Effects of the Additives Cerium and Lanthanum Oxides

It was noted in the introduction that the washcoat of the automobile catalysts contains several other oxides, mainly cerium and lanthanum oxide. 

Hardacre el al. (7 75, 174) investigated the properties, structure, and composition of cerium oxide films prepared by cerium deposition on Pt(lll), finding that the activity for CO oxidation is enhanced on Pt(lll) that is partially covered by ceria. It was suggested that new sites at the Pt-oxide interface become available for reaction. A remarkable observation is the high activity for CO oxidation when the Pt(lll) sample is fully encapsulated by ceria (Pt was undetectable by XPS and AES). It was proposed that an ultrathin, disordered ceria film becomes the active catalyst. It was also demonstrated by XPS and AES that Pt dramatically increases the reducibility of cerium oxide that is in intimate contact with Pt. This result suggests that intimate contact between the noble metal and oxide phases is indeed crucial to facile oxygen release from ceria. High-resolution electron microscopy demonstrated the presence of direct contact between ceria and noble metal for supported Pt-Rh catalysts (775). Hardacre et al. (173,174) related the catalytic activity of the ceria phase to partially reduced cerium oxide. 

In conclusion, there is overwhelming evidence for the beneficial effect of cerium oxide on the activity of the noble metal catalyst. However, the nature of the promoter effect of ceria is not fully understood. Most likely, the noble metal-cerium oxide interface is of crucial importance for some of the effects observed. More studies with model systems are needed for a better understanding of the promoter effects of ceria. 

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