Dynamics of Dissociative Chemisorption: Methane on Tungsten Single Crystal Surfaces
Lo, Tai-Chin
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https://hdl.handle.net/2142/77351
Description
Title
Dynamics of Dissociative Chemisorption: Methane on Tungsten Single Crystal Surfaces
Author(s)
Lo, Tai-Chin
Issue Date
1983
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Molecular
Language
eng
Abstract
Chemisorption of simple molecular gases on clean metal surfaces is usually rapid, with vanishingly small activation energies and large sticking coefficients. However, for more complicated molecules, such as saturated hydrocarbons, chemisorption is often a slow process which involves thermal excitation over relatively high barriers. We have studied one such system: the dissociation of methane on clean tungsten single crystal surfaces. The purpose of this study is to explore the nature of the elementary excitations which bring about dissociation and subsequent adsorption. Our effort has been concentrated upon studying the effect of thermal excitations of the gas as well as the importance of the crystal structure of the metal surface.
We have performed molecular beam studies on the (100), (211), (111), (411), and (811) planes of tungsten, using a Kelvin vibrating capacitor for contact potential measurements. It has been found that excitation of the gas alone is sufficient to bring about reaction with the tungsten surfaces. The barriers to dissociation of methane are sensitive to the atomic arrangements on the crystal surface. The empirical activation barrier of methane on W(100), the most unreactive crystal plane, is (TURN) 9.7 kcal/mole. This is about 30% higher than the empirical activation barrier of methane on W(211), which is (TURN) 7.5 kcal/mole.
The results are well described by a model invoking molecular adsorption of the methane molecule, followed by dissociation of those molecules which have sufficient vibrational energies of higher bending modes, in order to extend the H-H distance to a critical length beyond the equilibrium value.
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