Enhanced Rh-anchoring on the composite metal phosphate Y0.33Zr2(PO4)3 in three-way catalysis

Abstract

A series of binary metal phosphates with the general formula Ln_0.33Zr₂(PO₄)₃ (Ln = rare earths) was studied as support materials for Rh in three-way catalysis in comparison to the parent compound, ZrP₂O₇ (ZP). Among the investigated materials, the Y_0.33Zr₂(PO₄)₃ (YZP) support exhibited the most efficient anchoring of Rh nanoparticles and a higher activity of the catalyst persisted even after thermal ageing at 900 °C. Rh nanoparticles as large as 2 nm were found to persist on YZP; however, these nanoparticles grew into much larger agglomerates (∼10 nm) when supported on ZP. Rh K-edge extended X-ray absorption fine structure (EXAFS) was used to demonstrate that the observed difference in the stabilities of the nanoparticles was related to metal–support interactions via Rh–O–P bonding. Rh/YZP formed a more thermostable bidentate linkage compared to the monodentate linkage that formed in Rh/ZP. Thus, this bidentate linkage was preserved in the thermal ageing process. This type of interfacial bond was formed in Rh/YZP but did not form when Rh was supported on other Ln_0.33Zr₂(PO₄)₃ compounds, despite the similarity in the crystal structures of the binary metal phosphates. The lower crystallinity of YZP over that of the other binary metal phosphates facilitated the geometrical deformation of YZP, which may have contributed to the efficient anchoring via strong bonding of the PO₄ unit to Rh. By contrast, it was difficult for the less-flexible pyrophosphate (P₂O₇) unit, which was formed by corner-sharing two PO₄ tetrahedra to bind to Rh at the metal–support interface. This result may explain why Rh supported on ZP agglomerated into large grains among the Rh catalyst-supported metal phosphate materials.