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).     

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.     

TiO2-assisted degradation of acid orange 7 textile dye under solar light

Photocatalytic degradation of acid orange 7 (AO7) in aqueous systems was successfully achieved by the combination of TiO₂ with potassium persulphate under solar light using a photochemical reactor with recirculation. Degradation of AO7 involves color removal and mineralization. The employment of TiO₂ removed 85% of color from the 0.2 mM AO7 aqueous solution under solar light; while, 66% of color was abated using the persulphate ion as oxidant in the absence of TiO₂ under similar conditions in 2 h. However, over 90% of color removal was achieved by combining TiO₂ and the persulphate ion for the same solution under similar conditions. Color removal was faster at pH 3. Mineralization of AO7 was followed by measuring chemical oxygen demand (COD). Negligible COD abatement of the textile dye was observed in the absence of persulphate ions (S₂O₈²⁻) while over 70% of COD abatement was observed for the initial dye concentrations of 0.2–0.7 mM employing a mix of TiO₂–S₂O₈²⁻ under solar light.     

Solar photocatalytic degradation of methylene blue in carbon-doped TiO2 nanoparticles suspension

Carbon-doped TiO₂ nanoparticles were prepared by sol–gel auto-combustion method and characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Brunauer–Emmett–Teller method (BET), UV–vis diffuses reflectance spectroscopy (DRS). UV–vis diffuse reflectance spectra showed that carbon-doped TiO₂ exhibited obvious absorption in the visible light range. The visible light photocatalytic activity of carbon-doped TiO₂ was ascribed to the presence of oxygen vacancy state between the valence and the conduction bands because of the formation of Ti³⁺ species in the as-synthesized carbon-doped TiO₂. The sample calcined at 873 K showed the highest photocatalytic activity under solar irradiation. The effects of photocatalyst concentration, initial concentration of methylene blue, and pH value in aqueous solution were also presented.     

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.     

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⁻.     

Solar photocatalytic mineralization of isoproturon over TiO2/HY composite systems

The present investigation covers immobilization of titanium dioxide over HY support for the treatment of isoproturon pesticide. Catalysts are characterized by XRD, SEM–EDAX, TEM, BET surface area and UV–vis DRS. A detailed photocatalytic degradation study under solar light in aqueous suspensions with parameters like loading of TiO₂ over HY, amount of the catalyst, concentration of substrate, pH effect, durability of the catalyst and comparison between suspended TiO₂ and supported systems are reported. Mineralization of isoproturon is monitored by total organic carbon, chemical oxygen demand and a plausible mechanism is proposed for photocatalytic degradation based on degradation products.     

Solar photocatalytic degradation of a reactive azo dye in TiO2-suspension

The photocatalytic decolourisation and degradation of an azo dye reactive orange 4 (RO4) in aqueous solution with TiO₂-P25 (Degussa) as photocatalyst in slurry form have been investigated using solarlight. There is a significant difference in adsorption of dye on TiO₂ surface with the change in solution pH. The effect of various photocatalysts such as TiO₂-P25, TiO₂ (anatase), ZnO, CdS, Fe₂O₃, SnO₂ on the decolourisation and degradation has been studied. The order of reactivity of photocatalysts is TiO₂-P25>ZnO>TiO₂ (anatase). CdS, Fe₂O₃ and SnO₂ have negligible activity on RO4 decolourisation and degradation. The effects of various parameters such as catalyst loading, pH and initial concentration of the dye on decolourisation and degradation have been determined. The degradation was strongly enhanced in the presence of electron acceptors such as H₂O₂, (NH₄)₂S₂O₈ and KBrO₃. The effects of dye-assisting chemicals such as Na₂CO₃, NaCl have been carried out. Addition of these chemicals inhibits the removal rate. The photodecolourisation and degradation kinetics are discussed in terms of Langmuir–Hinshelwood kinetic model.     

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).     

Solar/TiO2 photocatalytic decomposition of β-blockers atenolol and propranolol in water and wastewater

The photocatalytic conversion of two β-blockers, namely atenolol and propranolol in aqueous TiO₂ suspensions was investigated. Irradiation was provided by a solar simulator equipped with 1 kW Xe-OP lamp, while emphasis was given on the effect of catalyst type and loading (50-3000 mg/L), substrate concentration (5-30 mg/L), initial solution pH (3-10), and the addition of H₂O₂ (0.07-1.4 mM) and oxygen on degradation in two matrices (i.e. pure water and treated municipal effluent). Of the various catalysts tested, Degussa P25 was highly active yielding up to about 80% conversion after 120 min of reaction. In general, conversion was favored at lower substrate concentrations, near-neutral pH values and in the absence of other organics (i.e. in pure water), while the addition of H₂O₂ did not accelerate kinetics which seem to follow the Langmuir-Hinshelwood model. Toxicity to D. magna was evaluated prior to and after photocatalytic treatment. Toxicity increased during the early stages of the reaction and then progressively decreased upon the elimination of the substrate and its reaction intermediates, with propranolol being more toxic than atenolol.     

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.     

Nanocrystalline TiO2 photosensitized with natural polymers with enhanced efficiency from 400 to 600 nm

TiO₂ sensitization for solar applications requires not only efficient but also stable and inexpensive sensitizers. Different condensed tannins extracted from bark wastes of tropical wood trees were studied as possible sensitizers of TiO₂. These natural polymers adhere strongly to the TiO₂ even from aqueous solutions. Absorption spectra are presented for 1 mM aqueous sensitizing solutions prepared with lyophilized condensed tannins which absorb light in the visible range. Spectral photocurrent measurements and I–V characterization show that no bias is required for electron injection to the TiO₂ from all studied condensed tannins. Incident photon to current efficiency (IPCE) analysis indicates that surface complexation originates absorption bands with different electron injection efficiencies. These play a dominant role in determining IPCE spectral shape. We propose that surface modification by the sensitizer changes the surface trap density, thereby decreasing recombination losses.     

Formation of multilayer-Eosin Y-sensitized TiO2 via Fe coupling for efficient visible-light photocatalytic hydrogen evolution

An efficient visible-light active photocatalyst of multilayer-Eosin Y-sensitized TiO₂ is prepared through linkage of Fe³⁺ between not only TiO₂ and Eosin Y but also different Eosin Y molecules to form three-dimensional polymeric dye structure. The multilayer-dye-sensitized photocatalyst is found to have high light harvesting efficiency and photocatalytic activity for hydrogen evolution under visible light irradiation (λ > 420 nm). On the optimum conditions (1:1 initial molar ratio of Eosin Y to Fe(NO₃)₃, initial 10 × 10⁻³ M Eosin Y, and 1.0 wt% Pt deposited by in situ photoreduction), its maximal apparent quantum yield for hydrogen evolution is 19.1% from aqueous triethanolamine solution (TEOA aq). The present study highlights linking between dye molecules via metal ions as a general way to develop efficient visible-light photocatalyst.     

Enhanced hydrogen generation using ZrO2-modified coupled ZnO/TiO2 nanocomposites in the absence of noble metal co-catalyst

Mesoporous ZrO₂-modified coupled ZnO/TiO₂ nanocomposites were prepared by a surfactant assisted sol–gel method. The photocatalytic performance of these materials was investigated for H₂ evolution without noble metal co-catalyst using aqueous methanol media under AM1.5 simulated light. The H₂ evolution was compared with coupled ZnO/TiO₂, TiO₂, ZnO and Degussa P25. The ZrO₂-modified nanocomposites exhibited higher H₂ generation, specifically 0.5 wt.% ZrO₂ loading produced 30.78 mmol H₂ g⁻¹ compared to 3.55 mmol H₂ g⁻¹ obtained with coupled ZnO/TiO₂. A multiple absorbance thresholds at 435 nm and 417 nm were observed with 0.5 wt.% ZrO₂ loading, corresponding to 2.85 eV and 2.97 eV band gap energies. The high surface area, large pore volume, uniform crystallite sizes and enhanced light harvesting observed in ZrO₂-modified nanocomposites were contributing factors for effective charge separation and higher H₂ production. The possible mechanism of H₂ generation from aqueous methanol solution over ZrO₂-modified nanocomposite is presented.     

Research on photocatalytic H2 production from acetic acid solution by Pt/TiO2 nanoparticles under UV irradiation

The Pt/TiO₂ particles have been prepared by the photodeposition of Pt on TiO₂ surface and characterized by X-ray diffraction. Photocatalytic H₂ production from acetic acid (HAc) over Pt/TiO₂ in aqueous solution has been studied at ambient temperature under UV irradiation. The effects of operational variables such as Pt loading, photocatalyst concentration, HAc concentration, and solution pH, have been systematically investigated. The optimum conditions for H₂ production from HAc by Pt/TiO₂ were Pt loading 1.0 wt.%, Pt/TiO₂ concentration 0.22 g/L, HAc concentration 6.52 g/L and pH 1.0. The H₂ yield is 0.27 mol-H₂/mol-HAc obtained under prolonged time irradiation. Experimental results showed that the photocatalytic H₂ production activity could be enhanced remarkably by depositing a suitable amount Pt on TiO₂ surface. Based on our results, a new process for H₂ production from biomass can be achieved by coupling fermentative H₂ production with photocatalytic H₂ production. The process also provides a method for degradation of organic pollutants with simultaneous H₂ production. A possible mechanism for photocatalytic decomposition of HAc over Pt/TiO₂ was also proposed.     

Oxygen evolution by the photo-oxidation of water

Photocatalytic oxygen evolution from TiO₂ suspended in water containing several electron acceptors was studied by measuring the increase in the concentration of dissolved oxygen with an oxygen electrode. The variation in the initial oxygen evolution rate expressed as a function of the concentration of electron acceptors showed either a maximum or a leveling off. The maximum rate (or the rate which levels off) was plotted against the standard redox potential of the electron acceptor, which showed the increase in the rate with rising redox potential. It is shown from this relationship that an irreversible redox system was not necessarily more efficient for oxygen evolution than a reversible system. The pH dependency of the oxygen evolution was discussed on the basis of the reaction mechanism of the electron acceptor with water sensitized by TiO₂, RuO₂ loading to TiO₂ enhanced the oxygen evolution, whereas Pt loading decreased the efficiency. Illumination of Pt and RuO₂ loaded TiO₂ suspension in the presence of a reduced species of a reduced species of electron acceptor (Fe⁺³) decreased the concentration of dissolved oxygen, to which the negative effect of Pt loading was partly attributed.     

Heterogeneous photocatalytic systems: Influence of some operational variables on actual photons absorbed by aqueous dispersions of TiO2

The photooxidation of phenol in aqueous solution in the presence of polycrystalline TiO₂ powder was used as "test" reaction in order to investigate the influence of incident light intensity and of some physical and chemical parameters on the actual absorbed photon flows and on the reactivity. The physical parameters investigated were the surface area and the size of particles while the chemical parameters investigated were the initial pH of the dispersion and the presence in the reacting medium of additives affecting the photoreactivity such as Cl⁻ and H₂O₂.     

Hydrogen production from water splitting over Eosin Y-sensitized mesoporous-assembled perovskite titanate nanocrystal photocatalysts under visible light irradiation

The photocatalytic water splitting is a promising process for producing H₂ from two abundant renewable sources of water and solar light, with the aid of a suitable photocatalyst. In this work, a combination of sensitizer addition and noble metal loading was employed to modify perovskite photocatalysts in order to achieve the enhancement of photocatalytic H₂ production under visible light irradiation. The dependence of the H₂ production on type of mesoporous-assembled perovskite titanate nanocrystal photocatalysts (MgTiO₃, CaTiO₃, and SrTiO₃), calcination temperature of photocatalyst, Pt loading, type and concentration of electron donor (diethanolamine, DEA; and triethanolamine, TEA), concentration of sensitizer (Eosin Y, E.Y.), photocatalyst dosage, and initial solution pH, was systematically studied. The experimental results showed that the 0.5 wt.% Pt-loaded mesoporous-assembled SrTiO₃ nanocrystal synthesized by a single-step sol-gel method and calcined at 650 °C exhibited the highest photocatalytic H₂ production activity from a 15 vol% DEA aqueous solution with dissolved 0.5 mM E.Y. Moreover, the optimum photocatalyst dosage and initial solution pH for the maximum photocatalytic H₂ production activity were found to be 6 g/l and 11.6, respectively.     

Photocatalysis of 4-chlorophenol mediated by TiO2 fixed to concrete surfaces

Titanium dioxide (TiO₂) powder was mixed with varying concrete sealer formulations and applied to prepared concrete surfaces within PVC batch reactors. A solution of 4-chlorophenol in de-ionized water, with an initial 4-chlorophenol concentration of 4 mg/L, was added to each open reactor. The photocatalytic efficiency of each TiO₂-sealer was determined through bulk solution sampling while a reactor was exposed to continuous UV light. Through analysis of mean 4-chlorophenol concentrations, the disappearance of 4-chlorophenol from solution was attributed solely to the presence of TiO₂ in the sealed concrete surface of a UV exposed reactor. The rate of disappearance increased when the mass percent of TiO₂ in the sealer was increased. The zero-order reaction rate constants ranged from 0.135 to 0.2873 mg L⁻¹ h⁻¹.     

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.