Immobilization of particles

Although the first reports of this approach involved studies with metal alloys [3] and minerals [4], within a few years the technique has been extended to a wide variety of research areas. As these findings have been summarized in several reviews [5-8] and also in a monograph [9], attention will be focused here on more recent developments, notably on the mechanical immobilization of particles on electrodes. Today, a huge amount of information is available for electrochemical systems comprising particles enclosed in polymer films or other matrices (see Refs [10-16]). Originally, the main aim of such particle enclosure was to achieve specific electrode properties (e.g., functionalized carbon/polymer materials as electrocatalysts [17, 18] solid-state, dye-sensitized solar cells [19]), rather than to study the electrochemistry of the particles. This situation arose mainly because the preparation of these composites was too cumbersome for assessing the particles properties. The techniques also suffered from interference caused by the other phases that constituted the electrode. 

Immobilization of Particles. If the particles involved are immobilized, sedimentation cannot occur. Neither can they encounter each other, whereby also aggregation is prevented. Since sedimentation or aggregation has generally to occur before fluid particles can coalesce, immobilization will also prevent coalescence. 

Table 17.2 indicates measures to be taken against physical instability of various kinds. To prevent particle motion, a very weak gel generally suffices. This is discussed in Section 13.3 under Immobilization of Particles. In several systems, the particles themselves can form the gel ... 

As we already said, the immobilization of particles on the surface is driven by the combined effects of capillary forces, both that drive meniscus deformation (and consequently particle confinement in the trapping structure) and that are exerted on the particles during meniscus break-up. Consequently, the assembly process must be sensitive to both the wetting properties of the colloidal suspension and the shape and dimension of the trapping structures. 

As the immobilization of particles on surfaces is due to the combination of geometrical confinement and capillary forces, it... 

These particles tend to sediment within a few minutes and are no longer sensitive to solvent fluxes created by evaporation. Nevertheless, the capillary forces exerted by the meniscus are sufficient to drag particles deposited on flat areas. This mechanism induces an accumulation of particles close to the contact line and finally leads to a selective immobilization of particles in the recessed areas of the template. Here also, the accumulation process is required to induce particle deposition. 

Electrodecantation or electroconvec tion is one of several operations in which one mobile component (or several) is to be separated out from less mobile or immobile ones. The mixture is introduced between two vertical semipermeable membranes for separating cations, anion membranes are used, and vice versa. When an electric field is apphed, the charged component migrates to one or another of the membranes but since it cannot penetrate the membrane, it accumulates at the surface to form a dense concentrated layer of particles which will sink toward the bottom of the apparatus. Near the top of the apparatus immobile components will be relatively pure. Murphy [J. Electrochem. Soc., 97(11), 405 (1950)] has used silver-silver chloride electrodes in place of membranes. Frilette [J. Phys. Chem., 61, 168 (1957)], using anion membranes, partially separated and Na, ... 

The next two examples illustrate more complex surface reaction chemistry that brings about the covalent immobilization of bioactive species such as enzymes and catecholamines. Poly [bis (phenoxy)-phosphazene] (compound 1 ) can be used to coat particles of porous alumina with a high-surface-area film of the polymer (23). A scanning electron micrograph of the surface of a coated particle is shown in Fig. 3. The polymer surface is then nitrated and the arylnitro groups reduced to arylamino units. These then provided reactive sites for the immobilization of enzymes, as shown in Scheme III. 

Successful applications of the oxygen-modified CNFs are reported on immobilization of metal complexes ]95], incorporation of small Rh particles [96], supported Pt and Ru CNFs by adsorption and homogeneous deposition precipitation ]97, 98], Co CNFs for Fischer-Tropsch synthesis ]99], and Pt CNFs for PEM fuel cells [100]. 

Catechin-immobilizing polymer particles were prepared by laccase-catalyzed oxidation of catechin in the presence of amine-containing porous polymer particles. The resulting particles showed good scavenging activity toward stable free l,l-diphenyl-2-picryl-hydrazyl radical and 2,2 -azinobis(3-ethylbenzothiazoline-6-sulfonate) radical cation. These particles may be applied for packed column systems to remove radical species such as reactive oxygen closely related to various diseases. 

Laboratory reactor for studying three-phase processes can be divided in reactors with mobile and immobile catalyst particles. Bubble (suspension) column reactors, mechanically stirred tank reactors, ebullated-bed reactors and gas-lift reactors belong the class of reactors with mobile catalyst particles. Fixed-bed reactors with cocurrent (trickle-bed reactor and bubble columns, see Figs. 5.4-7 and 5.4-8 in Section 5.4.1) or countercurrent (packed column, see Fig. 5.4-8) flow of phases are reactors with immobile catalyst particles. A mobile catalyst is usually of the form of finely powdered particles, while coarser catalysts are studied when placing them in a fixed place (possibly moving as in mechanically agitated basket-type reactors). 

E. S. Jensen, Mineralization-immobilization of nitrogen in. soil amended with low C N ratio plant residues with different particle sizes. Soil Biol. Biochem. 26 519 (1994). 

The development of combinations of two or more functional phases is one of the most promising approaches to develop novel materials with specific functional properties. It is known that the immobilization of nanosized noble metal particles... 

Common to all encapsulation methods is the provision for the passage of reagents and products through or past the walls of the compartment. In zeolites and mesoporous materials, this is enabled by their open porous structure. It is not surprising, then, that porous silica has been used as a material for encapsulation processes, which has already been seen in LbL methods [43], Moreover, ship-in-a-bottle approaches have been well documented, whereby the encapsulation of individual molecules, molecular clusters, and small metal particles is achieved within zeolites [67]. There is a wealth of literature on the immobilization of catalysts on silica or other inorganic materials [68-72], but this is beyond the scope of this chapter. However, these methods potentially provide another method to avoid a situation where one catalyst interferes with another, or to allow the use of a catalyst in a system limited by the reaction conditions. For example, the increased stability of a catalyst may allow a reaction to run at a desired higher temperature, or allow for the use of an otherwise insoluble catalyst [73]. 

Improvement of intraparticle mass transfer is the goal of some particle research efforts. One novel approach that has been recently tested is the co-immobilization of algae with bacteria the algae produced oxygen and the bacteria produced the desired product (Chevalier and de la Noue, 1988). Another method used microporous particles entrapped within alginate bead bioparticles to prevent excess biomass growth that could hinder intraparticle mass transfer (Seki et al., 1993). 

A continuous centrifugal bioreactor, in which cells are fluidized in balance with centrifugal forces, has been designed to allow high density cell cultivation and superior aeration without elutriation of the suspended cells (van Wie et al., 1991). Reactor performance was hampered by elutriation of biomass by evolved gas in an anaerobic fermentation, indicating that it may not be suitable in its present state for three-phase fermentations. Immobilization of the cells on denser particles may overcome this problem. 

Seki, M., Naito, K. I., and Furusaki, S., Effect of Co-Immobilization of Microporous Particles on the Overall Reaction Rate of Immobilized Cell Biocatalysts, J. Chem. Eng. Jpn., 26 662 (1993)... 

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