A Temperature Programmed Desorption Study on the Adsorption State of SO₂ on Iron(III) Hydroxide Oxides

Abstract

The evolution of sulfur oxides from the SO₂-adsorbed Fe (III) hydroxide oxides (α-, β-, and γ-FeOOH) was examined by the temperature programmed desorption (TPD) method in the temperature range between 30 and 600°C. TPD spectra of α-FeOOH and γ-FeOOH showed peaks at 130, 230, and 500°C (Figs.2 and 4). As for β-FeOOH, in addition to 130 and 230°C peaks, other two peaks appeared at 300 and 430°C corresponding to the structural transformation from FeOOH to α-Fe₂O₃ (Fig.3). The chemical analysis of the desorbed gases indicated that a portion of adsorbed SO₂ molecules is desorbed as SO₃ at 230 and 500°C, and that the evolved gas at 130°C consists of SO₂. The state of adsorption of SO2 was discussed with reference to the surface structures of each FeOOH crystal: SO₂ bound with the surface OH- evolves at 130°C and SO₂ bound with the surface O^2- at 230 and 500°C.
The TPD experiments have been carried out for SO₂ adsorbed on the sample pre-treated at the transformation temperature from FeOOH to α-Fe₂O₃. The results indicated that the surface O^2- is highly active to SO₂ in an intermediate state between the original FeOOH and the resultant Fe₂O₃, because the SO₃ content is considerably high (Fig.9) and the activation energy of desorption of SO₂ is low (Fig.10) at the transformation temperature.