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Low-Temperature Catalytic Oxidation of Carbon Monoxide over Palladium Metal, Hydrous Palladium Oxides, and Anhydrous Pal...

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Permanent Link: http://ufdc.ufl.edu/UFE0013045/00001

Material Information

Title: Low-Temperature Catalytic Oxidation of Carbon Monoxide over Palladium Metal, Hydrous Palladium Oxides, and Anhydrous Palladium Oxides
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0013045:00001

Permanent Link: http://ufdc.ufl.edu/UFE0013045/00001

Material Information

Title: Low-Temperature Catalytic Oxidation of Carbon Monoxide over Palladium Metal, Hydrous Palladium Oxides, and Anhydrous Palladium Oxides
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0013045:00001

Full Text












LOW-TEMPERATURE CATALYTIC OXIDATION OF CARBON MONOXIDE
OVER PALLADIUM METAL, HYDROUS PALLADIUM OXIDES, AND
ANHYDROUS PALLADIUM OXIDES















By

SEUNG-HOON OH


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


2005

































Copyright 2005

by

Seung-Hoon Oh

































This dissertation is dedicated to my parents, Byoung-Chul Oh and Jung-Ja Kim;
my wife, Min Chae; and my children, Anthony Jin-Wook and Ashley Jin-Sun for all their
love and support.















ACKNOWLEDGMENTS

I would like to thank my advisor, Dr. Gar B. Hoflund, for his guidance and

understanding and for giving me the opportunity to work in the area of catalysis. I also

appreciate the support of my colleague Michael Everett.

















TABLE OF CONTENTS



ACKNOW LEDGM ENTS ........................................ iv

LIST OF TABLES ............... ..................... ...................... ...... .. ........ vii

LIST OF FIGURES .................. ............................................... .............. viii

ABSTRACT................................. .............. xii

CHAPTER

1. IN TRODU CTION .................. .............................. ......... .. ..... ....... ................

1.1 CO Oxidation Mechanism over Metals and Metal Oxides............... ...............1..
1.2 CO Oxidation A application: CO 2 Laser................................................................3

2. CHEMICAL STATE OF PALLADIUM POWDER AND CERIA-SUPPORTED
PALLADIUM DURING LOW-TEMPERATURE CO OXIDATION........................7

2.1 Introduction............ ................................7
2.2 Experimental ........................................ .........9
2.3 Results and Discussions.................. ..... ......................10
2.3.1 Pd Powders .................... .....................10
2.3.2 Ceria-Supported Pd ................. ................................13
2.4 Summary......................................................... ...... ......... ........ 15

3. CARBON MONOXIDE OXIDATION OVER PALLADIUM METAL POWDER:
SLIGHTLY OXYGEN-RICH FEED STREAM.....................................................25

3.1 Introduction...................................... .................. ............ ........ 25
3.2 Experimental ........................................ .........26
3.3 R results and D discussion ................................................ ............... 28
3.4 Summary ......................................................... ..............40

4. LOW-TEMPERATURE CO OXIDATION OVER HYDROUS PALLADIUM
OXIDE ......................................................... .................53

4.1 Introduction......................................................... .............. ........ 53
4.2 Experimental ........................................ .........55










4 .3 R results and D iscu ssion ................................................................................... 56
4.4 Sum m ary .................................................................................. 62

5. NEAR-STOICHIOMETRIC CO OXIDATION OVER HYDROUS PALLADIUM
OXIDE FOR VARIOUS REACTION TEMPERATURES ......................................80

5.1 Introduction ......... ................ ........................... .............80
5.2 Experimental ........................... ............... .........8 1
5.3 R results and D discussion ................................................ ............... 82
5.4 Summary.............. ........... ..................88

LIST O F R EFER EN CE S ...................................................................................................98

BIOGRAPHICAL SKETCH ........................ ................. 102
















LIST OF TABLES


Table page

3-1. Feed stream compositions used in published studies of CO oxidation over
various supported noble metal catalysts.................... ...................42

3-2. Comparison of CO/02 concentration ratios obtained from GC area and the actual
MFC flow rates for various concentrations of CO and 02 in He..........................43

5-1. CO/02 feed ratios and 02 concentrations in He flow various reaction
temperatures. The total flow rate was 45 cc/min. The calculated relative 02
concentration is based on 1.0 % CO in the feed............................... ...........89
















LIST OF FIGURES


Figure page

1-1. Power output of a CO2 laser as a function of time............... ...............5

1-2. CO and 02 concentrations in a continuously operated high pulse repetition
frequencies CO2 TEA laser with and without a Pd/SnO2 catalyst............................6

2-1. CO conversion as a function of temperature for two different size Pd metal
powders.. ........................................................17

2-2. High-resolution XPS Pd 3d spectra obtained from Pd metal after oxidation using
0.5 % 02 at different temperatures ............................................... ....... 18

2-3. High-resolution XPS O is and Pd 3p spectra obtained from Pd metal after
oxidation using 0.5 % 02 at different temperatures...............................................19

2-4. High-resolution XPS Pd 3d spectra obtained from Pd metal oxidation after using
1.0 % CO and 0.5 % 02 at different temperatures ...................................... 20

2-5. High-resolution XPS O is and Pd 3p spectra obtained from Pd metal oxidation
using 1.0 % CO and 0.5 % 02 at different temperatures................ ...............21

2-6. CO conversion as a function of temperature over nanocrystalline ceria, a
physical mixture of 0.01 g of Pd metal powder and 0.09 g of nanocrystalline
ceria and 10 wt% Pd/n-CeO2 (impregnated). ............. .................. ............22

2-7. Conversion decay curves for 10 wt% Pd/n-CeO2 (impregnated) while operating
the catalytic reactor at 50 and 110 C. ................................................. 23

2-8. High-resolution XPS Pd 3d spectra obtained from fresh 10 wt% Pd/n-CeO2, 10
wt% Pd/n-CeO2 after 90 min reaction at 50 oC and after 90 min reaction at 110
C ........................................................................ 24

3-1. CO conversions over 52 mg of Pd metal powder as a function of (a) temperature
and CO/02 ratio (1.00, 2.00 and 2.88) and (b) time and CO/02 ratio (1.96 and
2.13)..............................................44

3-2. CO and 02 conversions over 52 mg of Pd metal powder as a function of time
before and after reaction gas bypass: (a) 210 oC, (b) 200 oC, (c) 195 oC, (d) 190
oC, (e) 185 oC and (f) 180 C .......................................................... 45









3-3. CO and 02 conversions over 52 mg of Pd metal powder as a function of
temperature and time before and after reaction gas bypass: (a) 205 'C, (b) 200
TC and (c) 190 C...............................................46

3-4. CO and 02 conversions over 52 mg of Pd metal powder as a function of time at
180 TC before and after reaction gas bypass.............................. ........ .......47

3-5. CO and 02 conversions over 52 mg of Pd metal powder as a function of
temperature and time before and after reaction gas bypass: (a) 205 'C, (b) 200
'C, (c) 195 TC and (d) 190 C .................. ................................48

3-6. CO and 02 conversions over 52 mg of Pd metal powder as a function of time at
170, 180, 190, 200 and 210 TC before and after reaction gas bypass....................49

3-7. CO and 02 conversions over 0.12 g of Pd metal powder as a function of time at
185 TC (a) before and (b) after a bypass.......... ...................................50

3-8. High-resolution XPS (a) Pd 3d, (b) 0 Is and Pd 3p and (c) C Is spectra.............51

3-9. CO and 02 conversions over 52 mg of Pd powder as a function of time and
temperature (a) 200 TC and (b) 190 and 195 TC before and after reaction gas
bypass ...................................... .................................. ........ 52

4-1. Comparison of CO and 02 conversions as a function of reaction time over 100
mg of hydrous PdO at 100 oC before and after annealing in He at 400 oC..............64

4-2. High-resolution XPS Pd 3d spectra obtained from fresh hydrous PdO, hydrous
PdO after drying in He at 150 oC for 120 min in the reactor and hydrous PdO
after drying in He at 400 oC for 120 min in the reactor............... ...............65

4-3. High-resolution XPS O is and Pd 3p spectra obtained from fresh hydrous PdO,
hydrous PdO after drying in He at 150 oC for 120 min in the reactor and hydrous
PdO after drying in He at 400 oC for 120 min in the reactor. ...............................66

4-4. Comparison of CO and 02 conversions as a function of reaction time at 25 TC
over 400 mg of hydrous PdO and anhydrous PdO................................. ...............67

4-5. High-resolution XPS Pd 3d spectra obtained from fresh hydrous PdO and fresh
anhydrous PdO. ....................................................68

4-6. High-resolution XPS O is and Pd 3p spectra obtained from fresh hydrous PdO
and fresh anhydrous PdO ............................................... ............... 69

4-7. High-resolution XPS Pd 3d spectra obtained from fresh hydrous PdO, hydrous
PdO after exposure to 1.0 % CO and 0.51 % 02 for 30 min in the catalytic
reactor at 25 TC and hydrous PdO after exposure to 1.0 % CO and 0.51 % 02 for
220 min in the catalytic reactor at 25 C ........................................ .....70









4-8. High-resolution XPS O is and Pd 3p spectra obtained from fresh hydrous PdO,
hydrous PdO after exposure to 1.0 % CO and 0.51 % 02 for 30 min in the
catalytic reactor at 25 TC and hydrous PdO after exposure to 1.0 % CO and 0.51
% 02 for 220 min in the catalytic reactor at 25 oC.................... .............. 71

4-9. CO and 02 conversions as a function of reaction time over 400 mg of hydrous
PdO at 100 TC for 205 min and 25 TC for 217 min ............. ..................................72

4-10. High-resolution XPS Pd 3d spectra obtained from fresh hydrous PdO, hydrous
PdO after exposure to 1.0 % CO and 0.51 % 02 for 220 min in the catalytic
reactor at 25 TC and hydrous PdO after exposure to 1.0 % CO and 0.52 % 02 for
205 min in the catalytic reactor at 100 C ........................................ .....73

4-11. High-resolution XPS O is and Pd 3p spectra obtained from fresh hydrous PdO,
hydrous PdO after exposure to 1.0 % CO and 0.51 % 02 for 220 min in the
catalytic reactor at 25 TC and hydrous PdO after exposure to 1.0 % CO and 0.52
% 02 for 205 min in the catalytic reactor at 100 oC........... ............................. 74

4-12. CO conversion as a function of reaction time over 400 mg of hydrous PdO in 1.0
% CO and 0.51 % 02 in He and 1.0 % CO in He both at a total flow rate of 45
cc/m in .............. ........ ......... .. .. ........................ 7 5

4-13. CO and 02 conversions as a function of reaction time over 400 mg of hydrous
PdO at 25 TC (216 min), 70 TC (77 min) and 100 TC (59 min) .............................76

4-14. CO conversion as a function of reaction time at 25 TC (216 min), 70 TC (70 min)
and 100 C (66 min) ...................... .......... ...... ... ........ 77

4-15. High-resolution Pd 3d spectra obtained from fresh hydrous PdO, hydrous PdO
after exposure to 1.0 % CO and 0.51 % 02 in He at the end of the temperature-
time program described in figure 4-14 and hydrous PdO after exposure to 1.0 %
CO only in He at the end of the temperature-time program described in figure 4-
14............................. .................... ........ 78

4-16. High-resolution O is and Pd 3p spectra obtained from fresh hydrous PdO,
hydrous PdO after exposure to 1.0 % CO and 0.51 % 02 in He at the end of the
temperature-time program described in figure 4-14 and hydrous PdO after
exposure to 1.0 % CO only in He at the end of the temperature-time program
described in figure 4-14............. ... ................ 79

5-1. 02 conversion as a function of reaction time and temperature (40 to 160 oC) over
100 mg of hydrous PdO ................. ..... ....... .............. 90

5-2. CO conversion as a function of reaction time and temperature (40 to 160 oC)
over 100 mg of hydrous PdO ....................... ................ 91

5-3. 02 conversion as a function of reaction time and temperature (160 to 220 oC)
over 100 mg of hydrous PdO ......................... .. .............. 92









5-4. CO conversion as a function of reaction time and temperature (160 to 220 'C)
over 100 m g of hydrous PdO ......................... ............ ... ... ..... .......... 93

5-5. 02 conversion as a function of reaction time and temperature (220 to 300 oC)
over 100 mg of hydrous PdO ................ ........ .. .............. 94

5-6. CO conversion as a function of reaction time and temperature (220 to 300 oC)
over 100 mg of hydrous PdO ........ ..................................95

5-7. Comparison of 02 conversions at 200 oC over 100 mg of metallic Pd, hydrous
PdO and anhydrous PdO under slightly 02-rich conditions..............................96

5-8. Comparison of CO conversions at 200 oC over 100 mg of metallic Pd, hydrous
PdO and anhydrous PdO under slightly 02-rich conditions.................97
















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

LOW-TEMPERATURE CATALYTIC OXIDATION OF CARBON MONOXIDE
OVER PALLADIUM METAL, HYDROUS PALLADIUM OXIDES, AND
ANHYDROUS PALLADIUM OXIDES


By

Seung-Hoon Oh

December 2005

Chair: Gar B. Hoflund
Major Department: Chemical Engineering

The chemical state of Pd at the surfaces of two sizes of Pd powders and ceria-

supported Pd during low-temperature CO oxidation has been studied using X-ray

photoelectron spectroscopy (XPS). During oxidation in 02 metallic Pd is converted into

PdO, and the thickness of the PdO layer increases with increasing reaction temperature.

A similar Pd oxidation process occurs while running the catalytic CO oxidation reaction,

but the extent of the Pd oxidation is less due to the presence of CO which is a reducing

agent. Catalytic CO oxidation data obtained from 10 wt% Pd supported on

nanocrystalline ceria powders indicate that there is a strong chemical interaction between

the ceria and the supported Pd. The Pd is present as PdO on the fresh catalyst. During

reaction, small amounts of Pd metal and PdO2 are formed at 50 oC while Pd metal and

only a small amount of PdO2 are present after running the reaction at 110 oC.

Stoichiometic oxygen concentration plays an important role during CO oxidation over









palladium metal. For 1.0 % CO at 200 oC, Pd metal exhibits 100 % CO conversion with

0.51 % 02 in the feedstream. However, the same Pd powder exhibits only 45 % CO

conversion with 0.47 % 02. CO oxidation catalytic enhancements from 5 to 100 % are

found for a short time period over the temperature range of 185 to 200 oC. During

feedstream bypass, the bicarbonate species dissociate into a hydroxyl group and a carbon

dioxide molecule. XPS data indicate that a carbon C Is peak due to bicarbonate is

present during the bypass step.

Hydrous palladium oxide (PdOH2O) exhibits 100 % CO conversion even at room

temperature. Existence of lattice water and its reaction with CO/02 are believed to be the

reason for the excellent CO oxidation activity. Hydrous PdO pretreated in He at 400 oC

does not exhibit any CO oxidation activity even at 100 oC while hydrous PdO exhibits

CO oxidation activity with only 1.0 % CO in the feed (no 02 feed). Pd 3d, Pd 3p and 0

Is XPS features indicate that the chemical state of palladium is shifted from oxide to

metal and that the intensity of the metallic peaks become more predominant at longer

reaction times and higher reaction temperatures. For 100 mg of hydrous PdO tested at

near-stoichiometric conditions (slightly 02 rich) and various reaction temperatures

ranging from 40 to 300 oC, the length of the period of initial 100 % CO conversion is

proportional to the temperature. At 40 TC 100 % CO conversion lasts 15 min compared

to 380 min at 200 oC. The optimum temperature for CO oxidation over hydrous PdO is at

220 TC at which complete conversion is maintained for more than 440 min.














CHAPTER 1
INTRODUCTION

Although carbon species constitute only 0.02 % of the mass of the earth's surface,

it is the second most abundant element in living organisms [McM95]. Carbon species are

found naturally as limestone, coal, petroleum, natural gas, and as carbon monoxide (CO)

and carbon dioxide (C02). Carbon monoxide is a colorless, odorless, tasteless toxic gas

that forms from incomplete oxidation of carbon during combustion.

2C (s) + 02(g) 4 2CO (g)

There are two ways to convert hazardous CO into less harmful compounds. The first is

the reductive reaction, which is the reverse of the above reaction. However, its

dissociation energy is so high at 1070.3 kcal/mol that the reverse reaction rate is

negligible at low reaction temperature (< 200 0 C) [Gre84]. The other is the oxidative

reaction where two moles of carbon monoxide react with one mole of oxygen to give

carbon dioxide.

2CO (g) + 02(g) 2CO2 (g)

CO oxidation can occur at very low temperatures and is an effective way to remove

carbon monoxide.

1.1 CO Oxidation Mechanism over Metals and Metal Oxides

Catalytic CO oxidation over various metal surfaces is considered to follow a

Langmuir-Hinshelwood (L-H) mechanism [McC02]:

02 + 2* 2 Oads

CO + COads









COads + Oads 4 CO2 + 2*

In this mechanism represents a free adsorption site on the metal surface. Under most

conditions the surface is primarily covered with CO. The rate of reaction is positive first

order in oxygen pressure and negative first order in CO pressure on a platinum single

crystal surface [Su97]. Sometimes an Eley-Rideal (E-R) mechanism is proposed for

catalytic CO oxidation. In this E-R mechanism oxygen molecules are first adsorbed and

then gas state CO molecules react directly with adsorbed oxygen:

02 + 2* 2 Oads

CO(g) + Oads CO2 + *

The proposed CO oxidation mechanism over metal oxides is different from that

over metals. Mars and van Krevelen proposed that the rate of oxidation of the reactant is

proportional to the fraction of active sites in the oxidized state and to the hydrocarbon (or

CO) partial pressure [Mar54]. The oxidation occurs in two steps;

2CO + 20cat 2CO2, cat + *

2CO2, cat 2CO2 (g)

02 + + 20cat

where depicts an oxygen vacancy site and Ocat is a surface lattice oxygen. First the

reactant is oxidized by surface lattice oxygen in the metal oxide. An oxygen vacancy is

generated thereby reducing the near metal ions to a lower oxidation state. In the second

step the surface metal atoms are re-oxidized by gas-phase oxygen. The oxidation over

metal oxides takes place via a redox mechanism in which the metal ion changes oxidation

state. The ability to change oxidation state determines the activity of the metal oxide in

the reaction.









1.2 CO Oxidation Application: CO2 Laser

The oxidation of CO at low temperature is important due to its application in

closed-cycle CO2 pulsed lasers. Carbon dioxide transversely excited atmospheric (TEA)

pressure lasers emit short pulses of infrared radiation and have attracted interest for their

use in remote-sensing applications from the ground, air and space. They can be used for

measuring global wind velocities, for imaging distant objects and for detecting airborne

particles. These applications require the laser to be self-contained and operate over

extended periods of time. The electrical discharge used to excite the CO2 pulsed laser,

however, causes the dissociation of CO2 to CO and 02 by electron (e) impact:

CO2 + e CO + 0 + e

CO2 + e CO + 0-

20 + M 02 + M

This loss of CO2 results in a decrease in the laser power. Furthermore, the buildup of

small amounts of 02 causes rapid power loss and contributes to the laser discharge

degeneration into localized arcs followed by laser failure. In a sealed-off CO2 laser, the

dissociation of CO2 proceeds until steady state is reached. Generally, about 60 to 80 % of

the CO2 is dissociated at this point.

In order to ensure high laser power output over extended periods of time, it is

necessary to recombine the CO and 02 rapidly to form CO2. Many commercial CO2

lasers are equipped with a gas recycling system in which the exhaust gas is pumped

through a catalyst bed heated to about 300 TC to reform CO2. Figure 1-1 shows that the

use of Pt/SnO2 catalysts, which are highly active for CO oxidation at low temperatures (<

100 oC) and pressures, can maintain high laser power output for extended periods of time

[Hof96a]. Figure 1-2 shows the concentration of CO and 02 versus the number of pulses






4


during continuous operation at 10 and 20 Hz pulse repetition frequencies, both with and

without a Pd/SnO2 catalyst [Sta83]. Without a catalyst the rise in the CO and 02

concentrations is rapid and approximately linear until the laser discharge deteriorates. In

the present of a Pd/SnO2 catalyst, the initial rate of rise is similar but as the temperature

of the catalyst increases, the CO and 02 levels reach a nearly constant level. Figure 1-2

also shows how the CO and 02 levels decrease with time after the laser is shut off while

circulating the gas through the catalyst as it cools.












Open cycle Illow through)
L ------------ ----------- h
(26 hd
Closed cycle wllh catalyst (fOg 1% PtISnOs o 100lC)


9-




,

t


10 4
Total pulses (pulse rate


10o
- 10 p.p.s.)


Figure 1-1.Power output of a CO2 laser as a function of time with and without a low-
temperature CO oxidation catalyst [Hof96a].










No catalyst


S/ CO 20 tH

//


Off
i


--.\


No of pulses Time (s)


Figure 1-2.CO and 02 concentrations in a continuously operated high pulse repetition
frequency CO2 TEA laser with and without a Pd/SnO2 catalyst. Initial gas
composition is 50 % C02, 33 % N2 and 17 % He [Sta83].














CHAPTER 2
CHEMICAL STATE OF PALLADIUM POWDER AND CERIA-SUPPORTED
PALLADIUM DURING LOW-TEMPERATURE CO OXIDATION

2.1 Introduction

The development of low-temperature carbon monoxide oxidation catalysts has been

an active research area for many years due to the many environmental and technological

applications including closed-cycle CO2 pulsed lasers, CO gas detection sensors, air-

purification, gas masks and polymer electrolyte fuel cells [Sch90, Sri98, San97, Oh93].

Many different types of catalysts have been tested for low-temperature CO oxidation.

Most of the catalysts that exhibit high activity consist of one or more noble metals

dispersed on a reducible oxide support (NMRO catalysts) [Sch90]. There are many

variables involved in the preparation, pretreatment, and reaction which all significantly

affect the catalytic activity and decay behavior of the different catalysts. Supported

platinum [Sta83] and gold [Har89, Har93, Gar9la] catalysts were originally proposed for

low-temperature CO oxidation. Since then, other noble metals including palladium,

rhodium and ruthenium have received attention for CO oxidation [Oh93]. A review of

platinum-based and gold-containing NMRO catalysts was published by Hoflund and

Epling [Hof96a] and a review of gold-containing catalysts was published by Haruta and

Date [HarOl].

Palladium-based catalysts which contain Pt and Rh are widely used for

automotive three-way catalysts which can simultaneously remove nitric oxide, carbon

monoxide and hydrocarbons. These catalysts have better durability for automotive









exhaust than platinum or rhodium. Automotive catalysts which contain only Pd have

been proposed recently as an alternative for the industry. Most Pd-containing catalysts

used in CO oxidation studies and surface characterization studies were supported on

metal oxides such as cobalt oxide, SiO2, TiO2, Al203, CeO2 or zeolites. The influence of

these different metal oxides on low-temperature CO oxidation has been reported by

Pavlova et al. [Pav96]. They found that the CO activity for Pd supported on different

metal oxides (SiO2, TiO2 and A1203) varies considerably and attribute this to varying

densities of surface defect centers which can stabilize CO and 02. Hoffmann et al.

[HofOl] have studied Pd-size effects for CO oxidation over A1203-supported Pd. They

found that two different Pd particle sizes (1.8 and 5.5 nm) exhibit different transient and

steady-state kinetics using molecular beam experiments.

Three catalysts which perform very well for low-temperature CO oxidation are

iron-oxide-supported Au [Har89], platinized tin oxide [Hof96a] and manganese oxide-

supported gold [Gar9lb, Gar9lc]. Au/MnOx is an excellent catalyst when the CO2

concentration is low [Hof95, Hof96b] while Fe-stabilized Pt/SnOx performs well in a

C02-rich environment [Hof95b]. Au supported on titania has drawn attention recently

from a fundamental viewpoint, but this catalyst is of little technological interest due to its

high activity decay rate [Kon04]. Pd supported on MnOx, CeO2, TiO2, CoOx and A1203

have been examined for CO oxidation activity [Oh05a]. Of these catalysts 10 wt%

Pd/CeO2 performed the best. In this present study the chemical state of Pd is determined

using X-ray photoelectron spectroscopy (XPS) for two sizes of Pd powders and 10 wt%

Pd supported on nanocrystalline ceria.









2.2 Experimental

Several types of catalysts were examined in this study. Two different sizes of Pd

metal powder (99.95 % purity on a metal basis) were purchased from Alfa Aesar. One

had a size range from 0.25 to 0.55 microns with a BET surface area of 10.6 m2/g, and the

other had a size range from 1 to 1.5 microns with a BET surface area of 1.08 m2/g. An

impregnated 10 wt% Pd/CeO2 catalyst (BET surface area of 43.2 m2/g) was prepared

using the incipient wetness method in which palladium nitrate (Pd(N03) H20) was added

to nano-crystalline ceria (CeO2, NanoPhase, Inc., BET surface area of 50.7 m2/g) using

purified water. The catalyst was dried for five hours in an oven at 100 oC and calcined in

air at 280 TC for three hours and granulated into fine particles using a mortar and pestle.

A 10 wt% Pd-CeO2 mixture was prepared by physically combining Pd metal (0.25 to

0.55 micron) and nano-crystalline ceria in a 1:9 weight ratio of Pd and ceria. This

mixture has a BET surface area of 46.7 m2/g.

The reaction studies were performed in a tubular reactor system operating at

atmospheric pressure. A specified mass of each catalyst was placed in a quarter-inch

diameter quartz U-shaped tube and supported by glass wool. The tube was then inserted

in a Thermolyne 21100 furnace. A K-type thermocouple was placed around the U-tube

at the center of the catalyst bed and connected to an auto-tuning PID controller. The

reaction temperature was maintained accurately inside the furnace within 1 TC. Ultra

high purity test gases obtained from Praxair were mixed to yield a composition of 1 %

CO and 0.5 % 02 in helium. The feed stream was slightly oxygen rich by less than 150

ppm due to small fluctuations of flow rates. The total flow rate was 45 cc/min. Initially,

the gas stream was bypassed and analyzed in a Hewlett Packard 5890A gas

chromatograph to check the feed concentration. A molecular sieve 5A column was









connected to a thermal conductivity detector (TCD) which kept the oven and TCD

temperatures essentially constant at 80 and 150 TC respectively. The reaction gas stream

and bypass gas line were connected to a Valco 6 port valve with a 1 cc sample loop. The

conversion of CO was calculated using the peak area differences between the unreacted

bypass and the effluent from the catalyst bed. During the reaction, care was taken to

allow the temperature to equilibrate before a sample of the outlet gas was analyzed.

Multiple samples were taken to ensure that the catalytic system had reached steady state.

These were then averaged.

Surface characterization studies were carried out on the catalysts before and after

reaction at various temperatures. The catalyst powders were pressed into aluminum cups,

inserted into the high vacuum chamber and characterized using XPS. XPS was

performed using a double-pass cylindrical mirror analyzer (DPCMA, PHI Model 25-255

AR) and pulse counting detection [Gil82] by operating the DPCMA in the retarding

mode using a pass energy of 25 eV for the high resolution spectra and 50 eV for the

survey spectra. The X-rays were generated using a Mg Ka X-ray source. Model

experiments were performed to insure that the brief exposure to air during the transfer

from the catalytic reactor to the characterization chamber did not alter the Pd chemical

state.

2.3 Results and Discussions

2.3.1 Pd Powders

The activity of Pd metal powders for CO oxidation as a function of temperature is

shown in figure 2-1. The Pd powders were initially given a reductive pretreatment in 3.5

% H2 at 100 TC for 1 hr to ensure that the Pd was metallic. Both the 0.25-0.55 micron

and the 1-1.5 micron powders exhibited poor CO oxidation activity below 150 TC. The









smaller size Pd powder is more active above 150 TC and reaches complete conversion at

175 TC while the larger size Pd powder oxidizes only 10 % of the CO at this temperature

and does not reach complete conversion until about 230 TC. The complete conversion of

CO corresponds to oxidation of 1.87 x 10 -5 gmoles/min which corresponds to areal rates

of 1.76 x 10 -6 gmoles/min-m2 and 1.73 x 10 5 gmoles/min-m2 for the small and large Pd

powder respectively. At 180 TC the areal rates are 1.76 x 10 -6 and 2.59 x 10 -6

gmoles/min-m2 so the larger particles are more than 47 % active as the smaller particles

on an area basis. The reason for this is not known, but it may be due to a size effect or to

surface morphological differences. The oxygen conversions over the whole temperature

range (not shown) are essentially identical to the carbon monoxide conversions.

The large Pd powder was then reduced in H2 as described above and then exposed

to 0.5 % 02 in He for 1 hr at 50, 200 and 300 TC. High-resolution XPS Pd 3d spectra

obtained after these treatments are shown in figure 2-2, and the corresponding O is and

Pd 3p spectra are shown in figure 2-3. The Pd 3d and Pd 3p features are characteristic of

clean Pd with no Pd oxide present. After the 02 treatment at 50 oC, all of the features are

altered. The Pd 3d5/2 and 3d3/2 peaks are shifted to higher binding energies (BEs) by

about 0.3 eV and they are broadened on the high-BE side. The Pd 3p3/2 peak is

broadened to both the low-BE and the high-BE sides. The broadening on the low-BE

side is due to the presence of an 0 Is feature from PdO, and the broadening on the high-

BE side is due to the formation of a PdO Pd 3p feature. These assertions are consistent

with the broadening on the high-BE sides of the Pd 3d features and the shift toward PdO

features in figure 2-2. The changes in these features occur to a greater extent after the 02

exposure at 200 TC. After the 02 exposure at 300 oC, the spectral changes are quite large









due to nearly complete conversion of Pd metal to PdO in the near-surface region. In

figure 2-2 the predominant features are PdO 3d peaks and the small shoulders on the low-

BE sides are small Pd metal features. Two predominant features are now present in

figure 2-3 due to PdO Pd 3p and PdO O is peaks. These data indicate that an oxide layer

forms on the Pd particles and that the thickness of this oxide layer depends upon the

temperature. Thus, diffusion of oxygen to the underlying Pd metal is the slow step in the

oxidation process.

XPS data were also obtained after running the catalytic reaction over the large Pd

powder at 50, 190, 240 and 300 TC. Before reaction the catalysts were reduced in 3.5%

H2 in He for 1 hr at 100 TC. High-resolution Pd 3d spectra and Pd 3p/O Is spectra are

shown in figure 2-4 and 2-5 respectively. Similar to the results for the 02 treatment, Pd

metal is oxidized during the catalytic reaction, but it is not oxidized as fast as during the

oxidation. This is most readily apparent by comparing the Pd 3d peaks obtained after the

300 TC 02 treatment in figure 2-2 and those obtained after running the catalytic reaction

at 300 TC in figure 2-4. The 02 treatment results in much more complete oxidation of the

Pd compared to the catalytic reaction. This is due to the fact the 02 treatment is purely

oxidative while the catalytic reaction is both oxidative and reductive due to the presence

of both 02 and CO. A significant portion of the 02 which would go toward oxidizing Pd

metal to PdO is instead reacted with the CO to produce CO2 resulting in a slower rate of

Pd oxidation.

Electron energy loss spectroscopy (ELS) has proven to be a useful technique for

distinguishing between Pd metal and PdO [Hag02]. Advantages of ELS over XPS is that

the primary beam can be focused to a small size, depth sensitivity can easily be changed









by varying the primary beam energy and chemical state differences are often more

apparent in ELS data. ELS has been used to study H2-reduced Pd powder after use as a

catalyst for methane oxidation [Hof03]. The results demonstrate that a shell of PdO

develops on the Pd particles and that metallic Pd lies only beneath this shell. This

implies that methane oxidation occurs on PdO. The fact that no Pd metal is present in the

PdO layer indicates that an oxidative-reductive cycle (Pdo <-- Pd+2) is not responsible for

low-temperature (< 300 'C) methane oxidation. A proposed mechanism is that a methane

C-H bond is broken, the C bonds to an 0 in PdO to form a methoxy species and the H

bonds to an 0 in PdO to form a hydroxyl species. The methoxy species then loses the

other H atoms to form hydroxyl groups which combine and desorb as water molecules.

The remaining C combines with 2 0 from PdO to form CO2 which desorbs. The last step

is for PdO containing 0 vacancies to adsorb 02 to form PdO.

2.3.2 Ceria-supported Pd

CO conversion versus temperature curves are shown in figure 2-6 for nano-

crystalline ceria, a physical mixture of nano-crystalline ceria and Pd metal and nano-

crystalline ceria impregnated with Pd (10 wt%). Nano-crystalline ceria exhibits the

lowest catalytic activity, but it does exhibit significant activity above 200 TC. Bare

oxides are generally believed not to exhibit much activity for catalytic reactions.

However, this is not the case for CO oxidation and methane oxidation. When Pd is

physically mixed with nano-crystalline ceria (0.010 g of Pd metal powder with size

ranging from 0.25 to 0.55 microns and 0.090 g of nano-crystalline ceria), the catalytic

activity is increased considerably achieving 100 % conversion below 250 TC. The third

curve in figure 2-6 indicates that 10 wt% impregnated Pd/n-CeO2 is much more active

than either the bare ceria or the physical mixture of Pd powder and ceria. It exhibits









activity at room temperature and reaches 100 % conversion below 100 TC before the

other two catalyst exhibit any activity. The data shown in figure 2-6 indicate that there is

a chemical interaction between ceria and the supported Pd which has a very large effect

on the catalytic properties.

Conversion decay curves are shown in figure 2-7 for the impregnated 10 wt%

Pd/n-CeO2 catalyst (170 mg) while operating the reaction at 50 and 110 TC. At 50 TC the

activity decays rapidly from 100 to about 40 % after 23 min and then stabilizes near this

value. Previous studies [Hof96a] have shown that this decay in activity is due to

accumulation of bicarbonate species on the surface. Elimination of these species by

heating or subjecting the catalyst to vacuum or an inert gas flow results in restoration of

its high catalytic activity. However, at 110 oC a high conversion (100%) is maintained

over the period tested. This fact indicates that at 110 oC the bicarbonate species

decompose to yield CO2 thereby maintaining a catalytically active surface.

XPS data have been obtained from the 10 wt0/o Pd/n-CeO2 catalyst before and

after running the CO oxidation reaction at 50 and 110 TC. High-resolution XPS Pd 3d

spectra are shown in figure 2-8. The Pd 3d feature obtained from the fresh catalyst is

narrow and well-defined with a BE characteristic of PdO. No other species including

metallic Pd or PdO2 are observable. The spectrum obtained after running the catalytic

reaction at 50 TC is much broader due to a shoulder on the low-BE side due to metallic Pd

and a shoulder on the high-BE side due to PdO2. A distinct O is peak (not shown) due to

bicarbonate is present. This is consistent with the decay in activity shown in figure 2-7.

The small amount of Pd metal present may be in the form of an alloy as CePd3. A small

shoulder is present on the Ce 3d5/2 peak (not shown) at 804.0 eV which may be due to









CePd3 [Mou95]. The Pd 3d spectrum obtained from the catalyst after running the

reaction at 110 oC indicates that Pd metal is still present and the peak at 804.0 eV is also

present (not shown) which supports the assertion that CePd3 is present. The peak due to

bicarbonate is not present in the O is spectrum (not shown) which is consistent with the

fact that the activity does not decay at 110 oC. Only a small amount of PdO2 is present.

Most of the Pd is present as PdO which is believed to the active catalytic species.

2.4 Summary

In this study the catalytic behavior toward low-temperature CO oxidation of two Pd

metal powders and ceria-supported Pd have been examined as a function of temperature.

XPS has been performed on these catalysts to determine the Pd chemical state before and

after running catalytic CO oxidation at different temperatures. For comparison oxidation

experiments also have been carried out on the larger Pd powder. Model studies have

been performed to insure that the brief exposure to air during transfer from the catalytic

reactor to the UHV characterization experiment did not alter the Pd chemical state.

In each case the Pd powder was reduced in hydrogen to form metallic Pd as the

starting point. The oxidation studies were carried out by exposing the Pd powder to 0.5

% 02 in He for 1 hr at 50, 200 and 300 TC. This treatment forms a layer of PdO on the Pd

particles, and the thickness of this layer depends upon the reactor temperature. At 300 TC

the PdO layer is so thick that the underlying Pd metal signal is quite small.

Catalytic CO oxidation was carried out over the small and large Pd powders. The

areal activity of the large powder is about 1.5 times that of the small powder. Similar

chemical-state changes occur as in the oxidation case. Metallic Pd is converted to a layer

of PdO but at a slower rate than for oxidation because CO is a reducing agent. A

proposed mechanism is that CO adsorbs to form a carbonate or bicarbonate species which









decompose to from CO2. Then oxygen chemisorbs at the vacancies. In this mechanism

the active surface is a PdO surface which has carbonate species and oxygen vacancies but

no Pd metal.

Reaction studies indicate that ceria exhibits catalytic activity for CO oxidation

above 200 TC. If impregnated with 10 wt0/o Pd, it becomes highly active between 0 and

100 TC due to a chemical interaction between ceria and Pd. There is no decay in activity

at 110 TC while there is significant decay at 50 oC due to accumulation of carbonate

species on the surface. XPS data indicate that PdO is the only Pd species on the surface

of the fresh catalyst. During reaction at 50 oC, both PdO2 and Pd metal form. Carbonate

also accumulates on the surface resulting in a rapid decay in activity. At 110 TC the

activity does not decay and carbonate does not accumulate on the surface. A smaller

amount of Pd metal is present on the surface as is a much smaller amount of PdO2. In

both cases the Pd metal may be present as a CePd3 alloy.












100-


-*- 0.25-0.55 micron Pd metal
-A- 1-1.5 micron Pd metal


100


150


200


250


Temperature (oC)








Figure 2-1.CO conversion as a function of temperature for two different size Pd metal
powders. The feed stream was 1.0 % CO and 0.5 % 02 in He at a total flow
rate of 45 cc/min.






18





Pd 3d Pd 3d5/2 Pd 3d5/2
PdO Pd0

Fresh *",
---- 50o I
--------- 200 oC .
- .. 300 C P .1 1
Ai'
*-, I I













1 1 1 I 1 1 1 II o I a
/













-Binding Energy (eV)




powders were pre-reduced with 3.5 % H2 in He at 100 for 60 min.'.
*I i










Pd 3d3 Pd 3d3






19




O 1s, Pd 3p



--- 50 OC 1/ \' PdO
i----200 C ,'' \ .
S----300C \ :.
C a/














Pd3p312 Pd3p 312. .
PdO Pd0


540 535 530 525
Binding Energy (eV)


Figure 2-3.High-resolution XPS 0 Is and Pd 3p spectra obtained from (a) Pd metal, (b)
Pd metal oxidized with 0.5 % 02 at 50 TC for 60 min in the reactor, (c) Pd
metal oxidized with 0.5 % 02 at 200 TC for 60 min in the reactor, and (d) the
Pd metal oxidized with 0.5 % 02 at 300 TC for 60 min in the reactor. All Pd
powders were pre-reduced with 3.5 % H2 in He at 100 TC for 60 min.




















S..----.300 C '. \


Z 3










4----....
.,.1


9. I I I j




I I I A









343 341 339 337 335

Binding Energy (eV)



Figure 2-4.High-resolution XPS Pd 3d spectra obtained from Pd metal and Pd powder
after exposure to 1 % CO and 0.5 % 02 for 60 min in the catalytic reactor at
50, 190, 240 and 300 TC. The Pd powder was reduced with 3.5 % H2 in He at
100 oC for 60 min.



















.......... 240 0C
S --- 3000C .








Pd 3 / Pd 3
PdO Pdo .
I i-
















540 535 530 525
Binding Energy (eV)



Figure 2-5.High-resolution XPS O Is and Pd 3p spectra obtained from Pd metal and Pd
powder after exposure to 1 % CO and 0.5 % 02 for 60 min in the catalytic
reactor at 50, 190, 240 and 300 TC. The Pd powder was reduced with 3.5 %
H2 in He at 100 TC for 60 min.













100-


/
*








*
x^


A-A-A-_A---













/
-0- impregnated 10 wt% Pd/n-CeO2
-A- physical mixing 10 wt% Pd/n-CeO2
-0- bare n-CeO2


Temperature (oC)


Figure 2-6.CO conversion as a function of temperature for nanocrystalline ceria, a
physical mixture of 0.010 g of Pd metal powder with sizes ranging from 0.25
to 0.55 microns and 0.090 g of nanocrystalline ceria and 10 wt% Pd/n-CeO2
(impregnated). The feed stream was 1.0 % CO and 0.5 % 02 in He at a total
flow rate of 45 cc/min.
















100- 0 *-- --*-*0-*



80-



O 60-


40
0
0
0 20- -I 50 OC
-0-110C

0-

0 20 40 60 80 100

On-Stream Time (minutes)





Figure 2-7.Conversion decay curves for 10 wt% Pd/n-CeO2 (impregnated) while
operating the catalytic reactor at 50 and 110 TC.




















Pd 3d2 a
35/2
PdO,














Pd 3d3 Pd 3d 3
PdO Pdo

346 344 342 340 338 336 334 332
Binding Energy (eV)


Figure 2-8.High-resolution XPS Pd 3d spectra obtained from the fresh 10 wt% Pd/n-
Ce02, the 10 wt% Pd/n-Ce02 after 90 min reaction at 50 TC and after 90 min
reaction at 110 TC. The feed stream was 1.0 % CO and 0.5 % 02 in He at a
total flow rate of 45 cc/min.
total flow rate of 45 cc/mmn.














CHAPTER 3
CARBON MONOXIDE OXIDATION OVER PALLADIUM METAL POWDER:
SLIGHTLY OXYGEN-RICH FEED STREAM

3.1 Introduction

Over the past several decades, a considerable effort has been made to develop

efficient low-temperature carbon monoxide oxidation catalysts because these catalysts

enhance an operation life cycle of closed-cycle carbon dioxide (C02) lasers by re-

oxidizing CO and 02 formed in the electric discharge back to CO2 in order to maintain a

high power output [Sch90]. However, these catalysts also can be used for other

applications such as CO gas sensing, air-purification, fuel cell CO removal and

automotive cold-start gas emission. Gold and the platinum metal group (platinum and

palladium) noble metals dispersed on a reducible metal oxide support exhibit high

activity for low-temperature CO oxidation [Sch90, Gar91a, Har89, Ves96].

Among these noble metals, palladium received attention as a catalyst due to its

excellent oxidative characteristics [And61, Cul83]. Although most Pd studies have

focused on catalytic methane oxidation, Pd-containing catalysts have been used as three-

way automotive catalysts since the early 1990s as a result of the high market price and

scarcity of rhodium, the technological advance of manufacturing lead- and sulfur- free

gasoline and the high oxidation activity of Pd [Ype98].

Table 3-1 shows that for conventional CO oxidation studies using only CO and 02

over noble metal catalysts (Pd, Pt, Au) were often carried out in oxygen-rich conditions

[Par99, DatOl, Lee02, Hof95a, Bi03, Zhu05, Per97]. Bekyarova et al. [Bek98] have









reported that the activation energy (Ea) of 0.5 wt% Pd/Ce0.6Zr0.402 for CO oxidation

under an oxygen-rich condition (P,, = 23 Torr and Poxygen= 23 Torr) is 82 kJ/mol, while

its activation energy under a CO-rich condition (Po = 23 Torr and Poxygen= 6 Torr) is 163

kJ/mol. Ea at the stoichiometric condition (Pco = 23 Torr and Poxygen= 11.5 Torr) is 175

kJ/mol. All of these data were measured in the temperature range of 500-530 K. These

activation energy differences between stoichiometric, CO-rich and 02-rich feed

composition imply that the CO oxidation activity of a supported Pd catalyst is strongly

dependent on the CO/02 ratio. When small amounts of noble metals are supported on

reducible metal oxides (A1203, TiO2, MnOx, CeO2, CoOx, etc.), the supports behave as an

oxygen reservoir. These reducible metal oxides can be categorized according to their

oxygen storage capacity (OSC): i.e. the ability to store and release oxygen for CO

oxidation. Ceria (CeO2) exhibits excellent OSC capacity among these metal oxides

[DupOl]. When the OSC characteristics are coupled with a stoichiometric CO/02 feed

stream (CO: 02 = 2: 1) for CO oxidation studies, the feed stream should be about two

hundred ppm rich in 02 to ensure reproducible conditions. Few studies have been

performed using this ppm level oxygen rich condition for CO oxidation over supported

noble metal catalysts.

In this chapter the effects of using a slightly oxygen-rich feed stream (1.0 % CO,

maximum 0.52 % 02 in He) over palladium metal powders during CO oxidation are

examined. Pretreatment effects using hydrogen and carbon monoxide are examined as

well as decay characteristics.

3.2 Experimental

Pd metal powder with a size range from 1 to 1.5 microns (99.95 % purity on a

metal basis, Alfa Aesar) was examined in this study. A 52 mg palladium powder sample









was loaded into a quarter-inch diameter quartz U-shaped tube and supported by glass

wool. Only one Pd sample was used for all of the entire experiments. The Pd powder

was repeatedly given a reductive pretreatment in 3.5 % H2 in He at various temperatures

initially (50 to 210 oC) for 0.5 to 1 hour to ensure that the chemical state of Pd was

initially metallic.

Ultra high purity grade (> 99.997 %) test gases obtained from Praxair were mixed

to yield a composition of 1 % CO and 0.51 to 0.52 % 02 in helium using MKS mass flow

controllers (MFC). MFCs are supposed to flow the exact amount of gas after calibrating

with a bubble soap flow meter. This can be confirmed by testing the gas mixture using

gas chromatography (GC). A comparison has been made between the MFC CO/02

values and the GC area values (table 3-2).



CO/02 ratio

Actual CO flow rate / Actual 02 flow rate

GC CO detection area / GC 02 detection area



If the CO/02 ratio is 02 rich (1.90) in the feed, the converted gas mixture contains

approximately 0.052 % 02 and 1.0 % CO in He while a CO-rich CO/02 ratio of 2.10

yields a converted gas mixture of 0.47 % 02 and 1.0 % CO in He. The total flow rate for

the experiment was 45 cc/min. Initially, the gas stream was bypassed and analyzed in a

Hewlett Packard 5890A GC to check the feed concentration. For product analysis a

molecular sieve 5A material packed column was connected to a thermal conductivity

detector (TCD). The GC oven and TCD temperatures were kept constant at 80 and 150









TC respectively. The reaction gas stream and bypass gas line were connected to a Valco 6

port valve with a 1 cc sample loop before entering the GC injector port. The conversion

of CO was calculated using the peak area difference between the unreacted bypass gas

mixture and the effluent gas from the catalyst bed.

The reactor tube which was operated at atmospheric pressure was inserted in a

Thermolyne 21100 furnace. A K-type thermocouple was placed around the U-tube

reactor at the center of the catalyst bed and connected to an auto-tuning PID controller.

The reaction temperature was maintained inside the furnace within 1 TC.

Surface characterization studies were carried out on the catalysts before and after

reaction at various temperatures. The catalyst powders were pressed into aluminum cups,

inserted into the high vacuum chamber and characterized using XPS. XPS was

performed using a double-pass cylindrical mirror analyzer (DPCMA, PHI Model 25-255

AR) and pulse counting detection by operating the DPCMA in the retarding mode using a

pass energy of 25 eV for the high resolution spectra and 50 eV for the survey spectra

[Gil82]. The X-rays were generated using a Mg Ka X-ray source. Model experiments

were performed to insure that the brief exposure to air during the transfer from the

catalytic reactor to the characterization chamber did not alter the Pd chemical state.

3.3 Results and Discussion

The CO oxidation activity of Pd metal powders as a function of temperature for

various CO/02 feed ratios are shown in figure 3-la, and the decay behavior is shown in

figure 3-lb. The CO conversion curves as a function of temperature indicate that the

oxygen-rich feed stream (1.0 % CO, 1.0 % 02) exhibits the best CO activity for the entire

temperature range tested. It reaches 100 % conversion at 180 TC while the CO

conversion for the stoichiometric ratio (1.0 % CO, 0.5 % 02) and CO-rich feed (1.45 %









CO, 0.5 % 02) at 180 oC are 26 and 15 % respectively. Complete CO conversion over Pd

powder for the stoichiometric feed is attained near 210 TC. Pd metal yields a maximum

CO conversion of 72% for the CO-rich feed stream. For a CO/02 feed ratio of 2.88, the

theoretical maximum CO conversion is -70%. This is within the error limits of the

experiment due to the accuracy of the MFCs.

Catalytic activity data can be displayed as either conversion versus temperature as

in figure 3-la or as conversion versus time. One disadvantage of conversion versus

temperature curves is that prior reaction at lower or higher temperatures can change the

surface chemistry and alter subsequent activity data. This can be monitored by collecting

multiple conversion versus temperature curves with temperature both increasing and

decreasing. If the history of the catalyst does not affect activity, then these curves will be

identical. If they are not identical, then the activity is changing as a function of time. In

figure 3-la the CO/02 feed ratio is changed from CO-rich to 02-rich. As shown in figure

3-lb, the Pd metal is sensitive to the oxidizing/reducing nature of the feed stream during

CO oxidation. When the feed is 1 % CO, 0.51 % 02 at 210 oC, the CO conversion

remains at 100 % for the entire 2.5-hr period. However, the conversion at the same

temperature is decreased to 42 % for a reducing feed stream of 1.0 % CO and 0.47 % 02.

Initially, the conversion of the CO-rich feed stream is 90 % which decreases to 55 %

during the first 20 min.

The CO and 02 conversions over Pd metal as a function of reaction time for

different reaction temperatures are shown in figure 3-2 for a slightly 02-rich feed stream.

The palladium powders (52 mg) were all pre-reduced with 3.5 % H2 in He at the reaction

temperature or higher for 30 min, and then CO conversion was measured. At 210 TC the









Pd powder exhibits stable and complete CO conversion and ~ 97 % 02 conversion (figure

3-2a). Since the feed is slightly 02 rich, a small amount of 02 remains unreacted at 100

% CO conversion. At a reaction temperature of 205 TC, the initial conversion is 40 % and

steadily decreases to about 23 % (figure 3-2b). The CO conversions decrease steadily as

the reaction temperature is decreases from 200 to 180 oC (figure 3-2c to 3-2f).

After measuring the CO and 02 conversions as a function of time, the feed gas

was bypassed at 20 min at each temperature. This means that the used (reacted) Pd

powders were maintained at constant temperature in the reactor without any gas flow.

After the oxygen-rich CO/02 gas mixture was re-introduced to the reactor, sudden

catalytic enhancements are found in the 185 to 200 oC temperature range. This increase

in Pd catalytic activity with time has been reported previously for methane oxidation in

an oxygen-rich feed stream [Rib94, Fuj98]. Ribeiro et al. [Rib94] tested a 10 at%

Pd/Zr02 catalyst for oxidation of 2 % CH4 in air. The methane oxidation rate increases

by about 2.5 times the initial rate at 700 oC after operating for about 1 hr. Fujimoto et al.

[Fuj98] used a 1 % Pd/Zr02 catalyst with a 2 % methane, 20 % 02 reaction mixture, and

the CH4 oxidation rate also increased by factor of two to five times at 280 TC.

For CO oxidation at temperatures ranging from 185 to 195 oC over Pd metal,

some type of surface chemical reaction must occur during the 20-min bypass period

which alters the catalytic behavior. Before the bypass the conversion ranges from 5 to 23

% over this temperature range even though the feed is oxygen rich. A possibility is that

H2 molecules from the pre-reduction treatment strongly interact with Pd metal sites and

hinder the adsorption of CO and/or 02 molecules. This site blocking phenomenon of the

Pd metal sites is also dependent on temperature. When the temperature is high enough (>









210 oC), the CO and 02 molecules adsorb and react at the Pd metal sites even in the

presence of hydrogen. However, over the temperature range from 180 to 200 oC, surface

hydrogen and CO react with surface oxygen to form bicarbonate species (HC03-), which

block reaction sites. The accumulation of bicarbonate species during reaction also

explains the decay in activity before the bypass.

During the bypass period, the flow of CO and 02 is stopped so new bicarbonate

species do not form. The existing bicarbonate species dissociate into adsorbed hydroxyl

groups and carbon dioxide which desorbs during the bypass. After the bypass the

conversion increases to nearly 100 % because sites are no longer blocked by bicarbonate

species over the temperature range from 185 to 200 oC. The length of the enhanced

catalytic activity after the bypass period is also proportional to the reaction temperature.

The enhanced 100 % conversion lasts more than 60 min at 200 oC, 20 min at 195 oC and

less than 5 min at 190 and 185 TC. The enhancement after the bypass is not observed at

180 TC. The short lifetime of the enhancement during 185 ~ 195 TC also may be

explained by site blockage by carbon monoxide on the palladium metal surface. Poppa

and Soria [Pop83] reported that CO thermal desorption spectra exhibit a maximum peak

at 470 K (197 oC) on a Pd (111) surface which indicates that some fraction of the surface

is covered by CO at temperatures below 198 TC. This fraction would increase as the

temperature is decreased thereby explaining the behavior shown in figure 3-2. Proposed

reaction mechanisms are,





PdO + H2 Pd-2H (ads)













Pd-2H (ads) + 02

Pd-OH + CO + O (ads)


Pd-OH (ads)

Pd-HCO3_ (ads)




Pd-HCO3 Pd-OH



<2nd stage CO oxidation during 185 ~ 200 'C>

Pd-OH + CO + O (ads) Pd-HCO3

Pd + CO 4 Pd-CO

Pd-HCO3 Pd-OH


+ CO2










+ CO2


Catalytic enhancements after the bypass stage are shown in figure 3-3 for Pd

metal powder after pre-reduction in 1 % CO in He for 1 h. Then a calibrated gas mixture

of 1.0 % CO and 0.51 to 0.52 % 02 in He was passed over the Pd powder bed. Similar to

Pd reduced with hydrogen, CO pre-reduced Pd metal initially exhibits a lower CO

conversion of 20 % at 205 TC and less than 10 % conversion at 190 oC. Re-introducing

the CO/02 reaction mixture after a 10-or 20-min bypass period results in a CO conversion

increase from about 20 to 100 %. At higher temperatures of 200 and 205 'C, complete

conversion remains for a long period of time, while 100 % conversion is maintained for

less than 5 min at 190 oC. This suggests that the enhanced activity of the CO pre-reduced

Pd metal is related to the desorption characteristics of CO. During the bypass at 200 oC,









most of the CO molecules desorb leaving a clean surface which achieves a higher

conversion. A simple reaction mechanism can also be proposed to explain these results,




Pdo +




CO + 1/2 02 over Pd-CO





Pd-CO


- Pd-CO (strong)





4- CO2 (low conversion)





-4 Pdo + CO


<2"d reaction stage>

CO +





Pd +

CO +


1/2 02 over clean Pd metal


CO

1/2 02 over Pd-CO


-* CO2





4- Pd-CO (accumulates)

4- CO2 (low conversion)


The data shown in figure 3-4a were obtained using different bypass conditions.

Pd powder (52 mg) was first reduced in 3.5 % H2 in He for 30 min at 180 oC. The initial

conversion decreases from 15 to about 7 % over a 75-min time period. During the first

bypass, the temperature was increased to 220 oC for 5 min and then cooled to the reaction









temperature of 180 TC. The overall bypass period was 42 min long without any gas flow.

After the bypass the initial CO conversion was 55 % for the first few minutes and then

rapidly decreased to about 5 % followed by a slow decay. The next bypass was carried

out at 180 TC for 40 min. After this bypass no increase in catalytic activity is observed

during a 50-min period. A second high-temperature bypass was performed in which the

temperature was raised to 230 TC for 10 min and then cooled down to 180 oC. The total

bypass period was 40 min. A high CO conversion of 65 % was measured immediately

after this bypass. The CO conversion rapidly decreases to about 4 %.

The next bypass was carried out using a He flow for 40 min at 180 oC followed by

a 15-min bypass without flowing He at 180 oC (figure 3-4b). A bypass with flowing He

has been shown to eliminate bicarbonate species from surfaces of Pt/SnOx and Au/MnOx

catalysts used for CO oxidation [Gar9la]. Unlike the previous 180-2C bypass without

flowing He, the conversion rises to nearly 100 % after the flow He bypass for a very short

period of time and decreases to a conversion of about 3 % over 30 min. This indicates

that bicarbonate species are removed by flowing He at 180 oC but that they accumulate

again on the surface during reaction at 180 oC. The sample was then pre-treated in H2 for

1 h at 180 TC followed by outgassing under flowing He for 30 min at 180 oC. Again, the

CO conversion is 100 % initially and drops to 6 % within 5 min.

The effects of flushing with helium after pre-reduction with hydrogen at various

temperatures are shown in figure 3-5. Although the results described above for reduction

of Pd metal powder in 3.5 % H2 at a reaction temperature of 190 TC initially exhibits less

than 20 % CO conversion (figure 3-2d), He outgassing for 10 min changes the Pd metal

CO oxidation performance. At 190 TC the conversion remains above 95 % for 30 min.









After 35 min the conversion drops to 15 % which is similar to the case without He

outgassing (figure 3-5d). Also, H2 reduction of Pd powder followed by He outgassing

produces 100% CO conversion which is maintained for over 2 hr at 195 to 205 TC (figure

3-5a to 3-5c). Hydrogen reduction only does not yield high conversions at 195 and 200

TC (figure 3-2b and 3-2c). These results were all obtained using a slightly 02-rich feed

stream. H2 TPD spectra in the literature [GorOO, SchOl] demonstrate that most of the H2

is desorbed at 127 TC for Pd (210) and Pd (110) surfaces. Therefore, surface hydrogen

which forms during the reduction is effectively removed by the He flow at temperatures

of 195 TC or higher. Subsurface hydrogen would also be removed by this process.

Proposed CO oxidation mechanisms for H2 reduction/He outgassing are;




PdO + H2 Pd





Pd-2H (ads)





2Pdo + 02 (excess)

CO + 1/202 over PdO





PdO + CO


-2H (ads)





- PdO + H2





- PdO

4- CO2 (high conversion)


-4 Pdo-CO









CO + /2 02 over Pdo-CO CO2 (low conversion)



The temperature effects of Pd powder for oxygen-rich CO oxidation are shown in

figure 3-6a. After Pd metal (52 mg) was reduced in H2 and outgassed at 190 oC in

flowing He, the CO conversion is in the 95 to 100 % range. When the reaction

temperature is reduced from 190 to 170 oC with the feed flowing through the catalyst bed,

the CO conversion drops to 6 %. After 40 min at 170 oC, the temperature is restored to

190 TC and the conversion is increased to only about 15 %. These two different CO

conversions at the same temperature of 190 TC indicate that bicarbonate species are

formed on reaction sites during reaction at 170 oC and are not removed when the reaction

temperature is increased to 190 oC in a continuous flow of CO and 02. After a 10-min

bypass period, the CO conversion increases to 100 %. After increasing the temperature

to 210 TC, complete CO conversion is maintained for 40 min. Decreasing the

temperature from 210 to 190 oC with feed flowing yields a high conversion over the next

60 min. This indicates that bicarbonate species do not accumulate at 190 or 210 oC.

For a feed of 1 % CO and 0.55 % 02 in He (Figure 3-6b, CO/02 ratio = 1.80), Pd

powder exhibits 100 % CO conversion after Pd reduction in H2 and outgassing in He.

During this period, oxygen is consumed and the un-reacted 02 should be 0.05 %. A

theoretical calculation of 02 conversion yields 90.9 % for complete oxidation. However,

the gas chromatograph TCD integrated area yields a maximum 02 conversion of 88 to 90

%. This 2 % difference may be experimental error due to calibration of the MFCs or due

to the presence of a small amount of oxygen which may oxidize some of Pd metal to

PdO. Two different CO oxidation mechanisms may be operating (1) CO is oxidized by









the Langmuir-Hinshelwood mechanism and (2) a small amount of PdO reacts with CO.

If palladium metal is oxidized during reaction in an 02-rich feed stream, then more

oxygen will be required relative to stoichiometric conversion of CO.

Fresh Pd metal was reduced with 3.5 % H2 in He at 100 oC for 60 min. Without a

bypass (figure 3-7a), 0.12g of Pd metal was prepared by reduction in 3.5 % H2 in He at

185 TC for 30 min followed by reaction under a CO and 02 gas feed stream (CO/02 =

1.94) for 30 min. The initial CO conversion is slightly above 40 % and it decays to about

20 % in 12 min. The 02 conversion is slightly less than the CO2 conversion as expected.

With a bypass (figure 3-7b), 0. 12g of Pd metal was given the same H2 reduction and CO

oxidation conditions (CO/02 = 1.94) for 35 min and then bypassed for 20 min without gas

flow and exposed to the CO and 02 feed stream (CO/02 = 1.91) for 20 min. The catalyst

behaves similarly before the bypass and then the CO conversion increases to 100 % after

the bypass with a lower 02 conversion.

Corresponding high-resolution XPS spectra of the Pd 3d, 0 Is, Pd 3p and C Is

features are shown in figure 3-8. The narrow XPS Pd 3d spectrum (Figure 3-8a)

indicates that a small amount of surface Pd meal is converted into PdO regardless of the

CO activity performance compared with fresh Pd metal. The PdO exhibits a shoulder

near the binding energy (BE) of Pd 3d5/2 and Pd 3d3/2 at 337 and 342 eV respectively

[Mou95]. The catalysts both with the bypass and without the bypass contain some PdO.

Hence, the existence of PdO cannot explain the large difference in catalytic behavior.

Further evidence of the formation of PdO is found in the corresponding XPS O is

spectra (figure 3-8b). Both catalysts with and without a bypass contain PdO based on the

presence of a shoulder at 529.2 eV. The fresh Pd metal exhibits a Pd 3p3/2 peak at a BE









of 532 eV. The Pd 3p3/2 shoulder of fresh metal is different from the catalysts with and

without a bypass. Both reacted catalysts exhibit a PdO shoulder at a BE of 533.4 eV.

Without a bypass the Pd powder, which has poor activity at 185 'C, has a larger shoulder

in figure 3-8b than those Pd powders which were given a bypass. This indicates that the

peak near a binding energy of 533 eV is not only due to PdO. Carbonate and bicarbonate

species typically have 0 Is BEs near 531 ~ 532 eV [Heu99]. Both carbonate species and

small amount of PdO 3p3/2 may yield the peak shape for the Pd powder which was not

given a bypass at 185 TC.

Proof of a bicarbonate (carbonate) species can also be confirmed from the XPS C

Is spectrum shown in figure 3-8c. The largest peak with a BE of 284.6 eV is due to CxHy

species which are adsorbed from the air and are present on all Pd powders. The C Is

feature obtained from the Pd powder without a bypass has a distinct peak with a BE of

282 eV which is not present on the other Pd powders. A peak at this BE is typically

characteristic of a carbide but it is most likely due to a reactive C species due to an

adsorbed CO molecule stripped of its oxygen atom. Contrary to this, peaks at this BE are

not present in the C Is spectra obtained from the Pd powders which were given a bypass.

This implies that an adsorbed carbon atom which can block the Pd active site is

effectively removed by the He outgas.

Another important feature is found at a BE near 290 eV which may be due to the

existence of a carbonate or bicarbonate species. Wu et al. [Wu04] reported that CO2 is

added to steel in order to reduce friction in tribological applications. They found that

carbonate and/or bicarbonate species are present in the surface layers using XPS. The

existence of Pd-CO3 or Pd-HCO3 at a BE of 289.8 eV without the bypass is apparent in









figure 3-8c. Pd metal which was given a bypass and exhibits 100 % conversion does not

contain any peak near a BE of 290 eV. Carbonate species are formed after Pd metal was

reduced in H2 and reacted under CO and 02. These carbonates prevent the adsorption of

CO and 02 molecules at the palladium metal surface. When the carbonates are

effectively removed during the bypass step, all of the Pd metal sites are available for the

adsorption of CO and 02 molecules thereby explaining the fact that CO oxidation activity

is abruptly enhanced after the bypass or outgassing with He.

Pd metal powder was reduced in hydrogen and tested for activity in a CO-rich

feed stream (CO/02=2.11) at 200 oC. As shown in figure 3-9a, its activity is stable at an

18 % conversion of CO. After a bypass at 200 oC for 20 min, the conversion does not

increase to 100 %. Instead, the CO conversion is increased from 18 to 25 % and then

decreased 20 %. Before the bypass more 02 is reacted then CO indicating that PdO

forms. The opposite is true after the bypass indicating that PdO is reduced to metallic Pd.

Also, bicarbonate species may dissociate during the bypass thereby freeing sites for

reaction. Pd metal powder was pre-oxidized with 0.5 % 02 in He at 190 and 195 TC

(figure 3-9b). Unlike the CO conversion after a hydrogen reduction, both exhibit a 100

% CO conversion for a short period of time before dropping to 12 and 15 % at 190 and

195 TC respectively. The 100 % conversion remains for 5 min at 190 oC after the

oxidation while the same conversion remains for about 20 min at 195 oC. This may

imply that more PdO sites are formed at the higher temperature and that PdO actively

oxidizes CO until either PdO is completely consumed or that Pd-CO adsorption species

cover all the available sites for CO oxidation.









3.4 Summary

In this study the catalytic enhancement of low-temperature CO oxidation over

palladium metal has been examined as functions of temperature and time. The CO

conversion for Pd metal is 100 % for oxygen-rich conditions (1.0 % CO, 1.0 % 02) at 200

TC while the same catalyst yields 65 % conversion for a feed stream at the stoichiometric

ratio (1.0 % CO, 0.50 % 02) and 28 % CO conversion for CO-rich conditions (1.45 %

CO, 0.5 % 02) at 200 oC. When the 02 concentration (based on 1.0 % CO) is reduced

from an 02-rich feed (0.51 % 02) to an 02-deficient feed (0.47 % 02) at 210 TC, the CO

conversion remains at 100 % for the 02-rich condition. However, for the same Pd metal

powder, the CO conversion decreases to 45 % over a 2.5 hr time period when the feed is

02-deficient.

The CO conversion steadily decreases from 200 to 180 oC yielding 20 to 5 %

conversions. After the initial Pd activity is stabilized, the Pd catalyst was bypassed for 20

min resulting in a catalytic enhancement to 100 % CO conversion for a short period of

time in the 185 to 200 oC temperature range. During the bypass the existing bicarbonate

species dissociate into hydroxyl groups and carbon dioxide, which desorbs during the

bypass. Surface characterization using XPS shows that bicarbonate species are found at a

binding energy of 290 eV from the Pd metal under the 185 TC, no-bypass conditions.

However, after a 20-min bypass, Pd metal exhibits 100 % CO conversion and the high-

resolution XPS C Is spectrum does not contain any significant peaks near 290 eV. When

Pd metal powder was tested using a CO-rich feed stream (1.0 % CO, 0.47 % 02), a 20

min bypass at 200 oC does not increase the CO conversions to 100 %. Similar catalytic

enhancements after the bypass period at 200 and 205 TC are found when the Pd metal is

pre-reduced with 1.0 % CO prior to CO oxidation. Flushing with helium after pre-







41


reduction with hydrogen at 190 to 205 TC shows that surface hydrogen during reduction

is removed by the He outgassing so that high CO conversion can be achieved even at 190

OC.









Table 3-1. Feed stream composition used in published studies of CO oxidation over
various supported noble metal catalysts


Authors Catalysts Metal loading CO/02 conditions


Park et al. [Par99]



Date et al. [DatOl]


Au/TiO2

Au/A1203

Au/TiO2


Lee et al. [Lee02] Au/A1203

Hoflund et al. [Hof95a]Au/MnOx

Pt/SnOx

Bi et al. [Bi03] Pd/NaZSM-5

Zhu et al. [Zhu05] Pd/CeO2-TiO2

Perry et al. [Per97] Pd/A1203


0.26-1.16 wt%



0.15-0.21 wt%



0.07 0.714 wt%

10 at%

19.5 at%

2.4 wt%

1 wt%

5 wt%


Pt/ A1203 1 wt%


1.0 vol% CO, 10 vol% 02

(or 5 vol% 02) in N2

1 vol% CO

in air (~ 20 % 02)

1 % CO, 0.5 % 02 in N2

1 % CO, 0.5 % 02,

0 ~ 16 % CO2 in He

2.5 % CO, 25 % 02 in N2

1 % CO, 1 % 02 in Ar

2 cc/sec CO, Icc/sec 02,

reactor pressure 1000 torr


wt%: weight percentage

at%: atomic percentage (mol. percentage)






43


Table 3-2. Comparison of CO/02 concentration ratios obtained from GC area and the
actual MFC flow rates for various concentrations of CO and 02 in He



Flow rate (cc/min) Gas chromatograph Area

CO 02 Flow ratio GC area ratio CO 02

0.46 0.21 2.19 2.18 771069 353000
0.50 0.25 2.00 2.11 772845 366468
0.49 0.26 1.88 1.97 746457 379213
0.46 0.25 1.84 1.94 748123 384980
0.46 0.26 1.77 1.78 696418 392171







44




100- (a)


80 -
AA-A-A
60


S40- 0
0

0 -E- C0/02=1.00
20- -0- CO/02=2.00
-A- CO/02=2.88



50 100 150 200 250 300
Temperature (OC)





100 --r- -0n n-a- -- ------ --


80 (b)

210 oC

60

0 -0 --"--o -o ___-o
c 40-
0
0

20- -0- CO/02=1.96
-0- C0/02=2.13

0-

0 30 60 90 120 150
On-Stream Time (min)




Figure 3-1.CO conversions over 52 mg of Pd metal powder as a function of (a)
temperature and CO/02 ratio (1.00, 2.00 and 2.88) and (b) time and CO/02
ratio (1.96 and 2.13). The total gas flow rate was 45 cc/min and the CO
concentration was approximately 1.0 %. The Pd metal powders were reduced
in 3.5 % H2 in He at reaction temperature for 30 min to form metallic Pd
before reaction.

















o- Cc-- 00o-----O o
CO0/,=1 96
(a)
210 C


0 30 60 90
On-Stream Time (min)


120 150


(C) -.0 \
195 C 0'




Bypass Bypass Bypass
20 mins

-O-,
--0-CO



C0/0,=1 93 C0/0 =1 94 CO/0=1 92 CO/0 =1 91


0 50 100 150 200 250
On-Stream Time (min)


(b)
200 C






CO/o =1 97


v8a:9


0 50 100 150 200
On-Stream Time (min)


(d)
190 C


Bypass
20 mins


CO/O =1 90


0 20 40 60
On-Stream Time (min)


(e)
185 C


Bypass
20 mins


C0/0=1 91


Bypass
20 mins





o-o-o=1 86
CO0/01 86


-- CO
-0- 1





CO/O =1 88


0 30 60 90 120 150 180
On-Stream Time (min)


(f)
180C


CO/02=1 94
'B^:=8


Bypass -0- CO
20 mins -0- 0


CO/02=1 92

8 8 9=


0 20 40 60 80 100
On-Stream Time (min)


Figure 3-2.CO and 02 conversions over 52 mg of Pd metal powder as a function of time

before and after reaction gas bypass: (a) 210 oC, (b) 200 oC, (c) 195 oC, (d)

190 oC, (e) 185 oC and (f) 180 oC. The total gas flow rate was 45 cc/min in

slightly 02-rich feed streams. The Pd powder was reduced in 3.5 % H2 in He

for 30 min at each temperature to form metallic Pd.


CO/02=1 99



- Bypass 20 mins


- CO
-o0- 02


80 100



















205 C


CO/0=1 98


Bypass
10 mins






-o- oo


0 30 60 90
On-Stream Time (min)


o-D-D--00-0--0--0

CO/0=1 86








-0- CO
-0-0,


120 150


(b)
200 C


Bypass
20 mlns


CO/0,=1 93


30 60 90
On-Stream Time (min)


(c)
190 C



Bypass
20 mlns









CO/02=1 92 C,

0 30 60 90
On-Stream Time (min)


Figure 3-3.CO and 02 conversions over 52 mg of Pd metal powder as a function of

temperature and time before and after reaction gas bypass: (a) 205 oC, (b) 200

oC and (c) 190 oC. The total gas flow rate was 45 cc/min, and the feed stream

was slightly oxygen rich. The Pd powder was reduced in 1.0 % CO in He for

30 min at 205 TC and 60 min at 200 and 190 TC to form metallic Pd before

reaction.


CO/0 =1 96


-0-CO
00


120 150


-- CO
-0-0


)2=1 89

120 150


IIIIII















(a)
180 C

Total bypass 42 mins,
raise to 220 'C, 5mins
cool down to 180 C







Stage (I)
CO/02=1 90


opar 8_5
06 88=


-0- CO
-0-0 o


100-



80-



S60-

0

2 40-
0

20-



0-


0 50 100 150 200 250

On-Stream Time (min)







(b)
180 C


He outgass
180 C,
40 mins,

Bypass
15 mins





Stage (IV)
CO/02=1.95
uc).C Q.C)-C


3.5 % H Red.
180 C,
60 mins,

He Outgassing
30 mins


Stage (V)
C0/0,=1.89


300 350 400 450 500

On-Stream Time (min)


300 350
360 3 0 0




















-D- CO
-o- O,


Stage (VI)
CO/0 =1.92




550 600


Figure 3-4.CO and 02 conversions over 52 mg of Pd metal powder as a function of time
at 180 oC before and after reaction gas bypass. The total gas flow rate was 45
cc/min, and the feed gas was slightly oxygen rich. The Pd metal powder was
reduced in 3.5 % H2 in He for 30 min at 180 oC to form metallic Pd before
reaction.


Bypass Total bypass 40 mins,
180 oC, raise to 230 oC, 10 mins,
40 mins cool down to 180 oC


0 1




Stage (II) Stage (III) Stage (IV)
CO/02=1 96 CO/02=1 95 CO/02=1 95



'S gi o- e O



















-c-D- -a-0-0---0
-o-o-o o-o-o-o-Oo-o-o


-o- CO
-0-O.


120 150


m-0-0-a-0-0-DO O-0-0-0-0-u-mm-0-m-0-0-0--


(b) CO/02=1 91
200 C


0- CO
-0-0O-


0 50 100
On-Stream Time (min)


150 200


---o-0-o-o-0-0-00oo

(c) CO/02=1 96
195 C


-00-0
-o--
-00-0 -


-0- CO
-0-0


0 30 60 90 120
On-Stream Time (min)


a-___

(d) CO/01 90
190 C


-0- CO
-o-02


o---


0 20 40 60 80
On-Stream Time (min)


Figure 3-5.CO and 02 conversions over 52 mg of Pd metal powder as a function of

temperature and time before and after reaction gas bypass: (a) 205 oC, (b) 200

TC, (c) 195 oC and (d) 190 oC. The total gas flow rate was 45 cc/min, and the

feed stream was slightly oxygen rich. The Pd powder was reduced in 3.5 %

H2 in He for 30 min at each temperature and then outgassed in He for 10 min

to form metallic Pd before reaction.


0 -0--0-
o--o-o-o-oo

(a) co/o,=1 88
205 C


0 30 60 90
On-Strean Time (min)




















1900C
C00/02=1.98


Bypass
1900C
10 min









1900C

170C
8898:8


-o-po-oC
010-0-00-0

1900C
CO/02=1.95


00-oo(D


2100C


0 50 100 150 200 250

On-Stream Time (min)


oED 0o-cE

0(?"0 0-C

190 C
C0/0,=1 80


180 C


190 C


OCOPOO-0O
200 'C









0


O-0-OO
0
o
1900C









(a)

-I- CO


300 350 400


08c%
190 C


Bypass
190 C
25 min


(b)

-0- CO
-0-0
2


I I I I I I I I
0 50 100 150 200 250 300 350

On-Stream Time (min)





Figure 3-6.CO and 02 conversions over 52 mg of Pd metal powder as a function of time
at 170, 180, 190, 200 and 210 oC before and after reaction gas bypass. The
total gas flow rate was 45 cc/min, and the feed gas was slightly oxygen rich.
The Pd powder was reduced in 3.5 % H2 in He for 30 min at 190 oC and then
outgassed in He for 30 min before reaction to produce metallic Pd before
reaction.


IIIIIIIIIIII














(a) Without bypass
Pd 0.12g at 185 oC


-0- CO


-o- O




O~-


2


10 20
On-Strean Time (min)


(b) With bypass
Pd 0.12g at 185 oC


CO0/O=1.94


--/0--=1.9
C0/0=1.91


Bypass
185 oC,
20 mins


-I- CO


0 20 40 60 81
On-Stream Time (min)


Figure 3-7.CO and 02 conversions over 0.12 g of Pd metal powder as a function of time
at 185 TC (a) before and (b) after a bypass. The total gas flow rate was 45
cc/min, and the feed gas was slightly oxygen rich. The Pd powder was
reduced in 3.5 % H2 in He for 30 min at 185 TC (no outgassing) before
reaction to produce metallic Pd.


CO/O =1.94





































346 344 342 340 338 336 334 332
Binding Energy (eV)





(c)
Ss COH from air
Cls Y
Fresh Pd metal
........... 185C w/o bypass
-185C w/ bypass

PC adsorbed C
PdCO






z I%
z ii


295 290 285 280
Binding Energy (eV)


(b)
O 1s, Pd 3p
-fresh
.......... 185C w/o bypass I'I
-- 185Cw/ bypass I
Ols
I I PdO



4,







p:i Pd 1- 1.
PdO Pd

0 535 530 5:
Binding Energy (eV)


Figure 3-8.High-resolution XPS (a) Pd 3d, (b) 0 Is and Pd 3p and (c) C Is spectra. The
spectra in each figure were obtained from Pd metal, after reaction for 36 min
with no bypass (figure 3-7a) and after reaction with a bypass at 75 min (figure
3-7b).














(a)
200 OC


100-


80-


S60-


S40

0

20-


0-


C00/0=2.14


0 50 100 150 200 250
On-Stream Time (min)


50 100
On-Strean Time (min)


Figure 3-9.CO and 02 conversions over 52 mg of Pd powder as a function of time and
temperature (a) 200 oC and (b) 190 and 195 oC before and after reaction gas
bypass. The feed gas was CO rich in (a) and 02 rich in (b). In (b) the Pd
powder was oxidized in 0.5 % 02 in He at 195 oC for 30 min after the initial
activity decay. The Pd powders were initially reduced in 3.5 % H2 in He at
200 oC for 30 min to produce metallic Pd before reaction.


Bypass
200 C
20 mins





0-0-0=-0-0
C/02,=2.11


100-


80-


S60-


| 40-
0

20-


0-














CHAPTER 4
LOW-TEMPERATURE CO OXIDATION OVER HYDROUS PALLADIUM OXIDE

4.1 Introduction

Palladium species have been widely used as catalysts for oxidation reactions such

as the complete oxidation of methane, automotive exhaust treatment and CO oxidation

[Epl99, Far92, Gan03, Gar89]. Although supported gold nano-particles are generally

regarded as superior catalysts for low-temperature CO oxidation including manganese

oxide-supported gold [Hof95c] and titania-supported gold [HarOl]. Palladium based

catalysts are also used for removal of CO over low-temperature ranges [Koc96, Zhu05].

To gain a better understanding of palladium-based catalysts for CO oxidation,

several points should be addressed. The first is identification of the active Pd species for

CO oxidation. Lyubovsky and Pfefferle [Lyu99] have studied the phase transformation

between PdO and Pd as a function of temperature and its relationship to catalytic activity

for methane oxidation. There are only a few studies relating to the active state of Pd for

CO oxidation. Hendriksen et al. [Hen04] tested a Pd (100) surface by switching

microflow between two different CO-rich and 02-rich conditions. They found that the

oxide formed on the Pd metal surface is more active than chemisorbed atomic oxygen

adsorbed on the surface. However, they used a very oxygen-rich feed (02 pressure > 1

bar, CO pressure < 0.1 bar) for the kinetic study and the oscillation reactions. Also they

did not determine the condition of the Pd surface near the stoichiometric condition (CO:

02 = 2:1). The second is the effect of water during CO oxidation. Ibashi et al. [Iba03]

demonstrated that H20 inhibits methane oxidation by decreasing the CH4 conversion up









to 550 TC for 10 wt% Pd/ZrO2. Date and Haruta [DatOl] reported that similar moisture

contents increase the CO oxidation rates by about 10 times for up to 200 ppm H20 in the

feed over a 0.9 wt% Au/TiO2 catalyst at 0 oC. They attributed this activity increase to

adsorption of water on the gold-containing catalyst surface. For CO oxidation over Pd

supported on NaZSM-5, Bi and Lu [Bi03] found that gas-phase H20 does not affect the

CO conversion. The effects of water vapor on CO oxidation are still being debated. The

water vapor effects discussed above were performed under 02-rich conditions (1 % CO in

air and 0.8 % CH4 in air), and the noble metals (Au and Pd) were supported on various

metal oxides (A12O3, TiO2, ZSM-5). By studying CO oxidation over pure noble metals

without a support material and close to the stoichiometric feed ratio (CO : 02 = 2 : 1), it

may be possible to understand how the CO and 02 molecules react over the noble metal

sites. Since pure gold metal is inactive for CO oxidation, Pd was chosen for these

studies. In this part of the study, hydrous and anhydrous PdO have been examined for

CO oxidation near stoichiometric condition in the low temperature reaction range

(<100oC).

Surface characterization was carried out using X-ray photoelectron spectroscopy

(XPS) in order to determine chemical state and compositional information. Epling and

Hoflund [Epl99] have studied methane oxidation over ZrO2-supported Pd catalysts and

characterized the catalyst before and after reaction using in situ XPS. The Pd chemical

state is primarily PdO before the reaction but is transformed to a mixture of PdO and

metallic Pd during reaction in 100 Torr of a 1: 2 ratio of CH4 : 02 mixture for 45 min at

180 TC. Nevertheless, the catalyst exhibits a stable catalytic activity versus time. The

reason for this is not clear but possibly a layer of PdO accumulates over the Pd metal









which forms. Datye et al. [DatOO] also proposed a similar transformation of Pd and PdO

over alumina-supported catalysts for methane oxidation. Their reaction temperature was

above 700 TC and their reaction feed condition was 0.5 % CH4 in air.

4.2 Experimental

Hydrous palladium (II) oxide (PdOH20, Pd 77.81 %, Alfa Aesar) and anhydrous

palladium (II) oxide (PdO, Pd 86.80 %, Alfa Aesar) were examined. A specific mass

(100 or 400 mg) of palladium oxide was loaded in a quarter-inch diameter quartz U-

shaped tube and supported by glass wool. Each sample has a BET surface area of 35.7

m2/g for hydrous PdO and 28.9 m2/g for anhydrous PdO. The tube was then inserted into

a Thermolyne 21100 furnace. A K-type thermocouple was placed around the U-tube at

the center of the catalyst bed and connected to an auto-tuning PID controller. The

temperature was increased at a rate of 15 oC/min from room temperature to 100 oC and

maintained accurately inside the furnace within 1 TC. Ultrahigh purity test gases

obtained from Praxair were mixed to yield a composition close to 1.0 % CO and 0.50 %

02 in helium. The feed stream was slightly oxygen rich by less than 200 ppm. The feed

composition was determined using the gas chromatograph (GC) integrated peak areas of

CO and 02. The total flow was 45 cc/min at STP. Initially, pure He gas was passed

through the catalyst bed for 5 min of flushing which the feed gas remained at room

temperature (25 oC) or was raised to 100 oC. The feed gas mixture was bypassed and

analyzed in a Hewlett Packard 5890A GC to determine the feed concentration. For

product analysis a molecular sieve 5A column was connected to a thermal conductivity

detector (TCD). The oven and TCD were kept essentially constant at 80 and 150 TC

respectively.









Surface characterization studies were carried out on the catalysts before and after

reaction at various temperatures. The catalyst powders were pressed into aluminum cups,

inserted into the ultrahigh vacuum chamber and characterized using XPS. XPS was

performed using a double-pass cylindrical mirror analyzer (DPCMA, PHI Model 25-255

AR) and pulse counting detection by operating the DPCMA in the retarding mode using a

pass energy of 25 eV for the high resolution spectra and 50 eV for the survey spectra

[Gil82]. The X-rays were generated using a Mg Ka X-ray source. Model experiments

were performed to insure that the brief exposure to air during the transfer from the

catalytic reactor to the characterization chamber does not alter the Pd chemical state.

4.3 Results and Discussion

The activity of 100 mg of hydrous PdO (PdO'H20) for CO oxidation as a function

of time at 100 oC is shown in figure 4-1. The feed gas was 1.0 % CO and 0.5 % 02.

Complete CO conversion is obtained over 30 min while the oxygen conversion is only 46

to 52 % 02 for this period. Theoretically, 100 % CO conversion requires almost 100 %

02 conversion for a feed at a near stoichiometric ratio. This indicates that oxygen from

the hydrous PdO must react with CO to produce CO2 at 100 oC. When pure Pd metal was

tested with 1 % CO and 0.51 % 02 in He at 300 oC, complete conversions of both CO and

02 were achieved. After 33 min the hydrous PdO activity gradually decreases over the

next 60 min and then becomes negligible.

In order to determine the effect of moisture from PdO H20, 100 mg of fresh

hydrous PdO was pre-treated by drying in He at 400 oC for 1 hour. It was then cooled

down to 100 oC in flowing He. The reaction-gas feed stream for the 400-2C calcined

PdO H20 was slightly 02-rich (1.0 % CO and 0.52 % 02). The initial CO conversion of









the dried PdO H20 is 20 % while the 02 conversion is only 10 %. The conversion

decreases to less than 5 % after 30 min of reaction time.

High-resolution XPS Pd 3d spectra obtained from fresh PdO H20 and helium pre-

treated PdO H20 at 150 and 400 TC are shown in figure 4-2. In all three cases the

predominant peaks are due to PdO and no feature due to metallic Pd are apparent. The

spectra obtained from the fresh and 400-2C pretreated hydrous PdO are quite similar even

though the catalytic behavior is very different. The 150-2C treated hydrous PdO has high

binding energy (BE) shoulders which may be due to increased amounts of water or

hydroxyl groups which migrate to the near-surface region during the 150 TC treatment.

These species most likely desorb during the 400 TC treatment. Corresponding high-

resolution XPS 0 Is and Pd 3p spectra are shown in figure 4-3. The 0 Is BEs of metal

oxides are generally in the range of 528 to 531 eV [Mou95]. For PdO the 0 Is peak is

located at 530.6 eV and the Pd 3p3/2 is located 533.6 eV. Pillo et al. [Pil97] reported that

when fresh PdO is annealed for 24 hrs at 100 oC followed by another 24 hrs at 200 oC

under UHV conditions the Pd 3 p3/2 peak shifts to its metallic value at 532.3 eV. Their

PdO sample is reduced to Pd metal during the 48 hr annealing while in this study hydrous

PdO does not exhibit any metallic feature at 532.3 eV after calcining in He at 150 oC for

2 hr. However, a metallic shoulder may be present after annealing at 400 oC in He for 2

hr, but a corresponding feature is not apparent in the Pd 3d spectrum. The Pd 3d

electrons arise from greater depth than the Pd 3p electrons due to their larger mean free

paths. Therefore, the very small amount of metallic Pd present lies near the surface. The

intensity of the O is peak at 530.6 eV gradually decreases as the calcinations temperature

increases.









A comparison of the CO oxidation rates at room temperature between PdO H20

and anhydrous PdO is shown in figure 4-4. Fresh hydrous PdO (0.40 g) was reacted at 25

TC with 1.0 % CO and 0.52 % 02 in He. Complete CO conversion was maintained over

77 min while the 02 conversion ranged from 41 to 44 % over the same time period. After

this period the CO activity decreases with reaction time until a CO conversion of 15 % is

reached after 217 min. Anhydrous PdO (0.40 g) was also tested with 1.0 % CO and 0.51

% 02 in He (figure 4-4). For a short period of time (less than 12 min), anhydrous PdO

also exhibits a high CO conversion (above 76 %). However, it rapidly deactivates and

yields a 13 % CO conversion after 19 min. The activity then decays slowly. The initial

oxygen conversion is 55 % at 5 min and 36 % at 12 min which are about one-half of the

corresponding CO conversions. The unusually high initial activity of anhydrous PdO is

most likely due to accumulation of moisture at the surface during air exposure for several

years in the laboratory. Anhydrous PdO purchased recently from Aldrich did not show

any CO oxidation activity even at 100 oC (figure not shown).

XPS spectra obtained from fresh hydrous PdO and fresh anhydrous PdO (Alfa

Aesar from figure 4-4) are shown in figures 4-5 and 4-6. The Pd 3d spectrum obtained

from hydrous PdO is quite similar to that obtained from anhydrous PdO except it is a few

tenths of an eV wider due to the presence of water in the crystalline lattice (figure 4-5).

In figure 4-6 the Pd 3p3/2 peaks lie at 533.6 eV which is also characteristic of an oxide

state for both hydrous and anhydrous PdO. The O is peak for anhydrous PdO is apparent

at 530.2 eV. Hoflund et al. [Hof03] also reported the same Pd 3d BE for hydrous PdO

and anhydrous PdO for their methane oxidation study.









The XPS spectra shown in figures 4-7 and 4-8 were obtained from hydrous Pd after

reaction for 30 and 220 min (0.40 g, slightly 02-rich feed and 25 'C). Compared with

fresh hydrous PdO, metallic Pd features form during reaction. After 30 min of reaction

they are just small shoulders on the low-BE side of the Pd 3d peaks, but they are quite

prominent after 220 min of reaction. The O is peak after 30 min reaction is similar to

that of the fresh hydrous PdO as expected, but it is significantly decreased after 220 min

of reaction. Increased structure is present in the metallic Pd 3p3/2 region after 30 min of

reaction and even more so after 220 min of reaction.

The effect of reaction temperature for hydrous PdO is shown in figure 4-9. At 100

TC 100 % CO conversion is maintained for 170 min and the 02 conversion is maintained

near 42%. At 25 TC 100 % CO conversion is maintained for only about 65 min and again

the 02 conversion is slightly less than one-half of the CO conversion over the time

studied. This behavior implies that less than one-half of the oxygen in the feed gas is

participating in CO oxidation so the rest of the required 02 molecules must come from

either PdO or H20 or both. When hydrous PdO is no longer active for CO oxidation,

most of the PdO is converted into Pd metal. The XPS data shown in figures 4-10 and 4-

11 indicate that Pd metal forms more rapidly at higher reaction temperatures even though

the catalyst still exhibits a 60 % CO conversion before taking the XPS data. Therefore,

the 0 Is peak is decreased relative to that from fresh PdO.

The moisture content of hydrous PdO (PdO xH20O) can be calculated from the Pd

content of 77.81 % and atomic weights to give x = 0.80. A proposed mechanism for CO

oxidation over hydrous PdO at temperatures lower than 100 TC is









(Reaction 1)

PdO xH20

PdO xH -- xOH- + xCO (g)

PdO xH -- xCOOH- + x/2 02 (g)

PdO xH+ -- xHCO3-


PdO xH --xOH-

PdO xH+ -- xCOOH-

PdO xH -- xHCO3

PdO xH --xOH- + xCO2(g)


xCO (g) + x/2 02 (g) 4 x CO2 (g)

x = 0.8 and repeating steps until PdO is converted into Pd metal,



(Reaction 2)

remaining CO in the feed stream can react with PdO directly,


1.2 (PdO xH20)


+ 1.2 CO (g) -*


1.2 (Pd xH20)


+ 1.2 CO2 (g)


1.2 CO (g) 4 1.2 C02 (g)

and Pd' xH20O is inactive for further CO oxidation at lower temperature. Combining

reaction (1) and reaction (2) gives


+ x/2 02 (g)


- 2 C02 (g)


with a total oxygen conversion of ~ 40 % due to the moisture content in hydrous PdO.

In order to check whether reaction (2) is valid, 400 mg of fresh hydrous PdO was

reacted with 1 % CO (no 02 in the feed gas) at room temperature. The data in figure 4-12


2 CO (g)









indicates that hydrous PdO can react with CO molecules even at 25 TC and exhibit a

maximum of ~ 97 % CO conversion for 20 min before gradually decreasing. Comparing

the two conversion versus time curves, the one without 02 deactivates faster and exhibits

lower CO conversion over the entire reaction time.

The conversions of CO and 02 for 400 mg of hydrous PdO are shown in figure 4-

13 for increasing reaction temperature under slightly 02-rich conditions. At 25 TC

hydrous PdO exhibits a 100 % CO conversion and a 44 % 02 conversion for 60 min

before gradually deactivating. The reaction temperature was then increased to 70 oC. At

this temperature the initial CO conversion is above 40 % for 15 min while the 02

conversion is less than 13 %. The low 02 conversion at 70 oC indicates that 02 in the

feed gas does not react to a significant extent by reaction (1) so another form of oxygen

reacts with CO for a short period of time at 70 oC. One possibility is that oxygen from

beneath the hydrous PdO surface reacts with CO. Hoflund et al. [Epl96] found that an

oxygen-exposed polycrystalline palladium surface does not form chemisorbed oxygen in

the outermost layer of atoms using ion scattering spectroscopy (ISS). However, an

exposure to CO induces subsurface oxygen formed during the 02 exposure to migrate to

the surface under a chemical driving force. McMillan et al. [McM05] also reported that

subsurface oxygen at a Pt (100) surface reacts with CO reaction at 267 TC using a feed

composition of CO/02 = 0.30 which is highly oxygen rich. At 100 TC an initial 36 % CO

conversion was found with no reaction of oxygen in the feed stream. The CO conversion

decreases rapidly to a few percent at about 65 min.

The CO conversion comparison for hydrous PdO (0.40 g) when either 0.51 % 02 or

no 02 is added to the feed is shown in figure 4-14. The CO conversions without 02 in the









feed exhibit faster deactivation and lower CO conversions for the entire reaction time at

all temperatures. Without 02 hydrous PdO yields a 9 % CO conversion after 216 min at

25 TC. Then the activity is increased to a CO conversion of 86 % after the temperature is

increased to 70 TC and 36 % at 100 oC. The area between the curves represents the

amount of 02 from the feed stream that is reacted.

High-resolution XPS spectra obtained from hydrous PdO reacted with and without

02 in the feed stream for over 6 hrs at 25, 70 and 100 TC are shown in figures 4-15 and 4-

16. The XPS Pd 3d spectra indicate that the chemical state of hydrous PdO converts to

metallic Pd during reaction. This alteration occurs more rapidly without 02 present in the

feed stream as expected. The XPS 0 Is and Pd 3p3/2 features behave in a manner

consistent with the XPS Pd 3d spectra. The 0 Is feature in PdO is diminished in size and

the Pd 3p3/2 feature is shifted to that of metallic Pd. These changes occur to a greater

extent for the catalyst run without 02 in the feed stream.

4.4 Summary

In this study hydrous palladium oxide (PdO xH20) was tested for CO oxidation as a

function of time at 25 and 100 TC. Fresh untreated hydrous PdO exhibits high CO

conversion at 100 oC, while hydrous PdO pretreated in He at 400 oC results in inactivity

for at 100 TC. During drying in He, lattice water species are removed from hydrous PdO

such that the CO catalytic activity of dehydrated PdO is similar to that of anhydrous PdO.

At 100 TC the time period for complete CO conversion for 0.40 g of catalyst is extended

to 170 min and then the CO conversion decreased rapidly. At 25 TC complete CO

conversion lasts only for 77 min and then smoothly decreases. The initial 40 % 02

conversion before deactivation can be explained by lattice oxygen in hydrous PdO

reaction with CO to form CO2. Direct reaction between CO and 0 from PdO xH20 was









confirmed when fresh hydrous PdO was tested for CO oxidation with 1 % CO and no 02

in He. Hydrous PdO exhibits more than 90 % CO conversion over 90 min. When the

reaction temperature is increased from 25 to 70 oC, the initial 02 conversion of 43 % at

25 TC decreases to only 13 %. This indicates that subsurface oxygen migrates to the

surface and with CO at high temperature. Hydrous PdO without 0.50 % 02 exhibits

faster deactivation and lower CO conversion than those with 02 in the feed. High-

resolution XPS data indicates that the chemical state of palladium during stoichiometric

CO oxidation changes from the oxide state to the metallic state and that the intensity of

the metallic Pd peak becomes more prominent with increasing reaction time or

temperature. This transition also occurs more rapidly when 02 is not added to the feed

gas.














EED-ED-m--o



0






o o-co-o,


100 0C, 0.10 g hydrous PdO


-0- CO, as entered
-o- O, as entered
*--- CO, He annealing, 400 oC, 1 h
0-o0--. 0, He annealing, 400 oC, 1 h

CO/O2 =1.94 for fresh hydrous PdO
CO/02, =1.92 for He-dried hydrous PdO


o -.......
o 0----8.::I-o-==6= .,o-o-----


60 90 120


On-Stream Time (minutes)





Figure 4-1.Comparison of CO and 02 conversions as a function of reaction time over 100
mg of hydrous PdO at 100 oC before and after annealing in He at 400 oC for 1
hr. The feed stream were slightly oxygen rich (1.0 % CO and 0.51 % 02
without annealing in He and 1.0 % CO and 0.52 % 02 with annealing in He)
at total flow rate of 45 cc/min.


100 -



80 -
































Pd 3d3, Pd 3d3/,,
PdO Pd
Pd 3d5/2 Pd 3d5/2
PdO Pd0


346 344 342 340 338 336 334 332
Binding Energy (eV)


Figure 4-2.High-resolution XPS Pd 3d spectra obtained from fresh hydrous PdO, hydrous
PdO after drying in He at 150 oC for 120 min in the reactor and hydrous PdO
after drying in He at 400 oC for 120 min in the reactor.






66





0 1s, Pd 3p Ols
PdO


/









Fresh
400 oC, 2 h, He drying





Figure 4-3.High-resolution XPS s and Pd 3p spectra obtained from fresh hydrous


PdO, hydrous PdO after drying in He at 150 for 120 min in the reactor and
PdO pdafter drying in He at 400 for 120 min in the reactor.



---- 150 0C, 2 h, He drying
.********** 400 0C, 2 h, He drying

540 535 530 525
Binding Energy (eV)


Figure 4-3.High-resolution XPS 0 Is and Pd 3p spectra obtained from fresh hydrous
PdO, hydrous PdO after drying in He at 150 0C for 120 min in the reactor and
hydrous PdO after drying in He at 400 0C for 120 min in the reactor.














100-



80-


, -a--a-- o-~an 250C reaction

-0- CO, Hydrous PdO
S-o- O, Hydrous PdO
--0-- CO, Anhydrous PdO
S o-- 02 Anhydrous PdO

SCO/0 = 1.95 for Hydrous PdO
O CO/O = 1.99 for Anhydrous PdO

Po-0-o--oo-o
0;O O0O\
0 0'




o:80-0--o:8 .B-0-0-0


On-Stream Time ( minutes )




Figure 4-4.Comparison of CO and 02 conversions as a function of reaction time at 25 TC
over 400 mg of hydrous PdO and anhydrous PdO. The feed stream were close
to stoichiometric (1.0 % CO and 0.51 % 02 for hydrous PdO and 1.0 % CO
and 0.50 % 02 for anhydrous PdO) at a total flow rate of 45 cc/min.






68





Pd 3d

Fresh Hydrous PdO
---------- Fresh Anhydrous PdO


>1





| |






Pd 3d3/2 Pd 3d, \
PdO Pd

Pd 3d5/2 Pd 3d5/2
PdO PdO

346 344 342 340 338 336 334 332
Binding Energy (eV)


Figure 4-5.High-resolution XPS Pd 3d spectra obtained from fresh hydrous PdO and
fresh anhydrous PdO.






69






O 1s, Pd 3p ols
PdO


/ >







CIO
i \ I V





C I
C \ / i I




Binding Energy (
II /













Figure 4-6.High-resolution XPS d s and Pd3p spectra obtained from fresh hydrous PdO
and fresh anhydrous PdO pd.



s.4
-- Fresh Hydrous PdO
Fresh Anhydrous PdO



540 535 530 525

Binding Energy (eV)



Figure 4-6.High-resolution XPS O is and Pd 3p spectra obtained from fresh hydrous PdO
and fresh anhydrous PdO.






70






Pd 3d



Pd 3d,, Pd 3d,, *
PdO Pd0
C,)



CI *







z -



Fresh '_
---- 25 C0 30 min
.......... 25 oC, 220 min P 5/2 5/2
PdO Pd0
p p p I p p p I p p p Ip I p.

346 344 342 340 338 336 334 332

Binding Energy (eV)



Figure 4-7.High-resolution XPS Pd 3d spectra obtained from fresh hydrous PdO, hydrous
PdO after exposure to 1.0 % CO and 0.51 % 02 for 30 min in the catalytic
reactor at 25 TC and hydrous PdO after exposure to 1.0 % CO and 0.51 % 02
for 220 min in the catalytic reactor at 25 oC.






71





O 1s, Pd 3p ois
PdO





I:
C I/





C r


F 4 -Pd s3oP 2 Pd 3ps
PdO pd0 0.



Fresh
----25GC rxn, 30 min
.......... 25 C rxn, 220 min


540 535 530 525
Binding Energy (eV)


Figure 4-8.High-resolution XPS 0 Is and Pd 3p spectra obtained from fresh hydrous
PdO, hydrous PdO after exposure to 1.0 % CO and 0.51 % 02 for 30 min in
the catalytic reactor at 25 TC and hydrous PdO after exposure to 1.0 % CO and
0.51 % 02 for 220 min in the catalytic reactor at 25 TC.














100-



80 -


72






0-oa-C-D-E3-DamI-EP--- o- 0-a-- --0 o


Hydrous PdO 0.40g

CO/02= 1.93 for 100 0C h
CO/0 = 1.95 for 25 "C 0 0


00


0D
O \0
-0- CO, 100 oC, 205 min 0. 0. \
o-0- 100 oC, 205 min *...


U.-0


--0 -- CO, 25 oC, 220 min


25 oC, 220 min


.o0---0 .-0


On-Stream Time (minutes)




Figure 4-9.CO and 02 conversions as a function of reaction time over 400 mg of hydrous
PdO at 100 'C for 205 min and 25 'C for 217 min. The feed streams contained
1.0 % CO and 0.52 % 02 at 100 oC and 1.0 % CO and 0.51 % 02 at 25 'C.
The total flow rate was 45 cc/min.


.--.. 0--.... 0 2,,






73






Pd 3d

Pd 3d Pd 3dP3
3/2 3/2
PdO PdN O d





C I
CIO
\1 I


-/ I \I I ^ \










---25Crxn,220min Pd 3d512 Pd 3d5/2
.. ... o100 "C rxn, 205 min PdO Pd0


346 344 342 340 338 336 334 332

Binding Energy (eV)



Figure 4-10.High-resolution XPS Pd 3d spectra obtained from fresh hydrous PdO,
hydrous PdO after exposure to 1.0 % CO and 0.51 % 02 for 220 min in the
catalytic reactor at 25 TC and hydrous PdO after exposure to 1.0 % CO and
0.52 % 02 for 205 min in the catalytic reactor at 100 oC.






74






O 1s, Pd 3p Ols
Fresh PdO
---- 25 0C rxn, 220 min "s
......... 100 oC rxn, 205 min /*" _


.lOC Pdo 2Omn ,. .\
C/)




P d 3 P d 3
S* lI













PdO, hydrous PdO after exposure to 1.0 % CO and 0.51 % 02 for 220 min in











the catalytic reactor at 25 T and hydrous PdO after exposure to 1.0 % CO and
0.52% 02 r 205 min in the catalytic reactor at 100 .





Figure 4-11.High-resolution XPS 0 Is and Pd 3p spectra obtained from fresh hydrous
PdO, hydrous PdO after exposure to 1.0 % CO and 0.51 % Pd for 220 min in
the catalytic reactor at 25 0C and hydrous PdO after exposure to 1.0 % CO and
0.52 % 02 for 205 min in the catalytic reactor at 100 0C.














100- -1-a-0-D-E-
S. PdO H2 00.40 g

80-



60 M\



c 40-
o



-0- 1.0 % CO, 0.51 % 02 s-uppl
-0- 1.0 % CO, no oxygen supply "
0-

0 50 100 150 200
On-Stream Time (minutes)




Figure 4-12.CO conversion as a function of reaction time over 400 mg of hydrous PdO in
1.0 % CO and 0.51 % 02 in He and 1.0 % CO in He both at a total flow rate of
45 cc/min.














100-



80-


Hydrous PdO 0.40 g
CO/02 = 1.99


25 oC


-0- CO
-o- O,


0 50 100 150 200 250 300 350 400
On-Stream Time (minutes)




Figure 4-13.CO and 02 conversions as a function of reaction time over 400 mg of
hydrous PdO at 25 TC (216 min), 70 TC (77 min) and 100 TC (59 min). The
feed stream contained 1.0 % CO and 0.503 % 02 in He at a total flow rate of
45 cc/min.














Hydrous PdO 0.40 g
1.0 % CO


25 oC


-0- with 0.51 % 02 supply
-0- without 02 supply


I I I I I. _


0 50 100 150 200

On-Stream Time


250

(minutes)


300 350 400I
300 350 400


Figure 4-14.CO conversion as a function of reaction time at 25 TC (216 min), 70 TC (70
min) and 100 TC (66 min). The feed stream contained 1.0 % CO and 0.51 %
02 in He in one case and 1.0 % CO only in He in the second case. The total
flow rate was 45 cc/min in both cases.


100-



80-


70 oC


100 C






78





Pd 3d


Pd 3d3/2 Pd 3d32 /
PdO Pd


I:


S Fresh













w/o oxygen, 371 min PdO Pd0
I \ 1







S I res \I I

----250C-700C-0 1OOC00
with oxygen, 387 min
********* 25 C-70C-100C Pd3d Pd 3d,,
w/o oxygen, 371 min PdO Pd

346 344 342 340 338 336 334 332
Binding Energy (eV)


Figure 4-15.High-resolution Pd 3d spectra obtained from fresh hydrous PdO, hydrous
PdO after exposure to 1.0 % CO and 0.51 % 02 in He at the end of the
temperature-time program described in figure 4-14 and hydrous PdO after
exposure to 1.0 % CO only in He at the end of the temperature-time program
described in figure 4-14.






79






O 1s, Pd 3p 01S
PdO



CI





Pd3 Pd 3p P1
Su





Z yPdO Pdo -


Fresh :

----250C-70OC-100oC
with oxygen, 387 min
**......... 25 OC 70 C -100 C
w/o oxygen, 371 min

540 535 530 525
Binding Energy (eV)



Figure 4-16. High-resolution 0 Is and Pd 3p spectra obtained from fresh hydrous PdO,
hydrous PdO after exposure to 1.0 % CO and 0.51 % 02 in He at the end of
the temperature-time program described in figure 4-14 and hydrous PdO after
exposure to 1.0 % CO only in He at the end of the temperature-time program
described in figure 4-14.














CHAPTER 5
NEAR-STOICHIOMETRIC CO OXIDATION OVER HYDROUS PALLADIUM
OXIDE FOR VARIOUS REACTION TEMPERATURES

5.1 Introduction

Pd catalysts are active for various oxidation reactions including methane oxidation

[Epl99, Bur96, Far92, Lyu99], CO oxidation [Sch90, Gar9lb, Zhu05, Fer99] and the NO

+ CO redox reaction [Alm99, HamOl]. For low-temperature CO oxidation, supported Pd

catalysts can be used for improving cold-start CO emission control [Oh93], maintaining

higher output power for closed-cycle CO2 lasers [Sch90], and selective CO removal from

streams of H2 for fuel cells [Oh93]. Most CO oxidation studies involving Pd to date have

examined Pd supported on reducible metal oxides such as CeO2 [Cos03], A1203 [Fer99],

SiO2 [Koc96], and TiO2 [Zhu05]. The role of a metal oxide support such as ceria for CO

oxidation is that ceria increases the thermal stability of an alumina support resulting in a

more durable catalyst [Oza90]. It also has an important property referred to as oxygen

storage capacity (OSC), which can produce a reverse spillover of oxygen from ceria to

the oxygen vacancies on a PdO surface during CO pulse transient oxidation [Cos03].

However, few studies have been carried out regarding how pure palladium powders

interact with carbon monoxide and oxygen near the stoichiometric ratio at low reaction

temperatures (< 200 'C). In most of the Pd-related CO oxidation studies, the supported

Pd catalyst activity was measured under 02-rich conditions (1% CO and 20% 02), and

the Pd was determined to be present as metal oxide clusters. Pd can be present in several

chemical states including Pd metal (Pd'), hydrous PdO (PdO H20), anhydrous PdO (PdO)









and oxygen-depleted PdO. Oh and Hoflund [Oh05b] have shown that metallic Pd is

converted into PdO during CO oxidation under stoichiometric conditions and that the

thickness of the PdO layer increases with increasing reaction temperature. When

conventional supported Pd catalysts are prepared using impregnation, the chemical state

of Pd is PdO [Epl99].

Another important factor for CO oxidation is the role of water. Date and Haruta

[DatOl] reported that the CO oxidation rate increases 10 times as the moisture content in

the feed is increased to 200 ppm H20 in the reactant gas (1 vol% CO in air) for Au/TiO2.

They also found that the moisture adsorbed on the catalyst surface is responsible for the

increase in the catalytic activity rather than water vapor in the gas phase. This is the basis

for studying hydrous palladium oxide (PdOxH20O) versus anhydrous PdO for low-

temperature CO oxidation. Assuming that moisture participates in or increases the CO

oxidation rate in the low-temperature range, information may be obtained regarding the

role of water-derived chemical species for CO oxidation. Cunningham et al. [Cun99]

suggest that CO oxidation may occur by reaction between hydroxyl group CO and OH

radicals over an icosahedral Au/Mg(OH)2 catalyst. They found that CO oxidation under

moist condition (10,000 ppb H20 in the gas inlet) enhances the CO oxidation rate and

that water moisture participates in CO oxidation as a promoter. However, they subjected

their Au/Mg(OH)2 catalyst to a reaction gas of 1 % CO in air so it is not clear if all of the

CO oxidation activity from the catalyst results only from the reaction between CO and

(OH)2.

5.2 Experimental

Hydrous palladium (II) oxide (PdOxH20, Pd 77.81 %) and metallic Pd metal

powders (Pd', 0.25-0.55 microns, Pd 99.95 %) were purchased from Alfa Aesar with









BET surface areas of 35.7 and 10.6 m2/g respectively. Anhydrous palladium(II) oxide

(PdO, Pd 99.998 %) was purchased from Aldrich. The catalytic oxidation of CO was

examined in a fixed-bed flow reaction system under atmospheric pressure. A gas stream

flow rate of 45 cc/min (1.0 vol% CO and 0.50 vol% 02 in He) was supplied through mass

flow controllers (MFC) into a quarter-inch quartz tube reactor. 100 mg of fresh hydrous

PdO or metallic Pd were supported by glass wool, and a K-type thermocouple was placed

around the U-tube reactor at the center of the catalyst bed. Initially, the feed stream

consisted of 100 % He flow for 15 min at room temperature in order to remove any

oxygen from the reactor. The furnace temperature was then raised at a rate of 8 oC/min

with He flowing. After stabilizing the temperature 1.0 % CO and 0.5 % 02 in He was fed

to the reactor. Reactants and products were analyzed by an on-line gas chromatograph

(HP 5890A, molecular sieve 5A) using a thermal conductivity detector (TCD). The

conversion of CO and 02 were calculated using the peak area difference between the

reactant gas mixture and the effluent product gas from the catalyst bed.

5.3 Results and Discussion

Theoretically, 1.0 mol of carbon monoxide and 0.50 mol of oxygen are the

stoichiometric ratio of CO and 02 for CO oxidation. However, it is impossible to attain a

stoichiometric condition due to the ppm level MFC operational oscillation and the 02

impurity levels in the CO and He gas cylinders. An attempt has been made to simulate a

near-stoichiometric ratio between CO and 02, and the experimental CO/02 feed

conditions are shown in Table 5-1. Based on a 1.0 vol% CO feed concentration in He, the

02 feed concentration ranged from 0.489 to 0.515 vol% in the reactant gas stream.

Multiple CO and 02 concentrations were determined from the integrated areas of the

TCD data before and after reaction.









The catalytic CO oxidation activity over hydrous PdO (PdO H20) as a function of

reaction time at low-temperatures (< 160 oC) is shown in figures 5-1 and 5-2. When 1.0

% CO and 0.504 % 02 in He was passed over 100 mg of fresh PdO at 40 oC, the 02

conversion increases from 15 to 40 % during the initial 6 min. The conversion steadily

decreases to 5 % after another 55 min of reaction of time. The CO conversion at 40 oC

exhibits an initial 80 % conversion then increases to 100 % for the next 10 min before

decreasing to 12 % over the next 60-min period. These data indicate that there is a large

conversion difference between CO and 02. Oh and Hoflund [Oh05c] found that 400 mg

of hydrous PdO exhibits a 100 % CO conversion and 40 to 47 % 02 conversion for 60

min reaction time at 25 TC and 170 min reaction time at 100 oC before the CO and 02

conversions decrease. They proposed a mechanism in which lattice moisture reacts with

carbon monoxide and oxygen molecules to form bicarbonate species. These decompose

to form CO2 which desorbs. This mechanism explains the consumption of only half of

the oxygen in the feed stream during the reaction.

As the reaction temperature increases from 40 to 160 oC for each 100 mg of

hydrous PdO, the time interval for 100 % CO conversion increases. At 50 TC, 100 % CO

conversion lasts 19 min while 100 % conversion is maintained for 110 min at 160 oC.

These extended high conversions are the result of subsurface water molecules migrating

to the surface at higher temperatures. Also the initial 02 conversions exhibit higher

values as reaction temperature increases. At 100 TC the initial 02 conversion is 47 % for

33 min while the 02 conversion at 160 oC is about 80 % for 100 min. When the feed

stream is slightly CO-rich (1.0 % CO and 0.492 % 02) at 150 oC, the CO activity rapidly

deactivates at 78 min and decays faster than that of a slightly 02-rich feed (1.0 % CO and









0.515 % 02) at 100 oC. This is due to the conversion of PdO into metallic Pd under CO-

rich conditions. Also, metallic Pd does not exhibit any CO oxidation activity at 150 OC

even under slightly 02-rich feed condition [Oh05b]. At 160 oC, 100 % CO and 75 to 80

% 02 conversions parallel each other for 108 min before deactivation. However, the CO

conversions are always higher than the 02 conversions which mean that oxygen from

PdO or H20 is being consumed during CO oxidation even during the deactivation stage.

The CO and 02 conversions are less than 1 % after 186 min at 160 oC.

The catalytic CO and 02 conversions over hydrous PdO in the temperature range

between 160 and 220 oC are shown in figures 5-3 and 5-4. The time period for complete

CO conversion is extended as reaction temperature increases so that 100 % CO

conversion is maintained for more than 445 min at 220 oC while complete conversion

occurs for only 190 min at 170 oC. Initial 02 conversion over hydrous PdO at 220 oC is

about 90 % during the 450 min period of 100 % CO conversion. A conversion of 90 %

02 can be obtained from three different mechanisms. First, some of the CO and 02 react

with H20 forming bicarbonate species which decompose to yield CO2 which desorbs.

The remaining un-reacted CO can react directly with PdO to produce CO2 and metallic

Pd. XPS data obtained from hydrous PdO which reacted with 1.0 % CO and 0.51 % 02,

indicate that most of the palladium oxide is converted into metallic Pd during reaction at

100 oC for 205 min [Oh05c]. At 220 oC unreacted oxygen molecules in the feed stream

can re-oxidize the Pd metal to PdO. A proposed mechanism for CO oxidation over

hydrous PdO (PdO xH20, x = 0.8) is:


(Reaction 1)









PdO xH20

PdO xH ----xOH" +

PdO xH ----xCOOH" +

PdO xH ----xHCO3


x CO(g)


x CO(g)

x/2 02(g)


+ x/2 02(g)


(Reaction 2)

1.2 (PdO xH20)


PdO xH ----xOH"

PdO xH ----xCOOH"

PdO xH ----xHCO3'

PdO xH ----xOH" + C02(g)


Sx CO2(g)


+ 1.2 CO(g) 1.2 (Pdo-xH20) + 1.2 C02(g)


1.2 CO(g)


1.2 C02(g)


(Reaction 3)

re-oxidizing Pd' metal sites

Pd"oxH20 +


1/2 02(g)


4- PdO xH20


1/2 02(g)


Combining gas-state components from (1) to (3), yields

2 CO(g) + (x+1)/2 02(g) 2 C02(g)









From equation (4), the nominal 02 conversion is calculated to be 90 % using this

mechanism. For a slightly 02-rich feed condition (0.502-0.512 % 02) 88 to 90 % 02

conversion at 220 TC is consistent with the calculated conversion.

The 02 and CO conversions as functions of reaction time and temperature (220-

300 oC) are shown in figures 5-5 and 5-6 respectively. The oxygen conversion over

hydrous PdO remains at 88 to 90 % for 450 min at 220 TC, and the CO conversion

remains at 100 % over this same time period. For a temperature of 240 TC, the initial 02

conversion is 87 to 88 % for 120 min and 80 to 85 % for the next 200 min. Also, the CO

conversion is 100 % for this 320 min and then drops rapidly to less than 5 % within a 20-

min period. The initial 100 % CO conversion period decreases as the reaction

temperature increases so that 100 % CO conversion occurs for 240 min at 260 TC, 160

min at 280 TC and 130 min at 300 oC. The initial 02 conversion also decreases as the

temperature increases, and the length of the high conversion period decreases. The 02

conversion is 81 to 85 % for 242 min at 260 TC, 76 to 80 % for 150 min at 280 TC and 65

to 76 % for 120 min at 300 oC.

The decrease in the initial 02 conversion and the decrease in the active time periods

for both CO and 02 conversions during CO oxidation is due to more rapid depletion of

surface moisture from hydrous PdO at higher reaction temperature. Reaction 1 cannot

occur after a certain time period at each temperature. Furthermore, the CO conversion is

about 11 % and the 02 conversion is about 10 % at 300 TC after deactivation. This

indicates that CO2 can form by reaction 3 without moisture present at 300 TC. This is

consistent with the fact that the reaction is stoichiometric under these conditions. A

blank run (no catalyst present) at 300 oC does not produce any CO2.











(Reaction 4)

Pdo + /202(g) PdO

PdO + CO(g) Pdo + C02(g)



CO(g) + 1/2 C02(g)



As shown in figures 5-7 and 5-8, 0.10g of pure Pd metal and a near-stoichiometric

feed ratio (1.0 % CO and 0.509 % 02) at 200 oC yields 100 % CO conversion and about

95 % 02 conversion for 630 min without exhibiting any signs of deactivation. The same

amount of hydrous PdO yields 100 % CO conversion and 79 to 87 % 02 conversion for

only 380 min before rapidly deactivating to a conversion less than 2 %. The reaction

mechanism for CO oxidation over metallic Pd is believed to follow reaction 4. With

regard to Pd content, 100 mg of hydrous PdO is equivalent to 77.8 mg of Pd metal. In a

previous study 52 mg of Pd metal was used for CO oxidation. A stable CO conversion

was obtained over a long period of reaction time indicating that a small amount of

hydrous PdO decomposition is not the reason for the observed low CO conversions. The

rapid deactivation of hydrous PdO may be explained by Pd sites agglomerating after loss

of water or some structural alteration of PdO which is not favorable for CO oxidation.

When 100 mg of anhydrous PdO was tested for CO oxidation, the CO and 02

conversions at 200 oC are low for the entire reaction time and are almost identical at 2 to

5 %. It seems that PdO would react with CO at 200 oC to form CO2 and metallic Pd, but

this is not the case. For Pd metal reacted with the same CO and 02 concentrations at 200