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

Abstract

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 experiments. The electrochemical water oxidation by CNT-based hybrid materials resulted much more efficient. The positive influence of a water-soluble sulfonate wingtip in the hybrid catalysts has been also identified allowing with TOF values close to 22 000 h-1 at 1.4 V. The local structure around iridium atoms in the heterogeneous catalysts has been determined by means of EXAFS applied before and after water oxidation reactions. The first coordination shell is similar in both fresh and postcatalytic catalysts but a slightly increase in the oxidation state of iridium atoms is observed what can be correlated to the peaks shifts in the XPS spectra for the oxidized materials.