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The low-temperature performance of NOx storage and reduction catalyst
Applied Catalysis B: Environmental 70 (2007) 198–204 www.elsevier.com/locate/apcatb
The low-temperature performance of NOx storage and reduction catalyst Naoki Takahashi *, Kiyoshi Yamazaki, Hideo Sobukawa, Hirofumi Shinjoh TOYOTA Central Research and Development Laboratories Inc., 41-1 Yokimichi Nagakute Nagakute-cho, Aichi 480-1192, Japan Available online 21 June 2006
Abstract The catalytic performance and the behavior of NOx storage and reduction (NSR) over a model catalyst for lean-burn gasoline engines have been mainly investigated and be discussed based on the temperature and reducing agents use in this study. The experimental results have shown that the NOx storage amount in the lean atmosphere was the same as the NOx reduction amount from the subsequent rich spike (RS) above the temperature of 400 8C, while the former was greater than the latter below the temperature of 400 8C. This indicated that when the temperature was below 400 8C compared with the NOx storage stage, the reduction of the stored NOx is somehow restricted. We found that the reduction efficiencies with the reducing agents decrease in the order H2 > CO > C3H6 below 400 8C, thus not all of the NOx storage sites could be fully regenerated even using an excessive reducing agent of CO or C3H6, which was supplied to the NSR catalyst, while all the NOx storage sites could be fully regenerated if an adequate amount of H2 was supplied. We also verified that the H2 generation more favorably occurred through the water gas shift reaction than through the steam reforming reaction. This difference in the H2 generation could reasonably explain why CO was more efficient for the reduction of the stored NOx than C3H6, and hinted as a promising approach to enhance the low-temperature performance of the current NSR catalysts though promoting the H2 generation reaction. # 2006 Elsevier B.V. All rights reserved. Keywords: Lean-burn engines; NOx storage and reduction catalyst; H2; Water gas shift reaction; Steam reforming reaction
1. Introduction Reducing CO2 emissions from motor vehicles is a very important effort for the global scale environmental protection; meanwhile it is also regarded as an additional benefit from the development of more fuel-efficient engines for the automobile industry. The lean-burn gasoline engine system has attracted a lot of attention for its remarkable potential to improve the fuel economy compared with the conventional stoichiometric operated engine systems. However, the wide use of the leanburn engine system is somewhat restricted by environmental regulations. This is mainly because the conventional three-way catalysts are not able to detoxify NOx in the lean-burn engine exhaust, in which oxygen is excessive. The NOx storage and reduction (NSR) catalyst system is proposed and already regarded as one of the most feasible and attractive solutions to this technical challenge [1,2].
* Corresponding author. Tel.: +81 561 63 6293; fax: +81 561 63 6150. E-mail address: [email protected] (N. Takahashi). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2005.10.029
A typical NSR catalyst consists of precious metals (mainly Pt), alkaline and alkaline earth metal oxides as NOx storage compounds (usually BaO), and a metal oxide as support (alumina). As shown in Fig. 1, the scheme of the NOx storage and reduction reaction on the NSR catalyst [1] in the NOx storage stage under an oxidative or lean-burn atmosphere, NOx (NO) is first oxidized to NO2 over precious metals, then react or combined with the NOx storage compounds, and finally stored as a nitrate ion. In the following reduction stage under a stoichiometric or reductive atmosphere (rich), the stored nitrate ion is released as NOx (NO or NO2) from the NOx storage compounds, and then reduced to nitrogen [3]. The engine control system periodically provides a fuel-rich spike (RS) containing reducing agents such as hydrogen (H2), carbon monoxide (CO) and hydrocarbons (HC) to convert the stored nitrate ions over the NSR catalyst [4]. The already reported NSR catalysts for the lean-burn gasoline engine are mainly operated at a temperature around 400 8C [5]. Erkfeldt et al. pointed out that decreasing the exhaust gas temperature of a lean-burn engines will be a very meaningful way to reduce the thermal loss, thus improving the
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Fig. 1. Outline of the NOx storage and reduction reaction.
fuel economy [6]. Therefore, corresponding with this improvement in the lean-burn engine for a higher fuel efficiency, the enhancement of the low-temperature performance of the NSR catalyst is very necessary and highly expected. It is reported that the NOx storage and reduction on the NSR catalyst apparently decrease if the reaction temperature goes below 400 8C [5]. It is well known that the reducing agents, such as CO and hydrocarbons, co-exist with oxygen, and the NOx in the exhaust, thus a series of reactions occur on the NSR catalyst. For further improving the low temperature activity and optimizing the formulation of the NSR catalysts, it is very necessary and meaningful to clarify which is the restricted stage, the NOx storage or the following reduction of the stored NOx in the entire NOx storage and reduction process? How significant is the influence on the performance and behavior of the NOx storage and reduction stages from the reducing agents contained in the exhaust? To answer these questions, the NOx storage and reduction on a model NSR catalyst with simulated exhaust gases was designed, and experimentally investigated on the basis of the temperature and the reducing agents in this research. 2. Experimental 2.1. Catalyst preparation The model NSR catalyst was prepared on a hexagonal cell monolithic substrate (62 cells/cm2) having a diameter of 30 mm and length of 50 mm [7]. This catalyst consisted of the oxide support (mainly alumina), ceria-zirconia-based oxygen storage material, barium and potassium oxides as the NOx storage compounds, and the supported metals of platinum and rhodium. The total loading amount of platinum and rhodium was 0.4 wt.% and that of barium and potassium was 5 wt.%. Nitric acid solution of Pt(NH3)2(NO2)2 and aqueous solution of RhCl3, barium acetate and potassium acetate were used for impregnation. Our previous studies showed that the change in the composition of the NSR catalyst would not qualitatively affect the basic features and behaviors of the NOx storage and reduction performance. This research is mainly studying the performance of the NSR catalysts instead of the catalyst itself, thus only one NSR catalyst was used in the experiments.
2.2. Measurement of NOx storage and reduction The NOx storage and reduction experiments were conducted using a conventional fixed-bed reactor [1,3,8] with simulated exhaust gases under atmospheric pressure. The fresh catalyst was first pretreated for 15 min at 500 8C with the NO free stoichiometric model gas, containing 0.28% CO, 0.16% H2, 0.09% C3H6, 0.62% O2, 14.25% CO2 and 5% H2O with N2 as the balance gas. Prior to the NOx storage/release testing, to clean away the residual NOx, the rich model gas (referred as RS1 in Table 1) was flowed through the catalyst for 5 min during cooling down or heating up to the test temperature. Thereafter, the NSR catalyst was exposed to the lean atmosphere until the NOx concentration in the outlet gas reached constant, and subsequently switched to 3-s RS under the lean atmosphere until the NOx concentration in the outlet gas reached constant. This NOx storage/release testing proceeded with the temperature stepping from 250 to 600 8C. The gas flow rate for each measurement was set as 30 000 cm3/min, corresponding to the GHSV of 51 500 h1. The NOx concentration in this research is taken as the integration of NO and NO2 in the gases, and is measured by the chemiluminescent NOx meter attached to the Horiba MEXA7100 evaluation system. As listed in Table 1, the gas compositions were determined according to the analysis results of the lean-burn gasoline exhaust, and C3H6 was used as the hydrocarbon species. 2.3. Influence on the behavior of NSR reaction by the reducing agents The gases as listed in Table 2 were used for investigating the influence on the reduction of the stored NOx by the reducing species in the RS. In these experiments, the in situ Table 1 Compositions of simulated exhaust gases for NOx storage and reduction performance
Lean 1 RS1
NO (ppm)
O2 (%)
H2 (%)
CO (%)
C3H6 (ppm)
CO2 (%)
H2O (%)
N2
400 400
7 0
0 1.6
0.01 6
200 1070
11 11
5 5
Balance Balance
200
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Table 2 The gas compositions used to investigate the effect of reducing agent type on the stored NOx reduction
Lean 2 RS2 RS3 RS4 Blank-RS
NO (ppm)
O2 (%)
H2 (%)
CO (%)
C3H6 (%)
CO2 (%)
H2O (%)
N2
400 400 400 400 400
7 0 0 0 0
0 6 0 0 0
0 0 6 0 0
0 0 0 0.67 0
11 11 11 11 11
5 5 5 5 5
Balance Balance Balance Balance Balance
pretreated NSR catalyst was alternately and periodically switched to a 60-s lean atmosphere and a 3-s RS. The CO and C3H6 concentrations of the reaction gases were measured using the non-dispersive infrared CO analyzer and a flame ionization HC detector equipped Horiba MEXA-7100, respectively. The H2 concentration in the reaction gas was measured by a sector-type mass spectrometer attached to a Horiba MSHA-1000L. 2.4. H2 generation over the NSR catalyst The gases in Table 3 were used for investigating the H2 generation over the NSR catalyst by the water gas shift (WGS) reaction or steam reforming (SR) reaction under the transient conditions. The lean and rich atmospheres were alternately and periodically switched every 30 s with the temperature decreasing from 400 to 200 8C at the rate of 10 8C/min. 3. Results and discussion 3.1. The performance of NOx storage and reduction with temperature As shown in Fig. 2, with the simulated gases in Table 1, the evolution of the NOx concentration in the outlet gas with time at 250 8C shows the storage and reduction stages of the entire NOx storage and reduction processes. When the lean atmosphere is switched on, the outlet NOx concentration gradually increased with time and then reached an approximately constant level around 1400 s. The difference in the NOx concentration between the inlet and outlet gases at this point could be attributed to the selective NOx reduction by HC on Pt of the NSR catalyst [8]. The shadow area ‘‘A’’ relates to the NOx amount stored on the catalyst. Upon the 3-s RS supplied to the NOx stored NSR catalyst, the NOx concentration in the outlet gas momently jumps higher than
Fig. 2. NOx concentration in the outlet and inlet gases using Lean 1 and RS1 gases at 250 8C: (—) outlet NOx concentration; ( ) inlet NOx concentration.
that in the inlet gas, quickly falls to nearly zero, and then gradually increased again with time. Apparently, this is closely related to the regeneration of the NOx storage sites or the reduction of the stored NOx by the reducing agents in the RS. The shadow area ‘‘B’’ relates to the amount of the regenerated NOx storage sites on the catalyst. In this study, the NOx storage amounts indicated by the shadow areas ‘‘A’’ and ‘‘B’’ are noted as ‘‘NOx storage amount’’ or ‘‘RS-NOx storage amount’’, respectively. The NOx storage amount corresponds to the NOx adsorption during the storage stage, while the RS-NOx storage amount corresponds to the reduction of the stored or adsorbed NOx during the reduction stage. Fig. 3 plots the ratios of the NOx storage amount and RSNOx storage amount to the NOx storage amount at 400 8C
Table 3 The gas compositions used to investigate hydrogen generation performance on the NSR catalyst O2 (%)
CO (%)
C3H6 (%)
H2O (%)
N2
Water gas shift Lean A 7 Rich A 0
0 6
0 0
10 10
Balance Balance
Steam reforming Lean B 7 Rich B 0
0 0
0 0.67
10 10
Balance Balance
Fig. 3. NOx storage performance with Lean 1 and RS1 gases vs. reaction temperature: (*) NOx storage amount; (*) RS-NOx storage amount.
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versus the reaction temperature. The ratio of the NOx storage amount on the catalyst showed a maximum value both at 300 and 400 8C, while the ratio of the RS-NOx storage amount had a maximum value at 400 8C. For the temperatures over 400 8C, the NOx storage amount are nearly the same as the RS-NOx storage amount. This suggests that the NOx storage sites on the catalyst are nearly completely regenerated upon the 3-s RS over 400 8C. For temperatures below 400 8C, the NOx storage amount on the catalyst is greater than the RS-NOx storage amount. The ratio of the RS-NOx storage amount to the NOx storage amount at 400 8C decreases as the reaction temperature is decreased, i.e., 71% at 300 8C and 47% at 250 8C. This result indicates that the NOx storage sites were not completely regenerated during the 3-s RS if the temperature is below 400 8C. The actual RSNOx storage amount under the actual exhaust atmosphere is probably lower than that in this research, because the RS with the actual lean-burn gasoline engines is one order of magnitude shorter than that in this experiment, i.e., less than 1 s. Based on these results, we can conclude that, compared with the storage stage, the reduction of the stored NOx is somewhat restricted or a kind of bottleneck for the entire NOx storage and reduction process when the reaction temperature is below 400 8C. Therefore, to improve the performance of the NSR catalysts, the efforts should be focus on promoting the reactions involved in the reduction stage instead of the NOx storage stage.
different below 400 8C. As already mentioned, the NOx reduction could occur under the lean atmosphere, thus, it is difficult to clarify the effect of the reducing agents based only on the analysis of the wave shape of the NOx in the outlet gas. Therefore, to remove the influence of the reducing agents or any possible selective NOx reduction, the lean gas without any reducing agents is necessary for further investigation. As summarized in Table 2, the simulated RS gases containing H2, CO or C3H6 are labeled as RS2, RS3 and RS4, respectively. A model gas without any reducing agents, consisting of NO, CO2 and H2O, with N2 as the balance gas was labeled the Blank-RS. Along with being alternately switched to a 60-s lean atmosphere and a 3-s RS at 250 8C, the NOx concentration evolutions in the outlet gases with time are shown in Fig. 5. For the convenience of the quantitative analysis, the concentration of the chemical equivalent is used in this study. The concentration of the chemical equivalent with the reducing agents is based on the O2 concentration, assuming the complete oxidation of the reducing agent, through the following reduction reactions (1)–(3):
3.2. The influence of the reducing agents on the reduction of the stored NOx
For RS2, RS3 and RS4 in Table 2, the concentration of the chemical equivalent with H2, CO and C3H6 was 3%. If all the NOx contained in the lean period (Lean 2 in Table 2) is totally stored on the NSR catalyst as nitrate ions, and then completely reduced to N2 during the RS, the required concentration of the chemical equivalent in the RS is estimated to be 1%. Therefore, the reducing agents (3%) supplied in Fig. 5 are excessive compared with the stored NOx amount. The RS removes the stored NOx and refreshes the NOx storage sites on the catalyst as described by reaction (4), and these regenerated NOx storage sites then regain their NOx storage ability:
The performance of the NOx storage and reduction on the NSR catalyst indicated that the reduction of the stored NOx is the restricted stage or bottleneck for the entire process. It is very important to clarify the individual reduction reaction involved in the NOx reduction stage, thus figure out the solutions to enhance the overall activity of the NSR catalyst. As shown in Fig. 4, the conversions of H2, CO and C3H6, obtained from Fig. 3 experiment, are approximately close to 100% at 400 8C, while the conversions apparently decrease at 250 and 300 8C, as H2 > CO > C3H6. These results indicated that the reduction activity or efficiency with these three reducing agents is
H2 þ ð1=2ÞO2 ! H2 O
(1)
CO þ ð1=2ÞO2 ! CO2
(2)
C3 H6 þ ð9=2ÞO2 ! 3H2 O þ 3CO2
(3)
MNO3 þ R ! NOx þ MOy þ ROz ðM : Ba or K; R : reducing agentsÞ
Fig. 4. Conversion of the reducing agents in the rich spike vs. reaction temperature: (*) H2; (*) CO; () C3H6.
(4)
The evolution of the NOx concentration under the lean atmosphere with different reducing agents are shown in Fig. 5. The NOx concentration in the outlet gas monotonically increased with time and finally reached a constant level with the Blank-RS (Fig. 5D). During the lean period, the increase in the NOx concentration in the outlet gas with the reducing agents decreases in the order no reducing agent Blank-RS > C3H6 > CO > H2. This indicates that the reduction abilities of the reducing agents decrease as H2 > CO > C3H6. With H2 used as the reducing agent, the evolution of the NOx concentration versus time for each NOx storage/reduction cycle was almost the same (Fig. 5A), and indicates that the RS with H2 can regenerate nearly all the NOx storage sites on the catalyst. As shown in Fig. 5, the NOx conversion with different reducing agents during the RS reaches a constant level when it gets close to the cycle end.
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Fig. 5. NOx concentration in the outlet gas at 250 8C vs. time using Lean 2 and RS: (A) alternating with the RS2 (H2); (B) alternating with the RS3 (CO); (C) alternating with the RS4 (C3H6); (D) alternating with the Blank-RS.
Besides the activity of the reducing agents, the amount of the reducing agents contained in the RS is also very important for the efficient reduction of the stored NOx and the fuel efficiency. As shown in Fig. 6, the NOx conversion in the seventh lean period is plotted versus the concentrations of the chemical equivalent with the reducing agents. When the concentration of the chemical equivalent is below 0.5%, the reducing gas species only has a slight influence on the NOx conversion. The NOx conversion apparently differs when the concentration of the chemical equivalent with the reducing agents ranges from 0.5 to 1.0%. In this concentration range,
Fig. 6. NOx conversion of the seventh lean period at 250 8C vs. the concentration of the chemical equivalent of the reducing agents in the RS: (*) H2; (*) CO; () C3H6; (~) no reducing agents.
supply of the reducing agents becomes the determining factor for the regeneration of the NOx storage sites. For the reducing agent of H2, the NOx conversion rises along with the H2 concentration and finally reached over 95%, which is apparently different from that with CO or C3H6 as the reducing agent. For CO and C3H6 used as the reducing agent, no effect from increasing the concentration of CO or C3H6 was observed in this concentration range, and the release of the stored NOx is the determining factor. In this case, the reduction of the stored NOx with H2 or CO is 2.5 or 1.5 times faster than that with C3H6, respectively, because the NOx conversion is proportional to the reaction rate in reaction (4). As indicated in Fig. 1, the release of the stored NOx needs the reducing agents to locate on the interface between the precious metals and the NOx storage compounds or on the surface of the NOx storage compounds. It is well known that the hydrogen atom spills over from the precious metals to the oxide supports on catalysts [9–11]. Therefore, the relative high mobility of the hydrogen atom on the catalyst surface probably facilitates the regeneration of the NOx storage sites with the H2 RS which results in its highest performance. In Figs. 2 and 5, the momentary sharp jumps of the NOx concentration upon RS are observed. Even when the Blank-RS is employed, the maximum NOx concentration in the outlet gas is also higher than the NOx concentration in the inlet gas (Fig. 5D). This result indicates that the momentary sharp jump is caused by the desorption or release of the stored NOx from the catalyst.
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Fig. 7. The emitted NOx concentration of the sixth RS at 250 8C vs. the concentration of the chemical equivalent of the reducing agents in the RS: (*) H2; (*) CO; () C3H6; (~) no reducing agents.
The difference in the NOx concentration in the outlet gas between the end point of the lean period and the peak point of RS, or the sharp jump in the NOx concentration at the switching point, is noted as the emitted NOx concentration hereafter. This emitted NOx concentration reveals the actual performance of the NSR catalyst. Fig. 7 shows the emitted NOx concentration of the sixth RS versus the concentration of the chemical equivalent of the reducing agents in the RS. The emitted NOx concentration can be quite high if the desorbed NOx is not fully or efficiently reduced to N2, as shown by reaction (5): NOx þ R ! ð1=2ÞN2 þ ROx
ðR : reducing agentsÞ
(5)
The emitted NOx concentration rises with the increasing concentration of the chemical equivalent with C3H6 up to 1.0%, while maintaining a somewhat stable value when the concentration is above 1.0%. The emitted NOx concentration gradually decreases with the increasing H2 concentration, while it varies only slightly with CO as the reducing agent. The effect of the reducing agents on the reduction of the released NOx is different from each other, and the reactivity decreases in the order H2 > CO > C3H6. Among the reducing agents investigated in this study, H2 had the highest activity for both the release of the stored NOx and the reduction of the released NOx. 3.3. H2 generation over the NSR catalyst at low temperatures The above results showed that H2 is the most active and efficient reducing agent among H2, CO and C3H6. In addition, to the inherent properties of these two molecules, is there any other factor that makes the difference between CO and C3H6? This will be very important for the optimization of the practical NSR catalyst. It is well known that H2 could be generated on catalysts in a rich atmosphere with CO and C3H6. For example, CO reacted with H2O on the Pt catalyst supported on cerium oxides to produce H2, i.e., the so-called water gas shift (WGS) reaction, as shown by reaction (6) [12–15]. Steam reforming (SR) is another reaction to form H2 through the reaction between HC and H2O on the Rh catalyst supported on the
203
Fig. 8. H2 concentration in the outlet gas vs. time when the lean and rich conditions were alternately switched every 30 s: ( ) water gas shift reaction; (—) steam reforming reaction. The dashed line represents the reaction temperature, which decreased at the rate of 10 8C/min.
thermal-resistant zirconium oxide in our NSR catalyst, as shown by reaction (7) [5]: CO þ H2 O ! CO2 þ H2
(6)
C3 H6 þ 6H2 O ! 3CO2 þ 9H2
(7)
As described in Section 2.1, the NSR catalyst contains the necessary compounds and works in the atmosphere for the WGS and SR reactions. The formed H2 on the NSR catalyst will facilitate the reduction of the stored NOx, which are one of the critical factors related to the performance of the NSR catalyst. Therefore, the H2 generation performance of the WGS and SR reactions were also investigated. The lean and rich atmospheres as listed in Table 3 were alternately and periodically switched every 30 s, and its temperature was decreased from 400 to 200 8C at the rate of 10 8C/min. Fig. 8 shows the H2 concentration in the outlet gas versus time. The temperature range was from 280 to 230 8C. The H2 concentration of the WGS reaction was higher than that of the SR reaction. Moreover, the response of the increasing H2 concentration with the WGS reaction was faster than that with the SR reaction, and the difference increased with the decreasing temperature. Thus, the difference in the H2 generation performance between the WGS and the SR reactions over the NSR catalyst seems to be one of the reasons why the stored NOx reduction activity of CO was higher than that of C3H6 as the RS reducing agent. Therefore, a higher reducing performance of the stored NOx on the NSR catalyst could be achieved if the H2 generation activity by the WGS and SR reactions is enhanced. 4. Conclusions The NOx storage and the subsequent reduction of the stored NOx at low temperatures over the NSR catalyst for lean-burn gasoline engines have been systematically investigated and discussed in this study. We can conclude that the reduction of the stored NOx was the restricted stage for the entire NOx storage and reduction process below 400 8C. At low temperatures, the activities of the reducing agents to reduce
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the stored NOx are different and decreases in the order H2 > CO > C3H6. A part of the NOx storage sites was not regenerated when an excess of CO or C3H6 was supplied to the NSR catalysts, while all the NOx storage sites could be fully regenerated when adequate H2 was supplied. The experimental results indicated that the release rate of the stored NOx was the determining factor for the reduction of the stored NOx when CO or C3H6 was used as the reducing agent. We also found that the H2 generation by the water gas shift reaction on the NSR catalysts was more apparent and efficient than that through the steam reforming reaction. It is regarded as one of the reasons why the activity of CO to reduce the stored NOx was higher than that of C3H6. Therefore, the promotion of the H2 generation activity by the water gas shift reaction is expected to be a promising approach to improve the performance of the NSR catalyst at low temperatures. Acknowledgement The authors would like to thank Dr. Fei Dong for the manuscript preparation.
References [1] N. Miyoshi, S. Matsumoto, K. Katoh, T. Tanaka, J. Harada, N. Takahashi, K. Yokota, M. Sugiura, K. Kasahara, SAE Technical Paper No. 950809, 1995. [2] W. Bo¨gner, M. Kra¨mer, B. Krutzsch, S. Pischinger, D. Voigtla¨nder, G. Wenninger, F. Wirbeleit, M.S. Brogan, R.J. Brisley, D.E. Webster, Appl. Catal. B: Environ. 7 (1995) 153. [3] N. Takahashi, H. Shinjoh, T. Iijima, T. Suzuki, K. Yamazaki, K. Yokota, H. Suzuki, N. Miyoshi, S. Matsumoto, T. Tanizawa, T. Tanaka, S. Tateishi, K. Kasahara, Catal. Today 27 (1996) 63. [4] N. Miyoshi, T. Tanaka, S. Matsumoto, TOYOTA Thec. Rev. 50 (2001) 28. [5] M. Takeuchi, S. Matsumoto, Top. Catal. 28 (2004) 151. [6] S. Erkfeldt, E. Jobson, M. Larsson, Top. Catal. 16/17 (2001) 127. [7] I. Hachisuka, T. Yoshida, H. Ueno, N. Takahashi, A. Suda, M. Sugiura, SAE Technical Paper No. 2002010732, 2002. [8] S. Matsumoto, Y. Ikeda, H. Suzuki, M. Ogai, N. Miyoshi, Appl. Catal. B: Environ. 25 (2000) 115. [9] R. Kramer, M. Andre, J. Catal. 58 (1979) 287. [10] R.R. Cavanagh, J.T. Yates Jr., J. Catal. 68 (1981) 22. [11] M. Machida, D. Kurogi, T. Kijima, J. Phys. Chem. B 107 (2003) 196. [12] T. Bunluesin, R.J. Gorte, G.W. Graham, Appl. Catal. B 15 (1998) 107. [13] B.I. Whittington, C.J. Jiang, D.L. Trimm, Catal. Today 26 (1995) 41. [14] T. Shido, Y. Iwasawa, J. Catal. 141 (1993) 71. [15] R.K. Herz, J.A. Sell, J. Catal. 94 (1993) 166.
The low-temperature performance of NOx storage and reduction catalyst Naoki Takahashi *, Kiyoshi Yamazaki, Hideo Sobukawa, Hirofumi Shinjoh TOYOTA Central Research and Development Laboratories Inc., 41-1 Yokimichi Nagakute Nagakute-cho, Aichi 480-1192, Japan Available online 21 June 2006
Abstract The catalytic performance and the behavior of NOx storage and reduction (NSR) over a model catalyst for lean-burn gasoline engines have been mainly investigated and be discussed based on the temperature and reducing agents use in this study. The experimental results have shown that the NOx storage amount in the lean atmosphere was the same as the NOx reduction amount from the subsequent rich spike (RS) above the temperature of 400 8C, while the former was greater than the latter below the temperature of 400 8C. This indicated that when the temperature was below 400 8C compared with the NOx storage stage, the reduction of the stored NOx is somehow restricted. We found that the reduction efficiencies with the reducing agents decrease in the order H2 > CO > C3H6 below 400 8C, thus not all of the NOx storage sites could be fully regenerated even using an excessive reducing agent of CO or C3H6, which was supplied to the NSR catalyst, while all the NOx storage sites could be fully regenerated if an adequate amount of H2 was supplied. We also verified that the H2 generation more favorably occurred through the water gas shift reaction than through the steam reforming reaction. This difference in the H2 generation could reasonably explain why CO was more efficient for the reduction of the stored NOx than C3H6, and hinted as a promising approach to enhance the low-temperature performance of the current NSR catalysts though promoting the H2 generation reaction. # 2006 Elsevier B.V. All rights reserved. Keywords: Lean-burn engines; NOx storage and reduction catalyst; H2; Water gas shift reaction; Steam reforming reaction
1. Introduction Reducing CO2 emissions from motor vehicles is a very important effort for the global scale environmental protection; meanwhile it is also regarded as an additional benefit from the development of more fuel-efficient engines for the automobile industry. The lean-burn gasoline engine system has attracted a lot of attention for its remarkable potential to improve the fuel economy compared with the conventional stoichiometric operated engine systems. However, the wide use of the leanburn engine system is somewhat restricted by environmental regulations. This is mainly because the conventional three-way catalysts are not able to detoxify NOx in the lean-burn engine exhaust, in which oxygen is excessive. The NOx storage and reduction (NSR) catalyst system is proposed and already regarded as one of the most feasible and attractive solutions to this technical challenge [1,2].
* Corresponding author. Tel.: +81 561 63 6293; fax: +81 561 63 6150. E-mail address: [email protected] (N. Takahashi). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2005.10.029
A typical NSR catalyst consists of precious metals (mainly Pt), alkaline and alkaline earth metal oxides as NOx storage compounds (usually BaO), and a metal oxide as support (alumina). As shown in Fig. 1, the scheme of the NOx storage and reduction reaction on the NSR catalyst [1] in the NOx storage stage under an oxidative or lean-burn atmosphere, NOx (NO) is first oxidized to NO2 over precious metals, then react or combined with the NOx storage compounds, and finally stored as a nitrate ion. In the following reduction stage under a stoichiometric or reductive atmosphere (rich), the stored nitrate ion is released as NOx (NO or NO2) from the NOx storage compounds, and then reduced to nitrogen [3]. The engine control system periodically provides a fuel-rich spike (RS) containing reducing agents such as hydrogen (H2), carbon monoxide (CO) and hydrocarbons (HC) to convert the stored nitrate ions over the NSR catalyst [4]. The already reported NSR catalysts for the lean-burn gasoline engine are mainly operated at a temperature around 400 8C [5]. Erkfeldt et al. pointed out that decreasing the exhaust gas temperature of a lean-burn engines will be a very meaningful way to reduce the thermal loss, thus improving the
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Fig. 1. Outline of the NOx storage and reduction reaction.
fuel economy [6]. Therefore, corresponding with this improvement in the lean-burn engine for a higher fuel efficiency, the enhancement of the low-temperature performance of the NSR catalyst is very necessary and highly expected. It is reported that the NOx storage and reduction on the NSR catalyst apparently decrease if the reaction temperature goes below 400 8C [5]. It is well known that the reducing agents, such as CO and hydrocarbons, co-exist with oxygen, and the NOx in the exhaust, thus a series of reactions occur on the NSR catalyst. For further improving the low temperature activity and optimizing the formulation of the NSR catalysts, it is very necessary and meaningful to clarify which is the restricted stage, the NOx storage or the following reduction of the stored NOx in the entire NOx storage and reduction process? How significant is the influence on the performance and behavior of the NOx storage and reduction stages from the reducing agents contained in the exhaust? To answer these questions, the NOx storage and reduction on a model NSR catalyst with simulated exhaust gases was designed, and experimentally investigated on the basis of the temperature and the reducing agents in this research. 2. Experimental 2.1. Catalyst preparation The model NSR catalyst was prepared on a hexagonal cell monolithic substrate (62 cells/cm2) having a diameter of 30 mm and length of 50 mm [7]. This catalyst consisted of the oxide support (mainly alumina), ceria-zirconia-based oxygen storage material, barium and potassium oxides as the NOx storage compounds, and the supported metals of platinum and rhodium. The total loading amount of platinum and rhodium was 0.4 wt.% and that of barium and potassium was 5 wt.%. Nitric acid solution of Pt(NH3)2(NO2)2 and aqueous solution of RhCl3, barium acetate and potassium acetate were used for impregnation. Our previous studies showed that the change in the composition of the NSR catalyst would not qualitatively affect the basic features and behaviors of the NOx storage and reduction performance. This research is mainly studying the performance of the NSR catalysts instead of the catalyst itself, thus only one NSR catalyst was used in the experiments.
2.2. Measurement of NOx storage and reduction The NOx storage and reduction experiments were conducted using a conventional fixed-bed reactor [1,3,8] with simulated exhaust gases under atmospheric pressure. The fresh catalyst was first pretreated for 15 min at 500 8C with the NO free stoichiometric model gas, containing 0.28% CO, 0.16% H2, 0.09% C3H6, 0.62% O2, 14.25% CO2 and 5% H2O with N2 as the balance gas. Prior to the NOx storage/release testing, to clean away the residual NOx, the rich model gas (referred as RS1 in Table 1) was flowed through the catalyst for 5 min during cooling down or heating up to the test temperature. Thereafter, the NSR catalyst was exposed to the lean atmosphere until the NOx concentration in the outlet gas reached constant, and subsequently switched to 3-s RS under the lean atmosphere until the NOx concentration in the outlet gas reached constant. This NOx storage/release testing proceeded with the temperature stepping from 250 to 600 8C. The gas flow rate for each measurement was set as 30 000 cm3/min, corresponding to the GHSV of 51 500 h1. The NOx concentration in this research is taken as the integration of NO and NO2 in the gases, and is measured by the chemiluminescent NOx meter attached to the Horiba MEXA7100 evaluation system. As listed in Table 1, the gas compositions were determined according to the analysis results of the lean-burn gasoline exhaust, and C3H6 was used as the hydrocarbon species. 2.3. Influence on the behavior of NSR reaction by the reducing agents The gases as listed in Table 2 were used for investigating the influence on the reduction of the stored NOx by the reducing species in the RS. In these experiments, the in situ Table 1 Compositions of simulated exhaust gases for NOx storage and reduction performance
Lean 1 RS1
NO (ppm)
O2 (%)
H2 (%)
CO (%)
C3H6 (ppm)
CO2 (%)
H2O (%)
N2
400 400
7 0
0 1.6
0.01 6
200 1070
11 11
5 5
Balance Balance
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Table 2 The gas compositions used to investigate the effect of reducing agent type on the stored NOx reduction
Lean 2 RS2 RS3 RS4 Blank-RS
NO (ppm)
O2 (%)
H2 (%)
CO (%)
C3H6 (%)
CO2 (%)
H2O (%)
N2
400 400 400 400 400
7 0 0 0 0
0 6 0 0 0
0 0 6 0 0
0 0 0 0.67 0
11 11 11 11 11
5 5 5 5 5
Balance Balance Balance Balance Balance
pretreated NSR catalyst was alternately and periodically switched to a 60-s lean atmosphere and a 3-s RS. The CO and C3H6 concentrations of the reaction gases were measured using the non-dispersive infrared CO analyzer and a flame ionization HC detector equipped Horiba MEXA-7100, respectively. The H2 concentration in the reaction gas was measured by a sector-type mass spectrometer attached to a Horiba MSHA-1000L. 2.4. H2 generation over the NSR catalyst The gases in Table 3 were used for investigating the H2 generation over the NSR catalyst by the water gas shift (WGS) reaction or steam reforming (SR) reaction under the transient conditions. The lean and rich atmospheres were alternately and periodically switched every 30 s with the temperature decreasing from 400 to 200 8C at the rate of 10 8C/min. 3. Results and discussion 3.1. The performance of NOx storage and reduction with temperature As shown in Fig. 2, with the simulated gases in Table 1, the evolution of the NOx concentration in the outlet gas with time at 250 8C shows the storage and reduction stages of the entire NOx storage and reduction processes. When the lean atmosphere is switched on, the outlet NOx concentration gradually increased with time and then reached an approximately constant level around 1400 s. The difference in the NOx concentration between the inlet and outlet gases at this point could be attributed to the selective NOx reduction by HC on Pt of the NSR catalyst [8]. The shadow area ‘‘A’’ relates to the NOx amount stored on the catalyst. Upon the 3-s RS supplied to the NOx stored NSR catalyst, the NOx concentration in the outlet gas momently jumps higher than
Fig. 2. NOx concentration in the outlet and inlet gases using Lean 1 and RS1 gases at 250 8C: (—) outlet NOx concentration; ( ) inlet NOx concentration.
that in the inlet gas, quickly falls to nearly zero, and then gradually increased again with time. Apparently, this is closely related to the regeneration of the NOx storage sites or the reduction of the stored NOx by the reducing agents in the RS. The shadow area ‘‘B’’ relates to the amount of the regenerated NOx storage sites on the catalyst. In this study, the NOx storage amounts indicated by the shadow areas ‘‘A’’ and ‘‘B’’ are noted as ‘‘NOx storage amount’’ or ‘‘RS-NOx storage amount’’, respectively. The NOx storage amount corresponds to the NOx adsorption during the storage stage, while the RS-NOx storage amount corresponds to the reduction of the stored or adsorbed NOx during the reduction stage. Fig. 3 plots the ratios of the NOx storage amount and RSNOx storage amount to the NOx storage amount at 400 8C
Table 3 The gas compositions used to investigate hydrogen generation performance on the NSR catalyst O2 (%)
CO (%)
C3H6 (%)
H2O (%)
N2
Water gas shift Lean A 7 Rich A 0
0 6
0 0
10 10
Balance Balance
Steam reforming Lean B 7 Rich B 0
0 0
0 0.67
10 10
Balance Balance
Fig. 3. NOx storage performance with Lean 1 and RS1 gases vs. reaction temperature: (*) NOx storage amount; (*) RS-NOx storage amount.
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versus the reaction temperature. The ratio of the NOx storage amount on the catalyst showed a maximum value both at 300 and 400 8C, while the ratio of the RS-NOx storage amount had a maximum value at 400 8C. For the temperatures over 400 8C, the NOx storage amount are nearly the same as the RS-NOx storage amount. This suggests that the NOx storage sites on the catalyst are nearly completely regenerated upon the 3-s RS over 400 8C. For temperatures below 400 8C, the NOx storage amount on the catalyst is greater than the RS-NOx storage amount. The ratio of the RS-NOx storage amount to the NOx storage amount at 400 8C decreases as the reaction temperature is decreased, i.e., 71% at 300 8C and 47% at 250 8C. This result indicates that the NOx storage sites were not completely regenerated during the 3-s RS if the temperature is below 400 8C. The actual RSNOx storage amount under the actual exhaust atmosphere is probably lower than that in this research, because the RS with the actual lean-burn gasoline engines is one order of magnitude shorter than that in this experiment, i.e., less than 1 s. Based on these results, we can conclude that, compared with the storage stage, the reduction of the stored NOx is somewhat restricted or a kind of bottleneck for the entire NOx storage and reduction process when the reaction temperature is below 400 8C. Therefore, to improve the performance of the NSR catalysts, the efforts should be focus on promoting the reactions involved in the reduction stage instead of the NOx storage stage.
different below 400 8C. As already mentioned, the NOx reduction could occur under the lean atmosphere, thus, it is difficult to clarify the effect of the reducing agents based only on the analysis of the wave shape of the NOx in the outlet gas. Therefore, to remove the influence of the reducing agents or any possible selective NOx reduction, the lean gas without any reducing agents is necessary for further investigation. As summarized in Table 2, the simulated RS gases containing H2, CO or C3H6 are labeled as RS2, RS3 and RS4, respectively. A model gas without any reducing agents, consisting of NO, CO2 and H2O, with N2 as the balance gas was labeled the Blank-RS. Along with being alternately switched to a 60-s lean atmosphere and a 3-s RS at 250 8C, the NOx concentration evolutions in the outlet gases with time are shown in Fig. 5. For the convenience of the quantitative analysis, the concentration of the chemical equivalent is used in this study. The concentration of the chemical equivalent with the reducing agents is based on the O2 concentration, assuming the complete oxidation of the reducing agent, through the following reduction reactions (1)–(3):
3.2. The influence of the reducing agents on the reduction of the stored NOx
For RS2, RS3 and RS4 in Table 2, the concentration of the chemical equivalent with H2, CO and C3H6 was 3%. If all the NOx contained in the lean period (Lean 2 in Table 2) is totally stored on the NSR catalyst as nitrate ions, and then completely reduced to N2 during the RS, the required concentration of the chemical equivalent in the RS is estimated to be 1%. Therefore, the reducing agents (3%) supplied in Fig. 5 are excessive compared with the stored NOx amount. The RS removes the stored NOx and refreshes the NOx storage sites on the catalyst as described by reaction (4), and these regenerated NOx storage sites then regain their NOx storage ability:
The performance of the NOx storage and reduction on the NSR catalyst indicated that the reduction of the stored NOx is the restricted stage or bottleneck for the entire process. It is very important to clarify the individual reduction reaction involved in the NOx reduction stage, thus figure out the solutions to enhance the overall activity of the NSR catalyst. As shown in Fig. 4, the conversions of H2, CO and C3H6, obtained from Fig. 3 experiment, are approximately close to 100% at 400 8C, while the conversions apparently decrease at 250 and 300 8C, as H2 > CO > C3H6. These results indicated that the reduction activity or efficiency with these three reducing agents is
H2 þ ð1=2ÞO2 ! H2 O
(1)
CO þ ð1=2ÞO2 ! CO2
(2)
C3 H6 þ ð9=2ÞO2 ! 3H2 O þ 3CO2
(3)
MNO3 þ R ! NOx þ MOy þ ROz ðM : Ba or K; R : reducing agentsÞ
Fig. 4. Conversion of the reducing agents in the rich spike vs. reaction temperature: (*) H2; (*) CO; () C3H6.
(4)
The evolution of the NOx concentration under the lean atmosphere with different reducing agents are shown in Fig. 5. The NOx concentration in the outlet gas monotonically increased with time and finally reached a constant level with the Blank-RS (Fig. 5D). During the lean period, the increase in the NOx concentration in the outlet gas with the reducing agents decreases in the order no reducing agent Blank-RS > C3H6 > CO > H2. This indicates that the reduction abilities of the reducing agents decrease as H2 > CO > C3H6. With H2 used as the reducing agent, the evolution of the NOx concentration versus time for each NOx storage/reduction cycle was almost the same (Fig. 5A), and indicates that the RS with H2 can regenerate nearly all the NOx storage sites on the catalyst. As shown in Fig. 5, the NOx conversion with different reducing agents during the RS reaches a constant level when it gets close to the cycle end.
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Fig. 5. NOx concentration in the outlet gas at 250 8C vs. time using Lean 2 and RS: (A) alternating with the RS2 (H2); (B) alternating with the RS3 (CO); (C) alternating with the RS4 (C3H6); (D) alternating with the Blank-RS.
Besides the activity of the reducing agents, the amount of the reducing agents contained in the RS is also very important for the efficient reduction of the stored NOx and the fuel efficiency. As shown in Fig. 6, the NOx conversion in the seventh lean period is plotted versus the concentrations of the chemical equivalent with the reducing agents. When the concentration of the chemical equivalent is below 0.5%, the reducing gas species only has a slight influence on the NOx conversion. The NOx conversion apparently differs when the concentration of the chemical equivalent with the reducing agents ranges from 0.5 to 1.0%. In this concentration range,
Fig. 6. NOx conversion of the seventh lean period at 250 8C vs. the concentration of the chemical equivalent of the reducing agents in the RS: (*) H2; (*) CO; () C3H6; (~) no reducing agents.
supply of the reducing agents becomes the determining factor for the regeneration of the NOx storage sites. For the reducing agent of H2, the NOx conversion rises along with the H2 concentration and finally reached over 95%, which is apparently different from that with CO or C3H6 as the reducing agent. For CO and C3H6 used as the reducing agent, no effect from increasing the concentration of CO or C3H6 was observed in this concentration range, and the release of the stored NOx is the determining factor. In this case, the reduction of the stored NOx with H2 or CO is 2.5 or 1.5 times faster than that with C3H6, respectively, because the NOx conversion is proportional to the reaction rate in reaction (4). As indicated in Fig. 1, the release of the stored NOx needs the reducing agents to locate on the interface between the precious metals and the NOx storage compounds or on the surface of the NOx storage compounds. It is well known that the hydrogen atom spills over from the precious metals to the oxide supports on catalysts [9–11]. Therefore, the relative high mobility of the hydrogen atom on the catalyst surface probably facilitates the regeneration of the NOx storage sites with the H2 RS which results in its highest performance. In Figs. 2 and 5, the momentary sharp jumps of the NOx concentration upon RS are observed. Even when the Blank-RS is employed, the maximum NOx concentration in the outlet gas is also higher than the NOx concentration in the inlet gas (Fig. 5D). This result indicates that the momentary sharp jump is caused by the desorption or release of the stored NOx from the catalyst.
N. Takahashi et al. / Applied Catalysis B: Environmental 70 (2007) 198–204
Fig. 7. The emitted NOx concentration of the sixth RS at 250 8C vs. the concentration of the chemical equivalent of the reducing agents in the RS: (*) H2; (*) CO; () C3H6; (~) no reducing agents.
The difference in the NOx concentration in the outlet gas between the end point of the lean period and the peak point of RS, or the sharp jump in the NOx concentration at the switching point, is noted as the emitted NOx concentration hereafter. This emitted NOx concentration reveals the actual performance of the NSR catalyst. Fig. 7 shows the emitted NOx concentration of the sixth RS versus the concentration of the chemical equivalent of the reducing agents in the RS. The emitted NOx concentration can be quite high if the desorbed NOx is not fully or efficiently reduced to N2, as shown by reaction (5): NOx þ R ! ð1=2ÞN2 þ ROx
ðR : reducing agentsÞ
(5)
The emitted NOx concentration rises with the increasing concentration of the chemical equivalent with C3H6 up to 1.0%, while maintaining a somewhat stable value when the concentration is above 1.0%. The emitted NOx concentration gradually decreases with the increasing H2 concentration, while it varies only slightly with CO as the reducing agent. The effect of the reducing agents on the reduction of the released NOx is different from each other, and the reactivity decreases in the order H2 > CO > C3H6. Among the reducing agents investigated in this study, H2 had the highest activity for both the release of the stored NOx and the reduction of the released NOx. 3.3. H2 generation over the NSR catalyst at low temperatures The above results showed that H2 is the most active and efficient reducing agent among H2, CO and C3H6. In addition, to the inherent properties of these two molecules, is there any other factor that makes the difference between CO and C3H6? This will be very important for the optimization of the practical NSR catalyst. It is well known that H2 could be generated on catalysts in a rich atmosphere with CO and C3H6. For example, CO reacted with H2O on the Pt catalyst supported on cerium oxides to produce H2, i.e., the so-called water gas shift (WGS) reaction, as shown by reaction (6) [12–15]. Steam reforming (SR) is another reaction to form H2 through the reaction between HC and H2O on the Rh catalyst supported on the
203
Fig. 8. H2 concentration in the outlet gas vs. time when the lean and rich conditions were alternately switched every 30 s: ( ) water gas shift reaction; (—) steam reforming reaction. The dashed line represents the reaction temperature, which decreased at the rate of 10 8C/min.
thermal-resistant zirconium oxide in our NSR catalyst, as shown by reaction (7) [5]: CO þ H2 O ! CO2 þ H2
(6)
C3 H6 þ 6H2 O ! 3CO2 þ 9H2
(7)
As described in Section 2.1, the NSR catalyst contains the necessary compounds and works in the atmosphere for the WGS and SR reactions. The formed H2 on the NSR catalyst will facilitate the reduction of the stored NOx, which are one of the critical factors related to the performance of the NSR catalyst. Therefore, the H2 generation performance of the WGS and SR reactions were also investigated. The lean and rich atmospheres as listed in Table 3 were alternately and periodically switched every 30 s, and its temperature was decreased from 400 to 200 8C at the rate of 10 8C/min. Fig. 8 shows the H2 concentration in the outlet gas versus time. The temperature range was from 280 to 230 8C. The H2 concentration of the WGS reaction was higher than that of the SR reaction. Moreover, the response of the increasing H2 concentration with the WGS reaction was faster than that with the SR reaction, and the difference increased with the decreasing temperature. Thus, the difference in the H2 generation performance between the WGS and the SR reactions over the NSR catalyst seems to be one of the reasons why the stored NOx reduction activity of CO was higher than that of C3H6 as the RS reducing agent. Therefore, a higher reducing performance of the stored NOx on the NSR catalyst could be achieved if the H2 generation activity by the WGS and SR reactions is enhanced. 4. Conclusions The NOx storage and the subsequent reduction of the stored NOx at low temperatures over the NSR catalyst for lean-burn gasoline engines have been systematically investigated and discussed in this study. We can conclude that the reduction of the stored NOx was the restricted stage for the entire NOx storage and reduction process below 400 8C. At low temperatures, the activities of the reducing agents to reduce
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the stored NOx are different and decreases in the order H2 > CO > C3H6. A part of the NOx storage sites was not regenerated when an excess of CO or C3H6 was supplied to the NSR catalysts, while all the NOx storage sites could be fully regenerated when adequate H2 was supplied. The experimental results indicated that the release rate of the stored NOx was the determining factor for the reduction of the stored NOx when CO or C3H6 was used as the reducing agent. We also found that the H2 generation by the water gas shift reaction on the NSR catalysts was more apparent and efficient than that through the steam reforming reaction. It is regarded as one of the reasons why the activity of CO to reduce the stored NOx was higher than that of C3H6. Therefore, the promotion of the H2 generation activity by the water gas shift reaction is expected to be a promising approach to improve the performance of the NSR catalyst at low temperatures. Acknowledgement The authors would like to thank Dr. Fei Dong for the manuscript preparation.
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