Photocatalyzed reaction of indole in an aqueous suspension of titanium dioxide

The photocatalyzed oxidation of indole (1) in an aqueous suspension of titanium dioxide has been investigated and an attempt has been made to identify the products formed during the photooxidation process by gas chromatographic–mass spectrometric (GC–MS) analysis. Photolysis of an aqueous solution of indole (1) in the presence of titanium dioxide and oxygen led to the formation of 2,3-dihydroindole-2-one (6) and 1H-indole-2,3-dione (7). A probable pathway for the formation of these products has been proposed.     

Photoinduced dissolution of titanium dioxide by oxalic acid

The photoinduced dissolution of titanium dioxide is observed upon the UV irradiation of aqueous solutions of degassed suspensions of this semiconductor in the presence of oxalic acid. A mechanism is proposed involving the photoinduced and dark dissolution of titanium dioxide with the formation of titanium(III) and titanium(IV) oxalates, respectively.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 6, pp. 545–547, November–December, 1994.     

Photocatalytic oxidation of methyl-orange in aqueous suspension: comparison of the performance of different polycrystalline titanium dioxide

The photocatalytic oxidation of methyl-orange (C14H14N3SO3Na) dye was carried out in aqueous suspensions of polycrystalline TiO2 irradiated with artificial light until its complete mineralization was achieved. The performances of two widely used semiconductor powders were studied for comparison purposes. The dependence of dye photo-oxidation rate on various experimental parameters, including substrate concentration, semiconductor amount, and pH was investigated by using both catalysts. The observed dye degradation rates followed pseudo-first order kinetics with respect to the substrate concentration under the experimental conditions used. The two investigated TiO2 powders (Degussa P25 and Merck) showed different photoactivities. TOC analyses confirmed the complete mineralization achievable using both photocatalysts. Three main transient products still maintaining the chromophoric azo group were identified prior to their transformation into other degradation products which are non-absorbing in the visible region.     

Photodegradation kinetics of aniline, 4-ethylaniline, and 4-chloroaniline in aqueous suspension of polycrystalline titanium dioxide

The photodegradation process of aniline, 4-ethylaniline, and 4-chloroaniline in aqueous suspension of irradiated TiO₂ (anatase) powder has been investigated. A batch photoreactor with immersed lamp irradiating in the near-UV region has been used for carrying out the reactivity experiments. For each substrate the influence of the initial concentration of organic compound and of pH on the degradation rate has been investigated. The main intermediates and the end products have been identified and determined, the total mass balance being satisfied. The kinetic modelling of the series of reactions occurring in the system has been satisfactorily carried out with a Langmuir-Hinshelwood rate law. The values of the parameters involved in the model have been also determined.     

Hot-fluid annealing for crystalline titanium dioxide nanoparticles in stable suspension

Titanium dioxide (TiO(2)) nanoparticles were synthesized by controlled hydrolysis of titanium alkoxide in reverse micelles in a hydrocarbon solvent. Upon annealing in situ in the presence of the micelles at temperatures considerably lower than those required for the traditional calcination treatment in the solid state, the TiO(2) nanoparticles became highly crystalline but still maintained the same physical parameters and remained in a stable suspension. Thus, the method has allowed the preparation of crystalline TiO(2) nanoparticles that are monodispersed in the same way as they are initially produced in the microemulsion. Effects of the fluid properties on the crystallization of nanoparticles are discussed.     

Sedimentation of titanium dioxide suspension in the presence of polyacrylamide flocculants

The kinetics of sedimentation of titanium dioxide suspension in aqueous and water-salt media is studied in the presence of high-molecular-weight flocculants — polyacrylamide and ionic (anionic and cationic) acrylamide copolymers. Data are obtained on the influence of flocculant concentration, the chemical nature of repeating units in polymer chains of acrylamide (co)polymers, and the order of the addition of active components on the sedimentation characteristics of titanium dioxide suspension.     

Effect of pH and temperature on degradation of dilute dihydroxybenzene, in aqueous titanium dioxide suspension irradiated by UV light

The degradation of p-dihydroxybenzene in a batch photocatalytic reactor, irradiated by a high intensity UV lamp, was investigated. In this experimental work, commercial anatase titanium dioxide (degussa P 25) was used as a catalyst. Various operating conditions such as initial concentration, pH of the solution, temperature, addition of H₂O₂, bubbling of oxygen, and catalyst loading were examined. A pseudo-first-order kinetics was observed with a rate constant ranging between 17.1 × 10⁻³ and 5.6 × 10⁻³ min⁻¹ which was decreasing with increasing initial concentration. H₂O₂ enhanced the degradation significantly. For the experimental conditions, there was no significant effect for the added catalyst dosage. The degradation rate of p-dihydroxybenzene increased to a certain amount with increasing temperature then decreased. Further experimentation is needed to verify this observation.     

Concentration factor and rheology of aqueous titanium dioxide dispersions

The rheological properties of aqueous titanium dioxide dispersions have been studied by rotational viscometry. It has been established that the studied disperse systems are highly structured, and their degree of structuring dramatically increases with the volume fraction φ of the solid phase. At solid phase contents φ ≥ 0.164, the breakage of the disperse structure is accompanied by the appearance of local discontinuities and an abnormal dependence of viscosity on shear stress. The results obtained have been analyzed in terms of the Uriev theory of aggregated systems. The radii of aggregates and maximum solid phase content φ_m that corresponds to the formation of a continuous disperse structure under the conditions of dynamic impact have been calculated.     

Photocatalytic degradation of dimethyl terephthalate in aqueous suspensions of titanium dioxide

The photocatalytic degradation of dimethyl terephthalate (DMT, 1) has been investigated in aqueous suspensions of titanium dioxide (TiO2) by monitoring the depletion of Total Organic Carbon (TOC) content as a function of irradiation time under a variety of conditions. The degradation kinetics were studied using different parameters such as pH, substrate and photocatalyst concentration, a different kind of titanium dioxide and the presence of electron acceptors such as H2O2, KBrO3 and (NH4)2S2O8 in addition to molecular oxygen. The photocatalytic activity was better for Degussa P25 as compared to the other photocatalysts. From the obtained kinetic data a detailed account of the influence of various parameters on the degradation rate for the mineralization of the compound is given. The degradation products were analyzed using the GC/MS technique and probable pathways for the formation of different products have been proposed.     

Adsorption of poly(acrylic acid) onto the surface of titanium dioxide and the colloidal stability of aqueous suspension

The adsorption of poly(acrylic acid) (PAA) in aqueous suspension onto the surface of TiO(2) nanoparticles was investigated. FTIR spectroscopic data provided evidence in support of hydrogen bonding and chemical interaction in the case of the PAA-TiO(2) system. Adsorption isotherms demonstrated that part of the PAA initially added to the suspension was adsorbed onto the TiO(2) surface, after which there was a gradual attainment of an adsorption plateau. The adsorption density of PAA was found to increase with an increase of PAA molecular weight, while it decreased with an increase of pH. The thickness of the PAA adsorption layer was calculated based on measurements of suspension viscosities in the absence and presence of PAA. It was shown that the thickness of the adsorption layer increased with the increase of pH, PAA molecular weight, and its concentration. The surface charge density, the diffuse charge density, and the zeta potential of TiO(2) varied distinctly after PAA adsorption. The shift of pH(iep) toward a lower pH value was observed in the presence of PAA. PAA was found to stabilize the suspension of TiO(2) nanoparticles through electrosteric repulsion. The influence of factors such as PAA molecular weight and its concentration on the colloidal stability of the aqueous suspension was also investigated.     

Adsorption of polymethacrylic acid from aqueous solutions on disperse titanium dioxide

The state of macromolecules of polymethacrylic acid adsorbed on the surface of disperse titanium dioxide was assessed using a combination of the differential concentration approach to the determination of adsorption and methods for determining the size of disperse adsorbents by dynamic light scattering and sedimentation analysis in the field of centrifugal forces. Three sections were found on the isotherm of adsorption: in the first, isolated islands of adsorbed macromolecules formed; in the second, layers of macromolecules with a different degree of deformation were observed; in the third, determining the adsorption of macromolecules is complicated by other accompanying processes, and assessing the state of macromolecules in the adsorption layer becomes difficult.     

Flocculation-induced homolysis of hydrogen peroxide in aqueous colloid solution of titanium dioxide nanoparticles

Thermal generation of oxygen and hydroxylated aromatic compounds by hydrogen peroxide, catalyzed by flocculation of titanium dioxide nanocrystallites aqueous suspension upon addition of hydrogen peroxide, is reported. The oxidation involves catalytic cleavage of a peroxide molecule followed by hydroxyl reaction with the organic solutes. The catalytic hydroxylation is associated with formation of TiO(2)-H(2)O(2) aggregates, which occurs within a specific range of [TiO(2)]/[H(2)O(2)] ratio. Comparison of the activation energy to literature values in the absence of nanoparticles indicates that flocculation induces an increase of the rate without decreasing the activation energy. This is, to the best of our knowledge, a unique case of nanoparticles catalysis driven by formation of a three-dimensional structure of the suspended particles.     

Thermal behaviour of titanium dioxide nanoparticles prepared by precipitation from aqueous solutions

Thermogravimetry-differential thermal analysis, emanation thermal analysis, mass spectrometry detection, Fourier transform infrared and XRD were used to characterize thermal behaviour of titanium dioxide photocatalyst precursors prepared by precipitation at various conditions from peroxotitanic acid sols. The transmission electron microscopy HRTEM technique was used to characterize the surface microstructure. The sols contained TiO₂ anatase particles of approximately 10 nm in diameter. During heating of the air dried samples, their chemical degradation took place giving rise to anatase. On further heating, the crystallization of anatase and formation of rutile phase was observed. To test the photocatalytic activity of the samples, the decomposition of 4-chlorophenol (4-CP) under ultraviolet and visible irradiation was monitored. It was shown that photocatalytic activities of the samples are comparable to the Degussa P25 photocatalyst reference material.     

Selective photocatalytic reduction of nitrate to nitrogen molecules in an aqueous suspension of metal-loaded titanium(IV) oxide particles

Nitrate was photocatalytically reduced to nitrogen molecules with a high selectivity in a basic aqueous suspension of palladium and copper-loaded titanium(IV) oxide powders in the presence of oxalate anion as a hole scavenger.     

Photocatalytic disproportionation of nitrite to dinitrogen and nitrate in an aqueous suspension of metal-loaded titanium(IV) oxide nanoparticles

Photocatalytic reaction of a nitrite ion in aqueous suspensions of bare and metal-loaded TiO(2) particles was examined without electron and hole scavengers under irradiation of UV light. In the bare TiO(2) system, disproportionation of NO(2)(-) to N(2) (or N(2)O) and NO(3)(-) with nitrogen balance (NB) and redox balance (ROB) close to unity within experimental errors was observed, although the reaction was slow. Palladium (Pd)-loaded TiO(2) particles exhibited an extraordinarily large rate of disproportionation of NO(2)(-) in their aqueous suspension, i.e. NO(2)(-) was almost completely converted to N(2) (or N(2)O) and NO(3)(-) even after only 3 h of photoirradiation, both the values of NB and ROB being close to unity. This result suggests that Pd loaded on TiO(2) particles acted as storage sites for photogenerated electrons and effectively transferred the electrons to NO(2)(-) and, therefore, that the reduction process in the photocatalytic disproportionation of NO(2)(-) was accelerated by Pd loaded on TiO(2). Effects of the amount of Pd and pH of the suspension on the reaction rate were also examined.     

Photoinduced oxygenation of tryptamines by aromatic amine N-oxides

Irradiation (1) (253-7 nm) of N_a,N_b-dimethyltryptaimne with pyridine N-oxide or benzo[c]cinnoline N-oxide in CH₂Cl₂ yielded 1,8-dimethyl-3a-hydroxy-1,2,3,3a,8-8a,- hexahydropyrrolo[2,3-b]indole (19), while with visible light N_b-(4-cyanobutadienyl)-N_a,N_b,- dimethyltryptamine (21) was obtained. This method was applied to trimethyltryptamine and the corresponding oxindole (34) and the N-formyl derivative (20) were obtained.     

Photocatalytic oxidation in aqueous titanium dioxide suspensions: the influence of dissolved transition metals

The influence of type, species distribution, standard reduction potential, and concentration of several transition metals on the rate of photocatalytic oxidation of toluene was investigated. A significant increase in reaction rate was observed in the presence of 10⁻⁵ M Cu(II), Fe(III), and Mn(II) at pH 3, with decreased rates at higher concentrations and pH values. There was no clear correlation between reaction rate and aqueous metal species distribution, nor did the oxidation states of Cu or Fe alter their effects on the reaction rate. Neither Ni(II) nor Zn(II) had a significant influence on the rate of organic oxidation. Negligible adsorption of metals onto TiO₂ was measured at the metal concentrations and pH values for which the highest reaction rates were observed, indicating that dissolved metals increase the reaction rate via a homogeneous pathway rather than a TiO₂ surface reaction. A mechanism involving formation of a reactive complex between the metal, the organic or its oxidation intermediate, and an oxygen-containing species is proposed to explain the experimental data. The rate of the photocatalytic reaction is described by a Langmuir—Hinshelwood rate form, modified to account for homogeneous catalytic pathways and decreased UV transmittance in the presence of dissolved metals.     

Foaming in chemical surfactant free aqueous dispersions of anatase (titanium dioxide) particles

Steady-state dynamic aqueous foams were generated from surfactant-free dispersion of aggregated anatase nanoparticles (in the micrometer size range). In order to tune the particle surfaces, to ensure a critical degree of hydrophobicity (so that they disperse in water and generate foam), the particles were subjected to low-temperature plasma treatment in the presence of a vapor-phase silane coupling agents. From ESCA it was shown that hydrophobization only occurred at a small number of surface sites. Foamability (foam generation) experiments were carried out under well-defined conditions at a range of gas flow rates using the Bikermann Foaming Column.1 The volume of the steady-state foams was determined under constant gas flow conditions, but on removing the gas flow, transient foams with short decay times (<5 s) were observed. The foamability of the steady-state foams was found to be dependent on (a) the time of plasma treatment of the particles (surface hydrophobicity), (b) the particle concentration in the suspension, and (c) the state of dispersion of the particles. High foamability was promoted in the neutral pH regions where the charged particles were highly dispersed. In the low and high pH regions where the particles were coagulated, the foamability was considerably reduced. This behavior was explained by the fact that the large coagula were less easily captured by the bubbles and more easily detached from the interface (during the turbulent foaming conditions) than individual dispersed particles.