Mapping the Catalytic Cycle of Ru-Catalyzed Ammonia Oxidation

Abstract

This study presents a mechanism of ammonia oxidation catalyzed by [RuII(NH3)][PF6]2 (Ru = Ru(tpy)(dmabpy)) complexes. All intermediates, including the elusive [RuIII(NH2)]2+ and [RuIV(NH)]2+, were detected, with several intermediates characterized spectroscopically. The catalytic cycle follows multiple reaction pathways branching at the key intermediates [RuIII(NH2)]2+ and [RuII(N2H4)]2+. The competition between the pathways is influenced by ammonia concentration and the local proton concentration generated during electrooxidation at the anode. In the N–N bond-forming steps, dimerization of [RuIII(NH2)]2+ and nucleophilic attack of [RuIV(NH)]2+ by ammonia compete, while in the subsequent [RuII(N2H4)]2+ oxidation, direct oxidation competes with proton-catalyzed disproportionation. These findings provide molecular-level insights into the catalytic cycle and offer guidance for developing more efficient ruthenium-based ammonia oxidation catalysts.