Theoretical Studies On The Modification Of The Silver Catalyst Surfaces And Its Effect To Ethylene Epoxidation

The value of simulation in metal surfaces stems from the roles that they play in both electrochemistry and catalysis. The silver surfaces are widely used to catalyze the partial oxidation of organic compounds, particularly the epoxidation of ethylene(C2H4) to ethylene oxide(C2H4O), which is an important industrial reaction. In this thesis, the interaction of adsorbed molecular with silver surface and the mechanism of ethylene epoxidation have been investigated by the ab initio pseudopotential total energy method based on density functional theory.The density functional theory(DFT) is the appreciable method for the metallic systems, which has been rapidly developed recently. So the DFT method has been performed in all of our studies. Up to date, cluster models are often used to simulate the metallic surfaces because of the convenience, and the structures of the substrate are mostly fixed at the ideal position as that of the bulk without considering the relaxation. This is not agreed with the experimental situations. Previous study in our lab has obtained that the influence of the relaxation to the metallic electronic properties is significant. The effect of the relaxation on the adsorption process has also been studied in previous study. It is found that the adsorption properties are greatly affected by the relaxation. So in this thesis, the supercell model is employed to simulate the metallic suface, which can consider the relaxation effect adequately.By using DFT method and supercell model, firstly, we studied the chlorine modified silver surfaces. From the experimental results, the introduction of the chlorine atom will enchance the reactivity and the selectivity of the epoxidation of ethylene. The properties of the chlorine modified Ag(111),Ag(110) and Ag(100) surfaces are investigated. The preferred adsorption site,adsorption energy,relaxation of the surface structure,work function change,density of states and the changes of the properties with the changes of coverage are studied in detail. From the structure and energetic properties, we can conclude that there exists strong interaction between the chlorine atom and the surface silver atoms. The changes of work functon valuesindicate that some electronic densities transfer from the silver surface to the chlorine atom. The modification of the chlorine atom to the surfaces is related to the surface atom packing, number of the nearest silver atoms and the coverage of the chlorine. By investigating the properties of Cl-modified Ag(111) surface with different coverage of chlorine, it is found that the structure of surface changes markedly when the coverage of chlorine is 1/3. The fcc hollow site is broadened obviously by the promotion of chlorine atom. Among all the silver surfaces, we find that the adsorption of chlorine atom obey the Langmuir isothermal rule with the low coverage condition. At higher coverage, the interaction between the chlorine atoms is appreciable. The strong repulsion between the chlorine atoms will weaken the interaction between the adsorbed chlorine atom and the silver surfaces.The chemisorption properties of oxygen on the silver surface are very important for the adsorption and epoxidation of ethylene on silver surface. In order to investigate the effect of chlorine on oxygen adsorption over Ag (111) surface, we have studied the oxygen adsorption on the Cl-modified Ag(111) surface. The Ag(111) _R30o surface cell is employed in the calculations because the optimal coverage of the chlorine atoms as modifier is about 1/3ML for the epoxidation of ethylene. The results show that the presence of chlorine on surface can enhance the adsorption energy for atomic oxygen. The modification of chlorine atom changes the electronic properties of the substrate and strengthens the bond between the oxygen atom and the surface. On the other hand, the calculated results indicate that the most stable adsorption site for both oxygen and chlorine atom on the silver surface is fcc hollow site. So the adsorption of chlorine will reduce the...