Cordierite honeycombs

Many different filter designs have been the subject of experimental studies on diesel soot combustion. In the early investigations, structured honeycomb filters made from cordierite, such as those applied for the three-way catalyst for the reduction of spark ignition engine gas emissions, were the focus of the experimental studies [29-39]. The experimental results with these filters were not promising, because the cordierite honeycomb filter did not withstand the thermal stress. Temperature peaks of almost 1200 °C were measured, after which the ceramic structure was partly melted or totally destroyed [29, 40],... 

For deNOxing lean exhaust gas within a broad temperature range, methods using multi-stage catalysts active in different temperature domains have been claimed, e.g. by Toyota and Tosoh Corporation (67) for the combination of Co-Ba/H-MFI and Cu/H-MFI on a cordierite honeycomb and by Mazda Motor... 

Ham et al. (99) have studied the effect of S02 in the temperature range 523-723K. Below 573 K a decrease of activity by 60% at 523 K and 20% at 573 K occurs, while above 573 K no inhibition was observed. During the tests at low temperature with S02, accumulation of ammonium sulfate and/or ammonium bisulfate occurs and may cause severe blocking. At higher temperatures these salts decompose and this may explain the absence of inhibition. Complete regeneration of deactivated catalysts can be achieved by heat treatment at 773 K. Over Cu-H-MOR bound with silica and deposited on a cordierite honeycomb, only 12% decrease of NO conversion was observed at 673 K after 350 h passivation in presence of 200... 

Another synthesis process proposed to receive benefits from operating with monolith catalysts is the conversion of methanol for gasoline production [16,17J. The catalyst used was the ZSM-5 zeolite. However, rather than binding the catalyst onto the wall by use of a washcoat, it was uniformly crystallized on the cordierite honeycomb (62 cells/cm ) wall surfaces (up to 30% by weight), similar to the method described in the patent assigned to Lachman and Patil [18]. The effects of methanol partial pressure on conversion and temperature on hydrocarbon selectivity were determined. Three regimes of mass transfer resistances are experienced in this reaction reactant transfer to the reactor walls within the monolith channels through the laminar flow, diffusion resistance at the surface between zeolite crystals on the walls, and diffusion into the zeolite molecular-size pores to the active sites within the crystals, where the reaction rate limit is anticipated. 

Ulla MA, Mallada R, Coronas J, Guiterrez L, Miro E, and Santamaria J. Synthesis and characterization of ZSM-5 coatings onto cordierite honeycomb supports. Appl Catal A Gen 2003 253(l) 257-269. 

Madhusoodana CD, Das RN, Kameshima Y, Yasumori A, and Okada K. Preparation of ZSM-5 thin film on cordierite honeycomb by solid state in situ crystallization. Micropor Mesopor Mater 2001 46(2-3) 249-255. 

Table 1 Percentage weight loss of cordierite honeycomb pieces (1" diameter x 1" length) after leaching in 1.5 N HNO, at 95°C for 2 hours...

Samples A to H were washcoated on leached cordierite honeycombs and sample I was extruded with 16% alumina binder. 

Figure 3. Optical micrographs of uncoated and washcoated cordierite honeycomb substrates. (a) Uncoated honeycomb, (b) 18.2% washcoat (80% silicalite + 20% silica binder), and (c) 30% washcoat (60% silicalite + 40% silica binder).

These results demonstrate that zeolite catalysts perform well in honeycomb form. This is similar to the well established use of noble metals/gamma alumina washcoats on cordierite honeycombs for automotive emission control. This geometrical form offers the advantages of low pressure drop, high geometric surface area, and short diffusion distance in a fixed bed reactor. 

Two types of catalytic converters are currently being used for meeting the passenger car emission standards in the U.S. three-way converters and dualbed converters. Both converters contain three-way catalysts, but with the dual-bed converter the three-way catalyst is followed by an air injection/ oxidation catalyst system. As for the earlier oxidation catalysts two forms of catalyst support are used pellets (thermally stable transitional alumina) and monoliths (cordierite honeycombs coated with a thin alumina washcoat). Figure 7 shows four catalytic converters currently being used by General Motors. 

Greene [90] investigated the oxidative destruction of methylene chloride by zeolite Y in the H form and ion exchanged with cerium and chromium. The zeolite was supported on a low surface area cordierite honeycomb monolith by application as a wash coat. Below 425°C the catalytic activity decreased in the order ... 

The monolithic samples were prepared by coating cordierite honeycombs (cell density 62 cells/cm wall thickness 20 pm) with an aqueous sluny of the desired washcoat oxide. After drying and subsequent calcination at 530°C for 2 h in air, the washcoated supports were impregnated with an aqueous solution of the platinum salt, dried and activated in hydrogen for 2h at 530°C. The total platinum loading was fixed at 50 g/ft3 Pt. 

Samples under study are commercial cordierite honeycomb catalysts. The catalytic washcoat is conventionally composed of ceria-promoted transition alumina-supported 5Pt/lRli (weight %). Samples are cut from original converters in the form of cylinders of 1 inch (diameter) X 3 inches (length) to meet the geometrical requirements of the experimental set-up (see below). 

For obtaining a monolithic catalyst with better mechanical and hydrothermal durability, ZSM-5 zeolite with different Si/Al ratios (60, 55, 40, 25, 15) have been synthesized in-situ on cordierite honeycomb substrate. SEM photos showed the different crystal size from sample to sample. Solid MAS NMR was used to determine Si/Al ratios and the amount of zeolite on the substrate by the peak intensity Si. Investigation of NO decomposition (2000 ppm NO in He) on the Cu-exchanged ZSM-5/cordierite monolithic catalysts was performed at 723 K, GHSV=10,000/h. Results proved that the TOF of NO decomposition on monolithic catalysts is comparable with pure zeolite catalysts. 

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. 

We consider shaping in Chapter 23, but mention some additional features here. The method used depends on the material. Either brute force or a plasticizer should be used. The classic example is pottery—we mold the clay. Then we have the alumina thermal conduction module (TCM), cordierite honeycombs, Si3N4 fishhooks, and carbide blades for kitchen knives. 

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

Extruded Cordierite Honeycomb Ceramics for Environmental Applications... 

Extruded cordierite honeycombs also have applications in other fields because of their unique material and structural properties such as high porosity, low thermal expansion, high geometric surface area, and low gas flow restriction [2]. Utilizing their porous ceramic wall as filters, extruded honeycombs can be used as trap oxidizers to eliminate toxic particulate matter from diesel engine exhaust. 

This paper describes the properties and applications of extruded cordierite honeycombs. Euture tasks to upgrade performance of extruded cordierite honeycombs are also discussed. 

Extrusion is the most widely used method of forming cordierite honeycombs. Other methods, such as corrugated paper dipped in ceramic slurry, combshaped panel lamination, and press forming have been investigated. However, from the view point of mass production, extrusion is the best method for forming honeycombs. 

Figure 13.1.1 shows the manufacturing process flow chart for cordierite honeycomb. The raw materials for cordierite are kaolin, alumina, and talc, which are pulverized to suitable sizes for extrusion. These raw material powders are mixed with organic binders and water, then kneaded. The kneaded clay mixture is extruded through a die, dried and cut to specified length. These dried bodies are then fired at around 1400°C. 

Table 13.1.2 shows the material properties of standard extruded cordierite honeycombs. Extruded cordierite honeycombs have an extremely low coefficient of thermal expansion from 40 to 800°C, hence, high thermal shock resistance is expected. The softening temperature is above 1410°C. Porosity is 35%, and mean pore diameter is 5 (jtm (typical value). 

EIGURE 13.1.1 Manufacturing process Flow chart of cordierite honeycombs. 

TABLE 13.1.2 Material Properties of Extruded Cordierite Honeycombs... 

---