- Email: [email protected]
A novel carbon-supported vanadium oxide catalyst for NO reduction with NH3 at low temperatures
Applied Catalysis B: Environmental 23 (1999) L229–L233
Letter
A novel carbon-supported vanadium oxide catalyst for NO reduction with NH3 at low temperatures Zhenping Zhu, Zhenyu Liu ∗ , Shoujun Liu, Hongxian Niu State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, PR China Received 9 March 1999; received in revised form 9 July 1999; accepted 9 July 1999
Abstract A novel activated carbon-supported vanadium oxide catalyst was studied for SCR of NO with NH3 at low temperatures (100 – 250◦ C). The effects of reaction temperature, preparation conditions and SO2 on SCR activity were evaluated. The results show that this catalyst has a high catalytic activity for NO–NH3 –O2 reaction at low temperatures. Preoxidation of the calcined catalyst helps improve catalytic activity. V2 O5 loading, other than calcination temperature, gives a significant influence on the activity. SO2 in the flue gas does not de-activate the catalyst but improves it. A stability test of more than 260 h shows that the catalyst is highly active and stable in the presence of SO2 . ©1999 Elsevier Science B.V. All rights reserved. Keywords: V2 O5 /AC catalyst; Preparation; Nitric oxide; Ammonia; Sulfur dioxide
1. Introduction A well-proven technique for NOx removal from stationary sources is the selective catalytic reduction (SCR) of NO with NH3 in the presence of oxygen [1,2]. For this process, V2 O5 /TiO2 or V2 O5 –WO3 /TiO2 catalyst [3] has been frequently used due to the high activity for NOx removal and low sensitivity towards SO2 poisoning. However, these catalysts suffer from many disadvantages [4]. Due to the high operating temperature (>350◦ C) needed, the SCR unit must be located at upstream of the de-sulfurizer and/or the particulate control device to utilize the high temperature of the flue gas, otherwise, heating of the flue gas may be necessary. Retrofit of a ∗ Corresponding author. Fax: +86-351-404-1153 E-mail address: [email protected] (Z. Liu)
SCR unit in the upstream of the sulfur and particulate removal device in an existing boiler system is costly because limited space and access in many power plants. Therefore, the development of new SCR catalysts which are active at temperatures much below 350◦ C but high enough for chimney ventilation is very important. Various catalysts have been found to have high SCR activities [5–11] in this temperature range. However, these catalysts are prone to de-activation by SO2 and hence restricted for practical application. Therefore, SO2 de-activation is a key problem in the development of low-temperature SCR catalysts. It was reported by Nishijima et al. [5] that SCR activity of copper salts depends on the nature of the support material, activated carbon results in higher catalytic activity than metal oxides do, such as silica and alumina, at low temperatures. Moreover, as is well established [1,3], vanadium oxide supported on titania
0926-3373/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 9 9 ) 0 0 0 8 5 - 5
L230
Z. Zhu et al. / Applied Catalysis B: Environmental 23 (1999) L229–L233
shows high activities and good tolerance to SO2 at temperatures higher than 350◦ C. For vanadium oxide catalysts, SO2 tolerance is also closely related to the property of support. For example, alumina can easily be deactivated by SO2 due to the formation of aluminum sulfate [1]. Considering these facts, activated carbon may serve as a good support for the vanadium catalyst because of possible high activities for SCR of NO at low temperatures and inability to react with SO2 . Little information can be found in this regard in the literature, although a similar catalyst of low activity was reported by Kasaoka et al. [12] for simultaneous removal of SOx and NOx at 130◦ C. In this paper, an activated carbon-supported vanadium oxide catalyst is studied for NO reduction with NH3 at 250◦ C and below. Effects of reaction temperature, catalyst preparation conditions and SO2 on SCR activity were presented.
in diameter. Four gas streams, 0.5% NO in Ar, 0.5% NH3 in Ar, 15% O2 in Ar and Ar, were used to formulate flue gas. Mass flow controllers were used to control the gas streams containing NO and NH3, and rotometers were used for O2 /Ar and Ar. In all the runs, the total gas flow rate was maintained at 300 ml/min. The feed gases were mixed in a chamber filled with glass wool before entering the reactor. For experiments invloving SO2 , a gas stream of 1600 ppm SO2 in Ar was used in place of Ar. To avoid the reaction between SO2 and NH3 occurring prior to the catalyst bed, in this case, NH3 /Ar was fed directly into the reactor bypassing the mixing chamber. The concentrations of NO, NO2 , SO2 and O2 both at the inlet and the outlet of the reactor were simultaneously monitored by an on-line Flue Gas Analyser (KM9006 Quintox, Kane International Limited) equipped with NO, NO2 , SO2 and O2 sensors.
2. Experimental
3. Results and discussion
2.1. Catalyst preparation
3.1. Effect of reaction temperature
The support, activated carbon (AC), was prepared from a commercial coal-derived semicoke (Datong Coal Gas Co., China) through steam activation at about 900◦ C. Before being used, the AC was oxidized with concentrated HNO3 (3 ml/gAC) at 60◦ C for 1 h, followed by filtration, washing with distilled water and drying at 120◦ C for 5 h. The resulting AC has a BET surface area of 560 m2 /g measured by N2 adsorption at 77 K. Vanadium oxide was supported on the AC by pore volume impregnation with an aqueous solution of ammonium metavanadate in oxalic acid, followed by overnight drying at 50◦ C and then at 120◦ C for 5 h, and by calcination in Ar for 8 h at desired temperatures. The V2 O5 loadings in the catalysts were determined by the concentration of the ammonium metavanadate used in the impregnation. Before use, the resulting catalysts were preoxidized in air at 250◦ C for 5 h unless a specific statement was given.
Fig. 1 shows NO conversion over AC (without oxidization with HNO3 ) and 5 wt.% V2 O5 /AC catalyst (calcined at 500◦ C) at reaction temperatures of 90–250◦ C. The NO conversion on the AC decreases
2.2. Activity test The SCR activities of the catalysts for NOx removal were carried out in a fixed-bed quartz reactor of 8 mm
Fig. 1. NO conversion vs. reaction temperature over 5 wt.% V2 O5 /AC and AC catalysts. Reaction conditions: 500 ppm NO, 560 ppm NH3 , 3.3% O2 , Ar balance; WHSV, 36 000 h−1 for 5 wt.% V2 O5 /AC, and 10 000 h−1 for AC.
Z. Zhu et al. / Applied Catalysis B: Environmental 23 (1999) L229–L233
L231
with increasing temperature. In the temperature range of greater than 120◦ C, the activities of the AC are very low, with NO conversions of less than 30%. In contrast, the activity of the V2 O5 /AC catalyst increases with increasing temperature, which results in a NO conversion of greater than 95% at 220◦ C. It is important to note that, in Fig. 1, the space velocity used for the V2 O5 /AC, 36 000 h−1 , is much greater than that for the AC, 10 000 h−1 . At the same space velocity, the superiority of V2 O5 /AC over AC should be much greater than that shown in Fig. 1. The different activity temperature patterns of the two catalysts in Fig. 1 suggest that the SCR reaction mechanism on V2 O5 /AC is different from that on AC. 3.2. Effect of catalyst preoxidization, calcination temperature and V2 O5 loading To obtain a highly active catalyst, the V2 O5 /AC catalysts were preoxidized in air at 250◦ C for 5 h. The effect of the preoxidation on catalytic activity was also studied. In the reaction conditions, as the same as that for 5 wt.% V2 O5 /AC catalyst in Fig. 1, the 500◦ C calcined 5 wt.% V2 O5 /AC catalyst without preoxidation shows relatively low initial activity, with a NO conversion of about 70%. With increasing reaction time its activity gradually increases and reaches a steady-state NO conversion of about 90% in about 15 h. This behavior was also observed for other V2 O5 /AC catalysts without preoxidation, especially for those calcined at temperatures above 500◦ C. The low initial activity of the non-preoxidized catalyst may result from the existence of some reduced vanadium species in the catalysts. It is possible that during catalyst calcination a portion of the vanadium species may be reduced by the AC, and that during the NO–NH3 –O2 reaction the reduced vanadium species are gradually oxidized by O2 (and/or NO) into active vanadium oxide and hence result in increased activity. Unlike the catalyst without preoxidation, the preoxidized catalyst shows a high and stable activity in the whole reaction period, with NO conversions of about 98% for the 5 wt.% V2 O5 /AC catalyst. This indicates that during catalyst preparation preoxidation is needed after calcination to improve catalytic activity. Therefore, all of the catalysts mentioned below are preoxidized in the way described above.
Fig. 2. The effect of V2 O5 loading on the activity of V2 O5 /AC calcined at 500◦ C. Reaction condition: 500 ppm NO, 560 ppm NH3 , 3.3% O2 , Ar balance; WHSV, 90 000 h−1 ; reaction temperature, 250◦ C.
The 5 wt.% V2 O5 /AC catalysts were prepared at different calcination temperatures, 300, 400, 450, 500, 550, 600◦ C. Their activity measurements showed that there is no obvious disparity within the studied range of 300–600◦ C. This result indicates that active vanadium oxide is easily formed through decomposition of the impregnated mixture of ammonium metavanadate and oxalic acid, even at temperatures as low as 300◦ C, and that sintering of active sites does not occur during calcination at higher temperatures as 600◦ C. Fig. 2 shows the dependence of catalytic activity on V2 O5 loading. With increasing V2 O5 loading from 0 to 5 wt.%, NO conversion increases from 15 to 80%, and maintains at about 80% in the V2 O5 loadings range of 5–13 wt.% and then drops to about 65% at V2 O5 loading of 17 wt.%. For the catalysts with lower V2 O5 loading such as 1 and 3 wt.%, their lower activities may be attributed to the lower coverage of vanadium on the AC surface. However, it is not easy to explain the nearly constant activity at V2 O5 loadings of 5–13 wt.% and the decreased activity of 17 wt.% V2 O5 /AC catalyst because XRD measurements of these catalysts show no diffraction signal of vanadium compounds, even for the 17 wt.% V2 O5 /AC catalyst, which suggests that the vanadium species in the catalysts are in small size and are highly dispersed on the AC surface. It may be associated with the complexity of the AC surface.
L232
Z. Zhu et al. / Applied Catalysis B: Environmental 23 (1999) L229–L233
Fig. 3. Effect of SO2 on the activity of V2 O5 /AC and AC catalysts. Reaction conditions: 500 ppm NO, 560 ppm NH3 , 3.3% O2 , Ar balance; WHSV, 90 000 h−1 ; reaction temperature, 250◦ C.
3.3. Effect of SO2 The effect of SO2 on SCR activity of the V2 O5 /AC catalyst is illustrated in Fig. 3 and compared with that of AC. When the feed stream contains no SO2, 1 wt.% V2 O5 /AC catalyst shows relatively low activity for NO–NH3 –O2 reaction, with NO conversion of about 58%. Upon addition of SO2 into the feed stream, the activity increases sharply and reaches a steady-state NO conversion of about 93% in about 4 h. Although the 5 wt.% V2 O5 /AC catalyst shows high activity without SO2 , the addition of SO2 in the feed does not increase its activity significantly, which results in a lower activity than that of the 1 wt.% V2 O5 /AC catalyst in the presence of SO2 . The activity of the AC support, however, is low and decreases quickly from 40% to about 12% in the first 4 h. Addition of SO2 into the feed stream only increases NO conversion by about 4% points but does not change the decreasing pattern of catalyst activity. These observations were found in every duplicate run and thus indicate that the V2 O5 /AC catalyst is significantly promoted by SO2 for the NO–NH3 –O2 reaction. During the experiments when SO2 was added into feed gas as shown in Fig. 3, outlet SO2 was also monitored continuously. It was found that for both 1 and 5 wt.% V2 O5 /AC catalysts the profile of outlet SO2 concentration is similar to that of NO conversion. That is, upon SO2 addition the outlet SO2 concentration
is initially very low but then gradually increases with time and reaches a constant value (nearly equal to the inlet value) in 4 h. This observation suggests that the promoting effect of SO2 may result from the adsorbed SO2 or the formed sulfate on the catalyst surface. The latter seems to be more favorable because SO2 is easy to oxidize on V2 O5 , which, as is well known, is a very active catalyst for SO2 oxidation. In fact, for the present catalyst, V2 O5 /AC, SO4 −2 was detected using FT-IR and XPS after the SCR reaction in the presence of SO2 , additionally, NH4 + was also observed by XPS. This suggests that the promoting effect of SO2 on the V2 O5 /AC catalyst may be due to the formation of SO4 −2 , which is formed from SO2 oxidation. As reported previously [13], a treatment of activated carbon by sulfate acid (or salt) greatly increases its ability for NH3 adsorption. In addition, Chen and Yang [14,15] reported that the addition of SO2 in reactants also increases the SCR activities for the V2 O5 /TiO2 and TiO2 catalysts at higher temperatures (>350◦ C) and showed that the promotion by SO2 is due to the formation of surface SO4 −2 which increases catalyst surface acidity. 3.4. A test for the stability of the catalyst To understand the stability of the V2 O5 /AC catalyst, in the presence of SO2 , the 1 wt.% V2 O5 /AC catalyst was tested in SCR reaction for more than 260 h under the conditions shown in Fig. 1 for the V2 O5 /AC catalyst, with the addition of 400 ppm SO2 . It showed high activity with NO conversion of more than 97% and showed no sign of de-activation. This excellent property of the V2 O5 /AC catalyst makes it a favorable candidate for flue gas NOx removal at low temperatures. It should be pointed out that after the test the catalyst shows a loss of only about 2% in weight, which suggests that the AC is stable although in the presence of V2 O5 , as a strong oxidation catalyst, at least under the employed reaction conditions and at the used catalyst composition. Acknowledgements The authors gratefully acknowledge the financial support from the Natural Science Foundation China, Shanxi Natural Science Foundation and State Key Laboratory of Coal Conversion.
Z. Zhu et al. / Applied Catalysis B: Environmental 23 (1999) L229–L233
References [1] H. Bosch, F. Janssen, Catal. Today 2 (1988) 369. [2] S. Morikawa, H. Yoshida, K. Takahashi, S. Kurita, Chem. Lett. (1981) 251. [3] M.D. Amiridis, I.E. Wachs, G. Deo, J.M. Jehng, D.S. Kim, J. Catal. 161 (1996) 247. [4] Y. Li, P.J. Battavio, J.N. Armor, J. Catal. 142 (1993) 561. [5] A. Nishijima, Y. Kiyozumi, A. Ueno, M. Kurita, H. Hagiwara, T. Sato, N. Todo, Bull. Chem. Soc. Jpn. 52 (1979) 3724. [6] N. Todo, A. Nishijima, A. Ueno, M. Kurita, H. Hagiwara, T. Sato, Y. Kiyozumi, Chem. Lett. (1976) 897. [7] F. Nozaki, K. Yamazaki, T. Inomata, Chem. Lett. (1977) 521.
L233
[8] L. Singoredjo, M. Slagt, J. Van Wees, F. Kapteijn, J.A. Moulijn, Catal. Today 7 (1990) 157. [9] L. Singoredjo, R. Korver, F. Kapteijn, J.A. Moulijn, Appl. Catal. B 1 (1992) 297. [10] D. Roberge, A. Raj, S. Kaliaguine, D.T. On, S. Iwamoto, T. Inui, Appl. Catal. B 10 (1996) L237. [11] H.E. Curry-Hyde, H. Musch, A. Baiker, Appl. Catal. 65 (1990) 211. [12] S. Kasaoka, E. Sasaoka, H. Iwasaki, Bull. Chem. Soc. Jpn. 62 (1989) 1226. [13] B.J. Ku, J.K. Lee, D. Park, H.K. Rhee, Ind. Eng. Chem. Res. 33 (1994) 2868. [14] J.P. Chen, R.T. Yang, J. Catal. 125 (1990) 411. [15] J.P. Chen, R.T. Yang, J. Catal. 139 (1993) 277.
Letter
A novel carbon-supported vanadium oxide catalyst for NO reduction with NH3 at low temperatures Zhenping Zhu, Zhenyu Liu ∗ , Shoujun Liu, Hongxian Niu State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, PR China Received 9 March 1999; received in revised form 9 July 1999; accepted 9 July 1999
Abstract A novel activated carbon-supported vanadium oxide catalyst was studied for SCR of NO with NH3 at low temperatures (100 – 250◦ C). The effects of reaction temperature, preparation conditions and SO2 on SCR activity were evaluated. The results show that this catalyst has a high catalytic activity for NO–NH3 –O2 reaction at low temperatures. Preoxidation of the calcined catalyst helps improve catalytic activity. V2 O5 loading, other than calcination temperature, gives a significant influence on the activity. SO2 in the flue gas does not de-activate the catalyst but improves it. A stability test of more than 260 h shows that the catalyst is highly active and stable in the presence of SO2 . ©1999 Elsevier Science B.V. All rights reserved. Keywords: V2 O5 /AC catalyst; Preparation; Nitric oxide; Ammonia; Sulfur dioxide
1. Introduction A well-proven technique for NOx removal from stationary sources is the selective catalytic reduction (SCR) of NO with NH3 in the presence of oxygen [1,2]. For this process, V2 O5 /TiO2 or V2 O5 –WO3 /TiO2 catalyst [3] has been frequently used due to the high activity for NOx removal and low sensitivity towards SO2 poisoning. However, these catalysts suffer from many disadvantages [4]. Due to the high operating temperature (>350◦ C) needed, the SCR unit must be located at upstream of the de-sulfurizer and/or the particulate control device to utilize the high temperature of the flue gas, otherwise, heating of the flue gas may be necessary. Retrofit of a ∗ Corresponding author. Fax: +86-351-404-1153 E-mail address: [email protected] (Z. Liu)
SCR unit in the upstream of the sulfur and particulate removal device in an existing boiler system is costly because limited space and access in many power plants. Therefore, the development of new SCR catalysts which are active at temperatures much below 350◦ C but high enough for chimney ventilation is very important. Various catalysts have been found to have high SCR activities [5–11] in this temperature range. However, these catalysts are prone to de-activation by SO2 and hence restricted for practical application. Therefore, SO2 de-activation is a key problem in the development of low-temperature SCR catalysts. It was reported by Nishijima et al. [5] that SCR activity of copper salts depends on the nature of the support material, activated carbon results in higher catalytic activity than metal oxides do, such as silica and alumina, at low temperatures. Moreover, as is well established [1,3], vanadium oxide supported on titania
0926-3373/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 9 9 ) 0 0 0 8 5 - 5
L230
Z. Zhu et al. / Applied Catalysis B: Environmental 23 (1999) L229–L233
shows high activities and good tolerance to SO2 at temperatures higher than 350◦ C. For vanadium oxide catalysts, SO2 tolerance is also closely related to the property of support. For example, alumina can easily be deactivated by SO2 due to the formation of aluminum sulfate [1]. Considering these facts, activated carbon may serve as a good support for the vanadium catalyst because of possible high activities for SCR of NO at low temperatures and inability to react with SO2 . Little information can be found in this regard in the literature, although a similar catalyst of low activity was reported by Kasaoka et al. [12] for simultaneous removal of SOx and NOx at 130◦ C. In this paper, an activated carbon-supported vanadium oxide catalyst is studied for NO reduction with NH3 at 250◦ C and below. Effects of reaction temperature, catalyst preparation conditions and SO2 on SCR activity were presented.
in diameter. Four gas streams, 0.5% NO in Ar, 0.5% NH3 in Ar, 15% O2 in Ar and Ar, were used to formulate flue gas. Mass flow controllers were used to control the gas streams containing NO and NH3, and rotometers were used for O2 /Ar and Ar. In all the runs, the total gas flow rate was maintained at 300 ml/min. The feed gases were mixed in a chamber filled with glass wool before entering the reactor. For experiments invloving SO2 , a gas stream of 1600 ppm SO2 in Ar was used in place of Ar. To avoid the reaction between SO2 and NH3 occurring prior to the catalyst bed, in this case, NH3 /Ar was fed directly into the reactor bypassing the mixing chamber. The concentrations of NO, NO2 , SO2 and O2 both at the inlet and the outlet of the reactor were simultaneously monitored by an on-line Flue Gas Analyser (KM9006 Quintox, Kane International Limited) equipped with NO, NO2 , SO2 and O2 sensors.
2. Experimental
3. Results and discussion
2.1. Catalyst preparation
3.1. Effect of reaction temperature
The support, activated carbon (AC), was prepared from a commercial coal-derived semicoke (Datong Coal Gas Co., China) through steam activation at about 900◦ C. Before being used, the AC was oxidized with concentrated HNO3 (3 ml/gAC) at 60◦ C for 1 h, followed by filtration, washing with distilled water and drying at 120◦ C for 5 h. The resulting AC has a BET surface area of 560 m2 /g measured by N2 adsorption at 77 K. Vanadium oxide was supported on the AC by pore volume impregnation with an aqueous solution of ammonium metavanadate in oxalic acid, followed by overnight drying at 50◦ C and then at 120◦ C for 5 h, and by calcination in Ar for 8 h at desired temperatures. The V2 O5 loadings in the catalysts were determined by the concentration of the ammonium metavanadate used in the impregnation. Before use, the resulting catalysts were preoxidized in air at 250◦ C for 5 h unless a specific statement was given.
Fig. 1 shows NO conversion over AC (without oxidization with HNO3 ) and 5 wt.% V2 O5 /AC catalyst (calcined at 500◦ C) at reaction temperatures of 90–250◦ C. The NO conversion on the AC decreases
2.2. Activity test The SCR activities of the catalysts for NOx removal were carried out in a fixed-bed quartz reactor of 8 mm
Fig. 1. NO conversion vs. reaction temperature over 5 wt.% V2 O5 /AC and AC catalysts. Reaction conditions: 500 ppm NO, 560 ppm NH3 , 3.3% O2 , Ar balance; WHSV, 36 000 h−1 for 5 wt.% V2 O5 /AC, and 10 000 h−1 for AC.
Z. Zhu et al. / Applied Catalysis B: Environmental 23 (1999) L229–L233
L231
with increasing temperature. In the temperature range of greater than 120◦ C, the activities of the AC are very low, with NO conversions of less than 30%. In contrast, the activity of the V2 O5 /AC catalyst increases with increasing temperature, which results in a NO conversion of greater than 95% at 220◦ C. It is important to note that, in Fig. 1, the space velocity used for the V2 O5 /AC, 36 000 h−1 , is much greater than that for the AC, 10 000 h−1 . At the same space velocity, the superiority of V2 O5 /AC over AC should be much greater than that shown in Fig. 1. The different activity temperature patterns of the two catalysts in Fig. 1 suggest that the SCR reaction mechanism on V2 O5 /AC is different from that on AC. 3.2. Effect of catalyst preoxidization, calcination temperature and V2 O5 loading To obtain a highly active catalyst, the V2 O5 /AC catalysts were preoxidized in air at 250◦ C for 5 h. The effect of the preoxidation on catalytic activity was also studied. In the reaction conditions, as the same as that for 5 wt.% V2 O5 /AC catalyst in Fig. 1, the 500◦ C calcined 5 wt.% V2 O5 /AC catalyst without preoxidation shows relatively low initial activity, with a NO conversion of about 70%. With increasing reaction time its activity gradually increases and reaches a steady-state NO conversion of about 90% in about 15 h. This behavior was also observed for other V2 O5 /AC catalysts without preoxidation, especially for those calcined at temperatures above 500◦ C. The low initial activity of the non-preoxidized catalyst may result from the existence of some reduced vanadium species in the catalysts. It is possible that during catalyst calcination a portion of the vanadium species may be reduced by the AC, and that during the NO–NH3 –O2 reaction the reduced vanadium species are gradually oxidized by O2 (and/or NO) into active vanadium oxide and hence result in increased activity. Unlike the catalyst without preoxidation, the preoxidized catalyst shows a high and stable activity in the whole reaction period, with NO conversions of about 98% for the 5 wt.% V2 O5 /AC catalyst. This indicates that during catalyst preparation preoxidation is needed after calcination to improve catalytic activity. Therefore, all of the catalysts mentioned below are preoxidized in the way described above.
Fig. 2. The effect of V2 O5 loading on the activity of V2 O5 /AC calcined at 500◦ C. Reaction condition: 500 ppm NO, 560 ppm NH3 , 3.3% O2 , Ar balance; WHSV, 90 000 h−1 ; reaction temperature, 250◦ C.
The 5 wt.% V2 O5 /AC catalysts were prepared at different calcination temperatures, 300, 400, 450, 500, 550, 600◦ C. Their activity measurements showed that there is no obvious disparity within the studied range of 300–600◦ C. This result indicates that active vanadium oxide is easily formed through decomposition of the impregnated mixture of ammonium metavanadate and oxalic acid, even at temperatures as low as 300◦ C, and that sintering of active sites does not occur during calcination at higher temperatures as 600◦ C. Fig. 2 shows the dependence of catalytic activity on V2 O5 loading. With increasing V2 O5 loading from 0 to 5 wt.%, NO conversion increases from 15 to 80%, and maintains at about 80% in the V2 O5 loadings range of 5–13 wt.% and then drops to about 65% at V2 O5 loading of 17 wt.%. For the catalysts with lower V2 O5 loading such as 1 and 3 wt.%, their lower activities may be attributed to the lower coverage of vanadium on the AC surface. However, it is not easy to explain the nearly constant activity at V2 O5 loadings of 5–13 wt.% and the decreased activity of 17 wt.% V2 O5 /AC catalyst because XRD measurements of these catalysts show no diffraction signal of vanadium compounds, even for the 17 wt.% V2 O5 /AC catalyst, which suggests that the vanadium species in the catalysts are in small size and are highly dispersed on the AC surface. It may be associated with the complexity of the AC surface.
L232
Z. Zhu et al. / Applied Catalysis B: Environmental 23 (1999) L229–L233
Fig. 3. Effect of SO2 on the activity of V2 O5 /AC and AC catalysts. Reaction conditions: 500 ppm NO, 560 ppm NH3 , 3.3% O2 , Ar balance; WHSV, 90 000 h−1 ; reaction temperature, 250◦ C.
3.3. Effect of SO2 The effect of SO2 on SCR activity of the V2 O5 /AC catalyst is illustrated in Fig. 3 and compared with that of AC. When the feed stream contains no SO2, 1 wt.% V2 O5 /AC catalyst shows relatively low activity for NO–NH3 –O2 reaction, with NO conversion of about 58%. Upon addition of SO2 into the feed stream, the activity increases sharply and reaches a steady-state NO conversion of about 93% in about 4 h. Although the 5 wt.% V2 O5 /AC catalyst shows high activity without SO2 , the addition of SO2 in the feed does not increase its activity significantly, which results in a lower activity than that of the 1 wt.% V2 O5 /AC catalyst in the presence of SO2 . The activity of the AC support, however, is low and decreases quickly from 40% to about 12% in the first 4 h. Addition of SO2 into the feed stream only increases NO conversion by about 4% points but does not change the decreasing pattern of catalyst activity. These observations were found in every duplicate run and thus indicate that the V2 O5 /AC catalyst is significantly promoted by SO2 for the NO–NH3 –O2 reaction. During the experiments when SO2 was added into feed gas as shown in Fig. 3, outlet SO2 was also monitored continuously. It was found that for both 1 and 5 wt.% V2 O5 /AC catalysts the profile of outlet SO2 concentration is similar to that of NO conversion. That is, upon SO2 addition the outlet SO2 concentration
is initially very low but then gradually increases with time and reaches a constant value (nearly equal to the inlet value) in 4 h. This observation suggests that the promoting effect of SO2 may result from the adsorbed SO2 or the formed sulfate on the catalyst surface. The latter seems to be more favorable because SO2 is easy to oxidize on V2 O5 , which, as is well known, is a very active catalyst for SO2 oxidation. In fact, for the present catalyst, V2 O5 /AC, SO4 −2 was detected using FT-IR and XPS after the SCR reaction in the presence of SO2 , additionally, NH4 + was also observed by XPS. This suggests that the promoting effect of SO2 on the V2 O5 /AC catalyst may be due to the formation of SO4 −2 , which is formed from SO2 oxidation. As reported previously [13], a treatment of activated carbon by sulfate acid (or salt) greatly increases its ability for NH3 adsorption. In addition, Chen and Yang [14,15] reported that the addition of SO2 in reactants also increases the SCR activities for the V2 O5 /TiO2 and TiO2 catalysts at higher temperatures (>350◦ C) and showed that the promotion by SO2 is due to the formation of surface SO4 −2 which increases catalyst surface acidity. 3.4. A test for the stability of the catalyst To understand the stability of the V2 O5 /AC catalyst, in the presence of SO2 , the 1 wt.% V2 O5 /AC catalyst was tested in SCR reaction for more than 260 h under the conditions shown in Fig. 1 for the V2 O5 /AC catalyst, with the addition of 400 ppm SO2 . It showed high activity with NO conversion of more than 97% and showed no sign of de-activation. This excellent property of the V2 O5 /AC catalyst makes it a favorable candidate for flue gas NOx removal at low temperatures. It should be pointed out that after the test the catalyst shows a loss of only about 2% in weight, which suggests that the AC is stable although in the presence of V2 O5 , as a strong oxidation catalyst, at least under the employed reaction conditions and at the used catalyst composition. Acknowledgements The authors gratefully acknowledge the financial support from the Natural Science Foundation China, Shanxi Natural Science Foundation and State Key Laboratory of Coal Conversion.
Z. Zhu et al. / Applied Catalysis B: Environmental 23 (1999) L229–L233
References [1] H. Bosch, F. Janssen, Catal. Today 2 (1988) 369. [2] S. Morikawa, H. Yoshida, K. Takahashi, S. Kurita, Chem. Lett. (1981) 251. [3] M.D. Amiridis, I.E. Wachs, G. Deo, J.M. Jehng, D.S. Kim, J. Catal. 161 (1996) 247. [4] Y. Li, P.J. Battavio, J.N. Armor, J. Catal. 142 (1993) 561. [5] A. Nishijima, Y. Kiyozumi, A. Ueno, M. Kurita, H. Hagiwara, T. Sato, N. Todo, Bull. Chem. Soc. Jpn. 52 (1979) 3724. [6] N. Todo, A. Nishijima, A. Ueno, M. Kurita, H. Hagiwara, T. Sato, Y. Kiyozumi, Chem. Lett. (1976) 897. [7] F. Nozaki, K. Yamazaki, T. Inomata, Chem. Lett. (1977) 521.
L233
[8] L. Singoredjo, M. Slagt, J. Van Wees, F. Kapteijn, J.A. Moulijn, Catal. Today 7 (1990) 157. [9] L. Singoredjo, R. Korver, F. Kapteijn, J.A. Moulijn, Appl. Catal. B 1 (1992) 297. [10] D. Roberge, A. Raj, S. Kaliaguine, D.T. On, S. Iwamoto, T. Inui, Appl. Catal. B 10 (1996) L237. [11] H.E. Curry-Hyde, H. Musch, A. Baiker, Appl. Catal. 65 (1990) 211. [12] S. Kasaoka, E. Sasaoka, H. Iwasaki, Bull. Chem. Soc. Jpn. 62 (1989) 1226. [13] B.J. Ku, J.K. Lee, D. Park, H.K. Rhee, Ind. Eng. Chem. Res. 33 (1994) 2868. [14] J.P. Chen, R.T. Yang, J. Catal. 125 (1990) 411. [15] J.P. Chen, R.T. Yang, J. Catal. 139 (1993) 277.