Investigation into support effects for Pt and Pd on LaMnO3

The catalytic properties of Pt and Pd supported on thin-film LaMnO3 were investigated. The LaMnO3 film consisted of an ALD-grown 0.5-nm-thick film supported on a high-surface-area MgAl2O4. Pt and Pd were added to the LaMnO3 films by Atomic Layer Deposition (ALD) and then aged by repeated oxidation and reduction cycles at 1073 K. X-Ray Diffraction (XRD), together with Scanning Transmission Electron Microscopy (STEM)/Energy Dispersive Spectroscopy (EDS) showed that Pt underwent extensive sintering during these redox treatments and formed a Pt-Mn alloy following reduction at 1073 K. Consistent with this, the activity of the supported Pt catalysts for CO oxidation decreased progressively with the number of redox cycles. In contrast to the results for Pt, STEM/EDS results indicated that Pd remained well dispersed on LaMnO3, even after five redox cycles. The Pd catalyst showed high activity for CO oxidation at lower temperatures due to a low activation energy and positive reaction order in CO. XRD results with Rietveld analysis for the Pd catalyst indicated the presence of very large crystallites for PdO on the oxidized catalyst and metallic Pd on the reduced catalyst, inconsistent with the high dispersions measured by STEM and the high CO-oxidation rates. Possible explanations for these observations are discussed.