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

Highlights: • 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. - Abstract: 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 ∼ 580 K for 2-line and 6-line FeOOH·nH₂O and at T ∼ 660 K 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 (∼480 K) 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