Catalyst washcoat

Fig. 1. The multiple scales in the catalytic monolith reactor (a) catalytic monolith (10 cm), (b) channel with catalyst washcoat on the walls (1mm), (c) SEM image of the washcoat layer (10 pm), (d) TEM image of meso-porous y-Al203 with dispersed Pt (200 nm).

It is generally accepted that thermal and especially hydrothermal treatment of aluminas and other catalytic materials results in deterioration of porous structure, i.e. increase in average pore radius and diminishing in specific surface area [1-4]. It is very important that such alumina materials as some catalyst washcoats and membranes have to be exploited at higher temperatures and at atmosphere of large humidity. Therefore it is necessary to improve their thermal and hydrothermal stabilization by application of new binder materials or additives. Such additives as silica, ceria or zirconia are known as thermal stabilizers. The aim of this work was to determine the influence of addition of the selected stabilizers on hydrothermal stability of alumina material in the temperature range 150 - 225 °C and time up to 72 hours. 

Figure 68. The function of platinum and rhodium on a fully formulated three-way catalyst washcoat in the conversion of CO, HC and NO, (monolith catalyst with 62 cells cm" three-way formulation, aged on an engine bench for 20 h engine bench test at a space velocity of 60000NIC h exhaust gas temperature 673K exhaust gas composition lambda 0.999, dynamic frequency I Hz amplitude 1 A/F). Reprinted with permission from ref. [34], (C 1991 Society of Automotive Engineers, Inc.

Figure 75. Secondary ion mass spectra (SIMS) showing the extent of the solid state reactions between alumina and ceria in a three-way catalyst washcoat (a) as a function of the type of precious metal and (b) as a function of the total precious metal loading (model catalyst on monoliths with 62 cells cm Pt, Pd or Rh loading 1.76 g 1 various total loadings at fixed Pt Rh ratio of 5 1 g/g model washcoat with 70 wt % alumina and 30 wt % ceria, after aging under air at 973 K).

Figure 86. Stability of the internal surface area of a three-way catalyst washcoat as a function of temperature and aging duration in an air atmosphere. Reprinted with permission from ref [34], CO 1991 Society of Automotive Engineers, Inc.

P. Pfeifer, K. Schubert, G. Emig, Preparation of copper catalyst washcoats for methanol steam reforming in microchannels based on nanoparticles, Appl. Catal. A Gen. 286 (2005) 175. 

In addition, there is pressure to reduce the cost of the catalyst. Washcoat aluminas have desired surface areas and porosities and are thermally stable. They are best produced by calcination of particular precursors, and aluminium isopropoxide or boehmite have been suggested to be useful materials to calcine [42]. Both of these precursors are not cheap, and less expensive raw materials would be desired,... 

Ceramic monoliths have proven themselves effective as substrates for catalyst washcoat and precious metal because they provide a relatively uniform porous surface. In the catalyst application process, the amount of alumina washcoat picked up depends upon the total porosity, as well as, the size distribution and shape of the pores within the wall. Likewise, the amount of precious metal picked up depends largely upon the amount of porous washcoat on the substrate. Catalyst coaters, therefore, have learned to optimize their process around typical properties of the substrate. However, through subtle variances in raw materials and process steps, variances in porosity occur piece to piece and lot to lot. 

The metal alloys for this purpose are available from a number of sources. Considerable research and development effort has been directed to the problem of application of the catalyst to the metal surface. The major problems to be overcome generally are considered to be those of cost to be competitive with existing materials, long term adhesion to catalyst washcoat to the metal surface and warping at high temps. 

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. 

Lead and Zinc. The x-ray images of a front core section (Figure 12) reveal the distributions of lead and zinc in the catalyst washcoat. Lead and zinc appeared to be thoroughly and somewhat randomly distributed in the washcoat there were also several pockets of very high concentrations of lead and zinc. 

The x-ray images presented in Figures 11, 12, and 13 indicate that the various contaminants were confined largely to the catalyst washcoat. That is, the contaminants did not penetrate to any significant extent into the cordierite substrate. The washcoat of the catalyst pictured in these micrographs was 30-40/x thick. 

Catalytic micromembrane device for high-purity hydrogen generation (a) micro-device unit with side-view (left) and cross section (right), (b) completed micro-device prior to assembly and (c) catalyst washcoat of microchannel with LaNiogBCooDsOs/AljOa (Wilhite eta ., 2006) (Copyright permission 2006 John Wiley and Sons). 

Figure 26.2 CuZnAl catalyst washcoatings prepared with a poly(vinyl alcohol) binder (left micrograph) and a Tylose binder (right micrograph).

Mathematical models for monolith reactors can be classified in several manners (see, e.g.. Ref. [3]), considering features such as dimensionaUty (one-dimensional (ID), two-dimensional (2D), or three-dimensional (3D) models) or the model multiscale nature (catalyst/washcoat-charmel-reactor, washcoat-channel, etc.). Most commonly, the behavior of a single monolith channel is described, assuming it to be representative of all channels in the whole reactor. This is highly desirable since the computational cost increases with the number of cells included in the simulations. However, it should be remarked that the single-channel analysis may not be valid for a number of cases ... 

Common cell densities used for heavy-duty diesel applications with vanadia SCR catalysts are 300 cpsi [19, 36] and 400 cpsi [6, 18, 35, 37, 40]. For these cell densities, the wall thicknesses for cordierite substrates range typically from 4 mil (100 pm) to 8 mil (200 pm) [18, 19, 37, 41]. In Fig. 3.12, with wall thickness is here considered the total wall thickness resulting from the substrate including the catalyst washcoat. The washcoat thickness for a coated vanadia SCR catalyst depends on the washcoat loading and could range from 20 to 100 pm [37]. For a washcoated SCR catalyst, the wall thickness could of course be reduced either by decreasing the inert substrate wall thickness or reduce the washcoat thickness as long as this does not effect the catalyst performance. 

The addition of nickel oxide to a catalyst washcoat can minimize the formation of Itydrogen sulfide during lean operation by absorbing some of the sulfur dioxide from exhaust gas. Nickel oxide can, therefore, store the sulfur dioxide as sulfate under reducing conditions and then release the sulfur dioxide under oxidizing conditions. 

In a previous paper the preparation of Pd/AEOs-based catalysts washcoated onto metallic foams was reported by some of us (Giani et al., 2006), using a sol-gel-type procedure. In this work, the development of a simpler method to deposit active washcoats of Ni/MgAl204 catalysts over FeCrAlloy (Fe 73 wt. %, Cr 20 wt. %, Al 5 wt. % and Y 2 wt. %) foams is described. 

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