Fabrication And Catalytic Performance Of Biological Hierarchically Micro-nanoporous Structure CeO₂ Materials

Cerium oxide is one kind of the most important rare earth oxide materials for its promising applications in three catalyst of automotive exhaust, solid oxide fuel cells, catalyst for organic wastewater treatment, oxygen storage materials and so on. The physical and chemical properties of cerium oxide are strongly dependent on their microstructures, including size, morphology, and specific surface area etc. The structure of hierarchical micro-nanoporous cerium oxide materials not only meet the requirements of micro structures for efficient adsorption, but also improve the catalytic capacity with increasing oxygen content of defects and catalytic activity sites on surface. Therefore, the study of the preparation for such materials is very important in reality.Bio-template method is a simple approach for synthesis of hierarchical micro/nanoporous oxide materials with biological natural structurce. In this dissertation, we employed bio-template method to prepare hierarchical micro/nanoporous ceria materials with biogenic morph-structures through facile reaction conditions with the assistance of leaves, rape pollen, clover stem, eggshell membrane, China rose petals and fungal hypha, which were chosen as the bio-template based on their natural abundance and low-cost. The novel hierarchical porous ceria materials were synthesized via the effective double-replications of the real microporous structure of biological cell membrane and the virtual cellular nanoporous with cerium source. Based on this research, the reaction processes and synthetic mechanisms of biological cerium oxide materials prapared by various templates were investigated, and the effect of ceria material structures and catalytic performance were discussed. The mian research results are as follows.1. The hierarchical micro/nanoporous ceria materials with big microns holes and nanoscale holes (1-4 nm) were synthesized using bamboo leaves as template and nitric acid cerium as cerium source. The optimized parameters were obtained through a quality ratio of precipitation agent to cerium salt of template. The materials were built from face centered cubic particles with the diameter of 5-7 nm and the surface area of 162 m2/g. In the reaction process, cerium ion was combined with the biological macromolecules with replacement of calcium ion, so that the internal nanopores of bamboo leaves were coped. Under the similar reaction conditions, the CeO2 materials with bionic morphology with the specific surface area of sample 64.4 m2/g.were synthesized using the maple leaf template, which consist of micron grade big hole and 2-4 nm holes,2. The hierarchical micro/nanostructure cerium oxide hollow microspheres were prepared using the rape pollen templates. The effect of quality ratio of template/cerium source and the addition amount of HMT on the structure of produce were investigated. The material with a 10μm hollow diameter and rich in 2-4nm diameter pores, specific surface area of about 168 m2/g was synthesized through the optimized process. Through biological nitrogen doping into the cerium oxide lattice, and nitrogen in the grid doped with distortion, the forbidden band width of materials were effectively reduced so that the scope of absorbing light by materials shifted to visible light.3. The ceria micron tubes with abundant nanopores were synthesized by clover stem templates. The influence of template pretreatment methods, the quality ratio of nitric acid cerium to stems and the temperature of hydrothermal reaction on the structure of the products has been studied, and the biomimetic synthesis reaction mechanism has been expounded. When the quality ratio of nitric acid cerium and stems was too high or too low, or the temperature of hydrothermal reaction was too hot or too cold, the precursor of ceria could not copy the structure of template. Under the optimum conditions, the products were flexible tube with bionic porous structure, which length for decades to hundreds of micron and diameter for dozens of nano to more than 10 microns, and contain diameter in 15 to 40 nm on the small pores, and the specific surface area of about 103 m2/g.4. Three-dimensional network structure of cerium oxide micron tube were synthesized by egg membrane template. The specific surface area was about 40 m2/g, the diameter about 500 nm, wall thickness about 250 nm, and the pores wall accumulated 1-5 nm pore of 6nm diameter contains cerium oxide particles. Moreover, the synthesis mechanism was explored that the cerium ion substitution calcium ions absorpt by biological protein, and forming cerium oxide micron tube with a three-dimensional network of hierarchical micro-nanoporous through biological mineralization.5. The China rose petals were used as templates to synthesize the scale-like structure of cerium oxide material, and the optimal reaction process was determined through the observation of different calcining temperature of synthetic material microstructure. Moreover, it turned out the cerium oxide nanosheet was about 400nm, width of about 200nm, about 8nm thick, and there had been 150 nm wide aperture pores among the chips that was rich in 1-2 nm and 4-5 nm double stage nanopores, and the specific surface area is about 86 m/g.6. The fungal hyphae was used as template, and the optimal reaction process was disccussed through examined the effect of calcination temperature and pH value on cerium oxide material moulding. The filamentous network structure with a large number of about 5 nm pores were accumulated by about 6nm CeO2 nanoparticle, and the specific surface area of product was about 75 m2/g. There were ordered strip microporous of 0.6-0.8μm length and 0.2μm width in the ordered macroporous cerium oxide hollow microsphere with about 20μm diameter and about 5-10μm short diameter, which had been sythesized in the optimal synthesis process by microbial diatoms as template.7. It was found that the different hierarchical biological micro-nanopores structure cerium oxide materials with Ce3+ and active oxygen content were tested by XPS, and there were highest content in the sample synthesized by China rose petals template. H2-TPR and XPS test results infered that the ceria materials having more Ce3+, its surface contain more reactive oxygen species. Hierarchical micro-nanostructure and large specific surface area could result in reduction temperature of active oxygen and bulk oxygen reduction. The low temperature of reactive oxygen and high temperature of body oxygen reduction peak were merger for the materials with biological nitrogen self-doping. The experiment of catalytic oxidation of CO and acid fuchsin catalytic decolorization comfirmed that the biological hierarchical micro-nanoporous cerium oxide materials possessed best catalytic efficiency than porous CeO2 microspheres and nonporous cerium oxide. The catalytic ability for CO would be advanced with temperature rise. In addition to the specific surface area, another reason was that the material with hierarchical micro-nanoporous structure had good adsorption activity and could capture and store oxygen, which provided reactive oxygen species for reaction.