The effect of platinum on the reducibility of Rh oxides on Ce-Zr modified alumina supported automotive catalysts

Abstract

The reducibility of Rh oxide on Rh-only and Pt-Rh catalysts was investigated. The catalyst washcoats were composed of alumina modified by La and Ce-Zr mixed oxide. The catalyst ageing was performed in air at 1000°C for 3 h. In order to study the synergistic effect of Pt and Rh on the reducibility of Rh oxide, the propensity of the noble metal oxides to reduction was investigated by exposing the fresh and aged catalysts to 400 mbar of H 2 at 300°C. XPS was used to observe the changes in chemical state of the noble metals and TEM and XRD were employed to determine the catalyst composition and morphology. Differences were revealed in the behaviour of Rh-only and Pt-Rh catalysts in H2-reduction. Rhodium supported on alumina-containing washcoats without Pt was found to react with the support at elevated temperatures to form a reduction-resistive oxide phase. On the Pt-Rh bimetallic catalyst Rh was found to be partly reducible after air ageing and the Rh 3d transition emerged with an unusually low binding energy after H 2-reduction. Pt-Rh alloy particles were also formed in air ageing. Copyright © 2004 John Wiley & Sons, Ltd.