Design of Active Centers in Ammonia Synthesis on Mo-Based Catalysts: A Theoretical Study

The strong Mo–N bond restrains the catalytic activity of metallic Mo in ammonia synthesis. In this study, the semi-empirical calculations in conjunction with the density functional theory calculations, Brønsted–Evans–Polanyi relationship and microkinetic modeling were used to evaluate the rate of ammonia synthesis on model active sites of Mo-based alloys, nitrides, and clusters with a modified Mo–N bond. It was found that active sites of binary alloys Mo_δMe_1−δ (0 ≤ δ ≤ 1; Me = Co, Pt, Ir, Rh) show the synergetic behavior. The sites of ternary Mo₃Me₃N (Me = Mo, Co, Pt, Ir) and Mo₂N-type nitrides revealed higher activities than sites on Mo planes due to an extra Mo bond with the lattice N atom. The sites of octahedral clusters Mo₃Me₃N (Me = Mo, Co, Ir, Pt) exhibited higher catalytic activity than the sites of nitrides because their Me–N bonds are weaker than Mo–N. It was also found that tetragonal Mo₂Me₂ (Me = Co, Pt, Ir) and bi-tetragonal clusters Mo₃Me₂ (Me = Co, Ir, Pt) are the best cases because their sites provide the optimal combination of local structure and thermodynamics. Catalytic activities of the most active sites, relative to the Fe–C₇ center, were found to change in the row 18.4 (threefold site Mo₂Ir₁ in cluster Mo₃Ir₂), 7.3 (Mo₂ in cluster Mo₂Ir₂), 3.9 (Mo₂Pt₁ in cluster Mo₃Ir₃N), 3.8 (M₃ on alloy Mo_0.78Ir_0.22), 2.0 [Mo₃Pt₁ on the plane (100) of Mo₃Pt₃N], 0.57 [Mo₃ on the plane (111) of Mo₂N], and 0.03 [Mo₄ on the plane Mo(110)–(1 × 2)]. The design of tailor-made catalytic sites suggested in this paper can probably be applied to other catalytic systems.