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19; iron oxide model catalysts: adsorption and catalysis
Abstract:
Epitaxial films of different iron oxide phases and of potassium iron oxide were grown onto Pt(111)
substrates and used for studying structure-reactivity 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
observed 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 reaction which is performed over iron oxide based catalysts, we find a
decreasing chemisorption strength of the reaction product molecule styrene if compared to ethylbenzene
when going from Fe3O4(111) over a-Fe2O3(0001) to KFexOy(111). Extrapolation of the adsorbate
coverages to the technical styrene synthesis reaction conditions using the Langmuir isotherm for
coadsorption 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
chemistry does not change across the pressure-gap and that the model systems simulate technical
styrene synthesis catalysts in a realistic way
