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Al Mixed Oxides
Science and Technology in Catalysis 1^ Copyright © 1999 by Kodansha Ltd.
61 Selective Reduction of NOxby Hydrocarbon over VIII-Metal/AI Mixed Oxides
Yasuyuki BANNO, Masaharu HATANO, Hiroo KINOSHITA Next Generation Catalyst Research Institute, Co., Ltd., Kanagawa Science Park, Sakado, Takatsuku, Kawasaki-shi, Kanagawa-ken 213-0012, Japan
Abstract The effects of preparation method and condition on group VHt metal/Al mixed oxide catalyst were investigated for the reduction of NOx by hydrocarbon in an oxygen-rich atmosphere. It was found that the NOx removal activities can be improved by applying the co-precipitation method and the activities were strongly dependent on the preparation conditions. Zr, Zn and Ga were found to be effective additives for Rh-Al oxide catalyst. l.INTRODUCTION The selective catalytic reduction (SCR) of NOx by hydrocarbons in the presence of oxygen has been extensively studied and various catalysts have been reported to be active. Among them, metal catalysts supported on alumina is well known as a potential catalyst for practical use because of high activity and high durability [1]. In the present work, we have focused on the performances of Co-Al oxide and Rh-Al oxide catalysts and have investigated the influence of preparation conditions and additives on the catalysts. 2.EXPERIMENTAL 2.1.Catalysts Preparation The catalysts were mainly prepared from co-precipitation of a solution containing group VIIIgroup metal and aluminum nitrates by ammonia aqueous solution. After filtering and drying in air at 383 K, the samples were calcined at 1073 K for 5 h. The ripening conditions (pH, temperature, time) of the suspension containing precipitate were varied to find the appropriate condition and to improve the catalytic activity. 2.2.H2-TPR The sample was submitted to pretreatment in 10% Oi/He flow at 873 K for 30 min. The sample was then cooled to 323 K, and heated to 1073 K at 5°C*min-i in 2% H2/He flow. H2 consumption was monitored by mass-spectrometer. 2.3.Catalytic Testing The catalytic tests were carried out using a fixed-bed flow reactor with simulated lean-bum exhaust gas. The gas mixture consisting of 1000 ppm NO, 1000 ppm C3H6, 6% O2, 1200 ppm CO,
379
380 Y.Banno etal.
10% CO2, 400 ppm H2 and 10% H2O in N2 was fed into the catalyst bed. Gas hourly space velocity was between 100000 and 200000 h-i. The measurement was done after a steady-state was obtained at each temperature. The gas composition was monitored by the analyzer equipped with chemiluminescence detector for NOx, FID detector for total hydrocarbon. 3.RESULTS AND DISCUSSION 3.1.Co-AI-oxide Fig. 1 shows the catalytic activity of various Co-Al oxides as a function of reaction temperature. It should be noted that composition and calcination temperature for every catalyst are the same. NOx and C3H6 conversions were strongly dependent on conditions of ripening precipitate in suspension. It is suggested that the ripening the precursor at high pH condition is preferable to NOx reduction. It also can be seen that the active catalyst for NOx reduction tends to be inactive for C3H6 conversion. These results strongly suggest that the properties of Co site on Co-Al-oxides are influenced by the pretreatment conditions of the precursor. In addition, it is suggested that the efficiency of hydrocarbon utilization for NOx reduction, which is one of the most important factors for SCR, can be optimized by controlling the preparation conditions. The XRD patterns for the catalysts are shown in Fig. 2. It was found that the crystallinity of CoAl-oxides was dependent on the preparation conditions, while the structure of each catalyst was similar spinel structure to one another. BET surface area of Co-Al-oxide (1), (2) and (3) were 125.1 m2g-i, 165.9 m2g-i and 169.8 m^g-i respectively. It seems reasonable to suppose that the result of surface area measurement is consistent with that of XRD analysis. Since the results of Fig. 1 and Fig. 2 indicates that the maximum conversion of NOx increased with increasing the crystallinity of the oxide, it may be that Co ion stabilized at a particular site of the oxide lattice is favorable for SCR of NOx. It is likely that Co exists at M site of MAI2O4 [2]. 3.2.Rh-Al-oxide Catalytic activities of SCR for supported rhodium catalysts (denoted as Rh/Al203) prepared by impregnation method and rhodium-aluminum mixed oxide catalyst without and with zirconium (denoted as Rh-Al-0x(9) and Rh-Zr-Al-0x(9) ) prepared by co-precipitation method were
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20 • • • • i • • • . 1 o " 0 fi^L 600 700 800 900 1000 Temperature / K •
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Fig. 1 Preparation condition effect on (a) NOx and (b) C3H6 conversions over Co-Al-oxide (Co /Al=l/15) catalysts at SV=200000 h-l. Ripening conditions; • pH=9, 353 K, 5 h; A pH=7, 353 K, 5 h; D pH=7, room temperature, 5 min.
1.2k 0.8k c/)
Q. O
0.4k 0.0k
Fig. 2 XRD patterns for Co-Al-oxides. Ripening conditions; (1) pH=9, 353 K, 5 h; (2) pH=7, 353 K, 5 h; (3) pH=7, room temperature, 5 min.
381
500
600 700 800 Temperature / K
673 773 873 973 1073 Temperature / K
900
Fig. 3 NOx conversions over ( D ) Rh/Al203
Fig. 4
(Rh/Al=l/160), (A) Rh-Al-0x(9) (Rh/Al=l
(Rh/Al=l/160), (A) Rh-Al-0x(9) (Rh/Al=l
TPR profiles for ( D ) Rh/Al203
/160), ( • ) Rh-Zr-Al-0x(9) (Rh/Zr/Al= 1/80/80)
/160), ( • ) Rh-Zr-Al-0x(9) (Rh/Zr/Al= 1/80/80).
atSV=100000h-i.
measured. The precipitation was treated at 353 K for 5 h (pH value is designated in a parenthesis). NOx conversions as a function of temperature for the catalysts are illustrated in Fig. 3. It is clear from the figure that Rh-Al-0x(9) showed higher NOx conversion than Rh/A^Oa of same Rh content. Furthermore, the addition of Zr to Rh-Al-0x(9) enhanced NOx reduction activity. Burch et al. [3,4] reported that the reduction characteristics of Rh species on Rh/Al203 and Rh/Zr02-Al203 catalyst systems influenced the activity of SCR. Thus, the catalysts tested in the experiment of Fig. 3 were characterized by H2-TPR. Fig 4 shows the TPR profiles normalized on the basis of the amount of rhodium for the catalysts. Rh/Al203, less active for SCR, showed large reduction peak starting from 673 K. Other catalysts prepared by co-precipitation method showed smaller reduction peak than Rh/A^Os, although the temperature at which the reduction begins did not change. Moreover, it should be noted that the activity of NOx removal was strongly correlated with the amount of H2 consumption observed in the TPR experiment. The results of Figs. 3 and 4 indicate that the rhodium in the catalyst which is hard to reduce is favorable for SCR. Since the easily reducible rhodium is known to be active for C3H6 combustion [4], rhodium species in Rh/Al203 would be undesirable for SCR because of the waste of C3H6 by the reaction between C3H6 and O2 in preference to selective reduction of NOx. In the case of mixed oxide catalysts such as Rh-Al-0x(9) and Rh-Zr-Al-0x(9), on the contrary, the lack of reducible rhodium depresses the C3H6 consumption independent of NOx reduction. Therefore, those catalysts can utilize C3H6 efficiently for SCR. Further studies were carried out for Rh-Zr-Al-Ox catalyst system by investigating the effect of 2.0k
fi-1.0k
600 700 800 Temperature / K
900
Fig. 5 NOx conversions over (D) Rh-Zr-AlOx(7) (Rh/Zr/Al=l/80/80), ( A ) Rh-Zr-Al-0x((9) (Rh/Zr/Al= 1/80/80), ( • ) Rb-Zn-Zr-Al-0x(7) (Rh/Zn /Zr/Al=l/10/70/80) at SV=100000 h-i.
0.0k'
Fig. 6 XRD patterns for (1) Rh-Zn-Zr-Al-0x(7) (Rh/Zn/Zr/Al=l/10/70/80), (2) Rh-Zr-Al-0x((9) (Rh/Zr/Al=l/80/80), (3) Rh-Zr-Al-0x(7) (Rh/Zr /Al=l/80/80).
382 Y.Banm etal.
600 700 800 Temperature / K
900
Fig. 7 NOx conversions over ( A ) Rh-Zr-AlOx(9) (Rh/Zr/Al=l/80/80), ( D ) Rh-Ga-Zr-AlOx((9) (Rh/Ga/Zr/Al=l/10/70/80). ( • ) Rh-GaZr-Al-0x(9) (Rh/Ga/Zr/Al= 1/20/60/80) at SV=100000h-i.
Fig. 8 XRD patterns for (1) Rh-Ga-Zr-Al-0x(9) (Rh/Ga/Zr/Al= 1/20/60/80), (2) Rh-Ga-Zr-AlOx((9) (Rh/GayZr/Al=l/10/70/80), (3) Rh-Zr-AlOx(9) (Rh/Zr/Al=l/80/80).
preparation conditions and additives. Fig. 5 shows the effect of pH for precipitation and zinc addition on NOx conversion. It is seen that the NOx conversion of Rh-Zr-Al-0x(7) was much lower than that of Rh-Zr-Al-0x(9). However, zinc addition restored the activity even if the catalyst was prepared at the same pH (=7) condition. In order to analyze these effects, XRD measurement was carried out for characterization of the catalysts. Fig. 6 shows the XRD patterns of the samples. The symbols represent diffraction peak for Y-AI2O3 and other peaks were assigned to cubic Zr02. XRD did not detect any Rh-oxide and Zn-oxide phase in this experiment. It is obvious that the active catalysts such as Rh-Zr-Al-0x(9) and Rh-Zn-Zr-Al-0x(7) exhibits clearly the diffraction peaks of AI2O3 phase, while AI2O3 phase completely disappeared in the case of less active catalyst such as Rh-Zr-Al-0x(7). These results indicate that the high pH condition and zinc addition exerted similar effect on the catalysts from the viewpoint of crystallinity of the oxides. Fig. 7 shows the effect of gallium addition to Rh-Zr-Al-Ox catalysts on the NOx conversion. Each catalyst was prepared at same conditions. It is clearly shown that maximum NOx conversion was enhanced by gallium addition, and the enhancement effect increased with increasing gallium content. Fig. 8 shows XRD patterns of the samples tested in the experiment of Fig. 7. The crystallinity of yAI2O3 increased, and the peak positions shifted to lower angle by gallium addition. In addition, it should be pointed out that the extent of the peak growth and the peak shift increased with increasing gallium content. Therefore, it is possible to suppose that the enhancement of NOx conversion was related to the change in crystallinity which modified the state of rhodium dispersed in the oxide. These correlations between the activity and crystalUnity observed for Rh-A-Zr-Al-Ox (A= additive) may be explained by the similar manner mentioned in the case of the Co-Al-oxide catalyst. 4.CONCLUSION Our research for Co-Al oxide and Rh-Al oxide catalysts has shown the possibility that the SCR activities can be improved by applying the co-precipitation method under proper conditions and choosing the appropriate additives such as zirconium, zinc and gallium. The points to notice are the stability of oxidation state of the metal ion and the properties of the oxide which stabilizes the active metal ions and its properties. 5.REFERENCES [1] RHamada, Catal. Today, 22 (1994) 21. [2] N. Okazaki, Y. Katoh, Y. Shiina, A. Tada and M. Iwamoto, Chem. Lett., (1997) 889. [3] R. Burch and P.K. Loader, Appl. Catal. A, 143 (1996) 317. [4] R. Burch, P.K. Loader and N.A. Cruise, Appl. Catal. A, 147 (1996) 375.
61 Selective Reduction of NOxby Hydrocarbon over VIII-Metal/AI Mixed Oxides
Yasuyuki BANNO, Masaharu HATANO, Hiroo KINOSHITA Next Generation Catalyst Research Institute, Co., Ltd., Kanagawa Science Park, Sakado, Takatsuku, Kawasaki-shi, Kanagawa-ken 213-0012, Japan
Abstract The effects of preparation method and condition on group VHt metal/Al mixed oxide catalyst were investigated for the reduction of NOx by hydrocarbon in an oxygen-rich atmosphere. It was found that the NOx removal activities can be improved by applying the co-precipitation method and the activities were strongly dependent on the preparation conditions. Zr, Zn and Ga were found to be effective additives for Rh-Al oxide catalyst. l.INTRODUCTION The selective catalytic reduction (SCR) of NOx by hydrocarbons in the presence of oxygen has been extensively studied and various catalysts have been reported to be active. Among them, metal catalysts supported on alumina is well known as a potential catalyst for practical use because of high activity and high durability [1]. In the present work, we have focused on the performances of Co-Al oxide and Rh-Al oxide catalysts and have investigated the influence of preparation conditions and additives on the catalysts. 2.EXPERIMENTAL 2.1.Catalysts Preparation The catalysts were mainly prepared from co-precipitation of a solution containing group VIIIgroup metal and aluminum nitrates by ammonia aqueous solution. After filtering and drying in air at 383 K, the samples were calcined at 1073 K for 5 h. The ripening conditions (pH, temperature, time) of the suspension containing precipitate were varied to find the appropriate condition and to improve the catalytic activity. 2.2.H2-TPR The sample was submitted to pretreatment in 10% Oi/He flow at 873 K for 30 min. The sample was then cooled to 323 K, and heated to 1073 K at 5°C*min-i in 2% H2/He flow. H2 consumption was monitored by mass-spectrometer. 2.3.Catalytic Testing The catalytic tests were carried out using a fixed-bed flow reactor with simulated lean-bum exhaust gas. The gas mixture consisting of 1000 ppm NO, 1000 ppm C3H6, 6% O2, 1200 ppm CO,
379
380 Y.Banno etal.
10% CO2, 400 ppm H2 and 10% H2O in N2 was fed into the catalyst bed. Gas hourly space velocity was between 100000 and 200000 h-i. The measurement was done after a steady-state was obtained at each temperature. The gas composition was monitored by the analyzer equipped with chemiluminescence detector for NOx, FID detector for total hydrocarbon. 3.RESULTS AND DISCUSSION 3.1.Co-AI-oxide Fig. 1 shows the catalytic activity of various Co-Al oxides as a function of reaction temperature. It should be noted that composition and calcination temperature for every catalyst are the same. NOx and C3H6 conversions were strongly dependent on conditions of ripening precipitate in suspension. It is suggested that the ripening the precursor at high pH condition is preferable to NOx reduction. It also can be seen that the active catalyst for NOx reduction tends to be inactive for C3H6 conversion. These results strongly suggest that the properties of Co site on Co-Al-oxides are influenced by the pretreatment conditions of the precursor. In addition, it is suggested that the efficiency of hydrocarbon utilization for NOx reduction, which is one of the most important factors for SCR, can be optimized by controlling the preparation conditions. The XRD patterns for the catalysts are shown in Fig. 2. It was found that the crystallinity of CoAl-oxides was dependent on the preparation conditions, while the structure of each catalyst was similar spinel structure to one another. BET surface area of Co-Al-oxide (1), (2) and (3) were 125.1 m2g-i, 165.9 m2g-i and 169.8 m^g-i respectively. It seems reasonable to suppose that the result of surface area measurement is consistent with that of XRD analysis. Since the results of Fig. 1 and Fig. 2 indicates that the maximum conversion of NOx increased with increasing the crystallinity of the oxide, it may be that Co ion stabilized at a particular site of the oxide lattice is favorable for SCR of NOx. It is likely that Co exists at M site of MAI2O4 [2]. 3.2.Rh-Al-oxide Catalytic activities of SCR for supported rhodium catalysts (denoted as Rh/Al203) prepared by impregnation method and rhodium-aluminum mixed oxide catalyst without and with zirconium (denoted as Rh-Al-0x(9) and Rh-Zr-Al-0x(9) ) prepared by co-precipitation method were
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60 40
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20 • • • • i • • • . 1 o " 0 fi^L 600 700 800 900 1000 Temperature / K •
•
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Fig. 1 Preparation condition effect on (a) NOx and (b) C3H6 conversions over Co-Al-oxide (Co /Al=l/15) catalysts at SV=200000 h-l. Ripening conditions; • pH=9, 353 K, 5 h; A pH=7, 353 K, 5 h; D pH=7, room temperature, 5 min.
1.2k 0.8k c/)
Q. O
0.4k 0.0k
Fig. 2 XRD patterns for Co-Al-oxides. Ripening conditions; (1) pH=9, 353 K, 5 h; (2) pH=7, 353 K, 5 h; (3) pH=7, room temperature, 5 min.
381
500
600 700 800 Temperature / K
673 773 873 973 1073 Temperature / K
900
Fig. 3 NOx conversions over ( D ) Rh/Al203
Fig. 4
(Rh/Al=l/160), (A) Rh-Al-0x(9) (Rh/Al=l
(Rh/Al=l/160), (A) Rh-Al-0x(9) (Rh/Al=l
TPR profiles for ( D ) Rh/Al203
/160), ( • ) Rh-Zr-Al-0x(9) (Rh/Zr/Al= 1/80/80)
/160), ( • ) Rh-Zr-Al-0x(9) (Rh/Zr/Al= 1/80/80).
atSV=100000h-i.
measured. The precipitation was treated at 353 K for 5 h (pH value is designated in a parenthesis). NOx conversions as a function of temperature for the catalysts are illustrated in Fig. 3. It is clear from the figure that Rh-Al-0x(9) showed higher NOx conversion than Rh/A^Oa of same Rh content. Furthermore, the addition of Zr to Rh-Al-0x(9) enhanced NOx reduction activity. Burch et al. [3,4] reported that the reduction characteristics of Rh species on Rh/Al203 and Rh/Zr02-Al203 catalyst systems influenced the activity of SCR. Thus, the catalysts tested in the experiment of Fig. 3 were characterized by H2-TPR. Fig 4 shows the TPR profiles normalized on the basis of the amount of rhodium for the catalysts. Rh/Al203, less active for SCR, showed large reduction peak starting from 673 K. Other catalysts prepared by co-precipitation method showed smaller reduction peak than Rh/A^Os, although the temperature at which the reduction begins did not change. Moreover, it should be noted that the activity of NOx removal was strongly correlated with the amount of H2 consumption observed in the TPR experiment. The results of Figs. 3 and 4 indicate that the rhodium in the catalyst which is hard to reduce is favorable for SCR. Since the easily reducible rhodium is known to be active for C3H6 combustion [4], rhodium species in Rh/Al203 would be undesirable for SCR because of the waste of C3H6 by the reaction between C3H6 and O2 in preference to selective reduction of NOx. In the case of mixed oxide catalysts such as Rh-Al-0x(9) and Rh-Zr-Al-0x(9), on the contrary, the lack of reducible rhodium depresses the C3H6 consumption independent of NOx reduction. Therefore, those catalysts can utilize C3H6 efficiently for SCR. Further studies were carried out for Rh-Zr-Al-Ox catalyst system by investigating the effect of 2.0k
fi-1.0k
600 700 800 Temperature / K
900
Fig. 5 NOx conversions over (D) Rh-Zr-AlOx(7) (Rh/Zr/Al=l/80/80), ( A ) Rh-Zr-Al-0x((9) (Rh/Zr/Al= 1/80/80), ( • ) Rb-Zn-Zr-Al-0x(7) (Rh/Zn /Zr/Al=l/10/70/80) at SV=100000 h-i.
0.0k'
Fig. 6 XRD patterns for (1) Rh-Zn-Zr-Al-0x(7) (Rh/Zn/Zr/Al=l/10/70/80), (2) Rh-Zr-Al-0x((9) (Rh/Zr/Al=l/80/80), (3) Rh-Zr-Al-0x(7) (Rh/Zr /Al=l/80/80).
382 Y.Banm etal.
600 700 800 Temperature / K
900
Fig. 7 NOx conversions over ( A ) Rh-Zr-AlOx(9) (Rh/Zr/Al=l/80/80), ( D ) Rh-Ga-Zr-AlOx((9) (Rh/Ga/Zr/Al=l/10/70/80). ( • ) Rh-GaZr-Al-0x(9) (Rh/Ga/Zr/Al= 1/20/60/80) at SV=100000h-i.
Fig. 8 XRD patterns for (1) Rh-Ga-Zr-Al-0x(9) (Rh/Ga/Zr/Al= 1/20/60/80), (2) Rh-Ga-Zr-AlOx((9) (Rh/GayZr/Al=l/10/70/80), (3) Rh-Zr-AlOx(9) (Rh/Zr/Al=l/80/80).
preparation conditions and additives. Fig. 5 shows the effect of pH for precipitation and zinc addition on NOx conversion. It is seen that the NOx conversion of Rh-Zr-Al-0x(7) was much lower than that of Rh-Zr-Al-0x(9). However, zinc addition restored the activity even if the catalyst was prepared at the same pH (=7) condition. In order to analyze these effects, XRD measurement was carried out for characterization of the catalysts. Fig. 6 shows the XRD patterns of the samples. The symbols represent diffraction peak for Y-AI2O3 and other peaks were assigned to cubic Zr02. XRD did not detect any Rh-oxide and Zn-oxide phase in this experiment. It is obvious that the active catalysts such as Rh-Zr-Al-0x(9) and Rh-Zn-Zr-Al-0x(7) exhibits clearly the diffraction peaks of AI2O3 phase, while AI2O3 phase completely disappeared in the case of less active catalyst such as Rh-Zr-Al-0x(7). These results indicate that the high pH condition and zinc addition exerted similar effect on the catalysts from the viewpoint of crystallinity of the oxides. Fig. 7 shows the effect of gallium addition to Rh-Zr-Al-Ox catalysts on the NOx conversion. Each catalyst was prepared at same conditions. It is clearly shown that maximum NOx conversion was enhanced by gallium addition, and the enhancement effect increased with increasing gallium content. Fig. 8 shows XRD patterns of the samples tested in the experiment of Fig. 7. The crystallinity of yAI2O3 increased, and the peak positions shifted to lower angle by gallium addition. In addition, it should be pointed out that the extent of the peak growth and the peak shift increased with increasing gallium content. Therefore, it is possible to suppose that the enhancement of NOx conversion was related to the change in crystallinity which modified the state of rhodium dispersed in the oxide. These correlations between the activity and crystalUnity observed for Rh-A-Zr-Al-Ox (A= additive) may be explained by the similar manner mentioned in the case of the Co-Al-oxide catalyst. 4.CONCLUSION Our research for Co-Al oxide and Rh-Al oxide catalysts has shown the possibility that the SCR activities can be improved by applying the co-precipitation method under proper conditions and choosing the appropriate additives such as zirconium, zinc and gallium. The points to notice are the stability of oxidation state of the metal ion and the properties of the oxide which stabilizes the active metal ions and its properties. 5.REFERENCES [1] RHamada, Catal. Today, 22 (1994) 21. [2] N. Okazaki, Y. Katoh, Y. Shiina, A. Tada and M. Iwamoto, Chem. Lett., (1997) 889. [3] R. Burch and P.K. Loader, Appl. Catal. A, 143 (1996) 317. [4] R. Burch, P.K. Loader and N.A. Cruise, Appl. Catal. A, 147 (1996) 375.