Process for coating a carrier structure

Coating processes – Interior of hollow article coating

Reexamination Certificate

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Details

C427S430100, C427S443200, C427S235000, C427S239000, C427S238000, C427S379000, C427S294000, C427S350000

Reexamination Certificate

active

06746716

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention provides a process for coating a cylindrical carrier structure with a coating suspension. In particular, the invention provides a process for coating carrier structures for catalysts, for example car exhaust gas catalysts.
The carrier structures for car exhaust gas catalysts are cylindrical with two end faces and an encasing face and a number of flow channels for the exhaust gases of the internal combustion engines which are parallel to the cylinder axis running from the first end face to the second end faces. The carrier structures are also called honeycomb structures.
The cross-sectional shape of the carrier structures depends on the requirements for building it into the vehicle. Carrier structures with a round, elliptical or triangular cross-section are widely used. The flow channels mostly have a square cross-section and are arranged in a tight grid over the entire cross-section of the carrier structure. Depending on the particular application, the channel density or cell density of the flow channels varies between 10 and 140 cm
−2
. Honeycomb structures with cell densities up to 250 cm
−2
and higher are under development.
Catalyst carrier structures which are obtained by the extrusion of ceramic materials are largely used for the treatment of car exhaust gases. Alternatively, catalyst carrier structures consisting of corrugated and rolled up metal foils are available. Currently, ceramic carrier structures with cell densities of 62 cm
−2
are still used extensively for the treatment of exhaust gases in private motor vehicles. In this case, the cross-sectional dimensions of the flow channels are 1.27×1.27 mm
2
. The thickness of the walls in such carrier structures is between 0.1 and 0.2 mm.
In general, very finely divided platinum group metals are used, the catalytic effect of which can be modified by compounds of base metals, to convert the harmful substances present in car exhaust gases, such as carbon monoxide, hydrocarbons and nitrogen oxides, into harmless compounds. These catalytically active components have to be deposited onto the carrier structures. However, it is not possible to ensure the required extremely fine distribution of catalytically active components by the deposition of these components on the geometric surface areas of the carrier structures. This applies equally to non-porous metallic and to porous ceramic carrier structures. A sufficiently large surface area for the catalytically active components can only be made available by applying a support layer made of finely divided high surface area materials to the internal faces of the flow channels. This process is called coating the carrier structure. Coating the outer encasing face of the carrier structures is undesirable and should be avoided in order to avoid the loss of valuable catalytically active materials.
A suspension of the finely divided high surface area materials in a liquid phase, usually water, is used to coat the carrier structures. Typical coating suspensions for catalytic applications contain, as high surface area support materials for the catalytically active components, for example active aluminum oxides, aluminum silicates, zeolites, silicon dioxide, titanium oxide, zirconium oxide and oxygen-storing components based on cerium oxide. These materials form the solids fraction of the coating suspension. In addition, soluble precursors of promoters or catalytically active noble metals from the platinum group in the Periodic System of Elements may also be added to the coating suspension. The solids content of typical coating suspensions is in the range between 20 and 65 wt. %, with respect to the total weight of the suspension. The density is between 1.1 and 1.8 kg/l.
A number of processes for depositing the support layer on the carrier structures using a coating suspension or slurry is known from the prior art. For coating purposes, the carrier structures, may be for example immersed in the coating suspension or the coating suspension may be poured over the carrier structures. Furthermore, there is the possibility of pumping or sucking the coating suspension into the channels in the carrier structures. In all cases, excess coating material has to be removed from the channels in the carrier structure under suction or by blowing out with compressed air. Any channels blocked with coating suspension are opened up by this means.
After the coating procedure, the carrier structure and support layer are dried and then calcined to solidify and fix the support layer to the carrier structure. Then the catalytically active components are introduced to the coating by impregnating with mostly aqueous solutions or precursor compounds of the catalytically active components. As an alternative, the catalytically active components may also be added to the coating suspension itself. Subsequent impregnation of the final support layer with catalytically active components is not required in this case.
An essential criterion for the coating process is the coating or loading concentration which can be achieved therewith in one working stage. This is understood to be the proportion of solids which remains on the carrier structure after drying and calcining. The coating concentration is given in grams per liter volume of the carrier structures (g/l). In practice, coating concentrations of up to 300 g/l are required in car exhaust gas catalysts. If this amount cannot be applied in one working stage with the process used, then the coating procedure has to be repeated, after drying and optionally calcining the carrier structure, often enough to achieve the desired loading. Frequently, two or more coating procedures using coating suspensions of different composition are performed. Catalysts which have several superimposed layers with different catalytic functions are obtained in this way.
Another criterion for the quality of a coating is its uniformity, in both the radial and axial direction of the carrier structure. Irregularities in the axial direction cause particular problems because they can lead at high target loadings to pressure losses which can no longer be tolerated.
It is known in the art that one way to achieve uniformity in the coating of a catalyst carrier structures in honeycomb form also called honeycomb structures is to vertically align the cylindrical axis of the honeycomb structure so the coating suspension is pumped into the channels through the lower end face of the honeycomb structure until it emerges from the upper end face. Then, the coating suspension is again pumped out downwards and excess coating suspension is removed from the channels by blowing out or under suction in order to avoid blocking the channels. Using this process, support layers are obtained which have high uniformity over the entire length of the honeycomb structure.
Other ways to coat ceramic honeycomb structures are known in the art. For example, a previously determined amount of a coating suspension is placed in a flat vessel and one end face of the honeycomb structure to be coated is dipped into the suspension. The previously determined amount of coating suspension corresponds to the target amount of coating for the honeycomb structures. Then the entire amount of coating suspension is pulled into the flow channels of the honeycomb structure under suction by applying a vacuum to the second end face. Since the previously determined amount of coating suspension corresponds to the target amount of coating for the honeycomb structures, no removal of excess coating suspension from the flow channels is required after introducing the coating suspension under suction. Coating is preferably performed in two steps, wherein in a first step 50 to 85% of the amount of coating required is introduced to the flow channels under suction from the first end face and the remaining amount of coating is introduced from the second end face of the honeycomb structure. This process results in a high degree of reproducibility for the coating concentration. However, the catalysts pr

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