Effects Of Pd-Ag Bimetallic Composition And Surface Properties On Acetylene Iiydrogenation:First-Principles Study

One of the main goals of the catalysis industry is to develop new materials that have novel catalytic properties, which can improve the properties of petrochemical industry products. Bimetallic materials are a class of important catalysts as they frequently exhibit much enhanced catalytic stabilities, activities and selectivity, as compared with their single-metal constituents. Bimetallic PdAg catalysts have been used for reactions as selective hydrogenation catalysts science1980s. The in-depth research about the characteristic properties of PdAg bimetallic surfaces has been extensively studied both theoretically and experimentally. Previous studies on PdAg systems have mainly focused on bulk alloys and its surfaces, while previous studies on PdAg surface alloys usually lack information about lateral distribution of the atoms at the surface, which is different in various morphology of the nanocatalyst. The detail information on the adsorption and reaction properties on different PdAg surface alloys is still an obvious gap in the fundamental research fields. It is necessary to understand the properties of palladium-silver bimetallic surfaces for different morphologies at molecular and electronic scales using computer simulation.In this dissertation, density functional theory simulation is applied to study the electronic properties of different low-index Pd-Ag bimetallic surfaces, adsorption properties on the Pd-Ag surface alloys, activity of the hydrogenation of acetylene on each surface, and detailed analysis of the selectivity of the hydrogenation of acetylene on (100) surface.The facets exposed on a bimetallic nanocatalysts have a strong correlation with the shape. Understanding the promotion effect of the guest metal in catalytic behaviors and building a correlation between structural and chemical parameters of bimetallic nanocatalysts and their catalytic performance will provide the crucial insights for rational design of robust catalysts. In this dissertation, the surface properties of three low-index PdAg surface alloys were investigated, and the effect of the silver atoms on the surface was explored. The main conclusions of the studies are summarized as follows.(i) Simulation studies show that the surface work function is decreased as the surface silver atomic content grows, which makes the electrons easy to emit due to the repulsion of silver atoms.(ii) After the surface alloying with Ag atoms, the closest surface, which should be the most stable surface for FCC crystal, become less stable. When the concentration of Ag atoms is greater than0.5, the stability of the surface becomes sequential in the order (110)>(100)>(111).(iii) Combining the density of states with d-band width and d-band center, it is clear that, although the density of states of surface palladium atom become localized due to the addition of silver atoms, the electric activity of Pd atoms on the surfaces is enhanced by Ag atoms and increased in the order (100)>(111)>(110).The catalytic reaction is determined by the properties of the catalyst surface, and the adsorption parameters of the reactants and products molecular is essential for the catalytic activity and selectivity. There are two critical factors that contribute to the modification of the electronic and chemical properties of bimetallic surfaces:the ligand effect and the ensemble effect. It’s worthy of attention and research to clear the effect on the adsorption properties of ethylene and acetylene influenced by the guest metal. In this dissertation, both acetylene and ethylene adsorption properties on the three kinds of low-index surface alloys are simulated. Molecules with different π electrons displays different effects by Ag atoms and facets. The ligand effect of the silver atoms is more significant for the (111) surface but not for (100) surface, which displays ensemble effect when the Ag atomic concentration grows; both of the ligand and ensemble effects is not obvious for the adsorption on (110) surface. Surface electron is transferred to acetylene and ethylene molecules after their adsorption, and the value is increased with the increasing of surface Ag atomic concentration. The adsorption is weakened because of the ensemble effect when the surface Ag atomic concentration is greater than50%.In addition, the influence of the surface composition on the reaction of acetylene hydrogenation is extremely important for the selective hydrogenation of acetylene, and plenty of investigations have been focused on the shape-sensitive catalytic performance. In this dissertation, the reaction of acetylene hydrogenation to ethylene is simulated on the three indices palladium-silver surface alloys. It is found that the acetylene hydrogenation reaction on (100) and the (110) surface shows more active than the (111) surface. For the (100) surface, a more detailed hydrogenation of acetylene and ethylene on the surface and hydrogen transfer reactions were simulated. By analyzing the energy barrier of the reaction system, the selectivity of ethylene increase when the content of silver atoms increased and the carbonaceous deposit can be reduced effectively.In summary, surface catalysis research of bimetallic nanoparticle catalysts is a very important part of the catalyst science. The studies in this dissertation focus only on the properties of PdAg bimetallic catalysts at the molecular scale. However, in consideration of the complexity of the bimetallic nanoparticle, the studies with molecular simulations by no means limited to above issues. The in-depth theoretical study of bimetallic nanoparticle surface not only provide the theoretical support for the development of new catalytic materials, but also improve the theoretical system for the bimetallic catalysis.