Coating processes – Interior of hollow article coating – Vacuum or pressure utilized
Reexamination Certificate
2000-10-27
2003-07-29
Hendrickson, Stuart L. (Department: 1754)
Coating processes
Interior of hollow article coating
Vacuum or pressure utilized
C502S439000
Reexamination Certificate
active
06599570
ABSTRACT:
The present invention concerns improvements in a manufacturing process, and more especially concerns apparatus and methods suitable for the manufacture of catalysts supported on monolithic supports and like products.
Large numbers of catalysts supported on high surface area monolithic supports are manufactured each year. One of the principal types of such catalyst is the catalytic convertor for the internal combustion engine, and the present invention has particular utility in relation to these. For conciseness, the following description will particularly describe the use of the invention for car catalysts, but it should be understood that the invention may find application in all similar types of catalyst, whether for vehicles (“car catalysts” is intended to include trucks, utility vehicles, buses, motorcycles etc), ships, stationary power sources or gas clean-up from other industrial processes. The present invention is also useful in the manufacturing of like products where a coating is applied to a carrier, and we mention particularly absorber/desorber coatings, many of which are used in exhaust emission control. Such products are not technically “catalysts”. We would mention particularly NO
X
absorbers, hydrocarbon absorbers, absorbers for sulphur compounds, and water vapour absorbers. As well as absorbers for gas treatment, absorbers may find use in heat pumps, eg air conditioning coolers.
The car catalyst is generally supported on an extruded ceramic, eg cordierite, or manufactured metal through-flow honeycomb substrate. The substrates are generally cylindrical, but may be oval or “racetrack” or a skewed oval, and have an effective continuous outer skin (if the support or substrate does not have a continuous skin, the present invention may be used if the support or substrate is located within a sleeve.) Such substrates arc very well known and are commercially available, and may have from 50 to 1200 cells/sq in. In order to increase the surface area for catalysis, it is commonplace to coat the substrate with a washcoat, comprising a slurry of high surface area particles such as alumina, optionally containing other components such as soluble and/or supported catalytically active platinum group metals (“POMs”), promoters such as ceria, zirconia, barium etc and other components or absorptive materials such as silica, zeolitc etc. In some cases, after the washcoat is applied and dried and/or fired to give an adherent coating, one or more catalyst layers is applied. This may be by impregnation of solutions of one or more PGMs, generally selected from one or more of platinum, palladium and rhodium, and/or soluble promoters and/or by applying more layers of the same or different types of catalytically active or absorptive washcoats. The coating processes in use are usually considered as confidential know-how, but are generally variations on immersing the substrate in the slurry or solution, or passing the substrates through a curtain or waterfall of the slurry or solution and using compressed air to blow the fluid into and through the substrate, ensuring coating of the cells and also ensuring that there are no or practically no blocked cells.
We have realised that such processes are not very flexible and have a number of other disadvantages. The slurry or solution is recycled, but the absorption characteristics of the substrate or washcoated substrate are such that certain components, eg finer particles or certain dissolved platinum group metal salts, deposit preferentially, thus changing the composition of the recycled fluid and accordingly altering some of the characteristics of the subsequently coated catalysts. We have accordingly realised that continuous processes may not provide the best answer for producing catalysts with identical performance and other characteristics. The present invention provides a batch or semi-batch process that offers very significant improvements in producing car catalysts, with advantages in consistency and control of raw materials, as well as offering catalyst design options not previously available for large scale manufacturing. It is believed that the present invention may have particular applicability in the coating of very high cell density substrates which are difficult to coat using conventional technology.
We are aware of some prior proposals which have some of the objectives of the present invention. For example, in U.S. Pat. No. 5,543,181, catalyst solution is sprayed into the cells of a honeycomb support through probes which are inserted into the cells. Such a system is mechanically complex and not capable of coping with very high cell density substrates. U.S. Pat. No. 4,931,419 discloses a method of soaking one end of a monolithic support in a specified volume of liquid, equal to the pore volume of the support and turning the monolithic support over to cause the liquid to drain through the support. Such a method still permits losses of valuable precious metal catalysts. U.S. Pat. No. 5,422,138 teaches a coating apparatus involving dipping the base of a monolithic support into a coating liquid then blowing excess liquid from the cells. WO 97/48500 describes a further catalyst coating system where a substrate is partially immersed in a coating liquid, a vacuum is applied to draw the liquid up into the cells of the substrate to a distance less than the length of the cells Such a system is not very flexible and still recycles coating liquid.
A further proposal is disclosed in GB 2,012,616, which discloses apparatus and methods for coating catalyst substrates using a measured charge of slurry which is drawn into and through the substrate. It is clear that the charge of slurry is such that it is in excess of that actually required to coat the substrate. It is not believed that this proposal was ever developed to the stage of commercial utility. We believe that the fact that excess slurry is used, which is recycled for re-use, results in the alteration of composition in time as various components of the slurry deposit preferentially onto the walls of the substrate.
The present invention provides a monolithic support coating apparatus, comprising means for dosing a predetermined quantity of a liquid component such quantity being such that it is substantially wholly retained within the intended support, liquid component containment means locatable on the top of a support to receive said quantity of liquid component and pressure means capable of withdrawing the liquid component from the containment means into at least a portion of the support.
The invention further provides a method of coating a monolithic support, comprising the steps (a) locating a containment means on top of a support, (b) dosing a predetermined quantity of a liquid component into said containment means, either in the order (a) then (b) or (b) then (a), and (c) by applying pressure or vacuum, drawing said liquid component into at least a portion of the support, and retaining substantially all of said quantity within the support. The method may, but need not, provide plug flow into the support.
Desirably, no more than 1 wt % of the liquid, more preferably no more than 0.5%, especially none of the liquid, is drawn through the substrate and expelled.
The liquid component may be any liquid used to coat a monolithic support, whether or not it contains catalytic component In general, this will be a slurry of particles, eg absorber or a washcoat slurry, or a solution of catalytically active components although for certain advanced design catalysts, a washcoat slurry may contain catalytically active particles or solutions. The actual composition of the liquid component is not important for the present invention.
Part of the present invention is a liquid component containment means which is located on top of a support to be coated. Whilst the design and function of this will be better appreciated from the specific description hereinafter, it should be understood to comprise at least a wall generally corresponding to the cross-sectional shape of the support. It may also comprise a p
Aderhold Dirk
Haynes Alan George
Spencer Michael Leonard William
Winterborn Duncan John William
Hendrickson Stuart L.
Johnson Matthey Public Limited Company
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