An integrated surface science approach towards metal oxide catalysis

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 alpha-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 pi-electron systems interact with Lewis acidic iron sites exposed on Fe3O4(111) and alpha-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 ethyl group of flat adsorbed ethylbenzene is dehydrogenated at Bronsted 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.