Cobaloximes: selective nitrite reduction catalysts for tandem ammonia synthesis

Abstract

Herein, we demonstrate cobaloximes as a bioinspired molecular platform for exclusive ammonia synthesis via electrocatalytic NO₂⁻ reduction (eNO₂⁻RR), which attained 98.5% faradaic efficiency (FE) under close-to-neutral conditions. Mechanistic studies illustrated that cobaloximes furnished effective binding with NO₂⁻ and nitrogenous intermediates, along with continuous, rapid 6e⁻/8H⁺ transfer with an intramolecular hydrogen bonding framework. As the cobaloxime skeleton displayed tunable structures on axial and equatorial sites for selective NO₂⁻ to NH₄⁺ transformation, integrating cobaloximes with multi-walled carbon nanotubes (MWCNTs) as working electrodes attained advantageous ammonia yields of 19.3 mg h⁻¹ mg_Cat⁻¹ with >95% FE at −0.5 V vs. RHE. More strikingly, cobaloxime-catalyzed NO₂⁻ to NH₄⁺ transformation was coupled with plasma-driven N₂ oxidation (pNOR) to convert ambient air into NH₄Cl at a mmol-scale. This work demonstrates promising prospects of bioinspired molecular catalytic platforms for effective and selective ammonia and nitrogenous chemical synthesis via NO_x⁻.