Structure and reactivity of iron oxide surfaces

Epitaxial films of different iron oxide phases and of potassium iron oxide were grown onto Pt(111) substrates and used for studying structure-reactivi ty correlations. The film morphologies and their atomic surface structures were characterized by scanning tunneling microscopy and low energy electron diffraction including multiple scattering calculations. The adsorption of water, ethylbenzene, and styrene was investigated by temperature programmed desorption and photoelectron spectroscopy. A dissociative chemisorption of water and a molecular chemisorption of ethylbenzene and styrene is observe d on all oxides that expose metal cations in their topmost layers, whereas purely oxygen-terminated FeO(111) monolayer films are chemically inert and only physisorption occurs. Regarding the technical styrene synthesis reacti on, which is performed over iron oxide based catalysts, we find a decreasin g chemisorption strength of the reaction product molecule styrene, if compa red to ethylbenzene, when going from Fe3O4(111) over alpha-Fe2O3(0001) to K Fe O-x(y)(111). Extrapolation of the adsorbate coverages to the technical s tyrene synthesis reaction conditions using the Langmuir isotherm for coadso rption suggests an increasing catalytic activity along the same direction. This result agrees with previous kinetic experiments performed at elevated gas pressures over the model systems studied here and over polycrystalline iron oxide catalyst samples. It indicates that the iron oxide surface chemi stry does not change across the pressure gap and that the model systems sim ulate technical styrene synthesis catalysts in a realistic way.