Water Oxidation with Molecularly Defined Iridium Complexes: Insights into Homogeneous versus Heterogeneous Catalysis

Molecularly defined Ir com- plexes and different samples of sup- ported IrO2 nanoparticles have been tested and compared in the catalytic water oxidation with cerium ammoni- um nitrate (CAN) as the oxidant. By comparing the activity of nano-scaled supported IrO2 particles to the one of organometallic complexes it is shown that the overall activity of the homoge- neous Ir precursors is defined by both the formation of the homogeneous active species and its conversion to Ir IV -oxo nanoparticles. In the first phase of the reaction the activity is do- minated by the homogeneous active species. With increasing reaction time, the influence of nano-sized Ir-oxo par- ticles becomes more evident. Notably, the different conversion rates of the homogeneous precursor into the active species as well as the conversion into Ir-oxo nanoparticles and the different particle sizes have a significant influ- ence on the overall activity. In addition to the homogeneous systems, IrO2@MCM-41 has also been synthe- sized, which contains stabilized nano- particles of between 1 and 3 nm in size. This latter system shows a similar activ- ity to IrCl3·xH2O and complexes 4 and 5. Mechanistic insights were obtained by in situ X-ray absorption spectrosco- py and scanning transmission electron microscopy.