Studies On The Preparation And The Photocatalytic Performances Of Highly Dispersed Layered Structure Catalysts

Heterogeneous catalysts generally suffer from serious agglomeration,and low dispersion of active site, which restricts their effective utilization.In this dissertation, the photocatalytic active site was highly dispersed bydistributing the Cr, Ti unit within layered double hydroxides （LDHs）matrix to enhance the efficiency of charge separation and improve thephotoconversion capability. The inorganic nanoparticles with surfaceactive defects were prepared through controlling the particle size. Thehierarchical assemblies were also achieved in micro scale by using thesenanoparticles as building blocks. Both experimental and theoreticalcalculations were carried out to study the crystal structure, energy levelcharacteristics and the interaction of intercalating metals, in order todemonstrate the structure-property correlation. Moreover, the applicationsin splitting water into H₂or O₂, visible-light catalysis, as well asantipathogen performance were explored. This study provided bothexperimental and computational basis for the development of novelhighly-dispersed catalysts. The main results of this dissertation are as follows:1. A family of visible-light responsive MCr–LDHs （M=Cu, Ni, Zn）photocatalyst was synthesized by a separate nucleation and aging steps（SNAS）, which displays remarkable photocatalytic degradation activityunder visible-light irradiation. The high dispersion of the CrO6unit inthe LDH matrix leads to the lower band gap and the resultingpronounced visible-light absorption. In addition, the abundant OHgroups on the surface of LDH accept photogenerated holes to yieldhighly-reactive hydroxyl radicals, accounting for the excellentphotocatalytic behavior. By virtue of the facile scale-up method and theintrinsic dispersion of the MO6octahedron unit, this approach can beextended for the preparation of other metal hydroxides/oxides for thewidely application in catalysis.2. Series of photocatalysts for water splitting into hydrogen wereprepared by distributing TiO6unit in a layered MTi–LDHs matrix（M=Ni, Zn, Mg）, which displays largely-enhanced H₂–productionphotocatalytic activity (314μmol·h^-1g^-1) as well as excellent recyclableperformance. Structure measurements reveal that a high dispersion ofTiO6octahedra in the LDH matrix was obtained by the formation ofM²⁺–O–Ti network, rather different from the aggregation state of TiO6in the inorganic layered material K₂Ti₄O₉. Furthermore, theelectron–hole recombination process was significantly depressed in the Ti-containing LDH materials relative to bulk Ti oxide, which isattributed to the abundant surface defects that serve as trapping sitesfor photogenerated electrons. A theoretical study based on DFTcalculation demonstrates that electronic structure of the TiO6unit wasmodified by adjacent MO6octahedron via covalent interaction,accounting for its superior water splitting behavior. Therefore, thedispersion strategy for TiO6unit within a2D inorganic matrix can beused to enhance the performance in photocatalysis and energyconversion.3. Ti3+doped NiZnTi-LDH nanosheets with a lateral dimension of4080nm were synthesised using a reverse microemulsion method,which exhibit very high visible-light photocatalytic activity forsplitting water into oxygen (2700μmol·h^-1g^-1) and antipathogenperformance. The photoluminescence spectra demonstrate that theelectron-hole recombination process was significantly depressed in theLDH nanosheets compared with bulk LDH material, which is attributedto the abundant surface defects serving as trapping sites forphotogenerated electrons. Electron Spin Resonance （ESR）measurement reveals that a high concentration of Ti³⁺sites aregenerated by creating nanoscized LDH platelets. This work provides afacile method for the preparation of LDH nanosheets containing a highconcentration of Ti3+active sites, which have potential applications in catalysis and solar energy utilization.4. Hierarchically macro/meso porous mixed metal oxides （MMO）framework with multilevel3D morphologies from nanoscale tomacroscale was fabricated by using a combination of an atomic layerdeposition （ALD） process in situ growth calcination process. It wasfound that the MMO framework can be used as an effective andrecyclable photocatalyst and exhibits higher photocatalytic activitythan the corresponding MMO powdered sample. This is due to thehierarchical structure of the biomorphic MMO framework along with ahigh specific surface area and wide pore size distribution. Our studydemonstrates an exciting approach for enhancing the chemicalperformance of conventional catalysts through biomimicking. It isanticipated that this new strategy can be employed to fabricate variousbio-templated LDH films as well as MMO frameworks.5. Various LDH films on different substrates （paper, sponge andcloth） were obtained by sol–gel deposition and subsequent in situgrowth procedure, which replicate the morphological properties of thetemplates with advantages of easy manipulation, low-cost, and largesurface area. The obtained films show higher adsorption capacity andexcellent ability to remove dye （20mg·g1LDHCongo Red）, comparedwith the corresponding powdered samples （11mg·g1LDHCongo Red）.By virtue of the facile sol–gel replication and in situ growth technique, it is expected that a wide variety of LDH films with complicatedmorphologies can be fabricated in a similar way. Therefore, this workprovides new possibilities for the rational design and fabrication ofhierarchical LDH films, which can be potentially applied in the fieldsof catalysts, adsorbents and membrane separation.