An all-inorganic, oxidatively and thermally stable, homogeneous water oxidation catalyst based on redox-active (vanadate(V)-centered) polyoxometalate ligands, Na10[Co4(H2O)2(VW9O34)2]·35H2O (Na101-V2, sodium salt of the polyanion 1-V2), was synthesized, thoroughly characterized and shown to catalyze water oxidation in dark and visible-light-driven conditions. This synthetic catalyst is exceptionally fast under mild conditions (TOF > 1 × 10(3) s(-1)). Under light-driven conditions using [Ru(bpy)3](2+) as a photosensitizer and persulfate as a sacrificial electron acceptor, 1-V2 exhibits higher selectivity for water oxidation versus bpy ligand oxidation, the final O2 yield by 1-V2 is twice as high as that of using [Co4(H2O)2(PW9O34)2](10-) (1-P2), and the quantum efficiency of O2 formation at 6.0 μM 1-V2 reaches ∼68%. Multiple experimental results (e.g., UV-vis absorption, FT-IR, (51)V NMR, dynamic light scattering, tetra-n-heptylammonium nitrate-toluene extraction, effect of pH, buffer, and buffer concentration, etc.) confirm that the polyanion unit (1-V2) itself is the dominant active catalyst and not Co(2+)(aq) or cobalt oxide.

Hydrogen sulfate salts of [Ru(1) 2 ] 2+ where 1 ¼ 4 0-(4-pyridyl)-2,2 0 :6 0 ,2 00-terpyridine and four N-alkylated derivatives [Ru(L) 2 ] 4+ were used as photosensitizers (l max $510 nm) for water oxidation in light driven reactions with peroxydisulfate as a sacrificial electron acceptor and Na 10 [Co 4 (H 2 O) 2 (a-PW 9 O 34) 2 ] (Co 4 POM) as the catalyst in sodium borate buffers at pH 8.0 and 9.0. The N-substituents investigated were benzyl (L + ¼ 2 +), ethyl (L + ¼ 3 +), allyl (L + ¼ 4 +) and 4-cyanobenzyl (L + ¼ 5 +). The O 2 yield in the presence of [Ru(L) 2 ] 4+ (L + ¼ 2 +-4 +) was comparable to that obtained in the presence [Ru(bpy) 3 ] 2+ (bpy ¼ 2,2 0bipyridine) using light sources with l max z 490 nm. The ruthenium(III) complexes [Ru(1) 2 ] 3+ and [Ru(L) 2 ] 5+ (L + ¼ 2 +-5 +) are rather unstable in acidic conditions and could not be isolated. The most efficient photosensitizers [Ru(L) 2 ] 5+ (L + ¼ 2 + and 4 +) were the least stable under weakly basic conditions (pH 9.0) with a half-life s 1/2 $ 10 ms. The stability of the complexes under photocatalytic turnover conditions is probably controlled by the rate at which ligand L + is oxidized by Co 4 POM in its highest oxidation state.

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We report a study on catalytic water oxidation by cobalt in oxygen ligand environments because such systems are as promising as any in the water oxidation component of solar fuel production. We have re-examined the catalytic activity of Co(II) in aqueous solution using either [Ru(bpy)3]3+ as a stoichiometric oxidant or in visible-light-driven reactions with persulfate as a sacrificial electron acceptor. In both systems a distinctive induction period is observed. A simple kinetic model is proposed that describes the experimental data well. The presence of an induction period is explained by relatively slow formation of the true catalyst from aquacobalt(II).

We have developed a model to study the kinetics of the redistribution of ions and molecules through a semipermeable membrane in complex mixtures of substances penetrating and nonpenetrating through a membrane. It takes into account the degree of dissociation of these substances, their initial concentrations in solutions separated by a membrane, and volumes of these solutions. The model is based on the assumption that only uncharged particles (molecules or ion pairs) diffuse through a membrane (and not ions as in the Donnan model). The developed model makes it possible to calculate the temporal dependencies of concentrations for all processing ions and molecules at system transition from the initial state to equilibrium. Under equilibrium conditions, the ratio of ion concentrations in solutions separated by a membrane obeys the Donnan distribution. The Donnan effect is the result of three factors: equality of equilibrium concentrations of penetrating molecules on each side of a membrane, dissociation of molecules into ions, and Le Chatelier's principle. It is shown that the Donnan distribution (irregularity of ion distribution) and accordingly absolute value of the Donnan membrane potential increases if: (i) the nonpenetrating salt concentration (in one of the solutions) and its dissociation constant increases, (ii) the total penetrating salt concentration and its dissociation constant decreases, and (iii) the volumes ratio increases (between solutions with and without a nonpenetrating substance). It is shown also that only a slight difference between the degrees of dissociation of two substances can be used for their membrane separation.