Evaluation of the efficiency of the photocatalytic one-electron oxidation reaction of aromatic compounds adsorbed on a TiO2 surface

The TiO2 photocatalytic one-electron oxidation mechanism of aromatic sulfides with a methylene bridging group (-(CH2)n-, n=0-4) between the 4-(methylthio)phenyl chromophore and the carboxylate binding group on the surface of a TiO2 powder slurried in acetonitrile (MeCN) has been investigated by time-resolved diffuse reflectance (TDR) spectroscopy. The electronic coupling element (H(DA)) between the hole donor and acceptor, which was estimated from the spectroscopic characteristics of the charge transfer (CT) complexes of the substrates (S) and the TiO2 surface, exhibited an exponential decline with the increasing of the methylene number of S. The determined decay factor (beta) of 9 nm(-1) also supports the fact that the 4-(methylthio)phenyl chromophore is separated from the TiO2 surface. The efficiency of the one-electron oxidation of S adsorbed on the TiO2 surface, which was determined from the relationship between the amount of adsorbates and the concentration of the generated radical cations, significantly depended on the H(DA) value, but not on the oxidation potential of S determined in homogeneous solution.     

Photocatalytic reactivity of surface platinized TiO2: substrate specificity and the effect of Pt oxidation state

Surface platinized TiO(2) (Pt/TiO(2)) has been frequently studied, but its photocatalytic reactivities reported in the literature are not consistent in some cases. To understand the discrepancies, the effects of Pt speciation on TiO(2) on the photocatalytic degradation (PCD) of a few chlorinated organic compounds (trichloroethylene (TCE), perchloroethylene (PCE), dichloroacetate, etc.) were investigated with several Pt/TiO(2) samples that were prepared differently. The oxidation state of Pt deposits was analyzed by X-ray photoelectron spectroscopy and was found to be the most important factor in determining the initial PCD rates of chlorinated organic compounds. TiO(2) with oxidized Pt species (Pt(ox)/TiO(2)) was less reactive than TiO(2) with metallic Pt (Pt(0)/TiO(2)) for all substrates tested. In particular, Pt(ox)/TiO(2) strongly inhibited the PCD of TCE and PCE whereas it was more reactive than pure TiO(2) for the PCD of other compounds. The photocurrents obtained with the Pt(ox)/TiO(2) electrode were lower than those with the Pt(0)/TiO(2) electrode, which was ascribed to the role of Pt(ox) species as a recombination center. It is proposed that TCE adsorbed on Pt(ox) chemically mediates the charge recombination through the redox cycle of TCE. The Pt effects in photocatalysis are highly substrate-specific and depend on the Pt-substrate interaction as well as the properties of Pt deposits.     

Some one-electron oxidation reactions of biheteroorganic derivatives with Ge-Hg and Ge-Li groupings

Conclusions 4-Bromo-2,4,6-tri-tert-butylcyclohexadien-2,5-one and the 2,4,6-tri-tert-butylphenoxyl radical function as a one-electron oxidizing agent in reactions with biheteroorganic compounds that contain the Ge-Hg and Ge-Li bonds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 685–688, March, 1978.     

Liquid-phase catalytic oxidation of methyl derivatives of biphenyl

Liquid-phase catalytic oxidation into acids by air was studied for the following hydrocarbons: isomers of cyclohexyltoluenes and cyclohexyl derivatives of para-xylene, mesitylene, pseudocumene, cyclopentyltoluene, cyclohexyladamantane, 4-methylbiphenyl, 2,4-, 2,5- and 3,4-dimethylbiphenyls, hydroxymethylbiphenyls, and hydroxymethylbenzenes. The oxidation of cyclohexyltoluenes involves a methyl group and proceeds without participation of the -CH bond of the cyclohexyl fragment in the oxidative conversions. The reactivity of the hydrocarbons increases in the order ortho < meta < para. Consecutive conversions of the methyl groups to carboxyls occur during the oxidation of dimethylbiphenyls. In 3,4- and 2,5-dimethylbiphenyls, the methyl groups in the para and ortho positions, respectively, are first oxidized, whereas the reactivity of both of the methyl groups in 2,4-dimethylbiphenyl is virtually the same. The mechanism of the oxidation of hydroxymethylbiphenyls and hydroxymethylbenzenes involves the formation of an unstable cation radical, which is then stabilized by emitting a proton, giving hydroxybenzyl, a more stable radical.Translated from Kinetika i Kataliz, Vol. 45, No. 6, 2004, pp. 872–876.Original Russian Text Copyright © 2004 by G. Koshel, S. Koshel, Postnova, Lebedeva, Kuznetsova, Belysheva, Yunkova.     

Regulation of one-electron oxidation rate of guanine by base pairing with cytosine derivatives

Effects of base pairing on the one-electron oxidation rate of guanine derivatives, guanine, 8-bromoguanine, and 8-oxo-7,8-dihydroguanine have been studied. The one-electron oxidation rate of guanine derivatives was determined by triplet-quenching experiments, using N,N'-dibutylnaphthaldiimide (NDI) in the triplet excited state (3NDI*) and fullerene (C(60)) in the triplet excited state ((3)C(60*)) as oxidants. In all three guanine derivatives studied here, acceleration of the one-electron oxidation was observed upon hydrogen bonding with cytosine, which demonstrates lowering of the oxidation potential of guanine derivatives by base pairing with cytosine. When a methyl or bromo group was introduced to the C5 position of cytosine, acceleration or suppression of the one-electron oxidation relative to the guanine:cytosine base pair was observed, respectively. The results demonstrate that the one-electron oxidation rate of guanine in DNA can be regulated by introducing a substituent on base pairing cytosine.     

Photocatalytic oxidation of n-octanol in aerated supercritical CO2 on hydrophobic TiO2

Supercritical CO₂ (scCO₂) has been used as a reaction medium for the photocatalytic oxidative degradation of n-octanol on a partially desilanized hydrophobic suspension of TiO₂ as photocatalyst. Hydrophobic sites on the catalyst surface are necessary to maintain a sustained suspension, and hence surface-mediated interfacial electron exchange, in this non-polar medium. The reaction rates for photooxidative degradation, ultimately to complete mineralization, depend only weakly on temperature and pressure of the supercritical fluid near the critical point. Product distributions were monitored in situ by on-line gas chromatographic analysis, which provides a convenient and rapid method for comparisons and optimization of the reaction conditions.     

A theoretical investigation on photocatalytic oxidation on the TiO2 surface

The TiO(2) photocatalytic oxidation mechanism was theoretically investigated by using long-range corrected time-dependent density functional theory (LC-TDDFT) with a cluster model of the anatase TiO(2)(001) surface. We found that LC-TDDFT with the cluster model quantitatively reproduces the photoexcitations of the TiO(2) surface by calculating the electronic spectra of a clean TiO(2) surface and one with oxygen defects. We calculated the electronic spectra of a molecularly adsorbed TiO(2) surface for the adsorptions of phenol, methanol, and methane molecules as typical organic molecules. We obtained the surprising result that the main peak of the phenol-adsorbed TiO(2) surface, which overlaps with the main peak of the clean TiO(2) surface, corresponds to charge transfers from the phenol molecule to the TiO(2) surface. This indicates that the TiO(2) photocatalytic oxidation proceeds through direct charge transfer excitation from the substrate molecules to the TiO(2) surface. In contrast, we found slight and no charge transfer for methanol and methane adsorption, respectively, in agreement with the experimental findings for their reactivities. In light of these results, we propose a new mechanism for heterogeneous TiO(2) photocatalytic oxidations.     

Photocatalytic activities of TiO2 loaded with NiO

A pretreated NiO-TiO₂ powder system is an active catalyst for photocatalytic decomposition of H₂O into H₂ and O₂ in aqueous alkaline solution (3 N NaOH) as well as under NaOH coating conditions.     

Photocatalytic oxidation surfaces on anatase TiO2 crystals revealed by single-particle chemiluminescence imaging

We report a single-particle approach for studying the photocatalytic oxidation reaction on individual anatase TiO(2) particles. Single-particle kinetic and imaging analyses of the chemiluminescence (CL) emitted from the probe dye molecules revealed that the oxidation reactivity is not significantly dependent on the crystal faces.     

纳米TiO2光催化降解聚乙二醇反应

纳米TiO2光催化降解聚乙二醇反应;纳米二氧化钛; 光催化; 聚乙二醇     

Photocatalytic reduction of nitrate over TiO2 and Ag-modified TiO2

This research work presents the efficiency of the TiO₂ and Ag–TiO₂ thin films prepared by the sol–gel method and coated onto the surface of 304 stainless steel sheets used in the photocatalytic nitrate reduction processes. The Ag–TiO₂ thin films had the weight by weight (w/w) ratio of Ag⁺/TiO₂ of 0.1% as Ag atom. The XRD results showed that the crystalline phase structure of TiO₂ on the Ag–TiO₂ thin films was anatase. The optical band gaps of the TiO₂ and 0.1% Ag–TiO₂ thin films were respectively 3.27 and 2.70 eV, while the surface of the prepared catalysts was hydrophobic with the respective average water contact angles of 94.8° and 118.5° for the TiO₂ and 0.1% Ag–TiO₂ thin films. The net efficiencies of photocatalytic nitrate reduction of TiO₂ and 0.1% Ag–TiO₂ were 41.4% and 70.0%, respectively. The loading of Ag only influenced the nitrate removal efficiency without affecting the stoichiometric ratio of formate to nitrate. The net stoichiometric ratio of formate to nitrate of all experiments was 2.8:1.0, which is close to the stoichiometric ratio of 2.5:1.0 of the nitrate reduction to nitrite and then to nitrogen gas.     

镧掺杂二氧化钛光催化性能的研究

通过在TiO2晶格中掺杂La2O3,考察不同的La2O3掺杂量对光催化活性的影响,找出了La2O3的最佳掺杂量.实验结果表明由于进入TiO2晶格中的La2O3使TiO2晶格发生畸变,减少了光生电子-空穴对的复合率,提高了催化剂的光催化效率.     

Photocatalytic TiO2 coatings on limestone

The application of photocatalytic coatings on stone has been investigated for providing surface protection and self-cleaning properties. Sol–Gel and hydrothermal processes were used to synthesise TiO₂ colloidal suspensions and coatings with enhanced photocatalytic activity without any thermal curing of the coated stone. The stone was a porous limestone (apulian sedimentary carbonatic, calcite stone). Films and powders prepared from TiO₂ sols were studied using X-ray diffraction to evaluate the microstructure and identify rutile and anatase phases. A morphological and physical characterisation was carried out on coated and uncoated stone to establish the changes of appearance, colour, water absorption by capillarity and water vapour permeability. The photocatalytic activity of the coated surface was evaluated under UV irradiation through NO _x and organics degradation tests. The performances of the synthesised TiO₂ sols were compared with commercial TiO₂ suspension. Since the coating doesn’t need temperature treatments for activating the photocatalytic properties, the nano-crystalline hydrothermal TiO₂ sols seem good candidate for coating applications on stone that cannot be annealed after the coating application.     

TiO2担载在新型载体上的光催化电极

介绍了一种将TiO₂光催化剂担载在新型载体上的光催化电极.新型载体是用炭质材料和聚四氟乙烯制备的,它具有电合成H₂O₂的功能.在电流密度为15mA/cm²时,光催化电极不仅具有高达80%的电合成H₂O₂的电流效率,而且该电极的电位处在0.02V(SCE)左右,使载体表面的TiO₂光催化剂获得了约+0.47V的阳极偏压(相对平带电势),对增加TiO₂光催化反应效率十分有益.在具有新型光催化电极的光反应体系中,水中的有机分子受到来自溶液中大量·OH自由基(H₂O₂被紫外光分解的中间产物)的均相氧化(即光化学氧化),以及来自TiO₂表面光生空穴的复相氧化(即光催化氧化).在光化学氧化和光催化氧化的联合作用下,有机分子的矿物化反应速度显著提高.     

PdO/TiO2 and Pd/TiO2 Heterostructured Nanobelts with Enhanced Photocatalytic Activity

Heterostructures play an important role not only in the manufacture of semiconductor devices, but also in the field of catalysis. Herein, we report the synthesis of PdO/TiO₂ and Pd/TiO₂ heterostructured nanobelts by means of a simple co‐precipitation method, followed by a reduction process using surface‐modified TiO₂ nanobelts as templates. The as‐obtained heterostructures were characterized by transmission electron microscopy, X‐ray photoelectron spectroscopy, and UV/Vis diffuse reflectance spectroscopy. PdO and Pd nanoparticles with a size of about 1.3 and 1.6 nm were assembled uniformly on the surface of TiO₂ nanobelts, respectively. Compared with TiO₂ nanobelts, PdO/TiO₂ and Pd/TiO₂ hybrid nanobelts exhibit enhanced photocatalytic activity upon UV and visible‐light irradiation. Photoelectrochemical technology was used to study the heterostructure effect on enhanced photocatalytic activity. Our mechanistic investigation revealed that energy‐band matching is the major factor in the observed enhancement of photocatalytic activity.     

超声协同TiO_2光催化脱除燃料油中有机硫的研究

采用溶胶凝胶法制备纳米TiO2光催化剂,引入超声作用,以空气中的氧气为氧化剂,正辛烷为模拟油品对燃料油中硫化物的脱除进行了研究。考察了光照强度、催化剂用量、反应时间、二苯并噻吩(DBT)初始浓度、超声功率等因素对TiO2光催化二苯并噻吩溶液降解效率的影响。结果表明,引入超声后DBT的降解率提高了10%左右,并在TiO2用量为2 g/L,通气量为800 mL/min,光照距离20 cm,DBT初始浓度为600 mg/L,反应时间为150 min,超声功率为500 W的条件下,DBT降解率达到了72.6%。     

Photocatalytic reduction of CO2 with H2O on TiO2 and Cu/TiO2 catalysts

Photoinduced reduction of CO₂ by H₂O to produce CH₄ and CH₃OH has been investigated on wellcharacterized standard TiO₂ catalysts and on a Cu²⁺ loaded TiO₂ catalyst. The efficiency of this photoreaction depends strongly on the kind of catalyst and the ratio of H₂O to CO₂. Anatase TiO₂, which has a large band gap and numerous surface OH groups, shows high efficiency for photocatalytic CH₄ formation. Photogenerated Ti³⁺ ions, H and CH₃ radicals are observed as reactive intermediates, by ESR at 77 K. Cu-loading of the small, powdered TiO₂ catalyst (Cu/TiO₂) brings about additional formation of CH₃OH. XPS studies suggest that Cu⁺ plays a significant role in CH₃OH formation.     

Photocatalytic degradation and adsorption of 2-naphthol on suspended TiO2 surface in a dynamic reactor

The photocatalytic oxidation of 2-naphthol has been investigated at room temperature in a dynamic photoreactor with system UV/O2 (air) and aqueous suspension of titanium dioxide TiO2 irradiated under a variety of conditions. The kinetics of disappearance of pollutant were affected by several operating parameters such as TiO2 mass, concentration of the substrate and reaction pH. The experiments were measured by high performance liquid chromatography. A Langmuir-Hinshelwood model was found to be accurate for photocatalytic degradation and indicates that adsorption of the solute on the surface of semiconductor particles plays a role in photocatalytic reaction.