Microstructure characterization and photocatalytic activity of mesoporous TiO2 films with ultrafine anatase nanocrystallites

A series of mesoporous TiO₂ films on borosilicate glass with ultrafine anatase nanocrystallites were successfully synthesized using a non-acidic sol gel preparation route, which involves the use of nonionic surfactant Tween 20 as template through a self assembly pathway. The microstructure of these TiO₂ films was characterized by XRD, SEM, HR-TEM, UV-Vis spectroscopy, and N₂ adsorption-desorption isotherm analysis. Their photocatalytic activities were investigated by using creatinine as a model organic contaminate in water. It was found that all mesoporous TiO₂ films prepared with Tween 20 exhibited a partially ordered mesoporous structure. The photocatalytic activity of the TiO₂ films could be remarkably improved by increasing Tween 20 loading in the sol at the range of 50% (v/v), which yielded large amount of catalyst (anatase) on the glass support and enhanced specific surface area. The optimum Tween 20 loading was 50% (v/v) in the sol, above which good adhesion between TiO₂ films and borosilicate glass could not be maintained. The final TiO₂ film (Tween 20: final sol = 50%,v/v) exhibits high BET surface area (∼ 120 m²/g) and pore volume (0.1554 cm³/g), ultrafine anatase nanocrystallinity (7 nm), uniform and crack free surface morphology, and improved photocatalytic activity.     

Synthesis, characterization and photocatalytic reactivity of Ti-containing micro- and mesoporous materials

Ti-oxides incorporated within the framework of microporous zeolites and mesoporous molecular sieves were found to exhibit high and unique photocatalytic activity for the decomposition of NO into N₂ and O₂ as well as the reduction of CO₂ with H₂O to produce CH₄ and CH₃OH. Spectroscopic investigations of these catalytic systems using photoluminescence, UV-vis, XAFS (XANES and FT-EXAFS) and IR analyses revealed that the charge transfer excited state of the isolated tetra-coordinated Ti-oxides plays a vital role in these photocatalytic reactions. The reactivity of such Ti-oxides was found to depend strongly on their local structures which were controlled by the unique framework structures of the micro- and mesoporous material supports.     

Investigation of the selective catalytic reduction of NO by NH3 on Fe-ZSM5 monolith catalysts

Photocatalytic nitric oxide (NO) decomposition and reduction reactions, using carbon monoxide (CO) as a reducing gas, have been investigated over Degussa P25 titanium dioxide photocatalysts, using a continuous flow reactor. The effects of thermal pretreatment temperature and reaction gas composition on the activity and selectivity of the decomposition and reduction reactions have been evaluated. Prepared materials were characterised by X-ray diffraction (XRD), N₂ physisorption, transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) and findings from these techniques were used to explain the observed photocatalytic properties. XRD and TEM results indicated that for the pretreatment temperatures used (70, 120, 200, 450 and 600 °C) there was no appreciable change in the phase composition and the original composition of ca. 77 vol.% anatase and 23 vol.% rutile was maintained even after treatment at 600 °C. It was found that the photocatalytic activity for both the decomposition and reduction reactions decreased with increasing pretreatment temperature. This was attributed to the removal of surface hydroxyl species that act as active sites for reaction. For the decomposition reactions the only products observed were nitrogen and nitrous oxide and the selectivity for nitrogen formation remained constant (ca. 23%) regardless of the pretreatment temperature. The presence of CO in the reaction gas had a dramatic effect on the selectivity of the reactions with nitrogen selectivities as high as 65% being observed. It was found that as the CO/NO ratio increased the selectivity for nitrogen formation increased.     

Kinetics of dye decolorization in an air–solid system

The photocatalytic decolorization of adsorbed organic dyes (Acid Blue 9, Acid Orange 7, Reactive Black 5 and Reactive Blue 19) in air was examined, applicable to self-cleaning surfaces and catalyst characterization. Dye-coated Degussa P25 titanium dioxide (TiO₂) and dye-coated photo-inert aluminum oxide (Al₂O₃) particles, both of sub-monolayer initial dye coverage, were illuminated with 1.3 mW cm⁻² of near-UV light. Visual evidence of color removal is reported with photographic images. Two methods, Indirect and Direct Analysis, were employed to quantitatively examine the decolorization kinetics of dyes using UV–visible transmission and diffuse reflectance spectroscopy, respectively. A decrease in dye concentration with time was observed with near-UV illumination of dye-coated TiO₂ powders for all dyes. Dyes did not photodegrade significantly on photo-inert Al₂O₃.

UV–visible spectroscopy data was used to model the kinetics of the photocatalytic degradation. Two first-order reactions in series provided the most convincing rate form for the photodegradation of dyes adsorbed to TiO₂, with a first step the conversion of colored dye to colored intermediate, and the second the conversion to colorless product(s). The first rate constant was of similar magnitude for all dyes, averaging k₁ = 0.13 min⁻¹. Similarly, for the second, k₂ = 0.0014 min⁻¹.     

Superhydrophilic Cu-doped TiO2 thin film for solar-driven photocatalysis

Nanosized Cu-doped TiO₂ film was prepared by the sol–gel spin coating technique. XPS analysis showed that Cu atoms had been successfully doped into TiO₂ lattice, which hence modified the surface chemical composition. As a result, the Cu-doped TiO₂ thin film possessed a superhydrophilic surface with a water contact angle (WCA) only 5.1° and exhibited excellent anti-fogging behavior. The Cu-doped TiO₂ thin film also exhibited a much better photocatalytic activity than the reference TiO₂ thin film, as evaluated by the degradation of 10 mg/L methylene blue (MB) solution under simulated solar-driven irradiation.     

A Review of Dehydration of Various Industrial Sludges

Wastewater characteristics and sludge generation potential of point source categories are reviewed critically. Novel industry-specific sludge dewatering/drying solutions necessary to establish a sustainable model are examined through a detailed literature survey. Knowledge of sludge properties is one of the most critical issues needed to design dewatering/drying equipment. This study focuses on industrial wastewater/sludge characterization. In addition, a comprehensive review of current drying models and technologies is also presented. A summary of the results derived from a novel thin-film-based photonic sludge dewatering/drying study is outlined as an alternative approach for industrial sludge control. Sludge was dried in a tubular quartz reactor (TQR), the inner surface of which was coated with a TiO₂ thin film. The TQR was irradiated with UV A, UV B, and UV C lamps. The consumed and generated energy fluxes through endergonic and exergonic reactions driven by photolysis and photocatalysis were investigated. In addition, the variations in sludge dewatering/drying characteristics were also examined and compared with conventional methods to evaluate the energy requirements.     

The dependence of the photocatalytic activity of TiO2/carbon nanotubes nanocomposites on the modification of the carbon nanotubes

Nanostructural TiO₂/modified multi-wall carbon nanotubes photocatalysts were prepared by hydrolysis of Ti(iso-OC₃H₇)₄ providing chemical bonding of anatase TiO₂ nanoparticles onto oxidized- or amino-functionalized multi-wall carbon nanotubes (MWCNT). The processes of functionalization of the MWCNT and the deposition of TiO₂ influence the photocatalytic activity of the synthesized nanocomposites. The phase composition, crystallite size, and the structural and surface properties of the obtained TiO₂/modified-MWCNT nanocomposite were analyzed from XRD, FEG-SEM, TEM/HRTEM and FTIR data, as well low temperature N₂ adsorption. In the photocatalytic study, the TiO₂/oxidized-MWCNT catalyst showed the highest and the TiO₂/amino functionalized-MWCNT catalysts somewhat lower degradation rates, indicating that the enhancement of photocatalysis was supported by the more effective electron transfer properties of the oxygen- than amino-containing functional groups, which support the efficient charge transportation and separation of the photogenerated electron-hole pairs.     

Synthesis of ternary zinc spinel oxides and their application in the photodegradation of organic pollutant

Ternary zinc spinel oxides such as Zn₂SnO₄, ZnAl₂O₄ and ZnFe₂O₄ were synthesized and characterized, and their activities in the photodegradation of phenol molecules were investigated. Zn₂SnO₄, ZnAl₂O₄ and ZnFe₂O₄ powders were synthesized by hydrothermal, metal–chitosan complexation and solvothermal routes, respectively. The face-centered cubic spinel structure of each material was confirmed by powder X-ray diffractometry (XRD) and its porous structure by N₂ adsorption–desorption isotherms. The characterization of spinels was complemented with Fourier transform infrared spectroscopy (FTIR) and X-rays fluorescence (XRF), revealing the formation of spinel structures with high purity. The photocatalytic activity in the degradation of phenol was observed only with Zn₂SnO₄ oxide. Mineralization degree of phenol molecules by Zn₂SnO₄ photocatalyst determined by total organic carbon analysis (TOC) reached 80% at 360 min under sunlight.     

α-Fe2O3/Pt hybrid nanorings and their enhanced photocatalytic activities

In this paper, α-Fe₂O₃ nanorings were synthesized and used as a support to further synthesize hybrid Pt/α-Fe₂O₃ nanorings. Transmission electron microscopy (TEM) analyses clearly suggest that Pt nanoparticles are deposited on the surface of α-Fe₂O₃ nanorings in a well-dispersed state. This metal–semiconductor hybrid system is expected to show high photocatalytic activity towards the degradation of environmental pollutants. Because of the Schottky contact between semiconductor and metal particles, the holes prefer to localize in energetically lower semiconductor, whereas the electrons can move through the heterointerface. This interfacial charge transfer and separation facilitate the redox reactions, results in a high photocatalytic activity. In our case, the as obtained α-Fe₂O₃/Pt hybrids exhibit enhanced photocatalytic performance with a degradation rate of 86.7% for methyl orange, which is much higher than that of pure α-Fe₂O₃ (33.2%).     

Photodegradation of phenol and 4-chlorophenol by BaO–Li2O–TiO2 catalysts

The catalytic photodegradation of phenol and 4-chlorophenol with white and UV light over TiO₂, BaTi₄O₉ and Hollandite catalysts has been studied in our laboratories. BaTi₄O₉ and Hollandite catalysts were prepared by solid state reaction at 900°C and 1200°C, respectively. All the catalysts were characterized by different techniques such as surface area measurements by the BET method, atomic absorption spectroscopy and XRD. Photodegradation reaction experiments were monitored by HPLC analysis. The reaction intermediates: hydroquinone and 1,4-benzoquinone were identified by GC–MS analysis. The photocatalytic activities of these catalysts in the degradation of phenol and 4-chlorophenol were evaluated in comparison with titanium oxide. Experimental results showed that BaTi₄O₉ and Hollandite catalysts exhibit small photocatalytic activity as compared with TiO₂.