Murat Kılıç

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.

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.

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.

Reactivity Indices for ortho/para Monosubstituted Phenols

Abstract The kinetics of the photocatalytic degradation reactions of twelve ortho/para mono-substituted phenols containing electron-donating or -withdrawing groups have been investigated experimentally. With the intention of determining the most suitable DFT reactivity descriptors, geometry optimizations of the compounds have been performed with the Density Functional Theory DFT at B3LYP/6-31G* level. In order to take the effect of solvent water into account, the calculations have been repeated for the optimized structures by using COSMO as the solvation model. Chemical hardness, softness, electronegativities, Fukui functions, local hardness and softness, local electrophilicities and softness differences for all phenol molecules have been calculated. Correlations between the apparent initial first-order rate constants determined in the experiments and the calculated DFT reactivity descriptors have been examined. Results show that the reactions investigated are electrophilic in nature. The local softness and softness differences correlate well with the reaction rates indicating that soft-soft interactions dominate in the photocatalytic degradation reactions of phenols.

Modeling of the Photocatalytic Degradation Reactions of Aromatic Pollutants: A Solvent Effect Model

Abstract The photocatalytic degradation reaction of 1,3-dihydroxybenzene has been modeled theoretically. With the intention of predicting the primary intermediates and the product distribution, geometry optimizations of the reactants, the product radicals and 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. 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. Three predictors have been determined for the prediction of the most probable transition state and the reaction path. A branching ratio for each of the reaction paths has been calculated and the most probable intermediate has been determined. Finally, the results obtained have been compared to the experimental results in order to assess the reliability of the proposed model.