C. W. GARLAND, R,. C. LORD,AND P. F. TROIANO
1188
An Infrared Study of High-Area Metal Films Evaporated in Carbon Monoxide
by C. W. Garland, R. C. Lord, and P. F. Troiano Department of Chemistry, and Spectroscopy Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts (Received October 1 , 1964)
A new technique has been developed for the preparation of porous, high-area, unsupported metal films which are suitable for an infrared absorption study of chemisorbed CO. In essence, the method involves the flash evaporation of a metal in the presence of CO gas. Spectra of CO chemisorbed on platinum, palladium, and rhodium have been obtained in this way. The results are compared with those reported previously for reduced, supported samples and for vacuum-evaporated films; in general, the bands observed in the 1800-21OO-~ni.-~ region are in good agreement with those seen on supported samples. In the case of platinum, two bands were also observed in the 300-500-cni.-’ region.
Introduction Infrared spectroscopy has been widely applied to the study of the cheniisorption of CO on transition metals. In all such studies, the main experimental problem has been that of preparing samples with high adsorptive capacity and acceptable infrared transmission. There have been two general approaches to this problemthe use of supported metal s a m p l e ~ l -and ~ of evaporated metal film^.^-^ Although both approaches have led to much valuable information about the nature of chemisorbed species, the techniques which are currently used in each of these approaches involve some undesirable features. With supported samples, the very presence of a supporting medium leads to difficulties in spectral interpretation. The effect of different supporting materials on the spectra of CO chemisorbed on nickel has been investigated by O’Neill and Yates.a This study showed large pressure-dependent variations in the ratio of linear to bridged CO species and significant changes in the strength of binding of both species, depending on whether silica, alumina, or titania supports were used. So far, however, no way has been devised for quantitatively assessing the effect of the presence of any of these supports on the spectrum of chemisorbed CO. A second complicating feature of the usual supported-sample preparation is the use of hydrogen for reducing the metal. This hydrogen may dissolve in the metal and thus constitute an impurity that may alter the electronic properties of the metal. Such electronic changes might then be expected to produce The Journal of Phusical Chemietry
observable spectral changes. This point has been well demonstrated in the case of CO adsorbed on nickel, for which Eischens4 has found pronounced changes in the ratio of linear to bridged CO species depending upon how thoroughly the hydrogen has been removed from the metal after reduction. Another limitation of most supported samples is infrared absorption by the supporting material, especially a t low frequencies. There have been even greater difficulties in the use of evaporated metal samples. In two of the techniques which have been developed to date the sample is exposed to air before CO is cheniisorbed.’,j I t is clear that such samples are contaminated to some unknown extent. Blyholder6 has evaporated metals onto infrared cell windows which were coated with vacuumpump oil to produce a highly dispersed evaporated deposit. This technique, however, produces filnis which may be contaminated and are, in a sense, supported. The work of Pickering and Eckstrom’ on the (1) R. P. Eischens and W. A. Pliskin, AdEan. Catalysis, 10, 1
(1958). (2) A. C. Yang and C . W. Garland, J . Phys. Chem., 61, 1504 (1957). (3) C. E. O’Neill and D. J. C. Yates. ibid., 6.5, 901 (1961). (4) R. P. Eischens in “The Surface Chemistry of Metals and Semiconductors,” H . C. Gatos, Ed., John Wiles and Sons, Inc., New York, N . Y., 1960, p. 521. (5) J. B. Sardisco, Perkin-Elmer Instrument .Veu)s, 15, No. 1. 13 (1963). (6) G. Blyholder, J. Chem. Phys., 36, 2036 (1962). (7) H . L. Pickering and H . C. Eckstrom. J. Phys. Chem., 63, 512 (1959). (8) R. A. Gardner and R. H . Petrucci, J. A m . Chem. SOC.,8 2 , 5051 (1960).
INFRARED STUDY OF HIGH-AREA METALFILMS EVAPORATED IN CARBON MONOXIDE
reflectance spectrum of CO adsorbed on vacuum-evaporated nickel and rhodium mirrors represents the best attempt to study “clean” metal surfaces. The spectra recorded for CO on their rather thick films were, however, quite different from those obtained on supported samples1f2and are presumably characteristic of adsorption on the bulk nietals rather than on higharea, small-particle deposits which are of greater catalytic interest. In view of the various difficulties mentioned above and the somewhat discordant results which have been obtained with different techniques, it seemed desirable to develop a new evaporation technique which would make it possible to record transmission spectra of CO Chemisorbed on high-area, unsupported metal samples which were free from contamination by either hydrogen or air. Obviously, mirror-like films would suffer from high reflection losses and would have adsorptive capacities which were too low to be of much use. Fortunately, the preparation of porous metal films analogous to those described by Beeck and his co-workerss and by Harris and Beasley’O offers a way of obtaining films which have good infrared transniission properties and high specific surface areas. A technique for preparing porous metal films suitable for infrared studies of chemisorbed CO has been developed. This technique involves evaporating a metal in the presence of CO gas and is, to our knowledge, conipletely new. Hayward and Trapnellll have briefly mentioned another, perhaps similar, method which is being investigated by Eberhagen, Hayward, and Tompkins. No details of their method have been published, and a comparison of our techniques and results with theirs is not yet possible. Spectra of CO adsorbed on platinum, palladium, and rhodium are presented in this paper, and a more detailed infrared study of CO chemisorbed on nickel is reported in the following paper. l 2 These transition metals were selected for their catalytic interest and also because infrared spectra had previously been obtained on these systems with one or more of the older methods cited previously.
Experimental Method Cell. The design of the infrared cell used in this work is illustrated in Figure 1. The body consists of a 34/45 standard-taper joint to which a short length of 22-mni. tubing has been attached. The wall of the 2%”. tubing was blown out in such a way as to produce two 15 X 20 mni. openings. The rims of these openings were ground optically flat and parallel so that CaF2 or CsI windows (A) could be sealed on with either clear glyptal or picein cement. The
1189
cm
I
Figure 1. Cell for infrared studiee of chemisorbed CO.
outer part of the standard-taper joint contains a well throcgh which heavy tungsten leads (C) are introduced into the cell. Evaporation Techiiques. Tungsten filaments (B) were prepared from wire in the form of a coil about 2 cm. in length and 5 nini. in diameter. The spacing between turns was approximately 2 nini. The wires a t the ends of the coil were brought up parallel to the coil axis. Two centimeters above the coil these wires were wound in a tight spiral to perniit a press-fit over the heavy tungsten leads in the cell. These filaments were charged with the appropriate metal for evaporation either by placing snlall loops of the metal on each of the turns of the tungsten coil or by ‘Lweaving’’ short lengths of metal wire along the coil. With nickel and palladium a few sniall loops of the wire on the filament were sufficient for several evaporations. Platinum and rhodium, on the other hand, required (9) 0. Beeck, A . E. Smith, and A. Wheeler, PTOC. Roy. SOC.(London),
A177, 62 (1940). (10) L. Harris and J. K. Beasley, J . O p t . SOC.A m . , 42, 134 (1952). (11) D. 0. Hayward and B . M. W. Trapnell. “Chemisorption.” 2nd Ed.. Butterworth, Inc.. Washington, D. C . , 1964. (12) C . W. Garland, R. C. Lord, and P. F. Troiano, J . Phys. Chem., 69, 1195 (1965).
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the weaving technique so as to produce a greater metal area during evaporation. During a typical run, the cell was assembled and evacuated at room temperature to a pressure of low6 nim. The filament was then slowly heated to drive off adsorbed or entrapped gases. (The temperature was raised in a series of small steps to prevent warping of the filantent coil.) Pumping and heating were continued until a pressure no greater than 5 x nim. was obtained a t a temperature just below the point a t which the metal would begin to melt and evaporate. During these final stages in the degassing procedure it was often necessary to turn the filament off for short periods of time to prevent cracking of the cell windows or rupturing of the window seals. After degassing, the filament was cooled to room temperature and the cell was pumped to a final pressure of nmi. Netal films were evaporated under vacuum or in the presence of small amounts of argon or GO. The evaporation was accomplished by presetting a power supply to give approximately 18 amp. through the filament for a potential drop of 20 v. The power was then switched on for short periods, and the metal was flash-evaporated onto the cell walls and windows. With nickel and palladium a single evaporation of about 2-4 sec. duration was sufficient to deposit a suitable film. Platinum and rhodium, on the other hand, required many 1-2-sec. evaporations to build up a deposit . Materials. The CO gas used in this study was Matheson chemically pure (min. 99.5% CO) and was used without further purification. Platinum, rhodium, and palladiuni wires, stated to be 99.9% pure, were obtained from Engelhard Industries Inc., Newark, N. J. Wire diameters ranged from 0.0127 to 0.0254 cm. Tungsten wire of 0.0508-cm. diameter was obtained from the General Electric Co. Recording of Spectra. The spectra of adsorbed gases were recorded on a Perkin-Elmer Model 421 doublebeam grating spectrometer in the region from 4000 to j50 cni.-]. Later in this work the range was extended to 300 cin.-l using a Model 521 Spectrometer. The spectrometers were flushed with dry nitrogen to remove atmospheric water vapor. No modifications in the instruinents were required, and the spectra were generally recorded under normal operating conditions. In a number of cases, spectra were recorded with the use of an accessory which permitted the yo transmission scale to be expanded as much as 20-fold. Scale expansions of fivefold proved extremely valuable in studying contours of relatively weak bands. Electron Micrographs. A number of Pt films were The Journal of Physical Chemistry
C. W. GARLAND, R. C. LORD,A N D P. F. TROIANO
inspected using a Siemens Elmiskop I electron microscope. The beam voltage was typically less than 80 kv., and photographs of the metal deposits were most commonly made a t a magnification of 30,OOOX. Metal films were evaporated onto Parlodion films cast over nickel electron-microscope grids. Several of these grids were laid on a window of the infrared cell, the cell was mounted in a horizontal position, and the metal evaporation was carried out in the usual manner. After evaporation, the grids were removed from the cell and rapidly placed in the microscope for study.
Results and Interpretation Metal films evaporated under high vacuum or in the presence of small amounts of argon were found to be unsuitable for infrared studies of adsorbed molecules. Thin films prepared a t loe6 nim. were mirror-like and almost opaque to infrared radiation. Subsequent additions of CO gas did not produce detectable bands ascribable to chemisorbed CO. Films evaporated in argon a t pressures between and 50 nim. were soot-like in appearance and transmitted much more infrared radiation than the vacuum-deposited films. Removal of the argon and admission of CO produced detectable bands in the region of 2000 cm.-'. Unfortunately, these bands were extremely weak and, thus, unsuitable for further study and interpretation. Another complication was the presence of interference fringes in the spectra of these samples. Films evaporated in the presence of CO gas were found to have desirable properties for infrared absorption studies. In this case reactive collisions in the gas phase are evidently quite efficient in producing a fine particle size and, hence, a high surface-to-volume ratio. P l a t i n u m Films. Results. Platinum films have been evaporated in CO a t pressures ranging from to 12 mm, and the resulting spectra have been recorded from 4000 to 300 cm.-'. The range from 2 to 12 mm. was found to be the most useful in preparing films which exhibit good infrared transmission and CO adsorptive properties. Films prepared a t these pressures were found to be relatively stable and showed no important spectral changes on prolonged standing (i.e., several days). Spectra of CO Chemisorbed on a Pt film evaporated in 2 mm. of CO are shown in Figure 2. Spectrum 2-1 shows the original spectrum of CO, which appears to consist of only two bands. The more intense of these occurs at 2053 f 2 cm.-', and the other, which is quite weak, appears a t 1840 f 10 cm.-l. The assignment of a large uncertainty to the frequency of the latter
INFRARED STIJDYOF HIGH-AREA METALFILMS EVAPORATED IN CARBON MONOXIDE
90
.
80
0
-4
3
0
E! 70
6C
2000
Frequency
1800 (cm-')
Figure 2. Spectra of CO chemisorbed on a Pt film: ( 1) original spectrum recorded after evaporation
of film in 2 mm. of CO gas; ( 2 ) sample pumped 15 min., (3) sample pumped 25 min.
band has been made because the band is sensitive to sample preparation, and, accordingly, its position varies froni film to film. In addition, the band is so weak m d broad that even with scale expansion it is difficult to locate the band center accurately. Although it is not apparent froni the spectra presented here, there is also some evidence of a band a t approximately 2080 f 5 cni.-'. The band a t 2053 cm.-', which is symnietrical a t high coverages, develops a definite asymmetry when large amounts of CO are desorbed by pumping, and a very weak shoulder can be observed at 2080 cm. -l in a number of spectra. The appearance of this shoulder cannot, however, be considered typical of the spectra recorded in this study. Spectra 2-2 and 2-3 show the results of pumping for 15 and 25 niin., respectively. Desorption causes a shift in the band maxima toward lower frequencies. For the sample used in Figure 2 the shift amounted to 15 cm. - l , and the background transmission a t 2000 cm.-' decreased from 42 to 25% after a total pumping time of 25 min. All I't films prepared in this study, regardless of whether evaporated in 2 or 12 nini. of CO, produced spectra similar to those of Figure 2 ; i e . , no new bands were observed when the CO pressure was varied. Occasional diff wences in band contours and frequencies
1191
owing to a strong Christiansen filter effectI3 have been noted. This effect is characterized by a rapid increase in the transmission when the main absorption band is first reached and by a shift in the bands to lower frequencies by as much as 20 cm.-'. No correlation between the CO pressure and the magnitude of this effect has been found. Platinum films have also been examined in the region from 600 to 300 cm.-' in an attempt to observe a frequency ascribable to the Pt-C stretching vibration. In most cases the transmission of the filnis was between 40 and 60% at 500 cni.-', and, in spite of several experimental daculties, bands ascribed to adsorbed CO have been observed. Bands in this region were found to be extremely weak and in all cases were recorded with expansion of the transmission scale. The results obtained on a film evaporated in 2 mm. of CO are presented in Figure 3. Here the spectrum consists of a weak broad band a t 570 f 5 em.-' and a weak but sharper band a t 477 f 2 cm.-'. All Pt films studied clearly showed the band a t 477 em.-', but in many cases the band a t 570 cm.-l was partially obscured by the sloping background. Spectral data are very difficult to obtain in this region during desorption, but it does appear that these bands are removed by pumping at about the same rate as those shown in Figure 2. In order to characterize the metal samples, electron micrographs have been obtained for several P t films evaporated in 2 mm. of CO. In all these films, the platinum appears to consist of small aggregates f f particles with diameters of approximately 45 A. Scanning such filnis with the microscope showed that the deposits were quite uniform and free from holes. This uniformity unfortunately made it extremely dif-
54
t
"r
L
2
L 600
5 00
400
Frequency (em-')
Figure 3. Low-frequency spectrum of CO chemisorbed on a Pt film evaporated in 2 mm. of CO gas.
(13) W. C. Price and A . N. Tetlow,
J. Chein. P h y s . ,
16, 1157 (1948).
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ficult to find and measure single isolated particles even on the thinnest films. Because of this and limited resolution, it is not unreasonable to assign an uncertainty of f 10 8 . to the particle diameter. Interpretation. Eischens and his co-workers' have reported the spectra of CO chemisorbed on a number of different types of Pt samples. Silica-supported samples show a relatively intense band at 2070 cm. -l and rather weak absorption near 1850 cm.-'. Alumina-supported samples, on the other hand, produce a broader band at 2050 em.-' and a more intense band near 1820 cm.-'. Spectra of CO adsorbed on Pt in KBr pellets and on evaporated films are also described in Eischens' review. The pelleted samples show bands at 2000 and 1850 c11i.-~,in addition to a number of bands resulting from COz and water contamination. Evaporated films (made in an apparatus designed for preparing electron microscope samples and thus presumably vacuum deposited) show a single strong band a t 2050 cm.-' and only a very slight indication of absorption in the region around 1800 cm.-'. Bands at 2000 cm.-' and above were assigned by Eischens, et al.,' to a linear CO species bonded via the carbon atom to a single metal atom. Bands in the 1800-1850-cm.-' region were assigned to a bridged carbonyl species between two adjacent metal atoms. The present results agree well with previous spectra of both supported and evaporated films although the relative band intensities agree more closely with those obtained on supported samples. The principal difference between our platinum deposits and supported samples is the ease with which our films "sinter" on desorption of CO (see also the more detailed data on nickel films12). As mentioned earlier, two low-frequency bands were found a t 570 and 477 cm.-'. Unfortunately, no data on supported samples or vacuum-evaporated films are available in this region for comparison. (The fact that silica and alumina have poor transmission in this region makes studies on supported samples virtually impossible.) Pelleted P t samples' have been examined and show a band a t 476 cni.-'. Force constant calculations1$ based on the isotopic frequency ratio for 12CO and 13C0on P t suggest that the Pt-C stretching frequency should lie in this region. Hence the band observed at 477 cni.-l is ascribed to the Pt-C stretching frequency. The band appearing at 570 cm.-' has not been previously reported and may represent a bending mode of chemisorbed CO. Since both the C-0 and Pt-C stretching frequencies are known, it is possible to calculate approximate force constants for bond stretching and obtain some idea of bond orders. If valence forces and a negligible interaction between the Pt-C and the C-O bonds are asThe Journal of Phgeical Chemietry
C. W. GARLAND, R. C. LORD,AND P. F. TROIANO
sumed, the calcuiation of the force constants can be made by analogy with Herzberg's treatment for the linear XYZ m01ecule.l~ If the mass of the Pt atom is further assumed to be infinite, values of 15.6 and 4.1 mdynes/A. are obtained for the C-0 and Pt-C bonds, respectively. The isotopic-shift measurements of Eischens'* indicated force constants of 16 and 4.5 mdynes/8. These values would indicate considerable double-bond character to the Pt-C bond, in agreement with the high degree of double-bond character found by Blyholdef for CO Chemisorbed on iron. Palladium Films. Results. The spectrum of CO chemisorbed on palladium has been studied with films evaporated in CO at pressures from 5 to 20 mm. Films prepared a t lower pressures were found to be unsuitable because they produced interference fringes in the region of interest. The most useful spectra were recorded on films which were somewhat thicker than the P t films and typically transmitted between 20 and 40y0 at 2000 cm.-'. The transniission of Pd films was found to increase with wave length faster than that of the other films. The spectrum of CO chemisorbed on a Pd film evaporated in 9 nun. of CO is shown in Figure 4. Here the spectrum consists of four bands located at 2085 f 5, 1970 f 5, 1910 f 5, and 1840 f 5 cni.-'. Regardless of the CO pressure used during evaporation, all
'
L -1 1 2000
I800 Frequency ( c m " )
Figure 4. Spectrum of CO chemisorbed on a Pd film evaporated in 9 mm. of CO gas. (14) R. P. Eischens, S. A. Francis, and W. A. Pliskin. J . P h w . Chem., 60, 194 (1956). (15) G. Herzberg, "Infrared and Raman Spectra," D. Van Nostrand Co.. Inc., Princeton, N. J., 1955.
INFRARED STUDYOF HIGH-AREA METALFILMS EVAPORATED IN CARBON NOSOXIDE
of the spectra obtained in this study are similar to that given in Figure 4 in that no new features appear as the pressure is varied. The band intensities are soniewhat variable, and the band at 1840 ciii.-l has been observed to be as intense as the 1910 cm.-' band. Desorption studies have been attempted on a nuniber of Pd films; however, transmission losses occurring during the desorption of CO have made it impossible to obtain iiieanirigful data. Studies in the region from 600 to 300 c m - ' were also undertaken without success. Interpretation. The spectrurii shown in Figure 4 is i n general agreement with those obtained by Eischens, et al., l 4 on silica-supported Pd samples. In their study, the spectrum at full coverage showed bands appearing a t 2074, 1930, 1895, and 1845 em.-'. The band above 2000 c n r ' was assigned to a linear CO species, and the three bands with frequencies below 2000 cm.-' were attributed to bridged CO species. There was no discussion of possible differences among these bridged species or anlorig the Pd sites on which they were adsorbed. It does appear, however, from the adsorption-desorption data presented by Eischens, et al., that the bridged species giving rise to their bands a t 1895 and 1845 em-' are most tightly bound and are probably due to adsorption on crystalline sites. This would be compatible with the results of a recent study For such films, of vacuuin-deposited Pd bands due to chemisorbed CO were observed a t 2062, 190.5,and 1869 cm. -l. Thus, in ternis of frequency and relative intensity, our bands at 2085, 1910, and 1840 cm.-' are in reasonable agreenien t with previous work. The appearance of a second band assigned to a bridged CO species and with a frequency around 1900 em.-' has also been observed in the case of rhodium (see next section) and nickel.12 This rather high frequency for a bridged CO group could be rationalized by the familiar assumption that the SI-C-11 bond angle is much smaller than usual. Such a reduction in angle is more likely due to a lengthening of the Sl-C bond (caused by a change in hybridization of the metal orbital) rather than a shortening of the nietal-metal distance in the surface. The greatest interpretive difficulty involves the weak band we have found at 1970 cni.-'. It would seem reasonable to relate this band to the 1930-cni.-l band observed on silica-supported samples, except for the considerable frequency shift and the great difference in intensities. Also, there is no third "bridge" band observed in the 1980-1970-cm.-l region for vacuuni-evaporated filnis. I t is difficult to explain a bridged CO frequency as high as 1970 ciii.-l, afid our results indicate that such an assignment niay also be
1193
incorrect for the 1930-cm.-' band on supported samples. One might speculate on the presence of Pd surface sites which adsorb CO to form species approaching the bonding indicated by Pd=C-0. Such sites and the associated band frequencies might well be very sensitive to the method of sample preparation. R h o d i u m Films. Results. Rhodium has been evaporated in CO at pressures ranging froni 3 to 12 mm., and the spectra of the resulting filiiis have been recorded froni 4000 to 300 em.-'. The transmission of these films was in the range froni 40 to 60% a t 2000 cni. - l , and the background was essentially flat throughout the region investigated. Spectra of CO cheniisorbed on Rh fi!nis evaporated in 3 nini. of CO are shown in Figure 5. These spectra were recorded with an adjustable shutter in the reference beani to compensate partially for absorption owing to the cell and the fi'm. Somewhat different spectra have been observed on Rh films which were evaporated a t the same CO pressure. The most typical spectrum is labeled 1 in Figure 5 and will be discussed first.
40
ct 1
u 2000
1800
Frequency l c m - ' )
Figure 5. Spectrum of CO chemisorbed on a Rh film evaporated in 3 mm. of CO gas: (1) most typica4 spec trum observed; ( 2 ) another spectrum observed on sever I samples.
B
On the high-frequency side of spectrum li-1 there is a small sharp band a t 2111 f 1 cni.-' which is most easily observed and well resolved when the film is examined within a few hours of the time of preparation. In many cases, on longer standing this band Volume 69,.Vumber 4
April 1965
1194
appears as only a weak shoulder. The main absorption band occurs initially at 2058 f 5 em.-' and after a time shifts to 2055 f 5 cm.-l. There appear to be three bands in the region from 1900 to 1800 em.-'. The first of these occurs at 1905 f 5 cm.-' and is often the niost intense band in this region. The next band appears a t 1852 f 2 cni.-' and has been found to vary somewhat in intensity from film to film. Finally, there is a band appearing here as a shoulder at 1817 f 5 em.-'. Although this latter band is most often observed as a weak shoulder, it has been clearly resolved in at least one spectrum. There are also sonie indications of another weak band a t 2075 f 5 cm.-I, which is usually masked by the main absorption band but which has been seen as a shoulder in several spectra. Spectruni 5-2 is an example of another spectrum which has been obtained on a number of Rh films. In this case the band a t 2111 cni.-' is missing, and the In the niain absorption band is at 2050 f 5 1900- to 1800-cni.-' region the most intense band now appears at 1885 f 5 em.-' rather than 1905 f 5 cni. -l. Although niost of our spectra agree with either spectrum 5-1 or 5-2, other contours have been observed in the 1900- to 1800-cm-' region. This is due to a wide variation in the relative intensity of the various bands, which indicates that the nature of the adsorption sites is quite sensitive to sample preparation. These Rh films have also been studied extensively in the region from 600 to 300 cni.-l. Although there are indications of weak absorption froni approxiniately 57.7 to 400 cix-1, the quality of the spectra obtained was quite poor, and no bands definitely ascribable to cheniisorbed CO have been observed. InteTpretatzon. The results presented above are quite similar to those obtained by Yang and Garland2 on alumina-supported Rh. They studied both sintered and unsintered samples ranging froni 2 to 16% Rh by weight. The sintered samples were heated to 400' and were free from residual water adsorbed on the support, while the unsintered saniples were never heated above 200" and contained some residual water. In their study a species consisting of two linear CO niolecules bonded to a single Rh atom was proposed to explain the presence of a doublet band appearing a t 2040 and 2108 em.-' on sintered (dry) samples. A band observed on sintered samples with a frequency between 2045 and 2062 ~ 1 n - l depending on the CO coverage was ascribed to a normal linear species with one CO per Rh atom. Finally, a band at 1925 em-' for sintered samples was interpreted in ternis of a bridged species consisting of a CO bonded to two Rh atoms each of which was also bonded to a linear CO. The Journal of Physical Chemistry
C. W. GARLAND, R. C. LORD,AND P. F. TROIANO
A comparison of their results with those obtained here on Rh films evaporated in CO shows that the spectra obtained by Yang and Garland a t interniediate coverages on 8% samples most closely resemble our spectruni 5-1. The only serious discrepancy appears to be associated with the absorption below 2000 em.-'. In this regard it should be pointed out that Yang and Garland also noted that the absorption in this region was quite sensitive to sample treatment and that this was the least reproducible region of the spectrum. It does, however, appear that the band assignnients proposed for alumina-supported Rh are generally applicable to the present results. It is, therefore, proposed that the band appearing a t 2111 em-' in spectrum 5-1 is one coniponent of a weak doublet band ascribed to a species consisting of two linear CO niolecules bonded to a single Rh atom. An important difference in the spectra obtained on supported and evaporated f i l m is the much greater intensity of the 2108-em. -' band on supported samples. Since this band is associated with highly dispersed Rh sites, it is apparent that the evaporated deposits are relatively more crystalline. The band we have observed in the 2050-2058-~m.-~region can be easily assigned to the normal linear CO species. .4s one would expect from the weakness of the 2111-c1n.-~.component, there is no appreciable absorption near 2040 cni.-'. Thus, our linear CO band has a symmetric contour in contrast to the results on supported samples. Unfortunately, we have no grounds for specific assignments of the several bands below 2000 cm.-'. A variety of bridged CO species seeins to exist for Rh as well as for Pd and S i . Presumably, this variety depends on the existence of distinct surface sites which are sensitive to the method of sample preparation. Small changes have been noted in spectra siniilar to spectrum 5-1 when the sample was left standing for sonie time. These changes, particularly the loss of the band at 21 11 cm. -l, are consistent with a slow sintering process occurring in the Rh films even a t room temperature.
Summary Results obtained on metal films evaporated in the presence of CO indicate that, for the niost part, these filnis produce spectra of cheniisorbed CO which are reinarkably similar to those obtained on supported metal samples. Spectral differences noted among the various types of samples can be largely interpreted in terms of support effects, variations in metal structure, and sintering. In general, there is niuch less correlation between our results and those obtained in previous
IXFHARED SPECTRUM OF CARBON MONOXIDE CHEMISORBED ON NICKELFILMS
studies using vacuuin-evaporation techniques. In contrast to our results, no bands were observed below 1980 c m - l for vacuuni-prepared Rh films. In the case of Pd and I’t, thc results are in somewhat better agreenient, but, again, there are differences noted in bands observed below 2000 c ~ i i . - ~ . The probleni of filii1 sintering represents the major difficulty in obtaining the adsorptioridesorption data that are necessary for a detailed characterization of
1195
surface species. A possible solution to this problem would involve cooling the substrate to a point where sintering could largely be eliminated. In spite of this difficulty, the technique does offer the distinct advantage of permitting one to obtain spectra of chemisorbed CO on clean, unsupported nietal samples. A detailed study of CO on nickel films, presented in the following paper,12will indicate the kind of results possible in a favorable case.
Infrared Spectrum of Carbon Monoxide Chemisorbed on Evaporated Nickel Films
by C. W. Garland, R. C. Lord, and P. F. Troiano Department of Chemistry and the Spectroscopy Laboratory, ~vassachusettsInstitute of Technology, Cambridge, Massachusetts (Received October 1 , 1964)
The infrared spectrum has been investigated for CO cheniisorbed on nickel films which were evaporated in the presence of CO gas. For these unsupported samples, two limiting types of spectra were observed, depending on the CO gas pressure. When nickel is evaporated in 2 nim. of CO, the film is a dispersed, patchy deposit of 65-A. particles. Films evaporated in 12 mni. of CO consist of 200-A. particles connected in an open, chain-like network. The corresponding spectra are discussed in terms of these structural differences.
Introduction and Method Infrared spectra of GO chemisorbed on nickel have been reported previously for both supported and evaporated films. The earliest study was that of Eischens, Pliskin, and Francis1 using a nonporous silica support. The saniple contained about 8% Ni by weight and a t full coverage showed infrared absorption bands a t 2074, 2041, 1926, and approximately 1870 c111.-l. A more detailed study is that by Yates and Garland2 on aluniina-supported Si. In their work both adsorption-desorption studies arid variations in the metal content of the samples were used in the characterization of the surface species. At full coverage on samples containing 10% S i by weight, bands were observed at 2082, 20.57, 2035, 1963, and 1915 c1n-l. Sainples con-
taining 25% Xi by weight showed a decrease in the relative intensity of the bands above 2000 cni.-I when compared with the 10% samples. The band at 2082 cni. -I showed the most pronounced loss in intensity. Studies of CO chemisorbed on evaporated Ni films have been much less detailed than the studies on supported films. Reflectance spectra of CO on vacuumevaporated Ni films have been recorded by Pickering and Eckstroi~i.~In their work a doublet at 2050 and 2060 cm-l is reported, as well as a weak band a t (1) 60, (2) (3)
R. P. Eischens, S. A. Francis, and W. A. Pliskin. J . Phys. Chem., 194 (1956). J. T. Yates and C. W. Garland, ihid., 65, 617 (1961). H. L. Pickering and H. C. Eckstrom, ibid., 63, 512 (1959).
Volume 69, S u m b e r 4
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