WO2024179456A1

WO2024179456A1 - Catalytic article for engine exhaust gas treatment

Info

Publication number: WO2024179456A1
Application number: PCT/CN2024/078777
Authority: WO
Inventors: Na Ting YANG; Min JIN; Wen Tao XU; Xiao Shuang YANG; Shau Lin CHEN; Cheng Hao SUN
Original assignee: Basf Corporation; Basf (China) Company Limited
Priority date: 2023-02-28
Filing date: 2024-02-27
Publication date: 2024-09-06

Abstract

A catalytic article for treating an exhaust stream, especially a TWC catalytic article, comprises a substrate comprising a plurality of walls extending longitudinally and a plurality of channels defined by the plurality of walls for the exhaust stream passing through, wherein the channels have corners at junctions of walls, a catalyst coating comprising a platinum group metal component in supported form on surfaces of the walls, consisting of a coating portion located at the corners and a coating portion located at remaining area of the walls, wherein the coating portion located at the corners includes cavities extending along the channels longitudinally. And a process for preparing the catalytic article, an exhaust treatment system comprising the catalytic article and a method for treating the exhaust stream are also disclosed.

Description

CATALYTIC ARTICLE FOR ENGINE EXHAUST GAS TREATMENT

FIELD OF THE INVENTION

The present invention relates to a catalytic article useful for treatment of engine exhaust gases, especially TWC catalytic article for saddle-riding-type vehicle engines, a process for preparing the catalytic article, and an exhaust treatment system comprising the catalytic article.

BACKGROUND OF THE INVENTION

Engine exhaust substantially consists of particulate matter and gaseous pollutants such as unburned hydrocarbons (HC) , carbon monoxide (CO) and nitrogen oxides (NO_x) . For stoichiometric engines operating near the optimum air/fuel ratio, such as gasoline engines, three-way conversion catalysts (hereinafter interchangeably referred to as TWC catalysts or TWC) are typically used to simultaneously oxidize unburnt hydrocarbons and carbon monoxide and reduce nitrogen oxides. TWC catalysts are known effective near stoichiometric conditions, under which the basic reactions including reduction and oxidation may be exemplified as follows,

2 NO + 2 CO → N₂ + 2 CO₂

2 CO + O₂ → 2 CO₂

2 C₂H₆ + 7 O₂ → 4 CO₂ + 6 H₂O.

TWC catalysts generally utilize one or more platinum group metals (PGMs) , e.g., rhodium (Rh) , platinum (Pt) , palladium (Pd) , ruthenium (Ru) , osmium (Os) and iridium (Ir) , as catalytically active species, which are typically supported on support particles of a refractory metal oxide and/or an oxygen storage capacity material. The support particles carrying the PGMs are generally coated on a ceramic or metallic honeycomb substrate to provide a TWC catalytic article. The TWC catalytic article may have a single coating layer or multiple different coating layers of TWC catalysts.

As one of TWC catalytic articles for various vehicles, a TWC catalytic article for saddle-riding-type vehicle particularly needs to exert high purification capacity since such a catalytic article has a small volume capacity due to the limited space for the catalytic article to be mounted, suffers a rapid atmospheric variation, and is easily exposed to a high temperature. When the TWC catalytic article has a single thick coating layer or multiple coating layers, it is difficult for the exhaust gas to diffuse into the interior part of the catalyst coating close to the substrate side. Moreover, when the TWC catalytic article is used under high space velocity, the exhaust gas hardly diffuses to the deep part of the catalyst coating, especially the thick catalyst coating portion at corners in exhaust channels of the substrate, and thus the catalyst performance cannot be sufficiently brought out.

To solve such a problem of insufficient diffusion of exhaust gases in a catalyst coating, some special catalyst compositions, configurations or structures for TWC catalytic articles have been proposed.

For example, WO2015/037613A discloses an exhaust gas purifying catalyst comprising a catalyst layer comprising two or more types of inorganic porous particles each having a different particle size, a catalytically active component and voids, wherein as a first characteristic, the voids satisfying a condition of L/2/ (πS) ^1/2 ≧ 2 occupy 50%or more by number of all the voids in the catalyst layer wherein S represents a void cross-sectional area, and L represents a void cross-sectional circumference; and as a second characteristic, in the void cross-sectional area in the catalyst layer, an average void radius, determined assuming that the void shape is a perfect circle, is 10 μm to 20 μm. It is described that the miscibility and diffusibility of the gas in the catalyst layer are improved and thereby excellent purifying performance can be exhibited.

US2017/0232425A1 discloses an exhaust gas purifying catalyst having a first catalyst layer which is formed on a surface of a substrate and a second catalyst layer which is formed on the upper side of the first catalyst layer, wherein the first catalyst layer comprises a precious metal, an OSC material and an alumina, and the OSC material and the alumina are comprised at a mass ratio of OSC material to alumina in the range of from 1: 7 to 1: 3, and the second catalyst layer comprises a precious metal, an OSC material and an alumina, and the OSC material and the alumina are comprised at a mass ratio of OSC material to alumina in the range of from 1: 1 to 10: 0, and wherein an average particle size (D₅₀) of the alumina in the first catalyst layer is 10 to 16 μm, and an average particle size (D₅₀) of the OSC material in the first catalyst layer is 3 to 12 μm. It is described that the gas diffusibility in the catalyst layer can be improved, the catalytic activity is rarely reduced, and the entire catalyst layer can be effectively utilized even in an internal-combustion engine which is used under an exhaust gas atmosphere where it is exposed to high temperature exhaust gas, and where a space velocity of the passed exhaust gas is extremely high.

WO2013118425A1 discloses a porous apatite catalyst layer containing apatite, in which a peak top is present within a void volume diameter range of from 100 nm to 1000 nm in logarithmic differentiation void volume distribution measured by a mercury intrusion porosimeter. It is described that a new catalyst structure is provided which is capable of maintaining gas diffusivity to a deep part of a catalyst layer even under condition that a gas flow rate is high.

WO2014156676A1 discloses a catalyst structure including a substrate, an upper catalyst layer, and a lower catalyst layer, the catalyst structure having a first peak or a second peak at a pore volume diameter of 10 nm to 50 nm and a pore volume diameter of 50 nm to 100 nm, respectively, in the logarithmic differential pore volume distribution analyzed by mercury intrusion porosimetry. It is described that the catalyst structure can increase gas diffusibility to the deep part of the catalyst layer and can sufficiently exhibit a function as a three-way catalyst.

There is still a need to provide a catalytic article which can exhibit improved interaction between feed gas and catalytically active surface, especially active surfaces at the corners in the channels of the substrate and enhance the performance of exhaust gas purification.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a catalytic article, especially a catalytic article for saddle-riding-type vehicle, which has improved overall catalytic performance in terms of abatement of HC, CO and NO_x, particularly improved abatement of NO_x, compared with conventional TWC catalysts for saddle-riding-type vehicle.

It has been surprisingly found that the object of the present invention was achieved by a catalytic article comprising a substrate comprising channels and a coating including cavities extending along the wall surfaces longitudinally at corners of the channels of the substrate.

Accordingly, in the first aspect, the present invention provides a catalytic article for treating an exhaust stream, especially a TWC catalytic article, comprising

- a substrate comprising a plurality of walls extending longitudinally and a plurality of channels defined by the plurality of walls for the exhaust stream passing through, wherein the channels have corners at junctions of walls,

- a catalyst coating comprising a platinum group metal component in supported form on surfaces of the walls, consisting of a coating portion located at the corners and a coating portion located at remaining area of the walls,

wherein the coating portion located at the corners includes cavities extending along the channels longitudinally.

In the second aspect, the present invention provides a process for preparing the catalytic article as described in the first aspect, which comprises

- applying a solution or suspension of a cavity-forming agent onto surfaces of the walls of a substrate and drying to provide a dried layer of the cavity-forming agent,

- providing a catalyst coating on the dried layer of the cavity-forming agent by applying a slurry comprising a platinum group metal component in supported form and optionally drying, and calcination,

wherein the cavity-forming agent is used in an amount of 0.05%to 9.0%by weight, based on the loading of the catalyst coating.

In the third aspect, the present invention provides an exhaust treatment system comprising the catalytic article as described herein located downstream of a stoichiometric engine, particularly a gasoline engine such as a saddle-riding-type vehicle engine.

In a fourth aspect, the present invention provides a method for treating an exhaust stream from a stoichiometric engine, which includes contacting the exhaust stream with the catalytic article or the exhaust treatment system as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1A shows microscope image and figures 1B and 1C show SEM images of the cross-section of the catalytic article according to Example 1.

Figures 2A shows microscope image and figures 2B and 2C show SEM images of the cross-section of the catalytic article according to Example 2.

Figures 3A shows microscope image and figures 3B and 3C show SEM images of the cross-section of the catalytic article according to Example 3.

Figures 4A shows microscope image and figures 4B and 4C show SEM images of the cross-section of the catalytic article according to Example 4.

Figures 5A shows microscope image and figures 5B and 5C show SEM images of the cross-section of the catalytic article according to Example 5.

Figures 6A and 6B show equivalent triangles approximately representing the cross-section of the cavities and equivalent triangles representing the cross-section of the catalyst coating respectively.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in detail hereinafter. It is to be understood that the present invention may be embodied in many different ways and shall not be construed as limited to the embodiments set forth herein.

As used herein, the singular forms “a” , “an” and “the” include plural referents unless the context clearly dictates otherwise. The terms “comprise” , “comprising” , etc. are used interchangeably with “contain” , “containing” , etc. and are to be interpreted in a non-limiting, open manner. That is, e.g., further components or elements may be present. The expressions “consists of” or cognates may be embraced within “comprises” or cognates.

As used herein, the terms “platinum group metal component” , “palladium component” , “platinum component” and “rhodium component” are intended to describe the presence of respective platinum group metal in any possible valence state, which may be for example metal or metal oxide as the catalytically active form, or may be for example metal compound, complex or the like which, upon calcination or use of the catalyst, decomposes or otherwise converts to the catalytically active form.

As used herein, the term “support” refers to a material in form of particles for receiving and carrying one or more platinum group metal (PGM) components, and optionally one or more other components such as stabilizers, promoters and binders.

Herein, any reference to a platinum group metal component in “supported form” is intended to mean that the platinum group metal component is supported on and/or in support particles.

Herein, any reference to an amount of loading in the unit of “g/ft³” or “g/in³” is intended to mean the weight of the specified component or coating layer per unit volume of the substrate or substrate part, on which they are carried.

As used herein, the term “catalyst coating” refers to a catalyst-comprising covering which is deposited on the surfaces of walls of a substrate which define channels for exhaust stream passing through. A catalyst coating may have a non-layered or layered configuration. For a non-layered configuration, the catalyst coating consists of a single coating layer; for a layered configuration, the catalyst coating consists of two or more coating layers wherein at least one is a catalyst-comprising coating layer. It is to be understood that a coating layer may be prepared by just one coating operation, or may be prepared by repeating a coating operation twice or more to attain a targeted loading. In the latter case, the coating layer will comprise more than one sub-layer having the same chemical composition and catalytic activity which may be distinguishable only with SEM analysis. Such a coating layer comprising more than one sub-layer having the same chemical composition and catalytic activity will be referred to as a single or one coating layer. Accordingly, when two or more coating layers are referred to herein, the coating layers will have different chemical compositions or catalytic activities from each other.

As used herein, the term “catalyst coating layer” particularly refers to a coating layer comprising a platinum group metal component in supported form.

Herein, any reference to a coating “on surfaces of the walls” is intended to mean the layer or coating is deposited on the walls which may be blank or have already carried one or more coating layers. Also, any reference to “applying ... on surfaces of the walls” is intended to mean the applying is subjected on the walls which may be blank or have already carried one or more coating layers.

As used herein, the term “solid content” is intended to refer to content of matters which are non-volatile under a calcination condition, expressed as a ratio of weights measured after and before a calcination process, for example at 500 ℃ for 1 hour.

As used herein, the terms “exhaust” , “exhaust gas” , “exhaust stream” and the like are used interchangeably with each other and refer to any engine effluents that may also contain particulate matter.

According to the first aspect of the present invention, a catalytic article for treating an exhaust stream is provided, which comprises

The substrate as used herein refers to a structure that is suitable for withstanding conditions encountered in an exhaust stream from combustion engines, on which catalyst compositions are carried, typically in the form of washcoat. The substrate is generally made of a refractory material such as ceramic or a metallic material.

Ceramic materials useful for constructing the substrate may include any suitable refractory material, e.g., cordierite, mullite, cordierite-alumina, silicon nitride, zircon mullite, spodumene, alumina-silica-magnesia, zircon silicate, sillimanite, magnesium silicates, zircon, petalite, alumina, and aluminosilicates.

Metallic materials useful for constructing the substrate may include any suitable heat resistant metals and metal alloys such as titanium and stainless steel as well as other alloys in which iron is a substantial or major component. Such alloys may contain one or more nickel, chromium, and/or aluminium, and the total amount of these metals may advantageously comprise at least 15%by weight of the alloy, for example 10 to 25%by weight of chromium, 3 to 8%by weight of aluminium, and up to 20%by weight of nickel. The alloys may also contain small or trace amounts of one or more metals such as manganese, copper, vanadium, titanium and the like. The surface of the metallic substrate may be oxidized at a high temperature, e.g., 950 ℃ or higher, to form an oxide layer on the surface of the substrate, improving the corrosion resistance of the alloy and facilitating adhesion of the washcoat layer to the metal surface.

Within the context of the present invention, a flow-through substrate is preferred, which has a plurality of fine, parallel gas flow channels extending from an inlet face to an outlet face of the substrate such that passages are open to fluid flow therethrough. The passages, which are essentially straight paths from their fluid inlet to their fluid outlet, are defined by walls on which the catalytic material is applied as a washcoat so that the gases flowing through the passages contact the catalytic material. The flow passages of the monolithic substrate are thin-walled channels, which can be of any suitable sizes and cross-sectional shapes with corners such as trapezoidal, rectangular, square, sinusoidal (S-shape) and hexagonal etc. Such structures may contain 60 to 900, or even more gas inlet openings (i.e., cells) per square inch of cross section. For example, the substrate may have 100 to 650, more usually 150 to 400 cells per square inch ( "cpsi" ) of the cross section. The wall thickness of flow-through substrates may vary, with a typical range of from 1 mil to 0.1 inches.

Each channel defined by walls of the substrate may have more than one, for example two, three, four, five or six corners, depending on the cross-sectional shape of the channels.

In some embodiments, the substrate for the catalytic article according to the present invention is a flow-through metallic substrate having channels of a cross-sectional shape with two or more corners, such as trapezoidal, rectangular, square, sinusoidal (S-shape) and hexagonal.

The catalyst coating of the catalytic article according to the present invention may consist of one coating layer or consist of two or more coating layers wherein at least one coating layer comprises a platinum group metal component in supported form, which is also referred to as a catalyst coating layer.

In some embodiments, the catalyst coating of the catalytic article according to the present invention may comprise or consist of two catalyst coating layers each comprising a platinum group metal component in supported form. For example, the catalyst coating of the catalytic article according to the present invention consists of a top catalyst coating layer comprising a first platinum group metal component in supported form and a bottom coating layer comprising a second platinum group metal component in supported form.

The catalyst coating on surfaces of the walls may be divided into two portions, wherein one portion is located on the surfaces of the walls at the corners of the channels (also referred to as coating portion located at the corners herein) and the other portion is located on the remaining surfaces of the walls.

The coating portion located at the corners may include cavities which continuously extend longitudinally throughout the coating portion located at the corners, cavities which extend longitudinally for a part of the length of the coating portion, or both.

Herein, the term “coating portion located at the corners” is intended to collectively refer to the coating portions at corners of the channels of the substrate which extend a certain length (e.g., full and part length) of the substrate.

It will be understood that the coating portion in a single channel may include two or more parts located in respective corners when the channel has more than one, for example two, three, four, five or six corners.

It will also be understood that two or more cavities may be included successively at a single corner in a channel over the full length of the coating, when the coating portion include cavities which extend longitudinally along the channels of the substrate over a part of the length of the coating.

The cavities may be of any cross-sectional shapes such as triangle, fan-shaped and off-round etc. The volume of the cavities may account for 0.5%to 35%, preferably 2.0 to 30%, based on the volume of the coating portion located at the corners. The percentage of the volume of the cavities may also be referred to as cavity volume ratio. The cavity volume ratio can be determined by calculation based on microscope image, as described in Examples hereinbelow.

There is no particular restriction to the platinum group metal (PGM) component useful for any catalyst coating in the catalytic article. Typically, the PGM component may be a rhodium (Rh) component, a platinum (Pt) component, a palladium (Pd) component, a ruthenium (Ru) component, an osmium (Os) component, an iridium (Ir) component or any combinations thereof, among which a Pt component, a Pd component, a Rh component or any combinations thereof are usually used.

Particularly, the catalyst coating of the catalytic article according to the present invention comprises a PGM component selected from a Rh component in combination with either or both of a Pt component and a Pd component. Those platinum group metal components may be comprised in the same one catalyst coating layer in the catalytic article. Alternatively, the PGM components may be arranged in different catalyst coating layers when the catalyst coating of the catalytic article comprises two or more catalyst coating layers comprising a PGM component in supported form.

For example, in some embodiments, the catalyst coating of the catalytic article comprises two catalyst coating layers each comprising a PGM component in supported form, wherein one catalyst coating layer comprises a Rh component in supported form and optionally either or both of a Pt component in supported form and a Pd component in supported form as the PGM component, and the other catalyst coating layer comprises a Pt component in supported form and optionally a Pd component in supported from as the PGM component.

In some particular embodiments, the catalyst coating of the catalytic article comprises or consists of a top catalyst coating layer comprising a PGM component in supported form and a bottom catalyst coating layer comprising a PGM component in supported form, wherein the top catalyst coating layer comprises a Rh component in supported form and optionally either or both of a Pt component in supported form and a Pd component in supported form as the PGM component, and the bottom catalyst coating layer comprises a Pt component in supported form and optionally a Pd component in supported from as the PGM component.

Particularly, the catalyst coating of the catalytic article comprises or consists of a top catalyst coating layer comprising a PGM component in supported form and a bottom catalyst coating layer comprising a PGM component in supported form, wherein the top catalyst coating layer comprises a Rh component in supported form, a Pt component in supported form and a Pd component in supported form as the PGM component, and the bottom catalyst coating layer comprises a Pt component in supported form and a Pd component in supported form as the PGM component.

As useful support materials for the PGM component in the catalytic article according to the present invention, refractory metal oxides, oxygen storage components and any combinations thereof may be mentioned.

The refractory metal oxide, a widely used support material for a PGM component in catalytic articles for exhaust treatment, is generally a high surface area alumina-based material, zirconia-based material or a combination thereof. Within the context of the present invention, “alumina- based material” refers to a material comprising alumina as a base and optionally a dopant. Similarly, “zirconia-based material” refers to a material comprising zirconia as a base and optionally a dopant.

Suitable examples of the alumina-based material include, but are not limited to alumina, for example a mixture of the gamma and delta phases of alumina which may also contain substantial amounts of eta, kappa and theta alumina phases, lanthana doped alumina, baria doped alumina, ceria doped alumina, zirconia doped alumina, ceria-zirconia doped alumina, lanthana-zirconia doped alumina, baria-lanthana doped alumina, baria-ceria doped alumina, baria-zirconia doped alumina, baria-lanthana-neodymia doped alumina, lanthana-ceria doped alumina, and any combinations thereof.

Suitable examples of the zirconia-based material include, but are not limited to zirconia, lanthana doped zirconia, yttria doped zirconia, neodymia doped zirconia, praseodymia doped zirconia, titania doped zirconia, titania-lanthana doped zirconia, lanthana-yttria doped zirconia, and any combinations thereof.

Particularly, the refractory metal oxide useful as the support may be selected from baria doped alumina, lanthana doped alumina, ceria doped alumina, zirconia doped alumina, lanthana-zirconia doped alumina, lanthana doped zirconia, and any combinations thereof.

Generally, the amount of the refractory metal oxide is 10 to 90%by weight, if used, based on the total weight of a single coating layer.

The oxygen storage component (OSC) refers to an entity that has a multi-valence state and can actively react with oxidants such as oxygen or nitrogen oxides under oxidative conditions or react with reductants such as carbon monoxide or hydrogen under reduction conditions. Typically, the oxygen storage component comprises one or more reducible rare earth metal oxides, such as ceria. The oxygen storage component may also comprise one or more of lanthana, praseodymia, neodymia, europia, samaria, ytterbia, yttria, zirconia and hafnia to constitute a composite oxide with ceria. Preferably, the oxygen storage component is selected from ceria-zirconia composite oxides and stabilized ceria-zirconia composite oxides.

Generally, the amount of oxygen storage component is from 15 to 85 %by weight, if used, based on the total weight of a single coating layer.

The support materials for different platinum group metal (PGM) components in a catalyst coating layer may be the same or different if two or more PGMs are comprised in the same catalytic coating layer. Also, the support materials for the same platinum group metal (PGM) component in different catalyst coating layers may be the same or different if two or more catalyst coating layers are comprised in the catalytic article. Further, more than one supports may be used for the same platinum group metal (PGM) component in one catalyst coating layer.

The catalytic article according to the present invention may comprise the catalyst coating layer (s) comprising a PGM component in supported form at a loading in the range of from 0.1 to 15.0 g/in³, or from 0.5 to 10.0 g/in³, or from 1.0 to 4.0 g/in³. When two or more catalyst coating layers each comprising a platinum group metal component in supported form are comprised in the catalyst coating of the catalytic article, the loading as described here refers to sum of all of such catalyst coating layers.

The catalyst coating layer (s) comprising a PGM component in supported form as described herein may comprise the PGM component at a total loading in the range of from 0.1 to 50.0 g/ft³, or from 0.25 to 40.0 g/ft³, or from 0.5 to 20.0 g/ft³, calculated as respective PGM elements.

The catalyst coating layer (s) may optionally comprise a stabilizer and/or a promoter as desired. Suitable stabilizer includes non-reducible oxides of metals selected from the group consisting of barium, calcium, magnesium, strontium and any combinations thereof. Preferably, one or more of barium oxide and magnesium oxide are used as the stabilizer. Suitable promoter includes non-reducible oxides of rare earth metals selected from the group consisting of lanthanum, praseodymium, yttrium, cerium, tungsten, neodymium, gadolinium, samarium, hafnium and any combination thereof.

The catalyst coating having the cavities as described herein may extend along the entire length of the walls of the substrate, or along only a part of the length of the walls of the substrate.

The catalyst coating layer (s) may be carried on the substrate in form of “washcoat” . The term “washcoat” has its usual meaning in the art and refers to a thin, adherent coating of a catalytic or other material applied to a substrate. A washcoat is generally formed by preparing a slurry containing a certain solid content (e.g., 15 to 60%by weight) of particles in a liquid medium, which is then applied onto a substrate, dried and calcined to provide a washcoat layer.

The catalytic article according to the present invention may be prepared by a process including coating a layer of cavity-forming agent before applying the catalyst coating.

Accordingly, in the second aspect, the present invention provides a process for preparing the catalytic article as described in the first aspect, which comprises

- applying a solution or suspension of a cavity-forming agent on surfaces of the walls of a substrate and drying to provide a dried layer of the cavity-forming agent,

As described herein above, the substrate on which the solution or suspension of a cavity-forming agent is applied may be a blank substrate or may have been precoated with any suitable bottom coating layers. The blank substrate is intended to mean a substrate carrying no coating before the solution or suspension of a cavity-forming agent is applied onto it.

The cavity-forming agent is preferably used in an amount of 0.2%to 4.0%by weight, more preferably 0.25%to 2.5%by weight, based on the loading of the catalyst coating.

The cavity-forming agent may be any organic or inorganic materials which can be burned-off and leave cavities during the calcination step providing a coating or coating layer. For example, the cavity-forming agent may be selected from organic materials such as natural and synthetic polymers, organic small molecule compounds, inorganic materials such as inorganic salts and carbon materials, cellulose-containing natural materials, and any combinations thereof.

Suitable natural and synthetic polymers as the cavity-forming agent may include, but are not limited to, polyether polyols such as polyethylene glycols and alkyl-capped derivatives thereof, styrenic homopolymers or copolymers such as polystyrenes, poly (meth) acrylic acids and ester derivatives thereof such as polymethyl methacrylate or crosslinked polymethyl methacrylate, celluloses, ether and ester derivatives of celluloses, polyvinyl alcohols, polyvinyl pyrrolidones and any combinations thereof.

Suitable organic small molecule compounds as the cavity-forming agent may include, but are not limited to, benzoic acid and derivatives thereof, carbamide (urea) , sugar crystals and any combinations thereof.

Suitable inorganic salts as the cavity-forming agent may include, but are not limited to, ammonium bicarbonate, magnesium carbonate, and any combinations thereof.

Suitable carbon materials as the cavity-forming agent may include, but are not limited to, carbon black, carbon fiber, graphite and any combinations thereof.

Suitable cellulose-containing natural materials as the cavity-forming agent may be granulated products from dried plants, which include, but are not limited to, sunflower, cotton, rice, wheat, sorghum, breadfruit tree, sugar cane, corn, bamboo and any combinations thereof. The granulated products may be obtained from various part of plants such as leaf, bark, straw, root, husk and any combinations thereof.

The cavity-forming agent is applied onto the surfaces of the walls of a substrate as a solution or suspension in a coating vehicle. The coating vehicle may be water, any suitable organic solvents or a mixture thereof, in which the cavity-forming agent may be soluble to provide a solution or insoluble to provide a suspension. In case of a suspension, the cavity-forming agent may be particles having a variety of geometries, including but are not limited to spheres, tablets, cylinders or fibers, and preferably having an average particle size D₅₀ in the range of from 1 to 50 μm, or from 10 to 30 μm, or from 15 to 20 μm.

As described hereinabove, the catalyst coating may consist of two or more coating layers. If the catalyst coating consists of two or more coating layers, the applying of the catalyst coating may be carried out by coating respective slurries to form the coating layers successively. It will be understood that at least one of the slurries as applied comprises a platinum group metal component in supported form to provide a catalyst coating layer. Preferably, each of the slurries as applied comprises a platinum group metal component in supported form to provide catalyst coating layers.

In some embodiments, the present invention provides a process for preparing a catalytic article as described in the first aspect, which comprises

- providing a catalyst coating by

i) applying a slurry comprising a first platinum group metal component in supported form onto the dried layer of the cavity-forming agent, drying and optionally calcining to form a first catalyst coating layer, and

ii) applying a slurry comprising a second platinum group metal component in supported form onto the first catalyst coating layer and optionally drying, and calcining to form a second catalyst coating layer,

Preferably, in the immediately above embodiments, the cavity-forming agent is used in an amount of 0.2%to 4.0%by weight, more preferably 0.25%to 2.5%by weight, based on the loading of the catalyst coating.

Particularly, the cavity-forming agent is used in an amount of 0.2%to 15.0%by weight, preferably 0.3%to 10.0%by weight, more preferably 0.4%to 3.5%by weight based on the loading of the first catalyst coating layer.

By such a process, a catalyst coating which comprises two catalyst coating layers and the cavities as specified hereinabove may be provided on the substrate. A catalyst coating comprising more catalyst coating layers and the cavities as specified hereinabove may also be provided on the substrate by applying a further slurry comprising a platinum group metal component in supported form.

The slurry may be prepared and applied in conventional ways. Generally, a slurry for a washcoat may be prepared by suspending finely divided particles of a catalyst (e.g., the PGM component in supported form) in an appropriate vehicle, e.g., water, to which a promoter, a binder, a stabilizer, a viscosity modifier and/or a surfactant may be added. The slurry may be comminuted/milled to result in substantially all of the solids having average particle sizes of higher than 10 microns, e.g., in the range of from 15 to 50 microns. The comminution/milling may be accomplished in a ball mill, a continuous Eiger mill, or any other similar equipment. The slurry generally has a pH of 2 to less than 9, and may be adjusted, if necessary, by adding an inorganic or an organic acid and/or base. The solids content of the slurry may be, e.g., 15 to 60 %by weight. The cavity-forming agent, when used, may be incorporated into the slurry at any timing during the preparation of the slurry, for example before the comminution/milling.

The obtained slurry may be applied on a substrate by dipping the substrate into the slurry, or otherwise coating the slurry onto the substrate, such that a desired loading of a coating layer will be deposited on the substrate. Thereafter, the coated substrate may be dried at a temperature in the range of from 100 to 300 ℃ and/or calcined by heating at a temperature in the range of from 350 to 650 ℃ for a period of time, for example 1 to 3 hours. Drying and calcination are typically done in air.

The applying, drying, and calcination processes may be repeated if necessary to achieve the final desired gravimetric amount of the catalyst washcoat layer on the support. The catalyst washcoat loading may be determined through calculation of the difference in the weights of the substrate after and before applying the washcoat.

The catalytic article according to the present invention may be used to treat exhaust streams from combustion engines of automobiles, especially gasoline engines. The catalytic article according to the present invention may particularly be effective to treat exhaust streams from saddle-riding type vehicle engines. Particularly, the catalytic article according to the present invention is a TWC catalytic article.

Accordingly, in the third aspect of the present invention, an exhaust treatment system is provided, which comprises the catalytic article as described herein located downstream of a stoichiometric engine, particularly a gasoline engine. In some embodiments, the exhaust treatment system is particularly useful for a saddle-riding type vehicle engine.

In the fourth aspect of the present invention, a method for treating an exhaust stream, particularly from a stoichiometric engine is provided, which includes contacting the exhaust stream with the catalytic article or the exhaust treatment system as described herein. Particularly, the present invention provides a method for treating an exhaust stream from a gasoline engine, preferably a saddle-riding type vehicle engine.

EMBODIMENTS

Various embodiments are listed below. It will be understood that the embodiments listed below may be combined with all aspects and other embodiments in accordance with the scope of the invention.

1. A catalytic article for treating an exhaust stream, especially a TWC catalytic article, comprising

2. The catalytic article according to Embodiment 1, wherein the cavities account for 0.5%to 35%by volume, based on the volume of the coating portion located at the corners.

3. The catalytic article according to Embodiment 2, wherein the cavities account for 2.0 to 30%by volume, based on the volume of the coating portion located at the corners.

4. The catalytic article according to any of preceding Embodiments, wherein the substrate is a flow-through substrate having channels of a cross-sectional shape with two or more corners, such as trapezoidal, rectangular, square, sinusoidal and hexagonal.

5. The catalytic article according to any of preceding Embodiments, wherein the substrate is a flow-through metallic substrate.

6. The catalytic article according to any of preceding Embodiments, wherein the PGM component is a Pt component, a Pd component, a Rh component or any combinations thereof.

7. The catalytic article according to any of preceding Embodiments, wherein the catalyst coating comprises or consists of two catalyst coating layers each comprising a platinum group metal component in supported from.

8. The catalytic article according to any of preceding Embodiments, wherein the catalyst coating comprises or consists of a top catalyst coating layer comprising a first platinum group metal component in supported form and a bottom coating layer comprising a second platinum group metal component in supported form.

9. The catalytic article according to any of preceding Embodiments, wherein the catalyst coating comprises or consists of two catalyst coating layers each comprising a PGM component in supported form, in which one catalyst coating layer comprises a Rh component in supported form and optionally either or both of a Pt component in supported form and a Pd component in supported form as the PGM component, and the other catalyst coating layer comprises a Pt component in supported form and optionally a Pd component in supported from as the PGM component.

10. The catalytic article according to any of preceding Embodiments, wherein the catalyst coating comprises or consists of a top catalyst coating layer comprising a Rh component in supported form and optionally either or both of a Pt component in supported form and a Pd component in supported form as the PGM component, and a bottom catalyst coating layer comprising a Pt component in supported form and optionally a Pd component in supported from as the PGM component.

11. The catalytic article according to Embodiment 10, wherein the top catalyst coating layer comprises a Rh component in supported form, a Pt component in supported form and a Pd component in supported form as the PGM component, and the bottom catalyst coating layer comprises a Pt component in supported form and a Pd component in supported form as the PGM component.

12. The catalytic article according to any of preceding Embodiments, which exhibits an improvement of NOx abatement of at least 10%higher than the NO_x abatement exhibited by a catalytic article having same catalyst coating composition but no cavities at coating portions located at the corners, as measured with catalytic articles upon aging in accordance with GB14622-2016, Type I.

13. A process for preparing a catalytic article according to any of preceding Embodiments, which comprises

- applying a solution or suspension of a cavity-forming agent on surfaces of the walls of a substrate and drying to provide a dried layer of the cavity-forming agent, and

14. The process according to Embodiment 13, wherein the cavity-forming agent is used in an amount of 0.2%to 4.0%by weight, based on the loading of the catalyst coating.

15. The process according to Embodiment 14, wherein the cavity-forming agent is used in an amount of 0.25%to 2.5%by weight, based on the loading of the catalyst coating.

16. The process according to any of Embodiments 13 to 15, wherein the cavity-forming agent is selected from organic materials such as natural and synthetic polymers, organic solid small molecule compounds, inorganic materials such as inorganic salts and carbon materials, cellulose-containing natural materials, and any combinations thereof.

17. The process according to Embodiment 16, wherein the cavity-forming agent is selected from polyether polyols such as polyethylene glycols and alkyl-capped derivatives thereof, styrenic homopolymers or copolymers such as polystyrenes, poly (meth) acrylic acids and ester derivatives thereof such as polymethyl methacrylate or crosslinked polymethyl methacrylate, celluloses, ether and ester derivatives of celluloses, polyvinyl alcohols, polyvinyl pyrrolidones and any combinations thereof.

18. The process according to Embodiment 17, wherein the cavity-forming agent is polyvinyl alcohols.

19. The process according to any of Embodiments 13 to 18, wherein the providing a catalyst coating is carried out by

ii) applying a slurry comprising a second platinum group metal component in supported form onto the first catalyst coating layer and optionally drying, and calcining to form a second catalyst coating layer.

20. The process according to Embodiment 19, wherein the calcining is carried out to form the first catalyst coating layer.

21. The process according to Embodiment 19 or 20, wherein the cavity-forming agent is used in an amount of 0.2%to 15.0%by weight, based on the loading of the first catalyst coating layer.

22. The process according to Embodiment 21, wherein the cavity-forming agent is used in an amount of 0.3%to 10.0%by weight, based on the loading of the first catalyst coating layer.

23. The process according to Embodiment 22, wherein the cavity-forming agent is used in an amount of 0.4%to 3.5%by weight, based on the loading of the first catalyst coating layer.

24. An exhaust treatment system, which comprises the catalytic article as defined in any of Embodiments 1 to 12 located downstream of a stoichiometric engine.

25. The exhaust treatment system according to Embodiment 24, wherein the stoichiometric engine is a gasoline engine, particularly a saddle-riding type vehicle engine.

26. A method for treating an exhaust stream, particularly from a stoichiometric engine, which includes contacting the exhaust stream with the catalytic article as defined in any of Embodiments 1 to 12 or the exhaust treatment system as defined in Embodiment 24 or 25.

27. The method according to Embodiment 26, wherein the exhaust stream is from a gasoline engine, preferably a saddle-riding type vehicle engine.

EXAMPLES

Aspects of the present invention will be more fully illustrated by the following examples, which are set forth to illustrate certain aspects of the present invention and are not to be construed as limiting thereof.

Preparation of Catalytic Articles

Example 1 (Sample S1, Comparative)

Bottom Coating Slurry

1.1 grams of 16%aqueous hexahydroxyplatinic acid diethanolamine ( (MEA) ₂Pt (OH) ₆) solution was impregnated onto a mixture of 89 grams of a powder of lanthana-zirconia composite oxide (7.5%La₂O₃) and 83 grams of a powder of ceria-zirconia composite oxide (30%CeO₂) via incipient wetness impregnation. 1.4 grams of 20%Pd nitrate solution was impregnated onto a mixture of 89 grams of a powder of lanthana-zirconia composite oxide (7.5%La₂O₃) and 83 grams of a powder of ceria-zirconia composite oxide (30%CeO₂) via incipient wetness impregnation. Then the obtained powders were added in a solution containing 413 grams of D.I. water, 99 grams of alumina, 19 grams of barium acetate, 1.6 grams of zirconium acetate, 3 grams of lanthanum nitrate, and 21 grams of barium sulfate with continuous stirring, with the pH being adjusted to 4.0 by nitric acid. After that, 2.6 grams of alumina binder was added and then milled to a D₉₀ of 40 microns.

Top Coating Slurry

0.9 grams of 16%aqueous hexahydroxyplatinic acid diethanolamine ( (MEA) ₂Pt (OH) ₆) solution was impregnated onto a mixture of 37 grams of a powder of lanthana-zirconia composite oxide (7.5%La₂O₃) and 52 grams of a powder of ceria-zirconia composite oxide (30%CeO₂) via incipient wetness impregnation. 0.3 grams of 20%Pd nitrate solution was impregnated onto a mixture of 37 grams of a powder of lanthana-zirconia composite oxide (7.5%La₂O₃) and 52 grams of a powder of ceria-zirconia composite oxide (30%CeO₂) via incipient wetness impregnation. Then the obtained powders were added in a solution containing 488 grams of D.I. water, 1.2 grams of 10%Rh nitrate solution, 52 grams of a powder of lanthana-zirconia composite oxide (7.5%La₂O₃) , 6 grams of barium hydroxide, 52 grams of alumina, 4 grams of barium sulfate with continuous stirring, with the pH being adjusted to 4.5 by nitric acid. After that, 10 grams of alumina binder was added and then milled to a D₉₀ of 30 microns.

The bottom coating slurry was coated onto a 300/2 (cpsi/mil) flow-through metallic substrate having a diameter of 40 mm and a length of 90 mm by immersing the substrate into the slurry, the substrate having channels with a cross-sectional shape of sinusoidal. The coated substrate was dried at 150 ℃ for 1 hour and then calcined at 500 ℃ for 2 hours. The bottom coating layer on each substrate was obtained with a washcoat loading of 2.07 g/in³ and the Pt and Pd loadings of the bottom coating layer are 1.25 g/ft³ and 2.00 g/ft³ respectively. Then, the top coating slurry was applied, dried at 150 ℃ for 1 hour and then calcined at 500 ℃ for 2 hours. The top coating layer was obtained with a washcoat loading of 1.43 g/in³ and the Pt, Pd and Rh loadings of the top coating layer are 1.25 g/ft³, 0.50 g/ft³ and 1.00 g/ft³ respectively. The morphology of the catalyst coating as obtained is shown in Figures 1A, 1B and 1C.

Example 2 (Sample S2, Inventive)

The preparation is same as that of Example 1 except that the substrate was precoated with 0.01 g/in³ of PVA (polyvinyl alcohol, CAS No. 9002-89-5, M_w = 89,000 to 98,000, density = 1.28 g/cm³ from Sinopharm Chemical Reagent Co., Ltd) using a 5 wt%aqueous PVA solution and dried at 150 ℃ for 30 min. The morphology of the catalyst coating as obtained is shown in Figures 2A, 2B and 2C.

Example 3 (Sample S3, Inventive)

The preparation is same as that of Example 2 except that the substrate is precoated with 0.03 g/in³ of PVA. The morphology of the catalyst coating as obtained is shown in Figures 3A, 3B and 3C.

Example 4 (Sample S4, Inventive)

The preparation is same as that of Example 2 except that the substrate is precoated with 0.05 g/in³ of PVA. The morphology of the catalyst coating as obtained is shown in Figures 4A, 4B and 4C.

Example 5 (Sample S5 Inventive)

The preparation is same as that of Example 2 except that the substrate is precoated with 0.07 g/in³ of PVA. The morphology of the catalyst coating as obtained is shown in Figures 5A, 5B and 5C.

Determination of Cavity Volume Ratio

The cavity volume ratio, i.e., the percentage of the volume of the cavities based on the volume of the coating portion located at the corners, is determined in accordance with the following procedure, assuming that the cavities extend the same length as the catalyst coating at each corner:

i) taking a microscope image of the cross-section of a catalytic article, including at least 120 corners, and drawing equivalent triangles approximately representing the cross-section of the cavity and equivalent triangles approximately representing the cross-section of the catalyst coating respectively at 120 corners in total, as illustratively shown in Figs. 6A and 6B for example;

ii) calculating the area of the equivalent triangle approximately representing the cross-section of cavity (Aera_cavity) and the area of the equivalent triangle approximately representing the cross-section of the catalyst coating (Aera_coating) ) at each corner, and dividing the Area_cavity by the Area_coating to provide the cavity ratios of the 120 corners; If there is no observable cavity at a corner, the cavity ratio at that corner will be zero (i.e., 0) ;

iv) calculating the average of the cavity ratios of the 120 corners, which is determined as the cavity volume ratio of the catalytic article.

Catalytic Performance Test

Test samples were prepared by accommodating respective catalytic articles as prepared in above Examples, in fresh state or upon aging in accordance with standard bench cycle (SBC) at a temperature of 820 to 940 ℃ and a lambda value of 0.9 to 1.1 for 18 hours, into a housing with an inlet and an outlet for passing the gas stream to be treated.

The test samples were measured for catalytic performance on a 125cc Yamaha motorcycle (ZY125T-15) . The emissions of the tail-pipe total hydrocarbons (THC) , CO and NO_x were measured using the World Motorcycle Test Cycle (WMTC) in accordance with GB14622-2016, Type I. The measurement of emissions includes following two phases in one test cycle:

P1: Cold start phase from 0 to 600 seconds, and

P2: Hot phase from 600 to 1200 seconds.

The exhausts from the two phases have following accumulative compositions under a fuel consumption of 2.17 L/100 km:

P1: 1.960 g/km CO, 0.447 g/km THC, 0.427 g/km NO_x, and

P2: 1.509 g/km CO; 0.353 g/km THC; 0.488 g/km NO_x.

Each sample was tested for three times to provide an average as the test result. Test results of emission are summarized in Tables 1 to 2 below.

Table 1. Emissions of THC, CO, NO_x of ZY125T-15 with samples S1 to S5

Table 2. Enhancement in THC, CO, NO_x reduction with samples S2 to S5 compared to sample S1.

The inventive samples S2 to S5 exhibit significantly improved abatement of NO_x compared with the comparative sample S1 which does not have cavity in the coating portion located at the corners. Particularly, the inventive samples exhibit an improvement of NO_x abatement of at least 10%higher than the NO_x abatement exhibited by the comparative sample.

Although slightly reduced CO or THC abatement performance was observed in some cases, it is clear that all inventive samples exhibit improved overall catalytic performance over the comparative sample.

Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It will be apparent to those of skill in the art that various modifications and variations can be made to the method and apparatus of the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention include modifications and variations that are within the scope of the appended claims and their equivalents.

Claims

A catalytic article for treating an exhaust stream, especially a TWC catalytic article, comprising

- a substrate comprising a plurality of walls extending longitudinally and a plurality of channels defined by the plurality of walls for the exhaust stream passing through, wherein the channels have corners at junctions of walls,

- a catalyst coating comprising a platinum group metal component in supported form on surfaces of the walls, consisting of a coating portion located at the corners and a coating portion located at remaining area of the walls,

wherein the coating portion located at the corners includes cavities extending along the channels longitudinally.
The catalytic article according to claim 1, wherein the cavities account for 0.5%to 35%by volume, preferably 2.0 to 30%by volume, based on the volume of the coating portion located at the corners.
The catalytic article according to any of preceding claims, wherein the substrate is a flow-through substrate having channels of a cross-sectional shape with two or more corners, such as trapezoidal, rectangular, square, sinusoidal and hexagonal.
The catalytic article according to any of preceding claims, wherein the substrate is a flow-through metallic substrate.
The catalytic article according to any of preceding claims, wherein the PGM component is a Pt component, a Pd component, a Rh component or any combinations thereof.
The catalytic article according to any of preceding claims, wherein the catalyst coating comprises or consists of two catalyst coating layers each comprising a platinum group metal component in supported from.
The catalytic article according to any of preceding claims, wherein the catalyst coating comprises or consists of a top catalyst coating layer comprising a first platinum group metal component in supported form and a bottom coating layer comprising a second platinum group metal component in supported form.
The catalytic article according to any of preceding claims, wherein the catalyst coating comprises or consists of two catalyst coating layers each comprising a PGM component in supported form, in which one catalyst coating layer comprises a Rh component in supported form and optionally either or both of a Pt component in supported form and a Pd component in supported form as the PGM component, and the other catalyst coating layer comprises a Pt component in supported form and optionally a Pd component in supported from as the PGM component.
The catalytic article according to any of preceding claims, wherein the catalyst coating comprises or consists of a top catalyst coating layer comprising a Rh component in supported form and optionally either or both of a Pt component in supported form and a Pd component in supported form as the PGM component, and a bottom catalyst coating layer comprising a Pt component in supported form and optionally a Pd component in supported from as the PGM component.
The catalytic article according to claim 9, wherein the top catalyst coating layer comprises a Rh component in supported form, a Pt component in supported form and a Pd component in supported form as the PGM component, and the bottom catalyst coating layer comprises a Pt component in supported form and a Pd component in supported form as the PGM component.
The catalytic article according to any of preceding claims, which exhibits an improvement of NOx abatement of at least 10%higher than the NO_x abatement exhibited by a catalytic article having same catalyst coating composition but no cavities at coating portions located at the corners, as measured with catalytic articles upon aging in accordance with GB14622-2016, Type I.
A process for preparing a catalytic article according to any of preceding claims, which comprises

- applying a solution or suspension of a cavity-forming agent on surfaces of the walls of a substrate and drying to provide a dried layer of the cavity-forming agent, and

- providing a catalyst coating on the dried layer of the cavity-forming agent by applying a slurry comprising a platinum group metal component in supported form and optionally drying, and calcination,

wherein the cavity-forming agent is used in an amount of 0.05%to 9.0%by weight, based on the loading of the catalyst coating.
The process according to claim 12, wherein the cavity-forming agent is used in an amount of 0.2%to 4.0%by weight, more preferably 0.25%to 2.5%by weight, based on the loading of the catalyst coating.
The process according to claim 12 or 13, wherein the cavity-forming agent is selected from organic materials such as natural and synthetic polymers, organic solid small molecule compounds, inorganic materials such as inorganic salts and carbon materials, cellulose-containing natural materials, and any combinations thereof.
The process according to claim 14, wherein the cavity-forming agent is selected from polyether polyols such as polyethylene glycols and alkyl-capped derivatives thereof, styrenic homopolymers or copolymers such as polystyrenes, poly (meth) acrylic acids and ester derivatives thereof such as polymethyl methacrylate or crosslinked polymethyl methacrylate, celluloses, ether and ester derivatives of celluloses, polyvinyl alcohols, polyvinyl pyrrolidones and any combinations thereof, preferably polyvinyl alcohols.
The process according to any of claims 12 to 15, wherein the providing a catalyst coating is carried out by

i) applying a slurry comprising a first platinum group metal component in supported form onto the dried layer of the cavity-forming agent, drying and optionally calcining to form a first catalyst coating layer, and

ii) applying a slurry comprising a second platinum group metal component in supported form onto the first catalyst coating layer and optionally drying, and calcining to form a second catalyst coating layer.
The process according to claim 16, wherein the calcining is carried out to form the first catalyst coating layer.
The process according to claim 16 or 17, wherein the cavity-forming agent is used in an amount of 0.2%to 15.0%by weight, preferably 0.3%to 10.0%by weight, more preferably 0.4%to 3.5%by weight based on the loading of the first catalyst coating layer.
An exhaust treatment system, which comprises the catalytic article as defined in any of claims 1 to 11 located downstream of a stoichiometric engine.
The exhaust treatment system according to claim 19, wherein the stoichiometric engine is a gasoline engine, particularly a saddle-riding type vehicle engine.
A method for treating an exhaust stream, particularly from a stoichiometric engine, which includes contacting the exhaust stream with the catalytic article as defined in any of claims 1 to 11 or the exhaust treatment system as defined in claim 19 or 20.
The method according to claim 21, wherein the exhaust stream is from a gasoline engine, preferably a saddle-riding type vehicle engine.