Desorption Kinetics of Methanol from Al₂O₃(0001) Studied Using Temperature-Programmed Desorption and Isothermal Desorption

Abstract

Temperature-programmed desorption (TPD) and isothermal desorption were used to investigate the desorption of methanol from Al2O3(0001) in ultrahigh vacuum. At low coverages, TPD traces for methanol displayed one broad peak that was attributed to monolayer desorption. A second, multilayer peak was observed at a lower temperature as the coverage was increased. The multilayer peak appeared at coverages well below the saturation coverage of the monolayer peak, implying that the multilayer was forming before the monolayer was completely full. Isothermal desorption studies were performed in the multilayer regime as a function of coverage and temperature. The coverage-dependent studies showed that multilayer desorption was of order n = 0.53 ± 0.12. The temperature-dependent studies showed that the multilayer activation barrier for desorption was 11.1 ± 0.5 kcal/mol with an n = 0.53 order preexponential of 3.1 × 1024 s-1. The approximately half-order desorption from the multilayer is interpreted in terms of a structured multilayer. In the monolayer regime, methanol adsorbs reversibly onto the Al2O3(0001) surface. The desorption peak is very broad, and quick temperature ramps to temperatures within the monolayer TPD peak show that a distribution of adsorption sites occur on the surface. The TPD traces were modeled assuming first-order desorption kinetics and using a Gaussian distribution of adsorption sites centered at a desorption activation barrier of 17.7 kcal/mol with a width (fwhm) of 3.0 kcal/mol. A first-order preexponential of 1013 s-1 was assumed in the model, and the results were in good agreement with the experimental data. The relatively high desorption activation barrier suggests a strong interaction between the methanol and the surface, most likely due to oxygen lone pair interactions with aluminum. The distribution of adsorption sites suggest that aluminum oxide surfaces, even in single crystals, are very inhomogeneous.