Preparation And Catalytic Performance Of Micro/Nano Hierarchically Structured Transition Metal Oxide Materials

The development of catalysts and adsorbents with excellent performance is one of the current research hots in the field of environmental remediation.As functional layered materials attracting much attention in recent years,layered double hydroxides ?LDHs? and flowerlike iron alkoxide based materials have become the preferred precursor materials of the catalysts and adsorbents for the removal of NOx and anionic azo dyes. Based on layered precursor method, the present thesis used the proper preparation method to construct the hierarchically structured transition metal based LDHs and alkoxide materials, and then obtained the morphology maintained hierarchically structured transition metal ?Fe, Co, Ni? based oxide materials after certain post treatments. The synthesis method is simple, economic and environmental-friendly. The structure, composition, morphology and surface property are systematically characterized. The catalytic performance of NO oxidation and the performance of Fenton oxidation degradation and adsorption towards anionic azo dyes were also investigated. The inherent relationship between the structure and performance is explored and the catalysis and adsorption mechanism in the current research system are revealed, offering certain theoretical basis for designing hierarchically structured transition metal based catalysis and adsorption materials with more excellent performance.The main results of the thesis are shown as follows:?1? Nanoflower-like Co3Al-HT and Co2NiAl-HT hydrotalcite-like precursors were synthesized by a facile co-precipitation method and nanoflower-like and nanoparticle-like cobalt based mixed metal oxides were obtained from thermal decomposition of these precursors at 500 and 800 ?,respectively. The mixed oxide catalysts mainly consisted of homogeneous and stable non-stoichiometric cobalt-based spinel phases Co?Co,Al?₂O₄ and Ni?Co,Al?₂O₄. The nanoflower-like catalysts Co3A10-500 and Co2NiAlO-500 possess cobalt-based spinel phases with smaller crystallite sizes ?11.0-16.5 nm?, larger surface areas (88.6-96.2 m2 g^-1 ) and mesoporous structure (pore size at the maximum probability: 8.61-8.68 nm; total pore volume: 0.69-0.78 cm3 g^-1)?The nanoparticle-like catalysts Co3Al0-800 and Co2NiAl0-800 possess cobalt-based spinel phases with obviously increased crystallite sizes?40.8-50.0 nm?, greatly decreased surface areas (17.2-19.3 m² g^-1) and the disappear of the mesoporous structure.?2? The catalysts Co3A10-500 and Co2NiAlO-500 presented excellent catalytic NO oxidation performance, with the maximum conversion efficiency ca. 88.8% and 87.6% ?285 ??, respectively, which were much higher than those of Co3A10-800 and Co2NiAl0-800 ?55.6% and 66.6% at 350 ??. The excellent catalytic NO oxidation performance of the catalysts calcined at 500? could be attributed to the much larger amount of surface active sites endowed by the hierarchically structured catalysts with smaller crystallite sizes of cobalt-based spinel phases, higher specific surface areas, and mesoporous structure. The nature of the active sites and NO oxidation pathways on the hierarchically structured cobalt based mixed oxides were revealed. Although Co2NiAlO-500 possessed slightly smaller amount ofactive sites (Co³⁺ and Ni³⁺ associated with surface adsorbed oxygen),compared to Co3AlO-500 with active sites (Co³⁺ associated with surface adsorbed oxygen), catalyst Co2NiAlO-500 possessed higher reducibility due to the dopant of Ni, thus possessing similar excellent NO oxidation performance and lower catalyst cost.?3? 3D flowerlike iron alkoxide ?Fe-EG? micro/nanostructure has been fabricated via a facile surfactant-free solvothermal method using NaOAc-3H₂O as alkali source based on low concentration and low temperature dissolution process. The obtained green colored 3D flowerlike Fe-EG micro/nanostructure is a polymeric ferrous glycolate with a chemical formula [Fe₂?OCH₂CH₂O?₂?HOCH₂CH₂OH?₂]_n. Upon quasi-in situ monitoring morphology and composition of the samples collected at varied reaction times,a coordination - ligand substitution - reduction - polymerization mechanism for Fe-EG is tentatively proposed. The 3D flowerlike Fe-EG can be converted to amorphous FeOOH with maintained flowerlike morphology via hydrolysis in water. The Fe-EG exhibited excellent Fenton degradation performance towards the azo dye Acid Orange 7 ?A07? in wastewater with H₂O₂ as oxidant(-99.7% A07 degradation in 40 min at pHO = 6.0, [H2O2]= 48.5 mM, and catalyst dosage of 0.2 g L^-1). The high dispersion of a large amount of Fe³⁺ on the surface of in situ formed 3D flowerlike amorphous FeOOH with large surface area and rich porous structure by the moderate interfacial hydrolysis process of Fe-EG benefits the adsorption of dye molecules and the formation of hydroxyl radicals, and believed to be the main contributors to the high performance of A07 dye degradation at neutral pH.?4? A series of nearly monodispersed Fe₃0₄ submicroparticles with tunable particle sizes were prepared by using the facile surfactant-free solvothermal method through controlling the concentration of single iron source FeCl₃�6H₂O. The obtained Fe₃0₄ submicro particles present single-crystal nature and strong ferromagnetic property with magnetization saturation values ranged in 54.3-88.7 emu�g^-1. A complexation-aggregation-phase transformation formation mechanism was firstly revealed for the obtained Fe₃0₄ submicro particles based upon the quasi-in-situ monitoring of the morphology and structure evolution of the samples collected at varied reaction time. The synthesis method is facile and environmental-friendly and can provide proper support for the assembly of the core-shell magnetic LDHs based hierarchically structured materials.?5? Magnetic core-shell hierarchically structured Fe₃O₄@MgFexAl-LDH?x = 0, 0.1, 0.5? materials were construct by a facile co-precipitation method and core-shell hierarchically structured mixed metal oxide materials were obtained from thermal decomposition of these precursors at proper temperature. The magnetic core is ?-Fe₂O₃ phase; the shell is mainly Mg?Al?O phase and the Fe₂O₃ phase is also included for the samples containing iron in the shell ?x = 0.1, 0.5?. The obtained hierarchically structured mixed oxides possess relatively high specific surface area (211.9 -95.4 m² g^-1 ), mesoporous structure and relatively strong magnetism (Ms = 20.7-22.7 emu g^-1). The obtained hierarchically structured materials exhibited excellent adsorption properties towards the anionic azo dye Congo red ?CR?, with the maximum adsorption capacity of 3980 mg g^-1, which is much higher than other magnetic hierarchical adsorbents reported in literatures. The adsorption process obeyed the pseudo-second-order rate equation and the adsorption isotherm obeyed the Langmuir adsorption model. The superhigh CR adsorption capacity can be attributed to the memory effect of the MgFeAl-LDHs, the electrostatic attraction between the oxide shell and CR, hydrogen bonding, coordination effect as well as large surface area and mesoporous structure endowed by the hierarchical structure.