Supported metals, small particles electron microscopy

For supported metal catalysts, no simple calculation is possible. A direct measurement of the metal crystallite size or a titration of surface metal atoms is required (see Example 1.3.1). TWo common methods to estimate the size of supported crystallites are transmission electron microscopy and X-ray diffraction line broadening analysis. Transmission electron microscopy is excellent for imaging the crystallites, as illustrated in Figure 5.1.5. However, depending on the contrast difference with the support, very small crystallites may not be detected. X-ray diffraction is usually ineffective for estimating the size of very small particles, smaller than about 2 nm. Perhaps the most common method for measuring the number density of exposed metal atoms is selective chemisorption of a probe molecule like H2, CO, or O2. 

Suppose you prepared an iron oxide catalyst supported on an alumina support. Your aim is to use the catalyst in the metallic form, but you want to keep the iron particles as small as possible, with a degree of reduction of at least 50%. Hence, you need to know the particle size of the iron oxide in the unreduced catalyst, as well as the size of the iron particles and their degree of reduction in the metallic state. Refer to Chapters 4 and 5 to devise a strategy to obtain this information. (Unfortunately for you, it appears that electron microscopy and X-ray diffraction do not provide useful data on the unreduced catalyst.)... 

To avoid sintering, the nanoparticles are isolated from each other by a small loading. For a support of circa lOOm g" , and metal loading of about 1 wt%, with ruthenium particles of about 2 nm in diameter one obtains a statistical repartition of each particle every 30 nm. This means that such metallic particles of ruthenium particles are at a random distance of about 30 nm from each other. Electron microscopy indicates that this is frequently the case. 

There are many ways in which small metal particles can be created and examined (Section 3.2). When the gold particles are supported, the first step is to determine their mean size and size distribution for this there is no real substitute for transmission electron microscopy (TEM). The various energetic and electronic properties then need to be examined, and the bases of the available experimental techniques will be briefly rehearsed in Section 3.3. Of particular interest is the point at which the change from metallic to nonmetallic behaviour occurs as size is decreased, because this corresponds very roughly to the point at which catalytic activity (at least for oxidation of carbon monoxide) starts to rise dramatically. Relevant experimental results and theoretical speculations are reviewed in Section 3.4. 

Since the number of atoms on the surface of a bulk metal or metal oxide is extremely small compared to the number of atoms in the interior, bulk materials are often too costly to use in a catalytic process. One way to increase the effective surface area of a valuable catalytic material like a transition metal is to disperse it on a support. Figure 5.1.5 illustrates how Rh metal appears when it is supported as nanometer size crystallites on a silica carrier. High-resolution transmission electron microscopy reveals that metal crystallites, even as small as 10 nm, often expose the common low-index faces commonly associated with single crystals. However, the surface to volume ratio of the supported particles is many orders of magnitude higher than an equivalent amount of bulk metal. In fact, it is not uncommon to use catalysts with 1 nm sized metal particles where nearly every atom can be exposed to the reaction environment. 

The results that have been obtained with the catalysts after reduction and passivation are the same as those after calcination, i.e. the textural and structural properties of the support material have completely been retained after the treatments (as determined with nitrogen physisorption. X-ray diflfiaction and transmission electron microscopy). Information concerning the metallic nickel particles has been obtained with X-ray diffraction and transmission electron microscopy. Diflractograms of the catalysts after passivation are shown in Fig. 8. The observed features are exactly the same as for the oxidic systems (Fig. 4) only very broad and low diffractions are visible for the catalyst ex citrate, whereas sharp, intense peaks with a broad onset are observed for the catalyst ex nitrate. Consequently the nickel particles of the catalyst ex citrate have resisted sintering during the reduction treatment, thereby conserving the high dispersion of the catalyst. These results were confirmed by transmission electron microscopy measurements (not shown) only very small nickel nanoparticles situated inside the mesopores were found for the catalyst ex citrate. 

Electron microscopy The use of pyrazinedicarboxylate-type precursors was found to generate the largest metal particles with a maximum size of 20 m, whereas the catalysts prepared from acetate precursors are so small that they are not detected by backscattered electron microscopy. EDX analysis of the (pzdc.PdBi/C) catalyst demonstrated the bimetallic nature of the metal particles. The SEM-EDX spectra of the used monometallic Ac.Pd/C catalyst tested in the presence of added dissolved bismuth, according to the procedure described elsewhere [8], gave evidence for bismuth deposition on the support and on Pd particles. 

The composition of isolated metallic particles was determined with a JEOL-GEM 2010 analytical microscope by measuring the intensities of the of Pd L and Mn K emissions. With this instrument, the size of the probe can be reduced down to Inm. On large particles( d s 10 nm), or when the analysis is performed with a large probe size, the Pd/Mn ratio is close to the nominal composition. But most of the particles with a diameter smaller than 5 nm have a lower Mn content than the nominal composition, as can be seen in Table 1. The remaining Mn is dispersed on the support probably under the form of small particles which are not observed by electron microscopy because of a too low contrast. 

The adherence of small particles of precious metals to the surface of the support can be assessed by transmission electron microscopy. It has been observed that mild ultrasonic treatment of the catalyst in a liquid, such as ethanol, can remove precious metal particles from the surface of the support. After applying a drop of the suspension resulting from the ultrasonic treatment on the carbon films used as specimen support the precious metal particles released from the support show up on the carbon support film. Especially dark-field techniques are useful to indicate the presence of precious metal particles on the carbon support film. 

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