AVS 51st International Symposium
    Surface Science Thursday Sessions
       Session SS1-ThA

Paper SS1-ThA6
Reactions of Substituted Hydrocarbons with Cerium Oxide Thin Films@footnote1@

Thursday, November 18, 2004, 3:40 pm, Room 210B

Session: Metal Oxides and Clusters IV: Oxide Surface Chemistry
Presenter: D.R. Mullins, Oak Ridge National Laboratory
Authors: D.R. Mullins, Oak Ridge National Laboratory
M.D. Robbins, Oak Ridge National Laboratory
T.S. McDonald, Oak Ridge National Laboratory
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Fully oxidized ceria surfaces are largely inactive with respect to the adsorption and reaction of most adsorbates under UHV conditions. Reduced surfaces, however, provide active sites where some adsorbates such as H@sub 2@O, NO and SO@sub 2@ can adsorb and react. Other molecules, such as CO, H@sub 2@ and C@sub 2@H@sub 4@ do not interact strongly with either an oxidized or a reduced surface. Based on the ethylene behavior, it appears that hydrocarbons may not interact strongly with ceria whereas on metals they frequently decompose. Substituted hydrocarbons may therefore bind to the surface through the heteroatom at the oxygen vacancy while the hydrocarbon part of the molecule may not interact strongly with the surface. We have recently completed a study of methanol and methanethiol adsorption on ceria as a function of temperature, exposure and Ce oxidation state. CH@sub 3@OH reacts at low temperatures with oxidized CeO@sub 2@ to produce H@sub 2@O at 200 K, and CH@sub 2@O and CH@sub 3@OH near 600 K. This leads to the reduction of the ceria. This is the first molecule we have examined that is capable of reducing a ceria film in UHV. Surprisingly, CH@sub 3@SH does not interact strongly with the CeO@sub 2@. It desorbs molecularly by 300 K and does not reduce the oxidized surface. On reduced ceria, the oxygen vacancies result in more methanol adsorption which undergoes more extensive decomposition producing CO and H@sub 2@ near 600 K. As the degree of ceria reduction increases, more H@sub 2@ and less H@sub 2@O are produced. Methanethiol does adsorb on the reduced surface producing CH@sub 3@S and OH. The C-S bond cleaves near 600 K and methyl reacts with the hydroxyls to produce CH@sub 4@. @FootnoteText@ @footnote 1@ Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.