Design of an inherently-stable water oxidation catalyst

Biswarup Chakraborty,Manoj Raula,Gal Gan-Or,Eyal Gadot,Ira A. Weinstock

doi:10.1038/s41467-018-07281-z

Biswarup Chakraborty, Manoj Raula + Show 3 more

Open Access

https://doi.org/10.1038/s41467-018-07281-z

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Journal: Nature Communications	Publication Date: Nov 20, 2018
Citations: 61	License type: open-access

Affiliation: Ben-Gurion University of the Negev

Abstract

While molecular water-oxidation catalysts are remarkably rapid, oxidative and hydrolytic processes in water can convert their active transition metals to colloidal metal oxides or hydroxides that, while quite reactive, are insoluble or susceptible to precipitation. In response, we propose using oxidatively-inert ligands to harness the metal oxides themselves. This approach is demonstrated by covalently attaching entirely inorganic oxo-donor ligands (polyoxometalates) to 3-nm hematite cores, giving soluble anionic structures, highly resistant to aggregation, yet thermodynamically stable to oxidation and hydrolysis. Using orthoperiodate (at pH 8), and no added photosensitizers, the hematite-core complex catalyzes visible-light driven water oxidation for seven days (7600 turnovers) with no decrease in activity, far exceeding the documented lifetimes of molecular catalysts under turnover conditions in water. As such, a fundamental limitation of molecular complexes is entirely bypassed by using coordination chemistry to harness a transition-metal oxide as the reactive center of an inherently stable, homogeneous water-oxidation catalyst.

Highlights

While molecular water-oxidation catalysts are remarkably rapid, oxidative and hydrolytic processes in water can convert their active transition metals to colloidal metal oxides or hydroxides that, while quite reactive, are insoluble or susceptible to precipitation
Their catalytically active transition metals are in equilibrium with trace concentrations in the aqueous solvent[20], which can lead to hydrolysis under nonoptimal conditions[18]
One reason for this heightened level of concern is that colloidal metal oxides can be extremely active[21,22,23], such that considerable efforts are required to prove that the molecular catalyst, and not its products of decomposition in water, are the kinetically competent species in catalytic water oxidation

Summary

Introduction

While molecular water-oxidation catalysts are remarkably rapid, oxidative and hydrolytic processes in water can convert their active transition metals to colloidal metal oxides or hydroxides that, while quite reactive, are insoluble or susceptible to precipitation. We propose using oxidatively-inert ligands to harness the metal oxides themselves This approach is demonstrated by covalently attaching entirely inorganic oxo-donor ligands (polyoxometalates) to 3-nm hematite cores, giving soluble anionic structures, highly resistant to aggregation, yet thermodynamically stable to oxidation and hydrolysis. As advances in ligand design have led to impressively rapid rates and longer catalyst lifetimes under turnover conditions, catalyst stability remains an ongoing topic of discussion and experiment[1,10,20] One reason for this heightened level of concern is that colloidal metal oxides can be extremely active[21,22,23], such that considerable efforts are required to prove that the molecular catalyst, and not its products of decomposition in water, are the kinetically competent species in catalytic water oxidation. As an inherently stable visible-light activated water-oxidation catalyst, 1 is capable of continuous operation for 7 days under turnover conditions (corresponding to 7600 turnovers) with no decrease in activity

Methods

Results

Conclusion