Patent Application
20180058286
The concepts described herein relate to internal combustion engines, and associated exhaust purification devices.
Internal combustion engines generate exhaust gases as byproducts of the combustion process, including nitrogen oxides (NOx), carbon monoxide (CO), and uncombusted or partially combusted hydrocarbons (HC). Emission control systems are employed to oxidize, reduce, filter and/or store and release various exhaust gas constituents prior to release into the atmosphere, and may achieve high efficiencies once reaching warmed up operating temperatures. However, such systems may be less efficient when operating at temperatures that are less than warmed up operating temperatures, such as may occur following a cold start.
Effective control of exhaust emissions at low temperatures is critical for emission compliance due to the generation of exhaust gas constituents during cold-start engine operation. Fuel-saving technologies such as lean burn engine operation, turbocharging, and other advanced combustion techniques may result in lower overall exhaust temperatures, further complicating low temperature emissions control.
An exhaust aftertreatment system for purifying an exhaust gas feedstream from an internal combustion engine that is disposed to operate at a lean air/fuel ratio is described, and includes an oxidation catalyst disposed upstream of a low-temperature NOx adsorber. The oxidation catalyst includes a zeolite catalyst including a base metal, a noble metal, and a zeolite disposed on a substrate, and the low-temperature NOx adsorber includes a zeolite catalyst and a supported platinum group metal catalyst.
The above features and advantages, and other features and advantages, of the present teachings are readily apparent from the following detailed description of some of the best modes and other embodiments for carrying out the present teachings, as defined in the appended claims, when taken in connection with the accompanying drawings.
One or more embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:
The components of the disclosed embodiments, as described and illustrated herein, may be arranged and designed in a variety of different configurations. Thus, the following detailed description is not intended to limit the scope of the disclosure, as claimed, but is merely representative of possible embodiments thereof. In addition, while numerous specific details are set forth in the following description in order to provide a thorough understanding of the embodiments disclosed herein, some embodiments can be practiced without some or all of these details. Moreover, for the purpose of clarity, certain technical material that is known in the related art has not been described in detail in order to avoid unnecessarily obscuring the disclosure. The drawings are in simplified form and are not to precise scale. Furthermore, the disclosure, as illustrated and described herein, may be practiced in the absence of any element which is not specifically disclosed herein. As employed herein, the term “upstream” and related terms refer to elements that are towards an origination of a flow stream relative to an indicated location, and the term “downstream” and related terms refer to elements that are away from an origination of a flow stream relative to an indicated location.
Referring to the drawings, wherein like reference numerals correspond to like or similar components throughout the several Figures,
The engine 10 may be any suitable internal combustion engine, and is preferably configured as a multi-cylinder compression-ignition engine that primarily operates at an air/fuel ratio that is lean of stoichiometry in one embodiment. The engine 10 may include a cylinder block having a plurality of cylinders and pistons therein (not separately shown), which, along with a cylinder head (also not separately shown), may define combustion chambers for internal combustion of a mixture of fuel and induction gases. The engine 10 may also include any suitable quantities of intake valves and exhaust valves disposed in the cylinder head for controlling flow of intake air and exhaust gases. The engine 10 preferably includes an exhaust manifold 12 that entrains exhaust gases that are output as a result of the combustion process and channels them into the exhaust aftertreatment system 20 for purification and expulsion into the atmosphere.
The exhaust aftertreatment system 20 preferably includes one or more additional aftertreatment devices that are disposed to oxidize, reduce, store, filter or otherwise treat the exhaust gas feedstream 15 of the engine 10. For cold start hydrocarbon control, an exhaust system may include hydrocarbon trapping components that employ zeolite materials. In such systems, the zeolite material adsorbs and stores hydrocarbons during the start-up period and releases the stored hydrocarbons when the exhaust temperature is high enough to desorb hydrocarbons. The desorbed hydrocarbons are preferably oxidized in downstream catalytic components. For cold start NOx control, especially under lean-burn engine operating conditions, an exhaust system may include NOx storage and release catalysts such as selective catalytic reduction (SCR) devices or NOx adsorbers to reduce NOx to nitrogen. Such catalysts adsorb NOx during warm-up and thermally desorb NOx at higher exhaust temperatures.
As shown, the exhaust aftertreatment system 20 preferably includes a diesel oxidation catalyst (DOC) 30 that is disposed upstream of a low-temperature NOx adsorber (LTNA) 40. In one embodiment, another exhaust purification device 50, such as an SCR device or a SCR device that is disposed on a substrate that includes a particulate filter (SCRF), is disposed downstream of the LTNA 40. When the exhaust purification device 50 is an SCR or an SCRF, a reductant injector system 42 is disposed upstream thereto. Design, implementation and operational control of reductant injector systems 42 and purification devices 50 such as SCRs and SCRFs are application-specific and known to one of ordinary skill in the art, and thus not described herein.
In one embodiment, the DOC 30 and the LTNA 40 are disposed in a close-coupled arrangement relative to the engine 10 and exhaust manifold 12. As used herein, the term “close-coupled” refers to a position of a device of the exhaust aftertreatment system 20 that is in as close proximity to the exhaust manifold 12 as is practicable in order to minimize loss of thermal energy from the exhaust gas feedstream prior to the exhaust gas reaching the device, e.g., DOC 30. In one embodiment, the close-coupled arrangement includes having the DOC 30 at a location that is less than about 1 meter downstream from the exhaust manifold 12 or turbocharger, and is preferably about 0.05 to about 0.5 meters. Furthermore, a close-coupled exhaust component is preferably located underhood in an engine compartment, although such an arrangement may not be practicable in some embodiments. As known to one skilled in the art, during engine start-up and engine operation under heavy load, exhaust aftertreatment devices that are arranged in a close-coupled position may be exposed to higher exhaust gas temperatures as compared to devices that are further downstream.
The DOC 30, 130 is configured to promote oxidation of several exhaust gas components in the exhaust gas feedstream in the presence of oxygen, which may be abundant in a lean exhaust environment. The exhaust components that may be oxidized include carbon monoxide (CO), gas phase hydrocarbons (HC), and a soluble organic fraction (SOF) of particulates. The DOC 30, 130 includes, for example, catalytic material that is supported on a surface of a substrate via washcoating and other processes that may include impregnation, adsorption, ion-exchange, etc. The substrate may be in the form of a structure having a multiplicity of flow channels arranged in parallel to an axial axis between an inlet and an outlet, wherein the flow channels are capable of retaining a washcoat that contains catalytic materials. The cross-sectional shape of the channels may be any suitable shape, including e.g., square, sinusoidal, triangular, rectangular, hexagonal, trapezoidal, circular, or oval.
In one embodiment, the substrate is fabricated from extruded ceramic materials such as cordierite. Alternatively, the substrate is fabricated from metal foil. When the substrate is formed from ceramic materials, it preferably has a honeycomb structure, and can be arranged as a flow-through device, or alternatively, as a wall-flow filter device that is able to remove particulate matter from the exhaust gas feedstream. The ceramic substrate may be fabricated from any suitable refractory material, e.g., alumina, silica, titania, ceria, zirconia, magnesia, zeolites, silicon nitride, silicon carbide, zirconium silicates, magnesium silicates, aluminosilicates, metallo aluminosilicates (such as cordierite and spudomene), or a mixture or mixed oxide of any two or more thereof. Cordierite, a magnesium aluminosilicate, and silicon carbide are preferred. The metallic substrates may be fabricated from any suitable metal, and in particular heat-resistant metals and metal alloys such as titanium and stainless steel as well as ferritic alloys containing iron, nickel, chromium, and/or aluminum in addition to other trace metals.
When the substrate is arranged as a wall-flow filter device, adjacent flow channels are blocked on alternate axial ends. This allows the exhaust gas feedstream to enter a channel from an inlet, flow through the channel walls, and exit the filter from a different channel leading to the outlet. Particulates in the exhaust gas stream may be thus trapped in the particulate filter and subsequently oxidized.
The catalytic material is preferably a zeolite catalyst that includes a base metal, a noble metal, and a zeolite. In one embodiment, the catalytic material further includes an oxygen storage capacity material disposed on the substrate. In one embodiment, the oxygen storage capacity material is ceria. Alternatively, the catalytic material may include, by way of non-limiting examples, an inorganic oxide selected from the group consisting of alumina, silica, titania, and zirconia. The noble metal may be selected from the group consisting of Pt, Pd, Rh, Ag, Au and Ir.
The LTNA 40, 140 may be composed as a zeolite catalyst and a supported platinum group metal catalyst that are disposed on a substrate. The zeolite catalyst preferably includes a base metal, a noble metal, and a zeolite. The base metal is preferably iron, copper, manganese, chromium, cobalt, nickel, tin, or mixtures thereof; more preferably, iron, copper, manganese, cobalt, or mixtures thereof. Iron is particularly preferred. The noble metal is preferably palladium, platinum, rhodium, silver, or mixtures thereof.
The zeolite may be any natural or a synthetic zeolite, including molecular sieves, and is preferably composed of aluminum, silicon, and/or phosphorus. The zeolite may have a three-dimensional arrangement of SiO4, AlO4, and/or PO4 that are joined by the sharing of oxygen atoms. The zeolite frameworks may be anionic, which are counterbalanced by charge compensating cations, including alkali and alkaline earth elements (e.g., Na, K, Mg, Ca, Sr, and Ba) and also protons. Other metals (e.g., Fe, Ti, and Ga) may be incorporated into the framework of the zeolite to produce a metal-incorporated zeolite. For example, iron may be substituted for aluminum in the framework of beta zeolite to produce an iron-beta zeolite (Fe-p zeolite).
In one embodiment, the zeolite is a beta zeolite, a faujasite (such as an X-zeolite or a Y-zeolite, including NaY and USY), an L-zeolite, a ZSM zeolite (e.g., ZSM-5, ZSM-48), an SSZ-zeolite (e.g., SSZ-13, SSZ-41, SSZ-33), a mordenite, a chabazite, an offretite, an erionite, a clinoptilolite, a silicalite, an aluminum phosphate zeolite (including metalloaluminophosphates such as SAPO-34), a mesoporous zeolite (e.g., MCM-41, MCM-49, SBA-15), a metal-incorporated zeolite, or mixtures thereof; more preferably, the zeolites are beta zeolite, ZSM-5 zeolite, Fe-p zeolite, or SSZ-33, or Y-zeolite. The zeolite is most preferably beta zeolite, ZSM-5 zeolite, Fe-p zeolite, or SSZ-33.
The zeolite catalyst may be prepared employing any suitable process. For example, the base metal and noble metal may be added to the zeolite to form the zeolite catalyst employing any suitable process. For example, a noble metal compound (such as palladium nitrate) and a base metal compound (such as iron nitrate) may be supported on the zeolite by processes that include impregnation, adsorption, ion-exchange, incipient wetness, precipitation, or the like. The noble metal compound and a base metal compound may be added to the zeolite simultaneously in one step, or sequentially in multiple steps.
The supported platinum group metal catalyst includes one or more platinum group metals (“PGM”) and one or more inorganic oxide carriers. The PGM may be platinum, palladium, rhodium, iridium, or combinations thereof, and most preferably platinum and/or palladium. The inorganic oxide carriers most commonly include oxides of Groups 2, 3, 4, 5, 13 and 14 elements. Useful inorganic oxide carriers preferably have surface areas in the range 10 to 700 m.sup.2/g, pore volumes in the range 0.1 to 4 mL/g, and pore diameters from about 10 to 1000 Angstroms. The inorganic oxide carrier is preferably alumina, silica, titania, zirconia, ceria, niobia, tantalum oxides, molybdenum oxides, tungsten oxides, or mixed oxides or composite oxides of any two or more thereof, e.g. silica-alumina, ceria-zirconia or alumina-ceria-zirconia. Alumina and ceria are particularly preferred.
The supported platinum group metal catalyst may be prepared employing any suitable process. Preferably, the one or more platinum group metals are loaded onto the one or more inorganic oxides employing any suitable process to form the supported PGM catalyst. For example, a platinum compound (such as platinum nitrate) may be supported on an inorganic oxide by processes that may include impregnation, adsorption, ion-exchange, incipient wetness, precipitation, or the like. Other metals may also be added to the supported PGM catalyst.
Preferably, the LTNA 40, 140 further includes a flow-through substrate or a filter substrate. In one embodiment, the zeolite catalyst and the supported platinum group metal catalyst are coated onto the substrate, and preferably deposited on the substrate using a washcoat procedure to produce the LTNA 40, 140.
The zeolite catalyst and the supported platinum group catalyst may be added to the substrate employing any suitable process. A representative process for preparing the LTNA 40 using a washcoat procedure is set forth below. It will be understood that the process below can be varied according to different embodiments. Also, the zeolite catalyst and the supported PGM catalyst may be added onto the substrate in any suitable order. Thus, the zeolite catalyst may be washcoated on the substrate prior to the supported PGM catalyst, or, alternatively the supported PGM catalyst may be washcoated on the substrate prior to the zeolite catalyst.
The pre-formed zeolite catalyst may be added to the substrate by a washcoating step. Alternatively, the zeolite catalyst may be formed on the substrate by first washcoating unmodified zeolite, a noble metal/zeolite or a base metal/zeolite onto the substrate to produce a zeolite-coated substrate. Noble metal and/or base metal may then be added to the zeolite-coated substrate, which may be accomplished by an impregnation procedure, or the like. In an alternative embodiment, the substrate is composed of the zeolite catalyst, and the supported platinum group metal catalyst is coated onto the zeolite catalyst substrate. The zeolite may be extruded to form the substrate, and is preferably extruded to form a honeycomb substrate. Extruded zeolite substrates and honeycomb bodies, and processes for making them, are known in the art. If a zeolite substrate is formed, the zeolite substrate is then subjected to an impregnation procedure if necessary to load noble metal and/or base metal to the zeolite substrate, followed by a washcoating step to washcoat the supported PGM catalyst.
In certain embodiments, the SCR 50, 150 is formulated as a part of a substrate that includes a particulate filter. In one example, the SCR 50, 150 is an SCR catalyst washcoat provided on a ceramic substrate, which may include a particulate filter. The reductant delivery system 42, 142 that is positioned upstream of the SCR 50, 150, respectively may include any suitable type of reductant injector or delivery device known in the art, including a urea or ammonia injector, and further including an air-assisted, liquid phase, or gas phase injector. The SCR 50, 150 is preferably used to convert oxides of nitrogen (NOx) into diatomic nitrogen (N2) and water (H2O).
Selective catalytic reduction (SCR) systems are devices that reduce NOx to N2 by reaction with nitrogen compounds (such as ammonia or urea) or hydrocarbons (lean NOx reduction). An SCR catalyst may include a vanadia-titania catalyst, a vanadia-tungsta-titania catalyst, or a metal/zeolite catalyst such as iron/beta zeolite, copper/beta zeolite, copper/SSZ-13, copper/SAPO-34, Fe/ZSM-5, or copper/ZSM-5. Particulate filters are devices that remove particulates from the exhaust gas feedstream 15. Particulate filters include catalyzed particulate filters and bare (non-catalyzed) particulate filters. Catalyzed particulate filters include metal and metal oxide components (such as Pt, Pd, Fe, Mn, Cu, and ceria) to oxidize hydrocarbons and carbon monoxide in addition to oxidizing particulate matter that is trapped by the particulate filter.
The detailed description and the drawings or figures are supportive and descriptive of the present teachings, but the scope of the present teachings is defined solely by the claims. While some of the best modes and other embodiments for carrying out the present teachings have been described in detail, various alternative designs and embodiments exist for practicing the present teachings defined in the appended claims.
