Temperature-programmed reduction and oxidation experiments with V2O5/TiO2 catalysts
Temperature-programmed reduction (TPR) and oxidation (TPO) experiments were performed with V2O5/TiO2 catalysts with vanadia loadings of 1 wt.% (Eurocat EL10V1) and 8 wt.% (Eurocat EL10V8) and with unsupported V2O5. It was possible to correlate the redox properties with the presence of different vanadia species: crystalline and polymeric vanadia species known to be present on EL10V8 were found to be the most difficult to reduce but the easiest to reoxidize. Monomeric vanadyl species known to be present on EL10V1 were the easiest to reduce but the most difficult to reoxidize. In a further set of experiments toluene was used as reducing agent under isothermal conditions at 623 K to probe the catalytic properties of the various vanadia species. In the subsequent TPO experiments the oxygen mass balance including O2, CO, CO2 and H2O was solved thus allowing us to distinguish between the oxidative degradation of adsorbates and the reoxidation of the catalyst. The O2 consumption at lower temperatures was shown to originate from the total oxidation of adsorbates, observed for all three samples, whereas the O2 consumption at higher temperatures, observed only for EL10V8 and V2O5, was caused by the reoxidation of these catalysts. Thus, monomeric vanadyl species were able to adsorb toluene dissociatively, but no oxygenated products were released into the gas phase. In contrast, fast oxygen insertion into toluene and desorption of oxygenates occurred when crystalline and polymeric vanadia species were present.