Thermal Transformations of Hydrated Titanium Dioxide with a Globular Structure of Aggregates

The phase and chemical transformations in hydrated titanium dioxide with a globular structure of aggregates, prepared by thermal hydrolysis of aqueous solution of oxotitanium(IV) sulfate and calcined in air at 200-1100°C, were studied. The effect of modifying additives (potassium, zinc, magnesium, and phosphorus compounds) and seed crystals of rutile-like structure on the kinetics and temperature of the polymorphous transition of anatase into rutile, morphology of crystals, and particle size of the resulting product was studied.     

Dissolution kinetics of titanium dioxide nanoparticles: the observation of an unusual kinetic size effect

Different types of industrially produced titanium dioxide nanoparticles and a precipitated titanium dioxide have been dissolved in aqueous NaCl solutions at temperatures of 25 and 37 degrees C. The titanium concentration in solution with regard to dependence on time has been determined up to 3000 h after starting the dissolution experiment. The effect of particle size, pH value, temperature, background electrolyte concentration, and mass concentration of titanium dioxide exposed to the liquid phase has been studied. The nanoparticles have been characterized by N2 physisorption measurements and XRD. The total dissolved titanium in solution has been determined by adsorptive stripping voltammetry (AdSV) and inductively coupled plasma mass spectrometry (ICP-MS). A new kinetic size effect has been observed. It turns out that this effect can be explained by applying an already existing phenomenological thermodynamic and kinetic model. The model describes all possible phenomena in a colloidal dispersion, nucleation, growth of particles, Ostwald ripening, and dissolution of particles using a uniform concept.     

Investigation of thermal power of reaction of titanium slag with sulphuric acid

Enriched titanium raw materials with high titanium content called titanium slags are received by the electrothermal reduction of ilmenite. Titanium slags are most frequently used in the titanium dioxide industry. The reaction of titanium slags with sulphuric acid is strongly exothermic and creates danger of thermal explosion. Kinetics of this reaction depends on the parameters such as temperature of initiation, sulphuric acid concentration and dimension of particles of titanium slag. The reaction of titanium slag with sulphuric acid was investigated at non-isothermal conditions in a special construction calorimeter. The observed thermal power changes in the calorimeter (“calorimeter run”), are the basis for estimation of reaction kinetics. A proposed model describing the thermal power changes and taking into account the moment of initiation of reaction is presented. The calorimetric investigations showed, that reaction rate of titanium slags with sulphuric acid depends on initial temperature of reaction, size of particles of titanium raw material and sulphuric acid concentration.     

DSC investigation of nanocrystalline TiO<Subscript>2</Subscript> powder

The aim of the article is to investigate the influence of particle size on titanium dioxide phase transformations. Nanocrystalline titanium dioxide powder was obtained through a hydrothermal procedure in an aqueous media at high pressure (in the range 25–100 atm) and low temperature (≤200 °C). The as-prepared samples were characterized with respect to their composition by ICP (inductive coupled plasma), structure and morphology by XRD (X-ray diffraction), and TEM (transmission electron microscopy), thermal behavior by TG (thermogravimetry) coupled with DSC (differential scanning calorimetry). Thermal behavior of nanostructured TiO₂ was compared with three commercial TiO₂ samples. The sequence of brookite–anatase–rutile phase transformation in TiO₂ samples was investigated. The heat capacity of anatase and rutile in a large temperature range are reported.     

Particle size effects on the thermal behavior of hematite

Hematite with different particle sizes was obtained through isothermal annealing and mechanochemical ball-milling methods. The hematite phase is very stable under air atmosphere. The thermal stabilities of hematite under argon atmosphere were characterized by thermal analysis studies up to 800 °C using a simultaneous DSC–TG technique. The lattice parameters a and c of hematite with different particle sizes were extracted from the Rietveld structural refinement of powder X-ray diffraction patterns. Decomposition of hematite into a lower oxidation state in inert argon atmosphere was studied by the TG experiments for the first time and the enthalpy associated with the decomposition reaction was determined from the DSC studies. Particle size has a strong effect on the thermal behavior of hematite samples. Ball-milled hematite samples with smaller particle size showed that the phase transformation was extended to higher temperature range with larger enthalpy. Hematite with larger average particle size showed higher stability under argon atmosphere.     

Kinetics of copper slag oxidation under nonisothermal conditions

The kinetics of air copper slag oxidation under nonisothermal conditions is studied using simultaneous TG–DTA at a varying heating rate of slag and flow rate of the oxidizing gas flux. The values of the kinetic parameters, activation energy and pre-exponential factor, have been determined based on: data from DTA by the methods of Kissinger and Ozawa; data from TG using an isoconversion method and the computation procedures of Ozawa–Flynn–Wall and Kissinger–Akahira–Sunose. No relationship between the kinetic parameters and the oxidation gas flow rate has been established. The changes of the phase composition with temperature are investigated by X-ray powder diffraction analysis on the basis of data obtained for the products formed at the different stages of the oxidation process. The morphology of the oxidized slag as well as the elements distribution is studied by electron microscopy and EDS analysis.     

Microwave-assisted chloride process for the production of titanium dioxide

The conditions for the formation of the particle-size and phase composition of titanium dioxide upon the oxidation of titanium tetrachloride in a stream of oxygen-containing microwave discharge plasma have been studied. The possibility of controlling the particle size of the resulting powder in the range of 50–100 nm by changing various operating parameters has been shown. The conditions for obtaining a nearly single-phase powder of the anatase or rutile modification have been found.     

Preparation of nanocrystalline anatase TiO2 using basic sol-gel method

Nanocrystalline titanium dioxide particles with anatase structure and high thermal stability have been synthesized using the basic sol-gel method. The particle size and morphology were refined under hydrothermal conditions in the presence of different concentrations of tetramethylammonium hydroxide (TMAH) at 210°C and 230°C. XRD and TEM analysis showed that the TiO₂ particles obtained were homogeneous and monodispersive at low contents of TMAH. All intense peaks, clearly observed in the XRD patterns, were assigned to the anatase phase and no rutile phase was observed. At high contents of TMAH, nanoscale small (10–30 nm) and larger (>100 nm) TiO₂ particles were one-pot synthesized. The nanocrystalline TiO₂ particles synthesized by this method have good thermal stability. With the sintering temperature of up to 650°C, all the XRD peaks maintained good agreement with the anatase reference data.     

Non-isothermal kinetic of oxidation of tungsten carbide

Tungsten carbide, WC, has shown dissimilar thermal behavior when it is heated on changeable heating rate and flow of oxidant atmosphere. The oxidation of WC to WO₃ tends to be in a single and slow kinetic step on slow heating rate and/or low flux of air. Kinetic parameters, on non-isothermal condition, could be evaluated to the oxidation of WC to heating rate below 15°C min⁻¹ or low flow of air (10 mL min⁻¹). The reaction is governed by nucleation and growth at 5 to 10°C min⁻¹ then the tendency is to be autocatalytic, JMA and SB, respectively.     

Carbon-containing nano-titania prepared by chemical vapor deposition and its visible-light-responsive photocatalytic activity

Ultraviolet and visible-light-responsive titania was synthesized and employed in the NO_x photomineralization. A thermal decomposition reaction of titanium isopropoxide was carried out with a metal-organic chemical vapor deposition (MOCVD), enabling continuous production of TiO₂ nanoparticles. Carbon-containing titanium dioxide with the anatase phase prepared at 500 °C under nitrogen atmosphere exhibited high photocatalytic activity for NO oxidation under visible-light illumination. Experimental results indicate that up to 48% removal of NO_x can be achieved in a continuous flow type of reaction system under visible-light illumination (green LED). The chamber temperature in this MOCVD process plays an important role in lattice structure formation, and also affected TiO₂ carbon content. The carbonaceous species on the TiO₂ surface, shown by X-ray diffractometry (XRD), and Raman, UV–vis, and X-ray photoelectron spectroscopies (XPS), is important to the visible-light absorption and visible-light-catalytic mineralization of NO_x.     

Nanocatalysts for photocatalytic air purification systems

Nanocatalysts containing platinum and palladium clusters have been synthesized on the basis of detonation nanodiamond, cubic silicon carbide, and titanium dioxide. Characteristic size of the Pt cluster was close to 4 nm on both nanodiamond (particle size 5 nm) and β-SiC (particle size 13 nm) supports. The catalysts show high catalytic activity in reactions of CO oxidation and photocatalytic oxidation of ethanol at room temperature and low concentrations (<100 mg m³). They are promising in photocatalytic air purification systems for domestic use.     

Influence of Gas Atmosphere on Removal of Sulphate Groups From Titanium Oxide

The studies were devoted to determination of the effect of gas atmosphere and its pressure on the second step of decomposition of hydrated titanium dioxide (HTD) promoted by sulfate groups. It has been found that thermal decomposition of HTD at temperatures above 300°C consists of a number of processes such as dehydroxylation, desulfuration, recrystallization and sintering of solid grains, photochemical processes (if the decomposition proceeds in the presence of light) and adsorption of gas phase components (in the presence of air or SO₂). Kinetic parameters characterizing this step of decomposition have been determined for processes carried out in vacuum and in argon or air atmospheres (at a pressure of 13.33hPa). The kinetic curves of decomposition carried out in the presence of gases capable of being adsorbed on the surface of partly dehydrated HTD are featured by local extrema due to simultaneous processes of decomposition and adsorption of gas components. This revised version was published online in August 2006 with corrections to the Cover Date.     

纳米氢氧化镁的合成及其形貌控制

用乙醇和水的混合溶剂热法合成氢氧化镁纳米材料,并用透射电镜(TEM)和X射线衍射(XRD)表征其形貌和结构,同时考察了镁源、温度、反应时间、反应物浓度和溶剂热体系对氢氧化镁纳米材料形貌的影响,探索其生长机理。镁源通过改变氢氧化镁纳米粒子的结晶习性从而影响形貌,温度和反应时间受热力学和动力学的控制使氢氧化镁纳米材料的生长从六个等价面的取向生长向各向同性生长转变,从而导致形貌由六边形片状结构向圆形变化,反应物浓度和溶剂热体系影响成核快慢,从而影响氢氧化镁纳米材料的晶型。     

Synthesis and properties of titanium glycolate Ti(OCH2CH2O)2

A new efficient method for the synthesis of extended micro-and nano-sized crystals (whiskers, fibers) of titanium glycolate Ti(OCH₂CH₂O)₂ has been suggested. The method implies the reaction of hydrated titanium dioxide with ethylene glycol on heating in air. Thermolysis of Ti(OCH₂CH₂O)₂ in air gives titanium dioxide as anatase (400–500°C) and rutile (T > 700°C), the morphology of titanium glycolate crystals being inherited by the oxide. The pseudocrystals of the thermolysis product in an inert gas medium (T = 500–950°C) represent aglomeration of nano-sized titanium dioxide particles and amorphous carbon. At temperatures up to 1300°C, the formation of the TiO_2?x C _x phase with a rutile structure is probable. In a wet air environment, titanium glycolate is partially hydrolyzed to give TiO _x (OCH₂CH₂O)_2?2x (OH)_2x ·xH₂O (0 ≤ x ≤ 1) and on keeping in water at room temperature, ethylene glycol is completely displaced from the crystals. This process is also not accompanied by changes in the particle morphology.     

Production of silicon from magnesium silicide

Kinetic methods and thermogravimetry were used to study the oxidation process of magnesium silicide in air in the temperature range 300–1000°C. The reaction products were identified by X-ray phase analysis. It was found that the reaction occurs in the temperature range 510–710°C to give silicon and magnesium oxide. With the temperature increasing further, silicon is oxidized to silicon dioxide.     

Heat release kinetics in the reaction of decane with nitric acid

The kinetics of the reaction of decane with nitric acid (25.07–75.53 %) at 57.8 to 119 °C in an acid-decane-gas triphasic system was studied. The main contribution to the rate of heat release is made by the oxidation of decane with nitrogen dioxide in the organic phase proceedingvia the mechanism of a degenerate branched-chain reaction. Nitration plays the role of a chain termination reaction. The acid phase is the source of NO₂, whose content increases with oxidation. The equilibrium of the nitrogen dioxide distribution in the triphasic system was analyzed. The kinetic law of the reaction, the dependences of the reaction constants of the initial and catalytic stages on temperature and the acid phase composition were determined. The results allow one to calculate the rate of heat release in the decane-HNO₃ system under any conditions of the process.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1986–1991, August, 1996.     

Study of the effect of ammonium sulfate additives on the structure and photocatalytic activity of titanium dioxide

Thermal hydrolysis of titanium hydrochloride in the presence of ammonium sulfate added in a Ti: SO ₄ ^2? = 20: 1 ratio was used to obtain samples of mesoporous titanium dioxide with anatase structure, stable in a wide temperature range. The phase composition, porous structure, and morphology of the synthesized titanium dioxide powders were examined by X-ray phase analysis, IR spectroscopy, scanning electron microscopy, and low-temperature adsorption of nitrogen. The role played by sulfate ions in how the microstructure of titanium dioxide is formed was determined. The effect of the hydrolysis temperature on the structure of titanium dioxide being obtained and on its photocatalytic activity in the reaction of decomposition of Methyl Orange dye was examined.     

X-ray diffraction and Mössbauer spectroscopy studies of LiFe0.5Ti1.5O4 – A new primitive cubic ordered spinel

LiFe_0.5Ti_1.5O₄ was synthesized by solid-state reaction carried out at 900 °C in flowing argon atmosphere, followed by rapid quenching of the reaction product to room temperature. The compound has been characterized by X-ray powder diffraction (XRD) and ⁵⁷Fe Mössbauer effect spectroscopy (MES). It crystallizes in the space group P4₃32, a = 8.4048(1) Å. Results from Rietveld structural refinement indicated 1:3 cation ordering on the octahedral sites: Li occupies the octahedral (4b) sites, Ti occupies the octahedral (12d) sites, while the tetrahedral (8c) sites have mixed (Fe/Li) occupancy. A small, about 5%, inversion of Fe on the (4b) sites has been detected. The MES data is consistent with cation distribution and oxidation state of Fe, determined from the structural data.The title compound is thermally unstable in air atmosphere. At 800 °C it transforms to a mixture of two Fe³⁺ containing phases – a face centred cubic spinel Li_(1+y)/2Fe_(5−3y)/2Ti_yO₄ and a Li_(z−1)/2Fe_(7−3z)/2Ti_zO₅ – pseudobrookite. The major product of thermal treatment at 1000 °C is a ramsdellite type lithium titanium iron(III) oxide, accompanied by traces of rutile and pseudobrookite.     

Particle size characterization of titanium dioxide in sunscreen products using sedimentation field-flow fractionation–inductively coupled plasma–mass spectrometry

Sedimentation field-flow fractionation–inductively coupled plasma–mass spectrometry (SdFFF-ICP-MS) was successfully applied to investigate particle size distribution of titanium dioxide (TiO₂) in sunscreen samples after hexane extraction to remove organic components from the samples. Three brands of sunscreen products of various sun protection factor (SPF) value were used as samples. Different particle size distribution profiles were observed for sunscreen samples of various brands and SPF values; however, the particle size distributions of titanium dioxide in most sunscreen samples investigated in this work were larger than 100 nm. The titanium dioxide concentrations were higher for the products of higher SPF values. By comparing the results obtained from online SdFFF-ICP-MS and those from the off-line ICP-MS determination of titanium after acid digestion, ICP-MS was found to effectively atomize and ionize the titanium dioxide particle without the need for acid digestion of the samples. Therefore, the online coupling between SdFFF and ICP-MS could be effectively used to provide quantitative information of titanium dioxide concentrations across particle size distribution profiles.     

Kinetics and reaction mechanism of isothermal oxidation of Iranian ilmenite concentrate powder

Thermal oxidation of commercial ilmenite concentrate from Kahnouj titanium mines, Iran, at 500–950 °C was investigated for the first time. Fractional conversion was calculated from mass change of the samples during oxidation. Maximum FeO to Fe₂O₃ conversion of 98.63 % occurred at 900 °C after 120 min. Curve fit trials together with SEM line scan results indicated constant-size shrinking core model as the closest kinetic mechanism of the oxidation process. Below 750 °C, chemical reaction with activation energy of 80.65 kJ mol^?1 and between 775 and 950 °C, ash diffusion with activation energy of 53.50 kJ mol^?1 were the prevailing mechanisms. X-ray diffraction patterns approved presence of pseudobrookite, rutile, hematite, and Fe₂O₃·2TiO₂ phases after oxidation of ilmenite concentrate at 950 °C.