The surface dynamics of vanadium oxide-based catalysts: the generation of the active species in V/P/O and V/O-TiO2

Vanadyl pyrophosphate catalysts having different P/V ratio were studied. In-situ surface characterization were carried out by means of Raman Spectroscopy, to reproduce the phase changes in function of the catalyst composition. It was found that surface composition changes as a function of gas-phase composition (N2 or air), steam partial pressure and catalyst P/V ratio [2,3]. For catalysts having a slight excess of P, the surface oxidation of VPP to δ-VOPO4 occurs at lower temperature (380°C). On the contrary, for sample prepared with stoichiometric P/V ratio, αI-VOPO4 forms. The excess of P favours the formation of δ-VOPO4 and improves its stability with the reaction mixture. The catalytic behaviours of these catalysts in n-butane oxidation to maleic anhydride differ greatly at temperatures ranging between 340 and 400°C. Specifically, a catalyst having P/V close to 1.0 is very active but poorly selective to maleic anhydride, whereas a catalyst having a slight excess of P, is moderately active but selective. The two catalysts, instead, gave similar performance at temperature lower than 340°C and higher than 400°C. This means that the formation of αI-VOPO4 is responsible for the lower selectivity to MA, while the excess of P favours the formation of δ-VOPO4 and improves its stability under reaction mixture, ensuring high selectivity to MA.