In-situ study of MCM-41-supported iron oxide catalysts by XANES and EXAFS

Our study focuses on the structural evolution of MCM-41-supported iron oxid e under the reducing environment of catalyst pretreatment and ethylbenzene dehydrogenation reaction. Powder X-ray diffraction (XRD) analysis showed th at the iron oxide is well-dispersed on the surface of the support with no d etectable diffraction peaks from iron oxide. X-ray absorption near edge str ucture (XANES) study indicates that iron oxide is being reduced during cata lyst pretreatment under flowing helium from alpha-Fe2O3 at room temperature to Fe3O4 at 425 degrees C. At 500 degrees C, the oxide species is reduced even further. Curve-fitting analysis of the extended X-ray absorption fine structure (EXAFS) radial distribution function (RDF) profile of the catalys t pretreated at 500 degrees C can be done with a basic tetragonal gamma-Fe2 O3 spinel structure. However, the cationic vacancies of the spinel on the o ctahedral position are almost filled with iron cations, indicating that the structure of the iron oxide species is approaching that of a ccp FeO, Stab ilization of the FeO-like structure formed at 500 degrees C is probably thr ough iron oxide-support interactions, especially via condensation of the ox ide terminal hydroxyl groups with the silanols of MCM-41. This distorted fo rm of iron oxide species is metastable and contains labile surface oxide an ions, which are probably responsible for the high initial catalytic activit y during ethylbenzene dehydrogenation reaction at 500 degrees C. In the pre sence of the reactant, however, the iron oxide is further reduced and metal lic iron is formed during the reaction. The formation of metallic iron is p robably through fragmentation of FeO particles, as shown by catalysis and E XAFS results. The reduction process contributes mainly to the deactivation of the catalyst. (C) 2000 Elsevier Science B.V. All rights reserved.