The new challenges for the development of NH<sub>3</sub>-SCR catalysts under new situation of energy transition in power generation industry

The new challenges for the development of NH₃-SCR catalysts under new situation of energy transition in power generation industry

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Feibin Wei^a, Yongfang Rao^e, Yu Huang^b^,^c^,^*, Wei Wang^b^,^c^,^d, Hui Mei^a^,**

Chinese Chemical Letters | 2024, 35(6) : 108931

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Chinese Chemical Letters | 2024, 35(6): 108931

• Review •

The new challenges for the development of NH₃-SCR catalysts under new situation of energy transition in power generation industry

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Feibin Wei^a, Yongfang Rao^e, Yu Huang^b^,^c^,^*, Wei Wang^b^,^c^,^d, Hui Mei^a^,**

Affiliations

^a Science and Technology on Thermostructural Composite Materials Laboratory, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China

^b Key Laboratory of Aerosol Chemistry and Physics, State Key Laboratory of Loess and Quaternary Geology (SKLLQG), Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China

^c Center of Excellence in Quaternary Science and Global Change, Chinese Academy of Sciences, Xi'an 710061, China

^d University of Chinese Academy of Sciences, Beijing 100049, China

^e Department of Environmental Science and Engineering, Xi'an Jiaotong University, Xi'an 710049, China

Published: 2024-06-15 doi: 10.1016/j.cclet.2023.108931

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Abstract

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NH₃-SCR was one of the most promising deNO_x technologies and it has been widely applied in industrial NO_x reduction. However, with the further development of energy transformation in power generation sector, the development of NH₃-SCR catalysts is facing some new challenges. It is becoming an urgent problem to solve low catalytic activity and stability of NH₃-SCR catalysts at the working condition of ultra-low temperature (≤ 200 ℃) and high concentrations of H₂O + SO₂ due to the gradual deployment of new energy power plants. Furthermore, the traditional coal-fired power plants would need flexible operation with the increasing share of renewable energy generation. The NH₃-SCR catalysts which were applied in coal-fired power industry would be requested to work in a wide temperature window from 200 ℃ to 500 ℃ in the near future. Therefore, in this review, we summarized the progress of NH₃-SCR catalysts in solving these different industrial problems in recent years. And the research directions which were deserved to be focused on the development of NH₃-SCR catalysts for the energy transition of power generation sector are proposed.

Key words

NH₃-SCR / NOx reduction / Power generation / Ultra-low temperature / Wide temperature windows

Cite this Article

Feibin Wei, Yongfang Rao, Yu Huang, Wei Wang, Hui Mei. The new challenges for the development of NH₃-SCR catalysts under new situation of energy transition in power generation industry[J]. Chinese Chemical Letters, 2024 , 35 (6) : 108931 - . DOI: 10.1016/j.cclet.2023.108931

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1 Introduction

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Nitrogen oxides (NO_x), as one of the main air pollutants, could cause some serious environmental problems such as acid rain, photochemical smog [1–3]. Furthermore, some researchers found that the nitrogen oxides could also damage people's lungs and contribute to severe lung disease in recent years [4–6]. Therefore, how to reduce the concentration of NO_x in the air is important for the treatment of air pollution. The industrial production was one of the main sources of NO_x in the air. According to the statistics [7–9], the industrial NO_x mainly comes from the flue gas of some industrial sectors which use fossil fuels and nitric acids (e.g., coal-fired power plants, municipal waste incineration, cement plants, steel plants). Therefore, in most countries of the world, the emission standard of NO_x in these industrial sectors was very strict (e.g., coal-fired power plants: USA: 135 mg/m³, EU: 200 mg/m³, China: 100 mg/m³) [10]. In order to meet the increasingly stringent emission standards of industrial NO_x, a lot of technologies have been applied to reduce the concentration of NO_x in flue gas from the industrial production. The general strategies for controlling NO_x emissions include selective non-catalytic reduction (SNCR), selective catalytic reaction (SCR), wet scrubbing, adsorption, electrochemical method and so on [11–16]. Among them, NH₃-SCR is becoming the most widely applied technology of NO_x removal in some industrial sectors due to its high efficiency and simplified operational procedure [17–20]. Generally, the main deNO_x reactions in NH₃-SCR are shown below (Eqs. 1–5) [21–23]:

(1)

(2)

(3)

(4)

(5)

The general definition about NH₃-SCR reactions is "Standard SCR" or "Fast SCR". And the fast SCR is completed more quickly than standard SCR when the molar ratio of NO/NO₂ is 1:1 [24].

For NH₃-SCR technology, the development of low-cost and high-performance SCR catalysts is the key to achieve highly efficient removal of NO_x. In the past few decades, V₂O₅–WO₃/TiO₂ catalysts have been widely used in industrial deNO_x on account of its high efficiency in the temperature window of 300–400 ℃ [25–30]. However, with the expanding of the application scenarios of NH₃-SCR, the traditional V₂O₅–WO₃/TiO₂ catalysts are unable to meet the need of NH₃-SCR under different working conditions in different industries. As shown in Table 1, we summarized and compared the main characteristics of the flue gas in several typical industries.

According to this, the central challenges for the development of NH₃-SCR catalysts in different industries were proposed. And compared with other industries, the NH₃-SCR technology is facing some new challenges which are deserved to be discussed carefully in energy generation industry due to the deep transformations of energy sector. Specifically, with the prosperous development of new energy generation sector, the technology of electricity generation is going through a transition from traditional coal-fired to new energy power generation [31]. And the traditional power plants would account for decreasing percentage of the total electricity generating capacity in the near future. The coal-fired power plants would operate under a low-load condition for a long time. Therefore, the temperature of flue gas exhausted from coal-fired boiler would drop to about 200 ℃. The activity of V₂O₅–WO₃/TiO₂ rapidly declines when the working temperature is below 300 ℃ or above 400 ℃. Thus, the catalytic activity of V₂O₅–WO₃/TiO₂ can not meet the demand of deNO_x efficiency with the temperature below 300 ℃. Therefore, how to develop novel SCR catalysts which could maintain high deNO_x efficiency in a wide temperature window from 200 ℃ to 400 ℃ is an urgent demand for the further application of NH₃-SCR technology in coal-fired power industry.

In addition, SO₂ poisoning for NH₃-SCR catalysts is another well-explored issue [32–35]. And it was generally accepted that the poisoning mechanism was the deposition of NH₄HSO₄ (ABS) and the sulfation of metal active sites [36]. As is reported, the deposition amount of ABS on catalyst surface would dramatically increase when the reaction temperature dropped to below 300 ℃ [32,37]. Therefore, another challenge for the development of NH₃-SCR catalysts in coal-fired power industry was how to improve SO₂ resistance of NH₃-SCR catalysts under low working temperature.

On the other hand, on account of the non-renewability of coals, people are trying to use renewable biomass energy to replace it as the main fuel to generate electricity. Thus, the technology of NH₃-SCR is facing an entirely different condition in new energy generation industry. In recent years, people come to recognize that municipal wastes could be used as a renewable biomass fuel [38–43]. Municipal waste incineration power is becoming an important new energy generation technology. The temperature of flue gas generated during municipal waste incineration was generally at 150–180 ℃. Therefore, developing NH₃-SCR catalyst which could work at ultra-low operation temperature (≤ 200 ℃) is highly demanded for municipal waste incineration. In addition, the flue gas of municipal waste incineration includes lots of water vapor (≥ 25%) and SO₂. Improving the H₂O + SO₂ resistance of NH₃-SCR catalysts was another challenge for deNO_x in municipal waste incineration industry.

The development of NH₃-SCR catalysts has received increasing attention in the past several decades [44,45]. As shown in Fig. 1, the number of publications on the topic of "SCR catalyst" increased dramatically in the past few decades. However, no reviews summarize the challenges of NH₃-SCR catalysts during the energy transition of power generation industry.

In this work, we detailly analyze the development of NH₃-SCR catalysts in electric industries (coal-fired power plants and municipal waste incineration). And subsequently, we further discuss that the general strategies on solving these problems for the different types of catalysts by giving some specific examples. At last, we also propose the future research challenges and directions of deNO_x in electric industries.

2 SCR catalysts in coal-fired power plants

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2.1 Broad temperature window (200–400 ℃)

NH₃-SCR technology has become one of the most efficient ways to remove NO_x in coal-fired power plants [46]. However, in order to combat climate change, the electric systems need turn to a low-carbon and sustainable development mode in the near future. The new energy generation plants are gradually replacing traditional power plants. Therefore, the traditional coal-fired power boilers would have to operate under a low load condition. The temperature of flue gas generated in coal-fired power boiler would decrease to about 250 ℃ under a low-load condition. However, traditional V₂O₅-TiO₂ only could maintain a high catalytic activity in a narrow temperature window (300–400 ℃). And it could not meet the demand of future coal-fired power plants. The development of new catalysts and modification of traditional V₂O₅-based catalysts which can work in a broad temperature window has been a pressing demand in coal-fired power industry.

As shown in Fig. 2, in general, the mechanism of standard NH₃-SCR reaction includes acid and redox circle. In terms of L-H mechanism, the NH₃ is absorbed on acid sites to generate NH₄⁺, and the active nitrates generated by the oxidation of NO further react with NH₄⁺, leading to the generation of NH₄NO₂/NH₄NO₃ species which degrade to N₂ and H₂O. In terms of E-R mechanism, the NH₃ is firstly absorbed on acid sites and dehydrogenated to -NH₂ while the high valence metal is reduced. Subsequently, the -NH₂ reacts with NO to form -NH₂NO which decomposes to N₂ and H₂O to complete the acid circle. The low valence metal is oxidized to high valence by O₂ and further to generate H₂O to complete the redox circle. As is reported [47], the acid and redox circle would significantly slow down with the decrease of reaction temperature. Therefore, how to build up the acid strength of active metal sites and improve their redox properties are the key to improve the catalyst activity of NH₃-SCR catalysts.

The development of catalysts generally focused on traditional V-based oxides, novel Ce/Fe-based oxides and ion-exchanged zeolites. And the general strategies were the surface modification, structural designment of catalyst and so on. According to our statistic in Table 2, most catalysts have achieved high NO_x removal efficiency in a broad-temperature window from 200 ℃ to 450 ℃ [48–69].

2.1.1 The modification of V-based catalysts

The traditional V₂O₅/TiO₂ catalyst work within a narrow temperature range between 300 ℃ and 400 ℃, nevertheless, its operating temperature could be broadened by modifying catalyst surface or using catalyst support. Zhao et al. [50] reported a S-N doped V₂O₅/TiO₂ catalyst and its NO_x removal efficiency achieved nearly 100% within a temperature range of 240–450 ℃. The NH₃-TPD and H₂-TPR curves indicated that the introduction of S-N built up the acid strength of V₂O₅/TiO₂ and enhanced the redox property of V₂O₅/TiO₂ catalyst. In their another publication, Ti³⁺ were introduced to V₂O₅/TiO₂ catalyst to improve its low-temperature catalysis performance [70]. Furthermore, Cao et al. [71] found that the co-doping of Ce⁴⁺ and Zr⁴⁺ could improve the denitration performance of V₂O₅-WO₃/TiO₂.

The broad temperature performance of V₂O₅-based catalysts could also be improved by modification of catalyst support. The modification of catalyst support can be classified into two groups: the modification of traditional TiO₂ support and the introduction of novel catalyst support.

Lian et al. [25] pretreated TiO₂ support at 850 ℃ and deposited vanadium species on its surface. They found that the NH₃-SCR performance of catalyst using pretreated TiO₂ as the support could achieve nearly 100% deNO_x efficiency at 240 ℃ and above. Si et al. [72] successfully increased the deNO_x efficiency of V₂O₅/TiO₂ under the working temperature of 250–350 ℃ by doping Sn into rutile TiO₂. Song et al. [73] successfully synthesized microporous TiO₂ as the support of V₂O₅ catalyst which enhanced the NO_x conversion to nearly 90% at the working temperature of 250 ℃ below.

For the design of novel catalyst support, Liu et al. [48] prepared ZrO₂ supports with diverse morphologies as the supports of V₂O₅ by self-assembly methods. And they found that the interaction between mesoporous ZrO₂ support and surficial vanadium species is beneficial for the increased proportion of low valence vanadium and absorbed oxygen. And the catalyst activity test indicated that the NO conversion over V/MZ (V₂O₅ deposited on mesoporous ZrO₂) achieved nearly 95% between 225 ℃ and 425 ℃.

2.1.2 The development of Fe-based catalysts

The Fe-based metal oxides as a promising alternative of V-based catalysts have become one of the most efficient catalysts in SCR catalysis due to the Fe²⁺/Fe³⁺ redox circle [74,75]. The development of Fe-based catalysts generally focused on the control of the morphology and structure as well as the modification of the catalyst surface. The overoxidation of NH₃ under high temperature is another concern for Fe-based catalysts. Wang et al. [49] successfully synthesized WO₃-FeO_x catalysts by a solvent-free method. The introduction of WO₃ restrained the growing of the Fe₂O₃ particles and inhibited the agglomeration of the particles. And the NO_x conversion over the catalyst could achieve 90% between 200 ℃ and 500 ℃. Liu et al. [76] also found that the addition of WO_x could improve the activity of Fe₂O₃ catalyst by supplying abundant surface reactive Lewis and Bronsted acid sites. The NO_x conversion could achieve 80% above from 300 ℃ to 450 ℃.

Han et al. [77] anchored Fe₂O₃–CeO₂ onto the mesoporous Al₂O₃ nanoarrays which were in situ created on the surface of Al-mesh. The Al₂O₃ nanoarrays supplied abundant sites for the anchoring of metal oxides catalysts. Furthermore, they also founded that the presence of Ce changed the charge distribution of Fe and the interaction between Ce and Fe benefited NH₃-SCR. Chen et al. [65] Introduce the single-atom Ce species to the surface of α-Fe₂O₃ to improve the content of Fe²⁺ and decrease the oxygen vacancy formation energy. NO conversation over the Ce-modified α-Fe₂O₃ was above 95% at a very low temperature of 175 ℃.

2.1.3 Metal ion-exchanged zeolite catalysts

The ion-exchanged zeolites are another high-efficiency substitute of traditional V₂O₅/TiO₂ catalysts. In recent years, Cu-based exchanged zeolites have been the most widely studied zeolite catalyst in NO_x removal because its active temperature window could cover low, medium and high temperature [18,20,78–83].

Sun et al. [84] investigated the influence of Si/Al ratio on NH₃-SCR activity of Cu-SSZ-13 zeolite, and found that the Si/Al ratio or Cu loadings could influence the hydrothermal stability and catalytic activity of Cu-SSZ-13 zeolite by controlling the formation of Cu²⁺–2Al and [Cu^Ⅱ(OH)]⁺–Al. Jin et al. [62] reported that the existence of Cu species on zeolite surface could promote the adsorption and activation of NH₃. And their NH₃-SCR activity test result indicated that the T₉₀ (temperature window of NO_x conversion ≥ 90%) temperature window was 200–350 ℃. Daya et al. [85] developed the kinetic model of the Cu^Ⅰ/Cu^Ⅱ redox circle during NH₃-SCR. Wang et al. [86] prepared Cu-ZSM-5 catalysts with different morphologies (nanosheets, nanoparticles and hollow spheres). They found that the NO conversion over nanosheet-like Cu-ZSM-5 (NS-Cu-CZSM-5) could achieved nearly 100% in the temperature window from 200 ℃ to 400 ℃. Furthermore, they also found that Cu-based zeolite contains CuO and CuO is bad for NO reduction. Several strategies have been reported in inhibiting the growth of CuO crystal. Zhang et al. [87] suppressed the generation of CuO by increasing the amounts of acidic sites in zeolite structure. They widened the T₉₀ of Cu exchanged zeolite to 200–600 ℃. Ma et al. [88] reported that the formation of CuO in Cu-SSZ could be restrained by surface Al modification. And the T₉₀ temperature range could be widened to 200–500 ℃. Yue et al. [57] successfully restrained the generation and growth of CuO grains by Fe modification. And the NO conversion over FeCu-SSZ-13 could achieved above 95% within the temperature range of 200–550 ℃. Except for Cu, some metal ions such as Fe and Ce also are attractive compensation ion for zeolite catalysts. Shi et al. [89] reported the NO_x conversion over In and Ce bimetallic catalyst supported by zeolite nearly achieved 100% within the temperature range of 250–600 ℃. Liu et al. [90] reported a Fe-Ni-W zeolite catalyst and investigated its NH₃-SCR activity in a wide temperature window of 200 ℃–850 ℃.

In recent years, some researchers are trying to synthesize and develop zeolites with different framework structure as NH₃-SCR catalysts. Tarach et al. [80] studied the influence of zeolite topology on NH₃-SCR activity and found that topology structure could affect the redox circle between Cu²⁺ and Cu⁰. Zhang et al. [91] reported a new CHA-type aluminoborosilicates and applied it to deNO_x reaction. The T₉₀ temperature range of the catalyst could be widened to 200–500 ℃. Chitac et al. [92] synthesized a small-pore SWY zeolite and further introduced Cu to zeolite framework structure. The NH₃-SCR light-off test indicated that the catalyst could achieve 90% NO_x conversion between 200 ℃ and 500 ℃.

In conclusion, in consideration of vanadium biotoxicity, developing novel vanadium-free catalysts working in a wide active temperature window has been a research hotspot. The critical factors which affect the performance of SCR catalysts are acid site strength and redox properties. The general strategies to improve acid site strength and redox property included doping modification, grain growth control and dominant crystal planes exposure. In recent years, some novel NH₃-SCR catalysts such as Fe-, Ce-based catalysts have been widely reported. However, the synthesis methods of these catalysts involve fancy instruments and require accurate parameter control. As a result, how to simplify the preparation procedures of the catalysts is critical for the mass production of novel vanadium-free deNO_x catalysts.

2.2 Improve the SO₂ resistance of SCR catalysts

Many studies have shown that the presence of SO₂ in the flue gas from coal-fired plants negatively influence the catalytic activity of SCR catalysts. The negative effects of SO₂ on SCR catalysts include physical deactivation and chemical deactivation. The physical deactivation was caused by the deposition of NH₄HSO₄ (ABS) over catalysts surface [32] while the chemical deactivation was the sulfation of active sites. In particular, the physical deactivation could be the main problem when the working temperature of deNO_x was below 300 ℃. ABS becomes a very viscous liquid (0.1–0.2 Pa·s) within the temperature range of 147–350 ℃ [93,94] and sticks to the surface of catalysts with coal ash, leading to the reactor channel being blocked. As described in Fig. 3, the generation of ABS proceed via three main steps: the adsorption of SO₂, the oxidation of SO₂ and the binding of NH₄⁺ and HSO₄⁻. The general strategies to improve the SO₂ resistance of catalysts focused on inhibiting the adsorption and oxidation of SO₂ and promoting the decomposition of ABS. In Table 3, We made a summary about the strategies to improve the SO₂ resistance of NH₃-SCR catalysts.

2.2.1 Inhibiting the adsorption of SO₂

Inhibiting the adsorption of SO₂ on the surface of catalysts was an important strategy to directly curb the generation of ABS [95–98]. The SO₂ adsorption could be inhibited by introducing some special catalytic promoters, constructing sacrificial sites, creating protection layer and so on. The catalytic promoter which could strengthen the acidity or weaken the alkalinity of active sites is beneficial for inhibiting SO₂ adsorption because SO₂ is atypical acidic oxide. Some nonmetal elements or metal elements with strong electronegativity are always good options. Kang et al. [99] found that the introduction of Fe into CeVO₄ could restrain the adsorption of SO₂ by increasing the types and amounts of acidic sites. The long-term NH₃-SCR test in the presence of 100 ppm SO₂ showed that the NO conversion over Fe_δCe_1-δVO₄ could maintain above 80% after 25 h. Tan et al. [100] successfully synthesized innovative CeO₂-SiO₂ mix oxide using coprecipitation method. The strong interaction of Ce-O-Si weaken the alkalinity of CeO₂ and further inhibited the adsorption of SO₂ on Ce. The catalytic stability tests in the presence of 500 ppm SO₂ indicated that the NO conversation over CeO₂–SiO₂ could still maintain about 90% after 40 h aging. Chen [101] introduced Co species into MnO_x and found that the NO_x conversion over Co-modified MnO_x could be kept at above 85% in the presence of 50 ppm SO₂ for 10 h (Fig. 4a). Park et al. [102] introduced Nb and Mo to VO_x-oxide and kept NOx conversion above 80% after 124 h in the presence of 500 ppm SO₂.

Moreover, creating protective layer or introducing sacrificial agents could also protect metallic active sites from the harm of SO₂ adsorption. Qi et al. [103] deposited a layer of iron sulfate on the surface of CeO₂/TiO₂ catalyst. The presence of surface iron sulfate cut off SO₂ adsorption pathway and inhibited the sulfation of Ce species (Fig. 4b). Fang et al. [104] doped Ce atoms into MnO₂ as the sacrificial sites of SO₂ adsorption and oxidation. Furthermore, they found that the generation of Ce₂(SO₄)₃ could supply extra adsorption sites for NH⁴⁺ to upgrade the reaction of NO_x reduction. Tang [97] prepared a protect layer of amorphous MoO_x on the surface of CeFeO_x and the NO_x conversion could maintain above 90% in the presence of SO₂ for 60 h.

2.2.2 Inhibiting the oxidation of SO₂

The SO₂ resistance of catalysts could also be improved by inhibiting the oxidation of SO₂ [105–109]. The key to restrain SO₂ oxidation is suppressing the electron transfer from SO₂ to metal active centers. Some researchers are trying to confining active metal species on nanoscale catalyst support to achieve this goal. Hu et al. [105] successfully prepared highly dispersed ceria particles by loading CeO₂ onto the surface of MoO₃ nanorod support. And the percentage of Ce³⁺ increased from 9% (bulk CeO₂) to 60% (CeO₂/MoO₃) (Fig. 4c). The catalyst could maintain nearly 100% NO conversion in the presence of 1000 ppm SO₂ for 20 h at 280 ℃. Xu et al. [110] confined active Cu species in TiO₂ nanotubes and successfully improve the Cu⁺/Cu²⁺ ratio. The NO_x conversion of the catalyst could keep above 80% after 100 ppm SO₂ poisoning test of 8 h. Wang et al. [111] synthesized Ce-based catalyst which used montmorillonite/TiO₂ composite as the support. The NO conversion over the catalyst still maintained above 80% in the presence of 100 ppm SO₂ after 5 h test.

Furthermore, constructing facile electron transfer pathway by introducing electron donor could also suppress the electron transfer between SO₂ and active metal sites. Wang et al. [106] doped Sm into MnCeTiO_x catalyst to inhibit SO₂ oxidation by Mn⁴⁺ and Ce⁴⁺. The electron transfer pathways of Sm²⁺ → Mn⁴⁺ and Sm²⁺ → Ce⁴⁺ suppressed the electron transfer between SO₂ and Mn⁴⁺/Ce⁴⁺ (Fig. 4d). Purbia et al. [107] doped CuO—CeO₂ nano-heterostructures to V₂O₅–CeO₂–WO₃/TiO₂ catalysts to improve SO₂ resistance of V₂O₅-based catalysts. They found that the facile electron transfer between Cu-Ce and V restrained the oxidation of SO₂ on the surface of active sites. Xu et al. [112] prepared a MnCe/GAC—CNTs catalyst with excellent SO₂ resistance. And the electron transfer between Ce and Mn restrained the oxidation of SO₂.

2.2.3 Promoting the regeneration of catalytic active sites

Because the decomposition temperature of ABS was above 350 ℃, the deposition of ABS on catalyst surface was unavoidable while the system was running below 300 ℃. Removing ABS from catalyst surface was critical for the regeneration of active sites covered by ABS. The general strategies are promoting the decomposition of ABS or inducing the migration of ABS [37,113–115].

Kwon et al. [36] found that the decomposition of ABS/AS was related with the oxygen vacancies and reactive oxygen species in catalysts (Fig. 4e). They added Sb-Ce to V₂O₅/TiO₂ and found that the introduction of Sb-Ce could improve the ratio of unstable O_α species. And further the decomposition temperature of ABS would decrease from 300 ℃ above to 264 ℃. Chen [114] successfully synthesized Fe₂O₃ on the surface of α-MoO₃ nanobelts (Fig. 4f). The α-MoO₃ with the structure of nanobelt could trap NH₄⁺ from ABS and promote the decomposition of ABS. Guo et al. [116] found that MoO₃ could capture NH₄⁺ to promote the decomposition of ABS. Guo et al. successfully prepared sub-monolayer MoO₃ on the surface of TiO₂ and found that the atomically dispersed O-Mo-O could effectively improve the decomposition of ABS on TiO₂. Chen et al. [117] prepared a TiO₂-support single-atom Mo catalyst and successfully decreased the decomposition temperature of ABS to 225 ℃.

Inducing the migrant of ABS is another effective strategy to remove the ABS on catalyst surface. Song et al. [118] physically mixed H-Y zeolite with V₂O₅/TiO₂ and used H-Y zeolite as a trapper of ABS to protect vanadium active sites. The regeneration and reusability test indicated that the VWTi + Z catalyst aged for 20 h by SO₂ could achieve almost the same performance with fresh catalyst after regeneration at 350 ℃. Jeon et al. [27] used alumina as the trapper of ABS to protect V₂O₅/WO₃-TiO₂ catalyst. And the catalyst could almost totally regeneration after calcining at 350 ℃.

Inhibiting AS/ABS deposition is critical to improve SO₂ resistance of catalysts at low temperature below 300 ℃. The general strategies adopted in previous studies were cutting off the generation pathway of AS/ABS and removing AS/ABS from catalyst surface. However, flue gas is a complex mixture which include water vapor, CO₂, particulates, alkali/alkaline/heavy metals, SO₂ and NO_x. And there are still lack of related studies about the poisoning mechanism of muti-pollutants coexistence. It is important for the developing of NH₃-SCR which have excellent poisoning resistance.

3 Recent progress of SCR catalyst in municipal waste incineration

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3.1 Ultra-low temperature SCR catalysts (≤ 200 ℃)

As a promising substitute of traditional power generation, biomass combustion power generation has gained increasing attention. The municipal waste incineration as a source of renewable biomass energy is gradually becoming an important power generation technology. However, the deNO_x treatment of flue gas in municipal waste incineration power plants restrained its development. The temperature of flue gas from municipal waste incineration power plants generally ranges between 100 ℃ and 200 ℃, and can not reach the ignition temperature of traditional V₂O₅-based catalysts. Compared with other types of deNO_x catalysts, MnO_x-based catalysts could exhibit excellent deNO_x efficiency under ultra-low temperature condition (≤ 200 ℃) [119–122]. Table 4 summarizes the NH₃-SCR catalysts working under ultra-low temperature condition. The modifications of MnO_x catalysts include structure or morphology modification of single MnO_x, construction of MnO_x-based composite oxides and the construction of core-shell structure [78,114,119,120,123–130].

The crystallinity is one of the most important factors which affect the activity of catalysts. The low crystallinity benefited the generation of oxygen defects on catalysts. Therefore, most studies focused on decreasing the crystallinity of MnO₂ to improve the catalytic activity of MnO₂. Yang et al. [125] successfully reduced the crystallinity of MnO₂ by a facile procedure of phase transformation. According to the HRTEM and XPS results of β-MnO₂, the considerable amounts of defects and the high concentrations of surface absorbed oxygen were detected, indicating that the concentrations of oxygen defects were increased in β-MnO₂ compared with that in γ-MnO₂. Furthermore, NO conversation over β-MnO₂ could maintain almost 100% NO conversation when the working temperature was decreased to 150 ℃. Zhang [126] successfully synthesized an amorphous MnO_x by in-situ deposition as the catalyst of deNO_x reaction, which exhibited excellent performance for low temperature deNO_x.

However, the working temperature windows of single MnO_x catalysts are very narrow. Lots of efforts have been devoted to broaden the working temperature windows of MnO_x catalysts. Construction of MnO_x-based composite oxides was an effective method to widen the working temperature of MnO_x. Some transition or lanthanide metals (Ce, Fe, Sm, Gd, etc.) have been introduced to combine with MnO_x. Wang [131] introduced Sm to MnO_x and subsequently erased Sm atoms from SmMnO_x by nitric acid. They found that the removal of Sm led to the exposure of Mn-O and Mn-Mn dual-functional sites and the increased Mn⁴⁺/Mn³⁺ ratio (Figs. 5a and b). They also found that the NO conversation over SmMnO₅ after being treated by nitric acid could maintain about 100% in a very wide temperature window from 80 ℃ to 210 ℃. Xu et al. [132] found doping Ti and Sm to MnO_x widened the optimal working temperature from 170–210 ℃ to 70–230 ℃. Geng et al. [127] successfully prepared W-Mn₃CeO_x which showed a good SCR activity at the temperature of 120–260 ℃.

In conclusion, Mn-based catalyst has become one of the most potential catalysts which could directly catalyze NH₃-SCR process under ultra-low temperature condition. The studies about widening active temperature window and improving N₂ selectivity of MnO_x-based catalysts have made a great progress in recent years. However, a lot of components including alkali/alkaline-earth/heavy metals (Mg, K, Na, Hg, etc.) and acidic compounds (HCl, HF, etc.), abundantly existed in the flue gas could cause the deactivation of Mn-based catalyst. Improving the poison resistance of Mn-based catalyst can be the focus of future studies.

3.2 SO₂ + H₂O resistance

The deNO_x catalysts composed by MnO_x-based oxides always show terrible stability in the presence of H₂O and SO₂. However, the combustion of municipal waste incineration produces a large number of water vapor (≥ 25%) and SO₂. MnO_x-based oxides has become one of the most promising deNO_x catalysts in municipal waste incineration industry. How to improve the stability of MnO_x-based catalysts in the presence of H₂O + SO₂ was another urgent issue which need be tackled for the development of deNO_x catalysts in municipal waste incineration industry [2,104,133–135]

Fig. 6 shows the deactivation mechanism of deNO_x catalysts in the presence of H₂O + SO₂. Water vapor can compete for active sites with reactants and aggravate the poisoning effect of SO₂.

The general strategies to improve H₂O + SO₂ resistance of MnO_x catalysts are also described in Fig. 6. Introducing sacrificial or protective layer was one of the most effective strategies to inhibit the influence of H₂O + SO₂. Huang et al. [63] constructed a TiO₂ nanocage on the surface of CeMnO_x by directional hydrolytic etching of MIL-125 (Fig. 7a). The as-prepared CeMnO_x exhibited excellent water vapor resistance under the protection of TiO₂ nanocages. Song et al. [136] also achieved protective effect by introducing Ce-Ti as the sacrificial sites of H₂O and SO₂ adsorption. Furthermore, improving the amounts of active sites was another effective strategy [134]. Raja et al. [137] modified Mn/TiO₂ catalyst by the introducing of CNTs and Cu. Their results demonstrated that the incorporation of CNTs and Cu increased the amount and acidity of active sites on the MnO_x. In addition, fabricating special structures or optimizing the pore channel of catalysts could also improve the H₂O + SO₂ resistance of deNOx catalysts. Shi et al. [134] successfully synthesized a MnCoO_x catalyst with the unique hollow nanotube structure (Fig. 7b). They found that the unique structure of catalyst well protected the active sites in the inner surface from the poisoning of H₂O + SO₂. The NO_x conversion over MnCoO_x could maintain above 80% for a long time in the presence of 10% H₂O and 100 ppm SO₂.

All in all, the general strategies for improving H₂O + SO₂ resistance of Mn-based catalyst focused on inhibiting the adsorption of vaporous H₂O for now. However, the liquefaction of water vapor would happen with the decrease of working temperature. And the liquid film of H₂O would cover the surface of catalyst and cut off reaction pathway. Therefore, in order to apply the Mn-based catalyst in the industrial practices in the future, the new problems caused by the state change of H₂O will be a concern.

4 Conclusion and perspective

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In conclusion, this review discussed the challenges SCR catalysts are facing in Coal-fired power plants which operate under low-load conditions and SCR catalysts are facing in municipal waste incineration. The studies on broadening working temperature range and improving the SO₂ resistance of SCR catalysts are summarized in coal-fired power plants which operate under low-load conditions and in municipal waste incineration comprehensively.

Although the studies on SCR catalysts have made a great progress, there are still some research directions which should be focused on:

(1) It is generally accepted that the reaction pathway of NH₃-SCR is controlled by L-H and E-R mechanism. However, which mechanism plays a leading role at specific working temperature have not been identified. In the future, the reaction mechanisms of NH₃-SCR should be studied at different working temperature. The related mechanism studies are the guidance of developing novel SCR catalysts which are adapted to specific working temperature.

(2) For now, the industrial application of powder NH₃-SCR catalysts is generally achieved by assembling monolithic catalysts. The main strategy is to coat catalysts on the surface of base materials. However, there are some problems which restrain the further development of monolithic catalysts in industry. The low utilization rate of catalyst coating results in limited deNO_x efficiency. Furthermore, the catalyst coating is easily to drop off due to low interface strength between powder catalysts and supports. These problems have been ignored during the development of catalysts for a long time. The following research directions are deserved to be focused on: how to choose suitable base materials and control the interface between base and catalyst coating; how to design and optimize the structure and reaction channel; the mass and heat transfer studies of the whole reactors; the construction of simulation models for screening and optimizing monolithic structure.

(3) Considering the complex components of industry flue gas, how to improve the catalytic performance of catalysts in terms of multi-pollutants degradation is a prospective research direction. Developing novel catalysts which could efficiently eliminate the multi-pollutants simultaneously is the optimal strategy to simplify the devices and decrease the costs.

Declaration of competing interest

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

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This work was financially supported by the National Natural Science Foundation of China (No. 52072306) and the National Key R&D Program of China (No. 2021YFB3701500), and the Fundamental Research Funds for the Central Universities (Nos. 3102019PJ008 and 3102018JCC002).

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Year 2024 volume 35 Issue 6

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doi: 10.1016/j.cclet.2023.108931

Receive Date：2023-04-25
Online Date：2025-11-21
Published：2024-06-15

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Received：2023-04-25
Revised：2023-07-23
Accepted：2023-08-13

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^a Science and Technology on Thermostructural Composite Materials Laboratory, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China

^b Key Laboratory of Aerosol Chemistry and Physics, State Key Laboratory of Loess and Quaternary Geology (SKLLQG), Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China

^c Center of Excellence in Quaternary Science and Global Change, Chinese Academy of Sciences, Xi'an 710061, China

^d University of Chinese Academy of Sciences, Beijing 100049, China

^e Department of Environmental Science and Engineering, Xi'an Jiaotong University, Xi'an 710049, China

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Table 1 The summary for composition and temperature of the flue gas in different industries.

Table 2 The summary about the literature data for widen operating temperature windows on NH₃−SCR reaction.

Table 3 The summary for the published works which improved SO₂ resistance by different strategies.

Table 4 The summary about the performance of catalysts under ultra-low temperature condition.

Fig. 1 Statistical data of articles on the topic of "SCR catalyst" published in the recent twenty five years. The data were compiled from the database of Web of Science Core Collection on May 18, 2023.

Fig. 2 Reaction mechanism of NH₃-SCR and general strategies for widening operating temperature window of SCR catalysts.

Fig. 3 The poisoning mechanism of SO₂, and general modification strategies of SO₂ resistance.

Fig. 4 Schematic illustration of (a, b) inhibiting adsorption of SO₂. Reprinted with permission [101]. Copyright 2023, Elsevier. Reprinted with permission [103]. Copyright 2022, American Chemical Society. (c, d) inhibiting oxidation of SO₂. Reprinted with permission [105]. Copyright 2021, American Chemical Society. Reprinted with permission [106]. Copyright 2020, American Chemical Society. (e, f) promoting regeneration of active sites. Reprinted with permission [36]. Copyright 2021, Elsevier. Reprinted with permission [114]. Copyright 2018, American Chemical Society.

Fig. 6 The poison mechanism of H₂O and general strategies for improving H₂O resistance.

Fig. 7 (a) Illustration of the hydrolysis etching and construction for CeMnTiO_x. Reprinted with permission [63]. Copyright 2022, Elsevier. (b) Schematic illustration of the formation of MnCoO_x oxides. Reprinted with permission [134]. Copyright 2021, Elsevier.

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