## 1 Introduction  
Nitrogen oxides (NO\({}_{\mathrm{x}}\)) are a major contributor to air pollution. It is estimated that about 500 million tons of NO\({}_{\mathrm{x}}\) are discharged into the atmosphere every year due to the natural nitrogen cycle. Of this, emissions from industrial and mobile sources linked to human activities make up roughly 10% of the total natural emissions [1, 2]. Despite being only a tenth of the natural emissions, these human-generated NO\({}_{\mathrm{x}}\) emissions pose significant environmental risks, such as acid rain and photochemical smog, especially in urban and industrial regions due to their high concentration [3]. Therefore, controlling the emission and treatment of nitrogen oxides is urgent [4]. Selective catalytic reduction (SCR) has become the most widely used method for flue gas denitrification due to its simple process, low cost, and high efficiency in removing nitrogen oxides [5, 6]. The catalyst is the most essential part of this system, with its activity, selectivity, and resistance to poisoning directly affecting the denitrification efficiency [7]. Currently, vanadium-titanium catalysts account for the largest share of the denitration catalyst market. The optimal operating temperature range for this type of catalyst is between 300 and 400 \({}^{\circ}\)C [8, 9, 10]. However, to minimize the energy consumption from the secondary heating of flue gas, the post-denitrification method is mainly used, which results in the vanadium-titanium catalyst's efficiency being inadequate at lower temperatures for practical applications. At the same time, the use of highly toxic vanadium pentoxide as an active component in vanadium-titanium catalysts leads to high costs associated with recyclingwaste catalysts [11]. Therefore, there has been increasing research interest in developing non-toxic, vanadium-free SCR catalysts suitable for low-temperature applications. In recent years, low-temperature SCR denitration catalysts that utilize Mn oxide (MnO\({}_{\text{x}}\)) as the main active component or as a promoter have emerged as a focal point in the research of low-temperature catalysts [11; 12; 13]. The Mn-based catalyst exhibits a superior redox performance because the Mn element has multiple variable valence states. During the SCR reaction, enhanced redox performance facilitates the formation of intermediate NH\({}_{2}\) and NO\({}_{2}\) substances, thereby promoting the denitrification process [14]. Additionally, Mn-based catalysts possess strong surface acidity, which aids in the adsorption and conversion of NH\({}_{3}\). These catalysts also demonstrate higher N\({}_{2}\) selectivity in lower temperature ranges compared to other metal oxide catalysts, making the SCR reaction more complete and minimizing the presence of intermediate products [15; 16]. These attributes contribute to the Mn-based catalyst's exceptional denitrification efficiency in low-temperature regions. Furthermore, modification or doping with other metal oxides significantly enhances the low-temperature H\({}_{2}\)O/SO\({}_{2}\) resistance of the optimized Mn-based composite catalyst, which can either inhibit sulfate formation during the SCR reaction in the presence of SO\({}_{2}\) or promote sulfate decomposition at low temperatures [17; 18]. This makes Mn-based catalysts suitable for processing flue gas with more complex components in practical applications. Moreover, Mn-based catalysts are more cost-effective and readily available compared to other low-temperature precious metal catalysts, thereby reducing the financial burden on enterprises during large-scale applications. Consequently, Mn-based multi-metal oxide catalysts are the most effective and stable option for low-temperature NH\({}_{3}\)-SCR of nitrogen oxides (NH\({}_{3}\)-SCR of NOx) [19; 20]. To enhance the low-temperature NH\({}_{3}\)-SCR efficiency of NOx, researchers have implemented various modifications to Mn-based catalysts.