Activating the N₂ molecule by transition-metal atoms: a high-throughput first-principles computational screening

The first step in designing a good catalyst for producing ammonia is to understand what activates the N₂ molecule, yet its origin is often unclear due to interactions between the catalyst and its support. Using density functional theory and high-throughput screening, we studied N₂ adsorption across 3680 transition metal-N₂ configurations (monomers, dimers, and trimers of the 3d and 4d groups) by sampling all molecular orientations. We identified the ground-state adsorption geometry and the d-orbital population of the transition metal as the key factors governing N₂ activation; side-on binding allows stronger activation than end-on binding. Among single transition-metal atoms, Zr is found to be the best candidate for activating N₂, while early transition-metal dimers composed of Sc, Ti, Y, and Zr activate the N₂ molecule most strongly, stretching the N-N bond from 1.1 Å to 1.6 Å. This results from a cooperative effect where the metal atoms in the monomer and dimer transfer electrons into the antibonding π* orbitals of N₂, significantly weakening the N-N bond. However, this improvement does not continue in larger clusters. Despite stronger binding and larger charge transfer, trimers are less effective than dimers because metal-metal interactions compete with metal-N₂ bonding, reducing the efficiency of π back-donation. These results provide simple guidelines for designing efficient catalysts.