Nevim San

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.

Determination of Lead in Drinking and Wastewater by Hydride Generation Atomic Absorption Spectrometry

ABSTRACT This work describes the optimization of hydride generation atomic absorption spectrometry for the determination of lead at trace concentrations. The parameters affecting the production, transport, and atomization of plumbane in a closed vessel were optimized. The concentrations of reagents affecting the generation of the lead hydride were also optimized to obtain high hydride generation efficiency. The atomization temperature, reaction period, pump speed, pump period, and flow rates of reducing agent and carrier gas were varied to maximize the sensitivity for lead. The accuracy of the method was evaluated by the analysis of certified reference materials. The method provided a linear dynamic range for lead from 2.0 to 40 µg/L with a limit of detection of 0.56 µg/L.

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.

Investigation of thermodynamic and surface characterisation of 4-[4-(2-ethylhexyloxy)benzoyloxy]benzoic acid thermotropic liquid crystal by inverse gas chromatography

The inverse gas chromatography method was used to obtain thermodynamic and surface properties of the thermotropic liquid crystalline material, 4-[4-(2-ethylhexyloxy)benzoyloxy]benzoic acid (EBBA). The retention diagrams of undecane, dodecane, tridecane, ethyl acetate, n-butyl acetate, iso-butyl acetate, toluene, ethylbenzene, iso-propylbenzene, n-propylbenzene and chlorobenzene on EBBA were plotted at temperatures between 443.2 and 468.2 K. The specific retention volume, , Flory–Huggins interaction parameter, , equation-of-state interaction parameter, , and effective exchange energy parameter, , were determined. The dispersive component of the surface free energy, , of the studied adsorbent surface was estimated using retention times of different nonpolar organic solvents in the infinite dilution region. The specific free energy of adsorption, the enthalpy of adsorption, , and the entropy of adsorption, , of solvents on liquid crystal were determined. The values of were correlated with the donor and the acceptor numbers of the probes to quantify the acidic and the basic parameters of the liquid crystal surface. The values obtained for and parameters indicated an acidic character for the surface of EBBA.

Nanopore detection of double stranded DNA using a track-etched polycarbonate membrane.

We investigate the resistive-pulse sensing of 50-bp DNA using track-etched polycarbonate (PC) nanopores and show the translocation dynamics originating from the electrophoretic transport of DNAs. Conically shaped PC nanopore membranes have been prepared with asymmetric chemical etching technique. We show the potential and concentration dependence of DNA translocation through a PC nanopore. We find that the translocation of DNA scales linearly with both potential and concentration. Additionally, the threshold potential is determined to complete the translocation. Finally, by investigating the current-pulse amplitudes of nanopores with different tip sizes, we show that the nanopore size can be successfully used to distinguish the DNA molecules. These results suggest great promise for the sensing of short DNAs and understanding the dynamics of the translocation process using chemically-etched PC nanopores.

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.

Effect of molecular properties on the photocatalytic degradation rates of dichlorophenols and dichloroanilines in aqueous TiO2 suspensions

The kinetics of the photocatalytic degradation of 10 different dichlorophenols and dichloroanilines have been investigated experimentally and theoretically. With the intention of finding certain molecular indices in order to determine the degradation rates of aromatic pollutants, geometry optimizations of the compounds have been performed with the semiempirical PM3 method. The molecular orbital calculations have been carried out by an SCF method using the RHF formalism. The correlations between the apparent first-order rate constants and the calculated molecular properties of the compounds have been examined. Four different structure–activity relationships have been developed expressing the logarithms of the rate constants in terms of the Hammett constants, electron densities of the substituents, the coefficients and the energies of the highest occupied molecular orbitals HOMO, and 1-octanol/water partition coefficients logu2009K OW.