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Review

Sulfur and Water Resistance of Carbon-Based Catalysts for Low-Temperature Selective Catalytic Reduction of NOx: A Review

by
Zhenghua Shen
1,2,
Shan Ren
3,*,
Baoting Zhang
4,
Weixin Bian
4,
Xiangdong Xing
1,2,* and
Zhaoying Zheng
1,2
1
School of Metallurgy Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China
2
Research Center of Metallurgical Engineering Technology of Shaanxi Province, Xi’an 710055, China
3
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China
4
Hanzhong Iron and Steel Co., Ltd., Shaanxi Iron and Steel Group, Hanzhong 724200, China
*
Authors to whom correspondence should be addressed.
Catalysts 2023, 13(11), 1434; https://0-doi-org.brum.beds.ac.uk/10.3390/catal13111434
Submission received: 23 September 2023 / Revised: 3 November 2023 / Accepted: 9 November 2023 / Published: 13 November 2023
(This article belongs to the Special Issue Nanotechnology in Catalysis, 2nd Edition)
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Abstract

:
Low-temperature NH3-SCR is an efficient technology for NOx removal from flue gas. The carbon-based catalyst designed by using porous carbon material with great specific surface area and interconnected pores as the support to load the active components shows excellent NH3-SCR performance and has a broad application prospect. However, overcoming the poor resistance of H2O and SO2 poisoning for carbon-based catalysts remains a great challenge. Notably, reviews on the sulfur and water resistance of carbon-based low-temperature NH3-SCR catalysts have not been previously reported to the best of our knowledge. This review introduces the reaction mechanism of the NH3-SCR process and the poisoning mechanism of SO2 and H2O to carbon-based catalysts. Strategies to improve the SO2 and H2O resistance of carbon-based catalysts in recent years are summarized through the effect of support, modification, structure control, preparation methods and reaction conditions. Perspective for the further development of carbon-based catalysts in NOx low-temperature SCR is proposed. This study provides a new insight and guidance into the design of low-temperature SCR catalysts resistant to SO2 and H2O in the future.

1. Introduction

Nitrogen oxide (NOx) mainly comes from iron and steel enterprises, electric power enterprises, and other fixed sources [1]. As one of the air pollutants, it not only reacts with hydrocarbons to form photochemical smog through ultraviolet irradiation, but it also reacts with water in the air to generate the main component of acid rain, causing a series of serious environmental problems [2,3,4]. This poses a serious threat to the human respiratory system. Therefore, it is an increasingly urgent task to remove NOx. Currently, several denitration technologies, including selective non-catalytic reduction (SNCR) [5,6,7], selective catalytic reduction (SCR) [8,9], and non-selective catalytic reduction (NSCR) [10,11], can be used to reduce NOx. Among them, selective catalytic reduction (SCR) for the removal of NOx is considered to be one of the most promising technologies which has been commercialized for both stationary and mobile NOx sources. The choice of catalyst is crucial for the application of SCR denitration technology. Depending on the efficiency of NOx removal, primarily the operating temperatures, process parameters, and catalyst regeneration, various catalysts including metal (titanium, vanadium, and iron, etc.) oxides [12], noble metals [13], zeolites [14], and activated carbon [15,16] have been developed for the NH3-SCR process. At present, V2O5/TiO2 [17,18] and V2O5-WO3/TiO2 [19] catalysts with high activity and sulfur resistance have been widely used in industrial denitration [20,21]. However, the traditional V-based catalyst temperature is 300–400 °C [22], which does not apply for most industrial flue gas cleaning with lower practical temperature. For example, in the iron and steel industry, the temperature of the sintering flue gas is generally 120–180 °C. It needs to be heated to the application temperature to satisfy the high temperature catalyst, a process that consumes a lot of energy. The supports, such as TiO2, Al2O3, ZrO2, SiO2, and carbon-based materials [23,24,25], have important influence on the denitration performance. Among them, carbon-based materials have gradually become a promising support for low-temperature SCR catalysts due to their large specific surface area, developed pore structure, and broad resources [26]. Compared with titanium, iron, aluminum, and other metal oxide catalysts, the surface of carbon-based catalysts is abundant in oxygen functional groups that can well adsorb NOx. In addition, the larger specific surface area can provide a place for the active metal to be highly dispersed. The catalysts with rare earth oxides and transition metal oxides as active components have excellent catalytic performance at low temperature, such as MnOx/ACs [23,27], CeOx/ACs [28,29], FeOx/ACs [30], etc. In the flue gas, these carbon-based catalysts display high low-temperature SCR activity in atmospheres free of SO2 and H2O. In particular, the valence state of Mn-based catalysts is variable and the redox ability is superb, which display good denitration ability. However, when the flue gas contains SO2 and H2O, the denitration activity of catalysts decreases greatly [31,32,33]. For example, the actual sintering flue gas contains about 10–13% water vapor and 300–1500 mg/Nm3 SO2 [34]. Under the conditions, the catalyst is prone to inactivate, which limits the practical application [35,36]. Therefore, it is necessary to develop the carbon-based catalysts with high SO2 and H2O resistance for low-temperature SCR technology.
Over time, the related scholars have conducted a lot of work on the sulfur and water resistance of low-temperature NH3-SCR catalysts and achieved remarkable results. In addition, the reviews in this area have been widely reported. A review by Zhang et al. comprehensively overviewed the progress of Mn-based catalysts in sulfur and water resistance and focused on analyzing the challenges and opportunities faced in the development of Mn-based catalysts [37]. A review by Xu et al. provided the reaction mechanism of Ce-based catalysts for low-temperature NH3-SCR, and the technology to improve the resistance to sulfur and water was emphasized [38]. The recent review by Tang et al. also summarized the research progress of Mn-based catalysts in improving the denitrification activity of NH3-SCR at low temperature and the resistance to sulfur and water, and the challenges and possible solutions for designing catalyst systems with high sulfur and water resistance were discussed in detail [39]. However, a review of sulfur and water resistance performance of carbon-based catalysts for low-temperature NH3-SCR is rarely reported. This paper has reviewed the research findings on carbon-based catalysts in sulfur and water resistance in recent years. The reaction behavior of the low-temperature SCR catalyst and the poisoning mechanism in the flue gas containing SO2 and H2O are summarized. For carbon-based catalysts, strategies to enhance the SO2 and H2O resistance performance are systematically introduced from the aspects of catalyst support, modification, structure control, and preparation methods. In addition, the application of theoretical calculation in the development of low-temperature SCR carbon-based catalysts resistant to SO2 and H2O is introduced. Finally, the possible development direction in this field is proposed, which plays a certain reference and guidance role for the design of low-temperature carbon-based catalysts resistant to SO2 and H2O in the future.

2. Reaction Mechanisms of Carbon-Based Catalysts for SCR Denitration

2.1. Reaction Mechanisms of Carbon-Based Catalysts

NO oxidation on carbon-based catalysts is a micropore filling process with NO as the adsorbate. NH3-SCR mainly uses NH3 as the reducing agent, which selectively reduces NOx to N2 and H2O at a temperature of 80–400 °C. The catalytic process lowers the activation energy of the chemical reactions [36]. Typical NH3-SCR reaction equations are shown in Equations (1) and (2).
4NO + 4NH3 + O2 → 4N2 + 6H2O
2NO2 + 4NH3 + O2 → 3N2 + 6H2O
If the O2 is absent, the reaction equations are shown as Equations (3)–(5).
6NO + 4NH3 → 5N2 + 6H2O
6NO2 + 8NH3 → 7N2 + 12H2O
NO + NO2 + 2NH3 → 2N2 + 3H2O
At present, there are two generally recognized mechanisms of the NH3-SCR reaction, Eley–Rideal (E-R) [40] and Langmuir–Hinshelwood (L-H) [41,42] mechanisms.
For the E-R mechanism, NH3-SCR of NOx occurs through Equations (6)–(8). NH3 is adsorbed and oxidized to NH2 by acid sites. Then, NH2 active intermediates react with the gas phase NO to form N2 and H2O.
NH3(g) → NH3(a)
NH3(a) + O(a) → NH2(a) + OH(a)
NO(g) + NH2(a) → NH2NO(a) → N2(g) + H2O
The NH3-SCR of NOx according to the L-H mechanism occurs via Equations (6), (9) and (10). NO reacts with O2 to generate adsorbed NO2, which reacts with coordinating NH3 to form N2 and H2O.
NO + O2(g) → NO2(a)
NO2(a) + 2NH3(a) + NO(g) → 2N2 + 3H2O
Carbon-based materials show larger specific surface area and abundant pore structure, which provide more active sites for the reaction. More gas phases NO and O2 are captured on the empty active sites. NO adsorbed on the carbon surface is oxidized to adsorbed NO2. The adsorbed NO2 further reacts with NH3 and NO to generate N2 and releases the active vacancies so that the whole reaction continues. Among them, the adsorption of NO2 on the AC surface involves the participation of NO, and the adsorbed NO2 oxidizes the AC surface, thereby promoting the subsequent NO2 adsorption.
For carbon-based catalysts, at a temperature of 80–200 °C, the NH3-SCR reaction mainly follows the L-H mechanism, or the two mechanisms cooperate. At a higher temperature above 200 °C, it mainly follows the E-R mechanism.

2.2. Poisoning Mechanisms of Carbon-Based Catalysts

The active temperature range of carbon-based NH3-SCR catalysts is rather low, about 100–150 °C, which avoids the clogging of pores on the surface of the catalyst due to excessive temperature. However, they are easily poisoned by H2O and SO2, leading to catalyst deactivation, which seriously affects their application and development in industry. Therefore, in order to improve H2O and SO2 tolerance, relevant scholars have conducted a lot of research on the mechanism of SO2 and H2O poisoning in the process of carbon-based catalyst denitration.

2.2.1. Poisoning of SO2

The catalytic performance of carbon-based catalysts is severely affected by SO2 [43,44,45,46]. The effects of SO2 poisoning can be divided into reversible inactivation and irreversible inactivation. The competitive adsorption of SO2 and NO on carbon-based catalysts is a reversible deactivation. In an irreversible situation, when the adsorbed SO2 reacts with the active components on the carbon-based catalysts’ surface, it generates sulfate, which occupies the active center, resulting in a decrease in available active sites and NOx conversion (Figure 1). For CeO2-WO3/TiO2 catalysts shown in Figure 2, the introduction of SO2 prevents the generation of active intermediates, such as NH2NO. In addition, SO2 reacts with NH3 or CeO2 to form (NH4)2SO4 and Ce2(SO4)3, respectively, which cover the active sites and inhibit the redox performance. This results in the irreversible inactivation. In addition, industrial flue gas usually contains other metals such as Cd, and SO2 binding with it also produces CdSO4 to cover the active sites, causing further deactivation of the catalyst. The production of sulfate on carbon-based catalysts’ surface covering the active sites and the vulcanization of the active metal are the two main forms of permanent deactivation. As shown in Figure 3, the addition of SO2 to a MnFeOx catalyst results in the production of MnSO4, FeSO4, and (NH4)2SO4, which inhibit both L-H and E-R mechanisms. Thus, the catalysts display poor SCR activity. When Sm is dropped, SO2 is preferred to combine with it. As a result, the active sites could be protected. The addition of another metal that preferentially reacts with SO2 is the main method to enhance the SO2 resistance of carbon-based NH3-SCR catalysts.

2.2.2. Poisoning of H2O

Water vapor deprives the catalyst surface of available active sites, thereby negatively affecting the SCR reaction [43,47]. Generally, the poisoning mechanism of H2O includes both reversible inactivation and irreversible inactivation. Among them, the competitive adsorption between H2O and NH3 (or NO) is the main reason for the reversible deactivation (Figure 4). When the adsorption of reactive gas molecules is lower, the NOx conversion rate also decreases. At a lower temperature, the thermal stability of hydroxyl groups is poor, and water has a significant effect on the catalyst activity, resulting in irreversible deactivation. However, when the temperature is higher than 200 °C, water vapor is not easily adsorbed on the catalyst surface, and the effect of water on the catalyst activity can be ignored. Water vapor affects the crystal shape, grain size, and specific surface area of carbon-based catalysts to a certain extent, which results in a reduction in catalytic activity. The adsorption of water molecules on a carbon-based catalyst surface competes with reaction gases for the active sites, reducing the active centers available and thus decreasing the activity of the catalyst.

2.2.3. Poisoning of SO2 and H2O

So far, the poisoning mechanism of H2O and SO2 of low-temperature SCR catalysts has been widely recognized, and the study of the mechanism of carbon-based catalysts resisting SO2 and H2O has made some progress [48,49]. It needs to be emphasized that some catalysts have good resistance to single SO2 or H2O, and the denitration rate could be maintained at a relatively high level. However, when SO2 and H2O coexist, the denitration rate decreases sharply. The main reason is that the reaction of H2O and SO2 produces H2SO4, which accelerates the sulfation of the active metal oxides. Therefore, we still need to conduct in-depth research on carbon-based catalysts with both sulfur and water resistance. As shown in Figure 5, when SO2 is introduced, it is absorbed on the MnOx surface and oxidized to SO3. When the sulphates accumulate to a certain amount, the formation of SO42− polymers result in a reduction in the surface area and the inhibition of redox property. Washing with water can remove SO42− polymers and restore the catalytic activity of active components.

3. Research Progresses of Carbon-Based Catalysts for SO2 and H2O Resistance

In order to improve the denitration activity and enhance the anti-toxic ability of the catalysts, the finding of suitable supports and active components is necessary [50]. The research progress of SO2 and H2O resistant carbon-based catalysts was introduced as follows [51,52].

3.1. Effects of Support

In industrial applications, in terms of thermal stability, mechanical strength, and large specific surface area, the supported catalyst is superior to the non-supported oxide catalyst. Selecting a suitable support is an important way to improve the anti-poisoning ability of carbon-based catalysts. In the presence of O2, SO2 will react with NH3 to form (NH4)2SO4, leading to the blockage of the pores of the catalyst, and H2O is adsorbed on the surface of the support to inhibit the adsorption of reaction gas. It is noted that the number of acid centers affects the adsorption performance of SO2. Therefore, an important method to enhance the resistance of SO2 and H2O is to improve the surface acidity of the support [53,54,55]. At present, the commonly used carbon support mainly includes activated carbon, carbon nanotubes, activated carbon fiber, and graphene. The carbon-based catalysts for SCR described above are summarized in Table 1.

3.1.1. Activated Carbon/Coke

Due to the advantages of abundant surface groups, large specific surface area, high porosity, and strong adsorption capacity, activated carbon/coke (AC) has attracted extensive attention and is widely used as catalyst support [51]. As shown in Figure 6, the reaction of NO on MnOx–Cu/AC catalysts follows both L-H and E-R mechanisms. For the E-R mechanism, NO in the gas phase reacts with NH3 activated by the acidic sites on the catalyst surface. For the L-H mechanism, NO and O2 in the gas phase interact and are oxidized to nitrate or nitrite intermediates by the catalyst, which then react with activated NH3. It is worth noting that regardless of the mechanism, the activation of NH3 at the acidic site is a key step for the reaction to proceed. The adsorption properties of AC are impacted by changing the polarity and/or acidity of surface functional groups. The main active sites on activated carbon are different kinds of oxygen-containing groups. Some researchers believe that for the V2O5/AC catalyst, in the presence of H2O, SO2 will react with NH3 and O2 to form ammonium sulfate, causing the pores of the catalyst to be blocked. As shown in Figure 7, after the introduction of SO2, the activity of the V2O5/AC catalyst decreased rapidly, then had a weak rise, and finally declined slowly to remain a stable value [53]. However, when the SO2-poisoned V2O5/AC catalyst was characterized, it is observed that only a small part of the pores on surface are blocked by sulfate. The generation of VOSO4 via the reaction of V2O5 and SO2 caused the NO conversion to drop sharply, which is the main reason for the deactivation of the catalyst. The sulfurization of active metal oxides by SO2 is the main cause of the irreversible inactivation of most carbon-based catalysts in sulfur-containing flue gas. For Mn–Ce/AC catalysts shown in Figure 8, SO2 reacted with NH3 to inhibit the reaction of NO and NH3 [54]. On the other hand, SO2 combined with manganese oxide and cerium oxide to form MnSO4 and (Ce)2(SO4)3, which caused permanent inactivation. The addition of V could improve the acidity of the catalyst surface and inhibit the combination of SO2 and NH3. In addition, the vulcanization of Mn–Ce solid solution is also prevented. Therefore, the sulfur resistance of the catalyst could be enhanced.
In addition, different types of activated carbons have different specific surface areas and pore volumes. Among them, Mn–Ce catalysts supported by charcoal and coal exhibit a large specific surface area and strong surface acidity, which leads to an increase in adsorbed oxygen and high dispersibility of metal oxides with high catalytic activity. Meanwhile, the surface properties of AC have an obvious influence on the structure and performance of the supported catalysts. Nitric acid treatment can enhance the surface acidity of activated carbon and the dispersibility of active components and can significantly improve the NO conversion and SO2 resistance of the catalyst [56]. However, AC catalysts are still prone to deactivation when SO2 reaches a certain level. Therefore, further efforts should be made to strengthen the SO2 resistance of SCR catalysts.

3.1.2. Carbon Nanotubes

As an allotrope of carbon, the pore size of carbon nanotubes (CNTs) can range from several nanometers to 100 nm. CNTs have adjustable nano hollow tubular structures, with a specific surface area of generally about 50–1300 m2/g and a tensile strength of 50–200 GPa [59,60]. CNTs are used as supports for NH3-SCR catalysts due to their high adsorption capacity of NOx and NH3 as well as good SO2 and H2O resistance (Figure 9). At present, the research on CNT supports mainly focuses on the morphology of CNTs and the direction of CNTs composite supports. Wang et al. [61] elucidated the effects of SO2 and water vapor on Ce/AC-CNTs catalysts. Obviously, when adding 50 ppm of SO2, the Ce/AC-CNTs catalyst had a small inhibition on NO conversion, which was only reduced by 4.8%. In addition, when H2O was introduced, the activity of Ce/AC-CNTs reduced by 21.7%, while the NO conversion of Ce/AC declined by 30.6%. CNTs can not only change the oxidation state of surface oxygen and active components but also increase the concentration of chemically adsorbed oxygen species. CNTs have a certain resistance to SO2 and H2O [62]. The morphology of CNTs is also related to the performance of the catalyst. When SO2 and H2O coexist, the denitration rate of multi-shell CNT catalysts is nearly 60% higher than that of ordinary CNT catalysts. Multi-shell CNTs can effectively inhibit the generation of surface sulfate species [63]. However, due to the limitations of the preparation process and scale of CNTs, the cost of CNTs is very high, which is known as “black gold”. The use of CNTs as catalyst supports is still in the stage of laboratory research and has not been industrialized.

3.1.3. Activated Carbon Fiber

Activated carbon fiber (ACF) is usually a porous fiber material that is activated by carbon fiber or various carbon-containing materials at high temperature. ACF has a large specific surface area which can reach 1000–1500 m2/g or even more than 2000 m2/g. Among the pores of ACF, the proportion of micropores accounts for more than 90%, most of which are distributed on the fiber surface with narrow pore size, uniform distribution, easy contact with adsorbates, and strong adsorption capacity for low-concentration adsorbates. Hence, it has a fast adsorption and desorption rate and excellent adsorption and separation performance.
ACF has a certain catalytic activity and strong adsorption capacity for NO, and the adsorbed NO and O2 in micropores can be converted into NO2. However, the removal efficiency of NO using ACF as a catalyst alone is low. The highest NO conversion of ACF in the range of 150–310 °C is only about 15%. When 10% CeO2 is added to ACF, the denitration activity is greatly improved. In the temperature range of 140–240 °C, more than 85% of the denitration rate can be achieved, which is higher than that of the same amount of the MnOx catalyst [64]. The catalytic activity of ACF can be greatly increased by modifying it with active components (Figure 10). However, the cost of catalysts supported by activated carbon fiber is also very high, and it has not been applied in industry at present.

3.1.4. Graphene

Compared with other carbonaceous supports, Graphene (GE), as a novel type of carbon nanomaterial, has a unique planar extension structure with a specific surface area of 100–2600 m2/g and conductivity of 106–108 S/m [65]. The excellent electronic properties can promote electron transfer in the redox process, which accelerates the catalytic reaction. The special structure of GE can disperse active components to improve the interaction, which produces more active oxygen species and active sites to enhance the low-temperature catalytic activity. TiO2-GE nanocomposite support shows uniform components, which is conducive to increasing the specific surface area of the carrier, changing its pore structure characteristics, and promoting the uniform loading of active ingredients. At 180 °C, the NO conversion of the CeOx–MnOx/TiO2-GE catalyst reaches 95% [66]. As a catalyst support, GE can also inhibit the sulfation of active components. Compared with the unsupported catalyst [67], the GE catalyst has a strong resistance ability for H2O and SO2 (Figure 11). The NO conversion is restored from 73% to 79% at 180 °C after the H2O and SO2 is stopped. However, the existing graphene preparation process is immature, which limits the large-scale application.

3.2. Effects of Modification

Many scholars have studied the influence of active components on the anti-toxicity and denitration performance of the catalysts [68,69]. At present, more research is focused on Mn-based catalysts [70] and Ce-based catalysts [71]. Additionally, there are other metal oxide catalysts such as V and Fe catalysts. This section mainly introduces the related research on the single modification, bimetallic modification, and polymetallic modification of metal oxides.

3.2.1. Single Metal Oxide-Modified Catalysts

Single metal-modified catalysts mainly include Mn-modified catalysts, Ce-modified catalysts, Fe-modified catalysts, V-modified catalysts, and other modified catalysts.

Single Mn-Modified Catalysts

Mn-based catalysts, as a research hotspot, have a variety of valence states and excellent redox ability, easily carry out the oxidation reaction, and have higher catalytic performance at low temperatures.
Generally, the higher valence state shows the better catalytic effect for Mn-based catalysts. The SCR activity decreases as follows: MnO2 > Mn5O8 > Mn2O3 > Mn3O4 [72]. MnOx-modified catalysts can increase the dispersity of amorphous states, the ratio of Mn4+/Mn3+, and the surface area and pore volume, which improves the NO conversion [73]. The performance of the catalyst is enhanced by adding Mn in an amount in the range of 10–20 wt.%. When the content of Mn increases to 25 wt.%, MnOx particles gather on the catalyst surface and the NO conversion decreases (Figure 12). However, SO2 has a serious toxicity to single Mn-based catalysts. The active Mn atoms are sulfated after the introduction of SO2, which leads to rapid deactivation. It is usually difficult to regenerate for the deactivated Mn-based catalysts.

Single Ce-Modified Catalysts

As an important rare earth metal oxide, CeO2 has strong redox performance and excellent oxygen storage/release ability. Between Ce4+ and Ce3+, the electron transfer promotes an increase in active oxygen species and accelerates the conversion of NO to NO2, thereby improving the activity of SCR. Therefore, CeO2 is usually used as an active component to improve NH3-SCR activity. Doping the rare earth element Ce effectively facilitates the generation of oxygen vacancies. The high concentration of oxygen vacancy is favorable for the adsorption of O2 and the further oxidation of NO, which is conducive to the subsequent reduction reaction [38]. In all temperature ranges, the maximum NO conversion of CeO2 nanoparticles is only 50%. The NO conversion on pure carbon nanotubes is extremely low. While carbon nanotubes are introduced as supports for CeO2, the NO conversion rate is increased significantly for all catalysts. In the temperature range of 250 °C to 370 °C, the CeO2/CNTs platinum catalysts display the best activity, corresponding to above 90% NO conversion. For CeO2/CNTs-PM catalysts, the maximum conversion of NO can only reach 80% below 380 °C [74]. The catalysts with a lower addition have less dispersed active sites, an incomplete catalytic reduction reaction, and low denitration efficiency. Appropriate CeO2 content can enhance the active sites on the catalyst surface and improve the catalytic performance. Excessive CeO2 loading causes the CeO2 particles to aggregate on the surface, which leads to a reduction in active sites. Compared with CeO2 nanoparticles, CeO2 nanotubes on the catalyst surface have more Ce and O atoms, more acidic centers, and stronger acidity, which is conducive to improve SCR performance [75]. The SO2 can preferentially bind with CeO2 to form Ce2(SO4)3, thereby reducing the generation of (NH4)2SO4 and NH4HSO4 to suppress catalyst deactivation (Figure 13) [76]. Therefore, CeO2-modified catalysts have broad prospects.

Single Fe-Modified Catalysts

Iron in Fe2O3 has a variety of valence states such as +2 and +3, which shows good redox properties. In addition, it has the characteristics of non-toxic and harmless, wide source, and low price. The mutual conversion between Fe2+ and Fe3+ can form unstable oxygen vacancies and lattice oxygen species with high transfer ability. Therefore, Fe is often used as the active component and support of catalysts and has attracted extensive attention among scholars [77,78,79]. For Fe-modified AC catalysts, when the molar ratio of Fe to AC was 0.10 [80], the performance was optimal, exhibiting 83.9% NO conversion at 240 °C. However, when the ratio was 0.15, the Fe2O3/AC catalyst displayed poor NO conversion. The influences of SO2, H2O, or SO2 + H2O on the NOx conversions of 8Mn6Fe/AC catalysts were investigated in [81]. The NOx conversion decreased by 12% when H2O was introduced into the reactor, due to a competitive adsorption between NH3 and H2O, and then, the activity remained stable. Fe was mainly dispersed in the form of γ-Fe2O3. After the introduction of Mn, the ratio of Fe3+/Fe2+ + Fe3+ changed only a little, and the value of Oβ/(Oβ + Oα) increased significantly. The doping of Mn increased the amount of chemisorbed oxygen with higher migration ability, which could oxidize NO to NO2 to form a “fast SCR” reaction, thus promoting the denitration performance and SO2 resistance of the catalysts (Figure 14).

Single V-Modified Catalysts

Vanadium is a rich material with low cost and high energy efficiency which can be loaded on the support to increase the activity and anti-poisoning ability of catalysts. In the field of NH3-SCR [82,83], traditional vanadium-titanium catalysts are widely applied, which makes more researchers explore vanadium-titanium catalysts that meet the anti-poisoning performance and low-temperature denitration performance.
V2O5/AC catalysts have a high SOx absorption capacity at 120–200 °C [84]. When the V addition is 5%, the NO conversion can reach 80% at 250 °C. After adding SO2, the NO conversion increases to more than 90%. For the abnormal phenomenon of increasing NO conversion, it is found that the interaction of AC with vanadium oxide activates the ammonium sulfate on the low-loaded V2O5/AC catalyst. The activated species react with NO, increase the acidity on the surface, and enhance the catalytic activity. Catalysts with V2O5 as the active component do not perform as well as Mn-based catalysts in low-temperature SCR reactions, but its sulfur resistance is obviously higher than Mn-based catalysts.

Other Single-Modified Catalysts

In addition to MnOx, CeO2, FeOx, and V2O5 modification, there are also some other modified catalysts which can also improve the denitration performance and poisoning resistance [85,86,87]. For nickel-supported carbon-based catalysts, appropriate pore structure and surface area can improve the SO2 resistance. Doping the transition metal oxide CrO3 on activated carbon can increase the formation of acid sites. Due to the valence change between Cr6+ and its low oxidation states, such as Cr5+, Cr3+, and Cr2+, the reaction rate of SCR is increased. When the mass ratio of Cr/AC is 2%, the NO removal efficiency is optimal, and the NO conversion rate is more than 90% at 125–150 °C. The NO conversion on the catalyst decreases after the addition of 100 ppm SO2. When 300 ppm SO2 is added, the NO conversion reduces to only about 5%. When 5% H2O is added, the NO conversion decreases by about 15%. The NO conversion decreases to 71.5% within a few minutes under the conditions of 100 ppm SO2 and 5% H2O [88]. The coexistence of H2O and SO2 leads to the formation and deposition of ammonium sulfate, which blocks active sites on the catalysts and inactivates the catalysts.
However, single-modified catalysts show weak surface acidity, relatively low activity, a narrow working temperature window, and poor resistance of SO2 under high temperature, which limit their practical application.

3.2.2. Bimetallic-Modified Catalysts

Bimetallic-modified catalysts mainly introduce Mn–Ce catalysts, Fe–Mn catalysts, V–Mn catalysts, and other bimetallic-modified catalysts.

Mn–Ce Catalyst

Manganese-containing catalysts show excellent catalytic activity, while SO2 resistance ability is poor. CeO2 has excellent redox properties and certain sulfur resistance. The addition of Ce can enhance the oxygen storage capacity of the catalyst and the capacity of NO oxidation to NO2. The NO conversion of MnOx–CeO2/AC catalysts is maintained at 92% at 210 °C after adding SO2, which has excellent resistance to SO2 toxicity [89]. The sulfation of active components is not obvious within a short time at 210 °C, and there is no obvious accumulation of NH4HSO4 in the reaction process. CeO2 could reduce the bond energy of NH4+ and SO42− and lower the decomposition temperature of NH4HSO4 to enhance the SO2 resistance. When the support is carbon nanotubes, the Mn–CeCNTs-R catalyst has good dispersibility of active components on its surface [90]. Under the conditions of 4 vol% H2O and 100 ppm SO2, it shows strong tolerance. With the coexistence of H2O and SO2, the NO conversion of the catalyst decreases by 13%. The NO conversion rate returns to 90% after stopping the addition of SO2 and H2O (Figure 15). Compared with the single metal-modified catalysts, the resistance to SO2 of Mn–Ce bimetallic catalysts is significantly improved.

Fe–Mn Catalyst

At low temperatures, Fe–Mn bimetallic oxides have high NO conversion and SO2 resistance. The NO conversion of Mn–FeOx/CNTs catalysts is close to 98% at 180 °C and decreases by 20% after adding SO2 [91]. By being in the form of amorphous oxides, Fe and Mn are highly dispersed on the support surface so that the catalyst has good redox performance. The NO conversion of Fe2O3@MnOx@CNTs catalysts decreases from 95% to 91% and recovers to 95% after SO2 and H2O removal, while MnOx@CNTs catalysts only recovered to 36% and could not be restored to the initial level [60]. The resistance of Fe–Mn bimetallic catalysts to SO2 and H2O is better than Mn–Ce bimetallic catalysts.

Other Bimetallic-Modified Catalysts

In addition to the above bimetallic catalysts, other bimetallic catalysts can also enhance resistance to H2O and SO2 performance. For instance, CeMo(0.3) hollow microsphere catalysts prepared with carbon microspheres as a template not only have the best low-temperature SCR performance [92] but have good stability and H2O resistance as well (Figure 16). For Fe2O3/AC catalysts, doping Ce can make γ-Fe2O3 evenly disperse on the AC surface, and the denitrification efficiency is significantly improved. Moreover, the sulfur resistance can be enhanced. The catalysts with a Ce/Fe mass ratio of 0.5:6 can achieve 94.1% NO conversion when 100 × 10−6 (vol) SO2 is added at 240 °C. When H2O is 5 wt.%, the NO conversion is stable at 86%. The introducing of slight vanadium oxide can enhance the dispersity of Fe species on Fe2O3/AC catalysts [93]. Whether it is V-modified or non-V-modified, in the presence of H2O and SO2, catalysts are severely deactivated. But at low space velocities, the inhibition caused by H2O and SO2 is reversible for 3%Fe-0.5%V catalysts. The addition of V species prevents the generation of sulfate by increasing the surface acidity, thereby enhancing the resistance to SO2.

3.2.3. Polymetallic-Modified Catalyst

Polymetallic-modified catalysts mainly introduce Fe–Mn–Ce catalysts, V–Mn–Ce catalysts, and other polymetallic-modified catalysts.

Fe–Mn–Ce Catalysts

Fe is often used as an additive to modify the catalyst. The effect of doping Fe on AC-supported Mn–Ce oxide catalysts are investigated for NH3-SCR. When the content of Fe is 5%, the NO conversion of Mn–Ce–Fe/AC is 90% at 125 °C and 12,000 h−1 space velocity [94]. As shown in Figure 17, the metal ions can enter the graphite crystal structure of AC and divide it into smaller graphene fragments. The doping of Fe can inhibit the decrease in surface area in the calcining process of the catalyst. In addition, the ratios of Mn4+/Mnn+ and Ce3+/Cen+ and the amount of adsorbed oxygen and acid increase significantly after Fe doping. The main reason is that the Fe species expose the active sites of the acid or influence the chemical state of Mn/Ce species. The performance and SO2 resistance of Mn–Fe–Ce/ACN catalysts is better than that of Mn/ACN catalysts. As shown in Figure 18, the surface acidity, reducibility, and surface chemisorbed oxygen are improved due to the addition of FeOx and CeO2, which promote the NH3-SCR performance [95]. In addition, the stronger surface acidity inhibits the adsorption of SO2 and the consumption of SO2 to adsorbed NH3. Moreover, a small amount of SO2 adsorbed on the catalyst surface reacts preferentially with CeO2 to protect the main active components MnOx and FeOx from sulfation.

V–Mn–Ce Catalyst

For Mn–Ce/AC catalysts, doping V2O5 can obviously increase the NO conversion and improve SO2 resistance. Whether it is V–Mn–Ce/AC or Mn–Ce/AC catalysts, their NO conversions are close to 98% at 200 °C. After adding SO2, the NO conversion decreases slightly to about 90% for V–Mn–Ce/AC catalysts, and there is little change after SO2 removal. For Mn–Ce/AC catalysts, the NO conversion drops significantly to around 68%, and it hardly restores after SO2 removal. In the coexistence of SO2 and H2O, the deactivation of V–Mn–Ce/AC is more pronounced and the inhibition is irreversible, compared to the presence of SO2 alone (Figure 19) [54]. Doping V2O5 enriches the chemically adsorbed oxygen and enhances the surface acidity, which accelerates the reaction of SCR. To some extent, the V2O5 clusters prevent SO2 forming and sulfating the dispersed Mn–Ce solid solution.

Other Polymetallic-Modified Catalysts

Novel core-shell structure SiO2@FeOx–CeOx/CNTs catalysts have good SO2 resistance and high stability. The introduction of 500 ppm SO2 shows little influence on the catalytic activity, and the NO removal rate is about 90% [96]. Fe–V2O5/TiO2-CNT catalysts also show excellent SCR performance. The NO conversion of the Fe3-V/Ti–C catalyst decreases from 96% to 91% after the addition of SO2 [97]. When SO2 is stopped, the activity recovered to 94% (Figure 20). In the coexistence of oxygen, water vapor, and sulfur dioxide, the NO conversion of the Cu–Ce–Fe–Co/AC catalyst remains at 100% for 240 min [98].

3.3. Application of Theoretical Calculation

As one of the most commonly used methods in computational chemistry, density functional theory (DFT) takes electron density instead of wave function as the research object. For DFT calculations [99,100,101,102], it can establish that quantity a SCR catalytic reaction is a fast process which cannot be understood by experimental characterization. The structure–activity relationships are revealed by simulating the adsorption process of active molecules and toxic molecules on the catalyst surface. The adsorption behavior of NO, NH3, and O2 on the FexOy/AC surface is investigated by DFT. On vacancies of the AC surface, FexOy clusters are stably adsorbed with accompanying charge redistribution. These are nitroso, nitro, and nitrite when NO is adsorbed on the surface of FexOy/AC (Figure 21) [103]. Through DFT analysis and calculation, the adsorption process of reaction gases on the Mn–MOF-74 metal-organic framework catalyst system can be obtained. It was found that the molecules had competitive adsorption at the Mn metal sites. Compared with H2O, SO2 can displace NH3 more easily, which explains the poisoning difference between H2O and SO2 [104]. In addition, DFT calculations have shown that the SCR activity of the FexOy/AC catalyst is 28% higher than that of the corresponding oxide catalyst. Carbon deposition increases both the amount of intermediate/strong acid centers and the reducibility of catalytic centers.

4. Other Strategies for Improving Sulfur and Water Resistance

In addition to selecting appropriate supports and active components for modification, it can also be improved by controlling the preparation method and reaction conditions of the catalyst for the SO2 and H2O resistance.

4.1. Preparation Methods

The successful support of metal oxide catalysts on adsorbents can be achieved by a variety of synthetic methods. So far, the preparation methods of denitration catalysts include the sol–gel method, citric acid method, precipitation method, ion exchange method, and impregnation method. The preparation method mainly affects the physical properties of the catalyst, including surface oxygen vacancy, pore volume, and specific surface area [105,106,107]. At present, the most common synthetic methods are impregnation and deposition precipitation. The enrichment of carbon support by completely immersing and mixing it with a metal precursor (i.e., copper nitrate solution) is called wet pore volume impregnation [108]. The MnOx–CeO2/P-CA catalysts were prepared via incipient wetness impregnation using Mn(NO3)2∙4H2O and Ce(NO3)3∙6H2O as precursors on phosphorus-doped carbon aerogels (P-CA) for NH3-SCR of NO. The hydrophilicity of the carbon carrier for the MnOx–CeO2/P-CA catalyst was improved, which was conducive to the dispersity of active components and enhanced the electronic interaction between MnOx and CeO2. Even with the presence of SO2, the catalyst can still adsorb and oxidize more NO, forming more NO complexes. And the suppression of SO2 to the SCR reaction is reduced through the L-H mechanism, which enhances the anti-SO2 ability (Figure 22).
Deposition precipitation means that the metal precursor (i.e., nitric acid) is dissolved with the precipitant, and the carbon support (bulk) is completely mixed and heated. The precipitation method could make the distribution of active components more uniform so they have better adsorption and oxidation properties for NO. However, the precipitation method generates more impurities, which affects the performance of the catalyst. The operation of the impregnation method is simple, while the dispersity of the obtained catalyst’s metal active particles is low.

4.2. Preparation and Reaction Conditions

The catalytic performance of catalysts is affected by reaction temperature, space velocity, initial NO concentration, O2 concentration, NH3 concentration, and SO2 concentration [98]. The NOx conversion of Mn–Ce/AC catalysts calcined in a N2, O2, and air atmosphere is 94%, 75.6%, and 85.6%, respectively [109]. The catalyst calcined under the N2 atmosphere could increase the dispersity of metallic oxides and enhance the surface acidity, reducing the oxidation and crystal formation of MnO2 which positively impacts the catalytic oxidation performance and sulfur tolerance on catalysts. In addition, the impacts of loadings and precursors on the activity of MnOx/ACs catalysts have been studied [110]. Mn3O4 using manganese acetate tetrahydrate as a precursor with a loading of 8% shows the best removal effect on NO, and the removal rate is 97% at 180 °C (Figure 23).

5. Conclusions and Perspectives

Low-temperature SCR technology is one of the most effective technologies for removing NOx from flue gas. Carbon-based catalysts show excellent low-temperature activity and are a promising SCR catalyst. However, the presence of SO2 and H2O in the flue gas can easily lead to catalyst poisoning and deactivation. This paper reviews the research progress of low-temperature SCR for NOx removal against SO2 and H2O of carbon-based catalysts. The low-temperature NH3-SCR reaction mechanism and SO2 or/and H2O poisoning mechanism are discussed, and the main strategies to enhance the SO2 and H2O resistance are summarized. In addition, the characteristics of common carbon-based materials are introduced. Moreover, the advantages and disadvantages of main low-temperature carbon-based SCR catalysts are evaluated comprehensively, and the application of DFT theoretical calculation in catalyst design and improvement of sulfur and water resistance is analyzed. Although great progress has been made in recent years, many catalysts can only be used under laboratory conditions and do not meet practical industrial application needs, so more in-depth research is needed.
(1)
At present, the mixing or doping of metal oxides to carbon-based catalysts has been extensively studied to enhance H2O and SO2 resistance. However, the mechanism of anti-sulfur and anti-water reactions of carbon-based catalysts has not been thoroughly explored.
(2)
When the carbon-based material is used as the carrier of the catalyst reaction, in addition to providing a larger specific surface area, the active site is increased and the activity is improved. In this process, whether the carbon-based materials participate in the reaction process or have a certain influence on the active components needs further study.
(3)
Many studies have focused on the effect of supports on the activity of carbon-based catalysts and their resistance to sulfur and water. However, the influences of different preparation processes on the catalytic performance and sulfur resistance of carbon-based material have not been fully considered and studied. The influence of various factors on the anti-sulfur and water-resistance of carbon-based catalysts needs to be further studied.
(4)
Mn–Ce carbon-based catalysts have excellent low-temperature NH3-SCR activity and certain resistance to sulfur and wate, and are considered as the most promising denitrification catalysts. However, the research of Mn–Ce carbon-based catalysts is mainly in the laboratory stage, and its original poisoning mechanism needs to be further explored.
(5)
In the actual industrial flue gas, in addition to SO2 and H2O, it also contains heavy metals, alkali metals, and dust. Therefore, the influence mechanisms of SO2/H2O and other harmful substances on carbon-based catalysts need to be studied. Moreover, corresponding anti-poisoning strategies should also be proposed.
(6)
Noble metal catalysts have excellent catalytic performance, but they cannot be used in large scale due to the high cost. The use of non-noble metal catalysts instead of noble metal has become the focus and general trend of research. However, most of the non-noble metal carbon-based catalysts possess poor stability and general sulfur and water resistance and are only used in the laboratory. It is necessary to develop non-precious metal carbon-based catalysts with excellent sulfur and water resistance under industrial conditions.
(7)
Each industrial flue gas containing NOx has its own characteristics, and a single catalyst is difficult to meet the needs of NOx removal in all flue gases. Therefore, high-performance NH3-SCR catalysts should be designed and developed according to the characteristics of the actual flue gas.
(8)
At present, carbon-based SCR catalysts have achieved corresponding results in improving sulfur and water resistance, but most catalysts are still inactivated after use for a period of time. In my opinion, the rapid and low-cost regeneration methods of deactivated catalysts and corresponding mechanisms are another topic for future research.

Author Contributions

Z.S.: Conceptualization, writing the original draft, and formal analysis; S.R.: writing, review, and editing, funding acquisition, and supervision; B.Z.: conceptualization; W.B.: formal analysis and investigation; X.X.: writing, review, and editing, funding acquisition, and supervision; Z.Z.: review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Science Basic foundation of China (No. 52174325), the Shaanxi Province key research and development plan project (2019TSLGY05-05), the Shaanxi Province innovation ability support plan (2023-CX-TD-53), and the Shaanxi Provincial Department of Education Key Laboratory Scientific Research Project (20JS072). And The APC was funded by 20JS072. The authors gratefully acknowledge their support.

Data Availability Statement

Not applicable.

Conflicts of Interest

Authors Baoting Zhang and Weixin Bian were employed by the company Hanzhong Iron and Steel Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. There are no conflict of interest.

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Figure 1. Reaction routes for fresh Fe/CNTs and SO2-Fe/CNTs catalysts. (Reproduced with permission from reference [43], Copyright 2018, MDPI (Basel, Switzerland)).
Figure 1. Reaction routes for fresh Fe/CNTs and SO2-Fe/CNTs catalysts. (Reproduced with permission from reference [43], Copyright 2018, MDPI (Basel, Switzerland)).
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Figure 2. Offset effects between Cd and SO2 over CeO2–WO3/TiO2 catalysts. (Reproduced with permission from reference [44], Copyright 2020, American Chemical Society (Washington, DC, USA)).
Figure 2. Offset effects between Cd and SO2 over CeO2–WO3/TiO2 catalysts. (Reproduced with permission from reference [44], Copyright 2020, American Chemical Society (Washington, DC, USA)).
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Catalysts 13 01434 g003
Figure 3. Mechanism model of Sm promoting SCR activity and SO2 resistance over MnFeOx catalysts. (Reproduced with permission from reference [46], Copyright 2022, Elsevier (Amsterdam, The Netherlands)).
Figure 3. Mechanism model of Sm promoting SCR activity and SO2 resistance over MnFeOx catalysts. (Reproduced with permission from reference [46], Copyright 2022, Elsevier (Amsterdam, The Netherlands)).
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Catalysts 13 01434 g004
Figure 4. Scheme of regular SCR reaction and H2O poisoning effect. (Reproduced with permission from reference [43], Copyright 2018, MDPI (Basel, Switzerland)).
Figure 4. Scheme of regular SCR reaction and H2O poisoning effect. (Reproduced with permission from reference [43], Copyright 2018, MDPI (Basel, Switzerland)).
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Catalysts 13 01434 g005
Figure 5. Proposed sulfur–treated poisoning and water–washed regeneration mechanism model. (Reproduced with permission from reference [48], Copyright 2021, MDPI (Basel, Switzerland)).
Figure 5. Proposed sulfur–treated poisoning and water–washed regeneration mechanism model. (Reproduced with permission from reference [48], Copyright 2021, MDPI (Basel, Switzerland)).
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Catalysts 13 01434 g006
Figure 6. NO removal mechanism of the MnOx–Cu/AC catalyst. (Reproduced with permission from reference [51], Copyright 2020, MDPI (Basel, Switzerland)).
Figure 6. NO removal mechanism of the MnOx–Cu/AC catalyst. (Reproduced with permission from reference [51], Copyright 2020, MDPI (Basel, Switzerland)).
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Figure 7. NO conversions and SO2 release concentrations during the SCR of NO over various V2O5/AC catalysts at 200 °C. (Reproduced with permission from reference [53], Copyright 2014, American Chemical Society (Washington, DC, USA)).
Figure 7. NO conversions and SO2 release concentrations during the SCR of NO over various V2O5/AC catalysts at 200 °C. (Reproduced with permission from reference [53], Copyright 2014, American Chemical Society (Washington, DC, USA)).
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Figure 8. Mechanism of SO2 tolerance over Mn–Ce(0.4)−V/AC catalysts. (Reproduced with permission from reference [54], Copyright 2019, Elsevier (Amsterdam, The Netherlands)).
Figure 8. Mechanism of SO2 tolerance over Mn–Ce(0.4)−V/AC catalysts. (Reproduced with permission from reference [54], Copyright 2019, Elsevier (Amsterdam, The Netherlands)).
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Catalysts 13 01434 g009
Figure 9. NOx conversion over Mn–Ce/TiO2 and Mn–Ce/TiO2–CNTs catalysts. (Reproduced with permission from reference [59], Copyright 2021, Elsevier (Amsterdam, The Netherlands)).
Figure 9. NOx conversion over Mn–Ce/TiO2 and Mn–Ce/TiO2–CNTs catalysts. (Reproduced with permission from reference [59], Copyright 2021, Elsevier (Amsterdam, The Netherlands)).
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Catalysts 13 01434 g010
Figure 10. Effects of H2O or/and SO2 on NOx conversion of (a) MnOx–CeO2 (8:1)/GR and (b) MnOx–CeO2 (2:1)/GR at 140 °C. (Reproduced with permission from reference [64], Copyright 2017, Elsevier (Amsterdam, The Netherlands)).
Figure 10. Effects of H2O or/and SO2 on NOx conversion of (a) MnOx–CeO2 (8:1)/GR and (b) MnOx–CeO2 (2:1)/GR at 140 °C. (Reproduced with permission from reference [64], Copyright 2017, Elsevier (Amsterdam, The Netherlands)).
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Figure 11. Effect of H2O and SO2 on NOx conversion over 7%MnOx/TiO2–0.8%GE catalysts. (Reproduced with permission from reference [67], Copyright 2014, American Chemical Society (Washington, DC, USA)).
Figure 11. Effect of H2O and SO2 on NOx conversion over 7%MnOx/TiO2–0.8%GE catalysts. (Reproduced with permission from reference [67], Copyright 2014, American Chemical Society (Washington, DC, USA)).
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Catalysts 13 01434 g012
Figure 12. Effect of Mn loading of MnOx/MWCNTs (60–100, 500) catalysts on catalytic activity. (Reproduced with permission from reference [68], Copyright 2021, Elsevier (Amsterdam, The Netherlands)).
Figure 12. Effect of Mn loading of MnOx/MWCNTs (60–100, 500) catalysts on catalytic activity. (Reproduced with permission from reference [68], Copyright 2021, Elsevier (Amsterdam, The Netherlands)).
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Catalysts 13 01434 g013
Figure 13. Influence of the SO2 concentration on the NOx reduction performance over sample of V2O5-WO3/TiO2. (Reproduced with permission from reference [76], Copyright 2012, Elsevier (Amsterdam, The Netherlands)).
Figure 13. Influence of the SO2 concentration on the NOx reduction performance over sample of V2O5-WO3/TiO2. (Reproduced with permission from reference [76], Copyright 2012, Elsevier (Amsterdam, The Netherlands)).
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Figure 14. XPS spectra for (A) Fe 2p, (B) Mn 2p, and (C) O 1 s of Fe2O3/AC (a) and Mn–Fe2O3/AC (b) catalysts. (Reproduced with permission from reference [81], Copyright 2018, Wiley-Blackwell (Hoboken, NJ, USA)).
Figure 14. XPS spectra for (A) Fe 2p, (B) Mn 2p, and (C) O 1 s of Fe2O3/AC (a) and Mn–Fe2O3/AC (b) catalysts. (Reproduced with permission from reference [81], Copyright 2018, Wiley-Blackwell (Hoboken, NJ, USA)).
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Figure 15. NH3-SCR performance of Mn–Ce@CNTs–R catalysts. (a) Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 3 vol%, N2 balance, and GHSV = 10,000 h−1. (b) Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 3 vol%, N2 balance, and GHSV = 10,000 h−1, [SO2] = 500 ppm. (c) Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 3 vol%, N2 balance, and GHSV = 10,000 h−1, [H2O] = 4 vol%. (d) Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 3 vol%, N2 balance, and GHSV = 10,000 h−1, [H2O] = 4 vol%, [SO2] = 500 ppm. (Reproduced with permission from reference [90], Copyright 2018, Royal Society of Chemistry (London, UK)).
Figure 15. NH3-SCR performance of Mn–Ce@CNTs–R catalysts. (a) Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 3 vol%, N2 balance, and GHSV = 10,000 h−1. (b) Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 3 vol%, N2 balance, and GHSV = 10,000 h−1, [SO2] = 500 ppm. (c) Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 3 vol%, N2 balance, and GHSV = 10,000 h−1, [H2O] = 4 vol%. (d) Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 3 vol%, N2 balance, and GHSV = 10,000 h−1, [H2O] = 4 vol%, [SO2] = 500 ppm. (Reproduced with permission from reference [90], Copyright 2018, Royal Society of Chemistry (London, UK)).
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Figure 16. (a) The study of thermal stability and (b) H2O resistance and SO2 tolerance at 250 °C on CeMo(0.3) hollow microspheres. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5%, balanced in N2. (Reproduced with permission from reference [92], Copyright 2016, Royal Society of Chemistry (London, UK)).
Figure 16. (a) The study of thermal stability and (b) H2O resistance and SO2 tolerance at 250 °C on CeMo(0.3) hollow microspheres. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5%, balanced in N2. (Reproduced with permission from reference [92], Copyright 2016, Royal Society of Chemistry (London, UK)).
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Catalysts 13 01434 g017
Figure 17. Reaction diagram of Mn–Ce/AC and Mn–Ce–Fe/AC catalysts for NO removing. (Reproduced with permission from reference [94], Copyright 2020, Elsevier (Amsterdam, The Netherlands)).
Figure 17. Reaction diagram of Mn–Ce/AC and Mn–Ce–Fe/AC catalysts for NO removing. (Reproduced with permission from reference [94], Copyright 2020, Elsevier (Amsterdam, The Netherlands)).
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Catalysts 13 01434 g018
Figure 18. Mechanism of better SO2 tolerance by doping FeOx and CeOx to Mn/CAN. (Reproduced with permission from reference [95], Copyright 2021, Elsevier (Amsterdam, The Netherlands)).
Figure 18. Mechanism of better SO2 tolerance by doping FeOx and CeOx to Mn/CAN. (Reproduced with permission from reference [95], Copyright 2021, Elsevier (Amsterdam, The Netherlands)).
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Catalysts 13 01434 g019
Figure 19. NO conversion over (a) Mn–Ce/AC and V–Mn–Ce/AC in the presence of SO2 and (b) combined SO2/H2O at 200 °C. (Reproduced with permission from reference [54], Copyright 2019, Elsevier (Amsterdam, The Netherlands)).
Figure 19. NO conversion over (a) Mn–Ce/AC and V–Mn–Ce/AC in the presence of SO2 and (b) combined SO2/H2O at 200 °C. (Reproduced with permission from reference [54], Copyright 2019, Elsevier (Amsterdam, The Netherlands)).
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Catalysts 13 01434 g020
Figure 20. Catalytic reduction of NOx over V/Ti and V–Fe3/Ti–C catalysts. (Reproduced with permission from reference [97], Copyright 2020, Springer Netherlands (Berlin, Germany)).
Figure 20. Catalytic reduction of NOx over V/Ti and V–Fe3/Ti–C catalysts. (Reproduced with permission from reference [97], Copyright 2020, Springer Netherlands (Berlin, Germany)).
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Catalysts 13 01434 g021
Figure 21. Configurations of FexOy/AC surface (Fe: golden, O: red, C: gray, H: white) and the electron density difference plots of FexOy supported on the AC surface (cyan: lose electron, yellow: obtained electron). (Reproduced with permission from reference [103], Copyright 2021, Royal Society of Chemistry (London, UK)).
Figure 21. Configurations of FexOy/AC surface (Fe: golden, O: red, C: gray, H: white) and the electron density difference plots of FexOy supported on the AC surface (cyan: lose electron, yellow: obtained electron). (Reproduced with permission from reference [103], Copyright 2021, Royal Society of Chemistry (London, UK)).
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Catalysts 13 01434 g022
Figure 22. The influence of (a) H2O, (b) SO2, and (c) SO2 + H2O on the NO conversion of 10%MnOx–CeO2/CA and 10%MnOx–CeO2/P–CA catalysts under GHSV of 2000 h−1 at 160 °C, and (d) SO2 + H2O resistance of 10%MnOx–CeO2/CA and 10%MnOx CeO2/P-CA catalysts under GHSV of 60,000 h−1 at 200 °C. (Reproduced with permission from reference [108], Copyright 2021, Wiley-Blackwell Publishing Ltd. (Weinheim, Germany)).
Figure 22. The influence of (a) H2O, (b) SO2, and (c) SO2 + H2O on the NO conversion of 10%MnOx–CeO2/CA and 10%MnOx–CeO2/P–CA catalysts under GHSV of 2000 h−1 at 160 °C, and (d) SO2 + H2O resistance of 10%MnOx–CeO2/CA and 10%MnOx CeO2/P-CA catalysts under GHSV of 60,000 h−1 at 200 °C. (Reproduced with permission from reference [108], Copyright 2021, Wiley-Blackwell Publishing Ltd. (Weinheim, Germany)).
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Catalysts 13 01434 g023
Figure 23. NOx conversion of samples with different loading amounts of Mn in the testing conditions: T = 120–180 °C, v(O2) = 3.0%, c(NO) = c(NH3) = 800 ppm, and GHSV = 10,000 h−1 [110]. (Reproduced with permission from reference [110], Copyright 2020, Elsevier (Amsterdam, The Netherlands)).
Figure 23. NOx conversion of samples with different loading amounts of Mn in the testing conditions: T = 120–180 °C, v(O2) = 3.0%, c(NO) = c(NH3) = 800 ppm, and GHSV = 10,000 h−1 [110]. (Reproduced with permission from reference [110], Copyright 2020, Elsevier (Amsterdam, The Netherlands)).
Catalysts 13 01434 g023
Table 1. The carbon-based catalysts for SCR.
Table 1. The carbon-based catalysts for SCR.
CatalystsActive Test Reaction ConditionsNOx Conversion/
Temperature		N2 Selectivity/
Temperature		SO2 Tolerance/
Water-Resistance		Ref.
Activated carbon/coke[NH3] = 550 ppm
[NO] = 500 ppm
[O2] = 5 vol%
[SO2] = 50 ppm
[H2O] = 10 vol%
balance of N2
GHSV = 50,000 h−1		>40%/
(150 °C)		->38%/
>32%		[51]
Carbon nanotubes[NH3] = 500 ppm
[NO] = 500 ppm
[O2] = 5 vol%
[H2O] = 3 vol%
balance of N2
GHSV = 60,000 h−1		>80%/
(200 °C)		>60%-[56]
Activated carbon fiber[NH3] = 500 ppm
[NO] = 500 ppm
[O2] = 5 vol%
balance of N2
GHSV = 40,000 h−1		>75%/
(150 °C)		--[57]
Graphene[NH3] = 500 ppm
[NO] = 500 ppm
[O2] = 7 vol%
[H2O] = 10 vol%
balance of N2
GHSV = 67,000 h−1		>90%/
(150 °C)		>90%>80%/
>95%		[58]
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Shen, Z.; Ren, S.; Zhang, B.; Bian, W.; Xing, X.; Zheng, Z. Sulfur and Water Resistance of Carbon-Based Catalysts for Low-Temperature Selective Catalytic Reduction of NOx: A Review. Catalysts 2023, 13, 1434. https://0-doi-org.brum.beds.ac.uk/10.3390/catal13111434

AMA Style

Shen Z, Ren S, Zhang B, Bian W, Xing X, Zheng Z. Sulfur and Water Resistance of Carbon-Based Catalysts for Low-Temperature Selective Catalytic Reduction of NOx: A Review. Catalysts. 2023; 13(11):1434. https://0-doi-org.brum.beds.ac.uk/10.3390/catal13111434

Chicago/Turabian Style

Shen, Zhenghua, Shan Ren, Baoting Zhang, Weixin Bian, Xiangdong Xing, and Zhaoying Zheng. 2023. "Sulfur and Water Resistance of Carbon-Based Catalysts for Low-Temperature Selective Catalytic Reduction of NOx: A Review" Catalysts 13, no. 11: 1434. https://0-doi-org.brum.beds.ac.uk/10.3390/catal13111434

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