Microporosity of heat-treated manganese dioxide

A structural and micro-pore analysis of a series of heat treated electrolytic manganese dioxide (EMD) samples has been conducted. In terms of crystal structure, the original EMD with gamma-MnO2 structure (orthorhombic unit cell) was found to progressively convert to beta-MnO2 (tetragonal unit cell) at elevated temperatures. The structural transition was kinetically limited, with the higher temperatures leading to a greater degree of transformation. The orthorhombic gamma-MnO2 unit cell was found to contract along the a and b axes, while along the c axis an expansion was observed only at the highest heat treatment temperatures. These changes occur as a result of manganese ion diffusion leading to the formation of a denser, more defect free material. The porosity of these heat treated EMD samples was also examined by N-2 gas adsorption combined with various interpretive techniques such as the Kelvin equation, MP method, Dubinin-Radushkevich method, Dubinin-Astakhov method and a more modem density functional theory based approach. Despite shortcomings associated with certain techniques, all clearly indicated that the EMD micro-pore volume decreased and the meso- and macro-pore volume increased as the heat treatment temperature was increased. This was justified as a result of manganese ion movement during the structural rearrangement causing the small pores to be progressively sintered shut, while the larger pores were formed as a result of stress-induced cracking in the denser final product. (c) 2006 Elsevier B.V. All rights reserved.