1 Transmission Infrared Spectroscopy for High Surface Area Oxides MICHAEL L. HAIR
Downloaded by 80.82.77.83 on November 7, 2017 | http://pubs.acs.org Publication Date: November 26, 1980 | doi: 10.1021/bk-1980-0137.ch001
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 vibrational spectra of adsorbed species and this symposium brings together a state-of-the-art survey of these techniques. In one's 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 b e 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 d u e to hydroxyl groups associated with the clay lattice and to adsorbed water which was slowly removed a s a function of drying.
0-8412-0585-X/80/47-137-001$05.00/0 © 1980 American Chemical Society
Bell and Hair; Vibrational Spectroscopies for Adsorbed Species ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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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 C O / N i 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.
Downloaded by 80.82.77.83 on November 7, 2017 | http://pubs.acs.org Publication Date: November 26, 1980 | doi: 10.1021/bk-1980-0137.ch001
Experimental In the application of transmission infrared spectroscopy to the study of surfaces it is important that high surface area materials b e used in order that the resultant spectrum contains a considerable contribution from the surface as distinct from the bulk of the sample. Typically, these samples have been pressed into thin self-supporting discs which are then inserted in the path of 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. A n excellent cell capable of temperatures up to 600°C under UHV conditions h a s been described (9).
Silica A classical paper on the adsorption of water on silica surfaces appeared from* the General Electric Laboratories in 1958 (10). O n e 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 a n d 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, 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* . In the case of the precipitated silica there is complete absorption between 3750 and 3000 c m " . 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 c m " has been removed by the evacuation and this is coincident with the removal (not shown) of a band at 1625 cm* . These are clearly due to the stretching and bending 4
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Bell and Hair; Vibrational Spectroscopies for Adsorbed Species ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
HAIR
Transmission IR Spectroscopy
CABOSIL 0.0
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BULKY SILICA
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Downloaded by 80.82.77.83 on November 7, 2017 | http://pubs.acs.org Publication Date: November 26, 1980 | doi: 10.1021/bk-1980-0137.ch001
10 1.5 0.0 0.1 0.2 (X3 0.4 0.5
degassed /2 hr at 500°C 1
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8 3
8 s
8 8
8 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-
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_ \ S i — O—H''' H / 3660 cm- 3500 cm1630 cmN
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Figure 2.
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