Received: August 6, 2007

In Final Form: November 4, 2007

Abstract:

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 WO_x and MoO_x domains supported together on alumina. The efficacy of UV-vis DRS as a tool to identify segregated MoO_x and WO_x 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 atoms/nm² and 0.5 W atoms/nm² show two distinct absorption edges at 3.60 and 4.13 eV, corresponding to spatially and compositionally segregated MoO_x and WO_x domains. At higher surface densities (2-8 total metal atoms/nm²) only one edge is observed, suggesting that MoO_x and WO_x are molecularly mixing and forming a unique metal oxide nanostructure with a band gap different from WO_x or MoO_x single metal oxide catalysts of comparable surface densities. The number of absorption edges and the edge energies obtained for MoO_x/WO_x-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 MoO_x domains and acidic character (diethyl ether formation) of WO_x domains and alumina. Binary MoO_x/WO_x-Al₂O₃ catalysts containing mixed metal atom surface densities of 2-4 Mo atoms/nm² and 2-6 W atoms/nm² show higher acetaldehyde selectivities than MoO_x-Al₂O₃ catalysts of the same Mo-atom surface density despite the poor redox character of WO_x. The presence of WO_x does not affect product selectivity in binary catalysts with MoO_x present in excess of monolayer coverage. Comparison of acetaldehyde selectivities over MoO_x/WO_x-Al₂O₃ to calculated selectivities based on an ideal noninteracting binary oxide catalyst in which the MoO_x and WO_x domains react with ethanol independently suggests a synergistic interaction between MoO_x and WO_x resulting in enhanced acetaldehyde selectivity in MoO_x/WO_x-Al₂O₃ catalysts.

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