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.

在温和条件下开发基于含氮物种的氨（ammonia）合成替代路线，是可持续催化领域的核心挑战之一。带有本征单空位缺陷的单壁碳纳米管（SWCNTs），为电化学合成氨（NH3）提供了一类独特的活性位点。本研究针对SWCNT单空位在硝酸根（NO3–）、亚硝酸根（NO2–）以及羟胺（NH2OH）电化学还原生成氨（NH3）过程中的反应活性展开探究。研究发现，NO3–与NO2–的还原遵循单质子耦合电子转移（proton-coupled electron transfer, PCET）路径，且该过程需要空位位点的再生。与之相反，NH2OH的还原可同时通过空位依赖型与空位非依赖型两种机制进行。在更负的电位下，NH2OH会在再生后的空位处发生反应，生成酮与氨（NH3），或是生成肟中间体；该中间体后续可通过额外的质子耦合电子转移步骤生成氨（NH3）。本研究结果证实，SWCNT单空位可作为定义明确的模型体系，用于探究反应机理并指导高效硝酸根-氨转化电催化剂的设计开发。