AVS 45th International Symposium
    Surface Science Division Thursday Sessions
       Session SS2-ThM

Paper SS2-ThM3
HCl Adsorption and Desorption on a Single-Crystal @alpha@-Al@sub 2@O@sub 3@(0001) Surface

Thursday, November 5, 1998, 9:00 am, Room 309

Session: Oxide Surface Chemistry
Presenter: C.E. Nelson, University of Colorado, Boulder
Authors: C.E. Nelson, University of Colorado, Boulder
J.W. Elam, University of Colorado, Boulder
S.M. George, University of Colorado, Boulder
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The adsorption and desorption of HCl on a single-crystal @alpha@-Al@sub 2@O@sub 3@(0001) surface were examined using laser induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. @alpha@Al@sub 2@O@sub 3@(0001) models Al@sub 2@O@sub 3@ particles generated by solid rocket motors and HCl interactions with these Al@sub 2@O@sub 3@ particles may affect the stratospheric ozone layer. The initial sticking coefficient for HCl on @alpha@Al@sub 2@O@sub 3@(0001) at 298 K was S~10@super -3@. The HCl sticking coefficient decreased nearly exponentially with HCl coverage and the HCl coverage saturated at @THETA@@sub HCl@=0.10x10@super 15@ molecules/cm@super 2@ after HCl exposures of >10@super 10@ Langmuir. HCl desorption from the @alpha@Al@sub 2@O@sub 3@(0001) surface occurred over a temperature range from 300 K to 650 K. This broad range suggests a distribution of surface sites with different binding energies. HCl desorption results versus HCl coverage prepared by progressively annealing a fully exposed @alpha@Al@sub 2@O@sub 3@(0001) surface confirmed a wide range of binding energies. Additional HCl desorption results versus HCl coverage prepared by varying the HCl exposure revealed that HCl dissociative adsorption randomly fills adsorption sites independent of their adsorption energy. These results also suggest that surface diffusion between the adsorption sites must be negligible. Modeling of the desorption results was consistent with surface site energies that range from 19 to 36 kcal/mole. These adsorption and desorption results predict that HCl will be stable on @alpha@Al@sub 2@O@sub 3@ rocket exhaust particles at stratospheric temperatures and pressures.