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 an atase, 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 mo nolayers (ML)]. The prepared samples were characterized by X-ray photo electron spectroscopy (XPS), V-51 magic-angle spinning (MAS) NMR and a surface potential (SP) technique and tested as catalysts in the oxida tive 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 bar e 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 conte nt. Monomeric and polymeric VOx species and V2O5 were detected by V-51 NMR on pure and K-doped catalysts. For the K-doped samples the polyme ric species were observed only at high V content and new tetrahedral V Ox species and traces of KVO3 appeared. It has been found that the pre sence of K on the TiO2 surface leads to (a) a decrease in the reducibi lity of the vanadia phase at low V content; (b) a decrease in the surf ace potential (electronic work function); (c) a decrease in acidity an d increase in basicity and (d) a decrease in the total activity for OD H 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 c ontent, higher amounts of vanadium (> 1 ML) are required to obtain the same catalytic performance. Polymeric [VOx] species seem to be more a ctive and selective in the ODH of propane than monomeric species or bu lk V2O5.