Comparison of Nitrogen Activation on Trinuclear Niobium and Tungsten Sulfide Clusters Nb 3 S n and W 3 S n (n=0-3): A DFT Study.

The reaction of N 2 with trinuclear niobium and tungsten sulfide clusters Nb 3 S n and W 3 S n (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 Nb 3 S n is generally higher than that of W 3 S n . 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 Nb 3 S n decreases because of the hindering effect of S atoms, while W 3 S and W 3 S 2 have the highest reactivity among four W 3 S n 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 Nb 3 S n than in W 3 S n , leading to the higher reactivity of Nb 3 S n . 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).