Volume 13, Issue 3

In the past, TiO 2 particles have been considered non-toxic because of both their chemical inertness and the conclusions of epidemiological studies dealing with μm-sized particles. Recently, the assignment of diseases to inhalation of atmospheric particles, and the important industrial development of nm-sized particles - among which TiO 2 particles are prominent, principally because of their photocatalytic properties - have induced new safety research. This article provides the results of in vivo experiments showing that TiO 2 particles inhaled or swallowed can go through the biological barriers and cause damage to various organs. Obviously, the smaller the particle size, the easier the translocation within the body. There are indications that the TiO 2 allotropic form and various surface properties also influence the biological impairments. Although the experimental methodologies can sometimes be criticized, a danger is pointed out. To evaluate the ensuing risk, the release of particles from TiO 2 - containing substances and materials is discussed. Precautions and recommendations are presented, even though the potential inhalation of TiO 2 -containing particles appears to be minor compared with that of particles emitted by combustions, with the obvious exception of workers manufacturing or using TiO 2 particles.

This article reports on an exploratory statistical analysis of the photocatalytic activity of titanium(IV) oxide (TiO 2 ; titania) using forty commercially available titania samples. Five photocatalytic reactions were examined as test reactions for the evaluation of photocatalytic activity of the samples: oxygen liberation along with silver deposition, methanol dehydrogenation, acetic acid decomposition, acetaldehyde decomposition and synthesis of pipecolinic acid from L-lysine. Six structural and physical properties of the samples were measured: specific surface area (BET), density of defective sites (DEF), primary particle size (PPS), secondary particle size (SPS) and existence of anatase (ANA) and rutile (RUT) phases. Correlations of photocatalytic activities for the five kinds of reactions with structural and physical properties of 35 (out of 40) titania samples were obtained, and photocatalytic activities were empirically reproduced by a linear combination of six properties with good to fair reliability. While a portion of the results could be interpreted using a conventional mechanism, significant activity dependence on properties, not yet disclosed, was suggested.

There are inconsistencies in the literature regarding the optimum particle size for TiO 2 in photocatalytic oxidation reactions in aqueous slurries. The apparent optimum particle size has been reported as low as 3 nm and ranges to greater than 30 nm. It was hypothesized that the inconsistencies reported in the literature stem from differences in the photocatalyst, the reactor system and the operating parameters used, among other factors. In this study, a mathematical model was developed to elucidate the effects of selected operating parameters on the apparent optimum particle size in aqueous-phase TiO 2 photocatalysis. The model utilizes five parameters, which were varied to best fit one set of experimental data: the photocatalytic oxidation of dimethyl methylphosphonate (DMMP) in water. For this set of experimental data, the apparent optimum particle size was approximately 25 nm, and the model parameters were set to fit this set of data. The sensitivity of the model to the model parameters and operating conditions were assessed, and the theoretical trends based upon model calculations were supported by observed trends of experimental data reported in the literature. It was found that the apparent optimum particle size can change significantly with TiO 2 concentration and depth of slurry perpendicular to the incident light. This supports that the light scattering model parameter, k scat , is the most significant model parameter that determines the apparent optimum particle size.

The TiO 2 photoassisted oxidation and ultimate mineralization of the cationic benzyldodecyldimethylammonium chloride (BDDAC) and the anionic sodium dodecylsulfate (DoS) surfactants in aqueous media were re-visited as part of our systematic examination of their cationic/anionic binary BDDAC/DoS complexes of variable compositions. The processes were monitored by surface tension measurements, by total organic carbon (TOC) assays, carbon dioxide evolution, and HPLC analyses. Some hydroxylated intermediates of the BDDAC system were identified by time-of-flight mass spectral (TOF-MS) techniques. Oxidative degradation of the anionic surfactant was significant involving prior adsorption of the species on the positively charged metal-oxide particle surface. By contrast, the degradation of the cationic surfactant was rather limited owing to a lesser extent of adsorption on the TiO 2 surface under otherwise identical conditions. The mineralization yield assayed by CO 2 evolution was ca. 80% for DoS and 47% for BDDAC after 180 min of UV irradiation. Evolution of CO 2 gas for DoS was 4-fold faster (k = 1.2 × 10 -2 min -1 ) than for BDDAC (k = 0.31 × 10 -2 min -1 ). Mineralization of the binary mixed system was also faster (k = 0.52 × 10 -2 min -1 ) than for the BDDAC systemalone, but 2-fold slower than for DoS alone. Initial intermediates identified by TOF-MS techniques were the mono-, di-, and tri-hydroxylated BDDAC species followed by several other intermediates with m/z greater than and less than the mass peaks of the two surfactant substrates.

The visible light activity of TiO 2 (Degussa P25) particles was improved by non-metal-doping. In order to characterize and describe the effect of anion-doping on the electronic and structural properties of TiO 2 , a combination of experimental structural methods and DFT calculations were used. A series of N, C and S-doped photocatalysts with different dopant contents were prepared by an incipient wet impregnation method, followed by calcination at three different temperatures for 1h, 3h and 5h. An obvious extension of the absorption to the visible region was observed by UV-DRS. The doped photocatalysts were characterized by XRD, XPS and Raman spectroscopy. The morphological structure of the photocatalysts was examined by HR-TEM. In the computational part of the study, a neutral, stoichiometric cluster Ti 9 O 27 H 18 cut from the anatase bulk structure and new models for the substitutional and interstitial anionic and cationic-doped TiO 2 were developed. The DFT calculations were carried out by the hybrid B3LYP functional, by using double-zeta, LanL2DZ basis set. The experimental results combined with DFT calculations, indicate that substitutional N, interstitial cationic C, and substitutional cationic S-doping of TiO2 occur in the as-prepared samples. The band-gap reduction arises from the contribution of N 2p, C 2p and S 3p orbitals to the O 2p and Ti 3d states in the VB of TiO 2 , as well as the presence of C and S induced mid-gap states in the band-gap. The photocatalytic activity of the doped TiO 2 photocatalysts was also determined by investigating the kinetics of the photocatalytic degradation of 4-nitrophenol under UV-A and solar light irradiation.

Nitrogen-doped TiO 2 nanoparticles were prepared by sol-gel method and characterized by XRD, BET, TG-DTA, FE-SEM, DLS, DRS, and XAS. UV-vis diffuse reflectance spectra showed that nitrogen doped TiO 2 exhibited obvious absorption in visible light range. With increasing calcination temperature, (1) the nitrogen adsorption- desorption isotherms shifted from a combination of types I and IV to type V; (2) the hysteresis loops shifted to the region of higher relative pressure, and the areas of the hysteresis loop decreased; (3) the pore size increased and pore volume decreased; (4) the absorbance increased; (5) the peak at 396.4 eV (O-Ti-N linkages within the crystalline lattice) increased; (6) calcination temp. > 600 °C, the peak at 396.4 eV decreased. With the increase in calcination temperature, the photocatalytic activity increased at first and then decreased. When the calcination temperature was at 600 °C, the photo-degradation rate was the highest. The substitutional N, the crystalline phase and the size of N-doped TiO 2 are the predominant factors to improve the photocatalytic activity of C 2 H 4 degradation.

The photocatalytic activity of titanium dioxide modified by surface fluorination (F-TiO 2 ) and by differently deposited gold nanoparticles (Au/TiO 2 ) was tested in the degradation of organic substrates, i.e. mainly formic acid and the azo dye Acid Red 1 (AR1), in comparison to that of unmodified titania. The effect of different noble metals deposition on TiO 2 was also investigated in the photocatalytic production of hydrogen from water solutions. Surface fluorination increases the rate of photocatalytic oxidation proceeding through a hydroxyl radical-mediated path. These radicals accumulated in higher amount on F-TiO 2 , because of the shielding effect of adsorbed fluoride. By contrast, the degradation of substrates implying direct interaction with the photogenerated holes occurred at a lower rate on F-TiO 2 , because of their hampered absorption on the fluorinated surface. The method employed to reduce Au(III) to metallic gold in the preparation of Au/TiO 2 photocatalysts was found to affect their photoactivity, also by modifying the surface properties of TiO 2 . The presence of gold on TiO 2 facilitates both the electron transfer to O2 and the mineralization of formic acid, proceeding mainly through direct interaction with photoproduced valence band holes. For substrates undergoing mainly oxidation through a hydroxyl radical mediated mechanism, the photogenerated holes may partly oxidize gold nanoparticles, which consequently act as recombination centers of photoproduced charge carriers. Photocatalytic hydrogen production tests, either from water or from methanol reforming, performed in a special set up, closed recirculation laboratory scale photoreactor employing titanium dioxide and noble-metals-modified titanium dioxide samples prepared by different techniques, confirmed the main role of noble metals nanoparticles on TiO 2 in facilitating electron transfer reactions also under anaerobic conditions. Higher hydrogen production rates were attained with the photocatalysts in contact with the vapor phase, i.e. in the absence of the liquid phase mass transfer limitations of aqueous suspensions. The activity of metal nanoparticles-containing photocatalysts prepared by one step flame spray pyrolysis was always higher than that of photocatalysts prepared by deposition on pre-formed gold nanoparticles on TiO 2 , most probably because of the better noble metal dispersion on the photocatalyst surface.

Photocatalysis belongs to one of the Advanced Oxidation Processes (AOP). These processes make possible the decomposition of organic compounds to carbon dioxide and water. The most useful photocatalyst is titanium dioxide which, to make it active, requires irradiation with suitable wavelengths. Pure titanium dioxide can be activated only by ultraviolet light irradiation (UV). For a long time, studies have been carried on modified titanium dioxide materials to obtain more effective photocatalysts with greater activity under UV light irradiation or to obtain photocatalysts which may be active also under visible light irradiation. One of the possible ways for modifying TiO 2 is carbon modification. This type of modification increases the adsorption capacity, and activation of the photocatalysts is then possible under visible light; moreover, the lifetime of the material is longer. What type of carbon-modified material is desired will depend on different preparation techniques so as to control the desired properties of the carbon-modified TiO 2 .

Organic synthesis through semiconductor photocatalysis has become an important research area in photochemistry in the last two decades. Significant examples of organic transformations employed for synthetic purposes are oxidation and reduction reactions, isomerization reactions, C-H bond activations and C-C and C-N bond forming reactions. In this review, the use of irradiated semiconductors and different operation conditions in the reduction of organic compounds is surveyed. Here, the photocatalytic reduction of nitroarenes, aldehydes, ketones and miscellaneous compounds will be discussed. This review is concluded with some perspectives of our group on the directions toward which future research may be aimed.

Various standards have been published in an effort to rationalize and unify the evaluation and quantification of photocatalytic activity. The experimental conditions stated in the published standards and test methods differ in many aspects that make it very difficult to compare different results. This paper focuses on the influence of the different parameters involved in the photocatalytic process. The photocatalytic active material consists of white mortar, surface coated by TiO 2 . The activity has been determined by quantifying the photocatalytic oxidation of NO. In the studied concentration interval the kinetics was estimated by a first order reaction. The influence on the activity by the relative humidity, temperature, irradiance, contaminant concentration and catalyst load has been studied. Most influent parameters are the catalyst load and light intensity until reaching plateau values at higher amount and intensity. Humidity influence the activity above 40% RH and an increased temperature lowers the activity. A global empirical correlation, including the mentioned variables, has been developed, allowing the determination of the photocatalytic activity at different environmental conditions (within the constraints of this research) and, when necessary, for comparison with reference conditions.