Metal crystallites, identification

Structure sensitive reactions for which the specific rate is highly dependent on the metal dispersion [6]. Boudart suggested that specific sites with special geometrical requirements are involved in structure sensitive reactions while all metal atoms of the surface are available for structure insensitive reactions. Such results obtained on metal supported catalysts with a polycrystalline structure for the metal crystallites corresponded to the breakthrough of Surface Science and the use of single crystals where site identification began to be possible. 

This material may be removed by reheating the sample to 850°, followed by the same annealing procedure outlined above. The X-ray diffraction pattern for 2H(a)-TaS2 may be used for identification. The following d values have been obtained for major low angle X-ray diffraction lines (and intensities) 6.05 (1) 3.025 (0.06) 2.8709 (0.32) 2.7933 (0.07) 2.3937 (0.80) and 2.3389 (0.04) A. Note that it is difficult to obtain the ideal intensities because of preferred orientation of the crystallites. This material is a superconducting metal with Te = 0.8 0.05°K.2... 

For many heterogeneous catalysts the presence of 2 nm crystallites may be deleterious to performance. Unfortunately the positive identification of 1 nm Pt crystallites on a high surface area 7-Al203 support is extremely difficult. Although EXAFS has been used, it is not a routine technique. High resolution electron microscopy has not been successful with low metals loading. 

XRD has been one of the most versatile techniques utilized for the structural characterization of nanocrystalline metal powders. The modem improvements in electronics, computers, and x-ray sources have allowed it to become a powerful indispensable tool for identilying nanocrystalline phases as well as crystal size and crystal strains. Comparison of the crystallite size obtained by the XRD dififractogram using the Scherrer formula with the grain size obtained from a TFM image allows identification of the mono- or polycrystalline nature of the nanoparticles. 

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