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Effect of the support for Pt catalysts on soot oxidation
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
1559
Effect of the Support for Pt Catalysts on Soot Oxidation Akira Obuchi, Junko Oi-Uchisawa, Ryuji Enomoto, Shetian Liu, Tetsuya Nanba and Satoshi Kushiyama Atmospheric Environmental Protection Department, National Institute for Resources and Environment, 16-30nogawa, Tsukuba 305-8569, Japan
Abstract The effect of the support for Pt catalysts on the oxidation of carbon black, a model diesel-exhaust soot were examined. Among eight kinds of Pt-supported metal oxides, Pt/Ta205 showed the highest activity towards the oxidation of carbon black in a model diesel exhaust, containing 02, 1-120, NO, and SO2 in N2. Pt catalysts supported on other non-basic metal oxides such as Nb2Os, WO3, SnO2, and SiO2 showed similar high activities. The cause of the high activity for these catalysts was investigated in relation to the roles of NO2 and SO3 (or H2SO4) in this catalytic process. 1. INTRODUCTION The emission of soot from diesel engines can be reduced by placing a filter device made of heat-resistive materials in the exhaust stream. To achieve this reduction, however, the soot collected in the filter must be continuously or periodically removed from the filter by combustion. Normally, solid carbon in the soot burns only above 600 ~ which can not be realized in the exhaust stream under typical engine operating conditions. Oxidation catalysts can lower this combustion temperature. Various kinds of metal oxides [1], mixed oxides (especially those containing vanadium, molybdenum and/or copper [2], and perovskites [3]), chlorides [4], and sulfates [5] have been reported to be active catalysts for carbon oxidation. Pt catalysts exhibit the highest activity at low temperatures [6,7]. Platinum is believed to promote soot oxidation indirectly, i.e., by oxidizing the NO, normally coexisting in the exhaust gas, to NO2, which subsequently oxidizes soot to CO and CO2. Furthermore, we have recently found that SO2 and 1-120 present in the reactant in addition to NO, substantially promote carbon oxidation in the presence of Pt/SiO2 [8]. SO3 or H2SO4 produced from SO2 oxidation on Pt surface is believed to catalyze the oxidation reaction of carbon by NO2. Water is believed to promote this reaction by decomposing intermediate species through hydrolysis [9]. Here we report the effect of the Pt-catalyst support and discuss the results, based on the above reaction mechanism. 2. EXPERIMENTAL Pt (0.3 wt%) was supported on the granular metal oxides, Ta205, Nb205, WO3, SnO2, SiO2, TiO2, A1203, and ZrO2, having sizes ranging from 0.15 to 0.25 mm. In the case of WO3 and Nb2Os, the oxide granules were prepared from a commercially available oxide in the
1560 form of powder, by kneading with aqueous ammonia, followed by drying, calcination at 600 ~ in air for 2 h, crushing the aggregates, and sieving. Pt was supported on these oxides by the incipient-wetness method using a solution of Pt(NH3)4(OH)2, followed by drying and calcination in air at 600 ~ The prepared catalyst samples were characterized by BET surface area and Pt dispersion, i.e., (Pt exposed) / (Pt total) expressed as a percentage, determined by CO pulse adsorption at 50 ~ or Pt particle-size observation by TEM. Furthermore, temperature-programmed desorption (TPD) of CO2 was carried out to evaluate the basicity of the support materials. After calcination in air at 600 ~ the sample was exposed to 100% CO~ for 10 min at 100 ~ purged with N2 and heated at a rate of 10 ~ under a N2 flow, during which the CO2 desorption rate was continuously measured. Temperature-programmed reactions (TPR) were carried out to evaluate the catalytic performance for carbon oxidation. Commercially available carbon black (CB; Nippon Tokai Carbon 7350F, primary particle size = 28 nm, BET surface area = 80 m2/g, elemental analysis C = 97.99 wt%, H =1.12 wt%, N = 0.06 wt%) was used as a model soot. The Pt catalyst (0.5 g) and CB powder (0.005 g) were mixed together with a spatula to attain "loose" contacts between the catalyst and carbon [10]. Reactant gases containing 1000-ppm NO, 100-ppm SO2, 7% 1-120, and 10% 02 in N2, were passed through the mixture of catalyst and CB at a flow rate of 0.5 dm3/min. The reactor temperature was raised by 10 ~ from 80 to 750 ~ and the concentrations of CO2, CO, and NO2 in the product gas were continuously measured using non-dispersive IR gas analyzers and a chemiluminescence-type NOx analyzer. In addition, isothermal reaction tests were carried out at 350 ~ under the same gaseous conditions to determine if the combustion of the CB is complete at this temperature. Similarly, transient response reactions were carried out at the same temperature to investigate the effect of SO2 on the catalytic NO oxidation, under the same gaseous conditions as TPR except that no CB was mixed with the catalyst sample, and the SO2 concentration was switched between 0 and 100 ppm. 3. RESULTS AND DISCUSSION The BET specific surface area and Pt dispersion of the catalyst samples are summarized in Table 1, as well as the source of the metal oxides and the pretreatment conditions for supporting Pt. The Pt dispersions on SiO2, ZrO2, A1203, and TiO2 exceeded
Table 1 Preparationmethod and physical properties of the Pt supported catalysts Support
Manufacturer/ Code
Pt preparation BET surface Pt dispersion condition ") area / m2.g1 / % SiO2 Wako chemicals / Wako-gel 100 H/A 434 13 ZIO 2 Daiichi-Kigenso H/A 44 129 A1203 SumitomoChemicals / KHS-24 H/A 152 14 TiO2 IshiharaSangyo/ ST-B11 H/A 29 21 SnO2 Rare Metallic A 3.0 3.8 WO3 Kanto Chemicals A 4.7 --- b) Nb205 WakoChemicals A 2.7 3.3 Ta205 WakoChemicals A 2.5 3.4 ")H" reduction in 3% H, in N~ at 400 ~ for 4h. A: calcination in air at 600 ~ for lh.
1561 10% even after a pretreatment of calcination in air at 600 ~ for 1 h. It is not clear why the Pt dispersion exceeded 100% in the case of ZrO2. This result suggests that more than one CO molecule were adsorbed on a Pt atom, such as a 'twin CO.' No Pt particles were visible on ZrO2 using TEM at a magnification as high as 3,000,000, which implies a very high Pt dispersion. On the other hand, the Pt dispersions for SnO2, Nb2Os, and Ta205 estimated by TEM were below 4%. The low surface area of these supports may cause the relatively low Pt dispersions. Figure 1(a) shows CO2-emission curves from the TPR experiments for the Pt catalysts supported on the various oxides examined. The CO emission was negligible in all the cases. For Pt/SiO2, the initial and peak temperatures were 280 and 500 ~ respectively, the former being defined as the temperature for which the CO2 concentration exceeds 100 ppm, with a shoulder located at 350 ~ The initial temperatures for Pt/ZrO2, Pt/Al203, and Pt/TiO2 were 100 to 130 ~ higher than that for Pt/SiO2. On the other hand, Pt/Ta2Os, Pt/Nb2Os, N/WO3, and Pt/SnO2 showed higher activities than that for Pt/SiO2 near 350 ~ although the initial temperatures were almost the same. Pt/Ta205 showed a CO2-emission rate nearly twice that for Pt/SiO2 at this temperature. Figure l(b) shows NO2-emission curves obtained during the same TPR experiments. NO2 production started at temperatures as low as 200 ~ for all the catalyst samples except Pt/ZrO2 and Pt/AI203, which indicates that the catalytic activity for NO oxidation to NO2 is very high for these materials. As the temperature rose, the NO2 concentration temporarily decreased near 280 ~ which almost coincides with the initial temperature for CB oxidation. This implies that NO2 oxidizes CB while simultaneously being reduced to NO under the TPR conditions. Above 450 ~ the NO2-emission concentration reached values predicted by the 2000
' li
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,
,
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300 400 500 Temperature / ~
600
700
100
I 200
I I I 300 400 500 Temperature / ~
I 600
700
Fig. 1 CO2 emission (a) and NO2 emission (b) curves from the TPR experiments evaluating the carbonblack oxidation activity of Pt catalysts supported on various oxides. For the experimental conditions, see the exoerimental section in the text.
1562 thermodynamic equilibrium among NO, NO2, and 02, thus decreasing with a rise in temperature. By contrast, in the case of Pt/ZrO2, NO2 production occurred only above 300 ~ and the concentration was also substantially lower than the equilibrium value predicted for higher temperatures. It is believed that Pt highly dispersed on ZrO2 decreases its catalytic oxidation activity, most likely by strongly interacting with the support. The behavior of Pt/TiO2 appears strange at first sight; while NO2 began to be produced at 230 ~ it was not consumed until 390 ~ Similar behavior was also observed in the case of Pt/AI203. These results suggest that factors other than NO2 production are critical in the catalytic oxidation of CB, which will be discussed later. Figure 2 shows time courses of the carbon oxidation rate at a constant temperature, 350 ~ (from 25 to 110 min), for the same Pt-supported oxides tested in the above TPR experiments. In the case of Ptffa205 and PtfNb2Os, which showed the highest performances in the TPR experiments, the CB was oxidized almost completely in 40 and 60 min at 3 50 ~ respectively. It took approximately 100 min for the CB to be removed in the Pt/SiO2 experiment. On the other hand, for Pt/Zr02 and Ptfrio2 the carbon oxidation rates were much lower, and it was necessary to raise the temperature to oxidize the carbon completely. Figure 3 shows results of transient responses of NO2 emission over different Pt catalysts at 350 ~ after the addition and subsequent cut-off of SO2. Carbon black was not mixed with the catalysts in this case. Except for Pt/ZrO2, the NO2 concentrations were 500 to 600 ppm before adding SO2. One hour after adding SO2, the NO2 concentrations produced by Pt supported on ZrO~, TiO2, A1203, SiO2, SnO2, WO3, Nb2Os, and TarO5 were higher in the stated order, which coincides with the order of the carbon oxidation rates, except for an exchange in the position of A1203 and TiO2. Following the addition of SO2, Pt/Ta~O5 showed a sudden decrease in NO2 of approximately 100 ppm, but with the cut-off of SO2, the concentration quickly returned to the original level. Similar results were obtained for Pt/Nb2Os, Pt/WO3, and Pt/SnO2. With Pt/SiO2, the response to adding SO2 was slower but the decrease became eventually more prominent; aider the SO2 cut-off, the NO2 concentration approached the original level with a slower recovery rate. Furthermore, in the case of Pt/TiO2, once SO2 was added, the NO2 concentration continuously decreased with a negligible 10001
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600 400
200 0
i 1000
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200 0
20
40
60
80 Time / min
100
120
0 140
Fig. 2 Timecourses of carbon-black oxidation at 350 ~ with Pt catalysts supported on Ta:Os, Nb2Os, SiO2, TiO2, and ZrO2. The reactant-gas compositionwas the same as that shown in Fig. 1.
1563 700
SO2 add
I
/
~
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t
SO2cut
600
Support: 500
0 .~,,t rd~
Ta O
-
400
SiO 300
-
eq
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-
---,..._~---
100 I 0
~ u,.-..,u~x_,-~.i3t~(3
~ .
ZIO I 50
100
150
I 200
I 250
Time / min Fig. 3 Transient responses of the rate of NO oxidation to NO2 over different Pt catalysts at 350 ~
after the addition and subsequentcut-offof SO2 in the reactant gas. The reactant-gascompositionwas the sameas that shown in Fig. 1, exceptfor SO2. recovery following the SO2 cut-off. Similar tendency was more remarkable for Pt/ZrO2. The total sulfur amount, which passed through the catalyst samples during this experiment, corresponded to 1.43 wt% of the catalyst sample. The amounts of sulfur present in Pt/Ta2Os, Pt/Nb2Os, Pt/WO3, Pt/SnO2, Pt/SiO2, Pt/TiO2, Pt/AI203, and Pt/ZrO2 measured after the experiment were 0.03, 0.05, 0.02, 0.05, 0.02, 0.99, 1.35, and 0.45 wt%, respectively. Only the last three samples strongly adsorbed sulfur. These results indicate that Pt/Ta2Os, Pt/Nb20~, Pt/WO3, and Pt/SnO2 have a strong resistance (with Pt/SiO2 having a more limited resistance) to poisoning by sulfur (SO3 or H2SO4 derived from SO2) for NO2 production, whereas Pt/TiO2, Pt/AI203, and Pt/ZrO2 lose their catalytic activity by poisoning at this temperature. Furthermore, for the latter three catalysts, SO3 (or H2SO4), which is a catalyst to promote the oxidation of carbon by NO2, is trapped by the supports and does not reach the carbon to be oxidized. Figure 4 shows the results of CO2 TPD experiments to evaluate the basicity of the supports. The CO2 emission was observed only for ZrO2, Al~O3 and TiO2, all of which demonstrated low activity as the support for Pt catalysts. Among these, ZrO2 showed the highest total amount of desorption, reaching 0.024-m01 CO2 per mol ZrO2. By contrast, the other five oxides did not adsorb CO2 at all. A high negative correlation between the basicity and activity of the support was found. SiO2 and the metal oxides with a lower basicity have a reduced affinity toward S03 (or H2S04), and the supported Pt is poisoned to a lesser degree. As a result, the SOa (or H2S04) produced can reach the carbon surfaces more easily, thereby promoting carbon oxidation. 4. CONCLUSION Eight kinds of Pt-supported metal oxides were tested and compared for their catalytic performance in the oxidation of carbon black, mechanically mixed with the catalyst sample; the tests used a model diesel exhaust gas containing 02, 1-120,NO, and SO2. Pt/Ta205 was the
1564 2500 ~
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2000 ~'
I I I
Ti~ ":'"1 --'C}~ SiO2 I
1500
1000 i~
r~
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500
100
200
300 400 Temperature
500
600
Fig. 4 CO2TPD fromPt/ZrO2,Pt/Al203, Ptfrio2, and Pt/SiO2. most active catalyst. Pt catalysts supported on other non-basic oxides, such as Nb2Os, WO3, SnO2, and SiO2, showed similar high activities. We attribute the high activity for these catalysts to their non-basicity and negligible affinity for SO3 (or H2SO4), which results in less poisoning of the supported Pt and in a smooth supply of SO3 (or H2SO4) to the carbon surface, the oxidation of which by NO2 is catalyzed by SO3 (H2SOa).
REFERENCES
1. J.EA.Neel~ M.Makkee, H.A.Moulijn, Appl. Catal. B, 8 (1996) 57. 2. A.Bellaloui, J.Varloud, P.Meriaudiau, V. Perrichon, E.Lox, M.Chevrier, C.Gauthier, EMathis, Catal. Today 29 (1996) 421. 3. Y.Teraoka, K. Nakano, S.Kagawa, W.F.Shangguan, Appl. Catal. B, 5 (1995) L181. 4. G.Mul, F.Kapteijn, J.A.Moulijn, Appl. Catal. B 12 (1997) 33. 5. Z. Zhao, A.Obuchi, J.Uchisawa, S.Kushiyama, Stud. Surf. Sci. Catal. 121 (1999) 387. 6. P. Hawker, N. Myers, G. Huthwohl, H. T. Vogel, B. Bates, L. Magnusson and P.Bronnenberg, SAE Technical Paper Series, 970182, 1997. 7. B. J. Cooper and J. E. Thoss, SAE Paper Series No. 890404 (1989). 8. J.Oi-Uchisawa, A.Obuchi, Z.Zhao and S.Kushiyama, Appl. Catal. B 18 (1998) L183. 9. J.Oi-Uchisawa, A.Obuchi, A.Ogata, R.Enomoto, S.Kushiyama, Appl. Catal. B 21 (1999) 9. 10. J. P. A. Neefl, O. P. Pruissen, M. Makkee and J. A. Moulijn, Appl. Catal. B, 12 (1997) 21.
1559
Effect of the Support for Pt Catalysts on Soot Oxidation Akira Obuchi, Junko Oi-Uchisawa, Ryuji Enomoto, Shetian Liu, Tetsuya Nanba and Satoshi Kushiyama Atmospheric Environmental Protection Department, National Institute for Resources and Environment, 16-30nogawa, Tsukuba 305-8569, Japan
Abstract The effect of the support for Pt catalysts on the oxidation of carbon black, a model diesel-exhaust soot were examined. Among eight kinds of Pt-supported metal oxides, Pt/Ta205 showed the highest activity towards the oxidation of carbon black in a model diesel exhaust, containing 02, 1-120, NO, and SO2 in N2. Pt catalysts supported on other non-basic metal oxides such as Nb2Os, WO3, SnO2, and SiO2 showed similar high activities. The cause of the high activity for these catalysts was investigated in relation to the roles of NO2 and SO3 (or H2SO4) in this catalytic process. 1. INTRODUCTION The emission of soot from diesel engines can be reduced by placing a filter device made of heat-resistive materials in the exhaust stream. To achieve this reduction, however, the soot collected in the filter must be continuously or periodically removed from the filter by combustion. Normally, solid carbon in the soot burns only above 600 ~ which can not be realized in the exhaust stream under typical engine operating conditions. Oxidation catalysts can lower this combustion temperature. Various kinds of metal oxides [1], mixed oxides (especially those containing vanadium, molybdenum and/or copper [2], and perovskites [3]), chlorides [4], and sulfates [5] have been reported to be active catalysts for carbon oxidation. Pt catalysts exhibit the highest activity at low temperatures [6,7]. Platinum is believed to promote soot oxidation indirectly, i.e., by oxidizing the NO, normally coexisting in the exhaust gas, to NO2, which subsequently oxidizes soot to CO and CO2. Furthermore, we have recently found that SO2 and 1-120 present in the reactant in addition to NO, substantially promote carbon oxidation in the presence of Pt/SiO2 [8]. SO3 or H2SO4 produced from SO2 oxidation on Pt surface is believed to catalyze the oxidation reaction of carbon by NO2. Water is believed to promote this reaction by decomposing intermediate species through hydrolysis [9]. Here we report the effect of the Pt-catalyst support and discuss the results, based on the above reaction mechanism. 2. EXPERIMENTAL Pt (0.3 wt%) was supported on the granular metal oxides, Ta205, Nb205, WO3, SnO2, SiO2, TiO2, A1203, and ZrO2, having sizes ranging from 0.15 to 0.25 mm. In the case of WO3 and Nb2Os, the oxide granules were prepared from a commercially available oxide in the
1560 form of powder, by kneading with aqueous ammonia, followed by drying, calcination at 600 ~ in air for 2 h, crushing the aggregates, and sieving. Pt was supported on these oxides by the incipient-wetness method using a solution of Pt(NH3)4(OH)2, followed by drying and calcination in air at 600 ~ The prepared catalyst samples were characterized by BET surface area and Pt dispersion, i.e., (Pt exposed) / (Pt total) expressed as a percentage, determined by CO pulse adsorption at 50 ~ or Pt particle-size observation by TEM. Furthermore, temperature-programmed desorption (TPD) of CO2 was carried out to evaluate the basicity of the support materials. After calcination in air at 600 ~ the sample was exposed to 100% CO~ for 10 min at 100 ~ purged with N2 and heated at a rate of 10 ~ under a N2 flow, during which the CO2 desorption rate was continuously measured. Temperature-programmed reactions (TPR) were carried out to evaluate the catalytic performance for carbon oxidation. Commercially available carbon black (CB; Nippon Tokai Carbon 7350F, primary particle size = 28 nm, BET surface area = 80 m2/g, elemental analysis C = 97.99 wt%, H =1.12 wt%, N = 0.06 wt%) was used as a model soot. The Pt catalyst (0.5 g) and CB powder (0.005 g) were mixed together with a spatula to attain "loose" contacts between the catalyst and carbon [10]. Reactant gases containing 1000-ppm NO, 100-ppm SO2, 7% 1-120, and 10% 02 in N2, were passed through the mixture of catalyst and CB at a flow rate of 0.5 dm3/min. The reactor temperature was raised by 10 ~ from 80 to 750 ~ and the concentrations of CO2, CO, and NO2 in the product gas were continuously measured using non-dispersive IR gas analyzers and a chemiluminescence-type NOx analyzer. In addition, isothermal reaction tests were carried out at 350 ~ under the same gaseous conditions to determine if the combustion of the CB is complete at this temperature. Similarly, transient response reactions were carried out at the same temperature to investigate the effect of SO2 on the catalytic NO oxidation, under the same gaseous conditions as TPR except that no CB was mixed with the catalyst sample, and the SO2 concentration was switched between 0 and 100 ppm. 3. RESULTS AND DISCUSSION The BET specific surface area and Pt dispersion of the catalyst samples are summarized in Table 1, as well as the source of the metal oxides and the pretreatment conditions for supporting Pt. The Pt dispersions on SiO2, ZrO2, A1203, and TiO2 exceeded
Table 1 Preparationmethod and physical properties of the Pt supported catalysts Support
Manufacturer/ Code
Pt preparation BET surface Pt dispersion condition ") area / m2.g1 / % SiO2 Wako chemicals / Wako-gel 100 H/A 434 13 ZIO 2 Daiichi-Kigenso H/A 44 129 A1203 SumitomoChemicals / KHS-24 H/A 152 14 TiO2 IshiharaSangyo/ ST-B11 H/A 29 21 SnO2 Rare Metallic A 3.0 3.8 WO3 Kanto Chemicals A 4.7 --- b) Nb205 WakoChemicals A 2.7 3.3 Ta205 WakoChemicals A 2.5 3.4 ")H" reduction in 3% H, in N~ at 400 ~ for 4h. A: calcination in air at 600 ~ for lh.
1561 10% even after a pretreatment of calcination in air at 600 ~ for 1 h. It is not clear why the Pt dispersion exceeded 100% in the case of ZrO2. This result suggests that more than one CO molecule were adsorbed on a Pt atom, such as a 'twin CO.' No Pt particles were visible on ZrO2 using TEM at a magnification as high as 3,000,000, which implies a very high Pt dispersion. On the other hand, the Pt dispersions for SnO2, Nb2Os, and Ta205 estimated by TEM were below 4%. The low surface area of these supports may cause the relatively low Pt dispersions. Figure 1(a) shows CO2-emission curves from the TPR experiments for the Pt catalysts supported on the various oxides examined. The CO emission was negligible in all the cases. For Pt/SiO2, the initial and peak temperatures were 280 and 500 ~ respectively, the former being defined as the temperature for which the CO2 concentration exceeds 100 ppm, with a shoulder located at 350 ~ The initial temperatures for Pt/ZrO2, Pt/Al203, and Pt/TiO2 were 100 to 130 ~ higher than that for Pt/SiO2. On the other hand, Pt/Ta2Os, Pt/Nb2Os, N/WO3, and Pt/SnO2 showed higher activities than that for Pt/SiO2 near 350 ~ although the initial temperatures were almost the same. Pt/Ta205 showed a CO2-emission rate nearly twice that for Pt/SiO2 at this temperature. Figure l(b) shows NO2-emission curves obtained during the same TPR experiments. NO2 production started at temperatures as low as 200 ~ for all the catalyst samples except Pt/ZrO2 and Pt/AI203, which indicates that the catalytic activity for NO oxidation to NO2 is very high for these materials. As the temperature rose, the NO2 concentration temporarily decreased near 280 ~ which almost coincides with the initial temperature for CB oxidation. This implies that NO2 oxidizes CB while simultaneously being reduced to NO under the TPR conditions. Above 450 ~ the NO2-emission concentration reached values predicted by the 2000
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,
t
Support:TiO
,
,
,
,
[[_bsupport: ~
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//
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1
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0 100
200
300 400 500 Temperature / ~
600
700
100
I 200
I I I 300 400 500 Temperature / ~
I 600
700
Fig. 1 CO2 emission (a) and NO2 emission (b) curves from the TPR experiments evaluating the carbonblack oxidation activity of Pt catalysts supported on various oxides. For the experimental conditions, see the exoerimental section in the text.
1562 thermodynamic equilibrium among NO, NO2, and 02, thus decreasing with a rise in temperature. By contrast, in the case of Pt/ZrO2, NO2 production occurred only above 300 ~ and the concentration was also substantially lower than the equilibrium value predicted for higher temperatures. It is believed that Pt highly dispersed on ZrO2 decreases its catalytic oxidation activity, most likely by strongly interacting with the support. The behavior of Pt/TiO2 appears strange at first sight; while NO2 began to be produced at 230 ~ it was not consumed until 390 ~ Similar behavior was also observed in the case of Pt/AI203. These results suggest that factors other than NO2 production are critical in the catalytic oxidation of CB, which will be discussed later. Figure 2 shows time courses of the carbon oxidation rate at a constant temperature, 350 ~ (from 25 to 110 min), for the same Pt-supported oxides tested in the above TPR experiments. In the case of Ptffa205 and PtfNb2Os, which showed the highest performances in the TPR experiments, the CB was oxidized almost completely in 40 and 60 min at 3 50 ~ respectively. It took approximately 100 min for the CB to be removed in the Pt/SiO2 experiment. On the other hand, for Pt/Zr02 and Ptfrio2 the carbon oxidation rates were much lower, and it was necessary to raise the temperature to oxidize the carbon completely. Figure 3 shows results of transient responses of NO2 emission over different Pt catalysts at 350 ~ after the addition and subsequent cut-off of SO2. Carbon black was not mixed with the catalysts in this case. Except for Pt/ZrO2, the NO2 concentrations were 500 to 600 ppm before adding SO2. One hour after adding SO2, the NO2 concentrations produced by Pt supported on ZrO~, TiO2, A1203, SiO2, SnO2, WO3, Nb2Os, and TarO5 were higher in the stated order, which coincides with the order of the carbon oxidation rates, except for an exchange in the position of A1203 and TiO2. Following the addition of SO2, Pt/Ta~O5 showed a sudden decrease in NO2 of approximately 100 ppm, but with the cut-off of SO2, the concentration quickly returned to the original level. Similar results were obtained for Pt/Nb2Os, Pt/WO3, and Pt/SnO2. With Pt/SiO2, the response to adding SO2 was slower but the decrease became eventually more prominent; aider the SO2 cut-off, the NO2 concentration approached the original level with a slower recovery rate. Furthermore, in the case of Pt/TiO2, once SO2 was added, the NO2 concentration continuously decreased with a negligible 10001
I
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------or---Pt/SiO = Pt/TiO2
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600 400
200 0
i 1000
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200 0
20
40
60
80 Time / min
100
120
0 140
Fig. 2 Timecourses of carbon-black oxidation at 350 ~ with Pt catalysts supported on Ta:Os, Nb2Os, SiO2, TiO2, and ZrO2. The reactant-gas compositionwas the same as that shown in Fig. 1.
1563 700
SO2 add
I
/
~
I
t
SO2cut
600
Support: 500
0 .~,,t rd~
Ta O
-
400
SiO 300
-
eq
200
-
---,..._~---
100 I 0
~ u,.-..,u~x_,-~.i3t~(3
~ .
ZIO I 50
100
150
I 200
I 250
Time / min Fig. 3 Transient responses of the rate of NO oxidation to NO2 over different Pt catalysts at 350 ~
after the addition and subsequentcut-offof SO2 in the reactant gas. The reactant-gascompositionwas the sameas that shown in Fig. 1, exceptfor SO2. recovery following the SO2 cut-off. Similar tendency was more remarkable for Pt/ZrO2. The total sulfur amount, which passed through the catalyst samples during this experiment, corresponded to 1.43 wt% of the catalyst sample. The amounts of sulfur present in Pt/Ta2Os, Pt/Nb2Os, Pt/WO3, Pt/SnO2, Pt/SiO2, Pt/TiO2, Pt/AI203, and Pt/ZrO2 measured after the experiment were 0.03, 0.05, 0.02, 0.05, 0.02, 0.99, 1.35, and 0.45 wt%, respectively. Only the last three samples strongly adsorbed sulfur. These results indicate that Pt/Ta2Os, Pt/Nb20~, Pt/WO3, and Pt/SnO2 have a strong resistance (with Pt/SiO2 having a more limited resistance) to poisoning by sulfur (SO3 or H2SO4 derived from SO2) for NO2 production, whereas Pt/TiO2, Pt/AI203, and Pt/ZrO2 lose their catalytic activity by poisoning at this temperature. Furthermore, for the latter three catalysts, SO3 (or H2SO4), which is a catalyst to promote the oxidation of carbon by NO2, is trapped by the supports and does not reach the carbon to be oxidized. Figure 4 shows the results of CO2 TPD experiments to evaluate the basicity of the supports. The CO2 emission was observed only for ZrO2, Al~O3 and TiO2, all of which demonstrated low activity as the support for Pt catalysts. Among these, ZrO2 showed the highest total amount of desorption, reaching 0.024-m01 CO2 per mol ZrO2. By contrast, the other five oxides did not adsorb CO2 at all. A high negative correlation between the basicity and activity of the support was found. SiO2 and the metal oxides with a lower basicity have a reduced affinity toward S03 (or H2S04), and the supported Pt is poisoned to a lesser degree. As a result, the SOa (or H2S04) produced can reach the carbon surfaces more easily, thereby promoting carbon oxidation. 4. CONCLUSION Eight kinds of Pt-supported metal oxides were tested and compared for their catalytic performance in the oxidation of carbon black, mechanically mixed with the catalyst sample; the tests used a model diesel exhaust gas containing 02, 1-120,NO, and SO2. Pt/Ta205 was the
1564 2500 ~
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2000 ~'
I I I
Ti~ ":'"1 --'C}~ SiO2 I
1500
1000 i~
r~
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500
100
200
300 400 Temperature
500
600
Fig. 4 CO2TPD fromPt/ZrO2,Pt/Al203, Ptfrio2, and Pt/SiO2. most active catalyst. Pt catalysts supported on other non-basic oxides, such as Nb2Os, WO3, SnO2, and SiO2, showed similar high activities. We attribute the high activity for these catalysts to their non-basicity and negligible affinity for SO3 (or H2SO4), which results in less poisoning of the supported Pt and in a smooth supply of SO3 (or H2SO4) to the carbon surface, the oxidation of which by NO2 is catalyzed by SO3 (H2SOa).
REFERENCES
1. J.EA.Neel~ M.Makkee, H.A.Moulijn, Appl. Catal. B, 8 (1996) 57. 2. A.Bellaloui, J.Varloud, P.Meriaudiau, V. Perrichon, E.Lox, M.Chevrier, C.Gauthier, EMathis, Catal. Today 29 (1996) 421. 3. Y.Teraoka, K. Nakano, S.Kagawa, W.F.Shangguan, Appl. Catal. B, 5 (1995) L181. 4. G.Mul, F.Kapteijn, J.A.Moulijn, Appl. Catal. B 12 (1997) 33. 5. Z. Zhao, A.Obuchi, J.Uchisawa, S.Kushiyama, Stud. Surf. Sci. Catal. 121 (1999) 387. 6. P. Hawker, N. Myers, G. Huthwohl, H. T. Vogel, B. Bates, L. Magnusson and P.Bronnenberg, SAE Technical Paper Series, 970182, 1997. 7. B. J. Cooper and J. E. Thoss, SAE Paper Series No. 890404 (1989). 8. J.Oi-Uchisawa, A.Obuchi, Z.Zhao and S.Kushiyama, Appl. Catal. B 18 (1998) L183. 9. J.Oi-Uchisawa, A.Obuchi, A.Ogata, R.Enomoto, S.Kushiyama, Appl. Catal. B 21 (1999) 9. 10. J. P. A. Neefl, O. P. Pruissen, M. Makkee and J. A. Moulijn, Appl. Catal. B, 12 (1997) 21.