Comparison of photocatalytic efficiencies of TiO2 in suspended and immobilised form for the photocatalytic degradation of nitrobenzenesulfonic acids

The photocatalytic efficiency of TiO₂ immobilised on various supports (glass, cement, red brick and inorganic fibres), using different techniques (sputtering, sol–gel dip-coating, patented method for inorganic fibres), are compared with the photocatalytic efficiency of TiO₂ Degussa P25 in suspension 2 g l⁻¹, for the degradation of 3-nitrobenzenesulfonic acid (3-NBSA) and 4-nitrotoluenesulfonic acid (4-NTSA). In all cases, the fixation of TiO₂ on solid supports appreciably reduces the photocatalytic efficiency. The best results were obtained with TiO₂ on inorganic fibres.     

Phototransformation of metobromuron in the presence of TiO2

The photocatalytic degradation of the herbicide metobromuron was studied, using TiO₂ photocatalysts (Degussa P25 and Millennium PC50, PC100, PC105 and PC500). Intermediate products were identified by means of liquid chromatography–mass spectrometry (LC–MS–MS) using electrospray (ES) interfacing technique. In order of importance, the main reaction pathways were demethoxylation of the urea chain, ring oxidation leading to ortho hydroxylation with respect to the N atom, substitution of Br by OH, ring oxidation with formation of the meta hydroxylated compound. Demethylation of the demethoxylated product occurred as a secondary reaction. Formation of bromide ions was observed too. In the presence of 2-propanol used as a hydroxyl radical scavenger, demethoxylation was drastically inhibited while the other reactions were only partly inhibited. It suggests that demethoxylation was exclusively due to the oxidation by hydroxyl radicals, whereas the other reactions involved positive holes too. The degradation rate of metobromuron varied in the order: PC50 < PC100 < PC105 < PC500 < Degussa P25. TiO₂ Degussa P25 was therefore the most efficient photocatalyst although its surface area was lower than that of most of Millennium TiO₂.     

Photocatalytic treatment of wastewater from cottonseed processing: Effect of operating conditions, aerobic biodegradability and ecotoxicity

The heterogeneous and homogeneous UV-A photocatalytic treatment of an acidic waste stream originating from cottonseed processing was investigated. Several TiO₂ samples were screened in heterogeneous slurry experiments and a commercially available TiO₂ anatase (Millennium PC 500) was found to be considerably more active than the rest. The effect of key operating conditions such as effluent dilution (COD from 800 to 8000 mg/L), Millennium TiO₂ loading (from 100 to 1500 mg/L), solution pH (from 2.6 to 10) and the water matrix (presence of effluent solids and dissolved oxygen) on treatment efficiency was also studied. In general, degradation decreased with increasing effluent concentration, increasing solution pH from acidic to neutral and alkaline conditions and in the absence of oxygen serving as electron acceptor. Experiments with 500 mg/L TiO₂ for 240 min led to nearly complete mineralization at 800 mg/L initial COD but higher catalyst loadings suppressed degradation. Homogeneous experiments with 100 mg/L Fe²⁺ or Fe³⁺ were also performed leading to about 50% COD reduction; this increased to about 65% when H₂O₂ was added, i.e. simulating a Fenton reaction. The original, un-treated effluent was very toxic to marine bacteria Vibrio fischeri and very slowly biodegradable aerobically as assessed by shake-flask tests. Nonetheless, a 10-fold dilution (i.e. 800 mg/L COD) eliminated completely ecotoxicity and improved substantially biodegradation rates. On the other hand, photocatalytically pre-treated samples were less biodegradable than the un-treated one. Deep photocatalytic oxidation resulted in decreased ecotoxicity to V. fischeri, while moderate photocatalytic oxidation increased toxicity probably due to the formation of toxic intermediates.     

New industrial titania photocatalysts for the solar detoxification of water containing various pollutants

A new series of titania industrial photocatalysts have been elaborated by Millennium Inorganic Chemicals and were denoted Millennium-PC/10, PC/25 and PC/50 with respective specific surface areas equal to 11, 23 and 43 m² g⁻¹. Their photocatalytic activities have been determined and compared in the solar pilot CPC-photoreactor at the Plataforma Solar de Almeria (PSA) (Spain) in the photocatalytic degradation of four different representative pollutants (4-chlorophenol, nitrobenzene, 2-chlorobenzoic acid and hydrobutanedioic (malic) acid), whose degradation pathways had previously been elucidated in laboratory experiments with artificial light, using titania Degussa P-25 as a reference photocatalyst. The study concerned the influence of (i) the nature of organic pollutants, (ii) the surface area and (iii) the concentration of suspended TiO₂. The affinity of the pollutants for TiO₂ and the presence of heteroatoms in the reactant molecules intervened on the activities of Millennium-PCs when compared to Degussa P-25. The higher the affinity of the organic pollutants for titania, the higher the efficiency of Millennium-PC/10 photocatalyst calibrated on the initial rate of pollutant disappearance. The degradation pathways were found similar for both photocatalysts and the primary steps of the degradation for the different types of molecules were discussed.

4-Chlorophenol (4-CP), a model pollutant for waste waters, was then chosen for the study of the influence of the surface areas and of the concentration of Millennium-PC photocatalysts. The initial apparent rate constants of 4-CP degradation in presence of all Millennium-PC catalysts were all higher than that obtained with Degussa P-25. When choosing the total organic carbon (TOC) disappearance rate as an overall kinetic parameter, Millennium-PC/10 and PC/25 appeared as less active than Degussa P-25, with longer solar exposures (by about 10%) necessary to obtain a total mineralisation. However, Millennium-PC/50 resulted as the best catalyst in all cases. The optimal slurry concentration (g l⁻¹) has been determined for each Millennium TiO₂ sample.

While the optimum of Degussa P-25 had previously been found equal to 0.2 g l⁻¹, higher amounts of Millennium-PC samples were required. A twice higher concentration (0.4 g l⁻¹) increased the activities by factors equal to 1.2 and 1.5 for Millennium-PC/50 and PC/10, respectively. The rate constants of disappearance of intermediates and of TOC were quantitatively affected by factors in agreement with a multiple consecutive reactions model. In any case, titania Millennium-PC/50 appeared as the best catalyst among all those tested, including Degussa P-25.     

Lewis acid and base sites at the surface of microcrystalline TiO2 anatase: relationships between surface morphology and chemical behaviour

The relationships between morphology and Lewis acid and base character of surface sites of two types of titania powders (TiO₂ P25 and TiO₂ Merck) were studied by HRTEM and FTIR spectroscopy of adsorbed molecules. Electron micrographs revealed that TiO₂ P25 microcrystals have a prismatic shape, mainly exposing (0 0 1) and (0 1 0) surface planes. TiO₂ Merck powder, which exhibits a significantly lower specific surface area, appeared constituted by large roundish microcrystals. FTIR spectra of adsorbed CO indicated that Ti⁴⁺ ions exposed on (0 0 1) and (0 1 0) faces of TiO₂ P25 particles are Lewis acid centres significantly stronger than those present on the surface of TiO₂ Merck microcrystals. As in both cases the exposed cations are coordinated to five oxygen anions, the observed differences in Lewis acidity are ascribed to some difference in the geometric arrangement of the O²⁻ ligands. Such difference in structure affects the basicity of these centres also. In fact, a fraction of O²⁻ ions on the surface of TiO₂ P25 behave as basic centres toward CO₂ linearly adsorbed on neighbour Ti⁴⁺ centres, and then Lewis acid–base pairs can be recognised. By contrast, no basic activity towards CO₂ was detected for the TiO₂ Merck sample.

The two titania powders exhibited different chemical behaviour in condition of high surface hydration also. Hydroxyl groups on the surface of hydrated TiO₂ P25 are able to transform benzaldehyde molecules in hemiacetalic-like species, whereas C₆H₅CHO molecules are only weakly perturbed by interaction with the OH groups on TiO₂ Merck particles. This feature could be related to the different photocatalytic behaviour in the oxidation of toluene in gas phase, where benzaldehyde was found as a relevant intermediate species.     

Photocatalytic properties of TiO2 modified with platinum and silver nanoparticles in the degradation of oxalic acid in aqueous solution

The commercially available TiO₂-catalyst (Degussa P25) was modified with nanosized platinum and silver particles by the photoreduction method to obtain different noble metal loading (0.5 and 1 wt.%). The characterization of the synthesized catalysts was carried out by the BET method, XPS, TEM and the adsorption of the model pollutant. The degradation of oxalic acid has been studied in aqueous solution photocatalyzed by band-gap-irradiated TiO₂, modified with nanosized platinum or silver particles. The photocatalytic activity of TiO₂, modified with noble metal, is approximately double that of the semiconducting support. The adsorption properties of the catalysts, as well as the noble metal content on the surface of the support, influence the efficiency of the photocatalytic process. The reaction rate of photocatalytic degradation of the oxalic acid follows a zero kinetic order according to the Langmuir–Hinshelwood model. The increase of the quantum yield of the photodestruction reaction of the studied model pollutant is due to the formation of Schottky barriers on the metal–semiconductor interface, which serve as efficient electron traps, preventing the electron–hole recombination.     

Study of the synergic effect of sulphate pre-treatment and platinisation on the highly improved photocatalytic activity of TiO2

An important improvement of the photocatalytic activity of sol–gel prepared TiO₂ has been achieved by sulphate pre-treatment, calcination at high temperature and further platinisation of the samples.

The presence of sulphuric acid clearly stabilised TiO₂ surface area against sintering, maintaining at the same time anatase phase until higher calcination temperatures than in non-sulphated samples. Platinisation of the samples with different nominal amounts of platinum (from 0.5 to 2.5 wt%) was performed and the influence of sulphate treatment on the dispersion and deposit size of platinum on the TiO₂ surface was studied.

Characterisation results and photocatalytic activity of these catalysts were compared with those of unmodified TiO₂. Simultaneously sulphated and platinised TiO₂ samples were highly active for phenol degradation, used as model reaction for the photocatalytic studies, having higher activities than only platinised or only sulphated samples. The activity of these samples were several orders of magnitude higher than that of the commercial TiO₂ Degussa P25 (platinised or unmodified) as well, with independence of the nominal amount of platinum of the samples.

A wide characterisation of the samples was performed and correlations between characterisation results and activity properties are reported.     

Effects of hydrothermal temperature and time on the photocatalytic activity and microstructures of bimodal mesoporous TiO2 powders

Bimodal nanocrystalline mesoporous TiO₂ powders with high photocatalytic activity were prepared by a hydrothermal method using tetrabutylorthotitanate (TiO(C₄H₉)₄, TBOT) as precursor. The as-prepared TiO₂ powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and N₂ adsorption–desorption measurements. The photocatalytic activity of the as-prepared TiO₂ powders was evaluated by the photocatalytic degradation of acetone (CH₃COCH₃) under UV-light irradiation at room temperature in air. The effects of hydrothermal temperature and time on the microstructures and photocatalytic activity of the TiO₂ powders were investigated and discussed. It was found that hydrothermal treatment enhanced the phase transformation of the TiO₂ powders from amorphous to anatase and crystallization of anatase. All TiO₂ powders after hydrothermal treatment showed bimodal pore-size distributions in the mesoporous region: one was intra-aggregated pores with maximum pore diameters of ca. 4–8 nm and the other with inter-aggregated pores with maximum pore diameters of ca. 45–50 nm. With increasing hydrothermal temperature and time, the average crystallite size and average pore size increased, in contrast, the Brunauer-Emmett-Teller (BET) specific surface areas, pore volumes and porosity steadily decreased. An optimal hydrothermal condition (180 °C for 10 h) was determined. The photocatalytic activity of the prepared TiO₂ powders under optimal hydrothermal conditions was more than three times higher than that of Degussa P25.     

Immobilized TiO2 photocatalyst during long-term use: decrease of its activity

When TiO₂ is immobilized on organic fibres, pumice stone or polymer film, the photocatalytic efficiency decreases slowly during long-term use. The efficiency of immobilized photocatalysts were tested on 500 ml of a 5×10⁻⁵ M solution of acid orange-7 (a classical azo dye) before and after treatment of 10⁻³ M solution of acid orange-7 during 4 weeks. It was observed that the efficiency was reduced approximately four, five and 10 times with polymer film containing TiO₂, TiO₂ on organic fibres and TiO₂ on pumice stone, respectively, after 4 weeks of use. Volumes treated were 40, 45 and 60 l, respectively. Nevertheless, the decomposition rate stays a little higher with TiO₂ on pumice stone than with the two other catalysts tested. The photocatalytic activity of immobilized TiO₂ was significantly reduced also during treated with wastewaters. For immobilized photocatalysts used, the decrease of activity is considered to be caused by the elimination of some particles from the catalyst surface during use and also by fouling of catalyst surface by the formation of by-products during the course of degradation process.     

Photocatalytic degradation of imazapyr in water: Comparison of activities of different supported and unsupported TiO2-based catalysts

Photocatalytic oxidation in aqueous phase of imazapyr, a systemic herbicide characterized by its high persistence and mobility in soils, has been investigated using two different titania photocatalysts: commercial Degussa P-25 and industrial Millennium PC-500. Despite potentially better textural and crystalline properties of PC-500, its photocatalytic activity is smaller than that of P-25 for that reaction. A less efficiency of PC-500 powders absorbing the irradiated photons, as well as a low substrate adsorption capacity, has been attributed as the main responsible for such behaviour. Variables such as concentration of imazapyr, amount of catalyst and the physical state of TiO₂ (as a powder or deposited on an Ahlstrom paper) have been studied to elucidate their influence on the efficiency of the photocatalyzed degradation of imazapyr. It has been determined that both adsorption capacities of the catalyst and the reaction rates are strongly affected by the above mentioned parameters. They are even able to modify both the mechanism and kinetic order of the process, by affecting the interfacial substrate–TiO₂ interactions.     

Photocatalytic oxidation of methanol using titania-based fluidized beds

Experiments determined methanol removal and catalyst elutriation rates during photocatalytic oxidation (PCO) of fluidized and packed beds of various titania-based catalysts. The study developed elutriation-resistant catalysts in which TiO₂ was precipitated from solution (p-TiO₂), or was coated on an Al₂O₃ support (TiO₂/Al₂O₃) and compared them to Degussa P-25 TiO₂. Although Degussa P-25 TiO₂ oxidized methanol effectively, it elutriated at a rate that was two orders of magnitude greater than those of p-TiO₂ and TiO₂/Al₂O₃. In addition, TiO₂/Al₂O₃ removed methanol at a significantly greater rate than did P-25, with p-TiO₂ being the least active. Fluidized beds produced greater PCO rates than did packed beds of P-25 and TiO₂/Al₂O₃. Fluidization enhancers, such as vibration and incorporation of a static mixer, improved the performance of the P-25 fluidized bed, but did not change the effectiveness of TiO₂/Al₂O₃ or p-TiO₂. The activity of TiO₂/Al₂O₃ decreased with increasing calcination temperature (over the temperature range 673–873 K). The optimum TiO₂ loading for TiO₂/Al₂O₃ was 30 wt.%.     

Enhanced photocatalytic degradation of maleic acid by Fe(III) adsorption onto the TiO2 surface

The role of Fe(III) on the TiO₂-assisted photocatalytic degradation of maleic acid has been investigated. The study on the kinetics of the removal of organic matter, as well as the identification of all the stable mineralization intermediates, demonstrates that the presence of Fe(III), adsorbed onto the TiO₂ surface, renders a more efficient process through a cleaner mineralization pathway compared to bare TiO₂. Furthermore, the behaviour of the system does not depend on the iron source. On the other hand, the study of the Fe(III) influence on the interaction of maleic acid with TiO₂ in the solid phase, by means of ATR–FT-IR technique, has been performed. From all the reported results, it is concluded that the Fe(III) effects are mainly due to surface phenomena. The enhanced photocatalytic activity is interpreted under different perspectives, including adsorbed Fe–maleic complexes, enhanced adsorption of oxygen and lower geminate recombination yields.     

Structural peculiarities of TiO2 and Pt/TiO2 catalysts for the photocatalytic oxidation of aqueous solution of Acid Orange 7 Dye upon ultraviolet light

TiO₂ (anatase) with different microstructure was synthesized by thermal hydrolysis of the titanyl sulfate and studied by X-ray powder diffraction, high resolution transmission electron microscopy and UV–vis diffuse reflectance spectroscopy. The effect of titanium dioxide structure, regular or distorted, on the photocatalytic degradation of Acid Orange 7 Dye (AO7) in water upon ultraviolet light was studied. It was found that synthesized TiO₂ possesses a relatively high reactivity when illuminated but also show different adsorption in the dark. The relationship between these behaviors depends on the real structure of the catalysts. Catalysts with a perfect structural ordering formed after heating at temperature higher than 500 °C show better photocatalytic performance. Small amount of Pt added into the TiO₂ structure was found to improve further the catalyst reactivity. Pt-modified titania catalysts oxidize AO7 more efficiently than P-25 Degussa TiO₂. Doping effect of Pt on the structural and photocatalytic properties of the samples is discussed.     

Syntheses of TiO2 photocatalysts by the molten salts method: Application to the photocatalytic degradation of Prosulfuron

Titania photocatalysts were synthesised by the molten salts method by reaction of a titanium precursor with three different alkali metal nitrates (LiNO₃, NaNO₃, KNO₃). The titania powders obtained have been characterised using TEM, XRD, BET and UV-Vis absorption spectroscopy. Their photoactivities were evaluated by degrading a commercial sulfonylurea herbicide, Prosulfuron^®. It has been found that the rate constants increased in the following order: Li2 prepared in KNO₃, with a rate constant 1.4 times smaller than that of P25. The effect of calcination was also studied, especially for the sample prepared in NaNO₃ (TiO₂[Na]). An improvement in photocatalytic activity was observed when TiO₂[Na] was calcined in the region of 700–800 °C. Even though the photocatalytic activities obtained did not exceed or even equal to that of Degussa P25, nevertheless, this study constitutes the first attempt to synthesise titania photocatalysts by the molten salts method.     

Synthesis of functionalized mesoporous TiO2 molecular sieves and their application in photocatalysis

Functionalized mesoporous TiO₂ molecular sieves were prepared by treating ordered mesoporous TiO₂ with phosphoric acid or ammonium sulfate at high temperature. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂ adsorption–desorption measurement, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectrometer (FT-IR). The photocatalytic activity of the samples was evaluated by photocatalytic decomposition of bromomethane (CH₃Br) in air. Results revealed that the functionalized TiO₂ samples preserved ordered mesostructure and exhibited enhanced physicochemical properties. The photocatalytic activity of the functionalized mesoporous TiO₂ sample was about three times higher than that of the pure mesoporous TiO₂. The concentrations of phosphoric acid and ammonium sulfate solutions used for the functionalization of TiO₂ greatly influenced the photocatalytic activity of the resultants materials. The optimal concentrations of phosphoric acid and ammonium sulfate solutions were 0.05 and 0.10 M, respectively. The enhanced photocatalytic performance of the functionalized mesoporous TiO₂ could be attributed to large specific surface area, high hydroxyl density, and enhanced surface chemical state.     

制备条件对Ni2P/TiO2-Al2O3催化剂加氢脱硫性能的影响

采用溶胶-凝胶法制备了TiO₂-Al₂O₃复合载体, 以柠檬酸为络合剂, 浸渍法制备了负载型Ni₂P/TiO₂-Al₂O₃催化剂前体, 程序升温H₂还原法制备了Ni₂P/TiO₂-Al₂O₃催化剂, 并用 X 射线衍射(XRD)、N₂吸附比表面积(BET)测定技术对催化剂的结构和性质进行了表征, 考察了浸渍方法、Ni/P摩尔比、Ni₂P负载量对其进行的二苯并噻吩(DBT)加氢脱硫(HDS)性能的影响。结果表明, 当Ni/P比低于1:1时, 能得到单一的Ni₂P物相;当Ni/P比为2:1时, 开始出现Ni₃P物相。采用Ni/P比为1:1、Ni₂P负载量为30%、采用共浸渍方法制备的Ni₂P/TiO₂-Al₂O₃催化剂具有最好的活性, 在360℃、3.0MPa、氢油比500 (体积比)、液时体积空速2.0h^-1的条件下反应4h时, 二苯并噻吩转化率为99.5%。     

Mechanistic approach of the combined (iron–TiO2) photocatalytic system for the degradation of pollutants in aqueous solution: an attempt of rationalisation

Kinetics of oxidative photodegradation of Monuron (3-(4-chlorophenyl)-1,1-dimethylurea) in different photocatalytic systems (iron, TiO₂ and combined system iron + TiO₂) were investigated and compared. The influence of iron addition on TiO₂ photocatalyst and of TiO₂ on the photocatalytic cycle Fe(III)/Fe(II) were carefully studied. A very positive effect of iron addition was observed. This phenomenon was more and more pronounced when TiO₂ concentration was lower. In a suspension of TiO₂ (24 mg L⁻¹) with addition of Fe(III) (3 × 10⁻⁴ mol L⁻¹) the measured rate constant was similar to that obtained in a suspension of TiO₂ with a concentration more than 20 times higher (500 mg L⁻¹). The mechanistic approach carried out in this study allows us to identify the main reactions governing the combined system and a photochemical cycle was proposed. The optimisation of the photocatalytic systems was obtained when each photocatalyst plays a specific role: Fe(III) as a main OH radicals source and TiO₂ as an oxidizing agent of Fe(II).     

Titanium dioxide-mediated photocatalysed degradation of few selected organic pollutants in aqueous suspensions

The photocatalytic degradation of maleic hydrazide (1), propham (2), tebuthiuron (3), propachlor (4), chlortoluron (5), thiram (6), phenoxyacetic acid (7), 2,4,5-trichlorophenoxy acetic acid (8), 4-chlorophenoxy acetic acid (9), uracil (10), 5-bromouracil (11) and bromothymol blue (12) have been investigated in aqueous suspensions of titanium dioxide (TiO₂) under a variety of conditions. The degradation was studied by monitoring the change in substrate concentration employing UV spectroscopic analysis technique and depletion in total organic carbon (TOC) content as a function of irradiation time. The degradation kinetics of the compounds were investigated under different conditions, such as types of TiO₂, pH, catalyst concentration, substrate concentration, temperature and in the presence of different electron acceptors, such as hydrogen peroxide (H₂O₂), potassium persulphate (K₂S₂O₈), ammonium persulphate (NH₄)₂S₂O₈ and potassium bromate (KBrO₃) besides molecular oxygen. TiO₂ Degussa P25 was found to be more efficient photocatalyst for the degradation of the model compounds as compared with other photocatalysts. The degradation products were analysed using GC/MS analysis technique and probable pathways for the formation of different products have been proposed.     

Synthesis, characterization and photocatalytic activity of N-doped TiO2 modified by platinum chloride

Structures and photocatalytic performance of N-doped TiO₂ modified by platinum chloride (PtCl_x/N-TiO₂) was investigated. It was found that the PtCl_x/N-TiO₂ forms anatase structure of TiO₂ involving nitrogen, chloride species and platinum ions (+IV) as major species, and it exhibits higher photocatalytic activity than either N-TiO₂ or PtCl_x/TiO₂ for the decomposition of acetic acid or acetaldehyde in aqueous solutions under visible light irradiation (λ > 420 nm). An enhancement of the photocatalytic activity on PtCl_x/N-TiO₂ has been proposed as a Z-scheme mechanism for charge separation between platinum chloride and N-TiO₂.