Emission control catalyst article having a PGM gradient structure

Abstract

The present invention provides an emission control catalyst article, which comprises a substrate; a bottom washcoat layer containing platinum group metals, which is coated on 60 to 100% of the length of the substrate; and a top washcoat layer containing platinum group metals, which is coated on 60 to 100% of the length of the substrate, such that the top washcoat covers at least 60% of the length of the bottom washcoat layer, wherein at least a portion of the top washcoat layer, the bottom washcoat layer, or both washcoat layers contains platinum group metals deposited within the washcoat layer, and wherein the platinum group metal gradient is such that the PGM concentration in the topmost portion of the washcoat layer is at least twice as high as the PGM concentration in the bottommost portion of the washcoat layer.

Description

TECHNICAL FIELD

[0001] The presently claimed invention relates to an emission control catalyst article that can be used to treat exhaust gases to reduce the pollutants contained therein. In particular, the presently claimed invention relates to an emission control catalyst article in which some or all of the platinum group metals are deposited such that a concentration gradient is created within and / or between the washcoat layers, and a method of preparing the catalyst article. BACKGROUND OF THE INVENTION

[0002] Three-way conversion (TWC) catalysts (hereinafter interchangeably referred to as three-way conversion catalysts, three-way catalysts, TWC catalysts, and TWC) have been used for many years to treat the exhaust gas stream of internal combustion engines. Generally, to treat or purify exhaust gases containing pollutants such as hydrocarbons, nitrogen oxides, and carbon monoxide, a catalytic converter containing a three-way conversion catalyst is used in the exhaust gas line of an internal combustion engine. Three-way conversion catalysts are generally known for oxidizing unburned hydrocarbons and carbon monoxide and reducing nitrogen oxides. Most commercially available TWC catalysts contain palladium as the main platinum group metal component, which is used in combination with smaller amounts of rhodium.

[0003] The catalyst is formed by coating a substrate with a PGM-containing slurry, and the coated layer can be in the form of a layered structure having a bottom layer and a top layer. The platinum group metals are uniformly coated on the substrate at a PGM loading within the range of 3 to 300 g / ft 3 . In another technique, the platinum group metals can be coated on the substrate in a partitioned manner.

[0004] However, it has been found that existing catalysts still need improvement in terms of washcoat architecture, PGM type and loading, and coating strategies and methods to achieve higher reduction of pollutants such as NOx, HC, and CO.

[0005] Therefore, it is envisioned to prepare a catalyst containing a PGM gradient within one or more washcoat layers covering 20 - 100% of the length of the catalyst substrate, such that the PGM concentration decreases from the topmost to the bottommost of the layer, and at least 20 - 80% of a given PGM type within the washcoat layer containing the PGM gradient is contained within the top 1 / 3 of the washcoat layer, to achieve improved reduction of pollutants such as NOx, HC, and CO. SUMMARY OF THE INVENTION

[0006] Accordingly, the present invention provides an emission control catalyst article comprising: a substrate having a first inlet axial end and a second outlet axial end,

[0007] A bottom carrier coating layer, which comprises a platinum group metal coated on 60% to 100% of the length of the substrate from the axial end of the first inlet to the axial end of the second outlet, and

[0008] A top carrier coating layer, which comprises a platinum group metal coated on 60 to 100% of the length of the substrate from the first axial end or the second axial end of the substrate, such that the top coating covers at least 60% of the length of the bottom carrier coating layer,

[0009] wherein at least a part of the top carrier coating layer, the bottom carrier coating layer or both carrier coating layers contains a platinum group metal deposited within the carrier coating layer, and wherein the platinum group metal gradient is such that the PGM concentration in the topmost part of the carrier coating layer is at least twice as high as the PGM concentration in the bottommost part of the carrier coating layer,

[0010] wherein the length of the part of the top carrier coating layer, the bottom carrier coating layer or both carrier coating layers ranges from 20 - 100% of the length of the substrate. Description of the Drawings

[0011] To provide an understanding of the embodiments of the present invention, reference is made to the accompanying drawings, which are not necessarily to scale and in which reference numerals refer to components of exemplary embodiments of the present invention. The drawings are merely exemplary and should not be construed as limiting the present invention. The above and other features, their nature and various advantages of the presently claimed invention will become more apparent when the following detailed description is considered in conjunction with the accompanying drawings:

[0012] Figure 1 is a schematic representation of the design of a catalyst article in an exemplary configuration according to some embodiments of the presently claimed invention.

[0013] Figure 2 illustrates the Rh gradient in the top coating of an exemplary inventive catalyst.

[0014] Figure 3A 、 3B and 3C contain figures that respectively show the comparative test results of the cumulative THC emissions, CO emissions and NOx emissions of various catalytic article materials in the mid-bed and tailpipe.

[0015] Figure 4A is a perspective view of a honeycomb substrate carrier, which may comprise a catalyst composition according to an embodiment of the presently claimed invention.

[0016] Figure 4B is relative to Figure 4A magnified and taken in a plane parallel to Figure 4A the end face of the substrate carrier, which shows Figure 4AAn enlarged view of the plurality of gas flow channels shown in

[0017] Figure 5 is a cross-sectional view of the enlarged portion relative to Figure 4A wherein the honeycomb substrate in Figure 4A represents the entire wall-flow filter substrate. DETAILED DESCRIPTION

[0018] The presently claimed invention will now be described more fully hereinafter. The presently claimed invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The language of the specification should not be construed to indicate any non-claimed element as essential to the practice of the disclosed materials and methods.

[0019] Unless otherwise indicated herein or clearly contradicted by context, the terms "a / an" and "the" and similar referential terms used in the context of describing the materials and methods discussed herein (especially in the context of the appended claims) are to be construed to cover both the singular and the plural.

[0020] The term "about" is used throughout the specification to describe and account for small fluctuations. For example, the term "about" means less than or equal to ±5%, such as less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.2%, less than or equal to ±0.1%, or less than or equal to ±0.05%. All numerical values herein are modified by the term "about" whether or not explicitly indicated. Values modified by the term "about" of course include the specific value. For example, "about 5.0" must include 5.0.

[0021] Unless otherwise indicated herein or otherwise clearly contradicted by context, all methods described herein may be performed in any suitable order. Unless otherwise stated, the use of any and all examples or exemplary language (e.g., "such as") provided herein is intended merely to better illustrate the materials and methods and does not pose a limitation on the scope.

[0022] The platinum group metals (PGM) refer to any component containing PGM (Rh, Pd, and Pt). For example, the PGM may be in the form of a zero-valent metal, or the PGM may be in the form of an oxide. Reference to "PGM component" allows the PGM to exist in any valence state. The terms "platinum (Pt) component", "rhodium (Rh) component", "palladium (Pd) component", etc. refer to the respective platinum group metal compounds, complexes, etc. that decompose or otherwise convert to a catalytically active form, typically a metal or metal oxide, upon calcination or use of the catalyst.

[0023] As used herein, the term "catalyst" or "catalyst composition" refers to a material that facilitates a reaction.

[0024] The term "catalytic article" or "catalyst article" or "catalyst" refers to an assembly in which a substrate is coated with a catalyst composition for facilitating a desired reaction. In one embodiment, the catalytic article is a layered catalytic article. The term layered catalytic article refers to a catalytic article in which a substrate is coated with a PGM composition in a layered manner. These compositions may be referred to as washcoats.

[0025] The term "NOx" refers to nitrogen oxides such as NO and / or NO 2 。

[0026] The term "PGM-gradient" refers to a controlled and systematic difference in the PGM concentration within a washcoat layer, where the PGM concentration in the upper portion of the layer is generally higher compared to the lower portion of the same layer. The term "rhodium gradient" refers to a controlled and systematic difference in the rhodium concentration within a washcoat layer, where the rhodium concentration in the upper portion of the layer is generally higher compared to the lower portion of the same layer.

[0027] In one aspect, the claimed invention provides an emission control catalyst article comprising: a substrate having a first inlet axial end and a second outlet axial end,

[0028] a bottom washcoat layer comprising a platinum group metal coated on 60% to 100% of the length of the substrate from the first inlet axial end to the second outlet axial end, and

[0029] a top washcoat layer comprising a platinum group metal coated on 60 to 100% of the length of the substrate from the first axial end or the second axial end of the substrate such that the top coat covers at least 60% of the length of the bottom washcoat layer,

[0030] wherein at least a portion of the top washcoat layer, the bottom washcoat layer, or both washcoat layers contains a platinum group metal deposited within the washcoat layer, where the platinum group metal gradient is such that the PGM concentration in the topmost portion of the washcoat layer is at least two times higher compared to the PGM concentration in the bottommost portion of the washcoat layer,

[0031] wherein the length of the portion of the top washcoat layer, the bottom washcoat layer, or both washcoat layers ranges from 20 - 100% of the substrate length.

[0032] In one or more embodiments, the specific features, structures, materials, or characteristics may be combined in any suitable manner.

[0033] The following are various embodiments. It should be understood that the embodiments listed below can be combined with all aspects and other embodiments according to the scope of the present invention.

[0034] In one embodiment, the total platinum group metal loading deposited in a gradient is 30% to 95% of the total platinum group metal loading present in the carrier coating layer, wherein the ratio of the total amount of platinum group metal deposited uniformly within the carrier coating to the amount of platinum group metal deposited in a gradient is from 10:1 to 1:10.

[0035] In one embodiment, there is provided an emissions control catalyst article comprising: a substrate having a first inlet axial end and a second outlet axial end,

[0036] a bottom carrier coating layer comprising platinum group metals coated on 60% to 100% of the length of the substrate from the first inlet axial end to the second outlet axial end, and

[0037] a top carrier coating layer comprising platinum group metals coated on 60 to 100% of the length of the substrate from the first axial end or the second axial end of the substrate such that the top coating covers at least 60% of the length of the bottom carrier coating layer,

[0038] wherein at least a portion of the top carrier coating layer, the bottom carrier coating layer, or both carrier coating layers contains platinum group metals deposited within the carrier coating layer, wherein the platinum group metal gradient is such that 50% or more of the platinum group metals are deposited in the topmost 1 / 3 of the carrier coating layer, as determined by electron probe microanalysis (EPMA) line scans from the topmost layer of the carrier coating layer containing the platinum group metal gradient to the substrate.

[0039] wherein the length of the top carrier coating layer, the bottom carrier coating layer, or the portion of both carrier coating layers ranges from 20 - 100% of the length of the substrate.

[0040] In one embodiment, there is provided an emissions control catalyst article comprising: a substrate having a first inlet axial end and a second outlet axial end,

[0041] a bottom carrier coating layer comprising platinum group metals coated on 60% to 100% of the length of the substrate from the first inlet axial end to the second outlet axial end, and

[0042] a top carrier coating layer comprising platinum group metals coated on 60 to 100% of the length of the substrate from the first axial end or the second axial end of the substrate such that the top coating covers at least 60% of the length of the bottom carrier coating layer,

[0043] wherein at least a portion of the top washcoat layer, the bottom washcoat layer, or both washcoat layers contains platinum group metals (PGMs) deposited within the washcoat layer, wherein the platinum group metal gradient is such that the PGM concentration in the topmost portion of the washcoat layer is at least two times higher than the PGM concentration in the bottommost portion of the washcoat layer,

[0044] wherein the length of the top washcoat layer, the bottom washcoat layer, or the portion of both washcoat layers ranges from 20 - 100% of the substrate length,

[0045] wherein the total platinum group metal loading deposited in a gradient is 30% to 95% of the total platinum group metal loading present in the washcoat layer, and wherein the ratio of the total amount of platinum group metals deposited uniformly within the washcoat to the amount of platinum group metals deposited in a gradient is from 10:1 to 1:10.

[0046] In one embodiment, there is provided an emission control catalyst article comprising: a substrate having a first inlet axial end and a second outlet axial end,

[0047] a bottom washcoat layer comprising platinum group metals coated on 60% to 100% of the length of the substrate from the first inlet axial end to the second outlet axial end, and

[0048] a top washcoat layer comprising platinum group metals coated on 60 to 100% of the length of the substrate from the first axial end or the second axial end of the substrate such that the top coating covers at least 60% of the length of the bottom washcoat layer,

[0049] wherein at least a portion of the top washcoat layer, the bottom washcoat layer, or both washcoat layers contains platinum group metals deposited within the washcoat layer, wherein the platinum group metal gradient is such that 50% or more of the platinum group metals are deposited in the topmost 1 / 3 of the washcoat layer within the top washcoat layer or the bottom washcoat layer, as determined by electron probe microanalysis (EPMA) line scans from the topmost layer of the washcoat layer containing the platinum group metal gradient to the substrate,

[0050] wherein the length of the top washcoat layer, the bottom washcoat layer, or the portion of both washcoat layers ranges from 20 - 100% of the substrate length,

[0051] wherein the total platinum group metal loading deposited in a gradient is 30% to 95% of the total platinum group metal loading present in the washcoat layer, and wherein the ratio of the total amount of platinum group metals deposited uniformly within the washcoat to the amount of platinum group metals deposited in a gradient is from 10:1 to 1:10.

[0052] In one embodiment, the length of the platinum group metal gradient ranges from 1.2 inches to 8 inches.

[0053] In one exemplary embodiment, an emission control catalyst article is provided, comprising: a substrate having a first inlet axial end and a second outlet axial end,

[0054] a bottom washcoat layer comprising platinum group metals coated on 60% to 100% of the length of the substrate from the first inlet axial end to the second outlet axial end, and

[0055] a top washcoat layer comprising platinum group metals coated on 60 to 100% of the length of the substrate from the first axial end or the second axial end of the substrate such that the top coat covers at least 60% of the length of the bottom washcoat layer,

[0056] wherein at least a portion of the top washcoat layer contains rhodium or a mixture of rhodium and platinum group metals, which is deposited in the washcoat layer as a rhodium gradient such that the rhodium concentration in the topmost portion of the washcoat layer is at least two times higher compared to the rhodium concentration in the bottommost portion of the washcoat layer,

[0057] wherein the length of the top washcoat layer, the bottom washcoat layer or portions of both washcoat layers ranges from 20 - 100% of the substrate length,

[0058] wherein the total platinum group metal loading deposited as a gradient is 30 to 95% of the total platinum group metal loading present in the washcoat layer,

[0059] wherein the ratio of the total amount of rhodium deposited uniformly in the washcoat to the amount of rhodium deposited as a gradient is in the range of 10:1 and 1:10.

[0060] In one exemplary embodiment, an emission control catalyst article is provided, comprising: a substrate having a first inlet axial end and a second outlet axial end,

[0061] a bottom washcoat layer comprising platinum group metals coated on 60% to 100% of the length of the substrate from the first inlet axial end to the second outlet axial end, and

[0062] a top washcoat layer comprising platinum group metals coated on 60 to 100% of the length of the substrate from the first axial end or the second axial end of the substrate such that the top coat covers at least 60% of the length of the bottom washcoat layer,

[0063] At least a portion of the carrier coating layer contains rhodium or a mixture of rhodium and platinum group metals, which is deposited in the carrier coating layer in a rhodium gradient such that 50% or more of the rhodium is deposited in the top 1 / 3 of the carrier coating layer, either the top carrier coating layer or the bottom carrier coating layer, as determined by electron probe microanalysis (EPMA) line scanning from the uppermost layer of the carrier coating layer containing the rhodium gradient to the substrate.

[0064] Wherein the length of the top carrier coating layer, the bottom carrier coating layer, or a portion of both carrier coating layers ranges from 20 - 100% of the length of the substrate.

[0065] Wherein the total platinum group metal loading deposited in a gradient is 30 to 95% of the total platinum group metal loading present in the carrier coating layer.

[0066] Wherein the ratio of the total amount of rhodium deposited uniformly in the carrier coating to the amount of rhodium deposited in a gradient is in the range of 10:1 and 1:10.

[0067] In one embodiment, the length of the rhodium gradient ranges from 1.2 inches to 8 inches.

[0068] The platinum group metal gradient is formed by coating a pre - loaded portion of the top carrier coating layer and / or the bottom carrier coating layer with an additional platinum group metal solution. In another embodiment, the platinum group metal gradient is formed by coating a substantially platinum group metal - free portion of the top carrier coating layer and / or the bottom carrier coating layer with a platinum group metal solution. The platinum group metals are selected from platinum, palladium, rhodium, and combinations thereof.

[0069] The platinum group metals are supported on a carrier selected from an oxygen storage component, an alumina component, a ceria component, a zirconia component, and combinations thereof. The alumina component includes alumina, lanthanum - alumina, ceria - alumina, ceria - zirconia - alumina, zirconia - alumina, lanthanum - zirconia - alumina, barium - alumina, barium - lanthanum - alumina, barium - lanthanum - neodymium - alumina, or combinations thereof. The oxygen storage component contains ceria - zirconia, ceria - zirconia - lanthanum, ceria - zirconia - yttrium, ceria - zirconia - lanthanum - yttrium, ceria - zirconia - neodymium, ceria - zirconia - praseodymium, ceria - zirconia - lanthanum - neodymium, ceria - zirconia - lanthanum - praseodymium, ceria - zirconia - lanthanum - neodymium - praseodymium, or any combination thereof. The zirconia component includes lanthanum - zirconia and barium - zirconia.

[0070] In one embodiment, at least 60% of the rhodium is deposited in a gradient such that it covers at least 50% of the substrate length, and the Rh concentration in the uppermost portion of the washcoat layer is twice that in the lowermost portion of the washcoat layer. In one embodiment, 50% or more of the platinum group metals within the top washcoat layer or the bottom washcoat layer are deposited in the uppermost 1 / 3 of the washcoat layer, as determined by electron probe microanalysis (EPMA) line scans from the uppermost layer of the washcoat layer containing the platinum group metal gradient to the substrate. In one embodiment, 55% to 95% of the platinum group metals within the top washcoat layer or the bottom washcoat layer are deposited in the uppermost 1 / 3 of the washcoat layer, as determined by electron probe microanalysis (EPMA) line scans from the uppermost layer of the washcoat layer containing the platinum group metal gradient to the substrate. In an exemplary embodiment, the platinum group metal gradient includes palladium.

[0071] In another exemplary embodiment, the platinum group metal gradient includes platinum. In yet another exemplary embodiment, the platinum group metal gradient includes rhodium.

[0072] In one embodiment, the bottom washcoat layer includes at least one alkaline earth metal oxide, the at least one alkaline earth metal oxide including barium oxide, strontium oxide, or any combination thereof, and the amount of the at least one alkaline earth metal oxide is from 1.0 to 20 wt% based on the total weight of the bottom washcoat.

[0073] In one embodiment, the substrate is a ceramic substrate, a metal substrate, a ceramic foam substrate, a polymer foam substrate, or a woven fiber substrate. In one embodiment, the substrate is a monolithic substrate or a honeycomb substrate.

[0074] The reference to a "monolithic substrate" or "honeycomb substrate" refers to a unified structure that is uniform and continuous from the inlet to the outlet.

[0075] According to one or more embodiments, the substrate of the catalytic article of the presently claimed invention can be composed of any material commonly used to prepare automotive catalysts and typically includes a ceramic or metal monolithic honeycomb structure.

[0076] The substrate typically provides a plurality of wall surfaces on which a washcoat including the catalyst composition described above is applied and adhered, thereby serving as a carrier for the catalyst composition.

[0077] Exemplary metal substrates include heat-resistant metals and metal alloys, such as titanium steel and stainless steel, and other alloys in which iron is a significant or major component. Such alloys may contain one or more of nickel, chromium, and / or aluminum, and the total amount of these metals may advantageously comprise at least 15 wt% of the alloy, such as 10 wt% to 25 wt% chromium, 3% - 8% aluminum, and up to 20 wt% nickel. The alloy may also contain minor or trace amounts of one or more metals, such as manganese, copper, vanadium, titanium, etc. The surface of the metal substrate can be oxidized at a high temperature (e.g., 1000 °C or higher) to form an oxide layer on the surface of the substrate, thereby improving the corrosion resistance of the alloy and promoting the adhesion of the carrier coating layer to the metal surface.

[0078] The ceramic materials used to construct the substrate can include any suitable refractory materials, such as cordierite, mullite, cordierite-alumina, silicon nitride, zircon mullite, spodumene, alumina-silica magnesia, zirconium silicate, sillimanite, magnesium silicate, zircon, petalite, alumina, aluminosilicate, etc.

[0079] Any suitable substrate may be employed, such as a monolithic flow-through substrate having a plurality of fine parallel gas flow channels extending from an inlet face of the substrate to an outlet face, such that the channels are open to fluid flow. The channels having a substantially straight path from inlet to outlet are defined by walls coated with a catalytic material as a washcoat such that the gas flowing through the channels contacts the catalytic material. The flow channels of the monolithic substrate are thin-walled trenches having any suitable cross-sectional shape, such as trapezoidal, rectangular, square, sinusoidal, hexagonal, oval, circular, etc. Such structures contain from about 60 to about 1200 or more gas inlet openings (i.e., "cells") per square inch of cross-section (cpsi), more typically from about 300 to 900 cpsi. The wall thickness of the flow-through substrate may vary, with typical ranges being between 0.002 inches and 0.1 inches. Representative commercially available flow-through substrates are cordierite substrates having 400 cpsi and a 6 mil wall thickness or 600 cpsi and a 4.0 mil wall thickness. However, it should be understood that the present invention is not limited to a particular substrate type, material, or geometry. In an alternative embodiment, the substrate may be a wall-flow substrate, wherein each channel is blocked at one end of the substrate body with a non-porous plug and alternate channels are blocked at the opposite end face. This requires the gas to flow through the porous walls of the wall-flow substrate to reach the outlet. Such a monolithic substrate may contain up to about 700 or more cpsi, such as from about 100 to 400 cpsi, and more typically from about 200 to about 300 cpsi. The cross-sectional shape of the cells may vary as described above. The wall thickness of the wall-flow substrate is typically between 0.002 and 0.1 inches. Representative commercially available wall-flow substrates are composed of porous cordierite, examples of which have 200 cpsi and a 10 mil wall thickness or 300 cpsi and an 8 mil wall thickness, and a wall porosity between 45% and 65%. Other ceramic materials such as aluminum titanate, silicon carbide, and silicon nitride, etc. are also used as wall-flow filter substrates. However, it should be understood that the present invention is not limited to a particular substrate type, material, or geometry. Note that in the case where the substrate is a wall-flow substrate, the catalyst composition may penetrate into the pore structure of the porous walls (i.e., partially or completely occlude the pore openings) in addition to being deposited on the surface of the walls. In one embodiment, the substrate has a flow-through ceramic honeycomb structure, a wall-flow ceramic honeycomb structure, or a metal honeycomb structure.

[0080] As used herein, the term "washcoat" has its ordinary meaning in the art as a thin adherent coating of a catalytic or other material applied to a substrate material, such as a honeycomb-type carrier member, which is porous enough to allow the treated gas stream to pass through. The washcoat is formed by preparing a slurry containing particles having a certain solids content (e.g., 15 - 60 wt%) in a liquid medium, then coating the slurry onto the substrate and drying to provide a washcoat layer.

[0081] As used herein and as described on pages 18 - 19 of Heck, Ronald and Farrauto, Robert, Catalytic Air Pollution Control (New York: Wiley - Interscience, 2002), a support coating layer comprises compositionally different material layers deposited on the surface of a monolithic substrate or on a support coating layer thereunder. In one embodiment, the substrate contains one or more support coating layers, and each support coating layer is different in some way (e.g., can be different in its physical properties, such as particle size or microcrystalline phase) and / or can be different in its chemical catalytic function.

[0082] A catalyst article can be “fresh,” which means it is new and has not been exposed to any heat or thermal stress for an extended period. “Fresh” can also mean that the catalyst was recently prepared and has not been exposed to any exhaust gases or high temperatures. Similarly, an “aged” catalyst article is not fresh and has been exposed to exhaust gases and high temperatures (i.e., greater than 500 °C) for an extended period (i.e., greater than 3 hours).

[0083] As used herein, the term “stream” broadly refers to any combination of flowing gases that may contain solid or liquid particulate matter.

[0084] As used herein, the terms “upstream” and “downstream” refer to the relative direction of flow from an engine to a tailpipe according to the engine exhaust stream, where the engine is in an upstream position and the tailpipe and any pollutant mitigation articles such as filters and catalysts are downstream of the engine.

[0085] Figure 4A and 4B Exemplary substrate 2 is shown in the form of a flow - through substrate that is coated with a support coating composition as described herein. Referring Figure 4A , exemplary substrate 2 has a cylindrical shape and a cylindrical outer surface 4, an upstream end face 6, and a corresponding downstream end face 8 that is the same as the upstream end face 6. Substrate 2 has a plurality of thin and parallel gas flow channels 10 formed therein. As Figure 4B shown, the flow channels 10 are formed by walls 12 and extend through substrate 2 from the upstream end face 6 to the downstream end face 8, and the channels 10 are unobstructed to allow fluid (e.g., an air stream) to flow longitudinally through substrate 2 via its gas flow channels 10. As in Figure 5More easily seen, wall 12 is sized and configured such that the flow channel 10 has a substantially regular polygonal shape. As shown, if desired, the carrier coating composition can be applied in multiple, different layers. In the illustrated embodiment, the carrier coating consists of a discrete first carrier coating layer 14 adhered to the wall 12 of the substrate member and a second discrete second carrier coating layer 16 coated over the first carrier coating layer 14. In one embodiment, the presently claimed invention is also practiced with two or more (e.g., 3 or 4) carrier coating layers and is not limited to the two-layer embodiment shown.

[0086] Figure 5 An exemplary substrate 2 is shown in the form of a wall-flow filter substrate, which is coated with a carrier coating layer composition as described herein. As Figure 5 shown, the exemplary substrate 2 has a plurality of channels 52. The channels are tubularly surrounded by an inner wall 53 of the filter substrate. The substrate has an inlet end 54 and an outlet end 56. Alternate channels are plugged at the inlet end with inlet plugs 58 and at the outlet end with outlet plugs 60 to form a relative checkerboard pattern at the inlet 54 and the outlet 56. An air flow 62 enters through an unblocked trench inlet 64, is blocked by the outlet plug 60, and diffuses to the outlet side 66 via the trench wall 53 (which is porous). The gas cannot return to the inlet side of the wall due to the inlet plug 58. The porous wall-flow filter for use in the present invention is catalytic because the walls of the element contain one or more catalytic materials thereon or therein. The catalytic material can be present alone on the inlet side of the element wall, alone on the outlet side, on both the inlet and outlet sides, or the wall itself can be composed entirely or in part of the catalytic material. The present invention includes the use of one or more layers of catalytic material on the inlet and / or outlet walls of the element.

[0087] On the other hand, a method for preparing an emission control catalyst article is also provided.

[0088] In one embodiment, the method includes providing a substrate having a first inlet axial end and a second outlet axial end, coating a bottom carrier coating layer containing a platinum group metal on 60% to 100% of the length of the substrate from the first axial end to the second axial end, and coating a carrier coating layer containing a platinum group metal on 60 to 100% of the length of the substrate from the first axial end or the second axial end of the substrate such that the top coating covers at least 60% of the length of the bottom carrier coating layer.

[0089] Wherein at least a portion of the top carrier coating layer, the bottom carrier coating layer, or both carrier coating layers contains a platinum group metal deposited within the carrier coating layer, wherein the platinum group metal gradient is such that the PGM concentration in the topmost portion of the carrier coating layer is at least twice as high as the PGM concentration in the bottommost portion of the carrier coating layer,

[0090] wherein the length of the top washcoat layer, the bottom washcoat layer, or a portion of both washcoat layers ranges from 20 to 100% of the length of the substrate.

[0091] In one embodiment, the method includes providing a substrate having a first inlet axial end and a second outlet axial end, coating a bottom washcoat layer comprising a platinum group metal thereon, which is coated on 60% to 100% of the length of the substrate from the first axial end to the second axial end, and coating a washcoat layer comprising a platinum group metal thereon, which is coated on 60 to 100% of the length of the substrate from the first axial end or the second axial end of the substrate, such that the top coating covers at least 60% of the length of the bottom washcoat layer.

[0092] wherein at least a portion of the top washcoat layer comprises rhodium or a mixture of rhodium and a platinum group metal, which is deposited in a rhodium gradient within the washcoat layer such that the rhodium concentration in the topmost portion of the washcoat layer is at least twice as high as the rhodium concentration in the bottommost portion of the washcoat layer.

[0093] wherein the length of the top washcoat layer, the bottom washcoat layer, or a portion of both washcoat layers ranges from 20 to 100% of the length of the substrate.

[0094] wherein the total platinum group metal loading deposited in a gradient is 30 to 95% of the total platinum group metal loading present in the washcoat layer.

[0095] wherein the ratio of the total amount of rhodium deposited uniformly within the washcoat layer to the amount of rhodium deposited in a gradient ranges from 10:1 to 1:10.

[0096] In one embodiment, the preparation of an emission control catalyst article includes a) preparing a bottom washcoat layer, which is coated on 60 to 100% of the length of the substrate from the inlet axial end to the outlet axial end, which includes obtaining a slurry comprising a platinum group metal impregnated on at least one carrier and coating the slurry over the entire length of the substrate; b) preparing a top washcoat, which is coated on 60% to 100% of the length of the substrate from the axial inlet end or the axial outlet end of the substrate such that the top coating covers at least 60% of the length of the bottom washcoat layer, which includes obtaining a slurry comprising a platinum group metal impregnated on a carrier and coating the slurry on at least 60% of the length of the bottom washcoat; and c) coating a portion of the top washcoat layer starting from the axial inlet end of the substrate with a platinum group metal solution, having a length of 1.2 to 8 inches, and then drying and calcining at a temperature of about 100 to 140 °C to obtain a PGM gradient.

[0097] The step of preparing the slurry includes techniques selected from incipient wetness impregnation, incipient wetness co-impregnation, and post-addition.

[0098] The incipient wetness impregnation technique, also known as capillary impregnation or dry impregnation, is commonly used to synthesize heterogeneous materials, namely catalysts. Typically, the active metal precursor is dissolved in an aqueous or organic solution, and then the metal-containing solution is added to a catalyst support that has a pore volume equal to the volume of the added solution. Capillary action draws the solution into the pores of the support. Adding a solution in excess of the pore volume of the support causes the transport of the solution to change from the capillary action process to a much slower diffusion process. 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 can be co-impregnated onto the catalyst support after appropriate dilution. Alternatively, during the slurry preparation process, the active metal precursor is introduced into the slurry via post-addition under stirring.

[0099] The support particles are typically dried sufficiently to adsorb substantially all of the solution to form a moist solid. An aqueous solution of a water-soluble compound or complex of the active metal is commonly used, such as rhodium chloride, rhodium nitrate, rhodium acetate, or combinations thereof, where rhodium is the active metal and palladium nitrate, tetraamine palladium, palladium acetate, or combinations thereof, where palladium is the active metal. After treating the support particles with the active metal solution, the particles are dried, for example, by heat treating the particles at an elevated temperature (e.g., 100 - 150 °C) for a period of time (e.g., 1 - 3 hours), and then calcined to convert the active metal into a more catalytically active form. An exemplary calcination process involves heat treating in air at a temperature of about 400 - 550 °C for 10 minutes to 3 hours. The above process can be repeated as needed to achieve the desired loading level of the active metal by impregnation.

[0100] The catalyst composition as described above is typically prepared in the form of catalyst particles as described above. These catalyst particles are mixed with water to form a slurry for coating a catalyst substrate, such as a honeycomb-type substrate. In addition to the catalyst particles, the slurry can optionally contain a binder, associative thickener, and / or surfactant (including anionic, cationic, non-ionic, or amphoteric surfactants) in the form of alumina, silica, zirconium acetate, colloidal zirconia, or zirconium hydroxide. Other exemplary binders include boehmite, γ-alumina, or δ / θ alumina, as well as silica sols. When present, the amount of the binder is typically about 1.0 wt% - 5.0 wt% of the total bulk coating load. An acidic or basic substance is added to the slurry to adjust the pH accordingly. For example, in some embodiments, the pH of the slurry is adjusted by adding ammonium hydroxide, aqueous nitric acid, or acetic acid. The typical pH range of the slurry is about 3.0 to 12.

[0101] The slurry can be milled to reduce the particle size and enhance particle mixing. The milling is done in a ball mill, a continuous mill, or other similar equipment, and the solids content of the slurry can be, for example, about 20 wt% - 60 wt%, more specifically about 20 wt% - 40 wt%. In one embodiment, the milled slurry is characterized by a D90 particle size of about 3.0 to about 40 microns, preferably 10 to about 30 microns, more preferably about 10 to about 15 microns. The D90 is determined using a dedicated particle size analyzer. The equipment employed in this embodiment uses laser diffraction to measure the particle size in a small volume of slurry. Typically, the D90 is in microns and means that, by number, 90% of the particles have a diameter less than the stated value.

[0102] The slurry is coated onto the catalyst substrate using any carrier coating technique known in the art. In one embodiment, the catalyst substrate is dip-coated one or more times in the slurry or otherwise coated with the slurry. Thereafter, the coated substrate is dried at an elevated temperature (e.g., 100 - 150 °C) for a period of time (e.g., 10 minutes - 3 hours) and then calcined by heating, for example, at 400 - 700 °C, typically for about 10 minutes to about 3 hours. After drying and calcining, the final carrier coating layer is considered to be substantially solvent-free. After calcination, the catalyst loading obtained by the above carrier coating technique can be determined by calculating the difference in weight between the coated and uncoated substrate. As will be apparent to those skilled in the art, the catalyst loading can be modified by changing the slurry rheology. Additionally, the coating / drying / calcining process used to produce the carrier coating can be repeated as needed to construct the coating to a desired loading level or thickness, which means that more than one carrier coating may be applied.

[0103] In certain embodiments, the coated substrate is aged by subjecting it to a heat treatment. In one embodiment, the aging is carried out at a temperature of about 850 °C to about 1050 °C in an environment of 10 vol% water under an alternating hydrocarbon / air feed for 50 - 75 hours. Thus, an aged catalyst article is provided in certain embodiments. In certain embodiments, particularly effective materials include metal oxide-based supports (including but not limited to substantially 100% cerium oxide supports) that retain a high percentage (e.g., about 95 - 100%) of their pore volume upon aging (e.g., aging at about 850 °C to about 1050 °C, 10 vol% water, in an alternating hydrocarbon / air feed for 50 - 75 hours).

[0104] In another aspect, there is provided an emissions system for an internal combustion engine, the system comprising a catalyst article according to the presently claimed invention.

[0105] According to the presently claimed invention, there is also provided a method of treating a gaseous exhaust stream containing hydrocarbons, carbon monoxide, and nitrogen oxides, the method comprising contacting the exhaust stream with a catalyst article or an emission system according to the presently claimed invention.

[0106] According to the presently claimed invention, there is also provided a method of reducing the levels of hydrocarbons, carbon monoxide, and nitrogen oxides in a gaseous exhaust stream, the method comprising contacting the gaseous exhaust stream with a catalyst article or an emission system according to the presently claimed invention to reduce the levels of hydrocarbons, carbon monoxide, and nitrogen oxides in the exhaust gas.

[0107] According to the presently claimed invention, there is also provided the use of a catalyst article according to the presently claimed invention for purifying a gaseous exhaust stream comprising hydrocarbons, carbon monoxide, and nitrogen oxides.

[0108] Examples

[0109] The following examples more fully illustrate aspects of the presently claimed invention, which are set forth to illustrate certain aspects of the invention and should not be construed as limiting the invention.

[0110] Example 1 Preparation of CC1 Reference Catalyst A

[0111] Reference catalyst A is a Pd / Rh catalytic article with a PGM loading of 52 g / ft 3 (Pt / Pd / Rh = 0 / 48 / 4). Catalyst A is a two-layer washcoat architecture coated on a cylindrical monolithic cordierite substrate having dimensions of 4.66” in diameter and 4.4” in length, a cell density of 900 cpsi, and a wall thickness of 2.5 mils.

[0112] Preparation of the bottom washcoat: 24 g / ft 3 of Pd (50 wt% of the total Pd) in the form of a palladium precursor solution was impregnated onto refractory alumina, and 24 g / ft 3 of Pd (50 wt% of the total Pd) in the form of a palladium precursor solution was impregnated onto a stabilized ceria-zirconia composite having approximately 40 wt% ceria. A slurry containing approximately 35.3 wt% refractory Al 2 O 3 , 50.0 wt% stabilized ceria-zirconia composite, barium acetate that yields 11.7 wt% BaO, zirconium acetate that yields 1.9 wt% ZrO 2 , and 1.1 wt% Pd was coated onto the substrate. After calcination in air at 550 °C for 1 hour, the washcoat loading of the bottom washcoat was approximately 2.59 g / in 3 .

[0113] Preparation of the top coat: 4 g / ft of Rh in the form of a rhodium precursor solution (100 wt% of total Rh) was impregnated onto refractory alumina. A slurry mixture containing approximately 84.8 wt% refractory Al 3 O, 15.0 wt% of a ceria - zirconia composite having approximately 50 wt% ceria, and approximately 0.2 wt% Rh was coated onto the bottom coat. After calcination in air at 550 °C for 1 hour, the carrier coat loading of the top coat was approximately 1.00 g / in 2 O 3 ... 3 . Figure 1 A reference Pd / Rh catalytic article is shown in

[0114] Example 2: Preparation of CC1 reference catalyst B

[0115] Reference catalyst B is a Pt / Pd / Rh catalytic article with a PGM loading of 49 g / ft 3 (Pt / Pd / Rh = 0 / 48 / 1). Catalyst B is a two - layer carrier coat structure coated on a cylindrical monolithic cordierite substrate having a diameter of 4.66” and a length of 4.4”, a cell density of 900 cpsi, and a wall thickness of 2.5 mils.

[0116] Preparation of the bottom coat: The bottom coat is the same as that of reference catalyst A.

[0117] Preparation of the top coat: 1 g / ft of Rh in the form of a rhodium precursor solution (100 wt% of total Rh) was impregnated onto refractory alumina. A slurry mixture containing approximately 84.9 wt% refractory Al 3 O, 15.0 wt% of a ceria - zirconia composite having approximately 50 wt% ceria, and approximately 0.1 wt% Rh was coated onto the bottom coat. After calcination in air at 550 °C for 1 hour, the carrier coat loading of the top coat was approximately 1.00 g / in 2 O 3 ... 3 . Figure 1 Catalyst B is shown in

[0118] Example 3: Preparation of CC1 inventive catalyst C

[0119] Inventive catalyst C is a Pt / Pd / Rh catalytic article with a PGM loading of 52 g / ft 3 (Pt / Pd / Rh = 0 / 48 / 4). Catalyst C is a two - layer carrier coat structure coated on a cylindrical monolithic cordierite substrate having a diameter of 4.66” and a length of 4.4”, a cell density of 900 cpsi, and a wall thickness of 2.5 mils.

[0120] Preparation of the bottom coating: The bottom coating is the same as that of reference catalyst A.

[0121] Preparation of the top coating: The top coating is the same as that of reference catalyst B.

[0122] Preparation of the Rh gradient within the top coating: From one end (the inlet end in this case) of the substrate that already contains the bottom and top coatings, with the Rh precursor solution, allow absorption of 3.77 g / ft 3 Rh (75% of the total Rh in the sample) to cover, for example, approximately 80% of the substrate length. Thus, an Rh gradient is formed with the top coating of the catalyst. In a subsequent step, the substrate is dried at 120 °C for 30 minutes and calcined in air at 550 °C for 1 hour. The resulting Rh-enriched region exhibits an Rh gradient where the Rh concentration decreases from the top to the bottom of the top coating. The gradient is set such that at least 50% of the Rh in the gradient of the part containing the top coating is located in the top 1 / 3 part of the top coating. Figure 1 Catalyst C is shown in

[0123] Example 4: Preparation of CC1, the catalyst D of the present invention

[0124] The catalyst D of the present invention is a Pt / Pd / Rh catalytic article with a PGM loading of 52 g / ft 3 (Pt / Pd / Rh = 0 / 48 / 4). Catalyst D is a two-layer washcoat architecture coated on a cylindrical monolithic cordierite substrate with dimensions of 4.66” in diameter, 4.4” in length, a cell density of 900 cpsi, and a wall thickness of 2.5 mils.

[0125] Preparation of the bottom coating: The bottom coating is the same as that of reference catalyst A.

[0126] Preparation of the top coating: The top coating is the same as that of reference catalyst B.

[0127] Preparation of the Rh gradient within the top coating: From the first axial end of the substrate that already contains the bottom and top coatings, with the Rh precursor solution, allow absorption of 11 g / ft 3 Rh (75% of the total Rh in the sample) to cover, for example, approximately 27% of the substrate length. Thus, an Rh gradient is formed with the top coating of the catalyst. In a subsequent step, the substrate is dried at 120 °C for 30 minutes and calcined in air at 550 °C for 1 hour. The resulting Rh-enriched region exhibits an Rh gradient where the Rh concentration decreases from the top to the bottom of the top coating. The gradient is set such that at least 50% of the Rh in the gradient of the part containing the top coating is located in the top 1 / 3 part of the top coating. Figure 1 Catalyst D is shown in

[0128] Example 5: Preparation of Catalyst E of the Present Invention

[0129] Catalyst E of the present invention is a Pt / Pd / Rh catalytic article with a PGM loading of 52 g / ft 3 (Pt / Pd / Rh = 0 / 48 / 4). Catalyst E is a two-layer washcoat architecture coated on a cylindrical monolithic cordierite substrate with a diameter of 4.66” and a length of 4.4”, a cell density of 900 cpsi, and a wall thickness of 2.5 mils.

[0130] Preparation of the bottom washcoat: The bottom washcoat is the same as that of reference catalyst A.

[0131] Preparation of the top washcoat: The top washcoat is the same as that of reference catalyst B.

[0132] Preparation of the Rh gradient within the top washcoat: From the first axial end of the substrate that already contains the bottom and top washcoats, with a Rh precursor solution, allow the absorption of 7.33 g / ft 3 Rh (75% of the total Rh in the sample) to cover, for example, approximately 41% of the substrate length. Thus, a Rh gradient is formed with the top washcoat of the catalyst. In a subsequent step, the substrate is dried at 120 °C for 30 minutes and calcined in air at 550 °C for 1 hour. The resulting Rh-enriched region exhibits a Rh gradient where the Rh concentration decreases from the top to the bottom of the top washcoat. The gradient is set such that at least 50% of the Rh in the gradient of the portion containing the top washcoat is located in the top 1 / 3 portion of the top washcoat. Figure 1 Catalyst E is shown in

[0133] Example 6: Preparation of UF Reference Catalyst F

[0134] Reference catalyst F is a Pd / Rh catalytic article with a PGM loading of 30 g / ft 3 (Pt / Pd / Rh = 0 / 25 / 5). Catalyst F is a single-layer washcoat architecture coated on a cylindrical monolithic cordierite substrate with a diameter of 4.16” and a length of 4.53”, a cell density of 400 cpsi, and a wall thickness of 4 mils.

[0135] Preparation of the washcoat: 5 g / ft 3 of Rh (100 wt% of the total Rh) in the form of a rhodium precursor solution is impregnated onto refractory alumina, and 25 g / ft 3 of Pd (100 wt% of the total Pd) in the form of a palladium precursor solution is impregnated onto a stabilized ceria-zirconia composite with approximately 30 wt% ceria. The washcoat containing approximately 28.3 wt% of refractory Al 2 O 3, 63.8 wt% stabilized cerium-zirconium composite, barium acetate that produces 4.5 wt% BaO, zirconium acetate that produces 2.9 wt% ZrO 2 is applied as a slurry onto a substrate along with 0.1 wt% Rh and 0.4 wt% Pd. After calcination in air at 550 °C for 1 hour, the washcoat loading of the washcoat is about 3.53 g / in 3 .

[0136] Example 7: Testing of the Catalyst

[0137] All the catalysts prepared in Examples 1 to 6 are schematically shown in Figure 1 . The PGM gradient concept is explained in more detail in Figure 2 using the inventive catalyst D as an example. In Figure 2 , the Rh wt% is provided on the Y-axis and the washcoat depth (μm) counted from the uppermost part of the top washcoat towards the substrate wall is provided on the X-axis.

[0138] All the catalysts are aged using the segment oil aging protocol and run using an engine setup such that the typical inlet temperature is about 950 °C and the typical catalyst bed temperature does not exceed about 1030 °C. The gas feed composition output from the engine alternates between rich and lean to simulate the typical operating conditions of a vehicle tested under the WLTC test protocol. All the CC1 catalysts are aged for 100 hours using the same conditions. Catalyst F is used as a common underfloor catalyst and aged for 100 hours using the same protocol but at the UF position, which results in a proportionate reduction in the effective temperature.

[0139] The emission performance is tested using a 2.0 L turbocharged engine in a vehicle having a CC+UF emission control system configuration operating under the WLTC test protocol. Each system is tested at least four times to ensure high experimental reproducibility and data consistency.

[0140] The benefits of using an Rh gradient in the catalyst (e.g., in the top washcoat) are shown in Fig. 3 (3A, 3B, and 3C). Compared to a reference with the same (0 / 48 / 4) PGM loading without a change in the washcoat carrier formulation, the inventive catalyst achieved improvements of about 12% THC, about 18% CO, and about 25% NOx in the middle bed in the WLTC test. This improvement is attributed to the PGM gradient-based architecture since the underlying washcoat is the same as that of the 0 / 48 / 1 reference catalyst B, and the effect of the increase in Rh in the washcoat is illustrated by the difference between catalyst A and catalyst B, which are identical except for the Rh concentration in the top washcoat having a uniform (substantially no Rh gradient).

[0141] Throughout this specification, references to "one embodiment", "certain embodiments", "one or more embodiments", or "an embodiment" mean that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the presently claimed invention. Thus, the appearances of the phrases "in one or more embodiments", "in certain embodiments", "in some embodiments", "in one embodiment", or "in an embodiment" in various places throughout this specification are not necessarily referring to the same embodiment of the presently claimed invention. Further, in one or more embodiments, the particular features, structures, materials, or characteristics may be combined in any suitable manner. All of the individual embodiments, aspects, and options disclosed herein may be combined in all variations, regardless of whether such features or elements are specifically combined in the description of a particular embodiment herein. The presently claimed invention is intended to be read as a whole such that any separable feature or element of the disclosed invention should be considered to be combinable in any of its aspects and embodiments, unless the context clearly indicates otherwise.

[0142] Although the embodiments disclosed herein have been described with reference to specific embodiments, it should be understood that these embodiments are merely illustrative of the principles and applications of the presently claimed invention. It will be apparent to those skilled in the art that various modifications and variations can be made to the methods and apparatuses of the presently claimed invention without departing from the spirit and scope of the presently claimed invention. Accordingly, the presently claimed invention is intended to cover modifications and variations that fall within the scope of the appended claims and their equivalents, and the embodiments described above are presented for purposes of illustration and not of limitation. All patents and publications cited herein are incorporated herein by reference for their specific teachings as mentioned, unless otherwise provided for a specific incorporation statement.

Claims

1. An emission control catalyst article, which comprises: a substrate having a first inlet axial end and a second outlet axial end, a bottom washcoat layer comprising a platinum group metal coated on 60% to 100% of the length of the substrate from the first inlet axial end to the second outlet axial end, and a top washcoat layer comprising rhodium and optionally a platinum group metal other than rhodium coated on 60 to 100% of the length of the substrate from the first inlet axial end or the second outlet axial end of the substrate, such that the top coat covers at least 60% of the length of the bottom washcoat layer, wherein at least a portion of the top washcoat layer contains rhodium deposited in a rhodium gradient within the top washcoat layer, the gradient being such that the rhodium concentration in the topmost portion of the top washcoat layer is at least twice as high as the rhodium concentration in the bottommost portion of the top washcoat layer, wherein the length of the at least a portion of the top washcoat layer ranges from 20 - 100% of the substrate length, wherein the total platinum group metal loading deposited in the gradient is 30 to 95% of the total platinum group metal loading present in the top washcoat layer, wherein the top washcoat layer contains rhodium deposited uniformly, and the ratio of the total amount of rhodium deposited uniformly within the top washcoat layer to the amount of rhodium deposited in the gradient ranges from 10:1 to 1:

10.

2. The catalyst article according to claim 1, wherein at least a portion of the top washcoat layer contains a mixture of rhodium and a platinum group metal other than rhodium deposited in a rhodium gradient within the top washcoat layer, the gradient being such that the rhodium concentration in the topmost portion of the top washcoat layer is at least twice as high as the rhodium concentration in the bottommost portion of the top washcoat layer.

3. The catalyst article according to claim 1 or 2, wherein the platinum group metal is selected from platinum, palladium, rhodium, and combinations thereof.

4. The catalyst article according to claim 1 or 2, wherein the platinum group metal is supported on a support selected from cerium oxide, alumina, lanthanum-alumina, cerium-alumina, cerium-zirconium-alumina, zirconium-alumina, lanthanum-zirconium-alumina, barium-alumina, barium-lanthanum-alumina, barium-lanthanum-neodymium-alumina, cerium-zirconium, cerium-zirconium-lanthanum, cerium-zirconium-yttrium, cerium-zirconium-lanthanum-yttrium, cerium-zirconium-neodymium, cerium-zirconium-praseodymium, cerium-zirconium-lanthanum-neodymium, cerium-zirconium-lanthanum-praseodymium, cerium-zirconium-lanthanum-neodymium-praseodymium, lanthanum-zirconium, barium-zirconium, and combinations thereof.

5. The catalyst article according to claim 3, wherein the platinum group metal is supported on a support selected from cerium oxide, alumina, lanthanum-alumina, cerium-alumina, cerium-zirconium-alumina, zirconium-alumina, lanthanum-zirconium-alumina, barium-alumina, barium-lanthanum-alumina, barium-lanthanum-neodymium-alumina, cerium-zirconium, cerium-zirconium-lanthanum, cerium-zirconium-yttrium, cerium-zirconium-lanthanum-yttrium, cerium-zirconium-neodymium, cerium-zirconium-praseodymium, cerium-zirconium-lanthanum-neodymium, cerium-zirconium-lanthanum-praseodymium, cerium-zirconium-lanthanum-neodymium-praseodymium, lanthanum-zirconium, barium-zirconium, and combinations thereof.

6. The catalyst article according to claim 1 or 2, wherein at least 60% of the rhodium is deposited in a gradient such that it covers at least 50% of the substrate length, and the Rh concentration in the topmost portion of the support coating layer is twice as high as that in the bottommost portion of the support coating layer.

7. The catalyst article according to any one of claims 1, 2, and 5, wherein 50% or more of the platinum group metal in the top support coating layer is deposited in the top 1 / 3 portion of the support coating layer, as determined by electron probe microanalysis (EPMA) line scanning from the topmost portion of the support coating layer containing the platinum group metal gradient to the substrate.

8. The catalyst article according to claim 3, wherein 50% or more of the platinum group metal in the top support coating layer is deposited in the top 1 / 3 portion of the support coating layer, as determined by electron probe microanalysis (EPMA) line scanning from the topmost portion of the support coating layer containing the platinum group metal gradient to the substrate.

9. The catalyst article according to claim 4, wherein 50% or more of the platinum group metal in the top support coating layer is deposited in the top 1 / 3 portion of the support coating layer, as determined by electron probe microanalysis (EPMA) line scanning from the topmost portion of the support coating layer containing the platinum group metal gradient to the substrate.

10. The catalyst article according to claim 6, wherein 50% or more of the platinum group metal in the top support coating layer is deposited in the top 1 / 3 portion of the support coating layer, as determined by electron probe microanalysis (EPMA) line scanning from the topmost portion of the support coating layer containing the platinum group metal gradient to the substrate.

11. The catalyst article according to claim 7, wherein 55% to 95% of the platinum group metal in the top support coating layer is deposited in the top 1 / 3 portion of the support coating layer, as determined by electron probe microanalysis (EPMA) line scanning from the topmost portion of the support coating layer containing the platinum group metal gradient to the substrate.

12. The catalyst article according to any one of claims 8 to 10, wherein 55% to 95% of the platinum group metal within the top support coating layer is deposited in the top 1 / 3 portion of the support coating layer, as determined by electron probe microanalysis (EPMA) line scanning from the topmost portion of the support coating layer having a platinum group metal gradient to the substrate.

13. The catalyst article according to any one of claims 1, 2, 5, and 8 to 11, wherein the bottom support coating layer comprises at least one alkaline earth metal oxide, the at least one alkaline earth metal oxide comprising barium oxide, strontium oxide, or any combination thereof, and the amount of the at least one alkaline earth metal oxide is 1.0 to 20 wt% based on the total weight of the bottom support coating.

14. The catalyst article according to claim 6, wherein the bottom support coating layer comprises at least one alkaline earth metal oxide, the at least one alkaline earth metal oxide comprising barium oxide, strontium oxide, or any combination thereof, and the amount of the at least one alkaline earth metal oxide is 1.0 to 20 wt% based on the total weight of the bottom support coating.

15. The catalyst article according to claim 7, wherein the bottom support coating layer comprises at least one alkaline earth metal oxide, the at least one alkaline earth metal oxide comprising barium oxide, strontium oxide, or any combination thereof, and the amount of the at least one alkaline earth metal oxide is 1.0 to 20 wt% based on the total weight of the bottom support coating.

16. The catalyst article according to claim 12, wherein the bottom support coating layer comprises at least one alkaline earth metal oxide, the at least one alkaline earth metal oxide comprising barium oxide, strontium oxide, or any combination thereof, and the amount of the at least one alkaline earth metal oxide is 1.0 to 20 wt% based on the total weight of the bottom support coating.

17. The catalyst article according to claim 1, wherein the substrate is a ceramic substrate, a metal substrate, a ceramic foam substrate, a polymer foam substrate, or a woven fiber substrate.

18. An exhaust system for an internal combustion engine, the system comprising the catalyst article according to any one of claims 1 to 17.

19. A method of treating a gaseous exhaust stream comprising hydrocarbons, carbon monoxide, and nitrogen oxides, the method comprising contacting the exhaust stream with the catalyst article according to any one of claims 1 to 17 or the exhaust system according to claim 18.

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