Effect of K2O on a Pd-Containing Catalytic Converter for Removing

motorcycles. It infers that PdRh/K2O/Al2O3-CeO2 is a promising catalytic converter for a two- ...... Ai, M. Activities for the Decomposition of Formic...
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Ind. Eng. Chem. Res. 1998, 37, 1260-1266

Effect of K2O on a Pd-Containing 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

Noble metals (Pt, Pd, and Rh) supported on Al2O3, K2O/Al2O3, CeO2/Al2O3, and K2O/CeO2/Al2O3 were prepared and characterized with respect to surface area, pore volume, and temperatureprogrammed desorption of CO2. The effects of K2O on the noble-metal catalysts for carbon monoxide and hydrocarbon oxidation were investigated. The reactions were carried out under the stoichiometric and oxygen-deficient conditions. Under the stoichiometric point, the Pdcontaining catalysts exhibit higher activity than the Pt-containing catalysts for both CO and C3H6 oxidation. Moreover, Pd/K2O/CeO2/Al2O3 is the most active catalyst among the powder catalysts in this study. Under the oxygen-deficient conditions and in the presence of water, the CO conversions on Pd/Al2O3 and Pd/CeO2/Al2O3 are significantly lower than those on Pt/Al2O3 and Pt/CeO2/Al2O3, respectively. In contrast, the Pd-containing catalysts exhibit higher C3H6 conversion than the Pt-containing catalysts. However, the CO conversions on the Pd-containing catalysts can be promoted by the addition of K2O. On the other hand, the test results of the monolithic catalysts revealed that the CO conversion on PdRh/K2O/Al2O3-CeO2 is quite close to that on PtRh/Al2O3-CeO2 under the simulative gases and the ECE-40 mode driving cycle test. PtRh/Al2O3-CeO2 is the typical composition of catalytic converters for two-stroke motorcycles. It infers that PdRh/K2O/Al2O3-CeO2 is a promising catalytic converter for a twostroke motorcycle. Introduction Motorcycles are used as the main source of transportation in many parts of the world, especially in Asian countries. 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 used on automobiles in the United States since 1975. The catalyst technology provides a significant and cost-effective method for reduction of hydrocarbon (HC), carbon monoxide (CO), and nitrous oxides (NOx). The composition of noble metals in the conventional three-way catalyst generally is Pt and Rh. The all-palladium autocatalyst had been successfully developed by Allied Signal (Thayer, 1993). Catalytic converters have been also massively employed to control exhaust emissions of two-stroke motorcycles in Taiwan since 1992 (Chen et al., 1992). The commercial catalytic converters used in Taiwan mainly contain Pt, Rh, CeO2, and Al2O3. The Pd-containing catalytic converter used to control exhaust emissions of a two-stroke motorcycle is very scarce. 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 (stoichiometric 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 concen* To whom correspondence should be addressed. Fax: 8863-4252296. E-mail: [email protected].

trations 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). Besides the fact that two-stroke motorcycles are normally running in oxygen deficiency, the conversions of CO and HC on a catalytic converter are constrained by the limited amount of oxygen in the exhaust gases. Nevertheless, the 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 amount of oxygen in the exhaust gases. The HC conversion on a catalytic converter increases with increasing reaction temperature, while this phenomenon contrasts with the CO conversion. Furthermore, the CO conversion on a catalytic converter is extremely low, even negative at the high-speed driving mode that represents an extremely oxygen-deficient and higher 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. Muraki (1991) reported that the CO conversion on the Pd-containing catalyst is inferior to that on the Ptcontaining catalyst under an oxygen-deficient condition. In addition, Muraki et al. (1986) reported that the partial oxidation of C3H6 takes place on Pd/Al2O3 to produce the extra CO emission under an oxygendeficient condition. The above results are detrimental to the Pd-containing catalyst to serve as a catalytic converter for two-stroke motorcycles. However, the testing conditions in the above reports are to simulate the exhaust emission of automobiles; the results cannot be directly extended to the exhaust emission of two-

S0888-5885(97)00355-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/12/1998

Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1261 Table 1. Compositions and Physical Properties of the Powder Catalysts

catalysta Pt/Al2O3 Pt/CeO2/Al2O3 Pd/Al2O3 PdCeO2/Al2O3 Pd/K2O/Al2O3 Pd/K2O/CeO2/Al2O3 a

MOx/support, wt % CeO2/Al2O3 ) 15 CeO2/Al2O3 ) 15 K2O/Al2O3 ) 5.5 K2O/(Al2O3 + CeO2) ) 5.5

pore BET volume, surface area, m2/g cm3/g 194 160 198 173 176 153

0.74 0.61 0.74 0.61 0.70 0.60

The Pt or Pd content is 0.4 wt % of the support.

stroke motorcycles. Even the CO removal capability of the Pd-containing catalyst is lower than that of the Ptcontaining catalyst. It is worth pursuing a method to improve the capability of the Pd-containing catalyst to remove CO emission from a two-stroke motorcycle, since Pd is abundant and significantly cheaper than Pt. In the previous reports (Lee and Chen, 1997a,b), the authors have reported that K2O can significantly improve the CO conversion for both Pt and Pd catalysts under oxygen-deficient conditions. The focus of this work was to develop a Pd-containing catalyst which can compete with the Pt-containing catalyst to control the exhaust emission from a two-stroke motorcycle. Previous reported results (Lee and Chen, 1997b) were included for comparison. The objective of this study was to investigate whether the CO conversion on the Pd-containing catalyst can be significantly promoted by the addition of K2O. 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 performance of the catalytic converter for removing emissions from a twostroke motorcycle was verified by the ECE-40 mode driving cycle test. Experimental Section Powder Catalyst Preparation. γ-Al2O3, CeO2/ Al2O3, K2O/Al2O3, and K2O/CeO2/Al2O3 powders 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, these materials were calcined in air at 500 °C for 3 h. Pt/Al2O3 and Pt/CeO2/ Al2O3 were prepared by incipient wetting of Al2O3 and CeO2/Al2O3 with the tetraammineplatinum nitrate aqueous solution. After being dried at 120 °C for 2 h, the catalyst was calcined at 500 °C for 3 h. Pd/Al2O3, Pd/ K2O/Al2O3, and Pd/CeO2/Al2O3 catalysts were prepared by a similar procedure, but the palladium nitrate aqueous solution was used. Pd/K2O/CeO2/Al2O3 was also prepared by the same procedure, except K2O was impregnated on the support prior to the noble-metal impregnation. The K2O content was 5.5 wt % of support. The noble-metal content on all powder catalysts was 0.4 wt % of support, as determined by ICPAES. The compositions of the catalysts are listed in Table 1. Monolithic Catalyst Preparation. The wash-coating slurries were made by ball-milling γ-Al2O3 or the mixture of CeO2 and γ-Al2O3 powder with psuedoboemite, respectively. The weight ratio of CeO2 to Al2O3 was controlled at 15%. The metallic monoliths with 100 cells/in.2 were wash-coated by the slurries. Following

Table 2. Compositions of the Monolithic Catalystsa

catalyst

MOx/Al2O3, wt %

PtRh/Al2O3-CeO2 PdRh/Al2O3 PdRh/Al2O3-CeO2 PdRh/K2O/Al2O3 -CeO2

CeO2/Al2O3 ) 15

a

CeO2/Al2O3 ) 15 CeO2/Al2O3 ) 15

K2O, g/in.3

noble metal, ratio by weight, g/ft3

0.11

Pt:Rh ) 5:1, 50 Pd:Rh ) 5:1, 50 Pd:Rh ) 5:1, 50 Pd:Rh ) 5:1, 50

The washcoat loading is approximately 2 g/in.3

Table 3. Feed Compositions of the Simulative Gases for the Powder Catalysts

CO, % C3H6, ppm O2, % CO2, % H2, % H2O, % N2

S)1

S ) 0.29 without water

S ) 0.29 with water

1.0 800 0.96 10 0.2 0 balance

1.72 4400 0.86 10 0.2 0 balance

1.72 4400 0.86 10 0.2 10 balance

drying at 120 °C for 2 h, the metallic monoliths were calcined in air at 500 °C for 3 h. The wash-coat loading was controlled at approximately 2 g/in.3 PdRh/Al2O3 and PdRh/Al2O3-CeO2 were prepared by co-impregnating with a mixture solution of palladium nitrate and rhodium nitrate. After being dried at 120 °C for 2 h, the catalyst was calcined at 500 °C for 3 h. PdRh/K2O/ Al2O3-CeO2 was also prepared by a similar procedure, except potassium was impregnated on the monolithic supports prior to the noble-metal impregnation. The K2O content was 0.11 g/in.3 In addition, a reference catalyst, PtRh/Al2O3-CeO2, was prepared by the similar procedure. The noble-metal content on all monolithic catalysts was 50 g/ft3 (Pd/Rh ) 5 or Pt/Rh ) 5). The compositions of the catalysts are listed in Table 2. Characterization of the Catalysts. The experiments of the temperature-programmed desorption of CO2 were carried out in a flow-type system, which is similar to that reported by Jones and McNicol (1986). A total of 200 mg of 40-60 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 and recorded by an on-line personal computer. The BET surface areas and pore volumes of the catalysts were measured by N2 sorption at liquid nitrogen temperature by Micromeritics ASAP 2400. The XRD patterns of the catalysts were measured by a Shimadzu diffractmeter (model XD-D1) with Cu KR radiator. Activity Measurement of the Powder Catalyst. The catalytic activity was measured using a continuous flow reaction apparatus (Lee and Chen, 1997a). 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 3. The flow rate of reaction gases was 2 L (NTP)/min. HC and CO conversions were simultaneously measured as a function of temperature (150∼600

1262 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 Table 4. Feed Compositions of the Simulative Gases for the Monolithic Catalyst A/F ratio

CO, % C3H6, ppm O2, % H2, % CO2, % H2O, % N2

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

C) under the steady-state condition. Before testing the catalytic 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]

Figure 1. CO2-TPD patterns of catalysts.

where [O2], [H2], [CO], and [C3H6] are the concentrations of O2, H2, CO, and C3H6, respectively, in the feed. Activity Measurement of the Monolithic Catalyst. The reaction system is the same as that reported by Lee and Chen (1997a). A monolithic catalyst, diameter of 2 cm and a length of 1.2 cm (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. HC and CO conversions on the monolithic catalysts were measured as a function of the A/F ratio of reaction gases. Table 4 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 GHSV is about 52 000 (NTP) h-1. Before testing the catalytic 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 percent and HC is presented as C3H6. The ECE-40 Mode Driving Cycle Test. The catalytic converter, L 6 × 4 cm, was canned beforehand inside the designated muffler of the target motorcycle. The target motorcycle chosen for this work was Taking 50, which is one of the popular two-stroke motorcycles in Taiwan with an engine volume of 50 cm3. The testing apparatus for the ECE-40 mode driving cycle test comprises a chassis dynamometer for the motorcycle (Meidan), a constant volume-sampling system (Horiba, CVS 9100), and a set of 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. The HC, CO, and NOx emissions from the target motorcycle were analyzed by the emission analyzing instruments. Results and Discussion Characterization of Catalyst. The BET surface areas and pore volumes of the catalysts are listed in

Table 1. The surface areas and pore volumes of Pt/Al2O3 and Pd/Al2O3 decreased slightly by adding CeO2 or K2O, due to the blockage of the micropore of -Al2O3. The basicities of these catalysts can be analyzed by the CO2-TPD. Figure 1 displays the CO2-TPD (temperature-programmed desorption) spectra of four catalysts, Pd/Al2O3, Pd/CeO2/Al2O3, Pd/K2O/Al2O3, and Pd/K2O/ CeO2/Al2O3. Besides the peak at 100 °C, no other desorption peak appears on Pd/Al2O3 and Pd/CeO2/ Al2O3. However, there are several desorption peaks on Pd/K2O/Al2O3. The major CO2 desorption peak on Pd/ K2O/Al2O3 is around 500 °C. This result indicates that the strength of basicity of Pd/K2O/Al2O3 is significantly greater than that of Pd/Al2O3, as expected. Similarly, the basicity of Pd/CeO2/Al2O3 can be increased by the addition of K2O. The major CO2 desorption peak on Pd/ K2O/CeO2/Al2O3 is around 530 °C. The strength of basicity of Pd/K2O/CeO2/Al2O3 is slightly greater than that of Pd/K2O/Al2O3 by the desorption temperature of CO2. In this study, the basic strength of Pd/CeO2/Al2O3 cannot distinguish from that of Pd/Al2O3. This is possible due to the fact that the basicity strengths of Pd/Al2O3 and Pd/CeO2/Al2O3 are weak. The difference of the basicity strength between them is not distinguished by the adsorbate of CO2, which is a weak acid. Nevertheless, the XRD analyzing results reveal the presence of bulk CeO2 and Al2O3 in Pd/CeO2/Al2O3. Tanabe (1981) reported that CeO2 manifests a more basic characteristic than Al2O3. Therefore, the strength of basicity of Pd/CeO2/Al2O3 might be greater than that of Pd/Al2O3. Comparing the Activities of the Pd-Containing and the Pt-Containing Catalysts for CO and C3H6 Oxidation under the Stoichiometric Point. The light-off temperatures (T50 defined as the temperature enough to achieve a conversion of 50%) of these catalysts are listed in Table 5. The light-off temperatures of Pd/ Al2O3 for CO and C3H6 oxidation are lower than those of Pt/Al2O3. Similarly, the light-off temperatures of Pd/ CeO2/Al2O3 for CO and C3H6 oxidation are lower than those of Pt/CeO2/Al2O3. The above results suggest that the activities of the Pd catalysts for CO and C3H6 oxidation are higher than those of the Pt catalysts at the stoichiometric condition. In addition, the activities of Pd/Al2O3 and Pt/Al2O3 for CO and C3H6 oxidation can be improved by the

Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1263 Table 5. Light-Off Temperatures of the Powder Catalysts T50,a °C catalyst

CO

HC

Pt/Al2O3 Pt/CeO2/Al2O3 Pd/Al2O3 Pd/K2O/Al2O3 Pd/CeO2/Al2O3 Pd/K2O/CeO2/Al2O3

279 188 231 181 175 166

315 273 244 189 198 178

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

addition of basic additives (CeO2 or K2O). The kinetics of CO and C3H6 oxidation on Pt/Al2O3 and Pd/Al2O3 may follow a Langmuir-Hinshelwood mechanism (Patterson and Kemball, 1963; Voltz et al., 1973; Yu Yao, 1984). The reaction occurs between adsorbed oxygen and an adsorbed reactant on the catalyst surface. The reaction rate increases with increasing oxygen concentration and is inhibited by CO and C3H6 strong adsorption. The addition of CeO2 or K2O on Pd and Pt catalysts can increase the basicity of the catalyst which could create an electron density on the catalyst surface. This might result in the reduction of the affinity of nucleophilic reagents, CO and C3H6 , to the catalyst surface. This can lessen the self-inhibition effect of CO and C3H6. Therefore, the light-off temperature of the catalyst containing CeO2 or K2O can be significantly decreased. In addition, Pd/K2O/CeO2/Al2O3 is the most active catalyst for CO and C3H6 oxidation among these catalysts at the stoichiometric condition. In contrast, Pt/ Al2O3 is the least active catalyst for CO and C3H6 oxidation. Comparing the CO and C3H6 Conversions on the Pd-Containing and the Pt-Containing Catalysts under an Oxygen-Deficient Condition and in the Presence of Water. Parts a and b of Figure 2 illustrate the reaction curves of Pd/Al2O3, Pd/CeO2/ Al2O3, Pt/Al2O3, and Pt/CeO2/Al2O3 catalysts for CO and C3H6 oxidation under S ) 0.29 and in the presence of water. The C3H6 conversions on the Pd-containing catalysts at 400 °C or higher are higher than those on the corresponding Pt-containing catalysts by at least 10%. In contrast, the CO conversions on the Pdcontaining catalysts at 400 °C or higher are significantly lower than those of the corresponding Pt-containing catalysts by at least 20%. The highest CO conversion on Pd/Al2O3 (ca. 40%) is close to that on Pt/Al2O3 . However, the CO conversion on Pd/Al2O3 gradually becomes lower than that on Pt/ Al2O3 above 350 °C. Moreover, the CO conversion on Pd/Al2O3 becomes negative above 550 °C due to the occurrence of the C3H6 partial oxidation taking place. Similarly, the difference of CO conversions between Pt/ CeO2/Al2O3 and Pd/CeO2/Al2O3 gradually increases with increasing the reaction temperature from 15% at 300 °C to 32% at 550 °C. The CO conversion on Pd/CeO2/ Al2O3 above 400 °C is only slightly higher than that on Pt/Al2O3 by 10%. The results verify that if Pt and Pd are supported on the same support, the CO conversion on the Pd catalyst is lower than that on the Pt catalyst under an oxygen-deficient condition. The results are in accordance with those reported by Muraki (1991). Effects of K2O on the Pd Catalysts for CO and C3H6 Oxidation under an Oxygen-Deficient Condition and in the Presence of Water. Figure 3 il-

Figure 2. CO and propylene oxidation on the Pd-containing and Pt-containing catalysts under S ) 0.29 and in the presence of water.

lustrates the effects of K2O on the Pd catalysts for CO and C3H6 oxidation under S ) 0.29 and in the presence of water. This reaction condition simulates the exhaust emissions of a two-stroke motorcycle at ca. 50 km/h. In addition to HC and CO being directly oxidized by oxygen, CO and HC can also be removed by the watergas shift (WGS) and steam reforming reactions, respectively (Barbier and Duprez, 1993). These reactions generally occur when oxygen is absent (Barbier and Duprez, 1993). Figure 4 displays the effect of water on CO and C3H6 conversions of these catalysts at 550 °C by the inducement of the WGS and steam reforming reactions. Moreover, oxygen in the reaction gases has been entirely consumed at 550 °C. Besides the highest CO conversion being double (from 40 to 80% at ca. 300 °C), the CO conversion on Pd/Al2O3 at 550 °C also increases from a negative value (-3%) to 49% by the addition of K2O, as shown in Figure 4. Figure 4 also displays that the promotional effect of K2O on Pd/Al2O3 for CO conversion is significantly greater than that of CeO2. This phenomenon has been reported by Lee and Chen (1997a) on the Pt/Al2O3 catalyst. Similarly, the CO conversion on Pd/CeO2/Al2O3 can also be enhanced by the addition of K2O. In addition to the highest CO conversion (86%) on Pd/K2O/CeO2/Al2O3 becoming close to that on Pt/CeO2/Al2O3, the difference of the CO conversions at 550 °C between them is also effectively lessened to ca. 10%, as shown in Figure 4. However, the C3H6 conversions on the Pd-containing catalysts are decreased by the addition of K2O, as shown in Figures 3 and 4.

1264 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998

Figure 5. CO and propylene conversions as a function of the A/F ratio of reaction gases with reactants, CO and C3H6.

Figure 3. Effect of K2O on the Pd catalysts for CO and propylene oxidation under S ) 0.29 and in the presence of water.

Figure 4. Effects of water on CO and C3H6 conversion at 550 °C under S ) 0.29 condition: 0, without water; 9, with water.

The effect of K2O on the CO conversions of Pd/CeO2/ Al2O3 and Pd/Al2O3 can be ascribed to the fact that it can promote not only the WGS reaction but also the CO conversions on the catalysts under an oxygen-deficient condition. Due to the participation of water in the reaction, the CO conversions on Pd/Al2O3, Pd/K2O/Al2O3, Pd/CeO2/Al2O3, and Pd/K2O/CeO2/Al2O3 are increased by 2, 18, 15, and 27%, respectively. Pd/Al2O3 exhibits the lowest activity for the WGS reaction; meanwhile, Pd/K2O/CeO2/Al2O3 exhibits the highest activity for the WGS reaction. Moreover, the effect of K2O on Pd/Al2O3 is superior to that of CeO2. CeO2 has been reported to be a good promoter for the WGS reaction (Kim, 1982; Diwell et al., 1991; Barbier and Duprez, 1993). 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 CO2 and H2 via the dehydrogenation reaction catalyzed by metals and basic metal oxides (Grenoble et al., 1981; Ai, 1977). Grenoble et al. (1981) also reported that there is an optimal interaction between the 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 the Pd catalysts not only reduces the interaction between CO and Pd but also enhances the decomposition rate of formate or formic acid via the dehydrogenation reaction by increasing the basicity of the catalyst. It could explain why the K2O-containing Pd catalysts exhibit higher activity for the WGS reaction. The above results verify that the addition of K2O can significantly promote the CO conversion on Pd/Al2O3 catalysts. In addition, the CO conversion on Pd/Al2O3 can be further promoted by the co-addition of CeO2 and K2O. Effect of A/F Ratio on the Monolithic Catalysts for CO and HC Oxidation. Figure 5 depicts the CO and C3H6 conversions of three monolithic catalysts, φ 2 × 1.2 cm, 100 cells/in.,2 measured as a function of A/F ratios of the simulative gases. The inlet temperature of reaction gases was controlled at 500 °C, that is, equivalent to the temperature of exhaust gases from a two-stroke motorcycle at ca. 50 km/h. Tables 2 and 4 illustrate the compositions of the monolithic catalysts and the simulative gases with various A/F ratios. The C3H6 and CO conversions of the catalysts decrease along with decreasing A/F ratios of reaction gases, as expected. Nevertheless, the effect of the A/F ratio on CO conversion is significantly greater than that on C3H6 conversion. For example, C3H6 and CO conversions on PdRh/Al2O3-CeO2 decrease from 100 and 100% at the stoichiometric point (A/F ) 14.63) to 75 and -12% 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 result verifies that the CO conversion is strongly affected by the A/F ratio of reaction gases. Moreover, the effect of A/F ratio on the CO conversion of the Pd-containing catalyst (e.g., PdRh/ Al2O3-CeO2) is greater than the Pt-containing catalyst (e.g., PtRh/Al2O3-CeO2). The difference of CO conversions between PdRh/Al2O3-CeO2 and PtRh/Al2O3-CeO2

Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1265 Table 6. ECE-40 Testing Result of the Metallic Catalytic Converters catalyst

CO, g/km

HC, g/km

NOx, g/km

dummy PtRh/Al2O3-CeO2 PdRh/Al2O3 PdRh/Al2O3-CeO2 PdRh/K2O/Al2O3-CeO2

5.72 4.50 (21.3%)a 6.77 (-18.4%) 6.26 (-9.4%) 4.60 (19.6%)

4.04 0.77 (80.9%) 0.78 (80.7%) 0.79 (80.4%) 1.22 (69.8%)

0.053 0.010 (81.1%) 0.021 (60.4%) 0.018 (66.0%) 0.022 (58.5%)

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

Figure 6. CO and HC conversions as a function of the A/F ratio of reaction gases with reactants, CO, C3H6, and C3H8.

is gradually magnified from 2% at A/F ) 14.2 to 15% at A/F ) 10.8. However, the disparity of the CO conversions between the Pd and Pt catalysts can be significantly lessened by the addition of K2O on the Pd catalyst. The CO conversion of PdRh/K2O/Al2O3-CeO2 at every A/F ratio is almost the same as that of PtRh/ Al2O3-CeO2. This verifies that K2O is a good additive for PdRh/Al2O3-CeO2 from the aspect of the CO removal. In addition, the C3H6 conversions on these catalysts are very close, except at A/F ) 10.8. On the other hand, the scavenging process is an inherent characteristic of the two-stroke motorcycle. The mechanism of the scavenging process is that the burnt exhaust gases in the engine are purged out by the fresh fuel gases. Some fuel gases will flow directly into the exhaust pipe from the exhaust port of the two-stroke engine. This results in a high HC emission. In addition, there are 40-50% alkanes in the unleaded gasoline sold in Taiwan. Therefore, there are some portions of alkanes in the exhaust gases of two-stroke motorcycles. To precisely simulate the exhaust emissions of twostroke motorcycles, propylene in the simulative gases, indicated in Table 4, was replaced by the mixture of propane and propylene (1:2). Figure 6 depicts the CO and HC conversions of three monolithic catalysts measured as a function of the A/F ratio of the new simulative gases. When compared to Figure 5, the HC conversions on these catalysts slightly decrease due to the existence of propane, since propane is more difficult to be oxidized than propylene (Yu Yao, 1980). The decreased extent is more significant for the catalyst containing K2O. From an analysis of the oxygen concentration in the effluent at A/F ) 14.63, it revealed that oxygen was not completely consumed upon reacting with reactants on PdRh/K2O/Al2O3-CeO2. This results in the fact that PdRh/K2O/Al2O3-CeO2 exhibits the lowest HC conversion (ca. 85%) among three catalysts. This can be explained that the oxidation mechanism of saturated hydrocarbons is different from that of unsaturated hydrocarbons. The initial adsorption of alkane on the catalyst is postulated as the rate-determining step of alkane oxidation (Patterson and Kemball, 1963). Increasing the basicity of the catalyst by adding K2O is likely to create an electron density on the catalyst surface. This would significantly lessen the feasibility of propane adsorption on the surface of the catalyst. This results in the fact that PdRh/K2O/Al2O3-CeO2 exhibits the lower activity for propane oxidation. Nevertheless, the propane conversion on PdRh/K2O/Al2O3-CeO2 can be increased by increasing the 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 participates in the reaction, the more energy is released. Therefore, the actual reaction temperature on the monolithic catalyst at A/F ) 14.2 is higher than that at A/F ) 14.63. This is the reason HC conversion of PdRh/ K2O/Al2O3-CeO2 at A/F ) 14.2 is higher than that at the stoichiometric point (A/F ) 14.63). In addition, PdRh/Al2O3-CeO2 exhibits a slightly higher HC conversion than PtRh/Al2O3-CeO2 does. On the other hand, the CO conversions of these catalysts are slightly increased due to propane participating in the reaction. Nevertheless, the difference of the CO conversions between PdRh/K2O/Al2O3-CeO2 and PtRh/Al2O3-CeO2 is still very small. The above experimental results suggest that PdRh/ K2O/Al2O3-CeO2 might be a promising catalytic converter to control exhaust emission of a two-stroke motorcycle. This feasibility is further verified by the ECE-40 mode driving cycle test. The ECE-40 Mode Driving Cycle Test. In addition to PdRh/Al2O3, the above three monolithic catalysts were prepared as the catalytic converters by supporting on the metallic monolith (φ 6 × 4 cm, 100 cells/in.2). The actual performances of these catalytic converters were verified by the ECE-40 mode driving cycle test. The ECE-40 test results of these catalysts are displayed in Table 6. PdRh/Al2O3 exhibits the highest HC and the lowest CO conversions. The CO conversion on PdRh/ Al2O3 is negative (-18.4%). It infers that some HCs are partially oxidized on PdRh/Al2O3 to produce the extra CO emission. Nevertheless, the CO conversion on PdRh/Al2O3 can be improved by the addition of CeO2. However, the CO emission from the target motorcycle installed with PdRh/Al2O3-CeO2 (6.26 g/km) is still significantly greater than that with PtRh/Al2O3-CeO2 (4.50 g/km). PtRh/Al2O3-CeO2 is the typical catalytic converter for a two-stroke motorcycle. Therefore, PdRh/ Al2O3-CeO2 is not a good catalytic converter for a twostroke motorcycle due to the poor capability in removing the CO emission (greater than the emission standard 4.5 g/km). However, the CO conversions on PdRh/Al2O-CeO2 can be increased by the addition of K2O, though the HC conversion will be decreased. Although the CO emission from the target motorcycle installed with PdRh/K2O/ Al2O3-CeO2 (4.60 g/km) is still greater than that with PtRh/Al2O3-CeO2 (4.50 g/km), the difference of CO emissions between them is quite small. Table 7 indicates that PdRh/K2O/Al2O3-CeO2 is a suitable catalytic converter for the two-stroke motorcycle with a lower CO emission. By installation with PdRh/K2O/Al2O3-CeO2, CO and HC emissions from the target motorcycle are 3.93 and 1.03 g/km, respectively, which can compile with the emission standards. The ECE-40 test results also show that HC emission from a two-stroke motorcycle can be effectively removed

1266 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 Table 7. ECE-40 Testing Result of the Two-Stroke Motorcycle with a Lower CO Emission catalyst

CO, g/km

HC, g/km

NOx, g/km

dummy PdRh/Al2O3 PdRh/Al2O3-CeO2 PdRh/K2O/Al2O3-CeO2

5.05 5.61 (-11.1%)a 5.25 (-4.0%) 3.93 (22.2%)

3.73 0.61 (83.6%) 0.74 (80.2%) 1.03 (72.4%)

0.052 0.011 (78.8%) 0.009 (82.7%) 0.010 (80.8%)

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

by the catalytic converter, and the CO emission is strongly affected by the emission characteristic of the target motorcycle. PdRh/K2O/Al2O3-CeO2 is a suitable catalytic converter for a two-stroke motorcycle with a lower CO emission. Conclusions 1. The surface areas and pore volumes of Pd/Al2O3 and Pt/Al2O3 are slightly decreased by the addition of K2O or CeO2. 2. The CO2-TPD results reveal that the basicity of Pd/Al2O3 or Pd/CeO2/Al2O3 can be increased by the addition of K2O. 3. Under the stoichiometric condition, the activities of the powder catalysts for CO and C3H6 oxidation follow the order Pd/K2O/CeO2/Al2O3 > Pd/K2O/Al2O3 = Pd/ CeO2/Al2O3 > Pd/Al2O3. Moreover, Pd catalysts exhibits a higher activity than Pt catalysts for CO and C3H6 oxidation, when they are supported on the same support. 4. When Pt and Pd are supported on the same support, the CO conversions on the Pd-containing catalysts are significantly lower than those on the Ptcontaining catalysts under an oxygen-deficient condition and in the presence of water. However, the CO conversions on Pd catalysts can be significantly promoted by the co-addition of K2O and CeO2. 5. The effect of the A/F ratio on the CO conversion of the monolithic catalysts is significantly greater than that on HC conversion. 6. The test results of the monolithic catalysts reveal that the CO conversion on PdRh/K2O/Al2O3-CeO2 is very close to that on PtRh/Al2O3-CeO2 under the simulative gases and the ECE-40 mode driving cycle. It infers that PdRh/K2O/Al2O3-CeO2 is a promising catalytic converter for a two-stroke motorcycle with a lower CO emission. Acknowledgment The authors thank the National Science Council of the Republic of China for financially supporting this work under Contract No. NSC-86-2214-E008-010.

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Received for review May 19, 1997 Revised manuscript received December 2, 1997 Accepted December 2, 1997 IE9703550