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

N dissociative adsorption is commonly the rate-determining step in thermal ammonia synthesis. Herein, we performed density functional theory (DFT) calculations to understand the N dissociation mechanism on models of unsupported Ru(0001) terraces, Ru B5 sites, and polar MgO(111)-supported Ru cluster mimicking a B5 site geometry, denoted (Ru(B5-like)/MgO(111)). The activation energy of N dissociative adsorption on the Ru(B5-like)/MgO(111) model ( = 0.33 eV) is much lower than that on the unsupported Ru(0001) terrace ( = 1.74 eV) and Ru B5 ( = 0.62 eV) models. The lower N dissociation barrier on Ru B5 sites is facilitated by the enhanced σ donation and π* back-donation between N(σ, π*) and Ru(d) orbitals resulting in the stronger activation of the molecular side-on N* dissociation precursor. The Ru(B5-like)/MgO(111) also exhibits enhanced σ donation because of the B5-like cluster geometry. Furthermore, the Ru cluster of the bare Ru(B5-like)/MgO(111) model is positively charged. This induced an unusual π donation from N(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(σ, π, π*) and Ru(d) resulted in an overall electron transfer to the adsorbed N 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(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 dissociation on the Ru(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* adsorption. These results demonstrate that an electron-deficient Ru catalyst are not always inhibited from donating electrons to adsorbed N. Rather, this study shows that the electron deficiency of Ru can promote π* back-donation and N activation. These new insights may therefore open new avenues to design supported Ru catalysts for nitrogen activation.