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TiO2 SCR catalyst by Eu modification: A mechanism study
Fuel 223 (2018) 385–393
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Full Length Article
Enhancement of the SO2 resistance of Mn/TiO2 SCR catalyst by Eu modification: A mechanism study
T
⁎
Jian Liua,b, Rui-tang Guoa,b,c, , Ming-yuan Lia,b, Peng Suna,b, Shu-ming Liua,b, Wei-guo Pana,b, Shuai-wei Liua,b, Xiao Suna,b a
School of Energy Source and Mechanical Engineering, Shanghai University of Electric Power, Shanghai, PR China Shanghai Engineering Research Center of Power Generation Environment Protection, Shanghai, PR China c Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, PR China b
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: SCR Mn/TiO2 catalyst Eu modification SO2 resistance
Mn/TiO2 catalyst is a promising candidate for future utilization in low-temperature NH3-SCR reaction, but its bad resistance to SO2 is still a great challenge for practical application. In this study, Eu was successfully used as the additive to improve its resistance to SO2 under SCR conditions, while the pretreatment of Mn/TiO2 and MnEu/TiO2 catalyst by SO2 + O2 had a strong deactivation effect on them. In situ DRIFT study clarified that the deactivation of Mn/TiO2-S (SCR + SO2), Mn/TiO2-S (SO2 + O2) and MnEu/TiO2-S (SO2+O2) were mainly originated from the inhibited adsorption of NH3 and NOx species, as well as the formation of a large amount surface sulfate species on them, which had a strong blacking effect on the SCR reactions over the three catalysts via both E-R and L-H routes. After the addition of Eu, SCR reaction over MnEu/TiO2 catalyst with the existence of SO2 took place through L-H pathway, accompanied by the generation of less surface sulfate species, which brought about the excellent SO2 tolerance of MnEu/TiO2 catalyst under SCR conditions.
1. Introduction As a byproduct generated in the combustion process of industrial fuels, NOx has touched off several issues to environment, including acid
⁎
precipitation, photochemical pollution and ozone layer destruction [1]. Till now, selective catalytic reduction of NOx with NH3 as the reductant has been used as the preferred technique for the industrial NOx abatement [2]. For this purpose, V-based catalyst (using W or Mo as the
Corresponding author at: School of Energy Source and Mechanical Engineering, Shanghai University of Electric Power, Shanghai, PR China. E-mail address: [email protected] (R.-t. Guo).
https://doi.org/10.1016/j.fuel.2018.03.062 Received 5 February 2018; Received in revised form 2 March 2018; Accepted 11 March 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
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the 2θ range from 10°to 80° (per 0.02°). The surface elements and their valence states over each catalyst was identified by XPS analysis (Thermal ESCALAB 250 spectrometer with Al Kα X-ray radiation, hν = 1486.6 eV). Refer to the C 1 s level at 284.8 eV, the banding energy shift celebration was performed. The deconvolution of XPS data was based on mixed Gaussian–Lorentzian functions, which was performed by the XPSPeak 4.1 software. To understand their redox properties, the catalysts were subjected to a temperature programmed reduction analysis using hydrogen as the reductant (H2-TPR) on a chemisorption analyzer (Autosorb-iQ-C, Quantachrome, USA). At first, 50 mg sample underwent a 1 h preprocess in N2 atmosphere at 400 °C. When performing the TPR test, the sample was under the heat treatment in the flow of 6% H2/N2 (30 mL/ min) with a temperature rising rate of 10 °C/min from 25 °C to 800 °C. The collection of H2 signals was performed on a thermal conductivity detector (TCD). NOx-TPD study was achieved in a reactor with the fixed-bed type (i.d. = 8 mm). Firstly, the sample was placed under pure Ar pretreatment at 450 °C for 1 h, then it was cooled to 25 °C. After this, the sample was treated in a mixed gas flow (600 ppm NO +5% O2/Ar, 30 mL/min) for 30 min obtain the adsorption equilibrium, then it was purged by Ar for another 1 h. Subsequently, NOx-TPD experiment was performed by warming the sample from room temperature to 500 °C in Ar flow (300 mL/min). The desorption of NOx was continuously monitored using a flue gas analyzer (Model 42i-HL, Thermo). The in situ DRIFT investigations were performed on a FTIR spectrometer (Nicolet iS50) with an MCT detector. In the DRIFT cell, the sample was under the 400 °C pretreatment with pure N2 for 1 h firstly, followed by a cooling process to scheduled temperature. The corresponding background spectra were recorded and subjected to an automatic subtraction process from the sample spectrum. The experimental atmosphere for DRIFT study was as follows: 500 ppm NH3/N2, or/and 500 ppm +5% O2/N2, or/and 100 ppm SO2/N2, with a flowrate of 300 mL/min.
assistant) was adopted as the dominant catalyst for several decades [3]. Owing to its special narrow temperature window (300–400 °C), SCR reactor using V-based catalysts should be located upstream of dust removal and desulfurization units [4], as a result, an inevitable poisoning process of SO2 and some fly ash components (such as Na, K) on this catalyst would happen [5]. Furthermore, the toxic effect of VOx to ecological environment is also detrimental to the utilization of V-based catalyst in future [6]. In response, the development of novel non-vanadium SCR catalyst may offer an effective solution to the problems mentioned above. It is well recognized that manganese oxides contain multivalent Mn species and labile lattice oxygens, which are commonly used as the catalyst in redox process including NH3-SCR reaction [7]. As reported in recent studies [8,9], MnOx supported on different carriers exhibited excellent performance for NO removal at low-temperature. However, the presence of a little amount of SO2 after the wet flue gas desulfurization (WFGD) devices is still an inducement for the failure of SCR catalyst using MnOx as the single active component [10–12]. Therefore, much effort is still needed to be devoted to enhancing its SO2 resistance. Recently, Ce and Pr had successively used as the additives for promoting the SO2 tolerance of Mn/TiO2 catalyst [13,14]. Our recent study also reported the similar effect of Eu [15]. However, the enhancement mechanism of Eu on the SO2 tolerance of Mn-based SCR catalyst is still uncertain. In present work, Eu addition showed an effective improvement effect on Mn/TiO2 catalyst to SO2 poisoning under SCR conditions. The role of Eu species for this purpose would be investigated and discussed based on the physico-chemical properties of the fresh and sulfated catalyst samples. 2. Experimental 2.1. Preparation of catalyst samples The fresh catalyst samples used in this work (denoted as Mn/TiO2 and MnEu/TiO2 respectively) were synthesized by sol-gel method, as described in our previous study [16]. The main elements in the catalysts including Mn, Eu and Ti were from manganese nitrate, europium nitrate and butyl titanate respectively, which were all supplied by Aladdin Reagent Inc., China with an analytical grade. When preparing Mn/TiO2 catalyst sample, 0.1 mol butyl titanate, 0.9 mol anhydrous ethanol, 0.9 mol water, 0.1 mol nitric acid and 0.016 mol manganese nitrate were mixed and fully stirred at 25 °C to obtain a yellowish sol. Then a 24 h drying treatment at 90 °C converted the sol into xerogel. After that, the xerogel was put into a 5 h air atmosphere calcination at 500 °C in a muffle furnace. Similarly, MnEu/TiO2 catalyst sample was prepared, with the Mn:Eu:Ti molar ratio of 0.12:0.04:1. Next then, catalyst samples were sulfated by exposing the fresh Mn/ TiO2 and MnEu/TiO2 catalysts in a mixed gas containing NO (600 ppm) +NH3 (600 ppm)+O2 (5%)+SO2 (100 ppm) and N2 balance (sulfated under SCR reactions) or SO2 (100 ppm)+O2 (5%) and N2 balance (sulfated by SO2 + O2 only) at 150 °C for 1 h. And the corresponding sulfated catalyst samples were denoted as Mn/TiO2-S (SCR + SO2), MnEu/TiO2-S (SCR + SO2), Mn/TiO2-S (SO2+O2) and MnEu/TiO2-S (SO2 + O2) respectively.
2.3. Catalytic performance evaluation Catalytic performances of the samples were investigated in a fixedbed quartz reactor with an 8 mm inner diameter. The feeding gas was composed of 600 ppm NH3, 600 ppm NO, 5% O2, 5% H2O, 100 ppm SO2 (when used), and Ar as balance. The reactions were executed with a 1L/ min gas flowrate and a GHSV of 108,000 h−1. A Nicolet iS50 FTIR spectrometer attached with a gas cell was used for effluent gas concentration analysis. Under the steady-state reaction conditions, the SCR performance parameters could be obtained by:
NOx conversion =
[NOx ]in −[NOx ]out × 100% [NOx ]in
(1)
2[N2 O]out ⎞ × 100% N2 selectivity = ⎛1− ⎝ [NOx ]in + [NH3]in −[NOx ]out −[NH3]out ⎠ ⎜
⎟
(2) In addition, NO oxidation activity over each catalyst sample was also evaluated under the similar experimental conditions. However, NH3 was excluded when performing NO oxidation test.
2.2. Characterization 3. Results and discussion The surface structure parameters of the catalysts were determined by nitrogen adsorption at −196 °C by means of a Quantachrome Autosorb-iQ-AG instrument. The Brunauer–Emmett–Teller (BET) model and the Barrett–Joyner–Halenda (BJH) model were used for the evaluation of specific surface area and the distribution of pore size of each sample. The crystal forms for each catalyst were determined based on X-ray diffraction method (Bruker D8 Advance, Germany, CuKα radiation with a wavelength of 0.154056 nm). The data collection was performed in
3.1. Catalytic performance The catalytic activities of the catalysts are shown in Fig. 1(A). A relatively high SCR performance was observed on Mn/TiO2 catalyst, over which about 80% NOx conversion was seen in 240–400 °C. After the addition of Eu, a distinct activity increase and a broadened temperature window for MnEu/TiO2 catalyst were presented, as also reported in our previous study [15]. The T90 window of MnEu/TiO2 386
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Fig. 2. SO2 tolerances of Mn/TiO2 and MnEu/TiO2 in NH3-SCR reaction.
Table 1 Textural properties of the fresh and sulfated catalysts. Samples
BET surface area (m2/g)
Total pore volume (cm3/g)
Average pore diameter (nm)
Mn/TiO2 MnEu/TiO2 Mn/TiO2-S (SCR + SO2) MnEu/TiO2-S (SCR + SO2) Mn/TiO2-S (SO2+O2) MnEu/TiO2-S (SO2+O2)
137 157 86.7 149
0.44 0.43 0.15 0.35
7.8 6.6 6.6 5.6
91.4 97.1
0.22 0.25
6.4 6.8
3.3. Physical and crystal properties (BET and XRD) Fig. 1. (A) SCR activities (B) N2 selectivities of the fresh and sulfated catalysts.
As listed in Table 1, the BET surface area of MnEu/TiO2 was 157 m2/g, an apparent increase was present compared with the same parameter for Mn/TiO2 catalyst as the result of Eu introduction. This phenomenon might be in virtue of the intensive interaction among the components of MnEu/TiO2 catalyst. On the contrary, the distinctly decreased BET surface areas were associated with the four sulfated catalyst samples. Noticeably, the MnEu/TiO2 catalyst sulfated under SCR reactions still exhibited a large specific surface area. Moreover, the total pore volumes of the four sulfated catalyst samples decreased a lot owing to the block by sulfate species during the sulfation process. XRD technique was used to identify the crystal structures of the fresh and sulfated catalysts, and the obtained results are presented in Fig. S1. In the XRD spectrum of each catalyst sample, only diffraction peaks belonging to anatase and rutile TiO2 were detected. These features revealed that Mn and Eu species were well dispersed on catalyst surface, which might exist as an amorphous or highly dispersed phase. It seemed that the Eu species in MnEu/TiO2 catalyst inhibited TiO2 crystal growth and decreased its crystallinity, as indicated by the decreased peak intensities in its spectrum. For the four sulfated catalyst samples, much stronger peak intensities of anatase and rutile TiO2 were present in their spectra, suggesting the formation of larger TiO2 particles during the sulfation process. However, the crystallinities of the sulfated MnEu/TiO2 catalyst samples were still lower than the corresponding sulfated Mn/TiO2 catalyst samples.
catalyst ranged from 200 to 400 °C, which was much wider than that of Mn/TiO2 catalyst (about 280–334 °C). Moreover, the presence of Eu also improved the N2 selectivity of Mn/TiO2 catalyst, as presented in Fig. 1(B). After the sulfation process under SCR conditions, the SCR activity of Mn/TiO2 decreased sharply, but MnEu/TiO2-S (SCR + SO2) catalyst still exhibited relatively high SCR activity, meaning that MnEu/ TiO2 catalyst possessed good SO2 resistance under SCR conditions. Unlike the catalyst samples sulfated under SCR conditions, the pretreatment with SO2 + O2 led to the strong deactivation of the two fresh catalyst samples, as reflected by their poor SCR performances presented in Fig. 1(A). Moreover, the decreased N2 selectivities were also found over Mn/TiO2-S (SO2 + O2) and MnEu/TiO2-S (SO2 + O2), as shown in Fig. 1(B).
3.2. SO2 tolerance The poisoning effect of SO2 on SCR catalyst is a well-recognized fact owing to the deposition of sulfate [17]. The SCR performances of Mn/ TiO2 and MnEu/TiO2 in the presence of SO2 at 150°Cwere tested and the results are illustrated in Fig. 2. From Fig. 2, the NOx conversion over MnEu/TiO2 catalyst decreased from 85.0% to about 70.1% after a 25 h SCR reaction process in the existence of 100 ppm SO2, while the NOx conversion over Mn/TiO2 catalyst decreased from 59.3% to 20.1% under the same experimental conditions. Therefore, the SO2 tolerance of MnEu/TiO2 catalyst was superior to that of Mn/TiO2 catalyst. A mechanistic investigation would be performed based on various characterization techniques.
3.4. Surface elements analysis (XPS) The valence state states of active species on the catalysts surface had an important impact on SCR reaction, which was identified by XPS analysis. The obtained results are depicted in Fig. 3 and Table 2. The XPS spectra of Mn 2p for the catalysts are illustrated in 387
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Fig. 3(A). The two main peaks at 642.0 and 653.8 eV represent Mn 2p3/ 2 and Mn 2p1/2 respectively. After a further peak-fitting deconvolution procedure, the Mn 2p XPS spectra could be attributed to three types of Mn species: Mn2+, Mn3+ and Mn4+ [18–20]. From the surface element concentrations listed in Table 2, Mn4+ concentration of Mn/TiO2 catalyst increased from 0.65 at.% to 0.93 at.% after the modification with Eu. Thus the presence of Eu promoted the formation of Mn4+ species. Evidently, the sulfation process would bring about the decrease of Mn4+ concentrations, as reflected by the corresponding values for the four sulfated samples. Previous studies had indicated that the high redox ability of Mn4+ administered to NO oxidation to NO2, which helped to open the “fast SCR” route for NOx removal due to the participation of NO2 [21–23]. Thus the beneficiation of Mn4+ species over MnEu/TiO2 catalyst should be an important incentive for its excellent SCR performance in 100–300 °C and high resistance to SO2. After the sulfurization process, the decreased Mn4+ concentrations over all the sulfated samples, which was unfavorable to the low-temperature SCR reactions over them, as illustrated in Fig. 1(A). Fig. 3(B) presents the O 1 s XPS spectra of the catalysts. The two subpeaks represented two kinds of oxygen species: lattice oxygen (B. E.≈529.5 eV, denoted as Oα) and surface adsorbed oxygen (B. E. ≈531.9 eV, denoted as Oβ), mainly existed in the forms of O22 − and O− in OH-like groups [7,24,25]. From Fig. 3(B), an obvious increase of Oβ peak was observed after the addition of Eu on Mn/TiO2 catalyst, as also reflected by the results given in Table 2, the Oβ concentration of MnEu/ TiO2 catalyst was 17.80 at.%, which was much greater than the corresponding value of Mn/TiO2 catalyst (16.18 at.%). In virtue of its high mobility, surface adsorbed oxygen played a critical role in the redox process in SCR reaction [26], which had a positive effect on “fast SCR” catalytic mechanism [27]. Furthermore, OH-like group could combine with NH3 to form NH+4 , which could further react with NO2 to produce N2 and H2O [28,29]. Moreover, a distinct drop of Oβ concentration was detected on the sulfated samples except MnEu/TiO2-S (SCR + SO2) catalyst, in accordance with their poor activities in 100–250 °C. Fig. 3(C) displays the S 2p XPS spectra of the four sulfated samples. From Fig. 3(C), two peaks at about 169.5 and 168.4 eV were present, which were assigned to HSO−4 and SO24− respectively [30,31]. It should be noticed that MnEu/TiO2-S (SCR + SO2) catalyst exhibited the lowest band intensity among the four sulfated catalyst samples, suggesting that the sulfation process under SCR conditions only generated traces of sulfate species on it. 3.5. H2-TPR The reducibility of each sample was evaluated by H2-TPR technique, as illustrated in Fig. 4. Three reduction peaks were found in each pattern. The first peak in Mn/TiO2 reduction pattern appeared at about 395 °C represented the reduction of MnO2/Mn2O3 to Mn3O4 [32,33], the second peak situated at about 574 °C might be resulted from the reduction of Mn3O4 to MnO [32], and the third one at about 649 °C indicated the reduction process of oxygen groups [34]. Therefore, H2TPR technique also revealed the presence of Mn2+, Mn3+ and Mn4+ species, as also obtained by XPS analysis. A peak shift to higher temperature was detected in the patterns of the four sulfated samples,
Fig. 3. XPS spectra of the fresh and sulfated catalysts.
Table 2 The concentrations of the atoms obtained by XPS analysis. Samples
Mn (at.%)
O (at.%)
Eu (at.%)
S (at.%)
Mn4+/Mn (%)
Oβ/O (%)
Mn4+ (at.%)
Oβ (at.%)
Mn/TiO2 MnEu/TiO2 Mn/TiO2-S (SCR + SO2) MnEu/TiO2-S (SCR + SO2) Mn/TiO2-S (SO2 + O2) MnEu/TiO2-S (SO2 + O2)
6.52 5.57 6.21 5.45 6.27 5.99
68.59 67.79 64.12 65.54 66.02 65.76
/ 1.36 / 0.98 / 1.18
/ / 4.92 2.50 3.86 3.29
10.02 16.74 8.61 12.35 8.83 11.46
23.59 26.26 19.46 24.92 21.03 21.94
0.65 0.93 0.53 0.67 0.55 0.68
16.18 17.80 12.48 16.33 13.88 14.43
388
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Fig. 4. H2-TPR patterns of the fresh and sulfated catalysts.
meaning the dropped reducibilities of them. Moreover, a new reduction peak at about 600 °C appeared in the profile of each sulfated catalyst sample, which could be attributed to the reduction of sulfate [35,36]. Noticeably, the sulfate reduction peaks in the profiles of MnEu/TiO2-S (SCR + SO2) and MnEu/TiO2-S (SO2 + O2) were weaker than that in the patterns of the other two sulfated samples, revealing the disposition of less sulfate species on them, which further confirming the results of XPS study. Although the sulfation process generate reducible sulfate species on the sulfated samples, the H2 consumption (HC) values of them are still low than that of the two fresh samples, as presented in Fig. 4. 3.6. Nox adsorption The NOx-TPD patterns for the catalysts are presented in Fig. S2. For Mn/TiO2 catalyst, only a NO desorption peak appeared at about 175 °C in its profile, which could be ascribed to the decomposition of chemicaladsorbed NOx [37]. For MnEu/TiO2 catalyst, two peaks were observed at about 143 and 348 °C, which could be assigned to weakly bound and strongly bound NO−x species respectively [37–39]. Besides that, more NO2 was desorbed from MnEu/TiO2 catalyst than that from Mn/TiO2 catalyst, proving the promoted NO oxidation after the modification with Eu. For the four sulfated catalyst samples, the situations were quite different. It seemed that the sulfation process under SCR conditions only had a weak inhibition effect on NO adsorption and the formation of NO2 over MnEu/TiO2 catalyst; therefore, there were still quite a number of adsorbed NO and NO2 species present over MnEu/TiO2-S (SCR + SO2) catalyst. For the other three sulfated catalyst samples, the adsorption of NO and the formation of NO2 over them were severely suppressed by SO2 pretreatment.
Fig. 5. NH3-adsorption DRIFT spectra for (A) Mn/TiO2 and (B) MnEu/TiO2 in 100–350 °C.
3.8. In situ DRIFT spectra 3.8.1. Adsorption of NH3 The recorded NH3-adsorption DRIFT spectra for Mn/TiO2 and MnEu/TiO2 catalysts are displayed in Fig. 5. Four NH3-adsorption bands could be detected at 1694, 1600, 1388 and 1159 cm−1 respectively, these bands included NH+4 species linked to Brønsted acid sites (1694 and 1388 cm−1) and NH3 species adsorbed on Lewis acid sites (1600 and 1159 cm−1) [41–45]. For the DRIFT spectra of MnEu/TiO2 catalyst, two different bands (1668 and 1442 cm−1) all originated from NH+4 species connected with Brønsted acid sites [41]. The enhanced band intensities in Fig. 5(B) revealed that more NH3 species were adsorbed on MnEu/TiO2 catalyst surface. Due to the desorption of ad-NH3 species at high temperature, all the band intensities decreased gradually with temperature. For the four sulfated catalyst samples, the adsorption of NH3 species were inhibited in various degrees, as presented in Fig. S4. It seemed that the sulfation process under SCR conditions only had a weak suppression effect on the adsorption of NH3 species over MnEu/ TiO2 catalyst; the band intensities in its DRIFT spectra were much higher than that in the DRIFT spectra of the other three sulfated samples.
3.7. NO oxidation As mentioned above, NO oxidation is an essential factor for the occurrence of “fast SCR” reaction [40]. Herein, NO oxidation performances over the catalysts were also investigated, as shown in Fig. S3. From Fig. S3, the favored NO oxidation over MnEu/TiO2 catalyst was evident. MnEu/TiO2-S (SCR + SO2) catalyst exhibited similar NO oxidation activity with Mn/TiO2 catalyst. On the contrary, the NO oxidation activities on the other three sulfated catalyst samples were distinctly suppressed, especially for that of the two sulfated Mn/TiO2 catalyst samples. Consequently, serious inhibition effect on the low-temperature SCR processes over them happened, as proven by the SCR activities shown in Fig. 1(A).
3.8.2. Co-adsorption of NO+O2 Fig. 6 exhibits the NOx-adsorption DRIFT spectra over Mn/TiO2 and 389
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Fig. 7. Transient reaction of NOx with preadsorbed NH3 over (A) Mn/TiO2 and (B) MnEu/TiO2 at 150 °C. Fig. 6. NOx-adsorption DRIFT spectra for (A) Mn/TiO2 and (B) MnEu/TiO2 in 100–350 °C.
similar trend for the reaction between NO + O2 and the preadsorbed NH3 species reappeared in the spectra of MnEu/TiO2 catalyst. Differently, it seemed that the adsorbed NH3 species over MnEu/TiO2 catalyst were of higher reactivity compared with that over Mn/TiO2 catalyst. For example, the band at 1600 cm−1 in Fig. 7(B) nearly vanished after the introduction of NO + O2 for 2 min; however, the corresponding band in Fig. 7(A) was still present under the same conditions, although the two bands were of similar intensities. Similar situation also happened on the band at 1159 cm−1. Thus both the adsorption and activation of NH3 species over MnEu/TiO2 catalyst were strengthened owing to the existence of Eu. As mentioned above, adsorbed NH3 species and gaseous NOx species were the major characters in the transient SCR reaction, verifying the applicability of Eley−Rideal (E-R) mechanism for it [28]. For the sulfated catalyst samples (Fig. S6), all the ad-NH3 species were active in the transient SCR reaction with a much lower reactivity, indicating the blockage effect of sulfation process on the NH3-SCR reactions over them through E-R pathway.
MnEu/TiO2 catalysts. Several characteristic NOx adsorption bands appeared in Fig. 6, which represented adsorbed NO2 (1606 cm−1), bidentate nitrate (1560 and 1552 cm−1), monodentate nitrate (1266 cm−1) and bridged nitrate (1247 cm−1) respectively [41,44–47]. It seemed that the band at 1606 cm−1 in Fig. 6(B) was more intensive than that in Fig. 6(A), further confirming the generation of more NO2 on MnEu/TiO2 catalyst, as discussed in Section 3.7. From Fig. S5, it seemed that the sulfation process under SCR conditions had little suppression effect on NOx adsorption over MnEu/TiO2 sample, as also concluded from the results of NOx-TPD analysis. However, the suppressed NOx adsorption over the other three sulfated catalyst samples was noticeable in their DRIFT spectra.
3.8.3. Transient reaction of NOx with preadsorbed NH3 To identify the function of adsorbed NH3 species in the SCR process on Mn/TiO2 and MnEu/TiO2 catalysts, the fresh catalyst samples were pretreated with NH3 for 0.5 h, followed by the exposure to 500 ppm NO +5% O2/N2, and the time-dependent DRIFT spectra were recorded and given in Fig. 7. From the DRIFT spectra of Mn/TiO2 (Fig. 7 (A)), several bands (1694, 1600 and 1159 cm−1) of adsorbed NH3 species appeared after the NH3 pretreatment. The introduction of NO + O2 led to the quick consumption of adsorbed NH3 species, their bands became weaker rapidly and completely disappeared in 5 min. From Fig. 7 (B), a
3.8.4. Transient reaction of NH3 with preadsorbed NOx The transient reaction of NH3 with preadsorbed NOx was also investigated by feeding the reactants into the IR cell in an opposite order, and the recorded DRIFT spectra for this process over Mn/TiO2 and MnEu/TiO2 catalysts are demonstrated in Fig. 8. From Fig. 8(A), NO + O2 pretreatment generated several adsorbed NOx species on Mn/ TiO2 catalyst, as reflected by the bands at 1602, 1553, 1487 and 390
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O2 (g) → 2O(ad)
(4)
NH3 (a) + O(ad) → NH2 (ad) + OH(ad)
(5)
NO(g) + NH2 (ad) → NH2 NO(ad)
(6)
NH2 NO(ad) → N2 (g) + H2 O
(7)
2) L-H mechanism:
O2 (g) → 2O−∗ (surface active sites)
(8)
NO(g) + O−∗ → NO2 (ad)
(9)
Mn4 +
NH3 (g) ⎯⎯⎯⎯⎯→ NH3 (ad)(over Lewis acid sites)
(10)
2NH3 (ad) + NO2 (ad) + NO(g) → 2N2 + 3H2 O
(11)
Combined with the characterization results, the modification of Mn/ TiO2 catalyst by Eu not only facilitated the oxidation of NO to NO2, but also facilitated the adsorption of NH3 species, which had an acceleration impact on the SCR reaction over MnEu/TiO2 catalyst via all the two routes mentioned above. As a contrast, the SCR reactions over Mn/ TiO2-S (SCR + SO2), Mn/TiO2-S (SO2 + O2) and MnEu/TiO2-S (SO2 + O2) through both E-R and L-H routes were seriously inhibited. It was noteworthy that the NH3-SCR reaction over MnEu/TiO2-S (SCR + SO2) mainly obeyed the L-H mechanism. 3.9. DRIFT study on the SO2 tolerance of the two fresh catalyst samples 3.9.1. Effect of SO2 on NH3 adsorption In this section, the two fresh catalyst samples were first exposed to NH3, next then SO2 was also added in the gas stream. The DRIFT spectra of this process were recorded (Fig. S8). From Fig. S8(A), two ad-NH3 bands appeared at 1600 and 1159 cm−1 and grew with time after the treatment with NH3. When SO2 was added in the gas, the two bands of adsorbed NH3 species still grew with time, meanwhile, two new bands of sulfate species (1283 and 1198 cm−1) [13,47] appeared and became stronger with time. Therefore, the presence of SO2 would promote the adsorption of NH3 species over Mn/TiO2 catalyst when no NOx was present in the gas atmosphere, which might be owing to the acidic nature of SO2. For the spectra of MnEu/TiO2 catalyst (Fig. S8(B)), four bands of ad-NH3 species were present. Similar promotion effect of SO2 on the adsorption of NH3 species over it could also be observed. It should be noticed that, there were no bands of adsorbed sulfate species present in the spectra of MnEu/TiO2 catalyst (Fig. S8(B)). Thus Eu decoration could prevent the formation of surface sulfate on Mn/TiO2 catalyst when no NOx species was present.
Fig. 8. Transient reaction of NH3 with preadsorbed NOx over (A) Mn/TiO2 and (B) MnEu/TiO2 at 150 °C.
1266 cm−1 respectively. All these bands quickly vanished after the introduction of NH3 for 2 min. Meanwhile, the formation and growth of ad-NH3 species bands were found, revealing the replacement of ad-NOx species by ad-NH3 species. Similar behavior of the adsorbed reactant species also appeared in Fig. 8(B). Therefore, all the ad-NOx species were participators in the transient SCR process over Mn/TiO2 and MnEu/TiO2 catalysts with similar reactivities, demonstrating the existence of Langmuir−Hinshelwood (L-H) mechanism [28]. For the four sulfated catalyst samples (Fig. S7), the situation was different. All the adsorbed NOx species over MnEu/TiO2-S (SCR + SO2) catalyst were quickly depleted in 2 min after the introduction of NH3, which was very similar with the spectra of the fresh MnEu/TiO2 catalyst. Therefore, the sulfation process under SCR conditions had rare suppression effect on the NH3-SCR reaction over MnEu/TiO2 catalyst though L-H pathway. On the contrary, the preadsorbed NOx species over the other three sulfated catalyst samples were still present after the importation of NH3 for 5 min, suggesting the NH3-SCR reactions over them through L-H route were blocked to a large extent. As discussed above, both E-R and L-H mechanisms played a part in the SCR processes over Mn/TiO2 and MnEu/TiO2 catalysts, as described by [21,48]:
3.9.2. Effect of SO2 on NOx adsorption Next then, the impact of SO2 on NOx adsorption over Mn/TiO2 and MnEu/TiO2 catalysts were investigated by DRIFT technique (Fig. S9). The treatment of Mn/TiO2 catalyst by NOx generated three bands of adNOx species at 1606, 1560 and 1266 cm−1 in its DRIFT spectra (Fig. S9(A)). It could be seen that the last band quickly vanished after SO2 feeding. And the intensity decrease could be observed for the other two bands. Therefore, SO2 could inhibit the adsorption of NOx species over Mn/TiO2 catalyst. In addition, several bands of sulfate species (1368, 1270 and 1176 cm−1) came into being in Fig. S9(A). For the spectra of MnEu/TiO2 catalyst (Fig. S9(B)), the trend was quite similar. But there were more adsorbed NOx species on MnEu/TiO2 compared with that on Mn/TiO2 catalyst, indicating that Eu modification could partially offset the inhibition effect of SO2 on NOx adsorption over Mn/TiO2 catalyst. Moreover, the formation of less sulfate species over MnEu/TiO2 catalyst could also be detected from the lower band intensities in its DRIFT spectra.
1) E-R mechanism: Mn4 +
NH3 (g) ⎯⎯⎯⎯⎯→ NH3 (ad)(over Lewis acid sites)
(3) 391
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which included adsorbed NO2 (1605 cm−1), NH+4 species (1454 cm−1), monodentate nitrate (1297 and 1123 cm−1) and NH3 species (1190 cm−1) [41–46,50,51]. With the feeding of SO2, three bands (1297, 1190 and 1123 cm−1) vanished, while the other two bands at 1605 and 1454 cm−1 were still present, accompanied with the decrease of band intensities with time. Under these conditions, the SCR process on MnEu/TiO2 could take place though L-H pathway. Moreover, several new bands could be observed after the introduction of SO2, which could be attributed to surface sulfate species with a single S]O band (1344 cm−1), bidentate sulfate (1278 cm−1) and surface or bulk-like sulfates (1127 cm−1) respectively [13,47]. In addition, the relatively low intensities of the bands owing to sulfate species on MnEu/TiO2 catalyst also suggested the formation of less sulfate species over it. 4. Conclusions The present study found that Eu modification improved the SO2 tolerance of Mn/TiO2 catalyst under SCR conditions, while the sulfation pretreatment of Mn/TiO2 and MnEu/TiO2 by SO2+O2 would strongly deactivate the two catalyst samples. Form the experimental and characterization results, the following conclusions could be arrived: (1) Eu modification suppressed the formation of sulfate species over Mn/TiO2 catalyst under SCR conditions. (2) The SCR process over MnEu/TiO2 catalyst in the presence of SO2 took place through L-H pathway. (3) The inhibited adsorption of reactants on Mn/TiO2-S (SCR + SO2), Mn/TiO2-S (SO2 + O2) and MnEu/TiO2-S (SO2 + O2) catalysts had a strong blockage effect on the SCR reactions over them via both ER and L-H routes. Acknowledgment This work was supported by the Natural Science Foundation of China (21546014) and the Natural Science Foundation of Shanghai, China (14ZR1417800). Appendix A. Supplementary data
Fig. 9. SO2-adsorption DRIFT spectra for (A) Mn/TiO2 and (B) MnEu/TiO2 catalysts under the SCR reactions (500 ppm NH3+500 ppm NO + 5% O2, balance N2, 100 ppm SO2 (when used)) at 150 °C.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2018.03.062.
3.9.3. SO2 poisoning experiments under NH3-SCR conditions Firstly, Mn/TiO2 and MnEu/TiO2 catalysts were pretreated by a mixture of 500 ppm NO, 500 ppm NH3, 5% O2 and balance N2 for 20 min. From Fig. 9(A), four bands appeared during the initial 20 min introduction of reactant gases over it. Based on previous literatures [28,41,42], these bands could be ascribed to adsorbed NO2 (1606 cm−1), monodentate nitrate (1517 and 1271 cm−1) and NH3 species over Lewis acid sites (1175 cm−1). Next then, Mn/TiO2 catalyst was purged by N2 at 150 °C, followed by the introduction of 100 ppm SO2 for 30 min. In this process, all the bands of adsorbed NH3 species and adsorbed NOx species quickly disappeared, however, several new bands (1618, 1431, 1292, 1214 and 1130 cm−1) came out and increased with time, along with the formation of ionic NH+4 over Brønsted acid sites (1431 cm−1) [42] and sulfate species (1618, 1292, 1214 and 1130 cm−1) [13,47]. In the sulfation process, some Lewis acid sites would be consumed due to the reaction between SO2 and metal ions, meanwhile, Brønsted acid sites with an S-OH form generated [49]. As a result, the band at 1431 cm−1 appeared. Therefore, the adsorption of reactants on Mn/TiO2 catalyst was basically cut off. The surface of Mn/ TiO2 catalyst was nearly under the bestrow of sulfate species. Under these circumstances, serious deactivation by SO2 happened, as shown in Fig. 2. For MnEu/TiO2 catalyst (Fig. 9(B)), the situation was quite different. Several bands formed at 1605, 1454, 1297, 1190 and 1123 cm−1 after the treatment of MnEu/TiO2 catalyst with the reactant gases,
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Full Length Article
Enhancement of the SO2 resistance of Mn/TiO2 SCR catalyst by Eu modification: A mechanism study
T
⁎
Jian Liua,b, Rui-tang Guoa,b,c, , Ming-yuan Lia,b, Peng Suna,b, Shu-ming Liua,b, Wei-guo Pana,b, Shuai-wei Liua,b, Xiao Suna,b a
School of Energy Source and Mechanical Engineering, Shanghai University of Electric Power, Shanghai, PR China Shanghai Engineering Research Center of Power Generation Environment Protection, Shanghai, PR China c Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, PR China b
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: SCR Mn/TiO2 catalyst Eu modification SO2 resistance
Mn/TiO2 catalyst is a promising candidate for future utilization in low-temperature NH3-SCR reaction, but its bad resistance to SO2 is still a great challenge for practical application. In this study, Eu was successfully used as the additive to improve its resistance to SO2 under SCR conditions, while the pretreatment of Mn/TiO2 and MnEu/TiO2 catalyst by SO2 + O2 had a strong deactivation effect on them. In situ DRIFT study clarified that the deactivation of Mn/TiO2-S (SCR + SO2), Mn/TiO2-S (SO2 + O2) and MnEu/TiO2-S (SO2+O2) were mainly originated from the inhibited adsorption of NH3 and NOx species, as well as the formation of a large amount surface sulfate species on them, which had a strong blacking effect on the SCR reactions over the three catalysts via both E-R and L-H routes. After the addition of Eu, SCR reaction over MnEu/TiO2 catalyst with the existence of SO2 took place through L-H pathway, accompanied by the generation of less surface sulfate species, which brought about the excellent SO2 tolerance of MnEu/TiO2 catalyst under SCR conditions.
1. Introduction As a byproduct generated in the combustion process of industrial fuels, NOx has touched off several issues to environment, including acid
⁎
precipitation, photochemical pollution and ozone layer destruction [1]. Till now, selective catalytic reduction of NOx with NH3 as the reductant has been used as the preferred technique for the industrial NOx abatement [2]. For this purpose, V-based catalyst (using W or Mo as the
Corresponding author at: School of Energy Source and Mechanical Engineering, Shanghai University of Electric Power, Shanghai, PR China. E-mail address: [email protected] (R.-t. Guo).
https://doi.org/10.1016/j.fuel.2018.03.062 Received 5 February 2018; Received in revised form 2 March 2018; Accepted 11 March 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
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the 2θ range from 10°to 80° (per 0.02°). The surface elements and their valence states over each catalyst was identified by XPS analysis (Thermal ESCALAB 250 spectrometer with Al Kα X-ray radiation, hν = 1486.6 eV). Refer to the C 1 s level at 284.8 eV, the banding energy shift celebration was performed. The deconvolution of XPS data was based on mixed Gaussian–Lorentzian functions, which was performed by the XPSPeak 4.1 software. To understand their redox properties, the catalysts were subjected to a temperature programmed reduction analysis using hydrogen as the reductant (H2-TPR) on a chemisorption analyzer (Autosorb-iQ-C, Quantachrome, USA). At first, 50 mg sample underwent a 1 h preprocess in N2 atmosphere at 400 °C. When performing the TPR test, the sample was under the heat treatment in the flow of 6% H2/N2 (30 mL/ min) with a temperature rising rate of 10 °C/min from 25 °C to 800 °C. The collection of H2 signals was performed on a thermal conductivity detector (TCD). NOx-TPD study was achieved in a reactor with the fixed-bed type (i.d. = 8 mm). Firstly, the sample was placed under pure Ar pretreatment at 450 °C for 1 h, then it was cooled to 25 °C. After this, the sample was treated in a mixed gas flow (600 ppm NO +5% O2/Ar, 30 mL/min) for 30 min obtain the adsorption equilibrium, then it was purged by Ar for another 1 h. Subsequently, NOx-TPD experiment was performed by warming the sample from room temperature to 500 °C in Ar flow (300 mL/min). The desorption of NOx was continuously monitored using a flue gas analyzer (Model 42i-HL, Thermo). The in situ DRIFT investigations were performed on a FTIR spectrometer (Nicolet iS50) with an MCT detector. In the DRIFT cell, the sample was under the 400 °C pretreatment with pure N2 for 1 h firstly, followed by a cooling process to scheduled temperature. The corresponding background spectra were recorded and subjected to an automatic subtraction process from the sample spectrum. The experimental atmosphere for DRIFT study was as follows: 500 ppm NH3/N2, or/and 500 ppm +5% O2/N2, or/and 100 ppm SO2/N2, with a flowrate of 300 mL/min.
assistant) was adopted as the dominant catalyst for several decades [3]. Owing to its special narrow temperature window (300–400 °C), SCR reactor using V-based catalysts should be located upstream of dust removal and desulfurization units [4], as a result, an inevitable poisoning process of SO2 and some fly ash components (such as Na, K) on this catalyst would happen [5]. Furthermore, the toxic effect of VOx to ecological environment is also detrimental to the utilization of V-based catalyst in future [6]. In response, the development of novel non-vanadium SCR catalyst may offer an effective solution to the problems mentioned above. It is well recognized that manganese oxides contain multivalent Mn species and labile lattice oxygens, which are commonly used as the catalyst in redox process including NH3-SCR reaction [7]. As reported in recent studies [8,9], MnOx supported on different carriers exhibited excellent performance for NO removal at low-temperature. However, the presence of a little amount of SO2 after the wet flue gas desulfurization (WFGD) devices is still an inducement for the failure of SCR catalyst using MnOx as the single active component [10–12]. Therefore, much effort is still needed to be devoted to enhancing its SO2 resistance. Recently, Ce and Pr had successively used as the additives for promoting the SO2 tolerance of Mn/TiO2 catalyst [13,14]. Our recent study also reported the similar effect of Eu [15]. However, the enhancement mechanism of Eu on the SO2 tolerance of Mn-based SCR catalyst is still uncertain. In present work, Eu addition showed an effective improvement effect on Mn/TiO2 catalyst to SO2 poisoning under SCR conditions. The role of Eu species for this purpose would be investigated and discussed based on the physico-chemical properties of the fresh and sulfated catalyst samples. 2. Experimental 2.1. Preparation of catalyst samples The fresh catalyst samples used in this work (denoted as Mn/TiO2 and MnEu/TiO2 respectively) were synthesized by sol-gel method, as described in our previous study [16]. The main elements in the catalysts including Mn, Eu and Ti were from manganese nitrate, europium nitrate and butyl titanate respectively, which were all supplied by Aladdin Reagent Inc., China with an analytical grade. When preparing Mn/TiO2 catalyst sample, 0.1 mol butyl titanate, 0.9 mol anhydrous ethanol, 0.9 mol water, 0.1 mol nitric acid and 0.016 mol manganese nitrate were mixed and fully stirred at 25 °C to obtain a yellowish sol. Then a 24 h drying treatment at 90 °C converted the sol into xerogel. After that, the xerogel was put into a 5 h air atmosphere calcination at 500 °C in a muffle furnace. Similarly, MnEu/TiO2 catalyst sample was prepared, with the Mn:Eu:Ti molar ratio of 0.12:0.04:1. Next then, catalyst samples were sulfated by exposing the fresh Mn/ TiO2 and MnEu/TiO2 catalysts in a mixed gas containing NO (600 ppm) +NH3 (600 ppm)+O2 (5%)+SO2 (100 ppm) and N2 balance (sulfated under SCR reactions) or SO2 (100 ppm)+O2 (5%) and N2 balance (sulfated by SO2 + O2 only) at 150 °C for 1 h. And the corresponding sulfated catalyst samples were denoted as Mn/TiO2-S (SCR + SO2), MnEu/TiO2-S (SCR + SO2), Mn/TiO2-S (SO2+O2) and MnEu/TiO2-S (SO2 + O2) respectively.
2.3. Catalytic performance evaluation Catalytic performances of the samples were investigated in a fixedbed quartz reactor with an 8 mm inner diameter. The feeding gas was composed of 600 ppm NH3, 600 ppm NO, 5% O2, 5% H2O, 100 ppm SO2 (when used), and Ar as balance. The reactions were executed with a 1L/ min gas flowrate and a GHSV of 108,000 h−1. A Nicolet iS50 FTIR spectrometer attached with a gas cell was used for effluent gas concentration analysis. Under the steady-state reaction conditions, the SCR performance parameters could be obtained by:
NOx conversion =
[NOx ]in −[NOx ]out × 100% [NOx ]in
(1)
2[N2 O]out ⎞ × 100% N2 selectivity = ⎛1− ⎝ [NOx ]in + [NH3]in −[NOx ]out −[NH3]out ⎠ ⎜
⎟
(2) In addition, NO oxidation activity over each catalyst sample was also evaluated under the similar experimental conditions. However, NH3 was excluded when performing NO oxidation test.
2.2. Characterization 3. Results and discussion The surface structure parameters of the catalysts were determined by nitrogen adsorption at −196 °C by means of a Quantachrome Autosorb-iQ-AG instrument. The Brunauer–Emmett–Teller (BET) model and the Barrett–Joyner–Halenda (BJH) model were used for the evaluation of specific surface area and the distribution of pore size of each sample. The crystal forms for each catalyst were determined based on X-ray diffraction method (Bruker D8 Advance, Germany, CuKα radiation with a wavelength of 0.154056 nm). The data collection was performed in
3.1. Catalytic performance The catalytic activities of the catalysts are shown in Fig. 1(A). A relatively high SCR performance was observed on Mn/TiO2 catalyst, over which about 80% NOx conversion was seen in 240–400 °C. After the addition of Eu, a distinct activity increase and a broadened temperature window for MnEu/TiO2 catalyst were presented, as also reported in our previous study [15]. The T90 window of MnEu/TiO2 386
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Fig. 2. SO2 tolerances of Mn/TiO2 and MnEu/TiO2 in NH3-SCR reaction.
Table 1 Textural properties of the fresh and sulfated catalysts. Samples
BET surface area (m2/g)
Total pore volume (cm3/g)
Average pore diameter (nm)
Mn/TiO2 MnEu/TiO2 Mn/TiO2-S (SCR + SO2) MnEu/TiO2-S (SCR + SO2) Mn/TiO2-S (SO2+O2) MnEu/TiO2-S (SO2+O2)
137 157 86.7 149
0.44 0.43 0.15 0.35
7.8 6.6 6.6 5.6
91.4 97.1
0.22 0.25
6.4 6.8
3.3. Physical and crystal properties (BET and XRD) Fig. 1. (A) SCR activities (B) N2 selectivities of the fresh and sulfated catalysts.
As listed in Table 1, the BET surface area of MnEu/TiO2 was 157 m2/g, an apparent increase was present compared with the same parameter for Mn/TiO2 catalyst as the result of Eu introduction. This phenomenon might be in virtue of the intensive interaction among the components of MnEu/TiO2 catalyst. On the contrary, the distinctly decreased BET surface areas were associated with the four sulfated catalyst samples. Noticeably, the MnEu/TiO2 catalyst sulfated under SCR reactions still exhibited a large specific surface area. Moreover, the total pore volumes of the four sulfated catalyst samples decreased a lot owing to the block by sulfate species during the sulfation process. XRD technique was used to identify the crystal structures of the fresh and sulfated catalysts, and the obtained results are presented in Fig. S1. In the XRD spectrum of each catalyst sample, only diffraction peaks belonging to anatase and rutile TiO2 were detected. These features revealed that Mn and Eu species were well dispersed on catalyst surface, which might exist as an amorphous or highly dispersed phase. It seemed that the Eu species in MnEu/TiO2 catalyst inhibited TiO2 crystal growth and decreased its crystallinity, as indicated by the decreased peak intensities in its spectrum. For the four sulfated catalyst samples, much stronger peak intensities of anatase and rutile TiO2 were present in their spectra, suggesting the formation of larger TiO2 particles during the sulfation process. However, the crystallinities of the sulfated MnEu/TiO2 catalyst samples were still lower than the corresponding sulfated Mn/TiO2 catalyst samples.
catalyst ranged from 200 to 400 °C, which was much wider than that of Mn/TiO2 catalyst (about 280–334 °C). Moreover, the presence of Eu also improved the N2 selectivity of Mn/TiO2 catalyst, as presented in Fig. 1(B). After the sulfation process under SCR conditions, the SCR activity of Mn/TiO2 decreased sharply, but MnEu/TiO2-S (SCR + SO2) catalyst still exhibited relatively high SCR activity, meaning that MnEu/ TiO2 catalyst possessed good SO2 resistance under SCR conditions. Unlike the catalyst samples sulfated under SCR conditions, the pretreatment with SO2 + O2 led to the strong deactivation of the two fresh catalyst samples, as reflected by their poor SCR performances presented in Fig. 1(A). Moreover, the decreased N2 selectivities were also found over Mn/TiO2-S (SO2 + O2) and MnEu/TiO2-S (SO2 + O2), as shown in Fig. 1(B).
3.2. SO2 tolerance The poisoning effect of SO2 on SCR catalyst is a well-recognized fact owing to the deposition of sulfate [17]. The SCR performances of Mn/ TiO2 and MnEu/TiO2 in the presence of SO2 at 150°Cwere tested and the results are illustrated in Fig. 2. From Fig. 2, the NOx conversion over MnEu/TiO2 catalyst decreased from 85.0% to about 70.1% after a 25 h SCR reaction process in the existence of 100 ppm SO2, while the NOx conversion over Mn/TiO2 catalyst decreased from 59.3% to 20.1% under the same experimental conditions. Therefore, the SO2 tolerance of MnEu/TiO2 catalyst was superior to that of Mn/TiO2 catalyst. A mechanistic investigation would be performed based on various characterization techniques.
3.4. Surface elements analysis (XPS) The valence state states of active species on the catalysts surface had an important impact on SCR reaction, which was identified by XPS analysis. The obtained results are depicted in Fig. 3 and Table 2. The XPS spectra of Mn 2p for the catalysts are illustrated in 387
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Fig. 3(A). The two main peaks at 642.0 and 653.8 eV represent Mn 2p3/ 2 and Mn 2p1/2 respectively. After a further peak-fitting deconvolution procedure, the Mn 2p XPS spectra could be attributed to three types of Mn species: Mn2+, Mn3+ and Mn4+ [18–20]. From the surface element concentrations listed in Table 2, Mn4+ concentration of Mn/TiO2 catalyst increased from 0.65 at.% to 0.93 at.% after the modification with Eu. Thus the presence of Eu promoted the formation of Mn4+ species. Evidently, the sulfation process would bring about the decrease of Mn4+ concentrations, as reflected by the corresponding values for the four sulfated samples. Previous studies had indicated that the high redox ability of Mn4+ administered to NO oxidation to NO2, which helped to open the “fast SCR” route for NOx removal due to the participation of NO2 [21–23]. Thus the beneficiation of Mn4+ species over MnEu/TiO2 catalyst should be an important incentive for its excellent SCR performance in 100–300 °C and high resistance to SO2. After the sulfurization process, the decreased Mn4+ concentrations over all the sulfated samples, which was unfavorable to the low-temperature SCR reactions over them, as illustrated in Fig. 1(A). Fig. 3(B) presents the O 1 s XPS spectra of the catalysts. The two subpeaks represented two kinds of oxygen species: lattice oxygen (B. E.≈529.5 eV, denoted as Oα) and surface adsorbed oxygen (B. E. ≈531.9 eV, denoted as Oβ), mainly existed in the forms of O22 − and O− in OH-like groups [7,24,25]. From Fig. 3(B), an obvious increase of Oβ peak was observed after the addition of Eu on Mn/TiO2 catalyst, as also reflected by the results given in Table 2, the Oβ concentration of MnEu/ TiO2 catalyst was 17.80 at.%, which was much greater than the corresponding value of Mn/TiO2 catalyst (16.18 at.%). In virtue of its high mobility, surface adsorbed oxygen played a critical role in the redox process in SCR reaction [26], which had a positive effect on “fast SCR” catalytic mechanism [27]. Furthermore, OH-like group could combine with NH3 to form NH+4 , which could further react with NO2 to produce N2 and H2O [28,29]. Moreover, a distinct drop of Oβ concentration was detected on the sulfated samples except MnEu/TiO2-S (SCR + SO2) catalyst, in accordance with their poor activities in 100–250 °C. Fig. 3(C) displays the S 2p XPS spectra of the four sulfated samples. From Fig. 3(C), two peaks at about 169.5 and 168.4 eV were present, which were assigned to HSO−4 and SO24− respectively [30,31]. It should be noticed that MnEu/TiO2-S (SCR + SO2) catalyst exhibited the lowest band intensity among the four sulfated catalyst samples, suggesting that the sulfation process under SCR conditions only generated traces of sulfate species on it. 3.5. H2-TPR The reducibility of each sample was evaluated by H2-TPR technique, as illustrated in Fig. 4. Three reduction peaks were found in each pattern. The first peak in Mn/TiO2 reduction pattern appeared at about 395 °C represented the reduction of MnO2/Mn2O3 to Mn3O4 [32,33], the second peak situated at about 574 °C might be resulted from the reduction of Mn3O4 to MnO [32], and the third one at about 649 °C indicated the reduction process of oxygen groups [34]. Therefore, H2TPR technique also revealed the presence of Mn2+, Mn3+ and Mn4+ species, as also obtained by XPS analysis. A peak shift to higher temperature was detected in the patterns of the four sulfated samples,
Fig. 3. XPS spectra of the fresh and sulfated catalysts.
Table 2 The concentrations of the atoms obtained by XPS analysis. Samples
Mn (at.%)
O (at.%)
Eu (at.%)
S (at.%)
Mn4+/Mn (%)
Oβ/O (%)
Mn4+ (at.%)
Oβ (at.%)
Mn/TiO2 MnEu/TiO2 Mn/TiO2-S (SCR + SO2) MnEu/TiO2-S (SCR + SO2) Mn/TiO2-S (SO2 + O2) MnEu/TiO2-S (SO2 + O2)
6.52 5.57 6.21 5.45 6.27 5.99
68.59 67.79 64.12 65.54 66.02 65.76
/ 1.36 / 0.98 / 1.18
/ / 4.92 2.50 3.86 3.29
10.02 16.74 8.61 12.35 8.83 11.46
23.59 26.26 19.46 24.92 21.03 21.94
0.65 0.93 0.53 0.67 0.55 0.68
16.18 17.80 12.48 16.33 13.88 14.43
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Fig. 4. H2-TPR patterns of the fresh and sulfated catalysts.
meaning the dropped reducibilities of them. Moreover, a new reduction peak at about 600 °C appeared in the profile of each sulfated catalyst sample, which could be attributed to the reduction of sulfate [35,36]. Noticeably, the sulfate reduction peaks in the profiles of MnEu/TiO2-S (SCR + SO2) and MnEu/TiO2-S (SO2 + O2) were weaker than that in the patterns of the other two sulfated samples, revealing the disposition of less sulfate species on them, which further confirming the results of XPS study. Although the sulfation process generate reducible sulfate species on the sulfated samples, the H2 consumption (HC) values of them are still low than that of the two fresh samples, as presented in Fig. 4. 3.6. Nox adsorption The NOx-TPD patterns for the catalysts are presented in Fig. S2. For Mn/TiO2 catalyst, only a NO desorption peak appeared at about 175 °C in its profile, which could be ascribed to the decomposition of chemicaladsorbed NOx [37]. For MnEu/TiO2 catalyst, two peaks were observed at about 143 and 348 °C, which could be assigned to weakly bound and strongly bound NO−x species respectively [37–39]. Besides that, more NO2 was desorbed from MnEu/TiO2 catalyst than that from Mn/TiO2 catalyst, proving the promoted NO oxidation after the modification with Eu. For the four sulfated catalyst samples, the situations were quite different. It seemed that the sulfation process under SCR conditions only had a weak inhibition effect on NO adsorption and the formation of NO2 over MnEu/TiO2 catalyst; therefore, there were still quite a number of adsorbed NO and NO2 species present over MnEu/TiO2-S (SCR + SO2) catalyst. For the other three sulfated catalyst samples, the adsorption of NO and the formation of NO2 over them were severely suppressed by SO2 pretreatment.
Fig. 5. NH3-adsorption DRIFT spectra for (A) Mn/TiO2 and (B) MnEu/TiO2 in 100–350 °C.
3.8. In situ DRIFT spectra 3.8.1. Adsorption of NH3 The recorded NH3-adsorption DRIFT spectra for Mn/TiO2 and MnEu/TiO2 catalysts are displayed in Fig. 5. Four NH3-adsorption bands could be detected at 1694, 1600, 1388 and 1159 cm−1 respectively, these bands included NH+4 species linked to Brønsted acid sites (1694 and 1388 cm−1) and NH3 species adsorbed on Lewis acid sites (1600 and 1159 cm−1) [41–45]. For the DRIFT spectra of MnEu/TiO2 catalyst, two different bands (1668 and 1442 cm−1) all originated from NH+4 species connected with Brønsted acid sites [41]. The enhanced band intensities in Fig. 5(B) revealed that more NH3 species were adsorbed on MnEu/TiO2 catalyst surface. Due to the desorption of ad-NH3 species at high temperature, all the band intensities decreased gradually with temperature. For the four sulfated catalyst samples, the adsorption of NH3 species were inhibited in various degrees, as presented in Fig. S4. It seemed that the sulfation process under SCR conditions only had a weak suppression effect on the adsorption of NH3 species over MnEu/ TiO2 catalyst; the band intensities in its DRIFT spectra were much higher than that in the DRIFT spectra of the other three sulfated samples.
3.7. NO oxidation As mentioned above, NO oxidation is an essential factor for the occurrence of “fast SCR” reaction [40]. Herein, NO oxidation performances over the catalysts were also investigated, as shown in Fig. S3. From Fig. S3, the favored NO oxidation over MnEu/TiO2 catalyst was evident. MnEu/TiO2-S (SCR + SO2) catalyst exhibited similar NO oxidation activity with Mn/TiO2 catalyst. On the contrary, the NO oxidation activities on the other three sulfated catalyst samples were distinctly suppressed, especially for that of the two sulfated Mn/TiO2 catalyst samples. Consequently, serious inhibition effect on the low-temperature SCR processes over them happened, as proven by the SCR activities shown in Fig. 1(A).
3.8.2. Co-adsorption of NO+O2 Fig. 6 exhibits the NOx-adsorption DRIFT spectra over Mn/TiO2 and 389
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Fig. 7. Transient reaction of NOx with preadsorbed NH3 over (A) Mn/TiO2 and (B) MnEu/TiO2 at 150 °C. Fig. 6. NOx-adsorption DRIFT spectra for (A) Mn/TiO2 and (B) MnEu/TiO2 in 100–350 °C.
similar trend for the reaction between NO + O2 and the preadsorbed NH3 species reappeared in the spectra of MnEu/TiO2 catalyst. Differently, it seemed that the adsorbed NH3 species over MnEu/TiO2 catalyst were of higher reactivity compared with that over Mn/TiO2 catalyst. For example, the band at 1600 cm−1 in Fig. 7(B) nearly vanished after the introduction of NO + O2 for 2 min; however, the corresponding band in Fig. 7(A) was still present under the same conditions, although the two bands were of similar intensities. Similar situation also happened on the band at 1159 cm−1. Thus both the adsorption and activation of NH3 species over MnEu/TiO2 catalyst were strengthened owing to the existence of Eu. As mentioned above, adsorbed NH3 species and gaseous NOx species were the major characters in the transient SCR reaction, verifying the applicability of Eley−Rideal (E-R) mechanism for it [28]. For the sulfated catalyst samples (Fig. S6), all the ad-NH3 species were active in the transient SCR reaction with a much lower reactivity, indicating the blockage effect of sulfation process on the NH3-SCR reactions over them through E-R pathway.
MnEu/TiO2 catalysts. Several characteristic NOx adsorption bands appeared in Fig. 6, which represented adsorbed NO2 (1606 cm−1), bidentate nitrate (1560 and 1552 cm−1), monodentate nitrate (1266 cm−1) and bridged nitrate (1247 cm−1) respectively [41,44–47]. It seemed that the band at 1606 cm−1 in Fig. 6(B) was more intensive than that in Fig. 6(A), further confirming the generation of more NO2 on MnEu/TiO2 catalyst, as discussed in Section 3.7. From Fig. S5, it seemed that the sulfation process under SCR conditions had little suppression effect on NOx adsorption over MnEu/TiO2 sample, as also concluded from the results of NOx-TPD analysis. However, the suppressed NOx adsorption over the other three sulfated catalyst samples was noticeable in their DRIFT spectra.
3.8.3. Transient reaction of NOx with preadsorbed NH3 To identify the function of adsorbed NH3 species in the SCR process on Mn/TiO2 and MnEu/TiO2 catalysts, the fresh catalyst samples were pretreated with NH3 for 0.5 h, followed by the exposure to 500 ppm NO +5% O2/N2, and the time-dependent DRIFT spectra were recorded and given in Fig. 7. From the DRIFT spectra of Mn/TiO2 (Fig. 7 (A)), several bands (1694, 1600 and 1159 cm−1) of adsorbed NH3 species appeared after the NH3 pretreatment. The introduction of NO + O2 led to the quick consumption of adsorbed NH3 species, their bands became weaker rapidly and completely disappeared in 5 min. From Fig. 7 (B), a
3.8.4. Transient reaction of NH3 with preadsorbed NOx The transient reaction of NH3 with preadsorbed NOx was also investigated by feeding the reactants into the IR cell in an opposite order, and the recorded DRIFT spectra for this process over Mn/TiO2 and MnEu/TiO2 catalysts are demonstrated in Fig. 8. From Fig. 8(A), NO + O2 pretreatment generated several adsorbed NOx species on Mn/ TiO2 catalyst, as reflected by the bands at 1602, 1553, 1487 and 390
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O2 (g) → 2O(ad)
(4)
NH3 (a) + O(ad) → NH2 (ad) + OH(ad)
(5)
NO(g) + NH2 (ad) → NH2 NO(ad)
(6)
NH2 NO(ad) → N2 (g) + H2 O
(7)
2) L-H mechanism:
O2 (g) → 2O−∗ (surface active sites)
(8)
NO(g) + O−∗ → NO2 (ad)
(9)
Mn4 +
NH3 (g) ⎯⎯⎯⎯⎯→ NH3 (ad)(over Lewis acid sites)
(10)
2NH3 (ad) + NO2 (ad) + NO(g) → 2N2 + 3H2 O
(11)
Combined with the characterization results, the modification of Mn/ TiO2 catalyst by Eu not only facilitated the oxidation of NO to NO2, but also facilitated the adsorption of NH3 species, which had an acceleration impact on the SCR reaction over MnEu/TiO2 catalyst via all the two routes mentioned above. As a contrast, the SCR reactions over Mn/ TiO2-S (SCR + SO2), Mn/TiO2-S (SO2 + O2) and MnEu/TiO2-S (SO2 + O2) through both E-R and L-H routes were seriously inhibited. It was noteworthy that the NH3-SCR reaction over MnEu/TiO2-S (SCR + SO2) mainly obeyed the L-H mechanism. 3.9. DRIFT study on the SO2 tolerance of the two fresh catalyst samples 3.9.1. Effect of SO2 on NH3 adsorption In this section, the two fresh catalyst samples were first exposed to NH3, next then SO2 was also added in the gas stream. The DRIFT spectra of this process were recorded (Fig. S8). From Fig. S8(A), two ad-NH3 bands appeared at 1600 and 1159 cm−1 and grew with time after the treatment with NH3. When SO2 was added in the gas, the two bands of adsorbed NH3 species still grew with time, meanwhile, two new bands of sulfate species (1283 and 1198 cm−1) [13,47] appeared and became stronger with time. Therefore, the presence of SO2 would promote the adsorption of NH3 species over Mn/TiO2 catalyst when no NOx was present in the gas atmosphere, which might be owing to the acidic nature of SO2. For the spectra of MnEu/TiO2 catalyst (Fig. S8(B)), four bands of ad-NH3 species were present. Similar promotion effect of SO2 on the adsorption of NH3 species over it could also be observed. It should be noticed that, there were no bands of adsorbed sulfate species present in the spectra of MnEu/TiO2 catalyst (Fig. S8(B)). Thus Eu decoration could prevent the formation of surface sulfate on Mn/TiO2 catalyst when no NOx species was present.
Fig. 8. Transient reaction of NH3 with preadsorbed NOx over (A) Mn/TiO2 and (B) MnEu/TiO2 at 150 °C.
1266 cm−1 respectively. All these bands quickly vanished after the introduction of NH3 for 2 min. Meanwhile, the formation and growth of ad-NH3 species bands were found, revealing the replacement of ad-NOx species by ad-NH3 species. Similar behavior of the adsorbed reactant species also appeared in Fig. 8(B). Therefore, all the ad-NOx species were participators in the transient SCR process over Mn/TiO2 and MnEu/TiO2 catalysts with similar reactivities, demonstrating the existence of Langmuir−Hinshelwood (L-H) mechanism [28]. For the four sulfated catalyst samples (Fig. S7), the situation was different. All the adsorbed NOx species over MnEu/TiO2-S (SCR + SO2) catalyst were quickly depleted in 2 min after the introduction of NH3, which was very similar with the spectra of the fresh MnEu/TiO2 catalyst. Therefore, the sulfation process under SCR conditions had rare suppression effect on the NH3-SCR reaction over MnEu/TiO2 catalyst though L-H pathway. On the contrary, the preadsorbed NOx species over the other three sulfated catalyst samples were still present after the importation of NH3 for 5 min, suggesting the NH3-SCR reactions over them through L-H route were blocked to a large extent. As discussed above, both E-R and L-H mechanisms played a part in the SCR processes over Mn/TiO2 and MnEu/TiO2 catalysts, as described by [21,48]:
3.9.2. Effect of SO2 on NOx adsorption Next then, the impact of SO2 on NOx adsorption over Mn/TiO2 and MnEu/TiO2 catalysts were investigated by DRIFT technique (Fig. S9). The treatment of Mn/TiO2 catalyst by NOx generated three bands of adNOx species at 1606, 1560 and 1266 cm−1 in its DRIFT spectra (Fig. S9(A)). It could be seen that the last band quickly vanished after SO2 feeding. And the intensity decrease could be observed for the other two bands. Therefore, SO2 could inhibit the adsorption of NOx species over Mn/TiO2 catalyst. In addition, several bands of sulfate species (1368, 1270 and 1176 cm−1) came into being in Fig. S9(A). For the spectra of MnEu/TiO2 catalyst (Fig. S9(B)), the trend was quite similar. But there were more adsorbed NOx species on MnEu/TiO2 compared with that on Mn/TiO2 catalyst, indicating that Eu modification could partially offset the inhibition effect of SO2 on NOx adsorption over Mn/TiO2 catalyst. Moreover, the formation of less sulfate species over MnEu/TiO2 catalyst could also be detected from the lower band intensities in its DRIFT spectra.
1) E-R mechanism: Mn4 +
NH3 (g) ⎯⎯⎯⎯⎯→ NH3 (ad)(over Lewis acid sites)
(3) 391
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which included adsorbed NO2 (1605 cm−1), NH+4 species (1454 cm−1), monodentate nitrate (1297 and 1123 cm−1) and NH3 species (1190 cm−1) [41–46,50,51]. With the feeding of SO2, three bands (1297, 1190 and 1123 cm−1) vanished, while the other two bands at 1605 and 1454 cm−1 were still present, accompanied with the decrease of band intensities with time. Under these conditions, the SCR process on MnEu/TiO2 could take place though L-H pathway. Moreover, several new bands could be observed after the introduction of SO2, which could be attributed to surface sulfate species with a single S]O band (1344 cm−1), bidentate sulfate (1278 cm−1) and surface or bulk-like sulfates (1127 cm−1) respectively [13,47]. In addition, the relatively low intensities of the bands owing to sulfate species on MnEu/TiO2 catalyst also suggested the formation of less sulfate species over it. 4. Conclusions The present study found that Eu modification improved the SO2 tolerance of Mn/TiO2 catalyst under SCR conditions, while the sulfation pretreatment of Mn/TiO2 and MnEu/TiO2 by SO2+O2 would strongly deactivate the two catalyst samples. Form the experimental and characterization results, the following conclusions could be arrived: (1) Eu modification suppressed the formation of sulfate species over Mn/TiO2 catalyst under SCR conditions. (2) The SCR process over MnEu/TiO2 catalyst in the presence of SO2 took place through L-H pathway. (3) The inhibited adsorption of reactants on Mn/TiO2-S (SCR + SO2), Mn/TiO2-S (SO2 + O2) and MnEu/TiO2-S (SO2 + O2) catalysts had a strong blockage effect on the SCR reactions over them via both ER and L-H routes. Acknowledgment This work was supported by the Natural Science Foundation of China (21546014) and the Natural Science Foundation of Shanghai, China (14ZR1417800). Appendix A. Supplementary data
Fig. 9. SO2-adsorption DRIFT spectra for (A) Mn/TiO2 and (B) MnEu/TiO2 catalysts under the SCR reactions (500 ppm NH3+500 ppm NO + 5% O2, balance N2, 100 ppm SO2 (when used)) at 150 °C.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2018.03.062.
3.9.3. SO2 poisoning experiments under NH3-SCR conditions Firstly, Mn/TiO2 and MnEu/TiO2 catalysts were pretreated by a mixture of 500 ppm NO, 500 ppm NH3, 5% O2 and balance N2 for 20 min. From Fig. 9(A), four bands appeared during the initial 20 min introduction of reactant gases over it. Based on previous literatures [28,41,42], these bands could be ascribed to adsorbed NO2 (1606 cm−1), monodentate nitrate (1517 and 1271 cm−1) and NH3 species over Lewis acid sites (1175 cm−1). Next then, Mn/TiO2 catalyst was purged by N2 at 150 °C, followed by the introduction of 100 ppm SO2 for 30 min. In this process, all the bands of adsorbed NH3 species and adsorbed NOx species quickly disappeared, however, several new bands (1618, 1431, 1292, 1214 and 1130 cm−1) came out and increased with time, along with the formation of ionic NH+4 over Brønsted acid sites (1431 cm−1) [42] and sulfate species (1618, 1292, 1214 and 1130 cm−1) [13,47]. In the sulfation process, some Lewis acid sites would be consumed due to the reaction between SO2 and metal ions, meanwhile, Brønsted acid sites with an S-OH form generated [49]. As a result, the band at 1431 cm−1 appeared. Therefore, the adsorption of reactants on Mn/TiO2 catalyst was basically cut off. The surface of Mn/ TiO2 catalyst was nearly under the bestrow of sulfate species. Under these circumstances, serious deactivation by SO2 happened, as shown in Fig. 2. For MnEu/TiO2 catalyst (Fig. 9(B)), the situation was quite different. Several bands formed at 1605, 1454, 1297, 1190 and 1123 cm−1 after the treatment of MnEu/TiO2 catalyst with the reactant gases,
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