On the role of monomeric vanadyl species in toluene oxidation over V2O5/TiO2 catalysts:: a kinetic study using the TAP reactor

V2O5/TiO2 catalysts with V2O5 loadings of I wt.% (Eurocat EL10V1) and 8 wt.% (Eurocat EL10V8) as well as the corresponding NH3-washed samples (EL10V1w, EL10V8w) exhibiting exclusively monomeric vanadyl species were used as model catalysts in order to elucidate the role of different vanadia species in toluene oxidation. The catalysts were characterized by elemental analysis and X-ray photoelectron spectroscopy. The catalytic performance in toluene oxidation over EL10V1 and EL10V1w was investigated using a temporal analysis of products (TAP) reactor system under high-vacuum conditions. When oxidizing toluene over the untreated sample EL10V1 the products benzaldehyde (BA), benzoic acid (BAc), maleic acid anhydride (MA), and COx, were detected. BA was observed at low temperatures followed by BAc at temperatures higher than about 500 K, and finally COx above 600 K indicating a consecutive oxidation mechanism. However, only the products BA and COx were observed for the EL10V1w catalyst. Previous DRIFTS and TPR/TPO studies had shown that the monomeric vanadyl species are able to adsorb toluene dissociatively, and that in the absence of gas-phase oxygen the desorption of oxygenated products from these species cannot occur, since the capability of fast oxygen insertion into toluene and desorption of oxygenates is associated with polymeric and crystalline vanadia species. Thus, a concerted mechanism for the selective oxidation of toluene in the presence of gas-phase oxygen over the ammonia-washed catalysts is suggested, i.e., the desorption of BA and the reoxidation of the monomeric vanadyl species are assumed to occur simultaneously. (C) 2004 Elsevier B.V. All rights reserved.