JP7656694B2 - Catalytic articles for filtering particulate matter and uses thereof - Patents.com - Google Patents

Description

The present disclosure relates to a catalytic article for treating exhaust gas emissions from a gasoline engine.

Spark ignition engines use a spark to ignite a mixture of hydrocarbons and air. In contrast, compression ignition engines ignite the hydrocarbons by injecting them into compressed air. Spark ignition engines can be fueled by gasoline fuel, gasoline fuel blended with oxygenates including methanol and/or ethanol, liquid petroleum gas, or compressed natural gas. Spark ignition engines can be stoichiometrically operated or lean burn operated.

Three-way catalysts (TWCs) typically contain one or more platinum group metals, particularly those selected from the group consisting of platinum, palladium, and rhodium. TWCs are intended to simultaneously catalyze the following three reactions: (i) the oxidation of carbon monoxide to carbon dioxide; (ii) the oxidation of unburned hydrocarbons to carbon dioxide and water; and (iii) the reduction of nitrogen oxides to nitrogen and oxygen.

These three reactions occur most efficiently when the TWC receives exhaust gas from an engine operating at or near the stoichiometric point. As is well known in the art, the amount of carbon monoxide (CO), unburned hydrocarbons (HC), and nitrogen oxides ( _NOx ) emitted when gasoline fuel is burned in a spark ignition (e.g., spark ignition) internal combustion engine is primarily affected by the air-fuel ratio in the combustion can. An exhaust gas having a stoichiometrically balanced composition is one in which the concentrations of oxidizing gases ( _NOx and _O2 ) and reducing gases (HC and CO) are substantially matched. The air-fuel ratio that produces this stoichiometrically balanced exhaust gas composition is typically given as 14.7:1.

Theoretically, in a stoichiometrically balanced exhaust gas composition, _O2 , _NOx , CO and HC should be completely converted to _CO2 , _H2O and _N2 (and residual _O2 ), which is the role of the TWC. Ideally, therefore, the engine should be operated in such a way that the air/fuel ratio of the combustion mixture produces a stoichiometrically balanced exhaust gas composition.

A way to define the compositional balance between oxidizing and reducing gases in the exhaust gas is the lambda (λ) value of the exhaust gas, which can be defined as the actual engine air-fuel ratio divided by the stoichiometric engine air-fuel ratio. A lambda (λ) value of 1 represents a stoichiometrically balanced (or stoichiometric) exhaust gas composition, a lambda value >1 represents an excess of _O2 and _NOx and the composition is described as "lean", and a lambda value <1 represents an excess of HC and CO and the composition is described as "rich". It is also common in the art to refer to the air-fuel ratio at which the engine operates as "stoichiometric", "lean" or "rich" depending on the exhaust gas composition that produces the air-fuel ratio: thus, the air-fuel ratio in a stoichiometrically operating or lean-burn gasoline engine.

It should be appreciated that the reduction of _NOx to _N2 using a TWC is less efficient when the exhaust gas composition is lean of stoichiometry. Similarly, the TWC is less capable of oxidizing CO and HC when the exhaust gas composition is rich. Thus, the challenge is to keep the composition of the exhaust gas entering the TWC as close to stoichiometric as possible.

Naturally, when the engine is at steady state, it is relatively easy to ensure that the air-fuel ratio is stoichiometric. However, when the engine is used to propel a vehicle, the amount of fuel required varies temporarily depending on the load demand placed on the engine by the driver. This makes it particularly difficult to control the air-fuel ratio so that stoichiometric exhaust gases are produced for the three-way conversion. In practice, the air-fuel ratio is controlled by an engine control unit that receives information about the exhaust gas composition from an exhaust gas oxygen (EGO) (or lambda) sensor, a so-called closed-loop feedback system. A characteristic of such a system is that the air-fuel ratio oscillates (or perturbs) between a slightly rich stoichiometric (or control set) point and slightly lean, due to the time lag associated with adjusting the air-fuel ratio. This perturbation is characterized by the amplitude and response frequency (Hz) of the air-fuel ratio.

The active components in a typical TWC include one or both of platinum and palladium in combination with rhodium supported on a high surface area oxide, or even palladium alone (no rhodium), and an oxygen storage component.

When the exhaust gas composition is slightly richer than the set point, a small amount of oxygen is needed to consume the unreacted CO and HC, i.e., to make the reaction more stoichiometric. Conversely, when the exhaust gas becomes slightly lean, excess oxygen needs to be consumed. This has been achieved by the development of oxygen storage components (OSCs) that release or absorb oxygen during perturbations. The most commonly used oxygen storage components in modern TWCs are cerium oxide (CeO ₂ ) or mixed oxides or composite oxides containing cerium, such as CeZr mixed oxide or CeZr mixed oxide composite oxide.

In addition to the emissions of carbon monoxide (CO), unburned hydrocarbons (HC), and nitrogen oxides ( _NOx ) that are emitted when spark-ignited combustion of gasoline fuel (all of which can be treated with TWC catalysts), particulate matter (PM) emissions must be considered.

Environmental PM is classified by most authors into the following categories based on their aerodynamic diameter (defined as the diameter of a 1 g/ ^cm3 density sphere with the same settling velocity in the air as the particle being measured): (i) particles with an aerodynamic diameter of less than 10 μm (PM-10); (ii) fine particles with a diameter of less than 2.5 μm (PM-2.5); (iii) ultrafine particles with a diameter of less than 0.1 μm (or 100 nm); (iv) nanoparticles, characterized by a diameter of less than 50 nm.

Since the mid-1990s, the particle size distribution of particulate matter emitted from internal combustion engines has received increasing attention due to the possible adverse health effects of fine and ultrafine particles. The concentration of PM-10 particulate matter in ambient air is regulated by US law. A new additional ambient air quality standard of PM-2.5 was introduced in the US in 1997 as a result of health studies that showed a strong correlation between human mortality and concentrations of fine particles smaller than 2.5 μm.

Attention is now shifting to nanoparticles, as they are understood to penetrate deeper into the human lungs than larger sized particulate matter and, as a result, are considered to be more harmful than larger particles, as inferred from research results on particulate matter in the 2.5-10.0 μm range.

The size distribution of diesel particulate matter has a well-established bimodal character that corresponds to particle nucleation and agglomeration mechanisms. Diesel PM is composed of many small particles that retain very little mass. Nearly all diesel particulate matter is significantly below 1 μm in size; it includes a mixture of fine particles (i.e., those falling under the 1997 US law), ultrafine particles, and nanoparticles.

It is understood that diesel particulate filters such as ceramic wall-flow monoliths can function with a combination of depth and surface filtration, with a filter cake occurring at a higher soot loading when the depth filtration capacity is saturated and the particulate layer begins to cover the filter surface. Deep filtration is characterized by a slightly lower filtration efficiency and less pressure drop than cake filtration.

PM produced by spark ignition engines has a very high percentage of ultrafine, with very little accumulation and coarse mode, compared to that produced by diesel (compression ignition) engines, which presents a challenge to removing it from the spark ignition engine exhaust to prevent emission to the atmosphere. In particular, because the majority of PM from spark ignition engines is relatively small compared to the size distribution of diesel PM, it is practically impossible to use filter substrates that promote surface-type cake filtration of spark ignition PM, because the relatively small average pore size of the required filter substrate would create impractically high backpressure in the system.

Furthermore, conventional wall-flow filters designed to trap diesel PM generally cannot be used to facilitate surface-type filtration of PM from spark-ignition engines to meet relevant emission standards because there is generally less PM in spark-ignition exhaust gas (and therefore less chance of soot cake formation), and spark-ignition exhaust gas temperatures are generally higher, which can result in faster removal of PM by oxidation, thus preventing increased PM removal by cake filtration. Deep filtration of spark-ignition PM in conventional diesel wall-flow filters is also difficult because the PM is significantly smaller than the pore size of the filter media. Thus, under normal operation, uncoated conventional diesel wall-flow filters have lower filtration efficiency when used with spark-ignition engines than with compression-ignition engines.

European emission regulations (Euro 6) from 1 September 2014 require the control of the number of particles emitted from both diesel and gasoline (spark ignition) passenger cars. For gasoline EU light duty cars, the permissible limits are: 1000 mg/km carbon monoxide; 60 mg/km nitrogen oxides (NO _x ); 100 mg/km total hydrocarbons (of which ≦68 mg/km non-methane hydrocarbons); and 4.5 mg/km particulate matter (for direct injection engines only). Euro 6 sets a standard limit of 6.0×10 ¹¹ per km for PM counts, although original equipment manufacturers may request a limit of 6×10 ¹² km ^-1 until 2017. In practical terms, the legislated particulate matter range is between 23 nm and 3 μm.

In the United States, on March 22, 2012, the California Air Resources Board (CARB) adopted new emissions standards for "LEV III" passenger cars, light trucks, and medium-duty vehicles, including a 3 mg/mile emission limit for model years 2017 and after, with the possibility of later implementation of a 1 mg/mile limit, provided that various interim reviews deem it feasible.

The new Euro 6 emissions standards present some difficult design problems for meeting gasoline emissions standards, particularly how to design a filter, or an exhaust system including a filter, to reduce PM gasoline (spark ignition) emissions and at the same time meet emission standards for non-PM pollutants such as one or more of oxides of nitrogen ( _NOx ), carbon monoxide (CO), and unburned hydrocarbons (HC), all at an acceptable backpressure (e.g., as measured at the maximum on-cycle backpressure of an EU drive cycle).

There have been some recent efforts to combine TWCs with filters to meet Euro 6 emissions standards.

WO2014125296 discloses a spark ignition engine including an exhaust system for a spark ignition internal combustion engine of a vehicle. The exhaust system includes a filter for filtering particulate matter from exhaust gas emitted from the spark ignition internal combustion engine of the vehicle coated with a three-way catalyst washcoat. The washcoat includes a platinum group metal and a plurality of solid particles, the plurality of solid particles including at least one base metal oxide and at least one oxygen storage component that is a mixed oxide or complex oxide including cerium. The mixed oxide or complex oxide including cerium and/or at least one base metal oxide has a median particle size (D50) of less than 1 μm.

US Patent Application Publication No. 20090193796 discloses an emission treatment system downstream of a gasoline direct injection engine for treating exhaust gases including hydrocarbons, carbon monoxide, nitrogen oxides, and particulate matter, the emission treatment system including a catalyzed particulate trap including a TWC catalyst coated on or in the particulate trap. In the description and examples provided, the catalyst coating (also referred to as layered or layered catalyst composite) is prepared from a slurry mixture of a solution of a desired precious metal compound and at least one support material (such as a finely divided high surface area, refractory metal oxide). The slurry mixture is ground, for example in a ball mill or other similar device, resulting in substantially all solids having a particle size less than about 20 μm, i.e., a mean diameter [known as "D50"] between about 0.1 and 15 μm. In the examples, the grinding by milling of alumina was performed such that 90% of the particles [known as "D90"] had a particle size between 8 and 10 μm. Comminution of the ceria-zirconia composite was achieved by milling to a D90 particle size of <5 μm.

In GB 2514875, the inventors investigated the use of washcoat compositions comprising milled Ce/Zr mixed oxide for use in three-way catalysts for coating filters such as those disclosed in US 20090193796 for low backpressure applications, where it was found that milling the Ce/Zr mixed oxide reduced backpressure with a reduction in the D50 of the cerium/zirconium mixed oxide, but at the same time significantly reduced the three-way catalyst activity, especially for CO and _NOx emissions.

It is therefore an object to provide an improved TWC catalyzed filter that overcomes the shortcomings of the prior art, or at least provides a commercially viable alternative thereto. More specifically, it is an object to provide a catalytic article that allows for the treatment of exhaust gases to address the emission of fine sub-23 nm particulate matter.

The present inventors have found that the problem of reduced three-way catalyst activity observed in GB 2514875 can be addressed. They have also found that by carefully selecting the particle size of the cerium/zirconium mixed oxide component and the inorganic oxide material, such as alumina, the backpressure can be reduced to a desirable level.

The benefits of this discovery are evident in addressing the health risks associated with ultrafine soot particles mentioned above, and have been fully referenced and discussed recently in the International Journal of Environmental Research and Public Health (Int. J. Environ. Res. Public Health, 2018, Vol. 15, p. 304). Indeed, the global acceptance of ultrafine particles and their impact on cardiopulmonary and central nervous system health issues is driving increased legislation and restrictions worldwide for automotive applications. Beyond today's emission limits, legislators are looking to further improve air quality, and one area to consider is particulate matter, currently measured as PN or particulate matter number in the Euro 6d standard.

Currently, the PN measurement requirement is for particles 23 nm and larger. One obvious aspect to consider tightening the legislation as measurement accuracy improves is to consider particle sizes below 23 nm. The described invention therefore helps to solve the technical challenges of the expected tightening of particulate matter emission laws beyond Euro 6d and provide the desired health benefit of reducing ultrafine particles in the air.

One aspect of the present disclosure relates to a catalyst article for treating exhaust gas from a spark ignition internal combustion engine, the catalyst article comprising a substrate that is a wall-flow filter having an inlet end, an outlet end, and an axial length L therebetween, a plurality of inlet channels extending from the inlet end, and a plurality of outlet channels extending from the outlet end, the plurality of inlet channels including a first catalyst composition extending from the inlet end or the outlet end over at least 50% of L, the plurality of outlet channels including a second catalyst composition extending from the outlet end or the inlet end over at least 50% of L, the first and second catalyst compositions overlap up to 80% of L, and the first and second catalyst compositions each independently include an oxygen storage component (OSC) derived from a CeZr mixed oxide sol having a D90 of less than 1.3 microns and a particulate inorganic oxide having a D90 of 1 to 20 microns.

Another aspect of the present disclosure relates to an exhaust gas treatment system including a catalytic article as described herein in fluid communication with an exhaust manifold of a spark ignition engine.

Another aspect of the present disclosure relates to a method for treating exhaust gas emitted from a spark ignition internal combustion engine, the method comprising contacting the exhaust gas with a catalyst article described herein.

Another aspect of the present disclosure relates to the use of the catalyst article of the present invention in an exhaust gas treatment system to reduce emissions of particles smaller than 23 nm.

FIG. 1 is a schematic diagram of one embodiment of a wall-flow filter as described herein. FIG. 2 is a schematic diagram of another embodiment of a wall-flow filter as described herein. 1 shows simulated WLTC engine bench data for particulate emissions, specifically 10-23 nm emissions, and in particular 10-23 nm particulate counts, for an exemplary catalyst article (#/km).

One aspect of the present disclosure relates to a catalyst article for treating exhaust gas from a spark ignition internal combustion engine, the catalyst article comprising a substrate that is a wall-flow filter having an inlet end, an outlet end, and an axial length L therebetween, a plurality of inlet channels extending from the inlet end, and a plurality of outlet channels extending from the outlet end, the plurality of inlet channels including a first catalyst composition extending from the inlet end or the outlet end over at least 50% of L, the plurality of outlet channels including a second catalyst composition extending from the outlet end or the inlet end over at least 50% of L, the first and second catalyst compositions overlap up to 80% of L, and the first and second catalyst compositions each independently include an oxygen storage component (OSC) derived from a CeZr mixed oxide sol having a D90 of less than 1.3 microns and a particulate inorganic oxide having a D90 of 1 to 20 microns.

In the following passages, different aspects of the invention are defined in more detail. Each aspect so defined can be combined with any other aspect or aspects, unless expressly indicated otherwise. In particular, any feature indicated as being preferred or advantageous can be combined with any other feature or features indicated as being preferred or advantageous.

The catalyst article comprises a substrate that is a wall-flow filter having an inlet end, an outlet end, and an axial length L therebetween, a plurality of inlet channels extending from the inlet end, and a plurality of outlet channels extending from the outlet end. Wall-flow filters are well known in the art and typically include a ceramic porous filter structure having a plurality of inlet channels and a plurality of outlet channels, each of which is defined in part by a ceramic wall of the porous structure, and each of which is separated from an outlet channel by a ceramic wall of the porous structure.

Typical lengths are 2-12 inches (5.1-30.5 cm) long, preferably 3-6 inches (7.6-15.2 cm) long. The cross section is preferably circular, and may typically have a filter diameter of 4.66 and 5.66 inches (11.8 cm and 14.4 cm). However, the cross section may also be dictated by the space on the vehicle where the filter needs to be secured.

The substrate may be a ceramic, such as silicon carbide, cordierite, aluminum nitride, silicon nitride, aluminum titanate, alumina, mullite, such as acicular mullite (see, for example, WO 01/16050), pollucite, cermets such as Al ₂ O ₃ /Fe, or a composite material comprising any two or more segments thereof. In embodiments in which the catalytic article of the present invention comprises a ceramic substrate, the ceramic substrate may be made of any suitable refractory material, such as alumina, silica, titania, ceria, zirconia, magnesia, zeolites, silicon nitride, silicon carbide, zirconium silicate, magnesium silicate, aluminosilicates and metalloaluminosilicates (such as cordierite and spodumene), or mixtures or mixed oxides of any two or more thereof. Cordierite, magnesium aluminosilicate, and silicon carbide are particularly preferred.

Substrates suitable for use in the present invention typically have a porosity of >40% or >50%, with porosities of 45-75%, such as 50-65% or 55-60%, being advantageously usable. Porosity is a measure of the percentage of void space within the substrate and is related to backpressure within the exhaust system. In general, the lower the porosity, the higher the backpressure.

Substrates suitable for use in the present invention typically have an average pore size of surface pores of 8 to 45 μm, for example 8 to 25 μm, 10 to 20 μm, or 10 to 15 μm. Pore size is well known to those skilled in the art, and suitable measurement techniques are known to those skilled in the art.

Each of the first and second catalyst compositions independently includes an oxygen storage component (OSC) derived from a CeZr mixed oxide sol having a D90 of less than 1.3 microns and a particulate inorganic oxide having a D90 of 1 to 20 microns.

Particle size distributions can be characterized by measurements of D10, D50, and D90. In each case, the numbers indicate the percentage amount of particles smaller than the stated value. In other words, a D90 of 100 microns means that 90% of the particles are smaller than 100 microns in diameter. Knowing the D10 and D90 allows one to define the range of particles in a particle distribution. Characterizing the particle size of a sample with D10 and D90 values generally defines the width of the particle size distribution. The closer these values are, the narrower the particle size distribution.

The particle size measurements required to obtain the D10, D50 and D90 values of the CeZr mixed oxide sol and/or particulate inorganic oxide can be obtained by laser diffraction particle size analysis using a Malvern Mastersizer 2000, which is a volume-based technique (i.e., D50 and D90 can also be referred to as D _V 50 and D _V 90 (or D(v,0.50) and D(v,0.90))) and applies the mathematical Mie theory model to determine the particle size distribution. The laser diffraction system works by determining the diameter of the particle based on a spherical approximation. For particle size measurements by laser diffraction particle size analysis, diluted samples were prepared by sonication in surfactant-free distilled water at 35 watts for 30 seconds.

In some embodiments, the D90 of the CeZr mixed oxide sol can be less than 1.2 microns, or less than 1.1 microns, or less than 1.0 microns, or less than 900 nm, or less than 800 nm, or less than 700 nm, or less than 600 nm, or less than 500 nm, or less than 400 nm, or less than 300 nm.

In some other embodiments, the D90 of the CeZr mixed oxide sol can be 1.2 microns to 1.3 microns, or 1.1 microns to 1.2 microns, or 1.0 microns to 1.1 microns, or 900 nm to 1.0 microns, or 800 nm to 900 nm, or 700 nm to 800 nm, or 600 nm to 700 nm, or 500 nm to 600 nm, or 400 nm to 500 nm, or 300 nm to 400 nm, or 200 nm to 300 nm, or 100 nm to 200 nm.

The CeZr mixed oxide sol may have a D50 of 100 nm to 700 nm, preferably 200 nm to 600 nm, more preferably 300 nm to 400 nm.

Particle size can also be characterized by obtaining the Z-average particle size of a sample. The Z-average is the intensity-weighted average hydrodynamic size of an ensemble collection of particles measured by dynamic light scattering (DLS). The Z-average is derived from a cumulant analysis of the measured correlation curve, where a single particle size is assumed and a single exponential fit is applied to the autocorrelation function. The particle size measurements required to obtain the Z-average particle size of CeZr mixed oxide sols and/or particulate inorganic oxides can be obtained by dynamic light scattering particle size analysis using a Malvern Zetasizer Nano. All testing is performed in dilute aqueous media and the harmonic mean hydrodynamic diameter of a sphere of equivalent diffusivity is determined by cumulant analysis of the time dependence of light scattered by randomly moving particles according to the Stokes-Einstein equation.

The Z-average particle size of the CeZr mixed oxide sol is preferably 150 nm to 350 nm, more preferably 230 to 310 nm.

The CeZr mixed oxide sol comprises CeZr mixed oxide (CeZr mixed oxide particles or CeZr mixed oxide nanoparticles) dispersed in an aqueous medium. The CeZr mixed oxide comprises ceria and zirconia. Although nominally described as "ceria" or "cerium dioxide" and "zirconia" or "zirconium oxide", it will be understood by those skilled in the art that the actual oxide anions present may comprise oxide anions or hydroxide anions, or mixtures thereof, such as hydrated oxide phases (e.g., oxyhydroxides).

Preferably, the concentration of CeZr mixed oxide in the CeZr mixed oxide sol ranges from 10 to about 50 weight percent ("wt %), more preferably from 15 to about 45 wt %, and most preferably from 20 to about 40 wt %, based on the weight of the CeZr mixed oxide sol.

The CeZr mixed oxide present in the CeZr mixed oxide sol may preferably contain ceria in an amount of about 5 to about 60 wt %, more preferably about 10 to about 50 wt %, and most preferably about 30 to about 40 wt %, based on the total weight of the CeZr mixed oxide.

The CeZr mixed oxide present in the sol may preferably contain zirconia in an amount of about 10 to about 75 wt %, more preferably about 30 to about 70 wt %, and most preferably about 50 to about 65 wt %, based on the total weight of the CeZr mixed oxide.

In one embodiment, the CeZr mixed oxide sol has a weight ratio of _ZrO2 to CeO2 of about 2: ₁ .

The CeZr mixed oxide present in the sol may contain rare earth oxides other than ceria. The rare earth oxides in the CeZr mixed oxide are selected from the group consisting of oxides of dysprosium (Dy), erbium (Er), europium (Eu), gadolinium (Gd), holmium (Ho), lanthanum (La), lutetium (Lu), neodymium (Nd), praseodymium (Pr), samarium (Sm), scandium (Sc), terbium (Tb), thulium (Tm), ytterbium (Yb), yttrium (Y), and mixtures thereof. The CeZr mixed oxide preferably contains an oxide of La, Nd, Pr, or Y, or a mixture thereof, more preferably an oxide of La or Nd, or a mixture thereof.

The amount of rare earth oxide in the CeZr mixed oxide in the CeZr mixed oxide sol may preferably be about 1 to about 25 wt %, more preferably about 2 to about 20 wt %, and most preferably about 5 to about 15 wt %, based on the total weight of the CeZr mixed oxide.

The CeZr mixed oxide present in the CeZr mixed oxide sol may include a transition metal oxide. The transition metal present in the CeZr mixed oxide is preferably selected from the group consisting of titanium (Ti), hafnium (Hf), vanadium (V), chromium (Cr), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), and mixtures thereof. More preferably, the transition metal is selected from the group consisting of Ti, Hf, and mixtures thereof. Most preferably, the transition metal is Hf.

CeZr mixed oxide sols suitable for the present invention can be prepared by known techniques, for example, WO 2020161459, U.S. Pat. No. 10,570,063, U.S. Pat. No. 9,820,917, U.S. Pat. No. 9,820,917, CN1371867A, H. He, Integrated Ferroelectrics, 161:1, 36-44, J. J. Gulicovski, S. K. Milonjic, and K. See Meszaros Szecsenyi, "Synthesis and Characterization of Stable Aqueous Ceria Sols, Materials and Manufacturing Processes," Vol. 24: 10-11, 2009, pp. 1080-1085.

The particulate inorganic oxide used in the present invention is preferably an oxide of an element of Groups 2, 3, 4, 5, 13, and 14. The particulate inorganic oxide is preferably selected from the group consisting of alumina, magnesia, lanthana, silica, neodymium, praseodymium, yttrium oxide, titania, niobia, tantalum oxide, molybdenum oxide, tungsten oxide, and mixed oxides or composite oxides thereof. More preferably, the particulate inorganic oxide is selected from the group consisting of alumina, magnesia, silica, lanthanum, neodymium, praseodymium, yttrium oxide, and mixed oxides or composite oxides thereof. Particularly preferably, the particulate inorganic oxide is alumina, lanthanum/alumina composite oxide, or magnesia/alumina composite oxide. One particularly preferred particulate inorganic oxide is lanthanum/alumina composite oxide.

Preferably, the particulate inorganic oxide has a surface area of at least 80 m ² /g, more preferably at least 150 m ² /g, and most preferably at least 200 m ² /g.

Generally, the particulate inorganic oxide has a D90 of 1 to 20 microns. Preferably, the particulate inorganic oxide has a D90 of 5 to 20 microns. More preferably, the particulate inorganic oxide has a D90 of 5 to 15 microns.

The first and second catalyst compositions are preferably both TWC compositions. TWC compositions are well known in the art and the specific components can be readily selected by one of ordinary skill in the art. TWCs typically include one or more platinum group metals (PGMs) provided on a high surface area support (i.e., an inorganic oxide) along with an oxygen storage component (OSC). The TWC composition is generally provided in a washcoat on the substrate.

Preferably the first and/or second catalyst composition has a weight ratio of OSC (mixed oxide derived from the CeZr mixed oxide sol) to particulate OSC to particulate inorganic oxide of from 10:1 to 1:10, preferably from 3:1 to 1:3. Preferably the loading of OSC in each composition is from 0.5 to 4 g/ ⁱⁿ³ , preferably from 1 to 3 g/ ⁱⁿ³ .

Preferably, the first and/or second catalyst composition further comprises a platinum group metal (PGM) component, preferably selected from the group consisting of platinum, palladium, rhodium, and mixtures thereof.

When a PGM component is present in the first and/or second catalyst composition, the PGM component is preferably supported on a particulate inorganic oxide.

The first and/or second catalyst composition may be from 1 to 100g/ft ³ of the PGM component, preferably from 5 to 80g/ft ³ , more preferably from 10 to 50g/ft ³ of the PGM component.

The first and/or second catalyst composition may further comprise an alkali or alkaline earth metal component. Preferably, the first and/or second catalyst composition comprises barium or strontium. Preferably, the barium or strontium, if present, is present in an amount of 0.1 to 15 wt%, more preferably 3 to 10 wt%, based on the total weight of the first and/or second catalyst composition. Preferably, the barium is present as a _BaCO3 composite material. Such material may be impregnated by any method known in the art, for example, incipient wetness impregnation or spray drying.

The catalyst articles of the present invention may include other components known to those skilled in the art. For example, the first and/or second catalyst compositions may further include at least one binder and/or at least one surfactant. When a binder is present, a dispersible alumina binder is preferred.

Preferably, the first and second catalyst compositions are substantially the same. They may be the same in composition but with different loadings. Preferably, to reduce manufacturing complexity, the first and second catalyst compositions may be the same.

The first and second catalyst compositions may be coated onto a substrate by a washcoating process. A washcoat slurry is prepared containing the CeZr mixed oxide sol, particulate inorganic oxide, and other components such as PGM, if used, and a suitable solvent, preferably water. Additional components such as binders, stabilizers, or promoters may also be incorporated. The slurry preferably contains about 10% to about 70% by weight solid material, more preferably about 20% to about 50% by weight solid material.

Many techniques can be used to coat a substrate with the washcoat slurry.

In one method of washcoating, the substrate may then be dipped into the washcoat slurry one or more times to deposit a desired loading of the composition onto the substrate, or the slurry may be coated onto the substrate. The washcoat slurry may at least partially penetrate the pores of the substrate during coating, depending on the particle size and viscosity of the washcoat slurry.

When coating with techniques such as Johnson Matthey's Precision Coating Process described in WO 1999/47260, which relies on vacuum or pressure drawing of the washcoat onto the substrate, it is possible to apply the coating to the inlet end of the outlet channel and the outlet end of the inlet channel. This technique not only provides the washcoat within the pores of the substrate, but also draws the material out to provide a coating on the wall of the substrate opposite where it was applied.

After the substrate is coated with the first and second compositions, the coated substrate is preferably dried and then calcined by heating at an elevated temperature to form the catalyst article. Preferably, the calcination is at 400-600° C. for about 1-8 hours.

According to the invention, the catalyst article has a plurality of inlet channels including a first catalyst composition extending from the inlet or outlet end over at least 50% of L, and a plurality of outlet channels including a second catalyst composition extending from the outlet or inlet end over at least 50% of L. The first and second catalyst compositions overlap by up to 80% of L. The first and second catalyst compositions preferably overlap by up to 20% of L, more preferably by up to 10% of L. One of the first and second compositions may provide a complete coating of the plurality of inlet or outlet channels.

If the overlap is at most 20%, the first catalyst composition can be provided on the inlet channel that extends a distance of at least 50% and at most 70% (of L) from the inlet end, while the second catalyst composition can be provided on the outlet channel that extends a distance of at least 50% and at most 70% from the outlet end. Since the overlap is at most 20%, if the first catalyst composition is, for example, 70%, the second catalyst composition can extend by 50%.

Preferably, the first and/or second catalyst composition is provided directly on the substrate.

The location of the first and second compositions on the substrate will depend on the coating technique used. The first and second compositions coated on the substrate may be in-wall, on-wall, or a combination of in-wall and on-wall. Preferably, at least a portion of the coating is provided on the wall.

In one embodiment, the inlet and/or outlet channels of the wall-flow filter have a wedge-shaped profile on the walls, with the thickest end of the wedge at the plug end of the channel, and the washcoat also resides in the wall. This arrangement differs from Applicant's WO 2017/056067, which discloses a catalytic wall-flow filter that includes a three-way catalytic washcoat disposed on the walls in a "wedge" shape when the wall-flow filter is viewed in longitudinal cross section. That is, the thickness of the wall coating tapers from the thickest end at the open channel end of the wall-flow filter.

When coating by vacuum infiltration as disclosed in WO 1999/47260, the washcoat composition is first contacted through the porous filter wall from the side of the wall of the wall-flow filter structure, using conditions that "pull" the particles in the washcoat such that some of them end up on the opposite (second) wall of the wall.

Another aspect of the present disclosure is an exhaust gas treatment system including a catalytic article as described herein in fluid communication with an exhaust manifold of a spark ignition engine.

Optionally, the exhaust system may include additional components. For example, in an exhaust system particularly applicable to lean-burn engines, a NOx trap may be placed upstream of the described catalytic article. NOx traps, also known as NOx absorption catalysts (NACs), are known, for example, from US Pat. No. 5,473,887, and are designed to adsorb nitrogen oxides (NOx) from lean (oxygen-rich) exhaust gases (lambda>1) during operation in a lean operating mode, and to desorb NOx when the oxygen concentration in the exhaust gas decreases (stoichiometric or rich operating mode). The desorbed NOx may be reduced to _N2 with a suitable reducing agent, for example gasoline fuel, promoted by catalytic components such as rhodium or ceria in the NAC itself or located downstream of the NAC.

After leaving the catalyst article described herein as desired, the exhaust gas stream can then be conveyed, if desired, through a suitable exhaust pipe to a downstream NOx trap to adsorb any remaining NOx emission pollutants in the exhaust gas stream. From the NOx trap through a further exhaust pipe, an SCR catalyst can be positioned to receive the outlet of the NOx trap and treat further emissions with a selective catalytic reduction catalyst that uses ammonia as a reductant to reduce the nitrogen oxides to form nitrogen and water. From the SCR catalyst, the exhaust pipe can be directed to a tailpipe and out of the system.

Another aspect of the present disclosure is a method for treating exhaust gas emitted from a spark ignition internal combustion engine, the method comprising contacting the exhaust gas with a catalyst article described herein. Preferably, the engine is a lean-burn gasoline engine. Additionally, the present disclosure can include a vehicle, such as a passenger car, that includes an engine described herein.

Another aspect of the present disclosure is the use of the catalyst article described herein in an exhaust gas treatment system to reduce emissions of particles less than 23 nm, and in particular to reduce emissions of particles between 10 and 23 nm. Preferably, the use of the catalyst article reduces the number of emissions of particles less than 23 nm by at least 20 percent, more preferably by at least 30 percent, and even more preferably by at least 40 percent.

Another aspect of the present disclosure is a method for treating exhaust gas emitted from a spark ignition internal combustion engine, comprising contacting the exhaust gas with a catalyst article described herein, the method being for reducing emissions of particles less than 23 nm, and in particular for reducing emissions of particles between 10 and 23 nm.

Another aspect of the present disclosure is the use of the catalyst article described herein in an exhaust gas treatment system to reduce emissions of particles less than 23 nm, and in particular to reduce emissions of particles between 10 and 23 nm.

The term "washcoat" is well known in the art and typically refers to an adherent coating that is applied to a substrate during the manufacture of a catalyst.

As used herein, the acronym "PGM" refers to "platinum group metals." The term "platinum group metals" generally refers to metals selected from the group consisting of Ru, Rh, Pd, Os, Ir, and Pt, preferably metals selected from the group consisting of Ru, Rh, Pd, Ir, and Pt. Generally, the term "PGM" preferably refers to metals selected from the group consisting of Rh, Pt, and Pd.

As used herein, the term "mixed oxide" generally refers to a mixture of oxides in a single phase, as conventionally known in the art. As used herein, the term "complex oxide" generally refers to a composition of oxides having two or more phases, as conventionally known in the art.

The term "loading" as used herein refers to a measurement in g/ ^ft3 on a metal weight basis unless other units are provided.

The invention will now be described with reference to the following non-limiting drawings:

Figure 1 illustrates one embodiment of the present invention. Figure 1 shows a cross-section of a wall-flow filter 1 made from a porous cordierite substrate. Wall-flow filter 1 has a length L extending between an inlet end 5 and an outlet end 10. Wall-flow filter 1 includes a number of inlet channels 15 and a number of outlet channels 20 (only one shown). The multiple inlet channels 15 and outlet channels 20 are plugged at their respective ends with plugs 21.

The inner surface 25 of the inlet channel 15 is coated with a wall-on coating 30 comprising a first catalyst composition. The coating 30 extends at least 50% of L from the inlet end 5 and has a substantially uniform thickness along the length of the channel.

The inner surface 35 of the outlet channel 20 is coated with a wall-on coating 40 comprising a second catalyst composition. The coating 40 extends at least 50% of L from the outlet end 10 and has a substantially uniform thickness along the length of the channel.

The overlap area 45 between coatings 30 and 40 is less than 20% of L.

In use, exhaust gas flow 50 (indicated by arrows) passes through the walls of the wall-flow filter 1 into the inlet channel 15 and out the outlet channel 20.

Figure 2 illustrates another embodiment of the present invention. Figure 2 shows a cross-section of a wall-flow filter 1 made from a porous cordierite substrate. Wall-flow filter 1 has a length L extending between an inlet end 5 and an outlet end 10. Wall-flow filter 1 includes a plurality of inlet channels 15 and a plurality of outlet channels 20 (only one shown). The plurality of inlet channels 15 and outlet channels 20 are plugged at their respective ends with plugs 21.

The inner surface 25 of the inlet channel 15 is coated with a wall-on coating 30 comprising a first catalyst composition. Additionally, the first catalyst composition is present within the wall structure of the inlet channel 15. The coating 30 extends at least 50% of L from the outlet end 10. The coating 30 is wedge-shaped, having a thicker amount at the outlet end 10. The coating 30 is applied according to the method of WO 1999/47260.

The inner surface 35 of the outlet channel 20 is coated with a wall-on coating 40 comprising a second catalyst composition. Additionally, the second catalyst composition is present within the wall structure of the outlet channel 20. The coating 40 extends at least 50% of L from the inlet end 5. The coating 40 is wedge-shaped, with a thicker amount at the inlet end 5. The coating 40 is applied according to the method of WO 1999/47260.

The overlap area 45 between coatings 30 and 40 is less than 20% of L.

In use, exhaust gas flow 50 (indicated by arrows) passes through the walls of the wall-flow filter 1 into the inlet channel 15 and out the outlet channel 20.

The invention will now be described with reference to the following non-limiting examples.

Materials All materials were commercially available and obtained from known sources unless otherwise stated.

Example 1
A three-way catalyst washcoat was prepared with a washcoat loading of 1.6 g/in3 containing 1.2 g/ ⁱⁿ³ CeZr mixed oxide (OSC) derived from a CeZr mixed oxide sol having a weight ratio of ZrO2 to _CeO2 of approximately 2: ₁ and a D90 of < ¹ μm, and 0.4 g/ ⁱⁿ³ La-stabilized alumina component having a D90 of 12 μm. The PGM loading was 22 g/ ^ft3 (Pt:Pd:Rh, 0:20:2).

The finished washcoat slurry was adjusted to an appropriate final washcoat solids content of approximately 24% for coating onto GPF substrates using Johnson Matthey's precision coating process described in WO 1999/47260. The substrate used was a commercially available cordierite GPF substrate with a nominal porosity of 63% and an average pore size of 17.5 μm, a diameter of 4.66 inches, a length of 6 inches, 300 cells/sq. inch, and a channel wall thickness of 8 thousandths of an inch. The coatings were applied from each end of the substrate, with each application covering 58% of the total substrate length to give a fully coated final product with no uncoated areas (16% overlap). The amount of washcoat slurry applied to the inlet and outlet channels was the same. The coated substrate was then dried and calcined by conventional methods known in the art. The product profile is shown in FIG. 2.

Example 2
Example 2 was prepared in the same manner as Example 1, except that the La-stabilized alumina component further had a D90 of 5 μm.

Comparative Example 3
A reference coated GPF was prepared by applying the latest TWC coating developed for GPF substrates. The washcoat loading was prepared at 1.6 g/in3 with 1.2 g/ ⁱⁿ³ OSC component (CeZr mixed oxide particles, not derived from sol), 0.4 g/ ⁱⁿ³ La-stabilized alumina component, and 22 g/ ^ft3 PGM loading (Pt:Pd:Rh, 0:20: ² ). In contrast to examples 1 and 2, here the D90 of both the OSC component and the La-stabilized alumina component is around 7 μm.

Uncoated 220/6 reference GPF
For further comparison with the present invention, an uncoated GPF reference was used. This is a commercially available GPF substrate that can be considered for use in such aftertreatment systems with the Euro 6PN limit of 6x10 ^11. The same catalyst diameter and length, and therefore catalyst volume, were selected as in Examples 1-3, 4.66 inches in diameter and 6 inches in length. However, to achieve the required filtration efficiency, such bare filters require a lower nominal porosity and average pore size. Because this can cause an undesirable increase in back pressure, the cells per square inch and wall thickness are typically thinner than substrates designed to be coated with a washcoat. The uncoated reference GPF had 220 cells per square inch and a wall thickness of 6 thousandths of an inch.

Filtration Performance Evaluation The above samples were evaluated for particulate matter emissions, specifically for emissions between 10-23 nm. Evaluation was performed on a simulated WLTC run on an engine bench equipped with a 2.0L gasoline direct injection engine. Particles were measured using a Combustion DMS500 particle analyzer.

The results are shown in Figure 3 and were an average of three WLTC tests completed for each example. The results show the advantages of Examples 1 and 2 compared to uncoated low porosity 220/6GPF and Comparative Example 3. In particular, Example 1 offers a much improved advantage over currently available alternatives.

Table 1 below shows the advantage of additional backpressure reduction for Examples 1 and 2 versus Comparative Example 3.

Claims (15)

1. A catalytic article for treating exhaust gas from a spark ignition internal combustion engine, comprising:
a substrate that is a wall-flow filter having an inlet end, an outlet end and an axial length L therebetween; a plurality of inlet channels extending from the inlet end; and a plurality of outlet channels extending from the outlet end;
the plurality of inlet channels comprises a first catalyst composition extending from the inlet end or the outlet end across at least 50% of the L, the plurality of outlet channels comprises a second catalyst composition extending from the outlet end or the inlet end across at least 50% of the L, the first and second catalyst compositions overlap up to 80% of the L;
A catalytic article, wherein the first and second catalytic compositions each independently comprise an oxygen storage component (OSC) derived from a CeZr mixed oxide sol having a D90 of less than 1.3 microns, and a particulate inorganic oxide having a D90 of 1 to 20 microns. The catalytic article of claim 1, wherein the CeZr mixed oxide sol has a D90 of less than 1.0 microns. The catalyst article of claim 1, wherein the first and second catalyst compositions overlap by up to 20% of the L. The catalyst article of claim 1, wherein the first and second catalyst compositions overlap by up to 10% of the L. 10. The catalytic article of claim 1, wherein the CeZr mixed oxide sol has a weight ratio of _ZrO2 to _CeO2 of about 2:1. The catalytic article of claim 1, wherein the particulate inorganic oxide is selected from the group consisting of alumina, magnesia, silica, lanthanum, neodymium, praseodymium, yttrium oxide, and mixed or composite oxides thereof. The catalytic article of claim 1 , wherein the first and/or second catalytic composition further comprises a platinum group metal (PGM) component. 8. The catalytic article of claim 7, wherein the platinum group metal (PGM) component is selected from the group consisting of platinum, palladium, rhodium, and mixtures thereof. The catalyst article of claim 1, wherein the first and/or second catalyst composition further comprises an alkali or alkaline earth metal component. The catalytic article of claim 1, wherein the first and second catalytic compositions are the same. 10. The catalyst article of claim 1, wherein the first and/or second catalyst composition has a weight ratio of the OSC to the particulate inorganic oxide of 3:1 . The catalytic article of claim 1, wherein the first and/or second catalytic composition is provided directly on the substrate. An exhaust gas treatment system comprising the catalytic article of claim 1 in fluid communication with an exhaust manifold of a spark ignition engine. A method for treating exhaust gas emitted from a spark ignition internal combustion engine, comprising contacting the exhaust gas with the catalytic article of claim 1. Use of the catalytic article of claim 1 in an exhaust gas treatment system to reduce emissions of particles less than 23 nm.