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.
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