A study of the adsorption sites on thoria by scanning transmission electron microscopy and fourier-transform infrared spectroscopy. Adsorption and desorption of water and methanol

A study of the adsorption sites of thoria has been made using scanning transmission electronic microscopy (STEM) and Fourier-transform infrared spectroscopy (FTIR). The STEM study showed that the thoria surface was composed mainly of (110)(211) and (111) faces in about equal proportions. The infrared study of activated ThO2 at 873 K indicated the existence of two remaining hydroxyl groups at 3660 and 3510 cm–1. With water adsorption, a third one appeared at 3745 cm–1 which was thermally less stable. Methanol adsorption gave rise to three kinds of species: (i) one reversibly adsorbed at room temperature and linked by hydrogen bonds in which thoria acts as a proton receiver; (ii) two methoxy species characterized by ν(CO) bands at 1127 cm–1(species I) and 1060 cm–1(species II). Species I desorption bands led to methanol whilst species II, much more thermally stable, gave methanol, dimethyl ether and carbon monoxide. Species I, which corresponds to the hydroxyl species at 3745 cm–1, are of the Th—OMe (Th—OH) type, and are formed on the (110) faces. Species II, which correspond to hydroxyl species at 3660 cm–1, are of the [graphic omitted] type, and are formed on the (211) faces. The third hydroxyl species, characterized by the ν(OH) band at 3510 cm–1, is probably due to species of the [graphic omitted] type appearing on (111) faces. The stability and reactivity of the methoxy species are shown to depend on their structure, and therefore on the local arrangement of the surface.