Layered Double Hydroxide-based Catalysts/Adsorbents: Nanostructure Design And Performance

This dissertation describes a systematic study on the design andpreparation of nanocatalysts/adsorbents based on the layered doublehydroxides （LDHs）. A series of highly-effective LDHs-based catalystshave been fabricated via tuning their nanostructure （preferred exposureof active facets, formation of specific defects, high degree of dispersion）;the relationship between its nanostructure and the catalytic performanceswas investigated. Moreover, the application of immobilized LDHs filmas structured adsorbents is one effective approach to achieve LDHsnano-adsorbents with high dispersion and high stability, which has beenused to remove heavy metal ions and anionic dyes from aqueoussolutions.The main results of this dissertation are as follows:（1） LDHs-based nanocatalystsZnO nanoplatelets with a high degree of exposure of （0001） facetsembedded on a hierarchical flower-like matrix have been prepared viathe in situ topotactic transformation process of the Zn/Al LDHsprecursor with hierarchical architecture. The resulting ZnO-based nanostructure possesses a low band gap （2.90eV） and displays superiorvisible-light photocatalytic activity, in comparison with theZn/Al mixed mental oxides （MMOs） powder, ZnO nanoplates and ZnOnanorods sample. The high visible-light photocatalytic behaviour of thiscatalyst is related to the presence of oxygen defects in the exposed （0001）facets. In addition, the material maintains a constant photocatalyticactivity over a number of consecutive cycles without peeling off oraggregation of ZnO nanoplatelets due to the strong anchoring effect.Ni nanocatalyst with high dispersion and high particle densityembedded on a flower-like Al₂O₃matrix was fabricated by a facileprocess involving in situ reduction of Ni/Al LDHs precursor withhierarchical architecture. HRTEM,（HAADF）STEM, PAS and EXAFSdemonstrate the existence of abundant surface vacancy clusters whichserve as active sites for CO₂methanation, accounting for the significantlyenhanced low-temperature activity of the supported Ni nanoparticles（CO₂conversion:99%at300°C; CH4selectivity:>99%）, much higherthan that of Ni/Al₂O₃material prepared via the traditional impregnationmethod （CO₂conversion:75%at350°C; CH4selectivity:>95%）. Inaddition, the supported Ni nanocatalyst exhibits high-stabilitysimultaneously, without sintering/aggregation of active species forlong-term employment.（2） LDHs-based nanoadsorbents The Mg/Al NO3LDHs films （the ab-plane parallel to the substrate）with strong adhesion to the substrate have been fabricated by theelectrophoretic deposition method （EPD） on aluminum substrate. It wasfound that the sorption capacity （qMAX） of the films reached79.4mgg^-1for Cr（VI） and222mgg^-1for RBBR respectively, much larger than thatof the corresponding LDHs powdered sample (67.6mgg^-1for Cr（VI） and192.3mgg^-1for RBBR, respectively). Furthermore, the LDHs filmexhibits excellent sorption-regeneration performances compared with thepowdered sample, which facilitates its repeatable and cyclic usage over along period.An Mg/Al CO3LDH hierarchical framework （LDH/Al-foam） as astructured adsorbent has been fabricated via the in situ growth process ofLDHs film （ab-plane perpendicular to the substrate） on the aluminumfoam, which exhibits a high dispersion of LDHs nanoplatelets and strongadhesion to the substrate. It was found that the sorption capacities （qMAX）of the LDHs framework reached27.8mgg^-1for Cr（VI） and212.8mgg^-1for RBBR, respectively, much larger than those of the correspondingLDHs powdered sample (21mgg^-1for Cr（VI） and166.7mgg^-1forRBBR). Furthermore, the LDH/Al-foam in this work exhibits theadvantages of large regeneration sorption capability, strong mechanicalrobustness, convenient manipulation as well as easy regeneration.Therefore, it is expected that the LDH/Al-foam can be potentially used as an efficient and recycling adsorbent in water treatment.