The effect of Pt–Rh synergism on the thermal stability of rhodium oxide on pure alumina and Ce–ZrO2-modified alumina-supported catalysts

Pt, Rh, and bimetallic Pt–Rh catalysts with pure alumina and Ce–ZrO2-modified alumina washcoats are studied by XPS and TEM. The synergistic effect of platinum and rhodium is shown to be capable of stabilizing rhodium against the formation of a reduction-resistive oxide phase on alumina-supported catalysts in high-temperature air aging. Rhodium is found to exist in two different states: reduction-resistive rhodium oxide in strong interaction with alumina support and reducible rhodium oxide on the surface of Pt–Rh bimetallic particles. The growth of the noble metal particle size in the aging treatment is observed to be more pronounced on the Ce–ZrO2-containing catalysts than on pure alumina-supported catalysts. Moreover, further increase of the particle size is detected on the Ce–ZrO2-containing catalysts after H2 reduction at 300 °C. This may be related to the ability of Ce–ZrO2 to release oxygen in reducing atmosphere.