Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
  • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
  • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
  • F01N3/10—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust
  • F01N3/18—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust characterised by methods of operation; Control
  • F01N3/20—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust characterised by methods of operation; Control specially adapted for catalytic conversion
  • F01N3/206—Adding periodically or continuously substances to exhaust gases for promoting purification, e.g. catalytic material in liquid form, NOx reducing agents
  • F01N3/208—Control of selective catalytic reduction [SCR], e.g. by adjusting the dosing of reducing agent
• • 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
  • B01D53/9413—Processes characterised by a specific catalyst
  • B01D53/9418—Processes characterised by a specific catalyst for removing nitrogen oxides by selective catalytic reduction [SCR] using a reducing agent in a lean exhaust gas
• • 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
  • B01D53/9481—Catalyst preceded by an adsorption device without catalytic function for temporary storage of contaminants, e.g. during cold start
• • 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
  • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
  • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
  • B01J23/54—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
  • B01J23/56—Platinum group metals
  • B01J23/58—Platinum group metals with alkali- or alkaline earth metals
• • 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
  • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
  • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
  • B01J23/54—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
  • B01J23/56—Platinum group metals
  • B01J23/63—Platinum group metals with rare earths or actinides
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
  • F01N13/00—Exhaust or silencing apparatus characterised by constructional features
  • F01N13/009—Exhaust or silencing apparatus characterised by constructional features having two or more separate purifying devices arranged in series
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
  • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
  • F01N3/02—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust
  • F01N3/021—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters
  • F01N3/033—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters in combination with other devices
  • F01N3/035—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters in combination with other devices with catalytic reactors
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
  • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
  • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
  • F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
  • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
  • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
  • F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
  • F01N3/0828—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents characterised by the absorbed or adsorbed substances
  • F01N3/0842—Nitrogen oxides
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
  • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
  • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
  • F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
  • F01N3/0871—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents using means for controlling, e.g. purging, the absorbents or adsorbents
  • F01N3/0885—Regeneration of deteriorated absorbents or adsorbents, e.g. desulfurization of NOx traps
• • 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
  • B01D2255/20707—Titanium
• • 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
  • B01D2255/20723—Vanadium
• • 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
  • B01D2255/20738—Iron
• • 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
  • B01D2255/20—Metals or compounds thereof
  • B01D2255/207—Transition metals
  • B01D2255/20776—Tungsten
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2255/00—Catalysts
  • B01D2255/50—Zeolites
  • B01D2255/502—Beta zeolites
• • 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
• • 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
  • B01D53/9431—Processes characterised by a specific device
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
  • F01N2370/00—Selection of materials for exhaust purification
  • F01N2370/02—Selection of materials for exhaust purification used in catalytic reactors
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
  • F01N2430/00—Influencing exhaust purification, e.g. starting of catalytic reaction, filter regeneration, or the like, by controlling engine operating characteristics
  • F01N2430/08—Influencing exhaust purification, e.g. starting of catalytic reaction, filter regeneration, or the like, by controlling engine operating characteristics by modifying ignition or injection timing
  • F01N2430/085—Influencing exhaust purification, e.g. starting of catalytic reaction, filter regeneration, or the like, by controlling engine operating characteristics by modifying ignition or injection timing at least a part of the injection taking place during expansion or exhaust stroke
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
  • F01N2510/00—Surface coverings
  • F01N2510/06—Surface coverings for exhaust purification, e.g. catalytic reaction
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
  • F01N2610/00—Adding substances to exhaust gases
  • F01N2610/02—Adding substances to exhaust gases the substance being ammonia or urea
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL-COMBUSTION ENGINES
  • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
  • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
  • F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
  • F01N3/0814—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents combined with catalytic converters, e.g. NOx absorption/storage reduction catalysts
• • 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
  • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
  • Y02A50/00—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE in human health protection, e.g. against extreme weather
  • Y02A50/20—Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters
• • 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

• the invention relates to an exhaust system for treating an exhaust gas from an internal combustion engine, and a method for treating exhaust gas from internal combustion engines.
• PCT Intl. Appl. WO 2008/047170 discloses a system wherein NO x from a lean exhaust gas is adsorbed at temperatures below 200°C and is subsequently thermally desorbed above 200°C.
• the NO x adsorbent is taught to consist of palladium and a cerium oxide or a mixed oxide or composite oxide containing cerium and at least one other transition metal.
• U.S. Appl. Pub. No. 201 1/0005200 teaches a catalyst system that simultaneously removes ammonia and enhances net NO x conversion by placing an ammonia-selective catalytic reduction ("NH 3 -SCR") catalyst formulation downstream of a lean NO x trap.
• NH 3 -SCR ammonia-selective catalytic reduction
• the NH 3 -SCR catalyst is taught to adsorb the ammonia that is generated during the rich pulses in the lean NO x trap.
• the stored ammonia then reacts with the NO x emitted from the upstream lean NO x trap, which increases NO x conversion rate while depleting the stored ammonia.
• PCT Intl. Appl. WO 2004/076829 discloses an exhaust-gas purification system which includes a NO x storage catalyst arranged upstream of an SCR catalyst.
• the NO x storage catalyst includes at least one alkali, alkaline earth, or rare earth metal which is coated or activated with at least one platinum group metal (Pt, Pd, Rh, or Ir).
• Pt, Pd, Rh, or Ir platinum group metal
• a particularly preferred NO x storage catalyst is taught to include cerium oxide coated with platinum and additionally platinum as an oxidizing catalyst on a support based on aluminum oxide.
• EP 1027919 discloses a NO x adsorbent material that comprises a porous support material, such as alumina, zeolite, zirconia, titania, and/or lanthana, and at least 0.1 wt% precious metal (Pt, Pd, and/or Rh). Platinum carried on alumina is exemplified.
• the invention is an exhaust system for treating an exhaust gas from an internal combustion engine.
• the system comprises a modified lean NO x trap (LNT), a urea injection system, and an ammonia-selective catalytic reduction (NH3-SCR) catalyst.
• LNT lean NO x trap
• NH3-SCR ammonia-selective catalytic reduction
• the modified LNT comprises platinum, palladium, barium, and a ceria-containing material, and has a platinum:palladium molar ratio of at least 3: 1 .
• the modified LNT stores NO x at temperatures below about 200°C and releases the stored NO x at temperatures above about 200°C.
• the invention is an exhaust system for treating an exhaust gas from an internal combustion engine.
• the system comprises a modified lean NO x trap (LNT).
• LNT modified lean NO x trap
• Lean NO x traps are well known in the art. Lean NO x traps are typically designed to adsorb NO x under lean exhaust conditions, release the adsorbed NO x under rich conditions, and reduce the released NO x to form N 2 .
• LNTs typically include a ⁇ -storage component, an oxidation component, and a reduction component.
• the ⁇ -storage component preferably comprises alkaline earth metals (such as barium, calcium, strontium, and magnesium), alkali metals (such as potassium, sodium, lithium, and cesium), rare earth metals (such as lanthanum, yttrium, praseodymium and neodymium), or combinations thereof. These metals are typically found in the form of oxides. Typically, platinum is included to perform the oxidation function and rhodium is included to perform the reduction function. These components are contained on one or more supports.
• the oxidation/reduction catalyst and the ⁇ -storage component are preferably loaded on a support material such as an inorganic oxide to form an LNT for use in the exhaust system.
• the modified LNT of the invention is designed to have a different function than known LNTs, in that they are designed to store NO x at temperatures below about 200°C and release the stored NO x at temperatures above about 200°C.
• the modified LNT comprises platinum, palladium, barium, and a ceria-containing material.
• the ceria-containing material is preferably ceria, ceria-zirconia, ceria- zirconia-alumina, or mixtures thereof. More preferably, the ceria-containing material is ceria.
• the modified LNT has a platinum:palladium molar ratio of at least 3: 1 , more preferably greater than 4: 1 .
• the modified LNT also comprises barium.
• the barium may be added to the modified LNT by any known means.
• a barium compound may be loaded onto the ceria-containing material or the support material by any known means, the manner of addition is not considered to be particularly critical.
• a barium compound (such as barium acetate) may be added to the ceria-containing material or the support material by impregnation, adsorption, ion- exchange, incipient wetness, precipitation, or the like.
• the modified LNT comprises at least 150 g/ft 3 loading of barium, and in some instances the modified LNT may comprise at least 400 g/ft 3 loading of barium.
• the modified LNT also comprises a support.
• the support is preferably an inorganic oxide, and more preferably includes oxides of Groups 2, 3, 4, 5, 13 and 14 elements.
• the support is an alumina, silica, titania, zirconia, magnesia, niobia, tantalum oxide, molybdenum oxide, tungsten oxide, a mixed oxide or composite oxide of any two or more thereof (e.g. silica- alumina, magnesia-alumina), and mixtures thereof.
• the support may also preferably contain cerium.
• Useful supports preferably have surface areas in the range 10 to 1500 m 2 /g, pore volumes in the range 0.1 to 4 mL/g, and pore diameters from about 10 to 1000 Angstroms. High surface area supports having a surface area greater than 80 m 2 /g are particularly preferred.
• the modified LNT of the invention stores NO x at temperatures below about 200°C and releases the stored NO x at temperatures above about 200°C.
• the modified LNT is preferably coated on a substrate.
• the substrate is preferably a ceramic substrate or a metallic substrate.
• the ceramic substrate may be made of any suitable refractory material, e.g., alumina, silica, titania, ceria, zirconia, magnesia, zeolites, silicon nitride, silicon carbide, zirconium silicates, magnesium silicates, aluminosilicates and metallo aluminosilicates (such as cordierite and spodumene), or a mixture or mixed oxide of any two or more thereof. Cordierite, a magnesium aluminosilicate, and silicon carbide are particularly preferred.
• the metallic substrate may be made of any suitable metal, and in particular heat-resistant metals and metal alloys such as titanium and stainless steel as well as ferritic alloys containing iron, nickel, chromium, and/or aluminum in addition to other trace metals.
• the substrate may be a filter substrate or a flow-through substrate, and is most preferably a flow-through substrate, especially a honeycomb monolith.
• the substrate is typically designed to provide a number of channels through which vehicle exhaust passes. The surface of the channels is loaded with the three-way catalyst.
• the modified LNT may be added to the substrate by any known means.
• the modified LNT material may preferably applied and bonded to the substrate as a washcoat, a porous, high surface area layer bonded to the surface of the substrate.
• the washcoat is typically applied to the substrate from a water- based slurry, then dried and calcined at high temperature.
• the modified LNT is added to the substrate as a single layer, although additional layers may be used.
• the exhaust system of the invention also comprises an ammonia-selective catalytic reduction (NH 3 -SCR) catalyst.
• the NH 3 -SCR catalyst may comprise any known NH 3 -SCR catalysts, which are well-known in the art.
• a NH 3 -SCR catalyst is a catalyst that reduces NO x to N 2 by reaction with nitrogen compounds (such as ammonia or urea).
• the NH 3 -SCR catalyst is comprised of a vanadia-titania catalyst, a vanadia-tungsta-titania catalyst, or a metal/zeolite.
• the metal/zeolite catalyst comprises a metal and a zeolite. Preferred metals include iron and copper.
• the zeolite is preferably a beta zeolite, a faujasite (such as an X-zeolite or a Y-zeolite, including NaY and USY), an L-zeolite, a ZSM zeolite (e.g. , ZSM-5, ZSM-48), an SSZ-zeolite (e.g.
• a ferrierite a mordenite, a chabazite, an offretite, an erionite, a clinoptilolite, a silicalite, an aluminum phosphate zeolite (including metalloaluminophosphates such as SAPO-34), a mesoporous zeolite (e.g. , MCM-41 , MCM-49, SBA-15), or mixtures thereof; more preferably, the zeolite is a beta zeolite, a ferrierite, or a chabazite.
• the NH 3 -SCR catalyst is preferably coated on a ceramic or a metallic substrate, as described above.
• the substrate is typically designed to provide a number of channels through which vehicle exhaust passes, and the surface of the channels will be preferably be coated with the NH 3 -SCR catalyst.
• the substrate for the NH 3 -SCR catalyst may be a filter substrate or a flow- through substrate.
• the NH 3 -SCR catalyst is coated onto a filter, which is known as an ammonia-selective catalytic reduction filter (NH 3 -SCRF).
• SCRFs are single-substrate devices that combine the functionality of an NH 3 -SCR and particulate filter. They are used to reduce NO x and particulate emissions from internal combustion engines.
• the system of the invention further comprises a urea injection system.
• the urea injection system preferably comprises a urea injector that injects urea into the exhaust gas stream upstream of the NH 3 -SCR catalyst and downstream of the modified LNT.
• the urea injection system will preferably consist of a nozzle to produce well defined droplets of urea solution.
• the droplet size is preferably less than 500 microns to allow rapid evaporation and urea decomposition.
• the injector pressure and pump rate will be such to allow effective mixing in the exhaust gas stream.
• the urea injection system will also preferably consist of a urea tank, transfer lines and possibly a heating system to avoid freezing of the urea solution.
• the urea injection system injects urea at temperatures above about 180°C.
• the urea injection system is configured to inject urea at temperatures above about 180°C.
• the urea injection system may further comprise a processor configured to inject urea at temperatures above about 180°C, wherein the processor is electrically coupled to a temperature sensor.
• the invention also includes a method for treating an exhaust gas from an internal combustion engine.
• the method comprises passing the exhaust gas over the modified LNT described above.
• the modified LNT removes oxides of nitrogen ( ⁇ ) from the exhaust gas at temperatures below about 200°C, and releases the ⁇ at temperatures above about 200°C.
• urea is injected into the exhaust gas downstream of the modified LNT, and the exhaust gas containing released NO x from the modified LNT and urea is passed over a NH 3 -SCR catalyst.
• the released NO x is converted to nitrogen by the reaction of ammonia (generated from urea) with NO x over the NH 3 -SCR catalyst.
• the released NO x is the NO x that is stored on the modified LNT at low temperatures and is then released at the higher temperatures, and also includes ⁇ that is passes over the NH 3 -SCR catalyst without being stored.
• the modified LNT is periodically subjected to a rich desulfation step.
• the presence of sulfur compounds in fuel may be detrimental to the modified LNT since the oxidation of sulfur compounds leads to sulfur oxides in the exhaust gas.
• sulfur dioxide can be oxidized to sulfur trioxide over the platinum group metals and form surface sulfates on the LNT surface (e.g. , barium oxide or barium carbonate reacts with sulfur trioxide to form barium sulfate). These sulfates are more stable than the nitrates and require higher temperatures (> 500°C) to desulfate.
• the modified LNT is typically subjected to a temperature above about 500°C in rich ainfuel ratio environment to accomplish sulfur removal.
• the desulfation is preferably performed by increasing exhaust temperatures through a post-injection of fuel.
• Desulfation strategies may include a single, continuous rich period, or a series of short rich air-to-fuel ratio pulses. The following examples merely illustrate the invention. Those skilled in the art will recognize many variations that are within the spirit of the invention and scope of the claims.
• EXAMPLE 1 PREPARATION OF MODIFIED LNTs
• a 400 cells per square inch (cpsi) flow-through cordierite substrate monolith is coated with a NO x absorber catalyst formulation comprising a single layer comprising 1 .5 g/in 3 Ce/magnesium-aluminate spinel, 3 g/in 3 particulate ceria, 94 g/ft 3 Pt, and 19 g/ft 3 Pd.
• the washcoat is coated on the virgin substrate monolith using the method disclosed in WO 99/47260, followed by drying for 30 minutes in a forced air drier at 100°C and then firing at 500°C for 2 hours.
• Comparative LNT 1 B is prepared according to the procedure for Modified LNT 1 A, with the exception that the NO x absorber catalyst formulation comprising a single layer contains 53.3 g/ft 3 Pt, and 26.6 g/ft 3 Pd.
• EXAMPLE 2 NO x STORAGE TESTING
• LNT 1 A (1 .6 L catalyst volume) is hydrothermally aged at 800°C for 5 hours, and is then tested over the NEDC drive cycles on a 1 .6 liter engine employing low pressure exhaust gas recirculation (EGR). No rich purging is employed during the testing.
• EGR exhaust gas recirculation
• LNT 1 A and Comparative LNT 1 B are subjected to a rich desulfation regeneration to test the effect on CO conversions.
• Comparative LNT 1 B (1 .6 liter catalyst volume), hydrothermally aged at 800°C for 5 hours, is tested over repeated lean NEDC drive cycles on a 1 .6 liter engine employing low pressure exhaust gas recirculation (EGR).
• EGR exhaust gas recirculation
• a series of three NEDC cycles with a 3-second rich purge on the 100 kph cruise are performed to reactivate the catalyst. Analysis shows that one 3- second rich purge is sufficient to re-activate the catalyst, as the tailpipe CO emission is reduced to 1 .8 g CO.
• a further 10 lean only NEDC cycles are completed to follow the CO deactivation again, and on the tenth cycle the tailpipe CO emission increases to 5.6 g CO.
• a 500- 550°C inlet temperature desulfation is performed for 10 minutes at lambda 0.95, and a third 10-cycle NEDC testing is performed.
• modified LNT 1 A of the invention shows good CO conversion following rich desulfation, where comparison LNT 1 B loses CO conversion following rich desulfation.

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