Rh Nanoparticle Anchoring on Metal Phosphates: Fundamental Aspects and Practical Impacts on Catalysis

Metal phosphates stabilize Rh nanoparticles on their surface via Rh-O-P bonds, in contrast to the Rh-O-M bonds formed on metal oxides (MO_x ). The local structure, electronic structure, and redox properties of Rh nanoparticles anchored on metal phosphates, and their practical impacts on catalysis, are reviewed based on recent publications from the author's research group. Because of the covalency of the Rh-O-P bond, Rh oxide is readily reduced to metallic Rh having a higher catalytic activity, whereas Rh oxide on metal oxide supports is more difficult to reduce with an increase of the anchoring strength. Furthermore, Rh metal shows a higher tolerance to reoxidation when supported on metal phosphates because the Rh-O-P bond is preserved under reducing atmospheres. The electron deficiency of Rh metal is another feature that affects its catalytic properties, and the extent of the electron deficiency can be tuned by replacing the metal in the metal phosphate with one of higher basicity. Further impacts on practical performance (thermal stability, poisoning stability, and lean NO_x purification) in automobile catalyst applications are also described.