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The oxidation of alkyl iodides on oxygen-covered Rh( 11 1) has been studied using temperature-programmed reaction and high-resolution electron energy loss spectroscopies. Ethyl and 2-propyl iodide are selectively oxidized to acetaldehyde and acetone with selectivities of -62% and -40%, respectively, on Rh( 11 1)p(2 x 1)-0 (00 = 0.5). Formation of ethene and propene are competing pathways. No CO or C02 is formed at this oxygen coverage. C-I bond breakage in the intact alkyl iodides is proposed to be the ratedetermining step in the formation of both the alkenes and the oxygenates. The resulting alkyl either rapidly dehydrogenates, eliminating the alkene, or adds oxygen, forming a transient alkoxide which subsequently eliminates the aldehyde or ketone via dehydrogenation. The selectivities for the different reaction pathways depend strongly on the oxygen coverage. On clean Rh( 1 1 l), nonselective decomposition to H2 and adsorbed carbon is the dominant pathway. At moderate oxygen coverages (00 < 0.3), CO, C02, alkene, and alkane formation predominate. The strong dependence of the product distributions on the oxygen coverage is attributed to the multifunctional role of oxygen on Rh( 11 1). Oxygen inhibits dehydrogenation such that selective P-H elimination and oxygen addition are enhanced at the expense of nonselective dehydrogenation leading to CO and C02. Carbon-iodine bond cleavage is also retarded by oxygen so that the resulting alkyl radical rapidly reacts to products at high oxygen coverage, allowing for direct oxygen addition to the alkyls. The strong dependence of product distribution on oxygen coverage has important implications for alkane oxidation on Rh catalysts and is in excellent agreement with recent studies of alkane oxidation over rhodium monoliths. Our results suggest that the oxygen coverage can be used to manipulate product distributions in alkane oxidation so as to enhance direct partial oxidation.

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The reactions of benzenethiol on clean Rh(111) have been studied using a combination of temperatureprogrammed reaction, high-resolution electron energy loss, and X-ray photoelectron spectroscopies. Benzenethiol adsorbs dissociatively on Rh(111) at 100 K, affording adsorbed phenylthiolate and hydrogen. The adsorption geometry of phenylthiolate is shown to depend on the coverage, with a low coverage favoring a parallel geometry of the phenyl ring. Phenylthiolate reacts by C-S bond breakage starting at 250 K, forming benzene. At saturation coverage (0.21 monolayers), the benzene is forced into the gas phase upon formation, because of molecular crowding on the surface. Comparison is made to the chemistry of benzenethiol on other transition metal surfaces, and there is no simple correlation between the metalsulfur bonding properties and the activity or selectivity for benzenethiol desulfurization.

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