Title

New synthetic routes to catalytically active manganite, K-OMS-2 and K-OMS-2/SiO2 and a preliminary study on the use of a continuous flow microwave technique in the synthesis of nanosized manganese and cerium oxides and cobalt and iron oxyhydroxides

Date of Completion

January 2008

Keywords

Chemistry, Inorganic|Energy

Degree

Ph.D.

Abstract

The work presented here comprises three main parts. The first part is a new route to nano-sized γ-MnOOH (manganite). γ-MnOOH has been synthesized via the reduction of KMnO4 with sucrose and MnSO 4 in acidic medium under refluxing conditions for four and six hours. Characterization of these manganite materials using XRD, FESEM, TEM, TGA and IR were done. The obtained manganite samples using the new route were compared against a conventionally prepared one where synthesis involved the oxidation of MnSO4 with H2O2 in basic medium. Two new synthetic methods were developed, one involving addition of KMnO4 into a solution of both sucrose and MnSO4 while the other involved addition of KMnO4 solution into sucrose followed by addition of MnSO4 (s). The latter method yielded smaller particles (up to 30 nm) than the former method (up to 80 nm) and the conventionally prepared manganite (up to 50 nm). The synthesized manganite materials exhibited promising characteristics when tested as electrocatalysts in O2 reduction. The larger particles gave higher peak currents in CV than smaller particles. When incorporated in Yardney's medium-sized lithium-air battery, the larger (up to 80 nm) particles gave higher specific capacity (up to 2.2 Ah/g), which corresponds to about 38% increase in specific energy of the battery when compared to a battery where no manganite was incorporated. ^ The second part of the research reported here involves the synthesis of K-OMS-2 and SiO2-supported K-OMS-2. K-OMS-2 (or OMS-2 from hereon) was successfully synthesized using a reflux method where, for the first time, oxone or KHSO5 was used as the oxidant. Nano-sized fibers with widths ranging from 8–40 nm were obtained. We also report for the first time the synthesis and catalytic activity of OMS-2 supported on SiO2 with surface area ranging from 228 to 513 m2/g and weight % OMS-2 ranging from 1.0 to 13.6%. An optimization study on styrene oxidation using OMS-2 and OMS-2/SiO2 as catalysts was done. Lowering the amount of catalyst and increasing the substrate used in the reaction significantly improved conversion and turnover frequency. Conversions of up to 81% and 19 h−1 TOF were achieved. These were the highest reports of conversion and TOF thus far for styrene oxidation using undoped OMS-2 and 24 hours reaction time. Selectivity towards the more valuable product, styrene oxide, significantly improved when OMS-2 was supported on SiO2. The weight % OMS-2 or dispersion of OMS-2 on the SiO2 support may be a key in optimizing OMS-2/SiO2 catalysts. The free-radical pathway is indeed involved in the oxidation of styrene and SiO2 contributes to the generation of epoxide-producing radicals as shown by kinetic and radical trapping experiments. Furthermore, the oxo-metal pathway may favor the production of benzaldehyde. ^ Finally, a preliminary study on the use of a continuous flow microwave (MW) technique to obtain inorganic nanomaterials is also presented in this thesis. Nanosized ϵ-MnO2, OMS-2, CeO2, CoOOH, and FeOOH were synthesized using the said microwave technique. Nanoplates and nanofibers of ϵ-MnO2 were obtained while mainly nanoplates of OMS-2 resulted in the use of the continuous flow MW technique. The obtained OMS-2 product was not pure. This OMS-2 was obtained with some ϵ-MnO 2. ϵ-MnO2 cannot be used as a precursor to OMS-2. Microspheres of ϵ-MnO2 can be obtained by using shorter reactors and more concentrated reactant solutions. Product yields of up to 24% were obtained in using the continuous flow MW reactor, which indicates that there are still some parameters that need to be further optimized to achieve a viable industrial process using the technique reported here. These parameters include reactor geometry, MW power, reactant concentrations, and the use of a carrier gas to alleviate clogging of the reactor. Lower product yields were obtained when the MW oven was replaced with a conventional one. ^