Polyoxometalate Cluster Based Single Atom Catalyst for NH3 Synthesis via an Enzymatic Mechanism

NH3 synthesis by electrochemical N2 reduction reaction (eNRR) under mild conditions has attracted much attention. Here, by means of 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 eNRR. We have systematically studied three reaction mechanisms, i.e., the distal, alternating, and enzymatic pathways, respectively, for 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 promote the eNRR selectivity. The high catalytic activity of Mo1/PMA for eNRR is attributed to the high spin density on Mo, enhanced N2 adsorption, stabilization of N2H* species, and the destabilization of NH2* species. The present work is further extended to investigate the kinetic analysis for conversion of N2 to ammonia on the Mo1/PMA via enzymatic mechanism. Our results expose that the calculated activation energy barrier for the protonation of N2 to form N2H4* species is kinetically and thermodynamically more favorable compared with other elementary steps. Moreover, microkinetic investigation predicts a maximum eNRR rate of 3.20 x 10-17 s-1 following the enzymatic pathway over the Mo1/PMA cluster. These results provide valuable guidance for NH3 synthesis by SAC under ambient temperatures with high efficiency and low cost.