Influence of Cobalt On the Textural, Redox and Catalytic Properties of Stoichiometric Vanadium Phosphate

Abstract The influence of coprecipitated cobalt on the catalytic, redox and textural properties of βVPO 5 is reported. Four catalysts were prepared in which the Co/V ratio was varied between 0 and 0.05 and the P/V ratio was held at unity. Cobalt strongly reduced the surface area of βVPO 5 and diminished reactivity toward hydrogen was observed as a monotonous feature as the cobalt content increased. By contrast this material showed complex kinetic behaviour during reoxidation from reduced states. When reoxidation was confined to surface layers a maximum in rate was observed for Co/V ratios of ca. 0.02. Bulk reoxidation was strongly sensitive to small amounts of cobalt but it was independent of cobalt content above Co/V = 0.01. Results of catalytic testing for n-butane partial oxidation to maleic anhydride indicate that conversion was at a maximum for the undoped material but selectivity showed an optimum value for Co/V of ca. 0.02. Above this value a sharp drop in selectivity was observed. A strong increase in areal reoxidisability was observed for Co/V above 0.02. X.P.S. data indicated that some surface segregation of cobalt had occured for Co/V = 0.05. Two domains of influence by cobalt on the catalytic activity of βVPO 5 were identified; one in which it influenced the solid state properties and increased selectivity, the other in which a surface enrichment in cobalt provoked total oxidation of n-butane.