Langmuir 1990,6, 1558-1566
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Reactions of Methanol on Rh(ll1) and Rh(lll)-(2X2)0 Surfaces: Spectroscopic Identification of Adsorbed Methoxide and ql-Formaldehyde Carl Houtman and Mark A. Barteau* Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received December 9,1989.I n Final Form: April 24, 1990 The reactions of methanol were examined on clean and oxygen-predosed Rh(ll1) surfaces under ultrahigh-vacuum conditions. Methoxy intermediates were formed at 140 K after methanol adsorption on the clean surface at 100 K. High-resolution electron energy loss spectroscopy (HREELS) indicated that these methoxides began to decompose to CO and Hz at 210 K. The addition of one-quarter monolayer of oxygen to the surface produced two principal effects on the reactions on the Rh(ll1) surface. The first was the alteration of reaction and desorption kinetics via lateral interactions with other adsorbed species. This change was manifested in both the stabilization of methoxide to higher temperatures than observed on the clean surface and the isolation of an adsorbed formaldehyde intermediate produced by methoxide dehydrogenation. Formaldehyde was not isolated after a methanol dose on the clean surface. The oxygen also changed the desorption kinetics of CO and H2.The second role of oxygen was direct reaction with surface species. An example was the direct transfer of the hydroxyl hydrogen of methanol to the surface oxygen, which enhanced the formation of methoxy intermediates at low temperature. These hydroxyls reacted with atomic hydrogen and desorbed as water at 230 K. The results of this study are qualitatively similar to the reactions of oxygenates on other Group VI11 metals, although the stabilities of adsorbates and the selectivities of their reactions vary among these metals.
Introduction Methanol formation and decomposition reactions on metals have been the topic of research for many years. With the advent of improved high-resolution electron energy loss spectroscopy (HREELS) in the last decade, these reactions have been explored in greater detail. Since the reaction studies of Wachs and Madix identified a methoxide on Cu(110) and Ag(llO),' numerous experiments have been conducted to gain an understanding of the reactions and stabilities of methanol and its products, such as methoxide, formaldehyde, formyl, and formate, on metal surfaces. Much of this work has been conducted with the goal of understanding CO hydrogenation chemistry. The relative stabilities of adsorbed methanol, methoxide, and formaldehyde depend strongly on the identity of the metal surface. These stabilities can also be modified by the addition of oxygen to the surface. For example, Sexton2 observed t h a t on t h e clean Pt(ll1) surface methanol decomposed directly to carbon monoxide and hydrogen a t 155 K, whereas with the addition of oxygen to the surface, the methoxide intermediate was observed spectroscopically at 170 K. In this case, no formaldehyde intermediates were isolated. This can be contrasted to experiments with methanol on the P d ( l l 1 ) ~ u r f a c e .As ~?~ on P t ( l l l ) , methanol on the Pd(ll1) surface does not form easily isolated methoxide intermediates on the clean surface but does on the oxygen-predosed ~ u r f a c e .These ~ two metals differ, however, in that methanol decomposition on Pd(ll1) yields isolable formaldehyde species on the clean surface5 rather than the complete dehydrogenation products observed on the Pt(ll1) surface. A final illustrative example is the decomposition of methanol on the Ru(001)e surface. On both the clean and oxygen(1)Wachs, I. E.;Madix, R. J. Surf. Sci. 1978,76,531. (2)Sexton, B. A. Surf. Sci. 1981,102, 271. (3) Davis, J. L.; Barteau, M. A. Surf. Sci. 1988,297, 123. (4) Gates, J. A.; Kesmodel, L. L. J. Catal. 1983,83,437. (5)Davis, J. L.; Barteau, M. A. Surf. Sci. In press. (6) Hrbek, J.; DePaola, R.; Hoffmann, F. M. Surf. Sci. 1986,166,361.
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predosed ruthenium surfaces, methanol yields methoxide intermediates upon warming, but a more striking difference is that 20% of the methoxide species decompose via C-0 cleavage on the clean surface. A similar pathway has also been proposed by Levis e t al.7 for t h e decomposition of methanol on the Pd(ll1) surface. This result, however, is controversial. Guo et did not observe C-0 bond breaking with methanol on Pd(ll1). Since Rh lies between Ru and Pd in the periodic table, one might expect it to exhibit similarities to both in the methanol decomposition reaction. T h i s issue was addressed previously by the T P D and EELS (in the electronic range) studies of Solymosi e t al.9J0 They suggested that methanol decomposition on the Rh(ll1) and R h ( l l l ) / O surfaces proceeds via a methoxide intermediate exclusively, b u t with the difficulty of interpreting electronic range EELS, there remain uncertainties regarding the identity of the intermediates in the decomposition sequence. More recently, Chuah et a1.l1 used pulsed field desorption mass spectrometry to study the decomposition on Rh field emitter tips. Chuah et al. concluded that stepwise hydrogen abstraction resulted in the methanol decomposition sequence methanol methoxide formaldehyde formyl carbon monoxide. They further concluded that the dehydrogenation of methoxide to formaldehyde was the rate-limiting step. The focus of the present study is the identification of the surface intermediates resulting from the reaction of methanol on the R h ( l l 1 ) surface by high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy ( X P S ) . Temperature-
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(7)Levis, R.J.; Zhicheng, J.; Winograd, N. J. Am. Chem. SOC. 1988, 110,4431. (8) Guo, X.;Hanley, L.; Yaks, J. T., Jr. J. Am. Chem. SOC. 1989,121, 3155. (9) Solymosi, F.;Berk6, A.; Tarnbczi, T. I. Surf. Sci. 1984,141, 533. (10) Solymosi, F.; Tarn6czi, T. I.; Berk6, A. J . Phys. Chem. 1984,88, 6170. (11)Chuah, G.-K.; Kruse, N.; Schmidt, W. A.; Block, J. H.; Abend, G. J . Catal. 1989,119,342.
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Reactions of Methanol on Rh(111) Surfaces
programmed desorption ( T P D ) and temperatureprogrammed HREELS were used to probe the kinetics of surface reactions. A comparison of these results with those from other metals serves to elucidate catalytic trends among the Group VI11 metals.
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Experimental Section The experiments were conducted in two chambers. The first was a diffusion-pumped p-metal vacuum chamber (VG Escalab), with a base pressure of 8 X lo-" Torr. This chamber was equipped with an ion gun, a Mg X-ray source, an electron gun, and a hemispherical electron energy analyzer. The analyzer was operated a t a pass energy of 20 eV, giving a resolution of 1.0eV fwhm of the Rh 3d lines. T h e second instrument, described previously,l* is a two-level stainless steel vacuum chamber, base pressure 2 X T o r r , equipped with a HREELS spectrometer (McAllister Technical Services) and four grid optics (Physical Electronics) for LEED and AES. Both instruments had a mass spectrometer (UTI lOOC) multiplexed with an IBM XT. The Rh(ll1) crystal was polished by using standard metallographic techniques and aligned by Laue X-ray backscattering to f0.5'. T h e crystal was spot-welded on two 0.5-mm tantalum wires that served as a support. Heating was achieved by passing current (30 A maximum) directly through the support wires. In the first chamber, the crystal was cooled to 165 K by thermal conduction through a copper braid connected to a liquid nitrogen reservoir. In the second chamber, the crystal could be cooled to 85 K by thermal conduction through a 1/4in. copper feedthrough which was immersed in liquid nitrogen. The temperature was monitored with chromel/alumel thermocouples spot-welded to the back of the crystal. The R h ( l l 1 ) crystal was cleaned by cycles of ion bombardment, oxygen TPD, and annealing to 1400 K. AES, HREELS, and oxygen T P D were used to judge the cleanliness of the crystal. The primary impurities prior to cleaning were B, C, S, and P. The methanol sample was stored in a glass tube connected to a stainless steel dosing line and purified by freeze/pump/thaw cycles. Formaldehyde was obtained by heating t o 60 "C paraformaldehyde which had been thoroughly outgassed in a glass tube. T h e extra dry oxygen was used as supplied by Matheson. These reagents were dosed to the crystal through a 1.5-mm stainless steel needle positioned within 2 cm of the front face of the crystal. An oxygen dose of 1.2 langmuirs (1 langmuir = lo+ Torr-s) from the background was found to produce a consistent ( 2 x 2 ) L E E D p a t t e r n corresponding t o onequarter monolayer of oxygen atoms on the surface. A T P D ramp rate of 4 K/s was used. This rate was constant (d~0.2K/s) to 1350 K. The electron beam energy for the HREELS experiments was 5 eV; this produced an elastic peak height of 3 X 105 CPS and a fwhm of 65 cm-l for reflection from the clean surface. For t h e temperature-programmed steps between t h e HREELS experiments, a ramp rate of 4 K/s was used to heat the crystal to the desired temperature. The power supply was then turned off and the maximum temperature recorded. This maximum was used a s t h e indicated temperature in t h e HREELS sequences below. The crystal was allowed to cool to the initial temperature before the HREEL spectrum was collected.
Results Reactions of Methanol on the R h ( l l 1 ) Surface. Temperature-programmed HREELS experiments clearly showed that methanol adsorbed molecularly on the clean Rh(ll1) surface at 99 K and dissociated upon warming. This sequence of HREEL spectra is shown in Figure 1. The 99 K spectrum of Figure 1was obtained after exposing the crystal to approximately 1 langmuir of methanol. All of the vibrational bands were assigned to molecular methanol on the basis of the close agreement with reference (12)Davis, J. L.; Barteau, M. A. Surf. Sci. 1989, 208, 383.
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Figure 1. HREELS after dosing approximately 1 langmuir of methanol a t 99 K and HREELS after subsequent heating to 180 and 218 K. An equivalent dose resulted in the desorption of 3.5 X 1014molecules/cm* of CO in TPD.
IR spectral3 and previously reported HREEL spectra of molecular methanol on other metals,4,6J4J5The assignment of this spectrum to molecular methanol was favored due to the two resolved 0-H modes: the 0-H bend a t 825 cm-l and the 0-H stretch a t 3230 cm-l. Comparison of this spectrum with a similar one after dosing methanol-dl (Figure 2) confirms that these are indeed the 0-H modes. The 0-D bend was found a t 600 cm-l and the 0-Dstretch at 2455 cm-l for adsorbed CH3OD. The ratios of the respective vibrational frequencies are 8251600 = 1.38and 323012455 = 1.32, which compare favorably with the theoretical value of the 1.39 obtained by the application of simple harmonic oscillator theory to the motion of hydrogen or deuterium relative to the rest of the methanol molecule. The other bands were easily assigned to the modes of methanol. Table I shows the mode assignments for both methanol and methanol-dl. The frequencies from reference IR spectra13are also shown for comparison. The expected characteristic methyl group vibrations were observed: the C-H stretch a t 2945 cm-l, the methyl deformation a t 1460 cm-l, and the methyl rock a t 1130 cm-'. A strong band at 1030 cm-l was assigned to the C-0 stretch of molecular methanol. Several other researchers have observed the methanol-surface stretch, but in the case of R h ( l l l ) ,this probably occurs below 200 cm-l and was obscured by the elastic peak. The only ambiguity in the assignments was the band at 620 cm-l, which appears in both the molecular methanol and the methoxide spectra. This band may be a molecular bending mode relative to the surface as suggested by Christmann and Demuth.14 Heating the adsorbed methanol or methanol-dl layers to 180 or 190 K, respectively, resulted in the formation of methoxide species. Since in the latter case the deuterium was lost to the surface to form a methoxide of identical isotopic composition to that derived from CHaOH, the (13) Shimanouchi, T. Tables of Molecular Vibrational Frequencies, Part 1 ; NSRDS-NBS6 Washington D.C., 1972. (14)Christmann, K.; Demuth, J. E. J. Chem. Phys. 1982, 76, 6308. (15) Bare, S. R.; Stroscio, J. A.; Ho, W. Surf. Sei. 1985, 150, 399.
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1560 Langmuir, Vol. 6, No. 10, 1990
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Figure 2. HREELS after dosing approximately 1 langmuir of methanol-dl at 99 K and HREELS after subsequent heating to 190 and 238 K. An equivalent dose resulted in the desorption of 3.6 X 1014 molecules/cm* of CO in TPD.
spectra of the methoxide species in Figures 1 and 2 are very similar. Both correspond to the fully hydrogenated methoxide. This dissociated state, rather than molecularly adsorbed methanol, was indicated by the absence of the characteristic 0-H/O-D bends a t 825/600 cm-l and the 0-H/O-D stretches a t 3230/2455 cm-', as well as by the addition of a new band at 345 cm-l. By comparing this vibrational frequency with that found for methoxide on other m e t a l ~ , ~ -one ~ J can ~ J ~assign this new mode to the oxygen-metal bond between the methoxide and the surface. Dissociation of methanol to methoxy species would be expected to release the hydroxyl hydrogen to the surface in the form of atomic hydrogen; however, since metalhydrogen bands are typically very weak and have not been observed on other methanol-dosed Group VI11 metals, it is not surprising no metal-hydrogen stretches were observed in this case. The other losses apparent in the 180 and 190 K spectra of Figures 1and 2, respectively, were assigned to adsorbed methoxide or carbon monoxide. The strongest band a t 1015 cm-I was assigned to the C-0 stretch of methoxide. The intensity of this band was consistent with the dipole activity of a bond perpendicular to the surface. The enhancement of this peak over that of the corresponding C-0 stretch of molecular methanol indicated that the C-0 bond axis was a t an angle to the surface when the hydroxyl hydrogen was present and that the bond became perpendicular when the molecule dissociated. The rest of the major bands have been assigned to modes of the methyl group of the methoxide, similar to those found in the molecular methanol spectra. The mode a t 2010 cm-l was produced by a small amount of decomposition to CO. On the clean surface, the methoxy species decomposed to CO upon heating to 218 K as shown in Figure 1, and there was no evidence in the HREEL spectra for the presence of other intermediates prior to t h e formation of adsorbed CO (CO(a) was fingerprinted by vibrations a t 480 and 2050 cm-'1. The sequential dehydrogenation of methoxide to CO must involve a formaldehyde intermediate. However, in parallel studies we have observed that formaldehyde adsorbed on
the Rh(ll1) surface began to decompose at 130 K.l6 Thus any formaldehyde formed from methoxide decomposition at 210 K would continue to dehydrogenate to CO. This conclusion is also supported by the work of Chuah et al." They concluded that the dehydrogenation of methoxide to formaldehyde was the rate-limiting step in the methanol decomposition sequence. In summary, H R E E L S experiments showed that the decomposition sequence began with molecular methanol bonded through the oxygen to the surface. This species reacted via the loss of the hydroxyl hydrogen to produce an adsorbed methoxide intermediate. This methoxide further dehydrogenated to yield carbon monoxide and atomic hydrogen as the sole surface species by 230 K. No other intermediates were isolated on the clean surface. The existence of the methoxide intermediate over a limited range of temperatures is further illustrated by plots of relative band intensities versus temperature. Figure 3 is one such plot for a 1-langmuir dose of methanol. The characteristic bands used to fingerprint each species were the 0-H bend at 825 cm-l for methanol, the M-0 stretch a t 345 cm-l for methoxide, and the C-0 stretch at 2050 cm-l for carbon monoxide. The relative intensities were obtained by integrating the respective bands and dividing these results by the integral of the elastic peak. Integration was possible because the bands were completely resolved, and it has the advantage over peak height analysis of eliminating the effects of band broadening common with hydrogenic modes. Since the prediction of transition crosssections for vibrational bands is difficult and only qualitative, comparison of intensities between various bands cannot give concentrations of species on the surface, but these intensities can be used to track the populations of individual species on the surface. It should be noted that, owing to the differences in the temperature ramps (intermittent vs continuous), there is not a n exact correspondence between T P H R E E L S a n d T P D experiments. In the case of methanol, the 0-H bend mode intensity decreased monotonically to zero over a wide temperature range (120-180 K). This broad but steady decrease was the result of two processes: desorption of molecular methanol (detected in TPD experiments) and dissociation of methanol to methoxide species. The conversion to methoxide intermediates was confirmed by the corresponding increase in the M-0 stretch intensity of the methoxide. As shown in Figure 3, the methoxide began to form between 140 and 160 K. Its concentration peaked at ca. 180 K, and it was removed completely from the surface by 220 K. The complete dehydrogenation of the methoxide, indicated by the decrease in M-0 vibration intensity, was matched by an increase in the C=O stretch intensity of adsorbed CO. This conversion exhibited a maximum rate at ca. 210 K, as illustrated by the slope of the CO stretch intensity versus temperature. TPD experiments complemented the results of the HREELS study. Methanol adsorbed at 99 K decomposed to carbon monoxide and hydrogen on the clean Rh(ll1) surface (Figure 4). These were the sole reaction products desorbed; no other products (e.g., CH20, CH4, or CH3OCH3) were detected during the TPD experiment. The H2/CO ratio from methanol TPD was determined to be 1.8/1 when corrected for mass spectrometer sensitivity. Since the shape and location of the hydrogen peak could be duplicated by exposure of the surface to H2, as shown in the inset of Figure 4,the Hz peak a t 320 K produced by methanol decomposition was limited by the kinetics (16) Houtman, C.; Barteau, M. A. Unpublished results.
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Reactions of Methanol on Rh(ll1) Surfaces
Table I. Methanol Vibrations (an-*)
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methanol
methanol41
3230 2945 1460 1130 1030 825 620
