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

Abstract

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. (Chemical Equation Presented).