Zekiye Çınar

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

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.

Prediction of primary intermediates and the photodegradation kinetics of 3-aminophenol in aqueous TiO2 suspensions

Abstract The kinetics of the photocatalytic degradation of 3-aminophenol (3-AP) has been investigated experimentally and theoretically. The reactions have been carried out in a batch-type photoreactor using TiO 2 P25 Degussa as the photocatalyst. The effects of the catalyst loading, the initial concentration of 3-AP and the electron acceptors; H 2 O 2 , K 2 S 2 O 8 and KBrO 3 on the degradation rate have been determined. With the intention of predicting the primary intermediates, geometry optimizations of the reactants, 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 four possible reaction paths have been calculated by means of the Transition State Theory and 1,4-dihydroxy-3-amino-cyclohexa-2,5-dienyl radical has been determined as the most probable primary intermediate.

Photocatalytic oxidation of dinitronaphthalenes: theory and experiment.

A combination of photocatalytic oxidation experiments and quantum mechanical calculations was used in order to describe the mechanism and the nature of the photocatalytic oxidation reactions of dinitronaphthalane isomers and interprete their reactivities within the framework of the Density Functional Theory (DFT). The photocatalytic oxidation reactions of three dinitronaphthalene isomers, 1,3-dinitronaphthalene, 1,5-dinitronaphthalene and 1,8-dinitronaphthalene in the presence of TiO(2) Degussa P-25 grade were investigated experimentally. The reactions were carried out in a Solarbox photoreactor equipped with a Xenon lamp. The removal of the individual substrates was followed by means of a gas chromatographic method. Nonpurgable organic carbon contents of the samples were determined by means of the catalytic oxidation method using Total Organic Carbon analyzer. With the intention of determining the best reactivity descriptors to explain the differences in the photocatalytic oxidation rates in terms of the molecular properties, geometry optimizations of the compounds were performed with the Density Functional Theory DFT at B3LYP/6-31G( *) level. In order to take the effect of adsorption on the oxidation rate, a cluster Ti(9)O(18) cut from the anatase bulk structure was modeled. The binding energies for the compounds were calculated by using the double-zeta basis set. Global hardness, softness, Fukui functions, local hardness-softness and local softness differences were calculated. The results show that the reactions investigated are orbital-controlled and electrophilic in nature. Local DFT descriptors reflect the reactivities of the dinitronaphthalene isomers better than the global ones, due to the differences in their adsorptive capacities.

A QSAR study on the kinetics of the reactions of aliphatic alcohols with the photogenerated hydroxyl radicals

Abstract The kinetics of the reactions of the OH radicals with eight straight-chain and cyclic aliphatic alcohols have been investigated theoretically, with the intention of determining the mechanism of H-abstraction. For 36 possible routes, 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 all the 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. Furthermore, two different QSARs expressing the logarithms of the rate constants in terms of the energies of the highest occupied molecular orbitals, charge densities, stretching frequencies and dissociation energies of the C–H bonds have been derived.

Enhancement and Modeling of the Photocatalytic Degradation of Benzoic Acid

Abstract The kinetics of the photocatalytic degradation of benzoic acid in the presence of TiO2 has been investigated experimentally and theoretically. The effects of catalyst loading, initial concentration of benzoic acid and the electron acceptors; H2O2, K2S2O8 and OXONE on the degradation rate have been examined. A pseudo-first order kinetic model has been used to describe the results. With the intention of predicting the primary intermediates and the product distribution, geometry optimizations of the reactants, product radicals and transition state complexes have been performed with the semi-empirical PM3 and DFT/B3LYP/6-31G* methods. Solvation effects have been computed by DFT calculations at the B3LYP/6-31G* level using COSMO as the solvation model. Based on the results of the quantum mechanical calculations, the rate constants of the four possible reaction paths have been calculated by means of Transition State Theory. A branching ratio for each of the reaction paths has been calculated and the product distribution for the degradation reaction has been determined. The results show that ortho-, meta- and para-hydroxybenzoic acids are formed as the primary intermediates in the photocatalytic degradation of benzoic acid.

The Role of Non-Metal Doping in TiO2 Photocatalysis

Abstract The visible light activity of TiO2 (Degussa P25) particles was improved by non-metal-doping. In order to characterize and describe the effect of anion-doping on the electronic and structural properties of TiO2, a combination of experimental structural methods and DFT calculations were used. A series of N, C and S-doped photocatalysts with different dopant contents were prepared by an incipient wet impregnation method, followed by calcination at three different temperatures for 1h, 3h and 5h. An obvious extension of the absorption to the visible region was observed by UV-DRS. The doped photocatalysts were characterized by XRD, XPS and Raman spectroscopy. The morphological structure of the photocatalysts was examined by HR-TEM. In the computational part of the study, a neutral, stoichiometric cluster Ti9O27H18 cut from the anatase bulk structure and new models for the substitutional and interstitial anionic and cationic-doped TiO2were developed. The DFT calculations were carried out by the hybrid B3LYP functional, by using double-zeta, LanL2DZ basis set. The experimental results combined with DFT calculations, indicate that substitutional N, interstitial cationic C, and substitutional cationic S-doping of TiO2 occur in the as-prepared samples. The band-gap reduction arises from the contribution of N 2p, C 2p and S 3p orbitals to the O 2p and Ti 3d states in the VB of TiO2, as well as the presence of C and S induced mid-gap states in the band-gap. The photocatalytic activity of the doped TiO2 photocatalysts was also determined by investigating the kinetics of the photocatalytic degradation of 4-nitrophenol under UV-A and solar light irradiation.

A Quantum Mechanical Approach to TiO2 Photocatalysis

Abstract Quantum mechanical techniques play a very important role in TiO2 photocatalysis. Recent advances in heterogeneous photocatalysis have produced a number of interesting surface phenomena, reaction products and various novel photocatalysts with improved properties than the standard TiO2 photocatalyst. Many of these phenomena have not been yet explained at the molecular level. Quantum mechanical calculations appear promising as a means of describing the mechanisms and the product distributions of the photocatalytic degradation reactions of organic pollutants in both gas and aqueous phases. Since quantum mechanical methods utilize the principles of particle physics, their use may be extended to the design of new photocatalysts. This paper presents the use of quantum mechanical techniques in TiO2 photocatalysis by using certain theoretical models. Surface active species responsible of the oxidation of organic compounds were determined to be hydroxyl radicals. The determination of the mechanisms and the product distributions of the photocatalytic degradation reactions in both gas and aqueous phases were explained by using phenols + OH reaction model. Bare and salicylic acid modified TiO2 cluster models were used to describe the design of novel photocatalysts. The relations between the degradability and molecular structure were discussed in terms of the DFT-based reactivity descriptors for monosubstituted phenol derivatives.

Modeling the photochemical transformation of nitrobenzene under conditions relevant to sunlit surface waters: Reaction pathways and formation of intermediates.

Nitrobenzene (NB) would undergo photodegradation in sunlit surface waters, mainly by direct photolysis and triplet-sensitized oxidation, with a secondary role of the *OH reaction. Its photochemical half-life time would range from a few days to a couple of months under fair-weather summertime irradiation, depending on water chemistry and depth. NB phototransformation gives phenol and the three nitrophenol isomers, in different yields depending on the considered pathway. The minor *OH role in degradation would make NB unsuitable as *OH probe in irradiated natural water samples, but the selectivity towards *OH could be increased by monitoring the formation of phenol from NB+*OH. The relevant reaction would proceed through ipso-addition of *OH on the carbon atom bearing the nitro-group, forming a pre-reactive complex that would evolve into a transition state (and then into a radical addition intermediate) with very low activation energy barrier.

Surface Modification of TiO2 with Ascorbic Acid for Heterogeneous Photocatalysis: Theory and Experiment

Abstract The photocatalytic activity of TiO2 particles was improved by surface modification with L(+)-ascorbic acid (AA), through the formation of a charge-transfer complex on the photocatalyst particles. In order to characterize and describe the effect of surface modification by AA on the electronic and structural properties of TiO2, a combination of experimental structural methods and DFT calculations were used. Surface modified photocatalysts with different AA contents were prepared by an incipient wet impregnation method and characterized by FTIR, XRD, SEM and UV-DRS. In the theoretical part of the study, a neutral, stoichiometric cluster Ti9O18 cut from the anatase bulk structure and a new model for the surface modified AA-Ti9O18 were developed. The DFT calculations were carried out by the hybrid B3LYP functional, which combines HF and Becke exchange terms with the Lee-Yang-Parr correlation functional by using double-zeta, LanL2DZ basis set. The formation process of the complex and its effect on the electronic structure of TiO2 were examined. The optimized geometries, the frontier-orbital energies, energy gaps, Mulliken charge distributions of the atoms on the surface were determined. The photocatalytic activity of the AA-modified TiO2 photocatalyst was also determined by investigating the kinetics of the photocatalytic degradation of hydroquinone, an environmentally important pollutant, in the presence of bare and surface modified TiO2.

Photodegradation kinetics of aniline in aqueous TiO2 suspensions

The kinetics of the photocatalytic degradation of aniline has been investigated experimentally. The reactions have been carried out in a batch‐type photoreactor using TiO2 P25 Degussa as the photocatalyst. The effects of the catalyst loading, H2O2, the initial concentration of aniline and the added Cu2+ ions on the degradation rate have been determined. With the intention of predicting the primary intermediates, geometry optimization of the aniline molecule has been performed with the semi‐empirical PM3 method. The molecular orbital calculations have been carried out by an SCF method using RHF formalism. Based on the results of the quantum mechanical calculations, two different primary intermediates have been determined.