Photogenerated OH Radicals Research Articles

Photo-generated hydroxyl radicals contribute to the formation of halogen radicals leading to ozone depletion on and within polar stratospheric clouds surface

Polar stratospheric clouds (PSCs), of which the surface is a dynamic liquid water layer and might consist of aqueous HNO3 and H2O2, is a well-known key meteorological condition contributing to the ozone hole in the polar stratosphere. PSCs has been considered to provide abundant surface for the heterogeneous reactions causing the formation of the Cl2 and HOCl, which are further photolyzed into Cl and ClO radicals leading to the ozone destruction. Here we demonstrated that the sunlight drives the massive and stable production of OH radicals in aqueous HNO3 and its main photo-induced byproduct HNO2. We also found that the photo-generated OH radicals in aqueous HNO3, HNO2 and H2O2 have the remarkable capability to react with the dissolved HCl, Cl− and Br− to form halogen radicals. In addition, we observed that the H2O2 can react with dissolved HCl and Br− in darkness to form and release Cl2 and Br2 gases, which could further be photolyzed into reactive halogen radicals whenever sunlight is available. All these findings suggest that, except for the well-known heterogeneous reactions, photochemical reactions involving the aqueous HNO3 and H2O2 on and within PSCs surface might constitute another important halogen activation pathway for ozone destruction. This study may shed deeper insights into the mechanism of halogen radicals resulting in ozone depletion in polar stratosphere.

Ultrafast photodegradation of isoxazole and isothiazolinones by UV254 and UV254/H2O2 photolysis in a microcapillary reactor

The photodegradation process of methylisothiazolinone (MIT), benzisothiazolinone (BIT), and isoxazole (ISOX) in ultrapure water and synthetic wastewater by means of UV254 photolysis and by UV254/H2O2 advanced oxidation process were investigated in a microcapillary photoreactor designed for ultrafast photochemical transformation of microcontaminants. For the first time, we estimated key photo-kinetic parameters, i.e. quantum yields (35.4 mmol·ein−1 for MIT, and 13.5 and 55.8 mmol·ein−1 for BIT at pH = 4–6 and 8, respectively) and rate constants of the reaction of photo-generated OH radicals with MIT and BIT (2.09·109 and 5.9·109 L mol−1·s−1 for MIT and BIT). The rate constants of the reaction of photo-generated OH radicals with ISOX in MilliQ water was also estimated (2.15·109 L mol−1·s−1) and it was in good agreement with literature indications obtained in different aqueous matrices. The models were extended and validated to the case of simultaneous degradation of mixtures of these compounds and using synthetic wastewater as an aqueous matrix. High resolution-accurate mass spectrometry analysis enabled identification of the main intermediates (BIT200, B200, saccharin, BIT166) and enabled proposal of a novel degradation pathway for BIT under UV254/H2O2 treatment. This study demonstrates an ultrafast method to determine key photo-kinetic parameters of contaminants of emerging concern in water and wastewater, which are needed for design and validation of photochemical water treatment processes of municipal and industrial wastewaters.

Mixed Ti-Zr metal-organic-frameworks for the photodegradation of acetaminophen under solar irradiation

Mixed Ti-Zr metal-organic-frameworks (MOFs) have been synthesized and tested as photocatalysts under solar-simulated radiation using acetaminophen (ACE) as target pollutant. These materials were obtained upon partial substitution of Ti by Zr atoms in the crystalline structure of NH2-MIL-125(Ti) MOF. The effect of the Ti:Zr molar ratio on their characteristics and catalytic behaviour has been analysed. Materials with high Zr relative amount (60–80%) showed amorphous structure and low solar-photocatalytic activity. In contrast, lower Zr proportions resulted in new MOFs with well-defined crystalline structure and high activity for the solar photocatalytic degradation of ACE, even higher than that of the bare Ti MOF. Results with scavengers allowed concluding that O2− radicals are the main reactive species, although photogenerated OH radicals and electrons also contribute to the degradation. The stability of the most active photocatalyst was confirmed upon three successive runs.

Photocatalytic abatement of trichlorethylene over Au and Pd–Au supported on TiO2 by combined photomineralization/hydrodechlorination reactions under simulated solar irradiation

The present work investigates the photocatalytical abatement of trichlorethylene (TCE) under simulated solar irradiation (AM 1.5). The employed catalytic materials, Au and Pd–Au nanoparticles supported on TiO2, were prepared by incipient wetness and by deposition–precipitation methods. It is found out that TCE is converted efficiently (>80%) to Cl− and CO2 over all scrutinized catalysts. Typically, the metal addition enhances to some extent the photooxidation activity of TiO2 via photogenerated OH radicals, electrons, and holes. The photocatalytic abatement of TCE over the same catalytic materials was examined in the presence of small amount of methanol. The idea was to convert TCE to Cl− and C2 (ethane and ethylene) over Au and Pd–Au/TiO2 catalysts by the H2 generated photocatalytically in situ. The experimental results evidenced that, over Pd–Au/TiO2 catalysts, hydrodechlorination (HDC) and photomineralization reactions of TCE occurs simultaneously. In contrast, Au/TiO2 is inactive to catalyze the hydrogenation of TCE. The results of TCE abatement by combined photooxidation/HDC reactions in presence of methanol are corroborated with H2 photocatalytic generation results, over the same catalytic materials, from water/methanol mixture. A reaction mechanism is advanced in light of experimental results. The photogenerated OH radicals, electrons, and holes participate in a complex reaction pathway to formation of oxygenated organic intermediates, Cl−, CO2, and H2.

OH Radical Formation at Distinct Faces of Rutile TiO2 Crystal in the Procedure of Photoelectrochemical Water Oxidation

It has been believed that the photogenerated OH radicals are major active species which cause photocatalytic oxidation of water. To investigate the actual contribution of OH radicals to the photocatalytic O2 generation, the amount of the OH radicals was measured for the three kinds of rutile TiO2 electrodes having (100), (110), and (001) crystalline surfaces. The current efficiencies for O2 generation measured with an oxygen sensor were almost 100% for all electrodes. However, the current efficiencies for OH radical formation estimated by means of a coumarin fluorescence probe method were less than 1%. Thus, it was experimentally elucidated that the contribution of OH radicals to the O2 production is negligibly small. The amount of OH radical production decreased in the order of (100) > (110) > (001), along with the increase in the efficiency of the O2 production. A plausible mechanism of OH radical formation as a byproduct in the O2 generation process was proposed.

Photocatalytic properties of sodium decatungstate supported on sol–gel silica in the oxidation of glycerol

Photoexcitation of Na4W10O32 dissolved in water produces a powerful oxidizing reagent that is able to form hydroxyl radicals from water. OH radicals formation has been demonstrated by EPR spin-trapping technique using DMPO as spin trap. The photooxidation of glycerol occurs through its reaction with OH radicals and is characterized by low selectivity and partial degradation of the substrate to CO2. Entrapment of Na4W10O32 inside a silica matrix by a sol–gel procedure gives a new heterogeneous photocatalyst characterized by the presence of micropores (7 and 13Å) and mesopores (30Å). Interaction between polyoxoanion and protonated silanol groups are strong and prevent any kind of leaching. The solid support affects the selectivity of the O2-assisted photooxidation process of glycerol. In particular, silica surface enhances alcohol adsorption improving its local concentration, so favouring its reaction with photogenerated OH radicals that leads to glyceraldehyde and dihydroxyacetone. At the same time, carbon dioxide is not formed, thus indicating that it is possible to control the high oxidizing power of sodium decatungstate through its heterogenization on a suitable support.

Sterilization of microorganisms on jet spray formed titanium dioxide surfaces

AbstractThe photocatalytic inactivation of microorganisms seeded on jet spray formed TiO2 surfaces was studied using ultra-violet LED illumination centered at a wavelength of 388nm. The surfaces were found to be highly effective at eradicating Pseudomonas aeruginosa, Staphylococcus aureus and Candida albicans cells, showing no obvious dependence on cell density. Efficiencies of killing were found to show a large variation between species with, in order of susceptibility, P. aeruginosa (gram-negative bacterium)>S. aureus (gram-positive bacterium)>C. albicans (yeast). A mechanism for inactivation based upon oxidation and reduction of cell membranes/walls by photo-generated OH radicals is supported. Those species with relatively thicker and complex cell wall structures (C. albicans and S. aureus) were destroyed more slowly than P. aeruginosa, which has a thinner cell wall. Electron microscopy examination showed complete mineralization of P. aeruginosa cells upon the TiO2 surface within 25min of illumination. The results presented illustrate the benefit that jet spray formed coatings could have upon the health service with examples including implants and sterile surfaces.

Effects of impurities containing phosphorus on the surface properties and catalytic activity of TiO 2 nanotube arrays

TiO 2 nanotube arrays were prepared by titanium anodic oxidation with either HF or H 3PO 4/NH 4F aqueous electrolyte solutions. The samples were characterized by means of X-ray diffraction (XRD), infrared spectroscopy (IR), Raman spectroscope, photoluminescence spectra (PL) and photocurrent response. Aqueous solutions of methylene blue or Cr(VI) ions were used as the target pollutants to compare catalytic activities of the two nanotube array types. The amorphous impurities containing phosphorus were confirmed by XRD and IR, for the sample synthesized with H 3PO 4/NH 4F electrolytes. They closed a portion of the active sites, acted as recombination centers of photo-generated charges, and were also involved in the negative reactions of competing photo-generated holes or OH radicals. The TiO 2 nanotube arrays formed in the H 3PO 4/NH 4F electrolytes exhibited a stronger fluorescence spectrum, a weaker photocurrent and a lower catalytic activity than the sample fabricated with HF electrolyte without phosphorus impurities.

RETRACTED: Characterization and mechanistic analysis of the visible light response of cerium and nitrogen co-doped TiO 2 nano-photocatalyst synthesized using a one-step technique

Cerium and nitrogen co-doped anatase TiO(2) nanoparticles were synthesized using a one-step technique via a modified sol-gel process and characterized by XRD, BET, DRS, Raman and XPS. The photocatalytic mechanism of the degradation of methylene blue (MB) under fluorescent light and visible light irradiation was studied. Co-doping cerium and nitrogen in the crystal lattice of TiO(2) narrowed the band gap from 2.40 eV (Ce-doped TiO(2)) to 2.21 eV (Ce/N co-doped TiO(2)). Ce(4+)/Ce(3+) pairs, oxynitride species and Ti-O-N and Ti-O-Ce bonds were determined by XPS. The recombination of photogenerated electron-hole pairs was inhibited due to the synergistic effect of doping with Ce(4+)/Ce(3+) ions and N atoms. The optimal doping ratio was 0.70% Ce and 0.70% N using MB photocatalytic degradation under fluorescent light and visible light irradiation (lambda>420 nm). The enhanced photocatalytic degradation under visible light irradiation was attributed to the increasing number of photogenerated OH radicals. The recombination of photogenerated e(-)-h(+) was attributed to be the key factor for the decrease in the photocatalytic degradation efficiency of MB.

Photooxidative mineralization of microorganisms-produced glycolipid biosurfactants by a titania-mediated advanced oxidation process

The photooxidative mineralizations of two microorganisms-produced glycolipid biosurfactants 4- O-(4′,6′-di- O-acetyl-2′,3′-di- O-alkanoyl-β- d-mannopyranosyl)- d-erythritol (MEL-A) and 1- O-(6′- O-acetyl-2′,3′-di- O-alkanoyl-β- d-mannopyranosyl)- d-erythritol (MEL-B) were examined by monitoring the temporal changes in UV absorption, the time profiles of CO 2 evolution and the changes in interfacial tension occurring by an advanced oxidation process in the presence of a metal-oxide (TiO 2). Features of their mineralization are compared to the photomineralization of the anionic dodecylbenzene sulfonate (DBS) surfactant carried out under otherwise identical conditions. The adsorption of surfactants on the TiO 2 surface and the positions of attack of the photogenerated OH radicals on the surfactants’ structure were assessed by molecular orbital (MO) calculations of partial charges and frontier orbital electron densities, respectively. The photodegradation of DBS was faster than the MELs as also evidenced by surface tension measurements, whereas the photomineralization of the anionic DBS surfactant was definitely slower than that of the MEL biosurfactants due to the hydrophobic alkyl chain in the DBS structure. Possible initial mechanistic stages of the photooxidation of MEL-A and MEL-B are proposed based on experimental data and comparison with MO calculations.

Characterization, activity and kinetics of a visible light driven photocatalyst: Cerium and nitrogen co-doped TiO2 nanoparticles

Abstract In order to effectively photocatalytically degrade azo dye under solar irradiation, anatase TiO 2 that was co-doped with cerium and nitrogen (Ti 1− x Ce x O 1− y N y ) nanoparticles (NPs) were synthesized using a one-step technique with a modified sol–gel process. The crystal structure and chemical properties were characterized using XRD, BET and XPS. Oxynitride species, Ce 4+ /Ce 3+ pairs, and Ti–O–N and Ti–O–Ce bonds were determined using XPS. The photocatalytic mechanism was investigated through methylene blue (MB) photocatalytic degradation using various filtered wavelengths of light ( λ > 365 nm, λ > 420 nm, λ > 500 nm, λ > 550 nm and λ > 600 nm) for a period of 10 h. Two experimental parameters were studied systematically, namely the atomic ratio of doped N to Ce and the irradiation wavelength number. The photocatalytic degradation of MB over Ti 1− x Ce x O 1− y N y NPs in aqueous suspension was found to follow approximately first-order kinetics according to the Langmuir–Hinshelwood model. The enhanced photocatalytic degradation was attributed to the increased number of photogenerated OH radicals.

Photocatalytic removal of pentachlorophenol by means of an enzyme-like molecular imprinted photocatalyst and inhibition of the generation of highly toxic intermediates

Pentachlorophenol (PCP) is a typical highly-toxic pollutant, and its direct photolysis and conventional photocatalysis may produce more toxic by-products such as dibenzodioxins. It is urgently needed to develop a photocatalytic process able to remove PCP without the generation of highly toxic by-products. To achieve this, enzyme-like molecular-imprinted photocatalysts were prepared by using structural analogues of PCP as pseudo templates. It was found that 2,4-dinitrophenol (DNP) was the best template among the tested analogues. The molecular imprinted polymer (MIP) coated P25 TiO2 photocatalyst DNP–P25 prepared with DNP as the template greatly accelerated the photocatalytic degradation of PCP and depressed the generation of toxic intermediates. It was confirmed that the amino groups at the footprint cavities provided a well-defined micro reaction environment, which made the benzene ring of the adsorbed PCP be better exposed to photo-generated reactive OH radicals, leading to easier cleavage of the benzene ring. Both the intermediate analysis and toxicity evaluation confirmed that the MIP-coated TiO2 can make the photocatalytic degradation a safe and green approach of removing PCP.

Photochemical behaviour of poly(ethylene oxide) (PEO) in aqueous solution: Influence of iron salts

The degradation of poly(ethylene oxide) (PEO) photoinduced ( λ ≥ 300 nm) by Fe(III) (6.0 × 10 −4 mol L −1) in aqueous solution in the presence of oxygen at 15 °C has been investigated in various experimental conditions of pH and concentrations: pH 3.1 and 7.8 and at three different concentrations of PEO: 0.1, 5.0 and 50.0 g L −1. The formation of oxidation products was followed by infrared analysis of deposit obtained by evaporation of aliquots of irradiated polymer solutions. The photooxidation of PEO aqueous solutions both in the presence and in the absence of Fe(III) leads to the formation of macromolecular formates and esters. Formate ions are formed by the hydrolysis of macromolecular formates leading to the acidification of the medium. The presence of Fe(III) modifies the rate of oxidation especially in weakly acidic medium. The photogenerated OH radicals from iron species accelerate the oxidation of PEO. Thermooxidation experiments of PEO in aqueous solution in the presence of Fe(III) at 50 °C are also reported and compared with photooxidation results.

Photoassisted mineralization of aromatic and aliphatic N-heterocycles in aqueous titanium dioxide suspensions and the fate of the nitrogen heteroatoms

The photoassisted degradation of aromatic heterocycles (pyrrole, imidazole, pyrazole, isoxazole, oxazole and thiazole) and N-containing alicycles (aliphatic heterocycles: pyrrolidine, 4-butanelactam and 5-pentanelactam) was examined in liquid–solid dispersions. Complete mineralization (TOC) of the aromatic heterocycles was attained within ca. 1h of UV irradiation of the TiO2/heterocycle system in acidic (pH 3), near-neutral (pH 6.0–7.6) and alkaline (pH 11) media. Mineralization kinetics were, in general, not appreciably influenced by the presence of acid but tended to be somewhat slower in alkaline media. N-alicycles were photomineralized more slowly than were the aromatics. The former could be mineralized in acidic and near-neutral media in less than 2h, but not in alkaline media in which pyrrolidine, 4-butanelactam and 5-pentanelactam (and for comparison 4-butanelactone) were not mineralized even after 3h of UV irradiation. Final products were, in all cases, CO2, NH4+ and NO3− ions and SO42− ions in the case of thiazole. Nitrogen (N2) gas was detected only during the photooxidation of pyrazole (contains two adjacent N heteroatoms) but not imidazole in which the N atom are separated by a C atom. The molar ratio NH4+/NO3− depended closely on the chemical structure of the substrates at the longer irradiation times. The site and mode of adsorption of these heterocycles onto the TiO2 particle surface were inferred from point charge calculations and the point of zero charge of the TiO2 (pHpzc ∼6.7); the position of elecrophilic attack by the photogenerated OH radicals was deduced from calculated frontier electron densities.

Sunlight-induced functionalisation reactions of heteroaromatic bases with aldehydes in the presence of TiO 2: A hypothesis on the mechanism

In previous studies we reported a new photocatalytic system involving polycrystalline TiO 2 for the selective functionalisation of heteroaromatic bases with ethers and amides. In order to extend the applications of this new reaction and to better understand the mechanism involved, we have examined aromatic and aliphatic aldehydes as acyl radical sources for the nucleophilic addition to protonated N-heteroarenes in acetonitrile as the solvent and TiO 2/H 2O 2 as the photocatalytic system. Acyl radicals may undergo decarbonylation to yield the corresponding alkyl radicals. Acyl/alkyl derivative ratios depend on the nature of the aldehydes, and present a different distribution from that obtained in corresponding redox reactions. Indeed, decarbonylation of primary aldehydes occurs in our system in significant amounts. For instance, the acyl/alkyl ratio in the case of the pivaloyl radical is 0.03 versus 1.2 for the redox reaction carried out under otherwise identical conditions of temperature and base concentration. Different polycrystalline TiO 2 samples were used and some differences in yields and product distribution were found. A mechanism is proposed on the basis of results obtained for which oxidation of the intermediate adducts is considered to occur on the surface of the photocatalyst TiO 2 either via direct involvement of valence band holes or indirectly via photogenerated OH radicals, whereas the additional oxidants H 2O 2 and O 2 (air) scavenge the photoelectrons produced.

Photocatalytic oxidation dynamics of acetone on TiO 2: tight-binding quantum chemical molecular dynamics study

The clarification of the excited states dynamics on TiO 2 surface is important subject for the design of the highly active photocatalysts. In the present study, we applied our novel tight-binding quantum chemical molecular dynamics method to the investigation on the photocatalytic oxidation dynamics of acetone by photogenerated OH radicals on the hydrated anatase TiO 2 surface. The elucidated photocatalytic reaction mechanism strongly supports the previous experimental proposal and finally the effectiveness of our new approach for the clarification of the photocatalytic reaction dynamics employing the large simulation model was confirmed.

Photocatalytic degradation of various types of dyes (Alizarin S, Crocein Orange G, Methyl Red, Congo Red, Methylene Blue) in water by UV-irradiated titania

The photocatalytic degradation of five various dyes has been investigated in TiO2/UV aqueous suspensions. It was attempted to determine the feasibility of such a degradation by varying the chemical structures, either anthraquinonic (Alizarin S (AS)), or azoic (Crocein Orange G (OG), Methyl Red (MR), Congo Red (CR)) or heteropolyaromatic (Methylene Blue (MB)). In addition to a prompt removal of the colors, TiO2/UV-based photocatalysis was simultaneously able to fully oxidize the dyes, with a complete mineralization of carbon into CO2. Sulfur heteroatoms were converted into innocuous SO42−ions. The mineralization of nitrogen was more complex. Nitrogen atoms in the −3 oxidation state, such as in amino-groups, remain at this reduction degree and produced NH4+ cations, subsequently and very slowly converted into NO3− ions. For azo-dye (OG, MR, CR) degradation, the complete mass balance in nitrogen indicated that the central NN azo-group was converted in gaseous dinitrogen, which is the ideal issue for the elimination of nitrogen-containing pollutants, not only for environmental photocatalysis but also for any physicochemical method. The aromatic rings were submitted to successive attacks by photogenerated OH radicals leading to hydroxylated metabolites before the ring opening and the final evolution of CO2 induced by repeated subsequent “photo-Kolbe” reactions with carboxylic intermediates. These results suggest that TiO2/UV photocatalysis may be envisaged as a method for treatment of diluted colored waste waters not only for decolorization, but also for detoxification, in particular in textile industries in semi-arid countries.

Photocatalytic degradation of polycarboxylic benzoic acids in UV-irradiated aqueous suspensions of titania.: Identification of intermediates and reaction pathway of the photomineralization of trimellitic acid (1,2,4-benzene tricarboxylic acid)

Three polycarboxylic (hemimellitic (Hem), trimellitic (Tri) and pyromellitic (Pyro)) acids, representative of industrial pollutants and of compounds from biomass, were degraded by heterogeneous photocatalysis. The three molecule disappearance rates followed the order Pyro>Hem>Tri. They obeyed a Langmuir–Hinshelwood mechanism. The two competitive initial steps of attack of the molecules corresponded (i) to a hydroxylation reaction induced by photogenerated OH radicals and (ii) by a decarboxylation (photo-Kolbe) reaction resulting from the direct attack of one carboxylic group by a positive photo-hole. A very careful analysis of the degradation intermediates was performed using high performance liquid chromatography (HPLC) and gas chromatograph/mass spectrometer (GC/MS), especially in the case of Tri. The loss of several carboxyl groups leading to benzoic acid formation was observed before the aromatic ring opening. Several aliphatic acidic fragments were detected, such as malonic and succinic acids. Interestingly, a condensation product was detected, which indicated that some carboxylic radicals could attack a Tri molecule and form a Pyr molecule. However, all these acid intermediates could be photodecomposed — in agreement with previous results from the laboratory — into CO2 with a complete carbon mass balance. A detailed degradation pathway could be proposed.Such a reaction demonstrated that complex aromatic water pollutants, originating either from industry or from biomass, could be totally mineralized and that they could produce clean water.

Fast photoreactions of oxygenates on tropospheric fly ash particles

Ethanol and methyl tert-butyl ether (MTBE) are finding increasing use as fuel additives in the U.S. because of governmental mandates, and the relative merits of each have therefore become the focus of intense debate. The ultimate fate of fugitive emissions of these species forms one aspect of this controversy, because ethanol has been implicated in reductions in air quality, while MTBE has been linked to human illness. Both species are known to react homogeneously with photogenerated OH radicals. Here we show that both can also photoreact on fly ash and related particulates present ubiquitously in urban tropospheres. The heterogeneous reactions involving MTBE ultimately form formaldehyde and acetone at rates comparable to purely homogeneous pathways, demonstrating for the first time the existence of a significant catalytic process on tropospheric particulates having widespread spatial distribution. Oxides of Ti and Fe drive the reaction.

Photoassisted oxidation of nitrite to nitrate over different semiconducting oxides

Abstract The photocatalytic oxidation of nitrite to nitrate in aqueous solutions containing different semiconducting metallic oxides of environmental concern was investigated. From the results obtained, TiO 2 and ZnO have been shown to be the most photoactive photocatalysts for nitrite photo-oxidation. For ZnO slurries, photogenerated H 2 O 2 was detected (larger amounts in the presence of nitrite than in its absence). In addition, in the presence of nitrite the photochemical instability of ZnO in aqueous suspensions was increased. From an analysis of the results it is concluded that the nitrite is oxidized by photogenerated OH radicals in the adsorption layer of the semiconductor particles following Langmuir-Hinshelwood kinetics.