Characterization of Mo‐V‐W Mixed Oxide Catalysts by ex situ and in situ X‐Ray Absorption Spectroscopy

Mixed oxides in the system Mo‐V‐W‐O are important catalysts for partial oxidation reactions, e.g . the partial oxidation of acrolein to acrylic acid. Multiphase catalysts of the overall composition Mo 8 V 2 W y O x were prepared by spray drying and crystallization. The charge states of the potential catalytic centers Mo, V and W were investigated by ex situ X‐ray absorption spectroscopy (XAS). In the as prepared samples the cations exist essentially as Mo 6+ , V 5+ and W 6+ . Exposure of the as prepared samples to acrolein results in a reduction of both Mo 6+ and V 5+ whereas W 6+ is inert. Re‐oxidation with oxygen causes a reversible oxidation of Mo and V. The kinetics of the reduction and re‐oxidation processes of the catalyst samples was studied at a temperature of 500 °C by in situ X‐ray absorption spectroscopy at the Mo K ‐edge with carbon monoxide and oxygen as reducing and oxidizing gases. The in situ experiments show that the reduction of Mo 6+ in CO is about one order of magnitude slower than the subsequent re‐oxidation in oxygen.