The Structural Transformation of Ferric Oxyhydroxides and Their Activity to Sulfur Dioxide

Abstract α-, β-, and γ-FeOOH calcined at various temperatures up to 500°C have been examined by determining adsorption isotherms in order to relate the reactivity to SO2 with the structural transformation. For each ferric oxyhydroxide, the amount of chemisorbed SO2 per unit surface area (Ac), a measure of reactivity to SO2, decreases with the progress of transformation into α-Fe2O3. At temperatures of calcination below 200°C the Ac value of β-FeOOH is larger than that of α-FeOOH or γ-FeOOH. It is shown by thermogravimetric analysis that the weight loss of β-FeOOH caused by heating is larger than the theoretical value corresponding to the dehydration of hydroxyl groups; 2FeOOH→Fe2O3+H2O. Changes in the X-ray diffraction patterns and Cl content of β-FeOOH with calcination indicate that β-FeOOH is transformed into α-Fe2O3 with elimination of inherently contained Cl at a higher temperature than other oxyhydroxides. The high reactivity to SO2 and large weight loss of β-FeOOH caused by heating have been discussed with respect to the particular crystal structure of β-FeOOH containing Cl and molecular H2O, in addition to hydroxyl groups.