Analysis of silica-supported molybdena by X-ray absorption spectroscopy: theoretical cluster studies

We have performed density-functional theory (DFT) calculations on oxygen core excitations in different molybdena-silica model clusters in order to analyze structural details of the molybdena species. Differently binding oxygen, characteristic for specific molybdena species and coordination of the corresponding molybdenum centers, tetrahedral, pentahedral, and octahedral, can be clearly distinguished in the theoretical spectra. Further, the oxygen of the silica support is found to contribute to the NEXAFS spectrum in an energy range well above that of the molybdena units. The theoretical results can be compared with X-ray absorption fine structure (NEXAFS) measurements near the O K-edge of molybdena model catalysts supported by SBA-15 silica. The experimental NEXAFS spectra exhibit a symmetric double-peak structure in the O 1s to Mo 4d/O 2p excitation range at 529-536 eV. The comparison with the present theoretical data gives clear indications that molybdena species with tetrahedral dioxo MoO4 units, singly or doubly connected with the silica substrate, dominate the experimental spectrum. This is different from results for MoO3 bulk material where octahedral MoO6 units exist and lead to a strongly asymmetric peak structure in the O K-edge NEXAFS spectrum, in agreement with the theoretical result.