US20160131001A1

US20160131001A1 - Catalyst design for selective-catalytic-reduction (scr) filters

Info

Publication number: US20160131001A1
Application number: US14/534,905
Authority: US
Inventors: Shouxian Ren; Wei Li
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2014-11-06
Filing date: 2014-11-06
Publication date: 2016-05-12
Also published as: DE102015118097A1

Abstract

Provided is an improved selective catalytic reduction filtering (SCRF) device that separates reduction of nitrogen oxides (NOx) from oxidation of soot, hydro-carbon (HC) and carbon monoxide (CO). The SCRF device has a diesel oxidation catalyst unit for oxidizing HC and CO and oxidizing diesel fuel to support DPF regenerations, and a SCR filtering unit, including at least one inlet channel and being connected to the diesel oxidation catalyst unit, for controlling (soot) emission, cleaning-up slipped HC and CO during a DPF regeneration, and reducing nitrogen oxides in the diesel exhaust gas. At least one inlet channel is coated with an ammonia-neutral oxidation catalyst, and at least one outlet channel is coated with a selected catalytic reduction catalyst.

Description

FIELD OF THE PRESENT TECHNOLOGY

The present technology relates generally to diesel exhaust emissions control. More specifically, the present technology related to simultaneously controlling nitrogen oxides (NOx), particulate matter (PM), carbon monoxide (CO) and hydrocarbons (HC) utilizing an innovatively designed SCR filter (SCRF).

BACKGROUND OF THE PRESENT TECHNOLOGY

In a diesel engine, the exhaust gas must be treated properly to remove harmful pollutants before being released to the atmosphere. The exhaust gas passes through a catalytic converter system that includes a DOC (diesel oxidation catalyst), a SCR filter (SCRF), and a selective catalytic reduction (SCR) catalyst. The DOC oxidizes carbon monoxide (CO) and hydrocarbons (HC), and nitric oxide (NO) to nitrogen dioxide (NO2). The DOC also behaves like a “diesel burner” to oxidize the injected diesel fuel to generate exotherm for supporting periodic soot oxidations or diesel particulate filter (DPF) regenerations. The SCRF is a combination of SCR and DPF technologies.
A diesel exhaust fluid (DEF) injection system injects urea solution into the exhaust for providing ammonia (NH3) to reduce nitrogen oxides (NOx) to harmless nitrogen and water in the presence of the SCR catalyst. Diesel exhaust contains relatively high levels of particulate matters (PM), which is also known as soot. The catalytic converter generally cannot remove elemental carbon, such as soot; soot is usually cleaned up by the DPF. The DPF needs to be regenerated by burning off the soot collected inside the DPF at temperatures greater than 500° C. The SCR is an individual catalytic converter that reduces the residual nitrogen oxides (NOx) by ammonia (NH3) from the exhaust gas.
The catalytic converter system containing the SCRF, though removing harmful emissions components, is not optimally efficient because of the low DPF regeneration efficiency, poor CO and HC dean-up activities of the SCRF during a DPF regeneration process, and potential contamination of the SCR catalyst by ash poisoning, HC coking, soot deposition, etc. The present technology is directed primarily to an improved exhaust gas treatment system through an innovative catalyst design of the SCRF.

SUMMARY OF EMBODIMENTS OF THE TECHNOLOGY

The present technology includes a highly efficient diesel exhaust gas treatment system containing a SCRF that enhances DPF regeneration efficiency, NOx reduction reliability, and CO and HC clean-up activities during DPF regenerations.
In one embodiment, the present technology includes a diesel exhaust gas treatment apparatus comprising a diesel oxidation catalyst unit for receiving diesel exhaust gas, and a selective catalyst reduction filtering (SCRF) unit, with at least one inlet channel and connected to the diesel oxidation catalyst unit, for oxidizing CO and HC, reducing nitrogen oxides (NOx), and control PM emission in the diesel exhaust gas, wherein at least one inlet channel is coated with an ammonia (NH3) neutral oxidation catalyst, and at least one outlet channel coated with an selective catalytic reduction (SCR) catalyst using ammonia (NH3) for NOx reduction
In another embodiment, the present technology includes a selective catalytic reduction filter (SCRF) comprising at least one inlet channel for receiving diesel exhaust gas, and at least one outlet channel connected to the plurality of inlet channels, wherein the at least one inlet channel is coated with an ammonia-neutral oxidation catalyst, and at least one outlet channel is coated with an selective catalytic reduction (SCR) catalysts.
In yet another embodiment, the present technology includes a method for improving reduction of nitrogen oxides (NOx) from diesel exhaust gas, comprising receiving the diesel exhaust gas, feeding the diesel exhaust gas with ammonia (NH3), or DEF (urea) solution as an ammonia source, into a selective catalytic reduction filtering device (SCRF) with multiple inlet channel coated with an ammonia-neutral oxidation catalyst and all corresponding outlet channels coated with a selective catalytic reduction catalyst, passing ammonia through the inlet channels to the outlet channels, and reducing nitrogen oxides (NOx), in the outlet channels of the SCRF.
In an additional embodiment, the present technology includes also a method for improving the DPF regeneration efficiency with an ammonia-neutral oxidation catalyst coated onto all inlet channels where soot (or PM) is continuously deposited and accumulated with time before carrying out a DPF regeneration. Moreover, with the given oxidation catalyst, residual CO and HC emissions can be oxidized to effectively reduce tailpipe CO and HC emissions during DPF regeneration.
Further features and advantages of the technology, as well as the structure and operation of various embodiments of the technology, are described in detail below with reference to the accompanying drawings. It is noted that the technology is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present technology and, together with the description, further serve to explain the principles of the technology and to enable a person skilled in the relevant art(s) to make and use the technology.

FIG. 1 is an illustration of a catalytic converter;

FIG. 2 is a schematic of a prior art SCR filter design;

FIG. 3 is a schematic of a SCR filter of the present technology; and

FIG. 4 is a chart illustrating performance of ammonia (NH3) neutral catalysts, which can be used in a SCR filter of the technology.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE TECHNOLOGY

While the present technology is described herein with illustrative embodiments for particular applications, it should be understood that the technology is not limited thereto. Those skilled in the art with access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the technology would be of significant utility. Features described in different embodiments described in the present specification may be combined.
FIG. 1 is an illustration of an after treatment system 100 for treating exhaust gas in a diesel engine. The given system, or a similar system, could be used for treating a lean-burn gasoline engine as well. The exhaust gas 102 is fed into or enters a diesel oxidation catalyst (DOC) 110, passes through a pipe to reach a selective catalytic reduction (SCR) filter (SCRF) 106. The SCRF is a combination of SCR and DPF technologies and controls both NOx and particulate matter (PM, also known as soot) emissions simultaneously. After being filtered by SCRF, the exhaust gas continues to the SCR catalyst 104, where more NOx are removed through reduction.
In the system 100, the SCR catalyst 104 reduces NOx using ammonia (NH3). The ammonia is introduced in the system 100 through a urea solution, e.g. a diesel exhaust fluid (DEF) solution, injected into the exhaust gas stream through a DEF injector 108. The urea solution mixes with the hot exhaust gas and produces ammonia (NH3), which is an agent for reducing NOx in the exhaust gas. Both the inlet and outlet channels of the SCRF 106 are coated with SCR catalyst; however, the DPF substrate of the SCRF 106 cannot be coated with excess of the SCR catalyst because the excessive coating will build up back pressure for the diesel engine. So generally, it is preferred to have a downstream SCR converter 104 installed. The downstream SCR converter will further reduce the residual NOx with ammonia.
During a DPF regeneration, DOC is used to oxidize the injected diesel fuel to generate exotherm (heat) for burning off soot accumulated in the inlet channels of a SCRF 106. However, some residual HC and CO will slip out of the DOC 110 when oxidizing the injected diesel fuel. As the exhaust gas mixed with HC and CO slips out from the DOC 110 and the NH3 produced from the urea solution moves through the exhaust system and goes into the SCRF 106, the SCRF 106 removes NOx from the exhaust gas. However, SCRF 106 does not effectively remove HC and CO because the SCRF 106 is not equipped with a catalyst agent to effectively oxidize HC and CO. For this purpose, a clean-up catalyst (not shown in FIG. 1) is needed at the tailpipe to remove residual HC and CO.
When the exhaust gas is injected with the urea solution through the DPF injector 108, ammonia produced from the urea solution will reduce NOx in the SCRF 106. As NOx is reduced, soot is also produced and accumulates in the inlet channels of SCRF 106. So, periodically DPF regeneration is conducted, during which diesel fuel is injected into the exhaust system 100 and the DOC 110 will combust the injected diesel fuel to burn off the soot. A typical SCR catalyst, however, is not effective for oxidizing soot during a DPF regeneration, and this results in an incomplete DPF regeneration. The inefficient DPF regeneration would in turn increase fuel consumption and increase the likelihood for uncontrolled DPF regenerations.
FIG. 2 is a schematic illustration of a conventional SCRF 200 for processing diesel exhaust. Exhaust gas 202 flows into the inlet channels 210 of the SCRF 200. The inlet channels 210 are coated with a SCR catalyst 206, such as a Zeolite-based SCR catalyst. The exhaust gas 202 is mixed with ammonia produced from the urea solution for reducing NOx in SCRF; in the meantime, the exhaust soot is deposited in the inlet channels of the SCRF 200 and the soot-filtered exhaust gas 202 exits through the outlet channel 208. The SCRF 200 with urea can simultaneously control NOx and PM emissions in diesel exhaust; however, a typical urea SCR catalyst, such as a zeolite-based SCR, is inactive for hydrogen-carbon (HC) and carbon monoxide (CO) oxidation at low exhaust temperature (<350 C). Moreover, a SCR catalyst is relatively inactive for catalyzing soot oxidation during a DPF regeneration, which results in low DPF regeneration efficiency.
Because of soot buildup inside the SCRF 200, the efficiency of NOx removal through a urea SCR catalyst coated on the inlet channels 210 may be reduced and the durability of SCR catalyst can be affected by contaminations from HC coking and ash depositions. The soot buildup also requires DPF regeneration to be conducted periodically. During the DPF regeneration, diesel fuel is injected into the exhaust gas and the soot is burned off. If the soot accumulation occurs at a higher rate, more frequent DPF regeneration would be needed and consequently, there is a greater likelihood of the uncontrolled DPF regeneration being affected by driving condition.
The shortcomings of the SCRF of FIG. 2 are overcome by an improved SCRF catalyst design according to the present technology. FIG. 3 is a schematic illustration of an improved SCRF300. The inlet channels 310 of this SCRF are coated with an ammonia-neutral oxidation catalyst (ANOC) 306. The ANOC 306 is basically inactive or non-selective for ammonia oxidation when temperature is below 400 C but still maintains HC and CO oxidation capability as a special type of DOC catalyst for temperature below 400 C. By coating the inlet channels 310 with an ANOC catalyst 306 washcoat, the ammonia produced via DEF (urea solution) injection can safely penetrate through the inlet channels 310 without being oxidized when temperature is lower than 400 C. The outlet channels 312 of the new SCRF are coated by a washcoat of urea SCR catalyst, such as a zeolilte-based SCR.
As illustrated in FIG. 3, the exhaust gas 302 with ammonia flows into the inlet channels 304 of the SCRF 300 and the ammonia can pass through the channels coated with ANOC 306 and reach the outlet channels 308. In the outlet channels 308, the ammonia reduces nitrogen oxides (NOx) of the exhaust gas in the presence of SCR catalyst 106 before the exhaust gas is released. The SCR catalyst is not consumed during reduction of NOx. As the exhaust gas flows into the inlet, coated with the ANOC 306, HC and CO oxidation occurs, as well as soot deposition, HC coking, and ash accumulations. The NOx in the exhaust gas passes through the inlet channels 310 substantially unaffected and reaches the outlet channels 312 that are coated by the SCR catalyst 106. The SCR NOx reduction activity occurs more reliably in the outlet channels because the NOx reduction occurs separately from the soot deposition, HC coking, and ash accumulation.
Because the reduction of NOx occurs in the outlet channels 312 when the exhaust gas passes through the outlet channels and the soot deposition, HC coking, and ash accumulation mostly occur in the inlet channels 310, the SCR washcoat, in the outlet channels 312, is prevented from being contaminated by ash and masked by HC coking. As a result, the reliability and durability of the SCRF 300 is enhanced.
In the improved SCRF catalyst design, the DPF regeneration efficiency is enhanced by an ammonia-neutral oxidation catalyst (ANOC) along with more effective HC and CO slip control. As a result, lower vehicle fuel consumption can be realized and DPF regeneration need to be conducted less frequently, thus reducing the possibility of DPF regeneration being affected by vehicle driving conditions, such as an uncontrolled DPF regeneration at an idling condition.
FIG. 4 is a chart 400 illustrating performance of ammonia-neutral catalysts related to the present technology. The X-axis indicates inlet temperature in degrees centigrade (C) of ammonia-neutral catalysts including a reference DOC catalyst. The left Y-axis indicates an ammonia (NH3) conversion percentage and the right Y-axis indicates a percentage by which nitrogen oxides (NOx) are remade via ammonia oxidation.
Bars 402 represent performance of a conventional DOC at different temperatures. Bars 404 represent performance of an improved ammonia-neutral oxidation catalyst (ANOC) at different temperatures. As can be seen, NOx remaking does not start until the temperature approaches about 400° C. for the given ANOC. In contrast, ammonia (NH3) conversion and NOx remaking start at a much lower temperature of approximately 250° C. for the typical DOC.
It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present technology as contemplated by the inventor(s), and thus, are not intended to limit the present technology and the appended claims in any way. It is within the scope of the technology that different embodiments described in the present specification may be combined.

Claims

What is claimed is:

1. An exhaust gas treatment apparatus comprising:

a diesel oxidation catalyst unit for receiving exhaust gas; and

a selective catalyst reduction filtering unit, having at least one inlet channel and being connected to the diesel oxidation catalyst unit, for oxidizing hydrocarbon and carbon monoxide, reducing nitrogen oxide, and controlling particulate matters emission in the exhaust gas,

wherein the at least one inlet channel is coated with an ammonia-neutral oxidation catalyst.

2. The exhaust gas treatment apparatus of claim 1, further comprising at least one outlet channel coated with a selective catalytic reduction catalyst.

3. The exhaust gas treatment apparatus of claim 2, wherein the selective catalytic reduction catalyst includes a zeolilte-based catalyst.

4. The exhaust gas treatment apparatus of claim 1, further comprising a selective catalytic reduction catalyst connected to the selective catalyst reduction filtering unit, for reducing the nitrogen oxides.

5. The exhaust gas treatment apparatus of claim 4, wherein the selective catalyst reduction unit is coated with a zeolilte-based catalyst.

6. The exhaust gas treatment apparatus of claim 1, further comprising a diesel exhaust fluid injector for injecting a urea solution into the exhaust gas.

7. The exhaust gas treatment apparatus of claim 6, wherein the diesel exhaust fluid injector is located between the diesel oxidation catalyst unit and the selective catalyst reduction filtering unit.

8. A selective catalytic reduction filter comprising:

at least one inlet channel for receiving exhaust gas; and

at least one outlet channel connected to the at least one inlet channel,

9. The selective catalytic reduction filter of claim 8, wherein the at least one outlet channel is coated with a selective catalyst reduction catalyst.

10. The selective catalytic reduction filter of claim 9, wherein the selective catalytic reduction filter is coated with a zeolilte-based catalyst.

11. The selective catalytic reduction filter of claim 8, wherein, when the selective catalytic reduction filter is in operation, nitrogen oxides in the exhaust gas pass through the at least one inlet channel substantially unaffected by the ammonia-neutral oxidation catalyst.

12. The selective catalytic reduction filter of claim 9, wherein, when the selective catalytic reduction filter is in operation, the exhaust gas is mixed with ammonia and nitrogen oxides in the exhaust gas is reduced when nitrogen oxides react with ammonia when the exhaust gas passes through the at least one outlet channel coated with the SCR catalyst.

13. The selective catalytic reduction filter of claim 8, wherein the exhaust gas is from a diesel engine.

14. The selective catalytic reduction filter of claim 8, wherein the exhaust gas is from a lean-burn gasoline engine.

15. A method for reducing nitrogen oxides from an exhaust gas, comprising:

feeding an exhaust gas with ammonia into a selective catalytic reduction filtering device comprising at least one inlet channel coated with an ammonia-neutral oxidation catalyst and at least one outlet channel coated with a selective catalytic reduction catalyst;

passing the ammonia through the at least one inlet channel to the at least one outlet channel; and

reducing, in the at least one outlet channel, nitrogen oxides in the exhaust gas.

16. The method of claim 15, further comprising injecting a urea solution into the exhaust gas to produce the ammonia.

17. The method of claim 15, further comprising oxidizing, in the at least one inlet channel, hydro-carbon and carbon-monoxide in the exhaust gas.

18. The method of claim 15, wherein reducing the nitrogen oxides occurs at temperatures below about 400C.

19. The method of claim 15, wherein the selective catalytic reduction catalyst comprises a zeolilte-based catalyst.

20. The method of claim 15, further comprising receiving the exhaust gas a diesel oxidation catalyst.