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 H 2 -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.

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Thermal decomposition of noble metal oxide phases formed during aging in air on industrially manufactured g-Al 2 O 3 -supported Rh catalysts was investigated. Some of the catalyst washcoats were modified by various stabilizers, promoters and oxygen storage components, such as La 2 O 3 , CeO 2 and Ce-Zr mixed oxide. In order to investigate the effect of aging, stabilizers and manufacturing method on the thermal stability of the noble metal oxide phases formed, a stepwise thermal decomposition in high vacuum (pressure <10 −7 Torr) was performed. X-ray photoelectron spectroscopy (XPS) was used to observe the changes in chemical state after each annealing cycle. Our results show marked differences in the rate of reduction between different Rh catalysts. Disposition of the Rh oxides towards reduction was also investigated by exposing the catalysts to 400 mbar of H 2 at 300 • C for 30 min. After 3 h of aging in air at 1000• C an irreducible rhodium oxide, which was identified to be either Rh 2 O 3 diffused into alumina or RhO 2 in intimate contact with the alumina surface, was formed regardless of the Ce-Zr mixed-oxide additive and manufacturing method of the catalyst.

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