Investigation of Rh-titanate (ATiO3) interactions on high-surface-area perovskites thin films prepared by atomic layer deposition
an example of application of ALD in the field of catalysis
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. Bulk ex-solution catalysts, synthesized by reduction of ATi0.98Rh0.02O3 (A=Ca, Sr, and Ba), were also examined for comparison. Scanning Transmission Electron Microcopy (STEM) and X-Ray Diffraction (XRD) demonstrated that each of the films had the perovskite structure and formed uniform coatings that remained stable upon redox cycling at 1073 K. Rh was also added by ALD and was found to interact very 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 CaTiO¬3 films showed the Rh remained well dispersed after repeated oxidations and reductions at 1073 K; however, the Rh appears not be reduced to the metallic form and was inactive for CO-oxidation reactions. Rh also interacted strongly with SrTiO3 films but could be reduced to form small metal particles that were active for CO oxidation upon reduction at 1073 K. There was no evidence for strong interactions between Rh and BaTiO3, and the reducibility and catalytic activities of the Rh/BaTiO3/MgAl2O4 were similar to that found on 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. Possible explanations for why Rh on CaTiO3 films show properties that are different from Rh ex-solved from bulk CaTiO3 and why Rh interacts differently on CaTiO3, SrTiO3, and BaTiO3 are discussed.