'Intelligent' Pt Catalysts Studied on High-Surface-Area CaTiO3 Films

摘要

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 (similar to 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 CaTi0.95Pt0.05O3 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.