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J. Phys. Chem. 1980, 84, 878-882
C reduced at 873 K. Since the Ni2+ions have two different environments (in the present sample about 15% are in the supercages with 40% of the La3+ions, and 85% are in the hexagonal prisms shielded from La3+ions) one may anticipate two different reducibilities. The La3+ions in the sodalite cages do not seem to play a part in the reduction mechanism of Ni2+ions located in hexagonal prisms since the corresponding activation energy remains the same with or without La3+ ions in the samp1e.l Alternatively, when La3+ ions are present in the supercages, a drop in Ni2+ions reducibility is observed. The influence of La3+ions in this case is indeed very important since, it spite of their accessibility,Ni2+ions located in sites I1 are less reducible than those sited in the prisms. We relate this fact to the formation of (La-0-Ni) associations inside the supercages. It must be emphasized also that the La3+ions may modify the redox character of the zeolitic support and stabilize the OH groups formed during the reduction run. All these factors may directly interfere in the reduction process. Presumably, the observed kinetics results from all these parameters.
Supplementary Material Available: A listing of the observed and calculated structure factors for samples A-C (3 pages). Ordering information is available on any current masthead page.
References and Notes (1) M. Briend-Faure, J. Jeanjean, M. Kermarec, and D. Delafosse, J . Chem. SOC., Faraday Trans. 1 , 74, 1538 (1978). (2) M. F. Guilleux, M. Kermarec, and D. Debfosse, J. Chem. SOC.,Chem. Commum., 102 (1977). (3) M. Briend-Faure, M. F. Guilleux, J. Jeanjean, D. Delafosse, G. Djega-Mariadassou, and M. Bureau-Tardy, Acta Phys. Chem., 24 (1-2), 99 (1978). (4) J. Jeanjean, D. Deiafosse, and P. Gallezot, J. Phys. Chem., 83, 2761 (1979). (5) D. H. Olson, G. T. Kokotailo, and J. F. Charnell, J. Colloid Interface Sci., 28, 305 (1968). (6) J. M. Bennett and J. V. Smith, Mater. Res. Bull., 3, 865 (1968). (7) J. M. Bennett and J. V. Smith, Mater. Res. Bull., 4, 7 (1969). (8) J. M. Bennett, J. V. Smith, and C. L. Angell, Mater. Res. Bull., 4, 77 (1969). (9) J. Scherzer, J. L. Bass, and F. D. Hunter, J. Phys. Chem., 79, 1194 (1975). (10) M. L. Costenoble, W. J. Mortier, and J. B. Uytterhoeven, J . Chem. SOC.,Faraday Trans. 1 , 73, 466 (1977). (11) M. L. Costenoble, W. J. Mortier, and J. B. Uytterhoeven, J . Chem. SOC.,Faraday Trans. 1 , 73, 477 (1977). (12) D. H. Olson, J. Phys. Chem., 72, 4366 (1968). (13) P. Gallezot, Y. Ben Taarit, and B. Imellk, J. Phys. Chem., 77, 652 (1973). (14) J. A. Rabo, C. L. Angeli, P. H. Kasai, and V. Schomaker, Discuss. Faraday SOC.,41, 328 (1966). (15) J. R. Feins and P. A. Mullen, Prepr., Div. Pet. Chem., Am. Chem. SOC.,89 (1970). (16) A. Abou-Kais, C . Mirodatos, J. Massardier, D. Barthomeuf, and J. Vedrine, J . Phys. Chem., 81, 397 (1977). (17) J. C. Conesa and J. Soria, J . Chem. SOC.,Faraday Trans. 1 , 75, 423 (1979).
Infrared Study of the Adsorption of CO and NO on Silica-Supported Pd and Pt-Pd Charles M. Grill and Richard D. Gonraler" Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 0288 1 (Received September 14, 1979) Publication costs assisted by the Petroleum Research Fund
The chemisorption of NO on silica-supportedPd and silica-supportedPt-Pd bimetallic clusters (nominal 1:l atomic ratio) has been studied with infrared spectroscopy. On supported Pd two bands are present. The band at 1735 cm-l is assigned to a linearly bound NO species with little back-bonding. The band at 1645 cm-' is assigned to an NO molecule linearly bound to a Pd atom of low surface coordination. On supported Pt-Pd bimetallic clusters these bands are shifted to 1655 and 1745 cm-l, and there is an additional band at 1760 cm-l which is assigned to NO linearly adsorbed on Pt. When CO is adsorbed on supported Pd, two main bands are observed in the infrared spectrum. The band at 1980 cm-l is assigned to multiply coordinated CO. A low-frequency shoulder on this band appears at 1900 cm-l. The band at 2070 cm-l which has a high-frequency component at 2090 cm-l is assigned to linearly bound CO. The linear species can be removed by evacuating at room temperature. When CO is adsorbed on the supported Pd-Pt bimetallic sample, all bands are shifted to higher frequencies by about 10 cm-l. CO can readily be removed from the Pd component of the bimetallic cluster by evacuating the sample at room temperature. The interaction of NO and CO over supported Pd was studied. We find that CO reacts with adsorbed NO and vice versa. A simple displacement of one by the other does not occur. The products of the reaction are COPand N20. We also observe no adsorbed isocyanate species.
Introduction In a previous study, the surface composition of silicasupported Pt-Ru bimetallic clusters was characterized with infrared spectroscopy.' In this study it was found that NO was selectively adsorbed on the Ru component of the bimetallic cluster whereas CO was equally selectively adsorbed on the Pt component. When relative absorbances of the CO and NO absorption bands against bulk metal concentration ratios were plotted, quantitative conclusions regarding surface concentrations could be made. This technique has several advantages over the more conventional spectroscopic techniques such as ESCA, SIMS, ISS, EXAFS, and AES in that (1)only the surface Iayer is sampled, (2) considerably more chemical information can 0022-3654/80/2084-0878$01 .OO/O
be obtained, (3) when applied to highly dispersed supported bimetallic clusters, techniques such as ESCA and AES essentially sample the entire metal sample, and (4) infrared spectroscopy is inexpensive and can be used to sample the surface at high pressures under reaction conditions, Its major disadvantages are that (1) extensive preliminary experiments to determine surface interactions of adsorbing species are usually required, (2) the method is not as accurate as the above-mentioned spectroscopic techniques, and (3) surface reconstruction may be induced by the chemisorbing species. Taking into account these factors, the development of infrared techniques capable of monitoring surface compositions under reaction conditions is a step which must 0 1980 American Chemical Society
Infrared Study of the Adsorption of CO and NO
The Journal of Physical Chemistry, Vol. 84, No. 8, 1980 879
be taken if we are to understand catalytic reactions over this very important class of catalysts. The subject of this study is twofold: (1)to investigate the adsorption of CO and NO over silica-supported Pd samples and (2) to determine the possible formation of silica-supported Pt-Pd bimetallic clusters by interpreting changes in injfrared spectral features which occur when CO and NO are eoadsorbed on a silica-supported Pt-Pd bimetallic sample (nominal Pd:Pt atomic ratio of 1:l). It is absolutely essential that in order to characterize the surface structure of a supported bimetallic cluster by using infrared spectroscopy, a thorough working knowledge of the adsorption of the gases used in the characterization be obtained. The adsorption of both CO and NO on silicasupported Pt2and silica-supported Ru3l4has been studied in previous reports; however, the adsorption of CO and NO and the surface interactions between the two adsorbed species over sidica-supported Pd are not presently available in the literature.
Experimental Methods Materials. The gases used in this study were subjected to the following purification treatments. CO (Matheson research grade) was purified by passing it through a liquid-N2 trap. :NO (Matheson technical grade) was purified by a standard vacuum distillation procedure in which only the middle cut was retained. Commercial Hz (Cranston Welding Supplies) was used in the reduction of the sample. I t was purified before use by first passing it through a Deoxo unit to convert O2impurities to H20 which was then removed by al molecular sieve backed up by a liquid-N2 trap. the purity of all gases was checked periodically on a Du Pont Model 104 mass spectrometer. The 5% Pd-silica samples used in this study were prepared as follows. Since PdClz is insoluble in H20, concentrated HC1 was added dropwise to a mixture of PdC1, (Ventron Alfa Products, Beverly, MA) and HzO. The mixture was maintained at 60 "C. Under these conditions soluble PdC1:- is formed. The solution of PdC1:was then mixed with Cab-o-Sil, grade M-5 (Cabot Corp., Boston, MA), to form a slurry. The bimetallic samples were prepared by mixing a solution of PdC1:- and a solution of H2PtC16-6H20(Engelhard Industries, NJ). The mixture was then added to Cab-0-Si1 as in the monometallic case. The slurry was then air-dried at room temperature for several days and stirred regularly during the drying process to retain uniformity. The dried catalyst was ground iinto a fine powder, less than 45 pm, and pressed into self-supporting disks 25 mm in diameter and less than 0.5 mm thick. The disk was placed in a stainless steel pellet hollder and suspended by a stainless steel wire in the infrared cell. The cell was similar in design to that of Brown and Gonzalez4and later refined by Ramamoorthy et al.5 Approximate dispersions were obtained by measuring hydrogen uptake at room temperature. In this procedure, a hydrogen isotherm is first obtained. Following evacuation at room temperature to remove weakly adsorbed hydrogen, a new isotherm is obtained. The difference between the two isotherms is taken as a measure of the strongly adsor bed hydrogen. Assuming an adsorption stoichiometry of one hydrogen atom per surface metal atom, it was found that 18 f 2% of the metal atoms was on the surface for all the samples reported in this study. Procedure. The following pretreatment procedure was used to reduce new disks: evacuation for 1 h at 350 "C, reduction in flowing H2 for 6 h at 350 "C, and evacuation for 2 h at 350 'C. The sample was then cooled to room temperature, and evacuation continued until the final
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I700 FREQUENCY, CM-'
Flgure 1. Stepwise adsorption of CO on 5% silica-supported Pd: (a) background; (b) first addition of CO; (c) second addition of CO; (d)'final 10 torr; (e) 15-min evacuation, pressure addition of CO, pressure II torr.
pressure was about torr (1torr = 133.3 N/m2). For disks previously reduced, the above procedure was followed except that the time in flowing H2was reduced to 4 h. The disk was reduced as above and then exposed to 30 torr of O2 at 300 "C for 1 min to produce an oxidized sample. It was then evacuated at 300 "C for 2 h. The final pressure, after cooling to room temperature, was again about lo4 torr. The cell was then transferred to the spectrometer and connected to a Pyrex vacuum system. From this system gases could be added to or evacuated from the cell. The spectra shown in Figures 2,3, and 5 were recorded at room temperature on a Perkin-Elmer Model 281 infrared spectrometer. For the spectra shown in Figures 1, 4, and 6, a Perkin-Elmer Model 521 infrared spectromleter was used. All spectra were obtained by using the double beam method. Here, 5% Pd-silica disks are placed in both the sample and the reference beams of the spectrometer. The absorption bands due to silica are thus canceled, ;and a relatively flat base line is obtained.
Results and Discussion Adsorption of CO. The infrared spectra of the stepvvise adsorption of CO on a silica-supported Pd sample having a nominal 5 % Pd content are shown in Figure 1. These spectra are in general agreement with the results of Palazov et ala6and Eischens et al.7-9 There are two major bands in these spectra. Eischens et al.7-9have assigned bands above 2000 cm-l to linearly bound CO and the bands below 2000 cm-l to bridge-bonded species. Although BlyholderlO has challenged this assignment, pointing out that bands below 2000 cm-l could very well be due to strongly backbonded linearly bound CO occurring at sites of low surface coordination such as crystal edges and corners, ithe bridge-bonded model is strongly supported by other experimental evidence. Palazov et aL6 found that when CO is adsorbed on PdO the bands assigned to multiply coordinated CO are no longer evident. This was attributed to a 10% increase in the Pd-Pd distance in PdO, making bridging very difficult. Further evidence which reinforces the bridged-bonding model comes from the recent work of Soma-Noto and Sachtler,ll who studied the adsorption
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Figure 2. Stepwise adsorption and desorption of NO on 5 % silicasupported Pd: (a) background; (b) total pressure of NO in cell = 8 X l o 4 torr; (c) total pressure of NO in cell = 5 X torr; (d) total pressure of NO in cell = 0.18 torr; (e) total pressure of NO in cell = 13 torr; (f) total pressure of NO in cell = 12 torr; (9) 1-min evacuation, 0.1 torr; (h) 1-h evacuation, pressure = 4 X lou4 torr. pressure
of CO on supported Pd-Ag clusters. As the concentration of Ag in these bimetallic clusters was increased, the relative intensities of the bands above 2000 cm-l increased at the expense of those below 2000 cm-l. These authors attributed this to a geometric effect in which the number of adjacent P d sites presumably necessary for multiply coordinated CO was diluted by Ag atoms. As shown in Figure 1, the absorption band due to multiply coordinated CO first appears at 1880 cm-l. As surface coverage is increased, an additional band forms at 1960 cm-l. At full surface coverage, the latter band shifts to 1980 cm-l, and the former becomes a shoulder at 1900 cm-l. Palazov et ala6have assigned the species absorbing at the lower frequency to a purely bridge-bonded CO and the species absorbing at the higher frequency to a bridge-bonded CO perturbed by linearly adsorbed CO. This linearly bound CO gives rise to a doublet absorbing at 2070 and 2090 cm-l. The band at 2070 cm-l is the first component of the doublet to appear, the one at 2090 cm-l appearing only a t relatively high CO pressures. There is an interesting parallel between CO adsorption on supported Pd and that on Pd single crystals. Using LEED and other high vacuum techniques, Ertl and Koch1* found that on Pd(ll1) CO adsorbed solely as the bridgebonded species. The pressures at which their study was torr. In the typical infrared made were less than 1 X experiment involving stepwise adsorption of CO, the first spectrum is taken at pressures in the 10-4-10-3 torr range. It seems reasonable, then, that the first bands to appear are those due to the multiply coordinated species. Adsorption of NO. The stepwise adsorption and desorption of NO are shown in Figure 2. A t a pressure of 8X torr, an infrared band appears at 1625 cm-*. This band reaches its maximum intensity at a pressure of 5 x torr, and its position shifts to 1645 cm-l. Further additions of NO have no effect on the intensity or the position of this band. torr, another band At NO pressures in excess of 5 X begins to appear. At a pressure of 13 torr, this band is more intense and symmetrical than the lower frequency band, and its position is shifted to 1735 cm-l. This higher-frequency band is highly pressure dependent, completely disappearing when the cell is evacuated to a pressure of 4 X torr. The NO species absorbing at 1645 cm-’ is strongly adsorbed as there is no detectable
Grill and Gonzalez
change in the intensity of this band on evacuation. Recently Ito et al.,I3 using reflectance infrared spectroscopy, studied the chemisorption of NO on Pd films. They observed bands at 1860 and 1805 cm-l. The band at 1860 cm-l was assigned to the symmetrical stretching vibration of an (NO), dimer. The antisymmetric stretching mode is predicted to have a frequency of about 1750 cm-l but was not observed with their experimental procedure. The band at 1805 cm‘l was assigned to a monomer species. When Kugler and Gryder14studied the adsorption of NO on silica-supported chromia, an (NO), dimer was observed. Since they used transmission infrared spectroscopy, both the symmetric and antisymmetric vibrational bands were observed at 1875 and 1745 cm-l, respectively. If a dimer were present on supported Pd, both bands should be present in the transmission infrared spectrum. Since we observe no band around 1875 crn-l, we conclude that a dimer does not form on silica-supported Pd. We therefore assign the band at 1745 cm-l to a linearly bound NO. Since it is weakly adsorbed, it appears that the NO species which gives rise to this band is not strongly back-bonded to surface Pd atoms. The band at 1645 cm-l is more difficult to assign. By analogy with inorganic complexes containing NO, this species has a bond order of about 2. Brown and Gonzalez3 observed a similar band on silica-supported Ru a t 1630 cm-l. This band was assigned to a bent NO species, In the inorganic literature, this type of ligand is often referred to as NO-; however, as pointed out by Cotton and Wilkinson,15this formalism is merely one of bookkeeping. It is more accurate to regard the bent NO ligand as an electrically neutral, sp2-hybridizedspecies. A single covalent bond is formed by pairing a d electron on NO with a CT electron on the metal. The nitrogen and oxygen atoms form a double bond, and a lone pair of electrons remains on the nitrogen atom, Since both the bent and the linear NO species form single bonds with the metal, they should be adsorbed with comparable strength to the surface. In fact, since sp2 orbitals form longer bonds than sp orbitals, one would expect the linear species to form a slightly stronger bond with the metal. These considerations are consistent with the results of Brown and Gonzalezn3On supported Pd, however, the species absorbing at 1645 cm-l appears to be much more strongly bound than the one absorbing a t 1735 cm-’. We therefore rule out a bent species as the assignment for the 1645-cm-’ band. A more feasible assignment for the species absorbing at 1645 cm-l is based on the concept originally espoused by Blyholder.lo The NO could be linearly bound with more back-bonding to surface Pd atoms than the species which absorbs at 1835 cm-l. This situation might arise when NO bonds to Pd atoms of lower surface coordination. At these sites, the Pd atoms would have more “dn” electrons available for back-bonding, resulting in a lower N-0 bond order. Since NO effectively forms a multiple bond with Pd, it would be more strongly adsorbed than the species which absorbs at 1735 cm-l. Because of the stronger Nmetal bond, we would expect these sites to be populated first. Since the 1645-cm-’ band is consistent with these three considerations, we favor the assignment of this band to a linear, highly back-bonded NO species. One further possibility is that the lower frequency band is due to a bridged-bonded species. Conrad et a1.16 studied the adsortion of NO on Pd(ll1) by using LEED and other high vacuum techniques. They found that a t the low pressures employed in their study (ca. torr) bridged NO was the exclusive surface species. By analogy with the
Infrared Study of the Adsorption of CO and NO
The Journal of Physical Chemistry, Vol. 84, No. 8, 1980 881
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Figure 3. Stepwise adsorption of NO on oxidized Pd: (a) background: (b) total pressure of NO in cell = 2 X torr; (c) total pressure of NO in cell = 2 X lo-* torr; (d) total pressure of NO in cell = 0.25 torr; (e) total pressure of NO in cell = 13.5 torr.
CO assignment previously discussed, the first bands to appear in the stepwise adsorption of NO might reasonably be assigned to a bridged species since a multiply coordinated NO molecule would also be more strongly adsorbed than a linear species. To explore further the possibility that a multiply coordinated NO might exist on the surface, experiments on oxidized Pd and on supported Pt-Pd bimetallic clusters were performed. Results on Oxidized Pd. Palazov et ale6have shown that when Pd is oxidized to PdO the intensity of the multiply coordinated bands of CO are depressed relative to those of the linear b,ands. In PdO the Pd atoms are too far apart to accommodate CO bridging species. If NO were to form bridged bonds on Pd, results similar to those of CO should be seen on oxidized Pd. The spectra for the stepwise adsorption of NO on oxidized P d are shown in Figure 3. At full-surface coverage the higher-frequency band has shifted to 1760 cm-l, and the lower-frequency one has shifted to 1665 cm-l. This is attributed to the electronegative oxygen atoms which tend to reduce the electron density at the Pd atoms, resulting in less back-bonding of electrons to the antibonding orbitals of NO. Neither the relative intensities of the two bands nor the pressure dependencies are significantly changed by the oxidation treatment. Thus the results on oxidized Pd argue against the assignment of a bridged species to the 1645-cm-l band. Results on Silica-Supported Pd-Pt Bimetallic Clusters. The spectra of the stepwise adsorption of CO on a silicasupported Pd--Pt bimetallic cluster (1:l atom ratio, total metal loading I= 6%) are shown in Figure 4. At full surface coverage there is a large broad band a t 2080 cm-l with a high-frequenc,y component a t 2095 cm-l. The high-frequency shoulder readily desorbs at room temperature, leaving a symmetrical band at 2075 cm-l. To assign these bands, recall that the linear CO species readily desorbs from supported Pd. Also, the linear species does not desorb from supported Pt at room temperature.2 Therefore, we assign the shoulder at 2095 cm-l to CO linearly adsorbed to Pd and the band at 2075 cm-l to linearly bound CO on Pt. CO does not form bridged species on silica-supported Pt. Therefore, we can assign the rather broad doublet at 1980 and 1890 cm-l to bridged CO on Pd. Note that the
2 200
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Figure 4. Stepwise adsorption of CO on silica-supported Pd-Pt, atomic ratio = 1:1, total metal loading = 6%: (a) base line; (b) first addition of CO; (c) second addition of CO; (d) third addition of CO, total pressure in cell N 20 torr; (e) 15-min evacuation, pressure torr. ’!
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Flgure 5. Stepwise adsorption of NO on silica-supported Pd-Pt, atomic ratio = 1:1, total metal loading = 6 % : (a) background; (b) total pressure of NO in cell = 8 X torr: (c) total pressure of NO in cell = 5 X torr; (d) total prassure of NO in cell = 21 torr.
intenstiy of the bridged Pd-CO band relative to the linear band is much smaller on the Pd-Pt bimetallic sample than on the monometallic Pd sample. A similar result was noted by Soma-Noto et al.ll on supported Pd-Ag samples. In both cases, the result can be explained as an “ensemble effect”. Dual Pd sites, necessary for bridging, are diluted in the bimetallic samples. Therefore, the ratio of bridged CO species to linear species is reduced in the bimetallic samples. The spectra showing the stepwise adsorption of NO on a silica-supported Pd-Pt bimetallic sample are shown in Figure 5. At full-surface coverage, we see a rather broad band centered at 7.750 cm-l and a low-frequency band a t 1655 cm-l. When NO was adsorbed on silica-supported Pt, only one band centered at 1760 cm-l was observed. This band has previously been assigned to linearly adsorbed NO on PtO2The band at 1750 cm-l on the bimetallic is thus assigned to linear NO on Pt and on I?d.
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Figure 6. Interaction of GO and NO over 5% silica-supported Pd: (a) background; (b) addition of 5 torr CO followed by 30-min evacuation, pressure lo-' torr; (c) addition of 0.2 torr NO to cell; (d) adsorption of 0.9 torr NO to cell.
The adsorbed NO species giving rise to the low-frequency band on the bimetallic sample was shifted to 1655 cm-l; however, the intensity of this band relative to the linear NO band on Pd clearly has not decreased. Again the evidence does not favor assigning the 1645-cm-l band to a bridge-bonded NO species. Interaction between CO and NO o n Silica-Supported Pd. Figure 6 shows the effect of adding NO(g) to CO preadsorbed on silica-supported Pd. When NO(g) is added, the intensities of the CO bands are decreased. Although not shown in Figure 6, after several hours the CO bands are completely displaced by NO. When CO(g) is added to preadsorbed NO, the intensity of the NO bands is also decreased (not shown). To determine whether there was a simple displacement of one gas by the other or whether a chemical reaction had occurred, we performed the following experiments. A reactor containing about 1 g of a powdered, reduced, silica-supported Pd sample was exposed to 17 torr of NO. The reactor was evacuated to a pressure of 0.1 torr and exposed to 20 torr of CO. A mass spectral analysis of the gas phase showed large amounts of COz and NzO. Following reduction of the powdered sample, 18 torr of CO was admitted to the reactor. The reactor was then evacuated to 6 X lo-, torr, and 20 torr of NO was added. Again, a mass-spectral analysis of the gas phase showed large amounts of COz and N20. These results show that one adsorbate is not merely displaced by the other. In both cases the following chemical reaction has occurred: CO
+ NO
-+
NzO
+ COZ
We also observe no evidence for an adsorbed isocyanate species in the region around 2180 cm-'. As such, this result is similar to the situation on supported Pt. On supported Ru the interaction between adsorbed NO and CO results in the formation of an adsorbed isocyanate speciesa2
Grill and Gonzalez
Conclusions The infrared spectrum of NO adsorbed on silica-supported Pd shows two bands. The higher-frequency band, occurring at 1735 cm-l, is intense and symmetric and is readily removed by evacuating at room temperature. This band cannot be assigned to an (NO), dimer because there is no band near 1850 cm-l. We therefore assign the species absorbing at 1735 cm-l to a linearly bound NO with little back-bonding. The species absorbing at 1645 cm-l is very tightly bound. The vibrational frequency suggests that this species has a bond order of about 2. We cannot assign this band to a bent NO species, because such a species would have about the same pressure dependence as the linearly adsorbed NO. We cannot assign it to a multiply coordinated species because the relative intensities of the two bands do not significantly change on oxidized Pd and on the Pd-Pt bimetallic sample. We therefore assign the 1645cm-l band to an NO species bound to a surface Pd atom having a relatively low coordination. Such a species would be strongly bound, would have a bond order of about 2, and would preferentially adsorb at low pressures. The species absorbing at 1645 cm-l meets all these criteria. In the interaction between adsorbed NO and CO, NzO and C 0 2 are formed. One adsorbate is not merely displaced by the other, and no surface isocyanate species is formed. Finally, since CO can be removed from the Pd component of a supported Pd-Pt bimetallic cluster by evacuating at room temperature, it appears that the most promising way in which infrared absorption measurements can be related to surface compositions is by taking relative absorbances of the CO infrared high-frequency absorption bands before and after evacuation. This method will only be reasonably accurate when the amount of multiply coordinated CO on the bimetallic sample is small. Preliminary measurements show this to be the case. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the support of this research. We also wish to acknowledge the National Science Foundation for the funds necessary to purchase a Perkin-Elmer Model 281 infrared spectrometer under Grant No. DMR 78-18917. References and Notes (1) P. Ramamoorthy and R. D. Gonzalez, J . Catal., 58, 188 (1979). (2) M. F. Brown and R. D. Gonzalez, J . Catal., 44,477 (1976). (3) M. F. Brown and R. D. Gonzalez, J . Catal., 47, 333 (1977). (4) M. F. Brown and R. D. Gonzalez, J . Phys. Chem., 80, 1731 (1976). (5) P. Ramamoorthy, R. D. Gonzalez, and A. Kosci, J. Appl. Spectrosc. (Engl. Trans/.), 33, 310 (1979). (6) A. Palazov, C. C. Chang, and R. J. Kokes, J. Catal., 36,338 (1975). (7) R. P. Eischens, W. A. Pliskin, and S . A. Francis, J . Chem. Phys., 22, 1786 (1954). (8) R. P. Eischens, W. A. Pliskin, and S. A. Francis, J. Phys. Chem., 60, 194 (1956). (9) H. P. Elschens and W. A. Pliskin, Adv. Catal., 10, 1 (1958). (10) G. Blyholder, J . Phys. Cbern., 88,2772 (1964). (11) Y. Soma-Noto and W. M. H. Sachtler, J. Catal., 32, 315 (1974). (12) G. Ertl and J. Koch in "Adsortpion-Desorption Phenomena", F. Ricca, Ed., Academic Press, New York, 1972,p 345. (13) M. Ito, S. Abe, and W. Suetaka, J . Catal., 57, 80 (1979). (14) E. L. Kugler and J. W. Gryder, J . Catal., 36, 152 (1975). (1 5) F. A. Cotton and G. Wllkinson, "Advanced Inorganic Chemistry", 3rd ed., Interscience, New York, 1972,pp 718-719. (16) H. Conrad, G. Ertl, J. Kuppers, and E. E. Latta, Surf. Sci., 65, 235 (1977).
