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Hydrogen Storage Properties of cobalt oxysulfide


Congress title :9th European Symposium on Thermal Analysis and Calorimetry (ESTAC9)
Congress location :Cracovie
Congress date :27/08/2006

Summary

  The irradiation of organic molecules produces mainly hydrogen. In the context of nuclear waste management, the release of hydrogen can generate safety problems. However, it has been shown that amorphous cobalt oxysulfide decreases this hydrogen production.
            When irradiating organic molecules with a proton beam, we showed that cobalt oxysulfide reacted neither as a catalyst of hydrogenation of unsaturated hydrocarbons, nor as a radical scavenger or a trap of energy, but was an hydrogen trap. We also observed that the hydrogen consumption by the cobalt oxysulfide was 1) the same whether the solid was irradiated or not, 2) proportional to the initial hydrogen pressure.
Because irradiations experiments produced low partial pressures of hydrogen and since the irradiation of the solid does not modify the hydrogen consumption, we performed experiments of "pure hydrogen sorption" using two types of experiments: on one hand, by thermogravimetry at ambient temperature, under a flow of helium and hydrogen at atmospheric pressure (H2 content being controlled from 42 to 500 hPa), on the other hand, under static atmosphere, in closed cells allowing to follow the evolution of the hydrogen pressure as a function of time (initial hydrogen pressure ranging between 100 and 1800 hPa). Moreover, the solid was characterized after contact with hydrogen (infra-red and Raman spectroscopy, X-ray diffraction).
All these experiments led us to propose a simple model of dissolution of hydrogen by the solid allowing to adjust the experimental curves with a good adequacy. This model describes the phenomenon of hydrogen sorption by the solid with no assumption of a reactional mechanism (adsorption, reaction, diffusion...) because it is based on only one step: H2 (gas) D H2 (solid) with the kinetic constants k and k' (K=k/k', equilibrium constant). We established two expressions giving the quantity of hydrogen (mole) fixed at time t for the two types of experiments. The fit of the experimental curves enables us to obtain the values of the kinetic constants k and k'. We observed a good invariability of the value of k but the value of k' is not constant. According to the results of characterization of the solid after hydrogen storage, an improvement of the model is proposed, taking into account a chemical reaction of transformation of the solid.
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