Investigation of Rh–titanate (ATiO3) interactions on high-surface-area perovskite thin films prepared by atomic layer deposition

Abstract

Thin, ∼1 nm films of CaTiO₃, SrTiO₃, and BaTiO₃ were deposited onto MgAl₂O₄ by Atomic Layer Deposition (ALD) and studied as catalyst supports for Rh. Scanning Transmission Electron Microcopy (STEM) and X-Ray Diffraction (XRD) demonstrated that the films had the perovskite structure and formed uniform coatings stable up to 1073 K. Rh, added by ALD, interacted strongly with CaTiO₃ and somewhat less strongly with SrTiO₃, while Rh on BaTiO₃ was similar to Rh on unmodified MgAl₂O₄. STEM measurements of Rh on CaTiO₃ films showed Rh remained well dispersed after repeated oxidations and reductions at 1073 K; however, the Rh was inactive for CO-oxidation. Rh formed small particles on SrTiO₃ films and was active for CO oxidation after reduction at 1073 K. The reducibility and catalytic activity of Rh/BaTiO₃/MgAl₂O₄ were similar to that of Rh/MgAl₂O₄. Evidence from CO-TPR, FTIR, and XPS all indicated that the degree of interaction between Rh and the three perovskite films can be ranked in the following order: Rh/CaTiO₃/MgAl₂O₄ > Rh/SrTiO₃/MgAl₂O₄ > Rh/BaTiO₃/MgAl₂O₄. Bulk ex-solution catalysts, synthesized by reduction of ATi_0.98Rh_0.02O₃ (A = Ca, Sr, and Ba), were also examined for comparison.