Molecular adsorption of the NCO, CNO and CON isomers on the Rh(001) surface : A standard DFT and DFT plus U calculations

The adsorption and dissociation of the NCO, CNO and CON isomeric molecules on Rh (001)-(3x3) are investigated using the standard density functional theory (DFT) and DFT with Hubbard U, i.e., DFT+U methods. The generalized gradient approximation (GGA) is used as the exchange-correlation functional. Also, three possible adsorption sites, which are hollow (H), bridge (B) and top (T) as well as two orientations, i.e., vertical and tilted, of the molecules on the Rh surface have been considered. We found that of the three isomers, the CNO molecule adsorbed at the hollow site of the Rh surface via its C atom end is the most stable for both methods. However, the inclusion of U parameter to GGA leads to the reduction in energies and slight changes in the relaxed adsorption geometries. Also, with the aid of nudged elastic band calculation (NEB), we study the minimum energy pathway (MEP) for dissociation of the CNO molecule into CN and O fragments. We obtain an activation barrier to the dissociation, E, of 0.97 eV with an enthalpy change Delta H = -1.25 eV. This dissociation process is found to be exothermic. Furthermore, the projected density of states (PDOS) calculations performed using the atomic structures corresponding to initial (IS), transition (TS), and the final states (FS) of the CNO dissociation process MEP show significant reduction and shifting of N-2p and O-2p states toward higher energies, as well as vanishing N-2p states. This reduction and vanishing of orbital states evidence the weakening and eventual breaking of N-O bond of CNO, which eventually led to the dissociation of the CNO molecule. This is further underscored by the total electronic density plot which shows a gradual decrease and eventual vanishing of charge density between the CN and O species as the dissociation process progresses.