J. Phys. Chem. 1993,97, 9161-9169
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Magnetic Resonance Studies of Ethene Adsorption on Alumina-Supported Platinum Surfaces Janet M. Griffiths3 Alexis T. Bell, and Jeffrey A. Reimer. Center for Advanced Materials, Lawrence Berkeley Laboratory, and Department of Chemical Engineering, University of California at Berkeley, Berkeley, California 94720-9989 Received: December 7 , 1992; In Final Form: June 17, 19938
Carbon-13 and deuterium magnetic resonance data for ethene adsorbed on high dispersion, alumina-supported platinum surfaces were obtained after adsorption at various temperatures and thermal treatments. Ethene is found to adsorb on 5% Pt/A1203 a t low temperature in a *-bonded form that yields a single carbon line shape at 70 ppm. Upon warming to room temperature, this species is partially converted to ethylidene and a second adsorbate, which has two protonated carbons that yield a single carbon line shape at 130 ppm. At 323 K, the predominant species on this catalyst is ethylidene, in contrast to observations reported for single-crystal and low dispersion metal-supported catalysts. Ethylidene appears to decompose into methane gas and surface carbon.
Introduction An important and challenging problem in catalytic chemistry is the characterization of adsorbed species on reactive surfaces. Identificationof the structure and dynamicsof adsorbates formed upon initial adsorption, as well as after temperature changes, provides a basis for understanding the detailed mechanisms of surface reactions. Ethene adsorption has been studied with particular interest owing to the importance of ethene-derived intermediates in cracking, hydrogenation, and hydrogenolysis reactions. Consequently,this olefin is selected often as a model for studyingsuch catalytic reactions. A survey of the vast surface science literature1-' for ethene adsorbed on single-crystal and supported Pt catalysts suggests that ethene adsorbs at low temperatures to form di-u-bondedand possibly r-bonded ethene. While *-bonded ethene may be removed by evacuation or hydrogenation, di-u-bonded ethene rearranges, with loss of a proton, to ethylidyne near room temperature. Ethylidyneremains as a predominant surface species on Pt( 111) from about 280 to 450 K. On supported Pt catalysts, ethylidyne may coexist with r-bonded and u-bonded ethene near room temperature. The *-bonded form of adsorbed ethylene is seen rarely on singlecrystal surfaces, although one EELS study has reported that *-bonded ethylene forms on Pt( 111) in the presence of coadsorbed oxygen atoms and can be hydrogenated readily to ethane.6 Reports of ethylideneon single-crystal platinum surfacesare rare; a recent TPD and HREELS study confirms the presence of ethylidene on Pt( 111) when potassium is coadsorbed.' Knowledge of the structure of an adsorbate layer can also provide insight into the pathway by which inactive carbon layers are formed as a consequence of adsorbate restructuring or decomposition. At elevated temperatures, hydrogenated carbon fragments formed on single-crystalsurfaces eventuallyrearrange into graphite.*-9 On supported metal catalysts, isolated and dehydrogenated carbon atoms are observed at similar temperatures.10 Adsorbed olefins have been identified specifically as precursors to inactive surface carbons which lead ultimately to catalyst d e a c t i v a t i ~ n . ~ J Although lJ~ deactivation due to olefins is well-documented in catalytic processes such as reforming, deactivationis poorly understood from a mechanisticstandpoint. The motivation for this work stems from the need to identify structures responsible for the formationof inactive surface carbon. Specifically, this work focuses on the surface chemistry of ethene Corresponding author. Current address: Francis Bitter National Magnet Laboratory and Dept. of Chemistry, M.I.T., Cambridge, MA 02139. Abstract published in Aduance ACS Abstracts, August 15, 1993.
adsorbed on alumina-supported platinum (Pt/A1203). Our approach has been to identify structures formed upon adsorption at low temperatures and to monitor changes in adsorbatestructure at elevated temperatures using techniques of 13C and 2H NMR spectroscopies. The 13C work differs from previous NMR10J3 studiesin that broad-line spectra were recorded on high dispersion samples in order to obtain chemical information from the resonance positions and line widths. We find that at low temperatures ethene adsorbs in a *-bonded configuration which has a singlecarbon line shape at 70 ppm. Upon warming to room temperature, this species is converted partially to ethylidene and a second adsorbate, which has two protonated carbons that yield a single carbon line shape at 130ppm. At 323 K, the predominant species on this catalyst is ethylidene. Upon further heating, ethylidene appears to decompose into methane gas and surface carbon which may be a precursor to deactivating carbon such as graphite. These results confirm the presence of *-bonded ethene on Pt/A1203 at low temperatures but suggest that ethylidene, and not ethylidyne, is the stable structure of adsorbed ethene at close to room temperature. The stability of these structures vis&vis those inferred from ultrahigh-vacuum studies of clean Pt surfaces may be rationalized in terms of the known physics of small particles.
Experimental Section Catalyst samples used in this study were prepared by incipientwetness impregnation of powdered y-alumina with a solution of 20% chloroplatinic acid.14 The T-alumina was obtained from Kaiser Chemical, and chloroplatinic acid was purchased from NOAH Chemical. After drying in air, the catalyst was heated slowly to 400 OC in a flowing mixture of 20% 02/80% He and held at this temperature for 4 h. Catalyst samples were analyzed for weight percent of Pt and Fe by X-ray fluorescence spectroscopy. Platinum content was 5% for all samples used here. Iron content ranged from 0.01 to 0.03% and originated mostly from the alumina support. Metal dispersion was measured by quantitative gas adsorption of Hz at room temperature. Both total and reversible adsorption curves were generated to 350 Torr, and the amount adsorbed was determined as the difference between the two adsorption curves extrapolated to zero pressure. A one-to-one stoichiometryof the hydrogen to metal surface site was assumed.15 Most samples showed a dispersion of 80% or higher, corresponding to particles with an average size of about 2 nmS16One sample appeared to have a dispersion of 100%based upon the hydrogen chemisorption measurement; this sample showed no Pt peaks in an X-ray
0022-3654/93/2097-9 161%04.00/0 0 1993 American Chemical Society
9162 The Journal of Physical Chemistry, Vol. 97, No. 36, 1993
diffraction pattern. High metal dispersion was deemed important for this work to minimize the magnetic properties of the small metal particles. To prepare the catalyst for chemisorption, approximately 0.5 g of catalyst was placed in a U-shaped quartz reactor. A 10mm-0.d. N M R coil was slipped around the reactor, which was then connected to the body of the N M R probe with ultra-Torr fittings. The probe assembly was connected to a vacuum system and degassed to 10-6 Torr. Catalyst was preheated in vacuum to 100 OC for about 1 h to assist in degassing. Ultrahigh-purity Hz, further purified by flowing through a heated Pd trap and molecular sieves, was used to reduce the catalyst at room temperature for about 1 h. Sample temperature was then increased slowly, over the course of 2 h, to 400 OC and held a t this temperature for 4 h. Afterward, the catalyst was evacuated for 8 h and subsequently cooled to room temperature or lower prior to use. Iron and other paramagnetic impurities can strongly influence the N M R line shapes; therefore, care was taken to minimize the introduction of such contaminants to the catalyst. The following procedure was used to introduce isotopically enriched gas to the catalyst. An adsorption isotherm was generated to determine the saturation uptake and to verify the spin count obtained from N M R experiments. Following adsorption, the sample was degassed to 1 o d Torr and the reactor back-filled with He to 800 Torr to minimize leakage of oxygen into the reactor. The reactor was then sealed by closing two vacuum valves located at the top of the N M R probe and removed from the vacuum system. Singly and doubly 13C-labeled ethene, enriched to 99%, and perdeuterated ethene, also enriched to 99%, were obtained from either MSD Isotope Inc. or Isotec, Inc., and used without further purification. Samples of 13C-labeled ethene adsorbed on 5% Pt/AlzO, were prepared at four different adsorption temperatures, 243, 263, 297, and 323 K. Samples prepared at 243 or 263 K were cooled to 77 K for acquisition of the N M R data, to avoid possible thermal restructuring of the adsorbate. These samples were subsequently warmed to room temperature and compared with samples prepared initially at 297 K. Samples prepared a t 297 or 323 K were examined first at the adsorption temperature and then at 77 K in order to investigate the effects of adsorbate motion on the I3C N M R line shape. To assess the thermal stability of the adsorbed structures, some samples were heated above room temperature. It is important to note that ethene chemisorption may be highly exothermic: reproducible NMR results require careful control of sample temperature during adsorption. For samples dosed with ethene at 243 and 263 K, we used the same proceduredescribed above with theexception that, following evacuation of the reduced catalyst, the reactor was placed in a dewared container and the temperature lowered with cold nitrogen gas. For samples prepared a t 323 K, the reactor was placed in a heater and the adsorption performed at warmer temperatures. Samples which required heating above 323 K ,were typically prepared at room temperature, sealed, and then heated in an oven for 3 h. Deuterium quadrupolar couplings were recorded for 2H-labeled ethene adsorbed on our catalyst. These samples were prepared at two different adsorption temperatures, 253 or 297 K, and cooled subsequentlyto 77 K to record N M R spectra. Deuterium samples were prepared as described above for 13C gases; however, for all deuterium experiments we sealed samples in 9-mm-0.d. N M R tubes. Immediately following preparation, these samples were stored at 77 K to minimize the exchange of hydrocarbon deuterons with surface adsorbed hydrogens and hydroxyl protons of the catalyst support. NMR spectra were acquired with a home-built spectrometer operating at 27.8 MHz for 2H and 45.6 MHz for 13C. An in-situ probe and 13C/lH double resonance circuit was constructed, similar to one described in ref 17. A separate single resonance
Griffiths et al. probe was used for *Hexperiments. Due to low spin counts and the small gyromagnetic ratios of 13C and 2H nuclei, the NMR spectra presented here required approximately 30 000 spin-echo accumulations for the l3C spectra and 200 000 quadrupolar-echo accumulations for the ZHspectra. Carbon- 13 experiments were based on a 9OX-7-1 8OY spin-echo sequence with the initial transverse carbon magnetization created by cross-polarization. Cross-polarization was used chiefly to minimize the observation of magnetization from *'A1 nuclei of the alumina support which overlap the 13Cfrequency a t this field strength.l8 A 7r pulse was used to refocus carbon magnetization and avoid the loss of signal from probe ringdown. Nominal crosspolarization contact time was 2 ms using a spin-locking field of 30 kHz. Signal cosubtraction was achieved by phase-alternating the proton 7r/2 prepulse. The proton spin-locking field was incremented to 8 G immediately following the spin-lock period for proton decoupling. A recycle delay of 2 s was selected for these experiments based upon proton spin-lattice relaxation times. Some experiments were performed with delayed decoupling or with variable cross-polarization contact time; these experiments are described later. All 13C spectra are referenced to TMS using the 6 scale where downfield is positive. Solid adamantane and hexamethylbenzene were used to determine probe sensitivity and to establish spin count references. Deuterium spectra were acquired using a quadrupolar-echo sequence consisting of two phase-alternated pulses of 4.6 ps separated by 50 ps. A recycle delay of 0.3 s or longer was selected for these experiments. For in-situ experiments, N M R pulses were set on the sample directly by introducing an excess of gas to the prepared catalyst. This was accomplished conveniently after generating an adsorption isotherm and before removing physisorbed molecules. Sufficient signal was available to set accurate pulses with the exact configuration of the coil, glass reactor, and catalyst in the reduced state. Excess gas was removed by evacuation, leaving only the chemically adsorbed state. For experiments using sealed samples, pulses were set with either an aqueous solution of W-enriched sodium formate or 2H20, sealed in a small tube, and encased in reduced Pt catalyst. In this fashion, pulses were set on a standard sample whose bulk conductivity matched that of our catalyst samples. Furthermore, the impedance of the probe circuit did not vary significantly from sample to sample. The HartmannHahn condition was then verified with a standard sample of hexamethylbenzene crystals ground and mixed with catalyst. Results Adsorption Isotherms. An adsorption isotherm for ethene, generated at 297 K, is shown in Figure 1. The saturationcoverage was determined by the difference between the total and reversible uptake extrapolated to zero pressure. The saturation coverage was found to be equivalent to one adsorbed molecule for four surface Pt atoms and is consistent with the saturation coverage reported for this molecule on Pt( 11l).l9 For our sample size (0.5 g), this coverage corresponds to 1.4 X 101913Cnuclei for ethene enriched isotopically at a single carbon atom. While the spin counts we obtained from magnetic resonance data are subject to quantification limitations inherent in N M R data obtained by cross-polarization, it is important to note that these spin counts were always consistent with the saturation coverage of irreversibly adsorbed ethene. The possibility of ethene adsorption on the alumina support was investigated by generating adsorption isotherms of catalyst prepared with weight loadings of 5%, 1%, and 0% Pt. These samples showed trends consistent with physical (reversible) and chemical (irreversible) adsorption on the metal-containing catalysts but physisorption only on the alumina (0% Pt) sample. Furthermore, 0% Pt samples resulted in no l3C N M R signal following degassing.
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TABLE I: Fitted Model Parameters for Perdeuterated Ethylene Adsorbed on Pt/AlZO3' adsorption temp (K) 130 kHz (%) 41 kHz (9%) 263 297
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a The reported frequenciesare the measured peak splittings of powder doublets.
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Figure 1. Adsorption isotherm for ethene at 297 K. Triangles show the
total uptake of ethene, and circles show the reversible uptake, recorded after evacuationof the sample. The lines are drawn as an aid to the eye. Saturation coverage is determined by the difference of the two curves extrapolated to zero pressure.
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Figure4. Carbon-13NMR spectrumof single-labeledethcne,condensed on the catalyst at 297 K and immediately frozen at 77 K for data
acquisition. Dots show the measured data.
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Frequency (kHz) Figure 2. Deuterium NMR spectrum of perdeuterated ethene adsorbed on F%/AlzOoat 253 K. The solid line represents a least-squaresfit to the data.
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PPm Figure 5. Carbon-13 NMR spectrum recorded followingadsorption of
ethene on 5% Pt/A1203 at 243 K. Dots show the measured data, and the solid line shows a Gaussian fit to the data.
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Frequency (kHz) Figure 3. Deuterium NMR spectrum of perdeuterated ethene adsorbed on R/A1203 at 297 K. The solid line represents a least-squares fit to the
data. Excess gas desorbed from the catalyst during adsorption isotherm experiments at room temperature was collected and analyzed by mass spectrometry; about 1% ethane and less than 1% butane were detected with the remainder being ethene gas. Deuterium N M R Results. Deuterium spectra recorded following adsorption of CzD4 a t 253 and 297 K are shown in Figures 2 and 3. Two distinct quadrupolar powder patterns are present a t both conditions with width between singularities of 130 and 41 kHz, respectively. For a ZH quadrupolar powder pattern with
no asymmetry, the width between singularities is related to the quadrupolar coupling by a multiplicative factor of 4/3.20 Theoretical quadrupolar powder patterns, convoluted with Gaussian broadening functions, were fit to the *Hspectra using a leastsquares algorithm. These results are also given in Figure 3. Intensities, corrected to account for the finite widths of zHpulses,21 are given in Table I. It is important to note that these sealed samples never exhibited spectral features associated with deuterium gas (Dz). Carbon-13NMR Results. The l3C spectrum of singly labeled ethene is presented in Figure 4. This spectrum was recorded following introduction of excess ethene gas to the catalyst and immediately cooling the sample to 77 K and should represent intact, frozen ethene. A Gaussian function was fit to these data to obtain the center of mass of the line shape. The center of mass of the line was found to be 126 ppm, which is consistent with the known chemical shift of ethene. The half-width at half-maximum (hwhm) was found to be 3.2 kHz. Figures 5-8 show the 13CNMR spectra recorded for irreversibly adsorbed ethene at 243, 263, 297, and 323 K, respectively. Gaussian functions were fit to the data, and the model parameters are given in Table 11. The spectrum recorded a t 77 K following
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9164 The Journal of Physical Chemistry, Vol. 97, No. 36, 1993
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TABLE II: Fitted Model Parameters for l3C Ethylene Adsorbed on 5 percent Pt/Alz03 at Various Temperatures from 243 to 323 K adsorption Gaussl hwhml Gauss2 hwhm2 Gauss3 hwhm3 temp(K) (ppm) (kH4 (ppm) (Hz) (ppm) (Hd 243 73 (100%) 3.0 263 70(40%) 3.0 90(40%) 2.0 201 (20%) 4.7 297 90(40%) 2.0 130(30%) 3.5 210(30%) 8.0 323 90(50%) 2.0 196(50%) 7.0
TABLE III: Fitted Model Parameters from Variable Cross-Polarization Contact Time Experiments for Ethylene Adsorbed at 297 K 600
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PPm Figure 6. Carbon-13 NMR spectrum recorded following adsorption of ethene on 5% R/Pt/A1203at 263 K. Dots show the measured data, and the solid line shows a sum of three Gaussian functions (dashed lines) which best fit the data.
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PPm Figure 7. Carbon-13 NMR spectrum recorded following adsorption of ethene on 5% Pt/A1203at 297 K. Dots show the measured data, and the
solid line shows a sum of three Gaussian functions (dashed lines) which best fit the data.
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PPm Figure 8. Carbon-13 NMR spectrum recorded following adsorption of ethene on 5% R/AlzO3 at 323 K. Dots show the measured data, and the solid line shows a sum of two Gaussian functions (dashed lines) which best fit the data. adsorption at 243 K (Figure 5) shows a broad line with an average resonance of 70 ppm. The spectrum recorded using a crosspolarization contact time of 80 ps exhibits similar features but with an overalldecreasein signalintensity. The spectrum recorded using a decoupling delay of 50 ps resulted in no signal, thereby suggesting that all the carbon nuclei experience strong proton dipolar coupling. The I3C spectrum recorded at 77 K after irreversibleadsorption of ethene at 263 K (Figure 6) shows additional features. One
80 85(50%)
1000 90(46%) 2000 90(40%)
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130(28%)
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component of the spectrum has the same 70 ppm average shift and approximate line width of the species observed for the sample prepared at 243 K. A second component has an average resonance of 90 ppm. A third component has an average resonance of 200 ppm and is a particularly broad resonance having a hwhm greater than 5 kHz. Variable contact cross-polarization experiments showed that recovery of all three components was possible with a contact time of 80 ps and yielded intensities about one-half the magnitude of those obtained with a 2-mscontact time. The spectrum recorded using a decoupling delay of 50 ps resulted in a reduction of intensity of all three carbon signalsand in particular the 70 ppm peak. Thus, all carbons experiencesignificant dipolar interactions;however,the 90 and 200 ppm peaks experienceproton dipolar coupling that is less than that of the 70 ppm peak. The spectrum recorded after irreversible adsorption of ethene at room temperature (297 K) is shown in Figure 7. Spectra having identical features were recorded when ethene was irreversibly adsorbed at a lower temperature (243 K) and then warmed to 297 K or when adsorption was carried out at 297 K. This spectrum is well fit by three Gaussians, centered at 90, 130, and 200 ppm. The latter resonance peak is particularly broad and is well fit by a Gaussian of 8.0 kHz hwhm. We obtained additional information by monitoring the relative contribution of each Gaussian line shape following changes in cross-polarization contact time. The results of least-squares fits to these line shape data for several different cross-polarization conditions are given in Table 111. These data show that all three components are recovered even when a short contact time is used although an overall decrease in signal intensity was also observed. Using a decoupling delay of 50 ws yielded a spectrum showing reduced intensity for all three carbon resonance peaks, with the 130 ppm peak most strongly attenuated. This set of results suggest that all three componentsexperiencestrong proton coupling,although the 90 and 200 ppm peaks experience proton coupling that is reduced compared to the 130 ppm peak. The 13Cspectrum obtained by preheating fresh catalyst to 323 K and performing the adsorption and subsequent evacuation of weakly adsorbed ethene at this temperature is given in Figure 8. A spectrum with identical features was obtained by taking a sample initially prepared at room temperature and heating it to 323 K for 3 h. In both circumstances, two distinct resonances are evident, and the spectra are well-fit by two equal intensity Gaussians centered at 90 and 200 ppm. The features of this spectrum could be reproduced, with some loss of overall signal intensity, using a short contact time of 80 ps. Furthermore, using a decoupling delay of 50 ps, we observed an overall decrease in signal intensity and a change in the relative intensity of the peaks at 90 and 200 ppm to 80% and 208, respectively. Motion can influence the shape and width of chemical shift line shapes. To determine whether our 13C line shapes were influenced by motional averaging, spectra were recorded of
Ethene Adsorption on Alumina-Supported Pt Surfaces I
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spin lock time (ms) Figure 9. Carbon-13 signal intensity as a function of proton spin-lock time. A biexponential fit is shown by the solid line. samples prepared at 297 and 323 K both at room temperature and with the sample cooled to 77 K. In both cases, however, these line shapes showed no change in shape or line width at the lower temperature. Spin-echo experiments were used to measure the 13C spin-spin relaxation time, T2. These experiments were performed for two separate samples, prepared by adsorption at 297 and 323 K.In both cases, a semilog plot of the intensity versus echo time yielded a straight line with T2 values of 300 and 250 ms, respectively. Features of these 13Cline shapes were found to be independent of echo time, and we conclude that spin-spin relaxation could not be used to isolate individual components of the 13C spectra. In some cases two ethene sampleswere prepared simultaneously, one enriched with l3C at a single carbon atom and one enriched at both carbon atoms. Identical carbon-13 experiments were performed on both samples, and the two resulting sets of spectra were compared. Samples prepared at two adsorption temperatures, 297 and 323 K,were examined in this fashion, and in both cases spectra from doubly-labeled ethene exhibited additional line broadening attributable to dipolar coupling with neighboring 13Cnuclei.22 The difference in second moments of these spectra should correspond to I3C-l3Cdipolar couplings. Furthermore, since the surface coverage is small, this dipolar coupling likely results from intramolecular couplings. For samples prepared at 297 K, we have determined a coupling (1.3 kHz) close to that expected for a single carbon bond length. A similar result was determined for samples at 323 K. For this calculation, the two carbon spins were taken to be "unlike" spins due to the large difference in chemical shifts.20 Although a fraction of adsorbed ethene molecules may fragment even at moderate temperatures,23 our spins count and isotherms measurements indicate we are observing in excess of 95% of the carbon nuclei. Consequently, our measurements indicate the predominant adsorbed species at 297 and 323 K contain carbonsarbon bonds, the lengths ofwhich are close to that of a single bond. Proton rotating frame relaxation (TI,) measurements, determined from I3Ccross-polarization intensities, were recorded for samples with ethene adsorbed at 323 K. The proton spin lock time was adjusted from 2 to 100 ms, and the carbon spin lock time was held at 2 ms. The carbon signal intensity was measured as a function of proton spin-lock time, and these results are given in Figure 9. The data are well represented by the sum of two exponentials with time constants corresponding to T I ,values of 5 and 130 ms. Heated Samples. Samples of adsorbed ethene were sealed in glass tubes and heated progressively in increments of approximately 50 K from 297 to 623 K for periods of 3 h. Due to the particularly weak NMR signals resulting from heated samples, all subsequent spectra were recorded at 77 K. A representative spectrum, shown in Figure 10,was recorded following heating of
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Figure 10. Carbon-13NMR spectrum (cross-polarizationtime is 2 ms) recorded following adsorption of ethene on 5% Pt/Al2O3 and subsequent heating to 523 K for 3 h. This spectrum was recorded at 77 K. Dots show the measured data, and the solid line shows a sum of two Gaussian
functions (dashed lines) which best fit the data.
a sample to 523 K. This spectrum shows a sharp resonance peak at 5 ppm which is likely due to methane gas (based upon its chemical shift and narrow line width). A second, broader resonance peak centered at 100 ppm is also evident. In sealed samples, it was possible to tip the catalyst out of the NMR coil region and record spectra of gas-phasecomponents;onour samples this experiment yields the 5 ppm (methane) resonance peak but not the broader resonance peak. A methane peak was observed in all samples heated above 373 K, however, the remainder of the spectrum was composed of broad, unresolved resonance peaks and was difficult to interpret in the temperature range 373-523 K. Samples heated to 523 K and higher show clear evidence of the resonance centered at 100 ppm. As samples were heated, we observed a dramatic decrease in the l3C signal intensity using conditionsof cross-polarization,suggestingthat carbon adsorbates dehydrogenate at elevated temperatures. In fact, a series of 13C spectra, recorded without cross-polarization(left-shiftedto remove the broad aluminum resonance signal), show similar features and signal intensitiesto spectra recorded using cross-polarization. Furthermore, we are able to use these spectra to determine that by 623 K one-half of the I3Csignal intensity is due to surface carbon and one-half is due to methane.
Discussion
NMR Considerations. To identify chemical species on the platinum surface, we must first consider the various factors influencing the interpretation of the NMR data presented above, namely, the 13Cresonance line shapes and line widths, the extent of dipolar coupling of 13Cnuclei to protons, and the quadrupolar line shapes for deuterons. Carbon-13 line shapes in solids are expected to be dominated by the anisotropic chemical shift interaction producing characteristic "powder pattern" line shapes which depend upon chemical bonding and hybridization. The 13Cresonances presented here, however, are best described by Gaussian functions which shows none of the characteristic singularities of chemical shift powder patterns. We surmise that the spectra reported here represent powder patterns obscured by line broadeningassociated with small metal particles on heterogeneous catalyst surfaces. Several sources of broadening are worthy of consideration, including chemical shift anisotropy, metal susceptibility, and distributions of isotropic chemical shifts. It should be noted, however, that incomplete or off-resonance decoupling could also contribute to the line width, although under our experimental conditionsproton decoupling was complete.
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An illustration of how broadening obscures chemical shift anisotropy is seen in the 13Cspectrum of intact, frozen ethene given in Figure 4. This spectrum is well-fit to a Gaussian, shows no powder pattern features, and has a second moment of 6.9 kHz2. The anticipated second moment for an isolated ethene molecule which has a well-defined powder pattern is 5.7 kHz2.24 The additional broadening can be attributed to bulk susceptibility due to the catalyst material and the physical shape of the reactor and surrounding probe components. Metal susceptibility broadening arises from local fields created by conduction electrons a t the surface of a particle in the presence of a magnetic field. The magnitude of this broadening depends on the metal type and particle size, the external field strength, and to a lesser extent on the adsorbate. For small spherical Pt particles the maximum magnetic susceptibility broadening is estimated to be about 100 ppm,25 corresponding to a second moment of 19.3 kHz2. Susceptibility broadening decreases by a factor of 4 for 2.2-nm particles and decreases to zero for monatomically dispersed meta1.26 Since the extent of broadening is related closely to particle size, the spectral broadening we observe also reflects the range of particle sizes that exist in heterogeneous catalysts. Furthermore, it is reasonable to expect a variation in broadening across a spectrum due to the orientational dependence of magnetic susceptibility. In this work, catalyst samples were prepared with dispersions corresponding to particles of 2.0-nm diameter on average; hence, the contribution of susceptibility to line width is important but small compared to low dispersion ((50%) catalysts.13b Knight shift broadening has been documented for CO adsorbed on Pt and arises from spatial variations of the Knight shift across a particle or from a distribution of particle size^.^^ For an adsorbed molecule, it is likely that a carbon nucleus bonded directly to a metal will experience a Knight shift, while a carbon nucleus that is further removed from the metal will not because the presence of conduction electrons at the nucleus is expected to decrease rapidly with distance from the surface. For surface structures we expect the Knight shift to contribute only to the carbon atoms bonded directly to the metal. The final broadening mechanism considered here is the distribution of isotropic shifts arising from variations in surface adsorption sites. This distribution has been proposed elsewhere to explain the large I3C line widths observed for 13C0adsorbed on P t / A 1 ~ 0 3 .It~is~ possible to correlate surface site heterogeneity with measurements of surface heats of adsorption; however, we did not conduct such measurements and thus have no way of quantifying surface site heterogeneity in our samples. Preliminary magic angle sample spinning data for our samples, however, show that I3C resonances are not narrowed by magic angle spinning. This is consistent with broadening which originates from a distribution of isotropic chemical shifts. In summary, we believe that the association of distinct carbon chemical bonding environments with a Gaussian function, as opposed to chemical shift anisotropy powder patterns, is justified given the various broadening mechanisms associated with adsorbates on dispersed catalysts. With this assumption we expect to beable touse thecenter ofmassofthevariouscarbon resonances for identification of carbon bonding configurations and the halfwidth at half-maximum (hmhm) to characterize the line width. In addition to carbon chemical shifts, the extent of dipolar coupling to protons is an important structural probe. Since the ability tocross-polarize relies on the magnitude of the C-H dipolar coupling, the cross-polarization experiment can be used to distinguish carbon types by measuring carbon signal intensities as a function of cross-polarization time using so-called ”variable contact cross-polarization” experiment^.^^ For hydrocarbons the approximate order of coupling strength is given by C < CH < CHI < CH3. The dipolar coupling for methyl carbons, however,
Griffiths et al. is reduced due to rapid rotation, and the realized coupling strength is closer in magnitude to that of a CH group. Strong proton decoupling was used to obtain the 13C spectra presented here; however, a variation of continuous decoupling, known as the delayed decoupling e ~ p e r i m e n twas , ~ ~ used to complement the variable contact cross-polarization experiments described above. In this experiment the proton decoupling field is turned off for a periodof time, typically 50-100 ps, immediately following cross-polarization. During the delay period, and prior to introducing a decoupling field, the transverse magnetization from protonated carbon atoms dephases rapidly, leaving essentially no magnetization for the u pulse to refocus. Consequently, the signal of protonated carbon atoms is strongly attenuated, while the signal of nonprotonated carbon atoms is unaffected and dominates the spectrum. The dynamics of this experiment can be discussed by grouping the various CH, moieties according to the magnitude of the C-H dipolar coupling, as done above for the cross-polarizationexperiment. Typically, C H and CH2 groups are strongly proton-coupled, and the carbon- 13 magnetization decays rapidly in the absence of decoupling. The magnetization from CH3 groups is affected mildly by delayed decouplingbecause the C-H coupling is scaled by rapid rotation. This method can be used to distinguish nonprotonated carbons from protonated carbons and, if sufficient spectral resolution is present, to assign C-H stoichiometry. Deuterium quadrupolar doublets obtained from covalent C-D bonds are characteristic of functionalgroup dynamics. Deuterons in static C-D bonds exhibit a quadrupolar doublet splitting of approximately 130 kHz, whereas methyl deuterons, which rotate rapidly (on the N M R time scale) about their C3 axis, exhibit a motionally averaged quadrupolar splitting of approximately 4 1 kHz. Species Identification. The 13C line shape recorded following adsorption of ethene at 243 K (Figure 5) is a broad featureless peak with an average resonance of 70 ppm, shifted substantially from the resonance position of intact ethene (126 ppm).31 This peak is well-fit by a single Gaussian, and since no changes in the features of this line shape were observed with delayed decoupling and variable cross-polarization experiments, it is likely that this spectrum results from a single carbon environment. Delayed decoupling and variable contact time cross-polarization experiments verify that the observed carbon is hydrogenated, although these experiments were not analyzed quantitatively to determine the C-H stoichiometry. Two structures consistent with these observations are u-bonded and di-a-bonded ethene. A commonly used spectroscopic reference for u-bound ethene, KC13Pt(C2Hd) (“Zeise’ssalt”), exhibits a carbon- 13chemical shift of 67 ppm.32-33 On this basis, we assign the 70 ppm peak to u-bound ethene. This assignment is consistent with our observations that the 70 ppm peak is easily removed, or converted to something else, with evacuation or warming to room temperature. The I3C spectrum obtained following adsorption at 263 K (Figure 6) shows evidenceof the u-bound species which resonates a t 70 ppm and two additional resonances at 90 and 200 ppm. These two peaks also appear in 13C spectra recorded following adsorption at 297 K and are particularly clear after adsorption at 323 K (Figure 8). Furthermore, the I3C-W coupling measured in doubly-labeled ethene adsorbed at 323 K corresponds to an internuclear distance of 0.16 nm, close to that of a carbonxarbon single bond. This leads us to conclude that the carbon nuclei associated with the peaks at 90 and 200 ppm belong to the same molecule. To assist in our assignment of the I3C spectra, we turn briefly to a discussion of the 2H spectra recorded after irreversible adsorption of ethene at 263 and 297 K. These data (Figures 2 and 3) are composed of two distinct powder doublets providing evidence for at least two deuterium environments. The broader doublet, with a peak splitting of 130 kHz, has a quadrupolar
Ethene Adsorption on Alumina-Supported Pt Surfaces coupling consistent with deuterons in static C-D bond environments at 77 K. The quadrupolar coupling associated with the 41 -kHz doublet is characteristic of rotating methyl groups. Furthermore, as shown in Table 11, the relative signal intensity of the 2H methyl doublet increases with adsorption temperature compared to the static C-D environment. From these results it is reasonable to propose that the methyl-containing adsorbate is favored at warmer temperatures. Deuterium gas or deuterium bound to either Pt metal or the oxide was never detected in our experiments, suggesting that the stoichiometry of four protons to each pair of carbon atoms is retained up to at least 323 K. Given that the deuterium results identify methyl groups as a large part of the total species population, we surmise that one feature in the carbon-13 spectra must arise from a methyl carbon. Methyl carbons usually resonate close to 23 ppm, although chemical shifts in the range of 3 5 4 5 ppm are reported for the ethylidyne ligand in clusters of Cr,34W,35and C O . The ~ ~ methyl carbon in ethylidyne and ethylidene ligands in Ru complexes are shifted downfield to about 45 However, Pt complexes generally exhibit larger shifts than Ru complexes, and there is considerable precedent for unusual carbon shifts for admolecules on metal s ~ r f a c e s . ~ ~We J 8 suggest that the 90 ppm peak reported here is associated with methyl carbon atoms of oneor more ethenederived species. Although this shift is clearly outside the range normally expected for methyl carbons, our assignment is wellsupported by the delayed decoupling,variable cross-polarization, and deuterium results presented here. Ethyl, ethylidyne, and ethylidene are methyl-containing adsorbates postulated to form from ethene on the Pt( 111) surface.38 No peaks are observed in the range expected for linear alkyl species in Pt complexes39 or on transition-metal surfaces.40 Furthermore, the carbon line widths of alkyl species, such as ethyl, are expected to be small because of motional averaging, again inconsistent with our observations. Hence, ethyl can be ruled out safely. Considering the two remaining structures, we suggest that the 90 ppm peak is associated with methyl carbon atoms of one of these adsorbatesand the 200 ppm peak is associated with a second carbon atom which is bonded directly to the metal surface. After irreversible adsorption at 323 K the 90 and 200 ppm carbon resonances could be reproduced quantitatively with a short (80 ps) contact time. This establishes that both carbons are bonded directly to protons. Indeed, variable contact crosspolarization experiments under identical conditions with hexamethylbenzene as a test molecule showed that the nonhydrogenated aromatic carbons were not polarized with a 8 0 - p contact ~ time.41 Delayed decoupling experiments show an increase in the relative intensity of the 90 ppm resonance compared to the resonance at 200 ppm. This is consistent with the decrease in C-H coupling expected for a rotating methyl carbon and suggests further that thecarbon atom bonded to themetal is alsoassociated with protons. Furthermore, proton rotating frame relaxation experiments (Figure 10) show two different relaxation time constants, consistent with two proton environments of differing local mobility. The spectral density function of the shorter TI, component is consistent with a correlation time faster than 20 kHz, as expected for methyl groups. The second value, 130 ms, corresponds to a slower relaxation mechanism and may result from a more complex type of motion yielding a less efficient relaxation mechanism. Taken together, these data demonstrate that both carbons are directly bonded to protons which in turn are experiencing different local mobilities. This is consistent only with the ethylidene structure. The strong downfield shift of the 200 ppm peak is, in fact, consistent with the shifts reported for carbons bonded to metals. For example, the bonding carbon in ethylidyne-Ru and ethylideneRu complexes are shifted to 219.3 and 142.95 ppm, respectively.37 The resonance position of the bonding carbon in ethylidyne in clusters of Cr, W, and Co are shifted downfield to
The Journal of Physical Chemistry, Vol. 97,No. 36, 1993 9167 230-360 Large resonance shifts have also been reported for carbon atoms adsorbed on supported metal catalysts. For example, the resonance position of carbidic carbon atoms is reported as 400 ppm on a Ru catalyst.42 The large line width reported here (second moment of 16.9 kHz*) is also consistent with the severe broadening expected for carbon atoms bonded directly to small metal particles. The 13Cspectrum recorded following adsorption of ethene at 297 K is composed of the two resonances attributed to ethylidene and a third resonance at 130 ppm. The resonance at 130 ppm only appears at the room temperature condition and coincides with the disappearanceof the 70 ppm resonance. Variable contact cross-polarization data show that individual contributions to the line shape do not change significantly with contact time; delayed decoupling results show the peak at 130 ppm to have the strongest dipolar coupling to protons, consistent with that expected for CH2 groups. Furthermore, the deuterium spectra shown in Figure 2 clearly show the fixed C-D line shapes anticipated from an ethene-like structure. Upon heating to 323 K, however, the resonance at 130 ppm disappears, and it is not evident from the data presented here whether this species leaves the surface as gas-phase ethene or converts to ethylidene. Finally, the doublylabeled carbon- 13 measurements suggest that this species has a carbon-carbon bond length close to that of a single bond. It is tempting to speculate that this species may be a " d i d form of bound ethene. It is important to note, however, that there is no precedent in the organometallic literature for this assignment. The deuterium spectra shown in Figures 2 and 3 possess features associated with both the ethylidene structure (90 and 200 ppm carbon resonances) and the other structures (70 and 130 ppm carbon resonances (vide supra)). Furthermore, as shown in Table 11,the relative signal intensity of the 2H methyl doublet increases with adsorption temperature compared to the static C-D environment. From theseresults, it is reasonable to propose that the ethylidene structure is favored at higher temperatures. This is consistent with the assignment of the 90 ppm carbon-13 peak to methyl groups which persist in the 13C spectra following adsorption at 263, 297, and 323 K. The results of the present investigation are consistent, in part, with previous studies of ethene interactions with Pt surfaces. Our NMR results suggest that a u-bound ethene specieswith chemical shift of 70 ppm is present on Pt/A1203 at low temperatures; warming to room temperature results in an ethene-like species with a chemical shift of 130 ppm that appears as the 70 ppm species disappears. This is consistent with both infrared studies on Pt/A120s43#"and HREELS studies conducted with Pt( 11 1).6 u-bonded ethene is reported to coexist with di-u ethene on supported Pt catalysts,434 and this adsorbate is weakly bound and desorbs readily or is removed upon hydrogenation. *-bonded ethene is rarely seen on single-crystal surfaces; however, EELS studies haveshown that this intermediatecoexistswithdi-uethene on the (2X 1) Pt( 110) surface6 but forms on Pt( 111) only in the presence of coadsorbed oxygen atoms6 Several studies of supported Pt catalysts have provided evidence for ethylidyne as the stable near room temperature adsorbate.13b*4s4749LEEDS, HREELS,SO and T D S 1 studies indicate the ethylidyne is the predominant stable adsorbate on Pt( 111) from 290 to 450 K. In most of these studies it is generally agreed that di-o-bondedethene is the precursor to ethylidyne, consistent with the data and interpretationpresented here. Our observations,however,strongly support the formation of ethylidene rather than ethylidyne, as discussed above. Relatively few studies have addressed the thermal decomposition of adsorbed ethene. An EELS study of Pt( 111) has shown that heating above 450 K causes the C-C bond of ethylidyne to break, producing a small quantity of hydrogenated fragments such as CH.9 Above 700 K, thesecarbon fragments lose hydrogen and rearrange into graphite.8~52TPD and EELS studies of the
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The Journal of Physical Chemistry, Vol. 97, No. 36, 1993
(1 X 1) Pt( 110) surfaces differ from Pt( 111) studies: ethane, methane, ethan- 1-yl-2-ylidyne,hydrogen, and adsorbed carbon are formed after adsorption of ethene at room temperature; after further heating, dehydrogenated “CZnspecies exists of the metal surface. Variable-temperature IR studies on Pd/SiOz have demonstrated that ethylidyne bands appear at about 150 K and remain until 400 K.53 At this temperature, introduction of more ethenegas produced little new ethylidyne, leading to the proposal that the surface may be covered by carbon fragments formed by ethylidyne decomposition. An NMR study which monitored the 13C-13C and W-1H couplings as a function of temperature has shown that ethylidyne decomposes to yield dehydrogenated and isolated carbon atomsolo It would appear that at elevated temperatures ethylidynedecomposesto form hydrogenated carbon fragments on Pt( 11l), while on supported Pt catalysts and some single-crystal surfaces there is evidence that isolated and dehydrogenated carbon atoms form instead. By analogy, we suspect that the thermal decomposition of ethylidene should produce surface carbon. Carbon-1 3 spectra recorded following heating samples above 323 K show evidence of methane gas and a complex resonance probably associated with decomposition products of the adsorbate. However, by 523 K, a distinct resonance centered at 100 ppm emerges which is extremely difficult to observe with cross-polarization,suggesting loss of directly bonded hydrogen. Although these spectra yield no further insight into the decomposition of ethylidene, we postulate that the first step in this process involves transfer of the proton from the surface carbon to the methyl carbon, followed by scission of the carbonsarbon bond. The carbon remaining on the metal will be an isolated, dehydrogenatedcarbon, consistent with our interpretation of the spectrum shown in Figure 9. The discrepancybetween the nature of hydrogenated carbon fragments formed on Pt( 111)and the isolated carbons formed on a supported catalysts may be important for understanding the mechanisms by which deactivating carbon is formed. The contact between our present conclusionsand those reported for single-crystal surfaces and dispersed catalysts is intriguing. Previous NMR studies1°J3have utilized lower dispersion samples, and most data were obtained without cross-polarization. We suggest that traditional high-resolution NMR techniques, such as cross-polarization, are effective only for catalytic systems containing metal particles which are extremely small such that attendant line-broadening mechanisms are minimal. Assuming that the ethylidyne structures reported previously and the present ethylidenestructures are correct, then we speculate why this could be based strictly upon particle size effects. Studies of platinum surfaces with co-adsorbed potassium7 reveal that ethylidene is the dominant surface structure. The authors of these studies suggest that co-adsorbed potassium increases the electron density on the Pt and thereby stabilizes r-bonded structures such as ?r ethene and ethylidene; similar arguments are invoked to explain the stabilization of A ethene with oxygen coadsorption.6 The NMR observationsreported here are consistent with the structures on these modified Pt surfaces as opposed to those reported for pristine surfaces. Does this suggest that, in contrast to (1 11) surfaces and low dispersion supported platinum, finely dispersed particles exhibit increased electron densities at their surfaces? While density functional theory calculations of Pt films show that low-energy surface densities of state are much larger than bulk values?) it is not clear how decreasing particle size would affect these densities of states. Platinum NMR studies of bulk55 and supported56~5~ catalysts exhibit line broadening consistent with surface electron spin density oscillations. Since decreasing particle size results in an increase in the surface-to-volume ratio, are the structures we observe vis-a-vis low dispersion catalysts indicativeof different chemistries of small particles? A systematic NMR study of ethene adsorbed on Pt particles of varying size and co-adsorbed atoms would be fruitful in elucidating this issue.
Further NMR studies, however, will be hampered by of our poor understanding of the interpretation of NMR shifts and line widths in the presence of metals. Conclusions In the study presented here, broad-line 13Cand *HNMR spectra are used to determine the structure of adsorbed ethene at various temperature conditions, and changes in those structures brought about upon heating. Our results demonstrate that ethene adsorbs on highly dispersed 5% Pt/A1203 in a a-bound configuration at low temperature. Upon warming to room temperature, this species is partially converted to ethylidene and possibly di-u-bonded ethene. Similar results have been reported for supported Pt catalysts, wherer-bondedetheneas wellas thedi-uandethylidyne species coexist at low temperatures. At 323 K the predominant species on this catalyst is ethylidene. This observation differs from the observation of ethylidyne in single-crystalstudies. Upon heating, ethylidene appears to decompose into methane gas and surface carbon. Thesecarbon atoms may be precursors to graphite or other forms of surface carbon. In contrast, hydrogenated carbon fragments are the products of ethylidyne decomposition on single-crystal Pt( 111). Acknowledgment. This work was supported by the donors of the Petroleum Research Fund, administered by the American Chemical Society, under Grant 19798-AC5. Facilities used in this work were provided by the Director, Office of Basic Sciences, Materials Sciences Division of the U S . Department of Energy, under Contract DE-AC03-76SF00098. The authors gratefully acknowledge the assistance of Phillip A. Armstrong with the deuterium experiments and Robert Giauque for the X-ray fluorescence measurements. References and Notes (1) Carter, E. A.; Koel, B. E. Surf.Sci. 1990, 223, 339-357. (2) Sheppard, N. Annu. Rev. Phys. Chem. 1988,39,589-644. (3) Bertolini, J. C.; Massardier, J. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: New York, 1984; Vol. 3b, pp 107-136. (4) Hatzikos, G. H.; Masel, R. I. In Catalysis 1987; Ward, J. W., Ed.; Elsevier: Amsterdam, 1988; p 883. (5) Yagasawki, E.; Masel, R. I. Surf. Sci. 1989, 222,430. (6) Steininger, H.; Ibach, H.; Lehwald, S.Surf.Sci. 1982, 117, 685. (7) Windham, R. G.; Koel, B. E. J. Phys. Chem. 1990,94, 1489. ( 8 ) Castner, D. G.; Sexton, B. A.; Somorjai, G. A. Surf.Sci. 1978, 71, 519. (9) Baro, A. M.; Ibach, H. J. Chem. Phys. 1981, 74, 4194. (10) (a) Ansermet, J.-Ph.; Slichter, C. P.; Sinfelt, J. H. Prog. NMR Spectrosc. 1990, 22, 401. (b) Wang, P.-K.; Ansermet, J.-P.; Slichter, C. P. Phys. Rev. Lett. 1985.55, 2731-2734. (1 1) Best, D. A.; Wojciechowski, B. W. J. Catal. 1972, 47, 11. (12) Corma, A.; Wojciechowski, B. W. Catal. Rev.-Sci. Eng. 1985.27, 11. (13) (a) Gay, I. D. J. Carol. 1987,108, 15. (b) Chin, Y.-H.; Ellis, P. D. J. Am. Chem. SOC.1993,115, 204. (14) Dorling, T. A.; Lynch, B. W. J.; Moss, R. L. J . Catal. 1971, 20, 19&201. (15) Wilson, G. R.; Hall, W. K. J . Caral. 1972, 24, 306. (16) Gates, B. C.; Katzer, J. R.; Schuit, G.C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979. (17) Haddix, G. W.; Reimer, J. A.; Bell, A. T. J. Caral. 1987, 106, 11 1. (18) Klug, C. A.; Slichter, C. P.; Sinfelt, J. H. J . Phys. Chem. 1991,95, 7033-7037. (19) Koestner, R. J.; Frost, J. C.; Stair, P. C.; Hove, M. A. V.; Somorjai, G. A. Surf.Sci. 1982, 116, 85. (20) Abragam, A. Principles of Nuclear Magnetism; Marshall, W. C., Wilkinson, D. H., Eds.; The International Series of Monographs on Physics; Oxford University Press: New York, 1961. (21) Bloom, M.; Davis, J. H.; Valic, M. I. Can. J. Phys. 1980, 58, 1510. (22) Broadening arises from I 9 T t nuclei is weak because of the small gyromagneticratio and low natural abundance (33%); hence, the anticipated broadening has a minor influence on the I3C line shape and is neglected. (23) Wang, P.-K.; Slichter, C. P.; Sinfelt, J. H. J. Phys. Chem. 1990,94, 1154-1 157. (24) Haeberlen, U. High Resolution NMR in Solids; Waugh, J. S., Ed.; Advances in Magnetic Resonance; Academic Press: New York, 1976. (25) Wang, P.-K.; et al. Science 1986, 234, 35.
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