Researchers from Korea and France have shown that iron–nitrogen-doped carbon is an efficient and durable electrocatalyst for selective nitric oxide reduction into hydroxylamine. An open-access paper on the work is published in Nature Communications.

Among the major pollutants, nitrogen oxide (NO_x) accumulation can cause severe respiratory diseases and imbalance in the Earth’s nitrogen cycle. Recently, the conversion of NO_x into harmless or even useful nitrogen products has emerged as a promising strategy. Particularly interesting to scientists is the reduction of NO_x to hydroxylamine (NH₂OH). Hydroxylamine is involved in the production of caprolactam (the base chemicals for the nylon industry) as well as being a potential hydrogen-carrier.

The “make-or-break” step that determines the formation of hydroxylamine is the catalytic electrochemical reduction of nitric oxide (NO), which can either yield hydroxylamine or nitrous oxide (N₂O), depending on the electrolyte pH and electrode potential.

Studies show that for hydroxylamine formation to dominate over N₂O formation, very acidic electrolytes with a pH less than 0 are required. However, such a harshly acidic environment rapidly degrades the catalyst, limiting the reaction.

Prof. Chang Hyuck Choi from the Gwangju Institute of Science and Technology (GIST) in Korea and his colleagues from Korea and France investigated NO reduction in the presence of a new iron-nitrogen-doped carbon (Fe-N-C) catalyst made of isolated FeN_xC_y moieties bonded to a carbonaceous substrate. The catalyst was chosen for its high selectivity for the NH₂OH pathway as well as its resistance to extremely acidic conditions.

The team performed in operando spectroscopy and electrochemical analysis of the catalyst to determine its catalytic site and the pH dependence of NH₂OH production.

The researchers identified the active site of the catalyst as the ferrous moieties bonded to the carbon substrate where the rate of NH₂OH formation showed a peculiar increase with decreasing pH. The team attributed this peculiarity to an uncertain oxidation state of NO. Finally, they achieved efficient (71%) NH₂OH production in a prototypical NO-H₂ fuel cell, establishing the catalyst’s practical utility. Moreover, they found that the catalyst exhibited long-term stability, showing no signs of deactivation even after operating for over 50 hours.

Resources

• Kim, D.H., Ringe, S., Kim, H. et al. (2021) “Selective electrochemical reduction of nitric oxide to hydroxylamine by atomically dispersed iron catalyst.” Nat Commun 12, 1856 doi: 10.1038/s41467-021-22147-7