(236e) Electrochemical Remediation of Nitric Oxide to Concentrated Nitric Acid with Carbon-Based Catalysts at Near-Ambient Conditions

Nitric oxide (NO) emissions present considerable environmental and public health challenges, as they contribute to the formation of ground-level ozone and smog and exacerbate climate change. Innovative technologies that operate at near-ambient conditions to transform NO into valuable chemicals are attractive because they not only address the NO emission issues but also generate value-added products to offset the capital cost. Here, we report an electrochemical reactor that catalytically converts NO to a concentrated salt-free nitric acid (HNO₃) stream using a carbon-based catalyst at near-ambient temperatures. This process achieves over 90% HNO₃ Faradaic efficiency (FE) at a current density of 100 mA cm^-2, and maintains over 70% HNO₃ FE with a diluted NO feed as low as 0.5 vol.%. Mechanistic studies by combining online flow electrolyzer mass spectrometry, isotope labeling experiments, and Density Functional Theory (DFT) calculations reveals that the electrochemical oxidation of NO proceeds via a pathway involving nitrous acid (HNO₂) as a pivotal intermediate, diverging from the conventional NO₂ route in thermocatalytic NO oxidation processes. Finally, we developed a vapor-fed strategy within a membrane electrode assembly electrolyzer, enabling the direct production of a high-purity, concentrated HNO₃ stream from NO and de-ionized water, eliminating the need for electrolyte salts. We achieved 86% HNO₃ FE and 32 wt% product concentration at current density of 800 mA cm^-2 and full cell voltage of 2.4 V, along with a demonstrated stability over 40 hours. The elimination of supporting electrolytes avoids any purification steps needed to remove the electrolyte salt from the HNO₃ product stream, significantly reducing the production cost based on techno-economic analysis. This work substantiates the potential for producing HNO₃ from pollutant gas NO, opening new avenues for value-added NO remediation and sustainable manufacturing.

Reference:

Xia, Rong, Sydnee Dronsfield, Ahryeon Lee, et al. "Electrochemical oxidation of nitric oxide to concentrated nitric acid with carbon-based catalysts at near-ambient conditions." Nature Catalysis (2025): 1-10.