Mechanistic understanding of N₂ 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 s donation and p* back-donation between N-2(s, p*) 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 s 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 p donation from N-2(p) to Ru(d) orbitals as revealed by analyses of the density of states and partial charge densities. The combined s and p donation resulted in an increased synergistic p* back-donation. The total interactions between N-2(s, p, p*) 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(s, p, p*) 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 p* back-donation and N-2 activation. These new insights may therefore open new avenues to design supported Ru catalysts for nitrogen activation.