Sol-gel synthesis and washing of amorphous g-FeO(OH) xerogels

Iron(III) oxyhydroxide xerogels were prepared by the sol-gel technology including a washing step for removal of a salt by-product. The synthesis involved an iron(III) nitrate nonahydrate as precursor, ethanol as solvent and ammonium hydroxide as gelation agent, following the procedure of an earlier work. As this chemical route originates ammonium nitrate as a by-product dispersed in the iron oxyhydroxide matrix, the removal of this salt by washing the gel before the drying stage is here studied. Two solvents were tested, namely water and ethanol, being the best washing efficiency (95%) achieved with water and two washing batches. Comparing the xerogels obtained without and with the washing step, the later only contained an insignificant amount of ammonium nitrate salt, as confirmed by Elemental Analysis, Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The iron phase in the washed xerogels was found to be g-FeO(OH), according to FTIR and Mossbauer spectroscopy results, and in consonance with the prevailing iron phase in the unwashed xerogels. The washed xerogels are amorphous and formed by large clusters of well connected nanocrystallites of iron oxyhydroxide. The washing step enhanced the clearing of mesopores, originating materials with a specific surface area of similar to 250 m2/g, 60 times higher than for unwashed xerogels.