What Is Tungsten Oxide?
Tungsten oxide is an inorganic compound composed of tungsten and oxygen.
There are various compounds, depending on the oxidation number of tungsten. One of the most common types of tungsten oxide is tungsten oxide (VI). Other known compounds include tungsten oxide (IV) and tungsten oxide (III).
Uses of Tungsten Oxide
Tungsten oxide is used industrially as a raw material for catalysts (such as visible light responsive photocatalysts) and tungsten metal, as well as an additive for ceramics, glass, and other sintered metals. It is also used as an additive for secondary batteries and as an electronic material. They can also be used as analytical additives for elements such as carbon, hydrogen, and nitrogen.
It is also used as a scintillator (a generic term for materials that emit fluorescence when exposed to radiation) for radiographic and nondestructive testing.
Properties of Tungsten Oxide
Tungsten oxide (VI) has a melting point of 2,683°F (1,473℃) and a boiling point of approximately 3,182° F (1,750℃). It is slightly soluble in water, soluble in alkali and ammonia water, and forms tungstates. Tungsten oxide (VI) is the final oxide of tungsten and is stable in air and an aqueous solution. However, any lower oxides can be produced by reducing agents.
It is stable in air at room temperature; it decomposes at 2,732-2,912°F (1,500 – 1,600°C) and is oxidized to tungsten oxide (VI). When scorched in a hydrogen stream, it is reduced to tungsten metal. Tungsten oxide (IV) has great electrical conductivity and is soluble in acid and potassium hydroxide solution, but insoluble in water.
Structure of Tungsten Oxide
Tungsten oxide (VI) is also called tungsten trioxide. Its chemical formula is WO3, and it is a yellow powder with a molar mass of 231.84 g/mol and a density of 7.16 g/cm3. The crystal structure of tungsten oxide (VI) varies with temperature. It is triclinic from -58°F (-50° C) to 62.6° F (17°C), monoclinic from 17 to 626°F (330°C), orthorhombic from 626-1,364°F (330 to 740°C), and tetragonal above 1,364°F (740°C).
Tungsten oxide (IV), also called tungsten dioxide, has the chemical formula WO2 and a molar mass of 215.84 g/mol. It is a bronze solid and its crystals are monoclinic, forming a distorted rutile-type structure of WO6 in octahedral coordination with short W-W bonds of 248 pm, each W center taking a d2 electron configuration.
The chemical formula for tungsten oxide (III) is W2O3 with a molar mass of 415.68 g/mol.
Other Information on Tungsten Oxide
1. Synthesis of Tungsten Oxide
Tungsten oxide (VI) is formed by heating tungsten metal, other tungsten oxide, and tungsten sulfides in air or oxygen. In addition, the reaction of CaWO4 or ash feldspar with hydrochloric acid produces tungsten acid, which decomposes into tungsten oxide (VI) when reacted with water at high temperatures. Furthermore, tungsten (VI) oxide can be synthesized by calcination of ammonium paratungstate under oxidizing conditions.
Tungsten oxide (IV) can be obtained by heating tungsten oxide (VI). Specifically, tungsten oxide (VI) is reduced by tungsten powder at 1,652° F (900°C) for 40 hours. The reaction proceeds with partial reduction via W18O49 in a mixed valence state as an intermediate in the reaction.
2. Other Compounds of Tungsten Oxide
In addition to WO3, WO2, and W2O3, other compounds of tungsten oxide have been reported, including W4O3, W3O, WO, W2O5, W3O8, W4O8, W5O9, and W5O14.
The color of tungsten oxide changes from gray, to brown, purple, blue, and yellow as the oxidation number increases. For example, blue-purple W2O5 is said to be the main component of tungsten blue.