Uniform particles of manganese compounds obtained by forced hydrolysis of manganese(II) acetate

Procedures for the preparation at low temperature (80 degrees C) of uniform colloids consisting of Mn3O4 nanoparticles (about 20 nm) or elongated alpha-MnOOH particles with length less than 2 mu m and width 0.4 mu m or less, based on the forced hydrolysis of aqueous manganese(II) acetate solutions in the absence (Mn3O4) or the presence (alpha-MnOOH) of HCl are described. These solids are only produced under a very restrictive range of reagent concentrations involving solutions of 0.2-0.4 mol dm(-3) manganese(II) acetate for Mn3O4 and of 1.6-2 mol dm(-3) Mn(II) and 0.2-0.3 mol dm(-3) HCl for alpha-MnOOH. The role that the acetate anions play in the precipitation of these solids is analyzed. It seems that these anions promote the oxidation of Mn(II) to Mn(III), which readily hydrolyze causing precipitation. The evolution of the characteristics of the powders with temperature up to 900 degrees C is also reported. Thus: Mn3O4 particles transform to Mn2O3 upon calcination at 800 degrees C; this is accompained by a sintering process. The a-MnOOH sample also experiences several phase transformations on heating. First, it is oxidized at low temperatures (250-450 degrees C) giving MnO2 (pyrolusite), which is further reduced to Mn2O3 at 800 degrees C. After this process the particles still retain their elongated shape.