Short summary
Novel diesel soot filters for removal of carbonaceous soot particles formed by diesel driven engines will be developed in this project. In this process, the contact between catalyst and soot is crucial for the oxidation and removal of soot. We aim at developing a special aerosol coating technique of the catalyst on the filter surface to improve the contact in comparison to existing filters where the catalyst is inside the filter. This will (a) position the catalyst mainly at the filter surface where also the soot deposits are located and (b) lead to a special structure of the catalyst coating which provides good contact between soot. Hence,, the amount of catalyst ma≠terial, as well as the pressure loss over the filter, is expected to de≠crea≠se. Finally, lower operation temperatures could be applied, thereby increasing fuel efficiency.
1. Project description
This project aims at studying and developing catalytic soot particulate filters for removal of toxic carbonaceous soot particles formed by diesel driven engines. We plan to apply novel preparation routes wherein flame-made aerosol nanoparticles are coated onto various filter substrates to form a highly porous and active catalytic deposit. This coating will consist of catalytic nanoparticles with a very large specific surface area ensuring a large number of active sites for the catalytic soot combustion to take place. Furthermore, the porous coating should decrease pressure-drop over the filter during operation. The most obvious advantage originates from the novel design of the catalyst particles on the filters. Due to the special deposition process, the catalytic material will be mainly deposited on the outside surface of the porous filter structure, which is exactly at the same position where soot will land during filtration. Therefore, so-called “tight” contact is expected to be obtained between the soot and the catalytic nanoparticles that is essential for lowering the temperature of the ensuing catalytic combustion. Since the catalyst is at the position where it is most needed, this should minimize the amount of it required. In total, the energy costs for running these filters should be significantly reduced, thereby leading to better fuel economy of the engine compared to current state-of-the-art filters. One key issue, however, is to obtain sufficient adhesion to the filter, as well as a high resistance towards thermal expansion during the operation, while at the same time optimizing deposition conditions that yield a high degree of soot-catalyst contact. To aide in this, computer modeling based on first principles will be performed.
2. Research focus
The research is divided into three main focus groups:
(1) Improvement of the soot-catalyst contact (“tight” contact) crucial for low temperature soot-combustion and minimization of the pressure-drop costs to improve fuel economy.
Improved soot-oxidation catalysis in soot particulate filters is expected to be generated by depositing an aerosol of flame-made catalytic metal-oxide nanoparticle agglomerates onto the filter substrate, by filtration of the aerosol through the filter substrate itself [1, 2]. The principle of this new approach is highlighted in Fig. 1.
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| Figure 1: Principle of aerosol coated filters. An aerosol containing the catalytic compound is deposited on a porous substrate (a) by filtration, which rapidly forms a highly porous agglomerated filter cake network/coating (b). Finally, during soot filtration (c), soot particles penetrate into the porous coating which yields “tight” soot-catalyst contact. |
Generation of the aerosol is achieved by using flame spray pyrolysis (FSP, cf. [3]). The resulting coating is expected to be highly porous (97 %), continuous and thin (approx. 5 mm) and with pore-sizes of approximately 500 nm [2] ensuring only a slight increase in the pressure-drop through the filter [4]. Most importantly, however, deposition of the catalyst occurs in the same manner as the soot. Once this deposition method works and has been optimised, a much larger area should be available compared to wash-coating and a “tighter” soot-catalyst contact is expected. Initially, catalytic combustion of model soot (carbon black) will be tested. This can be generated by combustion of acetylene [5]. Subsequently, the filters performance towards realistic engine-generated soot will be tested. The test method as suggested by van Setten et al. [6] will be followed as it resembles the actual filtration process including the regeneration step. The method consists of three steps: (a) Depositing a catalyst (here CeO2-based deposited by filtration, cf. Fig. 1) on the filter. (b) Depositing a soot cake on top of this coating. (c) Catalytic tests by temperature programmed oxidation (TPO) to determine the kinetics.
To gain more insight into the process, computer models based on first principles and computational fluid dynamics (CFD) will be developed parallel to experiments. This should be done on the basis of an existing validated code utilizing Langevin dynamics for particle tracking [2]. Recently, the effects of deposition on the flow-field has been taken into account. The flow-field is resolved during deposition using the Darcy equation in all three dimensions combined with a finite volume method to solve the large system of equations. Figure 2 shows the cross-section of the deposit morphology of 100 nm soot particles deposited during filtration at Pe = 10. The flow field is only calculated for the filter cake.
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| Figure 2: Cross-section of filter cake morphology for 100 nm soot particles filtered at Pe = 10. The flow direction is from the top and down (with z). The permeability of the deposit is seen as the colored contour background, where red and blue indicates a high and low permeability, respectively. The arrows show the direction of the flow. Only the flow-field outside the filter pore (“filter cake”) was calculated. A parabolic flow profile was assumed inside the pore, in accordance with Poiseuille’s law. Only a short part of the 10 mm long capillary shown as deposition mainly takes place in the filter cake. |
(2) Investigation of catalyst-filter adhesion and optimization of thermal stability
It is important that the coatings stick onto the filter while maintaining their highly porous structure without cracking. Therefore, we will investigate the resistance of the coatings to thermal expansion during operation and search for conditions to increase the adhesion onto the surface, which is typically dominated by weak Van der Waals forces for as-deposited coatings [4].
(3) Synthesis of novel catalytic materials to improve the catalytic performance of these filters.
State-of-art catalysts are typically based on CeO2 [7]. Doping with transition metals such as Zr, Fe, Mn have shown to increase the activity even further [8]. Due to the possibility of mixing several precursors, the FSP method is well-suited for testing various dopants effect on CeO2.
3. Bachelor/Master thesis
Any students interested in doing their bachelor and master thesis within the field (experimental or simulations, or both), may inquire for more details.
4. Contact
Post Doc. Tobias Dokkedal Elmøe
Building 229, room 166, Department of Chemical and Biochemical Engineering.
Telephone: +45 4525 2942
Email: .
5. References
1. Schimmöller, B., et al., Ceramic foams directly-coated with fame-made V2O5/TiO2 for synthesis of phthalic anhydride. Journal of Catalysis, 2006. 243(1): p. 82-92.
2. Elmøe, T.D., Deposition of nanoparticles on porous substrates, in Department of Chemical and Biochemical Engineering. 2008, Technical University of Denmark: Kgs. Lyngby.
3. Strobel, R. and S.E. Pratsinis, Flame aerosol synthesis of smart nanostructured materials. Journal of Materials Chemistry, 2007. 17(45): p. 4743-4756.
4. Elmøe, T.D., et al., Filtration of nanoparticles: Evolution of cake structure and pressure-drop. Journal of Aerosol Science, 2009. 40(11): p. 965-981.
5. Spicer, P.T., et al., Flame synthesis of composite carbon black-fumed silica nanostructured particles. Journal of Aerosol Science, 1998. 29(5-6): p. 647-659.
6. van Setten, B.A.A.L., et al., Realistic contact for soot with an oxidation catalyst for laboratory studies. Applied Catalysis B-Environmental, 2000. 28(3-4): p. 253-257.
7. Simonsen, S.B., et al., Ceria-catalyzed soot oxidation studied by environmental transmission electron microscopy. Journal of Catalysis, 2008. 255(1): p. 1-5.
8. Imanaka, N., et al., Novel catalysts for low-temperature combustion of diesel particulate matter. Journal of Materials Chemistry, 2009. 19: p. 208-210.