Elevada tolerância ao CO e alta estabilidade são necessário para o eletrocatalisador anódico de células a combustível de membrana protônica (PEMFC) para o melhoramento do desempenho do sistema operando com combustível reformado. Neste trabalho eletrocatalisadores suportados em carbono e carbetos (carbetos de molibdênio e tungstênio) foram estudados para a reação de oxidação de hidrogênio (ROH), a tolerância ao CO e a estabilidade no ânodo de PEMFC. Os materiais investigados incluem: PtMo/C, PtMo/C tratado-termicamente, Pt/Mo2C/C, PtMo/Mo2C/C, PtW/C, Pt/WC/C e Pt/C. As diferenças na morfologia dos eletrocatalisadores foram caracterizadas por redução com temperatura programada (TPR), difração de raios-x (XRD), microscopia eletrônica de transmissão (MET), energia dispersiva de raios-x (EDX), espectroscopia de absorção de raios-x (XAS), microscopia eletrônica de varredura (MEV) e espectroscopia de dispersão de comprimento de onda (WDS). As características e as atividades eletroquímicas dos electrocatalisadores foram avaliadas para ROH e tolerância ao CO por medidas de curvas de polarização em célula unitária e voltametria cíclica, sob a forma de elétrodos de difusão de gás. Espectrometria de massa (EMS) \"on-line\" e experimentos de stripping de CO foram realizados para avaliar o mecanismo de tolerância ao CO dos eletrocatalisadores. Voltametria cíclica conduzida até 5000 ciclos voltamétricos foi realizada para avaliar a estabilidade dos eletrocatalisadores. Tanto o eletrocatalisador de PtMo suportado em carbono e tratado a 600ºC, com tamanho médio de cristalito de 16,7 nm, como o eletrocatalisador de Pt suportado em carbeto de molibdênio mostraram atividade mais elevada para a oxidação de hidrogênio na presença de 100 ppm de CO e uma estabilidade melhorada, em comparação com os catalisadores de PtMo suportado em carbono e PtMo suportado em carbeto de molibdênio. Semelhantamente, melhor tolerância ao CO e maior estabilidade foram mostradas por Pt suportado em carbeto de tungstênio, como comparado com electrocatalisador de PtW suportado em carbono. Os resultados de voltametria cíclica e WDS mostraram que ocorre uma dissolução parcial de Mo e W e a sua migração/difusão do ânodo para o cátodo durante o período de ciclagem, o que é a maior causa das perdas em desempenho destes eletrocatalisadores. Os resultados mostraram que a atividade catalítica e a estabilidade podem ser melhoradas com o tratamento térmico, a despeito de um aumento do tamanho das partículas do catalisador, ou pela formação de carbetos nos suportes dos eletrocatalisadores. / Enhanced CO tolerance and stability of proton exchange membrane fuel cell (PEMFC) anode electrocatalysts is necessary for the improvement in the performance of PEM fuel cell system operating with reformate fuel. In this work carbon and carbides (molybdenum and tungsten carbides) supported electrocatalysts have been studied for the activity of hydrogen oxidation reaction (HOR), CO tolerance and stability in the anode of proton exchange membrane fuel cell. The materials investigated include: PtMo/C, PtMo/C heat-treated, Pt/Mo2C/C, PtMo/Mo2C/C, PtW/C, Pt/WC/C and Pt/C. Differences in electrocatalysts morphology were characterized by temperature programmed reduction (TPR), x-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), x-ray absorption spectroscopy (XAS), scanning electron microscopy (SEM) and wavelength dispersive spectroscopy (WDS). The electrochemical characteristics and activity of electrocatalysts were evaluated for the HOR and CO tolerance by single cell polarization measurements and cyclic voltammetry in the form of gas diffusion electrodes. Online mass spectrometry (OLMS) and CO stripping experiments were performed to evaluate the CO tolerance mechanism of the electrocatalysts. Cyclic voltammetry up to 5000 potential cycles was conducted to evaluate the electrocatalyst stability. It was observed that the carbon supported PtMo heat-treated at 600 °C with average crystallite size of 16.7 nm and Pt supported on Mo2C/C showed an enhanced stability and a good hydrogen electrooxidation activity in the presence of 100 ppm CO, as compared to as prepared carbon supported PtMo and molybdenum carbide supported PtMo electrocatalysts. Similarly, a better CO tolerance and improved stability was shown by tungsten carbide supported Pt electrocatalyst, as compared to carbon supported PtW electrocatalyst. Cyclic voltammetry and wavelength dispersive spectroscopy results showed that a partial dissolution of Mo and W and their migration/duffusion from the anode toward cathode take place during the cycling process, which is the major cause of performance losses of these electrocatalysts. On the basis of polarization measurements and cyclic voltammograms of both anodes and cathodes, it was concluded that the stability of these electrocatalysts may be improved either by heat-treatment or by the formation of carbides in the catalyst supports

Ce travail a pour objectif général de développer une compréhension approfondie de l’interaction du CO avec des catalyseurs anodiques dans les piles à combustible de type PEM (PEMFC), et d’évaluer son impact vis-à-vis de leur réactivité et stabilité lors de l’oxydation de l’hydrogène. Premièrement un modèle physique multi-échelle a été conçu pour simuler les performances de piles PEMFC alimentées par de l’hydrogène contenant des traces de CO. Il est basé sur la simulation Monte Carlo et la modélisation cinétique des étapes électrochimiques/chimie élémentaires. Une étude expérimentale de l’adsorption et de l’oxydation de CO simulant la technique d’ « O2 bleeding » a été utilisée pour mieux comprendre les mécanismes. Des catalyseurs de Pt ainsi que des bimétalliques PtxCoy et PtRu, supportés sur du carbone de grande aire spécifique, ont été étudiés. La spectroscopie IR (DRIFTS) et l’analyse QMS ont été utilisées pour l’étude de l’adsorption et oxydation de CO. Les défauts de surface, l’historique du catalyseur dans son interaction avec les différents gaz (H2, O2, CO), la température, la charge en Pt, la taille des particules, l’alliage de Pt avec Co ou Ru se sont révélé des paramètres clés dans la réactivité de CO avec O2. Le modèle multi-échelle a été appliqué aux catalyseurs Pt et PtxCoy. Les catalyseurs PtxCoy se révèlent plus tolérants au CO mais, en fonction du rapport Pt/Co, ils peuvent se dégrader par dissolution de Co comme démontré par nos expériences / The general objective of this work is to develop a deep understanding of the interaction of the CO with anodic catalysts in PEM Fuel Cells (PEMFCs), and to evaluate its impact on the reactivity towards the hydrogen oxidation and their stability. Firstly, a multiscale kinetic model is built up based on Monte Carlo simulation and kinetic modelling of elementary electrochemical/chemical steps as a tool to simulate the performance of PEMFCs fed with H2 containing CO traces. Experiments on CO adsorption and oxidation mimicking O2 bleeding were used to better understand the mechanisms. Monometallic Pt and bimetallic PtxCoy and PtRu catalysts supported on high surface area carbon were studied. CO adsorption and oxidation were investigated by means of DRIFT spectroscopy and QMS analysis. Defect sites (kink, edge), history of interaction with different gases (H2, O2, CO), temperature, Pt loading, particle size, alloying with Co or Ru are key parameters influencing the CO reactivity with O2. The multiscale kinetic model was applied to Pt and PtxCoy. PtxCoy nanocatalysts are shown to be highly CO tolerant but might degrade by Co dissolution in long term operation, depending on the Pt to Co ratio