Mechanosynthesis and process characterization of nanostructured manganese ferrite

Nanocrystalline MnFe2O4 particles were synthesized by a high-energy ball milling technique, starting from a manganosite (MnO) and hematite (α-Fe2O3) stoichiometric powder mixture. The mechanosynthesis process was performed at room temperature both in hardened steel and in tungsten carbide vials. X-ray powder diffraction quantitative phase analysis by the Rietveld method was used to study the chemical transformations promoted by the milling action. The nanocrystalline MnFe2O4 spinel phase begins to appear after 10 h of milling and reaches its maximum content (≈0.8 molar fraction) after 35 h of milling. A prolonged milling time induces a dramatic contamination of the powder mixture, when hardened stainless steel was adopted, due to metallic iron originating from vial and balls debris. Ball milling is able to induce a redox reaction between FeIII and metallic iron, transforming the MnFe2O4 spinel phase into a wüstite type (Fe, Mn)O phase. The yield of the hydrogen production reaction on synthetised materials is reported.