Electrochemically Driven Water-Oxidation Catalysis Beginning with Six Exemplary Cobalt Polyoxometalates: Is It Molecular, Homogeneous Catalysis or Electrode-Bound, Heterogeneous CoOxCatalysis?

A series of six exemplary cobalt-polyoxometalate (Co-POM) precatalysts have been examined to determine if they are molecular water-oxidation catalysts (WOCatalysts) or if, instead, they actually form heterogeneous, electrode-bound CoO x as the true WOCatalyst under electrochemically driven water-oxidation catalysis (WOCatalysis) conditions. Specifically, WOCatalysis derived from the following six Co-POMs has been examined at pH 5.8, 8.0, and 9.0: Co 4 (H 2 O) 2 (PW 9 O 34 ) 2 10– ( Co 4 P 2 W 18 ), Co 9 (H 2 O) 6 (OH) 3 (HPO 4 ) 2 (PW 9 O 34 ) 3 16– ( Co 9 P 5 W 27 ), ββ -Co 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 16– ( Co 4 P 4 W 30 ), Co(H 2 O)PW 11 O 39 5– ( CoPW 11 ), α 1 -Co(H 2 O)P 2 W 17 O 61 8– ( α 1 -CoP 2 W 17 ), and α 2 -Co(H 2 O)P 2 W 17 O 61 8– ( α 2 -CoP 2 W 17 ). The amount of Co(II) aq in 500 μM solutions of each Co-POM was measured after 3 h of aging as well as from t = 0 for pH = 5.8 and 8.0 by μM sensitive Co(II) aq -induced 31 P NMR line broadening and at pH = 9.0 by cathodic stripping. The amount of detectable Co(II) aq after 3 h for the six Co-POMs ranges from ∼0.25 to ∼90% of the total cobalt initially present in the Co-POM. For 12 out of 18 total Co-POM and different pH cases, the amount Co(II) aq detected after 3 h forms heterogeneous CoO x able to account for ≥100% of the observed WOCatalysis activity. However, under 0.1 M NaPi, pH 5.8 conditions for CoPW 11 and α 1 -CoP 2 W 17 where ∼1.5% and 0.25% Co(II) aq is detectable, the measured Co(II) aq cannot account for the observed WOCatalysis. The implication is that these two Co-POMs are primarily molecular, Co-POM-based, WOCatalysts under electrochemically driven, pH 5.8, phosphate-buffer conditions. Even for the single most stable Co-POM, α 1 -CoP 2 W 17, CoO x is still an estimated ∼76× faster WOCatalyst at pH = 5.8 and an estimated ∼740× faster WOCatalyst at pH = 8.