Monolithic Metallic Structures

The refractory metals for which CVD is commonly used to produce free-standing shapes are tungsten, niobium, rhenium, tantalum, molybdenum, and nickelb lb lb l (see Ch. 6). Shapes presently produced include rods, tubes, crucibles, manifolds, ordnance items, nozzles, and thrust chambers. They are usually deposited on a disposable mandrel of copper, molybdenum, or graphite which is subsequently machined off or removed chemically by etching. 

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Finally, applications of small monolithic metal structures orientated or randomly packed in larger reactors as catalysts for endothermic and exothermic reactions (e.g. selective oxidations) have been reviewed in Ref. 9. 

It is truly remarkable that catalysts can function so well in the exhaust of the modem highspeed vehicle. This fact has raised confidence in industry to use different monolithic (ceramic and metal) structures as supports for catalysts for other environmental applications such as diesel exhausts, power and chemical plants, restaurants, and even on widebody aircraft. 

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. 

Monoliths made out of metal were introduced to the market to some extent as an alternative to ceramic supports. As shown in Fig. 33, these supports consist of a metallic outer shell, in which a honeycomb-like metallic structure is fixed. The honeycomb is formed by alternate flat and corrugated thin metal foils (Fig. 34). These foils are made out of corrosion- and high-temperature-resistant steel, and are about 0.05 mm thick. 

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... 

The physical form of the support has to be chosen with a view to the type of reactor in which its use is intended. Silica and alumina are available as coarse granules or fine powders, and may be formed into various shapes with the aid of a binder (stearic acid, graphite) they can then be used in fixed bed reactors. For fluidised beds, or for use in liquid media, fine powders are required. Ceramic monoliths having structures resembling a honeycomb are used where (as in vehicle exhaust treatment) very high space velocities have to be used, but they are made of a non-porous material (a-alumina, muUite) and have to have a thin wash-coat of high area alumina applied, so that the metal can be firmly affixed. 

Pellets and ceramic monolithic substrate structures were initially involved in three-way catalytic converters for washcoat deposition, while metal foil monolithic substrates were also introduced since the late 1970s. TWCs manufactures were soon concentrated on cordierite (2Mg0-2Al203-5Si02) ceramic monoliths or on Fe-based alloys foil monoliths (iron-chromiimi—alimiimmi ferritic steels). Both options are used nowadays, although ceramic monoliths are preferably used, despite the several advantages of the latter [2]. 

Monolithic metallic alloys are among the most widely used structural materials. By reinforcing them with continuous fibers, discontinuous fibers, whiskers, and particles, new materials are created with enhanced or modified properties, such as higher strength and... 

There are only a few reports on nonaqueous sol-gel process applied for the formation of silica. The published literature describes the formation of monolithic silica structures [25], mixed silica metal oxide compounds [26], and the generation of a silica phase within a polymer matrix [27]. There are no reports that describe the synthesis of silica nanoparticles via the nonhydrolytic sol-gel process. One of the inherent problems might be that this type of reaction does not generate charged silica surfaces, which are usually required for the stabilization of the particles. Contrarily, a large variety of binary and ternary metal oxides can be formed. 

In these procedures, monoliths or, in general, metallic stmctures are dipped into suitable slurries, kept in the particle dispersion for a certain period of time and finally withdrawn. Once the metallic monolith is withdrawn it must be drained and the excess slurry eliminated. To form a thin oxide layer on the metal surface the metallic structure has to be dried and calcined to suitable temperatures. The procedure ends by evaluating the produced washcoated material by measuring three characteristics the specific obtained load (mg/cm ), the homogeneity of the coating (by optical or electron microscopy) and the adherence (by the ultrasonic bath test). Relevant parameters controlling this procedure are discussed below. 

Another important constraint comes from the pressure drop across the catalytic bed, which must be kept to a minimum to avoid a loss in engine power and performance. This requirement is satisfied by a very shallow pellet bed of no more than ten pellets deep, a monolithic structure with many open parallel channels, or a pile of metallic screens and saddles. 

The three principal catalyst bed configurations are the pellet bed, the monolith, and the metallic wire meshes. An open structure with large openings is needed to fulfill the requirement of a low pressure drop even at the very high space velocities of 200,000 hr-1. On the other hand, packings with small diameters would provide more external surface area to fulfill the requirement for rapid mass transfer from the g .s stream to the solid surface. The compromise between these two ideals results in a rather narrow range of dimensions pellets are from to 1 in. in diameter, monoliths have 6 to 20 channels/in., and metallic meshes have diameters of about 0.004 to 0.03 in. 

Metal monoliths show good thermal characteristics. A typical support with herringbone channels made from Fecralloy performed satisfactory in automotive applications [27]. Modeling showed that overall heat transfer was about 2 times higher than for conventional pellets [28,29]. Hence, there is potential for structured catalysts for gas-phase catalytic processes in multitubular reactors. 

Scientists from Politecnico di Milano and Ineos Vinyls UK developed a tubular fixed-bed reactor comprising a metallic monolith [30]. The walls were coated with catalytically active material and the monolith pieces were loaded lengthwise. Corning, the world leader in ceramic structured supports, developed metallic supports with straight channels, zig-zag channels, and wall-flow channels. They were produced by extrusion of metal powders, for example, copper, fin, zinc, aluminum, iron, silver, nickel, and mixtures and alloys [31]. An alternative method is extrusion of softened bulk metal feed, for example, aluminum, copper, and their alloys. The metal surface can be covered with carbon, carbides, and alumina, using a CVD technique [32]. For metal monoliths, it is to be expected that the main resistance lies at the interface between reactor wall and monolith. Corning... 

While conventional monoliths contain parallel channels, in practice, systems are often made from alternate layers that allow lighter structures with better mass transfer characteristics in gas-phase applications, see Figure 9.6 showing interconnected flow paths. They are usually made from metal, mostly Fecralloy , Kanthal , or stainless steel, and widely used in autocatalysts and in environmental... 

Figure 9.6 Structural metal monoliths (a) transversal structure, (b) SM design, and (c) LS design of EMITEC. (Reprinted from [12].)...

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