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, Bronsted-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 delta Me1-delta (0 <= delta <= 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-C-7 center, were found to change in the row 18.4 (threefold site Mo2Ir1 in cluster Mo3Ir2), 7.3 (Mo-2 in cluster Mo2Ir2), 3.9 (Mo2Pt1 in cluster Mo3Ir3N), 3.8 (M-3 on alloy Mo0.78Ir0.22), 2.0 [Mo3Pt1 on the plane (100) of Mo3Pt3N], 0.57 [Mo-3 on the plane (111) of Mo2N], and 0.03 [Mo-4 on the plane Mo(110)-(1 x 2)]. The design of tailor-made catalytic sites suggested in this paper can probably be applied to other catalytic systems.