Fibrous Structured Catalysts

Fibrous structured catalysts consist of threads that are knitted or woven into fabrics, felts, cloths, and so on or are formed by chemical or mechanical processing of ceramic or metal sheets. Figure 9.2a,b shows typical examples. A review can be found in [66]. 

Structure The polymers are produced as powders or as films on the electrodes. Most conductive polymers have a fibrous structure, each fiber consisting of hundreds of strands of polymer molecules. Techniques exist to control fiber preparation so as to obtain nanofibers expected to be particularly useful as catalyst substrates and in electronic applications (MacDiannid, 2000). 

The conversion of acetylene on an iron catalyst on Si02-support is a typical example. In this process, acetylene is thermally decomposed by leading it over a bed of catalyst within a quartz tube heated at about 700 °C (500-1000 °C, generally). Apart from the desired MWNT, there are also larger, fibrous structures and layers of amorphous graphene observed. These tend to coat the catalyst particles. The bamboo-like nanotubes (Section 3.3.4) usually obtained from this method are often covered with amorphous carbon too and, in parts, they are considerably curved. In addition to these bent species, there is also a spiral or helical structure... 

Another CTA preparation is the fibrous acetylation method. In this process part or all of the acetic acid is replaced with an inert diluent, e.g., toluene, benzene or hexane to maintain the cellulose fibrous structure during the reaction. Perchloric acid is usually employed as the catalyst in this process. This process is used exclusively for the preparation of cellulose triacetate. 

The more porous, fibrous structure of polyaniline (26) makes it a more attractive support than polypyrrole, and the high dispersion of the catalyst that can be achieved has produced current densities for methanol oxidation as high as 65 mA cm (26). The polyaniline support also appears to prevent the absorption of CO on Pt, resulting in less poisoning (28). 

Chemical Treatment of Paper. Test samples were treated with aqueous copper(II) or iron(II) sulfate solutions or with nonaqueous copper(II) or iron(III) acetylacetonate solutions. All chemical treatments were designed to obtain extensive and uniform penetration into the paper structure. To facilitate contact between paper and solution and to provide physical support, test samples were interleaved with fibrous sheets of nonwoven polyester. Sorption of metal species from aqueous media was achieved by immersion of paper samples into the solution of choice for 16-18 h. The metal-catalyst content of paper was varied by adjusting the solution concentration. The concentration ol the aqueous metal salt solutions was varied from 10 3 to 10 1 M. One liter of solution was used for every 25 sheets of paper. At the end of the treatment period, paper samples treated in aqueous media were washed with water. 

The configuration which a given monomer molecule assumes upon adsorption on the catalyst surface or on being complexed with the catalyst is related to the structure and/or nature of the effective sites. The influence of structure is seen in the differing catalytic activity of a-TiCls (layer structure) and (fibrous... 

Porosity is one of the most important continuum-scale parameters. It is defined as the fraction of the total volume that comprises void space e = Woid/ ktotai-Equivalently, the solid volume fraction ((f) = 1 - e) is generally used for fibrous materials or other open structures. The term microporosity implies that the particles in a porous medium are themselves porous, usually at a much smaller scale. A common example is porous catalyst in a packed-bed reactor. 

As an alternative to foams, fibrous materials in the form of tissues or filters may be used as structured catalysts [53,62-65]. Of particular interest for MSRs are sintered metal fibers (SMFs) [64,65]. These materials have open and homogeneous structures with porosities of 70-90% and high thermal conductivities, which ensure homogeneous temperatures in the catalytic bed. SMF materials consist of thin metallic fibers of 10-20 pm diameter. The small fiber diameters ensure high external fluid-solid mass transfer [66]. The fibers can be covered with a homogeneous layer of zeolitic material [66] or a washcoat, which can be impregnated with an active material [67,68]. Luther et al. [67] integrated the SMF catalyst in a... 

FIGURE 18 Examples of structured fibrous catalyst supports. 

V. Holler, I. Yuranov, L. Kiwi-Minsker, A. Renken, Structured multiphase reactors based on fibrous catalysts Nitrite hydrogenation as a case study, Catal. Today 69 (2001) 175. 

Fibrous proteins provide the structural material of many tissues in the body. They are the chief constituents of hair, cartilage, and muscles. Fibrous proteins consist of lengthwise bundles of polypeptide chains (a fiber). Globular proteins consist of polypeptide chains folded into a spherical shape they are found in the bloodstream where they transport and store various substances, act as antibodies (fight infections), act as enzymes (catalysts), and participate in the body s various regulatory systems. 

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