Water Oxidation Catalysis Beginning with 2.5 μM [Co₄(H₂O)₂(PW₉O₃₄)₂]^10–: Investigation of the True Electrochemically Driven Catalyst at ≥600 mV Overpotential at a Glassy Carbon Electrode

Evidence for the true water oxidation catalyst (WOC) when beginning with the cobalt polyoxometalate [Co₄(H₂O)₂(PW₉O₃₄)₂]^10– (Co₄–POM) is investigated at deliberately chosen low polyoxometalate concentrations (2.5 μM) and high electrochemical potentials (≥1.3 V vs Ag/AgCl) in pH 5.8 and 8.0 sodium phosphate electrolyte at a glassy carbon working electrodeconditions which ostensibly favor Co₄–POM catalysis if present. Multiple experiments argue against the dominant catalyst being CoO_<i>x</i> formed exclusively from Co²⁺ dissociated from the parent POM. Measurement of [Co²⁺] in the Co₄–POM solution and catalytic controls with the corresponding amount of Co­(NO₃)₂ cannot account for the O₂ generated from 2.5 μM [Co₄(H₂O)₂(PW₉O₃₄)₂]^10– solutions. This result contrasts with our prior investigation of Co₄–POM under higher concentration and lower potential conditions (i.e., 500 μM [Co₄(H₂O)₂(PW₉O₃₄)₂]^10–, 1.1 V vs Ag/AgCl, as described in Stracke, J. J.; Finke, R. G. <i>J. Am. Chem. Soc.</i> <b>2011</b>, <i>133</i>, 14872) and <i>highlights the importance of</i> <i>reaction</i> <i>conditions in governing the identity of the true, active WOC.</i> Although electrochemical studies are consistent with Co₄–POM being oxidized at the glassy carbon electrode, it is not yet possible to distinguish a Co₄–POM catalyst from a CoO_<i>x</i> catalyst formed via decomposition of Co₄–POM. Controls with authentic CoO_<i>x</i> indicate conversion of only 3.4% or 8.3% (at pH 8.0 and 5.8) of Co₄–POM into a CoO_<i>x</i> catalyst could account for the O₂-generating activity, and HPLC quantification of the Co₄–POM stability shows the postreaction Co₄–POM concentration decreases by 2.7 ± 7.6% and 9.4 ± 5.1% at pH 8.0 and 5.8. Additionally, the [Co²⁺] in a 2.5 μM Co₄–POM solution increases by 0.55 μM during 3 min of electrolysisfurther evidence of the <i>Co</i>_<i>4</i><i>-POM instability under oxidizing conditions</i>. Overall, this study demonstrates the challenges of identifying the true WOC when examining micromolar amounts of a partially stable material and when <i>nanomolar</i> heterogeneous metal-oxide will account for the observed O₂-generating activity