Transition metal heteropolyanions have been used to catalyze a variety of organic oxidations but have not previously been used for O2 generation, despite sharing some structural similarities with dioxoruthenium water-oxidation catalysts. In this study, we report that the di-Ru-substituted polyoxometalate (POM) [Ru2Zn2(H2O)2(ZnW9O34)2]14- can be used to catalyze the electrochemical generation of O2. By comparing the behavior of this compound to that observed using a mono-Ru-substituted POM catalyst, we show that adjacent Ru sites are necessary to observe O2 generation. These observations suggest a reaction pathway involving two Ru-bound oxygen species combining to form O2 and are consistent with the accepted mechanism of electrochemical oxygen evolution. Finally, analysis of the observed electrode kinetics yields a Tafel slope of roughly 120 mV, which is similar to values reported previously for perovskite anodes.

Cyclic voltammetry and chronoamperometry have been used to characterize the translational mobility of eight different redox probes trapped in hydrated silica gel monoliths and evaluate the extent of surface interactions. The redox probes, selected for their size and charge, were introduced into a silica sol prepared by the acid-catalyzed hydrolysis of tetramethoxysilane along with an ultramicroelectrode (r = 13 microns) and a Ag/AgCl reference/counter electrode. Relative changes in the magnitude of the Faradaic current, the half-wave potential, E1/2, and the apparent diffusion coefficient (D) were evaluated for each redox probe as the sol gelled, aged, and dried. Significant variations were observed in the diffusion rates between molecules of similar size and structure but varying ionic charge. Large shifts in the redox potential were also observed, with the direction of shift dependent on the entrapped reagent. These results demonstrate the importance of internal surface interactions versus surface confinement in wet and partially dried sol-gel glasses.

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