Characterization of Catalytic Materials by Wachs Israel

Author:Wachs, Israel.
Language: eng
Format: epub
Publisher: Momentum Press

Figure 5.2 Electron micrograph of an anisotropic sulfide—MoS2 platelets.

Morphology, Particle Size, and Surface Area

The morphology, particle size, and surface area of TMS are generally obtained using conventional methods such as electron microscopy, X-ray diffraction line-broadening, and nitrogen BET. In contrast to the isotropic class, major difficulties are encountered when characterizing materials of the anisotropic class. For example, molybdenum disulfide in a poorly crystalline state develops a morphology of its own, best described as “rag structure.” A quick look at an electron micrograph (Figure 5.3) makes clearly evident the difficulty of estimating the particle size. XRD may be used, but the line-broadening will be characteristic of the crystalline order, which depends upon the bending and folding of the layer. When the Debye-Scherrer equation is applied to the 002 diffraction peak, a reasonable value for the c-axis of the crystal is obtained for the stacking of the layers (within a factor of two), but a line-broadening analysis of the 103 or 110 peaks results in values along the a-axis between 70 and 100 Å, though the actual dimension can be as large as several microns.

Extended X-ray fine-structure spectroscopy (EXAFS) measurements have also been used for determining particle size, particularly in supported materials. The average coordination numbers of the metal atoms are generally used for estimating particle size. For isotropic materials, such estimations usually agree well with the results obtained by the preceding methods. In contrast, significant discrepancies can be observed for highly anisotropic materials. For molybdenum disulfide, the attenuation of the Mo–Mo peak in a sample treated at 900 °C is 50% of that observed for a single crystal. Therefore, the corresponding coordination number suggests particle sizes that are much smaller than the 20 000 Å particles estimated by the SEM micrograph (see Figure 5.3).

BET measurements are adequate in all cases and are suitable for surface areas up to 400 m2/g. After catalytic reaction, most poorly crystalline sulfides have surface areas in the 10–100 m2/g range. Most of the surface area of the anisotropic class is associated with the basal plane of the layer, which consists of a closely packed surface of sulfur atoms. If an infinite layer of MoS2 has a surface area of 327m2/g and if n is the number of stacked layers of large particles on the surface, then the surface area of the particles is 327/n because the surface area of relatively large particles is inversely proportional to their stacking. Also, the basal plane surface consists of a closely packed surface of sulfur atoms, each bonded to three metal atoms. The stability of the sulfur environment is in fact related to the weak van der Waals interaction between the layers. The basal plane exhibits a low reactivity and is of central interest to lubrication science. In contrast, the edge plane is highly reactive and is of primary importance for developing structure/function relationships in anisotropic systems. There is no easy way of measuring the edge plane area except when single microcrystals are used; the edge surface area is then directly estimated from SEM micrographs.

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