The overall topic of this thesis is within the field of catalysis, were model systems of different complexity have been studied utilizing a multipurpose Ultra High Vacuum chamber (UHV). The thesis falls in two different parts. First a simple model system in the form of a ruthenium single crystal is investigated. Second the development of a complex Cu/ZnO nanoparticle model system is described and gas-induced dynamical changes in the model system is investigated.
The ruthenium crystal serves as an extremely simple model for studying CO dissociation which is the rate limiting step of the methanation process. The Ru(0 1 54) surface is studied by means of Scanning Tunneling Microscopy (STM), Temperature Programmed Desoprtion (TPD), and Oxygen Titration (OT) experiments. Real space evidence of periodic features on every second monatomic step is observed via STM when the a clean ruthenium surface is exposed to 5·10-10 torr CO in a temperature range from 700 K to 400 K. These features are assigned to oxygen atoms from dissociated CO. After the dissociation experiment, the carbon coverage on the surface is measured by OT and is found to be equivalent with the theoretical step density of the Ru(0 1 54) surface. Furthermore, STM investigations indicate that the minimum required temperature for CO dissociation on ruthenium is 500 K.
The Cu/ZnO model system is created via oxidation and subsequent reduction of size selected CuZn alloy nanoparticles deposited on a rutile TiO2(110) single crystal. Dynamics of the Cu/ZnO nanoparticles is highly relevant to industrial methanol synthesis for which the direct interaction of Cu and ZnO nanocrystals synergistically boost the catalytic activity. The dynamical behavior of the nanoparticles under reducing and oxidizing environments were studied by means of ex situ X-ray Photoelectron Electron Spectroscopy (XPS) and in situ Transmission Electron Microscopy (TEM). The surface composition of the nanoparticles changes reversibly as the nanoparticles exposed to cycles of high-pressure oxidation and reduction (200 mbar). Furthermore, the presence of metallic Zn is observed by XPS as the nanoparticles are reduced. The Cu/ZnO nanoparticles are tested on a µ-reactor platform and prove to be active towards methanol synthesis, making it an excellent model system for further investigations into activity depended morphology changes.
|Place of Publication||Kgs. Lyngby|
|Publisher||Technical University of Denmark|
|Number of pages||196|
|Publication status||Published - 2014|