Adsorption mechanism of the N2 and NRR intermediates on oxygen modified MnN4–graphene layers – a single atom catalysis perspective

Abstract

In the present work the adsorption of N₂ and the nitrogen reduction reaction (NRR) intermediates have been investigated on oxygen modified MnN_xO_y (x + y = 4, x ≠ 0)/graphene layers through periodic density functional theory calculations. Various number of oxygen atoms substitute nitrogen atoms within the MnN_xO_y, 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 NN 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.