Unusual Redox Behavior of Rh/AlPO<sub>4</sub> and
Its Impact on Three-Way Catalysis
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Abstract
The
influence of the redox behavior of Rh/AlPO<sub>4</sub> on automotive
three-way catalysis (TWC) was studied to correlate catalytic activity
with thermal stability and metal–support interactions. Compared
with a reference Rh/Al<sub>2</sub>O<sub>3</sub> catalyst, Rh/AlPO<sub>4</sub> 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<sub><i>x</i></sub>) to Rh, and the metal showed a higher tolerance to reoxidation
when supported on AlPO<sub>4</sub> compared with Al<sub>2</sub>O<sub>3</sub>. 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 AlPO<sub>4</sub> promoted oxidative adsorption of C<sub>3</sub>H<sub>6</sub> as aldehyde, which showed a higher reactivity toward O<sub>2</sub>, as well as NO, compared with carboxylate adsorbed on Al<sub>2</sub>O<sub>3</sub>. 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