Theoretical Study Of The Adsorption Of Metal And Oxide Catalysts And The Reaction Mechanism Of Oxidative Dehydrogenation

Heterogeneous catalysis has attracted great interests in science, especially in industry. Many industrial process such as dehydrogenation, oxidation etc. are all based on heterogeneous catalysis. The most common catalysts include metal, oxide et al. Metal catalysts are used in real production firstly; oxide catalysts study are more complex due to various active sites on surface.To date, the Density Functional Theory (DFT) is a practical method to study 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 surface, which can consider the relaxation effect adequately.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 the first part of the thesis, 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 function values indicate 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 thecoverage 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.With the ever-increasing worldwide demand for olefins, alternative inexpensive ways to produce light olefins in industry are highly desired. As alkane is a source of inexpensive raw materials, the functionalization of light alkanes by selective oxidation has attracted considerable interests. Vanadium catalysts are important on selective oxidation of hydrocarbons. However, due to great oxidative ability, vanadium pentoxide could induce COx etc. easily. So, it is necessary to add something ( proper support) to increase activity and selectivity. Due to different VOx species structure and surroundings, different support-vanadium catalysts have different character; at the same time, supports' different acidity or basicity also induce the different catalysis properties. Thus, the study of catalysts' oxidative property, surface acidity have great effect on alkanes activation and transformation mechanism, and have great interesting in catalysts formation.In the second part of this thesis, the oxidative dehydrogenation (ODH) of propane on single crystal V₂O₅ (001) is studied by periodic density functional theory (DFT) calculations. The energetics and pathways for the propane to propene conversion are determined. We show that (i) the C-H bond of propane can be activated by both the terminal and the bridging lattice O atoms on surface with similar activation energies. At the terminal O site both the radical and the oxo-insertion pathways are likely for the C-H bond activation, while only the oxo-insertion mechanism is feasible at the bridging O site, (ii) Compared to that at the terminal O site, the propene production from the propoxide at the bridging O site is much easier due to the weaker binding of propoxide at the bridging O. It is concluded that single crystal V2O5 (001) is not a good catalyst due to the terminal O being too active to release propene. It is expected that an efficient catalyst for ODH reaction has to make a compromise between the ability to activate C-H bond and the ability to release propene.Then, we chose TiO₂ (anatase) as support to study VOx/ TiO₂ dispersion. Ourcalculation results indicate that the stability of monomer and dimmer on TiO₂ surface has the reverse order of the TiO₂ surface: (101) < (100) < (001). On the same surface, monomer are usually stable than dimmer. The ODH mechanism study on VOx/TiO₂(001) and (100) show: the interaction of support-catalyst have effect on catalyst character, especially stronger interaction which change catalysts character greatly; on TiO₂ (001) surface, the activity of monomer are better than dimmer, which indicate the activity decrease with loading increasing; on TiO₂ (100) surface, the results are opposite. In short, the interaction of support-catalyst and loading change could change O species property, even change relative activity order. Such change have complex and subtle effect on catalysts' activity and selectivity.