Abstract:
The emission of nitrogen oxides (NO
x), 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 (SO
2). NO
x 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 NO
x in industrial flue gas because of its high efficiency and stability, and it can be used to realize ultralow NO
x emission. A catalyst is a vital factor of the SCR technology. Commercial V
2O
5/TiO
2 catalysts have satisfactory tolerance to poisoning factors such as SO
2 and H
2O, 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 V
2O
5, 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 (MnO
x) 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 NO
x. However, Mn-based catalysts suffer poor resistance to H
2O/SO
2, making it difficult to achieve efficient and stable denitrification (i.e., deNO
x) over an extended period of time. They have poor N
2 selectivity and are prone to catalytic conversion of NO
x into the greenhouse gas N
2O. 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 vis-à-vis the aspects of low-temperature activity, N
2 selectivity, and stability. Elemental doping modification is the primary method for optimizing the N
2 selectivity and H
2O/SO
2 tolerance of these catalysts. In terms of comprehensive low-temperature activity, N
2 selectivity, and stability, the doping components should have satisfactory oxygen storage–release ability to provide abundant oxygen vacancies and high stability to disperse MnO
x and increase the tolerance to H
2O and SO
2; appropriate structural design can block the poisoning of H
2O and SO
2; in particular, surface hydrophobic modification can weaken the promotion effect of H
2O on poisoning of SO
2. In conclusion, this study indicates the ongoing research focuses and difficulties, which can provide references for future research.