Enhanced photocatalytic degradation of VOCs using Ln3+-TiO2 catalysts for indoor air purification

Photodegradation of selected PCBs in the presence of Nano-TiO2 as catalyst and H2O2 as an oxidant

Photodegradation of five strategically selected PCBs was carried out in acetonitrile/water 80:20. Quantum chemical calculations reveal that PCBs without any chlorine on ortho-positions are closer to be planar, while PCBs with at least one chlorine atoms at the ortho-positions causes the two benzene rings to be nearly perpendicular. Light-induced degradation of planar PCBs is much slower than the perpendicular ones. The use of nano-TiO₂ speeds up the degradation of the planar PCBs, but slows down the degradation of the non-planar ones. The use of H₂O₂ speeds up the degradation of planar PCBs greatly (by >20 times), but has little effect on non-planar ones except 2,3,5,6-TCB. The relative photodegradation rate is: 2,2′,4,4′-TCB > 2,3,5,6-TCB > 2,6-DCB ≈ 3,3′,4,4′-TCB > 3,4′,5-TCB. The use of H₂O₂ in combination with sunlight irradiation could be an efficient and “green” technology for PCB remediation.

Improved catalytic capability of mesoporous TiO2 microspheres and photodecomposition of toluene

A series of mesoporous titanium dioxide microspheres were calcined at various temperatures to improve their photocatalytic activity. The prepared catalysts were characterized by XRD, SEM, TEM, TG-DTA, UV-vis, and N(2) adsorption-desorption measurements, and their photocatalytic performances were investigated by photooxidation of gaseous toluene. As the results revealed, calcination temperatures obviously influenced the surface morphology and photocatalytic activity of the mesoporous TiO(2) microspheres. The noncalcined samples had a mesoporous structure of the anatase phase. The sample calcined at 400 °C showed a superior photocatalytic performance, which had a reaction rate constant 2-fold higher than that of P(25). The enhanced photoreactivity is possibly due to the synergetic effects of the mesoporous structure and light-transmittance ability of the catalysts. Two new reaction intermediates were discovered as well, and a tentative degradation pathway was proposed.

Photocatalytic degradation of gaseous acetone, toluene, and p-xylene using a TiO2 thin film

A nano-structured TiO(2) thin film immobilized on glass springs was prepared by the sol-gel method, and characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Acetone, toluene and p-xylene were chosen as common VOCs for a photocatalytic degradation study of both mixed and pure gases using the TiO(2) thin film. Addition of hydrogen peroxide promoted activation of the catalyst during acetone degradation. The effects of gas flow rate and UV light wavelength were investigated with the pure gases. Gas flow rate greatly influenced the degradation. The highest degradation rates were 77.7% (at 3 L/min) for acetone, 61.9% (at 3 L/min) for toluene, and 55% (at 7 L/min) for p-xylene. A UV light wavelength of 254 nm provided greater degradation of the VOCs than 365 nm UV light. The degradation rates of p-xylene and acetone in the gas mixture were lower than those of pure p-xylene and acetone. The opposite trend was observed for toluene. Acetone, both in the mixed gas and pure, had the highest degradation efficiency. Acetone, toluene and p-xylene degradation followed Langmuir-Hinshelwood kinetics.

Efficacy of photocatalytic HEPA filter on microorganism removal

This study assessed the application of photocatalytic oxidation (PCO) to the high efficiency particulate air (HEPA) filter for disinfection of airborne microorganisms. Experiments were conducted at two TiO2 loadings (1870 +/- 169 and 3140 +/- 67 mg/m(2)) on the HEPA filter irradiated with UV-A at the intensity of 0.85 +/- 0.18 or 4.85 +/- 0.09 mW/cm(2) under two relative humidity conditions (45 +/- 5% and 75 +/- 5%). Inactivation and penetration of four microorganisms were tested, including Aspergillus niger, Penicillium citrinum, Staphylococcus epidermidis, and Bacillus subtilis. It was found that microorganisms retained on a photocatalytic filter were inactivated around 60-80% and even 100% for S. epidermidis when the PCO reactions occurred. Lower penetration was also found from the photocatalytic filter for all airborne microorganisms. High humidity decreased photocatalysis efficacy. Increasing TiO2 loading or irradiance intensity did not substantially affect its disinfection capability.

PRACTICAL IMPLICATIONS

The high efficiency particulate air filter is used widely to remove particulates and microorganisms from the air stream. However, the filter may become a source of microbes if those retained microorganisms proliferate and re-entrain back into the filtered air. This study demonstrates that such a problem can be handled effectively by using photocatalytic reactions to inactivate those confined microorganisms. A 60-100% microbe reduction can be achieved for a wide variety of microorganisms to provide better indoor air quality for hospitals, offices, and domestic applications.

Reaction kinetics of photocatalytic degradation of sulfosalicylic acid using TiO2 microspheres

The photocatalytic (PC) degradation kinetics of sulfosalicylic acid (SSA) at different pH using TiO2 microspheres were elucidated by modeling. The resultant model had special consideration of adsorption and pH. The adsorption isotherms showed that the LC/MS(2)-identified intermediates were weakly adsorbed on the TiO2 microspheres, thus their adsorption was neglected in the modeling. By contrast, the SSA was significantly adsorbed, thus its adsorption retained as an item in the model. Consequently, a non-first-order model was obtained. Through the modeling, it was elucidated that the reaction rate increased non-linearly with the SSA adsorption equilibrium constant. Meanwhile, it was elucidated that a pH increase favored the hydroxyl radical production to accelerate the SSA degradation, while impeded the SSA adsorption to slower it, hence a neutral pH caused the fastest SSA degradation.

Chemical absorption process for degradation of VOC gas using heterogeneous gas-liquid photocatalytic oxidation: toluene degradation by photo-Fenton reaction

A novel process for degradation of toluene in the gas-phase using heterogeneous gas-liquid photocatalytic oxidation has been developed. The degradation of toluene gas by photo-Fenton reaction in the liquid-phase has experimentally examined. The photo-Fenton reaction in the liquid-phase could improve the overall toluene absorption rate by increasing the driving force for mass transfer and as a result enhance the removal of toluene in the exhaust gas. The toluene concentrations in the inlet gas were varied in the range from 0.0968 to 8.69 g m(-3) with initial hydrogen peroxide concentration of 400 mg l(-1) and Fe dose of 5.0 mg l(-1). It was found that toluene in the inlet gas was almost completely dissolved into water and degraded in the liquid-phase for the inlet toluene gas concentration of less than 0.42 g m(-3). The dynamic process of toluene gas degradation by the photo-Fenton reaction providing information for reaction kinetics and mass transfer rate was examined. Toluene removal kinetic analysis indicated that photo-Fenton degradation was significantly affected by H(2)O(2) concentration. The experimental results were satisfactorily described by the predictions simulated using the simplified tanks-in-series model combined with toluene removal kinetic analysis. The present results showed that the proposed chemical absorption process using the photo-Fenton heterogeneous gas-liquid photocatalytic oxidation is very effective for degradation of volatile organic gases.

Effects of peptizing conditions on nanometer properties and photocatalytic activity of TiO2 hydrosols prepared by H2TiO3

TiO2 hydrosols were prepared from metatitanic acid (H2TiO3) by chemical precipitation-peptization method under various peptizing conditions. The effects of peptizing conditions on nanosized properties and photocatalytic activity of TiO2 hydrosols were investigated. The crystal structure, crystallinity, particle size distribution, and transparency (T%) of as-obtained hydrosols were characterized by means of X-ray diffraction, transmission electron microscopy, light-scattering size analyzer, and UV-vis transmittance spectra. The results showed that the properties of hydrosols depended on peptizing conditions including a molar ratio of H+/Ti, temperature, and solid content. The photoactivity of TiO2 hydrosols was evaluated in terms of the degradation of rhodamine B (RhB) in aqueous solution, and formaldehyde (HCHO) and methyl mercaptan (CH3SH) in gaseous phase. The results showed that increase in H+/Ti ranging 0.19-0.75 led to the decrease in particle size and the increase in transparency. With increasing of temperature, particle sizes increased while the transparency and photoactivity decreased steadily when the temperature was higher than 65 degrees C. The particle size, transparency and photoactivity of the hydrosols hardly depended on solid content when it was not less than 2%. It should be confirmed that the hydrosols with higher crystallinity, smaller particle size and higher transparency could have the higher photoactivity for the degradation of RhB, CH3SH, and HCHO. In this study, the optimal peptizing conditions were determined to be H+/Ti=0.75, temperature=65 degrees C and solid content=2-6%.

Volatile organic compounds in indoor environment and photocatalytic oxidation: state of the art

Volatile organic compounds (VOCs) are the major pollutants in indoor air, which significantly impact indoor air quality and thus influencing human health. A long-term exposure to VOCs will be detrimental to human health causing sick building syndrome (SBS). Photocatalytic oxidation of VOCs is a cost-effective technology for VOCs removal compared with adsorption, biofiltration, or thermal catalysis. In this paper, we review the current exposure level of VOCs in various indoor environment and state of the art technology for photocatalytic oxidation of VOCs from indoor air. The concentrations and emission rates of commonly occurring VOCs in indoor air are presented. The effective catalyst systems, under UV and visible light, are discussed and the kinetics of photocatalytic oxidation is also presented.

Preparation, characterization and photocatalytic activities of boron- and cerium-codoped TiO2

Boron- and cerium-codoped TiO2 photocatalysts were synthesized using modified sol-gel reaction process and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), particle size distribution (PSD), diffuse reflectance spectra (DRS), and Brunauer-Emmett-Teller (BET). The photocatalytic activities were evaluated by monitoring the degradation of dye Acid Red B (ARB). The results showed that the prepared photocatalysts were mixed oxides mainly consisting of titania, ceria, and boron oxide. The structure of TiO2 could be transformed from amorphous to anatase and then to rutile by increasing calcination temperature; the transformation being accompanied by the growth of particle size without any obvious change in phase structure of CeO2. The XPS of B(1.6)Ce(1.0)-TiO2 prepared at 500 degrees C showed that a few boron atoms were incorporated into titania and ceria lattice, whereas others existed as B2O3. Cerium ions existed in two states, Ce3+ and Ce4+, and the atomic ratio of Ce3+/Ce4+ was 1.86. When boron and cerium were doped, the UV-Vis adsorption band wavelength showed an obvious shift toward the visible range (< or =526 nm). As the atomic ratio of Ce/Ti increased to 1.0, the absorbance edge wavelength increased to 481 nm. The absorbance edge wavelength decreased for higher cerium doping levels (Ce/Ti = 2.0). The particles size ranged from 122 to 255 nm with a domain at 168 nm (39.4%). The degradation of ARB dye indicated that the photocatalytic activities of boron- and cerium-codoped TiO2 were much higher than those of P25 (a standard TiO2 powder). The activities increased as the boron doping increased, whereas decreased when the Ce/Ti atomic ratio was greater than 0.5. The optimum atomic ratio of B/Ti and Ce/Ti was 1.6 and 0.5, respectively.

Heterogeneous photocatalytic decomposition of benzene on lanthanum-doped TiO2 film at ambient temperature

Lanthanum-doped anatase TiO2 thin films on glass prepared via a sol-gel process have been shown to have much higher photocatalytic activity for the degradation of gaseous benzene than pure anatase TiO2 thin film. The photodecomposition of benzene on both types of TiO2 films follows the first-order kinetics while the CO(2) and CO formation followed the zero-order kinetics. GC/MS identification of the intermediates produced during the photodegradation of benzene revealed that doping lanthanum into TiO2 thin film favors a cleavage of benzene ring. An optimal lanthanum amount with respect to photocatalytic activity was about 2.5 wt% (La2O3/TiO2).