Adsorption mechanism of the N₂ and NRR intermediates on oxygen modified MnN₄-graphene layers - a single atom...

In the present work the adsorption of N₂ and the nitrogen reduction reaction (NRR) intermediates have been investigated on oxygen modified MnNO (x + y = 4, x ≠ 0)/graphene layers through periodic density functional theory calculations. Various number of oxygen atoms substitute nitrogen atoms within the MnNO, with their effect on the layer stability, chemical bonding and N₂ adsorption being explored. As the oxygen amount increases in the porphyrin unit, Mn-O interactions weaken with reference to that of Mn-N, bonding orbitals become less populated while the antibonding orbitals between Mn-N-O atoms become partially occupied, as evidenced by the Crystal orbital Hamiltonian population (COHP) and integrated crystal orbital bond index (ICOBI) analyses. During N₂ adsorption on the different layers, the substitution of two and three nitrogen atoms by oxygen leads to the longest NN molecular bond length. Two main orientations for the N₂ molecules sorption have been investigated: side-on and end-on which are perpendicular and parallel to the surface normal, respectively. When the interaction of N₂ with MnNO₃ layer is considered, d-band center variation of the Mn with reference to the pre-adsorbed state is more obvious after side-on adsorption configuration. For the selected layers based on initial N₂ adsorption energies, the adsorption energies of nitrogen reduction reaction intermediates follow a trend based on the number of oxygen atoms in the porphyrin units. Charge density difference (CDD) maps and partial density of states (PDOS) analysis reveal that the interaction of N₂ with oxygen modified layers takes place through electron acception-donation mechanism between the partially occupied Mn-d orbitals and the 2p orbitals of the N₂ molecule. DDEC6-derived bond orders and atomic charges support the PDOS and adsorption/formation energy trends, and further clarify the bonding strengths of the atoms in the porphyrin units, as well as the Mn-N₂ interactions in the adsorbed systems.