Tuning the Electron Density of Rh Supported on Metal Phosphates for Three-Way Catalysis

The automotive three-way catalysis (TWC) performance of Rh supported on alkaline-earth and rare-earth phosphates was studied in comparison to that of Rh on aluminum phosphate (AlPO₄). The anchoring of Rh via interfacial Rh–O–P bonding in Rh/AlPO₄ leads to efficient Rh sintering suppression. However, the electron-withdrawing nature of the phosphate affords electron-deficient Rh, which has a negative impact on its catalytic activity under a reducing atmosphere due to a decrease in back-donation from the Rh <i>d</i>-orbitals to the antibonding π* orbitals of adsorbed CO and NO molecules. Notably, the extent of this electron deficiency could successfully be reduced by replacing AlPO₄ with alkaline-earth or rare-earth phosphates, and the Rh oxide formed on these phosphate supports was readily reduced to metallic Rh. This behavior is in complete contrast to that of corresponding metal oxide supports, because the higher basicity of these supports yields Rh oxides that are more difficult to reduce. Among the phosphate-supported catalysts investigated in the present study, Rh/LaPO₄ demonstrated the highest TWC performance after thermal aging under both oxidizing and reducing atmospheres. The effect of the higher basicity of LaPO₄ compared to that of AlPO₄ is most obvious in its improved catalytic activity for elementary CO–O₂, CO–H₂O, and CO–NO reactions. Importantly, this improvement is achieved while maintaining the activity toward C₃H₆ as an advanced feature of phosphate supports