Mn-Based Catalysts in the Selective Reduction of NO x with CO: Current Status, Existing Challenges, and Future Perspectives

: The technology for the selective catalytic reduction of NO x by CO (CO-SCR) has the capability to simultaneously eliminate CO and NO x from industrial flue gas and automobile exhaust, thus making it a promising denitrification method. The advancement of cost-effective and high-performing catalysts is crucial for the commercialization of this technology. Mn-based catalysts demonstrate enhanced catalytic efficiency under conditions of low temperature and low oxygen content when compared to other transition metal-based catalysts, indicating significant potential for practical applications. This review outlines the diverse Mn-based catalysts, including bulk or supported MnO x catalysts, bulk or supported Mn-based composite oxide catalysts, and the use of MnO x as dopants. Subsequently, the synthesis methods and catalytic mechanism employed by Mn-based catalysts are presented. The following section examines the impact of O 2 , H 2 O, and SO 2 on the catalytic performance. Finally, the potential and implications of this reaction are deliberated. This work aims to offer theoretical guidance for the rational design of highly efficient Mn-based catalysts in the CO-SCR reaction for industrial applications.


Introduction
NO x , including NO and NO 2 , constitutes a significant portion of atmospheric pollutants, which is primarily derived from vehicle exhaust and industrial emissions, notably from coke oven and coal-fired power plant emissions.NO x can cause severe environmental consequences, such as acid rain, photochemical smog, and ozone depletion [1][2][3], which present substantial hazards to human health.Currently, the prevalent industrial denitrification technique involves the selective reduction of NO x by NH 3 , known as NH 3 -SCR [4,5].However, this technology presents several concerns, including the potential for ammonia storage leakage, the formation of by-products that can lead to equipment blockage and corrosion, and the inherent toxicity of ammonia [6].Given the incomplete combustion of fuel, flue gas also contains a certain amount of CO.The use of CO as the reducing agents for the selective catalytic reduction of NO x (CO-SCR) obviates the necessity for extra reducing agents, thereby reducing costs.Moreover, this technology assists in the removal of NO x and CO emissions, enabling the concurrent elimination of both pollutants.Hence, CO-SCR technology shows promising potential for practical application [7].
The ideal CO-SCR reaction is presented in Equation (1).However, in real flue gas, there exists a high concentration of O 2 and H 2 O, along with a minor quantity of SO 2 .The presence of oxygen can lead to the preferential oxidation of NO and CO (Equations ( 2) and ( 3)), while the existence of H 2 O and SO 2 may cause catalyst poisoning; meanwhile, concurrent side reactions may occur, producing N 2 O (Equation ( 4)), consequently diminishing the selectivity for N 2 .

Bulk or Supported MnOx Catalysts
Manganese oxides, which are present in various crystal phases, are compounds characterized by multiple oxidation states (Mn 2+ , Mn 3+ , and Mn 4+ ), excellent redox capabilities, and a rich supply of active oxygen.The efficiency of pure manganese oxides in reducing NOx to N2 is ranked MnO2 > Mn5O8 > Mn2O3 > Mn3O4 > MnO.Highly oxidized manganese oxides have higher CO-SCR activity, and oxygen vacancies (OVs) produced in the reduction process serve as the adsorption site for NOx [22].These properties empower them to actively engage in and promote catalytic reactions.Therefore, manganese oxides hold great potential in the CO-SCR reaction (Table 1).The surface manganese oxide phases and redox properties of Mn oxide catalysts are crucial factors that significantly influence the overall catalytic activity [23].For the α-MnO2 nanorod catalyst, upon pretreatment with H2, Mn 4+ is first reduced to Mn 2+ .Subsequently, these Mn 2+ sites undergo rapid oxidation to Mn 3+ in the presence of NO.In situ studies showed that the α-MnO2 surface experiences significant recombination, forming a new surface active phase.Mn 3+ and OVs were the active sites [24].
Compared with other transition metal oxides, the low temperature activity of manganese oxides on support for CO-SCR is higher than that of other transition metal oxides.Boningari et al. [25] studied different titanium-supported transition metal-based catalysts and found that MnOx/TiO2 had better CO-SCR performance at low temperatures.In situ infrared spectroscopy showed that the Lewis acid site on the catalyst surface was responsible for the reaction, not the Brønsted acid site.The reduced Mn site served as the active site for activating NO.In Mn/TiO2 catalysts, the lattice oxygen proximal to Mn atoms facilitates the oxidation of CO to CO2, leading to the creation of OVs.On Mn/TiO2, the notable intermediate N2O can be reduced to N2, with an associated energy barrier of 0.51 eV, indicating a pronounced N2 selectivity.The Mn-OV pair present on the catalyst surface serves as the active site for the adsorption of NO and the catalysis of N-O bond cleavage [26].Moreover, the comparison of the activities of different transition metal

Bulk or Supported MnO x Catalysts
Manganese oxides, which are present in various crystal phases, are compounds characterized by multiple oxidation states (Mn 2+ , Mn 3+ , and Mn 4+ ), excellent redox capabilities, and a rich supply of active oxygen.The efficiency of pure manganese oxides in reducing NO x to N 2 is ranked MnO 2 > Mn 5 O 8 > Mn 2 O 3 > Mn 3 O 4 > MnO.Highly oxidized manganese oxides have higher CO-SCR activity, and oxygen vacancies (OVs) produced in the reduction process serve as the adsorption site for NO x [22].These properties empower them to actively engage in and promote catalytic reactions.Therefore, manganese oxides hold great potential in the CO-SCR reaction (Table 1).The surface manganese oxide phases and redox properties of Mn oxide catalysts are crucial factors that significantly influence the overall catalytic activity [23].For the α-MnO 2 nanorod catalyst, upon pretreatment with H 2 , Mn 4+ is first reduced to Mn 2+ .Subsequently, these Mn 2+ sites undergo rapid oxidation to Mn 3+ in the presence of NO.In situ studies showed that the α-MnO 2 surface experiences significant recombination, forming a new surface active phase.Mn 3+ and OVs were the active sites [24].
Compared with other transition metal oxides, the low temperature activity of manganese oxides on support for CO-SCR is higher than that of other transition metal oxides.Boningari et al. [25] studied different titanium-supported transition metal-based catalysts and found that MnO x /TiO 2 had better CO-SCR performance at low temperatures.In situ infrared spectroscopy showed that the Lewis acid site on the catalyst surface was responsible for the reaction, not the Brønsted acid site.The reduced Mn site served as the active site for activating NO.In Mn/TiO 2 catalysts, the lattice oxygen proximal to Mn atoms facilitates the oxidation of CO to CO 2 , leading to the creation of OVs.On Mn/TiO 2 , the notable intermediate N 2 O can be reduced to N 2 , with an associated energy barrier of 0.51 eV, indicating a pronounced N 2 selectivity.The Mn-OV pair present on the catalyst surface serves as the active site for the adsorption of NO and the catalysis of N-O bond cleavage [26].Moreover, the comparison of the activities of different transition metal oxides (Cu, Ni, Fe, Mn, and Cr) showed that the TiO 2 -supported manganese oxide has good NO reduction activity at 200 • C. The MnO x /TiO 2 catalyst showed highly N 2 selectivity even in the presence of oxygen.The results indicate that the high surface area and the reducibility of manganese oxide play an important role in the high activity for CO-SCR [27].

Bulk or Supported Mn-Based Composite Oxide Catalysts
When Mn forms a composite oxide with other metals, it frequently generates a synergistic effect, enhancing its catalytic performance beyond that of an individual Mn oxide (Table 1) [28].The presence of surface synergistic oxygen vacancies (SSOVs) within composite oxides facilitates the adsorption and activation of gas molecules, thereby enhancing catalytic performance.In comparison to pure MnO x , Cu-Mn catalysts demonstrate a higher propensity for oxidation and the generation of activated O species on their surface, thus promoting the adsorption of oxygen molecules.Mn-based catalysts doped with copper oxide comprise not only MnO x but also a linear Cu-CO active substance, which enhances the adsorption of NO x .An intriguing phenomenon is that the surface dispersed Cu x+ -O 2− -Mn y+ substance can be reduced to Cu + -□-Mn (4−x)+ active species with an elevation in adsorption temperature.The synergistic effect between Cu and Mn (Cu x+ -O 2− -Mn y+ ) plays a crucial role in the CO-SCR reaction (Figure 2a) [29].Yao et al. [44] synthesized CuO-MnO 2 /CeO 2 using the co-impregnation method, which exhibited a stronger interaction and an excellent reduction behavior.This process facilitated the generation of low-valence copper species (Cu + /Cu 0 ) and increased oxygen vacancies, particularly SSOVs (Cu + -□-Mn (4−x)+ ), during the reaction, which are conducive to the adsorption of CO and the dissociation of NO, respectively.In another study [30], it was found that the catalytic efficiency of CuO-Mn 2 O 3 /γ-Al 2 O 3 catalysts, after being pretreated with CO, exhibited superior performance compared to CuO/γ-Al 2 O 3 and Mn 2 O 3 /γ-Al 2 O 3 .The formation of SSOVs in the CuO-Mn 2 O 3 /γ-Al 2 O 3 catalyst occurs following CO pretreatment, leading to distinct adsorption properties compared to SOVs in CuO/γ-Al 2 O 3 and Mn 2 O 3 /γ-Al 2 O 3 catalysts.Exposed Cu + and Mn 2+ ions act synergistically as CO and NO adsorption sites, respectively, and the SSOVs function as a crucial bridge, facilitating the interaction between adjacent adsorbed CO and NO to promote the surface reaction.Therefore, SSOVs on the CO-CuMnAl catalyst exhibit superior catalytic activity, facilitating the reduction of NO by CO (Figure 2b) [30].Due to the difference in cation radius, the incorporation of Mn and Ni into MOF-74 leads to an improvement in the specific surface area, which is conducive to the formation of unsaturated coordination metal sites and Lewis acid sites for CO-SCR.In addition, the electron interaction between Ni and Mn in NiMn-MOF-74 facilitates a significant synergistic reaction (Mn 3+ + Ni 3+ ↔ Mn 4+ + Ni 2+ ).This synergistic effect not only enhances the formation of more active Mn 4+ components, but also promotes the generation of SSOVs, which exhibit higher catalytic activity compared to surface oxygen vacancies (SOVs) [31].

MnOx as Dopants
Mn is also a frequently utilized dopant in CO-SCR (Table 1).The introduction of Mn doping has been found to facilitate the generation of oxygen vacancies on the catalyst surface.This, in turn, enhances the adsorption and activation of NO, leading to an overall improvement in the catalyst's performance.When Cu 2+ and Fe 3+ combine to form a Febased composite oxide in an alkaline solution, the addition of a specific quantity of Mn facilitates its bonding with Fe.This process weakens the interaction between Cu and Fe, leading to the precipitation of the CuO phase.The Cu 2+ ions then selectively develop along the surface of CuO(110), leading to enhanced catalytic activity.At high temperatures (300-1000 °C), the presence of Mn 3+ can impede the conversion of γ-Fe2O3 to α-Fe2O3, thus achieving stable catalytic efficacy.The existence of Mn 4+ leads to the generation of reactive oxygen species, which can be readily reduced to create SSOVs during the reaction.The resulting SSOVs can promote the dissociation of NO and facilitate the oxygen transfer (Equation ( 5)).These factors ultimately result in the catalyst exhibiting good catalytic performance across a wide temperature range [35].Different metal cations M (M = Zr, Cr, Mn, Fe, Co, and Sn) were doped into a CuO/Ce20M1Ox catalyst to explore their impact on the catalyst's structure and activity in the CO-SCR reaction.It was found that the doping of Mn 4+ into the CeO2 lattice promoted the generation of oxygen vacancies and surface By adjusting the electronic configuration of manganese oxides, it is possible to alter their redox characteristics, subsequently impacting their catalytic efficiency.During the doping modification process, Co occupies the Mn site in the [MnO6] cell in the OMS-2 catalyst, forming a new [CoO6] crystal structure.The crystallinity of Co-doped OMS-2 decreases as the Co doping amount increases.Co substitutes for Mn, creating a novel active site.The active sites comprising Co 2+ , Co 3+ , Mn 3+ , and Mn 4+ adsorb CO and NO, catalyzing the conversion of these compounds into CO 2 and N 2 (Equations ( 9) and ( 10)).Compared with undoped OMS-2, the conversion of NO in the flue gas of Co 0.3 -OMS-2 reaches approximately 95% at 100-300 • C (Figure 2c) [32].In the case of the Cu x Mn 3−x O 4 catalyst, the introduction of copper ions into Mn 2 O 3 results in the formation of a spinel structure with a high density of lattice defects and oxygen vacancies.This modification increases the concentration of Mn 4+ , which has the capability to adsorb reactant molecules.Consequently, this process enhances the redox performance of the catalyst, increases the mobility of reactive oxygen species, and ultimately boosts its catalytic activity.Compared with CuO and Mn 2 O 3 , the catalytic activity of the Cu x Mn 3−x O 4 catalyst is enhanced.The strong synergy between binary metal oxides also results in its improved stability (Figure 2d) [33].Mn demonstrates a propensity to exhibit synergistic interactions with other metals, resulting in increased efficacy of catalysts operating at low temperatures.When Ce, Fe and Co were introduced into a Mn-based catalyst and supported on TiO 2 , the catalytic performance exhibited an enhancement compared to the performance of Mn alone.Under anaerobic conditions, the Mn-Ce-Fe-Co/TiO 2 catalyst achieved complete NO conversion at 200 • C. The enhanced performance of the modified Mn-based catalyst can be attributed to the increased oxygen mobility and the formation of Mn 4+ , resulting in an elevated rate of NO conversion [34].

MnO x as Dopants
Mn is also a frequently utilized dopant in CO-SCR (Table 1).The introduction of Mn doping has been found to facilitate the generation of oxygen vacancies on the catalyst surface.This, in turn, enhances the adsorption and activation of NO, leading to an overall improvement in the catalyst's performance.When Cu 2+ and Fe 3+ combine to form a Febased composite oxide in an alkaline solution, the addition of a specific quantity of Mn facilitates its bonding with Fe.This process weakens the interaction between Cu and Fe, leading to the precipitation of the CuO phase.The Cu 2+ ions then selectively develop along the surface of CuO(110), leading to enhanced catalytic activity.At high temperatures (300-1000 • C), the presence of Mn 3+ can impede the conversion of γ-Fe 2 O 3 to α-Fe 2 O 3 , thus achieving stable catalytic efficacy.The existence of Mn 4+ leads to the generation of reactive oxygen species, which can be readily reduced to create SSOVs during the reaction.The resulting SSOVs can promote the dissociation of NO and facilitate the oxygen transfer (Equation ( 5)).These factors ultimately result in the catalyst exhibiting good catalytic performance across a wide temperature range [35].Different metal cations M (M = Zr, Cr, Mn, Fe, Co, and Sn) were doped into a CuO/Ce 20 M 1 O x catalyst to explore their impact on the catalyst's structure and activity in the CO-SCR reaction.It was found that the doping of Mn 4+ into the CeO 2 lattice promoted the generation of oxygen vacancies and surface unsaturated metal cation.Hence, in comparison to Cu/CeO 2 and other metal cation-doped samples, Cu/CeMnO x demonstrated superior performance [45].When MnO x was doped into CuCeO x /γ-Al 2 O 3 , it significantly enhanced the catalytic activity at low temperatures, with maximum conversions of NO x and CO of 98% and 96%, respectively, at 200 • C. The robust interaction among Mn, Cu, and Ce oxides results in higher dispersibility and more coordination-unsaturated ions.In addition, this interaction enhances the formation of oxygen vacancies and acid sites, thereby enhancing the adsorption capacity for CO and NO x [36].The rise in the Fe 2+ /Fe 3+ ratio subsequent to the addition of an accelerant to the La-Fe/AC catalyst could be attributed to the supplementary synergistic impact of M (Mn and Ce) and Fe via the redox equilibrium (M 3+ + Fe 3+ ↔ M 4+ + Fe 2+ ).This phenomenon improves the redox cycle, promotes the formation of SSOVs, aids in the decomposition of NO, and accelerates the CO-SCR process.The presence of O 2 promotes the formation of C(O) complexes and enhances the activation of metal sites.The NO conversion of a Mn@La3-Fe1/AC catalyst reached 93.8% at 400 • C with 10% O 2 [37].Our group reported that the hierarchically interconnected porous (HIP) Mn x Co 3−x O 4 spinel, synthesized through a citric acid-assisted sol-gel method, demonstrated high catalytic efficiency for CO-SCR.The NO removal efficiency of the synthesized Mn 0.3 Co 2.7 O 4 was 87% at 180 • C, exhibiting a wide active temperature range (100-400 • C).Its superior activity can be attributed to the following factors: (i) the high oxidation states of Mn 3+ , Mn 4+ , and Co 3+ in the Co-O-Mn structure facilitates an effective redox cycle, creating adsorption sites for NO and CO (Equations ( 9) and ( 10)), and (ii) the HIP structure significantly enhances gas diffusion (Figure 3a) [46].exhibited an NO conversion of 82%, a CO conversion of 100%, and an N2 selectivity of 78% [38].In the Mn2O3-modified CuO/γ-Al2O3 catalyst prepared by Wan et al. [47], it was found that Mn2O3 can promote the dispersion of CuO on γ-Al2O3, forming a monomolecular layer.This enhances the reducibility of CuO (Cu 2+ + Mn 3+ ↔ Cu + + Mn 4+ ), thus boosting the adsorption capacity of CO on the CuO/γ-Al2O3 catalyst (Equation ( 9)).As a result, both NO conversion and N2 selectivity are enhanced.

Synthesis Methods
Various synthesis methods yield distinct structures, influencing factors like the particle size, the creation of specific morphologies, the generation of OVs, or the regulation of valence states of the active center.These factors, in turn, impact the catalytic activity [39].
In order to implement the catalyst in industrial settings, it is imperative to devise a synthesis approach that is easy to execute, amenable to upscaling, and cost-effective.Moreover, the catalyst's electronic structure can be modified through Mn doping to enhance its catalytic performance.Doping Mn into CeO 2 @Co 3 O 4 generates additional active sites and modulates the electronic structure of the catalyst (Figure 3b).When the O 2 concentration was 5 vol% and the temperature was 200 • C, the Mn-CeO 2 @Co 3 O 4 catalyst exhibited an NO conversion of 82%, a CO conversion of 100%, and an N 2 selectivity of 78% [38].In the Mn 2 O 3 -modified CuO/γ-Al 2 O 3 catalyst prepared by Wan et al. [47], it was found that Mn 2 O 3 can promote the dispersion of CuO on γ-Al 2 O 3 , forming a monomolecular layer.This enhances the reducibility of CuO (Cu 2+ + Mn 3+ ↔ Cu + + Mn 4+ ), thus boosting the adsorption capacity of CO on the CuO/γ-Al 2 O 3 catalyst (Equation ( 9)).As a result, both NO conversion and N 2 selectivity are enhanced.

Synthesis Methods
Various synthesis methods yield distinct structures, influencing factors like the particle size, the creation of specific morphologies, the generation of OVs, or the regulation of valence states of the active center.These factors, in turn, impact the catalytic activity [39].
In order to implement the catalyst in industrial settings, it is imperative to devise a synthesis approach that is easy to execute, amenable to upscaling, and cost-effective.Currently, alternative preparation methods distinct from conventional synthesis techniques have been reported in the literature.For instance, smooth spherical Fe x Mn y O catalysts with varying Fe contents were synthesized using the solvothermal method, and their CO-SCR activity was tested.The results indicated that the introduction of FeO x enhances the Fe 2+ + Mn 4+ ↔ Fe 3+ + Mn 3+ conversion, thereby facilitating oxygen cavitation and the production of reactive oxygen species [40].Also, a series of spherical Mn x -Fe 2 O 3 /C catalysts were synthesized using MnFe-MOF-74 as a precursor through the sacrif-membrane plate method, and they were used in the CO-SCR process.The characterization results revealed the formation of a redox reaction involving Fe 2+ and Mn 4+ ions at the Fe-O-Mn site, leading to the generation of Fe 3+ + Mn 3+ ions.This reaction significantly enhanced the adsorption and activation of NO molecules.DFT calculations revealed that the incorporation of Mn substantially enhances the local electron density at Fe-Mn sites, resulting not only in the formation of robust C-Fe/Mn and N-Fe/Mn bonds, but also in the effective weakening of N-O bonds [48].While these approaches enhance the catalytic performance of Mn-based catalysts in the CO-SCR reaction, they are associated with complex operational procedures and high expenses.
Several researchers have employed coprecipitation as a method to synthesize La-Cu-Mn-O catalysts with varying La concentrations.The incorporation of La has been shown to facilitate the diminution of the particle size of copper-manganese oxides, inhibit its agglomeration, improve the reducibility of the catalyst, and stimulate the augmentation of the exposed active sites and the reactivity between the reactants (NO and CO) [49].Although the co-precipitation method is straightforward, the utilization of La is costprohibitive, rendering it impractical for industrial applications.
Due to the complexity or high cost associated with the current synthesis methods, there is a need to establish a straightforward and effective synthesis approach that achieves catalytic activity for the preparation of the catalyst.

Reaction Mechanism
Generally, there exist three mechanisms for the CO-SCR reaction.The mechanisms are categorized into the Langmuir-Hinshelwood (L-H) mechanism, the Eley-Rideal (E-R) mechanism, and the Mars-Van Krevelen (MvK) mechanism.The L-H mechanism involves the adsorption of two reaction gas molecules on the catalyst's surface, where the reaction proceeds through the interactions between the adsorbed species [50][51][52].In contrast, the E-R mechanism entails the adsorption of a reactant gas on the catalyst's surface, with the reaction occurring through molecular interactions between the adsorbed species and the gasphase molecules [53][54][55].The MvK mechanism primarily pertains to reactions that involve active lattice oxygen [50,56,57].For Mn-based catalysts, many studies have identified the prevalent mechanisms governing the CO-SCR reaction as the L-H mechanism [31,33,38,58] and the E-R mechanism [23][24][25]49].To date, there is no literature reporting the involvement of the MvK mechanism in the CO-SCR reaction, where N 2 O acts as the principal reaction intermediate in this process.
The E-R mechanism is likely to occur when Mn predominantly exists in a low valence state in Mn-based catalysts.For the E-R mechanism, the formation of N 2 O can be described as follows: NO initially adsorbs on the catalyst to produce NO* (Equation ( 5)).Subsequently, NO* dissociates into N* and O* (Equation ( 6)), and N* reacts with another adsorbed NO to form N 2 O* (Equation ( 7)).Finally, the adsorbed N 2 O is reduced to N 2 and CO 2 by gaseous CO (Equation (8), where * denotes the adsorbed state).
The E-R mechanism is more likely to occur at low temperatures.Shan et al. [24] synthesized α-MnO 2 nanorods using a colloidal method and studied their catalytic behavior in the reduction of NO by CO.They found that the reduction of NO by CO on α-MnO 2 proceeds by initially generating N 2 O as an intermediate, which then dissociates into N 2 (Figure 4a).During the catalytic process, Mn 2+ is easily oxidized to Mn 3+ , which can subsequently adsorb and dissociate NO, thus forming the intermediate N 2 O.In situ investigations revealed the reconstruction of the α-MnO 2 surface, leading to the formation of a new active surface phase consisting of Mn 3+ and OVs serving as active sites.Among the various TiO 2 -supported transition metal-based catalysts, MnO x /TiO 2 shows the most promising CO-SCR activity at low temperatures.As illustrated in Figure 4b, the presence of NO on MnO x /TiO 2 inhibits the oxidation of CO to CO 2 .Instead, in situ Fourier transform infrared (FT-IR) studies found that the dissociation of NO takes place at the reduced Mn site, resulting in the generation of N 2 O. N 2 O acts as an intermediate and then reacts with CO to form N 2 [25].
When Mn in the Mn-based catalysts is present in a high valence state with numerous OVs, it leads to an increased adsorption capacity for NO and CO.This condition also facilitates the occurrence of the L-H mechanism.In the L-H mechanism, the formation process of N 2 O occurs as follows: NO and CO are adsorbed on the catalyst to form NO* and CO* (Equations ( 9) and ( 10)); then, NO* dissociated into N* and O* (Equation (11)).Subsequently, N* combines with another adsorbed NO to produce N 2 O* (Equation ( 12)), which is further reduced by adsorbed CO to yield N 2 and CO 2 (Equation ( 13 The presence of abundant variable valence states plays a crucial role in the catalytic activity and selectivity of Mn-based catalysts in the presence of oxygen.Mn 3+ ions exhibit efficient adsorption and dissociation of NO.Lewis acid sites are very important for the CO-SCR reaction of MnOx-based catalysts by enhancing the adsorption capability of CO molecules [41].For the V0-OMS-2 catalyst, the reaction mechanism can be described as follows: Mn 3+ (s)+ NO (g) → N2 (g) + Mn 4+ (s) + O2 (g) (14) Mn 4+ (s) + O2 (g) + CO (g) → CO2 (g) + Mn 3+ (s) (15) In summary, different Mn-based catalysts show different reaction mechanisms.When an N2O intermediate is generated, the selectivity of the CO-SCR reaction may be compromised if N2O, functioning as an intermediate, is unable to undergo further reduction by CO to form N2 and CO2.Therefore, it is necessary to modify the composition, structure, oxidation state, and acid site of the Mn-based catalysts to facilitate the conversion of N2O into N2.The diverse valence states of Mn play a crucial role in determining the catalytic activity and selectivity of the catalyst during the CO-SCR reaction.By adjusting the catalyst to achieve an appropriate oxidation state, it demonstrates improved resistance to oxygen.4c).The predominance of the bimolecular coupling mechanism in the CO-SCR reaction on the LaMnO 3 catalyst is attributed to the higher activation energy barrier for NO decomposition to N 2 O on the LaMnO 3 surface compared to that of the N 2 O 2 * formation [58].In the case of the NiMn-MOF-74 catalyst, the in situ FT-IR results showed that NO molecules preferentially adsorbed on the catalyst surface and dissociated to N* and O* under the action of surface SSOVs.This process weakens the competitive adsorption of NO and CO on the catalyst.The dissociated NO undergoes subsequent reactions with CO to form N 2 O intermediates, which are ultimately transformed into N 2 [31].
The presence of abundant variable valence states plays a crucial role in the catalytic activity and selectivity of Mn-based catalysts in the presence of oxygen.Mn 3+ ions exhibit efficient adsorption and dissociation of NO.Lewis acid sites are very important for the CO-SCR reaction of MnO x -based catalysts by enhancing the adsorption capability of CO molecules [41].For the V 0 -OMS-2 catalyst, the reaction mechanism can be described as follows: Mn 3+ (s)+ NO (g) → N 2 (g) + Mn 4+ (s) + O 2 (g) ( 14) In summary, different Mn-based catalysts show different reaction mechanisms.When an N 2 O intermediate is generated, the selectivity of the CO-SCR reaction may be compromised if N 2 O, functioning as an intermediate, is unable to undergo further reduction by CO to form N 2 and CO 2 .Therefore, it is necessary to modify the composition, structure, oxidation state, and acid site of the Mn-based catalysts to facilitate the conversion of N 2 O into N 2 .The diverse valence states of Mn play a crucial role in determining the catalytic activity and selectivity of the catalyst during the CO-SCR reaction.By adjusting the catalyst to achieve an appropriate oxidation state, it demonstrates improved resistance to oxygen.

Challenges
The concentrations of NO and CO in various industrial flue gases are different.Typically, the concentration of NO ranges between 500 ppm and 1600 ppm, whereas the CO concentration falls within the range of 1 vol% to 1.25 vol%.Besides CO and NO gases, there are also significant amounts of O 2 (≥5 vol%) and H 2 O (≥10 vol%), as well as a minor quantity of SO 2 (≤50 ppm), which have a discernible impact on the catalytic activity of the catalyst in the CO-SCR reaction.For instance, an excess of oxygen may lead to favoring of the oxidation of CO and NO.SO 2 tends to form sulfates on the catalyst's surface, which can obstruct the surface active sites.H 2 O can potentially compete with CO or NO for adsorption sites.On the other hand, the high catalytic activity required at low temperatures poses another challenge for the reaction.While the flue gas emission temperature in the power industry typically exceeds 350 • C, most industrial furnaces operate at lower temperatures, with flue gas emissions ranging between 200 and 230 • C. The flue gas temperature produced by industrial processes like steel sintering is typically below 150 • C. Hence, the catalytic efficiency of the catalyst in the CO-SCR reaction at low temperatures is of significant importance.Although the precious metal catalysts described in the literature exhibit commendable catalytic activity under oxygen-rich conditions, their performance notably diminishes at lower temperatures.In contrast, low-cost Mn-based catalysts demonstrate significant potential in catalyzing the CO-SCR reaction at low temperatures (Table 2).Further investigation in this field is anticipated to result in the creation of catalysts capable of functioning at reduced reaction temperatures in oxygen-rich environments.

O 2
Due to the abundance of O 2 , CO and NO may exhibit a preference for reacting with O 2 .This can lead to enhanced CO consumption while preventing the reduction of NO to N 2 .In particular, the hindrance of the CO-SCR reaction by an excess of O 2 (typically exceeding 5 vol.%) poses a significant challenge for the practical application of CO-SCR.On a Ni-Mn 2 catalyst, the presence of O 2 could inhibit the deNO x process.Under the condition of a high O 2 concentration, part of NO could be converted to NO 2 .Especially at high temperatures, the amount of NO 2 produced was greater than that of N 2 , leading to the poor N 2 selectivity of the Ni-Mn 2 catalyst [66].In the presence of 6% O 2 , the CO conversion of the Mn-Ce-Fe-Co/TiO 2 catalyst reached 92%.Nevertheless, the NO conversion by Mn-Ce-Fe-Co/TiO 2 decreased as the O 2 content increased.This could be elucidated by the observation that in the presence of O 2 , a portion of NO was oxidized to NO 2 instead of being converted to N 2 , consequently leading to reduced NO x conversion [34].At an O 2 concentration of 8%, the NO conversion by a 5Fe-10Mn/AC catalyst was high in the reaction's early stages.However, its stability was notably poor, decreasing to 42% at 230 • C after only 7 min [67].There are abundant variable valence states in Mn-based catalysts, and Mn 3+ has been shown to be able to effectively adsorb and activate NO and strengthen N-O bond cleavage.Thus, regulating the valence state of Mn in Mn-based catalysts may be a solution to their poor catalytic activity and selectivity under oxygen-rich conditions.
Currently, it has been established that the dispersion of nanoparticles into single atoms (SAs) and the manipulation of the charge state of SAs can enhance the resistance of catalysts to O 2 in the CO-SCR reaction to a certain degree [9,10,68].The underlying principle is that SACs possess an adaptable coordination structure, offering a favorable opportunity to modulate their catalytic efficacy [69].Theoretically, Mn displays a wide range of variable valence states, and by adjusting the coordination environment of Mn SAs, distinct valence states of Mn can be achieved.The distinctive valence of Mn affects its adsorption capacity for reaction gases differently than that of O 2 , potentially improving the O 2 resistance of Mn-based catalysts.While the utilization of Mn SAs in CO-SCR reactions has not been documented in the literature, studies have shown that Mn 4+ in MnO 2 exhibits a higher affinity for NO compared to O 2 , leading to a decrease in O 2 absorption [70].Theoretically, the dispersion of Mn species in Mn-based catalysts into single atoms and the exposure of more Mn 4+ species may enhance their O 2 resistance.

H 2 O
In actual flue gas, a significant quantity of water is typically present.The presence of water will compete with NO for adsorption sites, leading to a reduction in NO conversion and N 2 selectivity [71].However, the impact of water on the catalyst is reversible, and the catalyst can regain certain performance levels after the water is removed.For a CuMnO 2 nanosheet catalyst with the synergistic effect, it could achieve 100% NO conversion to N 2 at 310 • C.However, when 5% water vapor was introduced, the N 2 selectivity suddenly dropped from 90.2% to 10.1%, although the NO conversion increased from 89.2% to 100% (Figure 5a) [72].When MnO x was doped into CuO/CeO 2 , the Cu/CeMn-10:1 catalyst (the molar ratio of CuO/CeO 2 to MnO x was 10:1) showed the best catalytic activity in the NO + CO reaction, with 100% NO conversion and 83% N 2 selectivity at 220 • C in the absence of oxygen.After the introduction of 10 vol% H 2 O into the reaction system, both the NO conversion and N 2 selectivity decreased, especially the N 2 selectivity [73].The Mn-CeO 2 @Co 3 O 4 catalyst achieved 82% NO conversion and 78% N 2 selectivity at 200 • C with 5% O 2 present.When 10 vol.%H 2 O was introduced, the NO conversion decreased to 65% (Figure 5b).The main reason for this decline was identified as the competitive adsorption of NO and H 2 O [38].

SO2
Small quantities of SO2 are present in flue gas.In the case of Mn-based catalysts, their ability to tolerate sulfur is comparatively low.This is due to the formation of sulfides by SO2 on the catalyst surface.In comparison to undoped OMS-2, an Sb-doped Sb0.2-OMS-2 catalyst exhibited an increased proportion of OVs on the catalyst surface and enriched Lewis acid sites, thus facilitating the conversion of NO.The NO conversion improved from 60% in OMS-2 to 83% in Sb-doped Sb0.2-OMS-2 at 150 °C and from 80% to 95% at 300 °C.When SO2 was introduced, the NO conversion of OMS-2 plummeted.While the introduction of Sb led to some improvement in sulfur resistance, the conversion of NO also showed a continuous decline, and the sulfur resistance was still limited (Figure 5c), which required further improvement [72].Moreover, incorporating the metal Ho into OMS-2 also resulted in an enhancement in catalytic activity.Nonetheless, its resistance to sulfur was comparable to that of Sb0.2-OMS-2 (Figure 5d) [73].Likewise, the presence of SO2 could have a substantial impact on the catalytic performance of Mn-Ce-Fe-Co/TiO2 catalysts.When 50 ppm of SO2 was introduced, the NO conversion of Mn-Ce-Fe-Co/TiO2 experienced a notable decrease, with the lowest NO conversion being only 20% [34].

SO 2
Small quantities of SO 2 are present in flue gas.In the case of Mn-based catalysts, their ability to tolerate sulfur is comparatively low.This is due to the formation of sulfides by SO 2 on the catalyst surface.In comparison to undoped OMS-2, an Sb-doped Sb 0.2 -OMS-2 catalyst exhibited an increased proportion of OVs on the catalyst surface and enriched Lewis acid sites, thus facilitating the conversion of NO.The NO conversion improved from 60% in OMS-2 to 83% in Sb-doped Sb 0.2 -OMS-2 at 150 • C and from 80% to 95% at 300 • C. When SO 2 was introduced, the NO conversion of OMS-2 plummeted.While the introduction of Sb led to some improvement in sulfur resistance, the conversion of NO also showed a continuous decline, and the sulfur resistance was still limited (Figure 5c), which required further improvement [42].Moreover, incorporating the metal Ho into OMS-2 also resulted in an enhancement in catalytic activity.Nonetheless, its resistance to sulfur was comparable to that of Sb0.2-OMS-2 (Figure 5d) [43].Likewise, the presence of SO 2 could have a substantial impact on the catalytic performance of Mn-Ce-Fe-Co/TiO 2 catalysts.When 50 ppm of SO 2 was introduced, the NO conversion of Mn-Ce-Fe-Co/TiO 2 experienced a notable decrease, with the lowest NO conversion being only 20% [34].Indeed, as mentioned above, the addition of metal can enhance sulfur resistance to some degree; however, there remains significant potential for further enhancement.
By coordinating with a low-electronegativity atom, the central atom acquires a negative charge, thereby enhancing the resistance to O 2 and SO 2 of the catalyst to some degree.This is based on the fact that the negatively charged single atom can boost the adsorption of NO while reducing the adsorption of O 2 .Additionally, the adsorption capability of SO 2 is also diminished [9,69].

Conclusions and Perspectives
In recent years, CO-SCR technology has emerged as a prominent research focus in the denitrification field.Currently, the crucial aspect for the industrial implementation of CO-SCR technology lies in the development of cost-effective and high-performance catalysts.Mn-based catalysts offer advantages such as affordability and high activity at low temperatures.However, their development is hindered primarily by poor N 2 selectivity and stability in the presence of oxygen, as well as their limited water and sulfur resistance.In order to promote their industrialized application, it is crucial to tackle these issues.Subsequent research endeavors may be pursued with a focus on the following aspects: (i) Presently, numerous studies have been published on SACs of precious metals, such as those based on Ir, Pd, and Pt.The dispersion of transition metals into single atoms to form effective catalysts has also been documented.For instance, a single-atom Mn catalyst has been utilized in various reactions such as oxygen reduction reactions [74] and the photocatalytic removal of water-based organics [75].These reports provide support for the feasibility of synthesizing Mn SACs.However, currently, there is a lack of literature regarding the use of Mn SACs for the CO-SCR reaction.Previously, under the guidance of DFT, our research group successfully developed a catalyst consisting of Co SAs and CoO x nanoclusters (NCs) with dual active centers co-anchored on the CZO support, which achieved superior catalytic performance at a broad temperature range between 250 and 400 • C and under 5 vol% O 2 conditions [12].Therefore, by leveraging the benefits of the high low-temperature activity of Mn-based catalysts, there is the potential to obtain low-temperature performance.Furthermore, combining the Mn SAs with a support that possesses oxygen resistance may lead to outstanding low-temperature performance under oxygen-rich conditions.
(ii) Our group discovered that modifying the type and quantity of coordination atoms can alter their valence state in SACs.The adsorption behaviors of NO, CO, O 2 , and SO 2 on negatively charged SACs significantly differ from those on positively charged SACs or nanoparticle catalysts [9,10].For Mn-based catalysts, upon dispersing them into single atoms, the coordination type of the Mn atoms is adjusted to induce a negative charge, with the extent of negativity regulated by the quantity of coordinating atoms.By synthesizing negatively charged Mn SACs, it is possible to enhance their antioxidant properties and resistance to sulfur dioxide.
(iii) Various in situ characterization methods, such as in situ electron paramagnetic resonance (EPR), in situ transmission electron microscope (TEM), and in situ X-ray photoelectron spectroscopy (XPS), are frequently employed for defect identification of reaction intermediates and the changes in the structure and oxidation states of catalysts.Mn-based catalysts usually exhibit a significant number of defects, underscoring the critical necessity of accurately determining the defect type and density to gain insights into the underlying reaction mechanisms.Utilizing in situ techniques for observing the changes throughout the reaction process and integrating a variety of in situ characterization methods to thoroughly investigate the CO-SCR reaction process and elucidate the dynamic structure-activity relationship are crucial aspects.Hence, various in situ characterization techniques can be employed to observe reaction intermediates and clarify the reaction pathways for Mn-based catalysts, which is significant for designing more efficient Mn-based catalysts.
(iv) Our group employed solid-state ball milling technology to synthesize Co-based catalysts with excellent oxygen resistance and relatively high activity levels at low tem-peratures [65].The application of catalysts in industry necessitates simple preparation methods, superior catalytic performance, reproducibility, cost efficiency, and scalability for mass production.Hence, the development of straightforward, replicable, scalable, and promising preparation methodologies for Mn-based catalyst are imperative.

Figure 1 .
Figure 1.An overview of Mn-based catalysts.

Catalysts 2024 , 18 Figure 2 .
Figure 2. (a) Schematic illustration of the proposed mechanism for the catalytic oxidation of CO over Cu−Mn catalysts [29]; (b) proposed reaction mechanism for NO reduction by CO over CO−CuMnAl catalyst [30]; (c) NO conversion curves of OMS−2 prepared by Co doping method [32]; (d) schematic illustration of the proposed mechanism for the catalytic CO-SCR over CuxMn3−xO4 catalysts [33].

Figure 2 .
Figure 2. (a) Schematic illustration of the proposed mechanism for the catalytic oxidation of CO over Cu−Mn catalysts [29]; (b) proposed reaction mechanism for NO reduction by CO over CO−CuMnAl catalyst [30]; (c) NO conversion curves of OMS−2 prepared by Co doping method [32]; (d) schematic illustration of the proposed mechanism for the catalytic CO-SCR over Cu x Mn 3−x O 4 catalysts [33].

Figure 4 .
Figure 4. (a) Reaction mechanism on α-MnO 2 nanorod catalyst [24]; (b) reaction mechanism on MnO x /TiO 2 catalyst [25] (c) reaction mechanism on LaMnO 3 catalyst [58].For the LaMnO 3 catalyst, the Mn site on its surface serves as the primary active site for NO reduction.In this case, N 2 O 2 * generated through the NO coupling reaction acts as a crucial intermediate in the CO-SCR process.N 2 O 2 * subsequently reacts with chemisorbed CO to produce CO 2 and N 2 O* intermediates.The reaction path of CO-SCR on the LaMnO 3 surface consists of three sequential steps: 2NO* → N 2 O 2 *, N 2 O 2 * + CO* → N 2 O* + CO 2 , and N 2 O* + CO* → N 2 + CO 2 * (Figure4c).The predominance of the bimolecular coupling mechanism in the CO-SCR reaction on the LaMnO 3 catalyst is attributed to the higher activation energy barrier for NO decomposition to N 2 O on the LaMnO 3 surface compared to that of the N 2 O 2 * formation[58].In the case of the NiMn-MOF-74 catalyst, the in situ FT-IR results showed that NO molecules preferentially adsorbed on the catalyst surface and dissociated to N* and O* under the action of surface SSOVs.This process weakens the competitive adsorption of NO and CO on the catalyst.The dissociated NO undergoes subsequent reactions with CO to form N 2 O intermediates, which are ultimately transformed into N 2[31].

Table 1 .
Comparison of catalytic performances of the Mn-based catalysts in the literature.

Table 2 .
Comparison of catalytic properties of Mn-based catalysts and other transition metal catalysts.