Catalyst supports support structure

The introduction of automobile exhaust catalysts in the United States and elsewhere has produced a major market for platinum-type oxidation and reduction systems. An innovative consequence of this industry has been the development of ceramic honeycombed monoliths as catalyst supports. These structures contain long, parallel channels of less than 0.1 mm in diameter, with about SO channels per square centimeter. The monolith is composed of cordierite (2MgO - 2AI2O) SSiOj) and is manufactured by extrusion. A wash coat of stabilized alumina is administered prior to deposition of the active metal, either by adsorption or impregnation methods. 

The contemporary interest in metal organophosphonate coordination chemistry has received considerable impetus from the applications of such materials as sorbents, catalysts and catalyst supports. The structural chemistry of the metal organophosphonate system is extremely rich " and is represented by mononuclear coordination complexes, molecular clusters, " one-dimensional materials, and layered phases. Extensive investigations have been reported on layered phosphonates of divalent, " trivalent, tetravalent, " and recently hexavalent elements. Several examples of three dimensional metal organophosphonate frameworks have also been described since the first report by Bujoli. " ... 

Optimize the choice of metals and metal loadings for the CGO-based catalysts supported on structured forms, such as monoliths or foams. 

From a scientific standpoint, there is an interesting relationship between the fouling and poisoning mechanisms that are typical of silica deactivation, and the poisoning mechanism that more accurately describes phosphorous deactivation. It is of interest to carefully review the dependence and interplay of these processes as a function of the reactant, catalyst support, pore structure, metal loading, and oxidation conditions. 

Figure 8.40 The k ySk) extended X-ray absorption fine structure (EXAFS) signal, Fourier transformed and then retransformed after application of the filter window indicated, in (a) osmium metal and (b) a 1% osmium catalyst supported on silica. (Reproduced, with permission, Ifom Winnick, FI. and Doniach, S. (Eds), Synchrotron Radiation Research, p. 413, Plenum, New York, 1980)...

The metallic catalyst support can be in form of chips, open-mesh and reinforced wire structures, and staggered layers of metal screens or saddles. In one design, screens woven from metallic wires 0.01 to 0.03 in. diam are placed in a deep stack. In another design, metal foils 0.004 in. thick are perforated and expanded to form a screen, which is then rolled into a cylinder. See Fig. 12. 

Gulati, S.T., Makkee, M., and Setiabudi, A. (2006) Ceramic catalysts, supports and filters for diesel exhaust aftertreatment, in Structured Catalysts and Reactors, 2nd edn. Chapter 19 (eds A. Cybulski and J.A. Moulijn), CRC Taylor Francis, Boca Raton, p. 663. 

Amorphous silica, with pore sizes in the range 1-10 nm is a common support for base catalysts, whilst more structured pore sizes can be made by what is known as the sol-gel method. In this method a micelle is... 

Mesoporous carbon materials were prepared using ordered silica templates. The Pt catalysts supported on mesoporous carbons were prepared by an impregnation method for use in the methanol electro-oxidation. The Pt/MC catalysts retained highly dispersed Pt particles on the supports. In the methanol electro-oxidation, the Pt/MC catalysts exhibited better catalytic performance than the Pt/Vulcan catalyst. The enhanced catalytic performance of Pt/MC catalysts resulted from large active metal surface areas. The catalytic performance was in the following order Pt/CMK-1 > Pt/CMK-3 > Pt/Vulcan. It was also revealed that CMK-1 with 3-dimensional pore structure was more favorable for metal dispersion than CMK-3 with 2-dimensional pore arrangement. It is eoncluded that the metal dispersion was a critical factor determining the catalytic performance in the methanol electro-oxidation. 

As surface area and pore structure are properties of key importance for any catalyst or support material, we will first describe how these properties can be measured. First, it is useful to draw a clear borderline between roughness and porosity. If most features on a surface are deeper than they are wide, then we call the surface porous (Fig. 5.16). Although it is convenient to think about pores in terms of hollow cylinders, one should realize that pores may have all kinds of shapes. The pore system of zeolites consists of microporous channels and cages, whereas the pores of a silica gel support are formed by the interstices between spheres. Alumina and carbon black, on the other hand, have platelet structures, resulting in slit-shaped pores. All support materials may contain micro, meso and macropores (see text box for definitions). 

In ecent years the utility of extended X-ray absorption fine structure UXAFS) as a probe for the study of catalysts has been clearly demonstrated (1-17). Measurements of EXAFS are particularly valuable for very highly dispersed catalysts. Supported metal systems, in which small metal clusters or crystallites are commonly dispersed on a refractory oxide such as alumina or silica, are good examples of such catalysts. The ratio of surface atoms to total atoms in the metal clusters is generally high and may even approach unity in some cases. 

The results of the EXAFS studies on supported bimetallic catalysts have provided excellent confirmation of earlier conclusions (21-24) regarding the existence of bimetallic clusters in these catalysts. Moreover, major structural features of bimetallic clusters deduced from chemisorption and catalytic data (21-24), or anticipated from considerations of the miscibility or surface energies of the components (13-15), received additional support from the EXAFS data. From another point of view, it can also be said that the bimetallic catalyst systems provided a critical test of the EXAFS method for investigations of catalyst structure (17). The application of EXAFS in conjunction with studies employing ( mical probes and other types of physical probes was an important feature of the work (25). 

However, these techniques may not detect important phenomena taking place on the surface of or within the interior of individual Inm-to Ipm-diameter inorganic particles that are s3rnthesized specifically for their catalytic activity. AEM is an extremely useful technique for analysis of the individual heterogeneous catalyst particle and its relationship to various supporting materials. Structural and chemical analyses can be obtained from specimen regions nearly 1000 times smaller than those studied by conventional bulk analysis techniques. This high lateral spatial... 

Thin sections cut with a diamond knife microtome can be of great advantage in locating regions of catalyst where important chemical or structural changes take place during reaction. Comparison of equivalent areas of fresh and deactivated catalyst can be a difficult problem if the catalyst support does not have a uniform microstructure as in carbon supports produced from plant materials. Even when specimen selection and preparation are adequate, it may be difficult to know upon which image features to place the electron beam to solve the problem at hand. 

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