Ind. Eng. Chem. Res. 1995,34, 1952-1959
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Effect of Copper Content on Pt-Pd-CuOly-Alumina Catalysts for Motorcycle Soot Conversion Chin-Cheng Chien and Ta-Jen Huang* Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, R.O.C.
Catalytic combustion of motorcycle soot particulates over y-alumina-supported CuO, Pt, Pd, PtCuO, and Pd-CuO catalysts was studied. The catalyst coated with motorcycle soots was placed in a flow reactor to perform temperature-programmed oxidation. Results indicated that the CuO catalyst was quite effective for the catalytic combustion. The high activity of the CuO catalyst could be illustrated by a redox mechanism and an induced particle-motion mechanism. A higher copper content enhanced the reducibility of the copper oxide and induced a higher activity for catalytic combustion until the copper oxide content reached 5 wt %. A redispersion phenomenon of the CuO species was observed and was consistent with the induced particlemotion mechanism. Additionally, the effect of the noble metal additive was to promote the activity of the CuO species by a mechanism including dissociative adsorption and spillover of oxygen.
Introduction The emissions from motorcycles have serious impacts on urban air quality and human health (Hare et al., 1974; Walsh et al., 1990; Chen et al., 1992; Maji et al., 1992). Of these emissions, soot particulates have been a major pollution problem, especially those from twostroke motorcycles. The soot particulates from two-stroke motorcycles are produced primarily from motor oil mixed in gasoline fuel for engine lubrication (Hare et al., 1974). Soot particulates contain large amounts of mutagenic hydrocarbons and have a negative impact upon human health (Chiron, 1987; Gotze et al., 1991). A method of removing these particulates lies in employing a soot oxidation catalytic converter which can simultaneously oxidize particulates and gaseous pollutants. For catalytic combustion of soot particulates, the catalysts should be able t o oxidize particulates at temperatures below 300 "C since the temperature of the motorcycle exhaust is typically below 300 "C (Chen et al., 1992). The development of highly active catalysts for the combustion of motorcycle soot is therefore necessary. A number of transition metal oxides have been studied as catalysts for the oxidation of graphite by Mckee (1970a). The thermogravimetric results suggested that the catalytic effect involves the localized reduction of cupric oxide by graphite and subsequent reoxidation of the resulting metal (Mckee, 1970b). These results indicated that only those oxides which could be reduced by graphite to a lower oxide or metallic state appear to function as active catalysts (Amariglio and Duval, 1966; Mckee, 1970a,b). Therefore, CuO probably undergoes an oxygen transfer cycle at the point of contact between the catalyst and the graphite. They also found that CuO catalyst was highly mobile since the reaction temperature was well above the Tammann temperature. Note that the temperature at which lattices begin t o be appreciably mobile is termed the Tammann temperature, and that at which surface atoms become significantly mobile is termed the Huttig temperature (Satterfield, 1991). On the other hand, the catalytic channeling action could provide an alternative route for the migration of
* To whom correspondence
should be addressed.
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a small fraction of particles at temperatures much lower than the Tammann temperature (Baker and Shenvood, 1980). This driving force for its motion was considered t o be due to the unbalanced adhesion force acting upon the particle (only the front portion of the particle is in contact with the edge atoms of graphite) (Goethel and Yang, 1986). Additionally, CuO was demonstrated by Ahlstrom and Odenbrand (1990a) t o have a high activity in the combustion of hydrocarbons that desorb from the diesel soot upon heating. Watabe et al. (1983) studied the catalytic combustion of diesel soot by using a ceramic foam in combination with catalysts containing copper salt and found that the soot particulate was rapidly oxidized by mobile copper ions, showing worm-eatenlike spots, from TEM photograph. On the other hand, the CuO catalyst promoted with 0.055 wt % Pt was reported by Huang et al. (1987) to have high activity in the oxidation of carbon monoxide, even under the influence of sulfur dioxide up to 720 ppm. Carbon monoxide is an intermediate which occurs during the oxidation of the soot particulate and sulfur dioxide normally present in the motorcycle exhaust. Thus, the Pt-CuO catalysts apparently have promising potential for the combustion of the motorcycle soot. y-Alumina-supported CuO, Pt, Pd, Pt-CuO, and PdCuO catalysts were employed in this study. The Pd species was employed since it is usually employed as an active agent for automobile emission control (Hightower, 1976; Kummer, 1980). Additionally, Pd was reported by Yu-Yao (1975) to have substantially higher activity than Pt for the oxidation of carbon monoxide and ethylene. For the oxidation of various hydrocarbons in automobile exhaust, the Pt and Pd species are commonly employed simultaneously. This work was carried out to investigate the catalytic oxidation of the soot particulates from a two-stroke motorcycle. Since there is an almost complete lack of fundamental knowledge of the catalytic combustion of the motorcycle soot, the purpose of this study was to gain some knowledge of this aspect.
Experimental Section Catalyst Preparation. y-Alumina particles with a size of 40-80 mesh were used as the catalyst support, which were obtained from Strem Chemicals Inc. U.S.A. 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 6, 1995 1963 The surface area was 100 mzlg before impregnation. The CuOly-alumina catalysts were prepared by the wet impregnation (Satterfield, 1991) of y-alumina with cupric nitrate solution. Following drying a t 120 "C for 12 h, the catalyst was calcined in air at 500 "C. The Wy-alumina and Pdly-alumina catalysts were prepared by the dry impregnation (Satterfield, 1991) of the y-alumina support with solutions of chloroplatinic acid and palladium chloride, respectively. Following drying, the catalysts were calcined a t 550 "C first in a mixture of 50%hydrogen in nitrogen at a flow rate of 1 Umin for 4 h and then in flowing air. The Pt-CuOlyalumina and Pd-CuOly-alumina catalysts were prepared in a similar manner except that the CuOlyalumina catalyst was substituted in place of the support. The reducing atmosphere, of 50%hydrogen in nitrogen, was employed in the calcination process to eliminate the remaining C1 species and avoid the formation of CuC1,. The platinum and palladium contents were 0.1 wt %, while the copper oxide content was varied between 2.0 and 16.7 wt %. Catalyst Characterization. Temperature-programmed reduction (TPR) was performed with 10% hydrogen in argon over 0.1 g of catalyst in a conventional TPR setup (Jones and Mcnicol, 1986). The flow rate of this reducing gas was controlled a t 30 mumin. The temperature of the reactor was increased linearly a t a rate of 10 "Clmin from 25 t o 500 "C. This reactor was made of a quartz tube. The rate of hydrogen consumption was detected by a thermal conductivity detector (TCD) and then recorded by an on-line personal computer. Collection of Motorcycle Soot. A sample of 4 g of catalysts was coated with the motorcycle soot by being placed in the outlet of an exhaust pipe for 1h. In order to obtain a normal condition of motorcycle exhaust, the motorcycle was fixed upon a setup with a dynamometer and the load of the motorcycle engine was adjusted to be about 60 kg-W, i.e., one person's weight. The soot was collected after the motorcycle engine ran for 1 h and the engine speed was fixed at 2000 rpm. Since this coating operation may also result in adsorption of carbon dioxide, water, and hydrocarbon, the coated catalysts were dried at 120 "C for 12 h before the activity measurement. Preliminary investigations show that if the initial ratio of catalyst to soot is too low, it is difficult to distinguish the catalytic combustion from the noncatalytic combustion since soot also combusts in the absence of catalyst. Conversely, if the ratio of catalyst to soot is too high, it will be difficult t o detect the gaseous products. A 1h coating operation seems to give a proper catalyst-to-soot ratio. Characterizationof Motorcycle Soot. Motorcycle soot coated over y-alumina was analyzed by an elemental analyzer (Heraeous CHN-0-Rapid). Samples of about 40 mg were used in powder form after drying at 120 "C for 12 h. The desorption of the hydrocarbons of the two-stroke motorcycle soot was determined by a gas chromatograph (GC) equipped with a flame ionization detector (FID). A 0.2 g sample was placed in a flow reactor. The desorption of hydrocarbons was carried out between 100 and 700 "C by temperature-programmed desorption (TPD)with a heating rate of 10 "Clminin pure nitrogen or 2% oxygen in nitrogen at a flow rate of 150 mumin. Activity Measurements. A sample of 0.2 g of the coated catalysts was placed in a flow reactor. The
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T e m p era tur e ("C) Figure 1. TPD curves of two-stroke motorcycle sootly-alumina in nitrogen and 2% oxygen. ( 0 )2% oxygen; ( 0 )nitrogen.
reactor was made of a quartz tube with an inside diameter of 1.0 cm. The catalyst bed was heated according to the designated temperature program, and a gas mixture containing 2% oxygen and balance nitrogen was led through it at a flow rate of 100 m U min. The oxidation rate was determined at between 100 and 700 "C by temperature-programmed oxidation (TPO) with a heating rate of 10 "Clmin. Activity results for the soot conversion presented in this work were the amount of the produced carbon dioxide. The outflow of carbon monoxide was under consideration, but it was found to be negligible. The amount of the produced carbon dioxide was determined by a gas chromatograph equipped with a thermal conductivity detector after being separated by a Porapak Q column. The amount of carbon monoxide was monitored by a NDIR carbon monoxide analyzer (Beckman Model 880). The time resolution of the COz determination by the GC was 3 min. By means of repeating the experiment three times for each sample and picking data at different initial time, the data point of T,, was selected according to the most symmetric curve. Consequently, the time resolution ought t o be equal t o 1 min. The total amount of produced carbon dioxide during the TPO process was calculated by integrating the TPO curve. Results indicated that these integrated areas deviated from the average value within an error range of 20%.
Results and Discussion Characteristics of Two-StrokeMotorcycle Soot. Since the combustion process of the two-stroke engine is highly complex, the organic components of the motorcycle soot are as yet poorly defined. In this work, by elemental analysis, it is found that a typical two-stroke motorcycle sootly-alumina contains 15.0 wt % C, 2.3 wt % H,5.0 wt % 0, and a few trace elements, with y-alumina as the balance. The C/H atomic ratio of the soot is thus equal to 0.54. This indicates that the motorcycle soot contains large amounts of hydrocarbons and a significant portion of oxygenated compounds as well as small amount of carbon particles. Typical TPD curves of the motorcycle soot are shown in Figure 1. The magnitude of the FID response corresponds to the amount of the desorbed hydrocar-
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T e m p e r atur e ("C) Figure 2. TPO curves. (0) 8.2 wt 0.1 wt % Pt; (v)y-alumina.
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bons. It is noted that the difference of the amount of the desorbed hydrocarbons between TPD curves in 2% oxygen and in nitrogen exists at temperature above 350 "C. Furthermore, in oxygen-free atmosphere, a second peak of the FID response exists at 630 "C. This may imply that the motorcycle soot has a very complex composition. In 2% oxygen atmosphere, there is no second peak of the FID response. This indicates that a noncatalytic combustion may occur at over 350 "C. This is similar to the noncatalytic combustion of diesel soot occurring at over 400 "C (Ahlstrom and Odenbrand, 1989). Figure 2 demonstrated that y-alumina seems to have some activity for the combustion of the motorcycle soot. According to the report by Doorn et al. (1992) y-alumina was considered as having little or no catalytic effect for the combustion of diesel soot and the temperature of homogeneous combustion of the diesel soot was over 427 "C. Therefore, this activity ought to result from the activity of y-alumina and the noncatalytic combustion between the gas-phase oxygen zpd the desorbed hydrocarbons or between the carbon- and the oxygen-containing compounds of the motorcycle soot. Catalytic Combustion of Two-StrokeMotorcycle Soot. Temperature-programmedoxidation (TPO)of the motorcycle soot was carried out over a series of y-alumina-supported CuO, Pt, and Pd catalysts. A typical TPO result is shown in Figure 2. According t o this figure, the production of carbon dioxide follows a general scheme. As the temperature is increased to 160 "C, the oxidation of soot starts and the oxidation rate accelerates t o a maximum at a temperature, called maximum conversion temperature and denoted as T,,,, and suddenly decays as the unconverted soot cannot sustain the oxidation rate. This is consistent with that of the commonly accepted TPR test for catalyst characterization. In other words, T,, may represent the characteristic of the catalyst for its activity. In general, hydrocarbons are burned more easily than solid carbon. Consequently, one strategy for motorcycle soot oxidation is to ignite hydrocarbons with catalysts at lower temperature and subsequently to oxidize the particle carbon by the reaction heat. As shown in Figure 1,there is a large amount of desorbed hydrocarbons below 350 "C. This indicates that an effective catalyst should be active at temperatures below 350 "C.
Furthermore, the copper oxide catalyst has a lower T,, than those of the noble metal catalysts as also shown in Figure 2. Therefore, the copper oxide catalyst is considered t o be quite effective for the combustion of the two-stroke motorcycle soot. Two possible mechanisms could be suggested to explain this phenomenon: One mechanism is based on the redox properties of the transition metals as proposed by Mckee (1970a,b). Copper oxide is reduced by soot and reoxidized by gaseous oxygen. It was proposed that those oxides which could be reduced by graphite to a lower oxide or metallic state appear to function as active catalysts (Amariglio and Duval, 1966). Recently, Ciambell et al. (1993) also suggested that a redox mechanism is associated with the strong activity of the supported Cu/ V/K catalyst for carbon particle oxidation. The redox mechanism will be illustrated in detail and confirmed in a later section. The other mechanism is that the copper oxide exhibits the possibility of particle mobility and this may lead to a higher contact efficiency between the catalyst and the soot (Baker et al., 1981; Ahlstrom and Odenbrand, 1990a,b). Recently, Chu et al. (1993) found that for the CIOzN205 reaction catalytic activity of carbon gasification involves the motion of the catalyst particles. This has significance in two aspects: first, the catalyst motion itself is a strong reaction parameter and, second, the exposed channels act as the new active sites for noncatalytic reactions or for catalytic reaction if the catalyst particles are spread over the exposed surfaces. Baker and Shenvood (1980) reported that particle mobility on graphite can occur below the Tammann temperature as the catalytic channeling action induces motion into active particles. In this work, Tmaxfor soot oxidation over copper oxide catalyst is located between the Tammann temperature and the Huttig temperature of copper oxide. This may thus imply that the activity might be attributed to the particle mobility. The particle-motion mechanism will also be discussed and confirmed by the observation of CuO redispersion in a later section. Effect of Copper Content on Reducibility. The TPR test of CuO catalyst was employed to examine the catalyst characteristics for clarifying the relation between the activity and the catalyst characteristics. In Figure 3, typical TPR profiles of copper oxide catalysts are demonstrated. Two peaks are observed. This indicates that two types of copper oxide species are present. The first peak could correspond to the highly dispersive copper oxide, while the second peak might be attributed to the bulk copper oxide (Shimokawabe et al., 1983; Strohmeier, et al., 1985; Dumas et al., 1989). For the highly dispersive and isolated copper oxide, the crystal field is weaker owing to longer and weaker Cu2+-02- bonds, as revealed by the Cu2+ion adsorption band (Garbowski and Primet, 1991). Therefore, this type of copper oxide will be more easily reduced than bulk copper oxide, which has a stronger crystal field owing to shorter and stronger Cu2+-02- bonds (Strohmeier et al., 1985). As also shown in Figure 3, the reduction temperature of the highly dispersive copper oxide varies with the copper content. This suggests that the copper loading affects the reducibility significantly (Strohmeier et al., 1985; Solcova and Jiratova, 1993). This has been explained by the role of surface hydroxyl groups of alumina that are able to act as anchoring sites for some
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Temperature ( O C ) Figure 3. TPR curves of CuO. (a) 2.0 wt %; (b) 2.4 wt %; ( c ) 5.0 wt 70;(d) 8.2 wt % (e) 16.7 wt %.
cations (Garbowski and Primet, 1991; Chen and Zhang, 1992). During the calcination process the water removal leads t o isolated Cu2+ions in strong interaction with the support. Because of the density of the surface hydroxyl groups, the number of such isolated copper ions is limited. As soon as the number of introduced copper species exceeds the exchange capacity of the support, copper ions sinter into particles of bulk copper oxide (Dumas et al., 1989). According to the observation by Friedman et al. (1978), the saturation of alumina with copper is at a concentration of about 4 wt % for 100 m2/g of alumina BET surface area. In this work, the maximum percentage of the first TPR peak of copper oxide occurs at the 5 wt % CuO content (i.e., ca. 4 wt % Cu content), in agreement with the above observation. On the other hand, the interaction between support and metal oxide will also have a strong impact on the reducibility of the isolated copper oxide (Friedman et al., 1978; Chen and Zhang, 1992; Solcova and Jiratova, 19931, especially when the oxide loading is low (Yuen et al., 1982; Rethwisch and Dumesic, 1986). If the copper oxide content is below 5 wt %, i.e., under the limit of the ratio of the hydroxyl groups of alumina and copper ions, this interaction will be shared and weaken as the copper oxide loading increases. A higher copper content thus leads to a lower reduction temperature. However, if the copper oxide content is over 5 wt %, the interaction will remain the same owing t o the saturated surface CuO and consequently gives the results of Figure 4. Effect of Copper Content on Catalytic Combustion. The variation of the maximum conversion temperatures with the copper oxide content is demonstrated in Figure 5 . A higher copper oxide content leads to a lower T,, until the oxide content over 5 wt % has no effect on this temperature. This implies that the activity seems t o be associated with the catalyst characteristic (Ciambell et al., 1993). The tendency of TPO is similar t o that of TPR, as shown in Figure 4, and suggests that the catalytic activity of the copper oxide catalyst could be explained by a redox mechanism (Amariglio and Duval, 1966; Mckee, 1970a,b); i.e., copper content impacts upon the ability of catalyst to
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yield oxygen for soot oxidation and to recover oxygen from the gas phase for its reoxidation. The oxygen transfer cycle is important for the catalytic combustion. Holstein and Boudart (1983)proposed that the oxygen transfer cycle appears valid for the C-02 reaction due to the availability of the weakly bound oxygen in the bulk metal oxide or on the surface of a metal oxide. The other explanation for the oxygen transfer cycle is the redox mechanism by means of the localized reduction of cupric oxide by graphite and subsequent reoxidation of the resulting metal (Amariglio and Duval, 1966; Mckee, 1970a,b). Data shown in Figures 4 and 5 support the latter explanation. Moreover, the redox mechanism is also supported by Ciambell et al. (1993) for the observation of the strong activity of the supported C u N K catalyst for carbon particle oxidation. It is thus believed that the redox mechanism is related to the activity of the copper oxide catalysts. Redispersion of Copper Oxide. Figure 6 demonstrates the TPR profiles of the catalysts after TPO reaction, Compared to Figure 3, the first reduction peak of each used catalyst in Figure 6 is larger than that of the fresh catalysts in Figure 3 while the second reduction peak is smaller. This phenomenon is clearly seen from the comparison of Figures 3e and 6d. This suggests that the structure of these catalysts was varied
1956 Ind. Eng. Chem. Res., Vol. 34, No. 6, 1995
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Figure 8. Effect of CuO content on the ageufresh ratio of the first TPR peak.
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Figure 6. TPR curves of CuO after TPO reaction. (a) 2.4 wt %; (b) 5.0 wt %; (c) 8.2 wt %; (d) 16.7 wt %. 100
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by the TPO process. Since TPO was carried out upto a temperature of 700 "C while the fresh catalyst was calcined only at 500 "C,the TPO process may correspond to a process of aging. Figure 7 was obtained by calculating the area percentage of the first peak in the total TPR area of each catalyst from Figures 3 and 6. It is apparently seen that the first peak grew up after the TPO process. Since the first peak corresponds to the highly dispersive copper oxide as discussed in an above section, the catalysts might have undergone redispersion in the TPO process. In Figure 8, which is obtained from Figure 7, it is demonstrated that the ageafresh ratio of the first TPR peak's area varies with the copper oxide content, where the ratio is that of the first peak's area of the aged catalyst to that of the fresh catalysts. The minimum variation occurs at 5 wt % CuO. Note that for the fresh catalysts, the maximum percentage of the first peak also occurs at 5 wt % CuO. In other words, the 5 wt % CuO catalyst contains the minimum fraction of the bulk CuO species. Hence, the redispersion of the bulk CuO may
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Figure 9. Mechanism of induced particle motion for the catalytic combustion of motorcycle soot over CuO catalyst.
be minimum which gives the minimum variation in the ageafresh ratio. The above results reveal that the bulk copper oxide cracked t o become highly dispersive copper oxide during the TPO reaction. Therefore, it is verified that the phenomenon of redispersion of the copper oxide species appears in the catalytic combustion process. An induced particle-motion mechanism for the catalytic combustion of motorcycle soot over copper oxide catalysts was proposed to illustrate the redispersion phenomenon, as shown in Figure 9. The contact between the soot and the catalyst should be important for the catalytic combustion. The initial reaction thus occurs primarily on the interface between the catalyst and the soot particle, as shown in Figure 9a. Then the copper oxide particle will change shape owing to the attraction by the diminishing soot particle at the moderate temperature (between their Huttig temperature and Tammann temperature), as illustrated in Figure 9b. Indeed, Watabe et al. (1983)have reported that the diesel soot particulate was rapidly oxidized by
Ind. Eng. Chem. Res., Vol. 34, No. 6, 1995 1967 0 2
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T e m p e r a t ur e (' C) Figure 10. TPR curves. (a) F't-CuO; (b) Pd-CuO; (c) CuO.
mobile copper ion, showing worm-eaten-like spots, from a TEM photograph. Moreover, Baker and Shenvood (1980) found that the catalytic channeling action provides an alternative route for the migration of a small fraction of particles a t temperatures much lower than the Tammann temperatures. This driving force for its motion was considered to be due t o the unbalanced adhesion force acting upon the particle (only the front portion of the particle is in contact with the edge atoms of graphite) (Goethel and Yang, 1986). Consequently, the attracting force could induce the motion of the copper oxide particles. This motion will result in the large copper oxide particles being divided into smaller particles, as shown in Figure 9c. The induced particle-motion mechanism is therefore confirmed. This also suggests that the high activity of the copper catalyst for the catalytic combustion of the motorcycle soot may be attributed to the induced particle motion of copper oxides. As to the activity of metal oxides, the mechanism of particle mobility has been proposed t o account for the catalytic combustion of graphite (Baker and Shenvood, 1980; Watabe et al., 1983; Chu et al., 1993) and diesel soot (Ahlstrom and Odenbrand, 1990a,b). Effect of Noble Metal Additive to CuO Catalyst. The TPR profiles of the CuO catalysts with noble metal additive are demonstrated in Figure 10. It is observed that the 0.1 wt % Pt or Pd additive can only enhance the reducibility of the bulk copper oxide species but not shift the first TPR peak toward a lower temperature. Therefore, the first TPR peak, as shown in Figure 10, is due to the highly dispersed copper oxide and some bulk copper oxide with enhanced reducibility by Pt or Pd. This enhanced reducibility of the bulk copper oxide species has been explained by a mechanism involving hydrogen adsorption and spillover (Gentry et al., 1981). According to the new aspects of spillover effect (Weng and Delmon, 1992; Delmon, 1993; Inui, 1993),an oxygen spillover mechanism is proposed for the catalytic combustion of the motorcycle soot over noble metal catalysts as demonstrated in Figure 11. Since the Tammann temperature of noble metal is much higher than that of CuO, the induced particle motion of the noble metal particle should be negligible. Therefore, the noble metal
Figure 11. Mechanism of oxygen spillover for the catalytic combustion of motorcycle soot over noble metal catalyst.
particle could only be considered as a dissociation center for oxygen providing atomic oxygen which subsequently diffuse to the soot particle where reaction occurs. Indeed, Parera et al. (1983) have shown that the coke deposited on A1203 can be only partially eliminated by molecular oxygen but can be completely eliminated if Pt/A1203 catalyst is present. The explanation was that WAl2O3 activates molecular oxygen into atomic oxygen species which spill onto the A1203 surface for burning the deposited coke more efficiently than molecular oxygen. Recently, Baumgarten and his co-workers (Baumgarten and Schuck, 1988; Baumgarten and Dedek, 1990) found that the oxidation of coke deposited on A1203 can be considerably accelerated by catalysts such as Pt/ 4 2 0 3 , although there was no direct contact between the deposited coke (which was mainly situated in the pores of alumina) and the catalysts. The explanation was that oxygen activated by PtdA1203 migrates into the pores of alumina to burn out the deposited coke. Hence, the oxygen spillover mechanism instead of the induced particle-motion mechanism is applicable for the catalytic combustion of the motorcycle soot over the Pt or Pd catalyst. In Figure 12, the TPO curves of the catalytic combustion of the two-stroke motorcycle soot over CuO, PtCuO, and Pd-CuO catalysts are shown. According to this figure, the addition of the Pt or Pd species onto the CuO catalyst has almost no effect on 2'". Because T ,, represents the characteristic of the catalyst for its activity as mentioned above, this implies that the activity is primarily attributed to the redox mechanism and the induced particle-motion mechanism of the CuO species. On the other hand, the addition of 0.1 wt % Pt or Pd does result in an enhanced oxidation rate as also shown in Figure 12. This promoting effect may be accounted for by the oxygen spillover mechanism as mentioned above and supported by Ahlstrom and Odenbrand (1990b). Oxygen dissociatively adsorbs on the donor
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T e m p era tur e (' C) Figure 12. TPO curves. ( 0 )CuO; (v)Pt-CuO; (v)Pd-CuO. CuO content 16.7 wt %.
phase (noble metal) and then surface diffuses to the acceptor phase (copper oxide) where hydrocarbon oxidation takes place. Nevertheless, the atomic oxygen diffusion to copper oxide is limited by the distance between the noble metal phase and the copper oxide phase. The occurrence of oxygen spillover is expected to come from the situation when the Pt or Pd species is closely associated with the CuO species (Huang and Chien, 1993).
Conclusion The motorcycle soot was found to be composed mainly of hydrocarbons with a C/H atomic ratio of 0.54. The CuO catalyst with over 5 wt % CuO content displayed superior activity. The redox mechanism and the induced particle-motion mechanism could be suggested for the high activity of the CuO catalysts. A higher copper content increases the reducibility of copper oxide and induces a higher activity for catalytic combustion until the copper oxide content reaches 5 wt %. The redox mechanism was thus supported. In addition, the redispersion phenomenon of the CuO species may confirm the induced particle-motion mechanism. It was also observed that the redispersion of the 5 wt % copper oxide catalyst is minimum and this is due to its minimum bulk CuO species. On other other hand, the oxygen spillover mechanism seems to be valid for the catalytic combustion of soot over the noble metal species. Additionally, the effect of the noble metal additive is to promote the activity of the copper oxide species by a mechanism including dissociative adsorption and spillover of oxygen.
Acknowledgment This work was supported by the National Science Council of the Republic of China under Contract No. NSC81-0402-E007-535.
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Received for review October 7, 1994 Revised manuscript received March 8 , 1995 Accepted March 21, 1995@ IE940583J Abstract published in Advance ACS Abstracts, May 1, 1995. @
