730 Langmuir 1994,10, 730-733 Thermal Decomposition of Ethylene Oxide on Pd( 111): Comparison of the Pathways for the Selective Oxidation of Ethylene and Olefin Metathesis R. M. Lambert,+R. M. Ormerod,* and W. T. Tysoe*p% Department of Chemistry, University of Cambridge, Cambridge CB2 lEW, U.K., Department of Chemistry, University of Keele, Keele, Staffordshire ST5 5BG, U.K.,and Department of Chemistry and Laboratory for Surface Studies, University of Wisconsin, Milwaukee, Wisconsin 5321 1 Received August 3, 1993. In Final Form: December 16,199P The product distribution detected in the multimass temperature-programmeddesorption of a saturated overlayer of ethylene oxide adsorbed on Pd(ll1) at - 180 K indicates that it decomposes to yield ethylene and acetaldehyde. These observations are interpreted by postulating that ethylene oxide reacts to form an oxymetallocycle. This is proposed to thermally decompose in a manner analogous to carbometallocycles that form during olefin metathesis catalysis by the reaction between an alkene and a surface carbene. Thus, the metallocycle can decompose to yield ethylene and deposit adsorbed atomic oxygen or undergo a 8-hydrogen transfer to form acetaldehyde. Introduction Scheme 1 Ethylene epoxidation is a catalytic process of great industrial importance where the most selective catalysts are based on It has been unequivocally demon- T\ 2L + CAUI XCH, strated that both partial and total oxidation products arise from the reaction of ethylene with adsorbed atomic oxygen and desorbs intact at -175 K.g Ethylene oxide adsorbed and that the presence of dissolved subsurface oxygen on oxygen-precovered silver does thermally decompose enhances the selectivity to ethylene oxide.3~~It has been by reaction with oxygen, and this has been suggested as proposed that this facilitates electrophilic attack of a pathway to the formation of total oxidation products.3 ethylene by adsorbed atomic oxygen by reducing its negative charge. Chlorine is proposed to enhance the Once formed, it is likely that such an oxymetallocycle selectivity in a similar manner. can decompose via a number of pathways. Insights into possible decomposition routes can be gained by noting The nature of the surface intermediate formed by the similarity between oxymetallocycles formed by reaction reaction between O(&) and gas-phase ethylene remains of an alkene with adsorbed atomic oxygen and the elusive. This reaction could occur either by direct reaction metallocyclic intermediate formed by reaction between between ethylenic ?r-orbitals and adsorbed oxygen to yield an alkene and a surface carbene in the proposed adsorbed ethylene oxide directly or alternatively by (CH2(4 interaction with adsorbed oxygen and a vacant adjacent reaction pathway for olefin metathesis.lO The postulated metal site to form an oxymetallocycle intermediate. Such correspondence between the two reaction pathways is an oxymetallocycle has been proposed to form by the emphasized in Scheme 1where X is CH2(ah)for metathesis reaction of propylene with atomic oxygen on Rh(111),5 and O(ah)for epoxidation. and in the heterogeneous phase, they have been proposed The fate of the carbometallocycle (where X is CH2) is as intermediates in both epoxidatione and de-epoxidation' well documented. It can do the following: (i) Eliminate reactions. They have also been invoked to explain the an alkene by the reverse of the formation reaction (Scheme silver-catalyzed isomerization of ethylene oxide to ace- 1).loJ1In this context, ethylene desorption has been taldehydea8 Ethylene oxide formation would then occur observed in temperature-programmed desorption follow- via a reductive elimination of this species. This suggests ing ethylene oxide adsorption on Mo(ll0) although it that, under suitable conditions, ethylene oxide should should be noted that this was assigned to the direct adsorb on transition-metal surfaces by cleavage of the C-0 desorption of ethylene from ethylene oxide and the effect bond, forming an oxymetallocycle. It has, however, been attributed to the strong 0-Mo bond formed at the found that ethylene oxide adsorbs molecularly on silver surface.12 (ii) Reductively eliminate cyclopropane, par- ticularly on low oxidation state surfaces.13 An analogous * To whom correspondence should be addressed. reaction for the oxymetallocycle would yield ethylene + University of Cambridge. t University of Keele. (9) (a) Campbell, C. T.; Paffet, M. T. Surf. Sci 1986, 177, 417. (b) 8 University of Wisconsin. KrMer, B.; Benndorf, C. Surf. Sei. 1986, 178, 704. * Abstract published in Advance ACS Abstracts, February 1,1994. (10) (a) Herisson, J. L.; Chauvin, Y. Makromol. Chem. 1970,141,161. (1) Van Santen, R. A.; Kuipers, H. P. C. E. Adu. Catal. 1987,35,265. (b)Mocella, M. T.; Rovner, R.; Muetterties, E. L. J.Am. Chem. SOC.1976, (2) Zamerdijk, J. C.; Hall, M. W. Catal. Rev.-Sci. Eng. 1981,23,163. 98,4689. (c) Grubbs, R. H. hog.Znorg. Chem. 1978,24,1. (d) Calderon, (3) Grant, R. B.; Lambert, R. M. J. Catal. 1985,92, 364. N.; Lawrence, J. P.; Ofstead, E. A. Adu. Qrganomet. Chem. 1979,17,449. (4) Van Santen, R. A.; De Groot, C. P. M. J. Catal. 1986,98, 530. (e) Rapp6, A. K.; Goddard, W. A,, 111. J. Am. Chem. SOC.1982,104,448. (5) Xu, X.; Friend, C. M. J. Am. Chem. SOC.1991,113,6779. (11) (a) Grubbs, R. H.; Swethick, 5. J. J. Mol. Catal. 1980,8,25. (b) (6) Sharpless, K. B.; Teranishi, A. Y.; Bbkvall, J. E. J. Am. Chem. Farona, M. F.; Tucker, R. L. J. Mol. Catal. 1980,8, 85. SOC.1977,99, 3120. (12) Serafin, J. G.; Friend, C. M. J. Am. Chem. SOC.1989,111,6019. (7) Sharpless, K. B.; Umbreit, M. A,; Nieh, M. T.; Flood, T. C. J. Am. (13) (a)Theopold,K.H.;Bergman,R.G.J.Am. Chem.Soc. 1980,102, Chem. SOC.1972,94, 6538. 5698. (b) McQuillan, F. J.; Powell, K. G. J. Chem. SOC.,Dalton Tram. (8)Tan, S. A.; Grant, R. B.; Lambert, R. M. J. Catal. 1987,106, 54. 1972, 2123. 0743-7463/94/2410-0730~0~.50~00 1994 American Chemical Society Thermal Decomposition of Ethylene Oxide on Pd(ll1) Langmuir, Vol. 10, No. 3, 1994 731 Scheme 2 Scheme 2) can, in principle, also hydrogenate to form i methane. However, in this case, the probability of carbon CH,=CH, t XorCHl=X + CH, M M and hydrogen formation from methylene decomposition is likely to be much higher than that for rehydrogenation to methane. Formaldehyde predominantly decomposes on Pd(111) X 2 CHI with only -3 % desorbing molecularly.l8 Hydrogen (46 '5% ) \/M - and CO (51% ) are the primary desorption products. In this case, the hydrogen desorbs at 300 K compared with iii 340 K for hydrogen arising from acetaldehyde decompo- - CH,-CH=X sition. oxide. (iii) Undergo &hydrogen transfer,14 yielding, in Experimental Section the case of the carbometallocycle, propylene, and oxy- The experiments were carred out in a stainless steel ultrahigh metallocycle decomposition via this route would yield an vacuum chamber operating at a base pressure of <5 X 10-11 Torr, aldehyde or ketone. Acetone desorption has been detected which has been described in detail elsewhere.lB It contained a following propylene adsorption on Rh(lll)-(2X1)-05 four-grid retarding field analyzer for LEED/Auger measurements where a similar hydrogen-transfer mechanism was invoked and a computer-multiplexed mass spectrometer which was used and the formation of v2(C,0)-acetaldehyde has been to obtain temperature-programmed reaction data. A heating detected from ethylene oxide exposure to potassium- rate of 8 K/s was employed for these measurements. Blank promoted Ni(lll).15 In the absence of coadsorbed po- temperature-programmed desorption experiments were also tassium, ethylene oxide desorbs molecularly from Ni- carried out after leaving the sample in uacuo for an identical time as required to dose it to estimate the contribution to the (111).16 spectrum of any background contaminants. Minor amounts of It is hypothesized, on the basis of the analogy with the CO and H2 were detected, but these were negligible (<5%) chemistry of the carbometallocycle, that an oxymetallo- compared to the signal obtained by saturating the surface with cycle should decompose via route i in Scheme 2 to yield ethylene oxide. The Pd(ll1) single crystal was prepared by ethylene and deposit adsorbed atomic oxygen or evolve standard methods and cleaned by repeated cycles of argon ion formaldehyde and leave surface carbenes. It would react bombardment (800 K,500 eV, 0.05 A/m2) and annealing to 950 via route ii to desorb ethylene oxide, and pathway iii would K until no impurities could be detected by Auger spectroscopy. form acetaldehyde. These routes are summarized in Residual carbon was removed by repeated heating in 1 X 1o-B Scheme 2. Torr of 02 at 850 K, followed by annealing to 1200 K. This procedure was repeated until 02adsorption at 300 K resulted in To test this hypothesis, a transition metal is required no CO formation in the subsequent desorptionsweep. The sample that is sufficiently reactive to dissociatively adsorb eth- was resistively heated and could also be rapidly cooled by being ylene oxide, but where the various postulated reaction brought into contact with a movable liquid nitrogen-fiied products ethylene, formaldehyde, and acetaldehyde do reservoir. The minimum temperature attainable with the sample not decompose to a substantial extent on the surface or in contact with the cold finger was - 100 K. Ethylene oxide was, have well-documented decomposition pathways. The however, adsorbed at the maximum temperature (-180 K) surface chemistry of ethylene oxide has not been previously consistent with the formation of a chemisorbed overlayer. Gases investigated on palladium, although this surface will have were introduced into the chamber either by back-fiiing via a variable leak valve or by means of a calibrated capillary array a greater tendency to dissociate C-0 bonds than silver which was directed at the sample.