A molecular catalyst for water oxidation that binds to metal oxide surfaces.

Stafford W Sheehan,Robert H Crabtree,Gary W Brudvig,Ulrich Hintermair,Charles A Schmuttenmaer,Julianne M Thomsen

doi:10.1038/ncomms7469

Stafford W Sheehan, Robert H Crabtree + Show 4 more

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https://doi.org/10.1038/ncomms7469

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Abstract

Molecular catalysts are known for their high activity and tunability, but their solubility and limited stability often restrict their use in practical applications. Here we describe how a molecular iridium catalyst for water oxidation directly and robustly binds to oxide surfaces without the need for any external stimulus or additional linking groups. On conductive electrode surfaces, this heterogenized molecular catalyst oxidizes water with low overpotential, high turnover frequency and minimal degradation. Spectroscopic and electrochemical studies show that it does not decompose into iridium oxide, thus preserving its molecular identity, and that it is capable of sustaining high activity towards water oxidation with stability comparable to state-of-the-art bulk metal oxide catalysts.

Highlights

Molecular catalysts are known for their high activity and tunability, but their solubility and limited stability often restrict their use in practical applications
In the pursuit of combining the high efficiency of the molecular hom-water-oxidation catalysts (WOCs) with the stability of bulk metal oxides, we found that when an oxide material is immersed in an aqueous solution of the hom-WOC, the material rapidly and irreversibly chemisorbs some of the bluecoloured complex from the solution
Formation of a molecular monolayer is complete in 2 h at room temperature, with negligible absorption changes being observed at later times (Fig. 2b)

Summary

Introduction

Molecular catalysts are known for their high activity and tunability, but their solubility and limited stability often restrict their use in practical applications. Of the extensive library of available WOCs, molecular species show promise because of their high activity and tunability, as well as their ability to be integrated into sophisticated molecular assemblies[5,6,7,8,9,10,11,12] Their major drawback is their limited stability, with the best homogeneous systems providing turnover numbers in the thousands[13,14,15] to tens of thousands[16]. Cp*Ir precursors lacking a stable bidentate ligand anodically deposit amorphous IrOx on electrodes to give a heterogeneous WOC referred to as ‘blue layer’ (BL)[38] This BL, hydrated IrOx nanoclusters[40], and the homogeneous WOCs formed by the oxidative activation of our organometallic iridium precursors[41,42] all display a characteristic deep blue colour in their oxidized form, owing to an absorption feature near 600 nm. We show that this heterogenized molecular catalyst remains bound to the surface after extended use, eliminating the need for any linking moieties, while retaining its molecular identity and ligand-based tunability

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