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.     

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.     

Antireflecting–passivating dielectric films on crystalline silicon solar cells for space applications

Antireflecting–passivating TiO₂–SiO₂ double layers on crystalline silicon (Si) were optimized and characterized for space solar cells applications. In the numeric optimization, the MgF₂–glass–adhesive–TiO₂–SiO₂–Si structure was considered. In order to fabricate the TiO₂–SiO₂ double layer, titanium films were deposited on Si wafers in a vacuum chamber, and then, the sample was annealed in oxygen at high temperatures. Glasses with evaporated MgF₂ thin films were bonded to the TiO₂–SiO₂–Si samples so as to obtain the complete structure. A gain of up to 23.5% in the maximum power is demonstrated for simulated c-Si solar cells using the optimized structure. Characterization of the TiO₂–SiO₂–Si structure using transmission electron microscopy (TEM) and X-ray reflectivity (XRR) as well as optical characterization are presented.     

Sol–gel preparation and characterization of anti-reflective and self-cleaning SiO2–TiO2 double-layer nanometric films

Glass substrates were first coated with SiO₂ and then TiO₂ by dipping into sols which were prepared by two different methods involving complex formation and hydrolysis, using ethanol (EtOH) or butyl glycol (BG). Concentration of TiO₂ in the sols was kept at 0.1 and 0.5 wt%. Prepared coatings were investigated by field-emission scanning electron microscope (FESEM), atomic force microscope (AFM), hazemeter, UV–visible spectrophotometer and goniometer. Rhodamine B (RhB) photodegradation tests were performed in order to evaluate photocatalytic activity. Application of SiO₂ as the bottom layer increased the transmittance by 6% points, thereby compensated for the loss of transmittance caused by the TiO₂ self-cleaning top layer. Pencil-hardness values of the obtained coatings were in 5B–3H range. TiO₂ coatings obtained from sols containing 0.5% TiO₂ and BG solvent represented the highest photocatalytic activity, with a rate constant of 0.44 ppm⁻¹ h⁻¹ and a half period of 5.5 h. Self-cleaning surfaces were obtained while maintaining the anti-reflectance.     

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.     

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.     

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.     

Tri-layer antireflection coatings (SiO2/SiO2–TiO2/TiO2) for silicon solar cells using a sol–gel technique

Antireflection coatings (ARCs) have become one of the key issues for mass production of Si solar cells. They are generally performed by vacuum processes such as thermal evaporation, reactive sputtering, and plasma-enhanced chemical vapor deposition. In this work, a sol–gel method has been demonstrated to prepare the ARCs for the non-textured monocrystalline Si solar cells. The spin-coated TiO₂ single-layer, SiO₂/TiO₂ double-layer and SiO₂/SiO₂–TiO₂/TiO₂ triple-layer ARCs were deposited on the Si solar cells and they showed good uniformity in thickness. The measured average optical reflectance (400–1000 nm) was about 9.3, 6.2 and 3.2% for the single-layer, double-layer and triple-layer ARCs, respectively. Good correlation between theoretical and experimental data was obtained. Under a triple-layer ARC condition, a 39% improvement in the efficiency of the monocrystalline Si solar cell was achieved. These indicate that the sol–gel ARC process has high potential for low-cost solar cell fabrication.     

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.     

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.     

Production of hydrogen by methane catalytic decomposition over Ni–Cu–Fe/Al2O3 catalyst

Catalysts with high nickel concentrations 75%Ni–12%Cu/Al₂O₃, 70%Ni–10%Cu–10%Fe/Al₂O₃ were prepared by mechanochemical activation and their catalytic properties were studied in methane decomposition. It was shown that modification of the 75%Ni–12%Cu/Al₂O₃ catalyst with iron made it possible to increase optimal operating temperatures to 700–750 °C while maintaining excellent catalyst stability. The formation of finely dispersed Ni–Cu–Fe alloy particles makes the catalysts stable and capable of operating at 700–750 °C in methane decomposition to hydrogen and carbon nanofibers. The yield of carbon nanofibers on the modified 70%Ni–10%Cu–10%Fe/Al₂O₃ catalyst at 700–750 °C was 150–160 g/g. The developed hydrogen production method is also efficient when natural gas is used as the feedstock. An installation with a rotating reactor was developed for production of hydrogen and carbon nanofibers from natural gas. It was shown that the 70%Ni–10%Cu–10%Fe/Al₂O₃ catalyst could operate in this installation for a prolonged period of time. The hydrogen concentration at the reactor outlet exceeded 70 mol%.     

An ideal visible nanocomposite (Fe/GTiP) photoanode catalyst for treatment of antibiotics in water and simultaneous electricity generation in the photocatalytic fuel cell

The photocatalytic fuel cell (PFC) has been studied for the wastewater treatment and electricity generation by degrading antibiotic organic pollutant berberine chloride (BC). Through a simple chemical process Fe/GTiP anode and ZnIn₂S₄ cathode catalysts were prepared and loaded them on carbon fiber cloth. Up to 79% BC (10 mg/L) was removed with simultaneous electricity generation of 0.65 V within 90 min under pH-7 in PFC by using visible light (two 50-W halogen lamps). PFC is better with 79% BC removal and electricity generation than only 79% removal in photocatalysis (PC) without generating any clean energy. Under photocatalysis Fe/GTiP can remove 70% of BC, higher than 54% with GTiP and 12% with TiP at 50 mg catalyst/50 mL (10 mg/L BC). The photocatalytic performance of Fe/GTiP was also compared with commercial P25 and pure TiO_2. The obtained removal of 17.4% and 13.25% BC (10 mg/L) with P25 and TiO₂ proves that with more visible light absorption Fe/GTiP has significant photocatalytic effect than P25 and pure TiO₂. The impacts of external resistance, concentration of catalyst, pH, and electrolyte were investigated in the PFC. Removal of tetracycline hydrochloride (TC) (10 mg/L) followed the same trend as BC under photocatalysis with Fe/GTiP, GTiP and TiP (78%, 60% and 33% at pH-7). The removal of 89% TC (30 mg/L) in 90 min was also achieved with Fe/GTiP. The experimental study shows that Fe/GTiP visible light nanocomposite is ideal for removing antibiotics in water by photocatalysis or with simultaneous electricity generation through PFC.     

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.     

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.     

Solar photocatalytic degradation of gaseous formaldehyde by sol–gel TiO2 thin film for enhancement of indoor air quality

Solar photocatalytic degradation of formaldehyde in the gaseous phase has been investigated. The tested photoreactor is made of a borosilicate glass tube with the inner surface coated with a sol–gel TiO₂ thin film. In a pseudo-first-order Langmuir–Hinshelwood (L–H) model, the maximum reaction rate constant obtained is 0.148 min⁻¹ under an exposure to sunlight with solar UVA irradiance of 1.56 mW/cm². The solar photolysis effect is found to be negligible. It is also found that the sol–gel TiO₂ thin film has a lower apparent photonic efficiency of solar photocatalysis than a Degussa P25 TiO₂ coating. However, for the photonic efficiency taking into account the absorbed and scattered photons only and, in other words, excluding the transmitted photons, the thin film has a higher value. Based on a total of 28 measured data, an empirical-correlation equation has been developed to express the reactant residue with respect to the solar UVA irradiance and exposure time. A reasonable agreement between the correlation and experimental data is obtained. The findings of this investigation can be applied to design optimization of a honeycomb photoreactor made up of TiO₂-coated glass tubes or polygonal cells.     

Hydrogen production from water splitting under visible light irradiation using sensitized mesoporous-assembled TiO2–SiO2 mixed oxide photocatalysts

This work focused on hydrogen production from the photocatalytic water splitting under visible light irradiation using Eosin Y-sensitized mesoporous-assembled TiO₂–SiO₂ mixed oxide photocatalysts, of which the mesoporous-assembled TiO₂–SiO₂ mixed oxides with various TiO₂-to-SiO₂ molar ratios were synthesized by a sol–gel process with the aid of a structure-directing surfactant. The effects of SiO₂ content, calcination temperature, and phase composition of the mixed oxide photocatalysts were investigated. The experimental results showed that the TiO₂–SiO₂ mixed oxide photocatalyst with the TiO₂-to-SiO₂ molar ratio of 97:3 and calcined at 500 °C provided the maximum photocatalytic hydrogen production activity. The characterization results supported that the 0.97TiO₂–0.03SiO₂ mixed oxide photocatalyst (with the suitable SiO₂ content of 3 mol%) possessed superior physicochemical properties for the photocatalytic reaction as compared to the pure TiO₂, particularly higher specific surface area, lower mean mesopore diameter, higher total pore volume, and lower crystallite size, which led to an enhanced photocatalytic activity.     

Hydrothermally stabilized Fe(III) doped titania active under visible light for water splitting reaction

Fe³⁺ doped TiO₂ photocatalysts were prepared by hydrothermal treatment for the photocatalytic water splitting to produce stoichiometric hydrogen and oxygen under visible light irradiation. It was found that hydrothermal treatment at 110 °C for 10 h was essential for the synthesis of highly stabilized Fe³⁺ doped TiO₂ photocatalysts. The synthesized photocatalysts were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS) and BET surface area techniques. The doping of highly stabilized Fe³⁺ in the titania matrix leads to significant red shift of optical response towards visible light owing to the reduced band gap energy. Optimum amount of Fe³⁺ doped TiO₂, 1.0 wt% Fe/TiO₂, showed drastically improved hydrogen production performance of 12.5 μmol-H₂/h in aqueous methanol and 1.8 μmol-H₂/h in pure water, respectively. This Fe/TiO₂ photocatalyst was stable for 36 h without significant deactivation in the water splitting reaction.     

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 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.     

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.