Comparison of several titanium dioxides for the photocatalytic degradation of benzenesulfonic acids

The photocatalytic efficiency of several TiO₂ (namely Degussa P25 and Millennium PC50, PC100, PC105, PC500) used as suspensions are compared for the photocatalytic degradation of 3-nitrobenzenesulfonic acid (3-NBSA) and 2,5-anilinedisulfonic acid (2,5-ADSA). With 3-NBSA, P25 is clearly the most efficient and there is no apparent relationship between photocatalytic activity and specific surface area. This result is consistent with that obtained with phenol, but a contrast was noticed with 2,5-ADSA where PC500 is more efficient than P25 in spite of a higher adsorption of the latter. In the case of TiO₂ Millennium relative photonic efficiencies ζ_r are smaller with 3-NBSA than with 2,5-ADSA, that is consistent with the electron-withdrawing effect of nitro group. For the same reason the direct phototransformation in sunlight is much more rapid with of 2,5-ADSA than for 3-NBSA and 4-nitrotoluene-2-sulfonic acid.     

Comparative evaluation of Fe and TiO2 photoassisted processes in solar photocatalytic disinfection of water

A lost of culturability of bacteria Escherichia coli K12 was observed after exposition to a solar simulator (UV–vis) in a laboratory batch photoreactor. The bacterial inactivation reactions have been carried out using titanium dioxide (TiO₂) P25 Degussa and FeCl₃ as catalysts. At the starting of the treatment, the suspensions were at their “natural” pH. An increase in the efficiency in the water disinfection was obtained when some advanced oxidation processes such as UV–vis/TiO₂, UV–vis/TiO₂/H₂O₂, UV–vis/Fe³⁺/H₂O₂, UV–vis/H₂O₂ were applied. The presence of H₂O₂ accelerates the rate of disinfection via TiO₂. The addition of Fe³⁺ (0.3 mg/l) to photocatalytic system decreases the time required for total disinfection (<1 CFU/ml), for TiO₂ concentrations ranging between 0.05 and 0.5 g/l. At TiO₂ concentrations higher than 0.5 g/l the addition of Fe³⁺ does not significantly increase the disinfection rate. The systems: Fenton (H₂O₂/Fe³⁺/dark), H₂O₂/dark, H₂O₂/TiO₂/dark showed low disinfection rate. The effective disinfection time (EDT₂₄) was reached after 60 and 30 min of illumination for the Fe³⁺ and TiO₂ photoassisted systems, respectively. EDT₂₄ was not reached for the system in the absence of catalyst (UV–vis). The effect on the bacterial inactivation of different mixture of chemical substance added to natural water was studied.     

Characterization and photocatalytic activity of N-doped TiO2 prepared by thermal decomposition of Ti–melamine complex

Nitrogen doped spherical TiO₂ has been prepared by thermal decomposition of Ti–melamine complex in air atmosphere. A clear shift in the onset light absorption from UV region (<400) to visible region (>520 nm) has been observed for the N-doped samples. It has been deduced from the optical absorption spectra that the higher calcination temperature results in the decrease in the amount of N-doping. The XRD results revealed the phase transition of TiO₂ from anatase to rutile crystalline phase, starting at calcination temperature ≥600 °C. The electron microscopic images reveal the formation of spherical and flakes of TiO₂ nanocrystals (25 nm). The chemical nature of N in the N-TiO₂ has been evolved through X-ray photoelectron spectroscopy. The presence of different types of N species have been observed corresponding to different oxidation states and the presence of Ti–N and O–Ti–N have been confirmed from the observed binding energy values. Photocatalytic decomposition of methylene blue has been carried out both in the visible region and UV + visible region. In the visible region, N-TiO₂ showed higher activity compared to the undoped commercial TiO₂ (Degussa P25).     

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.     

Preparation of freestanding and crack-free titania–silica aerogels and their performance for gas phase, photocatalytic oxidation of VOCs

Freestanding and crack-free titania–silica aerogels with high titanium content (i.e., Ti/Si = 1) were successfully prepared by adjusting the hydrolysis of the two alkoxide precursors to a comparable rate during the sol–gel processing. Two titania–silica aerogels were prepared by ethanol and CO₂ supercritical drying methods. Well-dispersed, nanometer-sized anatase crystal domains (ca. 10 nm) were crystallized by high temperature, ethanol supercritical drying. The crystalline domains were solidly anchored to the aerogel network by Ti–O–Si bonds. Titania–silica aerogels prepared by CO₂ supercritical drying method were devoid of TiO₂ crystals. A molecular-level mixing was achieved and anatase TiO₂ was only crystallized with difficulty by high temperature calcination (1073 K). Both aerogels were mesoporous and displayed similar open pore structure that is readily accessible to reactant molecules. However, only the titania–silica aerogel with anatase TiO₂ prepared by ethanol supercritical drying was active for the gas phase, photocatalytic oxidation of volatile organic compounds (i.e., isopropanol and trichloroethylene). Catalysts prepared from Degussa P25 TiO₂ displayed lower activity under similar reaction conditions.     

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.%.     

Spectroscopic ellipsometry characterization of TiO2 thin films prepared by the sol–gel method

TiO₂ thin films were prepared on SiO₂/Si(100) substrates by the sol–gel process. XRD results indicate that the major phase of TiO₂ thin films is anatase. The surface morphology and cross-section are observed by FE-SEM. The surface of thin films is dense, free of cracks and flat. The average grain size is about 60–100 nm in diameter. The thickness of single layer TiO₂ thin films is about 60 nm, which increases with the concentration of solution. Ellipsometric angles ψ, Δ are investigated by spectroscopic ellipsometry. The optical constant and the thickness of TiO₂ thin films are fitted according to Cauchy dispersion model. The results reveal that the refractive index and the extinction coefficient of TiO₂ thin films in wavelength above 800 nm are about 2.09–2.20 and 0.026, respectively. The influences of processing conditions on the optical constants and thicknesses of TiO₂ thin films are also discussed.     

Low temperature preparation and visible light photocatalytic activity of mesoporous carbon-doped crystalline TiO2

A visible-light-active TiO₂ photocatalyst was prepared through carbon doping by using glucose as carbon source. Different from the previous carbon-doped TiO₂ prepared at high temperature, our preparation was performed by a hydrothermal method at temperature as low as 160 °C. The resulting photocatalyst was characterized by XRD, XPS, TEM, nitrogen adsorption, and UV–vis diffuse reflectance spectroscopy. The characterizations found that the photocatalyst possessed a homogeneous pore diameter about 8 nm and a high surface area of 126 m²/g. Comparing to undoped TiO₂, the carbon-doped TiO₂ showed obvious absorption in the 400–450 nm range with a red shift in the band gap transition. It was found that the resulting carbon-doped TiO₂ exhibits significantly higher photocatalytic activity than the undoped counterpart and Degussa P25 on the degradation of rhodamine B (RhB) in water under visible light irradiation (λ > 420 nm). This method can be easily scaled up for industrial production of visible-light driven photocatalyst for pollutants removal because of its convenience and energy-saving.     

Efficient destruction of pathogenic bacteria with AgBr/TiO2 under visible light irradiation

The photocatalytic inactivation of pathogenic bacteria in water was investigated systematically with AgBr/TiO₂ under visible light (λ > 420 nm) irradiation. The catalyst was found to be highly effective for the killing of Escherichia coli, a Gram-negative bacterium, and Staphylococcus aureus, a Gram-positive bacterium. The decomposition of the cell wall and cell membrane was directly observed by TEM and further confirmed by K⁺ leakage from the inactivated bacteria. A possible cell damage mechanism by visible light-driven AgBr/TiO₂ is proposed. In addition, the effects of pH, inorganic ions on bacterial photocatalytic inactivation were investigated. The electrostatic force interaction of the bacteria–catalyst is crucial for the efficiency of disinfection. Moreover, AgBr/TiO₂ supported on porous nickel showed much higher bactericidal activity than fixed P25 TiO₂ under visible or near UV light irradiation.     

Optimisation of degradation conditions of 1,8-diazabicyclo[5.4.0]undec-7-ene in water and reaction kinetics analysis using a cocurrent downflow contactor photocatalytic reactor

The photocatalytic degradation of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, a non-biodegradable nitrogenous organic compound) in water was optimised under UV radiation using titanium dioxide photocatalyst. The reactor used was a pilot scale cocurrent downflow contactor photocatalytic reactor (CDCPR), a system offering very high mass transfer efficiency. The effect of photocatalyst loading, initial substrate concentration, temperature, pH, and different combinations of UV, O₂, H₂O₂ and TiO₂ on the photocatalytic oxidation of DBU was investigated. The TiO₂ photocatalyst used was Degussa VP Aeroperl P25/20, a granulated form of Degussa P-25, recently developed to ameliorate downstream catalyst separation problems. The CDCPR was fitted with an internally and vertically mounted 1.0 kW UV lamp. The reactions were carried out at 40–60 °C and 1 barg, with the reactor being operated in closed loop recycle mode and suspended photocatalyst being re-circulated. Optimisation of reaction conditions using a combination of TiO₂, UV radiation and O₂ gave the most rapid degradation and mineralisation of the DBU in comparison with other processes. Under optimised conditions, 100% degradation of DBU was achieved in 45 min, with a quantum yield of 7.39, using a 1 kW lamp, 0.5 g/dm³ TiO₂, 100 mg/dm³ DBU, 1 barg, 50 °C and pH of 3.17. Investigating the reaction pathway and its modelling showed a first order dependency, incorporating the effect of first intermediates of degradation. The activation energy was found to be 54.68 kJ mol⁻¹ showing a significant influence of temperature on the photocatalytic degradation of DBU.     

Preparation of rough anatase films and the evaluation of their photocatalytic efficiencies

Sol–gel derived rough anatase films without controlled particle sizes were prepared by surfactant templating. The coating sol–gel was obtained by hydrolysis of Ti(OC₃H₇)₄ in ethanol/HNO₃ solution. The gel films, prepared by dipping glass substrates in surfactant solutions, were dried after immersion under an atmospheric pressure. The rough films of TiO₂ anatase were obtained after calcining at 500 °C. The resultant films were transparent, semitransparent or opaque and 136–402 nm thick. It was found that the TiO₂ films prepared from the sol–gel with surfactant showed a granular nanostructure, and they were composed of regular particles, for example; between 50 and 70 nm. The roughness of the films was found to depend on the surfactant concentration in the sol–gel solution and can show a roughness between 0.82 and near of 17 nm. The photocatalytic activity of the films for the degradation and mineralization of phenol, an industrial pollutant, in water and under 365 nm irradiation was improved by the surfactant modification. Kinetic analysis of degradation and mineralization of phenol in water were employed to evaluate the different TiO₂ films under the same experimental conditions. The global photonic efficiency for degradation and mineralization of phenol ξ_g, was calculated to facilitate comparison with a TiO₂ standard photocatalyst named Degussa P-25.     

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.     

Formation of active state in vanadium–titanium oxide system regarding to reaction of oxidation of β-picoline to nicotinic acid

Binary vanadia–titania catalysts comprising 5–75 wt.% of V₂O₅ and 95–25 wt.% of TiO₂, pretreated at the temperature ranging between 300 and 700°C, were studied as heterogeneous catalysts for oxidation of β-picoline at 250°C, and inlet concentrations of the following components (vol.%): 1% of 3-picoline, 20% of oxygen, 30% of steam. Nicotinic acid, 3-pyridinecarbaldehyde and CO₂ were the reaction products. The most active state for oxidation of 3-picoline into nicotinic acid was shown to result from formation of coherent interface between V₂O₅ and TiO₂ (anatase) crystallites. This state was generated at the temperature particular for each composition and persists below the temperature of the anatase to rutile transition.     

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.     

Photoelectrocatalytic and photocatalytic disinfection of E. coli suspensions by titanium dioxide

This paper reports studies of the photoelectrocatalytic and photocatalytic disinfection of E. coli suspensions by titanium dioxide in a sparged photoelectrochemical reactor.

Two types of titanium dioxide electrode have been used. ‘Thermal’ electrodes were made by oxidation of titanium metal mesh; ‘sol–gel’ electrodes were made by depositing and then heating a layer of titania gel on titanium mesh. Cyclic voltammetry was used to carry out an initial characterisation and optimisation of both electrode types. The best ‘thermal electrodes’—i.e. those with the highest photocurrents—were prepared by heating titanium mesh at 700 °C in air. For sol–gel derived electrodes, optimum performance was obtained by heating at 600 °C. These electrodes were then used, in a gas sparged reactor, to disinfect E. coli suspensions with an initial concentration of 10⁷ colony forming units (cfu) ml⁻¹. Films prepared by the oxidation of titanium metal were shown to be superior to sol–gel derived films. Direct experimental comparison demonstrates that the photoelectrochemical system is more efficient than photocatalytic disinfection effected by slurries of Degussa P25 titanium dioxide.

Since in practical systems the TiO₂ would be exposed to a variety of species additional to those that are targeted, we also examined the effects of H₂PO₄⁻ and HCO₃⁻ ions on the measured disinfection rates. Phosphate addition poisons both the electrode and particulate-slurry systems and is only partially reversible. By contrast, although bicarbonate addition affects all three systems, the effects are reversible.     

Effect of ammonium polyacrylate on rheology of anatase TiO2 nanoparticles dispersed in silicon alkoxide sols

Ammonium polyacrylate (NH₄PA) was introduced into powdered mixtures consisting of anatase-structured TiO₂ nanoparticles and silicon alkoxide precursors at the sol level, and the rheological behavior of the mixtures was examined under various solid loadings (φ=0.05–0.13 in volumetric ratios), shear rates (  s⁻¹) and NH₄PA concentrations. The alkoxide precursors were mixtures of tetraethyl orthosilicate (TEOS, Si(OC₂H₅)₄), ethyl alcohol (C₂H₅OH), H₂O and HCl in a constant [H₂O]/[TEOS] ratio of 11. The nanoparticle–sol mixtures generally exhibited a pseudoplastic flow behavior over the shear-rate regime examined. The NH₄PA appeared to serve as an effective surfactant which facilitates the suspension flow by reducing the flow resistance at low NH₄PA concentrations. At φ=0.10, a viscosity reduction ca. 85% was found at  s⁻¹ when the NH₄PA concentration was held at 2.5 wt.% of the solids. As the NH₄PA exceeded a critical level, e.g., [NH₄PA]≥3.0 wt.%, the NH₄PA acted as a catalyst which quickly turned the TiO₂–silica sol mixtures (φ=0.10) into a gelled structure, resulted in a pronounced increase of mixture viscosity. The maximum solids concentration (φ_m) of the mixtures was experimentally determined from a derivative of relative viscosity, i.e., (1−η_r^−1/2)–φ dependence. The estimated φ_m increased from 0.127 to 0.165 when NH₄PA of 0.5 wt.% was introduced into the TiO₂–silica sol mixtures.     

A study of anatase–supported Mn oxide as catalysts for 2-propanol oxidation

Several samples of TiO₂ (anatase)-supported Mn oxide XMn–TiO₂ (with X = 0.5, 1.0, 1.5 and 2.0, the theoretical monolayer fractions) have been prepared by a wet impregnation method and characterised by conventional techniques. Mn oxide was found to be well dispersed on the support surface as Mn³⁺species, while part of it is stabilised into the TiO₂ bulk as Mn⁴⁺species. This cation seem to hinder the support sintering and to favour the anatase to rutile phase transition at higher temperatures. Mn oxides supported on TiO₂ are stable and retain medium-high surface area in the 600–900 K range. They are less active than pure Mn oxides in the isopropanol oxidation but, above 600 K, allow complete oxidation of isopropanol to CO₂ only. FT–IR studies and the GC–MS analysis of some intermediate products allow us to confirm the 2-propanol total oxidation occurs through the previously proposed 2-propoxide/acetate/formate surface pathway.     

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.     

Kinetics of dye decolorization in an air–solid system

The photocatalytic decolorization of adsorbed organic dyes (Acid Blue 9, Acid Orange 7, Reactive Black 5 and Reactive Blue 19) in air was examined, applicable to self-cleaning surfaces and catalyst characterization. Dye-coated Degussa P25 titanium dioxide (TiO₂) and dye-coated photo-inert aluminum oxide (Al₂O₃) particles, both of sub-monolayer initial dye coverage, were illuminated with 1.3 mW cm⁻² of near-UV light. Visual evidence of color removal is reported with photographic images. Two methods, Indirect and Direct Analysis, were employed to quantitatively examine the decolorization kinetics of dyes using UV–visible transmission and diffuse reflectance spectroscopy, respectively. A decrease in dye concentration with time was observed with near-UV illumination of dye-coated TiO₂ powders for all dyes. Dyes did not photodegrade significantly on photo-inert Al₂O₃.

UV–visible spectroscopy data was used to model the kinetics of the photocatalytic degradation. Two first-order reactions in series provided the most convincing rate form for the photodegradation of dyes adsorbed to TiO₂, with a first step the conversion of colored dye to colored intermediate, and the second the conversion to colorless product(s). The first rate constant was of similar magnitude for all dyes, averaging k₁ = 0.13 min⁻¹. Similarly, for the second, k₂ = 0.0014 min⁻¹.     

H4SiW12O40-catalyzed oxidation of nitrobenzene in supercritical water: kinetic and mechanistic aspects

The catalytic effect of a heteropolyacid, H₄SiW₁₂O_40, on nitrobenzene (20 and 30 μM) oxidation in supercritical water was investigated. A capillary flow-through reactor was operated at varying temperatures (T=400–500 °C; P=30.7 MPa) and H₄SiW₁₂O₄₀ concentrations (3.5–34.8 μM) in an attempt to establish global power-law rate expressions for homogenous H₄SiW₁₂O₄₀-catalyzed and uncatalyzed supercritical water oxidation. Oxidation pathways and reaction mechanisms were further examined via primary oxidation product identification and the addition of various hydroxyl radical scavengers (2-propanol, acetone, acetone-d₆, bromide and iodide) to the reaction medium. Under our experimental conditions, nitrobenzene degradation rates were significantly enhanced in the presence of H₄SiW₁₂O₄₀. The major differences in temperature dependence observed between catalyzed and uncatalyzed nitrobenzene oxidation kinetics strongly suggest that the reaction path of H₄SiW₁₂O₄₀-catalyzed supercritical water oxidation (average activation E_a=218 kJ/mol; k=0.015–0.806 s⁻¹ energy for T=440–500 °C; E_a=134 kJ/mol for the temperature range T=470–490 °C) apparently differs from that of uncatalyzed supercritical water oxidation (E_a=212 kJ/mol; k=0.37–6.6 μM s⁻¹). Similar primary oxidation products (i.e. phenol and 2-, 3-, and 4-nitrophenol) were identified for both treatment systems. H₄SiW₁₂O₄₀-catalyzed homogenous nitrobenzene oxidation kinetics was not sensitive to the presence of OH√ scavengers.