DFT studies of Ni cluster on graphene surface: effect of CO2 activation

Abstract

CO₂ capture, storage, sequestration (CCS), and utilization (CCU) are regarded as the most effective way to slow the pace of global warming. In our present work, the structural stability and catalytic properties for adsorbing CO₂ on the surface of isolated Ni clusters and 5–8–5 monovacancy graphene (MGr) supported Ni clusters have been investigated using density functional theory. The results show that, with the increasing of atomic number, the structure of isolated Ni clusters tended to be stable and the structural parameters approached to that of metal crystal. In addition, when Ni clusters deposited on the MGr surface, the Ni–Ni bond generally elongated, indicating activation of the isolated metal clusters by graphene. Adsorption energies of CO₂ onto Ni₄ cluster and MGr were −1.80 and −1.35 eV, respectively, while the value reached −2.72 eV when CO₂ adsorbed on Ni₄ cluster modified with MGr (Ni_x/MGr). The phenomenon indicates that the adsorption capacity of CO₂ were significantly improved by depositing Ni clusters onto MGr surface and the system tended to be more stable. According to the analyses of Mulliken charge, electrostatic potential (EP) and partial density of states (PDOS), more electrons transferred from Ni_x cluster to the region of CO₂ and MGr when CO₂ adsorbed on Ni_x/MGr. The mechanism was proposed for CO₂ adsorption on Ni_x/MGr at the molecular level. It is of great significance to devise high activity catalyst of CO₂ storage and hydrocarbon production from CO₂.