Water Oxidation on MnOx and IrOx: Why Similar Performance?

The critical steps in water oxidation at a binuclear Mn (II–IV) oxide site are revisited. Ideal stabilities of intermediates are confirmed by comparing to results for a binuclear Ir (III–V) system. The latter in turn is known to be an excellent water oxidation catalyst. The inefficiency of the binuclear Mn (II–IV) site is owing to the high activation energy for the chemical step whereby Mn IV ═O double bonds on adjacent sites are broken prior to forming the Mn III —O—O—Mn III peroxy moiety. A rationale for Mn (II–IV) —Mn (III–V) mixed oxidation state for water oxidation catalysis, analogous to mixed transition metal oxide systems, is offered. Possible virtues of the kinetic stability of the binuclear Mn IV ═O moiety are discussed, utilizing its oxidizing power by sidestepping oxygen evolution.