Catalytic soot oxidation by platinum on sintered metal filters: Influence of the platinum quantity, particle size and location, and investigation of the platinum-soot contact

Kurzfassung

The great challenge for next years concerning the emission of diesel engines is to develop diesel particle filters (DPF) with catalytic regeneration systems. This thesis is focused on the global understanding of the diesel soot oxidation on sintered metal filters (SMF). Platinum is studied as reference catalyst. The first objective is to determine which of platinum quantity, platinum particle size, or platinum location exhibits the preponderant influence on the catalytic soot oxidation in the case of a model fuel borne catalyst (FBC) system. The second objective is to determine if it is possible to achieve the same catalytic effect on a platinum coated filter as the one obtained by using a FBC. The answers to these objectives should help to analyze the improvement potential of coated filters and FBCs in future. To answer these objectives, a method was developed to calculate the kinetic parameters of the thermal and the catalytic soot oxidation from temperature ramp experiments. The global activation energy of the catalytic soot oxidation does not differ from the one of the thermal soot oxidation. The catalytic effect was thus defined as the pre-exponential factor ratio between the catalytic and the thermal soot oxidation with the other kinetic parameters being kept constant, following an oxygen transfer mechanism. Temperature ramp experiments were performed in a thermogravimetric analysis (TGA) device for platinum-soot powder samples. A reactor was also designed and built to investigate the catalytic soot oxidation on sintered metal filters. Conversion curves obtained from reactor experiments do not allow to calculate kinetic parameters. For this case the catalytic effect was related to the temperature difference for 50% soot conversion between the thermal and the catalytic soot oxidation. Six different soot sources were used along this thesis. The kinetic parameters of each soot source were first investigated in the TGA device. Soot produced by pyrolyzing toluene exhibits the closest structure and composition to the reference diesel soot. Its oxidation behavior is also the closest to the reference diesel soot. But all soot sources were found to have reasonably similar composition, structure and thermal behavior to diesel soot. The catalytic soot oxidation on established DPF technologies was first investigated. Platinum-doped soot produced via FBC shows a catalytic effect, increasing with the platinum-to-soot mass ratio. But platinum coated filters do not display any catalytic effect. It was decided to deepen these results by investigating the catalytic effect of platinum on the soot oxidation with artificial platinum-doped soot samples, and with artificial platinum-soot contact scenarios on SMF. In the case of a model FBC, the catalytic effect was observed to increase linearly with the platinum-to-soot mass ratio, and the initial platinum specific surface area. This reinforces the theory of the oxygen transfer mechanism. Platinum particles sinter during the oxidation, but as the soot was itself consumed, this did not affect the platinum surface area related to the soot mass. By investigating the influence of the initial platinum particle size for soot samples containing same platinum quantity, a size optimum was found at a mean diameter of 3 nm. However, this size effect is low compared to the influence of the platinum quantity. The platinum location, embedded in the soot agglomerate or present on the soot surface does not influence the catalytic effect. The catalytic effect of three configurations modeling three contact possibilities on SMF was investigated. Filtering platinum and soot aerosol consecutively does only exhibit a marginal catalytic effect. But filtering platinum and soot aerosol simultaneously exhibits the same catalytic effect as filtering a platinum-doped soot aerosol. It suggests that it is possible to enhance the catalytic effect of coated filters by increasing uniformly the platinum particle density in the soot cake. The results of this thesis suggest that the use of platinum as FBC is limited by the cost of platinum, as the catalytic effect was found to be linear with the platinum-to-soot mass ratio. The only possibility to improve the performances of a platinum FBC for a fixed platinum quantity is to optimize the platinum particle size in the obtained platinum-doped soot, however with low influence on the catalytic effect. The cost problems of platinum FBC could principally be avoided by attaching a fixed platinum quantity on a coated SMF. The contact between filtered soot and platinum is today too low to establish a catalytic effect on a coated filter. However, it has been shown that if all platinum particles are uniformly distributed in the soot cake (simultaneously filtration), the catalytic effect is the same as for a platinum FBC. The enhancing of the platinum density in the soot on a coated filter is thus a great research challenge for next years.