Chromium (iii) Oxide: Structure, Nomenclature, Properties, Uses

The chromium (III) oxide or chromic is an inorganic green solid formed burning metal chromium (Cr) in oxygen (O 2 ), leaving the chromium oxidation state 3+. Its chemical formula is Cr 2 O 3In 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 Cr 2 O 3 hydrated (Cr 2 O 3 .nH 2 O) to completely remove the water. It is also obtained as a product of the calcination of chromium (VI) oxide (CrO 3 ). 

However, the best way to obtain it pure is by decomposition of ammonium dichromate (NH 4 ) 2 Cr 2 O 7 at 200 ºC. Industrially it is produced by reduction of solid sodium dichromate (Na 2 Cr 2 O 7 ) 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.

Structure

The α-Cr 2 O 3 oxide has the corundum-like structure. Its crystal system is hexagonal rhombohedral. It is isomorphic with α-alumina and α-Fe 2 O 3 .

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

Nomenclature

– Chromium (III) oxide.

– Green chromium oxide.

– Dichrome trioxide.

– Chromium Sesquioxide.

– Chromia.

– Eskolaíta: Chromium (III) oxide mineral.

– The hydrate: Cr 2 O 3 .nH 2 O (where n ≅ 2) is called chromium (III) oxide hydrate or Guignet Green.

Properties

Physical state

Crystalline solid.

Mohs hardness

9 (its crystals are extremely hard).

Molecular weight

151.99 g / mol.

Melting point

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

Density

5.22 g / cm 3

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; sSoluble in perchloric acid (HClO 4 ) at 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 Cr 2 O 3 and its hydrated form Cr 2 O 3 .nH 2 O are amphoteric, easily dissolving in acid to give aqua-ions [Cr (H 2 O) 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 SO 2 .

– It has an outstanding resistance to light because 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 Cr 2 O 3 .nH 2 O (where n ≅ 2) does not have thermal stability, its hydration water limits its applicability to less than 260 ºC. It has low dyeability and a 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 resistance to alkalis.

– Cr 2 O 3 is not classified as a hazardous material and is considered an inert fine powder. It is not subject to international transportation regulations.

– It does not irritate the skin or mucous membranes.

Applications

In the ceramics and glass industry

Due to its high resistance to heat and chemical resistance, calcined Cr 2 O 3 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 to protect against abrasive wear (when material removal is caused by particles moving across a surface).

In these cases, the application of a Cr 2 O 3 coating by plasma deposition generates a high resistance to abrasion.

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

In the refractory industry

It is used in the production of thermal 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. Also sIt is 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 for 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 2 O 3 is introduced as an impurity in the crystal lattice of α-Al 2 O 3 , 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 on alumina (Al 2 O 3 ) 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. It catalyzes the reaction of nitrogen (N 2 ) with hydrogen (H 2 ) to form ammonia (NH 3 ).

Due to its oxide-reduction capacity, acting together with chromium (VI) oxide, it plays an important role in the dehydrogenation of alkanes with CO 2 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.

On magnetic materials

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

Recent innovations

Pigments that have improved reflectance of the near IR have been obtained by doping nanoparticles of Cr 2 O 3 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 2 O 3 pigment .

This allows the doped Cr 2 O 3 to be classified as a “cold” pigment, as it is suitable for controlling heat accumulation.

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). CO 2 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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