Electrocatalytic oxygen evolution from water on a Mn(III-V) dimer model catalyst--a DFT perspective

A complete water oxidation and oxygen evolution reaction (OER) cycle is monitored by means of density functional theory (DFT). A biomimetic model catalyst, comprising a μ-OH bridged Mn(III-V) dimer truncated by acetylacetonate ligand analogs and hydroxides is employed. The reaction cycle is divided into four electrochemical hydrogen abstraction steps followed by a series of chemical steps. The former employ the tyrosine/tyrosyl redox couple acting as electron and proton sink, thus determining the reference potential. Stripping hydrogen from water leads to the formation of two highly unstable Mn(V)=O/Mn(IV)-O˙ moieties, which subsequently combine to form a μ-peroxy O-O bond. O(2) evolution results from subsequent consecutive replacement of the remaining Mn-O bonds by water. A Zener "spintronic" type mechanism for virtually barrierless O(2) evolution is found. The applicability of DFT is discussed and extended to include the rate-limiting steps in the OER. Rather than attempting to compute transition states where KS-DFT is unreliable, an upper bound for the activation barrier of the O-O bond formation step is estimated from the hessians of the relevant intermediates.