Synchronous Thermal Desorption and Decomposition of Ethanol on

J. Phys. Chem. 1995, 99, 17645-17649

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Synchronous Thermal Desorption and Decomposition of Ethanol on Rh{ 111) D. C. Papageorgopoulos, Q. Ge, and D. A. King* Department of Chemistry, Lensfield Road, Cambridge CB2 IEW, U.K. Received: July 17, 1995; In Final Form: September 15, 1995@

The effects of both conventional and laser-induced heating on the desorption and decomposition of ethanol have been studied on Rh{ 11l}. The results show that ethanol adsorbs on the surface molecularly. Ethoxide, generally considered as a stable intermediate in the course of ethanol decomposition, does not exist on the surface upon dosing at 200 K. In contrast, we find that decomposition of adsorbed ethanol occurs synchronously with desorption as ethanol. The surface decomposition reaction leaves adsorbed atomic H and CO on the surface, and these desorb (as HZand CO) at a temperature higher than that of the surface decomposition process. It is proposed that both desorption and surface decomposition occur through a common activated complex involving activation of the 0-H bond. Preadsorbed iodine causes blocking of ethanol and poisons the surface decomposition pathway relative to the desorption pathway.

Introduction Reactions of alcohols on transition metal surfaces are of great interest. First, the oxidation products, ketones and aldehydes, are important chemical intermediates in industry. Second, studies on the decomposition of alcohols provide an opportunity to probe the microscopic reverse of Fischer-Tropsch synthesis of alcohols from CO and H2. Investigations of methanol decomposition and of the high molecular weight alcohols suggest that alkoxide formation is the initial step in the decomposition process, with the alkoxides being the stable intermediates.' Xu and Friend, after studying 2-propanol and tert-butyl alcohol on Rh{ 11l}, concluded that the dissociated alcohol is bound to the surface through the oxygen atom.2 The P-C-H in adsorbed alkoxides is typically more labile than other C-H bonds. Elimination at the ,&position is more favorable because the C-H bond strength is reduced due to the adjacent electronegative oxygen. Additional dehydrogenation and, eventually, C-C bond activation rapidly follow. Therefore, CO and H2 would be kinetically favored in any process involving alcohol synthesis on clean Rh{ 11l}. TPD and HREELS studies of ethanol decomposition on the Pd{lll} surface suggested that ethoxide was formed upon adsorption at 170 K.3 Dehydrogenation of ethoxide produced adsorbed acetaldehyde at 200 K; above this temperature the reactions of adsorbed layers derived from ethanol and acetaldehyde on Pd{ 11l} were very similar in all respects. Gates et al. suggested a similar decomposition sequence for ethanol on Ni{ 11l}.4 Their results showed that C-H bond cleavage at the methylene carbon (Le., B-position) was the first step in the dehydrogenation of ethoxide. Barteau and co-workers examined the decomposition of ethanol and other primary alcohols on Rh{ 11l} by using TPD and HREELS.5-8 Their results precluded acetaldehyde as an intermediate in the ethanol decomposition process, relying on the fact that volatile hydrocarbon products were not observed. Instead, they suggested a surface oxymetallacycle as an intermediate in the sequence of the ethanol decomposition as a result of C-H scission at the y-position in the ethoxide. However, this appears to be inconsistent with the HREEL spectra of ethanol6 and ethylene oxide9 on the Rh{ 111) surface. A band at 1210 cm-l, observed in the temperature range @

Abstract published in Advance ACS Abstracts, November 1, 1995.

between 220 to 320 K for ethylene oxide, was not present in the ethanol HREEL spectra. Actually, there was no observable feature around this frequency even at temperatures higher than 250 K for ethanol on Rh{ 11l}. Iodine is of special interest since an iodine-promoted rhodium catalyst system has been widely utilized to produce acetic acid from methanol and CO by the Monsanto process.I0 Halogens adsorbed on metal surfaces generally form simple disordered and commensurate ordered chemisorbed overlayers at low and intermediate coverages without extensive lateral substrate restructuring. They serve as ideal adsorbates to probe the effects of electronegative coadsorbates on kinetics studies. This stems from the fact that, like alkali metals, halogens exhibit repulsive lateral interactions and hence tend to form two-dimensionally "dispersed" phases rather than densely packed two-dimensional islands. Laser-induced thermal desorption (LITD) has proved to be an effective technique in surface diffusion and reaction studies. In this paper the thermal desorption and decomposition of ethanol are studied using conventional and laser-induced heating, on both clean and iodine preadsorbed Rh{111). The primary motivation was to probe the stable intermediates in the course of ethanol decomposition by using LITD. Partially deuterated compounds of ethanol have been used in the experiments. Coadsorbed iodine was used to examine the influences of the electronegative coadsorbates on surface reactions.

Experimental Section All experiments were conducted in a diffusion and titanium sublimation pumped ultrahigh-vacuum chamber described previously, l3equipped with a rotatable hemispherical electrostatic analyzer, Masstorr DX-200 quadrupole mass spectrometer, and a four-grid LEED optics (Varian). The system was operated at a base pressure of mbar. The Rh{ 11l} crystal had been cut to within 0.5" of the respective plane, mechanically polished to a mirror finish, and bulk cleaned with a procedure described e1~ewhere.l~The cleanliness of the sample was checked with AES. The crystal was mounted on a low-temperature probe capable of resistive heating to approximately 1300 K and cooling to 80 K; the temperature was monitored by a chromel-alumel thermocouple spot-welded to the side of the crystal. Thermal desorption spectra were collected at a heating rate of 1.5 Us.

0022-3654/95/2099-17645$09.00/0 0 1995 American Chemical Society

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Figure 1. TPD spectra of ethanol (mass 31) from Rh{lll} with increasing exposures of ethanol at 200 K. The exposure is in langmuir.

Iodine was adsorbed by thermal decomposition of CH31 (Aldrich, 99.9% purity). CH31 was dosed at 200 K, followed by a subsequent flash to 550 K to remove hydrocarbon^.'^ AES indicated a spectroscopically pure iodine-covered surface, with surface carbon below the detection limit of the detector. The iodine 5 11 eV Auger peak-to-peak height was used as a monitor of surface coverage. Ethanol, its deuterated analogues (Aldrich, 99.9%), and methyl iodine were purified by repeated freeze/ pumplthaw cycles and were dosed through a capillary array doser at a distance of about 1 mm from the sample. The strongest fragments in the cracking pattem of ethanol and its deuterated compounds were used to follow the desorption of ethanol. Cleaning treatments with oxygen were regularly carried out between TPD and/or LITD runs. The laser pulses for the LITD experiments were provided by a Lambda Physik excimer laser operated at 308 nm (XeCl) and coupled to a pulse extenderheam profiling system.I3 The laser provided “top hat” light pulses with a temporal width of 120 ns in a 403 pm spot. The laser beam intensity was measured with an energy meter (C-25, Coherent). The laser peak temperature was calculated from the measured intensity using the surface heating model developed by Be~hte1.I~A laser intensity of 105 MW/cm2 gives a temperature rise of 1270 K within the irradiated spot. Laser-induced-desorption species were detected using the mass spectrometer in a time-of-flight (TOF) mode. The TOF mode enables us to distinguish between the mass 2 signal from the fragment of the cracking pattem of the ethanol and the H2 from recombinative desorption of atomic hydrogen. The mass spectrometer is interfaced to a multichannel scaling board synchronized with the scanning unit.

Results and Discussion Clean Rh{ 111) Surface. Dosing ethanol at temperatures below 200 K led to multilayer formation, as shown by Houtman and Barteau.6 Ethanol was the major product when the surface was heated. Figure 1 shows a series of TPD spectra after dosing at 200 K with increasing ethanol exposures. At very low exposure (4 x langmuir), there is only one ethanol desorption peak. With increasing exposure, another peak at a slightly higher temperature emerges and grows, this new peak growing faster than the low-temperature peak. The ethanol desorption pathway was not saturated at the exposure of 0.18 langmuir, shown by the uptake still increasing with further increasing exposures (higher exposures would cause pumping difficulties in the chamber and were avoided), and the onset of

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Figure 2. TPD spectra after exposing clean Rh{ 11l} to ethanol at 200 K. The spectra were normalized to the relative sensitivites of the mass spectrometer to each species. The dose leads to the saturation of the decomposition channel.

the desorption at this exposure is shifted upward in temperature by about 5 K. Both desorption and decomposition occur on the surface, with ethanol desorption being the major channel. The decomposition pathway was saturated at an exposure of -5 x langmuir. However, ethanol desorptiop can even be observed at 200 K for a very small dose, far below the saturation of the decomposition channel. Figure 2 shows the TPD spectra collected after exposing the surface with ethanol at 200 K at a heating rate of 1.5 Ws. The exposure here corresponds to the amount which saturated the decomposition pathway. The spectra were normalized for the relative sensitivity of the products to the mass spectrometer. At this coverage, ethanol desorption was detected in the temperature range 220-260 K and is the dominant channel (’80%). The only other products detected were hydrogen and carbon monoxide, both being desorption limited with peak temperatures corresponding to those of hydrogen and carbon monoxide from the clean surface.I4 After heating a carbon residue was left on the surface. Unlike the results of Houtman and Barteau,6 the desorption of hydrogen was confined to the temperature range 270-400 K. This hydrogen is desorption-limited, as stated above, which indicates that the dehydrogenation and decarbonylation of ethanol must be completed before hydrogen desorption. The broad feature of Houtman and Barteau’s hydrogen TPD spectrum could be due to a high background ethanol pressure as revealed by their ethanol desorption spectrum. Ethoxide has been considered as a stable intermediate in the process of ethanol decomposition.6 The recombinative reaction of alkoxides with atomic hydrogen to form the parent alcohol has been proposed to occur on transition metal surfaces.I6 In agreement with this proposal, Solymosi e t a1.I’ suggested that the hydrogenation of methoxy gave rise to the higher temperature desorption peak for methanol adsorption on Rh{ 111). However, the fact that no deuterated ethanol was detected after coadsorbing deuterium and ethanol on the surface ruled out the possibility that the recombination of ethoxide and hydrogen atom produces the parent molecule. A similar result was reported for methanol on Rh(ll1) by Houtman and Barteams The ethanol detected in the TPD spectra, therefore, can only be a result of desorption of the molecularly adsorbed ethanol, and we conclude that there are two different molecular adsorption states on the surface. The shift of the spectra to a higher temperature with increasing exposures suggests that the interactions between adsorbed ethanol are attractive. An attractive

Decomposition of Ethanol on Rh{ 11l} I

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Figure 3. Ethanol TPD spectra after exposing clean Rh( 11I} to the

same amount of four different isotopic compounds of ethanol at 200 K.

interaction could be due to the hydrogen bond.between adsorbed ethanol molecules. Partially deuterated compounds were used in order to examine the roles played by different groups in the ethanol chemisorption process. CH3CH20H, CD~CHZOH, CH~CDZOH, and CD~CDZOD were dosed onto the surface, and their thermal desorption spectra were collected. Figure 3 shows the TPD spectra after exposing the surface to the four different isotopic compounds at 200 K. For each compound, the exposure was controlled at -7 x lop2langmuir. While the fist three compounds exhibited the same desorption profiles and peak temperatures, the fourth one, where the hydrogen in the hydroxyl group is replaced with deuterium, leads to an increase of about 5 K in the peak temperature, with the higher temperature peak shifting more than the lower one as shown in Figure 3. While ethanol bonds to the surface via its oxygen atom, with the lone pair electrons involved in the process as suggested previously, the present results indicate that not only oxygen but also the hydroxyl hydrogen participates in the bonding of ethanol to the surface. Gates et al. observed a major kinetic isotope effect for methanol and ethanol on Ni{lll} with the H in the hydroxyl group substituted by D.43'8 They proposed a possible transition state controlling CH3O formation, with the CH30 sitting on a bridge site and the hydroxyl H atom approaching a 3-fold Ni site. The major contribution to the reaction coordinate is from the bending of the 0-H bond. Rapid heating achieved with laser irradiation has the potential to access energetically unfavorable channels if these channels are entropically favored. We have shown that acetate undergoes complete dehydrogenation and decarbonylation under conventional heating, decomposing into C02 and CH3 under laserinduced heating.I9 The LITD studies of methanol decomposition on Ni{100}20 and Ru(001}*' demonstrated that direct desorption of methanol is the major channel with laser-induced heating. Similarly, the direct desorption of ethanol, as distinct from surface decomposition, is the preferred channel at the high laser-induced heating rate, with all ethanol being desorbed intact. As expected, no decomposition is attributable to photoexcitation.22 On the other hand, the only laser desorption products from surface decomposed ethanol are H2 (D2 in the case of deuterated ethanol) and CO. Figure 4 shows the LITD spectra of ethanol and hydrogen after dosing ethanol onto the clean surface. it is evident that at temperatures below 220 K the ethanol LITD signal remains constant and there is not detectable HZLITD signal. Since H adatoms are readily detected by LITD,I4 we conclude that

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Temperature ( K ) Figure 4. LITD of ethanol and hydrogen after exposing clean Rh{ 11 1] to 70 x langmuir of ethanol at 200 K. The surface was ramped with a heating rate of 1.5 Ws, and the signal was normalized to 100. ethanol is molecularly adsorbed at temperatures below 220 K and that laser irradiation does not induce its decomposition. The ethanol LITD signal decreases monotonically with increase in temperature above 220 K. Ethanol desorption is completed at about 270 K, in good agreement with the TPD spectra. At a temperature just above 220 K, ethanol starts to dissociate, as shown by an increase in the H2 LITD signal with increase in temperature. The LITD spectrum of hydrogen further indicates that the dissociation of ethanol does not start until the desorption is initiated. Similar results have been observed for LITD of methanol on Ni{ and Ru(001}.21 From these LITD spectra we conclude that decomposition of surface ethanol is synchronous with desorption. The LITD results further prove that ethoxide is not formed on the surface, at least at temperatures below 220 K. The isotope experiment demonstrates that H adatodethoxy recombination does not contribute to ethanol desorption. Since the desorption and decomposition processes are coupled, we conclude that both processes proceed via a single transition state:

I Vibrational excitation of the 0-H group may control both processes, presumably involving excitation of the bending mode. If ethoxide is formed in the decomposition, it is not stable on the surface: it is dehydrogenated and decarbonylated into adsorbed hydrogen, CO, and surface carbon very rapidly. We also note that the observation of a VOH mode at -90 K6 can now be attributed to physisorbed ethanol. Upon heating, the YOH mode disappears at -150 K, indicating the desorption and, possibly, the transformation of the physisorbed state to the chemisorbed state. The chemisorbed species is adsorbed molecularly on the surface, and desorption from this state occurs at temperatures higher than 220 K. The inability to detect an 0 - H stretching mode for chemisorbed ethanol is attributed to the HREELS selection rules: the OH group must lie in the surface plane. Iodine Coadsorbed Surface. The adsorption of iodine on Rh{lll} has been well characterized by LEED.23 At the saturation coverage (6 = 0.33 ML of I), iodine forms an ordered (1/7 x 1/7)R3Oo overlayer. The ordering of iodine on Rh{ 111) is suggestive of net repulsive lateral interactions between adatoms. At low coverage, only a diffuse p (1 x 1) LEED

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Figure 5. Ethanol TPD spectra with increasing coverages of preadsorbed iodine.

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Figure 6. CO TPD spectra from Rh{ 11 l} with increasing I coverage langmuir of ethanol at 200 K. after exposing the surface to 70 x

structure can be observed. The ratio of the A E S peak-to-peak heights of I and Rh is used to follow the coverage of the iodine on the surface as a function of exposure. The influence of iodine on the adsorption of ethanol was examined by comparing ethanol TPD spectra from clean and from iodine precovered surfaces (Figure 5). Preadsorption of iodine on the surface decreases the uptake of the surface to ethanol. The ethanol yield from a 0.33 ML iodine covered surface is 50% of that from the clean surface. This indicates that the extent to which iodine adatoms block sites for ethanol adsorption. Ethanol TPD spectra also show a shift to a higher peak temperature for the iodine preadsorbed surface. Figure 6 displays mass 28 TPD spectra for different iodine coverages. With increasing iodine coverage, the CO desorption peak temperature shifts down. This effect has also been observed for iodine coadsorption with CO on Rh{ 11l}.I4 The weakening of the adsorbate-substrate bond is attributed mainly to the electrostatic effects of iodine adsorption. Comparing these spectra, we also find that the CO yield is decreased more than that of ethanol. This decrease demonstrates a selective poisoning effect of iodine on the rate of decomposition of ethanol, compared to the rate of desorption. This may well be a simple site-blocking effect, sites required for the decomposition products being blocked by neighboring iodine. Electronegative additives have been found to alter the selectivity between different reaction pathways in methanol adsorption and decomposition processes on transition metal surfaces.24 Generally, adsorption is blocked, and the decom-

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Temperature ( K ) Figure 7. Hydrogen LITD from the iodine preadsorbed surface with the iodine coverage of 0.11 monolayer after exposing to ethanol at 200 K. The surface was ramped with a heating rate of 1.5 IUS, and the signal was normalized to 100.

position to aldehyde (RCHO) species is favored in the presence of electronegative modifiers. LITD studies of the effects of sulfur on methanol decomposition on Ni{ 111}25and Ru{O01}26 showed that both site-blocking and electronic effects play roles in the process depending on the sulfur coverage. Preadsorbed oxygen on Rh{ 111) was found to modify the reactivity of the surface so that selective C-H bond breaking is inhibited while the more labile P-C-H bond is selectively cleaved, which leads to the formation of a~etaldehyde.~'Acetaldehyde further reacts with oxygen and produces acetate on the surface which decomposes into C02 and Hz. As shown in the TPD spectra, however, the presence of iodine on the surface does not alter the reaction pathway. It does, however, block adsorption and poison the decomposition channel. Both the uptake of ethanol and the decomposition yield are decreased by iodine. The influence of adsorbed iodine on the H2 LITD signal is shown in Figure 7. The results are similar to the clean surface, but consistent with ethanol desorption, the initial temperature at which the H:! LITD signal is detectable is shifted to a higher value (4 K). The different effects of oxygen and iodine, both of which are said to be electronegative modifiers, can be explained as follows. Oxygen not only acts as an electronegative additive but also participates in the reaction. During ethanol adsorption, surface oxygen extracts the hydroxyl hydrogen which promotes the dissociation of ethanol. Also, oxygen stabilizes ethoxide formed from the dissociation and reacts with acetaldehyde formed from further dehydrogenation of ethoxide. Iodine clearly does not react with ethanol, and the adsorption of iodine on Rh{ 111) actually decreases the work function by about 0.3 eV,** suggesting that it is not even electronegative. However, the effects of iodine on ethanol decomposition are similar to those of sulfur on methanol decomposition on Ni{ 111}25 and Ru{O01}.26 Iodine primarily acts by blocking sites for the decomposition channel.

Summary Thermal desorption and decomposition channels of ethanol on Rh{ 111) have been studied. We have shown that ethanol adsorbs molecularly on the surface through its hydroxyl group. At slow heating rates two competitive channels exist for adsorbed ethanol on Rh{ 111}, desorption and decomposition, with desorption being dominant. It is proposed that these two channels proceed through the same intermediate and that the

Decomposition of Ethanol on Rh( 11l} rate-limiting step could be the bending movement of the hydroxyl H atom. A small fraction of ethanol decomposes via some unstable intermediate which we could not identify in the present study. No hydrogen adatoms exist on the surface before ethanol desorption, even though no VOH mode was observed from the chemisorbed layer.6 Preadsorbed iodine does not alter the decomposition pathway but favors the desorption pathway.

Acknowledgment. The E.C. is acknowledged for a studentship (D.C.P) within the framework of the Human Capital and Mobility programme. Q.G. acknowledges grants from EPSRC and Unilever, and an equipment grant from EPSRC is also gratefully acknowledged. References and Notes (1) Netzer, F. P.; Ramsey, M. G. Crit. Rev. Solid State Mater. Sci. 1992, 17, 397 and references therein. (2) Xu, X.; Friend, C. M. Surj". Sei. 1992,260, 14. Xu, X.; Friend, C. M. Langmuir 1992, 8, 1103. (3) Davis, J. L.; Barteau, M. A. Surf.Sci. 1987, 187, 387. Davis, J. L.; Barteau, M. A. Surf. Sci. 1990, 235, 235. (4) Gates, S. M.; Russell, J. N.; Yates, J. T. Surf. Sei. 1986, 171, 111. (5) Houtman, C. J.; Barteau, M. A. Langmuir 1990, 6 , 1558. (6) Houtman, C. J.; Barteau, M. A. J . Catal. 1991, 130, 528. (7) Brown, N. F.; Barteau, M. A. Langmuir 1992, 8, 862. (8) Brown, N. F.; Barteau, M. A. In Selectivity in Catalysis; ACS Symp. Ser. 57; Davis, M. E., Saib, S. L., Eds.; American Chemical Society: Washington, DC, 1993: p 345. (9) Brown, N. F.; Barteau, M. A. Surf. Sci. 1993, 298, 6.

. I . Phys. Chem., Vol. 99, No. 49, 1995 17649 (10) Dekleva, T. W.; Forster, D. Adu. Catal. 1986,34,81 and references therein. (11) Hall, R. B. J . Phys. Chem. 1987, 91, 1007 and references therein. (12) George, S. M. In Investigations of Surfaces and Interfaces, Part A ; Rossiter, B. W., Baetzold, R. C., Eds.; John Wiley & Sons: New York, 1993; p 453. (13) Mann, S. S.; Seto, T.; Barnes, C. J.; King, D. A. Surf. Sci. 1992, 274, 129. (14) Mann, S. S. Ph.D. Thesis, Cambridge University, 1991. (15) Bechtel, J. H. J . Appl. Phys. 1975, 46, 1585. (16) Sexton, B. A.; Rendulic, K. D.; Hughes, A. E. Surf. Sei. 1982,121, 181. (17) Solymosi, F.; Berk6, A.; Tarn6czi, T. I. J . Chem. Phys. 1987, 87, 6745. ( 1 8 ) Gates, S. M.; Russell, J. N.; Yates, J. T. Surf. Sci. 1984, 146, 199. (19) Hoogers, G.;Papageorgopoulos, D. C.; Ge, Q.;King, D. A. Su$. Sci. 1995, 340, 23. (20) Hall, R. B.; Desantolo, A. M. Sutf Sci. 1984, 137, 421. (2 1) Deckert, A. A.; Brand, J. J.; Mak, C. H.; Koehler, B. G.; George, S. M. J . Chem. Phys. 1987, 87, 1936. (22) Stevens. P. G.:Hiebee. W. E.: Armstrong. R. T. J . Am. Chem. Soc. 1938, 60, 2658. (23) Barnes. C. J.: Wander, A. J.: King, D. A. Surf.Sci. 1993,281, 33. (24) Kiskinova, M. P. Poisoning and Promotion Catalysis Based on Surface Science Concepts and Experiments; Stud. Surf. Sci. Catal., Vol. 70; Delmon, B., Yates, J. T., Eds.; Elsevier: Amsterdam, 1992. (25) Hall, R. B.; Desantolo, A. M.; Grubb, S. G.J . Vac. Sci. Technol. 1987, A5, 865. (26) Deckert, A. A.; Arena, M. V.; Brand, J. J.; George, S. M. Surf. Sci. 1990, 226, 42. (27) Houtman, C. J.; Brown, N. F.; Barteau, M. A. J . Catal. 1994, 145, 31. (28) Solymosi, F.: KlivCnyi, G.J. Phys. Chem. 1994, 98, 8061. I

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