Decomposition of water by cerium oxide of δ-phase

Abstract

Reduced cerium dioxide (CeO 2−x) can reduce water, producing hydrogen at ⩾298 K. Kinetic studies were focused on the stoichiometric reaction of δ-phase cerium oxide (CeO 1.818) with water vapor. Different activation energies of 18.1 and 33.4 kJ mol −1 were observed for the reactions at the temperature ranges above and below ca. 453 K, respectively. Rate equations observed in the two temperature ranges were also different. These results strongly suggest that the rate-determining steps are different between the two temperature ranges. Rapid oxygen exchange observed between H 2 18O and lattice oxygen in cerium oxide of δ- phase at ⩾ 298 K indicated that neither the adsorption of water molecules not the diffusion of oxygen ions in the bulk of the oxide can be the rate-determining step. H 2D 2 exchange occurred rapidly at 373 K compared to the rate of water decomposition, suggesting that the recombination of hydrogen atoms on the surface is not rate- determining either. A tentative reaction mechanism was proposed to explain the results of the kinetic studies. The rate-determining step at high temperatures (>453 K) is the reduction of OH − by the six-coordinated Ce 3+ which is present in the nonstoichiometric cerium oxide, while that at low temperatures (<453 K) is the subsequent reduction of H + by the seven-coordinated Ce 3+.