• TABLE OF CONTENTS
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 Title Page
 Acknowledgement
 Table of Contents
 List of Figures
 List of Tables
 Introduction
 Literature review
 Experimental techniques
 Presentation and discussion of...
 Summary
 Bibliography
 Appendix
 Biography
 Copyright














Title: Relaxation studies of adsorption on thorium oxide.
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Title: Relaxation studies of adsorption on thorium oxide.
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Table of Contents
    Title Page
        Page i
    Acknowledgement
        Page ii
    Table of Contents
        Page iii
        Page iv
    List of Figures
        Page v
        Page vi
        Page vii
    List of Tables
        Page viii
        Page ix
    Introduction
        Page 1
        Page 2
        Page 3
    Literature review
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
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        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
    Experimental techniques
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
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        Page 31
        Page 32
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
        Page 39
        Page 40
        Page 41
    Presentation and discussion of results
        Page 42
        Page 43
        Page 44
        Page 45
        Page 46
        Page 47
        Page 48
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        Page 103
        Page 104
    Summary
        Page 105
        Page 106
        Page 107
        Page 108
    Bibliography
        Page 109
        Page 110
        Page 111
    Appendix
        Page 112
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    Biography
        Page 166
        Page 167
    Copyright
        Copyright
Full Text













RELAXATION STUDIES OF ADSORPTION ON

THORIUM OXIDE











By
KENNETH DARE LAWSON


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










UNIVERSITY OF FLORIDA
August, 1963















ACKNOWLEDGMENTS


To Dr. Wallace S. Brey, Jr., director of this project, the

author would like to express his sincere appreciation.

The author would also like to thank Dr. M. E. Fuller II,

unofficial co-director of the project, for both physical and

mental assistance.

Thanks are also extended to the Atomic Energy Commission

for financial aid received during the course of the investiga-

tion.















TABLE OF CONTENTS


ACKNOWLEDGMENTS . . . .

LIST OF FIGURES . . . .

LIST OF TABLES . . . .

Chapter


9 9 9...........9 9 9 9

. 9 9 9...........9 9

9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9


I INTRODUCTION . . . . . . . . . . ..

II LITERATURE REVIEW . . . . . . . . . .

A. Adsorption Studies by Magnetic Resonance
Techniques . . . . . . . . .

B. Adsorption Studies by Dielectric Measurements . .

III EXPERIMENTAL TECHNIQUES . . . . . . ... .

A. Samples . . . . . . . . . . .

B. Vacuum System and Techniques Used in the Nuclear
Resonance Experiments . . . . . .

C. Description and Procedures of the Magnetic
Resonance Studies . . . . . . . .

D. Determination of T and T2 . . . . . .

E. Measurement of Hf . . . . . . . . .

F. Systems and Techniques Used in the Dielectric
Experiments . . . . . . . . . .

IV PRESENTATION AND DISCUSSION OF RESULTS . . . . .

A. Magnetic Resonance Results . . . . . . .

B. Dielectric Results . . . . . . . . .

C. Comparison of Dielectric and Magnetic Resonance
Results . . . . . . . . . . .

iii


Page

ii

v

viii



1

4


4

15

22

22


25


29

31

32


33

42

42

82


103









Page

V SUMMARY . . . . . . . . . . . . 105

BIBLIOGRAPHY . . . . . . . .. . . . 109

APPENDIX A: Tables of Numerical Data . . . . . . 113

APPENDIX B: Theory of Nuclear Magnetic Resonance . . . 159

APPENDIX C: Theory of Dielectric Measurements . . . 163














LIST OF FIGURES


Figure Page

1. A Typical B. E. T. Isotherm Used for Determination
of the Surface Area . e . . . . . . . 26

2. Vacuum System Used in the N.M.R. Experiments . . 27

3. Determination of the Magnitude of H1 for Various
Attenuations . . . . . . . . . . 34

4. Vacuum System Used in the Dielectric Experiments . 35

5. Electrodes and Switching Device for the Dielectric
Cell . . . . . . . . . . . . 38

6. Assembled Dielectric Cell . . . . . . .. 40

7. Line Width of Water on Thorium Oxide Prepared from
Thorium Hydroxide . . . . . . . . . 48

8. The Observed Relaxation Times of Water on Thorium
Oxide Prepared from Thorium Hydroxide . . . . 51

9. Line Width of Water on Thorium Oxide Prepared from
Thorium Oxalate . . . . . . . . . 53

10. The Observed Relaxation Times of Water on Thorium
Oxide Prepared from Thorium Oxalate . . . . . 54

11. Line Widths of Various Adsorbates on Thorium Oxide . 56

12. The Observed Relaxation Times of Methanol on
Thorium Oxide . . . . . . . . . . 58

13. The Temperature Dependence of the Line Width of Water
on Thorium Oxide . . . . . . . . . 61

14. The Temperature Dependence of the Observed Relaxation
Times of Water on Thorium Oxide . . . . . . 62

15. Log of the Line Width versus the Reciprocal Temperature
for Water on Thorium Oxide Prepared from Thorium
Hydroxide . . . . . . . . . . . 65










Figure

16. Log of the Line Width versus the Reciprocal Tempera-
ture for Water on Thorium Oxide Prepared from Thorium
Oxalate . . . . . . . . . . . .

17. The Temperature Dependence of the Line Width of
Methanol on Thorium Oxide . . . . . . . .

18. The Temperature Dependence of the Line Width of
Ethanol Adsorbed on Thorium Oxide . . . . . .

19. The Temperature Dependence of the Line Width of
Butylamine Adsorbed on Thorium Oxide . . . . .

20. Presentations of the Absorption Line of Butylamine
Adsorbed on Thorium Oxide . . . . . . . .

21. Fluorine Spectrum of Hexafluoropropene Adsorbed on
Aluminum Oxide . . . . . . . . . .

22. The Temperature Dependence of the Line Widths of
56.0 mg. C3F6/g. A1203 . . . . . . . .

23. The Temperature Dependence of the Line Widths of
93.6 mg. C3F6/g. A120 . . . . . . . .

24. Capacitance Changes with Frequency for the Cell, Dry
Sample, and Sample with Adsorbed Water . . . .

25. Conductance of Water on Thorium Oxide Prepared from


Thorium Oxalate . . . . . .

26. Conductance of Water on Thorium Oxide
Thorium Hydroxide . . . . .

27. Capacitance of Water on Thorium Oxide
Thorium Hydroxide . . . . .

28. Capacitance of Water on Thorium Oxide
Thorium Oxalate . . . . .

29. Conductance of Water on Thorium Oxide
of 2.0 Megacycles . . . . .

30. Tangent of the Loss Angle of Water on
Prepared from Thorium Oxalate . .

31. Tangent of the Loss Angle of Water on
Prepared from Thorium Hydroxide . .


Page


Prepared from

Prepared from


Prepared from



at a Frequency
Thorium Oxide. . . .

Thorium Oxide
. . . . .

Thorium Oxide









Figure Page

32. Capacitance of Methanol on Thorium Oxide Prepared
from Thorium Hydroxide . . . . . . . . 99

33. Capacitance of Methanol on Thorium Oxide Prepared
from Thorium Oxalate ............... 100

34. Nuclear Relaxation Times as a Function of the
Correlation Time . . . . . . . . . 162


vii














LIST OF TABLES


Page

113


Summary of Sample Preparations and Properties . . .


2. Dependence of the Line
Times on the Amount of

3. Dependence of the Line
Times on the Amount of

4. Dependence of the Line
Times on the Amount of

5. Dependence of the Line
Times on the Amount of

6. Dependence of the Line
Times on the Amount of

7. Dependence of the Line
Times on the Amount of


Width and Observed Relaxation


Adsorbate

Width and
Adsorbate

Width and
Adsorbate

Width and
Adsorbate

Width and
Adsorbate

Width and


for Sample VI-D .

Observed Relaxation
for Sample VI-E .

Observed Relaxation
for Sample IX-A .

Observed Relaxation
for Sample IX-B .

Observed Relaxation
for Sample OX-3 .

Observed Relaxation


Adsorbate for Sample OX-4 . ..


8. Temperature Dependence of the Line Width and the
Observed Spin-spin Relaxation Time of Water Adsorbed
on Sample VI-D . . . . . . . . . .

9. Temperature Dependence of the Line Width and the
Observed Spin-spin Relaxation Time of Water Adsorbed
on Sample VI-E . . . . . . .. . .

10. Temperature Dependence of the Line Width and Observed
Relaxation Times of Water Adsorbed on Sample IX-A .

11. Temperature Dependence of the Line Width and Observed
Relaxation Times of Water Adsorbed on Sample IX-B .

12. Temperature Dependence of the Line Width and Observed
Relaxation Times of Water Adsorbed on Sample OX-3 .

13. Temperature Dependence of the Line Width and Observed
Relaxation Times of Methanol Adsorbed on Sample OX-3

14. Temperature Dependence of the Line Width and Observed
Relaxation Times of Methanol Adsorbed on Sample VI-D


viii


Table

1.









Table Page

15. Temperature Dependence of the Line Width and Observed
Relaxation Times of Ethanol Adsorbed on Sample VI-E . 131

16. Temperature Dependence of the Line Width and Observed
Relaxation Times of Butylamine Adsorbed on Sample VI-D 132

17. Apparent Activation Energies of Processes Contributing
to the Spin-spin Relaxation of Water on Thorium Oxide . 134

18. Line Widths of Thorium Hydroxide . . . . . . 135

19. Line Widths of Thorium Oxide in the Presence of Excess
Water . . . . . . . .. . . . . . 135

20. Line Width as a Function of the Coverage of CFaFb = CFxCF3
Adsorbed on Aluminum Oxide . . . . . . . 136

21. Observed Relaxation Times of CFaFb = CFxCF3 Adsorbed on
Aluminum Oxide . . . . . . . . . . 137

22. Chemical Shifts of CFaFb = CFxCF3 Adsorbed on Aluminum
Oxide . . . . . . . .. . . . 138

23. Temperature Dependence of the Chemical Shifts of
CFaFb = CFxCF3 Adsorbed on Aluminum Oxide . . . . 139

24. Temperature Dependence of the Line Widths of
CFaFb = CFxCF3 Adsorbed on Aluminum Oxide . . . . 140

25. Changes with Frequency in the Capacitance of the
Dielectric Cell and in the Capacitance of an RC Network 141

26. Dielectric Properties of Water Adsorbed on Sample X-B . 142

27. Dielectric Properties of Water Adsorbed on Sample OX-4 147

28. Dielectric Properties of Methanol Adsorbed on Sample
X-B . . . . . . . . . . . 152

29. Dielectric Properties of Methanol Adsorbed on Sample
OX-4 . . . . . . . . . . . . 154

30. Tangent of the Loss Angle of Water Adsorbed on Thorium
Oxide . . . . . . . . . . . . 156

31. Frequency Dependence of the Tangent of the Loss Angle
of Water and Methanol Adsorbed on Thorium Oxide . . 158















CHAPTER I

INTRODUCTION

"And ye shall know the truth and the truth shall
make you free."
John 8:32

The investigation of the mechanism of solid-gas interactions in

systems such as sorbed gases and/or liquids on solid substrates has

long been of considerable interest in several areas of scientific

research. One of the major areas of interest has been that of hetero-

geneous catalysis.

The ability of thorium oxide to catalyze a number of vapor phase

chemical reactions has long been known.1-5 However, up to the present

time the mechanisms of the various reactions, in terms of the behavior

of the oxide surface, are not completely understood. Several features

of the surface which must in some manner be related to the activity of

the catalyst toward a given reaction are: the surface area, the pore

structure, and the presence or absence of "active" sites on the surface.

Each of these features depends on the past history of the catalyst, and

thus, to obtain a catalyst having the desired properties, the history

must be controlled. In order to do this the processes which produce

the various properties must be understood.

This investigation is concerned with the interactions between the

surface of thorium oxide and adsorbed polar molecules. To determine

the nature of the interactions and the "degree of interaction" between










the surface and the adsorbates, several samples of thorium oxide,

differing in various properties, have been investigated by nuclear

magnetic resonance techniques and dielectric measurements.

In particular nuclear resonance techniques applied to nuclei in

sorbed liquids and/or gases give several kinds of information about

both the sorbed molecules and the adsorbing surface. As an example

the surface of the adsorbent may consist of sites on which the adsorbate

is adsorbed with different strengths. If this is true, different phases

may be distinguished by the presence of different sets of nuclear relax-

ation times. The accessibility of a catalyst and the influence of the

method of preparation upon its activity can also be investigated by

determination of the relaxation times of the sorbed molecules.

In the same manner dielectric measurements on sorbed systems can

also give pertinent information about the interactions between the

adsorbate and adsorbent through the determination of such quantities

as the capacitance and conductance changes.

In addition to the interest in thorium oxide arising from its use

as a catalyst there has been considerable interest in the relation of

its surface properties to its ability to cake. This interest rises

from its use as a breeder fuel in nuclear reactors.6

Yet another reason for investigating these particular systems is

simply the fact that there is no data of the type presented here

existing in the present scientific literature. Therefore, this study

improves and extends methods of studying solid-gas interactions in

relation to catalytic activity. This is especially true of nuclear

magnetic resonance techniques.7







3


In summary, the purpose of the present study is to investigate

solid-gas interactions by magnetic resonance and dielectric measure-

ments and to relate the results to the catalytic properties of the

solid. In addition, it is believed that the results will provide an

insight into the mechanism of the adsorption process itself.














CHAPTER II

LITERATURE REVIEW

"There is no new thing under the sun."
Ecclesiastes 1:9


A. Adsorption Studies by Magnetic Resonance Techniques

Although nuclear magnetic resonance (n.m.r.) has become a powerful

tool for molecular structure determination its application to the study

of gas-solid interactions has been somewhat limited.

In the infancy of the technique n.m.r. was used to determine the

water content of various materials8 and to observe in a qualitative way

the amount of water adsorbed on starch and egg albumin.9 Two lines

were observed in the study of water adsorbed on the albumins. These

consisted of a very narrow line superimposed on a broader line. The

broad line is attributed to the water in the solid and water "sorbed"

on the surface of solid produces the narrow line. The width of the

narrow line decreased as the amount of adsorbed was increased while the

broad component remained essentially constant.

Similar behavior was later observed by Tanaka and Yamagata10 for

water adsorbed on carbon. In this system the wide line is attributed

to the water in the first layer and the narrow line is attributed to

the water in the upper layers. A study of water sorbed on fibrous

materials11 has shown that there are two states of mobile water with a

boundary at about twenty per cent moisture content. These two states

show a different dependence of the line width upon the amount of water







5


in the states. Dielectric measurements of the same systemsl2,13 indi-

cate that in addition to these two states another phase is also present.

This latter phase is attributed to localized adsorbed water. It is

believed by the authors that the sensitivity of the n.m.r. apparatus

used in the above study was such that a line from this localized water

could not be obtained.

The water-titanium dioxide system has been studied by two groups

of workers. Mays and Brady14 studied the temperature dependence of the

line width and found that motion persists down to 77 K. This motion

is interpreted as molecules either "flopping about" at an active site

or jumping between these sites. It was concluded that for very low

coverages the water molecules are held to the surface by forces suffi-

ciently strong that formation of ice clusters is prevented even at 770 K.

However, when the coverage is increased to the point where multilayer

formation begins an ice-like structure results.

Fuschillo and Aston15 have also studied water and methane adsorbed

on rutile. For water adsorbed to the amount of about 0.1 monolayers the

line width is such as to indicate that the molecules are sufficiently

far apart so that little or no interaction is present among them. As

the coverage increases the width of the line decreases and at a coverage

of about two monolayers the width is about one-half of that found for the

0.1 monolayer coverage.

The line width for methane adsorbed on rutile was found to be much

less than that for water adsorbed on rutile. It was concluded that the

reorientation processes occur much more readily in this system than in

the water-rutile system.








6
Carbon tetrafluoride has been adsorbed on rutile and studied by

Stottlemeyer and coworkers.16 Rotation is found to persist in the

outer layer at all temperatures down to 200 K. Self-diffusion persists

down to 55 K. and, at a coverage of one-half a monolayer, down to 300 K.

One component of the absorption line, for a coverage of 3.24 layers,

is believed to be due to rotation of the molecules in the outer layer.

For this coverage it is believed that the inner layers are rigid.

O'Reilly and coworkers17 studied the residual protons in silica

gel and in a silica-alumina catalyst and found that the line widths are

essentially independent of temperature from 280 C. to -210* C. The

spin-spin relaxation time (see Appendix B for a discussion of relaxation

times) is of the order of 10-4 seconds for both samples. It was con-

cluded on the basis of chemical shift measurements that the protons were

present as hydroxyl groups rather than water molecules.

Graham and Phillips18 observed chemical shifts for several organic

compounds adsorbed on various types of carbon. This study indicates

that the adsorption frequencies of the organic material in the more

mobile part of an adsorbed film are much lower than those of the free

compounds. For a material having more than one resonance line, all the

lines are shifted by essentially the same amount. The product of the

observed shift and the thickness of the adsorbed layer, excluding the

first layer, was used to calculate a number which represents the thick-

ness of the material through which the perturbations of the surface

can be felt. This number is essentially constant for all combinations

of carbons and organic and corresponds to about seven molecular layers.

The shifts of the organic materials are of such a magnitude that

they must be attributed to the effect of paramagnetism in the surface










of the solid. However, previous to the adsorption of the organic

materials, magnetic susceptibility tests indicated that the bulk solids

were diamagnetic. Paramagnetic resonance techniques also indicated no

unpaired electrons in the carbons.

Pulsed or "spin-echo" nuclear resonance techniques have been used

by several groups of workers to study adsorbed materials. Zimmerman

and coworkersl9,20*21 have made an extensive investigation of the water-

silica gel system by this technique. For a coverage of greater than

two layers two distinct spin-spin relaxation times were observed in

the preliminary study.19 One of these remains nearly constant with

increasing coverage while the other increases sharply. The constant

component is associated with the water adsorbed in the monolayer while

the second component, which increases with coverage, is associated with

the water adsorbed on the monolayer. In the region between one-half

and two monolayers only one relaxation time is observed. This is taken

as an indication that protons in this condition have the same average

interactions with the local magnetic fields. For a coverage of less than

one-half a monolayer two values are again observed. The shorter of

these two is associated with the protons bound to impurity sites in

the silica gel.

A qualitative correlation was made between these results and the

dielectric results obtained by Kurosaki22 on the same system. The

changes in the slope of the plots of the apparent dielectric constant

against the amount of coverage occur near the points where the changes

occur in the spin-spin relaxation time.



*This work will be discussed in more detail in a later section.










In a later investigation20 measurements of the spin-lattice

relaxation time (T1) produced only one value below a coverage of one

monolayer. This value increases with increasing coverage. From the

data collected in these two studies the average life-time of a water

molecule in an adsorbed phase on silica gel is calculated to be approxi-

mately three milliseconds. From this calculation, it is concluded that

the value of T1 found is the value which is the average of the two

times for the protons in the different phases.

In a more refined study21 Zimmerman and Lasater again found two

values of the spin-spin relaxation time (T2) at low coverages. These

components remain constant up to a coverage of about two layers. At

this point the longer time commences to increase. This increase is

attributed to the effect of capillary condensation which occurs at this

coverage. The shorter time is associated with water molecules adsorbed

on high energy sites on the solid. An abrupt change in T2 which occurred

at about one-half a monolayer in the previous study20 was not observed

in this investigation. This is not considered an inconsistency, however,

because the small values of T2 found here would obscure this effect.

In the region from one-half to three-fourths of a monolayer the

spin-lattice relaxation time (T1) also has two components. All other

coverages produce only one value. The appearance of two phases in this

particular region is attributed to strong molecular interactions which

produce ordering of the molecules in the adsorbed phase. The spin-

lattice relaxation time measured over the total range of coverage up

to two layers exhibits a minimum between one-half and one monolayer.

This minimum is in the region where the two values are observed. It










was concluded that a single phase T1 will be observed if the average

lifetime of a water molecule in either of the two phases is much less

than the T1 of that phase. It was also concluded that the two phases

observed in the spin-spin relaxation data are identical to the two

phases which produce the two values of the spin-lattice relaxation

time.

Hickmott and Selwood23 have used spin-echo techniques to study

the proton reorientation process in several associated organic liquids

adsorbed on catalytic solids. For all liquids studied, methanol,

ethanol, water, and n-hexane, there is a linear increase in the spin-

lattice relaxation time as the amount of adsorbed material is increased.

Methanol adsorbed on gamma-alumina has a relaxation time slightly

greater than water but slightly less than ethanol. Hlexane has a relaxa-

tion time greater than any of the polar liquids. The hexane relaxation

time decreases when the alumina is completely dehydrated indicating a

greater interaction between the hexane and the dehydrated alumina surface.

Only one relaxation time was found in any particular measurement,

indicating only small variations in the environment of the individual

protons.

A qualitative explanation is offered to explain the behavior of

these systems. It is suggested that the first liquid adsorbed is

bound tightly to the solid surface and the protons exchange with lattice

vacancies and with protons incorporated in the alumina. As more liquid

is added a larger fraction of the total amount of liquid approaches

the liquid state and the relaxation times approach the values for the

bulk liquids. The smaller change noted for hexane indicates again that









hexane interacts only weakly with the surface of the solid. The fact

that dehydrated alumina produces a shorter relaxation time indicates

that hexane, being immiscible with water, can approach the solid sub-

strate closer after dehydration.

A study of the same liquids adsorbed on supported metal oxides

shows a similar behavior with the exception of water adsorbed on

supported copper oxide. This system shows a nearly constant change

in the relaxation time with the amount of water adsorbed regardless

of the initial concentration of paramagnetic ions. There is an indica-

tion that free cupric ions are present on the surface of the oxide which

are taken into the water and thus these ions rather than the solid

itself control the relaxation time. It was also found that those cata-

lysts and catalyst supports having the largest specific surface areas

were most effective in reducing the relaxation times.

The temperature dependence of the relaxation times of hydrogen

adsorbed on palladium has been studied by Burger.24 The spin-lattice

relaxation time (Tl) plotted as a function of temperature, shows a

minimum for all coverages of hydrogen while T2 exhibits a maximum at

low amounts of hydrogen. This maximum in T2 disappears as the amount

of hydrogen is increased. Similar studies on Pd-Fe alloys show that

T, has both a maximum and a minimum while T2 behaves in the same manner

as in the case of palladium. The minimum in the T1 curves is interpreted

as indicating that the protons are in rapid motion while the maximum in

the spin-spin relaxation time indicates that there is not a homogeneous

distribution of protons in the palladium.










The adsorption of water on alumina has been studied by Winkler.25,26

lHe found that the value of the spin-lattice relaxation time increases

linearly with increasing water coverage. Both the spin-spin and the

spin-lattice relaxation times show sharp breaks in plots of the relaxa-

tion times against the amount of adsorbed water. At higher coverages

the increase of relaxation times is greater for an incremental increase

in the amount of adsorbed water. The break points are associated with

the coverage necessary to completely fill the micropores and start

filling the macropores. The value of the correlation time found for

the first layer of adsorbed water is nearer to that of ice than it is

to that of liquid water.

In a study of water adsorbed on alumina by continuous excitation

n.m.r. Neikam27 observed a narrow line superimposed on a wide line at

very low water coverages. The narrow component does not change its

width or amplitude appreciably with addition of water. No reasonable

explanation of this component could be given. The broader component

of the line decreases in width with increasing water coverage. A plot

of line width against amount of water adsorbed shows two linear sections

separated by a region in which the line width changes only slightly

with addition of water. The second linear region, occurring at higher

coverages, is associated with rather loosly bound water molecules

adsorbed on the monolayer. This region is centered around the point

where the second layer should be completed. The first linear region

is attributed to those molecules which are adsorbed just after comple-

tion of the monolayer and are still interacting strongly with the

substrate. Whenever the water coverage was less than that necessary










for one monolayer the resonance signal could not be detected. The

transition region was postulated to be due to water molecules which

are adsorbed with their protons toward the monolayer and which thus

would not have as much motion as if adsorbed in the opposite direction.

The temperature dependence of the line widths shows two regions

with a sharp change in the slope at about -30 C. for low coverages

and at somewhat lower temperature for higher water coverages. This

abrupt change in line width is attributed to the "freezing out" of

motions which contribute to line narrowing.

The spin-spin relaxation process was treated as a first order

rate process and an apparent activation energy was obtained from plots

of the log of the line width against the reciprocal temperature.

Values around 1.7 kcal./mole were obtained for the region below the

transition temperature and values of less than one kcal./mole were

obtained for the region above the transition temperature. The values

for the upper region decrease with increasing water coverages. The

energies found indicate that the motions which were "frozen out" are

rotations.

Dielectric studies were made on the same systems and the results

compared with the n.m.r. results. Plots of the tangent of the loss

angle (see Appendix C) against the amount of water coverage exhibit

four linear regions. These regions appear to have little relation to

the regions in the line width curves, however. In all cases the loss

tangent appears to reach a maximum at about the same coverage where

the second linear region appeared in the line width curves.

The adsorption of methylamine and CH3ND2 on gamma-alumina has

been studied by Hirota and coworkers.28 It is concluded from chemical










shift measurements and from infrared spectroscopic data that the amine

is adsorbed in such a manner that the molecules are bonded by the amino

group to the solid surface. The deuterated material gave similar results

except that the line width is less for the same amount of material

adsorbed.

The same group of workers studied the adsorption of formic acid

on alumina and on silica gel.29 Only one signal is observed for low

coverages of the acid on silica and it is assigned to the hydrogen on

the carbon atom. This assignment was made by substituting deuterium

for the hydrogen on the carbon atom. At a coverage of about 2.7 layers

the signal from the acidic hydrogen is also observed. For formic acid

adsorbed on alumina the signal could not be detected at any coverage.

Infrared measurements indicate that the interactions between the acid

and the alumina substrate are very strong.

Several workers have studied water adsorbed on proteins by magnetic

resonance techniques. Brey and Fuller30 have studied water sorbed on

bovine serum albumin. This system shows a two component absorption

line. One component is very broad and apparently is independent of

temperature and unaffected by the presence or absence of sorbed water.

This component is evidently characteristic of the protein itself. As

water is added to the protein the line component of the adsorbed water

can be observed. The width is much less than that of the component

belonging to the protein protons. The water component decreases in

width with increasing temperature from about -95 C. to about 300 C.

Above this temperature range the width remains virtually constant for

high water coverages. For low water contents the width does not

reach a constant value.









Calculation of apparent activation energies for the reorientation

processes from the slopes of plots of the log of the line width against

the reciprocal temperature gives values ranging from one to six kcal./-

mole. For very low water coverages the energies are somewhat uncertain

but appear to be greater than six kcal./mole. As the coverage is

increased the energies decrease to about one kcal./mole but then at

even higher coverages larger values are again found. As the coverage

is increased even more the energy values decrease again. The maximum

in the energies, at the higher coverages, corresponds to an abrupt

change in the plot of the line width against the amount of adsorbed

water. These observations are explained by assuming that the first

water added is hydrogen bonded to the free polar groups in the protein

in such a way that one water molecule is attached to two groups. This

produces a rather large activation energy. As more water is added it

becomes necessary to have a one-to-one ratio of water molecules and

polar groups which results in a lower energy barrier to reorientation.

As the water coverage is increased still more it is assumed that some

type of hydration shell is formed causing another increase in the

activation energy. The constancy of the line width at high temperature

is unexplained but may result from magnetic field inhomogeneities

produced by the solid material.

Berendsen31 has studied the hydration of collagen in bovine

Achilles tendon. Measurements on partially dried samples show a water

absorption line separate from the collagen absorption line. The water

line was studied as a function of the angle between the magnetic field

direction and the fiber direction.










Three components of the water line are observed at some values of

the angle for all the samples studied except one which was exposed to

ten per cent relative humidity until equilibrium was reached. This

particular sample produces only a single component which is independent

of the angle. The separation and the angular dependence of the other

peaks allow certain hydration structures to be ruled out, at least in

this system. It is concluded that the hydration structures which are

not present are: (a) molecules bound to polar groups and rotating about

the bond axis, (b) water molecules bound to specific sites, by either

hydrogen or polar bonds, in such a way as to remain motionless and

having the direction of the proton interaction along the fiber direction,

and (c) free water molecules that translate in channels between the

collagen molecules.

The results are explained by assuming that the water molecules

form chains in the direction of the fibers. These chains reorient

about their lengths and have average lifetimes which are short com-

pared to the lifetime of a given spin state.

It is concluded that if the water has a three dimensional struc-

ture at high coverages it must have a large number of lattice defects

and is in a state somewhere between a solid and a liquid. The small

width of the observed lines makes this necessary.


B. Adsorption Studies by Dielectric Measurements

Extensive work has been done on the relation of the dielectric

properties of adsorbed systems to the nature of the adsorbed materials

and to the nature of the bonding between adsorbate and adsorbent.









Because of the large number of papers treating this subject only a

selected few will be discussed.

McIntosh and coworkers32 studied the dielectric properties of

ethylene oxide, butane, and ethyl chloride adsorbed on porous silica

gel. Curves obtained by plotting the change in capacitance against the

amount of adsorbed material consist of two or three linear segments.

Both butane and ethylene oxide produce curves with two linear

regions while three linear segments are found for ethyl chloride.

Comparison of the break points in these curves with the amount of

material necessary to produce one monolayer gave no correlation between

the two. However, several facts become apparent from the observed

results. These are: (a) all adsorbates appear to have two or more

portions having different dielectric constants. (b) The dielectric

constants of the adsorbed butane and ethyl chloride show a very small

temperature coefficient. (c) Adsorbed ethyl chloride has a dielectric

constant significantly lower than that of the bulk liquid. (d) Adsorbed

ethyl chloride behaves as a non-polar compound due either to restricted

mobility or to molecular association. (e) Butane appears to be adsorbed

in a liquid-like state.

A similar study33 with butane and ethyl chloride adsorbed on titanium

dioxide gave similar results. The temperature dependence of the capaci-

tance for adsorbed ethyl chloride is found to be essentially zero below

a monolayer coverage. This is interpreted to indicate the presence of

very rigid bonding in the monolayer. In a second investigation34 of

butane, ethyl chloride, and water adsorbed on silica gel no Debye type

dispersions were found down to temperature around -30* C. Mixtures of

water and ethyl chloride and water and butane were also studied and the









results indicate that both adsorbates share two types of sites on the

substrate. It is concluded from this study that water molecules undergo

oscillatory motion and ethyl chloride molecules, at least the first

quantities adsorbed, rotate freely.

In a more recent study35 McIntosh's group has investigated the

dielectric properties of methyl and ethyl chlorides and butane adsorbed

on porous vycor glass. Loss maxima are observed for low coverages of

methyl and ethyl chloride and are attributed to complexes being formed

between the hydroxyl groups in the glass and chlorides. Butane shows

no evidence of complex formation.

Two linear sections are observed in plots of capacitance change

against the amount of adsorbate with the second section having a lower

slope. It is concluded that the chlorides not included in the complexes

behave as an oscillatory system.

Heukelom and van Reijen36 have studied the adsorption of several

organic liquids and water on silica gel. For all materials, methanol,

water, methylethyl ketone, chlorobenzene, benzene, and diethyl ether,

high values of the real part of the dielectric constant are observed.

The values do not differ substantially among the several adsorbates or

for different amounts of the adsorbates. It is concluded that the

adsorbent is primarily responsible for the values of the dielectric

constant. Plots of the relaxation times as a function of the amount of

adsorbates indicate no special behavior at a coverage of one monolayer.

The value of the relaxation time decreases with increasing coverage for

all adsorbates.

Kurosaki22 has studied the dielectric behavior of water sorbed on

silica gel in the frequency range from two kilocycles to one megacycle.









Three different states of the sorbed water are found. In the first

stage of adsorption the very low specific polarization of the sorbed

water indicates that this water is firmly bound to the surface. In

the second stage, which ranges from about fifteen to ninety-five milli-

grams of water per gram of adsorbent, the water behaves as water

dissolved in an organic solvent. At high water coverages the water is

considered to be capillary-condensed and the value of the maximum loss

frequency indicates that hydrogen bonding is greater in this state than

in liquid water; in fact, it approaches that found in ice. The frequency

of the maximum loss increases as the amount of coverage increases.

Several groups of workers have studied the adsorption of various

liquids on alumina. Thorp37,38 has studied water, methanol, ethanol,

and benzene on alumina and on silica gel.

For water, ethanol, and methanol adsorbed on alumina, silica gel,

and mixed silica-ferric oxide gel37 a hysteresis phenomenon is observed

in both the adsorption isotherms and in the incremental capacitance

measurements. This phenomenon is attributed to the different electrical

properties of the molecules in the multilayers as compared to those in

the entirely capillary-condensed state which is present during desorp-

tion. In a study of water and benzene adsorbed on alumina,38 Thorp

found no hysteresis in the capacitance data for the water-alumina system.

This absence of hysteresis is attributed to strong adsorbate-adsorbate

interactions. The results obtained indicate that a layer of oriented

water molecules is physically adsorbed onto the substrate surface which

has been modified by some chemisorbed water molecules.

Benzene adsorbed entirely in the capillary-condensed state on both

alumina and silica gel has a dielectric constant which compares with










that of liquid benzene. The dielectric constant of a monolayer of

benzene on silica gel is lower than the value for the liquid but the

value for a monolayer on alumina is equal to that of benzene in the

solid state. These values are explained by assuming interactions

between the electrons of benzene and the hydroxyl groups in the

silica and by assuming close-packing of the benzene molecules on the

alumina surface.

More recently Baldwin and Morrow39 and Dransfeld and coworkers40

have studied the water-alumina system. Baldwin and Morrow39 measured

the dielectric constant and loss tangent as a function of temperature

in the frequency range from fifty cycles per second to one hundred

kilocycles per second. They found that water in the monolayer is very

strongly bound while water in the multilayers has sufficient freedom

to orient in the applied field. The polarization of the molecules in

the monolayer is very small and it is concluded that it does not arise

from dipole orientations. The polarization of molecules in the upper

layers is much larger and it is concluded that the orientation primarily

takes place by means of activated jumps of the molecules from one

equilibrium position to another on the substrate. A very narrow distri-

bution of relaxation times is found at the low frequencies and a much

broader distribution at the higher frequencies. Both the dielectric

constant and the loss factor are essentially temperature independent at

low coverages but quite temperature dependent at high coverages.

In order to ascertain the nature of the freezing process of sorbed

water Dransfeld and coworkers40 have studied the dielectric relaxation

of water on gamma -alumina as a function of temperature. In all cases









studied the amount of adsorbed water was sufficient for capillary

condensation to take place and the major portion of the adsorbed water

could be considered to be capillary-condensed. No abrupt change in the

relaxation rate is observed as the temperature is varied through the

normal freezing point of water. It is concluded that the solidifica-

tion of capillary-condensed water can be considered as a transition

between a supercooled liquid and a glass-like state.

Ebert41,42 has studied the adsorption of water on alumina rather

extensively. Three linear regions are observed when the tangent of the

loss angle is plotted against the amount of water adsorbed. The tempera-

ture dependence of the first region is very small but the other regions

show significant changes with temperature. This behavior is attributed

to the rigid bonding present in the monolayer compared to that in the

multilayers. The first two linear regions are attributed to the water

molecules in the monolayer.

In a later study42 Ebert found linear portions in the plots of the

apparent dielectric constant versus the amount of coverage. As the

frequency at which the measurements were made was increased the change

in the dielectric constant and in the loss factor with addition of

water became less. Both the apparent dielectric constant and the

dielectric loss decrease with decreasing temperature.

Neikam27 investigated the alumina-water system and found that plots

of the tangent of the loss angle versus the amount of adsorbate exhibit

four linear regions. In the first region the value of the tangent

remains essentially constant. This region ends at a water coverage

necessary for the completion of a monolayer. The other regions are









attributed to the water molecules being bound to the substrate by one,

two or three hydrogen bonds. The possibility of active sites on the

substrate is proposed as a explanation of the various types of adsorp-

tion. Heats of adsorption values indicate that all bonding after the

completion of the monolayer is rather weak. Plots of the change in

capacitance against the amount of adsorbed water exhibit no linear

regions. The dielectric results were compared with results from nuclear

magnetic resonance measurements on the same system. This comparison

was discussed in the previous section.














CHAPTER III

EXPERIMENTAL TECHNIQUES

"If we could first know where we are, and whither we
are tending, we could then better judge what to do,
and how to do it."
Lincoln


A. Samples

1. Preparation of thorium oxide samples

The thorium oxide samples used in this investigation were prepared

either by precipitation of the hydroxide from a thorium nitrate solu-

tion and then dehydrating thermally to the oxide or by direct thermal

decomposition of thorium oxalate.

The samples prepared by the precipitation method are designated

with a Roman numeral and a letter. The Roman numeral describes the

batch and the letter describes a subbatch which has been activated

under certain conditions. These samples were activated in a vycor tube

under vacuum after being powdered to 100 mesh. The temperature and

length of time of the activation process are given in Table 1 along

with other details of the preparation.

Several of the samples used in this study were prepared and used

in an earlier study of the catalytic activity of thorium oxide toward

the decomposition of ethyl alcohol.43

a. Batch VI. Two hundred and ten grams of thorium oxide was

dissolved in twelve hundred and fifty milliliters of water and to

this solution two hundred and fifty milliliters of fifty-eight per cent









ammonium hydroxide was added at a very rapid rate. The precipitate

was filtered in one portion, under suction, and washed with twenty-

eight hundred and fifty milliliters of water. The washings gave a

negative test to red litmus paper. The hydroxide was dried for twenty-

four hours at 1200 C. and powdered to 100 mesh before activation. The

details of the activation process are given in Table 1.

b. Batches I, IX, X and XI. Forty grams of thorium nitrate was

dissolved in five hundred milliliters of water and fifty milliliters of

fifty-eight per cent ammonium hydroxide was added with stirring. The

precipitates were filtered under suction and dried at 120" C. for

twenty-four hours. The details of the rate of precipitation, washing,

and activation are shown in Table 1. When the ammonium hydroxide was

added in one batch the time of addition is listed as zero.

c. Samples Oi-1 and 011-2. These samples were prepared in a

manner identical to that described above with the exception of a

different rate of addition of the hydroxide and the amount of washing.

Although these samples were used as the hydroxides, the surface areas

of activated portions were determined.

d. Sample N-2. One hundred grams of thorium nitrate was dissolved

in twelve hundred and fifty milliliters of water and one hundred and

twenty-five milliliters of fifty-eight per cent ammonium hydroxide was

added with stirring. The precipitate was filtered with suction and

washed with two hundred milliliters of 30-70 ammonium hydroxide-water

mixture. The precipitate was then dried for about twenty-four hours

at 1000 C.

e. Samples OX-3 and OX-4. These samples were prepared by the

thermal decomposition of thorium oxalate obtained from Fisher Scientific










Company. Attempts were made to activate the oxalate in vacuum and in

a nitrogen stream but in both cases the oxide obtained was contaminated

with a large amount of carbon. The method which gave best results was

as follows: The oxalate was placed in a vycor tube open at both ends

and the tube placed in and open-end electric furnace. A smaller vycor

tube containing a thermocouple was placed in the large tube so that the

thermocouple junction was adjacent to the oxalate sample. A small

blower was used to circulate air through the tube and to carry away

some of the water vapor for sample OX-3. This required a longer time

for the sample to reach 6000 C. In this manner the samples were

activated for six hours at 6000 C.

2. Preparation of alumina samples

a. Sample F-lA. The alumina used was Alcoa activated alumina

obtained in granules of eight to fourteen mesh. The material was used

as obtained without additional treatment with the exception of powdering

to 100 mesh. The activation was done under vacuum for a period of six

hours at a temperature of 6000 C.

3. Determination of surface areas

The surface areas of the samples were determined by nitrogen

adsorption using the method of Brunauer, Emmett, and Teller.44 Previous

to the adsorption of nitrogen the surface of the sample was cleaned by

heating under vacuum for one to two hours at 2000 C. This was done to

remove any sorbed material picked up in the process of handling and

transferring the sample.

The surface area of the sample was determined by using the linear

form of the adsorption isotherm. This equation is:









P/V(Po p) = 1/VmC + p(C l)/VmCpo [I]

where:

p = pressure of the gas

po = saturation vapor pressure of the gas

V = volume of gas adsorbed per gram of sample

Vm = volume of adsorbed gas necessary to form a

monolayer

C = a constant

A plot of p/po versus p/V(po p) yields a straight line from

which Vm and C can be determined from the values of the slope and the

intercept. In most cases the intercept value is very near zero and

only the slope need be used. Once the value of Vm is obtained the

surface area can be calculated. A typical plot is shown in Figure 1.


B. Vacuum System and Techniques Used in the
Nuclear Resonance Experiments

The vacuum system used for the preparation of the samples used

in the n.m.r. experiments is shown in Figure 2. The system is evacuated

by means of a Cenco Hyvac-2 vacuum pump and the pressure in the system

determined by means of the manometer shown at D. The system was cali-

brated, in terms of milligrams of adsorbate corresponding to a given

pressure in the system, in the following manner. A sample of activated

material, either thorium oxide or alumina, was allowed to adsorb an

increment of the adsorbate in question and the pressure change in the

system was observed. The sample was then weighed and the amount of

adsorbed material determined. A graph of pressure change in millimeters

versus milligrams of adsorbate was then constructed. From this graph










P
V(Po-P)


0.15








0.10 0








0.05









0 0.1 0.2 P/Po 0.3 0.4 0.5

Figure 1. A Typical B. E. T. Isotherm Used for Determination
of the Surface Area.





















U
Pump
B C D E F G

Figure 2. Vacuum System Used in the N. M. R. Experiments.
-4Is









the amount of material adsorbed could be determined by noting the

change in the pressure in the system. However, this graph was used

only as a guide and the actual amount of material adsorbed was deter-

mined by weighing the sample before and after each adsorption.

The sample tube containing the adsorbent is connected to the

system at A. The tube is fitted with a vacuum stopcock and is made

from a commercially available 5 mm. n.m.r. sample tube.

The various adsorbates were placed into the containers designated

B, E, and F. A drying tube filled with Drierite (G) was used to admit

air or nitrogen into the system. Each of the adsorbates were deaerated

by a process of freezing and pumping until no gas bubbles were visible

in the liquids. The container marked C is a vycor tube used for

activating the sample.

After being activated, the samples were placed in the sample tubes

and again heated to 400* C. to drive off any materials that might have

been adsorbed in the transfer process. To keep the samples in the tubes

during the degassing, a small piece of glass wool was placed in the top

of the sample tubes just beneath the stopcock. The amount of sample

in the tubes was determined by weighing the empty tube and then

reweighing after the sample had been placed into it and had been heated

to 400* C. In most cases about two grams of the material were sufficient

for a sample.

Whenever different adsorbates were used on the same sample care

was taken to insure that all the previous adsorbate had been removed.

This was done by heating the sample to a temperature just below its

activation temperature for six hours while under vacuum.









The temperature was kept somewhat lower than the original activation

temperature to prevent any change in surface area.


C. Description and Procedures of the Magnetic Resonance Studies

The magnetic resonance spectra in this study were obtained from

a Model 4300-2 Varian Associates spectrometer operated at a frequency

of fifty-six and four tenths megacycles. The spectra were recorded as

the derivative of the absorption signal on a Varian Model G-10 graphic

recorder. Several recordings were made for each measurement and the

values averaged. The rate of change of the magnetic field was deter-

mined by modulating the 56.4 mcs. oscillator with a low frequency sine

wave from a Hewlett-Packard oscillator which was monitored continuously

with an electronic counter. Either a sample of alumina saturated with

water or, in the case of the fluorine-containing materials, a sample

of sodium fluoride was used for calibration purposes. The modulation

produced side bands at known frequency differences from the main peak.

During all line width studies care was taken that excessive power

from the transmitter was not used while recording as this produces

broadening of the signal as a result of saturation. The power level

was adjusted until the effects of saturation could be detected and

then reduced several decibels below this point before recording the

signal. In addition, the amplitude of the sweep field used to modulate

the static magnetic field as it is being varied through resonance was

controlled in order to minimize artificial sweep broadening. While a

small magnitude of the sweep field must be used to prevent artificial

broadening a large magnitude is desired from the point of view of the

signal-to-noise ratio. In this study a sweep width which just produced









sufficient signal to record was used. This was usually one of the two

lowest available values. The lowest possible value of the line width

which would not be broadened excessively is estimated to be about one-

hundred and fifty cycles. This value is lower than most of the observed

line widths in this study. The frequency of the modulating field was

maintained at eighty cycles per second throughout the measurements.

Checks made at lower modulating frequencies indicated no change in the

line widths.

The temperature dependence of the line widths was studied by using

a Varian Model V-4340 variable temperature probe unit. The low tempera-

tures were obtained by passing a stream of dry nitrogen gas through a

copper coil immersed in a liquid nitrogen bath or a carbon dioxide-

acetone bath. Previous to passing through the cooling coil the stream

of nitrogen was passed through a tube containing calcium sulfate to

remove any water vapors. This prevents the formation of ice in the

cooling coil. The temperature was varied by regulating the nitrogen

flow rate by means of a needle valve.

The temperature of the samples was determined by means of a copper-

constantan thermocouple placed in the probe adjacent to the sample tube.

The thermocouple was used in conjunction with a Leeds and Northrup

potentiometer using a standard Weston Cell and a General Electric gal-

vanometer as a null point indicator. An equilibrium mixture of ice

and water was used as the reference point and the potentiometer

readings were converted to temperature units by use of standard cali-

bration tables.

The temperatures above the normal probe temperature (30 C.) were

obtained by using a Varian V-4343 heater control unit to heat the









nitrogen stream. The thermocouple arrangement described above was

used to determine the temperature. The accuracy of the temperature

is probably within one degree. For both high and low temperatures

several minutes were allowed for the sample to come to thermal equilib-

rium after a change in temperature.

At each temperature the spectra were recorded a number of times

and the measurements were averaged. With the exception of the studies

of the line widths as a function of temperature all spectra were obtained

at the temperature of the probe which is 300 C.

The chemical shifts of the adsorbed hexafluoropropene were deter-

mined in the following manner. The signal from the reference material,

in this case trifluoroacetic acid, was displayed on the oscilloscope

and the scope trace calibrated in terms of cycles per inch by applying

sidebands of known frequencies to the reference signal. The reference

was then removed from the spectrometer and replaced by the sample being

studied without changing any of the controls. The separation between

the reference and sample signals was then determined from the scaled

face of the scope. By repeating this a number of times a shift value

which is accurate to plus or minus thirty cycles can be obtained.


D. Determination of T1 and T2

Under certain conditions the spin-spin relaxation time (T2)*

is inversely proportional to the width of the resonance line. The

exact relationship is

Line width = 3-1/2/TT2 [II]




*For a discussion of relaxation times see Appendix B.









when the line is Lorentzian and

Line width = (2)-1/2/T2 [III]

when the line is Gaussian.* In both cases the width is measured

between the points of inflection.45

The spin-lattice relaxation time (Tl) can be obtained from a

knowledge of T2 and a knowledge of the magnitude of the alternating

field H1.46 By determining the value of H1 which produces the maximum

signal TI can be obtained. The maximum signal is obtained when the

condition

(yHi)2T1T2 = 1 [IV]

is fulfilled. In this expression y is the magnetogyric ratio which is

a constant for a given nucleus.


E. Measurement of Hi

In order to use the above expression to obtain T1 the values of

the alternating field H1l must be known for the various degrees of

attenuation which can be chosen. Values of Hi can be obtained by

modulating the magnetic field with a small audio frequency.46 Resonance

is obtained when the modulating angular frequency wm satisfies the

condition

m = 2 ( o 11)2 + y2f112 [V]

This condition is satisfied by two values of H and two signals will

be observed. The value of Hi for a given amount of attenuation can

be obtained by measuring the distance between the two signals and

plotting the square of this distance against the square of the modu-

lating frequency. This plot yields a straight line with an intercept



*It is more appropriate to call this a line-width parameter since a
true T2 cannot be defined in this manner for a Gaussian line.









of y2H12. Since precision attenuators are used in the commercial

spectrometers, a few values of Hi can be found in this manner and the

values for other attenuations can be calculated. A typical measure-

ment by this method is illustrated in Figure 3. The distance is

plotted in arbitrary units and the different plots are for different

attenuations.


F. Systems and Techniques Used in the Dielectric Experiments

1. Vacuum system

The vacuum system used in the dielectric experiments is shown

in Figure 4. The pressure in the system is measured by means of the

Zimmerli gauge B or the McLeod gauge F. The system is evacuated by

means of a mercury diffusion pump (G) in series with a Cenco Hyvac-2

vacuum pump (H1). The dielectric cell is connected to the system by

means of a ground glass ball joint at A. The adsorbate was stored in

the container C. At E there is a carbon dioxide-acetone cold trap to

prevent vapors from reaching the pumps. D is a gas burette used in

determining the volume of the system. The entire volume of the system

was obtained and this value used to determine the amount of material

adsorbed on the samples.

2. Dielectric Apparatus

Measurements of capacitance and conductance were made with a

General Radio Twin-T Type 821-A Impedance Bridge. The Bridge was

used in conjunction with a General Radio Oscillator Type 1330-A and

a Hallicrafter SK-62A receiver as a detector. A Heathkit model OP-1

Oscilloscope and a Heathkit Model V-4 vacuum-tube voltmeter were also

used in balancing the bridge. The scope was preceded by a sixty-cycle

filter network.









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i
t
S2



1
6 ///


S//
o / / /
0 2 4 6 8 10
f xl04 cps2

Figure 3. Determination of the Magnitude of H1 for Various
Attenuations.








































Vacuum System Used in the Dielectric Experiments.


Figure 4.









The Twin-T bridge circuit is a null instrument with a frequency

range from 460 kilocycles to 30 megacycles although frequencies some-

what higher and/or lower can be obtained. The frequency range of

the oscillator is 5 kilocycles to 50 megacycles and that of the

receiver is 540 kilocycles to 109 megacycles.

The apparatus is used to measure the impedence of the sample in

terms of its susceptance and conductance by a parallel substitution

method. The susceptance B is related to the capacitance, which is

read directly from the bridge. The relation is

B = w(C1 C2) [VI]

where w is the angular frequency, and C1 C2 is the change in capaci-

tance when the sample is placed in the bridge circuit. The conductance

G'is also obtained directly from the bridge by using the relation

G = Co(f/f)2 [VII]

where Co is the dial reading, f is the operating frequency, and fo is

the frequency at which the frequency range switch is set.

From the conductance and susceptance the dissipation factor or

loss tangent can be obtained. This factor (see Appendix C) can be

expressed as the ratio of the loss current to the charging current

and for this bridge circuit is given by

tan 6 = G/B [VIII]

which is equivalent to the expression given in Appendix C.

3. Dielectric Cell

In previous studies20 the available dielectric cells have had

several features which made it difficult to obtain results which were

completely reproducible. These difficulties were due in part to the









length of the leads on the cell and to the increase in volume of the

solid material whenever water was adsorbed. The expansion of the sample

with uptake of water caused the sample to push from between the plates

of the cell. In order to remove as many of these difficulties as possi-

ble a new cell was constructed for this study. To overcome the prob-

lem of the volume change of the sample with water uptake special

electrodes were constructed. One electrode is made up in the shape of

a dish and the other electrode is simply a perforated metal plate.

The outer diameter of the dish electrode is two inches and that of the

plate electrode is one and seven-eights inches. The plate electrode

is perforated to allow rapid equilibration of the water vapor and the

sample. Details of the two electrodes are shown in Figure 5.

The dish electrode is constructed of stainless steel and the

plate electrode of monel metal. Stainless steel was used for the dish

electrode because the edges had to be turned to give the desired shape.

This electrode was constructed by the Reliable Metal Spinning Co.,

Newark, New Jersey.

The cell is constructed in such a manner that it is possible to

use as many as three sets of the electrodes at once. In this study

only one set was used and these had a separation of 0.025 inches.

The electrodes are separated by means of Teflon spacers. If more than

one set of electrodes are used the sets are connected in parallel by

means of a series of 0.025 inch Teflon and steel washers. Once the

tier is completed the top electrode is held in place by a steel and a

Teflon nut.

The tier of electrodes (or in this case the single electrode) is

enclosed in a cylindrical glass container with hemispherical ends.











































A. Dish Electrode

B. Plate Electrode

C. Switching Device


Figure 5. Electrodes and Switching Device for the
Dielectric Cell.


I









The two halves of the glass container are connected by means of a

vacuum-tight flange joint. The top portion of the container is

fitted with a vacuum stopcock and a ball joint by which the cell is

connected to the vacuum system. The bottom glass section has two

nickel leads sealed into the glass. The electrodes themselves are

connected to the leads by screws and can be removed completely from

the cell. The assembled cell, with the exception of the external

bridge connections, is shown in Figure 6.

The cell is constructed in such a manner that it is not necessary

to have long leads from the cell to the bridge. Banana plugs are

connected to the electrode leads directly beneath the cell and this

allows the cell to be placed directly above the bridge.

In order not to remove the cell from the bridge during the entire

period measurements were being made, a special switching device was

constructed. This device is necessary since the cell must be removed

from the circuit in some manner during the initial balancing of the

bridge. A hole was drilled into the top of a banana plug and fitted

with a Teflon sleeve. One of the leads from the electrodes extends

into this sleeve. A hole was then drilled into the side of the

banana plug and through the Teflon sleeve. This hole was tapped and

fitted with a brass screw. This screw extends through the Teflon and

makes contact with the lead when tightened. Whenever it is necessary

to remove the cell from the bridge circuit the screw is simply with-

drawn until contact is broken. The reproducibility of the cell

properties was checked and found to be entirely satisfactory if the

screw is tightened snugly each time. The capacitance of the cell can




























































Figure 6. Assembled Dielectric Cell.









be reproduced to within 0.2 picofarads. A sketch of the switching

arrangement is also shown in Figure 5.

4. Loading of the dielectric cell

Before measurements of the dielectric properties were undertaken

the following procedure was carried out.

The entire cell assembly is weighed and then the activated sample

is placed into the dish electrode until all space between the two

electrodes is full. The cell assembly containing the sample with

some water which it had adsorbed from the atmosphere in the handling

process is now weighed. By comparing the weights of the empty and

filled cell, the weight of the "wet" sample is obtained. To prevent

any leakage it is necessary to seal the flange joint with Sealit

obtained from Fisher Scientific Company. The cell is now placed onto

the bridge and connected to the vacuum system described above. After

pumping on the sample for a period of 48 to 72 hours at a pressure of

about 10-5 millimeters of mercury the cell is closed off and removed

from the bridge and weighed again. The amount of dry sample is then

obtained from the difference in weights. Corrections are made for

the weight of the air contained in the cell during the original

weighing. Once the weight of the sample is found the cell is fastened

to the bridge and not removed until all measurements are completed.

The cell contained from six to eight grams of sample in most cases.














CHAPTER IV

PRESENTATION AND DISCUSSION OF RESULTS

"For my thoughts are not your thoughts, neither
your ways my ways."
Isaiah 55:8


A. Magnetic Resonance Results

1. General Comments

A macroscopic approach to the phenomenon of nuclear magnetic

resonance has been presented by Bloch47 by means of a set of phenom-

enological equations which describe the time behavior of the trans-

verse and longitudinal components of the magnetization. Even though

assumptions are made concerning the nature of the various relaxation

processes these equations appear to offer a simple and in many cases

a completely adequate manner of viewing these processes.

In the ideal cases the shapes of the absorption lines obtained

in nuclear resonance can be described by line shape functions which

are either Lorentzian or Gaussian. In actual practice the line shapes

may not be described exactly by either of these functions.

The solution of the Bloch equations leads to a shape function

which is Lorentzian. The Lorentzian line is characteristic of and

traditional in the theory of collisional- and radiation-broadened

spectral lines. The shape function which describes a Lorentzian line

(in nuclear resonance) is









g(u) = 2T2/[11 + (W wo)2T22 I1]

where T2 is the spin-spin, or in Bloch's nomenclature, the transverse

relaxation time. From absorption lines described by this shape func-

tion the two relaxation times can be obtained by the method described

in the previous chapter.

To justify the use of the described method of obtaining relaxa-

tion times the lines obtained in this investigation were tested by

the method of Pake and Purcell48 to determine to what extent they

vary from true Lorentzian or true Gaussian lines. This test consists

of determining the ratio of the large maximum to the small minimum

in the derivative presentation of the dispersion curve. For a

Lorentzian line the ratio of the two is 8:1. For a Gaussian line the

ratio is 3.5:1. The lines obtained from the adsorbed water in this

study have ratios which range from 4.9:1 to 7.5:1. This indicates,

as expected, that the lines are neither pure Lorentzian or pure

Gaussian but have a shape which is somewhere between the two. The

average of all the obtained ratios is 6.2:1 which possibly indicates

that the lines may best be described by a Lorentzian function.

While the relaxation times obtained here may not be the true

values characteristic of these systems it is believed that comparison

of the obtained values, for identical conditions, will afford valuable

information about the systems. It should be noted that the main points

of interest in this study are the changes and the differences in

relaxation times rather than absolute values.

One disadvantage of the method of determining relaxation used in

this study should be pointed out and kept in mind. Since the spin-

spin relaxation time is derived from the widths of the resonance line









it is affected by all factors which produce line broadening. In

addition to the "natural" broadening produced by the relaxation

processes "artificial" line broadening may result from (a) inherent

inhomogeneities in the magnetic field resulting from the properties

of the of the magnet itself, (b) inhomogeneities produced in the

magnetic field by the difference in the magnetic susceptibilities of

the solid and the surrounding medium, and (c) the effect of the

magnitude and frequency of the modulating field.

In this study the factor described in (a) is very small compared

to the line widths and does not present a problem. However, the

factors described in (b) and (c) are not always small compared to the

line widths and must be considered. As a result of the broadening

produced by factors (b) and (c) the observed spin-spin relaxation times

are not the true times needed to describe the relaxation processes

but are related to them in the following manner:

1/T2,obs = 1/T2 + l/'f2 [II]

The factor 1/T2 describes the total broadening produced by factors

(b) and (c).

An attempt was made to determine the magnitude of these two

factors. An excess of water was added to several thorium oxide samples

until a nearly homogeneous mixture was obtained. The line width of

the water was then measured with the results shown in Table 19. These

measurements were made using the smallest magnitude of the modulating

field possible on this particular spectrometer. The frequency of the

field was maintained at eighty cycles per second.

From the data in Table 19 it is noted that the minimum line width

obtained from any sample is 137 cycles per second. The major portion









of this value is considered to be the combined results of factors (b)

and (c) since the true line width of liquid water should be only a

few cycles at most. To determine what part of the observed widths

resulted from the inhomogeneity broadening, the same samples were

then run by the normal high-resolution technique to eliminate the

effect of the modulating field. In each case the line width was

reduced by about a factor of two. From these results it is concluded

that each of the factors (b) and (c) contributes about fifty or sixty

cycles to the observed line widths. The contribution from (b) remains

essentially constant from measurement to measurement but the contri-

bution from (c) depends upon the true line width and should approach

zero for lines with widths greater than about three or four hundred

cycles per second.

As mentioned previously the frequency of the modulating field

was maintained at eighty cycles per second. On previous measurements

this frequency was reduced to twenty cycles per second but there

was no reduction in the width of the line. It is believed that a

reduction in line width does not occur in this case because of the

larger effects of the magnitude of the modulating field and of the

field inhomogeneity.

No attempt will be made to correct the observed line widths and

relaxation times since definite correction factors cannot be obtained

but it should be kept in mind that the observed T2 values are smaller

than the true values and consequently the observed T1 values are

larger than the true values.









Prior to a discussion of the experimental results it may also

be informative to point out some characteristics of the surface of a

solid such as thorium oxide both before and during the adsorption

process. The physical condition of an adsorbing surface is very

difficult to determine even before an adsorbate has been added and

even more difficult after adsorption has taken place. However, one

fact is sure, the surface of a metal oxide is not homogeneous through-

out but must certainly consist of sections which are characteristic

of the oxide crystal structures, sections characteristic of the

hydroxide structure, and portions which have other characteristics.

After adsorption has taken place to some extent the already hetero-

geneous surface is made more so by the changes brought about by the

addition of the adsorbate. For example, the adsorption of water by

a metal oxide may result in ions being present on the surface as

well as water molecules in various states. As a result of these

changes in the surface with adsorption the upper layers of adsorbate

will not be adsorbed on the same kind of surface as were the first

layers. In the following discussion it should not be assumed that

the author has a definite geometrical arrangement of atoms or molecules

in mind when referring to the structure of the oxide surfaces.

One point should also be kept in mind when considering the sur-

face area of a material such as thorium oxide. Surface areas deter-

mined by using different adsorbates are not in general the same. For

example, the surface areas in this study were determined by nitrogen

adsorption and would probably not be the same if determined by the

adsorption of water or other materials.









2. Adsorption Studies

The adsorption of water has been studied on several different

thorium oxide samples. In Figure 7 the data for four different oxide

samples, all prepared from thorium hydroxide, is shown. In this figure

the line width is plotted as a function of the number of layers of

coverage. The surface areas of the samples are given in the figure.

The amount of water necessary for a monolayer is determined by taking

the surface area occupied by a water molecule as 14.0 A2. This value

was chosen from values found for water adsorbed on aluminum oxide27

and appears to be near the average of all values reported in the

literature. It should be pointed out that the area occupied by an

adsorbate molecule may depend to some extent on the nature of the

surface of the adsorbent.27

All the samples appear to show the same general behavior with

only small differences. In each case the line widths are large at

low coverages and decrease with increasing coverage until an essenti-

ally constant value is reached. This constant value is obtained only

after several layers of water have been adsorbed. The values of

the line widths at low coverages are of the same order of magnitude

as the line width of thorium hydroxide itself (Table 18). This

indicates that the first layers of water are bound quite tightly to

the surface. For most samples it is not possible to obtain an

absorption line whenever the coverage is less than one monolayer.

This is in part due to the small surface areas of the samples which

dictates the amount of water necessary for a monolayer and in part

to the degree of bonding of the first water layer. From the data






Line width


(kcs.)


ooo 14.1 m2/g.
,- 9.4 m.2/g.
0 00 28.9 m2g.
***oo31.1 m.2 /g.


4 8 12 1(
V/Vm


Line Width of Water on Thorium Oxide Prepared from Thorium Hydroxide.


2.0








1.0







0


Figure 7.









in Figure 7 it is apparent that the influence of the surface extends

out several molecular layers. Even at the highest coverages and after

application of the correction factor discussed in the previous section

the line widths do not approach that of pure liquid water. To indicate

the differences, liquid water has a line width of about one cycle per

second and ice has a line width of about sixty thousand cycles per

second.

Since the water coverage is plotted in terms of the number of

monolayers it is reasonable to expect that the curves would very nearly

superimpose; however, large differences are found. These differences

can be attributed to at least two things. One is that some residual

water remains on the samples after activation. This can be as high

as one to two per cent for an activation temperature of 6000 C. and

even higher for lower activation temperatures.49 In view of this it

should be noted that the samples having surface areas of 28.9 m.2/g.

and 31.1 m.2/g. were activated at lower temperatures than the other

two. Another explanation for the lower line widths of the sample

activated at the lower temperature may be in the condition of the

surface itself rather than in the amount of residual water. Since it

is apparent that these samples will not adsorb as many layers of water

as the samples activated at higher temperature even after considering

the residual water it appears quite reasonable to attribute part of

the differences in line widths to the condition of the surface.

A peculiar behavior was found for the sample having a surface

area of 28.9 m.2/g. (Sample VI-E). The line width was found to be

dependent upon the amount of elapsed time between the adsorption









process and the time of measurement. The break in the curve near

four monolayers occurred when the sample was allowed to stand for a

period of about thirty days between measurements. Similar behavior

was observed for other samples activated at 5000 C. but usually to a

lesser extent. Since the line width increases rather than decreases

with time it must be attributed to a very slow hydration process or

possibly some type of exchange process, say between water molecules

on sites of different energies. Another possibility is the diffusion of

water into the pore structure. Other evidence of this "time factor" has

been found in adsorption studies of thorium oxide.6

The behavior of the observed relaxation times of water adsorbed

on a thorium oxide sample prepared from thorium hydroxide (Sample IX-A)

is shown in Figure 8. The spin-spin relaxation time (T2) increases

with increasing water coverage and approaches a constant value. The

values (10 to 50 milliseconds) suggest that the water is rather tightly

bound to the surface and has restricted mobility even at the highest

coverage. For this sample there appears to be some difference in the

values obtained on adsorption from those obtained on desorption. On

other samples no differences were noted. The behavior on this partic-

ular sample may be related to the pore structure of the surface of

the oxide.

The spin-lattice relaxation time (T1) for this sample also increases

with increasing coverage but only slightly. This increase can probably

be taken as an indication that the adsorbed water approaches a more

"liquid-like" state as the amount of water increases. In some samples

the values found for T, are larger than the T1 value of liquid water










*** T2Desorption


oooT2Adsorption


(sec.)







x4
10


2


10


15 20
mg. H20/g.ThO2


25


30


I


35


The Observed Relaxation Times of Water on Thorium Oxide Prepared from
Thorium Hydroxide.


AAA T1


/7.
A A


Figure 8.


(se c.)







4 T1



2









but in light of the previous discussion of the magnitudes of the

observed T1's this is expected.

Several differences can be found between the data for the samples

prepared from thorium hydroxide and that for the samples prepared from

thorium oxalate. In Figure 9 results for the adsorption of water on

oxide samples prepared from thorium oxalate are shown. The most

prominent difference between these and the previous results is the

difference in the width of the lines. The lines from the samples

prepared from the oxalate are in general not as wide, especially at

very low coverages, as those found for samples prepared from the

hydroxide. This indicates that the interactions between the surface

and the adsorbed molecules must be weaker in these oxides. This could

indicate that catalysts prepared from thorium oxalate should have more

activity for such chemical reactions as the dehydration of alcohols

since they should not be as easily poisoned and the products of the

reaction should not be bound as tightly to the surface. In general

this is found to be true.

Both sets of data (Figures 7 and 9) indicate that the samples

having the largest surface are less efficient in producing relaxation,

i.e., smaller lines, for a given water coverage (in terms of the T2

relaxation). This is in accord with the catalytic behavior since

larger surface areas, all other factors being equal, usually indicate

greater activity.

The differences between the two kinds of oxides are reflected

quite strongly in the relaxation times. In Figure 10 the observed

relaxation times of water adsorbed on an oxide sample prepared from







oo o12.6 m2/g.
A A 37.0 m.g.


Figure 9. Line Width of Water on Thorium Oxide Prepared from Thorium Oxalate.


(kcs.)


0.8


0.6



0.4


0.2




0


2 4 6 8 10 1:
V/Vm







(sec.)



5



4
T2
x
104
3


mg. H20/g.ThO2


Figure 10.


The Observed Relaxation Times of Water on Thorium Oxide Prepared
from Thorium Oxalate.









thorium oxalate are shown (Sample OX-3). A comparison with Figure 8

shows that the spin-lattice relaxation time (Tl) of water adsorbed on

an oxide prepared from the oxalate is much smaller than that for water

adsorbed on an oxide prepared from the hydroxide. A comparison of the

T2 values also shows a difference but not as great as that for the TI

values. The smaller values of T, could result from the effect of

paramagnetic centers, on the surface of the oxalate oxides, perhaps

introduced into the oxide surface during the preparation. It has been

found by infrared studies50 that thorium oxide samples prepared from

thorium oxalate usually have a surface layer of thorium carbonate

which results from adsorbed carbon dioxide. It is quite conceivable

that the differences in the T, values can, at least in part, be

ascribed in the effect of this layer of carbonate. In general tempera-

tures greater than those used in the preparation of the samples used

here are necessary to completely remove the adsorbed carbon dioxide.

Several interesting facts become apparent whenever other adsor-

bates besides water are used. In Figure 11 the resulting line widths

of adsorbed water, methanol, ehtanol, and butylamine on a thorium

oxide sample (Sample VI-D) are shown. It is apparent that both water

and methanol behave in a similar manner but different from ethanol and

butylamine, which themselves behave somewhat differently. The line

widths of the latter compounds, on this sample, are correspondingly

less than the width of the methanol and water lines. This could

indicate that this oxide sample has a pore structure such that the

pores can accommodate the smaller molecules but not the larger ones.

Comparison of the results in Figure 11 with the results obtained from






(k cs.)



2.0


20


30 mg./g.


Line Widths of Various Adsorbates on Thorium Oxide.


000 H20

AAA CHOH

l000 C2H5OH

C4C4H9 NH2


1.0


AoA
0 0


Figure 11.









a sample activated at 500 C. (see Table 3) shows that the line widths

of the same adsorbates on the sample activated at the lower temperature

are all approximately the same. It is likely that the lower activation

temperature results in a more porous structure which is also indicated

by the larger surface area. The fact that the oxide samples appear

to have different degrees of interaction between the surface and various

adsorbates can possibly be used to gain knowledge of their catalytic

activity. As an example, the sample from which the data in Figure 11

is taken should not be a good catalyst for the dehydration of ethanol

since water is bound to the surface more tightly than ethanol.

The behavior of the line width of adsorbed butylamine differs

from that of adsorbed ethanol in that the changes with increasing

coverages are quite small. This can be seen in Figure 11 although the

small surface area of the sample allowed only three determinations.

The fact that the line width does not change appreciably with increas-

ing coverage can be attributed in part to the fact that the hydrogens

on the nitrogen are chemically shifted from the other hydrogens in

the molecule. At the higher coverages where the line width should

decrease this chemical shift prevents it to some extent.

It appears that the line widths of all the adsorbates are approach-

ing a constant limit which probably represents the result of the

artificial broadening mechanisms discussed previously.

The differences between water and the other adsorbates become

more apparent upon comparison of the observed relaxation times. In

Figure 12 the observed relaxation times of methanol adsorbed on thorium

oxide (Sample VI-D) are shown. Although the values of T2 are approx-

imately the same as those for adsorbed water, the values of T1 are










ooo T2


18


AAATT


24


mg./g.


The Observed Relaxation Times of Methanol on Thorium Oxide.


(s e c.)
24



21


I I(S


e c.)


T2
X
104


15


1




30


27


Figure 12.









much higher. Granting that the T, values are somewhat too large

because of the reasons discussed previously, it is apparent that the

true values are higher than those for the adsorbed water. The relax-

ation times of ethanol (see Tables 2 and 3) and butylamine (see Tables

2 and 4) are also higher than the values found for adsorbed water.

The spin-lattice relaxation times (Tl) of the alcohols differ in

one other respect from those of adsorbed water. The values for water

always increase as more water is added but the values for the alcohols

and butylamine increase in some cases and decrease in others although

the few points obtained in most cases make some of these trends

questionable.

The fact that the trend may be different for different samples

should not cause alarm since it has been found21 that T1 may show a

minimum if the values are obtained over a sufficient range of coverage.

It is quite possible that the coverages here may be on opposite sides

of the minimum and thus would show this particular behavior.

The larger values of the T1's for the alcohols and butylamine

indicate that these adsorbed molecules are not as sensitive to the

thermal motions which produce relaxation or that the adsorbed materials

behave more like a normal liquid. A liquid-like behavior could result

from the manner in which the molecules are adsorbed. If the adsorption

takes place through the polar groups the non-polar part of the molecules

would be free to move about and thus behave more like a liquid state

than if all parts of the molecules were strongly bound to the surface.

Further evidence of this type of interaction will be seen in a later

discussion of temperature effects upon the line widths and relaxation

times of these molecules.









3. Temperature Studies

The temperature dependence of the line width and relaxation times

has been studied for all adsorbates. A typical temperature dependence

of the line width of adsorbed water is shown in Figure 13 and other

data given in Tables 8 12. In Figure 13 the changes in line width

with temperature are plotted for three different water coverages. For

the lowest coverage, which is just less than two monolayers, the line

width begins to increase quite sharply at about 100 or 15 C. For

larger amounts of water the width remains essentially constant down

to about -300 to -50 C. The fact that the line width increases

before the normal freezing point of water, for low coverage, indi-

cates that some type of motion is being frozen out rather than the

normal formation of ice. In fact the data for the high coverages

indicates that if ice is formed at all it is formed much below the

normal freezing point of water. Several experiments have shown51 that

adsorbates tend to freeze at temperatures much below their normal

freezing points. The data in Figure 13 offers further proof of this

since the line width does not approach that of ice (60-80 kcs.) even

at -800 or -900 C.

The sharp increase in the line width at low water coverages

indicates that under these conditions it is the adsorbate-adsorbent

interactions which are primarily responsible for the line width.

At higher coverages the adsorbate-adsorbate interactions may also

play a substantial part in determining the relaxation times and line

width.

A typical temperature dependence of the observed relaxation

times of adsorbed water is shown in Figure 14. This data is for a












(kcs.)


7


6



L5 2 4.44
n
w4 0

d
t
h3


2



1


-110 -90


21.2


-70 -50 -30


Figure 13.


The Temperature Dependence
Water on Thorium Oxide.


of the Line Width of


10.8


-10


10


30







(sec.)


8



6
T,


4



2




-90


* oT1
o ooT2


-7


Figure 14.


0 -50

The Temperature
Thorium Oxide.


-30


-10


10


30


Dependence of the Observed Relaxation Times of Water on


50


(se c.)


4



3
T2
x
10









water coverage of 16.7 mg. H20/g. ThO2 adsorbed on sample IX-B. As

the temperature increases the observed value of T2 also increases

indicating an increased amount of mobility in the adsorbed molecules.

The observed values of T1 become less as the temperature increases

as is expected since the spin-lattice relaxation time is a measure

of the time for thermal equilibrium to be established between the

spin system and the thermal motions of the lattice. As the tempera-

ture increases more and more motions having components with frequencies

near the resonance frequency of the nuclei are present thus producing

more efficient T1 relaxation.

By considering the change in T2 with temperature, an apparent

activation energy for the processes responsible for the decay of

the transverse components of the magnetization can be obtained.

Theoretically, the activation energy is obtained by considering the

change in the correlation time (Tc) with temperature (see Appendix

B for a short discussion of the correlation time). The longer the

correlation time becomes the more rapid is the relaxation process

and the shorter T2 becomes. The decrease in T2 results in a wider

line width since the two are related inversely. Thus the line width

and the correlation time have the same directional behavior.

The correlation time is of course governed by thermally activated

motions and should follow an equation of the type

Tc To exp(Eact/kT) [III]

Plots of the log of the correlation time against the reciprocal of

the temperature should yield straight lines from which the activation

energy can be obtained from the slope. From the above discussion it









is apparent that a plot of the line width against l/T should also

yield straight lines from which the activation energy can be obtained.

Plots of this type for the two different kinds of thorium oxide samples

are shown in Figures 15 and 16. In each case a number of linear

segments are found for each water coverage. These segments range in

number from two to four. In Table 17 the energy values corresponding

to the various regions are listed.

Several features of Figures 15 and 16 and Table 17 are of interest.

One fact which is of considerable interest is the existence of differ-

ences between the two different kinds of oxides. The data from the

oxide samples prepared from thorium hydroxide shows linear portions

which have increasing slopes as the temperature decreases. The data

for the oxides prepared from the oxalate shows linear portions which

have alternating values of the slopes with decreasing temperature.

It can be seen from the figures that for low water coverage the first

linear portion, which has a small slope, is not present. The higher

the coverage becomes after this section is present the larger becomes

the temperature range covered by this segment. It can also be seen

from the data that the slope of the third linear segment, for the oxides

prepared from the hydroxide, becomes greater with addition of water.

At a very high water coverage this portion starts at a higher temper-

ature than it does for lower water coverages.

Upon first considering the data in the two figures and in Table

17 there appear to be inconsistencies. If the numbers are accepted

at face value it appears that those motions having the lowest activa-

tion energies are the first ones to be frozen out. This is certainly











oo8.20mg./g. AA 12.8 mg./g.

0024.9mg./g. oo33.1 mg./g.


2.9 d


A-
2.7

03-

3.2



Figure 15.


3.7 4.2 1 0O 4.7 5.2 5.7
T"C

Log of the Line Width versus the Reciprocal Temperature
for Water on Thorium Oxide Prepared from Thorium Hydroxide.


3.9



3.7


3.5




3.3



3.1












an 11.4 mg./g.
oo 29.3 mg./g.


4.2 _3 4.7
TC


Figure 16.


Log of the Line Width versus the Reciprocal Temperature
for Water on Thorium Oxide Prepared from Thorium Oxalate.


3.9



3.7


3.5



3.3



3.1


2.9



2.7




3.2


3.7


5.2









not reasonable if the numbers represent activation energies. However,

the data can be explained in a reasonable way and in a way which leads

to consistent results.

In the first linear region, which occurs when the line widths are

quite small, the widths are controlled not by the molecular motions but

by the mechanisms discussed previously. As evidence of this it should

be noted that when the water coverage is low this portion is not

present. This is because a low water coverage results in a wide line.

In other words the actual width of the line in the absence of the

artificial broadening is such that the effects of the modulating field

and the inhomogeneities are too small to broaden the line further. As

a consequence of the artificial broadening the line width is virtually

temperature independent and it is not until the line becomes suffi-

ciently broadened by the decrease in molecular motions does the observed

line width change. This is the beginning of the second portion of the

plots. In the second and the third, for the oxides prepared from the

oxalate, regions the obtained energies are more nearly true activation

energies of the various kinds of motions. For water on the oxides

prepared from the hydroxide there appears to be only one general kind

of motions since there is only one straight line portion in this

region. On the other hand the data for water on the oxides from the

oxalate shows two portions in this region. The first has the greatest

slope and represents the motions having the largest activation energies.

These are expected to freeze out at higher temperatures than are the

motions having lower activation energies.

The last linear portions of the plots, which appear to represent

motions having activation energies higher than the ones frozen out at









the higher temperatures, are in fact due to the beginning of an ice-

like structure. At this temperature the molecular motions become

sufficiently slow so that the water begins to take the ice structure

and line broadening occurs by dipole-dipole interactions. In general

the more water there is on the sample the more nearly like liquid it

becomes and the greater the amount of water which takes part in the

freezing process. This results in the increase in slope of this

portion of the plots with increasing water coverage.

The most important feature of this data is the fact that there

appear to be different kinds of motions present in the oxides prepared

from the oxalate while in the other samples there is indication of

only one general kind of motion. It is possible that there are

"active sites" of some type present in the oxides prepared from the

oxalate which result in exchange type motions in addition to the

normal rotations and vibrations.

Studies of the temperature dependence of the line widths of

adsorbed alcohols and butylamine have also been done. In Figure 17

the line width of adsorbed methanol is shown for the temperature

range from 300 to about -1150 C. For both coverages an apparent

maximum occurs in the line width near 0 C. This maximum is more

pronounced at the lower coverage. At temperatures below the point

where the maximum occurs the line width decreases to a value lower

than that found at room temperature and remains virtually constant

until the temperature reaches about -1150 C. This behavior appears

to be reasonable if it is assumed that the methanol molecules are

adsorbed onto the surface through interactions between the hydroxyl

groups and the surface. Since in methanol the chemical shifts of






(kcs.)


ooo 23.9 mg./g.

AAA 1 8.9 mg./g.


1.2


L
i 0.9 A
n
e
w


t
h


0.3



0
-110 -90 -70 -50 -30 -10 10 30
CFigure 17. The Temperature Dependence of the Line Width of Methanol on Thorium Oxide.

Figure 17. The Temperature Dependence of the Line Width of Methanol on Thorium Oxide.









the two kinds of hydrogens are not sufficiently far apart to observe

two distinct resonances, the line that is observed is made up of two

components. These two components come from the two different kinds of

hydrogens. At room temperature and at all temperatures above the

apparent maximum the observed line results from both of these

components. As the temperature is decreased the hydroxyl component

broadens out considerably since it is bound strongly to the surface

and at the temperature at which the maximum occurs begins to completely

disappear. Below this temperature only the component from the methyl

group is observed and this remains quite sharp since movement of the

methyl group is hindered only slightly by the surface. This "rotation"

of the methyl group persists until very low temperatures but at about

-115 C. the motions become sufficiently slow that dipole-dipole

interactions become a factor and cause the line to broaden. The data

in Figure 17 is for an oxide sample prepared from thorium hydroxide

but the same behavior is found for samples prepared from thorium

oxalate (see Tables 13 and 14).

In Figure 18 the variation with temperature of the line width of

adsorbed ethanol is shown. The general behavior is the same as that

of methanol. The difference between the two is attributed to the

presence of the methylene group in ethanol which interacts with the

surface in a manner different from either the methyl or the hydroxyl

group. The slight increase in line width between -50 and -80 C. is

attributed to the affect of the surface on the methylene group. This

interaction becomes greater as the temperature decreases.

The temperature dependence of the line width of adsorbed butyl-

amine is shown in Figure 19. For the higher coverage the behavior is






ooo 6.7 mg./g.


(kcs.)


0.8


0.6



0.4


0.2



0


-110


-90


-70


Figure 18. The Temperature Dependence of the Line Width of Ethanol Adsorbed on Thorium Oxide.


-50


-30


*C


-10


10


30






(kcs.) o 0 12.9 mg./g o
1.3 o o
AA 16.9 mg./g. o

0 0
A00
A 0 0 0
wA A o
i A A A
1.1 A A

w A 0




0.9 A o o A
A 0
A A
A OA

07 A
-110 -70 -30 10 50


Figure 19. The Temperature Dependence of the Line Width of Butylamine Adsorbed on Thorium Oxide.









similar to that of the alcohols except that two apparent maxima are

present. The first of these is attributed to the broadening of the

component of the line belonging to the NIl2 group. It is believed that

this group is interacting quite strongly with the surface in a manner

similar to the hydroxyl groups in the alcohols. The second apparent

maximum can possibly be attributed to the effects of the surface upon

the methylene group adjacent to the NH2 group. That these interactions

are different from the interactions of the methylene group of ethanol

is to be expected since the different geometries of the OH and Nil2

groups would cause the two groups to be in different positions relative

to the surface. The line width decreases quite sharply below the

second maximum before dipolar broadening begins. This is again expected

since the methyl group is now quite far removed from the surface.

The behavior of the line at the lower coverage is somewhat

different. While the values at the higher coverage are completely

reproducible over the entire temperature range there is a region

between 20 and -500 C. in which the line width of the lower water

coverage cannot be reproduced. Above and below this range the measure-

ments can be repeated well within the experimental error. The scatter

of the values in this region is attributed to the fact that the NH2

group interacts strongly with the surface producing a broad line which

is also chemically shifted from the line of the other hydrogens. Evi-

dence of this is shown in Figure 20. In this figure the absorption

line, here presented as the derivative, is shown at three different

temperatures. The line is not symmetrical at any temperature but

becomes less symmetrical in the temperature range mentioned above.










I I
2.0 kcs.


-3 3C.

Ho >


Figure 20. Presentations of the Absorption of Butylamine Adsorbed on Thorium Oxide.


300C.


-70C.










The increased width of the lower-field part of the line is attributed

to the component of the line belonging to the NiH2 group which is shifted

to lower field. The shift appears to be maximum in the region where the

line width is not reproducible and it is almost possible to pick out

the two components here. The measured line width in this region is

not a true measure of either of the components but more or less an

average of the two. Below this temperature region the component

belonging to the NH2 group becomes broader and the sharp component

belonging to the alkyl group, or part of the alkyl group, governs the

width of the line. The sharpness of the line at low temperatures

indicates that the alkyl part of the molecule interacts only weakly

with the oxide surface and has considerable freedom.

Because the line widths are made up of different components,

the behavior of the relaxation times of adsorbed alcohols and butyl-

amine with temperature cannot be readily interpreted. For the same

reason it is not possible to obtain an apparent activation energy as

in the case of adsorbed water. The observed relaxation times are

given in Tables 13 17, but aside from the fact that the T1 values

are much larger than the values found for water little information can

be gained from them. It should be noted however, that all the relaxa-

tion times as well as the line widths are averages. For water they

are the average of the relaxation times of the different kinds of

water molecules; those bound tightly to the surface as well as those

which have considerable movement. For the alcohols and butylamine

this is also true with the additional averaging of the relaxation

times of the various parts of the molecules. This averaging process

may in part account for the larger values of the relaxation times,










especially T1, of the alcohols and butylamine. Certainly part of the

larger T1 values is due to the mobility of the alkyl parts of the

adsorbed molecules.

4. Studies of C3F6 adsorbed on aluminum oxide

To study more fully the apparent differences in the interactions

between the surface of a solid and various parts of an adsorbed

molecule it was decided to study a material containing fluorine adsorbed

on an oxide. A fluorine-containing material was chosen because of the

magnitude of the chemical shifts of fluorine. These are sufficiently

large to allow the observation of the lines belonging to the different

groups in the molecule. Several attempts to adsorb an observable

amount of a fluorine-containing material on thorium oxide proved

unsuccessful apparently because of the small surface area of thorium

oxide. Success was attained, however, with a high surface area

aluminum oxide sample and hexafluoropropene as the adsorbate.

The F19 spectrum of the adsorbed hexafluoropropene is shown in

Figure 21. This molecule has four different kinds of fluorine atoms

and the lines belonging to the different kinds can be clearly distin-

guished. In no case is there overlap of the lines. The assignment

of the various bands can be made by considering the high-resolution

spectrum obtained from the liquid material.52 The assignments are as

follows: CF3 line, Fa line, Fb line, and Fx line, with increasing

magnetic field.

The widths of the various bands were obtained at several

different coverages and the temperature dependence studied at two

different coverages. The changes in the line width as a function of

the amount of adsorbed material are shown in Table 20. The widths













E CF
b 3


Ho ->


Fluorine Spectrum of Ilexafluoropropene Adsorbed on Aluminum Oxide.


Figure 21.









of the CF3 and Fa lines decrease with increasing coverage but the

widths of the Fb and Fx lines remain essentially constant over the

entire range of coverage. The lack of change in the latter lines is

attributed to the presence of spin-spin interaction between the two

atoms. This interaction results in a doubling of the lines with a

separation of about 120 cycles per second.

Although it is not entirely conclusive it appears that the lines

tend to increase in width at very high coverages. This behavior, if

it is real, may indicate that adsorbate-adsorbate interactions

become important at these coverages. This could arise, for example,

if one CF3 group is surrounded by molecules which are tightly adsorbed

and another CF3 group is surrounded by molecules which are bonded very

weakly to the surface. The two groups would thus be in different

environments.

The observed relaxation times are listed in Table 21. These times

reflect the changes described above. The small values of the T1's

along with the small line widths indicate that the interactions

between the adsorbed material and the surface are weak. This is also

indicated in the ease in which the hexafluoropropene is desorbed.

The chemical shifts of the adsorbed material as well as the

neat liquid are shown in Table 22. The shift of the CF3 group is

essentially the same as that for the liquid and remains nearly constant

over the entire range of coverage. All the other lines are shifted

toward higher applied field by about ten parts per million on the

average. These values are probably no more accurate than t 1 part

per million. The observed shifts indicate that the three fluorine









atoms are influenced by the surface to a greater extent than the CF3

group is. To explain this it must be assumed that the double bond

itself is affected by the surface in such a manner that the electron

distributions about the fluorine atoms are changed. The distribution

in the CF3 group would be changed to a lesser extent since it is one

more bond removed. It is difficult to separate an effect of this

type, however, from the effect of the surface upon the fluorine atoms

just from being in proximity with them.

The temperature dependence of the relative chemical shifts (CF3

line taken as zero) is shown in Table 23. No major changes in the

shift values occur over the entire temperature range although it

appears that the lines may tend to approach each other as the temper-

ature is decreased. The lack of change of the shifts indicates that

regardless of the temperature the adsorbed molecules must remain in

very nearly the same positions relative to the surface. The fact that

these are relative shifts makes definite statements about the shifting

of the entire spectrum impossible.

The temperature dependence of the line widths is shown in Figures

22 and 23 for two different coverages. For the higher coverage

(Figure 22) all lines show a similar behavior except the Fb line. For

the lower coverage (Figure 23) both the CF3 and the Fb lines show a

behavior different from the other two lines. For low coverages it

appears that the widths are essentially temperature independent until

the temperature becomes low enough for dipolar broadening to occur.

For the higher coverage the changes in the line widths are somewhat

greater and no reasonable explanation of the behavior can be given.










Fa, Fx

b C3Cc


CF3
3


10 -90


-G0 "C -30


Figure 22.


The Temperature Dependence of the Line Widths of 56.0 mg.
C3F6/g. A1203.


(kcs.)



0.2



0.1


0.3



0.2

0.3


0.2'
0.3



0.2




-1'


30










. F F
F CF


b 0 0 0 0 0
0



0 0 0
Ex 00
C
0


CF3
3


0 -90


-60


S-30


Figure 23. The Temperature Dependence
C3F6/g. A1203.


of the Line Widths of 93.6 mg.


(kcs)



0.2



0.1


0.3



0.2

0.3


0.21
0.3



0.2




-11


30









It is quite possible that the changes may be due to changes in the

various contributions to the line width. The dominant contribution

may very well change with temperature. The most important point to

be gained from this study is that different parts of an adsorbed

molecule interact in specific ways with the adsorbing surface and

that it is possible to observe this by magnetic resonance techniques.


B. Dielectric Results

1. General Comments

The investigation of the dielectric properties of any substance

involves a study of the polarization produced in the substance by

the influence of the applied electric field. If the applied field

is static, the dielectric properties can be described in terms of the

dielectric constant but whenever alternating fields are used the

dielectric loss must also be considered. The dielectric loss of a

substance is obtained by measuring the component of current in phase

with the applied potential. This loss current is the result of the

absorption of energy from the field and consequently any mechanism

which results in an energy absorption contributes to the dielectric

loss.

2. Adsorption of water

The investigation of the dielectric properties of the systems

reported here consists of the measurements of the changes in capaci-

tance (which is proportional to the dielectric constant) and the

conductance or loss current. In these measured quantities are

contributions from the thorium oxide sample, the adsorbate, and

vapors surrounding the solid particles. In addition the properties









of the cell and its connections are involved but these remain essen-

tially constant from sample to sample and will not effect any inter-

pretation made in regards to differences between samples and/or

adsorbates.

The capacitance changes with frequency are shown in Figure 24

for the empty cell, the "dry" sample, and a sample with adsorbed

water. The "dry" sample is the oxide sample which has been pumped

on for a period of time without the application of heat. It is very

probable that the oxide prepared from thorium hydroxide has a layer

of chemisorbed water on it even after pumping. It has been found in

preparing the n.m.r. samples that temperatures of 200* to 3000 C. are

required to completely remove all the water.

The capacitance of the empty cell is approximately twenty-four

picofarads and remains essentially constant over the entire frequency

range. A slight decrease is observed in the region around twenty

megacycles but this is attributed to the bridge circuit since it is

also present in the capacitance curve of a R-C network (see Table 25).

The cell shows no loss over the entire frequency range.

The capacitance of the "dry" sample also remains essentially

constant over the frequency range with the exception of the small

decrease near twenty megacycles. The data shown in Figure 24 is

for the oxide prepared from thorium oxalate (Sample OX-4). The

capacitance of the "dry" oxide sample prepared from thorium hydroxide

is higher by about twenty picofarads. This increase is attributed to

the water which remains on the sample after pumping.

The capacitance of the oxide sample with adsorbed water (Sample

OX-4) decreases with frequency over the range covered if the peculiar







(pfs.)


o o 13.64 mg. H20/g.ThO2


o o dry sample


60




50

AC

40




30




0


130

14) (pI Oqjo (p Op- aJ 00 ri 0 0--


10


(m cs.)


20


30


Figure 24.


Capacitance Changes with Frequency for the Cell, Dry Sample, and
Sample with Adsorbed Water.


v


0 0Ce


a a









behavior of the bridge at the higher frequencies is neglected. The

capacitance decrease becomes more obvious in terms of the dielectric

constant (C/Co). However, no additional information can be gained

from this transformation. This decrease is expected since the dielectric

constant decreases with frequency until at the high frequency limit

the optical dielectric constant is reached. The same trend is observed

at all water coverages on both samples (see Tables 26 and 27).

The behavior of the capacitance along with the frequency dependence

of the conductance indicates the presence of a dispersion region at a

frequency that is somewhere above thirty megacycles. The conductance

of the two samples is shown as a function of frequency in Figures 25

and 26. The conductance increases with frequency over the entire

frequency range and the increase becomes greater as the amount of

adsorbed water increases. The appreciable loss in this frequency

range indicates that the surface is producing some type of ordering

in the water layers. If the water remained as liquid water the region

of loss would appear at much higher frequencies.

A dispersion region has been found at low frequencies for the

alumina-water system39 but occurs in the kilocycle frequency range.

If the dispersion region is above thirty megacycles as suggested by

these results this would indicate that water is bound much tighter

to the alumina surface than to the thoria surface. This is accordant

with the values of the n.m.r. line widths of water on alumina and on

thoria.27

The conductance data of the two different kinds of oxides show

some differences. The conductance of water on the oxide sample

prepared from thorium hydroxide (X-B) (Figure 26) increases more






(/m hos)


16.10 mg./g.
0o 9.43 mg./g.


ee 5.35 mg./g.
00 0.00 mg./g.


10 f 20 30
(mcs.)


Conductance of Water on Thorium Oxide Prepared from Thorium Oxalate.


1200



900

G


300




0


Figure 25.






mhosos)




800



600

G


400



200


oa 1 5.76 mg./g.
D 0 5.28 mg./g.
o 0 0.00 mg./g.


0 0
0


0 10 f 20 30
(mcs.)

Figure 26. Conductance of Water on Thorium Oxide Prepared from Thorium Hydroxide.









rapidly at the higher frequencies than it does for water on the oxide

prepared from thorium oxalate (OX-4) (Figure 25). This may indicate

that the region of maximum loss is at a lower frequency for the

former oxides. This would again be consistent with the n.m.r. data

which also indicates that water is in general bound tighter to these

oxides.

Behavior of the tangent of the loss angle as a function of

frequency is shown in Table 31. The data for the oxide prepared from

thorium hydroxide roughly suggests the presence of a minimum in the

curve if the intrinsic perturbations of the bridge are ignored. The

data for the other sample is similar at the lower frequencies but

does not increase again at the higher frequencies. The decrease in

the loss tangent with frequency is the normal result of a mechanism

which produces a constant loss. This frequency dependence is shown

in equation IV, Appendix C.

The two sets of data can be reasonably interpreted in the

following manner. The dielectric loss results from at least two

mechanisms. In both oxides there is a component of conductance due

to an ohmic current which is attributed to ionic conductance. Evi-

dence of this component can be seen if the curves in Figures 25 and

26 are extrapolated to zero frequency. When this is done some

conductance is still present. This ionic component appears to be

greater in the oxide prepared from thorium oxalate and should be if

carbonate ions are present on this surface as postulated previously.

In addition to this ionic conductance it appears, from the large

increase in conductance with frequency, that dielectric loss due to

some other mechanism is present and that a region of dispersion is









being approached as the frequency is increased. This loss mechanism

is most probably due to dipole orientation. The dispersion region

appears to be nearer thirty megacycles for the oxide prepared from

the hydroxide than for the oxide prepared from thorium oxalate. This

is expected since water is bound tighter to the former oxides.

The changes in the capacitance with water coverage for several

frequencies shown in Figure 27 for the oxide sample prepared from

thorium hydroxide (X-B) and in Figure 28 for the sample prepared from

thorium oxalate (OX-4). In each case the amount of water which was

possible to adsorb prevents accurate deductions about its physical

state. When the amounts adsorbed on the dielectric samples are

compared to the amounts adsorbed on the n.m.r. samples a large differ-

ence is noted. This difference is attributed to the physical setup

of the dielectric cell in relation to the vacuum system. In order to

connect the cell directly to the bridge, thus eliminating long leads,

it was necessary that long glass plumbing be used from the vacuum

system to the cell. This resulted in a decrease in the rate of adsorp-

tion and for very high water coverages the time required to add an

increment became a matter of weeks.

The values of the capacitance of the "dry" samples indicate that

the oxide sample prepared from the hydroxide has some water remaining

on the surface even after it has been pumped on for a long period of

time. Few changes are observed in the slopes of the curves anywhere

in the range of available coverages. The only behavior which can be

considered a break occurs in the data in Figure 28 at very low cover-

ages. However, some indications of breaks are observed in the plots











(pfs.)



75



70



65

AC

G0



55



50




0


4 8 12 16
mg. H20/g.ThO2


20


Figure 27.


Capacitance of Water on Thorium Oxide Prepared from
Thorium Hydroxide.


1.0 me.








10.0 mc.



2 0.0 mc.











(pfs.)



60








50



AC




40


8 12
mg. H20/g.ThO2


20


Figure 28.


Capacitance of Water on Thorium Oxide Prepared from
Thorium Oxalate.


1.0 mcs.










20.0 mcs.


30




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