Synthesis and Redox Behavior of Nanocrystalline Hausmannite (Mn₃O₄)

Hausmannite (Mn 3 O 4 ) 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 Mn 3 O 4 nanoparticles are reduced to MnO and subsequently reoxidized in ramping temperature conditions. The temperature of Mn 3 O 4 to MnO reduction decreases as Mn 3 O 4 particle size decreases. On oxidation, 18 nm and smaller MnO nanoparticles formed the intermediate phase Mn 5 O 8 (MnO → Mn 3 O 4 → Mn 5 O 8 → Mn 2 O 3 ), while larger MnO particles oxidized to Mn 3 O 4 then directly to Mn 2 O 3 . Formation of Mn 3 O 4 occurred at lower temperature for smaller MnO nanoparticles. Further oxidation to Mn 2 O 3 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.