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

Abstract

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.