Mainly consisting of CeO2. Ceria is white when pure but usually pale yellow on account of the non stoichiometric phases. This non stoichiometry of ceria is exploited in the catalytic converters used to remove pollutants from motor vehicle exhaust emissions. These converters contain several catalytically active components, including platinum and other metals from groups 9 and 10, as well as ceria.

Ceria has several functions in the catalytic converter:

1 - the promotion of the water-gas shift reaction (CO + H2O -> CO2 +H2)

2 - enhancement of the NOx reduction capability of rhodium

3 - oxygen storage