Cerium(IV) oxide

Cerium(IV) oxide
Cerium(IV) oxide
IUPAC name
Cerium(IV) oxide
Other names
Ceric oxide,
Ceria,
Cerium dioxide
Identifiers
CAS number 1306-38-3 YesY, 
12014-56-1 (monohydrate)
PubChem 10219615
ChemSpider 8395107 YesY
Jmol-3D images Image 1
Properties
Molecular formula CeO2
Molar mass 172.115 g/mol
Appearance white or pale yellow solid,
slightly hygroscopic
Density 7.65 g/cm3, solid
7.215 g/cm3, fluorite phase
Melting point

2400 °C
Boiling point

3500 °C
Solubility in water insoluble
Structure
Crystal structure cubic (Fluorite)[1]
Related compounds
Related compounds Cerium(III) oxide
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Cerium(IV) 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.[2]

Cerium also forms cerium(III) oxide, Ce2O3, but CeO2 is the most stable phase at room temperature and under atmospheric conditions.

Contents

Applications

Cerium(IV) oxide is used in ceramics, 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".[3]

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 sunscreens, 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 nanoparticles, which can penetrate the body and reach internal organs, has been criticized as unsafe.[4]

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[5]

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 polarons. 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.[6]

As a catalyst

Ceria has been used in catalytic converters 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.

For water splitting

The cerium(IV) oxide-cerium(III) oxide cycle or CeO2/Ce2O3 cycle is a two step thermochemical water splitting process based on cerium(IV) oxide and cerium(III) oxide for hydrogen production.[7]

Defects

In the most stable fluorite 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.

References

  1. ^ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
  2. ^ ROBERT DAVID GREEN. CARBON DIOXIDE REDUCTION ON GADOLINIA-DOPED CERIA CA THODES. Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy. Dissertation Adviser: Dr. Chung-Chiun Liu. Department of Chemical Engineering CASE WESTERN RESERVE UNIVERSITY. May, 2009: http://etd.ohiolink.edu/send-pdf.cgi/Green%20Robert%20David.pdf?case1232574534
  3. ^ Properties of Common Abrasives (Boston Museum of Fine Arts)
  4. ^ "Suncream may be linked to Alzheimer's disease, say experts". Daily Mail (London). 24th August 2009. http://www.dailymail.co.uk/health/article-1208720/Suncream-linked-Alzheimers-disease-say-experts.html. Retrieved 2009-08-25. 
  5. ^ http://www.nanopartikel.info/cms/lang/en/Wissensbasis/Cerdioxid
  6. ^ S Deshpande, "Size dependency variation in lattice parameter and valency states in nanocrystalline cerium oxide." Applied Physics Letters 87 133113 (2005)
  7. ^ Hydrogen production from solar thermochemical water splitting cycles

External links