Radiolysis of confined water: Molecular hydrogen formation

The formation of molecular hydrogen in the radiolysis of water confined in nanoscale pores of well-characterised porous silica glasses and mesoporous molecular sieves (MCM-41) is examined. The comparison of dihydrogen formation by irradiation of both materials, dry and hydrated, shows that a large part of the H-2 comes from the surface of the material. The radiolytic yields, G(H-2) = (3 +/- 0.5) x 10(-7) mol J(-1), calculated using the total energy deposited in the material and the water, are only slightly affected by the degree of hydration of the material and by the pore size. These yields are also not modified by the presence of hydroxyl radical scavengers. This observation proves that the back reaction between H-2 and HO center dot is inoperative in such confined environments. Furthermore, the large amount of H-2 produced in the presence of different concentrated scavengers of the hydrated electron and its precursor suggests that these two species are for from being the only species responsible for the H-2 formation. Our results show that the radiolytic phenomena that occur in water confined in nonoporous silica are dramatically different to those in bulk water, suggesting the need to investigate further the chemical reactivity in this type of environment.