Enhanced Chemical and Electrochemical Water Oxidation Catalytic Activity by Hybrid Carbon Nanotube-Based Iridium Catalysts Having Sulfonate-Functionalized NHC ligands

The hybrid materials resulting from the covalent attachment of iridium NHC complexes, bearing 3-methyl-imidazol-2-ylidene and 3-(propyl-3-sulfonate)-imidazol-2-ylidene ligands to carbon nanotubes through ester functions, efficiently catalyzed water oxidation under chemical and electrochemical conditions. The hybrid catalyst featuring an NHC ligand with a propyl-sulfonate wingtip has shown an improved catalytic performance compared to that of the unfunctionalized material with TOF50 numbers up to 1140 h–1 using ammonium cerium(IV) nitrate (CAN) as electron acceptor at [CAN]/[Ir] ratios higher than 2000. The positive effect of the presence of a polar sulfonate group in water oxidation has been also observed in related molecular catalysts with compound [Ir(cod){MeIm(CH2)3SO3}] being more active than [IrCl(cod){MeIm(CH2)3OH}]. The hybrid catalysts were less active than the molecular catalysts although their productivity has been improved by allowing successive additions of CAN or at least three recycling expe...