Simulating Real World Soot-Catalyst Contact Conditions for Lab-Scale Catalytic Soot Oxidation Studies
In diesel soot oxidation studies, both well-defined model soot and a reliable means to simulate realistic contact conditions with catalysts are crucial. This study is the first attempt in the field to establish a lab-scale continuous flame soot deposition method in simulating the "contact condition" of soot and a structured diesel particulate filter (DPF) catalyst. The properties of this flame soot were examined by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM) for
... croscopy (TEM) for structure analysis, Brunauer-Emmett-Teller (BET) for surface area analysis, and thermogravimetric analysis (TGA) for reactivity and kinetics analysis. For validation purposes, catalytic oxidation of Tiki ® soot using the simulated contact condition was conducted to compare with the diesel particulates collected from a real diesel engine exhaust system. It was found that the flame soot is more uniform and controllable than similar samples of collected diesel particulates. The change in T 50 due to the presence of the catalyst is very similar in both cases, implying that the flame deposit method is able to produce comparably realistic contact conditions to that resulting from the real exhaust system. Comparing against the expensive engine testing, this novel method allows researchers to quickly set up a procedure in the laboratory scale to reveal the catalytic soot oxidation properties in a comparable loose contact condition. reactivity of Printex-U carbon black tends to be very similar to diesel soot, and it can usually generate good reproducibility. Until recently, the impact of soot nanostructure on reactivity has not drawn much attention [7-10]. Vander Wal et al. found by means of transmission electron microscopy (TEM) that the structural differences of soot/carbon at a nanoscale level (i.e., dimension and orientation of the graphitic layers) affect its oxidation reactivity [8, 9] . Other recent studies showed that the intrinsic oxidation rate of pyrolysis soot might vary from different fuels (such as benzene, ethanol, and acetylene), resulting in differences in soot nanostructure [7, 11, 12] . Moreover, it has been speculated that peculiar nanostructures, such as amorphous and shell/core formations, are present in the soot formation process  . Amorphous nanostructures are related to soot particles and have been identified mainly in less developed soot particles and condensed species [14, 15] . The study of soot nanostructure is important for both correlating reactivity and simulating non-catalytic/catalytic soot oxidation in exhaust systems. Comparisons of the kinetic parameters of non-catalytic soot oxidation with those of catalytic soot oxidation have been made by many researchers to evaluate the effectiveness of soot oxidation catalysts, and the obtained kinetics information can also reveal the mechanism of soot oxidation reactions. However, the reported activation energy for non-catalytic soot oxidation ranges from 92 to 211 kJ/mol since the employed experimental techniques and conditions vary from one group to another. Yezerets et al.  compared the oxidative reactivity of diesel soot and carbon black, and the activation energies were found to be 92 and 117 kJ/mol, respectively. In another study of soot samples from different engine conditions  , the activation energies for the soot with ash contents of 14 and 6.5% were found to be 126 and 146 kJ/mol, respectively. A recent study by Strzelec et al.  showed that the activation energy for diesel soot is 113 ± 6 J/mol. Hence, it seems the kinetic parameters for soot oxidation cannot simply be compared from one study to another due to the differences in the nature of the soot and the experimental conditions of the reactors (i.e., the mass and heat transfer limitations). In soot oxidation studies, both well-defined model soot and a reliable means to simulate realistic contact conditions with catalysts are crucial. In catalytic soot oxidation studies, the "contact condition" refers to the nature of the contact between the soot and catalyst, with the contact range varying from "tight" to "loose". The realistic contact condition obtained by in-situ collection on a DPF in an exhaust system is considered as "loose" contact [18, 19] . Replicating this condition in laboratory studies can be challenging. In lab-scale studies, thermogravimetric analysis (TGA) has usually been used to characterize soot oxidation behavior, permitting relatively straightforward examination of kinetics during heating  . When the catalysts under study are in a powder form, loose contact can be simulated by gently mixing the soot and catalyst powders with a spatula [5, 20] or gently shaking the sample container with catalyst/soot mixture [5, 21, 22] . However, these methods are not appropriate for study of catalysts incorporated onto complex support material shapes (e.g., porous cordierite/SiC filters). In the present study, it was anticipated that producing flame soot with a Tiki © torch oil lamp and capturing the soot directly onto a catalyst/support structure would yield a contact condition simulating realistic conditions. The properties of this flame soot were examined by means of XRD and TEM for structure analysis, BET for surface area analysis, and TGA for reactivity and kinetics analysis. The flame soot is thought to be more uniform and controllable than similar samples of collected diesel particulates. For validation purposes, catalytic oxidation of Tiki © soot using the simulated contact condition was compared with the diesel particulates collected from a real diesel engine exhaust system.