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 Al 2 O 3 was studied in relation to thermal aging in air. After air-aging at ≥900 °C, the Rh/Al 2 O 3 catalyst was more strongly deactivated compared with the Pd/Al 2 O 3 catalyst. As has been widely recognized, the solid-state reactions between Rh oxide and Al 2 O 3 decreased the active surface area and stabilized inactive Rh 3+ species. The activity was restored by the postreduction treatment with 20% H 2 /He at 200 °C, whereas a striking enhancement of activity was achieved by the reduction at 800–1100 °C, where substantial deactivation occurred for Pd/Al 2 O 3 . 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 Al 2 O 3 . The observed dispersion (as high as ∼20% after reduction at 1000 °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/ZrO 2 under both oxidizing and reducing atmospheres was similar to that of Pd/Al 2 O 3, 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 Al 2 O 3 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.