Mechanistic understanding of N2 activation: a comparison of unsupported and supported Ru catalysts

Abstract

N₂ dissociative adsorption is commonly the rate-determining step in thermal ammonia synthesis. Herein, we performed density functional theory (DFT) calculations to understand the N₂ dissociation mechanism on models of unsupported Ru(0001) terraces, Ru B5 sites, and polar MgO(111)-supported Ru₈ cluster mimicking a B5 site geometry, denoted (Ru₈(B5-like)/MgO(111)). The activation energy of N₂ dissociative adsorption on the Ru₈(B5-like)/MgO(111) model (E_a = 0.33 eV) is much lower than that on the unsupported Ru(0001) terrace (E_a = 1.74 eV) and Ru B5 (E_a = 0.62 eV) models. The lower N₂ dissociation barrier on Ru B5 sites is facilitated by the enhanced σ donation and π* back-donation between N₂(σ, π*) and Ru(d) orbitals resulting in the stronger activation of the molecular side-on N₂* dissociation precursor. The Ru₈(B5-like)/MgO(111) also exhibits enhanced σ donation because of the B5-like cluster geometry. Furthermore, the Ru cluster of the bare Ru₈(B5-like)/MgO(111) model is positively charged. This induced an unusual π donation from N₂(π) to Ru(d) orbitals as revealed by analyses of the density of states and partial charge densities. The combined σ and π donation resulted in an increased synergistic π* back-donation. The total interactions between N₂(σ, π, π*) and Ru(d) resulted in an overall electron transfer to the adsorbed N₂ from the Ru atoms in the B5-like site with no direct involvement of the MgO(111) substrate. Analyses of bond stretching vibrations and bond lengths show that the N₂(σ, π, π*) and Ru(d) interactions lead to a weaker N–N bond and stronger Ru–N bonds. These correspond to a lower barrier of N₂ dissociation on the Ru₈(B5-like)/MgO(111) model, where the highest red-shift of N–N vibration and the longest N–N bond length were observed after side-on N₂* adsorption. These results demonstrate that an electron-deficient Ru catalyst are not always inhibited from donating electrons to adsorbed N₂. Rather, this study shows that the electron deficiency of Ru can promote π* back-donation and N₂ activation. These new insights may therefore open new avenues to design supported Ru catalysts for nitrogen activation.