Vanadyl pyrophosphate catalysts: Surface analysis by XPS and LEIS

Different (VO)(2)P2O7 catalysts have been prepared from the same VO(HPO4). 0.5H(2)O precursor. These catalysts, characterized by XRD, have been compared with respect to their catalytic performances in the n-butane oxidation to maleic anhydride. The surface oxidation state of vanadium has been measured by XPS thanks to the decomposition of the V-2p3/2 peak or the difference of binding energy between the O-1s and V-2p3/2 levels. The relative percentage of V4+ and V5+ depends on the temperature of preparation and on the time of activation under the reaction mixture. The surface (P/V) atomic ratio, in the range 1.5-1.9 as measured by XPS, is not very sensitive to the method of preparation or to thermal treatments. This relatively high (P/V) ratio has been further substantiated by LEIS measurements using He-3(+)/He-4(+) isotopes for quantification. Besides a possible preferential deposit of carbonaceous species on the surface vanadium dimers, it has been proposed that the high (P/V) ratio measured on (VO)(2)P2O7 catalysts could be attributed, at least in parts, to the presence of vanadium vacancies within a few amorphous surface layers. (C) 1998 Elsevier Science B.V. All rights reserved.