Abstract
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(2p3/2) transition centroids, Vox=13.82−0.68,[O(1s)−V(2p3/2)]. A simple calculation is presented, which reveals that the layered structure of (VO)2P2O7can lead to an apparent 14% phosphorus enrichment when the surface is assumed to terminate with pyrophosphate groups. Exposure of β, γ, and δ VOPO4phases to increasing concentrations of butane in 10% O2/N2results 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 β-VOPO4increases by ca. 40%, whereas a much smaller increase is observed for the γ and δ phases. As prepared and under oxidizing conditions, the surface of β VOPO4has P/V<1.
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