Fourier transform photoacoustic and diffuse reflectance spectroscopies (FT-IR-PAS and -DRS) have been used in <i>situ</i> to monitor the reaction of SO<sub>2</sub> (1.5% in N<sub>2</sub>) with particulate CaCO<sub>3</sub> at temperatures up to 400°C (PAS) and 690°C (DRS). The PA spectra indicate that at 360°C SO<sub>2</sub> is physisorbed on the CaCO<sub>3</sub> surface. This species is readily desorbed by N<sub>2</sub> purging. Under continuous SO<sub>2</sub>/N<sub>2</sub> 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 SO<sub>3</sub><sup>−</sup> Reaction of the SO<sub>3</sub><sup>−</sup> with SO<sub>2</sub> to form SO<sub>4</sub><sup>−</sup> begins at 400°C and is essentially complete at 690°C. The DR data further indicate that for the SO<sub>4</sub><sup>−</sup>: (1) at temperatures between 400 and 640°C a monodentate C<sub>3ν</sub> species forms; (2) above 640°C the SO<sub>4</sub><sup>−</sup> ion has nearly <i>T<sub>d</sub></i> symmetry; and (3) upon subsequent cooling, the SO<sub>4</sub><sup>−</sup> exhibits the characteristics of a chelating <i>C</i><sub>2ν</sub> species.

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