WO1996001142A1 - Hybrid exhaust gas catalyst - Google Patents

Hybrid exhaust gas catalyst Download PDF

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Publication number
WO1996001142A1
WO1996001142A1 PCT/GB1995/001499 GB9501499W WO9601142A1 WO 1996001142 A1 WO1996001142 A1 WO 1996001142A1 GB 9501499 W GB9501499 W GB 9501499W WO 9601142 A1 WO9601142 A1 WO 9601142A1
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WO
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Prior art keywords
catalyst
hybrid
lean
exhaust gas
engine
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PCT/GB1995/001499
Other languages
French (fr)
Inventor
Haren Sakarlal Gandhi
Jeffrey Scott Hepburn
Original Assignee
Ford Motor Company Limited
Ford Werke Ag
Ford France S.A.
Ford Motor Company Of Canada Ltd.
Ford Motor Company
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Application filed by Ford Motor Company Limited, Ford Werke Ag, Ford France S.A., Ford Motor Company Of Canada Ltd., Ford Motor Company filed Critical Ford Motor Company Limited
Priority to AU27491/95A priority Critical patent/AU2749195A/en
Publication of WO1996001142A1 publication Critical patent/WO1996001142A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9445—Simultaneously removing carbon monoxide, hydrocarbons or nitrogen oxides making use of three-way catalysts [TWC] or four-way-catalysts [FWC]
    • B01D53/945—Simultaneously removing carbon monoxide, hydrocarbons or nitrogen oxides making use of three-way catalysts [TWC] or four-way-catalysts [FWC] characterised by a specific catalyst
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9404—Removing only nitrogen compounds
    • B01D53/9409—Nitrogen oxides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9459—Removing one or more of nitrogen oxides, carbon monoxide, or hydrocarbons by multiple successive catalytic functions; systems with more than one different function, e.g. zone coated catalysts
    • B01D53/9463—Removing one or more of nitrogen oxides, carbon monoxide, or hydrocarbons by multiple successive catalytic functions; systems with more than one different function, e.g. zone coated catalysts with catalysts positioned on one brick
    • B01D53/9468—Removing one or more of nitrogen oxides, carbon monoxide, or hydrocarbons by multiple successive catalytic functions; systems with more than one different function, e.g. zone coated catalysts with catalysts positioned on one brick in different layers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/024—Multiple impregnation or coating
    • B01J37/0244—Coatings comprising several layers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/024—Multiple impregnation or coating
    • B01J37/0246—Coatings comprising a zeolite
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/10—Noble metals or compounds thereof
    • B01D2255/102—Platinum group metals
    • B01D2255/1021—Platinum
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/10—Noble metals or compounds thereof
    • B01D2255/102—Platinum group metals
    • B01D2255/1023—Palladium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/10—Noble metals or compounds thereof
    • B01D2255/102—Platinum group metals
    • B01D2255/1025—Rhodium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/204—Alkaline earth metals
    • B01D2255/2042—Barium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/206—Rare earth metals
    • B01D2255/2063—Lanthanum
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/207—Transition metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/207—Transition metals
    • B01D2255/20761—Copper
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/50—Zeolites
    • B01D2255/504—ZSM 5 zeolites
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00—Road transport of goods or passengers
    • Y02T10/10—Internal combustion engine [ICE] based vehicles
    • Y02T10/12—Improving ICE efficiencies

Definitions

  • This invention relates to a hybrid exhaust gas catalyst, and more particularly to a catalyst for removing nitrogen oxides, hydrocarbons and carbon monoxide during engine operation utilising both stoichiometric and lean (oxygen rich) air/fuel combustion ratios.
  • TWCs three-way catalysts
  • HC, CO, and NO ⁇ three constituents
  • conventional three-way catalysts are restricted to operation in a very narrow window of A/F ratio around the stoichiometric point.
  • A/F ratios which are very near to the stoichiometric value.
  • Lean-burn engines operate at a substantially higher A/F ratios than stoichiometric engines, i.e., greater than about 14.7, more generally between about 19 and 27.
  • a dedicated lean-burn vehicle is projected to employ lean-burn operation during all driving modes except for full acceleration.
  • Another approach is to operate in the lean-burn mode only during idle, deceleration and select cruise modes.
  • lean-burn catalysts are those based on transition metal exchanged zeolites. Such lean-burn catalysts convert the nitrogen oxides (NO ⁇ ) by means of selective reduction by hydrocarbons present in lean-burn exhaust gases.
  • One such catalyst is disclosed in U.S. Patent 5,155,077 to Montreuil et al. entitled "Catalyst for Purification of Lean-Burn Engine Exhaust Gas”. It discloses as a catalyst a dual-phase zeolite having a transition containing zeolite phase and a metal containing oxide phase.
  • LNC lean N0 ⁇ catalyst
  • the present invention surprisingly has been found to overcome such deficiencies by providing a hybrid TWC/LNC catalyst which avoids the problem of delayed light-off of the series TWC during cold start of the engine. Further, this hybrid catalyst is more compact than such series systems and as such provides for relative ease of packaging compared to the series approach.
  • This invention is an hybrid catalyst for purification of stoichiometric/lean-burn engine exhaust gas.
  • the hybrid catalyst comprises a first catalyst consisting of a catalytic material capable of reducing nitrogen oxides (NO ⁇ v under lean-burn engine conditions, and a second catalyst consisting of a three-way catalyst.
  • the catalytic material may be an intimate mixture of the first catalyst and the second catalyst or a layered combination of them.
  • the layered combination is employed, with the first catalyst being layered on the second catalyst which is carried on a substrate, like cordierite.
  • the catalysts would be applied in washcoats.
  • the first catalyst preferably is transition metal exchanged high-silica zeolite, more preferably being copper-exchanged high silica ZSM5 and the three-way catalyst preferably comprises a noble metal being, most preferably, palladium.
  • the invention comprises a method of purifying such exhaust gas employing the hybrid catalyst disclosed above.
  • Fig. 1 is a graph showing the NO ⁇ conversion for an embodiment of the hybrid LNC/TWC of the present invention as compared to a conventional TWC as a function of A/F;
  • Fig. 2 is a graph showing cold start hydrocarbon conversion efficiency for a comparative series LNC + TWC catalyst as compared to an embodiment of a hybrid LNC/TWC as in the present invention.
  • the catalyst of this invention comprises a hybrid of two catalysts, a first catalyst which is a lean N0 ⁇ catalyst (LNC) which is capable of selectively reducing NO ⁇ with hydrocarbons in the presence of excess oxygen and a second catalyst which is a three-way catalyst (TWC) .
  • LNC lean N0 ⁇ catalyst
  • TWC three-way catalyst
  • the first catalyst functions primarily as a lean-burn conversion catalyst
  • the second catalyst functioning primarily as a stoichiometric conversion catalyst.
  • Each is however capable of converting some of the other exhaust gas components to a lesser extent.
  • the invention hybrid catalyst may be in the form of an intimate mixture of the first catalyst and the second catalyst, or in the form of a combination of layers of these two catalysts.
  • the hybrid catalyst invention comprises a layered combination of the TWC and the LNC, where the LNC is overcoated on the TWC (which is carried on a substrate like a cordierite monolith) . Locating the LNC in the outermost layer (such that the exhaust gas contacts the LNC first) maximises the availability of hydrocarbons for lean NO ⁇ catalysis, since the LNC has minimal oxidising activity.
  • the exhaust gas subsequently would be expected to diffuse through the LNC to contact the TWC for further catalytic activity toward the exhaust gas.
  • the catalysts are employed in the invention as part of a washcoat comprising these catalyst materials, individually in separate washcoat layers or in a mixture of the two catalysts in a single washcoat layer.
  • the lean NO ⁇ catalyst employed in the hybrid catalyst invention preferably is a transition metal-exchanged zeolite, desirably a high silica zeolite having a Si0 2 /Al 2 0 3 molar ratio which preferably exceeds about 10, and is more preferably up to about 60 (see U.S. Patent 4,297,328, which is expressly incorporated herein by reference for teaching of other zeolites or class or zeolites that may be used herein) .
  • transition metal used to exchange the preferred zeolite herein is limited to the group of copper, cobalt, nickel, chromium, iron, manganese, silver, zinc, calcium, and compatible mixtures thereof; transition metal as used herein includes the elemental metal itself as well as the metal oxide thereof.
  • the preferred transition metal is copper, and the preferred zeolite is a copper exchanged ZSM5 zeolite. The transition metal is provided into the zeolite by ion exchange.
  • a sodium, hydrogen, or ammonium zeolite is contacted by an aqueous solution of another cation, in this case an aqueous solution of a soluble transition metal compound, such as copper acetate, wherein replacement of the sodium, hydrogen or ammonium ion by copper ion takes place.
  • a soluble transition metal compound such as copper acetate
  • the zeolite is generally washed to remove excess surface transition metal compound. It is not necessary to do so, however.
  • the lean NO ⁇ catalyst used in the invention hybrid catalyst need not, however, necessarily be an ion-exchanged zeolite material.
  • suitable materials for the LNC portion of the hybrid catalyst could consist of a transition metal such as copper, cobalt, nickel, chromium, iron, manganese, silver, zinc, or precious metal such as platinum, palladium, rhodium, rhenium, osmium, iridium and compatible mixtures thereof dispersed within the pores of a support material such as alumina, silica, titania, zirconia.
  • the first catalyst is capable of selectively reducing nitrogen oxides with hydrocarbons in the presence of oxygen so that at least about 30% of the nitrogen oxides are converted, more preferably this amount is at least about 50%, under the operating temperatures of lean-burn engines.
  • the second catalyst of the hybrid catalyst of this invention is a three-way catalyst (TWC) which is suitable to simultaneously convert the components of exhaust gases, such as those from an internal combustion engine like hydrocarbons, carbon monoxide, and nitrogen oxides into more desirable species like carbon dioxide, water, and nitrogen under near stoichiometric engine conditions.
  • TWC three-way catalyst
  • Such catalysts are well known to those skilled in the art.
  • Exemplary of suitable three-way catalysts include conventional three-way catalysts containing either platinum and rhodium, palladium and rhodium, or palladium in combination with various promoters and stabilisers such as ceria, barium oxide, lanthanum oxide, or strontium supported on an high surface area carrier such as alumina. Still other TWCs which may be employed in this invention will be apparent to those skilled in the art in view of the present disclosure.
  • the first catalyst and the second catalyst are preferably employed in this invention as disclosed above as washcoat materials which are deposited directly onto a substrate.
  • the substrate is generally made of an electrically insulating material suitable for high temperature environments including, but not limited to, materials such as cordierite, mullite, etc.
  • the substrate may be in any suitable configuration, often being employed as a monolithic honeycomb structure, spun fibres, corrugated foils or layered materials. Still other materials and configurations useful in this invention and suitable in an exhaust gas system will be apparent to those skilled in the art in view of this disclosure.
  • the three-way catalyst and lean NO ⁇ catalyst materials can be applied to the substrate as a mixture or in two sequential but separate steps in a manner which would be readily apparent to those skilled in the art of catalyst manufacture.
  • the three-way catalyst material would be applied to the substrate first followed by a drying and calcination step then the lean NO ⁇ catalyst material would be applied directly on top of the previously deposited three-way catalyst material followed by a second drying and calcination.
  • the preferred outcome of the preparation procedure is a catalyst with an inner layer of three-way catalyst material and an outer most layer of lean NO ⁇ catalyst material.
  • a high silica zeolite ion-exchanged with copper was obtained from a commercial source as Cu-ZSM5.
  • the material contained 3% by weight and was in a powder form which was suitable for direct use.
  • the above material was ball milled then mixed with distilled water to produce a slurry.
  • the resulting slurry was applied as an outer layer to a 92in cordierite monolith which carried a palladium based three- way catalyst washcoat to obtain a 15 wt% loading of the copper zeolite.
  • the above three-way catalyst contained 110gm/ft 3 of palladium on alumina as a carrier and was obtained from a commercial source.
  • Fig. 1 shows a graph of NO ⁇ conversion efficiency as a function of A/F ratio for the hybrid LNC/TWC of this example compared to the TWC alone.
  • the data was obtained by using a simulated automobile exhaust gas produced by the combustion of isooctaine fuel in a laboratory pulse flame combustor.
  • the catalysts were tested at a space velocity of 20,000hr -1 and at a gas temperature of 500°C.
  • the invention hybrid LNC/TWC provides for significantly improved NO ⁇ conversion efficiency on the lean side of stoichiometry compared to the TWC alone.
  • Fig. 2 compares the cold start hydrocarbon conversions for the hybrid LNC/TWC " and the LNC+TWC in series (described in this example) during the first 120 seconds of the FTP test.
  • hydrocarbon conversion efficiency during cold starting of the engine is significantly better with the hybrid LNC/TWC.
  • the LNC+TWC in series With the LNC+TWC in series, light- off of the TWC is greatly delayed because the LNC acts as a heat sink which slows down the warming of the TWC to its necessary activation temperature.
  • cold start hydrocarbon conversion efficiency is less than desirable to attempt to meet projected government hydrocarbon emission standards.
  • This same effect is further reflected in the hydrocarbon tailpipe emissions for Bag One of the FTP test as shown in Table 1. Bag One tailpipe HC emissions are more than 50% higher with the LNC+TWC in series compared the hybrid LNC/TWC.
  • Table 1 also compares NO tailpipe emissions and NO ⁇ conversion efficiency for the hybrid LNC/TWC to the LNC+TWC in series.
  • NO ⁇ tailpipe emissions with the LNC+TWC in series are slightly lower. However, this difference is not deemed to be significant.
  • the availability of hydrocarbons for lean N0 ⁇ catalysis was maximised, thereby providing for nearly equivalent lean NO ⁇ conversion efficiencies compared to the LNC+TWC in series.
  • Feedgas 2.97 g/ml 2.75 g/ml 1.12 g/ml 1.07 g/ml

Abstract

This invention is directed to a hybrid catalyst for the purification of engine exhaust gas generated by an engine which is operated under both stoichiometric and leanburn (oxygen rich) air/fuel combustion ratios. The hybrid catalyst comprises a first catalyst consisting of a catalytic material capable of reducing nitrogen oxides under lean-burn engine operating conditions and a second catalyst consisting of a three-way catalyst. Preferably, the hybrid catalyst is a layered combination of the catalysts where the first catalyst is layered on the second catalyst which is carried on a substrate like cordierite.

Description

HYBRID EXHAUST GAS CATALYST
This invention relates to a hybrid exhaust gas catalyst, and more particularly to a catalyst for removing nitrogen oxides, hydrocarbons and carbon monoxide during engine operation utilising both stoichiometric and lean (oxygen rich) air/fuel combustion ratios.
The most commonly used catalysts for the purification of automobile engine exhaust are called three-way catalysts (TWCs) because they simultaneously oxidise hydrocarbons and carbon monoxide while reducing nitrogen oxides. In order to obtain efficient removal of all three constituents (HC, CO, and NOχ) , however, conventional three-way catalysts are restricted to operation in a very narrow window of A/F ratio around the stoichiometric point. As a result of this constraint, most automotive engines are presently operated at A/F ratios which are very near to the stoichiometric value.
At this time, however, there is a great deal of interest in the use of lean-burn engines due to the potential fuel economy benefits, estimated to be in the range of 6-10%. Lean-burn engines operate at a substantially higher A/F ratios than stoichiometric engines, i.e., greater than about 14.7, more generally between about 19 and 27. As a result, substantial research is presently being directed to catalysts which would function in lean- burn exhaust situations. A dedicated lean-burn vehicle is projected to employ lean-burn operation during all driving modes except for full acceleration. Another approach is to operate in the lean-burn mode only during idle, deceleration and select cruise modes.
Considerable success has been achieved in lean-burn catalytic oxidation of unburned hydrocarbons and carbon monoxide, but in lean-burn situations the conversion of the nitrogen oxides has proven to be a much more difficult problem. This is because the reducing substances in exhaust gases (such as carbon monoxide and hydrogen) tend to react more quickly with the free excess oxygen present in the exhaust gas than with the oxygen associated with nitrogen in the nitrogen oxide. Exemplary of lean-burn catalysts are those based on transition metal exchanged zeolites. Such lean-burn catalysts convert the nitrogen oxides (NOχ) by means of selective reduction by hydrocarbons present in lean-burn exhaust gases. One such catalyst is disclosed in U.S. Patent 5,155,077 to Montreuil et al. entitled "Catalyst for Purification of Lean-Burn Engine Exhaust Gas". It discloses as a catalyst a dual-phase zeolite having a transition containing zeolite phase and a metal containing oxide phase.
In order to provide for efficient NOχ removal under both stoichiometric and lean engine operation, one might consider employing a lean N0χ catalyst (LNC) in series with a conventional TWC; the LNC being positioned upstream of the TWC because the LNC needs hydrocarbons for the selective reduction of NO . In such a series approach, however, during cold start we have found that there is a significant delay in light-off of the TWC catalyst since the LNC catalyst acts as a heat sink, slowing down the warming of the TWC to it necessary light-off temperature. Hence the activity of the three-way catalyst particularly to oxidise exhaust components, e.g., hydrocarbons, is less than satisfactory.
The present invention surprisingly has been found to overcome such deficiencies by providing a hybrid TWC/LNC catalyst which avoids the problem of delayed light-off of the series TWC during cold start of the engine. Further, this hybrid catalyst is more compact than such series systems and as such provides for relative ease of packaging compared to the series approach.
This invention is an hybrid catalyst for purification of stoichiometric/lean-burn engine exhaust gas. The hybrid catalyst comprises a first catalyst consisting of a catalytic material capable of reducing nitrogen oxides (NOχv under lean-burn engine conditions, and a second catalyst consisting of a three-way catalyst. The catalytic material may be an intimate mixture of the first catalyst and the second catalyst or a layered combination of them. Preferably the layered combination is employed, with the first catalyst being layered on the second catalyst which is carried on a substrate, like cordierite. Generally the catalysts would be applied in washcoats. The first catalyst preferably is transition metal exchanged high-silica zeolite, more preferably being copper-exchanged high silica ZSM5 and the three-way catalyst preferably comprises a noble metal being, most preferably, palladium. According to another embodiment, the invention comprises a method of purifying such exhaust gas employing the hybrid catalyst disclosed above.
The invention will now be described further, by way of example, with reference to the accompanying drawings, in which:
Fig. 1 is a graph showing the NOχ conversion for an embodiment of the hybrid LNC/TWC of the present invention as compared to a conventional TWC as a function of A/F; and
Fig. 2 is a graph showing cold start hydrocarbon conversion efficiency for a comparative series LNC + TWC catalyst as compared to an embodiment of a hybrid LNC/TWC as in the present invention.
The catalyst of this invention comprises a hybrid of two catalysts, a first catalyst which is a lean N0χ catalyst (LNC) which is capable of selectively reducing NOχ with hydrocarbons in the presence of excess oxygen and a second catalyst which is a three-way catalyst (TWC) . In the present invention, the first catalyst functions primarily as a lean-burn conversion catalyst, and the second catalyst functioning primarily as a stoichiometric conversion catalyst. Each is however capable of converting some of the other exhaust gas components to a lesser extent.
As disclosed above, the invention hybrid catalyst may be in the form of an intimate mixture of the first catalyst and the second catalyst, or in the form of a combination of layers of these two catalysts. Preferably, the hybrid catalyst invention comprises a layered combination of the TWC and the LNC, where the LNC is overcoated on the TWC (which is carried on a substrate like a cordierite monolith) . Locating the LNC in the outermost layer (such that the exhaust gas contacts the LNC first) maximises the availability of hydrocarbons for lean NOχ catalysis, since the LNC has minimal oxidising activity. In such a layered arrangement of the invention hybrid catalyst, the exhaust gas subsequently would be expected to diffuse through the LNC to contact the TWC for further catalytic activity toward the exhaust gas. More preferably, the catalysts are employed in the invention as part of a washcoat comprising these catalyst materials, individually in separate washcoat layers or in a mixture of the two catalysts in a single washcoat layer.
The lean NOχ catalyst employed in the hybrid catalyst invention preferably is a transition metal-exchanged zeolite, desirably a high silica zeolite having a Si02/Al203 molar ratio which preferably exceeds about 10, and is more preferably up to about 60 (see U.S. Patent 4,297,328, which is expressly incorporated herein by reference for teaching of other zeolites or class or zeolites that may be used herein) . The transition metal used to exchange the preferred zeolite herein is limited to the group of copper, cobalt, nickel, chromium, iron, manganese, silver, zinc, calcium, and compatible mixtures thereof; transition metal as used herein includes the elemental metal itself as well as the metal oxide thereof. The preferred transition metal is copper, and the preferred zeolite is a copper exchanged ZSM5 zeolite. The transition metal is provided into the zeolite by ion exchange. Generally, a sodium, hydrogen, or ammonium zeolite is contacted by an aqueous solution of another cation, in this case an aqueous solution of a soluble transition metal compound, such as copper acetate, wherein replacement of the sodium, hydrogen or ammonium ion by copper ion takes place. It is advantageous to provide as much transition metal ion in the zeolite as possible since the amount of transition metal present in the zeolite is directly related to the catalytic activity of the first catalyst. Preferably this is at least 3% up to a maximum determined by the silica/alumina ration. After replacing the sodium, hydrogen, or ammonium ion with the metal ion, the zeolite is generally washed to remove excess surface transition metal compound. It is not necessary to do so, however.
The lean NOχ catalyst used in the invention hybrid catalyst need not, however, necessarily be an ion-exchanged zeolite material. Other suitable materials for the LNC portion of the hybrid catalyst could consist of a transition metal such as copper, cobalt, nickel, chromium, iron, manganese, silver, zinc, or precious metal such as platinum, palladium, rhodium, rhenium, osmium, iridium and compatible mixtures thereof dispersed within the pores of a support material such as alumina, silica, titania, zirconia. Preferably, the first catalyst is capable of selectively reducing nitrogen oxides with hydrocarbons in the presence of oxygen so that at least about 30% of the nitrogen oxides are converted, more preferably this amount is at least about 50%, under the operating temperatures of lean-burn engines.
The second catalyst of the hybrid catalyst of this invention is a three-way catalyst (TWC) which is suitable to simultaneously convert the components of exhaust gases, such as those from an internal combustion engine like hydrocarbons, carbon monoxide, and nitrogen oxides into more desirable species like carbon dioxide, water, and nitrogen under near stoichiometric engine conditions. Such catalysts are well known to those skilled in the art. Exemplary of suitable three-way catalysts include conventional three-way catalysts containing either platinum and rhodium, palladium and rhodium, or palladium in combination with various promoters and stabilisers such as ceria, barium oxide, lanthanum oxide, or strontium supported on an high surface area carrier such as alumina. Still other TWCs which may be employed in this invention will be apparent to those skilled in the art in view of the present disclosure.
The first catalyst and the second catalyst are preferably employed in this invention as disclosed above as washcoat materials which are deposited directly onto a substrate. The substrate is generally made of an electrically insulating material suitable for high temperature environments including, but not limited to, materials such as cordierite, mullite, etc. The substrate may be in any suitable configuration, often being employed as a monolithic honeycomb structure, spun fibres, corrugated foils or layered materials. Still other materials and configurations useful in this invention and suitable in an exhaust gas system will be apparent to those skilled in the art in view of this disclosure. The three-way catalyst and lean NOχ catalyst materials can be applied to the substrate as a mixture or in two sequential but separate steps in a manner which would be readily apparent to those skilled in the art of catalyst manufacture. Preferably, the three-way catalyst material would be applied to the substrate first followed by a drying and calcination step then the lean NOχ catalyst material would be applied directly on top of the previously deposited three-way catalyst material followed by a second drying and calcination. The preferred outcome of the preparation procedure is a catalyst with an inner layer of three-way catalyst material and an outer most layer of lean NOχ catalyst material.
Exam le
A high silica zeolite ion-exchanged with copper was obtained from a commercial source as Cu-ZSM5. The material contained 3% by weight and was in a powder form which was suitable for direct use. The above material was ball milled then mixed with distilled water to produce a slurry. The resulting slurry was applied as an outer layer to a 92in cordierite monolith which carried a palladium based three- way catalyst washcoat to obtain a 15 wt% loading of the copper zeolite. The above three-way catalyst contained 110gm/ft3 of palladium on alumina as a carrier and was obtained from a commercial source. Final drying was carried out at 120°C for three hours followed by calcination in air at 500°C for four hours. This preparation procedure provided for a hybrid catalyst according to this invention consisting of an inner most washcoat layer of three-way catalyst material and an outer most washcoat layer of a lean N0χ material. Additionally, for comparison to the present invention embodiment, a slurry containing another portion of the copper exchanged zeolite material of this example was applied to a blank cordierite monolithic carrier in accordance with the procedure used to prepare the example hybrid LNC/TWC. The resulting comparative single-phase zeolite catalyst was tested in series with a three-way catalyst (of the same example type described above) .
Fig. 1 shows a graph of NOχ conversion efficiency as a function of A/F ratio for the hybrid LNC/TWC of this example compared to the TWC alone. The data was obtained by using a simulated automobile exhaust gas produced by the combustion of isooctaine fuel in a laboratory pulse flame combustor. The catalysts were tested at a space velocity of 20,000hr-1 and at a gas temperature of 500°C. As shown in Fig. 1, the invention hybrid LNC/TWC provides for significantly improved NOχ conversion efficiency on the lean side of stoichiometry compared to the TWC alone.
Testing was also carried out using a 1.5L Honda Civic VTEC-E which is a dedicated lean-burn vehicle. Fig. 2 compares the cold start hydrocarbon conversions for the hybrid LNC/TWC "and the LNC+TWC in series (described in this example) during the first 120 seconds of the FTP test. As can be seen from Fig. 2, hydrocarbon conversion efficiency during cold starting of the engine is significantly better with the hybrid LNC/TWC. With the LNC+TWC in series, light- off of the TWC is greatly delayed because the LNC acts as a heat sink which slows down the warming of the TWC to its necessary activation temperature. As a result, cold start hydrocarbon conversion efficiency is less than desirable to attempt to meet projected government hydrocarbon emission standards. This same effect is further reflected in the hydrocarbon tailpipe emissions for Bag One of the FTP test as shown in Table 1. Bag One tailpipe HC emissions are more than 50% higher with the LNC+TWC in series compared the hybrid LNC/TWC.
Table 1 also compares NO tailpipe emissions and NOχ conversion efficiency for the hybrid LNC/TWC to the LNC+TWC in series. NOχ tailpipe emissions with the LNC+TWC in series are slightly lower. However, this difference is not deemed to be significant. By locating the zeolitic LNC in the outer most washcoat layer, the availability of hydrocarbons for lean N0χ catalysis was maximised, thereby providing for nearly equivalent lean NOχ conversion efficiencies compared to the LNC+TWC in series.
Table 1
Baσ One HC CVS C/H NO„
Hybrid Series Hybrid Series
Feedgas 2.97 g/ml 2.75 g/ml 1.12 g/ml 1.07 g/ml
Tailpipe 0.35 g/ml 0.52 g/ml 0.66 g/ml 0.57 g/ml
Efficiency 88% 80% 42% 46%

Claims

1. A hybrid catalyst for purification of stoichiometric and lean-burn engine exhaust gas, comprising: a hybrid of two catalysts, the first catalyst consisting of a catalytic material capable of reducing nitrogen oxides under lean-burn engine conditions, and the second catalyst consisting of a three-way catalyst.
2. A hybrid catalyst according to claim 1, wherein said first catalyst is selected from transition metal- exchanged high-silica zeolites.
3. A hybrid catalyst according to claim 2, wherein said transition metal is selected from the group consisting of Cu, Co, Ni, Cr, Fe, Mn, Ag, Zn, Ca, and compatible mixtures thereof.
4. A hybrid catalyst according to claim 1, wherein said three-way catalyst comprises a noble metal selected from the group consisting of palladium, platinum and rhodium, or palladium and rhodium.
5. A hybrid catalyst according to any one of the preceding claims, wherein said first catalyst and said second catalyst are in the form of: (1) an intimate- mixture thereof, or (2) a layer of said first catalyst carried on a layer of said second catalyst.
6. A hybrid catalyst according to any one of the preceding claims, wherein said hybrid catalyst is present on a substrate selected from cordierite or mullite.
7. A hybrid catalyst according to any one of the preceding claims, wherein said exhaust gas is that produced by an internal combustion engine.
8. A hybrid catalyst according to claim 7, wherein said engine is an automotive engine.
9. A hybrid catalyst according to claim 1, wherein the first catalyst consists of a copper-exchanged high- silica zeolite having a Si02/Al203 molar ratio which exceeds about 10, and the second catalyst consists of a three-way catalyst employed in an alumina washcoat, the second catalyst being a noble metal.
10. A hybrid catalyst according to claim 9, wherein said zeolite is a ZSM5 zeolite.
11. A hybrid catalyst according to claim 9 or 10, wherein said noble metal is palladium, platinum and rhodium, or palladium and rhodium.
12. A method for purification of stoichiometric and lean-burn engine exhaust gas, comprising: providing a hybrid of two catalysts, the first catalyst consisting of a catalytic material capable of reducing nitrogen oxides under lean-burn engine conditions, and the second catalyst consisting of a three- way catalyst, in the exhaust gas system of said stoichiometric/lean-burn engine.
13. A method according to claim 12, wherein said first catalyst is selected from transition metal-exchanged high-silica zeolites.
14. A method according to claim 13, wherein said transition metal is selected from the group consisting of Cu, Co, Ni, Cr, Fe, Mn, Ag, Zn, Ca, and compatible mixtures thereof.
15. A method according to any one of the preceding claims, wherein said three-way catalyst comprises a noble metal selected from the group consisting of palladium, platinum and rhodium, or palladium and rhodium.
16. A method according to any one of the preceding claims, wherein said first catalyst and said second catalyst are in the form of: (1) an intimate mixture thereof, or (2) a layer of said first catalyst carried on a layer of said second catalyst.
17. A method according to claim 12, wherein said hybrid catalyst is present on a substrate selected from cordierite or mullite.
18. A method according to claim 12, wherein said exhaust gas is that produced by an internal combustion engine.
19. A method according to claim 18, wherein said engine is an automotive engine.
PCT/GB1995/001499 1994-07-05 1995-06-26 Hybrid exhaust gas catalyst WO1996001142A1 (en)

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NL1003778C2 (en) * 1996-08-09 1998-02-12 Univ Delft Tech Process for the production of a composite catalyst.
EP0868940A1 (en) * 1997-03-31 1998-10-07 Mazda Motor Corporation Catalyst for purifying exhaust gas, process for manufacturing the catalyst and process for making use of the catalyst
US10746752B2 (en) 2009-11-13 2020-08-18 Ventana Medical Systems, Inc. Opposables and automated specimen processing systems with opposables

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
NL1003778C2 (en) * 1996-08-09 1998-02-12 Univ Delft Tech Process for the production of a composite catalyst.
WO1998006495A1 (en) * 1996-08-09 1998-02-19 Technische Universiteit Delft Method for preparing a composite catalyst
EP0868940A1 (en) * 1997-03-31 1998-10-07 Mazda Motor Corporation Catalyst for purifying exhaust gas, process for manufacturing the catalyst and process for making use of the catalyst
US10746752B2 (en) 2009-11-13 2020-08-18 Ventana Medical Systems, Inc. Opposables and automated specimen processing systems with opposables

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