Tuning the Nature of the Anion in Hydrated Layered Double Hydroxides for H2 Production under Ionizing Radiation
Résumé
Layered double hydroxides (LDHs) are adsorbent materials able to store significant amounts of anionic radionuclides under various levels of hydrations. In view of this storage application, it is crucial to understand their evolution under ionizing radiation, particularly as a function of their hydration state. We thus synthesized four LDHs, with Mg/Al molar ratio around 3.5 and four different anions: NO3-, CO32-, ClO4- and Cl- chosen for their contrasted behavior towards ionizing radiation. For all these materials, the initial interlamellar distance increases slightly within the first step of relative humidity (RH), 0-3%, and remains constant in the 3-74% RH range. Infrared spectroscopy reveals a lowering of the anion symmetry when it is confined in LDHs, caused by the interaction with surface OH groups and water molecules together with a decrease of degrees of freedom upon confinement. When RH increases, the environment of the anions becomes more and more symmetric. Under ionizing radiation, the nature of the anion in the interlamellar space and the hydration state control the H2 production. The dry LD containing nitrate anions leads to very low H2 yields as nitrate anions scavenge precursors of dihydrogen. Materials containing perchlorate (inert towards ionizing radiation) and carbonate (hydroxyl scavenger) anions lead to very similar H2 yields which increase in the 0-3% RH range, and then remain stable in the 3-74% range, with a maximum value approximately half of that obtained for bulk water. This shows that the material itself contains hydroxyl scavengers. The sample containing chloride anions produces the highest H2 amounts, with H2 yields twice higher than the yield measured in bulk water. This peculiar behavior can be linked to specific reactions involving confined chloride anions. This work shows that the anion of the LDH sample can be selected depending on the applications.