Abstract
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) 2P 2O 7 phase is obtained from crystalline precursors which show the X-ray pattern of VOHPO 4·1/2H 2O . The XPS spectra of 40 solids (precursors, catalysts, and model compounds) hinted the presence of low concentrations of V V centers on the surface of the bulk V IV 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 V V centers on the surface of the catalysts which may be initiated by strong oxygen chemisorption on exposed V IV sites of (VO) 2P 2O 7. It is also shown that the presence of excess phosphorus prevents the oxidation of the surface (and the bulk) to β-VOPO 4, a phase which has a deleterious effect on catalyst performance.
Published Version
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