TWC deactivation by lead:: A study of the Rh/CeO2 system

The effect of the incorporation of low amounts of Pb to a Rh/CeO2 catalyst on the typical reactions involved in Three Way Catalysts converters was investigated. The catalytic tests showed no deactivation for CO oxidation and NO reduction whereas for propylene oxidation the deactivation was evident and more intense with the increasing of Pb concentration. The interpretation of this behaviour is discussed on the basis of HREM combined with XEDS analysis, TPR and DRIFT spectroscopy of chemisorbed CO studies. The results provided clear evidence that Pb is not covering the Rh particles neither forming a new phase with Rh nor altering the dispersion of the Rh particles. They also suggest that a mixed oxide is formed in which lead cations are dissolved within the fluorite-like ceria structure. Thus, CeO2 support may act as sink of Pb preventing the formation of a Rh-Pb compound that explains why the CO oxidation and the NO reduction were only slightly affected. However, an electronic interaction between the Rh particles and the Pb-containing CeO2 support is evidenced by TPR and DRIFTS-CO chemisorption data: the Pb present in the Rh-CeO2 interphase as a result of the Pb dissolution in the ceria framework is proposed to be responsible for the electronic donation to Rh metal particles. The possibility that the electronic interaction and/or the modification of the oxygen storage capacity (OSC) of the ceria, both caused by the Pb, bring about the deactivation in the oxidation of propylene is also discussed. (c) 2005 Published by Elsevier B.V.