Efficient solar-driven catalytic water oxidation is one of the main challenges in solar-to-fuel conversion. In this
proceeding, we investigate three approaches for constructing electron acceptor -sensitizer - catalysts systems for
photocatalytic water oxidation and our current understanding of the relevant fundamental processes involved. We
demonstrated that an all-inorganic molecular water oxidation catalyst (WOC), [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10-
(or Ru4POM), catalyzed a homogenous O2 evolution system with 27% quantum efficiency in homogeneous solution in
the presence of sensitizer (Ru(bpy)3) and sacrificial electron donor.1 This suggests the feasibility of a heterogeneous
photoelectrochemical system in which the photoanode integrates all three components: electron acceptor,
photosensitizer, and WOC. We prepare a photocatalytic electrode based on Ru4POM and a dye-sensitized nanoporous
TiO2 film for efficient light-harvesting and charge separation. Ultrafast spectroscopic studies of this triadic
nanocomposite indicate efficient charge separation from the excited sensitizer to TiO2 and efficient regeneration of the
ground state of the dye. The latter can be attributed to Ru4POM oxidation by the photogenerated dye cation and has a
yield of > 80% within 1 ns.
New multi-cobalt-containing polyoxometalates (POMs) are soluble, fast and tunable water oxidation catalysts (WOCs).
We report additional studies of [Co4(H2O)2(PW9O34)2]10- (1), a very fast, soluble and oxidative stable WOC: new kinetics
data further indicate that Co2+ is not kinetically important in water oxidations catalyzed by 1. Second, we report a new
WOC, [{Co4(ì-OH)(H2O)3}(Si2W19O70)]11- that coexists in a 1:1 ratio in the solid state (2a and 2b), and while it is
oxidatively stable, it is not hydrolytically stable, rearranging to [Co(H2O)SiW11O39]6- in aqueous solution. All these
studies provide insights relating structural, electronic and other features of these WOCs to their reactivity and stability.
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