DFT study of carbon monoxide adsorption on α-Al2O3(0001)

The adsorption of CO on alpha-Al2O3(0001) was studied using the DFT-GGA computational method and on alpha-Al2O3 powder experimentally by Infra red spectroscopy. The core and valence level regions of alpha-Al2O3(0001) single crystal surface were also studied experimentally. Ar ions sputtering of the surface results in a slight but reproducible decrease in the XPS O2p lines in the valence band regions due to preferential removal of surface (and near surface) O atoms. Core level XPS O1s and Al2p further confirmed oxygen depletion with an associated surface stoichiometry close to Al2O2.9. The adsorption energy of CO was computed and found equal to 0.52 eV for theta = 0.25, it decreased to 0.42 eV at theta = 1. The IR frequency of nu CO was also computed and in all cases it was blue shifted with respect to gas phase CO. The shift, Delta nu, decreased with increasing coverage where it was found equal to 56 cm(-1) for theta = 0.25 and decreased to 30 cm(-1) for theta = 1. Structural analyses indicated that the change in the adsorption energy and the associated frequency shift is due to surface relaxation upon adsorption. Experimentally the adsorption of CO gave rise to one main IR peak at 2154 cm(-1) at 0.3 Torr and above. Two far smaller peaks are also seen at lower pressures of 0.03-0.2 Torr at 2189 and 2178 cm(-1). The isosteric heat of adsorption was computed for the IR band at 2154 cm(-1) and was found equal to 0.2 eV which did not change with coverage in the investigated range up to theta = 0.6. (C) 2011 Elsevier B.V. All rights reserved.