Periodic and non-periodic DFT modeling of CO reduction on the surface of Ni-doped graphene nanosheet

Carbon monoxide, as a major pollutant of the environment, could be reduced to hydrocarbons by using transition metal catalysts. Different catalysts produce different hydrocarbons, ranging from methane to heavier hydrocarbons. Some catalysts prefer methanation, while some others do Fischer-Tropsch process. Both kinds of catalysts suffer from coke formation on their surfaces. Coke is produced from direct dissociation of CO on the surface of a catalyst. In this paper, direct and indirect dissociation paths of CO on the Ni-doped graphene are studied by periodic and non-periodic DFT calculations. It is shown that direct CO dissociation has very high barrier energy of 5.32 eV which prevents coke formation on the catalyst. Indirect path investigation shows that HCO intermediate is more feasible than COH one. With the addition of one H to CO, barrier energy is reduced from 5.3 eV to 3.09 eV, and introducing one more H to form CH2O intermediate, the barrier energy is reduced to 1.5 eV. In addition, NBO and QTAIM theories are used to study the donor-acceptor charge transfers and nature of interactions. Data shows that there is a large charge donation and back-donation for CO adsorption on the surface of Ni-doped graphene. Electron density difference graphs and NBO/QTAIM charges show that, in all studied complexes, charge transfer is occurred from Ni to the adsorbed species.