Evaluation of crystalline structure and SO₂ storage capacity of a series of composition-sensitive De-SO₂ catalysts

A series of non-stoichiometric magnesium-aluminate solid solution spinels, used as sulfur-transfer catalysts in the fluid catalytic cracking units for SO_x emissions abatement, were prepared by using coprecipitation method and characterized with BET, TGA, AES, XRD and in situ IR techniques. It was found that both the crystalline structures and De-SO_x activities of magnesium-aluminate spinels are very sensitive to catalyst compositions. Several phase domains were produced by changing the mole ratio of alumina to magnesia in the preparation process. Owing to substitution of magnesium or aluminum ions in the spinel structure, that leads to a polarization of Al³⁺ or Mg²⁺ ions to adjacent oxygen lattices, the lattice cell parameter of spinel structure regularly varied with chemical compositions, producing a contraction or expansion effect in the lattice cell, that strongly affects De-SO_x activity. TG analysis showed that during SO₂ oxidative adsorption, most of sulfur species were captured on the surface and some sulfur species were stored in bulk of the solids. The IR and AES results confirmed that both surface and bulk-like sulfates with different H₂-reducibilities were formed on the catalysts, which is in good agreement with results of SO₂ monolayer adsorption measurement. Ten-cycle tests of SO₂ oxidative adsorption and reductive decomposition of the formed sulfate showed that the sample with X_Al = 0.33 is the optimum catalyst, its SO₂ capturing capacity reached 124.4 mg/g.