Aluminum washcoat

With reference to the formaldehyde reactor, assuming the parameters reported in Table 28.5, the simulations showed that the HCHO molar yield could be incremented from the 93.6%, reported for an optimized packed-bed reactor process,up to over 97% if aluminum-washcoated honeycombs with suitable design were loaded in the original reactor tubes. The optimal performance of the monolith catalysts originates... 

Catalyst Deactivation. Catalyst deactivation (45) by halogen degradation is a very difficult problem particularly for platinum (PGM) catalysts, which make up about 75% of the catalysts used for VOC destmction (10). The problem may weU He with the catalyst carrier or washcoat. Alumina, for example, a common washcoat, can react with a chlorinated hydrocarbon in a gas stream to form aluminum chloride which can then interact with the metal. Fluid-bed reactors have been used to offset catalyst deactivation but these are large and cosdy (45). 

FIGURE 25 Image of alumina washcoat on aluminum foam. Left closeup SEM left, aluminum middle, anodized layer (aluminum oxide, about 5 pm thick) right, washcoat (20 pm thick). Right image piece of solid foam from which part of the washcoat had been removed (broken off), thus showing the washcoat layer. 

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... 

Materials such as aluminum titanate and silicon carbide appear to be promising for high-temperature catalytic combustion. However, problems such as extrudability, the application of washcoats, and reaction with deposited washcoats are not solved yet. For instance, when hexa-aluminate, presented in the introduction to this section, was applied to silicon carbide monoliths, solid-state reactions occurred at 1200-1400 C [76], causing exfoliation of the coating and the formation of new phases. The application of an intermediate mullite layer was suggested as an approach to hinder these solid-state reactions. 

The chemical composition of the washcoat belongs to the core know-how of the catalyst manufacturers. The most common washcoats contain aluminum oxides, cerium oxides and zirconium oxides as major constituents. The minor constituents... 

The aluminum oxide is typically formed in the desired modification and with the desired chemical composition before it is added to the washcoat. 

Similarly, Fig. 75 shows the effect of both the amount and the type of precious metal on the extent of the solid state reaction between aluminum oxide and cerium oxide, leading to the formation of cerium aluminate. The overall effect of these interactions is that the catalytic activity and the durability of the precious metals vary as a function of the type of washcoat oxide they are deposited on [45]. 

Monoliths made of metal foils can also be used as substrates in combustion catalysts [19, 20]. The metal is generally an iron- or nickel-based steel containing small amounts of aluminum. The aluminum diffuses to the surface on heating and oxidizes to form an adherent alumina layer. This alumina layer gives the alloy high oxidation resistance and is essentially self-healing as it arises from diffusion from the bulk material. It also provides good adhesion for the alumina washcoat. 

Figure 10.12. AI solid-state NMR chemical shift data from a fresh Pd-only catalyst and from inlet, middle, and outlet sections of a high-mileage taxi-cab aged catalyst of the same formulation. Note the strong feature at 40 ppm in the vehicle-aged catalyst corresponding to hydrated aluminum phosphate. The broad features near 5 and 60 ppm are associated with octahedrally and tetrahedrally coordinated aluminum cations in the alumina washcoat. [30]...

A method for coating microchannel walls with layers as thick as 25 pm was developed by Stefanescu et al. [181]. The microreactor was built from FeCrAl (Aluchrom ). The metal surface was first chemically treated in several steps and afterward annealed at 1200 °C for 1 h to trigger the segregation of aluminum and the formation of an alumina layer on the metallic surface. An alumina washcoat was subsequently deposited from a slurry onto the microstructure and characterized by various physical methods. The authors varied the properties such as viscosity, particle size, and pH of the slurry. Acrylic acid, a component used as dispersant and binder, was found to be particularly important for the adhesion of the alumina layer. 

Within the current TWC catalyst washcoats, rhodium is susceptible to deleterious interactions with various components during a prolonged lean high temperature excursion. To elucidate the potentially detrimental rhodium compounds formed under such circumstances, unsupported rhodium oxides, rare earth metal rhodates, and aluminum rhodate are characterized and measured for catalytic activity. The intrinsic activities at 673K of NO, CO and CjHg conversions over various unsupported rhodium oxides species are basically structure insensitive. However, the intrinsic activities at the same temperature of both the rare earth metal rhodates and aluminum rhodate appear to be sensitive to their structure. The interaction between rhodium and the rare earths especially cerium, is found to be much stronger than that between rhodium and aluminum. 

Pb(P03)2 and Pb3(P04)2. In the exterior 20% of the washcoat (i.e. toward the external washcoat surface), however, the phosphorus concentration was disproportionately greater than the lead concentration. At the external surface of the washcoat, the phosphorus concentration was -— 180-fold that of lead. Under the conditions of the exhaust gas atmosphere, phosphorus could be present on the catalyst only in the form of stable phosphate compounds. The high phosphorus concentration in the external surface region of the washcoat indicated that chemical interaction between phosphorus and the washcoat material had occurred to form hydrothermally stable phosphates such as aluminum phosphates, e.g. A1(P03)3 and A1P04. X-ray analysis of this catalyst failed to confirm the presence of crystalline aluminum phosphates. It is therefore con-... 

It was reported in the literature and also demonstrated in this laboratory that both Co304 and copper chromite are poisoned by sulfur. This results from the accumulation of sulfate groups on the catalyst surface. The base metal sulfates and aluminum sulfate are very stable, and they decomposed to the oxide only at temperatures above 650°C (see Table IV and Figure 2). Above 650°C, activity was restored because of sulfate decomposition. When a base metal catalyst was subjected to high temperatures before being cooled down for a CVS test, it had good activity for a short period of time which was dependent on the sulfur content of the gasoline and the surface areas of the washcoat and base metal catalyst. 

Metal monoliths were obtained from Emitec (Germany). They were subjected to high-temperature treatment by the supplier. The cell density of the monoliths used is approximately 4(X) cpsi. The monoliths consist of an iron-chrome-aluminum alloy which provides the surface with a textured whisker structure after suitable treatment. These whiskers, shown in Figure 8, act as anchors for the washcoat when deposited onto the substrate. Tbe whiskers consist of aluminum oxide, completely covering the metal surface. This is shown by the data in Table 2, giving the results of EDX and XPS analyses of the whiskers-covered metal surface. 

A desiccant wheel assembly consists of i) a core that contains the desiccant materials, ii) a core/wheel support structure, iii) a drive system to rotate the wheel at a very low speed, and iv) a set of air seals to separate the process air stream from the regeneration air stream. The core has a monolith honeycomb type structure. Both sinusoidal and hexagonal channels are used in commercial equipment. Various types of materials have been used for support matrix that includes ceramic, glass fibers, and corrugated aluminum sheets. The desiccant materials may be washcoated, impregnated, or formed in situ on the support matrix. 

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