From Single Atom to Low-Nuclearity Cluster Immobilized Ti3C2TX MXene for Highly Efficient NO Electroreduction to NH3

Single-atom catalysts (SACs) have improved the performance of NO electroreduction to NH3 (NORR) to a high level. However, the influence of low-nuclearity clusters, which exist in SACs as well, on the catalytic performance is always overlooked. Herein, via density functional theory, we not only designed 9 single TMs, namely, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu, anchored on three Ti3C2O2-based models (Ti3C2O2-V0, Ti3C2O2-O1, and Ti3C2O2-Ti1) as catalysts for NORR but also further evaluated the impact of low-nuclearity clusters on optimal SAC. Our results identified Cu/Ti3C2O2-V0 and Ni/Ti3C2O2-Ti1 as promising SACs for the ultrahigh performance of NORR. Furthermore, NORR activity on both SACs diminished with the formation of the low-nuclearity cluster, especially for Ni clusters on Ti3C2O2-Ti1. From Ni single atoms to Ni clusters, the projected density of states increases significantly near the Fermi level, leading to stronger interactions between Ni clusters and reaction intermediates, thereby hindering hydrogenation steps and greatly slowing down the NORR on the Ni cluster. Meanwhile, a pH-dependent catalytic activity analysis revealed that the rate-determining step changed as the pH increased on Ni/Ti3C2O2-Ti1, and an acidic environment facilitates the effective NO-to-NH3 conversion on Cu/Ti3C2O2-V0. Therefore, this work not only predicts the highly efficient NORR catalyst but also reveals the weakening mechanism of low-nuclearity clusters that would guide future experimental research on SACs and metal clusters for NORR.