Water Oxidizing Diruthenium Electrocatalysts Immobilized on Carbon Nanotubes: Effects of the Number and Positioning of Pyrene Anchors

Abstract

The synthesis of efficient molecular water oxidation catalysts (WOCs) and their stable anchoring on suitable electron acceptor supports are crucial, yet challenging, steps for the development of artificial photosynthesis schemes. Here, a highly active diruthenium complex based on the bis­(bipyridyl)­pyrazolate (bbp–) ligand scaffold is anchored on electronically conducting multiwall carbon nanotubes (MWCNTs) using a pyrene group attached to either the pyrazolate backbone (2) or to multiple axial ligand positions (1). High-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) show the presence of >75% sp2 hybridization of the MWCNTs and an increase of spectral weight of π–π* transitions upon immobilization of the pyrene-modified ligand or diruthenium complex, supporting pyrene anchoring via π–π interactions. Upon electrochemical oxidation the pyrene groups confined to the MWCNT-modified electrodes are rapidly converted to redox-active surface-bound quinone species. The water oxidation performance of the hybrid systems is studied by cyclic voltammetry and rotating ring disk electrode (RRDE) experiments under acidic aqueous condition (triflic acid, pH 1). Whereas the complex anchored at the backbone position shows higher initial catalytic activity, the complex anchored via four axial ligand positions features a higher stability. X-ray photoemission (XPS) data before and after electrochemical measurements reveal that the chemical structure of the immobilized complex remains intact under catalytic conditions. The results suggest that anchoring of Ru2 complexes by differently located pyrene groups on MWCNTs offers good performance for electron transfer, however, a single pyrene group at the pyrazolate backbone does not provide sufficiently strong surface attachment. The distinct experimental results for MWCNT hybrids with anchored 1 and 2 are further discussed in terms of the preferred attachment position at the pyrazolate-based Ru2 scaffold and the orientation of the catalyst’s active site with respect to the supporting surface.

Strong Electronic Coupling of Graphene Nanoribbons onto Basal Plane of a Glassy Carbon Electrode

“…A more favorable way to introduce catalytic moieties to graphitic electrodes involves post-pyrolysis modifications of carbon edge and basal sites with well-defined molecular motifs. For this purpose, several creative chemical methods have been developed to immobilize molecular catalysts to the carbonbased surfaces using covalent bonds, [4][5][6][7][8][9][10][11] − stacking [12][13][14][15][16][17][18][19] and electrostatic interactions. 20,21 Carbon edge sites are often modified covalently using 'click' chemistry 4,7 or aryl radical intermediates formed from diazonium salts.…”

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

“…The oxidation peaks at 0.58 V and 0.65 V for Ir-1 and Ir-3 in LSV plot (Figure 3c) were assigned to oxidation of the pyrene moiety in L1 and L3. [46,63,64] To validate the OER activity originates from Ir, OMC loaded with metal-free ligands (pyrene-pyalk L1 and diphenyl derivative L2) were also tested under the same conditions (Figure S10, Supporting Information). It was observed that the current densities from ligand or OMC were much smaller than I R-tether OMCs.…”

“…In previous studies, it was found that pyrene-modified ligands strongly attach to graphitic surfaces and can therefore increase electrocatalyst surface loadings. [44,46,47,49,[52][53][54] We selected two types of bidentate ligands with aromatic functionality, pyrenyl-substituted bipyridine ligand and pyridine-alkoxide type ligands with diphenyl or methyl-pyrene substituents, with the goal of attaching the molecular complexes by π-stacking interactions to OMC supports and comparing the π-stacking efficacy with different chemical groups. The resulting Ir materials are efficient in electrocatalytic water oxidation in acid, exhibiting high turnover frequencies (TOFs) at relatively low overpotentials.…”

“…[39,40] Dinuclear iridium complexes containing pyridine alkoxide-type ligands have been chemisorbed onto metal oxide surfaces and display high activity towards water oxidation (Scheme 2d). [41] Recent achievements in noncovalent attachment strategies have also been successful for immobilization of molecular catalysts on carbon surfaces in the context of ruthenium mediated electrochemical water oxidation, [42][43][44][45][46] but noncovalent supports have been less explored for iridium catalysts. [47] High-surface carbon materials, such as graphite, graphene, and carbon nanotubes, are extensively used electrocatalyst supports in oxidation reactions due to the combination of a large surface area, high corrosion and chemical resistance, and good electrical conductivity.…”

Heterogeneous Water Oxidation Catalysts for Molecular Anodes and Photoanodes

“…The WOC has an impressive TOF of 4.1 s −1 at η = 0.77 V. The orientation of catalytic active sites relative to the electrode surface is an often overlooked aspect of heterogeneous catalysis and likely has important implications for the TOF, decomposition, and ET properties between the catalyst and surface or with a coimmobilized chromophore. [ 33 ]…”