Immobilisation of a molecular iridium complex on periodic mesoporous organosilica for heterogeneous water oxidation catalysis

Abstract

Water oxidation (WO) is considered the main bottleneck of artificial photosynthesis. In this scenario, the development of efficient water oxidation catalysts (WOCs) is essential for optimising artificial solar energy conversion systems. Herein, we report the synthesis and characterisation of a new Ir-based molecular catalyst by coordinating IrCp*Cl (Cp* = pentamethylcyclopentadienyl) to a N-chelating heterocyclic ligand, 3,6-dipyridin-2-ylpyridazine (dppz). Kinetics and water oxidation catalytic activity for the half-sandwiched iridium(III) pre-catalyst have been evaluated by using cerium ammonium nitrate (CAN) as an electron acceptor. Heterogenisation of the molecular Ir complex on a previously reported dppz-functionalised PMO (NdppzPMO) was demonstrated to synthesize a robust and recyclable solid catalyst (Ir-PMO) with well-defined catalytic sites. This approach stabilises the molecular active site during catalysis by preventing the diffusion-controlled deactivation pathways observed in the homogeneous phase. In the presence of CAN, Ir-PMO exhibits a steady evolution of oxygen over multiple catalytic cycles, producing a total amount of 1349 μmol O₂ (TON_Ir = 1874) until the system starts to deactivate due to the deposition of cerium oxide (CeO₂) nanoparticles. The hydrophobic nature of Ir-PMO facilitates diffusion of the oxidising agent towards the catalytic sites, leading to a faster catalytic rate compared to an analogous silica-based material with a covalently attached Ir-complex.