Kinetic Study of Carbon Monoxide and Propylene Oxidation on

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Ind. Eng. Chem. Res. 1997, 36, 1498-1506

Effect of Basic Additives on Pt/Al2O3 for CO and Propylene Oxidation under Oxygen-Deficient Conditions Chiou-Hwang Lee and Yu-Wen Chen* Department of Chemical Engineering, National Central University, Chung-Li, 32054 Taiwan, Republic of China

Pt catalysts supported on Al2O3, CeO2/Al2O3, Na2O/Al2O3, and K2O/Al2O3 were prepared and characterized with respect to surface area, CO chemisorption, temperature-programmed desorption (TPD) of CO2, and temperature-programmed reduction (TPR) of H2. The effects of basic additives on Pt/Al2O3 for carbon monoxide and propylene oxidation were investigated. The reactions were performed under the stoichiometric and oxygen-deficient conditions. The addition of basic additives slightly decreases the surface area of the catalyst and does not significantly change Pt dispersion. The addition of basic additives also influences the reducibility of Pt/ Al2O3. The basicity of the catalyst is in the order Pt-K2O/Al2O3 > Pt-Na2O/Al2O3 > Pt-CeO2/ Al2O3 > Pt/Al2O3. The promoted Pt/Al2O3 catalysts are much more active than the unpromoted one for CO and C3H6 oxidation under the stoichiometric point. Under oxygen-deficient conditions and in the absence of water, C3H6 conversions on all catalysts studied herein increase with increasing reaction temperature. Nevertheless, this phenomenon contrasts with CO conversion once oxygen is completely reacted. Pt/Al2O3 exhibits the highest C3H6 conversion and the lowest CO conversion among these catalysts, and the addition of CeO2, Na2O, and K2O on Pt/Al2O3 can promote the CO conversion. Under oxygen-deficient conditions and in the presence of water, the water-gas shift and steam re-forming reactions can take place and result in increases of CO and C3H6 conversions. Pt/Al2O3 is the most active catalyst for the steam re-forming reaction and the lowest active catalyst for the water-gas shift reaction among these catalysts. Nevertheless, the addition of basic additives on Pt/Al2O3 catalyst can significantly enhance the water-gas shift reaction that can reduce CO emission. The promotional effect is in the order K2O > Na2O > CeO2, that is the same order as the basicity of the promoted catalysts. Additionally, K2O could be a promising additive to a catalytic converter of a two-stroke motorcycle since it can significantly enhance CO conversion. Introduction Oxidative catalytic converters have been used to control exhaust emissions of automobiles in the United States since 1975. Mooney et al. (1975) and Koberstein and Pletka (1982) ever studied catalytic converters employed to control exhaust emissions of two-stroke motorcycles. Since 1986, motorcycles equipped with catalytic converters have been available in Switzerland and Austria (Laimbock and Landerl, 1990). However, Taiwan is the first country to massively employ catalytic converters to control exhaust emissions of two-stroke motorcycles. In order to reduce exhaust emissions of motorcycles, Taiwan EPA imposed in 1992 the most globally stringent emission standards. The test procedure is the same as ECE-15. 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). Complying with these standards requires installing catalytic converters in two-stroke motorcycles. The characteristics of exhaust emissions from twostroke motorcycles and those from automobiles differ greatly. Current two-stroke motorcycles run rich (oxygen deficiency). For two-stroke motorcycles, the air/fuel (A/F) ratio monotonously decreases with increasing the 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 * To whom correspondence should be addressed. Fax: 8863-4252296. S0888-5885(96)00414-9 CCC: $14.00

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). Mooney et al. (1975) reported that CO and HC concentrations in exhaust emissions of two-stroke motorcycles could exceed 5% and 5000 vol. ppm (C6), respectively. In addition, the conversions of CO and HC on a catalytic converter are constrained by the limited oxygen due to the fact that two-stroke motorcycles are normally running in oxygen deficiency. However, HC emission can effectively be 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. 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 that represents a more oxygen-deficient and higher temperature condition. Therefore, controlling 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, and CeO2. The effectiveness of cerium oxide is well established in three-way catalysts. Numerous papers have been published on the effects of CeO2 on autocatalyst. The main functions are to store oxygen under an © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1499

oxygen-excess condition (Yao and Yu Yao, 1984; Su et al., 1985; Harrison et al., 1988), to stabilize alumina (Harrison et al., 1988), to enhance noble metal dispersion (Yao et al., 1982; Diwell et al., 1991), and to promote the water-gas shift (WGS) and steam reforming reactions (Kim, 1982; Diwell et al., 1991; Barbier and Duprez, 1993). All of the above investigations examine the effects of CeO2 on autocatalysts, but none examine the effects on the catalytic converters for two-stroke motorcycles. Besides the fact that CeO2 can improve the Pt/Al2O3 activities for CO and C3H6 oxidation (Yu Yao, 1984), other basic oxides such as K2O also have an analogous effect (Kinoshita et al., 1983; Kang and Wan, 1994). Kang and Wan (1994) reported that the addition of K2O on Cr2O3-Co2O3/Al2O3 can enhance CO conversion but retard ethane oxidation. The light-off temperature of Pt/Al2O3 for C3H6 oxidation can also be reduced by the addition of K2O (Kinoshita et al., 1983). These results imply that CO and C3H6 oxidation can be enhanced by adding basic oxide, while the saturated hydrocarbon oxidation is retarded. It is suggested that increasing the basicity of the catalyst is likely to create electron density on the catalyst surface, thereby reducing the affinity of CO and C3H6 to the catalyst surface. Therefore, the catalyst activities for CO and C3H6 oxidation can be enhanced by suppressing the self-inhibition due to strong adsorption, while this phenomenon contrasts with the saturated hydrocarbon. It should be noted that all of the above investigations primarily concentrate on an oxygen-excess condition, instead of a severely oxygendeficient condition. Therefore, the results cannot extend to an oxygen-deficient condition. This work primarily concentrated on how to improve the CO conversion of a catalytic converter under oxygendeficient conditions. Besides oxidation, CO can be eliminated by the water-gas shift (WGS) reaction in the absence of oxygen. Although CeO2 is recognized as a good promoter for WGS reaction, basic metal oxides were also found to enhance WGS reaction (Ai, 1977; Amenomiya and Pleizier, 1982). Therefore, the effect of additives on the WGS reaction is another focus of this study. This work was carried out to investigate the effect of additives on Pt/Al2O3 for CO and C3H6 oxidation under oxygen-deficient conditions. The purpose of this study was to investigate whether the basic additives can selectively improve CO conversion. The catalytic activities of Pt/Al2O3, Pt-CeO2/Al2O3, Pt-Na2O/Al2O3, and Pt-K2O/Al2O3 were examined in terms of their capability to remove CO and C3H6 in simulative gases with various air/fuel ratios. Three sets of simulative gases, S ) 1.0, 0.31, and 0.17, were adopted to represent the equivalent A/F ratios of motorcycle emissions at idle speed, maximum speed, and full-throttle acceleration among the ECE-15 testing procedure, respectively. The effects of water on the WGS and steam re-forming reactions were investigated as well. Experimental Section Catalyst Preparation. γ-Alumina powders with a size of 40-60 mesh served as catalyst supports. CeO2/ Al2O3, Na2O/Al2O3, and K2O/Al2O3 supports were prepared by incipient wetting of γ-Al2O3 with an aqueous solution of cerium nitrate, sodium 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.

Table 1. Compositions of the Catalysts catalyst

M/Al ratio

M/Al2O3, wt %

Pt loading, wt %

Pt/Al2O3 Pt-CeO2/Al2O3 Pt-Na2O/Al2O3 Pt-K2O/Al2O3

Ce/Al ) 0.05 Na/Al ) 0.10 K/Al ) 0.10

13.7 4.5 7.6

0.4 0.4 0.4 0.4

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. The Pt-CeO2/Al2O3, Pt-Na2O/Al2O3 and Pt-K2O/Al2O3 catalysts were prepared by a similar procedure. The platinum content on all catalysts was 0.4 wt % of support as determined by ICP-AES. The compositions of the catalysts are listed in Table 1. Temperature-Programmed Reduction (TPR). The TPR apparatus is similar to that reported by Jones and McNicol (1986). A total of 100 mg of 40-60 mesh catalyst particles was packed into a quartz tube reactor and oxidized at 400 °C for 1 h by air. After oxidation, the catalyst was flushed and cooled under flowing argon (Ar). Next, 10% H2 in Ar flowed over catalyst at 25 °C for 30 min. The flow rate of gas was controlled at 25 mL/min. The temperature of the reactor was then increased linearly from 25 to 800 °C at a heating rate of 10 °C/min. Hydrogen consumption was detected by a thermal conductivity detector (TCD) and recorded by an on-line personal computer. Temperature-Programmed Desorption (TPD) of CO2. A CO2-TPD experiment was conducted in the TPR apparatus. A total of 100 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. Desorbing CO2 was monitored by a TCD and recorded by an on-line personal computer. 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, 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 Measurements. The catalytic activity was measured using a conventional continuous-flow reaction apparatus. A total of 150 mg of catalyst was placed in the reactor. The reactor was made of a quartz tube with an internal diameter of 0.8 cm. The catalyst bed was heated according to the designated temperature program. A schematic diagram of the experimental system was depicted in Figure 1. Catalytic activity was tested by several sets of simulative gases, the compositions of which are listed in Table 2. The flow rate of reaction gas was 2 L (NTP)/min, controlled by a mass flow controller (Brooks Model 5850). Water was fed by a liquid pump (LDC Analytical ContaMetric 3200) and evaporated in a preheated feed stream.

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Figure 1. Schematic diagram of the reaction system. M.F.C.: mass flow controller. L.P.: liquid pump. S.G.: standard gases. F1: reactor with temperature controller. T1, T2: thermal couple. C: cooler. B.M.: bubble meter. A.P.: air pump. F2: vaporizer. M: mixer. Table 2. Feed Compositions of the Simulative Gases S)1 CO, % C3H6, ppm O2, % CO2, % H2, % H2O, % N2

1.0 800 0.96 10 0.2 0 balance

S ) 0.31 S ) 0.17 S ) 0.31 S ) 0.17 with water with water 1.72 4400 0.9 10 0.2 0 balance

4.14 7000 0.9 10 0.2 0 balance

1.72 4400 0.9 10 0.2 10 balance

4.14 7000 0.9 10 0.2 10 balance

The stoichiometric number, S, employed to identify the redox characteristic of the model gas mixture is defined as

S)

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

When S < 1.0, S ) 1.0, and S > 1.0, the composition of the feed stream is net reducing, stoichiometric, and net oxidizing, respectively. The concentrations of C3H6, CO, and O2 were analyzed by flame ionization (GC-FID, HP5890A), nondispersive infrared (Rosemount, CO analyzer, Model 880), and magnetic susceptibility (Rosemount, O2 analyzer, Model 755), respectively. The conversion of C3H6 or CO was designated as [(Cin - Cout)/Cin] × 100%. The HC and CO conversions were measured as a function of temperature (150-600 °C) under steady-state conditions. Before testing the catalyst activity, the catalyst was reduced by 10% H2 in N2 (300 mL/min) at 450 °C for 30 min. Results and Discussion Characterization of Catalyst. The BET surface areas, pore volumes, and Pt dispersions of the catalysts

Table 3. Physical Properties and Light-Off Temperatures of the Catalysts

catalyst Pt/Al2O3 Pt-CeO2/Al2O3 Pt-Na2O/Al2O3 Pt-K2O/Al2O3

pore Pt BET surface area, volume, dispersion, cm3/g % m2/g 194 160 166 164

0.74 0.61 0.63 0.61

59.5 63.0 67.3 54.1

T50,a °C CO

HC

278 188 203 188

302 273 227 193

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.

are listed in Table 3. The surface area of the catalyst decreases by adding promoter, due to the blockage of the micropore of γ-Al2O3. Pt dispersions of all catalysts range from 54% to 67%. Blank experiments with supports have been conducted. The results reveal that the CO adsorption on supports is negligible. The addition of promoter changes Pt dispersion very slightly. All catalysts have similar metal dispersions. Therefore, the effect of metal dispersion might not be important in this study. The TPR profiles of these catalysts are depicted in Figure 2. The H2 consumption of Pt/Al2O3, which is multiplied by 33 times, is not obvious and relative broad. The major reduction peak of platinum oxide is around 220 °C. The H2 consumption of Pt/Al2O3 significantly increases by adding CeO2, Na2O, and K2O. The TPR pattern of Pt-CeO2/Al2O3 resembles that reported by Diwell et al. (1991). The reduction of the platinum component on Pt-CeO2/Al2O3 is more facile than that of platinum oxide dispersed on Al2O3. The reduction peak of platinum oxide on Pt-CeO2/Al2O3 catalyst has a maximum at 145 °C. The reduction peak around 265 °C for Pt-CeO2/Al2O3 would be associated with the reduction of CeO2 close to platinum (Diwell et al., 1991).

Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1501

Figure 2. H2-TPR patterns of catalysts.

The main reduction peak for K2O/Al2O3 is at 535 °C. However, the main reduction peak for Pt-K2O/Al2O3 is at 220 °C, and no peak appears at 535 °C. In addition, the H2 consumption of Pt-K2O/Al2O3 is much greater than that of Pt/Al2O3. This finding indicates that only a small amount of H2 is employed to reduce platinum oxide and most of H2 reacts with surface oxygen shared with potassium. Therefore, the peak at 220 °C could be assigned to the reduction of platinum oxide and oxygen shared with potassium. There might be a hydrogen spillover phenomenon on Pt-K2O/Al2O3 during the TPR process in which H2 adsorbed on Pt atom migrates to K2O in close proximity to Pt. The TPR profile of Pt-Na2O/Al2O3 is similar to that of Pt-K2O/ Al2O3. The basicities of these catalysts can be analyzed by the CO2-TPD spectra as shown in Figure 3. Besides the peak at 80 °C, no other desorption peak appears on Pt/ Al2O3 and Pt-CeO2/Al2O3. However, there are several desorption peaks on Pt-Na2O/Al2O3 and Pt-K2O/Al2O3. The major peaks on Pt-Na2O/Al2O3 are 500 and 595 °C, while the major peaks on Pt-K2O/Al2O3 are 550 and 665 °C. These results indicate that the strength of basicity of Pt-K2O/Al2O3 is stronger than that of PtNa2O/Al2O3, as expected. Although this study fails to clearly distinguish the strengths of basicity between Pt/ Al2O3 and Pt-CeO2/Al2O3, Tanabe (1981) reported that cerium oxide manifests a more marked basic character than Al2O3. Therefore, the order of the strength of basicity could be Pt-K2O/Al2O3 > Pt-Na2O/Al2O3 > Pt-CeO2/Al2O3 > Pt/Al2O3. Light-Off Temperature for Propylene and CO Oxidation at Stoichiometric Point and in the Absence of Water. Figure 4a illustrates the reaction curves of all catalysts for CO oxidation at the stoichiometric point (S ) 1). The light-off temperatures (T50 defined as the temperature enough to achieve a conversion of 50%) of these catalysts are listed in Table 3. Pt/ Al2O3 exhibits the lowest activity for CO oxidation among all catalysts studied herein due to the highest light-off temperature. Nevertheless, the light-off temperatures of other catalysts are extremely close. It

Figure 3. CO2-TPD patterns of catalysts.

Figure 4. CO and propylene oxidation at the stoichiometric point and in the absence of water: (a) CO; (b) propylene.

suggests that the promoted catalysts have similar activities for CO oxidation under the same reaction condition. Many researchers (Patterson and Kemball, 1963; Voltz et al., 1973; Yu Yao, 1984) reported that the

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kinetics of CO and C3H6 oxidation on Pt/Al2O3 catalyst may follow a Langmuir-Hinshelwood mechanism. Reaction occurs between adsorbed oxygen and adsorbed reactant on the catalyst surface. The reaction rate increases with increasing oxygen concentration and is inhibited by CO and C3H6. 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 strong adsorption on the catalyst surface. Therefore, the reaction rate of Pt/Al2O3 for CO oxidation at a low temperature is suppressed, whereas it can be improved by removing CO and C3H6 from the surface sites by O2 at a higher reaction temperature. This results in the fact that Pt/Al2O3 has a higher lightoff temperature for CO oxidation. Additionally, the Pt/ Al2O3 activity for CO oxidation can be enhanced by adding CeO2. Due to the addition of CeO2, the kinetic order of reaction with respect to CO can be switched from negative to positive and activation energy can also be reduced (Yu Yao, 1984). This results in the fact that the activity of Pt-CeO2/Al2O3 for CO oxidation is greater than that of Pt/Al2O3. The experimental result also reveals that the light-off temperature of Pt-CeO2/ Al2O3 is lower than that of Pt/Al2O3 by about 90 °C. On the other hand, the Pt/Al2O3 activity for CO oxidation can also be improved by addition of Na2O or K2O. According to Malinowski’s report (1985), depositing alkali metal on 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 addition of Na2O or K2O. Increasing the 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 activity of Pt-Na2O/Al2O3 or Pt-K2O/ Al2O3 for CO oxidation is significantly improved by effectively suppressing the self-inhibition of CO and C3H6. Figure 4b presents the reaction curves of these catalysts for C3H6 oxidation at the stoichiometric point. The light-off temperatures of these catalysts are listed in Table 3. Pt/Al2O3 exhibits the lowest activity for C3H6 oxidation among these catalysts because of the highest light-off temperature. This result implies that the activity of Pt/Al2O3 for C3H6 oxidation is still inhibited due to the strong adsorption of CO and C3H6 on the catalyst surface. Pt-CeO2/Al2O3 has a higher activity than Pt/Al2O3, because the addition of CeO2 effectively suppresses the adsorption strength of CO and C3H6 (Yu Yao, 1984). Additionally, the addition of Na2O or K2O on Pt/Al2O3 can also improve the reaction rate of C3H6 oxidation due to the electronic effect, which is the same as the effect of Na2O or K2O for CO oxidation. Moreover, the enhanced effect of Na2O or K2O on C3H6 oxidation is greater than that of CeO2. The results are in accordance with those in the literature (Kinoshita et al., 1983). On the basis of the above discussions, one can conclude that the addition of alkali-metal oxide can enhance the Pt/Al2O3 activities for CO and C3H6 oxidation by suppressing the self-inhibition of reactants at

Figure 5. CO and propylene oxidation at S ) 0.31 and in the absence of water: (a) CO; (b) propylene.

the stoichiometric point. In addition, the promotional effect of Na2O or K2O is superior to that of CeO2. S ) 0.31 and in the Absence of H2O. Parts a and b of Figure 5 depict the reaction curves of these catalysts for CO and C3H6 oxidation under an oxygen-deficient condition (S ) 0.31) and in the absence water. Since CO conversion of Pt/Al2O3 at 300 °C is only half those of other catalysts, one can expect that Pt/Al2O3 activity is still inhibited by strong adsorption of CO and C3H6. By the analyzed result of oxygen concentration beyond the catalyst, it revealed that oxygen was not completely consumed upon reacting with reactants on Pt/Al2O3 until 500 °C. For Pt/Al2O3, oxygen nearly only reacts with CO up to 350 °C. Therefore, CO conversion increases with 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 that increases insignificantly during reaction temperature, increasing from 350 to 500 °C. Above the reaction temperature (500 °C) that oxygen is entirely reacted, CO conversion on Pt/Al2O3 decreases with increasing reaction temperature. The highest CO conversion on Pt/Al2O3, indicated in Table 4, is less than 40%. 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 oxygen under an oxygen-deficient condition. When the C3H6 conversion increases, the CO conversion should be suppressed once oxygen is completely reacted. The experimental results reveal that Pt/Al2O3 exhibits the highest C3H6 conver-

Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1503 Table 4. Highest CO Conversions of the Catalysts at Various Conditions highest CO conversion

catalyst

S ) 0.31 without H2O

S ) 0.31 with H2 O

S ) 0.17 without H2O

S ) 0.17 with H2O

Pt/Al2O3 Pt-CeO2/Al2O3 Pt-Na2O/Al2O3 Pt-K2O/Al2O3

37 83 78 88

46 88 88 94

15 33 35 37

22 46 63 70

sion and the lowest CO conversion among these catalysts. Nevertheless, the CO conversion on Pt/Al2O3 can be significantly improved by adding CeO2, Na2O, or K2O. Besides the highest CO conversion becoming double, the CO conversions of the promoted catalysts at 550 °C are higher than that of Pt/Al2O3 by at least 25%, as shown in Figure 5a. On the promoted catalysts, oxygen is entirely reacted and mainly reacts with CO at reaction temperature below 350 °C. Pt-K2O/Al2O3 exhibits the highest CO conversion among these catalysts. In contrast, the C3H6 conversions on the promoted catalysts are 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 the temperature 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. All catalysts show that the C3H6 conversion increases along with increasing the reaction temperature under an oxygen-deficient condition (S ) 0.31). Meanwhile, CO conversion shows the reverse trend when oxygen is completely reacted. Nevertheless, the promoted Pt/Al2O3 catalysts still exhibit higher CO conversion than Pt/Al2O3. S ) 0.17 and in the Absence of H2O. Parts a and b of Figure 6 displays the reaction curves of these catalysts for CO and C3H6 oxidation under a more oxygen-deficient condition (S ) 0.17) and in the absence water. The effects of the additives on Pt/Al2O3 under S ) 0.17 without water show the same trend as that under S ) 0.31 without water. However, the enhanced effects of additives on Pt/Al2O3 activity are more significant under the severely oxygen-deficient condition (S ) 0.17). The onset temperature of Pt/Al2O3 increases by about 100 °C when the reaction condition switches from S ) 0.31 to S ) 017, while those of the modified catalysts remain unaffected. This result suggests that the Pt/ Al2O3 activity is inhibited more severely at a higher (CO + C3H6)/O2 ratio. The highest CO conversion on Pt/ Al2O3 (14%), shown in Table 4, is less than half those of other catalysts, and its CO conversion at 550 °C is about a third of those of other catalysts. Similarly, Pt/ Al2O3 still exhibits the highest C3H6 conversion among these catalysts. The above results confirm that under oxygen-deficient conditions a higher temperature is not beneficial to CO conversion once oxygen can completely be reacted. Nevertheless, the CO conversion can be improved by addition of basic additives. The promotional effect of K2O is superior to that of CeO2 or Na2O. S ) 0.31 and in the Presence of H2O. In order to simulate the emissions of two-stroke motorcycles, water

Figure 6. CO and propylene oxidation at S ) 0.17 and in the absence of water: (a) CO; (b) propylene.

was added into the reaction gases. Parts a and b of Figure 7 depict the reaction curves of CO and C3H6 oxidation on these catalysts under S ) 0.31 and in the presence of water which is equivalent to the exhaust emissions of two-stroke motorcycles at a speed of 50 km/ h. Additionally, the temperature of a catalytic converter is above 550 °C under this driving condition. Figure 8 summarizes the effects of water on CO and C3H6 conversions at 550 °C. The CO and C3H6 conversions on these catalysts can be enhanced by the existence of water that can induce the WGS and steam reforming reactions. 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 effect on improving CO conversion by the WGS reaction is more significant at a higher temperature (above 400 °C). The enhanced effect on CO conversion of Pt/Al2O3 by the WGS reaction is obviously inferior to that on Pt-CeO2/Al2O3. This result agrees with the fact that CeO2 is recognized as an excellent promoter for the WGS reaction (Kim, 1982; Diwell et al., 1991; Barbier and Duprez, 1993). The disparity of CO conversions between Pt-CeO2/Al2O3 and Pt/Al2O3 is markedly magnified. CO conversion on Pt/Al2O3 at 550 °C is 22%, that is, only a third of that on Pt-CeO2/ Al2O3 (65%). However, the enhanced effect of CeO2 is inferior to that of Na2O or K2O, as shown in Figure 8. The CO conversion on Pt-CeO2/Al2O3 at 550 °C increased by the WGS reaction is about 14%, that is, less than the 30% increase of Pt-Na2O/Al2O3 or Pt-K2O/

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Figure 7. CO and propylene oxidation at S ) 0.31 and in the presence of water: (a) CO; (b) propylene.

Figure 8. Effects of water on CO and C3H6 conversion at 550 °C under S ) 0.31 condition: (0) without water; (9) with water.

Al2O3. This result reveals that Na2O and K2O are superior to CeO2 in terms of functioning as a promoter for the WGS reaction. Moreover, the difference of CO conversions at 550 °C between Pt-CeO2/Al2O3 (65%) and Pt-K2O/Al2O3 (81%) is significantly magnified. 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 acid 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 Na2O or 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 addition of Na2O or K2O on Pt/Al2O3 can enhance the WGS reaction. The blank experiments with basic supports, CeO2/ Al2O3, Na2O/Al2O3, and K2O/Al2O3, were also conducted under the same reaction conditions but without oxygen. The experimental results reveal that CO conversion on the basic supports by the WGS reaction is not detectable. Furthermore, CO conversion on Pt/Al2O3 by the WGS reaction is also extremely low (below 5% at 550 °C) under the same conditions. These results verify that basic support or Pt alone cannot significantly enhance the WGS reaction. The high activity of the promoted catalysts for the WGS reaction can be contributed to the synergistic effect of basic support and active metal (Pt). On the other hand, C3H6 can also be eliminated by the steam re-forming reaction when oxygen is exhausted. Besides Pt/Al2O3 exhibiting the highest C3H6 conversion among these catalysts, the enhanced effect on C3H6 conversion of Pt/Al2O3 by steam re-forming reaction, indicated in Figure 8, is also the greatest among these catalysts. However, the enhanced effects on other catalysts are insignificant. This finding is in accordance with those in the literature (Barbier and Duprez, 1993; Rostrup-Nielson, 1973). Barbier and Duprez (1993) reported that the Pt-CeO2/Al2O3 activity for the steam re-forming reaction is less than that of Pt/Al2O3. Rostrup-Nielson (1973) reported that the activity of Ni catalyst for the steam re-forming reaction is inhibited by the addition of alkali-metal oxide. In addition, the blank experiments with basic supports, CeO2/Al2O3, Na2O/Al2O3, and K2O/Al2O3, are also conducted under the same reaction conditions but without oxygen. The C3H6 conversions on all supports by the steam re-forming reaction are not detectable. S ) 0.17 and in the Presence of Water. This reaction condition simulates the exhaust emissions of a two-stroke motorcycle at full-throttle acceleration that represents a more oxygen-deficient and higher temperature (above 550 °C) condition. Parts a and b of Figure 9 display the reaction curves of CO and C3H6 oxidation on catalysts at S ) 0.17 and in the presence of water. The CO conversion is heavily suppressed under this condition. The highest CO conversion on Pt/Al2O3 (20%) is less than half of that on Pt-CeO2/Al2O3 (43%), as shown in Table 4. When Na2O or K2O is added onto Pt/Al2O3, the highest CO conversions become 63 and 70%, respectively, which are higher than that of Pt-CeO2/Al2O3. Figure 10 illustrates the enhanced effects on CO and C3H6 conversions at 550 °C by the existence of water. The effects are similar to those at S ) 0.31 with water. The differences in CO conversions between Pt-CeO2/ Al2O3 and Pt/Al2O3 containing Na2O or K2O are more magnified under a more oxygen-deficient condition (S ) 0.17). This finding infers that alkali-metal oxides are

Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1505

Figure 9. CO and propylene oxidation at S ) 0.17 and in the presence of water: (a) CO; (b) propylene.

Why did one concentrate on investigating the CO and C3H6 conversions on all catalysts under oxygen-deficient conditions with or without water? In addition to reacting with oxygen, CO and C3H6 can also be eliminated by the WGS and steam re-forming reactions. Nevertheless, the WGS and steam re-forming reactions can only occur in the absence of oxygen, i.e., completely consumed. At lower speed conditions, the amount of oxygen in the exhaust gases of a two-stroke motorcycle is plentiful. Therefore, the CO conversion at this condition is dependent on the selective properties of the catalyst for CO oxidation. The vitals of this work are how to improve the CO conversion on Pt/Al2O3 catalysts under the oxygendeficient conditions. Controlling CO conversion is a critical factor in designing a catalytic converter for a two-stroke motorcycle. The experimental results are summarized in Figures 8 and 10 and Table 4. Pt/Al2O3 exhibits the highest C3H6 conversion and the lowest CO conversion among these catalysts under any oxygendeficient condition. In contrast, Pt-K2O/Al2O3 exhibits the highest CO conversion and the lowest C3H6 conversion under the same condition. The CO conversion on Pt-K2O/Al2O is higher than that on Pt-CeO2/Al2O3 under any oxygen-deficient condition, particularly under the circumstance of severe oxygen deficiency and with the existence of water. The results demonstrate that the effect of K2O on improving CO conversion is greater than that of CeO2. Moreover, the promotional effects of additives on CO conversion of Pt/Al2O3 by the WGS reaction is in the order K2O > Na2O > CeO2, which is the same as the order of basicity of catalyst. In addition, C3H6 conversion on Pt-Na2O/Al2O3 and Pt-K2O/Al2O3 resembles that on Pt-CeO2/Al2O3. Therefore, K2O can be a promising additive to a catalytic converter for a two-stroke motorcycle since it can significantly enhance CO conversion. Conclusions

Figure 10. Effects of water on CO and C3H6 conversion at 550 °C under S ) 0.17 condition: (0) without water; (9) with water.

superior to CeO2 in terms of functioning as a promoter for enhancing the WGS reaction. With respect to C3H6 oxidation, the result also resembles that at S ) 0.31 with water. Besides Pt/Al2O3 exhibiting the highest C3H6 conversion among these catalysts, the enhanced effect on C3H6 conversion on Pt/Al2O3 by the steam reforming reaction is the greatest among these catalysts. Nevertheless, the enhanced effects on other catalysts are insignificant. The above results clearly demonstrate that basic additives on Pt/Al2O3 can effectively enhance CO conversion due to promoting the WGS reaction. On the contrary, basic additives have the limited effect of promoting the steam re-forming reaction.

1. The addition of promoter on Pt/Al2O3 catalyst slightly decreases the surface area of catalyst and does not markedly change Pt dispersion. 2. The addition of promoter can influence the reducibility of platinum oxide on Al2O3 support. 3. The CO2-TPD results reveal that the basicity of the catalyst is in the order Pt-K2O/Al2O3 > Pt-Na2O/ Al2O3 > Pt-CeO2/Al2O3 > Pt/Al2O3. 4. The addition of Na2O, K2O, or CeO2 can enhance the Pt/Al2O3 activities for CO and C3H6 oxidation at the stoichiometric point. 5. Under an oxygen-deficient condition and in the absence of water, C3H6 conversions on all catalysts studied herein always increase with increasing reaction temperature. Nevertheless, CO shows the reverse trend once oxygen is completely reacted. Pt/Al2O3 exhibits the highest C3H6 conversion and the lowest CO conversion among these catalysts. The CO conversion on Pt/Al2O3 can be promoted by addition of CeO2, Na2O, or K2O on Pt/Al2O3. 6. 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 is the most active catalyst for the steam reforming reaction among these catalysts, while it is the lowest active catalyst for the WGS reaction. Addition of the basic promoters on Pt/Al2O3 catalysts can significantly enhance the WGS reaction. The promotional

1506 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997

effect is in the order of K2O > Na2O > CeO2, which is the same order as that of basicity of catalyst. 7. K2O could be a promising additive to a catalytic converter for a two-stroke motorcycle since it can significantly enhance CO conversion. Acknowledgment The authors thank the National Science Council of the Republic of China for financially supporting this work under Contract No. NSC-85-2214-E008-012. 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. Amenomiya, Y.; Pleizier, G. Alkali-Promoted Alumina Catalysts: II. Water-Gas Shift Reaction. J. Catal. 1982, 76, 345. 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. 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. Plat. 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. Kang, Y. M.; Wan, B. Z. Effects of acid or base Additives on Catalytic Combustion Activity of Chromium and Cobalt Oxides. Appl. Catal. A 1994, 114, 35. 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. 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. 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, 1985, 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. Rostrup-Nielsen, J. R. Activity of Nickel Catalyst for Steam Reforming of Hydrocarbons. J. Catal. 1973, 31, 173. Su, E. C.; Montreuil, C. N.; Rothschild, W. G. Oxygen Storage Capacity of Monolith Three-way Catalysts. Appl. Catal. 1985, 17, 75. Tanabe, K. Solid Acid and Base Catalysts. In CatalysissScience and Technology; Anderson, J. R., Boudart, M., Eds.; SpringerVerlag: Berlin, 1981; Vol. 2. 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. Yao, H. C.; Gandhi, H. S.; Shelef, M. Interactions of Base and Noble Metals with Insulator Supports. In Metal-Support and Metal-Additive Effects in Catalysis; Imelik, B., Naccache, C., Coudurier, G., Praliaud, H., Meriaudeau, P., Gallezot, P., Martin, G. A., Vedrine, J. C., Eds.; Elsevier Science Publishers B. V.: Amsterdam, The Netherlands, 1982; Vol. 11, p 159. Yao, H. C.; Yu Yao, Y. F. Ceria in Automotive Exhaust Catalysts: I. Oxygen Storage. J. Catal. 1984, 86, 254. Yu Yao, Y. F. The Oxidation of CO and Hydrocarbons over Noble Metal Catalysts. J. Catal. 1984, 87, 152.

Received for review July 18, 1996 Revised manuscript received January 8, 1997 Accepted January 13, 1997X IE960414U

X Abstract published in Advance ACS Abstracts, February 15, 1997.