Effect of KCl on selective catalytic reduction of NO with NH 3 over a V 2 O 5 /AC catalyst

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1 Available online at Catalysis Communications 9 (28) Effect of KCl on selective catalytic reduction of NO with NH 3 over a V 2 O 5 /AC catalyst Xianlong Zhang a,b, Zhanggen Huang a, Zhenyu Liu a,c, * a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 31, PR China b Graduate University of Chinese Academy of Sciences, Beijing 39, PR China c State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 29, PR China Received 24 April 27; received in revised form 5 September 27; accepted 7 September 27 Available online 14 September 27 Abstract Deactivation of a low temperature selective catalytic reduction (SCR) catalyst, 1 wt% V 2 O 5 /AC, by potassium chloride (KCl) is studied using BET and pore size distribution measurements, NH 3 sorption and temperature-programmed desorption (TPD), temperatureprogrammed surface reaction (TPSR). Results show that KCl deactivates 1 wt%v 2 O 5 /AC s SCR activity and the deactivation become severer at higher KCl loadings. The deactivation is not caused by pore plugging but by neutralization of acid sites on V 2 O 5 /AC. Weak acid sites are responsible for NH 3 activation and SCR of NO, and are preferentially deactivated by KCl. Ó 27 Elsevier B.V. All rights reserved. Keywords: V 2 O 5 /AC catalyst; Low temperature SCR; Deactivation; KCl 1. Introduction Nitric oxides (NO X ) emitted from thermal power plants are major air pollutants. Among various NO X emission control techniques, Selective catalytic reduction (SCR) of NO with NH 3 has been proven to be the most effective. The current commercial SCR catalysts are V 2 O 5 /TiO 2 based, which have to be used at temperatures higher than 3 C to avoid formation of NH 4 HSO 4 that deactivates the catalysts [1]. Low temperature SCR catalysts, resistant to NH 4 HSO 4 poisoning, have been studied because of energy savings and easy retrofitting to existing boiler systems [2]. A good example of the low temperature SCR catalyst is an activated carbon (AC)-supported vanadium oxide (V 2 O 5 /AC), which is very active in a temperature range of 18 2 C [3]. An important feature of this catalyst is its ability to catalyze reactions between NH 4 HSO 4 * Corresponding author. Address: State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 29, PR China. Tel.: ; fax: address: zyliu@sxicc.ac.cn (Z. Liu). and NO at temperatures below 2 C, and the presence of SO 2 promotes its SCR activity [4]. Fine fly ash has always been a concern to the SCR catalysts, it may plug the pores of the catalyst and react with the activate phases [5]. Alkali oxides and/or salts are major components in fly ash especially for biomass-fired boilers [6]. They tend to enrich on fine particulates with sizes smaller than 1 lm via a volatizing-condensation mechanism during combustion [7]. Among various alkali salts, KCl has been found to be a dominant form in some cases [8]. It is, therefore, important to understand the effect of KCl on the performance of V 2 O 5 /AC catalyst. Effect of alkali metals on commercial V 2 O 5 /TiO 2 catalyst for NO removal has been studied [9]. Deposition of potassium was widely believed to be the primary source leading to deactivation of V 2 O 5 /TiO 2. To a full-scale straw-fired grate boiler, NO conversion of a V 2 O 5 /TiO 2 catalyst dropped to about % after about 1 h operation, and an average K/V molar ratio of.3.5 in the catalyst was high enough for significant chemical deactivation [1]. Lietti [11] found that the deactivation was resulted from decreases in ammonia adsorption. Lisi [12] showed /$ - see front matter Ó 27 Elsevier B.V. All rights reserved. doi:1.116/j.catcom

2 X. Zhang et al. / Catalysis Communications 9 (28) that strong acid sites adsorbing ammonia at temperatures greater than 3 C were preferentially neutralized at K loadings less than.7 wt%, while medium and weak acid sites deactivated at higher alkali metals loadings. This information suggests that KCl may have profound negative effect on SCR activity of V 2 O 5 /AC catalyst, and the effect may be related to changes in surface acidity and in NH 3 adsorption. This paper investigates the effect KCl on a V 2 O 5 /AC catalyst for SCR of NO using BET, pore size distribution, NH 3 sorption, TPD (temperature-programmed desorption) and TPSR (temperature-programmed surface reaction) techniques. 2. Experimental 2.1. Catalyst preparation The activated carbon (AC) used was a commercial product from Shanxi Xihua Chem. Co. LTD, China. The AC was crushed into particles of.3.6 mm in diameter and subjected to proximate and ultimate analyses with accuracy of.1 wt% (Table 1). V 2 O 5 was supported to the AC by pore volume impregnation using an aqueous solution containing ammonium metavanadate and oxalic acid. The sample was then dried at C for 12 h and 11 C for 5 h, followed by calcination in Ar at C for 8 h and pre-oxidation in air at 2 C for 5 h. The catalyst used in this work contains 1 wt% V 2 O 5 and is termed V1/AC. KCl was loaded onto V1/AC also through impregnation, followed by drying at C for 12 h and at 11 C for 5 h. The KCl loaded catalysts are termed KxV1/AC, where x denotes weight percentage of KCl in the catalyst. It is important to note that although the state of KCl impregnated may be different from that of the solid KCl fine particulates; its effect on the catalyst is likely to be similar due to presence of water adsorbed on the catalyst from flue gases. Interaction of water with the solid KCl may result in a state of KCl similar to that formed in the impregnation Catalytic activity measurement SCR activity of the catalysts was measured in a fixed bed glass reactor of 15 mm in inner-diameter. The feed rates of NH 3 /Ar, NO/Ar and SO 2 /Ar were controlled by mass flow meters and the feed rates of Ar and air were controlled by Table 1 Proximate and Ultimate analyses of AC Proximate analyses (wt%) Ultimate analyses (wt%) M ad A ad V ad C ad H ad O ad N ad S t,ad M: moisture; A: ash; V: volatile matter; ad: air dry; t: total. rotameters. Water vapor was introduced by passing the Ar stream through a heated gas-wash bottle containing deionized water. The overall feed contains ppm NO, ppm NH 3, 3.4 vol% O 2, 2.6 vol% water vapor and balance Ar. The total flow was maintained at 3 ml/min all the time, corresponding to a space velocity of 5 h 1. The reaction temperature was varied between and 2 C. The concentration of NO and O 2 in the inlet and outlet of the reactor were measured on-line by a flue gas analyzer (KM96 Quintox, Kane International Limited). Conversion of NO, X NO, is defined by X NO ¼ C in C out % C in where C in and C out are NO concentrations in the inlet and outlet, respectively Adsorption and temperature-programmed desorption (TPD) of NH 3 To compare the number and strength of acid sites on V1/AC with or without KCl, adsorption of NH 3 and TPD were performed in a fixed bed glass reactor the same as in g catalyst was preheated to 3 C in a flow of Ar at ml/min for 1 h to eliminate water. The sample was then cooled to C and then exposed to a stream containing 1 ppm NH 3 and balance Ar. After the adsorption, the sample was purged with Ar for 3 min to eliminate physically adsorbed NH 3, and then heated to 8 C at a rate of 1 C/min in a flow of Ar at ml/ min. The outlet gas was monitored on-line by a mass spectrometer (Balzers QMS422) during the whole process. The release of NH 3 was monitored by tracing the signal of m/ e = Temperature-programmed surface reaction (TPSR) Reaction of ammonia pre-adsorbed on the catalysts (2.5 g, with or without KCl) with gaseous NO was studied using TPSR. The feed gas, at 3 ml/min, contains 7 ppm NO, 3.4 vol% O 2, 2.6 vol% water vapour and balance Ar. The temperature was ramped at a rate of 5 C/min from C to 2 C, maintained at 2 C for 1 min, and then ramped again from 2 C to 3 C. Concentrations of NO and O 2 in the outlet stream were measured simultaneously by the KM96 flue gas analyzer Nitrogen adsorption and desorption Nitrogen adsorption and desorption isotherms were measured at 77 K with a Micromeritics ASPA 2 to determine the textural properties of the catalyst. Surface area was derived from the BET model, total pore volume was derived from the amount of nitrogen adsorbed at a relative pressure close to unity, and average pore diameter was calculated from the total surface area and the pore volume assuming model.

3 844 X. Zhang et al. / Catalysis Communications 9 (28) Results and discussion 3.1. Textural properties It was reported that [1] a K/V molar ratio of.5 would cause significant deactivation to a V 2 O 5 /TiO 2 containing 3 wt% of V 2 O 5 (corresponding to1. 5 wt% K in the catalyst). In this work, therefore, K loadings of.1 2. wt% are used. The textural properties of the V1/AC catalysts with different KCl contents are shown in Table 2. As can be seen, KCl loadings of less than 1 wt% have little effect on pore structure of V1/AC catalyst, the surface areas are 73 ± 2 m 2 /g, the total pore volume are.527 ±.3 ml/g and the micro-pore volume are. ±.2 ml/g. For the catalyst with the highest potassium loading (2 wt% KCl), the surface area, total pore volume and micropore volume decrease to 661 m 2 /g,.5 ml/g and.384 ml/g respectively, suggesting blocking some of the pores of V1/AC Effect of KCl on V 2 O 5 /AC s SCR activity Fig. 1 shows effect of KCl loading on NO conversion, X NO, at different temperatures. In the absence of KCl, X NO increases from % to 8% when the SCR temperature is increased from C to2 C. An increase in KCl loading results in a decrease in X NO for all the temperatures. For catalysts containing more than.1 wt% K, X NO even decreases with an increase in temperature. It is important to note that in the temperature range of 2 Can increase in temperature promotes deactivation of the catalyst, while in the temperature range of 2 2 C an increase in temperature reduces the deactivation. Fig. 2 shows dependence of X NO and dx NO (minus difference in X NO due to addition of KCl) on KCl loading at 2 C. Apparently the relations are not linear and the most deactivation occurs at the lower KCl loadings. This behavior is understandable because at KCl loadings of higher than.8 wt% the catalyst is already deactivated to very low activities. Since the KCl loadings of less than 1 wt% do not cause significant changes in catalyst s physical properties (Table 2), the significant decrease in X NO may be caused by chemical interactions between KCl and the catalyst surface. It was reported that to a commercial V 2 O 5 /TiO 2 catalyst, potassium reduced the amount of acid sites on the surface and thus the catalyst s ability of NH 3 adsorption [12]. Since NH 3 adsorption is a key step in V 2 O 5 /AC catalyzed SCR reaction and the role of NO X NO V/AC K.1V1/AC K.5V1/AC K1.V1/AC K2.V1/AC Fig. 1. Temperature dependence of NO conversion for V1/AC catalysts with various KCl loadings. X NO X KCL Fig. 2. Effect of KCl loading on NO conversion over V1/AC catalyst at 2 C. adsorption is negligible [13], the deactivation of V1/AC by KCl is likely to be resulted from changes in acid sites Isothermal adsorption of NH 3 Amount of acid sites on a catalyst surface can be characterized by the amount of NH 3 adsorbed. Fig. 3 shows effluent NH 3 profiles measured by the MS during NH 3 adsorption on AC, V1/AC and K1.V1/AC. The AC has the lowest NH 3 adsorption capacity with a breakthrough time of only 2.5 min. V1/AC has the highest NH 3 adsorption capacity with a breakthrough time of 13.3 min, dx NO Table 2 Characteristics of V 2 O 5 /AC of different KCl loadings Sample BET surface area (m 2 /g) Total pore volume (ml/g) Micro pore volume (ml/g) V1/AC K.5V1/AC K1.V1/AC K2.V1/AC

4 X. Zhang et al. / Catalysis Communications 9 (28) MS Intensity (a.u.) Ar NH 3 /Ar Time (min) indicating adsorption of NH 3 on V 2 O 5 or on the AC surface modified by V 2 O 5. Compared to V1/AC, K1.V1/ AC shows a much smaller NH 3 adsorption capacity with a breakthrough time of 7.5 min. This may suggest that KCl interacts with V 2 O 5 and neutralizes its acid sites Desorption of NH 3 m/e=17 V1/AC K1.V1/AC Fig. 3. Effluent NH 3 profiles during isothermal adsorption of NH 3 at C on AC, K1.V1/AC and V1/AC. It is generally recognized that NH 3 adsorbed on V1/AC takes a number of forms [3]. To study distribution of NH 3 forms over the catalysts, TPD experiments were carried out to samples pre-adsorbed with NH 3 at C. Since only NH 3 is detected during the TPD, Fig. 4 shows only NH 3 profiles. The desorption signal of NH 3 is very weak for the AC (e) with no visible peaks. The NH 3 profile of V1/ AC (a) shows two large peaks at 175 and 68 C, and a small peak at 38 C. For the catalysts containing KCl, K.5V1/ AC (b), K1.V1/AC (c), and K2.V1/AC (d), the NH 3 profiles show a trend similar to that of V1/AC (a), indicating presence of similar acid sites in the catalysts. The effect of KCl is mainly on the peak sizes, especially to the peak at 175 C (from weak acid sites), which is close to disappear even at a KCl loading of.5 wt%. The NH 3 peaks at 68 and 38 C (from strong and medium acid sites) are also reduced by KCl. These results indicate that KCl neutralizes AC the acid sites on V1/AC for NH 3 adsorption especially the weak acid sites, other than the results reported by Khodayari [5] on commercial V 2 O 5 /TiO 2 catalyst Reaction of the adsorbed NH 3 and gaseous NO Temperature-programmed surface reaction (TPSR) experiments were carried out to compare reactions between NO and NH 3 adsorbed on catalysts with or without KCl, and the results are shown in Fig. 5. Behavior of V1/AC without pre-adsorbed NH 3 is also included to show the system lag at the beginning of TPSR, and reduction of NO by AC at temperatures higher than 23 C. Compared to that for V1/AC without NH 3 adsorption, the outlet NO concentrations for V1/AC adsorbed with NH 3 are much smaller at temperatures higher than 125 C, the outlet NO concentrations for K1.V1/AC adsorbed with NH 3, however, are slightly smaller in the same temperature range. These indicate that the NH 3 pre-adsorbed over V1/AC and K1V1/ AC reacts with gaseous NO at temperatures higher than 125 C, and the amount of NH 3 reacted over K1V1/AC is much less than that over V1/AC. This difference is consistent with the data in Fig. 3. During TPSR, no obvious release of NH 3 was found, indicating that the weakly adsorbed NH 3 may reacts with NO before desorption SCR reaction mechanism Since the stoichiometrical ratio of NO/NH 3 is one in the SCR reaction as indicated by reaction (1), the amounts of NH 3 consumed in TPSR can be determined from NO consumption curves in Fig. 5. The values determined are listed in Table 3 along with the amount of NH 3 adsorbed at C determined from Fig. 3. 4NO þ 4NH 3 þ O 2! 4N 2 þ 6H 2 O ð1þ Apparently, the AC adsorbs very small amounts of NH 3 and the adsorbed NH 3 is not very active for SCR of NO, resulting in only a 4.6% NH 3 conversion. In comparison, 8 3 Intensity (a.u.) m/e=17 e a b c d NO concentration (ppm) NO inlet 23 C fresh V1/AC NH 3 -K1.V1/AC NH 3 -V1/AC Fig. 4. NH 3 TPD profiles of V1/AC (a), K.5V1/AC (b), K1.V1/AC (c), K2.V1/AC (d) and AC (e) Time (min) Fig. 5. Temperature-programmed surface reaction between NO and NH 3 adsorbed on various catalysts.

5 846 X. Zhang et al. / Catalysis Communications 9 (28) Table 3 NH 3 adsorbed at C and reacted in TPSR in a temperature range of 2 C on various catalysts Sample NH 3 adsorbed (1 6 mol) NH 3 reacted in TPSR (1 6 mol) NH 3 remained (1 6 mol) Conversion of adsorbed NH 3 AC V1/AC K1.V1/AC V1/AC adsorbs much more NH 3, about 8 times as much as AC, and 57.1% of the adsorbed NH 3 is consumed in the TPSR. It is obvious that V 2 O 5 contributes most of the acid sites for NH 3 adsorption and SCR reaction. Presence of 1 wt% KCl (K1.V1/AC) reduces the amounts of NH 3 been adsorbed and consumed in the TPSR, yielding a NH 3 conversion of only 3.6%. It is important to note that the reductions in NH 3 by 1 wt% KCl is mol in TPSR and mol in adsorption, indicating that most of the acid sites inhibited by KCl are those capable of adsorbing NH 3 and active for SCR of NO. The data in Table 3 also show that not all the NH 3 adsorbed on the catalysts is capable to react with gaseous NO in a temperature range of 2 C. As indicated earlier, it is well recognized that NH 3 adsorption plays an important role in SCR of NO. To V 2 O 5 /TiO 2 based catalysts, the SCR reaction was proposed to occur between gaseous NO and NH 3 adsorbed on Brønsted acid sites [14] or NH 3 adsorbed on dual sites comprising a V OH surface group and an adjacent V@O group [15], and adsorption and partial oxidation of NH 3 on acid sites were believed to be key steps [16]. ToV 2 O 5 /AC catalyst, the data in Table 3 and Fig. 4 suggest that the weak acid sites are the sites of SCR reaction because neutralization of these sites by KCl results in deactivation in SCR activity. The data also indicates that the strong NH 3 adsorption sites on V1/AC are not involved in the SCR reaction. 4. Conclusions KCl deactivates activity of a low temperature V 2 O 5 /AC catalyst for SCR of NO. The deactivation is mainly caused by neutralization of weak acid sites on the catalyst surface. The weak acid sites are mainly on the V 2 O 5 phase and responsible for NH 3 adsorption and activation. Acknowledgement The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Nos and 2338), Chinese Academy of Sciences and Shanxi Natural Science Foundation (25113). References [1] H. Bosh, F. Janssen, Catal. Today 2 (1998) 369. [2] B.W.L. Jang, J.J. Spiver, M.C. kung, H.H. Kung, Energ. Fuels 11 (1997) 299. [3] Z. Zhu, Z. Liu, H. Niu, S. Liu, T. Hu, T. Liu, Y. Xie, J. Catal. 197 (21) 6. [4] Z. Huang, Z. Zhu, Z. Liu, Appl. Catal. B 39 (22) 361. [5] R. Khodayari, C.U.I. Odenbrand, Appl. Catal. B 3 (21) 87. [6] T.G. Brna, EPA//D-88/51, [7] T. Valmari, T.M. Lind, E.I. Kauppinen, Energ. Fuel 13 (1999) 39. [8] T. Valmari, T.M. Lind, E.I. Kauppinen, Energ. Fuel 13 (1999) 379. [9] F. Moradi, J. Brandin, M. Sohrabi, M. Faghihi, M. Sanati, Appl. Catal. B 46 (23) 65. [1] Y. Zheng, A.D. Jessen, J.E. Johnson, Appl. Catal. B (25) 253. [11] L. Lietti, P. Forzatti, G. Ramis, G. Busca, F. Bregani, Appl. Catal. B 3 (1993) 13. [12] L. Lisi, G. Lasorela, S. Malloggi, G. Russo, Appl. Catal. B (24) 251. [13] Z. Huang, Z. Zhu, Z. Liu, Q. Liu, J. Catal. 214 (23) 213. [14] M. Gaisor, J. Haber, T. Machej, T. Czeppe, J. Mol. Catal. 43 (1998) 359. [15] M. Inomata, A. Miyamoto, Y. Murakami, J. Catal. 43 (1998) 359. [16] U.S. Ozkan, Y.P. Cai, M.W. Kumthekar, J. Phys. Chem. 99 (1995) 2363.