Ceramic monolithic converters

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. 

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... 

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... 

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. 

Figure 1.2.1 Ceramic monolith catalytic converter. Adapted with permission from K. C. laylor. CllfIMTI-X ll. 20 (1990) 551. Copyright 19 -)0. Aniericaii Chemical SiK iety.

Most of the current converters consist of a flow-through ceramic monolith with its channel walls covered with a high-surface-area 7-AI2O3 layer (the washcoat) which contains the active catalyst particles. The monolith is composed of cordicrite, a mineral with the composition 2MgO 2AI2O3 5Si02. The chemical composition of a modern TWC is quite complex. In addition to alumina, the washcoat contains up to 30 wt% base metal oxide additives, added for many purposes. The most common additives are ceria and lanthana in many formulations BaO and Zr02 are used, and in some converters NiO is present. The major active constituents of the washcoat are the noble metis Pt, Pd, and Rh (typically 1-3 g). Most of the TWC systems in use today are still based on Pt and Rh in a ratio of about 10 1. 

An example of an automotive converter based upon a ceramic monolith is shown in Fig. 27. It consists essentially of three parts, see Fig. 28 ... 

Figure 27. Cutaway view of ceramic monolith based converters for the catalytic after-treatment of exhaust gases.

There exist numerous designs for the converter. The boundary conditions for the converter design are the space which is available in the vehicle platform to mount the converter and the overall volume of catalyst needed for the catalytic function. A single converter can be packed with one single or with more, typically two or sometimes three, single pieces of ceramic monolith. When multiple pieces are used, they are mounted with a well defined distance between them, to effect turbulent flow conditions at the inlet to each piece. 

Figure 28. Design principle of a ceramic monolith based converter for the catalytic aftertreatment of exhaust gases.

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. 

The increasing volume of air pollutants such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) has become a serious global environmental problem. Currently the three-way catalytic converters, which contain Rh, Pt and Pd noble metals supported on a ceramic monolith, are used for the simultaneously control of all the above pollutants from... 

D.Kattge, Advanced Canning Systems for Ceramic Monoliths in Catalytic Converters. SAE Paper 880284. 

Several different materials have been studied. Metallic monoliths have been used extensively since their first application for automobile converters. They allow very thin walls and have a very high thermal conductivity. However, their thermal expansion gives rise to some problems when looking at the coating and stability of the washcoat on the metallic surface, compared with the ceramic monolith. Furthermore, their maximum operation temperature is limited to 1200-1400 C, cf. Table 1. Probably, the maximum temperature is somewhat lower for long-time exposure. However, several ceramic monoliths that can stand higher thermal conditions have been developed, as reported in Table 1. 

The structured packing for column and ceramic monoliths has been in use for several decades. The latter is used in catalytic converters for automobile exhausts for the in-line conversion of CO and oxides of nitrogen into harmless oxides. It is also used for catalyst and adsorbent supports in process industries. They offer higher specific surface area and low pressure drop. A comprehensive review of monolith reactors as alternatives to trickle-bed, slurry, and slurry bubble column reactors is available in Roy et al. [65]. The monolith reactor... 

Washcoat technology was initially developed for the automobile catalytic converter, consisting of a ceramic monolith of many small parallel channels, in the mid-1970s. Monoliths have since then developed into a variety of different materials and configurations, essentially cordierite based [3]. 

Lampert presented a catalytic partial oxidation technique for sulfur compounds that was developed by the former Engelhard (now BASF) corporation [296]. The sulfur compounds of natural gas or liquefied petroleum gas were converted into sulfur oxides at a low 0/C ratio of 0.03 in a ceramic monolith over a precious metal catalyst. These sulfur oxides were then adsorbed downstream by a fixed adsorber bed, which contained adsorption material specific to sulfur trioxide and sulfur dioxide, which could trap up to 6.7 g sulfur per 100 g adsorbent. The partial oxidation was performed at a 250 °C monolith inlet temperature, the adiabatic temperature rise in the monolith amounted to 20 K. Light sulfur compounds usually present in natural gas and liquefied petroleum gas, such as carbon oxide sulfide, ethylmercaptane, dimethyl sulfide and methylethyl sulfide, could be removed to well below the 1 ppm level. Exposure of the monolith to an air rich fuel/air mixture at temperatures exceeding 150 °C had to be avoided. The same applied for contact with fuel in the absence of air regardless of the temperature. 

Figure 6.18.3 Design of a ceramic monolith based converter for the catalytic after treatment of exhaust gases from passenger c Adapted from Erti, Knoetzinger, and Weitkamp, 1999 photograph by courtesy of Umicore, Germany.

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]. 

T. Stary, O. olcova, P. Schneider, M. Marek Effective Diffirsivities and Pore-Transport Characteristics of Washcoated Ceramic Monolith for Automotive Catalytic Converter. Appl. Catal., B, (submitted 2005). 

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