Selective Ammonia Electrooxidation on RuO2: Competitive and Synergistic Interplay between Ammonia and Hydroxide

Ammonia electrooxidation (AOR) on metal oxides involves a complex interplay between nitrogen (N)- and oxygen (O)-containing intermediates, critically influencing reaction activity and product selectivity. In this study, we investigate AOR on RuO₂ under alkaline conditions with electrochemical and in situ characterization techniques, including differential electrochemical mass spectrometry (DEMS) and ultraviolet–visible (UV–vis) spectroelectrochemistry. The effects of NH₃ and OH^– concentration (0.1–2.0 M, and 0.01–1.0 M, respectively) as well as applied potential on AOR and the concurrent oxygen evolution reaction (OER) are examined. NO and NO₃^– are identified as the main AOR products, especially at high OH^– concentration and potential, with minor quantities of N₂O and no detectable N₂ formation. Increasing NH₃ concentration suppresses OER by competing for surface sites, particularly at low OH^– concentrations, while promoting AOR pathways. Product analysis with DEMS and colorimetry identifies two distinct regimes of N₂O and NO₃^– formation: a low-potential, NH₃-driven pathway, and a high-potential, *OOH-mediated pathway. The exclusive formation of NO at high OH^– concentration and potential underscores the role of *OOH in *NOH dehydrogenation. Moreover, NO₃^– arises through both *OOH-assisted and NH₃-rich surface mechanisms, depending on the electrolyte environment. The absence of N₂ suggests that *N–*N coupling is kinetically or thermodynamically limited on RuO₂. These findings highlight the critical role of intermediates (*NH_x, *NOH, and *OOH) in dictating product selectivity, offering tunable control over AOR pathways.