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Chemistry, Applied
In recent years, low-temperature SCR (Selective Catalytic Reduction) denitrification technology has been popularized in non-power industries and has played an important role in the control of industrial flue gas NOx emissions in China. Currently, the most commonly used catalysts in industry are V2O5-WO3(MoO3)/TiO2, MnO2-based catalysts, CeO2-based catalysts, MnO2-CeO2 catalysts and zeolite SCR catalysts. The flue gas emitted during industrial combustion usually contains SO2, moisture and alkali metals, which can affect the service life of SCR catalysts.
- selective catalytic reduction
- nitrogen oxides
- low temperature
1. Introduction
Nitrogen oxides (NOx), including NO and NO2, are considered as the main air pollutants from industrial and automobile exhausts, which have caused a lot of environmental problems, such as haze issues and ozone depletion. Until now, the selective catalytic reduction of NOx by ammonia (NH3-SCR) is accepted to be an effective method to eliminate NOx pollutants [1].
In SCR denitration technology, the most important thing is the catalyst that should possess the high activity, excellent sulfur oxides and water resistance abilities. The traditional V-based catalysts showed good deNOx performance at 300–420 °C, which have been well used to abate the flue gas from the power plants. Due to the wide applications of SCR technology, the NOx emissions from power industry have been well controlled, while NOx emission control in the non-power industry faces severe challenges due to the low temperature of the flue gas, which is usually below 300 °C. Therefore, it is difficult to use traditional SCR catalysts for the gas pollutant treatment in the non-power industry. In the past ten years, Chinese scholars and industries have made great efforts on denitration in the non-power industry and made remarkable achievements. Wang and Dong et al. [1,2] present the detailed information concerning NH3-SCR in some non-power industries, showing that the gas condition was more complex and fluctuated than that from the power plant.
2. Low-Temperature SCR Process
Usually, the flue gas temperature of the industrial process, such as in coking and steel sintering processes, is lower than 300 °C and contains many constituents with low concentrations, such as sulfur dioxide and alkali metal salts, and large amounts of water are also present. Thus, the SCR catalyst must be active in the low-temperature range (typically in the range of 160–300 °C) and stable under harsh gas conditions with good sulfur oxides and water resistance performance [3]. In the typical flue gas treatment system of power plants in China, which usually employ high-dust SCR system to control NOx emissions, the SCR reactor is located upstream of the particulate control devices and flue gas desulfurization (FGD) system. The so-called “medium- and high-temperature SCR” units can be operated in the temperature range of 280–400 °C. For NOx emission control in the non-power industry, especially in the coking and steel sintering industry, the “low-dust” or “tail-end” configuration of SCR technology should be adopted to decrease the impacts of SO2 and dust on the SCR catalyst. The low-temperature SCR reactor is located downstream of the particulate control devices and flue gas desulfurization (FGD) system. The SO2 concentration and dust amount in inlet of the SCR reactor should be lower than 35 and 5 mg/m3, respectively, to meet the ultra-low emission standards. In the key areas of coal-fired boilers, the value should be lower than 50 and 20 mg/m3. In this case, the operating temperature of the SCR unit can be decreased to 160 °C. Although SO2 and dust in the flue gas have very low values, the SCR catalyst needs to operate for three years at a low temperature, posing a severe challenge to SCR catalyst technology.
3. SCR Catalysts
Low-temperature SCR technology is an economic and effective process in abating the NOx pollutants emitted from the non-power industry. Based on the consensus of the advantages that low-temperature SCR technology possesses, in the last decade, considerable research in China have been devoted to developing catalysts that can work well under low-temperature conditions. The development and research in SCR catalysts have been reviewed and summarized [2,3]. As reported, many kinds of low-temperature SCR catalyst system have been proposed and investigated. The main low-temperature SCR catalyst systems include V2O5-WO3(MoO3)/TiO2, Mn complex oxides, CeO2-based and zeolite catalysts.
3.1. V2O5-WO3(MoO3)/TiO2
As a typical and efficient catalyst, a V2O5-WO3(MoO3)/TiO2 catalyst used in NH3-SCR technology has been commercialized for several decades. The typical commercial catalyst used in power stations presents low activity at low temperatures (below 300 °C) and cannot meet the need to abate NOx from non-power industries.
In the past decades, various transition-metal oxides have been researched as catalysts for NH3-SCR at low temperatures. In order to meet the need of activity, stability, and resistance of SO2 and H2O, plenty of methods have been tried to improve SCR performance.
The most direct and convenient method to improve the low temperature activity of V2O5-WO3(MoO3)/TiO2 catalyst is to appropriately increase the loading of V2O5. However, when V2O5 loading increases, the oxidative ability of the catalyst will be increased leading to the enhancement of SO2 conversion. This is not allowed for the low-temperature SCR technology. Therefore, the catalyst needs to coordinate the redox activity and the surface acid property, reduce the adsorption of SO2 on the catalyst surface and suppress the oxidation of SO2. In another way, the NH3 adsorbed on the Lewis acid sites (V5+-O) on V2O5-WO3(MoO3)/TiO2 catalysts can react with NO at low temperatures [4]. By increasing the surface acidity and inhibiting the oxidative ability over the SCR catalysts, the operating temperature window of the V2O5-MoO3/TiO2 catalyst is expanded to the range of 160–400 °C, which also shows acceptable SO2 and H2O resistance at low temperatures [5,6,7]. The V2O5-MoO3/TiO2 catalysts have been used intensively in denitration reaction projects of coking sintering, refuse incinerators and other non-electric industries in China.
Another way to improve the low-temperature SCR activity of the V2O5-WO3(MoO3)/TiO2 catalyst is through modification and doping by introducing other elements into the catalyst system, which is easy to achieve in practice due to the convenience in the preparation. For example, Zhang et al. [8] investigated the effect of Mn, Cu, Sb, and La doping on the SCR performance of the V2O5-WO3(MoO3)/TiO2 catalyst. The investigation showed that Mn and Cu could enhance the redox property and weak surface acidity, while Sb and La addition showed promotion in the amount of acid sites. Liang et al. [9] demonstrated that a 3% addition of CeO2 improved the NH3 adsorption performance, NO oxidation, and sulfur oxide and the water-resistance of the V2O5-WO3/TiO2 catalyst.
3.2. MnO2-Based Catalysts
Manganese-containing catalysts have been paid enough attention due to their variable valence states and excellent redox properties. However, for its poor N2 selectivity and easy deactivation by SO2, the catalyst only containing manganese oxide is extremely restricted in the SCR process. Mn-based composite oxides are popular and have proven to be effective catalysts with an enhanced SCR performance [10].
Over MnOx catalyst, NH3 species adsorbed on Lewis acid sites (Mn3+) were active at low temperatures. Bidentate nitrates were inactive at low temperatures (below ~225 °C), but active at higher temperatures [11].
Mn-based composite oxides possess excellent redox properties due to their various valence state, which are a benefit to enhance the process of NO oxidation to NO2. The formation of NO2 from NO oxidation is considered as a key factor in low-temperature activities because a certain concentration of NO2 gives an enhancement of the “Fast SCR” reaction at low temperatures. Chen et al. [12] proposed that the redox cycle between Cr5+ + 2Mn3+ ↔ Cr3+ + 2Mn4+ promoted the oxidization of NO to NO2 at low temperatures. Liu et al. [13] reported that an urchin-like MnCrOx catalyst possessed good NH3-SCR activity in the temperature range of 150–350 °C and improved SO2 resistance.
Gao et al. [14] discovered that CoMnOx showed high NH3-SCR activity at low temperatures and delayed the trend of SO2 poisoning. Zhao et al. [15] found that a lamellar CoMnOx composite oxide could provide more Lewis acid sites and surface oxygen species than those of CoMnOx nanoparticles. Wang et al. [16] reported that ballflower-like CoMnOx catalyst exhibited good SCR activity and N2 selectivity in the temperature range of 150–350 °C, showing a certain amount of SO2 resistance and durability.
The doped component was usually considered to give a promotion of surface lattice oxygen species. Sun et al. [17] investigated the NH3-SCR activity over the Nb-doped Mn/TiO2 catalysts with the optimum Nb/Mn molar ratio of 0.12. Rare earth elements were also used in the modifications. Liu et al. [18] and Xu et al. [19] investigated the effect of the introduction of Sm to Mn-TiOx catalysts. The introduced Sm could improve the dispersion of manganese oxide on the surface of the catalysts, resulting in increases in surface area, the amount of weak Lewis acid sites and surface oxygen.
Among these catalysts, spinel-type materials containing manganese attracted interest in SCR due to their special spatial structures and physical-chemical properties. Gao et al. [20] reported that a Zr4+ cation doped MnCr2O4 spinel, the zirconium incorporated in the crystal of MnCr2O4 produced higher levels of beneficial Mn3+, Mn4+ and Cr5+ species, and showed an increase in the acidity and redox ability.
However, these catalysts are very sensitive and exhibit unsatisfactory N2 selectivity [21]. The stability in the presence of SO2 and H2O in the flue gas is still a problem for MnO2-based catalysts.
3.3. CeO2-Based Catalysts
He et al. [22,23] reported that the crystal structure, crystallite size and catalytic NH3-SCR activity over the CeO2-based catalysts presented a regular change with the increase in CeO2 concentration. Particularly, the CeO2-TiO2 (1:1 in weight) catalyst with an amorphous structure showed a higher BET surface area and a stronger surface acidity than other samples. Meanwhile, favorable Ce3+ and the surface-adsorbed oxygen benefited the adsorption of NOx and NH3 molecules, which could enhance NH3-SCR activity.
In the past years, tungsten or molybdenum addition in ceria-based catalysts was paid some attention. Jiang et al. [24] demonstrated that the introduction of WO3 could improve SCR activity over the CeWTiOx catalysts due to the enhanced dispersion of Ce species over TiO2 and the amount of Ce3+ and chemisorbed oxygen. Li et al. [25] investigated the adsorption and reactivity of NH3 and NO over the CeWTiOx catalyst, showing that over 90% of NO conversion can be obtained in the temperature range of 250–500 °C. Liu et al. [26] reported that the WO3/TiO2@CeO2 core–shell catalyst present a synergistic effect of redox properties and acidity, which is in favor of the excellent NH3-SCR activity and better SO2 resistance. Kim et al. [27] found monomeric W in CeO2/TiO2 catalyst enhance the SCR reaction activity at low temperatures due to the increased NO adsorption and the formation of unstable NOx adsorption species. Dong et al. [28] presented that the coverage of MoO3 weakened the adsorption of nitrate species over the CeO2-TiO2 catalyst, giving an increase in the number of Brønsted acid sites. For the CeO2-based catalyst, cerium sulfate, which is formed in reaction with SO2 in the flue gas during the SCR process at high temperatures, has attracted wide attention. Fan et al. [29] showed that NH3-SCR reaction over CeO2/TiO2-ZrO2-SO42− mainly followed the L–H mechanism at a low temperature (250 °C) and the E–R mechanism at 350 °C.
Generally, for the limitation of sulfate formation and low temperature SCR performance, CeO2-based SCR catalyst does not seem an optimal choice for NOx elimination under low-temperature conditions at present.
3.4. MnO2-CeO2 Catalysts
Rare-earth metal oxides, such as Ce, have been frequently adopted to modify MnOx as an efficient low-temperature NH3-SCR catalyst due to their incomplete 4f and empty 5d orbitals [30]. Leng et al. [31] synthesized MnCeTiOx catalysts and compared the NH3-SCR activity over the samples with different Mn/Ce mol ratios in the low-temperature range. The results showed that the low-temperature SCR activity over MnCeTiOx compositions was greatly improved due to the incorporation of Mn, and the best performance (~100% NO conversion and above 90% N2 yields) across the temperature range of 175–400 °C at GHSV of 80,000 h−1.
In the published lectures, composite transition metal oxides usually showed a higher activity than single oxide materials. Zhang et al. [32] demonstrated the enhanced electron mobility effect that originated from MnOx and CeOx, which enhanced low-temperature deNOx efficiency. Compared to the single composition of CeO2, MnOx could increase the pore volume and pore diameter, and enhance the adsorption of NO and NH3 as well as in the concentrations of Ce3+ on the CeO2-MnOx catalyst, which is beneficial to increase the redox properties [33].
Yang et al. [34] studied SCR activity over the activated carbon supported Mn-Ce oxide catalysts modified by Fe, on which ca. 90% NO conversion was obtained at 125 °C with GHSV of 12,000 h−1. Zhu et al. [35] synthesized a 3D-structured MnOx-CeO2/reduced graphene oxide, giving a NOx conversion of 99% at 220 °C with GHSV of 30,000 h−1.
3.5. Zeolite SCR Catalysts
Ion-exchanged zeolite catalysts with small pores have been accepted as optimum SCR catalysts in NOx elimination from diesel engine exhaust. Among them, copper or iron exchanged zeolites with a chabazite (CHA) structure, such as Cu/SAPO-34 and Cu/SSZ-13, have received significant attention due to their excellent SCR performance, wide temperature window and thermal stability in harsh conditions [36]. Cu-SSZ-13 exhibits excellent SCR activity (>80% NO conversion) and N2 selectivity in a wide temperature range of 150−450 °C [37]. Cu/SAPO-34 prepared by a hard-template method using CaCO3 as template present NOx convention above 90% at 170–480 °C, even introducing 10% H2O [38]. A heterobimetallic FeCu-SSZ-13 zeolite with high crystallinity was prepared by an economic and sustainable one-pot synthesis strategy, which presents a wide reaction temperature window, excellent hydrothermal stability, high H2O and SO2 tolerance, and good gaseous hourly space velocity flexibility [39].
Zeolite catalysts for SCR has been developed rapidly these years, offering a great contribution to abate the NOx reduction. However, they may be not the optimum choice for NOx elimination in stationary sources due to the limitations of cost and synthesis strategy.
This entry is adapted from the peer-reviewed paper 10.3390/catal12030341
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