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The work presented in this thesis centers on the application of atomic-resolution transmission electron microscopy to study Platinum-alloy nanoparticles. In particular, the thesis focusses on the platinum-iron and platinum-gold systems. Additionally, few other complementary structures based on Pt thin films and nanowires are also characterized. These materials are studied in the context of their catalytic application towards the oxygen reduction reaction in polymer electrolyte membrane fuel cells (PEMFCs). Here we report on the detailed investigation of many structural and compositional aspects of these catalyst nanoparticles including lattice strain, the surface and bulk atomic-structure, the surface/bulk chemical composition, surface segregation, and atomic ordering. In some cases we have even looked beyond the traditional characterization approaches. For instance, instead of observing the particle structures before and after a particular treatment (e.g., heating and degradation tests), we have captured the dynamics of structural evolution over the entire course of such treatments. These investigations were useful in interpreting their catalytic performances, which opened new perspectives towards further optimization of their material structure on the atomic-level. / Thesis / Doctor of Philosophy (PhD)

There are three major problems affecting efficiency in the catalytic oxidation of ammonia for obtaining nitric acid in an industrial plant: the limited life of gauzes, the low efficiency of the catalyst after a few months of operation, and finally, the necessity of recovering as much quantity of precious metals as possible. The first point to study in order to control the process and extend the useful life of the catalyst should be the correct characterization of the deactivated system. Therefore, this research is a systematic study about the performance of the catalytic recovering systems in nitric acid plants for all industrial pressures. In addition, it intends to cover the need of updating the knowledge in this field. The results obtained in this study with the support of SEM, EDX, XPS and AAS techniques, allow us to reach the following conclusions: The extremely critical conditions in which the activation pretreatment takes place cause important structural variations in the material surface. Beyond that, significant PtO2 losses and subsequent enrichment in RhO2 occur in a campaign in a high pressure plant, and mean while a continuous surface reconstruction is taking place. In contrast, an enrichment in Rh0 occurs in a lower pressure plant. In all situations, the deactivation is associated to a decrease in the Platinum content, more active than Rhodium. The getter mechanism in the recovering gauzes is directional and consists in the absorption of PtO2 (or Pt0) on the surface of the Palladium-based wires. Next, the PtO2 reduces itself to Pt0 and forms the Pt-Pd alloy. The part of volatile Platinum and Palladium oxides which gest through the recovering pack, as well as the particles of Rh2O3 carried away mechanically, settle in the heat exchangers line and in the Platinum filter of the high pressure plant. The most part of impurities are associated to Fe, Ni, Cr, Cu and Mn,probably forming oxides. Finally, from the comparative analysis made between the diversity of plants studied, we can conclude that their different operating conditions have an extremely important influence in the performance of the catalytic and recovering systems used.