JOURNAL OF RARE EARTHS, Vol. 32, No. 10, Oct. 2014, P. 907
Rare earth containing catalysts for selective catalytic reduction of NOx with ammonia: A Review CHEN Lei (陈 磊)1, SI Zhichun (司知蠢)1, WU Xiaodong (吴晓东)1,2, WENG Duan (翁 端)1,2,, RAN Rui (冉 锐)2, YU Jun (於 俊)1 (1. Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China; 2. State Key Laboratory of New Ceramics & Fine Process, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China) Received 10 December 2013; revised 27 May 2014
Abstract: Increasingly stringent regulations in many countries require effective reduction and control of NOx emissions. To meet these limits, various methods have been exploited, among which the selective catalytic reduction of NOx using ammonia as the reductant (NH3-SCR) is the most favored technology. High catalytic activity, N2 selectivity and resistance to deactivation by sulfur, alkaline metals and hydrothermal conditions are the optimal properties of a successful SCR catalyst. Rare earth oxides, particularly CeO2, have been increasingly used to improve the catalytic activity and resistance to deactivation of deNOx catalysts, both modifying traditional vanadium catalysts, and also developing novel catalysts, especially for low temperature applications. This review summarized the open literature concerning recent research and development progresses in the application of rare earths for NH3-SCR of NOx. Additionally, the roles of rare earths in enhancing the performance of NH3-SCR catalyst were reviewed. Keywords: rare earths; ceria; transition metal oxide; deNOx; deactivation; NH3-SCR
In the past decades, nitrogen oxides (NOx: NO, NO2 and N2O), as major air pollutants, have attracted increased attention due to their environmental impact. Accordingly, NOx emission regulations have tightened for both mobile and stationary sources. Currently, the favored method of denoxification (deNOx) utilizes ammonia as a selective catalytic reductant (NH3-SCR), which, introduced in the late 1970s to control NOx emissions in the flue gases of coal-fuelled power plants and other industrial facilities. Both the V2O5-WO3/MoO3-TiO2 mixed oxide and transition-metal exchanged zeolite catalysts present excellent NH3-SCR performance. However, these commercial catalysts have intrinsic flaws to fulfill special applications, such as diesel exhaust purification, in which the deNOx catalyst requires ultra-high hydrothermal stability to tolerate the temperature induced by the regeneration of diesel particulate filter (DPF). Alternatively, some new deNOx applications, including waste treatment from cement and glass manufacture, require catalysts to function at much lower temperatures, particularly 100– 200 ºC, which falls below the optimal operational temperature of the vanadium and the zeolite based catalysts. Furthermore, vanadium byproducts formed during catalyst preparation and usage is hazardous to the environment and human health. Therefore, it is of great significance and interest to develop novel catalysts to replace or
reduce the vanadium loadings. Several reviews about the vanadium catalysts[1–4], transition metal ion exchanged zeolites[3–7] and NH3-SCR reactions can be found in the Refs. [1–7]. Recently, CeO2 and other rare earths oxides have also been used as the promoters to improve the thermal stability and catalytic activity of traditional vanadium and zeolite based SCR catalysts. CeO2 based SCR catalysts have been investigated due to oxygen storage and redox properties via the redox shift between Ce4+ and Ce3+. These can be divided into several groups, such as MnOx-CeO2, CeO2-TiO2, and ceria modified by acidic components (WO3, MoO3, Nb2O5, SO42–, PO43– or their composites). Study of ceria based NH3-SCR are currently focused on finding a new catalytic composite with fine catalytic performance, enhancing hydrothermal stability, increasing resistance to sulfur and alkali metals, elucidating the reaction mechanism of SCR and the possible interactions between ceria and other components. The following review will discuss the use of rare-earth oxides in SCR of NOx, including rare earth modified vanadium based catalysts, rare earth exchanged zeolites catalysts, CeO2-TiO2 catalysts, CeO2 based solid acids catalysts and MnO2-CeO2 catalysts. The common role of rare earths in these catalysts, the NH3-SCR mechanisms and the deactivation mechanism of ceria catalysts, will be summarized.
Foundation item: Project supported by National Natural Science Foundation of China (51202126), Postdoctoral Science Foundation of China (2012M520266) and Strategic Emerging Industry Development Funds of Shenzhen (JCYJ20120619152738634) * Corresponding author: WENG Duan (E-mail:
[email protected]; Tel.: +86+010+62772726) DOI: 10.1016/S1002-0721(14)60162-9
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JOURNAL OF RARE EARTHS, Vol. 32, No. 10, Oct. 2014
General fundamentals and mechanism of NH3-SCR catalysis
The “standard SCR” reaction stoichiometry (Eq. (1)), defines the reduction of NO by NH3 into H2O and N2. “Fast SCR” (Eq. (2)), first proposed in early 80s, proceeds at a much higher reaction rate than “standard SCR” and was developed to improve deNOx efficiency, especially at low temperatures[8,9]. Additional, unwanted non-selective reactions may also occur during practical applications of NH3-SCR (Eqs. (3)–(5)). Increasing the reaction temperature may improve the catalytic efficiency of catalysts, however, parallel reactions of SCR and side reactions result in an inverted bell-shaped deNOx efficiency vs. temperature curve. Ammonia can react selectively with NOx to give N2 or non-selectively to produce other NOx species. Therefore, the NOx conversion (Eq. (6)), temperature window (the temperature region for NOx conversions over 80%) and N2 selectivity (Eq. (7)) are always used in combination to evaluate the NH3-SCR activity of a catalyst. 4NO+4NH3+O2→4N2+6H2O (1) NO+NO2+2NH3=2N2+3H2O (2) 4NH3+3O2→2NO+6H2O (3) 4NH3+5O2→4NO+6H2O (4) 2SO2+O2→2SO3 (5) NO xinlet NO xoutlet NO x conversions (6) NO xinlet N 2 selectivity
NO xinlet NH 3 inlet NO xoutlet NO xinlet NH 3 outlet
(7)
From Eq. (1), it is reasonable to assume that improving the NH3 or NO adsorption/activation or oxygen mobility may facilitate high catalytic performance. The adsorption of NH3 and NO on the surface of SCR catalysts is mainly determined by their acidity. Lewis acid sites are the essential active sites for NH3-SCR catalysts, employing an electron lone pair from another molecule to stabilize one of its own atoms[1]. Brønsted acid sites, able to “donate” a proton (the hydrogen cation, or H+) on SCR catalysts are mainly hydroxyl groups. Acidic catalysts are characterized by the ability to adsorp/desorb of ammonia or pyridine at various temperatures. Side reactions (3, 4) are always induced by the undesired high redox capacity of catalysts, resulting in the consumption of NH3 at high temperatures, and thereby lead to a decrement of N2 selectivity. The deactivation of NH3-SCR catalysts in practical applications is mainly caused by side reaction (5) which may lead to sulfate deposition on catalysts and the subsequent loss of active sites.
2 Additives for V2O5-WO3-TiO2 catalyst It is known that the activity of a V2O5/WO3-TiO2 cat-
alyst increases notably with a rise in calcination temperature up to 600 ºC and then rapidly decreases, which is due to several structural and textural modifications occurring at high temperature over the support and the active phase, such as melting/crystallization of free V2O5, phase transformation of TiO2 from anatase to rutile, and consequent loss of BET surface area with changes in the chemical state of surface vanadium species[10–13]. Rare earths addition can strongly increase the thermal resistance of the catalysts via inhibiting rutilization and surface area loss via forming vanadates covering the external surface of anatase[11–14]. Rare earth addition was reported to cause little modification to the nature of the surface vanadyl and wolframyl species, however, leads to slightly reduced NH3SCR catalytic activity in a fresh state due to the lowered Lewis acidity of catalysts caused by rare earth, electron withdrawal[11]. Rare earth cations may act as additional Lewis acid sites for ammonia molecular adsorption; however, these ammonia species are not active as intermediates for SCR[11]. Rare earths (in particular Tb and Er) strongly increase catalytic activity after ageing (shown in Table 1), which might be related to higher thermal stability and the interaction between surface vanadium and Table 1 NOx conversion (%) of fresh and aged catalysts made out of REVO4/TWS [13] Reaction temp/ºC
V2O5 La Ce
Pr
Nd Sm Gd Tb Dy Er
Fresh 250
85
35
69
40
32
28
28
15
28
27
270
90
51
78
53
50
42
41
29
46
42
290
93
57
83
57
61
53
49
40
57
55
320
94
62
88
60
64
61
55
50
64
65
350
92
61
88
63
64
63
57
56
65
68
380
86
59
84
63
63
62
54
59
65
67
420
63
54
69
54
57
57
49
54
59
62
450
44
43
43
41
47
48
42
43
49
54
250
49
51
73
70
65
64
54
51
60
58
270
50
59
88
81
81
80
68
69
80
72
290
51
60
91
89
87
87
76
79
85
79
320
49
57
91
89
89
89
79
85
89
83
350
30
51
90
87
90
88
76
84
86
78
380
10
44
80
80
84
85
73
78
81
73
420
–
26
58
59
69
65
62
70
68
62
450
–
5
23
33
46
50
45
58
50
45
250
13
2
36
45
50
69
54
53
62
61
270
14
2
48
63
62
82
72
66
77
80
290
15
2
49
70
69
85
80
75
84
87
320
14
–
48
67
71
87
83
80
86
88
350
10
–
36
61
64
83
82
79
83
85
380
3
–
21
48
53
78
77
70
77
79
420
–
–
–
16
29
64
64
58
62
67
450
–
–
–
–
2
43
42
43
40
46
Aged 700 ºC/10 h
Aged 750 ºC/10 h
CHEN Lei et al., Rare earth containing catalysts for selective catalytic reduction of NOx with ammonia: A Review
rare earth elements[11,12]. The effect of ceria on the thermal deactivation is still in debate. A slight post-ageing deactivation of ceria modified catalyst was observed by Busca et al.[11] and Casanova et al.[12]. Similar results were also reported by Trovarelli et al.[13] with Tb, Er, Dy, Sm and Gd modified V2O5-WO3-TiO2-SiO2 catalysts showing excellent resistance to deactivation and high post-ageing activity, while Ce, La, Nd and Pr modified catalysts were more sensitive to the thermal treatment. This was deduced to be correlated to the periodic table position of rare earth elements and to the ability of rare earths to alter the characteristics of the V–O bond and the acid/base surface properties. However, Ce additive was reported to enhance the activity of V2O5-WO3-TiO2 catalyst in a wide temperature region due to the improved NOx adsorption on vanadium catalyst[14]. Ce mainly exists in the form of Ce3+ oxide in V0.1W6Ce10Ti catalysts, which is beneficial for the oxidation of NO to NO2, due to the stronger and more active Brønsted acid sites introduced[14].
3 CeO2 mixed with acidic components Pure ceria proved active in NO oxidation, NH3-SCR and selective catalytic oxidation (SCO) reactions. NH3 and NOx were found to adsorb on the ceria simultaneously, however, SCR activity was poor in the 200–500 ºC temperature region[15]. The scheme of ceria’s interaction with NOx and NH3 was proposed by Epling et al.[15] (Fig. 1). In the presence of NOx, NH3 reacted with adsorbed NOx species, again forming N2 at lower temperatures (250−450 °C), and while at higher temperature, a significant portion of the NH3 was oxidized to NO[15]. The combination of two metals in an oxide matrix can produce materials with novel structural and electronic properties which can lead to superior catalytic performance. However, it is generally accepted that NH3 activation and copious ammonia adsorption are the key factors in the NH3-SCR reaction. Therefore, various acidic components, such as VOx, SnOx, TiO2, WO3, MoO3, Nb2O5, SO42–, PO43– and composites of two or more thereof were used to improve the NH3-SCR activity of ceria by improving ammonia adsorption. In principle,
Fig. 1 Scheme of ceria’s interaction with NOx and NH3 [15]
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combination of highly redox active ceria with acidic components can contribute to superior catalytic performance in the NH3-SCR reaction, and scientific criteria are very much needed for choosing the optimal elements when designing a ceria catalyst. Oligomeric and polymeric VOx and CeVO4 are formed on the surface of CeO2 nanorods according to the surface density of vanadium atoms, which do not change the ceria lattice or the BET surface area[16]. However, these vanadium species decrease the reducibility and surface oxygen defects of the catalyst. Lewis acid sites mainly originate from CeO2 and polymeric VOx while CeVO4 significantly promotes Brønsted acid sites. At high temperatures, a portion of Lewis acid sites convert to Brønsted acid sites and both exhibit high reactivity. SnOx modified ceria catalysts were also reported to promote high NH3-SCR activity and high resistance to the poisoning effects of K2O and PbO from 100–400 °C[17]. This catalytic performance derived from the synergistic effect between CeO2 and SnO2 which results in co-adsorbed NH3 and NOx species on surface of catalysts. CeO2 mixed with TiO2 has been extensively reported in recent years, the loading amount of ceria on TiO2 varied due to different preparation methods and materials used, but in general a low loading amount of ceria (5 wt.%–20 wt.%) shows the optimum catalytic performance[18–35]. The CeO2-TiO2 catalyst with molar ratio of Ce/Ti=0.20, showed over 90% NOx conversion from 250 to 450 ºC under a GHSV of 20,000 h−1 in the presence of H2O, CO2, and C3H6 [18]. A dispersion capacity of about 6.98 Ce4+ ions/nm2 on TiO2 was reported by Zhu and Dong et al.[20]. CeO2-TiO2 catalysts with 5% Ce and above, synthesized by an impregnation method using an anatase type TiO2 and an aqueous solution of cerium nitrate, were reported to have high activity in the temperature range 275–400 ºC at a space velocity of 50,000 h–1 [24]. CeO2/TiO2 catalysts prepared by a single step sol-gel method with a mass ratio of CeO2/TiO2 of 0.6, yielded 98.6% NO conversion and 100% N2 selectivity at 250– 450 ºC and the gas hourly space velocity of 50,000 h−1 [27,28]. The synergistic effects between CeO2 and TiO2 can inhibit the growth of anatase TiO2 crystallite and thereby lead to a higher BET surface area of the catalyst, high surface concentrations of amorphous CeOx and increased chemisorbed oxygen and/or weakly bound oxygen species, all facilitating high catalytic activity[24–31]. Increased active oxygen on the surface of CeO2-TiO2 catalyst will promote NO2 formation which is beneficial for high SCR activity of the catalyst at low temperatures i.e. “fast SCR” reactions[27]. Amorphous CeO2-TiO2 mixed oxides were reported to show higher activity than the crystalline equivalents at low temperatures due to Ce−O−Ti short-range order species with the interaction between Ce and Ti in the atomic scale (Fig. 2)[31]. Sulfates form and preferentially diffuse from the sur-
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Fig. 2 Confirmation of Ce−O−Ti short-range order species[31]
face to a bulk phase during the poisoning process of CeO2-TiO2 catalysts[26]. SO2 reacts with the catalyst to form highly thermally stable Ce(SO4)2 and Ce2(SO4)3, resulting in the disruption of redox between Ce(IV) and Ce(III) and inhibition of the formation and adsorption of nitrate species[26]. Meanwhile, NH4HSO4 can deposit on the surface of the catalyst and block the active sites. The Ce/TiO2 catalyst was also observed to be deactivated significantly by loading of Na+, K+ or Ca2+ ions[32]. This decline in SCR activity is likely due to the enlargement of ceria nanoparticles, the reduced Ce4+/Ce3+ redox cycle rate and the change in the surface acidity after loading with Na+ or Ca2+ ions[32]. Wu et al.[33–35] reported a novel catalyst-titanate nanotube (TNT) confined CeO2 which showed a remarkable resistance to alkali metal poisoning during deNOx application. The tubular channel of H2Ti12O25 effectively shielded the main active phase, CeO2, from the poisons whilst the poisons (e.g., Na+) were also stabilized in the interlayer of H2Ti12O25 through ion exchange. CeO2-TiO2 catalysts have been modified by other transition metal oxides, such as manganese[36–41], iron[42], copper[43–45], zirconia[46] and fluorine[47,48] to improve their catalytic performance and deactivation resistance. It is notable that the low or high temperature activity of CeO2-TiO2 catalysts can be adjusted by these transition metal oxide additions. Wu et al.[36–41] investigated the catalytic behavior of Mn-Ce oxides supported on TiO2 and Al2O3, and further studied the SO2 poisoning effects of the Mn–Ce/TiO2 catalyst. The mechanism of SO2 poisoning was directly correlated to reaction temperature, with the active sites of the Mn–Ce/TiO2 catalyst seriously sulfated at 200 ºC, while the formation and deposition of (NH4)2SO3 and NH4HSO4 were the main causes of catalyst deactivation at 100 ºC. This kind of deactivation could be almost completely recovered after water-washing treatment. Wu et al. also studied the mechanism of Ca-modified Ce0.02Mn0.4/TiO2, the promotion of N2 selectivity induced by the reduced formation of NH and NO2 on catalyst[40]. Iron doping can enhance catalytic activity and resistance of Mn-Ce/TiO2 to H2O and SO2, which is ascribed to an increased specific surface area and NH3 and NOx adsorption capacity, as well as an improved dispersion and oxidation state of Mn and Ce on
JOURNAL OF RARE EARTHS, Vol. 32, No. 10, Oct. 2014
the surface of the catalysts[41]. An Fe-Ce/TiO2 catalyst with a Fe/Ti molar ratio of 0.2 has a high activity at low-temperatures and sulfur-poisoning resistance compared with Ce/TiO2 catalysts[42]. The introduction of Fe increases the amount of Ce3+ and chemisorbed oxygen species on the catalyst surface which generates increased ionic NH4+ and in situ formed NO2, respectively. Gao et al.[43–45] found that Ce-Cu-Ti oxide catalyst achieved >80% NO conversions in the temperature region of 230–400 ºC with high SO2-resistance. This was predominantly due to preservation of the redox ability and the increase of surface acidity after sulfation. H2O inhibits the SCR performance of the Ce-Cu-Ti catalyst at temperatures below 300 ºC due to competitive adsorption of H2O with NH3, however, H2O promotes SCR performance at temperatures above 300 ºC due to the inhibition of NH3 oxidation[45]. Zirconium additive was reported to improve both the catalytic activity and N2 selectivity of Ti0.8Ce0.2O2 complex oxides for NH3-SCR reaction by enhancing the oxygen storage capacity (OSC) and acidity of catalyst, refining the particle size and inhibiting the titania phase transformation from amorphous to anatase[46]. The enhancement mechanism of sulfur resistance is that SO2 oxidizes into SO3, thus formed SO42– enhances acidic sites at high temperatures[46]. Zhong et al.[47,48] proposed that addition of F could enhance redox potential, resulting in increased active adsorbed NH3 species, thus enhancing catalytic activity for NH3-SCR of NOx. CeO2 mixed with WO3 [49–59], MoOx [60,61] and NbOx [62–64] have been under extensively studied in the past few years. The case of WO3 is complex as several polymorphs, such as the monoclinic and orthorhombic phases, have been detected on contact with ceria. The structure evolution of tungsten oxides on CeO2 is greatly influenced by its content. It was reported by Li et al.[49] that surface WOx species are isopolytungstate coupled with microcrystalline WO3 at low WO3 content, whereas both crystalline WO3 and Ce2(WO4)3 are the dominant surface species at intermediate and high WO3 content. A reaction mechanism for WO3-CeO2 catalysis was proposed by Peng et al.[49] (see Fig. 3), in which gaseous NH3 bonds to CeO2 (on Ce3+) or crystalline WO3 (on Wn+) as Lewis acids and strongly bonds to Ce2(WO4)3 (on W–O–W or W=O) as Brønsted acids[49]. The gaseous NO or weakly adsorbed NO2 species can react with activated NHx species. Similar results were proposed that Lewis acid sites originate from CeO2 and some of the unsaturated Wn+ cations of crystalline WO3. Brønsted acid sites were formed on the W–O–W or W=O sites of Ce2(WO4)3 [51]. However, the reducibility of the WOx-CeO2 catalysts was reported to be decreased by the strong interaction between ceria and tungsten oxide[49,55]. After severe hydrothermal aging at 760 ºC, the NH3-SCR activity of WOx-CeO2 catalyst was sharply decreased compared to fresh catalyst because of
CHEN Lei et al., Rare earth containing catalysts for selective catalytic reduction of NOx with ammonia: A Review
Fig. 3 Reaction mechanism of CeO2-WO3 catalyst[49]
the greatly reduced redox potential caused by the formation of cerium tungstate, although the acidity of aged WOx-CeO2 catalyst was still high[55]. For the fresh catalyst, the interaction between ceria and tungsten oxide promotes the activation of gaseous oxygen to compensate for the lattice oxygen consumed in the NH3-SCR reaction at low temperatures[30]. The alkaline poison of CeO2-WO3 mainly arises from decreased reduction activity and number of Brønsted acid sites rather than the acidic strength[51]. Hot water washing is also a convenient and effective method to regenerate alkali metalpoisoned CeO2-WO3 catalysts. Li et al.[57,58] reported that manganese doped CeO2-WO3 catalysts show improved low temperature NH3-SCR activity. Theoretical studies suggest that oxygen vacancies can easily form on the MnCeW (110) surface, resulting in more facile NH3 adsorption and higher activity. Kröcher et al.[59] reported Ce0.6Zr0.4O2 dispersed on the surface of acidic WO3/ZrO2 via solution combustion, showed the best NOx reduction efficiency among variously synthesized catalysts. Li et al.[60,61] reported that SCR activities of MoO3CeO2 catalysts are enhanced by both Lewis and Brønsted acidities provided by surface cerium atoms and amorphous MoO3 structures, respectively. Strong interactions between CeOx and MoOx in the catalyst could be the main reason for the excellent NH3-SCR catalytic performance and strong resistances to high space velocity as well as SO2 and H2O poisoning[61]. Kröcher et al.[62] reported a niobia-ceria based catalyst that may be applied for both the SCR of NOx as well as the catalytic regeneration of DPF in diesel engines. Both Brønsted acid (Nb–OH) and Lewis acid sites (Nb=O) can be introduced on ceria by niobium oxides[63], compensating for the loss of acid sites on WOx-TiO2 support by the impregnation of alkaline, which significantly improves the ammonia adsorption on Nb–Ce/WOx–TiO2 catalysts at high temperatures[63]. The concentration of oxygen vacancies can also be increased by charge imbalances between Nb5+ and Ce4+ on Nb5+–O2– –Ce4+ interfaces[64]. Large amounts of surface active oxygen and acid sites imbue a wide NH3-SCR operation temperature window for Nb–Ce catalysts[62–64]. Recently, increasingly positive effects of sulfur on ceria catalysts have been reported. The improvement in
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SCR activity by sulfation may originate from increased active oxygen species and enhanced acidity for NH3 chemisorption[65–73]. The interactions between sulfates and NiO-CeO2-ZrO2 solid solution lead to an enhanced Lewis acidity of the due to the electron withdrawing effect of sulfates from Nin+ [65–67]. The addition of sulfates decreases the amount of readily available oxygen on the catalyst, however, promotes lattice oxygen mobility via the formation of Nin+–O–Sn+ bonds in CeO2-ZrO2 mixed oxides[65]. Ammonia oxidation is inhibited by decreased surface active oxygen by sulfation, while the adsorption of ammonia on the catalyst is promoted by sulfate-derived acid sites[68]. Sites for NH2 adsorption on CeO2 and oxidizing agents were separated following sulfation, resulting in an intuitive inhibition of catalytic oxidization from NH2 to NO[68]. The severe decrease of SCR activity of sulfated ceria after aging at 760 °C may occur due to the decomposition of sulfate at temperatures higher than 650 °C [72]. Sulfation treatment of zirconia supports was reported to greatly enhance the surface acidity of CeO2/ZrO2 catalysts, which is critical for enhanced resistance to alkali metal ions[69,70]. Alkali species preferentially interact with strongly acidic supports leaving active metal sites intact. Ce–P–O catalysts were reported to show high deNOx activity (NO conversion>90%) within the temperature range 300–550 °C at GHSV of 20,000 h–1 [73,74]. The Ce–P–O catalyst exhibited greater resistance to deactivation by K2O poisoning than the commercial V–W–Ti catalyst. Weng et al.[75] reported a Ce0.75Zr0.25O2–PO43− catalyst which showed over 80% NOx conversion at 250–450 ºC. The catalyst still presented high NH3-SCR performance at 300–400 °C after hydrothermal aging at 760 °C for 48 h and could be regenerated completely by treating in air at 650 °C after sulfur aging. Phosphates improved the ammonia adsorption and decreased the ammonia oxidation on the catalyst, leading to a high NH3-SCR activity and a high N2 selectivity.
4 MnO2-CeO2 catalysts The low temperature activity of MnOx-CeO2 catalysts for SCR of NO with NH3 was first reported by Qi et al. and Yang et al.[76–78], which initiated extensive study over the last decade[3,76–90]. In MnOx-CeO2 catalysts, CeO2 mainly acts as support for dispersal of and interaction with manganese oxides, leading to more facile active oxygen species. Preparation method has significant impact on catalytic activity, which does not correlate with the BET surface area of catalysts. The optimal molar ratio Mn/(Mn+Ce) has not yet been consistently achieved, and MnOx-CeO2 catalysts prepared by different methods present highly disparate temperature regions for maximal NO conversion. The most active catalyst, with a molar Mn/(Mn+Ce)
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ratio of 0.3, yielded nearly 100% NO conversion at 120 ºC at a high space velocity of 42,000 h–1 [76]. The oxygen vacancy formed in the CeO2 lattice by the incorporation of Mn atoms would adsorb and activate molecular oxygen species. The initial step of the NH3SCR reaction on MnOx-CeO2 catalysts was reported to be the adsorption of NH3 onto Lewis acid sites of the catalyst, followed by reaction with nitrite species with intermediate of NH2NO to produce N2 and H2O[76,78]. Similar results were reported by Eigenmann et al.[80], indicating that nitrogen formation follows an Eley-Rideal type mechanism at 100–150 ºC, where adsorbed ammonia reacts with NOx in the gas phase, and adsorbed NOx shows no significant reactivity. Wet-impregnation, coprecipitation and solid-state synthesis techniques lead to clustered MnOx-like species in the ceria matrix, while the solution combustion route yields a highly dispersed form of Mn species which enhances the redox behavior of the catalyst[79]. Liu et al.[81] reported that Mn-Ce mixed oxide catalysts prepared by a surfactant template method yielded nearly 100% NOx conversion in the temperature range of 100–200 ºC, its resistance against H2O and SO2 is higher than that prepared by conventional co-precipitation method. Further study by Shi et al.[82] confirmed that the mechanism of the NH3-SCR reaction on MnOx/CeO2 catalysts involves a [NH3...NO–] complex as an intermediate (Fig. 4), the decomposition of which, into N2 and H2O, is the rate-limiting step[82]. Additionally, The role of oxygen in the SCR reaction is to re-oxidize the reduced catalyst surface, thereby completing the catalytic cycle. Wu et al.[83] investigated SOx poisoning effects on pure and Mn doped ceria and proposed that the introduction of Mn to ceria enhanced the thermal stability of surface sulfate, which greatly increased catalyst sensitivity to SOx poisoning. The influence of various dopants, including niobium[84,85], tungsten[86,87], zirconium[88,89] and stannum[90], on the low temperature activity of MnOx-CeO2 catalysts has been widely studied. Clearly increased catalytic activity and greatly superior nitrogen selectivity were obtained from the niobium-doped catalyst in comparison with the MnOx-CeO2 reference system[84,85]. At 200 ºC, the deNOx efficiency was >80% while the N2 selectivity reached >96%. In contrast, a decrease of the SCR activity was observed when iron, zirconium or tungsten ox-
ides were added to MnOx-CeO2[84]. However, a strong and irreversible deactivation of niobium-doped catalyst occurred after exposure to SO2. This sharp decrease in low temperature NH3-SCR activity by the addition of an acidic component remains a challenge to obtaining high N2 selectivity from MnOx-CeO2 catalysts. A MnOx(0.6)/ Ce0.5Zr0.5O2 catalyst, synthesized by impregnating Mn(NO3)2 in precipitated CeuZr1–uO2, exhibited high NO conversion at 100–220 ºC at a space velocity of 30,000 h–1, which arose from improved redox properties, active oxygen, acidity and BET surface area by Ce0.5Zr0.5O2 support[88,89]. Li et al.[90] reported that low temperature activity of MnOx-CeO2 can be improved significantly by Sn-modification due to increased Lewis acid sites and the tolerance to SO2 sulfation and H2O can be enhanced by sacrifice of Sn sites[90]. A comparison of the catalytic activities of various cerium compounds is shown in Fig. 5. It is noticeable that the low or high temperature activity of CeO2 catalysts can be adjusted by the addition of various compounds.
Fig. 4 Reaction scheme for low-temperature SCR on MnOx/ CeO2 catalyst[82]
Fig. 5 General comparison of catalytic activities of CeO2 containing NH3-SCR catalysts
5 Cerium on other support Apart from mixed oxide catalysts, ceria on other supports, especially zeolites[91–103] and carbon[104–108] are under extensive study. Kooten et al.[92] reported that asolid state ion-exchanged catalyst is very active at temperatures between 400–600 ºC in presence of 10 vol.% water. The presence of water shifted the maximum NO conversion to higher temperatures, due to a reversible inhibition of active sites by water. In the absence of water in the feed, the reported catalyst showed excellent catalytic activity in the temperature region of 300–500 ºC. The activity of Cen+ exchanged zeolites were greatly determined by the topology of the zeolite. Presence of large cages and a high framework charge density allows the formation of CeOx clusters[94]. Additionally, the decrease of the number of free coordination sites may lead to a low utilization of cerium ions. Therefore, the formation of CeOx
CHEN Lei et al., Rare earth containing catalysts for selective catalytic reduction of NOx with ammonia: A Review
clusters is more difficult in a high silica zeolite such as mordenite or ZSM-5, where cation sites are more remotely separated. The preference of rare earth metal ions for positions in hexagonal prisms and sodalite cages restricts accessibility to reactants[95]. The main challenge of cerium-zeolite catalysts is their thermal stability, which is again greatly dependent on the topology of zeolite. For instance, oligomers are more active in the unselective oxidation of the reductant, which limits the temperature window of selective NO reduction[91]. Accordingly, many research projects to date have examined the effect of additives, such as Ce, La, Sm, Ho, Ga, and Ba on the stability of metal-loaded zeolites[92,97–101]. La and Ce show the strongest stabilizing effect, however, none of these elements enhance stability to a satisfactory level. The pore diameters and Si/Al ratios of zeolites also influence the stability of cerium exchanged zeolites. Ce-beta and Ce-Y are reported to be more stable than Ce-ZSM, while Ce-beta has better catalytic activity[92]. The insertion of cerium in mordenite and ZSM-5 led to a decrease in their degree of crystallinity and BET surface area. Most CeIV ions were embedded in the framework of mordenite, where they performed as extra-framework CeO2 whereas in ZSM-5 CeIV ions where incorporated as cerium silicate, which may induce the decrease of crystallinity and BET surface area of zeolite[102]. Delahay et al.[103] reported that Fe–Ce–ZSM-5 catalysts prepared along a new synthetic route exhibit very high NO conversion in a wide temperature window of 250–550 ºC, even in the presence of H2O and SO2. Similarly, further study reported that a Mn–Ce/ZSM-5 catalyst prepared in an aqueous phase at 423 K exhibits a broad temperature window (517–823 K) for high NO conversions from the NH3-SCR reaction even in the presence of H2O and SO2 [104]. Both the zeolite matrix and the over-exchanged quantities of manganese and cerium contribute to a complex structure with microporous-mesoporous characteristics and specific surface properties. Carbon materials, such as carbon nanotubes[105,106], and active carbon[107,108], have also been used as high surface area, high pore volume supports, which leads to highly dispersed active sites beneficial for the NH3-SCR reaction. Wang et al.[105] reported that CeO2 catalysts supported on HNO3-treated carbon nanotubes (TCNTs) show high NO conversion in the medium temperature range 250–400 °C. The activity of Ce/TCNTs was attributed to the enlarged surface area created by HNO3 treatment, the suitable crystal size of CeO2 and the high dispersion of ceria on the CNTs surface. Similar studies carried out on CeO2/ACF catalysts, in which surface active carbon fibers (ACF) were modified by low-temperature oxygen plasma and nitric acid, both of which did greatly improved low-temperature SCR catalytic activity. Zheng et al.[106] studied the NH3-SCR per-
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formance of Mn–Ce mixed oxide catalysts supported on carbon nanotubes (CNTs), which showed more than 90% NO conversion at 120–180 °C at a high space velocity of 42,000 h−1. Zhan and colleagues[108] also investigated activated carbon honeycomb supported manganese and CeO2 in the temperature region 80−200 °C. The addition of CeO2 improved the distribution of MnOx and enhanced the oxidation of NO to NO2, producing more absorbed NO3– on the catalyst surface for subsequent reduction into N2 by NH3. CeO2 catalysts on other supports, such as Al2TiO5TiO2-SiO2 (ATS)[109], CeO2-ZrO2-Al2O3 [110], and Zr-delaminated-clays[111,112] have also been investigated. Zhu et al.[109] reported TiO2-ZrO2-CeO2/ATS ceramic NH3- SCR catalysts show high catalytic activities in the temperature range 200–420 ºC, and investigated different regeneration methods for deactivated ceramic catalysts used for deNOx in glass manufacturing where alkali, alkali earth metals and sulfides are in high concentration. Results showed that washing with H2SO4 was the most effective regeneration method and the sulfuric acid concentration of the washing solution was an important factor. Shen et al. demonstrated that Zr–Ce–PILC was a potential support for NH3-SCR, reaching 96% NOx conversion at 200 °C when doped with/on Mn[111]. The alkali metal deactivation of Mn-CeOx/Zr-delaminated-clay has also been studied. The method, washing the Na and K poisoned catalysts by water, was found to be efficient for catalyst regeneration.
6 Conclusions and perspectives Here we have reviewed the current status of the application of rare earths, especially CeO2, for SCR of NOx with ammonia. CeO2 catalysts, both pure and combined with transition metal oxides, are reported to present high NH3-SCR performance in a broad temperature region of 100–500 °C. Advantages compared with traditional vanadium and zeolite catalysts include low temperature activity, high hydrothermal stability, and resistance to sulfur and alkaline deactivation. In summary, the roles of CeO2 in SCR catalysts are as follows: (1) Provide oxygen storage and redox properties via the redox shift between Ce4+ and Ce3+; (2) Disperse and interact with other transition metal oxides to generate increased active sites; (3) Promote the NOx adsorption and oxidation to NO2 or nitrites/nitrates, thereby inducing the “fast SCR” reaction; (4) Improve the hydrothermal stability of SCR catalysts via formation of solid solutions or cerium salts on the boundary of different phases. To meet increasingly stringent emission restrictions, greater efforts should be made to promote the practical use of CeO2 catalysts, especially for industrial applica-
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tions. Future investigations will be aimed at fully exploiting the potential of rare earth metals in deNOx catalysts to improve their long term efficiency, N2 selectivity and resistance to deactivation. Specific areas of future research include: (1) Structural and component design of rare earth based deNOx catalyst; (2) The effect of strong interactions between cerium and other components on catalytic performance; (3) The NH3-SCR reaction pathway on CeO2 catalysts; (4) The deactivation mechanism of CeO2 catalyst.
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