Progress of performance optimization for Mn-based SCR catalysts at low temperature

The emission of nitrogen oxides (NOx), the primary air pollutant in China, reached 8.96 million tons in 2022, considerably higher than the emissions of volatile organic compounds, particulate matter, and sulfur dioxide (SO2). NOx emission control is the focus and challenge with respect to air pollution management in China. Selective catalytic reduction (SCR) is widely employed to control the emission of NOx in industrial flue gas because of its high efficiency and stability, and it can be used to realize ultralow NOx emission. A catalyst is a vital factor of the SCR technology. Commercial V2O5/TiO2 catalysts have satisfactory tolerance to poisoning factors such as SO2 and H2O, and the operating temperature is generally in the high-temperature range of 300–420 ℃. Although the catalysts can be more effectively adapted to the medium-temperature range of 200–300 ℃ by increasing the loading amount of V2O5, their low-temperature activity is poor at temperatures less than 200 ℃. The development of efficient and stable catalysts for SCR at low temperatures can prevent the high energy consumption associated with flue gas reheating, resulting in considerable energy saving and carbon reduction benefits. Manganese oxides (MnOx) exhibit remarkable redox properties due to variable chemical states and abundant lattice defects, and they have considerably strong surface acidity, showing satisfactory low-temperature activity in the reaction of catalytic reduction of NOx. However, Mn-based catalysts suffer poor resistance to H2O/SO2, making it difficult to achieve efficient and stable denitrification (i.e., deNOx) over an extended period of time. They have poor N2 selectivity and are prone to catalytic conversion of NOx into the greenhouse gas N2O. Modification and enhancement of Mn-based catalysts have been extensively researched in recent years, which has expedited the pace of their industrial application. This study summarizes the latest research progress on reaction mechanism, elemental doping, and structure design of Mn-based catalysts in the aspects of low-temperature activity, N2 selectivity, and stability. Elemental doping modification is the primary method for optimizing the N2 selectivity and H2O/SO2 tolerance of these catalysts. In terms of comprehensive low-temperature activity, N2 selectivity, and stability, the doping components should have satisfactory oxygen storage–release ability to provide abundant oxygen vacancies and high stability to disperse MnOx and increase the tolerance to H2O and SO2; appropriate structural design can block the poisoning of H2O and SO2; in particular, surface hydrophobic modification can weaken the promotion effect of H2O on poisoning of SO2. In conclusion, this study indicates the ongoing research focuses and difficulties, which can provide references for future research.