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Small addition of potassium to V/Ti-oxide catalyst (K:V=0.19), consisting of 3.7 monolayer VOx, increased activity and selectivity in partial oxidation of toluene. In order to elucidate the nature of vanadia species formed on the surface of V/Ti-oxide upon potassium doping, the catalysts were studied by transient kinetics method. The transient product responses during toluene oxidation by the oxygen present in the catalyst were compared for K-doped and non-doped samples. The formation of CO2 decreased and formation of benzaldehyde increased with addition of potassium. This suggests a lower surface concentration of electrophilic oxygen (O-, O2-), which is usually responsible for the deep oxidation, and a higher concentration of nucleophilic oxygen (O2-), responsible for the partial oxidation. The catalysts were characterised by means of HRTEM, FT-Raman spectroscopy and 51V NMR. Potassium addition introduces a disorder in the crystalline structure of bulk V2O5 particles resulting in better spreading of V2O5 over TiO2 surface. The interaction of V2O5 with TiO2 was facilitated upon K-doping, leading to the increased formation of monomeric vanadia species, which are the active sites in toluene partial oxidation to benzaldehyde.
Andreas Züttel, Thi Ha My Pham, Kangning Zhao, Youngdon Ko, Liping Zhong, Manhui Wei
Davide Ferri, Pierdomenico Biasi, Filippo Buttignol