Adsorption and desorption of HCl on a single-crystal α-Al2O3(0001) surface

The adsorption and desorption of HCl on a single-crystal α-Al2O3(0001) surface were examined using laser-induced thermal desorption (LITD) and temperature-programmed desorption (TPD) techniques. α-Al2O3(0001) is a model for the surface of Al2O3 exhaust particles generated by solid rocket motors. HCl adsorption on these Al2O3 particles may alter the surface reactivity and affect heterogeneous atmospheric chemistry. Absolute HCl surface coverages on α-Al2O3(0001) were determined using calibrated HCl LITD signals. The reactive sticking coefficient for HCl adsorption on α-Al2O3(0001) at 298 K was S∼10−3 at the lowest HCl coverages. The HCl reactive sticking coefficient decreased nearly exponentially with HCl coverage. The HCl coverage saturated at ΘHCl=1.0×1014 molecules cm−2 after HCl exposures of >2×109 L. HCl desorption from the α-Al2O3(0001) surface occurred over a wide temperature range from 300 to 650 K. This broad temperature range suggests a distribution of surface sites with different binding energies. HCl desorption was investigated versus HCl coverage by annealing an α-Al2O3(0001) surface that was previously saturated by a large HCl exposure. These results confirmed a wide range of binding energies and suggested that surface diffusion between the adsorption sites must be negligible. Additional HCl desorption results versus HCl coverage, prepared by varying the HCl exposure, revealed that HCl adsorption randomly fills the adsorption sites independent of their binding energies. Modeling of the HCl desorption results was consistent with surface binding energies that ranged from 19 to 36 kcal mol−1. These adsorption and desorption results predict that α-Al2O3 rocket exhaust particles will be partially covered with a stable HCl adlayer at stratospheric temperatures and pressures.