Effects of Metal Oxide Domain Size, Dispersion, and Interaction in Mixed WO_x/MoO_x 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.

Selective Autooxidation of Ethanol over Titania‐Supported Molybdenum Oxide Catalysts: Structure and Reactivity

“…This value is lower than that reported for isolated Mo species ( cf. 4.3 eV for Na 2 MoO 4 wherein Mo exists as a monomeric species31), meaning that even the MoO x sites in 1 are not entirely monomeric. The decrease in AEE with increasing surface density indicates a stronger interaction between the MoO x species, and consequently a growth in domain size.…”

Electronic structures and water reactivity of mixed metal sulfide cluster anions

“…1 Molybdenum (Mo) and tungsten (W), both group VI transition metals, show a variety of unique chemical and electronic properties in their oxides. Catalytic and electrochromic properties of mixed Mo-W oxides have been studied 26,27 for potential applications. 28 Since sulfur is in the same group in the periodic table as oxygen, it is of interest to study the geometrical and electronic properties of transition metal sulfides for comparison with previously studied oxide counterparts.…”

Property and Activity of Molybdates Dispersed on Silica Obtained from Various Synthetic Procedures

“…Molybdenum containing catalysts are receiving great interest because of their importance both in environmental catalysis, such as combustion of soot, and in many industrial reactions, such as hydrodesulfuration, dehydrogenation of alkanes, partial oxidation of methanol to formaldehyde, and metathesis of olefins, among others [1][2][3]. The efficiency of these catalysts is strongly related to the amount and, in particular, the dispersion of the Mo-phase [4,5] which in its turn can be very influenced by the preparation method of the sample [6,7]. To improve the activity-selectivity behavior of the Mo-based catalysts in given reactions and the Mo-dispersion, new Mo-precursors and preparation methodologies are expected helping the development of even more performing catalysts.…”