AVS 47th International Symposium
    Surface Science Monday Sessions
       Session SS1-MoA

Paper SS1-MoA7
Co-adsorption and Reaction of Ethylene and NO over Rh Loaded Ceria Surfaces

Monday, October 2, 2000, 4:00 pm, Room 208

Session: Model Catalysts at High Pressures
Presenter: S.H. Overbury, Oak Ridge National Laboratory
Authors: D.R. Mullins, Oak Ridge National Laboratory
S.H. Overbury, Oak Ridge National Laboratory
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In ceria containing emission control catalysts, such as three-way convertors in automobiles, the ceria plays an important role in oxygen exchange in both reduction and oxidation reactions. It is important to understand this role in catalytic reaction pathways. We have used soft x-ray photoelectron spectroscopy and TPD to study the co-adsorption and reaction of ethylene and NO on model catalysts surfaces composed of Rh deposited onto ceria films of controlled oxidation state. Studies focused on the effect of the oxidation state of the ceria upon the reaction pathways. The reactions occur predominantly on the Rh particles but are affected by oxygen spillover, interfacial reactions and modulation of the Rh reactivity by the ceria. Following adsorption at 100 K and subsequent programmed temperature increase, the ethylene decomposes in stages on the Rh particles leading to hydrogen evolution and leaving a reactive C species capable of reducing the ceria to produce CO. Correspondingly, NO reacts with both Rh and ceria. NO oxidizes the ceria resulting in N@sub 2@ evolution. NO dissociation occurs on Rh to an extent dependent upon the oxidation state of the ceria support. There is little interaction between co-adsorbed NO and ethylene when the ceria support is fully oxidized, except for site competition on Rh at low temperature. As temperature is increased water and N@sub 2@ are evolved followed by higher temperature CO evolution. However, for highly reduced ceria, water desorption is eliminated in favor of H@sub 2@, due to oxygen spillover onto ceria. Also, there is strong interaction between the N and C species on Rh which stabilizes them with respect to evolution of CO and N@sub 2@. Except for the absence of isocyanate and the presence of hydrogen and its branching between H@sub 2@ and water, the results are similar to reactions between CO and NO. Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, USDOE.