The economically important dehydrogenation of ethylbenzene (EB) to styrene (ST) is run at 870 K over potassium promoted iron oxide catalysts in the presence of steam. Here we present the continuation of model catalysis studies using thin epitaxial films (10-20 nm) of magnetite Fe3O4, hematite Fe2O3 and potassium ferrites KFexOy of different composition. They allow the application of surface science methods for pre- and post-reaction surface characterization. The phases are identified from the

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... ED pattern and Auger (AES) spectrum. Catalytic conversions are measured using an in-situ stagnation point micro-flow reactor. The standard feed consists of EB: H2O=1:10 (molar). The partial pressures of reactive gases (p(EB) and p(H2O) are 3.3 and 33 respectively, the rest working pressure of 1 bar is He. The standard total flow rate is 25 ml/min. In addition, O2 can be introduced or H2O can be switched off. The low film thickness is an advantage; the whole bulk material is essentially in thermodynamic equilibrium at reaction conditions. The nature of the films also excludes pore diffusion effects. When starting with Fe2O3 the high initial conversion drops within about 80 min by about an order of magnitude. Simultaneously, the film is reduced to Fe3O4 and covered by coke. Without water in the feed, the deactivation behaviour is similar but substrate reduction proceeds towards metallic iron and coking is heavier. This confirms that water is not involved in the catalytic reaction but it prevents substrate reduction beyond Fe3O4 and help in coke gasification. We conclude further that the final low activity is connected to coke. Addition of O2 prevents both reduction and coking and the high initial conversion is maintained. The necessary amount of O2 corresponds to that necessary to oxidize the produced H2 and to gasify the coking products. Initial deactivation of the promoted catalyst is much slower and occurs mainly by coking which is counteracted by surface K, possibly as K2CO3. K accumulates at the surface but is in equili [...]