A Review on Selective Catalytic Reduction of NO x by NH 3 over Mn – Based Catalysts at Low Temperatures : Catalysts , Mechanisms , Kinetics and DFT Calculations

It is a major challenge to develop the low–temperature catalysts (LTC, <250 ◦C) with excellent efficiency and stability for selective catalytic reduction (SCR) of NOx by NH3 from stationary sources. Mn-based LTC have been widely investigated due to its various valence states and excellent redox performance, while the poisoning by H2O or/and SO2 is one of the severe weaknesses. This paper reviews the latest research progress on Mn-based catalysts that are expected to break through the resistance, such as modified MnOx–CeO2, multi-metal oxides with special crystal or/and shape structures, modified TiO2 supporter, and novel carbon supporter (ACF, CNTs, GE), etc. The SCR mechanisms and promoting effects of redox cycle are described in detail. The reaction kinetics will be a benefit for the quantitative study of Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) mechanisms. This paper also introduces the applications of quantum-chemical calculation using density functional theory to analyze the physic-chemical properties, explicates the reaction and poisoning mechanisms, and directs the design of functional catalysts on molecule levels. The intensive study of H2O/SO2 inhibition effects is by means of the combination analysis of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT), and the amplification of tolerance mechanisms will be helpful to design an excellent SCR catalyst.


NO x Emissions and Legislations
According to BP's Energy Outlook (2016), fossil fuels will continue to remain the dominant source of energy powering the world's economy, supplying 60% of the energy increase out to 2035.It will be increased with oil (+63%), natural gas (+193%) and coal (+5%) accounting for 53% of the growth in demand.Unfortunately, the combustion of fossil fuels inevitably leads to the production of various air pollutants, such as sulfur dioxide, nitrogen oxide (NO x ), particulate matter, heavy metal, volatile organic compounds, etc. [1].
NO x is a generic term for mono-nitrogen oxides, namely NO and NO 2 , which are produced during combustion at high temperatures (above 1350 • C).NO x (x = 1, 2) emitted to air are largely responsible for the ozone decline in middle to high latitudes from spring to fall, and for the acid rain perturbing the ecosystems and the cause of biological death of lakes and rivers.Peroxyacetylene nitrates (PAN) can also be formed from nitric oxide and contribute significantly to global photo-oxidation pollution [1,2].In addition, NO x species are also harmful to the human body, which can diffuse through the alveolar cells and the adjacent capillary vessels of the lungs and damage the alveolar structures and their functions throughout the lungs, provoking both lung infections and respiratory allergies like bronchitis, pneumonia, etc. [1,3].
From 1970 to 2004, America was the largest country for contributing NO x emissions, followed by Organization for Economic Co-operation and Development (OECD)-Europe regions (see Figure 1A).The NO x emissions of both countries were decreased by year since the 1990s, which benefited from the legislations and policies of NO x limitation and technologies of reducing NO x .For example, the maximum allowable NO x emission rates of American are set at 0.45 b/mmBtu for tangentially fired boilers, and 0.50 b/mmBtu for dry bottom wall-fired boilers (The Clean Air Act, Sec.407, USA, 2004).For China, the NO x emissions were increased by years at that time, which resulted in China becoming the largest contributor in 2008.Since 2011, the NO x emission of China was also decreased year by year (in Figure 1B), which was attributed to the latest emission standard (GB 13223-2011, China, 2011), in which the emission limits are 100 mg/m 3 for new case of coal-fired power plants, 100 mg/m 3 for natural gas-fired boilers and 50 mg/m 3 for natural gas-fired turbines.The legislations and regulations policies in China for limiting NO x emissions are getting to be more stringent, such as limiting the ultra-low emission to 50 mg/m 3 for thermal-power boilers and the special emission to 150 mg/m 3 for sinter, pelletizing and coking furnaces in the following 13th Five Year Plan (2016-2020).
Catalysts 2017, 7,199 2 of 31 NOx is a generic term for mono-nitrogen oxides, namely NO and NO2, which are produced during combustion at high temperatures (above 1350 °C).NOx (x = 1, 2) emitted to air are largely responsible for the ozone decline in middle to high latitudes from spring to fall, and for the acid rain perturbing the ecosystems and the cause of biological death of lakes and rivers.Peroxyacetylene nitrates (PAN) can also be formed from nitric oxide and contribute significantly to global photooxidation pollution [1,2].In addition, NOx species are also harmful to the human body, which can diffuse through the alveolar cells and the adjacent capillary vessels of the lungs and damage the alveolar structures and their functions throughout the lungs, provoking both lung infections and respiratory allergies like bronchitis, pneumonia, etc. [1,3].
From 1970 to 2004, America was the largest country for contributing NOx emissions, followed by Organization for Economic Co-operation and Development (OECD)-Europe regions (see Figure 1A).The NOx emissions of both countries were decreased by year since the 1990s, which benefited from the legislations and policies of NOx limitation and technologies of reducing NOx.For example, the maximum allowable NOx emission rates of American are set at 0.45 b/mmBtu for tangentially fired boilers, and 0.50 b/mmBtu for dry bottom wall-fired boilers (The Clean Air Act, Sec.407, USA, 2004).For China, the NOx emissions were increased by years at that time, which resulted in China becoming the largest contributor in 2008.Since 2011, the NOx emission of China was also decreased year by year (in Figure 1B), which was attributed to the latest emission standard (GB 13223-2011, China, 2011), in which the emission limits are 100 mg/m 3 for new case of coal-fired power plants, 100 mg/m 3 for natural gas-fired boilers and 50 mg/m 3 for natural gas-fired turbines.The legislations and regulations policies in China for limiting NOx emissions are getting to be more stringent, such as limiting the ultra-low emission to 50 mg/m 3 for thermal-power boilers and the special emission to 150 mg/m 3 for sinter, pelletizing and coking furnaces in the following 13th Five Year Plan (2016-2020).

NOx Abatement and Demand of LT-SCR Technique
As mentioned above, there is a need to control NOx emissions due to the adverse environmental and human health effects.The NOx emissions are mainly from stationary source (combustion, industry and other processes), transportation and miscellaneous source.To reduce NOx, many control technologies, including combustion control and flue gas denitrification, had been utilized in thermal power plants and industry boilers.Combustion controls are source-control approaches for reducing NOx generation by altering or modifying the combustion conditions during the combustion

NO x Abatement and Demand of LT-SCR Technique
As mentioned above, there is a need to control NO x emissions due to the adverse environmental and human health effects.The NO x emissions are mainly from stationary source (combustion, industry and other processes), transportation and miscellaneous source.To reduce NO x , many control technologies, including combustion control and flue gas denitrification, had been utilized in thermal power plants and industry boilers.Combustion controls are source-control approaches for reducing NO x generation by altering or modifying the combustion conditions during the combustion process, such as low NO x burners, fuel re-burning and flue gas recirculation [4].In spite of this, the NO x emissions are still too high to meet the emission limits.Selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), and hybrid SNCR-SCR technologies are the three major post-combustion processes for NO x abatement [5,6].Among them, selective catalytic reduction is a method of converting nitrogen oxides, also referred to as NO x , with the aid of a catalyst into diatomic nitrogen (N 2 ) and water (H 2 O).A gaseous reductant, typically anhydrous ammonia, aqueous ammonia or urea, is added to a stream of flue or exhaust gas and adsorbed onto a catalyst [7].The NH 3 -SCR technique has been widely used for the purification of NO x from industrial stationary sources due to its outstanding efficiency and strong stability, which can achieve over 90% NO x conversion [8].The NH 3 -SCR method contains the following reactions: [9][10][11].In recent years, new De-NO x technologies, including non-thermal plasma (NTP) [12,13] and pre-oxidation combined absorption [14,15], have been studied in the theoretical stage facing the problems of by-product and energy consumption for NTP, quantitative oxidation and great mass-transfer for pre-oxidation combined absorption, etc.
An overview of the historical development of NH 3 -SCR technology and catalysts is available by Smirniotis et al. [3] SCR of NO x using ammonia was patented in the United States by the Englehard Corporation in 1957.It is well known that the catalyst is the key to the SCR technique.The early SCR catalysts were comprised of platinum or platinum group (noble) metals, such as Co, Ni, Fe and Cr oxides, which were inadequate for the need of a high-temperature range in which explosive ammonium nitrate forms.In the 1960s, the advancements of SCR technology were carried forward in Japan and the United States on the inexpensive and highly durable V 2 O 5 /TiO 2 catalyst, which demonstrated adequate efficacy at medium temperatures.However, vanadium catalysts have a lack of high thermal durability and a high catalyzing potential to oxidize SO 2 into SO 3 , which can be extremely destructive due to its acidic properties.To solve the above shortcomings, mixed oxides of V 2 O 5 and WO x or MoO x supported on TiO 2 were commonly used as the addition of WO 3 and MoO 3 could improve the thermal stability, durability and hinder the oxidation of SO 2 to SO 3 .
At present, the V 2 O 5 -WO 3 (MoO 3 )/TiO 2 catalyst has been widely applied in the industry due to the excellent NO x removal efficiency at a high temperature (300-400 • C) [16,17].Traditional NH 3 -SCR units are operated downstream before the particle removal units.Although the soot-blowers are adopted to clean away the dust from the surface, the catalysts will be inevitably poisoned and deactivated by the toxic material in the dust, such as the alkalis/alkaline-earth metals, phosphorus and heavy metals [16,18].The following focus of air protection is actively promoting the pollution control of flue gas in steel, cement, glass and other industries.However, the traditional SCR technique is not applicable in these plants due to the burdensome operating costs for heating the low-temperature flue gas (<250 • C).Therefore, it has become a hot topic to explore the development of the catalysts with excellent efficiency and strong stability that can be placed in low-temperature locations behind dust removal and desulfurizer units [4].

Catalysts for LT-SCR
In recent years, the low-temperature catalysts, including precious metals (Pt, Pd, Rh, etc.) [19,20] and transition metals (Mn, Fe, Ni, Cr, V, Co, Cu, Ce, etc.) [21][22][23][24][25][26][27][28][29], have been widely researched.Precious metal catalysts are usually applied to reducing NO x from moving sources but are limited in stationary sources due to the shortcomings of high costs, narrow working temperature window, oxygen inhibition effect and sensitivity to SO 2 or other gases.Research on the LT-SCR catalysts in stationary sources mainly focus on the transition metal oxides.Table 1 displays the catalytic performance of mixed metal oxides for low-temperature SCR of NO by NH 3 .It can be found that Mn-based catalysts have a superior property for SCR at low temperatures that have become the major research objects.It has been accepted that the variable valence states and excellent redox ability of Mn-based catalysts are favorable to the low-temperature SCR activity [30], which are also significantly affected by the crystal structure, crystallinity, specific surface area, oxidation state, active oxygen, active sites and acidity on the surface, etc. [31][32][33][34][35][36][37][38][39][40][41][42].Mn-FeO x Co-precipitation (500 MnO 2 /TiO 2 Impregnating (400 Mn/ZSM-5 Ion-exchange (300 Table 2 gives a comparison of industrial V 2 O 5 -WO 3 /TiO 2 , novel low-temperature V 2 O 5 -WO 3 /TiO 2 , best Mn-based and zeolite catalysts, including the parameters of operation temperature, SCR activity, degradation performance, and ammonia slippage.It is noteworthy that the V 2 O 5 -WO 3 /TiO 2 catalyst has been applied successfully in commercial plants with the NO x removal efficiency above 90%, including the novel vanadium-based catalyst for the removal of NO x in the low-temperature flue gas (200-300 • C) from coking industries.However, the deposition and blockage effects by semi coking tar and ammonium bisulfate have become the problems associated with these industrial low-temperature SCR catalysts for the post-combustion removal of NO x during the cold start and low idle conditions.In addition, the V 2 O 5 -WO 3 /TiO 2 catalyst is toxic for the environment and humans and will eventually become hazardous wastes after devitalization.Although the degradation performance is inferior to vanadium-based catalysts, the SCR activity of Mn-based catalysts is quite high such that the NO x removal efficiency at low-temperature regions of 150 to 300 • C is almost 100%, which is promising for application on NO x purification in the low-temperature flue gases with low-concentration H 2 O and SO 2 such as the flue gases from coking after desulfurization, daily-use glass industries and gas-fired boilers.Li et al. [52] reviewed metal oxide catalysts, (Fe/Cu) zeolite and reaction mechanism of NH 3 -SCR and pointed out that the resistance of H 2 O and SO 2 on Mn-based metal oxide is a big challenge for its application at low temperatures.Fu et al. [4] focused on the reaction mechanism and kinetics of supported SCR catalysts at 100-300 • C including Mn-based catalysts reported before 2013, and believed that the exploration of surface chemical reaction process may be a direction to develop low-temperature supported catalysts for removing NO x .Shan et al. [53] proposed that modified V 2 O 5 based catalysts have attracted much attention for applications on removing NO x from stationary sources while using Cu-containing zeolites for diesel vehicles.
Over the last four years, some Mn-based catalysts are expected to break through the resistance to H 2 O and SO 2 , such as the modified MnO x -CeO 2 , multi-metal oxides with special crystal or/and shape structures, modified TiO 2 supporter, and novel carbon supporter (ACF, CNTs, GE), etc. Fortunately, the application of quantum chemical calculation gives an insight into the molecular and atomic level by density functional theory (DFT) for studying reaction/poisoning mechanisms and designing catalyst with special construction.Therefore, in this article, the research progress on NH 3 -SCR of NO x over Mn-based catalysts at low temperatures is reviewed.We focus on the above catalysts in the latest reports that present a relatively good resistance to H 2 O or/and SO 2 .The SCR mechanisms and reaction kinetics are also discussed.In addition, the typical applications of quantum chemistry are introduced and the further research directions of Mn-based catalysts are also proposed.

Mn-Based Catalysts
Pure MnO x catalysts suffer significantly from the inhibition or/and poisoning effects of H 2 O and SO 2 in real conditions.To enhance the stability and resistance, Mn-based multiple metal oxides and supported catalysts are widely studied.

Composite Oxides
Although the composite oxides including Cu-Mn [54,55], Sn-Mn [56], Fe-Mn [57][58][59], Nb-Mn [60], Li-Mn [61], Eu-Mn [62] and Ni-Mn [63] catalysts showed good SCR performances (activity and N 2 selectivity), the resistance to H 2 O or/and SO 2 were not performed or were not satisfactory at low temperatures below 250 • C. MnO x -CeO 2 catalysts have an excellent SCR performance due to the synergistic mechanism between Mn and Ce that enhance the quantity, acidity of acid sites and the ability to store/release oxygen [22,[64][65][66][67][68][69][70].Ralph T. Yang's team obtained about 95% NO conversion at 120 • C in 4 h on Ce-Mn-O x catalyst prepared by a citric acid method in the presence of 100 ppm SO 2 and 2.5% H 2 O at a gas hourly space velocity (GHSV) of 42,000 h −1 [22].Up to now, this result has not been repeated by others.Mn-Ce-O x prepared by Liu et al. [26] using a surfactant-template reached a NO x conversion above 90% at 150-200 • C in the condition of 5 vol % H 2 O and 50 ppm SO 2 at 64,000 h −1 .However, the effect of high concentration H 2 O and SO 2 on catalytic activity needs to be further researched.Chang et al. [71] found that the SCR activity of Mn(0.4)CeO x catalyst by the co-precipitation method soon decreased to 18% from 86% at 110 • C after introducing 100 ppm SO 2 at 35,000 h −1 .Furthermore, multicomponent MnO x -CeO 2 by Fe [64], Zr [64], Nb [72], Sn [71,73], W [74] or novel catalysts synthesized by template [26,75] were investigated to further improve the SCR performance and resistance to H 2 O or/and SO 2 .Chang et al. [71,73] found the NO conversion of Sn(0.1)Mn(0.4)CeOx could be maintained at ca. 70% at 110 • C after introducing 100 ppm SO 2 and 12 vol % H 2 O.They concluded that the sulfation of Sn-modified MnO x -CeO 2 might form Ce(III) sulfate that could enhance the Lewis acidity and improve NO oxidation to NO 2 during NH 3 -SCR at T > 200 • C. Ma et al. [74] reported that W 0.1 Mn 0.4 Ce 0.5 mixed oxides have an excellent low-temperature activity and N 2 selectivity, which might benefit from the weakened reducibility and increased number of acid sites.They proposed that the decreased SO 2 oxidation activity, as well as the reduced formation of ammonium/manganese sulfates, were the main reasons for the high SO 2 resistance of this catalyst (60% NO x conversion with 60 ppm SO 2 at 150 • C and 300,000 h −1 ).In our previous study [76], we found that the NO x conversion of Ni 1 Mn 4 O 5 and Co 1 Mn 4 O 5 can be 78% in the presence of 150 ppm SO 2 at 175 • C, which benefited from the suppression of surface ammonium bisulfate and metal-sulphation, the trifling impacts of SO 2 on the reaction pathways of bidentate nitrate species and the ER mechanisms in which adsorbed NH 3 -species were activated and reacted with gaseous NO or NO 2 .

Spinel Crystal Catalysts
Recently, the specific structural multi-metal oxides have been investigated to removal NO x .Zhang et al. [77] found that the NO conversion of perovskite-type BiMnO 3 catalyst decreased from 85.5% to 82.7% during the following 9 h after introducing 5% H 2 O and 100 ppm SO 2 and proposed that the more Lewis acid sites and a high concentration of surface oxygen on perovskite-type BiMnO 3 were responsible for its better performance.Perovskite-type NiMnO 3 catalyst prepared by Wan et al. [24] kept the NO conversion at 90% for the following 20 h when 10 vol % H 2 O was added, and remained at 82% for 5 h in the case of 100 ppm SO 2 at 230 • C at 64,000 h −1 .
Chen et al. [78] proposed that the efficient electron transfer between Cr and Mn in the spinel crystal of CrMn 1.5 O 4 was thought to be the reason for a good SCR performance of Cr(0.4)-MnO x catalyst, which lost 15% of its initial activity in 4 h after introducing 100 ppm SO 2 under 120 • C at 30,000 h −1 .The last two years, the Mn x Co 3−x O 4 spinel (i.e., Mn 2 CoO 4 and MnCo 2 O 4 ) catalyst has been widely studied for SCR, which showed an excellent SCR activity, N 2 selectivity and H 2 O/SO 2 durability [79][80][81][82][83]. Qiu et al. [80] successfully synthesized the porous bimetallic Mn x Co 3−x O x catalysts by the nano-casting method, and found that the NO conversion of MnCo 2 O 4 spinel was near 98% with less than 50 ppm N 2 O at 150-300 • C. It also showed a high H 2 O and SO 2 resistance that the NO x conversion was stabilized at 86% in 12 h at 200 • C at 50,000 h −1 after introducing 5% H 2 O and 100 ppm SO 2 .Qiao et al. [79] prepared porous Mn 2 Co 1 O x spinel catalyst by a one-step combustion method, and they found that the NO conversion was almost unaffected by 10 vol % H 2 O and kept at 92% in the presence of 100 ppm SO 2 at 200 • C at 30,000 h −1 .It is accepted that the excellent SCR performance and H 2 O or/and SO 2 resistance were mainly attributed to the orderly mesoporous spinel structures with strong interaction of Mn and Co cations, which can provide a larger surface area, abundant active surface oxygen species, and Lewis acid sites for the adsorption and activation of reaction gases [80,81].
It is worth introducing that Zhang et al. [83] synthesized the hollow porous Mn x Co 3−x O 4 nanocages and nanoparticles as SCR catalysts that derived from Mn 3 [Co(CN) 6 ] 2 •nH 2 O with a structure of nanocube-like metal-organic frameworks via a self-assemble method.Compared to Mn x Co 3−x O 4 nanoparticles (seen Figure 2), Mn x Co 3−x O 4 nanocages presented a much better catalytic activity at low-temperature regions, higher N 2 selectivity, more extensive operating-temperature window and higher H 2 O or/and SO 2 tolerance.The NO conversion of Mn x Co 3−x O 4 nanocages was 99% in the absence of 8 vol %H 2 O and decreased slightly after introducing 200 ppm SO 2 at 175 • C, demonstrating that this catalyst was the best catalyst of H 2 O or SO 2 -resistant, and the inhibitions were reversible.The coexistence of H 2 O and SO 2 induced a 10% decrease in the NO conversion and recovered to 96% after cutting off H 2 O and SO 2 .They concluded that the uniform structure/distribution and strong interaction of Mn and Co provided a larger surface area and more active sites, and enhanced the catalytic cycle.However, for all this, the 5 h of SO 2 and steam poisoning studies are not good enough to understand the tolerance effect.The stability and tolerance to H 2 O and SO 2 in the long-term and large-scale of this catalyst need to be further investigated.
Liu et al. [23] had rationally designed and originally developed three-dimensional hierarchical MnO 2 @NiCo 2 O 4 core-shell nanowire arrays with nickel foam, which took the advantages of the high surface area of Ni-Co nanowires for achieving high loading and dispersion of manganese oxides, as well as the synergistic catalytic effect between Ni, Co and Mn multiple oxides, resulting in an excellent low-temperature catalytic performance and superior H 2 O resistance (promoting effect) in a long-term stable operation.

Specific Structure/Shape Catalysts
Metal-organic frameworks (MOFs) are very attractive in separation and catalysis due to the advantages of high porosity, large surface areas, and versatility of their structures, such as MnCe@MOF [84], MIL-100 (Fe-Mn) [85], MOF-74(Mn, Co) [86], etc. Jiang et al. [86] found that, in their reaction process, NO conversion on Mn-MOF-74 decreased with the introduction of 5 vol % H2O and 100 ppm SO2 and almost recovered when gas was cut off.However, for Co-MOF-74, SO2 has almost no effect on the catalytic activity (65% NO conversion) at 220 °C.
Meng et al. [87] found that the novel fluffy structural Co-Mn-O, Fe-Mn-O, Ni-Mn-O catalysts had high degree crystal splitting, high surface areas, abundant surface acid sites and large active surface oxygen, which were essential for the enhancement of their catalytic activities.Novel 3D flower-like NiMnFe mixed oxides prepared by Li et al. [88] using an in situ hydrothermal method showed an excellent catalytic performance, which was attributed to the synergistic effect between Ni, Mn and Fe species.Chen et al. [89] found that the Fe(0.4)MnOxcatalyst with a crystal phase of Fe3Mn3O8 showed excellent SCR performances (98.8% NOx conversion with 100% N2 selectivity) and good tolerance to 5% H2O and 100 ppm SO2 (a 13% decrease in the NOx conversion in 4 h) at 120 °C at 30,000 h −1 .MnO2 doped Fe2O3 hollow nanofibers were prepared using an electrospinning method by Zhan et al. [90], which reached an NO conversion of 82% in 9 h at 150 °C after introducing 8 vol % H2O and 200 ppm SO2.
It is noteworthy that Li et al. [91] synthesized the Mn2O3-doped Fe2O3 hexagonal microsheets, which obtained a 98% NO conversion at 200 °C at 30,000 h −1 .Meanwhile, good tolerance to 15% H2O or 100 ppm SO2 were obtained, maintaining the NO conversion at 92% and 85% in 100 h, respectively.It suggests that coupled Mn2O3 nanocrystals played a key role and produced a possible redox mechanism in the low-temperature SCR process.

Metal Oxides as the Carrier
It has been reported that the SCR activity was followed in the order: MnOx/TiO2 > MnOx/γ-Al2O3 > MnOx/SiO2 > MnOx/Y-ZrO2 [92].Moreover, the higher surface MnO2 concentration it is, the higher

Specific Structure/Shape Catalysts
Metal-organic frameworks (MOFs) are very attractive in separation and catalysis due to the advantages of high porosity, large surface areas, and versatility of their structures, such as MnCe@MOF [84], MIL-100 (Fe-Mn) [85], MOF-74(Mn, Co) [86], etc. Jiang et al. [86] found that, in their reaction process, NO conversion on Mn-MOF-74 decreased with the introduction of 5 vol % H 2 O and 100 ppm SO 2 and almost recovered when gas was cut off.However, for Co-MOF-74, SO 2 has almost no effect on the catalytic activity (65% NO conversion) at 220 • C.
Meng et al. [87] found that the novel fluffy structural Co-Mn-O, Fe-Mn-O, Ni-Mn-O catalysts had high degree crystal splitting, high surface areas, abundant surface acid sites and large active surface oxygen, which were essential for the enhancement of their catalytic activities.Novel 3D flower-like NiMnFe mixed oxides prepared by Li et al. [88] using an in situ hydrothermal method showed an excellent catalytic performance, which was attributed to the synergistic effect between Ni, Mn and Fe species.Chen et al. [89] found that the Fe (0.4) MnO x catalyst with a crystal phase of Fe 3 Mn 3 O 8 showed excellent SCR performances (98.8% NO x conversion with 100% N 2 selectivity) and good tolerance to 5% H 2 O and 100 ppm SO 2 (a 13% decrease in the NO x conversion in 4 h) at 120 • C at 30,000 h −1 .MnO 2 doped Fe 2 O 3 hollow nanofibers were prepared using an electrospinning method by Zhan et al. [90], which reached an NO conversion of 82% in 9 h at 150 It is noteworthy that Li et al. [91] synthesized the Mn 2 O 3 -doped Fe 2 O 3 hexagonal microsheets, which obtained a 98% NO conversion at 200 • C at 30,000 h −1 .Meanwhile, good tolerance to 15% H 2 O or 100 ppm SO 2 were obtained, maintaining the NO conversion at 92% and 85% in 100 h, respectively.It suggests that coupled Mn 2 O 3 nanocrystals played a key role and produced a possible redox mechanism in the low-temperature SCR process.

Metal Oxides as the Carrier
It has been reported that the SCR activity was followed in the order: MnO [92].Moreover, the higher surface MnO 2 concentration it is, the higher the Lewis acidity will be.The redox properties of TiO 2 -supported MnO x catalyst were important factors in achieving better DeNO x performance at low temperatures [93].Kang et al. [92] concluded the deactivation caused by H 2 O and/or SO 2 was significantly affected by these kind of supports.They found that TiO 2 contributed to the resistance to SO 2 , but the NO x conversion also decreased to 68% in 4 h after introducing 100 ppm SO 2 and 10 vol % H 2 O at 150 • C. Smirniotis et al. [93] compared TiO 2 -supported, Al 2 O 3 -supported and SiO 2 -supported manganese oxide catalysts for the low-temperature SCR reaction and indicated the presence of primarily Lewis acid sites in Mn/TiO 2 .However, there is only Brönsted acidity in Al 2 O 3 -supported and SiO 2 -supported catalysts.Wu et al. [94] reported that SO 2 has an obvious poisoning-effect on SCR activity of Mn/TiO 2 , which was decreased from 93% to 30% in the presence of 100 ppm SO 2 for 6.5 h at 150 • C. TiO 2 has been widely used as the support of Mn-based catalysts due to its stable physical and chemical properties.The SCR activity, selectivity and resistance are greatly affected by the precursors, preparation method, amorphous phase, surface dispersity and acidity, and the interaction between Mn and Ti, etc. [44,[95][96][97][98].
For enhancing the resistance to H 2 O and SO 2 , the composite-oxides, such as Mn confined titania nanotubes (Mn/TNT) [99], M-Mn/TiO 2 (M = Fe [100][101][102][103][104], Co [105][106][107], Ce [105,108], Ni [109], W [110] etc.), Fe-Mn/Ti-Zr [111], MnO x /Fe-Ti [112], MnO x /CeO 2 -TiO 2 [113], Mn-Ce/PG (palygorskite) [114], [118], SO 4 2− -Mn-Co-Ce/Ti-Si [119], etc., were investigated.Among these reports, Zhu et al. [104] reported that Fe 0.3 Ho 0.1 Mn 0.4 /TiO 2 catalyst presented a broad operating temperature window in the range of 60-200   C over the course of 5 h.Qiu et al. [119] found that the SO 4 2− -Mn-Co-Ce/Ti-Si catalyst has an excellent SO 2 durability at low temperatures.However, these catalysts show a relatively good resistance for a short time.Because of its pre-sulfating of dopants for protecting MnO x , the downward trend of NO x conversion makes it difficult to determine whether the activity will be reduced or not as the reaction time goes on.[21,109,[120][121][122][123] found that Mn/TiO 2 anatase (Hombikat), Mn-Ni(0.4)/TiO 2 and Mn-Ce(5.1)/TiO 2 catalysts, with external surface manganese oxide clusters, promoted Mn 4+ , nano-size metal oxide crystallites, and surface labile oxygen species seemed to be the reasons for high de-NO x efficiency at low temperatures.These catalysts also demonstrated impressive catalytic activity by exhibiting stable conversions within hundreds of hours to 7 and 10 vol % H 2 O vapors at 175 and 200 • C. The addition of H 2 O in the feed gas does not seem to create an eternal alteration to the surface manganese species most probably due to the existence of high redox potential pairs of Mn and Ni or Ce in the structure, which represent an efficient active catalyst.Furthermore, Smirniotis et al. [99,122] designed Mn loaded elevated surface texture hydrated titania and MnO x confined interweaved titania nanotubes to improve SCR activity in the temperature regime of 100-300 • C.These studies indicated that the novel titania nanotubes prepared by alkaline hydrothermal treatment could provide enormous surface area and a unique nano-tubular structure, which can be beneficial for the higher dispersion of the active species and greatly promote the deNO x potential of MnO x -based formulations with less pronounced coagulation under the tested reaction temperatures.
Deng et al. [102] investigated MnO x /TiO 2 (anatase) nano-sheets (NS) with a preferentially exposed (001) facet, in which it was possible to enhance the low-temperature SCR activity of the catalysts by tailoring the preferentially exposed facet of TiO 2 .It is worthwhile to note that Wang et al. [110] reported that the W(0.25)-Mn(0.25)-Ti(0.5)catalyst presented an obvious improvement in the sulfur-resistant, obtaining a NO x conversion of 100% from 140 • C to 260 • C in the presence of 100 ppm SO 2 and 10 vol % H 2 O, which may be favorable in practical application.Guo et al. [124] found that the modification of TiO 2 support by W would lead to lower crystallinity, higher reducibility and surface acidity, accompanied by the presence of more chemisorbed oxygen over its surface and resulting in promoting the NH 3 -SCR reaction over the Mn/TiWO x catalyst.

Molecular Sieves as the Carrier
According to the literature, many ion (Fe, Cu, Ce, Mn) exchanged zeolites were reported to be active in NH 3 -SCR reaction.Among them, Fe, Cu, Ce exchanged ZSM-5, SAPO, CHA, SSZ, SBA seem to be particularly interesting and have been extensively studied to remove NO x from moving sources.In the early years, the MnO x /NaY [125], M-Mn/USY (Fe, Ce) [126,127], M-Mn/ZSM-5 (Fe, Ce) [128,129] and Mn/SAPO-34 [130] were reported for SCR.The NO x conversion of Mn-Ce/ZSM-5 by Carja et al. [128] reached above 75% in a broader temperature range (244-549 • C) with a GHSV of 332,000 h −1 and the catalytic activity was stable at 275 • C even in the presence of H 2 O and SO 2 .Lin et al. [127] reported that the NO conversion of a 10%Mn-8%Fe/USY catalyst began to decline rapidly and eventually stabilized at around 60% 180 • C as 10% H 2 O and 100 ppm SO 2 were added to the reaction gas.Qi et al. [126] found that the NO conversion on 14%Ce-6%Mn/USY decreased slowly to 80% from 98% at 150 • C in 4 h.However, regrettably, the NO conversion showed a downward trend when the SO 2 and H 2 O were turning off.Yu et al. [131] prepared the MnO x /SAPO-34 by conventional impregnation and an improved molecularly designed dispersion method for the low-temperature SCR, which obtained the 90% NO x conversion at above 200 • C. Furthermore, the resistance to SO 2 and H 2 O was not ideal.After nearly two years, attention on Mn-based molecular sieves for SCR in stationary sectors is not adequate, which may be due to the relatively high-temperature windows and unsatisfactory resistance to H 2 O or/and SO 2 .

Carbon Materials as the Carrier
Carbon materials including active carbon (AC), active carbon fiber (ACF) and carbon nanotubes (CNTs), and graphene (GE) have been widely utilized in air purification and separation due to its large number of well-distributed microspores, high adsorption speed and high surface area.Catalysts such as MnO x /AC [132][133][134], MnO x /AC/C [135], Mn-Ce/ACH (honeycomb) [136,137], Mn-Ce/ACF [138] and triple-shell hollow structured CeO 2 -MnO x /carbon-spheres [139] on had been reported for SCR, but they were significantly suffered from the poisoning of H 2 O and SO 2 .

Carbon Nanotubes (CNTs)
Reports have shown that CNTs were a good carrier to support active metal oxides for SCR reactions, such as MnO x /MWCNTs (multi-walled) [140,141], Mn-Ce/CNTs [142][143][144][145], Mn-Fe/CNTs [146], MnO 2 -Fe 2 O 3 -CeO 2 -Ce 2 O 3 /CNT [147], Mn-Ce/TiO 2 -CNT [148], etc.More importantly, the CNT-based catalysts presented good capacity for H 2 O and SO 2 resistance.Pourkhalil et al. [141] found that the NO x conversion over MnO x /FMWNTs decreased from 97% to 92% within 6 h after adding 100 ppm SO 2 and 2.5 vol % H 2 O at 200 • C at 30,000 h −1 .Zhang et al. [144] reported that the MnCe/CNTs catalyst has a stable NO conversion of 78% with 100 ppm SO 2 and 4 vol % H 2 O at 300 • C. Furthermore, Zhang et al. [149] demonstrated a core-shell structural de-NO x catalyst with a high SO 2 -tolerance and stability, which was that CNT-supported MnO x and CeO x NPs coated with mesoporous TiO 2 sheaths (mesoTiO 2 @MnCe/CNTs).In this design, the meso-TiO 2 sheaths could prevent the generation of ammonium/manganese sulfate species from blocking the active sites.It was indicated that CNT supported Mn-based catalysts can be investigated to achieve higher stability and better activity in the presence of SO 2 and H 2 O at low temperatures.Fang et al. [150] reported the nano-flaky MnO x on carbon nanotubes (nf-MnO x @CNTs) synthesized by a facile chemical bath deposition route that has an excellent performance in the low-temperature SCR of NO to N 2 with NH 3 and also presented a favorable stability and H 2 O resistance.Cai et al. [151] further synthesized a multi-shell Fe 2 O 3 @MnO x @CNTs.They found that the formation of a multi-shell structure induced the enhancement of the active oxygen species, reducible species as well as adsorption of the reactants, which brought about excellent de-NO x performance.Moreover, the Fe 2 O 3 shell could effectively suppress the formation of the surface sulfate species, leading to the desirable SO 2 resistance to the multi-shell catalyst.Hence, the synthesis protocol could provide guidance for the preparation and elevation of Mn-based catalysts.

Graphene (GE)
Graphene, a new carbon nanomaterial, has the exceptional properties of a large theoretical specific surface area, flexible structure, high electron mobility and excellent conductivity.Thus, it was used as a carrier for SCR to promote the electron transfer between various Mn cations because of the efficient electron gain and loss in GE bringing the improvement in the redox performance [152][153][154][155]. Xiao et al. [152] found that MnO x -CeO 2 /Graphene (0.3 wt %) was an environmentally-benign catalyst for controlling NO x , which exhibited excellent NH 3 -SCR activity and strong resistance against H 2 O and SO 2 .We found that the MnO x /TiO 2 -GE catalyst showed high NH 3 -SCR activity and N 2 selectivity, and good stability during low-temperature SCR at a high GHSV of 67,000 h −1 [153].To improve the SCR activity and resistance, we further investigated a series of Ce-Mn/TiO 2 -GE catalysts by the sol-gel and ultrasonic impregnation methods [154].It was found that the 7 wt % (Ce(0.3)-MnOx )/TiO 2 -0.8 wt % GE catalyst exhibited an improved resistance to H 2 O and SO 2 at 180 • C comparing with the markedly decreased NO x conversion within the first 90 min but maintained at about 80% after adding 10 vol % H 2 O and 200 ppm SO 2 to the system.In particular, hydrophobic groups on the surface of GE were not conducive to H 2 O adsorption.Although this catalyst displays a relatively good stability, H 2 O and SO 2 tolerance in our small-scale lab, the long-term and large-scale sulfur-tolerance properties of the catalyst need to be further investigated.
In recent years, as shown in Table 3, elements doped composite (active component/support) catalyst, which is in order to protect the active component by pre-sulfurization, has been extensively studied and achieved good low-temperature SCR activity, as well as a certain effect on water resistance and sulfur resistance, such as Sn [71,73], Cr [78,156], Ni [24,76], Fe [154,157], Ce [22,64,66], and Co [21,79,83].In addition, many synthesis methods have been widely used in the preparation of non-supported Mn-based catalysts such as co-precipitation method [54,64], citrate method [78], redox method [40,56], hydrolysis method [158], combustion method [68,158], and (inorganic/organic) template synthesis method [26,80,83].These catalysts had achieved good low-temperature SCR activity and a certain effect on water resistance and sulfur resistance.Currently, the catalysts with high H 2 O and SO 2 resistant capacity in the reports are better prepared by unconventional methods, such as homogeneous MnO x -CeO 2 pellets prepared by a one-step hydrolysis process [158], Sn-MnO x -CeO 2 catalyst prepared by ultrasound-assisted co-precipitation method [71,73], CrMn 1.5 O 4 catalyst with spinel structure prepared by citrate method [78], and Mn x Co 3−x O 4 catalyst prepared by the template synthesis method [79,80,83].It can be learned that the synthesis method has a great impact on the structure of the catalyst, while the structure of the catalyst is closely related to the physical and chemical properties of the catalyst.Therefore, regarding the synthesis method and the structure of the catalyst as starting points, it is one of the research directions of low-temperature SCR catalyst-that of preparing special structured Mn-based catalyst with high low-temperature SCR activity and high H 2 O and SO 2 resistant capacity.Catalysts 2017, 7, 199 13 of 32

Adsorption Behavior of Reactants
It is well-known that the adsorption and activation behaviors of gaseous on the surface of catalyst are the important processes in the gas-solid phase catalytic reaction [160].In particular, it is difficult for gaseous NO to be adsorbed without O 2 due to the non-activated sites.The adsorption capacity of NO with O 2 has a qualitative increase, resulting in the final form of nitrite, nitrate, and NO 2 -congaing species.Kijlstra et al. [161] investigated the co-adsorption behaviors of NO with O 2 over MnOx/Al 2 O 3 catalyst using the Fourier Transform infrared spectroscopy (FTIR) technique and summarized the thermal stabilities of various NO x -absorbed species in the following order: nitrosyl < linear nitrites, monodentate nitrites, bridged nitrites < bridged nitrates < bidentate nitrates, as shown in Table 4.
Here, the temperature for thermal stability of absorbed species was defined by the temperature range as one's infrared spectra reduced with in situ increasing of temperature.The author found that, as for NO adsorption, nitrosylic species formed at Mn 3+ sites in the absence of O 2 were very unstable in its presence, which were no active reactive intermediates over the catalyst.In contrast, bidentate nitrates were formed during heating at the expense of less stable nitrites and nitrates.Wherein the nitrites with lower thermal stability can sustainably react with adsorbed NH 3 , it is conducive to low-temperature activity of the catalyst while the nitrates with higher thermal stability will be sustained and steady on the surface of catalysts resulting in the activity decline.Qi et al. [64,66] proposed the NO adsorption over MnO x -CeO 2 catalyst, which is in the form of initiative anionic nitrosyl (NO − ) to nitrite or nitrate species, whose transformation rate on MnO x was much faster than that on CeO 2 .Compared to the nitrate and nitrite, NO 2 was easy to desorb from the catalyst.The nitrite and nitrate transformed into more stable forms (>80 • C) and weakened at the temperature >150 • C.
Ramis et al. [162] found that NH 2(ads) formed by H-abstraction of coordinated NH 3 on Lewis acid sites was the intermediate in both SCR reaction and NH 3 oxidation to N 2 , and -NH (ads) and -N (ads) formed by depth sequential oxidation of NH 2(ads) were the intermediates to N 2 O and NO.Furthermore, Kijlstra et al. [31] found gaseous NH 3 can be absorbed on Brønsted acid sites (Mn-O-H) to NH 4 + ions and Lewis acid sites (Mn 3+ ) to coordinated NH 3 for the subsequent transformation to NH 2 species, and O 2 appeared to be important in assisting in the formation of reactive NO x -absorbed species and for H-abstraction from adsorbed NH 3 .

Reaction Pathways
The adsorption and further H-abstraction of NH 3 (sequential oxidation) are regarded as the key factors in the NH 3 -SCR reaction.Ramis et al. [162] proposed various reaction pathways of NH 3 at NO + O 2 atmosphere, as shown in Figure 3. Here, no more detailed description is needed.In the NH 3 -SCR system, it is accepted that the function of both Lewis and Brönsted acid sites are affected significantly by temperature, and that NH 4 + adsorbed on Brönsted acid sites play a major role in NH 3 -SCR reaction at higher temperature, while coordinated NH 3 linked to Lewis acid sites play a decisive role at a slightly lower temperature [163].
Catalysts 2017, 7, x FOR PEER REVIEW 14 of 31 and O2 appeared to be important in assisting in the formation of reactive NOx-absorbed species and for H-abstraction from adsorbed NH3.

Reaction Pathways
The adsorption and further H-abstraction of NH3 (sequential oxidation) are regarded as the key factors in the NH3-SCR reaction.Ramis et al. [162] proposed various reaction pathways of NH3 at NO + O2 atmosphere, as shown in Figure 3. Here, no more detailed description is needed.In the NH3-SCR system, it is accepted that the function of both Lewis and Brönsted acid sites are affected significantly by temperature, and that NH4 + adsorbed on Brönsted acid sites play a major role in NH3-SCR reaction at higher temperature, while coordinated NH3 linked to Lewis acid sites play a decisive role at a slightly lower temperature [163].It has become a consensus that there are two mechanisms of low-temperature NH3-SCR reaction [163].One is the Eley-Rideal (ER) mechanism that the gaseous NO first reacts with activated NH3absorbed species to form intermediates and then decomposes into N2 and H2O.Another is the Langmuir-Hinshelwood (LH) mechanism that the gaseous NO are absorbed on basic sites and further combine with the adjacent activated NH3 species to form N2 and H2O.Qi et al. [64,66] studied the NH3-SCR mechanism on the MnOx-CeO2 catalyst and proposed that a gaseous NH3 molecule was first adsorbed to form activated NH3 (coordinated NH3 and -NH2).The ER mechanism was considered as a typical mechanism that the -NH2 species reacted with gaseous NO to generate nitrosamine (NH2NO) and ultimately decomposed into N2 and H2O.In addition, the LH mechanism was also observed in which the absorbed NO was oxidized to NO2 and HNO2, which further reacted with coordinated NH3 to form NH4NO2 and ultimately decomposed to N2 and H2O.Many researchers [26,64,66,83,112,164,165] also confirmed that the coexistence of the LH mechanism and the ER mechanism are prevalent in the NH3-SCR reaction at low temperatures.

Langmuir-Hinshelwood Mechanism
The LH mechanism can be approximately expressed as: gaseous NH3 and NO are absorbed on the surface through Reactions ( 1) and ( 2), respectively.It is generally agreed that the SCR reaction starts with the adsorption of NH3.Physically adsorbed NO can be oxidized by M n+ to form nitrite and nitrate through Reactions ( 3) and ( 4), respectively.Furthermore, these species react with adsorbed NH3 to form NH4NO2 and NH4NO3 by Reactions ( 5) and ( 6) then decomposed to N2 and N2O, respectively.Finally, the reduced M n+ can be rapidly regenerated by O2 (i.e., Reaction ( 7)): It has become a consensus that there are two mechanisms of low-temperature NH 3 -SCR reaction [163].One is the Eley-Rideal (ER) mechanism that the gaseous NO first reacts with activated NH 3 -absorbed species to form intermediates and then decomposes into N 2 and H 2 O. Another is the Langmuir-Hinshelwood (LH) mechanism that the gaseous NO are absorbed on basic sites and further combine with the adjacent activated NH 3 species to form N 2 and H 2 O. Qi et al. [64,66] studied the NH 3 -SCR mechanism on the MnO x -CeO 2 catalyst and proposed that a gaseous NH 3 molecule was first adsorbed to form activated NH 3 (coordinated NH 3 and -NH 2 ).The ER mechanism was considered as a typical mechanism that the -NH 2 species reacted with gaseous NO to generate nitrosamine (NH 2 NO) and ultimately decomposed into N 2 and H 2 O.In addition, the LH mechanism was also observed in which the absorbed NO was oxidized to NO 2 and HNO 2 , which further reacted with coordinated NH 3 to form NH 4 NO 2 and ultimately decomposed to N 2 and H 2 O.Many researchers [26,64,66,83,112,164,165] also confirmed that the coexistence of the LH mechanism and the ER mechanism are prevalent in the NH 3 -SCR reaction at low temperatures.

Langmuir-Hinshelwood Mechanism
The LH mechanism can be approximately expressed as: gaseous NH 3 and NO are absorbed on the surface through Reactions ( 1) and ( 2), respectively.It is generally agreed that the SCR reaction starts with the adsorption of NH 3 .Physically adsorbed NO can be oxidized by M n+ to form nitrite and nitrate through Reactions ( 3) and ( 4), respectively.Furthermore, these species react with adsorbed NH 3 to form NH 4 NO 2 and NH 4 NO 3 by Reactions ( 5) and ( 6) then decomposed to N 2 and N 2 O, respectively.Finally, the reduced M n+ can be rapidly regenerated by O 2 (i.e., Reaction ( 7)):

Eley-Rideal Mechanism
The SCR reaction through the ER mechanism can be approximately described as that the adsorbed NH 3 can be activated to NH 2 (through Reaction (8)) and subsequently oxidized to NH by M n+ .Generally, the reaction products of gaseous NO with NH 2 and NH (i.e., Reactions (10) and ( 11)) are N 2 and N 2 O, respectively: NH 2(ads) NH 2(ads) +NO (gas) Afterwards, the dual LH-ER mechanism was proposed by Chen et al. [166] on Ti 0.9 Mn 0.05 Fe 0.05 O 2−δ catalyst.Monodentate nitrate M (n−1) -O-NO 2 is involved in the reaction with adsorbed NH 4 + (i.e., Reaction ( 12)) or NH 3 on neighboring acid sites to produce an active intermediate M ).The adsorbed NO x -NH 3 complex finally reacts with gaseous NO to form N 2 and H 2 O through Reactions ( 15) and ( 16).Furthermore, Hadjiivanov et al. [167] and Jiang et al. [168] found that bidentate nitrate could be transformed to monodentate nitrates, producing new Brønsted acid sites for more NH 4 + .Long et al. [169] and Liu et al. [170] also reported that monodentate nitrate species could be converted to the intermediate complex with coordinated NH 3 , which further decomposed or reacted with gas NO via ER mechanism to N 2 .The latter can be considered as the association mechanisms of ER and LH [76]: The better and potential performances of TiO 2 supported metal oxides in the NH 3 -SCR reaction were investigated in-depth by the group of Smirniotis from the early 2000s [21,109,[120][121][122][123].They gave a clear picture of the SCR reaction pathways drawn by exploring the surface interactions of isotopic-labeled reactants, using transient response analysis and in situ FTIR studies, as shown in Figure 4 [171].As for the surface of MnO 2 /TiO 2 , they proposed that the reactions followed a Mrks-van-Krevelen-like mechanism through the formation of nitrosamide and azoxy intermediates, in which the lattice oxygen has a direct effect on the mechanism for the instantaneous oxidation of NH 3 , and the products and active intermediates were mainly due to a coupling of one nitrogen atom from ammonia and another one from nitric oxide (2 15 NO + 2NH 3 + 0.5O 2 → 2 14 N-15 N +3H 2 O).
isotopic-labeled reactants, using transient response analysis and in situ FTIR studies, as shown in Figure 4 [171].As for the surface of MnO2/TiO2, they proposed that the reactions followed a Mrksvan-Krevelen-like mechanism through the formation of nitrosamide and azoxy intermediates, in which the lattice oxygen has a direct effect on the mechanism for the instantaneous oxidation of NH3, and the products and active intermediates were mainly due to a coupling of one nitrogen atom from ammonia and another one from nitric oxide (2 15 NO + 2NH3 + 0.5O2 → 2 14 N-15 N +3H2O).(Reprinted with permission from (Ref.[83]).Copyright (2012) Clearance Center, Inc.).

Effects of H2O and SO2
Kijlstra et al. [31,32] found that H2O has the effects of reversible physical and irreversible chemical competitive adsorption with NO and NH3.Xiong et al. [172] proposed that the effects of H2O were not only attributed to the competition adsorption, but also to the decrease of oxidation ability and the inhibition of interface reaction over the Mn-Fe spinel.
Park et al.
[173] found that SO2 was easily oxidized to SO3 and eventually formed (NH4)2SO4 and NH4HSO4 or other ammonium nitrates via the reaction with NH3 and H2O at low temperatures (<200 °C).Kijlstra et al. [32] proposed that the deactivation of Mn-based catalysts by SO2 was mainly due to the formation of MnSO4 on the surface.Many reports have demonstrated the above arguments [31,32,40,157,174].
In our study, we found that the adsorption of NOx to monodentate nitrite species over Mn-based catalysts were significantly inhibited by SO2, while the bidentate nitrate species had no influences from SO2, which further transformed to monodentate nitrate producing new Brønsted acid sites for adsorbing NH 4+ [76].Liu et al. [157,174] have made a careful investigation of the SCR mechanism and H2O/SO2 inhibition effects over the Fe-Mn-TiO2 catalyst using in situ DRIFTS.They found the impacts of H2O on the adsorption of NH3 and NO gaseous were moderate and reversible such that SO2 significantly reduced the adsorption capacity of NH3, suppressing the formation of nitrate/nitrite species (NH4NO3 and NH4NO2), which were the active intermediates in the SCR reaction.In addition, the accumulative deposition of sulfate species on the surface further interrupted the Langmuir-Hinshelwood reaction pathway, resulting in an unrecoverable reduction of SCR activity.(Reprinted with permission from (Ref.[83]).Copyright (2012) Clearance Center, Inc.).

Effects of H 2 O and SO 2
Kijlstra et al. [31,32] found that H 2 O has the effects of reversible physical and irreversible chemical competitive adsorption with NO and NH 3 .Xiong et al. [172] proposed that the effects of H 2 O were not only attributed to the competition adsorption, but also to the decrease of oxidation ability and the inhibition of interface reaction over the Mn-Fe spinel.
Park et al.
[173] found that SO 2 was easily oxidized to SO 3 and eventually formed (NH 4 ) 2 SO 4 and NH 4 HSO 4 or other ammonium nitrates via the reaction with NH 3 and H 2 O at low temperatures (<200 • C).Kijlstra et al. [32] proposed that the deactivation of Mn-based catalysts by SO 2 was mainly due to the formation of MnSO 4 on the surface.Many reports have demonstrated the above arguments [31,32,40,157,174].
In our study, we found that the adsorption of NO x to monodentate nitrite species over Mn-based catalysts were significantly inhibited by SO 2 , while the bidentate nitrate species had no influences from SO 2 , which further transformed to monodentate nitrate producing new Brønsted acid sites for adsorbing NH 4+ [76].Liu et al. [157,174] have made a careful investigation of the SCR mechanism and H 2 O/SO 2 inhibition effects over the Fe-Mn-TiO 2 catalyst using in situ DRIFTS.They found the impacts of H 2 O on the adsorption of NH 3 and NO gaseous were moderate and reversible such that SO 2 significantly reduced the adsorption capacity of NH 3 , suppressing the formation of nitrate/nitrite species (NH 4 NO 3 and NH 4 NO 2 ), which were the active intermediates in the SCR reaction.In addition, the accumulative deposition of sulfate species on the surface further interrupted the Langmuir-Hinshelwood reaction pathway, resulting in an unrecoverable reduction of SCR activity.Ramis et al. [162], Tang et al. [165] and Yang et al. [164] believed that the depth dehydrogenation oxidation of NH 3 to the form of NH 2 intermediate could react with gaseous NO to form N 2 , while NH or N species could react with NO to only give N 2 O.It has been proven that gaseous NH 3 molecules could be adsorbed to the active Mn sites through coordination bond of Mn-NH 3 without cleavage of N-H bonds (Reactions (8) and ( 9)) [175], while adsorbing dissociatively on surface active oxygen species successively strip all hydrogen atoms to form adsorbed nitrogen atom species, which are responsible for the generations of N 2 , N 2 O and NO [178].Thus, the selectivity to N 2 O, predominantly derived from the activation of NH 3 and three N-H bonds, must be cleaved by surface oxygen species of to form N 2 O.

By
Furthermore, it has been reported that the Mn-O bond energies of manganese oxides play an important role in the SCR reaction [165,179] in which the active oxygen species of MnO x with lower Mn-O bond energy can facilitate the cleavage of more N-H bonds in NH 3 molecule to form more adsorbed nitrogen atom species, which lead to greater amounts of deep oxidation by-product [165].Therefore, this is helpful to develop novel catalysts for NH 3 -SCR of NO x with low selectivity to N 2 O.

Promoting Effect of Redox Cycles
Ce can enhance the low-temperature activity of MnO x catalyst due to its redox shift between Ce 4+ and Ce 3+ [26,163,180] [73].It is widely accepted that O α (chemisorbed oxygen) species are more active than O β (lattice oxygen) species due to the higher mobility [73,157].Moreover, the higher percentage of O α species is conducive to the prior oxidation of NO to NO 2 , leading to the enhancement of the "fast-SCR" reaction (4NH 3 + 2NO + 2NO 2 → 4N 2 + 6H 2 O).
In general, Mn-based catalysts have variable valence states and good redox capability at low temperatures, which are associated with the crystal structure, crystallinity, specific surface area, oxidation state, active oxygen, active sites and acidity on the surface, etc. [52].As mentioned above, many reports [3,9,10,[20][21][22][23] have indicated that the ER mechanism and LH mechanism are prevalent in low-temperature NH 3 -SCR reaction on Mn-based catalysts.However, the study of active sites remains contentious between Mn 3+ active sites [31,55,73,182] and Mn 4+ active sites [26,43,54,79,158,183].The synergistic effect of promoter about the pending interactions and synergies between Mn and promoter M elements needs to be further studied, including the surface acidity and oxidation ability of adsorption sites under multi-element effect.In addition, the contribution of ER mechanism and LH mechanism of NH 3 -SCR reaction system needs to be further defined.

Macro-Kinetics
Assuming the reaction was free of diffusion limitation, the intrinsic rate of NO conversion (R NO ) as a function of reactant concentrations can be expressed as follows [22]: where R NO is the rate of NO conversion, k is the rate constant, and x, y, z are the reaction orders of NO, NH 3 and O 2 , respectively.It has become a consensus that the SCR reaction is approximately zero-order with respect to NH 3 and first order with respect to NO over MnO x -CeO 2 [22], MnO x /TiO 2 [184], and Mn 2 O 3 -WO 3 /γ-Al 2 O 3 catalysts [185], etc.In spite of the different reaction-order of O 2 , it is accepted that oxygen plays an important role in the SCR reaction, and the NO x conversion shows nearly no change when O 2 concentration is more than 1-2%.

Micro-Kinetics
Both LH and ER mechanism exist in the low temperature SCR system over Mn-based catalysts as previously described.In recent years, Junhua Li's groups [112,164,186] made several breakthroughs about kinetics with the identification of LH and ER mechanisms of low-temperature SCR catalysts.The detailed summaries are described as follows.
It is well known that the concentration of NH 3 and NO in the gas phase is sufficiently high for the surface to be saturated with adsorbed NH 3 and NO, so the concentration of NH 3 [NH 3(ads) ] and NO [NO (ads) ] adsorbed on the surface at a specific temperature is invariable and can be described as Reactions (19) and (20), respectively: where, k 1 , k 2 and c acid are a constant and the acidity on the catalyst, respectively.k 1 and k 2 would rapidly decrease with the increase of reaction temperature.

LH Mechanism
The kinetic equations of N 2 and N 2 O formation through the LH mechanism can be approximately described as: where k 3 , k  6)) can be approximately described as Reactions ( 23) and (24), respectively: where , and [NH 3(ads) ] are the reaction kinetic constants of Reactions ( 5) and ( 6), and the concentrations of nitrite, nitrate and NH 3 adsorbed on the surface, respectively.
The kinetic equations of nitrite and nitrate formation (i.e., Reactions (3) and ( 4)) can be approximately described as Reactions ( 25) and (26): where k 7 , k 8 , [M n+ =O] and [NO (ads) ] are the reaction kinetic constants of Reactions ( 3) and ( 4), and the concentrations of M n+ and NO adsorbed on the surface, respectively.Meanwhile, the concentration of M n+ on the surface can be regarded as a constant at the steady state, as it can be rapidly recovered through Reaction (7).It can be implied from Reactions ( 23)-( 26) that the formation of NH 4 NO 2 and NH 4 NO 3 is approximately not related to the concentrations of gaseous NO and NH 3 .Hinted at by Reactions ( 24) and ( 25), the formations of N 2 and N 2 O via LH mechanism are approximately independent of gaseous NO concentration.

ER Mechanism
The kinetic equations of N 2 and N 2 O formation through the ER mechanism (i.e., Reactions (10) and ( 11)) can be described as Reactions ( 27) and ( 28): where k 9 , k 10 , [NH 2 ], [NH] and [NO (g) ] were the reaction rate constants of Reactions (10) and (11), the concentrations of NH 2 and NH on the surface, and gaseous NO concentration, respectively.
The reaction kinetic equations of NH 2 and NH formation can be described as Reactions ( 29) and (30): where k 11 , k 12 , [M n+ =O] and [Mn 4+ =O] were the reaction kinetic constant of Reactions (8) and ( 9), and the concentrations of M n+ and Mn 4+ on the surface, respectively.At the steady state of reaction, NH concentration will not be varied.According to Reactions ( 28) and ( 30), the variation of NH concentration can be described as follows: Thus, and

Total Reaction Kinetic Equations
Taking the contributions of both LH and ER mechanism into account, the kinetic equations of NO reduction and N 2 O formation can be approximately described as follows: where k NO , k SCR-ER , k SCR-LH and k NSCR are the rate of NO reduction, the reaction rate constant of the SCR reaction (i.e., N 2 formation) through the ER mechanism, the reaction rate constant of the SCR reaction through the LH mechanism, and the reaction rate constant of the NSCR reaction (i.e., N 2 O formation), respectively.During the SCR reaction, the above kinetic constants can be obtained through the steady-state kinetic study involved in differential and integral calculus.It is a developing trend of micro-kinetics that is beneficial for defining the contribution of ER and LH mechanism to the whole SCR reaction.

New Insight from DFT Calculation
Computer-simulation based on quantum chemistry theory has become a visual tool for the chemical material research.In recent years, the application of quantum chemical calculation using density functional theory (DFT) has been developed rapidly in the field of SCR for the mechanisms of reaction and poisoning.A typical application of quantum chemistry includes three aspects as below.

Analysis of Material Properties
Li et al. [187] calculated orbital energy and surface charge of MnO 2 crystal with different planes and found that the activity of O 2 adsorption was in the sequence of (110) > ( 111) > (001), which was closely related to the Highest Occupied Molecular Orbital (HOMO) energy and surface residual electrons.Yu and Liu et al. [188,189] further found the adsorption capacity on beta-MnO 2 (110) was in the sequence of hydroxyl > water > hydroxyl radical > oxygen.Peng et al. [190] prepared CeO 2 catalysts with different metal doped and studied NH 3 adsorption on the catalyst in experimental and computational methods simultaneously of.CeO 2 doped with four metals (Fe, Mn, La and Y) were prepared by co-precipitation, and the corresponding slab model were constructed by DFT calculations to investigate the influences of dopants on the adsorption of NH 3 molecules.A higher reducibility, a larger quantity of labile surface oxygen and a greater extent of surface distortion are responsible for the high NH 3 activation of the Fe-Ce and Mn-Ce, which is higher than the La-Ce and Y-Ce samples.Moreover, Fe-Ce and Mn-Ce can provide more NH 3 adsorption sites on the Lewis acid sites while the Mn dopant can also provide more Brønsted acid sites, which could react with NH 4+ .The Mulliken charges indicated that the lone pair electrons of nitrogen can transfer to Fe or Mn cations, whereas this process is less favorable for the La and Y cations.Fe and Mn dopant atoms can significantly change the catalytic surfaces of CeO 2 (bond lengths and angles), forming more oxygen defects.These distortions of structure resulted in the enhancement of the catalysts reducibility.
Tang's team [36,38] calculated the reaction between NO and NH 3 on the surface, as well as the impact of H 2 O on the constructed oxygen-rich and oxygen-depleted plane (110) of alpha-MnO 2 based on topology.Duan et al. [191] calculated and analyzed the effect of Ni and Co that doped on MnO 2 through DFT calculation in multiple perspectives.These studies provide clues for a further understanding of the reaction mechanism.

Visualization of Reaction and Poisoning Mechanisms
The SCR reaction mechanism over Mn-CeO 2 catalysts has been investigated by DFT calculations [192,193].Song et al. [193] investigated the NH 3 -SCR of NO mechanism over a mode of Mn cations doped into the CeO 2 (111) surface by DFT + U calculations.They found that NH 3 was referentially adsorbed on the Lewis acid Mn sites and further dissociated one of its N-H bonds to form the key NH 2 intermediate.NO adsorption on this NH2 intermediate resulted in the form of nitrosamine (NH2NO) that could then undergo further N-H cleavage reactions to form OH groups.They proposed that the redox mechanism involved doped Mn, as Lewis acid sites for ammonia adsorption and O vacancies in the ceria surface makes N 2 O decompose into the desired N 2 product.Peng et al. [194,195] studied the alkali metals and As 2 O 5 poisoning resistant capacity of V 2 O 5 -WO 3 -TiO 2 by employing DFT theoretical approaches.They found that the alkali atom mainly influences the active site V species rather than W oxides, and the decrease of acidity after poisoned might directly reduce the catalytic activity calculated by the density of states (DOS) and projected DOS (PDOS) [194].Phil et al. [196] inferred that the catalyst of high resistance to SO 2 poisoning should be combined with sulfate weakly using quantum chemical calculation, which means that the bonding strength of metal atom and oxygen atom is weaker.In this theory, they selected the transition metal Sb and prepared Sb-V 2 O 5 /TiO 2 catalyst of high resistance to H 2 O and SO 2 poisoning.
Maitarad et al. [192] constructed the Mn-CeO 2 (110) model on the basis of a combination of experiment and calculation.DFT calculations demonstrated that the catalyst has excellent catalytic activity and NO and NH 3 can be adsorbed on the catalyst surface.Lu et al. [197] proposed that the formation of bidentate sulfates were mainly generated on the plane (110) of the CeO 2 surface during the SCR process by means of density of states, electron localization functions and charge density difference.Liu et al. [198] investigated the SO 2 poisoning mechanism of Mn x Ce 1−x O 2 catalyst used CeO 2 as the model by the method of calculation.By calculating the electronic structure, density of states and the energy barrier that SO 2 needs to overcome in the formation of sulfates of CeO 2 and Mn x Ce 1−x O 2 being obtained, it was noted that the CeO 2 deactivation was due to the formation of CeSO 4 rather than Ce 2 (SO 4 ) 3 , which was acidized by SO 2 .For the Mn x Ce 1−x O 2 catalyst, it was easier to be poisoned since the introduction of Mn enhanced the thermal stability of the surface sulfate.

Design of Functional Materials
Cheng et al. [199], using DFT, found that surface oxygen vacancy was beneficial for the catalytic activity due to the decrease of exposed Mn atom valence and reduction of reaction energy barrier.Peng et al. [194] proposed that more oxygen vacancies will generate more empty orbits for lone pair electrons, resulting in more NH 3 adsorption and higher redox property of the catalyst.Therefore, it is a novel way to improve the catalytic activity through the oxygen defects.Phil et al. [196] believed that the catalyst with weak bond energy of Metal-SO 2 or sulfate will be highly resistant to SO 2 .Thus, they selected the transition metal Sb using quantum chemical calculation and prepared Sb-V 2 O 5 /TiO 2 catalyst with high resistance to H 2 O and SO 2 .
Plainly, the quantum chemical calculation contributes a lot to the further research of catalytic mechanisms and phase characteristics of catalyst surface.It needs more in-depth theoretical investigation to help us understand the reaction mechanism of NH 3 -SCR catalysts and SO 2 poisoning mechanism at the molecule level, which will benefit preparation of the catalysts with higher activity and resistance.

Conclusions
There is strong interest in developing excellent efficiency, strong stability, and high resistance catalysts for SCR in the low-temperature range (<250 • C) for the removal of NO x from stationary sources such as coal-fired power plants, steel, cement, glass and other industries.Because of the various valence states and excellent redox performance at low temperatures, Mn-based catalysts have aroused widespread concern regarding the low temperature NH 3 -SCR, while the poisoning effects by H 2 O or/and SO 2 are some of the weaknesses.Many types of research have been done to study the SCR mechanisms, reaction kinetics and poisoning effects.It is accepted that the phenomena of LH and ER mechanisms or the coexistence of both in the SCR process are the major theories.The inhibition mechanisms of H 2 O and SO 2 on activity are mainly due to the competitive adsorption with NH 3 and NO, reduction of interface reaction, deposition of ammonium sulfate and sulfurization of active components.Many works still remain to be done to study the inhibition mechanism of H 2 O and SO 2 to the catalyst, especially for the impacts on the catalytic reaction paths and bulk phase structure of catalysts.Quantum chemical calculation using density functional theory has been introduced at the molecule level to analyze the catalyst' properties, explicit reaction and poisoning mechanisms, and directs the design of a functional catalyst.
In recent years, many Mn-based catalysts have been widely investigated to improve the H 2 O or/and SO 2 resistance, such as modified MnO x -CeO 2 , multi-metal oxides with the structure of special crystal or/and shape (such as perovskite-type, spinel, nanocages, hollow nanofibers, hexagonal microsheets, MOFs and core-shell, etc.), modified TiO 2 supporter and novel carbon supporter (ACF, CNTs, GE), etc.These catalysts show a relatively good resistance because of their pre-sulfating of dopants for protecting MnO x , which are expected to break the resistance to H 2 O or/and SO 2 .Despite these great efforts that have been achieved, it is still a challenge to develop low temperature de-NO x catalysts with a high tolerance and stability in the long-term and at a large-scale.We believe that the intensive study of H 2 O and SO 2 inhibition effects through the combination analysis of in situ DRIFTS and DFT and the amplification of tolerance mechanism will be helpful to guide the design of excellent SCR catalysts with special crystal or/and shape structures.
Because of the stringent regulations on NO x emissions and disadvantages of commercial V-based catalysts, the catalysts (techniques) have already received significant attention with the promising De-NO x performance at low temperatures (<250 • C), including the non V-based catalysts for low temperature SCR, novel design of catalysts for the direct decomposition without any agent, quantitative catalytic oxidation for pre-oxidation combined absorption and development of novel NTP-assisted catalysis technology.Mn-based catalysts have attracted considerable attention for utilization in low-temperature SCR of NO x in the presence of excess of oxygen and sulfur dioxide with a broad temperature window, without sacrificing too much of the efficiency.

Figure 1 .
Figure 1.NOx Emissions of countries and regions by time series (A) *, and that of China and American from 2011 to 2014 (B) #. (* data from "Emission Database for Global Atmospheric Research" by the Joint Research Centre of European Commission; # data from "Bulletin of China's Environment State" by the Ministry of Environmental Protection of China, and "Air Pollutant Emissions Trends Data" by the Environmental Protection Agency of USA).

Figure 1 .
Figure 1.NO x Emissions of countries and regions by time series (A) *, and that of China and American from 2011 to 2014 (B) #. (* data from "Emission Database for Global Atmospheric Research" by the Joint Research Centre of European Commission; # data from "Bulletin of China's Environment State" by the Ministry of Environmental Protection of China, and "Air Pollutant Emissions Trends Data" by the Environmental Protection Agency of USA).

Figure 3 .
Figure 3. Proposed reaction scheme for NH3 stage-oxidation by O2 and by NO in the NH3-SCR process by Ramis et al. [162].

Figure 3 .
Figure 3. Proposed reaction scheme for NH 3 stage-oxidation by O 2 and by NO in the NH 3 -SCR process by Ramis et al. [162].

) N 2 O
mainly originates from the reaction of NO and NH 3 (Reaction (17)) in which gaseous NO react with adsorbed NH x and N species to give N 2 and N 2 O via the ER mechanism [165,175].The postulation that one nitrogen atom of N 2 O comes from NO and another from NH 3 has been directly substantiated by the isotopic labeling experiments by Singoredio et al. [176] and Janssen et al. [177] 4 , [M (n−1)+ −O−NO−NH 3 ], and [M (n−1)+ −O−NO 2 −NH 3 ] are the decomposition rate constants of NH 4 NO 2 and NH 4 NO 3 , and the concentrations of NH 4 NO 2 and NH 4 NO 3 on the surface, respectively.The kinetic equations of NH 4 NO 2 and NH 4 NO 3 formation (i.e., Reactions (5) and (

Table 1 .
Review of catalysts and the catalytic performance in NH 3 -SCR of NO x at low temperatures.

Table 2 .
Parameter comparisons of industrial vanadium-based, best Mn-based and zeolite catalysts.

Table 3 .
Research results of SO 2 and H 2 O resistance over kinds of manganese based catalysts in literature.

Table 4 .
Thermal stabilities of various NO x -absorbed species.