Photocatalytic Properties of Titania Pillared Clays by Different Drying Methods

Titania-Clay Mineral Composites for Environmental Catalysis and Photocatalysis

The use of titania-based composite materials in the field of heterogeneous catalysis and photocatalysis has a long and rich history. Hybrid structures combining titania nanoparticles with clay minerals have been extensively investigated for nearly four decades. The attractiveness of clay minerals as components of functional materials stems primarily from their compositional versatility and the possibility of using silicate lamellae as prefabricated building blocks ready to be fitted into the desired nanoconstruction. This review focuses on the evolution over the years of synthetic strategies employed for the manufacturing of titania–clay mineral composites with particular attention to the role of the adopted preparative approach in shaping the physical and chemical characteristics of the materials and enabling, ultimately, tuning of their catalytic and/or photocatalytic performance.

Supercritical Carbon Dioxide-Based Processes in Photocatalytic Applications

Conventional methods generally used to synthesize heterogeneous photocatalysts have some drawbacks, mainly the difficult control/preservation of catalysts’ morphology, size or structure, which strongly affect the photocatalytic activity. Supercritical carbon dioxide (scCO₂)-assisted techniques have recently been shown to be a promising approach to overcome these limitations, which are still a challenge. In addition, compared to traditional methods, these innovative techniques permit the synthesis of high-performance photocatalysts by reducing the use of toxic and polluting solvents and, consequently, the environmental impact of long-term catalyst preparation. Specifically, the versatility of scCO₂ allows to prepare catalysts with different structures (e.g., nanoparticles or metal-loaded supports) by several supercritical processes for the photocatalytic degradation of various compounds. This is the first updated review on the use of scCO₂-assisted techniques for photocatalytic applications. We hope this review provides useful information on different approaches and future perspectives.

A fluidized-bed reactor for the photocatalytic mineralization of phenol on TiO2-coated silica gel

TiO₂ photocatalysis represents a promising class of oxidation techniques that are intended to be both supplementary and complementary to the conventional approaches for the removal of refractory and trace organic contaminants in water and air. Powdered TiO₂ dispersion systems employed in most studies require an additional separation step to recover the catalyst from the effluent water, which represents a major drawback for large scale applications. The optimization of photocatalytic treatment systems involves merging the benefits of catalyst immobilization on a retainable support, thus eliminating the need for downstream catalyst separation, maximization of photon-exposed catalyst area, and continuous operation.

Aiming to integrate such conditions into a single system, a bench-scale annular photo-reactor with concentric UV-C lamp was built to study the photocatalytic mineralization of phenol on fluidized silica gel beads coated with sol-gel-synthetized TiO₂. Reactor efficiency was investigated for different silica particle diameters (224, 357 and 461 μm), fluidized-bed concentrations in the bulk liquid (5, 10, 20 and 30 g L⁻¹), initial phenol concentrations in the aqueous solution (0.25 mmol L⁻¹ to 4.0 mmol L⁻¹), and single and multiple sol-gel depositions. Then, the resulting optimum reactor configuration was compared to that of the same process on suspended Degussa P25 TiO₂ nanoparticles under similar experimental conditions. The latter is expected to be more efficient, but post-treatment catalyst recovery, being an energy intensive process, represents a major limitation for large scale applications. Process efficiency was measured as a function of the accumulated energy necessary for the mineralization of 50% of the initial dissolved chemical oxygen demand (COD), or, Q_0.5. Results showed that for any given mass of fluidized bed material, photo-oxidation efficiency increases with decreasing particle size (even for bed concentrations with similar equivalent surface area), decreasing initial phenol concentrations, and increasing number of sol-gel coatings. It was found that, for any given particle size and contaminant mass, there is an optimum bed concentration of 20 g L⁻¹ for which Q_0.5 reaches a minimum. Finally, under the optimum configuration, the fluidized-bed reactor efficiency is only 30% lower than that of photocatalysis on suspended TiO₂ nanopowder, thus making the proposed fluidized system a viable alternative to slurry-TiO₂ reactors.

Photocatalytic and adsorption properties of TiO2-pillared montmorillonite obtained by hydrothermally activated intercalation of titanium polyhydroxo complexes

We report on a new approach for the synthesis of TiO₂-pillared montmorillonite, where the pillars exhibit a high degree of crystallinity (nanocrystals) representing a mixture of anatase and rutile phases. The structures exhibit improved adsorption and photocatalytic activity as a result of hydrothermally activated intercalation of titanium polyhydroxo complexes (i.e., TiCl₄ hydrolysis products) in a solution with a concentration close to the sol formation limit. The materials, produced at various annealing temperatures from the intercalated samples, were characterized by infrared spectroscopy, differential scanning calorimetry (DSC)/thermogravimetric analysis (TGA), X-ray diffraction, dynamic light scattering (DLS) measurements, and liquefied nitrogen adsorption/desorption. The photocatalytic activity of the TiO₂-pillared materials was studied using the degradation of anionic (methyl orange, MO) and cationic (rhodamine B, RhB) dyes in water under UV irradiation. The combined effect of adsorption and photocatalysis resulted in removal of 100% MO and 97.5% RhB (with an initial concentration of 40 mg/L and a photocatalyst-sorbent concentration of 1 g/L) in about 100 minutes. The produced TiO₂-pillared montmorillonite showed increased photocatalytic activity as compared to the commercially available photocatalyst Degussa P25.

Optimizing photo-mineralization of aqueous methyl orange by nano-ZnO catalyst under simulated natural conditions

BACKGROUND

Photo-degradation of organic contaminants into non-hazardous mineral compounds is emerging as a strategy to purify water and environment. Tremendous research is being done using direct solar light for these purposes. In this paper we report on optimum conditions for complete mineralization of aqueous methyl orange using lab-prepared ZnO nanopowder catalyst under simulated solar light.

RESULTS

Nano-scale ZnO powder was prepared in the lab by standard methods, and then characterized using electronic absorption spectra, photolumenscence emission (PL) spectra, XRD, and SEM. The powder involved a wurtzite structure with ~19 nm particles living in agglomerates. Photo-degradation progressed faster under neutral or slightly acidic conditions which resemble natural waters. Increasing catalyst concentration increased photodegradation rate to a certain limit. Values of catalyst turn over number and degradation percentage increased under higher light intensity, whereas the quantum yield values decreased. The photocatalytic efficiency of nano-ZnO powders in methyl orange photodegradation in water with solar light has been affected by changing the working conditions. More importantly, the process may be used under natural water conditions with pH normally less than 7, with no need to use high concentrations of catalyst or contaminant. The results also highlight the negative impact of possible high concentrations of CO2 on water purification processes. Effects of other added gaseous flows to the reaction mixture are also discussed.

CONCLUSION

ZnO nano-particles are useful catalyst for complete mineralization of organic contaminants in water. Photo-degradation of organic contaminants with ZnO nano-particles, methyl orange being an example, should be considered for future large scale water purification processes under natural conditions.

Recent advances in synthesis and applications of clay-based photocatalysts: a review

Clay-based photocatalysts with high adsorbability and special structures have attracted extensive attention because of their applications in environment and energy fields.

ZnO nanoparticles immobilized on flaky layered double hydroxides as photocatalysts with enhanced adsorptivity for removal of acid red G

Flaky layered double hydroxides (FLDH) composed of cross-linked nanoflakes were prepared by the reconstruction of their oxides in alkali solution. The effect of reconstruction temperatures on the physicochemical properties was investigated. FLDH with a specific surface area of as high as 217 m(2)/g was obtained at a reconstruction temperature of 6 °C, and its derived flaky mixed metal oxides (FMMO) had a specific surface area of 249 m(2)/g. The ZnO nanoparticles were homogeneously deposited on the surface of the FLDH by coprecipitation. After calcination at 500 °C for 2 h, the ZnO-coated FLDH was transformed into ZnO-coated flaky mixed metal oxides (FMMO). The powders were characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscope, N(2) adsorption-desorption isotherm, UV-vis diffuse reflectance spectroscopy, and Fourier transform infrared spectroscopy. In the presence of FLDH as a support, the ZnO nanoparticles were of about 10 nm in size and showed higher photocatalytic decomposition of acid red G than bare ZnO powder prepared under similar experimental conditions. It should be noted that the ZnO-coated FMMO combined excellent adsorption with photocatalytic activity. The flaky structure of mixed metal oxides appears to play important roles in the adsorption and photodecomposition process.

Degradation of methyl orange by composite photocatalysts nano-TiO2 immobilized on activated carbons of different porosities

Composite photocatalysts TiO(2) immobilized on granular activated carbons with different porosities (TiO(2)/AC) were prepared by a novel approach, dip-hydrothermal method using peroxotitanate as precursor. The TiO(2)/AC composites were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and the nitrogen absorption. Their photocatalytic activity was evaluated by degradation of methyl orange (MO). The results showed that nano-TiO(2) particles of anatase type were well deposited on the activated carbon surface. The porosity of activated carbon had significant influence on the adsorption, the amount of TiO(2) deposited on the external surface of AC and the activity of composite photocatalysts. The composite TiO(2)/AC made from proper mesoporosity AC exhibited higher catalytic activity than the mixture of powdered TiO(2) with AC. Furthermore, the mechanism of synergistic effect of AC adsorption and TiO(2) photocatalysis was discussed.

Highly thermostable anatase titania-pillared clay for the photocatalytic degradation of airborne styrene

Airborne styrene is a suspected human carcinogen, and traditional ways of mitigation include the use of adsorption technologies (activated carbon or zeolites) or thermal destruction. These methods presenttheir own shortcomings, i.e., adsorbents need to be regenerated or replaced regularly, and relatively large energy inputs are required in thermal treatment. Photocatalysis offers a potentially sustainable and clean means of controlling such fugitive emissions of styrene in air. The present study demonstrates a new type of well-characterized, highly thermostable titania-pillared clay photocatalysts for airborne styrene decomposition in a custom-designed fluidized-bed photoreactor. This photocatalytic system is found to be capable of destroying up to 87% of 300 ppmV airborne styrene in the presence of ultraviolet (UV) irradiation. The effects of relative humidity (RH: 0 or 20%) are also studied, together with the arising physical structures (in terms of porosity and surface characteristics) of the catalysts when subjected to relatively high calcination temperatures of 1000-1200 degrees C. Such a temperature range may be encountered, e.g., in flue gas emissions (1). It is found that relative humidity levels of 20% retard the degradation efficiencies of airborne styrene when using highly porous catalysts.

A combined study by XRD, FTIR, TG and HRTEM on the structure of delaminated Fe-intercalated/pillared clay

Fe-PILC samples were synthesized by the reaction between Na(+)- and/or Ca(2+)-montmorillonite (Mt) and base-hydrolyzed solutions of Fe(III) nitrate. Different from the known usual microporous pillared structure, a meso-microporous delaminated structure containing intercalated or pillared fragments was found in the respective resulting Fe-intercalated or -pillared clays. XRD patterns of Na(+)-Mt-based Fe-intercalated/pillared clays show one large d-spacing above 6.4 nm corresponding to the mesoporous delaminated part, whereas another d-spacing of ca. 1.5 nm was indicative of the microporous pillared part. Fe-intercalated/pillared clays based on Ca(2+)-Mt lead to similar results, but with a d-spacing less than 6 nm and a second low intense d-spacing less than 1.5 nm. In the delaminated Fe-intercalated clays, NO(-)(3) anions were retained even after thorough washing process. They play as counterions to neutralize the positive-charged iron aggregates in the delaminated structure, and can be exchanged by heteropolyanions as [PW(12)O(40)](3-). The delaminated Fe-pillared clays show good thermal stability at 500 degrees C and exhibit at this temperature dramatically higher specific surface area and porosity than the starting montmorillonites. However, calcination at a higher temperature leads to the formation of nanocrystalline hematite. Air-drying after ethanol extraction (EAD) method has an advantage over air-drying (AD) method in preserving the delaminated structure.

Low-temperature synthesis and photocatalytic activity of TiO2 pillared montmorillonite

TiO2 pillared montmorillonite (PILM) was synthesized by hydrolyzing TiO2 sol into the interlayers of montmorillonite (MMT) at low temperatures. It is novel that all the as-prepared catalysts without calcination show a high photocatalytic activity for the degradation of acid red G (ARG) under UV light irradiation. The as-prepared powders were characterized by XRD, TEM, UV-vis diffuse reflectance spectroscopy, BET, and FT-IR. The X-ray diffraction analysis indicated that the (001) plane of MMT in the composites disappeared and that the layered structure became disordered, which also was confirmed by the TEM photographs. The UV absorption edge of the composites shows a red-shift in comparison to that of pure TiO2 particles. The obtained catalyst has the highest photocatalytic activity when the composite temperature is 70 degrees C, which could be attributed to the synergetic effects of the adsorbability of MMT and the photocatalytic property of TiO2 in it.

Photocatalytic oxidation of organic pollutants on titania-clay composites

TiO2/Ca-montmorillonite composites were prepared by wet grinding in an agate mill. Positively charged TiO2 nanoparticles are bound to the surface of the negatively charged montmorillonite layers via heterocoagulation; the clay mineral is used as adsorbent and support for the photooxidation process. Aquatic solution of 0.5mM phenol was degraded by irradiation with UV-VIS light (lambda=250-440 and 540-590 nm) in suspensions of TiO2-clay composites and significant photodegradation was observed at 40-60% TiO2/Ca-montmorillonite compositions. Synergistic effect was detected at solid/liquid interface for degradation of phenol and at solid/gas interface in the recycling flow reactors for photooxidation of ethanol and toluene vapors.

Synthesis and characterisation of clay-supported titania photocatalysts

This study examines the use of laponite, a synthetic smectite, which forms exfoliated silicate layers when dispersed in water, as an inorganic support for titania nanocrystals. Titania nanocrystals are prepared prior to addition to the clay dispersion, by a sol-gel synthesis incorporating a microwave hydrothermal step. The characteristics of the resultant structure such as titania phase, crystallite size, and particulate size are examined via X-ray diffraction (XRD), transmission electron spectroscopy (TEM), and infrared spectroscopy.

Degradation of methylene blue in aqueous dispersion of hollow titania photocatalyst: Optimization of reaction by peroxydisulfate electron scavenger

The submicrometer-sized titania hollow spheres have been synthesized by employing sulfonated polystyrene latex particles as a template in sol-gel method. Photocatalytic activity of the hollow spheres was investigated by focusing on the effect of electron scavengers in the photocatalytic decomposition of methylene blue (MB). Photocatalytic activities of titania hollow spheres were significantly enhanced by the addition of peroxydisulfate as an electron scavenger. The rate constant for photodecomposition of MB was increased more than two times by the addition of peroxydisulfate. The significant increase in the photodecomposition rate seems to be related to electrons scavenging as well as production of additional oxidizing species. It was found that the optimum concentration of peroxydisulfate was 10 mM. Further increase in the concentration of peroxydisulfate is not recommended due to the saturation of reaction rate by the excess amount of SO(4)(2-) species.

Photocatalytic remediation of gamma-hexachlorocyclohexane contaminated soils using TiO2 and montmorillonite composite photocatalyst

TiO2 and montmorillonite composite photocatalysts were prepared and applied in degrading gamma-hexachlorocyclohexane (gamma-HCH) in soils. After being spiked with gamma-HCH, soil samples loaded with the composite photocatalysts were exposed to UV-light irradiation. The results indicated that the photocatalytic activities of the composite photocatalysts varied with the content of TiO2 in the order of 10% < 70% < 50% < 30%. Moreover, the photocatalytic activity of the composite photocatalysts with TiO2 content 30% was higher than that of the pure P25 with the same mass of TiO2. The strong adsorption capacity of the composite photocatalysts and quantum size effect may contribute to its increased photocatalytic activities. In addition, effect of dosage of composite photocatalysts and soil pH on gamma-HCH photodegradation was investigated. Pentachlorocyclohexene, trichlorocyclohexene, and dichlorobenzene were detected as photodegradation intermediates, which were gradually degraded with the photodegradation evolution.

Photocatalytic degradation of methylene blue by a combination of TiO2 and activated carbon fibers

Photocatalytic degradation of methylene blue (MB) in aqueous solution was investigated using TiO2 immobilized on activated carbon fibers (ACFs). The TiO2 and ACF combination (TiO2/ACF) was prepared by using epoxy as the precursor of the link between TiO2 and ACFs, followed by calcination at 460 degrees C in a N2 atmosphere. The TiO2/ACF composite prepared was easier to handle than the original TiO2 powder in suspension. More significantly, the TiO2/ACF composite can be used repeatedly without a decline in photodegradation ability. After six cycles, the amount of MB removal for the TiO2/ACF composite was still slightly higher than that for fresh P25 TiO2 in suspension. Through measurement of chemical oxygen demand in the solution and the concentration of ammonium generated during degradation of MB, it was confirmed that MB molecules are mineralized instead of adsorbed by ACFs.

Characterization of Pore Structure and Coordination of Titanium in TiO2 and SiO2–TiO2 Sol-Pillared Clays

Titania sol-pillared clay (TiO(2) PILC) and silica-titania sol-pillared clay (SiO(2)-TiO(2) PILC) were synthesized by the sol-gel method. Supercritical drying (SCD) and treatment with quaternary ammonium surfactants were used to tailor the pore structure of the resulting clay. It was found that SCD approach increased the external surface area of the PILCs dramatically and that treatment with surfactants could be used to tailor pore size because the mesopore formation in the galleries between the clay layers follows the templating mechanism as observed in the synthesis of MCM-41 materials. Highly mesoporous solids were thus obtained. In calcined TiO(2) PILC, ultrafine crystallites in anatase phase, which are active for photocatalytic oxidation of organics, were observed. In SiO(2)-TiO(2) PILCs and their derivatives, titanium was highly dispersed in the matrix of silica and no crystal phase was observed. The highly dispersed titanium sites are good catalytic centers for selective oxidation of organic compounds. Copyright 2001 Academic Press.

A Kinetic Study on Photocatalytic Oxidation of Phenol in Water by Silica-Dispersed Titania Nanoparticles

Photocatalytic oxidation of phenol in water was carried out with nanoparticles of silica-titania mixtures, which were synthesized under different temperatures and silica-to-titania ratios. The crystal size of TiO(2) (in anatase phase) was determined to be in the nanometer range and it increased with increasing autoclaving temperature. Furthermore, there was no obvious relationship between the ize and the SiO(2)/TiO(2) ratio at the same preparation temperature. A specific reaction rate constant (k(s)) was used for comparison of photocatalytic activity of different samples. It was found that k(s) decreases with increasing anatase size and TiO(2) concentration. A kinetic model was developed to describe the effect of the crystal size and titania concentration on the reactivity of the SiO(2)-TiO(2) samples. Copyright 2000 Academic Press.