is alloyed.

Summary

A platinized tin oxide catalyst (commercially avail-

able from Engelhard Industries for low-temperature CO

oxidation) has been examined using surface analytical tech-

niques, including ISS, AES and XPS, before and after










pretreatment by annealing in 40 Torr of CO for 1 hour at

75, 100, 125 and 175 C. The results have been correlated

with catalytic activity data. The nonpretreated sample

consists primarily of oxidic Sn (SnOz, SnO and Sn(OH)x)

with a very small amount of Pt present as Pt-O-Sn, Pt(OH)z,

PtO and PtOz species. Reduction results in loss of both 0

and OH- from the Sn and produces metallic Sn. The extent

of these processes increases as the pretreatment tempera-

ture increases. The chemical state of the Pt changes with

pretreatment temperature. At or below 100 OC, the

predominant forms are Pt-O-Sn and Pt(OH)2. As the reduc-

tion temperature increases, more Pt(OH)2 forms. This fact

suggests that Pt(OH)z plays an important role in the low-

temperature catalytic oxidation of CO. The results

demonstrate that the application of surface analytical

techniques in studies of real catalysts can provide infor-

mation that is useful in understanding catalytic behavior.














CHAPTER 5
CHARACTERIZATION STUDY OF SILICA-SUPPORTED PLATINIZED
TIN OXIDE CATALYSTS USED FOR LOW-TEMPERATURE CO
OXIDATION: EFFECT OF PRETREATMENT TEMPERATURE


Introduction

Research has indicated that platinized tin oxide

(Pt/SnOx) is an efficient CO oxidation catalyst at condi-

tions which correspond to steady-state CO2 laser operation

[7,8,23,28]. As discussed in the previous chapter, the

performance of Pt/SnOx may be further enhanced following

pretreatment in CO at elevated temperatures. However, for

CO reduction temperatures above about 175 C, a sharp, tem-

porary decrease in activity is observed initially [26].

This induction period is believed to be the result of sur-

face dehydration caused by combination of surface hydroxyl

groups and desorption of water. No significant induction

period results when Pt/SnOx is humidified either after CO

pretreatment or during the reaction itself and optimum

pretreatment times exist which are consistent with the

hypothesis involving surface dehydration. The data ob-

tained thus far suggest that COz is produced through decom-

position of a bicarbonate species formed from adsorbed CO,

a surface hydroxyl group and lattice oxygen [86].










Acknowledging the importance of surface hydroxyl

groups, there has been considerable effort directed towards

the development of a platinized tin oxide catalyst which is

supported on a silica substrate (Pt/SnOx/SiOz) [27,87].

Hygroscopic silica may improve the performance of Pt/SnOx

surfaces by preventing extensive surface dehydration and

consequent activity loss. Experiments have been conducted

wherein the catalyst performance has been optimized with

respect to several variables including pretreatment proce-

dures [27]. Superior performance is realized by using a

reductive pretreatment in 5% CO/He at 125 C for 1 hour.

As pretreatment temperatures approach 250 C, there is a

significant decrease in the observed activity. The op-

timized Pt/SnOx/SiOz catalyst represents a significant im-

provement over commercially available Pt/SnOx with respect

to low-temperature CO oxidation activity and performance

decay.

In order to understand these experimental observa-

tions, a characterization study has been carried out on

silica-supported Pt/SnOx before and after reduction in CO

at 125 and 250 OC. This study utilized ion scattering

spectroscopy (ISS) and X-ray photoelectron spectroscopy

(XPS) to examine the concentrations and chemical states of

species present at these surfaces and the changes which oc-

cur during reduction.










Experimental

The catalyst prepared for this study consisted of a

thin layer of platinum and tin oxide dispersed on a silica

gel substrate [27,87]. The silica gel was impregnated with

tin oxide by evaporation of a stirred solution of tin metal

powder and silica gel in concentrated nitric acid at

150 C. Subsequently, Pt was precipitated from an aqueous

solution of platinum tetraamine dihydroxide and formic acid

with heating followed by drying at 150 OC. The final

product was heated in air at 150 C for 4 hours.

As-prepared, silica-supported Pt/SnOz samples were in-

serted into an ultrahigh vacuum (UHV) system (see Appendix)

which has an ultimate pressure near 10-11 Torr. After ini-

tial surface characterization, the samples were transferred

into a preparation chamber connected to the UHV system and

reduced in 10 Torr of CO for 2 hours at 125 or 250 OC.

Heating was accomplished using a platform heating element

[75] which did not dissociate the reducing gas. Sample

temperatures were measured using a thermocouple attached to

the stainless-steel sample support block. After reduction

each sample was returned to the UHV analytical chamber

without air exposure for further characterization.

Energy analysis for the ISS and XPS experiments was

accomplished using a Perkin-Elmer PHI Model 25-270AR

double-pass cylindrical mirror analyzer (CMA). The CMA









contained an internal, movable aperture which varied the

polar acceptance angle for incoming particles. The ISS

spectra were collected in the nonretarding mode using a

1470-scattering angle, which was fixed by the experimental

geometry, and pulse counting detection [51]. A 100-nA, 1-

keV 4He* primary ion beam was defocused over a 1-cm2 area

to minimize sputter damage. Survey and high-resolution XPS

spectra were recorded with Mg Ka excitation in the retard-

ing mode using 50- and 25-eV pass energies respectively.

Results and Discussion

Survey XPS spectra taken from one silica-supported

Pt/SnOx catalyst sample before and after reduction in 10

Torr of CO at 125 C appear in Figure 5-1, and XPS survey

spectra taken from another sample before and after reduc-

tion at 250 OC appear in Figure 5-2. The two catalyst

samples examined were similar in that both were randomly

dispensed from the same catalyst batch. The spectra shown

in Figures 5-1(a) and 5-2(a) were taken from the air-

exposed surfaces. They exhibit predominant peaks due to Sn

and O while peaks due to Pt are present but less discern-

ible. Differences between Figures 5-1(a) and 5-2(a) with

regard to the O/Sn ratio suggest that the as-prepared

catalyst samples lack uniformity in average surface com-

position. Nevertheless, important information may still be

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sample before and after reduction. A thick tin oxide sur-

face layer (> 50 angstroms) is indicated by the lack of Si

XPS signals arising from the silica substrate. Only trace

amounts of C and surface contaminants are detected. Reduc-

tion in CO results in changes in the Sn and O signals as

shown in Figures 5-1(b) and 5-2(b). The spectra indicate

that the surface region of the sample reduced at 125 OC be-

comes oxygen-enriched while depletion of surface O occurs

in the sample reduced at 250 oC. A possible mechanism

responsible for this result is discussed below.

An ISS spectrum taken from an air-exposed, silica-

supported Pt/SnOx surface before reduction is shown in

Figure 5-3(a), and ISS spectra taken after reduction in CO

at 125 and 250 C appear in Figures 5-3(b) and 5-3(c),

respectively. Figure 5-3(a) reveals distinct Pt and O fea-

tures and a Sn peak which appears as a shoulder on the Pt

peak. Peaks due to small amounts of contaminants such as C

and Na are also present as are some peaks due to charging.

Peaks due to charging shift in energy as the sample is

rotated so they can be distinguished easily from elemental

peaks, which appear at energies near those predicted by the

elastic binary collision model [52,88]. After catalyst

reduction at 125 OC, Figure 5-3(b) indicates that the sur-

face composition has changed considerably. The Sn and Pt

peaks have increased in size significantly, and the Sn peak





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has become more prominent and well defined in size with

respect to the Pt. As shown in Chapter 1, this behavior is

characteristic of Pt/Sn alloy formation. A few peaks due

to migration of substrate impurities such as Na and Mg also

appear after the reduction as does a Ti peak from the

sample holder. After reduction at 250 C the Sn and Pt

peaks are greatly reduced in size due to coverage by con-

taminants (primarily Na and Mg) in the silica which migrate

to the surface during reduction. Due to the stability of

silica at these fairly low reduction temperatures, silica-

containing species most likely do not migrate over the Pt

and Sn. Since ISS is essentially sensitive to the outer-

most layer of surface atoms, the data in Figure 5-3 suggest

that the 250 OC reduction results in physical coverage

encapsulationn) of most of the surface Pt and Sn. This en-

capsulation hypothesis is confirmed by ISS data taken after

the reduced surface was lightly sputtered. As shown in

Figure 5-4, the sputtering process uncovered significant

amounts of underlying Pt and Sn. Although peak energy

shifts and surface charging features appear in these

spectra, the peaks which correspond to Pt and Sn remain

prominent and meaningful.

The data presented thus far suggest that reduction at

both 125 and 250 C promotes movement of O, Sn and

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concentration in the near-surface region is probably

determined by the rates of several mechanisms. One is

migration of subsurface 0 to the near-surface region during

the reduction at elevated temperature under the chemical

potential produced by the presence of the reducing gas.

Another is loss of surface O through reaction with the

reducing gas and subsequent desorption as COz. As stated

above, O loss may also occur through OH- combination fol-

lowed by desorption of HzO. At 125 C the rate of migra-

tion of subsurface O to the surface apparently is greater

than the rate of O loss by reaction to form COz and H20,

which results in an increase in the near-surface O con-

centration as observed in the XPS data. Similar observa-

tions have been made during the reduction of a TiOz(001)

surface [841. However, at 250 OC the rate of O loss

through reaction is greater than the rate of 0 migration to

the surface, resulting in a decrease in the O concentration

in the near-surface region.

As shown in Figure 5-5, the Pt 4f XPS spectra exhibit

broad features suggesting that multiple Pt oxidation states

are present both before and after reduction. According to

a standard reference [80] and previous studies from this

laboratory [32,89], Figure 5-5(a) indicates that a mixture

of primarily PtO2 and PtO with smaller amounts of Pt(OH)z

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After reduction, peak positions shift toward lower binding

energies as the surface Pt species are partially reduced.

After reduction at 125 OC the predominant species present

is Pt(OH)z with an increased amount of metallic Pt.

However, only very small amounts of PtO and PtOz remain

after the 125 C reduction. The extent of reduction is

much greater after the 250 OC reduction. The Pt is

primarily in metallic form with a relatively small amount

of Pt(OH)z. Little or no PtO or PtOz is present. There

has been some debate concerning the stability of PtO, but

its existence is well substantiated in numerous studies

[32,80,89-94].

Tin can be present in several forms which include

SnOz, SnO, Sn hydroxides, Sn suboxides, Sn-O-Pt species

132,891, metallic Sn and Sn alloyed with Pt. Using XPS, it

is not difficult to distinguish between metallic Sn and Sn

oxides because their Sn 3d binding energies differ by about

1.8 eV, and Asbury and Hoflund have observed XPS features

due to suboxides [601. However, it is difficult to distin-

guish between SnOz and SnO using XPS, but this can be done

using valence-band XPS [50,953 or electron energy-loss

spectroscopy (ELS) [56,96]. Paparazzo and co-workers 197]

claim that there is a 0.18 eV difference in the Sn 3d bind-

ing energies of SnO and SnOz. Valence-band XPS (50,95] and

electron-stimulated desorption (ESD) [501 can be used to










identify Sn hydroxides. These species also yield a high-

binding-energy shoulder on the O Is XPS peak [50,81], but

no studies have reported any influence on the Sn 3d XPS

peaks. Studies of Pt/Sn alloys [55,58] suggest that the Sn

3d XPS alloy features differ from those of metallic Sn, but

a systematic study of Sn 3d shifts due to alloying has not

been reported.

The Sn 3d XPS spectrum taken before reduction is shown

in Figure 5-6(a), and spectra taken after reduction in CO

at 125 and 250 OC are shown in Figures 5-6(b) and 5-

6(c), respectively. Small changes in the Sn 3d peak shapes

occur during annealing in CO. Since a large number of Sn

species may be present and their contributions to the Sn 3d

XPS features are not well understood, it is only possible

to analyze these peaks in a qualitative manner. Before

reduction, the Sn 3d XPS peaks are due predominantly to

SnOz. However, they are not symmetrical, and the symmetry

on the low-binding-energy side indicates that some of the

Sn on the as-prepared catalyst may be present as suboxides

and SnO and possibly as metal. The Sn 3d peaks become

slightly wider with increasing reduction temperature as in-

dicated in Figure 5-6. Based on the data of Paparazzo and

co-workers [97], this is probably due to partial conversion

of SnOz to SnO. After reduction at 250 C it appears that

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as well suggesting that some of the Sn oxides are reduced

to metallic Sn. Metallic Sn probably forms during reduc-

tion at 125 OC, but the amount is so small that it is not

detected by XPS. Recall that similar changes in the Sn 3d

peak shape were observed during reduction of platinized tin

oxide surfaces in Chapters 1 and 3. Some, but probably not

all, of the metallic Sn formed alloys with Pt. Paffett and

Windham [55] have found that Pt/Sn alloy formation occurs

at temperatures as low as 175 OC in their study of Sn

deposited on Pt(lll). Recently, tin oxide films (no Pt)

have been reduced in CO under conditions similar to those

used in this study [98]. The XPS data indicate that metal-

lic Sn does not form in this case. Thus, it appears that

Pt promotes the formation of metallic Sn during reduction

in CO. The XPS data in Figure 5-6 also indicate that the

extent of reduction is greater at 250 OC than at 125 C.

It is interesting to relate the results of this

characterization study with those obtained from the commer-

cial Pt/SnOx catalyst (not supported on silica) inves-

tigated in Chapter 4. Both catalysts require a reductive

pretreatment for optimization of catalytic performance, but

increasing the reduction temperature affects the two

catalysts differently. The silica-supported catalyst ex-

amined in this study yields an optimum CO oxidation ac-

tivity after a 125 C pretreatment. The commercial Pt/SnOx










catalyst exhibits the same activity over a broad range of

pretreatment temperatures (125 to 225 oC) [26]. The XPS

data in this study and the characterization study in Chap-

ter 4 agree that Pt is present predominantly as hydroxides

after a pretreatment which yields high activity. This fact

is consistent with the observation that the catalytic ac-

tivity of the commercial Pt/SnOx catalyst is enhanced by

the presence of water vapor in the reaction gas mixture

[26]. Another similarity is that the data obtained from

both catalysts indicate that a Pt/Sn alloy forms during

reduction. This fact apparently explains why Pt supported

on tin oxide is greatly superior as a catalyst for CO

oxidation at low temperatures compared to Pt supported on

alumina or other nonreducible oxides.

Another factor relating to catalytic activity which

must be considered is encapsulation of the active Pt and Sn

species. This process does not occur when reducing the

commercial Pt/SnOx catalyst at temperatures up to 175 OC.

The impurities in the silica-supported Pt/SnOx catalyst

which cover the Pt and Sn species during the 250 OC reduc-

tion may be responsible for the loss in catalytic activity

since the encapsulation does not occur during the 125 C

reduction after which the catalytic activity remains high.

The catalyst surface produced during the reduction

step is not the same surface responsible for the long-term










catalytic behavior. Significant changes occur at the

catalyst surface during the initial reaction period. Based

on activity data [26], transient behavior may occur for

hours after which time the activity goes through a maximum

and then slowly decays. The changing activity must corre-

late with changes in composition and chemical states of

species at the surface, but this relationship has not been

determined yet.

Summary

The changes induced at an optimized, silica-supported

platinized tin oxide surface by annealing at 125 and 250 C

in 10 Torr of CO for 2 hours have been determined using ISS

and XPS. The reduction at 125 C results in enrichment of

the near-surface region in O, reduction of Pt oxides with

the formation of Pt hydroxide, and the reduction of SnOz to

SnO and metallic Sn which mostly alloys with Pt. Different

results are obtained from the surface reduced at 250 OC in

that the extent of the Sn oxide reduction is greater, the

predominant form of the Pt is metallic, impurities in the

silica migrate over the active Pt and Sn species, and the O

content of the near-surface region is reduced. The dif-

ferences found after reduction at the two temperatures may

explain the fact that a 125 OC pretreatment results in a

surface which is active for CO oxidation at low temperature






81



whereas a 250 OC pretreatment results in a catalytically

inactive surface.














CHAPTER 6
CATALYTIC BEHAVIOR OF NOBLE METAL/REDUCIBLE OXIDE
MATERIALS FOR LOW-TEMPERATURE CO OXIDATION:
COMPARISON OF CATALYST PERFORMANCE


Introduction



The catalytic oxidation of CO near ambient tempera-

tures has many important applications. As described pre-

viously, closed-cycle COz lasers produce CO and Oz in the

laser discharge resulting in a rapid loss of output power.

This problem can be overcome by incorporating a low-

temperature CO oxidation catalyst into the laser system

which converts the dissociated products back into CO2

17,8,22,23]. Also, air purification devices often contain

catalysts to oxidize dangerous levels of toxic CO [17,99-

101]. Such devices are utilized in fire safety equipment

and in underground mines as respiratory aids.

Consequently, the development of low-temperature CO

oxidation catalysts has received considerable attention [7-

171. Although significant progress has been made with

regard to understanding the reaction mechanism, there

remains a need for the development of catalysts which ex-

hibit higher activities for prolonged periods at low tem-

peratures (typically less than 100 C) and in the diverse










range of oxidation environments which are encountered.

Factors which determine oxygen availability and gaseous im-

purities often have a pronounced effect on catalyst perfor-

mance. CO oxidation in ambient air has the advantages of

excess Oz and low COz concentrations which facilitate the

reaction considerably. Consequently, numerous materials

are known to oxidize CO in excess Oz at low temperatures

[9-11,13,141, but complications due to the presence of

humidity and/or air pollutants are often detrimental to

their activity. In COz lasers, CO and 02 are present in

small stoichiometric quantities in a large amount of COz.

Although the catalytic reaction benefits from the fact that

the lasers usually operate at temperatures somewhat above

ambient (25-100 OC), catalytic CO oxidation is difficult

under these conditions.

Recently, Haruta and co-workers [15,161 prepared

supported-gold catalysts on various base-metal oxides in-

cluding MnOx, FezOs, CoasO, NiO and CuO and determined

their catalytic activities toward the oxidation of Hz

and/or CO. Most of the CO oxidation tests were performed

using 1 vol% CO in dry air. At 0 C, Au/FezOs and Au/NiO

maintain essentially 100% CO conversion under the flow con-

ditions used over a 7-day test period. Similar performance

was also observed for Au/FezOs and Au/CosO* at 30 C in 76%

relative humidity. Therefore, these catalysts appear to be









quite useful in air purification devices, but activities in

the presence of air contaminants were not determined.

These catalysts may be useful in COe lasers even though the

reaction conditions are quite different as described above.

It is interesting to note that Haruta and co-workers

[15,161 apparently did not examine the behavior of Au/MnOx

toward CO oxidation.

Catalysts consisting of Pt and/or Pd supported on tin

oxide have been researched extensively for use in COz

lasers [7,8,22-26]. Although these materials can exhibit

considerable CO oxidation activity in this application,

there are complications which must be overcome. Acceptable

activity is observed only after the catalyst undergoes a

reductive pretreatment. Unfortunately, such pretreatments

may lead to considerable induction periods often lasting

several days during which the observed activity declines

before reaching a maximum [26]. Even after acceptable ac-

tivity is recovered these materials exhibit a steady decay

in performance over time.

The purpose of the present study is to explore the be-

havior of materials other than platinized tin oxide as

catalysts for low-temperature CO oxidation, particularly

with regard to COz laser applications. Several materials

were synthesized and screened for CO oxidation activity

using small concentrations of stoichiometric CO and z0 in










He and temperatures between 30 and 75 OC. The tests were

run for periods as long as 18000 minutes in order to ob-

serve the induction and decay characteristics of the

catalysts.

Catalysts Preparation

A review of the literature provided a basis for selection

of support materials examined in this study which include

iron oxide (Fe2Os), nonstoichiometric manganese oxides

(MnOx), and ceria (CeOx) where x is between 1 and 2. The

materials investigated were synthesized using established

impregnation and coprecipitation techniques [1021. The

samples prepared include MnOx, Pt/MnOx, Ag/MnOx, Pd/MnOx,

Cu/MnOx, Au/MnOx, Ru/MnOx, Au/CeOx and Au/FezOa.

The MnOx was used as-received from the Kerr-McGee Com-

pany, USA. It was prepared by the electrolytic oxidation

of manganous sulfate and has BET surface area of 74

m2/gram. The MnOx served as a sample itself as well as an

impregnation support for other materials.

Two Pt/MnOx samples (0.2 wt% Pt) were prepared by im-

pregnation of MnOx using an aqueous solution of NazPt(OH)e.

Sample #1 was dried in air at 280 OC for 4.5 hours whereas

sample #2 was dried in air at 75 C for 3 hours. A Pd/MnOx

catalyst (0.2 wt% Pd) was prepared by impregnating MnOx

with an aqueous solution of PdClz. The product was dried

in air at 280 C for 4.5 hours.










Two Ag/MnOx samples were prepared, one containing 0.2

wt% Ag and another containing 1.0 wt% Ag. Impregnation of

MnOx with Ag was accomplished using a solution prepared by

dissolving AgO in NH4OH. The products were dried in air at

280 C for 4.5 hours.

A sample which contained admixtures of CuO and MnOx

was prepared from the products of several procedures. Pro-

cedure A involved coprecipitation from aqueous solutions of

CuSO4 + sucrose and KMnO4. The precipitate was washed with

water and dried in air at 105 OC for 15 hours. The Cu:Mn

molar ratio was approximately 1.4. In procedure B, a por-

tion of the former product was dried in air at 280 C for 2

hours. In procedure C, MnOx was precipitated from aqueous

solutions of sucrose and KMnO4. The precipitate was washed

and dried as outlined in procedure A. The final product

consisted of an admixture of 0.4 grams from procedure A,

0.7 grams from procedure B, 0.3 grams from procedure C and

0.2 grams of commercial CuO powder.

A technique in which Mn(OH)z was precipitated in the

presence of fine Ru powder was utilized to prepare a 2 wt%

Ru/MnOx sample. An aqueous solution of Mn(NOs)2 was added

dropwise to a stirred mixture of Ru powder in NHeOH. The

resulting product was dried and calcined at 400 OC for 2

hours.










Three supported Au samples were synthesized via

coprecipitation from aqueous HAuClI and the nitrate of the

corresponding support metal. The composition of the

materials is approximately 5 at% Au/MnOx, 20 at% Au/CeOx

and 5 at% Au/FezO3 on a Au/metal basis. In each case the

appropriate precursor solutions were added dropwise to a

stirred solution of NazCOs at room temperature. After

washing with hot water (80 oC) and drying, the pre-

cipitates were calcined in air at 400 OC for 4 hours. Two

Au/FezOs samples were prepared which differed only in the

temperature of the wash water utilized (25 C and 80 C).

Experimental

The reactor used to test the CO oxidation activity of

the catalysts is located at NASA Langley Research Center in

Hampton, VA, and has been described previously [27,103].

Screening of catalysts for CO oxidation has typically been

performed using a test gas consisting of a few percent CO

in air (excess Oz), and the catalytic behavior under

stoichiometric CO and Oz and in the presence of COz has of-

ten not been determined. Since the catalytic behavior can

vary considerably under different environments as described

above, it is necessary to perform such experiments. All

tests were conducted using 0.15 grams of catalyst and a

reaction gas mixture consisting of 1 vol% CO, 0.5 vol% 02

and 2 vol% Ne in He at a total pressure of 1 atmosphere










flowing at 10 standard cm3 per minute (sccm). The reaction

temperatures investigated were 75, 50 and 30 OC as noted.

The conversions are quite high under these conditions which

corresponds to operating the reactor in an integral mode.

In most cases the catalysts were tested as prepared

without additional pretreatments. Unless noted otherwise,

each catalyst was loaded into the reactor and exposed to

flowing He for about 1 hour as the reaction temperature

stabilized. Then the He flow was changed to the reaction

gas mixture and product sampling was begun. At predeter-

mined time intervals, an automated sampling valve directed

a small fraction of the reaction products to a gas

chromatograph (GC) for analysis of %COz yield, %CO loss and

%0 loss, and the results were plotted versus time.

Results and Discussion

During these initial activity screening experiments,

emphasis is placed upon characteristics of the overall CO

oxidation activity curves with respect to temperature and

time. An appropriate catalyst for use in COz lasers must

not only exhibit high activity at low temperatures (25-

100 C) but also maintain acceptable activity over a

lifetime of up to 3 years [6]. Since a catalyst cannot be

practically tested for a 3-year lifetime, its activity

profile must be extrapolated with reasonable confidence.

Nevertheless, it is necessary to exercise caution when










evaluating potential catalysts for COz lasers because a

catalyst which exhibits the best activity initially might

succumb to decay mechanisms which render it inferior after

extended use. Consequently, a catalyst exhibiting only

marginal activity initially may become the optimal choice

if the corresponding activity decay remains negligible.

Carbon monoxide oxidation activity curves for several

MnOx-based catalysts appear in Figure 6-1. Initially, MnOx

and Cu/MnOx exhibit the highest CO oxidation activities al-

though their performance rapidly deteriorates. However,

after about 2000 minutes the reaction curve for Cu/MnOx ap-

pears to approach a steady-state conversion with negligible

activity decay. The MnOx sample may approach a more active

steady-state conversion but more extensive testing is

required to be certain. Even though the Pt/MnOx #1 and

Ag/MnOx samples display superior activity throughout most

of the test period, extrapolation of the data in Figure 6-1

indicates that Cu/MnOx may be the optimal catalyst in a

long-term test.

The data in Figure 6-1 also depict an interesting com-

parison between the catalytic activities of Pt/MnOx #1

(dried for 4.5 hours at 280 C) and Pt/MnOx #2 (dried for 3

hours at 75 OC). The poor activity exhibited by Pt/MnOx #2

may be the result of incomplete removal of surface im-

purities (such as Na, Cl or OH-) associated with



























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the impregnation step. However, as found in previous

studies of MnOz and MnOz-CuO catalysts [104-1063, the inac-

tivity is most likely the result of incomplete surface ac-

tivation. Many MnOx-based catalysts usually require heat-

ing between 100-200 OC in air or Oz to produce an active

surface. The heat treatments apparently activate the sur-

face through the creation of reactive sites via partial

surface reduction, depletion of adsorbed water or surface

hydroxyl groups, and/or concurrent micropore generation.

An interesting observation is that the reaction

profiles of Pt/MnOx #1 and Ag/MnOx are remarkably similar.

This is unexpected based on the different catalytic

properties of Pt and Ag. It is possible that this behavior

results primarily from exposure of MnOx to similar basic

solutions followed by drying in air at 280 OC for 4.5

hours. The activity curve for pure MnOx (as-received and

common to both samples) is quite different in character,

which is consistent with this hypothesis. Nevertheless,

both materials performed well during the 10000-minute test

period oxidizing 70-80% of available CO at 75 OC.

As mentioned above, Pt/SnOx catalysts have received

considerable attention for use in COz lasers. A comparison

of CO oxidation performance between Pt/MnOx #1 (see Figure

6-1) and a commercial Pt/SnOx catalyst manufactured by

Engelhard Industries (see Chapter 4) is shown in Figure































































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6-2. At 75 OC, the Pt/MnOx #1 sample exhibits superior

activity after approximately 2500 minutes of reaction. Due

to the limited reaction data for the Pt/SnOx sample, fur-

ther comparisons require data extrapolation. Assuming that

the indicated trends continue, Pt/MnOx #1 represents the

optimal catalyst over an extended time period. This also

is true for Ag/MnOx which behaves identically to Pt/MnOx

#1.

Figure 6-2 represents a valid comparison because the

sample size and experimental parameters used were identical

in both tests. It should be noted, however, that the

Pt/SnOx was pretreated in a 5% CO/He stream at 225 OC for 1

hour prior to activity testing. As discussed in previous

chapters of this research, such reductive pretreatments

significantly enhance the performance of Pt/SnOx catalysts

[26]. The fact that no pretreatments were used for the

MnOx-based catalysts is an advantage. Furthermore, since

the precious metal loading for the Pt/MnOx #1 and Ag/MnOx

samples is only 0.2 wt% (compared with 2 wt% for the

Pt/SnOx catalyst), there also appears to be an economic ad-

vantage over the Engelhard Pt/SnOx catalyst. Of course,

the Ag/MnOx catalyst is the least costly.

The fact that reductive pretreatments activate Pt/SnOx

catalysts provided motivation to investigate the effects of

similar pretreatments on Pt/MnOx catalysts. Two










pretreatment conditions were used in which the Pt/MnOx #1

sample was exposed to 5% CO/He for 1 hour at 125 and

225 C. The effects on catalytic performance are shown in

Figure 6-3. It is clear that the pretreatments are

detrimental to the CO oxidation activity of Pt/MnOx. In

fact, the observed activity of Pt/MnOx decreases with in-

creasing pretreatment temperature, a trend opposite to that

which is observed for Pt/SnOx catalysts [261. A possible

explanation may involve the reducibility of the MnOx and

SnOx supports. It appears that catalysts based on these

materials require a certain degree of surface reduction for

optimal activity. There is evidence that a completely

dehydroxylated or an entirely oxygenated MnOx surface is

not active toward low-temperature CO oxidation [104,105].

Similarly, surface hydroxyl groups are believed to be in-

strumental in the CO oxidation mechanism over Pt/SnOx [26].

Given the relative instability of MnOx with respect to

SnOx, such an optimum degree of surface reduction most

likely results from milder pretreatments than those used to

generate the data shown in Figure 6-3. In fact, heat

treatments in air or Oz appear to be more beneficial for

MnOx CO oxidation catalysts [12,104,105,107,108]. Although

the CO reductive pretreatments at 125 and 225 OC are ap-

propriate for Pt/SnOx, they apparently are too severe for

Pt/MnOx.




Full Text

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81L\(56n-"6QQQ"O+?LO?OO fUL6% R7%%6



HIGH-PERFORMANCE CO OXIDATION CATALYSTS
ENGINEERED FOR COz LASERS
By
STEVEN DWAYNE GARDNER
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1990

ACKNOWLEDGMENTS
I wish to express my sincere thanks to Professor Gar
B. Hoflund whose instruction and guidance made this re¬
search possible. We have shared countless discussions
through which his knowledge and optimism have served to
motivate me both personally and professionally. I would
also like to acknowledge the efforts of David R. Schryer,
Billy T. Upchurch, Erik J. Kielin and Jacqueline Schryer at
NASA Langley Research Center who provided the catalytic ac¬
tivity measurements. As exemplified during several NASA-
sponsored workshops, their contributions have had an enor¬
mous impact on the quality of this entire research project.
Support of this research under NASA grant NAG-I-794 is also
gratefully acknowledged. Finally, I wish to thank my wife,
Carmen, for her encouragement and forbearance during the
seemingly endless years of graduate study. Throughout the
exhausting hours of research, she has been a constant
reminder of what is most important in my life.
ii

TABLE OF CONTENTS
page
ACKNOWLEDGMENTS i i
ABSTRACT iv
CHAPTERS
1 INTRODUCTION 1
COz Lasers in Remote Sensing Applications... 1
CO2 Lasers and Low-Temperature CO Oxidation
Catalysis 5
2 EVIDENCE OF ALLOY FORMATION DURING REDUCTION
OF PLATINIZED TIN OXIDE SURFACES 10
Introduction 10
Experimental 11
Results and Discussion 13
Summary 2 3
3 SURFACE CHARACTERIZATION STUDY OF THE REDUC¬
TION OF AN AIR-EXPOSED PtaSn ALLOY 24
Introduction 24
Experimental 25
Results and Discussion 26
Summary 3 7
4 EFFECT OF PRETREATMENT ON A PLATINIZED TIN
OXIDE CATALYST USED FOR LOW-TEMPERATURE
CO OXIDATION 3 9
Introduction 39
Experimental 41
Results and Discussion 42
Summary 61
5 CHARACTERIZATION STUDY OF SILICA-SUPPORTED
PLATINIZED TIN OXIDE CATALYSTS USED FOR
LOW-TEMPERATURE CO OXIDATION: EFFECT OF
PRETREATMENT TEMPERATURE 6 3
Ü1

Introduction 63
Experimental 65
Results and Discussion 66
Summary 80
6 CATALYTIC BEHAVIOR OF NOBLE METAL/REDUCIBLE
OXIDE MATERIALS FOR LOW-TEMPERATURE CO
OXIDATION: COMPARISON OF CATALYST
PERFORMANCE 8 2
Introduction 82
Catalyst Preparation 85
Experimental 87
Results and Discussion 88
Summary 105
7 COMPARISON OF THE PERFORMANCE CHARACTERISTICS
OF Pt/SnOx AND Au/MnOx CATALYSTS FOR LOW-
TEMPERATURE CO OXIDATION 107
Introduction 107
Experimental 108
Results and Discussion 110
Summary 121
8 CATALYTIC BEHAVIOR OF NOBLE METAL/REDUCIBLE
OXIDE MATERIALS FOR LOW-TEMPERATURE CO
OXIDATION: SURFACE CHARACTERIZATION OF
Au/MnOx 123
Introduction 123
Experimental 126
Results and Discussion 128
Summary 153
9 SUMMARY 154
APPENDIX DESCRIPTION OF THE ULTRAHIGH VACUUM SUR¬
FACE ANALYSIS SYSTEM 158
REFERENCES 162
BIOGRAPHICAL SKETCH 172
iv

Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
HIGH-PERFORMANCE CO OXIDATION CATALYSTS
ENGINEERED FOR COz LASERS
By
STEVEN DWAYNE GARDNER
December, 1990
Chairman: Gar B. Hoflund
Major Department: Chemical Engineering
The low-temperature CO oxidation activity of numerous
materials has been evaluated in order to develop efficient
catalysts for use in C02 lasers. The materials were
screened for activity in small, stoichiometric concentra¬
tions of CO and O2 at temperatures near 55 °C. An Au/MnOx
catalyst was synthesized which exhibited exceptional CO
oxidation activity while maintaining negligible performance
decay over a period of at least 70 days. The data suggest
that Au/MnOx has potential applications in air purification
and CO gas sensing as well.
Extensive surface characterization data from Pt/SnOx
and Au/MnOx catalysts are reported which relate surface
composition and chemical state information to corresponding
CO oxidation activity data. Ion scattering spectroscopy
(ISS), Auger electron spectroscopy (AES), angle-resolved
v

Auger electron spectroscopy (ARAES) and X-ray photoelectron
spectroscopy (XPS) were utilized to observe the behavior of
these surfaces as a function of numerous pretreatments
which alter their catalytic activity. The results suggest
that Pt(OH)z and Pt/Sn alloy formation may play a key role
in the CO oxidation mechanism on Pt/SnOx surfaces. A PtaSn
alloy was subsequently characterized before and after Hz
reduction to study its surface characteristics.
Surface characterization of Au/MnOx and MnOx was per¬
formed in order to elucidate the CO oxidation mechanism.
The spectral data yield evidence that the enhanced CO
oxidation activity of Au/MnOx is related to Mn present
primarily as MnaO* with substantial amounts of water or
hydroxyl groups. The spectra are consistent with very
small Au particles which may exist in an oxidized state.
The behavior of Au/MnOx and MnOx toward an inert pretreat¬
ment suggests the possibility of a Au-MnOx interaction.
vi

CHAPTER 1
INTRODUCTION
This research is a collection of studies whose main
objective is to deveiop 1ow-1empera ture CO oxidation
catalysts which can be effectively utilized in COz lasers.
The studies themselves represent systematic efforts to syn¬
thesize and evaluate new catalysts for this application and
to gain fundamental information about the low-temperature
CO oxidation mechanism. It will become apparent during the
course of the investigations, however, that the results
have potential applications beyond COz lasers.
In order to provide an understanding of the basis for
this research, this introductory section highlights impor¬
tant aspects of COz laser operation and identifies the need
for low-temperature CO oxidation catalysts. This is fol¬
lowed by a review which summarizes previous, related re¬
search on COz lasers utilizing CO oxidation catalysis. Af¬
terwards the individual studies themselves are presented.
The research concludes with a summary of the overall
results with recommendations for future research.
COz Lasers in Remote Sensing Applications
The advent of pulsed lasers has made it possible to
utilize pulsed laser energy in a radar fashion (laser radar
1

2
or lidar) with applications in altimetry, ranging, pollu¬
tion detection, atmospheric chemistry, and weather monitor¬
ing [1-5]. Recently, the National Aeronautics and Space Ad¬
ministration (NASA) has undertaken a project to develop a
laser atmospheric wind sounder (LAWS) which will incor¬
porate a COz Doppler lidar [6]. The LAWS system will
operate from a satellite platform in earth orbit and
measure global wind velocities for a period of up to three
years. It is anticipated that the data obtained from LAWS
will enhance the understanding of atmospheric dynamics and
aid in weather prediction. The same techniques may be
employed to detect dangerous wind shear near airports. A
COz lidar was chosen because the technology is mature and
the corresponding COz laser radiation is safe to the eye.
However, as described below, there are significant problems
which need to be solved before COz lidars can be operated
in space or other remote locations.
Carbon dioxide lasers are commonly operated in an
open-cycle mode. That is, fresh COz laser gas flows con¬
tinuously through the laser from an external storage tank
and is subsequently exhausted to the environment. Unat¬
tended operation for an extended period of time, therefore,
requires large, massive reservoirs of COz laser gas. Con¬
sequently, this method of operation is undesirable for COz
lidars which will be deployed in remote locations such as

3
earth orbit. An additional complication arises for COz
lidars which must transmit signals long distances through
the atmosphere. The atmosphere contains a significant con¬
centration of COz (approximately 300 parts per million) and
HzO both of which absorb COz laser radiation. As a result,
the COz lidar beam may experience considerable attenuation
as it propagates through the atmosphere [4,6]. A solution
is to use a COz laser gas which contains a COz isotope such
as C18Oz. Relative to a common-isotope COz (C16Oz) laser
which operates at a wavelength of 10.6 micrometers, a laser
containing C18Oz emits radiation at 9.1 micrometers which
interacts less with atmospheric constituents. However,
C180z is much more expensive than COz and therefore open-
cycle laser configurations which use C18Oz are not economi¬
cal .
An alternative to open-cycle COz laser operation is
known as closed-cycle operation [7]. A COz laser operating
in a closed-cycle mode is initially charged with a COz gas
mixture and subsequently sealed. This configuration is ad¬
vantageous because it eliminates the need for an external
COz storage tank and the COz gas mixture itself is con¬
served. Unfortunately, there is a serious problem with COz
lasers which precludes closed-cycle operation. During the
lasing process, COz in the laser gas is dissociated into
stoichiometric amounts of CO and Oz. The gradual loss of

4
COz steadily decreases the power output of the laser. Per¬
haps most critical, however, is the increase in Oz con¬
centration which can lead to arcing and ultimately to laser
failure. An overall solution is to incorporate a CO oxida¬
tion catalyst into the working confines of the COz laser to
recombine the CO and Oz into COz. Successful closed-cycle
operation is achieved by continuously recirculating the
laser gas across a suitable catalyst bed. The resulting
configuration is compact and portable and permits the COz
laser to be utilized in remote locations for an extended
period of time.
The requirements for an effective COz laser catalyst
are rather stringent [7,8]. An appropriate catalyst must
exhibit exceptional CO oxidation activity under the condi¬
tions which correspond to steady-state COz laser operation.
That is, the catalyst must maintain activity at tempera¬
tures near or below 50 °C in a gas mixture which contains
small, stoichiometric concentrations of CO and Oz and as
much as 50% COz. In addition, the use of Cl8Oz requires
that the catalyst preserve the integrity of the isotopic
laser gas mixture. The catalyst should also be resistant
to dusting which could damage the delicate optics within
the laser.

5
COz Lasers and Low-Temperature CO Oxidation Catalysis
Few catalysts have been researched and targeted
specifically for use in COz lasers. Ordinarily, CO oxida¬
tion catalysts are screened for activity using test gases
which consist of small concentrations of CO in air (excess
Oz). As a result, the performance of most CO oxidation
catalysts remains to be evaluated in the COz laser environ¬
ment. Excess Oz facilitates CO oxidation and therefore
numerous catalysts perform well under these conditions even
at low temperatures (less than 100 °C) [9-17]. However, CO
oxidation in stoichiometric concentrations of CO and Oz is
much more difficult. Consequently, under these conditions
only a limited number of catalysts are available which ex¬
hibit significant low-temperature CO oxidation activity.
The following paragraphs summarize previous research
directed towards testing and evaluating low-temperature CO
catalysts in COz laser environments.
Hopcalite (a mixed-oxide catalyst consisting of ap-
-U â– â–  1-
proximately 60% MnOz and 40% CuO with trace amounts of
other oxides) has been tested in a sealed COz laser with
limited success [18]. Appreciable CO oxidation activity
was realized only when excess CO (approximately 0.5%) was
introduced into the COz laser gas mixture. This is due
largely to the fact that the CO oxidation rate over
hopcalite is approximately first-order with respect to CO

6
concentration [11,12,19]. Contrary to the COz laser's low
tolerance for Oz, a small excess in CO concentration is not
harmful to the COz laser performance. The hopcalite
catalyst was able to sustain laser performance for a period
of up to 10s pulses with no indication of failure. In or¬
der to satisfy the requirements for LAWS, the laser must be
capable of at least 109 pulses [6]. Therefore, more exten¬
sive testing is needed to verify the performance of hop¬
calite. However, considering the activity decay which has
previously been observed for hopcalite catalysts [19], it
is doubtful that hopcalite will provide a sufficient
lifetime for the laser.
It has been shown that a heated Pt wire may be used to
recombine CO and Oz in atmospheres similar to those in
sealed COz lasers [20,21]. Activity measurements between
700 and 1300 °C indicated significant recombination rates
which were independent of the CO and Oz partial pressures.
Nevertheless, this overall approach to CO and Oz recombina¬
tion is undesirable in that it is fragile and requires ad¬
ditional energy input to heat the Pt catalyst. In addi¬
tion, the power dissipated in the laser may cause thermal
expansion of the laser components resulting in decreased
performance. It is much more advantageous to utilize a
catalyst which is effective near the steady-state

7
temperatures of the COz laser, particularly for space
applications where there are weight and power constraints.
Catalysts consisting of Pt and/or Pd supported on SnOx
(Pt/SnOx, Pd/SnOx, Pt-Pd/SnOx) have received considerable
attention with respect to CO oxidation in COz lasers
[7,8,22-26]. As a result, several interesting observations
have been made regarding their performance characteristics.
It has been demonstrated that optimum Pt/SnOx performance
results from Pt concentrations between 15 and 20 wt% [27].
Further enhancements in activity are realized by pretreat¬
ing the Pt/SnOx surfaces with CO or Ht at elevated tempera¬
tures (near 125 to 225 °C) [26]. It has also been deter¬
mined that relative to Pt/SnOx and Pd/SnOx alone, bimetal¬
lic catalysts consisting of Pt-Pd/SnOx yield better perfor¬
mance [27,28]. The performance of a 2 wt% Pt/SnOx catalyst
has been verified during actual closed-cycle COz laser
operation. Utilizing an external catalyst bed containing
150 grams of 2 wt% Pt/SnOx at 100 °C, the C02 laser main¬
tained 96% of its maximum power output for at least 28
hours with no indication of failure [29].
Although Pt/SnOx and Pd/SnOx appear to be promising
catalysts for the C02 laser application, they are not yet
totally compatible with long-term laser operation in space.
Isotopic reaction studies [30] have indicated that about
15% of the total amount of O2 which participates in CO

8
oxidation over Pt/SnOx originates from the Pt/SnOx surface
itself. This is likely to be the case for Pd/SnOx and Pt-
Pd/SnOx as well. Since these SnOx-based surfaces or¬
dinarily contain 16Oz, the isotopic integrity of a C18Oz
laser gas mixture would be destroyed. However, a procedure
has been developed to label Pt/SnOx with 180z yielding
Pt/Sn180x [30]. A preliminary test (lasting 17 days) has
indicated that C180 oxidation over the 1 8Oz-1 abe 11ed
Pt/SnOx catalyst yields essentially 100% C18Oz within ex¬
perimental error. Nevertheless, the tests need to be per¬
formed over longer periods to ensure isotopic com¬
patibility. The long-term performance characteristics of
Pt/SnOx, Pd/SnOx and Pt-Pd/SnOx represent another area for
concern. In the presence of stoichiometric CO and Oz near
ambient temperatures, all of these SnOx-based catalysts ex¬
hibit significant decay in CO oxidation activity over ex¬
tended time [27,28]. Consequently, their performance is
currently unacceptable in long-term COz laser applications
such as LAWS.
A study has recently evaluated the performance of a
sealed COz laser containing a Au catalyst [31] . The inter¬
nal wall of the laser's discharge tube was coated with a
thin, discontinuous Au film to recombine the products of
COz dissociation. The activity of the Au film appears to
result from the presence of atomic oxygen in the laser

9
discharge. However, the presence of a conductive Au film
in the laser amplification volume can create serious
problems under certain conditions. Care must be taken to
ensure that the Au film is sufficiently discontinuous (Au
islands) so as to avoid interference with the laser's
electrical requirements. In addition, the Au surface area
must be kept low to minimize stray reflection lasing which
reduces power output. Therefore, the disadvantages as¬
sociated with coating such a large surface with Au under
conditions which minimize (but not eliminate) the drawbacks
noted above render this approach highly cost-ineffective
and inefficient.
Considering the previous research on CO oxidation
catalysis in COz lasers, much of the present research has
been directed toward SnOx-based materials such as Pt/SnOx.
Chapters 1 through 4 describe surface characterization
studies of Pt/SnOx and Pt-Sn alloy surfaces. These studies
were performed to learn more about the low-temperature CO
oxidation mechanism over Pt/SnOx catalysts. Chapter 5
presents results of activity screening experiments designed
to identify new low-temperature CO oxidation catalysts for
the C02 laser application. Finally, Chapters 6 and 7
report activity and surface characterization data for a
Au/MnOx catalyst whose exceptional activity was identified
in the former screening experiments.

CHAPTER 2
EVIDENCE OF ALLOY FORMATION DURING REDUCTION
OF PLATINIZED TIN OXIDE SURFACES
Introduction
Platinized tin oxide surfaces (Pt/SnOz or Pt/SnOx)
have been proven to be efficient, low-temperature, CO
oxidation catalysts which makes them useful in COz laser
applications [7,8,23]. However, relatively little is un¬
derstood about the oxidation of CO over Pt supported on
reducible oxides. In order to understand the mechanism, it
is necessary to determine the surface composition, chemical
states of the surface species, and types of interactions
between the surface species. It also appears that surface
hydrogen has a large influence on the catalytic behavior of
these surfaces [26].
Alumina-supported Pt/Sn bimetallic catalysts are im¬
portant in hydrocarbon reforming processes [32-34]. The
addition of Sn generally leads to prolonged catalyst
lifetime through an improved resistance toward coke forma¬
tion. Furthermore, the reaction selectivity is enhanced
toward aromatization which results in an increase in the
reformate octane number.
10

11
It is important to understand the nature of the Pt/Sn
interaction in both the CO oxidation catalysts and the
reforming catalysts. This point has been controversial for
many years with regard to Pt/Sn reforming catalysts. Many
studies suggest that a Pt/Sn alloy is formed during reduc¬
tion [35,36] while others suggest that tin is present as an
oxide [37-42]. Recent studies by Davis and coworkers
[43,44] are particularly convincing in demonstrating that
an alloy forms during reduction using X-ray photoelectron
spectroscopy (XPS) and in-situ X-ray diffraction. The dif¬
fraction patterns indicate that the composition of the al¬
loy is PtSn, but other alloys such as PtsSn may form at
different metal loadings.
The purpose of the present study is to investigate the
Pt/Sn interaction on a platinized tin oxide surface similar
to those used in CO oxidation. Auger electron spectroscopy
(AES), ion scattering spectroscopy (ISS), and XPS have been
used to examine the platinized tin oxide surface before,
during and after reduction by vacuum annealing and after
exposure of the reduced surface to oxygen at room tempera¬
ture .
Experimental
A tin oxide film was prepared by thermal hydrolysis of
Sn4+ from a solution containing 3 M SnCl« and 1.5 M HC1
[45]. The solution was sprayed onto a titanium foil which

12
had been heated and maintained at 500 °C in air. The
resulting tin oxide film was about 5000 angstroms thick and
it contained significant amounts of hydrogen [46-50].
Platinum was deposited by impregnating the tin oxide film
with a saturated chloroplatinic acid solution. The film
was then rinsed with distilled water and calcined in air at
500 °C for one hour.
The sample was inserted into an ultrahigh vacuum (UHV)
system (base pressure of 10'11 Torr) which is described in
detail in the Appendix. The UHV system contained a double¬
pass cylindrical mirror analyzer (CMA) (Perkin-Elmer PHI
Model 25-270AR) used for AES, ISS and XPS. The AES experi¬
ments were performed in the nonretarding mode using a 3-
keV, 10-uA primary beam with a 0.2-mm spot diameter. The
ISS spectra were collected in the nonretarding mode using a
147°-scattering angle and pulse counting detection [51]. A
1-keV, 100-nA 4He* primary beam was defocused over a 1-cm-
diameter area to minimize sputter damage. The XPS spectra
were collected in the retarding mode using a 25-eV pass
energy and Mg Ka excitation. The sample was heated
radiantly using a tungsten filament and its temperature was
measured with an optical pyrometer.

13
Results and Discussion
An AES spectrum and an ISS spectrum taken from the as-
prepared surface are shown in Figure 2-1. The AES spectrum
exhibits peaks due to Pt, Sn, 0, Cl and C. The large Pt
feature at 64 eV indicates that the surface contains a high
concentration of Pt which is consistent with the ISS
spectrum shown in Figure 2-1(b). Several features appear
in the ISS spectrum. The Sn peak is a shoulder on the Pt
peak and the peak at 0.6 E/Eo is due to contamination
(probably Na). The secondary ions apparently desorb with a
threshold at 0.12 E/Eo. This suggests that charging is oc¬
curring on this surface which shifts the elemental features
to higher E/Eo than predicted by the binary collision model
[52]. A very small 0 peak is at 0.49 E/Eo and the peak at
0.28 E/Eo may be due to C. A crude estimate of the elemen¬
tal cross sections [53] and surface composition suggests
that the outermost layer of atoms contains about 60 at% Pt.
Next the sample was annealed under vacuum in incre¬
ments of 50 °C from 250 to 450 °C for 30 minutes at each
temperature. The ISS spectra taken after annealing at 300
and 450 °C are shown in Figures 2-2(a) and 2 — 2(b), respec¬
tively. Several changes have occurred after annealing the
as-prepared sample at 300 °C. The Sn shoulder has become a
more prominent peak and its size has increased relative to
the Pt peak. Charging is no longer apparent on this

KINETIC ENERGY (eV)
Figure 2-1. The (a) AES and
the as-prepared sample after
system and pumpdown.
(b) ISS spectra taken from
insertion into the vacuum
N(E)

G?
z
|i njrri]iii|ij[|iii[iiijiiT|ni|ni[iTr|
ISS
(a)
x 16
Pt
Liiiinliulmlinli ii luiluil iiiliiil.
0.1
0.5
E/En
0.9
TnrnTiTTnnnpnnmnTTinniTniTi
ISS
(b)
iiu
0.1
0.5
E/E.
0.9
Figure 2-2. The ISS spectra taken after annealing the
o o
the sample in vacuum at (a) 300 C and (b) 450 C.

16
surface so the peak positions are approximately those pre¬
dicted by the elastic binary scattering equation. Both the
contamination peak at 0.52 E/Eo and the 0 peak at 0.39 E/Eo
are reduced in intensity, and the C peak is no longer ap¬
parent. Furthermore, the background intensity is sig¬
nificantly decreased from that observed in Figure 2-1(b).
A possible explanation is discussed below.
The trends observed in annealing to 300 °C are con¬
tinued by annealing to higher temperatures. Figure 2-2(b)
shows that the surface formed after annealing at 450 °C is
very different from the as-prepared surface. In comparing
Figures 2-2(a) and 2-2(b), the Sn peak is increased with
respect to the Pt and is even more well defined. The con¬
taminant peak is no longer present and the O peak ap¬
parently is further decreased in intensity. Also, the in-
elastically scattered ions composing the background are
greatly decreased in intensity. Several similarities are
observed between the ISS spectrum shown in Figure 2-2(b)
and the spectra taken from PtaSn surfaces in previous
studies [see Reference 54, Figure 5(b)]. Both surfaces
produce comparable inelastic backgrounds and display
similar Pt, Sn and O features with regard to peak shapes
and relative peak heights. The small inelastic background
is characteristic of spectra taken from metallic surfaces
where the electron mobility is sufficiently high to rapidly

17
neutralize inelastically scattered ions. The fact that the
ISS spectrum taken from a reduced platinized tin oxide sur¬
face closely resembles an ISS spectrum taken from a Pt/Sn
alloy surface (rather than an ISS spectrum taken from a
nonreduced platinized tin oxide surface) supports the as¬
sertion that a Pt/Sn alloy forms during reduction of the
platinized tin oxide surface.
The AES spectrum shown in Figure 2-3(a) and the Sn 3d
XPS spectrum shown in Figure 2-3(b) were also taken from
the sample after annealing at 450 °C. Comparing Figures 2-
1(a) and 2-3(a) shows that the Pt-to-Sn peak height ratio
essentially remains constant at a value of 1.12 during the
annealing. However, the Sn-to-0 peak height ratio in¬
creases from 1.10 to 1.37 during annealing by either migra¬
tion of 0 below the near-surface region or desorption of 0
from the surface. These facts suggest an increased inter¬
action between Sn and Pt in the near-surface region. The
Sn 3d XPS spectrum shown in Figure 2 — 3(b) has a dominant
peak at a binding energy of 486.4 eV. This feature is due
to Sn present as an oxide, but it is difficult to determine
the precise nature of this oxide from the Sn 3d features as
discussed previously [32]. The arrows in Figure 2—3(b) in¬
dicate shoulders which are apparent at an approximate bind¬
ing energy of 484.5 eV. This binding energy suggests that

BINDING ENERGY (eV)
Cu
Z
Figure 2-3. The (a) AES spectrum and (b) XPS spectrum
O
taken after annealing the sample under vacuum at 450 C.

19
Sn is also present in metallic or alloyed form. Only the
oxidic feature at 486.4 eV is present before annealing the
sample so tin oxide is reduced to metallic or alloyed Sn
during the vacuum annealing, which is a reductive process.
A study by Paffett and Windham [55] provides evidence that
the form of the Sn is alloyed rather than metallic. They
deposited metallic Sn on a Pt(lll) surface and showed that
an alloy forms during annealing at or above 150 °C. The
450 °C annealing temperature used in the present study then
should result in alloy formation. Thus, the ISS, AES and
XPS results are all consistent with the assertion that a
Pt/Sn alloy is formed during the reduction of platinized
tin oxide.
Similar annealing treatments have been carried out on
tin oxide films which did not contain Pt [45,48,50,56]. In
these studies vacuum annealing to 450 °C or above did not
result in reduction of the tin oxide to metallic Sn. Water
of hydration or hydroxyl groups are removed from the sur¬
face by vacuum annealing and a portion of the SnOz is
reduced to SnO, but metallic Sn does not form. In fact, a
tin oxide surface which has been reduced by argon-ion sput¬
tering is actually enriched in 0 by vacuum annealing
through migration of bulk 0 to the surface [56]. These ob¬
servations demonstrate the important role which Pt plays in

20
reducing the Sn to metallic form which then alloys with the
Pt.
Next, the oxidative behavior of the surface was ex¬
amined by exposing the sample to 10'7 Torr of oxygen for 1
hour at room temperature. The ISS and XPS spectra taken
from this surface are shown in Figures 2-4(a) and 2 — 4(b),
respectively. The expanded 0 feature in the ISS spectrum
indicates that the amount of 0 in the outermost surface
layer of atoms increases by about 30% during the oxygen ex¬
posure. By comparison of Figures 2—2(b) and 2-4(a), it can
be seen that the oxygen exposure has increased the height
of the Sn peak with respect to the height of the Pt peak.
This behavior is characteristic of Pt/Sn alloys as
described in previous studies [54,57]. The differences be¬
tween the ISS spectra shown if Figures 2—2(b) and 2-4(a)
are quite similar to those obtained by exposing a PtaSn al¬
loy surface to low-pressure oxygen at room temperature [see
Reference 57, Figures 5(a) and 5(b)]. Also, there is a
very slight reduction in the size of the shoulders on the
XPS Sn 3d peaks. The extent of this reduction is so small
that it is necessary to hold an expanded version of Figure
2—3(b) over Figure 2-4(b) in order to observe the decrease
in the size of the shoulders. The nonreactive behavior of
this surface toward oxygen as detected by XPS is also
characteristic of Pt/Sn alloy surfaces [57,58]. Using XPS,

Figure 2-4. The (a) ISS and (b) XPS spectra taken after
exposing the 450 °C-reduced surface to 10 Torr of oxygen
for 1 hour at room temperature.

22
Cheung [58] found that Sn alloyed with Pt exhibits greater
resistance toward oxidation than metallic Sn. It appears
that the Pt/Sn alloy bond causes the Sn to be less suscep¬
tible to oxidation. Therefore, this finding also supports
the assertion that the shoulders observed in the XPS Sn 3d
spectra are due to alloyed Sn rather than elemental Sn, but
quantification of the rates of oxygen adsorption on a Pt/Sn
alloy and polycrystalline Sn has not yet been carried out.
It is interesting to compare the oxidative and reduc¬
tive properties of platinized titania (Pt/TiOz) with those
of platinized tin oxide presented in this study since both
titania and tin oxide are reducible, conductive oxides. A
recent study by Hoflund and co-workers [59] of platinized
titania shows that the outermost layer always exhibits a
large ISS peak due to O. Reduction of this surface results
in encapsulation of the Pt by TiO, and oxygen exposure
forms TiOz which migrates back off the Pt. This behavior
is quite different than that for the platinized tin oxide
surface for which the 0 lies underneath the Sn and Pt. It
is interesting to note that oxygen exposure of a polycrys¬
talline Sn surface results in incorporation of O beneath an
outermost layer of Sn atoms [60]. One possible explanation
for the dramatically different behavior of platinized tin
oxide and platinized titania is that tin oxide is more
readily reduced to metal than titania in the presence of

23
Pt. Reduction of a platinized tin oxide surface by anneal¬
ing under vacuum then results in the formation of a Pt/Sn
alloy rather than encapsulation of the Pt by reduced oxidic
support species as observed on platinized titania.
Summary
The Pt/Sn interaction at a platinized tin oxide
catalyst surface has been examined using ISS, AES and XPS
after reduction of the surface by vacuum annealing and ex¬
posure of the reduced surface to oxygen. After reduction
XPS shows that metallic or alloyed Sn is formed, AES shows
an increase in the Sn-to-0 peak height ratio and the ISS
spectrum becomes similar to that taken from a Pt/Sn alloy
surface. Furthermore, exposure to oxygen at room tempera¬
ture induces changes similar to those found for a PtsSn al¬
loy surface. These facts all support the assertion that a
Pt/Sn alloy is formed during reduction.

CHAPTER 3
SURFACE CHARACTERIZATION STUDY OF THE REDUCTION
OF AN AIR-EXPOSED PtaSn ALLOY
Introduction
Platinum/tin systems are important catalytic materials
for hydrocarbon reforming [33-40,61-66] and low-temperature
CO oxidation [7,8,22,23]. The presence of Sn alters the
catalytic behavior of Pt so it is important to understand
the nature of the Pt-Sn interaction. In the previous chap¬
ter it was shown that a Pt/Sn alloy forms when a platinized
tin oxide film is reduced by annealing in vacuum. This
result suggests the importance of characterizing Pt/Sn al¬
loy surfaces since these surfaces may be responsible for
the unique catalytic properties of Pt/Sn systems. Early
studies by Bouwman and co-workers [67-70] show that the
surface compositions of PtSn and PtaSn samples are readily
altered by ion sputtering, annealing in vacuum, exposure to
oxygen or exposure to hydrogen. More recent studies by
Hoflund and co-workers [54,57,71,72] have focused primarily
on the surface enrichment in Sn of PtaSn alloy surfaces
caused by annealing in vacuum or oxygen exposure. These
studies utilized angle-resolved Auger electron spectroscopy
(ARAES) [73], high-energy-resolution Auger electron
24

25
spectroscopy (HRAES) [74], scanning Auger microscopy (SAM),
ion scattering spectroscopy (ISS) and angle-resolved X-ray
photoelectron spectroscopy (ARXPS).
Relatively little work has been done on the reduction
of oxidized Pt/Sn alloy surfaces. The purpose of the
present study is to examine an air-exposed PtsSn alloy sur¬
face before and after reducing the surface by annealing
hydrogen. Results from ISS, ARAES and XPS are presented.
Experimental
Details of the method used to prepare the PtsSn sample
appear in a related study [71] . The sample was stored in
argon until use. It was inserted into the UHV system (see
Appendix) and sputter-cleaned until all C and 0 contamina¬
tion was removed. Then the sample was exposed to air and
reinserted into the vacuum system. After the oxidized
sample was characterized, it was moved into a preparation
chamber attached to the UHV system and reduced under 1 Torr
of hydrogen at 300 °C for 1 hour. The heating was carried
out using a special heating system [75] which did not ex¬
pose any hot spots to the hydrogen which would cause dis¬
sociation to atomic hydrogen. Thus, the effects of the
reduction are due only to exposure to molecular hydrogen.
The sample temperature was measured using an optical
pyrometer. After reduction the sample was moved back into

26
the UHV system without air exposure for further charac¬
terization .
The ARA.ES, ISS and XPS experiments were performed
using a Perkin-Elmer PHI Model 25-270AR double-pass
cylindrical mirror analyzer (CMA) containing an internal
electron gun and movable aperture as the charged particle
energy analyzer. The ARAES experiments were performed in
the nonretarding mode using a 3-keV, 10-uA primary beam
from an external, glancing incidence electron gun. The
electron beam incidence angle was approximately 20° off the
alloy surface plane. Rotation of the 90°-slotted aperture
allowed for selection of emission angles of 75° to obtain
more bulk-sensitive spectra and 20° to obtain more surface-
sensitive spectra. The ISS spectra were also collected in
the nonretarding mode using a 147°-scattering angle and
pulse counting detection [51]. Sputter damage was mini¬
mized through the use of a 1-keV, 100-nA 4He* primary beam
defocused over an area of about 1 cm2. Both survey and
high-resolution XPS spectra were collected with Mg Ka ex¬
citation in the retarding mode using 50- and 25-eV pass
energies, respectively.
Results and Discussion
An ISS spectrum taken after sample cleaning and air
exposure is shown if Figure 3—1(a). This spectrum contains
a predominant peak due to Sn and very small peaks due to Pt

Figure 3-1. The ISS spectra taken from the polycrystalline
Pt3Sn sample after (a) sputter cleaning and exposing to air
and (b) reducing the air-exposed sample under 1 Torr of
hydrogen for 1 hour at 300 C.

28
and 0. The 0 peak is shifted from its appropriate E/Eo
value of 0.4 to 0.5, the background is fairly large and no
ions are detected below an E/Eo of 0.26. These facts indi¬
cate that the surface is oxidic in nature and that charging
is occurring. This ISS spectrum is quite similar to that
taken from an oxygen-exposed Sn surface [60].
Annealing in hydrogen dramatically alters the composi¬
tion of the outermost layer of surface atoms as seen in the
ISS spectrum of Figure 3-1 (b). The Pt peak has grown in
size nearly to that of the Sn peak, the O peak is sig¬
nificantly decreased in size, the O peak appears at its
predicted E/Eo value of 0.4 indicating that charging is no
longer a problem and the inelastic background is reduced
indicating that the outermost surface layer is more metal¬
lic in nature. Figures 3-l(a) and 3—1(b) were collected
using the same instrument settings and are scaled ac¬
curately with respect to each other. Thus, the Sn peak
height does not change significantly during the reduction.
The ISS cross section of 0 is much smaller than that of Pt
[53] . This suggests that the decrease in the number of 0
atoms may roughly correspond to the increase in the number
of Pt atoms in the outermost surface layer. It is probable
that the 0 atoms are removed from the surface layer through
water formation during the reduction. Although
speculative, it is possible that the Pt which migrates to

29
the surface fills the vacancies left by the 0. The chemi¬
cal interaction between the Pt and hydrogen provides a
driving force for the Pt migration as discussed by Bouwman
and co-workers [67] much like the chemical interaction be¬
tween the Sn and 0 provides a driving force for Sn migra¬
tion to the surface during an oxygen exposure
[54,57,72,73]. These studies also demonstrate that anneal¬
ing a Pt/Sn alloy surface in vacuum rather than hydrogen
enriches the near-surface region in Sn and not in Pt. A
hydrogen environment is necessary to enrich the near¬
surface region in Pt.
Figure 3-2(a) shows a bulk-sensitive AES spectrum and
Figure 3-3(a) shows a surface-sensitive AES spectrum taken
from the cleaned and air-exposed sample. There are Sn, C
and 0 peaks appearing in these spectra but no peaks due to
Pt appear. This fact provides clear evidence that a fairly
thick (>30 angstroms) layer of tin oxide forms over the Pt-
rich region during the air exposure. The same conclusion
has been reached in a previous study by Hoflund and Asbury
[72] using angle-resolved XPS. The O/Sn peak-height ratios
in Figures 3-2(a) and 3-3(a) are 0.61 and 0.58, respec¬
tively. These comparable values indicate that the composi¬
tion of the tin oxide layer is fairly uniform with depth.
The ratios are also typical of those obtained from tin
oxide surfaces [50]. The C peak is probably due to

100 300 500
100 300 500
KINETIC ENERGY (eV)
KINETIC ENERGY (eV)
Figure 3-2. Bulk-sensitive AES spectra taken from the
(a) air-exposed and (b) reduced Pt3Sn surfaces.

100 300 500
KINETIC ENERGY (eV)
100 300 500
KINETIC ENERGY (eV)
Figure 3-3. Surface-sensitive ARAES spectra taken from
the (a) air-exposed and (b) reduced Pt3Sn surfaces.

32
adsorption of hydrocarbons during the air exposure and al¬
ways appears after exposing these samples to air.
Figures 3-2(b) and 3 — 3(b) show the bulk-sensitive and
surface-sensitive AES spectra, respectively, taken after
the reduction. Two changes due to the reduction are ob¬
served by comparing the bulk-sensitive AES spectra shown in
Figures 3 — 2(a) and 3 — 2(b). The O/Sn ratio is substantially
reduced and Pt migrates to the near-surface region during
the reduction. Both of these facts are consistent with the
ISS spectrum shown in Figure 3-1(b) even though the bulk-
sensitive AES probes more deeply beneath the surface than
ISS. Comparing the surface-sensitive spectrum shown in
Figure 3 — 3(b) and the bulk-sensitive spectrum shown in
Figure 3-2 (b), it is seen that more Pt and less O are con¬
tained in the near-surface region than at greater depths.
This observation supports the assertion that Pt replaces 0
which has been removed during the reduction. It is also
possible to qualitatively describe the depth composition
profiles of the Pt and O from these spectra. Initially,
the 0 had a fairly uniform concentration through the oxidic
region (of greater depth than that probed by the bulk-
sensitive AES). After the reduction the O concentration is
low at the surface and increases at greater depths. The Pt
is more concentrated at the surface and less so at greater
depths. It is seen that the increase in Pt and decrease in

33
0 in the near-surface region is large by comparing the
surface-sensitive spectra shown in Figures 3-3(a) and 3-
3(b) .
The XPS survey spectra taken before and after the
reduction are shown in Figures 3-4(a) and 3-4(b), respec¬
tively. Peaks due to Sn, 0, Pt, C, and Ta are observed in
the spectra. The Ta peak appears because a Ta strip was
used to hold the sample on the mounting block. As observed
by the presence of the large Pt 4f peaks in Figure 3-4(a),
XPS probes more deeply than bulk-sensitive AES. This Pt
signal originates from a Sn-depleted region which lies
beneath the tin oxide layer [57,72]. After the reduction
the Pt peaks are increased in size and the height of the 0
Is peak is decreased. These results are consistent with
both the ISS and AES spectra even though XPS probes more
deeply than the other two techniques.
High-resolution XPS spectra of the Sn 3d, Pt 4f and 0
Is features before (A(a), B(a) and C(a), respectively) and
after reduction (A(b), B(b) and C(b), respectively) are
shown in Figure 3-5. The Sn 3d peaks shown in A(a) have a
binding energy of 486.4 eV which corresponds to that of
oxidic Sn (SnO, SnOz or Sn hydroxides) [32,50]. However,
these peaks are broad and contain a shoulder at about 485.0
eV which is due either to alloyed or to metallic tin lying
beneath the oxide layer [72]. After the reduction most of

Figure 3-4. The XPS survey spectra taken from the (a) air-
exposed and (b) reduced Pt3Sn surfaces.

Figure 3-5. High-resolution XPS spectra taken from the (a)
air-exposed and (b) reduced Pt3Sn surfaces showing the (A)
Sn 3d peaks, (B) Pt 4f peaks and (C) 0 Is peak. The shoulder
emphasized by the dotted line in A(b) is oxidic Sn.
U>
Ln

36
the Sn is in the form of alloyed or metallic Sn as
evidenced by the peak position of about 485.3 eV. This
value is larger than the Sn 3d binding energy of 484.6 eV
for metallic Sn. Since a detailed deconvolution of these
peaks has not been attempted in this study, a rigorous as¬
signment of binding energies to species is not made. Fur¬
ther work on this topic is in progress. A higher binding
energy shoulder at approximately 486.4 eV (shown as a
dotted line) is also present after the reduction. This is
consistent with the AES spectra shown in Figures 3—2(b) and
3-3(b), which show that most of the 0 lies beneath the sur¬
face .
The Pt 4f peaks shown in Figure 3-5B(a) lie at a bind¬
ing energy of 71.5 eV which is about 0.6 eV larger than
that of metallic Pt. The width and shapes of these peaks
suggest that only one chemical form of Pt contributes to
this spectrum. As discussed above, this Pt is probably an
Sn-depleted alloy, but the reason for the binding energy
difference from metallic Pt is not currently understood.
After reduction the Pt 4f binding energy is 71.7 eV.
Again, it appears that only one chemical form of Pt is
present. One possibility is that the Sn-depleted Pt alloy
has a binding energy of 71.5 eV and that this binding
energy increases to 71.7 eV as the Sn concentration in¬
creases after reduction of the alloy. Two studies [55,58]

37
suggest that the Sn and Pt peaks obtained from an alloy are
shifted to higher binding energy. This and other pos¬
sibilities are being investigated.
Figure 3-5C(a) shows the XPS O Is peak taken from the
air-exposed alloy surface and Figure 3-5C(b) shows the 0 Is
peak taken from the reduced alloy surface. The width and
shape of the peak in Figure 3-5C(a) indicates that two
forms of 0 are present on the air-exposed sample: an
oxidic form with a binding energy at 530.5 eV and hydroxyl
groups with a binding energy at about 531.5 eV. The reduc¬
tion process removes the hydroxyl groups leaving only the
oxidic form beneath the surface.
Summary
An air-exposed, polycrystalline PtaSn surface has been
examined using ISS, ARAES and XPS before and after reduc¬
tion by heating at 300 °C under 1 Torr of hydrogen for 1
hour. Before reduction the surface was covered with a
fairly thick (about 40 angstroms), uniform layer of tin
oxide and hydroxide. Beneath this layer was a Pt-rich
region. The ISS data shows that the outermost atomic layer
contains mostly Sn and 0 with only a very small amount of
Pt present.
Reduction results in loss of 0 and enrichment in Pt of
the near-surface region. The ISS data shows (1) that the
outermost atomic layer is strongly enriched in Pt through

38
migration of Pt to the surface, (2) that the outermost
atomic layer becomes more metallic and (3) that 0 is
removed from the outermost atomic layer during the reduc¬
tion. The ARAES data suggest that the Pt migrates to the
surface from the Pt-rich region by moving through vacancies
left by the 0 which reacted with the hydrogen and desorbed,
thus strongly enriching the outermost layer in Pt. The XPS
data shows that most of the tin oxide is reduced to metal¬
lic form and probably is present as alloyed Sn. The reduc¬
tion results in loss of hydroxyl groups but a subsurface,
oxidic form remains under the reductive conditions used.
It is likely that reduction under the conditions used in
this study but for longer periods would result in complete
removal of the 0 leaving a Pt-rich alloy at the surface.

CHAPTER 4
EFFECT OF PRETREATMENT ON A PLATINIZED TIN OXIDE
CATALYST USED FOR LOW-TEMPERATURE CO OXIDATION
Introduction
It has been observed that the performance of
platinized tin oxide catalysts used for low-temperature CO
oxidation can be enhanced through reductive pretreatments
prior to reaction [26]. For the pretreatment and reaction
conditions used (see Figure 4-1), the unpretreated catalyst
exhibits the lowest long-term activity. Pretreatment in CO
at 100 °C greatly enhances the long-term activity, and a
further enhancement occurs following CO pretreatment at
125 °C. Utilizing higher pretreatment temperatures up to
225 °C yields no change in the catalytic activity compared
to the pretreatment at 125 °C. In order to understand how
platinized tin oxide catalysts function in converting CO
and 0? to COz at low temperatures, it is necessary to
characterize the composition and chemical states of the
species present at these surfaces. This has been ac¬
complished in this study using several surface charac¬
terization techniques, including ion scattering spectros¬
copy (ISS), X-ray photoelectron spectroscopy (XPS) and
Auger electron spectroscopy (AES) to examine the surface of
39

Figure 4-1. Catalytic activity of the Engelhard Pt/SnOx
catalyst for CO oxidation as a function of time and
pretreatment temperature.

41
platinized tin oxide before and after pretreatment in CO as
a function of the pretreatment temperature. It is an¬
ticipated that these studies will lead to a better under¬
standing of this catalytic process and eventually to the
development of improved catalysts for this application.
Experimental
The platinized tin oxide catalyst was acquired from
Engelhard Industries in powdered form. It has an average
particle size of 1 urn and a BET surface area of 6.9
mz/gram. For the characterization studies, the powder was
pressed into thin disks approximately 1 cmz in diameter.
These were inserted directly into the ultrahigh vacuum
(UHV) system (see Appendix) for pretreatment and charac¬
terization. Pretreatments were performed by moving the
sample into a preparation chamber (base pressure of 10"7
Torr), introducing CO at a pressure of 40 Torr and heating
the sample at the prescribed temperature for 1 hour. Then
the sample was allowed to cool while the preparation cham¬
ber was pumped down to 10"7 Torr before moving the sample
into the main analytical chamber (base pressure of 10"11
Torr) for analysis by ISS, XPS and AES. The samples were
heated at 75, 100, 125 and 175 °C in the preparation cham¬
ber using a custom sample heater [75], which did not expose
the reducing gas to hot spots that would have dissociated
Two different as-prepared samples were analyzed
the CO.

42
without prereduction and the results were found to be
reproducible. A new sample was prepared and introduced
into the analysis chamber for each reduction temperature
studied.
The ISS, XPS and AES data were taken using a double¬
pass cylindrical mirror analyzer (CMA) (Perkin-Elmer PHI
Model 25-270AR) as the charged-particle energy analyzer.
The ISS spectra were collected in the nonretarding mode
using a 147°-scattering angle and pulse counting detection
[51]. A 1-keV, 100-nA 4He+ primary ion beam was defocused
over a 1-cm-diameter area and spectra were collected as
quickly as possible (typically 90 s) to minimize damage
from sputtering. The AES experiments were performed in the
nonretarding mode using a 3-keV, 10-uA primary electron
beam with a 0.2-mm spot diameter. The XPS experiments were
performed in the retarding mode using Mg Ka excitation.
Survey XPS spectra were collected using a 50-eV pass energy
whereas a 25-eV pass energy was utilized for obtaining
elemental lineshape information.
Results and Discussion
An XPS survey spectrum taken from the as-received
Engelhard platinized tin oxide catalyst is shown in Figure
4-2. The peaks of significant size are due only to 0 and
Sn, and no peaks due to C or other typical contaminants ap¬
pear. In fact, this spectrum is essentially identical to

Figure 4-2. The XPS survey spectrum taken from the as-
received Engelhard Pt/SnOx catalyst.

KINETIC ENERGY (eV)
Figure 4-3. The AES spectrum taken from the as-received
Engelhard Pt/SnOx catalyst.

45
an XPS spectrum obtained from a very clean tin oxide sur¬
face. It is most interesting that the predominant Pt 4f
peaks are so small that they cannot be discerned readily in
this spectrum. A corresponding AES spectrum taken from the
same surface is shown in Figure 4-3. Again, the
predominant features are due to Sn and 0, and this spectrum
is similar to one obtained from a clean tin oxide surface.
However, a feature due to Pt appears at a binding energy
near 64 eV. It can be described as an edge rather than a
typically shaped AES peak. Similar Pt features have been
observed in a study of the electro-chemisorption of Pt on
tin oxide [76]. This feature is more prominent than the Pt
XPS peaks in Figure 4-2. In agreement with the XPS data,
no contaminant peaks are apparent in this AES spectrum. As
the catalyst is reduced at various temperatures, very small
changes are observed in the XPS and AES spectra correspond¬
ing to Figures 4-2 and 4-3, but taken from the reduced sur¬
faces. Therefore, these survey spectra are not shown.
Ion scattering spectroscopy is a particularly useful
technique for examining catalytic surfaces because it is
very highly surface sensitive (outermost one or two atomic
layers). The ISS spectra taken before (a) and after (b-e)
reduction are shown in Figure 4-4. The spectrum shown in
(a) consists of peaks due to 0, Sn and Pt, and the high in¬
elastic background is characteristic of ISS spectra taken

E/Eo
Figure 4-4. The ISS spectra taken from (a) an unpretreated
sample and samples reduced at (b) 75, (c) 100, (d) 125 and
(e) 175 C in 40 Torr of CO for 1 hour.

47
from nonmetallic surfaces. In such a case, inelastical ly
scattered ions are not efficiently neutralized since the
electron mobility at a non-metallic surface is so low that
these ions contribute to the background signal. A previous
study by Asbury and Hoflund [60] showed that O penetrates
beneath the surface during room-temperature oxygen exposure
of polycrystalline Sn. This suggests that the fairly large
O peak in (a) is due to O associated with the Pt and/or
perhaps with hydroxyl groups attached to Sn that may lie
above the surface [50] .
The ISS spectra shown if Figure 4-4(b-e) were obtained
from samples reduced in 40 Torr of CO for 1 hour at 75,
100, 125 and 175 °C respectively. All four spectra have
two characteristics in common. Firstly, the pretreatments
have resulted in negligible inelastic backgrounds, which is
indicative of the formation of surfaces with a metallic na¬
ture. Secondly, the O peak is no longer discernible after
the reductions, which is also consistent with the observa¬
tion that the surfaces appear to be metallic. The reduc¬
tive pretreatment results in an increase in the Sn/Pt
ratio, and the extent of this increase is greater at higher
reduction temperatures. As shown in chapter 1, an increase
in the ISS Sn/Pt ratio during reduction of platinized tin
oxide surfaces has been found to be indicative of alloy
formation. All of the ISS spectra shown in Figure 4-4 were

48
taken using the same instrument settings, but the maximum
peak heights vary considerably. Although the variation is
not understood, it could be due to changes in ion
neutralization probability, surface morphological changes
or changes in the concentration of surface hydrogen, which
have been shown to alter the ISS signal strength [77,78].
The Sn 3d XPS spectra and Sn MNN AES spectra taken
before and after reduction are shown in Figures 4-5 and 4-
6, respectively. Before pretreatment (air-exposed sample)
the Sn 3ds/z lineshape and peak position (486.4 eV) indi¬
cate that Sn is present in the +2 or +4 oxidation states
most likely as SnO, Sn(OH)z, SnOz or Sn(OH)« and that
metallic Sn is absent. As discussed by Hoflund and co¬
workers [32], it is not possible to distinguish between
these species based on the XPS Sn 3d peaks, but more
specific information can be gained about these species
using electron energy-loss spectroscopy (ELS) [48,56],
valence band XPS [48,50], electron-stimulated desorption
(ESD) [49,50] or secondary ion mass spectrometry (SIMS)
[46,47,79]. The metallic XPS Sn 3ds/z peak appears at an
energy of 484.6 eV [80]. When a small amount of metallic
Sn and a relatively large amount of tin oxides or
hydroxides are present, a slight broadening appears on the
low-binding-energy sides of the oxidic XPS Sn 3d features.
This is the case for the 175 °C-reduced sample as shown in

BINDING ENERGY (eV)
Figure 4-5. The XPS Sn 3d peaks obtained from (a) an
unpretreated sample and (b) a sample reduced at 175 C in
40 Torr of CO for 1 hour. The circled regions indicate
crossing points.

Figure 4-6. The AES Sn MNN peaks obtained from (a) an
unpretreated sample and samples reduced at (b) 75, (c) 100,
(d) 125 and (e) 175 °C in 40 Torr of CO for 1 hour.
Ln
o

51
Figure 4-5. Samples reduced at lower temperatures also
yield this metallic shoulder, and the amount of metallic Sn
produced generally increases as the annealing temperature
increases with the most being produced at 175 °C. However,
the amounts of metallic Sn produced at 75 and 100 °C ap¬
pear similar. Similar observations can be made by con¬
sidering the AES Sn MNN peaks shown in Figure 4-6. The
high-kinetic-energy oxidic peak lies at 430 eV, while the
high-kinetic-energy metallic peak lies at 423 eV. The
peaks shown in Figure 4-6(a) are characteristic of oxidic
Sn. When metallic Sn is present, the height of the split¬
ting between the two primary peaks decreases. This is ob¬
served in spectra (b)-(e) taken from the reduced samples.
In agreement with the XPS Sn 3d spectra, the extent of
metallic Sn formation is greater at elevated reduction tem¬
peratures with the maximum amount of metallic Sn being
produced at 175 °C. Further reduction, either for pro¬
longed periods or at higher temperatures than used in this
study, would undoubtedly result in the production of in¬
creased amounts of metallic Sn.
The XPS 0 Is peaks are shown in Figure 4-7. These
were taken before (a) and after (b) reduction at 175 °C.
The peak shown in (a) exhibits a distinct asymmetry on the
high-binding-energy side due to the presence of hydroxyl
groups, which are responsible for a shoulder at

Figure 4-7. The XPS O Is peaks obtained from (a) an
unpretreated sample and (b) a sample reduced at 175 °C
in 40 Torr of CO for 1 hour.

53
approximately 531.8 eV. It is also possible that the very
small shoulder near 533.0 eV is due to adsorbed water.
This assignment is consistent with results obtained in the
study of hydrated polycrystalline tin oxide films by
Tarlov and Evans [81]. Pretreatment at 75 °C slightly
reduces the amounts of adsorbed water and hydroxyl groups
present while pretreatment at 100 °C or above eliminates
the adsorbed water and further reduces the concentration of
hydroxyl groups. The 175 °C pretreatment results in the
lowest surface hydroxyl group concentration. It is inter¬
esting to note that these hydroxyl species are strongly
bonded to the surface and require annealing at 600 °C in
vacuum for nearly complete removal [48,50,82].
The O content of the near-surface region is decreased
by the pretreatment process. The AES and XPS O/Sn atomic
ratios obtained at the various pretreatment temperatures
are listed in Table 4-1. A decrease in the O/Sn ratio is
caused by loss of adsorbed water, a decrease in hydroxyl
group concentration, reduction of tin oxides and hydroxides
to metallic Sn and reduction SnOz to SnO. However, the
relative importance of these factors cannot be assessed
completely from the types of data taken in this study. The
amount of Pt present on these surfaces is so small that
changes in the Pt oxidation state, which are discussed
below, would not affect the O/Sn atomic ratios presented in

Table 4-1. The O/Sn atomic ratios versus pretreatment
temperature.
a
b
Pretreatment
AES
XPS
Untreated
1.32
1.56
75 C
1.08
1.45
100 °C
1.05
1.39
125 °C
1.04
1.33
175 °C
1.05
1.21
'calculated using methods described in
reference 128.
b , ,
Calculated using methods described in
reference 80.

55
Table 4-1. The AES and XPS results in Table 4-1 are dif¬
ferent with respect to both magnitude and trend. The AES
data indicate that the near-surface region loses about 20%
of its 0 regardless of the reduction temperature. The XPS
O/Sn atomic ratios are considerably larger than the AES
values and decrease monotonically as the pretreatment tem¬
perature increases. Either the XPS values differ from the
AES values due to inaccuracies in the tabulated cross sec¬
tions or the variation is due to the fact that AES and XPS
probe different volumes of the near-surface region. If the
difference were due only to inaccurate cross sections, then
the XPS O/Sn ratios obtained from the reduced samples would
not show such a large variation with reduction temperature.
Thus, the variation is due to the fact that XPS probes more
deeply than AES, which is consistent with mean-free-path
arguments also. The kinetic energies of the XPS 0 Is and
Sn 3d electrons are 723 eV and approximately 770 eV,
respectively, while the kinetic energies of the AES 0 and
Sn electrons are near 510 eV and 430 eV, respectively. The
corresponding mean free paths (x) are about 6 angstroms for
XPS and 4 angstroms for AES [83] . Since most of the XPS
and AES electrons originate within a depth of approximately
3(t), XPS probes about 18 angstroms beneath the surface
while AES probes at a depth of about 12 angstroms. Then,
the data in Table 4-1 indicate that the near-surface region

56
probed by AES contains less O than the region probed by XPS
for the untreated sample and samples reduced at 75-175 °C.
Reduction at any of the temperatures used causes the AES
O/Sn ratio to drop from 1.32 to about 1.06 whereas the XPS
O/Sn ratio decreases monotonically as the reduction tem¬
perature increases. The essentially constant AES value
probably results from competing processes, that is, 0 leav¬
ing the surface as COz during the reduction and 0 migrating
to the near-surface region from farther beneath the sur¬
face. This is consistent with the trend of the XPS data
implying that subsurface reduction takes place to a greater
extent at higher temperatures. A similar phenomenon has
been observed previously for the reduction of a TiOz (001)
surface [84].
The XPS Pt 4f peaks obtained from the unpretreated
samples are shown in Figure 4-8(a), and the peak assign¬
ments used in this study are listed in Table 4-2. Most of
these assignments were taken from a standard reference
[80], but the 72.3 eV feature has been assigned as Pt-O-Sn
in previous studies of platinized tin oxide surfaces
[32,56]. The spectrum shown in Figure 4-8(a) indicates
that very little metallic Pt is present and that the Pt
species consist mostly of Pt-O-Sn, Pt(OH)z and Pt oxides.
In fact, a spectrum very similar to this one has been taken

Figure 4-8. The XPS Pt 4f peaks obtained from (a) an
unpretreated sample and samples reduced at (b) 75, (c)
100, (d) 125 and (e) 175 °C in 40 Torr of CO for 1 hour.

Table 4-2. The XPS Pt 4f peak assignments.
Species
Binding Energy (eV)
Pt° (bulk)* 70.9
Pt° (crystalline)6 71.3
Pt-0-Snb 72.3
Pt (OH) 2° 72.8
Pto“ 74.2
Pt02“ 74.9
a
Assignments taken from Reference 80.
>
Assignments taken from References 32 and 89.

59
from a co-deposited platinum/tin oxide film after calcining
in air at 725 K for 1.5 hours [32].
The XPS Pt 4f spectra taken after pretreating at 75
and 100 °C are shown in Figures 4-8(b) and 4-8(c), respec¬
tively. These two spectra are quite similar in that very
little metallic Pt is present and the predominant species
consist of Pt-O-Sn and Pt(OH)z. The metallic Pt apparently
is in the form of small crystallites. A shoulder due to
PtOz may also be present in both spectra, but this shoulder
is smaller after the 100 °C reduction. The relative
amounts of Pt-O-Sn and Pt(OH)z are similar in both spectra.
Pretreating at a temperature 25 °C higher produces a sig¬
nificant change in the Pt species present, as shown in
Figure 4 — 8(d). The predominant Pt species after the 125 °C
reduction are Pt(OH)z and Pt-O-Sn in approximately equal
amounts. Also, a small amount of metallic Pt is present
after this pretreatment, and features due to Pt oxides do
not appear.
Reduction at 175 °C produces a further and more
pronounced shift in the Pt chemical state toward Pt(OH)z as
shown in Figure 4 — 8(e) . The Pt-O-Sn feature is now a
shoulder on the Pt(OH)z peak, and the amount of metallic Pt
present is decreased compared to the lower temperature
reductions. The spectrum shown in Figure 4-8(a), taken
from the unpretreated sample, exhibits a prominent feature

60
at 79.8 eV binding energy. Since the splitting between the
Pt 4 f 7 / z and Pt 4fs/z peaks is approximately 3.35 eV, this
feature does not correspond to the Pt 4fs/2 peak of any of
the species listed in Table 4-2. The 75 °C pretreatment
reduces the size of this feature, and it does not appear
after the 100 or 125 °C pretreatment. Therefore, it be¬
haves like an oxidic feature and reappears in Figure 4-
8(e) .
It is interesting to compare the spectral information
contained in Figure 4-8 with the kinetic information con¬
tained in Figure 4-1. The nonpretreated catalyst exhibits
a low activity compare to any reduced catalyst. This
correlates with the facts that the Pt is predominantly
oxidic on the unpretreated catalyst and that significant
changes in the Pt chemical state occur during the reductive
pretreatments used in these studies. As the pretreatment
temperature increases, more of the Pt is converted into
Pt (OH) z . This fact suggests that the Pt(OH)z plays an im¬
portant role in the conversion of CO and O2 into COz at
low temperatures. In this discussion a relationship is
being drawn between the long-term catalytic behavior (past
the first 500 minutes of reaction) and the chemical state
of the Pt after the pretreatment but before the onset of
reaction (time=0 in Figure 4-1). The initial catalytic be¬
havior is quite complex [26], and changes may occur in the

61
Pt chemical state during this period. Consequently,
characterization studies are in progress in which the state
of the catalytic surface is being examined as the reaction
is run for various periods of time. These studies should
lead to an understanding of the chemical changes respon¬
sible for the unusual, initial catalytic behavior and the
long-term decay in catalytic activity.
A small amount of metallic Pt forms during both the
125 and 175 °C pretreatment, and metallic Sn is also
present as stated above. Paffett and Windham [55] have
deposited layers of Sn on Pt(lll) and found that alloy for¬
mation during annealing at or above 150 °C is strongly sug¬
gested by their data. Also, Fryberger and Semancik [85]
have found that Pd deposited on Sn02 (110) alloys with the
Sn even at room temperature. It is anticipated that alloy
formation occurs in the Engelhard catalyst under the
pretreatment conditions used in this study. Since the
amount of Pt present on this surface is small, it is likely
that most of the metallic Sn is not alloyed while probably
all of the Pt is alloyed.
Summary
A platinized tin oxide catalyst (commercially avail¬
able from Engelhard Industries for low-temperature CO
oxidation) has been examined using surface analytical tech¬
niques, including ISS, AES and XPS, before and after

62
pretreatment by annealing in 40 Torr of CO for 1 hour at
75, 100, 125 and 175 °C. The results have been correlated
with catalytic activity data. The nonpretreated sample
consists primarily of oxidic Sn (SnOz , SnO and Sn(OH)x)
with a very small amount of Pt present as Pt-O-Sn, Pt(OH)z,
PtO and Pto2 species. Reduction results in loss of both O
and OH" from the Sn and produces metallic Sn. The extent
of these processes increases as the pretreatment tempera¬
ture increases. The chemical state of the Pt changes with
pretreatment temperature. At or below 100 °C, the
predominant forms are Pt-O-Sn and Pt(OH)z. As the reduc¬
tion temperature increases, more Pt(OH)2 forms. This fact
suggests that Pt(OH)2 plays an important role in the low-
temperature catalytic oxidation of CO. The results
demonstrate that the application of surface analytical
techniques in studies of real catalysts can provide infor¬
mation that is useful in understanding catalytic behavior.

CHAPTER 5
CHARACTERIZATION STUDY OF SILICA-SUPPORTED PLATINIZED
TIN OXIDE CATALYSTS USED FOR LOW-TEMPERATURE CO
OXIDATION: EFFECT OF PRETREATMENT TEMPERATURE
Introduction
Research has indicated that platinized tin oxide
(Pt/SnOx) is an efficient CO oxidation catalyst at condi¬
tions which correspond to steady-state COz laser operation
[7,8,23,28]. As discussed in the previous chapter, the
performance of Pt/SnOx may be further enhanced following
pretreatment in CO at elevated temperatures. However, for
CO reduction temperatures above about 175 °C, a sharp, tem¬
porary decrease in activity is observed initially [26].
This induction period is believed to be the result of sur¬
face dehydration caused by combination of surface hydroxyl
groups and desorption of water. No significant induction
period results when Pt/SnOx is humidified either after CO
pretreatment or during the reaction itself and optimum
pretreatment times exist which are consistent with the
hypothesis involving surface dehydration. The data ob¬
tained thus far suggest that COz is produced through decom¬
position of a bicarbonate species formed from adsorbed CO,
a surface hydroxyl group and lattice oxygen [86].
63

64
Acknowledging the importance of surface hydroxyl
groups, there has been considerable effort directed towards
the development of a platinized tin oxide catalyst which is
supported on a silica substrate (Pt/SnOx/SiOz) [27,87].
Hygroscopic silica may improve the performance of Pt/SnOx
surfaces by preventing extensive surface dehydration and
consequent activity loss. Experiments have been conducted
wherein the catalyst performance has been optimized with
respect to several variables including pretreatment proce¬
dures [27]. Superior performance is realized by using a
reductive pretreatment in 5% CO/He at 125 °C for 1 hour.
As pretreatment temperatures approach 250 °C, there is a
significant decrease in the observed activity. The op¬
timized Pt/SnOx/SiOz catalyst represents a significant im¬
provement over commercially available Pt/SnOx with respect
to low-temperature CO oxidation activity and performance
decay.
In order to understand these experimental observa¬
tions, a characterization study has been carried out on
silica-supported Pt/SnOx before and after reduction in CO
at 125 and 250 °C. This study utilized ion scattering
spectroscopy (ISS) and X-ray photoelectron spectroscopy
(XPS) to examine the concentrations and chemical states of
species present at these surfaces and the changes which oc¬
cur during reduction.

65
Experimental
The catalyst prepared for this study consisted of a
thin layer of platinum and tin oxide dispersed on a silica
gel substrate [27,87]. The silica gel was impregnated with
tin oxide by evaporation of a stirred solution of tin metal
powder and silica gel in concentrated nitric acid at
150 °C. Subsequently, Pt was precipitated from an aqueous
solution of platinum tetraamine dihydroxide and formic acid
with heating followed by drying at 150 °C. The final
product was heated in air at 150 °C for 4 hours.
As-prepared, silica-supported Pt/Sn02 samples were in¬
serted into an ultrahigh vacuum (UHV) system (see Appendix)
which has an ultimate pressure near 10'11 Torr. After ini¬
tial surface characterization, the samples were transferred
into a preparation chamber connected to the UHV system and
reduced in 10 Torr of CO for 2 hours at 125 or 250 °C.
Heating was accomplished using a platform heating element
[75] which did not dissociate the reducing gas. Sample
temperatures were measured using a thermocouple attached to
the stainless-steel sample support block. After reduction
each sample was returned to the UHV analytical chamber
without air exposure for further characterization.
Energy analysis for the ISS and XPS experiments was
accomplished using a Perkin-Elmer PHI Model 25-270AR
double-pass cylindrical mirror analyzer (CMA). The CMA

66
contained an internal, movable aperture which varied the
polar acceptance angle for incoming particles. The ISS
spectra were collected in the nonretarding mode using a
147°-scattering angle, which was fixed by the experimental
geometry, and pulse counting detection [51]. A 100-nA, 1-
keV 4He+ primary ion beam was defocused over a 1-cm2 area
to minimize sputter damage. Survey and high-resolution XPS
spectra were recorded with Mg Ka excitation in the retard¬
ing mode using 50- and 25-eV pass energies respectively.
Results and Discussion
Survey XPS spectra taken from one silica-supported
Pt/SnOx catalyst sample before and after reduction in 10
Torr of CO at 125 °C appear in Figure 5-1, and XPS survey
spectra taken from another sample before and after reduc¬
tion at 250 °C appear in Figure 5-2. The two catalyst
samples examined were similar in that both were randomly
dispensed from the same catalyst batch. The spectra shown
in Figures 5-l(a) and 5-2(a) were taken from the air-
exposed surfaces. They exhibit predominant peaks due to Sn
and 0 while peaks due to Pt are present but less discern¬
ible. Differences between Figures 5-l(a) and 5-2(a) with
regard to the O/Sn ratio suggest that the as-prepared
catalyst samples lack uniformity in average surface com¬
position. Nevertheless, important information may still be
obtained with respect to data taken from each catalyst

Figure 5-1. The XPS survey spectra taken from the silica-
supported Pt/SnOx surface (a) before and (b) after CO
reduction at 125 C.

Figure 5-2. The XPS survey spectra taken from the silica
supported Pt/SnOx surface (a) before and (b) after CO
reduction at 250 C.

69
sample before and after reduction. A thick tin oxide sur¬
face layer (> 50 angstroms) is indicated by the lack of Si
XPS signals arising from the silica substrate. Only trace
amounts of C and surface contaminants are detected. Reduc¬
tion in CO results in changes in the Sn and 0 signals as
shown in Figures 5-l(b) and 5 — 2(b). The spectra indicate
that the surface region of the sample reduced at 125 °C be¬
comes oxygen-enriched while depletion of surface 0 occurs
in the sample reduced at 250 °C. A possible mechanism
responsible for this result is discussed below.
An ISS spectrum taken from an air-exposed, silica-
supported Pt/SnOx surface before reduction is shown in
Figure 5-3(a), and ISS spectra taken after reduction in CO
at 125 and 250 °C appear in Figures 5 — 3 (b) and 5-3(c),
respectively. Figure 5-3(a) reveals distinct Pt and 0 fea¬
tures and a Sn peak which appears as a shoulder on the Pt
peak. Peaks due to small amounts of contaminants such as C
and Na are also present as are some peaks due to charging.
Peaks due to charging shift in energy as the sample is
rotated so they can be distinguished easily from elemental
peaks, which appear at energies near those predicted by the
elastic binary collision model [52,88]. After catalyst
reduction at 125 °C, Figure 5—3(b) indicates that the sur¬
face composition has changed considerably. The Sn and Pt
peaks have increased in size significantly, and the Sn peak

Figure 5-3. The ISS spectra taken (a) from the as-prepared
sample, (b) after reduction at 125 C and (c) after
reduction at 250 C.

71
has become more prominent and well defined in size with
respect to the Pt. As shown in Chapter 1, this behavior is
characteristic of Pt/Sn alloy formation. A few peaks due
to migration of substrate impurities such as Na and Mg also
appear after the reduction as does a Ti peak from the
sample holder. After reduction at 250 °C the Sn and Pt
peaks are greatly reduced in size due to coverage by con¬
taminants (primarily Na and Mg) in the silica which migrate
to the surface during reduction. Due to the stability of
silica at these fairly low reduction temperatures, silica-
containing species most likely do not migrate over the Pt
and Sn. Since ISS is essentially sensitive to the outer¬
most layer of surface atoms, the data in Figure 5-3 suggest
that the 250 °C reduction results in physical coverage
(encapsulation) of most of the surface Pt and Sn. This en¬
capsulation hypothesis is confirmed by ISS data taken after
the reduced surface was lightly sputtered. As shown in
Figure 5-4, the sputtering process uncovered significant
amounts of underlying Pt and Sn. Although peak energy
shifts and surface charging features appear in these
spectra, the peaks which correspond to Pt and Sn remain
prominent and meaningful.
The data presented thus far suggest that reduction at
both 125 and 250 °C promotes movement of O, Sn and
subsurface impurities contained in the silica. The 0

0.1 0.5 0.9
E/E,
Figure 5-4. The ISS spectrum taken after reduction at 250 °C
(Figure 5-3c) and subsequent sputtering with 2-keV He ions
for 5 minutes using a primary beam current of 10 fih over a
1-cm area.

73
concentration in the near-surface region is probably
determined by the rates of several mechanisms. One is
migration of subsurface 0 to the near-surface region during
the reduction at elevated temperature under the chemical
potential produced by the presence of the reducing gas.
Another is loss of surface 0 through reaction with the
reducing gas and subsequent desorption as COz. As stated
above, 0 loss may also occur through OH" combination fol¬
lowed by desorption of HzO. At 125 °C the rate of migra¬
tion of subsurface 0 to the surface apparently is greater
than the rate of 0 loss by reaction to form COz and HzO,
which results in an increase in the near-surface 0 con¬
centration as observed in the XPS data. Similar observa¬
tions have been made during the reduction of a TiOz(OOl)
surface [84]. However, at 250 °C the rate of 0 loss
through reaction is greater than the rate of O migration to
the surface, resulting in a decrease in the 0 concentration
in the near-surface region.
As shown in Figure 5-5, the Pt 4f XPS spectra exhibit
broad features suggesting that multiple Pt oxidation states
are present both before and after reduction. According to
a standard reference [80] and previous studies from this
laboratory [32,89], Figure 5-5(a) indicates that a mixture
of primarily PtOz and PtO with smaller amounts of Pt(OH)z
and metallic Pt is present on the air-exposed surfaces.

BINDING ENERGY (eV)
Figure 5-5. The Pt 4f XPS spectra taken (a) from the as-
prepared, silica-supported Pt/SnOv surface, (b) after CO
reduction at 125 C and (c) after CO reduction at 250 C.

75
After reduction, peak positions shift toward lower binding
energies as the surface Pt species are partially reduced.
After reduction at 125 °C the predominant species present
is P t(OH) 2 with an increased amount of metallic Pt.
However, only very small amounts of PtO and PtOz remain
after the 125 °C reduction. The extent of reduction is
much greater after the 250 °C reduction. The Pt is
primarily in metallic form with a relatively small amount
of Pt (OH) z . Little or no PtO or PtOz is present. There
has been some debate concerning the stability of PtO, but
its existence is well substantiated in numerous studies
[32,80,89-94].
Tin can be present in several forms which include
SnOz, SnO, Sn hydroxides, Sn suboxides, Sn-O-Pt species
[32,89], metallic Sn and Sn alloyed with Pt. Using XPS, it
is not difficult to distinguish between metallic Sn and Sn
oxides because their Sn 3d binding energies differ by about
1.8 eV, and Asbury and Hoflund have observed XPS features
due to suboxides [60]. However, it is difficult to distin¬
guish between SnOz and SnO using XPS, but this can be done
using valence-band XPS [50,95] or electron energy-loss
spectroscopy (ELS) [56,96]. Paparazzo and co-workers [97]
claim that there is a 0.18 eV difference in the Sn 3d bind¬
ing energies of SnO and SnOz. Valence-band XPS [50,95] and
electron-stimulated desorption (ESD) [50]
can be used to

76
identify Sn hydroxides. These species also yield a high-
binding-energy shoulder on the O Is XPS peak [50,81], but
no studies have reported any influence on the Sn 3d XPS
peaks. Studies of Pt/Sn alloys [55,58] suggest that the Sn
3d XPS alloy features differ from those of metallic Sn, but
a systematic study of Sn 3d shifts due to alloying has not
been reported.
The Sn 3d XPS spectrum taken before reduction is shown
in Figure 5-6(a), and spectra taken after reduction in CO
at 125 and 250 °C are shown in Figures 5 — 6 (b) and 5-
6(c), respectively. Small changes in the Sn 3d peak shapes
occur during annealing in CO. Since a large number of Sn
species may be present and their contributions to the Sn 3d
XPS features are not well understood, it is only possible
to analyze these peaks in a qualitative manner. Before
reduction, the Sn 3d XPS peaks are due predominantly to
Sn02 . However, they are not symmetrical, and the symmetry
on the low-binding-energy side indicates that some of the
Sn on the as-prepared catalyst may be present as suboxides
and SnO and possibly as metal. The Sn 3d peaks become
slightly wider with increasing reduction temperature as in¬
dicated in Figure 5-6. Based on the data of Paparazzo and
co-workers [97], this is probably due to partial conversion
of SnOz to SnO. After reduction at 250 °C it appears that
the low-binding-energy shoulder near the baseline increases

Figure 5-6. The Sn 3d XPS spectra taken from the silica-
supported Pt/SnOv surface (a) before and (b) after CO
reduction at 125 C and (c) after CO reduction at 250 C.

78
as well suggesting that some of the Sn oxides are reduced
to metallic Sn. Metallic Sn probably forms during reduc¬
tion at 125 °C, but the amount is so small that it is not
detected by XPS. Recall that similar changes in the Sn 3d
peak shape were observed during reduction of platinized tin
oxide surfaces in Chapters 1 and 3. Some, but probably not
all, of the metallic Sn formed alloys with Pt. Paffett and
Windham [55] have found that Pt/Sn alloy formation occurs
at temperatures as low as 175 °C in their study of Sn
deposited on Pt(lll). Recently, tin oxide films (no Pt)
have been reduced in CO under conditions similar to those
used in this study [98]. The XPS data indicate that metal¬
lic Sn does not form in this case. Thus, it appears that
Pt promotes the formation of metallic Sn during reduction
in CO. The XPS data in Figure 5-6 also indicate that the
extent of reduction is greater at 250 °C than at 125 °C.
It is interesting to relate the results of this
characterization study with those obtained from the commer¬
cial Pt/SnOx catalyst (not supported on silica) inves¬
tigated in Chapter 4. Both catalysts require a reductive
pretreatment for optimization of catalytic performance, but
increasing the reduction temperature affects the two
catalysts differently. The silica-supported catalyst ex¬
amined in this study yields an optimum CO oxidation ac¬
tivity after a 125 °C pretreatment. The commercial Pt/SnOx

79
catalyst exhibits the same activity over a broad range of
pretreatment temperatures (125 to 225 °C) [26]. The XPS
data in this study and the characterization study in Chap¬
ter 4 agree that Pt is present predominantly as hydroxides
after a pretreatment which yields high activity. This fact
is consistent with the observation that the catalytic ac¬
tivity of the commercial Pt/SnOx catalyst is enhanced by
the presence of water vapor in the reaction gas mixture
[26]. Another similarity is that the data obtained from
both catalysts indicate that a Pt/Sn alloy forms during
reduction. This fact apparently explains why Pt supported
on tin oxide is greatly superior as a catalyst for CO
oxidation at low temperatures compared to Pt supported on
alumina or other nonreducible oxides.
Another factor relating to catalytic activity which
must be considered is encapsulation of the active Pt and Sn
species. This process does not occur when reducing the
commercial Pt/SnOx catalyst at temperatures up to 175 °C.
The impurities in the silica-supported Pt/SnOx catalyst
which cover the Pt and Sn species during the 250 °C reduc¬
tion may be responsible for the loss in catalytic activity
since the encapsulation does not occur during the 125 °C
reduction after which the catalytic activity remains high.
The catalyst surface produced during the reduction
step is not the same surface responsible for the long-term

80
catalytic behavior. Significant changes occur at the
catalyst surface during the initial reaction period. Based
on activity data [26], transient behavior may occur for
hours after which time the activity goes through a maximum
and then slowly decays. The changing activity must corre¬
late with changes in composition and chemical states of
species at the surface, but this relationship has not been
determined yet.
Summary
The changes induced at an optimized, silica-supported
platinized tin oxide surface by annealing at 125 and 250 °C
in 10 Torr of CO for 2 hours have been determined using ISS
and XPS. The reduction at 125 °C results in enrichment of
the near-surface region in 0, reduction of Pt oxides with
the formation of Pt hydroxide, and the reduction of SnOz to
SnO and metallic Sn which mostly alloys with Pt. Different
results are obtained from the surface reduced at 250 °C in
that the extent of the Sn oxide reduction is greater, the
predominant form of the Pt is metallic, impurities in the
silica migrate over the active Pt and Sn species, and the O
content of the near-surface region is reduced. The dif¬
ferences found after reduction at the two temperatures may
explain the fact that a 125 °C pretreatment results in a
surface which is active for CO oxidation at low temperature

81
whereas a 250 °C pretreatment results in a catalytically
inactive surface.

CHAPTER 6
CATALYTIC BEHAVIOR OF NOBLE METAL/REDUCIBLE OXIDE
MATERIALS FOR LOW-TEMPERATURE CO OXIDATION;
COMPARISON OF CATALYST PERFORMANCE
Introduction
The catalytic oxidation of CO near ambient tempera¬
tures has many important applications. As described pre¬
viously, closed-cycle COz lasers produce CO and Oz in the
laser discharge resulting in a rapid loss of output power.
This problem can be overcome by incorporating a low-
temperature CO oxidation catalyst into the laser system
which converts the dissociated products back into COz
[7,8,22,23]. Also, air purification devices often contain
catalysts to oxidize dangerous levels of toxic CO [17,99-
101]. Such devices are utilized in fire safety equipment
and in underground mines as respiratory aids.
Consequently, the development of low-temperature CO
oxidation catalysts has received considerable attention [7-
17]. Although significant progress has been made with
regard to understanding the reaction mechanism, there
remains a need for the development of catalysts which ex¬
hibit higher activities for prolonged periods at low tem¬
peratures (typically less than 100 °C) and in the diverse
82

83
range of oxidation environments which are encountered.
Factors which determine oxygen availability and gaseous im¬
purities often have a pronounced effect on catalyst perfor¬
mance. CO oxidation in ambient air has the advantages of
excess O2 and low COz concentrations which facilitate the
reaction considerably. Consequently, numerous materials
are known to oxidize CO in excess Oz at low temperatures
[9-11,13,14], but complications due to the presence of
humidity and/or air pollutants are often detrimental to
their activity. In COz lasers, CO and Oz are present in
small stoichiometric quantities in a large amount of CO2.
Although the catalytic reaction benefits from the fact that
the lasers usually operate at temperatures somewhat above
ambient (25-100 °C), catalytic CO oxidation is difficult
under these conditions.
Recently, Haruta and co-workers [15,16] prepared
supported-gold catalysts on various base-metal oxides in¬
cluding MnOx, FezOs , Co a O« , NiO and CuO and determined
their catalytic activities toward the oxidation of Hz
and/or CO. Most of the CO oxidation tests were performed
using 1 vol% CO in dry air. At 0 °C, Au/FezOa and Au/NiO
maintain essentially 100% CO conversion under the flow con¬
ditions used over a 7-day test period. Similar performance
was also observed for Au/FezOa and Au/Co30« at 30 °C in 76%
relative humidity. Therefore, these catalysts appear to be

84
quite useful in air purification devices, but activities in
the presence of air contaminants were not determined.
These catalysts may be useful in COz lasers even though the
reaction conditions are quite different as described above.
It is interesting to note that Haruta and co-workers
[15,16] apparently did not examine the behavior of Au/MnOx
toward CO oxidation.
Catalysts consisting of Pt and/or Pd supported on tin
oxide have been researched extensively for use in COz
lasers [7,8,22-26]. Although these materials can exhibit
considerable CO oxidation activity in this application,
there are complications which must be overcome. Acceptable
activity is observed only after the catalyst undergoes a
reductive pretreatment. Unfortunately, such pretreatments
may lead to considerable induction periods often lasting
several days during which the observed activity declines
before reaching a maximum [26]. Even after acceptable ac¬
tivity is recovered these materials exhibit a steady decay
in performance over time.
The purpose of the present study is to explore the be¬
havior of materials other than platinized tin oxide as
catalysts for low-temperature CO oxidation, particularly
with regard to COz laser applications. Several materials
were synthesized and screened for CO oxidation activity
using small concentrations of stoichiometric CO and Oz in

85
He and temperatures between 30 and 75 °C. The tests were
run for periods as long as 18000 minutes in order to ob¬
serve the induction and decay characteristics of the
catalysts.
Catalysts Preparation
A review of the literature provided a basis for selection
of support materials examined in this study which include
iron oxide (FezOa), nonstoichiometric manganese oxides
(MnOx), and ceria (CeOx) where x is between 1 and 2. The
materials investigated were synthesized using established
impregnation and coprecipitation techniques [1021. The
samples prepared include MnOx, Pt/MnOx, Ag/MnOx, Pd/MnOx,
Cu/MnOx, Au/MnOx, Ru/MnOx, Au/CeOx and Au/FezOa.
The MnOx was used as-received from the Kerr-McGee Com¬
pany, USA. It was prepared by the electrolytic oxidation
of manganous sulfate and has BET surface area of 74
mz/gram. The MnOx served as a sample itself as well as an
impregnation support for other materials.
Two Pt/MnOx samples (0.2 wt% Pt) were prepared by im¬
pregnation of MnOx using an aqueous solution of NazPt(OH)s.
Sample #1 was dried in air at 280 °C for 4.5 hours whereas
sample #2 was dried in air at 75 °C for 3 hours. A Pd/MnOx
catalyst (0.2 wt% Pd) was prepared by impregnating MnOx
with an aqueous solution of PdClz. The product was dried
in air at 280 °C for 4.5 hours.

86
Two Ag/MnOx samples were prepared, one containing 0.2
wt% Ag and another containing 1.0 wt% Ag. Impregnation of
MnOx with Ag was accomplished using a solution prepared by
dissolving AgO in NH«OH. The products were dried in air at
280 °C for 4.5 hours.
A sample which contained admixtures of CuO and MnOx
was prepared from the products of several procedures. Pro¬
cedure A involved coprecipitation from aqueous solutions of
CuS04 + sucrose and KMnO«. The precipitate was washed with
water and dried in air at 105 °C for 15 hours. The Cu:Mn
molar ratio was approximately 1.4. In procedure B, a por¬
tion of the former product was dried in air at 280 °C for 2
hours. In procedure C, MnOx was precipitated from aqueous
solutions of sucrose and KMn0 4. The precipitate was washed
and dried as outlined in procedure A. The final product
consisted of an admixture of 0.4 grams from procedure A,
0.7 grams from procedure B, 0.3 grams from procedure C and
0.2 grams of commercial CuO powder.
A technique in which Mn(OH)z was precipitated in the
presence of fine Ru powder was utilized to prepare a 2 wt%
Ru/MnOx sample. An aqueous solution of MniNOa)? was added
dropwise to a stirred mixture of Ru powder in NH4OH. The
resulting product was dried and calcined at 400 °C for 2
hours.

87
Three supported Au samples were synthesized via
coprecipitation from aqueous HAuCl* and the nitrate of the
corresponding support metal. The composition of the
materials is approximately 5 at% Au/MnOx, 20 at% Au/CeOx
and 5 at% Au/FezOa on a Au/metal basis. In each case the
appropriate precursor solutions were added dropwise to a
stirred solution of NazCOa at room temperature. After
washing with hot water (80 °C) and drying, the pre¬
cipitates were calcined in air at 400 °C for 4 hours. Two
Au/FezOa samples were prepared which differed only in the
temperature of the wash water utilized (25 °C and 80 °C).
Experimental
The reactor used to test the CO oxidation activity of
the catalysts is located at NASA Langley Research Center in
Hampton, VA, and has been described previously [27,103].
Screening of catalysts for CO oxidation has typically been
performed using a test gas consisting of a few percent CO
in air (excess Oz), and the catalytic behavior under
stoichiometric CO and Oz and in the presence of COz has of¬
ten not been determined. Since the catalytic behavior can
vary considerably under different environments as described
above, it is necessary to perform such experiments. All
tests were conducted using 0.15 grams of catalyst and a
reaction gas mixture consisting of 1 vol% CO, 0.5 vol% Oz
and 2 vol% Ne in He at a total pressure of 1 atmosphere

88
flowing at 10 standard cm3 per minute (seem). The reaction
temperatures investigated were 75, 50 and 30 °C as noted.
The conversions are quite high under these conditions which
corresponds to operating the reactor in an integral mode.
In most cases the catalysts were tested as prepared
without additional pretreatments. Unless noted otherwise,
each catalyst was loaded into the reactor and exposed to
flowing He for about 1 hour as the reaction temperature
stabilized. Then the He flow was changed to the reaction
gas mixture and product sampling was begun. At predeter¬
mined time intervals, an automated sampling valve directed
a small fraction of the reaction products to a gas
chromatograph (GC) for analysis of %COz yield, %CO loss and
%Oz loss, and the results were plotted versus time.
Results and Discussion
During these initial activity screening experiments,
emphasis is placed upon characteristics of the overall CO
oxidation activity curves with respect to temperature and
time. An appropriate catalyst for use in COz lasers must
not only exhibit high activity at low temperatures (25-
100 °C) but also maintain acceptable activity over a
lifetime of up to 3 years [6]. Since a catalyst cannot be
practically tested for a 3-year lifetime, its activity
profile must be extrapolated with reasonable confidence.
Nevertheless, it is necessary to exercise caution when

89
evaluating potential catalysts for CO2 lasers because a
catalyst which exhibits the best activity initially might
succumb to decay mechanisms which render it inferior after
extended use. Consequently, a catalyst exhibiting only
marginal activity initially may become the optimal choice
if the corresponding activity decay remains negligible.
Carbon monoxide oxidation activity curves for several
MnOx-based catalysts appear in Figure 6-1. Initially, MnOx
and Cu/MnOx exhibit the highest CO oxidation activities al¬
though their performance rapidly deteriorates. However,
after about 2000 minutes the reaction curve for Cu/MnOx ap¬
pears to approach a steady-state conversion with negligible
activity decay. The MnOx sample may approach a more active
steady-state conversion but more extensive testing is
required to be certain. Even though the Pt/MnOx #1 and
Ag/MnOx samples display superior activity throughout most
of the test period, extrapolation of the data in Figure 6-1
indicates that Cu/MnOx may be the optimal catalyst in a
long-term test.
The data in Figure 6-1 also depict an interesting com¬
parison between the catalytic activities of Pt/MnOx #1
(dried for 4.5 hours at 280 °C) and Pt/MnOx #2 (dried for 3
hours at 75 °C). The poor activity exhibited by Pt/MnOx #2
may be the result of incomplete removal of surface im¬
purities (such as Na, Cl or OH") associated with

100 n
i
90 i
\
l
» A
80 -i
^ A
A
701
g0QQtaQaQa
,0 AAA
| A A a a a a
O' 60-
1
O
â– 
? 50 -
■ T=75 °C
â– 
* 40 -
■„ a Pt/MnOx#1
" o Ag/MnO„
30 -
¿ MnOx
20 -
â–  Cu/MnO*
10 -
k * Pt/MnO*#2
i
O 4A*A^**t** T T 1 1 1 T T
O 2000 4000 6000 8000 10000
Time (min)
Figure 6-1. The CO oxidation activity of 0.2 wt% Pt/MnOx
#1, 0.2 wt% Ag/MnOx, MnOx, 60 at% Cu/MnOx and 0.2 wt%
Pt/MnOx #2 at 75 °C as a function of time.

91
the impregnation step. However, as found in previous
studies of MnOz and MnOz-CuO catalysts [104-106], the inac¬
tivity is most likely the result of incomplete surface ac¬
tivation. Many MnOx-based catalysts usually require heat¬
ing between 100-200 °C in air or Oz to produce an active
surface. The heat treatments apparently activate the sur¬
face through the creation of reactive sites via partial
surface reduction, depletion of adsorbed water or surface
hydroxyl groups, and/or concurrent micropore generation.
An interesting observation is that the reaction
profiles of Pt/MnOx #1 and Ag/MnOx are remarkably similar.
This is unexpected based on the different catalytic
properties of Pt and Ag. It is possible that this behavior
results primarily from exposure of MnOx to similar basic
solutions followed by drying in air at 280 °C for 4.5
hours. The activity curve for pure MnOx (as-received and
common to both samples) is quite different in character,
which is consistent with this hypothesis. Nevertheless,
both materials performed well during the 10000-minute test
period oxidizing 70-80% of available CO at 75 °C.
As mentioned above, Pt/SnOx catalysts have received
considerable attention for use in COz lasers. A comparison
of CO oxidation performance between Pt/MnOx #1 (see Figure
6-1) and a commercial Pt/SnOx catalyst manufactured by
Engelhard Industries (see Chapter 4) is shown in Figure

100
90
80
70
8"60
2
® 50
* n
40
30
20
10
0
0 2000 4000 6000 8000 10000
Time (min)
Figure 6-2. Comparison of CO oxidation activity between
0.2 wt% Pt/MnOx #1 and a commercial Pt/SnOx catalyst at 75 C.
□ °
~3
£D¿ppnn ddoddDqDdD[1D^^
T=75 °C
a 0.2% Pt/MnOx
A 2% Pt/SnOx (Engelhard)
n i i i i i r
VO
bO

93
6-2. At 75 °C, the Pt/MnOx #1 sample exhibits superior
activity after approximately 2500 minutes of reaction. Due
to the limited reaction data for the Pt/SnOx sample, fur¬
ther comparisons require data extrapolation. Assuming that
the indicated trends continue, Pt/MnOx #\ represents the
optimal catalyst over an extended time period. This also
is true for Ag/MnOx which behaves identically to Pt/MnOx
#1.
Figure 6-2 represents a valid comparison because the
sample size and experimental parameters used were identical
in both tests. It should be noted, however, that the
Pt/SnOx was pretreated in a 5% CO/He stream at 225 °C for 1
hour prior to activity testing. As discussed in previous
chapters of this research, such reductive pretreatments
significantly enhance the performance of Pt/SnOx catalysts
[26]. The fact that no pretreatments were used for the
MnOx-based catalysts is an advantage. Furthermore, since
the precious metal loading for the Pt/MnOx #1 and Ag/MnOx
samples is only 0.2 wt% (compared with 2 wt% for the
Pt/SnOx catalyst), there also appears to be an economic ad¬
vantage over the Engelhard Pt/SnOx catalyst. Of course,
the Ag/MnOx catalyst is the least costly.
The fact that reductive pretreatments activate Pt/SnOx
catalysts provided motivation to investigate the effects of
similar pretreatments on Pt/MnOx
catalysts.
Two

94
pretreatment conditions were used in which the Pt/MnOx #1
sample was exposed to 5% CO/He for 1 hour at 125 and
225 °C. The effects on catalytic performance are shown in
Figure 6-3. It is clear that the pretreatments are
detrimental to the CO oxidation activity of Pt/MnOx. In
fact, the observed activity of Pt/MnOx decreases with in¬
creasing pretreatment temperature, a trend opposite to that
which is observed for Pt/SnOx catalysts [26]. A possible
explanation may involve the reducibility of the MnOx and
SnOx supports. It appears that catalysts based on these
materials require a certain degree of surface reduction for
optimal activity. There is evidence that a completely
dehydroxylated or an entirely oxygenated MnOx surface is
not active toward low-temperature CO oxidation [104,105].
Similarly, surface hydroxyl groups are believed to be in¬
strumental in the CO oxidation mechanism over Pt/SnOx [26].
Given the relative instability of MnOx with respect to
SnOx, such an optimum degree of surface reduction most
likely results from milder pretreatments than those used to
generate the data shown in Figure 6-3. In fact, heat
treatments in air or 02 appear to be more beneficial for
MnOx CO oxidation catalysts [12,104,105,107,108]. Although
the CO reductive pretreatments at 125 and 225 °C are ap¬
propriate for Pt/SnOx, they apparently are too severe for
Pt/MnOx.

100
90
80
70
« 60
O
O
2 50
£
* 40
30
20
10
0
Figure 6-3. The CO oxidation activity of 0.2 wt% Pt/MnOx #1
at 75 C as a function of pretreatment in 5 vol% CO/He at
125 and 225 °C.
Pt/MnO,
un pretreated
"â– â– â– â– â– â– â– â– m
*A
pretreated 1 hr at 125 °C
pretreated 1 hr at 225 °C
AAaAAaaa*AAAAAAaAAAAAaAAAAAAAaAaaaaAAAAa,
—I 1 1 1 1 r
2000 4000 6000
Time (min)
—i 1 1—
8000 10000

96
Additional insight on the pretreatment effects may be
gained by considering the initial reaction characteristics
with regard to CO2 production, CO consumption and Oz con¬
sumption (determined by GC analysis). These data are shown
in Figures 6-4, 6-5 and 6-6 for Pt/MnOx catalysts which
were not pretreated, pretreated at 125 °C and pretreated at
225 °C, respectively. For unpretreated Pt/MnOx #1, Figure
6-4 shows that a considerable amount of catalyst surface Oz
is utilized in COz formation during the early stages of
reaction because the Oz loss is much lower than the COz
production. The participation of catalyst Oz during CO
oxidation has been observed for MnOx catalysts previously
[11,12,19,108]. After a 125 °C pretreatment, the catalyst
activity is decreased, and this decrease is accompanied by
a decrease in the utilization of catalyst Oz as shown in
Figure 6-5. Since the curves now nearly coincide, the
early stages of CO oxidation on this pretreated surface ap¬
pear catalytic in nature with only gas-phase Oz being util¬
ized. The data of Figure 6-6 obtained after pretreating in
CO at 225 °C indicate essentially opposite behavior to that
shown in Figure 6-4. That is, the surface appears to have
been reduced to a point where gas-phase Oz is utilized not
only in COz formation but in catalyst regeneration as well.
Even though the catalyst surface acquires Oz from the gas
phase, this fresh surface Oz does not appear to participate

80
70 -
60 -
*
Tj
# so
40 -
30
Pt/MnOx#1
8
8
â–¡
O
â–¡ â–¡
o o
â–¡
o
â–¡ o
â–¡ o
&
A
A A
A
A
A
â–¡
% Yield CO 2
0
% Loss CO
a
A
% Loss 0 2
pretreatment: none
20
Time (min)
40
60
Figure 6-4. The % C02 yield at 75 C with corresponding
% loss CO and 0, for unpretreated 0.2 wt% Pt/MnO #1.

40
38
36
34
*
2 30
£
* 28
26
24
22
20
Pt/MnO, #1
â–¡
% Yield CO 2
o
% Loss CO
6
A
% Loss02
0
â–¡
A
•
« D □
6 S
â–¡ â–¡
* 2
pretreatment: 1 hr in 5% CO/He at 125 °C
20
Time (min)
"IT
40
60
Figure 6-5. The % C02 yield at 75 C with corresponding
% loss CO and 02 for 0.2 wt% Pt/MnOx #1 pretreated in 5
vol% CO/He for 1 hour at 125 °C.

60
50
40
1
5 30
É
*
20
10
0
0 20 40 60
Time (mln)
Figure 6-6. The % C02 yield at 75 C with corresponding
% loss CO and 02 for 0.2 wt% Pt/MnOx #1 pretreated in 5
vol% CO/He for 1 hour at 225 °C.
Pt/MnOx #1
A
â–¡
% Yield CO 2
0
% Loss CO
A
% LossO2
A
â–¡
O
A
A
a o o â–¡
O o o o
É
O
a â–¡
O o
pretreatment: 1 hr in 5% CO/He at 225 °C
t i i 1 r

100
in the reaction or restore the catalytic activity which was
lost during the pretreatment.
A consistent interpretation of the data in Figures 6-4
through 6-6 may be realized by invoking a REDOX (reduction-
oxidation) mechanism for CO oxidation. The active surface
in Figure 6-4 appears to reach a situation wherein both
surface and gas-phase Oz participate in the reaction. The
active surface is partially reduced after the first 30
minutes of reaction, and the extent of reduction depends
upon the relative rates of surface reduction by CO and
reoxidation by gas-phase Oz. During the CO pretreatments,
the surface can be reduced to such an extent that catalyst
Oz is not available for reaction. Therefore, the resulting
surfaces are not as active toward CO oxidation. These data
suggest that it might be possible to determine optimal
pretreatment conditions and that these optimal conditions
would be less severe than the ones used in this study. The
exact form(s) of the active surface oxygen species remains
to be determined.
Figure 6-7 shows the CO oxidation performance of
Au/CeOx, Au/FezOa #1 and Au/FezOa #2 at 75, 50 and 30 °C.
Several important features appear in these activity curves.
The Au/CeOx exhibits very high activity at 75 °C oxidizing
greater than 80% of the available CO. Also, the reaction
profile exhibits negligible decay over 10000 minutes. This

100
90
80
70
60
| 50
* 40
30
20
10
0
0 4000 8000 12000 16000
Time (min)
Figure 6-7. The CO oxidation activity of 20 at% Au/CeOx,
5 at% Au/Fe203 #1 and 5 at% Au/Fe203 #2 at 75, 50 and
30 C as a function of time.
8
75 °C
â–  Au/CeOx
A Au/Fe203 #1
♦ Au/Fe203 #2
A A
A A a A A
A A A 4 a A
A A A A a
50 °C
aaaaaaaaa
30 °C
v .. ♦ ♦ * ♦♦♦♦♦♦♦♦♦
A ♦ ♦ *
TOT

102
represents a significant improvement over the performance
of Pt/MnOx #1 and Ag/MnOx shown in Figure 6-1. At 50 °C,
Au/CeOx continues to perform well maintaining a CO2 yield
near 43%.
Figure 6-7 also provides an interesting comparison be¬
tween Au/Fe2O3 #1 (washed with hot water) and Au/FezOa #2
(washed with cold water). The activity of Au/FezOa #1 is
clearly superior although some decay in performance is evi¬
dent. Surface Cl is generally believed to inhibit low-
temperature CO oxidation [28] . Therefore, the difference
in activity of the two samples may be attributable to
poisoning by surface Cl (originating from the gold precur¬
sor HAuCIa) which is not as effectively removed by washing
with cold water compared to hot water. Nevertheless, it is
interesting to note that the activity of Au/FezOa #2
steadily increases with time (negative or inverse decay).
This behavior may be a consequence of some surface process
which removes the surface Cl as the reaction proceeds.
It is interesting to compare the performance of
Au/FezOa HI with that of a Au/FezOa catalyst investigated
by Haruta and co-workers [15,16]. They observed that
Au/Fe20a is essentially 100% efficient in oxidizing 1 vol%
CO in air even below 0 °C. The lower activities found in
this study apparently are due to the difficulties involved

103
in oxidizing CO in a stoichiometric mixture as described
above.
Carbon monoxide oxidation activity curves for a second
set of MnOx-based materials appear in Figure 6-8. The data
indicate that Au/MnOx is clearly the most active catalyst
examined in this study. At 75 °C, Au/MnOx sustains nearly
100% COz yield over a 10000-minute period, and excellent
activity is also observed at 50 and 30 °C. At all tempera¬
tures the activity profiles are exceptional in that they
exhibit negligible decay over the entire test period.
Figure 6-8 also shows the activity curves for two
Ag/MnOx samples (0.2 wt% Ag and 1.0 wt% Ag). As stated
above, the two samples were prepared in a similar manner
differing only in Ag content. The data indicate that small
Ag loadings result in better catalytic behavior. Both
reaction profiles are similar up to 1000 minutes of reac¬
tion after which the 1 wt% Ag/MnOx sample exhibits ac¬
celerated decay in activity.
Although Pt/MnOx and Ru/MnOx are the least active
catalysts according to the data in Figure 6-8, significant
conversions are nevertheless observed. However, both of
these materials and the others used in this study probably
would be improved by optimizing preparation and pretreat¬
ment techniques.

100
90
80
70
CM 60
8
2 50
£
# 40
30
20
10
0
0 3000 6000 9000 12000 15000 18000
Time (min)
Figure 6-8. The CO oxidation activity of 5 at% Au/MnOx,
0.2 wt% Pd/MnOx, 2 wt% Ru/MnOx, 0.2 wt% Ag/MnOx and 1 wt%
Ag/MnOx at 75, 50 and 30 C as a function of time.
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104

105
Summary
Selected materials have been prepared and tested as
low-temperature CO oxidation catalysts for long-term use in
COz lasers. These materials were prepared utilizing im¬
pregnation and coprecipitation techniques and include MnOx ,
Pt/MnOx, Ag/MnOx, Pd/MnOx, Cu/MnOx, Ru/MnOx, Au/MnOx,
Au/CeOx and Au/FezOa. Each was tested for CO oxidation ac¬
tivity in low concentrations of stoichiometric CO and Oz at
temperatures between 30 and 75 °C. Although most of the
materials exhibit significant CO oxidation activity,
Au/MnOx and Au/CeOx are exceptionally active. At 75 °C,
Au/MnOx sustains nearly 100% COz yield for 10000 minutes
with no evidence of activity decay under the test condi¬
tions used. Exceptional activities are also observed at 50
and 30 °C. Many of the catalysts tested perform better
than a platinized tin oxide catalyst either with regard to
activity, decay characteristics or both. For example,
Cu/MnOx has a lower activity than several of the catalysts
tested, but it shows negligible decay making it a potential
candidate for long-term applications. A Pt/MnOx catalyst
and a Ag/MnOx catalyst both exhibit similar and higher ac¬
tivities but decay more rapidly than Cu/MnOx and less
rapidly than the commercially available platinized tin
oxide. Pretreatment in CO at 125 and 225 °C decreases the
activity of Pt/MnOx. Optimization studies of preparative

106
and pretreatment variables need to be performed in order to
further increase the performance of low-temperature CO
oxidation catalysts.

CHAPTER 7
COMPARISON OF THE PERFORMANCE CHARACTERISTICS OF
Pt/SnOx AND Au/MnOx CATALYSTS FOR LOW-TEMPERATURE
CO OXIDATION
Introduction
Long used as a research tool to study heterogeneous
catalysis, the catalytic oxidation of CO is being utilized
in an increasing number of practical applications. Carbon
monoxide oxidation catalysts are often an integral com¬
ponent of pollution control devices designed to reduce in¬
dustrial and automotive emissions. Air purification
devices (for respiratory protection) [17,99-101] and CO gas
sensors [85,101,109-112] commonly employ CO oxidation
catalysis. Soon the catalytic oxidation of CO will be
utilized in orbiting closed-cycle COz lasers [5-8] used for
weather monitoring or in other remote sensing applications.
Sealed COz lasers must incorporate a CO oxidation catalyst
to recombine stoichiometric concentrations of CO and Oz
which are produced during the lasing process because both
COz loss and Oz build-up can degrade the performance of the
laser.
Therefore, there is a growing demand for CO oxidation
catalysts which are more effective and versatile. Research
directed toward the development of long-life, sealed COz
107

108
lasers [7-8] has produced several new materials which ac¬
tively catalyze CO oxidation near ambient temperatures. In
1983 Stark and co-workers [23] identified platinized tin
oxide (Pt/SnOx) as a good catalyst for this application.
However, the data given in the previous chapter indicate
that gold supported on manganese oxide (Au/MnOx) may per¬
form even better than Pt/SnOx. The purpose of this study
is to compare the long-term CO oxidation performance of
several Pt/SnOx and Au/MnOx catalysts and evaluate their
potential utility in the former applications.
Experimental
Details regarding sample preparation and the CO oxida¬
tion reactor have been described elsewhere [27,103] and in
the previous chapters of this research. The samples
prepared for this study include 19.5 wt% Pt/SnOx (15.8 at%
Pt based on Pt and Sn content), 14.5 wt% Pt/SnOx/SiOz (11.4
at% Pt based on Pt and Sn content) and 10 at% Au/MnOx
(based on Au and Mn content). The SnOx support was
prepared by dissolving Sn powder in nitric acid and heating
to dryness at 150 °C. For the sample containing SiOz, this
was done in the presence of deaerated SiOz particles.
Deposition of Pt was accomplished by reducing tetra-
aminoplatinum (II) hydroxide with formic acid in the
presence of the deaerated SnOx and SnOx/SiOz supports. The
resulting slurry was dried in air at 150 °C. The Au/MnOx

109
sample was prepared via coprecipitation from aqueous tetra-
chloroauric acid, manganese (II) nitrate and sodium car¬
bonate. The precipitate was washed with hot water, dried
in air at 110 °C, and calcined at 400 °C for 4 hours.
Since the final Mn oxidation state is unknown at this
point, the Au/MnOx composition is based on the molar ratio
of Au:Mn in the precursor solutions. A 2 wt% Pt/SnOx (1.6
at% Pt based on Pt and Sn content) catalyst was obtained
from Engelhard Industries. The experiments were conducted
at 35 or 55 °C using 50-150 mg of catalyst in powdered
form. Unless noted otherwise, the reactor feed contained 1
vo1 % CO, 0.5 vo1 % Oz and 2 vol% Ne (for GC calibration) in
helium flowing at 10 seem and 1 atmosphere of total pres¬
sure. No COz was present in the reactor feed.
The experimental procedures varied depending upon
whether the catalysts were pretreated prior to the onset of
reaction. The unpretreated samples were exposed to 10 seem
of helium for 1 hour as the reactor temperature stabilized.
Pretreated samples were subjected to the following sequence
of events: (1) heating to the desired pretreatment tem¬
perature in 10 seem of helium for 1 hour, (2) exposure to
the pretreatment gas mixture (10 seem) for 2 hours and (3)
cooling to the reaction temperature in 10 seem of helium
for 1 hour. In each case the helium flow was subsequently
replaced with the reaction gas mixture and product sampling

no
was initiated. At predetermined time intervals an
automated sampling valve directed a 1-milliliter portion of
the reaction products to a gas chromatograph (GC) for quan¬
titative analysis of moles of COz formed (referenced to
0 °C and 1 atmosphere) per second per gram of catalyst
(moles COz/s/g). The results were plotted versus time to
yield the CO oxidation activity curves for each catalyst.
Results and Discussion
Air purification and CO detection commonly involve the
oxidation of small concentrations of CO in air (excess Oz)
which may contain substantial amounts of water and/or other
pollutants. Temperature extremes range from near ambient
for respiratory aids to above 150 °C for CO gas sensors
[110]. Carbon monoxide oxidation in COz lasers is charac¬
terized by small, stoichiometric concentrations of CO and
Oz with a large partial pressure of COz at temperatures
near 50 °C. Such diverse environments can have a dramatic
effect on the performance of low-temperature CO oxidation
catalysts. For example, a Au/FezOa catalyst has recently
been shown to exhibit excellent CO oxidation activity in
air (both wet and dry) near ambient temperature [15,16].
However, the research presented in Chapter 6 indicates that
its performance is unacceptable in a COz laser environment.

Ill
Apparently, CO oxidation on these supported catalysts is
much more difficult when excess Oz is not available.
The CO oxidation activities of the 10% Au/MnOx, 19.5%
Pt/SnOx and 2% Pt/SnOx samples are shown in Figure 7-1 as a
function of time for unpretreated surfaces (open symbols)
and surfaces pretreated with a 5 vol% CO/He mixture at
50 °C (solid symbols). The unpretreated Pt/SnOx catalysts
exhibit similar overall reaction profiles which are maximum
at the onset of reaction but steadily decay over time. The
data are consistent with previous experiments which indi¬
cate that maximum Pt/SnOx activity occurs near 15 to 20 wt%
Pt under similar experimental conditions [27]. The
unpretreated Au/MnOx catalyst exhibits remarkable CO oxida¬
tion activity which is approximately an order of magnitude
greater than that of the 19.5% Pt/SnOx sample after 6000
minutes of reaction. The fact that Au/MnOx exhibits supe¬
rior CO oxidation activity without the need for pretreat¬
ment is critical in many applications including respiratory
protection.
It has previously been shown that CO pretreatments en¬
hance the activity of Pt/SnOx surfaces toward low-
temperature CO oxidation [26]. This fact is clearly il¬
lustrated by the data in Figure 7-1. Furthermore, a CO
pretreatment at 50 °C significantly enhances the activity
of Au/MnOx as well, and the effect is most dramatic for

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â–¡ â–¡â–¡â–¡â–¡â–¡
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T=55 °C
D â–  10%Au/MnOx
0 • 19.5% Pt/SnOx
A A 2% Pt/SnOv
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A A
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O O
A A
A A
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aaaaaaaaaa
AAAAAAAAAA
1000 2000 3000 4000
Time (min)
5000
6000
7000
Figure 7-1. The CO oxidation activity of 10 at% Au/MnOx,
19.5 wt% Pt/SnOx and 2 wt% Pt/SnOx at 55 °C without pre¬
treatment (open symbols) and pretreatment in 5 vol% CO/He
at 50 C (solid symbols).
112

113
Au/MnOx where the CO conversion increases by approximately
10%. It is interesting to note that while the activity of
each sample is enhanced as a result of the CO pretreatment,
the overall character of the activity profiles remains un¬
affected.
As indicated in Figure 7-2, increasing the CO
pretreatment temperature from 50 °C (open symbols) to
125 °C (solid symbols) further enhances the long-term ac¬
tivity of these catalysts. The activity profile of the
19.5% Pt/SnOx catalyst is affected most by the increased
pretreatment temperature. However, characteristics of an
induction phenomenon (initial steep decline in activity
followed by an increase and then a slow decline) soon be¬
come apparent in the activity profiles of both 2% Pt/SnOx
and 19.5% Pt/SnOx. Although similar induction phenomena
have been observed in a previous study of Pt/SnOx [26],
identical CO pretreatments at 125 °C did not result in any
significant induction period during reaction at 75 or
85 °C. The fact that an induction period ensues during
reaction at 55 °C is consistent with the hypothesis that
the induction results from temporary surface dehydration
caused by the reductive pretreatment [26] because
replenishment of catalyst surface moisture via bulk diffu¬
sion would be less facile at 55 °C relative to 75 or 85 °C.
Although the induction period exhibited by 19.5% Pt/SnOx is

•f
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T=55°C
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0 • 19.5% Pt/SnOv
A â–²
2% Pt/SnO,
O o
A A A
o O O
Ooo
o O O O o
^AAAAAAAAAAAAAAAAAAAAAA
1000 2000 3000 4000
Time (min)
5000
6000
7000
Figure 7-2. The CO oxidation activity of 10 at% Au/MnOx,
19.5 wt% Pt/SnOx and 2 wt% Pt/SnOx at 55 C after pre¬
treatment in 5 vol% CO/He at 50 °C (open symbols) and
125 C (solid symbols).
114

115
the least severe, it spans a much greater time period
(approximately 5300 minutes) relative to the 2% Pt/SnOx
sample. Nevertheless, beyond the induction period the CO
oxidation activity of both Pt/SnOx samples steadily decays
with time. On the other hand, subsequent to CO pretreat¬
ment at 125 °C, the Au/MnOx catalyst continues to exhibit
its characteristic reaction profile exhibiting negligible
activity decay. While reductive pretreatments are benefi¬
cial with regard to the low-temperature CO oxidation ac¬
tivity of Au/MnOx, they have been shown to be detrimental
to the CO oxidation performance of Pt/MnOx under similar
conditions (see Chapter 6).
In addition to reductive pretreatments, pretreatments
in an oxidizing atmosphere were also investigated. As
shown in Figure 7-3, a pretreatment in 5 vol% 02/He at
50 °C (solid symbols) decreases the CO oxidation activity
of all the samples relative to no pretreatment (open
symbols). The effects of oxygen pretreatment on Au/MnOx
are the most significant; however, the characteristics of
the overall activity profile are retained and activity
decay remains negligible. The results for 2% Pt/SnOx are
once again consistent with a previous study which was per¬
formed using considerably different experimental conditions
[26]. However, the relative degree of activity decline is
much less under the conditions of this study. The effects

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DoaDüQODQaDDDDQODD
T=55 °C
° ■ 10%Au/MnOx
° • 19.5% Pt/SnOx
* A 2% Pt/SnOv
i • • • • * n s u i n i i e i h ¡
4 A W W W W W A 1 i , i A A 1 W W .
0 1000 2000 3000 4000 5000 6000 7000
Time (min)
Figure 7-3. The CO oxidation activity of 10 at% Au/MnOx,
19.5 wt% Pt/SnOx and 2 wt% Pt/SnOx at 55 °C without pre¬
treatment (open symbols) and pretreatment in 5 vol% 02/He
at 50 C (solid symbols).
116

117
of the oxygen pretreatment are noticeable only during the
first 500 minutes of reaction after which the unpretreated
and pretreated curves essentially coincide. Similar be¬
havior is also evident for 19.5% Pt/SnOx where the majority
of the activity decrease occurs during the initial stages
of reaction. After 6500 minutes of reaction, the effect of
the oxygen pretreatment is much less apparent.
Although the performance criteria of low-temperature
CO oxidation catalysts may vary considerably among dif¬
ferent applications, there is a common need for catalysts
which maintain high activity over extended time periods
without experiencing a significant decay in performance.
The operational lifetime of catalysts utilized in
respiratory aids is of course a critical factor to be con¬
sidered. Catalysts for C02 lasers should be expected to
perform well for at least 3 years [6-8]. In CO gas sensor
applications, low-temperature operation is desirable
without the need for periodic flash heating in order to re¬
store catalyst activity [110]. Considering these important
performance criteria, the data in Figure 7-4 indicate that
Au/MnOx is a promising candidate for all these applica¬
tions. Even after 70 days near ambient temperature,
Au/MnOx continues to exhibit excellent CO oxidation perfor¬
mance with negligible activity decay. The optimized 14.5%
Pt/SnOx/SiOz catalyst does not perform nearly so well. It

4.0
3.0
2.0
1.0
0
(100% conversion)
-
Temperature
- â– 
variations
• #
•
•
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• •
•
•
• •• •••
•
••••
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-
T=35 °C
â–  10%Au/MnOx
J i I. i
• 14.5%Pt/Snq(/SiQ2
—i l i 1 i l■ i.i.i,
0 10 20 30 40 50 60 70 80 90
Time (days)
Figure 7-4. The CO oxidation activity of 10 at% Au/MnOx
(unpretreated) and 14.5 wt% Pt/SnOx/Si02 (pretreated in 5
vol% CO/He at 125 °C) at 35 °C.
118

119
experiences a significant induction period which lasts ap¬
proximately 1 week followed by considerable activity decay
which precludes its use in long-term applications.
Further understanding of the Au/MnOx catalyst can be
gained by varying the CO/Oz concentration ratio in the
reaction gas mixture. As shown by the data in Figure 7-5,
the composition of the test gas has a marked effect on the
activity profile of Au/MnOx. Overall performance is sig¬
nificantly enhanced in an oxygen-rich atmosphere whereas
activity in a CO-rich reaction mixture is diminished con¬
siderably. Similar results have been witnessed for Pt/SnOx
catalysts as well [27]. The data in Figure 7-5 are impor¬
tant with regard to understanding the variations in
catalyst performance with respect to compositional varia¬
tions and indicate that excess CO should be avoided if pos¬
sible. However, in the majority of low-temperature CO
oxidation applications oxygen is present in at least
stoichiometric concentrations relative to CO. Therefore,
Au/MnOx should continue to perform well in many applica¬
tions. The mechanism responsible for the activity decay in
a CO-rich reaction mixture is not fully understood at this
time. During CO pretreatment of these catalysts, GC
analysis suggests that the surfaces are progressively
reduced with corresponding COz formation. It is possible
that excess CO in the reaction gas acts to further reduce

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5 0.4
10% Au/MnOx - 1% CO, 1% 02
i
8
T=55 °C a 1%C0, 0.5% 02
0.2
• 1.2% CO, 0.5% 02
0
i i i i i.i,i,
0 2000 4000 6000 8000 10000
Time (min)
Figure 7-5. The CO oxidation activity of 10 at% Au/MnO„
(unpretreated) at 55 C as a function of test gas compo¬
sition (balance He).
120

121
the surface resulting in the subsequent loss of CO oxida¬
tion activity.
Under similar experimental conditions the performance
data obtained in this study compare favorably with activity
data for other low-temperature CO oxidation catalysts. All
of the Pt/SnOx catalysts prepared for this study have pre¬
viously been shown to be superior to proprietary catalysts
manufactured by General Motors and Teledyne [27]. Further¬
more, the activity of 19.5 wt% Pt/SnOx and 14.5 wt%
Pt/SnOx/SiOz appears to be comparable to numerous other
Pt/SnOx, Pd/SnOx and Pt-Pd/SnOx catalysts for similar reac¬
tion times [27,28,113]. However, long-term activity decay
continues to be a problem for all of the SnOx-based
materials. Consequently, Au/MnOx remains the optimum
catalyst under the test conditions used in this study.
Summary
This comparison study reveals that a 10 at% Au/MnOx
catalyst exhibits superior low-temperature CO oxidation
performance relative to several Pt/SnOx catalysts under
similar conditions of stoichiometric CO and Oz near ambient
temperatures. The activity of both Au/MnOx and Pt/SnOx is
enhanced subsequent to CO pretreatments at 50 and 125 °C
whereas an Oz pretreatment at 50 °C results in an activity
decrease. Au/MnOx exhibited a remarkable activity profile
wherein the activity decay remained essentially negligible

122
for reaction times up to 70 days. The versatility of
Au/MnOx has been demonstrated further with respect to reac¬
tion mixtures containing excess oxygen. As a result, this
study implies that Au/MnOx is a promising catalyst for many
practical applications of low-temperature CO oxidation in¬
cluding COz lasers, air purification and CO gas sensing.

CHAPTER 8
CATALYTIC BEHAVIOR OF NOBLE METAL/REDUCIBLE OXIDE
MATERIALS FOR LOW-TEMPERATURE CO OXIDATION: SURFACE
CHARACTERIZATION OF Au/MnOx
Introduction
The two previous studies (see Chapters 6 and 7) of
low-temperature CO oxidation on noble metal/reducible oxide
(NMRO) materials have identified several promising
catalysts for use in COz lasers, chemical sensors and air
purification devices. By far the most active materials ex¬
amined in these studies consist of gold supported on man¬
ganese oxide (Au/MnOx). The reaction data indicate that Au
and MnOx interact synergistically exhibiting remarkable
long-term CO oxidation activity near ambient temperatures
with negligible activity decay. However, little is under¬
stood about the nature of the Au-MnOx synergism and the
corresponding CO oxidation mechanism on these seemingly
complex surfaces.
It is well documented that the catalytic properties of
Au can be significantly altered in the presence of support
materials. Reviews of gold in catalysis [114,115] cite
numerous investigations of Au particles dispersed on sup¬
ports such as MgO, AlzOa and SiOz. Relative to more
massive forms of Au [116], supported Au particles often
123

124
exhibit a marked increase in surface reactivity, par¬
ticularly toward oxygen-bearing molecules. It has been
postulated that this enhanced reactivity results from an
interaction between the Au and the support material. For
example, in studies utilizing extended X-ray absorption
fine structure spectroscopy (EXAFS), interatomic distances
assigned to Au-0 species have been measured on Au/MgO and
Au/AlzOs surfaces [117,118]. Infrared spectroscopy of
Au/MgO has indicated CO adsorption bands which appear at
significantly lower wavenumbers than those previously ob¬
served on Au/SiOz and Au films [119]. Reaction experiments
have revealed that Au/SiOz and Au/MgO undergo extensive
isotopic oxygen exchange whereas under comparable condi¬
tions metallic Au, SiOz and MgO do not [120]. Data from
Mflssbauer spectroscopy and X-ray photoelectron spectroscopy
are consistent with Au-support interactions as well [121-
123]. While the exact nature of the Au-support interaction
(if it indeed occurs) remains somewhat speculative, it ap¬
pears to be a unique property which is dependent upon the
choice of support. In fact, a recent study has indicated
that characteristics of strong metal-support interactions
(SMSI) which are often observed for Group (VIII) metals
supported on TiOz, VzOa and NbzOs are absent on Au/TiOz
surfaces [124]. Nevertheless, the possibility of SMSI

125
remains for Au supported on metal oxides which are perhaps
more readily reducible [125].
Although postulations of a "support effect" appear to
be well substantiated on a considerable number of Au
catalysts, there is evidence that the Au particle size may
also affect the reactivity of these surfaces. Experiments
with numerous Au catalysts including Au/FezOa, Au/CozOa,
Au/NiO, Au/AlzOa and Au/SiOz [16] have revealed an overall
trend wherein CO oxidation activity increases with decreas¬
ing Au particle size. These results are consistent with
the observation that small Au particles exhibit an in¬
creased affinity for Oz [126]. However, some exceptions to
the former trend were observed which would appear to sig¬
nify the importance of the support material as well. It is
possible that ultrafine Au particles are a prerequisite for
Au-support interaction [127]. Under this assumption the
reactivity of supported Au may ultimately be traced to the
method of preparation. Indeed, relative to preparation via
impregnation, precipitation techniques yield smaller Au
particles and more active catalytic surfaces [16,126].
It is evident that the increased reactivity of sup¬
ported Au particles has prompted renewed interest in Au as
a major catalyst constituent. While there has been con¬
siderable research on the interaction of Au with relatively
inactive supports, it is apparent that Au supported on

126
reducible oxides has received little attention. This is
probably due to the fact that the catalytic activity of
these materials has only recently been discovered [16,110].
This chapter describes a detailed surface characterization
study which has been undertaken to elucidate the mechanism
of low-temperature CO oxidation on Au/MnOx. Auger electron
spectroscopy (AES), ion scattering spectroscopy (ISS) and
X-ray photoelectron spectroscopy (XPS) were utilized to in¬
vestigate Au/MnOx surfaces in order to examine the chemical
states of the surface species and the nature of their in¬
teraction. It is anticipated that these studies will aid
in improving the performance of Au/MnOx while yielding im¬
portant information which may be applied to other NMRO
catalytic surfaces as well.
Experimental
The catalytic surfaces investigated in this study were
10 at% Au/MnOx (on a Au:Mn basis) and a bare MnOx support.
Details regarding their preparation have been described in
the previous two chapters. Briefly, the samples were
prepared via coprecipitation from HAuCl* and/or Mn(N03)z.
The appropriate precursor solutions were added dropwise to
a stirred solution of NazCOa at room temperature. After
washing and drying the crushed precipitates were calcined
in air at 400 °C for 4 hours.

127
The as-prepared samples were pressed into tin specimen
cups and inserted into an ultrahigh vacuum (UHV) system
(base pressure of 10-11 Torr) for initial surface charac¬
terization (see Appendix). In order to correlate the sur¬
face characterization data to the CO oxidation activity
curves (see Chapters 6 and 7), the samples were sub¬
sequently transferred to a preparation chamber connected to
the UHV system and pretreated in 760 Torr of He for 1 hour
at 55 °C. The samples were heated on a custom platform
heating element [75]. After pretreatment the samples were
returned to the UHV analytical chamber without air exposure
for further characterization.
Energy analysis for the ISS, AES and XPS experiments
was accomplished using a Perkin-Elmer PHI Model 25-270AR
double-pass cylindrical mirror analyzer (CMA). The CMA
utilized an internal, movable aperture which varied the
polar acceptance angle for incoming particles. The ISS
spectra were collected in the nonretarding mode using a
147°-scattering angle (fixed by the experimental geometry)
and pulse counting detection [51]. A 100-nA, 1-keV 4He*
primary ion beam was defocused over a 1-cm2 area to mini¬
mize sputter damage. The AES experiments were performed in
the nonretarding mode using a 3-keV, 10-uA primary electron
beam with a 0.2-mm spot diameter. Survey and high-
resolution XPS spectra were recorded with Mg Ka excitation

128
in the retarding mode using 50- and 25-eV pass energies,
respectively. The XPS binding energies were referenced to
SnOz (486.4 eV) which was always present on the surface of
the tin specimen cups. The amount of Sn contribution to
the XPS spectra was kept small to ensure that the oxygen
present was not due to significant amounts of SnOx.
Results and Discussion
The ISS spectra taken from the Au/MnOx and MnOx
samples appear in Figures 8-1 and 8-2, respectively. The
spectra yield interesting information regarding the com¬
position of the outermost atomic layers of (a) the air-
exposed surfaces and (b) the surfaces pretreated in He at
55 °C. Both figures reveal prominent spectral features due
to the constituent atoms of Au, Mn and O. However, a sig¬
nificant surface concentration of Na is also indicated.
The Na originates from aqueous Na2CC>3 which was used in the
preparation of both samples. Given the significant
presence of Na on these surfaces, it is interesting that
Figure 8-1 indicates essentially a negligible concentration
of Cl (originating from the HAuCl* precursor solution) on
Au/MnOx. Chlorine features, if present, would appear near
0.66 E/Eo. It has not been determined if Cl was present on
Au/MnOx prior to washing, though it is possible that wash¬
ing with hot water is more effective in removing Cl from
these surfaces relative to Na. It has been suggested that

Figure 8-1. The ISS spectra of 10 at% Au/MnOx (a) before
and (b) after He pretreatment at 55 C for 1 hour.
129

Figure 8-2. The ISS spectra of MnOx (a) before and (b)
after He pretreatment at 55 C for 1 hour.
130

131
Cl should be avoided in the preparation of Pt/SnOx low-
temperature CO oxidation catalysts since the use of Cl-
containing precursors decreases the observed activity [28].
Considering the ISS spectra in Figure 8-1 as well as the
Au/MnOx reaction data in Chapters 6 and 7, the preparation
method used in this study appears to avoid significant com¬
plications due to Cl contamination from HAuCl*. The
relationship between surface Na and the low-temperature CO
oxidation activity of Au/MnOx has not yet been inves¬
tigated.
Figures 8-1 and 8-2 also indicate that the surface
composition of Au/MnOx and MnOx changes significantly as a
result of the He pretreatment at 55 °C. The relative
amounts of Na on both catalysts are subsequently reduced.
However, the spectra indicate an opposite trend with
respect to surface 0 concentration. That is, after He
pretreatment the O/Mn concentration ratio on Au/MnOx
slightly increases whereas the O/Mn concentration ratio
decreases considerably on MnOx. Furthermore, as shown in
Figure 8-1, there is a concurrent increase in surface Au
concentration on Au/MnOx. While these ISS data are consis¬
tent with an increased interaction between Au and MnOx, ad¬
ditional information from AES and XPS experiments is help¬
ful in determining the characteristics of these surfaces.

132
The AES spectra of Au/MnOx and MnOx both before (a)
and after (b) He pretreatment appear in Figures 8-3 and
8-4, respectively. Distinct Mn and 0 features appear
whereas features due to Au, Cl and Na are present but some¬
what less discernible. The AES spectra are in many ways
consistent with the ISS data. Firstly, the spectra indi¬
cate that the Na concentration is greater on the MnOx sur¬
face and that the amounts on both samples decrease sub¬
sequent to He pretreatment. The previous trends concerning
the surface 0 and Au concentrations are observed also.
After He pretreatment, Figure 8-3 indicates that the con¬
centration of Au and O on the Au/MnOx surface increases
relative to Mn whereas Figure 8-4 indicates that the rela¬
tive surface concentration of O decreases on MnOx. In ad¬
dition, Figure 8-3 depicts an increase in Au concentration
after the Au/MnOx has been pretreated. Contrary to the ISS
data in Figure 8-1, however, the AES spectra in Figure 8-3
reveal visible features due to Cl. These AES spectra indi¬
cate that the amount of Cl remains small relative to Na
(the AES cross section for Cl is nearly 5 times that of Na
[128]) and that the Cl is essentially confined to a region
slightly beneath the Au/MnOx surface. This is due to the
fact that AES probes a region which consists of ap¬
proximately the top 20 angstroms of the surface whereas ISS
is essentially sensitive to the outermost one or two atomic

100 300 500 700 900
KINETIC ENERGY (eV)
Figure 8-3. The AES spectra of 10 at% Au/MnOv (a) before
O X
and (b) after He pretreatment at 55 C for 1 hour.
133

Figure 8-4. The AES spectra of MnOx (a) before and (b)
after He pretreatment at 55 C for 1 hour.
134

135
layers. In a manner similar to that observed for Na, the
amount of Cl decreases after He pretreatment. Considering
the AES spectra in Figures 8-3(b) and 8 — 4(b), an estimate
of elemental cross sections [128] and surface composition
suggests an O/Mn concentration ratio which corresponds ap¬
proximately to MnaO*. Similarly, the data in Figure 8-3(b)
indicate a gold concentration near 11 at% based upon Au and
Mn only. This compares favorably with a Au concentration
of 10 at% based upon Au and Mn contained in the preparation
solutions.
In order to understand the CO oxidation mechanism on
Au/MnOx, it is necessary to determine not only the surface
composition but also the chemical states of the surface
species. In addition, knowledge of the Au and Mn chemical
states with respect to both Au/MnOx and MnOx should provide
insight into the synergistic interaction between Au and
MnOx. X-ray photoelectron spectroscopy is a particularly
useful technique for obtaining chemical state information
and it complements the data which is available from ISS and
AES experiments. Thus far the ISS and AES data have indi¬
cated that Au/MnOx and MnOx each respond differently to a
pretreatment in He at 55 °C. The surface of MnOx becomes
depleted of 0 whereas the surface of Au/MnOx is enriched
not only with O but with Au as well. It is important to
determine whether the 0 enrichment is due to a genuine

136
change in the Mn and/or Au oxidation state(s) or simply the
result of an increased surface concentration of Au which is
present in an oxidized state.
Manganese-oxygen systems have previously been studied
using XPS techniques [129-133]. Unfortunately, XPS peak
binding energies alone are not sufficient to accurately
determine the chemical state of Mn in manganese oxides.
That is, the Mn 2p features are relatively broad for MnO*
compounds and consequently, shifts in binding energy gener¬
ally occur within experimental error of the XPS instrument.
This problem is compounded by the fact that manganese
oxides are often complex and may contain Mn which is
present in several chemical states together with con¬
siderable amounts of water [134]. As a result, the
reported Mn 2pa/z binding energies for manganese oxides
have been largely inconsistent [129-133]. However, addi¬
tional information is available from XPS which has proven
useful in determining the Mn oxidation state. This infor¬
mation is obtained from the location of Mn 2p shake-up
lines, the extent of Mn 3s multiplet splitting, XPS
valence-band structure and XPS 0 Is spectra [129-133].
The Mn 2p XPS spectra of Au/MnOx and MnOx taken (a)
before and (b) after pretreatment in He at 55 °C appear in
Figures 8-5 and 8-6, respectively. Although the spectra
for both samples appear similar in overall character there

Figure 8-5. The XPS Mn 2p spectra of 10 at% Au/MnOx (a)
before and (b) after He pretreatment at 55 C for 1 hour.
137

Figure 8-6. The XPS Mn 2p spectra of MnOx (a) before and
(b) after He pretreatment at 55 C for 1 hour.
138

139
are visible differences between them. Figure 8-5 indicates
that the Mn 2ps/z peaks of Au/MnOx remain essentially sta¬
tionary after pretreatment with a binding energy near 641.1
eV. In addition, several features appear on the high bind¬
ing energy side of the 2p peaks. In Figure 8-5(a), a small
shoulder is indicated approximately 4.4 eV from the Mn
2p3/2 peak and two small shake-up peaks appear which are
shifted about 5.5 eV from their respective 2p peaks. Sub¬
sequent to the He pretreatment, Figure 8-5(b) suggests that
the two shake-up peaks remain whereas the 2pa/z shoulder
becomes more apparent on the 2pw? peak. Unlike many of
the Mn 2p spectra reported in the literature
[129,130,132,133], the Mn 2p spectra recorded in this work
exhibited a high background signal above 660 eV binding
energy. As a result it is difficult to identify shake-up
structures which might correspond to MnzOa, MnsO« or Mn02.
These structures, if present, would be located near 664 eV
binding energy [133]. Although several features resembling
shake-up structures appear in this proximity, it is specu¬
lated that such features are artifacts of the high-
intensity background given their number and randomness.
The small shake-up structures appearing about 5.5 eV from
the 2p peaks are consistent with MnO presence. However,
the position of the shoulders on the 2p peaks themselves
does not correspond to previously measured Mn 2p binding

140
energies or shake-up peak separations [129-133]. Although
these features are not fully understood, they may be an in¬
dication of the complexity of these MnOx surfaces. This
assumption is consistent with the small magnitude of the
shake-up peaks. It has been suggested that strong shake-up
lines are observed on these types of surfaces only when
stoichiometric oxides are present [131]. Accordingly the
spectra in Figure 8-5 (and 8-6) would appear to indicate
multiple Mn oxidation states. Indeed, spectra similar to
those in Figures 8-5 and 8-6 exhibiting numerous shoulders
on the Mn 2p peaks have been observed for discontinuous
films consisting of mixtures of Mn metal and MnO [132].
As indicated in Figure 8-6, relative to Au/MnOx, the
Mn 2p peaks of MnOx appear at slightly higher binding
energy (about 641.4 eV) and they too remain essentially
stationary as a result of the He pretreatment. Further¬
more, the shoulders on the high binding energy side of the
2p peaks appear more distinct and numerous, again perhaps
an indication of several Mn chemical states. Small shake-
up structures are visible approximately 5 eV from the 2p
peaks which within experimental error are consistent with
MnO presence. Similar to the Mn 2p lines of Au/MnOx, the
Mn 2p spectra in Figure 8-6 exhibit a high-intensity back¬
ground above 660 eV which hinders the identification of
potential shake-up lines in this region.

141
The XPS Mn 2p binding energies measured for Au/MnO*
and MnOx are near those which have been reported for both
MnO and MnaCU [133]. Therefore, the existence of multiple
Mn oxidation states in Figures 8-5 and 8-6 is further sub¬
stantiated. However, a shake-up structure approximately
10-11 eV above the Mn 2pi/2 peak has been shown to be in¬
dicative of Mn a 0 4 presence [129-133]. Given the data in
Figures 8-5 and 8-6, the appearance of such a feature can¬
not be confirmed. Observation of XPS Mn 2p spectra of
MnaO* [129] reveals that the magnitude of the shake-up
structures are such that they could become insignificant
against a high-intensity background. The shake-up struc¬
tures should be further attenuated if additional forms of
Mn are present on a complex surface [131,133]. Therefore,
it is possible that the shake-up features are present yet
too small to be resolved in this work.
It has been demonstrated that information regarding
the Mn oxidation state may be realized from the extent of
Mn 3s multiplet splitting [129,131-133]. Unfortunately,
XPS Mn 3s spectra may only be obtained for MnOx because the
Mn 3s peaks of Au/MnOx are obscured by the Au 4f peaks. In
order to compensate for excessive surface charging in the
region of 0-100 eV binding energy, the Mn 3s spectra were
referenced to the valence band of Mn at approximately 4 eV
below the Fermi level [132,133]. The surface charging is

142
not a severe limitation in this case since the magnitude of
the Mn 3s splitting is measured relative to the Mn 3s sig¬
nal itself- The Mn 3s spectra of MnO* (a) before and (b)
after He pretreatment appear in Figure 8-7. The broad ap¬
pearance of the spectra, particularly after He pretreat¬
ment, is consistent with the presence of multiple Mn chemi¬
cal states. In addition, it appears that the multiplet
splitting features also contain contributions from dif¬
ferent oxidation states of Mn. As a result, it is dif¬
ficult to determine a representative value of the extent of
Mn 3s multiplet splitting. The spectrum in Figure 8-7(a),
which is the most easily resolved, indicates two sig¬
nificant features which are separated by approximately 5
eV. This value is close to that which has been measured
for Mn3O4 and MnzOa [129].
Figures 8-8 and 8-9 show valence-band XPS spectra for
Au/MnOx and MnOx (a) before and (b) after He pretreatment
at 55 °C, respectively. The spectra indicate at least two
major peaks which are located near 4 and 7 eV binding
energy. The former is attributed to the localized 3d
states of Mn and the latter is due to 0 2p electrons
[130,132,1331. The shape and relative intensity of these
two features yields insight into the nature of the Au/MnOx
and MnOx surfaces. As indicated by Figures 8-8(a) and
8-9(a), the XPS valence-band spectra of the air-exposed

Figure 8-7. The XPS Mn 3s spectra of MnOx (a) before and
(b) after He pretreatment at 55 C for 1 hour.
143

BINDING ENERGY (eV)
Figure 8-8. The valence-band XPS spectra of 10 at% Au/MnO„
(a) before and (b) after He pretreatment at 55 C for 1 hour.
144

BINDING ENERGY (eV)
Figure 8-9. The valence-band XPS spectra of MnOx (a) before
and (b) after He pretreatment at 55 C for 1 hour.
145

146
Au/MnOx and MnOx surfaces exhibit different characteris¬
tics. The spectrum for Au/MnOx resembles spectra cor¬
responding to the intermediate formation of MnzOa on MnO
while the spectrum for MnOx appears to be more indicative
of MnzOa [132,133]. Figures 8—8(b) and 8-9(b) suggest that
significant changes occur in the surface composition of
Au/MnOx and MnOx as a result of the He pretreatment. Rela¬
tive to the Mn 3d peak, the intensity of the O 2p peak in¬
creases on Au/MnOx and decreases on MnOx. This is consis¬
tent with the ISS and AES data which indicate that the sur¬
face of Au/MnOx becomes oxygen-enriched whereas the MnOx
surface becomes depleted of oxygen. The spectrum in Figure
8—8(b) is similar to that of Mn2Ü3 while the spectrum in
Figure 8 — 9(b) resembles that of MnO [132,133].
It is apparent that the assignment of a single Mn
oxidation state on the Au/MnOx and MnOx surfaces would not
be consistent with the data obtained in this study thus
far. That is, with respect to previous investigations of
MnOx systems [129-133] the XPS data in the present study
have indicated characteristics which correspond to MnO,
Mn a 0 4 as well as MnzOa. If Mn is indeed present as several
phases of manganese oxide, perhaps evidence of different Mn
oxidation states may be realized from the XPS 0 Is spectra.
The 0 Is XPS spectra of Au/MnOx and MnOx (a) before and (b)
after He pretreatment at 55 °C are shown in Figures 8-10

147

Figure 8-11. The XPS O Is spectra of MnO„ (a) before and
o *
(b) after He pretreatment at 55 C for 1 hour.
148

149
and 8-11, respectively. The binding energies are near
529.1 eV for Au/MnOx and approximately 529.3 eV for MnOx
and they remain essentially constant. The spectral binding
energies and peak shapes are similar to 0 Is spectra re¬
corded for MnaO« [129]. Subsequent to the He pretreatment,
Figures 8 — 10(b) and 8 — 11(b) reveal that the high binding
energy shoulder increases for Au/MnOx and slightly
decreases for MnOx. Similar peak shoulders have been ob¬
served in XPS 0 Is spectra of Pt/SnOx surfaces and they are
believed to be due to hydroxyl groups and adsorbed water as
discussed in Chapter 4. Tin oxide, like MnOx, is a
reducible metal oxide whose surface composition is often
quite complex [48,60]. Therefore, the data in Figures 8-10
and 8-11 may indicate that relative to pretreated MnOx, the
pretreated surface of Au/MnOx contains a greater concentra¬
tion of hydroxide species and adsorbed water. This assump¬
tion is consistent with the hypothesis that hydroxyl groups
and surface moisture are important in the low-temperature
CO oxidation mechanism on Pt/SnOx and perhaps other NMRO
surfaces [26]. Recall that the data in Chapters 4 and 5
for Pt/SnOx surfaces indicated that optimum CO oxidation
performance coincides with a maximum in surface Pt(OH)z
concentration.
Given the data above it is possible that the increased
0 concentration on the surface of pretreated Au/MnOx may be

150
due to Au which is present in an oxidized state, perhaps as
Au hydroxide. Therefore, it was anticipated that informa¬
tion from the Au 4f spectra of Au/MnOx might prove benefi¬
cial. The XPS Au 4f spectra of Au/MnOx (a) before and (b)
after He pretreatment appear in Figure 8-12. The Au 4f?/2
peak of the air-exposed Au/MnOx surface is located near
85.3 eV binding energy whereas upon He pretreatment the
peak shifts to approximately 85.7 eV. However, when the
spectra are superimposed they appear essentially identical
in overall peak shape and peak width. Such behavior is in¬
dicative of differential surface charging and as a result
the binding energies of the spectra in Figure 8-12 may not
be representative of the true chemical state of Au.
Nevertheless, the binding energies are between those
reported for metallic Au at 83.8 eV 180] and AU2O3 at 86.3
eV [16]. A literature search was unable to locate an es¬
tablished binding energy for Au hydroxide.
There is additional information to consider with
respect to Figure 8-12 and its interpretation. Investiga¬
tions of Au deposited on A1 zOa and Si02 [122,135] have
reported XPS Au 4f peaks which are shifted to higher bind¬
ing energy (as high as 85 eV) when small Au particles (or
islands) with dimensions on the order of 30 angstroms or
less are present. When the Au particle dimensions became
larger, the Au 4f peaks were progressively shifted toward

151

152
the binding energy of bulk metallic gold. These observa¬
tions have been attributed to matrix effects of the support
material and perhaps real differences in the electronic
structure of small Au particles relative to bulk Au. The
latter is consistent with the increased reactivity of small
Au particles. Since the Au particle size distribution has
not been measured for the Au/MnOx sample, relationships be¬
tween the Au particle size and the Au 4f binding energy
cannot be assessed in the present study. On the contrary,
Haruta and co-workers [16] have studied a Au/FezOa catalyst
which was prepared by a coprecipitation technique similar
to the one used in this study. The surface was charac¬
terized by ultrafine Au particles (mean diameter of ap¬
proximately 36 angstroms) and the measured Au 4f?/z binding
energy was 83.9 eV. Furthermore, no detectable differences
were observed between the Fe 2p and 0 Is spectra of the
Au/FezOa surface and the bare FezOa support. The Mn 2p and
0 Is spectra of Au/MnOx and MnOx in the present study,
however, do exhibit significant differences and this may
correlate with the increased binding energies observed in
the Au 4f spectra of Figure 8-12. These differences are
consistent with reaction data in Chapter 6 which indicate
that Au/MnOx is superior to Au/FezOa with respect to
measurements of low-temperature CO oxidation activity using
stoichiometric CO and Oz.

153
Summary
Numerous Au/MnOx and MnOx surfaces used for low-
temperature CO oxidation have been characterized before and
after pretreatment in He at 55 °C using ion scattering
spectroscopy (ISS), Auger electron spectroscopy (AES) and
X-ray photoelectron spectroscopy (XPS). The surfaces are
complex and appear to contain multiple phases of Mn with an
average composition estimated near MnaO*. Significant con¬
centrations of Na with much smaller amounts of Cl remain on
the surfaces as a result of the preparation. In addition,
hydroxylated species and adsorbed water appear to be
present. Subsequent to the He pretreatment, the surface of
Au/MnOx is enriched in 0 and Au whereas the surface of MnOx
becomes oxygen-depleted. Coincidentally, the XPS spectra
before and after pretreatment exhibit significant dif¬
ferences which may be indicative of a Au-MnOx interaction.
However, the extreme heterogeneous nature of the Au/MnOx
and MnOx surfaces precludes an exact determination of the
Mn and Au chemical states present. The relative surface
concentrations observed for Au, Mn, 0, Na and Cl must
correlate with the exceptional low-temperature CO oxidation
activity exhibited by Au/MnOx but such a correlation has
not been investigated in this study.

CHAPTER 9
SUMMARY
Research has been performed with the objective of
developing novel catalysts for low-temperature CO oxidation
in COz lasers. Initial experiments were designed to learn
more about the low-temperature CO oxidation mechanism on
Pt/SnOx, a catalyst previously known to exhibit significant
activity in this application. The experiments utilized ion
scattering spectroscopy (ISS), Auger electron spectroscopy
(AES) and X-ray photoelectron spectroscopy (XPS) to examine
the chemical species present on Pt/SnOx surfaces as a func¬
tion of numerous pretreatments, and the results were corre¬
lated to catalytic activity data. The results indicate
that pretreatments which produce optimum CO oxidation ac¬
tivity alter the surface composition of Pt/SnOx such that
Pt(OH)z becomes the dominant Pt species. In addition, a
measurable amount of Sn is reduced to metallic form which
appears to alloy with Pt. Similar results were obtained
for a Pt/SnOx/SiOz catalyst, although pretreatment at an
elevated temperature beyond the optimum promoted extensive
encapsulation of surface Pt and Sn with impurities con¬
tained in the SiOz substrate.
154

155
These results provided motivation for investigating
Pt/Sn alloy surfaces themselves. A PtaSn alloy was charac¬
terized with ISS, XPS, AES and ang 1 e-r e so 1 ved Auger
electron spectroscopy (ARAES) before and after reduction in
H2 at 300 °C. The air-exposed surface consisted of a
uniform layer of SnOx and Sn(OH)x over a Pt-rich sublayer.
The data suggest that during reduction in H2, Pt diffuses
to the surface through vacancies left in the matrix by O
which reacted with the Hz and desorbed. Furthermore, most
of the Sn is reduced to metallic Sn which probably alloys
with the Pt.
In another phase of the research, numerous materials
were synthesized and screened for low-temperature CO oxida¬
tion activity in stoichiometric amounts of CO and Oz. The
most active catalysts tested were Au/CeOx and Au/MnOx. In
particular, the Au/MnOx catalyst exhibited remarkable CO
oxidation activity near ambient temperature, surpassing
that of Pt/SnOx by approximately one order of magnitude.
The long-term activity of Au/MnOx was sustained for periods
greater than 70 days with negligible decay in performance.
The experiments indicate that Au/MnOx, and perhaps Au/CeOx,
are promising catalysts for use in COz lasers as well as
air purification and CO gas sensing applications. The ac¬
tivities need to be tested over a wider range of CO oxida¬
tion environments.

156
Using ISS, AES and XPS, Au/MnOx and MnOx surfaces were
subsequently characterized in order to investigate the CO
oxidation mechanism. The surfaces appeared to be extremely
heterogeneous containing several chemical forms of Mn with
substantial amounts of water or hydroxyl groups. The
spectra were consistent with MnaO« identification whereas
Au appeared to be present in very small dimensions, pos¬
sibly in an oxidized state. The behavior of Au/MnOx and
MnOx toward an inert pretreatment suggests the possibility
of a Au-MnOx interaction.
Although Au/MnOx was the most active catalyst dis¬
covered in the screening experiments, its performance has
yet to be optimized. That is, significant increases in CO
oxidation activity may be realized by pretreating the sur¬
face prior to reaction and/or optimizing the preparation
procedures. Variables which should be considered include
the overall Au content and the concentration of surface
impurities. The Au/MnOx samples prepared in this study al¬
ways contained considerable amounts of Na originating from
the preparation procedure. It is therefore important to
determine if surface Na plays a key role in the CO oxida¬
tion mechanism on Au/MnOx. In addition to Na, other ele¬
ments should be screened as promoters for low-temperature
CO oxidation on Au/MnOx.

157
Finally, the experimental data from this research have
established correlations between CO oxidation activity and
the surface composition of Pt/SnOx and Au/MnOx which are
valid only during the onset of reaction. However, data in¬
dicate that the surfaces of Pt/SnOx and Au/MnOx undergo
considerable changes during long-term CO oxidation activity
measurements. Therefore, future research should be
oriented toward surface characterization studies which re¬
late the surface composition to the activity data over a
much broader time scale. Such information may provide in¬
sight into the exceptional decay properties of Au/MnOx
relative to Pt/SnOx while providing further knowledge of
the CO oxidation mechanism on these surfaces.

APPENDIX
DESCRIPTION OF THE ULTRAHIGH VACUUM SURFACE
ANALYSIS SYSTEM
A schematic (top view) of the uitrahigh vacuum (UHV)
system which was utilized in this research appears in
Figure A-1 . The apparatus is composed of several in¬
dividual chambers each of which may be isolated to perform
a specific function. A preparation chamber is available
for numerous surface treatments including exposure to
selected gaseous atmospheres (low and high pressure),
sample heating and metal deposition. In addition, the
system consists of two analytical chambers which are
equipped to perform an extensive number of surface analyti¬
cal techniques as listed in Table A-l. Two long-stroke
manipulators provide an effective means of specimen trans¬
fer among the chambers without exposing the surfaces to
air.
The UHV system itself utilizes turbo molecular pumping
throughout in series with dual-stage, rotary-vane mechani¬
cal pumps. In addition, the analytical chambers employ Ti
sublimation and ion pumping to attain ultimate pressures
near 10*11 Torr. A PHI Model 25-270AR double-pass
cylindrical mirror analyzer (CMA) is mounted vertically in
158

159
the main chamber as indicated in Figure A-l. A specimen
manipulator provides accurate sample placement beneath the
CMA while enabling heating and cooling via feedthroughs in
its flange. Numerous chamber ports are available which may
be used to direct various source beams to the CMA focal
point. The UHV system main chamber is currently equipped
with an ion gun, two electron guns (one mounted coaxially
within the CMA and one mounted off-axis for angle-resolved
Auger electron spectroscopy (ARAES)) and a Mg Ka X-ray
source.
The UHV surface analytical system has proven to be
very effective in studying catalytic surfaces of varying
degrees of heterogeneity. The complimentary nature of the
analytical techniques allows the determination of the com¬
position and chemical states of elements in a region con¬
sisting of the outermost atomic layers to near 50 angstroms
beneath the surface. In particular, the system has enabled
the catalytic surfaces to be characterized as a function of
numerous reductive and oxidative surface pretreatments
(performed in the preparation chamber without intermittent
exposure to air) which may then be correlated with
catalytic activity data. Indeed, this type of information
has been a powerful tool for the elucidation of the low-
temperature CO oxidation mechanism on Pt/SnO* and Au/MnOx
catalysts as indicated by the present research.

Turbopump
Figure A-l. A schematic of the UHV surface analysis system.
160

161
Table A-l. The experimental techniques available
from the UHV surface analysis system.
Cylindrical Mirror Analyzer
X-ray Photoelectron Spectroscopy
Ultraviolet Photoelectron Spectroscopy (UPS)
Auger Electron Spectroscopy (AES)
Scanning Auger Microscopy (SAM)
Electron Energy-Loss Spectroscopy (EELS)
Electron-Stimulated Desorption (ESD)
Ion Scattering Spectroscopy (ISS)
Depth Profiling
Others
A. Work Function Measurements
B. Temperature-Programmed Desorption (TPD)
C. Isotope Exchange Experiments
D. Secondary Ion Mass Spectroscopy (SIMS)
E. Gas dosing
F. Heating to 1800 K
G. Cooling to 10 K
H. Inverse Photoelectron Spectroscopy (IPS)
I. Metal Deposition
J. High-Pressure Treatment
a
Can be performed in the angle-resolved mode

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BIOGRAPHICAL SKETCH
The author was born on December 19, 1961, in Gadsden,
Alabama, to James H. and R.J. Gardner. He attended
Anniston High School and graduated in May, 1980, with
valedictorian honors. He subsequently enrolled at the
University of Alabama in Huntsville (UAH) where he was
awarded a B.S.E. degree in chemical engineering in June,
1985. The author chose to continue studying at UAH under
the direction of Professor James E. Smith where he re¬
searched heterogeneous catalysis and CO hydrogenation. He
was awarded an M.S.E. degree in chemical engineering in
June, 1987. The following fall the author enrolled at the
University of Florida where he continued his graduate
studies in catalysis and surface science under the guidance
of Professor Gar B. Hoflund. The author will receive his
Ph.D. in chemical engineering during December, 1990, after
which he will join the chemical engineering faculty at
Mississippi State University as Assistant Professor.
172

I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
Gar B. HoflundH Chairman
Professor of Chemical
Engineering
I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
YA
Timothy ¿r./Anderson
Professor of Chemical
Engineering
I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
Professor of Materials
Science and Engineering
I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degr
Herbert A. Laitinen
Graduate Research
Professor of Chemistry

I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the deqree of Doctor of Philosophy.
Vaneica Y. You/ig
Associate Professc
Chemistry
This dissertation was submitted to the Graduate
Faculty of the College of Engineering and to the Graduate
School and was accepted as partial fulfillment of the
requirements for the degree of Doctor of Philosophy.
December, 1990
Winfred M. Phillips
Dean, College of Engineering
Madelyn M. Lockhart
Dean, Graduate School

3 1262 08553 ®188



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