Solid-state kinetics and catalytic behavior of selective oxidation catalysts from time-resolved XAFS investigations

Time-resolved measurements are required to elucidate time-dependencies of the electronic and geometric structure of a catalyst under changing reaction conditions. Monitoring the evolution of the bulk structure of a catalyst under changing conditions reveals the solid-state kinetics of the corresponding reaction. X-ray absorption spectroscopy (XAS) permits to reveal quantitative phase composition and average valence together with the evolution of the local structure. Hence, combining time-resolved XAS with simultaneous catalysis measurements may elucidate correlations between catalytic performance, the catalyst state under reaction conditions, and its solid-state kinetics. Here, results from time-resolved in situ XAS investigations of various molybdenum-based selective oxidation catalysts are compared and discussed. Model systems (i.e. α-MoO₃, hexagonal MoO₃ supported on SBA-15, and H₄PVMo₁₁O₄₀]) suitable to distinguish structural effects and promotion by additional metal centers have been studied under changing reaction conditions. Correlations between reduction and oxidation solid-state kinetics and catalytic performance reveal the dependence of the selectivity of the catalyst on its electronic structure. In particular the re-oxidation kinetics and the average valence under reaction conditions appear to be determined by the defect structure of the underlying catalyst bulk.