Modification Of Biological Structure Of Rice Husk And Its Catalytic Performance

Owning to the well-defined morphologies, controllable electronic transportproperties and high specific surface area, multi-dimensional semiconductors catalystwith hierarchically porous have been of growing interest for the potential applicationon the purification of automobile exhaust gas and the degradation of organic wastewater, etc. The architecture and controllable microstructures is heavily demanded asthe physical and chemical properties of catalyst are strongly dependent on theirporous structures and microstructures. Although the progress in this aspect has beenachieved in some extent, it still needs to further work on the control of catalystmorphology, high cost, complicated elaboration, etc. Inspired from natural materialswhich have been optimized in nature for millions of years, morph-genetic catalystmaintains the with their intrinsic hierarchical, multi-dimensional and multi-levelstructure of natural materials and has the unique characteristic.Rice husk, an agricultural by-product of the rice milling industry, containssubstantial concentration of organic substances and silica, and possesses hierarchicalstructures to satisfy the requirements for the novel functional materials. However, thehierarchical porous structure of rice husk is vulnerably collapsed after heat treatmentor other procedures due to its weak mechanical properties. Most studies are stillfocused on the exploration and application of its mineral components (organicsubstances and silica). Basing on the previous studies, the influence of externalconditions on hierarchical porous structure of morph-genetic materials was firstinvestigated in this work. Furthermore, using chemical and physical synthesismethods, nano-material with one dimensional modified three dimensional biomorphicporous structures was synthesized to optimize the function basing on rice husk. Theelaboration parameters, microstructure and photocatalytic property were studied toreveal the formation mechanism and the theory basis. The main contents andconclusions are shown as follows:(1) The influence of acid, temperature on hierarchical porous structure of morph-genetic materials which were dipped in oxide sol was investigated. The porousstructure of rice husk is vulnerably collapsed when the concentration of hydrochloricacid is above1wt.%, or when temperature of heat treatment was higher than1000°Cfor3h, whereas TiO2or ZrO2grain film was formed after sol impregnation and wasstill maintained at1400°C.(2) A systematical study employing rice husk as a biotemplate for the fabricationof morph-structured TiO2by sol-gel method was carried out. The result showed thatthis method not only maintained the rice husk structure but also simultaneouslyintroduced TiO2nanoparticles on the porous wall of samples to get goodphotocatalytic performance that the ability of RhB degradation of under visible lightis better than those prepared via classic routes. For example, nearly98%RhB wasphotodegraded in80min in this sample. The reason should be attributed to that thehierarchical structural feature provide a large specific surface area containing activesites and the presence of active carbon and the effective nitrogen and Zn dopingduring synthesis to increase the specific surface area of sample in further while theself-N doping also did the contribution during the photocatalytic reaction.(3) ZnO/SiO2photocatalysis with multi-dimensional and multi-level weresynthesized using rice husk as template by zinc solution immersion process. Both theevolution of microstructure and the photocatalysis performance of samples werestudied to discuss the formation mechanism and relation between sample’s structureand property. The result showed that regular hexagon and spire shape ZnOsubmicronrods which have an average diameters of400-2500nm and length of3-5μm were synthesized on the surface and in the pores of rice husk SiO2by zincsolution rice husk treated at300℃，400℃or500℃in a nitrogen atmosphere andthen at450℃or550℃in an air atmosphere. The specific surface area of theas-prepared samples are about6-14m2g-1. The zinc chloride concentration andtreating temperature have important effects on morphology, size, quantity and statusof ZnO submicronrods. Nearly90%10μMRhB was photodegraded in120min underultraviolet light.(4) Hybrid one-dimensional (3D) carbon nanostructures on three-dimensional (3D) hierarchical porous SiO2（RHx-y）were obtained from rice husk as a biomorphicstructural template and raw materials while R-Ni was as catalyst precursor throughin-situ catalytic pyrolysis reaction. This nanostructured materials were coated withtitanium by means of a sol-gel technique and then were calcined at varioustemperatures （TiO2/RHx-y）. The microstructure and catalysis performance ofsamples were characterized as well. The result showed that amount of CNTs wasobtained at1200°C, whereas the SiC and Si2N2O nanowires were obtained at1300°C.The specific surface area of1D/3D nano-structured samples was larger than that ofrice husk ash, which was about36-90m2g-1,5-10times of the rate of the rice huskash. TiO2/RHx-y had also shown excellent photocatalytic capability in the degradationof RhB (10μM) just in80-100min comparing with pure TiO2which requires140mins to photodegrad80%10μMRhB under ultraviolet light.(5) Fan-shaped SiO2micro-nanowires hybrid biomorphic porous SiO2@Ccomposites with bio-inspired hierarchical porous structure and a high accessiblesurface area were prepared by in-situ molten salt template method. The result showedthat a large amount of coral-like, spindle-like and fan-like structures with numeroustiny branches sprouting from the central nanowires were grown in the inter epidermisporous and on the surface of rice husk SiO2under heat treated at1200°C. The specificsurface area of the as-prepared samples are about826m2g-1-1025m2g-1.(6) The absorption mechanism was discussed basing on the simulating the wastewater treatment by1D/3D nano-structured samples (CS-RH) which was employed asorganic absorbers and acted as a bio-template to obtain anatase TiO2(xTiO2/CS-RH-y).The result showed that the adsorption of CS-RH reached the maximum value (248mg/g) when the initial RhB concentration was280mg/L. The adsorption of RhB ontothe CS-RH could be described by the Freundlich isotherm and thepseudo-second-order model. The results of thermodynamic calculations suggested theadsorption process was a spontaneous and exothermic physisorption. The activationenergy was45450J/mol. xTiO2/CS-RH-y shown excellent adsorption andphotocatalytic capability that the average reaction rate of the specimens was0.02521min-1and0.02456min-1, respectively which was16.5-17times of the pure TiO2. It demonstrated that the synergistic reaction occurred between TiO2andmulti-dimensional supporters to improve the catalysis performance. The studiedshowed that this degradation procedure was a first-order reaction. The high specificsurface area and pore volume benefited the RhB absorption while the hierarchicalporous structure could enhance the optical absorption efficiency. At the meantime, the1D nanostructure of materials accelerated the movement of photoinduced holes to thesurface. All of structural features contributed a high efficiency of photocatalysisreaction of our composited materials.