Effects of Metal Oxide Domain Size, Dispersion, and Interaction in Mixed WO<i>_x</i>/MoO<i>_x</i> Catalysts Supported on Al₂O₃ for the Partial Oxidation of Ethanol to Acetaldehyde

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/nm 2 and 0.5 W atoms/nm 2 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/nm 2 ) 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 -Al 2 O 3 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 -Al 2 O 3 catalysts containing mixed metal atom surface densities of 2−4 Mo atoms/nm 2 and 2−6 W atoms/nm 2 show higher acetaldehyde selectivities than MoO x -Al 2 O 3 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 -Al 2 O 3 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 -Al 2 O 3 catalysts.