Variation of the surface properties of hematite upon heat treatment evidenced by inverse gas chromatography and temperature programmed desorption techniques: Influence of surface impurities and surface reconstruction

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, Delta G(a)(CH2), measured by IGC using a homologous series of n-alkanes, and the TPD curve reveals similarities, In particular, the increase either of the rate of departure of impurities (SO2 from a polluted hematite sample) or the rate of water release upon heating a pure hematite sample is accompanied by a significant increase of Delta G(a)(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 degrees C, This points to reconstruction or relaxation of the surface layer upon heat treatment. Determination of the changes in nano-morphology and acid/base properties complements this study. A layer of oxyhydroxide possibly exists on hematite surfaces. (C) 1998 Academic Press.