Chemical transport reaction

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Gold crystals grown by chemical transport using chlorine as the transport agent. Gold-crystals.jpg
Gold crystals grown by chemical transport using chlorine as the transport agent.
Schematic diagram of the CVT process. Point A is the reaction between the starting materials and the transport agent to form volatile intermediates. These intermediates then are free to move around the inside of the tube via diffusion or convection (point B), and when they reach point C some of the gaseous species react to form solid prodcts. CVT DiagramCVTDiagram.png
Schematic diagram of the CVT process. Point A is the reaction between the starting materials and the transport agent to form volatile intermediates. These intermediates then are free to move around the inside of the tube via diffusion or convection (point B), and when they reach point C some of the gaseous species react to form solid prodcts.

In chemistry, a chemical transport reaction describes a process for purification and crystallization of non-volatile solids. [1] The process is also responsible for certain aspects of mineral growth from the effluent of volcanoes. The technique is distinct from chemical vapor deposition, which usually entails decomposition of molecular precursors (e.g. SiH4 → Si + 2 H2) and which gives conformal coatings. The technique, which was popularized by Harald Schäfer, [2] entails the reversible conversion of nonvolatile elements and chemical compounds into volatile derivatives. [3] The volatile derivative migrates throughout a sealed reactor, typically a sealed and evacuated glass tube heated in a tube furnace. Because the tube is under a temperature gradient, the volatile derivative reverts to the parent solid and the transport agent is released at the end opposite to which it originated (see next section). The transport agent is thus catalytic. The technique requires that the two ends of the tube (which contains the sample to be crystallized) be maintained at different temperatures. So-called two-zone tube furnaces are employed for this purpose. The method derives from the Van Arkel de Boer process [4] which was used for the purification of titanium and vanadium and uses iodine as the transport agent.

Contents

Crystals of titanium grown using the Van Arkel-de Boer process with I2 as the transport agent. Titan-crystal bar.JPG
Crystals of titanium grown using the Van Arkel-de Boer process with I2 as the transport agent.

Cases of the exothermic and endothermic reactions of the transporting agent

Transport reactions are classified according to the thermodynamics of the reaction between the solid and the transporting agent. When the reaction is exothermic, then the solid of interest is transported from the cooler end (which can be quite hot) of the reactor to a hot end, where the equilibrium constant is less favorable and the crystals grow. The reaction of molybdenum dioxide with the transporting agent iodine is an exothermic process, thus the MoO2 migrates from the cooler end (700 °C) to the hotter end (900 °C):

MoO2 + I2⇌ MoO2I2 ΔHrxn < 0 (exothermic)

Using 10 milligrams of iodine for 4 grams of the solid, the process requires several days.

Alternatively, when the reaction of the solid and the transport agent is endothermic, the solid is transported from a hot zone to a cooler one. For example:

Fe2O3 + 6 HCl ⇌ Fe2Cl6+ 3 H2O ΔHrxn > 0 (endothermic)

The sample of iron(III) oxide is maintained at 1000 °C, and the product is grown at 750 °C. HCl is the transport agent. Crystals of hematite are reportedly observed at the mouths of volcanoes because of chemical transport reactions whereby volcanic hydrogen chloride volatilizes iron(III) oxides. [5]

Halogen lamp

A similar reaction like that of MoO2 is used in halogen lamps. The tungsten is evaporated from the tungsten filament and converted with traces of oxygen and iodine into the WO2I2, at the high temperatures near the filament the compound decomposes back to tungsten, oxygen and iodine. [6]

WO2 + I2⇌ WO2I2, ΔHrxn < 0 (exothermic)

Related Research Articles

Van Arkel–de Boer process Process for the commercial production of pure titanium and zirconium

The van Arkel–de Boer process, also known as the iodide process or crystal-bar process, was the first industrial process for the commercial production of pure ductile titanium, zirconium and some other metals. It was developed by Anton Eduard van Arkel and Jan Hendrik de Boer in 1925. Now it is used in the production of small quantities of ultrapure titanium and zirconium. It primarily involves the formation of the metal iodides and their subsequent decomposition to yield pure metal.

Chlorine monofluoride Chemical compound

Chlorine monofluoride is a volatile interhalogen compound with the chemical formula ClF. It is a colourless gas at room temperature and is stable even at high temperatures. When cooled to −100 °C, ClF condenses as a pale yellow liquid. Many of its properties are intermediate between its parent halogens, Cl2 and F2.

Iodine pentafluoride is an interhalogen compound with chemical formula IF5. It is one of the fluorides of iodine. It is a colorless liquid, although impure samples appear yellow. It is used as a fluorination reagent and even a solvent in specialized syntheses.

Titanium tetraiodide Chemical compound

Titanium tetraiodide is an inorganic compound with the formula TiI4. It is a black volatile solid, first reported by Rudolph Weber in 1863. It is an intermediate in the van Arkel–de Boer process for the purification of titanium.

Zirconium(IV) iodide Chemical compound

Zirconium(IV) iodide is the chemical compound with the formula ZrI4. It is the most readily available iodide of zirconium. It is an orange-coloured solid that degrades in the presence of water. The compound was once prominent as an intermediate in the purification of zirconium metal.

Titanium(II) chloride Chemical compound

Titanium(II) chloride is the chemical compound with the formula TiCl2. The black solid has been studied only moderately, probably because of its high reactivity. Ti(II) is a strong reducing agent: it has a high affinity for oxygen and reacts irreversibly with water to produce H2. The usual preparation is the thermal disproportionation of TiCl3 at 500 °C. The reaction is driven by the loss of volatile TiCl4:

Gallium(III) iodide Chemical compound

Gallium(III) iodide is the inorganic compound with the formula GaI3. A yellow hygroscopic solid, it is the most common iodide of gallium. In the chemical vapor transport method of growing crystals of gallium arsenide uses iodine as the transport agent. In the solid state, it exists as the dimer Ga2I6.

Tungsten dichloride dioxide Chemical compound

Tungsten dichloride dioxide, or Tungstyl chloride is the chemical compound with the formula WO2Cl2. It is a yellow-colored solid. It is used as a precursor to other tungsten compounds. Like other tungsten halides, WO2Cl2 is sensitive to moisture, undergoing hydrolysis.

Jan Hendrik de Boer Dutch physicist and chemist

Jan Hendrik de Boer was a Dutch physicist and chemist.

Potassium amide Chemical compound

Potassium amide is an inorganic compound with the chemical formula KNH2. Like other alkali metal amides, it is a white solid that hydrolyzes readily. It is a strong base.

Vanadium(II) iodide Chemical compound

Vanadium(II) iodide is the inorganic compound with the formula VI2. It is a black micaceous solid. It adopts the cadmium iodide structure, featuring octahedral V(II) centers. The hexahydrate is also known. It forms purple crystals.

Titanium(II) iodide Chemical compound

Titanium(II) iodide is the inorganic compound with the formula TiI2. It is a black micaceous solid. It adopts the cadmium iodide structure, featuring octahedral Ti(II) centers. It arises via the reaction of the elements:

The telluride iodides are chemical compounds that contain both telluride ions (Te2−) and iodide ions (I−). They are in the class of mixed anion compounds or chalcogenide halides.

Nitride fluorides containing nitride and fluoride ions with the formula NF4-. They can be electronically equivalent to a pair of oxide ions O24-. Nitride fluorides were discovered in 1996 by Lavalle et al. They heated diammonium technetium hexafluoride to 300 °C to yield TcNF. Another preparation is to heat a fluoride compound with a nitride compound in a solid state reaction. The fluorimido ion is F-N2- and is found in a rhenium compound.

Transition metal chloride complex Coordination complex

In chemistry, a transition metal chloride complex is a coordination complex that consists of a transition metal coordinated to one or more chloride ligand. The class of complexes is extensive.

Vanadium(V) chloride chlorimide Chemical compound

Vanadium (V) chloride chlorimide is a chemical compound containing vanadium in a +5 oxidation state bound to three chlorine atoms and with a double bond to a chlorimide group (=NCl). It has formula VNCl4. This can be also considered as a chloroiminato complex.

A chloride nitride is a mixed anion compound containing both chloride (Cl−) and nitride ions (N3−). Another name is metallochloronitrides. They are a subclass of halide nitrides or pnictide halides.

Arsenidogermanates are chemical compounds that contain anions with arsenic bonded to germanium. They are in the category of tetrelarsenides, pnictidogermanates, or tetrelpnictides.

Carbide chlorides are mixed anion compounds containing chloride anions and anions consisting entirely of carbon. In these compounds there is no bond between chlorine and carbon. But there is a bond between a metal and carbon. Many of these compounds are cluster compounds, in which metal atoms encase a carbon core, with chlorine atoms surrounding the cluster. The chlorine may be shared between clusters to form polymers or layers. Most carbon chloride compounds contain rare earth elements. Some are known from group 4 elements. The hexatungsten carbon cluster can be oxidised and reduced, and so have different numbers of chlorine atoms included.

Carbide iodides are mixed anion compounds containing iodide and carbide anions. Many carbide iodides are cluster compounds, containing one, two or more carbon atoms in a core, surrounded by a layer of metal atoms, encased in a shell of iodide ions. These ions may be shared between clusters to form chains, double chains or layers.

References

  1. ↑ Michael Binnewies, Robert Glaum, Marcus Schmidt, Peer Schmidt "Chemical Vapor Transport Reactions – A Historical Review" Zeitschrift für anorganische und allgemeine Chemie 2013, Volume 639, pages 219–229. doi : 10.1002/zaac.201300048
  2. ↑ Günther Rienäcker, Josef Goubeau (1973). "Professor Harald Schäfer". Zeitschrift für anorganische und allgemeine Chemie . 395 (2–3): 129–133. doi:10.1002/zaac.19733950202.
  3. ↑ Schäfer, H. "Chemical Transport Reactions" Academic Press, New York, 1963.
  4. ↑ van Arkel, A. E.; de Boer, J. H. (1925). "Darstellung von reinem Titanium-, Zirkonium-, Hafnium- und Thoriummetall". Zeitschrift für anorganische und allgemeine Chemie (in German). 148 (1): 345–350. doi:10.1002/zaac.19251480133.
  5. ↑ P. Kleinert, D. Schmidt (1966). "Beiträge zum chemischen Transport oxidischer Metallverbindungen. I. Der Transport von α-Fe2O3 über dimeres Eisen(III)-chlorid". Zeitschrift für anorganische und allgemeine Chemie . 348 (3–4): 142–150. doi:10.1002/zaac.19663480305.
  6. ↑ J. H. Dettingmeijer, B. Meinders (1968). "Zum system W/O/J. I: das Gleichgewicht WO2, f + J2, g = WO2J2,g". Zeitschrift für anorganische und allgemeine Chemie . 357 (1–2): 1–10. doi:10.1002/zaac.19683570101.
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