Desorption of H2O from a hydroxylated single-crystal α-Al2O3(0001) surface

The desorption of H2O from a hydroxylated single-crystal alpha-Al2O3(0001) surface was studied using laser-induced thermal desorption (LITD) and temperature-programmed desorption (TPD) techniques. alpha-Al2O3(0001) models the surface of exhaust particles from solid rocket motors. H2O desorption from alpha-Al2O3(0001) will determine hydroxyl coverages that will influence Al2O3 surface chemistry. H2O desorption from the hydroxylated alpha-Al2O3(0001) surface occurred over a temperature range from 300 to 500 K. This broad distribution suggests a variety of hydroxyl surface sites with different binding energies. H2O desorption results versus hydroxyl coverage prepared by progressively annealing a fully hydroxylated alpha-Al2O3(0001) surface confirmed a wide range of hydroxyl binding energies. Additional H2O desorption results versus hydroxyl coverage prepared by varying the H2O exposure revealed that H2O dissociative adsorption randomly fills the adsorption sites independent of their adsorption energy. These results argued that hydroxyl surface diffusion between the various adsorption sites must be negligible. Modeling of the desorption results was consistent with hydroxyl desorption energies that range from E-d = 23 to E-d = 41 kcal mol(-1). The H2O desorption results predict that hydroxyl groups will be stable on alpha-Al2O3 rocket-exhaust particles under stratospheric conditions. (C) 1998 Published by Elsevier Science B.V. All rights reserved.