La sûreté du risque hydrogène généré par radiolyse de matériaux organiques lors d'une phase de transport de déchets est une problématique majeure dans le domaine du nucléaire. L'utilisation de piégeurs irréversibles de H2 est envisagée afin de limiter le risque encouru. Le but de ce travail est d'étudier l'un de ces piégeurs, le mélange MnO2/Ag2O, afin de mieux comprendre le phénomène de piégeage. Dans un premier temps, différents paramètres influant sur la cinétique e piégeage ont été étudiés. Il a ainsi été déterminé que, parmi les différentes variétés allotropiques d'oxydes de manganèse, la nsutite possède la meilleure cinétique de piégeage de H2. La surface spécifique du piégeur améliore également la cinétique. La teneur massique en Ag2O dans le piégeur (promoteur du piégeage) a été déterminée à 13 %. La chimisorption de H2, requise pour l'application visée, a été mise en évidence grâce à des techniques de caractérisation telles que la magnétométrie, la spectroscopie infrarouge et de perte d'énergie des électrons. L'irréversibilité du piégeage et la régénération du piégeur après protonation ont été mis en évidence respectivement sous N2 et sous air à 150 °C. Le mécanisme d'insertion du proton dans MnO2 a été déterminé par analyse de fonction de distribution de paires, grâce à l'élaboration d'un modèle permettant de simuler le matériau réel, très complexe. La transformation d'Ag2O en Ag2CO3, lors de la préparation du piégeur sous eau, a également pu être mise en évidence grâce à cette technique d'analyse innovante.

This work computationally investigates the partial oxidation of ethylene (i.e. ethylene epoxidation) using periodic Density Functional Theory (DFT) on slab models that represent the catalyst surfaces. The mechanical aspects of the reaction were investigated on silver surfaces, which are industrially applied catalysts, for a wide range of surface models varying from metallic surfaces with low oxygen coverage to oxide surfaces. For comparison, the metallic and oxide phases of copper and gold were also studied. On these surfaces, the reaction paths and the transition states along these paths for the selective and non-selective reaction channels were obtained using the climbing image nudged elastic band (CI-NEB) method. In order to answer the question &ldquo / what is the relation between the surface state and the ethylene oxide selectivity?&rdquo / metallic (100), (110) and (111) surfaces of Cu, Ag and Au / and, (001) surfaces of Cu2O, Ag2O and Au2O oxides were studied and compared. For the studied metallic surfaces, it was found that the selective and non-selective reaction channels proceed through the oxametallacycle (OMC) intermediate, and the product selectivity depends on the relative barriers of the these channels, in agreement with the previous reports. However for the studied metallic surfaces and oxygen coverages, a surface state that favors the ethylene oxide (EO) formation was not identified. The studied Au surfaces did not favor the oxygen adsorption and dissociation, and the Cu surfaces favored the non-selective product (acetaldehyde, AA) formation. Nevertheless, the results of Ag surfaces are in agreement with the ~50% EO selectivity of the un-promoted silver catalyst. The catalyst surface in the oxide state was modeled by the (001) surfaces of the well defined Cu2O, Ag2O and Au2O oxide phases. Among these three oxides, the Cu2O is found not to favor EO formation whereas Au2O is known to be unstable, however selective for epoxidation. The major finding of this work is the identification of a direct epoxidation path that is enabled by the reaction of the surface oxygen atoms, which are in two-fold (i.e. bridge) positions and naturally exist on (001) oxide surfaces of the studied metals. Among the three oxides studied, only Ag2O(001) surface does not show a barrier for the formation of adsorbed epoxide along the direct epoxidation path. Moreover, the overall heat of reaction that is around 105 kJ/mol agrees well with the previous reports. The single step, direct epoxidation path is a key step in explaining the high EO selectivities observed. Also for the oxide surfaces, the un-selective reaction that ends up in combustion products is found to proceed through the OMC mechanism where aldehyde formation is favored. Another major finding of this study is that, for the studied oxide surfaces two different types of OMC intermediates are possible. The first possibility is the formation of the OMC intermediate on oxygen vacant sites, where the ethylene can interact with the surface metal atoms directly. The second possibility is the formation of a direct OMC intermediate, through the interaction of the gas phase ethylene with the non-vacant oxide surface. This occurs through the local surface reconstruction induced by the ethylene. The effect of Cl promotion was also studied. Coadsorption of Cl is found to suppress the oxygen vacant sites and also the reconstruction effects that are induced by ethylene adsorption. Thus, by preventing the interaction of the ethylene directly with the surface metal atoms, Cl prevents the OMC formation, therefore the non-selective channel. At the same time Cl increases the electrophilicity of reacting surface oxygen. The direct epoxidation path appears to be stabilized by coadsorbed oxygen atoms. Thus, we carry the discussions on the silver catalyzed ethylene epoxidation one step further. Herein we present that the EO selectivity will be limited in the case of metallic catalyst, whereas, the oxide surfaces enable a direct mechanism where EO is produced selectively. The role of the Cl promoter is found to be mainly steric where it blocks the sites of non-selective channel.