Abstract:
A catalyst composition is disclosed comprising a support, a substantially sulfur-free stabilizer and a catalytic metal. Also disclosed is the catalytic converter comprising this new catalyst. Methods of making the catalyst composition and the catalytic converter are also described. The lack of added sulfur combined with basic pH application provide a catalyst with excellent stability for high temperature applications such as close coupled catalysts.

Description:
TECHNICAL FIELD  
         [0001]    This disclosure relates to the purification of contaminants from exhaust gases. More particularly, this disclosure relates to a substantially sulfur-free catalyst composition for the treatment of automotive exhaust gases and the catalytic element including this catalyst.  
         BACKGROUND OF THE INVENTION  
         [0002]    In order to meet exhaust gas emissions standards, the exhaust gases emitted from an internal combustion engine are treated prior to emission into the atmosphere. Exhaust gases are passed through a catalytic element such as a catalytic converter to remove undesirable emission components such as unburned hydrocarbons, carbon monoxide, and nitrogen oxides. In order to meet strict emission standards, the catalytic converter can be located in a close-coupled configuration, i.e., closer to the engine than the standard under the floor configuration. The location of a catalytic converter in the close-coupled configuration allows for reduction of emissions during the period immediately after starting the engine from ambient temperatures, also known as cold start. A close-coupled catalytic converter can either be the only catalytic converter or can be a small volume catalytic converter used in conjunction with a traditional under the floor catalytic converter. A close-coupled catalytic converter is disclosed in U.S. Pat. No. 6,044,644 to Hu et al.  
           [0003]    The catalyst in a catalytic converter is exposed to the high temperature exhaust gas that exits the engine after the engine has warmed up. Because the temperature of the exhaust gas is proportionally hotter depending on the distance from the engine, a close-coupled catalyst can be exposed to temperatures as high as 1100° C. Exposure to temperatures up to 1000° C. or higher can lead to instability of the catalyst, that is, for example the catalyst can lose adhesion to its substrate thus becoming ineffective. Thus there remains a need for catalyst formulations that are stable under temperatures in excess of 1000° C. for use in close-coupled configurations.  
         SUMMARY OF THE DISCLOSURE  
         [0004]    Disclosed herein is an exhaust emission treatment device, comprising a shell, a substrate comprising catalyst composition, wherein the catalyst composition comprises a catalyst support, a catalytic metal, a stabilizer comprising less than or equal to 0.1 wt % sulfur based on the total weight of the catalyst composition and a support element disposed between the shell and the substrate. Preferred stabilizers are selected from the group consisting of barium aluminate, barium stabilized aluminate, barium-manganese aluminate, barium-magnesium aluminate, barium-cerium-magnesium aluminate, strontium-lanthanum-manganese aluminate, lanthanum-magnesium aluminate, lanthanum-neodymium-magnesium aluminate, lanthanum stabilized aluminate, lanthanum oxide, lanthanum hydroxide, and complexes, compounds, and combinations comprising at least one of the foregoing stabilizers.  
           [0005]    The above described and other features are exemplified by the following drawing and the detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0006]    [0006]FIG. 1 is a partially cut-away perspective view of a catalytic converter. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0007]    A catalyst composition is disclosed which is disposed within or on a catalytic device, e.g., an exhaust emission treatment device, such as a converter that catalyzes the oxidation of pollutants such as unburned hydrocarbons, carbon monoxide and nitrogen oxides. An exemplary catalyst composition comprises a catalyst support, a catalytic metal and a substantially sulfur-free stabilizer. This composition is typically disposed on a substrate.  
         [0008]    As used in the present disclosure, catalyst support refers to a material on which the stabilizers and catalytic metal are deposited. The catalyst support provides a large surface for the catalyst active sites. The preferred surface area can be about 150 to about 200 meters squared per gram (m 2 /g) or higher. The catalyst support is typically a metal or metal oxide, such as alumina (e.g., alpha alumina, gamma alumina, delta alumina, and the like), ceria, zirconia, silica, titanium and the like, as well as cermets, oxides, compounds, and combinations comprising at least one of the foregoing supports. An exemplary catalyst support is a refractory oxide such as aluminum oxide.  
         [0009]    It has been discovered that, at high temperatures, such as 1,000° C. and greater, sulfur can cause instability of an aluminum oxide support. Without being held to theory, it is believed that at high temperatures, barium sulfate decomposes and the sulfate then attacks a metal substrate. This attack by sulfate causes loss of adhesion of the alumina scale to the substrate and thus loss of the catalytic metal. Consequently, the catalyst composition is substantially sulfur-free; i.e., the only sulfur present is as trace contaminants from the other components (e.g., a substrate impurity that migrates to the catalyst composition). This low sulfur content results in better adhesion of the catalyst to the substrate at a variety of temperatures. Preferably, a substantially sulfur-free catalyst composition contains less than or equal to about 0.1 weight percent (wt %) sulfur, based upon the total weight of the catalyst composition, with both the stabilizer and the catalyst composition being substantially sulfur free.  
         [0010]    In a moving vehicle, the temperature of the exhaust gas can reach temperatures of 1,000° C. and higher. At such temperatures, the catalyst support material can undergo thermal degradation leading to volume shrinkage and loss of exposed catalyst surface area. The purpose of a stabilizer is thus to stabilize the catalyst support from thermal degradation. Stabilizers can include alkaline earth, alkali or rare-earth metals, oxides, complexes, compounds, and combinations comprising at least one of the foregoing stabilizers. Preferred stabilizers include lanthanum, scandium, germanium, yttrium, cerium, boron, barium, magnesium, strontium, calcium, praseodymium, neodymium and the like, as well as oxides, complexes, compounds, and combinations comprising at least one of the foregoing stabilizers. Preferred stabilizers include barium aluminate, barium stabilized aluminate, barium-manganese aluminate, barium-magnesium aluminate, barium-cerium-magnesium aluminate, strontium-lanthanum-manganese aluminate, lanthanum-magnesium aluminate, lanthanum-neodymium-magnesium aluminate, lanthanum stabilized aluminate, lanthanum oxide, lanthanum hydroxide, and the like, as well as oxides, complexes, compounds, combinations comprising at least one of the foregoing stabilizers, with lanthanum compounds and complexes more preferred, and barium lanthanum aluminum oxide even more preferred.  
         [0011]    An exemplary stabilizer comprises barium aluminate. A preferred barium aluminate compound is barium in the crystal structure of aluminum, otherwise known as barium stabilized aluminate. A method to obtain barium stabilized aluminate is to mix aluminum isoproproxide and barium isoproproxide and then calcine to obtain aluminum with barium in the crystal structure of aluminum. A preferred amount of barium in the barium stabilized aluminate is about 0.05 wt % to about 6 wt %; and more preferably about 0.5 wt % to about 3 wt %, based upon the total weight of the barium stabilized aluminate.  
         [0012]    The stabilizer can also, for example, comprise lanthanum stabilized aluminate. Lanthanum stabilized aluminate can be formed by mixing aluminum isoproproxide and lanthanum isoproproxide and calcining the mixture. Other preferred lanthanum compounds are lanthanum oxide, lanthanum hydroxide, or any soluble lanthanum compound that can be deposited on aluminum oxide at a pH greater than or equal to about 7.0.  
         [0013]    An exemplary stabilizer is barium lanthanum aluminum oxide. A method of forming barium lanthanum aluminum oxide is to deposit on aluminum oxide the ammonium form of colloidal aluminum hydroxide, barium aluminate and a soluble lanthanum compound and then calcine the mixture. An exemplary solvent is water. Another method to form barium lanthanum aluminum oxide is to combine barium aluminate, lanthanum oxide and aluminum oxide and to calcine to form barium lanthanum aluminum oxide. Yet another method to form barium lanthanum aluminum oxide is to combine barium aluminate powder in water with lanthanum hydroxide and then dry and calcine. An barium-aluminum hydroxide binder is then added in water to form the barium lanthanum aluminum oxide. The purpose of the binder is to prevent the dissolution and redistribution of particles in the stabilizer.  
         [0014]    The amounts of the various stabilizer components are described as a weight percentage of the stabilizer. The stabilizer comprises about 88 wt % to about 96 wt %, with about 88 wt % to about 94 wt % of the barium stabilized aluminate preferred. An aluminum hydroxide binder can comprise less than or equal to about 2 wt % of the final stabilizer. A lanthanum compound can be present in an amount of about 4 wt % to about 10 wt %, with about 4 wt % to about 6 wt % preferred.  
         [0015]    In addition to the stabilizer, the catalyst composition comprises catalytically active metal. Exemplary catalytic metals include metals, such as platinum, palladium, rhodium, iridium, ruthenium, nickel, cobalt, iron, manganese, chrome, copper, and the like, as well as oxides, alloys, and combinations comprising at least one of the foregoing catalyst materials. Exemplary chemical forms of the catalytic metals that are preferred include platinum amine hydroxide, palladium amine hydroxide, or other forms that can be added at a pH greater than or equal to about 7.0. The amount of catalytic metal employed in the catalyst composition can be up to about 5 wt % or so, depending upon the application. Typically about 0.01 wt % to about 1 wt % catalytic metal is employed, with about 0.1 to about 0.75 wt % preferred, and about 0.25 wt % to about 0.5 wt % especially preferred. A preferred amount of catalytic metal is about 0.01 wt % to about 0.5 wt % palladium or platinum, and optionally about 0.01 wt % to about 0.05 wt % rhodium.  
         [0016]    The catalyst composition is typically disposed on a substrate to form a substrate supported catalyst. The catalyst substrate can have a size and geometry chosen to optimize the surface area in a given exhaust emissions control device design. The catalyst substrate can have a honeycomb geometry, with the cells being any multi-sided (polygonal) or rounded shape, with substantially square, triangular, pentagonal, hexagonal, heptagonal, or octagonal or similar geometries preferred due to ease of manufacturing and increased surface area. Some possible materials for a catalyst substrate include metals (e.g., sintered metal materials, and the like), ceramics, cermets, and combinations comprising at least one of the foregoing materials, in the form of a foil, monolith, honeycomb, or the like. A metallic substrate can be comprise, for example, ferrous materials (e.g., stainless steel, steel, and the like), nickel, iron, chrome, titanium, and the like, as well as alloys, mixtures, cermets, and combinations comprising at least one of the foregoing metals. Some specific examples of the metal substrates include chrome-nickel alloys, iron alloys, nickel plated stainless steel, and the like, with a preferred substrate being a metal foil with an alumina scale. Suitable ceramic materials include cordierite, cordierite-alpha alumina, silicon nitride, zirconia, mullite, spodumene, alumina-silica magnesia, zirconium silicate, sillimanite, magnesium silicates, petalite, alpha alumina aluminosilicates, and cermets and combinations comprising at least one of the foregoing ceramics.  
         [0017]    The catalyst composition is disposed on and/or throughout the catalyst substrate via various techniques. Possible techniques include spraying, dipping, washcoating, imbibing, impregnating, physisorbing, chemisorbing, precipitating, or otherwise applying the catalyst composition to the substrate. One method for applying the catalyst to the substrate is to spray, dip a metal foil into the catalyst composition and then roll the foil to form a monolithic substrate. Another method is to roll the metal foil into a monolithic substrate, and then to dip or otherwise apply the catalyst onto the substrate.  
         [0018]    In order to enhance the adhesion of the catalyst composition to the substrate, the pH of the catalyst composition is preferably adjusted to a basic pH, i.e., a pH of greater than or equal to about pH 7.0. Preferably the pH is greater than or equal to about 9.0, with a pH of greater than or equal to about pH 10.0 more preferred.  
         [0019]    An example of applying the catalyst composition to the substrate comprises first calcining a barium lanthanum alumina stabilizer onto an alumina-coated substrate. The pH of the mixture of the catalytic metal and stabilizer is adjusted to preferably great than or equal to 9.0. The mixture is then sprayed, dipped, or otherwise coated onto the alumina (i.e., the support). Alternatively, the support is mixed with the catalytic metal and stabilizer to form a slurry. The pH of the slurry is adjusted, then the slurry is applied to the substrate. Once the catalyst composition is on the substrate, it is calcined.  
         [0020]    The substrate supported catalyst can be employed in various exhaust emission treatment devices, such as catalytic converters, catalytic absorbers, diesel particulate traps, non-thermal plasma conversion devices, and the like. Many of these devices comprise the catalyst composition supported on a substrate disposed within a shell, with a mat support optionally disposed between the substrate and the shell, depending on the particular type of device. Referring now to FIG. 1, a catalytic converter  10  is illustrated. Catalytic converter  10  comprises an outer shell  12 , a mat support material  16  and a catalyst substrate  14 .  
         [0021]    The shell  12  is usually a protective metal layer that is disposed around the catalyst substrate  14  and mat support  16 . The shell is of a shape and size that is suitable to contain the substrate and to protect it from such operating conditions as severe mechanical shocks. The choice of material for the shell depends upon the type of exhaust gas, the maximum temperature reached by the catalyst substrate, the maximum temperature of the exhaust gas stream, and the like. Suitable materials for the shell can comprise any material that is capable of resisting under-car salt, temperature and corrosion. Ferrous materials, for example, are employed such as ferritic stainless steels. Ferritic stainless steels can include stainless steels such as, e.g., the 400-Series such as SS-409, SS-439, and SS-441, with grade SS-409 generally preferred.  
         [0022]    Also similar materials as the shell, end cone(s), end plate(s), exhaust manifold cover(s), and the like, can be concentrically fitted about the one or both ends and secured to the shell to provide a gas tight seal. These components can be formed separately (e.g., molded or the like), or can be formed integrally with the shell using a methods such as, e.g., a spin forming, or the like.  
         [0023]    Located between the shell  12  and the catalyst substrate  14  is a support element  16 , typically in the form of a mat, pellets or the like. The support element both holds the catalytic substrate in place, and in some cases, insulates the shell from exhaust gas temperatures and the exothermic catalytic reaction occurring in the substrate  14 . The support element material, which enhances the structural integrity of the catalyst substrate by applying compressive radial forces about it, reducing its axial movement, and retaining it in place, is disposed around the catalyst substrate to form a support element/catalyst substrate subassembly. The support material can either be an intumescent material (e.g., a material comprising a vermiculite component that expands with heating), or a non-intumescent materials (i.e., a material that does not contain vermiculite), as well as materials which include a combination of both. The intumescent and non-intumescent materials can comprise ceramic materials, and other materials such as organic binders and the like, and combinations comprising at least one of the foregoing materials.  
       EXAMPLE  
       [0024]    A first formulation with 4 wt % barium sulfate and a second formulation with a lanthanum hexaaluminate and substantially no sulfur were coated onto a metal monolith. After 10 hours of 1,000° C. aging, virtually all of the formulation containing 4 wt % barium sulfate was no longer adherent to the metal substrate. After the same aging process, none of the lanthanum hexaaluminate composition had lost adhesion to the metal substrate.  
         [0025]    A pH 3.0 lanthanum hexaaluminate formulation with substantially no sulfate was coated onto a metal monolith at pH 3.0. After 48 hours of 1,000° C. aging, 32% of the formulation was no longer adherent to the metal substrate. A pH 9.0 lanthanum hexaaluminate formulation with substantially no sulfate was coated onto a metal monolith at pH 9.0. After the same aging process, none of the lanthanum hexaaluminate composition deposited at pH 9.0 had lost adhesion to the metal substrate. Thus, deposition of the substantially sulfur-free catalyst at pH 9.0 or above significantly improves catalyst adhesion to the substrate.  
         [0026]    The sulfur-free catalyst composition, and therefore the exhaust emission treatment device, comprises improved heat-resistance at temperatures greater than about 1,000° C. compared to similar devices that are not sulfur-free (i.e., compositions comprising greater than about 0.1 wt % sulfur). The key features of the catalyst are the lack of added sulfur and application of the catalyst to the substrate at basic pH. Both of these features contribute to the heat stability of the catalyst. Under conditions where the sulfur-containing catalysts are no longer adherent to a substrate, the substantially sulfur-free catalyst has no loss in adhesion.  
         [0027]    While the disclosure has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.