Exhaust after-treatment system for a lean burn internal combustion engine

Abstract

An exhaust gas after-treatment system having a NOx storage material and a separate HC and CO oxidation section, such oxidation section having an oxidation catalyst substantially free of the NOx storage material.

Description

TECHNICAL FIELD

This invention relates to exhaust after-treatment systems and more particularly to exhaust after-treatment systems for lean burn internal combustion engines.

BACKGROUND AND SUMMARY

As is known in the art, precious metal three-way catalysts are generally used as a means for removing pollutants from the exhaust gas of an internal combustion engine. These three-way catalysts remove CO, HC, and NO_x simultaneously from engine exhaust gases under stoichiometric conditions. However, under lean fuel conditions, which are desired for optimal fuel efficiency, the three-way catalyst is ineffective for the removal of NO_x. Accordingly, to achieve NO_x control under fuel lean conditions, exhaust after-treatment systems have included a lean NO_x trap (LNT).

An LNT has 3 essential components:

• • 1) a NO_x storage medium (also called compound or component). Prototypically, this is barium. Barium never exists by itself; it will always be present in the form of a compound in the trap, e.g., barium carbonate. Other storage components are those of the alkali metal group (especially potassium and cesium) and other alkaline earth elements besides Ba (e.g., strontium and magnesium).
  • 2) a NO oxidation component. NO_x is present in engine exhaust gases as a mixture of NO and NO₂. It is stored as a nitrate species (NO₃). To convert to the nitrate form, both the NO and NO₂ must be oxidized (i.e. reacted with oxygen from the exhaust gas). Platinum is the prototypical metal for doing that, but other metals have oxidation capability.
  • 3) a reducing component. Regeneration of the trap involves driving the exhaust gas to rich conditions (i.e. excess of reductant species such as carbon monoxide, hydrogen, and hydrocarbons) and reacting the adsorbed nitrate back to nitrogen. This is similar to the way NO_x is treated in a three-way catalyst. Rhodium is the prototypical element for NO_x reduction and it is used in most LNTs for the purpose of regenerating the trap.

Those are the three main components. Additionally, a high surface support phase is used such as alumina over which all the components are dispersed to create finally divided, small particles of all the active components. Various stabilizers and so-called oxygen storage materials are often added as well.

An additional function of the Pt in the LNT is to combust reductants such as CO, H₂, and HC to release heat needed to raise the operating temperature of the LNT to the high temperature levels required for removal of stored sulfur.

Thus, the LNT includes material to oxidize the CO and HC and material to store NO_x. Presently, however, the performance of NO_x trap technology is limited in several respects. NO_x trap performance is affected by the relatively narrow operating temperature window of current trap formulations. At temperatures outside this window, the system may not operate efficiently and NO_x emissions can increase.

Both three-way catalysts and lean NO_x traps (LNT) are generally inefficient at ambient temperatures and must reach high temperatures before they are activated. Typically, contact with high-temperature exhaust gases from the engine elevates the temperature of the catalyst or LNT. The temperature at which a catalytic converter can convert 50% of CO, HC, or NO_x is referred to as the “light-off” temperature of the converter.

During start up of the engine, the amount of CO and HC in the exhaust gas is typically higher than during normal engine operation. While a large portion of the total emissions generated by the engine is generated within the first few minutes after start up, the catalysts are relatively ineffective because they will not have reached the “light-off” temperature. In other words, the catalysts are the least effective during the time they are needed the most.

As noted above, in order to achieve NO_x control in lean burn engines, exhaust after-treatment systems have included an additional NO_x storage device often referred to as a lean NO_x trap (LNT). Presently, however, the performance of NO_x trap technology is limited in several respects. NO_x trap performance is affected by the operating temperature and requires a relatively narrow temperature-operating window of the exhaust gases. At temperatures outside this window, the system will not operate efficiently and NO_x emissions will increase. Exposure to high temperature will also result in permanent degradation of the NO_x trap capacity.

The LNT is purged periodically to release and convert the oxides of nitrogen (NO_x) stored in the trap during the preceding lean operation. To accomplish the purge, the engine has to be operated at an air-to-fuel ratio that is rich of stoichiometry. As a result of the rich operation, substantial amounts of feedgas carbon monoxide (CO) and hydrocarbons (HC) are generated to convert the stored NO_x. Typically, the purge mode is activated on the basis of estimated trap loading. That is, when the estimated mass of NO_x stored in the trap exceeds a predetermined threshold, a transition to the purge mode is initiated. The rich operation continues for several seconds until the trap is emptied of the stored NO_x, whereupon the purge mode is terminated and the normal lean operation is resumed. The end of the purge is usually initiated by a transition in the reading of the HEGO sensor located downstream of the trap, or based on the model prediction of the LNT states. Since the engine is operated rich of stoichiometry during the purge operation, the fuel economy advantage of the lean operation is lost.

In addition to normal trap regeneration, the LNT may also be subjected to a much higher temperature regeneration process for the removal of stored sulfur (typically temperatures in excess of 600 degrees Celsius). Furthermore, if the LNT is contained in an exhaust system that also contains a diesel particulate filter (DPF), the LNT may also be subjected to temperatures in excess of 500 degrees Celsius during regeneration of the DPF (i.e. removal of accumulated carbonaceous (i.e. soot) material via combustion with oxygen in the exhaust gas). Both of these processes can result in permanent, gradual deterioration in NO_x trap performance—more so even than normal trap regeneration to remove stored NO_x.

More particularly, as noted above, a LNT has both functions of oxidation of HC and CO, etc. and storage/reduction of NO_x. In a conventional LNT, as shown in FIG. 1, an oxidation material (namely platinum, Pt) used to oxidize the HC and CO is included along with additional components such as rhodium (Rh), used for NO_x reduction, and barium (Ba) used to store the NO_x. The inventors have discovered that the exposure of the lean NO_x trap (LNT) to temperatures in the range of 600 to 700 degrees Celsius, especially under the oxidizing conditions required for DPF regeneration, can cause the deterioration of the LNT especially its “light off” function, and largely reduces its low temperature NO_x reduction efficiency. The inventors speculate that it is one or more of the major components of the LNT (i.e., such as rhodium (Rh) and barium (Ba)) that interacts with the Pt in a deleterious way following the high temperature operation of the LNT required for de-sulfurization and/or DPF regeneration (if such DPF is serially connected in the system). For example, it is known that Rh and Pt can form alloys, and it may turn out that the high temperature conditions required for LNT desulfurization and/or DPF regeneration causes the Pt and Rh to alloy in the LNT in such a way that the oxidation activity of the Pt is adversely affected.

In accordance with the present invention, an exhaust gas after-treatment system is provided having a NO_x storage material in a NO_x storage section and an HC and CO oxidation catalyst in a separate HC and CO oxidation section, such oxidation section being substantially free of the NO_x storage material.

In one embodiment the oxidation section is substantially free of Rh.

With such an arrangement, the HC and CO oxidation catalyst is physically separated from the NO_x storage material. Thus, the oxidation catalyst used in the oxidation section will not become adversely affected by any alloying or other types of interactions with components contained in the NO_x storing section.

In one embodiment, the oxidation catalyst is Pt for generating heat required to “light off”. Thus, while it is known that Pt is an effective NO_x oxidation catalyst, the negative effects described above of using the Pt completely in conjunction with the NO_x storage material such as Ba and reducing components such as Rh are avoided by separating part of the Pt out into a separate oxidation (combustion) catalyst preceding the NO_x storage section.

In one embodiment, an exhaust gas after-treatment system is provided. The system includes in one section thereof, a NO_x oxidation component, a NO_x storage component, and a NO_x reduction component, and, in a separate section thereof, a catalytic HC and CO combustion section substantially free of the NO_x storage component and the NO_x reduction component.

In accordance with another feature of the invention, a method is provided for treating exhaust gas produced by an internal combustion engine. The method includes oxidizing hydrocarbons and carbon monoxide present in the exhaust gas and storing NO_x in the exhaust gas; wherein the oxidizing and NO_x storing are performed as separate, sequential processes on the exhaust gas.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram of an after-treatment system coupled to the exhaust of an internal combustion engine, such after-treatment system having a Lean NO_x Trap (LNT) according to the prior art;

FIG. 2 is a diagram of an after-treatment system coupled to the exhaust of an internal combustion engine, such after-treatment system providing NO_x storage and HC and CO oxidation according to the invention;

FIG. 3 is a diagram of an after-treatment system coupled to the exhaust of an internal combustion engine, such after-treatment system providing NO_x storage and HC and CO oxidation according to another embodiment of the invention;

FIG. 4 are curves showing NO_x conversion percentage as function of LNT temperature with and without deterioration by a de-SO_x treatment of the trap at 600 degrees Celsius for 16 hours; and

FIG. 5 are curves showing the effect of an HC and CO oxidation section separate from a NO_x storage section according to the invention with the prior art, each of three curves therein showing the functional relationship between NO_x conversion percent as a function of temperature, one of the curves being associated with an exhaust gas after-treatment system having an HC and CO oxidation section separate from a NO_x storage section according to the invention, another one of the curves being associated with a LNT according to the prior art, and the third one of the curves being associated with an LNT which has not been deteriorated;

FIGS. 6 and 7 are curves showing the inlet and the catalyst middle temperatures for the two tests showed in FIG. 5 at 200 degrees Celsius, which are the deteriorated LNT (1″ long) and the same LNT (1″ long) plus a ⅛″ thick diesel oxygen catalyst (DOC) mounted in front of the it.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring now to the drawing and initially to FIG. 2, a block diagram of an exhaust gas after-treatment system 10 coupled to an internal combustion engine 12, here a diesel engine. The exhaust gas after-treatment system 10 has two separate sections 14, 16. The first section 14 is used to combust reductants such as CO, H₂, and HC and is substantially free of the NO_x storage component and the NO_x reduction component. Here, the first section 14 contains platinum, for example, as the active combustion component. The second section 16 provides NO_x storage and includes: a NO_x oxidation component, here for example, platinum, Pt; a NO_x storage component, here for example, barium, Ba, and a NO_x reduction component, here for example, rhodium, Rh. The first section 14 is upstream of the second section 16.

In FIG. 2, the second section 16 is in a separate housing from the first section 14. The first and second sections 14, 16 are then physically attached by any convenient means, such as welding the two sections together. Note that as drawn, the exhaust gas after-treatment system 10 is comprised of cylindrical flow-through devices. Such devices are nominally monolithic honeycomb type structure catalysts containing the active components dispersed on either ceramic or metallic type substrates of various cell densities, wall thicknesses, length, shape (e.g., round, oval, or racetrack). Furthermore, sections 14 and 16 can either be separated from one another as shown in the diagram or butted against one another. In FIG. 3, the first and second sections 14, 16 are contained on the same substrate body via a process known as zone-coating wherein two different catalyst washcoat formulations are coated on different regions of the substrate body. In both embodiments, the first section 14 is used to combust reductants such as CO, H₂, and HC and is substantially free of the NO_x storage component and the NO_x reduction component and the second section 16 provides NO_x storage and includes: a NO_x oxidation component; a NO_x storage component; and a NO_x reduction component.

It is noted that, in FIGS. 2 and 3, the exhaust gases from the engine 12 pass sequentially, i.e., serially, through the first section 14 and the second section 16. Thus, a method is provided for treating exhaust gases from an internal combustion engine. The method includes oxidizing hydrocarbons and carbon monoxide in the exhaust gas and storing NO_x in the exhaust gas; wherein the oxidizing and storing are performed as separate, sequential processes on the exhaust gas after-treatment device.

Both the oxidation section and the NO_x storing section contain Pt, in various proportions, with the Pt providing a CO and HC oxidation catalyst in the oxidation section and primarily as a NO_x oxidation catalyst in the NO_x storing section second section. The ratio of the volume of the oxidation section to the NO_x storing section, ranges from 1/10 to 1 and more preferably from 1/10 to ⅓.

With the exhaust gas after-treatment system of either FIG. 2 or FIG. 3 the NO_x reduction efficiency is improved over the system of FIG. 1 at low temperature. More particularly, the inventors have observed that frequent de-sulfurization of the diesel lean NO_x trap (LNT) at 600 to 700 degrees Celsius can cause the deterioration of the LNT especially its light off function, and largely reduces its low temperature NO_x reduction efficiency as shown in FIG. 4, which contains two NO_x conversion vs. catalyst inlet temperature curves tested over core (1″ diameter with 1″ length) samples at 30,000 s.v./hr (Note that s.v. refers to space velocity, a term commonly used to characterize the amount of gas flow through the catalyst body in relation to the volume of the catalyst body; e.g. cubic feet of gas flow per hour divided by the cubic feet of volume of the catalyst body based on external dimensions. The space velocity therefore carries the units of inverse time, e.g., 1/hr. With regard to space velocity, it is also a convenient measure for matching laboratory-scale experiments as reported here to larger scale applications such as would be practiced on a vehicle. Hence, the 1″ diameter by 1″length laboratory samples used in the laboratory at relatively low gas flow rates may translate into a 6″ diameter by 6″ length catalyst unit on a vehicle at much higher flow rates. The exact dimensions could be adjusted to yield the same s.v. in both cases, however, and those skilled in the art will recognize that the s.v. can vary between about 5000/hr to 50,000/hr under conditions experienced in automotive diesel exhaust). The diesel oxidation catalyst formulation is much more stable in the de-sulfurization temperature range (600 to 700 degrees Celsius) than the LNT.

Referring specifically to the embodiment shown in FIG. 3, the zone coating of an oxidation formulation (i.e., the first section 14) in a small area of the inlet of the monolith body or attachment of a small piece of diesel oxidation catalyst in front of the second section 16 (FIG. 2), helps to maintain the “light-off” property of the aged LNT. The rich condition in the diesel LNT vehicle operation is unique from gasoline (TWC, or LNT) or diesel SCR with about 1% oxygen in rich condition (gasoline exhaust contains much lower levels of oxygen for an equivalent degree of richness). Consequently, much more reaction heat, or exothermic temperature rise, can be generated in the diesel case. With good “light-off” function, a LNT catalyst temperature can be raised an additional 30 to 80 degrees Celsius utilizing the embodiments of FIGS. 2 and 3, which can make quite a big impact on the low temperature NO_x reduction efficiency.

FIG. 5 are curves showing the effect of an HC and CO oxidation section separate from a NO_x storage section according to the invention compared with the prior art, each of three curves therein showing the functional relationship between NO_x conversion percent as a function of temperature, curve 20 is associated with an exhaust gas after-treatment system having an HC and CO oxidation section separate from a NO_x storage section according to the invention, curve 22 is associated with a LNT according to the prior art, and curve 24 is associated with an LNT according to the prior art which has not been deteriorated;

Here, a one eighth inch long diesel oxidation catalyst, first section 14 (1″ diameter) is attached in front of a one inch long aged second section 16 (i.e., the same piece as shown in FIG. 4 deteriorated by the de-sulfurization) Addition of the small section of diesel oxidation catalyst improved the NO_x reduction from 10% to 70% with the same inlet temperature of 200 degrees Celsius. This specific diesel oxidation catalyst was aged in a much more severe condition (670 degrees Celsius for 64 hrs) than the LNT catalyst and also, it has the same Pt loading per unit volume as the LNT.

Since the exhaust temperature of a light duty diesel vehicle is usually in the range of 150 to 250 degrees Celsius, improving the low temperature NO_x reduction efficiency will have a large impact on the overall vehicle NO_x reduction efficiency.

The inventors have concluded that the main reason that the NO_x reduction efficiency of the LNT is improved by a ⅛ volume of diesel oxidation catalyst with same precious metal loading per unit volume in front of it, is the “light-off” function of the diesel oxidation catalyst, which raised the LNT operation temperature with the same catalyst inlet temperature by burning the CO, HC and H₂ in the rich condition during the lean/rich cycle since there is about 1% oxygen in the diesel LNT rich condition. FIGS. 6 and 7 show the inlet (curve 30) and the catalyst middle temperatures (curve 32) for the two tests shown in FIG. 5 at 200 degrees Celsius, which are the deteriorated LNT (1″ long) and the same LNT (1″ long) plus a ⅛″ thick diesel oxidation catalyst (DOC) attached in front of it. Obviously, the ⅛″ DOC helped raise the LNT middle temperature by about 35 degrees Celsius, thus resulting in much higher NO_x conversion.

The zone coating of a DOC formulation at the inlet of a catalyst will function similarly as attaching a same volume of DOC catalyst in front of the catalyst.

A number of embodiments of the invention have been described. It should be noted that the hydrocarbon and carbon monoxide oxidation material might includes Pt and/or other oxidation catalyst material. Further, the NO_x storing material might include Ba, or Cs, Na, K, Sr, and/or any other similar material for storing and releasing NO_x in operating temperature range of diesel exhaust gases. Still further, it should be noted that both the oxidation section and the NO_x storing section contain Pt, in various proportions, such that the Pt is utilized primarily as a CO and HC oxidation catalyst in the oxidation section and primarily as a NO_x oxidation catalyst in the NO_x storing section. Also, the oxidation section might include one or more CO and HC oxidation components with the oxidation section being substantially free of the NO_x storage component(s) and the NO_x reduction component(s). Thus, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.

Claims

1. An exhaust gas after-treatment system for an internal combustion engine, comprising:

a lean NOx trap, such lean NOx trap comprising: an oxidation section having an oxidation material for oxidizing hydrocarbons and carbon monoxide in the exhaust gas; and a NOx storage section, such NOx storage section having a NOx storing material for storing NOx in the exhaust gas;

wherein the oxidation material in the oxidation section is physically separated from the NOx storing material in the NOx storage section.

2. The exhaust gas after-treatment system recited in claim 1 wherein the hydrocarbon and carbon monoxide oxidation material includes Pt.

3. The exhaust gas after-treatment system recited in claim 2 wherein the NOx storing material includes Ba, Cs, Na, K, or Sr.

4. The exhaust gas after-treatment system recited in claim 2 wherein the physical separation between the two sections is provided by coating the two sections on separate pieces of catalyst material.

5. The exhaust gas after-treatment system recited in claim 2 wherein the physical separation between the two sections is provided by zone-coating both sections on the same catalyst body.

6. The exhaust gas after-treatment system recited in claim 1 wherein both the oxidation section and the NOx storing section contain Pt, in various proportions, with the Pt providing a CO and HC oxidation catalyst in the oxidation section and primarily as a NOx oxidation catalyst in the NOx storing section second section.

7. The exhaust gas after-treatment system recited in claim 1 wherein the ratio of the volume of the oxidation section to the NOx storing section, ranges from 1/10 to 1 and more preferably from 1/10 to ⅓.

8. An exhaust gas after-treatment system, comprising:

a second section having therein: a NOx oxidation component; a NOx storage components; and a NOx reduction components; and

a first section for oxidizing hydrocarbons and carbon monoxide in the exhaust gas. such first section being physically separate from the second section, such first section being substantially free of the NOx storage component and the NOx reduction component.

9. The system recited in claim 9 wherein the first section is upstream of the second section.

10. A method for treating exhaust gas produced by an internal combustion engine, comprising:

oxidizing hydrocarbons and carbon monoxide in the exhaust gas;

storing and reducing NOx in the exhaust gas;

wherein the oxidizing and storing/reducing functions are performed as separate, sequential processes on the exhaust gas.

11. An exhaust gas after-treatment system for an internal combustion engine, comprising:

a lean NOx trap, such lean NOx trap comprising: an oxidation section having an oxidation material for oxidizing hydrocarbons and carbon monoxide in the exhaust gas; and a NOx storage section, such NOx storage section having a NOx storing material for storing NOx in the exhaust gas and noble metal components for both oxidizing NO during NOx storage and reducing released NOx during trap regeneration; and

wherein the oxidation material in the oxidation section is physically separated from the NOx storing material and noble metal components in the NOx storage section.

12. The exhaust gas after-treatment system recited in claim 11 wherein the hydrocarbon and carbon monoxide oxidation material includes Pt and/or other oxidation catalyst material.

13. The exhaust gas after-treatment system recited in claim 12 wherein the NOx storing material stores and releases NOx in an operating temperature range of diesel exhaust gases.

14. The exhaust gas after-treatment system recited in claim 11 wherein both the oxidation section and the NOx storing section contain Pt, in various proportions, such that the Pt is utilized primarily as a CO and HC oxidation catalyst in the oxidation section and primarily as a NOx oxidation catalyst in the NOx storing section.

15. The exhaust gas after-treatment system recited in claim 11 wherein the ratio of the volume of the oxidation section relative to the storing section ranges from 1/10 to 1 and more preferably from 1/10 to ⅓.

16. An exhaust gas after-treatment system, comprising:

a second section having therein: one or more NOx oxidation components; one or more NOx storage components; and one or more NOx reduction components; and

a first section for oxidizing hydrocarbons and carbon monoxide in the exhaust gas. such first section being physically separate from the second section, such first section being substantially free of the NOx storage component(s) and the NOx reduction component(s).

17. The system recited in claim 16 wherein the first section is upstream of the second section.