Unusual Redox Behavior of Rh/AlPO₄ and Its Impact on Three-Way Catalysis

The influence of the redox behavior of Rh/AlPO₄ on automotive three-way catalysis (TWC) was studied to correlate catalytic activity with thermal stability and metal–support interactions. Compared with a reference Rh/Al₂O₃ catalyst, Rh/AlPO₄ exhibited a much higher stability against thermal aging under an oxidizing atmosphere; further deactivation was induced by a high-temperature reduction treatment. In situ X-ray absorption fine structure experiments revealed a higher reducibility of Rh oxide (RhO_<i>x</i>) to Rh, and the metal showed a higher tolerance to reoxidation when supported on AlPO₄ compared with Al₂O₃. This unusual redox behavior is associated with an Rh–O–P interfacial linkage, which is preserved under oxidizing and reducing atmospheres. Another effect of the Rh–O–P interfacial linkage was observed for the metallic Rh with an electron-deficient character. This leads to the decreasing back-donation from Rh <i>d</i>-orbitals to the antibonding π* orbital of chemisorbed CO or NO, which is a possible reason for the deactivation by high-temperature reduction treatments. On the other hand, surface acid sites on AlPO₄ promoted oxidative adsorption of C₃H₆ as aldehyde, which showed a higher reactivity toward O₂, as well as NO, compared with carboxylate adsorbed on Al₂O₃. A precise control of the acid–base character of the metal phosphate supports is therefore a key to enhance the catalytic performance of supported Rh catalysts for TWC applications