Porous/hollow structures

Since in some cases the previously described alloy, dealloyed, and controlled-crystal-face catalysts also develop porous/hollow structures, it is of particular interest to determine to what extent the hollow structure affects the high ORR activities seen in those catalysts. Focus points for future research should include (i) developing scalable synthesis techniques and (ii) determining whether the surface and bulk diffusion rates of Pt in these hollow structures, relative to the fuel cell life, are sufficiently slow for this type of catalyst to be practical. 

The hollow fine fiber configuration (refer to Figure 51) consists of a bundle of porous hollow fine fibers. These fibers are externally coated with the actual membrane and form the support structure for it. Both ends of each fiber are set in a single epoxy tube sheet, which includes an 0-ring seal to match the inside diameter of the pressure vessel. 

Severin and coworkers reported (146) the reaction of tris(2-aminoethyl)amine and 4-formylphenylboronic acid with penta-erythritol to give, via multicomponent assembly, the boronic acid based macrobicyclic cage 35 (Fig. 25). The cage has the form of an ellipsoid with a diameter of 20.5 A and binds two Cud) ions in a fashion similar to the smaller tren-based cryptands. The reversible formation of boronic esters has also been employed to build other hollow structures such as nanotubes (147) and porous covalent organic frameworks (148,149). 

The detection of other molecules, such as ammonia, requires the use of a porous catalytic metal. To obtain a gas response from the NH3 molecule, it is believed that active sites of triple points are required where the molecules are in contact with the metal, insulator, and ambient [30, 31]. It has been shown that gas species such as hydrogen atoms or protons also diffuse out onto the exposed oxide surface in between the metal grains [Figure 2.1(b)] [32, 33]. Furthermore, Lofdahl et al. have performed experiments that provide clear evidence that hydrogen atoms or protons also diffuse under the metal from the triple point [34]. The hollow structure of the metal surface facing the insulator has been revealed by Abom et al. [35]. 

Besides hollow channels, porous lamellar structures have also been prepared via the comb-shaped supramolecules route. PS-fo-P4VP diblock copolymers were taken that, together with zinc dodecylbenzenesulfonate [187],... 

The most simple form is a single, uniformly structured wall of a certain material, the so-called symmetric, stand-alone membranes. Examples are dense metal or oxide tubes and porous hollow fibres. To obtain sufficient mechanical strength, single-walled symmetric systems usually have a considerable thickness. 

As a self-supported cylinder, hollow fiber membrane is required to withstand high transmembrane pressure without collapsing. Modulus of elasticity is a crucial parameter for the calculation of the collapse pressure of a given fiber. With a much more porous overall structure, asymmetric hollow fibers specifically require a high modulus of elasticity to avoid collapse of the... 

An alternative approach is illustrated in Figure 11.20b and involves a catalytic coating of the porous inner structure of the filter elements. In addition, there is potential here for increasing the catalyst capacity by filling the free hollow cylindrical volume of the filter element with catalytic particles or foam of high active surface area. 

Fig. 3.17 Schematic diagrams for the preparation of hollow structures using the (a) self-assembled hydrothermal/sol-vothermal reaction, (b) Ostwald ripening of porous secondary particles, and (c) solid evacuation by the KirkendaU effect (Reprinted with permission from Lee (2009), Copyright 2009 Elsevier)...

Membranes can be thought of as special types of films that provide specific end use characteristics. Membrane technology has replaced some conventional techniques for separation, concentration or purification [78]. Applications include desalination, dialysis, blood oxygenators, controlled release drug delivery systems and gas separation. Processing of polymer films and membranes is well known to affect the morphology, which in turn affects the physical and mechanical properties. As is true for all films, membrane separation properties are based on both the chemical composition and the structure resulting from the process. Membranes are produced in two major forms, as flat films and as porous hollow fibers, both of which will be discussed in this section. 

The preparation of porous hollow silica nanoparticles for a drug delivery applications was described by Chen et al. The authors used the time release profile of cefradine as a test example. Caruso et al. " prepared hollow silica spheres and silica-polymer layered composite spheres. A controlled pore structure remained after decomposition of the organic material and possible applications in medical and pharmaceutical applications are described in their publication. Another drug release example was described by Chamay et al., ° who incorporated ibuprofen, a well... 

The pores were either partially or completely filled with polypeptide (Figure 3(b) and (c), respectively) depending on the polymerization time. At 115 °C and for Ih deposition time, hollow structures were obtained with a wall thickness around 80nm. A further increase in polymerization time up to 2h converted these nanotubes into solid nanorods (Figure 3(c)). Besides the polymer formation inside the pores, a thick polypeptide film covered the surfaces of the porous alumina templates. The excess film on top of the alumina templates was removed with a Microtome Ultra Cutter. These nanorods were released from the template... 

The third liquid-membrane technique is the hollow fiber contained liquid membrane (HFCLM). In the SLM/ ILM technique, the liquid membrane is in contact with the feed liquid/feed gas and the strip liquid/permeate gas. The membrane liquid may be lost by solubilization/volatiliza-tion in addition, there may be irreversible reactions with extractants, complexing agents inside the liquid membrane, which could reduce the performance over time. The HFCLM structure can take care of such problems. In this structure (Figure 8.1.50), the membrane module (cylindrical or otherwise) has two sets of porous hollow fiber membranes. The shell-side interstitial space between the fibers is filled with a liquid acting as the membrane. If this membrane liquid wets the membrane pores and is therefore present in the pores, the pressure conditions in the feed liquid and the sweep/strip fluid should be such (higher) that the membrane liquid is not dispersed in the feed/sweep/stilp fluids. If the membrane liquid does not wet the membrane pores, but the feed/sweep fluids do, then the membrane liquid pressure should be higher than the feed/sweep fluid pressures. 

In the fabrication of FIFMMRs, the porous hollow fiber membranes with asymmetric structures are first prepared through a phase-inversion/ sintering technique, which has been described in Chapter 2 of this book and in many recent publications [31]. The microstructure of the hollow fibers can be tailored as expected for different applications by modulation of the suspension composition and the spinning parameters [32, 33]. Aran et al. [5,21] developed porous AI2O3 FIFMMRs with various geometrical parameters for gas-liquid-solid (G-L-S) reactions. The Pd-AhOa catalyst... 

Rhodium nanoparticles encapsulated in a hollow porous carbon were used as catalyst in the hydrogenation of 3-hydroxypyridines to obtain 3-hydroxypiperidines [158]. The porous wall structure of the carbon shell presents channels... 

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