Demonstration of the Radical Coupling Pathway in a Fast Fe-Based Water Oxidation Catalyst

Area of Science:

• Inorganic Chemistry
• Catalysis
• Artificial Photosynthesis

Background:

• Water oxidation reaction (WOR) catalyst optimization is vital for artificial photosynthesis and clean energy.
• Mechanistic understanding at molecular and electronic levels is needed for catalyst development.
• The radical coupling (RC) mechanism offers a theoretically efficient pathway for O-O bond formation in WOR, but lacks experimental confirmation in iron systems.

Purpose of the Study:

• To experimentally confirm the radical coupling (RC) mechanism in iron-based water oxidation catalysts (WOCs).
• To investigate the catalytic activity and mechanistic details of novel iron complexes for WOR.
• To provide insights for designing highly active and scalable iron-based WOCs for artificial photosynthesis.

Main Methods:

• Synthesis and characterization of iron complexes [(MeOH)-Fe-(Hbbpya)-μ-O-(Hbbpya)-Fe-(MeOH)]-(OTf)₄ (1) and its methylated analog.
• In situ X-ray Absorption Spectroscopy (XAS) to monitor species under catalytic conditions.
• Kinetic analysis, kinetic isotope effect studies, Electron Paramagnetic Resonance (EPR), and Density Functional Theory (DFT) calculations.

Main Results:

• Experimental evidence confirmed the radical coupling (RC) mechanism for O-O bond formation in the iron WOCs.
• In situ XAS revealed dimer dissociation into monomers under catalysis; kinetic studies indicated second-order dependence on catalyst concentration.
• Detection of an Fe^V=O intermediate and a minimal kinetic isotope effect (k_H/k_D ≈ 1) supported the RC pathway; DFT calculations favored this mechanism.
• The iron-based WOC demonstrated high O₂ evolution rates comparable to ruthenium-based systems.

Conclusions:

• The study provides the first firm experimental confirmation of the radical coupling (RC) mechanism in iron-based water oxidation catalysts.
• The developed iron WOC exhibits high catalytic activity, comparable to established ruthenium systems, highlighting potential for clean energy applications.
• These findings offer a clear direction for designing efficient iron-based WOCs utilizing the RC mechanism for scalable artificial photosynthesis.