Effects of metal oxide domain size, dispersion, and interaction in mixed WOx/MoOx catalysts supported on Al2O3 for the partial oxidation of ethanol to acetaldehyde

摘要

UV-visible diffuse reflectance spectroscopy (UV-vis DRS) and the ethanol oxidation probe reaction were used to investigate the structure and function of binary transition metal oxide catalysts containing dispersed WOx and MoOx domains supported together on alumina. The efficacy of UV-vis DRS as a tool to identify segregated MoOx and WOx surface domains along with their growth into larger and interacting binary oxides is demonstrated for the first time. UV-vis absorption edge analysis of physical mixtures of single oxide catalysts indicates that spatially segregated domains of different composition result in multiple edges in the UV-vis absorption spectra. Binary oxide catalysts containing 0.5 Mo atomsm(2) and 0.5 W atomsm(2) show two distinct absorption edges at 3.60 and 4.13 eV, corresponding to spatially and compositionally segregated MoOx and WOx domains. At higher surface, densities (2-8 total metal atomsm(2)) only one edge is observed, suggesting that MoOx and WOx are molecularly mixing and forming a unique metal oxide nanostructure with a band gap different from WOx or MoOx single metal oxide catalysts of comparable surface densities. The number of absorption edges and the edge energies obtained for MoOx/WOx-Al2O3 catalysts are independent of the sequence of metal oxide deposition during catalyst preparation, indicating that the two metal oxides are compositionally well dispersed at all surface densities. Ethanol partial oxidation reactions over single oxide catalysts confirm the primarily redox nature (acetaldehyde formation) of MoOx domains and acidic character (diethyl ether formation) of WO, domains and alumina. Binary MoOx/WOx-Al2O3 catalysts containing mixed metal atom surface densities of 2-4 Mo atomsm(2) and 2-6 W atomsm(2) show higher acetaldehyde selectivities than MoOx-Al2O3 catalysts of the same Mo-atom surface density despite the poor redox character of WOx The presence of WOx does not affect product selectivity in binary catalysts with MoOx present in excess of monolayer coverage. Comparison of acetaldehyde selectivities over MoOx/WOx-Al2O3 to calculated selectivities based on an ideal noninteracting binary oxide catalyst in which the MoOx and WOx domains react with ethanol independently suggests a synergistic interaction between MoOx and WOx resulting in enhanced acetaldehyde selectivity in MoOx/WOx-Al2O3 catalysts.