AVS 49th International Symposium
    Surface Science Tuesday Sessions
       Session SS3-TuA

Paper SS3-TuA2
The Interaction and Reactivity of Hydrocarbons on Hydroxylated @gamma@-Al@sub 2@O@sub 3@/NiAl(100)

Tuesday, November 5, 2002, 2:20 pm, Room C-112C

Session: Metal/Oxide Surfaces
Presenter: K.A. Layman, University of California, Irvine
Authors: K.A. Layman, University of California, Irvine
J.C. Hemminger, University of California, Irvine
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@gamma@-Al@sub 2@O@sub 3@ is an important industrial catalyst and catalyst support. The surface OH groups on bulk @gamma@-Al@sub 2@O@sub 3@ influence the Brönsted acidity and metal dispersion on these catalysts. However, the detailed understanding of A l@sub 2@O@sub 3@ surfaces has been limited because of the complexity of the surface hydroxyl groups. We are able to grow highly-ordered and well-defined thin films of hydroxylated @gamma@-Al@sub 2@O@sub 3@ which model bulk @gamma@-Al@sub 2@O@sub 3@ by exposing the NiAl(100) substrate to 100 L H@sub 2@O at 1000 K. Unlike bulk @gamma@-Al@sub 2@O@sub 3@, these films exhibit a single OH stretch at 3711 cm@super -1@ (FWHM approximately 100 cm@super -1@). This frequency is indicative of non-interacting OH groups bonded to 2 or 3 Al atoms. We have used HREELS to study the interaction and reactivity of hydrocarbons, such as acetonitrile, pyrazine, 2,6-dimethylpyridine, and ammonia, on our thin films of hydroxylated @gamma@-Al@sub 2@O@sub 3@ following adsorption at 140 K. These molecules interact with surface OH groups, forming acid-base complexes. Complex formation causes the OH bond strength to decrease and the OH stretch to shift to lower frequency. We have observed that the magnitude of the OH shift depends on both the basicity and adsorption orientation of these hydrocarbons. Correlation of the OH frequency shift with the gas basicity of the adsorbate allows us to assign a pK@sub a@ to the surface OH groups. The hydrocarbons can also interact with the surface Al@super 3+@ cations. In the case of ammonia, this interaction leads to disruption of an N-H bond and the formation of surface NH@sub 2@ and OH species.