Geometric and Electronic Structure Sensitivity of Methyl and Methylene Reactions on Î±-Cr2O3 and Î±-Fe2O3 surfaces
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Structural and electronic effects in hydrocarbon reactions over metal oxides have been examined by comparing the reactions of methyl (-CH3) and methylene (=CH2) fragments on three different oxide single crystal surfaces: Î±-Cr2O3(10-12), Î±-Cr2O3(0001), and Î±-Fe2O3(10-12). The intermediates have been generated through the decomposition of halogenated hydrocarbons. The primary reactions of methyl and methylene over Î±-Cr2O3 are methyl dehydrogenation to methylene, and methylene coupling (C-C bond formation) to ethylene (CH2=CH2). The different surface geometric structures of Î±-Cr2O3(10-12) and (0001) lead to an increase in the activation barrier for methylene surface migration, a critical step in the coupling reaction, of 5.9 kcal/mol over the (0001) surface. For methyl dehydrogenation, differences in the local site pair (cation/anion) geometry and the proximity of surface lattice oxygen to the methyl group do not result in a significant difference in the barrier for dehydrogenation, suggesting that the surface anions play a minor role in the dehydrogenation of methyl on these surfaces. Electronic differences in the Fe3+ (d5) and Cr3+ (d3) cations on structurally-similar Î±-Cr2O3(10-12) and Î±-Fe2O3(10-12) surfaces lead to major differences in reaction selectivity. Î±-Cr2O3(10-12) is nonreducible under the reaction conditions of this study, but Î±-Fe2O3(10-12) is highly reducible due to the difference in the d electron configuration. Hydrocarbons are formed over Î±-Cr2O3(10-12), but nonselective oxidation products (CO2, CO, H2O) are formed over the stoichiometric Î±-Fe2O3(10-12) surface along with surface reduction. Reduction of the Î±-Fe2O3(10-12) leads to a shift in the product selectivity towards formaldehyde (CH2O) and ethylene. For the limited number of systems examined in this study, examples of geometric structure sensitive (methylene coupling) and structure insensitive (methyl dehydrogenation) reactions have been found on Î±-Cr2O3, and electronic effects are observed for the reactions on Î±-Cr2O3(10-12) and Î±-Fe2O3(10-12). For the structure sensitive reaction, the differences in surface geometry impact the reactions kinetics over Cr2O3 but not the types of products formed, while the electronic differences give rise to dramatic changes in the selectivity associated with the very different products formed over Î±-Cr2O3(10-12) and Î±-Fe2O3(10-12).
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