Ceramic monolithic structures

Ceramic monolithic structures made from inverted opal silicon carbonitride (see Figure 6.6) were presented by Mitchell et al. [460]. The inverse opal structure was achieved using polystyrene templates. They prepared monoliths of 74% porosity, typically 350-pm wide, 100-pm high and 3-mm long. Propane steam reforming was then successfully performed in the reactor (see Section 7.1.2). 

Figure 9.1 Monoliths and arrayed structures, (a) Ceramic monoliths with parallel channels. (Reprinted from [4].)...

Catalysts can be metals, oxides, sulfides, carbides, nitrides, acids, salts, virtually any type of material. Solid catalysts also come in a multitude of forms and can be loose particles, or small particles on a support. The support can be a porous powder, such as aluminium oxide particles, or a large monolithic structure, such as the ceramics used in the exhaust systems of cars. Clays and zeolites can also be solid catalysts. 

Initially, packed beds were also used. They, however, were no success, and at present monoliths are applied exclusively. This should not be misunderstood. Monolith means literally a single stone. However, metal-based analogues are also included in the definition of monolith. In fact, for catalytic converters in cars, in addition to ceramics, metal-based monoliths have been and still are used. A major advantage of metal was the thin wall thickness that could be achieved. Later, industry succeeded in manufacturing ceramic structures of comparable wall thickness. In view of their higher resistance against corrosion, ceramic monoliths are now more generally applied than metal ones. 

Comparing the performance of the micro structured reactor with a ceramic monolith at 230 °C reaction temperature and a GHSV of 300 000 h 1, the conversion in the micro reactor was 94%, whereas 86% was found for the monolith, which was attributed to the improved heat and mass transfer in the metallic micro-structures (see Figure 2.89). The GHSV value of 500 000 h 1 corresponds to a dry gas flow rate of440 Ndm3 h 1. However, the stability of the catalyst coated on the monolith was superior to that of the catalyst coated on the micro structured stainless-steel plates. 

Multi-channel ceramic monoliths (Fig. 7.4) are now the primary choice as support structures to carry the active catalytic species for cleaning emissions from various sources of pollution.5 Figure 7.4 shows the shapes used for both automotive and stationary pollution abatement applications. 

Besides ceramic monoliths, metallic monoliths are available (64). In comparison with ceramic monoliths, metallic monoliths can be produced in more advanced structures, for example, to create turbulence in the flow in the channels (65). Several structured catalyst supports, such as solid foams or Sulzer packings, are usually made from metal. The surface area of the metal itself will be usually too low for practical applications. 

The microstructure of monoliths is important particularly with ceramic monoliths when the chemical nature and structure of the crystalline and glass phases, together with the pore structure, determine the thermal expansion, thermal conductivity, melting point, surface area, and strength of the... 

Honeycomb structures offered to the market have a similar cell density as the ceramic ones. After welding inlet and outlet cones to the outer shell, the metallic monolithic converter can be inserted directly into the exhaust gas pipe, which means that the canning procedure used for the ceramic monoliths is not needed anymore. 

Finally, it should be noted that metallic monoliths allow for some additional design freedom over ceramic monoliths. Numerous examples of this are found in the literature (Fig. 35) [28]. One example shows a metallic structure consisting of a macroscopic corrugated foil and a microscopic corrugated foil. The microscopic corrugation considerably increases the geometrical surface area of the structure. A further example shows a metallic monolith in which the channels interconnect to... 

Ceramic and metallic monolith structures have a geometrical surface area in the range 2.0-4.0m T support volume. This is much too low to adequately perform the catalytic conversion of the exhaust gas components. Therefore, these structures are coated with a thin layer of a mixture of inorganic oxides, some of which have a very high internal surface area. This mixture is called the washcoat (Fig. 36). 

The preparation and use of active catalysts coated on a structure packing was further studied as an attractive replacement for conventional catalysfs in randomly packed beds or slurry reactions. A method was developed in which catalytically active and selective BEA coatings could be prepared onto ceramic monoliths constituted either of pure silica or cordierite (Figure 4.7a) and mefallic wire gauze packings (Figure 4.7b). ... 

Two basic catalyst structures were used, distinguished by the configuration of the catalyst support. The two support types are alumina pellets and alumina coated ceramic monoliths (Figure 2). The pellets are approximately 1/8th inch in diameter and are composed of thermally stable transitional alumina. The monoliths are made of a ceramic material such as cordierite (2Mg,2Al203,5Si02). 

A typical three way catalyst consists of a honeycomb monolith structure of a ceramic material such as cordierite, Al Mg2Si50(g (ref. 2). The ceramic surface is provided with a layer of high surface area alumina as a washcoat which then will act as a substrate for the active ingredients. The thickness of the washcoat is usually not uniform but varies in the range 10-150jum (ref. 3). The washcoat may amount to 5-15 wt% of the monolith, and may provide 15-30 m /g of surface area (ref. 9). 

The bare monolithic structures used were made of cordierite (a ceramic material consisting of magnesia, silica and alumina in the ratio of 2 5 2). The main characteristics are shown in Table 1. 

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