| Abstract |
The rapid development of civilization and industrial activities has led to a large amount of pollutants being disposed into the environment either intentionally or accidentally, including toxic metals with a great health concern. Chromium is a heavy metal with variable oxidation states and hexavalent chromium, Cr(VI), is the most toxic form. In aquatic systems, chromium exists mostly in the hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)) states. Anionic Cr(VI) species are far more mobile and toxic than Cr(III) and difficult to remove from water, since aqueous Cr(III) can be readily precipitated as Cr(OH)3 or Cr2O3 in a basic solution. Thus, the presence of Cr(VI) in the aquatic environment is a serious environmental concern because of its high toxic and non-degradable properties. The World Health Organization (WHO) recommended a maximum allowable concentration of 50 μg/L for Cr(VI) in drinking water. Moreover, as a consequence of its toxicity, Cr(VI) has also been categorized as a Group I human carcinogen by the International Agency for Research on Cancer (IARC). Therefore, finding an effective way for remediation of Cr(VI)-contaminated solutions is undoubted of high priority in the field of environmental and health protection.
This thesis focuses on synthesis, structural characterization and environmental applications of high-surface-area mesoscopic architectures composed of tightly connected ultrasmall spinel ferrite nanoparticles. In particular, we found that mesoporous spinel ferrite nanoparticle assemblies (MeFe2O4 or MeFO MNAs, Me=Zn, Mn, Ni, Cd and Co) can efficiently suppress electron-hole recombination, manifesting an exceptional activity and magnetic recyclability in photocatalytic reduction of aqueous Cr(VI). Revealed by transmission electron microscopy, N2 physisorption, and X-ray scattering studies, the resulting materials, which were obtained through a block copolymer-assisted cross-linking aggregation of colloidal nanoparticles, show a 3D interconnected nanoporous structure with large internal surface and exhibit small grain composition (ca. 6–7 nm in size). In addition, a suitable combination of UV–visible/NIR diffuse reflectance spectroscopy and electrochemical impedance spectroscopy (EIS) studies indicated that the electronic band structure of these mesoporous materials fits the electronic requirements for both Cr(VI) reduction and water oxidation under UV and visible light irradiation. Among spinel ferrite nanocrystal assemblies, ZFO MNAs present the highest activity, readily operating without additional sacrificial reagents in photocatalytic detoxification of aqueous Cr(VI), which together with transient gas analysis and fluorescence spectroscopy results suggest a competitive formation of oxygen and hydroxyl radicals at the catalyst surface. These findings provide an essential tool for the delineation of the electronic structure-catalytic property relationship in spinel ferrite nanostructures offering intriguing possibilities for designing new nanoscale photocatalytic systems for efficient environmental pollution purification and energy conversion. Moreover, in an effort to further improve the photocatalytic performance of ZFO assemblies, we suggest the synthesis of binary mesoporous networks consisting of ZFO and MFO (x% MFO-ZFO MNAs, x = 4, 6.5, 8.5 and 12.5 wt%) nanoparticles as promising catalysts for detoxification of Cr(VI) aqueous solutions. By tuning the chemical composition and electronic band structure of constituent nanocrystals, the 6.5 wt% MFO-loaded ZFO mesoporous catalyst impart outstanding photocatalytic Cr(VI) reduction activity in the presence of phenol. Mechanistic studies with UV–visible/NIR diffuse reflectance spectroscopy and electrochemical impedance spectroscopy indicate that the performance enhancement of this catalyst predominantly arise from the appropriate band edge positions for Cr(VI) reduction and phenol oxidation. The remarkable activity and durability of the 6.5% MFO-ZFO MNAs implies the great possibility of implementing these new composite catalysts into a realistic Cr(VI) detoxification of contaminated water.
Additional subject of the present research is the synthesis of mesoporous Mn3O4 nanoparticle assemblies and investigation of their catalytic activity in oxidation of various saturated and unsaturated hydrocarbons. The successful synthesis of this material highlights the general applicability of the proposed polymer-assisted aggregating self-assembly method to produce high-surface-area mesoporous networks of cross-linked metal oxide nanoparticles. The resulting material possesses a network structure of interconnected 6–7 nm-sized Mn3O4 nanoparticles and has a hight accessible surface area (ca. 90 m2/g) and uniform pores (ca. 6.6 in size). These assembled Mn3O4 networks demonstrate great potential for application in catalytic oxo-functionalization of various aromatic and cyclic alkenes as well as aryl alkanes with tert-butyl hydroperoxide as mild oxidant. Through comparative studies, the high catalytic activity and stability of these Mn3O4 assemblies arise from the unique 3D open-pore structure, large internal surface area and uniform mesopores.
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