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Review

Preparation and Performance of Cerium-Based Catalysts for Selective Catalytic Reduction of Nitrogen Oxides: A Critical Review

by
Ming Cai
,
Xue Bian
*,
Feng Xie
*,
Wenyuan Wu
and
Peng Cen
School of Metallurgy, Northeastern University, NO.3-11 Wenhua Road, Shenyang 110819, China
*
Authors to whom correspondence should be addressed.
Catalysts 2021, 11(3), 361; https://doi.org/10.3390/catal11030361
Submission received: 17 February 2021 / Revised: 5 March 2021 / Accepted: 7 March 2021 / Published: 10 March 2021
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Abstract

:
Selective catalytic reduction of nitrogen oxides with NH3 (NH3-SCR) is still the most commonly used control technology for nitrogen oxides emission. Specifically, the application of rare earth materials has become more and more extensive. CeO2 was widely developed in NH3-SCR reaction due to its good redox performance, proper surface acidity and abundant resource reserves. Therefore, a large number of papers in the literature have described the research of cerium-based catalysts. This review critically summarized the development of the different components of cerium-based catalysts, and characterized the preparation methods, the catalytic performance and reaction mechanisms of the cerium-based catalysts for NH3-SCR. The purpose of this review is to highlight: (1) the modification effect of the various metal elements for cerium-based catalysts; (2) various synthesis methods of the cerium-based catalysts; and (3) the physicochemical properties of the various catalysts and clarify their relations to catalytic performances, particularly in the presence of SO2 and H2O. Finally, we hope that this work can give timely technical guidance and valuable insights for the applications of NH3-SCR in the field of NOx control.

1. Introduction

NOx emissions from automobile exhausts and stationary sources pose a serious threat to environment. In 2017, the national NOx emission reached 17,852,200 tons, of which the NOx emissions from industrial sources was 6,459,000 tons, accounting for 36.2% of the total NOx emissions, and the NOx emissions from thermal power plants ranked first among the total NOx emissions from the key industrial enterprises under investigation [1]. Meanwhile, many countries have issued a number of laws and measures to strictly control NOx emissions, such as the New Sources Performance Standard of the United States, the Large Combustion Plant Directive: H 2001/80/EC of the European Union, the Air Pollution Prevention Law of Japan, the Atmospheric Environment Preservation Law of South Korea and the Thermal Power Plant of China (GB13223-2011).
The most promising approach to reduce NOx emissions is the selective catalytic reduction of NOx with NH3. The V2O5–WO3/TiO2 and V2O5–MoO3/TiO2 commercial catalysts were conventionally developed for NH3-SCR, because of their excellent catalytic performance and strong stability [2,3,4,5]. However, the poor catalytic temperature window (300~400 °C) and the toxicity of vanadium also bring difficulties for the disposal of the waste catalysts, which limit the future development of the vanadium-based catalysts [6,7]. Therefore, non-vanadium-based NH3-SCR catalysts currently attract significantly more attention in this field.
Apparently, China is the country with the most abundant rare earth mineral resources in the world, with not only with large reserves, but more importantly with complete mineral species and relatively low costs [8,9]. If rare earth oxide is applied to the research and development of SCR catalysts, it can develop the high efficiency deNOx from industrial sources and automobile exhausts at low costs, which is the technical route for the preparation of SCR catalysts in accordance with China’s national conditions. Especially, CeO2 plays a key role in the treatment of automobile exhausts, and also has certain significance for the abatement of particulate matter [10,11].
Up to now, CeO2 as the main active component and promoter of NH3-SCR catalysts has been widely studied [12,13,14]. In general, CeO2 is an acid-based substance, which has a large number of Lewis acid sites and a few Brönsted acid sites. CeO2 is as an oxygen reservoir, which stores and releases oxygen via the redox shift between Ce4+ and Ce3+ under oxidizing and reducing conditions. Besides, CeO2 exhibited an excellent SCR activity in the presence of SO2 at 300–500 °C [15,16,17,18]. Furthermore, the most important properties of suitable surface acidity and good redox ability play a significant role in SCR performance [19,20]. Therefore, cerium-based catalysts were widely studied in NH3-SCR reaction [21,22]. In this paper, the research progress of cerium-based NH3-SCR catalysts made in recent years is summarized, including cerium-based bimetallic oxide catalysts, cerium-based multiplex oxide catalysts and cerium-based molecular sieve catalysts.

2. Cerium-Based Bimetallic Oxide Catalysts

CeO2 enhances redox performance of the catalysts, which is vital for the catalytic reaction. Obviously, CeO2 is responsible for the oxygen storage through the redox reaction, and Ce3+ increases the amount of unstable surface oxygen holes and oxygen free radicals [23,24,25,26]. However, the SCR performance of pure CeO2 catalyst is poor, so many researchers have focused on synthesizing different composite catalysts for promoting NH3-SCR activity and extending the operating temperature windows. Therefore, the performance of cerium-based catalysts is continuously optimized by adding different metal oxides [27,28].

2.1. CeO2–TiO2 Catalyst

Firstly, the CeO2–TiO2 catalyst has been widely concerned due to good redox performance and its high specific surface area on the surface of catalyst [29,30]. As is known to all, TiO2 is an optimal support of NH3-SCR catalysts with strong Lewis acidity and good SO2 durability. Meanwhile, active components can be uniformly dispersed on its surface, consequently increasing the number of surface active sites [31,32]. Generally, the preparation methods of the CeO2–TiO2 catalyst directly affect the strong interaction between CeO2 and TiO2 and the dispersion state of CeO2 on the catalyst’s surface. The former mainly increases the specific surface. The latter directly affects the content of Ce3+ on the surface, thereby determining the redox performance of the catalysts. For example, Gao et al. [33,34] systematically compared the CeO2–TiO2 catalysts obtained by impregnation method, sol–gel method and coprecipitation method. The results found that the catalyst prepared by the sol–gel method showed up to 93–98% NOx conversion at 300–400 °C. More specifically, the good deNOx performance might be attributed to the strong interaction between CeO2 and TiO2, shown in Figure 1. In addition, it can be also observed from Figure 2 that the primary particle size of CeTi (sol–gel) was less than 10 nm; meanwhile, these pictures revealed that CeO2 was well dispersed on the surface of TiO2, thereby improving the catalytic activity of the catalysts. Similarly, Duan et al. [35] found that CeO2 can be uniformly dispersed on the TiO2; moreover, the levels of CeO2 in the catalyst did not change the crystal structure of the anatase. Obviously, the preparation methods of the sol–gel have great impact on the strong molecular interaction and surface characteristics of catalysts, which determine the NH3-SCR performance. Besides, Huang et al. [36] obtained a series of CeO2/TiO2 catalysts with organic additives by the ball milling method. The results indicated that the addition of citric acid into the ball milling process could significantly change the proceedings of the precursor mixture decomposition, enhancing the dispersion and reducibility of the CeO2 and the surface acidity as well as the surface microstructure.
Furthermore, the presence of CeO2 in the CeO2–TiO2 catalyst can effectively enhance the catalytic activity and thermostability of TiO2. Especially, ceria atoms can inhibit the crystalline grain growth and the collapse of the small channels generated by calcination [37]. In addition, some studies have found that the sulfate formed in the presence of SO2 is unstable on the surface of TiO2 and is easily decomposed; thus, TiO2 has high SO2 durability performance [38]. The function of the CeO2–TiO2 catalyst has been evaluated by Fei et al. [39]. Particularly, the Ce0.5Ti0.5 catalyst exhibited the best catalytic activity and extraordinary H2O/SO2 durability (Figure 3 and Figure 4). Furthermore, the mechanism of NH3-SCR over CeaTi1−a catalysts was confirmed in Figure 5, where NH3, as the main active intermediate, reacted with NO to produce N2 and H2O in the E-R mechanism (Pathway 1). Besides, for the L-H mechanism, a large amount of Ce3+ species and high surface adsorbed oxygen reacted with adsorbed NH3 through the “fast SCR”, (Pathway 2). Subsequently, NO firstly adsorbed on the active sites and reacted with O− to form the intermediate, and lastly reacted with the adsorbed NH3 to form N2 and H2O (Pathway 3). Meanwhile, the effect of the loading sequence of CeO2 and TiO2 on the catalytic activity was investigated by Zhang et al. [40]. They also found that the TiO2/CeO2 catalyst not only showed good low-temperature activity at 150~250 ℃, but also showed great SO2 resistance performance with the existence of 200 ppm SO2 at 300 ℃. Actually, a large amount of CeO2 will actively react with SO2 in priority, avoiding the interaction between the SO2 and Ce–O–Ti active species; thereby the active species can completely exhibit great deNOx performance, as shown in Figure 6. Additionally, some investigators have done some work on the influence of different precursors of CeO2 and TiO2 on the catalysts’ performance. For instance, Yao et al. [41] synthesized CeO2/TiO2 catalyst with anatase, brookite and rutile TiO2 as support. The catalyst with rutile TiO2 exhibited great NH3-SCR activity owing to the large amount of acid sites, surface Ce3+ content, and surface adsorbed oxygen species. However, the H2O/SO2 durability performances of CeO2/TiO2 catalyst with rutile TiO2 need to be further studied and improved. The abovementioned reports suggested that CeO2 and TiO2 exhibited more acid sites and higher dispersion than the pure CeO2, which significantly enhanced the catalytic activity of the catalysts. SO2 and H2O showed a promotion on NOx reduction over Ce/TiO2 catalyst at higher temperature, whereas they show a great inhibitory effect at low temperature [42].

2.2. CeO2–MnO2 Catalyst

Up to now, the Mn-based catalyst has been extensively investigated in the literature. It was found to have superior low temperature activity due to its rich variable valence states (MnO, Mn3O4, Mn5O8, Mn2O3, and MnO2) and huge surface area [43,44,45]. The presence of the Mn4+ species and its redox process are important for the excellent NH3-SCR activity at low temperatures and for N2 selectivity [46,47,48,49]. At the same time, the thermal stability, chemisorbed oxygen and the concentration of Oα species on the surface will be improved by MnOx and CeO2 [50]. However, the obstacle to the application of Mn-based catalysts is the poor performance of resisting H2O and SO2 [51]. The addition of CeO2 can enhance resistance to H2O and SO2 to a certain extent [52,53]. For example, Qi et al. [54,55] introduced the CeO2–MnOx, which catalyst showed great H2O/SO2 durability. Moreover, Mn ions entered the lattice of CeO2 and a large number of chemisorbed oxygen species were released to the surface, thereby enhancing the NH3-SCR activity of the catalysts.
Besides, Ce and Mn can present different valence states under suitable preparation methods and reaction conditions. Furthermore, the strong interaction between CeO2 and MnO2 will make the catalysts show excellent low-temperature activity and improve the redox performance of the catalysts. For example, the different preparation methods of the MnOx–CeO2 catalyst have been analyzed by Shen et al. [56]. It was demonstrated that the hydrolysis process method suggested higher SCR activity in the temperature range of 80–260 °C; meanwhile, this catalyst showed higher Mn4+/Mn3+, Ce4+/Ce3+ ratio, higher specific area and higher Oa/Op ratio. Apart from some mature preparation methods, many researchers have made innovative works about preparation methods; Yao et al. [57] reported that the MnOx–CeO2 catalyst prepared by the hydrothermal treatment method revealed the best NH3-SCR performance and good resistance to SO2 and H2O (Figure 7a,b). The XRD patterns and Raman spectra characterization were shown in Figure 7c,d, where it was demonstrated that CeO2 and MnOx had a strong interaction under the conditions of high temperature and high pressure. Furthermore, Mnn+ entered into the lattice of CeO2 to form Mn–O–Ce solid solution, which enhanced the SCR performance of the catalysts. Andreoli et al. [58] prepared CeO2–MnOx catalysts by the solution combustion synthesis method, and this catalyst exhibited a higher NOx conversion of more than 90% at 120–350 °C. Besides, Liu et al. [59] synthesized the MnOx–CeO2 catalyst by the surfactant-template (ST) method and coprecipitation (CP) method, and the XRD demonstrated that smaller mixed oxide particles were obtained by the surfactant-template method. The smaller particles could contribute to improving the SCR performance, as shown in Figure 8a. Meanwhile, more reducible subsurface and bulk oxygen were clearly observed in the H2-TPR, as shown in Figure 8b. However, one problem relating to the Mn based catalyst is that its application at low temperature is a big challenge. The main problem is that the N2 selectivity will decrease significantly at high temperature. Meanwhile, the catalytic activity will still be inhibited by H2O and SO2, which cannot meet the requirements of industrial production [60].

2.3. Other CeO2–MOx Catalysts

In addition to the above composite metal oxides, other metal oxides as the main active components in NH3-SCR catalysts have been extensively studied for low–medium temperatures. For example, the addition of WO3 [61,62], Sn2O5 [63], MoO3 [64], CuOx [65] and NiO [66] into Ce-based catalysts can improve the redox performance, surface acidity and the adsorption of NH3 on the catalyst surface. Meanwhile, the addition of ZrO2 [67], CoO [68] can enhance the specific surface area, thermal stability and the resistance to H2O/SO2 of cerium-based catalysts. The modification of cerium-based catalysts by doping WO3 showed excellent de-NOx performance. For instance, Wang et al. [61] reported that the WO3 was deposited on CeO2 nanoparticles. This catalyst also exhibited the highest SCR activity below 300 °C, excellent H2O/SO2 resistance and good NH3 adsorption at 125–450 °C. Generally, the presence of WO3 provided more surface lattice oxygen O2- and acid sites at lower temperatures, which benefits the catalytic activity for NH3-SCR. At the same time, the CeO2–WO3 catalyst has been reported on by Liu et al. [62]. The results suggested that the presence of W provided more acid sites, thereby generating additional chemisorbed oxygen, weakly adsorbed oxygen species and concentrations of Ce and Ce3+ on the surface of the catalyst, shown in Figure 9. Liu et al. [63] investigated the performance of a CeO2–SnO2 catalyst for NH3-SCR. The results revealed that the high catalytic performance of this catalyst was attributed to the synergetic effect between Ce and Sn species, which enhanced the redox ability, the Lewis acidity and the adsorption and activation of NH3 species, thereby contributing to improving the NH3-SCR performance.
Besides, the catalytic performance of the CeO2–MoO3 catalyst has been investigated by Peng et al. [64]. This catalyst showed good NH3-SCR performance. Moreover, the Ce atoms and amorphous MoO3 structure provided a large number of Lewis acid sites and Brönsted acid sites on the catalyst surface. Atribak et al. [67] reported the performance of the CeO2–ZrOx catalyst at high temperature, and the results indicated the addition of Zr provided excellent thermal stability and more specific surface area of the catalysts. Apart from the traditional CeO2–MOx catalysts, the single-atom catalysts have also showed great potential in the NH3-SCR. Especially, adding a second late-transition metal into cerium-based catalyst as single atom could have great potential in the automobile exhaust field [69,70].

3. Cerium-Based Multiplex Oxide Catalysts

Cerium-based multiplex oxide catalysts are particularly outstanding owing to making up for the shortcomings of some single or bimetallic catalysts on NH3-SCR activity. For CeO2/TiO2 catalyst, such as Mn, W and Mo are introduced to further improve the redox performance, the surface acidity and the H2O/SO2 durability of the catalysts. Therefore, the effects of cerium-based multiplex oxide catalysts on de-NOx performance were mainly studied from the aspects of preparation methods, preparation conditions and additive doping modification.

3.1. Ce–Mn/TiO2 Catalyst

Apparently, MnOx has many changeable valence states. Its oxides can be converted to each other, which shows excellent catalytic activity at low temperature [71,72,73]. Meanwhile, CeO2 can reduce the loss of specific surface area and pore volume during calcination, which improves the oxygen storage capacity and redox performance of the catalysts. Besides, the interaction between MnOx and CeO2 can form Mn–O–Ce solid solution, thereby improving the adsorption and activation properties of NH3 [74]. For example, Liu et al. [75] developed the Mn–Ce/TiO2 catalyst by hydrothermal method. It was also found that the environmentally benign Mn–Ce/TiO2 catalyst exhibited excellent NH3-SCR activity and good resistance to H2O and SO2 with a wide temperature window. Meanwhile, this result showed that the dual redox cycles (Mn4+ + Ce3+ ↔ Mn3+ + Ce4+, Mn4+ + Ti3+ ↔ Mn3+ + Ti4+) might play a key role in the catalytic reaction, which facilitated the adsorption and activation of NH3, as shown in Figure 10. The structure and properties of 8% Mn–Ce/TiO2-PILC catalyst has been analyzed by Shen et al. [76]. The catalyst suggested rich mesoporous structure and large specific surface area. More specifically, it could be demonstrated that Ce modified Mn–Ce/TiO2-PILC catalyst enhanced the dispersion of Mn on the surface.
Compared with CeO2–MnOx catalyst, the resistance to H2O and SO2 of the Ce-Mn/TiO2 catalyst has been enhanced to some extent [77]. CeO2 can significantly inhibit the deposition of (NH4)2SO4 and NH4HSO4 on the catalyst surface, which is the fundamental reason for the improvement of SO2/H2O resistance [78]. For instance, that SO2 poisoning and regeneration of the Mn–Ce/TiO2 catalyst have been reported by Sheng et al. [79]. This catalyst showed good resistance to SO2; however, the deactivation of the Mn–Ce/TiO2 poisoned by SO2 still occurred. Then, Peng et al. [80] reported the influence of Ce addition on the potassium poisoning of the MnOx/TiO2 catalyst, and found that K can reduce the surface acidity and reduction performance of the catalyst. However, the presence of CeO2 can provide a certain number of Lewis acid sites, shown in Figure 11a; meanwhile, CeO2 enhanced the reducibility of Mn/Ti and maintained the redox performance of the SCR catalysts after potassium poisoning, shown in Figure 11b.
Eventually, from the reported work on the Ce–Mn/TiO2 catalyst, it is not difficult to find that the rich variable valence states of Mn show excellent NH3-SCR activity at low temperature. However, its SCR performance resistance to H2O/SO2 still needs to be further strengthened.

3.2. Ce–W/TiO2 Catalyst

Apparently, different aspects of W modified cerium-based catalysts have been widely studied, and highly dispersed WO3 is beneficial to improve the catalytic effect of the whole catalyst [81,82]. WO3, as a stabilizer and promoter, significantly increased the specific surface area, Ce3+/Ce4+ ratio and surface acid sites of the catalysts, consequently enhancing the adsorbed oxygen on the surface and the activated oxygen species [83,84]. Firstly, Chen et al. [85] developed Ce/TiO2 and W–Ce/TiO2 catalysts by the impregnation method. They also found that W–Ce/TiO2 catalyst showed better de-NOx performance. As shown in Figure 12, the presence of W provided more acid sites on the catalyst surface, and accelerated the reaction between NH4NO3 and NO to achieve a superior low-temperature activity. Then, Guo et al. [86] found that the CeO2–WO3/TiO2 catalyst showed good catalytic activity. Pretreated TiO2 made the surface active substances have higher dispersion. The addition of WO3 also enhanced the surface acidity and surface chemisorption oxygen. Meanwhile, the influence of WO3 intervention on the catalytic performance of MnCeW/m-TiO2 catalyst has been investigated by Zha et al. [87]. This catalyst showed excellent deNOx performance and N2 selectivity under the conditions of wide temperature window and high space velocity. Particularly, in in situ DRIFTs, as shown in Figure 13, it found that the addition of WO3 enhanced more Brönsted acid sites on the surface at high temperature. Additionally, some researchers have reported some innovative preparation methods. For example, Katarzyna et al. [88] prepared WO3/CeOx–TiO2 catalyst by the flame-spray synthesis method, and Figure 14 suggests the interpretation of the mechanism of particle formation during flame-spray synthesis method. This method further strengthened the interaction of WO3, CeO2 and TiO2. Meanwhile, the presence of WO3 increased Ce3+ and surface acidity on the catalyst surface to a great extent. The highly dispersed WO3 enhanced the Ce–O–W reaction and Ce–O–Ti reaction, and consequently improved the performance of the NH3-SCR catalysts. Besides, the addition of WO3 improved the thermal stability of the catalysts at 550–600 °C [89], and a large cerium oxide phase and more TiO2 crystal formation can be avoided in the catalytic reaction process [90].
Generally, the ratio of CeO2 and WO3 has great effect on the redox ability and surface characteristics of Ce–W/TiO2 catalysts, which also determines the NH3-SCR performance. For instance, the Ce0.2W0.2Ti catalyst with Ce/W molar ratio of 1:1 has been synthesized by Shan et al. [91]. It also showed that the best NH3-SCR catalytic performance and 100% N2 selectivity; above 90% of NO conversion was maintained from 275 °C to 450 °C. Besides, in our previous research, we have done some work on the Ce–W/TiO2 catalyst for the NH3-SCR reaction and analyzed the influence of the active components CeO2 and WO3 content on the de-NOx performance of the catalysts, and found that the 30Ce4W/TiO2 catalyst showed up to 90% NOx conversion at the widest temperature range of 310 °C. More specifically, the results show that a higher proportion of Ce4+, more chemisorption of oxygen and high specific surface area were key for the excellent NH3-SCR activity of this catalyst [92].

3.3. Ce–Mo/TiO2 Catalyst

Undoubtedly, it is not difficult to find that the addition of Mo can remarkably improve the performance of SCR catalysts. Especially, CeO2 and MoO3 can be highly dispersed on the surface of TiO2 carrier. Furthermore, Mo doping increases the Ce3+ content, creates more abundant Brönsted acid sites, and increases the oxygen vacancy and adsorbed oxygen substances on the catalyst surface [93,94]. Additionally, the presence of MoO3 could effectively enhance the SO2 and H2O resistance of the catalysts at low temperature [95]. For example, Li et al. [96] prepared an Mo-doped MoO3/CeO2–TiO2 (MoO3/CT) catalyst. The catalyst showed good low temperature activity and excellent SO2/H2O resistance performance (Figure 15a,b). More specifically, the addition of MoO3 increased the Brönsted acid sites on the catalyst surface, shown in Figure 16. Then, the influence of MoO3 modified CeO2–TiO2 catalyst on the NH3-SCR performance was systematically investigated by Liu et al. [97],, who suggested that having more Brönsted acid sites was conducive to the adsorption of NH3. Furthermore, MoO3 can inhibit the formation of sulfates; thereby the catalyst simultaneously showed excellent SO2/H2O resistance. Besides, Ye et al. [98] prepared CeO2–MoO3/TiO2 catalysts by different kinds of methods, and found that the catalyst prepared by sol–gel method exhibited the widest reaction temperature window of 250–475 °C.
Additionally, some researchers added active components to CeO2–MoO3/TiO2 catalysts to enhance the deNOx performance of the catalyst. For instance, the NH3-SCR performance of a new type of CeO2–MoO3–WO3/TiO2 catalyst has been reported by Jiang et al. [99]. The result exhibited that the NO conversion was 93.8~98.9% at a GHSV of 90,000 h−1 and a temperature window of 275–450 °C. The presence of WO3 and MoO3 increased the adsorption capacity of NH3, the redox performance, the amount of Ce3+ and the chemisorption of oxygen on the surface. At the same time, the interaction between CeO2, MoO3, WO3 and TiO2 might play an increasingly vital role in the improvement of catalytic performance. Zhang et al. [100] developed the catalytic performance of the CeFMoTiOx catalyst, which not only exhibited higher than 90% NO conversion at 240–420 °C, but also presented superior H2O/SO2 durability. The results demonstrated that the presence of MoO3 improved the dispersity of CeO2 on the catalyst surface. The introduction of F increased the oxygen vacancy, consequently improved the redox performance of CeO2. Meanwhile, the Ti–F bond played a key role in the SCR reaction. Eventually, the poisoning mechanism of As on CeO2–MoO3/TiO2 catalyst has been analyzed by Li et al. [101], as shown in Figure 17. The results exhibited that As2O5 would directly weaken the specific surface area, surface acidity and redox performance. However, after the addition of Mo, the stronger interaction between Mo and As can alleviate the effects of surface CeO2 poisoning to a certain extent, so as to recover the redox performance and Brönsted acid sites of the CeO2–MoO3/TiO2 catalyst.

3.4. Other Cerium-Based Multiplex Oxide Catalysts

In addition to the above multiplex oxide catalysts, many research papers have reported that CeO2 was combined with other transition metal oxides to form NH3-SCR catalysts, such as Sn2O5 [102], VOx [103], CuO [104], Nb2O5 [105], ZrO2 [106] and CoO [107]. They can simultaneously enhance the surface acidity, redox performance and SO2/H2O resistance of the SCR catalysts. For instance, Zhang et al. [97] prepared CeSnTiOx catalysts by the solvothermal method. The results suggest that the Sn doped catalysts showed better low-temperature activity, exhibiting an extraordinarily wide operation window ranging from 180 to 460 °C. Meanwhile, the H2-TPR and XPS spectra results verified that the addition of Sn2O5 improved the interaction between CeO2 and SnO2 and the redox ability of the catalysts. Then, a novel V2O5/CeTiOx catalyst was introduced by Lian et al. [103]. The addition of VOx enhanced catalytic activity, N2 selectivity and the resistance to SO2 and H2O. Then, Li et al. [104] studied Cu modified Ce/TiO2 catalyst, and found that the catalyst with a Cu/Ce molar ratio of 0.005 showed the best low-temperature activity and excellent SO2 resistance performance. By means of XRD, BET, Raman, XPS and NH3-TPD, it was demonstrated that the presence of CuO increased the amount of the surface adsorbed oxygen and Ce3+ species and created more Brönsted acid sites on the catalyst surface. Furthermore, the in situ DRIFT results demonstrated that CuO doping enhanced the adsorption capacity of NH3.
Besides, Jawaher et al. [105] prepared Nb5-Ce40/Ti10 catalyst by sol–gel method, and found that the catalyst showed up to 95% NOx conversion at 200 °C. The addition of Nb strengthened the surface acidity. Meanwhile, the strong interaction between Ce and Ti to form the Ce–O–Ti solid solution and the high dispersion of Nb2O5 can improve the NH3-SCR activity. However, the presence of Nb2O5 will greatly decrease the specific surface area of the catalysts. Then, Zr modified Ce–W/TiOx catalyst was analyzed by Zhao et al. [106]. The presence of Zr enhanced more acidic sites, oxygen vacancies and adsorbed oxygen species on the surface, which showed the best NH3-SCR catalytic activity and thermal stability. Liu et al. [107] found that the Co–Ce/TiO2 catalyst exhibited good low-temperature activity, widened the temperature window and reacted quickly under the mechanism of L-H and E-R. Furthermore, the different particle sizes of Co2+ and Ce4+ promoted the Ce3+ ratio and surface adsorption oxygen. Besides, Li et al. [108] prepared Ho-doped Mn–Ce/TiO2 catalyst by impregnation method. The results indicated that the catalyst with Ho/Ti of 0.1 presented excellent catalytic activity with the NO conversion of more than 90% at 140–220 °C (Figure 18a,b). The characterization results showed that Ho increased the specific surface area and led to higher levels of chemisorbed oxygen, as shown in Figure 19; meanwhile, the presence of Ho inhibited the sulfation on the surface to some extent.

4. Molecular Sieve Catalysts

Apart from the ceria-based composite oxide SCR catalysts, the excellent activity and high N2 selectivity of molecular sieves are also considered as the most promising SCR catalysts [109,110]. Especially, molecular sieve catalyst has strong stability, toxicity resistance and wide reaction temperature range [111]. Among the molecular sieve catalysts, ZSM-5, Beta, USY and other carriers exhibit good adsorption capacity, moderate surface acidity and flexible reaction temperature window. Peculiarly, ZSM-5 exhibits stable crystal structure, good specific surface area, abundant acid sites and great thermal stability [112,113,114]. For example, Krishna K et al. [115] prepared the Ce/ZSM-5 catalyst by ion exchange method. The results suggested that CeO2 is closely bound to ZSM-5, which provided more active sites to transform NOx. Then, Liu et al. [116] prepared CeO2-modified Cu/ZSM-5 catalyst by a wetness impregnation method, and found the presence of CeO2 enhanced the NH3-SCR activity of the catalyst at low temperature. However, this catalyst had a poor catalytic performance at high temperature. Additionally, Dou [117] analyzed that the addition of Ce can inhibit the crystallization of Cu and increase the dispersion of active component, which made the catalyst show better de-NOx performance at 148–427 °C. The Fe–ZSM-5@CeO2 catalyst has been investigated by Chen et al. [118]. The catalyst showed excellent NH3-SCR activity and N2 selectivity, mainly due to the construction strategy of Fe–ZSM-5@CeO2 to increase the redox performance and active oxygen species of the catalyst, shown in Figure 20. Subsequently, the surface Ce4+ and active oxygen species over Fe–ZSM-5@CeO2 promoted the adsorption and activation of NO, shown in Figure 21. Carja et al. [119] studied the Mn–Ce/ZSM-5 catalyst and the results exhibited good NH3-SCR activity in the presence of H2O and SO2. More importantly, the synergistic interaction of ZSM-5 and Ce, Mn promoted microporous-mesoporous characteristics and specific surface properties of catalysts. Besides, Liu et al. [120] reported CuCe0.75Zr0.25Oy/ZSM-5 catalyst. The reaction temperature window was widened to 175–468 °C. According to XRD and TEM results, the presence of Zr increased the dispersion of Cu and inhibited the crystallization of Cu, and XPS and H2-TPR analysis demonstrated that Cu ions entered the lattice of ZrO2 or CeO2.
In addition, both the β and USY zeolite catalysts have been mentioned slightly less often than the ZSM-5 zeolite catalyst. However, there are still some valuable studies to be found; for example, Liu et al. [121] reported the coating of CeO2 shells on the surface of MoFe/Beta catalyst, as shown in Figure 22, and found that the presence of the CeO2 shells enhanced the resistance to SO2 and H2O and high thermal stability. This was mainly due to the fact that both chemisorbed oxygen species and specific surface area were increased after the coating of the CeO2 shells (Figure 23 and Figure 24). Then, Huang et al. [122] reported Mn–Ce catalysts with β, ZSM-5 and USY molecular sieves as carriers, respectively, by the impregnation method and studied the de-NOx performance of the catalysts at low temperature. The results showed that the three zeolite supported Mn–Ce catalysts have good low temperature activity, and the Mn–Ce/USY catalyst showed up to 90% NOx conversion at 107 °C. The MnOx is mainly distributed on the catalyst surface in an amorphous structure. Meanwhile, the weak acid on the catalyst surface played a major role in the reaction.
Finally, it is worth considering that the addition of active components and promoters can increase redox property for cerium-based SCR catalysts. Furthermore, the oxidation reaction of the catalysts was enhanced. However, the oxidation of SO2 was simultaneously increased in the catalytic reaction process, thereby resulting in the formation of sulfate on the surface and inhibiting the NH3-SCR activity of the catalysts. Therefore, the question of the resistance to SO2, H2O needs to be further investigated. Finally, for all the above types of catalysts, the denitration performance of the catalysts under different preparation methods and conditions was described in Table 1.

5. Conclusions and Perspectives

In conclusion, cerium-based catalysts have been deeply studied due to their high deNOx performances and low costs. The catalytic performance of cerium-based catalysts mainly depends on surface acidity, specific surface area, redox performance and resistance to H2O and SO2. The current study indicated the better catalytic performance of cerium-based bimetallic oxides than pure CeO2 in NH3-SCR. Furthermore, the multiplex oxide catalysts present a wider operation temperature widow and great low-temperature activity than the bimetallic oxide catalysts. This is attributed to the synergistic interaction between active components and promoters, the enhancement of the acid sites and the redox properties. Moreover, not only the addition of the other metal oxides can modify the performance of cerium-based catalysts, but different synthesis methods can also enhance the dispersion of the active species and the interaction of the different active components, the cerium-based bimetallic oxide catalysts, the cerium-based multiplex oxide catalysts and cerium-based molecular sieve catalysts are still the research directions in NH3-SCR field in the future. Some researchers have done fruitful work in the fields of the synthesis method, modification and catalytic mechanism of cerium-based catalysts. Nevertheless, some aspects need to be further investigated. First of all, at low temperature, the performance of catalysts is still inhibited by H2O and SO2. Due to that, the improvement of the SO2/H2O resistance of cerium-based catalysts is still the main research direction. Secondly, traditional synthesis methods of catalysts need to be further studied and new synthesis methods need to be explored in order to expose more active sites on the catalyst surface and enhance the interaction between the active components. Furthermore, in order to provide the excellent performance of cerium-based catalysts, it is necessary to further achieve the optimal ratio of the active components. Additionally, for the cost of the catalysts, some metal oxides have high costs, which cause substantial obstacles to their actual production. Therefore, to ensure the excellent catalytic performance of cerium-based catalysts, the active components with low costs should be selected.

Author Contributions

M.C., X.B.: literature search, writing—original draft preparation and editing; F.X., W.W., P.C.: writing—review and editing; X.B., F.X.: funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

The work described above was supported by the Major State Basic Research Development Program of China (973 Program) (no. 2012CBA01205) and National Natural Science Foundation of China (no. 51274060).

Data Availability Statement

The data presented in this review are from published sources.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. H2-TPR profiles of the catalysts: (a) CeTi (sol–gel method), (b) CeTi (impregnation method), (c) CeTi (coprecipitation method) [33,34].
Figure 1. H2-TPR profiles of the catalysts: (a) CeTi (sol–gel method), (b) CeTi (impregnation method), (c) CeTi (coprecipitation method) [33,34].
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Figure 2. HR-TEM micrographs of the catalysts: (a) CeTi (sol–gel method), (b) CeTi (impregnation method) and (c) CeTi (coprecipitation method) [33,34].
Figure 2. HR-TEM micrographs of the catalysts: (a) CeTi (sol–gel method), (b) CeTi (impregnation method) and (c) CeTi (coprecipitation method) [33,34].
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Figure 3. NH3-SCR activity of TiO2, CeO2 and CeaTi1−a catalysts. Conditions: [NH3] = [NO] = 1000 ppm, [O2] = 3 vol.%, N2 as balance gas, total flow rate = 500 mL·min−1 [39].
Figure 3. NH3-SCR activity of TiO2, CeO2 and CeaTi1−a catalysts. Conditions: [NH3] = [NO] = 1000 ppm, [O2] = 3 vol.%, N2 as balance gas, total flow rate = 500 mL·min−1 [39].
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Figure 4. Effect of (a) SO2, (b) H2O and (c) SO2 + H2O on NH3-SCR activity over Ce0.5Ti0.5 catalyst at 300 °C. Conditions: [NH3] = [NO] = 1000 ppm, [SO2] = 100 ppm, [H2O] = 10 vol.%, [O2] = 3 vol.%, N2 as balance gas [39].
Figure 4. Effect of (a) SO2, (b) H2O and (c) SO2 + H2O on NH3-SCR activity over Ce0.5Ti0.5 catalyst at 300 °C. Conditions: [NH3] = [NO] = 1000 ppm, [SO2] = 100 ppm, [H2O] = 10 vol.%, [O2] = 3 vol.%, N2 as balance gas [39].
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Figure 5. Proposed NH3-SCR reaction mechanism over CeaTi1−a catalysts [39].
Figure 5. Proposed NH3-SCR reaction mechanism over CeaTi1−a catalysts [39].
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Figure 6. Proposed adsorption model of SO2: (a) CeO2, (b) Ce/Ti and (c) Ti/Ce [40].
Figure 6. Proposed adsorption model of SO2: (a) CeO2, (b) Ce/Ti and (c) Ti/Ce [40].
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Figure 7. (a) NO conversion of the synthesized MnOx–CeO2 catalysts; (b) the H2O resistance of MnCe-HTM catalyst at 200 °C; (c) XRD patterns and of the synthesized catalysts and; (d) Raman spectra of the synthesized catalysts [57].
Figure 7. (a) NO conversion of the synthesized MnOx–CeO2 catalysts; (b) the H2O resistance of MnCe-HTM catalyst at 200 °C; (c) XRD patterns and of the synthesized catalysts and; (d) Raman spectra of the synthesized catalysts [57].
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Figure 8. (a) XRD patterns of Mn5Ce5 (CP) (A) and Mn5Ce5 (ST) (B) catalysts and (b) H2-TPR profiles for Mn5Ce5 (CP) (A) and Mn5Ce5 (ST) (B) catalysts [59].
Figure 8. (a) XRD patterns of Mn5Ce5 (CP) (A) and Mn5Ce5 (ST) (B) catalysts and (b) H2-TPR profiles for Mn5Ce5 (CP) (A) and Mn5Ce5 (ST) (B) catalysts [59].
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Figure 9. XPS spectra of (a) Ce3d and (b) W4f over CeO2–WO3 catalysts [62].
Figure 9. XPS spectra of (a) Ce3d and (b) W4f over CeO2–WO3 catalysts [62].
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Figure 10. Dual redox cycles for the activation of NO and NH3 [70].
Figure 10. Dual redox cycles for the activation of NO and NH3 [70].
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Figure 11. (a) NH3-TPD profiles of fresh and poisoned catalysts in the range of 75–400 °C and (b) H2-TPR profiles of fresh and poisoned catalysts in the range of 150–700 °C [80].
Figure 11. (a) NH3-TPD profiles of fresh and poisoned catalysts in the range of 75–400 °C and (b) H2-TPR profiles of fresh and poisoned catalysts in the range of 150–700 °C [80].
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Figure 12. NH3-NO/NO2 SCR reaction routes on CeTi and CeWTi catalysts [85].
Figure 12. NH3-NO/NO2 SCR reaction routes on CeTi and CeWTi catalysts [85].
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Figure 13. In situ DRIFTs of NH3 desorption over (a) MnCeW/m-TiO2 and (b) MnCe/m-TiO2 catalysts as a function of temperature [87].
Figure 13. In situ DRIFTs of NH3 desorption over (a) MnCeW/m-TiO2 and (b) MnCe/m-TiO2 catalysts as a function of temperature [87].
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Figure 14. Schematic representation of a feasible mechanism of WO3/CeOx–TiO2 nanoparticles formation during flame-spray synthesis [88].
Figure 14. Schematic representation of a feasible mechanism of WO3/CeOx–TiO2 nanoparticles formation during flame-spray synthesis [88].
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Figure 15. (a) NO conversions on these MoO3/CeO2–TiO2 (Mo/CT) catalysts with different Mo and (b) H2O + SO2 resistance of the CeO2–TiO2 (CT) and Mo/CT catalysts at 250 °C [96].
Figure 15. (a) NO conversions on these MoO3/CeO2–TiO2 (Mo/CT) catalysts with different Mo and (b) H2O + SO2 resistance of the CeO2–TiO2 (CT) and Mo/CT catalysts at 250 °C [96].
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Figure 16. NH3 adsorption in situ DRIFTS of (a) CeO2–TiO2 (CT) and (b) MoO3/CeO2–TiO2 (Mo/CT) catalyst [96].
Figure 16. NH3 adsorption in situ DRIFTS of (a) CeO2–TiO2 (CT) and (b) MoO3/CeO2–TiO2 (Mo/CT) catalyst [96].
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Figure 17. The performance of the arsenic resistance on MoO3 doped CeO2/TiO2 catalyst for SCR of NOx with ammonia [101].
Figure 17. The performance of the arsenic resistance on MoO3 doped CeO2/TiO2 catalyst for SCR of NOx with ammonia [101].
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Figure 18. (a) Comparison of catalytic performance of different catalysts and (b) effect of SO2 + H2O on NO conversion over Mn0.4Ce0.07/TiO2 and Mn0.4Ce0.07Ho0.1/TiO2 catalysts [108].
Figure 18. (a) Comparison of catalytic performance of different catalysts and (b) effect of SO2 + H2O on NO conversion over Mn0.4Ce0.07/TiO2 and Mn0.4Ce0.07Ho0.1/TiO2 catalysts [108].
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Figure 19. SEM images of two catalysts (a) Mn0.4Ce0.07Ho0.1/TiO2, (b) Mn0.4Ce0.07/TiO2, (c) Mn0.4Ce0.07Ho0.1/TiO2 and (d) Mn0.4Ce0.07/TiO2 [108].
Figure 19. SEM images of two catalysts (a) Mn0.4Ce0.07Ho0.1/TiO2, (b) Mn0.4Ce0.07/TiO2, (c) Mn0.4Ce0.07Ho0.1/TiO2 and (d) Mn0.4Ce0.07/TiO2 [108].
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Figure 20. The PDA (Personal Digital Assistant)-assisted route for Fe-ZSM-5@CeO2 preparation [118].
Figure 20. The PDA (Personal Digital Assistant)-assisted route for Fe-ZSM-5@CeO2 preparation [118].
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Figure 21. The proposed mechanism of NH3-SCR over Fe–ZSM-5@CeO2 [118].
Figure 21. The proposed mechanism of NH3-SCR over Fe–ZSM-5@CeO2 [118].
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Figure 22. Schematic illustration of the formation of MoFe/Beta@CeO2 core-shell catalyst [121].
Figure 22. Schematic illustration of the formation of MoFe/Beta@CeO2 core-shell catalyst [121].
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Figure 23. XRD patterns of the catalysts: (a) MoFe/Beta@CeO2, (b) CeMoFe/Beta, (c) MoFe/Beta and (d) CeO2 [121].
Figure 23. XRD patterns of the catalysts: (a) MoFe/Beta@CeO2, (b) CeMoFe/Beta, (c) MoFe/Beta and (d) CeO2 [121].
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Figure 24. (A) Nitrogen adsorption–desorption isotherms and (B) the size distribution curves of the catalysts: (a) MoFe/Beta@CeO2, (b) CeMoFe/Beta, (c) MoFe/Beta and (d) CeO2 [121].
Figure 24. (A) Nitrogen adsorption–desorption isotherms and (B) the size distribution curves of the catalysts: (a) MoFe/Beta@CeO2, (b) CeMoFe/Beta, (c) MoFe/Beta and (d) CeO2 [121].
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Table
Table 1. The denitration performance of different catalysts.
Table 1. The denitration performance of different catalysts.
CatalystsMethodTemperature Window/°CNOx Conversion/%Gas Hourly Space Velocity (GHSV)/h−1Refs.
CeO2/TiO2
CeO2/TiO2		Sol–gel
Dry ball milling		300–400
180		93–98%
84.6%		50,000 h−1
GHSV of 30,000 h−1		[31,32]
[34]
MnOx–CeO2One-step hydrolysis process180Over 90%GHSV of 30,000 h−1[53]
CeO2–WO3Two-step hydrothermal impregnation300–450100%GHSV of 60,000 h−1[58]
CeO2–SnO2Hydrothermal280–425Over 90%GHSV of 128,000 h−1[60]
Mn–Ce/TiO2Hydrothermal150–350Over 90%GHSV of 64,000 h−1[70]
Ce–W/TiO2Sol–gel precipitation210–460Over 90%GHSV of 150,000 h−1[81]
Ce–Mo/TiO2Sol–gel250–475Over 90%GHSV of 90,000 h−1[94]
MnCeW/TiO2Impregnation140–340Over 95%GHSV of 40,000 h−1[83]
Ce–Cu/ZSM-5Wet impregnation210–320Over 90%GHSV of 100,000 h−1[112]
MoFe/Beta@
CeO2		Wet impregnation225–600Over 90%GHSV of 50,000 h−1[117]
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Cai, M.; Bian, X.; Xie, F.; Wu, W.; Cen, P. Preparation and Performance of Cerium-Based Catalysts for Selective Catalytic Reduction of Nitrogen Oxides: A Critical Review. Catalysts 2021, 11, 361. https://doi.org/10.3390/catal11030361

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Cai M, Bian X, Xie F, Wu W, Cen P. Preparation and Performance of Cerium-Based Catalysts for Selective Catalytic Reduction of Nitrogen Oxides: A Critical Review. Catalysts. 2021; 11(3):361. https://doi.org/10.3390/catal11030361

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Cai, Ming, Xue Bian, Feng Xie, Wenyuan Wu, and Peng Cen. 2021. "Preparation and Performance of Cerium-Based Catalysts for Selective Catalytic Reduction of Nitrogen Oxides: A Critical Review" Catalysts 11, no. 3: 361. https://doi.org/10.3390/catal11030361

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