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Surface Mobility in Chemical Reactions and Catalysis

  • J. J. Fripiat
  • H. Van Damme
Part of the NATO Advanced Science Institutes Series book series (NSSB, volume 86)

Abstract

When the surface of a solid is exposed to a mixture of gases that would not react in the gas phase, adsorption may promote the reaction kinetics for at least two reasons: i) it increases the local concentration in reagents in the surface phase and ii) it brings one of the reagents or possibly both, in a state which decreases the activation energy barrier because of the presence of surface active sites. These are the two simple basic principles on which catalysis is founded. In spite of this apparent simplicity there are however a large number of problems behind the notion of catalytic activity. They may be listed at follows:
  1. 1)

    What is the physical meaning of the “active site” concept? Since it has to promote a reagent molecule into an activated state, adsorption on an active site must provoke some change in the bonds within the adsorbed species.

     
  2. 2)
    What is the role of surface diffusion? Suppose that molecule A during its residence time (τR) ont the active site is brought into an activated state A*. Yet in order for the schematic reaction
    $${{A}^{*}} + B \to C $$
    (I, 1)
    to occur during that time τR, reagent B must have sufficient mobility to collide with A*. This collision may result from B (from the gas phase) impinging the surface site where A* is formed or from B diffusing onto the surface in a random walk. If the surface was an ideal infinite plane, the number of collisions of molecules B from the gas phase and the surface could compete with the number of collisions due to surface diffusion. However most of the catalysts are made of small crystals with external and (porous) internal surfaces with large specific areas. An ensemble of such small particles is characterized by a more or less narrow distribution of pores with diameters in the range, say, between 5 and 100 Å. It may thus be expected that the main mechanism contributing to the collisions B →A* is due to surface diffusion.
     

Keywords

Kcal Mole Correlation Time Surface Diffusion Silanol Group Proton Mobility 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Plenum Press, New York 1983

Authors and Affiliations

  • J. J. Fripiat
    • 1
  • H. Van Damme
    • 1
  1. 1.Centre de Recherche sur les Solides à Organisation Cristalline ImparfaiteC.N.R.S.OrléansFrance

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