Spinel structured CoaMnbOx mixed oxide catalyst for the selective catalytic reduction of NOx with NH3

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A highly efficient catalyst of a CoaMnbOx mixed oxide prepared by the co-precipitation method was developed for the selective catalytic reduction (SCR) of NOx with ammonia. With an increase in the Mn content, the catalytic activity of the CoaMnbOx mixed oxide exhibited a volcano-type tendency, and when Co/Mn molar ratio reached 7:3 (Co7Mn3Ox), the operation temperature for achieving >80% NOx conversion was 170°C (116–285°C window). The formation of spinel structured MnCo2O4 is highly important, and the presence of the spinel structure in Co7Mn3Ox contributes to the increase in active sites and thermal stability and promotes SO2 and/or H2O resistance. In comparison with MnOx or CoOx alone, the CoaMnbOx catalysts possess improved redox properties and more surface acid sites due to synergistic effects between the Co and Mn species. Among the CoaMnbOx catalysts with different Co/Mn molar ratios, a higher NH3 and NO+O2 adsorption ability was found for the Co7Mn3Ox catalyst, originating from its MnCo2O4.5 phase and higher surface area, which leads to the higher activity of the Co7Mn3Ox catalyst. In situ DRIFTs indicated that bridging nitrate and bidentate nitrate are the intermediate species in the NH3-SCR reaction, and the high NO adsorption ability and improved redox properties of the Co7Mn3Ox catalyst are beneficial for the formation of nitrate species on the catalyst surface. Furthermore, NH3 species adsorbed at Lewis acid sites taken part in SCR reaction, while the reactivity of NH3 species adsorbed at Brönsted acid was not definitized.