Infrared study of acid–base properties of thorium dioxide

Abstract

Adsorption of CO₂, C₅H₅N, CH₃OCH₃ and CD₃OCD₂H on ThO₂ has been studied by Fourier-transform infrared spectroscopy. CO₂ adsorption gives rise to several types of species: polydentate carbonates, bidentate carboxylates, bidentate carbonates, hydrogenocarbonates, monodentate carbonates and linear CO₂ species. The carbonate species have been identified on the basis of the splitting of the v₃ vibration and of thermal-stability considerations. These results show that thorium dioxide is a basic oxide. Considering the v_a(CO₂) mode of linear CO₂ species, the v_8a mode of pyridine, the v(CH) band of CD₃OCD₂H and the v(COC) modes of CH₃OCH₃, it is concluded that some Th⁴⁺ ions are very weak Lewis-acid sites. The OH groups giving rise to the 3740 cm^–1 band are basic (some are involved in the formation of hydrogenocarbonate species), while some of the OH groups corresponding to the 3655 cm^–1 band are very weak proton donors.