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Controlling the Molecular Structure and Reactivity of Supported Metal Oxide Catalytic Active Sites

Edward Lee and Israel E. Wachs. Chemical Engineering, Lehigh University, 7 Asa Drive, Sinclair Laboratory, Rm 117, Bethlehem, PA 18015

The surface molecular structures of dehydrated model supported catalysts (e.g., CrO3/SiO2, MoO3/SiO2, and WO3/SiO2) have been determined, for the first time, by employing in-situ Raman, UV-Vis and O-18 isotopic labeling. Raman spectroscopy confirmed that the supported metal oxide phases were 100% dispersed on the SiO2 support at low surface coverage. These model catalysts were found consist of both monoxo (O=M(-O)4) and dioxo ((O=)2M(-O)2 surface molecular structures. The monoxo and dioxo symmetric M=O vibrations are observed in the 1000-1020 cm-1 and 970-985 cm-1 regions, respectively. The monoxo and dioxo surface MOx species were also confirmed by complementary UV-Vis diffuse reflectance spectroscopy (DRS) because of theie different electronic features. The isotopic O-18 labeling and reduction studies identified the vibrations of the terminal M=O and bridging M-O-Si bonds. The reduction kinetics of the dioxo species was found to be faster than for the monoxo species.

The distribution of monoxo and dioxo surface MOx molecular structures could be controlled by the addition of surface AlOx, TiOx, and ZrOx species. The surface AlOx and TiOx additives were found to possess AlO4/AlO5 and TiO4/TiO5 coordination by solid-state NMR and XANES, respectively. According to literature reports, the Zr cations on SiO2 should possess 5-fold or higher coordination. The surface MOx species were found to preferentially self-assemble on the more reactive surface AlOx, TiOx, and ZrOx sites over the relatively inert SiO2 surface. For the supported MoOx and WOx species, the addition of these surface metal oxide additives was generally found to transform the surface dioxo species to surface monoxo species. The opposite trend was found for upon the addition of the surface metal oxide additives for the supported CrOx species, with dioxo species enhanced over monoxo species. These molecular structural changes were also reflected in the corresponding UV-Vis DRS electronic spectra. For the bilayered catalyst systems, two LMCT (ligand to metal charge transfer) transitions are present for the supported CrOx system, but only a single LMCT transition is present for the supported MoOx and WOx catalyst systems.

The structural modifications with AlOx, TiOx, and ZrOx metal oxide additives lead to significant acid activity enhancements. The acidity of the supported metal oxides was chemically probed with CH3OH-TPSR, where the formation of dimethyl ether (DME-CH3OCH3) from CH3OH dehydration occurs on acidic catalytic surface sites. For exclusively monoxo WOx species, the rate constant for DME, krds, were enhanced by greater than two orders of magnitude over dioxo species. The monoxo/dioxo WOx combination were not found to enhance the acidic activity and may even moderately suppress the acidity. The rate constant for surface redox products, primarily yielding HCHO, was found to be independent of the surface structure. Further investigations of CrOx and MoOx will reveal the structure-reactivity behavior of these active sites.

These fundamental studies have shown for the first time that there are multiple surface M+6Ox species, dioxo and monoxo, present on the SiO2 support and that their relative surface concentrations can be tuned with the addition of secondary metal oxide additives (the bilayered catalysts). The structural studies coupled with catalytic reactivity of these novel model catalysts allows for the establishment of molecular/electronic structure–activity/selectivity relationships.