DRIFT Study of the SO2 Effect on Low-Temperature SCR Reaction

Mar 2, 2010 - SO2 would deactivate the low-temperature SCR (selective catalytic reduction) catalysts and reduce NO removal. In this study, Fe(0.1)−M...
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J. Phys. Chem. C 2010, 114, 4961–4965

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DRIFT Study of the SO2 Effect on Low-Temperature SCR Reaction over Fe-Mn/TiO2 B. Q. Jiang,†,‡ Z. B. Wu,*,‡ Y. Liu,‡ S. C. Lee,§ and W. K. Ho§ College of EnVironmental Science and Engineering, Zhejiang Gongshang UniVersity, Hangzhou 310012, China, Department of EnVironmental Engineering, Zhejiang UniVersity, Hangzhou 310027, China, and Department of CiVil and Structural Engineering, The Hong Kong Polytechnic UniVersity, Hung Hom, Kowloon, Hong Kong ReceiVed: August 12, 2009; ReVised Manuscript ReceiVed: January 5, 2010

SO2 would deactivate the low-temperature SCR (selective catalytic reduction) catalysts and reduce NO removal. In this study, Fe(0.1)-Mn(0.4)/TiO2 prepared by sol-gel method was selected to carry out the in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) investigation for revealing the mechanism of the SO2 effect on the SCR reaction. The DRIFT spectroscopy showed that SO2 could be adsorbed on the surface of the catalyst as the bidentate mononuclear sulfate. This type of sulfate would retard the formation of NO complex on the surface of catalyst, resulting in the decrease of NO adsorption. For NH3 adsorption, the adsorption of SO2 had little effect on the coordinated NH3, but would increase the amount of NH4+ because of the formation of new Brønsted acid sites. Therefore, besides the deposition of ammonium sulfates, the competitive adsorption between SO2 and NO on the active sites of the catalysts also contributed to the poisoning effect of SO2 on the SCR reaction. When sulfate was formed on the catalyst, much less NO could be adsorbed and take part in the SCR reaction. 1. Introduction The major technology for the removal of nitrogen oxide emissions from stationary sources is selective catalytic reduction (SCR) of NOx by NH3. In power plants, the conventional SCR technology, with the reaction temperature 573-673 K, is widely used. However, considering the problem of SO2 and dust deactivation, there has been a rising interest in developing a low-temperature (353-523 K) SCR catalyst for the removal of NOx in recent years since by using this kind of catalyst, the deNOx reactor can be moved downstream of the desulfurization scrubber and/or particulate control device, where most of the SO2 and dust are removed and then the deactivation effect can be weakened. Flue gases after FGD (flue gas desulfurization) still contain small amounts of SO2. Thus, the sulfur tolerance of the catalyst should be considered for practical usage. For the conventional SCR reaction process, V2O5-WO3/TiO2 is usually used, and the reaction usually occurs via an Eley-Rideal mechanism, where gaseous NO directly reacts with an activated ammonia surface complex.1 In this reaction process, SO2 has a beneficial effect on the activity due to the formation of new Brønsted acid sites.2 For low-temperature SCR, many kinds of catalysts have been investigated,3–5 and a Langmuir-Hinshelwood reaction mechanism involving a surface NO complex was suggested partially contributing to the DeNOx reaction.6 Although many studies have been done for the poisoning effect of SO2 on SCR catalysts,7–11 the mechanism of the SO2 effect on the lowtemperature SCR reaction is still not in consensus. Some authors considered the deactivation could be attributed to the formation of metal sulfate on the catalysts (such as MnOx/Al2O3).8 And when other catalysts (CuO/Al2O3) were investigated, the authors believed that the deactivation was due to the formation of * To whom correspondence should be addressed. E-mail: [email protected]. Phone: (86)571-87952459. Fax: (86)571-8793088. † Zhejiang Gongshang University. ‡ Zhejiang University. § The Hong Kong Polytechnic University.

NH4HSO4 and (NH4)2SO4, which would decrease the BET surface area of the catalysts.9 Furthermore, some literature reported that the SCR activity of some catalysts such as MnOx/ TiO2 could be restored after cutting off SO2 feeding, since the poisoning effect of SO2 was mainly the result of the competitive adsorption of NO and SO2.10,11 It seems that the SO2 effect on the SCR reaction is different for different catalytic systems. Manganese oxides attract interest with their high SCR activity at low temperature, since these oxides contain various types of labile oxygen, which is necessary to complete a catalytic cycle.12,13 In our previous studies,14,15 it was found that the catalysts based on Fe-Mn/TiO2 had high catalytic activity for removing NO at low temperature, and NO was removed via notonlytheEley-RidealmechanismbutalsotheLangmuir-Hinshelwood mechanism. In this paper, the DRIFT (in situ diffuse reflectance infrared Fourier transform spectroscopy) study was carried out to reveal the influence of SO2 on the SCR reaction by using Fe-Mn/TiO2. 2. Experimental Methods 2.1. Catalyst Preparation. The catalyst was prepared by sol-gel method. All chemicals used were of analytical grade. Butyl titanate (0.1-0.3 mol), ethanol (0.8 mol), water (0.6 mol), acetic acid (0.3 mol), manganese nitrate (0.04-0.12 mol), and ferric nitrate (0.01-0.03 mol) were mixed under vigorous stirring, forming a transparent red sol. After being stable at room temperature for several days, the sol transformed to a gel. The gel was dried at 378 K and transform to a porous solid. Then the solid was crushed and sieved to 50-90 mesh. After that the solid was calcined at 773 K for 6 h in air in a tubular furnace. The catalyst was denoted as Fe(0.1)-Mn(0.4)/TiO2, which meant that the molar ratio of Fe to Ti was 0.1, and the molar ratio of Mn to Ti was 0.4. 2.2. Catalytic Activity Measurement. The SCR activity measurement was carried out in a fixed-bed, stainless steel, flow reactor. The experiments were performed under atmospheric pressure at 423 K. The reactor consisted of a steel tube of 1-cm

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Figure 1. The effect of SO2 on NO conversion for Fe(0.1)-Mn(0.4)/ TiO2.

i.d. in which 4 mL of catalyst was placed. The typical reactant gas composition was as follows: 1000 ppm NO, 1000 ppm NH3, 200 ppm SO2, 3% O2 (v/v), and balance N2. The total flow rate was 2000 mL/min. The tubing of the reactor system was heat traced to prevent the formation and deposition of ammonium nitrate at the reaction temperature. The concentrations of NO, NO2, SO2, and O2 were monitored by a flue gas analyzer (KM9006 Quintox Kane International Limited). 2.3. Transient DRIFT Experiments. FTIR spectra were acquired using in situ DRIFT cell equipped with a gas flow system. The DRIFT measurements were performed with Nicolet 6700 FTIR spectrometers at 4 cm-1 resolution with 64 coadded scans. In DRIFT cell, the gas flow rate was 30 mL/min. The catalyst was treated at 773 K in He environment for 2 h, then cooled to 423 K. All the spectra were recorded at this temperature. The background spectrum was recorded with the flowing of He and was subtracted from the sample spectrum. The final differential sample spectra were calculated by Kubelka-Munk function. When the adsorption of NH3 and/or NO on the sample pretreated by SO2 was investigated, the spectra were recorded after catalyst exposure to SO2. First, the catalyst was pretreated at 773 K in He for 2 h. When the cell was cooled to 423 K, 500 ppm SO2 + 3% O2 was introduce to the cell, and then the catalyst was swept by He for 30 min. The background spectrum was recorded with flowing He and was subtracted from the sample spectrum. The final differential sample spectra were also calculated by Kubelka-Munk function. 3. Results 3.1. Effect of SO2 on NO Conversion. Figure 1 shows the influence of SO2 on NO conversion for Fe(0.1)-Mn(0.4)/TiO2 at 423 K. When SO2 did not exist in the system, NO conversion could be maintained at about 100% (step a). After 200 ppm SO2 was introduced, NO conversion decreased rapidly from 100% to about 80% in 35 min (step b), and then decreased gradually with time (step c). This result was in agreement with the previous study.16 The different NO conversion variation trends (steps b and c) with the introduction of SO2 should be controlled by a different mechanism. In previous works,9,17 it had been found that the gradual decrease (step c) of NO conversion was probably due to the deposition of ammonium sulfate. However, less research has focused on the mechanism of the rapid decrease step (step b). Therefore, in this paper,

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Figure 2. DRIFT spectra of Fe(0.1)-Mn(0.4)/TiO2 exposed to 500 ppm SO2 + 3% O2 for various times: (a) 5 min; (b) 10 min; (c) 20 min; (d) 30 min.

DRIFT was used to study the SO2 poisoning mechanism during this process (step b). 3.2. SulfurDioxideAdsorption.SO2 uptakeonFe(0.1)-Mn(0.4)/ TiO2 in the presence of O2 was observed first. Figure 2 shows the spectra when the sample was exposed to SO2 + O2/He for different times. Several bands at 1428, 1344, 1278, 1149, 1050, and 976 cm-1 were detected, and the intensity increased with time. The band at 1428 cm-1 was previously found by Watson and Ozkan,18 but the band assignment was not given. According to other studies,19,20 this band could be assigned to the SO3 species, and the band at 1344 cm-1 was the ν(SdO) vibration of surface sulfate species with only one SdO band. After the catalyst had been exposed to SO2 for 5 min, the bands at 1278, 1149, 1050, and 976 cm-1 gradually appeared. According to previous studies,21,22 these bands could be assigned to the stretching motion of adsorbed bisulfate or sulfate on the surface of the sample. On the basis of the study of Peak et al.,23 there were two infrared sulfate vibrations that were accessible to FTIR investigation, which were the nondegenerate symmetric stretching ν1 band and the triply degenerate asymmetric stretching ν3 band. The ν1 band appeared around 975 cm-1. And when the bidentate sulfate complex was formed on the surface of the sample, the ν3 band would split into three bands. Therefore, the bands at 1278, 1149, and 1050 cm-1 were assigned to bidentate sulfate on Fe(0.1)-Mn(0.4)/TiO2. 3.3. Effect of SO2 on NO Adsorption. Figure 3 shows the effect of SO2 on NO adsorption on Fe(0.1)-Mn(0.4)/TiO2. As shown in Figure 3a, when the sample was treated in 1000 ppm NO + 3% O2 for 30 min, there were several bands assigned to the NO complex. The band at 1625 cm-1 was assigned to NO2.24 And the bands at 1580 and 1275 cm-1 were attributed to bidentate nitrate13 and monodentate nitrate,25 respectively. The band at 1245 cm-1 could be assigned to the bridged nitrate.26 Furthermore, there was a small peak at about 1550 cm-1, which could also be assigned to bidentate nitrate on the surface of the sample.27 When SO2 was added to this system, the spectra exhibited considerable variation. After the sample had been exposed to SO2 for 5 min, three bands at 1278, 1149, and 1050 cm-1 attributed to bidentate sulfate were detected, and the band at 1344 cm-1 also appeared. The intensity of these bands increased with time. With the formation of sulfate, the intensity of the bands assigned to the NO complex decreased. Although it was

SO2 Effect on SCR Reaction over Fe-Mn/TiO2

Figure 3. DRIFT spectra of Fe(0.1)-Mn(0.4)/TiO2 under different conditions: (a) 1000 ppm NO + 3% O2 for 30 min; (b-e) 1000 ppm NO + 500 ppm SO2 + 3% O2; (b) 5 min; (c) 10 min; (d) 20 min; (e) 30 min.

Figure 4. DRIFT spectra of Fe(0.1)-Mn(0.4)/TiO2 under different conditions: (a) 1000 ppm NH3 + 3% O2 for 30 min; (b-e) 1000 ppm NH3 + 500 ppm SO2 + 3% O2; (b) 5 min; (c) 10 min; (d) 20 min; (e) 30 min.

difficult to determine the band around 1250 cm-1 because the nitrate and sulfate appeared in the same region, the bands assigned to nitrate in the region of 1500-1600 cm-1 had almost vanished after the sample had been exposed to SO2 for 20 min (Figure 3 d). Furthermore, the amount of NO2 also decreased. After the exposure to SO2 for 30 min (Figure 2 e), the intensity of the band at 1625 cm-1 had decreased to less than half compared to Figure 3a. 3.4. Effect of SO2 on NH3 Adsorption. When the sample was purged by 1000 ppm NH3 + 3% O2 for 30 min, several bands at 1598, 1443, and 1170 cm-1 and a broadband in the range of 1850-1640 cm-1 were detected (Figure 4). The bands at 1598 and 1170 cm-1 could be assigned to coordinated NH3 on Lewis acid sites.28,29 The bands at 1443 cm-1 and in the range of 1850-1640 cm-1 were attributed to NH4+ species on Brønsted acid sites.30 Furthermore, there were two small bands at 966 and 930 cm-1, which contributed to weakly adsorbed or gas-phase NH3. When SO2 was added to the cell, the bands at 1250, 1150, and 1041 cm-1 attributed to bidentate sulfate were formed rapidly. And the adsorption of NH3 on Fe(0.1)-Mn(0.4)/TiO2 changed. The bands at 966 and 930 cm-1 were still visible after

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Figure 5. DRIFT spectra of Fe(0.1)-Mn(0.4)/TiO2 pretreated by SO2 exposed to NO + 3% O2 for various times: (a) 5 min; (b) 10 min; (c) 20 min; (d) 30 min.

SO2 was added, indicating that SO2 would not reduce the amount of weakly adsorbed or gas-phase NH3. Although the band at 1170 cm-1 was overlapped by the band at 1150 cm-1 assigned to sulfate, the intensity of the band at 1598 cm-1 did not decrease after SO2 was introduced. Therefore, SO2 had little effect on the formation of coordinated NH3 on the surface of the sample. Simultaneously, the intensity of the bands assigned to NH4+ increased greatly, which indicated that when the sample was treated by SO2, new Brønsted acid sites would be formed on the surface, which enhanced the formation of NH4+. 3.5. NO and NH3 Adsorption on the Sample Pretreated by SO2. In this part of the study, the sample was pretreated by SO2 and purged by He for 30 min. The spectra were recorded with SO2 background. When NO + O2 was introduced to the sample pretreated by SO2 (Figure 5), there were negative bands at 1428 and 1278 cm-1, which were assigned to the displacement of the SO3 species and sulfate, respectively. This implied that NO and SO2 were adsorbed competitively on the surface of sample. After NO was introduced, the band at 1625 cm-1 assigned to NO2 was formed immediately, and the band at 1198 cm-1 could be assigned to the shift of bridged nitrate.31 However, the intensity of these bands was much lower than that in Figure 3a. In this procedure, the bands assigned to bidentate nitrates and monodentate nitrates (at 1580, 1550, and 1275 cm-1) were not found, indicating that the adsorption of SO2 on the surface would inhibit the formation of nitrates. When NH3 was introduced to the sample pretreated by SO2, the bands at 1598 and 1210 cm-1 appeared rapidly, which were attributed to the formation of coordinated NH3 on Lewis acid sites. And the bands at 1443 cm-1 and in the range of 1850-1640 cm-1 attributed to NH4+ species were detected, which showed that a large amount of Brønsted acid sites existed. The amount of NH4+ in Figure 6 was much higher than that in Figure 4a. Therefore, the new Brønsted acid sites were associated with the surface sulfate on the sample. Furthermore, the negative band at 1344 cm-1 was due to the displacement of the sulfate species on the surface, indicating that NH3 was more strongly adsorbed on the surface than some of the sulfate species. 4. Discussion As shown in Figure 2, when SO2 was adsorbed on the catalyst, the major bands were assigned to bidentate sulfate on the

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Figure 6. DRIFT spectra of Fe(0.1)-Mn(0.4)/TiO2 pretreated by SO2 exposed to NH3 + 3% O2 for various times: (a) 5 min; (b) 10 min; (c) 20 min; (d) 30 min.

surface. For SO2 adsorption, the bidentate sulfate could still be divided into the bidentate binuclear sulfate and the bidentate mononuclear sulfate. The typical three splitted ν3 bands of bidentate binuclear sulfate were between 1050 and 1250 cm-1, and if the bidentate mononuclear sulfate was formed, the bands would shift to higher wavenumbers.23 In Figure 2, the three bands were at 1278, 1149, and 1050 cm-1. Thus, the main production of SO2 adsorption was the bidentate mononuclear sulfate, as shown in Figure 7 c. It could be seen in Figure 3 that when the sample was exposed to both NO and SO2, part of the NO complex were displaced by sulfate. The amount of NO2 decreased sharply and the bidentate and monodentate nitrates almost vanished. This result was supported again in Figure 5, when NO was introduced to the sample pretreated by SO2, although some of the SO3 species and sulfate were desorbed in this procedure, the amount of NO2 formed was much lower than that in Figure 3, and the bands assigned to bidentate and monodentate nitrate could not be detected. Therefore, NO and SO2 were adsorbed competitively on the sample, and the adsorption ability of SO2 was much

Jiang et al. higher than that of NO. When SO2 was adsorbed onto the sample, it would occupy the active sites on Fe(0.1)-Mn(0.4)/ TiO2 (Figure 7c). The reaction in Figure 7b would be hindered, and the formation of NO complex was reduced. Compared to NO, the effect of SO2 on NH3 adsorption was different. Figure 4 shows that the amount of coordinated NH3 did not decrease after SO2 was introduced to the cell, and the formation of NH4+ was greatly promoted. When the sample was pretreated by SO2 (Figure 6), the coordinated NH3 could still be formed, and the intensity of bands assigned to NH4+ was much higher than that shown in Figure 4a. According to the research of Galvez et al.,32 the adsorption of NH3 could be divided into coordinated NH3 formed on Lewis metallic sites and NH4+ formed on the oxygen surface groups (Brønsted acid sites). The results of Figures 4 and 6 indicated that the formation of bidentate mononuclear sulfate would not reduce the amount of coordinated NH3 (Figure 7d). However, the amount of NH4+ increased greatly, indicating that a number of new Brønsted acid sites were formed by the sulfatization.33 When the sulfate was formed, SdO had a covalent double bond and had a much stronger affinity to electrons as compared with that of metal sulfate, and the Lewis acid strength of metal ions would become stronger. When a water molecule was bonded to the Lewis acid site, the Lewis acid site would be transformed to the Brønsted acid site,34 and then enhance the formation of NH4+, as shown in Figure 7e. Therefore, during the SCR reaction, the effect of SO2 was more significant on NO adsorption rather than NH3 adsorption. Combining the research of Pena et al.35 and our previous study,15 there were two reaction pathways for low-temperature SCR based on Fe-Mn/TiO2. One reaction pathway took place between the coordinated NH3 and gas-phase NO. In this paper, it could be known that the coordinated NH3 was little influenced by SO2, and the SCR reaction could still occur via this pathway. The other SCR reaction pathway took place between NH4+ and NO complex. When SO2 existed in the system, although the formation of NH4+ was enhanced due to the increase of the Brønsted acid sites, NO adsorption on the catalyst decreased. Thus, little NO could be removed via this reaction pathway. This resulted in the rapid decrease of NO conversion in the early period after the addition of SO2 (Figure 1b).

Figure 7. The proposed mechanism of SO2 deactivation effect on the SCR reaction.

SO2 Effect on SCR Reaction over Fe-Mn/TiO2 Therefore, it had been known that the sharp decrease after the addition of SO2 (Figure 1b) was due to the competitive adsorption of NO and SO2, which reduced the amount of NO complex on the surface of sample, leading to the inhibition of the reaction between NH4+ and NO complex. And according to the previous study,9 the gradual decrease of NO conversion may be concerned with the deposition of ammonium sulfate or ammonium bisulfate. Thus, the SO2 poisoning effect on the lowtemperature SCR reaction could be attributed to the combination effect of the above two types of mechanisms. 5. Conclusions In this paper, the effect of SO2 on the low-temperature SCR reaction was investigated by DRIFT. The main conclusions were drawn as follows: SO2 was adsorbed on the surface of catalyst as the bidentate mononuclear sulfate. This kind of sulfate could produce new Lewis acid sites on the catalysts, which would be transformed to the Brønsted acid site when the water molecule was bonded. For NH3 adsorption, the adsorption of SO2 had little effect on the coordinated NH3, but would increase the amount of NH4+ because of the formation of new Brønsted acid sites. The mononuclear sulfate would retard the NO adsorption on the surface of catalyst. With the formation of sulfate on the catalyst, much less NO complex could be formed and take part in the SCR reaction, resulting in the decrease of NO conversion. Therefore, the competitive adsorption between SO2 and NO was an important factor contributed to the poisoning effect of SO2 on the SCR reaction. Acknowledgment. The project is financially supported by the National Natural Science Foundation of China (NSFC50878190), and the Natural Science Foundation of Zhejiang Province (Y5090163). The authors would also like to acknowledge the support by the Zhejiang University Joint Supervision Scheme with Hong Kong PolyU. References and Notes (1) Nova, I.; Lietti, L.; Tronconi, E.; Forzatti, P. Catal. Today 2000, 60, 73–82. (2) Khodayari, R.; Odenbrand, C. U. I. Appl. Catal., B 2001, 30, 87– 99. (3) Kang, M.; Park, E. D.; Kim, J. M.; Yie, J. E. Catal. Today 2006, 111, 236–241.

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