Theoretical Insights into Superior Nitrate Reduction to Ammonia Performance of Copper Catalysts

Nitrate reduction to ammonia (NRA) is critical and attractive for environmental remediation and energy conservation. Copper represents one of the most promising non-noble-metal NRA electrocatalysts while its intrinsic catalytic activity of facets and pH influence remain unclear. Using density functional theory calculations, nitrate reduction to ammonia pathways are evaluated on low-index crystal surfaces, Cu(111), Cu(100), and Cu(110), at different pH. Systematic thermodynamic and kinetic analysis indicates that the pathway *NO3- -> *NO3 -> *NO2 -> *NO -> *NOH -> *NHOH -> *NH -> *NH2 -> *NH3 -> NH3(g) is the most probable in all pH ranges, ending a long-standing debate on NRA pathways. Both the catalytic deoxygenation and hydrogenation processes in NRA are substantially affected by pH. Thus, the rate-determining steps and overpotentials exhibit pH-dependent characteristics. Besides, it is found that the pH influences the competition between the hydrogen evolution reaction (HER) and NRA. By considering NRA and HER on different surfaces, we found that Cu(100) and Cu(111) contribute most to NRA other than Cu(110). Specifically, in near-neutral and alkaline environments, Cu(111) exhibits the best NO3- to NH3 performance, while Cu(100) is more effective in a strong acidic environment. This result rationalizes recent experimental observations. The NRA activity differences of copper surfaces are attributed to the local coordination environment and electronic states of surface atoms. Thanks to a stereospecific Cu-Cu couple, both strong *NOH adsorption and weak *NH3 adsorption are realized on Cu(111) and Cu(100), facilitating superior NRA.

Automated summary

This study investigated the catalytic performance of nitrate reduction to ammonia on copper crystal surfaces and found that Cu(100) and Cu(111) contribute most to NRA, with different optimal pH conditions. The research clarified the NRA pathway and highlighted the influence of pH on the catalytic processes.