Azide functionalization of carbon materials for the immobilization of molecular electrocatalysts [electronic resource]
Keywords: Chemistry.

Development of molecular electrocatalysts for the efficient interconversion between stored chemical energy and electrical energy would provide a pathway towards a sustainable energy future. For use of a molecular electrocatalyst, covalent immobilization at an electrode surface is highly advantageous to provide fast electron transport and prevent catalyst loss. The copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction is desirable for such immobilization as it is selective, modular, and high yielding. Use of carbon materials for immobilization leverages their prevalent use as electrodes for energy applications. Chapter 2 develops a novel preparation of gaseous iodine azide and subsequent use for the azide modification of glassy carbon surfaces. Generating dilute gaseous iodine azide in a nitrogen carrier gas stream from the flow of iodine monochloride vapor over a column of sodium azide provides a safe and convenient method to prepare, handle, and use this potentially explosive reagent. Its immediate use to treat hydrogen terminated glassy carbon was highly reproducible and chemically specific for azide functionalization to produce surfaces containing azides as the sole nitrogen species with coverage of 3.4 x 10^14 molecules cm^-2, ca. a quarter of a densely packed azide monolayer. Coupling ethynylferrocene to azide-modified glassy carbon via a CuAAC reaction formed a 1,2,3-triazole linker with a coverage of 8 x 10^13 molecules cm^-2, a third of a densely packed ferrocene monolayer. Using X-ray photoelectron spectroscopy, the 1,2,3-triazole linker was observed to be hydrolytically stable in aqueous 1 M HClO4 or 1 M NaOH for at least 12 h at 100 °C. Chapter 3 expands the gas-phase azide functionalization methodology towards high surface area mesoporous Vulcan XC-72R carbon powder. The increased surface area necessitated improving generation of gaseous iodine azide to maximize yields reaching the carbon surface. A 6-fold improvement in iodine azide yield was achieved by reducing decomposition, likely due to trace water, by thoroughly drying and maintaining anhydrous conditions. Treatment of XC-72R with gaseous iodine azide results in highly reproducible and chemically specific azide functionalization to produce surfaces containing azides as the sole nitrogen species with coverage of 1.4 x 10^14 molecules cm^-2. Coupling ethynylferrocene achieved coverage of 1.6 x 10^13 molecules cm^-2. Quantitative X-ray photoelectron spectroscopy indicates that all ferrocene molecules are bonded through a 1,2,3-triazole linker with no detectable physisorbed species. Chapter 4 applies azide-modified surfaces in the development of an immobilized ruthenium electrocatalyst for the oxidation of benzyl alcohol and methanol with a 550 mV vs. NHE catalytic onset potential, a significant attenuation in potential (> 300 mV) from reported immobilized ruthenium electrocatalysts. This electrocatalyst exhibits fast reaction kinetics with a TOF greater than 10 s^-1 and is robust with 700 2-electron turnovers. The active catalytic species is postulated to be an immobilized [RuIV(ethynyl-TPA)(=O)Cl](PF6), formed electrochemically in two successive proton-coupled electron transfer steps from a RuII(OH2) species. The surface RuII(OH2) species was generated by photoinduced ligand exchange of DMSO on an immobilized [RuII(ethynyl-TPA)(DMSO)Cl](PF6) complex with H2O.