Vue microscopique de l'adsorption d'hydrogène dans des matériaux nano-argileux de taille variable

Hydrogen is the carbon-free fuel par excellence because its combustion emits only water. At present, the limiting point to a massive deployment of technologies using hydrogen for energy purposes is its storage. Technologies based on sorption are among the most thoroughly explored. Little research has been done on clays for hydrogen storage, while the recent discovery of high hydrogen contents in clay-rich rocks shows their potential for it. The aim of the present study is to investigate, at the microscopic level, H₂ adsorption mechanisms on synthetic 2:1 trioctahedral smectites of nanometric dimensions, specifically Laponite, and its precursor, pre-Laponite. Starting with the hydrogen adsorption isotherms, we observed that pre-Laponite exhibits a higher H₂ sorption capacity than Laponite at 40K and 77K at 1bar. The first part of the thesis focuses on the characterization of synthesized samples. ICP-AES showed that Laponite and pre-Laponite share a similar molecular formula, while X-ray scattering revealed similar structure and that pre-Laponite exhibits reduced long-range order. IR spectroscopy points towards similar local order in both materials. X-ray absorption near-edge spectroscopy performed at the Si and Mg K-edges supports this conclusion.. ²⁹Si solid-state NMR provided additional insights, revealing three distinct silicon environments in both samples, with variations in signal area that correlate with differences in the edge-to-volume ratio between the two materials. Nitrogen adsorption isotherms further highlighted differences in textural properties: pre-Laponite exhibits a composite pore structure with both microporosity and macroporosity, whereas Laponite is primarily microporous with mesoporous contributions. The second part of the thesis concerns H₂ adsorption. The adsorption isotherms at 40 K were fitted with Langmuir and Freundlich contributions. Grand-Canonical Monte-Carlo simulations allowed one to infer that the Langmuir model, accounting for the strongest binding sites with monolayer saturation, describes adsorption within the interlayer, while the Freundlich model accounts for adsorption at the external surface. Inelastic neutron scattering experiments were undertaken to probe the para-to-ortho rotational transition of H₂, the energy of which is highly sensitive to its environment. At 40K, three adsorption sites were identified in pre-Laponite. The first could correspond to a hydrogen dissociation site, while the other two are physiosorption sites in the interlayer space. The pressure dependence of the 001-peak intensity in the neutron diffraction diagrams of Laponite and pre-Laponite reveals that interlayer H₂ forms a dense phase. Wide-angle neutron scattering, combined with atomistic simulations, was used to probe the structuration of D₂ (used in place of H₂ for improved signal-to-noise ratio) within this interlayer region. This thesis work contributes to a deeper understanding of H₂ adsorption mechanisms in layered clay materials, particularly highlighting the enhanced sorption capacity of pre-Laponite, and demonstrates how adsorption isotherms and neutron scattering techniques complement each other.