Method of manufacturing a catalytic composite useful for the treatment of gaseous waste products
Gaseous waste products resulting from the burning or combustion of hydrocarbonaceous fuels such as gasoline and fuel oils, comprise hydrocarbons, carbon monoxide and oxides of nitrogen as products of combustion or imcomplete combustion and, when discharged directly to the atmosphere, pose a serious health problem. While exhaust gases from other hydrocarbonaceous fuel burning sources, such as industrial furnaces, are a substantial pollution factor, the exhaust gases of automotive internal combustion engines are a principal source of pollution. Of the various methods which have been proposed for the control of exhaust gases from automotive internal combustion engines, the incorporation of a catalytic converter in the exhaust system holds the most promise for meeting the increasingly rigid standards set by the responsible governmental agenices. 
     It has heretofore been shown that a catalytically active component impregnated on the outer surface of a support or carrier material provides a particularly useful catalytic composite for the conversion of hydrocarbons, carbon monoxide and oxides of nitrogen contained in the hot exhaust gases from an internal combustion engine. For example, see U.S. Pat. No. 3,367,888 which provides a method of impregnating a catalytic component on the outer surface of a support or carrier material. Briefly, the method comprises commingling a refractory inorganic oxide carrier material with a catalytic component and with a sulfur-containing carboxylic acid whereby said component is deposited on the outer surface of the carrier material without any substantial penetration thereof. 
     There are a number of factors which affect the activity, activity stability and physical stability of a catalytic composite, and the factors are generally peculiar to the environment in which the catalytic composite functions. For example, the automotive internal combustion engine is commonly operated over a wide range of speed and load conditions including idling, cruising, accelerating and decelerating conditions, and the combustion efficiency varies accordingly. Consequently, the environment in which the catalytic composite must function will comprise not only variations in the concentration of pollutants at any given time, but also the variations in temperature and space velocity. Thus, in the treatment of noxious exhaust gases from an internal combustion engine, the catalytic composite must function in an environment of changing gaseous hourly space velocities in the range of from about 10,000 to about 100,000 or more, and temperature variations of from about 95° to about 1100° C. or more, and must exhibit high activity and physical strength over an extended period equivalent to about 50,000 miles or more of automotive operation. 
     While the refractory inorganic oxides typically employed as a support or carrier material are exceptionally distant to physical degradation in a more conventional application, they experience shrinkage upon continued exposure to the extreme temperatures encountered in the treatment of exhaust gases from an internal combustion engine, and the ultimate effect is breakage and undue loss of catalyst. This is particularly true with respect to gamma-alumina which is commonly employed as a catalyst support or carrier material. Upon continued exposure to the extremely high temperatures involved, said alumina tends to assume a more thermally stable phase of lower surface area, for example, theta-, and even alpha-alumina, as well as a high density through shrinkage. 
     A catalytic composite comprising a refractory inorganic oxide carrier material, a platinum group metal component and an alkaline earth metal component, and particularly adapted to the oxidation of noxious exhaust gases from an internal combustion engine, is disclosed in U.S. Pat. Nos. 3,388,077 and 3,409,390. The inclusion of an alkaline earth metal component, which may be calcium, strontium, and/or barium, is disclosed as useful to enhance the physical stability of the catalytic composite as well as the activity and activity stability thereof. Subsequent associated work has revealed the order of impregnation to have a significant influence on catalyst activity. Thus, when the alkaline earth metal component is impregnated on the carrier material either prior to or simultaneously with the platinum group metal component, an inferior catalyst is obtained, most likely as a result of poor distribution of the platinum group metal component on the carrier material. On the other hand, when the platinum group metal component is impregnated on the carrier material prior to the alkaline earth metal component, an intermediate high temperature calcination is required to obviate leaching of the expensive platinum group metal component from the carrier material in the course of the subsequent alkaline earth metal impregnation operation. It will be appreciated that the required intermediate high temperature calcination is a substantial factor in determining the cost of this highly useful exhaust gas conversion catalyst. 
     It is therefore an object of this invention to present a novel method of impregnating a catalytic component on the outer surface of a carrier material at conditions to improve the activity, activity stability and physical stability of the catalytic composite. 
     It is a further object of this invention to present a method of impregnating a porous refractory carrier material with a Group VIII metal component and an alkaline earth metal component at conditions to obviate the need of an intermediate high temperature calcination. 
     In one of its broad aspects, the present invention embodies a method of manufacturing a catalytic composite which comprises commingling a refractory inorganic oxide carrier material in an impregnating solution with a sulfur-containing carboxylic acid and a Group VIII metal component in an acid/metal mole ratio of from about 1 to about 3, and effecting the deposition of said Group VIII metal component on the outer surface of said carrier material; prior to calcining the impregnated carrier material, further impregnating the same with a solution of an alkaline earth metal salt of an organic acid selected from the group consisting of the calcium, strontium and barium salts thereof, and evaporating said solution to dryness in contact with the impregnated carrier material; and thereafter calcining the impregnated carrier material. 
     Other objects and embodiments of this invention will become apparent in the following detailed specification. 
     The refractory inorganic oxides suitable for use as a support or carrier material include naturally occurring materials, for example clays and silicates such as fuller&#39;s earth, Attapulgus clay, feldspar, halloysite, montmorillonite, kaolin, and diatomaceous earth -- frequently referred to as siliceous earth, diatomaceous silicate, kieselguhr, etc., and the naturally occurring materials may or may not have been activated prior to use by one or more treatments including drying, calcining, steaming and/or acid treatment. Synthetically prepared refractory inorganic oxides like alumina, silica, zirconia, boria, thoria, magnesia, titania, chromia, etc., or composites thereof, particularly alumina in combination with one or more refractory inorganic oxides, for example, alumina-silica, alumina-zirconia, alumina-chromia, and the like are especially suitable. Alumina is a preferred refractory inorganic oxide, and the alumina may be any of the various hydrous aluminum oxides or alumina gels including alpha-alumina monohydrate (boehmite), alpha-alumina trihydrate (gibbsite), beta-alumina trihydrate (bayerite), and the like. Activated aluminas, such as have been thermally treated to eliminate substantially all of the water and/or hydroxyl groups commonly associated therewith, are particularly useful. Preferably, the alumina is an activated alumina with a surface area of from about 100 to about 500 square meters per gram, especially gamma-alumina and eta-alumina resulting from the thermal treatment of boehmite alumina and bayerite alumina respectively, generally at a temperature of from about 400° to about 850° C. The alumina may be employed in any suitable shape or form including spheres, pills, extrudates, granules, cakes, briquettes, rings, etc., and particularly low density spheres such as are continuously manufactured by the oil drop method described in detail in U.S. Pat. No. 2,620,314. 
     As heretofore stated, the refractory inorganic oxide support or carrier material is commingled with an impregnating solution of a Group VIII metal component and a sulfur-containing carboxylic acid -- said carboxylic acid effecting impregnation of the Group VIII metal component on the outer surface of the support or carrier material. Group VIII metal compounds suitable for use in an impregnating solution, that is, compounds of iron, nickel, cobalt, platinum, palladium, ruthenium, rhodium, osmium and iridium, include nickel nitrate, nickel sulfate, nickel chloride, nickel acetate, nickel formate, cobaltous nitrate, cobaltous sulfate, ferric chloride, ferric nitrate, ferric sulfate, chloroplatinic acid, platinum tetrachloride, ammonium chloroplatinate, dinitrodiamino platinum, palladium chloride, chloropalladic acid, rhodium trichloride, ruthenium tetrachloride, osmium trichloride, iridium trichloride, and the like. Of the Group VIII metals, the noble or platinum group metals are preferred. While platinum per se has long been known to catalyze the oxidation of noxious exhaust gases to effect substantially complete conversion of the combustible pollutants contained therein, it has been found that the required amount of the relatively expensive platinum can be substantially reduced utilizing a palladium promoter, and it has been further found that substantially less total noble metals, palladium and platinum, is required to obtain substantially complete conversion than is the case with platinum alone. The total noble metals concentration and the palladium/platinum weight ratio for a given concentration are factors which exert a strong influence on the activity stability of the catalytic composite of this invention. The activity stability of a catalytic composite containing from about .0025 to about 2.0 wt. % or more total noble metals is improved utilizing a palladium/platinum weight ratio in the range of from about 9:1 to about 1:4. In any case, the total noble metals in the range of from about 0.0025 to about 2.0 wt. % or more, utilizing a palladium/platinum weight ratio of from about 9:1 to about 1:4, is appreciably less than would otherwise be required of platinum alone to achieve an equivalent conversion. A total noble metals concentration in the lower range, say from about .0025 to about 1.0 wt. % is particularly effective in combination with the palladium/platinum weight ratio of from about 2:1 to about 4:1. 
     The sulfur-containing carboxylic employed herein is substantially as described in U.S. Pat. No. 3,367,888, that is, preferably a thio or mercapto carboxylic acid such as for example thiomalic acid, thioglycolic acid, mercaptopropionic acid, and the like. The quantity to be employed herein is based on the moles of Group VIII metal present, and is suitably employed in a mole ratio of from about 1 to about 3 therewith. 
     Impregnation of the carrier material can be effected utilizing impregnating techniques known to the art. Thus, a particulate carrier material can be immersed in a common aqueous solution of chloroplatinic acid and chloropalladic acid or palladium chloride, said solution further containing a selected sulfur-containing carboxylic acid, preferably thiomalic acid, and the solution thereafter evaporated to dryness in contact with the carrier material. For example, a given volume of 1/16-1/8 inch alumina spheres is immersed in a substantially equal volume of an impregnating solution in a steam-jacketed rotary dryer -- the impregnating solution comprising chloroplatinic acid, chloropalladic acid and a requisite quantity of thiomalic acid. The spheres are tumbled in the impregnating solution for a brief period at ambient temperature, after which steam may be applied to the dryer jacket to expedite evaporation of the solution and recovery of substantially dry impregnated alumina spheres, or the solution may be drained or decanted from the spheres. 
     The catalytic composite of this invention further includes an alkaline earth metal component selected from the group consisting of calcium, strontium and barium to improve the activity, activity stability and physical stability of the catalytic composite. Of the alkaline earth metal components, barium is preferred. The optimum alkaline earth component is a function of the density of the refractory inorganic oxide employed as a support or carrier material. It will be appreciated that the refractory inorganic oxides of higher density suffer less shrinkage at higher temperatures than do those of lower density, and the optimum alkaline earth metal content will therefore increase in the range of from about 1 to about 20 wt. % with decreasing density. 
     Pursuant to the present invention, leaching of the Group VIII metal component from the carrier material is substantially obviated by further impregnating the carrier material with a solution of an alkaline earth salt of an organic acid, and maintaining the solution in contact with the carrier material while it is evaporated to dryness. While the practice of evaporating the impregnating solution in contact with the carrier material obviates leaching of the Group VIII metal component therefrom, the practice precludes the use of certain alkaline earth metal compounds heretofore preferred. Thus, while an aqueous solution of an alkaline earth metal salt of the strong acid, and especially an aqueous solution of an alkaline earth metal oxide or hydroxide, has heretofore been a preferred impregnating solution, when said solutions are evaporated and concentrated in contact with the carrier material, the final catalyst product is comprised of an excessive amount of catalyst fines. However, when the Group VIII metal-impregnated carrier material is impregnated with a solution of an alkaline earth metal salt of an organic acid and the solution evaporated to dryness in contact with the carrier material pursuant to the present invention, the final catalyst product is recovered substantially free of catalyst fines. Suitable alkaline earth metal salts of an organic acid for use herein include the various water-soluble salts, and particularly those containing up to about 10 carbon atoms, for example calcium formate, calcium acetate, calcium propionate, calcium butyrate, calcium isobutyrate, strontium formate, strontium acetate, barium formate, barium acetate, barium propionate, barium butyrate, and the like. 
     The dried Group VIII metal-impregnated carrier material is conveniently further impregnated with the alkaline earth metal salt solution utilizing the aforementioned steam-jacketed rotary evaporator. Thus, a volume of the alkaline earth metal salt solution, for example an aqueous barium acetate solution, is added to a substantially equal volume of the impregnated carrier material contained in said steam-jacketed rotary evaporator. Again, the impregnated carrier material is advantageously tumbled in the solution for a brief period of about 30 minutes at ambient temperature after which steam is applied to the dryer jacket to expedite the evaporation process. The resulting dried composite is subsequently calcined in air at a temperature of from about 200° to about 1000° C. or more. 
     The following example is presented in illustration of the method of this invention and is not intended as an undue limitation on the generally broad scope of the invention as set out in the appended claims. 
    
    
     EXAMPLE 
     In the preparation of a catalytic composite in accordance with the method of this invention, 200 grams of 1/8 inch gamma-alumina spheres having an average bulk density of about 0.3 grams per cubic centimeter and a surface area of about 180 square meters per gram were immersed in a 600 milliliter aqueous chloroplatinic acid solution contained in a steam-jacketed rotary evaporator. The impregnating solution contained 0.80 grams of thiomalic acid and the equivalent of 1.0 grams of platinum. The spheres were tumbled in the solution for about 10 minutes at room temperature, and the solution was thereafter drained from the spheres. Next, 600 milliliters of an aqueous barium acetate solution containing 18.0 grams of barium was added, and the spheres were tumbled therein at room temperature for about 30 minutes. The barium acetate solution was then evaporated substantially to dryness under the influence of steam applied to the dryer jacket. The dried spheres were subsequently calcined in a stream of air for 1 hour at 315° C. and then for 2 hours at 535° C. to yield a catalytic composite containing 0.9 wt. % platinum and 9.0 wt. % barium. There was no apparent leaching of the platinum component from the carrier material during the barium impregnation operation.