Fe 3− xMn xO 4 catalysts: phase transformations and carbon monoxide oxidation

Abstract

In this work, the synthesis, characterization and transformation of Fe 3− x Mn x O 4 ( x=0–0.53) spinels have been studied by Mössbauer spectroscopy, powder X-ray diffraction (XRD), thermal analyses (TG and DSC) and temperature programmed reduction (TPR) experiments. Mössbauer spectroscopy and XRD lattice parameters ( a o) showed the presence of pure spinel phases with Mn incorporation mainly in the octahedral site replacing Fe 2+. Upon thermal treatment in air, the spinels Fe 3− x Mn x O 4 are oxidized to an Mn containing maghemite. At higher temperatures the maghemite is converted to the hexagonal α-Fe 2O 3 hematite phase. DSC analyses showed that the presence of manganese in the maghemite structure strongly decreases the transition temperature to hematite. The effect of these phase transformations on the catalytic carbon monoxide oxidation was investigated. It has been observed that the presence of Mn on the catalysts Fe 3− x Mn x O 4 does not significantly affect the catalytic activity at lower temperature. On the other hand, at temperatures higher than 200 °C the magnetites are oxidized to maghemite/hematite and a great increase in the catalytic activity is observed. Catalytic experiments with the different iron oxides Fe 3O 4 (magnetite), γ-Fe 2O 3 (maghemite) and α-Fe 2O 3 (hematite) showed that at lower temperatures, magnetite is the most active phase whereas at higher temperatures, hematite showed the highest activity.