Interaction of some molecules with complex surfaces

Detta är en avhandling från Science

Sammanfattning: This thesis investigates the adsorption, decomposition and reactions of small molecules on flat, oxidised and vicinal surfaces of late transition metals. The main technique for these studies is high resolution core level spectroscopy (HRCLS). All the studies presented in the thesis are performed in Ultra High Vacuum (UHV). The articles can be divided into three groups. The first three articles deal with co-adsorption on flat and stepped surfaces. The first article is focused on the adsorption thermodynamics that determines the adsorption sites. Adsorption of CO is performed on two surfaces vicinal to the Rh(111) surface which exhibit different step geometry to investigate the influence of the local step geometry on the adsorption process. The experiments proceed further to study CO co-adsorption with H2. The measurements and conclusions are supported by Density Functional Theory (DFT) calculations. The second article deals with the CO co-adsorption with oxygen and a kinetic analysis of the reaction occurring between CO and oxygen as function of the temperature. In this system, the two adsorbates create separate domains on the surface and the reaction proceeds with two different mechanisms depending on the temperature. The time evolution of the reactions has been followed by in-situ high resolution core level spectroscopy. The third article studies the co-adsorption of ethanol (C2H5OH) and oxygen on Rh(111). Due to a larger number of atoms the ethanol molecule allows a rich chemistry on the surface. Via high resolution core level spectroscopy it is shown that acetate (CH3COO ?Rh) is formed during the ethanol oxidation process. This is confirmed by Infra Red Reflection Absorption Spectroscopy. The second group of articles, article four and five, investigates the interaction of small hydrocarbons such as ethanol and ethylene (C2H4) with flat and stepped Rh surfaces. Article four studies the adsorption of pure ethanol on Rh(111) and Rh(553). Here use is made of the HRCLS chemical sensitivity to identify the hydrocarbon species on the surfaces. DFT calculations proved to be an invaluable tool in this identification. Clear differences in the ethanol dissociation on a flat and a vicinal surface are observed. The fifth article investigates the affects of the steps and their local geometry on the decomposition of ethylidyne (C2H3) generated on the surface from ethylene. Rh(111) is used as reference to compare to the two different step geometries present on Rh(553) and Rh(322). Clear indications that the local step geometry influences the ethylidyne decomposition are found. The last group of articles, article six and seven, investigates the interaction between CO and H2 respectively and a re-oxidised Rh(111) surface. The reduction of the oxide thin film is measured in real time while the reductive reagent is introduced in the chamber. Both articles compare the microscopic information obtained from scanning tunnelling microscope and the laterally averaged information typical of HRCLS.