Reduction Of Ti Research Articles

Microstructure and Dielectric Properties of Ni<sup>2+</sup>-Doped (Ba,Sr)TiO<sup>3</sup> Ceramics

Ba0.62Sr0.38TiO3 ceramics of perovskite structure are prepared by solid reaction method with the addition of x wt% NiO and their dielectric properties are investigated. It is found that Ni2+ can enter the B site in ABO3 perovskite when doping concentration is less than 0.6 wt%, whereafter, Ni2+ ions most segregates on the grain boundaries to inhabit the grain growth. Due to the decrease of grain size and the impurity, the dielectric constants are decreased. Decrease of dissipation factor is attributed to appearance of the oxygen vacancy and disappearance of the reduction of Ti4+ ion. It is concluded that the diffuse phase transition is linked to the adulteration of Ni2+. The disordering, mechanical stress in the grain and the oxygen vacancies are produced because of the adulteration of Ni2+, and the diffuse phase transition increases with the level of doping increasing, leading to that the peaks of dielectric constants becomes slower and flatter.

Active Site Formation in MgCl2−Supported Ziegler−Natta Catalysts. A Density Functional Theory Study

All types of chemical reactions (reduction, alkylation, and complexation of Ti species by organoaluminum compounds) leading to formation of active sites in heterogeneous Ziegler−Natta catalysts have been studied within density functional theory. A generally accepted mechanism of active site formation was found to be less preferable as compared to the alternative ways considered. Based on the calculation implemented, a whole active site formation scenario was postulated for the different active center precursors on the MgCl2 surface. The mechanism proposed allows us to rationalize the domination of Ti3+ over Ti2+ under reduction of Ti4+ surface species by AlR3, the absence of an electron spin resonance signal for the TiCl4/MgCl2 + AlR3 system with low Ti content, the stronger reduction ability of Al(i-Bu)3 than AlEt3, the deactivation effect of AlR2Cl, and the reactivation ability of AlR3.

Structure, Conductivity, and Thermal Expansion Studies of Redox Stable Rutile Niobium Chromium Titanates in Oxidizing and Reducing Conditions

Materials from the solid solution with general formula Ti1−2xCrxNbxO2 (0<x≤0.5) were synthesised and their structure, conductivity, redox behavior, and thermal expansion studied in air and under reducing conditions of up to 10−20 atm p(O2). Their rutile structure (P42/mnm) is retained after reduction, though TEM analysis showed that crystallographic shear planes were formed on reduction to accommodate oxygen vacancies. The conductivity of these materials at 900 °C in air was 1.5 − 1.7 × 10−2 S cm−1, increasing to >20 S cm−1 in 5% H2/Ar. The lowest conductivity was observed for samples low in titanium content, indicating that the reduction of Ti4+ to Ti3+ is primarily responsible for the high electronic conductivity in these materials. All samples showed very slow reduction kinetics, and true equilibrium was not achieved even after >50 h in flowing 5% H2/Ar at 900 °C, suggesting that the equilibrium conductivity of these materials at 10−20 atm may be significantly higher. Conductivity studies as a function of p(O2) showed complex redox behavior with indications of a number of step transitions and very slow redox kinetics. The thermal expansion coefficient was in the range of 5.6 × 10−6 − 8.6 × 10−6 K−1, which changed little between reduced and fully oxidized samples.

Influence of gaseous medium on crystallization of cordierite glasses, synthesized by exposure to concentrated solar radiation

Presented are results of study of the influence of gaseous media on the crystallization of cordierite glasses. Partial reduction of Ti4+ to Ti3+ was found to occur in the nitrogen medium. The influence of the nitrogen medium was found to be similar to increase of the titanium dioxide concentration and the degree of dispersion of the glass powders.

Mechanism of the interfacial reaction between cation-deficient La0.56Li0.33TiO3 and metallic lithium at room temperature

We used x-ray diffractometry (XRD), x-ray photoelectron spectrometry (XPS), and secondary-ion mass spectrometry (SIMS) to investigate the mechanism of the interfacial room-temperature (RT) chemical reaction between cation-deficient La0.56Li0.33TiO3 solid electrolytes and metallic lithium anodes in all-solid-state lithium batteries. A stoichiometric mixture of La2O3, Li2CO3, and TiO2 powders was calcined at 1250 °C for 8 h to obtain a single perovskite structure of La0.56Li0.33TiO3. When this La0.56Li0.33TiO3 sample and lithium were placed in contact at room temperature for 24 h, the phase of the La0.56Li0.33TiO3 remained unchanged. The XPS results indicate that 12% of the tetravalent Ti4+ ions were converted into trivalent Ti3+ ions. The valence conversion and degree of conversion were limited by the structural rigidity of the host crystal. Our SIMS analysis suggests the existence of a local electric field near the contact surface and indicates that the 6Li+ isotope ions were inserted into the specimen through the effect of this field. The change in the electrical properties of La0.56Li0.33TiO3 supports this mechanism for the interfacial reaction. The ionic conductivities of the grain and total grain boundary decreased and increased, respectively, after the insertion of Li+, and the total electronic conductivity increased as a result of the presence of intervalence electron hopping between mixed Ti3+/Ti4+ states. The mechanism of the lithium-activated RT interfacial reaction is associated with the reduction of Ti4+ transition metal ions from tetravalent to trivalent states and the local-electric-field-induced Li+ insertion into La3+/Li+-site vacancies of La0.56Li0.33TiO3.

Characterization of ramiform precipitates formed on SiO2–TiO2 gel coatings by electric field hot water treatment

Abstract The shape of the precipitates on sol–gel derived SiO2–TiO2 coatings at the negative electrode changed from granular to ramiform by applying an electric field to the substrates during a hot water treatment, whereas such changes in the shape of titania nanocrystals with the electric field were not observed at the positive electrode. The granular and ramiform precipitates were identified as anatase (TiO2) and hydrated titania (n(TiO2) · mH2O), respectively. The ramiform shape of the titania precipitates became significant with increasing the applied voltage, while the coatings gradually became dark-colored due to the reduction of Ti4+ to Ti3+. The coatings with ramiform precipitates showed an excellent wettability for water.

Titanium reduction processes in oxide glasses under electronic irradiation

Borosilicate and disilicate glasses doped with 0–2mol% of TiO2 have been irradiated with 2MeV electrons. Reduction of Ti4+ is attested by the presence of an intense electronic paramagnetic resonance (EPR) signal around g=1.92 corresponding to Ti3+, not observed in samples that were not irradiated. Moreover, two distinct sites of Ti3+ have been detected in each glass composition. Ti–K-edge X-ray absorption near edge structure (XANES) experiment and more precisely the pre-edge analysis of non-irradiated samples indicates the presence of four, five and six coordinated Ti(IV) in borosilicate glass compositions. In contrast, the average Ti(IV) coordination in DS composition is a mixture between fivefold and sixfold. In irradiated glasses, the Ti–K-edge energy (as well as the pre-edge position) shows a shift to smaller energies, which we suggest is caused by Ti3+. Comparison between EPR, Raman and XANES results in a coherent description of Ti4+ reduction process in the glass samples.

Li Insertion in Sol−Gel Prepared Mn-Doped TiO2 Studied by Electron Spectroscopy in Ultrahigh Vacuum

The properties of a sol−gel prepared Mn-modified TiO2 film have been studied with X-ray absorption spectroscopy (XAS) and photoelectron spectroscopy (PES) using synchrotron radiation. The chemical composition and oxidation state of the elements have been determined. The manganese content estimated by PES of the Mn-modified film is about 10% and both Mn2+ and Mn3+ are observed. Addition of Mn is found to modify the valence band edge. The Mn 3d states are found to extend about 1 eV into the TiO2 band gap region. It is demonstrated that lithium insertion into the sol−gel film can be performed in a stepwise fashion in situ under ultrahigh vacuum (UHV) conditions. Lithium is distributed evenly throughout the entire film and leads to reduction of Mn3+ to Mn2+ followed by reduction of Ti4+ to Ti3+. The XAS and PES measurements give fully consistent results regarding the amount of inserted lithium.

Dielectric properties of nanocrystalline TiO2 prepared using spark plasma sintering

Ultra fine rutile powders (below 50 nm) were prepared via the sol-gel process and bulk type TiO2 specimens were fabricated using spark plasma sintering (SPS). The TiO2 specimen sintered at a low temperature (720∘C) exhibited a highly relative density (97%) and a nano-sized grain structure (200 nm). Dielectric properties of spark plasma sintered TiO2 specimens including dielectric constants (k) and losses (tan δ) were measured. The TiO2 specimen, obtained by SPS, showed a high dielectric constant (∼780) and a low tan δ (∼0.005), and a relaxation behavior at 1 MHz. After the subsequent annealing process of the TiO2 specimen in O2 flow, the dielectric constants remarkably decreased (k = 100s). These dielectric properties of nanocrystalline TiO2 specimens prepared by SPS were discussed in terms of space charges produced by the reduction of Ti4+ ions and crystallographic orientations of grains.

PHYSICAL PROPERTIES OF MN-DOPED Bi3.65La0.47Ti3O12.17 CRYSTALS

ABSTRACT Acceptor dopants prevent the reduction of Ti4+ to Ti3+ by neutralizing the donor action of the oxygen vacancies. Bi3.6La0.4Ti3O12 thin film has a low dielectric constant and the smallest dielectric loss among La-modified bismuth titanate materials. We grew Bi4.3Ti3O12.4, Bi3.65La0.47Ti3O12.17 and Mn-doped Bi3.65La0.47Ti3O12.17 crystals. The remanent polarization of Mn-doped Bi3.65La0.47Ti3O12.17 crystal indicated a higher value, and the coercive field indicated a lower value. From the XPS results, we suggested that oxygen vacancies of Bi3.65La0.47Ti3O12.17 might be decreased with Mn doping. The band gap of the crystals was about 3.11, 3.25 and 3.2 eV for Bi4.3Ti3O12.4, Bi3.65La0.47Ti3O12.17 and Mn-doped Bi3.65La0.47Ti3O12.17 crystals, respectively.

Propylene polymerization using MgCl2/ethylbenzoate/TiCl4 catalyst: Determination of titanium oxidation states

AbstractThe chemical interaction of the catalyst MgCl2/ethylbenzoate/TiCl4 with the cocatalysts triethylaluminum and trisobutylaluminum was investigated to establish a relationship between the titanium oxidation states and the catalytic activity, polymer isotacticity, and polymer molecular weight in propylene polymerizations. This interaction was studied using different Al : Ti molar ratios by measuring the changes of the titanium oxidation states at different polymerization times. Both hydrogen and alkyl aluminum caused a reduction of Ti4+ species to lower oxidation states species Ti3+ and Ti2+. However, the Ti4+ species reduction appeared to be incomplete. It was found that the Ti4+ species undergoes a severe reduction as the Al : Ti molar ratio increases from 50 to 230 as overreduction takes place. This change of the Ti3+ species percentage with time was found to correlate with the rate–time profiles of propylene polymerization. From this observation, it would be fair to conclude that the trivalent titanium species is more likely to be the active titanium species for propylene polymerization than the aforementioned catalyst system. On the other hand, hydrogen addition was found to cause an increase in Ti3+ species. The increases in both hydrogen amount and/or Al : Ti molar ratio were found to cause a decrease in both molecular weight and polypropylene isotactic index. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 56–62, 2004

The effect of titania polymorph on the strong metal-support interaction of Pd/TiO2 catalysts and their application in the liquid phase selective hydrogenation of long chain alkadienes

In situ EPR and IR investigation by using CO as probe molecules show that even pre-reduced by H2 at lower temperature results in SMSI for anatase titania supported palladium catalyst, but not for rutile titania supported palladium catalyst, which is attributed that the Ti3+ ions produced by reduction of Ti4+ are fixed in the surface lattice of TiO2, as rutile titania is more thermodynamically and structurally stable than anatase titania so that the Ti3+ ions fixed in the surface lattice of anatase TiO2 is easier to diffuse to surface of palladium particle than one in the surface lattice of rutile TiO2. Anatase titania supported palladium catalyst 0.075% Pd/TiO2 (A) pre-reduced by H2 at lower temperature has higher selectivity of alkenes for the liquid phase selective hydrogenation of long chain alkadienes than rutile titania supported palladium catalysts 0.075% Pd/TiO2 (R). For titania (rutile or anatase) supported palladium catalysts, the elevation of pre-reduction temperature from 200 to 450 °C gives rise to sharp change of catalytic properties, especially for selectivity of alkenes. The very different catalytic properties between 0.075% Pd/TiO2 (R) and 0.075% Pd/TiO2 (A) catalyst pre-reduced at lower temperature, and the rapid increasing of selectivity of 0.075% Pd/TiO2 (A) and 0.075% Pd/TiO2 (R) with the elevation of pre-reduction temperature are reasonably explained by the presence of SMSI both for anatase titania supported palladium catalyst pre-reduced at lower temperature, and titania (rutile and anatase) supported palladium catalyst pre-reduced at higher temperature.

Can fresnoite (Ba2TiSi2O8) incorporate Ti3 when crystallizing from highly reduced melts?

Abstract The crystallization of fresnoite (Ba2TiSi2O8) in supercooled melts can be triggered by the reduction of Ti4+ to Ti3+. We address the question of whether Ti3+ is incorporated into the fresnoite crystal structure during such electrochemically induced crystallization. Fresnoite crystals grown from a variety of highly reduced melts were studied by electron energy-loss spectroscopy in order to distinguish between Ti3+ and Ti4+ contents in the corresponding phases. Although the presence of Ti3+ strongly promotes crystallization, Ti3+ is not incorporated into the fresnoite crystal lattice in a significant amount (less than 10at.%) but, at sufficiently high Ti3+ concentrations, forms a previously unreported barium oxotitanate(III) containing trivalent titanium only. Since, in the fresnoite crystal structure, Ti4+ solely adopts an unusual fivefold co-ordination, we also discuss the energy-loss near-edge structure (ELNES) of the Ti L2,3 ionization edge and compare it with the Ti L2,3 ELNES of tetrahedrally and octahedrally coordinated Ti4+.

Can fresnoite (Ba2TiSi2O8) incorporate Ti3+when crystallizing from highly reduced melts?

Abstract The crystallization of fresnoite (Ba2TiSi2O8) in supercooled melts can be triggered by the reduction of Ti4+ to Ti3+. We address the question of whether Ti3+ is incorporated into the fresnoite crystal structure during such electrochemically induced crystallization. Fresnoite crystals grown from a variety of highly reduced melts were studied by electron energy-loss spectroscopy in order to distinguish between Ti3+ and Ti4+ contents in the corresponding phases. Although the presence of Ti3+ strongly promotes crystallization, Ti3+ is not incorporated into the fresnoite crystal lattice in a significant amount (less than 10at.%) but, at sufficiently high Ti3+ concentrations, forms a previously unreported barium oxotitanate(III) containing trivalent titanium only. Since, in the fresnoite crystal structure, Ti4+ solely adopts an unusual fivefold co-ordination, we also discuss the energy-loss near-edge structure (ELNES) of the Ti L2,3 ionization edge and compare it with the Ti L2,3 ELNES of tetrahedrally and octahedrally coordinated Ti4+.

Study of the electronic structure of TiO2(110) and Cs/TiO2(110) with metastable impact electron spectroscopy and ultraviolet photoemission spectroscopy (HeI)

We report the metastable impact electron spectroscopy (MIES) (He∗1s2s) and ultraviolet photoemission spectroscopy (UPS) (Hei) spectra for clean TiO2(110) and Cs/TiO2(110). The cesium coverage is varied at room temperature between zero and saturation. Simulations of the MIES spectra for clean and cesiated TiO2(110), based on presently available information on the electronic structure of the clean and cesiated surfaces, facilitate the interpretation of the spectra. The MIES results for low Cs coverages are compatible with fully ionic adsorption of Cs whereby the 6s electron of Cs is transferred to the substrate. This causes reduction of Ti4+ cations as detected with UPS. For larger coverages the charge transfer is incomplete as judged from the formation of temporary negative He−∗(1s2s2) ions during the interaction of the metastable He atoms with the cesiated surface. The ionicity, i.e. the charge transfer per adsorbed Cs atom, decreases from practically unity to about 12% at saturation. At saturation a rearrangement of the oxygen in the surface layer takes place with time. The results suggest that the Ti and Cs ions compete for the bonding to the oxygen.

Novel Doping Mechanism for Very‐High‐Permittivity Barium Titanate Ceramics

Barium titanate (BaTiO3) can be doped with La3+ ions via partial substitution for Ba2+ ions; charge balance is maintained by the creation of Ti4+ vacancies. Samples processed in an atmosphere of 1 bar O2 and a temperature of 1350°C are insulating and free from electronic defects associated with either O2 loss or reduction of Ti4+ to Ti3+. The Curie temperature (Tc) decreases approximately linearly as the lanthanum content increases and, at the same time, an increase in the permittivity (ɛ′) maximum at Tc occurs. For the composition Ba1‐xLaxTi1‐x/4O3, where x= 0.05, ɛ′ has a maximum value of 19000 at 18°C, compared with a typical value of 10000 at 130°C in undoped BaTiO3 ceramics. This value is the highest value reported for A‐site‐doped BaTiO3 and is linked to the mechanism of combined A‐site doping and Ti‐vacancy creation.

Absorption spectra and luminescence of oxygen-deficient centres in a titanosilicate glass synthesised in a steady laser plume. I. Absorption spectra

The absorption spectra of titanosilicate glasses with the titanium concentration ∼ 0.1 — 3% by weight were investigated in the wavelength range 225 — 460 nm. These glasses were synthesised in a steady laser plume in an atmosphere of air or nitrogen at normal pressure. Synthesis in these two atmospheres gave rise to Ti ions of different valence in the glasses. For the same Ti concentration the intensity of the characteristic absorption bands of titanium in its reduced states (Ti2+, Ti3+) with maxima at λ ∼ 278 and 306 nm were 3 — 6 times higher when the synthesis took place in nitrogen. The enhancement of the 278 nm band was approximately half that of the band at 306 nm. Clear-cut strong absorption in the UV range at 260 — 280 nm as well as strong absorption in the visible range at 400 — 500 nm, observed for the first time for titanium-doped silica glass, were the results of a record-high concentration of Ti3+ ions, compared with the concentrations found in bulk glass samples prepared by other technologies such as melting, reduction of Ti4+ ions, and plasma hydrolysis.

The roles of Grignard reagent in the Ziegler-Natta catalyst for propylene polymerization

Grignard reagent PhMgCl was added during the preparation of the catalyst system MgCl2/di-n-butyl phthalate (DNBP)/TiCl4—AlEt3/diphenyl dimethoxyl silane (DPDMS) to improve its performance. It was found that PhMgCl could enhance both the activity of the catalyst and the isotacticity of the products, but decreased the Ti content of the catalysts under the same preparation conditions. The polymerization kinetics showed that PhMgCl accelerates the decay in the same time that it increased the initial and final polymerization rates. By means of UV-vis spectroscopy, electron spin resonance (ESR) spectroscopy, and the Ti content determination of the catalysts, the multiple roles of PhMgCl were disclosed: reduction of Ti4+ to Ti3+, association with MgCl2 to replace part of TiCl4 in aspecific active sites, and complexing with the original active sites to form new active sites. © 1997 John Wiley & Sons, Inc. J Appl Polm Sci 65:925–930, 1997

The roles of Grignard reagent in the Ziegler–Natta catalyst for propylene polymerization

Grignard reagent PhMgCl was added during the preparation of the catalyst system MgCl2/di-n-butyl phthalate (DNBP)/TiCl4—AlEt3/diphenyl dimethoxyl silane (DPDMS) to improve its performance. It was found that PhMgCl could enhance both the activity of the catalyst and the isotacticity of the products, but decreased the Ti content of the catalysts under the same preparation conditions. The polymerization kinetics showed that PhMgCl accelerates the decay in the same time that it increased the initial and final polymerization rates. By means of UV-vis spectroscopy, electron spin resonance (ESR) spectroscopy, and the Ti content determination of the catalysts, the multiple roles of PhMgCl were disclosed: reduction of Ti4+ to Ti3+, association with MgCl2 to replace part of TiCl4 in aspecific active sites, and complexing with the original active sites to form new active sites. © 1997 John Wiley & Sons, Inc. J Appl Polm Sci 65:925–930, 1997

Pressure- and temperature-effects on exchange-coupled-pair bands in electronic spectra of some oxygen-based iron-bearing minerals

The special properties of spin-allowed transitions of Fe2+, exchanged-coupled in pairs with Fe3+, (ECP-effect) are studied in single crystals of vivianite, phlogopite, biotite, elbaite and schorl under changing temperature and pressure in the ranges 79≤T[K]≤597 and 10-4≤P[GPa]≤8. The two Fe2+dd-transitions, known to be subject to ECP-effects, occur at 11000 to 14000 (II) and 8400 to 9150 cm-1 (III), depending on the structural matrix. With pressure, band energies shift to higher values, while temperature has the opposite effect. Δv is nearly the same in all cases, decrease on temperature, and increase with pressure. Δα/ΔT or Δα/ΔP have similar values for bands II and III in all minerals studied. These observations are interpreted in terms of geometrical and vibrational changes of the octahedra, involved in the pair effects, on changing P and T. They clearly separate the ECP-bands from ordinary dd-transitions and also from IVCT-bands. A unique pressure effect in the spectral range of 17000 to 26000 cm-1 was found in schorl: a band system that immensly gains intensity on pressure. Two explanations are suggested: (a) traces of Ti3+, exchange-coupled to Fe2+ show the above pressure effect typical of ECP, (b) there occurs pressureinduced reduction of Ti4+ in Y-positions, induced by Fe2+ in connected Y-octahedra, whereby OH in trans-configuration of Y-octahedra promote this process.