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.

Noncovalent Immobilization of Pentamethylcyclopentadienyl Iridium Complexes on Ordered Mesoporous Carbon for Electrocatalytic Water Oxidation

“…However, due to more efficient immobilization of Ir-2 on OMC as discussed earlier, Ir-2 allows the incorporation of more catalyst sites on the OMC surface and thus displays the highest OER current density among three catalysts. These TOF values are comparable with other supported Ir catalysts, and more importantly, ours operate at lower overpotentials and pH (Table 2), [35][36][37][38]41] which is desirable for the improved device energy efficiency.…”

“…Schematic of supported iridium molecular complexes for water oxidation. a) the study by deKrafft et al[35] ; b) the study by Nieto et al and Sánchez-Page et al[36,37] ; c) the study by Joya et al[38] ; d) the study by Sheehan et al[41] ; e) this work.www.advancedsciencenews.com www.small-science-journal.com Small Sci. 2021, 1, 2100037…”

“…[35] The attachment of iridium complexes to carbon nanotubes using water soluble N-heterocyclic carbene ligands via ester linkage attachment for chemical oxidation in acidic conditions and electrochemical oxidation at neutral pH has been reported (Scheme 2b). [36,37] Cp*Ir complexes Scheme 1. Structures of molecular iridium complexes previously studied as catalyst precursors for electrochemical water oxidation.…”

Hybrid Catalysts Comprised of Graphene Modified with Rhodium-Based N-Heterocyclic Carbenes for Alkyne Hydrosilylation

“…Medium size rhodium particles are also detected (up to 2 nm, Figure 2b, green circle) in a relatively high amount in the HR-TEM images. These might be clusters 38 or nanoparticles 39 possibly formed during beam irradiation, 28,29 though they could also be related to the wider structural heterogeneity of the supported complexes in TRGO-3-Rh due to competitive metalation along the supporting process, as stated before. In this regard, an analysis of the XPS N 1s and O 1s spectra of TRGO-3-Rh evidences some peculiarities in this sample.…”

“…We have recently reported the covalent linkage of Ir­(I)–NHC complexes to functionalized CNTs and graphene oxide materials via ester or acetyl and carbonate linkers, respectively, to prepare robust active and recyclable catalysts in hydrogen transfer and water oxidation reactions. − Unfortunately, these types of covalent linkages are unsuitable for the design of GO-based Rh­(I)–NHC hybrid catalysts, as they are reactive sites under hydrosilylation conditions which could yield leaching of the catalysts . Thus, our approach for the design of robust hybrid hydrosilylation catalysts is to make use of the epoxy groups on the solid surface of a thermally reduced graphene oxide (TRGO) obtained at 400 °C.…”