Microstructured Catalytically Active Materials

The introduction of the catalyst presents one of the main problems in using MSRs for heterogeneously catalyzed reactions. There are some examples of reactors that are constructed directly from the catalytically active material. Kestenbaum et al. [145] used silver foils for the construction of a microchannel reactor for the partial oxidation of ethene to oxirane. A similar concept was proposed by Fichtner et al. [91,146], These authors used a microstructured rhodium catalyst for the partial oxidation of methane to syngas. This reaction can be considered as a coupling of the exothermic oxidation and the endothermic reforming of methane, which occur at different reaction rates. In such a case, the formation of a pronounced axial temperature profile can be avoided through the use of a material with high thermal conductivity. The reactor... 

An efficient oxidation catalyst, OMS-1 (octahedral mol. sieve), was prepared by microwave heating of a family of layered and tunnel-structured manganese oxide materials. These materials are known to interact strongly with microwave radiation, and thus pronounced effects on the microstructure were expected. Their catalytic activity was tested in the oxidative dehydrogenation of ethylbenzene to styrene [25]. 

The general requirements for an SOFC anode material include [1-3] good chemical and thermal stability during fuel cell fabrication and operation, high electronic conductivity under fuel cell operating conditions, excellent catalytic activity toward the oxidation of fuels, manageable mismatch in coefficient of thermal expansion (CTE) with adjacent cell components, sufficient mechanical strength and flexibility, ease of fabrication into desired microstructures (e.g., sufficient porosity and surface area), and low cost. Further, ionic conductivity would be beneficial to the extension of... 

Hence, catalysis related challenges for SOFC cathode are the development of cathode specifications, i.e., material and microstructure, having high catalytic activity for oxygen reduction at 600 °C, high electron and ion conductivity, and a low sensitivity for poisoning by volatile Cr species. Again, as for the anode, cost and compatibility related requirements have to be considered. 

When combining the separator and the reactor functions into one compact physical unit, factors related to catalysis need to be considered in addition to those related to selective separation discussed in previous chapters. The selection of catalyst material, dispersion and heat treatment and the strategic placement of catalyst in the membrane reactor all can have profound impacts on the reactor performance. The choice of membrane material and its microstructure may also affect the catalytic aspects of the membrane reactor. Furthermore, when imparting catalytic activity to inorganic membranes, it is important to understand any effects the underlying treatments may have on the permeability and permselectivity of the membranes. 

Bulk nanostructured materials are soUds with nanosized microstructure. Their basic units are usually nanoparticles. Several properties of nanoparticles are useful for applications in electrochemical sensors [67], However, their catalytic behavior is one of the most important. The high ratio of surface atoms with free valences to the total atoms has led to the catalytic activity of nanostructured SEs being used in electrochemical reactions. The catalytic properties of nanoparticles could decrease the overpotential of electrochemical reactions and even provide reversibility of redox reactions, which are irreversible at the bulk metal SE [68], Multilayers of conductive nanoparticles assembled on electrode surfaces produce a high porous surface with a controlled microenviromnent. These structures could be thought of as assemblies of nanoelectrodes with controllable areas. 

Abu-Surrah et al. [29] also reported on highly active 2,6-bis(arylimino) pyridine iron(II)- and cobalt(II)-based ethylene polymerization catalysts which lack the ortho alkyl substituents on the aryl groups. Modifications of the steric bulkiness of the aromatic groups in the tridentate ligands influenced not only the catalytic activity, but also the molecular weight, and for the first time the microstructure of the resulted material (Scheme 3.14) [28]. 

Taking into account all these factors, it has been found that the most appropriate supports for PFMFCs catalysts are carbon blacks of ca. 250 m g BFT surface area, and the most widely used is Vulcan XC-72R commercialized by Cabot. ° ° Due to the importance of the surface chemistry of these supports, and its influence in the supported active metal phase, ° ° different chemical modifications of the support have been also investigated. The chemical nature of the carbon surface produces different electronic interactions between the noble metals and the carbon support, and affects the metal particle morphology, and influences the catalytic activity. Additionally, during the last few years, new alternative materials to carbon blacks have also been used, especially on the basis of their porous structure (nanotubes, mosoporous carbons) or their microstructure (nano- and microfibers, and microspheres). 

In recent years there has been tremendous interest in porous ceramics because of their applications as filters, membranes, catalytic substrates, thermal insulation, gas-burner media and refractory materials. These are due to their superior properties, such as low bulk density, high permeability, high temperature stability, erosion/corrosion resistance and excellent catalytic activity. One branch of this field is porous SiC ceramics, owing to their low thermal expansion coefficient, high thermal conductivity and excellent mechanical properties. However, it is difficult to sinter SiC ceramics at moderate temperatures due to their covalent nature. In order to realize the low temperature fabrication of porous SiC ceramics, secondary phases may be added to bond SiC. Oxidation bonded porous SiC ceramics have been found to exhibit good thermal shock resistance owing to the microstructure with connected open pores. 

A viable process for manufacturing polyolefin-clay nanocomposifes by in situ polymerization requires adequate catalytic activity, desirable polymer microstructure, and physical properties including processibility, a high level of clay exfoliation fhaf remains stable under processing conditions and, preferably, inexpensive catalysf components. The work described in the previous two sections focused on achieving in situ polymerization with clay-supported transition metal complexes, and there was less emphasis on optimization of polymer properties and/or clay dispersion. Since 2000, many more comprehensive studies have been undertaken that attempt to characterize and optimize the entire system, from the supported catalyst to the nanocomposite material. The remainder of this chapter covers work published in the past decade on clay-polyolefin nanocomposites of ethylene and propylene homopolymers, as well as their copolymers, made by in situ polymerization. The emphasis is on the catalyst compositions and catalyst-clay interactions that determine the success of one-step methods to synthesize polyolefins with enhanced physical properties. 

Depending on the philosophy of the microdesign, the microstructure itself can be seen as a catalyst support or solely as a reactor wall. The catalytically active species can be used as raw material for microfabrication or can be deposited by conventional... 

An important factor for selectivity can be the microstructured material, for example when catalytically active species or supports are deposited on metals. Even... 

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