A theoretical evaluation of possible transition metal electro-catalysts for N₂reduction

TLDR

The study examines the feasibility of electrochemical ammonia synthesis at ambient temperature and pressure. Density functional theory combined with the computational standard hydrogen electrode was used to compute free‑energy profiles and activity trends for N₂ reduction on various transition‑metal surfaces in acidic electrolyte, assuming activation barriers scale with elementary‑step free‑energy differences. The calculations predict that Mo, Fe, Rh, and Ru are the most active catalysts, yet hydrogen evolution competes and lowers faradaic efficiency; early transition metals such as Sc, Y, Ti, and Zr bind N more strongly than H, enabling higher ammonia yields at 1–1.5 V vs SHE, and defect‑free surfaces outperform stepped ones.

Abstract

Theoretical studies of the possibility of forming ammonia electrochemically at ambient temperature and pressure are presented. Density functional theory calculations were used in combination with the computational standard hydrogen electrode to calculate the free energy profile for the reduction of N2 admolecules and N adatoms on several close-packed and stepped transition metal surfaces in contact with an acidic electrolyte. Trends in the catalytic activity were calculated for a range of transition metal surfaces and applied potentials under the assumption that the activation energy barrier scales with the free energy difference in each elementary step. The most active surfaces, on top of the volcano diagrams, are Mo, Fe, Rh, and Ru, but hydrogen gas formation will be a competing reaction reducing the faradaic efficiency for ammonia production. Since the early transition metal surfaces such as Sc, Y, Ti, and Zr bind N-adatoms more strongly than H-adatoms, a significant production of ammonia compared with hydrogen gas can be expected on those metal electrodes when a bias of 1 V to 1.5 V vs. SHE is applied. Defect-free surfaces of the early transition metals are catalytically more active than their stepped counterparts.