Volume 10, Issue 1

A traditional and popular Japanese house decorating lime tile, commercially available “Limix”, was used to photodecompose the formaldehyde after coating the tile with a titania sol, which solves the problem of sick-building syndrome. A total of five kinds of lime tile samples with titania sol coating and without coating were examined by using a flow-type photoreactor based on the JIS standard to test their photocatalytic activities. The photocatalytic mechanism in the coated lime tile was also analyzed by Electron Paramagnetic Resonance Spectroscopy (EPR). The results show that a high removal efficiency of formaldehyde was achieved when using the lime tile combined with commercially available zeolite compared to the lime tile immobilized titania photocatalyst. The EPR spectrum indicates that oxygen centered radicals and surface trapped holes are present in the titania sol, but it is much different from the spectrum observed in the pure anatase and Degussa P25 titania. It means that there are different characteristics among these systems on the photocatalysis mechanism.

The presented studies have focused on the application of photocatalysis in the degradation of azo dye Direct Green 99 (DG99) under artificial solar light irradiation. The comparison of three different TiO 2 photocatalysts was performed: TiO 2 Aeroxide Ⓡ P25 (Degussa, Germany) and prepared photocatalysts based on the metatytanic acid (TiO(OH) 2 ) (Police, Poland) before and after modification with ammonia. TiO 2 -P25 and TiO(OH) 2 -100 showed higher adsorption of DG99 than TiO(OH) 2 -300NH 3 . After adsorption the samples were transferred to a new aqueous solution of DG99 and then the process of decomposition under artificial solar light was carried out. The prepared photocatalysts showed much higher photoactivity than the commercial P25. Extending the time of irradiation to 24 h caused complete decolorisation of DG99 on TiO(OH) 2 -100 and on TiO(OH) 2 -300NH 3 and 52% on P25. The in situ measurements of UV-VIS/DR spectra of the photocatalysts revealed that regeneration of the surface of the spent samples under artificial solar light irradiation is possible, the time needed for the regeneration of those photocatalyst differed among the samples, being longer for TiO(OH) 2 -100 than for TiO(OH) 2 -300NH 3 .

Anatase type TiO 2 , ST-01 (Ishihara Sangyo Co. Ltd, Japan) was modified by mixing and heating with the powder of iron(II) oxalate in air and argon to obtain Fe-TiO 2 and Fe-C-TiO 2 photocatalysts, respecttively. TiO 2 as-received and the prepared photocatalysts were used for the phenol decomposition under UV irradiation with an addition of H 2 O 2 as a Fenton reagent. A different mechanism of phenol oxidation on Fe-TiO 2 and Fe-C-TiO 2 samples was observed: on the former, the radical reaction was dominant, whereas on the latter, a complex reaction with iron occurred. A different interaction of phenol molecules and its oxidation products with the surface of TiO 2 , Fe-TiO 2 and Fe-C-TiO 2 samples was observed by FTIR studies. Formed upon the phenol decomposition, hydroquinone is more likely adsorbed on the surface of Fe-C-TiO 2 than on TiO 2 or Fe-TiO 2 and is suggested that it affects the rate of phenol decomposition by the reduction of iron(III) to iron(II) and formation of a ternary HQ-Fe-H 2 O 2 complex.

Three types of titania supported materials including titanium dioxide and silicon dioxide composite, titania-coated activated carbon and titania-coated glass beads were prepared and used as photocatalysts to remove toluene from an air stream. Their surface areas were analysed. TEM image reveals titania-silica composites were nanostructured aggregates. XRD was used to determine their crystalline phase which was 100% anatase for the titania component. A fixed bed reactor was designed and built in the laboratory, the toluene with initial concentration of 300 ppm (1149 mg/m 3 ) was fed into the reactor, the destruction efficiencies of toluene were determined by the gas analyser. It was also found that TiO 2 -SiO 2 aggregates with high surface area (421.1 m 2 /g) achieved high destruction efficiencies. The combined effects of adsorption and photocatalysis were further studied by comparing the performance of pure activated carbon (surface area of 932.4 m 2 /g) and TiO 2 coated activated carbon with BET surface area of 848.4 m 2 /g. It was found that the TiO 2 coated activated carbon demonstrated comparable results to pure activated carbon, and most importantly, the TiO 2 -coated activated carbon can be effectively regenerated by UV irradiation, and was reused as adsorbent. The experimental result of titania-coated glass beads demonstrated a steady degradation efficiency of 15% after a period of 17 hours. It helped to understand that photocatalysis degradation ability of the TiO 2 was constant regardless of the adsorption capacity of the catalysts. This photocatalytic property can be used to degrade the adsorbed toluene and regenerate the catalyst. This study revealed that if the experiments were designed to use adsorption to remove toluene and followed by regeneration of adsorbent using photocatalysis, it could achieve a very high removal efficiency of toluene and reduce the regeneration cost of saturated adsorbent.

The immobilization technique is important to extend the application field of a photocatalyst. The titanium plate surface was changed into a TiO 2 thin film by the anodizing process. The anodized photocatalyst reactivity increased with relative humidity, but excessive humidity led to a decrease of the reaction rate. Production of CO 2 is fully indicative of the oxidation process that was shown to prevail in different organic compounds examined. The electrochemical effect enhanced the decomposition rate of the organic compounds, but excessive applied voltage decreased the reaction rate. In a comparative study of the sol-gel coated versus anodized TiO 2 , the anodized TiO 2 in the presence of the anodic potential discharge showed much better decomposition rate than the sol-gel coated TiO 2 .

The kinetics of the photocatalytic degradation of benzoic acid in the presence of TiO 2 has been investigated experimentally and theoretically. The effects of catalyst loading, initial concentration of benzoic acid and the electron acceptors; H 2 O 2 , K 2 S 2 O 8 and OXONE on the degradation rate have been examined. A pseudo-first order kinetic model has been used to describe the results. With the intention of predicting the primary intermediates and the product distribution, geometry optimizations of the reactants, product radicals and transition state complexes have been performed with the semi-empirical PM3 and DFT/B3LYP/6-31G* methods. Solvation effects have been computed by DFT calculations at the B3LYP/6-31G* level using COSMO as the solvation model. Based on the results of the quantum mechanical calculations, the rate constants of the four possible reaction paths have been calculated by means of Transition State Theory. A branching ratio for each of the reaction paths has been calculated and the product distribution for the degradation reaction has been determined. The results show that ortho-, meta- and para-hydroxybenzoic acids are formed as the primary intermediates in the photocatalytic degradation of benzoic acid.

The kinetics of the photocatalytic degradation reactions of twelve ortho/para mono-substituted phenols containing electron-donating or -withdrawing groups have been investigated experimentally. With the intention of determining the most suitable DFT reactivity descriptors, geometry optimizations of the compounds have been performed with the Density Functional Theory DFT at B3LYP/6-31G* level. In order to take the effect of solvent water into account, the calculations have been repeated for the optimized structures by using COSMO as the solvation model. Chemical hardness, softness, electronegativities, Fukui functions, local hardness and softness, local electrophilicities and softness differences for all phenol molecules have been calculated. Correlations between the apparent initial first-order rate constants determined in the experiments and the calculated DFT reactivity descriptors have been examined. Results show that the reactions investigated are electrophilic in nature. The local softness and softness differences correlate well with the reaction rates indicating that soft-soft interactions dominate in the photocatalytic degradation reactions of phenols.

The photocatalytic degradation reaction of 1,3-dihydroxybenzene has been modeled theoretically. With the intention of predicting the primary intermediates and the product distribution, geometry optimizations of the reactants, the product radicals and transition state complexes have been performed with the semi-empirical PM3 method. The molecular orbital calculations have been carried out by an SCF method using RHF or UHF formalisms. Solvation effects have been computed by DFT calculations at the B3LYP/6-31G* level using COSMO as the solvation model. Based on the results of the quantum mechanical calculations, the rate constants of the four possible reaction paths have been calculated by means of Transition state Theory. Three predictors have been determined for the prediction of the most probable transition state and the reaction path. A branching ratio for each of the reaction paths has been calculated and the most probable intermediate has been determined. Finally, the results obtained have been compared to the experimental results in order to assess the reliability of the proposed model.

The effect of the variables: initial pH (4-10), concentration of the dye (3-17 ppm), irradiation wavelength (254-360 nm) and sustrate: H 2 O 2 molar ratio (1:1-1:9) on the photocatalytic degradation of malachite green using TiO 2 immobilized on glass Raschig rings was studied. The analysis was planned as a fractional factorial design in order to determine the best affordable experimental conditions to diminish the concentration of this highly toxic and persistent dye, and to use the data as a starting point for the removal of this compound from polluted industrial waters present in industrial salmon cultures found in southern lakes of Chile. Interestingly, the best conditions were achieved when the solution was irradiated with 360 nm wavelength under alkaline conditions arising from the results a negligible effect of the H 2 O 2 , making possible to consider the use of solar light to carry on this process.

Removal of natural organic matter (NOM) is always a concern in water treatment, particularly the formation of chlorinated organic compounds such as trihalomethane (THM) - toxic by-products resulting from the chlorination and disinfection of residual organic compound. TiO 2 photocatalysis has been exhibited as a promising technology for complete mineralization of various organic matter in water treatment. In this study, the adsorption and photodegradation of humic acids (HA) by homemade nano-structured TiO 2 was investigated. It was found that high HA adsorption on the homemade nano-structured TiO 2 was achieved at low pH condition and increased cation strength. The adsorption isotherm of HA on TiO 2 was well modeled with Langmuir and Freundlich model. The adsorption kinetics of HA on TiO 2 was found to follow a two-stage process: diffusion-controlled process and attachment-controlled process. Lower pH values and increased Ca 2+ concentration would enhance the adsorption kinetic of HA on TiO 2 . However high adsorption of HA on TiO 2 did not show significant effect upon the photo-degradation rate of HA.

Τhe polyoxometalate anions (POM) PW 12 O 40 3- and SiW 12 O 40 4- are proposed as efficient photocatalysts for the reductive degradation of the azo dyes Acid Orange 7 (AO), Naphthol Blue Black (NB) and Disperse Blue 79 in aqueous or ethanol solutions. The process involves absorption of light by polyoxometalates, oxidation of an organic sacrificial electron donor, for instance propan-2-ol or ethanol, and the subsequent reductive destruction of the azo dye by reduced POM with the concomitant recycling of the POM catalyst to the oxidized form. Preliminary results show that the dyes are reduced to aromatic amine derivatives. Increase of catalyst and/or propan-2-ol concentration accelerates the photodegradation of the dyes until a saturation value, while an optimum concentration of the added dye is usually required, due to an inner filter effect. The kind of POM catalyst dictates the efficiency of dye decoloration, following the order PW 12 O 40 3- > SiW 12 O 40 4- in one-pot photolysis experiments ((dye/propan-2-ol or ethanol/POM). The POM that is more efficient in photooxidizing the organic electron donor is also more efficient in reducing the azo dye due to the greater amounts of reduced catalyst that are formed. The presence of electron scavengers suppresses the degradation rate of dyes according to the trend Cu 2+ > O 2 > Ni 2+ .

CdO + CdTiO 3 thin films were prepared on glass substrates by means of the sol-gel technique. Two Ti/Cd ratios, 0.54 and 0.67, in the precursor solutions were studied. After the dipping process, the samples were sintered in an open atmosphere during one hour at selected temperatures (T s ) in the range of 350-600 °C. The variation of T s was in steps of 50 °C. X-ray diffraction, UV-Vis optical absorption, and atomic force spectroscopy techniques were employed for characterization. The photocatalytic activity (PA) was measured by means of the photobleaching of an aqueous solution of methylene blue (MB) after irradiation with an UV lamp. The MB concentration was determined by using optical absorption spectra, measured before and after the photobleaching process, and the Lambert-Beer principle. The best PA was obtained for films sintered at 500 °C for both Ti/Cd ratios studied, as for this T s value the PA was very similar.

The photocatalytic activity of Zn 2 TiO 4 + ZnO thin films by the photobleaching of methylene blue (MB) in aqueous solution, under an UV irradiation, is studied. The films were prepared on glass slide substrates by the sol-gel technique starting from a simple novel precursor solution, which is based on the preparation of both ZnO and TiO 2 precursor solutions separately, and subsequent mixing at room temperature. In order to change the films’ composition, several precursor solutions were prepared, each with different Ti/Zn ratio in the solution. The Ti/Zn ratio was in the 0.18 to 2.13 range and for each composition; two sintering temperatures were used, 450 °C and 550 °C. X-ray results show that the films are mainly constituted of the Zn 2 TiO 4 compound (cubic phase). The best photocatalysis was found for a Ti/Zn ratio of 0.5 after an irradiation time of 5 hours.

Two photoactive TiO 2 -supported materials are considered for toluene degradation in the gas phase. A kinetic study is performed over these two catalysts. The mechanisms involved are well represented by the Langmuir-Hinshelwood (L-H) relation at initial time. The kinetic rates and adsorption constants are then calculated to compare the catalysts. Particular attention focuses on the role of the humidity on the photocatalytic activity. Conversion rates of the toluene into carbon dioxide are measured and reveal a singular difference between the catalysts. Benzaldehyde and benzoïc acid are identified as major by-products in the gas phase. The production of stable intermediates is also shown. It is concluded that not only the presence of water vapor but also the characteristics of the titanium materials have to be taken into account to explain the kinetic behavior of photocatalytic oxidation.

The present work follows a study that permitted to enlighten the influence of TiO 2 coating types on the role of water vapor during Methyl Ethyl Ketone photocatalytic oxidation. For catalysts constituted of a film of TiO 2 , the formation of a water layer at the catalyst surface was suggested. Consequently, for this kind of catalysts, not only the adsorption of pollutants on TiO 2 but also their dissolution in water may influence their degradation. Further experiments were then performed to further examine this aspect. A photocatalyst composed of a thin film of TiO 2 was used. Three VOCs having different aqueous solubilities were tested at various initial concentrations and a relative humidity of either 0 or 30%. The experimental results were modelled by the Langmuir-Hinshelwood relation at the initial time considering that the surface reaction was the limiting step of the reaction. For sufficiently low initial concentrations (C 0 ), the adsorption term (K A C 0 ) in the denominator could be neglected. The constants k′ = k s K A (k s is the Langmuir-Hinshelwood kinetic constant and K A is the equilibrium adsorption constant) were determined by linear regressions. The influence of the aqueous solubility was then assessed by the behavior of the ratio k′ humid /k′ dry .

Most fruits and vegetables have to be stored in cold and controlled atmospheres. Vegetables and climacteric fruits are very sensitive to the presence of ethylene gas, which acts as a ripening hormone and plays a regulatory role in many plant metabolic processes. Produce and flowers display ethylene receptors at their surface. Their actuation promotes ethylene production by the fruit itself, accelerating its ripening and aging. One way to moderate ethylene production is to slow down the metabolism by keeping the produce in a controlled atmosphere (2% oxygen, 2-5% carbon dioxide, 93-96% nitrogen). In this study, the control of ethylene concentration is performed in ambient air by using a photocatalytic converter, which mineralises ethylene into water and carbon dioxide. This device appears, therefore, as a flexible and inexpensive alternative to the catalytic converter for small producers.