Photoinduced reactivity of titanium dioxide

A surface science perspective on TiO2 photocatalysis

The hydroxyl radical (•OH) is one of the most powerful oxidizing agents, able to react unselectively and instantaneously with the surrounding chemicals, including organic pollutants and inhibitors. The •OH radicals are omnipresent in the environment (natural waters, atmosphere, interstellar space, etc.), including biological systems where •OH has an important role in immunity metabolism. We provide an extensive view on the role of hydroxyl radical in different environmental compartments and in laboratory systems, with the aim of drawing more attention to this emerging issue. Further research on processes related to the hydroxyl radical chemistry in the environmental compartments is highly demanded. A comprehensive understanding of the sources and sinks of •OH radicals including their implications in the natural waters and in the atmosphere is of crucial importance, including the way irradiated chromophoric dissolved organic matter in surface waters yields •OH through the H2O2-independent pathway, and the ...

Environmental Implications of Hydroxyl Radicals ((•)OH).

Anatase TiO2 nanocrystallines (17–29 nm) were successfully synthesized by the metal–organic chemical vapor deposition method (MOCVD). Moderate manipulation of system parameters of MOCVD can control the particle size. The electro-optical and photocatalytic properties of the synthesized TiO2 nanoparticles were studied along with several commercially available ultra-fine TiO2 particles (e.g., 3.8–5.7 nm). The band gap of the TiO2 crystallines was determined using the transformed diffuse reflectance technique according to the Kubelka–Munk theory. Results showed that the band gap of TiO2 monotonically decreased from 3.239 to 3.173 eV when the particle size decreased from 29 to 17 nm and then increased from 3.173 to 3.289 eV as the particle size decreased from 17 to 3.8 nm. The results of band gap change as a function of particle size agreed well with what was predicted by the Brus’ equation, i.e., the effective mass model (EMM). However, results of the photocatalytic oxidation of 2-chlorophenol (2-CP), showed that the smaller the particle size, the faster the degradation rate. This is attributed in part to the combined effect of band gap change relative to the spectrum of the light source and the specific surface area (or particle size) of the photocatalysts. The change of band gap due to particle size represents only a small optical absorption window with respect to the total spectrum of the light source, i.e., from 380 to 400 nm versus >280 nm. Consequently, the gain in optical property of the larger particles was severely compromised by their decrease in specific surface area. Our results clearly indicated the importance of specific surface area in controlling the photocatalytic reactivity of photocatalysts. Results also showed that the secondary particle size grew with time due mainly to particle aggregation. The photocatalytic rate constants decreased exponentially with increase in primary particle size. Primary particle size alone is able to predict the photocatalytic rate as it is closely related to the electro-optical properties of photocatalysts.

http://www.physics.udel.edu/~ismat/paper1/20063.pdf

Size dependency of nanocrystalline TiO2 on its optical property and photocatalytic reactivity exemplified by 2-chlorophenol

Nano-Co 3 O 4 was prepared by precipitation method and was successfully applied as heterogeneous catalyst to activate peroxymonosulfate (PMS) and degrade a model compound Acid Orange 7 (AO7). The catalyst exhibits spherical morphologies with minor particle agglomeration, small particle average size (20 nm) and high specific surface area (18 m 2 /g). The degradation kinetics of AO7 induced by nano-Co 3 O 4 /PMS system was investigated at both acidic and neutral pH conditions. The heterogeneous character of PMS activation with nano-Co 3 O 4 is more pronounced at neutral pH as indicated by fast degradation rate of AO7 and low dissolved Co ion. The catalyst presented a long-term stability through using the catalyst for multiple runs in the degradation of AO7. The main degradation intermediates of AO7 identified by GC/MS and LC/MS were 4-hydroxybenzenesulfonic acid, 1,2-naphthalenedione, coumarin, phthalic anhydride, phthalimide and 2-formyl-benzoic acid. Proposed degradation pathways were elucidated in light of the analyzed degradation products and frontier electron density theory.

Performance of nano-Co3O4/peroxymonosulfate system: Kinetics and mechanism study using Acid Orange 7 as a model compound

The kinetics of the photocatalytic degradation of 4-nitrophenol (4-NP) in the presence of TiO2 has been investigated experimentally and theoretically. The effects of the catalyst loading, the initial concentration of 4-NP, H2O2 and the added Cu2+ ions on the degradation rate have been examined. A pseudo-first order kinetic model has been used to describe the results. A linear dependence of the rate constant upon the reciprocal of the initial 4-NP concentration has been obtained. The addition of H2O2 increases the reaction rate while Cu2+ ions has a detrimental effect. With the intention of predicting the primary intermediates, geometry optimizations of the reactants, the products and the transition state complexes have been performed with the semi-empirical PM3 method. The molecular orbital calculations have been carried out by an SCF method using RHF or UHF formalisms. Based on the results of the quantum mechanical calculations, the rate constants of the two possible reaction paths have been calculated by means of the transition state theory, and 1,2-dihydroxy-4-nitro-cyclohexadienyl radical which then forms 4-nitrocatechol has been determined as the most probable primary intermediate by the application of three different theoretical shortcut methods.

Photocatalytic degradation of 4-nitrophenol in aqueous TiO2 suspensions: Theoretical prediction of the intermediates

Photocatalytic degradation of cefazolin over N-doped TiO2 under UV and sunlight irradiation: Prediction of the reaction paths via conceptual DFT

This study has the goal of determining the most probable reaction path and the product distribution for the photo-oxidative degradation of toluene in aqueous media. A combination of experimental and quantum mechanical methods were used to elucidate the effect of water solvent on the reaction rate and on the subsequent formation of the primary intermediates. In the experimental part of the study, the formation yields of hydroxylated intermediates and of benzaldehyde were measured by HPLC, in the presence of nitrate under UVB irradiation as the OH source. Modeling of the reaction paths was performed with density functional theory (DFT) calculations, to investigate the most plausible mechanism for the initial OH attack and to determine the identities of the primary intermediates. Rate coefficients for all the reaction paths were computed by the Transition State Theory (TST) to obtain the product distribution. The effect of solvent water was investigated by using COSMO as the solvation model. The experimental results combined with DFT calculations indicate that ortho -addition to finally give o -cresol is the dominant reaction path for gas and aqueous media. The presence of a dielectric medium such as water has a stabilizing effect that decreases the overall energy for this mechanism. Finally, the significance for surface waters of the reaction between toluene and OH was studied by use of a recently developed photochemical model that foresees the lifetime of a compound upon reaction with OH, as a function of the reaction rate constant, the chemical composition of the surface water layer, and the water column depth.

Photo-oxidative degradation of toluene in aqueous media by hydroxyl radicals

This study evaluates the potential of the enzyme horseradish peroxidase in the decolorization of one common industrial azo dye, naphthol blue black. Studies are carried out to understand the process parameters such as pH, temperature and reaction time. The enzymatic decolorization of the dye was examined by UV-Vis spectrophotometer and LC-MS measurements. Temperature and pH conditions were optimized for obtaining high azo-dye decolorization. Azo-dye removal at a pH range 4-6 was found to be the highest for all temperatures. After 5 minutes of treatment, the color removal of dye was ca. 80-90%. The LC-MS and spectrophotometric analyses indicated that the decolorization of the azo dye with enzyme was due to the reduction of the azo bonds. This study verifies the viability of the use of the horseradish peroxidase in the decolorization of naphthol blue black.

Decolorization of naphthol blue black using the horseradish peroxidase.

The kinetics of the photocatalytic degradation of meta -cresol ( m -CR) in the presence of TiO 2 has been investigated experimentally and theoretically. The effects of the catalyst loading and the initial concentration of m -CR on the degradation rate have been examined. A pseudo-first order kinetic model has been used to describe the results. A linear dependence of the rate constant upon the reciprocal of the initial m -CR concentration has been obtained. With the intention of predicting the primary intermediates and the product distribution, geometry optimizations of the reactants, the product radicals and the transition state complexes have been performed with the semiempirical PM3 method. The molecular orbital calculations have been carried out by an SCF method using RHF or UHF formalisms. Based on the results of the quantum mechanical calculations, the rate constants of the six possible reaction paths have been calculated by means of the transition state theory. Three predictors have been determined for the prediction of the most probable transition state and the reaction path. Branching ratio for each of the reaction paths has been calculated and the most probable intermediate has been determined to be 1,6-dihydroxy-3-methyl-cyclohexadienyl radical which then forms 3-methylcatechol. The results show that m -methylphenoxyl and m -hydroxybenzyl radicals which cause the formation of polyaromatics are not produced in the photodegradation of m -CR.

An experimental and theoretical investigation of the photocatalytic degradation of meta-cresol in TiO2 suspensions: a model for the product distribution

Arzu Hatipoglu

Papers

Photocatalytic degradation of 4-nitrophenol in aqueous TiO2 suspensions: Theoretical prediction of the intermediates

Photocatalytic degradation of cefazolin over N-doped TiO2 under UV and sunlight irradiation: Prediction of the reaction paths via conceptual DFT

Photo-oxidative degradation of toluene in aqueous media by hydroxyl radicals

Decolorization of naphthol blue black using the horseradish peroxidase.

An experimental and theoretical investigation of the photocatalytic degradation of meta-cresol in TiO2 suspensions: a model for the product distribution