Catalytic Reduction of NO by Hydrocarbon in Oxidizing Atmosphere

Chapter 5

Catalytic Reduction of NO by Hydrocarbon in Oxidizing Atmosphere Downloaded by CALIFORNIA INST OF TECHNOLOGY on November 22, 2017 | http://pubs.acs.org Publication Date: February 23, 1994 | doi: 10.1021/bk-1994-0552.ch005

Importance of Hydrocarbon Oxidation H. W. Jen and H. S. Gandhi ChemicalEngineeringDepartment, Ford Research Laboratories, Ford Motor Company, Mail Drop 3179, 20000 Rotunda Drive, Dearborn, MI 48121-2503

The reduction of NO by C H in excess O was studied over Cu/ZSM5, Cu/γ-Al2O3, Pd/ZSM5, and Au/γ-Al O . The catalyst with higher activity of C H -oxidation incurred the reduction of NO at lower temperature. The effects of various reaction parameters were also examined in detail, especially for Cu/γ-Αl O . Under the integral reaction conditions studied, the effects of P(C H ) and P(O ) on the conversion of NO varied with temperature. There is little effect of P(NO) on the conversions of C H and NO. The activity profiles against temperature also varied with space velocity for Cu/ZSM5 catalyst. The interrelated effects of the reaction parameters suggests that the evaluation of lean-NO catalysts cover a broad range of reaction conditions. These observed results can be explained by relating the reactivity pattern of NO-reduction to the activity of hydrocarbon oxidation. 3

6

2

2

3

3

6

2

3

3

3

6

2

6

x

Automobiles equipped with three-way-catalysts (TWC's) operate at an air/fuel ratio (A/F) around the stoichiometric value. Catalytic reduction of NO to N2 decreases precipitously as A/F increases to an excess of 02 in the exhaust (lean condition). There have been commercial efforts to apply lean-burn engine technology for enhancing fuel economy and lowering pollutants (1-2). Even with lean-burn engines most cars may still need catalysts to further lower the emissions in order to meet the coming regulation standards. However, no manufacturer has yet utilized a catalyst specifically developed to decrease N O emission under lean conditions. Thus, a catalyst, other than the current TWC's, is a requisite for the application of lean-bum engine technology to production vehicles. Direct decomposition of NO is too slow to be a viable method for removing N O from automobile exhaust (3), even over recently developed Cu-based zeolite catalysts (4-8). The selective reduction of NO by NH3 has been used for stationary exhaust, but the method is not practical for automobiles. Recently, the reduction of NO by hydrocarbons under lean conditions has been reported for Cu-based zeolites (911). This reaction has also been examined on non-Cu/zeolite catalysts (12-20). x

x

0097-6156/94/0552-0053$08.00/0 © 1994 American Chemical Society Armor; Environmental Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

Downloaded by CALIFORNIA INST OF TECHNOLOGY on November 22, 2017 | http://pubs.acs.org Publication Date: February 23, 1994 | doi: 10.1021/bk-1994-0552.ch005

54

ENVIRONMENTAL CATALYSIS

Cu/ZSM5 is among the most active catalysts studied (21-22). These results prompt our interest in developing catalysts that under lean conditions catalyze the reduction of NO by residual hydrocarbons in the exhaust. In contrast to these recent reports, a previous study (23) showed that the activities of Cu/ZSM5 catalysts under slightly lean conditions were not high enough for practical application on automobiles. This contrast is understood now because the activity is dependent on the concentration of 02 (U). In the process of evaluating potential catalysts, we also found that the activities were strongly dependent on reaction conditions. Similar work by others has shown that the relative order of the activities for two catalysts can be reversed by changing the temperature or space velocity (10,16-17). These results suggest that the activity measurement or comparison should cover a broad range of reaction conditions. An applicable explanation for the interrelated effects can serve as a guideline for the evaluation process and for establishing reaction mechanism. At present there is no reported study exploring in detail the interdependence of the reaction parameters. In this report, the effects of temperature, space velocity, and the concentration of each reactant on the catalytic reduction of NO by C3H6 were examined. The NOconversion for various catalysts with different activities of hydrocarbon oxidation were compared. The oxidation of hydrocarbon is proposed as a key step to explain the experimental results. Experimental All the catalysts used in this study were powders. Cu/ZSM5 (Si/Al = 15, 2.47 wt% Cu) was obtained from UOP. Οι/γ-Αΐ2θ3 (2 wt%) was prepared by wet-impregnation of γ-Αΐ2θ3 (Degussa, 60-80 mesh) with Cu(N03)2 solution, drying at 70°C, and calcination at 450°C for 4 hours. Pd/ZSM5 (5.4 wt%) was prepared by ion-exchange of H-ZSM5 (UOP, Si/Al = 15) with Pd(NH3)4Cl2 in a solution of 3 χ 10 " M, washing the powder after the exchange, and drying at 100°C. Au/y-Al203 (2 wt%) was prepared by wet-impregnation of γ-Αΐ2θ3 with HAuCl4 solution, drying at 100°C, and calcination at 500°C. Prior to activity measurements, the catalysts were generally pretreated in situ at 550°C in a flow of 20% O2/N2. The activity under steady state reaction conditions was measured. For the activity measurement, a mixture of reaction gases was passed through the catalyst powder which was sandwiched between two layers of quartz wool in a 0.75 inch I.D. quartz reactor tube. Two buttons of ceramic monolith were used to hold the catalyst bed in place. The downstream button was drilled through to fit a thermocouple tube (O.D. = 1/16 inch) with the tip of the tube touching the bottom of the catalyst bed. The standard reaction mixture flowing at 3 1/min contained 500 ppm NO, 500 ppm C3H6, 3% 02, and N2 as balance to achieve atmospheric pressure. The concentrations of NO, C3H6, and occasionally O2 were monitored continuously. The change in the concentration was used to calculate the conversion of each reactant. The detectors were Beckman Model 951 for NO, Model 400A for C3H6, and Model OM11EA for 02- Because of the presence of excess 02, the NO-detector was set to measure the sum of NO and NO2. For the Al203-based catalyst about 1 gram sample was used to obtain reasonable conversion of NO. For the ZSM5 based catalyst about 0.5 g sample was used and the conversion of NO was generally substantial. The back pressure from using a larger load of ZSM5 based catalyst would be too high for the reactor system used in this study. It was realized that packing a uniform and continuous layer of catalyst bed from the 0.5g sample was sometimes not successful. Some reactions were 2

Armor; Environmental Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


5.

JEN & GANDHI

NO Reduction by Hydrocarbon in Oxidizing Atmosphere

repeated over different loads of Cu/ZSM5 catalysts and different activities were measured, but the reactivity patterns were generally found to be the same. Therefore, this report is mainly focused on the effect of reaction parameters on the reactivity patterns over catalysts. The data in each table or figure were obtained from the same load of catalyst. The experimental data were obtained mostly under integral reaction conditions since a substantial conversion of NO can only be achieved with a high conversion of C3H6. The values in Table I were obtained by simply plotting the logarithm of the integral reaction rate against the logarithm of the initial concentration of a reactant. Thus, the calculated dependence on the initial reactant concentration, P , in Table I are empirical. Table I is used only to show the broad features of the reactions, e.g. the effects of temperature and the initial reactant concentration, because many reports in the literature show the effect of a parameter on the activity only at one set of reaction conditions. No attempts are made to use the apparent kinetic parameters to develop a quantitative reaction model. 0

Results Cu/ZSM5. Under the standard reaction condition, the NO and C3H6 conversions increased with the temperature up to 530°C (Figure 1). The conversions were low at temperatures below 400°C. Thus, a reasonable Arrhenius plot was made from a separate experiment in the 300 - 400 °C range (Figure 2) resulting in the apparent activation energies (E ) of 11± 3 kcal/mol for NO conversion and 10 ± 2 kcal/mol for C3H6. The result indicates a possibly common rate determining step. One of the reason why the rate is used in Figure 2 ais that the correct rate equation is not known c and the simple relationship, R = k[NO][C3H6]^[02], is unlikely the right rate law based on the result of NO conversion vs. temperature. Bennett and coworkers (25) report a similar E for C3H6 conversion but their E for NO conversion is about twice of our result. They obtained E from the rate constants of a simple rate equation derived from a spinning basket reactor system. This can be the reason of difference. In addition, the catalysts used may be different since their catalyst would yield only 6% NO-conversion at 400 °C with 5% O2, 800ppm C3H6 and 500ppm NO (the flow is equivalent to 3.1 1/min over one gram catalyst). In the range from 1.5% to 4.5% O2, there was virtually no effect of O2 concentration on the NO conversion in agreement with other reports (77, 24). At 3% O2, the conversion of NO increased monotonously with the partial pressure of C3H6. 66 The empirical relationship, Rate(NO) «= P0(C3H6)°- , was obtained at 475 °C in the range from 100 to 500 ppm C3H6. The similar effect of P(C3H6) has been reported by others (14, 25). The effect of space velocity in Figure 3 was obtained in a different flow reactor equipped with G. C. to analyze the effluents. The profile of NO-conversion was shifted to higher temperature by applying higher space velocity, but the activity converged around 500 °C. In contrast to many reports (10-11,26), the NO-conversion profiles against temperature are not convex up to 600 °C. Neither is the result in Figure 1. This is due to the difference in the catalysts or the reaction conditions used. The lower activity in Figure 1 is probably due to the inappropriate packing of catalyst bed. a

a

a

a

Cu/y-Al203. In contrast to Cu/ZSM5, a maximum at 420 °C was observed in the NO conversion over the CU/V-AI2O3 catalyst, while the conversion of C3H6 rose to 100% with increasing temperature (Figure 4). As in the case of Cu/ZSM5, the


55



Ο

Έ Φ >

c ο Ο

350

400

450

500

550

Reaction Temperature (°C)

Figure 1. Effect of Temperature on NO- and C3H6-Conversion over 0.5 g Cu/ZSM5 in Standard Reaction Mixture . . . . . . -

I

'

'

1

I

1

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1000/Τ (Κ-1) Figure 2. Arrhenius Plot for Cu/ZSM5 in Standard Reaction Mixture




JEN & GANDHI

φ Α ι®ι Δ φ Δ ιΩι Δ φ

100

—Δ—NO · 0.5 l/min - - -Δ- - - C H Ο NO . 1 l/min — 0 - - C H

80

3

3

6

6

60 40 20 -

200

300

400

500

600

700

REACTION TEMPERATURE (°C)

Figure 3. Effect of Row Rate on The Activity over Cu/ZSM5 (0.15g,

1000 ppm NO, 500 ppm C3H6,3% 02) 100 80 c ο 'w ω >

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20 -

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• 400

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500

550

600

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T(°C) Figure 4. Effect of Temperature on NO- and C3H6-Conversion over lg G1/AI2O3 in Standard Reaction Mixture


58


Arrhenius plot was made at low temperatures resulting in E = 15 ± 3 kcal/mol for NO conversion and 13 ± 2 kcal/mol for C3H6. The effect of 02 concentration is shown in Figure 5 for three temperatures selected below and above the temperature for maximum NO conversion (Tmax) in Figure 4. At 300°C, the conversion of NO increased monotonously with increasing O2 concentration. At 350°C or 482 °C, the NO-conversion increased sharply by the increase of O2 concentration from 0% to 0.2% and decreased by the additional increase of O2 from 0.2% to 3%. The decrease in the profile is steeper at 482 °C than at 350 °C. It is noted that there was substantial conversion of NO at 0% 02 and 482°C indicating the direct reaction of NO and C3H6 at high temperature. At all temperatures, the conversion of C3H6 increased following the increase in 02 concentration. The dependence of the NO-conversion on the initial concentration of O2 is shown in Table I. The empirical dependence on P (02) for the conversion rate of NO or C3H6 varies with temperature.


a

0

Table I. Rate Dependence on Initial Reactant Concentration over Cu/y-Al203 b (Rate(RE)oc {P (NO)}a Rate(RE) ~{P0(C3H6)}, Rate(RE) ~ {P (02)} ) c

0

τ cc) 300

0

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C3H6 350

NO

C3H6 482

NO

C3H6

a 1.0 0.0 Ν. M. Ν. M. 1.0 0.0

b

c

0.25 0.61 0.44 0.74 0.74

0.21

0.93

0.39 -0.09 0.18 -0.36 0.05

Ν. M. : not measured. The conversion of NO increased with C3H6-concentration (Figure 6), like the results for Cu/ZSM5. It is noted that the NO conversion at 350°C in 3% 02 was larger than that at 482°C for P(C3H6) < 250 ppm. Neither the NO nor the C3H6 conversion was affected by a change in P(NO) over the range of 500 - 2500 ppm (Figure 7). When expressed in terms of reaction rate, the rate dependence on P(NO) is 1 for the NO reaction and 0 for the C3H6 reaction at 300°C or 482°C (Table I). Apparently, the rate of C3H6 converted was invariant with the rate of NO reaction at constant temperature, O2 concentration, and C3H6 concentration . The experimentally determined dependencies on the reactant concentrations are summarized in Table I. As stated previously, these values are not necessarily the true reaction orders. They are used only to show the empirical relationship between the reaction rates and the reaction parameters under the conditions studied here. Pd/ZSM5. This catalyst is comparatively active for hydrocarbon oxidation with 100% C3H6 conversion above 350°C in 3% O2, while 15% is the maximum conversion for NO at 270°C (Figure 8) which is much lower than 420 °C for Cu/γAI2O3. The effect of P(02) on the conversion of NO varies with temperature (Figure 9). The conversion of NO increased at 208 °C but decreased at 372 °C as the O2 concentration was increased from 1.5% to 6%.




JEN & GANDHI

100 80 482 °C C H --0-NO 3

0

350 °C

6

300 °C --0--

200 400 600 800 10001200 14001600

P(C H ), ppm 3

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Figure 6. Effect of C3H6-Concentration on NO- and C3H6Conversion over lg CU/AI2O3 (500 ppm NO, 3% 02,3 l/min)




100

500

1000

1500

2000

2500

3000

P(NO), ppm

Figure 7. Effect of NO-Concentration on NO- and C3H6-Conversion over lg CU/AI2O3 (500 ppm C3H6, 3 % 02, 3 l/min)

400

Figure 8. Effect of Temperature on NO- and C3H6-Conversion over 0.6 g Pd/ZSM5 in Standard Reaction Mixture


5.

JEN & GANDHI


Αιι/γ-Αΐ2θ3· The catalyst is much less active for the oxidation of C3H6 (Figure 10) in comparison to the other catalysts described above. The conversion of NO was significant only at or above 450 °C with a maximum value of 5%.


Discussions The experimental results have shown that the catalytic conversion for NO under lean conditions is not only dependent on the type of catalyst but is also very sensitive to reaction conditions. This implies that the relative activities of different catalysts are dependent on the experimental conditions. For example, the activity for Pd/ZSM5 at 270°C is 15% which is larger than that for Cu/ZSM5, but above 400°C Cu/ZSM5 is much more active. Sato et al. (10) reported that AI2O3 was more active than Cu/ZSM5 at a space velocity below 10,000 hr 1, but the relative activity was reversed at space velocities above 10,000 h r . The interrelated effects of reaction parameters are clearly illustrated by the variation of the empirical rate dependencies with temperature for Οι/γ-Αΐ2θ3 (Table I). Thus, we propose that the evaluation of potential catalysts needs to incorporate a wide range of reaction parameters and encompass conditions close to those in actual engine exhaust. Scheme A is proposed to help explain the observed results. This simple scheme lists the feasible reactions between the reactants without involving any detailed surface interactions. Our results can not be used to either confirm or refute any of the possible mechanistic features that others have suggested, e.g. NO adsorption (27-29), NO2 formation (13, 14), NO decomposition (30), or reduction-oxidation of the catalyst (31-32). The scheme is similar to that suggested by Hamada et al. (13, 14). However, the importance of hydrocarbon oxidation is stressed here and only NO is involved without the need to invoke any other N O intermediate species. The complete oxidation of one C3H6 molecule requires nine oxygen atoms. The amount of NO converted is not sufficient to supply the oxygen for the C3H6 conversion in our results that substantial conversion of NO was detected under lean conditions. The majority of C3H6 was converted by reacting with 02- C3H6 can react directly with NO at 482 °C over Οι/γ-Αΐ2θ3 (Figure 5) or at 372 °C over Pd/ZSM5 (Figure 9) as evidenced by the NO conversion in the absence of O2. The direct reduction of NO by C3H7OH was reported (24) over Cu/ZSM5 at 482 °C. At 500 °C, little conversion of NO was observed over Cu/ZSM5 (11,24). Thus, the occurrence of the direct reduction is related to the reaction temperature, the catalyst, and the reductant used. However, the direct reduction can not be used to explain why, by increasing the 02 concentration from 0% to 0.2%, the NO conversion at 482°C increased from 40% to 62 % as the C3H6 conversion increased from 10% to 82% (Figure 5), Nor can it explain why at lower temperatures (e.g. 300°C or 350°C in Figure 5) the NO conversion reached significant values only in the presence of 02- Also, there was no effect of P(NO) on the conversion of C3H6 (Figure 7). Thus, 02 has to be involved in the selective reduction of NO under lean conditions and NO is not directly involved in the determining step of C3H6 oxidation. These observations lead to a simple explanation that NO is reduced by reacting with an intermediate generated from the incomplete oxidation of C3H6 as shown in Scheme A. The empirical relationship of Rate(NO) P (NO)l agrees with this simple explanation, too. The intermediate can be further oxidized by 02 with no chance of reacting with NO. The complete oxidation of C3H6 by 02 is then a side reaction competing with NO reduction. The effect of P(02) on the conversion of NO in Figure 5 implies that 1

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2

3

4

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0 Content (%) Figure 9. Effect of 02-Content on NO- and C3H6-Conversion over 0.6 g Pd/ZSM5 (500 ppm NO, 500 ppm C3H6,3 l/min) 2

200

300

400

500

600

700

e

τ (Q Figure 10. Effect of Temperature on NO- and C3H6-Conversion over 0.75g AU/AI2O3 in Standard Reaction Mixture


5.

JEN & GANDHI


the conversion of NO may be increased by decreasing P(02) at high temperature and vice versa. The detrimental effect of increasing P(02) at high temperature indicates that the competition of the side reaction increases with temperature. This increasing competition explains the observation of a convex profile in the plot of NO-conversion against temperature (Figure 4). The temperature of the maximum activity, T x , for the selective reduction of NO by hydrocarbon is related to the oxidation of hydrocarbon. Over a catalyst, the value of T x would decrease as the combustibility of the hydrocarbon used increases, as shown by Hamada et al. (14) for the order of Tmax: CH3OH « C2H5OH « C3H7OH < C3H6 < C3H8. For the catalysts with various activities of hydrocarbon oxidation, the order of Tmax values should be the reverse order of the oxidation activity. Our results agrees with the expectation: T = 270 °C for Pd/ZSM5 (figure 8), 420 °C for CU/V-AI2O3 (Figure 4), or ~600°C for Au/y-Al203 (Figure 10). Thus, one important feature of the catalyst for the selective reduction of NO under lean conditions is to promote the incomplete oxidation of the reductant used. The catalyst with high activity for the complete oxidation of hydrocarbon would result in low concentration of intermediate(s) and low NO conversion. The catalyst with low activity for the oxidation would need high temperature to generate substantial concentration of intermediate(s) for NO reduction, which may poses a problem for practical application. Scheme A basically agrees with that proposed by Hamada et al. (13,14) and that suggested by Bennett et al. (25). However, we present more data to support the scheme and stress the importance of hydrocarbon oxidation. In a recent paper (33), Sasaki et al. propose that the reduction of NO by C3H8 over H-ZSM5 involves the direct reaction of NO2 with C3H8 under lean conditions. By the estimation from their report the direct interaction alone can not explain why the amount of oxygen provided by the NO2 conversion is less than the demand for C3H8 oxidation. Thus, O2 has to be involved in the overall reaction scheme. In addition, the direct interaction of hydrocarbon with NO2 can not explain our result that the conversion of C3H6 was unchanged as the NO conversion increased linearly with P(NO) (Figure 10). It has been shown that CO or H2 is unlikely the reactive intermediate under lean conditions (22,34). There have been reports about the reactive intermediates for the reduction of NO: carbon species (25), oxygenates (24), isocyanate (-CNO) (35), and C(O) species on carbon surface (36). Our current results can not confirm any one of them. However, it is proposed that the stable oxygenates (such as aldehydes, alcohols, and ketones) may not be the reactive intermediates because these oxygenates resulted in lower reduction temperature for NO than the corresponding hydrocarbons (14) or the effect of P(02) on the NO conversion was different between the two types of reductants (24). Under our reaction conditions, it was found that the there was little NO conversion over H-ZSM5 which turned black after the reaction up to 600 °C. Black color was not observed for Q1/AI2O3, while the color was seen on the Cu/ZSM5 powders after the reaction. Therefore, the presence of carbon deposit does not warrant a good activity for NO conversion. The reactive intermediates is produced in the process of incomplete oxidation of the hydrocarbon before being further oxidized. We can only speculate that the carbon deposit generated over Cu/ZSM5 catalyst during the reaction may act as another source of reductant for NO. The requirement of high temperature to generate reactive intermediates from the oxidation of the deposit may be one reason why the NO conversion over the Cu/ZSM5 catalyst was flat over a wide range of high temperatures (Figure 1 and 3), in contrast to the result over CU/AI2O3. If the reaction profile was extended to higher temperature in Figure 1 or 3, the convex curve might be observed. The above speculation is consistent with our results that the m a


m a

m a x


63

64

ENVIRONMENTAL CATALYSIS SCHEME A

N , CO 2

x>

H0 2

Vf Downloaded by CALIFORNIA INST OF TECHNOLOGY on November 22, 2017 | http://pubs.acs.org Publication Date: February 23, 1994 | doi: 10.1021/bk-1994-0552.ch005

HC

+ Oo (Π-1)

CO

x>

H0 2

+ NO [HClox

(ΙΠ-2)

N , CO , H 0 2

x

2

[HC]QX : intermediates from partial oxidation of hydrocarbon

reactivity pattern for the reaction is closely related to the oxidation of reductant. Further work is needed to isolate and identify the intermediate(s). Also, the explanation linked the intermediate(s) to the experimental observations has to be rationalized. Conclusion 1. Empirical rate dependencies on the reactants were obtained for Cu/y-Al203 and the numbers varied with temperature. The catalytic activity for the reduction of NO under lean conditions is dependent on not only the type of catalyst but also the reaction conditions. The evaluation of potential catalysts should be done for a broad range of conditions encompassing the operating range of lean-burn engines. 2. A simple scheme is proposed for the selective reduction of NO under lean conditions. One key factor in the reaction is the oxidation of the hydrocarbon. The experimental results can be explained by the balance between the complete oxidation and the partial oxidation of the hydrocarbon to produce reactive intermediates. Acknowledgment It is greatly appreciated that J. S. Hepburn provided a reactor system for the activity measurements and gave many suggestions. Discussions with M . Shelef, R. W. McCabe, K. Otto, and A. D. Logan are also appreciated.

Literature Cited 1.

Automotive Fuel Economy: How Far Should We Go? ; National Research

Council; National Academy Press: Washington, D.C., 1992, pp 217-226. Kummer, J. T., In Fuel Economy; Hillard, J. C.; Springer, G. S., Eds.; Plenum: New York, NY, 1984; pp 35-90. 3. Shelef, M.; Otto, K.; Gandhi, H. Atm. Environ. 1969, 3 , 107. 4. Iwamoto, M.; Hamada, H. Catal. Today. 1991, 10 , 57. 2.


5. JEN & GANDHI NO Reduction by Hydrocarbon in Oxidizing Atmosphere 65 5.

Iwamoto, M.; Furukawa, H.; Mine, Y.; Uemura, F.; Mikuriya, S.; Kagawa, S. J.

6.

Chem. Soc., Chem. Commun. 1991, 272. L i , Y.; Hall, W. K. J. Phys. Chem. 1990, 94, 6145.


7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

Truex, T. J.; Searles, R. Α.; Sun, D. C. Platinum Metal Rev. 1992, 36, 2. Kagawa, S.; Ogawa, H.; Furukawa, H.; Teraoka, Y. Chem. Lett. 1991, 407. Held, W.; Konig, Α.; Richter, T.; Puppe, L. SAE; Paper #900496. Sato, S.; Yu-u, Y.; Yahiro, H.; Mizuno, N.; Iwamoto, M. Appl. Catal. 1991, 70, L1. Iwamoto, M.; Yahiro, H.; Shundo; S., Yu-u, Y.; Mizuno, N. Appl. Catal. 1991, 69, L15. Hamada, H.; Kintaichi, Y.; Sasaki, M.; Ito, T.; Tabata, M. Appl. Catal. 1991, 64, L1. Hamada, H.; Kintaichi, Y.; Sasaki, M.; Ito, T.; Tabata, M. Appl. Catal. 1991, 70, L15. Hamada, H.; Kintaichi, Y.; Sasaki, M.; Ito, T.; Tabata, M. Appl. Catal. 1991, 75, L1. Hamada, H.; Kintaichi, Y.; Tabata, M.; Sasaki, M.; Ito, T. Chem. Lett. 1991, 2179. Torikai, Y.; Yahiro, H.; Mizuno, N.; Iwamoto, M. Catal. Lett. 1991, 9, 91. Torikai, Y.; Yahiro, H.; Mizuno, N.; Iwamoto, M. Chem. Lett. 1991, 1859. Misono, M.; Kondo, K. Chem. Lett. 1991, 1001. Kikuchi, E.; Yogo, K.; Tanaka, S.; Abe, M. Chem. Lett. 1991, 1063.

20. Li, Y.; Armor, J. N. Appl. Catal. B: Environmental. 1992, 1, L31.

21. Konno, M.; Chikakisa, T.; Murayama, T.; Iwamoto, M. SAE; Paper #920091. 22. Iwamoto, M.; Mizuno, N. J. Automobile Eng. Part D: Proc. Inst. Mech. Eng. 1993, 207, 23.

23. Rirscher, J. S.; Sandner, M. R. U. S. Patent 4 297 328, 1981. 24. Montreuil, C. N.; Shelef, M. Appl. Catal. B: Environmental. 1992, 1, L1.

25. Bennett, C. J.; Bennett, P. S.; Golunski, S. E.; Hayes, J. W.; Walker, A. P. Appl. Catal. A: General. 1992, 86, L1.

26. Sato, S.; Hirabayashi, H.; Yahiro, H.; Mizuno, N.; Iwamoto, M. Catal. Lett. 1992, 12, 193. 27. Shelef, M . Catal. Lett. 1992, 15, 305.

28. Giamello, E.; Murphy, D.; Magnacca, G.; Morterra, C.; Shioya, Y.; Nomura, T.; Anpo, M. J. Catal. 1992, 136, 510. 29. Iwamoto, M.; Yahiro, H.; Mizuno, N.; Zhang, W. X.; Mine, Y.; Furukawa, H.; Kagawa, S. J. Phys. Chem. 1992, 96, 9360. 30. Li, Y.; Hall, W. K. J. Phys. Chem. 1990, 94 (16), 6145.

31. 32. 33. 35. 36.

Hall, W. K.; Valyon, J. Catal. Lett. 1992,15,311. d'Itri, J. L.; Sachtler, W.M.H. Catal. Lett. 1992,15,289. Sasaki, M.; Hamada, H.; Kintaichi, Y.; Ito, T. Catal. Lett. 1992, 75, 297. Ukisu, Y.; Sato, S.; Muramatsu, G.; Yoshida, K. Catal. Lett. 1992, 16, 11. Tomita, Α.; Yamada, H.; Kyotani, T.; Radovic, L. Energy Fuels. 1993, 7(1), 85.

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Catalytic Reduction of NO by Hydrocarbon in Oxidizing Atmosphere

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