Evaluation of magnesium aluminate spinel as a sulfur dioxide transfer catalyst

Abstract

Infrared spectroscopy and vacuum microbalance techniques have been used to study the sulfation of magnesium aluminate spinel, MgAl 2O 4, by oxidation of sulfur dioxide in excess oxygen at temperatures from 300 to 550°C. The results have been compared with those previously obtained for sulfated Al 2O 3, TiO 2, ZrO 2 and MgO with a view to evaluating which material would be a superior SO x transfer catalyst. For low doses of sulfur dioxide, surface sulfates on spinel are mainly formed which are covalently bonded to the surface but, at higher doses, ionic sulfates are incorporated into the bulk of the spinel. At least two types of surface sulfate species have been identified and that formed preferentially at low coverages has the probable structure (−0) 3SO, as was found previously for SO 2 oxidation on Al 2O 3 and TiO 2. All sulfate species are thermally stable to evacuation up to 800°C but are removed between about 800–900°C. They are also readily removed following reduction in excess hydrogen in a relatively narrow temperature range from about 550 to 640°C. On a very low surface area spinel, only bulk sulfate is formed. Relative to the other oxides studied, sulfated spinel has a high thermal stability, the sulfates are easily reduced in hydrogen and it has a high sorption capacity by virtue of its ability to form bulk sulfate species (not observed under the same conditions on Al 2O 3, TiO 2, or ZrO 2). These favorable characteristics account for the use of spinel as a transfer catalyst for reducing SO x emissions.