Metal oxides as heterogeneous catalysts for oxygen evolution under photochemical conditions
Metal oxides, in the form of dispersed powders, have been tested as potential catalysts for the four-electron oxidation of water to O2 under photochemical conditions. The most efficient catalysts were found to be IrO2, Co3O4, RuO2, NiCo2O4, Rh2O3 and Mn2O3 and, in particular, high activity was observed with IrO2. Comparison of the oxide structure with its observed rate of O2 generation under standard conditions has allowed formulation of a few general requisites for an effective catalyst. Samples of iridium oxide deposited onto the surface of a second (inert) oxide were tested for their O2-evolving capability. The efficiency of the system depended markedly upon the nature of the support. Materials that favour formation of small deposits of iridium oxide (e.g. ZnO, MgO, TiO2) are the best supports, whilst O2 production is almost completely inhibited with acidic supports. Many metal oxides can be prepared in the form of hydrates of variable composition. These materials also function as O2-evolving catalysts, the efficiency of the process depending upon any thermal pretreatment. This finding is explained in terms of changes in structure and composition of the oxide that occur upon heating.