Unveiling the Nature of High Catalytic Activity of the Zr1@Mo2TiC2 Single-Atom Catalyst for N2-to-NH3 Thermal Conversion

Abstract

Two-dimensional (2D) MXene nanomaterial-supported single-atom catalysts (SACs) have attracted extensive attention due to their high stability and catalytic performance in ammonia synthesis. Herein, density functional theory (DFT) calculations were performed to systematically investigate the structural stability and electronic properties of M₁@Mo₂TiC₂ SACs. Among these SACs, Zr₁@Mo₂TiC₂ was screened as the most stable SAC, on which N₂ can be directly activated well, akin to its activation on Fe(211) C7 (iron atoms with seven nearest neighbors) and Ru(0001) B5 sites, and H₂ can also be adsorbed dissociatively. Further calculations indicated that N₂ can be directly converted to NH₃ on Zr₁@Mo₂TiC₂ through a dissociation mechanism with a low energy barrier of 1.13 eV in the rate-determining step (RDS). Moreover, microkinetic simulations showed that the turnover frequency (TOF) of ammonia synthesis on Zr₁@Mo₂TiC₂ is as high as 1.01 × 10^–2 s^–1 site^–1 at 51 bar and 700 K. The nature of high catalytic activity stems from the effective σ donation from N₂(3σ_g) → Zr(4d) and the three π back-donation from Mo(4d) → N₂(1π_g), yielding the well-activated *N₂ with the N–N bond order of 1.5 on the Zr₁Mo₃ single-cluster site, which is effectively converted into NH₃. Our work provides a theoretical understanding of the stability and catalytic mechanism of Zr₁@Mo₂TiC₂ and guidance for further designing and fabricating MXene-based metal SACs for N₂ fixation.