Mechanistic studies of the photocatalytic oxidation of trichloroethylene with visible-light-driven N-doped TiO2 photocatalysts

Visible-light-driven TiO2 photocatalysts doped with nitrogen have been prepared as powders and thin films in a cylindrical tubular furnace under a stream of ammonia gas. The photocatalysts thus obtained were found to have a band-gap energy of 2.95 eV. Electron spin resonance (ESR) under irradiation with visible light (lambda > or = 430 nm) afforded the increase in intensity in the visible-light region. The concentration of trapped holes was about fourfold higher than that of trapped electrons. Nitrogen-doped TiO2 has been used to investigate mechanistically the photocatalytic oxidation of trichloroethylene (TCE) under irradiation with visible light (lambda > or = 420 nm). Cl and O radicals, which contribute significantly to the generation of dichloroacetyl chloride (DCAC) in the photocatalytic oxidation of TCE under UV irradiation, were found to be deactivated under irradiation with visible light. As the main by-product, only phosgene was detected in the photocatalytic oxidation of TCE under irradiation with visible light. Thus, the reaction mechanism of TCE photooxidation under irradiation with visible light clearly differs markedly from that under UV irradiation. Based on the results of the present study, we propose a new reaction mechanism and adsorbed species for the photocatalytic oxidation of TCE under irradiation with visible light. The energy band for TiO2 by doping with nitrogen may involve an isolated band above the valence band.     

Pathway of oxygen incorporation from O2 in TiO2 photocatalytic hydroxylation of aromatics: oxygen isotope labeling studies

The hydroxylation process is the primary, and even the rate-determining step of the photocatalytic degradation of aromatic compounds. To make clear the hydroxylation pathway of aromatics, the TiO(2) photocatalytic hydroxylation of several model substrates, such as benzoic acid, benzene, nitrobenzene, and benzonitrile, has been studied by an oxygen-isotope-labeling method, which can definitively assign the origin of the O atoms (from oxidant O(2) or solvent H(2)O) in the hydroxyl groups of the hydroxylated products. It is found that the oxygen source of the hydroxylated products depends markedly on the reaction conditions. The percentage of the products with O(2)-derived hydroxyl O atoms increases with the irradiation time, while it decreases with the increase of substrate concentration. More intriguingly, when photogenerated valence-band holes (h(vb)(+)) are removed, nearly all the O atoms (>97?%) in the hydroxyl groups of the hydroxylated products of benzoic acid come from O(2), whereas the scavenging of conduction-band electrons (e(cb)(-)) makes almost all the hydroxyl O atoms (>95?%) originate from solvent H(2)O. In the photocatalytic oxidation system with benzoic acid and benzene coexisting in the same dispersion, the percentage of O(2)-derived hydroxyl O atoms in the hydroxylated products of strongly adsorbed benzoic acid (ca. 30?%) is much less than in that of weakly adsorbed benzene (phenol) (>60?%). Such dependences provide unique clues to uncover the photocatalytic hydroxylation pathway. Our experiments show that the main O(2)-incorporation pathway involves the reduction of O(2) by e(cb)(-) and the subsequent formation of free (?)OH via H(2)O(2), which was usually overlooked in the past photocatalytic studies. Moreover, in the hydroxylation initiated by h(vb)(+), unlike the conventional mechanism, the O atom in O(2) cannot incorporate into the product through the direct coupling between molecular O(2) and the substrate-based radicals.     

Mechanistic studies on the formation of glycosidase-substrate and glycosidase-inhibitor covalent intermediates

Glycoside hydrolases catalyze the breaking of the glycosidic bond. This type of bond fashioned between two monosaccharides is very stable, and the polymers created are involved in multiple cellular processes, being crucial to life. In this article, computational methods were used to study the first step of the mechanism of reaction of retaining glycoside hydrolases in atomic detail. The systems modeled included a simplified reaction center and a small substrate/inhibitor. Using DFT calculations we were able to corroborate and provide molecular-level detail to the dissociative mechanism proposed in the literature. The role of the hydrogen bridge between the nucleophile and the C(2)--OH group of the ring was also investigated. Therefore, we concluded that this bridge is responsible for lowering the activation barrier by 5.1 kcal mol(-1) with functional BB1K/6-311+G(2d,2p), and the absence of the bridge explains, at least in part, the inhibitory effect of fluoro-substituted glycosides in the -2 position. The hydrogen bridge could also be involved in favoring the ring distortion verified in the transition state, and the dissociative character of the reaction mechanism. Using the NBO method, point atomic charges were calculated. In the transition state, the positive charge generated in the sugar ring is distributed nearly equally between the anomeric carbon and the ring oxygen, through a partial double bond involving the two atoms.     

Development of bacteriorhodopsin analogues and studies of charge separated excited states in the photoprocesses of linear polyenes

Development of bacteriorhodopsin (bR) analogues employing chromophore substitution technique for the purpose of characterizing the binding site of bR and generating bR analogues with novel opto-electronic properties for applications as photoactive element in nanotechnical devices are described. Additionally, the photophysical and photochemical properties of variously substituted diarylpolyenes as models of photobiologically relevant linear polyenes are discussed. The role of charge separated dipolar excited states in the photoprocesses of linear polyenes is highlighted.     

FT-IR study of the nature and stability of NOx surface species on ZrO2, VOx/ZrO2 and MoOx/ZrO2 catalysts

The adsorption of NO, NO/O₂ mixtures and NO₂ on pure ZrO₂ and on two series of catalysts supported on ZrO₂, one containing vanadia and the other molybdena (ZV and ZMo, respectively), has been investigated. The V and Mo surface contents of the latter were ≤3 atoms nm⁻² and ≤5 atoms nm⁻², respectively. All samples had been previously submitted to a standard oxidation treatment. On all samples, only extremely minor amounts of NO_x surface species are formed by NO interaction at room temperature (RT). NO_x surface species are formed in greater amounts on pure ZrO₂ when NO and O₂ are coadsorbed at RT; they are mainly nitrites, small amounts of nitrates, and small amounts of (O₂NO−H)^δ− species; when ZrO₂ is warmed to 623 K in the NO/O₂ mixture, nitrites decrease, nitrates and (O₂NO−H)^δ− species increase. The same NO_x species as on the ZrO₂ surface free from V (or Mo) are formed on ZV (or ZMo) samples with surface V (or Mo) density <1.5 atoms nm⁻²; however, they occur in decreased amount with increasing V (or Mo) coverage. On ZV samples with a surface V density of 1.5–3 atoms nm⁻² (or ZMo samples with a surface Mo density of 1.5–5 atoms nm⁻²) when NO and O₂ are coadsorbed at RT, there is formation of small amounts of nitrites, nitrates (both on ZrO₂ surface free from V (or Mo) and at the edges of V- or Mo-polyoxoanions) and NO₂ ^δ+ species, associated with V⁵⁺ (or Mo⁶⁺) of very strong Lewis acidity; when samples are warmed up 623 K in the NO/O₂ mixture, nitrites disappear, nitrates increase, NO₂ ^δ+ species remain constant or slightly decrease. When NO₂ is allowed into contact at RT with oxidized samples, surface situations almost identical to those obtained for each sample warmed to 623 K in NO/O₂ mixture is reached. The NO_x surface species stable at 623 K, the temperature at which catalysts show the best performance in the selective catalytic reduction (SCR) of NO by NH₃, are nitrates, both on ZrO₂ and on polyvanadates or polymolybdates at high nuclearity. On the contrary, nitrites and NO₂ ^δ+ species are unstable at 623 K.     

Mechanistic studies on the stereoselective formation of beta-mannosides from mannosyl iodides using alpha-deuterium kinetic isotope effects

Stereoselective synthesis of beta-mannosides is one of the most challenging linkages to achieve in carbohydrate chemistry. Both the anomeric effect and the C2 axial substituent favor the formation of the axial glycoside (alpha-product). Herein, we describe mechanistic studies on the beta-selective glycosidation of trimethylene oxide (TMO) using mannosyl iodides. Density functional calculations (at the B3LYP/6-31+G(d,p):LANL2DZ level) suggest that formation of both alpha- and beta-mannosides involve loose S(N)2-like transition-state structures with significant oxacarbenium character, although the transition structure for formation of the alpha-mannoside is significantly looser. alpha-Deuterium kinetic isotope effects (alpha-DKIEs) based upon these computed transition state geometries match reasonably well with the experimentally measured values: 1.16 +/- 0.02 for the beta-linkage (computed to be 1.15) and 1.19 +/- 0.05, see table 2 for the alpha-analogue (computed to be 1.26). Since it was unclear if beta-selectivity resulted from a conformational constraint induced by the anomeric iodide, a 4,6-O-benzylidine acetal was used to lock the iodide into a chairlike conformation. Both experiments and calculations on this analogue suggest that it does not mirror the behavior of mannosyl iodides lacking bridging acetal protecting groups.     

Mechanistic studies on the formation of silver nanowires by a hydrothermal method

Silver (Ag) nanowires were fabricated from silver chloride (AgCl) by the hydrothermal method. The successful formation of Ag nanowires relied on the low solubility of AgCl as a precursor and the structural change of glucose to polymer on the Ag nanowire (protective layer). The Ag(+) ion concentration in the reaction solution containing AgCl was initially low, but after a reaction time of over 12 h, Ag(+) gradually reduced to Ag metal. Transmission electron microscope, Raman spectrometery, and X-ray photoelectron spectroscopy revealed that the surface of the obtained Ag nanowires possessed a carbon-rich layer with a carboxyl group, and the Ag(+) ion coordinated with the carboxyl group of this layer. The difference in the surface-free energy of Ag crystals changed the crystal growth rate that impelled the anisotropic growth of the Ag particles. By examining various reaction conditions, it was determined that the ratio of Cl(-) to Ag(+), reaction temperature, and reaction time are important factors for successful preparation of Ag nanowires. Under the reaction condition that the molar ratio of Cl(-) to Ag(+) at 160 °C for 24 h is above equimolar concentration, uniform Ag nanowires were successfully prepared.     

Quantum theory of chemical bond formation processes in condensed systems: studies towards exploration of electrochemical dark and photoprocesses on semiconductors

Quantum theoretical models of chemical bond formation and partial charge transfer processes in condensed systems, and their extension to processes in electronic non-equilibrium are investigated in this paper with a view to further exploration of electrochemical and photoelectrochemical kinetics on semiconductors. Electrochemical dark and photocurrents on n-III-V-semiconductors are correlated with calculated transition probabilities for atom-group transfer over larger distances (80 to 240 pm), leading to a first estimate of potential surface shapes compatible with experiment. Some specific problems connected with transition probability calculations for heavy-particle transfer in strong anharmonic potentials are considered in detail, including approximation of Franck-Condon transitions in arbitrary potentials by their classical limit.     

Preparation of high surface area ZrO2 and ZrO2-Y2O3 from the alkoxide

In order to obtain a catalyst support with a high surface area, ZrO₂ and ZrO₂-Y₂O₃ were prepared by the hydrolytic decomposition of the corresponding isopropoxide dissolved in benzene. The hydrolysis was carried out at 80°C using an excess amount of distilled water in flowing dry nitrogen. The precipitates thus obtained were dried at 100°C followed by calcination at 500°C in air or nitrogen for 1 h. The specific surface areas for both of the ZrO₂ and ZrO₂-Y₂O₃ increased with increasing amount of water added for hydrolysis, and the surface areas for ZrO₂-Y₂O₃ increased with increasing yttrium content. A ZrO₂ having a surface area of 130 m²/g was produced, and a stabilized tetragonal ZrO₂ with 15 mol% Y³⁺ having a surface area of 200 m²/g was produced. Furthermore, despite the difference in the ZrO₂ and ZrO₂-Y₂O₃ crystal structures, the lattice-strain of ZrO₂ has been unequivocally related to the surface area.     

Investigations on the determination of NO with galvanic ZrO2 solid electrolyte cells

In order to demonstrate the usefulness of gassymmetrical galvanic ZrO₂ solid electrolyte cells for the determination of NO, cell tensions caused by catalytically different active electrodes and thereby different oxygen partial pressures in N₂/NO mixtures with a low oxygen concentration (<50 vol.-ppm) were measured in the temperature range of 500 to 800°C. Separate investigations demonstrated that the catalytic activity of Pt decreases with increasing sinter temperature (1000 to 1300°C), but is higher than the activity of perovskite type oxides La_1?xSr_xMeO₃ (x=0.1.... 0.5; Me=Co, Mn, Fe, Cr). The activity of the perovskites decreases in the order Co>Mn ≈ Fe>Cr. Between different Pt electrodes changes of cell tension were measured when alternating between pure and NO containing nitrogen (e.g. 19.7 mV with 0.4 vol.-% NO). Cells with a Pt electrode and a perovskite electrode produce higher changes of the cell tension, but without going to zero in pure nitrogen due to own oxygen potentials of the mixed oxides.     

Mechanistic studies of the biosynthesis of 2-thiosugar: evidence for the formation of an enzyme-bound 2-ketohexose intermediate in BexX-catalyzed reaction

The first mechanistic insight into 2-thiosugar production in an angucycline-type antibiotic, BE-7585A, is reported. d-Glucose 6-phosphate was identified as the substrate for the putative thiosugar biosynthetic protein, BexX, by trapping the covalently bonded enzyme-substrate intermediate. The site-specific modification at K110 residue was determined by mutagenesis studies and LC-MS/MS analysis. A key intermediate carrying a keto functionality was confirmed to exist in the enzyme-substrate complex. These results suggest that the sulfur insertion mechanism in 2-thiosugar biosynthesis shares similarities with that for thiamin biosynthesis.     

Mechanistic studies on the formation of Pt(II) hydroformylation catalysts in imidazolium-based ionic liquids

The kinetics of the formation of the active species cis-[Pt^II(PPh₃)₂Cl(SnCl₃)] and cis-[Pt^II(PPh₃)₂(SnCl₃)₂] from the hydroformylation catalyst precursor cis-[Pt^II(PPh₃)₂Cl₂] in the presence of SnCl₂, was studied in two different imidazolium-based ionic liquids. A large range of different chlorostannate melts consisting of 1-butyl-3-methyl-imidazolium cations and [Sn_xCl_y]^{(−y + 2x)} anions with varying molar fraction of SnCl₂, were prepared and characterized by ¹H and ¹¹⁹Sn NMR. The observed chemical shifts point to major changes in the composition of the anionic species within the melt. The second ionic liquid employed, viz., 1-butyl-3-methyl-imidazolium-bis(trifluormethylsulfonyl)amide was prepared in a colorless quality that enabled its application in kinetic studies. The concentration and temperature dependence of the substitution of Cl⁻ by [SnCl₃]⁻ to yield cis-[Pt^II(PPh₃)₂Cl(SnCl₃)], could be studied in detail. Theoretical (DFT) calculations were employed to model the reaction progress and to resolve the role of the ionic liquid in the activation of the catalyst. The available results are presented and a plausible mechanism for the formation of the catalytically active species is suggested.     

Study on photocatalytic oxidation for determination of the low chemical oxygen demand using a nano-TiO2-Ce(SO4)2 coexisted system

A new photocatalytic system, nano-TiO₂-Ce(SO₄)₂ coexisted system, which can be used to determine the low chemical oxygen demand (COD) is described. Nano-TiO₂ powders is used as photocatalyst in this system. The measuring method is based on direct determination of the concentration change of Ce(IV) resulting from photocatalytic oxidation of organic compounds. The mechanism of the photocatalytic oxidation for COD determination was discussed and the optimum experimental conditions were investigated. Under the optimum conditions, a good calibration graph for COD values between 1.0 and 12 mg l⁻¹ was obtained and the LOD value was achieved as low as 0.4 mg l⁻¹. When determining the real samples, the results were in good agreement with those from the conventional methods.     

Preparation of photocatalytic nano-ZnO/TiO2 film and application for determination of chemical oxygen demand

A composite nano-ZnO/TiO₂ film as photocatalyst was fabricated with vacuum vaporized and sol-gel methods. The nano-ZnO/TiO₂ film improved the separate efficiency of the charge and extended the range of spectrum, which showed a higher efficiency of photocatalytic than the pure nano-TiO₂ and nano-ZnO film. The photocatalytic mechanism of nano-ZnO/TiO₂ film was discussed, too. A new method for determination of low chemical oxidation demand (COD) value in ground water based on nano-ZnO/TiO₂ film using the photocatalytic oxidation technology was founded. This method was originated from the direct determination of the Mn(VII) concentration change resulting from photocatalytic oxidation of organic compounds on the nano-ZnO/TiO₂ film, and the COD values were calculated from the absorbance of Mn(VII). Under the optimal operation conditions, the detection limit of 0.1 mg l⁻¹, COD values with the linear range of 0.3-10.0 mg l⁻¹ were achieved. The results were in good agreement with those from the conventional COD methods.     

Mechanistic studies on the formation and reactivity of dioxygen adducts of diiron complexes supported by sterically hindered carboxylates

Dioxygen activation by enzymes such as methane monooxygenase, ribonucleotide reductase, and fatty acid desaturases occurs at a nonheme diiron active site supported by two histidines and four carboxylates, typically involving a (peroxo)diiron(III,III) intermediate in an early step of the catalytic cycle. Biomimetic tetracarboxylatodiiron(II,II) complexes with the familiar "paddlewheel" topology comprising sterically bulky o-dixylylbenzoate ligands with pyridine, 1-methylimidazole, or THF at apical sites readily react with O(2) to afford thermally labile peroxo intermediates that can be trapped and characterized spectroscopically at low temperatures (193 K). Cryogenic stopped-flow kinetic analysis of O(2) adduct formation carried out for the three complexes reveals that dioxygen binds to the diiron(II,II) center with concentration dependences and activation parameters indicative of a direct associative pathway. The pyridine and 1-methylimidazole intermediates decay by self-decomposition. However, the THF intermediate decays much faster by oxygen transfer to added PPh(3), the kinetics of which has been studied with double mixing experiments in a cryogenic stopped-flow apparatus. The results show that the decay of the THF intermediate is kinetically controlled by the dissociation of a THF ligand, a conclusion supported by the observation of saturation kinetic behavior with respect to PPh(3), inhibition by added THF, and invariant saturation rate constants for the oxidation of various phosphines. It is proposed that the proximity of the reducing substrate to the peroxide ligand on the diiron coordination sphere facilitates the oxygen-atom transfer. This unique investigation of the reaction of an O(2) adduct of a biomimetic tetracarboxylatodiiron(II,II) complex provides a synthetic precedent for understanding the electrophilic reactivity of like adducts in the active sties of nonheme diiron enzymes.     

Influence of Zn(II) adsorption on the photocatalytic activity and the production of H2O2 over irradiated TiO2

The photocatalytic activity of semiconductor oxides, in particular TiO₂ powders or colloids, is a complex function of bulk (light absorption and scattering, charge carrier mobility and recombination rate) and surface (structure, defects and reconstruction, charge, presence of adsorbate, surface recombination centers) properties. Among surface modifications, the inner sphere surface complexation of metal cations can change the surface charge of the metal oxide, thus changing the surface activity coefficient of ionic substrates, the band edge positions, as well as the mechanism and kinetic of interfacial electron transfer by blocking surface trapping sites for photogenerated carriers (≡Ti?OH). In this work we show that in anatase/water systems under band-gap irradiation, both the organic substrate (formate) oxidation initiated by photogenerated valence band holes and the formation of hydrogen peroxide from O₂ reduction (by conduction band electrons) is strongly influenced by the presence of Zn²⁺ cations. Depending on the pH, the formate oxidation rate can be enhanced or nearly completely inhibited. The observed result can be rationalized by considering the fraction of ≡Ti?OH surface sites blocked by inner sphere complexation of Zn²⁺ as a function of pH. When this fraction is low, the more positive surface charge favors formate oxidation, whereas when the fraction is high the almost complete blockage of ≡Ti?OH surface sites by Zn²⁺ stops almost entirely formate oxidation. Interestingly, the surface complexation of Zn²⁺ is accompanied by an increasing production of H₂O₂ during formate degradation in the presence of O₂. Zn(II) cations are not complexed by peroxide/superoxide species derived from O₂ reduction. When ≡Ti?OH sites are blocked by Zn²⁺, the complexation on the TiO₂ surface of peroxide/superoxide species is inhibited, hindering their further transformation. The results presented demonstrate that the combined effect of pH and surface complexation of redox inert cations greatly influences both the oxidative and reductive processes during the photocatalytic process over TiO₂.     

Mechanistic studies on fast ligand substitution reactions of Pt(II) in different ionic liquids: role of solvent polarity and ion-pair formation

The effect of several imidazolium-based ionic liquids on the mechanism of a classical ligand substitution reaction of [Pt(terpyridine)Cl] (+) with thiourea was investigated. A detailed kinetic study as a function of the nucleophile concentration and temperature was undertaken under pseudo-first-order conditions using stopped-flow techniques. Polarity measurements were performed for the employed ionic liquids on the basis of solvatochromic effects, and they show similarities with conventional polar solvents. Density-functional theory calculations (RB3LYP/LANL2DZp) were employed to predict the ion-pair stabilization energy between the ionic components of the ionic liquids and/or between the anions of the ionic liquids and the cationic Pt (II) complex. These data illustrate how the anions of the ionic liquids can affect the investigated substitution reaction. In general, the substitution mechanism in ionic liquids was found to have an associative character similar to that in conventional solvents. The observed deviations reflect the influence of the ionic liquid on the interaction between the anionic component of the liquid and the positively charged complex.     

Effects of simultaneously doped and deposited Ag on the photocatalytic activity and surface states of TiO2

Ag-TiO2 catalysts with different Ag contents were prepared via a sol-gel method in the absence of light. Based on the characterizations of XRD, photoluminescence (PL), surface photovoltage spectroscopy (SPS), field-induced surface photovoltage spectroscopy (FISPS), and XPS as well as the evaluation of the photocatalytic activity for degrading rhodamine B(RhB) solutions, it was found that the Ag dopant promoted the phase transformation as well as had an inhibition effect on the growth of anatase crystallite. The PL and SPS intensities were decreased with increasing Ag content, indicating that the Ag dopant could effectively inhibit the recombination of the photoinduced electrons and holes. However, the active sites capturing the photoinduced electrons reduced, while the Ag content exceeded 5 mol %. At rather low Ag dopant concentrations, the migration and diffusion of Ag+ ions were predominant, while at rather high Ag dopant concentrations, the migration, diffusion, and reduction of Ag ions simultaneously occurred. The Ag-TiO2 photocatalysts with appropriate content of Ag (Ag species concentration is from about 3 to 5 mol %) possessed abundant electron traps so as to be favorable for the separation of the photoinduced electron-hole pairs, which could greatly enhance the activity of the photocatalysts. From the results of FISPS measurements, it could be found that the impurity bands and abundant surface states were introduced into the interfacial layer of TiO2 because of Ag simultaneously doping and depositing, which could improve the absorption capability for visible light of the photocatalysts.     

Mechanistic studies on the formation of trifluoromethyl sulfur pentafluoride, SF5CF3--a greenhouse gas

The formation of SF5CF3(X1A'), through the radical-radical recombination of SF5(X2A1) and CF3(X2A1), was observed for the first time in low-temperature sulfur hexafluoride-carbon tetrafluoride matrices at 12 K via infrared spectroscopy upon irradiation of the ices with energetic electrons. The nu1 fundamentals of the SF5(X2A1) and CF3(X2A1) radicals were monitored at 857 and 1110 cm-1, respectively; the newly formed trifluoromethyl sulfur pentafluoride molecule, SF5CF3(X1A'), was detected via its absorptions at 846 and 1160 cm-1. This formation mechanism suggests that a source for this potentially dangerous greenhouse gas might be the recombination of SF5(X2A1) and CF3(X2A1) radicals on aerosol particles in the terrestrial atmosphere.     

Mechanistic and kinetic studies on the photoinitiated polymerization of tetrahydrofuran

Polymerization of tetrahydrofuran (THF) (bulk) promoted by a free radical source, 2,2-dimethoxy-2-phenyl acetophenone (DMPA), in the presence of 4,4′-di-(methylphenyl)iodonium hexafluorophosphate was studied in detail. Rate dependence of the polymerizing system on both DMPA and the iodonium salt was studied dilatometrically. Rate saturation for the iodonium salt was attributed to the formation of iodobenzene, which may react with propagating cations to form an iodonium salt. DMPA was shown to be an efficient transfer agent in the cationic polymerization of THF. A transfer constant for DMPA was estimated to be 2.8 and, whilst the precise details of the overall mechanism cannot be deduced from the present results, it may result in functionalized polytetrahydrofuran chains having photochemically active end groups.