JOURNAL OF MATERIALS SCIENCE 38 (2 0 0 3 ) 2815 – 2824 Characterisation of K, Na, and Li birnessites prepared by oxidation with H 2 O 2 in a basic medium. Ion exchange properties and study of the calcined products O. PRIETO, M. DEL ARCO, V. RIVES ∗ Departamento de Qu´ ımica Inorg ´ anica, Universidad de Salamanca, 37008-Salamanca, Spain E-mail: vrives@usal.es Birnessites containing Na, K or Li in the interlayer have been prepared by oxidation of Mn(II) cations with H 2 O 2 in a basic medium with different alkaline cation/Mn molar ratios. The solids prepared have been characterised by elemental chemical analysis, powder X-ray diffraction, thermal analyses (differential thermal analysis and thermogravimetric analysis), FT-IR spectroscopy and surface texture assessment by adsorption of N 2 at -196 ◦ C. Crystalline birnessites are obtained for A/Mn ratios (A = K, Li) larger than 3.4, but MnO(OH) has been also identificed when such a ratio is smaller than 3.4. Ion exchange is topotactic, but is not complete for exchanging Na, K, or Mg for pre-existing Li. The solids are stable up to 400 ◦ C, and formation of spinels and solids with tunnel structures is observed at this temperature. Li-containing birnessites are transformed to LiMn 2 O 4 spinel at 400 ◦ C, and co-crystallization of bixbyte (Mn 2 O 3 ) is observed at higher temperatures. Bixbyte and cryptomelane are formed at 500 ◦ C for the K-containing birnessites. C  2003 Kluwer Academic Publishers 1. Introduction Birnessite manganese oxides have a layered structure, formed by [MnO 6 ] octahedra sharing edges, with al- kaline cations and water molecules between the lay- ers, Fig. 1 [1–11]. The formula is usually written as A x MnO 2±y · zH 2 O, where A stands for the alkaline metal cation. The oxidation state of manganese is be- tween 3.6 and 3.8 [1, 9, 12–14]. The interlayer spacing is close to 7 ˚ A, but can increase up to 10 ˚ A upon hy- dration, forming a hydrated phase known as busserite. A dehydrated form with a spacing of 5.5–5.6 ˚ A has been also isolated [9, 12, 13, 15–17]. While birnessite contains a single layer of water molecules between the octahedra layers, busserite contains two layers of water molecules. Busserite is not stable at room temperature and is easily transformed into birnessites, although Suib et al. [18] have reported formation of stable busserites. Because of the relatively easy exchange of the in- terlayer cations and the diversity of oxidation states of Mn in these compounds, birnessites find applications in several fields, e.g., ion exchange, batteries and het- erogeneous catalysis [13, 19, 20]. Upon appropriated thermal treatments, birnessites give rise to formation of mixed oxides with tunnel or spinel structures, and these materials find wide applications; for example, Mn-Li spinels find application in rechargeable batteries [3, 12, 21–25]. Todorokite, a 3 × 3 spinel structure, is ∗ Author to whom all correspondence should be addressed. obtained by hydrothermal treatment of birnessites and cryptomelane, a 2 × 2 tunnel structure, is obtanied by calcination of birnessites [26]. We here report on the preparation and characterisa- tion of birnessites containing K, Na or Li in the in- terlayer prepared by oxidation of Mn(II) cations with H 2 O 2 in a basic medium. The effect of the initial A/Mn ratio on the nature of the solids obtained is studied. In addition, we study also their ion exchange ability, as well as the nature of the solids obtained by calcina- tion. The synthesis, characterization and applications of porous manganese oxides has been recently reviewed by Suib et al. [27]; several methods (including precipi- tation, ion-exchange, hydrothermal and sol-gel routes) are discussed about their effect on the precise nature of the oxides finally formed, but the H 2 O 2 -oxidation process is not discussed. 2. Experimental 2.1. Preparation of the samples All reagents were from Fluka (Switzerland) and the gases from L’Air Liquide (Spain). In a typical ex- periment, birnessites containing Li, Na, or K in the interlayer region have been prepared by the follow- ing procedure: 100 ml of a 0.86 M solution of AOH also 3% (v/v) H 2 O 2 , is dropwise added to 50 ml of a 0022–2461 C  2003 Kluwer Academic Publishers 2815