J. Phys. Chem. 1995, 99, 6024-6028
6024
Comparative IR Study of Ethylene Adsorption on a PtRh Alloy and Monometallic Pt and Rh Catalysts Supported on A1203. Identification of Alloy-Specific Binding Sites Hugo Celio and Michael Trenary* Department of Chemistry, 845 W Taylor Street, University of Illinois at Chicago, Chicago, Illinois 60607
Heinz J. Robota' Allied-Signal Research and Technology, 50 E Algonquin Road, Des Plaines, Illinois 6001 7 Received: June 15, 1994; In Final Form: January 23, 1995@
The surfaces of a series of PtRh bimetallic catalysts, supported on y-AL03 (Degussa-C), were characterized by the IR spectrum of n-bonded ethylene. We have compared results for ethylene adsorbed on 3% Pt/A1203, 1.6% Rh/A1203, nonalloy W A 1 2 0 3 , and alloy W A l 2 O 3 catalysts. We find that the C% stretch frequency can reveal the presence of PtRh alloy particles. The C=C stretch frequency increases by ~ 6 0 - 8 0 cm-' for n-bonded ethylene adsorbed on PtRh alloy sites compared with n-bonded ethylene adsorbed on monometallic Pt or Rh particles. The definitive correlation of n-bonded ethylene bands with PtRh alloy particles was achieved by using X-ray diffraction, scanning transmission electron microscopy, and analytical electron microscopy to classify the bimetallic catalysts as alloy or nonalloy. Once established, the correlation permits detection of alloy particles that are too small to be characterized by noninfrared techniques.
Introduction
The sensitivity of the IR frequencies of chemisorbed molecules to the nature of the adsorption site can be used to probe properties of the substrate. The fact that the C-0 stretch of chemisorbed CO depends on adsorption site' (atop, 2- and 3-fold bridge) combined with its large IR intensity, makes CO an obvious choice as a probe Associated with the strong IR signal of CO are strong shifts in frequency with coverage that complicates association of a given frequency with a particular adsorption site. This is less of a problem with hydrocarbons, which have much weaker IR intensities and very small frequency shifts with coverage. Also, the properties of chemisorbed hydrocarbons are of more relevance to heterogeneous catalytic reactions involving hydrocarbons. In a recent study5 we reported IR spectra for n-bonded ethylene adsorbed on the following A1203supported metals: Pt, Rh, Ir, Ru, and Pd. Although n-bonded ethylene has several characteristic IR peaks, the most intense one for each metal is due to a mode involving both C=C stretch and 6,(CH2)scissor motion.6 The exact frequency of the peak depends on the metal and varies from 1188 to 1239 cm-' among the five metals listed above. For reasons discussed more fully below, we refer to this band as the C=C stretch. In the previous study5 we reported this band at 1204 cm-' for W&03 and at 1222 cm-' for Rh/A1203. In this paper we compare the C=C stretch of n-bonded ethylene on a PtRh alloy catalyst with monometallic Pt and Rh catalysts. While many methods can be used to characterize monometallic catalyst particles, one important but difficult issue is the structure of the particles. Additional structural questions pertain to alloy particles. One would like to know how the constituent metals are distributed between the surface and the interior of a particle and how the two metals are distributed in two dimensions on the particle's surface. We do not address such issues here. Instead, we examine the extent to which the chemisorption
T To whom correspondence should be addressed. Present address: Allied Signal Environmental Catalysts, PO Box 580970, Tulsa, OK 74158-0970. @Abstractpublished in Advance ACS Absrracrs, April 1, 1995. '
properties of the alloy particles, as manifested in the IR spectrum of n-bonded ethylene, differ from a simple mixture of monometallic Pt and Rh particles. This relates to the important issue of whether there is a synergistic effect associated with the PtRh alloy or if the alloy will exhibit properties that are simply an addition of Pt and Rh proper tie^.^ Our results support the former in that a form of n-bonded ethylene exists on the PtRh alloy that is not present on monometallic Pt or Rh. The use of PtRh bimetallic automotive catalysts has stimulated much basic research on the surface properties of alloy single crystals. Studies of the pto,75w.25(100)8-10and Pto.10&.SO( 11l ) " ~ 'surfaces ~ have shown that the top layer is platinum enriched compared with the bulk, although the enrichment can be altered and even reversed by chemisorption. An alloy single crystal can also be formed through the deposition of Rh onto a Pt ~ubstrate.'~ Despite the extensive past literature on PtRh alloys, there have been no previous studies of the sort described here. Metal weight loadings in our experiments were chosen to be low enough to minimize agglomeration yet high enough to give strong IR signals. This precludes following established preparation methods known to give PtRh alloy particles as these methods involve lower metal weight loadings than we need. Consequently, it was crucial to characterize our bimetallic catalysts as carefully as possible. As we show here, slight variations in the method of preparation of a bimetallic catalyst can determine whether alloy formation occurs or not. We note that the goal of this study was not to establish why a particular preparation method leads to alloy particles while another method does not. Rather, it was to correlate the presence of PtRh alloy particles as detected with noninfrared techniques with the infrared spectrum of adsorbed ethylene. Because catalyst characterization was such an essential component of this work, it is described in detail in the next section. This is followed by the presentation and discussion of the ethylene IR results. Experimental Section Preparation of Catalysts. IR spectra of ethylene on six different catalyst samples are reported here. The catalysts were
0022-365419512099-6024$09.0010 0 1995 American Chemical Society
Identification of Alloy-Specific Binding Sites prepared and characterized with techniques other than infrared spectroscopy at Allied-Signal while the infrared measurements were carried out at the University of Illinois at Chicago. The hydrogen (99.999% pure) used in the pretreatments, the 5% H2/ N2 mixture used for the reduction treatment, and the ethylene (99.99% pure) were purchased from Matheson Gas Products. Hydrazine monohydrate (98% pure) was purchased from Aldrich and was used without further purification. The 3% WAl203 (catalyst 1) and 1.6% MA1203 (catalyst 2) samples had approximately equal metal mole fractions and were prepared by impregnation of Degussa Alumina-C ( y AlzO3) with aqueous solutions of H2PtCl6 and RhCl3*H2O by the general procedure described earlierS5 After drying at 100 "C the solids were transferred to a quartz tube furnace and calcined at 400 "C in flowing air followed by reduction in flowing 5% H2/N2 at 500 "C. A nonalloy F'tRh/Al2O3 catalyst (catalyst 3) was prepared by first impregnating A1203 with H2PtCls(aq) to achieve a Pt weight loading of 2%. This was followed by calcination in flowing air, while the temperature was ramped at 5 " C h i n from 25 to 350 "C where it remained for 2 h. After cooling overnight to room temperature, the reduction step was carried out by flowing 5% H2/N2, while ramping to 400 "C and holding at 400 "C for 4 h. The solid was then reimpregnated with RhClyH20(aq) to obtain a Rh loading of 1 wt %. It was reduced without further calcination in flowing 5% H2/N2, while heating at 1 " C h i n to 150 "C. After 4 h, the sample was further heated to 400 "C at 1 "C/min and kept at 400 "C for 4 h. The preparation of bimetallic catalysts 4-6 began with simultaneous impregnation of A1203 with H2PtCl6(aq) and RhC13*HzO(aq). The final catalysts contained 2 wt % Pt and 1 wt % Rh. Catalyst 4 was reduced without calcination in flowing 5% H2/N2, while the temperature was ramped to 400 "C and held at this temperature for 2 h. Catalyst 5 was calcined in flowing air, while ramping at 5 "C/min to 120 OC and was kept at 120 "C for 1 h. This was followed by a second ramping to 400 "C and was kept at 400 "C for 2 h. It was reduced with flowing hydrogen in the same way as catalyst 4. Catalyst 6 was calcined in the same way as catalyst 5 but was reduced at 95 "C with a 0.08 M hydrazine solution (N2H4'H20) and dried at 95 "C. NonInfrared Characterization. The techniques of analytical electron microscopy (AEM) in conjunction with scanning transmission electron microscopy (STEM) and powder X-ray diffraction (XRD) are useful in characterizing PtRh bimetallic catalyst^.'^ For catalysts containing Pt, XRD is sensitive to particles 23.5 nm. Although the strongest Pt diffraction lines are obscured by A1203lines, a scan over the region of the Pt(31 1) peak revealed no deviation from the normal A1203 pattern indicating that few Pt particles 13.5 nm were present in the 3% WA12O3 catalyst. Particles as small as 2.0 nm can be detected with STEM, and their elemental composition determined with AEM by analyzing the electron beam induced X-ray fluorescence. Examination with STEM of 3% Pt/A1203 and 1.6% Rh/A1203 catalysts prepared in a wayI5 similar to those used here revealed that most Pt and Rh particles had sizes of 2-2.5 nm. When particles of sufficient size are present, the Pt(311) diffraction peak is quite useful. Particles large enough to detect with XRD were found in catalyst 3, and these particles were not alloyed with Rh since the position of the Pt(311) peak was identical to that of monometallic platinum. Unfortunately no information can be obtained from Rh diffraction because the Rh XRD peaks are obscured by ,41203 lines. The fact that no alloy particles were detected with XRD does not, of course, preclude the possibility that alloy particles too small to detect
J. Phys. Chem., Vol. 99, No. 16, 1995 6025 with XRD are also present in catalyst 3. The fact that larger Pt particles are found in catalyst 3 with 2% Pt than in catalyst 1 with 3% Pt is readily explained by differences in preparation procedure. Before impregnation with the Rh solution, the Pt in catalyst 3 is likely highly dispersed. However, during addition of Rh a hydroxylated alumina surface is produced on which the Pt particles have enough mobility for some agglomeration to occur during the subsequent reduction step. The XRD and STEM results for catalyst 4 were inconclusive due to the difficulty of finding particles large enough for analysis. However, the IR data presented below indicate that catalyst 4 does not contain alloy particles. In contrast to catalyst 4, XRD, STEM, and AEM provided substantial proof that catalyst 5 contained particles of the PtRh alloy. The use of STEWAEM showed that most of the particles contained both platinum and rhodium in various ratios, a few particles contained platinum only, but no particles contained only rhodium. Catalyst 6 was characterized with XRD only, which showed the Pt(311) peak shifted by 1". Since complete alloy formation would yield a shift of 2-3", a 1" shift indicates the presence of particles where the two metals are not as completely mixed as in a true 1:1 PtRh alloy.I6 The particles of catalyst 6 would thus be expected to contain alloy sites and sites associated with nonalloy Pt and Rh. Infrared Measurements. A general description of the experimental apparatus for the infrared studies has been given p r e v i ~ u s l y . ~Briefly, *'~ an IR cell following a design of Yates and co-workers'7~1* that consists of a double-sided stainless steel flange, modified to include a Nz(1) cooled sample holder and a chromel-alumel thermocouple, with two flange-mounted CaF2 windows was used. All samples consisted of a conventional pressed disk of catalyst pressed into a hole in a 0.05 mm thick mica sheet that is then clamped tightly to a CaF2 disk. Spectra were obtained with a commercial FI'IR using a MCT (mercurycadmium-telluride) detector. IR data were taken at a resolution of 4 cm-' by averaging 512 scans. After being prepared as described above, the samples were mounted in the IR cell for a pretreatment with H2. The cell was evacuated to 2 x Torr, and the catalysts were reduced at 300 "C under 100 Torr of H2 for 1 h. The IR cell was evacuated at 300 "C to lo-' Torr and then cooled to 25 "C. These finishing conditions ensured stable, reproducible catalysts for the C2H4 adsorption experiments. Results and Discussion
Figure l a shows the IR spectrum obtained on WAl2O3 under an ambient pressure of 1 Torr of ethylene at room temperature. The IR bands correspond to two distinct species: ethylidyne and n-bonded ethylene. The small feature at 1290 cm-' is a spectral artifact probably due to a miscancellation of an A1203 peak and was not reproducible. The corresponding spectrum for MA1203 is shown in Figure 2a and the assignments for the bands are given in Table 1. The results are essentially the same as reported earlier for catalysts prepared by similar q~ethods.~ Except for the symmetric CH3 bend of ethylidyne, most of the bands have measurably different frequencies on the two metals. However, because of their high intensity, particularly on Pt, and wide separation, the C-C stretch band at 1205 cm-I on WAlpO3 and at 1230 cm-' on RhlA1203 were chosen to characterize the Pt and Rh sites of the mixed metal samples. We note that labeling the ethylene peaks in the 1200-1300 cm-' region as "C=C stretches" is an oversimplification. In C2&(g) the C=C stretch is at 1623 and the 6,(CH2) scissor is at 1342 cm-'. This would suggest that our 1205 and 1501 cm-' bands of C~H4/Pt/Al203have assignments opposite to those of
Celio et al.
6026 J. Phys. Chem., Vol. 99, No. 16, 1995 a)
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wavenumbers (cm-') Figure 1. (a) IR spectrum obtained with 1 Torr of C2&(g) over W ,41203. (b) Line-shape analysis of the band at 1205 cm-'.
wavenumbers (cm-') Figure 2. (a) IR spectrum obtained with 1 Torr of Cz&(g) over Rh/ A1203. (b) Line-shape analysis of the band at 1230-1260 cm-l.
Table 1. However, for ethylene complexed to metalsI9 or chemisorbed on metal surfaces? the normal modes with fundamentals in the 1200-1600 cm-I region contain appreciable amounts of both C-C stretch and d,(CH2) scissor motions. M a ~ l o w s k yconsiders '~ this point in detail and shows that the strength of the ethylene-metal interaction determines which motion dominates which normal mode and that there is a crossover in assignment of the two bands. Although his analysis supports our assignment, it would be more precise, for example, to state that the band that appears at 1205 cm-' on WA1203 is the fundamental of a normal mode with more, possibly only slightly more, C=C stretch than d,(CH2) character. Figures l b and 2b show fits of the C=C stretch peaks. The objective of these fits is to obtain a good empirical description of the IR band for the monometallic particles for comparison with the band shapes obtained with the bimetallic catalysts. The physical origin of the line shapes is a complicated subject beyond the scope of this paper. In Figure l b the band is fit20 to a single asymmetric component with a maximum at 1205 cm-' and a fwhm of 26 cm-I. The fitting function is a threeparameter exponentially modified Gaussian.*O The equivalent band on the Rh/A1203 sample has a more complicated shape
but is well described by the sum of two separate components. The first component is a four-parameter Pearson function20 centered at 1230 cm-I and the second is a Gaussian centered at 1260 cm-I. The 1260 cm-' component is present in all spectra of n-bonded ethylene on Rh but the relative amounts of the 1230 and 1260 cm-' peaks varies among the samples examined. Figure 3a shows the spectrum of a mixed metal catalyst prepared in a way that gave particles that XRD showed were nonalloy. As expected the spectrum consists entirely of peaks seen in either the WAlzO3 or MA1203 spectra of Figures 1 and 2. However, the spectrum of Figure 3a is not simply the sum of the spectra in Figures l a and 2a as is evident from a comparison of the intensity scales in Figures 1-3. The fact that the mole fraction of metal is higher for the sample of Figure 3 does not fully account for the intensity of the ethylidyne bands compared with the n-bonded ethylene bands. Rather, the amount of n-bonded ethylene relative to ethylidyne on the Pt particles has decreased significantly in the mixed-metal case. We have generally found that the n-bonded ethylene to ethylidyne ratio shows large variations in response to slight variations in the sample preparation method. The fact that the
TABLE 1: Frequencies for Ethylidyne and n-Bonded Ethylene on Rh/A1203 and WA1203 Rh/A1203
C-H stretch region
ethylidyne
n-bonded ethylene
ethylidyne
2885
2966 3080
2897 2943
2939
W.41203 n-bonded ethylene 2955
2998 3018 3013
overtone of asym CH3 bend C-C stretch region CH2 scissors, CH3 deformation region
2191 1116 1341 1411
1230
1510
2803 1124 1342 1424
1205 1501
Identification of Alloy-Specific Binding Sites
J. Phys. Chem., Vol. 99,No. 16, 1995 6027
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wavenumbers (cm-') Figure 3. (a) IR spectrum obtained with 1 Torr of CZ&(g) over a 1: 1 PtRh/A1203 bimetallic catalyst which does not contain alloy particles. (b) Line-shape analysis showing that the 1200-1260 cm-] band is a simple sum of bands due to C 2 a on Pt and Rh particles.
coverage of n-bonded ethylene is less on the Pt particles of the mixed metal catalysts but the frequencies are essentially the same illustrates the advantages of using a probe molecule with IR bands that do not undergo large frequency shifts with coverage. In Figure 3b we show that the 1200-1260 cm-' region is well fit by three peaks. Peak 1 is the exponentially modified Gaussian used to fit the 1205 cm-' band of the monometallic WAl2O3 sample of Figure 1. Peaks 2 and 3 are the Pearson and Gaussian peaks used to fit the -1230 and -1260 cm-' components of the WAl2O3 sample of Figure 2. The spectra of Figures 1-3, and in particular the 1200- 1260 cm-' regions, are well described as due to a simple mixture of Pt and Rh particles. This is evident from a comparison of the fitting parameters of Figures lb, 2b, and 3b. Figure 4 shows the results obtained on catalyst 6, a mixedmetal catalyst that XRD clearly shows contains PtRh alloy particles. The spectrum has the same general appearance as the nonalloy mixed metal sample of Figure 3 with the ethylidyne peaks dominating. The ethylidyne peaks have essentially the same frequencies as in Figure 3 and show no sensitivity to the presence of alloy formation. The n-bonded ethylene peaks are also similar to those of Figure 3 with the obvious exception of the prominent peak at 1292 cm-l. The 1200-1292 cm-' region is fit to four peaks as shown in Figure 4b. As the peak fit parameters show, peaks 1-3 are essentially the same three peaks observed on the nonalloy PtRh sample of Figure 3. Although the (peak 3)/(peak 2) area ratio is higher for the alloy sample than for the nonalloy cases, the association of these two peaks with Rh sites that are unperturbed by alloy formation is unambiguous. The fact that the C-C stretch is most sensitive to alloy formation is in accord with the fact that its frequency
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wavenumbers (cm-I) Figure 4. (a) IR spectrum obtained with 1 torr of CZ&(g) over a 1:l PtRh/A1203 alloy catalyst. (b) Line-shape analysis of 1200-1290 cm-' region showing an additional band at 1290 cm-I associated with ethylene on alloy particles.
showed the highest variability among the different metals previously ~ t u d i e d . ~ The results described above establish that the presence of a C-C stretch of n-bonded ethylene near 1290 cm-' can serve as an indicator of PtRh alloy formation. This is further supported by the results of Figure 5 in which a series of spectra are compared directly in both the 1000-1600 cm-' region and the C-H stretch region of 2700-3200 cm-I. The sharp bands in the latter region are due to gas-phase ethylene. Spectra a and b are of the HA1203 and WAl2O3 samples of Figures 1 and 2, spectrum c is of the bimetallic nonalloy sample of Figure 3, and spectrum f is of the alloy of Figure 4. Spectrum g was obtained on a fraction of catalyst 6 subjected to additional calcining in air at 500 "C for 5 h followed by an H2 pretreatment at 190 "C. The resulting particles were large enough to characterize with XRD. A Pt(311) diffraction peak, indicative of monometallic platinum, was detected. This shows that the metals had segregated or "dealloyed". A comparison o f f and g shows that the dealloying procedure results in the disappearance of the 1290 cm-' band. This is further support for associating this band with adsorption on alloy sites. Spectrum e is for catalyst 5 , which was prepared in the same way as catalyst 6 of spectrum f except that the final reduction was done under flowing hydrogen at 400 "C rather than with hydrazine at 95 "C. Clear evidence for the presence of PtRh alloy particles in catalyst 5 was obtained by XRD and STEWAEM. The 1290 cm-' band in spectrum e supports this. In the cases described so far, when noninfrared techniques show the presence (absence) of alloy particles, IR shows the presence (absence) of the 1290 cm-' band. However, IR data can just as easily be obtained for cases where the particle size is too small for XRD or STEWAEM to be used. Under these
Celio et al.
6028 J. Phys. Chem., Vol. 99, No. 16,1995
phase ethylene and to purely di-a-bonded ethylene, respectively. The nu parameters for ethylene on WAl2O3, WA1203, and the alloy sites of PtRh/AlzOs are 0.49, 0.42, and 0.30, respectively. This would then imply that ethylene has a weaker interaction with the PtRh alloy sites than it does with either metal alone. The strength of the interaction is thought to correlate with the ability of the metal to back-donate electron density into the 2n* orbital of ethylene, which is apparently reduced at alloy sites. Unfortunately, there is no independent measure of the electronic properties of Pt and Rh in the alloy. Nevertheless, the mere fact that the alloy possesses chemisorption properties distinct from either metal alone is one of the two significant conclusions of this study. The other is that the IR spectrum of a probe molecule can be used to detect the presence of alloy particles in bimetallic catalysts under conditions where other more conventional methods cannot be used. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work. References and Notes I100 1200 1300 1400 1500
2700 2800 2900 3000 3100 3200
Wavenumbers (cm-’) Figure 5. Direct comparison of spectra obtained with 1 Torr of C2&(g). Dashed vertical lines identify n-bonded ethylene peaks. (a) 3% WA1203; (b) 1.6% Rh/A1203; (c) nonalloy PtRh/Al+&; (d) PtRh/AlzO3 with particles too small to detect alloy formation by non-IR methods; (e) PtRh/A1203 containing alloy particles; (f) PtRh/Al203 with alloy particles prepared by a method different from (e); (g) same PtRh/Alz03 catalyst used in (f) after being dealloyed.
circumstances, the presence of the 1290 cm-’ band provides a unique way to establish whether alloy formation has occurred. Spectrum d of Figure 5 represents such a case. The sample was characterized with XRD and STEWAEM but the results were inconclusive as the particles were too small to be detected by these methods. The absence of the 1290 cm-’ band in spectrum d indicates that this catalyst does not contain alloy particles. This example also illustrates how sensitive the IR spectrum is to the exact method used to prepare the sample; none of the other bimetallic catalysts were prepared in exactly the same way and none have exactly the same IR spectrum. For example, the ratio of the -1260 to e1230 cm-’ components of n-bonded ethylene on the Rh sites is higher in spectrum d than for any of the other catalysts. It is possible to make some inferences from the IR spectra about the strength of the ethylene-metal interaction at the alloy sites. Stuve and Madix2’ proposed assigning a nu parameter to adsorbed ethylene based on the positions of the C=C stretch and d,(CHz) bands where values of 0 and 1 correspond to gas-
(1) Sheppard, N.; Nguyen, T. T. The Vibrational Spectra of Carbon Monoxide Chemisorbed on the Surfaces of Metal Catalysts-A Suggested Scheme of Interpretation. In Adv. Infrared Raman Spectrosc. 1978, 5, 67. (2) Hollins, P. Surf: Sci. Rep. 1992, 16, 51. (3) Brandt, R. K., Sorbello, R. S., Greenler, R. G. Surf: Sci. 1992,271, 605. (4) Brandt, R. K., Hughes, M. R.; Bourget, L. P.; Truszkowska, K.; Greenler, R. G. Surf: Sci. 1993, 286. 605. (5) Mohsin, S. B.; Trenary M.; Robota, H. J. J. Phys. Chem. 1991,95, 6657. (6) Sheppard, N . Annu. Rev. Phys. Chem. 1988 39, 589. (7) Oh, Se. H.; Carpenter, J. E. J. Catal. 1986, 98, 178. (8) Van Delft, F. C. M. J. M.; Siera, J.; Nieuwenhuys, B. E. Surf: Sci. 1989, 208, 365. (9) Van Delft, F. C. M. J. M.; Van Langeveld, A. D.; Nieuwenhuys, B. E. Surf: Sci. 1987, 189/190, 1129. Van Langeveld; A. D. Niemantsverdriet, J. W. Surf: Sci. 1986, 178, 880. (10) Yamada. T.: Hirano, H.: Tanaka, K.: Siera. J.: Nieuwenhuvs, B. E. Surf: Sci. 1990, 226, 1. (1 1) Beck, D. D.; DiMaggio, C. L.; Fisher, G. B. Sutf Sci. 1993,297, 293. (12) Ng, K. Y. S.; Belton, D. N.; Schmeig, S. J.; Fisher, G. B. J. Catal. 1994, 146, 394. (13) Taniguchi, M.; Kuzembaev, E. K.; Tanaka, K. Surf: Sci. 1993,290, L711. (14) Robota H. J; Broach R. W.; Sachtler J. W. A,; Bradley S. A. Ultramicroscopy 1987, 22, 149. (15) Mohsin, S. B.; Trenary M.; Robota, H. J. J. Phys. Chem. 1988, 92, 5229. (16) Darling, A. S. Plarinum Mer. Rev. 1961, 5, 58. (17) Beebe, T. P.; Gelen P.; Yates, Jr., J. T. Surf: Sci. 1984, 148, 526. (18) Wang, H. P.; Yates, Jr., J. T. J. Phys. Chem. 1984, 88, 852. (19) Maslowsky, Jr., E. Vibrational Spectra of Organometallic Compounds; Wiley: New York, 1977. (20) The oeaks were fit with a commercial software Dackpape. “Peak Fit’;, Jandel, kan Rafel, CA. (21) Stuve, E. M., Madix, R. J. J. Phys. Chem. 1985, 89, 3183.
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