Ab initio study on the mechanism of hydrogen release from the silicate surface in the presence of water molecule

The mechanism of the reaction of a water molecule with surface isolated hydroxyl of silica and silicates is investigated by ab initio molecular orbital calculations within a model reaction, H2O+H3SiOH, to figure out the role of water molecules in tritium release process. The hydrogen release from the surface isolated hydroxyl is found to proceed as a result of the Si–O bond breaking. Namely, tritium would be released by the hydroxyl-exchange reaction between water and surface hydroxyl of silicate, not by the hydrogen-exchange reaction. Prior to the exchange reaction, water molecules are found to prefer adsorbing as proton acceptors in the water – silanol complex, whereas the hydroxyl-exchange reaction occurs from a complex with a different form, in which the water molecule adsorbs as a proton donor. The overall potential energy barrier is calculated as 24.4 kcal mol−1 for the hydrogen release from the silicate in the presence of water molecules.