4.3 Thermal soot oxidation in a TGA device
4.3.2 Differences between synthetic and diesel soot
is that the preheating affects the soot structure by closing micropores. The fraction of adsorbed species in LPW3 diesel soot was too high to applied the Slovak method on the normal oxidation (without preheating). In the following preheating ramps are performed for these two soot sources to avoid the influence of adsorbed species on the catalytic soot oxidation.
SDG soot and reference diesel soot also probably contain some adsorbed hydro-carbons, but not enough to explain the great part of soot that disappears during the preheating ramp. Both of them also contain a high fraction of oxygen. It was assumed that the carbon bonded to oxygen can be oxidized during the preheat-ing (C=O, C-O-C, C-O-H and some aromatic structures). The normal oxidation could not be modeled with the Slovak method. Furthermore, the application of this method to determine the kinetic parameters of the oxidation after a preheating is also questionable. A very low activation energy was calculated for SDG soot, and a high activation energy was calculated for the reference diesel soot [15]. Performing isotherm experiments could help to determine the validity of the calculated kinetic parameters. In the following, to study the catalytic effect of platinum on SDG soot, only the temperature T50 is considered. Regarding the presence of adsorbed species, SDG is the soot that models the reference diesel soot the best.
0 0.2 0.4 0.6 0.8 1
200 300 400 500 600 700 800
Temperature T [°C]
Conversion X
PrintexU soot Vulcan soot SDG soot Pyrolysis soot Diesel soot LPW3 soot
Figure 4.13: Conversion curves for the six soot sources after preheating to 700°C
information on the curve slope. A straight-forward dependence between the soot composition, the soot structure and the conversion curve shape parameters T50 and Ea could not be found.
It had been expected that soot with small primary particle diameter (determined on TEM micrographs) oxidizes at lower temperatures or with a lower activation en-ergy. However, if SDG soot presents the lowest diameter and the lowest activation energy, it is not a systematic trend. Comparing two synthetic soot sources with each other, PrintexU soot and Vulcan soot, Vulcan soot presents the lower diame-ter, and also the higher temperature T50 and activation energy Ea. The structure is thus not the only parameter controlling the oxidation. Regarding the soot composi-tion, Vulcan soot presents the highest carbon fraccomposi-tion, and the highest temperature T50. It could be expected that soot without impurities oxidizes at higher tempera-tures or with a higher activation energy than soot containing impurities. However, this is also not a systematic trend. Comparing pyrolysis soot with LPW3 diesel soot gives a contra-example. Pyrolysis soot presents the higher carbon fraction but lower energy of activation and temperature T50. No systematic trend regarding the influence of the soot structure and composition on the conversion curves shape was found. However, both of them play a certain role on it. Indeed, the soot present-ing the closest conversion curve shape to the one of the reference diesel soot is the pyrolysis soot. Pyrolysis soot is also the only soot that presents both structure and composition parameters close to the reference diesel soot.
It was discussed that not only the soot structure, but also the soot nanostructure (length and curvature of the graphene segments in the soot primary particles) de-termines the soot oxidation [58]. It was found that soot presenting fullerene like structures such as SDG soot oxidizes at lower temperatures than well-graphitized soot [55, 56]. As Vulcan soot is an amorphous furnace carbon black, it is pre-sumably more graphitized than the flame soot PrintexU for example, and it could explain why Vulcan soot oxidizes at high temperatures. The difference between the reference diesel soot and LPW3 diesel soot - two soot sources produced by diesel engines - can not be explained yet. The reference diesel soot was produced on a recent engine (2004) and LPW3 diesel soot on an old one (1996). It was argued that soot emitted from recent engines that fulfill the Euro 4 standard presents a more fullerene like nanostructure than soot emitted from old engines [55, 56]. This could explain why the LPW3 diesel soot oxidizes at higher temperatures than the reference diesel soot.
From the actual soot comparison, it can be concluded that pyrolysis soot models the reference diesel soot the best from a structure and an elemental composition point of view. Considering the presence of adsorbed species, SDG soot models the reference diesel soot the best. However, excepting Vulcan soot, all synthetic soot types (PrintexU, SDG and pyrolysis) present conversion curves comprised between the ones of the two diesel soot types (the reference diesel soot and LPW3 diesel soot). The shape of the conversion curve for Vulcan soot is additionally very closed to the one of LPW3 diesel soot, displaying a temperature shift lower as 20 K. It is consequently assumed that all the future results obtained on the synthetic soot types could be applied on real diesel soot.
Catalytic soot oxidation by platinum using established DPF technologies
The aim of this chapter is to investigate the catalytic effect of platinum on the oxi-dation of diesel soot (LPW3 and reference diesel soot), using two established diesel particle filter (DPF) technologies: a fuel borne platinum catalyst (FBC) and a plat-inum coated sintered metal filter (SMF). The catalytic effect of the platplat-inum FBC was investigated by thermogravimetric analysis (TGA), and observed to increase with the platinum quantity in the soot. This dependence will be discussed in more details in Chapter 6. The catalytic effect of the platinum coated SMF was investi-gated in the reactor presented in Chapter 3. An intermediate fibre supporting the platinum catalyst was applied onto the SMF. The fibre itself played a positive role on the soot oxidation but the platinum did not exhibit any catalytic effect, even with very high platinum quantities. The contact between platinum and soot particles was presumably too small, so that the possibility to achieve the same catalytic effect on a coated filter as using a FBC will be investigated in Chapter 7.
5.1 Fuel Borne Catalyst
In this part, the catalytic effect of a platinum FBC is investigated. Platinum-doped soot produced with different FBC concentrations was sampled from the LPW3 diesel engine and characterized by elemental analysis and TEM micrographs. This allows the determination of platinum quantity, location (in the soot agglomerate or on the soot surface), and particle size. The catalytic effect of the sample was deter-mined from conversion curves obtained by TGA experiments. The catalytic effect was determined as A = k00,cat / k00,th, as defined in Chapter 2. Over the studied range, the catalytic effect increases with the total platinum quantity. However, due
to the sample characteristics, no conclusion concerning the form of this increase, the influence of the platinum particle size and influence of the platinum location could be made. This will be investigated in Chapter 6.