Chromium (III) oxide structure, nomenclature, properties, uses

4716
Anthony Golden

The chromium (III) oxide Chromic oxide is a green inorganic solid that is formed by burning chromium metal (Cr) in oxygen (Otwo), leaving the chromium with a 3+ oxidation state. Its chemical formula is CrtwoOR3. In nature it is found in the mineral Eskolaíta. No usable natural deposits of chromium (III) oxide are known.

It can be prepared among other ways by heating CrtwoOR3 hydrated (CrtwoOR3.nHtwoO) to completely remove the water. It is also obtained as a product of the calcination of chromium (VI) oxide (CrO3). 

Chromium (III) oxide pigment. FK1954 [Public domain]. Source: Wikipedia Commons

However, the best way to obtain it pure is by decomposition of ammonium dichromate (NH4)twoCrtwoOR7 at 200 ° C. Industrially it is produced by reduction of sodium dichromate (NatwoCrtwoOR7) solid with sulfur.

When it is finely divided, it has a bright green color with a yellowish tinge. But if the particles are larger, it exhibits a bluish tint. Chromic oxide is the most stable green pigment known. Its thermal and chemical resistance make it a valuable ceramic colorant.

It is used in industrial coatings, varnishes, in the construction industry, in jewelry, as a colorant in cosmetics or pharmaceutical products, among other applications.

Article index

  • 1 Structure
  • 2 Nomenclature
  • 3 Properties
    • 3.1 Physical state
    • 3.2 Mohs hardness
    • 3.3 Molecular weight
    • 3.4 Melting point
    • 3.5 Density
    • 3.6 Solubility
    • 3.7 pH
    • 3.8 Refractive index
    • 3.9 Other properties
  • 4 Uses
    • 4.1 In the ceramics and glass industry
    • 4.2 In industrial coatings
    • 4.3 In the refractory industry
    • 4.4 Under construction
    • 4.5 As a pigment in various applications
    • 4.6 In jewelry
    • 4.7 In catalysis of chemical reactions
    • 4.8 In the manufacture of chromium
    • 4.9 On magnetic materials
    • 4.10 Recent innovations
  • 5 References

Structure

Α-Cr oxidetwoOR3 It has the corundum type structure. Its crystal system is hexagonal rhombohedral. It is isomorphic with α-alumina and α-FetwoOR3.

Eskolaite, a natural mineral of chromium (III) oxide, has the structure shown below:

Crystalline structure of the mineral Eskolaíta. https://upload.wikimedia.org/wikipedia/commons/e/ee/Eskolaite_structure.jpg. Source: Wikipedia Commons

Nomenclature

- Chromium (III) oxide.

- Chromium oxide green.

- Dichrome trioxide.

- Chromium Sesquioxide.

- Chromia.

- Eskolaíta: Chromium (III) oxide mineral.

- Hydrate: CrtwoOR3.nHtwoO (where n ≅ 2) is called Chromium (III) Oxide Hydrate or Guignet Green.

Chromium (III) oxide hydrate. W. Oelen [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]. Source: Wikipedia Commons

Properties

Physical state

Crystalline solid.

Mohs hardness

9 (its crystals are extremely hard).

Molecular weight

151.99 g / mol.

Melting point

It melts at 2435 ° C, but begins to evaporate at 2000 ° C, forming clouds of green smoke..

Density

5.22 g / cm3

Solubility

When it has been heated to high temperatures, it is practically insoluble in water (3 micrograms / L at 20ºC); insoluble in alcohols and acetone; slightly soluble in acids and alkalis; s Soluble in perchloric acid (HClO4) to 70%, in which it decomposes.

pH

6.

Refractive index

2,551.

Other properties

- If it is calcined strongly it becomes inert towards acids and bases. Otherwise the CrtwoOR3 and its hydrated form CrtwoOR3.nHtwoOr they are amphoteric, easily dissolving in acid to give aqua-ions [Cr (HtwoOR)6]3+, and in concentrated alkali to form "chromites".

- When calcined, it is chemically resistant to acids, alkalis, and high temperatures. It is extremely stable to SOtwo.

- It has outstanding resistance to light due to the fact that its crystals have opacity, high UV attenuation and transparency to visible light..

- It is an extremely hard material, it can scratch quartz, topaz and zirconium.

- Its hydrate CrtwoOR3.nHtwoO (where n ≅ 2) does not have thermal stability, its hydration water limits its applicability to less than 260 ºC. Low dyeability and limited range of shades.

- But this hydrate has a very clean and bright blue-green hue. It is semi-transparent, has low opacity, excellent lightfastness and alkali resistance.

- The crtwoOR3 It is not classified as a hazardous material and is considered an inert fine powder. It is not subject to international transport regulations.

- Does not irritate the skin or mucous membranes.

Applications

In the ceramics and glass industry

Due to its high heat resistance and chemical resistance, CrtwoOR3 calcined is used as a colorant or vitrifiable pigment in the manufacture of ceramics, in porcelain enamels and glass mixtures.

In industrial coatings

Chromium (III) oxide ceramic provides excellent resistance against most corrosive environments. All this through the mechanism of exclusion of the substrate from the environment that surrounds it..

For this reason, it is used in coatings to prevent corrosion of many materials, being applied by thermal spraying (atomization or hot spray)..

It is also used as protection against abrasive wear (when material removal is caused by particles moving across a surface).

In these cases, the application of a coating of CrtwoOR3 through plasma deposition, generates high resistance to abrasion.

The two previous cases are useful, for example, in gas turbine engines in the aerospace industry..

In the refractory industry

Used in the production of thermally and chemically resistant bricks, facing materials and alumina-based refractory concrete.

In construction

Since it is extremely resistant to atmospheric conditions, light and heat, it is applied as a granulated rock colorant for asphalt roofs, concrete cement, high quality industrial coatings for exteriors, steel constructions and facade coatings (emulsifiable paints)..

As a pigment in various applications

It can withstand vulcanization conditions and does not degrade, which is why it is used in rubber pigmentation.

Because it is non-toxic, it is used as a pigment for toys, cosmetics (especially its hydrate), plastics, printing inks, paints that come into contact with food and pharmaceutical products..

In the pigment industry it is used as a raw material to produce penetrating dyes containing chromium and in pigments based on mixed metal oxide phases. It is also used as a colorant in paints for coating coils.

Its hydrate has a transparency that allows the formulation of polychromatic finishes in the automotive industry (metallic finishes of automobiles).

Due to its unique characteristic of reflecting infrared radiation (IR) in a similar way to chlorophyll in plants, under infrared light it looks like foliage. For this reason, it is widely used in camouflage paints or coatings for military applications..

In jewelry

It is used as a colorant for synthetic gems. When Cr is introducedtwoOR3 as an impurity in the crystal lattice of α-AltwoOR3, as in the semi-precious mineral ruby, the color is red instead of green.

It is also used as a grinding and polishing agent for its high hardness and abrasive properties..

In catalysis of chemical reactions

Supported in alumina (AltwoOR3) or other oxides, it is used in organic chemistry as a catalyst, for example, in the hydrogenation of esters or aldehydes to form alcohols and in the cyclization of hydrocarbons. Catalyzes the nitrogen reaction (Ntwo) with hydrogen (Htwo) to form ammonia (NH3).

Due to its oxidation-reduction capacity, acting together with chromium (VI) oxide, it plays an important role in the dehydrogenation of alkanes with CO.two to produce propene and isobutene, since the deactivation-reactivation cycle of the catalyst is easily executable. It is also used as a catalyst in inorganic chemistry.

In the manufacture of chrome

It is used in the aluminothermic production of pure chromium metal. For this, it must be heated to 1000 ºC in order to increase its grain size..

Preparation of chromium metal by aluminothermic reduction of chromium (III) oxide. Rando Tuvikene [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]. Source: Wikipedia Commons

On magnetic materials

It has been added in small quantities to magnetic materials in audio and video tapes, imparting a self-cleaning effect to the sound heads..

Recent innovations

Pigments having improved near IR reflectance have been obtained by doping Cr nanoparticlestwoOR3 with salts of elements belonging to the group of rare earths, such as lanthanum and praseodymium.

By increasing the concentration of these elements, the near infrared solar reflectance increases without affecting the green color of the Cr pigment.twoOR3.

This allows CrtwoOR3 doped as a “cold” pigment, as it is suitable for controlling heat build-up.

Applied to ceilings, automobiles and upholstery, among other applications, it achieves a high reflectance of IR sunlight, which allows to considerably reduce the increase in heat in the environments.

References

  1. Cotton, F. Albert and Wilkinson, Geoffrey. (1980). Advanced Inorganic Chemistry. Fourth Edition. John Wiley & Sons.
  2. Kirk-Othmer (1994). Encyclopedia of Chemical Technology. Volume 19. Fourth Edition. John Wiley & Sons.
  3. Ullmann's Encyclopedia of Industrial Chemistry. (1990). Fifth Edition. Volume A7 and A20. VCH Verlagsgesellschaft mbH.
  4. American Elements. (2019). Chromium (III) Oxide. Recovered from americanelements.com.
  5. National Library of Medicine. (2019). Chromium (III) Oxide. Recovered from: pubchem.ncbi.nlm.nih.gov.
  6. Dorfman, Mitchell R. (2012). Thermal Spray Coatings. In Handbook of Environmental Degradation of Materials. Chapter 19. Recovered from sciencedirect.com.
  7. Takehira, K. et al. (2004). COtwo Dehydrogenation of Propane over Cr-MCM-41 Catalyst. In Studies in Surface Science and Catalysis 153. Recovered from sciencedirect.com.
  8. Selvam Sangeetha et al. (2012). Functional pigments from chromium (III) oxide nanoparticles. Dyes and Pigments 94 (2012) 548-552. Recovered from sciencedirect.com.

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