| Transition metals are generally adopted in carbon dioxide methanation to overcome the kinetic hurdle.Among them,nickel is widely used since it has high catalytic conversion and low activation energies to dissociate CO2.However,the metal sintering,the formation of mobile nickel sub-carbonyls,and carbon deposits limit the application in catalytic reactions.Ni-based catalytic has been recognized as an effective response to those problems in laboratories.La2O3,an alkaline support,has been investigated to prevent metal sintering by avoiding particle coalescence.High catalytic activity and almost 100%methane selectivity exhibited in Ni/La2O3 at 208～380℃ have also been observed in the previous experimental results.However,most existing literature on Ni-based catalysts mainly performed at nanoparticles supported on oxides surface to simulate CO2 methanation process,while the diffusion properties at the interface structures were often neglected for structural complexity.Nevertheless,the extra adsorption site formation and the corresponding possible diffusion path of the Ni/La2O3 interface have not yet been seriously addressed,although they immensely affect catalysis properties.In chapter 1,we introduce the background of carbon dioxide methanation and the physicochemical properties of nickel metal,as well as the research progress on nickel-based catalysts.In chapter 2,a brief introduction of the interface definition and theoretical simulation methods of interface materials has been provided.Besides,the density functional theory based first-principles code VASP,and the CI-NEB method for transition states have also been briefly introduced.In chapter 3,a systematic study on the construction of the Ni/La2O3 interface,structural stability and electronic properties are discussed,as well as the choice of an appropriate side surface of the interface for the following methanation simulation.The optimal structure of Ni/La2O3 is obtained by minimizing the lattice mismatch between the lattice constant and the cell shape.The cluster expansion and strain application methods are mainly used to obtain the common lattice constant.The translation method is employed to obtain the optimal relative position structure of atoms and the spacing of the interface layer.By analyzing the partial-wave state density of some atoms at the interface,the main bonding at the interface and the factors affecting the stability of the interface are determined.Additionally,four low-index surfaces of the interface structure are tested to set up a surface where carbon dioxide methanation is most likely to occur.In chapter 4,we introduce carbon dioxide methanation simulations on the Ni/La2O3 interface.Twelve molecules with their adsorption structure and eight hydrogenation processes among the methanation are simulated.Compared with the pure nickel surface,the adsorption of chemical molecules on the interface material are much stronger（except H,H2,H2O and CH4）,as the La2O3 side is more energetically favored.Also,the diffusion rates of atoms on the catalyst surfaces are compared to confirm that the interface has a prior catalytic activity and a faster catalytic reaction rate.Finally,we summarize and discuss further researches in this field.The results show that interface material has higher catalytic activity,which is in excellent agreement with the experimental results.Associated with theoretical analysis,more insights of experimental results are revealed,providing theoretical support for further experiments related to the properties of Ni/La2O3 interface and synthesis of catalyst. |
