Influence of the phosphorus/vanadium ratio on the solid state chemistry and redox properties of vanadium phosphate-based catalysts

The influence of the P V atomic ratio on the redox properties of two series of V-P-O catalysts was investigated. The catalysts were calcined at 773 or 923 K and the P V ratio was varied in the range 0.94 to 1.07. The 4+ oxidation state of vanadium was highly stabilized during calcination at 773 K by a slight excess of phosphorus. This stabilization was less pronounced at the higher calcination temperature. The influence of the P V ratio on the reactivity toward hydrogen of the catalysts calcined at 773 K was difficult to establish because the phase composition of these samples varied over a large range. It was established for the series calcined at 923 K that excess phosphorus, far from activating reduction of vanadium, tended to inhibit this process. Excess phosphorus also inhibited reoxidation of the bulk of these catalysts but tended to have little effect on the reoxidation of near surface layers. The catalytic activity in n-butane partial oxidation to maleic anhydride was compared with the redox behavior and a model for selective oxidation is presented which envisages excess phosphorus as regulating the flow of oxygen anions from the bulk to the surface. The primary interaction is postulated as being between the hydrocarbon and oxygen anions associated with a V⁵⁺ related phase.