Study of the uncatalyzed and catalyzed soot combustion in the exhaust of last generation combustion enginesKinetic analysis and optimization of the catalytic formulations

Abstract

Against a backdrop of growing environmental concern and awareness due to the urgent need to reduce CO2 emissions into the atmosphere, the aim is to implement increasingly restrictive measures in one of the biggest contributors to these greenhouse gas emissions, the road transport sector. In this case, one of the paths being followed to decarbonize transport in the short and medium term is the use of vehicles equipped with the latest generation of diesel and gasoline engines, due to their greater energy efficiency. However, increasing engine efficiency inevitably means lowering the temperature of the exhaust gases emitted. Because of this drop in temperature, more active catalytic formulations in exhaust gas after-treatment systems will be needed for pollution control, such as the emission of particulate matter, so that they can continue to meet the strict pollutant emission limits imposed by current or future vehicle legislation. On the other hand, in order to avoid the use of platinum group metals (PGMs) because of their high cost and sharp price fluctuations, new catalysts with alternative formulations (without noble metals) are being considered for exhaust gas after-treatment systems. In this case, ceria-based catalysts are seen as promising candidates, due to their remarkable catalytic activity in these processes. CeO2, (or ceria), has been found to exhibit high activity in oxidation reactions, which is crucial for the removal of pollutants from exhaust gases. Additionally, the doping of ceria with certain cations (such as Pr) has been shown to improve the physico-chemical properties and the reducibility. The use of ceria-based catalysts could thus provide a cost-effective and sustainable solution for reducing emissions from vehicles, aligning with the broader goal of reducing the environmental impact of the road transport sector. In accordance with the above-mentioned problems, the main objective of this Doctoral Thesis is the development of new active catalytic formulations based on ceria/praseodymia mixed oxides or (pure oxides), specifically designed without the inclusion of noble metals in their formulations. These catalysts will be applied to a range of processes aimed at the removal of soot present in exhaust gases, with a particular emphasis on their application within post-treatment systems for the latest generation internal combustion engines, including both diesel and gasoline engines (GDI engines). To fully understand and establish the soot combustion process, the study begins by investigating the physico-chemical properties of different soots (real versus commercial soot). This is where the novelty of the contribution lies, as a novel attempt was made to deal with a real GDI (Gasoline Direct Injection) soot sample, which represents a breakthrough from previous studies that have primarily focused on diesel soot. Building on existing research, the Thesis explores the areas of diesel soot combustion and GDI soot combustion as a general concept. The detailed comparison of physicochemical properties between real soots (GDI soot, D-soot) and commercial soot (mainly Printex-U) revealed that the microstructure of the investigated soot samples is qualitatively similar, with notable differences in textural properties (BET surface area). Interestingly, the real GDI soot displayed lower orderliness compared to Printex-U. Significant differences in carbon content and heteroatom levels were observed, with the real soots presenting higher ash content and volatile matter than model soots. The volatile matter/fixed carbon content variations in real soot samples were linked to engine type and operating conditions. Overall, Printex-U's properties are closer to those of GDI soot than those of D-soot. Since the objectives cover soot removal reactions related to exhaust after-treatment, different families of catalysts have been synthesized and characterized for this purpose. Firstly, the soot removal tests have been performed through a study at a fundamental level and under simplified reaction conditions in order to understand and evaluate the processes that occur, in the context of the diesel system, with Pr-rich CexPr1-xO2-δ mixed oxides catalysts, with different nominal content in Ce and Pr (where x = 0, 0.2, 0.3, and 1). The catalyst samples were characterized using several techniques such as XRD, Raman spectroscopy, HR-TEM, N2 adsorption-desorption at −196°C, XPS analysis, O2-TPD, H2-TPR, and work function measurements. Pr-rich compositions, ranging from Ce0.3Pr0.7O2-δ to PrO2-δ, significantly increased total evolved O2 amounts and enhanced catalyst reducibility. However, a deterioration in the textural properties of the catalysts was noted, being particularly important for the pure praseodymia under the synthesis route conducted. The catalytic activity tests were investigated under two contact modes of soot and catalyst: loose and tight. The results revealed that the catalytic performance is associated with the surface contact in tight contact mode and with the combination of surface/subsurface/bulk oxygen mobility and the BET surface area in loose contact mode. Notably, the T10 and T50 parameters were achieved at much lower temperatures than those of the uncatalyzed soot combustion, even under loose contact mode. Specifically, the 50% conversion was achieved at 511°C and 538°C for Ce0.3Pr0.7O2 and Ce0.2Pr0.8O2, respectively. Although no direct correlation between catalytic activity and work function was observed, a significant relationship emerged between work function values and the formation of oxygen vacancies, whatever the conditions used for these measurements. On the other hand, the ability to generate a high population of oxygen vacancies at low temperatures, rather than the direct activation of gas-phase O2, influences the catalytic performance of Pr-doped ceria catalysts, highlighting the importance of surface/subsurface oxygen vacancy generation, which was the parameter that showed a better correlation with the catalytic activity, whatever the soot conversion value or the mode of contact considered. The next step in this research work was the study of the behavior of uncatalyzed combustion of GDI soot and Printex-U. For this purpose, a matrix of TG-MS experiments for uncatalyzed soot combustion was approached by combining three representative temperatures (500-525-550°C) with three different O2 concentrations (0.25-1.3-16.6%) to make explicit the influence of the gas reactant. The tested range emulates real working conditions from very small O2 concentration governed by the lambda control to high O2 concentration representative of fuel cut off events in which passive filter regeneration is promoted in GDI exhaust system. As an overall result, the GDI soot exhibited higher reactivity than model soot in whatever condition tested, especially at low temperature and oxygen concentration and the soot combustion rates reach higher conversion values in the trend: 500<525<<550°C, mainly at very low O2 concentrations. Furthermore, some of the most interesting ceria-praseodymia catalysts used in the previous chapter were also selected, (Ce0.3Pr0.7O2 and Ce0.2Pr0.8O2 mixed oxides, as well as pure praseodymia), in an attempt to investigate their ability for the catalyzed soot combustion, by evaluating the combustion of soot at 500°C under 1.3% of oxygen concentration under loose contact mode. The Ce-Pr catalysts showed significantly higher reactivity in soot combustion compared to the uncatalyzed reactions, particularly Ce0.3Pr0.7O2/Ce0.2Pr0.8O2 mixed oxides, which exhibited notable activity enhancement, and the catalytic response of GDI soot is more practically advantageous compared to model soot, as it exhibits enhanced selectivity towards CO2 exceeding 96%, with Ce0.3Pr0.7O2 identified as the most active catalyst. On the other hand, the benefits of conducting isothermal ex On the other hand, the benefits of conducting isothermal experiments at the temperatures of interest (instead of temperature-programmed reactions) are many, opening up the possibilities of conducting kinetic modeling (to describe the reaction rates) and obtaining the kinetic parameters of interest. For this reason, and assuming a kinetic controlled shrinking core model, (internal and external diffusion are neglected), the work demonstrated that this model can be used for the analysis of the combustion of soot (by only measuring weight losses as a function of time). At constant temperature, the resulting model can be integrated analytically which provides a closed expression to determine the kinetic parameters: rate constant [k(k0, Ea)] and reaction order (n). The model was applied to the combustion of soot without catalyst. To that end two different algorithms (or models) were developed (called M1 and M2). The first one (M1) follows a sequential approach in which first the kinetic constants at different temperatures and the reaction order are determined, and then in a second stage, the pre-exponential factor and activation energy are calculated assuming an Arrhenius type equation. In the second algorithm (M2) all the kinetic parameters are determined simultaneously. The results for the uncatalyzed oxidation showed a reaction order that was slightly different from the theoretical predictions (0.67). As expected, the activation energy values belonging to GDI soot were lower than those of Printex-U. In contrast, for the catalyzed soot oxidation, the reaction order (n) orders were greater than 1, indicating the involvement of multiple reactions. Interestingly, the kinetic constants parameter (k) for catalyzed soot combustion was significantly higher, especially with GDI soot, indicating that the catalyst significantly accelerates the combustion rate. Finally, in an attempt to go further into the optimization of the catalysts’ formulations, a series of ceria and ceria-praseodymia (Ce0.3Pr0.7O2) based catalysts incorporating different contents of copper and/or manganese-based active phases were synthesized by incipient wetness impregnation (IWI) method and characterized by several techniques. The catalytic activity tests were evaluated in the combustion of soot under an inert atmosphere, simulating the most demanding conditions encountered in the direct injection (GDI) engine exhaust. The results showed that ceria-praseodymia based catalysts are much more effective for soot removal than ceria-based catalysts, regardless of the active phase incorporated. In addition, copper was found to be a more effective active phase than manganese in promoting the reducibility of the catalyst and its activity in this reaction. Finally, the relevance of carrying out these reactions under the loose contact mode with respect to the tight contact mode has been erified, as different orders of catalytic activity are found. The family of ceria-praseodymia based catalysts under loose contact mode exhibits a good correlation between the oxygen vacancies created at low temperatures and the catalytic activity. In addition, they demonstrate superior soot removal capabilities by efficiently transferring active oxygen to the soot surface, with the best catalyst being the one with the highest Cu content, 3Cu-CePr.