Metal nanoclusters on high surface area oxide supports are important catalysts for many applications. To a first approximation, the properties of each system depend on the size of the nanoclusters. It is therefore of great importance to determine the cluster size accurately. One of the methods to investigate individual nanoclusters is high-angle annular dark-field imaging (HAADF) in scanning transmission electron microscopy (STEM). Although a spherical aberration corrected STEM has a good spatial resolution, its intense probe can force nanoclusters move around the surface of the support to make the size measurement difficult. On other hand, a conventional STEM with a less intense probe can avoid the cluster movement at the expense of resolution as well as signal intensity. In this abstract, we discuss our first attempts to quantify the size distributions from STEM images and describe a method that produces reasonable results even for the smallest clusters when the image has a low signal-to-noise (S/N) ratio. Decaosmium carbido carbonyl (Os 10 C(CO) 24 ) and tritantalum (Ta 3 ) clusters were prepared on MgO and silica supports, respectively, by surface-mediated organometallic synthesis [1-2]. These clusters were examined using JEOL JEM-2500SE (S)TEM operated at 200 kV with the probe size of 0.2 nm and the collection angle of 35-90 mrad. Images were taken at a magnification of 4,000,000x and 6,000,000x for Os 10 C(CO) 24 and Ta 3 clusters, respectively. Fig. 1(a) shows a raw HAADF-STEM image for Os 10 C(CO) 24 clusters on a MgO support. The pixel size in the image is 0.063 nm. It is very difficult to measure the cluster size from the raw image because of the noisy intensity profile as shown in Fig. 1(b). Although noise in the image represents the primary difficulty, the size distribution of the intensity profile will also be modified by any blurring effect in the final image -initial probe size, beam broadening, cluster orientation effects. We can smooth the noise in the image by broadening the image using a Gaussian blur, in which each pixel is convoluted with a Gaussian (normal) distribution function with a standard deviation (σ value). When the σ value is small, the noise is not completely smoothed as shown in the profiles (Figs. 1(c) and (d)). When the σ value is large enough, however, the noise is smoothed out so that the cluster size can be measured with the profiles as shown in Figs. 1(e)-(g). Fig. 2 shows a size distribution histogram for twenty clusters in a blurred image with a σ value of 0.25 nm. The averaged sizes of Os 10 C(CO) 24 clusters in the Gaussian blurred images are plotted in Fig. 3(a) as a function of σ value. The error-weighted linear fit intercepts the vertical axis at the value of 0.61 0.11 nm. This value corresponds to the cluster size without Gaussian blur and is slightly larger than the Os-Os-Os atomic distance in Os ± 10 C(CO) 24 clusters (0.57 nm) that obtained with
Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008
Most industrial catalysts are high-surface-area solids such as amorphous or crystalline oxides, onto which an active component (often clusters of a metal, metal oxide, or metal sulfide, often including high-Z contrast elements) is dispersed in the form of very small clusters or particles [1,2]. So far, metal clusters on support have been restricted almost entirely to group 8 metals but recently supported early transition metal catalysts exemplified by tantalum clusters on SiO 2 have been reported [3]. Catalyst performance is sensitive to cluster size, because the surface structure, electronic properties and cluster-support interactions depend on this size. The location of metal particles and their orientation with respect to the support material are also important in determining catalytic properties. For example, partial coverage of the metal by an amorphous oxide can influence the catalytic activity and selectivity [2].In this investigation, a silica-supported tantalum catalyst was investigated to determine the detailed 3D morphology of the nanoparticles, the degree of encapsulation of the metal cluster by the support, the location of the clusters on the support material, and the size and distribution of the clusters. For this purpose, tomography based on Scanning Transmission Electron Microscopy (STEM) was performed on a 200kV JEOL 2500SE TEM/STEM microscope. Because of the sensitivity of the image to the atomic number, the Z-contrast technique of STEM provides images of the nanoclusters on the oxide support with a high resolution (figure 1). To provide a 3D reconstructed volume, a tilt range of images from -70° to +70° with a 2° increment processed and analyzed with the Composer and Visualizer software package (JEOL Ltd.) [4].Previously reported results characterizing the sample by extended X-ray absorption fine structure (EXAFS) spectroscopy showed an average Ta-Ta coordination number of 4.8 for the clusters that had been treated only in inert atmospheres, with an average Ta-Ta distance corresponding to a chemical bond between the Ta atoms. Accordingly, a preliminary model of the tantalum clusters was suggested to be a single layer (raft) of approximately 40 Ta atoms.However, when the sample was treated in air and characterized by STEM, the tomography results showed that the shapes of the clusters tended to be almost spherical rather than raft-like. Tomography reconstruction results shown in figures 2A and B also show that the nanoclusters are not evenly distributed within the SiO 2 support. Thus, being partially encapsulated, the tantalum nanoparticles would not be expected to show the same catalytic activity as such clusters positioned on the support [5].
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