Preparation, ion-exchange, and electrochemical behavior of Cs-type manganese oxides with a novel hexagonal-like morpholo
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ngzhu Zhao School of Science, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China
Kenta Ooi National Institute of Advanced Industrial Science and Technology, Takamatsu, Kagawa 761-0395, Japan (Received 7 November 2006; accepted 29 January 2007)
Cs-type layered manganese oxide with a novel hexagonal-like morphology (Cs–BirMO) was prepared by a solid-state reaction procedure. The Cs+ extraction and alkali–metal ion insertion reactions were investigated by chemical analyses, x-ray analyses, scanning electron microscopy observation, Fourier transform-infrared spectroscopy, thermogravimetric differential thermal analyses, pH titration, and distribution coefficient (Kd) measurements. A considerable percentage (88%) of Cs+ ions in the interlayer sites were topotactically extracted by acid treatment, accompanied by a slight change of the lattice parameters. Alkali–metal ions could be inserted into the interlayer of the acid-treated sample (H–BirMO), mainly by an ion-exchange mechanism. The pH titration curve of the H–BirMO sample showed a simple monobasic acid toward Li+, Rb+, and Cs+ ions, and dibasic acid behavior toward Na+ and K+ ions. The order of the apparent capacity was K+ > Li+ ≈ Na+ ≈ Rb+ ≈ Cs+ at pH < 6. The Kd study showed the selectivity sequence of K+ > Rb+ > Na+ > Li+ for alkali–metal ions at the range of pH Rb+ > Cs+ in the range of pH >10; the sequence hardly agrees with the increasing order of the effective ionic radii of metal ions. The weaker acidic site is suitable for relatively small ions, owing to a steric effect in a state of high metal ion loading. Cs+, which has a large ionic radius, can only be adsorbed on the stronger acidic sites and causes a low apparent capacity above pH 6. These results suggest that the H–BirMO sample has two kinds of adsorption sites: one is a moderately acidic site that is exchangeable below pH 6; and the other is a very weak acidic site that is only exchangeable in a strong basic solution.
strengthens at a higher loading of metal ions in the pH titration study, so it does not reflect exactly the intrinsic affinity between the exchange site and metal ions, but reflects partly the steric interaction between the loaded metal ions. The pH titration is rather suitable for studying the overall exchange behaviors from microamounts to macroamounts of loading. From this standpoint, the Kd measurement will be suitable to evaluate the ion-sieve property of the inorganic ion exchanger. The equilibrium Kd values of alkali–metal ions on H–BirMO are plotted as a function of the solution pH are shown in Fig. 8. The logarithms of the Kd values increase nearly linearly with increasing pH in the pH range studied. The selectivity sequence is K+ > Rb+ > Na+ > Li+ for alkali–metal ions in the range of pH < 5. The selectivity sequence is similar to those of hollandite-type manganese oxide (K+ > Rb+ Ⰷ Na+, Cs+ >Li+) but is different from those for other kinds of manganese oxide samples6,37,44 (Cs+ > Rb+ > K+ > Li+ > Na+ for spinel-type, and Cs+ > Rb+ > K+ > Na+ > Li+ for todoro
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