Synthesis and Redox Behavior of Nanocrystalline Hausmannite (Mn3O4)

Abstract

Hausmannite (Mn3O4) nanoparticles have been prepared by mixing aqueous solutions of manganese nitrate and hexamethylenetetramine from 20 to 80 °C. Activation energy for the particle formation increases from 0.5 to 0.8 kJ/mol with nitrate concentration. Nanoparticles (18−41 nm) with a faceted structure are prepared by this method. We describe synchrotron in-situ time-resolved XRD experiments in which Mn3O4 nanoparticles are reduced to MnO and subsequently reoxidized in ramping temperature conditions. The temperature of Mn3O4 to MnO reduction decreases as Mn3O4 particle size decreases. On oxidation, 18 nm and smaller MnO nanoparticles formed the intermediate phase Mn5O8 (MnO → Mn3O4 → Mn5O8 → Mn2O3), while larger MnO particles oxidized to Mn3O4 then directly to Mn2O3. Formation of Mn3O4 occurred at lower temperature for smaller MnO nanoparticles. Further oxidation to Mn2O3 required higher temperatures for the initially smaller MnO nanoparticles, indicating that the kinetics of forming the new oxide phases is not controlled by diffusion, where smaller distance favors faster reaction, but by nucleation barrier.