A novel phosphotungstic acid-supported single metal atom catalyst with high activity and selectivity for the synthesis of NH3 from electrochemical N2 reduction: a DFT prediction

Abstract

The electrochemical reduction of N₂ to generate NH₃ (NRR) under ambient conditions is a promising alternative to the industrial Haber–Bosch process which requires high temperature and pressure. NRR electrocatalysts are needed to overcome the slow kinetics due to the high energy barrier for NN bond cleavage. Another main challenge is suppressing the competing hydrogen evolution reaction (HER) which results in poor NRR selectivity. Here, we report the development of a novel and cost-efficient electrocatalyst—a phosphotungstic acid (PTA)-supported single metal (M) atom (M-PTA, M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, and Cd) which exhibits substantial stability—via a comprehensive theoretical screening formula of over 20 different d-block metals (M-PTA, M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, and Cd). It is interesting to mention that this class of catalysts has a greatly suppressed HER selectivity with 17 of the 20 candidates falling in the N₂-dominant region. Further study demonstrates that Mo-PTA, Tc-PTA and Ru-PTA only demand an energy input of 0.42 eV, 0.24 eV and 0.34 eV for the first hydrogenation step of N₂. Detailed analysis of NRR mechanisms show that it follows the distal mechanism with an overpotential of 0.26 V on Mo-PTA. This work provides DFT guidelines for developing stable electrocatalysts through experiments for catalyzing the NRR with high reactivity and selectivity.