Synthesis and surface characterization of todorokite-type microporous manganese oxides: implications for shape-selective oxidation catalysts

Various todorokite-type Mn(IV)-oxides, namely, Mg-, Ni- and Cu-todorokite, were synthesized via hydrothermal treatment of corresponding buserites. The bulk chemical and phase compositions, crystalline structure, particle morphology and thermal stability, were determined by means of atomic absorption spectroscopy, X-ray powder diffractometry, infrared spectroscopy, and thermogravimetry. The surface chemical composition, texture and structure were assessed using X-ray photoelectron spectroscopy, nitrogen sorptiometry, and scanning and high-resolution transmission electron microscopy. The results revealed that (i) the bulk is crystalline and thermally stable up to 200 degreesC, (ii) nature of the decomposition residues is dependent on type of the divalent metal ion contained, (iii) the crystallites are nano-sized fibrils and expose low-index (0 0 2) crystal facet, (iv) the surface chemical composition reflects that of the bulk, and (v) the surface texture constitutes moderate specific areas, slit-shaped mesopores associated with particle interstices, and micropores assuming surface accessibility for the bulk structural tunnels of the test todorokites. Thus, application of such test oxides as shape-selective oxidation catalysts has been rendered worth attempting. (C) 2003 Published by Elsevier Inc.