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Al2O3: Inhibition effect by surface nitrate
Science and Technology in Catalysis 1998 Copyright © 1999 by Kodansha Ltd.
59 Selective Catalytic Reduction of NO over Ag/AbOs: Inhibition Effect by Surface Nitrate
Makoto YAMAGUCHI, Ichiro GOTO, Zheng Ming WANG and Mikio KUMAGAI Institute of Research and Innovation, 1201 Takada, Kashiwa, Chiba 277-0861, Japan Abstract Catalytic activity of Ag/Al20 for selective NOx reduction by hydrocarbons was studied. The activity decreased drastically when NO/hydrocarbon concentration ratio in the feed exceeded a certain value, in contrast to the cases of Al203and C0/AI2O3. Supported silver on alumina was characterized as Ag^ under oxdizing condition. High temperature calcination was effective to enhance the activity in spite of the decrease in surface area, indicating the formation of specific surface state of supported silver and alumina. Amount of Br0nsted acid sites increased by addition of silver as probed by NH adsorption. IR spectra of adsorbed species were measured under simulated feed gas and peaks assigned to nitrate ion were found to be much stronger on Ag/A\^Oy This nitrate species adsorbed on active sites inhibits the reaction of NOx and hydrocarbon to form nitrogen. A reaction scheme on alumina catalysts was delineated and further prospect was mentioned. 1. INTRODUCTION Catalytic removal of NOx from exhausts has been paid much attention in recent years. Among various kinds of metal oxide catalysts, silver alumina has been extensively studied because of its high NOx reduction activity by oxygen containing compounds as reductants [1]. It was also reported that the catalyst is relatively stable under the feed gas containing water and sulfur dioxide [2]. This character of Ag/Al^Og has induced recent further studies for application to diesel exhausts using hydrocarbons as reductants [3-8]. However, its catalytic activity should be improved for commercial application, especially in the lower temperature region since the activity decreases abruptly [5]. Furthermore, the activity was strongly dependent on NO/HC(hydrocarbon) ratio, which is unfavorable for application to automobile exhausts whose composition changes significantly along with the changes in engine operation. In this study, catalytic activity of Ag/Alfi^ was tested under various feed composition. Supported state of silver on alumina was also studied since there are relatively feew reports on that topic compared to the case of other metal-alumina catalysts as Co/Al^Og [9-11]. 2. EXPERIMENTAL Metal supprted alumina catalysts were prepared by impregnation of y-alumina (Mizusawa Kagaku, GB) with O.OS-'O.SM (O.IM was typical) metal nitrate solutions at 90°C. Dried samples were calcined in air at 500~1000°C. Fixed bed flow reactors (bed volume: 5 and 40ml) were used for activity tests. Conversions were calculated from NO and HC concentrations of inlet and outlet flows measured with chemiluminescent NOx analyzer and TOC meter or GC, respectively. Differential heat of adsorption was measured by Tokyo Riko HAC-450G microcalorimeter. Small amount of ammonia was introduced to the sample at 200°C. IR spectra were measured by Shimadzu FTIR-8100 spectrometer with self supporting catalyst disks held in a glass reactor cell. 3. RESULTS AND DISCUSSION 3.1.NOX conversion over alumina catalysts Figure 1 shows temperature dependence of NO conversion over alumina catalysts. Temperature at 371
372 M. Yamaguchi et al. C0/AI2O3 (450°C)
100 H
Ag/Al203 (400°C)
loo 80
o
60 h
60
I 9
6
40
c
20
400 500 Temperature / °C
500 1000 NO Concentration / ppm
Figure 1. Temperature Dependence of NO and Propane Conversion
Figure 2. NO Concentration Dependence of NO and Propane Conversion
NO: lOOOppm, Propane:! lOOppm, 02:10%, H2O:0%, SV:18000h',reactor:5cc
Propane: 1 lOOppm, 02:10%, H2O:0%, SV:18000h' reactor: 5cc
maximum activity shifted to lower value by addition of metals. It should be mentioned that the activity over Ag/AljOg decreased significantly at 400^^0, in contrast to the case of Co/Al^G.. Figure 2 shows the concentration dependence of NO conversion over those catalysts at temperature sUghtly below the maximum activity. In the case of Aip^ and Co/Al^Og, NO and HC conversion slightly decreased as NO concentration increased with propane concentration fixed in the absence of water in the feed. On the contrary, NO conversion over Ag/Al (X decreased abruptly as the NO concentration increased. When HC concentration was varied with NO concentration fixed, NOx conversion again decreased when NO/HC ratio exceeded --0.5. Theese phenomena were also observed when other C^-C^ alkane/alkene were used as reductants. Concentration dependence over AI2O3 and Co/Al O3 can be qualitatively explained by LangmuirHinshelwood type rate equation assuming the reaction oetween adsorbed NOx and reductant as the rate determining step. In contrast, conversion over Ag/Al203 cannot be simply explained by that scheme. Since both NO and HC conversions decreased in parallel, it was assumed that some inhibition effect might occur when NO/HC ratio becomes high.
200| I I • I I I I I I I
500°C
700°C
900^^0 •hi
111111111111
200 250 300 350 400 450 500 Wavelength / nm Figure 3. UV-Vis Diffuse Reflectance Spectra
0
100 200 300 400 500 Weight of adsorbed NH^ / |imol/g
Figure 4. Differential Heat of Adsorption of NH adsorption : 200°C Numbers indicate amount of silver on alumina in wt%
373
3.2.Characterization of silver alumina In the case of C0/AI2O3, it has been already revealed that highly dispersed Co^^ ion is acitive for NOx reduction and its dispersion is affected by calcination temperature[7-9]. In contrast, supported state of silver on alumina has not been well understood and we have studied supported state of silver on alumina. As already reported, excess loading of silver lowers its activity because hydrocarbons are oxidized without reacting with NOx. Thus we have mainly analyzed catalysts with silver loading of ~2.5wt% which showed the highest activity. Their TEM images and XRD patterns did not show any tendency of formation of silver metal and/or oxide particles. In UV-visible diffuse reflectance spectra a peak appeared at 240nm as shown in Figure 3 which is assigned to Ag^ ion when pretreated under oxidizing condition at 500°C. These results indicate that silver is dispersively supported as Ag^ ion in those highly active Ag/Al O^ catalysts under reaction conditions. We nave also studied the effect of calcination temperature. In the present case, calcination at 800°C was effective to increase NOx reduction activity, although the change is much smaller than that for Co/Al O3 and no apparent phase change of supported silver was detected. However, the peak of Ag^ in UV visible spectra changed in its width as shown in Figure 3, indicating the change in the coordination of Ag"^ at higher temperature. As the relevance of surface acidity to NOx reduction activity of metal oxide catalysts has been pointed out, we have examined the effect of supported silver on the acidity of alumina in a quantitative manner by microcalorimetry. Differential heat of adsorption of ammonia was measured for Ag/Al203 catalysts pretreated in air at 500°C. Ammonia was introduced at 200°C to the sample. As shown in Figure 4, the amount of strong Lewis acid sites (q^ >150kJ/mol) decreased, while moderate acid sites (q^-lOOkJ/mol) which has been assigned to Br0nsted acid sites increased by addition of'-2.6wt% silver. IR spectra of OH stretching region of pretreated samples and after adsorption ammonia both indicated the formation of Br0nsted acid sites. The amount of Br0nsted acid sites decreased as excess amount of silver is loaded and this trend was in parallel with the NOx reduction activity. These effects of silver were similar to the case of alkaline metals [10], although the formation of Br0nsted acid was more pronounced in the present case. The formation of Br0nsted acid sites can be interpreted by the coordination of Ag^ ions to type I and II hydroxyls and this ability of coordination may be attributed to the participation of d orbitals on the bond formation of silver which is absent in the case of alkaline atoms. 3.3.Reaction of adsorbed species Reactivity of adsorbed NOx species was investigated by temperature programmed desorption after feeding NO+O^ gas at lOO^C to preatreated alumina catalysts. The peak appeared in a desorption curve (m/e=30) at ~500°C in the case of AI2O3 which has been assigned to decomposed products (NO) of surface NOx species. The peak shifted to 540°C in the case of Ag/ Al O3. indicating that silver was effective to convert NOx species to more stable form such as nitrate ions. Figure 5 shows IR spectra of adsorbed species over alumina catalysts exposed to C2H4/NO/O2 flow. At lower temperatures, peaks at 1050, 1250 and 1570cm^ which were assigned to nitrate (NO3) were observed. A peak tentatively assigned to NO2 was also observed at 1590cm^ in the case of AI2O3. However, this peak was not observed on Ag/Al O., while peaks of nitrate were stili observable at 550''C. As the tempereature rises, peaks of nitrate become weaker and peaks
X)
<
2200
2000
1600
1400
1200
1000
Wavenumber / cm'^
Figure 5. IR Spectra of Adsorbed Species on Alumina Catalysts (upper) AI2O3 (lower) Ag/Al203 feed: C H 0.2%, 0 2 10%, He balance, 60SCCM 2
i
374 M. Yama"uchi et al.
asigned to nitrite (NO ) (1080, 1230,1320 and 1480cm 0 NO3-S RH C and formate (HCOO) (1390 and 1590cm 0 appeared in both cases. A faint peak of surface isocyanate (Al-NCO) was also observable at 2240cm^ in both catalysts and an 02 NO2-S RCOO-S H2O additional peak at 2140cm^ on Ag/Al203was assigned to NO isocyanate on silver (Ag-NCO). Scheme 1 summarizes possible reactions on Ag/Al203. Adsorbed nitrite ions are converted to nitrate ions at Ag^ 02 (RNO2-S) HCOO-S sites by surface oxygen possibly due to weaker Ag-0 bonding. They remain on the active sites and inhibit further C02 reaction. If sufficient reductants are fed to the surface, nitrate H20 H 2 0 K ^ l ' ' NO2 ions are removed by reductants while they are oxidized to NCO-S ^ / form carboxylate. This is indicated by the increase of HC conversion along with the increase of NO concentration CO2 CO: when NO/HC ratio is low enough (Fig.2) and silver may be (NH3-S)->,^/N02 active for this reaction. However, if the NO/HC ratio is high, HC is insufficient for the removal of nitrate and thus the NO conversion decreases. The concomitant decrease of HC ^ . . ^^2 H2O conversion may indicate that it is not hydrocarbons itself ^cneme but partially oxidized ones on the surface which is reactive for the removal of nitrate. A key step in the scheme is the formation of organic nitro compounds by the reaction of nitrite and carboxylate. However, no nitro compound has not been actually detected on the catalyst surface under simulated feed gas in the present study. It is presumed that the nitro compounds are highly reactive that it further decomposes to yield isocyanate whose peaks were observed.
0)( ^)(
0
J0
Jl
0V
V,
4. CONCLUSION Selective catalytic reduction of NOx was examined with several metal alumina catalysts. Both NO and hydrocarbon conversion decreased drastically over Ag/Al203 as NO/HC ratio increased, in contrast to the cases of A1^03 and C0/AI2O3. Charaterization of Ag/Al203 indicated no formation of small particles of metallic or oxide phases of silver and silver is dispersed as Ag^ on the surface. Specific surface state is formed by the calcination at higher temperature (~800'^C) which is active for NOx reduction. The amount of Lewis acid sites decreased and the amount of Br0nsted acid sites increased by addition of small amount of silver and the effect was explained by surface model of alumina. IR spectra of adsorbed species on alumina catalysts were measured and it was found that peaks of surface nitrate were stronger on Ag/A\fl^ than AI2O3 and C0/AI2O3, which may inhibited the reaction over Ag/A\Py Although further studies are necessary to clarify the inhibition effect of nitrate ions and the role of silver, weak Ag-0 bonding may be closely related to the formation of nitrate. Thus it is expected that if secondary elements on Ag/AlX)-make oxygen atoms adjacent to silver less reactive, the formation of nitrate ions is suppressed and NOx reduction activity is improved in the lower temperature region. A further study is now in progress based on this working hypothesis. Acknowledgment This work was supported by Petroleum Energy Center (PEC) under the sponsorship of New Energy and Industrial Technology Development Organization (NEDO). References [I] T.Miyadera, Appl.Catal.BiEnronmental, 2 (1993) 199. [2] T.Miyadera and K.Yoshida, Chem.Lett. 1993, 1483. [3] N.Aoyama, K.Yoshida, A.Abe and T.Miyadera, Catal.Lett. 43 (1997) 249. [4] M.Haneda, Y.Kintaichi, M.Inaba and H.Hamada, Bull.Chem.Soc.Jpn. 70 (1997) 499. [5] K.A.Bethke and H.H.Kung, J.Catal. 172 (1997) 93. [6] T.E.Hoost, R.J.Kudla, K.M.Collins and M.S.Chattha, Appl.Catal.B:Environmental 13 (1997) 59. [7] H-W.Jen, Catal.Today, 42 (1998) 37. [8] M.Haneda, Y.Kintaichi, M.Inaba and H.Hamada, Catal.Today, 42 (1998) 127. [9] H.Hamada, Y.Kintaichi, M.Inaba, M.Sasaki, T.Ito, M.Tabata, T.Yoshinori, K.Miyamoto and H.Tsuchida, Shokubai (Catalyst), 36 (1994) 116. [10] N.Okazaki, S..Tsuda, M.Iwamoto and A.Tada, Shokubai (Catalyst), 38 (1996) 446. [II] J.Yan, M.C.Kung, W.M.H.Sachtler and H.H.Kung, J.Catal. 172 (1997) 178. [12] J.Shen, R.D.Cortright, Y.Chen and J.A.Dumesic, J.Phys.Chem. 98 (1994) 8067.
59 Selective Catalytic Reduction of NO over Ag/AbOs: Inhibition Effect by Surface Nitrate
Makoto YAMAGUCHI, Ichiro GOTO, Zheng Ming WANG and Mikio KUMAGAI Institute of Research and Innovation, 1201 Takada, Kashiwa, Chiba 277-0861, Japan Abstract Catalytic activity of Ag/Al20 for selective NOx reduction by hydrocarbons was studied. The activity decreased drastically when NO/hydrocarbon concentration ratio in the feed exceeded a certain value, in contrast to the cases of Al203and C0/AI2O3. Supported silver on alumina was characterized as Ag^ under oxdizing condition. High temperature calcination was effective to enhance the activity in spite of the decrease in surface area, indicating the formation of specific surface state of supported silver and alumina. Amount of Br0nsted acid sites increased by addition of silver as probed by NH adsorption. IR spectra of adsorbed species were measured under simulated feed gas and peaks assigned to nitrate ion were found to be much stronger on Ag/A\^Oy This nitrate species adsorbed on active sites inhibits the reaction of NOx and hydrocarbon to form nitrogen. A reaction scheme on alumina catalysts was delineated and further prospect was mentioned. 1. INTRODUCTION Catalytic removal of NOx from exhausts has been paid much attention in recent years. Among various kinds of metal oxide catalysts, silver alumina has been extensively studied because of its high NOx reduction activity by oxygen containing compounds as reductants [1]. It was also reported that the catalyst is relatively stable under the feed gas containing water and sulfur dioxide [2]. This character of Ag/Al^Og has induced recent further studies for application to diesel exhausts using hydrocarbons as reductants [3-8]. However, its catalytic activity should be improved for commercial application, especially in the lower temperature region since the activity decreases abruptly [5]. Furthermore, the activity was strongly dependent on NO/HC(hydrocarbon) ratio, which is unfavorable for application to automobile exhausts whose composition changes significantly along with the changes in engine operation. In this study, catalytic activity of Ag/Alfi^ was tested under various feed composition. Supported state of silver on alumina was also studied since there are relatively feew reports on that topic compared to the case of other metal-alumina catalysts as Co/Al^Og [9-11]. 2. EXPERIMENTAL Metal supprted alumina catalysts were prepared by impregnation of y-alumina (Mizusawa Kagaku, GB) with O.OS-'O.SM (O.IM was typical) metal nitrate solutions at 90°C. Dried samples were calcined in air at 500~1000°C. Fixed bed flow reactors (bed volume: 5 and 40ml) were used for activity tests. Conversions were calculated from NO and HC concentrations of inlet and outlet flows measured with chemiluminescent NOx analyzer and TOC meter or GC, respectively. Differential heat of adsorption was measured by Tokyo Riko HAC-450G microcalorimeter. Small amount of ammonia was introduced to the sample at 200°C. IR spectra were measured by Shimadzu FTIR-8100 spectrometer with self supporting catalyst disks held in a glass reactor cell. 3. RESULTS AND DISCUSSION 3.1.NOX conversion over alumina catalysts Figure 1 shows temperature dependence of NO conversion over alumina catalysts. Temperature at 371
372 M. Yamaguchi et al. C0/AI2O3 (450°C)
100 H
Ag/Al203 (400°C)
loo 80
o
60 h
60
I 9
6
40
c
20
400 500 Temperature / °C
500 1000 NO Concentration / ppm
Figure 1. Temperature Dependence of NO and Propane Conversion
Figure 2. NO Concentration Dependence of NO and Propane Conversion
NO: lOOOppm, Propane:! lOOppm, 02:10%, H2O:0%, SV:18000h',reactor:5cc
Propane: 1 lOOppm, 02:10%, H2O:0%, SV:18000h' reactor: 5cc
maximum activity shifted to lower value by addition of metals. It should be mentioned that the activity over Ag/AljOg decreased significantly at 400^^0, in contrast to the case of Co/Al^G.. Figure 2 shows the concentration dependence of NO conversion over those catalysts at temperature sUghtly below the maximum activity. In the case of Aip^ and Co/Al^Og, NO and HC conversion slightly decreased as NO concentration increased with propane concentration fixed in the absence of water in the feed. On the contrary, NO conversion over Ag/Al (X decreased abruptly as the NO concentration increased. When HC concentration was varied with NO concentration fixed, NOx conversion again decreased when NO/HC ratio exceeded --0.5. Theese phenomena were also observed when other C^-C^ alkane/alkene were used as reductants. Concentration dependence over AI2O3 and Co/Al O3 can be qualitatively explained by LangmuirHinshelwood type rate equation assuming the reaction oetween adsorbed NOx and reductant as the rate determining step. In contrast, conversion over Ag/Al203 cannot be simply explained by that scheme. Since both NO and HC conversions decreased in parallel, it was assumed that some inhibition effect might occur when NO/HC ratio becomes high.
200| I I • I I I I I I I
500°C
700°C
900^^0 •hi
111111111111
200 250 300 350 400 450 500 Wavelength / nm Figure 3. UV-Vis Diffuse Reflectance Spectra
0
100 200 300 400 500 Weight of adsorbed NH^ / |imol/g
Figure 4. Differential Heat of Adsorption of NH adsorption : 200°C Numbers indicate amount of silver on alumina in wt%
373
3.2.Characterization of silver alumina In the case of C0/AI2O3, it has been already revealed that highly dispersed Co^^ ion is acitive for NOx reduction and its dispersion is affected by calcination temperature[7-9]. In contrast, supported state of silver on alumina has not been well understood and we have studied supported state of silver on alumina. As already reported, excess loading of silver lowers its activity because hydrocarbons are oxidized without reacting with NOx. Thus we have mainly analyzed catalysts with silver loading of ~2.5wt% which showed the highest activity. Their TEM images and XRD patterns did not show any tendency of formation of silver metal and/or oxide particles. In UV-visible diffuse reflectance spectra a peak appeared at 240nm as shown in Figure 3 which is assigned to Ag^ ion when pretreated under oxidizing condition at 500°C. These results indicate that silver is dispersively supported as Ag^ ion in those highly active Ag/Al O^ catalysts under reaction conditions. We nave also studied the effect of calcination temperature. In the present case, calcination at 800°C was effective to increase NOx reduction activity, although the change is much smaller than that for Co/Al O3 and no apparent phase change of supported silver was detected. However, the peak of Ag^ in UV visible spectra changed in its width as shown in Figure 3, indicating the change in the coordination of Ag"^ at higher temperature. As the relevance of surface acidity to NOx reduction activity of metal oxide catalysts has been pointed out, we have examined the effect of supported silver on the acidity of alumina in a quantitative manner by microcalorimetry. Differential heat of adsorption of ammonia was measured for Ag/Al203 catalysts pretreated in air at 500°C. Ammonia was introduced at 200°C to the sample. As shown in Figure 4, the amount of strong Lewis acid sites (q^ >150kJ/mol) decreased, while moderate acid sites (q^-lOOkJ/mol) which has been assigned to Br0nsted acid sites increased by addition of'-2.6wt% silver. IR spectra of OH stretching region of pretreated samples and after adsorption ammonia both indicated the formation of Br0nsted acid sites. The amount of Br0nsted acid sites decreased as excess amount of silver is loaded and this trend was in parallel with the NOx reduction activity. These effects of silver were similar to the case of alkaline metals [10], although the formation of Br0nsted acid was more pronounced in the present case. The formation of Br0nsted acid sites can be interpreted by the coordination of Ag^ ions to type I and II hydroxyls and this ability of coordination may be attributed to the participation of d orbitals on the bond formation of silver which is absent in the case of alkaline atoms. 3.3.Reaction of adsorbed species Reactivity of adsorbed NOx species was investigated by temperature programmed desorption after feeding NO+O^ gas at lOO^C to preatreated alumina catalysts. The peak appeared in a desorption curve (m/e=30) at ~500°C in the case of AI2O3 which has been assigned to decomposed products (NO) of surface NOx species. The peak shifted to 540°C in the case of Ag/ Al O3. indicating that silver was effective to convert NOx species to more stable form such as nitrate ions. Figure 5 shows IR spectra of adsorbed species over alumina catalysts exposed to C2H4/NO/O2 flow. At lower temperatures, peaks at 1050, 1250 and 1570cm^ which were assigned to nitrate (NO3) were observed. A peak tentatively assigned to NO2 was also observed at 1590cm^ in the case of AI2O3. However, this peak was not observed on Ag/Al O., while peaks of nitrate were stili observable at 550''C. As the tempereature rises, peaks of nitrate become weaker and peaks
X)
<
2200
2000
1600
1400
1200
1000
Wavenumber / cm'^
Figure 5. IR Spectra of Adsorbed Species on Alumina Catalysts (upper) AI2O3 (lower) Ag/Al203 feed: C H 0.2%, 0 2 10%, He balance, 60SCCM 2
i
374 M. Yama"uchi et al.
asigned to nitrite (NO ) (1080, 1230,1320 and 1480cm 0 NO3-S RH C and formate (HCOO) (1390 and 1590cm 0 appeared in both cases. A faint peak of surface isocyanate (Al-NCO) was also observable at 2240cm^ in both catalysts and an 02 NO2-S RCOO-S H2O additional peak at 2140cm^ on Ag/Al203was assigned to NO isocyanate on silver (Ag-NCO). Scheme 1 summarizes possible reactions on Ag/Al203. Adsorbed nitrite ions are converted to nitrate ions at Ag^ 02 (RNO2-S) HCOO-S sites by surface oxygen possibly due to weaker Ag-0 bonding. They remain on the active sites and inhibit further C02 reaction. If sufficient reductants are fed to the surface, nitrate H20 H 2 0 K ^ l ' ' NO2 ions are removed by reductants while they are oxidized to NCO-S ^ / form carboxylate. This is indicated by the increase of HC conversion along with the increase of NO concentration CO2 CO: when NO/HC ratio is low enough (Fig.2) and silver may be (NH3-S)->,^/N02 active for this reaction. However, if the NO/HC ratio is high, HC is insufficient for the removal of nitrate and thus the NO conversion decreases. The concomitant decrease of HC ^ . . ^^2 H2O conversion may indicate that it is not hydrocarbons itself ^cneme but partially oxidized ones on the surface which is reactive for the removal of nitrate. A key step in the scheme is the formation of organic nitro compounds by the reaction of nitrite and carboxylate. However, no nitro compound has not been actually detected on the catalyst surface under simulated feed gas in the present study. It is presumed that the nitro compounds are highly reactive that it further decomposes to yield isocyanate whose peaks were observed.
0)( ^)(
0
J0
Jl
0V
V,
4. CONCLUSION Selective catalytic reduction of NOx was examined with several metal alumina catalysts. Both NO and hydrocarbon conversion decreased drastically over Ag/Al203 as NO/HC ratio increased, in contrast to the cases of A1^03 and C0/AI2O3. Charaterization of Ag/Al203 indicated no formation of small particles of metallic or oxide phases of silver and silver is dispersed as Ag^ on the surface. Specific surface state is formed by the calcination at higher temperature (~800'^C) which is active for NOx reduction. The amount of Lewis acid sites decreased and the amount of Br0nsted acid sites increased by addition of small amount of silver and the effect was explained by surface model of alumina. IR spectra of adsorbed species on alumina catalysts were measured and it was found that peaks of surface nitrate were stronger on Ag/A\fl^ than AI2O3 and C0/AI2O3, which may inhibited the reaction over Ag/A\Py Although further studies are necessary to clarify the inhibition effect of nitrate ions and the role of silver, weak Ag-0 bonding may be closely related to the formation of nitrate. Thus it is expected that if secondary elements on Ag/AlX)-make oxygen atoms adjacent to silver less reactive, the formation of nitrate ions is suppressed and NOx reduction activity is improved in the lower temperature region. A further study is now in progress based on this working hypothesis. Acknowledgment This work was supported by Petroleum Energy Center (PEC) under the sponsorship of New Energy and Industrial Technology Development Organization (NEDO). References [I] T.Miyadera, Appl.Catal.BiEnronmental, 2 (1993) 199. [2] T.Miyadera and K.Yoshida, Chem.Lett. 1993, 1483. [3] N.Aoyama, K.Yoshida, A.Abe and T.Miyadera, Catal.Lett. 43 (1997) 249. [4] M.Haneda, Y.Kintaichi, M.Inaba and H.Hamada, Bull.Chem.Soc.Jpn. 70 (1997) 499. [5] K.A.Bethke and H.H.Kung, J.Catal. 172 (1997) 93. [6] T.E.Hoost, R.J.Kudla, K.M.Collins and M.S.Chattha, Appl.Catal.B:Environmental 13 (1997) 59. [7] H-W.Jen, Catal.Today, 42 (1998) 37. [8] M.Haneda, Y.Kintaichi, M.Inaba and H.Hamada, Catal.Today, 42 (1998) 127. [9] H.Hamada, Y.Kintaichi, M.Inaba, M.Sasaki, T.Ito, M.Tabata, T.Yoshinori, K.Miyamoto and H.Tsuchida, Shokubai (Catalyst), 36 (1994) 116. [10] N.Okazaki, S..Tsuda, M.Iwamoto and A.Tada, Shokubai (Catalyst), 38 (1996) 446. [II] J.Yan, M.C.Kung, W.M.H.Sachtler and H.H.Kung, J.Catal. 172 (1997) 178. [12] J.Shen, R.D.Cortright, Y.Chen and J.A.Dumesic, J.Phys.Chem. 98 (1994) 8067.