The relationship between microstructure and photocatalytic behavior in lanthanum-modified 2D TiO2 nanosheets upon annealing of a freeze-cast precursor.

Titanium dioxide modified with 3 wt% La was prepared via a green freeze-casting method, and its photocatalytic activity was tested in terms of its ability to degrade 4-chlorophenol (4-CP) and remove total organic carbon (TOC). Under annealing conditions, the freeze-cast precursor was transformed into an La-modified anatase with a well-defined 2D TiO2 nanosheet morphology. Rietveld refinement of the X-ray diffraction patterns confirmed the substitutional nature of the La cation that induced local structural variations and involved subtle ion displacement in the TiO2 lattice due to the ionic size effect. Despite nearly identical tetragonal structures, replacement of Ti with La alters the photocatalytic activity through a reduction in band gap energies and an increase in charge carrier mobility. Material annealed at 650 °C exhibited the highest photocatalytic performance and achieved efficient TOC removal. Upon annealing at 800 °C, nanoscale lanthanum-enriched regions were generated due to the diffusive migration of La cations and phase transition from anatase to rutile. The La3+ cation, acting as a structural promoter, supported 2D TiO2 growth with well controlled crystallite size, surface area and porosity. La3+ could be regarded as a potential electronic promoter that can reduce the band gap of 2D TiO2 nanosheets and can provide a signature of the electron transfer and carrier charge separation. Both methods, kinetics of degradation of 4-CP and TOC, provided similar results, revealing that the photocatalytic activity under UV light irradiation increased in the order 950C < 500 °C < 800 °C < 650 °C < TiO2-P25.

Mechanism and kinetics of photochemical transformation of ketoprofen and its degradation intermediates.

Ketoprofen, 2-(3-benzoylphenyl)-propionic acid, a widely used non-steroidal anti-inflammatory drug, is considered as an important water pollutant. Kinetics and mechanism of its photolytic transformation in aqueous solutions was studied experimentally and partial reaction steps were modelled by means of quantum chemistry methods. While the rate of ketoprofen photolysis was not significantly affected by its acid-base equilibrium, a marked influence of pH on the subsequent degradation reactions was observed. At pH 1.3, two oxygenated primary products were identified, that underwent fast photolysis. Deprotonated form of ketoprofen was transformed preferentially to ethylbenzophenone and further degradation proceeded substantially slower. Oxygen participated on photolytic processes both as a reactant and the triplet state quencher. The active involvement of water molecules in the reaction mechanism was investigated by comparative experiments in acetonitrile. The phototransformation mechanism proposed based on the experimental data corresponded well with the theoretical results.

Photocatalytic behavior of nanosized TiO2 immobilized on layered double hydroxides by delamination/restacking process.

INTRODUCTION: Efficient immobilization of TiO(2) nanoparticles on the surface of Mg(2)Al-LDH nanosheets was performed by delamination/restacking process. EXPERIMENTAL PART: The structural and textural properties of as-prepared nanocomposite were deeply analyzed using different solid-state characterization techniques such as: X-ray powder diffraction, Fourier transform infrared spectroscopy, and Raman spectroscopies, chemical analysis, X-ray photoelecton spectroscopy, N(2) adsorption-desorption, and electronic microscopy. RESULTS AND DISCUSSION: The photocatalytic properties of immobilized TiO(2) nanoparticles on Mg(2)Al were investigated using the photodegradation of two model pollutants: Orange II and 4-chlorophenol, and compared with pure colloidal TiO(2) solution. CONCLUSION: It appears that Orange II photodegradation was systematically faster and more efficient than 4-chlorophenol photodegradation regardless of the medium pH. Moreover under slightly basic conditions, even if the TiO(2) photocatalytic efficiency decreases, photodegradation performed in presence of easily recovered TiO(2)/Mg(2)Al(1.5) nanocomposite gives rise to comparable or better results than pure TiO(2).

Secondary processes in atmospheric pressure chemical ionization-ion trap mass spectrometry: a case study of orotic acid.

Atmospheric pressure chemical ionization is known for producing unusual artifacts of the ionization process in some cases. In this work, processes occuring in atmospheric pressure chemical ionization/MS of orotic acid that afforded ions accompanying protonated and deprotonated orotic acid molecules in the spectra were studied. Two processes ran in parallel in the ion source: decarboxylation of neutral orotic acid and collision-induced dissociation of its protonated or deprotonated form. A procedure discerning pre-ionization decomposition and post-ionization dissociation by manipulating ion source parameters was proposed. Experiments with isotopically labeled solvents confirmed ion-molecule reactions of the product of collision-induced dissociation of protonated orotic acid with solvent molecules in the ion source and even under vacuum in the ion trap.

Degradation of 4-chlorophenol in aqueous solution photoinduced by Fe(III)-citrate complex.

The 4-chlorophenol (4-CP) degradation photoinduced by Fe(III)-citrate complex was investigated upon irradiation at 365 nm at different pHs (2.0, 3.0, 4.0 and 6.0). The best efficiency for 4-CP degradation in the presence of Fe(III)-citrate was at pH 3.0 with a value of quantum yield equal to 0.026. This effect is mainly attributed to speciation of Fe(III)-citrate complex which is very sensitive to the pH. Quantum yields of 4-CP degradation, Fe(III)-citrate disappearance and Fe(II) formation were carried out at wavelength 365 nm at different pHs (3.0 and 6.0). The effect of the pH is less pronounced for the quantum yields of 4-CP than for Fe(III)-citrate complex and Fe(II) quantum yields which are species involved in the initial photochemical process. The effect of oxygen, isopropanol and Fe(III)-citrate concentrations on the quantum yields of 4-CP degradation and also on kinetics of 4-CP disappearance was also studied. Indeed, 4-CP requires the presence of oxygen to be degraded. The disappearance of 4-CP was totally inhibited without oxygen dissolved in the solution. Oxygen is essential for the formation of oxidative species and, as a consequence, for the degradation of the pollutant.

Mesoporous films of TiO(2) as efficient photocatalysts for the purification of water.

Variably thick mesoporous TiO(2) films were prepared by alternately dip- and spray-coating using sol containing titanium(IV) alkoxide, concentrated HCl and suitable block-copolymer as a structure directing agent. The film thickness was controlled by varying the number of deposited layers. Their porosity was homogeneous: surface area and pore volume increased linearly with increasing number of deposited layers. Photoactivity of the films was tested employing photocatalytic degradation of 10(-4) M 4-chlorophenol in a 60 mL photoreactor under UV irradiation at 365 nm. The incident light intensity was 1 mW cm(-2). The degradation rate constants increased with increasing number of layers, proportional to their absorbances. However, the progressive improvement of the photocatalytic performance slightly decreased with the increasing number of layers. The spray-coating layers exhibited lower photocatalytic performance due to their smaller thicknesses.

Effect of dissolved ozone or ferric ions on photodegradation of thiacloprid in presence of different TiO2 catalysts.

Combining TiO(2) photocatalysis with inorganic oxidants (such as O(3) and H(2)O(2)) or transition metal ions (Fe(3+), Cu(2+) and Ag(+)) often leads to a synergic effect. Electron transfer between TiO(2) and the oxidant is usually involved. Accordingly, the degree of synergy could be influenced by TiO(2) surface area. With this in mind, the disappearance of thiacloprid, a neonicotinoid insecticide, was studied applying various photochemical AOPs and different TiO(2) photocatalysts. In photocatalytic ozonation experiments, synergic effect of three different TiO(2) photocatalysts was quantified. Higher surface area resulted in a more pronounced synergic effect but an increasing amount of TiO(2) did not influence the degree of the synergy. This supports the theory that the synergy is a consequence of adsorption of ozone on the TiO(2) surface. No synergy was observed in photocatalytic degradation of thiacloprid in the presence of dissolved iron(III) species performed under varied experimental conditions (concentration, age of iron(III) solution, different TiO(2) films, usage of TiO(2) slurries). This goes against the literature for different organic compounds (i.e., monuron). It indicates different roles of iron(III) in the photodegradation of different organic molecules. Moreover, TiO(2) surface area did not affect photodegradation efficiency in iron(III)-based experiments which could confirm absence of electron transfer between TiO(2) photocatalyst and iron(III).

Mass spectrometric study of selected precursors and degradation products of chemical warfare agents.

Selected precursors and degradation products of chemical warfare agents namely N,N-dialkylaminoethane-2-ols, N,N-dialkylaminoethyl-2-chlorides and some of related N-quaternary salts were studied by means of electrospray ionization-multiple tandem mass spectrometry (ESI-MS(n)). Proposed structures were confirmed with accurate mass measurement. General fragmentation patterns of these compounds are discussed in detail and suggested processes are confirmed using deuterated standards. The typical processes are elimination of alkene, hydrogen chloride, or water, respectively. Besides, elimination of ethene from propyl chain under specific conditions was observed and unambiguously confirmed using exact mass measurement and labelled standard. The potential of mass spectrometry to distinguish the positional isomers occurring among the studied compounds is reviewed in detail using two different MS instruments (i.e. ion trap and hybrid quadrupole-time of flight (Q-TOF) analyzer). A new microcolumn liquid chromatography (microLC)/MS(n) method was designed for the cases where the resolution based solely on differences in fragmentation is not sufficient. Low retention of the derivatives on reversed phase (RP) was overcome by using addition of less typical ion pairing agent (1 mM/l, 3,5-dinitrobenzoic acid) to the mobile phase (mixture water : acetonitrile).

Aggregation of methylene blue in frozen aqueous solutions studied by absorption spectroscopy.

The paper presents a qualitative as well as quantitative spectroscopic study of methylene blue (MB) aggregation that occurs upon freezing the aqueous solutions over a wide concentration range. The Gaussian curve analysis and the multivariate curve resolution-alternating least squares method were used to determine the number and concentration of chemical species responsible for the overlaying absorption visible spectra measured. The results show the extent of aggregation for the concentrations above 10(-7) mol L(-1), being dependent on the freezing rate and the initial concentration. While the local concentration of MB at the grain boundaries of polycrystalline ice increased by approximately 3 orders of magnitude upon fast freezing at 77 K compared to the liquid phase, the concentration raised at least by 6 orders of magnitude upon slow freezing at 243 K. Since enhancement of the local concentration of solutes plays an important role in (photo)chemical transformations in solid aqueous media, this work helps to understand how the initial conditions control the course of the process. The results are relevant in other interdisciplinary fields, such as environmental chemistry, cosmochemistry, or geochemistry.

Mineralisation of Monuron photoinduced by Fe(III) in aqueous solution.

The degradation of Monuron (3-(4-chlorophenyl)-1,1-dimethylurea) photoinduced by Fe(III) in aqueous solution has been investigated. The rate of degradation depends on the concentration of Fe(OH)2+, the most photoreactive species in terms of *OH radical formation. These *OH radicals are able to degrade Monuron until total mineralisation. The primordial role of the speciation of Fe(III)-hydroxy complex in aqueous solution, for the efficiency of the elimination of pollutant, was shown and explained in detail. The formation of Fe(II) in the irradiated solution was monitored and correlated with the total organic carbon evolution. Degradation photoproducts were identified and a mechanism of degradation is proposed.

Degradation of supramolecular complexes photoinduced by iron(III): the influence of the host on the reactivity of the guest.

Fe(III)-photoinduced degradation of two different supramolecular host-guest complexes, naphthalene with alpha-cyclodextrin and 4-chloroaniline with p-sulfonic-calix[4]arene; was studied in aqueous solution. The stoichiometries, equilibrium constants and conformations of these complexes were characterized. In both cases, the degradation of the guest could not be accounted for by simple competition kinetics of OH radical attack on the guest and host molecules, respectively. This observation was explained by the occurrence of subsequent radical reactions of transients resulting from an OH attack onto the host molecule, which are assumed to be able to induce additional degradation reactions of the guest. An inhibiting influence of the calix[4]arene host on the abundance of oligomeric degradation intermediates of 4-chloroaniline was observed.