Cerium(IV) oxide
Encyclopedia
Cerium oxide, also known as ceric oxide, ceria, cerium oxide or cerium dioxide, is an oxide
of the rare earth metal cerium
. It is a pale yellow-white powder with the chemical formula CeO2.
Cerium(IV) oxide is formed by the calcination
of cerium oxalate
or cerium hydroxide.
Powdered ceria is slightly hygroscopic and will also absorb a small amount of carbon dioxide
from the atmosphere
.
Cerium also forms cerium(III) oxide
, Ce2O3, but CeO2 is the most stable phase at room temperature and under atmospheric conditions.
s, to sensitize photosensitive glass
, as a catalyst and as a catalyst support, to polish
glass and stones, in lapidary
as an alternative to "jeweller's rouge". It is also known as "optician's rouge".
It is also used in the walls of self-cleaning ovens as a hydrocarbon catalyst during the high-temperature cleaning process.
While it is transparent for visible light, it absorbs ultraviolet
radiation strongly, so it is a prospective replacement of zinc oxide
and titanium dioxide
in sunscreen
s, as it has lower photocatalytic
activity. However, its thermal catalytic properties have to be decreased by coating the particles with amorphous silica or boron nitride
.
The use of these nanoparticle
s, which can penetrate the body and reach internal organs, has been criticized as unsafe.
Cerium oxide has found use in Infrared filters
, as an oxidizing species in Catalytic converters and as a replacement for Thorium dioxide
in incandescent mantles
s. However, doped ceria has an extended electrolytic region (area of predominant ionic conductivity), over that of ceria, that allows its use as an electrolyte
in SOFCs. Substituting a fraction of the ceria with gadolinium
or samarium
will introduce oxygen vacancies in the crystal without adding electronic charge carriers. This increases the ionic conductivity and results in a better electrolyte.
Under reducing
conditions, those experienced on the anode
side of the fuel cell
, a large amount of oxygen vacancies within the ceria electrolyte can be formed. Some of the cerium(IV) oxide is also reduced
to cerium(III) oxide
under these conditions which consequently increases the electronic conductivity of the material. The lattice constant of ceria increases under reducing
conditions as well as with decreasing nanocrystal
size in nanocrystalline
ceria, as a result of reduction of the cerium cation from a 4+ to a 3+ state in order to charge compensate for oxygen vacancy formation.
s in automotive applications. Since ceria can become non-stoichiometric
in oxygen content (i.e. it can give up oxygen without decomposing) depending on its ambient partial pressure
of oxygen, it can release or take in oxygen in the exhaust stream of a combustion engine. In association with other catalysts, ceria can effectively reduce NOx
emissions as well as convert harmful carbon monoxide
to the less harmful carbon dioxide
. Ceria is particularly interesting for catalytic conversion economically because it has been shown that adding comparatively inexpensive ceria can allow for substantial reductions in the amount of platinum needed for complete oxidation of NOx and other harmful products of incomplete combustion.
Due to its fluorite structure, the oxygen atoms in a ceria crystal are all in a plane with one another, allowing for rapid diffusion as a function of the number of oxygen vacancies. As the number of vacancies increases, the ease at which oxygen can move around in the crystal increases, allowing the ceria to reduce and oxidize molecules or co-catalysts on its surface. It has been shown that the catalytic activity of ceria is directly related to the number of oxygen vacancies in the crystal, frequently measured by using X-Ray Photoelectron Spectroscopy to compare the ratios of Ce3+ to Ce4+ in the crystal.
Ceria can also be used as a co-catalyst in a number of reactions, including the water-gas shift and steam reforming of ethanol or diesel fuel into hydrogen gas and carbon dioxide (with varying combinations of rhodium oxide, iron oxide, cobalt oxide, nickel oxide, platinum, and gold), the Fischer-Tropsch reaction, and selected oxidation (particularly with lanthanum). In each case, it has been shown that increasing the ceria oxygen defect concentration will result in increased catalytic activity, making it very interesting as a nanocrystalline co-catalyst due to the heightened number of oxygen defects as crystallite size decreases—at very small sizes, as many as 10% of the oxygen sites in the fluorite structure crystallites will be vacancies, resulting in exceptionally high diffusion rates.
or CeO2/Ce2O3 cycle is a two step thermochemical
water splitting
process based on cerium(IV) oxide and cerium(III) oxide
for hydrogen production
.
phase of ceria, it exhibits several defects depending on partial pressure of oxygen. The primary defects of concern are oxygen vacancies and small polarons (electrons localized on cerium cations) because these two are located in the "useful" range of ceria. In the case of oxygen defects, the increased diffusion rate of oxygen in the lattice causes increased catalytic activity as well as an increase in ionic conductivity, making ceria interesting as a fuel cell electrolyte in solid-oxide fuel cells.
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
of the rare earth metal cerium
Cerium
Cerium is a chemical element with the symbol Ce and atomic number 58. It is a soft, silvery, ductile metal which easily oxidizes in air. Cerium was named after the dwarf planet . Cerium is the most abundant of the rare earth elements, making up about 0.0046% of the Earth's crust by weight...
. It is a pale yellow-white powder with the chemical formula CeO2.
Cerium(IV) oxide is formed by the calcination
Calcination
Calcination is a thermal treatment process applied to ores and other solid materials to bring about a thermal decomposition, phase transition, or removal of a volatile fraction. The calcination process normally takes place at temperatures below the melting point of the product materials...
of cerium oxalate
Cerium oxalate
Cerium oxalate is the inorganic cerium salt of oxalic acid. It is a white crystalline solid with the chemical formula of Ce23. It may be formed by the reaction of oxalic acid and cerium oxide.Cerium oxalate is used as an antiemetic....
or cerium hydroxide.
Powdered ceria is slightly hygroscopic and will also absorb a small amount of carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
from the atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
.
Cerium also forms cerium(III) oxide
Cerium(III) oxide
Cerium oxide is an oxide of the rare earth metal cerium. It has chemical formula Ce2O3, and is gold-yellow in color.-Exhaust catalysts:...
, Ce2O3, but CeO2 is the most stable phase at room temperature and under atmospheric conditions.
Applications
Cerium(IV) oxide is used in ceramicCeramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
s, to sensitize photosensitive glass
Photosensitive glass
Photosensitive glass is a crystal-clear glass that belongs to the lithium-silicate family of glasses, in which an image of a mask can be captured by microscopic metallic particles in the glass when it is exposed to short wave radiations such as ultraviolet light. Photosensitive glass was first...
, as a catalyst and as a catalyst support, to polish
Polishing
Polishing is the process of creating a smooth and shiny surface by rubbing it or using a chemical action, leaving a surface with a significant specular reflection In some materials polishing is also able to reduce diffuse reflection to...
glass and stones, in lapidary
Lapidary
A lapidary is an artist or artisan who forms stone, mineral, gemstones, and other suitably durable materials into decorative items such as engraved gems, including cameos, or cabochons, and faceted designs...
as an alternative to "jeweller's rouge". It is also known as "optician's rouge".
It is also used in the walls of self-cleaning ovens as a hydrocarbon catalyst during the high-temperature cleaning process.
While it is transparent for visible light, it absorbs ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
radiation strongly, so it is a prospective replacement of zinc oxide
Zinc oxide
Zinc oxide is an inorganic compound with the formula ZnO. It is a white powder that is insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber , lubricants, paints, ointments, adhesives, sealants,...
and titanium dioxide
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
in sunscreen
Sunscreen
Sunblock is a lotion, spray, gel or other topical product that absorbs or reflects some of the sun's ultraviolet radiation on the skin exposed to sunlight and thus helps protect against sunburn...
s, as it has lower photocatalytic
Photocatalysis
In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalysed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity depends on the ability of the catalyst to create electron–hole pairs,...
activity. However, its thermal catalytic properties have to be decreased by coating the particles with amorphous silica or boron nitride
Boron nitride
Boron nitride is a chemical compound with chemical formula BN, consisting of equal numbers of boron and nitrogen atoms. BN is isoelectronic to a similarly structured carbon lattice and thus exists in various crystalline forms...
.
The use of these nanoparticle
Nanoparticle
In nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties. Particles are further classified according to size : in terms of diameter, coarse particles cover a range between 10,000 and 2,500 nanometers. Fine particles are sized...
s, which can penetrate the body and reach internal organs, has been criticized as unsafe.
Cerium oxide has found use in Infrared filters
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
, as an oxidizing species in Catalytic converters and as a replacement for Thorium dioxide
Thorium dioxide
Thorium dioxide , also called thorium oxide is a white, crystalline powder. It was formerly known as thoria or thorina. It is produced mainly as a by-product of lanthanide and uranium production.[1]...
in incandescent mantles
Gas mantle
An incandescent gas mantle, gas mantle, or Welsbach mantle is a device for generating bright white light when heated by a flame. The name refers to its original heat source, existing gas lights, which filled the streets of Europe and North America in the late 19th century, mantle referring to the...
As a fuel cell electrolyte
In the doped form (it comes from cerium and oxygen), ceria is of interest as a material for solid oxide fuel cells or SOFCs because of its relatively high oxygen ion conductivity (i.e. oxygen atoms readily move through it) at intermediate temperatures (500-800 °C). Undoped and doped ceria also exhibit high electronic conductivity at low partial pressures of oxygen due to the formation of small polaronPolaron
A polaron is a quasiparticle composed of a charge and its accompanying polarization field. A slow moving electron in a dielectric crystal, interacting with lattice ions through long-range forces will permanently be surrounded by a region of lattice polarization and deformation caused by the moving...
s. However, doped ceria has an extended electrolytic region (area of predominant ionic conductivity), over that of ceria, that allows its use as an electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
in SOFCs. Substituting a fraction of the ceria with gadolinium
Gadolinium
Gadolinium is a chemical element with the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. It is found in nature only in combined form. Gadolinium was first detected spectroscopically in 1880 by de Marignac who separated its oxide and is credited with...
or samarium
Samarium
Samarium is a chemical element with the symbol Sm, atomic number 62 and atomic weight 150.36. It is a moderately hard silvery metal which readily oxidizes in air. Being a typical member of the lanthanide series, samarium usually assumes the oxidation state +3...
will introduce oxygen vacancies in the crystal without adding electronic charge carriers. This increases the ionic conductivity and results in a better electrolyte.
Under reducing
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
conditions, those experienced on the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
side of the fuel cell
Fuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...
, a large amount of oxygen vacancies within the ceria electrolyte can be formed. Some of the cerium(IV) oxide is also reduced
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
to cerium(III) oxide
Cerium(III) oxide
Cerium oxide is an oxide of the rare earth metal cerium. It has chemical formula Ce2O3, and is gold-yellow in color.-Exhaust catalysts:...
under these conditions which consequently increases the electronic conductivity of the material. The lattice constant of ceria increases under reducing
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
conditions as well as with decreasing nanocrystal
Nanocrystal
B. D. Fahlman has described a nanocrystal as any nanomaterial with at least one dimension ≤ 100nm and that is singlecrystalline.-Summary:More properly, any material with a dimension of less than 1 micrometre, i.e., 1000 nanometers, should be referred to as a nanoparticle, not a nanocrystal...
size in nanocrystalline
Nanocrystalline
A nanocrystalline material is a polycrystalline material with a crystallite size of only a few nanometers. These materials fill the gap between amorphous materials without any longe range order and crystalline materials with a clear three dimensional long range order.X-ray diffraction is commonly...
ceria, as a result of reduction of the cerium cation from a 4+ to a 3+ state in order to charge compensate for oxygen vacancy formation.
As a catalyst
Ceria has been used in catalytic converterCatalytic converter
A catalytic converter is a device used to convert toxic exhaust emissions from an internal combustion engine into non-toxic substances. Inside a catalytic converter, a catalyst stimulates a chemical reaction in which noxious byproducts of combustion are converted to less toxic substances by dint...
s in automotive applications. Since ceria can become non-stoichiometric
Non-stoichiometric compound
Non-stoichiometric compounds are chemical compounds with an elemental composition that cannot be represented by a ratio of well-defined natural numbers, and therefore violate the law of definite proportions. Often, they are solids that contain crystallographic point defects, such as interstitial...
in oxygen content (i.e. it can give up oxygen without decomposing) depending on its ambient partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of oxygen, it can release or take in oxygen in the exhaust stream of a combustion engine. In association with other catalysts, ceria can effectively reduce NOx
Nitrogen oxide
Nitrogen oxide can refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:* Nitric oxide, also known as nitrogen monoxide, , nitrogen oxide* Nitrogen dioxide , nitrogen oxide...
emissions as well as convert harmful carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
to the less harmful carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
. Ceria is particularly interesting for catalytic conversion economically because it has been shown that adding comparatively inexpensive ceria can allow for substantial reductions in the amount of platinum needed for complete oxidation of NOx and other harmful products of incomplete combustion.
Due to its fluorite structure, the oxygen atoms in a ceria crystal are all in a plane with one another, allowing for rapid diffusion as a function of the number of oxygen vacancies. As the number of vacancies increases, the ease at which oxygen can move around in the crystal increases, allowing the ceria to reduce and oxidize molecules or co-catalysts on its surface. It has been shown that the catalytic activity of ceria is directly related to the number of oxygen vacancies in the crystal, frequently measured by using X-Ray Photoelectron Spectroscopy to compare the ratios of Ce3+ to Ce4+ in the crystal.
Ceria can also be used as a co-catalyst in a number of reactions, including the water-gas shift and steam reforming of ethanol or diesel fuel into hydrogen gas and carbon dioxide (with varying combinations of rhodium oxide, iron oxide, cobalt oxide, nickel oxide, platinum, and gold), the Fischer-Tropsch reaction, and selected oxidation (particularly with lanthanum). In each case, it has been shown that increasing the ceria oxygen defect concentration will result in increased catalytic activity, making it very interesting as a nanocrystalline co-catalyst due to the heightened number of oxygen defects as crystallite size decreases—at very small sizes, as many as 10% of the oxygen sites in the fluorite structure crystallites will be vacancies, resulting in exceptionally high diffusion rates.
For water splitting
The cerium(IV) oxide-cerium(III) oxide cycleCerium(IV) oxide-cerium(III) oxide cycle
The cerium oxide–cerium oxide cycle or CeO2/Ce2O3 cycle is a two step thermochemical process based on cerium oxide and cerium oxide for hydrogen production...
or CeO2/Ce2O3 cycle is a two step thermochemical
Thermochemistry
Thermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the...
water splitting
Water splitting
Water splitting is the general term for a chemical reaction in which water is separated into oxygen and hydrogen. Efficient and economical water splitting would be a key technology component of a hydrogen economy. Various techniques for water splitting have been issued in water splitting patents in...
process based on cerium(IV) oxide and cerium(III) oxide
Cerium(III) oxide
Cerium oxide is an oxide of the rare earth metal cerium. It has chemical formula Ce2O3, and is gold-yellow in color.-Exhaust catalysts:...
for hydrogen production
Hydrogen production
Hydrogen production is the family of industrial methods for generating hydrogen. Currently the dominant technology for direct production is steam reforming from hydrocarbons. Many other methods are known including electrolysis and thermolysis...
.
Defects
In the most stable fluoriteFluorite
Fluorite is a halide mineral composed of calcium fluoride, CaF2. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon...
phase of ceria, it exhibits several defects depending on partial pressure of oxygen. The primary defects of concern are oxygen vacancies and small polarons (electrons localized on cerium cations) because these two are located in the "useful" range of ceria. In the case of oxygen defects, the increased diffusion rate of oxygen in the lattice causes increased catalytic activity as well as an increase in ionic conductivity, making ceria interesting as a fuel cell electrolyte in solid-oxide fuel cells.