Energetics of C-N coupling reactions on Pt(111) and Ni(111) surfaces from application of density-functional theory.

We applied density-functional theory (DFT) with the projector-augmented-wave method (PAW) to investigate systematically the energetics of C-N coupling reactions on Pt(111) and Ni(111)surfaces. Our approach includes several steps: the adsorption of reactants and products (CHx, NHy and CHxNHy, x = 0-3, y = 0-2), movement of molecular fragments on the surface, and then C-N coupling. According to our calculations, the energies (ignoring the conventional negative sign) of adsorption of CHx and NHy on Pt(111)/Ni(111) surfaces decrease in the order C > CH > CH2 > CH3 and N > NH > NH2, with values 7.41/6.91, 6.97/6.52, 4.58/4.39, 2.19/2.01 eV and 5.10/5.49, 4.12/4.79, 2.75/2.87 eV, respectively. Regarding the adsorption energies among CHxNHy, the adsorption energy of CNH2 species is the highest on the Pt(111) surface, whereas on the Ni(111) surface CH3N is the most stable. The C-N coupling barriers differ on the two metallic surfaces despite the structures of initial, transition and final states being similar. On the Pt(111) surface, the coupling reaction of CH2 + NH2 has the smallest barrier, whereas CH + NH2 is the most favorable on the Ni(111) surface. The detailed local density of states (LDOS) and electron-localization functions (ELF) were investigated to rationalize the calculated outcomes.