ABO<sub>4</sub> as an Active Catalyst Structure for\nDirect Partial CH<sub>4</sub> Oxidation as Identified through Screening\nof Supported Catalysts

In the present study, 76 different metal-oxide-supported-transition-metal\ncatalysts were prepared using 11 different metal oxides (MgO, Al<sub>2</sub>O<sub>3</sub>, SiO<sub>2</sub>, TiO<sub>2</sub>, V<sub>2</sub>O<sub>5</sub>, ZrO<sub>2</sub>, Nb<sub>2</sub>O<sub>5</sub>, MoO<sub>3</sub>, Ta<sub>2</sub>O<sub>5</sub>, WO<sub>3</sub>, and La<sub>2</sub>O<sub>3</sub>) and seven 3d metals (V, Mn, Fe, Co, Ni, Cu,\nand Zn). The 76 supported catalysts, along with 11 single metal oxides,\nwere screened to identify catalytically active lattice oxygen structures\nfor the partial oxidation of CH<sub>4</sub> to formaldehyde and methanol.\nFe/MoO<sub>3</sub>, Fe/V<sub>2</sub>O<sub>5</sub>, and particularly\nFe/Nb<sub>2</sub>O<sub>5</sub> were found to be highly effective.\nStructural analysis of the active Fe sites in the 11 supported Fe\ncatalysts was performed using high-energy-resolution-fluorescence-detected\nFe K-edge X-ray absorption near-edge structure spectroscopy, revealing\nthat FeNbO<sub>4</sub>, FeMoO<sub>4</sub>, and FeVO<sub>4</sub> species\nin Fe/Nb<sub>2</sub>O<sub>5</sub>, Fe/MoO<sub>3</sub>, and Fe/V<sub>2</sub>O<sub>5</sub>, respectively, are responsible for their partial-oxidation\nactivities. In contrast, Fe<sub>2</sub>O<sub>3</sub> species formed\nin Fe/Al<sub>2</sub>O<sub>3</sub>, Fe/SiO<sub>2</sub>, Fe/Ta<sub>2</sub>O<sub>5</sub>, and Fe/WO<sub>3</sub> were found to be active for\ncomplete oxidation to CO<sub>2</sub> than partial oxidation, as were\nthe MgFe<sub>2</sub>O<sub>4</sub>, LaFeO<sub>3</sub>, and TiFe<sub>2</sub>O<sub>5</sub> species formed in Fe/MgO, Fe/La<sub>2</sub>O<sub>3</sub>, and Fe/TiO<sub>2</sub>, respectively, and the interstitial\nsolid solution of Fe<sup>3+</sup> in ZrO<sub>2</sub> generated in\nFe/ZrO<sub>2</sub>. Furthermore, while the Fe<sub>2</sub>O<sub>3</sub> species in Fe/WO<sub>4</sub> are ineffective for partial oxidation,\nFeWO<sub>4</sub> prepared by a hydrothermal method exhibits high selectivity\nfor partial oxidation. Additionally, previous studies have shown that\nCuWO<sub>4</sub> and CuMoO<sub>4</sub> are active for partial CH<sub>4</sub> oxidation. Accordingly, the ABO<sub>4</sub> structure (where\nA is a 3d metal and B is a group 5 or 6 metal) is indicated as a viable\ndesign basis for the development of catalysts for partial CH<sub>4</sub> oxidation.