Structural Analysis of Silica-Supported Molybdena Based on X-ray Spectroscopy: Quantum Theory and Experiment

Abstract

Oxygen core excitations in different molecular molybdena silica models are evaluated using density-functional theory (DFT). These results can be compared with in situ X-ray absorption fine structure (NEXAFS) measurements near the O K-edge of molybdena model catalysts supported on SBA-15 silica, used for exploratory catalytic activity studies. The comparison allows an analysis of structural details of the molybdena species. The silica support is found to contribute to the NEXAFS spectrum in an energy range well above that of the molybdena units, allowing a clear separation between the corresponding contributions. Different types of oxygen species, O(1) in terminal MdO bonds, O(2) in interphase Mo—O—Si bridges and in Mo—O—Mo linkages, as well as O(2) in terminal Mo—O—H groups can be distinguished in the theoretical spectra of the molybdena species with molybdenum in tetrahedral (dioxo species), pentahedral (monooxo species), and octahedral coordination. The experimental NEXAFS spectra exhibit a pronounced double-peak structure in the O 1s to Mo 4dO 2p excitation range of 529 536 eV. Comparison with the present theoretical data gives clear indications that dioxo molybdena species with tetrahedral MoO4 units can explain the experimental spectrum. This does not fully exclude species with other Mo coordination, like pentahedral. However, the latter are believed to exist in the present samples in much smaller amounts. The experimental NEXAFS spectrum for the supported molybdena species differs substantially from that for MoO3 bulk material with octahedral MoO6 units where the observed asymmetric peak structure is also reproduced by the calculations

A study of molybdena catalysts in ethanol oxidation. Part 1. Unsupported and silica‐supported MoO3

“…Thus, both IR and UV-visible spectroscopic data suggest that, besides the formation of external MoO 3 particles, part of molybdenum could be entering the silica matrix as tetrahedrallike molybdates. This was observed previously in molybdena containing silica glasses 38 and is in agreement with early EXAFS and Raman studies, 39 and with more recent X-ray absorption and theoretical studies 40 on similar MoO 3 /SiO 2 catalytic systems.…”

Monitoring Silica Supported Molybdenum Oxide Catalysts at Work: A Raman Spectroscopic Study

“…This value is in good agreement with the dioxo/monooxo ratio of 3.8 obtained for the 0.8 Mo nm −2 catalyst as discussed above. Also, in a recent NEXAFS study on silica‐SBA‐15‐supported molybdenum oxide catalysts, dioxo species were considered the dominant species 21…”

“…14 Dispersed molybdenum oxide surface species are considered the active sites in supported molybdenum oxide catalysts; for illustration various structures proposed in the literature are depicted in Figure 1. Structural differences may arise from different coordination (tetrahedral, square pyramidal, octahedral) and/or nuclearity (monomer, dimer/oligomer) of the molybdenum oxide species 15–21. An important aspect of the discussion on the molybdenum oxide structure in the literature concerns the presence of tetrahedral dioxo (structures a, d) and square pyramidal monooxo species (structures b, e) during the catalytic reaction.…”

Acid sites on silica-supported molybdenum oxides probed by ammonia adsorption: Experiment and theory

“…Density functional theory (DFT) calculations were performed to simulate Lewis and Brønsted acidity of the silica-supported molybdenum oxides by applying cluster models adapted from previous studies [42][43][44][45][46]. The Becke three-parameter hybrid functional with Lee-Yang-Parr correlation functional (B3LYP) together with Ahlrichs' triple-zeta split-valence basis set augmented by polarization functions (def2TZVP or def2TZVPP, respectively) were chosen for the calculations [47,48].…”