Catalytic converters, ceramic honeycombs

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 second method used to reduce exliaust emissions incorporates postcombustion devices in the form of soot and/or ceramic catalytic converters. Some catalysts currently employ zeolite-based hydrocarbon-trapping materials acting as molecular sieves that can adsorb hydrocarbons at low temperatures and release them at high temperatures, when the catalyst operates with higher efficiency. Advances have been made in soot reduction through adoption of soot filters that chemically convert CO and unburned hydrocarbons into harmless CO, and water vapor, while trapping carbon particles in their ceramic honeycomb walls. Both soot filters and diesel catalysts remove more than 80 percent of carbon particulates from the exliatist, and reduce by more than 90 percent emissions of CO and hydrocarbons. 

FIGURE 13.37 The catalytic converter of an automobile is made from a mixture of catalysts bonded to a honeycomb ceramic support. 

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. 

A billion cars and coimting, himdreds of millions of them with catalytic converters—this application is a landmark success of catalytic science and technology. Automobile catalytic converters are mostly monoliths— like ceramic honeycombs with porous catalyst layers on their inner wall surfaces. These monoliths are the most widely used structured reactors, the topic addressed by Moulijn, Kreutzer, Nijhuis, and Kapteijn. In contrast to the classical reactors containing discrete particles of catalyst and characterized by random and chaotic behavior, structured reactors are characterized by regular structures and predictable laminar flow. Structured reactors can be designed in full detail up to the local surroimdings of the... 

Because of the complex nature of the reactions that take place in the converter, a mixture of catalysts is used. The most effective catalytic materials are transition metal oxides and noble metals such as palladium and platinum. A catalytic converter typically consists of platinum and rhodium particles deposited on a ceramic honeycomb, a configuration that maximizes the contact between the metal particles and the exhaust gases. In studies performed during the last ten years researchers at General Motors have shown that rhodium promotes the dissociation of NO molecules adsorbed on its surface, thereby enhancing the conversion of NO, a serious air pollutant, to N2, a natural component of pure air. 

The microstructure of ceramic honeycombs not only affects physical properties like CTE, strength, and structural modulus, but has a strong bearing on substrate/washcoat interaction, which, in turn, affects the performance and durability of the catalytic converter [28-30]. The coefficient of thermal expansion, strength, fatigue, and structural modulus of the honeycomb substrate (which also depend on cell orientation and temperature) have a direct impact on its mechanical and thermal durability [22). Finally, since all of the physical properties are affected by washcoat formulation, washcoat loading, and washcoat processing, they must be evaluated before and after the application of washcoat to assess converter durability [28-30]. 

How a catalytic converter works A typical catalytic converter consists of particles of platinum and rhodium deposited on a ceramic structure that is like a honeycomb. The platimun and rhodium catalyze reactions that remove pollutants such as nitrogen monoxide (NO), carhon monoxide (CO), and unhurned hydrocarbons. When nitrogen monoxide binds to the rhodimn surface, it breaks down to oxygen and nitrogen. The bound oxygen reacts with carbon monoxide, which has also become bound to the rhodimn surface. The reaction produces carbon dioxide. The oxidation of unburned hydrocarbons produces carbon dioxide and water. 

Reduced thermal mass came from changes to the catalyst support. The catalyst materials are deployed as a thin coating on a ceramic honeycomb substrate. The surface area for the deployment of the catalytic coating, and the mass of the substrate both contribute to the overall performance of the catalytic converter. The surface area was increased by reducing the thickness of the channel walls, thereby increasing the flow channel density and reducing the thermal mass of the substrate. 

A catalytic converter consists of a honeycomb ceramic structure coated in finely divided AI2O3 (the washcoat). Fine particles of catalytically active Pt, Pd and Rh are dispersed within the cavities of the washcoat and the whole unit is contained in a stainless steel vessel placed in sequence in the vehicle s exhaust pipe. As the exhaust gases pass through the converter at high temperatures, redox reactions... 

Catalytic converters are composed of a monolithic support surrounded by a mat and tightly packed into a stainless steel housing. The monolith has a honeycomb structure traversed by straight, square channels and consists of a ceramic or sometimes metallic material, coated with the so called washcoaL To carry out the catalytic reactions, PGE are dispersed throughout the washcoat, which typically has a thickness of about 10-30 pm at the walls and 100-150 pm at the comers of the support channels and contains as major constituents alumina and other oxides. 

Typical ceramic materials produced on a co-rotating twin screw extruder are for example catalyst carriers. They are commonly shaped into granules for use as bulk material in reactors in the chemical industry or into honeycombs for catalytic converters in automobiles exhaust systems (Fig. 12). After extrusion, the catalyst carriers are cut oversized in the lineal direction, dried and then cut to the proper length. Afterwards the binder is removed and the carriers are calcinated or sintered. Finally, to provide them with catalytic properties, they are impregnated with an active film in a bath [Fri76]. 

The honeycomb of the catalytic converter provides a large surface area to support the catalyst particles and ensures that the gases pass close to these particles. When used as filters the pore (channel) diameter is engineered to suit the purpose. The synthetic bone needs the pores to encourage the intergrowth of regrowing natural bone, but if the pores are too large then the ceramic is weakened. One technique used to provide the porous structure and... 

The new mesoporous materials have extremely high surface-to-volume ratios. An example of these materials is MCM41, which was invented by DuPont. A simple structure that can be manufactured in the laboratory is illustrated in Eigure 15.14. The structure initially contained a periodic array of polymer spheres. When close packed, these spheres leave 26% of the volume empty. We can then infiltrate a liquid into these pores, burn out the spheres, and convert the liquid to a polycrystalline ceramic. Another synthesized porous ceramic is the cordierite honeycomb structure used to support the Pt catalyst in automobile catalytic converters. In this case the cylindrical pores are introduced mechanically in the extrusion process. 

FIGURE 37.10 Looking through two ceramic extruded cordierite honeycomb substrates for catalytic converters. 

A catalytic converter consists of a honeycomb ceramic structure coated in finely divided AI2O3 (the washcoat). Fine particles of catalytically active Pt, Pd and Rh are... 

These catalytic converters contain a high surface area, a honeycombed ceramic or stainless steel core that is coated with silica and alumina, called a washcoat. Precious metal catalysts, such as platinum, palladium, and rhodium, are added as a suspension to the washcoat. As the hot gases pass through the catalytic converter, they are converted by the catalysts to the reduced or oxidized products. 

Catalytic Converter. A device to remove noxious components from the hot exhaust gases of diesel and petrol engines, by passing them over a suitable catalyst to promote their breakdown. Cordierite ceramic honeycombs are the usual catalyst carriers. 

The interior structure of a catalytic converter is usually made of a ceramic honeycomb with a surface coating of metal catalyst particles. The honeycomb has many holes for... 

The ceramic honeycomb inside a catalytic converter is coated with a metal catalyst. 

Ceramics in the Transportation Sector Catalytic converters reduced the pollution from automobiles by 90% by the mid-1970s. The ceramics used in these devices were in the form of a honeycomb. 

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