Rare Earths in Noncracking Catalysts

Since 1962 rare earths have been used to stabilize zeolite cracking catalysts for the petroleum industry ( 1, 2 ). Until recently this application to catalysis has been the only commercially significant one. Currently, however, a number of new applications of potential commercial significance are appearing. One of the most important of these is the use of cerium in catalysts for automobile exhaust emission control. We will emphasize this application in our review without neglecting other applications. The rare earth oxides have a number of distinguishing properties important in catalytic applications. The oxides are basic ( 3 ) compared to alumina, lanthanum oxide (La 2 O 3 ) being the most basic. The oxides also have good thermal stability, a valuable characteristic in most industrial applications. Some rare earths including cerium, praseodymium, and terbium form non-stoichiometric oxides ( 4 ), an important property shared by many good oxidation catalysts. These mixed valence state compounds are typically polymorphic. Cost and abundance