Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

TiO2 photocatalysis and related surface phenomena

http://publicationslist.org/data/meng.chong/ref-18/sdarticle.pdf

Recent Developments in Photocatalytic Water Treatment Technology: A Review

Photoinduced reactivity of titanium dioxide

Fujishima and Honda (1972) demonstrated the potential of titanium dioxide (TiO2) semiconductor materials to split water into hydrogen and oxygen in a photo-electrochemical cell. Their work triggered the development of semiconductor photocatalysis for a wide range of environmental and energy applications. One of the most significant scientific and commercial advances to date has been the development of visible light active (VLA) TiO2 photocatalytic materials. In this review, a background on TiO2 structure, properties and electronic properties in photocatalysis is presented. The development of different strategies to modify TiO2 for the utilization of visible light, including non metal and/or metal doping, dye sensitization and coupling semiconductors are discussed. Emphasis is given to the origin of visible light absorption and the reactive oxygen species generated, deduced by physicochemical and photoelectrochemical methods. Various applications of VLA TiO2, in terms of environmental remediation and in particular water treatment, disinfection and air purification, are illustrated. Comprehensive studies on the photocatalytic degradation of contaminants of emerging concern, including endocrine disrupting compounds, pharmaceuticals, pesticides, cyanotoxins and volatile organic compounds, with VLA TiO2 are discussed and compared to conventional UV-activated TiO2 nanomaterials. Recent advances in bacterial disinfection using VLA TiO2 are also reviewed. Issues concerning test protocols for real visible light activity and photocatalytic efficiencies with different light sources have been highlighted.

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A review on the visible light active titanium dioxide photocatalysts for environmental applications

Heterogeneous photocatalysis is a process of great potential for pollutant abatement and waste treatment. In order to improve the overall performance of the photoprocess, heterogeneous photocatalysis is being combined with physical or chemical operations, which affect the chemical kinetics and/or the overall efficiency. This review addresses the various possibilities to couple heterogeneous photocatalysis with other technologies to photodegrade organic and inorganic pollutants dissolved in actual or synthetic aqueous effluents. These combinations increase the photoprocess efficiency by decreasing the reaction time in respect to the separated operations or they decrease the cost in respect of heterogeneous photocatalysis alone, generally in terms of light energy. Depending on the operation coupled with heterogeneous photocatalysis, two categories of combinations exist. When the coupling is with ultrasonic irradiation, photo-Fenton reaction, ozonation, or electrochemical treatment, the combination affects the photocatalytic mechanisms thus improving the efficiency of the photocatalytic process. When the coupling is with biological treatment, membrane reactor, membrane photoreactor, or physical adsorption, the combination does not affect the photocatalytic mechanisms but it improves the efficiency of the overall process. The choice of the coupling is related to the type of wastewater to be treated. A synergistic effect, giving rise to an improvement of the efficiency of the photocatalytic process, has been reported in the literature for many cases.

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The combination of heterogeneous photocatalysis with chemical and physical operations: A tool for improving the photoprocess performance

The production of ammonia from dinitrogen and water vapor at mild conditions of temperature and pressure on Fe{sup 3+}-doped titanium dioxide powders under UV radiation has been studied in a continuous photoreactor working in the gas-solid regime. A net activity decline was observed after a few hours of irradiation; this decline did not depend on the reactor temperature, the powder composition, or the amounts of produced ammonia. The highest activity is found when no excess iron is segregated at the surface; overall turnover numbers for dinitrogen reduction as high as six electrons per iron atom can then be reached before powder deactivation, showing the catalytic character of the participation of Fe in this process. An IR investigation of the active and spent specimens revealed that the irradiation determines the almost complete disappearance of the OH groups from the powder surface. Furthermore, an ESR study of all the powders showed that bulk Fe{sup 3+} ions are better electron traps than Ti{sup 4+} ions so that, when a UV photon generates a hole-electron pair, the electron can be stabilized on the iron ions.

Dinitrogen photoreduction to ammonia over titanium dioxide powders doped with ferric ions

Nanostructured TiO2 samples with primary particle size in the 4−20 nm range were prepared by either hydrothermal (H) or thermal (T) treatment of an amorphous precursor, and their behavior under UV illumination at 77 K was studied by means of EPR spectroscopy. The samples of the H series present the smallest crystallite size and after irradiation in a vacuum show some Ti3+ centers. In contrast, under these conditions only weak signals associated with oxygenated radicals are observed for the T samples. However, when oxygen is preadsorbed, several oxygenated complexes (O-, O2-, O2H•, and O3-) are photogenerated in proportions that depend on the characteristics of the material. Subsurface O- species are exclusively detected in the case of the samples of the H series, whereas ozonide radicals and surface O- are stabilized on materials with larger crystalline domains. These oxygenated complexes are thermally unstable, and they disappeared after warming to room temperature in the case of the T samples, but they ...

EPR Study of the Surface Characteristics of Nanostructured TiO2 under UV Irradiation

Visible light-activated nanosized doped-TiO2 photocatalysts

NO adsorption on CeO2 pre-outgassed at different temperatures (Tv) has been studied by EPR and FTIR. The results are compared with those obtained by measuring the products of the reaction, performed at 323 K, between NO and CeO2 samples activated by previous heating under N2 flow. EPR results indicate that, for Tv 573 K, NO molecules are adsorbed in radical form at 77 K and react at 298 K with surface oxygen vacancies generating diamagnetic products. FTIR spectra show correspondingly the formation of hyponitrites, nitrites and nitrates upon NO adsorption at 298 K on CeO2 outgassed at different temperatures. The hyponitrites can decompose at ambient temperatures to produce N2O; the process leads to modification (probably reoxidation) of the surface centres initially present. This can be followed by O2 adsorption and EPR spectroscopy, which shows that O2– adsorbed on the modified vacancies is less stable than on CeO2 without NO adsorbed, its reaction leading in some cases to the formation of peroxynitrate-type radicals. The reaction at 323 K of NO on the pretreated CeO2 surface produced N2O for pretreatment temperatures TF 573 K, in amounts increasing with TF. The results indicate that surface centres containing associated oxygen vacancies are active sites for this process.

Javier Soria

Papers

The combination of heterogeneous photocatalysis with chemical and physical operations: A tool for improving the photoprocess performance

Dinitrogen photoreduction to ammonia over titanium dioxide powders doped with ferric ions

EPR Study of the Surface Characteristics of Nanostructured TiO2 under UV Irradiation

Visible light-activated nanosized doped-TiO2 photocatalysts

NO reaction at surface oxygen vacancies generated in cerium oxide