In situ spectroscopic investigations of MoOx/Fe2O3 catalysts for the selective oxidation of methanol

Abstract

Multicomponent oxide shell@core catalysts have been prepared, affording overlayers of MoO_x on Fe₂O₃. This design approach allows bulk characterization techniques, such as X-ray Absorption Fine Structure (XAFS), to provide surface sensitive information. Coupling this approach with in situ methodologies provides insights during crucial catalytic processes. Calcination studies were followed by a combination of XAFS and Raman, and demonstrate that amorphous multi-layers of MoO_x are first converted to MoO₃ before formation of Fe₂(MoO₄)₃. However, a single overlayer of O_h Mo units remains at the surface at all times. In situ catalysis studies during formaldehyde production identified that Mo⁶⁺ was present throughout, confirming that gas phase oxygen transfer to molybdenum is rapid under reaction conditions. Reduction studies in the presence of MeOH resulted in the formation of reduced Mo–Mo clusters with a bonding distance of 2.6 Å. It is proposed that the presence of the clusters indicates that the selective conversion of MeOH to formaldehyde requires multiple Mo sites.