Cerium(IV) oxide

Production

Cerium occurs naturally in bastnaesite and monazite ores mixed with other rare-earth elements. After extraction into aqueous base, cerium is separated by adding an oxidant and adjusting pH, exploiting the low solubility of CeO2 and the resistance of other rare earths to oxidation. Cerium(IV) oxide is produced by calcining cerium oxalate or cerium hydroxide. Cerium(III) oxide, which also forms, oxidizes in air to cerium(IV) oxide.

Structure and defect behavior

Cerium oxide adopts the fluorite structure with 8-coordinate Ce4+ and 4-coordinate O2−. At high temperatures it releases oxygen, forming a non-stoichiometric, anion-deficient form CeO(2−x) (0 < x < 0.5) that retains the fluorite lattice; this release becomes appreciable above 500 °C. Oxygen vacancies are measured by X-ray photoelectron spectroscopy comparing Ce3+ to Ce4+ ratios. In the molten state, local Ce-O coordination drops to predominantly 6-fold.

The primary defects are oxygen vacancies and small polarons (electrons localized on cerium cations). Increasing defect concentration raises oxide anion diffusion rate and ionic conductivity, making ceria suitable as a solid electrolyte in solid-oxide fuel cells. Undoped and doped ceria also show high electronic conductivity at low oxygen partial pressures due to cerium reduction forming small polarons.

Natural occurrence

Cerium(IV) oxide occurs naturally as the mineral cerianite-(Ce), a rare example of a tetravalent cerium mineral. Other such minerals are stetindite-(Ce) and dyrnaesite-(La). Cerianite-(Ce) formation relates to the cerium anomaly, where easily oxidized Ce separates from other rare-earth elements that remain trivalent.

Applications

The principal industrial use of ceria is for polishing, especially chemical-mechanical planarization, where it has displaced iron oxide and zirconia; hobbyists know it as "opticians' rouge". It decolorizes glass by converting green-tinted ferrous impurities to nearly colorless ferric oxides. In heterogeneous catalysis, it catalyzes the water-gas shift reaction and oxidizes carbon monoxide; its reduced derivative Ce2O3 reduces water to release hydrogen.

This interconvertibility is used in self-cleaning oven walls as a hydrocarbon oxidation catalyst and in gas mantles. Ceria serves as a sensor in catalytic converters, controlling the air-exhaust ratio to reduce NOx and carbon monoxide emissions. Its ionic and electronic conduction makes it suitable for solid oxide fuel cells. The CeO2/Ce2O3 cycle is a two-step water splitting process for hydrogen production.

In optics, it polishes glass through mechanical abrasion and chemical interaction, and can form a protective layer. It is also used in infrared filters and as a replacement for thorium dioxide in incandescent mantles.