Water Oxidation Catalysis Beginning with Co₄(H₂O)₂(PW₉O₃₄)₂^10– 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 O₂ beginning with the cobalt polyoxometalate Co₄(H₂O)₂(PW₉O₃₄)₂^10– (Co₄POM) and the chemical oxidant ruthenium­(III)­tris­(2,2′-bipyridine) (Ru­(III)­(bpy)₃³⁺). This specific water oxidation system was first reported in a 2010 <i>Science</i> paper (Yin et al. <i>Science</i> <b>2010</b>, <i>328</i>, 342). Under standard conditions employed herein of 1.0 μM Co₄POM, 500 μM Ru­(III)­(bpy)₃³⁺, 100 μM Ru­(II)­(bpy)₃²⁺, pH 7.2, and 0.03 M sodium phosphate buffer, the highest O₂ yields of 22% observed herein are seen when Ru­(II)­(bpy)₃²⁺ is added prior to the Ru­(III)­(bpy)₃³⁺ oxidant; hence, those conditions are employed in the present study. Measurement of the initial O₂ evolution and Ru­(III)­(bpy)₃³⁺ reduction rates while varying the initial pH, [Ru­(III)­(bpy)₃³⁺], [Ru­(II)­(bpy)₃²⁺], and [Co₄POM] indicate that the reaction follows the empirical rate law: −d­[Ru­(III)­(bpy)₃³⁺]/d<i>t</i> = (<i>k</i>₁ + <i>k</i>₂)­[Co₄POM]_soluble[Ru­(III)­(bpy)₃³⁺]/[H⁺], where the rate constants <i>k</i>₁ ∼ 0.0014 s^–1 and <i>k</i>₂ ∼ 0.0044 s^–1 correspond to the water oxidation and ligand oxidation reactions, and for O₂ evolution, d­[O₂]/d<i>t</i> = (<i>k</i>₁/4)­[Co₄POM]_soluble[Ru­(III)­(bpy)₃³⁺]/[H⁺]. Overall, at least seven important insights result from the present studies: (i) Parallel WOC and Ru­(III)­(bpy)₃³⁺ self-oxidation reactions well documented in the prior literature limit the desired WOC and selectivity to O₂ in the present system to ≤28%. (ii) The formation of a precipitate from ∼2 Ru­(II)­(bpy)₃²⁺/3 Co₄(H₂O)₂(PW₉O₃₄)₂^10– with a <i>K</i>_sp = (8 ± 7) × 10^–25 (M⁵) greatly complicates the reaction and interpretation of the observed kinetics, but (iii) the best O₂ yields are still when Ru­(II)­(bpy)₃²⁺ is preadded. (iv) CoO_<i>x</i> is 2–11 times more active than Co₄POM under the reaction conditions, but (v) Co₄POM is still the dominant WOC under the Co₄POM/Ru­(III)­(bpy)₃³⁺ 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 Co₄POM/Ru­(III)­(bpy)₃³⁺ system