the reaction conversion was calculated from the reaction product concentrations (CO and CO2) measured at the outlet of the reactor. As the emissivity of filter does not remain constant during an entire experiment (depending on the presence of soot or not), a temperature shift was observed for conversion superior to 0.5 by comparing conversion curves obtained in TGA device and in the reactor. No kinetic parameters could be calculated from conversion curves obtained in the re-actor. Thus the catalytic effect was related in this case as the temperature difference between thermal and catalytic soot oxidation at a conversion of 0.5, which is pro-portional to the logarithm of the catalytic effect A. However, this definition does not allow to compare the catalytic effect obtained for different soot sources with each other.
The kinetic parameters of the thermal soot oxidation were first studied in the TGA device. The catalytic effect achieved using established diesel particle filter (DPF) technologies with fuel borne catalyst and coated filter was then investigated. Af-ter that, platinum-doped soot samples (modeling the one obtained using a FBC) with varying platinum-to-soot mass ratio, varying platinum particle size and vary-ing platinum location were produced. The sample characterization was performed with elemental analysis (EA), transmission electron microscopy (TEM) and CO-chemisorption. The influence of the three key parameters on the catalytic soot oxidation was investigated in the thermogravimetric analysis device. Finally, the catalytic soot oxidation by platinum on a sintered metal filter with three different contact configurations was investigated in the designed reactor, to determine if it is possible to achieve the same catalytic effect on a platinum-coated filter as using a platinum FBC.
which both does not contain any adsorbed species, were comparable with literature for these types of soot [15, 50]. Soot obtained by pyrolyzing toluene and LPW3 diesel soot both contain adsorbed hydrocarbons. Their activation energies after a desorption ramp (159 kJ/mol and 173 kJ/mol, respectively) were comparable with activation energies reported in literature for various soot types [15]. Soot from a spark discharge generator (SDG) and diesel soot provided by a micro-cogenerator were both assumed to contain oxygen bonded to carbon. The activation energy of SDG soot after a preheating ramp (78 kJ/mol) was lower than the one already observed for such soot sources [56]. The activation energy of diesel soot after a preheating ramp (206 kJ/mol) was found to be a little high in comparison with activation energies reported in literature for general soot sources [15]. The validity of the use of the Slovak method for these two soot sources was questionable.
Diesel soot provided by a micro-cogenerator was chosen as reference for diesel soot. By comparing the shapes of the conversion curves, it was found that pyrol-ysis soot models the reference diesel soot the best. Pyrolpyrol-ysis soot also exhibits structure and composition closest to diesel soot. The differences between the struc-ture, composition and oxidation of both diesel soot (issued from a LPW3 engine or from the micro-cogenerator) underline the relativity of this result, and the fact that the future results obtained studying all synthetic soot sources can be further applied on diesel soot.
8.2.2 Catalytic soot oxidation by platinum using established DPF technologies
The catalytic effect of platinum using two established diesel particle filter technolo-gies (a platinum fuel borne catalyst and a platinum-coated sintered metal filter) was studied. The catalytic effect of the platinum FBC was investigated by thermogravi-metric analysis, and observed to increase with the platinum quantity in the soot (0.002 - 0.007 mg Pt / mg soot). The catalytic effect of the platinum-coated SMF was investigated in the reactor test bench. An intermediate fibre supporting the plat-inum catalyst was applied onto the SMF. The fibre itself lowers the soot oxidation temperature, but the platinum did not exhibit any catalytic effect, even with very high platinum quantities (3.2 - 8.2 mg Pt / mg soot). The contact between platinum and soot particles was presumably to low.
The dependence of the catalytic effect on the platinum quantity, and the influence of the platinum particle size and location has to be investigated in more details in the case of the FBC technology. Furthermore, the possibility of the contact
enhancement between platinum and soot on a coated SMF to achieve the same catalytic effect than the one obtained using a FBC has to be investigated too.
8.2.3 Influence of the platinum loading using model FBC systems
The catalytic effect A was observed to linearly increase with the platinum-to-soot mass ratio, for a fixed platinum particle size, and for more than 0.0005 mg Pt / mg soot. This was confirmed by plotting the catalytic effect as a function of the initial platinum surface area. Assuming the oxygen transfer mechanisms [18, 19], it was expected that the catalytic effect was linearly dependent on the platinum surface area. Platinum nano-particulate agglomerates can sinter together between 300°C and 700°C [63]. The influence of the platinum particle sintering during the oxida-tion was investigated. Platinum particle size increases with progressing conversion, but the platinum surface area related to the mass of soot remains constant.
The influence of the initial platinum particle size on the catalytic effect at a con-stant platinum-to-soot mass ratio was investigated too. As previously determined, it was expected that the catalytic effect increases with a decreasing platinum par-ticle size (increasing platinum surface area). However, a size optimum was found at a mean platinum particle diameter of 3 nm. This optimum can be explained by the superposition of two contrarily trends: the expected trend, and a decrease of the catalytic effect with decreasing platinum particle size for very small particles. Size optimum was already observed in other systems such as the CO oxidation on gold nanoparticles. Quantum size effects as well as geometric size effects have been ad-vanced to explain it [67, 68]. This trend is however secondary in comparison with the influence of the platinum-to-soot mass ratio.
The catalytic effect is independent of the platinum location, embedded inside the soot agglomerate, or present on the soot surface. Oxygen is supposed to diffuse through the soot to the embedded platinum particles. This result was confirmed by studying the catalytic effect of platinum-doped soot obtained using a FBC.
Platinum-doped soot issued from a FBC exhibit the same catalytic effect as syn-thetic platinum-doped soot samples, increasing linearly with the platinum-to-soot mass ratio. This result is not in contradiction with the results obtained in literature using FBC, were platinum was found to be inactive [30]. Indeed, the FBC quanti-ties used in literature probably lead to lower platinum quantity than the 0.0005 mg Pt / mg soot found here as the minimum to detect any catalytic effect.
8.2.4 Effect of the platinum-soot contact on coated filters
The catalytic effect of a platinum-coated SMF with a presumably enhanced contact area between platinum and soot was previously investigated with the intermediate fibre. However, no catalytic effect was detected. At the present research state, platinum-coated filter does not exhibit a sufficient contact between platinum and soot to achieve the same catalytic effect as using a FBC.
The catalytic effect of three configurations modeling three contact possibilities on SMF was investigated. The contact obtained using a FBC was modeled by the filtration of a platinum-doped soot aerosol. The conventional coated filter configu-ration was produced by the consecutive filtconfigu-ration of a platinum and a soot aerosol.
And finally, the contact obtained with a FBC was approximated on a filter by the si-multaneous filtration of a platinum and a soot aerosols. These three configurations were produced for two different soot sources and lead to the same results, inde-pendently of the soot source. The simultaneous filtration exhibits a high catalytic effect, comparable with the one of the platinum-doped soot filtration, and increas-ing with the platinum quantity. Only a very low catalytic effect was observed for the consecutive filtration, enhanced when the soot aerosol was filtered before the platinum aerosol. These results suggest that it is possible to enhance the catalytic activity of coated filters by increasing uniformly the platinum particle density in the soot cake.