Cerium(IV) oxide, also known as ceric oxide, ceric dioxide, ceria, cerium oxide or cerium dioxide, is an oxide of the rare-earth metalcerium. It is a pale yellow-white powder with the chemical formula CeO2. It is an important commercial product and an intermediate in the purification of the element from the ores. The distinctive property of this material is its reversible conversion to a nonstoichiometric oxide.
Cerium occurs naturally as a mixture with other rare-earth elements in its principal ores bastnaesite and monazite. After extraction of the metal ions into aqueous base, Ce is separated from that mixture by addition of an oxidant followed by adjustment of the pH. This step exploits the low solubility of CeO2 and the fact that other rare-earth elements resist oxidation.
Cerium also forms cerium(III) oxide, Ce 2O 3, which is unstable and will oxidize to cerium(IV) oxide.
Structure and defect behavior
Cerium oxide adopts the fluorite structure, space group Fm3m, #225 containing 8-coordinate Ce4+ and 4-coordinate O2−. At high temperatures it releases oxygen to give a non-stoichiometric, anion deficient form that retains the fluorite lattice. This material has the formula CeO(2−x), where 0 < x < 0.28. The value of x depends on both the temperature, surface termination and the oxygen partial pressure. The equation
has been shown to predict the equilibrium non stoichiometry x over a wide range of oxygen partial pressures (103–10−4 Pa) and temperatures (1000–1900 °C).
The non stoichiometric form has a blue to black color, and exhibits both ionic and electronic conduction with ionic being the most significant at temperatures > 500 °C.
In the most stable fluorite phase of ceria, it exhibits several defects depending on partial pressure of oxygen or stress state of the material.
The primary defects of concern are oxygen vacancies and small polarons (electrons localized on cerium cations). Increasing the concentration of oxygen defects increases the diffusion rate of oxide anions in the lattice as reflected in an increase in ionic conductivity. These factors give ceria favourable performance in applications as a solid electrolyte in solid-oxide fuel cells. Undoped and doped ceria also exhibit high electronic conductivity at low partial pressures of oxygen due to reduction of the cerium ion leading to the formation of small polarons. Since the oxygen atoms in a ceria crystal occur in planes, diffusion of these anions is facile. The diffusion rate increases as the defect concentration increases.
The presence of oxygen vacancies at terminating ceria planes governs the energetics of ceria interactions with adsorbate molecules, and its wettability. Controlling such surface interactions is key to harnessing ceria in catalytic applications.
Catalysis and surface activity
The primary emerging application of applied CeO2 materials is in the field of catalysis. Surfaces of ceria, in its most stable fluorite phase, are dominated by the lower energy (111) planes, which tend to exhibit lower surface energy. The reaction most commonly catalysed by cerium(IV) is the water gas shift reaction, involving the oxidation of carbon monoxide. Ceria has been explored towards the catalysis of various hydrocarbon conversion reactions including CO2 methanation and the catalytic oxidation of hydrocarbons such as toluene.
The surface functionality of CeO2 stems largely from its intrinsic hydrophobicity, a trait that is common among rare earth oxides. Hydrophobicity tends to impart resistance to water-deactivation at the surfaces of catalysts and thus enhances the adsorption of organic compounds. Hydrophobicity, which can be conversely seen as organophilicity, is generally associated with higher catalytic performance and is desired in applications involving organic compounds and selective synthesis.
The interconvertibility of CeOx materials is the basis of the use of ceria for an oxidation catalyst. One small but illustrative use is its use in the walls of self-cleaning ovens as a hydrocarbon oxidation catalyst during the high-temperature cleaning process. Another small scale but famous example is its role in oxidation of natural gas in gas mantles.
A glowing Colemanwhite gas lantern mantle. The glowing element is mainly ThO2 doped with CeO2, heated by the Ce-catalyzed oxidation of the natural gas with air.
Cerium oxide nanoparticles (nanoceria) have been investigated for their antibacterial and antioxidant activity.
Ceria is of interest as a material for solid oxide fuel cells (SOFCs) because of its relatively high oxygen ion conductivity (i.e. oxygen atoms readily move through it) at intermediate temperatures (500–650 °C) and lower association enthalpy compared to Zirconia system.
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