Mechanistic understanding of N 2 activation: a comparison of unsupported and supported Ru catalysts.

N 2 dissociative adsorption is commonly the rate-determining step in thermal ammonia synthesis. Herein, we performed density functional theory (DFT) calculations to understand the N 2 dissociation mechanism on models of unsupported Ru(0001) terraces, Ru B5 sites, and polar MgO(111)-supported Ru 8 cluster mimicking a B5 site geometry, denoted (Ru 8 (B5-like)/MgO(111)). The activation energy of N 2 dissociative adsorption on the Ru 8 (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 N 2 dissociation barrier on Ru B5 sites is facilitated by the enhanced σ donation and π* back-donation between N 2 (σ, π*) and Ru(d) orbitals resulting in the stronger activation of the molecular side-on N 2 * dissociation precursor. The Ru 8 (B5-like)/MgO(111) also exhibits enhanced σ donation because of the B5-like cluster geometry. Furthermore, the Ru cluster of the bare Ru 8 (B5-like)/MgO(111) model is positively charged. This induced an unusual π donation from N 2 (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 N 2 (σ, π, π*) and Ru(d) resulted in an overall electron transfer to the adsorbed N 2 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 N 2 (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 N 2 dissociation on the Ru 8 (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 N 2 * adsorption. These results demonstrate that an electron-deficient Ru catalyst are not always inhibited from donating electrons to adsorbed N 2 . Rather, this study shows that the electron deficiency of Ru can promote π* back-donation and N 2 activation. These new insights may therefore open new avenues to design supported Ru catalysts for nitrogen activation.