In Situ Fourier Transform Infrared Diffuse Reflectance and Photoacoustic Spectroscopy Characterization of Sulfur-Oxygen Species Resulting from the Reaction of SO2 with CaCO3

Fourier transform photoacoustic and diffuse reflectance spectroscopies (FT-IR-PAS and -DRS) have been used in situ to monitor the reaction of SO2 (1.5% in N2) with particulate CaCO3 at temperatures up to 400°C (PAS) and 690°C (DRS). The PA spectra indicate that at 360°C SO2 is physisorbed on the CaCO3 surface. This species is readily desorbed by N2 purging. Under continuous SO2/N2 exposure at 400°C and above, the physisorbed species is shown by the DR data to be converted first to oxygen-bound and then to nonspecifically bound pyramidal SO3−. Reaction of the SO3= with SO2 to form SO4− begins at 400°C and is essentially complete at 690°C. The DR data further indicate that for the SO4−: (1) at temperatures between 400 and 640°C a monodentate C3 v species forms; (2) above 640°C the SO4− ion has nearly Td symmetry; and (3) upon subsequent cooling, the SO4− exhibits the characteristics of a chelating C2 v species.