Metal porous structure, control

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

Although with the selective removal proecdurc the active component and the support are usually verv intimately mixed, it is difficult to control the porous structure and/or the mechanical strength ol the result ing eatalyst bodies Nonetheless the procedure is difficult to beat for the production ol highly loaded supports The most well known example of selective removal, the preparation of Raney metals, where alu minum is selectively removed, leaves behind almost exclusively the desired active metal... 

The most commonly used hard templates are anodic aluminum oxide (AAO) and track-etched polycarbonate membranes, both of which are porous structured and commercially available. The pore size and thickness of the membranes can be well controlled, which then determine the dimension of the products templated by them. The pores in the AAO films prepared electrochemically from aluminum metals form a regular hexagonal array, with diameters of 200 nm commercially available. Smaller pore diameters down to 5 nm have also been reported (Martin 1995). Without external influences, capillary force is the main driving force for the Ti-precursor species to enter the pores of the templates. When the pore size is very small, electrochemical techniques have been employed to enhance the mass transfer into the nanopores (Limmer et al. 2002). 

The presence of metal atoms in porous organosilicon polymers gives rise to active sites necessary for specific adsorption interactions. The use of various types reactants as well as variation of reactant ratios and synthesis conditions allows for the production of new adsorbents with controllable porous structures and selective adsorption properties. 

Although it does not exhaust the entire range of porous materials, the list attempts to cover those that can be described in terms of extended porous structures and whose electrochemistry has been extensively studied. In addition, since 1990 there has been a growing interest in the preparation of nanostructures of metal and metal oxides with controlled interior nanospace, whereas a variety of nanoscopic poro-gens such as dendrimers, cross-linked and core-corona nanoparticles, hybrid copolymers, and cage supramolecules are currently under intensive research (Zhao, 2006). Several of such nanostructured systems will be treated along the text, although, for reasons of extension, the study in extenso of their electrochemistry should be treated elsewhere. 

Concerning the two-layer model, the thickness and properties of each layer depend on the nature of the electrolyte and the anodisation conditions. For the application, a permanent control of thickness and electrical properties is necessary. In the present chapter, electrochemical impedance spectroscopy (EIS) was used to study the film properties. The EIS measurements can provide accurate information on the dielectric properties and the thickness of the barrier layer [13-14]. The porous layer cannot be studied by impedance measurements because of the high conductivity of the electrolyte in the pores [15]. The total thickness of the aluminium oxide films was determined by scanning electron microscopy. The thickness of the single layers was then calculated. The information on the film properties was confirmed by electrical characterisation performed on metal/insulator/metal (MIM) structures. 

The syntheses of materials with tailored properties have been proposed to improve the performance of the fuel cells [12, 34,35] by increasing the dispersion of the metal, decreasing the formation of agglomerates of catalyst nanoparticles, and raising the diffusion of species from/to the electroactive area. Several synthesis routes produce a carbon with tailored porous structure and composition by controlling the properties of the carbon precursor. The fabrication method to obtain a porous material, in general, can be enclosed in three steps ... 

The fuel processing reactors contain different types of flow internals in order to maximize contact surfaces or increase flow turbulence. These internals, made of metal or ceramic, have either an ordered chaimel structure or a chaotic character. Fully resolving the small-scale porous structures is extremely time consuming and computationally intensive. These structures are therefore simplified and represented as porous bodies. A porous structure is not modeled in FLUENT as a solid body penetrated by a fluid, but is only taken into account in terms of its flow interactions and a heat balance averaged over the control volume. Taking the porosity y into account, the Navier-Stokes equations can be written as... 

Other Uses. Carbon blacks are used to make highly porous structures for catalyst supports. Most supports are metal oxides rather than carbon, but in certain circumstances a chemically inert support is required. The open structure of the carbon black aggregates allows control of the pore size distribution within the consolidated carbon body. 

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