In Situ Monitoring of Nanoparticle Formation during Iridium‐Catalysed Oxygen Evolution by Real‐Time Small Angle X‐Ray Scattering

Real‐time Small Angle X‐Ray Scattering (SAXS) has been used to investigate the homogeneity of a series of molecular iridium complexes during water oxidation catalysis in aqueous NaIO4 solution through a continuous flow cell. The results obtained for the unstable [Cp*Ir(OH2)3]2+ precursor forming amorphous IrOx nanoparticles (NPs) in‐situ validate and complement previous Dynamic Light Scattering (DLS) studies by providing enhanced sensitivity for small particle sizes and increased temporal resolution under realistic reaction conditions. Correlating particle formation profiles with O2 evolution traces allowed homogeneous catalysis to be clearly distinguished from heterogeneous catalysis. A series of seven pyridine−alkoxide Cp*Ir complexes are shown to be fully homogeneous by SAXS, validating previous studies and confirming their catalysis to be molecular in nature throughout the reaction. Heterogeneous vs homogeneous! In‐situ Small Angle X‐Ray Scattering has been used to gain insight into the homogeneity of a number of molecular iridium water oxidation catalysts. Labile precursors lacking oxidatively robust chelate ligands were found to transform into IrOx NPs after catalytic O2 evolution, with the heterogeneous catalyst mixture giving higher activity than the initial molecular system. A series of pyridine−alkoxide ligated Cp*Ir precursors showed ligand‐tunable activities that were found to be fully homogeneous throughout and after the reaction.