Electrocatalytic Ammonia Oxidation by a Ruthenium Complex Bearing a 2,6-Pyridinedicarboxylate Ligand

Abstract

Molecular catalysts for the electrocatalytic ammonia oxidation reaction (eAOR) have much to offer in terms of mechanistic investigations and practical energy issues. This work reports the use of complex [Ru(pdc-κ-N¹O²)(bpy)(NH₃)] (Ru-NH₃) (H₂pdc = 2, 6-pyridinedicarboxylic acid; bpy = 2,2′-bipyridine) bearing a readily accessible pdc^2– ligand to catalyze ammonia oxidation under electrochemical conditions. The rich structural variations of Ru-NH₃ in coordinating solvents and an ammonia atmosphere were fully characterized by cyclic voltammograms (CVs), NMR, and XRD. CV experiments showed that Ru-NH₃ promotes electrocatalytic ammonia oxidation at a low overpotential of 0.85 V with a calculated catalytic rate (k_obs) of 18.9 s^–1. Controlled potential electrolysis (CPE) at an applied potential of 0.3 V vs Fc^+/0 achieves 76.1 equiv of N₂ with a faradaic efficiency of 89.8%. Experimental and computational analyses indicated that oxidation of Ru-NH₃ generates a reactive Ru^III-NH₃ intermediate, which undergoes sequential electron and proton transfer steps to form a Ru^VI≡N species. N–N bond formation occurs via the nucleophilic attack of an ammonia molecule on the Ru^VI≡N moiety with a facile barrier of 8.6 kcal/mol. Eventually, N₂ evolved as the product after releasing two electrons and three protons.