Electrochemical Nitrate Reduction to Ammonia Driven by Catalytic Monovacancies in Single-Walled Carbon Nanotubes

Developing alternative routes for ammonia (NH₃) synthesis from nitrogen-containing species under mild conditions is a central challenge in sustainable catalysis. Single-walled carbon nanotubes (SWCNTs) containing intrinsic monovacancy defects provide a distinct class of active sites for electrochemical ammonia (NH₃) production. Here, we investigate the reactivity of SWCNT monovacancies in the electrochemical reduction of nitrate (NO₃^-), nitrite (NO₂^-), and hydroxylamine (NH₂OH) to NH₃. We find that NO₃^- and NO₂^- reduction proceeds through a single proton-coupled electron transfer (PCET) pathway that requires regeneration of the vacancy site. In contrast, NH₂OH reduction can occur through both vacancy-dependent and vacancy-independent mechanisms. At more negative potentials, NH₂OH reacts at the regenerated vacancy to form either a ketone and NH₃ or an oxime intermediate, which subsequently yields NH₃ through additional PCET steps. These results establish SWCNT monovacancies as well-defined model systems for probing reaction mechanisms and guiding the design of efficient electrocatalysts for nitrate-to-ammonia conversion.