Dynamical Coupling during the course of Catalysis Processes on the Well-defined Surfaces of Noble Metals

Dr. Weixin Huang(1996-2001)
Directed by Prof. Dr. Xinhe Bao
Abstract
Present address:
Department of Inorganic Chemistry
Fritz-Haber-Institut der MPG, Germany
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 spillover, 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's 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 NOx is decomposition. Noble metals possess a high activity to decompose NOx, but the resulting Oad on the noble metal surfaces will inhibit the decomposition of NOx, therefore only above the temperature at which the resulting Oad can desorb from the noble metal surfaces, usually around 1000k, can the noble metals behave the high activity to decompose NOx. At the other hand, silver has a poor activity to decompose NOx and Oad on the silver surface can desorb around 600K. Therefore we propose an idea to realize the high activity of noble metals to decompose NOx at relatively low reaction temperature by the addition of silver. During the course of NOx decomposition, the concentration gradient of Oad between noble metals and Ag will result from their different ractivities, and Oad 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 Oad 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 Photoemission Electron Microscopy (PEEM). PEEM is a newly developed surface technique around 1990s 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+Oad reaction, coupling occurs between (110) plane and its adjacent planes due to the concentration gradients through Oad diffusion. The chemical wave is imaged by PEEM on the Pd (110) plane, which largely accelerate the CO+Oad reaction on the Pd (110) plane.
  3. Studying the growth of Silver thin film on the Pt (110)-(1x2) surface. At 300K the growth of Ag thin film follows the SK mode, i.e., layer-by-layer growth up to 1.5 monolayer (ML) followed by 3D island growth. During the course of the initial 1.5ML layer-by-layer growth, the first 0.5ML silver atoms fill in the missing-row sites of the Pt(110)-(1x2) surface and form a pseudo Pt(110)-(1x1)-0.5ML Pt-0.5ML Ag surface structure, then the subsequent 0.5ML and the last 0.5ML silver atoms locate on the silver sites and the platinum sites of the pseudo (1x1) surface, respectively. At 600K, the formation of AgPt surface alloys follows the layer-by-layer growth of Ag thin film up to 2.25ML.
  4. Investigating the adsorption and decomposition of NO and NO2 on the Ag/Pt (110) surface. Linearly and bridged bound NOad coexist on the Pt (110) surface upon NO adsorption, at RT, linearly bound NOad dominate; NOad can partially decompose when heated. The pre-adsorption of oxygen on Pt (110) can inhibit the bridge adsorption of NO and decrease the activation energy for. At RT, NO2 totally decompose on the Pt (110) surface, and there exist the strong repulsive interactions between the resulting NOad and Oad, which decrease the activation energy for NOad desorption and inhibit the bridged bound NOad. Upon NO2 decomposition at 500K, a new oxygen adsorption state, defined as a-Oads, emerges on the Pt (110) surface, whose formation can be attributed to the decomposition of NO2 on the (110) domain experiencing surface reconstruction induced by NOad. The addition of a small amount of Ag can obviously weaken the Pt-O bond and decrease the desorption of Oad from Pt (110).
  5. Preparing the Ag/Pt (110) bimetallic surface with AgPt interface and studying the adsorption, diffusion and reaction of CO and O2 on it. Upon oxygen adsorption on CO-saturated Ag/Pt (110) bimetallic surface, coupling occurs between Pt and Ag due to their different reactivities, in which Oad diffuse from the Pt surface to the Ag surface and participate the CO oxidation on the Ag surface, resulting in the formation of the chemical wave on the Ag surface. Similarly, coupling also occurs between Pt and Ag during the course of CO adsorption on O-saturated Ag/Pt (110) bimetal surface, the chemical wave of COad is imaged to propagate on the Ag surface, triggered by the diffusion of COad from Pt to Ag. During the course of CO oxidation, the diffusion of Oad and COad are also observed between the Pt surface and Ag surface.

      According to the above results, we propose an equation to describe the diffusion rate of the chemical wave triggered by the coupling between adjacent surfaces during the course of reactions. In summary, on the Ag/Pt (110) bimetallic surface, Oad can
diffuse from the strong bound Pt surface to weak bound Ag surface in the form of the chemical wave during the course of reaction, which implies that the idea of eliminating the Oad on Pt through the coupling between Pt and Ag is plausible. However, there is a much long distance between the model catalysts and the real catalysts, the extension of the idea to the real catalysts is concerned with the reaction system, reaction condition and the preparation of the real catalysts.
Adsorption and Diffusion of CO and Oxygen on Ag-Pt (110)