Identifying MnVII-oxo Species during Electrochemical Water Oxidation by Manganese Oxide

Biaobiao Zhang,Quentin Daniel,Lizhou Fan,Tianqi Liu,Qijun Meng,Licheng Sun

doi:10.1016/j.isci.2018.05.018

Biaobiao Zhang, Quentin Daniel + Show 4 more

Open Access

https://doi.org/10.1016/j.isci.2018.05.018

Copy DOI

Abstract

SummaryIdentifying surface active intermediate species is essential to reveal the catalytic mechanism of water oxidation by metal-oxides-based catalysts and to develop more efficient catalysts for oxygen-oxygen bond formation. Here we report, through electrochemical methods and ex situ infrared spectroscopy, the identification of a MnVII = O intermediate during catalytic water oxidation by a c-disordered δ-MnOx with an onset-potential-dependent reduction peak at 0.93 V and an infrared peak at 912 cm−1. This intermediate is proved to be highly reactive and much more oxidative than permanganate ion. Therefore, we propose a new catalytic mechanism for water oxidation catalyzed by Mn oxides, with involvement of the MnVII = O intermediate in a resting state and the MnIV−O−MnVII = O as a real active species for oxygen-oxygen bond formation.

Highlights

The continuous extraction of electrons and protons from water is a key step in sustaining life on Earth, and research on such processes is crucial for developing renewable energy systems via artificial photosynthesis (Walter et al, 2010)
Water oxidation occurs at the Mn4CaO5 cluster in photosystem II (PSII), with a low overpotential of around 160 mV and high rate of 100–400 sÀ1(Cox et al, 2013; Najafpour et al, 2016; Umena et al, 2011; Limburg et al, 1999)
Mn complexes and oxides have been developed as promising water-oxidation catalysts (WOCs) (Najafpour et al, 2016; Limburg et al, 1999), the activity gaps between artificial catalysts and the Mn4CaO5 cluster are large (Najafpour et al, 2016)

Summary

Introduction

The continuous extraction of electrons and protons from water is a key step in sustaining life on Earth, and research on such processes is crucial for developing renewable energy systems via artificial photosynthesis (Walter et al, 2010). MnIV = O, and MnV = O species are widely accepted as key intermediates for both Mn4CaO5 clusters and synthetic catalysts (Cox et al, 2013; Zahran et al, 2016; Najafpour et al, 2016; Jin et al, 2017) These mechanisms do not reflect the unique redox chemistry of Mn (five valences, varying from MnII to MnVII, incorporation of many disproportionation and comproportionation reactions, moderate oxidation potentials from MnII to MnVII) and the fact that low-valent MnIII species and high-valent MnO4À species are usually observed during water-oxidation catalysis (Takashima et al, 2012; Yagi and Narita, 2004; Limburg et al, 1997, 1999). This new information on water oxidation with a Mn-based catalyst might help designing more efficient Mn-based WOCs for artificial photosynthesis

Methods

Results

Discussion

Conclusion