Cerous oxide, Ce₂O₃

Cerium sesquioxide or cerous oxide, Ce₂O₃, so readily oxidises to form cerium dioxide that cerous salts of volatile oxyacids yield ceria when ignited in the air. The sesquioxide can be prepared by the reduction of the dioxide with hydrogen at 150 atmos. pressure and at a temperature of about 2000° C. In small quantities and mixed with much ceria and a little carbon, it is produced by the ignition of cerous oxalate in vacuo or in hydrogen. It is also produced in poor yield by heating ceria with zinc at the boiling-point of the metal, but it cannot be separated from the other products of the reaction.1 The simplest method yet known for its preparation consists in heating ceria with metallic calcium and dissolving out the lime and excess of calcium in aqueous ammonium chloride at -10° C. The sesquioxide is thus obtained as a yellowish-green powder which readily absorbs oxygen even at ordinary temperatures and burns easily when gently heated.