Metallic monolithic converters

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. 

D.K.S. Chen, C.E. Cole, Numerical simulation and experimental verification of conversion and thennal response for a Pt/Rli metal monolithic converter, SAE Paper 890798 (1989). 

In general, both cordierite and metallic monoliths are unsuitable as catalytic supports. To process a monolith into an active monolithic catalyst, a layer of porous catalytic support must be deposited on the walls between channels. y-Alumina appeared to be the most effective support for automotive catalysts. The alumina layer is deposited by sol-gel technique (so called washcoating). Adherence of 7-alumina to cordierite is relatively strong. However, to form the stable 7-alumina layer on a metallic surface, we need to use an appropriate alloy that is appropriately processed before the layer is deposited. Stainless steel containing chromium, aluminum, and yttrium subjected to thermal treatment under oxidizing conditions meets requirements of automotive converters. Aluminum in the steel is oxidized to form 7-alumina needles (whiskers) protruding above the metal... 

Figure 14 Comparison of mcthanation rates and selectivity for converting CO to CH for ruthenium- and nickel-catalyzed metal monoliths. (Circles and squares ruthenium triangles nickel.) (From Ref. 15, reprinted with permission of Gulf Publishing Co.)...

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. 

An automotive catalyst, that is, a catalytic converter for internal combustion engines, can be regarded as a packed bed reactor, in which the active metals (the most important ones being Pt, Pd, and Rh) reside on a carrier substance, which in turn is attached to a ceramic or a metallic monolith structure. A catalytic converter for internal combustion engines is illustrated in Figure 5.4 [ 1 ]. Similar monolith structures can also be utilized in conventional industrial processes such as catalytic hydrogenations [4]. 

The radial velocity distribution in the monolith and the pressure drop across the converter depend on the shape of the divergent and of the monolith sections. Furthermore, mass and heat transfer and hydrodynamic properties depend on the shape of the monolith channels. Ceramic monoliths are generally made of square, circular or triangular channels. In metallic monoliths, channels are manufactured by rolling up a thin corrugated metal sheet. 

Some catalyst supports rely on a relatively low surface area stmctural member coated with a layer of a higher surface area support material. The automotive catalytic converter monolith support is an example of this technology. In this appHcation, a central core of multichanneled, low surface area, extmded ceramic about 10 cm in diameter is coated with high surface area partially hydrated alumina onto which are deposited small amounts of precious metals as the active catalytic species. 

Diffusion effects can be expected in reactions that are very rapid. A great deal of effort has been made to shorten the diffusion path, which increases the efficiency of the catalysts. Pellets are made with all the active ingredients concentrated on a thin peripheral shell and monoliths are made with very thin washcoats containing the noble metals. In order to convert 90% of the CO from the inlet stream at a residence time of no more than 0.01 sec, one needs a first-order kinetic rate constant of about 230 sec-1. When the catalytic activity is distributed uniformly through a porous pellet of 0.15 cm radius with a diffusion coefficient of 0.01 cm2/sec, one obtains a Thiele modulus y> = 22.7. This would yield an effectiveness factor of 0.132 for a spherical geometry, and an apparent kinetic rate constant of 30.3 sec-1 (106). 

The catalytic converter on a car uses a precious-metal-based, solid catalyst, usually in the form of a monolith, to convert unburned hydrocarbons and carbon monoxide to carbon dioxide. Many different reactants are converted to two products CO2 and water. 

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... 

Monolithic catalysts have found a wide range of applications in the removal of pollutants from air, especially in the automotive industry. Specifically, the demand for large surface to small volume, high conversions achieved for low retention times, and low pressure drop led to the development of monolithic supports. More information on automotive catalytic converters has been given in Chapter 1. Usually, a thin layer of alumina is deposited onto a monolith for keeping the precious metal used for air pollutants abatement dispersed. The oxidations that take place are highly exothermic and the reaction rates achieved are in turn high. Hence, the reactants diffuse only a small distance... 

Since 1981, three-way catalytic systems have been standard in new cars sold in North America.6,280 These systems consist of platinum, palladium, and rhodium catalysts dispersed on an activated alumina layer ( wash-coat ) on a ceramic honeycomb monolith the Pt and Pd serve primarily to catalyze oxidation of the CO and hydrocarbons, and the Rh to catalyze reduction of the NO. These converters operate with a near-stoichiometric air-fuel mix at 400-600 °C higher temperatures may cause the Rh to react with the washcoat. In some designs, the catalyst bed is electrically heated at start-up to avoid the problem of temporarily excessive CO emissions from a cold catalyst. Zeolite-type catalysts containing bound metal atoms or ions (e.g., Cu/ZSM-5) have been proposed as alternatives to systems based on precious metals. 

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