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• W2069972007 abstract "The molecular structures and surface chemistry of mixed metal oxide heteropolyoxo vanadium tungstate (H3+xPW12−xVxO40 with x = 0, 1, 2, and 3) Keggin nanoparticles (NPs), where vanadium is incorporated into the primary Keggin structure, and supported VOx on tungstophosphoric acid (TPA, H3PW12O40), where vanadium is present on the surface of the Keggin unit, were investigated with solid-state magic angle spinning 51V NMR, FT-IR, in situ Raman, in situ UV−vis, CH3OH temperature-programmed surface reaction (TPSR), and steady-state methanol oxidation. The incorporated VOx unit possesses one terminal V═O bond, four bridging V−O−W/V bonds, and one long V−O−P bond in the primary Keggin structure, and the supported VOx unit possesses a similar coordination in the secondary structure under ambient conditions. The specific redox reaction rate for VOx in the Keggin primary structure is comparable to that of bulk V2O5 and the more active supported vanadium oxide catalysts. The specific acidic reaction rate for the WOx in the TPA Keggin, however, is orders of magnitude greater than found for bulk WO3, supported tungsten oxide catalysts, and even the highly acidic WO3−ZrO2 catalyst synthesized by coprecipitation of ammonium metatungstate and ZrO(OH)2. From CH3OH-TPSR and in situ Raman spectroscopy it was found that incorporation of vanadium oxide into the primary Keggin structure is also accompanied by the formation of surface VOx species at secondary sites on the Keggin outer surface. Both CH3OH-TPSR and steady-state methanol oxidation studies demonstrated that the surface VOx species on the Keggin outer surface are significantly less active than the VOx species incorporated into the primary Keggin structure. The presence of the less active surface VOx sites in the Keggins, thus, decreases the specific reaction rates for both methanol oxidation and methanol dehydration. During methanol oxidation/dehydration (O2/CH3OH = 2.17, T = 225 °C), in situ UV−vis diffuse reflectance spectroscopy revealed that vanadium oxide is primarily present as the V+5 cation, which reflects the Mars−van Krevelen redox mechanism and rapid reoxidation by molecular O2. The bulk TPA Keggin structure becomes more disordered and less thermally stable as the vanadium content increases. Although surface polyaromatic carbon forms during methanol oxidation on the Keggin surfaces, its influence on the reaction kinetics seems minimal as the carbon content diminishes as the vanadium oxide content increases and the reaction temperature is raised. No relationships were found between the electronic structure (UV−vis Eg values) and TOFredox and TOFacid (TOF = turnover frequency) kinetics, which reflect the complexity of H3+xPW12−xVxO40 Keggins. The overall catalytic performance of the H3+xPW12−xVxO40 Keggin materials results from a complex interplay among the presence of redox vanadium (as secondary surface VOx species and substituted VOx sites in the primary Keggin NP structure), structural disorder of the Keggin NPs, exposed surface acid and redox sites, and coke deposition. These new insights reveal that the Keggin heteropolyoxometallates are much more complex than originally thought and that care must be taken in using Keggins as model mixed metal oxide NPs in catalytic kinetic and theoretical studies because their surface and bulk structures are dynamic under the reaction conditions." @default.
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• W2069972007 date "2009-10-06" @default.
• W2069972007 modified "2023-09-28" @default.
• W2069972007 title "Surface and Bulk Aspects of Mixed Oxide Catalytic Nanoparticles: Oxidation and Dehydration of CH₃OH by Polyoxometallates" @default.
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• W2069972007 doi "https://doi.org/10.1021/ja904957d" @default.
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