Geometric and Electronic Effects Contributing to N₂ Dissociation Barriers on a Range of Active Sites on Ru Nanoparticles

Abstract

Density functional theory calculations have been used to calculate activation barriers for N2 dissociation on a range of possible active sites on ruthenium (Ru) nanoparticles, including step, edge, and planar sites. Variations in activation barriers are rationalized through a combination of geometric and electronic effects. The lowest-energy barriers are obtained using a favorable five-fold Ru atom surface feature, although surfaces with four-fold Ru atom features are also able to offer appreciably low barriers. The degree of undercoordination of the various sites has been quantified using generalized coordination numbers and is found to largely account for differences in activity between sites with the same geometry. Transition state energies have been converted to rate constants at typical industrial conditions using harmonic transition state theory. It is found that the most active site is the so-called B5 step site, commonly accepted to be the active site for N2 dissociation. However, several other step sites also offer competitive reactivity, within an order of magnitude at industrial conditions. Further, several edge sites, which have been previously identified as present on representative nanoparticles, have been found to give appreciably low barriers; the most active edge site is only 20 times less active than the most active step site, lending to the possibility of other sites contributing to the industrial formation of NH3. Even the quasi-planar (101̅1) site exhibits a modest transition state energy, which may contribute to the formation of NH3 on large nanoparticles, given its likely very high abundance.