H₂ Production in the Radiolysis of Water on CeO₂ and ZrO₂

The production of H2 in the radiolysis of water adsorbed on micron-sized particles of CeO2 and ZrO2 has been examined. Radiation chemical yields of H2 increase substantially with decreasing number of adsorbed water layers when the yield is determined with respect to the energy deposited directly by γ-rays to the water. These yields reached values of 20 and 150 molecules of H2 per 100 eV for one to two water layers on CeO2 and ZrO2, respectively, compared to 0.45 molecule/100 eV in bulk liquid water. The yields of H2 determined with respect to the total energy deposited in both the oxide and water were found to have a smaller, but observable, dependence on the amount of water adsorbed. Radiolysis of ZrO2 with γ-rays produced about 5 times more H2 than CeO2 for the equivalent amount of water adsorbed. The results suggest that the increase in H2 production is due to the transfer of energy, possibly by an exciton, from the oxide to the water. O2 production was at least an order of magnitude less than H2. The yield of H2 in the 5 MeV helium ion radiolysis of water on CeO2 is the same as with γ-rays, but the results with ZrO2 are substantially lower. The H2 yields with helium ion radiolysis may be nearly independent of the type of oxide.

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