This study presents a systematic investigation of the thermodynamic properties of free and γ-

Platinum nanoparticles (NPs) on γ-Al 2 O 3 support is known as an active catalytic system [1]. Recently, in situ X-ray Absorption Spectroscopy (XAS) measurements of this system showed negative thermal expansion, where the nearest neighbor Pt-Pt bond distances decreased with increasing temperature. This unusual phenomena must involve the charge transfer interactions between the Pt particle and the γ-Al 2 O 3 support [2]. Preliminary theoretical simulations revealed that the energetically favorable Pt nanoparticle structures, including their bond-lengths, depend quite sensitively on orientation, surface structure and defects on the γ-Al 2 O 3 surface. For example, O vacancies tend to pin the Pt nanoparticle and significantly change their structure, including a decrease of their bond-lengths with increasing temperatures. Therefore detailed knowledge of the atomic structure of both Pt particles and γ-Al 2 O 3 , as well as their structural correlations, is necessary. Although high-angle annular dark-field (HAADF) technique is widely used in imaging heterogeneous catalytic materials because of the high contrast of the heavy metal relative to the low-Z support, the information from the support is limited, see Fig. 1 (Fig. 1 is a HAADF image which shows Pt particles with an average size of 2.9 nm). Phase contrast given by high-resolution transmission electron microscopy (HRTEM) provides sub-nano information of both the Pt and γ-Al 2 O 3 simultaneously. 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 in order to remove the ligands to form metallic nanoparticles [2]. The sizes of Pt particle were controlled by the loading amount, where 1 wt% produced an average Pt sizes of ~1nm and heavy loading of 5 wt% produced sizes of ~2.9 nm [2]. 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, naturally dried. The HRTEM observations were carried out with JEM 2100FEG S/TEM, operated at 200 kV. Fig. 2 is an HRTEM image taken from the 1 nm Pt sample. We noted that the crystallinity of the Pt particles is size-dependent. The 1 nm Pt particles or smaller did not show clear evidence for crystallinity. The lack of uniform bond-lengths and order is supported by XAS and theoretical simulations. The 2.9 nm Pt sample showed F.C.C. structure, and a = 0.39 nm, where a few particles contained twin boundaries. The HREM image in Fig. 3 is selected to show orientation correlation of a Pt particle with its support. The Pt particle with [110] zone axis located on the γ-Al 2 O 3 with [100] axis, rotated with an angle of ~25.5° between Pt (002) and γ-Al 2 O 3 (004) planes. Fig. 4 is a profile-view of an edge-on Pt particle on γ-Al 2 O 3 , and the Pt particle shows in spherical shape with faceted surfaces (not indexed due to the its off-alignment to the incident beam). It is clearly seen that the Pt particle was well contacted to its support, with atomic level clean interface. The miss-orientation of Pt ...

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 ...

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Pt nanoparticles dispersed on γ-alumina is one of the most widely used heterogeneous catalysis systems used in commercial chemical and energy industries, including petroleum refining [1]and, hence, has been investigated extensively as a model catalyst system to elucidate structure-catalytic activity and selectivity relationships. Our specific research interest is to understand γ-Al 2 O 3 support affects on the structure and chemistry of the Pt catalyst. Several recent researchers reported that the support determines the structure of the metal catalysts, including size, uniformity and 3-dimensional morphology. For example, recent theoretical simulations revealed that defects in the γ-Al 2 O 3 stabilize the Pt nanoparticles. These simulations are conducted on ideal single crystal γ-Al 2 O 3 , whereas commercial γ-Al 2 O 3 is polycrystalline, irregular in shape, and contains impurities (Fig. 1). In order to directly link experiments with theory necessitates the creation of a well-defined, single crystal gamma alumina film. Oxide terraces can be obtained and used as support for metal clusters in model catalytic systems [2]. Previous investigators demonstrated that epitaxial γ-Al 2 O 3 thin film forms on single crystal β-NiAl by oxidation [3], Fig.1b.In this research, NiAl alloys are used to grow ultrathin γ-Al 2 O 3 layers under well-controlled oxidation conditions. Morphology becomes flatter but more discontinuous during temperature decreasing. Here, we present our results of the oxidation of β-NiAl(110) as a function of oxidation temperature (750-950°C), time and air flow. The oxide films were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). Plan-view TEM samples were prepared by scratching the oxide off of the surface and placed onto a holey C grid. Fig. 2 and 3 are the TEM results after NiAl was oxidized for 1 hr at 950 and 850 , respectively. The selected area electron diffraction pattern (SAED) confirmed that the oxide is γ-Al 2 O 3 , not another phase of alumina (e.g. theta, delta, alpha). The XRD results confirm that epitaxial (111) γ-Al 2 O 3 plane grows on (110)NiAl substrates (Fig.4). The surface morphology of the oxide films has been examined by SEM (Fig.5). With decreasing temperature, the morphology of the γ-Al 2 O 3 film has become flatter but more discontinuous. The transformation kinetics is accelerated with higher air flowrate. A peculiar ridge network morphology is created which is believed to be a vestige of high diffusivity paths of oxides growth.