Controlled Synthesis of Manganese (oxihydr)oxides in Aqueous Solution by Thermolysis
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Controlled Synthesis of Manganese (oxihydr)oxides in Aqueous Solution by Thermolysis S. Cassaignon, M. Koelsch, J-P. Jolivet Laboratoire de Chimie de la matière condensée, CNRS UMR7574 Université Pierre et Marie Curie - Tour 54 - 4, place jussieu - 75252 Paris Cedex 05 ABSTRACT Nanometric particles of manganese (oxihydr)oxides (Hausmannite, Manganite, Nsutite, Pyrolusite and Vernadite) have been synthesized by thermohydrolysis of MnCl2 and/or KMnO4 in aqueous medium. The modification of various chemical parameters (pH and precursors concentration, nature of the reducer or oxidizer, temperature and duration of the thermolysis) allow the control of the nature, the size and the morphology of the obtained particles. INTRODUCTION Manganese exists under numerous degrees of oxidation (from 0 to VII) and forms a great number of solid compounds, in particular oxides and oxihydroxides which are used in many domains: components of batteries, metallurgy, pigments, catalysts [1-3]. The properties of these materials depend closely on their crystalline structure as well as size and morphology of the particles. The objects with nanometric size are of specific interest, because of the high surface to volume ratio and effects related to the small size of the crystalline domains. Because of the variety of manganese oxides (composition, structure), their conditions of synthesis must be strictly controlled. Most of the syntheses of manganese (oxihydr)oxides found in literature involve organic compounds [4] or template structures [5]. In aqueous medium, manganese (oxihydr)oxides are usually obtained from molecular complexes of Mn(II) or Mn(VII), the most stable oxidation states in solution [6,7]. This work deals with the precipitation of manganese (oxihydr)oxides by thermohydrolysis of MnCl2 or KMnO4 in aqueous solution, and with the influence of various chemical medium parameters (pH and concentration, nature of the reducer or oxidizer) on the nature, the size and the morphology of the obtained particles. EXPERIMENTAL Various manganese oxides were prepared by varying synthesis conditions. MnCl2, 4H2O (Fluka) or KMNO4 (Prolabo) were used as precursors. Three different protocols were followed. Protocol 1: MnCl2, 4H2O was dissolved, at room temperature, in deionized water ([Mn(II)] = 0.1 mol.L-1) with pH between 1 and 9 (depending on the experiments), the pH being fixed with hydrochloric acid or sodium hydroxide. Hydrogen peroxide (H2O2, 30 w-% in water, Aldrich) or ammonium persulfate (NH4)2S2O8, Prolabo) was added in order to control the oxidation of the precursor. The amount of oxidizer added corresponded to a molar ratio Mn(II):Ox = 1:1 and 1:10 for H2O2 and 1:1, 0.1:0.1 (the initial Mn(II) concentration has been divided by 10), 1:5 and 2:1 for S2O82-. Protocol 2: KMnO4 (Prolabo) was dissolved, at 20°C, in deionized water ([Mn(VII)] = 0.1 mol.L-1) at pH 7 or 9. H2O2 or ammonium thiosulfate (NH4)2S2O3 (Prolabo) was added as oxidizer. The molar ratio was Mn(VII):H2O2 = 2:3 and 2:30 and Mn(VII):S2O32- = 2:6. Protocol 3: KMnO4 was
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