XPS Studies of de Surface Oxidation States on Vanadium-Phosphorus-Oxygen (VPO) Equilibrated Catalysts

LAURA CORNAGLIA, CARLOS SÀNCHEZ , EDUARDO LOMBARDO

APPLIED CATALYSIS A-GENERAL

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 1995 vol. 127 p. 125 - 138

0926-860X

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)2P207 phase is obtained from crystalline precursors which show the Xray pattern of VOHPO4.1/2H20. 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 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 TM sites of (VO)2P207. It is also shown that the presence of excess phosphorus prevents the oxidation of the surface (and the bulk) to/~-VOPO 4, a phase which has a deleterious effect on catalyst performance.