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Chemical reactivity and structural determination of metal and gaseous adsorbates on Cu{100} using TPD and LEED

Younis Ahmed, Hamid M. (2003) Chemical reactivity and structural determination of metal and gaseous adsorbates on Cu{100} using TPD and LEED. Master of Science thesis, Dublin City University.

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Abstract

The structures formed by adsorbing thin-film platinum, formic acid and oxygen on Cu{ 100} single crystal are investigated by quantitative low-energy electrondiffraction (LEED) and Temperature Programmed Reaction Spectroscopy (TPRS) Symmetrized Automated Tensor LEED (SATLEED) calculations are used to determine the structure of the formed surface alloys and overlayers. TPRS was used to probe the surface reactivity of the systems studied while surface composition was obtained using Auger Electron spectroscopy (AES). The decomposition of a formate intermediate from a clean Cu(100) surface has been monitored through the use of TPD Spectroscopy CO2 evolution was observed at -440K. The presence of repulsive lateral interactions between the adsorbates on the surface has been identified, through the shift of the peak temperature Tp to lower values, as the surface was exposed to increasing amounts of formic acid. The Tp for 0 5L was observed at 458K,while that for SOL appears at 443K This shift means that the decomposition energy is reduced by 6%. It has also been evidenced that platinum has a destabilising effect on the formate intermediate. The peak temperature (Tp) for the CO2 desorption spectra from copperplatinum model surfaces, appear around 40K lower than those from clean copper. This suggests a much less stable surface alloy compared to the clean surface. In activation energy terms, this destabilisation can be expressed as a 13% decrease in the energy required for the formate to decompose. It was also observed that desorption is much more rapid from the copper-platinum than from clean copper surfaces. More study is needed to fully understand these results. A Cu{100}-c(2x2)-Pt surface alloy structure formed by deposition of 1ML mono layer of Pt and thermal processing to 550K is shown to correspond to a coppercapped bimetallic surface localised alloy with a sub-surface ordered c(2x2) CuPt layer. The layerwise compositional profile has been extracted via ATA modelling resulting in an almost pure outermost copper monolayer with only a small Pt impurity concentration (10± 10 at%) Layers 3 and 4 contained higher Pt concentrations of 20+20 and 30±30 at% respectively. Substitution of platinum into the selvedge results in a significant expansion in the surface mterlayer spacmgs relative to clean Cu{100} and switches the weak oscillatory relaxation of clean Cu{100} to a strongly and non-uniformly expanded interlayer separation. The outermost three mterlayer spacings are strongly expanded to 1 84+0 02A (+1 9±1 1%), 1 91+0 03A (+5 8±1 7%) and 1 89±0 03A (+4 7±1 7%) respectively. A slight rippling in the c(2x2) CuPt underlayer of amplitude 0 03±0 04A, with Pt atoms rippled outwards towards the vacuum interface within the composite layer occurs. A Pt/Cu{100}-(2x2)-0 surface alloy structure, formed by deposition of a high Pt loading and thermal processing in an oxygen atmosphere, is shown to correspond to an oxygen overlayer on a copper-capped bimetallic surface localised alloy with an ordered c(2x2) CuPt monolayer in layers 2 and 4. The selvedge structure within the LEED probing depth strongly resembles the {100} surface of the LI2 phase of the bulk Cu3Pt alloy. Substitution of platinum into the selvedge results in a significant expansion in the surface mterlayer spacmgs relative to Cu{100}-(2V2xV2)R45°-0 due to the larger metallic radius of Pt and switches the weak oscillatory relaxation of Cu{100}-(2>/2xV2)R450-0 to a strongly and non-umformly expanded mterlayer separation. The outermost three interlayer spacmgs are expanded with a slight rippling in the first CuPt underlayer with Pt atoms rippled outwards towards the vacuum interface within the composite layer.

Item Type:Thesis (Master of Science)
Date of Award:2003
Refereed:No
Supervisor(s):Barnes, Colin
Uncontrolled Keywords:Copper Absorption and adsorption; Low energy electron diffraction; Gases Absorption and adsorption
Subjects:Physical Sciences > Chemistry
DCU Faculties and Centres:DCU Faculties and Schools > Faculty of Science and Health > School of Chemical Sciences
Use License:This item is licensed under a Creative Commons Attribution-NonCommercial-No Derivative Works 3.0 License. View License
ID Code:18281
Deposited On:27 May 2013 16:01 by Celine Campbell. Last Modified 04 Oct 2013 15:54

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