Manganese dioxide structural effects on its thermal decomposition

In this work, the influence of the structure, composition and morphology on the amount of mass lost from γ-MnO2 during heat treatment, and the activation energy for this process (from iso-conversional kinetic analysis), was determined using a statistical approach. It was found that the differential thermogravimetric (DTG) data for electrolytic manganese dioxide (EMD) samples typically show two regions of mass loss; water bound to surface sites and structural water at or near the surface removed between ∼120 and 200 °C, and bulk hydroxyl groups lost in the range ∼200–400 °C. The composition and morphology had the greatest effect on the amount of mass lost; namely, BET surface area and percentage Mn(III) for the first process (r2 = 0.92) and percentage Mn(IV), percentage Mn(III) and cation vacancy fraction for the second (r2 = 0.89). The activation energy for the first processes was most affected by BET surface area, c0 and crystallite size in the (2 2 1) direction (r2 = 0.89). Relatively poor correlations were found between the amount of water lost and the activation energy for the second process and the material properties measured in this work. This likely relates to the slow kinetics of the first mass loss which complicates the relationships existing for the second.

► The work describes the kinetics of mass loss from a range of γ-MnO2 samples under heat treatment.
► γ-MnO2 has a wealth of structural variety due in part to structural water.
► Understanding the loss of this water during heat treatment is therefore critical for determining material performance.
► Here we report the effects of material structure and morphology on mass loss during heat treatment.