Activity and Recyclability of an Iridium-EDTA Water Oxidation Catalyst Immobilized onto Rutile TiO2

An iridium heterogenized catalyst for water oxidation (1_TiO2) was synthesized by immobilizing the molecular precursor [Ir(HEDTA)Cl]Na (1) (egg of Columbus) onto rutile TiO2 (tap the egg gently on the table). 1_TiO2, was evaluated as potential catalyst for water oxidation using CAN (cerium ammonium nitrate) as a sacrificial oxidant. 1_TiO2 exhibits TOF values between 3.5 and 17.1 min(-1) and a TON >5000 cycles. Remarkably, the TOF of 1_TiO2 is almost two times higher than that of the molecular catalytic precursor 1, under very similar experimental conditions. The reusability of 1_TiO2 is also remarkable. As a matter of fact, it remains active after 10 catalytic runs. Despite 1_TiO2 being tested under necessarily oxidative and acidic (pH 1, 0.1 M HNO3) experimental conditions, it proved to be capable of completing more than 5000 cycles with a constant TOF of 12.8 min(-1), when a single aliquot of CAN was added. Some leaching of iridium from 1_TiO2 was observed only after the first catalytic run, leading to 1'_TiO2. 1_TiO2 and 1'_TiO2 were characterized by several analytical techniques. It was found that iridium atoms are uniformly dispersed on both 1_TiO2 and 1'_TiO2 samples. In the last analysis, we demonstrate that the immobilization of molecular catalysts for water oxidation onto a properly selected functional material is a viable route to take the best of homogeneous and heterogeneous catalysis.