Tuning the Nature of the Anion in Hydrated Layered Double Hydroxides for H-2 Production under Ionizing Radiation

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 similar to 3.5 and four different anions: NO3-, CO32-, ClO4-, and Cl- and chosen for their contrasted behavior toward 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 H-2 production. The LDH containing nitrate anions leads to low H-2 yields, as nitrate anions scavenge precursors of dihydrogen. Materials containing perchlorate (inert toward 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 H-2 amounts, with H-2 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.