A SUBNANOMETER STRUCTURAL STUDY OF PT-RH CATALYSTS SUPPORTED ON CE DOPED SIC

In this paper, the HRTEM technique has been applied to the structural study of Pt-Rh catalysts supported on Ce doped SiC. The results on cer ium doped active carbon and on cerium doped silicon carbide synthezise d before its use as catalyst support are also reported. It was found t hat the Ce doped SiC catalyst support contains a new compound which is a Ce amorphous phase. The presence of such a compound within the supp ort is thought to have the advantage of contributing to the improvemen t of the activity of the catalysts, At present, the HRTEM study on red uced Pt-Rh/SiC-Ce catalyst gives direct evidence of a selective associ ation of Pt and Ph metals with the cerium oxide additive. This associa tion, on the one hand, shows that cerium oxide is not inert towards no ble metals, and on the other hand, it gives rise to a noble metal-ceri um oxide composite to which the reduction is linked. The formation of an active interface, or a buffer oxygen layer which can be defined as a surface that serves as a common boundary between noble metals and ce rium oxide, must be then considered in order to interpret the improvem ent in catalytic activities. After the ageing treatment, some structur al changes were observed, particularly the formation a Rh suboxide pha se formed by three to four monoatomic layers. Finally, it seems that t he main oxidation and reduction processes occur through the active int erface where the cerium oxide is easily able to change from cerium plu s four (Ce4+) to cerium plus three (Ce3+) oxidation state. The metal, will act only as a donor or acceptor of electrons. It must be emphasis ed that the SiC catalyst support showed a high chemical inertia since no structural modifications were observed on the noble metals caused b y SiC.