Local Chemical Environment Dependent Nitrate-Reduction-to-Ammonia Performance on Cu-Based Electrocatalysts

The active component of copper-based materials for electrocatalytic nitrate reduction to ammonia (NRA) remains unclear due to the susceptibility of oxidation of copper. Using density functional theory calculations, NRA pathways are evaluated on low-index crystal surfaces Cu₂O (111), CuO (111), and Cu (111) at different pH. Cu₂O (111), with abundant undercoordinated Cu atoms on the surface, shows easier adsorption of NO₃^- than Cu (111) or CuO (111). NRA on CuO (111) is hindered by the large ΔG of adsorption of NO₃^- and hydrogenation of *NO. Thus, Cu (111) and Cu₂O (111) contribute most to the NRA activity while CuO (111) is inert. Three key steps of NRA on copper-based catalysts are identified: adsorption of NO₃^-, *NO → *NOH/*NHO, and *NH₃ desorption, as the three can be rate-determining steps depending on the local environment. Moreover, previous experimentally detected NH₂OH on copper-based catalysts may come from the NRA on Cu₂O (111) as the most probable pathway on Cu₂O (111) is NO₃^- → *NO₃ → *NO₂ → *NO → *NHO → *NHOH → *NH₂OH → *NH₂ → *NH₃ → *NH₃(g). At high reduction potential, CuO would be reduced into Cu, so the effective active substance for NRA in a strong reduction environment is Cu.