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

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In the present study, 76 different metal-oxide-supported-transition-metal catalysts were prepared using 11 different metal oxides (MgO, Al 2 O 3, SiO 2, TiO 2, V 2 O 5, ZrO 2, Nb 2 O 5, MoO 3, Ta 2 O 5, WO 3, and La 2 O 3 ) 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 CH 4 to formaldehyde and methanol. Fe/MoO 3, Fe/V 2 O 5, and particularly Fe/Nb 2 O 5 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 FeNbO 4, FeMoO 4, and FeVO 4 species in Fe/Nb 2 O 5, Fe/MoO 3, and Fe/V 2 O 5, respectively, are responsible for their partial-oxidation activities. In contrast, Fe 2 O 3 species formed in Fe/Al 2 O 3, Fe/SiO 2, Fe/Ta 2 O 5, and Fe/WO 3 were found to be active for complete oxidation to CO 2 than partial oxidation, as were the MgFe 2 O 4, LaFeO 3, and TiFe 2 O 5 species formed in Fe/MgO, Fe/La 2 O 3, and Fe/TiO 2, respectively, and the interstitial solid solution of Fe 3+ in ZrO 2 generated in Fe/ZrO 2 . Furthermore, while the Fe 2 O 3 species in Fe/WO 4 are ineffective for partial oxidation, FeWO 4 prepared by a hydrothermal method exhibits high selectivity for partial oxidation. Additionally, previous studies have shown that CuWO 4 and CuMoO 4 are active for partial CH 4 oxidation. Accordingly, the ABO 4 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 CH 4 oxidation.