Synthesis, characterization, and catalytic activity of cryptomelane nanomaterials produced with industrial manganese...

Industrial manganese sulfate from manganese mines has been utilized to synthesize cryptomelane in a simple and feasible route. K-birnessite precursor was prepared by air oxidation of the mixture of MnSO4 and KOH solutions under alkaline conditions, and then transformed to cryptomelane under a heating process. The effects of OH- concentration, airflow rate, liquid reaction temperature, stirring rate, liquid reaction time, washing condition, calcination time, and temperature were investigated. X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) revealed that cryptomelane prepared under optimal conditions had a tetragonal symmetry and the particles were mostly in short lathy form with sizes of 20-30 nm. The average pore size and BET surface area of cryptomelane examined by N2 adsorption methods were 24.15 nm and 32.21 m2/g, respectively. X-ray photoelectron spectroscopy (XPS) studies demonstrated that the average oxidation state of manganese in cryptomelane was about 3.9 in comparison with prephase K-birnessite of 3.4. The synthesized cryptomelane sample showed improved catalytic activity for decomposition of hydrogen peroxide as compared with natural cryptomelane, but lower than those synthesized with hydrothermal and sol-gel methods. The results of this investigation will provide fundamental information for developing a large-scale production process for transforming manganese sulfate to cryptomelane.