Materials Science Forum Vol. 734

Abstract: Photo catalytic reduction of carbon dioxide or artificial photo synthesis to yield hydrogen and hydrocarbons like methane, methanol etc., has emerged as a subject/process of intensive study due to its potential applications towards abatement of atmospheric CO₂ levels and conversion to fuels and chemicals. This Chapter provides a comprehensive picture of the process that has posed several scientific and technological challenges, like activation of most stable molecules-CO₂ and water, extremely low conversion rates, complex reaction pathways involving multi electron transfer steps and short catalyst life. All the major aspects/developments on this process like, the salient features and technological aspects, thermodynamic and kinetic characteristics, various types of photo-active catalysts-, like, titania based catalysts and titania with various dopants and modifiers, various metal oxides/sulfides/nitrides/ layered titanates, binary and ternary oxides of Nb, Ta, Ga & In mixed oxide catalysts, metal complexes, and supra molecular catalysts-, sensitization by macro cylic ligands, influence of process parameters, catalyst structure-property-activity correlations, aspects of deactivation of catalysts, reaction mechanistic aspects and sequential surface reaction pathways, recent trends and future directions have been covered. Design and development of efficient catalyst systems and achieving higher yield of desired products (higher selectivity) and extending the catalyst life are the key issues being pursued by the researchers. The process is in nascent stage and further improvements are needed as CO₂ conversion rates are extremely small, with products formed in terms of 1-10 micro moles/hr. One of the means of improving the process efficiency is to carry out electrochemical reduction of CO₂ using solar electric power, with an integrated Photo electrochemical cell (PEC). Yet another option is to reduce CO₂ to methanol with hydrogen produced using solar powered PEC.

Abstract: Recent years have seen a surge of interest in the application of solar energy for water disinfection by using nanostructured photocatalysts elaborately designed and fabricated. Photocatalysis has its unique advantage for utilizing sunlight to drive the disinfection process. The highly reactive oxygen species (ROS) serve as the main oxidants and are capable of inactivating microorganisms, including viruses, bacteria, spores and protozoa. This chapter presents an overview of current research activities that center on the preparation, characterization and application of highly efficient photocatalysts for water disinfection under both UV and visible light irradiation. It is organized into two major parts. One is the development of TiO₂-based photocatalysts including surface noble metal modified, ion doped, dye-sensitized, and composite TiO₂. The other part is the introduction of new types of photocatalysts and advanced technologies that have recently fascinated the scientific community. Particular attention is given to the pioneering fields such as graphene-based photocatalysts, plasmonic-metal nanostructures and naturally occurring photocatalysts. Finally, we conclude with a discussion of what major advancements are needed to move the field of photocatalytic water disinfection forward.

Abstract: Different types of Li-doped ZnO (LDZ) (Li = 0 to 10%) powders were prepared by following a novel pyro-hydrolysis route at 450°C, and were thoroughly characterized by means of thermo-gravimetry (TG), differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared (FT-IR) spectroscopy, Fourier-transform Raman spectroscopy (FT-Raman), diffuse reflectance spectroscopy (DRS), ultra-violet visible (UV-vis.) spectroscopy, BET SA, and zeta potential (ζ) measurements. Photocatalytic activity of these powders was evaluated by means of methylene blue (MB) degradation experiments conducted under the irradiation of simulated and natural solar light. Characterization results suggest that both pure and LDZ powders are quite thermally stable up to a temperature of 700°C and possess band gap (BG) energies in the range of 3.16 to 3.2 eV with a direct band to band transition and ζ values of-31.6 mV to-56.4 mV. The properties exhibited by LDZ powders were found to be quite comparable to those exhibited by p-type semi-conducting LDZ powders. In order to study the kinetics of MB degradation reaction under the irradiation of simulated solar light, the Li (0.2 to 10%) and Al (0.5%) co-doped ZnO powders were also synthesized and employed for this purpose. The photocatalytic degradation of MB over Al and Li co-doped ZnO powders followed the Langmuir-Hinshelwood (L-H) first order reaction rate relationship. The 10% Li and 0.5% Al co-doped ZnO exhibited highest photocatalytic activity among various powders investigated in this study.

Abstract: The wastewater from the coir, pharmaceutical, leather, paper and pulp industries is contaminated with water-soluble poly phenolic compounds (tannins). Among various tannins, tannic acid is a typical hydrolysable tannin prevalent in wastewater. The degradation of tannic acid using TiO₂ nanoparticles as photocatalyst was investigated. The effect of catalyst concentration, pH of aqueous suspension and also electron acceptors such as hydrogen peroxide (H₂O₂) and ozone (O₃) on the degradation of tannic acid was studied. The degradation of tannic acid was found to be more efficient and complete in the presence of UV/TiO₂/O₃ compared to UV/TiO₂/H₂O₂. The kinetics of degradation was observed to follow first order rate equation which indicates that the mineralization process is diffusion controlled.

Abstract: The present investigation is about fabrication of single-crystalline ceria (CeO₂) nanoparticle by a hydrothermal route. High surface area CeO₂ was synthesized with transformation of morphology from nanofibers to nanocubes in response to processing conditions. A steady variation of average nanocrystallite size ca. in the range 3.0-16.9 nm and a range of band gap energy from 2.6 to 2.9 eV were measured. The surface area of the nanoparticles varied in the range 16.0136.1 m²/g and the variation in surface area is attributed to the nature of packing of particles. The ceria nanofibers could generate 870.5 µmol of H₂ in 3 h of irradiation.

Abstract: Recently, numerous semiconducting materials and its composites are studied for their photocatalysis applications. These materials having different size, shape and controlled morphology in micro, meso and nanoscale exhibits various important surface features having remarkable applications in photocatalytic degradation of toxic pollutants, hydrogen production and adsorbent for wastewater treatment. However different methods are followed to synthesis semiconductors, metal supported/loaded semiconductors, heterostructures, graphene based semiconductors and other newer materials. In addition, the surface morphologies of these materials and composites for its photo catalytic processes can be explained. Finally the photophysical properties of semiconductor and composite materials with unusual texture will be summarized.

Abstract: In the recent years most of the studies are confined to the mixing of ZrO₂, SiO₂, WO₄ or ceria with titania in different composition so as to stabilize anatase phase, maintain high surface area and increase visible light absorption for better photocatalytic activity. Method of synthesis also helps in effective doping and enhancing surface area of the resultant materials. Nonmetal doping of oxide semiconductor materials facilitates the visible light application of photocatalysis. Based on the recent literature this review elaborately discuss on the development of titania based mixed oxide catalyst with or without different doping for visible light application. In addition this review deals with critical analysis of these materials towards photocatalytic oxidation of organics and reduction of pollutants like toxic metal ions and nitrates.

Abstract: The rate of 1,4-dichlorobenzene (1,4-DCB) degradation in the aqueous phase was investigated under direct photolysis or photocatalysis in the presence of TiO₂ thin film prepared by reactive sputtering using a metal Ti target and a reaction sputtering atmosphere of argon and oxygen. The prepared thin films were analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). XPS confirmed the presence of completely oxidized TiO₂ films whereas XRD showed that the films contained a mixture of rutile and anatase phases with rutile being approximately 30% of the total volume. Two lamps, both of the same power but different wavelength range were employed as irradiation sources. Photocatalysis showed faster removal of 1,4-DCB as compared to direct photolysis. The complete degradation was attained using the freshly prepared TiO₂ sample. The intermediate produced during the photocatalysis was benzoquinone. Photolysis using visible irradiation was relatively slower and both benzoquinone and hydroquinone were formed as intermediates. Higher initial degradation rates were observed when the same film was re-used, most probably due to the effect of washing of the TiO₂ thin films surface with methanol.

Abstract: The sol gel synthesis method was used to prepare carbon and nitrogen doped titanium dioxide (TiO₂) photo-catalyst using titanium tetrachloride (TiCl₄) as the precursor. Doping was carried out to modify the absorption band edge of titanium dioxide. To avert the problem associated with use of powder TiO₂, the photo-catalyst was immobilized on glass support using tetraethyl orthosilicate (TEOS), which served as a binder and precursor for silicon dioxide (SiO₂). The prepared photo-catalytic materials were characterized by FT-IR, XRD, TEM, BET and DRS. The photo-catalytic efficiency of titanium dioxide immobilized on glass support was evaluated using the degradation of methyl orange (MeO) and phenol red (PRed) under ultraviolet and visible light irradiation. Doping with carbon and nitrogen, and incorporation of silicon dioxide into the titanium dioxide matrix allowed utilization of visible light by the prepared TiO₂-SiO₂ nanocomposites. Photo-degradation tests were carried out for doped and undoped photo-catalyst. An increased rate of photo-oxidation of methyl orange and phenol red was observed under visible light irradiation as compared to UV light irradiation.