Structure and reactivity of thin-film oxides and metals

The structure and reactivity of thin-film oxides of molybdenum and Co metal supported on oxidized Mo(110) is discussed. Reactions of interest in heterogeneous oxidation catalysis, in particular hydrocarbon oxidation is the focus of the work. A combination of electron energy loss, infrared, and X-ray photoelectron is used to characterize the structures of the oxides and Co films. Oxidation conditions are used to control the nature of the oxygen coordination sites available as well as the thickness and morphology of the oxide. Accordingly, the reactivity of specific types of oxygen coordination sites was investigated. In the case of the Co overlayers, thermal treatment was used as a means of varying the structure and morphology of the metal supported on the oxidized Mo. Oxygen bound to Mo(110) in low-symmetry, high-coordination sites was found to play an important role in the hydrocarbon oxidation process. For example, gaseous methyl radicals selectively add to oxygen in these sites, but not to terminal oxygen. In the microscopic reverse of methyl radical oxidation, vacancies at high-coordination sites are necessary for methanol reaction to methoxy to occur. The site-specific oxidation chemistry is modeled in selected cases using first-principles electronic structure calculations. The reactions of alcohols on various Co thin films were also investigated. The selectivity for alcohol reaction is altered by electronic and structural modification of the film. The reactions of ethanol and methanol were used to illustrate these principles. (C) 1999 Elsevier Science B.V. All rights reserved.