Cerium (IV) oxide structure, properties, uses

The cerium (IV) oxide u ceric oxide is a white or pale yellow inorganic solid that is produced by the oxidation of cerium (Ce) by oxygen to its 4+ valence. The chemical formula of ceric oxide is CeO_two and is the most stable oxide of cerium.

Cerium (Ce) is an element of the series of lanthanides, which are included in the group of rare-earths. Natural source of this oxide is the mineral bastnasite. In the commercial concentrate of this mineral the CeO_two can be found in an approximate proportion of up to 30% by weight.

The CEO_two can be easily obtained by heating cerium (III) hydroxide, Ce (OH) in air or oxygen₃, or any salt of cerium (III), such as oxalate, carbonate or nitrate.

The CEO_two Stoichiometric can be obtained by high temperature reaction of cerium (III) oxide with elemental oxygen. The oxygen must be in excess and sufficient time must be allowed to complete the conversion of the various non-stoichiometric phases that are being formed..

These phases comprise multi-colored products with the CeO formula_x (where x varies between 1.5 and 2.0). They are also called CeO_2-x, where x can have a value of up to 0.3. The CEO_two It is the most widely used form of Ce in the industry. It has a low toxicity classification, especially due to its poor solubility in water..

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 Refractive index
  • 3.8 Other properties
• 4 Uses
  • 4.1 - In the metallurgical industry
  • 4.2 - In the glass industry
  • 4.3 - In enamels
  • 4.4 - In zirconium ceramic
  • 4.5 - In catalysts for vehicle emission control
  • 4.6 - In catalysis of chemical reactions
  • 4.7 - In biological and biomedical applications
  • 4.8 - Other uses
• 5 References

Structure

Stoichiometric cerium (IV) oxide crystallizes in the fluorite-like cubic lattice (CaF_two), with 8 O ions^two- in a cubic structure coordinated with 4 Ce ions⁴⁺.

Nomenclature

- Cerium (IV) oxide.

- Ceric oxide.

- Cerium dioxide.

- Ceria.

- Stoichiometric cerium oxide: material made entirely of CeO_two.

- Non-stoichiometric cerium oxide: material formed by mixed oxides from CeO_two up to the CeO_1.5

Properties

Physical state

Pale yellow solid. Color is sensitive to stoichiometry and the presence of other lanthanides. Non-stoichiometric oxides are often blue.

Mohs hardness

6-6.1 approximately.

Molecular weight

172.12 g / mol.

Melting point

2600 ºC approximately.

Density

7.132 g / cm³

Solubility

Insoluble in hot and cold water. Soluble in concentrated sulfuric acid and concentrated nitric acid. Insoluble in dilute acids.

Refractive index

2.2.

Other properties

The CEO_two It is an inert substance, it is not attacked by strong acids or alkalis. However, it can be dissolved by acids in the presence of reducing agents, such as hydrogen peroxide (H_twoOR_two) or tin (II), among others, generating solutions of cerium (III).

It has high thermal stability. Does not undergo crystallographic changes during usual heating intervals.

Its hydrated derivative (CeO_two.nH_twoO) is a yellow and gelatinous precipitate that is obtained by treating solutions of cerium (IV) with bases.

The CEO_two it is poorly absorbed from the gastrointestinal tract so it has no toxic effects.

Applications

- In the metallurgical industry

The CEO_two used on the electrodes of certain welding technologies, such as inert gas tungsten arc welding.

The oxide is finely dispersed throughout the tungsten matrix. At low voltages these CeO particles_two give greater reliability than tungsten alone.

- In the glass industry

Glass polishing

Cerium oxide is the most efficient polishing agent for most commercial glass compositions. The CEO_two has almost completely replaced other polishing oxides, such as Fe_twoOR₃, silica, and ZrO_two, due to its higher polishing speed and cleaning, which increase as the degree of purity of the oxide increases.

Commercial glass polishes based on cerium oxide powders have defined particle sizes and controlled dispersibility in aqueous systems..

The glass polishing process requires water and what is removed or reformed is a softer hydrated surface layer. The polishing agent should have a Mohs hardness of approximately 6.5, close to the hardness of most glass.

Cerium oxide in water contains the Ce (IV) / Ce (III) pair that with its oxidation-reduction reactions can provide chemical assistance during the breakdown of the glass silicate network.

The CEO_two with a high degree of purity it is used to treat mirrors, television sets, ophthalmic lenses and precision optical material.

Glass discoloration

The CEO_two can discolor soda-lime glass for bottles, jugs and the like. Ce (IV) oxidizes Fe (II) impurities, which provide a bluish-green color, to Fe (III) which gives a 10 times weaker yellow color.

Radiation resistant glass

The addition of 1% CeO_two al glass suppresses discoloration or darkening of the glass caused by the bombardment of high energy electrons in TV glasses. The same is true of glass used in windows in hot cells in the nuclear industry, as it suppresses discoloration induced by gamma rays..

The suppression mechanism is believed to depend on the presence of Ce ions⁴⁺ and Ce³⁺ in the glass net.

Photosensitive glasses

Some glass formulations can develop latent images which can then be converted to a permanent structure or color..

This type of glass contains CeO_two which absorbs UV radiation and releases electrons in the glass matrix.

After treatment, the growth of crystals of other compounds in the glass is generated, creating detailed patterns for electronic or decorative uses..

- In enamels

Due to its high refractive index, CeO_two is an opacifying agent in enamel compositions used as protective coatings on metals.

Its high thermal stability and its unique crystallographic shape throughout the entire range of temperatures reached during the glazing process, make it suitable for use in porcelain enamels..

In this application the CeO_two Provides the desired white coating during enamel burnout. It is the ingredient that provides opacity.

- In zirconium ceramic

Zirconium ceramic is a thermal insulator and is used in high temperature applications. It requires an additive to have high strength and toughness. Adding CeO_two zirconium oxide produces a material with exceptional toughness and good resistance.

CeO-doped zirconium oxide_two used in coatings to act as a thermal barrier on metal surfaces.

For example, in aircraft engine parts, these coatings protect from the high temperatures to which metals would be exposed..

- In catalysts for vehicle emission control

The CEO_two It is an active component in the removal of pollutants from vehicle emissions. This is largely due to its ability to store or release oxygen depending on the conditions around it..

The catalytic converter in motor vehicles is located between the engine and the exhaust gas outlet. It has a catalyst that must oxidize unburned hydrocarbons, convert CO to CO_two, and reduce nitrogen oxides, NO_x, to N_two me_two.

In addition to platinum and other catalytic metals, the main active component of these multifunctional systems is CeO_two.

Each catalytic converter contains 50-100 g of CeO_two finely divided, which serves several functions. The most important ones are:

Acts as a stabilizer for high surface area alumina

High surface area alumina tends to sinter, losing its high surface area during high temperature operation. This is delayed by the presence of CeO_two.

It behaves as an oxygen buffer-releaser

Due to its ability to form non-stoichiometric oxides CeO_2-x, cerium (IV) oxide provides elemental oxygen of its own structure during the oxygen lean / fuel rich cycle period.

Thus, the oxidation of unburned hydrocarbons coming from the engine and the conversion of CO into CO can continue._two, even when gaseous oxygen is insufficient.

Then, in the oxygen-rich cycle period, it takes up the oxygen and re-oxidizes, recovering its stoichiometric form CeO_two.

Others

It works as an enhancer of the catalytic capacity of rhodium in the reduction of nitrogen oxides NO_x to nitrogen and oxygen.

- In catalysis of chemical reactions

In the catalytic cracking processes of refineries, CeO_two acts as a catalytic oxidant that aids in the conversion of SO_two to SO₃ and promotes the formation of sulfates in specific traps of the process.

The CEO_two improves the activity of the iron oxide-based catalyst used to obtain styrene starting from ethylbenzene. This is possibly due to the positive interaction between the oxide-reduction pairs Fe (II) - Fe (III) and Ce (III) - Ce (IV).

- In biological and biomedical applications

CeO nanoparticles have been found_two work by scavenging free radicals, such as superoxide, hydrogen peroxide, hydroxyl and nitric oxide radical.

They can protect biological tissues from radiation-induced damage, laser-induced retinal damage, increase the life span of photoreceptor cells, reduce spinal injuries, reduce chronic inflammation, and promote angiogenesis or blood vessel formation..

Additionally, certain nanofibers containing CeO nanoparticles_two have been shown to be toxic against bacterial strains, being promising candidates for bactericidal applications.

- Other uses

The CEO_two It is an electrical insulating material due to its excellent chemical stability, high relative permittivity (it has a high tendency to polarize when an electric field is applied) and a crystalline lattice similar to silicon..

It has found application in capacitors and damping layers of superconducting materials.

It is also used in gas sensors, solid oxide fuel cell electrode materials, oxygen pumps and oxygen monitors..

References

1. Cotton, F. Albert and Wilkinson, Geoffrey. (1980). Advanced Inorganic Chemistry. Fourth Edition. John Wiley & Sons.
2. Bailar, J.C .; Emeléus, H.J .; Sir Ronald Nyholm and Trotman-Dickenson, A.F. (1973). Comprehensive Inorganic Chemistry. Volume 4. Pergamon Press.
3. Kirk-Othmer (1994). Encyclopedia of Chemical Technology. Volume 5. Fourth Edition. John Wiley & Sons.
4. Ullmann's Encyclopedia of Industrial Chemistry. (1990). Fifth Edition. Volume A6. VCH Verlagsgesellschaft mbH.
5. Casals, Eudald et al. (2012). Analysis and Risk of Nanomaterials in Environmental and Food Samples. In Comprehensive Analytical Chemistry. Recovered from sciencedirect.com.
6. Mailadil T. Sebastian. (2008). Alumina, Titania, Ceria, Silicate, Tungstate and other materials. In Dielectric Materials for Wireless Communication. Recovered from sciencedirect.com.
7. Afeesh Rajan Unnithan, et al. (2015). Scaffolds with Antibacterial Properties. In Nanotechnology Applications for Tissue Engineering. Recovered from sciencedirect.com.
8. Gottardi V., et al. (1979). Polishing the surface of a glass investigated with a nuclear technique. Bulletin of the Spanish Society of Ceramics and Glass, Vol. 18, No. 3. Recovered from boletines.secv.es.