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DE-112013007862-B4 - Closely coupled SCR system

DE112013007862B4DE 112013007862 B4DE112013007862 B4DE 112013007862B4DE-112013007862-B4

Abstract

System for the treatment of NOx- containing exhaust gases from an engine, the system comprising the following: a flow monolith with a first catalytic composition for selective catalytic reduction of NO x and with a first volume; a closely coupled particulate matter filter with a second catalytic composition for the reduction of particulate matter and the selective catalytic reduction of NO x and with a second volume; wherein the first volume is 10% to 75% of the second volume; wherein the flow monolith is in fluid communication with the fine dust filter, and is installed upstream of it, and wherein the first and the second catalytic composition comprise a base metal-supported aluminosilicate molecular sieve or a silicoaluminophosphate molecular sieve, and wherein the first and the second catalytic composition are different; and furthermore an oxidation catalyst comprising at least one platinum group metal selected from platinum, palladium or a combination of platinum and palladium, wherein the platinum group metal is applied to a high surface area primer component selected from aluminum oxide, zeolite, silicon dioxide, non-zeolite silicon dioxide-aluminum oxide, cerium oxide, zirconia, titanium dioxide or a mixed or compound oxide containing both cerium oxide and zirconia, wherein the oxidation catalyst is arranged upstream of the flow monolith and is adapted to release a gas stream having a ratio of NO to NO 2 of 4:1 to 1:3 per volume.

Inventors

  • Paul Richard Phillips
  • James Alexander Wylie

Assignees

  • JOHNSON MATTHEY PUBLIC LIMITED COMPANY

Dates

Publication Date
20260513
Application Date
20131017
Priority Date
20121018

Claims (18)

  1. A system for treating NOx- containing exhaust gases from an engine, comprising: a flow-through monolith with a first catalytic composition for selective catalytic reduction of NOx and with a first volume; a closely coupled particulate filter with a second catalytic composition for particulate reduction and selective catalytic reduction of NOx and with a second volume; wherein the first volume is 10% to 75% of the second volume; wherein the flow-through monolith is in fluid communication with the particulate filter and is installed upstream of it, and wherein the first and second catalytic compositions comprise a base metal-supported aluminosilicate molecular sieve or a silicoaluminophosphate molecular sieve, and wherein the first and second catalytic compositions are different; and further an oxidation catalyst comprising at least one platinum group metal selected from platinum, palladium or a combination of platinum and palladium, wherein the platinum group metal is applied to a high surface area primer component selected from aluminum oxide, zeolite, silicon dioxide, non-zeolite silicon dioxide-aluminum oxide, cerium oxide, zirconia, titanium dioxide or a mixed or compound oxide containing both cerium oxide and zirconia, wherein the oxidation catalyst is arranged upstream of the flow monolith and is adapted to release a gas stream having a NO to NO₂ ratio of 4:1 to 1:3 per volume.
  2. System according to Claim 1 , in which the volume ratio is 1:10 to 1:2.
  3. System according to Claim 1 , in which the volume ratio is 1:6 to 1:4.
  4. System according to Claim 1 , wherein the flow monolith is an extruded catalyst brick.
  5. System according to Claim 4 , wherein the fine dust filter is a non-reactive substrate coated and/or impregnated with the second catalytic composition.
  6. System according to Claim 5 , in which the substrate is primarily made of either cordierite or metal.
  7. System according to Claim 1 , in which the flow monolith has a lower heat capacity in relation to the fine dust filter.
  8. System according to Claim 1 , in which the flow monolith has a lower specific heat capacity in relation to the fine dust filter.
  9. System according to Claim 8 , wherein the flow monolith has a specific heat capacity that is 20 to 80% of the specific heat capacity of the particulate filter.
  10. System according to Claim 9 , wherein the flow monolith has a specific heat capacity that is 35 to 65% of the specific heat capacity of the particulate filter.
  11. System according to Claim 1 , wherein the flow-through monolith has an SCR catalyst loading that is greater than an SCR catalyst loading on the particulate filter.
  12. System according to Claim 11 , wherein the flow monolith has an SCR catalyst loading of 3 to 15 g/in 3 .
  13. System according to Claim 1 , wherein the second catalytic composition for the selective catalytic reduction of NO x is coated and/or impregnated on a downstream side of the particulate filter.
  14. System according to Claim 1 , wherein the second catalytic composition is for the selective catalytic reduction of NO x on an upstream side of the fine dust filter is coated and/or impregnated.
  15. System according to Claim 1 , in which the fine dust filter is located 0.01 to 0.25 meters downstream of the flow monolith.
  16. System according to Claim 15 , further comprising a source of reducing agent injection, in fluid communication with and arranged between the flow monolith and the fine dust filter.
  17. A method for treating an engine exhaust stream containing NOx and soot, comprising: contacting the engine exhaust stream with an oxidation catalyst comprising at least one platinum group metal selected from platinum, palladium, or a combination of platinum and palladium, wherein the platinum group metal is applied to a high surface area primer component selected from aluminum oxide, zeolite, silicon dioxide, non-zeolite silicon dioxide-aluminum oxide, cerium oxide, zirconia, titanium dioxide, or a mixed or compound oxide containing both cerium oxide and zirconia, wherein the oxidation catalyst is adapted to release a gas stream having a NO to NO2 ratio of 4:1 to 1:3 per volume; contacting the gas stream in the presence of a reducing agent with a flow-through monolith having a first SCR catalyst composition loading and a first volume to generate an intermediate gas stream; The process involves bringing the intermediate gas stream into contact with a tightly coupled catalytic particulate filter with a second SCR catalyst composition loading and a second volume to capture some of the soot and produce a clean gas stream; oxidizing the portion of soot at a soot oxidation temperature to renew the catalytic particulate filter; heating the catalytic tightly coupled flow monolith to an SCR start-up temperature before the catalytic particulate filter is heated to an SCR start-up temperature; and maintaining, under low-load conditions, the soot oxidation temperature of the catalytic particulate filter for a longer period of time compared to a catalytic particulate filter with a volume equal to the combined first and second volumes, wherein the first volume is 10% to 75% of the second volume.
  18. Procedure according to Claim 17 , wherein a system according to Claim 1 is used.

Description

AREA OF INVENTION This invention relates to the exhaust gas purification of internal combustion engines. BACKGROUND OF THE INVENTION One of the most harmful components of vehicle exhaust is NOx, which includes nitrogen oxide (NO), nitrogen dioxide ( NO₂ ), and nitrous oxide ( N₂O ). NOx emissions are particularly problematic for lean-burn engines, such as diesel engines. To reduce the environmental impact of NOx in exhaust gases, it is desirable to eliminate these undesirable components, preferably through a process that does not produce any other harmful or toxic substances. In addition to emitting NOx gases, lean-burn engines have the disadvantage, due to their combustion characteristics, of producing particulate matter or soot, to which a variety of organic substances can be absorbed, including unburned hydrocarbons and sulfuric acid, which is produced by the oxidation of sulfur dioxide derived from sulfur compounds present in the fuel or lubricating oils. Diesel engine exhaust tends to contain more soot compared to gasoline engines. Since exhaust gases from lean combustion engines contribute to air pollution, treatment systems are essential to minimize the harmful environmental impacts resulting from operating a lean combustion engine. Two methods are commonly used to reduce pollutants in the exhaust gases of lean-burn engines. The first method converts NOx in diesel exhaust into less harmful substances, a process also known as selective catalytic reduction (SCR). An SCR process involves the conversion of NOx, in the presence of a catalyst and a reducing agent, typically anhydrous ammonia, aqueous ammonia solution, or urea, into elemental nitrogen ( N₂ ) and water. The second method reduces soot emissions by passing the soot-laden exhaust gas through a particulate filter. However, the accumulation of soot particles on the filter can cause an undesirable increase in the exhaust system's backflow while it is in operation, thus reducing efficiency. To renew the filter, the accumulated carbon-based soot must be removed, for example, by regularly burning off the soot through passive or active oxidation at high temperatures. From the WO 99 / 39 809 A1 The combination of several separate components in an exhaust system is known, including an SCR catalyst, to treat, among other things, particulate matter and nitrogen oxides. For example, an exhaust stream from an engine, combined with a reducing agent, may first flow through a flow-through monolith containing the SCR catalyst to reduce NOx, and then the gases are further treated downstream to remove particulate matter by passing them through a particulate filter. The disadvantage of such designs is that an increase in the number of exhaust aftertreatment components increases the overall cost of the exhaust system as well as its overall volume and weight, which is particularly detrimental for motor vehicles. The heavier a motor vehicle's overall exhaust system is, the more fuel the vehicle will need to carry it. To counteract the aforementioned disadvantages, exhaust systems have been designed with a single component that can reduce NOx and particulate matter. The US patent publication US 2010 / 0 180 580 A1 Disclosure reveals a system for combining an SCR catalyst with a wall-flow filter. Wall-flow filters contain multiple adjacent parallel channels, each closed at one end, with the closure occurring at the opposite ends of adjacent channels in an alternating pattern. The closure of alternating channel ends prevents the gas entering the inlet side of the filter from flowing straight through and exiting the channel. Instead, the gas enters the front of the substrate and travels to approximately the middle of the channels, where it is forced over the channel walls before exiting through the outlet side of the substrate. The catalyst is typically applied to the walls of the wall-flow filter as an aqueous mixture or primer and then calcined to adhere to the surface of the walls. The disadvantage of certain SCR wall-flow filters is that a limited amount of catalyst can be applied to the surface. A thick primer will constrict the channels and, in some cases, the pores, inhibiting gas flow and contributing to backflow, thus negatively impacting the system's effectiveness. A known method for reducing backflow involves limiting the amount of primer applied. Less primer results in less catalyst and reduced efficiency. of the filter to convert NOx. Ultimately, coating the surface of a wall-flow filter with an SCR catalyst contributes to the filter's overall weight. Increased mass will require more time and energy to heat the wall-flow filter to the temperatures necessary for catalyst activation, which is a significant disadvantage during start-up when the engine has not yet reached its normal steady-state operating temperature. To increase the heating rate of an SCR wall-flow filter, the filter can be positioned close to the engine. A well-known method to counteract the d