Water Radiolysis in Exchanged-Montmorillonites: The H₂ Production Mechanisms

The radiolysis of water confined in montmorillonites is studied as a function of the composition of the montmorillonite, the nature of the exchangeable cation, and the relative humidity by following the H₂ production under electron irradiation. It is shown that the main factor influencing this H₂ production is the water amount in the interlayer space. The effect of the exchangeable cation is linked to its hydration enthalpy. When the water amount is high enough to get a basal distance higher than 1.3 nm, then a total energy transfer from the montmorillonite sheets to the interlayer space occurs, and the H₂ production measured is very similar to the one obtained in bulk water. For a basal distance smaller than 1.3 nm, the H₂ production increases with the relative humidity and thus with the water amount. Lastly, electron paramagnetic resonance measurements evidence the formation of a new defect induced by ionizing radiation. It consists of a hydrogen radical (H₂ precursor) trapped in the structure. This implies that structural hydroxyl bonds can be broken under irradiation, potentially accounting for the observed H₂ production