Titanium dioxide

• Titanium dioxide
• Titansäureanhydrid
• Rutile
• Anatase
• Brookite
• E 171
• C. I. Pigment White 6
• C. I. 77891

• TiO2: 13463-67-7
• Anatase: 1317-70-0
• Brookite: 12188-41-9
• Rutile: 1317-80-2

White crystalline powder

Fixed

Rutile 4.24 g · cm -3

1855 ° C

2900 ° C

• Insoluble in water, organic solvents, dilute bases and dilute acids
• Soluble in hot concentrated sulfuric acid, hydrofluoric acid, and the molten alkali metal hydroxides and alkali metal carbonates

Optically anisotropic, birefringent or biaxial

Template: Infobox chemical / molecular formula search is not possible

Titanium ( IV) oxide (titanium dioxide ) is the IV -valent titanium oxide. In addition to this polymorphic oxide there are a number of non-stoichiometric suboxide of titanium, so-called Magneli phase and the titanium (III ) oxide and titanium ( II) oxide.

Titanium dioxide is produced as a white pigment, a wide range of applications, so four to five million tons per year worldwide. The main application is in the field of coatings such as paints and coatings, followed by coloring plastics and laminated papers. Colored products also typically contain white pigments in order to achieve a high hiding power.

• 4.1 Physical Properties 4.1.1 Other modifications
• 4.1.2 Optical Properties
• 4.1.3 Dielectric Properties

• 5.1 pigment
• 5.2 photocatalyst
• 5.3 Other Applications

History

After Titanium was discovered in 1791 by William Gregor in ilmenite, Heinrich Klaproth recognized the titania in rutile. The industrial use began when the excellent suitability has been recognized as a white, non-toxic pigment in 1908 in Norway and the USA. From 1916, the pigment was made ​​already commercially under the name Kronos titanium white. Until 1938, titanium white was produced only in the anatase modification, but then increasingly in the rutile modification because their photocatalytic activity is lower and the weathering of the products made from them is correspondingly higher. The white pigment based on the rutile modification is also referred to as Rutilweiß.

More than half of the production quantity is used in coating materials, followed by polymers. 70 % of world production are produced by five manufacturers, which in addition to the leader, DuPont (USA ), nor the company Cristal Global ( Saudi Arabia), Huntsman (USA), Kronos (USA) and Tronox (USA) are ranked among the world's largest producers. The regions that consume most titanium dioxide, are North America, Europe and China. Overall, a third in Europe - Africa - Middle East, America and the Asia-Pacific region is consumed each.

Occurrence

Titanium ( IV) oxide is present in nature in three modifications:

• Rutile is a tetragonal mineral usually of prismatic habit. The crystal structure belongs to the space group 136, which corresponds to the Hermann- Mauguin symbol P42/mnm. The rutile TiO2 has a density of 4.26 g/cm3. The name rutile comes from the Latin rutilus - reddish, in allusion to the color produced by iron impurities.
• Anatase forms tetragonal crystals holoedrische ( holoedrisch means the highest symmetric group within a crystal system ) in the tetragonal so 4 / m 2 / m 2 / m. It crystallizes in the space group 141, ie I41/amd. Anatase transforms itself at 700 ° C, depending on the atmosphere and foreign ions irreversibly into rutile. The density of anatase is 3.88 g/cm3.
• Brookite is orthorhombic minerals and crystallizes in the space group 61, Pbca. Brookite also goes below the melting point in rutile and has a density of 4.12 g/cm3. Technically, the brookite has no meaning.

Production and representation

Titanium dioxide may be in the laboratory by solvolysis (hydrolysis) of Ti (IV) - compounds are prepared, including:

Or metal alkoxides such as titanium tetraisopropoxide. As the titanic acid esters of lower n- alkanols react too violently, the use of isopropanol or t- Butanolester recommends. :

The resulting Titanoxohydrat formally TiO (OH ) 2 or TiO2xH2O is converted by calcination in anatase or rutile, with pure, fully annealed titanium dioxide always results in the rutile lattice. The combustion of titanium ( IV) chloride with oxygen is rarely used on a laboratory scale. Very pure titanium dioxide can be readily obtained by hydrolysis of purified TiCl4.

Since the bulk of the TiO2 pigment is used as the industrially produced, disrupting the coloring ions, such as iron. ( FeTiO3 ) or titanium-containing slag from the electro-reduction of ilmenite are used for the sulphate process usually ilmenite as ores. This slag, just like the alluvial deposits of rutile can also be used in more sophisticated chloride process. Both methods increase the purity of the titanium oxide significantly. The sum of the coloring ions is generally less than 200 ppm in the sulphate process, especially niobium subordinate iron, and less than 50 ppm in the chloride process, niobium and iron.

In the industrial production of titanium dioxide from ilmenite by the sulphate process dilute sulfuric acid is produced (diluted sulfuric acid ), which is recycled usually after concentration for the digestion of ilmenite. In some countries this dilute acid is conducted to date in part or dumped into rivers and oceans. The production by the chloride process, mainly from rutile ore or TiO2 slag, on the other hand does not create waste acid. The chlorine used remains largely in the process cycle. The resulting method in both iron salts are used, among other things, to chromate reduction in cements, waste water treatment and biogas plants.

Single crystals

Rutile crystals are generally produced by the Verneuil method and used in the optical system owing to the high refractive indices as a coupling prisms or diamond imitations. Chance also the zone melting method is used, while the Czochralki process is described as unsuitable.

The preparation of anatase crystals can not be made from the melt. Here CTR methods are used.

Properties

Physical Properties

The melting point of titanium is 1855 ° C, the compound is thermally stable. Titanium dioxide is also chemically inert. It is resistant to light, inexpensive, previous studies indicate that non-toxic and therefore the most important white pigment and also authorized in food ( E171).

Further modifications

In addition to the three natural modifications eight synthetic modifications are known, three of which are metastable (monoclinic, tetragonal and orthorhombic ) and five high-pressure modifications ( α - PbO2, baddeleyite, Cotunnit - and orthorhombic and cubic structures). The modification with Cotunnit structure was of L. Dubrovinsky et. al. as the hardest known oxide having a Vickers hardness of 38 GPa and a bulk modulus of 431 GPa (compared to diamond has 442 GPa to 446 GPa) described under atmospheric pressure. Later studies came to different results with lower values ​​for hardness 7-20 GPa, thus softer than oxides such as corundum Al2O3 and rutile. and the bulk modulus ( ≈ 300 GPa).

Optical Properties

The indices of refraction are very high, however, depending on the crystal form of the titanium dioxide and the wavelength of light ( optical dispersion ). Here, the ordinary ray of rutile produces the highest refractive index at a birefringence An = 0.29.

From coloristic point of view, titanium dioxide due to the high refractive index of the highest opacity of all white pigments and the same time an excellent tinting strength. The teilchengrößenabhängige maximum of the opacity of rutile is at a grain size of about 200 nm to 300 nm depending on the application and reference, number-based or mass-based size distribution.

Titanium dioxide is a semiconductor, so that the valence band is filled and unfilled the conduction band. The band gap is dependent on the modification. Photons having energy greater than the band gap are absorbed. UV- light may be absorbed from the appropriate wavelength and thus an ultraviolet protection can be produced. Due to short wavelength light irradiation, electrons are lifted from the valence band to the conduction band, leaving a hole. The size of the band gap is dependent on the crystal direction and additionally in the range of nanoparticulate material of the particle size.

Dielectric properties

Titanium dioxide has a relatively high dielectric constant. For rutile it is ε = 111 in the direction of the crystallographic a - and ε = 257 along the c -axis. Other sources give smaller values ​​, with the values ​​of measured parameters, such as frequency and temperature dependent. Applications include high-k dielectrics.

Chemical Properties

Of the titanium oxide, the titanium (IV) dioxide is the most common connection. It is chemically inert and can only be solved in hot sulfuric acid, hydrofluoric acid and hot alkalis. It is partly a starting material for the preparation of titanates. When illuminated with UV light photocatalytic radical reactions can take place.

Use

Titanium dioxide is mainly used as white pigment use and is in the Colour Index under CI And C.I. Pigment White 6 77891 listed. It is chemically stable, non-toxic and to be found under the designation E 171 as a food additive, for example, in toothpaste, chewing gum and cough drops, and under CI 77891 as a pigment in cosmetics. Even in oil painting it will partially use. Among the technical applications of titanium dioxide, which account for around 80 percent of total consumption, including paints and coatings, plastics and textiles; It is used (tablets) even in the manufacture of paper to achieve a high degree of whiteness, as well as UV blockers in sun creams and brightener in medicaments.

Pigment

Titanium dioxide having particle sizes in the range of 200 nm to 300 nm is used because of the large refractive index difference from the majority of organic substances as a pigment. The size range results from the Mie theory. The particle size is influenced firstly the opacity and on the other the hue, finely divided pigments appear bluish. The most important applications are around 60 % market share coating materials and 25 % polymer applications.

Pure titanium dioxide comes here except as E171 hardly used, since in addition to the UV - protective effect through the TiO2 held light-induced chemical radical reactions. By a functionalization of the pigment grains, this effect is reduced and at the same time, typically achieves an improvement in the color properties by simple dispersion. Some applications, such as fibers or cement applications use, despite the higher photochemical activity anatase pigments, while the majority of applications makes use of rutile pigments.

Photocatalyst

Many manufacturers offer photocatalysts on TiO2 - based. These are usually anatase, anatase - rutile mixtures or doped titanium dioxides with various applications. Photocatalysis is a heterogeneous catalysis, wherein the gaseous or dissolved substances under UV illumination by radical reaction or carrier transition react to titanium dioxide, or other materials. By the illumination with UV light having an energy larger than the band gap, or by the less efficient stimulation of the impurity doping are free charge carriers, electrons in the conduction band and holes in the valence band is produced. In general, these charge carriers recombine pairs very quickly, but can be done by the band bending in the surface of a charge carrier separation. This generally react with adsorbed oxygen and water to hydroxy and peroxy Radiakalen. As a rule, except in direct charge transitions to adsorbates, the radicals react with adsorbed organics. The reaction paths until complete mineralization can be very complex and require many photons suggestions.

For outdoor use, as an example may be mentioned the photocatalytic self-cleaning, usually the UV component of sunlight is 1.5 ASTM exploited by about 3 %, at most about 35 W/m2. However, the sunlight is less than this maximum value, and in 2011 in Germany in the middle 1134 kWh/m2, ie 130 W/m2 and corresponding to 4 W/m2 UV radiation. Indoor applications usually fall out less favorable to one of the UV component is very low or when doped catalysts, reaction rate is low. The parameters in the Photokatalayse are different defined quantum yields, typical values ​​can be specified almost as many parameters included in the catalysis, but usually orders of magnitude from one reaction to 1000 photons are called. Another problem is that the make the photocatalytic reactions no distinction between the organic binder matrix and the pollutants, inadequate binder systems therefore tend to early chalking.

Other Applications

In the production of special optical glasses TiO2 is to influence the optical dispersion, Abbe used. Titanium dioxide in the anatase modification is the main component of the catalysts which are used for industrial denitrification of flue gases according to the SCR process. In the semiconductor properties of the titanium dioxide based dye solar cell ( Grätzel cell). With the help of titanium dioxide have managed to produce memristors. Titanium dioxide is also used as a main component of the ceramic dielectric in class 1 ceramic capacitors.

Risks

Titanium dioxide is not classified as toxic and not as wassergefährend.

A biological meaning, see also Titan is not yet known; however, result, very high concentrations of nanoparticles, ie particles with less than 100 nm, in the lungs immune responses. The immune response is discussed with the possibility of an inflammation- based cancer risk, which is often applied to nanoparticulate TiO2 less than 100 nm is tested and pigmentary TiO2 greater than 200 nm is used as a sample application and the economy's output.

In a group of 56 people that were selectively because of problems selected with titanium implants showed 21 people a positive reaction in the MELISA test ( lymphocyte transformation test ) with TiO2, while all 54 people in the group were tested by patch test, tested negative. In a study by the University of North Carolina has been found that titanium dioxide nanoparticles are toxic to microglia brain cells in mice.

In experiments of biologists at the University of Koblenz -Landau with Daphnia (water fleas), significant effects were partially observed despite low titanium dioxide concentrations in the water: The concentrations used up to 2 mg / L were in the experiment by a factor of up to about 1000 over the found in the environmental concentration of ng / L to a few micrograms / L. The primary effect was made with an accumulation of particles on the exoskeleton of the water fleas in the experimental group with deadly consequences. The next generation of Daphnia showed in the studies of a non- interpreted and analyzed mechanism of action also damage. These studies are partly in direct contradiction to work with even much higher concentrations to 50 mg / L.

Proof

In the cold, freshly precipitated titanium dioxide is amphoteric and soluble in dilute mineral acids. A disruption is carried out with potassium bisulfate in a porcelain crucible. Is then dissolved in cold water with a little sulfuric acid. With a few drops of hydrogen peroxide to form the yellow ( alkaline) to yellow- orange (acidic, photo) [Ti (O2) aq ] 2 cation. With hydrochloric acid and zinc ( granule ) to nascent hydrogen, the Ti ( IV) to red-violet [Ti (H2O ) 6] 3 is reduced.