- Structure
- Nomenclature
- Properties
- Physical state
- Mohs hardness
- Molecular weight
- Melting point
- Density
- Solubility
- Refractive index
- Other properties
- Applications
- - In the metallurgical industry
- - In the glass industry
- Glass polishing
- Radiation resistant glass
- Photosensitive glasses
- - In enamels
- - In zirconium ceramic
- - In catalysts for the control of vehicle emissions
- Acts as a stabilizer for high surface area alumina
- It behaves as an oxygen buffer-releaser
- Others
- - In catalysis of chemical reactions
- - In biological and biomedical applications
- - Other uses
- References
The cerium oxide (IV) oxide or ceric is a white or pale yellow solid inorganic produced by the oxidation of cerium (Ce) for oxygen to its valence 4+. The chemical formula of ceric oxide is CeO 2 and it 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, CeO 2 can be found in an approximate proportion of up to 30% by weight.
A sample of cerium (IV) oxide. Picture taken August 2005 by User: Walkerma. {{PD-self}} Source: Wikipedia Commons
CeO 2 can be easily obtained by heating cerium (III) hydroxide, Ce (OH) 3, or any salt of cerium (III), such as oxalate, carbonate or nitrate, in air or oxygen.
Stoichiometric CeO 2 can be obtained by the elevated 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 multicolored products of the formula CeO 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. CeO 2 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.
Bastnasite mineral sample. Rob Lavinsky, iRocks.com - CC-BY-SA-3.0 Source: Wikipedia Commons
Structure
The stoichiometric cerium (IV) oxide crystallizes in the fluorite-like cubic lattice (CaF 2), with 8 O 2- ions in a cubic structure coordinated with 4 Ce 4+ ions.
Cerium (IV) oxide crystalline structure. Benjah-bmm27 Source: Wikipedia Commons
Nomenclature
- Cerium (IV) oxide.
- Ceric oxide.
- Cerium dioxide.
- Ceria.
- Stoichiometric cerium oxide: material formed entirely by CeO 2.
- Non-stoichiometric cerium oxide: material formed by mixed oxides from CeO 2 to 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 / cm 3
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
CeO 2 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 2 O 2) or tin (II), among others, generating cerium (III) solutions.
It has high thermal stability. It does not undergo crystallographic changes during usual heating intervals.
Its hydrated derivative (CeO 2.nH 2 O) is a yellow and gelatinous precipitate that is obtained by treating solutions of cerium (IV) with bases.
CeO 2 is poorly absorbed from the gastrointestinal tract so it has no toxic effects.
Applications
- In the metallurgical industry
CeO 2 is used in 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 2 particles give greater reliability than tungsten alone.
- In the glass industry
Glass polishing
CeO 2 can discolor soda-lime glasses for bottles, jugs and the like. Ce (IV) oxidizes Fe (II) impurities, which provide a bluish-green color, to Fe (III) which confers a yellow color 10 times weaker.
Radiation resistant glass
The addition of 1% CeO 2 to the glass suppresses the 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 gamma-ray-induced discoloration.
The suppression mechanism is believed to depend on the presence of Ce 4+ and Ce 3+ ions in the glass lattice.
Photosensitive glasses
Some glass formulations can develop latent images that can then be converted to a permanent structure or color.
This type of glass contains CeO 2 which absorbs UV radiation and releases electrons into 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 2 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 glazes.
In this application CeO 2 provides the desired white coating during enamel burnout. It is the ingredient that provides opacity.
- In zirconium ceramic
Zirconia ceramic is a thermal insulator and is used in high temperature applications. It requires an additive to have high strength and toughness. Adding CeO 2 to zirconia produces a material with exceptional toughness and good strength.
CeO 2- doped zirconium oxide is 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.
Jet engine. Jeff Dahl, Spanish translation by Xavigivax Source: Wikipedia Commons
- In catalysts for the control of vehicle emissions
CeO 2 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 2, and reduce nitrogen oxides, NO x, to N 2 and O 2.
Catalytic converter for exhaust gases from the internal combustion engine of a motor vehicle. Ahanix1989 at English Wikipedia Source: Wikipedia Commons
Besides platinum and other catalytic metals, the main active component of these multifunctional systems is CeO 2.
Each catalytic converter contains 50-100 g of finely divided CeO 2, 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 2.
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 2 can continue, even when oxygen gas is insufficient.
Then, in the oxygen-rich cycle period, it takes up oxygen and re-oxidizes, recovering its stoichiometric form CeO 2.
Others
It works as an improver 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 2 acts as a catalytic oxidant that helps in the conversion of SO 2 to SO 3 and promotes the formation of sulfates in specific traps of the process.
CeO 2 improves the activity of the iron oxide-based catalyst that is used to obtain styrene starting from ethylbenzene. This is possibly due to the positive interaction between the Fe (II) - Fe (III) and Ce (III) - Ce (IV) oxide reduction pairs.
- In biological and biomedical applications
CeO 2 nanoparticles have been found to act 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 2 nanoparticles have been shown to be toxic against bacterial strains, being promising candidates for bactericidal applications.
- Other uses
CeO 2 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
- Cotton, F. Albert and Wilkinson, Geoffrey. (1980). Advanced Inorganic Chemistry. Fourth Edition. John Wiley & Sons.
- Dance, JC; Emeléus, HJ; Sir Ronald Nyholm and Trotman-Dickenson, AF (1973). Comprehensive Inorganic Chemistry. Volume 4. Pergamon Press.
- Kirk-Othmer (1994). Encyclopedia of Chemical Technology. Volume 5. Fourth Edition. John Wiley & Sons.
- Ullmann's Encyclopedia of Industrial Chemistry. (1990). Fifth Edition. Volume A6. VCH Verlagsgesellschaft mbH.
- Casals, Eudald et al. (2012). Analysis and Risk of Nanomaterials in Environmental and Food Samples. In Comprehensive Analytical Chemistry. Recovered from sciencedirect.com.
- Mailadil T. Sebastian. (2008). Alumina, Titania, Ceria, Silicate, Tungstate and other materials. In Dielectric Materials for Wireless Communication. Recovered from sciencedirect.com.
- Afeesh Rajan Unnithan, et al. (2015). Scaffolds with Antibacterial Properties. In Nanotechnology Applications for Tissue Engineering. Recovered from sciencedirect.com.
- 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.