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J . Phys. Chem. 1989, 93. 8072-8080
A typical phase diagram showing this behavior is presented in Figure 9. Compound B in this figure is the same as compound B shown in Figure 7. We have also reported a similar result when a highly tilted smectic C* phase was mixed with a low tilt angle C* p h a ~ e . ~However, J~ these two examples involve materials that have sufficiently different chemical and physical properties such that the injection of an unusual phase is not an extraordinary event. Yet, in study 3 we have two compounds that have a very similar chemical nature. Their dipolar natures, polarizabilities, spatial configurations, transitions, and transition temperatures are almost identical. The factor which is different is the steric shape; the molecular length of one of the components is much longer than that of the other. This in itself does not appear to be an important enough factor to cause the injection of a B phase into the phase diagram. However, when coupled with the fact that both materials do not have strongly polar groups at the ends of their aromatic cores, the mismatch of the molecular lengths together with a weak dipolar interaction of the McMillan type25 could lead to the destabilization of tilted phases. This is apparently the case with this particular mixture. For example, if one of the components had possessed a single terminal alkoxy group such an injection of a B phase might not have occurred. The results obtained in this study, like those in previous investigations:6 point in an indirect way to the possibility that phase (24) Budai, J.; Pindak, R.; Davey, S.C.; Goodby, J. W. J . Phys. (Paris) Lerr. 1984, 45, L1053. (25) McMillan, W. L. Phys. Reu. 1975, I I A , 365.
formation is controlled by steric factors, but at the same time the steric packing is responsible for allowing the dipolar forces to act in a limited number of ways so as to generate a particular pattern of phases. Thus the steric interaction can be seen as somewhat of a long-range force, whereas the dipolar interaction can be considered as more of a short-range force.
Conclusion We have shown that certain physical properties vary linearly in binary mixtures with respect to changing concentration, while at the same time other related properties vary in a nonideal fashion. We suggest that this is due to differences between the range and nature of steric interactions as opposed to dipolar interactions. We also use the unusual optical and electrical properties of ferroelectric liquid crystals as tools to further investigate the nature of phase diagrams. Registry No. (S)-CIOH210-C6H4-~COO-C6H4-p-O(CH2)2CH(CH,)(CH2),CH(CHj)2, 102853-03-2; (S)-CloH210-C6H4-p-CooC6H4-p-OCH2CH(CH,)C2H,, 74109-50-5; ( S ) - C ~ H ~ ~ O - C ~ H ~ - P - C O O C~H~-/PCOO(CH~)~CH(CH,)(CH~)~CH(CH,)~, 102852-96-0; (S)C~,H~,O-C~H~-~-COO-C~H~-~-COOCH~CH(CH(CH~)C~H 90937-62-5; ( S ) - C , ~ H ~ I - C ~ H ~ - ~ - C ~ H ~ - ~ C(CH2)2CH(CH3) O O - C ~ H ~ -(CHJ ~ - ( 3CH(CH3)2, 122800-94-6; (S)-Cg,H,,-C6H4-p-C6H4-p-coo-c6H4-pCHzCH(CH&Hj, 701 16-35-7;C9Hi9-C6H4-p-N=CH-C6H4-p-CH= N-C,H4-p-C9H 19, 74324-26-8. (26) Diele, S.; Pelzl, G.; Weissflog, W.; Demus, D. Liq. Crysf. 1988, 3, 1047.
Role of Coverage in Determining Adsorbate Stability: Phenol Reactivity on Rh( 111) Xueping Xu and C. M. Friend* The Department of Chemistry, Harvard University, Cambridge, Massachusetts 021 38 (Received: February 14, 1989; In Final Form: June 16, 1989)
The reaction of phenol on Rh( 111) has been studied by use of temperature programmed reaction and X-ray photoelectron spectroscopies under ultrahigh vacuum. Phenol undergoes competing molecular desorption and 0-H bond cleavage to form adsorbed phenoxy below 300 K. Phenoxy quantitatively reacts to form carbon monoxide (400-500 K) and stoichiometric amounts of surface carbon and dihydrogen. 0-H bond cleavage of phenol occurs at temperatures as low as 120 K, but no C-H bond cleavage occurs until above 350 K. The decomposition kinetics of the adsorbed phenoxy are strongly dependent on its coverage. At low coverage phenoxy decomposes below 400 K to form adsorbed CO which desorbs near 5 0 0 K, while at saturation coverage, phenoxy decomposes above 450 K to CO, a large fraction of which is evolved directly into the gas phase at 495 K. Comparison of the reactivity of phenoxy on Rh(ll1) with previous studies of Mo(l10) suggests that the strength of the metal-oxygen bond results in different selectivities on the two surfaces. On Mo( 1 lo), all C-0 bonds are broken by 450 K leaving an oxygen overlayer on the surface whereas no C-0 bond breaking is induced by the Rh( 1 11) surface.
Introduction Comparison of the reactions of structurally related organic molecules on transition-metal surfaces is important in understanding factors that control the selectivity and activity for metal-catalyzed reactions. In general, the bond strengths within molecules and those between adsorbed species and the metal surface control reactivity. For example, the ring strain and size has been shown to control the reaction selectivity and kinetics for cyclic sulfide desulfurization induced by Mo( 1lO).l The primary motivation for the study of the reactivity of phenol on Rh( 11 1) is to understand the mechanism of Rh-catalyzed deoxygenation in order to provide insight into the related hydrodeoxygenation reactions that occur during the catalytic treatment of liquid fuel feedstock? Although only a small amount of oxygen is present in conventional crudes, the main source of commercial fuels, a substantial amount of oxygen is present in ( I ) Roberts, J. T.; Friend, C. M. J. Am. Chem. SOC.1987, 109, 7899. (2) Furies, E. Caral. Rev.-Sci. Eng. 1983, 25(3), 421.
0022-365418912093-8072$01.50/0
synthetic liquids which have been identified as a potential source of commercial fuels. Liquids containing large quantities of 0containing compounds are rather unstable, and phenolic groups account for most of the oxygen in synthetic crudes derived from coal, shale, and tar sands. Thus, the reaction of phenol on metal surfaces is of particular interest because it serves as a model for metal-induced deoxygenation catalysis. An important aspect of this work is the study of periodic trends in reactivity for transition-metal-catalyzed processes. For many catalytic reactions, there is a maximum in the activity for catalytic processes as a function of the period of the metal in the catalyst, usually referred to as a “volcano” relationship. Although not explicitly studied, periodic trends may also be anticipated for heteroatom removal reactions, such as deoxygenation, based on the observed trends in hydrodesulfurization activity of transition-metal s ~ l f i d e s .While ~ sulfided Mo is the commercially used catalyst, sulfides of Ru and Rh3s4exhibit the highest activity for (3) Pecoraro, T. A,; Chianelli, R. R. J . Caral. 1981, 67, 430.
0 1989 American Chemical Society
Phenol Reactivity on Rh( 11 1)
The Journal of Physical Chemistry, Vol. 93, No. 24, 1989 8073
E. 1. J . Catal. 1984, 86,226. (5) Serafin, J. G.; Friend, C. M. Surf. Sci. 1989, 209, L163. (6) Robins, J. In Structural Adhesives: Chemistry and Technology; Hartshorn, S. R., Ed.; Plenum: New York, 1986. (7) Lu, F.; Salaita, G.N.; Davidson, L. L.; Stern, D. A.; Wellner, E.;
diative heating was used in temperature programmed reaction spectroscopy, and electron bombardment heating was employed during crystal cleaning. A feedback loop maintained linear heating in the range of 0-100 K/s. Unless otherwise noted, experiments were performed at a heating rate of 10 f 1 K/s. The crystal was cleaned by repetitive cycles of 1-keV, FA Ar+ bombardment and heating in dioxygen at 1200 K followed by annealing 15 s at 1500 K under vacuum. The surface cleanliness was monitored by Auger electron spectroscopy and X-ray photoelectron spectroscopy. Temperature programmed reaction spectroscopy of oxygen adsorbed on the surface was also used to check for carbon not detected by Auger electron spectroscopy. If the crystal were contaminated by carbon, CO formation should be observed at high temperature (>1000 K) during the temperature programmed reaction of adsorbed oxygen.I0 The Rh( 11 1) surface prepared in this way shows a sharp (1 X 1) low-energy electron diffraction (LEED) pattern. Boron was also not detectable after the cleaning procedure. Before adsorption of reactants, the clean crystal was rapidly heated to 600 K to desorb background CO, H20, and H2 and finally cooled to 120 K within 5 min. This procedure minimized the background adsorption. Phenol was obtained from Mallinckrodt. All isotopically labeled phenol compounds, phenol-I3C, 2,4,6-C6H2D30D,and C6D50H, were obtained from Cambrid e Isotope Laboratories. All compounds were stored with a 3- molecule sieve to minimize water impurities. At the beginning of each day of experimentation, the samples were subjected to several freeze-pump-thaw cycles and used without further purification. Dideuterium (99.5%) and dioxygen (research purity), obtained from Matheson, were used without further purification. The purity of phenol compounds was monitored with mass spectrometry by backfilling the chamber to a pressure of -5 X Torr. Phenol, phenol-13C, and phenol-d5 were >99% isotopically pure; however, the deuterium of the -OD group of 2,4,6-C6H2D,0D was found to undergo €3-D exchange with the walls of storage bottle. Therefore, the isotopic purity of the -OD group in 2,4,6-C6H2D30Dvaried. Multilayer sublimation peaks were used as a standard for purity of the -OD group. All samples were dosed through a leak valve with a directed doser located -0.1 in. in front of the crystal. The exposures are expressed in dosing time at the same leak valve position which resulted in monolayer coverage after a =45-s exposure. During dosing the chamber pressure increase was less than 1 X Torr, thus minimizing adsorption and/or reaction on the chamber walls. The crystal was kept below 130 K during dosing unless otherwise noted. Reaction products were monitored by an UTI-100C mass spectrometer interfaced with an IBM PC. Two programs were employed in the experiments." One program was used to search for reaction products by monitoring more than 100 masses. The other was used to monitor up to ten selected products to achieve better temperature resolution and signal-to-noise ratios. X-ray photoelectron spectra were taken with a Perkin-Elmer PHI-5300 ESCA system. A 300-W, 15.0-keV Mg X-ray source was positioned at ~ 6 0 'with respect to the surface normal, and the electron energy analyzer was normal to the Rh(ll1) surface. The electron energy analyzer was operated in high-resolution mode with a pass energy of 17.9 eV and a resolution of 0.1 eV. Data were acquired simultaneously for Rh(3d), C(ls), and O(1s) with acquisition times of 1,6, and 12 min, respectively. The total data acquisition time was 20 min. The acquisition time and signalto-noise ratio were optimized in the experiments to have a reasonable signal-to-noise ratio with minimal background gas adsorption. All spectra were collected with the crystal maintained at 100 K. The X-rays do not induce any desorption or reaction under these conditions. Spectra were acquired after heating the sample to a specific temperature at a rate of 10 K/s and cooling to 100 K. All binding energies are referred to the Fermi level with the Rh 3dSi2peak at 307.2 eV taken as a standard. Strong
Frank, D. G . ;Batina, N.; Zapien, D. C.; Walton, N.; Hubbard, A. T. Lang.muir ..-. 1988. . .- -, 4., 637. -- . (8) Serafin, J. G.; Friend, C. M. J . A m . Chem. SOC.1989, 111, 4233. (9) Baldwin, E. K.; Friend, C. M. J . Phys. Chem. 1985, 89,2576.
(10) Fisher, G . B.; Schmieg, S. J. J . Vac. Sci. Technol. A 1983,1, 1064. (1 1) Liu, A. C.; Friend, C. M. Rev. Sci. Instrum. 1986, 56, 15 19.
dibenzothiophene hydrodesulfurization. This periodic behavior in hydrodesulfurization activity has been primarily attributed to the intermediate strength of the metalsulfur bond in the respective metal sulfides, and similar trends are likely for deoxygenation. Rhodium (1 11) was chosen in order to compare the mechanism and kinetics for deoxygenation of phenol to Mo( 1 lo), which has been previously studied.j These two surfaces are the most thermodynamically stable planes and have the most similar geometric structure of these two metals. However, Rh and Mo have very different electronic structures, and their respective bulk oxides have substantially different metal-oxygen bond strengths. A comparison of phenol reactivity on these metals will therefore yield insight into the role of these factors. The reactivity of phenol on metal surfaces is of additional interest because adhesives containing phenolic groups are used in the metal bonding.6 This is the first in a series of investigations of phenol reactivity on clean and modified Rh( 11 1). Phenol has been previously studied under ultrahigh vacuum on Pt(lll),7Mo(110),5and oxidized Mo(110).* On both Pt(ll1) and Mo( 1 lo), the 0-H bond is cleaved below 300 K, forming adsorbed phenoxy. On Pt( 11 l), phenoxy forms an ordered (3 X 3) layer with a packing density of -0.1 molecule/surface atom and the phenyl ring is proposed to orient parallel to the surface based on the packing density. The reactivity of the phenoxy on Pt( 11 1) was not reported. On clean Mo( 110) phenol decomposes to dihydrogen, surface oxygen, and hydrocarbon fragments with C-O bond cleavage occurring at about 400 K, whereas on oxidized Mo( 110) (0, = 0.33), surface oxygen stabilizes phenoxy with respect to C-H and C-C bond cleavage up to 650 K, where it then reacts to form gaseous phenol and surface carbon and oxygen. The molecular orientation of phenoxy is not known on either clean or oxidized Mo(1 lo), but the saturation coverage in the two cases suggests that a parallel orientation is unlikely. Near edge X-ray absorption fine structure studies of phenoxy are planned to determine the ring orientation since the geometry will affect the kinetics and bond activation. Contrary to the comparative desulfurization rates on Rh and Mo, the C-0 bond in phenol is never broken on R h ( l l l ) , as demonstrated by this study. Instead, gaseous C O is formed as the only oxygen-containing product from the decomposition of surface phenoxy. These observed differences are rationalized, in part, in terms of the relative metal-oxygen bonds strengths on Mo(ll0) and Rh(ll1). Experimental Section
Experiments were performed in a stainless steel ultra-highTorr devacuum chamber with a base pressure of -1 X scribed in detail previo~sly.~The chamber is equipped with electron optics for retarding field Auger electron spectroscopy and low-energy electron diffraction, an X-ray photoelectron spectrometer, and a quadrupole mass spectrometer mounted in a differentially pumped liquid nitrogen cooled cryoshield. A Rh( 1 1 1) single crystal rod, -1/4-in. diameter, was obtained from Metal Crystals Ltd. and was cut into -1-mm-thick disks by spark erosion. The crystals were oriented to within 0.5' of the (1 1 1) plane using Laue X-ray back-diffraction and polished successively with 400,600, and 800 grit S i c powder and with 6-, 1-,and 1/ 4 - ~ mdiamond paste. The Rh( 11 1) disk was spot-welded at its edge to a 0.020-in.-diameter Ta wire which is thermally connected to a Cu block allowing cooling to 100 K. The temperature was measured with a W-5% Re/W-26% Re thermocouple spot-welded on the top side of the crystal edge. A tungsten filament mounted behind the crystal was used for heating. Ra(4) Chianelli, R. R.; Pecoraro, T. A,; Halbert, T. R.; Pan, w. H.; Stiefel,
w
8074
The Journal of Physical Chemistry, Vol. 93, No. 24, 198'9
100 200 300 400 500 600 700 800 900
Temperature ( K 1
Figure 1. Temperature programmed reaction spectra for phenol-I3Con Rh( 11 1 ) adsorbed a t 120 K. The exposure was 60 s, sufficient for multilayer growth. Products were monitored at m / e = 100 for phenol"C, m / e = 2 for H2, and m / e = 29 for "CO. H2evolution from benzene adsorbed on Rh( 11 1) and 'TO and '*CO evolution from phenol-"C coadsorbed with I2CO (0 = 0.15 monolayer) on Rh(l11) are also shown for comparison.
substrate photoemission is present in both the C( 1s) and O(1s) spectral regions.12 In the C(1s) region, peaks are observed at 289.8 and 287.2 eV, which arise from the excitation of Rh(3d512) electrons by satellite X-rays. In the oxygen region, the energy-loss feature of the R h ( 3 ~ , / transition ~) at 521.6 eV contributes a strong background. Therefore, background subtraction of the X-ray photoelectron spectra using software supplied with the spectrometer was necessary. The spectra reported here have had the photoelectron contribution from the substrate subtracted from them. A curve-fit routine supplied with the spectrometer was used to fit the spectra with Gaussian-Lorentzian curves. The curve-fit parameters used are presented with the data.
Results Reaction of Phenol at Saturation Coverage. Phenol irreversibly decomposes on Rh( 1 11) forming the gaseous products, dihydrogen and carbon monoxide, and surface carbon as is shown in Figure 1. The data depicted in Figure 1 were obtained with phenol-13C so that 13C0is evolved during temperature programmed reaction, precluding background CO evolution from contributing to the product signal. Notably, no hydrogenolysis to form benzene or other volatile hydrocarbon products is observed. At exposures above those where no additional products are formed, desorption of a weakly bound phenol state, possibly due to a second layer, and sublimation of phenol multilayers are also observed. At exposures of greater than 45 s, the yields of HZand CO do not change, and thus 45 s is defined as a saturation exposure. A comprehensive search for gaseous products was performed by monitoring all masses between 2 and 100 amu and only H2, CO, and phenol were observed. At saturation exposure dihydrogen evolution commences at 250 K with three peaks observed at 310 (PI), 490 (p2),and 690 K (p3). The amount of PI-dihydrogen formed at reaction saturation is approximately one-sixth (0.16 f 0.02) of the total dihydrogen produced, suggesting Pl-dihydrogen is derived from 0-H bond cleavage. Dihydrogen evolution from clean Rh( 11 1) has been studied previously, reproduced in our experiments (data not shown) and modeled as a complex second-order desorption process.13 At low hydrogen coverage the maximum rate of dihydrogen evolution for hydrogen atom recombination occurs at 380 K from clean (12) Muilenberg, G. E., Ed. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1979. (13) Yates, J . T.;Thiel, P. A,: Weinberg, W. H. Surf: Sci. 1979, 84, 427.
Xu and Friend Rh( 11 1). As the hydrogen atom coverage increases, the dihydrogen evolution peak shifts to 270 K. Therefore, the rate of PI-dihydrogen evolution from phenol decomposition is probably limited by hydrogen atom recombination kinetics on the Rh( 11 1) surface, suggesting that 0-H bond breaking occurs below 3 10 K. This contention is supported by isotopic labeling and X-ray photoelectron data described below. The dihydrogen peaks at high temperature (p2 and p3) from phenol are very similar to those from reaction of benzene on this surface (Figure l),I43l5except that the P2-H2peak at 490 K is sharper and shifted to higher temperature in the decomposition of phenol. The p2- and @3-H2peaks are too high in temperature to be rate limited by hydrogen atom recombination and must, therefore, be due to decomposition of a surface hydrocarbon fragment. Carbon monoxide (CO) from phenol evolves at 495 K, slightly higher in temperature than the P2-dihydrogen evolution at saturation phenol coverage. The rate of CO evolution must be either reaction limited or a convolution of reaction- and desorptionlimited processes. Carbon monoxide on clean Rh( 1 1 1) has been previously studied and is molecularly adsorbed, occupying two different adsorption sites, one atop and one bridging.I6 At high coverage, CO desorbs in peaks at 400 and 510 K, temperatures in the same range as CO evolution from the decomposition of phenol. At low coverages (0 C 0.5), C O binds at the atop sites and desorbs in a peak at 510 K. At higher coverage (0 > OS), CO begins to occupy the bridging sites, leading to the desorption at 400 K. The saturation coverage is 0.75 C O molecule per Rh atom. The CO evolution following a saturation phenol exposure has an initial steep rise and is clearly different from that obtained for CO adsorbed on Rh( 1 11). Temperature programmed reaction after coadsorption of phenol-13C with a small amount of I2C0 shown in Figure 1 illustrates that 13C0and l2C0 evolution shapes are different, suggesting that CO evolution from phenol is not desorption limited at saturation. Furthermore, the presence of a small amount of coadsorbed C O did not measurably alter the phenol reaction kinetics. The reaction of oxygen on carburized Rh( 11 1) was also investigated in this study to examine the recombination kinetics of carbon and oxygen on Rh( 11 1). The carbide surface is prepared by dosing a saturation coverage of benzene at low temperature and then annealing to 800 K. Oxygen was subsequently adsorbed at 120 K. Two reaction products were detected: CO and C 0 2 . C 0 2 is evolved between the temperatures of 350 and 600 K, and CO evolves above 550 K. The fact that C O is not formed until 550 K from oxidation of the surface carbide is evidence that the CO formed from phenol is due to C-C bond cleavage with retention of the C-O bond and not recombination of surface carbon and oxygen. Phenol sublimation is observed at a peak temperature of ~ 2 1 0 K in a peak denoted a2 in Figure 1, corresponding to an approximate heat of desorption of 16.1 kcal/mol17 in good agreement with the heat of sublimation of 16.2 kcal/mol.I* The onset of phenol desorption in the a2peak is observed for exposures of 45 s, which is defined as saturation exposure. Also consistent with assignment of the cy2 peak as sublimation of solid phenol are the facts that its intensity continuously increases as a function of exposure, there is no H-D exchange in the a2peak (see below), and the total amount of H2 formation does not increase as the a2 phenol peak is populated. In addition to the a2 multilayer (14) Koel, B. E.; Crowell, J. E.; Mate, C. M.; Somorjai, G. A. J . Phys. Chem. 1984, 88, 1988. (15) Koel, B. E.; Crowell, J. E.; Bent, B. E.; Mate, C. M.; Somorjai, G. A. J . Phys. Chem. 1986, 90, 2949. (16) DeLouise, L. A.; Winograd, N. Surf: Sci. 1984, 138, 417. (17) a2 is a zero-order desorption process, as expected for sublimation, because its desorption line shape is independent of the coverage and heating rate. For zero-order desorption, the desorption rate constant is equal to the desorption rate and is, therefore, proportional to the mass spectrometer intensity assuming infinite pumping speed. The heat of desorption is thus obtained from an Arrhenius plot of the phenol desorption signal versus temperature. A consequence of the zero-order sublimation is that the peak temperature shifts with the size of multilayers, as is observed here. (18) Balson, E. W. Trans. Faraday SOC.1947, 43, 54.
The Journal of Physical Chemistry, Vol. 93, No. 24, 1989 8075
Phenol Reactivity on Rh( 11 1)
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F i 2. Temperatureprogrammed reaction spectra of a multilayer dose of C6DsOH on Rh( 11 1) adsorbed at a temperature of 120 K. The product parent ions were monitored: C6D50H ( m / e = 99), CO ( m / e = 28), D2 ( m / e = 4), HD ( m / e = 3), and H2( m / e = 2). The heating rate was 10 K/s.
sublimation peak, there is a second reversible phenol desorption peak, a l ,a t 240 K, which is first observed at an exposure of 30 s. In contrast to the multilayer peak, the a1phenol peak does undergo H-D exchange when phenol is adsorbed on a deuterium-precovered surface and the H2 integrated intensity increases during population of the aIstate. Therefore, the relatively weakly bound a,phenol state communicates with the surface and participates in reactive events. Temperature-programmed reaction of C6DSOHdemonstrates that the 0-H bond is broken at temperatures below 350 K, yielding a chemisorbed phenoxy intermediate. As shown in Figure 2, dihydrogen is exclusively evolved below 400 K in the Dl peak whereas D2 is evolved in the f12 and D3 peaks at 490 and 710 K, respectively. Only trace amounts of H D are formed exclusively in the f12 peak. The absence of D2 or H D formation a t temperatures below 450 K further demonstrates that no C-D bond cleavage occurs up to this temperature. The minor amount of B2-HD formation is attributed either to a minor amount of H D exchange into the phenol or phenoxy or to an isotopic impurity of C6DSOH. The temperature of the P2-D2peak is the same as that for f12-H2from phenol-& at saturation. The D3 (710 K) dideuterium peak formed from C6DSOHis 20 K higher than the dihydrogen evolution from CsHSOH(690 K), suggesting that C-H bond breaking is important in determining the rate of decomposition of the hydrocarbon species present above 600 K. X-ray photoelectron spectroscopy was used to identify surface intermediates formed from the reaction of phenol on Rh( 11 1). Oxygen (1s) and carbon (1s) X-ray photoelectron data for a saturation exposure of phenol heated to different temperatures are shown in Figure 3, and the curve-fitting parameters are summarized in Table I. At 100 K there is an O(1s) peak at 533.0 eV, which is attributed to phenol multilayers, with a tail at 530.7 eV. At 250 K, after the desorption of molecular phenol, there are two O(1s) peaks with binding energies of 530.6 and 532.0 eV. Heating decreases the intensity of the 532.0-eV peak and increases the 530.6-eV peak intensity. Thus, the two O(1s) binding energies are clearly due to two chemically distinct species, not final state effects. In the temperature range of 300-400 K, only the 530.5-eV O(1s) peak, attributed to surface phenoxy, is detected. The 532.0-eV peak is assigned as intact phenol. The oxygen (1s) binding energy of phenoxy is in agreement with previous studies. For example, methoxy on Cu( 110) has an O(1s) binding energy of 530.7 eVI9 and phenoxy on Mo(ll0) has an O(1s) binding energy of 530.8 eV.S The 530.7-eV peak is visible for smaller phenol exposures at 120 K, indicating the presence of phenoxy on the surface upon adsorption, a result that is consistent with deuterium preadsorption experiments to be described below. The saturation coverage of phenoxy at 300 K on the surface is 0.12 ~~
___
(19) Bowker, M.; Madix, R. J. Surf. Sci. 1980, 95, 190.
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figure 3. X-ray photoelectron spectra of phenol multilayers as a function of annealing temperature. (a) Oxygen (1s) and (b) carbon (1s) data for annealing temperatures Of (i) 100 K, the adsorption temperature;(ii) 250 K;(iii) 300 K;(iv) 400 K;and (v) 450 K.
TABLE I: X-ray Photoelectron Binding Energies and Curve-Fitting Parameters for Phenol Multilayers as a Function of Annealing Temoerature O(W 100 K binding energy, eV 533.0 area, counts.eV/s 32800 fwhm, eV 1.6 250 K binding energy, eV 532.1 area, counts-eV/s 1800 fwhm, eV 1.5 300 K binding energy, eV
area, counts.eV/s fwhm, eV 400 K binding energy, eV area, counts-eV/s fwhm, eV 450 binding energy, eV 532.1 area, counts.eV/s 850 fwhm, eV 1.4
(31s)
530.8 285.9 730 11300 1.2 1.3 530.6 285.7 2300 1800 1.2 1.2 530.5 285.7 3800 1900 1.2 1.2 530.5 285.8 3700 1800 1.2 1.2 530.5 286.2 285.3 2700 720 1700 1.2 1.2 1.2
284.2 57600 1.3 284.1 6500 1.1 284.1 7100 1.2 284.1 6800 1.2 283.9 5800 1.2
f 0.03 per rhodium atom. This coverage is obtained by comparing
the O(1s) intensity for a saturation coverage of phenoxy to that of a well-ordered Rh( 111)-p(2X2)-0 surface, which has an oxygen coverage of 0.25.20 Carbon (1s) X-ray photoelectron data are also consistent with only phenoxy being present on the surface in the range of 300-400 K. At temperatures between 100 and 400 K, two C(ls) peaks with binding energies of 284.1 and 285.7 eV and a relative intensity of 5:1 are observed as is shown in Figure 3b. A similar set of peaks is observed for phenol multilayers with binding energies of 284.7 and 286.4 eV and for phenoxy on Mo( 110) with binding energies of 284.1 and 285.6 eV.S The higher binding energy peak (285.7 eV) is associated with the carbon atom directly bound to oxygen, and the low binding energy peak (284.1 eV) is attributed to the five other ring carbons. The splitting in the C( 1s) binding energies is 1.6 eV, essentially the same as that for phenol multilayers and phenoxy on Mo( 1lo), which both have splittings of 1.7 eV. Similarly, a splitting of 1.8 eV is observed for the two inequivalent carbon atoms in gas-phase 1,3,5-ben~enetriol.~~ The C( 1s) X-ray photoelectron data also clearly show that C O is not present in significant amounts in this temperature regime since the C( 1s) binding energy for atop and bridge-bonded CO was previously measured to be 286.2 and 285.4 eV.16 Heating to temperatures above 400 K leads to the population of a new state with an O(1s) binding energy of 532.1 eV as shown in Figure 3a(v). Notably, no broadening of the O(is) peak to (20) Wong, P. C.; Hui, K. C.; Zhou, M. Y.; Mitchell, K. A. R. Surf. Sci. 1986, 165, L21. (21) Gelius, U.; Heden, P. F.; Hedman, J.; Lindberg, B. J.; Manne, R.; Nordberg, R.; Nordling, C.; Siegbahn, K. Phys. Ser. 1970, 2, 70.
Xu and Friend
8076 The Journal of Physical Chemistry, Vol. 93, No. 24, 1989
lower binding energy is observed above 400 K, indicating that a significant amount of atomic oxygen, which has a binding energy of 529.5 eV, is not formed in this temperature range. Above 500 K, after CO evolution is complete, no surface oxygen is detectable with X-ray photoelectron spectroscopy (data not shown). This observation is consistent with the temperature programmed reaction results, which suggested that the C-0 bond of phenoxy remained intact throughout the reaction. Thus, during the phenol decomposition process, atomic oxygen is never formed; the C-O bond does not break. Previous X-ray photoelectron studies of CO on Rh(l1 l), which was reproduced in this work, evidenced two states with O(1s) binding energies of 532.1 and 530.9 eV attributed to atop and bridge-bonded CO, respectively.I6 Thus, the peak observed at 532.1 eV after heating to 450 K is attributed to CO on the surface formed from decomposition of phenol. All CO, then, desorbs from the surface intact at 495 K. The O( 1s) peak observed at 530.5 eV after annealing to 450 K is attributed to remaining intact phenoxy. Phenoxy decomposes in the range of 400-500 K, forming CO as the only oxygen-containing product. The X-ray photoelectron data demonstrate that some fraction of the phenoxy yields adsorbed CO, but based on the temperature programmed reaction experiments described above, some CO is probably evolved directly into the gas phase at 490 K. C( 1s) X-ray photoelectron data obtained after heating to above 400 K are also consistent with the decomposition of phenoxy to form surface C O and one or more hydrocarbon fragments. Heating to 450 K yields the spectrum shown in Figure 3b(v) with a slight broadening to higher binding energy. Although not unique, the best fit to the data is given by three peaks with binding energies of 286.2, 285.3, and 283.9 eV. The 286.2-eV peak is attributed to adsorbed CO formed from the reaction of phenoxy, while the 285.3-eV peak is assigned as the carbon bound directly to oxygen in unreacted phenoxy. The 283.9-eV peak is a convolution of the five carbons not bound to oxygen in unreacted phenoxy and a stoichiometric amount of hydrocarbon fragment(s) formed when the phenoxy decomposes to CO. A C( 1s) binding energy of =284 eV is generally observed for adsorbed hydrocarbons with intact C-C and C-H bonds. With heating to 500 K (data not shown), above the CO desorption temperature, the 285.7-eV peak, characteristic of intact phenoxy, vanishes and only the peak near 284 eV, characteristic of hydrocarbon fragments, is observed. Heating to 800 K, where all hydrogens have been evolved, a single C( 1s) peak centered at 284 eV is observed that is significantly broader than the others. The full width at half-maximum of C(1s) is 1.8 eV for phenol after heating to 800 K, compared to 1.3 eV for phenol multilayers and 1.2 eV for phenoxy. This broad C(1s) peak probably corresponds to carbon polymers formed on the surface as suggested previously for benzene on Rh( 11 l).15 The C( 1s) intensity after heating to 800 K is 5/6 of the total C(1s) intensity at 300 K, consistent with stoichiometric elimination of CO during temperature programmed reaction. Only minor amounts of reversible H-D-exchange processes were observed during reaction of phenol (13C6H50H)in the presence of preadsorbed deuterium. Figure 4 is the temperature programmed reaction spectra of phenol with surface deuterium (0, = 0.6) on Rh( 11 1). Preadsorbed deuterium inhibited phenol reaction on Rh(ll1): the maximum amounts of CO and H2 formation and the C(KLL)/Rh(MNN) Auger intensity ratio after temperature programmed reaction decreased linearly as a function of the deuterium coverage. The deuterium coverage was estimated by integrating the amount of deuterium evolved during temperature programmed reaction compared to that from saturation coverage of deuterium on clean Rh(ll1). The peak temperatures for C O and Hzare essentially the same as for reaction in the absence of deuterium for a given phenoxy coverage. A new unresolved dihydrogen peak is observed at 260 K for H2, HD, and D,. This low-temperature desorption is also observed in temperature programmed reaction of benzene with preadsorbed deuterium2, and is attributed to a weakly bound state of H or D (22) Xu, X.; Friend, C. M. Manuscript in preparation.
100
ZOO
300
400 500 600 Temperature ( K 1
700
800
900
Figure 4. Temperature programmed reaction spectra for a saturation exposure of phenol-”C reacted in the presence of preadsorbed deuterium on Rh(l11). The deuterium coverage is 0.6 of saturation, the adsorption temperature is 120 K, and the heating rate is 10 K/s.
atoms which recombine on the surface. This state is populated when phenol or benzene is present on the surface but is not present if only H or D atoms are adsorbed. Another dihydrogen desorption peak is observed at 320 K and is attributed to the recombination of H or D atoms analogous to the clean surface: most of the deuterium isotopes of dihydrogen are formed at low temperature (below 400 K). At high phenoxy coverage (i.e., low OD), only minor amounts of H D are formed in the &-dihydrogen peak and no Dz is formed above 400 K, the temperature range where hydrocarbon fragments decompose. These results demonstrate that only a minor amount of reversible C-H bond cleavage occurs in these fragments. Interestingly, the amount of reversible C-H bond activation in the adsorbed phenoxy intermediate is considerably less than for benzene on the Rh( 11 1) surface where H D and D2 are formed in the high-temperature dihydrogen formation peaks when benzene is reacted in the presence of surface deuterium.22 Surprisingly, deuterium is incorporated into the al-phenol desorption at 240 K. Deuterium incorporation is clearly evident from a comparison of the relative intensities of the a1and a2p k s for phenol-doand -di parent ions at 100 and 101 amu, respectively, in Figure 4. The phenol-dl intensity is less than 0.5% of the a, (multilayer) peak whereas the ai is about 5% phenol-d,. The observation of deuterium incorporation into the a1peak is in sharp contrast to behavior of phenol on Mo( 110) where phenol desorbs at 240 K but does not incorporate deuterium. The low temperature of the ai-phenol peak is suggestive of a weakly bound second layer, but the observed H-D exchange shows that some reversible 0-H bond breaking occurs below 240 K on Rh(ll1). The origin of this peak could be due either to hydrogen bonding of the phenol which could facilitate H-D exchange or to low-temperature phenoxy recombination with surface hydrogen at high coverages. Coverage Dependence: Reaction of Phenol at Subsaturation Exposures. The reactivity and the stability of surface intermediates derived from the reaction of phenol are dependent on their coverage as evidenced by the temperature programmed reaction spectra shown in Figure 5. At low coverage (0.3 saturation, Figure 5(i)), dihydrogen is evolved at peak temperatures of 360 (pi) and 450 K (p2) with a high-temperature tail extending to 700 K. As the phenol exposure is increased, the Pi-H2 peak increases in intensity and shifts to lower temperature. The & dihydrogen formation peak gradually shifts to higher temperature with higher intensity and narrows with increasing phenol exposure. In the same exposure regime (0.6 saturation) the high-temperature dihydrogen evolution tail increases in intensity, developing into a partially resolved peak at 680 K denoted as p3. The kinetics for CO evolution are also coverage dependent. At low coverage (0.3 saturation), CO desorbs at 5 15 K. As the phenol exposure increases, the CO evolution peak temperature shifts, reaching its low limit of 495 K at 0.6 saturation. The kinetics for C O formation are in fact changed with coverage. At low coverage CO is formed at temperatures below 495 K but remains
The Journal of Physical Chemistry, Vol. 93, No. 24, 1989 8077
Phenol Reactivity on Rh( 11 1)
100 300 500 700 100 300 500 700 100 300 500 700 900 Temperature K 1
Figure 5. Temperature programmed reaction spectra for phenol on Rh(1 11) as function of exposure. (a) phenol-"C ( m / e = 100); (b) dihydrogen ( m / e = 2); (c) c a r b o d 3 C monoxide ( m / e = 29). The exposures are (i) 10 s (0.3saturation), (ii) 20 s (0.6saturation), (iii) 30 s (0.9 saturation), and (iv) 60 s (1.3 saturation), respectively. TABLE 11: X-ray Photoelectron Binding Energies and Curve-Fitting Parameters for 0.5 Saturation Exposure of Phenol as a Function of Annealing Temperature
100 K binding energy, eV 532.2 area, counts.eV/s 770 fwhm, eV 1.4 300 K binding energy, eV
530.7 670 1.2 530.5 area, counts.eV/s 1800 fwhm, eV 1.2 400 K binding energy, eV 532.2 530.5 area, counts.eV/s 600 1000 fwhm, eV 1.2 1.2 450 K binding energy, eV 532.2 530.6 area, counts.eV/s 1400 500 fwhm, eV 1.3 1.2
285.8 500 1.2 285.8 500 1.2 286.2 285.7 300 400 1.2 1.2 286.2 285.5 740 220 1.2 1.2
284.1 1800 1.2 284.1 2400 1.1 284.0 1500 1.1 283.8 1600 1.2
on the surface until 5 15 K, where desorption takes place. This is apparent since C O evolution from phenol at low coverage is very similar to that from small amounts of CO adsorbed on clean Rh(l11). Confirming evidence that a significant amount of CO is formed on the surface below 400 K based on X-ray photoelectron spectroscopy is presented below. Recall that, a t saturation coverage, C O evolution is largely reaction limited. Temperature programmed reaction of 2,4,6-C6H2D30Dand C6DsOH further demonstrates the coverage dependence of the reaction kinetics. Recall that, at saturation coverage, no isotope effect is observed in the &-dihydrogen evolution at 490 K. In contrast, at low exposure (15 s = 0.3 saturation), a significant isotope effect is observed in the &dihydrogen evolution kinetics: B2-H2 is formed at 450 K, and &-D2 evolves at 460 K from 2,4,6-C6H2D30D. This suggests that, at coverage less than half-saturation, C-H bond breaking is involved in the rate-limiting step of &-Hz evolution, while a t saturation coverage, C-H bond breaking does not affect the rate of P2-H2evolution. Note that at saturation the leading edges of CO and B2-H2peaks coincide. X-ray photoelectron data, shown in Figure 6 for 0.5 saturation exposure, offer further evidence for coverage-dependent CO formation kinetics. The curve-fitting parameters for the data shown are summarized in Table 11. We note that the signalto-noise ratio is rather poor in these data because of the lower coverage, so that only qualitative interpretation is made; the curve fits are to serve as a guide in the interpretation. At 100 K there are clearly two O(1s) peaks observed with binding energies of 532.2 and 530.7 eV. The 530.7-eV peak is attributed to surface phenoxy while the peak with a binding energy of 532.2 eV is probably due to intact phenol. There are also C( 1s) peaks observed at 285.8 and 284.1 eV. The presence of carbon with a binding energy of 285.8 eV is indicative of intact C-0 bonds and is consistent with the presence of a mixture of phenoxy and phenol.
517
535
533
631
629
I
I ,
627 290 288 B1ndlng Energy ( a V )
286
284
282
I
280
Figure 6. (a) Oxygen (Is) and (b) carbon (1s)X-ray photoelectron spectra for half-saturation exposure of phenol on Rh( 1 1 1) after annealing to temperatures of (i) 100 K, the adsorption temperature; (ii) 300 K; (iii) 400 K; and (iv) 450 K.
Heating to 300 K yields X-ray photoelectron data consistent with the presence of only phenoxy on the surface. A single O(1s) peak is observed at 530.5 eV and two C( 1s) peaks a t 285.8 and 284.1 eV are detected after heating to 300 K. The O(1s) binding energy is the same within experimental error as that measured for phenoxy at saturation on Rh( 111). Furthermore, the binding energies of the C( 1s) peaks after heating to 300 K are essentially the same a t the two coverages studied and the relative intensity of the 285.5- and 284.1-eV C(1s) peaks are nearly 15. We further note that the absence of the 532.2-eV peak a t 300 K precludes the assignment of 532.2-eV peak at 100 K as adsorbed CO since CO does not desorb between 100 and 300 K. X-ray photoelectron data demonstrate that the rate of CO formation is more rapid at lower phenoxy coverage. Carbon monoxide is clearly present on the surface after the 0.5 saturation phenoxy layer is heated to above 350 K. For example, after heating to 400 K, a new O( 1s) peak at 532.2 eV (Figure 6a(iii)) appears and a broadening in the C( 1s) peak to higher binding energy is observed (Figure 6b(iii)). This is in sharp contrast to saturation coverage, where no COformation was detected a t 400 K . In addition to CO there is also still unreacted phenoxy at 400 K for the 0.5 saturation layer with characteristic O(1s) and C( 1s) binding energies of 530.5 and 285.7 eV, respectively. Further heating to 450 K results in an increase in the amount of CO on the surface, as determined from the increase in the intensities of the 532.2-eV O(1s) and 286.2-eV C( 1s) peaks which are characteristic of atop CO. The relative intensity of the O(1s) peaks at 532.2 and 530.5 eV are a measure of the relative amount of CO and phenoxy present on the surface. At 450 K the intensity ratios for the 532.2- and 530.5-eV peaks are 2.8 at 0.5 saturation exposure compared to 0.3 at saturation exposure, indicating that the relative amount of CO produced at 450 K is greater by nearly an order of magnitude at 0.5 of saturation exposure. We note that there are no substituents other than small amounts of atomic hydrogen on the surface when the phenoxy first begins to react; only phenoxy is present. Small amounts of preadsorbed or postadsorbed hydrogen do not change the phenoxy reaction kinetics at 0.5 saturation. Clearly, the coverage of the phenoxy intermediate alters its own stability, with more facile C-C bond cleavage occurring at lower coverage. Notably, however, the overall selectivity for C O formation is unchanged a t 0.5 of saturation; C O is still the only oxygen-containing product formed from phenoxy. Specifically, no residual oxygen is detected on the surface by X-ray photoelectron spectroscopy or Auger electron spectroscopy after temperature programmed reaction, and the amount of C O produced at 0.5 saturation is 0.5 of that at saturation within experimental accuracy. In addition, temperature programmed reaction of phenol-13C shows that the "CO yield parallels the H2 yield and the amount of residual carbon after reaction to 800 K is always 5/6 of the total amount of carbon from phenoxy at 300 K.
8078 The Journal of Physical Chemistry, Vol. 93, No. 24, 1989
Figure 7. Proposed reaction scheme for phenol on Rh(l11).
The extent of reversible C-H bond activation is also dependent on coverage. As discussed above, preadsorbed deuterium (8, = 1/4) does not react with phenoxy a t saturation coverage, based on the fact that there is only a minor amount of H D and no D, produced at temperatures above 400 K during temperature programmed reaction. In contrast, at low phenoxy coverages (0.5 saturation) relatively more H D and Dz are evolved above 400 K during temperature programmed reaction in the presence of 0.25 monolayer of deuterium. Although the extent of H-D exchange at low phenol coverage is small compared to that of benzene, it is significantly greater than at high coverage, demonstrating that the rate of reversible C-H bond activation is somewhat more rapid a t lower phenoxy coverage.
Discussion Both temperature programmed reaction and X-ray photoelectron spectroscopy data show that phenol undergoes competing molecular desorption and 0-H bond cleavage to form surface phenoxy on Rb(l11) as shown in the scheme in Figure 7. Some 0-H bond dissociation occurs upon adsorption a t 120 K at all coverages, but molecular adsorption is also observed. By 300 K, the only oxygen-containing species present on the surface is phenoxy for all coverages studied which coexists with a stoichiometric amount of atomic hydrogen. The maximum (saturation) phenoxy coverage attainable is 0.12, similar to that on Mo(110). The adsorbed phenoxy reacts to form CO, H2, and surface carbon as shown schematically in Figure 7 at all coverages, although the kinetics for reaction are strongly dependent on the phenoxy coverage. Importantly, the C-0 bond of phenol never breaks; deoxygenation is achieved by formation of gaseous CO. At saturation coverage, the rate of phenoxy decomposition on Rh( 1 11) is proposed to be controlled by C-C bond breaking of the phenyl ring. This is proposed on the basis of the absence of an isotope effect in the reaction of deuterio- and hydrophenol; if the rate-limiting step of phenoxy decomposition is the C-H bond cleavage, an isotope effect would be expected in the dihydrogen evolution (p2). At low coverage, the decomposition of phenoxy occurs at lower temperature and an isotope effect of dihydrogen evolution (8,) is observed. However, this isotope effect does not necessarily suggest that C-H bond breaking plays a role in the initial phenoxy decomposition; it most likely reflects the decomposition of hydrocarbon fragments derived from phenoxy after CO formation. Thus, it is reasonable to propose the initial phenoxy decomposition is via same mechanism for all coverages, C-C bond cleavage of phenyl ring. In organometallic complexes containing alkyl-substituted aryloxy ligands, C-H bond scission of bulky alkyl substituents
Xu and Friend is usually the first step in decomposition, not C-C bond cleavage.23 No C-H bond activation occurs, however, for unsubstituted phenoxy in [(dippe)RhI2(p-H)(p-0-C6H5)(dippe = 1,2-bis(diisopropy1phosphino)ethane) in which the phenoxy is ligated solely via the oxygen atom.24 As discussed below, the phenyl ring of phenoxy is probably perpendicular to the surface, making this a reasonable structural analogue. Of course, the absence of other metal centers in the complex that can interact with the phenyl ring is expected to lead to different reactivity in the complex and on the surface. Interestingly, the precursor to the [(dippe)Rhj2(p-H)(p-O-C6H5),[(dippe)Rh],(r-H), reacts with an excess of phenol to produce a mononuclear Rh complex, (dippe)Rh(q5-C6H@)*2C6H@H, in which the phenoxy has isomerized to an enolate bound to the metal center via interaction of the .Ir-system of the phenyl ring and hydrogen bonded to two other phenol molecules. In the reaction of phenoxy on the Rh( 1 1 1) surface to form CO, a significant amount of structural reorganization is required since the oxygen-Rh bond must be broken and new carbon-Rh interactions must be formed to form CO and adsorbed hydrocarbon fragments. An enolate isomer such as that observed in the Rh complex may, indeed, be a plausible intermediate in the surface reaction. The interaction of the phenyl a-orbitals with the surface would facilitate C-C bond breaking and the presence of a C-O double bond in the enolate should favor CO formation. Furthermore, the observation of hydrogen-bonding interactions in the (dippe)Rh(q5-C6H50)-2C6H50H referred to above as well in (PMe3)3RhO(tolyl)(HOC6H5(CH3))z5 also demonstrate the plausibility of hydrogen bonding of aryl alcohols, such as phenol, as suggested for the al-phenol state on Rh( 11 1). Indeed, the hydrogen bond enthalpy for the above tolyloxy complex in cyclohexane was measured to be -14.0 f 0.4 kcal/mol; surprisingly large for a hydrogen-bonding interaction and comparable to the enthalpy estimated of 14 kcal/mol for the a,-phenol desorption.26 The reactions of phenol are qualitatively similar to methanol on Rh(l1 l), which forms dihydrogen and carbon monoxide on Rh( 11 1) via stepwise hydrogen abstraction from absorbed m e t h o ~ y . ~ ’ -Methoxy ~~ is significantly less stable than phenoxy on Rh(l1 l), decomposing at temperatures near 200 K compared to above 400 K for phenoxy at saturation coverage. The coverage dependence of methoxy stability on Rh( 11 1) was not reported. The difference in the stability of methoxy and phenoxy may be a direct consequence of the type of bonds that must be cleaved during decomposition. Carbon-hydrogen bond cleavage must occur in the rate-limiting step for C O formation from methoxy decomposition whereas C-C bond cleavage must play a role in determining the rate of CO formation from phenoxy reaction. The average C-H single bond energy is 98.7 kcal/mol compared to average C = C double bond energies of 146 k ~ a l / m o I . ~Therefore, ~ the relative stability of methoxy and phenoxy may simply reflect the different types of bond breaking that limit the rate of decomposition. Both intermediates are expected to be bound via the oxygen atom, although interaction of the ring and the surface is possible for phenoxy. Therefore, a significant amount of reorganization must occur along the reaction path in both cases since the C O bond must reorient after carbon monoxide formation. An important finding of this work is the dependence of phenoxy stability with respect to decomposition on its coverage. Specifically, the rate of C-C bond cleavage to form CO is more rapid at low phenoxy coverages. At low coverages, phenoxy decom(23) Chisholm, M. H.; Rothwell, I. P. Compr. Coord. Chem., 1987, 2, 359, and references therein. (24) Fryzuk, M. D.; Jang, M.-L.; Jones, T.; Einstein, F. W. B. Can. J . Chem. 1986. 64. 174. (25) Kegiey, S . E.; Schaverien, C. J.; Freudenberger, J. H.; Bergman, R. G . J . Am. Chem. SOC.1987, 109, 6563. (26) (a) Assuming first-order desorption and a preexponential factor of 10”. (b) Redhead, P. A. Vacuum 1962, 12, 203. (27) Weimer, J. J.; Chuah, G.K.; Abend, G.;Kruse, N.; Block, J. H. Catal. Lett. 1988, I , 361. (28) Kruse, N.; Chuch, G. K.; Abend, G.;Cocke, D. L.;Bloch, J. H. Surf. Sci. 1987, 189, 832. (29) Solymosi, F.; Berko, A.; Tarnoczi, T. I. Surf. Sci. 1984, 141, 533. (30) Kemp, D. S.; Vellaccio, F. Organic Chemistry; Worth: New York, 1980; p 1058.
Phenol Reactivity on Rh( 1 1 1 )
The Journal of Physical Chemistry, Vol. 93, No. 24, 1989 8079
position commences at temperatures below 400 K, forming adsorbed CO, which is desorbed at near 510 K. At saturation coverage phenoxy decomposes near 490 K, with most of the CO evolved directly into the gas phase at 495 K. Notably, the coverage dependence of the CO formation kinetics is due to a self-modification of the stability of phenoxy on the Rh( 1 1 1) surface since the temperature required for the onset of CO formation reflects the initial rate of phenoxy decomposition in the absence of CO or other adsorbed species. Even the surface hydrogen formed from 0-H bond dissociation in phenol has recombined and desorbed from the surface as Hz by 375 K. An analogous coverage dependence has been observed in the stability of phenoxy and its S analogue, phenyl thiolate, adsorbed on Mo( 1 lo), suggesting that self-stabilization is general for molecules of the form -XC6H5.5*31,32 On both Mo(ll0) and Rh( 1 1 l), the C-C bond cleavage in phenoxy occurs more readily at low coverage, even though the product distributions are dramatically different for Mo and Rh. Interestingly, the kinetically important steps in phenoxy decomposition on both Mo( 110) and Rh(ll1) are qualitatively different than for the S analogue, phenyl thiolate. In phenyl thiolate, coverage-dependent reaction kinetics are also observed, but it is the rate of C-S bond cleavage that is affected rather than C-C bond cleavage. This difference is attributed at least in part to the relative stabilities of the C-0 and C-S bonds, as discussed in related studies of phenol on Mo( 1 and benzenethiol on Rh( 1 1 l).33 Elucidating the origin of the coverage dependence in the decomposition of phenoxy on Rh( l l l ) requires further investigation. A transformation of the adsorption structure driven by coverage, blocking of sites necessary for C-H or C-C bond activation or CO adsorption, or modification of the electronic structure of neighboring Rh atoms by phenoxy all may be important in determining the coverage dependence of the reaction kinetics. Although the adsorption structure of phenoxy on Rh( 1 1 1) is not known, the absence of reversible C-H bond activation in the phenoxy suggests that the phenyl ring is not oriented parallel to the surface. Futhermore, all of the phenoxy intermediates would have to be azimuthally oriented on the surface in order to accommodate all phenoxy in a parallel orientation for the saturation coverage of 0.12 whereas a perpendicular orientation does not require azimuthal order. A parallel orientation is expected to result in extensive reversible C-H bond activation based on related studies of benzene on Rh( 1 1 1).22 Extensive H-D exchange occurs when benzene, which is bound parallel to the surface plane,14J5q34-37 reacts with surface deuterium on Rh( 1 1 1).z2 In contrast, only minimal exchange is observed into the ring of phenoxy in the presence of surface deuterium on Rh( 1 1 1). Furthermore, a large isotope effect in the lowest temperature dihydrogen formation peak from benzene at 450 K suggests that C-H bond breaking is kinetically important in decomposition whereas no significant isotope effect for &-H2 formation from phenoxy decomposition is observed. Although the origin of the difference in the reactivity of benzene and phenoxy is unknown, it is reasonable to assert that it is due to a qualitative difference in ring orientation. In parallel-bound benzene all the C-H bonds are accessible to the surface, allowing for facile reversible and irreversible C-H cleavage. A perpendicular ring orientation, where most C-H bonds are inaccessible to the surface, may account for the absence of H-D exchange in phenoxy at high coverage. Structural determination using the near edge X-ray absorption fine structure method will determine if the reactivity difference is due to a perpendicular ring orientation. (31) Roberts, J. T.; Wiegand, B. C.; Friend, C. M. Manuscript in preparation. (32) Roberts, J. T.; Friend, C. M. J . Chem. Phys. 1988, 88, 7172. (33) Xu, X.; Friend, C. M. Manuscript in preparation. (34) Bertel, E.; Rosina, G.; Netzer, F. P. Surf.Sci. 1986, 172, L515. (35) Neumann, M.; Mark, J. U.; Bertel, E.; Netzer, F. P. Surf. Sci. 1985, 155, 629. (36) Van Hove, M. A.; Lin, R. F.; Somorjai, G. A. J . A m . Chem. SOC. 1986, 108, 2532. (37) Van Hove, M. A.; Lin, R. F.; Somorjai, G. A. Phys. Reu. Lett. 1983, 51, 778.
The coverage dependence in the rate of CO formation may also be related to the availability of sites for adsorption. At saturation coverage, the adsorbed phenoxy may effectively block sites for C-H, C-C bond activation and/or CO adsorption. This would be consistent with the conclusion that most CO formed is evolved directly into the gas phase at saturation coverage after phenoxy decomposition and CO desorption has commenced. At low coverage, the surface is not fully covered with phenoxy so that sites for C O coordination are available and CO formation can proceed readily before desorption commences. The fact that a small amount (
