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The mysterious cerium oxide: why is it so important in industry?
Cerium(IV) oxide, also known as cerium oxide, cerium dioxide or ceric acid, is an oxide of the rare earth metal cerium. Its chemical formula is CeO2 and its appearance is light yellow white powder. Cerium oxide is not only an important commercial product, but also plays an intermediate role in the process of extracting cerium from ores. Its unique feature is its reversible conversion to non-stoichiometric oxides, which makes it valuable for industrial applications.
Production
Cerium exists in nature in the form of oxides, usually mixed with other rare earth elements. Its main ores include busnaite and solitary mica. When cerium metal is extracted from these ores, the cerium ions are first transferred to the aqueous phase, then an oxidizing agent is added, and finally the pH is adjusted to separate the cerium. This step takes advantage of the low solubility of cerium oxide (CeO2) and the antioxidant properties of other rare earth elements.
Features
CeO2 is the most widely studied oxide of cerium and is the most oxidized form of cerium. The strong mixed state formed by 4f electrons and O 2p electrons delocalizes the 4f electrons, thereby forming a wide dispersion band.
The structure of cerium oxide is characterized by a fluorspar structure with space group Fm3m (#225). In high-temperature environments, cerium oxide releases oxygen, forming an oxygen-deficient, non-stoichiometric form that retains the properties of the fluorspar crystal lattice. The chemical formula of this material is CeO(2−x), and the x value is affected by temperature, surface termination, and oxygen partial pressure in the range 0 < x < 0.28.
Defect Chemistry
In the most stable cerium oxide fluorite phase, the main defects are related to oxygen vacancies and small polarons (electrons localized to the cerium cation). The increased concentration of oxygen vacancies increases the diffusion rate of oxide anions in the crystal lattice. This makes cerium oxide have good solid electrolyte properties in solid oxygen fuel cells.
Natural existence
Cerium oxide exists in nature in the form of cerianite and is one of the few tetravalent cerium minerals. The occurrence of ceria is associated with certain unusual examples of cerium, in which cerium is oxidized and separated from other rare earth metals that are still trivalent.
Application
Main industrial applications of cerium oxide include polishing agents for chemical mechanical planarization (CMP) and use in color-removing glasses. It has the function of converting green iron ion impurities into almost colorless iron oxides.
Cerium oxides have gained widespread attention in catalytic reactions and are capable of catalyzing water gas conversion reactions, oxidizing carbon monoxide, and reducing water by releasing hydrogen.
Other niche and emerging applications
Catalyst
Cerium oxides also have important applications in heterogeneous catalysis. It can be used to catalyze hydrocarbon reactions in self-cleaning stoves and to control gas emissions in automobile exhaust catalytic converters.
Energy and Fuel
Due to its significant ionic and electronic conductivity, cerium oxide is suitable for use as a hybrid conductor, especially in solid-state oxygen fuel cells.
Optics
Cerium oxide is valued in the optical industry for its excellent polishing ability, which can effectively remove minor scratches and imperfections on glass surfaces.
Welding
Cerium oxide is also used as an additive for titanium alloy electrodes in argon arc welding to improve arc stability and efficiency.
Security
Cerium oxide nanoparticles are being investigated for their antibacterial and antioxidant activities and are expected to become alternatives to zinc oxide and titanium dioxide in sunscreens.
Do we have any other untapped potential for future applications of cerium oxide?
