Monolith ceramic

In the latter twentieth century, spent automotive catalysts have emerged as a significant potential source of secondary Pt, Pd, and Rh. In North America, it has been estimated that 15.5 metric tons per year of PGM from automotive catalysts are available for recycling (22). However, the low PGM loading on such catalysts and the nature of the ceramic monoliths used have required the development of specialized recovery techniques as well as the estabhshment of an infrastmcture of collection centers. These factors have slowed the development of an automotive catalyst recycling iadustry. 

The most widely used exhaust control device consists of a ceramic monolith with a thin-waHed open honeycomb stmcture. The accessible surface of this monolith system is iacreased by applyiag a separate coatiag, a wash coat, of a high surface area material such as gamma-alumiaa with the catalyticaHy active species impregaated iato this washcoat. The catalyst aeeds to oxidize hydrocarboas, coavert CO to CO2, and reduce NO. The whole system forms a catalytic converter that, suitably encased, is placed between the engine and the muffler/silencer unit. 

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). 

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

A ceramic monolith catalyst support, cordierite, consisting of silica, alumina and magnesium oxide. The purpose of this is to provide support, strength and stability over a wide temperature range. 

In many cases supports are shaped into simple cylinders (1-5 mm in diameter and 10-20 mm in length) in an extrusion process. The support powder is mixed with binders and water to form a paste that is forced through small holes of the desired size and shape. The paste should be sufficiently stiff such that the ribbon of extmded material maintains its shape during drying and shrinking. When dried, the material is cut or broken into pieces of the desired length. Extrusion is also applied to make ceramic monoliths such as those used in automotive exhaust catalysts and in DeNOx reactors. 

The three-way catalyst, consisting of Pt and Rh particles supported on a ceramic monolith, represents a remarkably successful piece of catalytic technology. It enables the removal of the three pollutants CO, NO and hydrocarbons by the following overall reactions (Tab. 10.3) ... 

Zeolite catalysts and adsorbents have also been incorporated into monolithic contactors by several routes, including extruded zeoHte/binder composites [70], wash-coated ceramic monoliths [71] and corrugated thin-sheet monoliths [72]. 

Figure 7-16 A highly simplified sketch of an automohile engine and catalytic converter with typical gas compositions indicated before and after the automotive catalytic converter. The catalytic converter is a tube wall reactor in which a noble-metal-impregnated wash coat on an extruded ceramic monolith creates surface on which reactions occur.

There are a number of examples of tube waU reactors, the most important being the automotive catalytic converter (ACC), which was described in the previous section. These reactors are made by coating an extruded ceramic monolith with noble metals supported on a thin wash coat of y-alumina. This reactor is used to oxidize hydrocarbons and CO to CO2 and H2O and also reduce NO to N2. The rates of these reactions are very fast after warmup, and the effectiveness factor within the porous wash coat is therefore very smaU. The reactions are also eternal mass transfer limited within the monohth after warmup. We wUl consider three limiting cases of this reactor, surface reaction limiting, external mass transfer limiting, and wash coat diffusion limiting. In each case we wiU assume a first-order irreversible reaction. 

The modem catalytic converter installed on most automobiles is a washcoat consisting of precious metal oxides, supported on a ceramic monolith. After passage of the... 

The use of a monolithic stirred reactor for carrying out enzyme-catalyzed reactions is presented. Enzyme-loaded monoliths were employed as stirrer blades. The ceramic monoliths were functionalized with conventional carrier materials carbon, chitosan, and polyethylenimine (PEI). The different nature of the carriers with respect to porosity and surface chemistry allows tuning of the support for different enzymes and for use under specific conditions. The model reactions performed in this study demonstrate the benefits of tuning the carrier material to both enzyme and reaction conditions. This is a must to successfully intensify biocatalytic processes. The results show that the monolithic stirrer reactor can be effectively employed in both mass transfer limited and kinetically limited regimes. 

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. ... 

Recently, the use of Rh supported on washcoated alumina monoliths has attracted interest for ATR of higher hydrocarbons.Reyes et al carried out ATR of n-C6 in monolithic catalysts containing Rh as an active component. A maximum H2 yield of 170% was obtained from the reforming of n-C6 at an O/C ratio of 1, a S/C of 1, preheat temperature of 700°C, and GHSV of 68,000 h Brandmair et al. also carried out ATR of n-C6 over Rh supported on ceramic monoliths at similar conditions, and reported that the Rh catalyst provided better performance over time. 

The oxidation of gaseous ethanol (first-order reaction) was studied in a spinning basket reactor with Pt/Al203 on a ceramic monolith as catalyst. The inlet concentration of ethanol... 

So far, MR studies of reactors of relevance to catalytic processes have been restricted to fixed beds and ceramic monoliths. Recently, the first reports of MR being... 

Fro. 37. Two successive 2-D xz images of two-phase flow through the parallel channels of a ceramic monolith rated at 400 cpsi, for a gas flow rate of 200 cm min (a) 74ms after excitation, (b) 220 ms after r.f. excitation. In-plane image resolution is 393 pm (x) x 783 pm (z). Reprinted from reference (84) with permission of Springer Science and Business Media. 

Fig. 38. Line excitation SEMI-RARE images of two-phase flow in a ceramic monolith rated at 200 cpsi. The signal intensity shows how far the initially excited water molecules have traveled in the period between line excitation and the image acquisition. The gas flow rate was (a) 0, (b) 100, (c) 200, and (d) iOOcin inin. Images acquired 78ms after r.f. excitation are shown. A 5mm-high slice of spins was initially excited along the direction of flow. The fleld-of-view is 50 mm (x) x 25 mm (z).

A 15-fold glass tube parallel-packed bed reactor has been described [28-30], which is close to conventional catalyst testing equipment. The same authors also reported a 64-fold ceramic block reactor and a ceramic monolithic reactor for the screening of up to 250 catalysts in parallel. The individual catalysts were coated... 

Fig. 4.3 Right x-y-z positioning device for scanning individual catalyst-coated micro-channels of a ceramic monolith right plant set-up for high-throughput experiments at the ACA Berlin-Adlershof.

Converters for cars are usually ceramic monoliths and occasionally metal based. Without much exaggeration, they can be claimed to be one of the major successes of recent decades in the area of chemical engineering and catalysis. In the beginning, the catalytic converter was placed underbody, where sufficient space was available and where the temperature was expected to be mild. There was no need... 

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