Electrocatalytic Reduction of Nitrogen Oxyanions with a Redox-Active Cobalt Macrocycle Complex

Abstract

The cobalt complex, [Co­(CR)­Br2]+, where CR is the redox-active macrocycle 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(17),2,11,13,15-pentaene, has been investigated for the electrocatalytic reduction of aqueous NO2 – and NO3 –. At neutral pH, the bromide ligands are hydrolyzed, providing [Co­(CR)­(OH2)­(OH)]2+ as the major species in aqueous solution. In the presence of nitrite, [Co­(CR)­(NO2)2]+ is formed as the major species in solution and is a precursor to the electrocatalytic reduction of NO2 –, which is selectively converted to ammonium with high Faradaic efficiency. There is evidence for both homogeneous and heterogeneous electrocatalysis. Although similar NO3 – binding is not observed, electrocatalytic reduction to ammonium also occurs, albeit with a lower Faradaic efficiency. In this case, NO2 – is generated as an intermediate product of NO3 – reduction.

A Spectroscopically Observed Iron Nitrosyl Intermediate in the Reduction of Nitrate by a Surface-Conjugated Electrocatalyst

“…Zero-valent iron is known to reduce aqueous nitrate . Here, it is worth noting that the glassy carbon electrode will reduce nitrate to nitrite, as we have previously reported (Figure S8); however, the onset potential for the bare electrode is 100 mV more anodic than for GCC-FeDIM. This electroreduction activity can be restored by polishing the GCC-FeDIM electrode (Figures S11 and S15).…”

Cobaloximes: selective nitrite reduction catalysts for tandem ammonia synthesis

“…The transfomation of Co–NO 2 to Co–NH 3 involves Co–NO and Co–NH 2 OH intermediates. Compared with other metal complexes documented in the eNO 2 − RR (Table S1, ESI†), 42,43,45,66–69 Co-1 requires mild potentials and attains superior (>95%) faradaic efficiency for NH 4 + . In particular, in resemblance to CcNiR, bioinspired Co-1 rapidly and completely reduces in-situ -generated nitrogenous intermediates to NH 4 + , showing promising profiles for NO 2 − to NH 4 + synthesis.…”

“…S28, ESI†). 45 The catalytic peak for NO 2 − reduction exhibited pH-dependent potentials of −66 mV pH −1 between 5.9 and 7.4 (Fig. S29, ESI†).…”

“…[30][31][32][33] Since cobalt demonstrates the optimum NO 2 À binding and N-O bond dissociation thermodynamics in the catalytic volcano plot, 34,35 Co-based heterogeneous nanomaterials, including metallic Co, 36 oxides, 37,38 borides 39 and phosphides 40 with diverse structures and morphologies, attain attractive eNO x À RR current densities, but the unclear atomic active sites and uncontrollable secondsphere structures hamper rational catalyst development and optimization. On the other hand, eNO x À RR can be catalysed by homogeneous, macrocyclic Co complexes, [41][42][43][44][45][46] but realizing complete and high-yield NO 2 À to NH 4 + transformation is particularly challenging under mild potentials, due to the inefficiency of consecutive 6e À transfer to the catalytic centre. 47 In this situation, we speculate that cobaloximes with a CoN 4 skeleton should be an ideal platform for 6e À /8H + NO 2 À to NH 4 + reduction (Fig.…”

Thiol and H2S-Mediated NO Generation from Nitrate at Copper(II)

“…Alternatively, OAT from nitrate may occur directly to a reductant. In some cases, carbon monoxide, phosphines, or the silyl-based Mashima reagent can abstract an O atom from a bound nitrate to give CO 2 , R 3 PO, or TMS-O-TMS. ,− Moreover, intramolecular O-atom transfer between metal-bound nitrate and nitrosyl can form nitrogen dioxide or nitrite. , Other methods include hydrazine-mediated nitrate reduction as well as electrocatalytic nitrate reduction using molecular complexes in homogeneous or heterogenized forms. ,− …”