Supported Tantalum Oxide Catalysts: Synthesis, Physical Characterization, and Methanol Oxidation Chemical Probe Reaction

Supported tantalum oxide catalysts on Al2O3, TiO2, ZrO2, and SiO2 supports were prepared by the incipient wetness impregnation method. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and X-ray absorption near edge structure (XANES) under hydrated and dehydrated conditions. The Al2O3-, TiO2-, and ZrO2-supported tantalum oxide catalysts possess similar surface TaOx molecular structures, consisting primarily of polymerized surface TaO5/TaO6 species at high surface coverage. The surface Ta atom density at monolayer coverage are also similar and were found to be 4.5, 6.6, and 6.3 Ta atoms/nm2 on the Al2O3, TiO2, and ZrO2 supports, respectively. The SiO2-supported tantalum oxide catalyst is very different and consists of isolated TaO4 species with a much lower maximum surface Ta density, 0.7 Ta atoms/nm2, due to the lower concentration and reactivity of the silica hydroxyls. The catalytic properties of the surface TaOx species were chemically probed with the methanol oxidation reaction. The Al2O3-, TiO2-, and ZrO2-supported tantalum oxide catalysts, possessing monolayer surface coverage of surface TaOx species, were found to exhibit 100% dimethyl ether originating from surface acidic sites. The TOFacidic values varied by almost 2 orders of magnitude, reflecting the influence of the specific oxide support on the surface TaOx acidic sites through the bridging Ta−O−support bond. In contrast, the surface TaOx species on SiO2 were found to possess redox characteristics rather than acidic characteristics. Thus, the specific reactivity and selectivity of the surface TaOx species strongly depend on the specific oxide support ligand and its effect on the bridging Ta−O−support bond.