Synthesis and characterization of high specific surface area vanadium carbide; Application to catalytic oxidation

A method initially developed for the synthesis of high surface area carbides using volatile metallic precursors was successfully adapted to the preparation of high surface area vanadium carbide using a liquid precursor. The method consists of reacting metallic oxide vapours with activated charcoal to form the corresponding carbide which keeps the shape of the high surface area charcoal (shape memory synthesis). In the present extension of the method, the vanadium oxide V₂O₅ is liquid instead of being gas; V₂O₅ melts and spreads over the carbonaceous frame (1150 m² g^-1) as soon as it is heated to its melting point. Due to its good wetting properties, V₂O₅ covers the charcoal surface homogeneously. By further thermal treatment, the oxidic layer is reduced and carburized by the carbon, yielding a vanadium carbide which retains a very high dispersion. The composite product, still containing some carbon in its bulk, covered by carbide, exhibits a surface area greater than 200 m² g^-1. After careful passivation in air the vanadium carbide presents a superficial amorphous layer of 4 to 30 nm. This superficial layer contains oxygen. The pure vanadium carbide is air sensitive at room temperature and slowly reoxidises. This material is then tested as catalyst for the controlled oxidation of butane into maleic anhydride. Under rich mixture butane/oxygen V₈C₇ is stable but not selective yielding large amounts of CO and CO₂; under lean mixture the catalyst is transformed into different vanadium oxycarbides and oxides, one of these oxycarbides being very active and selective when compared to a conventional VPO catalyst.