Abstract
This thesis presents the results of my research work on the effects of ageing on the structure and functionalities of a tri-metal (Pt-Pd-Rh) commercial Lean NOX Trap (LNT) catalyst. The investigation has been focused on the correlation between the real-world ageing and artificial or rapid-ageing procedures. Samples exposed to hydrothermal, vehicle and dynamometer ageing have been fully characterized, and their performance tested. In addition, this work includes analyses on samples exposed to de-greening procedures, aimed to induce a stabilized state of the catalysts.Chapter I introduces the main topic of the thesis with a general overview of the legislation on vehicle pollution and the main technologies for diesel emission control. Afterwards, the chapter reports a comprehensive literature review on the formulation, reaction mechanisms and main degradation processes of the LNT catalysts.
Chapter II reports the results of the characterization techniques adopted in order to understand the catalysts structural and chemical changes. The characterization techniques involved were: XRD, XPS, XAFS, ICP and BET. The most significant changes observed were: sulfur poisoning, Pt sintering, ceria sintering and Pd-Pt alloying. Sulfur poisoning was exclusively observed on samples exposed to exhaust gasses.
Chapter III illustrates the experimental procedures and results of the spatially resolved experiments. This technique allowed to observe the evolution of reactants and products along the catalysts channels during lean-rich cycling experiments. A preliminary set of NOX storage experiments has been performed to assess the specific number of NOX storage sites available on each catalyst. The findings indicated that aged samples were characterized by a lower NOX storage capacity, compared to the fresh sample. This resulted in an increased area of catalyst required by the aged catalyst to achieve complete NOX storage during lean-rich cycles. By the end of the lean phase, different distribution of stored NOX were observed on the samples, according to their respective NOX storage capacities. As a results, different speciation of the products, such as N2 and NH3, was observed along the catalysts during the regeneration of the trap. In addition, the rapid development of the H2 reduction front through the aged catalyst hinted at a decrease of the oxygen storage capacity properties.
Chapter IV describes the experimental procedures and results of bench-scale performance experiments. This set of experiments has been designed for the evaluation of the catalysts performances in presence of complex gaseous mixtures. The aim was to con rm what previously observed on spatially resolved experiments in a wider range of temperatures and conditions close to real-world. The NOX storage capacity of the catalysts and the regeneration properties of the samples were evaluated and correlation with spatially resolved experiments and characterization analyses were observed.
Finally, a conclusive chapter reports an overview of the findings of this thesis work and future outlooks.
| Date of Award | Dec 2021 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Alexandre Goguet (Supervisor) & Geoffrey McCullough (Supervisor) |