SYNTHESIS OF MANGANESE OXIDES BY REDUCTION OF KMNO4 WITH KBH4 IN AQUEOUS-SOLUTIONS

Reduction of KMnO4 with KBH4 in aqueous solutions has been investigate d systematically at ambient temperatures as a function of reaction pH and the volume of borohydride. The products are characterized by X-ray diffraction, transmission electron microscopy, atomic absorption spec troscopy, redox titrations to determine the oxidation state of mangane se, and thermogravimetric analysis. The reduction gives both binary Mn O2-delta and ternary KxMnyOz oxides that are hydrated and nanocrystall ine. The composition of the reduction products is influenced by the re action pH as well as the volume of borohydride. While acidic condition s tend to stabilize products with predominantly Mn4+ due to a dispropo rtionation of Mn3+, nearly neutral conditions could access Mn3+ with d elta approximate to 0.5 in MnO2-delta. While no potassium-containing K xMnyOz is obtained in highly acidic solutions with pH approximate to 1 , the compound becomes accessible with increasing pH, and a maximum x approximate to 0.5 could be obtained in highly basic solutions with pH approximate to 11. The as-prepared samples lose most of the mater bel ow 200 degrees C and tend to lose oxygen at higher temperatures. (C) 1 998 Academic Press.