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

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 prole for the reduction of N 2 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 dierence 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 eciency 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 signicant 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.

Recent progress towards the electrosynthesis of ammonia from sustainable resources

“…According to the calculations of Skúlason et al, 55 no catalytic activity is expected above -0.5 V vs.…”

“…Heterogeneous catalysts that bind nitrogen atoms to their surfaces more strongly than they bind hydrogen atoms may reduce hydrogen evolution, but this seems likely to require employing catalysts that are not optimal for the nitrogen reduction reaction itself, such as the early transition metals. 55 Then again,…”

“…55 Volcano diagrams were created showing the most active surfaces, with Mo, Fe, Rh, and Ru predicted to be the most active for NH 3 generation.…”

Electrochemical Synthesis of Ammonia: A Low Pressure, Low Temperature Approach

“…20,30,31 Most recently, Licht and co-authors demonstrated significantly higher ammonia production rates and faradaic efficiencies in a molten hydroxide electrolyte cell with a nano-Fe 2 O 3 catalyst and at slightly elevated temperatures (105 °C-200 °C). 13,18 Initial theoretical modeling efforts by Howalt, Skúlason and co-authors 32,33 (Fig. 4, volcano plot representation of theoretical predictions) suggested that while ruthenium may be optimal for nitrogen reduction, several nonprecious metals such as iron, nickel, and cobalt might be useful especially in combination.…”

Rational design of metal nitride redox materials for solar-driven ammonia synthesis

“…Employing these trends for the computational screening [3,59] of advanced metal nitride redox materials is outlined. Figure 7a decomposes the electronic interaction of the metal dopant, as an example for doped Mo 2 N, into the metalprojected s-, p-and d-orbitals.…”