First-principles mechanistic study on nitrate reduction reactions on copper surfaces: Effects of crystal facets and pH

As a beneficial technology for nitrate removal and ammonia synthesis, electrocatalytic nitrate reduction reaction (NO3RR) has received much attention in experimental and theoretical studies. Previous work suggests that Cu is one of the most active electrocatalysts for NO3RR. However, the fundamental reaction mechanism, governing the activity and selectivity of Cu towards NO3RR with various facets and pH, remains elusive. Herein, a hybrid model is developed by combining the explicit water layer and the implicit solvation scheme to simulate the water/Cu interface based on theoretical density functional theory simulations. The hybrid model is used to elucidate the effects of crystal facets and pH on NO3RR mechanism over Cu. Our simulation results illustrate that NO3RR in acidic media proceeds via the formation of *NOH to produce NH3 on both (1 0 0) and (1 1 1) facets. While in alkaline media, Cu(1 0 0) prefers the formation of NH2OH via *NHO at low overpotentials, and Cu(1 1 1) kinetically favors the formation of NH3. In general, the (1 0 0) facet is more active towards NO3RR than the (1 1 1) facet, especially at higher pH, due to lower thermodynamic and kinetic barriers. Our results provide deeper mechanistic insights into the effects of both crystal facets and pH on the electrocatalytic activity and product selectivity of Cu towards NO3RR. The hybrid model developed here can be applied to other electrochemical reactions at water/metal interfaces.