Electrocatalytic oxygen evolution from water on a Mn(iii–v) dimer model catalyst—A DFT perspective

Abstract

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.

Redox and electrochemical water splitting catalytic properties of hydrated metal oxide modified electrodes

“…31 is analogous to those depicted for various homogeneous catalyst systems. 316,323,324 This is not unexpected given the very dispersed and somewhat tenuous nature of the catalytically active hydrous oxide layer. A common feature of these reaction schemes is that the initial catalytic step involves the deprotonation of a metal coordinated water molecule.…”

“…In this way our mechanistic thinking is guided by the earlier work of Kobussen and Broers, 296,301 but also takes into account recent developments in DFT calculations 270,271,335 and homogeneous OER catalysis. 323,324 The results of a comprehensive kinetic analysis of pathway D are summarised in Table 3 where the diagnostic parameters for each step, when it is assumed to be rate-determining, are presented for low and high surface coverage of intermediates. By way of an example, we now outline a sample kinetic analysis for the primary steps in pathway D. First, we note that the first step in the reaction sequence occurs rapidly and need not be included in the steady-state kinetic analysis.…”

“…In actual fact, the degree of radical character has been shown to depend on the length of the metal oxo bond with M(V)QO being more stable for shorter bond lengths. 322,323 In the case of Fe, it is likely that the metal oxo species involves an Fe(V) metal centre. Indeed, a recent variable temperature mass spectrometry investigation has identified an Fe(V) oxo species as the catalytic centre in a biomimetric non-heme Fe complex.…”

Hydrous Nickel Oxide: Redox Switching and the Oxygen Evolution Reaction in Aqueous Alkaline Solution

“…This scheme is analogous to those depicted for various different photocatalyst systems. [51][52][53] A common feature of these schemes is that the starting point for the OER catalytic cycle is usually represented as a metal coordinated water molecule. However, in the strongly alkaline conditions used in this system it is likely that a significant proportion of these coordinated water molecules will be deprotonated.…”

Redox Switching and Oxygen Evolution at Hydrous Nickel Oxide Films in Aqueous Alkaline Solution

“…This scheme is analogous to those depicted for various different photocatalyst systems (54)(55)(56). A common feature of these schemes is that the starting point for the OER catalytic cycle is usually represented as a metal coordinated water molecule.…”