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.     

Photoanodic reactions occurring at nanostructured titanium dioxide films

The reasons of unusually large differences observed in photocurrent efficiencies for the oxidation of various organic and inorganic substrates at nanostructured TiO₂ photoelectrodes are discussed. The “redox cycling”, where a product of the hole transfer acts, in turn, as scavenger for the photogenerated electrons, appears as a frequent cause of weak photocurrents. Such a recombination mechanism operates not only under open-circuit conditions but also in the presence of large anodic bias applied to the conducting support of the TiO₂ electrode. Experiments conducted in the presence of both, an efficient hole scavenger, formate ion, and an electron acceptor, methyl viologen dication, showed that, in a major part of the anodically polarized nanostructured TiO₂ film, the quasi-Fermi level of electrons remains actually close to the conduction band edge potential. Importantly, addition to the solution of an electron acceptor causes large drop of the photocurrent both under weak and intense UV illumination generated by an argon-ion laser. On the other hand, a large number of organic molecules undergoing essentially irreversible photooxidation (e.g., to form CO₂ and H₂O) generate high photocurrents at the nanostructured TiO₂ electrodes. Such reactions, when occurring in the presence of large concentrations of the reactants, may involve direct hole transfer.     

Photosensitized degradation in water of the phenolic pesticides bromoxynil and dichlorophen in the presence of riboflavin, as a model of their natural photodecomposition in the environment

Within the context of environmentally friendly methods for the elimination of surface-water pollutants, the photodegradation of the phenolic pesticides bromoxynil (BXN) and dichlorophen (DCP) under simulated natural conditions has been studied. The work was done in the presence of the visible-light absorber photosensitizer riboflavin (Rf), usually present in trace quantities in natural waters. Under aerobic conditions, an efficient photooxidation of both pesticides was observed. The relatively intricate photochemical mechanism involves pesticide and oxygen consumption and, to a lesser extent, Rf degradation. The kinetic and mechanistic study supports that both H(2)O(2) and singlet molecular oxygen, O(2)((1)Δ(g)), are involved in the process. Kinetic data for the O(2)((1)Δ(g))-mediated oxidation indicate that BXN and DCP are photodegraded with this species faster than the parent compound phenol, very frequently employed as a model for aquatic contaminants, likely due to their lower pK(a) values. This observation allows the design of phenolic pesticides with different photodegradation rates under environmental conditions.     

Deactivation of riboflavin triplet-excited state by phenolic antioxidants: mechanism behind protective effects in photooxidation of milk-based beverages

The water-soluble plant polyphenols rutin, (+)-catechin, and (–)-epigallocatechin gallate were each found to be efficient quenchers for the triplet-excited state of riboflavin in aqueous solution using nano-second laser flash photolysis. The deactivation followed second-order kinetics with rates close to the diffusion control: (–)-epigallocatechin gallate (1.70±0.2×10⁹ l mol^–1 s^–1), (+)-catechin (1.44±0.04×10⁹ l mol^–1 s^–1), and rutin (9.7±0.2×10⁸ l mol^–1 s^–1) at 25 °C and pH = 6.8. No synergetic effects for mixtures of the phenolic compounds were found. Hydrogen atom transfer is calculated to be more exergonic than electron transfer, suggesting the hydrogen atom transfer mechanism to be operative. The efficient deactivation by the plant polyphenols makes this type of compounds of importance as protectors against light-induced oxidation in flavored milk-based beverages.     

Synthesis of hollow TiO2@g-C3N4/Co3O4 core-shell microspheres for effective photooxidation degradation of tetracycline and MO

Traditional bulk photocatalysts often experience serious charge recombination and poor visible-light capturing, resulting in inefficient photocatalytic activity. However, proper nanostructure design usually helps to increase the activity of composite photocatalysts. Here, hollow TiO₂@g-C₃N₄/Co₃O₄ core-shell microspheres are first reported. The hollow structure of the heterostructured will directionally separate the photogenerated carriers, and the photogenerated holes transferred to the surface will be further captured by Co₃O₄ to achieve an exposed oxidized surface. The novel multi-stage hollow microspheres can simultaneously achieve effective transfer of photogenerated carriers and extended light absorption. Benefiting from these structural and compositional characteristics, the optimized TiO₂@g-C₃N₄/Co₃O₄ nanospheres have excellent photodegradation activity for tetracycline and MO. Under simulated sunlight, the degradation rates of TC (10 mg/L) and MO (25 mg/L) at 60 min are 91.6% and 97.8%, respectively. At the same time, high activity is maintained after multiple cycles of testing. Possible transfer paths for photogenerated carriers have also been proposed. This work will provide more inspiration for the design of multi-stage hollow photocatalytic systems.     

Phototransformation of water-soluble polymers. Part II: photooxidation of poly(ethylene oxide) in aqueous solution

Poly(ethylene oxide) (PEO) was irradiated in aqueous solution under long wavelengths (λ>300 nm, 20 °C) and in presence of oxygen. The photooxidation of PEO was studied by IR spectrophotometry, viscometry and size exclusion chromatography. The formation of the oxidation photoproducts was studied by infrared analysis of films obtained by evaporation of aliquots of irradiated aqueous solutions. The photoproducts were identified by chemical derivatization treatments coupled with infrared measurements. Viscosimetry and SEC analysis showed that photooxidation was leading to a dramatic decrease of the molecular weights. The influence of the pH of the aqueous solutions was also examined. Unexpected results were obtained for the pH 12 solutions, indicating a strong inhibition of the oxidation.

Comparison with the results obtained in the case of PEO irradiated in the solid state showed that no direct transposition of the knowledge concerning the behavior of the solid polymer could be made.     

New approaches in wood coating stabilization

A major drawback in the use of wood materials for construction and decorative applications is its sensitivity to light. Numerous researchers have shown that the ultraviolet (UV) as well as fractions of the visible (VIS) spectrum of solar radiation cause most of the chemical modification and mechanical breakdown of exposed wood. To insure the long life of wood substrates, they are usually coated with various decorative and protective finishes. The protection effect depends strongly on the opacity, i.e., the degree of pigmentation of the coatings. However, the use of UV absorbers (UVA) and hindered amine light stabilizers (HALS) both in the coating system and also in a direct wood impregnation step can improve the light stability considerably. Here the knowledge of the light sensitivity of wood is crucial for an optimal photoprotection concept for coatings and the wood substrate. This paper highlights findings of a systematic investigation of the light sensitivity of pine wood (Pinus radiata). Furthermore, it describes the correlation between quantity and type of organic UVA and HALS as well as the lignin stabilizer concept and the degree of pigmentation of the coating. These findings are of great importance for an efficient and economic use of light stabilizers in film-forming wood coating systems for pale wood species and allow a cost-effective stabilization. Presented at the 5th International Woodcoatings Congress: Enhancing Service Life, organized by the Paint Research Association (PRA), in Prague, Czech Republic, October 17–18, 2006.     

Photocleavage of plasmid DNA by dibenzothiophene S-oxide under anaerobic conditions

The ability of dibenzothiophene S-oxide (1) to photochemically induce strand breaks in plasmid DNA was studied under anaerobic conditions. DNA cleavage is monitored by the conversion of closed circular pUC19 DNA (form I) to the nicked (form II) and linear forms (form III) using densitometer digital imaging of ethidium stained gels. In buffered aqueous–acetonitrile (9:1) solutions the single-strand cleavage is efficient and does not require an alkaline reaction workup. Photodeoxygenation of 1 in buffered aqueous–acetonitrile (9{:}1) solutions containing benzene led to the production of phenol. The effect of solvent deuteration does not support the involvement of ¹O₂ from a sulfoxide dimerization reaction nor a sensitized photooxygenation reaction. The results are interpreted in terms of a sulfoxide photodeoxygenation via oxygen atoms [O(³P)] where oxidation of DNA can lead to single-strand breaks. Since the reaction of O(³P) atoms with water itself is endoergic, we propose that hydroxyl radicals do not intervene in the DNA cleaving reaction.     

Solar photocatalysis: oxidation of aniline on CdS

The photooxidation of aniline to azobenzene in ethanol on CdS with natural sunlight and UV irradiation (365 nm) was studied as a function of [aniline], catalyst loading, airflow rate, solvent composition, etc. The metal sulfide shows sustainable catalytic activity. The product formation is larger with illumination at 254 nm than at 365 nm. Electron donors like triphenylphosphine, diphenylamine and hydroquinone favor the photocatalysis. Singlet oxygen quencher, azide ion does not inhibit the oxidation. The photocatalysis occurs in protic and aprotic solvents. The mechanism of photocatalyzed oxidation is discussed and the product formation examined using a kinetic equation.     

Photooxidation of nitrosylmyoglobin at low oxygen pressure. Quantum yields and reaction stoechiometries

Photooxidation of the nitrite-cured meat pigment, nitrosylmyoglobin, in aqueous solution saturated with a 20% CO₂/80% N₂ gas mixture with varying oxygen contents (0.1, 0.5 or 1.0%) was found to depend linearly on the oxygen content for both visible (436 nm) and UV-light (366 nm). Quantum yields were similar for the two wavelengths of excitation in agreement with previous findings at higher oxygen pressures. The reaction stoechiometry for photooxidation was different from that of thermal oxidation (investigated at the same oxygen pressures) with a unity MbFe(II)NO/O₂ ratio in the thermal reaction and a ratio larger than one for the photooxidation. For cured meat products packed in modified atmospheres light exposure may be even more harmful for the oxidative stability than expected from the concentration of residual oxygen.