Automotive catalyst monolithic

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. 

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. 

The most widely known pollution control catalysts are those for auto emission control. Automotive catalysts can be of two types—monoliths and pellets. Monoliths now dominate the market. Pollution control catalysts are also used to control diesel emissions. 

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... 

With thermal durability under control, the mechanical durability takes on a major focus to ensure total durability. To this end, it is necessary to ascertain high mechanical strength of the coated monolith, build in a resilient packaging system, and ensure positive and moderately high mounting pressure to guard against vibrational and impact loads. Much like automotive catalysts, the D(Xs can continue to function catalytically even in... 

Not all catalysts need the extended smface provided by a porous structure, however. Some are sufficiently active so that the effort required to create a porous catalyst would be wasted. For such situations one type of catalyst is the monolithic catalyst. Monolithic catalysts are normally encountered in processes where pressure drop and heat removal are major considerations. Typical examples include the platinum gauze reactor used in the ammonia oxidation portion of nitric acid manufacture and catalytic converters used to oxidize pollutants in automobile exhaust. They can be porous (honeycomb) or non-porous (wire gauze). A photograph of a automotive catalytic converter is shown in Figure CD 11-2. Platinum is a primary catalytic material in the monolith. 

Fig. 4.1. Illustration of the structure gap for an automotive three-way catalysts (TWO) depicted by a schematic drawing of a monolithic, supported catalyst. The lower right image shows a TEM picture of aged automotive catalyst 12-nm Pt on AI2O3. Note, the presence of small Pt single crystals. Well-defined model catalysts such as those made by electron-beam lithography (EBL lower middle image) are used to mimic real catalyst and bridge the gap between single crystal studies (bottom left) and real-life catalysts...

There has been a long debate about the relative advantages of metallic monoliths over ceramic monoliths. From the standpoint of automotive catalyst manufacturing, durability and performance, the following aspects are of relevance [27] ... 

Figure 40 shows the conversion of CO, HC and NO e at typical automotive catalyst operation conditions for a precious metal based catalyst on a ceramic monolith with an extremely low precious metal loading, and for a precious metal free catalyst in which the same ceramic monolith support was used but with a washcoat consisting of a typical base metal catalyst formulation. The extremely poor con-... 

As mentioned before, the heat-up of the automotive catalyst only by the exhaust gas needs approx. 1 min. In order to shorten the start-up period an electrically heated pre-catalyst can be used which is located in front of the main catalyst (Figure 1). The presently used EHC is a two-brick design. It consists of a short metallic monolith which is heated by the car battery and a second, larger but unheated monolith. This second monolith enlarges the catalytic surface area and ensures the mechanical stability of the whole EHC construction. The design of this EHC was obtained by optimization based upon extensive experimental and simulation studies... 

The selective heating of the active areas of the monolith by the CHC-concept can also be demonstrated experimentally. An automotive catalyst was aged artificially in a way that the first 3 cm were completely deactivated. With this aged catalyst the cold-start experiment from Figure 7 was repeated. 

Noble metal catalysts are highly active for the oxidation of carbon monoxide and therefore widely used in the control of automobile emissions. Numerous recent studies on noble metal-based three-way catalysts have revealed characteristics of good thermal stability and poison resistance(l). Incorporation of rare earth oxides as an additive in automotive catalysts has improved the dispersion and stability of precious metals present in the catalyst as active components(2). Monolith-supported noble-metal catalysts have also been developed(3). However, the disadvantages of noble metal catalysts such as relative scarcity, high cost and requirement of strict air/fuel ratio in three-way function have prompted attention to be focused on the development of non-noble metal alternatives. 

The present embodiment of an automotive catalyst consist of a monolithic support made of a high-melting ceramic material, cordierite, typically having 64 square cells per square centimeter cross-sectional area, with the walls between the cells being 150 pm thick. The walls are coated with a high surface "washcoat" having a BET area of 80-100 m /g. Since the... 

Thus, recent automotive catalyst development tends toward depositing platinum in a thin washcoat of alumina on low-surface-area, ceramic monoliths or in egg shell layers on the exterior of pellets. 

Certain factors are analyzed to determine their effects on automotive catalyst activity. At operating gas velocities, spherical catalysts were more active than monolithic catalysts at comparable catalyst volumes and metals loadings. Palladium was the most active catalyst metal. Platinum in a mixed platinum palladium catalyst stabilizes against the effects of lead poisoning. An optimum activity particulate catalyst would contain about 0.05 wt % total metals on a gamma-alumina base with a platinum content of 0.03-0.04 wt % and a palladium content of 0.01-0.02 wt %. A somewhat thick shell of metals located near the outer surface of the particle provides better catalyst activity than a shell type distribution of metals. 

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]. 

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. 

J. Howitt, Thin Wall Ceramics as Monolithic Catalyst Supports, SAE 910611, Society of Automotive Engineers, Warrendale, Pa., 1991. 

Serious research in catalytic reduction of automotive exhaust was begun in 1949 by Eugene Houdry, who developed mufflers for fork lift trucks used in confined spaces such as mines and warehouses (18). One of the supports used was the monolith—porcelain rods covered with films of alumina, on which platinum was deposited. California enacted laws in 1959 and 1960 on air quality and motor vehicle emission standards, which would be operative when at least two devices were developed that could meet the requirements. This gave the impetus for a greater effort in automotive catalysis research (19). Catalyst developments and fleet tests involved the partnership of catalyst manufacturers and muffler manufacturers. Three of these teams were certified by the California Motor Vehicle Pollution Control Board in 1964-65 American Cyanamid and Walker, W. R. Grace and Norris-Thermador, and Universal Oil Products and Arvin. At the same time, Detroit announced that engine modifications by lean carburation and secondary air injection enabled them to meet the California standard without the use of catalysts. This then delayed the use of catalysts in automobiles. 

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. 

At the heart of an automotive catalytic converter is a catalyzed monolith which consists of a large number of parallel channels in the flow direction whose walls are coated with a thin layer of catalyzed washcoat. The monolith catalyst brick is wrapped with mat, steel shell and insulation to minimize exhaust gas bypassing and heat loss to the surroundings. 

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. 

Figure 10.5. Monolith, washcoat and noble metal particles in an automotive exhaust catalyst.

As with the automotive exhaust converter, the SCR catalyst is designed to handle large flows of gas (e.g. 300 N s for a 300 MW power plant) without causing a significant pressure drop. Figure 10.12 shows a reactor arrangement with about 250 m of catalyst in monolithic form, sufficient for a 300 MW power plant. 

New reactor technologies are currently under development, and these include meso- and micro-structured reactors or the use of membranes. Among meso-structured reactors, monolithic catalysts play a pre-eminent role in environmental applications, initially in the cleaning of automotive exhaust gases. Beside this gas-solid application, other meso-structures such as membranes [57, 58], corrugated plate or other arranged catalysts and, of course, monoliths can be used as multiphase reactors [59, 60]. These reactors also offer a real potential for process intensification, which has already been demonstrated in commercial applications such as the production of hydrogen peroxide. 

Ceramic honeycomb monoliths are porous macro-structured supports consisting of parallel channels. On the walls a thin layer of active material can be applied (Figure 1). Honeycomb catalyst supports were originally developed for use in automotive... 

When a material used for the dispersion of the active agents is bonded to a support, it is called washcoat. A characteristic example is the case of automotive monolithic catalysts, where the monolith is die support and a thin film of alumina attached to the monolith constitutes the washcoat, the phase where the catalytically active metals are dispersed. In contrast to these supported catalysts, there are some catalytic materials that... 

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... 

Transfer coefficients in catalytic monolith for automotive applications typically exhibit a maximum at the channel inlet and then decrease relatively fast (within the length of several millimeters) to the limit values for fully developed concentration and temperature profiles in laminar flow. Proper heat and mass transfer coefficients are important for correct prediction of cold-start behavior and catalyst light-off. The basic issue is to obtain accurate asymptotic Nu and Sh numbers for particular shape of the channel and washcoat layer (Hayes et al., 2004 Ramanathan et al., 2003). Even if different correlations provide different kc and profiles at the inlet region of the monolith, these differences usually have minor influence on the computed outlet values of concentrations and temperature under typical operating conditions. 

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