Aberration-Corrected Transmission Electron Microscopy and In Situ XAFS Structural Characterization of Pt/γ-Al2O3 Nanoparticles

Aberration-corrected (AC) STEM, AC TEM and in situ X-ray absorption fine structure spectroscopy (XAFS) were used to characterize the Pt clusters present on a 0.35 wt % Pt on γ-alumina support after reduction in hydrogen at 700 °C. STEM high-angle annular dark field imaging shows that cluster formation takes place at temperatures up to approximately 350 °C, and this is followed by gradual growth in cluster size for heat treatments in hydrogen up to 700 °C. The STEM data show that after 700 °C reduction the Pt clusters are present in a narrow size distribution centered at 0.88 nm, and using a method involving a redistribution of the Pt atoms using a high electron dosage in the STEM, it is shown that the clusters are present in two-dimensional morphology. This conclusion is verified using intensity line scans. The in situ extended X-ray absorption fine structure data are in good agreement with these observations. High-resolution AC–TEM, which uses a broad coherent electron beam, and can thus offer advantages relative to STEM for structure determination of fine clusters, supported by image simulations of through-focus series, were used to analyze the structures of Pt particles. The structures determined by using AC–TEM are consistent with STEM and EXAFS data in having a flat two-dimensional morphology. Comparison of AC–STEM and AC TEM data for the same 700 °C reduced sample suggests that parallel-beam TEM mode of imaging may be advantageous because of the less pronounced beam-induced structural rearrangements that occur when imaging with a fine STEM probe.