Characterization of VPO catalysts by X-ray photoelectron spectroscopy

Methods are described for determining the P/V ratio and average vanadium oxidation state in vanadium phosphates using X-ray photoelectron spectroscopy. The determination of P/V ratios are based on a calibration derived from five organometallic complexes containing vanadium and phosphorus. The vanadium oxidation state is shown to be related to the splitting between O(1s) and V(2p(3/2)) transition centroids, V-ox = 13.82-0.68 [O(1s)-V(2p(3/2))]. A simple calculation is presented, which reveals that the layered structure of (VO)(2)P2O7 can lead to an apparent 14% phosphorus enrichment when the surface is assumed to terminate with pyrophosphate groups. Exposure of beta, gamma, and delta VOPO4 phases to increasing concentrations of butane in 10% O-2/N-2 results in transformation of the phases to vanadium pyrophosphate, as evidenced by the shifts in the P(2p) photoelectron transitions. During the transformation, the X-ray photoelectron spectroscopy P/V ratio in beta-VOPO4 increases by ca. 40%, whereas a much smaller increase is observed for the gamma and delta phases. As prepared and under oxidizing conditions, the surface of beta VOPO4 has P/V < 1. (C) 1996 Academic Press, Inc.