METHOD OF MAKING PRECIOUS METAL OXIDATION CATALYSTS

Abstract: A catalyst construction and method of making is disclosed for treating automotive exhaust gases from a fossil fueled internal combustion engine. The construction comprises: (a) a substrate; (b) a coating on said substrate having comingled first and second phases, said first phase comprising ceria and precious metal in microscale intimacy and said second phase comprising precious metal substantially devoid of ceria and separated from the ceria in said first phase on a macroscale. The phases may be in particle form with (i) the first phase particles resulting from the codeposition of CeO₂, precious metal, and Al₂O₃ from an aqueous solution containing dissolved salts of CeO₂ and precious metal and suspended particles of Al₂O₃; and (ii) the second phase particles resulting from the codeposition of precious metal and Al₂O₃ from an aqueous solution containing a dissolved salt of the precious metal and suspended particles of Al₂O₃. Alternatively, the substrate has Al₂O₃ thereon to increase its surface area, and the two-phase coating is adhered to said Al₂O₃, said first phase resulting partially from the discontinuous deposition of CeO₂ onto said Al₂O₃ and partially from the continuous deposition of precious metal over said discontinuous CeO₂ and over the exposed Al₂O₃; the precious metal in immediate microscale contact with the CeO₂ constitutes the first phase and the precious metal not immediately in microscale contact with the CeO₂ constitutes the second phase.

Description: This invention relates to a process of making a precious metal catalyst system for treating automotive exhaust gases. The use of transition-type aluminas as a suitable support for automotive exhaust gas catalysts encounter at least one very significant problem. That is, as the temperature increases during use of such alumina supports, they undergo a phase change to alpha alumina. This instability is undesirable because it results in a severe loss of surface area and physical strength. These changes, in turn, lessen the effectiveness of the catalysts because of shrinkage, which causes lower activity and loss of catalyst due to attrition. Several ingredients have heretofore been commercially employed as alumina stabilizers, namely, barium oxide, lanthanum oxide, and cerium oxide. These oxides are usually promoted by impregnating the catalyst substrate with a solution in which is dissolved a salt of such metals for such oxides. Calcination of such impregnated substrate results in a washcoat of the desired metal oxide. Of such stabilizers, ceria also has the ability to absorb and store oxygen on the catalyst when an exhaust gas temporarily becomes oxygen-excess and to release oxygen when it becomes oxygen-short. Thus, if an insufficiency of oxygen, required for oxidation of CO or HC, should occur for a moment in a reaction gas, when ceria and a precious metal are intimately deposited on alumina, the reaction of the gas can be carried out by oxygen release from the cerium. This oxygen transfer characteristic of ceria further enhances the catalytic effectiveness of precious metals, particularly at and adjacent stoichiometric conditions. To utilize these dual advantages of ceria, there has been developed two distinct manners of use of ceria for oxidation catalysts: (i) use of finely divided CeO₂ as a codeposit with a small quantity of precious metal resulting in a microscale intimacy between the elements in the resulting coating, represented by U.S. patents 3,850,847; 4,448,895; 3,993,572; and 4,367,162; and (ii) a first deposition of CeO₂ onto the substrate followed by a separate deposition of the usually small quantity of precious metal, but still resulting in a microscale intimacy between CeO₂ and the precious metal, such as represented in U.S. patents 4,448,895; 4,591,580; 4,407,735; 4,476,246; 4,426,319; 4,153,579; 3,903,020; 4,157,316; 4,331,565; 4,189,404; 4,283,308; and 4,294,726. As part of the research leading to this invention, it has been discovered that the use of cerium oxide with precious metals in microscale intimacy results in two problems: (a) a lack of tolerance to lead poisoning, and (b) the suppression of catalytic activity of saturated hydrocarbons. With respect to the increase in lead poisoning, this is not only a problem for engines supplied with lead-containing gasoline supplies, such as is prevalent in Europe and Australia, but also for engines adapted to run on leadfree gasoline since lead is typically present in such latter gasolines in trace amounts. To convert lead oxide to lead sulphate in exhaust gases requires oxidation of SO₂ or SO₃; the presence of SO₃ converts lead oxide to harmless lead sulphate. But the presence of SO₃ is dependent upon the catalytic oxidation of SO₂ by the precious metal. Unfortunately, the microscale intimacy of ceria with the precious metal suppresses such catalytic activity. With respect to the suppression of catalytic activity for saturated hydrocarbons, this phenomena appears to be the result of the inability of ionic PdO to oxidise saturated hydrocarbons, this phenomena appears to be the result of the inability of ionic PdO to oxidise saturated hydrocarbons and the inhibition of the reduction of PdO to Pd by the presence of CeO₂. EP-A-0,251,708 discloses a method of making a catalyst composition comprising a platinum group metal catalytic component and an activated alumina. The method comprises applying a coating of alumina to a substrate, calcining the alumina-coated substrate, dispersing platinum group metal component in alumina, dispersing a stabiliser-precurser onto the coating on the substrate and thereafter calcining the coating. EP-A-0,262,962 discloses a catalyst including a catalytically inert refractory support, an inside layer containing cerium oxide and platinum and an outside layer containing zirconium compound and rhodium. EP-A-0,171,151 discloses a catalyst including rhodium, rare earth oxide and platinum and/or palladium. EP-A-0,152,052 discloses a method of producing a catalyst which comprises coating a monolithic support with an aqueous slurry composition comprising alumina, cerium hydroxide and at least one element selected from a group consisting of platinum, palladium and rhodium. EP-A-0,203,525 discloses a catalyst having deposited on a monolithic honeycomb carrier, a composition including alumina modified cerium compound, a precious metal such as platinum, palladium or rhodium and active alumina. Accordingly, a primary object of this invention is to provide a catalyst construction for automotive emissions, derived from fossil fuels, that will retain the advantages of ceria use and eliminate problems associated with ceria use. According to the present invention there is provided a method of making a catalyst system, for treating exhaust gases, said catalyst comprising an alumina-containing coated substrate, CeO₂ in an amount of from 2 to 20% by weight and precious metal in an amount of from 0.05 to 0.5% by weight of the final coated substrate, the method comprising, depositing onto said substrate a two-phase coating in which said phases are in particle form with (i) the first phase resulting from the deposition of CeO₂ and precious metal onto Al₂O₃ particles from an aqueous solution containing dissolved salts of CeO₂ and said precious metal and suspended particles of Al₂O₃, and (ii) the second phase resulting from the deposition of precious metal onto Al₂O₃ particles from an aqueous solution containing a dissolved salt of said precious metal and suspended particles of Al₂O₃, and devoid of CeO₂, the second phase being deposited onto the first phase. Advantageously, the precious metal is selected from the platinum group, and particularly from the group consisting of platinum, palladium, rhodium, ruthenium, iridium, osmium, and mixtures thereof. Preferably the ratio of Al₂O₃/CeO₂ varies between 1:1 to 20:1. Preferably, the weight ratio of the first phase to the second phase is in the range of 4:1 to 1:4. Advantageously, the deposition of the first phase particles for the method is carried out by (i) dissolving in deionized water a salt of cerium and a salt of the precious metal and in which is suspended Al₂O₃, (ii) drying and calcining the solution to produce a collection of Al₂O₃ particles impregnated with CeO₂ and precious metal, (iii) forming an aqueous slurry of the impregnated collection of first phase particles, and (iv) dipping the substrate in the slurry. Preferably, the wet-coated substrate is dried by heating to 75-100°C, preferably 75°C for 3-5 hours. The second phase is prepared in a similar mode except that a salt of cerium is absent in solution for impregnating the Al₂O₃. A second slurry is made using the dried and calcined precious metal impregnated Al₂O₃ particles. The previously coated substrate is dipped in this second slurry, dried at 75-100°C, preferably 75°C for 3-5 hours. The coated assembly may then be calcined by heating to 800°C for 3-5 hours, preferably four hours. A method of using precious metal catalysts in the treatment of automotive exhaust gases, comprises exposing such exhaust gases to a comingled two-phase catalyst construction embodying the invention. The exhaust gases contain unsaturated hydrocarbons and lead oxide; the temperature of the exhaust gases, under treatment, is in the range of 250-1000°C and at 15,000-100,000 Hr⁻¹ space velocity. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which : Figure 1 is a graphical illustration of the effect of temperature on conversion efficiency of propane (a saturated hydrocarbon). To disclose how a dual-phase catalyst construction can be provided having microscale intimacy between ceria and the precious metal in one phase and microscale separation between the precious metal and ceria in the other phase, the following best mode for the construction and its method of making is presented. Exhaust Gases to be Treated Exhaust gases to be treated by the catalyst construction hereunder are emitted by internal combustion engines fueled by fossil fuel (gasoline or diesel fuel). In such exhaust gases, there will be found saturated hydrocarbons, such as propane, and a presence of lead varying in degree with the country of method of producing the fuel for the engine. The gases from such engines are delivered to the catalyst construction and pass therethrough at a space velocity advantageously in the range of 15,000-100,000 Hr⁻¹ and at a temperature in the range of 250-1000°C. Substrate A suitable catalyst substrate having a high surface area to volume ratio, preferably a monolithic cordierite cellular body having at least 300 cells per square inch and a fresh BET area of at least 15 m²/g and a six mil wall thickness, is used. To achieve the high surface area to volume ratio of the substrate, it is conventional to apply a washcoat or slip of Al₂O₃ to such substrate. Such Al₂O₃ coating may also contain additional components selected from the group consisting of titania, zirconia, silica, magnesia, strontium oxide, and calcium oxide. Such coating will typically comprise between 3-25% by weight of the finished refractory supported catalyst. The Al₂O₃ washcoat promotes a high surface area, typically about 20-30 m²/g. To stabilize such Al₂O₃ and inhibit it from changing to alpha alumina at heated temperatures, stabilizing agents selected from the group consisting of CeO₂, La₂O₃, or BaO have been placed in intimate relationship with the Al₂O₃. CeO₂ is most desired because it not only acts as a stabilizer for Al₂O₃, but promotes exhaust gas reactions and acts as an oxygen storer for precious metal catalysts, and may even alleviate some sulphur poisoning. Unfortunately, it has been discovered that CeO₂ (i) promotes an intolerance to lead by acting as an inhibitor of the conversion of SO₂ to SO₃, the latter being so necessary to conversion of lead oxide to lead sufphate, and (ii) inhibits the oxidation of saturated hydrocarbons by slowing down the reduction of precious metal oxides to their elemental stage. To maintain the above advantages of CeO₂ in conjunction with Al₂O₃, and eliminate the above disadvantages, a two-phase catalyst coating is used. The substrate or two-phase coating may also contain other surface extenders, stabilizers, or promoters as long as they do not functionally inhibit the desired effects of CeO₂ and the precious metals. The refractory substrate may, however, be physically formed as discrete parts, such as pellets or extrudates, and may be selected from other materials such as mullite, zirconia, titania, spinel, silica, silica/alumina, or alumina. The monolith construction is preferred because it is more durable by avoiding the shifting of discrete parts due to thermal changes of the container which may result in the destructive grinding of such parts, such as pellets. The substrate may also be a metal requiring similar coating techniques. First Phase An aqueous slip or slurry is prepared with alumina particles impregnated with cerium oxide and precious metal. The Al₂O₃ particles in each of the phases will act as the surface extender provided in prior art constructions by an Al₂O₃ washcoat. The monolith substrate is then preferably dipped in such slip or slurry to wet the monolith and coat it with the ceria and precious metal impregnated alumina particles. The coating should be applied so that it provides a coating in the range of 18-40 g/ft³, preferably about 20 g/ft³. The alumina is preferably derived from agglomerated gamma alumina powder having a high surface area (180 m²/g BET area). Such alumina typically has an average particle size of about 20nm, a density of about 2.9 g/cm³, and a purity greater than 99.6% alumina. The cerium oxide is impregnated into the alumina preferably by forming a slurry of the alumina powder in deionized water to which is added a water solution A of Ce(NO₃)₃-6H₂O. Advantageously, such solution A is prepared by adding about 69 grams of the cerium salt to about 320 grams of distilled water and mixing thoroughly. Such solution A is then added to an alumina slurry which has been prepared by suspending a mixture of about 168 grams of the powdered alumina in about 1000 grams of distilled water; solution A is blended into such slurry and mixed thoroughly. Other water soluble salts may, of course, be used in amounts to achieve the equivalent amount of cerium hydroxide precipitate as here illustrated. The uniform, adhered coating of cerium hydroxide on the alumina granules was obtained by the addition of ammonium hydroxide in amounts sufficient to achieve complete precipitation of the cerium. The precipitation occurs in accordance with the following reaction: Ce(NO₃)₃+3NH₄OH → Ce(OH)₃+3NH₄NO₃ The resulting aqueous suspension of coated alumina granules is then filtered and the residue thoroughly washed using distilled water and dried at about 75°C for about two hours to produce a dry cake. The resulting alumina granules in such cake, coated by the hydrated oxide of cerium Ce₂O₃ x H₂O, constitutes a partially impregnated alumina particle needed for phase one. The dry cake is crushed to pass a 48 mesh screen and ball milled in water to form a slurry. About 740 grams of the crushed and ball milled impregnated alumina granules is mixed with about 1100 grams of distilled water, to which may be added a small amount of a dispersant or a binder, to form a second slurry. A solution B is prepared using a salt of a precious metal selected from the group consisting of palladium, platinum, rhodium, ruthenium, iridium, osmium, and mixtures thereof. The precious metal may be added as a salt in the form of a nitrate, chloride, tetramine, or nitrate. To about 75 grams of distilled water, about 13 grams of chloroplatinic acid (containing about 39.5% platinum) is added. This solution B is then added, preferably by drops, to the second slurry and mixed thoroughly. Such mixing was continued for at least 15 minutes and the viscosity was allowed to stabilize by letting the slurry stand for several hours. Such slurry thus becomes the medium for a first dipping of the monolith. Excess slurry is blown off and the wet coating is dried at about 75°C in ambient air for about two hours. Alternatively, the solutions A and B may be added to a single slurry containing suspended Al₂O₃, avoiding the two-step impregnation of the Al₂O₃ particles. Moreoever, deposition or impregnation may also be carried out by spraying or other equivalent means. Second Phase A slurry of suspended Al₂O₃ particles is prepared to which is added solution B. About 700 grams of Al₂O₃ and about 88 grams of solution B are utilized. The coated substrate is dipped in the second phase slurry and dried. The finished catalyst should have a composition by weight of .05-.50% precious metal, 2-20% CeO₂, and 20-40%, preferably 25-30% Al₂O₃, with the coating being 22-61% by weight of the finished catalyst. The ratio of Al₂O₃/CeO₂ should be in the range of 1:1 to 20:1. The above catalyst construction provides a two-phase precious metal catalyst construction with a first phase comprised of alumina particles impregnated with ceria and precious metal in microscale intimacy (due to the codeposition of the ceria and precious metal together from a slurry or slip preparation), and a second phase comprised of precious metal impregnated into a separate collection of alumina particles (which, when deposited over the previously coated alumina particles, forms a phase where the precious metal is not only substantially devoid of ceria but is separated from ceria in the first phase on a macroscale). Microscale intimacy is herein defined to mean atomic proximity between Ce and the precious metal. Macroscale separation is defined herein to mean that in no instance in the matrix of the second phase can the nearest atomic neighbor of an atom of the precious metal be cerium. The first phase is effective to provide oxidation of automotive exhaust gases; the precious metal in the first phase oxidizes aromatic and olefinic hydrocarbons, all of which are unsaturated, as well as carbon monoxide. As indicated earlier, such a microscale intimacy between the ceria and precious metal inhibits the oxidation reaction of SO₂ to SO₃ which is a necessary constituent to oxidize lead oxide to a harmless constituent of lead sulphate. Thus, the first phase is not effective in oxidizing lead oxide and so is intolerant to lead poisoning of the gaseous constituents. The ceria in the first phase operates very effectively to enhance the oxidation capabilities of the precious metal catalyst and acts as an oxygen storage mechanism so that during excess oxygen conditions the ceria oxidizes to higher valance, and, in a reducing condition, is able to give up such stored oxygen to continue oxidation during conditions which are slightly less than stoichiometric. The second phase has precious metal separated on a macroscale from the ceria in the first coating and thus is capable of oxidizing saturated hydrocarbons (such as C₃H₈, propane), and is also effective in promoting the oxidation of SO₂ to SO₃ to facilitate completion of the oxidation of lead oxide. Test Examples Several tests were undertaken to corroborate the intended effects of this invention. First, a model reaction analysis was undertaken wherein a first catalyst sample coating was prepared similar solely to the first phase of this invention, including precious metal (platinum), ceria, and gamma alumina. The deposited ingredients of such first phase were in an amount of 3.7 weight percent for cerium, .7% for platinum, and 15-20% for gamma alumina. The other sample coating was comprised essentially of the second phase of this invention, namely, a precious metal (platinum) and gamma alumina. In this second sample, the platinum was applied in an amount of .7 weight percent. Each of these coating samples was applied as a slip coating to a cordierite cellular ceramic monolith. The coated monolith samples were subjected to a simulated exhaust gas flow at a level of about SV=60,000 Hr⁻¹; the exhaust gas contained 1000ppm of propane C₃H₈ and 2% oxygen. As shown in Figure 1, the sample equivalent to the first phase demonstrated a low efficiency for oxidizing propane, and particularly at lower temperatures; the sample equivalent to the second phase had a high capability of conversion over a broader range of temperatures. As shown in Table I, sample 2 (equivalent to the first phase) had a relatively poor capability of oxidizing SO₂ to SO₃ at various temperatures as indicated. This was not true of sample 1 (equivalent to the second phase) containing simply platinum devoid of CeO₂, which had a much higher efficiency for converting SO₂ at the same various temperatures. This demonstrates the need for macroscale separation of PM and CeO₂ for preventing lead poisoning. SO₂ OXIDATION (%) OVER Pt AND Pt/Ce CATALYSTS Sample Loading T°C 400 450 500 550 1 Pt (0.07 wt. %) 80 88 92 82 2 Pt (0.07 wt. %) + Ce (3.7 wt. %) 52 54 56 57 In another series of samples, the high temperature oxidizing effects of a three-way catalyst performance was examined. The precious metal loadings for this series are as indicated in Table II. TABLE II HIGH TEMPERATURE OXIDIZING EFFECTS ON THREE WAY CATALYST PERFORMANCE Pretreatment: 940°C in air (4 hrs.) after pulsator aging for 13,000 simulated miles. Sample PM=Pt/Rh (5/1) TWC % Conversion (R = 1.0) %CO %C₃H₆ %C₃H₈ 450°C 450° 450° 550°C 3 [PM/CeO₂]/Al₂O₃ +PM/Al₂O₃ (20 G PM/ft³) 96 100 70 76 4 [PM/CeO₂]/[Al₂O₃] (35 G PM/ft³) 91 100 33 45 5 [PM/CeO₂]/[Al₂O₃] +PM (30 G PM/ft³) 90 100 31 42 Sample 3 contains a first phase coating of precious metal and cerium oxide in microscale intimacy, impregnated onto Al₂O₃, and precious metal impregnated onto Al₂O₃ in a second phase coating thereover . The precious metal (PM) included platinum and rhodium in a 5:1 ratio, which is characteristic for a three-way catalyst. Sample 4 contained only a first phase coating consisting of cerium oxide and precious metal (in microscale intimacy) impregnated onto Al₂O₃, and with a higher precious metal loading. In a fifth sample, precious metal and ceria (in microscale intimacy) were impregnated onto Al₂O₃ to form a first phase, and precious metal was coated onto the first phase without Al₂O₃ to promote microscale intimacy in another manner with the first phase. All of the precious metal catalyst loadings were pretreated in air by heating at 943°C for four hours after having been subjected to a pulsator aging at 13,000 simulated miles. Pulsator aging is defined herein to mean durability testing of catalyst with fuel containing lead, phosphorus, and sulphur. This is a conventional test. As shown in Table II, the carbon monoxide had its highest conversion efficiency for sample 3 and the ability to convert unsaturated hydrocarbons (C₃H₆, such as propylene) was excellent, and the saturated hydrocarbons (C₃H₈, such as propane) also were at their highest conversion efficiencies. This is in contrast to samples 4 and 5, where the CO conversion efficiency dropped and the saturated hydrocarbon conversion efficiency dropped dramatically. Without some portion of the precious metal being separated on a macroscale from CeO₂, poor oxidation of saturated HC occurs.

Claims: A method of making a catalyst system, for treating exhaust gases, said catalyst comprising an alumina-containing coated substrate, CeO₂ in an amount of from 2 to 20% by weight and precious metal in an amount of from 0.05 to 0.5% by weight of the final coated substrate, the method comprising, depositing onto said substrate a two-phase coating in which said phases are in particle form with (i) the first phase resulting from the deposition of CeO₂ and precious metal onto Al₂O₃ particles from an aqueous solution containing dissolved salts of CeO₂ and said precious metal and suspended particles of Al₂O₃, and (ii) the second phase resulting from the deposition of precious metal onto Al₂O₃ particles from an aqueous solution containing a dissolved salt of said precious metal and suspended particles of Al₂O₃, and devoid of CeO₂, the second phase being deposited onto the first phase. A method as claimed in claim 1, in which the weight ratio of the first phase to said second phase is in the range of 4:1 to 1:4. A method as claimed in claim 1 or 2, in which said precious metal is selected from the group consisting of platinum, palladium, rhodium, ruthenium, iridium, osmium and mixtures thereof. A method as claimed in any one of claims 1 to 3, in which the ratio of Al₂O₃/CeO₂ varies between 1:1 to 20:1. A method as claimed in any one of the preceding claims, in which the deposition of said first phase particles is carried out by (i) dissolving a salt of cerium and a salt of said precious metal in deionised water in which is suspended Al₂O₃, (ii) drying said solution to produced a collection of Al₂O₃ particles impregnated with CeO₂ and precious metal, (iii) forming an aqueous slurry of said impregnated collection of first phase particles, and (iv) dipping said substrate in said slurry. A method as claimed in claim 5, in which deposition of said second phase particles is carried out by (i) dissolving a salt for said precious metal in deionized water in which is suspended Al₂O₃, (ii) drying said solution to produce a collection of Al₂O₃ particles impregnated with said precious metal, (iii) forming an aqueous slurry of said collection of second phase particles, (iv) dipping said coated substrate in said slurry, and (v) drying and calcining said doubly coated substrate. A method as claimed in claim 6, in which said drying is carried out by heating to 75-100°C for 3-5 hours, and said calcining is carried out by heating to 800°C for 3-5 hours. A method as claimed in claim 1, in which said CeO₂ is present in an amount of 4-8% by weight of said coated substrate and said precious metal is present in an amount of .05-.25% by weight of the coated substrate. A method as claimed in claim 8, in which Al₂O₃ is present on said substrate in an amount of 25-30% by weight of the coated substrate and the ratio of Al₂O₃/CeO₂ is in the range of 3:1 to 7:1.

Inventors: GANDI, HAREN G., WATKINS, WILLIAM L.H.
Applicants: FORD FRANCE S. A., FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1988
Main classification: B01J23
IPC codes: B01D53, B01J37, B01J23