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Hydroxide Oxidation and Peroxide Formation at Embedded Binuclear Transition Metal Sites; TM=Cr, Mn, Fe, Co

Journal article
Authors Michael Busch
Elisabet Ahlberg
Itai Panas
Published in Physical Chemistry Chemical Physics
Volume 13
Pages 15062-15068
ISSN 1463-9076
Publication year 2011
Published at Department of Chemistry
Pages 15062-15068
Language en
Links xlink.rsc.org/?doi=C1CP20487D
Keywords Water oxidation, electrocatalysts, test rig, magnesium oxyhydroxide support, binuclear site, activation energy, catalysis
Subject categories Quantum chemistry, Other Chemistry Topics, Other Chemical Engineering, Surface engineering

Abstract

Key steps in electro-catalytic water oxidation on binuclear Transition Metal (TM) sites are addressed. These comprise (a) two one-electron oxidation steps of TM-OH moieties to form the corresponding two TM=O oxy-groups, and (b) a chemical step whereby the two oxy-species form a TM-O-O-TM peroxy-bridge. A test rig representing a generic low crystal field oxide support is described and employed. The energetics for said reactions are compared for homo-nuclear Cr(IIIV), Mn(III-V), Fe(II-IV) and Co(II-IV) sites. The uniqueness of the TyrO/TyrOH reference potential for driving said oxidation steps is demonstrated. Hydroxide oxidation on binuclear Mn and Co candidates require an overpotential of approximately 0.5 V relative to the chosen reference potential. Correspondingly, the subsequent O-O bond formation becomes strongly exothermic of the order of 1 eV. Hydroxide oxidation on binuclear CrCr and FeFe systems are exothermic by 0.2-0.4 eV relative to the TyrO/TyrOH reference potential. Consequently, the chemical step for transforming the TM=O moieties to the peroxo species is found to be endothermic on the order of 0.7 eV. These findings suggest that hetero-binuclear sites containing one TM from each class would be optimal. The validity of this concept is demonstrated for the FeCo binuclear site. The results are discussed in the context of experimental observations, which display preference for mixed oxide systems.

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