Structure Sensitivity of Ethylene Adsorption on Platinum (Dehydrogenation)
Hatzikos, George Harry
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https://hdl.handle.net/2142/69780
Description
Title
Structure Sensitivity of Ethylene Adsorption on Platinum (Dehydrogenation)
Author(s)
Hatzikos, George Harry
Issue Date
1987
Department of Study
Chemical Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Abstract
Theoretical and experimental work is conducted to investigate the structure sensitivity of ethylene adsorption on platinum surfaces. Temperature Programmed Desorption (TPD), Electron Energy Loss Spectroscopy (EELS), Low-Energy Electron Diffraction (LEED) and Auger spectroscopy are applied to study the adsorption of ethylene on the square (1 x 1)Pt(100) and on the pseudo-hexagonal (5 x 20)Pt(100). A model based on the conservation of orbital symmetry on surfaces provides interesting predictions.
The model suggests that conservation of symmetry may be the dominant factor and not thermodynamics in determining the stability of CnHx hydrocarbon intermediates on platinum. On the hexagonal surfaces of Pt(111) and (5 x 20)Pt(100) ethylidyne ($\equiv$C-Ch3) should be the stable intermediate while the formation of pi-bound ethylene (H2C=CH2) and vinylidene (=C=CH2) should be symmetry forbidden. On the square (1 x 1)Pt(100) ethylidyne formation is forbidden while the formation of vinylidene and pi-bound ethylene is allowed.
The two reconstructions of Pt(100), the pseudo-hexagonal (5 x 20) and the square (1 x 1) maintain their symmetry upon ethylene adsorption as it is verified by LEED. The TPD spectra for hydrogen (H2) and ethylene (C2H4) depend strongly on the initial structure of Pt(100). On the (5 x 20), H2 desorbs at 330K, 380K and 480K. The 330K peak is reaction limited and it is attributed to the formation of ethylidyne while the 480K peak to the decomposition of ethylidyne to CHx fragments. The EELS spectra for the (1 x 1) and (5 x 20) at 120K are similar suggesting no structure sensitivity. However at higher temperatures the EELS spectra of C2H4 for the (5 x 20) compare favorably with those for ethylidyne.
The TPD and EELS data for C2H4 on the (1 x 1) are different from the data on the (5 x 20). The EELS spectra at T = 330K indicate the formation of vinylidene (=C=CH2) and of an acetylenic species (HCCH). In support of the vinylidene species, the EELS spectra compare favorably with the IR spectra of a ruthenium inorganic cluster where a vinylidene molecule is perpendicularly bonded. In addition, TPD data show that 50% of the total H2 desorbs with a single broad peak at 350K proposing a C2H2 stoichiometry for the surface intermediates on the (1 x 1)Pt(100).
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