Abstract

Fourier transform photoacoustic and diffuse reflectance spectroscopies (FT-IR-PAS and -DRS) have been used in <i>situ</i> 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 C_3ν species forms; (2) above 640°C the SO₄⁻ ion has nearly <i>T_d</i> symmetry; and (3) upon subsequent cooling, the SO₄⁻ exhibits the characteristics of a chelating <i>C</i>_2ν species.

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