Role of Metal Centers in Metal-Organic Frameworks for Nitrogen Reduction Reaction: A First-Principles Study

The impact of central metal elements in the open metal sites of metal-organic frameworks (MOFs) for the nitrogen reduction reaction was examined through first-principles (FP) calculations for M 2(dobdc) and M 2(dsbdc) with and without N2, where M = V, Cr, Mn, Fe, Co, Ni, Mo, Ru, dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate, and dsbdc4- = 2,5-disulfidobenzene-1,4-dicarboxylate. The structure optimization for MOFs including adsorbed N2, as determined by the FP calculations, revealed that the N-N distance in the N2 molecule adsorbed on metal elements such as V, Cr, Mo, and Ru exceeds that in the isolated N2 molecule. Conversely, the N-N distance remained unchanged for metal elements with stable divalent oxidation states such as Mn, Fe, Co, and Ni. The adsorption energies of N2 on V2(dobdc) and Ru2(dobdc) are -146.6 and -72.9 kJ/mol, respectively, which are lower compared with the adsorption energy on Fe2(dobdc) of -29.1 kJ/mol. In addition, the M-N distances for M = V and Ru have decreased to 2.086 & Aring; and 1.873 & Aring;, respectively, which are shorter than the Fe-N distance of 2.350 & Aring;. These results suggest that N2 can chemically adsorb on V and Ru. Analysis of the partial charge density indicated that the formation of bonding states between the d epsilon orbital of the metal element and p orbital of nitrogen, as well as the formation of antibonding states between nitrogen atoms, is crucial in decreasing the M-N distance, and increasing the N-N distance of adsorbed N2.