“Intelligent” Pt Catalysts Studied on High-Surface-Area CaTiO₃ Films

CaTiO₃-supported Pt is sometimes referred to as an “Intelligent” catalyst because Pt can reversibly leave or enter the perovskite lattice following high-temperature reduction or oxidation; however, slow egress–ingress kinetics associated with large perovskite crystallites make these systems impractical. In the present work, thin films (∼1 nm) of CaTiO₃ were deposited onto MgAl₂O₄ and then examined as catalyst supports for Pt and Pd. While Pd/CaTiO₃/MgAl₂O₄ showed adsorption and CO-oxidation properties that were essentially the same as Pd/MgAl₂O₄, the Pt/CaTiO₃/MgAl₂O₄ catalyst exhibited evidence for strong support interactions. Pt/CaTiO₃/MgAl₂O₄ showed high activity for CO oxidation following reduction at 1073 K, even though CO adsorption was suppressed, but the catalysts were dramatically less active after oxidation at 1073 K and reduction at 773 K. Both Pt/CaTiO₃/MgAl₂O₄ and a catalyst formed by ex-solution of CaTi_0.95Pt_0.05O₃ exhibited very low rates for toluene hydrogenation in comparison to Pt/MgAl₂O₄. Scanning transmission electron microscopy (STEM) and energy-dispersive spectroscopy (EDS) showed that the CaTiO₃ films uniformly covered the MgAl₂O₄ surface after both reduction and oxidation at 1073 K. Pt particles on reduced Pt/CaTiO₃/MgAl₂O₄ exhibited an unusual rhombohedral shape and may be flat, a further indication of strong interactions between the metal and the support. Low-energy ion scattering (LEIS) indicated that high-temperature reduction caused a restructuring of the CaTiO₃. The implications of these results for understanding catalysts formed by ex-solution of metals from a perovskite lattice are discussed.