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1.
The influence of the modification of the TiO2 surface with F? ions on the physicochemical properties of the catalysts and efficiency in the photooxidation of gaseous molecules under atmospheric conditions and under UV irradiation was studied by IR spectroscopy. The fluorine-containing samples adsorb more water molecules than unmodified TiO2. The amount of adsorbed water increases with increasing content of surface fluoride ions. The fluorination of the TiO2 surface first leads to the substitution of the terminal OH groups by F? ions and to an increase in acidity of the bridging acidic OH groups remained on the surface. The modification also results in the structural rearrangement of the surface involving defective and surface Ti4+ sites. Fluorine modification increases the activity of TiO2 in the photocatalytic oxidation of ethanol, acetaldehyde, acetic acid, and acetone. At the same time, benzene and H2S are oxidized more rapidly on unmodified TiO2. The presence of fluorine on the TiO2 surface exerts almost no effect on the oxidation rate of chlorine-containing substrates C3H7Cl and C2H4Cl2. 相似文献
2.
The formation of a series of oxygen-centred radicals on different TiO 2 samples (P25 and two different rutile materials) under various conditions was investigated using X-band c.w. Electron Paramagnetic Resonance (EPR) spectroscopy. The radicals were formed either on thermally-reduced TiO 2, or by UV irradiation of the oxide under an oxygen atmosphere. The nature and stability of the radicals was also explored as a function of surface hydration. On thermally reduced TiO 2, containing surface and bulk Ti 3+ centres, oxygen adsorption at 300 K results in the preferential formation and stabilisation of O 2
- anions on the P25 surface, but O - and O 3
- anions are generated on the rutile surfaces. Superoxide anions (O -) and trapped holes (O 2
-) were also identified after photo-irradiation of the thoroughly dehydrated TiO 2 samples under oxygen. The O - anions were only visible at low temperatures under continuous irradiation, while the O 2
- anions were stable for days at 300 K. By comparison, on fully hydrated surfaces, no stable oxygen centred radicals could be detected on P25, while O 2
- anions were easily observed on the rutile surfaces. On partially hydrated P25, the O -, O 2
- and HO 2 anions were detected after UV irradiation at 77 K; all radicals decayed upon warming to 298 K. On partially hydrated rutile, the O - and O 2
- anions were detected and, unlike the case for P25, were found to be stable for days under the same conditions. The results illustrate the varied formation and stability of the oxygen centred radicals on TiO 2 surfaces depending on the pretreatment conditions. 相似文献
3.
Based on first-principles electronic structure calculations we find that the bridging oxygen vacancies on the (1 1 0) surface is more favorable and may be responsible for the unexpected ferromagnetism in undoped rutile TiO 2. Our results show that the ferromagnetism largely originates from the d orbitals of low-charge-state Ti ions converted from Ti 4+ ions induced by the surface oxygen vacancies. The second-nearest neighbors of these ions (fivefold coordinated Ti) also contribute to the total magnetic moments. The spins induced by the local oxygen vacancies form a ferromagnetic arrangement. 相似文献
4.
Several possible adsorption sites and adsorption geometries of CO 2 on small rutile fragments were studied by Extended Hückel Molecular Orbital (EHMO) calculations. The parameters for the rutile part were optimised to reproduce the experimental rutile bulk structure and were tested in several small clusters up to [(TiO 2) 31(OH) 32] 32??6H 2O, a 175 atoms cluster. It was found that the average experimental bond legth can be reproduced with good accuracy. However the slight distortion of the TiO 6 octahedra is calculated with the wrong sign (four long and two short Ti?O bonds). The agreement for the angle α O-Ti-O is less satisfactory. The study shows that CO 2 can adsorb on fivefold coordinated surface titanium sites as well as surface oxygen sites. This means that CO 2 can act as either Lewis base or acid. In the case of binding as a Lewis base, CO 2 can adsorb linearly forming a single Ti?OCO bond, or interact with two neighboring Ti 4+ sites. A chelating structure forming two Ti?O bonds was found to be weakly stable at the most. When CO 2 behaves as a Lewis acid, a carbonate-like structure is formed by interaction with either terminal oxygen ions or bridging oxygen centers. 相似文献
5.
A series of tungsten‐doped Titania photocatalysts were synthesized using a low‐temperature method. The effects of dopant concentration and annealing temperature on the phase transitions, crystallinity, electronic, optical, and photocatalytic properties of the resulting material were studied. The X‐ray patterns revealed that the doping delays the transition of anatase to rutile to a high temperature. A new phase W yTi 1‐yO 2 appeared for 5.00 wt% W‐TiO 2 annealed at 900 °C. Raman and diffuse reflectance UV–Vis spectroscopy showed that band gap values decreased slightly up to 700 °C. X‐ray photoelectron spectroscopy showed that surface species viz. Ti 3+, Ti 4+, O 2?, oxygen‐vacancies, and adsorbed OH groups vary depending on the preparation conditions. The photocatalytic activity was evaluated via the degradation of methylene blue using LED white light. The degradation rate was affected by the percentage of dopants. The best photocatalytic activity was achieved with the sample labeled 5.00 wt% W‐TiO 2 annealed at 700 °C. 相似文献
7.
We have investigated the photoinduced decomposition of formaldehyde (CH 2O) on a rutile TiO 2(100)-(1×1) surface at 355 nm using temperature-programmed desorption. Products, formate (HCOO -), methyl radical (CH 3·), ethylene (C 2H 4), and methanol (CH 3OH) have been detected. The initial step in the decomposition of CH 2O on the rutile TiO 2(100)-(1×1) surface is the formation of a dioxymethylene intermediate in which the carbonyl O atom of CH 2O is bound to a Ti atom at the five-fold-coordinated Ti 4+ (Ti 5c) site and its carbonyl C atom bound to a nearby bridge-bonded oxygen (O b) atom, respectively. During 355 nm irradiation, the dioxymethylene intermediate can transfer an H atom to the O b atom, thus forming HCOO - directly, which is considered as the main reaction channel. In addition, the dioxymethylene intermediate can also transfer methylene to the O b row and break the C-O bond, thus leaving the original carbonyl O atom at the Ti 5c site. After the transfer of methylene, several pathways to products are available. Thus, we have found that O b atoms are intimately involved in the photoinduced decomposition of CH 2O on the rutile TiO 2(100)-(1×1) surface. 相似文献
8.
Two carbonyl complexes of rhenium, [HRe(CO) 5] and [CH 3Re(CO) 5], were used to probe surface sites of TiO 2 (anatase). These complexes were adsorbed from the gas phase onto anatase powder that had been treated in flowing O 2 or under vacuum to vary the density of surface OH sites. Infrared (IR) spectra demonstrate the variation in the number of sites, including Ti +3? OH and Ti +4? OH. IR and extended X‐ray absorption fine structure (EXAFS) spectra show that chemisorption of the rhenium complexes led to their decarbonylation, with formation of surface‐bound rhenium tricarbonyls, when [HRe(CO) 5] was adsorbed, or rhenium tetracarbonyls, when [CH 3Re(CO) 5] was adsorbed. These reactions were accompanied by the formation of water and surface carbonates and removal of terminal hydroxyl groups associated with Ti +3 and Ti +4 ions on the anatase. Data characterizing the samples after adsorption of [HRe(CO) 5] or [CH 3Re(CO) 5] determined a ranking of the reactivity of the surface OH sites, with the Ti +3? OH groups being the more reactive towards the rhenium complexes but the less likely to be dehydroxylated. The two rhenium pentacarbonyl probes provided complementary information, suggesting that the carbonate species originate from carbonyl ligands initially bonded to the rhenium and from hydroxyl groups of the titania surface, with the reaction leading to the formation of water and bridging hydroxyl groups on the titania. The results illustrate the value of using a family of organometallic complexes as probes of oxide surface sites. 相似文献
9.
The phase constitution and phase structure of Zn 0.5Ti 1−xZr xNbO 4 ceramics were analyzed by multiphase structure refinement. The diffraction patterns of Zn 0.5Ti 1−xZr xNbO 4 showed that the ixiolite phase ZnTiNb 2O 8 and rutile phase Zn 0.15Nb 0.30Ti 0.55O 2 were obtained. The distortion of oxygen octahedron, bond valence of Ti-site and volume of oxygen octahedron were calculated. For the main phase ZnTiNb 2O 8, with substitution of Zr 4+ for Ti 4+, the distortion of oxygen octahedron and the bond valence of Ti-site increased. The increase of Ti-site bond valence led to a harder rattling of Ti cations of the specimens. As a result, the contribution of the rattling effect to the polarizabilities of the specimens decreased, and subsequently the dielectric constant decreased. With increasing fraction of Zn 0.15Nb 0.30Ti 0.55O 2, the Qf value decreased. The decrease of τf was mainly attributed to the increase of Ti-site bond valence. With the Ti-site bond valence increasing, the bonding strength between oxygen ion and Ti 4+ ion became stronger, and the dilution of the average ionic polarizability decreased. 相似文献
10.
The reactivity of specific sites on rutile TiO 2(110)-(1×1) surface and anatase TiO 2(001)-(1×4) surface has been comparably studied by means of high resolution scanning tunneling microscopy. At the rutile TiO 2(110)-(1×1) surface, we find the defects of oxygen vacancy provide distinct reactivity for O 2 and CO 2 adsorption, while the terminal fivefold-coordinated Ti sites dominate the photocatalytic reactivity for H 2O and CH 3OH dissociation. At the anatase TiO 2(001)-(1×4) surface, the sixfold-coordinated terminal Ti sites at the oxidized surface seem to be inert in both O 2 and H 2O reactions, but the Ti-rich defects which introduce the Ti 3+ state into the reduced surface are found to provide high reactivity for the reactions of O 2 and H 2O. By comparing the reactions on both rutile and anatase surfaces under similar experimental conditions, we find the reactivity of anatase TiO 2(001) is actually lower than rutile TiO 2(110), which challenges the conventional knowledge that the anatase (001) is the most reactive TiO 2 surface. Our findings could provide atomic level insights into the mechanisms of TiO 2 based catalytic and photocatalytic chemical reactions. 相似文献
11.
Tri(phosphonio)methanide dication 3 2+, prepared from a trifluoromethylsulfanylphosphonium dication ( 1 2+) via an intramolecular electrophilic aromatic substitution reaction, is an unexpected P‐based, water‐resistant Lewis acid that is capable to selectively and reversibly bind fluoride ions from organic/aqueous biphasic solution. The formed complex is an unusual fluorophosphorane ( [3‐F] OTf). The multiple donor–acceptor interactions of 3 2+ that are crucial for the fluoride fixation have been elucidated by quantum chemical calculation. Compound [3‐F] OTf can also be used as a convenient anhydrous fluoride ion source and was probed as a suitable catalyst of the silylotrifluoromethylation of various aldehydes. 相似文献
12.
Irradiations of Ni/TiO 2 catalyst by UV in hydrogen at 77 K produced not only Ni + ions on the catalyst surface, but also Ni 3+ and Ti 3+ species in bulk or near the interface between nickel and titania. These photo-generated species were detected and characterized by low temperature electron paramagnetic resonance (EPR) spectroscopy. Relative spin concentrations of the photogenerated paramagnetic species (Ni n+ and Ti 3+) varied with the nickel content in titania. A high nickel content in the sample resulted in a high peak intensity ratio of Ni n+ to Ti 3+. It was found that the photoinduced self-redox reaction of Ni 2+ ions to form Ni + and Ni 3+ ions has a priority over the photoreduction of Ti 4+ to Ti 3+ ions. The characteristic EPR spectrum of the Ni 3+ (3d 7) ions with g 1 = 2.268, g 2 = 2.237, and g 3 = 2.045 indicates that the Ni 3+ ions are most likely located in the substitutional sites of TiO 2, possibly near the surface rutile phase. The Ni + species (3d 9) with g 4 = 2.130 and g 1 = 2.063 are on the surface of TiO 2. Both Ni + and Ni 3+ ions are quite stable in hydrogen. The Ni 3+ ions seem to be responsible for anchoring the nickel ions onto titania and stablizing the Ni + species on the surface. The Ni + ions are thus free from oxygen poisoning and still show a high activity toward olefin oligomerization. 相似文献
13.
ESR analysis of 1–20 wt. % V 2O 5 samples heated at 700–950°C has revealed the formation of V 4+ ion clusters upon V 4+ ion stabilization in Ti 4+ substitution points of rutile. 相似文献
14.
Ti 1–xFe xO 2 / Fe 2O 3 ( x = 0.3, 0.6, and 0.7 wt%) composites were prepared by solid state reaction of the oxides TiO 2 (rutile phase) and Fe 2O 3 at 550 °C. The following techniques were applied for the characterization of the composites: X‐ray powder diffraction, Mössbauer spectroscopy, SEM, energy dispersive X‐ray spectroscopy and adsorption of nitrogen. The anatase/rutile/hematite ratio and the abundance of Fe 3+ were quantified. The results indicate that Fe 3+ substituted Ti 4+ in the rutile structure and that the α‐Fe 2O 3 phase was predominantly on the surface of the crystalline Ti 1–xFe xO 2 powders. A substantial increase of the materials density, with respect to rutile, favoured the application of the composites in photocatalytic experiments. The performance of the solids upon the photodegradation of aqueous solutions of carbofuran was evaluated. The Lewis sites created in the composites correlated directly with the photodegradation rate constant of carbofuran and the decrease of the total organic carbon content in the treated solutions. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
15.
Thermogravimetric measurements were conducted on undoped, Nb 5+ or Ta 5+ donor-doped, and Al 3+ acceptor doped rutile at 1000°C and 10 5–10 −7 Pa oxygen activity. Nb 5+ or Ta 5+ doped rutile was counterdoped by Al 3+ in order to identify the role of Al 3+ acceptor and its effect on donor doped rutile. Our results show that both dopants enter the lattice substitutionally for Ti 4+, that Al 3+ is as effective an acceptor as Nb 5+ or that Ta 5+ is effective as a donor, and the compensation occurs on a one-to-one basis. Earlier reports indicate that Al 3+ is at best 50–70% efficient as an acceptor. 相似文献
16.
A numerical analysis of the 1H MAS NMR sideband pattern of the bridging OH groups in various zeolites of the aluminosilicate type (LTA, faujasite, erionite, mordenite, pentasil) and of the silicoaluminophosphate type (SAPO-5, −17, −34, −37) yields H-Al distances of the hydroxyl protons to the adjacent framework aluminium nucleus which cover a range between 0.234 and 0.252 nm. From the results, a relation between the H-Al distance and the size of the oxygen rings (6−, 8−, 10− and 12-membered oxygen rings) could be derived. Using 29Si CP/MAS NMR it was shown that the bridging OH groups in zeolites of the aluminosilicate type are located at the Si( nAl) sites with the largest possible number n. While the second coordination sphere of the T positions of the bridging OH groups in zeolites of the silicoaluminophosphate type (Si/(Al + P + Si) < 0.12) is occupied by the same number of phosphorus and aluminium atoms, the bridging OH groups in zeolites of the aluminosilicate type with medium framework aluminium densities are located at the Si( nAl) sites with n = 3, 2 and 1. 相似文献
17.
The adsorption of lithium atoms on rutile TiO 2(110) single crystals was studied with metastable‐induced electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS(HeI)) between 130 K and room temperature. Some auxiliary measurements on W(110) required for data interpretation are also reported. At 130 K ionic adsorption at titania prevails up to 0.3 monolayer equivalents (MLE) as judged from the weak Li(2s) emission in MIES for these exposures. The reduction of the Ti 4+ cation is manifested by the growth of an occupied bandgap state in UPS: the alkali s‐electron is transferred to a near‐surface cation, thereby reducing it to Ti 3+ 3d. The transfer of the s‐electron is responsible for the observed work function decrease up to ~0.5 MLE coverage. From the analysis of the UPS Ti 3+ 3d signal, as well as from the Li(2s) emission, it is concluded that the degree of ionicity of the adsorbed Li decreases from 100% at 0.3 MLE to 40% at 0.7 MLE. Above 0.5 MLE the MIES spectra are dominated by an Li(2s)‐induced peak indicating the presence of Li with an at least partially filled 2s orbital. At temperatures above 160 K this peak is almost absent. Excluding Li desorption at these temperatures, we suggest that Li moves into or below the rutile TiO 2(110) surface above 160 K. Lithium insertion into the surface and intercalation are discussed. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
18.
This study presents the experimental and theoretical study of highly internally Al‐doped TiO 2 nanoparticles. Two synthesis methods were used and detailed characterization was performed. There were differences in the doping and the crystallinity, but the nanoparticles synthesized with the different methods share common features. Anatase to rutile transformation occurred at higher temperatures with Al doping. X‐ray photoelectron spectroscopy showed the generation of oxygen vacancies, which is an interesting feature in photocatalysis. In turn, the band‐gap energy and the valence band did not change appreciably. Periodic density functional calculations were performed to model the experimentally doped structures, the formation of the oxygen vacancies, and the band gap. Calculation of the density of states confirmed the experimental band‐gap energies. The theoretical results confirmed the presence of Ti 4+ and Al 3+. The charge density study and electron localization function analysis indicated that the inclusion of Al in the anatase structure resulted in a strengthening of the Ti?O bonds around the vacancy. 相似文献
19.
Determinations of the [Ti(IV)]/[Ti(III) ratio in solutions of titanium(IV) chloride equilibrated with H 2(g), at 25°C in 3 M (Na)Cl ionic medium, have indicated the predominance of the Ti(OH) 22+ species in the concentration ranges 0.5 ? [H +] ? 2 M and 1.5 x 10 ?3 ? [Ti(IV)] ? 0.05 M. From the equilibrium data the reduction potential has been evaluated Ti(OH) 22+ + 2 H + + e ? Ti 3+ + 2H 2O, EoH = (7.7 ± 0.6) x 10 ?3 V. The acidification reactions of Ti(OH) 22+ were also studied in 12 M(Li)Cl medium at 25°C by measuring the redox potential of the Ti(IV)/Ti(III) couple as a function of [H +]. The potentiometric data in the acidity range 0.3 ? [H +] ? 12 M have been explained by assuming Ti 4+ + e ? Ti 3+, Eo = 0.202 ± 0.002 V Ti 4+ + H 2O ? TiOH 3+ + H +, log Ka1 = 0.3 ± 0.01 Ti 4+ + 2H 2O ? Ti(OH) 22+ + 2H +, log Ka1Ka2 = 1.38 ± 0.05. 相似文献
20.
The reaction of TiO 2 with Sb 2O 4 for temperatures in the range of 650 to 1000°C indicates that Sb can dissolve in rutile up to the proportion of 7 mole% in an Sb 2O 4 equivalent. With increasing content of Sb, the unit cell volume increased gradually. From the results on the binding energies of 530.6 eV forSb(3d 32) and 540.1 eV forSb(3d 52), it is expected that Sb was substituted as Sb 3+ ion for Ti 4+ in the rutile structure. The ac resistivities of rutile with Sb are in the range of 10 2 to 10 5 ohm · m at room temperature. The frequency and temperature dependencies of the dielectric constant can be elucidated by the ionic polarization model coupling Sb with oxygen vacancy. The activation energy, giving a loss maximum, was determined to be 0.15–0.17 eV over a frequency range of 10 4 to 10 7 Hz. 相似文献
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