| The decomposition of methane with four H atoms produces the high percentage hydrogen, meanwhile, obtained the carbon nanomaterials, which has a lot of potential application. Thus, the catalytic decomposition of methane has become the hotspots in the field of industrial catalysis. To design high efficiency catalysts and reveal its basic mechanism have an important means of enhancing the methane decomposition efficiency. It is well known that the nickel is abundant in the world and low-cost. The most important is that the catalytic activity of nickel for the decomposition of methane is comparable with some noble metal. Thus the nickel has become the focus of industrial application and scientific investigation. However, the nickel catalyst is easy to inactive for the carbon deposition. A useful way to solve the problem is to choose the appropriate catalyst support. The spinel MgAl2O4 material expresses much more stability and heat-resistant than some other oxides, which has been widely used as catalyst support. For further improving the stability and activity of the catalysts, it was necessary to undestand the interaction between the catalyst and support, and on this basis the methane reaction mechanism is carefully investigated. This work provides theoretical guidance for designing and synthesizing the efficient catalyst.In this paper, we selected the spinel MgAl2O4 supported the Ni4 cluster as the catalyst for the methane decomposition. With the density functional theory, we established the surface model, and investigated the interaction between the cluster and support; the microscopic reaction mechanism of the CH4 molecule reaction on Ni4/MgAl2O4 is further studied. We analyzed the support effect for the catalytic activity, and studied the CH4 decomposition mechanism on surface with the atomic and electronic level.In the chapter one, we reviewed the research background and significance. We first introduced the development and research progress of methane catalytic reaction. Then introduced the catalyst, and pointed out the advantages and disadvantages of the catalytic materials. Finally, the research motivation and contents of the dissertation was proposed and presented the significance of this study.In the chapter two, we briefly introduced the basic theory of quantum chemistry, and presented the density functional theory method and its development. The main target in DFT investigation was to explore good approximation for exchange-correlation functional. As the DFT research continues, DFT theory developed from Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA) and self-consistent field theory. Then, the quantum chemistry software CASTEP used in this work was introduced carefully.In the chapter three, a systematic first principles study of CH4 preliminary reaction (CH4â†'CH3+H) on Ni cluster supported MgAl2O4 (100), (110) and (111) surfaces had been performed. We mainly calculated and analyzed the interaction between the cluster and support, and the reaction between the adsorbate and catalyst. The interaction between the Ni4 and MgAl2O4 was crystal facet dependence. The Ni atoms pointed to the surface O3c atoms to form the Ni-O3c bond. The supported surface model possessed a symmetry structure. For the (111) surface, the surface Mg, Al and O atoms all participated in the cluster-support interaction, which resulted into the most favorable Ni4 supported surface configuration. We further studied the CH4 adsorption and dissociation on the top and interface site of the three surfaces. We found the effect of surface metal site (Mg or Al) promoted the adsorption stability, which was site dependence. The adsorption on interface site was much more favorable than that on top site. Compared the adsorption energies of the three surfaces, the adsorption was more favorable on (100) surface thermodynamically. The potential barriers was calculated and found the methane dissociation overcome the lower activation energy on (100) surface, which suggested the C-H bond could be easy activated on (100) surface.The previous chapter concluded the MgAl2O4 (100) surface expressed the best stability and catalytic activity. On the basis, we conducted DFT to calculate the sequent dissociation of CHX on Ni4/MgAl2O4 (100) perfect and defective surfaces and study the influence of surface defect state for the catalytic activity. According to the CHx adsorption energies, the adsorption stability increased with the methane sequent dissociation, while the adsorption energy of C atom was most favorable. The CH4, CH3 and CH2 adsorption on perfect surface was more favorable; the CH and C adsorption on defective surface was much more favorable. In addition, the result of potential barriers showed the perfect surface was more likely to active the C-H bond, thus the sequent dissociation was able to proceed easily. The establishment of oxygen vacancy lowered the reaction barrier of CH dissociation, which was easy to produce the C atom. Thus, we proposed two schemes for the CH4 dissociation on perfect and defective surfaces, which were related with the production of hydrogen and carbon nanomaterials.In the chapter five, we summarized the main conclusions and innovation of this dissertation, and proposed the further studies in the related fields. |
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