Surface texture of microcrystalline tunnel-structured manganese(IV) oxides: Nitrogen sorptiometry and electron microscopy studies

Three different modifications of manganese(IV) oxide, viz. cryptomelane, nsutite and todorokite-like, were synthesized by hydrothermal methods. The bulk chemical composition, phase composition, crystalline structure and particle morphology of the resulting materials were determined by thermogravimetry, atomic absorption spectroscopy, X-ray diffractometry, infrared spectroscopy and scanning electron microscopy. The surface chemical composition, texture and structure were assessed using X-ray photoelectron microscopy, nitrogen sorptiometry and high-resolution electron microscopy. The results highlighted the hydrothermal conditions under which such tunnel-structured modifications of manganese(IV) oxide can be successfully synthesized. Moreover, they revealed that (i) the bulk was microcrystalline, (ii) the crystallites were either fibrils (cryptomelane and nsutite) or rod-like (todorokite) with low-index exposed facets, (iii) the surface chemical composition mostly reflected that of the bulk and (iv) the surface texture was linked with high specific areas, slit-shaped mesopores associated with particle interstices and micropores which allowed surface accessibility to the bulk tunnels of the test oxides. The application of such test oxides as shape-selective oxidation catalysts appears worthy of investigation.