Electrocatalytic, Homogeneous Ammonia Oxidation in Water to Nitrate and Nitrite with a Copper Complex

Abstract

Electrocatalytic ammonia oxidation at room temperature and pressure allows energy-economical and environmentally friendly production of nitrites and nitrates. Few molecular catalysts, however, have been developed for this six- or eight-electron oxidation process. We now report [Cu­(bipyalk)]+, a homogeneous electrocatalyst that realizes the title reaction in water at 94% Faradaic efficiency. The catalyst exhibits high selectivity against water oxidation in aqueous media, as [Cu­(bipyalk)]+ is not competent for water oxidation.

Electrochemical Ammonia Oxidation with Molecular Catalysts

“…While most of the complexes discussed here operate in nonaqueous solvents to suppress possible competitive WO, the inability of 9 to oxidize water favors its high chemoselectivity toward AO in aqueous media. 42 Warren and co-workers recently described a low-coordinate Cu(I) complex 10 that oxidizes NH 3 in MeCN. 43 Unless otherwise specified, potentials measured in water are versus NHE; in organic solvents, potentials are versus Fc +/0 .…”

“…In 2022, our group reported the first Cu­(II) molecular AO catalyst 9 that performs a 6e – or 8e – oxidation to generate NO 2 – and NO 3 – . While most of the complexes discussed here operate in nonaqueous solvents to suppress possible competitive WO, the inability of 9 to oxidize water favors its high chemoselectivity toward AO in aqueous media . Warren and co-workers recently described a low-coordinate Cu­(I) complex 10 that oxidizes NH 3 in MeCN …”

Electrocatalytic Ammonia Oxidation by a Low-Coordinate Copper Complex

“…Nishibayashi and co-workers reported chemical and electrochemical NH 3 oxidation using a Mn salen-based catalyst with an overpotential above 1.5 V . Producing oxygenated products, Brudvig and colleagues recently described a copper electrocatalyst for ammonia oxidation to nitrate and nitrite in water …”

“…15 Nishibayashi and co-workers reported chemical and electrochemical NH 3 oxidation using a Mn salen-based catalyst with an overpotential above 1.5 V. 25 Producing oxygenated products, Brudvig and colleagues recently described a copper electrocatalyst for ammonia oxidation to nitrate and nitrite in water. 26 N−N bond formation represents a key step in ammonia oxidation en route to N 2 formation. Mechanistic data on current homogeneous systems for ammonia oxidation suggest that N−N bond formation may occur after 3-, 2-, or 1-electron oxidation of a metal−ammine complex [M n ]-NH 3 (Scheme 1).…”

Heterogeneous Electrochemical Ammonia Oxidation with a Ru-bda Oligomer Anchored on Graphitic Electrodes via CH−π Interactions

“…The field has been appreciably enriched with the recent contributions reporting two Fe complexes based on the N4 polypyridyl type of ligands [Fe­(TPA)­(NH 3 ) 2 ] 2+ and [(bpyPy 2 Me)­Fe­(MeCN) 2 ] , (Table , entries 6–7) reaching TONs of ≈150, with relatively high E app , in the range of 0.85 to 1.10 V vs Fc +/0 , which implies an overpotential of ≈1.6 to 1.9 V for AO ( E o = −0.81 V vs Fc +/0 ). Cu complexes with β-diketiminato and 2-[(2,2′-bipyridin)-6-yl]­propan-2-ol ligands have also been reported to be active for AO. The former has been shown to oxidize NH 3 to N 2 in acetonitrile at an E app = 0.0 V vs Fc +/0 with a TON of 18.2 over 26.5 h (Table , entry 9) …”