The surface chemistry of hydrocarbon partial oxidation catalysis

The catalytic oxidation of simple alkyl moieties and alcohols on nickel surfaces has been studied by a combination of surface-science and catalytic techniques on increasingly more complex model systems. Initial work was carried out on oxygen-treated nickel single-crystal surfaces under ultrahigh vacuum conditions. The emulation of sub-stoichiometric nickel oxides was corroborated by a number of physical and chemical means, in particular by the use of carefully chosen probe molecules. The mechanism for partial oxidation was then determined. In the case of alkyl groups, an initial facile oxygen insertion was determined to lead to the formation of surface alkoxides, and to a subsequent dehydrogenation of those intermediates at the beta position to yield aldehydes or ketones. Alcohols were found to dehydrogenate by the same β-hydride elimination step, and surface OH groups to enhance that partial oxidation pathway. High selectivity towards dehydrogenation was observed on most nickel-based catalysts, except when alumina was used as a support, where the new acid sites favored dehydration instead. Total oxidation appears to be a secondary reaction on the initial dehydrogenation products, not a parallel pathway.