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

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(3σg) → Zr(4d) and the three π back-donation from Mo(4d) → N2(1πg), 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.