Vibrational Spectroscopies for Adsorbed Species

295 Pages·1980·4.79 MB·English

by  Alexis T. Bell and Michael L. Hair (Eds.)

Vibrational Spectroscopies
for Adsorbed Species
Alexis T. Bell, EDITOR
University of California, Berkeley
Michael L. Hair, EDITOR
Xerox Research Center of Canada
Based on a symposium
sponsored by the Division of
Colloid and Surface Chemistry
at the 178th Meeting of the
American Chemical Society,
Washington, D.C.,
September 12-13, 1979.
137
ACS SYMPOSIUM SERIES
AMERICAN CHEMICAL SOCIETY
WASHINGTON, D. C. 1980
In Vibrational Spectroscopies for Adsorbed Species; Bell, A., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
Library of Congress CIP Data
Vibrational spectroscopies for adsorbed species.
(ACS symposium series; 137 ISSN 0097-6156)
Includes bibliographies and index.
1. Vibrational spectra—Congresses. 2. Infrared
spectrometry—Congresses. 3. Surface chemistry—Con
gresses.
I. Bell, Alexis T., 1942- . II. Hair, Michael L.,
1934- . III. American Chemical Society. Division
of Colloid and Surface Chemistry. IV. Title: Adsorbed
species. V. Series: American Chemical Society. ACS
symposium series; 137.
QC454.V5V534 541.3'453 80-21181
ISBN 0-8412-0585-X ACSMC8 137 1-295 1980
Copyright © 1980
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In Vibrational Spectroscopies for Adsorbed Species; Bell, A., et al.;
ACS Symposium Series; AWmaesrihcianng Cthoenm, Dic.aCl. S2o0c0ie3ty6: Washington, DC, 1980.
ACS Symposium Series
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Advisory Board
David L. Allara W. Jeffrey Howe
Kenneth B. Bischoff James D. Idol, Jr.
Donald G. Crosby James P. Lodge
Donald D. Dollberg Leon Petrakis
Robert E. Feeney F. Sherwood Rowland
Jack Halpern Alan G Sartorelli
Brian M. Harney Raymond B. Seymour
Robert A. Hofstader Gunter Zweig
In Vibrational Spectroscopies for Adsorbed Species; Bell, A., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
FOREWORD
The ACS SYMPOSIUM SERIES was founded in 1974 to provide
a medium for publishin
format of the Serie parallel g
IN CHEMISTRY SERIES except that in order to save time the
papers are not typeset but are reproduced as they are sub
mitted by the authors in camera-ready form. Papers are re
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In Vibrational Spectroscopies for Adsorbed Species; Bell, A., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
PREFACE
The identification of species adsorbed on surfaces has preoccupied chem
ists and physicists for many years. Of all the techniques used to deter
mine the structure of molecules, interpretation of the vibrational spectrum
probably occupies first place. This is also true for adsorbed molecules, and
identification of the vibrational modes of chemisorbed and physisorbed
species has contributed greatly to our understanding of both the underlying
surface and the adsorbed molecules. The most common method for deter
mining the vibrational modes of a molecule is by direct observation of
adsorptions in the infrared region of the spectrum. Surface spectroscopy
is no exception and by fa
ture refer to the infrared
time, the main approach has been the use of conventional transmission IR
and work in this area up to 1967 has been summarized in three books.
The first chapter in this volume, by Hair, presents a necessarily brief over
view of this work with emphasis upon some of the developments that have
occurred since 1967.
One of the major advances in the past decade has been the maturation
of the electronic revolution. This has had its effect on surface spectroscopy,
with regard to instrumentation for transmission IR, but particularly for
sensitivity gains that have made reflectance techniques the preferred alterna
tive for fundamental studies. In the transmission mode, the commercial
development of the Fourier transform IR spectrometer has led to signifi
cant advantages in the determination of the vibrational spectra of adsorbed
species. This is covered in the chapter by Bell.
In the transmission mode, IR spectroscopy is limited by sensitivity
requirements to the study of surfaces exhibiting relatively high surface area,
i.e., porous oxides and supported metals. A major advance in the past
decade has been achieved by using reflection techniques. Modulation of
the incident beam or ellipsometry usually are employed and optimization
of these techniques now enables the IR spectra of highly absorbing mole
cules to be obtained on single crystal surfaces. As a result, it has become
possible to compare vibrational data with data obtained from other surface
diagnostic techniques (i.e., LEED and AES). Chapters describing the
determination of these reflection absorption spectra are given by Allara,
Pritchard, and Dignam.
Surface wave spectroscopy provides yet another means for obtaining
absorption spectra of species present on a single crystal surface. In this
case, IR radiation is coupled to the sample in such a way that it propagates
vii
In Vibrational Spectroscopies for Adsorbed Species; Bell, A., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
laterally along the surface for a finite distance before it is decoupled and
detected. Use of this technique permits a larger absorption to be achieved
than normally is obtained by means of reflection techniques. The theory
and practice of surface wave spectroscopy are discussed by Bell.
Raman spectroscopy is a complementary technique that yields the
same vibrational information as is obtained in conventional IR absorption
spectroscopy. Here, however, the exciting radiation is in the visible region
of the spectrum. Application of this technique to surfaces has been difficult
because of practical problems associated with fluorescence. This problem
now has been essentially resolved, and recent progress in the application
of Raman spectroscopy to surfaces is reviewed by Morrow.
Conventionally, in IR spectroscopy the measured absorptions are
caused by the fundamental vibrations of the atoms within a molecule. In
many cases, important information can be obtained by examining the
overtones of these fundamental vibrations. This can be done in the trans
mission mode, but elegan
discussed by Klier.
The final chapters of this book review the progress of three recently
developed techniques that provide information about the vibrational states
of adsorbed molecules. Perhaps the most important of these techniques is
electron energy loss spectroscopy that, despite its inherent low resolution,
gives valuable information on vibrational modes that are either inactive in
the IR, or inaccessible because of experimental difficulties. The applica
tions of this technique are discussed in two chapters by Somorjai and
Weinberg. The review of new experimental techniques concludes with
presentations on inelastic electron tunneling spectroscopy and neutron
scattering by Kirtley and Taub.
In assembling this collection of review/progress articles, the editors
have tried to provide an update of all techniques used to determine the
vibrational structure of molecules adsorbed on surfaces. The symposium
itself provided a forum whereby the leading workers in the field could
interact and rationalize their various approaches. No single technique will
ever give all the information required to describe an adsorbed molecule
and the recent developments relating the vibrational spectroscopies and
the UHV techniques are, thus, particularly exciting. Further overlap
between these techniques will undoubtedly occur and inevitably lead to a
deeper understanding of the molcular structure of adsorbed species.
Department of Chemical Engineering ALEXIS T. BELL
Berkeley, California
Xerox Research Center of Canada MICHAEL L. HAIR
Mississauga, Ontario, Canada
May 19, 1980
viii
In Vibrational Spectroscopies for Adsorbed Species; Bell, A., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
1
Transmission Infrared Spectroscopy for
High Surface Area Oxides
MICHAEL L. HAIR
Xerox Research Centre of Canada, 2480 Dunwin Drive,
Mississauga, Ontario, L5L 1J9, Canada
Interest in the vibrational spectra of adsorbed molecules is at least 40 years
old. The past ten years have seen the development of many novel techniques
for determining the vibrationa
brings together a state-of-the-ar
ethusiasm for the recent advances made in any subject there is a tendency to
forget the parent technique and its steady contribution to our knowledge. In
this case, the parent is simple transmission infrared spectroscopy. This paper,
therefore, is an attempt to briefly present an overview of some of the
developments which have occurred in the application of transmission infrared
spectroscopy to surface studies with emphasis upon results generated in the
past 10 years. For more detailed information on work published prior to 1967
the reader is referred to three texts which have appeared on this subject (1-3).
Because of the maturity of the method the advances in technique are
incremental rather than revolutionary. Perhaps the major new developments
have been in the instrumental area where the ready availability of the Fourier
Transform instruments has led to its introduction to surface studies. The ease
of obtaining spectra and the advantages associated with the direct computation
of data will be discussed in a separate paper (4).
Transmission IR still remains the best method for examining insulating oxide
surfaces and over the past decade there has developed a considerable
understanding of many surfaces, particularly those of silica, alumina, molecular
sieves and complex catalysts. The objective of this paper, therefore, will be to
demonstrate how some of the recent advances have been made. Clearly it is
not possible to discuss all the materials studied by transmission IR and the
author has chosen to use the surface properties of silica to illustrate the type of
understanding that is now available.
Some History
The first application of transmission infrared spectroscopy to the study of
adsorbed species appears to be the work of Buswelf et al in 1938 (5). Those
authors pressed a montmorillonite clay into a disc which was then "dried" at
various temperatures. The spectra they obtained bear a remarkable similarity to
many others that have been produced in the literature over the next forty years:
the authors were clearly able to resolve bands due to hydroxyl groups
associated with the clay lattice and to adsorbed water which was slowly
removed as a function of drying.
0-8412-0585-X/80/47-137-001$05.00/0
© 1980 American Chemical Society
In Vibrational Spectroscopies for Adsorbed Species; Bell, A., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
2 VIBRATIONAL SPECTROSCOPIES
Almost twenty years elapsed before the next advances. In the 1950's the
Russian School (Terenin, Yaroslavski, Kiselev) (6) studied the structure of
porous glass and made the first assignments for hydroxyl groups on such
surfaces; the Cambridge School (Shephard, Little, Yates) (7) investigated the
process of physical adsorbtion and the rotational motion of physcially adsorbed
molecules; and the American workers (Eischens, Francis, Pliskin) (8) applied
the technique to supported metal catalysts and initiated the application of this
technique to the study of catalysts. The work of Eischens and co-workers on
CO/Ni is remarkable in that the spectra obtained (and the interpretation) have
stood the test of time and are confirmed most elegantly by some of the studies
on single crystal surfaces which will be presented in later chapters.
Experimental
In the application of transmission infrared spectroscopy to the study of
surfaces it is important that high surface area materials be used in order that
the resultant spectrum contains a considerable contribution from the surface as
distinct from the bulk of th
pressed into thin self-supportin
the infrared radiation in the spectrometer. Developments in this area in the
past decade have been aimed mainly at quantifying the infrared data and two
basic types of vacuum cell have emerged: those in which the sample is moved
in and out of the beam into a furnace above the spectrometer, and those in
which the furnace is built around a static sample holder which is permantly
held in the beam. The latter is clearly a more desirable system but suffers from
the experimental disadvantages that the furnace must be constructed within the
confines of the infrared spectrometer, thus giving rise to problems associated
with the cooling of the infrared transmitting windows. An excellent cell capable
of temperatures up to 600°C under UHV conditions has been described (9).
Silica
A classical paper on the adsorption of water on silica surfaces appeared
from* the General Electric Laboratories in 1958 (10). One set of spectra from
this paper are redrawn in Fig. 1. The results clearly demonstrate the difference
between the two forms of high surface area silica commonly found in the
laboratory: the silica which is precipitated from solution and is widely used as
a dessicant and the finely divided silica (Cabosil, Aerosil, etc.) which is
prepared by flame oxidation of SiCI at elevated temperatures. Experimentally,
4
MacDonald made pressed discs of each of the forms of silica, placed them in a
simple IR cell in the beam of a spectrometer as described earlier and recorded
the spectra. The solid black lines (a) are the spectra recorded at room
temperature and the difference between the two silicas is readily apparent. The
Cabosil spectrum shows distinct structure with bands being readily observed at
3747, 3660 and 3520 cm*1. In the case of the precipitated silica there is
complete absorption between 3750 and 3000 cm"1. On evacuating the samples
at room temperature changes are seen in the spectra which can be related to
the removal of physically adsorbed water from the surface. This is done by
comparing (a) and (b). A broad band (d) centered around 3400 cm"1 has been
removed by the evacuation and this is coincident with the removal (not shown)
of a band at 1625 cm*1. These are clearly due to the stretching and bending
In Vibrational Spectroscopies for Adsorbed Species; Bell, A., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
HAIR Transmission IR Spectroscopy
CABOSIL BULKY SILICA
0.0 I I I I I I I tfTTT" I I I I III I
0.1
0.2
0.3
0.4
0.5
10
1.5
0.0
0.1
0.2 7--}
(X3 degassed 1/2 hr at 500°C
0.4
0.5
1.0
1.5
8 8 8 8 8
3 s 8 S §
(cm-1)
Journal of Physical Chemistry
Figure 1. The IR spectra of Cab-O-Sil and bulky silica: (a) before degassing; (b)
after degassing for 3 h at 30°C in vacuo; (c) difference between (a) and (b); (d) de­
gassed 30 min at 500°C in vacuo; (e) degassed 8.5 h at 940° C in vacuo (10)
3747 cm-1
H
V 150°C.
+ H0
2
_\ Sis— O—H''' NH
/ 3660 cm-1 3500 cm-1
1630 cm-1 11
<400°C
REVERSIBLE
800°C
IRREVERSIBLE
~7S,S\
^H20 Ο +H20
Figure 2. Schematic of hydration-dehydration on a silica surface
In Vibrational Spectroscopies for Adsorbed Species; Bell, A., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
4 VIBRATIONAL SPECTROSCOPIES
motions of molecular water. On heating the sample to higher temperatures
further changes can be observed. In both samples the band at 3660 cm*1
becomes more distinct as the heating temperature is raised until it eventually is
seen as a shoulder on the 3747 cm*1 band. After the sample has been
degassed at 940°C in vacuum, both the precipitated silica and the Cabosil
exhibit identical spectra which consist of one very narrow, sharp band at
3747 cm*1.
One important observation which can be drawn from these spectra is the
fact that, at least at low coverages, the removal of molecular water from the
surface of the silica does not particularly affect the intensity of the band at
3747 cm"1. Thus, it can be concluded that the water is not specifically
interacting with this group during the adsorption at low coverages and is
therefore sitting on other parts of the surface. (This view is widely held, but
definitive evidence is not available. Interaction certainly occurs at higher partial
pressure but this would be expected in a random rather than a specific
adsorption process.)
The interpretation of th
universally accepted althoug
and fine structure. Thus, the band at 3747 cm*1 is assigned to a hydroxl group
attached to a silicon atom on the surface. The OH group is situtated so that it
is vibrating freely and is unperturbed by its neighbours. The band at 3660 cm*1
is attributed to surface hydroxyl groups which are sufficiently close together
that they are hydrogen bonded to each other and are thus perturbed from the
3747 cm"1 position. It is usually accepted that the frequency shift is related to
the strength of the Η-bonding interaction, the larger shifts corresponding to
stronger interaction and it is noted that dehydration proceeds from the lower
frequency end. Other absorptions in this region are attributed to molecular
water adsorbed on the surface. The dehydration which occurs as the
temperature of the sample is raised clearly proceeds by a mechanism in which
the Η-bonded SiOH groups are removed from the surface, the more strongly
Η-bonded groups being removed initially.
Rehydration of the surface is found to be reversible only if the
pretreatement temperature is kept below about 400°C. Above that temperature
a restructuring of the surface apparently occurs. The removal of the H-bonded
groups removes the adsorption site for water thus giving rise to a surface which
does not adsorb water very readily at low pressure (Fig. 2).
Sample Preparation
One of the major experimental problems in studying surfaces by the
transmission techniques has always been the sample preparation. Indeed, the
quality of the spectra was limited by the quality of the pressed disc and good
transmission below 1300 cm"1 was not possible. This precluded observation of
the region where many bending modes would be expected and prevented
complete assignment of surface structures. After much experience it is now
possible to prepare very thin, high quality discs of Cabosil and other oxides
(10 mg/cm2) and the resultant increase in available information is illustrated by
the work of Morrow and his co-workers on Cabosil (11-14). Thus, Morrow and
Cody were able to obtain the spectra shown in Fig. 3 in the region between 800
and 1000 cm*1. The effect of heating the sample is clearly shown and can be
In Vibrational Spectroscopies for Adsorbed Species; Bell, A., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1980.