Z-Contrast Tomography of Nanoscale Heterogeneous Catalysts

Abstract

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].