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 proc essing (at 100 degreesC and 120 kPa) of redox reaction between aqueous solu tions of NaMnO4 and MnSO4 (at pH similar to1) in the presence of Ca2+ ion a dditives. Atomic absorption spectrometry and thermogravimetry found the mat erial bulk to have a non-stoichiometric chemical composition almost identic al to (Na0.03Ca)Mn5.4O17.7.0.9 H2O. Moreover, X-ray powder diffractometry a nd 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 det ermined by X-ray photoelectron spectroscopy, whereas nitrogen sorptometry a nd electron microscopy were used to assess the surface texture, structure a nd morphology. The results showed the surface to expose Mngreater than or e qual to4+, Ca2+ and O2- sites, OH groups and H2O molecules. The material pa rticles 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 m ean 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 th e micropores. most likely to accessible bulk structural tunnels (less than or equal to0.69 nmX0.69 mn). Hence, application of this and like tunnel-str uctured Mn(IV)-oxides, e.g. alpha-, beta- and gamma -MnO2, as shape-selecti ve catalysts is worth attempting. (C) 2001 Elsevier Science Ltd. All rights reserved.