AVS 46th International Symposium
    Surface Science Division Monday Sessions
       Session SS2-MoA

Paper SS2-MoA4
Dissociation and Desorption of NO and N@sub 2@ on Rh(100) and Rh(111), Effect of Surface Structure on Elementary Reaction Steps

Monday, October 25, 1999, 3:00 pm, Room 607

Session: Reactions on Metals
Presenter: M.J.P. Hopstaken, Eindhoven University of Technology, The Netherlands
Authors: M.J.P. Hopstaken, Eindhoven University of Technology, The Netherlands
J.W. Niemantsverdriet, Eindhoven University of Technology, The Netherlands
Correspondent: Click to Email
Removal of NO@sub x@ is an important issue in automotive exhaust catalysis. On the molecular level, dissociation of NO on rhodium is the crucial step in the reduction of NO by CO, H@sub 2@ or hydrocarbons. Here we use TPD/SIMS to study reactions of NO on Rh(100) and compare with similar results on Rh(111).@footnote 1@ On both surfaces the reaction rates of the different elementary steps depend highly on coverage. At low coverage, dissociation of NO is completed around 250 K and 340 K for the Rh(100) and the Rh(111) surface, respectively. When the surface is saturated with NO, dissociation only starts when NO desorbs i.e. when empty sites become available. However, inhibition of NO dissociation at higher coverages cannot be explained by site blocking alone. Monte Carlo simulations strongly indicate that the activation energy for dissociation of NO on Rh(111) increases when other adsorbates such as N, O, and NO are present. At low coverages we find an activation energy of 45 kJ/mole for dissociation of NO on Rh(100), which is 20 kJ/mole lower than on Rh(111). Desorption of NO from both surfaces proceeds at similar rates. However, formation of N@sub 2@ is slower on Rh(100). Apparently, bond breaking of NO is faster and formation of a N-N bond is slower on the more open Rh(100) surface. This can be explained on the basis of surface chemical reactivity theories.@footnote 2@ Unexpectedly, the oxidation of CO to CO@sub 2@ proceeds much faster on Rh(100) than on Rh(111). This, together with the higher rate of dissociation and lower rate of N@sub 2@ formation, has important consequences for the kinetics of the CO + NO reaction on the different rhodium surfaces. @FootnoteText@ @footnote 1@ H.J. Borg, J.P.C.-J.M. Reijerse, R.A. van Santen and J.W. Niemantsverdriet, J. Phys. Chem. 101 (1994) 10052. @footnote 2@ R.A. van Santen, J.W. Niemantsverdriet, Chemical Kinetics and Catalysis, Plenum Press, New York, 1995.