Dynamical Coupling During The Course Of Catalysis Processes On The Well-defined Surfaces Of Noble Metals

Diffusion is one of the most common phenomena. During the course of heterogeneous catalysis, the diffusion of surface adsorbed species between the different parts of the catalysts is defined as spiltover, which has an influence on the kinetics of heterogeneous catalysis. For the reaction system, temporal-spatial pattern will form if the diffusion process is locally coupled with the reaction, such as the chemical wave. The chemical wave of the diffusing species propagates much faster than the diffusing species following the Fick抯 law. The chemical wave usually appears in the reaction systems behaving nonlinear kinetics, such as the BZ reaction, the oscillatory reaction of CO oxidation and NO reduction on the Pt single crystals. However, for the heterogeneous catalysis system, if the coverage gradient of surface adsorbed species formed between adjacent parts of the catalysts due to their different catalytic activities is large enough to trigger the diffusion, and the diffusion process is locally coupled with surface reaction, then the chemical wave can also form and has a great influence on the surface involved. The coupling between adjacent surfaces in the form of the chemical wave provide a way to design the heterogeneous catalysts. Nitric oxides are one of the main air pollutants. The cleanest way to eliminate NO~ is decomposition. Noble metals possess a high activity to decompose NOR, but the resulting 0ad on the noble metal surfaces will inhibit the decomposition of NOR, therefore only above the temperature at which the resulting O~ can desorb from the noble metal surfaces, usually around 1000k, can the noble metals behave the high activity to decompose NOR. At the other hand, silver has a poor activity to decompose NO,, and O~ on the silver surface can desorb around 600K. Therefore we I,? Vi Al~trEt propose an idea to realize the high activity of noble metals to decompose NO~ at relatively low reaction temperature by the addition of silver. During the course of NO~ decomposition, the concentration gradient of O~ between noble metals and Ag will result from their different ractivities, and O~ on the noble metal surfaces may diffuse to the Ag surface and be cleaned off on the Ag surface. Based on this idea, the surface techniques were employed to study the O~ diffusion between Pt and Ag on the Ag/Pt (110) model catalysts. The following research works have been conducted in the dissertation: 1. Setting up the Pbotoemission Electron Microscopy (PEEM). PEEM is a newly developed surface technique around I 990s and it images the local work function distribution on the two-dimensional metal surface. PEEM can reflect the slight work function change with high contrast and in situ and in real time monitor the dynamical processes on the metal surface following work function change, such as adsorption and desorption, diffusion, reaction. PEEM set up in our laboratory possesses a 200nm lateral resolution and 33ms temporal resolution. 2. Investigating the adsorption, diffusion and reaction of oxygen on the defected Pd (100) surface with (110) and (111) microfacets. Various kinds of oxygen species form on different Pd planes upon oxygen adsorption, which behave different reactivities towards CO. In the course of CO+O~ reaction, coupling occurs between (110) plane and its adjacent planes due to the concentration gradients through...