Nature and mechanism of formation of sulfate species on copper/alumina sorbent-catalysts for sulfur dioxide removal

The nature, thermal stability, and reducibility in H of sulfate species on Cu/Al2O3 and their mechanism of formation during interaction of the sorbent-catalyst with SO2-contg. flows were studied in a flow microreactor and by Fourier-transform IR spectroscopy. Spectra of the sulfate species formed on Al2O3, CuO, CuAl2O4, and CuO/Al2O3 samples either by impregnation with various amts. of sulfates or by direct sulfation with SO2 + O2 are compared. The results indicate that, on pure Al2O3, 2 types of surface sulfate species form, one more stable at low surface coverage attributed to a type with 1 double S:O bond and the second less stable and more easily decompd. 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 Cu aluminate leads to 3 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 anal. of the formation of sulfate species on Cu/Al2O3 sorbent-catalysts suggests the following mechanism: in the presence of O, Cu performs catalytically the first step of oxidn. of SO2 to SO3, which then forms a stable surface sulfate at either the Cu site or the Al site. During the first cycle of interaction of the sorbent catalyst with the SO2-contg. flow, a sulfate linked mainly to Al sites forms in an amt. (∼300-400 μmol/g) equiv. to the limiting value of sulfate species on pure Al2O3. This species is more stable against redn. than the other sulfate species and is not reduced by H at 420°. During the first cycle of redn., Cu aluminate sites are reduced to metallic Cu, which, in the consecutive step of interaction with the SO2 + O2-contg. flow, give rise to the formation of surface species, mainly on Cu, that are completely regenerated in the consecutive treatment with H at 420°. [on SciFinder(R)]