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

Supported tantalum oxide catalysts on Al 2 O 3, TiO 2, ZrO 2, and SiO 2 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 Al 2 O 3 -, TiO 2 -, and ZrO 2 -supported tantalum oxide catalysts possess similar surface TaO x molecular structures, consisting primarily of polymerized surface TaO 5 /TaO 6 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/nm 2 on the Al 2 O 3, TiO 2, and ZrO 2 supports, respectively. The SiO 2 -supported tantalum oxide catalyst is very different and consists of isolated TaO 4 species with a much lower maximum surface Ta density, 0.7 Ta atoms/nm 2, due to the lower concentration and reactivity of the silica hydroxyls. The catalytic properties of the surface TaO x species were chemically probed with the methanol oxidation reaction. The Al 2 O 3 -, TiO 2 -, and ZrO 2 -supported tantalum oxide catalysts, possessing monolayer surface coverage of surface TaO x species, were found to exhibit 100% dimethyl ether originating from surface acidic sites. The TOF acidic values varied by almost 2 orders of magnitude, reflecting the influence of the specific oxide support on the surface TaO x acidic sites through the bridging Ta−O−support bond. In contrast, the surface TaO x species on SiO 2 were found to possess redox characteristics rather than acidic characteristics. Thus, the specific reactivity and selectivity of the surface TaO x species strongly depend on the specific oxide support ligand and its effect on the bridging Ta−O−support bond.