Catalytic Water Oxidation Mechanisms and Molecular Catalysts

Catalytic water oxidation, in which water is converted to molecular oxygen with concurrent release of protons and electrons, constitutes the crucial oxidative half‐reaction in artificial photosynthesis and renewable fuel production. This four‐electron, four‐proton process demands catalysts that operate at low overpotentials, exhibit high turnover frequencies and numbers, and maintain structural integrity under oxidative conditions. Molecular catalysts—typically coordination complexes of ruthenium, iridium or first‐row transition metals—offer precise control over electronic and steric environments through ligand design, facilitating mechanistic dissection of key steps such as O–O bond formation. Two principal mechanistic pathways dominate: water nucleophilic attack (WNA) on a high‐valent metal–oxo intermediate and bimolecular coupling of two metal–oxo units (I2M). In recent years, strategies to lower energy barriers have centred on proton‐coupled electron transfer (PCET), redox‐active (non-innocent) ligands, second-sphere interactions and heterogenisation of molecular species on conductive oxide supports. Advances in these areas have driven progress towards scalable, robust systems for solar-driven water splitting and sustainable fuel generation.

Research from Nature Portfolio

A heterogenised iridium complex has been engineered to bind directly to metal oxide surfaces without ancillary linkers, yielding a water oxidation catalyst with minimal overpotential and sustained turnover frequency on conductive electrodes. Spectroscopic and electrochemical analyses confirm retention of the discrete molecular structure, rather than transformation into bulk oxide, and demonstrate operation with stability comparable to state-of-the-art inorganic catalysts.

A ruthenium complex bearing a tetradentate sulfonate ligand and two picoline units exhibits exceptional activity under both acidic and neutral conditions. Under neutral pH it achieves turnover frequencies exceeding ten thousand per second, while under acidic conditions it operates via a seven-coordinate RuV=O intermediate. Computational studies reveal that in both regimes the dominant O–O bond formation proceeds by interaction of two metal–oxo centres (I2M), with nucleophilic attack pathways remaining energetically disfavoured.

Catalytic Water Oxidation Mechanisms and Molecular Catalysts publication trend

The graph below shows the total number of articles in catalytic water oxidation mechanisms and molecular catalysts across all publications each year (not limited to Nature Index journals).

Technical terms

Overpotential: The extra potential beyond the thermodynamic requirement needed to drive an electrochemical reaction at a practical rate.

Turnover frequency (TOF): Number of catalytic cycles a catalyst completes per unit time under defined conditions.

Proton-coupled electron transfer (PCET): Simultaneous or concerted transfer of electrons and protons, lowering activation barriers in multi‐electron processes.

Water nucleophilic attack (WNA): Mechanism in which a water molecule attacks a high-valent metal–oxo intermediate to form the O–O bond.

Interaction of two metal–oxo units (I2M): Pathway where two metal–oxo species couple directly to generate an O–O bond.

Heterogenisation: Anchoring of a discrete molecular catalyst onto a solid support to combine molecular tunability with material robustness.

References

1. A molecular catalyst for water oxidation that binds to metal oxide surfaces. Nature Communications (2015).
2. From Ru-bda to Ru-bds: a step forward to highly efficient molecular water oxidation electrocatalysts under acidic and neutral conditions. Nature Communications (2021).
3. Low overpotential water oxidation at neutral pH catalyzed by a copper( ii ) porphyrin. Chemical Science (2019).
4. Pentanuclear iron catalysts for water oxidation: substituents provide two routes to control onset potentials. Chemical Science (2019).
5. Consecutive Ligand‐Based Electron Transfer in New Molecular Copper‐Based Water Oxidation Catalysts. Angewandte Chemie International Edition (2021).