INFRARED SPECTRA OF ADSORBED GASES By V. CRAW FORD^ (THE UNIVERSITY MEDFORD MASS. U.S.A.) 1. Introduction THE infrared spectrometer has become firmly established as a useful and important tool for investigating the adsorption of gases on solids. Work published during the last ten years has described the results obtained in studies of both physical adsorption and chemisorption. It is the object of this Review to illustrate how spectroscopic methods may be used to con- tribute towards elucidating certain general problems of adsorption and for this purpose examples will be chosen from some of the solid-gas systems which have been studied. The traditional distinction between physical adsorption and chemisorption will be observed although it is becoming increasingly evident that in certain instances there are no clear-cut criteria to enable such a distinction to be made with certainty.2. Physical Adsorption (A) Spectroscopic Results to be expected from Perturbing Effects of Surface Forces on Adsorbed Molecules and vice versa.-When a gas molecuIe approaches the surface of a solid interaction between the mole- cule and the surface occurs the interaction energy U(r) being a function of the distance r from the surface. Irrespective of its functional form U(P) satisfies the following conditions U=O a t r = o o and U=cx>atr=O If the adsorption minimum in the potential curve U(r) plotted against r is located at values of r where there is practically no overlap between the wave functions of the adsorbed molecules and the lattice of the solid then physical adsorption has occurred.In this case the perturbing effects of the surface might be manifested in the following ways. (i) The surface forces distort one “side” of the adsorbed molecule more than the other and this induced asymmetry might be expected to result in the occurrence of new bands in the spectrum of the adsorbate i.e. bands which for reasons of symmetry are forbidden in the spectrum of the isolated unperturbed adsorbate molecules. Further this effect should be particularly marked in the case of highly symmetrical molecules and depending on the degree of distortion the degeneracy of vibrations may either be reduced or completely destroyed. (ii) It is well known that in passing from gaseous to condensed phases the frequencies of the corresponding bands are diminished. Since in physical t Present address Physical Chemistry Laboratories Imperial College of Science and Technology S.Kensington London S.W.7. 378 CRAWFORD INFRARED SPECTRA OF ADSORBED GASES 379 adsorption the perturbing effects of the surface forces on the adsorbate molecule are comparable to those of the surrounding molecules in the liquid state the frequency shifts would be expected to be of a similar order of magnitude to that observed in passing from the gas phase to the liquid state. Indeed the magnitude of the frequency shift might perhaps prove useful in distinguishing between physical adsorption and chemisorption in doubtful cases. (iii) In carrying out a statistical thermodynamic study of the adsorbed phase a model is chosen and various thermodynamic quantities of interest are computed for certain assumed degrees of freedom of the adsorbed molecule.Comparison of the computed with the experimental quantities then permits deductions to be made concerning the degrees of freedom lost by the adsorbate in passing from the gaseous to the adsorbed phase. It might be expected that study of band shapes and in particular high- resolution studies would provide direct information about the degrees of freedom of molecules in the adsorbed state. (iv) Condonl has studied the infrared spectrum induced by electrical fields and as might have been expected found the selection rules for the induced spectrum to be the same as those for Raman spectra. It was also shown that the intensity of bands induced by electrostatic forces should depend on the square of the field strength. Study of the infrared spectra of adsorbed molecules on ionic adsorbents should then permit determination of the field strength prevailing at the adsorption equilibrium distance.(v) It has been suggested2 that in a previous study of the adsorption of ammonia on barium fluoride hydrogen bonding between adsorbate molecules on the surface was neglected. Since hydrogen bonding has been very successfully studied by infrared spectroscopy it might be expected that the same technique would provide direct information on the occurr- ence of hydrogen bonding in the adsorbate on the surface. (vi) Statistical and thermodynamic studies of the adsorbed phase almost invariably neglect the perturbation of the adsorbate on the adsorbent. Nevertheless the presence of adsorbate molecules would be expected to perturb the adsorbent surface.This effect should be easily detectable spectroscopically if the surface possesses chemical functional groups which terminate the bulk structure of the solid e.g. OH on a silica surface. Thus when a molecule is adsorbed near the OH group the motions of the atoms of the group are perturbed and consequently a shift in the group vibration frequency might be expected. (B) Experimental Requirements.-Having indicated the type of informa- tion which might be obtained from a study of the infrared spectra of adsorbed molecules we must consider whether such spectra are in fact physically realisable. The intention here is not to discuss details of the types of cell and methods of sample preparation that have been used but E. U. Condon Phys. Rev. 1932,41,759. R. M. Dell and R.A. Beebe J. Phys. Chem. 1955,59 754. 380 QUARTERLY REVIEWS rather to direct attention to the special combination of circumstances which make it possible to obtain well-defined spectra. Since a layer of liquid 1-100 p thick is sufficient to give measurable absorption bands a monolayer 10 A thick would require 1,000 traversals of infrared radiation to give measurable spectra. Owing to surface scatter- ing however radiation is lost on each traversal. This loss due to scattering by individual particles can be greatly diminished by using particles of a size less than the wavelength of the incident radiation; but decreasing the particle size increases the specific surface of the adsorbent and so the means used to reduce scattering has the simultaneous beneficial effect of ensuring that a correspondingly greater amount of gas is adsorbed at a given pressure.It is this fortunate combination of factors which makes possible the spectroscopic study of adsorbed molecules. In practice however it is found that a layer of powder has a marked residual scattering even though the particles of which it composed satisfy the size criterion for minimum scattering. This is undoubtedly due to the formation of agglomerates having an effective size greater than the wavelength of the incident radiation. On the other hand adsorbents in the form of gels or porous glasses seem to scatter much less radiation than a powder of equivalent path length and specific surface. The reason for this it has been ~uggested,~ is that the individual surfaces are separated from each other more often than in the case of a layer of powder.In addition to losses due to scattering possible additional losses may occur because of absorption by the adsorbent. However since many solids do not exhibit strong absorption in certain regions of the spectrum this leaves certain frequency ranges available for study. The major requirements then for obtaining well-defined spectra are these (i) That there be a sufficiently high concentration of adsorbed gas in the path of the incident radiation; this can be secured by using an adsorbent of sufficiently high specific surface. (ii) Scattering by particles of the adsorbent should be eliminated or at least diminished as much as possible and this can be achieved by using particles of a size smaller than the wavelength of the incident radiation.(iii) The adsorbent should be transparent to infrared raaiarion. The remainder of this section will now be devoted to experimental evidence illustrating and confirming the expectations enumerated in section 2(A). (C) Perturbation of the Surface by the Adsorbate.-As indicated pre- viously perturbation of the surface by the adsorbate should be detected very easily if the surface possesses functional groups. Indeed the very first infrared absorption study of a solid-gas system4 was with one demonstrat- ing just this effect. a N. Sheppard Spectrochim. Acta 1959 14 249. N. G. Yaroslavsky and A. N. Terenin Doklady Akad. Nauk S.S.S.R. 1949 66 885. CRAWFORD INFRARED SPECTRA OF ADSORBED GASES 38 1 In their pioneering studies on the infrared spectra of physically adsorbed molecules Yaroslavsky and Terenin4 used as adsorbent a porous glass-like silica.Sirice it was several mm. thick and absorbed the long-wavelength radiation their work was confined to the near-infrared region (5000- 10,000 cm.-l). A band was found at 7325 cm.-l and assigned to the first overtone of the fundamental frequency of the surface OH groups. The location of this band was subsequently confirmed by Yaroslavsky5 and by Yaroslavsky and Karyakh6 On the adsorption of gases and vapours this band is broadened reduced in intensity (sometimes disappearing) and displaced to lower frequencies. The effect on the surface OH groups of gases which are physically adsorbed is demonstrated nicely by the work of Yaroslavsky and Karyakin6 on the adsorption of nitrogen on porous glass. After degassing at SO" the sharp peak at 7325 cm.-l was obtained.The adsorbent at - 180" was then exposed to 1 atm. pressure of nitrogen and the spectra were recorded at 1 10 20 and 120 min. respectively after the admission of the nitrogen. After 20 min. the 7325 cm.-l band disappeared and was replaced by one at 7257 cm.-l the latter band appearing 1 min. after the admission of nitrogen. The 7325 cm.-l band was undetectable after 20 min. but the 7257 cm.-l band increased in intensity between 20 and 120 min. Since the original band at 7325 was obtained on raising the temperature to 20" this suggests that the shift of the OH frequency is caused by physical adsorption of nitrogen. Similar results were obtained on adsorption of oxygen except that in this case the admission of oxygen at -180" resulted in (i) the instan- taneous disappearance of the 7325 cm.-l and appearance of the 7257 cm.-l band and (ii) the finding that the 7325 cm.-l band is only restored on heating to 200".Sidorov using transparent plates of glass obtained the spectra not only of surface groups but also of several different types of molecule in the adsorbed phase. Carbonyl-containing adsorbates such as acetone and benzaldehyde cause the OH overtone band to be widened increased in intensity and displaced to lower frequency the displacements being 370 and 290 cm.-l respectively for acetone and benzaldehyde. Similar effects on the first overtone band of OH have been reported by Filimono~.~*~ For example exposure of silica gel to the vapour of chlorobenzene and nitromethane produced shifts of 140 and 160 cm.-l respectively.Some very comprehensive studies of the silica surface have been carried out by McDonald.lO The adsorbent used was Aerosil 2491 or Cabosil which is a pure fumed silica. Before degassing the adsorbent showed a N. G. Yaroslavsky Zhur. Jiz. Khim. 1950,24 68. N. G. Yaroslavsky and A. V. Karyakin Doklady Akad. Nauk S.S.S.R. 1952 85 A. N. Sidorov Doklady Akad. Nauk S.S.S.R. 1954 95 1235. V. N. Filimonov Optika i Spektroskopiya 1956 1 450. V. N. Filimonov and A. N. Terenin Doklady Akad. Nauk S.S.S.R. 1956,109,982. 1103. lo R. S. McDonald J. Amer. Chem. Soc. 1957 79 850. 3 382 QUARTERLY REVIEWS sharp band at 3749 and a broader one at 3400 cm.-l. On out-gassing the former increased and the latter decreased in intensity until after prolonged evacuation at 300-350” only the 3749 cm.-l band remained.This was assigned as the 0-1 stretching frequency of OH groups oriented in such a way as to be incapable of interacting with their surroundings. The effect on this band of physically adsorbed rare-gas atoms and non-polar molecules was then investigated and in every case the OH frequency diminished as shown in Table 1. TABLE 1. Perturbation of free OH frequency of Aerosil by various adsorbates. Adsorbate None Argon Krypton Xenon Nitrogen Oxygen Methane Polarisabili ty (A”> - 1-65 2.54 4.13 1 *76 1 *60 2.60 OH Frequency (cm.-l) 3749 3741 3733 3730 3725 3737 3717 Frequency shift (cm.-l) 8 16 19 24 12 32 - McDonald sought a correlation between the frequency shifts produced by the various adsorbates and their polarisabilities. In the case of the rare gases there appeared to be such a correlation for argon krypton and xenon which have polarisabilitiesll of 1.65 2.54 and 4.1 3 A3 produced frequency shifts of 8 16 and 19 cm.-l respectively.However oxygen and nitrogen show no such correlation. These molecules have approximately the same polarisabilityll (1 -60 and 1 -76 pi3 respectively) yet produce frequency shifts of approximately 12 and 24 cm.-l respectively. Frohnsdorff and Kingtonf2 have suggested that thareason for this is to be found in the difference in the quadrupole moments of these molecules (0,<0*1 A2;13 N2 < 0.5 A2 14). Very approximate calculations based on various assumed positions and orientations of the quadrupole show that interaction between proton and quadrupole moment can satisfactorily account for the fre- quency difference provided the effective protonic charge is not less than -0.2 x e.s.u.The suggestion that the quadrupole moment of nitrogen cannot be ignored is reinforced by a study of the intensities of the OH bands. Thus McDonald’s high-pressure measurements show that the free OH band disappears at higher relative pressures for oxygen than for nitrogen indicating that the latter molecule perturbs the OH group more strongly than oxygen does. This is in accordance with the fact that l1 Landolt-Bornstein “Zahlenwerte und Funktionen” Vol. I Part 3 1951 p. 510. la G. J. C. Frohnsdorff and G. L. Kington Trans. Faraday SOC. 1959,55 1173. Is W. V. Smith and R. Howard Phys. Rev. 1950,79 132. l4 C. H. Tomes and A. L. Schawlow “Microwave Spectroscopy” McGraw-Hill New York 1955 p. 365. CRAWFORD INFRARED SPECTRA OF ADSORBED GASES 383 the quadrupole of nitrogen interacts with the field gradient of the OH group giving a higher heat of adsorption.0.0 0 0 g 0.1 0' 0-2 n $ 0.3 >" 0-4 ,o 0.5 1-0 1.5 v) .- CI 0 m 3700 3700 3700 3700 F r aq u enc y ( c m:' ) FIG. 1 . Infrared absorption due to SiOH of Cabosil (12.5 mg.lcm2 pressed at 12,000 Ib./in.2). (a) Before degassing; peak at 3747 cm.-'. (b) After degassing for 30 min. at 500" in vacuo; peak at 3748 cm?. (c) After degassing for 15 min. at 940" in vacuo; peak at 3749 cm.-'. (d) After degassing for 8.5 hr. at 940" in vacuo; peak at 3750 cm.-'. 3700 3700 3700 F r Q q u Q n c y (c m:' ) FIG. 2. Infrared absorption due to SiOH of Mallinckrodt Special Bulky Silicic Acid (12.5 mg./cm.2 pressed at 12,000 1b./in.2). (a) Degassed for 30 min.at 500" in vacuo; peak at 3740 cm.-'. (b) Degassed for 15 min. at 940" in vacuo; peak at 3748 cm.-'. (c) Degassed for 8.5 hr. at 940" in vacuo; peak at 3748 cm.-l. (Figs. 1 and 2 are reproduced with permission from R. S. McDonald J. Phys. Chern. 1958 62 1168.) More recently McDonald15 described an infrared study of silanol groups on the surface of two varieties of pure amorphous silica Mallin- ckrodt Special Bulky Silicic Acid (MSBS) which is a pure precipitated silica and Cabosil. The results indicate that OH on the surface of silica exists in different states. Thus the shift and narrowing of the residual SiOH band of MSBS is much more pronounced than for Cabosil as in- dicated in Figs. 1 and 2. l5 R. S. McDonald J. Phys. Chern. 1958 62 1168. 384 QUARTERLY REVIEWS The breadth of the band after degassing at 500" (Fig.2a) shows that silanol groups of MSBS interact with each other much more than those of Cabosil which has not been degassed at all. However degassing at 940" in a vacuum for eight hours destroyed hydrogen bonding and left ap- preciable amounts of isolated silanol groups on the surface of both silicas. As a result of the degassing the absorption band of surface silanol groups was narrowed and displaced to higher frequency. This would indicate that the groups destroyed during outgassing had an environment markedly different from those which remained after outgassing. Very recently Folman and Yates16 studied effects due to hydrogen bonding between physically adsorbed molecules and the OH groups present on the surface of porous silica glass.In this case the surface OH groups show a very strong absorption even after evacuation for eleven hours at 450". The band is not only much wider than the normal OH band but it is also highly asymmetric. This implies that the band is composite consisting of a narrow band at approximately 3740 crn.-I due to isolated OH groups and another broader band at a lower frequency. Since hydrogen bonding is known to produce broad bands it can be inferred that the broadening in the observed band is probably due to OH groups hydrogen-bonded to adjacent groups. On adsorption of sulphur dioxide chloroform acetone and ammonia a new broad band appeared in each case and at a frequency lower than that attributed to OH on the free surface. The results obtained are shown in Table 2. TABLE 2.Shifts in OH frequencies produced by various adsorbates on the surface of porous silica glass. Adsorbate Temp. so2 24 O CH3Cl 24 (CM3)2CO 25 75 135 NH3 25 75 100 150 Average displacement (cm.-l) 115 110 330 305 270 820 750 710 640 In each case the new band appearing at lower wave-number is attributed to perturbation of the surface OH groups by the adsorbate as a result of hydrogen-bond formation. At room temperature the magnitude of the perturbation of the surface OH groups increases in the order CH,Cl < SO < (CH,)&O < NH, the last compound forming a particularly M. Folman and D. J. C. Yates Proc. Roy. SOC. 1958 A 246 32. CRAWFORD INFRARED SPECTRA OF ADSORBED GASES 385 strong hydrogen bond. Later work17 showed that within experimental error dv is independent of coverage and that vt the half-width of the perturbed OH band increases with increasing temperature.In the case of acetone and ammonia where measurements were made at several temperatures the magnitude of the shift decreases with rise in temperature. This behaviour is similar to that displayed by solutions in which hydrogen bonding is present. (D) Perturbation of the Adsorbate by the Surface.-The infrared absorp- tion spectra of methane ethylene acetylene and hydrogen adsorbed on porous glass were investigated by Sheppard and Yates.18 In addition to obtaining the spectra they simultaneously determined the coverage. This work provided the first definite infrared spectroscopic evidence which clearly showed the perturbing effect of surface forces on adsorbed mole- cules and for this reason will be considered in some detail.Their results are shown in Table 3 which lists not only the frequencies for the adsorbed state but also those for the corresponding liquids and gases. It will be noted that (i) Where data are available the frequencies in the adsorbed phase are either lower than or equal to those of the corresponding liquids; thus the perturbing effects of the surface forces are greater than those of the surrounding molecules in the liquid. (ii) All the shifts in frequency are small and approximately 2 % of the gaseous value indicating that no change in chemical species could have occurred as a result of adsorption. TABLE 3. Frequencies (in cm.-l) of bands of adsorbed molecules. Molecule Gas Liquid Adsorbed vgas- vadsorbed CH v3 3018.8” 301 8 C2H4 vQ 3105*5c 3105 vll 2989.5c 2980 v1 2916~5~ (R) - vl 3019*3c (R) - C2H2 v3 3287c - 4160*2d (R) - H2 Vl 3006 12.8 2899 17.5 3 100 5.5 2980 9.5 3010 9.3 3240 47 4131 29.2 a D.R. J. Boyd H. W. Thompson and R. L. Williams Proc. Roy. SOC. 1952 A b B. P. Stoicheff C. Cumming C. E. St. John and H. L. Welsh J. Chern. Phys. 1952 c G. Herzberg “Infrared and Raman Spectra of Polyatomic Molecules,” New York d G. Herzberg Canad. J. Res. 1950 28 A 144. 213 42. 20 498. Van Nostrand 1945. As indicated previously when a molecule is adsorbed it is necessarily distorted to some extent. If the adsorbate is a highly symmetric poly- atomic molecule this reduction in its symmetry on transference from the l7 M. Folman and D. J. C. Yates J. Phys. Chzm. 1959,63,183. Is N. Sheppard and D. J. C. Yates Proc. Roy. SOC.1956 A 238 69. 386 QUARTERLY REVIEWS gaseous to the adsorbed phase allows vibrations to appear in the spectra which previously were forbidden. Also vibrations which are degenerate in the isolated gas molecule may have the degeneracy lifted or completely destroyed depending on the extent to which the symmetry is reduced. Thus in the case of methane the totally symmetric vibration vl which occurs at 2916 crn.-l in the Raman spectrum is as expected unobserved in the infrared spectra of the gaseous and the liquid phases. In the adsorbed phase however a band occurs in the infrared spectrum at 2899 cm.-l and is assigned to vl. The v3 band is a triply degenerate C-H stretch and if on adsorption the symmetry of the molecule were reduced from Td (free gas) to CSv in the adsorbed phase v3 would be split into two components of symmetry species E (doubly degenerate) and A (non-degenerate) whose intensities would be approximately in the ratio 2:l.If however transference of methane from gas to adsorbed phase should reduce its symmetry from Td to CZv the degeneracy of v3 would be completely destroyed yielding three bands of approximately equal intensity. Sheppard and Yates in fact obtained a single broad but fairly symmetrical band. (i) Rotational degrees of freedom. It is well known that the bands of a spherical-top molecule consist of P Q and R branches each branch consisting of lines which correspond to transitions between quantised rotational levels of the molecule. However a sharp fine structure will be seen only as long as the molecules can absorb radiation without interrup- tion.This can be accomplished for instance by increasing the pressure of the absorbing gas in which case the individual lines become broadened. When this becomes sufficiently great the individual rotational lines merge but the merging may have very little effect on the overall shape of the band. Sheppard and Yates then attempted to obtain some information about rotational degrees of freedom from the effects of rotational motions on the shape of the v3 band of adsorbed methane. Three possibilities which in principle can be distinguished spectroscopically are conceivable namely no free rotation in the adsorbed state; free rotation about one axis probably perpendicular to the surface; and three degrees of rotational freedom. No fine structure was observed in the spectrum of adsorbed methane at W O K whereas the spectrum of the gas phase exhibited clear fine structure.This however does not necessarily rule out the possibility of free rotation as the individual rotational energy levels may have merged. Consequently detailed calculations as follows were made of the shapes of the bands to be expected for methane adsorbed in the three ways suggested. Case I. The molecule may be so strongly adsorbed that the rotational degrees of freedom present in the gas phase become torsional oscillations on adsorption. Since rotational fine structure is absent most of the intensity occurs in a single peak and so the overall shape of the band should be well represented by a Lorentz-type curve viz., CRAWFORD INFRARED SPECTRA OF ADSORBED GASES 387 In &/I) = a / [ ( v - vo)2 + b2] where vo is the centre of the band 2b is its half-width and a/b* = Case II.Two degrees of rotational freedom are lost on adsorption the remaining one being about an axis perpendicular to the plane of the surface. In this case the rotational energy levels are given by In (IO/I)v()- E,(K) = K2h2/8n21 where K = 0 1,2 - the selection rules being AK = & 1 for perpendicular bands and AK = 0 for parallel bands. As usual the relative intensities depend largely on the factor exp [-E,(K)/kET] the statistical weights being 1 if K = 0 and 2 if K>O. Case ZII. No rotational degrees of freedom are lost on adsorption. In this case the energy levels are given by the familiar expression E,(J) = J(J + l)h2/8n21 where J = 0 1 2 the relative intensities of the rotational lines being roughly proportional to exp [-E,(J”)/kT) times the mean value of the statistical weights for the upper and lower levels [(W + 1)2].The results of the calculation showed that Case 111 can very definitely be ruled out for it allocates too much intensity to the P and R branches. So Cases I and I1 remain for consideration. Unfortunately the experi- mental evidence available does not permit a clear-cut and decisive distinc- tion to be made between these two alternatives. Since the magnitude of the induced dipole moment is proportional to the polarisability for a fixed field strength and since the polarisability ellipsoid of methane is a sphere the induced dipole moment will be independent of the orientation of the molecule. Hence the band shape is independent of rotational motions and the spectrum should consist of a single peak corresponding to a Q branch.On the basis of the free rotational model the width of the vl band should be less than that of v3 for the latter consists of P Q and R branches. Thus a simple com- parison of the relative widths of the two bands might seem to favour the free-rotational model. On the other hand the greater width of the vs band could possibly also be due to an unresolved splitting of the three-fold degenerate vibration due to surface forces. (iii) Hydrogen. The absorption band of adsorbed hydrogen was found to be symmetrical with a half-width of about 21 cm.-l. Unfortunately the shape of the band is not decisively helpful in distinguishing between models I (no free rotation on the surface) and I1 (free rotation about an axis perpendicular to the surface).The Raman spectrum of hydrogen has been obtained several timesfe and consists of a very strong Q branch together with widely spaced but lD F. Rasetti Phys. Rev. 1929 34 367. (ii) vl Band. 388 QUARTERLY REVIEWS weaker rotational wings. These rotational branches do not appear in the infrared spectrum of adsorbed hydrogen but it would be invalid to con- clude from this that the hydrogen molecule in the adsorbed state has completely lost its rotational degrees of freedom. The absence of the rotational structure could be due to insufficient hydrogen on the surface of the adsorbent. Crawford and Dagg20 had obtained the spectrum of hydrogen at high pressure under the influence of electrostatic fields and confirmed Condon’s prediction that the intensity of bands induced by electrostatic fields varies as the square of the field strength.Sheppard and Yates combined their intensity measurements with the data of Crawford and Dagg and com- puted the field strength to be about 7 x lo6 v/cm. at the equilibrium adsorption distance. This figure is of the right order of magnitude and clearly shows that the calculation of field strengths outside the adsorbent surface from intensity measurements is along the right lines. 3. Chemisorption If the minimum in the plot of U(r) against distance occurs at sufficiently small values of r so that overlap of wave functions cannot be neglected chemisorption has occurred. In this case the forces which hold the mole- cules to the surface are of an exchange nature and result in the formation of new bonds.Here several problems are of interest and it is worth inquiring what contributions might be expected from the application of infrared techniques. In the case of chemisorption these techniques were pioneered by Eischens and his colleagues.21*22 The objective is to obtain a metal adsorbent which (i) will cause as little radiation as possible to be lost by scattering and (ii) have a specific surface such as to ensure that sufficient gas is chemisorbed to give a well-defined spectrum. Since metals are very good absorbers of infrared radiation this is an additional source of loss and might seem to make impossible the application of infrared techniques to a study of chemisorption on metals. The absorption coefficient however is dependent on the size of the metal particle and so infrared study of chemisorption is rendered possible because of the happy circumstance that the means used to diminish scattering of radiation and increase the specific surface simultaneously make the adsorbent transparent to infrared radia- tion.Thus Eischens et al. 21,22 have shown that metal particles of size 3 x 10-2p are opaque but that satisfactory spectra can be obtained pro- vided the gas is chemisorbed on metal particles ,(10-2,u. In practice the the metal particles are dispersed in a non-porous silica support the particles of which are in the range 1.5-2-0 x 10-2p. (A) Surface Heterogeneity.-Probably the most significant single pro- perty of adsorption is the energetics of the process. The differential heat of 2o M. F. Crawford and I. R. Dagg Phys. Rev. 1953 91 1569.21 R. P. Eischens W. A. Pliskin and S. A. Francis J. Chem. Phys. 1954,22 1786. 22 R. P. Eischens W. A. Pliskin and S. A. Francis J . Phys. Chem. 1956,60 194. CRAWFORD INFRARED SPECTRA OF ADSORBED GASES 389 chemisorption falls markedly with coverage this variation of heat with coverage being quite complex in some cases. The fall in heat has been attributed to surface heter~geneity~~ and repulsive interactions between adsorbed species.24 Since interactions are never large it seems that in those cases where there is a marked fall in initial heat this is to be at- tributed to the heterogeneity of the surface. In particular the chemisorption of carbon monoxide on metals has been abundantly studied and by analogy with the structure of metallic carbonyls carbon monoxide chemisorption might be expected to take place by a single-site mechanism.At the surfaces of nickel25 and tungsten some evidence for this had been advanced but recently alternative possi- bilities have been suggested.28 Thus on molybdenum and rhodihm films the chemisorption of carbon monoxide was found to be almost exactly equal to that of hydrogen and on tantalum it was approximately equal to that of oxygen suggesting that on these metals the carbon monoxide molecule covers two sites. On iron and tungsten the chemisorption was respectively 1.2 and 1.4 times that of hydrogen thus implying mixed one- and two-site chemisorption. Observation of the infrared spectrum of chemisorbed carbon monoxide and variation in the spectrum with cover- age might be expected to provide some information on the heterogeneity of metal surfaces towards carbon monoxide.A good example of the study of surface heterogeneity by observing variations in the infrared spectra of a chemisorbed species with coverage is provided by the work of Eischens Pliskin and Francis,22 on the spectra of carbon monoxide chemisorbed on silica-supported palladium. The results are shown in Fig. 3 spectra A-E being recorded at progressively higher coverages. In interpreting the spectra the following considerations are pertinent. (a) With increasing coverage all bands might grow at the same relative rate. The implication of this would be that the position of the band was not a function of the bond strength and that the multiple band spectrum is not due to surface heterogeneity. (b) The bands might grow at different rates and this would be good evidence that the surface was heterogeneous.In this case the bands appearing first would be associated with the most strongly bonded carbon monoxide. (c) It is conceivable that as the coverage increases new bands might appear while simultaneously bands formed at low coverages might dis- appear. This would indicate that the structure of the chemisorbed carbon monoxide was a function of coverage. 23 H. S. Taylor J. Phys. Chem. 1926 30 145. 24 J. K. Roberts Proc. Roy. SOC. 1935 A 152 445. 25 0. Beeck A. E. Smith and A. Wheeler Proc. Roy. SOC. 1940 A 177,62. 26 B. M. W. Trapnell Proc. Roy. SOC. 1951 A 206,39. 27 Sir Eric Rideal and B. M. W. Trapnell Proc. Roy. SOC. 1951 A 205,409. 28 M. A. H. Lanyan and B. M. W. Trapnell Proc. Roy. SOC. 1955 A 227,387. 390 QUARTERLY REVIEWS Referring again to Fig.3 it will be noticed that as coverage increases new bands appear at 1835 1887,2062 and 1923 crn.-I. The last two bands increase in intensity with increasing coverage the 2062 cm.-l band growing proportionately more than the 1923 cm.-l band. On desorption the bands disappear in the reverse order of their appearance indicating that the species responsible for the 2062 cm.-l band is the least tightly bound. 100 n s u 9 0 t 0 Y) ul .- .- 5 C 8 0 t- 7 0 4.8 5 . 0 5.2 5.4 5.6 5.8 Wavelengt h ( p ) FIG. 3. Efect of increasing surface coverage on the spectrum of carbon monoxide chemi- (Reproduced by permission from R. P. Eischens S. A. Francis and W. A. Pliskin sorbed on palladium. (Coverage increases from A to E.) J. Phys. Chem. 1956 60 194.) The bands formed are assigned by analogy with the spectra of metal carbonyls and their spectra are of two types.Carbonyls of nickel,29 cobalt,32 manganese,33 and rhenium,33 in which the carbon monoxide is bound to the metal atom via the carbon have bands in the 2083-2000 cm.-l region. On the other hand in the case of dicobalt ~ctacarbonyl,~~ iron nonaca~bonyl,~~ and iron tetra~arbonyl~~ bands occur in the 1852-1 8 18 cm.-l region and these are attributed to carbon monoxide bridging two metal atoms. On this basis then the bands found for carbon monoxide chemisorbed on palladium indicate that the gas occurs on the surface bound in two ways (i) linearly to a single metal atom as Pd-CEO and (ii) bridged between two metal atoms i.e. thecarbonis bonded to two metal atoms as in (A). The spectroscopic evidence then is that the surface B.L. Crawford and P. C. Cross J. Chem. Phys. 1938 6 525. R. K. Sheline and K. S. Pitzer J. Amer. Chem. SOC. 1950 72 1107. R. K. Sheline J . Amer. Chem. Soc. 1951,73 1615. J. W. Cable R. S. Nyholm and R. K. Sheline J. Amer. Chem. SOC. 1954,76 3373. 89 G. 0. Brumm M. A. Lynch and W. Sesny J. Amer. Chem. SOC. 1954,76 3831. CRAWFORD INFRARED SPECTRA OF ADSORBED GASES 39 1 is heterogeneous and in particular indicates the presence of two types of site. 0 In the case of carbon monoxide on platinum the spectrum shows only a single intense band which occurs in the short-wavelength region and is attributed to Pt -C_O. With increasing coverage no additional bands appear but the one intense band does shift its position. Since in this case increasing coverage does not result in a multiplicity of bands it is im- possible to say whether the observed shift is due to heterogeneity or inter- action between molecules.As previously mentioned linearly chemisorbed carbon monoxide on palladium produced a band at 2070 ern? which was readily removed on desorption. In contrast the linearly chemisorbed gas on platinum is strongly bonded and so the band position for linear carbon monoxide on different metals is no indication of the chemisorption bond strength. Further spectroscopic evidence relating to the heterogeneity of the adsorbent surface is provided by the work of Yang and GarlandM on the infrared spectra in the region 1700-4000 cm.-l of carbon monoxide on rhodium surfaces. The metal in this instance was supported on a high-area alumina and spectra were taken of carbon monoxide chemisorbed on both sintered and unsintered surfaces.Not only were simpler spectra obtained in the case of the unsintered surface but in addition the behaviour of the bands on desorption was different for the two types of surface. Here then is additional spectroscopic corroboration of a fact which has long been known viz. that the character of an adsorbent surface varies with the conditions under which it has been prepared. In this particular case the difference was shown to be due to the presence of adsorbed water on the surface of the unsintered sample. Assignment of the spectra obtained and their variation with coverage would indicate that on the adsorbents used there were present at least three types of site. The type of site and the band obtained from carbon monoxide chemisorbed on it are indicated in Table 4.More recent spectroscopic evidence for the heterogeneity of an adsorbent surface is provided by the work of Terenin and roe^^^ on the spectra of nitric oxide adsorbed on transition metals and their oxides. (i) Metals. Iron chromium and nickel were obtained in the disperse state by adsorbing the corresponding metal carbonyl on alumina gel and then decomposing it in vacuo at 150". The spectra obtained for nitric oxide chemisorbed on the metals dispersed in alumina are summarised in Table 5. 34 A. C. Yang and C. W. Garland J. Phys. Chern. 1957,61,1504. 35 A. Terenin and L. Roev Spectrochim. Acta 1959 946. 392 QUARTERLY REVIEWS In interpreting these results the following facts are pertinent.In the presence of an electron acceptor nitric oxide is easily converted into the NO+ ion in which case the vibration frequency of 1876 cm.-l for the nitric oxide molecule in the gaseous state is displaced to 2000-2400 ~ r n . - l . ~ ~ TABLE 4. Location of bands (in cm.-l) in spectra of Coon rhodium. 0 0 111 II C C oc co \/ Sample -Rh- I /\ preparation Coverage -Rh- >Rh-Rh< unsintered All 2027 2095 - - 2% Rh 8% or 16% Rh Low - - 2040 unsintered High 2027 2095 2055 1905 8% or 16% Rh Low - - 2045 - sintered High 2040 2108 2062 1925 TABLE 5. Metal Frequency Desorption behaviour Iron 2008vs Disappears on desorption at 20" Frequencies (in cm.-l) of bands of NO adsorbed on Fe Cr Ni 1735w 1805w } Disappear after 1 hour at 150" Disappear on desorption at 350" 1698w 1660w 1625w Nickel 1850vs 1735w 1698w 1660w 1625w Chromium 2010 after long desorption at room Disappear after heating 1735 1698 1660 lrn Alternatively in the presence of an electron donor nitric oxide is con- verted into the NO- anion which has a vibration frequency located at 1000-1 100 ~rn.-l.~' In addition with transition-metal atoms and ions 36 W.R. Angus and A. H. Leckie Proc. Roy. Soc. 1935 A 149 327. 37 L. N. Short Rev. Pure Appl. Chem. (Austruliu) 1954 4 41. CRAWFORD INFRARED SPECTRA OF ADSORBED GASES 393 nitric oxide forms different types of covalent and co-ordinate bonds which are characterised by definite vibration freq~encies.~~ For example the characteristic frequency of N=O is known39 to be located in the range 1700-1 870 cm.-l and so the occurrence of a band in this region would be indicative of covalent bonding.With this background then it is evident that the very strong band at 2008 cm.-l in the case of iron indicates an ionised state of the chemisorbed molecule which might perhaps be attached to the surface as *NO+ the asterisk representing a bond to the surface. Judged by the ready removal of this band on desorption this kind of chemisorption is weak. Nevertheless the nitric oxide molecule undergoes a pronounced change in its electronic structure which is reflected in the large shift of its vibration frequency on chemisorption. In addition to this type of binding the bands at 1805 and 1735 cm.-l are characteristic of a double bond between nitrogen and oxygen and the nitric ,oxide molecule might perhaps be bound to the surface as ,N=O. The spectra in the case of chromium like that of iron indicate the occurrence of more than one type of binding.(ii) Oxides. Here Fe,O and Cr,O were used as gels but NiO was prepared by the thermal decomposition of the nitrate. The spectrum of nitric oxide chemisorbed on Fe203 gel is summarised in Table 6. TABLE 6. Frequencies (in cm.-l) of bands of NO adsorbed on Fe,O gel. Treatment 1927 1865 1806 1770 1738 On standing in contact . with NO (12 hr.) Increase in intensity Desorption at 20" Dis- Relative Dis- Relative appears decrease appears decrease in intensity in intensity On admission of Disappears Disappears oxygen Desorption on heating and a broad intense band appears at 1620 cm.-l Disappear at 150" v L- I Bands at 1698 1660 and 1625 cm.-l disappear at 350" The bands here may be classified into two types according to their behaviour on desorption.Thus the group of bands above 1700 cm-l are removed at 150" whereas the group below 1700 cm.-l disappear only at 350". That this classification corresponds to two different types of adsorp- tion centre is reinforced by the fact that oxygen acts only on the first 38 W. Hieber and A Jahn 2. Naturforsch. 1958 13b 195. 39 W. G. Burns and H. J. Bernstein J . Chem. Phys. 1952 20 380. 394 QUARTERLY REVIEWS group of bands. Tarte's results40 indicate that the absorption bands of 0-N=O are found in the range 1610-1690 cm.-l and so the bands 1700 1665 and 1625 cm.-l are presumably to be ascribed to a covalent bonding of nitric oxide with oxygen atoms. Nitric oxide on silica and alumina gives bands below 1700 cm.-l but only in the case of the transition-metal oxides are bands found above 1700 cm.-l.Hence the bands above 1700 cm.-l are attributed to adsorption on metal cations of the oxides. In the case of transition-metal oxides then adsorption can occur on both cations and anions and binding to the former may be either ionic or covalent. It is noteworthy that in no case is there any spectroscopic evidence for the formation of NO-. In this respect the behaviour of nitric oxide is very similar to that of carbon monoxide which is always chemisorbed with electron transfer from the gas to the solid. (B) Mechanism of Chemisorption.-During chemisorption the adsorbate undergoes chemical change which usually results in its dissociation into independent fragments. Consequently any discussion of the mechanism of chemisorption should consider not only the nature of the surface bonding but also the nature of the new species formed during chemi- sorption and it is with regard to the latter that infrared studies might prove most useful.For example the chemisorption and hydrogenation of ethylene has proved particularly perplexing and any information derived from a spectroscopic study of the chemisorption would be pertinent in unravelling this particular heterogeneous r e a ~ t i o n . ~ l * ~ ~ Chernisorption and hydrogenation of ethylene. When ethylene is catalytically hydrogenated there also occurs an exchange reaction between a hydrogen atom of ethylene and gaseous hydrogen43 which is readily demonstrated by the use of deuterium instead of light hydrogen. Farkas and consider these two reactions as occurring independently of each other and picture the hydrogenation as consisting of simultaneous addition of two atoms of the same hydrogen molecule adsorbed on the surface of the catalyst to a presumably physically adsorbed ethylene molecule C2H4 + H* + H* + C2HG On the other hand46 they regard the catalytic exchange as involving a dissociative mechanism according to which the hydrocarbon is split on the surface of the catalyst into a hydrogen atom and a hydrocarbon radical 40 P.Tarte J. Chem. Phys. 1952 10 1570. I1 D. D. Eley Quart. Rev. 1949 3 209. J. K. Laidler Catalysis 1954 1 168. 43 A. Farkas L. Farkas and Sir Eric Rideal Proc. Roy. SOC. 1934 A 146 630. 44 A. Farkas and L. Farkas Trans. Furuduy Soc. 1937 33 827. 45 A. Farkas and L. Farkas Trans. Furuduy SOC. 1939,35 906,941.46 A. Farkas and L. Farkas J. Amer. Chem. SOC. 1938,60 22. CRAWFORD INFRARED SPECTRA OF ADSORBED GASES 395 and subsequent reunion of the radical with a deuterium atom. The dis- sociative mechanism for exchange may be formulated as H CH2 II D CH2 II C2H4 4 CH + I CH + I -+ CHa=CHD Ni Li I Ni Ni In contrast to this view Horiuti and Polanyi*’ consider ethylene to be adsorbed “associatively” i.e. by opening of the double bond. Addition of a hydrogen atom then leads to formation of the “half-hydrogenated state”. This may either lose one hydrogen atom or take up a second thus leading to either exchange or hydrogenation. According to this picture exchange and hydrogenation are alternative processes of the same primary reaction viz. that resulting in the half-hydrogenated state.The views of Horiuti and Polanyi may be written CHZ - CH2 D CH2D CHZ-CHD H I I ’ ’ + ki 4 CH2 -+ Ni ?!Ji + l!Ji Ni NI C,H* -+ I Ji Here the deuterium atom adds to give an ethylnickel radical which breaks up to form an adsorbed ethylene molecule and liberate a hydrogen atom. Since these views were propounded much effort has been devoted to the hydrogenation and exchange reactions of ethylene with the object of discriminating between the dissociative and the associative mechanisms or some modification of them. Thus Twigg and Sir Eric Ridea148 claimed that their results on ethylene hydrogenation and deuterium exchange at nickel surfaces could best be explained if the double bond opened on chemi- sorption giving a complex bound by two-point attachment. Subsequent work49 seemed to confirm this result.During the course of time it has become clear that the reaction is far from simple but the efforts of Beeck and his colleague^^^^^^^^^ have done much to elucidate the nature of the chemisorption of ethylene. By using nickel films it was shown that if ethylene is carefully admitted so as never to build up an excess pressure one ethylene molecule is adsorbed per four nickel sites. Admission of excess of ethylene leads to appearance of ethane in the gas phase. Since the hydrogen for this hydrogenation could only have come from the ethylene the data were taken to mean that the primary adsorption is a dissociation of ethylene into acetylenic residues (occupying two sites) and two hydrogen atoms (each occupying one site). Excess of ethylene immediately removes pairs of *H atoms leaving pairs of empty 47 I.Horiuti and M. Polanyi Truns. Furuduy Suc. 1934 30 1164. 48 G. H. Twigg and Sir Eric Rideal Proc. Roy. Suc. 1939 A 171 55. 49 G. K. T. Corn and G. H. Twigg Pruc. Roy. Suc. 1939 A 171,70. 50 0. Beeck Rev. Mud. Phys. 1945 17 61. 61 0. Beak Discuss. Furday Suc. 1950 8 118. 396 QUARTERLY REVIEWS sites and the formation of ethane. The recent work of Jenkins and Sir Eric Ridea152 has confirmed the results of Beeck et al. that most of the surface is covered with acetylenic and polymerised acetylenic radicals i.e. that ethylene undergoes dissociative adsorption. At the present time there is no general agreement as to the mechanism of the hydrogenation or even of the way in which ethylene is chemisorbed. The reason for this is due partly to the complicated nature of the reaction and partly to the difficulty of devising a "one-result" experiment which will enable an unambiguous decision to be made in favour of one of the competing points of view.Consequently the Reviewer looks forward to the contribution which infrared spectroscopy of the surface phase can make towards the solution of this stubborn problem. The work of Eischens and P l i ~ k i n ~ ~ . ~ ~ shows that in the case of ethylene chemisorbed on Cabosil-supported nickel either dissociative or associative chemisorption may occur depending on the prevailing experimental conditions. Fr e 4 nc y (c m:' ) 3000 2 5 0 0 1500 1400 ' I 1 I 100- 9 0 B - n .- v) v) I- 9 0 - A d 6o - 3-5 4.0 6 . 5 7 . 0 Wave I ongt h ( p ) FIG. 4. Infrared spectra of (A) ethylene chemisorbed on hydrogen-covered nickel and (Figs.4 5 and 6 are reproduced by permission from R. P. Eischens and W. A. Pliskin (a) The adsorbent was a Cabosil-supported nickel (9-2 weight %) which had been reduced at 350" and cooled and the hydrogen had been pumped out for five minutes at room temperature. It is assumed that under these conditions the nickel remains covered with adsorbed hydrogen. The spectrum obtained when ethylene is chemisorbed on such a surface is shown in Fig. 4A. The C-H stretching bands occur in the region 2967- (B) the same after treatment with hydrogen. Adv. Catalysis 1958 10 1. 52 G. T. Jenkins and Sir Eric Rideal J. 1955 2490. 53 R. P. Eischens and W. A. Pliskin A h . Catalysis 1958 10 1. 54 R. P. Eischens and W. A. Pliskin J . Chem. Phys. 1956 24 482. CRAWFORD INFRARED SPECTRA OF ADSORBED GASES 39' 2857 cm.-l in saturated hydrocarbons and above 3003 cm.-l in olefins.Consequently the bands in the 2890 cm.-l region of spectrum A are assigned to C-H stretch vibrations for groups in which there are no double bonds on the carbon. The band at 1447 cm.-l is assigned to C-H deforma- tion which changes the H-C-H angle. Its occurrence shows that there are at least two hydrogen atoms on the carbon. Hence the combined presence of bands characteristic of C-H stretch and HCH deformation implies that in this case the chemisorption is associative. When the chemisorbed ethylene H2$-5H2 (asterisks representing a bond to the adsorbent) shown in spectrum A of Fig. 4 is treated with 2 mm. of hydrogen at 25" spectrum B results. Treatment with hydrogen therefore produces a shift of the 2890 cm.-l band whereas the 1447 cm.-l band is increased in intensity and displaced to 1458 cm.-l.The most significant change however is in the appearance of the small but sharp band at 1379 cm.-l. This is assigned to the symmetrical CH deformation and the increase in 1447 cm.-l intensity is explained as being due to a super-position of the CH asymmetrical deformation on the CH deforma- tion. These effects imply the presence of the half-hydrogenated state CH,CH, i.e. ethyl radicals. Thus the information contained in Fig. 4 provides spectroscopic evidence for the theory of Horiuti and Polanyi.*' (b) When ethylene is adsorbed on "bare" nickel obtained by evacuation of the hydrogen at 350" for hour or on hydrogen-covered nickel at 150" spectrum A of Fig.5 is obtained. By comparing Figs. 4 and 5 it will be seen that the intensity of the C-H bands in A of Fig. 5 is small compared with those of the associatively chemisorbed ethylene shown in A of Fig. 4. When ethylene chemisorbed on bare nickel is treated with hydrogen at 35" the band intensities increase as shown in Fig. 5B and this would seem to indicate that the species A is the spectrum of dissociatively chemisorbed ethylene. Pickering and E c k ~ t r o m ~ ~ recently obtained infrared spectra in reflec- tion of ethylene adsorbed on rhodium and nickel films. The metals were evaporated on to the mirrors of a multiple reflection cell,56 and scans taken before and after adsorption. It was found that when ethylene is added in excess to new rhodium mirrors it is adsorbed with dissociation to produce ethane.The spectrum of acetylene chemisorbed at 35" on either a hydrogen- covered or a bare nickel surface is shown in Fig. 6. This spectrum is similar to that of Fig. 4B which was assigned to the half-hydrogenated ethylene. The same spectrum was also obtained when acetylene was chemisorbed on a deuterium-covered surface. It seems then as if chemisorption of acetylene could involve a self-hydrogenation process and Pliskin and Eischens point out that the infrared evidence is consistent with the work 55 H. L. Pickering and H. C . Eckstrom J. Phys. Chem. 1959 63. 512. 56 J. U. White J . Opt. SOC. Amer. 1942 32 285. 398 QUARTERLY REVIEWS Frcrquency (cm-') 2800 6o t 3.2 3.3 3.4 3.5 3-6 Wavelength ( p ) FIG. 5. Spectrum of (A) ethylene chemisorbed on bare nickei and (B) the same afrer treatment with hydrogen.F r equ en c y (c m? 3000 2 5 0 0 1500 I 4 0 0 I I I J 3.0 4.0 6 - 5 7 - 0 Wove I engt h ( p Fro. 6. Spectrum of (A) acetylene chemisorbed on nickel and (B) the same after treat- ment with hydrogen. CRAWFORD INFRARED SPECTRA OF ADSORBED GASES 399 of Douglas and Rabin~vitch.~’ These authors found that deuteroacetylene was self-hydrogenated on nickel supported on kieselguhr to a mixture of deuterated ethylenes the greater proportion of which is C2D,. This seems to indicate that the effect is independent of any hydrogen which might have remained on the surface as a result of preparation of the catalyst. However since residual surface hydrogen after reduction is always a problem the present conclusion must be that evidence for self-hydrogena- tion is indicative rather than rigorously convincing.(C) Effect of the Support.-Almost all the work to date has been on adsorbents which have been supported on silica or alumina. That the support is not inert has been known for some time. For example S e l ~ o o d ~ ~ noticed that the concentration of electron holes in nickel oxide films can be increased if the films are supported on y-alumina. Similarly HiittigS9 showed that zinc oxide and ferric oxide as single oxides are poor catalysts for some hydrogenation-dehydrogenations yet when they are combined their catalytic activity is markedly increased. Independent spectroscopic evidence of this activity of the support is provided by Terenin and roe^^^ for the case of oxide adsorbents. Thus nitric oxide adsorbed on nickel oxide gives a band at 1805 cm.-l but when nitric oxide is chemisorbed on nickel oxide dispersed in alumina gel this band is shifted to 1850 cm?.More pronounced changes are found in the spectrum of nitric oxide chemisorbed on mixed Fe,O,-Al,O gel. In addition to the bands at 1927 1938 and 1806 cm.-l found with ferric oxide gel alone new bands appear at 1980 and 2125 cm.-l indicating the presence of new adsorption centres. This is not very surprising for since the oxide adsorbents and supports are semiconductors the support can be expected to affect the nature and concentration of defects in the oxide adsorbent. The “inert” support is also known to affect the catalytic activity of a metal adsorbent dispersed in it. Confirmatory spectroscopic evidence for this is provided by Terenin and R o ~ v ~ ~ who deposited iron from iron pentacarbonyl on aluminium oxide and also on zinc oxide and nickel oxide.The nitric oxide band at 2008 cm.-l for the Fe203-A1,03 system was found at 1985 cm.-l for the Fe-NiO system indicating that there is indeed an interaction between the support and chemisorbed nitric oxide via the iron atom. When nitric oxide is adsorbed on iron dispersed on zinc oxide instead of the one band there are now two one being at 2040 and the other at 1915 cm.-l. It is evident that the zinc oxide is far frominert and the support can markedly influence the metal sites. Eischens and PliskinM show that for chemisorption of carbon monoxide on supported metal adsorbents changing the carrier can bring about 67 J. E. Douglas and R. S. Rabinovitch J. Amer. Chem.Soc. 1952 74 2486. 68 P. W. Selwood Bull. SOC. chirn. France 1949 489. G. F. Hiittig Discuss. Faraday Suc. 1950 8 215. O0 J. Sheridan J. 1945 470. 400 QUARTERLY REVIEWS very marked changes in the spectrum of chemisorbed carbon monoxide. The effects observed by them are shown in Fig. 7. Changing the carrier from silica to y-alumina results in (i) a displacement to lower frequency of the band due to linear carbm monoxide and (ii) a large increase in the intensity of the band due to bridging carbon monoxide. Fig. 8 shows that a similar result is obtained when hydrogen is added to carbon monoxide chemisorbed on platinum supported on silica.21 In this J) .,_,---- 6 0 - 0 b I I 20 00 8 0 - 6 0 - 4 0 - I I 4.5 5 . 0 5 . 5 Wavelenqth ( A ) FIG. 7 . Spectrum of carbon monoxide chemisorbed on (A) silica-supported and (B) (Figs.7 and 8 are reproduced by permission from R. P. Eischens and W. A. Pliskin alumina-supported platinum. Adv. Catalysis 1958 10 1. case however the effects are not nearly as marked as those demonstrated in Fig. 7. Since no bands are produced in the 3700 cm.-l region (OH) or near 2800 cm.-l (C-H region) this negative evidence might be taken as indicat- ing that no significant amounts of HO)C'Pt H or o\ H/C-Pt are formed. In addition fdrmation of these structures would diminish the intensity of the band in the 2040 cm.-l region due to linear carbon monoxide. This occurs to a slight extent but is accompanied by a simultaneous increase in CRAWFORD INFRARED SPECTRA OF ADSORBED GASES 40 1 the band at approximately 1840 cm.-l due to bridging carbon monoxide.It seems then as if addition of hydrogen has not decreased the amount of carbon monoxide chemisorbed but has converted some linearly bonded carbon monoxide into the bridging form. The material reviewed here indicates the potential value of studying the infrared spectra of adsorbed gases. So far results have been obtained almost exclusively for gases either physically adsorbed on high-area silica or chemisorbed on silica-supported metals and metal oxides. Since spectro- scopic evidence has been presented which indicates that the support is not inert it becomes imperative to determine to what extent the observed effects can be attributed to interaction between the adsorbate and the adsorbent. This will have to be done before the spectroscopic results obtained for gases adsorbed on supported metals can usefully be compared with the results obtained on evaporated metal films. Despite this it can safely be predicted that infrared spectroscopic studies will increase in the future and advances will probably come in the follow- ing three directions viz. (i) increasing the frequency range available for study (ii) the obtaining of high-resolution spectra and (iii) the obtaining of fairly reliable intensity measurements. Attainment of the last objective will not be easy but a start has been made in this direction.61 The writing of this Review was supported in part by the U.S. Air Force. 61 L. H. Little J. Phys. Chem. 1959 63 1616.