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Abstract

Photocatalytic activities of a series of well available commercial TiO2 powders in both suspended and immobilized forms were determined by kinetic measurements of the degradation of acid orange 7 as model compound. Although the powders showed lower photoactivity in the immobilized than suspended form, the differences among particular materials considerably varied. The porosity of the photocatalysts and related adsorption/desorption phenomena strongly influence the photocatalytic degradations occurring in aqueous suspension. The adsorption of the azo dye has a significant effect on observed concentration changes during irradiation and must be taken in account in the evaluation of degradation rates in suspensions. No direct correlation between photocatalyst material properties and photoactivity was found. The paper brings a convincing discussion that, with only minor exceptions, it is not possible to form any general conclusions on the performance of a standard type photocatalyst, even if a model reactant is used.

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Abstract

The photocatalytic degradation of an organophosphorus insecticide chlorpyrifos (O,O-diethyl O-3,5,6-trichloro-2 pyridyl phosphorothioate) in aqueous suspensions of nano ZnO and nano TiO2 under sunlight is reported in this study. Nanocrystals of ZnO (34.3 nm) were prepared by the wet chemical method and TiO2 nanocyrstals (7.5 nm) were synthesized from titanium tetraisopropoxide by hydrolysis and peptization. The synthesized nanocrystals were characterized by X-ray diffraction, FT-IR, SEM–EDS and UV–Visible analytical techniques. The degradation of the insecticide was studied by monitoring the change in substrate concentration employing UV–Visible spectroscopy. The influence of catalyst loading, substrate concentration and pH were studied. The intermediates of the degradation process were identified by GC–MS. The synthesized nano ZnO and nano TiO2 demonstrated high photocatalytic activity under solar light. The results show that nanocrystals of TiO2 exhibit a better photocatalytic activity on the degradation of Chlorpyrifos than nanocrystals of ZnO.

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Abstract

PdCl2(bipy) was found to be an efficient and stable catalyst in biphasic medium (organic-glycol) for the Heck reaction. The kinetics of the Heck coupling of styrene with iodobenzene using the same catalyst was studied in a biphasic medium in a temperature range of 393–413 K. The rate was found to have a first order dependence tending to a fractional order, on the iodobenzene as well as catalyst concentration. The rate was found to have a complex dependence on the styrene concentration and passes through a maximum, showing typical substrate inhibition kinetics. The rate had a first order dependence on the base (morpholine) concentration. The trends observed for the influence of the different parameters on the activity of the catalyst are in agreement with the established mechanism for Heck reaction. An empirical rate model has been proposed to fit the observed rate data. The activation energy was found to be 72.91 kJ/mol. This is the first time that kinetic modeling of Heck reaction in a biphasic medium (organic-glycol) has been attempted.

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Abstract

Vapor-phase selective hydrogenation of crotonaldehyde was carried out over Ir/TiO2 catalysts with different Ir contents prepared by an impregnation method. The catalysts were characterized by X-ray powder diffraction (XRD), temperature-programmed reduction (TPR), diffuse reflectance infrared Fourier transform spectra of CO adsorption (CO-DRIFTS), NH3 temperature-programmed desorption (NH3-TPD), Raman spectroscopy and temperature-programmed oxidation (TPO). It was found that with increasing Ir content in Ir/TiO2, both the activity (TOF) and selectivity to crotyl alcohol first increased and then slightly decreased. The 3 % Ir/TiO2 catalyst showed the highest activity, (with a TOF of 9.3 × 10−3 s−1) and the highest selectivity to crotyl alcohol (74.6 %) in the hydrogenation of crotonaldehyde. The results of CO-DRIFTS indicated that the reduced catalyst contains a mixture of Ir0 and Irδ+. It was concluded that the catalytic performance of the catalysts depended on the strength of surface acidity and the Ir particles size for the selective hydrogenation of crotonaldehyde to crotyl alcohol.

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Abstract

Sol–gel Co/SiO2 catalysts were prepared using Co–en (ethylenediamine) complex precursors with different cobalt introduction order. More highly reduced catalysts were obtained by gelling the silica support followed by depositing cobalt nanoparticles on silica surface, and then catalysts exhibited higher FT activity and concomitant higher C18+ selectivity. Meanwhile, both series of catalysts prepared with en/Co molar ratio of 2/1 exhibited higher activity due to their high cobalt reduction.

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Abstract

Pure and doped samples of sodium oxalate (Na2C2O4) were subjected to pre-compression and their thermal decomposition kinetics was studied at five different temperatures in the range 783–803 K under isothermal conditions by thermogravimetry. The pre-compressed samples decomposed in two stages governed by different rate laws; the Prout–Tompkins model best describes the acceleratory stage of the decomposition while the decay region is best fitted with the contracting cylinder model as in the case of uncompressed sodium oxalate samples. The rate constants k 1 and k 2 of the acceleratory and deceleratory stages of the thermal decomposition were dramatically decreased on pre-compression. However, the activation energies, evaluated by model fitting kinetic method, E 1 and E 2 for the respective stages of decomposition remained unaltered by pre-compression. The results favor ionic diffusion mechanism proposed earlier on the basis of doping studies.

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Reaction Kinetics, Mechanisms and Catalysis
Authors: Haiying Wang, Yucheng Yang, Jianhong Wei, Ling Le, Yang Liu, Chunxu Pan, Pengfei Fang, Rui Xiong, and Jing Shi

Abstract

Nitrogen-doped titanium dioxide (TiO2) nanotube arrays were synthesized by anodization in ethylene glycol electrolyte and annealing in ammonia at 500 °C. Detailed analysis showed that the nitrogen-doped titania nanotubes were pure anatase of ordered structure, with a crystallite size of 8.5 nm. The doping nitrogen atoms were induced on the interstitial sites and substitutional sites and the ratio of oxygen vacancies increased to 27.15 %, resulting in an add-on peak in the absorption spectrum and extended the absorption from 387 to 618 nm. The photocatalytic activity of the nitrogen-doped TiO2 nanotubes was evaluated by photocatalytic degradation of methyl blue under visible light irradiation. Significant improvement of photocatalytic activity under visible light irradiation was observed. We assumed the nitrogen doping induced the effect produced by nitrogen atoms, Ti3+ cations and oxygen vacancies and the size effect of the TiO2 crystallite should be responsible for the effective photocatalytic activity.

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Abstract

Nitrogen-doped titanium dioxide (N–TiO2) powders were synthesized by hydrolysis and used as a support for doping with various metals, such as, Fe, Cr, Ni, and Pt. Aqueous solutions of metal salts were used as a metal source and metals were deposited on N–TiO2 powders. Ni–N–TiO2 catalysts with various nickel concentrations were studied in detail. X-ray diffraction and diffuse reflectance spectrophotometry were used for the characterization of the photocatalysts. The Ni–N–TiO2 photocatalysts were then tested in photocatalytic water splitting under visible light. The optimum dopant concentration was found to be 10 μmol Ni/g N–TiO2 for Ni–N–TiO2. The photocatalyst, Ni–N–TiO2, has shown a stable and high activity, 490 μmol of H2 gcat −1 h−1 for the first 6 h of operation.

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Abstract

In this study, it was shown that a possible explanation of increasing photocatalytic activity with temperature may be the fact that with increasing water temperature, the amount of hydroxyl radicals in water also increases, because the ionic product of water increases with an increase in temperature. For measurements of the amount of hydroxyl radicals, the fluorescence technique was used. Terephthalic acid was used as a hydroxyl radical scavenger. After inducing of TiO2, positive holes in the valance band may react with OH ions and produce OH radicals, a strong oxidizing agent.

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Abstract

Cs-promoted Ag/α-Al2O3 catalysts prepared via thermal decomposition of Ag oxalate or reduction of Ag nitrate have been studied. XRD and hydrogen titration showed an increased Ag dispersion in oxalate-derived catalysts, and TEM revealed a domain structure of Ag particles, with the size of the particles being essentially independent of the conditions of thermal treatments. According to TEM and XRD, the Cs content (500–1,500 ppm) did not influence Ag particle sizes but high Cs loading stabilized the domains during high-temperature treatments. EDX did not reveal enrichment of Ag surface with Cs and indicated predominantly uniform distribution of the promoter, at least in high-loading samples. The yield of butadiene oxide was a strong function of Cs content, and typical volcano-shaped dependence has been observed for catalysts at the steady state. However, the catalysts approached the steady state quite differently, the changes in the yield with time-on-stream being dependent on the Cs loading and catalyst pretreatments. While high-loading samples displayed some activation, the low-loading ones showed deactivation. The reasons are discussed.

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