Redox property of tungstated-zirconia analyzed by time resolved in situ UV–vis spectroscopy

Kinetic analysis of surface W species on a series of tungstated-zirconia catalysts has been carried out using time resolved in situ UV–vis spectroscopy. The effects of WO3- and Pt-loading on the reduction–oxidation process between surface W6+ and W(6−δ)+ species, which is correlated to the solid acid properties of tungstated-zirconia, are discussed. The rate of reduction of surface W6+ in 0.5% H2/He gradually increased with WO3-loading. The rate of re-oxidation process in pure He increased with WO3-loading and showed the maximum value at the theoretical monolayer coverage at around 25 wt.% of WO3; however, that value was decreased by the further addition of WO3. Based on Raman and UV–vis spectroscopy results, the increase in the redox rates was correlated to the growth of polytungstate domain size, and the lower re-oxidation rate at higher WO3-loading was related to the formation of crystalline WO3. The very small amount of Pt(0.02 wt.%)-loading drastically improved both the reduction and re-oxidation rates, and the activation energy significantly decreased from 43 to 9 kJ mol−1 for the reduction process, and from 62 to 11 kJ mol−1 for the re-oxidation. Further Pt-loading induced a deleterious effect on the rate and degree of reduction.

Kinetic analysis of surface W species on tungstated-zirconia has been carried out using time resolved in situ UV–vis spectroscopy to clarify the reduction–oxidation process of surface W, which is the origin of protonic acid on tungstated-zirconia. The rate of reduction (kR) gradually increased with WO3-loading, while the rate of re-oxidation (kO) showed the maximum value at the theoretical monolayer coverage. The very small amount of Pt(0.02 wt.%)-loading drastically improved both the reduction and re-oxidation rates.Figure optionsDownload as PowerPoint slide