H 2O and HCl adsorption on single crystal α-Al 2O 3(0001) at stratospheric temperatures

• Published in: Applied surface science Vol. 171; no. 1; pp. 21 - 33
• Main Authors: Nelson, C.E., Elam, J.W., Tolbert, M.A., George, S.M.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 2001
• Subjects: H 2O; HCl; Laserinduced thermal desorption; Temperature programmed desorption technique; α-Al 2O 3; Temperature programmed desorption technique; HCl; α-Al 2O 3; H 2O; Laserinduced thermal desorption
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/S0169-4332(00)00536-5

The adsorption of H 2O and HCl on single crystal α-Al 2O 3(0001) at stratospheric temperatures was studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The α-Al 2O 3(0001) surface was used to model stratospheric Al 2O 3 exhaust particles from solid rocket motors. H 2O and HCl coverages may influence heterogeneous chemistry on Al 2O 3 particles at stratospheric temperatures of ∼200–270 K. Compared with earlier measurements at 298–300 K, the saturation coverages of both H 2O and HCl on α-Al 2O 3(0001) were approximately two times higher at 210 K. The saturation coverages of H 2O and HCl at 210 K were Θ H 2O =4.4×10 14 molecules/cm 2 and Θ HCl=2.0×10 14 molecules/cm 2. The sticking coefficients for both H 2O and HCl on α-Al 2O 3(0001) at 210 K were also higher than the corresponding sticking coefficients at 298–300 K. The sticking coefficients (S) for both H 2O and HCl at 210 K were ∼0.08 at the lowest coverages. The TPD spectra revealed that H 2O and HCl desorption occurred over a broad range of temperatures. These results indicate that H 2O and HCl adsorb onto a wide distribution of binding sites on the α-Al 2O 3(0001) surface. The LITD and TPD results predict that α-Al 2O 3 rocket exhaust particles will be covered with chemisorbed H 2O and HCl under stratospheric conditions. Additional H 2O and HCl exposure can also replace the adsorbed HCl and H 2O species, respectively, by exchange reactions.