A First-principles Study Of Graphene-based Composite Materials In Photocatalytic Reduction Of CO₂

Due to the widely use of fossil fuels, the concentration of CO₂in the air hasincreased rapidly year by year, which has became a serious global problem. On theother hand, as the huge consumption of coal, oil and natural gas also caused worldwideenergy shortage. Therefore, the effective recovery and utilization of CO₂have a dualsignificance to resolve energy and environmental issues. So far, converting CO₂intohigh value-added chemical products or organic fuel by photocatalytic reaction is themost promising and effective one in many conversion methods.The rapid development of quantum chemistry theoretical study and the rapidincrease in computer performance make treating complex catalytic system moreaccurately as possible. In this paper, adsorption and activation of CO₂and catalyticreduction process for metal-graphene composite systems were studied byfirst-principles calculations, and then screen the catalyst by theoretical calculation andanalysis.The adsorption and activation of CO₂is a crucial step in the reaction system of theCO₂photocatalysis. The different activated modes and states of CO₂determine theroute of the reaction and the final products. Metal-graphene system was taken as theresearch object in this study. Density functional theory （DFT）, combined with localdensity approximation （LDA） and PWC functional, was employed to study the changesin the geometry structure, energy, charge distribution and density of states （DOS） ofthe systems before and after absorption of CO₂on them. The results show that theelectrons are transferred from the M-graphene system to CO₂, which is eventuallyactivated by negative charge. The Cu-G system is most effective to activate CO₂inthese three complexes. The bond length of CO₂increased by6and14pm, respectively,and the bond angle of O-C-O decreased to122°. Furthermore, the first ionizationenergy and electron affinity of metal clusters and graphene play a decisive role in theelectron transfer. Compared with the first ionization energy of graphene, the largerelectron affinity of metal clusters, the more electrons transferred from graphene tometal cluster.The catalytic reduction of CO₂occurs when they adsorbed and activated on theCu-G. The catalytic reduction pathways of Cu-G system and Cu cluster system ascatalysts reducing CO₂were studied by DFT calculation with B3LYP hybrid functional in this chapter. We mainly discussed the catalytic reaction pathway of the two systems,the changes of the geometry, the energy of ground states and excited states energy, theelectronic structure and other properties. The results show that the Cu-G system meetsthe requirment of catalyst in the photocatalytic reduction of CO₂from the reactionthermodynamics and kinetics, without considering the excitation probability and thelifetime of the excited state. Reaction requires the wavelength of excitation light lessthan604.8nm, and need to overcome a reaction barrier with91.6kJ·mol^-1, thenreduceing the CO₂to HCOOH. The catalytic processes of Cu cluster along the reactionpath in ground state and excited statewere needed to overcome125.4kJ·mol^-1and146.7kJ·mol^-1barrieres, respectively, so it is not suitable for photocatalytic reductionof CO₂as a catalyst.