Regulating surface oxygen species on copper (I) oxides via plasma treatment for effective reduction of nitrate to ammonia

Electrochemical nitrate reduction (NO 3 - RR) to synthesize ammonia is considered to be a promising strategy to enable artificial nitrogen cycle. Great efforts have been devoted to improving the efficiency and selectivity of the electrocatalysts for NO 3 - RR. Herein, we demonstrate that tuning the oxygen chemical environment via Ar plasma treatment is an effective approach to improve the NO 3 - RR activity of Cu 2 O. Combining synchrotron-based X-ray absorption spectroscopy and other advanced spectroscopy techniques, we find that plasma treatment can effectively promote the formation of oxygen vacancies and hydroxyl groups on Cu 2 O surface. In-situ diffuse-reflectance infrared Fourier transform spectroscopy and density functional theory calculation further reveal that oxygen vacancies and hydroxyl groups facilitate the adsorption of nitrate and proton transfer on the Cu 2 O surface, thus leading to improved ammonia selectivity. Our results clarify the critical role of surface oxygen species for NO 3 - RR and can guide the design of other electrocatalysts via surface engineering.