AVS 45th International Symposium
    Surface Science Division Tuesday Sessions
       Session SS2-TuM

Paper SS2-TuM10
Surface Species and Desorption Products from NO Exposure on Clean and (Rh,Pt) Loaded Surfaces of Highly Oriented CeO@sub 2@ Characterized by SXPS and TPD

Tuesday, November 3, 1998, 11:20 am, Room 309

Session: Noble Metal Catalysis
Presenter: S.H. Overbury, Oak Ridge National Laboratory
Authors: S.H. Overbury, Oak Ridge National Laboratory
D.R. Mullins, Oak Ridge National Laboratory
D.R. Huntley, Oak Ridge National Laboratory
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Adsorption and reaction of NO on ceria, both with and without Rh or Pt additions, was studied by soft x-ray photoelectron spectroscopy and temperature programmed desorption. The results demonstrate the complexities and interactions which may occur in supported catalysts involving reducible substrates. Growth in UHV of thin films of CeO@sub 2@ on a Ru(0001) substrate allowed creation of oriented films with variable oxidation state, quantitatively determined by valence band and Ce 4d photoemission. Following NO exposure various N containing surface species were observed by N 1s photoemission, and their distribution depended upon surface oxidation state, exposure and adsorption temperature. These species included N@sub 2@O, NO, NO@super +@ and three states believed to be associated with NO@super -@ or atomic forms of N. Occurrence of adsorbed N@sub 2@O at 100 K indicates that NO dissociation and reaction occurs readily at low temperature. N@sub 2@ is evolved during adsorption from above 100 K to at least 600 K indicating that some NO direct decomposition occurs over a wide range of temperatures and coverages. Adsorption of NO at 150 K is predominantly molecular while exposure to NO at 400 K leads to a low binding energy atomic state which desorbs at temperatures above 500 K. NO adsorbed on oxidized ceria yields a molecular state at a higher binding energy than on reduced ceria and thus is associated with NO@super +@. Adsorption of NO on reduced ceria is oxidative resulting in strongly non-Langmuirian adsorption behavior. Deposition of Rh or Pt on these surfaces alters the observed NO chemistry, and the extent of reduction of the ceria in turn modulates the activity of the noble metal for NO dissociation. Greater dissociation probability of NO and increased stability of the resulting N atoms is observed for the noble metal deposited on a reduced compared to a fully oxidized support. Research sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy