OOH Radicals Research Articles

Photocatalytic Degradation of Surfactants. XX. Photooxidation of Sodium Butylnaphthalenesulfonates

The photocatalyzed degradation of surface-active and surface-inactive naphthalene-sulfonate derivatives has been investigated in UV-irradiated air-equilibrated heterogeneous TiO2 dispersions. The dynamics for the photodegradation of sodium butylnaphthhalene-1-sulfonate (B-1-NS) and sodium naphthalene-1-sulfonate (1-NS) are compared to those of sodium dodecylbenzenesulfonate (DBS) and sodium benzenesulfonate (BS). The adsorption behavior of the surfactants on the TiO2 catalyst, cleavage of the aromatic ring, mineralization to carbon dioxide, formation of sulfate ions, and the temporal changes of the surface activity (surface tension) against UV irradiation time were probed experimentally to provide inferences on the photoinitiated mechanism. The position(s) of ·OH and/or ·OOH radicals attack on the surfactant structure was inferred from theoretical considerations on the basis of molecular orbital calculations of frontier electron densities; similarly, the adsorption behavior of the surfactants on the TiO2 catalyst surface was deduced from calculations of point charges of each of the atoms in the surfactant structure.

Photocatalyzed degradation of polymers in aqueous semiconductor suspensions: V. Photomineralization of lactam ring-pendant polyvinylpyrrolidone at titania/water interfaces

The mechanism of photo-oxidation of the water-soluble polyvinylpyrrolidone (PVP) having a pendant five-membered lactam ring is complicated by the manner by which the macromolecule adsorbs on the TiO 2 particle surface in a heterogeneous dispersion. Experimental results and computer simulation of the initial process(es) infer three major steps for the photodegradation of the PVP structure. The first step is adsorption (or coagulation) of PVP on TiO 2 particles as evidenced by the size distribution of TiO 2 particles by dynamic light-scattering and by the electric charge on the TiO 2 particle surface assayed by ζ-potential measurements. Molecular orbital simulations of initial processes were calculated at the AM1 level using the MOPAC system available in the CAChe package. The second step, namely attack of PVP by OH and/or OOH radicals, involves cleavage of the PVP main chain and opening of the lactam ring in the PVP structure probed by temporal UV spectral changes and by a decrease of the molecular weight using gel permeation chromatographic methods. Additional details of the photo-oxidation mode of the lactam ring was examined by a detailed examination of the photo-oxidation of the model compound 2-pyrrolidone to ascertain formation of OH radical adducts, opening of the lactam ring, and identification of intermediates by HPLC, and 13C- and 1H-NMR methods. The final major step in the mechanism involves generation and subsequent conversion of the primary amine (methylamine from opening of the lactam ring) to yield ultimately NH 4 + and NO 3 − ions, and conversion of the propanoic acid to acetic and formic acids and then to CO 2. The effects of the extent of polymerization and variation in light intensity were examined using PVP samples having different hardness factors (hf) of 15 and 30, and different light intensities (namely, 1, 2, 3 and 4 mW cm −2), respectively.

Mechanistic study of the TiO2-assisted photodegradation of squarylium cyanine dye in methanolic suspensions exposed to visible light

Details of the degradation pathway(s) of squarylium cyanine (SQ) dye in methanolic TiO2 dispersions under visible light irradiation were probed in an attempt to more clearly understand the photodegradation mechanism of dye under visible radiation. For SQ, degradation begins by cleavage of the CC double bond to yield ultimately the final product 1-sulfopropyl-3,3-dimethyl-5-bromoindolenium-2-one. A considerable amount of H2O2 is also formed during degradation through reduction (but not through dismutation) of the superoxide radical anion. In comparison with the previous results in aqueous media, it is evidenced for the first time that the pathway for the TiO2-assisted photodegradation of SQ using visible light irradiation is initiated by a series of complex reactions involving the dye cation radical SQ•+ with dissolved dioxygen and does not implicate any of the photogenerated active oxygen species such as •OH, O2•− and •OOH radicals. These results clarify the previous equivocal conceptions on the details of the reaction between dye cationic radical and oxygen species.

Hydrogen peroxide synthesis over metallic catalysts

The energetics of the elementary reactions involved in the synthesis of hydrogen peroxide over noble metal catalysts have been investigated using the bond order conservation-Morse potential approach (BOC-MP). Enthalpy changes as well as activation energies for forward and reverse reactions were calculated for the different elementary steps which may occur by reaction of H 2 and O 2 on the noble metal series Pd, Pt, Ag and Au. According to the BOC-MP model, under low surface coverage conditions the reaction proceeds along a water channel in which hydroxyl radicals are the main intermediates. No hydrogen peroxide is produced under these conditions. The model indicates that the formation of hydroxyl radicals should be blocked and adsorbed hydroxyls should be destabilized for the reaction to proceed along the hydrogen peroxide channel in which OOH radicals play a fundamental role. In the metal series investigated the production of H 2O 2 is more favorable on gold and silver than on the other metals. Experimental conditions such as hydrogen and oxygen loading of the catalyst are simulated and the role of promoters is discussed.

New theoretical value of the enthalpy of formation of the OOH radical

The enthalpy of formation of the OOH radical has been estimated by using as working chemical reactions HOO + H2 → OH + H2O and HOO + H2O → HOOH + OH, and calculating the energies at the MP4SDTQ(FC)/TZ(2df,p)//MP2(FULL)/6-31G(d,p) level of theory, with spin projection and post-MP4 corrections. The value obtained, ΔH f,298K = 16·8 ± 2·5 kJ mol-1, is in excellent agreement with the latest experimental value.

Untersuchungen zur Thiol/En‐Polymerisation: elektronenspinresonanzspektroskopischer Nachweis spontaner Radikalbildung

AbstractBy means of ESR (electron spin resonance) spectroscopic investigations it is shown, that in dithiol/diene‐mixtures, in the absence of daylight and oxygen, radicals are formed spontaneously. Using the spin trapping technique and nitrosodurene (ND) as the trapping agent the existence of sulfur and carbon radicals could be proved unambiguously. Oxygen has no significant influence on the reaction. Spin trapping experiments have shown, that the aliphatic dithiols investigated do not form any traceable quantity of RS. and .OOH radicals in the presence of air. Radicals are detected only after addition of an ene‐compound.

Ultrasonic magnetic nuclear resonance review: Akust. Zh., 8, No. 4, p. 383 (1962)

Herein, we have developed a method for ultra–thinning MnO2 nanosheets by H2O swelling followed by a cetyltrimethylammonium bromide (CTAB)–intercalation exfoliation strategy. The thickness of the obtained U–MnO2 nanosheets was about 1–2 layers. As–prepared U–MnO2 exhibited higher turnover frequency (TOF) value of H2 production from alkaline formaldehyde solution at room temperature as compared to that of pristine MnO2 (TOF per surface Mn atom: 2.7 vs. 1.1 h–1). With the ultra–thinning process, abundant surface oxygen vacancies (VO) on U–MnO2 were demonstrated by X–ray photoelectron spectroscopy and extended X–ray absorption fine structure analysis, which could couple molecular O2 and benefit for the breaking of CH bonds from formaldehyde with the generation of OOH radical. Detailed reaction–pathway calculations showed that the O2 assisted dehydrogenation of alkaline HCHO solution were thermodynamically favored on U–MnO2 with lower energy barrier of 0.32 eV as compared to that of 1.25 eV on pristine MnO2. This work not only provides an applicable method for synthesizing ultrathin MnO2 nanosheets but also gives an evidence towards more essential understanding of hydrogen evolution reaction from alkaline formaldehyde solution at atomic level.

PHOTOLYSIS AND PHOTO‐OXIDATION OF AMINO ACIDS AND PEPTIDES—III. EFFECT OF IONIZING RADIATION ON CYSTINE AND RELATED AMINO ACIDS

SummaryIrradiation of cystine (RSSR, R = NH2CH(COOH)CH2‐‐), cysteine (RSH), S‐methycysteine (RSCH,) or lanthionine (RSR) in aqueous solution with ionizing radiation (X‐or y‐rays) yields products similar to those previously shown to be produced by ultraviolet light irradiation. The range and yields of products are affected by factors such as radiaion dosage, pH and the presence of air. Cystine readily yields products derived both from S‐S bond fission (e.g. cysteic acid) and from C‐S fission (e.g. alaninethiosulphuric acid (RSSO3H)). The initial products from cysteine are mainly cystine and alanine. With radiation doses up to 105 rad, S‐methylcysteine and lanthionine usually give low yields of the corresponding sulphoxides, but with higher doses (106‐107 rad) C‐S fission occurs yielding other amino acids such as alanine.On irradiation of the four amino acids in the solid state, ionizing radiation, unlike 2537Å light, effects only slight conversion to other amino acids. However electron spin resonance (ESR) spectra indicate that for cystine and cysteine similar trapped free radicals are obtained with both forms of electromagnetic radiation. The relative ease of S‐bond fission of these amino acids on irradiation in aqueous solution as compared with solid state irradiation is compatible with accepted views on indirect action of H, OH and OOH radicals.