Key intermediate of water oxidation and oxygen reduction identified

The research group of Prof. Antoni Llobet used a combination of techniques to trap a superoxido intermediate involved in the water oxidation reaction.

Water oxidation is often considered the bottleneck of the water splitting process. Years ago, Prof. Llobet was among the first chemists to achieve this reaction using a bimetallic ruthenium molecular catalyst. Later, he also described an analogous catalyst based on cobalt, an earth abundant metal that is preferred from an environmental and economic point of view. Now, he led the team of researchers that discovered a derivative superoxido dicobalt complex that provides unique information for the understanding of the mechanism of the reaction.

Raman resonance spectra of the peroxido and superoxido cobalt complexes.

Scientists have used a myriad of techniques to ‘capture’ this elusive intermediate. Among them, they used Raman spectroscopy to identify the characteristic peaks associated with the O-O bond vibration of peroxido and superoxido species. Llobet and co-workers used labelled O₂ to further confirm the existence of this complex. The team also carried out DFT calculations, EPR and X-ray absorption spectroscopy analyses that supported their suggested mechanism.

To demonstrate that the oxygen atoms coordinated to the cobalt were ultimately the same involved in the oxygen evolution step, researchers used different oxygen isotopologues and on-line mass spectrometry. When pushing the reaction forward using an external oxidiser, they observed that the isotopic profile of O₂ gas matched the ratios in the labelled starting materials.

These results are important to better understand the mechanism behind the oxygen evolution reaction, and may provide a model of the behaviour of cobalt oxide. Furthermore, they demonstrate for the first time that the same catalyst can be used for the water oxidation reaction as well as for its inverse reaction: the reduction of O₂ to water, which is critical for construction of fuel cell devices.

Structural and spectroscopic characterization of reaction intermediates involved in a dinuclear Co-Hbpp water oxidation catalyst.
C. Gimbert-Suriñach, D. Moonshiram, L. Francàs, N. Planas, V. Bernales, F. Bozoglian, A. Guda, L. Mognon, I. López, M.A. Hoque, L. Gagliardi, C.J. Cramer and A. Llobet.
J. Am. Chem. Soc., 2016, DOI: 10.1021/jacs.6b08532