Tantalum oxide-based catalysts: Synthesis, molecular structures and catalysis

There are many catalytic applications of tantalum oxide based-materials, such as selective oxidation, hydrocarbon conversion, hydration/dehydration, photochemistry, etc. The fundamental understanding of the relationships between catalysis and the surface molecular structures is limited. This research mainly focused on metal oxide supported tantalum oxide and tantalum oxide supported metal oxide catalysts. The catalysts were characterized by Raman spectroscopy, XPS and/or XANES, and the catalytic properties were chemically probed with the methanol oxidation reaction.; The dehydrated Al₂O₃, TiO₂ and ZrO ₂ supported tantalum oxide catalysts possess similar surface TaOx molecular structures, consisting primarily of polymerized surface TaO₅/TaO ₆ species at high surface coverage, and exhibit 100% acidic properties. The dehydrated SiO₂ supported tantalum oxide catalyst consists of isolated TaO₄ species and possesses redox characteristics. The specific oxide support influences the selectivity and the TOF values.; The molecular structures of the surface species of the tantalum oxide supported metal oxide (V₂O₅, Nb₂O₅, CrO₃, MoO₃, WO₃ and Re₂O ₇) catalysts were different under hydrated and dehydrated conditions. Under hydrated conditions, the surface metal oxide species possesses structures similar to the corresponding metal oxide ionic species present in acidic solutions (ReO₄−, Cr₂O₇ 2−, Mo₈O₂₆4−, HW ₁₂O₄₂11−, V₁₀O₂₈ 6− and Nb₂O₅·nH₂O). Under dehydrated conditions, only one surface ReOx species is present and for CrO₃, MoO₃, WO₃, V₂O ₅ and Nb₂O₅ supported on tantalum oxide, two species may be present: surface isolated and/or polymerized species. The surface vanadium, chromium and rhenium oxide species behave as surface redox sites, while the surface tungsten and niobium oxide species are surface acidic sites. The surface molybdenum oxide species possess both acidic and redox characteristics. No surface basic sites were found on any of the tantalum oxide supported metal oxide catalysts. The relative redox activity and selectivity of the various tantalum oxide supported metal oxides reflect the catalytic properties of the pure metal oxides. Comparison within the Group Five, tantalum oxide most resembles niobium oxide, possessing similar catalytic properties with both supported catalysts or being as a support.