Catalyst coating washcoating

Firstly, there are technical reasons concerning catalyst and reactor requirements. In the chemical industry, catalyst performance is critical. Compared to conventional catalysts, they are relatively expensive and catalyst production and standardization lag behind. In practice, a robust, proven catalyst is needed. For a specific application, an extended catalyst and washcoat development program is unavoidable, and in particular, for the fine chemistry in-house development is a burden. For coated systems, catalyst loading is low, making them unsuited for reactions occurring in the kinetic regime, which is particularly important for bulk chemistry and refineries. In that case, incorporated monolithic catalysts are the logical choice. Catalyst stability is crucial. It determines the amount of catalyst required for a batch process, the number of times the catalyst can be reused, and for a continuous process, the run time. 

The x-ray micrograph of the cross section of a flow channel wall of a washcoated cordierite monolith in Figure 6 reveals how the catalyst support washcoat is distributed on the external surfaces (sides) of the flow channels and throughout the macroporous interior region of the cordierite monolithic substrate. The light areas are the washcoat support material, and the dark areas are cordierite. In the micrograph, the exterior walls (sides) of the channel are vertical. The washcoat on the right wall is noticeably thicker than that on the left wall. It is quite typical that the thickness of the washcoat on the exterior walls varies considerably. The external washcoat is macroporous. The washcoat support material has penetrated into the macroporous structure of the cordierite substrate and has coated its internal surfaces. 

Catalyst samples containing noble metals and different washcoats were studied. A flat metal foil was coated with the washcoat, thus allowing the analysis of a real exhaust catalyst. The washcoat layer on the metal substrate comprised AI2O3 or Ce02-Al203. The noble metal loading of Pt, Rh, Pd and Pt-Rh on the washcoat was in the range of 1.4 to 2.9 %. The thickness of the washcoat was typically 15-40 pm. 

Monolith reactors are composed of a large number of parallel channels, all of which contain catalyst coated on their inner walls (Figure 1.9 [1]). Depending on the porosity of the monolith structure, active metals can be dispersed directly onto the inner channel walls, or the catalyst can be washcoated as a separate layer with a definite thickness. In this respect, monolith reactors can be classified among PER types. However, their characteristic properties are notably dhferent from those of the PBRs presented in Section 1.2.1. Monolith reactors offer structured, well-defined flow paths for the reactive flow, which occurs through random paths in PBRs. In other words, the residence time of the reactive flow is predictable, and the residence time distribution is narrow in monoliths, whereas in a PBR, different elements of the reactive mixture can pass through the bed at different rates, resulting in a wider distribution of residence times. This is a situation that is crucial for reactions where an intermediate species is the desired product and has to be removed from the reactor before it is converted into an undesired species. 

A related topic is the evaluation of internal diffusion limitations in catalyst coatings supported on microchannels. Experimentally, the two most common tests are variation of temperature and catalyst layer thickness. Both quantities are sometimes given as criteria for the chemical regime (specification of a temperature or thickness below which kinetics controls). For example, some authors [127, 131] consider internal diffusion neghgible for t <50pm. Kapteijn et al. [132] concluded that washcoat layers thicker than this would lead to mass transfer control in the washcoat, when testing square channel... 

Catalyst coating design. More efficient calculation of the effectiveness factor in monolith channels is possible for nonuniform washcoat geometries and nonlinear kinetics [94]. Trade-offs in reactor performance arising from catalyst loading and washcoat thickness were also considered [43]. 

Once the low surface area monolith body has been produced, a catalyst carrier needs to be deposited onto the monolith, which may be achieved in most instances by washcoating (see Section 4.1.3). The materials of ceramic monoliths are very compatible with catalyst coatings. They do not migrate into the catalyst coatings and neither do the active species of the precious metal catalysts migrate into the monolith body [57]. 

The first step to washcoat a metalhc substrate is to prepare a stable slurry of the catalyst to be deposited. Next, nature and roughness of the substrate surface must be adequate in order to fix the catalyst coating. Finally, the metallic monolith is washcoatedby immersing and withdrawing in the slurry followed by the elimination of the excess. In the following, we will discuss some of the most important variables controlling every step. 

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. 

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. 

Among the non-traditional routes for formation of catalyst and catalyst/carrier coatings, the most prominent way is the washcoat route followed by wet impregna-... 

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. 

Lenz and Aicher reported the experimental results obtained with an autothermal reformer fed with desulfurized kerosene employing a metallic monolith coated with alumina washcoat supporting precious metal catalysts (Pt and Rh) [78]. The experiments were performed at steam-to-carbon ratios S/C = 1.5-2.5 and... 

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 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. 

Figure 22 plots the failure temperature, measured in a cyclic thermal shock test [29], for ceramic catalyst supports as a function of their axial TSP values, which were controlled by modifying either the substrate, the washcoat, or the substrate/washcoat interaction. There is an excellent correlation between the failure temperature and the TSP value. Most automakers call for a failure temperature in excess of 750 C, although this may depend on the size of the catalyst and inlet pipe. Thus, a TSP value of more than 0.4 is required for the coated substrate. Finally, Fig. 22 shows that the washcoat may reduce the failure temperature of the catalyst support by 100-200 C, a trade-off the automakers are well aware of. 

Several catalysts with the above washcoats were thermally cycled at successively higher temperatures until failure occurred. An acoustic technique was employed to detect invisible fractures. The failure temperature (7 ) obtained in this manner is plotted against the TSP value in Fig. 22 for each of these coated monoliths. Also included in these data... 

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