Supported Particles

In these experiments the correlation between catalytic activity and particle size was investigated using ethane-l,2-diol oxidation as a model reaction. In the presence 

For the supported gold, difficulties in calculating the exposed metal area and possible metal-support interactions prevent accurate correlations, which can be more easily obtained in the absence of support, as discussed in the next section. 

As a practical conclusion, gold particles over 10 nm are almost inactive in ethane-1,2-diol oxidation. 

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Fig. 9. Schematic representation of a catalyst for ethylene oxide synthesis (not to scale). The porous support particle consists of microparticles held together...

Other immobilization methods are based on chemical and physical binding to soHd supports, eg, polysaccharides, polymers, glass, and other chemically and physically stable materials, which are usually modified with functional groups such as amine, carboxy, epoxy, phenyl, or alkane to enable covalent coupling to amino acid side chains on the enzyme surface. These supports may be macroporous, with pore diameters in the range 30—300 nm, to facihtate accommodation of enzyme within a support particle. Ionic and nonionic adsorption to macroporous supports is a gentle, simple, and often efficient method. Use of powdered enzyme, or enzyme precipitated on inert supports, may be adequate for use in nonaqueous media. Entrapment in polysaccharide/polymer gels is used for both cells and isolated enzymes. 

The next level is that of small catalytically active particles, with typical dimensions of between 1 and 10 nm, and inside the pores of support particles (pm range). The questions of interest are the size, shape, structure and composition of the active particles, in particular of their surfaces, and how these properties relate to catalytic reactivity. Although we will deal with heterogeneous catalysis, the anchoring of catalytic... 

Therefore, in many fundamentally oriented studies the complex catalyst is replaced by a simplified model, which is better defined. Such models range from supported particles from which all promoters have been removed, via well-defined particles deposited on planar substrates, to single crystals (Fig. 4.1). With the latter we are in the domain of surface science, where a wealth of informative techniques is available that do not work on technical catalysts. 

Because XPS is a surface sensitive technique, it recognizes how well particles are dispersed over a support. Figure 4.9 schematically shows two catalysts with the same quantity of supported particles but with different dispersions. When the particles are small, almost all atoms are at the surface, and the support is largely covered. In this case, XPS measures a high intensity Ip from the particles, but a relatively low intensity Is for the support. Consequently, the ratio Ip/Is is high. For poorly dispersed particles, Ip/Is is low. Thus, the XPS intensity ratio Ip/Is reflects the dispersion of a catalyst on the support. Several models have been reported that derive particle dispersions from XPS intensity ratios, frequently with success. Hence, XPS offers an alternative determination of dispersion for catalysts that are not accessible to investigation by the usual techniques used for particle size determination, such as electron microscopy and hydrogen chemisorption. 

Electron microscopy is a rather straightforward technique to determine the size and shape of supported particles [S. Amelinckx, D. van Dyck, J. van Landuyt and G. van Tendeloo, Handbook of Microscopy (1997), VCH, Weinheim]. Electrons have characteristic wavelengths of less than 1 A, and come close to monitoring atomic detail. Figure 4.13 summarizes what happens when a primary electron beam of energy between 100 and 400 keV hits a sample ... 

Transmission electron microscopy is one of the techniques most often used for the characterization of catalysts. In general, detection of supported particles is possible, provided that there is sufficient contrast between particles and support - a limitation that may impede applications of TEM on well-dispersed supported oxides. The determination of particle sizes or of distributions therein is now a routine matter, although it rests on the assumption that the size of the imaged particle is truly proportional to the size of the actual particle and that the detection probability is the same for all particles, independent of their dimensions. 

However, there may be good reasons why a catalyst should not consist of particles that are too small, as we saw in the beginning of this chapter, e.g. to avoid pressure gradients in the reactor. Based on an analysis such as the above, one can decide whether it makes sense to use support particles that contain a homogeneous distribution of the catalytic phase. With expensive noble metals, one might perhaps decide to use an egg-shell type of arrangement, where the noble metal is only present on the outside of the particles. 

In addition to obtaining Information about the size, relative mass, and structure of the platinum crystallites, the STEM can provide a qualitative evaluation of the metal distribution from support particle to support particle. In general, the distribution of platinum was more uniform on alumina than silica, however, optimal uniformity was not achieved. This observation was based on wide variations In Pt/Sl and Pt/Al ratios measured by EDS. 

Synthesis conditions can to be tuned to enable a homogeneous or a peripheral distribution of M° nanoclusters through the body of the support particles. 

The presence of shielding compounds interferes with subsequent processes, as the formation of metal-support interactions is able to stabilize supported particles. Moreover, the shielding effect of the colloid protectors prevents the contact of metal particles with the reacting molecules, thus avoiding the use of unsupported colloidal particles as a catalytic system [11]. 

Stabilizer Stabilizer/Au Particle (wt/wt) dimension (sol) Support Particle dimension (supported)... 

Among various methods to synthesize nanometer-sized particles [1-3], the liquid-phase reduction method as the novel synthesis method of metallic nanoparticles is one of the easiest procedures, since nanoparticles can be directly obtained from various precursor compounds soluble in a solvent [4], It has been reported that the synthesis of Ni nanoparticles with a diameter from 5 to lOnm and an amorphous-like structure by using this method and the promotion effect of Zn addition to Ni nanoparticles on the catalytic activity for 1-octene hydrogenation [4]. However, unsupported particles were found rather unstable because of its high surface activity to cause tremendous aggregation [5]. In order to solve this problem, their selective deposition onto support particles, such as metal oxides, has been investigated, and also their catalytic activities have been studied. 

Figure 7. Au particles deposited on different supports (a) a-FeOOH, (b) p-FeOOH, (c) Z1O2 (A) (rough surface), (d) ZrOi (B) (smooth surface), and (e) Xi02 particles. Support particles were also prepared by the authors.

Supporting particles Size (pm) Structure Specific surface area (m /g) Yield (mol%) Size of... 

Table 2. Effect of support particle on the size of precursor and metal particles of Pt.

Figure 2. TCS-D strategy. A resin layer is produced outside each support particle. Metallation-reduction of the polymer shell will lead to size-controlled metal nanoclusters. Gentle thermal degradation of the organic shell will lead to size-controlled metal nanoclusters, expected to be evenly dispersed on the support particles surface.

Zhdanov VP, Kasemo B. 2002. Kinetics of electrochemical reactions from single crystals to nm-sized supported particles. Surf Sci 521 L655-L661. 

Where u, is the mobile phase velocity at the column outlet, Fg the column volumetric flow rate, and Ag the column cross-sectional area available to the mobile phase. In a packed bed only a fraction of the column geometric cross-sectional area is available to the mobile phase, the rest is occupied by the solid (support) particles. The flow of mobile phase in a packed bed occurs predominantly through the interstitial spaces the mobile phase trapped within the porous particles is largely stagnant (37-40). 

Transmission IR (TIR) spectroscopy if the solid in question is IR transparent over an appreciable range of wavelength. This is often used on supported metal catalysts, where the large metallic surface area permits a high concentration of adsorbed species to be sampled. The sample consist typically of 10-100 mg of catalyst, pressed into a self-supporting disk of approximately 1 cm2 and a few tenths of a mm in thickness. The support particles should be smaller than the wavelength of the IR radiation, otherwise scattering losses become important. 

Sharma, S., Ramakrishna, C., Desai, J.D., and Batt, N.M., Anaerobic biodegradation of petrochemical wastewater using biomass support particles, Appl. Microbiol. Biotechnol., 40, 768-771, 1994. 

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