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KINETICS, CATALYSIS, AND REACTION ENGINEERING Effect of SO2 on the Low-Temperature Selective Catalytic Reduction of Nitric Oxide with Ammonia over TiO2, ZrO2, and Al2O3 Fuyu Notoya, Caili Su, and Eiji Sasaoka* Faculty of Environmental Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan
Shigeru Nojima Hiroshima R &D Center, Mitsubishi Heavy Industries Ltd, 4-6-22 Kannonshin-machi, Nishi-ku Hiroshima-shi 733-8553, Japan
The selective catalytic reduction (SCR) of NO to N2 by NH3 over TiO2, ZrO2, and Al2O3 was studied at 90 °C using a feed stream that was simulated to represent the effluent from a coal combustion boiler. The effects of SO2 on the reduction of NO were studied. It is interesting that the SCR reaction was interrupted when SO2 was absent from the feed gas. This indicated that SO2 participated in the reduction process of NO at low temperature. A mechanism for this lowtemperature SCR process was proposed, which was based on the reaction results and studies of NO, NH3, and SO2 temperature-programmed desorption (TPD) profiles under different experimental conditions. Among the three catalysts, ZrO2 and TiO2 had higher activities than Al2O3. The selectity to N2 over the different catalysts changed in the sequence of ZrO2 > TiO2 > Al2O3. The activities of these three catalysts decayed with reaction time as a result of the accumulation of ammonium sulfate and nitrate salts plugged in the pores of the catalysts. However, the deactivated catalysts could be easily regenerated by washing with water. Introduction The selective catalytic reduction (SCR) process for NOx reduction with ammonia has already been applied for flue gases from power plants in which natural gas, oil, and coal were used as fuels. The active component of the commercial catalyst is V2O5, and TiO2 is commonly used as a support material for the catalyst.1-6 The chemistry of the process involves the following reactions:7
4NH3 + 4NO + O2 f 4N2 + 6H2O 2NH3 + NO2 + NO f 2N2 + 3H2O The conventional SCR process is usually set just after a boiler because it requires temperature above 300 °C. The feed ratio of NH3 to NO is controlled at 1.0 to prevent the leakage of NH3, as NH3 has some negative effects on the apparatus of the preheaters and electrical dust precipitator. If the SCR process can be operated at a temperature lower than 100 °C, it can be set behind the electrical dust precipitator and just before a wettype desulfurization process. In this case, the leakage of NH3 can be used as a source of reagent for the * To whom correspondence should be addressed. Telephone: 086-251-8900. Fax: 086-251-8900. E-mail: sasaokae@ cc.okayama-u.ac.jp.
desulfurization process. Robie et al.8 reported that a lowtemperature SCR process is economical for power plants. Furthermore, a low-temperature SCR process can be applied not only to flue gas from an incinerator but also to the exhause gas from an automobile. Previously, the low-temperature SCR of NO has been reported,9-13 but the temperature was still above 100 °C. The SCR of NO by NH3 at a temperature lower than 100 °C has been rarely studied.14-16 The purpose of this study is to investigate the effect of SO2, one type of coexisting gas in the practical processes, on the reduction of NO with NH3 at low temperature (90 °C). The activity, selectivity, and desorption of different reactants over ZrO2, TiO2, and Al2O3 were compared. A mechanism for the reaction at low temperature is proposed on the basis of the experimental results and compared with the mechanism proposed for the higher-temperature process.17 Experimental Section Preparation of Catalysts. TiO2, ZrO2, and Al2O3 catalysts were prepared by a precipitation method using Ti(SO4)2, ZrOCl2, and Al(NO3)3 solutions (ZrO2, 10 wt %; TiO2 and Al2O3, 20 wt %) and NH3 aqueous solution (7 wt % for ZrO2 and 14 wt % for TiO2 and Al2O3). The total amount of NH3 added was 10% excess of the theoretical amount required for precipitation. The precipitation was carried out by adding the raw salt
10.1021/ie000972f CCC: $20.00 © 2001 American Chemical Society Published on Web 07/24/2001
Ind. Eng. Chem. Res., Vol. 40, No. 17, 2001 3733 Table 1. Bulk Densities, Specific Surface Areas, Average Activities, and Selectivities of the Catalysts average amount bulk BET average amount of N2 formeda density surface of NO removeda catalyst (g/mL) area (m2/g) (mol m-2 h-1) × 107 (mol m-2 h-1) × 107 TiO2 Al2O3 ZrO2
0.67 0.72 1.01
83.5 279.4 75.8
7.6 2.5 7.4
4.6 1.2 5.7
a Reaction conditions: inlet gas 250 ppm NO, 500 ppm NH , 3 800 ppm SO2, 5% O2 4.4% H2O, 14% CO2, balance N2; reaction -1 temperature 90 °C; SV 12 000 h ; reaction time 20 h.
solution to the NH3 aqueous solution with vigorous stirring at room temperature. The product of the precipitation was filtered, washed, dried at 110 °C for 25 h, and then calcined in air at 500 °C for 1 h. The last product obtained was crushed and sieved to 1.0 mm. The BET surface areas and bulk densities of the samples are listed in Table 1. Apparatus and Procedure. The SCR process was carried out using a flow-type packed-bed tubular reactor system under atmospheric pressure at 90 °C. The reactor consisted of a quartz tube of 1.5 cm i.d. in which 1.0 mL of catalyst was packed. In these experiments, a mixture of NO (250 ppm), SO2 (800 ppm), NH3 (500 ppm), O2 (5%), H2O (10 or 4.4%), CO2 (10%), and N2 (or He was used as the balance gas to analyze the formation of N2 in the products) was fed into the reactor at 200 cm3 STP/min. The composition of the inlet gas used in this work was approximately the same as the composition of the effluent gas from a coal combustor. The inlet and outlet NO concentrations were measured using a chemiluminescent NO analyzer (Yanako Corporation, model ES-7) after NO2 was converted to NO. The conversion of NO2 to NO was carried out using a KI solution containing H2SO4 (aq).18 The amount of NO2 can be calculated by comparing the concentrations of NO in the outlet gas before and after the conversion of NO2. The fractional removal of NOx can be expressed as
fractional removal of NOx ) ([NOx]in - [NOx]out)/[NOx]in The possible products related to the conversion of NO in this low-temprature SCR process were N2, NH4NO3, and NO2 and a very small amount of N2O, which could be neglected. The formation of N2 was confirmed when He was used as the balance gas and the conversion of NOx to N2 was analyzed using a GC (Yanako G3800) equipped with a thermal conductivity detector and a column of molecular sieve 5A. The conversion of NOx to N2 was calculated as
conversion of NOx to N2 (%) )
[N2]formed [NOx]in
× 100%
Unfortunately, the selectivity of N2 could not always be measured online because of the limit of utilization of He as the balance gas for a long time. Therefore, an average selectivity of N2 during the reaction time (20 h) was calculated as
average selectivity of N2 (%) ) [NO]decreased - [NO3-]formed - [NO2]formed [NO]decreased
× 100%
Figure 1. Catalytic activities of ZrO2, TiO2, and Al2O3 for NO reduction. Inlet gas 250 ppm NO, 500 ppm NH3, 800 ppm SO2, 5% O2, 4.4% H2O, 14% CO2, balance N2; reaction temperature 90 °C; SV 12 000 h-1.
The salts that accumulated over the catalysts during the experiments were extracted with hot water (ca. 80 °C) until no more SO42- and NO3- were detected by an ion chromatograph. The total amounts of SO42- and NO3- were measured using the ion chromatograph. The regeneration of the catalysts was carried out by washing with 50 mL of water 5 times at 90 °C. The washed samples were dried at the same temperature. TPD Procedure. The temperature-programmed desorption (TPD) profiles of adsorbed gases from the catalysts were examined using a temperature-programmed desorption apparatus equipped with a quadrupole mass spectrometer. In the TPD experiments, 0.1 g of the catalyst was packed in the reactor. The gas or the mixture of gases intended to be adsorbed was fed into the reactor at a rate of 30 cm3 STP/min at 90 °C for 2 h. Then, a flow of He was fed at 20 cm3 STP/min until the baseline of the mass spectrometer was stable. The sample was heated to the expected temperature at a rate of 3 °C/min when the temperature was below 200 °C but at a rate of 10 °C/min when the temperature was above 200 °C. Results and Discussion Activity of the Catalysts and Selectivity to N2. The fractional removals of NOx (within 20 h) and the average selectivities to N2 during the reaction time over ZrO2, TiO2, and Al2O3 are shown in Figures 1 and 2. Figure 1 shows that the fractional removal of NO over the three catalysts (same volume) varied in the sequence ZrO2 > Al2O3 > TiO2 at 90 °C. Considering the different bulk densities of the samples, we calculated the average amounts of NO removal and the selectivities of N2 over a unit surface area of the catalysts during the reaction time (Table 1). The results show that ZrO2 had a higher selectivity to N2 than did TiO2 and Al2O3. Because the reaction was carried out at very low temperature (90 °C), it must be confirmed that the decrease of NO was not caused by the vapor-phase reaction or the adsorption and deposition of NO on the surface of the catalysts. If the removal of NO was due to the vapor reaction, the concentration of NO in the outlet gas, in the case without catalysts, would change with the concentration of NH3, O2, or SO2. However, no such change was observed in our experiments. Additional evidence to rule out the vapor-phase reaction
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Figure 2. Average selectivities to N2 during the reaction time of 20 h over ZrO2, TiO2, and Al2O3. Inlet gas 250 ppm NO, 500 ppm NH3, 800 ppm SO2, 5% O2, 4.4 or 10% H2O, 14% CO2, balance N2; reaction temperature 90 °C; SV 12 000 h-1; reaction time 20 h.
is that the fractional removal of NO changed greatly over different catalysts. For example, a very small amount of NO was converted over SiO2 (400 °C) were probably caused by the raw material of Al2O3. In contrast, for TiO2, it is clear that there were two kinds of surface species of NOad, which desorbed at about 150 and 250 °C. These results indicate that NO adsorbed molecularly on the catalysts or formed species that mainly decomposed to NO by heat; only a small desorption peak of NO2 (m/z 46) was observed. NH3 TPD profiles are shown in Figure 11. In addition to the main peaks at about 100 °C, the desorption of NH3 from all three catalysts extended over a wide temperature range, but no other peaks were observed during NH3 desorption. Usually, the oxidation of NH3 occurs in the higher-temperature SCR process, but in our experiments at 90 °C, the oxidation of NH3 was not observed. Comparing the desorption profiles of NO and NH3 from the different catalysts, it is obvious that NO desorbed at higher temperatures than NH3 over all three catalysts. This indicated that NO had stronger interactions with the catalysts than NH3. It has been proposed that the mechanism of the SCR process of NO by NH3 follows an Eley-Rideal mechanism,17 which means that adsorbed NH3 reacts with gaseous NO. However, in this low-temperature process, both NO and NH3 probably adsorbed and then reacted with each other via a Langmuir-Hinshelwood mechanism. Judging from this proposal, the higher activities of ZrO 2 and TiO2 than Al2O3 might be due to their acidic-basic properties, which can activate both NH3 and NO, but not to what has been proposed for the high-temperature process that NH3 is adsorbed and activated over acidic sites and then reacts with gaseous NO. Figure 12 shows the SO2 and O2 TPD profiles from samples that were pretreated with SO2, CO2, O2, H2O, and N2 at 90 °C for 2 h. The desorption temperature of SO2 from the catalysts varied from high to low in the sequence ZrO2 > TiO2 > Al2O3. It is known that Al2O3 is a typical acidic metal oxide, so the adsorption of SO2 on Al2O3 was weaker than that on the acidic-basic oxides, ZrO2 and TiO2. No desorptions of other species were observed, except for a very small amount of oxygen desorbed. We have deduced from the results in Figure 5 that SO2 participated in the reaction without adsorption, so that, although SO2 had a stronger adsorption on ZrO2 than on TiO2 and Al2O3, this might not be the reason that ZrO2 had a higher activity than Al2O3.
Ind. Eng. Chem. Res., Vol. 40, No. 17, 2001 3737
Figure 13. TPD profiles of SO2, NOx, and N2O from a used catalyst (ZrO2). Constituents of the pre-purged gas: 250 ppm NO, 500 ppm NH3, 800 ppm SO2, 5% O2, 4.4% H2O, 14% CO2, balance N2. Reaction temperature 90 °C; SV 12 000 h-1.
Figure 12. TPD profiles of SO2 from ZrO2, TiO2, and Al2O3. Constituents of the pre-purged gas: 800 ppm SO2, 5% O2, 10% H2O, 14% CO2, balance N2.
TPD Profiles of SO2 and NOx from the Catalysts Treated with NO-SO2-NH3. To identify the adsorbed species on the catalyst surface at low temperature, TPD profiles of SO2 and NOx from the catalysts pretreated with a NO-SO2-NH3 coexisting feed gas were performed in a temperature range of 30-200 °C. The catalysts for the TPD experiments were treated with a system of 250 ppm NO, 800 ppm SO2, 500 ppm NH3, 5% O2, 10% CO2, 4.4% H2O, and the balance N2. The temperature of the treatment was 90 °C for 10 min, followed by cooling to 40 °C to prevent decomposition of surface species. As a typical result of the TPD study, TPD profiles of the used ZrO2 catalyst are shown in Figure 13. The desorptions of NO, N2O, and NO2 and N2 were observed above ca. 140 °C, and SO2 desorption peaks were observed at about 90 °C. No desorption peak of NOx and N2 was observed under ca. 140 °C. The peaks of NOx above ca. 140 °C might be due to decomposition of NH4NO3, which has been considered as one of the intermediates in the higher-temperature SCR process.17 However, it can be speculated that the intermediate species for the reduction of NO to N2 at low temperature (90 °C) might be other very unstable species that had already decomposed during the cooling process. Decomposition of NH4NO3 and NH4NO2 over the Metal Oxides. Based on the idea that NH4NO3 was an intermediate in the high-temperature SCR process of NO by NH3,17 we tried to determine whether NH4NO3 was still an intermediate at low temperature. TiO2supported NH4NO3 (24.4 wt % NH4NO3) was prepared by the impregnation method, and its TPD spectra were studied. Furthermore, another possible intermediate, NH4NO2, was also supported over TiO2 (20.4 wt % NH4NO2), and its TPD profiles were compared with those of NH4NO3/TiO2. NH4NO2 was prepared using Ba(NO2)2 (aq) and (NH4)2SO4. As shown in Figure 14, the beginning temperatures of the desorptions of N2O and NO
Figure 14. Decomposition of NH4NO3 and NH4NO2 over TiO2.
from NH4NO3/TiO2 were coincident with those of the TPD profiles in Figure 13. However, NH4NO2 was mainly decomposed to N2 and NO at temperatures below 100 °C (Figure 14). This suggests that the reduction of NO to N2 probably occurs via an intermediate like NH4NO2. To confirm the catalytic activity of the three oxides for the decomposition of NH4NO2, TPD spectra of metal-oxide-supported NH4NO2 and nonsupported NH4NO2 were compared, as shown in Figure 15. It was confirmed that NH4NO2 could be catalytically decomposed to N2 and NO over all three metal oxides; although the selectivities to N2 and NO were unknown, it was obvious that N2 was one of the main product from NH4NO2 decomposition. Mechanistic Consideration. To clarify the mechanism of NO reduction with NH3 at low temperature, many more studies will be needed, especially in situ
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NOad + O2ad ) [NO‚‚‚O2]ad
(1)
[NO‚‚‚O2]ad + SO2 ) SO3ad + NO2ad
(2)
NOad + NO2ad ) N2O3ad
(3)
N2O3ad + H2O ) 2H+ + 2NO2-
(4)
H+ad + NH3 ) NH4+ad
(5)
NH4+ad + NO2-ad f NH4NO2ad f N2 + 2H2O
(6)
2NH4NO2ad f 2NO + N2O + H2O + 3H2
(7)
Judging from the above mechanism, when either SO2 or NH3 is absent from the reaction feed stream, the reduction of NO should be interrupted, as observed in the experiments reported here. Conclusion The presence of SO2 is indispensable to the SCR of NO at low temperature (90 °C). SO2 is assumed to contribute to the production of active surface oxygen, which is significant for the reduction of NO by NH3. The mechanism of low-temperature reduction of NO to N2 is proposed to occur through an intermediate of NH4NO2, which can be decomposed into N2 and H2O below 100 °C. Acknowledgment We gratefully acknowledge that this work was supported by the Ministry of Education, Science and Culture, Japan, through the Grant in Aid for Research 1058093. Figure 15. Comparison of the thermal decomposition (without catalyst) and catalytic decomposition of NH4NO2.
Literature Cited
studies. Here, a mechanism can be proposed according to the reaction data and TPD profiles. Two mechanisms can be considered. One of them is the mechanism that has already been reported17 for the higher-temperature SCR process: NO combines with surface NH4+ and then produces N2 in the presence of surface oxygen. In this case, active surface oxygen plays an important role in the reaction. For the lowertemperature process, because the results indicate that the presence of SO2 is critical, it is probable that SO2 contributes to the production of active surface oxygen and the the active surface oxygen interacts with surface NOad and formed NO2ad. Then, NO2ad could interact with NH4+ad and formed NH4NO2ad, which would decompose to form N2 and H2O (eq 6), or NO, N2O, H2O, and H2 (eq 7). Oxygen plays a significant role in the reaction. We have confirmed using ZrO2 as the catalyst that, if oxygen is absent from the reactants, the reaction does not occur. Further research is currently being performed in our laboratory on this point. SO3ad formed in the reaction could interact with NH4+ad in the presence of O2 and formed NH4HSO4 or (NH4)2SO4. NH4+ ad could interact with NO3- to form NH4NO3. These salts would plug the pores of the catalysts and cause the deactivation of the catalysts. The supposed mechanism for the SCR of NO at low temperature can be expressed as follows:
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Ind. Eng. Chem. Res., Vol. 40, No. 17, 2001 3739 (11) Muniz, J.; Marban, G.; Fuertes, A. B. Low-Temperature Selective Catalytic Reduction of NO over Modified Activated Carbon Fibres. Appl. Catal. B: Environ. 2000, 27, 27. (12) Broer, S.; Hammer, T. Selectivity Catalytic Reduction of Nitrogen Oxides by Combining a Nonthermal Plasma and a V2O5WO3/TiO2 Catalyst. Appl. Catal. B: Environ. 2000, 28, 101. (13) Muniz, J.; Marban, G; Fuertes, A. B. Low-Temperature Selective Catalytic Reduction of NO over Polyarylamide-based Carbon Fibres. Appl. Catal. B: Environ. 1999, 23, 25. (14) Chen, J. P.; Yang, R. T.; Buzanowski, M. A. Cold Selective Catalytic Reduction of Nitric Oxide for Flue Gas Application. Ind. Eng. Chem. Res. 1990, 29, 1431. (15) Yashikawa, M.; Yasutake, A.; Mochida, I. Low-Temperature Selective Catalytic Reduction of NOx by Metal Oxides Supported on Active Carbon Fibers. Appl. Catal. A: Gen. 1998, 173 (2), 239.
(16) Mochida, I.; Korai, Y.; Shirahama, M.; Kawano, S.; Hada, T.; Seo, Y.; Yoshikawa, M.; Yasutake, A. Removal of SOx and NOx over Activated Carbon Fibers. Carbon 2000, 38 (2), 227. (17) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Chemical and Mechanistic Aspects of the Selective Catalytic Reduction of NOx by Ammonia over Oxide Catalysts: A Review. Appl. Catal. B: Environ. 1998, 18, 1. (18) Nojima, S.; Suzumura, H.; Hirano, M.; Sasaoka, E. Catalytic Reduction of NOx over NiSO4 Supported on Al2O3 at Low Reaction Temperature. J. Chem. Soc. Jpn., Chem. Ind. Chem. 2000, 9, 621.
Received for review November 16, 2000 Revised manuscript received May 17, 2001 Accepted May 30, 2001 IE000972F
