WO1997030193A1

WO1997030193A1 - Catalyst on metal substrate by simultaneous electrophoretic deposition of catalyst and catalyst adhesion material

Info

Publication number: WO1997030193A1
Application number: PCT/US1997/000578
Authority: WO
Inventors: Semyon D. Friedman; Rasto Brezny; Poovatholil F. Francis
Original assignee: Engelhard Corporation
Priority date: 1996-02-13
Filing date: 1997-01-13
Publication date: 1997-08-21
Also published as: AU1578897A

Abstract

Flexible metal foil catalyst members (7) suitable for use in catalytic devices for combustion engine emission control are prepared by electrophoretic deposition using an aqueous slurry (17) of catalyst support layer material and catalyst. The deposited layer (8) is of uniform thickness and stable surface area. The catalyst members (7) from the invention are especially suitable for use in automotive applications, and more especially in electrically-heated catalytic converters.

Description

CATALYST ON METAL SUBSTRATE BY SIMULTANEOUS

ELECTROPHORETIC DEPOSITION OF CATALYST

AND CATALYST ADHESION MATERIAL

Field of the Invention

This invention relates to a method and apparatus for electrophoretic deposition of catalyst onto thin metal structures . More particularly, it relates to the simultaneous electrophoretic deposition of a catalyst support material and a catalytically-active noble metal onto thin metal foils.

Background of the Invention

Catalytic converters are devices for converting noxious exhaust components into less toxic agents.

Catalytic converters are especially employed to reduce the amount of atmospheric pollution resulting from combustion or manufacture. Since the middle 1970s, catalytic converters have been required equipment for the treatment of exhaust from automobile engines. Automobile catalytic converters in particular converter hydrocarbon waste fumes into carbon dioxide, water and nitrogen, and trap and oxidize other incompletely combusted products, such as particulate carbon. These converters are also used industrially to reduce plant emissions of certain regulated gases, such as carbon monoxide.

Catalytic converters conventionally comprise a substrate structure, a catalyst support layer bonded thereto, and one or more catalytically-active agents impregnated upon the catalyst support layer. These converters are found in many shapes and sizes, their design depending on the end use of the converter, the cost structure of the particular converter market, and the availability of materials and fabrication techniques. Substrate structures house the catalyst and provide conduits for the exhaust fumes which are to be chemically scrubbed. They are generally shaped in a fashion so as to maximize the available surface area for catalytic reaction while maintaining adequate mechanical properties. Structures are not infrequently formed with corrugated substrates, fashioned in the form of repeated "honeycomb" cells, or composed of a plurality of reticulated shapes with straight-through openings. Conventionally, substrate structures have been fabricated from ceramic material which provides the advantage of high heat stability and non-reactivity with respect to the catalyst support layer and the catalyst. Ceramic suffers, however, from friability when fabricate din the form of thin walls. Further as disclosed in U.S. Patent No. 3,719,739 to Thompson (issued March 6, 1973) , ceramic takes a relatively long time to warm up to catalyst operating temperatures and catalyst bound to such supports often give high pressure drops . The disadvantages of ceramic-based substrates led researchers in the 1970s to reinvestigate metal-based substrates such as those disclosed by Suter et al, in U.S. Patent No. 2,658,742, issued November 10, 1953. Substrate structures made of thin metals have the advantage that they can be made in large cross-sections, as would be needed for the treatment of large gas flows. Since a metal substrate can be made thinner than a ceramic substrate, a metal substrate has the further advantage of providing a higher open frontal area, and a lower pressure drop throughout the structure. Lastly, manufacture of substrate structures from a metal which can be rolled down to a foil provides comparatively easier fabrication of the structure into any shape desired. It was not until recently that thin-metal substrates received significant commercial acceptability for employment in catalytic converters. Early metal substrates frequently were rejected by the marketplace owing to employment of metals which reacted chemically with the catalyst utilized in the catalytic converter (causing the catalyst to lose its activity) , an inability of the metallic structure to withstand high temperatures (an automobile catalytic converter often is subject to temperatures which approach about 2000°F) , an inability to tolerate frequent heating and cooling, an incapacity to handle the vibrations to which many catalytic converters are subject (in particular, automotive catalytic converters) , or non-resistance to oxidation, nitridation or carburization. The development of metal compositions such as ferritic stainless steel (as described in U.S. Patent Nos. 4,414,023 and 4,601,909) and Haynes 214 (as described in U.S. Patent No. 4,671,931) , which resist temperature-induced and vibrational fracturing, which are relatively non-corroding, and which are substantially non-reactive with many commonly-used catalysts, aided significantly in the acceptance of such support systems.

The wider acceptance of metallic substrate structures is also due in part to the widespread-use of heating elements in catalytic converters. Heating is used to improve catalytic performance prior to engine warm-up as many converters fail to efficiently catalyze the conversion of noxious agents to less toxic materials when the exhaust entering the converter is relatively cold (e.g., high conversion rates of carbon monoxide to carbon dioxide require a minimum temperature of approximately 390°F, while the conversion of methane or natural gas to carbon dioxide and water requires a minimum temperature somewhere about 1000°F) . In general, heating elements work more efficiently when coupled to thin metal foil substrates as opposed to ceramic substrates.

Converter substrates are generally coated with a catalyst support layer. The catalyst support layer is used as a support for the catalyst, and therefore, must have the property of both bonding to the substrate and the catalyst itself. Typically, such catalyst support layer is formed by repeatedly dipping the substrate into a slurry which contains particles of the catalyst support material and drying and/or calcining the resulting coated product. Catalyst support layers manufactured by such a "dipping" technique are customarily referred to as "washcoat" . Catalytic converter support layers are generally comprised of such substances as activated alumina (Al₂0₃) (U.S. Patent No. 4,601,999) , silicas, and mixed oxide powders of silica, vanadia and titania (U.S. Patent No. 5,272,125) . The catalyst support layer should in general have a high amount of available surface area while being coherent to the substrate. Cohesion of the catalyst support material is especially important where the catalyst member is to be exposed to gas flows, thermal shock, etc. over the service life of the engine. Porosity characteristics of the layer are of importance in terms of the overall efficiency of the converter. In electrically-heated designs, the catalyst support layer may also act as an insulator to protect against short circuiting between the various metal foil members in the converter. In thin metal converters, the catalyst support layer is preferably placed upon the substrate after the substrate in an oxidizing atmosphere results in an adherent self-healing oxide diffusion barrier which prevents further oxidation and thus protects the metal core. The barrier also prevents base metal in the core from diffusing into the catalyst support layer.

Several catalysts may be applied to the catalyst support layer, the catalyst (s) being employed in any given converter being dependent upon the chemical conversion desired and the temperature range over which the converter will operate. For example, noble metals such as platinum and palladium are often used in the treatment of auto exhaust to promote oxidation of unburned or partially oxidized hydrocarbons and to promote the reduction of nitrogen oxides because of their durability at high temperatures. Metal oxide catalysts, such as those formed with metals of Groups V and VI of the Periodic Table, are frequently employed for vapor phase catalytic oxidation of organic compounds.

Catalysts are conventionally adhered to the catalyst support layer by means of liquid-carrier impregnation. Catalytic converters may employ catalyst members either as monoliths or as a plurality of separate members. The manufacture of metal foil catalytic converters involves several special considerations, particularly if the metallic structure is corrugated.

Adhesion of the catalyst support layer to the substrate is especially problematic in metal foil catalytic converters since metal foil is generally much more flexible than the catalyst support layer. "Dipping" of the metallic substrate into the catalyst support material frequently leads to a catalyst support layer with poor adhesion characteristics resulting in delamination of the layer from the foil during handling, assembly, or use. For mass market applications, such as the automotive industry, even infrequent delaminations can become an intolerable problem in terms of quality control and performance reliability. Corrugated metal foils further suffer form a tendency of the corrugations to resist uniform coating with the catalyst support material. ashcoat applied to corrugated converter structures by "dipping" often preferentially collects in the negative radius of curvature areas of the structure (valleys or crevices) . This preferential collection result sin a catalyst support layer of non-uniform thickness. Non-uniformity of thickness can result in non-optimal use of the available surface area in the catalyst member, waste of precious metals in areas of excessive coating thickness, restriction of gas flow past the catalyst member in the converter, etc. Non-uniform coatings are also prone to delamination from the metal substrate by cracking or chipping.

Various efforts have been made to improve the results obtained by "dipping" . Often, expensive auxiliary ingredients are added to the slurry containing the catalyst support material to improve its ability to coat the metallic substrate. In some instances, the substrate itself is pre-treated to make it more amenable to coating. These efforts have not been successful in overcoming the problems of inadequate adherence of the catalyst support material to the metallic substrate and of non-uniform thickness of the catalyst support layer.

In the search for better metallic-substrate based catalytic converters, some attempts have been made to use electrophoretic deposition to deposit catalyst support materials onto the metal substrate. To date, however, it is not believed that electrophoretic deposition has been used to produce a commercially-viable catalyst support layer having acceptable characteristics (e.g., porosity, cohesion, adhesion, etc.) on a flexible metal substrate suitable for use in automotive exhaust applications. In co-pending U.S. Patent Application No. 08/276,126, a system employing electrophoretic deposition which provides for an acceptable metallic-substrate based catalytic converter is described. Such system encompasses contacting a metal foil with an aqueous slurry of catalyst support material, placing an electrode in contact with said slurry, applying an electric field between the metal foil and the electrode whereby the foil becomes a cathode and the electrode an anode, maintaining the electric field for a time sufficient to cause deposition of at least some of the catalyst support material onto the metal foil, removing the coated foil from the electric field, drying the coating foil, impregnating the dried coated foil with a catalytic species, and calcining the impregnated foil.

We have now discovered that the intermediate step of drying the catalyst support layer, and catalyst impregnation step, which are described in said co-pending application, can be dispensed within a system employing electrophoretic deposition if the catalyst species is added directly to the catalyst support material slurry. This discovery provides a means for significantly improving the production of rate of catalytic foil by electrophoretic deposition, by way of avoiding two time- consuming steps in the manufacturing process. The discovery further significantly reduces the cost involve din the production of such foils by reducing the utilization of expensive heat lamps, negating the need for the preparation of separate catalyst-containing slurrys for impregnation into the dried catalyst support layer, and in reducing waste product which may result from inadequate impregnation of catalyst into an otherwise suitably-manufactured substrate coated with a catalyst support layer. This discovery also provides a eans for more uniform dispersion of the catalyst throughout the catalyst support layer. Post-impregnation of catalyst into the adhesion layer often result sin a concentration gradient from the surface of the bulk whereas the presence of catalyst in the catalyst support material slurry results in a homogenous distribution of catalyst within the catalyst support layer.

Summary of the Invention The invention provides for the more homogenous, efficient, economical and streamlined manufacture of catalytically-active, metallic members for use in catalytic converters. Such members have a suitably porous and adherent catalyst support coating of substantially uniform thickness, said support layer having at least one catalytic agent uniformly dispersed throughout, and a flexible metal substrate. The invention further provides a new process using electrophoretic deposition, preferably aqueous electrophoretic deposition, to form a catalyst- impregnated coating on metal substrate surfaces.

The invention encompasses a method of forming catalytically-active, metal substrate members, such members comprising (1) a metal foil substrate having first and second primary surfaces and an edge surface, and (2) a porous catalyst support layer coated on at least the first primary surface and a catalyst impregnated in the catalyst support layer, the method comprising: (a) preparing a slurry of particles of a catalyst and catalyst support material, the slurry pH being such that particles have a surface charge, preferably the particles having a positive surface charge; (b) contacting a metal foil having a first and second primary surface and edge surface with the slurry whereby at least a portion of the first primary surface is in contact with the slurry contacting the metal foil;

(c) placing an electrode in contact with the slurry in an electrophoretic deposition cell;

(d) contacting the foil within an electrical contact, such contact being coupled to a power source;

(e) applying an electric field between the foil and the electrode whereby an electrical potential is crated on the foil which electrical potential is opposite to the electrical potential on the particles in the slurry;

(f) maintaining the electric field for a time sufficient to cause deposition of at least some of the particles of the catalyst and catalyst support material on the first primary surface thereby forming a foil coated with catalyst support material impregnated with catalyst;

(g) removing the coated foil from the electric field and from contact with the slurry;

(h) drying the coated foil; and (i) optionally calcining the coated foil.

The method of the invention may be conducted as batch or continuous operations . These and other aspects of the invention will be described in further detail below.

Brief Description of the Drawings

Figure 1 shows a conceptual view of a corrugated metal foil substrate with the foil thickness exaggerated for ease of illustration. Figure 2 shows a cross-sectional view of corrugated metal foil of Figure 1 coated according to the invention.

Figure 3 illustrates a cross-sectional view of a system employing the method of the present invention. Figure 4 is a perspective view of an embodiment of an electrophoretic deposition cell.

Figure 5 is a block diagram of the method of the present invention.

Detailed Description of the Invention

The catalytically-active metal substrate member, of the invention contains a suitably porous and adherent catalyst support layer of substantially uniform thickness, with one or more catalytic agents dispersed throughout, such layer being deposited on a metal foil substrate. The catalyst support layer/catalyst layer is situated on the metal foil substrate either directly or with an intervening thin oxide coating.

Referring to Figure 1 , metal foil substrate 1 has primary surfaces 2 and edge surface 3. The foil may also have corrugations 4 on some or all regions. Corrugations are typically employed as a means of increasing the available surface area for catalytic activity, to alter the gas flow through the converter, etc. In many instances, it may also be desirable to have non- corrugated regions 5 for purposes such as brazing, electrical contact or mechanical attachment. If desired, the foil may also contain perforations 6 which may be useful in handling the foil during processing as well as for alignment and assembly in the converter device. The invention is not limited to any specific foil design or shape. If desired, the foil may be treated as a continuous length of foil which is subsequently cut into individual foil substrates (or catalyst members) at a desired stage in the overall process of manufacture.

The substrate's dimensions will generally be dictated by converter design considerations, commercial availability of metal stock, etc. Typical foils are described in U.S. Patent Nos. 5,272,876; 4,838,067; 4,601,999; and 4,414,023. Foil thickness 7 for most catalytic converter designs ranges from 0.02-0.25 mm. Thinner foils are generally preferred since they can provide increased available surface area per unit volume. The primary surface dimensions of the foil are also largely dependent on design considerations, handling considerations, etc.

While the invention is not limited to specific metal foil compositions, ferritic or nickel alloys are generally preferred. Preferably, the metal foil composition contains at least a minor amount of a metal which, when oxidized will act to facilitate adherence of the catalyst support layer material to be deposited. Thus, for alumina-containing catalyst support layer materials, metal foil compositions which contain aluminum are preferred. Typical foil compositions often contain combinations of aluminum, chromium, nickel and/or iron with minor amounts of other elements. Now referring to Figure 2 , there is shown a cross- sectional view of corrugated metal foil of Figure 1 coated according to the invention. Catalyst-impregnated catalyst support layer 8 comprises catalyst dispersed throughout catalyst support material. Catalyst- impregnated catalyst support layer 8 may be in direct contact with metal foil primary surface 2. Preferably, however, layer 8 is in direct contact with thin oxide film 9 which directly contacts metal foil primary surface 2. Layer 8 may cover the entire surface of metal foil 1 or may cover only selected regions of the foil. If desired, layer 8 may cover one primary surface or both primary surfaces. Preferably, corrugated regions 4 are entirely covered with layer 8. Layer 8 may be present as a continuous layer or may be interrupted such that exposed regions of the metal foil are surrounded by continuous regions which are covered by layer 8.

Catalyst-impregnated catalyst support layer 8 preferably has substantially uniform thickness 10 in all regions where it is present on metal foil substrate 1. In some instances, it may be possible to have layer 8 exist in different discrete thicknesses such that step¬ like differences 11 in thickness would exist. By us of electrophoretic deposition, as discussed below, it is possible to limit the variation in thickness such that there is less than about 10%, (more preferably about 5% or less, and most preferably about 2%) difference in thickness (measured as the greatest support thickness divided by the smallest support thickness) across the entire layer 8 (except for possible step-wise differences) .

Catalyst support layer material preferably comprises one or more metal oxides . The metal oxide is preferably selected from the group consisting of alumina, ceria, baria, titania, zirconia, lanthanum oxide, other rare earth oxides, and mixtures thereof. The catalyst support layer may be designed to contain variations of composition and/or porosity through the thickness of the layer. Preferably, the loading of support on the substrate is about 5-80 mg per square inch of coated surface, more preferably about 15-50 mg/in², and most preferably about 15-30 mg/in².

Catalyst-impregnated catalyst support layer 8 preferably has (i) a surface area of about 100-300 m²/g based on the weight of the ceramic oxide contained therein, more preferably about 150-250 m²/g, and most preferably about 200-250 m²/g, (ii) micropore volume of about 0.5-1.0 cc/g, more preferably about 0.70-1.0 cc/g, and most preferably about 0.8-1.0 cc/g and (iii) a thickness of about 10-60μm, more preferably about 20- 40μm, and most preferably about 20-30μm. The coherence of the deposited particles is preferably such that the coating is not chalky. The adhesive strength of the catalyst support layer is preferably such that the substrate can be flexed during normal processing and use without delamination of the support layer.

If present, the thin oxide film may cover all or portions of the metal substrate surface. The film preferably intervenes between the metal substrate and the catalyst support layer over the entire area of the catalyst support layer. There may be instances in which portions of the substrate are covered by the catalyst support layer, but not the thin oxide film layer. There may also be instances where the thin oxide film exists on portions of the metal substrate without any coverage by the catalyst support layer.

The thin oxide film layer preferably has a thickness of about 50-5000 A, more preferably about 100-2000 A, and most preferably about 500-2000 A. Preferably, the thickness of thin oxide film layer is substantially uniform. The thin oxide film composition preferably contains a metal oxide which is also present in the catalyst-catalyst support material to be deposited or which facilitates adherence of the catalyst support layer material to the substrate. Thus, for alumina-containing catalyst support layer materials, the thin oxide film compositions preferably contain alumina. The thin oxide film is preferably formed by oxidation of the underlying metal foil. In such cases, the film will contain oxides by oxidation of the underlying metal foil. In such cases, the film will contain oxides of the metals in the foil. The ratio of molar metal oxides in the foil may vary from the molar ratio of the corresponding metals in the oxide film due to differences in diffusion rates of the various metals .

Catalytically active species are dispersed uniformly throughout and on the catalyst support layer 8 which is typically porous. The catalytically active species may be any known species or combination thereof. Typically, precious metals such as platinum, palladium and/or rhodium are used. The loading of catalyst is preferably at levels conventionally used in the art (e.g., 20-200 g/ft³) . In some instances, due to the uniformity of the catalyst support layer thickness, it may be possible to use less catalyst for the same effective activity level as would be achieved using other coating techniques. Referring now to Figure 3 , there is shown a schematic of the present invention whereby a catalyst support layer with catalyst dispersed throughout 10 is deposited in one-step by electrophoretic deposition in electrophoretic cell 11 onto thin metal substrate 1. Thin metal substrate 1 is rolled from de-reeling station 16 through mixture 17 of catalyst support material and catalyst and into electrophoretic cell 11 also containing mixture 17. Excess slurry on the electrically disposed thin metal substrate is removed by means of air knives 12. Coated thin metal substrate 18 is then dried by dryer 13 and subsequently passed through calciner 14.

After passing through calciner 14, the coated, dried and calcined thin metal substrate is wound on take-up station 15. Referring now to Figure 4 , there is shown a perspective view of an embodiment of electrophoretic deposition cell 11. Electrophoretic deposition cell 11 comprises housing 19, which may be fabricated from polypropylene, electrode 25 which act as anodes to thin metal substrate 1 (not shown) when thin metal substrate 1 is electrically coupled to become a cathode, main slurry chamber 22 for retaining the catalyst-catalyst support material slurry, overflow chamber 23 for housing slurry overflow from main slurry chamber 22 and shunting the overflow through slurry outlet 26, thin metal substrate inlet port 20 (partial view) , thin metal substrate exit port 21, and rubber seal 24 for aiding in removing excess slurry from the electrophoretically disposed thin metal substrate.

Referring now to Figure 5, there is shown a block diagram of the method of the present invention. As shown, the method of the invention preferably includes at least the following steps: (a) preparing an aqueous slurry comprising particles of catalyst support material and a catalytically active metal, the slurry pH being such that the particles have a positive surface charge, (b) contacting at least a portion of a first primary surface of a metal foil with the slurry, (c) placing an electrode in contact with the slurry, (d) contacting the foil with an electrical contact, such contact being coupled to the negative terminal of a power source, (e) applying an electric field between the foil and the electrode whereby the foil becomes a cathode and the electrode becomes an anode,

(f) maintaining the electric field for a time sufficient to cause deposition of at least some of the catalyst support material particles on the first primary surface of the metal foil to form a foil coated with the catalyst support material impregnated with catalyst, (g) removing the coated foil from the electric field and from contact with the slurry, (h) drying the coated foil, and (i) optionally, calcining the foil. Metal foils typically come from the supplier in wound rolls. Depending on the method of foil manufacture, the foil may have residual stresses associated with the working of the metal. Metal foil stock also may contain an extremely thin surface oxide coating (i.e., <30 A) . If desired, the foil may be treated with an initial annealing step such as that disclosed in U.S. Patent 4,711,009 in order to lessen the amount of residual stress.

If the foil is to be corrugated, the corrugating would typically follow the annealing (if done) . Corrugation may be performed by any suitable method known in the art to form whatever corrugation pattern is desired. The foil is then preferably treated to remove any lubricant (associated with the corrugation process) and is annealed to remove stresses from the corrugation step. The lubricant may be removed by washing with an appropriate solvent or detergent. More preferably, however the lubricant is burned off in the initial stages of the subsequent annealing step. While the process of the invention can be practiced with an untreated foil, preferably the foil is pre¬ treated at some point before the electrophoretic deposition step to enhance the adherence of the subsequently deposited catalyst-impregnated catalyst support material layer. While abrasion of the surface has been disclosed in the prior art as enhancing adhesion, a preferred pretreatment is to grow a thin oxide film on the foil surface by firing the foil in a mildly oxidizing atmosphere for a brief period of time.

The oxidizing treatment is preferably carried out at about 800-950°C (more preferably about 875-955°C and most preferably about 900-925°C) in an atmosphere having an oxygen partial pressure of about 0.1-0.3 at (preferably about 0.2 atm) for about 0.5-3 minutes (preferably about 1-2 min.) . If desired, the milder or more severe oxidizing environments can be used with appropriate changes in firing time. The oxide film thickness is preferably about 50-5000 A, more preferably about 100- 2000 A, and most preferably 500-2000 A. The oxide film grows by oxidation of metals in the metal foil as they diffuse to the surface. Depending on the oxidizing conditions, the actual oxide composition of the film may differ from the metal foil bulk significantly in terms of the proportions of the various metals in the oxides as compared with the actual foil composition. Thus, for example, where aluminum is present in the foil in a minor proportion, aluminum oxide may nevertheless form the bulk of the oxide film since aluminum has a comparatively high diffusion rate. While the mechanism by which the film improves adhesion has not been fully understood, it appears that the adhesion improvement is greatest when the oxide film contains, at its outermost surface, a predominant amount of an oxide which is also used in the catalyst-impregnated catalyst support layer. Preferably, the thin oxide film is of substantially uniform thickness over the entire surface of the metal substrate.

The deposition conditions are preferably selected so as to deposit the desired amount of catalyst support material and catalyst in a very short period of time. The total deposition time is preferably 15 seconds or less, more preferably 5 seconds or less. The use of extremely short deposition times has been found to enhance the adhesion strength and coherence of the deposit while minimizing disruptive effects associated with electrolysis of water at the foil surface. Surprisingly, it has been found that the use of high deposition current density actually acts to minimize the adverse effects of electrolysis while producing a deposit having good porosity, adhesion and cohesion properties.

The deposition is preferably carried out at constant current density where a batch deposition process is employed, or at constant voltage where a continuous length of foil is passed through a deposition bath. It is possible to vary both voltage and current density if desired. In general, it is preferred to use either constant current or constant voltage so the amount of deposition can be controlled by control of the deposition time. Since the effective resistivity of the deposition electrode increases with the amount of deposit, a constant current density mode would require increasing voltage over the time of deposition. Correspondingly, if the voltage is held constant, the current density would decrease with the time of deposition.

The current density preferably ranges from about 0.12-5 amp/in², more preferably 0.3-3 amp/in², and most preferably 0.3-1.0 amp/in². The applied voltage necessary to achieve this current density will depend on the resistivity of the deposition bath, the resistivity of the deposition electrode, the mobility and charge of the particles, etc. Typically, the applied voltage is about 0.1-70 volts depending on these various factors. The deposition rate is preferably such that about 5-50 mg/in² (more preferably about 15-40 mg/in², and most preferably 15-30 mg/in²) is deposited. Preferably, the total deposition is accomplished in about 5 seconds or less deposition time. The deposition may be accomplished in a single run or may be accomplished in several shorter runs .

Preferably, the particles in the slurry (other than those actually being deposited) are maintained in a dispersed state throughout the deposition by agitation of some sort. The slurry composition used in the invention preferably uses water as the dispersion medium. However, it is within the scope of the invention to use an organic solvent system, e.g., an organic alcohol, although the organic solvent system might require a higher voltage for electrical deposition than the aqueous system.

Aqueous slurry compositions used in the invention preferably contain the catalyst support layer particles, the catalyst itself, deionized water, and a pH adjusting agent. The slurry may also include aluminum hydroxide colloidal particles which are believed to improve the coherence of catalyst support layer particles. The pH is adjusted so as to ensure the desired polarity of surface charge on the catalyst support layer particles and catalyst. The pH is advantageously adjusted by addition of a mineral acid such as nitric acid. For deposition of materials such as alumina on the cathode, the pH is preferably about 2-5, more preferably about 2.5-3.5. Solids content of the slurry is also preferably kept at about 15-50 wt% . After the desired deposit has been achieved, the coated foil is removed from the bath. Excess slurry is removed from the foil to avoid segregation of residual slurry which clings to the foil. The removal of excess slurry may be performed by using air knives, a rinsing bath or other known means. The foil is then dried. Preferably, the foil is also calcined. The drying and calcining conditions used may be any conventional conditions such as those disclosed in U.S. Patent 4,711,009.

If any masking has been applied to the foil, the masking is preferably removed by oxidation during the calcination.

The resulting catalyst-coated foil can then be assembled into a desired converter design.

The invention, however, is further illustrated by the following example. It should be understood that the invention is not limited to the specific details of the example.

Example 1 A catalyst slurry containing palladium as a catalytically-active agent and comprised also of mixed oxides (alumina, ceria) was adjusted by addition of acetic acid solution to a pH of about 2.9 and solids content of 27.5%. The slurry was then placed in an agitated deposition bath.

A continuous aluminum-chromium-iron alloy foil strip (Alpha IV sold by Allegheny Ludlum Corp.) was treated at 900°C and 0.2 atm oxygen for one minute to form thin oxide film on both sides of the foil. The foil was then passed through the deposition bath and an electrical field was applied such that the foil was made to be a cathode with the other electrodes (already in contact with the deposition bath) as anodes. The effective length of the foil cathode in the deposition batch was about one foot. The foil was passed though the bath at a line speed of 10 ft/min. The deposition was performed at a constant voltage of about 25 volts at a cathode-anode separation of 2 inches. The average current density over the foil was about 0.5 amp/in². The foil with the resulting deposit was rinsed in deionized water and excess liquid was removed using air knives. The resulting catalyst support layer impregnated with palladium catalyst was dried and calcined at 950°C for 30 seconds.

The resulting catalytically-active metal substrate strip was analyzed by x-ray fluorescence, the elements in the specimen being identified by the wavelength of spectral lines emitted, and the concentrations of such elements being determined by the intensities of the lines. Cross-sectional microprobe mapping was subsequently undertaken to characterize the distribution of the components in the deposited layer, and porosity was determined by mercury porosimetry. Such measurements, in conjunction with catalyst activity testing, evidenced a catalyst-coated foil strip with a substantially uniform catalyst coating, and having porosity and catalytic activity comparable to that found in commercially available palladium-coated foil catalytic converter strips .

Claims

WHAT IS CLAIMED IS:

1. A method of forming a catalytically-active metal substrate member comprising: (a) preparing slurry comprising particles of one or more catalysts and a catalyst support material,

(b) contacting a metal foil having a first and second primary surface and an edge surface with said slurry whereby at least a portion of said first primary surface is in contact with said slurry,

(c) placing an electrode in contact with said slurry,

(d) contacting the foil with an electrical contact, such electrical contact being coupled to a power source, (e) applying an electric field between said foil and said electrode whereby an electrical potential is created on the foil which potential is opposite to the electrical potential on the particles in the slurry, (f) maintaining said electric field for a time sufficient to cause deposition of at least some of said catalyst support material particles and said one or more catalysts on at least said first primary surface thereby forming a foil coated with catalyst support material impregnated with catalyst, (g) removing said coated foil from said electric field and from contact with said slurry, (h) drying said coated foil, and (i) optionally, calcining the coated foil.

2. The method of Claim 1 wherein the slurry is an aqueous slurry.

3. The method of Claim 1 wherein the catalyst support material is a metal oxide.

4. The method of Claim 1 wherein the one or more catalysts are catalytically active materials.

5. The method of Claim 1 wherein said metal foil is pre-treated to make at least a portion of said primary surface more capable of adhering said deposited particles .

6. The method of Claim 5 wherein said pretreatment comprises contacting said foil with an oxidizing atmosphere to form metal oxide film on said foil surface.

7. The method of Claim 5 wherein said pretreatment comprises heating said foil to about 800-950°C for about

0.5-3.0 minutes in an atmosphere having an oxygen partial pressure of 0.1-0.3 atm.

8. The method of Claim 1 wherein said contacting step (a) comprises immersing said foil in said slurry.

9. The method of Claim 1 wherein, prior to said contacting step (a) , at least a portion of said first primary surface is coated with a masking layer adapted to prevent adhesion of electrophoretically deposited particles on said masked portion.

10. The method of Claim 9 wherein said masking is removed after step (d) to yield an uncoated surface portion on said foil.

11. The method of Claim 1 wherein said slurry is agitated during step (d) .

12. The method of Claim 1 wherein said contacting step (a) comprises passing said foil through said slurry.

13. The method of Claim 12 wherein said slurry is j flowed countercurrent to the direction of foil passage.

14. The method of Claim 12 wherein said foil is in the form of a continuous ribbon which is cut into individual members after said deposition.

15. The method of Claim 1 wherein said slurry has a pH such that said particles have a positive charge.

16. The method of Claim 1 wherein said coated foil is dried at 50-150°C.

17. The method of Claim 1 wherein said calcining is performed at less than 600°C.

18. The method of Claim 1 wherein in electrical field is applied between the foil and the electrode whereby the foil becomes a cathode and the electrode becomes an anode.

19. A catalytically-active metal substrate member comprising a metal foil substrate having first and second primary surfaces and an edge surface and having at least on porous catalyst support layer on at least one surface of the metal film substrate wherein said porous catalyst support layer comprises metal oxide particles having one or more catalytically-active metal uniformly dispersed throughout said metal oxide particles.

20. The catalytically-active metal substrate member of Claim 19 wherein said catalyst support layer is of substantially uniform thickness and has a (i) surface area of about 100-300 m²/g on the weight of said metal oxide, (ii) pore volume of about 0.5-1.0 cc/g and (iii) a thickness of about 15-60μm.

21. The catalytically-active metal substrate member of Claim 19 or 20 wherein said metal oxide is selected from the group consisting of alumina, ceria, baria, zirconia, titania, lanthanum oxide and mixtures thereof.

22. The product produced by the method of Claim 1.