Influence of p/v ratio on the phase composition and catalytic activity of vanadium phosphate based catalysts

The influence of the bulk P/V ratio of V-P-O catalysts on their phase composition, surface P/V ratio and catalytic activity for n-butane partial oxidation is reported. Catalysts were prepared by reduction of V2O5 in lactic acid followed by addition of O-H3PO4. Excess lactic acid was evaporated off and the solid precursor calcined in air at 773 K. The bulk P/V ratio was varied in the range 0.94 to 1.07. This range was sufficient to bring about large variations in the nature of the final catalyst obtained. Catalysts with tow P/V ratios formed as βVPO5 with vanadium in the +5 oxidation state. Those with high P/V ratios formed in a phase which we label β in which vanadium was in the +4 oxidation state. Scanning electron microscopy indicated that the catalysts increased in crystallinity as the P/V ratio decreased and EPMA indicated that the P/V ratio was homogeneous throughout each catalyst. Overall conversion of n-butane was at a maximum for P/V close to unity whereas selectivity was highest for the highest P/V ratio studied. The XPS study indicated that the surface P/V ratio did not change in the range of bulk compositions in which selectivity increased from ca. 10 to 50%. Arguments are presented which suggest that it is the solid state properties of these catalysts which largely determine selectivity and that the primary interaction of the hydrocarbon occurs with the aid of a V+5 related surface phase.