Transient kinetics of toluene interaction with V/Ti-oxides in anaerobic conditions

Toluene interaction with the catalysts consisting of 0.35, 0.62, 0.75 and 3.7 monolayers (ML) of VOx supported on anatase–titania, containing potassium, was studied by transient response techniques at 523–673 K. FT-Raman spectroscopy under dehydrated conditions was used to determine the state of vanadia. K-perturbed (1020 cm-1) and K-doped (990 cm-1) monomeric vanadia species as well as “amorphous” KVO3 (960–940 cm-1) were found at vanadia coverage less than a monolayer. Bulk V2O5 (994 cm-1) was present only in the 3.7 ML V/TiO2 catalyst as a dominant species. Benzaldehyde (BA), total oxidation products and surface carbon-containing species were the main products of the toluene interaction. The proposed reaction network involves five steps and two types of oxygen sites. Both the BA and CO2 formation increased with the concentration of vanadia. The former is determined mainly by nucleophilic-lattice oxygen that is involved in the monolayer vanadia species. The latter as well as the formation of the main part of surface carbon-containing species increased much more steeply being dependent, probably, from electrophilic oxygen abundant in polymerised vanadia species and V2O5. The performed kinetic modelling satisfactorily describes the response curves of BA, CO2 and toluene obtained during the toluene interaction with the pre-oxidised 0.35–0.75 ML V/TiO2 catalysts. The presence of bulk V2O5 in the 3.7 ML V/TiO2 catalyst seems to provide some change in the reaction mechanism demanding a modification of the reaction scheme.