The invention relates to improvements in controlling the emission of particulates from diesel engines and provides methods and apparatus to that end.
Diesel engines are the most preferred for heavy-duty applications and light-duty trucks due to their high torque and superior fuel economy. By virtue of their fuel economy they also result in decreased CO2 emissions as compared to other engines. Unfortunately, diesel engines contribute significantly to urban and global air pollution through the emissions, particularly soot or carbon particulates (PM) and NOx. There is a recognized tradeoff between PM and NOx—when one is decreased, the other tends to increase.
Particulates (soot) can be collected on a filter, and active and passive filter regeneration strategies are being used in practice to burn soot. During most of the diesel engine operation period, the exhaust gas temperatures are below 300° C.—too low for initiating continuous uncatalyzed soot oxidation with O2 or NO2 (See Kimura, K, Alleman, T, L, Chatterjee, S, Hallstrom, K, SAE paper 2004-01-0079, Detroit 2004). However, from energy considerations and system design, an ideal particulate removal unit should minimize the temperature for continuous or induced regeneration of the soot filter.
The use of catalysts has the potential of decreasing the soot oxidation temperature sufficiently to provide for passive trap regeneration. Currently, the two most popular technologies to decrease the necessary temperature for soot oxidation are i) catalyzed soot filters that convert NO to NO2 which in turn oxidizes soot (See R. Allensson, Goersmann, Cavenius, Phillips, Uusimak, A. J, A. P. Walker, SAE paper 2004-01-0072, Detroit 2004), and ii) fuel-borne catalysts, that oxidize soot mainly with O2 as well as to some extent with NO (See T. Campenon, P. Wouters, G. Blanchard, P. Macaudiere, T. Seguelong, SAE paper 2004-01-0071, Detroit 2004). Soot oxidation with oxygen is insignificant in catalyzed soot filters due to the poor contact between catalyst and soot (See J. P. A. Neeft, M. Makkee, J. A. Moulijn, Chemical Engineering Journal 64 (1996) 295). In the catalyzed soot filter applications, the soot is mainly oxidized by NO2, where Pt is one of the primary components which generates significant amounts of NO2 at low temperatures. Unfortunately, catalyzed soot filters lack the desired durability, and the presence of SO2 further leads to sulfate formation (particulates) and deactivation of the catalyzed soot filter.
Diesel particulate filters (DPFs) can be regenerated by either injecting a fuel to increase the temperature of filter or employing an FBC alone. Using a fuel borne catalyst the problem of poor contact between catalyst and soot can be overcome and permit the use of uncatalyzed soot filters to capture and oxidize soot. Depending on the type of fuel borne catalyst used, soot can be oxidized with O2 or with O2+NO2 (See T. Campenon, P. Wouters, G. Blanchard, P. Macaudiere, T. Seguelong, SAE paper 2004-01-0071, Detroit 2004; S. J. Jelles, R. R. Krul, M. Makkee, J. A. Moulijn, Catalysis Today 53 (1999) 623; and S. J. Jelles, R. R. Krul, M. Makkee, J. A. Moulijn, G. J. K. Acres, J. D. Peter-Hoblyn, SAE 1999-01-0113). The significant advantage of fuel borne catalysts can be realized in the presence of SO2, which do not influence the soot oxidation behavior of the catalyst.
Ce and Ce—Fe fuel borne catalysts oxidize soot mainly by utilizing the ‘lattice oxygen’ and decrease the soot oxidation temperature by about 100° C. (See T. Campenon, P. Wouters, G. Blanchard, P. Macaudiere, T. Seguelong, SAE paper 2004-01-0071, Detroit 2004). Though enough NO is present in the feed gas, the rate of NO oxidation to NO2 over Ce or Ce—Fe fuel borne catalysts is not efficient and therefore the more powerful oxidant (NO2) cannot be extensively generated, leading to insignificant NO impact on soot oxidation. Bimetallic fuel borne catalysts containing ultra low concentrations of Pt—Ce is shown to decrease the balance point temperature to around 275 to 300° C. (See S. J. Jelles, R. R. Krul, M. Makkee, J. A. Moulijn, Catalysis Today 53 (1999) 623; S. J. Jelles, M. Makkee, J. A. Moulijn, Topics in Catalysis 16 (2001) 269; and S. J. Jelles, R. R. Krul, M. Makkee, J. A. Moulijn, G. J. K. Acres, J. D. Peter-Hoblyn, SAE 1999-01-0113). This is the lowest balance point achieved among the many combinations of fuel additives and catalyzed soot filters studied so far. The additional benefit by using Pt—Ce fuel borne catalyst is that, it forms Pt catalyst coating on the exhaust gas system and on the filter, which is able to significantly oxidize NO to NO2 and therefore further decreasing the balance point temperature. Further advantages of using Pt—Ce fuel borne catalysts include the resistance to sulfur poisoning, even using fuel containing 500 ppm of sulfur, the filter did not suffer from filter plugging or sulfate formation. (See, S. J. Jelles, R. R. Krul, M. Makkee, J. A. Moulijn, Catalysis Today 53 (1999) 623; S. J. Jelles, R. R. Krul, M. Makkee, J. A. Moulijn, G. J. K. Acres, J. D. Peter-Hoblyn, SAE 1999-01-0113; and B. A. A. L. van Setten, M. Makkee, J. A. Moulijn, Catal. Rev. Sci. Eng. 43 (2001) 489) Therefore, Pt—Ce fuel borne catalyst will have significant advantage over catalyzed soot filter like systems where the soot oxidation mainly depends on the generation of NO2 over catalysts which are sulfur sensitive (also, SO2 is oxidized to SO3 very efficiently ultimately leading to the emissions of sulfate PM). Using the ultra low dosage of Pt—Ce (<8 ppm) fuel borne catalyst the frequency of filter cleaning could be reduced significantly due to less ash accumulation.
Recently, diesel soot containing fuel borne ceria catalyst was characterized and a micro kinetic approach was followed to study the impact of the surface oxygen complex (SOC) reactivity with O2 (See L. Retailleau, R. Vonarb, V. Perrichon, E. Jean, D. Bianchi, Energy Fuels 18 (2004) 872; D. Bianchi, E. Jean, A. Ristori, R. Vonarb, Energy Fuels 19 (2005) 1453; and R. Vonarb, A. Hachimi, E. Jean, D. Bianchi, Energy Fuels 19 (2005) 35). It was found that a cerium additive decreased the ignition temperature by about 90 K compared with uncatalyzed soot oxidation, and part of the activity is ascribed to Ce2O2S like phase, formed from the decomposition of Ce2(SO4)3. On the other hand it is shown by temporal analysis of products that, CeO2 lattice oxygen is involved in soot oxidation with O2, when CeO2 is in tight contact with Printex-U soot, which can be considered as a mimic of the fuel borne catalyst (See A. Bueno-Lopez, K. Krishna, M. Makkee, J. A. Moulijn, J. Catal. 230 (2005) 237). Ce(IV)O2 or CeO2 based catalysts supply the lattice oxygen to soot, thus increasing the rate of soot oxidation; and the gas phase oxygen will replace the thus formed vacant sites on Ce(III)Ox.
Soot oxidation was also studied with NO+O2, over soot containing fuel borne ceria catalysts as well as by externally adding CeO2 to soot (See S. J. Jelles, R. R. Krul, M. Makkee, J. A. Moulijn, Catalysis Today 53 (1999) 623; S. J. Jelles, M. Makkee, J. A. Moulijn, Topics in Catalysis 16 (2001) 269; and A. Setiabudi, J. Chen, G. Mul, M. Makkee, J. A. Moulijn, Applied Catalysis B: Environmental 51 (2004) 9). The main reaction in such a process is NO oxidation to NO2, wherein the NO2 formed is a powerful oxidant than O2. However most of these studies are performed in loose contact mode and not with CeO2 and soot in tight contact and NO+O2 as an oxidant. Soot oxidation in the presence of Co—K—Ba/CeO2 catalysts (in tight contact with soot) with feed gas containing NO has also shown that surface nitrogen containing species are involved in oxidizing soot at much lower temperatures (See V. G. Milt, C. A. Querini, E. E. Miro, M. A. Ulla, J. Catal. 220 (2003) 424).
Ce and Pt—Ce fuel borne catalysts are extensively studied by Jelles et al. (See S. J. Jelles, R. R. Krul, M. Makkee, J. A. Moulijn, Catalysis Today 53 (1999) 623; S. J. Jelles, M. Makkee, J. A. Moulijn, Topics in Catalysis 16 (2001) 269; and S. J. Jelles, R. R. Krul, M. Makkee, J. A. Moulijn, G. J. K. Acres, J. D. Peter-Hoblyn, SAE 1999-01-0113). It has been found that Pt—Ce fuel borne catalysts are very active in soot oxidation and have shown lowest balance point among the catalysts known so far (275-300° C.). It is observed that these fuel borne ceria catalysts are more active after an initial induction period of a catalyzed trap. During this induction it is proposed that, platinum coats the walls of the trap and catalyses the oxidation of NO to NO2. The thus formed NO2 is more reactive towards Pt—Ce-soot compared with Fe-soot and Cu-soot. Furthermore, it is postulated that, NO2 decomposes over CeO2 to form active oxygen, ‘O’, which oxidizes soot efficiently. Fe and Cu do not seem to catalyze such oxygen transfer reactions.
There is a current need for new insights on mechanistic aspects for very high efficiency of Pt—Ce fuel borne catalysts, compared with other fuel borne catalysts/catalyzed soot filter systems and to employ them to design particulate filters with improved efficiency, and this patent application discloses such improvements. Desirably, this knowledge could aid in providing traps with improved regeneration characteristics, which could preferably retain increased levels of ultrafine particles without disadvantageous sacrifices in fuel economy or DPF size.
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