Alcohol surface chemistry in ultrahigh vacuum and at high pressure
Zhang, Ruiming
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Permalink
https://hdl.handle.net/2142/22154
Description
Title
Alcohol surface chemistry in ultrahigh vacuum and at high pressure
Author(s)
Zhang, Ruiming
Issue Date
1992
Doctoral Committee Chair(s)
Gellman, A.J.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Physical
Language
eng
Abstract
As models for the boundary layer films formed on lubricated metal surfaces, the surface chemistry of a set of straight chain alcohols on the clean and pre-oxidized Ag(110) surfaces have been examined. On the clean Ag(110) surface, the alcohols from methanol to pentanol were found to adsorb reversibly. XPS results showed that the alkyl chains were oriented roughly parallel to the surface rather than forming densely packed films of a Langmuir-Blodgett type.
On the pre-oxidized Ag(110) surface, alcohols were observed to react with atomic oxygen on the surface to form water and surface alkoxides. The alkoxides are more strongly bound to the surface than the alcohols and decompose to yield mainly aldehydes. The fact that all alkoxides decompose within a narrow temperature range (270-300 K) suggests that there is a common reaction step initiating the decomposition process.
In an effort to narrow the gap between the ultra-high vacuum surface science and real catalytic processes, a new in-situ high pressure experimental method which combines work function measurements with temperature programmed desorption ($\Delta\Phi$ - TPD) has been developed. Using this new technique, the influence of gas phase hydrogen on the desorption of CO and decomposition of methanol on Ni(111) surface has been examined. Under high ambient hydrogen pressure, the desorption temperature of CO is decreased from the temperature observed under ultra-high vacuum conditions. On the Ni(111) surface, it is found that when the hydrogen pressure is increased from 1 $\times$ 10$\sp{-9}$ torr to 10 torr, the peak temperature for CO desorption shifts from 435 K down to 320 K. This implies approximately a 7 kcal/mol decrease in the activation energy of desorption.
In the presence of gas phase hydrogen, methoxide formed from the adsorption of methanol on the Ni(111) surface was found to recombine with the hydrogen atoms on the surface to produce methanol, while in ultra-high vacuum methoxide was found to decompose completely to CO and hydrogen. These experimental results clearly indicate the strong influence of the gas phase on surface reaction mechanisms and kinetics, and therefore the importance of high pressure surface chemistry.
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