Water Oxidation Catalysis Beginning with Co₄(H₂O)₂(PW₉O₃₄)₂¹⁰– When Driven by the Chemical Oxidant Ruthenium(III)tris(2,2′-bipyridine): Stoichiometry, Kinetic, and Mechanistic Studies en Route to Identifying the True Catalyst

Stoichiometry and kinetics are reported for catalytic water oxidation to O 2 beginning with the cobalt polyoxometalate Co 4 (H 2 O) 2 (PW 9 O 34 ) 2 10– (Co 4 POM) and the chemical oxidant ruthenium(III)tris(2,2′-bipyridine) (Ru(III)(bpy) 3 3+ ). This specific water oxidation system was first reported in a 2010 Science paper (Yin et al. Science 2010, 328, 342). Under standard conditions employed herein of 1.0 μM Co 4 POM, 500 μM Ru(III)(bpy) 3 3+, 100 μM Ru(II)(bpy) 3 2+, pH 7.2, and 0.03 M sodium phosphate buffer, the highest O 2 yields of 22% observed herein are seen when Ru(II)(bpy) 3 2+ is added prior to the Ru(III)(bpy) 3 3+ oxidant; hence, those conditions are employed in the present study. Measurement of the initial O 2 evolution and Ru(III)(bpy) 3 3+ reduction rates while varying the initial pH, [Ru(III)(bpy) 3 3+ ], [Ru(II)(bpy) 3 2+ ], and [Co 4 POM] indicate that the reaction follows the empirical rate law: −d[Ru(III)(bpy) 3 3+ ]/d t = ( k 1 + k 2 )[Co 4 POM] soluble [Ru(III)(bpy) 3 3+ ]/[H + ], where the rate constants k 1 ∼ 0.0014 s –1 and k 2 ∼ 0.0044 s –1 correspond to the water oxidation and ligand oxidation reactions, and for O 2 evolution, d[O 2 ]/d t = ( k 1 /4)[Co 4 POM] soluble [Ru(III)(bpy) 3 3+ ]/[H + ]. Overall, at least seven important insights result from the present studies: (i) Parallel WOC and Ru(III)(bpy) 3 3+ self-oxidation reactions well documented in the prior literature limit the desired WOC and selectivity to O 2 in the present system to ≤28%. (ii) The formation of a precipitate from ∼2 Ru(II)(bpy) 3 2+ /3 Co 4 (H 2 O) 2 (PW 9 O 34 ) 2 10– with a K sp = (8 ± 7) × 10 –25 (M 5 ) greatly complicates the reaction and interpretation of the observed kinetics, but (iii) the best O 2 yields are still when Ru(II)(bpy) 3 2+ is preadded. (iv) CoO x is 2–11 times more active than Co 4 POM under the reaction conditions, but (v) Co 4 POM is still the dominant WOC under the Co 4 POM/Ru(III)(bpy) 3 3+ and other reaction conditions employed. The present studies also (vi) confirm that the specific conditions matter greatly in determining the true WOC and (vii) allow one to begin to construct a plausible WOC mechanism for the Co 4 POM/Ru(III)(bpy) 3 3+ system.