Transformation of Nanostructured Schwertmannite and 2-Line-Ferrihydrite into Hematite

Nanostructured ferric oxides and sulfated ferric oxides are of high interest in catalysis, e.g. for hydrocarbon isomerization. Sulfated ferric oxides are often prepared by immersion of ferric oxides in sulfate containing solutions which technique is facile to conduct, yet poorly reproducible. This problem is circumvented by sulfate incorporation into ferric oxide particles during precipitation. This way of intrinsic sulfation generated ferric oxides with defined properties. The precipitation reaction is accomplished by using a microjet mixer, in which ferric sulfate is brought to reaction with ammonia, what allows for forming schwertmannite and ferrihydrite qualities with defined sulfate content. As ferric oxide synthesis is conducted at ambient conditions, while catalysis mainly occurs at elevated temperatures, it is important to study structural as well as chemical changes during heating. Thus these materials were analysed in terms of structural and chemical changes during heating. In this course it was found that both materials are transformed into hematite, the crystallization degree as well as specific surface properties of which depend both on annealing temperature and the precursor used. This transformation was accompanied by dehydration and liberation of sulfur trioxide.