Washcoat deposition

Fig. 9. Details of a monolithic catalyst showing washcoat deposited on a substrate.

Combined with the use of precious metal catalyzed washcoats deposited on the walls, microchannel reactors can realize nearly 10 times reduction in reactor size compared with that of a process that utilizes catalyst particles. The washcoat thickness is usually less than lOOjit and provides greater structural stability. This stability arises from smaller thermal expansion ratios and lower temperature gradients. 

C. Agraflotis, A. Tsetsekou, The effect of processing parameters on the properties of y-aluminia washcoats deposited on ceramic honeycombs, J. Mater. Sci. 2000, 35, 951-960. 

Figure 6. Amount of washcoat deposited versus fraction of solid material in the dipping slurry for different methods for removal of excess slurry. The figure legend rpm denotes the centrifuge speed used.

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]. 

CAMET control catalyst was shown to obtain 80% NO reduction and 95% carbon monoxide reduction in this appHcation in the Santa Maria, California cogeneration project. The catalyst consists of a cormgated metal substrate onto which the active noble metal is evenly deposited with a washcoat. Unlike the typical 20 on titania turbine exhaust catalysts used eadier in these appHcations, the CAMET catalyst is recyclable (52). 

The process has been commercially implemented in Japan since 1977 [1] and a decade later in the U.S., Germany and Austria. The catalysts are based on a support material (titanium oxide in the anatase form), the active components (oxides of vanadium, tungsten and, in some cases, of molybdenum) and modifiers, dopants and additives to improve the performance, especially stability. The catalyst is then deposited over a structured support based on a ceramic or metallic honeycomb and plate-type structure on which a washcoat is then deposited. The honeycomb form usually is an extruded ceramic with the catalyst either incorporated throughout the stmcture (homogeneous) or coated on the substrate. In the plate geometry, the support material is generally coated with the catalyst. 

Generally, the preparation of washcoated structured catalysts is governed by several parameters, such as the nature and particle size of the precursor powder, loading of powder, nature and concentration of dispersants, temperature of the slurry, use of binders in the slurry and deposition of a primer layer on the monolith. 

Values of effectiveness factors in washcoat layers with non-uniform thickness around the channel perimeter have been studied by Hayes et al. (2005). However, the applicability of (even the generalized) effectiveness factor approach is quite limited in complex systems with competing reactions, surface deposition of reaction components, non-linear rate laws and under transient operating conditions (e.g. periodically operated NSRC). Typically, the effectiveness factor method can be used for more accurate prediction of CO, H2 and HC oxidation light-off and conversions in DOC. 

Complete characterization of poisoned catalysts, of course, requires much more than chemical analysis. For example, the interaction of poisons with catalyst constituents and with each other has been studied by X-ray diffraction and by electron microscopy, the morphology of the poison deposits by optical methods, the distribution within the catalyst pellets and washcoats by the microprobe, and the distribution of poison on the surface of the active metals by Auger spectroscopy. 

Fig. 10. Fracture cross section normal to honeycomb channels at inlet surface deposits grow outward from washcoat surface. [From Bomback et al. (35).] (Reprinted with permission from Environmental Science and Technology. Copyright by the American Chemical Society.)...

The relative penetration of lead, zinc, phosphorus, and sulfur into the washcoat is seen in Fig. 12 at a comer where the washcoat is relatively thick. Lead and sulfur penetrate over 100 /urn, phosphorus between 30 and 50 /tm, and zinc less than 2 p.m. All of the elements, except zinc, penetrate down into the deep crack and laterally into the washcoat. This supports again the supposition that zinc arrives at the surface in particulates, while lead, phosphorus, and sulfur arrive as gaseous entities. However, the penetration profiles do not prove the modes of deposition conclusively. Contaminant concentrations expressed in terms of lead... 

Refractory high surface area oxides are deposited from slurries onto the walls of the channels that make up monoliths in order to provide an adequate surface area to support the active catalytic species. Washcoats such as AI2O3 and TiC>2 are commonly used for pollution abatement applications (auto exhaust, stationary NO abatement, etc.) where the monolith is usually a ceramic. Metal monoliths are finding increasing use however, they represent only a small percentage of the total monoliths used. Optical microscopy enables one to see that the catalyzed washcoat follows the contour of the ceramic surface. Figure 7 shows the AI2O3 washcoat-ceramic interface for a typical auto exhaust catalyst. In this case, no evidence of loss of adhesion between washcoat and ceramic can be seen. 

The characteristic length of the washcoat section to be simulated is 10 pm thus, we may consider constant temperature profile on this scale. Since the volume diffusion in the macro-pores is much faster than the Knudsen diffusion in the meso-pores of the y-Al203 particles, we may further assume that the CO and 02 gas concentrations in the macro-pores are constant within the simulated washcoat section. For the surface-deposited components CO and O, a zero diffusivity is used, i.e., Df1 — 0. For gaseous CO and 02, the effective diffusivi-ties are based on the Knudsen diffusivity in the meso-pores (with diameter T0nm) of the y-Al203. 

A bare monolithic structure can be coated with a catalyst support layer in several ways. Figure 21 shows a SEM image of a typical commercial cordierite monolith structure. Washcoating can be done by (partly) filling the pores of the macroporous walls with the washcoat material or by depositing a washcoat as a layer on top of the walls. These methods are shown schematically in Figure 22. 

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... 

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