Photocatalytic degradation of organophosphorus pesticides using floating photocatalyst TiO2 · SiO2/beads by sunlight

In this paper, the floating TiO₂ · SiO₂ photocatalyst beads are prepared by the dip-coating method, which use hollow glass microbeads as the carrier and titanium tetraisopropoxide [Ti(iso-OC₃H₇)₄] and ethyl silicate as the raw materials. The feasibility of photocatalytic degradation of organophosphorus pesticides using TiO₂ · SiO₂ beads as a floating photocatalyst by sunlight is studied. The results show that the best heat treatment condition for TiO₂ · SiO₂ beads is at 650 °C for 5 h. Apart from heat treatment temperature and time, the amount of SiO₂ also influences the photocatalytic activity of TiO₂ · SiO₂ beads. The optimum amount of SiO₂ is 0.20 (molecular fraction). 0.65 × 10⁻⁴ mol/dm³ of four organophosphorus pesticides of three structures can be completely photocatalytically degraded into after 420 min illumination by sunlight. The effects of parameters such as the amount of TiO₂ · SiO₂ beads, initial pH and metal ions on the photocatalytic degradation of the organophosphorus pesticides are also studied. The possible mechanisms of photocatalytic degradation of phosphate ester pesticides are proposed. After 120 h illumination by sunlight, there is no significant loss of the photocatalytic activity of TiO₂ · SiO₂ beads.     

Photocatalytic H2 production from water splitting under visible light irradiation using Eosin Y-sensitized mesoporous-assembled Pt/TiO2 nanocrystal photocatalyst

Sensitized photocatalytic production of hydrogen from water splitting is investigated under visible light irradiation over mesoporous-assembled titanium dioxide (TiO₂) nanocrystal photocatalysts, without and with Pt loading. The photocatalysts are synthesized by a sol–gel process with the aid of a structure-directing surfactant and are characterized by N₂ adsorption–desorption analysis, X-ray diffraction, UV–vis spectroscopy, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray analysis. The dependence of hydrogen production on the type of TiO₂ photocatalyst (synthesized mesoporous-assembled and commercial non-mesoporous-assembled TiO₂ without and with Pt loading), the calcination temperature of the synthesized photocatalyst, the sensitizer (Eosin Y) concentration, the electron donor (diethanolamine) concentration, the photocatalyst dosage and the initial solution pH is systematically studied. The results show that in the presence of the Eosin Y sensitizer, the Pt-loaded mesoporous-assembled TiO₂ synthesized by a single-step sol–gel process and calcined at 500 °C exhibits the highest photocatalytic activity for hydrogen production from a 30 vol.% diethanolamine aqueous solution with dissolved 2 mM Eosin Y. Moreover, the optimum photocatalyst dosage and initial solution pH for the maximum photocatalytic activity for hydrogen production are 3.33 g dm⁻³ and 11.5, respectively.     

Low-temperature synthesis of TiO2 nanoparticles and preparation of TiO2 thin films by spray deposition

Low-temperature (180–240 °C) synthesis of nanocrystalline titanium dioxide (TiO₂) by surfactant-free solvothermal route is investigated. Titanium iso-propoxide is used as the precursor and toluene as the solvent. Different precursors to solvent weight ratios have been used for the synthesis of TiO₂ nanoparticles. For the weight ratios 15/100, 25/100 and 35/100 the X-ray diffractograms show the formation of nanocrystalline TiO₂. The X-ray diffraction and transmission electron microscopy studies shows that the product has anatase crystal structure (for temperatures <200 °C) with average particle size below 15 nm. The films deposited by spray deposition method using these nanoparticles show the crystalline and porous nature of the films. The present method of deposition also avoids the post-treatment (sintering) of the films. The nanoparticles thus prepared and the films can be used for gas sensing and biological applications and also as photo-electrodes for dye-sensitized solar cells.     

The preparation of TiO2–nitrogen doped by calcination of TiO2·xH2O under ammonia atmosphere for visible light photocatalysis

The investigation on incorporating nitrogen group into titanium dioxide in order to obtain powdered visible light-active photocatalysts is presented. The industrial hydrated amorphous titanium dioxide (TiO₂·xH₂O) obtained directly from sulphate technology installation was modified by heat treatment at temperatures of 100–800 °C for 4 h in an ammonia atmosphere. The photocatalysts were characterized by UV–VIS–DR and XRD techniques. The UV–VIS–DR spectra of the modified catalysts exhibited an additional maximum in the VIS region (, ) which may be due to the presence of nitrogen in TiO₂ structure. On the basis of XRD analysis it can be supposed that the presence of nitrogen does not have any influence on the transformation temperature of anatase to rutile. The photocatalytic activity of the modified photocatalysts was determined on the basis of decomposition rate of phenol and azo-dye (Reactive Red 198) under visible light irradiation. The highest rate of phenol degradation was obtained for catalysts calcinated at 700 °C (6.55%), and the highest rate of dye decomposition was found for catalysts calcinated at 500 and 600 °C (ca. 40–45%). The nitrogen doping during calcination under ammonia atmosphere is a very promising way of preparation of photocatalysts which could have a practical application in water treatment system under broader solar light spectrum.     

Preferential oxidation of CO in H2 stream on Au/CeO2–TiO2 catalysts

A series of Au catalysts supported on CeO₂–TiO₂ with various CeO₂ contents were prepared. CeO₂–TiO₂ was prepared by incipient-wetness impregnation with aqueous solution of Ce(NO₃)₃ on TiO₂. Gold catalysts were prepared by deposition–precipitation method at pH 7 and 65 °C. The catalysts were characterized by XRD, TEM and XPS. The preferential oxidation of CO in hydrogen stream was carried out in a fixed bed reactor. The catalyst mainly had metallic gold species and small amount of oxidic Au species. The average gold particle size was 2.5 nm. Adding suitable amount of CeO₂ on Au/TiO₂ catalyst could enhance CO oxidation and suppress H₂ oxidation at high reaction temperature (>50 °C). Additives such as La₂O₃, Co₃O₄ and CuO were added to Au/CeO₂–TiO₂ catalyst and tested for the preferential oxidation of CO in hydrogen stream. The addition of CuO on Au/CeO₂–TiO₂ catalyst increased the CO conversion and CO selectivity effectively. Au/CuO–CeO₂–TiO₂ with molar ratio of Cu:Ce:Ti = 0.5:1:9 demonstrated very high CO conversion when the temperature was higher than 65 °C and the CO selectivity also improved substantially. Thus the additive CuO along with the promoter and amorphous oxide ceria and titania not only enhances the electronic interaction, but also stabilizes the nanosize gold particles and thereby enhancing the catalytic activity for PROX reaction to a greater extent.     

Development of copper-doped TiO2 photocatalyst for hydrogen production under visible light

The advantage of copper doping onto TiO₂ semiconductor photocatalyst for enhanced hydrogen generation under irradiation at the visible range of the electromagnetic spectrum has been investigated. Two methods of preparation for the copper-doped catalyst were selected – complex precipitation and wet impregnation methods – using copper nitrate trihydrate as the starting material. The dopant loading varied from 2 to 15%. Characterization of the photocatalysts was done by thermogravimetric analysis (TGA), temperature programmed reduction (TPR), diffuse reflectance UV-Vis (DR-UV-Vis), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD). Photocatalytic activity towards hydrogen generation from water was investigated using a multiport photocatalytic reactor under visible light illumination with methanol added as a hole scavenger. Three calcination temperatures were selected – 300, 400 and 500 °C. It was found that 10 wt.% Cu/TiO₂ calcined at 300 °C for 30 min yielded the maximum quantity of hydrogen. The reduction of band gap as a result of doping was estimated and the influence of the process parameters on catalytic activity is explained.     

Improving hydrogen production via water splitting over Pt/TiO2/activated carbon nanocomposite

Mesoporous TiO₂/AC, Pt/TiO₂ and Pt/TiO₂/AC (AC = activated carbon) nanocomposites were synthesized by functionalizing the activated carbon using acid treatment and sol–gel method. Photochemical deposition method was used for Pt loading. The nano-photocatalysts were characterized using XRD, SEM, DRS, BET, FTIR, XPS, CHN and ICP methods. The hydrogen production, under UV light irradiation in an aqueous suspension containing methanol has been studied. The effect of Pt, methanol and activated carbon were investigated. The results show that the activated carbon and Pt together improve the hydrogen production via water splitting. Also methanol acts as a good hole scavenger. Mesoporous Pt/TiO₂/AC nanocomposite is the most efficient photocatalyst for hydrogen production compared to TiO₂/AC, Pt/TiO₂ and the commercial photocatalyst P25 under the same photoreaction conditions. Using Pt/TiO₂/AC, the rate of hydrogen production is 7490 μmol (h g catal.)⁻¹ that is about 75 times higher than that of the P25 photocatalyst.     

Oxidative photoelectrochemical technology with Ti/TiO2 anodes

Rutile and anatase TiO₂ films have been grown on Ti plates by thermal (500–800°C) and anodic oxidation followed by thermal annealing (400–500°C), respectively. The photoelectrochemical efficiency of these photoanodes, evaluated by current density measurements in the photooxidation of 4-methoxybenzyl alcohol in deaerated CH₃CN, has been determined. The photocurrent efficiency increases with the thickness of the TiO₂ rutile film up to 1 μm (the most efficient thickness). At the wavelengths furnished by the irradiation apparatus similar thicknesses of anatase and rutile films show nearly the same efficiencies. Anodic bias produces similar relative increases of current intensity in both crystalline forms.     

Direct solar water splitting cell using water, WO3, Pt, and polymer electrolyte membrane

A solar water splitting cell composed of WO₃, Polymer Electrolyte Membrane (PEM) and Pt was constructed for producing hydrogen from deionized water in sunlight. Spectral responsivity measurements under various temperatures and bias voltages were conducted for the cell using the Incident Photon to Current Efficiency (IPCE) method. For comparison, a known WO₃ Photo Electro Chemical (PEC) cell containing H₃PO₄ electrolyte, WO₃/H₃PO₄/Pt, was tested using the same test method. The WO₃/PEM–H₂O/Pt cell showed better Quantum Efficiency (QE) performance compared to that obtained from the cell with the chemical electrolyte. For the first time, spectral responsivity of photo water splitting process without bias power was unveiled in the new WO₃ cell, demonstrating the self-sustained photo electrolysis capability. Bias voltage effect on Solar to Hydrogen (STH) conversion efficiency was dramatic in the range from 0.2 V to 1.2 V and suppressions of STH were observed when high bias voltages were applied. In addition, a strong temperature effect on the energy conversion efficiency at high bias voltage was observed in the cell containing PEM–H₂O, revealing that the STH at 54 °C is nearly five times that at 14 °C.     

Effect of the sintering temperature on the photocatalytic activity of ZnO+Zn2TiO4 thin films

ZnO+Zn₂TiO₄ thin films were obtained by the sol–gel method, the precursor solutions were prepared using two Ti/Zn ratios: 0.49 and 0.69. The films were deposited on glass slide substrates and sintered at temperatures in the 200–600 °C range in increments of 50 °C, with the goal of studying the dependence of the photocatalytic activity (PA) on the annealing temperature. The films were characterized by X-ray diffraction and UV–Vis spectroscopy. The PA was evaluated by measuring the UV–Vis absorption spectra of the methylene blue in aqueous solution before and after photobleaching, using the Lambert–Beer's principle. The higher photocatalytic activities were obtained from the films with sintering temperature around 450 °C, for both Ti/Zn ratios studied.     

Electrochromic properties of heat-treated thin films of CeO2–TiO2–ZrO2 prepared by sol–gel route

CeO₂–TiO₂–ZrO₂ thin films were prepared using the sol–gel process and deposited on glass and ITO-coated glass substrates via dip-coating technique. The samples were heat treated between 100 and 500 °C. The heat treatment effects on the electrochromic performances of the films were determined by means of cyclic voltammetry measurements. The structural behavior of the film was characterized by atomic force microscopy and X-ray diffraction. Refractive index, extinction coefficient, and thickness of the films were determined in the 350–1000 nm wavelength, using nkd spectrophotometry analysis.Heat treatment temperature affects the electrochromic, optical, and structural properties of the film. The charge density of the samples increased from 8.8 to 14.8 mC/cm², with increasing heat-treatment temperatures from 100 to 500 °C. It was determined that the highest ratio between anodic and cathodic charge takes place with increase of temperature up to 500 °C.     

Efficient photochemical hydrogen production under visible-light over artificial photosynthetic systems

Photosynthesis of green plants provides an effective blueprint for transform solar energy into useful hydrogen energy. Thereinto, their hierarchical structures are favorable to the light-harvesting. Meanwhile, the functional components (light-harvesting pigments) can absorb visible wavelengths of sunlight, and offer reaction center for the energy transform. Inspired by these, we contrive an artificial photosynthetic system for the high efficiency of H₂-production rate by introducing a similar functional structure (reticular hierarchical structure) and component (CdS/Pt–TiO₂). The CdS/Pt–TiO₂ with hierarchically reticular structure is prepared by transforming wings into TiO₂ via a sol–gel process, and depositing Pt and CdS nanoparticles onto the TiO₂ substrate by photoreduction and chemical bath deposition method, respectively. Contributing to the couple effect of reticular hierarchical structure and ternary hybrid composition, CdS/Pt–TiO₂ nanocomposites exhibit high visible-light photocatalytic H₂-production rate (12.7% apparent quantum efficiency obtained at 420 nm). This concept provides a new horizon to exploit solar energy for sustainable energy by imitating the photosynthesis process from structure and ingredients.     

Floating photocatalysts based on TiO2 grafted on expanded polystyrene beads for the solar degradation of dyes

In this work, a highly active, low cost, simple and robust floating photocatalyst based on TiO₂ P25 grafted on expanded polystyrene (EPS) beads was developed. SEM and TG analyses showed that ca. 18 wt% of TiO₂ can be permanently grafted on the surface of EPS particles. This floating photocatalyst showed high efficiency for the degradation of three different dyes, i.e. methylene blue, indigo carmine and drimaren red, with UV (Hg 245 nm) or solar irradiation under constrained conditions, i.e. non-stirring and no-oxygenation. Under the same conditions pure TiO₂ showed very low activities. The floating photocatalyst can be reused for at least four consecutive times without any significant decrease on the discoloration and TOC removal after each reuse.     

Visible-light-driven TiO2 catalysts doped with low-concentration nitrogen species

Visible-light-driven nitrogen-doped TiO₂ was synthesized using a novel nitrogen-ion donor of hydrazine hydrate. Low-concentration (0.2 at%) nitrogen species and Ti³⁺ were detected in the TiO₂-based photocatalyst by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) spectroscopy. The trace amount of Ti–N would contribute to the minor band-gap narrowing of about 0.02 eV. Those nitrogen-containing species, especially the NO₂²⁻ species, form surface states, which make the catalysts possible to degrade 4-chlorophenol (4-CP) under visible irradiation (λ>400 nm). Moreover, Ti³⁺ species induce oxygen vacancy states between the valence and the conduction bands, which would also contribute to the visible response. The photocatalytic activity of the nitrogen-doped TiO₂ catalyst was thought to be the synergistic effect of nitrogen and Ti³⁺ species. The catalysts showed higher photocatalytic activity for degradation of 4-CP than pure TiO₂ under not only visible but also UV irradiation. The visible response and the higher UV activity of the nitrogen-doped TiO₂ make it possible to utilize solar energy efficiently to execute photocatalysis processes.     

TiO2 and TiO2–SiO2 thin films and powders by one-step soft-solution method: Synthesis and characterizations

Simple soft-solution method has been developed to synthesize films and powders of TiO₂ and mixed TiO₂–SiO₂ at relatively low temperatures. This method is simple and inexpensive. Furthermore, reactor can be designed for large-scale applications as well as to produce large quantities of composite powders in a single step. For the preparation of TiO₂, we used aqueous acidic medium containing TiOSO₄ and H₂O₂, which results in a peroxo-titanium precursor while colloidal SiO₂ has been added to the precursor for the formation of TiO₂–SiO₂. Post annealing at 500 °C is necessary to have anatase structure. Resulting films and powders were characterized by different techniques. TiO₂ (anatase) phase with (1 0 1) preferred orientation has been obtained. Also in TiO₂–SiO₂ mixed films and powders, TiO₂ (anatase) phase was found. Fourier transform infrared spectroscopy (FTIR) results for TiO₂ and mixed TiO₂–SiO₂ films have been presented and discussed. The method developed in this paper allowed obtaining compact and homogeneous TiO₂ films. These compact films are highly photoactive when TiO₂ is used as photo anode in an photoelectrochemical cell. Nanoporous morphology is obtained when SiO₂ colloids are added into the solution.     

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.     

Temperature dependence and the oscillatory behavior of the opto-electronic properties of a dye-sensitized nanocrystalline TiO2 solar cell

A dye-sensitized TiO₂ solar cell was developed and characterized. The I–V (current–voltage) characteristics were studied at different temperatures from −40°C to 80°C. The opto-electronic properties of the cell depend on factors like ambient temperature and the time constants of the redox processes at the cell interfaces. The temperature dependence of V_oc and I_sc were clearly demonstrated. I_sc increased with increasing temperature above room temperature, where as V_oc increased with decreasing temperature below room temperature. The opto-electronic properties showed oscillatory behavior especially at low temperatures, which may be attributed to the different velocities of the redox processes occurring at the TiO₂/dye, dye/electrolyte and the electrolyte/counter electrode interfaces.     

Photocatalyzed N-demethylation and degradation of methylene blue in titania dispersions exposed to concentrated sunlight

S imulated wastewaters that con tain methylene blue (MB) were bleached in a photocatalytic aqueous TiO₂ dispersion illuminated by concentrated sunlight using a parabolic round concentrator reactor (PRCR). The kinetic analysis was carried out well when the temporal concentration variation was a function of the concentrated light energy irradiated. The p hotocatalyzed N-demethylation of MB takes place concurrently with photocatalytic decomposition of MB by pseudo- first-order kinetics. The dependence of the photo- decomposition kinetics on the initial concentration of MB is consistent with the Langmuir– Hinshelwood model. Elimination of TOC (total organic carbon) also occurs by pseudo- first-order kinetics prior to full bleaching of the aqueous TiO₂ dispersion, after which the TOC level decreases only slightly. Also, compared with the open to air, the photodegradation of MB is not influenced by molecular oxygen bubbl ing continuously through the reactant suspensions during illumination . TiO₂ loadings and flow rates markedly a ffect the degradation o f MB. Under concentrated sunlight, t he relative photonic efficiency of MB photodegradation i s ζ_rel=0.49 (relative to phenol) . T he efficiency ζ_rel for the degradation of MB is independent of photoreactor geometry (cylindrical bottle reactor versus round-bot tomed flask), of light sources (solar light concentrator versus a Hg lamp) and of the operating mode used (flow versus batch operation).     

Enzymatic hydrogen production by light-sensitized anodized tubular TiO2 photoanode

Preliminary experiments with a slurry system of enzyme and powdery photocatalyst mixed in one compartment suggested that the electron transfer from light-sensitized photocatalyst to enzyme is the rate-determining step. Hence, in this study an anodized tubular TiO₂ electrode (ATTE) on a titanium substrate was examined as a photoanode in an anodic cell for enzymatic hydrogen production in a cathodic cell. Anodization of Ti foil in a two-electrode electrochemical cell followed by annealing in an O₂ atmosphere led to the formation of a tube-shaped TiO₂ arrays, destroyed tube arrays, or spongelike TiO₂ dense film. Samples were proven based on methylene blue (MB) discoloration to be photocatalytically active. The rate of photocatalytic hydrogen production in one of the samples (20 V–25 °C in a mixed electrolyte/350 °C–5 h) was 40 μmol/(h cm²) with a 0.1 M Na₂S electrolyte in one compartment reactor system, while the enzymatic hydrogen production rate with light-sensitized photoanode was 30 μmol/(h cm²) in the cathodic compartment with an oxygen production rate of 15 μmol/(h cm²) in the anodic compartment. These results confirmed the successful evolution of stoichiometric H₂ and O₂ separately. For the system with a sample (20 V–5 °C in 0.5% HF/650 °C–5 h), a hydrogen production rate was ca. 43 μmol/(h cm²) in the cathodic compartment and an oxygen production rate was ca. 20 μmol/(h cm²) in the anodic compartment. X-ray diffraction (XRD) results clearly indicated that the samples showing the highest evolution rate were composed of both anatase and rutile, while those made of either anatase or rutile showed a lower evolution rate. Higher annealing temperatures increased the thickness of the oxide barrier layer and obstructed the charge transfer to the back contact.     

Influence of platinum on catalytic activity of polycrystalline WO3 employed for phenol photodegradation in aqueous suspension

A series of polycrystalline WO₃ solids loaded with platinum (0.5; 1.0; 2.0 and 3.0 wt%) were prepared and characterised by X-ray diffraction, diffuse reflectance spectroscopy, scanning electron microscopy, specific surface areas determination, X-ray photoelectron spectroscopy. XPS and X-ray results indicate that the samples loaded with Pt up to 1 wt% contained on the surface mainly Pt(2+) species, while those loaded with higher amounts contained Pt(0). Moreover, the samples were employed for a probe reaction, i.e. phenol photooxidation in aqueous suspension, and their photoactivity was compared with that of two TiO₂ commercial samples, mainly in the anatase phase. All the Pt–WO₃ samples were more photoactive than the bare WO₃ for the partial photooxidation of phenol and particularly those containing 2 and 3 wt% of platinum were also more photo active that the TiO₂ samples used for comparison. On the contrary, as indicated by the total organic carbon analyses, the mineralisation (complete photooxidation) of phenol was faster on TiO₂ samples. Cathecol, quinone and muconic acid were found as main intermediates, both in the presence of Pt–WO₃ and TiO₂ samples, while hydroquinone was found only when TiO₂ samples were used.