Metallic monolith

The ceramic oxide carrier is bonded to the monolith by both chemical and physical means. The bonding differs for a ceramic monolith and a metallic monolith. Attrition is a physical loss of the carrier from the monolith from the surface shear effects caused by the exhaust gas, a sudden start-up or shutdown causing a thermal shock as a result of different coefficients of thermal expansion at the boundary between the carrier and the monolith, physical vibration of the cataly2ed honeycomb, or abrasion from particulates in the exhaust air (21) (see Fig. 6d). 

An alternate form of catalyst is pellets. The pellets are available in various diameters or extruded forms. The pellets can have an aluminum oxide coating with a noble metal deposited as the catalyst. The beads are placed in a tray or bed and have a depth of anywhere from 6 to 10 inches. The larger the bead (1/4 inch versus 1/8 inch) the less the pressure drop through the catalyst bed. However, the larger the bead, the less surface area is present in the same volume which translates to less destruction efficiency. Higher pressure drop translates into higher horsepower required for the oxidation system. The noble metal monoliths have a relatively low pressure drop and are typically more expensive than the pellets for the same application. 

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... 

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

A structured ruthenium catalyst (metal monolith supported) was investigated by Rabe et al. [70] in the ATR of methane using pure oxygen as oxidant. The catalytic activity tests were carried out at low temperature (<800 ° C) and high steam-to-carbon ratios (between 1.3 and 4). It was found that the lower operating temperature reduced the overall methane conversion and thus the reforming efficiency. However, the catalyst was stable during time on-stream tests without apparent carbon formation. 

Lenz and Aicher reported the experimental results obtained with an autothermal reformer fed with desulfurized kerosene employing a metallic monolith coated with alumina washcoat supporting precious metal catalysts (Pt and Rh) [78]. The experiments were performed at steam-to-carbon ratios S/C = 1.5-2.5 and... 

Metal monoliths with internal heat exchange capabilities are obtained by assembling single side-coated flat and corrugated sheets as illustrated in Figure 12.2 [17,18]. 

On the other hand, following the development of hybrid combustor configurations that prevent operation of the catalyst module at temperatures above 900-1000 °C, the major drawback of metallic monoliths, namely the limited maximum operating temperature, has been overcome. Accordingly, honeycombs made of metal foils have been adopted in GT catalytic combustors in view of their excellent thermal shock resistance and thermal conductivity properties [9]. In addition, metallic substrates are a promising option for the fabrication of microcombustors. 

Weiland et al observed that a small amount of Pt metal present in the Rh-based catalyst could significantly improve the catalyst activity for ATR of gasoline range fuels. They claimed that the role of Pt is to enhance oxidation activity, whereas Rh provides high SR activity. The Rh-Pt/alumina catalyst used in the study was supported on monolithic honeycombs and had a Rh to Pt ratio of 3-10 by weight. The geometry (metal monolith, ceramic monolith, or ceramic foam) of the support did not affect the product composition. ... 

The differences in reactions at different reactor positions was studied by Springmann et al. who reported product compositions for ATR of model compounds as a function of reactor length in a metal monolith coated with a proprietary noble metal containing Rh. As expected, the oxidation reactions take place at the reactor inlet, followed by the SR, shift, and methanation reactions. Figure 32 shows the product concentration profiles for a 1-hexene feed, which are typical results for all the fuels tested. These results show that steam, formed from the oxidation reactions, reaches a maximum shortly after the reactor inlet, after which it is consumed in the shift and reforming reactions. H2, CO and CO2 concentrations increase with reactor length and temperature. In this reactor, shift equilibrium is not reached, and the increase in CO with distance from the inlet is the net result of the shift and SR reactions. Methane is... 

Metal monoliths can be shaped rather freely. A good example is given in Figure 4 (9), where it can be seen that in these parallel-channel systems the structure of the channels is such that the turbulence increases. The reasoning behind that is the wish to counteract the low mass transfer rates associated with laminar flow in the thin channels of the monolith. 

Figure 4 Shaped channels in metal monoliths in order to increase mass transfer in gas-phase applications.

Figure 12 Industrial unit for SRC containing metal monolith units.

Metallic monoliths made of both rhodium ([HCR 1]) and FeCrAlloy (72.6% Fe, 22% Cr and 4.8% Al ([HCR 3]) carrying micro channels of 120 pm x 130 pm cross-section at various length (5 and 20 mm) were applied. The monoliths were prepared of micro structured foils by electron beam welding. After bonding, the FeCrAlloy was oxidized in air at 1 000 °C for 4 h to form an a-alumina layer, which was verified by XRD. Its thickness was determined as < 10 pm by SEM/EDX. The alumina layer was impregnated with rhodium chloride and alternatively with a nickel salt solution. The catalyst loading with nickel (30 mg) was much higher than that with rhodium (1 mg) (see Table 2.4). The amount of rhodium on the catalyst surface was determined as 3% by XPS. 

An early application of a combined steam reformer/catalytic combustor on the meso scale was realized by Polman et al. [101]. They fabricated a reactor similar to an automotive metallic monolith with channel dimensions in the millimeter range (Figure 2.65). The plates were connected by diffusion bonding and the catalyst was introduced by wash coating. The reactor was operated at temperatures between 550 and 700 °C 99.98% conversion was achieved for the combustion reaction and 97% for the steam reforming side. A volume of < 1.5 dm3 per kW electrical power output of the reformer alone was regarded as feasible at that time, but not yet realized. 

Mayer et al. [43] compared their results generated at a micro structured monolith (see Section 2.4.3) with literature data [137]. The degree of conversion and the hydrogen selectivity of the rhodium monolith outperformed both metal-coated foam monoliths, Pt-Rh gauzes and extruded monoliths. This was partially attributed to the higher activity of the rhodium monolith, but also to the lower cross-sectional channel area of the metallic monolith, which reduced mass transfer limitations, and to the improved heat conductivity. 

Refractory high surface area oxides are deposited from slurries onto the walls of the channels that make up monoliths in order to provide an adequate surface area to support the active catalytic species. Washcoats such as AI2O3 and TiC>2 are commonly used for pollution abatement applications (auto exhaust, stationary NO abatement, etc.) where the monolith is usually a ceramic. Metal monoliths are finding increasing use however, they represent only a small percentage of the total monoliths used. Optical microscopy enables one to see that the catalyzed washcoat follows the contour of the ceramic surface. Figure 7 shows the AI2O3 washcoat-ceramic interface for a typical auto exhaust catalyst. In this case, no evidence of loss of adhesion between washcoat and ceramic can be seen. 

Sakabe, Y., Minai, K. and Wakino, K. (1981) High-dielectric constant ceramics for base metal monolithic capacitors, Jpn. J. Appl. Phys. 20 Suppi, 147-50. 

For the control of carbon monoxide, hydrocarbon, and nitrogen oxide emissions from automobiles, oval-shaped extruded cordierite or metal monolith catalysts are wrapped in ceramic wool and placed inside a stainless steel casing (Fig. 19-18a). The catalytic metals are Pt-Rh or Pd-Rh, or combinations. Cell sizes typically ranges between 400 and 600 cells per square inch. The catalysts achieve over 90 percent reduction in all three pollutants. 

Alternatively it may take the form of a ceramic or metallic monolith, of which a variety of physical shapes is available monoliths are now widely used as supports for the active catalyst, which lines the channels which permeate the structure. They find particular application for the control of exhaust from vehicles powered by internal combustion or diesel engines. If the catalyst particles are small enough, a fast flow of reactants causes the bed to expand and the particles to move about like molecules in a liquid. We then have a fluidised bed reactor, which affords a more uniform temperature profile than is possible in fixed bed reactors, and is therefore more apposite to strongly exothermic reactions. 

Monoliths are mainly produced by extrusion, although other methods are applied, in particular for the production of metal monoliths from thin corrugated sheets. The size of the channels and the wall thickness can be varied independently, and the optimal values depend on the particular application. Therefore, an optimum can be established between the amount of the solid phase (catalyst loading), the void space in the monolith, and the wall thickness. As a consequence of the extrusion process and the use of plasticizers, the channel walls are not completely dense but possess a macroscopic porosity, t)q)ically 30-40%. Thus, the thermal expansion properties can also be adjusted. 

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. 

A sol-gel coating procedure for silica on metallic monoliths was developed by Zwinkels et al. (74), who used colloidal silica sols together with potassium waterglass. Various procedures resulted in washcoats 20-50 pm thick, with surface areas of 60-140 m /g. Coating with the colloidal silica without binder material resulted in a thickness of only a few micrometers. 

FIGURE 44 Metal monoliths allowing splitting and mixing of flows (734). 

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