LAYERED CATALYSTS FOR PURIFYING EXHAUST GAS STREAMS AND METHODS OF MAKING THE SAME

A method of manufacturing a layered catalyst for purifying an exhaust gas stream includes introducing a mixture of colloidal ceria, alumina particles, and a liquid medium into a drying chamber via an atomizer to form atomized droplets of the mixture. A drying gas is introduced into the drying chamber such that the atomized droplets contact the drying gas, the liquid medium is removed from the atomized droplets, and ceria nanoparticles deposit on the alumina particles to form composite catalyst support particles. A catalyst precursor including a rhodium precursor and colloidal ceria is applied to the composite catalyst support particles. The composite catalyst support particles and the catalyst precursor are heated to form the layered catalyst. The layered catalyst includes an alumina substrate, a ceria nanoparticle layer extending substantially continuously over the alumina substrate, and a rhodium catalyst layer including an atomic dispersion of rhodium adsorbed on the ceria nanoparticle layer.

INTRODUCTION

The present disclosure relates to catalysts for purifying exhaust gas streams from combustion processes and to methods of manufacturing layered catalyst structures including mixed metal oxide support materials loaded with platinum group metal (PGM) elements (e.g., platinum, rhodium, palladium, et al.) as catalysts.

Exhaust gases from combustion processes typically contain a variety of combustion reaction by-products, including unburned hydrocarbons (HC), carbon monoxide (CO), nitric oxide (NO), and nitrogen dioxide (NO2), with NO and NO2collectively referred to as nitrogen oxides or NOx. It may be desirable to reduce or control the emission of HC, CO, and/or NOxfrom various combustion processes into the ambient environment.

Exhaust gas treatment systems for internal combustion engines of automotive vehicles may include a so-called three-way catalyst (TWC) disposed in a path of an exhaust gas stream from the engine, which is designed to simultaneously convert HC, CO, and NOxin the exhaust gas stream to CO2, N2, and H2O prior to discharge. Such three-way catalysts oftentimes include one or more platinum group metal elements supported on a thermally and mechanically stable, high surface area porous support material, which may comprise alumina (Al2O3) and ceria (CeO2).

SUMMARY

A method of manufacturing a layered catalyst for purifying an exhaust gas stream comprises multiple steps. In step (a), a mixture of colloidal ceria, alumina particles, and a liquid medium is introduced into a drying chamber via an atomizer to form atomized droplets of the mixture. In step (b), a drying gas is introduced into the drying chamber such that the atomized droplets contact the drying gas, the liquid medium is removed from the atomized droplets by evaporation, and ceria nanoparticles deposit on surfaces of the alumina particles to form composite catalyst support particles. In step (c), a catalyst precursor comprising a rhodium precursor and colloidal ceria is applied to the composite catalyst support particles. In step (d), the composite catalyst support particles and the catalyst precursor are heated to form the layered catalyst. The layered catalyst comprises an alumina substrate, a ceria nanoparticle layer extending substantially continuously over the alumina substrate, and a rhodium catalyst layer comprising an atomic dispersion of rhodium adsorbed on the ceria nanoparticle layer.

The rhodium catalyst layer may constitute, by weight, greater than or equal to about 0.05% to less than or equal to about 0.3% of the layered catalyst.

The composite catalyst support particles may comprise, by weight, greater than or equal to about 1% to less than or equal to about 10% ceria and greater than or equal to about 90% to less than or equal to about 99% alumina.

The layered catalyst may comprise, by weight, greater than or equal to about 5% to less than or equal to about 15% ceria and greater than or equal to about 85% to less than or equal to about 95% alumina.

The colloidal ceria in the mixture of step (a) may comprises ceria nanoparticles having a D50 diameter of greater than or equal to about 5 nanometers to less than or equal to about 20 nanometers. A weight ratio of the alumina particles to the ceria nanoparticles in the mixture of step (a) may be greater than or equal to about 12.5:1 to less than or equal to about 50:1.

The liquid medium may comprise water.

The drying gas may comprise air and may be introduced into the drying chamber at a temperature of greater than or equal to about 100 degrees Celsius to less than or equal to about 250 degrees Celsius.

The alumina particles may be jet milled to achieve a desired particle size distribution. The jet-milled alumina particles may have a D90 particle diameter of greater than or equal to about 5 micrometers to less than or equal to about 9 micrometers.

The composite catalyst support particles may have a D90 particle diameter of greater than or equal to about 6 micrometers to less than or equal to about 10 micrometers.

After formation of the composite catalyst support particles in step (b), the composite catalyst support particles may not be milled, ground, or otherwise treated to reduce the particle size thereof.

In step (c), the composite catalyst support particles may be impregnated with the catalyst precursor using an incipient wetness impregnation technique.

The composite catalyst support particles and the catalyst precursor may be heated in step (d) at a temperature of greater than or equal to about 350 degrees Celsius to less than or equal to about 800 degrees Celsius.

The mixture of step (a) may be substantially free of a binder.

The rhodium precursor may comprise rhodium nitrate, a rhodium amine complex, a rhodium hydrate complex, or a combination thereof. Heating the composite catalyst support particles and the catalyst precursor in step (d) may releases gases or vapors of nitrogen, nitrogen oxides, ammonia, and/or water. The catalyst precursor may have a pH of greater than or equal to about 5 to less than or equal to about 12.

The atomic dispersion of rhodium may comprise rhodium ions and/or rhodium atoms disposed at the location of surface defect sites in the ceria nanoparticle layer.

The colloidal ceria in the catalyst precursor of step (c) may comprise ceria nanoparticles having a D50 diameter of greater than or equal to about 5 nanometers to less than or equal to about 20 nanometers. A weight ratio of the ceria nanoparticles to rhodium in the catalyst precursor may be greater than or equal to about 15:1 to less than or equal to about 100:1.

In a method for removing hydrocarbon, carbon monoxide, and nitrogen oxides from an exhaust gas stream of a gasoline-powered internal combustion engine, the exhaust gas stream may be passed over the layered catalyst.

An exhaust gas treatment device may include the layered catalyst. In such case, the layered catalyst may be deposited on wall surfaces of a monolithic substrate defining a plurality of flow-through passages extending therethrough.

A method of manufacturing a layered catalyst for purifying an exhaust gas stream comprises multiple steps. In step (a), a mixture of colloidal ceria, alumina particles, and a liquid medium are introduced into a drying chamber via an atomizer to form atomized droplets of the mixture. The alumina particles have a D90 particle diameter of greater than or equal to about 5 micrometers to less than or equal to about 9 micrometers. The colloidal ceria comprise ceria nanoparticles having a D50 diameter of greater than or equal to about 5 nanometers to less than or equal to about 20 nanometers. In step (b), a drying gas is introduced into the drying chamber such that the atomized droplets contact the drying gas, the liquid medium is removed from the atomized droplets by evaporation, and the ceria nanoparticles deposit on surfaces of the alumina particles to form composite catalyst support particles having a D90 particle diameter of greater than or equal to about 6 micrometers to less than or equal to about 10 micrometers. The drying gas is introduced into the drying chamber at a temperature of greater than or equal to about 100 degrees Celsius to less than or equal to about 250 degrees Celsius. In step (c), a catalyst precursor comprising a rhodium precursor and colloidal ceria is applied to the composite catalyst support particles. In step (d), the composite catalyst support particles and the catalyst precursor are heated to form the layered catalyst. The layered catalyst comprises an alumina substrate, a ceria nanoparticle layer extending substantially continuously over the alumina substrate, and a rhodium catalyst layer comprising an atomic dispersion of rhodium adsorbed on the ceria nanoparticle layer. The rhodium catalyst layer constitutes, by weight, greater than or equal to about 0.05% to less than or equal to about 0.3% of the layered catalyst.

The composite catalyst support particles may comprise, by weight, greater than or equal to about 1% to less than or equal to about 10% ceria and greater than or equal to about 90% to less than or equal to about 99% alumina. The layered catalyst may comprise, by weight, greater than or equal to about 5% to less than or equal to about 15% ceria and greater than or equal to about 85% to less than or equal to about 95% alumina.

The atomic dispersion of rhodium may comprise rhodium ions and/or rhodium atoms disposed at the location of surface defect sites in the ceria nanoparticle layer.

DETAILED DESCRIPTION

The present disclosure relates to methods of manufacturing a rhodium (Rh)-, ceria (CeO2)-, and alumina (Al2O3)-containing layered catalyst for purifying exhaust gas streams from combustion processes. During the manufacturing process, composite catalyst support particles are prepared by depositing a thin ceria nanoparticle layer on surfaces of alumina particles using a spray drying technique. The spray drying technique does not result in a substantial increase in the size of the composite catalyst support particles, as compared to that of the alumina particles. The ceria nanoparticle layer provides the composite catalyst support particles with a relatively high defect density, as compared to that of the alumina particles. Then, a rhodium catalyst layer comprising an atomic dispersion of rhodium is formed on the composite catalyst support particles by impregnating the composite catalyst support particles with a rhodium and colloidal ceria-containing catalyst precursor and then calcining the particles. During the impregnation process, rhodium in the catalyst precursor adsorbs on the ceria nanoparticle layer at the location of surface defect sites and the high defect density in the ceria nanoparticle layer is believed to increase the stability of the rhodium in the rhodium catalyst layer. Without intending to be bound by theory, it is believed that the colloidal ceria in the catalyst precursor enhances physical interactions between the rhodium and the composite catalyst support particles during the impregnation process, which helps the rhodium in the rhodium catalyst layer adhere to the composite catalyst support particles.

In practice, the Rh/CeO2/Al2O3layered catalyst may help catalyze the conversion of unburned hydrocarbons (HC), carbon monoxide (CO), nitric oxide (NO), and nitrogen dioxide (NO2) in exhaust gas streams from combustion processes to carbon dioxide (CO2), nitrogen (N2), and water (H2O) prior to discharging the exhaust gas streams to the ambient environment. In addition, the Rh/CeO2/Al2O3layered catalyst may exhibit exceptional aging resistance, with the disclosed Rh/CeO2/Al2O3layered catalyst exhibiting relatively high catalytic activity even after aging, e.g., after the catalyst structures are subjected to a lean-rich cycling aging protocol.

A method of manufacturing layered catalyst particles10(FIG.7) may include a first stage in which composite catalyst support particles12(FIGS.2and5) are prepared. Each of the composite catalyst support particles12may comprise an alumina substrate14and a ceria nanoparticle layer16. In a second stage, a rhodium catalyst layer18(FIG.7) is deposited on an exterior surface20of each of the composite catalyst support particles12to form the layered catalyst particles10.

Referring now toFIG.1, the composite catalyst support particles12may be manufactured using a spray drying process, which may include one or more of the following steps. The spray drying process may be performed using a spray drying apparatus22comprising a drying chamber24, a separator26, and a collector28. In a first step, a liquid feedstock30may be prepared that comprises a mixture of alumina particles32(FIG.2), colloidal ceria, and a liquid medium.

The alumina particles32may comprise, consist essentially of, or consist of alumina and may provide the composite catalyst support particles12with exceptional thermal and mechanical stability. The alumina particles32may have a relatively narrow particle size distribution, which may be achieved by jet milling alumina until a desired average particle size and particle size distribution is achieved. The alumina particles32may have a D90 particle diameter of greater than or equal to about 5 micrometers to less than or equal to about 10 micrometers. For example, the alumina particles32may have a D90 particle diameter of greater than or equal to about 6 micrometers and less than or equal to about 9 micrometers, or optionally less than or equal to about 8 micrometers. In some aspects, the alumina particles32may have a D90 particle diameter of about 7.9 micrometers. The alumina particles32may have a D50 particle diameter of greater than or equal to about 2 micrometers and less than or equal to about 4.5 micrometers, optionally less than or equal to about 4 micrometers, or optionally less than or equal to about 3.5 micrometers. In some aspects, the alumina particles32may have a D50 particle diameter of about 3.48 micrometers. The alumina particles32may have a D10 particle diameter of greater than or equal to about 1 micrometer and less than or equal to about 2 micrometers, optionally less than or equal to about 1.75 micrometers, or optionally less than or equal to about 1.6 micrometers. In some aspects, the alumina particles32may have a D10 particle diameter of about 1.52 micrometers. The alumina particles32may have a BET surface area of about 80 m2/g and a pore volume of about 0.6 mL/g.

The amount of the alumina particles32in the liquid feedstock30may be selected to achieve a target alumina loading on the layered catalyst particles10. For example, the amount of alumina particles32in the liquid feedstock30may be selected so that the amount of alumina in the layered catalyst particles10constitutes, by weight, greater than or equal to about 85% or optionally about 87% to less than or equal to about 95% or optionally about 93% of the layered catalyst particles10. In aspects, the amount of alumina in the layered catalyst particles10may constitute, by weight, about 90% of the layered catalyst particles10.

The colloidal ceria comprises a plurality of ceria nanoparticles40(FIG.2). The ceria nanoparticles40may comprise, consist essentially of, or consist of ceria and may have a D50 diameter of greater than or equal to about 5 nanometers to less than or equal to about 20 nanometers, or greater than or equal to about 10 nanometers to less than or equal to about 15 nanometers. The weight ratio of the alumina particles32to the ceria nanoparticles40in the liquid feedstock30may be selected to achieve a desired weight ratio of alumina to ceria in the composite catalyst support particles12. For example, the weight ratio of the alumina particles32to the ceria nanoparticles40in the liquid feedstock30may be greater than or equal to about 12.5:1 to less than or equal to about 50:1. In some aspects, the weight ratio of the alumina particles32to the ceria nanoparticles40in the liquid feedstock30may be about 20:1.

The liquid medium used to prepare the liquid feedstock30may comprise water (H2O). The liquid feedstock30may be substantially free of a binder, e.g., an organic or inorganic polymer binder. The liquid feedstock30may be prepared by stirring the alumina particles32, the colloidal ceria (including the ceria nanoparticles40), and the liquid medium together at about ambient temperature (e.g., about 25° C.).

The liquid feedstock30may be introduced into the drying chamber24via an atomizer34to form atomized droplets36of the liquid feedstock30. A drying gas38may be introduced into the drying chamber24. The drying gas38may comprise air or an inert gas. The drying gas38may be introduced into the drying chamber24at a temperature of greater than or equal to about 100 degrees Celsius to less than or equal to about 250 degrees Celsius. For example, the drying gas38may be introduced into the drying chamber24at a temperature of greater than or equal to about 120 degrees Celsius to less than or equal to about 180 degrees Celsius. When the drying gas38is introduced into the drying chamber24, the atomized droplets36may contact the drying gas38and the liquid medium may be removed from the atomized droplets36by evaporation. At the same time, the ceria nanoparticles40may deposit directly on surfaces of the alumina particles32(FIG.2) to form the ceria nanoparticle layer16of the composite catalyst support particles12. The ceria nanoparticle layer16may comprise, consist essentially of, or consist of ceria. The ceria nanoparticle layer16may have a BET surface area in a range of from about 50 square meters per gram (m2/g) to about 300 m2/g and a pore volume in a range of from about 0.05 milliliters per gram (mL/g) to about 0.3 mL/g.

The composite catalyst support particles12, the drying gas38, the evaporated liquid medium, and any other gaseous materials introduced into or produced in the drying chamber24during formation of the composite catalyst support particles12may be discharged from the drying chamber24and introduced into the separator26, which may comprise a cyclone. In the separator26, the solid composite catalyst support particles12may collect at a lower end of the separator26and the gaseous materials may be discharged from the separator26at an upper end thereof. The solid composite catalyst support particles12may be collected in the collector28.

As shown inFIG.2, in each of the composite catalyst support particles12, the alumina substrate14may be defined by one or more of the alumina particles32and the ceria nanoparticle layer16may be defined by a plurality of the ceria nanoparticles40. The ceria nanoparticles40may deposit on surfaces of the alumina particles32at a thickness of greater than or equal to about 50 nanometers to less than or equal to about 200 nanometers or optionally less than or equal to about 100 nanometers. The exterior surface20of each of the composite catalyst support particles12may be at least partially or entirely defined by the ceria nanoparticles40. For example, in some aspects, in each of the composite catalyst support particles12, the ceria nanoparticle layer16defined by the ceria nanoparticles40may completely encapsulate the alumina substrate14. The composite catalyst support particles12may comprise, by weight, greater than or equal to about 1% or optionally about 3% to less than or equal to about 10% or optionally about 7% ceria and greater than or equal to about 90% or optionally about 93% to less than or equal to about 99% or optionally about 97% alumina. For example, the composite catalyst support particles12may comprise, by weight, about 5% ceria and about 95% alumina.

Referring now toFIGS.3and4, the composite catalyst support particles12may have a relatively narrow particle size distribution. The composite catalyst support particles12may have a D90 particle diameter of greater than or equal to about 6 micrometers or optionally about 7 micrometers to less than or equal to about 10 micrometers, optionally less than or equal to about 9 micrometers, or optionally less than or equal to about 8.5 micrometers. In some aspects, the composite catalyst support particles12may have a D90 particle diameter of about 8.1 micrometers. The composite catalyst support particles12may have a D50 particle diameter of greater than or equal to about 1.5 micrometers or optionally about 2 micrometers to less than or equal to about 5 micrometers, or optionally about 4 micrometers. In some aspects, the composite catalyst support particles12may have a D50 particle diameter of about 3.4 micrometers. The composite catalyst support particles12may have a D10 particle diameter of greater than or equal to about 1 micrometers to less than or equal to about 2 micrometers. The particles sizes and particle size distribution of the composite catalyst support particles12may be obtained without having to mill, grind, or otherwise treat the composite catalyst support particles12after formation thereof.

Referring now toFIGS.5,6, and7, the rhodium catalyst layer18may be deposited on the composite catalyst support particles12by impregnating the ceria nanoparticle layer16of the composite catalyst support particles12with a catalyst precursor42and then subjecting the composite catalyst support particles12and the catalyst precursor42to a heat treatment. The ceria nanoparticle layer16may be impregnated with the catalyst precursor42by applying the catalyst precursor42directly to the exterior surface20of each of the composite catalyst support particles12. The catalyst precursor42may comprise a mixture of a rhodium precursor, colloidal ceria, and an aqueous medium.

The rhodium precursor may comprise rhodium or a rhodium-containing compound dissolved or dispersed in the aqueous medium of the catalyst precursor42. Examples of rhodium-containing compounds include rhodium salts, e.g., rhodium(III) nitrate, Rh(NO3)3; rhodium(III) chloride, RhCl3; rhodium(II) acetate, Rh2(OOCCH3)4; and/or rhodium(III) sulfate, Rh2(SO4)3and rhodium amine complexes, e.g., rhodium(III) pentaamine trinitrate, [Rh(NH3)5](NO3)3; pentaamminechlororhodium dichloride, [RhCl(NH3)5](Cl)2; rhodium(III) pentaamminechloro sulfate, [RhCl(NH3)5]SO4; and/or pentaammine(nitrito-n)rhodium dinitrate, [Rh(NH3)5](NO2)(NO3)2. In aspects, the rhodium compound may be provided in the form of a hydrate complex, e.g., rhodium(III) nitrate dihydrate (Rh(NO3)3·(H2O)n] and/or rhodium(III) chloride hydrate [RhCl3·(H2O)n], where n is an integer in the range of 1 to 3. The amount of rhodium in the catalyst precursor42may be selected to provide the layered catalyst particles10with a target rhodium loading. For example, the amount of rhodium in the catalyst precursor42may be selected so that the amount of rhodium in the layered catalyst particles10constitutes, by weight, greater than or equal to about 0.05% to less than or equal to about 0.3% of the layered catalyst particles10. In aspects, the amount of rhodium in the layered catalyst particles10may constitute, by weight, about 0.2% of the layered catalyst particles10.

Like the colloidal ceria in the liquid feedstock30used to prepare the composite catalyst support particles12, the colloidal ceria in the catalyst precursor42may comprise a plurality of ceria nanoparticles. Like the ceria nanoparticles in the colloidal ceria in the liquid feedstock30, the ceria nanoparticles in the colloidal ceria in the catalyst precursor42may comprise, consist essentially of, or consist of ceria and may have a D50 diameter of greater than or equal to about 5 nanometers to less than or equal to about 20 nanometers, or greater than or equal to about 10 nanometers to less than or equal to about 15 nanometers. The inclusion of colloidal ceria in the catalyst precursor42may help enhance certain desirable physical interactions between the ceria and the rhodium in the layered catalyst particles10, which may help form a strong bond between the rhodium in the rhodium catalyst layer18and the ceria in the ceria nanoparticle layer16. In addition, the high defect density in the ceria nanoparticle layer16is believed to aid in the absorption of a highly stable dispersion of rhodium atoms and/or rhodium ions on the ceria nanoparticle layer16at the location of surface defect sites, which may help prevent the formation and/or agglomeration of rhodium particles and/or clusters.

The amount of ceria nanoparticles in the catalyst precursor42may be selected to achieve a target ceria loading on the layered catalyst particles10. For example, the amount of ceria nanoparticles in the catalyst precursor42may be selected so that the amount of ceria in the layered catalyst particles10constitutes, by weight, greater than or equal to about 5% or optionally about 7% to less than or equal to about 15% or optionally about 13% of the layered catalyst particles10. In aspects, the amount of ceria in the layered catalyst particles10may constitute, by weight, about 10% of the layered catalyst particles10.

The weight ratio of the ceria nanoparticles to the rhodium in the catalyst precursor42may be greater than or equal to about 15:1 to less than or equal to about 100:1 or optionally less than or equal to about 50:1. In some aspects, the weight ratio of the ceria nanoparticles to the rhodium in the catalyst precursor42may be about 25:1.

The aqueous medium may comprise water (H2O). In some aspects, the aqueous medium may comprise an aqueous nitric acid solution (HNO3) or an aqueous ammonium hydroxide (NH4OH) solution. The catalyst precursor42may be formulated to exhibit a pH of greater than or equal to about 5 to less than or equal to about 12 at a temperature of about 25° C. In the catalyst precursor42, rhodium may be present in the form of positively charged Rh3+cations and/or positively charged Rh-containing complexes balanced by anions, e.g., of NO3−, Cl−, CH3COO−, and/or SO42−. Examples of positively charged Rh-containing complexes include [Rh(NH3)5]3+, [RhCl(NH3)5]2+, and/or [Rh(NH3)6]3+. The catalyst precursor42may be formulated to promote the adsorption of isolated Rh3+ions on the exterior surface20of each of the composite catalyst support particles12, as explained in U.S. patent application Ser. No. 17/749,894 titled Rhodium-Containing Layered Catalyst Structures and Methods of Making the Same filed May 20, 2022, the contents of which are incorporated herein by reference.

The ceria nanoparticle layer16of the composite catalyst support particles12may be impregnated with the catalyst precursor42using a wet impregnation technique or a dry or incipient wetness impregnation technique. If a wet impregnation technique is used, the volume of the catalyst precursor42applied to the composite catalyst support particles12will be greater than a calculated pore volume of the composite catalyst support particles12. If a dry or incipient wetness impregnation technique is used, the volume of the catalyst precursor42applied to the composite catalyst support particles12may be substantially equal to a calculated pore volume of the ceria nanoparticle layer16or of the composite catalyst support particles12.

After the ceria nanoparticle layer16of the composite catalyst support particles12is impregnated with the catalyst precursor42, the composite catalyst support particles12and the catalyst precursor42may be subjected to a heat treatment to deposit the rhodium catalyst layer18on the composite catalyst support particles12and form the layered catalyst particles10. The composite catalyst support particles12and the catalyst precursor42may be heat treated to remove the aqueous medium, the negatively charged anions, e.g., NO3−, Cl−, CH3COO−, and/or SO42, and reaction byproducts therefrom (e.g., by evaporation), and deposit the rhodium catalyst layer18on the ceria nanoparticle layer16. The heat treatment may include heating the composite catalyst support particles12and the catalyst precursor42in an oxygen O2-containing environment (e.g., air) at a temperature of greater than or equal to about 350° C. to less than or equal to about 800° C. for a duration of greater than or equal to about one (1) hour to less than or equal to about 5 hours. In aspects, the heat treatment may comprise heating the composite catalyst support particles12and the catalyst precursor42in an O2-containing environment at a temperature of about 500° C. for a duration of about 2 hours to form the rhodium catalyst layer18on the ceria nanoparticle layer16.

During the heat treatment, the ceria nanoparticles in the catalyst precursor42may be deposited on and incorporated into the structure of the ceria nanoparticle layer16. In aspects where rhodium is present in the catalyst precursor42in the form of positively charged rhodium-containing complexes, the positively charged rhodium-containing complexes may decompose during the heat treatment. Chemical compounds that may be released in gas or vapor form from the composite catalyst support particles12and the catalyst precursor42during the third heat treatment may include nitrogen (N2), nitrogen oxides (e.g., N2O, NO2, and/or NO), ammonia (NH3), and/or H2O. The rhodium catalyst layer18may comprise an atomic dispersion of Rh3+ions, Rh atoms, and optionally a plurality of sub-nanometer sized Rh particles absorbed on the ceria nanoparticle layer16at the location of surface defect sites in the ceria nanoparticle layer16. When present, the sub-nanometer sized Rh particles in the rhodium catalyst layer18may have a D50 particle diameter of less than or equal to about one (1) nanometer. The rhodium catalyst layer may be substantially free of Rh particles having diameters greater than or equal to about 1 nanometer and may be substantially free of clusters of Rh particles having diameters greater than or equal to about 1 nanometer.

Referring now toFIG.8, an exhaust gas treatment device50may be used to position a plurality of the layered catalyst particles10in a path of an exhaust gas stream52from a combustion process (not shown) to help catalyze the conversion of unburned hydrocarbons (HC), carbon monoxide (CO), nitric oxide (NO), and nitrogen dioxide (NO2) in the exhaust gas stream52to carbon dioxide (CO2), nitrogen (N2), and water (H2O). In such application, the layered catalyst particles10may be referred to as a three-way catalyst. The exhaust gas treatment device50may include a housing54having an inlet56in which the exhaust gas stream52is received, an outlet58through which a treated gas stream60is discharged, and a monolithic substrate62supported within the housing54. The monolithic substrate62may include walls having wall surfaces that define a plurality of flow-through passages extending therethrough between the inlet56and the outlet58of the housing54. A plurality of the layered catalyst particles10may be positioned in the path of the exhaust gas stream52by depositing the layered catalyst particles10in the form of a slurry or washcoat on the wall surfaces of the monolithic substrate62such that, when the exhaust gas stream52flows through the flow-through passages in the monolithic substrate62, the unburned hydrocarbons, carbon monoxide, nitric oxide, and/or nitrogen dioxide in the exhaust gas stream52pass in direct contact with the layered catalyst particles10deposited on the wall surfaces of the monolithic substrate62and are converted to carbon dioxide, nitrogen, and water to form the treated gas stream60.

The HC, CO, and NO conversion efficiency of the layered catalyst particles10may be evaluated by exposing a volume of the layered catalyst particles10to a simulated exhaust gas stream including CO, NO, C3H6, and C3H8. The simulated exhaust gas flow may be heated from an initial temperature of 100° C. to a temperature of 450° C. to determine the HC, CO, and NO conversion efficiency of the layered catalyst particles10over a range of temperatures. The term “T50” refers to the temperature at which the layered catalyst particles10achieved 50% conversion efficiency. After initial formation of the layered catalyst particles10, the layered catalyst particles10may have a T50 for CO oxidation of about 230° C., a T50 for NO reduction of about 268° C., and a T50 for C3H6oxidation of about 290° C.

The layered catalyst structure10may exhibit exceptionally high catalytic activity, even after the layered catalyst structure10is subjected to a simulated gasoline engine exhaust gas stream using a lean-rich cycling aging protocol, wherein the composition of the simulated exhaust gas stream is repeatedly cycled between fuel-lean and fuel-rich simulated engine exhaust conditions. Such lean-rich cycling aging protocols may be performed at temperatures greater than about 950° C. for durations greater than 20 minutes. After the layered catalyst structure10is subjected to a lean-rich cycling aging protocol, the layered catalyst structure10may have a T50 for CO oxidation of about 315° C., a T50 for NO reduction of about 370° C., and a T50 for C3H6oxidation of about 387° C.

As used herein, the terms “composition” and “material” are used interchangeably to refer broadly to a substance containing at least the preferred chemical constituents, elements, or compounds, but which may also comprise additional elements, compounds, or substances, including trace amounts of impurities, unless otherwise indicated. An “X-based” composition or material broadly refers to compositions or materials in which “X” is the single largest constituent of the composition or material on a weight percentage (%) basis. This may include compositions or materials having, by weight, greater than 50% X, as well as those having, by weight, less than 50% X, so long as X is the single largest constituent of the composition or material based upon its overall weight. When a composition or material is referred to as being “substantially free” of a substance, the composition or material may comprise, by weight, less than 5%, optionally less than 3%, optionally less than 1%, or optionally less than 0.1% of the substance.