A polyoxometalate cluster-based single-atom catalyst for NH3 synthesis via an enzymatic mechanism

Abstract

NH₃ synthesis by the electrochemical N₂ reduction reaction (eNRR) under mild conditions has attracted much attention. Here, by means of the first principles calculations, we propose a new strategy using a transition metal single-atom catalyst (SAC) anchored on a phosphomolybdic acid (PMA) cluster as a heterogeneous catalyst for the eNRR. We have systematically studied three reaction mechanisms, i.e., distal, alternating, and enzymatic pathways, respectively, for the eNRR on a Mo₁/PMA SAC via a six-proton and six-electron process, and found that the preferred mechanism is the enzymatic pathway with the smallest overpotential (η) of 0.19 V. N₂ is first strongly adsorbed on Mo₁/PMA and then dissociated by the subsequent protonation process. In addition, we found that Mo₁/PMA can impede the hydrogen evolution reaction (HER) process and thus promotes the eNRR selectivity. The high catalytic activity of Mo₁/PMA for the eNRR is attributed to the high spin density on Mo, enhanced N₂ adsorption, stabilization of the N₂H* species, and the destabilization of the NH₂* species. The present work is further extended to investigate the kinetics of the conversion of N₂ to ammonia on Mo₁/PMA via an enzymatic mechanism. Our results expose that the calculated activation energy barrier for the protonation of N₂ to form the N₂H₄* species is kinetically and thermodynamically more favorable compared with other elementary steps. These results provide valuable guidance for NH₃ synthesis using SACs at ambient temperature with high efficiency and low cost.