Patent Publication Number: US-2005135977-A1

Title: Multi-part catalyst system for exhaust treatment elements

Description:
TECHNICAL FIELD  
      This invention relates generally to catalytic exhaust treatment elements and, more particularly, to catalytic exhaust treatment elements that include multi-part catalyst systems.  
     BACKGROUND  
      Internal combustion engines can produce exhaust streams that include various gases and combustion products. Some of these gases, such as nitrogen oxide gases (NOx) including, for example, nitrogen monoxide (NO) and nitrogen dioxide (NO 2 ), can contribute to environmental pollution in the form of acid rain and other undesirable effects. As a result, many regulations have been imposed on engine manufacturers in an attempt to reduce the levels of NOx emitted into the atmosphere.  
      NOx removal from the exhaust streams of lean burn engines can be especially challenging. Lean burn engines, which may include diesel engines as well as certain spark ignited engines, may operate with an excess of oxygen. Specifically, in a lean burn engine, more oxygen may be supplied to the engine than is necessary to stoichiometrically consume the fuel admitted to the engine. As a result, the exhaust streams of these lean burn engines may be rich in oxygen, which can limit the available techniques suitable for NOx removal.  
      To reduce the NOx concentrations in the exhaust stream of lean burning engines, a number of lean-NOx catalysts have been developed that may selectively reduce NOx in oxygen rich exhaust streams with hydrocarbon reductants. These lean-NOx catalytic systems may depend on the presence of sufficient levels of hydrocarbon species to be fully effective. The amount of hydrocarbons available in the exhaust streams of many lean burning engines can be low. Therefore, in some applications including as active catalytic systems, a hydrocarbon compound such as diesel fuel, for example, may be introduced into the exhaust stream in order to promote reduction of NOx compounds.  
      Several lean-NOx catalysts have been developed that include alumina in some form. Alumina is known as a durable material, and it has shown promise as a catalyst for lean-NOx reactions at high temperatures. Nevertheless, even alumina-based catalysts have proven problematic. For example, many catalysts or catalytic systems that have been used with lean burn engines suffer from low NOx conversion efficiencies, inadequate catalyst durability, low thermal stability, narrow effective temperature ranges, and NOx selectivity limited to only certain compounds.  
      In an attempt to address the shortcomings of lean-NOx catalysts, various catalyst configurations and compositions have been proposed. For example, U.S. Pat. No. 6,284,211 (“the &#39;211 patent”) describes a multi-component NOx-reducing catalyst that includes a silver oxide-based catalyst formed on one part of an exhaust gas cleaner and a tungsten and/or vanadium oxide-based catalyst formed on another part of the exhaust gas cleaner. Despite its multi-component catalyst, the exhaust gas cleaner of the &#39;211 patent may still suffer from one or more problems including low NOx conversion efficiencies, inadequate catalyst durability, low thermal stability, narrow effective temperature ranges, and NOx selectivity limited to only certain compounds.  
     SUMMARY OF THE INVENTION  
      One aspect of the present invention includes an exhaust treatment element that has a substrate and a first catalyst layer including a first promoter disposed on the substrate. The exhaust treatment element may also have a second catalyst layer including a second promoter disposed on the first catalyst layer.  
      A second aspect of the present invention includes a method of making an exhaust treatment element including supplying a substrate and forming a first catalyst layer including a first promoter on the substrate. A second catalyst layer including a second promoter may be formed on the first catalyst layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic illustration of an exhaust treatment system according to an exemplary embodiment of the present invention.  
       FIG. 2  is a pictorial representation of an exhaust treatment element according to an exemplary embodiment of the invention.  
       FIG. 3  is a diagrammatic partial cross-sectional representation of an exhaust treatment element including a single layer catalyst according to an exemplary embodiment of the invention.  
       FIG. 4  is a diagrammatic partial cross-sectional representation of an exhaust treatment element including a multi-layer catalyst according to an exemplary embodiment of the invention.  
       FIG. 5  is a graph that plots NOx conversion percentage as a function of temperature for various exhaust treatment elements in an exhaust stream containing NO.  
       FIG. 6  is a graph that plots NOx conversion percentage as a function of temperature for various exhaust treatment elements in an exhaust stream containing NO 2 . 
    
    
     DETAILED DESCRIPTION  
       FIG. 1  illustrates an exemplary exhaust system  10  that may include an exhaust treatment element  11  for treating an exhaust stream  12  transferred through exhaust conduit  13 . In one embodiment of the invention, exhaust stream  12  may be produced by a lean burn internal combustion engine  14 , which may be a diesel engine, a spark ignited engine, or any other type of engine that may be operated with an excess of oxygen. Further, engine  14  may operate in either a stationary role (e.g., power plants, generators, etc.) or in a mobile capacity (e.g., vehicles, moving equipment, etc.). As a common trait of many lean burn engines, the excess oxygen present during combustion may yield NOx in the exhaust stream. Exhaust treatment element  11  may be provided in system  10  to convert at least some of the NOx from exhaust stream  12  into more benign compounds such as nitrogen gas (N 2 ), carbon dioxide, and water vapor, for example. These compounds may then be expelled into the atmosphere through an exhaust conduit  15 . Exhaust system  10  may also include a reservoir  17  for housing a supplemental reductant that may be added to exhaust stream  12  through fluid inlet  16 .  
       FIG. 2  illustrates exhaust treatment element  11  according to an exemplary embodiment of the invention. Exhaust treatment element  11  may be cylindrical, as shown, or any other suitable shape depending on a particular application. A plurality of channels  20  may be formed in exhaust treatment element  11 . Channels  20  may extend through the entire length of exhaust treatment element  11  and allow the passage of exhaust stream  12  through exhaust treatment element  11 . Further, catalyst components that may aid in the conversion of NOx in exhaust stream  12  may be deposited on the walls of channels  20 . Exhaust treatment element  11  may include a substrate  30  with channels  20  extending therethrough in a honeycomb pattern. The term “honeycomb,” as used herein, may refer to a structure in which channels  20  have cross sections that are hexagonal, rectangular, square, circular, or any other shape. Substrate  30  may be a ceramic or metallic substrate and may include at least one of alumina, cordierite, titania, and FeCr. Other materials, however, may also be used to form substrate  30 .  
       FIG. 3  provides a diagrammatic partial cross-sectional, magnified view (i.e., looking at substrate  30  primarily through a single channel  20 ) of one embodiment of exhaust treatment element  11 . A series catalyst system  32  may be formed on substrate  30 . Series catalyst system  32  may include two or more catalysts of differing material composition formed on separate regions of substrate  30 . For example, in one exemplary embodiment, series catalyst system  32  may include a first catalyst disposed on a first region  37  ( FIG. 2 ) of substrate  30 . Series catalyst system  32  may also include a second catalyst disposed on a second region  38  ( FIG. 2 ) of substrate  30 . When exhaust treatment element  11  is placed in exhaust stream  12 , first region  37  may be located in exhaust stream  12  in a position upstream with respect to second region  38 , for example.  
      The first catalyst located in region  37  may include metal catalytic promoters such as, for example, tin, indium, gallium, germanium, molybdenum, vanadium, or any combination thereof, dispersed within a catalyst support material. Any other promoter that exhibits catalytic chemical behavior (e.g., partial oxidation of hydrocarbons) to the materials listed may also be used in the first catalyst in region  37 . The catalyst support material may include, for example, at least-one of γ-alumina, zeolite, aluminophosphates, hexaluminates, aluminosilicates, zirconates, titanosilicates, and titanates. In one exemplary embodiment, the first catalyst may include tin dispersed within the catalyst support material in an amount of about 5% to about 15% by weight. In certain embodiments, the catalyst support material may be γ-alumina, and the tin may be included in the first catalyst in an amount of about 9% to about 11% by weight.  
      In one embodiment, the second catalyst disposed in region  38  may include a metal catalytic promoter (e.g., silver, silver oxide, silver nitrate, or any other material that exhibits catalytic behavior similar to silver) dispersed within a catalyst support material. The catalyst support material may include at least one of γ-alumina, zeolite, aluminophosphates, hexaluminates, aluminosilicates, zirconates, titanosilicates, and titanates. The silver may be included in the second catalyst in an amount of about 0.5% to about 4% by weight. In certain embodiments, the catalyst support material may be γ-alumina, and the silver may be included in the second catalyst in an amount of about 1.5% to about 2.5% by weight.  
      Another embodiment of the invention may include two or more catalyst layers formed on substrate  30 , where each layer includes a different material composition.  FIG. 4  provides a partial cross-sectional, magnified view (i.e., looking at substrate  30  primarily through a channel  20 ) of one embodiment of a layered catalyst system  44 . For example, in one exemplary embodiment, a layered catalyst system  44  may include a first catalyst layer  45  disposed on substrate  30 . Layered catalyst system  44  may also include a second catalyst layer  46  disposed on first catalyst layer  45 . First catalyst layer  45  may cover substantially all of substrate  30 , or any portion thereof, and second catalyst layer  46  may cover at least a portion of first catalyst layer  45 .  
      In one embodiment of the invention, first catalyst layer  45  may include silver, silver oxide, silver nitrate, or any other material that exhibits catalytic behavior similar to silver, dispersed within a catalyst support material. The catalyst support material may include at least one of γ-alumina, zeolite, aluminophosphates, hexaluminates, aluminosilicates, zirconates, titanosilicates, and titanates. The silver may be included in first catalyst layer  45  in an amount of about 0.5% to about 4% by weight. In certain embodiments, the catalyst support material may be γ-alumina, and the silver may be included in first catalyst layer  45  in an amount of about 1.5% to about 2.5% by weight.  
      Second catalyst layer  46  may include metal catalytic promoters such as, for example, tin, indium, gallium, germanium, molybdenum, vanadium, any combination thereof, and any other materials exhibiting similar catalytic chemical behavior, dispersed within a catalyst support material. The catalyst support material may include, for example, at least one of γ-alumina, zeolite, aluminophosphates, hexaluminates, aluminosilicates, zirconates, titanosilicates, and titanates. In one exemplary embodiment, second catalyst layer  46  may include tin dispersed within the catalyst support material in an amount of about 5% to about 15% by weight. In certain embodiments, the catalyst support material may be γ-alumina, and the tin may be included in second catalyst layer  46  in an amount of about 9% to about 11% by weight.  
      Preparation of exhaust treatment element  11  may be accomplished in a variety of ways. An alumina honeycomb or cordierite substrate  30  may be supplied, and the catalysts of series catalyst system  32  and the catalyst layers of layered catalyst system  44  may be formed on substrate  30  using a washcoating technique, for example. As noted above, the catalysts of catalyst systems  32 ,  44  can include at least two components; i.e., a catalyst support material and a metal promoter. In one embodiment, the catalyst support material may be loaded with the metal promoter prior to the washcoating process. Alternatively, in another embodiment, the catalyst support material may be washcoated without first being loaded with the metal promoter. For example, the metal promoter may be loaded into the catalyst support material after the catalyst support material has already been deposited.  
      The catalyst support material may be formed using a variety of techniques. For example, powders of γ-alumina, zeolite, aluminophosphates, hex aluminates, aluminosilicates, zirconates, titanosilicates, titanates, or any other suitable catalyst support material may be produced using sol gel, incipient wetness, or precipitation techniques.  
      The catalyst support material in powder form may be dispersed in a solvent including water, for example, to form a slurry. Other solvents may be used depending on the requirements of a particular application. This slurry can be used in a washcoating process to deposit the catalyst support material onto a selected surface (e.g., substrate  30  and/or first catalyst layer  45 ). Specifically, the slurry may be applied to the surface in such a way that at least some of the catalyst support material in the slurry may be transferred to the selected surface. In one embodiment, the selected surface may be fully or partially immersed in the slurry. Alternatively, the slurry may be applied to the selected surface by brushing, spraying, wiping, or any other suitable method. After applying the slurry containing the catalyst support, the slurry may be allowed to dry leaving the catalyst support material deposited on the selected surface.  
      Loading of a metal promoter into the catalyst support material may be accomplished using, for example, an incipient wetness impregnation technique. Other techniques for dispersing the metal promoter material in the catalyst support material, however, may also be suitable. In the incipient wetness technique, the catalyst support material may be brought into contact with a slurry of the metal promoter by, for example, full or partial immersion in the metal promoter slurry. Alternatively, the metal promoter slurry may be applied by brushing, spraying, wiping, dripping, or any other suitable technique. In one embodiment of the invention, the amount of metal promoter slurry applied to the catalyst support material may be equal to or greater than a total pore volume of the catalyst support material.  
      Where the catalyst support material has not yet been deposited on a selected substrate, the catalyst support material, by itself, may be contacted with the metal promoter slurry. For example, a pipette may be used to introduce the metal promoter slurry to the catalyst support material. A ball mill may also be used to promote homogeneous mixing of the catalyst support material and the metal promoter slurry.  
      The metal promoter slurry may be formed by dissolving a metal precursor into a solvent such as water, for example. In one embodiment of the invention, the metal promoter may be silver or tin, and the metal precursors may include tin or silver nitrates, acetates, chlorides; carbonates, sulfates, or any other suitable precursors. Contacting the catalyst support material with the metal promoter slurry may have the effect of dispersing the metal promoter, e.g., tin or silver, into the catalyst support material.  
      Exhaust treatment element  11  may be subjected to additional processing steps including, for example, drying and/or calcining to remove volatile components. Drying may include placing exhaust treatment element  11  in a furnace at a particular temperature and for a particular amount of time. For example, exhaust treatment element  11  may be dried at a temperature of from about 100° C. to about 200° C. for several hours. Calcining may proceed for several hours at temperatures of greater than about 500° C. It will be appreciated that any particular time-temperature profile may be selected for the steps of drying and calcining without departing from the scope of the invention.  
      Exhaust treatment element  11  may aid in the reduction of NOx from exhaust stream  12  ( FIG. 1 ). The lean-NOx catalytic reaction is a complex process including many steps. One of the reaction mechanisms, however, that may proceed in the presence of exhaust treatment  11  can be summarized by the following reaction equations: 
 
NO+O 2 →NOx   (1) 
 
HC+O 2 →oxygenated HC   (2) 
 
NOx+oxygenated HC+O 2 →N 2 +CO 2 +H 2 O   (3) 
 
      The catalyst of region  37  ( FIG. 2 ) and second catalyst layer  46  ( FIG. 4 ), which may include tin dispersed within a catalyst support material, may catalyze the reaction of equation (2). Specifically, the presence of tin in these catalysts may aid in the reformation of hydrocarbon reducing agents to produce activated, oxygenated hydrocarbons such as aldehyde and acrolein. Ultimately, these oxygenated hydrocarbons may combine with NOx compounds to form organo-nitrogen containing compounds. Over a silver containing catalyst, such as first catalyst layer  45 , these materials may decompose to isocyanate (NCO) or cyanide groups and eventually yield nitrogen gas (N 2 ) through a series of reactions, which are summarized by equations (1)-(3).  
      The catalyst of region  38  ( FIG. 2 ) and first catalyst layer  45  ( FIG. 4 ), which may include silver dispersed within a catalyst support material, may catalyze the reduction of NOx to N 2  gas, as shown in equation (3). The multi-part catalyst systems  32 ,  44  of the present invention may exhibit a synergistic effect derived from its components. For example, the tin-containing catalyst can promote the formation of oxygenated hydrocarbons, which are consumed in the reaction catalyzed by the silver-containing catalyst. Thus, the catalyst components of multi-part catalyst systems  32 ,  44  can work together to increase the efficiency the NOx reduction reaction.  
      While not necessary, a supplemental hydrocarbon reductant may be introduced into exhaust stream  12  ( FIG. 1 ) in order to aid in the production of oxygenated hydrocarbons, as represented by equation (2). Supplemental reductants may include propene, ethanol, diesel fuel, or any other suitable compounds. As illustrated in  FIG. 1 , exhaust system  10  may include a fluid inlet  16  disposed on exhaust conduit  13  for introducing a supplemental reductant. Further, the supplemental reductant may be stored in a reservoir  17 . In one embodiment of the invention, a supplemental reductant consisting of diesel fuel may be supplied to exhaust stream  12 . In this embodiment, reservoir  17  may coincide with the fuel tank of a vehicle.  
       FIG. 5  is a graph that plots NOx conversion % as a function of temperature for NO reduction over various catalysts. Curve  51  includes data for a catalyst of 10% tin by weight dispersed in alumina; Curve  52  includes data for a catalyst of 2% silver by weight dispersed in alumina; Curve  53  includes data for a catalyst formed by physically mixing 10% tin and 2% silver by weight in an alumina support material; Curve  54  includes data for one embodiment of the multi-part catalyst system of the present invention (e.g., one catalyst component including 10% by weight of tin dispersed in alumina and a separate catalyst component including 2% by weight of silver dispersed in alumina). The exhaust stream flowed over each of the catalysts included 0.1% NO, 0.1% propene, 9% O 2 , and 7% H 2 O at a space velocity of 30,000 h −1 . As shown in  FIG. 5 , the NO conversion efficiency of the multi-part catalyst system (Curve  54 ) is significantly higher than the single component catalysts (Curve  51  and Curve  52 ) or the physical mixture catalyst (Curve  53 ).  
       FIG. 6  is a graph that plots NOx conversion % as a function of temperature for NO 2  reduction over various catalysts. Curve  61  includes data for a catalyst of 10% tin by weight dispersed in alumina; Curve  62  includes data for a catalyst of 2% silver by weight dispersed in alumina; Curve  63  includes data for a catalyst formed by physically mixing 10% tin and 2% silver by weight in an alumina support material; Curve  64  includes data for one embodiment of the multi-part catalyst system of the present invention (e.g., one catalyst component including 10% by weight of tin dispersed in alumina and a separate catalyst component including 2% by weight of silver dispersed in alumina). The exhaust stream flowed over each of the catalysts included 0.1% NO 2 , 0.1% propene, 9% O 2 , and 7% H 2 O at a space velocity of 30,000 h −1 . As shown in  FIG. 6 , the NO 2  conversion efficiency of the multi-part catalyst system (Curve  64 ) is significantly higher than the single component catalysts (Curve  61  and Curve  62 ) or the physical mixture catalyst (Curve  63 ).  
      Industrial Applicability  
      The disclosed multi-part lean-NOx catalyst systems may be useful in any of a wide variety of applications where reduction of NOx from exhaust streams would be desirable. A multi-part lean NOx catalyst may provide a synergy effect in the reduction of NOx compounds. Specifically, the NOx reduction performance of the multi-part catalyst system may be greater than the NOx reduction performance of any of the catalyst components, or mixtures thereof, taken separately. The catalyst systems of the present invention have demonstrated NOx conversion efficiencies for both NO and NO 2  of about 80% or greater.  
      Further, the disclosed multi-part catalyst systems may offer high deNOx conversion efficiencies and broad operating temperature windows in the presence of various reductants. The catalysts may also exhibit resistance to poisoning or deactivation from the presence of SO 2  in an exhaust stream.  
      It will be apparent to those skilled in the art that various modifications and variations can be made in the described catalyst systems without departing from the scope of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the invention being indicated by the following claims and their equivalents.