In-situ electron imaging of oxidation and reduction of rutile (TiO2) nanocrystals and interaction with palladium

Metal nanoparticles supported on oxides exhibit excellent catalytic properties for chemical reactions that impact several industries, for example, Pd/CeO2 is used in the catalytic converters of automobiles for environmental remediation, Rh/ZrO2 is used for fuel refinement, and Pt/TiO2 is an excellent photocatalyst. Since chemical reactions take place at surfaces and interfaces, the catalytic reactions are highly dependent on the oxide surface structure and the interface with metal nanoparticles. Further, the oxides undergo the oxidation and reduction reactions as environment changes, the oxide surface structure also evolves dynamically during chemical reactions. The dynamic changes include the shape of nanocrystals, surface reconstruction, and the formation and rearrangements of defects. Here, we demonstrate that such structural changes can be directly observed using the new and improved environmental transmission electron microscopy (ETEM) methods. We focus on the surface and interface of TiO2 nanocrystals and Pd nanoparticles supported on TiO2 and observe their structural dynamics by direct electron imaging. In our experimental setup, we flow the gas directly into the sample area in a TEM column, while we heat the sample using a heating stage to the desired temperature. To reduce the TiO2 nanocrystals, high temperature heating under the TEM column vacuum of several 10-5 Pa is used. To oxidize the TiO2 nanocrystals, high purity oxygen gas is introduced into the TEM column to reach the pressure higher than 1x10-3 Pa at the same sample temperature. Under these reduction and oxidation conditions, the surface reconstruction of TiO2 nanocrystals is observed, as well as the defect evolution in the TiO2 nanocrystals, and changes on the surface of Pd nanoparticles supported on the TiO2 nanocrystals. To determine the shape and surface structure of the TiO2 nanocrystals, the high resolution electron microscopy (HREM) image analysis technique is developed based on the high-throughput multislice HREM image simulations and template matching. Using this technique, we determine the crystals orientation, the crystal thickness, and the image defocus from a single HREM image. In our study of TiO2 surface, we identified the TiO2 (110) surface has 2x1 reconstruction with the Ti3O5 sub-stoichiometry during reduction. The TiO2 (110) surface has a different reconstruction of 1x1 structure during oxidation. The reconstructions on other TiO2 surfaces are also observed. To visualize the defect evolution in TiO2 nanocrystals, we introduce a large amount of defects in the TiO2 nanocrystals by electron beam irradiation at 550 °C in vacuum. The electron beam induced reduction leads the creation of the planar defects, called crystallographic shear planes (CSPs). The CSPs transform from {101} to {211} orientations when the oxygen partial pressure is increased. Ex-situ observation of the CSP structures using scanning TEM (STEM) show that the {211} CSPs contain a high concentration of the titanium interstitials. The titanium interstitial diffusion mediated mechanism of CSP rotation is proposed. Pd nanocrystals of 1 to 2 nm size on TiO2 nanocrystals support of the average 50 nm size are examined to study interfacial interaction. The over-layer formation on the surface of the Pd nanocrystals is identified. The over-layer formation shows a support-facet dependency, which we attribute to the surface reducibility of the TiO2. We further suggest that the over-layer formation is due to the titanium interstitial defects migrating to the Pd nanocrystal surface during reduction.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste