Assessment and influence of operational parameters on the TiO2 photocatalytic degradation of sodium benzene sulfonate under highly concentrated solar light illumination

Sodium benzene sulfonate (BS) was decomposed in aqueous TiO₂ dispersions under highly concentrated solar light illumination to examine the photocatalytic characteristics of a parabolic round concentrator (PRC) reactor to degrade the pollutant without visible light absorption. The effects of such operational parameters as initial concentration, volume of the aqueous BS solution, oxygen purging, and TiO₂ loading on the kinetics of decomposition of BS were investigated. An effective photodegradation necessitates a suitable combination of initial volume and concentration of BS solution. Relative to atmospheric air, oxygen purging significantly accelerates the degradation process at high initial concentrations of BS (0.40 mM or 1.0 mM). Optimal TiO₂ loading was 9 g l⁻¹, greater than previously reported. Elimination of TOC (total organic carbon) followed pseudo first-order kinetics in the initial stages of the photodegradation process. The relative photonic efficiency for the photodegradation of BS is ζ_rel=1.0.     

Solar photocatalysis, photodegradation of a commercial detergent in aqueous TiO2 dispersions under sunlight irradiation

A commercial detergent whose major components are an anionic surfactant and a fluorescent whitening agent can be photodegraded in aqueous TiO₂ dispersions under irradiation with concentrated sunlight in the presence of air. The degradation process followed apparent first-order kinetics in terms of the total sunlight energy impinging on the photoreactive system. The effects of (a) TiO₂ loading, (b) circulation flow rate, and (c) pH of the reactant solution on the kinetics of decomposition of the detergent were examined. Under the prevailing conditions, the optimal operational parameters for this detergent were, respectively: TiO₂ loading, 6 g l⁻¹; circulation flow rate, 4.9 l min⁻¹; and pH, 4.9. The rate of increase of the surface tension was greater than the rate of decrease of the concentration of the detergent. This study adds to our knowledge base in the effective use of sunlight irradiation to detoxify wastewaters containing undesirable detergents.     

Degradation of toluene and acetaldehyde with Pt-loaded TiO2 catalyst and parabolic trough concentrator

Photocatalytic degradation of volatile organic compounds (VOCs) with Pt-loaded TiO₂ was analyzed at elevated temperatures in the laboratory experiments (40–190 °C) and in the field experiments (30–230 °C). The temperature of catalyst coated on the sunlight receiver was easily elevated to around 200 °C by parabolic trough concentrator (1 m × 1 m). When gaseous toluene (15 ppm) or acetaldehyde (400 ppm) was passed through the reactor, 79% of toluene or 93% of acetaldehyde was removed continuously. In the similar condition, bare TiO₂ was rapidly deactivated by the formation of byproducts. The combination of sunlight concentrator and Pt–TiO₂ catalyst exhibited the enhancement of complete degradation of VOCs, the inhibition of deactivation, and the reactivation of photocatalyst. The contributions of photocatalytic and catalytic activities of Pt–TiO₂ were analyzed by using UV lamp and electric heater. Acetaldehyde is thermocatalytically degraded by photodeposited Pt on TiO₂ at 70–190 °C without UV irradiation, however the UV irradiation is necessary for the complete oxidation of acetaldehyde into CO₂. It is inferred that the degradation of VOCs is enhanced by the combined effect of Pt thermocatalyst and Pt–TiO₂ photocatalyst.     

Solar photocatalytic decomposition of Probenazole in water with TiO2 in the presence of H2O2

The photocatalytic decomposition of Probenazole in water using TiO₂/H₂O₂ under sunlight illumination is studied. The addition of H₂O₂ is effective for the improvement of photocatalytic decomposition of Probenazole with TiO₂. Furthermore, the operating conditions, such as photocatalyst dosage, temperature, pH, sunlight intensity and illumination time are also optimized. The kinetics of photocatalytic decomposition follow a pseudo–first–order kinetic law, and the rate constant is 0.129 min^?1. The activation energy (E_a) is 11.34 kJ/mol. The photocatalytic decomposition mechanism is discussed on the basis of molecular orbital (MO) simulation for frontier electron density.     

Solar photocatalytic degradation of azo dye: comparison of photocatalytic efficiency of ZnO and TiO2

The photocatalytic activity of commercial ZnO powder has been investigated and compared with that of Degussa P25 TiO₂. Laboratory experiments with acid brown 14 as the model pollutant have been carried out to evaluate the performance of both ZnO and TiO₂ catalysts. Solar light was used as the energy source for the photocatalytic experiments. These catalysts were examined for surface area, particle size and crystallinity. The effect of initial dye concentration, catalyst loading, irradiation time, pH, adsorption of acid brown 14 on ZnO and TiO₂, intensity of light and comparison of photocatalytic activity with different commercial catalysts were studied. The progress of photocatalytic degradation of the acid brown 14 has been observed by monitoring the change in substrate concentration of the model compound employing HPLC and measuring the absorbance in UV–Visible spectrophotometer for decolourisation. The photodegradation rate was determined for each experiment and the highest values were observed for ZnO suggesting that it absorbs large fraction of the solar spectrum and absorption of more light quanta than TiO₂. The complete mineralisation was confirmed by total organic carbon (TOC) analysis, COD measurement and estimation of the formation of inorganic ions such as NH₄⁺, NO₃⁻, Cl⁻ and SO₄²⁻.     

Preparation of highly efficient sunlight driven photodegradation of some organic pollutants and H2 evolution over rGO/FeVO4 nanocomposites

FeVO₄ and rGO/FeVO₄ nanocomposites were synthesized by one-step hydrothermal method. The structural and morphological properties of the prepared samples were investigated by XRD, TEM, EDS, XPS, FTIR, Raman, UV–Vis DRS and PL techniques. XRD, XPS and Raman techniques displayed that graphene oxide (GO) was successfully reduced to reduced graphene oxide (rGO) under hydrothermal conditions. Also, the addition of rGO enhanced the visible light absorption, oxygen vacancies, phase transformation of FeVO₄ from triclinic to monoclinic phase at high amount of rGO and reduced the band gap energy of FeVO₄. The photoactivity of the synthesis catalysts was investigated for the photodegradation of Malachite green, phenol, Methylene blue and Rhodamine B under natural sunlight. Also, the catalysts were applied in H₂ evolution from water splitting. The results showed that, the rGO amount was proven to be a crucial factor affecting the structural properties and photocatalytic performance of FeVO₄. The mineralization (TOC), photodegradation mechanism and photocatalytic reaction kinetics were discussed.     

Photocatalytic degradation of organic compounds in aqueous solution by a TiO2-coated rotating-drum reactor using solar light

This paper deals with the degradation of aqueous phenol by a newly proposed rotating-drum reactor coated with a TiO₂ photocatalyst, in which TiO₂ powders loaded with Pt are immobilized on the outer surface of a glass-drum. The reactor can receive solar light and oxygen from the atmosphere effectively. It was shown experimentally that phenol can be decomposed rapidly by this reactor under solar light: with our experimental conditions the phenol with an initial concentration of 22.0 mg/dm³ was decomposed within 60 min and was completely mineralized through intermediate products within 100 min. The photonic efficiency under solar light was shown to take the value 0.00742 mol-C/Einstein. The photocatalytic decomposition processes of phenol by this reactor were also discussed on the basis of the Langmuir–Hinshelwood kinetic model.     

Degradation of lincomycin in aqueous medium: Coupling of solar photocatalysis and membrane separation

The photocatalytic oxidation of a common antibiotic, the lincomycin was carried out in aqueous suspensions of polycrystalline TiO₂ Degussa P25 irradiated by sunlight. In order to improve the performance of the lincomycin degradation a hybrid system consisting of a solar photoreactor with the photocatalyst in suspension coupled with a membrane module, used to confine both photocatalyst and pollutants in the reaction environment, was tested.A preliminary study was carried out in order to determine some kinetics parameters of the drug photodegradation. The influence of initial substrate concentration on the lincomycin photooxidation rate was investigated. The photooxidation rate follows a pseudo-first order kinetics with respect to the lincomycin concentration under the used experimental conditions. The presence of the membrane reactor allows the catalyst separation and to operate in continuous mode as the membranes rejection for lincomycin and its oxidation products was quite high.     

Photoreduction of CO2 to fuels under sunlight using optical-fiber reactor

An optical-fiber reactor is employed to photocatalytically reduce CO₂ with H₂O to fuels under UVA artificial light and concentrated natural sunlight. The optical fiber is coated with gel-derived TiO₂–SiO₂ mixed oxide-based photocatalysts. Fe atom is found to insert into the TiO₂–SiO₂ lattice during sol–gel process, resulting in the full visible light absorption as well as the effect on product selectivity of the derived catalyst. Under UVA, ethylene is mainly produced on Cu–Fe/TiO₂ catalyst with the quantum yield of 0.0235%, whereas Cu–Fe/TiO₂–SiO₂ catalyst is observed to favor methane production with the quantum yield of 0.05%. Meanwhile, the overall energy efficiency is found to be much higher on Cu–Fe/TiO₂–SiO₂ (0.0182%) than on its Cu–Fe/TiO₂ counterpart (0.0159%). There is only methane evolved over both bare TiO₂–SiO₂ and Cu–Fe/TiO₂–SiO₂ catalysts under natural sunlight with the production rates of 0.177 and 0.279 μmol/g-cat h, respectively. For the former catalyst, the increase in light intensity is not found to compensate the inherent electron–hole recombination in the TiO₂–SiO₂–acac catalyst, whereas the superior photoactivity of Cu–Fe/TiO₂–SiO₂ catalyst under natural sunlight could be ascribed to its full absorption of visible light.     

Enhanced photocatalytic performance of hollow spherical CdS QDs@ZrO2–TiO2 composites with double Z-scheme heterostructures

In this paper, CdS QDs@ZrO₂–TiO₂ hollow spherical composites with double Z-scheme heterostructure modified by CdS QDs were prepared by the sol–gel method and vacuum impregnation method using polystyrene (PS) microspheres synthesized by the self-assembly technique as templates. A series of characterization results show that CdS QDs@ZrO₂–TiO₂ composites treated by PS microspheres have the structure of hollow spheres with uniform size and orderly arrangement. Moreover, the double Z-scheme heterojunction formed between CdS QDs, ZrO₂, and TiO₂ in the composite can optimize the charge transfer path and improve the charge separation efficiency. Results of the photodegradation and the photo-splitting water of CdS QDs@ZrO₂–TiO₂ composites show that, compared with other systems, CdS QDs@ZrO₂–TiO₂ composites have the highest photocatalytic degradation rate of crystal violet, and meanwhile, the photocatalytic hydrogen production rate of the composite under simulated sunlight is more than 100 times that of TiO₂. The good absorption of visible light, the successful construction of double Z-scheme heterojunction, and the unique hollow sphere structure of CdS QDs@ZrO₂–TiO₂ composites are the key factors for enhanced photocatalytic performance.     

Recent progress in metal-doped TiO2, non-metal doped/codoped TiO2 and TiO2 nanostructured hybrids for enhanced photocatalysis

Titanium dioxide remains a benchmark photocatalyst with high stability, low cost, and less toxicity, but it is active only under UV light; thus, in practical applications using visible light, its catalytic reactions are stalled. To enhance its catalytic activity under visible light, non-metal/codoped TiO₂ structures are being studied. These structures improve the photocatalytic activity of TiO₂ in visible light by reducing its energy bandgap. This might be useful in wastewater treatment for the photocatalytic degradation of organic contaminants under visible and UV light irradiation. In this intensive review, we describe recent developments in TiO₂ nanostructured materials for visible-light driven photocatalysis, such as (i) mechanistic studies on photo-induced charge separation to understand the photocatalytic activity and (ii) synthesis of non-metal doped/codoped TiO₂ and TiO₂ nanostructured hybrid photocatalysts. Furthermore, the effects of various parameters on their photocatalytic efficiency, photodegradation of various organic contaminants present in wastewater, and photocatalytic disinfection are delineated.     

Photodegradation of methyl tertiary butyl ether (MTBE) vapor with immobilized titanium dioxide

A study on the photocatalytic degradation of methyl tertiary butyl ether (MTBE) vapor was performed with respect to reaction parameters, kinetics, and mechanism. A conventional TiO₂-coated plate system was used for this initial work on vaporized MTBE. On irradiating with UVBLB (5 mW/cm²), 0.6 mg/cm² of TiO₂ (P25) photocatalyst showed the highest reaction rate (0.85 μmol/min) for 25.76 μmole of MTBE (1000 ppmv). The numerical values obtained depended predictably with respect to light intensity, initial concentration, and quantity of P25. The photocatalytic degradation of MTBE followed Lanmuir-Hinshelwood kinetics with a linearity of 0.989. As far as the mechanism of degradation is concerned, attack by generated hydroxyl radicals on the methoxy group in the MTBE structure dominated and proceeded through tert-butyl formate (TBF) as an intermediate, instead of methyl group to proceed through producing 2-methoxy-2-methyl propionaldehyde (MMP). TBF finally degrades to CO₂ through acetone, which was shown easily converted to CO₂. This study shows that the use of sunlight seems to be possible given adequate tools for concentrating the light.     

Visible light-induced degradation of blue textile azo dye on TiO2/CdO–ZnO coupled nanoporous films

The photodegradation of a typical textile blue azo dye, followed by UV–VIS spectra analysis, has been carried out successfully under white light illumination on TiO₂/CdO–ZnO nanoporous coupled thin films. A relatively fast degradation occurs in dye solutions with concentrations of 100 mg/l (pH=3), at temperatures of 85°C, and with the aid of 400 mg/l hydrogen peroxide. Photodegradation also occurs on nanoporous TiO₂ films but with significant lower efficiency than on TiO₂/CdO–ZnO coupled nanoporous films. Dye photodegradation does not occur on TiO₂/CdO or TiO₂/ZnO nanoporous films, suggesting that both CdO and ZnO components are required on the sensitization of TiO₂ nanoporous films. A combined effect of new sensitizing interband states (response to white illumination) and/or rectification phenomena (improved charge separation) may be responsible of the higher photocatalytic activity of the TiO₂/CdO–ZnO nanoporous films. Similarly, the alternative route for visible degradation, the photosensitized degradation mechanism, could also benefit from the coupled nanoporous films due to a higher driving force for electron injection (dye oxidation).     

Morphological, optical and photocatalytic properties of TiO2–Fe2O3 multilayers

Morphological, optical and photocatalytic properties of TiO₂, Fe₂O₃ and TiO₂–Fe₂O₃ samples (formed by 1, 3 and 5 coatings) were studied. The layers were deposited on glass substrate by the sol–gel method. The catalytic activity of the samples was studied by the photodecomposition of methylene blue (MB) under visible light illumination. The FTIR results indicate that all samples present surface OH radicals that are bound either to the Ti or Fe atoms. This effect is better visualized at larger number of coatings in the TiO₂–Fe₂O₃/glass systems. Also, two mechanisms are observed during the photodecomposition of the MB.     

Highly broadband plasmonic Cu film modified Cu2O/TiO2 nanotube arrays for efficient photocatalytic performance

To develop an efficient photocatalyst electrode for solar energy harvesting and photocatalysis application in the visible region, broadband plasmonic Cu film combined with Cu₂O/TiO₂ nanotube arrays heterojunction (Cu film/Cu₂O/TiNT) has been successfully fabricated by anodization combined with electrodeposition method. Interestingly, linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and UV–Vis diffuse reflectance spectroscopy reveal that the combined consequence of both Cu film and Cu₂O in the as-synthesized ternary composite considerably enhances light absorption in the visible spectral. This activity is attributed to the more efficient charge separation/transportation and the presence of Cu film with strong plasmon resonance (SPR) effect. Moreover, the combined effects of both Cu film and Cu₂O on TiNT approved highest catalytic current density and highest photocatalytic activity on methylene blue (MB). The efficiency and the rate of MB photodegradation over the Cu film/Cu₂O/TiNT were found to be triple compared to TiNT. Within only 30 min of reaction time, the photodegradation of MB reaches nearly 100%.     

Visible-light-assisted photocatalytic degradation of gaseous formaldehyde by parallel-plate reactor coated with Cr ion-implanted TiO2 thin film

Sol–gel nano titanium dioxide (TiO₂) thin film can be activated by the ultraviolet (UV) radiation available in sunlight to perform solar photocatalysis. The useful spectral range can be extended from UV to visible light by implantation of metal ion into the TiO₂ lattice. As a result, the solar visible light can be utilized more efficiently to enhance the solar photocatalysis. In this study, visible-light-assisted photocatalytic glass reactors were built by parallel borosilicate glass plates coated on the upper surfaces with sol–gel TiO₂ thin films implanted with chromium (Cr) ion. The properties of the Cr/TiO₂ thin films were fully characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermal gravity (TG) analysis, scanning-electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis. In the performance tests, a metal halide lamp was used as an external light source to resemble the solar visible spectral radiation. The performance of a Cr/TiO₂ photoreactor was measured in terms of its photocatalytic degradation of gaseous formaldehyde in a single pass of contaminated air flowing through the photoreactor. The experimental results demonstrated the promise of using light-transmitting glass substrate to allow transmission and distribution of light from an external source to achieve solar photocatalysis. In the design of a parallel-plate photoreactor, it is important to properly control the Cr ion loading so that each Cr/TiO₂-coated glass plate absorbs a portion of the incident light for its photocatalytic activation and allows light transmission available for the remaining coated plates.     

Photocatalytic oxidation of nitrite by sunlight using TiO2 supported on hollow glass microbeads

The feasibility of photocatalytic oxidation of nitrite using TiO₂ supported on hollow glass microbeads as a photocatalyst by sunlight was studied. The results showed that 1.1×10⁻⁴ mol/dm³ of nitrite can be completely photocatalytically oxidized after 120-min illumination by sunlight. The conversion of nitrite was increased rapidly by adding a small amount of H₂O₂, Fe³⁺ and Pd²⁺. The effects of parameters such as the amount of TiO₂/beads, air flow, initial pH, dichromate, SO₄²⁻, NO₃⁻, Cl⁻ and organic compounds on the photocatalytic oxidation of nitrite were also studied. The possible mechanisms of photocatalytic oxidation of nitrite were investigated. After 150-h illumination by sunlight, there was no significant loss of the photocatalytic activity of TiO₂/beads.     

Green synthesis of well dispersed TiO2/Pt nanoparticles photocatalysts and enhanced photocatalytic activity towards hydrogen production

Deposition of Pt NPs with preferred dispersion and morphologies on TiO₂ have been the focus of studies in photocatalytic and photoelectrochemical hydrogen production. Green synthesis of TiO₂/Pt NPs nanocomposites with narrow size distribution of Pt NPs still remain a challenge. Herein, we report that sucrose is highly efficient for the preparation of well-dispersed TiO₂/Pt NPs photocatalysts. Moreover, the sucrose could act as an electron donor, showing higher hydrogen production activity under simulated sunlight than pure water. The as-synthesized photocatalysts have been characterized by techniques of transmission electron microscopy (TEM), energy dispersive X-ray spectrometer (EDX), and diffuse reflectance spectroscopy (DRS). Compared with TiO₂/Pt NPs photocatalysts prepared through conventional photodeposition, the photocatalysts as prepared showed higher photocatalytic efficiency. Moreover, the catalyst could be reused easily without apparent degradation of their original photocatalytic activities. This approach presents a promising and low-cost strategy to improve the photocatalytic performance of TiO₂ from biomass.     

Photocatalytic splitting of seawater and degradation of methylene blue on CuO/nano TiO2

The main objective of this study was to prepare effective photocatalysts for splitting of seawater for solar fuel – H₂ and degradation of seawater organic pollutants such as dyes. To enhance photocatalytic activities, CuO is supported on nano TiO₂ (CuO/nano TiO₂). By X-ray absorption near edge structure (XANES) spectroscopy, CuO clusters are found on nano TiO₂. The 2.5% CuO/nano TiO₂ has greater activities in photocatalytic splitting of water and seawater than nano TiO₂ by 9.9 and 7.8 times, respectively. Interestingly, the 2.5% CuO/nano TiO₂ is also very active for photocatalytic splitting of water and seawater contaminated with dyes such as methylene blue (MB) (10 ppm). Under a 5-h irradiation of the UV–Vis light, about 99% of MB is degraded while 3.1 μmol/h g cat of H₂ are generated from seawater in the photocatalysis process.     

Solar-based photoreduction of methyl orange using zeolite supported photocatalytic materials

A new class of novel photocatalysts has been prepared by supporting TiO₂ on the zeolite matrix by various routes of synthesis. Different transition metals like cobalt, nickel, and ruthenium have been incorporated in these photocatalysts, alongwith molybdenum based heteropolyacid (HPA) to improve the photocatalytic activity of these materials. Photoreduction of methyl orange under solar radiation was compared with photoreduction in presence of artificial visible light illumination to evaluate their photocatalytic activity. The quantity of methyl orange photoreduced by the cobalt containing photocatalyst was about 2.40 mg/g of TiO₂ under the influence of sunlight as compared to 4.111 mg/g of TiO₂ under artificial visible light illumination. However, the efficiency of the photocatalyst is high as compared to P25 TiO₂ under solar light (0.508 mg/g of TiO₂). The high photocatalytic activity of these materials is due to the synergistic effect of incorporation of transition metals in combination with TiO₂ and HPA supported by the zeolite matrix. These materials are being evaluated for photocatalytic water splitting.