Synthesis of polyoxometalate nano-architectures for environmental applications

• Main Author: Lua, Shun Kuang
• Other Authors: Dong Zhili
• Format: Theses and Dissertations
• Language: English
• Published: 2017
• Subjects: DRNTU::Engineering::Materials::Material testing and characterization; DRNTU::Engineering::Environmental engineering::Water treatment; DRNTU::Engineering::Materials::Composite materials
• Online Access: http://hdl.handle.net/10356/72467
• Institution: Nanyang Technological University

Polyoxometalates are unique molecular metal oxides containing early transition elements in their highest oxidation state. They have highly tuneable structure which allows their design from an atomic level, thus their potential in various applications in a wide field of research is made viable. The structural and physical properties are important understanding required in the toolbox of researchers for the design of polyoxometalates based materials. Due to their highly soluble nature, the heterogenization of polyoxometalates will be favourable from the perspective of sustainable development. The facile separation of catalyst will improve the feasibility of polyoxometalates for environmentally sustainable applications, making the synthesis and design of polyoxometalates based nano-architecture a necessary endeavour. In the first part of the study, polyoxometalates (POMs) of the series H(3+n)[PVnMo(12-n)O40] (PVMA) were prepared and investigated to assist in the design of POMs based materials. Basic characterizations of PVMAs were performed to understand the structural and physiochemical characteristics. Two simple design approaches: (1) surfactant encapsulation of POMs and (2) surface modification of metal oxide for immobilization of POMs were adopted with the aim of obtaining a heterogenizing PVMA. In order to further investigate the redox characteristic of PVMA, PVMA-2 was chosen for incorporation with TiO2 for investigation of redox properties. The TiO2 nanoplates immobilized with PVMA-2 via (3-aminopropyl)triethoxysilane as a coupling agent showed synergistic effect of PVMA-2 on TiO2 surface and the potential of such material in application as anodes in lithium ion battery. This also provides insights for design of POMs immobilized materials for environmental applications. In the second part of the study, amine functionalized high surface area titanate nanotube was modified with PVMA-2. The synthesis utilizes the high surface area of titanate nanotubes for immobilization of PVMA-2, obtaining heterogeneity and high surface area in the as-prepared material for application as a photocatalyst. Structural characteristics and physical properties of PVM-X%S-TNT (X = 2, 5, 10) was characterized by various technique herein. PVM-5%S-TNT was found to be optimized for the photocatalytic degradation of MB with H2O2 added as an oxidant. The photocatalytic efficiency of the material was also tested against rhodamine B, methyl orange and acid orange 7. The results provided useful insights on the mechanism for photocatalytic degradation of organic dye by PVMA-2. In the final part of this study, a series of surfactant encapsulated heteropolyanion (SEH-n) based on H(3+n)[PVnMo(12-n)O40] (PVMAs) was prepared. The morphological optimisation of the SEHs was studied by the control of solvent polarity and PVMA to surfactant ratio used. Investigation by TEM revealed the formation of particles with multi-lamellar nano-structure in the SEHs. The performance of SEHs as catalysts for the removal of bisphenol‒A under ambient conditions was evaluated. Factors influencing the performance of the SEH‒n are the relative stability of the Keggin structure and electron accepting property. The hydrophobic property of the nano‒sized SEHs provides good aqueous stability and allows excellent recoverability of the catalyst from the aqueous solution after treatment. The studies show a potential for further development, design and understanding of polyoxometalates based nano-architecture which can find a niche in environmental application. The recommendations for future work are communicated in the concluding chapter.