Coating processes – Measuring – testing – or indicating
Reexamination Certificate
2001-03-22
2003-04-15
Barr, Michael (Department: 1762)
Coating processes
Measuring, testing, or indicating
C427S230000, C427S238000, C427S430100, C427S372200
Reexamination Certificate
active
06548105
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention pertains to a method for partially coating a cylindrical carrier body with a coating suspension. The invention pertains, in particular, to a method for coating carrier bodies for catalysts—for example, automobile exhaust gas catalysts.
Carrier bodies for automobile exhaust gas catalysts have a cylindrical shape with two end faces and an exterior surface jacket, and are provided with a series of flow channels for the exhaust gases of the internal combustion engine which lie parallel to the cylindrical axis extending from the first end face to the second end face. These carrier bodies are also referred to as honeycomb bodies.
The cross-sectional shape of the carrier body depends on the installation requirements of the motor vehicle. Carrier bodies with round, elliptical or triangular cross sections are broadly utilized. The flow channels usually have a square cross section and are arranged tightly adjacent to one another over the entire cross section of the carrier body. Depending on the type of application, the channel or cell density of the flow channels varies between 10 and 120 cm
−2
. Honeycomb bodies with cell densities up to 250 cm
−2
and more are being developed.
Catalyst carrier bodies obtained by extruding ceramic masses are primarily used for purifying automobile exhaust gases. Alternatively, catalyst carrier bodies consisting of metal foils that are corrugated and subsequently wound are also available. Today, ceramic carrier bodies with cell densities of 62 cm
−2
are still predominantly used for purifying exhaust gases of passenger cars. The cross-sectional dimensions of the flow channels are 1.27×1.27 mm
2
in this case. The wall thicknesses of such carrier bodies lie between 0.1 and 0.2 mm.
Dispersed metals of the platinum group, the catalytic effect of which may be altered by compounds of base metals, are most frequently utilized for converting the harmful substances contained in automobile exhaust gases, e.g., carbon monoxide, hydrocarbons and nitrogen oxides, into harmless compounds. These catalytically active components need to be deposited on the carrier bodies. However, it is impossible to ensure the required dispersion of the catalytically active components on the geometric surfaces of the carrier body by depositing these components. This applies to non-porous metallic carrier bodies as well as porous ceramic carrier bodies. A sufficiently large surface for the catalytically active components can only be provided by applying a support layer of fine-particle, high surface area materials onto the inner surfaces of the flow channels. This process is referred to in the following as the coating of the carrier body. A coating of the outer envelope, or exterior surface jacket, of the carrier body is undesirable and should be prevented in order to avoid losses of valuable catalytically active materials.
A suspension of the fine-particle, high surface area materials in a liquid phase, usually water, serves for coating the carrier bodies. Various methods for depositing the support layer on the carrier body, by utilizing the coating suspension or slurry, are known from the state of the art. The coating is, for example, realized by immersing the carrier body in the coating suspension or by pouring the coating suspension over the carrier body. It is also possible to pump or attract the coating suspension by suction into the channels of the carrier body. Excess coating material always needs to be removed from the channels of the carrier body by means of suction or compressed air. This also ensures that channels which might have become clogged with coating suspension are opened.
After the coating process, the carrier body and the support layer are dried and subsequently calcined, in order to solidify and fix the support layer on the carrier body. Subsequently, catalytically active components are introduced into the coating by means of an impregnation with usually aqueous solutions of precursor compounds of the catalytically active components. Alternatively, the catalytically active components can be added into the coating suspension. In this case, a subsequent impregnation of the finished support layer with catalytically active components is not necessary.
One essential criterion of the coating method is that the coating or charging concentration can be achieved in one cycle. This refers to the portion of solids which remains on the carrier body after the drying and calcining processes. The coating concentration is expressed in grams per liter of the volume of the carrier body (g/L). In practical applications, coating concentrations up to 300 g/L are required for automobile exhaust gas catalysts. If this quantity cannot be applied in one cycle with the respectively utilized method, the coating process must be repeated after drying and, if applicable, calcining the carrier body until the desired concentration is reached.
DE 40 40 150 C2 describes a method in which catalyst carrier bodies having a honeycomb shape can be uniformly coated with a support layer, or with a catalytically active layer, over their entire length. In the following description, catalyst carrier bodies with a honeycomb shape are also referred to as honeycomb bodies. According to the method described in DE 40 40 150 C2, the cylinder axis of the honeycomb body is vertically aligned for the coating process. Subsequently, the coating suspension is pumped into channels through the lower end face of the honeycomb body until it emerges at the upper end face. The coating suspension is then pumped off downward and excess coating suspension is removed from the channels by means of suction or compressed air in order to prevent clogging of the channels. Support layers that have an adequate uniformity over the entire length of the honeycomb body can be obtained with this method.
U.S. Pat. No. 4,550,034 and U.S. Pat. No. 4,609,563 describe a method for coating ceramic honeycomb bodies in which a predetermined quantity of a coating suspension is filled into a flat container and the honeycomb body to be coated is immersed into the suspension with one of its end faces. The predetermined quantity of the coating suspension corresponds to the desired coating quantity for the honeycomb body. Subsequently, the entire quantity of the coating suspension is attracted by suction into the flow channels of the honeycomb body by applying a vacuum to the second end face. Since the predetermined quantity of the coating suspension corresponds to the coating quantity required for the honeycomb body, no removal of excess coating suspension from the flow channels takes place after the coating suspension is introduced into the flow channels. The coating process is preferably carried out in two steps, with 50-85% of the required coating quantity being attracted by suction from the first end face in the first step, and with the remaining coating quantity being attracted by suction into the flow channels from the second end face of the honeycomb body.
A high reproducibility of the coating concentration can be achieved with the method described in these two patents. However, the thickness of the coating along the honeycomb body has a significant gradient in the catalysts manufactured in this manner. Also, the preferred coating of the honeycomb body in two steps is unable to sufficiently improve the uniformity of the coating along the honeycomb body.
Certain applications require catalysts that have regions with different catalytic activities along the catalyst carrier body. For example, EP 0 410 440 B1 describes a catalyst that consists of two partial catalysts—namely a catalyst on the inflow side which serves for achieving a selective catalytic reduction of nitrogen oxides by means of ammonia or a compound that supplies ammonia and an oxidation catalyst on the outflow side. In this case, the oxidation catalyst is applied in the form of a coating onto a section of the one-piece reduction catalyst that is fully extruded in a honeycomb shape, which section is situated o
Kiessling Ralph
Piroth Josef
Barr Michael
DMC2 Degussa Metals Catalysts Cerdec AG
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