Abstract
The influence of heat treatment on the surface properties of hematite samples of different origins was examined by inverse gas chromatography (IGC) and temperature programmed desorption (TPD) techniques. Comparison of the curve relating the evolution of the free energy of adsorption of a methylene group, ΔGa(CH2), measured by IGC using a homologous series ofn-alkanes, and the TPD curve reveals similarities. In particular, the increase either of the rate of departure of impurities (SO2from a polluted hematite sample) or the rate of water release upon heating a pure hematite sample is accompanied by a significant increase of ΔGa(CH2). Rehydration experiments of heat-treated hematite samples show that restoring the characteristics of the initial, nonheated oxide surface becomes quasi-impossible for samples treated at above 350°C. This points to reconstruction or relaxation of the surface layer upon heat treatment. Determination of the changes in nanomorphology and acid/base properties complements this study. A layer of oxyhydroxide possibly exists on hematite surfaces.
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