A highly effective catalyst of Sm-Mn mixed oxide for the selective catalytic reduction of NOx with ammonia: Effect of the calcination temperature

• Sm-Mn mixed oxide is reliably a catalyst with higher performance for removing NOx with NH3 compared with MnOx.
• Calcination temperature can affect the structure and physical and chemical properties of Sm-Mn mixed oxide.
• The root of the change in the activity of Sm-Mn mixed oxide is the transformation of MnOx phase.

For low temperature selective catalytic reduction (SCR) of NOx with ammonia, the highly effective Sm-Mn mixed oxide (Sm-Mn-O, the molar ratio Sm/Mn is 0.1) catalysts was developed. The effect of the calcination temperature on the structure, physicochemical and catalytic properties of the Sm-Mn-O catalysts was investigated in detail. The results showed that the Sm-Mn-O catalyst calcined at 450 °C exhibited the highest activity, and after been calcined at >550 °C its catalytic activity markedly decreased. With an increase in the calcination temperature, its surface area firstly increased and then decreased, and the poorly crystallized MnO2 in Sm-Mn-O was transformed to highly crystalized Mn2O3 phase. These changes influenced the adsorption ability of the Sm-Mn-O catalyst for NH3 or NO + O2. After calcination at >550 °C, the contents of Mn4+ and adsorbed oxygen (OS) on the surface of Sm-Mn-O decreased obviously, resulting in the decrease in its catalytic properties for the NO oxidation and transformation of bridged nitrite to active intermediate of bidentate nitrate. Conversely, its catalytic activity for NH3 oxidation was enhanced after being calcined at high temperature. These adverse factors cause the decrease in the SCR activities of Sm-Mn-O calcined at >550 °C.

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