Structure-reactivity correlations across the pressure-gap studied on epitaxial iron oxide model catalyst films

Epitaxial model catalyst films of different iron oxide phases and of potassium doped iron oxide were grown onto Pt(111) substrates in order to study the styrene synthesis reaction. The adsorption of ethylbenzene and styrene was investigated by thermal desorption spectroscopy (TDS), and catalytic activities were measured at high gas pressures in a newly designed single-crystal flow reactor. A molecular chemisorption of ethylbenzene and styrene is observed on all three oxide films, where the chemisorption strength of the product molecule styrene decreases if compared to the educt molecule ethylbenzene when going from Fe3O4(111) over a-Fe2O3(0001) to KFexOy(111). Based on desorption energies and frequency factors determined from the TDS data a Langmuir extrapolation of the adsorbate coverages to the technical styrene synthesis reaction conditions was performed. It suggests an increasing catalytic activity when going from Fe3O4(111) over a-Fe2O3(0001) to KFexOy(111), because less reactive sites get blocked by the styrene product molecule along this way. This is observed in the high pressure reactivity studies, indicating that the iron oxide surface chemistry does not change significantly across the pressure-gap. We conclude that the product desorption is the rate determining step for this reaction