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Adsorption of CO,, C,H,N, CH30CH3 and CD30CD,H on Tho, has been studied by Fourier-transform infrared spectroscopy. CO, adsorption gives rise to several types of species : polydentate carbonates, bidentate carboxylates, bidentate carbonates, hydrogenocarbonates, monodentate carbonates and linear CO, 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,(CO,) mode of linear CO, species, the vsa mode of pyridine, the v(CH) band of CD30CD,H and the v(C0C) modes of CH30CH3, it is concluded that some Th4+ ions are very weak Lewis-acid sites. The OH groups giving rise to the 3740cm-l band are basic (some are involved in the formation of hydrogenocarbonate species), while some of the OH groups corresponding to the 3655 cm-l band are very weak proton donors.

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