Preliminary Study of H₂O Adsorption on Cr₂O₃ and Al₂O₃ Surfaces by Ab Initio Cluster Calculations

Adsorption and desorption of water over Cr-oxide and alumina are of great concern with the contamination of stainless steel and gas chromatographic isotope separation. However,the mechanisms of those phenomena have not been fully clarified. A preliminary ab inito study on the behavior of water molecules on the (001) surfaces of α-Cr₂O₃ and α-Al₂O₃ was carried out for small clusters of Me4O4 (Me=Cr or Al) by use of the Gaussian 03 package. It was found that water molecules can be adsorbed molecularly and dissociatively on these surfaces. Molecular adsorption of a H₂O molecule takes place on a metal atom of the cluster as Me-OH₂, with the bond length of about 2.0 Å. Dissociative adsorption occurs by forming Me-OH (on-top site) or Me2-OH (bridge site) and O-H bonds. There appeared a feature that the bond length of Me2-OH is greater than that of Me-OH. Calculated adsorption energies were examined by comparing with the values estimated from the thermal desorption spectra of water for α-Cr₂O₃ and γ-alumina. The ab initio calculations coincided quite well with the observed thermal desorption spectrum for α-Cr₂O₃, whereas they did not for γ-alumina. Further studies are required to clarify the structures of adsorbed species, adsorptiron energies and kinetic processes