Abstract:
Durability of emissions catalysts in vehicular and other emissions control systems is significantly increased by positioning, prior to the catalyst-laden catalytic converter, at least one poison trap consisting of a low back pressure structure having a coating which reacts with non-metal or amphoteric oxides to form non-volatile reaction products.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention pertains to exhaust systems for internal combustion (“IC”) engines, particularly to increasing the longevity of catalytic systems employed in IC engines as part of their emissions control system.  
           [0003]    2. Background Art  
           [0004]    Exhaust emissions from IC engines are regulated in virtually all industrialized countries, with the aim of limiting emissions of unburnt or partially burnt hydrocarbons (“HC”), nitrogen oxides (“NO X ”), and carbon monoxide (“CO”). It was found early on that changes in such items as combustion chamber geometry, fuel/air flow paths, type of fuel, and air/fuel ratio all affect emissions. However, obtaining the mandated emissions levels required far more emissions limitation than these variables could provide. Further, not only are some changes in the foregoing variables detrimental to power output and fuel economy, but moreover, the effect of these variables on emissions is often conflicting. For example, increasing the air/fuel ratio generally lowers CO and HC, but increases NO x .  
           [0005]    Thus, all present day automotive engines employ, in lieu of or in addition to other measures, a catalytic system located in the exhaust path. The principle component of such a system is typically termed a “catalytic converter,” and contains one or more catalytic elements which lower HC, CO and/or NO x . Some catalyst systems are also designed to reduce particulate matter from diesel engines.  
           [0006]    The individual catalysts may be coated onto ceramic or metal spheres, on metal screen or honeycomb, but most often are supplied coated onto a ceramic or metal honeycomb element termed a “monolith.” Also included in this category are monoliths applied as diesel particulate filters (DPFs). A single converter or multiple converters may be used. The catalyst may be supplied either directly to the monolith as a component of a “washcoat” which is subsequently calcined, or may be deposited on a previously applied and calcined washcoat. As many as four or five monoliths may be placed in succession in the exhaust stream depending on the particular application. Emissions requirements have become increasingly stringent, requiring development of both new catalysts and higher catalyst loadings. In addition to absolute emissions standards, emissions control system longevity, or “durability” requirements have also been extended. This maintenance of operation over extended periods has also challenged catalyst development, and has required still further increased catalyst levels.  
           [0007]    Catalyst durability is limited by numerous reactions which can occur in the varied temperature and fuel/air stoichiometric environments in which the catalysts operate. For example, it was recognized quite early that lead, formerly supplied as an octane booster in fuel as tetraethyl lead, is a serious catalyst poison. It thus has been removed from modern day fuels. However, numerous trace elements still come into contact with the exhaust catalyst, some unavoidably so, and several of these are known to decrease catalyst durability. Not all these are derived from the fuel. For example, zinc dialkyldithiophosphates have been long used as antioxidants and/or high pressure lubricant additives in motor oils. Especially with modern high speed engines, increased piston/wall clearances and decreased sealing allow increased entry of oil into the combustion chamber, where oil additives, or their combustion byproducts, subsequently pass into the exhaust stream. Products which are specifically problematic are acidic or amphoteric oxides such as P x O y , ZnO, and SO x . It has been proposed in Res. Disc.  39017 , p.  650 , October 1996, to trap acidic emissions products by incorporating basic metal oxides into the catalyst converter monolith washcoat. However, the presence of additional components in the emissions catalyst washcoat has several notable drawbacks: first, it partially obscures the active catalyst, thus requiring increased catalyst loading; second, the increased thickness of the washcoat increases exhaust back pressure, lowering volumetric efficiency and engine power. Additionally, with some catalysts, the additional metal oxide components may react with catalyst components, altering their catalytic activity.  
           [0008]    Earlier, it was proposed in Japanese applications JP 55 151109 and JP 56 044411, to insert an alumina-containing phosphorus trap in the oil recirculation system to remove suspect components from the oil being recirculated, and thus protect the exhaust catalyst. However, such systems are inefficient in the degree of protection achieved, may become rapidly fouled, and may remove desirable antioxidant from the oil.  
           [0009]    It would be desirable to provide a means whereby acidic catalyst poisons which lower emissions catalyst durability can be effectively removed without requiring increased catalyst loading to compensate for reduced catalyst activity. It would be further desirable to increase catalyst durability with little or no effect on engine exhaust back pressure.  
         SUMMARY OF THE INVENTION  
         [0010]    It has now been surprisingly discovered that catalyst durability may be significantly extended by positioning a flow-through element in the exhaust gas path prior to the catalyst of the catalytic converter, this element being coated with a basic oxide which traps acidic contaminants. By means of such devices, catalyst longevity may be increased from 25% to 50% or more at low cost, without any significant effect on power or fuel economy, and without requiring additional catalyst loading. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 illustrates one embodiment of a preferred tubular poison trap in accordance with the present invention, and its preferred location in a catalytic converter;  
         [0012]    [0012]FIG. 1 a  illustrates an enlarged view of a tubular poison trap in accordance with the present invention;  
         [0013]    [0013]FIG. 2 illustrates an embodiment of the present invention wherein the poison trap is coated onto the interior of an engine manifold;  
         [0014]    [0014]FIG. 3 a  illustrates a “monolith” poison trap element;  
         [0015]    [0015]FIG. 3 b  illustrates the poison trap of FIG. 3 a  inserted into an exhaust pipe of an IC engine;  
         [0016]    [0016]FIG. 4 illustrates a poison trap of enlarged cross-section located in a separate exhaust system canister;  
         [0017]    [0017]FIGS. 5 a  and  5   b  illustrate the positioning of the poison trap components on the same first most monolith of the catalytic converter; and  
         [0018]    [0018]FIG. 6 illustrates one embodiment of a conical poison trap in accordance with the present invention.  
         [0019]    [0019]FIG. 7 illustrates an embodiment of the present invention wherein the poison trap is located in the engine manifold. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    The durability increasing devices of the present invention may be termed “poison traps.” These poison traps, and their active ingredient, contain a basic or amphoteric compound, preferably a metal oxide, which is capable of reacting with gaseous acidic catalyst poisons to remove these from the exhaust stream. It is believed that the poison trap functions by reacting with volatile acidic contaminants to form a substantially non-volatile compound which may be, without limitation, a salt, a mixed oxide, a ceramic material, or a glass. However, the exact mode of operation is irrelevant so long as catalyst durability is increased.  
         [0021]    The basic or amphoteric compounds must be capable of reacting with and thereby trapping oxides of phosphorus, i.e., in the manner described above, and may be, without limitation, oxides of the metals of Groups I and II of the Periodic Table of the Elements, or oxides (non-lniiting) of Ti, Zr, Mn, Fe, Co, Ni, Zn, Ga, Al, Si, Sn, Bi, Y, La, Ce, and/or Pr. Silicates, carbonates, and other compounds capable of reacting with P x O y  may also be used. In addition, compounds which, under high temperature operating conditions form such compounds, i.e., are “precursors,” may be used as well. Many acetates and nitrates of the various metals are suitable precursors. A simple test may be applied to determine whether any particular metal oxide, metal compound, or their precursors, including those not mentioned previously, are suitable. In such a test, hot exhaust gas from an IC engine, containing a minor amount of P x O y  is passed through a poison trap employing the proposed metal oxide or metal compound. The poison trap may be followed by a catalytic converter. If the poison trap does not absorb a measurable amount of P x O y , or alternatively, if the catalyst life of the catalytic converter is not increased as compared to an identical catalyst tested under identical conditions but without the poison trap, then the metal oxide, metal compound, or precursor thereof is not effective as a poison trap at the P x O y  concentration and metal oxide or metal compound concentration used with that particular catalyst.  
         [0022]    The poison trap must be positioned in the exhaust stream prior to the catalyst elements of the catalytic converter. The poison trap may be located in the engine exhaust passages, more preferably in the exhaust manifold, and most preferably in the exhaust header or in the catalytic converter itself, prior to the emissions catalyst-laden substrate (monolith). By the term “exhaust valve or port” is meant the point of exit of hot exhaust gases from the engine combustion chamber, i.e., the exhaust valve of a four stroke, valved engine, or the port of a two stroke engine.  
         [0023]    The poison trap may constitute a tubular or conical poison trap, may constitute a coating on one of the exhaust components located prior to the exhaust catalyst itself, or may itself be a “monolith”, preferably one of relatively large cell size such that exhaust back pressure is not unacceptably increased. In particular, the exhaust back pressure should not be increased by more than 50%, preferably not more than 25%, and most preferably 10% or less or unmeasurably, over the exhaust back pressure measured without the poison trap. Combinations of such poison traps may be used. The poison trap, in general, is termed herein a “flow through element” which is inclusive of the various forms of the poison trap as disclosed herein.  
         [0024]    Most preferably, the tubular poison trap is a HOT TUBE™, a trademark of Degussa, consisting of a hollow tube of ferrous metal, preferably stainless steel, having numerous holes in the wall thereof, and located appropriately in the exhaust stream and coated throughout with poison trap components. More than a single tubular poison trap may be used, in parallel or in series with other tubular poison traps. A suitable geometrical configuration of a tubular poison trap is described in the assignee&#39;s U.S. Pat. No. 5,916,128 (Jun. 29, 1999), as shown in FIG. 1 a  herein. However, other configurations, such as conical-shaped, are possible. It is preferred that the geometry of the tubular poison trap and any holes, slits, etc., in the tube impart some turbulence to the exhaust stream to maximize contact of the hot gases with the metal oxides on the tubular poison trap&#39;s surface.  
         [0025]    The preferred location of the tubular poison trap is in the converter itself, prior to the first catalyst monolith. A preferred configuration is shown in FIG. 1, wherein the tubular poison trap is secured, preferably by welding, to a flange or an extension thereof at one end.  
         [0026]    An alternative embodiment involves creating a poison trap of the exhaust manifold itself, by coating the interior passages of the manifold with a suitable poison trap washcoat, and preferably firing the washcoat to bond it thoroughly to the manifold. Such an embodiment is shown in FIG. 2.  
         [0027]    A further embodiment employs a ceramic or metal honeycomb. In preferred embodiments, the honeycomb has a cell size greater than that employed in a conventional monolith, such that little if any effect on back pressure is realized. Preferably, the cell density is about 20-400 cells per square inch, and more preferably about 50-300 cells per square inch. Such a monolith  30  is illustrated in FIG. 3 a , and may be located in the exhaust manifold catalytic converter. Also, such a “monolith,” if metal, may be positioned directly in the exhaust stream as shown in FIG. 3 b , or may be located in a separate canister as disclosed in FIG. 4. If contained in a separate canister, the cell size may be made smaller and the overall diameter larger, to avoid increasing back pressure. It should be noted that the monolith  50  may be metal or ceramic when located in a separate cannister.  
         [0028]    The term “discrete” as used herein with respect to the durability enhancing flow-through element, means that the durability enhancing coating is spaced apart from the first emissions catalyst monolith. The actual location may be, for example, any of those previously described. In addition to those locations where the poison trap is physically distinct from the emissions catalyst, the poison trap ingredients may also be located on the same monolith as the first catalytic element, but preceding the emissions catalyst. Such a monolith may be prepared by positioning the majority of the active catalytic elements, generally precious metals such as platinum, rhodium, and/or palladium, on the rearmost portion of the first monolith, with the frontmost portion serving as the poison trap. Such a monolith may be prepared by coating, preferably by washcoating, the monolith with a suitable poison trap washcoat, followed by coating only the rearmost portion of the monolith with a suitable emissions catalyst-containing washcoat.  
         [0029]    [0029]FIG. 5 a  illustrates a coated monolith wherein the frontmost portion of the monolith  42  is coated with poison trap components  44 , while the rearmost portion is coated with a conventional emissions catalyst-containing washcoat  46 . In this embodiment, components  44  and  46  preferably abut each other or have a slight gap between them. While this and other monoliths are shown to have the coating on the outer surface, it should be understood that the coatings could be, and preferably are, on the inner, or cell surfaces, of the monolith, either in addition to, or in lieu of being on the outer surface.  
         [0030]    In an alternative embodiment shown in FIG. 5 b , the entire monolith  52  is coated with a slurry containing the poison trap components  54 , dried and calcined. Next, a second coat containing catalysts components  56  is applied over a rearmost portion of the monolith, and thus a rear most portion of the poison trap components, to coat the rear most portion of the coated monolith for example, the rear half or two thirds of the coated monolith, with catalyst components. In this manner, the poison trap precedes the catalytic element as in other embodiments. However, because these components are calcined prior to application of the catalyst-containing washcoat, they have little or no effect on the emissions activity of the catalyst per se, except to prolong the life thereof.  
         [0031]    The active ingredients of the poison trap are preferably applied as a washcoat onto the supporting substrate, and subsequently calcined, or may be applied to a previously applied and calcined substrate. Any conventional application procedure may be employed. For example, a washcoat of hydrophilic fumed silica, alumina, ceria, titania, or mixtures thereof, optionally with preferably, basic substances such as alkali metal hydroxides, alkaline earth hydroxides and the like may be applied to the substrate and calcined. If the calcined washcoat does not already contain the basic metal oxide or metal compound, then the calcined washcoat may be dipped or sprayed with a solution or dispersion (if solid) of the metal compound, followed by drying and preferably calcining. In some cases, the final calcination, or conversion of precursor to metal oxide or metal compound, etc., may take place in situ, i.e., when the element becomes exposed to hot exhaust gases.  
         [0032]    In FIG. 1, the catalytic converter  1  comprises a can  7 , preferably of stainless steel, that encloses multiple “bricks” or “monoliths” of emissions-catalyst laden ceramic honeycomb elements, for example three elements  9 ,  11 , and  13 . The catalytic converter  1  is connected to the engine exhaust stream at  3 , by any convenient means, and to the tailpipe and any resonator, etc. present, at  15 . Located in the converter, preferably welded to a flange, is tubular poison trap  5 , coated throughout with poison trap components. The location of tubular poison trap  5  is prior to catalytic monoliths which might be poisoned by acidic vapors of the combustion process, particularly oxides of phosphorous, and less preferably, sulfur. As best shown in FIG. 1 a , the tubular poison trap  5  comprises a hollow cylinder  12  having the poison trap components  17  coated thereon. The hollow cylinder  12  is formed of a cylindrical wall  14  and includes a series of perforations  16  that are preferably circular, oval, or the like in shape.  
         [0033]    In an alternative embodiment illustrated in FIG. 6, a conical poison trap  60  having an essentially conical shape is shown secured, preferably by welding, to the catalytic converter or the exhaust pipe. The conical poison trap  60  comprises a conical member  22  having the poison trap components coated throughout thereon. The conical member  22  is formed of a conical wall and includes a series of perforations  16  therein, and could comprise a typical flow distributor used in a catalytic converter. Though not shown for clarity, it should be understood that the surfaces that define the perforations  16  in FIGS. 1, 1 a  and  6  are also coated with poison trap components.  
         [0034]    In FIG. 2 is illustrated an engine exhaust manifold  20  having a port  21  through which exhaust gases pass, the manifold exhaust port being terminated by a flange  23  containing bolt holes  25  adapted to receive a bolt to fasten the exhaust pipe to the engine manifold. On the interior surface  24  of the manifold is coated the poison trap components  27  of the present invention.  
         [0035]    [0035]FIG. 3 a  illustrates a “monolith” poison trap element  30  of relatively large cross-sectional area cells  33 . Preferably, to minimize back pressure, element  30  has a cell density of 20-400 cells per square inch, and more preferably about 50-300 cells per square inch. The poison trap  30  may be secured to an exhaust header or pipe by conventional means, such as by welding, as shown in FIG. 3 b , where  40  is the internal combustion engine,  41  the exhaust manifold,  43  the exhaust pipe containing the monolithic poison trap  30 , through which exhaust gases travel to catalytic converter  45 . While the poison trap  30  is shown in FIG. 3 a  as being generally cylindrical, it should be understood that the trap  30  may have many different configurations, such as oval-shaped. While the poison trap  30  in FIG. 3 a  is shown as having the poison trap components  34  coated on its outer surface for clarity, it should be understood that the poison trap components could be, and preferably are, coated on the cell surfaces, i.e., interior surfaces of the trap  30 , either in addition to, or in lieu of, being coated on the outer surface.  
         [0036]    [0036]FIG. 4 illustrates a poison trap  50  of enlarged cross-section, located in a separate canister  53  located between the exhaust manifold  55  of the engine and the catalytic converter  57 . The poison trap  50  may be a monolith of small cell size due to its larger diameter, as the exhaust back pressure will be substantially unaffected. Distribution vanes  59  or other equivalent devices may aid in directing the gas flow over a greater portion of the flow through element  50 .  
         [0037]    In an alternative embodiment shown in FIG. 7, the poison trap comprises a plurality of substrates  72 , preferably metallic monoliths, that are coated with poison trap components and located in the exhaust manifold  77  prior to the catalytic converter  79 .  
         [0038]    As the location of the poison trap is not critical, so long as it is located prior to catalytic elements which are susceptible to poisoning, it may conveniently take many forms. For example, a quite turbulent location is on the vanes and/or interior housing of the driven side of an exhaust turbocharger on engines so-equipped. It should be noted that if an emissions system contains a monolith coated with a catalyst system which is not subject to poisoning by acidic oxides, such a monolith or catalytic element may precede the poison trap, the latter then preceding one or more catalytic elements which are affected by acidic oxides.  
         [0039]    By the term “flow through” is meant a portion of the exhaust system of an internal combustion engine downstream from the exhaust valves or exhaust ports thereof, through which the exhaust of the internal combustion engine passes. By the terms “exhaust catalyst” or “emissions catalyst” is meant a component which is designed to lower the amount of CO, HC, particulate matter and/or NO x , emitted by an internal combustion engine, preferably exclusive of the “poison trap” of the present invention. By the terms “a” or “an” is meant “one or more” unless otherwise indicated.  
         [0040]    Those skilled in the art are familiar with the emissions catalysts and washcoats suitable for use with them. Reference may be had, for example, to U.S. Pat. Nos. 5,371,056; 5,610,117; and 6,103,660, which are incorporated herein for this purpose. These same washcoats, less all or substantially all the precious metal components, may be used for the poison trap washcoat. Additional poison trap components may be added to the washcoat prior to or after deposition.  
       EXAMPLE 1  
       [0041]    γ-aluminum oxide, stabilized with 2-4% by weight lanthanum (specific surface area 140 m 2 /g), is added to de-ionized water and the mixture wet pulverized to form a slurry. The solids content is 45% by weight. A monolithic metal carrier possessing 400 cells per square inch is immersed in the slurry so as to completely wet it. Excess slurry is then removed by blowing air through the carrier, after which it is dried (120° C., 2 h) and calcined in air (600° C., 2 h).  
       EXAMPLE 2  
       [0042]    A coating dispersion is prepared by mixing lanthanum-stabilized γ-aluminum oxide and barium acetate in a 10:1 weight ratio in de-ionized water. The solids content is 35% by weight. A hot tube™ of 6.5″ length and 1.5″ diameter is partially immersed in the slurry, such that the entire perforated portion of the tube is coated. Excess slurry is removed by draining, after which the tube is dried and calcined as above.  
       EXAMPLE 3  
       [0043]    An aqueous coating dispersion is prepared by mixing de-ionized water with γ-aluminum oxide (specific surface area 180 m 2 /g) and titanium dioxide (anatase, specific surface area 95 m 2 /g). The ratio of aluminum oxide to titanium dioxide is 2:1 by weight and the solids content is 40%. The mixture is wet pulverized to form a slurry and then used to coat a monolithic carrier by immersion. Excess slurry is allowed to drain from the carrier, after which it is dried in air (120° C., 1 h) and calcined (500° C., 2 h).  
       EXAMPLE 4  
       [0044]    A carrier is coated with γ-aluminum oxide as described in Example 1. The dried and calcined carrier is impregnated with magnesium by immersion in a 0.5 M aqueous solution of magnesium acetate, after which the body is dried (120° C., 1 h) and calcined (500° C., 2 h).  
       EXAMPLE 5  
       [0045]    A coating dispersion is prepared from lanthanum-stabilized yaluminum oxide, barium acetate and de-ionized water as described in Example 2. A ceranic monolith possessing 600 cells per square inch is partially immersed in the slurry, such that the front 1.5″ of the monolith (in the axial direction) is coated with the slurry. After drying and calcining, a palladium-containing catalyst washcoat is applied to the uncoated portion of the monolith by immersion of that portion in a slurry prepared according to the example given in U.S. Pat. No. 6,103,660. The monolith is dried at 120° C. in air and calcined for 2 hours at 600° C.  
         [0046]    While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.