Effect of Support on a Catalytic Converter for ... - ACS Publications

Dec 1, 1997 - Therefore, K2O could be a promising additive to a catalytic converter of a two-stroke motorcycle since it can significantly enhance CO ...
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Ind. Eng. Chem. Res. 1997, 36, 5160-5169

Effect of Support on a Catalytic Converter for Removing CO and HC Emissions from a Two-Stroke Motorcycle Chiou-Hwang Lee and Yu-Wen Chen* Department of Chemical Engineering, National Central University, Chung-Li, 32054 Taiwan, Republic of China

The effect of support on the noble metal catalysts for carbon monoxide and hydrocarbon oxidation was investigated. The reactions were performed under the stoichiometric and oxygen-deficient conditions. Under the stoichiometric point, the activities of the powder catalysts for CO and C3H6 oxidation are in the order Pt/K2O/Al2O3 > Pt/Al2O3 > Pt/Al2O3-SiO2, that is, the same as the order of basicity of these catalysts. Under an oxygen-deficient condition, Pt supported on an acidic support, Al2O3-SiO2, exhibits the higher C3H6 conversion and the higher activity for the steam re-forming reaction. In contrast, Pt supported on a basic support, K2O/Al2O3, exhibits the higher CO conversion and the higher activity for the water-gas shift reaction. The order of activity of the powder catalysts for the water-gas shift reaction is the same as the order of basicity of these catalysts. On the other hand, the testing results of the monolithic PtRhcontaining catalysts by the simulative gases and the ECE-40 mode driving cycle also reveal the same trend as that of the Pt powder catalysts. Furthermore, the addition of K2O on PtRh/ Al2O3-CeO2 not only increases the basicity of the catalyst but also significantly reduces the CO emission under the ECE-40 mode driving cycle test. Therefore, K2O could be a promising additive to a catalytic converter of a two-stroke motorcycle since it can significantly enhance CO conversion. Introduction Motorcycles are used as the main source of transportation in many parts of the world. The number of motorcycles on the road continues to grow rapidly. As a result, pollution from motorcycles has become a serious problem. This is a common phenomenon for many Asian countries where the exhaust emissions from motorcycles are a major contributor to air pollution. Catalytic converters have been successfully used on automobiles in the United States since 1975. The catalyst technology provides a significant and costeffective method for reduction of hydrocarbon (HC), carbon monoxide (CO), and nitrous oxides (NOx). Similarly, catalytic converters were ever studied to control exhaust emissions from two-stroke motorcycles (Mooney et al., 1975; Koberstein and Pletka, 1982; Laimbock and Landerl, 1990). Nevertheless, Taiwan is the first country to massively employ catalytic converters to control exhaust emissions from two-stroke motorcycles. In order to reduce exhaust emissions of motorcycles, the Taiwan EPA in 1992 has imposed the most globally stringent emission standards. The test procedure of the driving cycle is the same as the ECE-40 mode. The phase II emission standards (imposed in 1992) for CO and HC plus NOx are 4.5 and 3.0 g/km (Chen et al., 1992). In order to comply with the emission standards, all new two-stroke motorcycles sold in Taiwan need to install a catalytic converter to reduce the unburned HC and CO. Other Asian countries including Japan, Thailand, Malaysia, India, and Indonesia are expected to follow Taiwan’s lead in setting up tough emission requirements for motorcycles. Run rich (oxygen deficiency) is an inherent characteristic of two-stroke motorcycles. For two-stroke motorcycles, the air/fuel (A/F) ratio monotonously decreases with increasing engine speed from A/F > 14.63 (sto* To whom correspondence should be addressed. Fax: 8863-4252296. Tel.: 886-3-4227151, ext. 4203. S0888-5885(97)00296-0 CCC: $14.00

ichiometric point) at the idle speed to A/F < 11.0 at a high engine speed or heavy load. Whereas for automobiles, the A/F ratio fluctuates around the stoichiometric point. This results in the fact that CO and HC concentrations emitted by two-stroke motorcycles are higher than those by automobiles. Nevertheless, NOx emitted by two-stroke motorcycles is scarce (less than 0.1 g/km). In addition, the conversions of CO and HC on a catalytic converter are constrained by the limited amount of oxygen due to the fact that two-stroke motorcycles are normally running in oxygen deficiency. However, HC emission can be effectively controlled by the catalytic converter. Unlike HC, the CO emission is strongly affected by reaction conditions, e.g., temperature and A/F ratio. Under oxygen-deficient conditions, HC competes with CO to react with the limited oxygen in the catalytic converter. The HC conversion on a catalytic converter increases with increasing reaction temperature, while this phenomenon contrasts with CO conversion. Moreover, CO conversion on a catalytic converter is extremely low, even negative at the high-speed driving mode which represents a more oxygen-deficient and higher reaction temperature condition. Therefore, how to control CO emission is a critical factor in designing a catalytic converter for a two-stroke motorcycle in order to comply with the emission regulations. Commercial catalytic converters for two-stroke motorcycles manufactured in Taiwan generally contain Pt, Rh, CeO2, and Al2O3. The effectiveness of cerium oxide is well-established in three-way catalysts. Numerous papers have been published on the effects of CeO2 on an autocatalyst (Yao and Yu Yao, 1984; Su et al., 1985; Harrison et al., 1988; Yao et al., 1982; Diwell et al., 1991; Kim, 1982; Barbier and Duprez, 1993). All of the above investigations examine the effects of CeO2 on autocatalysts, but none on the catalytic converters for two-stroke motorcycles. Table 1 shows the similarities and differences between autocatalysts and catalytic converters for two-stroke motorcycles. Besides the fact that CeO2 can improve the Pt/Al2O3 © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5161 Table 1. Comparing Auto Catalysts with the Catalysts for Two-Stroke Motorcycles automotive

two-stroke motorcycle

test procedure HC concentration in the exhaust gases CO concentration in the exhaust gas A/F ratio of the exhaust gases

FTP-75 less then 600 ppm (C6) high fluctuates around the stoichiometric point

noble metal promoter critical factor in designing the catalytic converter

generally Pt and Rh CeO2, NiO, La2O3, and BaO 1. low light-off temperature

ECE-40 greater than 5000 ppm (C6) at the higher speed higher; even greater than 5% at the higher speed decreases along with increasing speed from A/F > 14.63 at idle speed to A/F < 11.0 at full-throttle acceleration Pt and Rh CeO2 1. simultaneously reduce HC and CO emissions

2. simultaneously reduce the HC, CO, and NOx emissions

2. effectively reduce the CO emission under the severely oxygen-deficient conditions

activities for CO and C3H6 oxidation (Yu Yao, 1984), other oxides such as K2O also has an analogous effect (Kinoshita et al., 1983; Ishikawa et al., 1994). The lightoff temperature of Pt/Al2O3 for C3H6 oxidation can be reduced by the addition of K2O (Kinoshita et al., 1983). The activity of the Pt catalyst for C3H8 oxidation can also be enhanced by an acidic support (Ishikawa et al., 1994). It should be noted that all of the above investigations primarily concentrate on an oxygen-excess condition, instead of a severely oxygen-deficient condition. Therefore, the results cannot extend to an oxygendeficient condition. This work was carried out to investigate the effect of support on the noble metal catalyst for CO and HC oxidation under oxygen-deficient conditions. The purpose of this study was to investigate whether the acidbase property of the catalyst support can affect the selectivity of the catalyst for CO and HC oxidation. The catalytic activities of the powder and monolithic catalysts were examined in terms of their capability to remove CO and HC in simulative gases with various A/F ratios. Moreover, the actual performances of the catalytic converters for removing emissions from a twostroke motorcycle are verified by the ECE-40 mode driving cycle. Experimental Section Powder Catalyst Preparation. γ-Al2O3, Al2O3SiO2, CeO2/Al2O3, and K2O/Al2O3 powder with a size of 40-60 mesh served as catalyst supports. CeO2/Al2O3 and K2O/Al2O3 supports were prepared by incipient wetting of γ-Al2O3 with an aqueous solution of cerium nitrate and potassium nitrate, respectively. Following drying at 120 °C for 2 h, those materials were calcined in air at 500 °C for 3 h. Pt/Al2O3 was prepared by incipient wetting of γ-Al2O3 with a tetraammineplatinum nitrate solution. After drying at 120 °C for 2 h, the catalyst was calcined at 500 °C for 3 h. Pt/CeO2/ Al2O3, Pt/K2O/Al2O3, and Pt/Al2O3-SiO2 catalysts were prepared by a similar procedure. In addition, Pt/K2O/ CeO2/Al2O3 and Pt/K2O/Al2O3-SiO2 were also prepared by the same procedure, except K2O was impregnated on the respective support prior to the noble metal impregnation. Moreover, the K2O content was 5.5 wt % of support. The platinum content on all powder catalysts was 0.4 wt % of support as determined by an inductively coupled plasma-atomic emission spectrophotometer (ICP-AES). The compositions of the catalysts are listed in Table 2. Monolithic Catalyst Preparation. The wash-coating slurries were made by ball-milling γ-Al2O3, Al2O3SiO2, and Al2O3-CeO2 powder with psuedo-boehmite, respectively. Prior to the ball-milling process, Al2O3CeO2 powder was prepared by incipient wetting of γ-Al2O3 with an aqueous solution of cerium nitrate.

Table 2. The Compositions and Physical Properties of the Powder Catalysts

catalysta

MOx/support (wt %)

Pt/Al2O3 Pt/Al2O3-SiO2 Pt/K2O/Al2O3 Pt/CeO2/Al2O3 Pt/K2O/Al2O3-SiO2 Pt/K2O/CeO2/Al2O3

SiO2/Al2O3 ) 18 K2O/Al2O3 ) 5.5 CeO2/Al2O3 ) 15 K2O/(Al2O3+SiO2) ) 5.5 K2O/(Al2O3+CeO2) ) 5.5

a

BET surface pore area volume (m2/g) (cm3/g) 194 281 180 160 256 147

0.74 0.63 0.69 0.61 0.59 0.55

The Pt loading is 0.4 wt % of the support.

Table 3. Compositions of the Monolithic Catalystsa

catalyst PtRh/Al2O3 PtRh/K2O/Al2O3 PtRh/Al2O3-SiO2 PtRh/K2O/Al2O3-SiO2 PtRh/Al2O3-CeO2 PtRh/K2O/Al2O3-CeO2 a

MOx/Al2O3 (wt %)

K 2O (g/cm3)

noble metal ratio by weight (g/L)

Pt:Rh ) 5:1, 1.77 0.0067 Pt:Rh ) 5:1, 1.77 SiO2/Al2O3 ) 18 Pt:Rh ) 5:1, 1.77 SiO2/Al2O3 ) 18 0.0067 Pt:Rh ) 5:1, 1.77 CeO2/Al2O3 ) 15 Pt:Rh ) 5:1, 1.77 CeO2/Al2O3 ) 15 0.0067 Pt:Rh ) 5:1, 1.77

The washcoat loading is approximately 0.122 g/cm3.

Moreover, the metal ratio of Ce to Al was controlled at 5%. Then, the metallic monoliths with 15.5 cells/cm2 were washcoated by the slurries. Following drying at 120 °C for 2 h, the metallic monoliths were calcined in air at 500 °C for 3 h. The washcoat loading was controlled at approximately 0.122 g/cm3. PtRh/Al2O3, PtRh/Al2O3-CeO2, and PtRh/Al2O3-SiO2 were prepared by co-impregnating with a mixture solution of tetraammineplatinum nitrate and rhodium nitrate. After the catalyst was dried at 120 °C for 2 h, it was calcined at 500 °C for 3 h. PtRh/K2O/Al2O3, PtRh/K2O/Al2O3SiO2, and PtRh/K2O/Al2O3-CeO2 were also prepared by a similar procedure, except potassium oxide was impregnated on the monolithic supports prior to the noble metal impregnation. The K2O content was 0.0067 g/cm3. The noble metal content on all monolithic catalysts was 1.77 g/L (Pt/Rh ) 5). The compositions of the catalysts are listed in Table 3. Temperature-Programmed Desorption (TPD) of CO2. A CO2-TPD apparatus is similar to that reported by Jones and McNicol (1986). A total of 200 mg of 4060 mesh catalyst particles was packed into a quartz tube reactor and dehydrated at 200 °C in flowing helium for 1 h. After the sample cooled to room temperature, the catalyst was flushed with pure CO2 for 30 min. Weakly bound CO2 was removed by flushing with helium for 30 min. The reactor was heated at 10 °C/min from 25 to 800 °C. The desorption of CO2 was monitored by a thermal conductivity detector (TCD) and recorded by an on-line personal computer.

5162 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 Table 4. Feed Compositions of the Simulative Gases for the Powder Catalysts

CO (%) C3H6 (ppm) O2 (%) CO2 (%) H2 (%) H2O (%) N2 a

S)1

S ) 0.31 without water

S ) 0.31 with water

1.0 800 0.96 10 0.2 0 balance

1.72 4400 0.9 10 0.2 0 balance

1.72 4400 0.9 10 0.2 10 balance

The total flow rate of the reaction gases is 2 L/min.

CO Chemisorption and BET Surface Area Measurement. The dispersion of platinum was measured through the pulse adsorption of CO in a flow of the carrier gas (He). At first, the catalyst was calcined at 400 °C in air for 30 min and then reduced in flowing H2 at 400 °C for 1 h. After reduction, CO pulses were injected at 3 min intervals at room temperature until the quantity of the exit CO pulse reached a steady value. The dispersion of platinum was calculated from the total CO uptake by assuming a stoichiometry of CO/Pt ) 1. The BET surface area and pore volume of the catalyst were measured through N2 adsorption at liquid nitrogen temperature by Micromeritics ASAP 2400. Activity Measurement of the Powder Catalyst. The catalytic activity was measured using a conventional continuous flow reaction apparatus, that is, the same as that reported by Lee and Chen (1997). A total of 150 mg of catalyst was placed in the quartz reactor with an internal diameter of 0.8 cm. The catalyst bed was heated according to the designated temperature program. The catalytic activity was tested by several sets of simulative gases, the compositions of which are listed in Table 4. The flow rate of the reaction gas was 2 L (NTP)/min. The HC and CO conversions were measured as a function of temperature (150-600 °C). The reaction temperature was corresponding to the center of the powder catalyst bed. Before testing the catalyst activity, the catalyst was reduced by 10% H2 in N2 (300 mL/min) at 450 °C for 30 min. In addition, the stoichiometric number, S, employed to identify the redox characteristic of the model gas mixture is defined as the following equation:

S)

2[O2] [H2] + [CO] + 9[C3H6]

Activity Measurement of the Monolithic Catalyst. The reaction system is the same as that reported by Lee and Chen (1997). A monolithic catalyst, 2 cm diameter and 1.2 cm length (L 2 × 1.2 cm), was sealed in the quartz reactor with an internal diameter of 2.2 cm. The inlet temperature of reaction gases was controlled at 500 °C. The HC and CO conversions of the monolithic catalysts were measured as a function of the A/F ratio of reaction gases. Table 5 illustrates the compositions of the simulative gases with various A/F ratios. The flow rate of reaction gases was 3.3 L (NTP)/min. The value of gas hourly space velocity (GHSV) is about 52 000 (NTP) h-1. Before testing the catalyst activity, the monolithic catalyst was conditioned in the simulative gases with A/F ) 14.63 (stoichiometric point) at 500 °C for 30 min. The A/F ratio of the reaction gases is calculated according to the following equation reported by Cho (1988).

A/F ) 14.63/[1 + 0.02545(CCO + CH2 + 9CHC 2CO2 - CNO)] The concentrations of CO, NO, O2, and HC are expressed in volume percentage and HC is presented as C3H6. The ECE-40 Mode Driving Cycle Test. The catalytic converter, L 6 × 4 cm, was beforehand to be canned inside the designated muffler of the target motorcycle. The target motorcycle chosen at this work was Taking 50, that is, a popular two-stroke motorcycle with engine volume 50 cm3 in Taiwan. The testing apparatus for the ECE-40 mode driving cycle comprises a chassis dynamometer for a motorcycle (Meiden), a constant volume sampling system (Horiba, CVS 9100), and a set of the emission analyzing instruments (Horiba, MEXA 9200). The target motorcycle, retrofitted with a muffler with a desired catalytic converter, was tested according to the testing procedure of the ECE-40 mode driving cycle. Then, the HC, CO, and NOx emissions from the target motorcycle were analyzed and calculated by the emission analyzing instrument. Results and Discussion Characterization of the Catalyst. The BET surface areas and pore volume of the catalysts are listed in Table 2. The surface area of the catalyst decreases by adding CeO2 or K2O, due to the blockage of the micropore of γ-Al2O3. Pt dispersions of all catalysts range from 39 to 63%, indicated in Table 6. Blank experiments with supports have been conducted. The results reveal that the CO adsorption on supports is negligible. The addition of a promoter changes the Pt dispersion slightly. The basicities of these catalysts can be analyzed by the CO2-TPD spectra. Figure 1 displays the CO2-TPD spectra of four catalysts, Pt/Al2O3, Pt/K2O/Al2O3, Pt/K2O/ Al2O3-SiO2, and Pt/K2O/CeO2/Al2O3. Besides the peak at 80 °C, no other desorption peak appears on Pt/Al2O3. However, there are several desorption peaks on Pt/K2O/ Al2O3. The major peaks on Pt/K2O/Al2O3 are 120, 145, 290, and 510 °C. This result indicates that the strength of basicity of Pt/K2O/Al2O3 is significantly stronger than that of Pt/Al2O3, as expected. Moreover, the CO2-TPD spectra reveal that the basicity of Pt/Al2O3-SiO2 or Pt/ CeO2/Al2O3 can be increased by the addition of K2O. Even the strength of the basicity of Pt/K2O/CeO2/Al2O3 is similar to that of Pt/K2O/Al2O3. Although Pt/Al2O3SiO2 was not analyzed by CO2-TPD, it is generally accepted that Al2O3-SiO2 is an acidic support (Ishikawa et al., 1994). Therefore, the order of the strength of basicity could be expressed as follows: Pt/K2O/Al2O3 = Pt/K2O/CeO2/Al2O3 > Pt/K2O/Al2O3-SiO2 > Pt/Al2O3 > Pt/Al2O3-SiO2. Light-Off Temperature for Propylene and CO Oxidation at the Stoichiometric Point and in the Absence of Water. Parts a and b of Figure 2 illustrate the reaction curves of three catalysts for CO and C3H6 oxidation at the stoichiometric point (S ) 1). The lightoff temperatures (T50 defined as the temperature enough to achieve a conversion of 50%) of these catalysts are listed in Table 6. The light-off temperatures of Pt/K2O/ Al2O3 for CO and C3H6 are the lowest among the three catalysts. It suggests that Pt/K2O/Al2O3 exhibits the highest activity for CO and C3H6 oxidation among these catalysts. Even the activity of Pt/K2O/Al2O3 is higher than that of Pt/CeO2/Al2O3 for C3H6 oxidation. Never-

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5163 Table 5. Feed Compositions of the Simulative Gases for the Monolithic Catalystsa A/F ratio CO (%) C3H6 (ppm) O2 (%) H2 (%) CO2 (%) H2O (%) N2 a

14.63

14.2

13.6

12.5

11.9

10.8

1.0 1650 1.35 0.2 10 10 balance

1.55 4400 2.25 0.2 10 10 balance

1.55 4400 1.35 0.2 10 10 balance

3.2 6500 1.35 0.2 10 10 balance

4.33 8100 1.35 0.2 10 10 balance

4.33 13500 1.35 0.2 10 10 balance

The total flow rate of the reaction gases is 3.3 L/min; the size of the metallic monolith is L 2.0 × 1.2 cm.

Table 6. The Pt Dispersions and Light-Off Temperatures of the Powder Catalysts T50a (°C) catalyst

Pt dispersion (%)

CO

HC

Pt/Al2O3 Pt/Al2O3-SiO2 Pt/K2O/Al2O3 Pt/CeO2/Al2O3

60 39 50 63

278 340 200 188

302 370 208 273

a T 50 is defined as the temperature enough to achieve a conversion of 50%; testing condition is at the stoichiometric point and in the absence of H2O.

Figure 2. CO and propylene oxidation under the stoichiometric point and in the absence of water: (a) CO; (b) Propylene.

Figure 1. CO2-TPD patterns of the catalysts.

theless, Pt/Al2O3-SiO2 exhibits the lowest activities for CO and C3H6 oxidation due to the highest light-off temperature. The support seems to have a strong effect on the activity of the Pt catalyst, because the difference in activity is much larger than that in the Pt dispersion. The C3H6 conversion on Pt/K2O/Al2O3 is higher than that on Pt/Al2O3-SiO2 at 300 °C by a factor of 20, while the dispersion is only higher by a factor of less than 2. Many researchers (Patterson and Kemball, 1963; Voltz et al., 1973; Yu Yao, 1984) reported that the kinetics of CO and C3H6 oxidation on the Pt/Al2O3 catalyst may follow a Langmuir-Hinshelwood mechanism. Reaction occurs between the adsorbed oxygen and adsorbed reactant on the catalyst surface. The reaction rate increases with increasing oxygen concen-

tration and is inhibited by increasing CO and C3H6 concentrations. Yu Yao (1984) reported that the Pt/Al2O3 activities for CO and unsaturated hydrocarbon oxidation heavily depend on the O2 partial pressure, with kinetic orders of 1-2, and are inhibited by unsaturated hydrocarbon and CO, with kinetic orders of -0.6 to -1. The negative order suggests that the reaction rate is inhibited by CO and C3H6 due to the strong adsorption on the catalyst surface. Therefore, the reaction rate of Pt/Al2O3 for CO and C3H6 oxidation at a low temperature is suppressed, whereas it can be improved by removing CO and C3H6 from the surface sites by O2 at higher reaction temperatures. This results in the fact that Pt/Al2O3 has a high light-off temperature for CO and C3H6 oxidation. Nevertheless, the Pt/Al2O3 activities for CO and C3H6 oxidation can be improved by the addition of K2O. According to Malinowski’s report (1985), depositing an

5164 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997

alkali metal on a catalyst can enhance the basic strength of the surface by crowding electrons toward the oxide ions. The CO2-TPD results also demonstrate that the basicity of Pt/Al2O3 can be markedly increased by the addition of K2O. Increasing basicity of Pt/Al2O3 by adding alkali metal oxide is likely to create an electron density on the Pt/Al2O3 surface, which will reduce the affinity of nucleophilic reagents, CO and C3H6, to the catalyst due to the electronic effect. Therefore, the Pt/ K2O/Al2O3 activities for CO and C3H6 oxidation are significantly improved by effectively suppressing the self-inhibition of CO and C3H6. This results from the fact that Pt/K2O/Al2O3 has lower light-off temperatures for CO and C3H6 oxidation. In contrast to the promotional effect of a basic metal oxide (K2O) on Pt/Al2O3, the light-off temperatures of Pt/Al2O3-SiO2 for CO and C3H6 oxidation are higher than those of Pt/Al2O3 by about 70 °C. This indicates that Pt supported on an acidic support, Al2O3-SiO2, exhibits a lower activity than Pt/Al2O3 . The Pt/Al2O3SiO2 activities for CO and C3H6 oxidation are severely self-inhibited by reactants. This result is apparently contrary to the result reported by Ishikawa et al. (1994). Ishikawa et al. (1994) reported that the activity of the Pt catalyst for propane oxidation could be improved by increasing the acidic strength of the support. This disparity might be explained that the reaction mechanism of the saturated hydrocarbon oxidation is different from that of the unsaturated hydrocarbon oxidation. The initial adsorption of alkane on the Pt catalyst is postulated as the rate-determining step for alkane oxidation (Patterson and Kemball, 1963). When Pt is supported on an acidic support, the electron density on the Pt catalyst surface might be decreased. Therefore, It might significantly increase the feasibility of alkane adsorption on the catalyst surface and result in the higher activity for alkane oxidation. In contrast to alkane oxidation, C3H6 oxidation on Pt catalyst may follow a LangmuirHinshelwood mechanism (Patterson and Kemball, 1963; Voltz et al., 1973; Yu Yao, 1984). When Pt is supported on an acidic support, it also increases the feasibility of alkene adsorption on the catalyst surface. Nevertheless, it will lessen the opportunity for oxygen to be adsorbed on the catalyst surface. Therefore, this results in that the Pt/Al2O3-SiO2 activity for C3H6 oxidation is severely self-inhibited by C3H6 due to the strong adsorption. As described above, one can find that the activities of Pt catalysts for oxidation of nucleophilic reagents, CO and C3H6, are in the order Pt/K2O/Al2O3 > Pt/Al2O3 > Pt/Al2O3-SiO2, that is, the same order as the basicity of these catalysts. On the basis of the above discussion, one can conclude that the addition of K2O can enhance the Pt/Al2O3 activities for CO and C3H6 oxidation at the stoichiometric point by suppressing the self-inhibition of reactants. However, this phenomenon is contrary to Pt supported on an acidic support. CO and C3H6 Oxidation under an OxygenDeficient Condition (S ) 0.31). Parts a, b, and c of Figure 3 depict the reaction curves of three catalysts for CO and C3H6 oxidation under an oxygen-deficient condition (S ) 0.31) with or without water. This reaction condition simulates the exhaust emissions from a two-stroke motorcycle at a speed of 50 km/h. Under an oxygen-deficient (S ) 0.31) condition and in the absence of water, the CO conversions of Pt/K2O/ Al2O3, Pt/Al2O3, and Pt/Al2O3-SiO2 at 300 °C are 72, 29, and 12%, respectively. By the analyzed result of oxygen concentration beyond the catalyst, it revealed

Figure 3. CO and propylene oxidation under S ) 0.31: (a) Pt/ Al2O3; (b) Pt/Al2O3-SiO2; (C) Pt/K2O/Al2O3.

that oxygen was not completely consumed upon reacting with reactants on Pt/Al2O3 and Pt/Al2O3-SiO2 until 500 °C or higher. Nevertheless, oxygen was entirely reacted with reactants on Pt/K2O/Al2O3 below 450 °C. From the above experimental data, one can expect that the activities of Pt/Al2O3 and Pt/Al2O3-SiO2 are still inhibited by strong adsorption of CO and C3H6. Moreover, the inhibited effect on Pt/Al2O3-SiO2 activity is more severe than that on Pt/Al2O3. For these catalysts, oxygen nearly only reacts with CO up to 350 °C. The CO conversion increases with

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5165

Figure 4. Effects of water on CO and C3H6 conversions at 550 °C under the reaction condition (S ) 0.31): 0, without water; 9, with water.

increasing reaction temperature below 350 °C. Above 350 °C, oxygen starts to react with C3H6, and then C3H6 conversion increases with increasing reaction temperature. This phenomenon contrasts with CO conversion which increases insignificantly during reaction temperature increasing from 350 to 500 °C. Above the reaction temperature that oxygen can be entirely consumed, CO conversions on these catalysts decrease with increasing reaction temperature. The highest CO conversions on Pt/Al2O3 (37%) and Pt/Al2O3-SiO2 (22%) are less than that on Pt/K2O/Al2O3 (74%). In general, the higher the reaction temperature, the higher the CO and hydrocarbon conversions under an oxygen-excess condition. However, C3H6 competes with CO to react with the limited amount of oxygen under an oxygen-deficient condition. When the C3H6 conversion increases, the CO conversion should be suppressed if oxygen is completely reacted. This explains that Pt/Al2O3 and Pt/Al2O3-SiO2 exhibit higher C3H6 conversions and lower CO conversions than Pt/K2O/Al2O3. Moreover, the CO conversion on Pt/Al2O3 is apparently higher than that on Pt/Al2O3SiO2. On the other hand, the CO conversion on Pt/Al2O3 can be significantly improved by adding K2O. Besides the highest CO conversion becoming double, the CO conversion of the promoted catalyst at 550 °C is higher than that of Pt/Al2O3 by 31%, as shown in Figure 4. Pt/ K2O/Al2O3 exhibits the highest CO conversion among these catalysts, but the C3H6 conversion on Pt/K2O/ Al2O3 is less than that on Pt/Al2O3. Even though there is a sufficient amount of oxygen available to react with both CO and C3H6 below 350 °C, oxygen nearly only reacts with CO. Obviously, the reaction rate of CO oxidation exceeds that of C3H6 oxidation below 350 °C. However, the reaction rate of C3H6 oxidation gradually increases with increasing reaction temperature. In addition, C3H6 competes with CO to react with the limited oxygen at temperatures above 350 °C. The experimental results indicate that the increase of the reaction rate of C3H6 oxidation by increasing reaction temperature is greater than that of

CO oxidation. It implies that the activation energies of the catalysts for HC oxidation are higher than those for CO oxidation. Therefore, all catalysts show that the C3H6 conversion increases along with increasing the reaction temperature under an oxygen-deficient condition (S ) 0.31) and in the absence of water. Meanwhile, CO conversion shows the reverse trend when oxygen is completely reacted. In addition, the CO conversion decrease can also be caused by the partial oxidation of C3H6 to produce the extra CO emission. One confirms that the partial oxidation of C3H6 really takes place on these catalysts at 400 °C or higher under S ) 0.31 but without CO and water. The amount of CO produced by the partial oxidation of C3H6 is insignificant when oxygen is present. Nevertheless, the produced amount of CO gradually increases along with an increase of reaction temperature when oxygen is completely reacted. Moreover, the amount of CO produced on Pt/Al2O3 or Pt/ Al2O3-SiO2 (ca. 0.3%) at 550 °C is greater than that on Pt-K2O/Al2O3 (0.18%). This result is still in agreement with the above one. The amount of CO in reaction gases eliminated by the catalysts decreases along with an increase of the C3H6 conversion. On the other hand, the CO and C3H6 conversions on these catalysts can be enhanced by the existence of water that can induce the water-gas shift (WGS) and steam re-forming reactions, as shown in Figures 3 and 4. The above reactions take place after oxygen is completely consumed (Barbier and Duprez, 1993). Not only the highest CO conversion but also the CO and C3H6 conversions at 550 °C are increased in the presence of water. The enhanced effects on CO conversions of these catalysts by the WGS reaction are also the same as the order of bascity of these catalysts. The CO conversion on Pt/K2O/Al2O3 at 550 °C increased by the WGS reaction is about 27%, that is, higher than the 4% increase of Pt/Al2O3. Therefore, the disparity of CO conversions between Pt/K2O/Al2O3 and Pt/Al2O3 is markedly magnified. The CO conversion on Pt/Al2O3 at 550

5166 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997

°C is 22%, that is, less than a third of that on Pt/K2O/ Al2O3 (76%). However, the enhanced effect on Pt/ Al2O3-SiO2 by the WGS reaction is limited, as shown in Figure 4. This result reveals that a basic support might be superior to an acidic support in terms of functioning as a support for the WGS reaction. Grenoble et al. (1981) proposed a bifunctional mechanism for the WGS reaction on the supported noble metal catalysts. CO adsorbs on the metal and migrates toward the support so as to form a formate intermediate, and then the intermediate is decomposed. On the other hand, a formic acid or formate is proposed as the intermediate for the WGS reaction (Grenoble et al., 1981; Amenomiya, 1979). The intermediate can be decomposed into CO and water via the dehydration reaction catalyzed by an acidic oxide such as Al2O3. Meanwhile, it can also be decomposed into CO2 and H2 via the dehydrogenation reaction catalyzed by metals and basic metal oxides (Grenoble et al., 1981; Ai, 1977). Grenoble et al. (1981) reported that there is an optimal interaction between noble metal and CO. The interaction should not be too strong to allow CO to migrate toward the active sites of the support. The addition of K2O on Pt/Al2O3 not only could reduce the interaction between CO and Pt but also might enhance the decomposition rate of formate or formic acid via the dehydrogenation reaction by increasing the basicity of the catalyst. This could explain why the activity of Pt/K2O/ Al2O3 for the WGS reaction is better than those of Pt/ Al2O3 and Pt/Al2O3-SiO2. The blank experiment with support, K2O/Al2O3, was also conducted under the same reaction condition but without oxygen. The experimental results reveal that CO conversion on the support by the WGS reaction is not detectable. Furthermore, the CO conversion on Pt/Al2O3 by the WGS reaction is also extremely low (below 5% at 550 °C) under the same condition. These results verify that K2O or Pt alone cannot significantly enhance the WGS reaction. The high activity of Pt/K2O/Al2O3 for the WGS reaction can be contributed to the synergistic effect of the basic support and the active metal (Pt). On the other hand, Pt/Al2O3 and Pt/Al2O3-SiO2 exhibit higher C3H6 conversions and higher activities for the steam re-forming reaction than Pt/K2O/Al2O3, indicated in Figure 4. From the above experimental results, one can conclude that Pt supported on a basic support can exhibit the higher CO conversion and the higher activity for the WGS reaction under an oxygen-deficient condition and in the presence of water. As shown in Figure 4, K2O is superior to CeO2 in terms of functioning as a promoter of Pt/Al2O3 for the WGS reaction. In contrast, Pt supported on an acidic support can exhibit the higher HC conversion and the higher activity for the steam reforming reaction under the same reaction condition. The major emissions from two-stroke motorcycles are HC and CO. If the above result might be applied to design a catalytic converter for a two-stroke motorcycle, the designer can adjust the selectivity of a catalytic converter for CO or HC oxidation by supporting noble metals on a suitable support. Therefore, the designer can easily design a suitable catalytic converter depending on the characteristic of the exhaust gases (higher HC or CO emission) from the target motorcycle. The below experiments will verify the feasibility of the above interpretation. Effect of A/F Ratio on the Monolithic Catalysts for CO and HC Oxidation. Parts a and b of Figure 5 depict the CO and C3H6 conversions of six monolithic

Figure 5. CO and propylene conversions as a function of A/F of reaction gases with reactants, CO and C3H6: (a) CO; (b) Propylene.

catalysts, L 2 × 1.2 cm, 15.5 cells/cm2, measured as a function of the A/F ratio of the simulative gases. The inlet temperature of reaction gases is controlled at 500 °C, that is, equivalent to the temperature of exhaust gases from a two-stroke motorcycle at ca. 50 km/h. Tables 3 and 5 illustrate the compositions of the monolithic catalysts and the simulative gases with various A/F ratios, respectively. The C3H6 and CO conversions on the catalysts decrease along with decreasing of the A/F ratio of reaction gases, as expected. Nevertheless, the effect of the A/F ratio on CO conversion is significantly greater than that on HC conversion. For example, HC and CO conversions on PtRh/Al2O3-SiO2 decrease from 100 and 100% at the stoichiometric point (A/F ) 14.63) to 60 and -15% at A/F ) 10.8, respectively. The negative value of CO conversion is due to the partial oxidation of C3H6 taking place on the catalyst. This verifies that CO conversion is heavily affected by the A/F ratio of reaction gases. In addition, the difference of CO conversions between these catalysts is gradually magnified from 5% at A/F ) 14.2 to 30% at A/F ) 10.8. Moreover, the CO conversion on the monolithic catalysts is still affected by the acidbase property of the catalyst support. The CO conversion on the monolithic catalysts follows the order PtRh/ K2O/Al2O3 > PtRh/Al2O3 g PtRh/Al2O3-SiO2, that is, the same as the order of basicity of these catalysts. The CO conversion of PtRh/K2O/Al2O3 is slightly higher than that of PtRh/Al2O3-CeO2, which is a typical composition of the catalytic converter for a two-stroke motorcycle. One can expect that the addition of K2O on the catalyst support can increase its basicity by CO2-TPD experi-

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5167 Table 7. The ECE-40 Test Results of the Metallic Catalytic Converters catalyst

CO (g/km)

HC (g/km)

NOx (g/km)

dummy PtRh/Al2O3 PtRh/Al2O3-SiO2 PtRh/K2O/Al2O3 PtRh/Al2O3-CeO2 PtRh/K2O/Al2O3-CeO2

5.72 5.42 (5.2%)a 6.20 (-8.4%) 4.03 (29.5%) 4.18 (26.9%) 3.05 (46.7%)

4.04 0.67 (83.4%) 0.58 (85.6%) 1.06 (73.8%) 0.96 (76.2%) 1.23 (69.6%)

0.053 0.013 (75.5%) 0.010 (81.1%) 0.011 (79.2%) 0.008 (84.9%) 0.013 (75.5%)

a The value in the parenthesis represents the conversion of HC, CO, or NOx.

Figure 6. CO and HC conversions as a function of A/F of reaction gases with reactants, CO, C3H6, and C3H8: (a) CO; (b) Propylene and Propane.

mental result and also promote the CO conversion on the catalyst. The experimental result verifies that the CO conversion on PtRh/Al2O3-CeO2 is really increased by above 10% at A/F ) 10.8 due to the addition of K2O. However, the order of these catalysts for C3H6 oxidation is contrary to the order of these catalysts for CO oxidation. The differences of HC conversions among these catalysts are lower than 10%. On the other hand, the scavenging process is an inherent characteristic of two-stroke motorcycles. The mechanism of the scavenging process is that the burnt exhaust gases in the engine are purged out by the fresh fuel gases. Some fresh fuel gases will flow directly into the exhaust pipe from the exhaust port of a two-stroke engine. This results in a high HC emission. In addition, there is 40-50% alkane in the unleaded gasoline sold in Taiwan. Therefore, there are some portions of alkane in the exhaust gases of two-stroke motorcycles. In order to precisely simulate the exhaust emissions of two-stroke motorcycles, propylene in the simulative gases, indicated in Table 5, is replaced by the mixture of propane and propylene (1:2). Figure 6 depicts the CO and HC conversions of the six monolithic catalysts measured as a function of the A/F ratio of the new simulative gases. Compared to part a of Figure 5, the HC conversions of these catalysts are slightly decreased due to the existence of propane. It can be attributed to that propane has more difficulty oxidizing than propylene (Yu Yao, 1980). The decreased extent is more signifi-

cant for the catalysts containing K2O. By the analyzed result of the oxygen concentration beyond the catalyst at A/F ) 14.63, it was revealed that oxygen was not completely consumed upon reacting with reactants on the catalysts containing K2O. This results from the fact that these catalysts exhibit lower HC conversions. Moreover, PtRh/K2O/Al2O3-CeO2 exhibits the lowest HC conversion (80%) among all catalysts studied in this work. This might be attributed to the reaction mechanism of the saturated hydrocarbon oxidation being different from that of the unsaturated hydrocarbon oxidation. Adsorption of alkane on the catalyst is postulated as the determining step of alkane oxidation (Patterson and Kemball, 1963). When the basicity of the catalyst is increased by the addition of K2O, it will significantly lessen the feasibility of propane adsorption on catalysts. This results in the K2O-containing catalysts exhibiting a lower activity for propane oxidation. Nevertheless, the propane conversion on these catalysts can be increased by increasing reaction temperature. The oxygen concentration is 2.25% at A/F ) 14.2 higher than 1.35% at A/F ) 14.63. The more oxygen that participates in the oxidation reaction, the more energy is released. Therefore, the actual reaction temperature on the monolithic catalysts at A/F ) 14.2 is higher than that at A/F ) 14.63. This is the reason why HC conversions of these catalysts at A/F ) 14.2 are higher than those at the stoichiometric point (A/F ) 14.63). In addition, the HC conversions on these monolithic catalysts follow the order PtRh/Al2O3-SiO2 g PtRh/Al2O3 > PtRh/Al2O3-CeO2 > RtRh/K2O/Al2O3-SiO2 > RtRh/ K2O/Al2O3 > PtRh/K2O/Al2O3-CeO2, that is, contrary to the order of basicity of these catalysts. Moreover, the differences of HC conversions among these catalysts are obviously magnified to 20%. On the other hand, the CO conversions of these catalysts are slightly increased (ca. 5%) due to the propane participation. Nevertheless, the order of these catalysts for CO conversion does not change. The ECE-40 Driving Cycle Test. Except for RtRh/ K2O/Al2O3-SiO2, the other five catalysts are prepared as the catalytic converter by supporting on the metallic monolith (L 6 × 4 cm, 15.5 cells/cm2). Moreover, the actual performances of these catalytic converters were tested by the ECE-40 mode driving cycle. The ECE-40 test results of these catalysts are displayed in Table 7. PtRh/Al2O3-SiO2 and PtRh/Al2O3 exhibit higher HC conversion and lower CO conversion. Even the CO conversion on PtRh/Al2O3-SiO2 is negative (-8.4%). It implies that the partial oxidation of HC takes place on PtRh/Al2O3-SiO2. The CO emission from the target motorcycle installed with PtRh/Al2O3 or PtRh/Al2O3SiO2 exceeds the emission standard (CO 4.5 g/km). Therefore, they cannot serve as a catalytic converter for a two-stroke motorcycle due to the poor capability to remove CO emission. Nevertheless, PtRh/K2O/Al2O3 exhibits the optimal conversions for HC and CO, i.e., higher CO conversion (29.5 %) and lower HC conversion

5168 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997

(73.8%). Moreover, the CO and HC conversions on PtRh/K2O/Al2O3 are close to those on PtRh/Al2O3-CeO2, that is, a typical catalytic converter for a two-stroke motorcycle. The CO conversions on these catalytic converters follow the order RtRh/K2O/Al2O3 > PtRh/ Al2O3 > PtRh/Al2O3-SiO2, that is, the same as the order of the basicity of the catalyst support. Nevertheless, the HC conversions on these catalytic converters are contrary to the order of the basicity of the catalyst support. From the above discussion, one can infer that increasing the basicity of the catalytic converter might increase the selectivity of the catalytic converter for CO oxidation. The experimental data really display that the CO conversion on PtRh/Al2O3-CeO2 is significantly promoted by the addition of K2O, but its HC conversion is lessened. Therefore, PtRh/K2O/Al2O3-CeO2 exhibits the highest CO conversion (46.7%) and the lowest HC conversion (69.6%) among these catalytic converters studied herein. It verifies that K2O is a promising additive to the catalytic converter for a two-stroke motorcycle due to significantly promoting the CO conversion on the catalyst. Even the ECE-40 test results of these catalytic converters cannot quantitatively correlate with those of the monolithic catalysts by the simulative gases. Nevertheless, the trends of the experimental results of monolithic catalysts and catalytic converters are identical. The experimental results clearly verify that the acid-base property of the catalyst support can affect the selectivity of the catalytic converter for CO and HC oxidation. If the basicity of catalyst support is increased by the addition of K2O, the catalytic converter will exhibit the higher CO conversion and the lower HC conversion, e.g., PtRh/K2O/Al2O3 and PtRh/K2O/Al2O3CeO2. Similarly, if noble metals are supported on an acidic support, the catalytic converter will exhibit higher HC conversion and lower CO conversion, e.g., PtRh/ Al2O3-SiO2. In addition, the ECE-40 test results verify that HC emission from a two-stroke motorcycle can be easily controlled by the catalytic converter. Nevertheless, the CO emission is strongly affected by the emission characteristic of the target motorcycle. Nevertheless, the PtRh/K2O/Al2O3-CeO2 might be a suitable catalytic converter for a two-stroke motorcycle with a higher CO emission. Conclusions 1. The addition of K2O or CeO2 on the Pt/Al2O3 catalyst slightly decreases the surface area of the catalyst and does not markedly change the dispersion of Pt. 2. The CO2-TPD results reveal that the basicity of the powder catalyst is in the following order: Pt/K2O/ Al2O3 = Pt/K2O/CeO2/Al2O3 > Pt/K2O/Al2O3-SiO2 > Pt/ Al2O3 > Pt/Al2O3-SiO2. The basicity of Pt/CeO2/Al2O3 or Pt/Al2O3-SiO2 can be increased by the addition of K2O. 3. Under the stoichiometric point, the activities of the powder catalysts for CO and C3H6 oxidation follow the order Pt/K2O/Al2O3 > Pt/Al2O3 > Pt/Al2O3-SiO2 , that is, the same as the order of basicity of these catalysts. 4. Under an oxygen-deficient condition and in the absence of water, C3H6 conversions on all powder catalysts studied herein always increase with increasing reaction temperature. Nevertheless, the CO conversion shows the reverse trend once oxygen is completely reacted. Pt/Al2O3 and Pt/Al2O3-SiO2 exhibit higher

C3H6 conversions and lower CO conversions, that is, contrary to Pt/K2O/Al2O3. 5. Under an oxygen-deficient condition and in the presence of water, CO and C3H6 conversions can be increased by the WGS and steam re-forming reactions. Pt/Al2O3 and Pt/Al2O3-SiO2 exhibit higher activities for the steam re-forming reaction, while they exhibit lower activities for the WGS reaction. The addition of K2O on Pt/Al2O3 catalysts can significantly enhance the WGS reaction. The activities of the catalysts for the WGS reaction also follow the order of basicity of these powder catalysts, i.e., Pt/K2O/Al2O3 > Pt/Al2O3 > Pt/Al2O3SiO2. 6. The effect of the A/F ratio on CO conversion of the monolithic catalyst is significantly greater than that on HC conversion. Moreover, the experimental results reveal that the HC conversions on the monolithic catalysts follow this order: PtRh/Al2O3-SiO2 g PtRh/ Al2O3 > PtRh/Al2O3-CeO2 > RtRh/K2O/Al2O3-SiO2 > RtRh/K2O/Al2O3 > PtRh/K2O/Al2O3-Ce2O, that is, contrary to the order of CO conversion. 7. The test results of the ECE-40 mode driving cycle reveal that the CO conversion on PtRh/Al2O3-CeO2 can be significantly promoted by the addition of K2O. It verifies that K2O is a promising additive to the catalytic converter for a two-stroke motorcycle. Acknowledgment The authors acknowledge the National Science Council of the Republic of China for financially supporting this work under Contract No. NSC-86-2214-E008-010. Literature Cited Ai, M. Activities for the Decomposition of Formic Acid and the Acid-Base Properties of Metal Oxide Catalysts. J. Catal. 1977, 50, 291. Amenomiya, Y. Active Sites of Solid Acidic Catalysts: III. Infrared Study of the Water Gas Conversion Reaction on Aluimina. J. Catal. 1979, 57, 64. Barbier, J., Jr.; Duprez, D. Reactivity of Steam in Exhaust Gas Catalysis: I . Steam and Oxygen/Steam Conversions of Carbon Monoxide and Propane over PtRh Catalysts. Appl. Catal., B 1993, 3, 61. Chen, H. W.; Hsiao, H. C.; Wu, S. J. Current Situation and Prospects of Motorcycle Pollution Control in Taiwan, Republic of China. SAE Paper No. 922176, 1992. Cho, B. K. Performance of Pt/Al2O3 Catalysts in Automobile Engine Exhaust with Oscillatory Air/Fuel Ratio. Ind. Eng. Chem. Res. 1988, 27, 30. Diwell, A. F.; Rajaram, R. R.; Shaw, H. A.; Truex, T. J. The Role of Ceria in Three-Way Catalysts. In Catalysis and Automotive Pollution Control II; Crucq, A., Ed.; Elsevier Science Publishers B.V.: Amsterdam, The Netherlands, 1991; Vol. 71, p 139. Grenoble, D. C.; Estadt, M. M.; Ollis, D. F. The Chemistry and Catalysis of the Water Gas Shift Reaction: 1. The Kinetics over Supported Metal Catalysts. J. Catal. 1981, 67, 90. Harrison, B.; Diwell, A. F.; Hallett, C. Promoting Platinum Metals by Ceria. Metal-Support Interactions in Autocatalysts. Platinum Met. Rev. 1988, 32 (2), 73. Ishikawa, A.; Komai, S.; Satsuma, A.; Hattori, T.; Murakami, Y. Solid Superacid as the Support of a Platinum Catalyst for LowTemperature Catalytic Combustion. Appl. Catal., A 1994, 110, 61. Jones, A.; McNicol, B. D. Temperature-Programmed Reduction for Solid Materials Characterization; Marcel Dekker: New York, 1986. Kim, G. Ceria-promoted Three-Way Catalysts for Auto Exhaust Emission Control. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 267. Kinoshita, H.; Suzuki, Y.; Saruhashi, S.; Sato, M. Exhaust gas purifying catalyst. U.S. Patent 4,369,132, 1983.

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5169 Koberstein, E; Pletka, H. D. Exhaust Purification with Small 2-Stroke EnginessA Challenge for Catalytic Systems. SAE Paper No. 820279, 1982. Laimbock, F. J.; Landerl, C. J. 50 cc Two-Stroke Engines for Mopeds, Chainsaws and Motorcycle with Catalysts. SAE Paper No. 901598, 1990. Lee, C. H.; Chen, Y. W. Effect of Basic Additives on Pt/Al2O3 for CO and Propylene Oxidation under Oxygen-Deficient Conditions. Ind. Eng. Chem. Res. 1997, 36, 1498. Malinowski, S. Modification of the Acidity and Basicity of the Surface of Oxide Catalysts. In Catalysis by Acids and Bases; Imelik, B., Naccache, C., Coudurier, G., Ben Tarrit, Y., Vedrine, J. C., Eds.; Elsevier Science Publishers B.V.: Amsterdam, The Netherlands, 1985l Vol. 20, p 57. Mooney, J. J.; Hansel, J. G.; Hoyer, R. D. Catalytic Control of TwoStroke Motorcycle Exhaust Emissions. SAE Paper No. 750910, 1975. Patterson, W. R.; Kemball, C. The Catalytic Oxidation of Olefins on Metal Films. J. Catal. 1963, 2, 465. Su, E. C.; Montreuil, C. N.; Rothschild, W. G. Oxygen Storage Capacity of Monolith Three-Way Catalysts. Appl. Catal. 1985, 17, 75. Voltz, S. E.; Morgan, C. R.; Liederman, D.; Tacob, S. M. Kinetic Study of Carbon Monoxide and Propylene Oxidation on Platinum Catalysts. Ind. Eng. Chem. Prod. Res. Dev. 1973, 12 (4), 294.

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Received for review April 24, 1997 Revised manuscript received August 12, 1997 Accepted August 12, 1997X IE970296Q

X Abstract published in Advance ACS Abstracts, October 1, 1997.