Washcoat Composition

The surface of the honeycomb is covered by the alumina washcoat, which can be stabilized against sintering at high temperature by the addition of more refractory materials such as barium oxide, calcium oxide, magnesium oxide or lanthanum oxide. A very thin surface layer of the washcoat, up to about 10-30 xm, is applied although the thickness increases to about 150pm at the comers of the square channels. 

Ceria and zirconia have been shown by electron microprobe spectroscopy to combine the preparation of the catalyst, forming a very thermally stable phase, and platinum group metals deposit preferentially on the alumina. The washcoat has often been applied and fired with more alumina before the platinum group metals are impregnated. Alternatively, the washcoat and metals can be applied at the same time. In either case, the conditions for the deposition can be adjusted to provide a variable surface layer of alumina and the oxides, which absotb trace poisons and protect the active metals. 

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It is difficult to separate the influence of the precious metals from the influence of the washcoat composition. Each of the precious metals platinum, palladium and rhodium, which have a specific task, interact with each other during the use and the aging of the catalyst, so that their influence on the catalyst performance is in most cases not additive. Figure 68 compares the performance of a fully formulated Pt/Rh catalyst to the performance of catalysts having either Pt or Rh alone in the same loadings and on the same washcoat as the fully formulated catalyst. 

The washcoat composition is quite different to that used for three-way catalysts. The washcoat oxides used are chosen so as to ensure a minimal adsorption both of the soluble organic fraction and of the sulfur oxides SO2 and SO3. The properties of three different oxides in these respects are shown in Figs. 103 and 104. The composition of the washcoat is, together with the choice of precious metal formulation. 

Nevertheless the comparison of the activity of the catalyst supported on alumosilicate ceramics with that of the catalyst on cordierite with the same content of active component (0,4%Pt + 0,2%Pd) and the same washcoat composition (5%Zr02-t7-Al203) gives same results. 

Table 3 The nine catalysts used in the tests with the target washcoat composition, precious...

Numerous permutations in composition exist, but usually the precise composition, particularly that of the washcoat, is a commercial secret. Detailed accounts of the three-way catalyst have been given by Heck and Farrauto [R.M. Heck and R.J. Farrauto, Catalytic Air Pollution Control, (2002, 2" Edition), WUey, New York.]. Here we briefly describe the functions of the catalyst ingredients. 

Figure 4.11 reveals that Pt is present on the surface of the catalyst as an oxide, in combination with hydrocarbon species (a contaminant during sample preparation) and as a chloride (derived from the Pt precursor, chloroplatinic acid). The results show the composition of the washcoat to be Pt and Rh on alumina and ceria. 

Several length-scales have to be considered in a number of applications. For example, in a typical monolith reactor used as automobile exhaust catalytic converter the reactor length and diameter are on the order of decimeters, the monolith channel dimension is on the order of 1 mm, the thickness of the catalytic washcoat layer is on the order of tens of micrometers, the dimension of the pores in the washcoat is on the order of 1 pm, the diameter of active noble metal catalyst particles can be on the order of nanometers, and the reacting molecules are on the order of angstroms cf. Fig. 1. The modeling of such reactors is a typical multiscale problem (Hoebink and Marin, 1998). Electron microscopy accompanied by other techniques can provide information on particle size, shape, and chemical composition. Local composition and particle size of dispersed nanoparticles in the porous structure of the catalyst affect catalytic activity and selectivity (Bell, 2003). 

Most of the current converters consist of a flow-through ceramic monolith with its channel walls covered with a high-surface-area 7-AI2O3 layer (the washcoat) which contains the active catalyst particles. The monolith is composed of cordicrite, a mineral with the composition 2MgO 2AI2O3 5Si02. The chemical composition of a modern TWC is quite complex. In addition to alumina, the washcoat contains up to 30 wt% base metal oxide additives, added for many purposes. The most common additives are ceria and lanthana in many formulations BaO and Zr02 are used, and in some converters NiO is present. The major active constituents of the washcoat are the noble metis Pt, Pd, and Rh (typically 1-3 g). Most of the TWC systems in use today are still based on Pt and Rh in a ratio of about 10 1. 

Most likely, the higher the degree of homogeneity in the washcoat material, the higher the resistance to sintering. Sol-gel synthesis provides a method to produce mixed oxide materials with homogeneity on the atomic scale [87] and should thus be explored further, especially with respect to novel compositions. 

Figure 8 Steam reforming of hexane at flow rates of 2 0 and 0 64 Ib/hr of water and hexane, respectively Axial bed-temperature and composition profiles for a metal monolith (250 cells/in consisting of Kanthal support/7-Al203 washcoat/NiO catalyst, and a packed bed of Girdler G-9(X pellets (j in. X in ) of alumina impregnated with nickel. (From Ref. 9.)...

A large number of permutations in composition exists. Usually the precise composition, particularly that of the washcoat, is secret. Here, we describe the function of the different catalyst ingredients [42,43]. 

Eqs. (1), (2), and (5) must be solved simultaneously to obtain the methane conversion and temperature profiles for the monolith. Gas phase composition jg and temperature Tg are calculated for each volume element from the mass and heat balances, Eqs. (1) and (2), and then substituted in the washcoat heat balance, Eq. (5). At the inlet of the monolith, where both the composition and the temperature of the gas phase are known, Eq. (5) can be solved independently. 

Another important application for zeolite nanoparticles is the preparation of zeolite matrix composite films. Micrometer-sized crystals have long been used in these types of films, using techniques such as washcoating and sol-gel processing. However, the use of nanoparticles in these films appears to offer many unparalleled advantages in terms of achievable film... 

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. 

The precious metal composition is typically uniform in the radial and axial directions of the monolith structure, although different designs have been described in the patent literature and have even been used in some selected applications. However, much more common is a nonuniform distribution of the precious metals within the washcoat layer. One - macroscopic - example of nonuniform distribution is that the amount of one precious metal component decreases from the part of the washcoat that is in contact with the gas phase towards the part of the washcoat that is in contact with the monolith wall and eventually vice-versa for the second precious metal component. Another - microscopic - example of nonuniform distribution within the washcoat is that each precious metal component is selectively deposited on different washcoat components. These nonuniformities are intentional and are desirable for kinetic reasons or because of specific beneficial interactions between the precious metals and the washcoat oxides. The type of nonuniformity that can be achieved depends strongly on the production procedure of the catalyst. 

The catalyst formulation is fixed by the composition and the properties of its washcoat components, by the amount of the various precious metals used, and by the catalyst preparation procedure. The composition and the properties of the washcoat are the key factors that govern the performance and the durabihty of the catalyst. 

Figure 66. Influence of the washcoat loading of a ceramic monolith on the conversion of NO t (monolith catalyst with 62cells cm" three-way formulation with Reprinted with permission from ref. [34], 1991 Society of Automotive Engineers, Inc. Pt 1.42gl" , Rh 0.28gl" after aging on an engine bench 20 h engine bench test space velocity 60000N1E h exhaust gas temperature 723 K exhaust gas composition lambda 0.999 dynamic frequency 1 Hz amplitude 1 A/F). Reprinted with permission from ref [34], 1991 Society of Automotive Engineers, Inc.

Figure 69. Effect of platinum, palladium and rhodium at an equimolar loading on the temperature needed to reach 50% conversion of butene and butane, as a function of the exhaust gas oxygen current (monolith catalyst with 62 cells cm y-Al203 washcoat fresh precious metal loading 8.8 mmol 1" model gas light-off test at a space velocity of 60000N11 h model gas composition is stoichiometric at 1.0 vol % O2). Reprinted with permission from ref [30], (g) 1994 Society of Automotive Engineers, Inc.

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