NATURE AND MECHANISM OF FORMATION OF SULFATE SPECIES ON COPPER ALUMINA SORBENT CATALYSTS FOR SO2 REMOVAL

The nature, thermal stability, and reducibility in H2 of sulfate species on copper-on-alumina and their mechanism of formation during interaction of the sorbent-catalyst with SO2-containing flows were studied in a flow microreactor and by Fourier-transform infrared spectroscopy. Spectra of the sulfate species formed on Al2O3, CuO, CuAl2O4, and CuO/Al2O3 samples either by impregnation with various amounts of sulfate salts or by direct sulfation with SO2 + O2 are compared. The results indicate that, on pure alumina, two types of surface sulfate species form, one more stable at low surface coverage attributed to a type with only one double S = O bond and the second less and stable and more easily decomposed by water vapor, attributed to a SO3 group linked to an Al-O pair site or to an oligomer species as S2O7. Sulfation of CuO leads to bulklike CuSO4, whereas sulfation of copper aluminate leads to three types of sulfate species, one linked to Al3+ ions, another to Cu2+ ions, and the third to sulfate species in interaction both with Al3+ and Cu2+ ions. The latter species does not appear in the spectra of Cu supported on Al2O3. The analysis of the formation of sulfate species on copper-on-alumina sorbent-catalysts suggests the following mechanism: in the presence of gaseous oxygen, copper performs catalytically the first step of oxidation of SO2 to SO3 which then forms a stable surface sulfate at either the copper site of the aluminum site. During the first cycle of interaction of the sorbent catalyst with the SO2-containing flow, a sulfate linked mainly to aluminum sites forms in an amount (about 300-400-mu-mol/g) equivalent to the limiting value of sulfate species on pure Al2O3. This species is more stable against reduction than the other sulfate species and is not reduced by H2 at 420-degrees-C. During the first cycle of reduction, copper aluminate sites are reduced to metallic copper which, in the consecutive step of interaction with the SO2+O2 containing flow, give rise to the formation of surface species mainly on copper that are completely regenerated in the consecutive treatment with H2 at 420-degrees-C.