Catalysts hierarchical structure

In all of these systems, chemical reactions have a hierarchical structure. What we normally think of as one reaction is actually composed from several elementary steps, which are often called, collectively, the mechanism of the reaction. For processes with a solid catalyst, these steps describe the interaction of the catalyst s active sites with the fluid-phase reactants. For biochemical reactions, there are association and dissociation steps that involve complexes of the enzyme and the substrates. In all cases, unstable, short-lived intermediates may be formed and destroyed in a rapid succession of steps the overall reaction we observe might not... 

Aligned multiwall CNT arrays were synthesized as a basis for a microstructured catalyst, which was then tested in the Fischer-Tropsch reaction in a microchannel reactor [269]. Fabrication of such a structured catalyst first involved MOCVD of a thin but dense A1203 film on a FeCrAlY foam to enhance the adhesion between the catalyst and the metal substrate. Then, multiwall CNTs were deposited uniformly on the substrate by controlled catalytic decomposition of ethene. Coating the outer surfaces of the nanotube bundles with an active catalyst layer results in a unique hierarchical structure with small interstitial spaces between the carbon bundles. The microstructured catalyst was characterized by the excellent thermal conductivity inherent to CNTs, and heat could be efficiently removed from the catalytically active sites during the exothermic Fischer-Tropsch synthesis. 

As well as the crystal structure of the microporous catalysts, the secondary mesoporosity is also important, because molecular transport to and from the active sites is favoured in these materials. In steamed Y the mesoporosity and extra-framework aluminium results in a very active catalyst for cracking. Designed hierarchical structures, in which nanoparticles of zeolites are joined together to and connected by a secondary mesopore system for the same reason are discussed further in Chapter 10. 

Several sessions were included in scientific program. The session Bio-inspired Polymers included 11 oral presentations. Self-assembly of an aquaporin mimic, tailoring surface properties with polymer brushes, bioinspired block copolymers, hierarchically structured conjugated polymers via supramolecular self-assembly, natural polymeric composites with mechanical function, macromolecular oxidation catalysts based on miniemulsion polymerization and some other problems were discussed on this session. 

Only recently, carbon nanotubes and graphene attracted increasing attention as catalyst support in fuel cells and other applications due to their favorable properties. Also the use of hierarchical structures, which are obtained by templating [74,75], as well as mesoporous carbons has received a proper share of interest. For the broad application of these more sophisticated carbon structures, however, their cost needs to be significantly reduced. Among these promising carbon-based materials, the use of CNTs as support in electrocatalysis will be described in some detail in the following. 

To introduce additional functionality and to allow for a wider variety of structural features, branching of the primary CNF/CNT with secondary, usually thirmer ones, has been performed [113,114]. Primary as well as secondary carbon nanostructures were grown in CVD processes. After electrodepositing iron as growth catalyst and subsequent CNT growth, secondary CNTs were grown in a similar fashion to yield hierarchically structured layers. The principles of such an approach are visualized in Figure 10.22. It was shown with impedance spectroscopy and other techniques, that the electrochemical properties of such composites are superior to those of unbranched CNF. 

The proper design and organization of hierarchically structured electrodes optimize the bioelectronic performance and enhance mass transport of fuel to the biocatalysts. By using nanostructured materials at the interface, the relative surface area for catalyst loading is increased compared with bulk materials and the distance between catalyst and the conductive surface may be decreased to enhance DET. Nanostructured modification or functionalization of 2D surfaces may enhance power output however, the essentially planar surface can limit scalability. 

Y. Xia, Z. Shi and Y. Lu, Gold microspheres with hierarchical structure/ conducting polymer composite film preparation, characterization and application as catalyst. Polymer 51, 1328-1335 (2010). 

The search for better catalysts has been facilitated in recent years by molecular modeling. We are seeing here a step change. This is the subject of Chapter 1 (Molecular Catalytic Kinetics Concepts). New types of catalysts appeared to be more selective and active than conventional ones. Tuned mesoporous catalysts, gold catalysts, and metal organic frameworks (MOFs) that are discussed in Chapter 2 (Hierarchical Porous Zeolites by Demetallation, 3 (Preparation of Nanosized Gold Catalysts and Oxidation at Room Temperature), and 4 (The Fascinating Structure... 

The subgroups were determined based on a hierarchical decomposition strategy for a FCCU (Ramesh et al, 1992). The FCCU was decomposed into four separate units feed.system, catalyst.system, reactor/regenera-tor.system, and separation.system as shown in Fig. 32. Each of these units was further divided into more detailed functional, structural, or behavioral... 

In view of catalytic potential applications, there is a need for a convenient means of characterization of the porosity of new catalyst materials in order to quickly target the potential industrial catalytic applications of the studied catalysts. The use of model test reactions is a characterization tool of first choice, since this method has been very successful with zeolites where it precisely reflects shape-selectivity effects imposed by the porous structure of tested materials. Adsorption of probe molecules is another attractive approach. Both types of approaches will be presented in this work. The methodology developed in this work on zeolites Beta, USY and silica-alumina may be appropriate for determination of accessible mesoporosity in other types of dealuminated zeolites as well as in hierarchical materials presenting combinations of various types of pores. 

The inherent limitations of the use of zeolites as catalysts, i.e. their small pore sizes and long diffusion paths, have been addressed extensively. Corma reviewed the area of mesopore-containing microporous oxides,[67] with emphasis on extra-large pore zeolites and pillared-layered clay-type structures. Here we present a brief overview of different approaches to overcoming the limitations regarding the accessibility of catalytic sites in microporous oxide catalysts. In the first part, structures with hierarchical pore architectures, i.e. containing both microporous and mesoporous domains, are discussed. This is followed by a section on the modification of mesoporous host materials with nanometre-sized catalytically active metal oxide particles. 

Figure 4.2.2 Hierarchical catalyst structure with the schematic representation of active sites.

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