GB2505511A

GB2505511A - A catalyst converter

Info

Publication number: GB2505511A
Application number: GB1215717.8A
Authority: GB
Inventors: Emilie Reynier; Andrea De Filippo; Rahul Mital
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2012-09-03
Filing date: 2012-09-03
Publication date: 2014-03-05
Also published as: GB201215717D0; CN103670615A

Abstract

A catalyst converter of a lean oxides of nitrogen (NOx) trap (281) comprising a catalyst substrate coated with a wash-coat with at least two active zones, wherein a rear zone comprises two active components; an oxidation catalyst and an adsorbent, and a minimized oxygen storage capacity (OSC) and wherein a zone immediately before the rear zone comprises an oxidation catalyst, a reduction catalyst, an adsorbent and an oxygen storage capacity. Advantageously the zone immediately before the rear zone has the active components of a standard LNT and therefore is able to produce ammonia by the NOx oxidation during rich combustion mode, and the rear zone is able to maintain ammonia since the low oxygen storage capacity reduces NH3 oxidation and the absence of a reduction catalyst minimises the competitive NH3 consumption reaction in reducing NOx.

Description

A WASI-ICOAT TECHNOLOGYFORA LEAN NO TRAP CATALYST

OFAN INTERNAL COMBUSTION ENGINE

TECHNICAL FIELD

The present disclosure relates to a new washcoat technology for a Lean NOx Trap catalyst of an internal combustion engine, to enhance ammonia production.

BACKGROUND

It is known that the exhaust gas after-treatment systems of a Diesel engine can be provided, among other devices, with a Lean NOx Trap (LNT).

A Lean NOx Trap (LNT) is provided for trapping nitrogen oxides NOx contained in the exhaust gas and is located in the exhaust line.

A LNT is a catalytic device containing PGM (Platinum Group Metal) catalysts and adsorbents, such as barium based elements, which provide active sites suitable for binding the nitrogen oxides (NOx) contained in the exhaust gas, in order to trap them within the device itself.

Lean NOx Traps (LNT) are subjected to periodic regeneration processes, whereby such regeneration processes are generally needed to release and reduce the trapped nitrogen oxides (NOx) from the LNT.

The LNT are operated cyclically, for example by switching the engine from lean-bum operation to operation whereby an excess amount of fuel is available, also referred as rich operation or regeneration phase. During normal operation of the engine, the NO are stored on a catalytic surface. When the engine is switched to rich operation, the NO stored on the adsorbent site react with the reductants in the exhaust gas and are desorbed and converted to nitrogen and ammonia, thereby regenerating the adsorbent site of the catalyst.

In the art are also known exhaust gas treatment systems for the emissions reduction and in particular of particulates and oxides of nitrogen (NOx) from the diesel engine exhaust gas. These systems are provided with after-treatment devices installed along the exhaust line of the engine and typically comprise a diesel particulate filter (DPF) for control of particulates, and selective catalytic reduction (SCR) system for NO control.

It is also known in the art, in some exhaust system configuration, to inject a reagent (catalyst) fluid in the exhaust line of the diesel engine in order to reduce emissions by means of the afore-mentioned aftertreatment devices. In particular, hydrocarbon based reagents, generally indicated as HG, like the same diesel fuel used for fuelling the engine, are injected in the exhaust line in order to promote the regeneration of diesel particulate filter (DPF) with the buming of soot accumulated therein. Furthermore, a fluid catalyst such as urea, or ammonia, or a combination thereof (generally in a water solution) are also injected into the exhaust line of the diesel engine in order to promote the reduction of nitrogen oxides (NOX) in the selective catalytic reduction system (SCR).

Hydrocarbon (HC) and the urea catalyst are injected into the exhaust gas produced by the engine by means of two separate injectors installed in the exhaust line.

Such a configuration, due to the number of components, and in particular the number of injectors needed for delivery hydrocarbon and urea in the exhaust line, leads to an high system complexity, high costs of production, and also to a reduced installation flexibility of the exhaust gas treatment system.

A possible simplification of such complex exhaust system is to combine a Lean NOx Trap (LNT) upstream to a NH3 storage device (a SCR, selective catalytic reduction system or a SCRF, a particulate filter comprising a selective catalytic reduction system).

The big potential of such architecture would be from one side the reduction of the after-treatment device numbers and on the other side the fact that an external urea/ammonia injector would not be needed anymore or, at least, the external urea requirements would be much lower. In fact, during rich operation, the NO stored on the adsorbent site react with the reductants in the exhaust gas and are desorbed and converted to nitrogen and * ammonia too.

The generated ammonia from the LNT could be advantageously used for the downstream SCR/SCRF to remove excess NOx, while the filter removes particulate matter. In such a way, no urea injector is needed (see fig. 3), thus allowing a cheaper exhaust system architecture, or in any case, lower external urea requirements. It has to be said the ammonia generation must be carefully controlled: a lower amount of it is insufficient to help pertorming the complete nitrogen reduction without any extemal catalyst injection, while a higher amount causes an ammonia slip downstream the 5CR or the SCRF. This can be obtained by improving the control strategies, and the present applicant has already filed a patent application on this topic, or alternatively the technology of the catalyst.

Therefore a need exists for a new technology of the LNT catalyst which optimizes ammonia generation during the rich phase, i.e. during the regeneration process of the LNT.

An object of this invention is to provide a new LNT catalyst, which uses a new washcoat technology optimizing the ammonia generation.

Another object of the invention is an exhaust architecture, combining a Lean NOx Trap (LNT) upstream to a SCR or a SCRF. In particular, the optimization is related to the new LNT technology, improving the generation of the ammonia.

These objects are achieved by a product, by an exhaust system, by an engine, by an automotive system provided with an electronic control unit able to control the fuel injections, having the features recited in the independent claims.

The dependent claims delineate preferred andfor especially advantageous aspects.

SUMMARY

An embodiment of the disclosure provides a catalyst converter of a lean NOx trap comprising a catalyst substrate coated with a washcoat with at least two active zones, wherein a rear zone comprises two active components, an oxidation catalyst and an adsorbent, and a minimized oxygen storage capacity and wherein a zone immediately before the rear zone comprises an oxidation catalyst, a reduction catalyst, an adsorbent and an oxygen storage capacity.

An advantage of this embodiment is that it provides a catalyst with a zone, immediately before the rear one, having the active components of a standard" LNT and therefore able to produce ammonia by the NOx oxidation during rich combustion mode, and a rear zone able to maintain ammonia, since the law oxygen storage capacity reduces NH3 oxidation and the absence of a reduction catalyst minimes the competitive NH3 consumption reaction in reducing NOx.

According to an aspect of the invention, said oxidation catalyst of the rear zone comprises Palladium said oxidation catalyst comprises Palladium.

An advantage of this aspect is that the presence of Palladium favorites the oxidation of CO and IC.

According to another aspect of the invention, said oxidation catalyst comprises Platinum but only in the zone immediately before said rear zone.

An advantage of this aspect is that the presence of Platinum favorites the oxidation of the NOx, thus producing ammonia in the zone immediately before the rear one. On the contrary, Platinum is not needed in the rear zone, where ammonia should not be produced, but preserved, and this allows a remarkable cost saving of the washcoat.

According to a further aspect of the invention, said reduction catalyst comprises Rhodium but only in the zone immediately before said rear zone.

An advantage of this aspect is that the presence of rhodium favorites the reduction of the NOx, during the regeneration under rich combustion mode, in the zone immediately before the rear one. On the contrary, Rhodium i not needed in the rear zone, where ammonia should be preserved, since its absence minimizes the competitive NH3 consumption reaction in reducing NOx. Avoiding Rhodium in the rear zone also allows a remarkable cost saving of the washcoat.

According to a still further aspect of the invention said adsorbent comprises Barium oxide.

An advantage of this aspect is that the presence of the adsorbent favorites the storage of NOx during the engine normal operating conditions, i.e. during lean combustion mode.

According to another embodiments, the invention -provides a Lean NOx trap for trapping nitrogen oxides and then reduce and release nitrogen, comprising a catalyst converter, having a catalyst substrate coated with a washcoat coated with two active zones, a front one and a rear one, wherein the rear zone is realized according to an aspect of the previous embodiment and wherein the front zone is realized like said zone immediately before the rear zone, according to an aspect of the previous embodiment.

An advantage of this embodiment is that the Lean NOx trap can perform, with the front zone, its usual functionalities, while the rear zone is mainly devoted to enhance and preserve the ammonia generation.

According to still another embodiment, the invention provides an aftertreatment system of an internal combustion engine comprising at least two after-treatment devices, the aftertreatment devices being at least a lean NOx trap, according to the previous embodiment, and a selective catalytic reduction system or a selective catalytic reduction system comprising a particulate filter.

An advantage of this embodiment is that the aftertreatment system is optimized to reduce NOx emissions, by balancing the performances of the two devices and providing ammonia for the SCR/SCRF without the need of any external ammonia or urea supply.

A further embodiment of the invention provides an internal combustion engine of an automotive system equipped with an aftertreatrnent system according to the previous embodiment.

A still further embodiment of the invention provides an automotive system comprising an electronic control unit configured for controlling an aftertreatment system of an internal combustion engine according to the previous embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

The various embodiments will now be described, byway of example, with reference to the accompanying drawings, in which: Figure 1 shows an automotive system.

Figure 2 is a section of an internal combustion engine belonging to the automotive system of figure 1.

Figure 3 is a schematic view of the engine and the after-treatment system according to the invention.

Figure 4 is a schematic view of the after-treatment system according to the invention.

Figure 5 is a schematic view of the LNT catalyst combined washcoat, according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Some embodiments may include an automotive system 100, as shown in Figures land 2, that includes an internal combustion engine (ICE) 110 having an engine block 120 defining at least one cylinder 125 having a pisLton 140 coupled to rotate a crankshaft 145.

A cylinder head 130 cooperates with the piston 140 to define a combustion chamber 150.

A fuel and air mixture (not shown) is disposed in the combustion chamber 150 and ignited, resulting in hot expanding exhaust gasses causing reciprocal movement of the piston 140. The fuel is provided by at feast one fuel injector 160 and the air through at least one intake port 210. The fuel is provided at high pressure to the fuel injector 160 from a fuel rail 170 in fluid communication with a high pressure fuel pump 180 that increase the pressure of the fuel received a fuel source 190. Each of the cylinders 125 has at least two valves 215, actuated by a camshaft 135 rotating in time with the crankshaft 145. The valves 215 selectively allow air into the combustion chamber 150

B

from the port 210 and alternately allow exhaust gases to exit through a port 220. In some examples, a cam phaser 155 may selectively vary the timing between the camshaft 135 and the crankshaft 145.

The air may be distributed to the air intake port(s) 210 through an intake manifold 200.

An air intake duct 205 may provide air from the ambient environment to the intake manifold 200. In other embodiments, a throttle body 330 may be provided to regulate the flow of air into the manifold 200. In still other embodiments, a forced air system such as a turbocharger 230, having a compressor 240 rotationally coupled to a turbine 250, may be provided. Rotation of the compressor 240 increases the pressure.and temperature of the air in the duct 205 and manifold 200. An intercooler 260.disposed in the duct 205 may reduce the temperature of the air. The turbine 250 rotates by receiving exhaust gases from an exhaust manifold 225 that directs exhaust gases from the exhaust ports 220 and through a series of vanes prior to expansion through the turbine 250. The exhaust gases exit the turbine 250 and are directed into an exhaust system 270. This example shows a variable geometry turbine (VGT) with a VGT actuator 290 arranged to move the vanes to alter the flow of the exhaust gases through the turbine 250. In other embodiments, the turbocharger 230 may be fixed geometry andlor include a waste gate.

The exhaust system 270 may include an exhaust pipe 275 having one or more exhaust aftertreatment devices 280. The aftertreatment devices may be any device configured to change the composition of the exhaust gases. Some examples of aftertreatment devices 280 include, but are not limited to, catalytic converters (two and three way), oxidation catalysts, lean NOx traps 281, hydrocarbon adsorbers, selective catalytic reduction (SCR) systems 282, particulate filters (DPF) or a combination of the last two devices, Le.

selective catalytic reduction system comprising a particulate filter (SCRF) 283. Other embodiments may include an exhaust gas recirculation (EGR) system 300 coupled between the exhaust manifold 225 and the intake manifold 200. The EGR system 300 may include an EGR cooler 310 to reduce the temperature of the exhaust gases in the S EGR system 300. An EGR valve 320 regulates a flow of exhaust gases in the EGR system 300.

The automotive system 100 may further include an electronic control unit (ECU) 450 in communication with one or more sensors andlor devices associated with the ICE 110 and equipped with a data carrier 40. The ECU 450 may receive input signals from various sensors configured to generate the signals in proportion to various physical parameters associated with the ICE 110. The sensors include, but are not limited to, a mass airflow and temperature sensor 340, a manifold pressure and temperature sensor 350, a combustion pressure sensor 360, coolant and oil temperature and level sensors 360, a fuel rail pressure sensor 400, a cam position sensor 410, a crank position sensor 420, exhaust pressure and temperature sensors 430, an EGR temperature sensor 440, and an accelerator pedal position sensor 445. Furthermore1 the ECU 450 may generate output signals to various control devices that are arranged to control the operation of the ICE 110, including, but not limited to, the fuel injectors 160, the throttle body 330, the EGR Valve 320, the VGT actuator 290, and the cam phaser 155. Note, dashed lines are used to indicate communication between the ECU 450 and the various sensors and devices, but some are omitted for clarity.

Turning now to the ECU 450, this apparatus may include a digital central processing unit (CPU) in communication with a memory system and an interface bus. The CPU is configured to execute instructions stored as a program in the memory system, and send and receive signals to/from the interface bus. The memory system may include various storage types including optical storage, magnetic storage, solid state storage, and other non-volatile memory. The interface bus may be configured to send, receive, and modulate analog and/or digital signals to/from the various sensors and control devices.

The program may embody the methods disclosed herein, allowing the CPU to carryout out the steps of such methods and control the ICE 110.

Turning back to exhaust system 270, the proposed invention relies on the optimization of the after-treatment system, which features the combination of two NOx after-treatment devices. The combined system consists (Fig. 3) of an upstream LNT 281 and a downstream SCR 282 or SCRF (SCR on DPF) 283 positioned in all possible close-coupled and/or under floor configurations.

Preferably, the first catalyst could be positioned as close as possible to the exit of the turbocharger to take advantage of the high temperature conditions which are beneficial for both LNT and SCR/SCRF.

The LNT reduces engine-out exhaust gas constituents (CO and HC) with high efficiency and stores NOx during lean operating conditions. During rich operating conditions, the NOx is released and converted.

However during rich operating conditions, NH3 (ammonia) is also generated even though this is not typically monitored when the LNT catalyst is used as stand alone or with a downstream DPF. The amount of generated ammonia depends on the specific 1].

management of the rich combustion conditions, as for example the air/fuel ratio value, the duration of each single DeNOx events and the temperature conditions.

In general, as shown in Fig. 4, the ammonia production by the LNT during the rich combustion mode could be monitored by means of a specific sensor (Le. NH3/NOx dual sensor) 284 placed upstream the SCRF. In this way, it will be possible to quantify the amount of rJN3 arriving at the SCRF, independently of the LNT technology.

A second specific sensor 285, placed downstream of the SCRF, monitors the amount of NH3 slipping through the SCRF. Thus, combining the information from the two sensors, it will be possible to monitor the present ammonia production stored into the SCRF.

Depending on the calibration status and the extensive knowledge and characterization of the LNT washcoat, the present ammonia production could eventually be predicted and mapped in rich combustion mode. Thus, the sensor 284 upstream the SCRISCRF could be avoided.

The purpose of this invention is to design a new washcoat technology on the LNT. As known, the LNT is based on a catalytic converter consisting of a catalyst substrate or core, which is usually a ceramic monolith with a honeycomb structure. The monolith is coated with a complex substance, the so-called washcoat. A washcoat is a carrier for the catalytic materials and is used to disperse the materials over a high surface area.

Aluminum oxide, titanium dioxide, silicon dioxide, or a mixture of silica and alumina can be used. The catalytic materials (precious metals, such as platinum, palladium and rhodium and also barium oxide, in the LNT specific case) are suspended in the washcoat prior to applying to the core. A zoning concept for this new washcoat should be a preferred embodiment: in fact, in addition to the standard" LNT 281 functionality, this new washcoat shall enhance NH3 production, under defined calibration and controls operation, to let the SCRISCRF 282, 283 work properly. More in detail, the necessary S functions of a "standard' LNT are: convert hydrocarbons (MC), carbon oxide (CO), trap NIOx, generate exothermicity, convert NOx and release sulphur species in rich combustion mode In addition to them, the feature of the new washcoat technology should be to produce more ammonia. -A known washcoat for the LNT catalyst combines three active components: an oxidation catalyst, such as platinum and palladium (Pt-Pd), to enhance the oxidation reactions of CO, HC and NO (to NO2, to maintain the optimized NO:NO2 ratio for the downstream SCRISCRF), an adsorbent, such as barium oxide (BaO), to store NOx engine out, and a reduction catalyst, such as rhodium (Rh) to reduce NOx during the regeneration under rich combustion mode. In fact, in rich mode, the NOx released from the BaO sites are converted to N2. Furthermore, the standard LNT washcoat also comprises an oxygen storage capacity (OSC), which provides the needed oxygen for the oxidation reactions.

It has to be noted that, during the DeNOx regeneration under rich combustion mode, the produced N2, coupled with H2, will be the reagent to produce NH3. In addition, the released NOx can directly react with HC or H2 to produce NH3. Thus, the amount of the barium oxide can be optimized to enhance the NH3 production for both types of reactions.

Once produced, the ammonia can be further oxidized (due to available oxygen, released form the OSC, and enhanced by the presence of platinum) or reduced: the presence of rhodium could improve the reaction of the NOx (slipped from the catalyst) to N2 with the consequent ammonia consumption. To minimize these further reactions, a new washcoat concept has been defined. It is characterized as a zoned coating, where at least the zone immediately before the rear one has to be designed to convert the emissions as a standard LNT catalyst. In Fig. 5 an example of such zoned coating washcoat is shown. It comprises two zones (but in other embodiments the zones can be more than two) and the zone immediately before the rear zone, which in the example of Fig. 5 is the front zone, is designed to convert the emissions as a standard LNT catalyst. Therefore, it combines three active components: an oxidation catalyst (Pt-Pd), an adsorbent (BaO) and a reduction catalyst (Rh). Also the OSC can be equivalent to the standard LNT catalyst.

In the rear zone, the washcoat is designed to take advantage of the reactions (and exothermal), which are generated in the front zone. It comprises two active components only: an oxidation catalyst and an adsorbent (BaO). The oxidation catalyst, preferably, should only be palladium, to improve the oxidation of CO and HC, while the amount of platinum can be reduced to zero, to lower the activity for NH3 oxidation. The reduction catalyst (Rh) can also be reduced to zero to minimize the competitive NH3 consumption reaction (the NOx slipped from the LNT can be converted later to N2 by means of the stored NH3, thanks to the presence of the downstream SCRISCRF. The OSC must also be minimized to reduce NH3 oxidation.

The present new washcoat does not impact on the LNT manufacturing process: in fact the coating of such lean NOx trap can be done by using standard coating technology.

Summarizing the catalyst characteristics to get more ammonia (NH3) production are a low oxygen storage capacity (OSC), in order to reduce Nl-13 oxidation, a low or null rhodium content, to minimize the competitive NH3 consumption reaction and a zoned coating, to control and enhance H2 production on the front zone, to reduce OSC on the rear zone and to control NO to N02 oxidation, maintaining the optimized NO:N02 ratio for the downstream SCRJSCRF.

The optimization of the LNT washcoat design, in addition to the use of an aftertreatment system as the one in Fig. 4 i.e. LNT and SCR/SCRF and the proper aftertreatment controls, will have the following benefits: a) reduce the hardware cost of the SCRISCRF engine management system: infact the current/standard urea injection system will not be required anymore: the ammonia will be produced in-situ in vapour phase; b) reduce NOx emission in the engine exhaust stream due to enhancement of NOx storage at low temperature by means of the LNT and to the additional NOx removal by means of the downstream SCR/SCRF; c) lower the cost of the LNT catalyst by reducing the rhodium content, since the overall NOx conversion efficiency can be obtained leveraging both LNT and SCR/SCRF volumes; d) lower the cost of the LNT catalyst by reducing the platinum amount; e) allow a higher temperature at the SCR/SCRF inlet, compared to an underfloor SCR, and consequently a better NOx conversion efficiency; f) reduce H2S emissions (rotten eggs smell), by converting LNT secondary emissions of H2S on the SGR/SCRF catalyst during the DeSOx regeneration, g) reduce packaging: compared to a known solution like close coupled DOC/DPF and underfloor SCR, this solution offers a reduced number of substrates and canning boxes.

h) reduce C02 emissions and backpressure: this more compact packaging will also be lighter, i) enhance further conversion of hydrocarbons, by means of reactions to produce ammonia (NH3).

While at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration in any way: Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing at least one exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.

REFERENCE NLJNIBERS

data carñer automotive system internal combustion engine 120 engine block cylinder cylinder head camshaft piston 145 crankshaft combustion chamber cam phaser fuel injector fuel rail 180 fuel pump fuel source intake manifold 205 air intake pipe 210 intake port 215 valves 220 port 225 exhaust manifold 230 turbocharger 240 compressor 245 turbocharger shaft 250 turbine 260 intercooler 270 exhaust system 275 exhaust pipe 280 aftertreatment devices 281 lean NOx trap 282 selective catalytic reduction (SCR) system 283 selective catalytic reduction systems comprising a particulate filters (SGRF) 284 NH3/NOx dual sensor upstream the SCR/SCRF 285 NH3/NOx dual sensor downstream the SCR/SCRF 290 VGT actuator 300 exhaust gas recirculation system 310 EGR cooler 320 EGR valve 330 throttle body 340 mass airflow and temperature sensor 350 manifold pressure and temperature sensor 360 combustion pressure sensor 350 coolant temperature and level sensors 385 lubricating oil temperature and level sensor 390 metal temperature sensor 400 fuel rail pressure sensor 410 cam position sensor 420 crank position sensor 430 exhaust pressure and temperature sensors 440 EGR temperature sensor 445 accelerator position sensor 446 accelerator pedal 450 ECU

Claims

CLAIMS1. A catalyst converter of a lean NOx trap (281) comprising a catalyst substrate coated with a washcoat with at least two active zones, wherein a rear zone comprises two active components, an oxidation catalyst and an adsorbent, and a minimized oxygen storage capacity (OSC) and wherein a zone immediately before the rear zone comprises an oxidation catalyst, a reduction catalyst, an adsorbent and an oxygen storage capacity (OSC).
2. Catalyst converter according to claim 1, wherein said oxidation catalyst of the rear zone comprises Palladium (Pd).
3. Catalyst converter according to claim 1 or 2, wherein said oxidation catalyst comprises Platinum (Pt) but only in the zone immediately before said rear zone.
4. Catalyst converter according to one of the previous claims, wherein said reduction catalyst comprises Rhodium (Rh) but only in the zone immediately before said rear zone.
5. Catalyst converter according to one of the previous claims, wherein said adsorbent comprises barium oxide (BaO)
6. Lean NOx trap (281) for trapping nitrogen oxides and then reduce and release nitrogen, comprising a catalyst converter, having a catalyst substrate coated with a washcoat with two active zones, a front one and a rear one, wherein the rear zone is realized according to claim 1, 2 or 5 and wherein the front zone is realized like said zone immediately before the rear zone, according to one of the claims 1-5.
7. Washcoat of a catalyst converter of a lean NOx trap (281), according one ofthe claims 1-5.
8. Aftertreatment system of an internal combustion engine (110) comprising at least two after-treatment devices (280), the after-treatment devices being at least a lean NOx trap (281), according to claim 6, and a selective catalytic reduction system (282) or a selective catalytic reduction system comprising a particulate filter (283).
9. Internal combustion engine (110) of an automotive system (100) equipped with an aftertreatment system according to claim 8.
10. Automotive system (100) comprising an electronic control unit (450) configured for controlling an aftertreatment system of an internal combustion engine (110) according to claim 9.