| Photocatalytic degradation of harmful organic pollutants in the air and the water using semiconductor particles, such as titanium dioxide (TiO2), is one of the most widely studied methods. Adsorption of a compound onto a photocatalyst surface is one of the key reaction steps in photocatalysis. Unfortunately, nanoparticles tend to aggregate in suspension. The main drawback of immobilization of semiconductor particles on the carrier is the difficulty to regenerate the carrier. Mesoporous TiO2fibers with large surface area would combine the advantages of both adsorption and photocatalytic techniques, thus improving the efficiency of pollution removal.In this paper, Si-doped TiO2fibers were synthesized by sol-gel method. A new type of materials used for wast-water treatment was prepared through chemical modification and different heat treatment process. The samples possessed various functional groups on the surface and owned a hierarchical mesoporous structure. The enriched dyes on the surface of fibers can be degradated via photo-catalysis/-sensitization process thus regenerated the TiO2fiber in-suit. The adsorption behavior of MO, X-3B and K-2BP to the surface of TiO2was investigated. The prepared samples were deeply characterizated by XRD, FT-IR, PL, UV-Vis DRS, SEM, XPS and N2adsorption-desorption. The effects of optical characteristics, crystal structure, and surface defects were carefully examined and validated by direct evidence. Such studies can give insights into the mechanistic aspects of preparation of photocatalyst for practical use. This paper was divided into three parts as follows:In the first part, the surface characteristics of acid-modified continuous TiO2fibers, the adsorption of dyes, and the photocatalytic degradation of dye pollutants under UV and visible light irradiation were investigated. After HF extraction of silicon, a hierarchical mesoporous structure is produced. The-OH groups located on the surface were more resistant to exchange with F-because of the inductive effect on surface Ti4+centers originating from adjoining electronegative F-ions. After protonation of the HF-modified fibers by HNO3treatment, the photodegradation percentage of X-3B on the TiO2fibers reached98%in45min under visible light irradiation, and the first-order rate constant increased by a factor of163.4. The high visible light activity was mainly attributed to a synergetic effect of the hierarchical mesoporous structure and the strong adsorption of X-3B upon the protonated surface. The adsorption isotherms showed that the adsorption of X-3B on acid-modified TiO2followed the Langmuir model with a maximum adsorption capacity of19.03mg/g;6-fold higher than the unmodified TiO2. The nature of the X-3B sorption was spontaneous and thermodynamically favorable. The sorption kinetics was best described by the pseudo-second-order model.In the second part, the process of enrichment of anionic azo-dye on the surface of TiO2fiber and followed by photosensitization degradation under ambient air condition was extensively investigated. The three anionic dyes used were Methyl Orange (MO), Reactive Brilliant Red X-3B and Reactive Brilliant Red K-2BP, increasing in number of sulfonic groups, respectively. The dye as sensitizer adsorption on TiO2surface strongly dependent on pH with a maximum adsorption capability achieved at pH3. The Langmuir model shows that the higher charged dyes had a higher affinity of adsorption. The adsorption of dye proceeds via chemisorption via the sulfonic acid (-SO3H) surface groups. Reactivity of the dye-adsorbed photocatalyst could be restored after visible light regeneration and5%HNO3elution. The pore structure of regenerated TiO2fibers was more regular and no noticeable change of the specific surface areas. Optical measurements reveal that the electron injection and recombination process between the dye and TiO2plays an important role in photosensitized degradation. The adsorption ability of regenerated TiO2fibers for50mg/L K-2BP was almost the same (74-76%) after repeated five times. Thus, the process of "adsorption-photosensitized regeneration-reuse" can be established.In the third part, TiO2fiber photocatalysts were synthesized by a sol-gel process using tetrabutyl titanate as Ti source and tetraethyl orthosilicate as Si source followed by calcination at700℃in N2, O2, steam and air, respectively. The synthesized catalysts contain only anatase phase, and N2-calcinated sample showed an obvious lattice distortion. A hydrophilic surface can be observed in steam-and air-calcinated fiber due to the large amount of surface Ti-OH. The added Si can be identified as oxygen deficient Si3+species in Si-O-Ti bonds, and this bonding results in a more strength fiber surface. For fiber obtained in steam, Ti2p made a red shift of0.2eV to the lower bonding energy is observed, indicating the presence of Ti3+. Calcination in hypoxic atmosphere could avoid the harmful effects of carbonization, which was helpful for increasing surface area and pore volume. Toward X-3B degradation under UV light from aqueous solution, the catalysts exhibited activities in the order, according to calcination atmosphere, of steam> air> O2> N2. The photocatalytic results showed that the steam-fibers excel in photo-absorbance properties exhibited highest photocatalytic activity and the degradation rate of X-3B (30mg/L) reached99%(kapp=0.014) after210min under UV irradiation. |
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