DFT study of carbon monoxide adsorption on α-Al_2O_3(0001)

摘要

The adsorption of CO on α-Al_2O_3(0001) was studied using the DFT-GGA computational method and on tx-A1_2O_3 powder experimentally by Infra red spectroscopy. The core and valence level regions of α-Al_2O_3(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 Sevel XPS Ols and A12p further confirmed oxygen depietion with an associated surface stoichiometry close to Al_2O_(2.9). The adsorption energy of CO was computed and found equal to 0.52 eV for θ= 0.25, it decreased to 0.42 eV at θ= 1. The IR frequency of vCO was also computed and in all cases it was blue shifted with respect to gas phase CO. The shift, Av, decreased with increasing coverage where it was found equal to 56 cm~(-1) for θ = 0.25 and decreased to 30 cm~(-1) for θ = 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 θ = 0.6.