Rhodium-containing layered catalyst structure and method for producing the same

Abstract

Rhodium-containing layered catalyst structures and methods for making the same are disclosed. Layered catalyst structures for purifying an exhaust gas stream include a catalyst support and a rhodium catalyst layer including rhodium ions and / or atomic dispersions of rhodium atoms adsorbed on an outer surface of the catalyst support. The catalyst support includes an alumina base, a first ceria layer disposed on and extending substantially continuously over the alumina base, and a second colloidal ceria layer formed directly on the first ceria layer over the alumina base.

Description

[0001] INTRODUCTION

[0002] This section provides background information relating to the present disclosure, which is not necessarily prior art. TECHNICAL FIELD

[0003] The present disclosure relates to catalysts for purifying exhaust gas streams from combustion processes, and to methods of making layered catalyst structures comprising a mixed metal oxide support material loaded with a rhodium catalyst.

[0004] Exhaust gases from combustion processes typically contain a variety of combustion reaction byproducts, including unburned hydrocarbons (HC), carbon monoxide (CO), nitric oxide (NO), and nitrogen dioxide (NO2), where NO and NO2 are collectively referred to as nitrogen oxides or NOx. x It can be desirable to reduce or control the emission of HC, CO, and / or NOx x from various combustion processes into the surrounding environment.

[0005] Exhaust gas treatment systems for automotive internal combustion engines can include a so-called three-way catalyst (TWC) disposed in the exhaust gas stream path from the engine, which catalyst is designed to simultaneously convert HC, CO, and NOx x in the exhaust gas stream to CO2, N2, and H2O prior to emission. Such three-way catalysts often contain one or more platinum group metal (PGM) elements (e.g., platinum, rhodium, palladium, etc.) supported on a thermally and mechanically stable, high surface area porous support material, which can include alumina (AI2O3). SUMMARY

[0006] This section provides a general summary of the present disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

[0007] A layered catalyst structure for purifying an exhaust gas stream is disclosed. The layered catalyst structure includes a catalyst support and a rhodium catalyst layer. The catalyst support includes an alumina base, a first ceria layer disposed on and extending substantially continuously over the alumina base, and a second colloidal ceria layer formed directly on the first ceria layer over the alumina base. The rhodium catalyst layer comprises atomic dispersions of rhodium ions and / or rhodium atoms adsorbed on an outer surface of the catalyst support.

[0008] The atomic dispersions of rhodium ions and / or rhodium atoms can be arranged on the outer surface of the catalyst support at locations of surface defect sites in the second colloidal ceria layer.

[0009] The rhodium catalyst layer can be substantially free of Rh particles having a diameter greater than or equal to about 1 nanometer, and can be substantially free of clusters of Rh particles having a diameter greater than or equal to about 1 nanometer.

[0010] After oxidation treatment at a temperature of about 500 °C and after CO adsorption at a temperature of about 60 °C, the layered catalyst structure can exhibit, using diffuse reflectance infrared Fourier transform spectroscopy, a predominant peak at a wavenumber of about 2101 cm -1 and about 2031 cm -1 .

[0011] After oxidation treatment at a temperature of about 500 °C and after CO adsorption at a temperature of about 60 °C, the layered catalyst structure can not exhibit, using diffuse reflectance infrared Fourier transform spectroscopy, a visually discernible peak at a wavenumber of about 2070 cm -1 or about 1870 cm -1 .

[0012] The rhodium catalyst layer can constitute greater than or equal to about 0.1 % to less than or equal to about 2 % by weight of the layered catalyst structure.

[0013] In aspects, the rhodium catalyst layer can constitute greater than or equal to about 0.2 % to less than or equal to about 1 % by weight of the layered catalyst structure.

[0014] The first ceria layer can have a BET surface area of greater than or equal to about 30 m 2 / g to less than or equal to about 150 m 2 / g and a pore volume of greater than or equal to about 0.2 mL / g to less than or equal to about 1.5 mL / g. The second colloidal ceria layer can have a BET surface area of greater than or equal to about 50 m 2 / g to less than or equal to about 180 m 2 / g and a pore volume of greater than or equal to about 0.2 mL / g to less than or equal to about 1.5 mL / g. The BET surface area of the second colloidal ceria layer can be greater than the BET surface area of the first ceria layer.

[0015] The first ceria layer and the second colloidal ceria layer can collectively constitute greater than or equal to about 5 % to less than or equal to about 40 % by weight of the layered catalyst structure.

[0016] A method of removing hydrocarbons, carbon monoxide, and nitrogen oxides from an exhaust gas stream from a gasoline-powered internal combustion engine can include passing the exhaust gas stream over the layered catalyst structure.

[0017] A method of making a layered catalyst structure for purifying an exhaust gas stream is disclosed. In the method, a rhodium-containing precursor solution is applied to an outer surface of a catalyst support. The catalyst support includes an alumina base, a first ceria layer disposed on and extending substantially continuously over the alumina base, and a second colloidal ceria layer deposited on the first ceria layer over the alumina base, such that the outer surface of the catalyst support is defined by the second colloidal ceria layer. The rhodium-containing precursor solution includes positively charged rhodium ions or positively charged rhodium-containing complexes in an aqueous medium. The catalyst support and the rhodium-containing precursor solution are heated to evaporate the aqueous medium and form an atomic dispersion of the rhodium ions on the outer surface of the catalyst support.

[0018] In applying the rhodium-containing precursor solution to the outer surface of the catalyst support, a net negative charge can be provided to the outer surface of the catalyst support, and the positively charged rhodium ions and / or positively charged rhodium-containing complexes can be electrostatically adsorbed onto the outer surface of the catalyst support.

[0019] The positively charged rhodium ions and / or positively charged rhodium-containing complexes can be electrostatically adsorbed onto the outer surface of the catalyst support at locations of surface defect sites in the second colloidal ceria layer.

[0020] The method can further include dissolving or dispersing a rhodium-containing compound in an aqueous solution to form the rhodium-containing precursor solution. The rhodium-containing compound can include rhodium nitrate, rhodium chloride, rhodium acetate, a rhodium ammine complex, a rhodium hydrate complex, or combinations thereof.

[0021] The catalyst support can be impregnated with the rhodium-containing precursor solution using a wet impregnation technique or a incipient wetness impregnation technique.

[0022] The catalyst support and the rhodium-containing precursor solution can be heated in an oxygen O2-containing environment at a temperature greater than or equal to about 350 °C to less than or equal to about 800 °C.

[0023] Heating the catalyst support and the rhodium-containing precursor solution can release a gas or vapor of nitrogen, nitrogen oxides, ammonia, chlorine, acetone, carbon dioxide, water, or combinations thereof.

[0024] The aqueous medium can include an aqueous ammonium hydroxide solution or a nitric acid solution.

[0025] The rhodium-containing precursor solution can have a pH greater than or equal to about 5 to less than or equal to about 12.

[0026] The following technical solutions are disclosed in the present invention:

[0027] Solution 1. A layered catalyst structure for purifying an exhaust gas stream, the layered catalyst structure comprising:

[0028] A catalyst support comprising an alumina substrate, a first ceria layer disposed on and extending substantially continuously over the alumina substrate, and a second colloidal ceria layer formed directly on the first ceria layer over the alumina substrate; and

[0029] A rhodium catalyst layer comprising atomic dispersions of rhodium ions and / or rhodium atoms adsorbed on an outer surface of the catalyst support.

[0030] Scheme 2. The layered catalyst structure according to Scheme 1, wherein the atomic dispersions of rhodium ions and / or rhodium atoms are arranged on the outer surface of the catalyst support at locations of surface defect sites in the second colloidal ceria layer.

[0031] Scheme 3. The layered catalyst structure according to Scheme 1, wherein the rhodium catalyst layer is substantially free of Rh particles having a diameter greater than or equal to about 1 nanometer, and is substantially free of clusters of Rh particles having a diameter greater than or equal to about 1 nanometer.

[0032] Scheme 4. The layered catalyst structure according to Scheme 1, wherein after oxidation treatment at a temperature of about 500 °C and after CO adsorption at a temperature of about 60 °C, the layered catalyst structure exhibits, using diffuse reflectance infrared Fourier transform spectroscopy, dominant peaks at wave numbers of about 2101 cm -1 and about 2031 cm -1 .

[0033] Scheme 5. The layered catalyst structure according to Scheme 1, wherein after oxidation treatment at a temperature of about 500 °C and after CO adsorption at a temperature of about 60 °C, the layered catalyst structure does not exhibit, using diffuse reflectance infrared Fourier transform spectroscopy, visually discernible peaks at wave numbers of about 2070 cm -1 or about 1870 cm -1 .

[0034] Scheme 6. The layered catalyst structure according to Scheme 1, wherein the rhodium catalyst layer comprises greater than or equal to about 0.1% to less than or equal to about 2% by weight of the layered catalyst structure.

[0035] Scheme 7. The layered catalyst structure according to Scheme 1, wherein the rhodium catalyst layer comprises greater than or equal to about 0.2% to less than or equal to about 1% by weight of the layered catalyst structure.

[0036] Scheme 8. The layered catalyst structure according to Scheme 1, wherein the first ceria layer has a surface area greater than or equal to about 30 m 2 / g to less than or equal to about 150 m 2a BET surface area of greater than or equal to about 50 m2 / g to less than or equal to about 180 m2 / g and a pore volume of greater than or equal to about 0.2 mL / g to less than or equal to about 1.5 mL / g, and wherein the BET surface area of the second colloidal ceria layer is greater than the BET surface area of the first ceria layer. 2 a BET surface area of greater than or equal to about 50 m2 / g to less than or equal to about 180 m2 / g and a pore volume of greater than or equal to about 0.2 mL / g to less than or equal to about 1.5 mL / g, and wherein the BET surface area of the second colloidal ceria layer is greater than the BET surface area of the first ceria layer. 2 a BET surface area of greater than or equal to about 50 m2 / g to less than or equal to about 180 m2 / g and a pore volume of greater than or equal to about 0.2 mL / g to less than or equal to about 1.5 mL / g, and wherein the BET surface area of the second colloidal ceria layer is greater than the BET surface area of the first ceria layer.

[0037] Scheme 9. The layered catalyst structure according to Scheme 1, wherein the first ceria layer and the second colloidal ceria layer collectively comprise greater than or equal to about 5% to less than or equal to about 40% by weight of the layered catalyst structure.

[0038] Scheme 10. A method for removing hydrocarbons, carbon monoxide, and nitrogen oxides from an exhaust stream of a gasoline-powered internal combustion engine, the method comprising passing the exhaust stream over the layered catalyst structure of Scheme 1.

[0039] Scheme 11. A method for preparing a layered catalyst structure for purifying an exhaust stream, the method comprising the steps of:

[0040] (a) applying a rhodium-containing precursor solution to an outer surface of a catalyst support, the catalyst support comprising an alumina substrate, a first ceria layer disposed on and extending substantially continuously over the alumina substrate, and a second colloidal ceria layer deposited on the first ceria layer over the alumina substrate, such that the outer surface of the catalyst support is defined by the second colloidal ceria layer, wherein the rhodium-containing precursor solution comprises positively charged rhodium ions or positively charged rhodium-containing complexes in an aqueous medium; and

[0041] (b) heating the catalyst support and the rhodium-containing precursor solution to evaporate the aqueous medium and form an atomic dispersion of the rhodium ions on the outer surface of the catalyst support.

[0042] Scheme 12. The method according to Scheme 11, wherein, upon application of the rhodium-containing precursor solution to the outer surface of the catalyst support in step (a), a net negative charge is provided to the outer surface of the catalyst support, and the positively charged rhodium ions and / or positively charged rhodium-containing complexes are electrostatically adsorbed onto the outer surface of the catalyst support.

[0043] Scheme 13. The method according to Scheme 12, wherein the positively charged rhodium ions and / or positively charged rhodium-containing complexes are electrostatically adsorbed onto the outer surface of the catalyst support at the location of surface defect sites in the second colloidal ceria layer.

[0044] Scheme 14. The method according to Scheme 1, further comprising:

[0045] dissolving or dispersing a rhodium-containing compound in an aqueous medium to form a rhodium-containing precursor solution of step (a),

[0046] wherein the rhodium-containing compound comprises rhodium nitrate, rhodium chloride, rhodium acetate, a rhodium ammine complex, a rhodium hydrate complex, or a combination thereof.

[0047] Scheme 15. The method according to Scheme 11, wherein the catalyst support is impregnated with the rhodium-containing precursor solution in step (a) using a wet impregnation technique or a incipient wetness impregnation technique.

[0048] Scheme 16. The method according to Scheme 11, wherein the catalyst support and the rhodium-containing precursor solution are heated in step (b) at a temperature greater than or equal to about 350 °C to less than or equal to about 800 °C in an oxygen, O2, containing environment.

[0049] Scheme 17. The method according to Scheme 11, wherein heating the catalyst support and the rhodium-containing precursor solution in step (b) releases a gas or vapor of nitrogen, nitrogen oxides, ammonia, chlorine, acetone, carbon dioxide, water, or a combination thereof.

[0050] Scheme 18. The method according to Scheme 11, wherein the aqueous medium comprises an aqueous ammonium hydroxide solution or a nitric acid solution.

[0051] Scheme 19. The method according to Scheme 11, wherein the rhodium-containing precursor solution has a pH greater than or equal to about 5 to less than or equal to about 12.

[0052] Other applicable fields will be apparent from the description provided herein. The description in this summary and the specific examples are intended merely as illustrations and are not intended to limit the scope of the disclosure. BRIEF DESCRIPTION OF DRAWINGS

[0053] The drawings described herein are for illustration only and are not intended to limit the scope of the disclosure.

[0054] Figure 1 is a schematic depiction of steps in a method of making a layered catalyst structure, wherein an Al203substrate is impregnated with a Ce-containing precursor solution.

[0055] Figure 2 is a schematic depiction of an Al203substrate after a first heat treatment has been applied to the Al203substrate and the Ce-containing precursor solution so as to form a first CeO2layer on the surface of the Al203substrate. Figure 1 is a schematic depiction of an Al203substrate after a first heat treatment has been applied to the Al203substrate and the Ce-containing precursor solution so as to form a first CeO2layer on the surface of the Al203substrate.

[0056] Figure 3 is a schematic depiction of steps in a method of making a layered catalyst structure, wherein an Al203substrate is impregnated with a Ce-containing precursor solution. Figure 2A schematic depiction of an Al2O3 substrate and a first CeO2 layer, wherein the first CeO2 layer is impregnated with a Ce-containing colloidal suspension.

[0057] Figure 4 It includes Figure 3 The Al2O3 substrate and the first CeO2 layer and by means of the Al2O3 substrate and the first CeO2 layer Figure 3 A schematic depiction of a catalyst support consisting of an Al2O3 substrate, a first CeO2 layer, and a Ce-containing colloidal suspension subjected to a second heat treatment to form a second colloidal CeO2 layer on the Al2O3 substrate above the first CeO2 layer.

[0058] Figure 5 yes Figure 4 A schematic depiction of a catalyst support in which a second colloidal CeO2 layer of the catalyst support is impregnated with a solution containing Rh precursor.

[0059] Figure 6 yes Figure 5 An enlarged view of the surface of the second colloidal CeO2 layer, depicting the negatively charged –O atoms on the surface of the second colloidal CeO2 layer during impregnation. - The group (shown as a circle ○) and the positively charged Rh in the Rh precursor solution 3+ Electrostatic interactions between ions or Rh-containing complexes (shown as square □).

[0060] Figure 7 It includes Figure 5 catalyst support and through the Figure 5 A schematic depiction of a layered catalyst structure in which a Rh catalyst layer is formed directly on the surface of a second colloidal CeO2 layer on the catalyst support after a third heat treatment is applied to the catalyst support and the Rh precursor solution.

[0061] Figure 8 The image depicts a volume of light collected using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Figure 7 Absorbance (au) vs. wavenumber (cm) measured for CO adsorption on a layered catalyst structure -1 The spectrum of ).

[0062] In the various views of the accompanying drawings, the corresponding reference numerals indicate the corresponding parts. Detailed Implementation

[0063] The exemplary embodiments are provided so that the present disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific compositions, ingredients, methods, etc. to provide a thorough understanding of the exemplary embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that the exemplary embodiments can be embodied in many different forms and that they should not be construed as limiting. In some exemplary embodiments, well-known methods, well-known structures, and well-known technology are not described in detail.

[0064] The terminology used herein is for the purpose of describing illustrative embodiments only and is not intended to be limiting. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises," "comprising," "including," and "having" are inclusive and therefore specify the presence of stated features, integers, compositions, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. Although the open-ended term "comprising," is to be understood as a non-limiting term used to describe and claim various embodiments of the present disclosure, in certain aspects the term can alternatively be understood to be the more restrictive term "consisting of" or "consisting essentially of." Thus, for any given embodiment reciting compositions, materials, components, elements, features, integers, operations, and / or process steps, the present disclosure also specifically includes embodiments, which are modified, re-structured, or otherwise changed from the given embodiment, as well as embodiments, which include one, some, or all of the described compositions, materials, components, elements, features, integers, operations, and / or process steps in conjunction with one or more like compositions, materials, components, elements, features, integers, operations, and / or process steps, not specifically described. In the case of "consisting of," the alternative embodiment does not include any additional compositions, materials, components, elements, features, integers, operations, and / or process steps, while in the case of "consisting essentially of," such an embodiment does not include any additional compositions, materials, components, elements, features, integers, operations, and / or process steps which materially affect the basic and novel characteristics of the described embodiment.

[0065] Any of the method steps, processes, and operations described herein are not to be interpreted in a strict order unless explicitly stated otherwise and are not to be interpreted in accordance with the order of discussion. It is also to be understood that additional or alternative steps can be employed, unless otherwise indicated.

[0066] When a component, element, or layer is referred to as being "on", "engaged", "connected", or "coupled" to another component, element, or layer, it can be directly on, engaged, connected, or coupled to the other component, element, or layer or intervening components, elements, or layers can be present. In contrast, when an element is referred to as being "directly on", "directly engaged", "directly connected", or "directly coupled" to another element or layer, there are no intervening components, elements, or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., "between" versus "directly between", "adjacent" versus "directly adjacent", etc.). The term "and / or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items.

[0067] Although the terms first, second, third, etc. can be used herein to describe various steps, elements, components, regions, layers and / or sections, these steps, elements, components, regions, layers and / or sections should not be limited by these terms. These terms are only used to distinguish one step, element, component, region, layer or section from another step, element, component, region, layer or section. Unless the context clearly indicates otherwise, as such, terminology such as "first", "second", and other ordinal terminology is used herein when referring to a step, element, component, region, layer or section to indicate that one of the steps, elements, components, regions, layers or sections is different from the other steps, elements, components, regions, layers or sections. The first step, element, component, region, layer or section discussed below can thus be termed a second step, element, component, region, layer or section without departing from the teachings of the example embodiments.

[0068] Spatially relative terms, such as "front", "back", "side", "under", "underneath", "below", "lower", "upper", and the like, can be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms can be intended to encompass different orientations of the device or system in use or operation in addition to the orientation depicted in the figures.

[0069] Throughout this disclosure, numerical values represent approximate measurement or range limits and include slight deviations and embodiments generally having the listed values as well as embodiments exactly having the listed values. Except in the Examples provided at the end of the DETAILED DESCRIPTION, all numerical values of parameters (e.g., of quantities or conditions) in this specification, including the attached claims, are to be interpreted as approximately the value that they would be read as having, even if that value is an integer or otherwise not a range of values. The approximation extends to all values, whether they are numerical or not, unless the context clearly indicates otherwise. The term "about" means plus or minus 10% of the value to which it pertains, unless otherwise explicitly indicated. For example, "about" can include a variation smaller than or equal to 5%, optionally smaller than or equal to 4%, optionally smaller than or equal to 3%, optionally smaller than or equal to 2%, optionally smaller than or equal to 1%, optionally smaller than or equal to 0.5%, and in certain aspects, optionally smaller than or equal to 0.1% of the value to which it pertains.

[0070] Further, the disclosure of ranges includes all values and further subdivisions between the stated range limits, including the range limits and subranges therein.

[0071] As used herein, the terms "composition" and "material" are used interchangeably to refer to a substance that contains at least the preferred chemical constituent, element, or compound, but can also include additional elements, compounds, or substances, unless otherwise indicated.

[0072] Exemplary embodiments will now be more fully described with reference to the accompanying drawings.

[0073] The present disclosure relates to a layered catalyst structure containing rhodium (Rh), ceria (Ce02), and alumina (AI2O3) for purifying an exhaust gas stream from a combustion process, and to a method of making the Rh / Ce02 / AI2O3 layered catalyst structure. The layered catalyst structure includes a catalyst support including an AI2O3 substrate, a first Ce02 layer deposited on the AI2O3 substrate, and a second colloidal Ce02 layer including a plurality of colloidal Ce02 particles deposited on the AI2O3 substrate over the first Ce02 layer. An atomic dispersion of rhodium (Rh) forms as a catalyst layer on exposed surfaces of the high surface area colloidal Ce02 particles.

[0074] In practice, the disclosed Rh / Ce02 / A1203 layered catalyst structure can help catalyze the conversion of unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen monoxide (NO), and nitrogen dioxide (NO2) in an exhaust gas stream from a combustion process to carbon dioxide (C02), nitrogen (N2), and water (H20) prior to the exhaust gas stream being emitted to the surrounding environment. Moreover, the disclosed Rh / Ce02 / A1203 layered catalyst structure can exhibit excellent aging resistance, with the disclosed Rh / Ce02 / A1203 layered catalyst structure exhibiting a relatively high catalytic activity even after aging, such as after being subjected to a lean- rich cycle aging procedure.

[0075] Reference will now be made to Figure 1 , a method of making the layered catalyst structure 10 Figure 7 may include a first step in which the catalyst support 12 Figure 4 is prepared and a second step in which a rhodium (Rh) catalyst layer 14 Figure 7 is deposited on the surface 32 of the catalyst support 12 to form the layered catalyst structure 10. In the first step, an alumina (A1203) substrate 18 defining an outer surface 20 can be provided. The A1203 substrate 18 is porous and provides the layered catalyst structure 10 with excellent thermal and mechanical stability. The A1203 substrate 18 can include an A1203 powder including a plurality of A1203 particles having a D50 particle size of greater than or equal to about 20 microns to less than or equal to about 70 microns. For example, the plurality of A1203 particles in the A1203 powder can have a D50 particle size of greater than or equal to about 40 microns to less than or equal to about 60 microns, or about 50 microns. The A1203 substrate 18 can have a BET surface area of about 80 m2 / g and a pore volume of about 0.6 mL / g. 2 / g of BET surface area and about 0.6 mL / g of pore volume.

[0076] As shown in Figure 1 , the A1203 substrate 18 can be impregnated with a cerium (Ce)-containing precursor solution 22 by applying the Ce-containing precursor solution 22 directly to the outer surface 20 of the A1203 substrate 18. The Ce-containing precursor solution 22 can include a cerium salt dissolved or dispersed in an aqueous medium. The cerium salt can include a cation of cerium (Cei 3+ ) and a nitrate (N03 – ), sulfate (S04 2– ), carbonate (C03 2 − ), citrate, halide (e.g., F – , or Cl –), an alcoholate, a phenolate, an acetate, a benzoate, an oxalate, an acetylacetonate, and / or a carboxylate. The aqueous medium can comprise water (H2O). The amount of cerium in the Ce-containing precursor solution 22 can be selected to achieve a target cerium loading on the AI2O3 substrate 18. In aspects, the amount of cerium in the Ce-containing precursor solution 22 can comprise greater than or equal to about 5% to less than or equal to about 30% by weight of the Ce-containing precursor solution 22. For example, the Ce-containing precursor solution 22 can comprise cerium in an amount of about 10% by weight of the Ce-containing precursor solution 22. The AI2O3 substrate 18 can be impregnated with the Ce-containing precursor solution 22 using a dry impregnation or incipient wetness impregnation technique, wherein the volume of the Ce-containing precursor solution 22 applied to the AI2O3 substrate 18 is substantially equal to the calculated pore volume of the AI2O3 substrate 18.

[0077] Referring now to Figure 2 After applying the Ce-containing precursor solution 22 to the outer surface 20 of the AI2O3 substrate 18, the AI2O3 substrate 18 and the Ce-containing precursor solution 22 are subjected to a first heat treatment to remove the aqueous medium and the anion of the cerium salt (e.g., by evaporation) and deposit a first CeO2 layer 24 on the outer surface 20 of the AI2O3 substrate 18. The first heat treatment can include heating the AI2O3 substrate 18 and the Ce-containing precursor solution 22 in an oxygen (O2)-containing environment (e.g., air) at a temperature greater than or equal to about 550 °C to less than or equal to about 1050 °C for a duration greater than or equal to about one (1) hour to less than or equal to about 5 hours to form the first CeO2 layer 24 on the outer surface 20 of the AI2O3 substrate 18. In aspects, the first heat treatment can include heating the AI2O3 substrate 18 and the Ce-containing precursor solution 22 in the O2-containing environment at a temperature of about 800 °C for a duration of about 2 hours to form the first CeO2 layer 24 on the outer surface 20 of the AI2O3 substrate 18.

[0078] The first CeO2 layer 24 is porous and extends substantially continuously over the outer surface 20 of the AI2O3 substrate 18. The first CeO2 layer 24 can have a BET surface area greater than or equal to about 30 m 2 / g to less than or equal to about 150 m 2 / g and a pore volume greater than or equal to about 0.2 mL / g to less than or equal to about 1.5 mL / g. In aspects, the impregnation and heat treatment steps can be repeated to achieve a target CeO2 loading on the AI2O3 substrate 18.

[0079] Referring now to Figure 3After forming the first Ce02layer 24, the first Ce02layer 24 is impregnated with a cerium (Ce)-containing colloidal suspension 26, such as by directly applying the Ce-containing colloidal suspension 26 to the first Ce02layer 24 over the outer surface 20 of the AI2O3substrate 18. The Ce-containing colloidal suspension 26 can include a plurality of colloidal Ce02particles suspended in an aqueous medium. The colloidal Ce02particles can consist essentially of Ce02and 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 aqueous medium can include water (H2O) and optionally an acid, such as acetic acid, which can help the colloidal Ce02particles remain suspended in the aqueous medium of the Ce-containing colloidal suspension 26. The amount of colloidal Ce02particles in the Ce-containing colloidal suspension 26 can be selected to achieve a target colloidal Ce02particle loading. In aspects, the colloidal Ce02particles can constitute greater than or equal to about 10% to less than or equal to about 30% by weight of the Ce-containing colloidal suspension 26. In aspects, the first Ce02layer 24 can be impregnated with the Ce-containing colloidal suspension 26 using a dry impregnation or incipient wetness impregnation technique, in which the volume of the Ce-containing colloidal suspension 26 applied to the first Ce02layer 24 is substantially equal to the calculated pore volume of the first Ce02layer 24.

[0080] Referring now to Figure 4 A second thermal treatment is applied to the AI2O3substrate 18, the first Ce02layer 24, and the Ce-containing colloidal suspension 26 to form a second colloidal Ce02layer 28 directly on the first Ce02layer 24. The AI2O3substrate 18, the first Ce02layer 24, and the second colloidal Ce02layer 28 collectively comprise the catalyst support 12, with the second colloidal Ce02layer 28 defining an outer surface 32 of the catalyst support 12. During the second thermal treatment, the AI2O3substrate 18, the first Ce02layer 24, and the Ce-containing colloidal suspension 26 are heated to remove the aqueous medium (such as by evaporation) and deposit the second colloidal Ce02layer 28 directly on the first Ce02layer 24 over the AI2O3substrate 18. The second thermal treatment can include heating the AI2O3substrate 18, the first Ce02layer 24, and the Ce-containing colloidal suspension 26 in an oxygen (O2)-containing environment (such as 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 second thermal treatment can include heating the AI2O3substrate 18, the first Ce02layer 24, and the Ce-containing colloidal suspension 26 in the O2-containing environment at a temperature of about 550 °C for a duration of about 2 hours to form the second colloidal Ce02layer 28 over the AI2O3substrate 18.

[0081] The second colloidal Ce02layer 28 is porous and can chemically and mechanically bond to the first Ce02layer 24, for example, via lattice matching. The second colloidal Ce02layer 28 can consist essentially of Ce02and can have a BET surface area of about 50 m2 / g to about 180 m2 / g and a pore volume of about 0.2 mL / g to about 1.5 mL / g. In aspects, the impregnation and heat treatment steps can be repeated one or more times to achieve a target colloidal Ce02particle loading on the AI2O3substrate 18. 2 2 The second colloidal Ce02layer 28 is porous and can chemically and mechanically bond to the first Ce02layer 24, for example, via lattice matching. The second colloidal Ce02layer 28 can consist essentially of Ce02and can have a BET surface area of about 50 m2 / g to about 180 m2 / g and a pore volume of about 0.2 mL / g to about 1.5 mL / g. In aspects, the impregnation and heat treatment steps can be repeated one or more times to achieve a target colloidal Ce02particle loading on the AI2O3substrate 18.

[0082] The first Ce02layer 24 can facilitate adhesion of the second colloidal Ce02layer 28 to the surface 20 of the AI2O3substrate 18 and can extend substantially continuously over the surface 20 of the AI2O3substrate 18. The second colloidal Ce02layer 28 can be formed on the AI2O3substrate 18 over the first Ce02layer 24 and can provide the catalyst support 12 with a relatively high number of surface defect sites. Defect sites on the outer surface 32 of the catalyst support 12, for example, can occur at the location of oxygen vacancies, interstitial atoms, lattice substitutions, dislocations, grain boundaries, or impurities in the second colloidal Ce02layer 28. Without intending to be bound by theory, it is believed that the relatively high surface area and high pore volume of the second colloidal Ce02layer 28 compared to the first Ce02layer 24 can provide the second colloidal Ce02layer 28 with a relatively high surface defect density compared to the first Ce02layer 24. Further, it is believed that the most chemically reactive sites on a metal oxide surface occur at the location of surface defect sites. Thus, forming the second colloidal Ce02layer 28 on the AI2O3substrate 18 over the first Ce02layer 24 can increase the surface defect density of the catalyst support 12 and can provide a layered catalyst structure 10 with improved catalytic activity.

[0083] Referring now to Figure 5 After forming the second colloidal Ce02layer 28, a rhodium (Rh)-containing precursor solution 30 is applied to the outer surface 32 of the catalyst support 12 such that the Rh-containing precursor solution 30 at least partially impregnates the catalyst support 12 by infiltrating the pores of the catalyst support 12. As Figure 5 ​As shown, the Rh-containing precursor solution 30 can be directly applied to the surface 32 of the catalyst support 12, such that the Rh-containing precursor solution 30 wets the surface of the second colloidal CeO2 layer 28 and penetrates its pores. The Rh-containing precursor solution 30 may contain rhodium or a rhodium-containing compound dissolved or dispersed in an aqueous medium. Examples of rhodium-containing compounds include rhodium salts, such as rhodium(III) nitrate, Rh(NO3)3; rhodium(III) chloride, RhCl3; rhodium(II) acetate, Rh2(OOCCH3)4; and / or rhodium(III) sulfate, Rh2(SO4)3, and rhodium amine complexes, such as pentaamminerhodium(III) trinitrate, [Rh(NH3)5](NO3)3; pentaamminerhodium dichloride, [RhCl(NH3)5](Cl)2; pentaamminerhodium(III) sulfate, [RhCl(NH3)5]SO4; and / or pentaamminerhodium dinitrate, [Rh(NH3)5](NO2)(NO3)2. In several respects, the rhodium compound can be provided in the form of a hydrated complex, such as rhodium(III) nitrate dihydrate [Rh(NO3)3·(H2O)] n ] and / or hydrated rhodium(III) chloride [RhCl3·(H2O)] n ],in n It is an integer from 1 to 3.

[0084] The Rh-containing precursor solution 30 can be prepared by dissolving or dispersing rhodium or a rhodium-containing compound in an aqueous medium. In some aspects, the aqueous medium may contain or consist essentially of water (H₂O). In some aspects, the aqueous medium may include an aqueous solution of nitric acid (HNO₃) or an aqueous solution of ammonium hydroxide (NH₄OH). The Rh-containing precursor solution 30 can be formulated to exhibit a pH of approximately 5 to approximately 12 at a temperature of approximately 25°C. In the Rh-containing precursor solution 30, rhodium can be present as positively charged Rh... 3+ Cations and / or anions, such as NO3- - Cl - CH3COO - and / or SO4 2- It exists in equilibrium as a positively charged Rh-containing complex. Examples of positively charged Rh-containing complexes include [Rh(NH3)5]. 3+ [RhCl(NH3)5] 2+ and / or [Rh(NH3)6] 3+ The amount of rhodium in the Rh precursor solution 30 can be selected to give the layered catalyst structure 10 the target Rh loading.

[0085] 30g of Rh precursor solution was prepared to facilitate the separation of Rh. 3+ Adsorption of ions on the outer surface 32 of the catalyst support 12. For example... Figure 6As shown, upon application of the Rh-containing precursor solution 30 to the surface 32 of the catalyst support 12, the functional groups on the surface of the second colloidal Ce02layer 28 will deprotonate and convert to negatively charged species 34 (shown as circles o), resulting in a net negative charge on the surface 32 of the catalyst support 12. In aspects, the functional groups on the surface 32 of the catalyst support 12 can comprise hydroxyl groups (-OH), and the hydroxyl functional groups can deprotonate and convert to negatively charged -O - species on the surface of the second colloidal Ce02layer 28. Without intending to be bound by theory, it is believed that positively charged Rh 3+ ions (and / or positively charged Rh-containing complexes) 36 (shown as squares □) in the Rh-containing precursor solution 30 will be Coulombically attracted to the negatively charged species 34 on the surface of the second colloidal Ce02layer 28 and electrostatically adsorb onto the surface 32 of the catalyst support 12 at the negatively charged species 34. Without intending to be bound by theory, it is believed that deprotonation of the functional groups and formation of the negatively charged species 34 can be facilitated at the location of surface defect sites in the second colloidal Ce02layer 28.

[0086] The catalyst support 12 can be impregnated with the Rh-containing precursor solution 30 using a wet impregnation technique or a dry impregnation or incipient wetness impregnation technique. If a wet impregnation technique is used, the volume of the Rh-containing precursor solution 30 applied to the surface 32 of the catalyst support 12 will be greater than the calculated pore volume of the catalyst support 12. If a dry impregnation or incipient wetness impregnation technique is used, the volume of the Rh-containing precursor solution 30 applied to the surface 32 of the catalyst support 12 will be substantially equal to the calculated pore volume of the catalyst support 12.

[0087] Referring now to Figure 7 , a third heat treatment is applied to the catalyst support 12 and the Rh-containing precursor solution 30 in order to deposit the Rh catalyst layer 14 directly on the surface 32 of the catalyst support 12 (i.e., directly on the surface of the second colloidal Ce02layer 28) and form the layered catalyst structure 10. During the third heat treatment, the catalyst support 12 and the Rh-containing precursor solution 30 can be heated in order to remove the aqueous medium, the negatively charged anions (e.g., N03 - , CI - , CH3COO - , and / or S04 2) and reaction by-products (e.g., by evaporation), and deposit a Rh catalyst layer 14 directly on the surface 32 of the catalyst support 12. In aspects where rhodium is present in the Rh-containing precursor solution 30 in the form of positively charged Rh-containing complexes, the catalyst support 12 and Rh-containing precursor solution 30 can be heated to decompose the positively charged Rh-containing complexes. Compounds that can be released from the catalyst support 12 and Rh-containing precursor solution 30 in gaseous or vapor form during the third heat treatment can include nitrogen (N2), nitrogen oxides (e.g., N2O, NO2, and / or NO), ammonia (NH3), chlorine (CI2), acetone, carbon dioxide (CO2), sulfur dioxide (SO2), and / or H2O.

[0088] The third heat treatment can include heating the catalyst support 12 and Rh-containing precursor solution 30 in an O2-containing environment (e.g., air) at a temperature greater than or equal to about 350 °C to less than or equal to about 800 °C for a duration greater than or equal to about one (1) hour to less than or equal to about 5 hours. In aspects, the third heat treatment can include heating the catalyst support 12 and Rh-containing precursor solution 30 in an O2-containing environment at a temperature of about 500 °C for a duration of about 2 hours to form the Rh catalyst layer 14 directly on the surface 32 of the catalyst support 12.

[0089] The Rh catalyst layer 14 can include Rh 3+ atoms, and optionally a plurality of sub-nanometer Rh particles deposited directly on the surface 32 of the catalyst support 12 (i.e., on the surface of the second colloidal CeO2layer 28). Without intending to be bound by theory, it is believed that the positively charged Rh 3+ ions (and / or positively charged Rh-containing complexes) 36 in the Rh-containing precursor solution 30 can be anchored to the surface 32 of the catalyst support 12 at specific locations of the negatively charged species 34, such as at specific locations of surface defect sites in the second colloidal CeO2layer 28. Also, during the third heat treatment, these strong electrostatic interactions can inhibit Rh 3+ ions 36 from migrating on the surface 32 of the catalyst support 12, which in turn can promote the deposition of isolated Rh 3+ ions (and / or positively charged Rh-containing complexes) 36 in the Rh-containing precursor solution 30 can be anchored to the surface 32 of the catalyst support 12 at specific locations of the negatively charged species 34, such as at specific locations of surface defect sites in the second colloidal CeO2layer 28. Also, during the third heat treatment, these strong electrostatic interactions can inhibit Rh 3+ ions and / or Rh atoms on the catalyst support 12, rather than forming and / or aggregating relatively large Rh particles, Rh particle clusters, and / or RhO particles. In aspects, when present, sub-nanometer Rh particles in the Rh catalyst layer 14 can have a D50 particle size less than or equal to about one (1) nanometer.

[0090] The Rh catalyst layer 14 can consist essentially of rhodium. The Rh catalyst layer 14 can constitute greater than or equal to about 0.1% or about 0.2%, less than or equal to about 2.0% or about 1.5%, or about 0.1% to about 2.0%, or about 0.2% to about 1.5%, by weight of the layered catalyst structure 10. In some aspects, the Rh catalyst layer 14 can constitute about 0.5% or about 1.0%, by weight, of the layered catalyst structure 10.

[0091] The second colloidal Ce02layer 28 and the first Ce02layer 24 can consist essentially of Ce02, and the second colloidal Ce02layer 28 and the first Ce02layer 24 can collectively constitute greater than or equal to about 5% to less than or equal to about 40%, by weight, of the layered catalyst structure 10. For example, the second colloidal Ce02layer 28 and the first Ce02layer 24 can constitute about 30%, by weight, of the layered catalyst structure 10.

[0092] The layered catalyst structure 10 can be a substantially spherical particle having a diameter greater than or equal to 200 microns to less than or equal to 500 microns. A plurality of the layered catalyst structures 10 can be positioned in the path of an exhaust gas stream from a combustion process to aid in the conversion of unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NO) and nitrogen dioxide (N02) in the exhaust gas stream to carbon dioxide (C02), nitrogen (N2) and water (H20). The plurality of the layered catalyst structures 10 can be positioned in the path of the exhaust gas stream, for example, by filling a duct or passageway with the layered catalyst structures 10 that fill the volume or by depositing the layered catalyst structures 10 in slurry form on the wall surface of a flow-through monolithic substrate. Prior to positioning the plurality of the layered catalyst structures 10 in the path of the exhaust gas stream, the layered catalyst structures 10 can be sieved such that in practice the layered catalyst structures 10 exhibit a D50 particle diameter of about 245 microns to about 450 microns.

[0093] The HC, CO and NO conversion efficiency of the layered catalyst structure 10 can be evaluated by exposing a volume of the layered catalyst structure 10 to a simulated exhaust gas stream comprising CO, NO, C3H6 and C3H8. The simulated exhaust stream can be heated from an initial temperature of 100 °C to a temperature of 450 °C in order to determine the HC, CO and NO conversion efficiency of the layered catalyst structure 10 over a range of temperatures. The term “T50” refers to the temperature at which the layered catalyst structure 10 achieves a 50% conversion efficiency. Upon initial formation of the layered catalyst structure 10, the layered catalyst structure 10 can have a T50 for CO oxidation of about 242 °C, a T50 for NO reduction of about 328 °C and a T50 for C3H6 oxidation of about 335 °C.

[0094] The layered catalyst structure 10 exhibits extremely high catalytic activity, even after being subjected to a lean-rich cycle aging process using simulated gasoline engine exhaust gas streams, wherein the composition of the simulated exhaust gas streams is repeatedly cyclical under lean and rich fuel conditions. This lean-rich cycle aging process can be carried out at temperatures above approximately 450°C for more than 20 minutes. After undergoing the lean-rich cycle aging process, the layered catalyst structure 10 can have a T50 of approximately 338°C for CO oxidation, approximately 343°C for NO reduction, and approximately 375°C for C3H6 oxidation.

[0095] Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) can be used to confirm and / or verify the separation of Rh. 3+ Formation of ions on the surface of the second colloidal CeO2 layer 28 of the layered catalyst structure 10. Prior to DRIFTS, a volume of the layered catalyst structure 10 can be exposed to an oxidation feed containing 10 vol% O2 in an inert carrier gas at approximately 500 °C. Subsequently, a DRIFTS spectrum of CO adsorption is obtained by exposing a volume of the layered catalyst structure 10 to a feed of 1000 ppm CO in an inert carrier gas at approximately 60 °C.

[0096] like Figure 8 As shown, the absorbance (au) 100 vs. wavenumber (cm²) of CO adsorption collected from a layered catalyst structure 10 of a certain volume is measured. -1 The DRIFTS spectrum of 200 nm depicts the area at approximately 2101 cm⁻¹. -1 and approximately 2031 cm -1 The dominant spectral band at that location. For example, such as... Figure 8 As shown, the DRIFTS spectrum of CO adsorption collected on a layered catalyst structure 10 of a certain volume can be depicted at a value less than or equal to approximately 2111 cm⁻¹. -1 Or approximately 2106 cm -1 ≥2091 cm -1 Or approximately 2096 cm -1 or approximately 2111 cm -1 Up to approximately 2091 cm -1 or approximately 2106 cm -1 Up to approximately 2096 cm -1 The first dominant band within the range. Furthermore, the DRIFTS spectrum of CO adsorption collected on a layered catalyst structure 10 of a given volume can be depicted at a range less than or equal to approximately 2041 cm⁻¹. -1 Or approximately 2036 cm -1greater than or equal to about 2021 cm -1 or about 2026 cm -1 or about 2041 cm -1 to about 2021 cm -1 or about 2036 cm -1 to about 2026 cm -1 to about 2026 cm -1 and about 2031 cm -1 The presence of a dominant band at about 2070 cm 3+ -1 and about 1870 cm -1 -1 indicates carbonyl groups on isolated Rh -1 ions adsorbed on the surface of the second colloidal Ce02layer 28 of the layered catalyst structure 10. Notably, Figure 8 The DRIFTS spectra shown do not depict a dominant band or a visually discernible band at about 2070 cm -1 -1 or about 1870 cm -1 -1. A dominant band at about 2070 cm -1 -1 would indicate the presence of clusters of nanometer-sized Rh particles, and a dominant band at about 1870 cm -1 -1 would indicate the presence of Rh nanoparticles having a diameter greater than or equal to about 3 nanometers on the surface of the second colloidal Ce02layer 28 of the layered catalyst structure 10. In practice, it is more advantageous to deposit isolated Rh 3+ ions on the surface of the second colloidal Ce02layer 28 than to form Rh nanoparticles and clusters of Rh nanoparticles. Without intending to be bound by theory, it is believed that the formation of isolated Rh 3+ ions increases the catalytic activity of the layered catalyst structure 10 without increasing the Rh loading (i.e., the weight fraction of the Rh catalyst layer 14). Moreover, it is believed that the formation of isolated Rh 3+ ions increases the number of Rh 3+ ions and / or Rh atoms exposed to the exhaust gas stream compared to the number of Rh atoms exposed to the exhaust gas stream when the Rh catalyst layer 14 includes Rh nanoparticles and clusters of Rh nanoparticles. In the layered catalyst structure 10 of the present disclosure, the isolated Rh 3+ ions and / or isolated Rh atoms can comprise greater than or equal to about 80% or about 90% by weight of the Rh catalyst layer 14. At the same time, the Rh catalyst layer 14 can be substantially free of Rh nanoparticles having a diameter greater than or equal to about 3 nanometers and can be substantially free of clusters of nanometer-sized Rh particles.

[0097] The foregoing description of the implementation embodiments has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed. The various elements or features of a particular implementation are generally not limited to that implementation unless expressly so limited and, where applicable, can interchange other elements or features with other implementation even if not specifically shown or described. Modifications can be made to these implementations in various ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

Claims

1. A layered catalyst structure for purifying waste gas streams, said layered catalyst structure comprising: The catalyst support includes an alumina substrate, a first cerium dioxide layer disposed on the alumina substrate and continuously extending above the alumina substrate, and a second colloidal cerium dioxide layer formed directly on the first cerium dioxide layer above the alumina substrate. and A rhodium catalyst layer comprising an atomic dispersion of rhodium ions and / or rhodium atoms adsorbed on the outer surface of a catalyst support. The first cerium dioxide layer is composed of CeO2, and the second colloidal cerium dioxide layer is also composed of CeO2.

2. The layered catalyst structure according to claim 1, wherein the atomic dispersion of rhodium ions and / or rhodium atoms is arranged on the outer surface of the catalyst support, located at the position of surface defect sites in the second colloidal cerium dioxide layer.

3. The layered catalyst structure according to claim 1, wherein the rhodium catalyst layer does not contain Rh particles with a diameter greater than or equal to 1 nanometer, and does not contain clusters of Rh particles with a diameter greater than or equal to 1 nanometer.

4. The layered catalyst structure according to claim 1, wherein after oxidation treatment at 500°C and CO adsorption at 60°C, the layered catalyst structure exhibits a high spectral density at 2101 cm⁻¹ using diffuse reflectance infrared Fourier transform spectroscopy. -1 and 2031 cm -1 The dominant peak at the wavenumber.

5. The layered catalyst structure according to claim 1, wherein after oxidation treatment at 500°C and CO adsorption at 60°C, the layered catalyst structure does not exhibit a high CO adsorption rate at 2070 cm⁻¹ using diffuse reflectance infrared Fourier transform spectroscopy. -1 Or 1870 cm -1 The visually discernible peak at the wavenumber.

6. The layered catalyst structure according to claim 1, wherein the rhodium catalyst layer constitutes more than or equal to 0.1% and less than or equal to 2% of the layered catalyst structure by weight.

7. The layered catalyst structure according to claim 1, wherein the rhodium catalyst layer constitutes more than or equal to 0.2% and less than or equal to 1% of the layered catalyst structure by weight.

8. The layered catalyst structure according to claim 1, wherein the first cerium dioxide layer has a thickness greater than or equal to 30 μm. 2 / g to less than or equal to 150 m 2 The second colloidal cerium dioxide layer has a BET surface area of ​​≥0.2 mL / g and a pore volume of ≥1.5 mL / g, wherein the second colloidal cerium dioxide layer has a BET surface area of ​​≥50 m² / g and a pore volume of ≥0.2 mL / g and ≤1.5 mL / g. 2 / g to less than or equal to 180 m 2 The BET surface area is greater than or equal to 0.2 mL / g and the pore volume is less than or equal to 1.5 mL / g, wherein the BET surface area of ​​the second colloidal cerium dioxide layer is greater than the BET surface area of ​​the first cerium dioxide layer.

9. The layered catalyst structure according to claim 1, wherein the first cerium dioxide layer and the second colloidal cerium dioxide layer together constitute more than or equal to 5% and less than or equal to 40% of the layered catalyst structure by weight.

10. A method for removing hydrocarbons, carbon monoxide, and nitrogen oxides from exhaust gas streams of a gasoline-powered internal combustion engine, the method comprising passing the exhaust gas streams through a layered catalyst structure of claim 1.

11. A method for preparing a layered catalyst structure for purifying waste gas streams, the method comprising the following steps: (a) Applying a rhodium-containing precursor solution to the outer surface of a catalyst support comprising an alumina substrate, a first cerium dioxide layer disposed on and continuously extending above the alumina substrate, and a second colloidal cerium dioxide layer deposited on the first cerium dioxide layer above the alumina substrate, such that the outer surface of the catalyst support is defined by the second colloidal cerium dioxide layer, wherein the rhodium-containing precursor solution contains positively charged rhodium ions or positively charged rhodium-containing complexes in an aqueous medium; and (b) The catalyst support and the rhodium-containing precursor solution are heated to evaporate the aqueous medium and form an atomic dispersion of rhodium ions on the outer surface of the catalyst support. The first cerium dioxide layer is composed of CeO2, and the second colloidal cerium dioxide layer is also composed of CeO2.

12. The method according to claim 11, wherein, When the rhodium-containing precursor solution is applied to the outer surface of the catalyst support in step (a), a net negative charge is provided to the outer surface of the catalyst support, and positively charged rhodium ions and / or positively charged rhodium-containing complexes are electrostatically adsorbed onto the outer surface of the catalyst support.

13. The method according to claim 12, wherein positively charged rhodium ions and / or positively charged rhodium-containing complexes are electrostatically adsorbed onto the outer surface of the catalyst support at the locations of surface defect sites in the second colloidal cerium dioxide layer.

14. The method of claim 11, further comprising: The rhodium-containing compound is dissolved or dispersed in an aqueous solution to form the rhodium-containing precursor solution of step (a). The rhodium-containing compound mentioned above includes rhodium nitrate, rhodium chloride, rhodium acetate, rhodium amine complexes, rhodium hydrate complexes, or combinations thereof.

15. The method according to claim 11, wherein the catalyst support is impregnated with a rhodium-containing precursor solution in step (a) using a wet impregnation technique or a pre-wet impregnation technique.

16. The method according to claim 11, wherein the catalyst support and the rhodium-containing precursor solution are heated in an oxygen-containing O2 environment at a temperature greater than or equal to 350°C and less than or equal to 800°C in step (b).

17. The method according to claim 11, wherein heating the catalyst support and the rhodium-containing precursor solution in step (b) releases a gas or vapor of nitrogen, nitrogen oxides, ammonia, chlorine, acetone, carbon dioxide, water, or a combination thereof.

18. The method according to claim 11, wherein the aqueous medium comprises an aqueous solution of ammonium hydroxide or a nitric acid solution.

19. The method of claim 11, wherein the rhodium-containing precursor solution has a pH greater than or equal to 5 and less than or equal to 12.

Images