Comparative study of SO2 adsorption on metal oxides

Abstract

SO₂ adsorption on different metal oxides (MgO, CeO₂, ZrO₂, MgAl₂O₄, TiO₂-anatase, TiO₂-rutile, Al₂O₃ and Na–Al₂O₃) has been studied using various techniques (thermogravimetry, temperature-programmed desorption, IR spectroscopy). Several types of species are formed and the results are discussed in terms of their thermal stability. Weakly adsorbed species can result from the coordination of SO₂ on Lewis acid sites such as Al₂O₃ or TiO₂. However, such interactions are quite weak. On more basic oxides, such as Na–Al₂O₃, SO₂ acts as an electron acceptor and adsorbs on either weakly basic O 2^– sites or on the basic OH^– groups. In this latter case, the formation of hydrogen sulfite species is suggested. More strongly adsorbed species occur on all the oxides. These are characterized by strong absorption bands between 1100 and 800 cm^–1, and are due to sulfite species. Several types generally occur. Gravimetric measurements allow us to compare the thermal stability and the amount of the different types of adsorbed SO₂ species. The quantity of SO₂ irreversibly adsorbed after evacuation at T < 370 K gives rise to an evaluation of the number of basic sites and therefore to a scale of basicity of the different oxides. The results are compared with those obtained using other probe molecules such as carbon dioxide or hexafluoroisopropanol. A special case is that of ceria for which it is observed that heating under vacuum transforms sulfite species into sulfates, preventing the use of TPD for the determination of its relative basicity.