Formation Rate Of OH Research Articles

Efficient bifunctional piezocatalysis of Au/BiVO4 for simultaneous removal of 4-chlorophenol and Cr(VI) in water

Piezocatalysis induced by polarization of piezoelectric materials is considered as an emerging technology in environmental cleanup. In this work, a piezoelectric material, Au nanoparticles modified BiVO4 (Au/BiVO4) is synthesized and its piezoelectric response is observed by piezo-response force microscopy. Au/BiVO4 is used as a bifunctional piezocatalyst for simultaneous removal of 4-chlorophenol (4-CP) and hexavalent chromium (Cr(VI)) upon ultrasonic vibration, with 91% and 83% of removal efficiency in 120 min, respectively. The piezocatalytic performance of Au/BiVO4 increases with the increase of ultrasonic power and the decrease of pH. The main active species for 4-CP oxidation and Cr(VI) reduction are determined to beOH and H2O2, respectively, since a significant formation rate ofOH is estimated by a probe method, and a high amount of H2O2 accumulation is directly detected. This study provides understanding on the piezocatalysis mechanism and brings new insights for development of piezocatalysis for multifunctional environmental applications.

In-cloud formation of secondary species in iron-containing particles

Abstract. The increase in secondary species through cloud processing potentially increases aerosol iron (Fe) bioavailability. In this study, a ground-based counterflow virtual impactor coupled with a real-time single-particle aerosol mass spectrometer was used to characterize the formation of secondary species in Fe-containing cloud residues (dried cloud droplets) at a mountain site in southern China for nearly 1 month during the autumn of 2016. Fe-rich, Fe-dust, Fe-elemental carbon (Fe-EC), and Fe-vanadium (Fe-V) cloud residual types were obtained in this study. The Fe-rich particles, related to combustion sources, contributed 84 % (by number) to the Fe-containing cloud residues, and the Fe-dust particles represented 12 %. The remaining 4 % consisted of the Fe-EC and Fe-V particles. It was found that above 90 % (by number) of Fe-containing particles had already contained sulfate before cloud events, leading to no distinct change in number fraction (NF) of sulfate during cloud events. Cloud processing contributed to the enhanced NFs of nitrate, chloride, and oxalate in the Fe-containing cloud residues. However, the in-cloud formation of nitrate and chloride in the Fe-rich type was less obvious relative to the Fe-dust type. The increased NF of oxalate in the Fe-rich cloud residues was produced via aqueous oxidation of oxalate precursors (e.g., glyoxylate). Moreover, Fe-driven Fenton reactions likely increase the formation rate of aqueous-phase OH, improving the conversion of the precursors to oxalate in the Fe-rich cloud residues. During daytime, the decreased NF of oxalate in the Fe-rich cloud residues was supposed to be due to the photolysis of Fe-oxalate complexes. This work emphasizes the role of combustion Fe sources in participating in cloud processing and has important implications for evaluating Fe bioavailability from combustion sources during cloud processing.

Photocatalytic behavior of colored mortars containing TiO2 and iron oxide based pigments

This paper analyses the effect of adding iron based pigments to produce photocatalytic colored mortars (Ph-CM) evaluating nitrogen oxide air purification-NOx, Rhodamine B self-cleaning-RhB and formation rate of hydroxyl radicals-OH. All Ph-CM tested shifted towards lower energy values with respect to the absorption-edge of the TiO2-mortar samples without pigments. However, different UV–Visible photoactivity behavior was obtained in function of the type and content of pigment. The results indicate that even though a decrease in the photocatalytic efficiency can occur, in some cases, an interfacial electron transfer between the conduction band of iron pigment and the photocatalytic mortar could also take place, resulting in an effective inhibition of electron-hole recombination, higher formation rate of OH, and thereby higher photocatalytic activity. This electron transfer ability directly depends on the band-edge position of pigment-cement heteroestructure and the iron to pigment ratio.

Formation of hydroxyl radicals and kinetic study of 2-chlorophenol photocatalytic oxidation using C-doped TiO2, N-doped TiO2, and C,N Co-doped TiO2 under visible light.

This work reports on synthesis, characterization, adsorption ability, formation rate of hydroxyl radicals (OH(•)), photocatalytic oxidation kinetics, and mineralization ability of C-doped titanium dioxide (TiO2), N-doped TiO2, and C,N co-doped TiO2 prepared by the sol-gel method. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and UV-visible spectroscopy were used to analyze the titania. The rate of formation of OH(•) for each type of titania was determined, and the OH-index was calculated. The kinetics of as-synthesized TiO2 catalysts in photocatalytic oxidation of 2-chlorophenol (2-CP) under visible light irradiation were evaluated. Results revealed that nitrogen was incorporated into the lattice of titania with the structure of O-Ti-N linkages in N-doped TiO2 and C,N co-doped TiO2. Carbon was joined to the Ti-O-C bond in the C-doped TiO2 and C,N co-doped TiO2. The 2-CP adsorption ability of C,N co-doped TiO2 and C-doped TiO2 originated from a layer composed of a complex carbonaceous mixture at the surface of TiO2. C,N co-doped TiO2 had highest formation rate of OH(•) and photocatalytic activity due to a synergistic effect of carbon and nitrogen co-doping. The order of photocatalytic activity per unit surface area was the same as that of the formation rate of OH(•) unit surface area in the following order: C,N co-doped TiO2 > C-doped TiO2 > N-doped TiO2 > undoped TiO2.

Controlled synthesis and facets-dependent photocatalysis of TiO2 nanocrystals

Titanium dioxide (TiO2) is a wide band gap semiconductor that has been extensively used in several environmental applications including degradation of organic hazardous chemicals, water splitting to generate hydrogen, dye sensitized solar cells, self cleaning agents, and pigments. Herein we demonstrate the synthesis of TiO2 nanocrystals (NCs) with the shapes of ellipsoids, rods, cuboids, and sheets with different exposed facets using a noncorrosive and nontoxic chemical (i.e. diethanolamine) as the shape controlling agent, unlike hydrofluoric acid commonly used. The TiO2 NCs of diverse shapes with different exposed facets were tested for photocatalytic hydroxyl radical (OH•) formation, which determines their photocatalytic behavior and the results were compared with the standard P-25 Degussa. The formation rate of OH• per specific surface area was found to be >6 fold higher for rod-shaped TiO2 NCs than that of commercial Degussa P25 catalyst. The highest photocatalytic activity of rod-shaped TiO2 NCs is ascribed to the unique chemical environment of {010} exposed facets which facilitates the electron/hole separation in presence of {101} facets.

Global modeling of SOA: the use of different mechanisms for aqueous-phase formation

Abstract. There is growing interest in the formation of secondary organic aerosol (SOA) through condensed aqueous-phase reactions. In this study, we use a global model (IMPACT) to investigate the potential formation of SOA in the aqueous phase. We compare results from several multiphase process schemes with detailed aqueous-phase reactions to schemes that use a first-order gas-to-particle formation rate based on uptake coefficients. The predicted net global SOA production rate in cloud water ranges from 13.1 Tg yr−1 to 46.8 Tg yr−1 while that in aerosol water ranges from −0.4 Tg yr−1 to 12.6 Tg yr−1. The predicted global burden of SOA formed in the aqueous phase ranges from 0.09 Tg to 0.51 Tg. A sensitivity test to investigate two representations of cloud water content from two global models shows that increasing cloud water by an average factor of 2.7 can increase the net SOA production rate in cloud water by a factor of 4 at low altitudes (below approximately 900 hPa). We also investigated the importance of including dissolved Fe chemistry in cloud water aqueous reactions. Adding these reactions increases the formation rate of aqueous-phase OH by a factor of 2.6 and decreases the amount of global aqueous SOA formed by 31%. None of the mechanisms discussed here is able to provide a best fit for all observations. Rather, the use of an uptake coefficient method for aerosol water and a multi-phase scheme for cloud water provides the best fit in the Northern Hemisphere and the use of multiphase process scheme for aerosol and cloud water provides the best fit in the tropics. The model with Fe chemistry underpredicts oxalate measurements in all regions. Finally, the comparison of oxygen-to-carbon (O / C) ratios estimated in the model with those estimated from measurements shows that the modeled SOA has a slightly higher O / C ratio than the observed SOA for all cases.

The effect of carbon content on the structure and photocatalytic activity of nano-Bi2WO6 powder

Carbon-doped nano-Bi2WO6 photocatalysts with different carbon contents were successfully prepared by a facile hydrothermal method at 433K, and characterized by XRD, BET, XPS, TEM, HR-TEM, SEM, EDX, photoluminescence (PL) and UV–vis spectroscopy. The photocatalytic activity of C-Bi2WO6 was evaluated from the analysis of the photodegradation of methylene blue (MB). The results show that the C-Bi2WO6 photocatalysts exhibit an orthorhombic Bi2WO6 phase. With the increase of carbon content, the absorption intensity of C-Bi2WO6 samples increases in the 410–650nm region and the absorption edge has a little shift to long wavelength. Compared with reference C-TiO2 or pure Bi2WO6, 0.47% C-Bi2WO6 shows obviously higher UV and visible light photocatalytic activities. The remarkably enhanced photocatalytic activities can be mainly attributed to the fact that the proper carbon doped into nano-Bi2WO6 increases its BET surface area, enhances the absorption in the 410–650nm region, improves the separation efficiency of photogenerated electrons and holes, and enhances the formation rate of OH on its surface.

CHEMICAL ANALYSIS OF A DIFFUSE CLOUD ALONG A LINE OF SIGHT TOWARD W51: MOLECULAR FRACTION AND COSMIC-RAY IONIZATION RATE

Absorption lines from the molecules OH+, H2O+, and H3+ have been observed in a diffuse molecular cloud along a line of sight near W51 IRS2. We present the first chemical analysis that combines the information provided by all three of these species. Together, OH+ and H2O+ are used to determine the molecular hydrogen fraction in the outskirts of the observed cloud, as well as the cosmic-ray ionization rate of atomic hydrogen. H3+ is used to infer the cosmic-ray ionization rate of H2 in the molecular interior of the cloud, which we find to be zeta_2=(4.8+-3.4)x10^-16 per second. Combining the results from all three species we find an efficiency factor---defined as the ratio of the formation rate of OH+ to the cosmic-ray ionization rate of H---of epsilon=0.07+-0.04, much lower than predicted by chemical models. This is an important step in the future use of OH+ and H2O+ on their own as tracers of the cosmic-ray ionization rate.

Quantitative characterization of hydroxyl radicals produced by various photocatalysts

Hydroxyl radicals ( OH) have been deemed to be the major active species during the photocatalytic oxidation reaction. In this study, OH produced on various semiconductor photocatalysts in aqueous solution under Xenon lamp irradiation was quantitatively investigated by the photoluminescence (PL) technique using coumarin (COU) as a probe molecule. The results indicated that the formation rate of OH on the surface of irradiated commercial Degussa P25 (P25) was much higher than that of other semiconductor. The pH values of the solution and phase structure of TiO 2 significantly influenced the production rate of OH. The acidic pH environment of the solutions and bi-phase structure (anatase and rutile) of TiO 2 were beneficial to enhancing the formation rate of OH. In addition, the formation rate of OH on anatase TiO 2 and P25 was much faster than that of OH on the other semiconductors (such as rutile TiO 2, ZnO, WO 3, CdS, Bi 2WO 4 and BiOCl, etc.). A new concept “OH-index” was first proposed to compare photocatalytic activity of photocatalysts, which would provide new insight into the investigation of semiconductor photocatalysts.

Water-vapor-based source of UV radiation

The results of studying a capacitive discharge in water vapor are reported. This discharge is an effective and environmentally friendly source of UV radiation due to hydroxyl OH (A2Σ+ → X2Π transition) in the wavelength range 280–330 nm. It is shown that, at E/N 210 Td, ionization dominates over dissociation. The formation rate of excited OH* radicals (A2Σ+) in the water-vapor discharge is much higher than the formation rate of emitting states in hydrogen and oxygen atoms. As E/N grows, the ratio between the intensities of OH lines and atomic hydrogen Balmer series decreases. According to estimates, E/N values optimal for the OH radical line excitation fall into the range 250–400 Td.

Effect of phase structures on the formation rate of hydroxyl radicals on the surface of TiO 2

To study the relationship between the phase structures of TiO 2 and the photoinduced hydroxyl radicals ( OH), TiO 2 nanocrystallines were synthesized by a hydrolysis-precipitate method using tetrabutylorthotitanate (TBOT) as precursor, and then calcined at 450, 600, 700, 800 and 900 °C for 2 h, respectively. The calcined samples were characterized by X-ray diffraction and N 2 sorption. The formation rate of OH on the surface of UV-illuminated TiO 2 was detected by the photoluminescence (PL) technique using terephthalic acid as a probe molecule. The results show that with increasing calcined temperatures, the amorphous (Am) TiO 2 precursor begins to turn into anatase (A) at 450 °C and rutile (R) phase appears at 600 °C, which is completely turned into the rutile phase at 900 °C. The BET specific surface areas of the catalyst decrease as the calcined temperatures increase. TiO 2 sample calcined at 600 °C, with a mixed phase of anatase and rutile, shows the highest OH formation rate, and the order of the OH formation rate is as follows: A+R>A>R>Am. Phase structures of TiO 2 play a more important role than specific surface areas in the OH formation rate. Two phase structure of anatase and rutile with a proper ratio is beneficial to the OH formation due to decrease of the combination rate of photo-generated electrons and holes. Our experimental result implies that the mixed phase of anatase and rutile can markedly enhance the photocatalytic activity of TiO 2.

Photodissociation of 3-Bromo-1,1,1-trifluoro-2-propanol at 193 nm: Laser-Induced Fluorescence Detection of OH(ν′′ = 0, J′′)

Photodissociation of 3-bromo-1,1,1-trifluoro-2-propanol (BTFP) has been investigated at 193 nm, employing the laser photolysis laser-induced fluorescence technique. The nascent OH product was detected state selectively, and the energy released into translation, rotation, and vibration of the photoproducts has been measured. OH is produced mostly vibrationally cold, with a moderate rotational excitation, which is characterized by a rotational temperature of 640 +/- 140 K. However, an appreciable amount of the available energy of 36.1 kcal mol(-1) is released into translation of OH (15.1 kcal mol(-1)). OH product has no preference for a specific spin-orbit state, Pi(3/2) or Pi(1/2). However, between two Lambda-doublet states, Pi(+) and Pi(-), the OH product has a preference for the former by a factor of 2. A mechanism of OH formation from BTFP on excitation at 193 nm is proposed, which involves first the direct C-Br bond dissociation from a repulsive state (n(Br)sigma*(C-Br)) as a primary process. The primary product, F(3)C-CH(OH)-CH(2), with sufficient internal energy undergoes spontaneous C-OH bond dissociation, through a loose transition state. The formation rate of OH is calculated to be 5.8 x 10(6) s(-1) using Rice-Ramsperger-Kassel-Marcus unimolecular rate theory. Experimental results have been supported by theoretical calculations, and energies of various low-energy dissociation channels of the primary product, F(3)C-CH(OH)-CH(2), have been calculated.

Photocatalytic reactivity for O2− and OH radical formation in anatase and rutile TiO2 suspension as the effect of H2O2 addition

Abstract Effect of crystalline structure, anatase and rutile, on the production of OH and O2 − by TiO2 photocatalytic reaction was investigated. The OH radical free from the TiO2 surface was monitored by the fluorescence intensity of 2-hydroxyl terephthalic acid produced by the reaction with terephthalic acid. Superoxide radical was detected by the chemiluminescence probe method with luminol. Formation rate of OH with rutile photocatalysts was significantly lower than that with anatase photocatalysts. By the addition of H2O2, the formation rate of OH was significantly increased for rutile and for anatase mixed with rutile by 10–20%, while pure anatase showed an opposite tendency. We suggest that the adsorption structure of H2O2 on the rutile TiO2 surface is preferable to produce OH . In photocatalytic production of O2 −, rutile surpassed anatase in stabilizing the produced O2 −. On H2O2 addition, anatase surpassed rutile in the photocatalytic activity to produce O2 − from H2O2.

Light-induced formation of hydroxyl radicals in fog waters determined by an authentic fog constituent, hydroxymethanesulfonate

The determination of the photo-production rate of hydroxyl radical (OH) in atmospheric liquids is of fundamental importance to an understanding of atmospheric aquatic chemistry. Recently, several studies have been performed to examine the photo-chemical formation rate of OH in cloud and fog waters using a free radical quenching technique with addition of a relatively large concentration of organic compounds as an OH scavenger. The addition of free-radical scavenger chemicals may significantly alter the nature of the sample water and its OH production rate. In this paper, an authentic constituent, hydroxymethanesulfonate, is proposed as a free radical probe for the measurement of photo-chemical generation rate of OH in fog water. At 313 nm, an apparent quantum yield for the production of OH in a fog water was found to be 0.012±0.001, indicating that aqueous-phase photo-chemical processes could represent a significant and may be a dominant source of OH in atmospheric liquids.

Properties of O2•- and OH• Formed in TiO2 Aqueous Suspensions by Photocatalytic Reaction and the Influence of H2O2 and Some Ions

We have investigated the effect of H2O2 on the behavior of O2•- and OH• produced in photocatalysis of aqueous TiO2 suspensions by means of luminol chemiluminescence probing and terephthalic acid fluorescence probing, respectively. The reduction of O2 by photoinduced conduction band electrons (e-) was increased by the addition of H2O2, since the consumption of photoinduced valence band holes (h+) in the oxidation of H2O2 caused the repression of e-−h+ recombination. After the end of the light irradiation, the amount of O2•- decreased based on the fractal-like kinetics at the heterogeneous surface of the TiO2 particle. The decay process might be caused by trapped h+, which cannot react with water and then remains on the TiO2 particle after the irradiation. The energy level of the trapped h+ was estimated to be above the redox potential of SCN-, since it could react with the adsorbed H2O2 and I- ions but not with other ions such as SCN-, Br-, and Cl-. The formation rate of OH• was increased by the addition of H2O2, indicating the direct reduction of adsorbed H2O2 on the TiO2 surface. The quantum efficiencies of the formation of O2•- and OH• were increased by 3.0 and 3.6 times by the addition of 0.2 mM H2O2.

Precision Estimates of the Predissociation Rates of the OH A2Σ State (v ⩽ 2)

The lifetimes of 108 different rotational levels of the OH A state (υ = 0-2) have been measured at high spectral resolution using the High Frequency Deflection technique. From these lifetimes the variation of the electronic transition moment is deduced as well as the predissociation rates for levels above the dissociation limit and the formation rate of OH through inverse predissociation.