Effect of potassium addition to the TiO2 support on the structure of V2O5/TiO2 and its catalytic properties in the oxidative dehydrogenation of propane

Vanadium oxide has been deposited by a grafting technique onto TiO2 anatase, both pure and doped with potassium [(1.2 and 2.5) atoms nm–2]. The V content varied between 0.1 and 20 atoms nm–2[0.01–2 V2O5 monolayers (ML)]. The prepared samples were characterized by X-ray photoelectron spectroscopy (XPS), 51V magic-angle spinning (MAS) NMR and a surface potential (SP) technique and tested as catalysts in the oxidative dehydrogenation (ODH) of propane and propan-2-ol decomposition, a probe reaction for acid–base properties. From the XPS and SP data it has been inferred that VOx are located beside the K centres on the bare surface of TiO2 with the lower K content sample, whereas they cover the K-doped fraction of the surface for the sample with higher K content. Monomeric and polymeric VOx species and V2O5 were detected by 51V NMR on pure and K-doped catalysts. For the K-doped samples the polymeric species were observed only at high V content and new tetrahedral VOx species and traces of KVO3 appeared. It has been found that the presence of K on the TiO2 surface leads to (a) a decrease in the reducibility of the vanadia phase at low V content; (b) a decrease in the surface potential (electronic work function); (c) a decrease in acidity and increase in basicity and (d) a decrease in the total activity for ODH of propane. The pattern of the activity and selectivity changes with the total V content depends on the amount of K on the support surface: with K < 1 ML, the maximum activity and selectivity is observed at a vanadium content corresponding to ca. 40% of a V2O5 ML. At higher K content, higher amounts of vanadium (> 1 ML) are required to obtain the same catalytic performance. Polymeric [VOx] species seem to be more active and selective in the ODH of propane than monomeric species or bulk V2O5.