STUDY OF THE NATURE AND STABILITY OF ACTI VE-SITES IN VANADIUM PHOSPHATE CATALYSTS

The nature and stability of the superficial active sites in VPO cataly sts are studied using the XRD, ESR, XPS and IR adsorption modes of ads orbed pyridine. The catalysts are first activated under either pure ni trogen or nitrogen and a butane-air mixture. The sites present at the surface are Lewis (V4+) and Bronsted (P-OH) types. These sites are sta bilized in catalysts prepared in organic media. This stability is due to the large cohesion of VO6 pseudooctaedra in the bi-dimensional fram ework at the surface. However, for catalysts prepared in aqueous media , the Lewis acid sites are less stable and have a tendency to be oxidi zed to V5+ under a flow of butane-air mixture, leading to the formatio n of various VOPO4 phases. Excess phosphorus at the surface is necessa ry to stabilize the pyrophosphate framework so that the appearance of V5+ sites, which are essential to the V4+/V5+ redox mechanism, will be reversible.