Characterisation of Basic Sites on Ga₂O₃, MgO, and ZnO with Preadsorbed Ethanol and Ammonia—IR Study

Abstract

The effect of adsorption of ethanol and ammonia on the basicity of Ga2O3, MgO, and ZnO was examined via IR studies of CO2 adsorption. Ethanol reacts with OH groups on Ga2O3, and MgO, forming ethoxyl groups. The substitution of surface hydroxyls by ethoxyls increases the basicity of the neighbouring oxygen. The ethoxyl groups that also form on ZnO do not contain surface OH groups, but the mechanism of their formation is different. On ZnO, ethoxy groups are formed by the reaction of ethanol with surface oxygens. The presence of ethoxyls on ZnO decreases the basicity because some surface oxygens are already engaged in the bonding of ethoxyl groups. The effect of ammonia adsorption on basicity is different for each oxide. For Ga2O3, ammonia adsorption increases the basicity of neighbouring oxygen sites. Ammonia is not adsorbed on MgO; therefore, it does not change the basicity of this oxide. Ammonia adsorbed on ZnO forms coordination bonds with Zn sites; it does not change the number of basic sites but changes how carbonate species are bonded to surface sites.

Keywords

• Ga₂O₃
• MgO
• ZnO basic sites
• CO₂ adsorption
• ethoxy groups
• ammonia adsorption