engleski

Characterization of precipitates formed by slow or forced hydrolysis of Fe(ClO4)3 solutions

Slow and forced hydrolyses of Fe3+ salt solutions have been studied from different standpoints. Hydrolysis of Fe3+ ions plays an important role in natural processes, corrosion of iron (steel), synthesis of the polymorphs of Fe(III)-oxyhydroxides and oxides, as well as in the synthesis of many Fe-bearing materials for advanced technologies. The kinetics of the Fe3+ hydrolysis in aqueous solutions and the properties of solid hydrolytic products strongly depend on the nature of anion involved. Majority of studies was related to the hydrolysis of FeCl3, Fe(NO3)3 or Fe2(SO4)3 salt solutions, whereas a much smaller number was related to the Fe(ClO4)3 hydrolysis. In the present work we have focused on the formation and properties of precipitates formed by slow or forced hydrolyses of Fe(ClO4)3 aqueous solutions. The precipitates were investigated using 57Fe Mössbauer and FT-IR spectroscopies, as well as TEM and FE SEM. Slow (long time) hydrolysis yielded g-FeOOH (5&middot ; 10-3 M Fe(ClO4)3), or a mixture of g-FeOOH as the dominant phase and a-FeOOH (1&middot ; 10-2 M Fe(ClO4)3). Forced hydrolysis at 90 oC of the solution 0.1 M Fe(ClO4)3 + 0.1 M HClO4 yielded a-Fe2O3 with a small amount of  -FeOOH upon 72 h of hydrolysis, and a-Fe2O3 as a single phase upon 21 days of hydrolysis. An FE SEM photograph of the latter sample showed a layered substructure of a-Fe2O3 particles. The corresponding 57Fe Mössbauer spectrum showed a slightly decreased Bhf value in relation to monolithic a-Fe2O3 particles, and a distribution of hyperfine magnetic fields as a consequence of the layered substructure. Autoclaving of 0.1 M Fe(ClO4)3 or 0.1 M Fe(ClO4)3 + 0.1 M HClO4 solutions at 160 oC yielded a-Fe2O3 as a single phase. The a-Fe2O3 particles precipitated at 160 oC in the presence of HClO4 also showed a layered substructure. In the absence of HClO4, the nanosized a-Fe2O3 particles with a size interval between ~15 and ~60 nm were obtained. The presence of foreign substances during the hydrolysis of Fe(ClO4)3 solutions influenced the phase composition and particle size/morphology of the solid hydrolytic products. For example, the FT-IR spectrum of the precipitate formed by a forced hydrolysis of the 0.1 M Fe(ClO4)3 solution containing 0.1 % sodium dodecyl sulphate showed a-FeOOH as a single phase, whereas superparamagnetic a-FeOOH was detected by Mössbauer spectroscopy.

hydrolysis; Fe2O3; gamma-FeOOH; alpha-FeOOH; Mossbauer; FE SEM

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