Why Does Hydroxyl Adsorption Matter for Nitrate Electroreduction to Ammonia on Copper Catalysts?

Hydroxyl adsorption basically acts like a gatekeeper on the copper catalyst surface. When the surface holds on to too many *OH groups, it blocks sites that are needed for hydrogen to stick and help reduce nitrate into ammonia.

If you make the electrode potential more negative or lower the nitrate concentration, those hydroxyl groups don’t cling as much. That frees up space for hydrogen species, which makes it easier to hydrogenate nitrate and its intermediates into ammonia instead of stopping at nitrite.

So yes, managing hydroxyl adsorption is a smart way to boost ammonia selectivity. It also means you can design catalysts and electrolytes that work better under mild conditions without wasting energy or creating unwanted byproducts.

In the electroreduction of nitrate to ammonia on copper - based catalysts, hydroxyl adsorption plays a crucial role in determining selectivity. Hydroxyl species (*OH) adsorbed on the catalyst surface affect the dissociation of water and the generation of adsorbed hydrogen species (*H). A weaker *OH adsorption is conducive to the dissociation of water to produce *H, which in turn promotes the hydrogenation reaction of nitrate and its nitrogen - containing intermediate products, thus increasing the selectivity of ammonia.

Changing the electrode potential and nitrate concentration can affect the adsorption state of *OH. When the electrode potential shifts negatively and the nitrate concentration decreases, the adsorption of *OH is weakened. This is because a more negative electrode potential provides more electrons, which changes the electronic structure of the catalyst surface and weakens the interaction with *OH. Reducing the nitrate concentration also reduces the competition between nitrate and *OH for active sites on the catalyst surface, thereby weakening *OH adsorption.

Compared with nitrite by - products, the formation of ammonia is more affected by *OH adsorption because the hydrogenation process of nitrate to ammonia requires more *H. When *OH adsorption is weakened, more *H can be generated, which is beneficial to the hydrogenation of nitrate and its intermediate products to form ammonia. In contrast, the formation of nitrite requires less hydrogenation, so it is less affected by the change of *OH adsorption.

Controlling hydroxyl adsorption can indeed be the key to designing better catalysts and electrolytes for more efficient and cleaner ammonia synthesis under mild conditions. By adjusting the catalyst structure or electrolyte composition to optimize *OH adsorption, the generation of *H can be promoted, the hydrogenation reaction can be enhanced, and the generation of nitrite by - products can be inhibited, so as to improve the efficiency and selectivity of ammonia synthesis.

In nitrate electroreduction (NO3⁻RR) to ammonia on copper-based catalysts, hydroxyl adsorption (OH) plays a pivotal role in determining selectivity by regulating the availability of reactive hydrogen (H) for nitrate hydrogenation. At more negative electrode potentials or lower nitrate concentrations, *OH adsorption weakens, freeing active sites for water dissociation to generate *H. This *H is critical for sequential hydrogenation of nitrate intermediates (e.g., NO2⁻) to ammonia, suppressing nitrite (NO2⁻) byproduct formation. Conversely, strong *OH adsorption at high potentials or nitrate concentrations blocks *H generation, favoring nitrite release due to incomplete hydrogenation.

The electrode potential and nitrate concentration directly modulate *OH coverage. For example, on Cu nanocube catalysts, negative potential shifts reduce *OH stability, while diluted nitrate solutions limit *OH accumulation from nitrate decomposition. This mechanistic insight explains why ammonia selectivity improves under these conditions. Controlling *OH adsorption thus serves as a descriptor for rational catalyst/electrolyte design. For instance, optimizing Cu surface morphology or alkaline electrolyte pH could fine-tune *OH binding strength, enhancing *H supply for efficient ammonia synthesis at low overpotentials.

This principle bridges theory and practice: in wastewater treatment, dilute nitrate streams coupled with moderate potentials could maximize ammonia yield while minimizing energy-intensive nitrite separation. Such strategies align with sustainable NH3 synthesis, replacing the Haber-Bosch process under mild conditions.