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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
| ABSTRACT - Internet Electron. J. Mol. Des. December 2002, Volume 1, Number 12, 620-635 |
Density Functional Study of Ethylene Oxidation on Ag(111) Surface.
Mechanism of Ethylene-Oxide Formation and Complete Oxidation with
Influence of Subsurface Oxygen
Hisayoshi Kobayashi, Katsumi Nakashiro, and Tomoatsu Iwakura
Internet Electron. J. Mol. Des. 2002, 1, 620-635
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Abstract:
Ag catalysts oxidize ethylene to epoxide. This reaction is one of the
most important reactions in chemical industry. Although the catalysts
have been intensively improved for years, small portion of ethylene
leads to combustion. The reaction mechanisms including the role of
molecular and atomic oxygen are not yet clear, and have been
investigated using the density functional method using a Ag5 cluster
model. Industrial catalysts include several dopants as co-catalysts, and
one of them is oxygen atom under the surface layer. The effects of
subsurface oxygen have also been examined with a Ag5O cluster. The
hybrid type density functional method with Gaussian basis set was used
for computations. In the reaction between ethylene and oxygen
molecule, the activation energy for ethylene epoxidation was much
lower than those for acetaldehyde formation and for hydrogen
abstraction from ethylene, which led to complete oxidation. This
suggests that the oxygen molecule should oxidize ethylene to epoxide
with a very high selectivity (almost 100%). The oxygen atom, which
was formed by dissociative adsorption of oxygen molecule or as a
"by-product" when one oxygen atom of the molecule was consumed,
oxidizes ethylene into epoxide and acetaldehyde with a similar
selectivity, since the difference in activation energies is very small
(3.7 kcal/mol). The oxygen atom may, therefore, play a main role in ethylene
epoxidation, because successive oxidation by both atoms of adsorbed
oxygen molecule means to give higher selectivity at zero conversion
than upper limit (85.7%) according to the active oxygen molecule
mechanism. Subsurface oxygen slightly increased a difference in
activation energies for epoxidation and acetaldehyde formation (up
to 4 kcal/mol) with the atomic oxygen mechanism, suggesting a slight
enhancement of the epoxidation selectivity, while adsorbed oxygen
molecule maintained a very high selectivity again (difference was from
24 to 22 kcal/mol). On the Ag surface containing subsurface oxygen, the
oxidizing agent for ethylene epoxidation was, therefore, supposed to be
adsorbed oxygen atom on Ag catalysts, similar to a Ag surface without
subsurface oxygen.
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