Determination of hydroxyl radical rate constants in a continuous flow system using competition kinetics

It is important to determine reaction kinetics of the hydroxyl radical (OH) with various water pollutants for understanding advanced oxidation processes (AOPs). Hence, a simple competition kinetics in a continuous flow system was employed to determine the second-order rate constants of OH with 14 organic and inorganic solutes selected in this study. In this method, p-nitrosodimethylaniline (PNDA) was specifically employed as a well-known reference probe for OH, which gave a competitive relationship between the PNDA and each solute over OH radicals. PNDA decay with OH radicals obeyed reaction kinetics in a first-order as long as the initial concentrations of H₂O₂ and PNDA were less than 30 μM and 2 μM, respectively. The second-order rate constants of OH radical with 14 solutes obtained in this study were found to be consistent with literature values using pulse radiolysis method.     

Measurement of the hydroxyl radical formation from H2O2, NO3−, and Fe(III) using a continuous flow injection analysis

Production of hydroxyl radical (OH) is of significant concern in engineered and natural environment. A simple in situ method was developed to measure OH formation in UV/H₂O₂, UV/Fe(III), and UV/NO₃^? systems using trapping of OH by benzoic acid (BA) and measuring fluorescence signals from hydroxylated products of BA. Method development included characterization of OH trapping mechanism and measurement of quantum yields (Φ_OH) for OH. The distribution of OHBA isomers was in the order of o-OHBA > p-OHBA > m-OHBA, although it changed with the H₂O₂ concentration and light intensity. This supports that OH attacks dominantly on the benzene rings. The quantum yields for OH formation in the UV/H₂O₂ process were 1.02 and 0.59 at 254 and 313 nm, which were in good agreement with the literature values, confirming that the method is suitable for the measurement of OH production from UV/H₂O₂ processes. Using the continuous flow method developed, quantum yields for OH in UV/H₂O₂, UV/Fe(III), and UV/NO₃^? systems were measured varying the initial concentration of OH precursors. The Φ_OH values increased with increasing concentrations of H₂O₂, Fe(III), and NO₃^? and approached constant values as the concentration increased. The Φ_OH values were 0.009 for H₂O₂ at 365 nm, showing that OH production is not negligible at such high wavelength. The Φ_OH values during the photolysis of Fe(OH)²⁺ (pH 3.0) and Fe(OH)₂⁺ (pH 6.0) at 254 nm were 0.34 and 0.037, respectively. The Φ_OH values for NO₃^? approached a constant value of 0.045 at 254 nm at the initial concentration of 10 mM.     

Novel BiOI/BiOBr heterojunction photocatalysts with enhanced visible light photocatalytic properties

A novel heterojunction photocatalyst BiOI/BiOBr was synthesized by a simple modified deposition–precipitation method. Several characterization tools including XRD, SEM, HRTEM and UV–vis DRS were employed to study the phase structures, morphologies and optical properties of the samples. BiOI/BiOBr exhibited higher photocatalytic activity than single BiOI and BiOBr for the degradation of methyl orange (MO) under visible light (λ > 420 nm). This result can be due to the formation of the heterojunction between BiOI and BiOBr, which can separate photogenerated carriers efficiently. The photocatalytic mechanism study demonstrates that O₂⁻ and h⁺ are the main reactive species while OH can be negligible.     

Mechanism of photocatalytic oxidation of guanine by BiOBr under UV irradiation

This article analyzes the mechanism details of guanine photocatalytic oxidation on the surface of BiOBr nanoparticles under UV irradiation. The reaction of O₂⁻ with radical species, generated from primary oxidation of guanine by hole, is proposed to be the dominant cause of guanine damage. The present work demonstrates that BiOBr photocatalysis provided good model to investigate the reaction of substrate radical with the in situ formed O₂⁻. It also implicates that the frequently-used biomarker for DNA damage 8-oxo-7,8-dihydro-2-deoxyguanosine (8-oxodG) may be ineffective in the situation that damage is caused by O₂⁻.     

New long-wavelength ethanolamino-substituted hypocrellin: photodynamic activity and toxicity to MGC803 cancer cell

To improve hydrophilicity and photoactivity of the new type of therapeutic agent, hypocrellin, a novel long-wavelength ethanolamino-substituted hypocrellin B (EAHB) was synthesized and its molecular structure was characterized by IR, NMR, MS, and UV–vis spectrometers, and EAHB had strong absorption at the phototherapeutic window (600–900 nm). Illumination of deoxygenated DMSO solution containing EAHB generated a strong electron paramagnetic resonance (EPR) signal, which was assigned to the semiquinone anion radical of EAHB (EAHB⁻). Absorption measurements displayed that the absorptive bands at 632 and 565 nm (shoulder) arose from the semiquinone anion radical (EAHB⁻) and the absorptive bands at 519 and 450 nm (shoulder) belonged to hydroquinone (EAHBH₂), which were formed via the decay of EAHB⁻ in water-contained solution. Superoxide anion radical (O₂⁻) was produced via electron transfer from EAHB⁻ (the precursor) to ground state oxygen. The presence of NADH, a bio-electron donor, significantly enhanced production of EAHB⁻ and O₂⁻. Singlet oxygen O₂ (¹Δ_g) could be produced via energy transfer from triplet EAHB to ground state oxygen molecules. The quantum yield of O₂ (¹Δ_g) and the relative quantum yield of O₂⁻ of EAHB were 0.15 and 0.76, respectively, with the parent compound hypocrellin B (HB) as the standard. It was inferred that Type I pathway was possibly a major photodynamic mechanism of EAHB. The study on photobiological action of EAHB on MGC803 cancer cells revealed that EAHB kept the same good phototoxic ability as HB but reduced 4 times cytotoxicity than HB, and also its photopotentiation factor increased 4-folds.     

Au@TiO2 nanocomposites for the catalytic degradation of methyl orange and methylene blue: An electron relay effect

Au@TiO₂ nanocomposites were used for the catalytic degradation of methyl orange and methylene blue by NaBH₄. A detail pathway for step by step reduction, oxidation and complete mineralization of intermediates into the respective end-products was established by UV–vis spectroscopy, chemical oxygen demand, ion chromatography and cyclic voltammetry (CV). CV studies confirmed that the dyes were reduced and oxidized to the end-products by NaBH₄ in the presence of Au@TiO₂ nanocomposites and O₂, OH and HO₂ radicals generated in situ. Results suggest that Au@TiO₂ nanocomposites not only assist in the decolorization of dyes, but also promote their complete mineralization into harmless end-products.     

Transformation of hydroquinone to benzoquinone mediated by reduced graphene oxide in aqueous solution

The mediation effect of reduced graphene oxide (rGO) on the oxidative transformation of 1,4-hydroquinone (H₂Q) to 1,4-benzoquinone (BQ) in aqueous solution was investigated using a batch method and electron paramagnetic resonance. The results showed that the autoxidation of H₂Q was spin-restricted and extremely slow in acidic and neutral pH range, but this process can be dramatically accelerated when rGO was added. In the presence of 33.3 mg L⁻¹ rGO, more than 76.0% of H₂Q was oxidized to BQ within 36 h. The enhancement effects of rGO were attributed to the combined contribution of the high chemical reactivity of graphenic edges and defects on rGO and the high electron conductivity of graphene basal surface of rGO. It is proposed that dissolved oxygen reacted with graphenic edges and defects of rGO to produce surface-bound oxygen intermediates, which capture H atoms from the phenolic hydroxyl groups of H₂Q and facilitate the generation of semiquinone radical (SQ⁻). The generated SQ⁻ continued to transfer an electron to molecular oxygen to yield superoxide radical (O₂⁻) and BQ. As a chain-carrying radical, O₂⁻ further reacted with H₂Q to produce SQ⁻ and H₂O₂.     

New mononuclear CuII and ZnII complexes capable of stabilizing phenoxyl radicals as models for the active form of galactose oxidase

Herein, we describe the synthesis and crystal structure of mononuclear Cu^II and Zn^II complexes containing the new pentadentate H₂L ligand (H₂L = N-(methyl)-N^′-(2-pyridylmethyl)-N,N^′-bis(3,5-di-tert-butyl-2-hydroxybenzyl)-1,3-propanediamine. The [Cu^II(L)] and [Zn^II(L)] complexes and their one-electron oxidized [Cu^II]⁺ ([1]⁺) and [Zn^II]⁺ ([2]⁺) compounds exhibit electrochemical and spectroscopic (UV–Vis and EPR) features with relevant insight into the oxidizing half-reaction of the metalloenzyme galactose oxidase.     

Electroreductive dimerization of acetyl compounds in dimethylformamide

Seven acetyl compounds were cathodically electrolyzed in dimethylformamide containing TBAI and LiI to give corresponding hydrodimers in good yields respectively. On the basis of the rate constant of dimerization determined by use of digital simulation, it was found that the dimerization proceeded through anion radical (R⁻)/ion-pair (R⁻Li⁺) coupling in all the cases that a linear relationship existed between the logarithm of the rate constant and electron spin density at the carbonyl carbon.     

Dynamic IR properties of Fe(CO)3(η4-2,5-norbornadiene) and its radical anion

A variable temperature specular reflectance cell for thin layer spectroelectrochemistry (SEC) in the infrared and UV–Vis regions was employed to study the reduction of neutral Fe(CO)₃(η⁴-NBD) (1) (NBD=2,5-norbornadiene) to its radical-anion Fe(CO)₃(η⁴-NBD) (1⁻). Together 1 and 1⁻ comprise an unusual case where a dynamic contribution to IR spectral features can be observed in one redox state (1), but not the other (1⁻).     

In situ FT-IR study and evaluation of toluene abatement in different plasma catalytic systems over metal oxides loaded γ-Al2O3

Silver and nickel based catalysts with γ-Al₂O₃ support were synthesized via impregnation method for the removal of toluene in the presence of continuous discharge or adsorption- discharge plasma. An elaborate FT-IR analysis confirmed the formation of transient species during the plasma-catalytic oxidation of toluene. TOF-MS analysis indicated benzoic acid was not an important pathway of aromatic rings opening. NiO/Al₂O₃ catalyst displayed a higher enhancement of CO₂ selectivity and ozone decomposition ability. Moreover, the active component (O and OH) areas would increase during the discharge process in adsorption-discharge plasma system that were conducive to the removal of the organic byproducts.     

A model study for the breaking of cyanogen out of CNx within DFT

To explain the nitrogen low rate in CVD CN_x layers, and their amorphisation, a release of gaseous species containing N was proposed. Previous modelling studies based on local density functional theory use pyrite type CN₂ as starting model to assess the release of molecular nitrogen. This model is completed here to investigate another mechanism of release of cyanogen as another gaseous species. The theoretical pyrite-like C₂N wherein C and N atoms form dumbbells in octahedral sites of a carbon face-centred sub-lattice is proposed as likely to model it. It demonstrates a good compressibility (B₀ = 272 GPa) and a mechanical instability, which supports both release of atoms and the amorphisation of layers. However, the variation of interatomic distances under strain leads to propose the formation of a radical species CN. This is enforced by the consideration of the chemical bonding and its evolution in the model with the E_COV and ELF functions. The release of nitrogen in molecular cyanogen is consequently substituted by a mechanism which implies a CN radical species, which is likely to recombinate at the CVD layers surfaces.     

Sonocatalytic performance of AgBr in the degradation of organic dyes in aqueous solution

AgBr sonocatalysts were prepared by a simple method. The AgBr sonocatalysts exhibited excellent sonocatalytic activities against the decolorization of methylene blue, rhodamine B, and methyl orange. By determining the content of OH radicals in the ultrasonic degradation of organic dyes, many OH radicals were detected, which may be responsible for the high sonodegradation rate over AgBr under ultrasonic radiation. Based on the effects of the initial dye concentration on the sonodegradation, we also discussed and analyzed the kinetic data, and the results are consistent with the first-order kinetic rate equation.     

Nonlinear optical properties in tetrametallic Fe3Mn complexes with pseudo-C3 symmetry

New type of 3D chromophores [{CpFeC₅H₄CRCH4-py}₃Mn(CO)₃]BF₄ (1) with weakly interaction subchromophore were synthesized and found to display improved nonlinearity compared with their 1D reference systems [(CpFeC₅H₄CRCH4-py)Mn(CO)₅]BF₄ (2).     

Preparation of high-temperature stable SiBCN fibers from tailored single source polyborosilazanes

Bulk SiBCN ceramics derived from polyborosilazanes of the type [B(C₂H₄SiRNH)₃]_n (1a, R = CH₃; 2a, R = H; C₂H₄ = CHCH₃, CH₂CH₂) exhibit an exceptional structural stability at high temperature. Therefore, such quaternary systems are of great scientific and technical interest as fibrous reinforcements intended for high-temperature applications. In this context, the design of novel polyborosilazanes, which display properties tailored for the preparation of SiBCN fibers, is studied. Boron-modified polysilazanes of the type [B(C₂H₄SiRNCH₃)₃]_n (1b, R = CH₃; 2b, R = H) are prepared via aminolysis of the tris(dichlorosilylethyl)boranes B(C₂H₄SiRCl₂)₃ (1, R = CH₃; 2, R = H). It is shown that the functionalisation of the precursors with NCH₃ units improves their processability (i.e. solubility) compared to that of their ammonolysed analogs [B(C₂H₄SiRNH)₃]_n (1a, R = CH₃; 2a, R = H). In addition to the influence of the NCH₃ units, the presence of the SiCH₃ functions in such polymers offers the best potential for the preparation of fibers by melt-spinning. As-spun fibers are then converted under controlled atmosphere into high-temperature stable SiBCN fibers according to the polymer-derived ceramic route.     

New insights into the influence of activated carbon surface oxygen groups on H2O2 decomposition and oxidation of pre-adsorbed volatile organic compounds

In this study, the influence of the surface oxygen groups of activated carbons (ACs) on the decomposition of H₂O₂ and the consequent OH radicals generation is investigated. The oxidation of pre-adsorbed volatile organic compounds by H₂O₂ is also studied. Four ACs, with low percentage of inorganic matter (<0.2%), similar textural properties but differing in their surface oxygen content were evaluated. The surface oxygen groups of the ACs were characterised by using appropriate characterisation techniques (temperature programmed desorption and X-ray photoelectron spectroscopy). The kinetic curves of H₂O₂ decomposition were very similar for all the ACs. However, different profiles in the production of OH radicals were observed. OH radicals generation seemed to be promoted by low surface oxygen contents. Oxidation of two volatile organic compounds (VOCs) of different polarity, methyl ethyl ketone (MEK) and toluene, pre-adsorbed onto the ACs was finally investigated. H₂O₂ was used as oxidising agent. Both VOCs presented similar maximum oxidation rates, around 70%, in spite of their different hydrophobicity. Some evidences are provided supporting that oxidation of pre-adsorbed VOCs can take place in the inner pore structure of the ACs.     

Unsymmetrical cyrhetrenyl and ferrocenyl azines derived from 5-nitrofurane: Synthesis,structural characterization and electrochemistry

A new series of unsymmetrical cyrhetrenyl and ferrocenyl azines that were monosubstituted [(η⁵-C₅H₄)–C(R)N–NCH(5-NO₂–2-C₄H₂O)]M {with MRe(CO)₃ and RH (1a) or RMe (1b); MFe(η⁵-C₅H₅) and RH (2a) or RMe (2b)} and disubstituted [Fe{(η⁵-C₅H₄)–C(Me)N–NCH(5-NO₂–2-C₄H₂O)}₂] (3a) were prepared by condensation reactions of the corresponding organometallic hydrazone [(η⁵-C₅H₄)–C(R)N–NH₂)]M with 5-nitro-2-furaldehyde. The ¹H and ¹³C{¹H} NMR spectra indicated that these compounds adopted an (E,E)-configuration about the ˃CN − bond and an s-trans conformation about the N1–N2 bond, and this result was confirmed by X-ray crystallographic analyses of 1a and 2b. The opposite electronic effects of the organometallic fragments correlate with the co-planarity of the [(η⁵-C₅H₄)–C(R)N–NCH(5-NO₂–2-C₄H₂O)] system, the reduction potential of the nitro group (E_1/2) and the chemical shifts of the iminic carbons.     

Crystalline MoVO based complex oxides as selective oxidation catalysts of propane

Three single crystalline MoVO based oxides, MoVO, MoVTeO and MoVTeNbO, all of which have the same orthorhombic layer-type structure with the particular arrangement of MO₆ (M = Mo, V, Nb) octahedra forming slabs with pentagonal, hexagonal, and heptagonal rings in (1 0 0) plane, were synthesized by hydrothermal method and their catalytic performance in the selective oxidation of propane to acrylic acid were compared in order to elucidate the roles of constituent elements and crystal structure in the course of the propane oxidation. It was observed that the rate of propane oxidation was almost the same over all three catalysts, revealing that Mo and V, which were indispensable elements for the structure formation, were responsible for the catalytic activity for propane oxidation. The Te-containing catalysts showed much higher selectivity to acrylic acid than the MoVO catalyst. Since propene was formed as a main product at low conversion levels over every catalyst, it can be concluded that Te located in the central position of the hexagonal ring promoted the conversion of intermediate propene effectively to acrylic acid. The catalyst with Nb occupying the same structural position of V clearly showed the improved selectively to acrylic acid particularly at high conversion region, because the further oxidation of acrylic acid to CO_x was greatly suppressed. These conclusions were further supported by the additional studies of the determination of activation energy and catalytic oxidations of intermediate products of the propane oxidation.     

Preparation and characterization of carboximidate iron(II) complexes

The reactions of C₅H₄NCHNNHC(O)Ph (1) with Fe(II) chloride gave [Fe₂(C₅H₄NC(OEt)NNHC(O)Ph)₂(μ-OEt)₂Cl₂]) (2) in ethanol and [Fe₂(C₅H₄NC(OMe)NNHC(O)Ph)₂(μ-OMe)₂Cl₂] (3) in methanol as well as [Fe(C₅H₄NCHNNHC(O)Ph)Cl₂] (4) in tetrahydrofuran, respectively. The X-ray diffraction analysis reveals their structures and complex 4 is proposed as an intermediate of formation of complexes 2 and 3.