Ethylene decomposition and hydrogenation on platinum single crystal surfaces
Backman, Arthur Leonard
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Permalink
https://hdl.handle.net/2142/22663
Description
Title
Ethylene decomposition and hydrogenation on platinum single crystal surfaces
Author(s)
Backman, Arthur Leonard
Issue Date
1990
Doctoral Committee Chair(s)
Masel, Richard I.
Department of Study
Chemical and Biomolecular Engineering
Discipline
Chemical and Biomolecular Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Physical
Engineering, Chemical
Language
eng
Abstract
The adsorption and decomposition of ethylene on the Pt(210) crystal surface is examined using TPD and EELS. At 100 K, ethylene adsorbs molecularly to form a type 2 $\pi$-bound complex. Upon heating to 250 K, some of the $\pi$-bound complex desorbs and also reacts to form ethane. However, most of the $\pi$-bound species is stable to room temperature. Further heating results in the decomposition of the $\pi$-bound species to form a mixture of ethylylidyne, methyl groups, surface carbon and hydrogen. Ethylylidyne decomposes to form a mixture of ethynyl, methylidyne, hydrogen and adsorbed surface carbon while the methyl groups decompose to form methylene and methylidyne. Further decomposition of these intermediates results in hydrogen liberation as well as more surface carbon and a C$\sb2$ species.
At high ethylene exposures, a weakly $\pi$-bonded species forms in addition to the $\pi$-bonded complex at low temperatures. The weakly $\pi$-bound species can also be formed at low temperatures by pre-exposing the Pt(210) crystal to hydrogen before ethylene exposure. This species is less stable than the $\pi$-bound complex since around 195 K, it either desorbs or reacts to form ethane.
A new high pressure/low pressure reaction system is developed to investigate ethylene hydrogenation on the Pt(111) and (5x20)Pt(100) crystal surfaces around atmospheric pressure. Two limiting reaction rates are observed for reactions initiated over an atomically clean surface. Both XPS and EELS results indicate a build up of a carbonaceous overlayer as a function of reaction time suggesting that the decrease in rate results from the increase of the carbonaceous overlayer. Both limiting reaction rates are larger for reactions over the Pt(111) than on the (5x20)Pt(100) implying that higher concentrations of the adsorbed overlayer are present on the (5x20)Pt(100) surface.
On Pt(111), ethylidyne and an ethyl species are identified using EELS following atmospheric pressure hydrogenation experiments. Post reaction treatments with pure hydrogen indicate that the ethyl species can be reacted off the surface whereas ethylidyne is stable. In addition, carbon monoxide is found to adsorb only after the hydrogen treatment. These results suggest that the reaction occurs on the platinum surface and not on top of the ethylidyne layer.
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