Effect of doping of TiO2 support with altervalent ions on physicochemical and catalytic properties in oxidative dehydrogenation of propane of vanadia-titania catalysts

Vanadia phase (one monolayer) was deposited on TiO2 anatase doped with Ca2+, Al3+, Fe3+ and W6+ ions and the catalysts thus obtained (VMeTi) were characterized by XPS, work function technique, decomposition of isopropanol (a probe reaction for acido-basic properties) and tested in oxidative dehydrogenation of propane. The doping of the TiO2 Support modifies physicochemical and catalytic properties of the active vanadia phase with respect to the undoped TiO2. The specific activity in the propane oxydehydrogenation decreases in the order: VFeTi > VWTi > VTi > VAlTi > VCaTi (3), whereas the selectivity to propene follows the sequence: VWTi < VTi < VFeTi < VAM < VCaTi (4). No clear correlation has been observed between the activity of the catalysts and the acido-basic properties. On the other hand, the rate of the isopropanol dehydration (the acidity) decreases in the same order as the selectivity increases (4). The increase in the selectivity with the decrease in the acidity is ascribed to the easier desorption of propene from the less acidic surface, preventing the consecutive total combustion of propene. The selectivity to propene can be also correlated with the work function values which decrease in the series: VWTi > VTi > VFeTi > VAM > VCaTi. This implies that the lower is the surface energy barrier for transfer of electrons from the catalyst to the reacting molecules the higher is the selectivity to the partial oxidation product. It is argued that owing to the decrease in this energy barrier the reoxidation step in the catalytic reaction, involving such a transfer: O2+ 4e --> 2O(2-) is fast, thus, preventing the presence of intermediate non-selective electrophilic oxygen species on the surface. (C) 2002 Elsevier Science B.V. All rights reserved.