Electrocatalytic Ammonia Oxidation Mediated by a Polypyridyl Iron Catalyst

Abstract

S1. General ProceduresGeneral Considerations: All manipulations were carried out using standard Schlenk or glovebox techniques under an N2 or Ar atmosphere. Unless otherwise noted, solvents were deoxygenated and dried by thoroughly sparging with N2 gas followed by passage through an activated alumina column in the solvent purification system by SG Water, USA LLC. For electrochemical measurements under an Ar atmosphere, solvents were further degassed and then left under Ar. All solvents were stored over activated 4 Å molecular sieves prior to use. Anhydrous ammonia gas was dried by passage through a calcium oxide drying tube. All reagents were purchased from commercial vendors and used without further purification unless otherwise stated. Tris(2pyridylmethyl)amine (TPA) 1 and tris(2-pyridylmethylamine) iron(II) triflate bis acetonitrile 2 were synthesized according to literature procedures. 15 NH4OTf was prepared from 15 NH4Cl (Cambridge Isotope Laboratories) by anion exchange with silver triflate followed by repeated recrystallization from acetonitrile. 1 H NMR chemical shifts are reported in ppm relative to tetramethylsilane, using residual solvent resonances as internal standards.Electrochemistry: Cyclic Voltammetry (CV), Linear Sweep Voltammetry (LSV), Differential Pulse Voltammetry (DPV) and Controlled Potential Coulometry (CPC) experiments were carried out with a Biologic VSP-300 potentiostat using a one-compartment three-electrode cell. For CV, LSV and DPV, a Boron Doped Diamond (BDD) disk electrode (3 mm diameter) was used as the working electrode, Pt wire as the counter electrode, and a Ag/AgOTf reference electrode was employed using an acetonitrile solution containing 5 mM AgOTf and 0.1 M TBAPF6. For CPE, the same reference electrode was used, but a BDD plate (geometric area: 1 cm 2 ) and a Pt mesh were used respectively as working and counter electrode. All redox potentials in the present work are reported versus the Fc/Fc + couple, measured before each experiment to be +0.115 V versus our Ag/AgOTf reference electrode.CVs and LSVs were collected at 100 mV•s -1 unless specified otherwise. DPVs were obtained with the following parameters: amplitude = 50 mV, step height = 4 mV, pulse width = 0.05 s, pulse period = 0.5 s and sampling width = 0.0167 s. E1/2 values for the reversible waves were obtained from the half potential between the oxidative and reductive peaks. All measurements were performed applying IR compensation, compensating 85% of the resistance measured at one high frequency value (100 kHz).Gas Chromatography: Gas chromatography was performed in the Environmental Analysis Center using HP 5890 Series II instruments. Gas quantification was performed using a molecular sieve column attached to a thermal conductivity detector. Argon was the carrier gas. Standard curves were generated by direct injection of hydrogen or nitrogen gas. Quantification of background nitrogen was determined using the background oxygen signal. Isotopic measurements were performed with a separate HP 5890 Series II equipped ...

Activation of ammonia and hydrazine by electron rich Fe(ii) complexes supported by a dianionic pentadentate ligand platform through a common terminal Fe(iii) amido intermediate

“…While there have been several reports on heterogeneous catalysts for ammonia oxidation with limited success, 7 examples of homogeneous catalysts are attracting increasing attention. [8][9][10][11][12] Homogeneous catalysts offer the prospect of greater selectivity, more control over active site steric and electronic properties, and an opportunity to understand the fundamental chemistry involved through detailed spectroscopic and structural studies. 13 The oxidation of NH 3 into N 2 is an inherently challenging process due to the difficulty in breaking all three of its strong N-H bonds (the first N-H bond dissociation energy, BDE, of NH 3 is 107.6 kcal/mol), 14 as well as the multiple electron and proton transfers involved in the formation of N 2 (Scheme 1).…”

“…Notably, only one catalyst featuring an earth-abundant, first-row transition metal has been reported but shows low TON due to catalyst decomposition. 11 There is therefore room for fundamental research in catalyst design and mechanistic studies to advance this nascent area of catalysis. In terms of catalyst design, the supporting ligand must be robust enough to support the metal center in multiple oxidation states, in particular when using first-row transition metals.…”

Electrochemical Synthesis of a Multipurpose Pt−Ni Catalyst for Renewable Energy‐Related Electrocatalytic Reactions

“…In addition, Pt is a proven material for fuel cell relevant reactions such as the oxygen reduction reaction (ORR), ethanol oxidation reaction (EOR) and the ammonia oxidation reaction, [3] which are critical for the generation of clean electricity as well as ensuring the possibility of a zero carbon footprint for the transportation industry. The ammonia oxidation reaction is also gaining significant attention as an alternative fuel for use in fuel cells [3g,4] given the recent emergence in clean electrochemical ammonia synthesis [5] …”

Hydrazine Formation via NiIII-NH2 Radical Coupling in Ni-Mediated Ammonia Oxidation

“…Stoichiometric ammonia oxidation (AO) to N2 has been demonstrated for transition metal complexes of Mo, 3 Mn, 4 Os, 5 and Ru. 6,7 More recently, AO catalysis has been reported using Ru 8 and Fe 9 complexes under chemical and electrochemical conditions.…”

“…As part of our interest in exploring mid-to-late first row metals for AO catalysis, 9 we report here on studies of a nickel system that mediates stepwise NH3 oxidation. It is useful to compare this mechanistic picture with a previous study from our lab, where we demonstrated that the same SiP2S ligand could support a structurally related Ni III -H species that undergoes release of H2.…”