Mechanism of reduction in hydrogen atmosphere and thermal transformation of synthetic ferrihydrite nanoparticles

[Display omitted] •Magnetic measurements used to study thermal transformation of ferrihydrite.•Reduction mechanisms of ferrihydrite were monitored by TPR, MS, and in-situ XRD.•Nanocrystalline Fe was produced by the reducing ferrihydrite in H2.•The presence of SiO2 in ferrihydrite alters the reduction pathway of ferrihydrite. The thermal transformation under vacuum and the reduction behavior in hydrogen atmosphere of 2- and 6-line ferrihydrite (FeOOH·nH2O) as well as 2-line FeOOH·nH2O deposited onto silica are reported. The investigation methods include magnetization measurements, temperature programmed reduction, in-situ X-ray diffraction and Mössbauer spectroscopy. The thermal transformation of FeOOH·nH2O to hematite (α-Fe2O3) was monitored through changes in the magnetization as a function of temperature; it appears to proceed through the loss of the lattice water and sintering accompanied by improved crystallinity and structural changes. Such a transformation is initiated at T∼580K for 2-line and 6-line FeOOH·nH2O and at T∼660K for 2-line FeOOH·nH2O/SiO2, i.e., the presence of SiO2 appears to inhibit the transformation. SiO2 also tends to prevent the increase of the crystallite size above a certain threshold value. Reduction reactions are initiated at relatively lower temperatures (∼480K) implying that a gaseous environment facilitates the thermal dehydration/dehydroxylation process. Three different reduction mechanisms of FeOOH·nH2O to metallic iron (α-Fe) are observed: a two-stage process via magnetite (Fe3O4) as an intermediate phase for 2-line FeOOH·nH2O, a three-step reduction involving Fe3O4 and wüstite (FeO) as intermediate phases for 2-line FeOOH·nH2O/SiO2 and a thermal transformation to α-Fe2O3 followed by a two-step reduction via Fe3O4 intermediate for 6-line FeOOH·nH2O. It is inferred that SiO2 interacts with Fe species to form Fe–O–Fe structures which tend to inhibit the thermal transformation and resist the reduction.