Comparison of Nitrogen Activation on Trinuclear Niobium and Tungsten Sulfide Clusters Nb3Sn and W3Sn (n=0-3): A DFT Study

The reaction of N-2 with trinuclear niobium and tungsten sulfide clusters Nb3Sn and W3Sn (n=0-3) was systematically studied by density functional theory calculations with TPSS functional and Def2-TZVP basis sets. Dissociations of N-N bonds on these clusters are all thermodynamically allowed but with different reactivity in kinetics. The reactivity of Nb3Sn is generally higher than that of W3Sn. In the favorite reaction pathways, the adsorbed N-2 changes the adsorption sites from one metal atom to the bridge site of two metal atoms, then on the hollow site of three metal atoms, and at that place, the N-N bond dissociates. As the number of ligand S atoms increases, the reactivity of Nb3Sn decreases because of the hindering effect of S atoms, while W3S and W3S2 have the highest reactivity among four W3Sn clusters. The Mayer bond order, bond length, vibrational frequency, and electronic charges of the adsorbed N-2 are analyzed along the reaction pathways to show the activation process of the N-N bond in reactions. The charge transfer from the clusters to the N-2 antibonding orbitals plays an essential role in N-N bond activation, which is more significant in Nb3Sn than in W3Sn, leading to the higher reactivity of Nb3Sn. The reaction mechanisms found in this work may provide important theoretical guidance for the further rational design of related catalytic systems for nitrogen reduction reactions (NRR).