HREM, EXAFS and MD Studies on Size-dependent Crystallinity of Pt Nanoparticles Supported on Gamma-Al2O3

Abstract

The catalytic system of nanoscale Pt particles on γ-Al 2 O 3 support is widely applied for oxidation of hydrocarbon and CO, in fuel cells, and as a catalyst microsensor. A novel phenomenon of negative thermal expansion (NTE) was found in this system as Pt particle sizes reduced to ~1 nm [1]. The novel size effects may result from the change of atomic structure. We employed high-resolution transmission electron microscopy (HREM) to observe thousands of individual Pt particles with a size range from sub-up to 5 nm, to gain the statistics of the crystallinity versus Pt particle sizes; with extended X-ray absorption fine-structure spectroscopy (EXAFS) we measured the general order-disorder trends of Pt-Pt bond length distributions from samples with different average sizes. The first-principle molecular dynamics (MD) simulation was applied to Pt 37 /γ-Al 2 O 3 system to find its stable structures. The samples were prepared by impregnating the Pt 2+ precursor, Pt(NH 3 ) 4 (OH) 2 ⋅H 2 O, on γ-Al 2 O 3 , reducing in H 2 gas at 573 K to remove the ligands[1]. The Pt particle sizes were controlled by the loading amount, where 1 wt% produced an average Pt size of ~1nm, 3 wt% produced an average size of 2.1 nm and heavy loading of 5 wt% produced a average size of ~2.7 nm. The TEM samples were prepared by spreading a drop of Pt/γ-Al 2 O 3 suspension in ethanol onto an ultra-thin C-grid, and dried in vacuum. The HREM observations were carried out with JEM 2100FEG S/TEM, operated at 200 kV. In order to enhance the contrast of ~1 nm Pt particles to the γ-Al 2 O 3 support and C-film, focal series reconstruction technique, a software package (HREM Research Inc.) was employed and the TEM images were filtered at zero loss energy with Gatan GIF Tridiem. Fig. 1 shows representative HREM images, Pt nanoparticles adopting disordered structure in (a) and a particle forming an FCC structure in (b). Fig. 1(c) is the enlarged image of the particle in (b) and its FFT in (d) indicated [110] orientation of the crystalline Pt particle was parallel to the beam. Through observations of many different-size Pt particles, we found that all Pt particles smaller than 1 nm adopted disordered structure; Pt particles larger than 2.5 nm possess crystalline structure. There was a transition regime of sizes from 1.1 to 2.4 nm, in which more than 80% Pt particles tend to form disordered structure and less than 20% particles tend to form crystalline structure. The static disorder parameters of Pt-Pt bond lengths were measured in temperature-resolved EXAFS experiments using samples same as those observed by TEM. All the measurements were conducted in inert atmosphere (He) or in H 2 for protection from the oxidation of metals. The results of static disorder parameters (variance) from both He and H 2 atmospheres in Fig. 2 shows the trend of increasingly disordered distribution of Pt-Pt bond lengths with decreasing sizes of Pt NPs. This general trend from the ensemble of Pt NPs is well consistent with the statistics of HREM observations from many individual Pt ...

Characterization of Al2O3 in High-Strength Mo Alloy Sheets by High-Resolution Transmission Electron Microscopy

“…Alumina (Al 2 O 3 ) (Su et al, 2012), an important structural engineering material, is widely used in many industry fields owing to the advantages of high hardness, excellent chemical stability, and good wear resistance. Al 2 O 3 has more than ten kinds of known polymorphs, of which γ -Al 2 O 3 (Li et al, 2009), β -Al 2 O 3 (Subasri & Näfe, 2008), and α -Al 2 O 3 (Tavoosi et al, 2008; Zaki et al, 2012) are the most common ones. Although Al 2 O 3 with different structures has different natures, all of the polymorphs are transformed into α -Al 2 O 3 above 1,300°C.…”