Particle characteristics and reduction behavior of synthetic magnetite.

Abstract: Two samples (S1 and S2) of magnetite were synthesized, using two different methods, and characterized by means of X-ray powder diffractometry, infrared and Mössbauer spectroscopy, N2 sorptiometry and electron microscopy. Particles of sample-S1 were found to be loosely agglomerated, micro-sized spheroids (200–350nm) composed almost solely of highly aggregated (fused) crystallites (size averaged at 35nm) of cubic-Fe3O4. In contrast, particles of sample-S2 were strongly agglomerated, nano-sized spheroids (25–30nm) composed of slightly aggregated crystallites (size averaged at 11nm) of cubic-Fe3O4 and noncrystalline domains made-up of FeO(OH) species. Temperature-programed reduction (TPR) profiles obtained for the two samples were similar in monitoring two peaks at >450°C assignable to a two-step reduction of Fe3O4 (→FeO→Fe), but different in monitoring a peak at<450°C only for the reduction of FeO(OH) (→Fe3O4) contained in sample-S2. However, curve fitting analysis of the TPR profiles and molecular stoichiometry calculations based on amounts of hydrogen consumed revealed that the two-step reduction of Fe3O4 is not straightforward. That is by resolving two consecutive pathways for each step and, hence, nonstoichiometric intermediate products whose composition was found to be critically controlled by the composition of the reducing gas atmosphere (5 or 80% H2/N2) and characteristics of the starting sample particles (chemical and phase composition, and, but to lesser extents, the agglomeration and average size). [Copyright &y& Elsevier]