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キーワード:
iron oxides, surface structure, temperature-programmed reaction, model catalysis, pressure gap, ethylbenzene
19: iron oxide model catalysts: adsorption and catalysis
要旨:
The function of a metal oxide catalyst was investigated by an integrated approach, combining a variety of surface science techniques in ultrahigh vacuum with batch reactor conversion measurements at high gas pressures. Epitaxial FeO(111), Fe3O4(111) and a?Fe2O3(0001) films with defined atomic surface structures were used as model catalysts for the dehydrogenation of ethylbenzene to styrene, a practized selective oxidation reaction performed over iron oxide based catalysts in the presence of steam. Ethylbenzene and styrene adsorb onto regular terrace sites with their phenyl rings oriented parallel to the surface, where the p-electron systems interact with Lewis acidic iron sites exposed on Fe3O4(111) and a?Fe2O3(0001). The reactant adsorption energies observed on these films correlate with their catalytic activities at high pressures, which indicates that the surface chemical properties do not change significantly across the pressure gap. Atomic defects were identified as catalytically active sites. Based on the surface spectroscopy results a new mechanism was proposed for the ethylbenzene dehydrogenation, where the upward tilted ethylgroup of flat adsorbed ethylbenzene is dehydrogenated at Brønsted basic oxygen sites located at defects and the coupling of the phenyl ring to Fe3+ terrace sites determines the reactant adsorption-desorption kinetics. The findings are compared to kinetic measurements over polycrystalline catalyst samples, and an extrapolation of the reaction mechanism found on the model systems to technical catalysts operating under real conditions is discussed. The work demonstrates the applicability of the surface science approach also to complex oxide catalysts with implications for real catalysts, provided suitable model systems are available.
