XPS studies of the surface oxidation states on vanadium-phosphorus-oxygen (VPO) equilibrated catalysts Original Research Article

The oxidation states of vanadium on the surface of VPO catalysts which show the characteristic diffraction pattern of vanadyl pyrophosphate have been investigated by X-ray photoelectron spectroscopy (XPS). The (VO)2P2O7 phase is obtained from crystalline precursors which show the X-ray pattern of VOHPO4·1/2H2O. The XPS spectra of 40 solids (precursors, catalysts, and model compounds) hinted the presence of low concentrations of VV centers on the surface of the bulk VIV catalysts. To further investigate this matter, the solids were oxidized or reduced in a pretreatment chamber attached to the spectrometer. These studies confirmed the presence of VV centers on the surface of the catalysts which may be initiated by strong oxygen chemisorption on exposed VIV sites of (VO)2P2O7. It is also shown that the presence of excess phosphorus prevents the oxidation of the surface (and the bulk) to β-VOPO4, a phase which has a deleterious effect on catalyst performance.