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

Abstract

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 °C, (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.