Characterization of Alumina-Supported Single and Binary W and Mo Oxide Catalysts Using Ultraviolet-Visible Diffuse Reflectance Spectroscopy

Ultraviolet-visible (UV-Vis) absorption spectroscopy provides information on the size of nanostructured metal oxide domains on the surface of supported catalysts. For mixed metal oxides, it may additionally allow for detection of spatially isolated domains. The goal of this research was to investigate surface density and level of interaction for single and mixed W and Mo oxide domains. Catalysts with surface oxide concentrations from 0.5-8.0 metal atoms/nm² were prepared on γ?alumina using conventional incipient wetness impregnation. UV-vis diffused reflectance spectra (DRS) were obtained for single metal oxide catalysts, co-impregnated binary catalysts, and physical mixtures. Reflectance measurements converted to pseudo-absorbance were used to calculate the edge energy of the surface domains. Both single metal oxides show decreasing edge energies as surface concentrations increase, approaching bulk oxide edge energies near 8 metal atoms/nm². In physical mixtures of the two catalysts, two distinct absorption edges were observed, corresponding to non-interacting surface domains and indicating the efficacy of UV-vis DRS for detecting spatially segregated surface domains. Co-impregnated binary oxide catalysts show two distinct absorption edges below a surface density of ~4 metal atoms/nm², indicating compositionally isolated domains, but only a single edge above ~4 metal atoms/nm², indicating the two oxide species are locally interacting.