Manganese cryptomelane-type oxides: A thermo-kinetic and morphological study

In the present work, a detailed study on structural and textural properties, as well as a thermo-kinetic characterization of two manganese cryptomelane-type oxides, was carried out. A suitable methodology was stated for determining the several stages taking place during the surface molecules' removal. Thus, further insights were provided about the chemical nature of the sites present at the solid's surface. Relative strengths and the number of basic sites at the surface were estimated. The results show that the removal of the adsorbed (H2O, CO2) molecules from the surface is essential to disclose the oxide surface features. The morphological properties and thermo-kinetic characterization indicate the existence of at least three types of basic sites, owing activation energy values ranging from 140 to 190 kJ/mol. The dispersion observed in the activation energy data, during the CO2 loss, suggests that the interaction occurs on different sites. Therefore, both samples showed a heterogeneous surface; this is related to the energetic nature of the sites, as well as the basic strengths of the surface manganese cryptomelane oxides.