Low ammonia-emmiting mobile exhaust aftertreatment system

Abstract

Systems and methods are disclosed for the removal of hydrocarbons, nitrogen oxides, and carbon monoxide. The systems are further configured to reduce ammonia formulation while increasing the removal of ammonia that is formulated during the removal of other pollutants. The system includes an upstream three-way catalyst positioned on a first substrate and comprising a bilayer washcoat structure and a downstream catalyst positioned on a second substrate and comprising one of a three-way catalyst or a four-way catalyst.

Description

LOW AMMONIA-EMMITING MOBILE EXHAUST AFTERTREATMENTSYSTEMBACKGROUND

[0001] In metropolitan areas, the gasoline-powered mobile source accounts for approximately 50% of ammonia emission, which contributes to formation of particulate matter (PM) in the atmosphere. Three-way catalysts and systems, during abating hydrocarbons (HC), nitrogen oxides (NOx), and carbon monoxide (CO) pollutants of engine-out exhausts for gasoline vehicles, unavoidably emit ammonia as a secondary emission. Mobile emission regulations continue to set limits for the ammonia emissions from vehicles.

[0002] A state-of-the-art exhaust aftertreatment system for a gasoline-powered vehicle typically consists of at least one upstream catalyst and one downstream catalyst. The upstream catalyst is a conventional three-way catalyst (TWC), placed in a first close-coupled (CC1) position near the engine-out manifold. The downstream catalyst is either a TWC or four-way catalyst (FWC), which is located either in a second close-coupled (CC2) or underfloor (UF) position. The upstream CC1 TWC experiences most engine lambda perturbations, and therefore forms ammonia under rich-biased conditions. The downstream CC2 (or UF) TWC (or FWC) can largely wipe out the lambda fluctuations broken through from the upstream catalyst, and hence converts a moderate or substantial amount of ammonia formed by the CC1 TWC.

[0003] There exists a need for an improved exhaust aftertreatment system to reduce ammonia formulations and emissions while maintaining reduction of pollutants such as hydrocarbons, nitrogen oxides, and carbon monoxide.SUMMARY

[0004] In one embodiment, an exhaust aftertreatment system for removing HC, NOx, and CO from an exhaust stream of an internal combustion engine comprises an upstream three-way catalyst (TWC) deposited on a first substrate and including a bilayer washcoat structure; and a downstream catalyst deposited on a second substrate and including one of a TWC or a fourway catalyst (FWC), wherein the upstream TWC includes a topcoat including at least 75 wt.% Rh of the total Rh loading of the upstream TWC and a bottomcoat including at least 75 wt.% Pd of the total Pd loading of the upstream TWC, and wherein the downstream catalyst includesa Rh loading of about 0.5 g / ft3to about 20 g / ft3and a Pd loading wherein the Rh loading is higher than the Pd loading.

[0005] In some embodiments, the upstream TWC includes a total ceria loading of about 0.75 g / in3to about 1.50 g / in3.

[0006] In some embodiments, the upstream TWC includes a total ceria loading of about 0.90 g / in3to about 1.30 g / in3.

[0007] In some embodiments, the downstream catalyst includes a lower total loading of ceria than the upstream TWC.

[0008] In some embodiments, the upstream TWC further includes a Pt loading of about 1 g / ft3to about 100 g / ft3.

[0009] In some embodiments, the bottomcoat of the upstream TWC is deposited on a surface of the first substrate and the topcoat of the upstream TWC is deposited on a surface of the bottomcoat.

[0010] In some embodiments, the downstream catalyst further includes a Pt loading of about 1 g / ft3to about 30 g / ft3.

[0011] In some embodiments, the downstream catalyst is essentially free of Pd.

[0012] In some embodiments, the first substrate includes a flow-through honeycomb structure with a cell density of about 400 cpsi to about 900 cpsi.

[0013] In some embodiments, the second substrate includes a flow-through honeycomb structure with a cell density of about 400 cpsi to about 900 cpsi.

[0014] In some embodiments, the second substrate includes a wall-flow filter honeycomb structure with a cell density of about 200 cpsi to 600 cpsi.

[0015] In some embodiments, the upstream TWC is positioned in a first close-coupled position.

[0016] In some embodiments, the downstream catalyst is positioned in one of a second close- coupled position or an underfloor position.

[0017] In some embodiments, the upstream TWC includes a first alumina component and a first oxygen storage component (OSC), wherein the first OSC includes ceria, zirconia, and a first dopant including yttria, lanthana, praseodymia, neodymia, and combinations thereof.

[0018] In some embodiments, the first alumina component and the first OSC are present in a weight ratio of about 1 : 5 to 3 : 1.

[0019] In some embodiments, the downstream TWC includes a second alumina component and a second OSC, wherein the second OSC includes ceria, zirconia, and a second dopant including yttria, lanthana, praseodymia, neodymia, and combinations thereof.

[0020] In some embodiments, the second alumina component and the second OSC are present in a weight ratio of about 1 : 5 to 3 : 1.

[0021] In some embodiments, the exhaust aftertreatment system further comprises a third catalyst positioned downstream of the downstream catalyst, wherein the third catalyst includes: a platinum group metal component, a third alumina component; and a third OSC including ceria, zirconia, and a third dopant selected from yttria, lanthana, praseodymia, neodymia, and combinations thereof.

[0022] In one embodiment, a method of removing HC, NOx, and CO from an exhaust stream comprises providing a catalyst system comprising an upstream TWC deposited on a first substrate and including a bilayer washcoat structure and a downstream catalyst deposited on a second substrate and including one of a TWC or a FWC; contacting the upstream TWC with an exhaust stream including HC, NOx, and CO; and contacting the downstream catalyst with the exhaust stream, wherein the upstream TWC includes a topcoat including at least 75 wt.% Rh of the total Rh loading of the upstream TWC and a bottomcoat including at least 75 wt.% Pd of the total Pd loading of the upstream TWC, and wherein the downstream catalyst includes a Rh loading of about 0.5 g / ft3to about 20 g / ft3and a Pd loading wherein the Rh loading is higher than the Pd loading.

[0023] In some embodiment, the method further comprises forming ammonia at the upstream TWC to form an ammonia-including exhaust stream; and removing the ammonia from the exhaust stream by contacting the ammonia-including exhaust stream with the downstream catalyst in an amount of at least about 40% of the formed ammonia.BRIEF DESCRIPTION OF DRAWINGS

[0024] FIG. 1 depicts an illustrative diagram of a substrate for an exhaust aftertreatment system in accordance with an embodiment.

[0025] FIG. 2 depicts an illustrative diagram of an upstream TWC in accordance with an embodiment.

[0026] FIGS. 3A-3B depict illustrative diagrams of exhaust aftertreatment systems in accordance with several embodiments.

[0027] FIGS. 4A-4F depict graphical representations of the HC, NOx, and CO removal and ammonia formation data of catalysts in accordance with several embodiments.

[0028] FIGS. 5-6 depict HC, NOx, and CO removal and ammonia formation data of catalysts in accordance with several embodiments.DEFINITIONS

[0029] As used herein, the term “about” when immediately preceding a numerical value means a range of plus or minus 10% of that value, for example, “about 50” means 45 to 55, “about 25,000” means 22,500 to 27,500, etc., unless the context of the disclosure indicates otherwise, or is inconsistent with such an interpretation.

[0030] As used herein, the terms “upstream” and “downstream” refer to relative directions according to the flow of an engine exhaust gas stream from an engine towards a tailpipe. For example, an upstream catalyst is positioned between an engine-out manifold and a downstream catalyst, whereas a downstream catalyst is positioned between an upstream catalyst and a tailpipe.

[0031] As used herein, the term “close-coupled position” (CC) refers to a position to place a catalyst adjacent to an engine-out manifold. For example, an upstream catalyst may be positioned in a first close-coupled position (CC1) to receive engine-out exhaust directly from the engine. A downstream catalyst may be positioned in a second close-coupled position (CC2) with a small gap next to the upstream catalyst in CC1.

[0032] As used herein, the term “underfloor position” (UF) refers to a position to place a catalyst further away from an engine-out manifold, typically underneath the main body of a vehicle. For example, a downstream catalyst may be positioned in an underfloor position with a big gap to an upstream catalyst in a first close-coupled position.

[0033] The terms “exhaust stream,” “engine exhaust stream,” “exhaust gas stream,” and the like refer to any combination of flowing engine effluent gas that may also contains solid and / or liquid species, including but not limiting to hydrocarbon residues due to incomplete combustion, oxides of nitrogen, carbon monoxide, carbon dioxide, ammonia, soot, water, unreacted oxygen, and / or nitrogen. Such terms refer as well as to the effluent downstream of one or more other catalyst system components as described herein.

[0034] The term “NOx” refers to nitrogen oxide compounds, such as NO and / or NO2.

[0035] The platinum group metal (PGM) component refers to any component that includes a PGM (e.g., Rh, Pd, and Pt). For example, the PGM may be in a metallic form, with zero valence, or the PGM may be in an oxide form. Reference to “PGM component” allows for the presence of the PGM in any valence state. The terms “platinum (Pt) component,” “rhodium (Rh) component,” “palladium (Pd) component,” and the like refer to the respective platinum group metal compound, complex, or the like which, upon calcination or use of the catalyst, decomposes or otherwise converts to a catalytically active form, usually the metal or the metal oxide.

[0036] A “support” or “support material” in a catalytic material or catalyst composition or catalyst washcoat refers to a material that receives metals (e.g., PGMs), stabilizers, promoters, binders, and the like through precipitation, association, dispersion, impregnation, or other suitable methods. A support material usually but not necessarily has a relatively high BET surface area. For example, a refractory alumina is regarded as a support.

[0037] "BET surface area" has its usual meaning of referring to the Brunauer-Emmett-Teller method for determining surface area by N2-adsorption measurements. Unless otherwise specifically stated, all references herein to the “specific surface area” or “surface area” refers to the BET surface area.

[0038] As used herein, the term “oxygen storage component” (OSC) refers to an entity that has a multi-valence state and can actively react with reductants such as carbon monoxide (CO) and / or hydrogen under reductive conditions to release oxygen and then react with oxidants such as oxygen or nitrogen oxides under oxidative conditions to store oxygen. Examples of oxygen storage components include ceria composites optionally doped with early transition metal oxides, including zirconia, yttria, lanthana, praseodymia, neodymia, europia, samaria, ytterbia, and combinations thereof. Oxygen storage components may be used as support materials for PGM in TWC and FWC catalysts.

[0039] As used herein, the term “substrate” refers to the monolithic material onto which the catalyst composition is placed, typically in the form of a washcoat containing a plurality of particles containing a catalytic composition thereon. Reference to “monolithic substrate” or “honeycomb substrate” means a unitary structure that is homogeneous and continuous from inlet to outlet. A substrate can be a flow-through type for TWC applications or a wall-flow filter type for FWC applications.

[0040] As used herein, the term “three-way catalyst” or “TWC” refers to a catalyst or catalytic article that can simultaneously abate HC, NOx, and CO pollutants of an exhaust stream from an internal combustion engine.

[0041] The term “four-way catalyst” or “FWC” refers to a catalyst or catalytic article that can simultaneously abate HC, NOx, and CO pollutants as well as remove particulate matter or soot emitted by an internal combustion engine.

[0042] As used herein, the term “washcoat” has its usual meaning in the art of a thin, adherent coating of a catalytic or other material applied to a substrate material which permits the passage of the gas stream being treated. A washcoat is formed by preparing a slurry containing a solid content (e.g., 10-60 % by weight) of particles in a liquid vehicle, which is then coated onto a substrate and dried to provide a washcoat layer.

[0043] As used herein, a washcoat layer includes a compositionally distinct layer of material disposed on the surface of a monolithic substrate or an underlying washcoat layer. In one embodiment, a substrate may contain one or more washcoat layers, and each washcoat layer is different in some way (e.g., PGM package, support material, particle size, or crystallite phases) and / or may differ in the chemical catalytic functions. As used herein, the terms “topcoat” and “bottomcoaf ’ each refer to a washcoat layer.

[0044] The present disclosure is not to be limited in terms of the particular embodiments described in this application, which are intended as illustrations of various aspects. Many modifications and variations can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. Functionally equivalent methods and apparatuses within the scope of the disclosure, in addition to those enumerated herein, will be apparent to those skilled in the art from the descriptions. Such modifications and variations are intended to fall within the scope of the appended claims. The present disclosure is to be limited only by the terms of the appended claims, along with the full scope of equivalents to which such claims are entitled. It is to be understood that this disclosure is not limited to particular methods, reagents, compounds, compositions or biological systems, which can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.

[0045] As used in this document, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. Nothing in this disclosure is to be construed as an admission that the embodiments described in this disclosure are not entitled to antedate such disclosure by virtue of prior invention. As used in this document, the term “comprising” means “including, but not limited to.”

[0046] While various compositions, methods, and devices are described in terms of "comprising" various components or steps (interpreted as meaning "including, but not limited to"), the compositions, methods, and devices can also "consist essentially of' or "consist of' the various components and steps, and such terminology should be interpreted as defining essentially closed-member groups.

[0047] With respect to the use of substantially any plural and / or singular terms herein, those having skill in the art can translate from the plural to the singular and / or from the singular to the plural as is appropriate to the context and / or application. The various singular / plural permutations may be expressly set forth herein for sake of clarity.

[0048] It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (for example, bodies of the appended claims) are generally intended as “open” terms (for example, the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those skilled in the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to embodiments containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (for example, “a” and / or “an” should be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should be interpreted to mean at least the recited number (for example, the bare recitation of "two recitations," without other modifiers, means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (for example, “ a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and / or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (for example, “ a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and / or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and / or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”

[0049] In addition, where features or aspects of the disclosure are described in terms of Markush groups, those skilled in the art will recognize that the disclosure is also thereby described in terms of any individual member or subgroup of members of the Markush group.

[0050] As will be understood by one skilled in the art, for any and all purposes, such as in terms of providing a written description, all ranges disclosed herein also encompass any and all possible subranges and combinations of subranges thereof. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as “up to,” “at least,” and the like include the number recited and refer to ranges which can be subsequently broken down into subranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member. Thus, for example, a group having 1-3 cells refers to groups having 1, 2, or 3 compounds. Similarly, a group having 1-5 compounds refers to groups having 1, 2, 3, 4, or 5 compounds, and so forth.DETAILED DESCRIPTION

[0051] This disclosure is not limited to the particular systems, devices, and methods described, as these may vary. The terminology used in the description is for the purpose of describing the particular versions or embodiments only and is not intended to limit the scope.Systems

[0052] Exhaust aftertreatment systems can be assembled for the removal of hydrocarbons (HC), nitrogen oxides (NOx), and carbon monoxide (CO) from engine exhaust streams of internal combustion engines. The exhaust aftertreatment system may comprise a catalyst system comprising one or more catalyst articles. The one or more catalyst articles may comprise an upstream three-way catalyst (TWC) and a downstream catalyst. Each of the one or more catalyst articles may comprises a catalyst component positioned on a substrate. In some embodiments, the catalyst components comprise one or more of a platinum group metal component, an oxygen storage component, an alumina component, dopants, and binders. The upstream TWC may comprise a bilayer washcoat structure positioned on the surface of a first substrate. The downstream catalyst may comprise one of a TWC or a four-way catalyst (FWC) positioned on a second substrate. The downstream catalyst may comprise a Rh loading. In some embodiments, the upstream TWC is configured to remove HC, NOx, and CO from an exhaust stream while limiting the formation of ammonia. In some embodiments, the downstreamcatalyst is configured to remove HC, NOx, and CO from an exhaust stream and to remove ammonia formed at the upstream TWC. In some embodiments, the exhaust aftertreatment systems are configured to further remove ammonia from the treated exhaust streams.

[0053] FIG. 1 depicts an illustration of a substrate 100 for the removal of hydrocarbons (HC), nitrogen oxides (NOx), and carbon monoxide (CO) from engine exhaust streams. In some embodiments, the substrate 100 comprises a plurality of cells 101 configured to allow a gaseous stream to pass through the substrate 100 from a front side 102 to a back side 103. The substrate 100 may comprise any structure effective for the removal of pollutants from an exhaust stream. In some embodiments, the substrate 100 comprises a honeycomb structure. In some embodiments, the honeycomb structure comprises one of a flow-through honeycomb structure or a wall-flow filter honeycomb structure. In some embodiments, the honeycomb structure comprises a cell density of about 200 cells per square inch (cpsi), about 300 cpsi, about 400 cpsi, about 500 cpsi, about 600 cpsi, about 700 cpsi, about 750 cpsi, about 800 cpsi, about 900 cpsi, about 1000 cpsi, or any value or range of values between any two of these values. In some embodiments, the substrate comprises cordierite.

[0054] FIG. 2 depicts an illustration of a catalyst article comprising a bilayer washcoat structure 201 / 202 positioned on a substrate 205. In some embodiments, the catalyst article is an upstream TWC. The upstream TWC comprises a plurality of cells 204 configured to allow a gaseous stream to flow through the substrate 205. The bilayer washcoat structure 201 / 202 comprises a bottomcoat 201 positioned on a surface of the substrate 205 and a topcoat 202 on a surface of the bottomcoat 201. In some embodiments, the bottomcoat 201 is deposited on a surface of the substrate 205 and the topcoat 202 is deposited on a surface of the bottomcoat 201. The substrate 205 may comprise any material effective for the removal of pollutants from an exhaust stream. In some embodiments, the substrate 205 comprises cordierite. In some embodiments, the substrate 205 comprises a plurality of walls 203 positioned between each of the plurality of cells 204 configured to allow for a uniform flow of a gaseous stream to flow through the substrate 205.

[0055] The substrate 205 may comprise any material and structure effective for the removal of pollutants from an exhaust stream. In some embodiments, the substrate 205 comprises a honeycomb structure. In some embodiments, the honeycomb structure comprises a cell density of about 200 cells per square inch (cpsi), about 300 cpsi, about 400 cpsi, about 500 cpsi, about 600 cpsi, about 700 cpsi, about 750 cpsi, about 800 cpsi, about 900 cpsi, about 1000 cpsi, or any value or range of values between any two of these values. In some embodiments, thehoneycomb structure has a cell density of about 400 cpsi to about 900 cpsi. In some embodiments, the substrate comprises cordierite.

[0056] The upstream TWC may be present on the substrate 205 in any amount effective to allow to the exhaust stream to flow through the substrate 205 and to remove pollutants from the exhaust stream. In some embodiments, the washcoat loading of the upstream TWC is present in an amount of about 2.0 g / in3, about 2.1 g / in3, about 2.2 g / in3, about 2.3 g / in3, about 2.4 g / in3, about 2.5 g / in3, about 2.6 g / in3, about 2.7 g / in3, about 2.8 g / in3, about 2.9 g / in3, about3.0 g / in3, about 3.1 g / in3, about 3.2 g / in3, about 3.3 g / in3, about 3.4 g / in3, about 3.5 g / in3, about3.6 g / in3, about 3.7 g / in3, about 2.8 g / in3, about 3.9 g / in3, about 4.0 g / in3, about 4.1 g / in3, about4.2 g / in3, about 4.3 g / in3, about 4.4 g / in3, about 4.5 g / in3, about 4.6 g / in3, about 4.7 g / in3, about4.8 g / in3, about 4.9 g / in3, about 5.0 g / in3, or any value or range of values between any two of these values.

[0057] In some embodiments, the upstream TWC comprises a bottomcoat 201 and a topcoat 202. In some embodiments, the bottomcoat 201 and the topcoat 202 are present in a weight ratio of about 4: 1, about 3: 1, about 2: 1, about 1 : 1, about 1 :2, about 1 :3, about 1 :4, or any value or range of values between any two of these values.

[0058] In some embodiments, the upstream TWC comprises one or more of alumina, ceria, zirconia, yttria, barium oxide, and lanthana. In some embodiments, the upstream TWC comprises an alumina component and an oxygen storage component (OSC). In some embodiments, the OSC comprises one or more of ceria, zirconia, and a dopant comprising yttria, lanthana, praseodymia, neodymia, and combinations thereof. The alumina component and the OSC may be present in the upstream TWC in any ratio effective for the removal of pollutants from an exhaust stream. In some embodiments, the alumina component and the OSC are present in a weight ratio of about 10: 1, about 9: 1, about 8:1, about 7: 1, about 6: 1, about 5: 1, about 4: 1, about 3: 1, about 2: 1, about 1 : 1, about 1 :2, about 1 :3, about 1 :4, about 1 :5, about 1 :6, about 1 :7, about 1 :8, about 1 :9, about 1 : 10, or any value or range of values between any two of these values. In some embodiments, the alumina component and the OSC are present in a weight ratio of about 1 : 5 to about 5: 1, about 1 : 5 to about 3 : 1 , or about 1 : 3 to about 3: 1.

[0059] In some embodiments, the upstream TWC comprises one or more ceria-containing composites. The upstream TWC may comprise any ceria loading effective for the removal of pollutants from an exhaust stream. In some embodiments, the upstream TWC comprises a ceria loading of about 0.60 g / in3, about 0.65 g / in3, about 0.70 g / in3, about 0.75 g / in3, about 0.80 g / in3, about 0.85 g / in3, about 0.90 g / in3, about 0.95 g / in3, about 1.00 g / in3, about 1.05 g / in3, about 1.10 g / in3, about 1.15 g / in3, about 1.20 g / in3, about 1.25 g / in3, 1.30 g / in3, about 1.35 g / in3, about1.40 g / in3, about 1.45 g / in3, about 1.50 g / in3, about 1.55 g / in3, about 1.60 g / in3, about 1.65 g / in3, about 1.70 g / in3, about 1.75 g / in3, about 1.80 g / in3, about 1.85 g / in3, about 1.90 g / in3, about 1.95 g / in3, about 2.00 g / in3, or any value or range of values between any two of these values. In some embodiments, the upstream TWC comprises a ceria loading of greater than about 0.75 g / in3, greater than about 0.90 g / in3, greater than about 1.10 g / in3, or greater than about 1.25 g / in3. In some embodiments the upstream TWC comprises a ceria loading of about 0.75 g / in3to about 1.50 g / in3, about 0.90 g / in3to about 1.30 g / in3, or about 1.00 g / in3to about 1.10 g / in3.

[0060] In some embodiments, the upstream TWC comprises a platinum group metal component comprising a Rh loading and a Pd loading. The upstream TWC may comprise any amount of the Rh loading effective for the removal of pollutants from an exhaust stream. In some embodiments, the Rh loading is present in the upstream TWC in an amount of about 1 g / ft3, about 2 g / ft3, about 3 g / ft3, about 4 g / ft3, about 5 g / ft3, about 6 g / ft3, about 7 g / ft3, about 8 g / ft3, about 9 g / ft3, about 10 g / ft3, about 15 g / ft3, about 20 g / ft3, about 25 g / ft3, about 30 g / ft3, about 35 g / ft3, about 40 g / ft3, about 45 g / ft3, about 50 g / ft3, or any value or range of values between any two of these values. In some embodiments, the Rh loading is present in the upstream TWC in an amount of about 1 g / ft3to about 50 g / ft3, about 2 g / ft3to about 20 g / ft3, or about 3 g / ft3to about 10 g / ft3.

[0061] In some embodiments, the Rh loading is positioned in the topcoat 202 in an amount as compared to the total Rh loading of the upstream TWC of about 75 wt. %, about 80 wt. %, about 85 wt. %, about 90 wt. %, about 95 wt. %, about 98 wt. %, about 99 wt. %, about 100 wt. %, or any value or range of values between any two of these values. In some embodiments, the Rh loading is positioned in the topcoat 202 in an amount as compared to the total Rh loading of the upstream TWC of at least 75 wt. %, at least 80 wt. %, or at least 90 wt. %.

[0062] The upstream TWC may comprise any amount of the Pd loading effective for the removal of pollutants from an exhaust stream. In some embodiments, the Pd loading is present in the upstream TWC in an amount of about 1 g / ft3, about 5 g / ft3, about 10 g / ft3, about 20 g / ft3, about 30 g / ft3, about 40 g / ft3, about 50 g / ft3, about 60 g / ft3, about 70 g / ft3, about 80 g / ft3, about 90 g / ft3, about 100 g / ft3, about 125 g / ft3, about 150 g / ft3, about 200 g / ft3or any value or range of values between any two of these values. In some embodiments, the Pd loading is present in the upstream TWC in an amount of about 1 g / ft3to about 200 g / ft3, about 10 g / ft3to about 150 g / ft3, or about 20 g / ft3to about 100 g / ft3.

[0063] In some embodiments, the Pd loading is positioned in the bottomcoat 201 in an amount as compared to the total Pd loading of the upstream TWC of about 75 wt. %, about 80 wt. %,about 85 wt. %, about 90 wt. %, about 95 wt. %, about 98 wt. %, about 99 wt. %, about 100 wt. %, or any value or range of values between any two of these values. In some embodiments, the Pd loading is positioned in the bottomcoat 201 in an amount as compared to the total Pd loading of the upstream TWC of at least 75 wt. %, at least 80 wt. %, or at least 90 wt. %.

[0064] In some embodiments, the platinum group metal component further comprises a Pt loading. The Pt loading is present in the upstream TWC in an amount of about 1 g / ft3, about 5 g / ft3, about 10 g / ft3, about 20 g / ft3, about 30 g / ft3, about 40 g / ft3, about 50 g / ft3, about 60 g / ft3, about 70 g / ft3, about 80 g / ft3, about 90 g / ft3, about 100 g / ft3, or any value or range of values between any two of these values. In some embodiments, the Pt loading is present in the upstream TWC in an amount of about 1 g / ft3to about 100 g / ft3, about 5 g / ft3to about 75 g / ft3, or about 10 g / ft3to about 50 g / ft3.

[0065] In some embodiments, a catalyst system may comprise a second catalyst article. In some embodiments, the second catalyst article comprises a downstream TWC. In some embodiments, the downstream catalyst comprises one of a TWC or a four-way catalyst (FWC). In some embodiments, the FWC comprises three-way catalytic components deposited on a soot filter. The downstream catalyst is positioned on a second substrate. In some embodiments, the bottomcoat 201 is deposited on a surface of the second substrate. The second substrate may comprise any material and structure effective for the removal of pollutants from an exhaust stream. In some embodiments, the second substrate comprises a honeycomb structure. In some embodiments, the second substrate comprises cordierite.

[0066] In some embodiments, the second substrate may comprise any material and structure of a flow-through type effective for the removal of pollutants from an exhaust stream. In some embodiments, the flow-through honeycomb structure comprises a cell density of about 200 cells per square inch (cpsi), about 300 cpsi, about 400 cpsi, about 500 cpsi, about 600 cpsi, about 700 cpsi, about 750 cpsi, about 800 cpsi, about 900 cpsi, about 1000 cpsi, or any value or range of values between any two of these values. In some embodiments, the flow-through honeycomb structure has a cell density of about 400 cpsi to about 900 cpsi.

[0067] In some embodiments, the second substrate may comprise any material and structure of a wall-flow filter type effective for the removal of pollutants from an exhaust stream. In some embodiments, the wall-flow filter honeycomb structure comprises a cell density of about 200 cells per square inch (cpsi), about 300 cpsi, about 400 cpsi, about 500 cpsi, about 600 cpsi, about 700 cpsi, about 800 cpsi, or any value or range of values between any two of these values. In some embodiments, the wall-flow filter honeycomb structure has a cell density of about 200 cpsi to about 600 cpsi.

[0068] The downstream catalyst may be present in any amount effective to allow to the exhaust stream to flow through the channels or walls of the downstream catalyst and to remove pollutants from the exhaust stream. In some embodiments, the washcoat loading of the downstream catalyst is present in an amount of about 2.0 g / in3, about 2.1 g / in3, about 2.2 g / in3, about 2.3 g / in3, about 2.4 g / in3, about 2.5 g / in3, about 2.6 g / in3, about 2.7 g / in3, about 2.8 g / in3, about 2.9 g / in3, about 3.0 g / in3, about 3.1 g / in3, about 3.2 g / in3, about 3.3 g / in3, about 3.4 g / in3, about 3.5 g / in3, about 3.6 g / in3, about 3.7 g / in3, about 2.8 g / in3, about 3.9 g / in3, about 4.0 g / in3, about 4.1 g / in3, about 4.2 g / in3, about 4.3 g / in3, about 4.4 g / in3, about 4.5 g / in3, about 4.6 g / in3, about 4.7 g / in3, about 4.8 g / in3, about 4.9 g / in3, about 5.0 g / in3, or any value or range of values between any two of these values.

[0069] In some embodiments, the downstream catalyst comprises one or more of alumina, ceria, zirconia, yttria, barium oxide, and lanthana. In some embodiments, the downstream catalyst comprises an alumina component and an oxygen storage component (OSC). In some embodiments, the OSC comprises ceria, zirconia, and a dopant comprising yttria, lanthana, praseodymia, neodymia, and combinations thereof. The alumina component and the OSC may be present in the downstream catalyst in any ratio effective for the removal of pollutants from an exhaust stream. In some embodiments, the alumina component and the OSC are present in a weight ratio of about 10: 1, about 9: 1, about 8: 1, about 7: 1, about 6: 1, about 5: 1, about 4: 1, about 3: 1, about 2: 1, about 1 : 1, about 1 :2, about 1 :3, about 1 :4, about 1 :5, about 1 :6, about 1 :7, about 1 :8, about 1 :9, about 1 : 10, or any value or range of values between any two of these values. In some embodiments, the alumina component and the OSC are present in a weight ratio of about 1 : 5 to about 5: 1, about 1 : 5 to about 3 : 1 , or about 1 : 3 to about 3: 1.

[0070] In some embodiments, the downstream catalyst comprises one or more ceria-containing composites. The downstream catalyst may comprise any ceria loading effective for the removal of pollutants from an exhaust stream. In some embodiments, the downstream catalyst comprises a ceria loading of about 0.30 g / in3, about 0.40 g / in3, about 0.50 g / in3, 0.60 g / in3, about 0.70 g / in3, about 0.80 g / in3, about 0.90 g / in3, about 1.00 g / in3, about 1.10 g / in3, about 1.20 g / in3, 1.30 g / in3, about 1.40 g / in3, about 1.50 g / in3, or any value or range of values between any two of these values. In some embodiments the downstream catalyst comprises a ceria loading of about 0.40 g / in3to about 1.40 g / in3, about 0.50 g / in3to about 1.20 g / in3, or about 0.60 g / in3to about 1.00 g / in3. In some embodiments, the downstream catalyst comprises a ceria loading lower than the ceria loading of the upstream TWC.

[0071] In some embodiments, the downstream catalyst comprises a Rh loading. The downstream catalyst may comprise any amount of the Rh loading component effective for theremoval of pollutants from an exhaust stream. In some embodiments, the Rh loading is present in the downstream catalyst in an amount of about 0.3 g / ft3, about 0.5 g / ft3, about 1 g / ft3, about 2 g / ft3, about 3 g / ft3, about 4 g / ft3, about 5 g / ft3, about 6 g / ft3, about 7 g / ft3, about 8 g / ft3, about 9 g / ft3, about 10 g / ft3, about 15 g / ft3, about 20 g / ft3, about 25 g / ft3, about 30 g / ft3, about 35 g / ft3, about 40 g / ft3, about 45 g / ft3, about 50 g / ft3, or any value or range of values between any two of these values. In some embodiments, the Rh loading is present in the downstream catalyst in an amount of about 0.5 g / ft3to about 20 g / ft3, about 1 g / ft3to about 15 g / ft3, or about 2 g / ft3to about 10 g / ft3.

[0072] In some embodiments, the downstream catalyst is essentially free of Pd with a Pd loading less than about 0.1 g / ft3. In some embodiments, the downstream catalyst further comprises a Pd loading. The downstream catalyst may comprise any amount of the Pd loading component effective for the removal of pollutants from an exhaust stream. In some embodiments, the Pd loading is present in the downstream catalyst in an amount of about 0.3 g / ft3, about 0.5g / ft3, about 1 g / ft3, about 2 g / ft3, about 3 g / ft3, about 4 g / ft3, about 5 g / ft3, about 6 g / ft3, about 7 g / ft3, about 8 g / ft3, about 9 g / ft3, about 10 g / ft3, about 11 g / ft3, about 12 g / ft3, about 13 g / ft3, about 14 g / ft3, about 15 g / ft3, about 16 g / ft3, about 17 g / ft3, about 18 g / ft3, about 19 g / ft3, about 20 g / ft3, or any value or range of values between any two of these values. In some embodiments, the Pd loading of the downstream catalyst is lower than the Rh loading of the downstream catalyst.

[0073] In some embodiments, the downstream catalyst comprises a Pt loading. The downstream catalyst may comprise any amount of the Pt loading component effective for the removal of pollutants from an exhaust stream. In some embodiments, the Pt loading is present in the downstream catalyst in an amount of about 1 g / ft3, about 2 g / ft3, about 3 g / ft3, about 4 g / ft3, about 5 g / ft3, about 6 g / ft3, about 7 g / ft3, about 8 g / ft3, about 9 g / ft3, about 10 g / ft3, about 15 g / ft3, about 20 g / ft3, about 25 g / ft3, about 30 g / ft3, about 35 g / ft3, about 40 g / ft3, about 45 g / ft3, about 50 g / ft3, or any value or range of values between any two of these values. In some embodiments, the Pt loading is present in the downstream catalyst in an amount of about 1 g / ft3to about 30 g / ft3, about 2 g / ft3to about 20 g / ft3, or about 5 g / ft3to about 15 g / ft3.

[0074] The downstream catalyst may comprise any structure effective for the removal of pollutants from an exhaust stream. In some embodiments, the downstream catalyst comprises at least one layer. The downstream catalyst may comprise any number of layers effective for the removal of pollutants from an exhaust stream. In some embodiments, the downstream catalyst comprises 1, 2, or 3 layers. In some embodiments, each of the at least one layer comprises a platinum group metal component.

[0075] FIGS. 3A and 3B depict illustrative diagrams of exhaust aftertreatment systems. The exhaust aftertreatment systems comprise an engine 301 and a first catalyst article. In some embodiments, the first catalyst article is positioned in a first close-coupled position 302. In some embodiments, the first close-coupled position 302 is directly downstream from the engine 301. In some embodiments, the first catalyst article comprises the upstream TWC.

[0076] In some embodiments, the exhaust aftertreatment system further comprises a second catalyst article. In some embodiments, the second catalyst article is positioned in a second close-coupled position 303. In some embodiments, the second close-coupled position 303 is directly downstream from the first close-coupled position 302. In some embodiments, the second catalyst article is positioned in an underfloor position 304. In some embodiments, the second catalyst article comprises the downstream catalyst. In some embodiments, the downstream catalyst is one of a TWC or an FWC.Methods of Use

[0077] Methods can be assembled to remove HC, NOx, and CO from an exhaust stream using the above-described systems.

[0078] The method comprises providing a catalyst system comprising an upstream TWC positioned on a first substrate and comprising a bilayer washcoat structure. In some embodiments, the upstream TWC comprises similar properties as the upstream TWC disclosed above. In some embodiments, the first substrate comprises similar properties as the first substrate disclosed above. The catalyst system further comprises a downstream catalyst positioned on a second substrate and comprising one of a TWC or an FWC. In some embodiments, the downstream catalyst comprises similar properties as the downstream catalyst disclosed above. In some embodiments, the second substrate comprises similar properties as the second substrate disclosed above.

[0079] The method further comprises contacting the upstream TWC with an exhaust stream comprising HC, NOx, and CO. This allows the upstream TWC to remove HC, NOx, and CO from the exhaust stream. The exhaust stream may be from any source. In some embodiments, the exhaust stream is from an internal combustion engine. In some embodiments, the upstream TWC is configured to remove the HC, NOx, and CO from the exhaust stream. In some embodiments, the upstream TWC is configured to remove each of HC, NOx, and CO from the exhaust streams in an amount as compared to the total amount of the corresponding pollutants in the exhaust stream of about 60 %, about 65 %, about 70 %, about 75 %, about 80 %, about 85 %, about 90 %, about 95 %, about 98 %, about 99 %, or any value or range of values between any two of these values.

[0080] The method further comprises contacting the downstream catalyst with an exhaust stream comprising HC, NOx, and CO. In some embodiments, the downstream catalyst is configured to remove the HC, NOx, and CO from the exhaust stream. In some embodiments, the downstream catalyst is configured to remove each of HC, NOx, and CO from the exhaust streams in an amount as compared to the corresponding remaining pollutants in the exhaust stream of about 20 %, about 30 %, about 40 %, about 50 %, about 60 %, about 70 %, about 80%, about 90 %, about 95 %, about 98 %, about 99 %, or any value or range of values between any two of these values.

[0081] In some embodiments, the method further comprises forming ammonia at the upstream TWC to form an ammonia-comprising exhaust stream. In some embodiments, the method further comprises removing the ammonia from the ammonia-comprising exhaust stream by contacting the ammonia-comprising exhaust stream with the downstream catalyst. In some embodiments, the downstream catalyst is configured to remove the formed ammonia in an amount of about 40 wt. %, about 45 wt. %, about 50 wt. %, about 55 wt. %, about 60 wt. %, about 65 wt. %, about 70 wt. %, about 75 wt. %, about 80 wt. %, about 85 wt. %, about 90 wt. %, about 95 wt. %, about 98 wt. %, about 99 wt. %, about 99.9 wt. %, or any value or range of values between any two of these values.Methods of Manufacture

[0082] Methods can be assembled to manufacture the above-described catalyst articles for the removal of HC, NOx, and CO from an exhaust stream. In some embodiments, a preparation process of a catalytic article is provided. The preparation process is described below for building a bilayer washcoat structure on a substrate. Nevertheless, those skilled in the art can modify such a process for making any catalyst article with a one, two, or three-layered washcoat structure.

[0083] The catalyst preparation process includes the following steps: A bottomcoat slurry is prepared which is deposited on a substrate. A topcoat slurry is prepared and deposited on the bottomcoat. Calcination is performed at a temperature ranging from 400 to 700°C with a duration of 10 minutes to 3 hours. The steps of preparing the bottom coat and the topcoat may involve a technique such as incipient wetness impregnation, incipient wetness coimpregnation, and post-addition. In some embodiments, the process includes a step of calcination of the substrate coated with the bottomcoat before coating with the topcoat. In some embodiments, the calcination is carried out at a temperature ranging from 400 to 700°C with a duration of 10 minutes to 3 hours.

[0084] Incipient wetness impregnation techniques, also called capillary impregnation or dry impregnation are commonly used for the synthesis of heterogeneous materials, i.e., catalysts. In some embodiments, an active metal precursor is dissolved in an aqueous or organic solution and then the metal-containing solution is added to a catalyst support containing the same pore volume as the volume of the solution that was added. Capillary action draws the solution into the pores of the support. Solution added in excess of the support pore volume causes the solution transport to change from a capillary action process to a diffusion process, which is much slower. The catalyst is dried and calcined to remove the volatile components within the solution, depositing the metal on the surface of the catalyst support. The concentration profile of the impregnated material depends on the mass transfer conditions within the pores during impregnation and drying. Multiple active metal precursors, after appropriate dilution, may be co-impregnated onto a catalyst support. In some embodiments, an active metal precursor is introduced to a slurry via post-addition under agitation during the process of a slurry preparation.

[0085] In some embodiments, the method comprises treating support particles with the active metal solution. In some embodiments, the support particles are dry enough to absorb substantially all of the solution to form a moist solid. In some embodiments, aqueous solutions of water-soluble compounds or complexes of the active metal are utilized, such as rhodium chloride, rhodium nitrate, rhodium acetate, or combinations thereof where rhodium is the active metal and palladium nitrate, palladium tetra amine, palladium acetate, or combinations thereof where palladium is the active metal. In some embodiments, the method further comprises drying the particles. In some embodiments, the particles are dried by heat treating the particles at elevated temperature, such as 100- 150°C, for a period of time, such as 1-3. The method may further comprise calcining the particles to convert the active metal to a more catalytically active form. In some embodiments, the calcination process comprises heat treatment in air at a temperature of about 400-700°C for 10 min to 3 hours. The above process can be repeated as needed to reach the desired level of active metal impregnation.

[0086] In some embodiments, the catalyst particles are mixed with water to form a slurry for purposes of coating a catalyst substrate, such as a honeycomb-type substrate. In addition to the catalyst particles, the slurry may contain a binder in the form of alumina, silica, zirconium acetate, colloidal zirconia, or zirconium hydroxide, associative thickeners, and / or surfactants including anionic, cationic, non-ionic or amphoteric surfactants. Other exemplary binders include boehmite, gamma-alumina, or delta / theta alumina, as well as silica sol. In some embodiments, the binder is used in an amount of about 1-5 wt.% of the total washcoat loading.Addition of acidic or basic species to the slurry is carried out to adjust the pH accordingly. In some embodiments, the pH of the slurry is adjusted by the addition of ammonium hydroxide, aqueous nitric acid, or acetic acid. In some embodiments, the pH of the slurry is about 3 to about 12.

[0087] The slurry may be milled to reduce the particle size and enhance particle mixing. The milling may be accomplished in a ball mill, continuous mill, or other similar equipment, and the solids content of the slurry may be about 10 wt. % to about 60 wt. %. In some embodiments, the solids content of the slurry is about 20 wt. % to about 40 wt. %. In some embodiments, the post-milling slurry is characterized by a D90 particle size of about 5 to about 30 microns, about 8 to about 20 microns, or about 10 to about 15 microns. The D90 is determined using a dedicated particle size analyzer. The equipment employed in this example uses laser diffraction to measure particle sizes in small volume slurry. The D90, typically with units of microns, means 90% of the particles by number have a diameter less than that value.

[0088] The slurry is coated on the catalyst substrate using any washcoat technique known in the art. In some embodiments, the catalyst substrate is dipped one or more times in the slurry or otherwise coated with the slurry. Thereafter, the coated substrate is dried at an elevated temperature, such as 100-150 °C, for a period, such as 10 min - 3 hours. The coated substrate may be calcined by heating at a temperature between about 400-700 °C, for a time between about 10 minutes to about 3 hours. Following drying and calcining, the final washcoat coating layer is viewed as essentially solvent-free. After calcining, the catalyst loading obtained by the above described washcoat technique can be determined through calculation of the difference in coated and uncoated weights of the substrate. As will be apparent to those of skill in the art, the catalyst loading can be modified by altering the slurry rheology. In addition, the coating / drying / calcining process to generate a washcoat can be repeated as needed to build the coating to the desired loading level or thickness, meaning more than one washcoat may be applied.EXAMPLESExample 1 : Powder Samples of PGM on 1 : 1 AI2O3 / OSC

[0089] A binary powder support material (1 : 1 AI2O3 / OSC) was prepared by physically mixing a 4% lanthana-stabilized y-alumina (fresh BET surface area: 150 m2 / g) and a ceria-zirconia based OSC (CeO2 / ZrO2 / Y2O3 / La2O3 = 40% / 50% / 5% / 5%, fresh BET surface area: 64 m2 / g) at a weight ratio of 1 : 1. This binary support material was loaded with 2.0% Pt, 2.0% Pd, 0.5% Rh, 1.0% Pt-1.0% Pd, 1.0% Pt-0.25% Rh, or 1.0% Pd-0.25% Rh by the standard method ofincipient wetness impregnation. The PGM precursors were platinum nitrate (CAS: 18496-40- 7, Umicore AG & Co KG), palladium nitrate (CAS: 32916-07-7, Umicore AG & Co KG), rhodium nitrate (CAS: 10139-58-9, Umicore AG & Co KG), and a mixture thereof. The resulting wet powders were dried at 120°C and then calcined at 550°C for 2 hr in the air to give samples 1 A-F as follows: (1 A) 2.0% Pt / 1 : 1 AI2O3 / OSC, (IB) 2.0% Pd / 1 : 1 AI2O3 / OSC, (1C) 0.5% Rh / 1 : 1 AI2O3 / OSC, (ID) 1.0% Pt-1.0% Pd / 1 : 1 AI2O3 / OSC, (IE) 1.0% Pt-0.25% Rh / 1 : 1 AI2O3 / OSC, and (IF) 1.0% Pd-0.25% Pd / 1 : 1 AI2O3 / OSC.Example 2: Aging and Testing of Powder Samples

[0090] The PGM-deposited powder catalysts of samples 1 A-F were individually dispersed in deionized water to give a solid percentage of approximately 30%. The slurries were thoroughly mixed and then dried at 120°C under agitation. The dried slurries were calcined at 550°C for 2 hours. The resulting powders were crushed and sieved to collect particles of a diameter of 250-500 pm for aging and testing.

[0091] Aging was carried out at 1000°C for 5 hours under cyclic lean / rich conditions in the presence of 10% steam. Each lean / rich aging cycle included 10 minutes of air (lean) and 10 minutes of 4% EE balanced with N2 (rich).

[0092] TWC light-off tests were performed on a 48-fold high-throughput reactor unit at = 1.00 + / - 0.05 with an oscillating feed of one second lean gas and one second rich gas. Lean gas at X = 1.05: 0.7% CO, 0.22% H2, 1.8% O2, 3000 ppmCi (C3H6:C3H8 = 2: 1), 1500 ppm NO, 14% CO2, 10% H2O, and balance N2. Rich gas at X = 0.95: 2.33% CO, 0.77% H2, 0.7% O2, 3000 ppmCi (C3H6:C3H8 = 2: 1), 1500 ppm NO, 14% CO2, 10% H2O, and balance N2. The sample amount is 105 mg which was diluted to a volume of 1 mL with corundum of the same particle sizes. The gas hourly space velocity (GHSV) was set to 70K h'1. The catalytic conversions were measured at constant temperatures from 200-450°C at a 25°C intervals.

[0093] For samples 1A-F, curves of HC, NOx, and CO conversions as well as NH3 formation are plotted in FIGS. 4A-F. The NH3 formation is defined as the percentage of the detected amount of ammonia with respect to the feed amount of NO. 2.0% Pt / 1 : 1 AI2O3 / OSC (Sample 1A, FIG. 4A) lit off HC, NOx, and CO at relatively high temperatures. Since NH3 was made during NOx light-off, its formation was also delayed to high temperatures for this specific sample. These results revealed that Pt alone had low to moderate activities for TWC reactions. In contrast, 2.0% Pd / 1 :1 AI2O3 / OSC (Sample IB, FIG. 4B) and 0.5% Rh / 1 : 1 AI2O3 / OSC (Sample 1C, FIG. 4C) gave good HC, NOx, and CO light-off activities. The NH3 formation of the Rh sample 1C was notably lower than that of the Pd sample IB, indicating that Rh has a substantially better N2 selectivity than Pd during NOx abatement. The HC, NOx, and COactivities of 1.0% Pt-1.0% Pd / 1 : 1 AI2O3 / OSC (Sample ID, FIG. 4D) were between these of samples 1A and IB. The NH3 formation of the Pt-Pd sample remained relatively high and comparable to that of the Pd counterpart. 1.0% Pt-0.25% Rh I 1 : 1 AI2O3 / OSC (Sample IE, FIG. 4E) displayed good HC, NOx, and CO light-off activities as well as low NH3 formation. In comparison with IE, 1.0% Pd-0.25% Rh / 1 : 1 AI2O3 / OSC (Sample IF, FIG. 4F) also gave good HC, NOx, and CO light-off performances but substantially higher NH3 formation. A combination of Rh and Pt as the PGM package gives good TWC activities as well as low NH3 formation, even at a reduced Rh loading. During TWC catalyst and system designs, the Rh component should be deliberately formulated to a specific layer or zone for better NH3 control. Example 3

[0094] Example 3 comprises a Pt / Pd / Rh-based upstream catalyst E3-TWC-1 and a Pd / Rh- based downstream catalyst E3-TWC-2. E3-TWC-1 has a bilayer washcoat structure which was coated on a monolith cordierite substrate having dimensions of 4.66” in diameter and 3.81” in length, a cell density of 800 cpsi (cells per square inch), and a wall thickness of 2.5 mils. The total washcoat loading is 3.57 g / in3and the total PGM loading is 120 g / ft3(Pt / Pd / Rh = 59 / 59 / 2). The catalyst contains 0.77 g / in3ceria. The bottom layer comprises 59 g / ft3Pd deposited onto a ceria-zirconia mixed oxide (40% CeCh), 59 g / ft3Pt deposited to a refractory ceria-alumina composite (CeCE / AhCE = 10 / 90), and barium oxide. The washcoat loading of the bottom layer is 2.57 g / in3. The top layer comprises 2 g / ft3Rh deposited onto a refractory ceria-alumina composite (CeCE / AhCh = 10 / 90). The washcoat loading of the top layer is 1.00 g / in3. E3-TWC- 2 has a single layer washcoat structure which was coated on a monolith cordierite substrate having dimensions of 5.20” in diameter and 3.96” in length, a cell density of 400 cpsi, and a wall thickness of 6.5 mils. The total washcoat loading is 2.82 g / in3and the total PGM loading is 12 g / ft3(Pt / Pd / Rh = 0 / 10 / 2). The catalyst contains 0.45 g / in3ceria. The catalyst comprises 10 g / ft3Pd deposited onto a ceria-zirconia mixed oxide (40% CeCE), 2 g / ft3Rh deposited to a refractory alumina, and barium oxide.Example 4

[0095] Example 4 comprises a Pt / Pd / Rh-based upstream catalyst E4-TWC-1 and a Pt / Rh-based downstream catalyst E4-TWC-2. E4-TWC-1 is the same to E3-TWC-1 as described in Example 3. E4-TWC-2 has a single layer washcoat structure which was coated on a monolith cordierite substrate having dimensions of 5.20” in diameter and 3.96” in length, a cell density of 400 cpsi, and a wall thickness of 6.5 mils. The total washcoat loading is 2.76 g / in3and the total PGM loading is 12 g / ft3(Pt / Pd / Rh = 10 / 0 / 2). The catalyst contains 0.62 g / in3ceria. The catalyst comprises 8 g / ft3Pt and 1 g / ft3Rh deposited onto a ceria-zirconia mixed oxide (40%CeCh), 2 g / ft3Pt and 1 g / ft3Rh deposited to a refractory ceria-alumina composite (CeCh / AhCh = 10 / 90), and barium oxide.Example 5: Engine Aging and Testing

[0096] The example catalyst systems 3 and 4 were mounted in steel converter cans and aged in an exhaust pipeline of a gasoline engine which was operated under exothermic 4-mode aging cycles. The aging duration is 82 hours at a maximum bed temperature of about 935°C for TWC- 1 and about 900°C for TWC-2. The aged catalytic converters were tested in a close-coupled position on a Daimler 4-cylinder gasoline engine with an engine displacement of 2.0 L. The test drive cycle was WLTC, following the certified procedures and tolerances.

[0097] TABLE 1 summarizes the WLTC tailpipe emission data of THC (total hydrocarbons), NOx, CO, and NH3 acquired on the Daimler engine. The system of Example 3 comprises a downstream TWC-2 with a Pd / Rh ratio of 10 / 2, which is regarded as a comparative example. The system of Example 4 comprises a downstream TWC-2 with a Pt / Rh weight ratio of 10 / 2 and is thus regarded as an inventive example. Compared to Example 3, Example 4 displayed almost equivalent THC emissions but moderately better NOx and CO emissions. In addition, Example 4 gave a significantly lower NH3 emission relative to Example 3. The data supported that, for NH3 control, the Rh / Pt PGM package for the downstream catalyst is desired over the Rh / Pd PGM package with a substantial amount of Pd.TABLE 1Example 6: Catalyst Preparation

[0098] Samples 1-7 are provided to demonstrate that, for a bilayer upstream TWC, allocation of more Rh and less Pd in topcoat results in less NH3 generation. The loadings of PGM and washcoat components are summarized in FIG. 5.

[0099] Sample 1 (El) is a Pd / Rh-based TWC catalyst of a bilayer washcoat structure, which was coated on a monolith cordierite substrate having dimensions of 5.20” in diameter and 2.36” in length, a cell density of 600 cpsi, and a wall thickness of 2.5 mils. The total washcoat loading is 4.16 g / in3and the total PGM loading is 57.5 g / ft3(Pt / Pd / Rh = 0 / 55 / 2.5). The catalyst has 0.99 g / in3ceria. The bottom layer comprises 10 g / ft3Pd deposited onto a refractory alumina and a ceria-zirconia mixed oxide. The washcoat loading of the bottom layer is 1.61 g / in3. Thetop layer comprises 45 g / ft3Pd and 2.5 g / ft3Rh. Pd was deposited onto a refractory alumina and a ceria-zirconia mixed oxide, and Rh was deposited onto a doped ZrCh and a ceria-zirconia mixed oxide. The washcoat loading of the top layer is 2.55 g / in3.

[0100] Sample 2 (E2) is a Pd / Rh-based TWC catalyst of a bilayer washcoat structure, which was coated on a monolith cordierite substrate having dimensions of 5.20” in diameter and 2.36” in length, a cell density of 600 cpsi, and a wall thickness of 2.5 mils. The total washcoat loading is 4.16 g / in3and the total PGM loading is 57.5 g / ft3(Pt / Pd / Rh = 0 / 55 / 2.5). The catalyst has 0.92 g / in3ceria. The bottom layer comprises 44 g / ft3Pd deposited onto a refractory alumina and a ceria-zirconia mixed oxide. The washcoat loading of the bottom layer is 2.57 g / in3. The top layer comprises 11 g / ft3Pd and 2.5 g / ft3Rh. Pd was deposited onto a refractory alumina and a ceria-zirconia mixed oxide, and Rh was deposited onto a doped ZrCh and a ceria-zirconia mixed oxide. The washcoat loading of the top layer is 1.59 g / in3.

[0101] Sample 3 (E3) is a Pt / Pd / Rh-based TWC catalyst of a bilayer washcoat structure, which was coated on a monolith cordierite substrate having dimensions of 5.20” in diameter and 2.36” in length, a cell density of 600 cpsi, and a wall thickness of 2.5 mils. The total washcoat loading is 3.92 g / in3and the total PGM loading is 62.5 g / ft3(Pt / Pd / Rh = 20 / 40 / 2.5). The catalyst has 0.85 g / in3ceria. The bottom layer comprises 5 g / ft3Pd deposited onto a refractory alumina and a ceria-zirconia mixed oxide. The washcoat loading of the bottom layer is 1.47 g / in3. The top layer comprises 20 g / ft3Pt, 35 g / ft3Pd, and 2.5 g / ft3RhPt. and Rh were deposited onto a ceriazirconia mixed oxide, and Pd was deposited onto a refractory alumina and a ceria-zirconia mixed oxide. The washcoat loading of the top layer is 2.45 g / in3.

[0102] Sample 4 (E4) is a Pt / Pd / Rh-based TWC catalyst of a bilayer washcoat structure, which was coated on a monolith cordierite substrate having dimensions of 5.20” in diameter and 2.36” in length, a cell density of 600 cpsi, and a wall thickness of 2.5 mils. The total washcoat loading is 3.98 g / in3and the total PGM loading is 62.5 g / ft3(Pt / Pd / Rh = 20 / 40 / 2.5). The catalyst has 0.92 g / in3ceria. The bottom layer comprises 32 g / ft3Pd deposited onto a refractory alumina and a ceria-zirconia mixed oxide. The washcoat loading of the bottom layer is 2.52 g / in3. The top layer comprises 20 g / ft3Pt, 8 g / ft3Pd, and 2.5 g / ft3Rh. Pt and Rh were deposited onto a ceria-zirconia mixed oxide, and Pd was deposited onto a refractory alumina and a ceria-zirconia mixed oxide. The washcoat loading of the top layer is 1.46 g / in3.

[0103] Sample 5 (E5) is a Pt / Pd / Rh-based TWC catalyst of a bilayer washcoat structure, which was coated on a monolith cordierite substrate having dimensions of 5.20” in diameter and 2.36” in length, a cell density of 600 cpsi, and a wall thickness of 2.5 mils. The total washcoat loading is 4.16 g / in3and the total PGM loading is 62.5 g / ft3(Pt / Pd / Rh = 20 / 40 / 2.5). The catalyst has0.91 g / in3ceria. The bottom layer comprises 10 g / ft3Pd deposited onto a refractory alumina and a ceria-zirconia mixed oxide. The washcoat loading of the bottom layer is 1.60 g / in3. The top layer comprises 20 g / ft3Pt, 30 g / ft3Pd, and 2.5 g / ft3Rh. Pt and 33% Rh was deposited onto a ceria-zirconia mixed oxide, 67% Rh was deposited onto a doped ZrCh, and Pd was deposited onto a refractory alumina and a ceria-zirconia mixed oxide. The washcoat loading of the top layer is 2.56g / in3.

[0104] Sample 6 (E6) is a Pt / Pd / Rh-based TWC catalyst of a bilayer washcoat structure, which was coated on a monolith cordierite substrate having dimensions of 5.20” in diameter and 2.36” in length, a cell density of 600 cpsi, and a wall thickness of 2.5 mils. The total washcoat loading is 3.85 g / in3and the total PGM loading is 62.5 g / ft3(Pt / Pd / Rh = 20 / 40 / 2.5). The catalyst has 0.92 g / in3ceria. The bottom layer comprises 20 g / ft3Pt and 40 g / ft3Pd. Pd was deposited onto a refractory alumina and a ceria-zirconia mixed oxide, and Pt was deposited onto to ceriaalumina composite. The washcoat loading of the bottom layer is 2.65 g / in3. The top layer comprises 2.5 g / ft3Rh deposited onto a lanthanum doped alumina and a ceria-zirconia mixed oxide. The washcoat loading of the top layer is 1.2 g / in3.Example 7: Engine Aging and Emission Testing

[0105] The samples E1-E6 were mounted in steel converter cans and aged in an exhaust pipeline of a gasoline engine which was operated under exothermic 4-mode aging cycles. The duration of the aging is 100 hours at a maximum bed temperature of about 980°C. The aged catalytic converters were tested on an engine (2.0T) bench or a vehicle (1 5T) on the WLTC drive cycle.

[0106] The WLTC tailpipe emission data for TWC El and E2 tested on the engine bench are provided in TABLE 2. El represents a TWC design in which 81.8% of total the total Pd is in the top layer. Example 2 is a TWC design in which 80% of the total Pd is allocated in the bottom layer. In other designs, all Rh is located in the top layer. In comparison with El, E2 exhibited slightly lower THC and CO emissions, marginally higher NOx emissions, and a substantially lower NH3 emission. As the two washcoat designs possess similar loadings in washcoat and CeO2, the design feature of less Pd in top layer of E2 is deduced to account for the reduced NH3 emission.TABLE 2

[0107] TABLE 3 summarizes the tailpipe emissions of THC, NOx, CO, and NH3 acquired on the vehicle on WLTC testing cycle. E3 is a Pt / Pd / Rh-based TWC in which 87.5% of the total Pd and all Rh is in the top layer. In comparison, E4 is a Pt / Pd / Rh-based TWC in which 80% of the total Pd is in the bottom layer while 20% Pd and all Rh are in the top layer. Example 4 gave improved tailpipe THC, CO, and NOx emissions, as well as low formation of NH3. E5 is a Pt / Pd / Rh-based TWC in which 75% of the total Pd, and all Pt and Rh are in the top layer. In comparison, E6 is a Pt / Pd / Rh-based TWC in which all Pd and Pt are in the bottom layer while all Rh is in the top layer. E6 displayed improved tailpipe THC, CO, and NOx emissions, and a significantly lower NH3 emission. Based on these data, to reduce ammonia formation on an upstream TWC, it is advantageous to allocate the majority of Rh in the topcoat and place the majority of Pd in the bottomcoat for a bilayer TWC design.TABLE 3Example 8: Catalyst Preparation

[0108] Samples 8-11 are provided to demonstrate the effect of the ceria loading of TWC on the NH3 formation. The loadings of washcoat components and PGM are summarized in TABLE 4.TABLE 4was coated on a monolith cordierite substrate having dimensions of 1” in diameter and 3” in length, a cell density of 750 cpsi, and a wall thickness of 2.5 mils. The total washcoat loading is 4.11 g / in3and the total PGM loading is 33 g / ft3(Pt / Pd / Rh = 0 / 30 / 3). The catalyst has 1.0g / in3ceria. In addition to PGM, the washcoat comprises a refractory alumina, a ceria-zirconia mixed oxide, and barium oxide.

[0110] Sample 9 (E9) has the same washcoat components to E8, except that the ceria loading is 0.8 g / in3. Sample 10 (E10) has the same washcoat components to E8, except that the ceria loading is 0.6 g / in3. Sample 11 (El 1) has the same washcoat components to E8, except that the ceria loading is 0.4 g / in3.Example 9: Reactor Testing[OHl] Fresh samples of E8-E11 were mounted in a steel lab reactor and tested on a simulated lambda jump cycle at a bed temperature of 450°C. Gas compositions and lambda values are summarized in TABLE 5. The target lambda values were achieved by adjusting oxygen feed concentrations. After stabilization of the gas feed at lambda 1.00, five repeated jump cycles, with lambda being switched back-and-forth from 1.35 to 0.96, were conducted for each sample.TABLE S

[0112] FIG. 6 shows NH3 emissions during lambda jump cycle testing for E8-E11. At lambda 1, very littleNFE generation was found for all samples. For E8 with a CeCh loading of 1.Og / in3, essentially no NH3 was generated when lambda was switched from 1.35 to 0.96 on all 5 cycles. For E9 with 0.8g / in3CeCh, there was a delay for NH3 generation at a small quantity during lambda-cycling. As to E10 with 0.6 g / in3CeCh, NH3 formation during the rich period was significantly more than that of E9. Further lowering CeCh to 0.4 g / in3for El l, even more NH3 was created during lambda-cycling. This comparison indicates that ammonia is typically generated under rich conditions and a higher ceria loading on TWC can effectively reduce the ammonia formation. It is thus desired to have a ceria loading greater than 0.6 g / in3or greater than 0.8 g / in3, to suppress NH3 formation on an upstream CC1 TWC.

[0113] Various of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art, each of which is also intended to be encompassed by the disclosed embodiments.

Claims

CLAIMS1. An exhaust aftertreatment system for removing HC, NOx, and CO from an exhaust stream of an internal combustion engine, the exhaust aftertreatment system comprising: an upstream three-way catalyst (TWC) deposited on a first substrate and comprising a bilayer washcoat structure; and a downstream catalyst deposited on a second substrate and comprising one of a TWC or a four-way catalyst (FWC), wherein the upstream TWC comprises a topcoat comprising at least 75 wt. % Rh of the total Rh loading of the upstream TWC and a bottom coat comprising at least 75 wt. % Pd of the total Pd loading of the upstream TWC, and wherein the downstream catalyst comprises a Rh loading of about 0.5 g / ft3to about 20 g / ft3and a Pd loading wherein the Rh loading is higher than the Pd loading.

2. The exhaust aftertreatment system of claim 1, wherein the upstream TWC comprises a total ceria loading of about 0.75 g / in3to about 1.50 g / in3.

3. The exhaust aftertreatment system of claim 1, wherein the upstream TWC comprises a total ceria loading of about 0.90 g / in3to about 1.30 g / in3.

4. The exhaust aftertreatment system of claim 2, wherein the downstream catalyst comprises a lower total loading of ceria than the upstream TWC.

5. The exhaust aftertreatment system of claim 1, wherein the upstream TWC further comprises a Pt loading of about 1 g / ft3to about 100 g / ft3.

6. The exhaust aftertreatment system of claim 1, wherein the bottomcoat of the upstream TWC is deposited on a surface of the first substrate and the topcoat of the upstream TWC is deposited on a surface of the bottomcoat.

7. The exhaust aftertreatment system of claim 1, wherein the downstream catalyst further comprises a Pt loading of about 1 g / ft3to about 30 g / ft3.

8. The exhaust aftertreatment system of claim 1, wherein the downstream catalyst is essentially free of Pd.

9. The exhaust aftertreatment system of claim 1, wherein the first substrate comprises a flow-through honeycomb structure with a cell density of about 400 cpsi to about 900 cpsi.

10. The exhaust aftertreatment system of claim 1, wherein the second substrate comprises a flow-through honeycomb structure with a cell density of about 400 cpsi to about 900 cpsi.

11. The exhaust aftertreatment system of claim 1, wherein the second substrate comprises a wall-flow filter honeycomb structure with a cell density of about 200 cpsi to 600 cpsi.

12. The exhaust aftertreatment system of claim 1, wherein the upstream TWC is positioned in a first close-coupled position.

13. The exhaust aftertreatment system of claim 1, wherein the downstream catalyst is positioned in one of a second close-coupled position or an underfloor position.

14. The exhaust aftertreatment system of claim 1, wherein the upstream TWC comprises a first alumina component and a first oxygen storage component (OSC), wherein the first OSC comprises ceria, zirconia, and a first dopant comprising yttria, lanthana, praseodymia, neodymia, and combinations thereof.

15. The exhaust aftertreatment system of claim 14, wherein the first alumina component and the first OSC are present in a weight ratio of about 1 :5 to 3 : 1.

16. The exhaust aftertreatment system of claim 1 , wherein the downstream TWC comprises a second alumina component and a second OSC, wherein the second OSC comprises ceria, zirconia, and a second dopant comprising yttria, lanthana, praseodymia, neodymia, and combinations thereof.

17. The exhaust aftertreatment system of claim 16, wherein the second alumina component and the second OSC are present in a weight ratio of about 1 :5 to 3 : 1.

18. The exhaust aftertreatment system of claim 1 further comprising a third catalyst positioned downstream of the downstream catalyst, wherein the third catalyst comprises: a platinum group metal component,a third alumina component; and a third OSC comprising ceria, zirconia, and a third dopant selected from yttria, lanthana, praseodymia, neodymia, and combinations thereof.

19. A method of removing HC, NOx, and CO from an exhaust stream, the method comprising:Providing a catalyst system comprising an upstream TWC deposited on a first substrate and comprising a bilayer washcoat structure and a downstream catalyst deposited on a second substrate and comprising one of a TWC or a FWC; contacting the upstream TWC with an exhaust stream comprising HC, NOx, and CO; and contacting the downstream catalyst with the exhaust stream, wherein the upstream TWC comprises a topcoat comprising at least 75 wt. % Rh of the total Rh loading of the upstream TWC and a bottom coat comprising at least 75 wt. % Pd of the total Pd loading of the upstream TWC, and wherein the downstream catalyst comprises a Rh loading of about 0.5 g / ft3to about 20 g / ft3and a Pd loading wherein the Rh loading is higher than the Pd loading.

20. The method of claim 19 further comprising: forming ammonia at the upstream TWC to form an ammonia-comprising exhaust stream; and removing the ammonia from the exhaust stream by contacting the ammonia-comprising exhaust stream with the downstream catalyst in an amount of at least about 40% of the formed ammonia.

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