SO₂ adsorption and thermal stability and reducibility of sulfates formed on the magnesium-aluminate spinel sulfur-transfer catalyst

Magnesium-aluminate spinel used as a sulfur-transfer catalyst in the fluid catalytic cracking units for SO_x emission control was prepared by the precipitation method. The crystalline structure, textural property, and surface dehydroxylation of the sample were characterized by thermogravimetry-derivative thermogravimetry (TG-DTG), differential thermal analysis (DTA), X-ray diffraction (XRD), liquid N₂ adsorption-desorption and infrared spectroscopy (IR) measurements. The behavior of SO₂ adsorption and oxidation on the surface of catalyst was evaluated with IR from 50 °C to 600 °C. Particularly, the thermal stability and H₂-reducibility of the formed sulfite or sulfate during SO₂ adsorption or oxidation were tested under various conditions. In the absence of oxygen in the feed mixture, weak physically adsorbed SO₂ species and surface sulfite were identified. In the case of SO₂ oxidative adsorption, both surface sulfate and bulk-like sulfate were formed. When the sulfated sample was reduced with hydrogen, the surface sulfite and sulfates were completely removed below 550 °C in vacuum. The bulk-like sulfate, however, showed a high ability to resist H₂-reduction, which indicates that the reducibility of bulk-like sulfate formed on magnesium-aluminate spinel must be enhanced when it is used as a sulfur-transfer catalyst.