ABO4 as an Active Catalyst Structure for Direct Partial CH4 Oxidation as Identified through Screening of Supported Catalysts

Abstract

In the present study, 76 different metal-oxide-supported-transition-metal catalysts were prepared using 11 different metal oxides (MgO, Al₂O₃, SiO₂, TiO₂, V₂O₅, ZrO₂, Nb₂O₅, MoO₃, Ta₂O₅, WO₃, and La₂O₃) and seven 3d metals (V, Mn, Fe, Co, Ni, Cu, and Zn). The 76 supported catalysts, along with 11 single metal oxides, were screened to identify catalytically active lattice oxygen structures for the partial oxidation of CH₄ to formaldehyde and methanol. Fe/MoO₃, Fe/V₂O₅, and particularly Fe/Nb₂O₅ were found to be highly effective. Structural analysis of the active Fe sites in the 11 supported Fe catalysts was performed using high-energy-resolution-fluorescence-detected Fe K-edge X-ray absorption near-edge structure spectroscopy, revealing that FeNbO₄, FeMoO₄, and FeVO₄ species in Fe/Nb₂O₅, Fe/MoO₃, and Fe/V₂O₅, respectively, are responsible for their partial-oxidation activities. In contrast, Fe₂O₃ species formed in Fe/Al₂O₃, Fe/SiO₂, Fe/Ta₂O₅, and Fe/WO₃ were found to be active for complete oxidation to CO₂ than partial oxidation, as were the MgFe₂O₄, LaFeO₃, and TiFe₂O₅ species formed in Fe/MgO, Fe/La₂O₃, and Fe/TiO₂, respectively, and the interstitial solid solution of Fe³⁺ in ZrO₂ generated in Fe/ZrO₂. Furthermore, while the Fe₂O₃ species in Fe/WO₄ are ineffective for partial oxidation, FeWO₄ prepared by a hydrothermal method exhibits high selectivity for partial oxidation. Additionally, previous studies have shown that CuWO₄ and CuMoO₄ are active for partial CH₄ oxidation. Accordingly, the ABO₄ structure (where A is a 3d metal and B is a group 5 or 6 metal) is indicated as a viable design basis for the development of catalysts for partial CH₄ oxidation.