Surface texture, morphology and chemical composition of hydrothermally synthesized tunnel-structured manganese(IV) oxide

A brownish black Mn(IV)-oxide powder was synthesized by a hydrothermal processing (at 100 degreesC and 120 kPa) of redox reaction between aqueous solutions of NaMnO4 and MnSO4 (at pH similar to1) in the presence of Ca2+ ion additives. Atomic absorption spectrometry and thermogravimetry found the material bulk to have a non-stoichiometric chemical composition almost identical to (Na0.03Ca)Mn5.4O17.7.0.9 H2O. Moreover, X-ray powder diffractometry and infrared spectroscopy found the bulk also to be weakly crystallized into microcrystallites assuming a 3X3 tunnel structure of todorokite-like [(Na,Ca,Mn)Mn3O7. xH(2)O] Mn(IV)-oxide, The surface chemical composition was determined by X-ray photoelectron spectroscopy, whereas nitrogen sorptometry and electron microscopy were used to assess the surface texture, structure and morphology. The results showed the surface to expose Mngreater than or equal to4+, Ca2+ and O2- sites, OH groups and H2O molecules. The material particles were shown to assume a rod-Eke morphology of nanodimensions, and to consist of rod-like crystallites (5-6 nm wideX50-70 nm long). The surface area was found to amount to 62 m(2)/g, and the pore structure to consist of meso and micropores. The mesopores were slit-shaped and quite uniform in mean pore diameter ((D) over bar (p) = 28 nm), and the micropores were of (D) over bar (p) less than or equal to2 nm. Backscattered electron microscopy imaging facilitated relating the mesopores to particle interstices, and the micropores. most likely to accessible bulk structural tunnels (less than or equal to0.69 nmX0.69 mn). Hence, application of this and like tunnel-structured Mn(IV)-oxides, e.g. alpha-, beta- and gamma -MnO2, as shape-selective catalysts is worth attempting. (C) 2001 Elsevier Science Ltd. All rights reserved.