New Insights into the Electrochemical Nitrate Reduction Reaction on Cu(111) from Theoretical Calculations

Abstract

Ammonia production via renewable electricity-driven electrochemical processes at ambient conditions is preferable to traditional energy- and carbon-intensive Haber–Bosch processes. Nitrate (NO 3 – ), a common water pollutant with mild N–O bond strength, is an ideal nitrogen precursor for the generation of NH 3 . An atomistic understanding of the reaction mechanisms of the electrochemical nitrate reduction reaction (NO 3 RR) on model catalytic surfaces is needed for the bottom-up design of efficient catalysts to develop NO 3 – to NH 3 conversion technologies. Here, we perform first-principles density functional theory (DFT) calculations to get an atomistic understanding of the NO 3 RR on Cu(111) under conditions similar to experiments. Our calculations provide a deeper mechanistic insight into the NO 3 RR pathways and show that the NO 3 RR proceeds via the formation of *NO 3 → *NO 2 -*O → *NO 2 -*OH → *NO 2 → *NO-*O → *NO-*OH → *NO → *NHO → *NHOH → *NH-*OH → *NH → *NH 2 → *NH 3 → * + NH 3 intermediates with *NHO formation identified as the potential determining step (PDS). The charge extrapolated-constant potential (CE-CP) and grand canonical-density functional theory (GC-DFT) methods are implemented to compute the potential-dependent reaction energetics of the NO 3 RR along the most favorable pathway. The results show that the computational hydrogen electrode (CHE) model overestimates the free energy change (Δ G ) of electrochemical steps in the anodic region ( U higher than 0 V vs U RHE ) and underestimates it in the cathodic region ( U lower than 0 V vs U RHE ) for electrochemical reduction reaction steps. Furthermore, in general, GC-DFT calculated free energy changes are lower compared with values obtained using CHE and CE-CP methods. Importantly, our constant potential ( U ) calculations illustrate that in alkaline conditions, the NO 3 RR is facilitated at U above −0.49 V, and the HER is favorable at U below −0.49 V, providing a key insight into the selection of the U and pH range suitable for selective NO 3 RR. Thus, our extensive DFT calculations provide new insights into the reaction pathways and key steps of the NO 3 RR. The U -dependent reaction energetics outline a rational selection of operating conditions (pH, U range) to selectively convert NO 3 – to NH 3, minimizing the undesired side products. These fundamental key insights will be critical in designing Cu-based catalysts for the selective NO 3 RR.

Topics & Concepts