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

Abstract

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 x ) 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 d-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.

Lean NOx reduction over Rh/ZrP2O7 catalyst under steady-state and perturbation conditions

“…The metallic Rh species on ZrP 2 O 7 are therefore more resistant against reoxidation compared to those on ZrO 2 . The present result is in accordance with our previous XANES/XPS and H 2 -TPR/O 2 -TPO results on the same catalyst systems [15], and very similar to the reduction/reoxidation behavior of Rh/AlPO 4 catalyst as revealed by in situ EXAFS technique [14]. The higher stability of metallic Rh against reoxidation is considered to be a common feature of these metal phosphate supports and is associated with via RhO P interfacial linkage via surface phosphate units.…”

“…Rh/ZrO 2 showed a greater CO/Rh value than Rh/ZrP 2 O 7 , although their S BET values were comparable. However, the negligible CO/Rh value for Rh/ZrP 2 O 7 is closely associated with weak CO chemisorption, because metallic Rh supported on phosphates has an electron-deficient character, which leads to the decreasing back-donation to chemisorbed CO as reported previously [14]. This effect seems to be more obvious when Rh particles are grown by thermal aging and Rh metal dispersion in such a case is difficult to evaluate by a pulse chemisorption technique.…”

“…1c. The metal phosphates contain more acid sites on their surface than corresponding metal oxides, which is one plausible reason for different oxidative intermediates [14].…”

“…The catalytic activity and stability of catalysts is strongly dependent on the interactions at the metal-support interface. We have previously studied various metal phosphates, such as AlPO 4 , ZrP 2 O 7 , and LaPO 4 , as thermally stable supports for Rh catalysts [11][12][13][14][15][16][17][18][19]. These materials produce optimum interactions with the Rh species via Rh O P bonding and enable high dispersion of Rh anchored onto the phosphate unit on the surface [13,14,17].…”

“…is more reducible and the thus formed metallic Rh exhibits a higher tolerance against reoxidation [14]. More interestingly, this unusual redox behavior of the Rh catalysts causes a positive impact on their lean NO x purification (deNO x ) performance.…”