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

Paper SS1-ThA7
Adsorption and Reaction of Acetaldehyde and Methanol on Stoichiometric and Defective Mixed-Metal Oxide Surfaces

Thursday, November 18, 2004, 4:00 pm, Room 210B

Session: Metal Oxides and Clusters IV: Oxide Surface Chemistry
Presenter: L.Q. Wang, Pacific Northwest National Laboratory
Authors: L.Q. Wang, Pacific Northwest National Laboratory
S.A. Azad, Pacific Northwest National Laboratory
K.F. Ferris, Pacific Northwest National Laboratory
C.H.F. Peden, Pacific Northwest National Laboratory
M.H. Engelhard, Pacific Northwest National Laboratory
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The adsorption and reaction of oxygenated hydrocarbons on metal oxide surfaces are of much interest from both fundamental and practical perspectives. The reactivity of these catalytic processes largely depends on the characteristics of the oxide catalysts defined by their surface structures, acid-base properties and surface defects. Oxygenated hydrocarbons are often used as fuels and fuel additives , and they may be formed as a result of incomplete combustion of fuel in the engine. To efficiently reduce these toxic exhaust products, it is especially helpful to have a fundamental understanding of the adsorption and reaction of oxygenated hydrocarbons on metal oxide surfaces. In this presentation, we examined the interactions of acetaldehyde and methanol with stoichiometric and defective SrTiO@sub 3@(100) surfaces using x-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and first-principles density-functional calculations. The results obtained from methanol and acetaldehyde on SrTiO@sub 3@(100) surfaces are compared with our results on Ce@sub 0.8@Zr@sub 0.2@O@sub 2@(111) surfaces and with the previous results on single crystal TiO@sub 2@ surfaces. Both acetaldehyde and methanol adsorb mostly non-dissociatively on the stoichiometric SrTiO@sub 3@(100) surface that contains predominately Ti@super 4+@ cations. Theoretical calculations predict weak adsorption of acetaldehyde and methanol on TiO@sub 2@-terminated SrTiO@sub 3@(100) surfaces, in agreement with the experimental results. The stronger binding of acetaldehyde and methanol on TiO@sub 2@ surfaces than on SrTiO@sub 3@(100) surfaces is attributed to the more covalent nature of the Ti@super 4@@super +@ cation sites in the mixed-metal oxides and the unique surface structure due to the absence of the bridging oxygen atoms on the TiO@sub 2@-terminated SrTiO@sub 3@(100). The Ti@super 4@@super +@ sites on the stoichiometric SrTiO@sub 3@(100) surface are not sufficiently active for surface reactions such as aldol condensation, as opposed to the Ti@super 4@@super +@ ions on the TiO@sub 2@ (001) surface. However, decomposition and redox reactions for both methanol and acetaldehyde occur in the presence of surface defects created by Ar@super +@ sputtering. The decomposition products following reactions of acetaldehyde on the defective surface include H@sub 2@, C@sub 2@H@sub 4@, CO, C@sub 4@H@sub 6@ and C@sub 4@H@sub 8@. Reductive coupling to produce C@sub 2@H@sub 4@ and C@sub 4@H@sub 8@ is the main reaction pathway for decomposition of acetaldehyde on the sputter reduced SrTiO@sub 3@(100) surface. Adsorption of CH@sub 3@OH on the reduced SrTiO@sub 3@(100) surface produces the decomposition products of H@sub 2@, CO, and CH@sub 4@. As compared with SrTiO@sub 3@(100) surfaces, Ce@sub 0.8@Zr@sub 0.2@O@sub 2@(111) surfaces exhibit enhanced adsorption and reactivity for methanol and acetaldehyde. Both acetaldehyde and methanol mostly adsorb dissociatively on the oxidized Ce@sub 0.8@Zr@sub 0.2@O@sub 2@(111) surfaces. The formation of furan was surprisingly observed on reduced Ce@sub 0.8@Zr@sub 0.2@O@sub 2@(111) surfaces following the adsorption of acetaldehyde.