Exploration of reverse micelle synthesis of carbon-supported LaMnO ₃

Reverse micelle (RM) based synthesis of carbon-supported perovskite type oxide (LaMnO₃) was investigated. By using cyclohexane as oil phase, six kinds of nonionic surfactants were tested for the formation of the revere micelle dispersions containing aqueous solutions of mixed nitrates of La ³⁺ and Mn²⁺ (RM-N) and tetramethylammonium hydroxide (RM-A) at the water/surfactant molar ratio (R_w) of 3. RM-A was found to be more difficult to form compared with RM-N or the RM containing pure water. At a temperature range of 5-25°C, it was given only by the surfactants with hydrophilic-lipophilic balance (HLB) values of 10.0-10.9. Phase diagrams of oil-surfactant-alkaline solution (or pure water) were constructed at 5 and 15°C for the best two surfactants, i.e., poly-(oxyetylene) ₆-nonylphenyl ether and poly-(oxyetylene)₅-lauryl ether. Mixing RM-N and RM-A together gave the RM containing a mixed hydroxides-precursor (RM-P), which was further converted into carbon-supported LaMnO₃ through a series of treatments including the addition of carbon powder and calcination in N₂ atmosphere at 600°C. The size of RM-P as well as that of the LaMnO₃ grains supported on carbon could be controlled well by selecting R_w. The carbon-supported LaMnO₃ proved to be highly active for the electrochemical reduction of oxygen.