A Review of Mn-Based Catalysts for Abating NOx and CO in Low-Temperature Flue Gas: Performance and Mechanisms

Mn-based catalysts have attracted significant attention in the field of catalytic research, particularly in NOx catalytic reductions and CO catalytic oxidation, owing to their good catalytic activity at low temperatures. In this review, we summarize the recent progress of Mn-based catalysts for the removal of NOx and CO. The effects of crystallinity, valence states, morphology, and active component dispersion on the catalytic performance of Mn-based catalysts are thoroughly reviewed. This review delves into the reaction mechanisms of Mn-based catalysts for NOx reduction, CO oxidation, and the simultaneous removal of NOx and CO. Finally, according to the catalytic performance of Mn-based catalysts and the challenges faced, a possible perspective and direction for Mn-based catalysts for abating NOx and CO is proposed. And we expect that this review can serve as a reference for the catalytic treatment of NOx and CO in future studies and applications.


Introduction
As the primary pollutants emitted from sintering, the coking and cement industries, etc., nitrogen oxides (NO x ) and carbon monoxide (CO) are harmful to both the environment and human health, often leading to acid rain, photochemical smog, ozone depletion, global warming, and so on [1,2].Consequently, many scholars have focused their efforts on exploring effective strategies for removing these pollutants [3][4][5][6].Among the available technologies, selective catalytic reduction (SCR) using NH 3 as the reducing agent was identified as an effective and reliable method for mitigating NO x emissions [7].Additionally, CO catalytic oxidation is also considered as a pivotal method for reducing hazardous exhaust emissions [8].However, the lack of good-performance catalysts restricts the extensive utilizations of NH 3 -SCR and CO catalytic oxidation technologies.
For the NH 3 -SCR reaction, V 2 O 5 -WO 3 (MoO 3 )/TiO 2 catalysts have been commercially employed in power plants due to their excellent de-NO x performance within the temperature range of 300-400 • C. Nevertheless, the narrow working temperature window and poor SCR activity at lower temperatures have constrained the application of this method in low-temperature flue gas denitrification.Additionally, Cu-CHA catalysts have been commercially used in vehicles due to their superior SCR activity and hydrothermal stability [9].Nevertheless, Cu-CHA catalysts are sensitive to sulfur oxides, resulting in a significant decrease in SCR activity at lower temperatures [10,11].As for CO catalytic oxidation, noble metal catalysts, including Au [12], Pt [13,14], Pd [15], etc., have been extensively employed to remove CO.Noble metal catalysts display a higher intrinsic activity for the conversion of CO and demonstrate good resistance to sulfur poisoning at lower temperatures [16].However, noble metal catalysts also present some disadvantages.For instance, they are chemically sensitive and might degrade rapidly in the presence of impurities [17].Furthermore, they also show poor catalytic activity at lower temperatures [18].Comparing transition metal and noble metal catalysts, it has been found that they could control CO almost equally, provided that the transition metal catalysts are used in larger volumes [19].Therefore, transition metal catalysts have been considered as a promising alternative [20,21].Among the transition metal oxide catalysts, Mn-based catalysts have attracted much attention due to their good low-temperature redox property [22].However, Mn-based catalysts are easily poisoned by SO 2 and H 2 O at lower temperatures and suffer from a low N 2 selectivity at higher temperatures in the NH 3 -SCR reaction, as well as a narrow operating window [23].Therefore, great efforts have been made to improve the catalytic performance and widen the temperature window [24,25].Although some review articles have discussed the NH 3 -SCR reaction over Mn-based catalysts, a systematic summary of Mn-based catalysts for NO x catalytic reductions and CO catalytic oxidation is still missing.Hence, this review aimed to present a comprehensive review for the progress of Mn-based catalysts for both NH 3 -SCR reactions and CO catalytic oxidation processes.In this review, the catalytic performance, reaction mechanisms, and interaction effects of the catalytic process for NO x and CO are summarized.Furthermore, the crucial factors which influence the catalytic activity of Mn-based catalysts for NO reduction and CO oxidation are systematically identified.The structure diagram of this review is elucidated in Figure 1.
Molecules 2023, 28, x FOR PEER REVIEW 2 of 22 they are chemically sensitive and might degrade rapidly in the presence of impurities [17].Furthermore, they also show poor catalytic activity at lower temperatures [18].Comparing transition metal and noble metal catalysts, it has been found that they could control CO almost equally, provided that the transition metal catalysts are used in larger volumes [19].Therefore, transition metal catalysts have been considered as a promising alternative [20,21].Among the transition metal oxide catalysts, Mn-based catalysts have attracted much attention due to their good low-temperature redox property [22].However, Mnbased catalysts are easily poisoned by SO2 and H2O at lower temperatures and suffer from a low N2 selectivity at higher temperatures in the NH3-SCR reaction, as well as a narrow operating window [23].Therefore, great efforts have been made to improve the catalytic performance and widen the temperature window [24,25].Although some review articles have discussed the NH3-SCR reaction over Mn-based catalysts, a systematic summary of Mn-based catalysts for NOx catalytic reductions and CO catalytic oxidation is still missing.
Hence, this review aimed to present a comprehensive review for the progress of Mn-based catalysts for both NH3-SCR reactions and CO catalytic oxidation processes.In this review, the catalytic performance, reaction mechanisms, and interaction effects of the catalytic process for NOx and CO are summarized.Furthermore, the crucial factors which influence the catalytic activity of Mn-based catalysts for NO reduction and CO oxidation are systematically identified.The structure diagram of this review is elucidated in Figure 1.

NOx Removal
As low-temperature SCR catalysts, manganese oxide catalysts have exhibited great potential for NOx reduction due to their diverse crystal structures and diverse metallization valences.However, pure MnOx catalysts have the disadvantages of poor anti-SO2 and H2O catalytic activities and an inferior N2 selectivity, which restrict their further applications.To address these shortcomings, some researchers have made efforts to enhance the SCR catalytic performance by forming other mixed oxides, such as transition or rare earth metal oxides, regulating the morphology and structure, and introducing suitable supports for the catalysts [26,27].

NO x Removal
As low-temperature SCR catalysts, manganese oxide catalysts have exhibited great potential for NO x reduction due to their diverse crystal structures and diverse metallization valences.However, pure MnO x catalysts have the disadvantages of poor anti-SO 2 and H 2 O catalytic activities and an inferior N 2 selectivity, which restrict their further applications.To address these shortcomings, some researchers have made efforts to enhance the SCR catalytic performance by forming other mixed oxides, such as transition or rare earth metal oxides, regulating the morphology and structure, and introducing suitable supports for the catalysts [26,27].
2.1.1.Single Mn Oxide Catalysts for NOx Removal (1) Effect of crystal phase and morphology The SCR catalytic performance of manganese oxide catalysts is influenced by key factors, including the oxidation state, crystallinity, specific surface area, and m ogy [28][29][30][31].And these factors exert crucial influences on the catalysts' catalytic mance to varying degrees.Manganese oxides are composed of octahedral [MnO which could form diverse arrays of tunnels and layered structures through the sh corners or edges [32,33].These crystalline MnO2 materials (α-, β-, δ-, γ-, λ-, and ε can be categorized into three primary groups based on their structures: 1D, 2D, mesh structures, respectively.The 1D tunnel structures include α-, β-, and γ-Mn of which feature different tunnel arrangements: 1D (1 × 1) (2 × 2), (1 × 1), and (1 × tunnel, respectively.ε-MnO2, which is similar to γ-MnO2, exhibits a tunnel structu highly disordered manganese lattice points and an irregular tunnel shape.δ-MnO its 2D-layered structures formed by the shared side of the [MnO6] octahedron.other hand, λ-MnO2 shows a typical spinel structure, characterized by a 3D (1 × 1 arrangement [34,35].Figure 2 illustrates the schematics of different MnO2 cryst tures, suggesting their unique arrangements and configurations [36].It has been reported that crystal structures could significantly influence the N version efficiency of MnO2 catalysts [37].Dai et al. [38] suggested that α-MnO2 c display a higher de-NOx activity compared to δ-MnO2 catalysts, primarily due being assigned a higher abundance of surface chemisorbed oxygen species.Yang e indicated that the NH3 species adsorbed at Lewis sites on β-MnO2 catalysts exhibi reactivity with O2, resulting in less N2O formation and a lower NO conversion.catalytic performance of γ-, α-, and δ-MnO2 catalysts were also investigated (Fi Furthermore, a range of MnO2 catalysts with various morphologies, including M norods, nanospheres, nanowire, nanotubes, nanoflower, mesoporous MnO2 nan and 3D mesoporous MnO2, were synthesized for removing NOx [40][41][42][43].It has been reported that crystal structures could significantly influence the NO x conversion efficiency of MnO 2 catalysts [37].Dai et al. [38] suggested that α-MnO 2 catalysts display a higher de-NO x activity compared to δ-MnO 2 catalysts, primarily due to them being assigned a higher abundance of surface chemisorbed oxygen species.Yang et al. [39] indicated that the NH 3 species adsorbed at Lewis sites on β-MnO 2 catalysts exhibited poor reactivity with O 2 , resulting in less N 2 O formation and a lower NO conversion.And the catalytic performance of γ-, α-, and δ-MnO 2 catalysts were also investigated (Figure 3).Furthermore, a range of MnO 2 catalysts with various morphologies, including MnO 2 nanorods, nanospheres, nanowire, nanotubes, nanoflower, mesoporous MnO 2 nanosheets, and 3D mesoporous MnO 2 , were synthesized for removing NO x [40][41][42][43].Li et al. [44] synthesized MnO 2 nanomaterials with various morphologies via the hydrothermal method, as illustrated in Figure 4.The results indicated that the NO conversion efficiency decreased in the following order: nanospheres > nanosheets > nanorods for MnO 2 , and the removal efficiency of NO for the MnO 2 nanosphere catalyst could reach nearly 100% over the temperature range of 200 to 350 • C. Gao et al. [45] successfully synthesized mesoporous MnO 2 catalysts by employing KIT-6, SBA-15, and MCM-41 mesoporous silica as the templates.Among them, the mesoporous MnO 2 -KIT-6 catalyst, characterized by 3D cubic channels, exhibited the highest NH 3 -SCR activity and the widest temperature window.
Molecules 2023, 28, x FOR PEER REVIEW 4 of 22 as illustrated in Figure 4.The results indicated that the NO conversion efficiency decreased in the following order: nanospheres > nanosheets > nanorods for MnO2, and the removal efficiency of NO for the MnO2 nanosphere catalyst could reach nearly 100% over the temperature range of 200 to 350 °C.Gao et al. [45] successfully synthesized mesoporous MnO2 catalysts by employing KIT-6, SBA-15, and MCM-41 mesoporous silica as the templates.Among them, the mesoporous MnO2-KIT-6 catalyst, characterized by 3D cubic channels, exhibited the highest NH3-SCR activity and the widest temperature window.(2) Effect of valence state Molecules 2023, 28, x FOR PEER REVIEW 4 of 22 as illustrated in Figure 4.The results indicated that the NO conversion efficiency decreased in the following order: nanospheres > nanosheets > nanorods for MnO2, and the removal efficiency of NO for the MnO2 nanosphere catalyst could reach nearly 100% over the temperature range of 200 to 350 °C.Gao et al. [45] successfully synthesized mesoporous MnO2 catalysts by employing KIT-6, SBA-15, and MCM-41 mesoporous silica as the templates.Among them, the mesoporous MnO2-KIT-6 catalyst, characterized by 3D cubic channels, exhibited the highest NH3-SCR activity and the widest temperature window.(2) Effect of valence state (2) Effect of valence state Each valence state of manganese is associated with a stable oxide, including MnO, Mn 2 O 3 , Mn 5 O 8 , Mn 4 O 3 , and MnO 2 .The good de-NO x activity of MnO x catalysts at lower temperatures could be attributed to the presence of the multivalent oxidation state of manganese and high mobility of lattice oxygen [46][47][48].Additionally, Yang et al. [48] conducted a study on the pathways of N 2 O formation during an NH 3 -SCR reaction over different valence states of manganese oxide catalysts, as depicted in Figure 5.It could be observed that the MnO 2 catalysts yielded more N 2 O at lower temperatures.In situ DRIFTS results indicated that the most of the N 2 O was generated from the SCR reaction on the MnO 2 catalysts at lower temperatures, and the N 2 O amounts produced via the NH 3 oxidation of the catalyst increased with the rising temperature.Wan et al. [49] emphasized the significance of Mn 4+ species as the primary active species in the NH 3 -SCR reaction.These findings were also confirmed by the studies of Zhang et al. [50] and Pappas et al. [51].
Molecules 2023, 28, x FOR PEER REVIEW 5 of 22 Each valence state of manganese is associated with a stable oxide, including MnO, Mn2O3, Mn5O8, Mn4O3, and MnO2.The good de-NOx activity of MnOx catalysts at lower temperatures could be attributed to the presence of the multivalent oxidation state of manganese and high mobility of lattice oxygen [46][47][48].Additionally, Yang et al. [48] conducted a study on the pathways of N2O formation during an NH3-SCR reaction over different valence states of manganese oxide catalysts, as depicted in Figure 5.It could be observed that the MnO2 catalysts yielded more N2O at lower temperatures.In situ DRIFTS results indicated that the most of the N2O was generated from the SCR reaction on the MnO2 catalysts at lower temperatures, and the N2O amounts produced via the NH3 oxidation of the catalyst increased with the rising temperature.Wan et al. [49] emphasized the significance of Mn 4+ species as the primary active species in the NH3-SCR reaction.These findings were also confirmed by the studies of Zhang et al. [50] and Pappas et al. [51].

Composite MnOx Catalysts for NOx Removal
As for the stringent emission standards for NOx, relying solely on a single MnOx catalyst in the NH3-SCR system is not sufficient to meet the discharge requirements.Mixing or doping MnOx with other metal oxides can enhance the catalytic activity of MnOx catalysts, owing to the synergistic effects achieved by combining Mn and other metal elements.Transition metal oxides, such as Fe2O3, CeO2, Co3O4, CuO, etc., have been widely employed for the modification of MnOx catalysts [52][53][54].The incorporation of other metal oxides usually generates abundant active sites, rich oxygen vacancies, and an excellent redox capacity, all of which contribute to the enhancement of the catalytic performance [55][56][57].For instance, Shi et al. [58] synthesized Mn-based bimetallic transition oxide catalysts and investigated the impact of various transition metals on the catalytic activity of pure MnOx catalysts.As illustrated in Figure 6, the Co-MnOx catalyst achieved about a 95% NO conversion efficiency at 100 °C.This superior catalytic activity of the Co-MnOx catalyst was likely attributed to the unique manganese-rich surface activity.Kang et al. [59] synthesized a Ni-doped MnOx catalyst using the solvent-free doping method and

Composite MnO x Catalysts for NO x Removal
As for the stringent emission standards for NO x , relying solely on a single MnO x catalyst in the NH 3 -SCR system is not sufficient to meet the discharge requirements.Mixing or doping MnO x with other metal oxides can enhance the catalytic activity of MnO x catalysts, owing to the synergistic effects achieved by combining Mn and other metal elements.Transition metal oxides, such as Fe 2 O 3 , CeO 2 , Co 3 O 4 , CuO, etc., have been widely employed for the modification of MnO x catalysts [52][53][54].The incorporation of other metal oxides usually generates abundant active sites, rich oxygen vacancies, and an excellent redox capacity, all of which contribute to the enhancement of the catalytic performance [55][56][57].For instance, Shi et al. [58] synthesized Mn-based bimetallic transition oxide catalysts and investigated the impact of various transition metals on the catalytic activity of pure MnO x catalysts.As illustrated in Figure 6, the Co-MnO x catalyst achieved about a 95% NO conversion efficiency at 100 • C.This superior catalytic activity of the Co-MnO x catalyst was likely attributed to the unique manganese-rich surface activity.Kang et al. [59] synthesized a Ni-doped MnO x catalyst using the solvent-free doping method and evaluated its SCR performance.The result suggested that Ni doping could enhance the medium-temperature activity, obtaining a remarkable 100% NO conversion rate at 100-200 • C. Jiang et al. [60] suggested that the incorporation of Zr could improve the catalytic activity and SO 2 tolerance of Mn-based catalysts.The characterization results revealed that the introduction of Zr strengthened the interaction between Zr and the active sites, resulting in the amorphous structure of the catalysts.Moreover, in situ DRIFTS studies displayed that the addition of Zr promoted the L-H reaction pathways at lower temperatures.Long et al. [61] indicated that the co-doping of Nb and Fe optimized the low-temperature SCR activity and N 2 selectivity of MnCeO x catalysts, as depicted in Figure 7.
Molecules 2023, 28, x FOR PEER REVIEW 6 of 22 evaluated its SCR performance.The result suggested that Ni doping could enhance the medium-temperature activity, obtaining a remarkable 100% NO conversion rate at 100-200 °C.Jiang et al. [60] suggested that the incorporation of Zr could improve the catalytic activity and SO2 tolerance of Mn-based catalysts.The characterization results revealed that the introduction of Zr strengthened the interaction between Zr and the active sites, resulting in the amorphous structure of the catalysts.Moreover, in situ DRIFTS studies displayed that the addition of Zr promoted the L-H reaction pathways at lower temperatures.
Long et al. [61] indicated that the co-doping of Nb and Fe optimized the low-temperature SCR activity and N2 selectivity of MnCeOx catalysts, as depicted in Figure 7.   Molecules 2023, 28, x FOR PEER REVIEW 6 of 22 evaluated its SCR performance.The result suggested that Ni doping could enhance the medium-temperature activity, obtaining a remarkable 100% NO conversion rate at 100-200 °C.Jiang et al. [60] suggested that the incorporation of Zr could improve the catalytic activity and SO2 tolerance of Mn-based catalysts.The characterization results revealed that the introduction of Zr strengthened the interaction between Zr and the active sites, resulting in the amorphous structure of the catalysts.Moreover, in situ DRIFTS studies displayed that the addition of Zr promoted the L-H reaction pathways at lower temperatures.
Long et al. [61] indicated that the co-doping of Nb and Fe optimized the low-temperature SCR activity and N2 selectivity of MnCeOx catalysts, as depicted in Figure 7.

Supported MnO x Catalysts for NO x Removal
The choice of suitable supports has a profound impact on shaping the crystalline and catalytic performance of NH 3 -SCR catalysts.An ideal support not only provides a large specific surface area for the efficient dispersion of the active components, but also creates a favorable environment for catalytic reactions to occur.To date, extensive efforts have been dedicated to exploring various support materials, including TiO 2 [62,63], Al 2 O 3 [64][65][66], CeO 2 [67][68][69], SiO 2 [70], ZrO 2 [71,72], and active carbon (AC) [21,73], as potential supports for the immobilization of MnO x catalysts.
(1) TiO 2 as support TiO 2 as a support has demonstrated an excellent resistance to SO 2 [74].Moreover, it can interact with MnO x catalysts to enhance the dispersion of Mn species [75,76].Smirniotis et al. [77] investigated the impact of different TiO 2 phases on the SCR catalytic activity of MnO 2 /TiO 2 catalysts.The result indicated that MnO x /TiO 2 (hombikat, anatase) displayed the highest SCR activity, attributed to the larger specific surface area and abundant acid sites of TiO 2 (hombikat, anatase).Li et al. [74] prepared Mn-Ce/TiO 2 -NS and Mn-Ce/TiO 2 -NP catalysts, utilizing anatase TiO 2 with exposed {001} crystal faces (TiO 2 -Ns) and anatase TiO 2 with exposed {101} crystal faces (TiO 2 -NP) as supports, respectively.The Mn-Ce/TiO 2 -NS catalyst exhibited higher SCR activity than that of the Mn-Ce/TiO 2 -NP catalyst, even in the presence of SO 2 , as illustrated in Figure 8.This was due to the anatase TiO 2 {001} facets potentially preferentially reacting with SO 2 , thus avoiding the inactivation of the active sites.The choice of suitable supports has a profound impact on shaping the crystalline and catalytic performance of NH3-SCR catalysts.An ideal support not only provides a large specific surface area for the efficient dispersion of the active components, but also creates a favorable environment for catalytic reactions to occur.To date, extensive efforts have been dedicated to exploring various support materials, including TiO2 [62,63], Al2O3 [64][65][66], CeO2 [67][68][69], SiO2 [70], ZrO2 [71,72], and active carbon (AC) [21,73], as potential supports for the immobilization of MnOx catalysts.
(1) TiO2 as support TiO2 as a support has demonstrated an excellent resistance to SO2 [74].Moreover, it can interact with MnOx catalysts to enhance the dispersion of Mn species [75,76].Smirniotis et al. [77] investigated the impact of different TiO2 phases on the SCR catalytic activity of MnO2/TiO2 catalysts.The result indicated that MnOx/TiO2 (hombikat, anatase) displayed the highest SCR activity, attributed to the larger specific surface area and abundant acid sites of TiO2 (hombikat, anatase).Li et al. [74] prepared Mn-Ce/TiO2-NS and Mn-Ce/TiO2-NP catalysts, utilizing anatase TiO2 with exposed {001} crystal faces (TiO2-Ns) and anatase TiO2 with exposed {101} crystal faces (TiO2-NP) as supports, respectively.The Mn-Ce/TiO2-NS catalyst exhibited higher SCR activity than that of the Mn-Ce/TiO2-NP catalyst, even in the presence of SO2, as illustrated in Figure 8.This was due to the anatase TiO2 {001} facets potentially preferentially reacting with SO2, thus avoiding the inactivation of the active sites.(2) Al2O3 as support A large surface area, abundant acid sites, and a superior mechanical property makes Al2O3 an outstanding support for SCR catalysts.Yao et al. [78] synthesized MnOx catalysts with different supports, and the influence of these supports on the physicochemical properties and denitration performance of the catalysts was evaluated.The results indicated that the MnOx/γ-Al2O3 catalyst exhibited a strong NOx adsorption capacity and had abundant Mn 4+ species, resulting in a higher SCR activity in the entire reaction temperature (Figure 9).Furthermore, comparative studies on Mn-Ce oxides supported on TiO2 and Al2O3 for NH3-SCR at low temperatures were conducted by Jin et al. [79].The results demonstrated that the Mn-Ce/Al2O3 catalyst showed a relatively higher SCR activity than the Mn-Ce/TiO2 catalyst at the temperature range of 80-150 °C, primarily due to the Mn-Ce/Al2O3 catalyst having more acid sites.Li et al. [80] synthesized supported catalysts of FeOx and MnOx that were co-supported on aluminum-modified CeO2 for a low-temperature NH3-SCR reduction of NOx.It was observed that the Fe-Mn/Ce1Al2 catalyst achieved over a 90% NO conversion at 75-250 °C and displayed superior SO2 resistance compared to the Fe-Mn/CeO2 catalyst.The improved catalytic performance could be ascribed to the larger surface area, and the enhanced reducibility was due to the introduction of Al2O3.(2) Al 2 O 3 as support A large surface area, abundant acid sites, and a superior mechanical property makes Al 2 O 3 an outstanding support for SCR catalysts.Yao et al. [78] synthesized MnO x catalysts with different supports, and the influence of these supports on the physicochemical properties and denitration performance of the catalysts was evaluated.The results indicated that the MnO x /γ-Al 2 O 3 catalyst exhibited a strong NO x adsorption capacity and had abundant Mn 4+ species, resulting in a higher SCR activity in the entire reaction temperature (Figure 9).Furthermore, comparative studies on Mn-Ce oxides supported on TiO 2 and Al 2 O 3 for NH 3 -SCR at low temperatures were conducted by Jin et al. [79].The results demonstrated that the Mn-Ce/Al 2 O 3 catalyst showed a relatively higher SCR activity than the Mn-Ce/TiO 2 catalyst at the temperature range of 80-150 • C, primarily due to the Mn-Ce/Al 2 O 3 catalyst having more acid sites.Li et al. [80] synthesized supported catalysts of FeO x and MnO x that were co-supported on aluminum-modified CeO 2 for a low-temperature NH 3 -SCR reduction of NO x .It was observed that the Fe-Mn/Ce 1 Al 2 catalyst achieved over a 90% NO conversion at 75-250 • C and displayed superior SO 2 resistance compared to the Fe-Mn/CeO 2 catalyst.The improved catalytic performance could be ascribed to the larger surface area, and the enhanced reducibility was due to the introduction of Al 2 O 3 .(3) Carbon materials as support Carbon materials, including carbon nanotubes (CNTs), activated carbon (AC), activated carbon fiber (ACF), and graphene (GR), have been identified as attractive supports for SCR catalysts [81,82].Su et al. [83] synthesized a range of MnOx catalysts supported by CNTs and assessed their SCR catalytic performance.The results showed that the catalyst with MnOx introduced into the CNT channels demonstrated superior SCR activity compared to the MnOx on the outside surface of the CNTs.Xiao et al. [84] reported that the denitration performance of a MnOx-CeO2/GR catalyst was better than that of a MnOx-CeO2 catalyst even in the presence of SO2 and H2O, as displayed in Figure 10.The result suggested that the introduction of GR altered the composition of the Mn species, thereby exerting a notable influence on the electron mobility.Jiang et al. [85] revealed that the introduction of AC into the catalyst resulted in an enhancement in the NO conversion efficiency.Table 1 summarizes the research results of Mn-based catalysts for NO catalytic reduction in recent years.(3) Carbon materials as support Carbon materials, including carbon nanotubes (CNTs), activated carbon (AC), activated carbon fiber (ACF), and graphene (GR), have been identified as attractive supports for SCR catalysts [81,82].Su et al. [83] synthesized a range of MnO x catalysts supported by CNTs and assessed their SCR catalytic performance.The results showed that the catalyst with MnO x introduced into the CNT channels demonstrated superior SCR activity compared to the MnO x on the outside surface of the CNTs.Xiao et al. [84] reported that the denitration performance of a MnO x -CeO 2 /GR catalyst was better than that of a MnO x -CeO 2 catalyst even in the presence of SO 2 and H 2 O, as displayed in Figure 10.The result suggested that the introduction of GR altered the composition of the Mn species, thereby exerting a notable influence on the electron mobility.Jiang et al. [85] revealed that the introduction of AC into the catalyst resulted in an enhancement in the NO conversion efficiency.Table 1 summarizes the research results of Mn-based catalysts for NO catalytic reduction in recent years.(3) Carbon materials as support Carbon materials, including carbon nanotubes (CNTs), activated carbon (AC), activated carbon fiber (ACF), and graphene (GR), have been identified as attractive supports for SCR catalysts [81,82].Su et al. [83] synthesized a range of MnOx catalysts supported by CNTs and assessed their SCR catalytic performance.The results showed that the catalyst with MnOx introduced into the CNT channels demonstrated superior SCR activity compared to the MnOx on the outside surface of the CNTs.Xiao et al. [84] reported that the denitration performance of a MnOx-CeO2/GR catalyst was better than that of a MnOx-CeO2 catalyst even in the presence of SO2 and H2O, as displayed in Figure 10.The result suggested that the introduction of GR altered the composition of the Mn species, thereby exerting a notable influence on the electron mobility.Jiang et al. [85] revealed that the introduction of AC into the catalyst resulted in an enhancement in the NO conversion efficiency.Table 1 summarizes the research results of Mn-based catalysts for NO catalytic reduction in recent years.

CO Removal 2.2.1. Single MnO x Catalysts for CO Oxidation
Manganese oxide catalysts with different crystal structures and morphologies have exhibited significant differences in their catalytic performance for CO catalytic oxidation [86].Xu et al. [87] reported that an α-MnO 2 nanowire catalyst exhibited higher catalytic activity than a β-MnO 2 catalyst, which was attributed to the α-MnO 2 catalyst possessing a remarkable oxidation ability.Frey et al. [88] prepared non-stoichiometric MnO x catalysts and studied the relationship between their micro-structural correlation and catalytic activity for CO oxidation.The results revealed that the excellent catalytic activity of the non-stoichiometric MnO x catalyst could be attributed to the presence of nanocrystals at the ending of the nanorods.Additionally, earlier studies have indicated that the catalyst's reactivity is linked to the ability of Mn to form different oxidation states, such as the redox of Mn 2+ /Mn 3+ or Mn 3+ /Mn 4+ , as well as the mobility of lattice oxide species [89,90].Composite oxides consist of two or more active components, and the interaction between different active species can modify their dispersion state, ultimately leading to an enhanced catalytic activity and the stability of the catalysts.In comparison to a pure MnO x catalyst, composite MnO x catalysts have preferable crystal structures and redox properties, which exhibit higher catalytic activity in CO catalytic oxidation [91].Pan et al. [92] found that CO conversion efficiency on MnO x catalysts was significantly enhanced after introducing copper oxides.And the improved CO catalytic activity of CuMnO x catalysts was related to the resonance system of Cu 2+ +Mn 3+

Composite MnOx Catalysts for CO Oxidation
Composite oxides consist of two or more active components, and the inter tween different active species can modify their dispersion state, ultimately lead enhanced catalytic activity and the stability of the catalysts.In comparison to a p catalyst, composite MnOx catalysts have preferable crystal structures and redo ties, which exhibit higher catalytic activity in CO catalytic oxidation [91].Pan found that CO conversion efficiency on MnOx catalysts was significantly enha introducing copper oxides.And the improved CO catalytic activity of CuMnO was related to the resonance system of Cu 2+ +Mn 3+ ⇆ Cu + +Mn 4+ and the efficient of CO onto Cu 2+ and Mn 4+ species.Zhang et al. [93] prepared a range of MnOx-C lysts with varying Mn/Ce molar ratios and studied the catalytic activity for CO oxidation.It could be observed that a Mn1Ce1 catalyst showed a better catalyt mance and wider operating temperature window than pure MnOx and CeO2 ca 2.2.3.Supported MnOx Catalysts for CO Oxidation Loading MnOx on the support materials, such as TiO2 [94,95], Al2O3 [96], [68], provided a prospective practical application of Mn-based catalysts in CO oxidation.Dong et al. [94] designed a Mn3O4/TiO2 catalyst grown in situ on a mesh substrate for CO catalytic oxidation.As shown in Figure 11, the Mn3O4/TiO achieved nearly complete CO conversion (100%) at a relatively low temperature surpassing the performance of some noble metal catalysts.Li et al. [97] CuMn/Al2O3 catalysts employing ordered mesoporous Al2O3 as a support.The r gested that the ordered mesoporous Al2O3 led to catalysts with higher specific s eas and large pore volumes, as well as more surface activity species, thereby e the CO catalytic oxidation activity of the catalyst.A comprehensive summary search results of Mn-based catalysts for CO catalytic oxidation are presented in

MnOx-CeO2
Co-precipitation 1% CO, 20% O2, Ar as balance, 75,000 h Cu + +Mn 4+ and the efficient oxidation of CO onto Cu 2+ and Mn 4+ species.Zhang et al. [93] prepared a range of MnO x -CeO 2 catalysts with varying Mn/Ce molar ratios and studied the catalytic activity for CO catalytic oxidation.It could be observed that a Mn 1 Ce 1 catalyst showed a better catalytic performance and wider operating temperature window than pure MnO x and CeO 2 catalysts.

Supported MnO x Catalysts for CO Oxidation
Loading MnO x on the support materials, such as TiO 2 [94,95], Al 2 O 3 [96], and CeO 2 [68], provided a prospective practical application of Mn-based catalysts in CO catalytic oxidation.Dong et al.
[94] designed a Mn 3 O 4 /TiO 2 catalyst grown in situ on a titanium mesh substrate for CO catalytic oxidation.As shown in Figure 11, the Mn 3 O 4 /TiO 2 catalyst achieved nearly complete CO conversion (100%) at a relatively low temperature of 160 • C, surpassing the performance of some noble metal catalysts.Li et al. [97] prepared CuMn/Al 2 O 3 catalysts employing ordered mesoporous Al 2 O 3 as a support.The result suggested that the ordered mesoporous Al 2 O 3 led to catalysts with higher specific surface areas and large pore volumes, as well as more surface activity species, thereby enhancing the CO catalytic oxidation activity of the catalyst.A comprehensive summary of the research results of Mn-based catalysts for CO catalytic oxidation are presented in Table 2.

Composite MnOx Catalysts for CO Oxidation
Composite oxides consist of two or more active components, and the interaction between different active species can modify their dispersion state, ultimately leading to an enhanced catalytic activity and the stability of the catalysts.In comparison to a pure MnOx catalyst, composite MnOx catalysts have preferable crystal structures and redox properties, which exhibit higher catalytic activity in CO catalytic oxidation [91].Pan et al. [92] found that CO conversion efficiency on MnOx catalysts was significantly enhanced after introducing copper oxides.And the improved CO catalytic activity of CuMnOx catalysts was related to the resonance system of Cu 2+ +Mn 3+ ⇆ Cu + +Mn 4+ and the efficient oxidation of CO onto Cu 2+ and Mn 4+ species.Zhang et al. [93] prepared a range of MnOx-CeO2 catalysts with varying Mn/Ce molar ratios and studied the catalytic activity for CO catalytic oxidation.It could be observed that a Mn1Ce1 catalyst showed a better catalytic performance and wider operating temperature window than pure MnOx and CeO2 catalysts.

Supported MnOx Catalysts for CO Oxidation
Loading MnOx on the support materials, such as TiO2 [94,95], Al2O3 [96], and CeO2 [68], provided a prospective practical application of Mn-based catalysts in CO catalytic oxidation.Dong et al. [94] designed a Mn3O4/TiO2 catalyst grown in situ on a titanium mesh substrate for CO catalytic oxidation.As shown in Figure 11, the Mn3O4/TiO2 catalyst achieved nearly complete CO conversion (100%) at a relatively low temperature of 160 °C, surpassing the performance of some noble metal catalysts.Li et al. [97] prepared CuMn/Al2O3 catalysts employing ordered mesoporous Al2O3 as a support.The result suggested that the ordered mesoporous Al2O3 led to catalysts with higher specific surface areas and large pore volumes, as well as more surface activity species, thereby enhancing the CO catalytic oxidation activity of the catalyst.A comprehensive summary of the research results of Mn-based catalysts for CO catalytic oxidation are presented in Table 2.

Simultaneous Removal of NO x and CO
Nitrogen oxides and carbon monoxide coexist in the emissions of some plants, including coal-fired power plants, the steel industry, coking plants, and the cement industry.The development of bifunctional catalysts was of great importance for effectively removing both NO x and CO simultaneously.Manganese-based catalysts exhibit a range of oxidation states and unstable oxygen species, which play a crucial role in enhancing the adsorption and activation of NO x and CO on the catalyst surface [98,99].In our earlier study [100], we found that γ-MnO 2 catalysts exhibit higher catalytic activity for both NO reduction and CO oxidation compared to α-, β-, and δ-MnO 2 catalysts (see Figure 12).And the outstanding catalytic performance of the γ-MnO 2 catalyst could be assigned to its remarkable redox property and abundant active sites, which promote the adsorption and activation of NO and CO molecules.Zheng et al. [101] prepared CuMnO x bifunctional catalysts and evaluated their catalytic performance for NO reduction and CO oxidation.As shown in Figure 13, a Cu 1 Mn 1 catalyst exhibited excellent activity for removing NO and CO simultaneously, achieving nearly 100% NO conversion and 96% CO conversion at 125 • C, respectively.Gui et al. [102] reported a bifunctional catalyst of Mn 2 Cu 2 Al 1 O x which possessed dual active sties and was highly active for both NH 3 -SCR and CO oxidation reactions.The results indicated that CO was more easily adsorbed on the Cu active sites, while NH 3 was more inclined to absorb on the Mn active sites, which enabled the simultaneous occurrence of NO catalytic reduction and CO oxidation on the catalyst surface.Guo et al. [53] synthesized a CuMn-HZSM-5 catalyst via the impregnation method.The optimized catalyst achieved a 90% NO removal efficiency and nearly a 100% CO conversion rate at 200 • C. The results of the bifunctional catalysts for removing both NO x and CO simultaneously are concluded in Table 3.

Simultaneous Removal of NOx and CO
Nitrogen oxides and carbon monoxide coexist in the emissions of some plants, including coal-fired power plants, the steel industry, coking plants, and the cement industry.The development of bifunctional catalysts was of great importance for effectively removing both NOx and CO simultaneously.Manganese-based catalysts exhibit a range of oxidation states and unstable oxygen species, which play a crucial role in enhancing the adsorption and activation of NOx and CO on the catalyst surface [98,99].In our earlier study [100], we found that γ-MnO2 catalysts exhibit higher catalytic activity for both NO reduction and CO oxidation compared to α-, β-, and δ-MnO2 catalysts (see Figure 12).And the outstanding catalytic performance of the γ-MnO2 catalyst could be assigned to its remarkable redox property and abundant active sites, which promote the adsorption and activation of NO and CO molecules.Zheng et al. [101] prepared CuMnOx bifunctional catalysts and evaluated their catalytic performance for NO reduction and CO oxidation.As shown in Figure 13, a Cu1Mn1 catalyst exhibited excellent activity for removing NO and CO simultaneously, achieving nearly 100% NO conversion and 96% CO conversion at 125 °C, respectively.Gui et al. [102] reported a bifunctional catalyst of Mn2Cu2Al1Ox which possessed dual active sties and was highly active for both NH3-SCR and CO oxidation reactions.The results indicated that CO was more easily adsorbed on the Cu active sites, while NH3 was more inclined to absorb on the Mn active sites, which enabled the simultaneous occurrence of NO catalytic reduction and CO oxidation on the catalyst surface.Guo et al. [53] synthesized a CuMn-HZSM-5 catalyst via the impregnation method.The optimized catalyst achieved a 90% NO removal efficiency and nearly a 100% CO conversion rate at 200 °C.The results of the bifunctional catalysts for removing both NOx and CO simultaneously are concluded in Table 3.    Understanding the pathways and mechanisms of NO x catalytic reductions over the catalysts was of significance in attaining efficient denitrification.In the NH 3 -SCR reaction, the primary pathways of NO x reduction could be outlined using Equations ( 1)-(5): Among them, Reaction (1) was referred to as the "standard SCR" reaction, containing a stoichiometry with identical amounts of NO and NH 3 .In the presence of NO 2 , Reaction (2) proceeded at a higher rate compared to the "standard SCR", so it was defined as "fast SCR".When an excess of NO 2 (NO 2 /NO > 1) was present in the flue gas, Reactions (3) and ( 4) happened.Reaction (5), between NH 3 and NO, proceeded in an oxygen-free or low-oxygen atmosphere.
The Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms are commonly accepted pathways in the NH 3 -SCR reaction [105], as illustrated in Figure 14.As for the E-R mechanism, ammonia molecules are initially adsorbed at the acid sites on the catalyst surface, leading to the formation of intermediates, such as -NH 2 species and adsorbed NH 3 species.Subsequently, these intermediates react with gaseous NO and NO 2 , ultimately resulting in the generation of N 2 and H 2 O.The reaction process can be described using Equations ( 6)-( 10): NH 3(gas) → NH 3(ads) (6) NH 2(ads) +NO (gas) → N 2 +H 2 O ( 9) monly accepted pathways in the NH3-SCR reaction [105], as illustrated in Figure 14.As for the E-R mechanism, ammonia molecules are initially adsorbed at the acid sites on the catalyst surface, leading to the formation of intermediates, such as -NH2 species and adsorbed NH3 species.Subsequently, these intermediates react with gaseous NO and NO2, ultimately resulting in the generation of N2 and H2O.The reaction process can be described using Equations ( 6)-( 10 Marbán et al. [106] found that Mn3O4/AC catalysts primarily follow the E-R mechanism in the NH3-SCR reaction, in which NO2 and, to a lesser extent, NO react with surfaceactive NH3 species.Xu et al. [107] also proposed that the SCR reaction over a MnOx catalyst proceeds via the E-R mechanism, in which the adsorbed NH3 species could react with the gaseous NO.Chen et al. [52] confirmed that the E-R mechanism plays a more significant role in the SCR reaction over MnFeOx catalysts by employing the transient reaction experiments.Marbán et al. [106] found that Mn 3 O 4 /AC catalysts primarily follow the E-R mechanism in the NH 3 -SCR reaction, in which NO 2 and, to a lesser extent, NO react with surface-active NH 3 species.Xu et al. [107] also proposed that the SCR reaction over a MnO x catalyst proceeds via the E-R mechanism, in which the adsorbed NH 3 species could react with the gaseous NO.Chen et al. [52] confirmed that the E-R mechanism plays a more significant role in the SCR reaction over MnFeO x catalysts by employing the transient reaction experiments.
For the L-H mechanism, NO was adsorbed on the active sites of the catalyst to form NO x adsorbed species.Then, the adsorbed NH 3 reacted with the adsorbed NO x species to produce N 2 and H 2 O.The specific processes are shown in Figure 15.In general, the L-H mechanism is easier to proceed with than the E-R mechanism owing to its low activation energy [108].Kijlstra et al. [66] proposed that the Mn 3+ site over the MnO x /Al 2 O 3 catalyst was the center of Lewis acid sites, and -NH 2 species were generated via the deamination of adsorbed NH 3 reacting with gaseous NO and adsorbed NO at the same time.That is, both the L-H and E-R mechanisms occurred.Wei et al. [109] explored the mechanism of a Mn/TiO 2 catalyst in the NH 3 -SCR reaction via a series of experiments and DFT calculations.The result showed that the catalytic reaction pathway on the catalysts consisted of two fundamental steps, as illustrated in Figure 16.
energy [108].Kijlstra et al. [66] proposed that the Mn 3+ site over the MnOx/Al2O3 catalyst was the center of Lewis acid sites, and -NH2 species were generated via the deamination of adsorbed NH3 reacting with gaseous NO and adsorbed NO at the same time.That is, both the L-H and E-R mechanisms occurred.Wei et al. [109] explored the mechanism of a Mn/TiO2 catalyst in the NH3-SCR reaction via a series of experiments and DFT calculations.The result showed that the catalytic reaction pathway on the catalysts consisted of two fundamental steps, as illustrated in Figure 16.NOx adsorbed species.Then, the adsorbed NH3 reacted with the adsorbed NOx species to produce N2 and H2O.The specific processes are shown in Figure 15.In general, the L-H mechanism is easier to proceed with than the E-R mechanism owing to its low activation energy [108].Kijlstra et al. [66] proposed that the Mn 3+ site over the MnOx/Al2O3 catalyst was the center of Lewis acid sites, and -NH2 species were generated via the deamination of adsorbed NH3 reacting with gaseous NO and adsorbed NO at the same time.That is, both the L-H and E-R mechanisms occurred.Wei et al. [109] explored the mechanism of a Mn/TiO2 catalyst in the NH3-SCR reaction via a series of experiments and DFT calculations.The result showed that the catalytic reaction pathway on the catalysts consisted of two fundamental steps, as illustrated in Figure 16.

Mechanisms of CO Oxidation on Mn-Based Catalysts
The catalytic oxidation of CO is one of the most representative prototype reactions in heterogeneous catalysis, and attracts significant interest due to its extensive applications in the environmental and energy fields.At present, the proposed mechanisms for CO catalytic oxidation mainly encompass the L-H, E-R, and Mars-van Krevelen (MvK) mechanisms [110][111][112], as depicted in Figure 17.
The L-H mechanism, as presented in Figure 17a, involves the following key steps: CO reacts with OH-on the catalyst surface, leading to the formation of formate or carbonate species.Subsequently, the adsorbed formate or carbonate species decompose to produce CO 2 and H 2 .Then, the presence of metal catalysts facilitates the preferential adsorption of CO and promotes the easier breaking of C-H bonds in formate species.During this reaction process, the lattice oxygen does not participate in the catalytic oxidation, and the reaction occurs through the adsorption and reaction of CO and O 2 on the catalyst surface [111,113].Dey et al. [114] proposed that CuMnO x catalysts predominantly follow the L-H mechanism in the CO catalytic oxidation process, primarily involving the reaction of surface-activated oxygen species with adsorbed CO species to produce CO 2 .
ically, transition metal ions exhibit excellent electron conductivity, which facilitates efficient electron transfer during the redox process.Additionally, the mobility of lattice oxygen in the catalyst ensures the re-oxidation of the reduced surface, thereby enabling the regeneration of the active sites.Xu et al. [98] proposed that CO catalytic oxidation over an α-Mn2O3 nano catalyst is dominated by the L-H mechanism at lower temperatures, and turns to the MvK mechanism at higher temperatures, as shown in Figure 18.Morgan et al. [112] found a significant predominance of the MvK mechanism and a relatively minor involvement of the L-H mechanism for CO catalytic oxidation over both undoped and gold-doped CuMnOx catalysts, and the introduction of gold clearly facilitated the MvK mechanism.The E-R mechanism is displayed in Figure 17b.The mechanism involves the reaction that occurs between gaseous CO molecules and chemisorbed oxygen species (atomic oxygen and molecular oxygen).The MvK mechanism is also known as the redox mechanism, as illustrated in Figure 17c.In this mechanism, the catalyst surface exhibits a preference for the adsorption of activated CO molecules, which then react with lattice oxygen, resulting in the formation of CO 2 and the creation of oxygen vacancies on the catalyst surface [115].Subsequently, gaseous oxygen enters the oxygen vacancies and reacts with the partially reduced catalyst, replenishing its oxidation capacity [116].The redox reaction mechanism involves two types of active sites: (1) active metal cation sites, which are responsible for oxidizing the reactants, and (2) active sites for the reduction of molecular oxygen.Typically, transition metal ions exhibit excellent electron conductivity, which facilitates efficient electron transfer during the redox process.Additionally, the mobility of lattice oxygen in the catalyst ensures the re-oxidation of the reduced surface, thereby enabling the regeneration of the active sites.Xu et al. [98] proposed that CO catalytic oxidation over an α-Mn 2 O 3 nano catalyst is dominated by the L-H mechanism at lower temperatures, and turns to the MvK mechanism at higher temperatures, as shown in Figure 18.Morgan et al. [112] found a significant predominance of the MvK mechanism and a relatively minor involvement of the L-H mechanism for CO catalytic oxidation over both undoped and gold-doped CuMnO x catalysts, and the introduction of gold clearly facilitated the MvK mechanism.

Interactions between Simultaneous NO x Catalytic Reduction and CO Catalytic Oxidation on Mn-Based Catalysts
The simultaneous removal of NO x and CO from industrial fumes involves complex interactions between the NO x reduction and CO oxidation processes, resulting in some favorable or unfavorable consequences.Nevertheless, these interactions between multiple reactants were determined by various factors, such as the reaction temperature, the concentration of the reactants, and the catalyst properties.Understanding the interactions between various reactants and the influence of reaction conditions on the synergistic removal efficiency was of great significance in designing the catalysts for the simultaneous removal of NO x and CO.

Effect of CO Oxidation on NO x Reduction
Gaining insight into how CO oxidation reactants influence NO x reductions is crucial for improving the efficiency of NO x removal during the joint removal process.Nevertheless, there is currently no unanimous consensus regarding whether CO catalytic oxidation promotes or inhibits NO x reduction.Some researchers have proposed that CO catalytic oxidation promotes NO x reduction.For instance, Zeng et al. [117] confirmed that the CO oxidation reaction has a positive effect on the NO x reduction reaction.This advantageous effect could be attributed to the heat that is generated during CO catalytic oxidation, which acts as an ideal heat source to increase the flue gas temperature, thus enhancing the SCR catalytic activity at lower temperatures.Guo et al. [53] demonstrated that the introduction of CO could improve the removal efficiency of NO by facilitating NO adsorption on preadsorbed sites.The adsorbed CO serves as a reducing agent, converting NO to N 2 , thereby providing an alternative reaction pathway in the SCR process.Nevertheless, some scholars have suggested that CO catalytic oxidation has an inhibitory effect on NO x reduction.Gui et al. [102] found that the presence of CO has an adverse effect on the NH 3 -SCR catalytic activity of Mn 2 Cu 1 Al 1 O x catalysts.This was mainly due to the competitive adsorption of NH 3 and CO on the active sites.Similarly, Liu et al. [118] observed a noteworthy reduction in NO conversion efficiency in the presence of CO.The decline was ascribed to the simultaneous adsorption of NO and CO on a Mn/Ti catalyst, resulting in a competitive adsorption between CO and NO, as depicted in Figure 19.

Interactions between Simultaneous NOx Catalytic Reduction and CO Catalytic Oxidation on Mn-Based Catalysts
The simultaneous removal of NOx and CO from industrial fumes involves complex interactions between the NOx reduction and CO oxidation processes, resulting in some favorable or unfavorable consequences.Nevertheless, these interactions between multiple reactants were determined by various factors, such as the reaction temperature, the concentration of the reactants, and the catalyst properties.Understanding the interactions between various reactants and the influence of reaction conditions on the synergistic removal efficiency was of great significance in designing the catalysts for the simultaneous removal of NOx and CO.

Effect of CO Oxidation on NOx Reduction
Gaining insight into how CO oxidation reactants influence NOx reductions is crucial for improving the efficiency of NOx removal during the joint removal process.Nevertheless, there is currently no unanimous consensus regarding whether CO catalytic oxidation promotes or inhibits NOx reduction.Some researchers have proposed that CO catalytic oxidation promotes NOx reduction.For instance, Zeng et al. [117] confirmed that the CO oxidation reaction has a positive effect on the NOx reduction reaction.This advantageous effect could be attributed to the heat that is generated during CO catalytic oxidation, which acts as an ideal heat source to increase the flue gas temperature, thus enhancing the SCR catalytic activity at lower temperatures.Guo et al. [53] demonstrated that the introduction of CO could improve the removal efficiency of NO by facilitating NO adsorption

Effect of NH 3 -SCR Atmosphere on CO Oxidation
For the simultaneous removal of NO x and CO in the NH 3 -SCR system, the CO conversion rate displays notable distinctions when compared to the individual CO catalytic oxidation reaction, suggesting that NO x might participate in the CO catalytic oxidation reaction.Zheng et al. [101] found that the CO conversion in a coordinated experiment over a Cu 1 Mn 2 catalyst was higher than in a separate experiment, suggesting that NO played a facilitating role in the CO catalytic oxidation reaction.Guo et al. [53] indicated that the adsorption of NO on a CuMn-HZSM-5 catalyst surface generated NO 2 − and N 2 O 2 − species, which served as key intermediates for the oxidation of CO to CO 2 , as depicted in Figure 20.

Effect of NH3-SCR Atmosphere on CO Oxidation
For the simultaneous removal of NOx and CO in the NH3-SCR system, the CO conversion rate displays notable distinctions when compared to the individual CO catalytic oxidation reaction, suggesting that NOx might participate in the CO catalytic oxidation reaction.Zheng et al. [101] found that the CO conversion in a coordinated experiment over a Cu1Mn2 catalyst was higher than in a separate experiment, suggesting that NO played a facilitating role in the CO catalytic oxidation reaction.Guo et al. [53] indicated that the adsorption of NO on a CuMn-HZSM-5 catalyst surface generated NO2 − and N2O2 − species, which served as key intermediates for the oxidation of CO to CO2, as depicted in Figure 20.

Conclusions and Perspectives
This review provided an in-depth summary of the research progress of Mn-based catalysts in the elimination of NOx and CO.The catalytic performance, reaction mechanisms, and influence factors of Mn-based catalysts for eliminating NOx and CO were summarized.Pure MnOx catalysts exhibit a good catalytic activity for NOx catalytic reduction and CO oxidation, but with a narrow operating window and poor resistance to toxic substances.The modification of MnOx catalysts through the incorporation of other metal

Conclusions and Perspectives
This review provided an in-depth summary of the research progress of Mn-based catalysts in the elimination of NO x and CO.The catalytic performance, reaction mechanisms, and influence factors of Mn-based catalysts for eliminating NO x and CO were summarized.Pure MnO x catalysts exhibit a good catalytic activity for NO x catalytic reduction and CO oxidation, but with a narrow operating window and poor resistance to toxic substances.The modification of MnO x catalysts through the incorporation of other metal oxides has been demonstrated to enhance the catalytic activity and widen the operating window.Moreover, the introduction of supports, such as Al 2 O 3 , TiO 2 , and carbon materials, is also an effective strategy for improving the catalytic activity in NH 3 -SCR and CO catalytic oxidation reactions.Despite significant advancements in Mn-based catalysts for the removal of NO x and CO, there remains a pressing need for further in-depth research to develop catalysts with a higher catalytic activity for NO x reduction and CO oxidation in industrial flue gas conditions.The following aspects could be considered in the future: (1) Mn-based catalysts exhibit a poor N 2 selectivity in the NH 3 -SCR reaction.This is primarily ascribed to the strong oxidizing property of Mn-based catalysts, resulting in the non-selective reduction of NH 3 on the catalyst surface, thereby producing a large amount of the by-products, N 2 O.Further research should focus on improving the N 2 selectivity.For enhancing the SCR catalytic properties, it is imperative to inhibit the non-selective catalytic reduction of NH 3 , thus enhancing the utilization rate of NH 3 .
(2) The resistance to SO 2 and H 2 O of Mn-based catalysts is insufficient in both the NH 3 -SCR and CO catalytic oxidation reactions.In future studies, scholars should concentrate their efforts on optimizing the active components and developing new structures and morphologies to avoid catalyst deactivation.Furthermore, a crucial

Figure 1 .
Figure 1.Structure diagram of this review.

Figure 1 .
Figure 1.Structure diagram of this review.

Figure 5 .
Figure 5. (a) SCR activity, (b) N2O concentration and N2 selectivity of catalysts at different temperature, (c) N2O concentration and NO conversion, and (d) N2O concentrations and N2 selectivity of catalysts at different NO and O2 concentrations [48].

Figure 5 .
Figure 5. (a) SCR activity, (b) N 2 O concentration and N 2 selectivity of catalysts at different temperature, (c) N 2 O concentration and NO conversion, and (d) N 2 O concentrations and N 2 selectivity of catalysts at different NO and O 2 concentrations [48].

Figure 6 .
Figure 6.Influence of different metal oxides as additives on the catalytic activity of a MnO x catalyst: (a) CoO x , MnO x , and Co-MnO x ; (b) NiO x , MnO x , and Ni-MnO x ; (c) CuO x , MnO x , and Cu-MnO x ; (d) Cu-MnO x , Ni-MnO x , and Co-MnO x [58].

Figure 8 .
Figure 8.(a) SCR performance in the presence of SO 2 and (b) N 2 selectivity for two kinds of Mn-Ce/TiO 2 catalysts [74].

Figure 11 .
Figure 11.(a) CO conversion with temperature and (b) the CO oxidation stability of the catalyst [95].

Figure 11 .
Figure 11.(a) CO conversion with temperature and (b) the CO oxidation stability of the Mn/TiO2 catalyst [95].

Figure 11 .
Figure 11.(a) CO conversion with temperature and (b) the CO oxidation stability of the Mn/TiO 2 catalyst [95].

3 . 3 . 1 .
Mechanisms and Interactions of NO x Catalytic Reduction and CO Catalytic Oxidation Pathways and Mechanisms of NO x Catalytic Reduction on Mn-Based Catalysts

Figure 15 .
Figure 15.Schematic diagram of the L-H mechanism in the NH3-SCR reaction over the catalysts.

Figure 15 .
Figure 15.Schematic diagram of the L-H mechanism in the NH 3 -SCR reaction over the catalysts.

Figure 15 .
Figure 15.Schematic diagram of the L-H mechanism in the NH3-SCR reaction over the catalysts.

Figure 19 .
Figure19.Inhibition mechanism of CO on the NH 3 -SCR reaction over a Mn/Ti catalyst[118].

Figure 20 .
Figure 20.Reaction mechanisms for the selective catalytic reduction of NO and CO oxidation over a CuMn-HZSM-5 catalyst [53].

Figure 20 .
Figure 20.Reaction mechanisms for the selective catalytic reduction of NO and CO oxidation over a CuMn-HZSM-5 catalyst [53].

Table 1 .
Details of catalytic performance, preparation methods, and reaction conditions for Mnbased catalysts removing NOx.

Table 1 .
Details of catalytic performance, preparation methods, and reaction conditions for Mnbased catalysts removing NOx.

Table 1 .
Details of catalytic performance, preparation methods, and reaction conditions for Mn-based catalysts removing NO x .

Table 2 .
Details of catalytic performance, preparation methods, and reaction conditio based catalysts removing CO.

Table 2 .
Details of catalytic performance, preparation methods, and reaction conditions for Mnbased catalysts removing CO.

Table 2 .
Details of catalytic performance, preparation methods, and reaction conditions for Mn-based catalysts removing CO.

Table 3 .
Details of catalytic performance, preparation methods, and reaction conditions for Mn-based catalysts removing NO x and CO.