Preparation Of SiO₂（AG）/ZnO And Its Adsorptive And Photocatalytical Properties

ZnO is a semiconductor with direct wide band gap of3.37eV. Owing to its good biocompatibility and environmental safety, ZnO has received enormous attention to be a promising photocatalyst for degradation of environmental pollutants. It is well known that the structure, morphology and sufacial character, et al. of the catalyst are confessed to have great effects on their properties and applications. In recent years, different morphologies of ZnO have been synthesized, but there are few report about the effects of the morphology of ZnO on the photocatalytic activity of ZnO. In this dissertation, we systematically investigated this kind of effects on the photocatalytic activity of ZnO due to the morphological differences. Considering that the adsorption process of organic compounds in ZnO had a significant impact to the photocatalytic activity during the process of the ZnO photocatalytic degradation of organic compounds, we prepared SiO2aerogel/ZnO (SiO2(AG)/ZnO) composite materials to improve the ZnO adsorption to the organic compounds. Additionally, it was of important to investigate the effect of the adsorption of organic compounds in the photocatalyst on photocatalytic activity.We conducted the comparison experiments of photocatalytic degradation of different kinds of organic compounds such as nitrobenzene, phenol and methylene blue using tetrapod-like ZnO whisker (T-ZnO), nano-sized ZnO (n-ZnO) and micro-sized ZnO (c-ZnO) as the catalysts which had different morphologies. The results showed that T-ZnO had the best photocatalytic activity against the mentioned organic compounds, and c-ZnO exhibited the second, n-ZnO displayed the worst activity. Firstly, ZnO with different morphologies had different oxygen vacancy concentration in their crystals, which directly caused different amount of-OH generation and lead different photocatalytic activity. The oxygen vacancy concentration of T-ZnO was significantly higher than those in c-ZnO and n-ZnO. Accordingly, T-ZnO generated more-OH in its suspension, displaying better photocatalytic activity than the other two kinds of ZnO. Secondly, the process of-OH generating could restrain the dissolving Zn2+which was reacted by the photogenerated holes and the oxygen atoms of the ZnO surface. T-ZnO generated the most amount of-OH, and had the lowest Zn2+concentration in photocatalytic reaction process, resulting in the best light stability. Thirdly, T-ZnO had a special four needle-like structure, whose needles’tip were in nano-scale, which let it exhibit features of nanomaterials. Meanwhile, it overcame the shortcomings that nanomaterials usually had such as aggregation and worse dispersal. The n-ZnO had higher amount of oxygen vacancy concentration than c-ZnO, but it displayed worse photocatalytic activity. This was because n-ZnO had smaller particle size, resulting in stronger inclination of aggregating, which lead to decrease of photocatalytic activity, even the oxygen vacancy concentrations of these two types of ZnO were similar.By using sol-gel method, we prepared SiO2(AG) and SiO2(AG)/T-ZnO with different loading amount by solvent exchange under ambient pressure. The result of material characterization shows that SiO2(AG) sample is the micro-porous structure formed by numerous fine particles, whose particle size distributed evenly, Its specific surface area is902m2/g, and the average pore size is8.91nm, and the pore volume is2.01ml/g. Further experiment illustrates SiO2(AG) is hydrophobic. SiO2(AG) can be transformed from hydrophobic to hydrophilic after500℃treatment. The specific surface area and the pore volume of SiO2(AG)/T-ZnO composite materials increase as the amount of the loaded SiO2(AG) increase. The composite materials preserve the original material characteristics of SiO2(AG) or T-ZnO. As it is compared to T-ZnO, SiO2(AG)/T-ZnO have no obviously change in the UV absorptions.We systematically studied the materials’absorption performance of organic compounds.The results show that, in the absorption of nitrobenzene, phenol and methylene blue, the hydrophobic and hydrophilic interaction of adsorbent and adsorbate plays a dominant role, and the SiO2(AG) which is hydrophobic has good adsorption capability on hydrophobic and insoluble organic compounds nitrobenzene, though it shows few absorption capabilities on hydrophilic and soluble organic compounds phenol and methylene blue. After500℃treatment, SiO2(AG) turns hydrophilic, and it shows good adsorption capacities on phenol and methylene blue, and the adsorption capacity on nitrobenzene drops significantly. The adsorption performance of SiO2(AG)/T-ZnO for nitrobenzene improved significantly compared to T-ZnO. After500℃treatment, its adsorption performance on phenol and methylene blue also improved significantly.We studied the photocatalytic degradation kinetics of organic compounds, and explored the effect of the improvement of adsorption performance for organic compounds photocatalytic degradation of nitrobenzene, phenol and methylene blue. The result shows that SiO2(AG)/T-ZnO having8.0%loading of SiO2(AG) on T-ZnO has the best photocatalytic activity for nitrobenzene. SiO2(AG)/T-ZnO having8.0%loading of SiO2(AG) shows better photocatalytic activity for phenol and methylene blue after500℃treatment. The process of SiO2(AG)/T-ZnO and T-ZnO photocatalytic reactions for nitrobenzene, phenol and methylene blue display a pseudo first-order kinetics process. Considering the initial reaction kinetics, the reactions of nitrobenzene and phenol are fitted to the Langmuir-Hinshelwood kinetics model, but methylene blue is unfitted. The experimental result also shows that catalyst improves the performance of organic compounds adsorption which beneficial to the photocatalytic degradation of organic pollutants. The high colorimetry of dye wastewater with high concentration affects the light absorption and utilization of catalyst, which makes photocatalytic activity decrease.We deeply analyzed the factors that affect the photocatalytic reaction of nitrobenzene wastewater using SiO2(AG)/T-ZnO having8.0%loading of SiO2(AG) on T-ZnO as the catalyst. The results show that, at the room temperature, the apparent rate constant of nitrobenzene decreases as the initial concentration increases. The best quantity of SiO2(AG)/T-ZnO is2.0g/1for the nitrobenzene solution of24.0mg/1. And the degradation ratio of nitrobenzene is as high as82.1%and there are few effects on degradation ratio by reaction temperature. The nitrobenzene degradation ratio increases to85.8%from82.1%by adding30.0%H2O21.5.0ml/1. It is also found that the common anions of industrial wastewater, such as Cl-,SO42-make nitrobenzene degradation ratio decrease significantly, SO42-exhibit greater restraints than Cl-, but NO-3has no obvious effects on nitrobenzene degradation ratio. As to the treatment of mixed wastewater containing nitrobenzene and methylene blue, SiO2(AG)/T-ZnO preferentially adsorbs hydrophobic nitrobenzene, and makes it degrade preferentially. The hydrophilic methylene blue has small effects on the photocatalytic degradation of nitrobenzene. Conversely, the photocatalytic degradation of methylene blue is restrained significantly. In addition, it is experimentally proved that, after3times of usage, SiO2(AG)/T-ZnO exhibits photocatalytic performance almost the same as the original sample.