Exhaust After Treatment System

Abstract

In an exemplary embodiment of the present invention, an exhaust treatment system for reducing constituents in the exhaust gas of an internal combustion engine is provided. A ceramic monolith is disposed within the exhaust treatment system and has exhaust flow passages extending therethrough that are defined by longitudinally extending walls therebetween. A first catalyst composition for catalytic reduction of oxides of nitrogen in the exhaust gas is applied to a first portion of the exhaust flow passages. A second catalyst composition for catalytic oxidation of hydrocarbons and oxides of nitrogen in the exhaust gas is applied serially downstream to a second, portion of the exhaust flow passages. The second catalytic composition is configured to reduce ammonia and hydrocarbon slip past the first catalytic composition.

Description

FIELD OF THE INVENTION

Exemplary embodiments of the present invention are related to exhaust treatment systems for internal combustion engines.

BACKGROUND

Diesel engine exhaust gas is a heterogeneous mixture which contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and NO_x, as well as condensed phase materials (liquids and solids) that constitute particulate matter. Catalyst compositions, and substrates on which the catalysts are disposed may be provided in a diesel engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated components. Diesel exhaust emission treatment systems may include one or more of a precious metal containing diesel oxidation catalyst (“DOC”), a diesel particulate trap or filter “(DPF”), and a Selective Catalytic Reduction (“SCR”) catalyst device for the reduction of NO_x.

One exhaust treatment technology in use for high particulate matter reduction is the DPF. There are several known filter structures that have displayed effectiveness in removing the particulate matter from diesel exhaust gas such as ceramic honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal fibers, etc. Ceramic wall flow filters have experienced significant acceptance in automotive applications. The filter is a physical structure for removing particulates from exhaust gas and, as such, accumulating particulates will have the effect of increasing the exhaust system backpressure experienced by the engine. To address backpressure increases caused by the accumulation of exhaust gas particulates, the DPF is periodically cleaned, or regenerated. Regeneration involves the burning of accumulated particulates in what is typically a high temperature (>600 C) environment that may result in an increase in the levels of NO_x components in the exhaust gas stream. The DPF may include an SCR catalyst which, with the assistance of upstream injected ammonia (NH₃) in the form of gas, liquid or contained in a urea solution, will convert the NO_x to nitrogen (“N₂”).

One method of generating the temperatures required in the exhaust system for regeneration of the DPF is to deliver excess HC to an oxidation catalyst disposed upstream of the DPF. In the oxidation catalyst, HC is oxidized, resulting in an exothermic reaction that raises the exhaust gas temperature to levels required for DPF regeneration thereby burning or oxidizing the trapped particulate matter and cleaning the trap.

During the regeneration event, some excess HC may pass through the oxidation catalyst to the DPF. Since HC is a regulated exhaust gas constituent, release to the atmosphere should be avoided in order to meet applicable regulations. Similarly, the quantity of ammonia injected into the exhaust gas stream should be limited to that required for complete NO_x conversion. However circumstances may occur in which some ammonia is not consumed by the SCR catalyst activity and passes through the DPF. The release of unconverted ammonia is undesirable.

Accordingly, it is desirable to provide an exhaust system configuration that will reduce the levels of unconverted HC, CO and NH₃ in the exhaust gas stream resulting from the operation of the DPF.

SUMMARY OF THE INVENTION

In an exemplary embodiment of the present invention, an exhaust gas treatment system for reducing constituents in the exhaust gas of an internal combustion engine is provided. A ceramic monolith is disposed within the exhaust gas treatment system and has exhaust flow passages extending therethrough that are defined by longitudinally extending walls therebetween. A first catalyst composition for catalytic reduction of oxides of nitrogen in the exhaust gas is applied to a first portion of the exhaust flow passages. A second catalyst composition for catalytic oxidation of hydrocarbons, carbon monoxide and ammonia in the exhaust gas is separately applied, serially downstream to a second portion of the exhaust flow passages. The second catalytic composition is configured to reduce hydrocarbon, carbon monoxide and ammonia slip past the exhaust treatment system.

In another exemplary embodiment of the present invention, an exhaust gas treatment system for reducing constituents in the exhaust gas of an internal combustion engine comprises a ceramic wall flow monolith filter disposed within the exhaust gas treatment system having exhaust flow passages extending therethrough that are defined by longitudinally extending walls therebetween. A first subset of exhaust flow passages have open inlets and closed outlets to define inlet passages. As second subset of exhaust flow passages have closed inlets and open outlets to define outlet passages. Exhaust gas enters the ceramic wall flow monolith filter through the inlet passages and migrates through, as is filtered by the longitudinally extending walls to exit the filter through the outlet passages. A first catalyst composition that is operable, with an ammonia reductant, top catalytically reduce oxides of nitrogen in the exhaust gas is applied to a first portion of the ceramic wall flow monolith filter. A second catalyst composition for catalytic oxidation of hydrocarbon, carbon monoxide and ammonia in the exhaust gas is separately applied, serially downstream to a second portion of the ceramic wall flow monolith filter.

In yet another exemplary embodiment of the present invention, an exhaust gas treatment system for reducing constituents in the exhaust gas of and internal combustion engine include an oxidation catalyst and a particulate filter downstream of the oxidation catalyst for removal of particulates from the exhaust gas. A first catalyst composition, operable to catalytically reduce oxides of nitrogen in the exhaust gas is applied to a first portion of the particulate filter and a second catalyst composition for catalytic oxidation of hydrocarbon, carbon monoxide and ammonia in the exhaust gas is applied serially downstream to a second portion of the particulate filter.

The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features, advantages and details appear, by way of example only, in the following detailed description of embodiments, the detailed description referring to the drawings in which:

FIG. 1 is a schematic view of an exhaust treatment system for an internal combustion engine; and

FIG. 2 is an axial, sectional view that schematically illustrates a portion of a ceramic monolith embodied in the exhaust treatment system of FIG. 1.

DESCRIPTION OF THE EMBODIMENTS

Referring to FIG. 1, an exemplary embodiment of the invention is directed to an exhaust gas treatment system 10 for the reduction of regulated exhaust constituents of an internal combustion engine, such as diesel engine 12. The exhaust gas treatment system 10 includes an exhaust conduit 14 that transports exhaust gas from the diesel engine 12 to the various exhaust treatment components of the exhaust gas treatment system. The exhaust treatment components may include a Diesel Oxidation Catalyst (“DOC”) 16. The DOC may include a flow through ceramic monolith (not shown) that is has an oxidation catalyst disposed thereon. The oxidation catalyst may be applied as a wash coat and may contain precious group metals such as platinum (Pt), palladium (Pd), rhodium (Rh) or other suitable oxidizing catalysts, or combination thereof. The DOC 16 is useful in treating unburned gaseous and non-volatile HC and CO, which are combusted to form carbon dioxide and water.

Downstream of the DOC 16, a reductant may be injected into the exhaust gas 20 in the exhaust conduit 14. NH₃ in the form of a gas, a liquid or an aqueous urea solution may be used as the reductant and may be mixed with air in the injector nozzle 18 to aid in the dispersion of the injected spray. The exhaust gas stream containing the added NH₃ passes through an SCR device, in this case a DPF having an SCR catalyst applied thereto. The DPF/SCR 22 is configured to filter the exhaust gas to remove carbon and other particulates and to reduce the NO_x levels resident in the exhaust gas stream. The DPF/SCR 22 just described is typically referred to as a 2-way device as a result of its ability to treat or remove more than one exhaust component.

The DPF/SCR 22 may be constructed with a ceramic wall flow monolith filter 23, FIG. 2, that has a plurality of longitudinally extending passages 24 defined by longitudinally extending walls 26. The passages 24 include a subset of inlet passages 28 that have an open inlet end 30 and a closed outlet end 32, and a subset of outlet passages 34 that have a closed inlet end 36 and an open outlet end 38. Exhaust gas entering the DPF/SCR 22 through the inlet end 30 of the inlet passages 28 is forced to migrate through the associated longitudinally extending walls 26 to the outlet passages 34. It is through this wall flow mechanism that the exhaust gas 20 is filtered of carbon and other particulates. The filtered particulates 40 are deposited on the longitudinally extending walls 26 of the inlet passages 28 and, over time, will have the effect of increasing the exhaust backpressure experienced by the diesel engine 12.

In an exemplary embodiment of the exhaust gas treatment system 10, an SCR catalyst composition 42 preferably contains a zeolite and one or more base metal components such as iron (Fe), cobalt (Co), copper (Cu) or vanadium (V) which can operate efficiently to convert the NO_x constituents in the exhaust gas 20 across the operating range of the DPF/SCR 22. The SCR catalyst composition 42 may be applied to the longitudinally extending walls 26 of the inlet passages 28, the outlet passages 34, or both, of the ceramic wall flow monolith filter 23. Due to the porous nature of the ceramic wall flow monolith filter 23, the SCR catalyst may also be applied within the walls of the substrate to increase the contact time between the exhaust gas 20 and the SCR catalyst composition 42. The DPF/SCR 22 operates as an effective particulate filter and SCR system that is useful for remediation of the NO_x in the engine exhaust gas while at the same time removing particulate mater therefrom.

To address exhaust backpressure issues caused by particulate accumulation, the DPF/SCR 22 is periodically cleaned, or regenerated. Regeneration involves oxidation or burning of the accumulated particulate matter 40 in what is typically a high temperature (>600 C) environment. In an exemplary embodiment, excess HC is delivered to the DOC 16 for oxidation therein. The exothermal reaction caused by the oxidation of the HC will raise the temperature of the exhaust gas 20 upstream of the DPF/SCR 22 to required regeneration levels thereby burning or oxidizing the trapped particulate matter and cleaning the trap.

During the operation of the DPF/SCR 22 and, particularly during the regeneration event, some excess HC and CO may pass through the oxidation catalyst to the DPF/SCR 22. Since HC and CO are regulated exhaust gas constituents their release to the atmosphere should be minimized in order to meet applicable regulations. Similarly, the quantity of NH₃ injected into the exhaust gas 20 should be limited to that required for complete NO_x conversion. However circumstances may occur in which some NH₃ is not converted by the SCR catalyst activity and passes through the DPF/SCR 22. The release of unconverted NH₃ is undesirable.

The unintentional passage of HC, CO and/or NH₃ through the DPF/SCR 22 is referred to as “slip”. To reduce the slip of HC, CO and/or NH₃ through the DPF/SCR 22, an additional catalyst 44 containing precious group metals such as platinum (Pt), palladium (Pd), rhodium (Rh) or other suitable oxidizing catalyst or combination thereof is applied serially downstream of the SCR catalyst composition 42, FIG. 2, adjacent to the open outlet ends 38 of the outlet passages 34 of the DPF/SCR 22. In a preferred embodiment, and to improve the flow through characteristics of the DPF/SCR 22, the additional catalyst 44 is applied separately to the DPF/SCR and is not layered over the SCR catalyst composition 42. When excess HC, CO and/or NH₃ enters or “slips” into the zone defined by the downstream catalyst 44 they are oxidized to non-regulated and desired elements. As a result, the HC, CO and/or NH₃ slips in the exhaust gas exiting the DPF/SCR 22 are treated, without the additional cost or complexity of a separate, stand-alone catalyst device downstream of the DPF/SCR 22 for HC, CO and NH₃ slip control.

While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents maybe substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the present application.

Claims

1. An exhaust gas treatment system for reducing constituents in the exhaust gas of an internal combustion engine comprising:

a ceramic monolith disposed within the exhaust gas treatment system having exhaust flow passages extending therethrough defined by longitudinally extending walls therebetween;

a first catalyst composition, for catalytic reduction of oxides of nitrogen in the exhaust gas, applied to a first portion of the exhaust flow passages; and

a second catalyst composition, for catalytic oxidation of hydrocarbon, carbon monoxide and ammonia in the exhaust gas, separately applied, serially downstream to a second, portion of the exhaust flow passages.

2. The exhaust gas treatment system of claim 1, wherein the ceramic monolith is a wall flow monolith filter comprising:

longitudinally extending passages including a first subset of inlet passages having an open inlet end and a closed outlet end and a second subset of outlet passages having a closed inlet end and an open outlet end, wherein the exhaust gas enters the wall flow monolith filter through the inlet end of the passages and migrates through the longitudinally extending walls to the outlet passages to remove particulates from the exhaust gas.

3. The exhaust gas treatment system of claim 1, wherein the first catalyst composition comprises a reduction catalyst containing zeolite and one or more base metal components and the second catalyst composition comprises an oxidation catalyst containing one or more precious group metals.

4. The exhaust gas treatment system of claim 3, wherein the reduction catalyst base metals comprise iron, cobalt, copper, vanadium or combinations thereof.

5. The exhaust gas treatment system of claim 3, wherein the oxidation catalyst precious group metals comprise platinum, palladium, rhodium or combinations thereof.

6. An exhaust gas treatment system for reducing constituents in the exhaust gas of an internal combustion engine comprising:

a ceramic wall flow monolith filter disposed within the exhaust gas treatment system;

exhaust flow passages extending through the ceramic wall flow monolith filter defined by longitudinally extending walls therebetween;

a first subset of exhaust flow passages having an open inlet end and a closed outlet end defining inlet passages;

a second subset of exhaust flow passages having a closed inlet end and an open outlet end defining outlet passages, wherein exhaust gas enters the ceramic wall flow monolith filter through the inlet passages and migrates through, and is filtered by, the longitudinally extending walls to exit the ceramic wall flow monolith filter through the outlet passages;

a first catalyst composition, operable with an ammonia reductant to catalytically reduce oxides of nitrogen in the exhaust gas, applied to a first portion of the ceramic wall flow monolith filter; and

a second catalyst composition, for catalytic oxidation of hydrocarbon, carbon monoxide and ammonia in the exhaust gas, separately applied, serially downstream to a second portion of the ceramic wall flow monolith filter.

7. The exhaust treatment system of claim 6, wherein the first catalyst composition comprises a reduction catalyst containing zeolite and one or more base metal components and the second catalyst composition comprises an oxidation catalyst containing one or more precious group metals.

8. The exhaust treatment system of claim 7, wherein the reduction catalyst base metal comprises iron, cobalt, copper or vanadium or combinations thereof.

9. The exhaust treatment system of claim 7, wherein the oxidation catalyst precious group metal comprises platinum, palladium, rhodium or combinations thereof.

10. An exhaust gas treatment system for reducing constituents in the exhaust gas of an internal combustion engine comprising:

an oxidation catalyst disposed within the exhaust gas treatment system;

a particulate filter downstream of the oxidation catalyst to remove particulates from the exhaust gas;

a first catalyst composition, operable to catalytically reduce oxides of nitrogen in the exhaust gas, applied to a first portion of the particulate filter; and

a second catalyst composition, for catalytic oxidation hydrocarbon, carbon monoxide and ammonia in the exhaust gas, applied serially downstream to a second portion of the particulate filter.

11. The exhaust gas treatment system of claim 10, wherein the first catalyst composition comprises a reduction catalyst containing zeolite and one or more base metal components and the second catalyst composition comprises an oxidation catalyst containing one or more precious group metals.

12. The exhaust treatment system of claim 11, wherein the reduction catalyst base metals comprise iron, cobalt, copper or vanadium or combinations thereof.

13. The exhaust treatment system of claim 11, wherein the oxidation catalyst precious group metals comprise platinum, palladium, rhodium or combinations thereof.