Thermostable Rh Metal Nanoparticles Formed on Al2O3 by High-Temperature H2 Reduction and Its Impact on Three-Way Catalysis

The influence of high-temperature H-2 reduction treatment on Rh and Pd catalysts supported on Al2O3 was studied in relation to thermal aging in air. After air-aging at >= 900 degrees C, the Rh/Al2O3 catalyst was more strongly deactivated compared with the Pd/Al2O3 catalyst. As has been widely recognized, the solid-state reactions between Rh oxide and Al2O3 decreased the active surface area and stabilized inactive Rh3+ species. The activity was restored by the postreduction treatment with 20% H-2/He at 200 degrees C, whereas a striking enhancement of activity was achieved by the reduction at 800-1100 degrees C, where substantial deactivation occurred for Pd/Al2O3. A mechanistic interpretation is proposed based on local structural characterization, which explains these contrasting thermal behaviors. The high-temperature reduction treatment produced active and thermostable Rh metal nanoparticles, which were highly dispersed on Al2O3. The observed dispersion (as high as similar to 20% after reduction at 1000 degrees C) is among the highest for supported Rh catalysts reported in the literature. This is in complete contrast to the rapid sintering of Pd and other precious metals (Ru and Pt) into large metal agglomerates greater than 50 nm. Because the thermal behavior observed for Rh/ZrO2 under both oxidizing and reducing atmospheres was similar to that of Pd/Al2O3, the stability of metal nanoparticles depended not only on metal species but also on the interactions with support materials. An important implication of this study is that Al2O3 is a very efficient support for anchoring Rh metal nanoparticles via interfacial Rh-O-Al bonding under strong reducing conditions, in contrast to the well-known incompatibility with Rh oxide under oxidizing conditions.