Effect of humidity on the photocatalytic degradation of trichloroethylene in gas Phase over TiO2 thin films treated by different conditions

The effect of humidity on the photocatalytic degradation reaction of trichloroethylene (TCE) in gas phase was investigated by using pretreated TiO₂ sol-gel films. It was observed that the photocatalytic activity of the TiO₂ films depended more greatly on the pretreatment conditions, for example, UV pre-illumination, than on the moisture content. When the relative humidity was over 50%, the reaction rates decreased regardless of treatment conditions of the photocatalyst. The photocatalytic decomposition rate of TCE increased with the increase of light intensity. However, the influence of humidity on the reaction rate was less significant under the increased light intensity. The intermediates and byproducts of the reaction were not changed in different humidity conditions.     

Structure and photocatalytic performance of magnetically separable titania photocatalysts for the degradation of propachlor

A magnetic photocatalyst was prepared by modification of TiO₂ nanoparticles (Degussa P25) with nanocrystalline γ-Fe₂O₃ nanoparticles through a protective lining made up of two oppositely charged polyelectrolytes. As-prepared magnetically separable photocatalysts differing in γ-Fe₂O₃ loading (3, 8, 13, 20 and 30 wt.%) were characterized by XRD, TEM, thermal analysis, Mössbauer and magnetic measurements. The photocatalytic efficiency of the nanocomposite catalysts was evaluated using a chloroacetanilide herbicide (propachlor) in water as model compound. The primary degradation of propachlor followed pseudo-first-order kinetics according to the Langmuir–Hinshelwood model. Generally, all magnetic photocatalysts exhibit good catalytic activity towards organic pollutants, do not suffer from photodissolution and can be reused several times without any decrease in their photocatalytic activity.     

TiO2, TiO2/Ag and TiO2/Au photocatalysts prepared by spray pyrolysis

TiO₂, TiO₂/Ag and TiO₂/Au photocatalysts exhibiting a hollow spherical morphology were prepared by spray pyrolysis of aqueous solutions of titanium citrate complex and titanium oxalate precursors in one-step. Effects of precursor concentration and spray pyrolysis temperature were investigated. By subsequent heat treatment, photocatalysts with phase compositions from 10 to 100% rutile and crystallite sizes from 12 to 120 nm were obtained. A correlation between precursor concentration and size of the hollow spherical agglomerates obtained during spray pyrolysis was established. The anatase to rutile transformation was enhanced with metal incorporations and increased precursor concentration. The photocatalytic activity was evaluated by oxidation of methylene blue under UV-irradiation. As-prepared TiO₂ particles with large amounts of amorphous phase and organic residuals showed similar photocatalytic activity as the commercial Degussa P25. The metal incorporated samples showed comparable photocatalytic activity to the pure TiO₂ photocatalysts.     

Evaluating the effectiveness of photocatalysts based on titanium dioxide in the degradation of the dye Ponceau 4R

The performance of different titanium dioxide (TiO₂) catalysts and a composite based on the association between TiO₂ P25 and Zinc Phthalocyanine (TiO₂/ZnPc 1.6%) was evaluated in the photocatalytic degradation of the dye Ponceau 4R (C.I. 16255). The results show that the composite presents a better performance than the other, reaching about 50% mineralization of the dye in 120 min of reaction, around three times higher than that observed under the action of pure TiO₂ P25, although this catalyst presents a quantum yield of hydroxyl radical generation about three times higher than that estimated for the composite. This result seems to be a consequence of the synergism between the electronically excited ZnPc aggregates and the TiO₂ surface.A parallel study, based on methods from quantum mechanics also suggested the most feasible routes for the photodegradation of the dye in the absence of the catalyst.     

Phorate degradation by TiO2 photocatalysis: Parameter and reaction pathway investigations

The photocatalytic degradation of phorate in aqueous suspensions was examined with the use of titanium dioxide (TiO₂) as a photocatalyst. About 99% of phorate was degraded after UV irradiation for 60 min. The photodegradation of phorate followed pseudo-first-order kinetics and parameters such as pH of the system, TiO₂ dosage, and presence of anions were found to influence the reaction rate. To obtain a better understanding of the mechanistic details of this TiO₂-assisted photodegradation of phorate with UV irradiation, the intermediates of the processes were separated, identified, and characterized by the solid-phase microextraction (SPME) and gas chromatography/mass spectrometry (GC/MS) techniques. The probable photodegradation pathways were proposed and discussed. To the best of our knowledge, this is the first study that reports the degradation pathways of phorate. The electrical energy consumption per order of magnitude for photocatalytic degradation of phorate was also calculated and showed that a moderated efficiency (E_EO = 96 kWh/(m³ order)) was obtained in TiO₂/UV process.     

Inorganic gels as precursors of TiO2 photocatalysts prepared by low temperature microwave or thermal treatment

A simple procedure for preparing active TiO₂ photocatalysts is presented. The starting materials were unusual TiO₂ gels formed from TiCl₄. The use of microwaves for a very short time enhanced the TiO₂ crystallinity preventing an increase of particle size and minimizing the decrease of specific surface area. This result makes this preparation very attractive. The formation of the gels was monitored through measurements of viscosity. All the samples were characterized by means of X-ray diffraction, diffuse reflectance spectroscopy and BET specific surface area measurements. The photoactivity of the samples was evaluated using the photodegradation of 4-nitrophenol in liquid–solid regimen and gaseous 2-propanol as probe reactions. Commercial TiO₂ Degussa P25 was used for the sake of comparison.     

Design and Development of Highly Efficient Rectangular Column Structured Titanium Oxide Photocatalysts Anchored onto Silica Sheets

The purpose of this study is to design and develop highly efficient photocatalysts that can be anchored onto a substrate, and to this end we have prepared a titanium oxide photocatalyst using a wet process or a dry process. The results of this study led to the successful development of highly efficient rectangular column structured titanium oxide photocatalysts which could be anchored onto silica sheets.     

TiO2 nanotubes and CNT–TiO2 hybrid materials for the photocatalytic oxidation of propene at low concentration

In this work, we investigated titanium dioxide (TiO₂) nanotubes and CNT–TiO₂ hybrid materials for the photocatalytic oxidation (PCO) of propene at low concentration (100 ppmv) in gaseous phase. The materials were prepared via sol–gel method using sacrificial multi-walled carbon nanotubes (CNT) as templates and subsequent heat treatments to obtain the desired crystalline phase (anatase, rutile or a mixture of both) and eventually to remove the carbon template. We also studied rutile nanotubes for the first time and demonstrate that the activity strongly depends on the crystalline composition, following rutile < anatase < anatase/rutile mixture. The enhanced activity of the anatase–rutile mixture is attributed to the decrease in the electron–hole pair recombination due to the multiphasic nature of the particles. The key result of this work is the exceptional performance of the CNT–TiO₂ hybrid, which yielded the highest observed photocatalytic activity. The improved performance is attributed to synergistic effects due to the hybrid nature of the material, resulting in small anatase crystalline sizes (CNT act as heat sinks) and a reduced electron–hole pair recombination rate (CNTs act as electron traps). These results demonstrate the great potential of hybrid materials and stimulate further research on CNT-inorganic hybrid materials in photocatalysis and related areas.     

Photocatalytic activity of TiO2-based materials for the oxidation of propene and benzene at low concentration in presence of humidity

This paper complements previous studies devoted to the photocatalytic oxidation of a low-concentration propene gaseous stream in absence of humidity using agglomerated TiO₂-based materials. These prepared agglomerated materials have shown very good oxidation activities and complete selectivity towards CO₂. The present paper analyses the role of humidity on propene oxidation, which has not been studied before, and extends the use of the prepared agglomerated photocatalysts to low concentration benzene oxidation, both in absence and presence of humidity. The obtained results have shown that humidity must be totally avoided, or kept as low as possible, to achieve high propene conversions. In the case of benzene, some insight to controversial published results is given; very low conversions together with benzene cracking on the surface of the photocatalyst occur in absence of humidity. However, the introduction of humidity leads to high conversions and avoids benzene cracking. The performance of the agglomerated photocatalyst containing a high surface area activated carbon (TiO₂/C1) must be underlined in terms of activity.     

Photocatalytic degradation of trichloroethylene over TiO<Subscript>2</Subscript>/SiO2 in an annulus fluidized bed reactor

The effects of superficial gas velocity (Ug), wavelength and intensity of ultraviolet (UV) light, oxygen and H₂O concentration on the photocatalytic degradation of TCE (Trichloroethylene) over TiO₂/SiO₂ catalyst have been determined in an annulus fluidized bed photoreactor. The key factor in determining the performance of the annulus fluidized bed photoreactor is found to be an optimum superficial gas velocity (Ug) that provides the optimum UV lighttransmit through the proper size of bubbles in the photoreactor. The degradation efficiency of TCE increases with light intensity but decreases with wavelength of the UV light and H₂O concentration in the fluidized bed of TiO₂/silica-gel photocatalyst. The optimum concentration of O₂ for TCE degradation is found to be approximately 10%. The annulus fluidized bed photoreactor is an effective tool for high TCE degradation with efficient utilization of photon energy. This paper is dedicated to Professor Dong Sup Doh on the occasion of his retirement from Korea University.     

TiO2/ZnO Supported on Sepiolite: Preparation,Structural Characterization,and Photocatalytic Degradation of Flumequine Antibiotic in Aqueous Solution

In this study, TiO₂, ZnO, TiO₂/ZnO (Ti/Zn), and TiO₂/ZnO/Sep (Ti/Zn/Sep) catalysts have been synthesized using sol–gel and chemical precipitation method. Their photocatalytic performances have been compared using Flumequine (FLQ) antibiotic. X-ray diffraction (XRD), Fourier transform infrared spectra (FTIR), scanning electron microscopy (SEM), N₂-adsorption, and the determination of a zero point charge has been used to characterize the synthesized catalysts. The degradation studies showed that the catalytic efficiency of Ti/Zn/Sep is higher than that for other catalysts. The operational parameters such as pH, initial FLQ concentration, and catalyst dosage were evaluated. UV–vis and high-resolution mass spectroscopy (HRMS) analyses were used to determine the degradation efficiency and products. ZnO played a major role in the FLQ degradation process, and sepiolite contributed to adsorption of FLQ on the catalyst surface enormously. The catalysts exhibited 11%, 23%, 63%, and 85% degradation efficiency for ZnO, TiO₂, Ti/Zn, and Ti/Zn/Sep in the decomposition of FLQ, respectively.     

The kinetics of photocatalytic degradation of trichloroethylene in gas phase over TiO2 supported on glass bead

In this investigation, a packed bed filled with coated titanium dioxide glass beads to study the kinetics of photocatalytic degradation of trichloroethylene under irradiation of 365 nm UV light. In the range of 100–500 ml/min of flowrate, the reaction rate for 6 μM TCE increased with an increasing flowrate upto 300 ml/min, while was not affected by the flowrate at the values higher than 300 ml/min. For moisture in the range of 9.4–1222.2 μM, the reaction rate of TCE was decreased with an increasing humidity. The adsorbed water on the catalyst surface could compete with the adsorption of TCE on the sites. The reaction rate of 6 μM TCE increased as the light intensity increased, and was proportional to the 0.61 order of the light intensity. Among the three L–H bimolecular form models, the experimental data had the best fit for one of models:

     

Operando FTIR study of the photocatalytic oxidation of acetone in air over TiO2–ZrO2 thin films

Operando FTIR spectroscopy has been used to study the photocatalytic oxidation of acetone vapors over semiconductors films containing TiO₂ and ZrO₂. Preparation of these coatings was carried out by dipping a silicon wafer in stable sols containing particles of TiO₂, Ti_1−xZr_xO₂, or a mixture of ZrO₂ and TiO₂. These differences in chemical composition and phase homogeneity were selected in order to determine their effect on the photocatalytic performance. A transmission cell specifically designed for in situ studies of photocatalytic coatings was utilized for the FTIR experiments under reaction conditions. In contrast with investigations with powdered photocatalysts, the use of thin films guarantees that the whole semiconductor is irradiated, and for that reason purely photochemical reactions are monitored. Acetone adsorption takes place molecularly and is higher on the Ti_1−xZr_xO₂ coating. This fact is very likely related to the higher specific surface of the samples containing Zr. However, the maximum photocatalytic rate for acetone degradation corresponds to the films composed by a binary mixture of TiO₂ and ZrO₂. On the other hand, remarkable differences on the type and concentration of intermediates appearing as a result of the photocatalytic oxidation of acetone are found for the coatings studied. A simple kinetic model was applied to analyze the evolution of both gas phase and surface species. The parameters obtained indicate that each specific surface process is affected in a different way by the variation in the composition of the photoactive films.     

Effect of M-doping (M = Fe,V) on the photocatalytic activity of nanorod rutile TiO2 for Congo red degradation under the sunlight

Nanorods TiO₂, Fe-TiO₂ (3 and 2 at.% Fe), V-TiO₂ (5 at.% V) were prepared by a low temperature method and characterized by powder X-ray diffraction, thermal analysis, transmission electron microscope and BTE surface area analysis. The as-prepared samples were evaluated as catalysts for photodegradation of Congo red aqueous solution under the sunlight. Nanorods Fe-doped TiO₂ shows higher adsorption and also higher photocatalytic degradation of Congo red solution compared to pure nanorods TiO₂ rutile. A higher activity is obtained when the amount of doped Fe is 2 at.%, compared to 3 at.%. However, nanorods V-TiO₂ does not show neither adsorption nor photodegradation activity of Congo red solution.     

Combined TiO2/SiO2 mesoporous photocatalysts with location and phase controllable TiO2 nanoparticles

Combined TiO₂/SiO₂ mesoporous materials were prepared by deposition of TiO₂ nanoparticles synthesised via the acid-catalysed sol–gel method. In the first synthesis step a titania solution is prepared, by dissolving titaniumtetraisopropoxide in nitric acid. The influences of the initial titaniumtetraisopropoxide concentration and the temperature of dissolving on the final structural properties were investigated. In the second step of the synthesis, the titania nanoparticles were deposited on a silica support. Here, the influence of the temperature during deposition was studied. The depositions were carried out on two different mesoporous silica supports, SBA-15 and MCF, leading to substantial differences in the catalytic and structural properties. The samples were analysed with N₂-sorption, X-ray diffraction (XRD), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM) to obtain structural information, determining the amount of titania, the crystal phase and the location of the titania particles on the mesoporous material (inside or outside the mesoporous channels). The structural differences of the support strongly determine the location of the nanoparticles and the subsequent photocatalytic activity towards the degradation of rhodamine 6G in aqueous solution under UV irradiation.     

Capability of coupled CdSe/TiO2 heterogeneous structure for photocatalytic degradation and photoconductivity

Highly ordered TiO₂ nanotube arrays (TiO₂-NTAs), with a uniform tube size on titanium substrate, were obtained by means of reoxidation and annealing. A composite structure, CdSe quantum dots@TiO₂ nanotube arrays (CdSe QDs@TiO₂-NTAs), was fabricated by assembling CdSe quantum dots into TiO₂-NTAs via cyclic voltammetry electrochemical deposition. The X-ray diffractometer (XRD), field-emission scanning electron microscope (SEM), and transmission electron microscope (TEM) were carried out for the determination of the composition and structure of the tubular layers. Optical properties were investigated by ultraviolet-visible spectrophotometer (UV-Vis). Photocurrent response under visible light illumination and photocatalytic activity of samples by degradation of methyl orange were measured. The results demonstrated that the photo absorption of the composite film shifted to the visible region, and the photocurrent intensity was greatly enhanced due to the assembly of CdSe QDs. Especially, photocurrent achieved a maximum of 1.853 μA/cm² after five voltammetry cycles of all samples. After irradiation under ultra violet-visible light for 2 h, the degradation rate of composition to methyl orange (MO) reached 88.20%, demonstrating that the CdSe QDs@TiO₂-NTAs exhibited higher photocatalytic activity.     

Photocatalytic degradation of toluene over doped and coupled (Ti,M)O2 (M = Sn or Zr) nanocrystalline oxides: Influence of the heteroatom distribution on deactivation

In this work, we have studied the photocatalytic oxidation (PCO) of toluene over Sn- and Zr-doped TiO₂, and coupled TiO₂/SnO₂ and TiO₂/ZrO₂ catalysts. The TiO₂ sample doped with Sn (8% of metal ions) is composed by the anatase and rutile phases of TiO₂, while the Zr-doped sample (same dopant content) contains only the anatase phase. The coupled photocatalysts are formed, in addition to the phases present in their doped counterparts, by a segregated MO₂ phase (M: Sn or Zr). For the photocatalytic degradation of toluene, higher rates in the stationary state are obtained with the coupled catalysts with respect to the doped ones and the TiO₂ references (both synthetic and Degussa P25). In the case of the coupled photocatalysts, these higher rates are due to the absence of the deactivation that does occur for the rest of samples. Fresh and used photocatalysts have been studied by FTIR and EPR spectroscopies and by solid/liquid extraction in methanol, followed by GC/MS analysis. The obtained results lead us to conclude that, while structural and electronic modifications, due to the guest cations, are responsible for the high activity of doped samples observed in previous studies for a reaction not causing catalyst deactivation (methylcyclohexane PCO), other factors are crucial for the PCO of toluene. For this reaction, there is a relationship between surface water, adsorbed intermediates and resistance to deactivation, and thus the modifications in the amount and arrangement of surface water molecules caused by the second oxide may be the cause of the high degradation rate obtained with the coupled TiO₂/SnO₂ and TiO₂/ZrO₂ photocatalysts.