Layered catalyst structure and preparation method thereof

Abstract

A layered catalyst structure for purifying an exhaust gas stream includes a catalyst support and a palladium catalyst layer comprising an atomic dispersion of palladium ions electrostatically adsorbed to an outer surface of the catalyst support. The catalyst support includes 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 over the first ceria layer above the alumina substrate. The palladium catalyst layer is formed on the outer surface of the catalyst support by applying a palladium-containing precursor solution to the outer surface of the catalyst support and subsequently heating the catalyst support and the palladium-containing precursor solution. The palladium-containing precursor solution comprises a positively charged palladium complex in an aqueous medium and has a pH greater than the zero charge point of the second colloidal ceria layer.

Description

[0001] Government funding

[0002] This invention was made with Government support under Agreement No. DE-EE0009196 awarded by the U.S. Department of Energy. The Government may have certain rights in this invention. Technical Field

[0003] The present invention relates to a layered catalyst structure and a preparation method thereof. Background Art

[0004] This section provides background information related to the present disclosure which is not necessarily prior art.

[0005] The present disclosure relates to catalysts for purifying exhaust gas streams from combustion processes and to methods of preparing layered catalyst structures comprising a mixed metal oxide support material loaded with a palladium catalyst.

[0006] 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), of which NO and NO2 are collectively referred to as nitrogen oxides or NO x It may be desirable to reduce or control HC, CO and / or NO from various combustion processes x Release to the surrounding environment.

[0007] Exhaust treatment systems for automotive internal combustion engines may include a so-called three-way catalyst (TWC) positioned in the path of the exhaust gas stream from the engine, the catalyst being designed to convert HC, CO, and NO in the exhaust gas stream into x Simultaneously converting CO2, N2, and H2O. Such three-way catalysts often comprise 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 may comprise alumina (Al2O3). Summary of the Invention

[0008] This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

[0009] A method for preparing a layered catalyst structure for purifying an exhaust gas stream is disclosed. In various aspects, the method comprises the steps of: (a) applying a palladium-containing precursor solution to the outer surface of a catalyst support, and (b) heating the catalyst support and the palladium-containing precursor solution to form an atomic dispersion of palladium ions on the outer surface of the catalyst support. The catalyst support comprises an alumina substrate, a first ceria layer disposed on and extending substantially continuously above the alumina substrate, and a second colloidal ceria layer deposited on the first ceria layer above the alumina substrate, such that the outer surface of the catalyst support is defined by the second colloidal ceria layer. The palladium-containing precursor solution comprises a positively charged palladium complex in an aqueous medium. The palladium-containing precursor solution has a pH greater than the point of zero charge of the second colloidal ceria layer. The catalyst support and the palladium-containing precursor solution are heated in step (b) to evaporate the aqueous medium, decompose the positively charged palladium complex, and form an atomic dispersion of palladium ions on the outer surface of the catalyst support.

[0010] The aqueous medium may include aqueous ammonium hydroxide solution.

[0011] The palladium-containing precursor solution may have a pH of greater than or equal to about 11 to less than or equal to about 12.

[0012] The positively charged palladium complex may comprise a tetraamminepalladium(II) complex.

[0013] When the palladium-containing precursor solution is applied to the outer surface of the catalyst support in step (a), a net negative charge can be provided to the outer surface of the catalyst support, and the positively charged palladium complex can be electrostatically adsorbed onto the outer surface of the catalyst support. In various aspects, the positively charged palladium complex can be electrostatically adsorbed onto the outer surface of the catalyst support at the locations of surface defect sites in the second colloidal ceria layer.

[0014] The method may further include dissolving or dispersing a palladium salt in an aqueous ammonium hydroxide solution to form the palladium-containing precursor solution of step (a). In various aspects, the palladium salt may include at least one of palladium nitrate, tetraamminepalladium dinitrate, tetraamminepalladium dichloride, tetraamminepalladium acetate, tetraamminepalladium sulfate, or a combination thereof.

[0015] The catalyst support may be impregnated with the palladium-containing precursor solution in step (a) using either a wet impregnation technique or an incipient wetness technique.

[0016] The catalyst support and palladium-containing precursor solution may be heated in step (b) at a temperature of greater than or equal to about 350°C to less than or equal to about 800°C.

[0017] Heating the catalyst support and the palladium-containing precursor solution in step (b) may release nitrogen, nitrogen oxides, ammonia and / or water gas or vapor.

[0018] Another method for preparing a layered catalyst structure for purifying an exhaust gas stream is disclosed. In various aspects, the method can include the following steps, in the order described: (a) applying a cerium-containing precursor solution to an outer surface of an alumina substrate; (b) heating the alumina substrate and the cerium-containing precursor solution at a first temperature to form a first ceria layer on the outer surface of the alumina substrate, the first ceria layer extending substantially continuously over the outer surface of the alumina substrate; (c) applying a ceria-containing colloidal suspension to the alumina substrate over the first ceria layer; and (d) heating the alumina substrate, the first ceria layer, and the ceria-containing colloidal suspension at a second temperature to deposit colloidal ceria particles directly on the first ceria layer and form a catalyst support comprising the alumina substrate, the first ceria layer, and a second colloidal ceria layer overlying the first ceria layer on the alumina substrate. (e) applying a palladium-containing precursor solution to the outer surface of the catalyst support, the palladium-containing precursor solution comprising a positively charged palladium complex in an aqueous medium and having a pH greater than the point of zero charge of the second colloidal ceria layer, and (f) heating the catalyst support and the palladium-containing precursor solution at a third temperature to evaporate the aqueous medium, decompose the positively charged palladium complex, and form an atomic dispersion of palladium ions on the outer surface of the catalyst support.

[0019] The cerium-containing precursor solution of step (a) may comprise a cerium salt in an aqueous medium. The alumina substrate and the cerium-containing aqueous precursor solution may be heated in step (b) at a first temperature of greater than or equal to about 550°C to less than or equal to about 1050°C in an oxygen-containing ambient.

[0020] The ceria-containing colloidal suspension of step (c) may comprise a plurality of colloidal ceria particles in an aqueous medium. The colloidal ceria particles may have a D50 diameter of greater than or equal to approximately 5 nanometers to less than or equal to approximately 20 nanometers. The alumina substrate, the first ceria layer, and the ceria-containing colloidal suspension may be heated in step (d) at a second temperature of greater than or equal to approximately 350° C. to less than or equal to approximately 800° C. in an oxygen-containing ambient.

[0021] In step (e), the aqueous medium may include an aqueous ammonium hydroxide solution, and the palladium-containing precursor solution may have a pH of greater than or equal to about 11 to less than or equal to about 12.

[0022] The positively charged palladium complex may comprise a tetraamminepalladium(II) complex.

[0023] When the palladium-containing precursor solution is applied to the outer surface of the catalyst support in step (e), a net negative charge can be provided to the outer surface of the catalyst support, and the positively charged palladium complex can be electrostatically adsorbed onto the outer surface of the catalyst support.

[0024] The catalyst support and the palladium-containing precursor solution may be heated in step (f) in an oxygen-containing environment at a third temperature of greater than or equal to about 350° C. to less than or equal to about 800° C. Heating the catalyst support and the palladium-containing precursor solution in step (f) may release nitrogen, nitrogen oxides, ammonia, and / or water gas or vapor.

[0025] A layered catalyst structure for purifying an exhaust gas stream is disclosed. The layered catalyst structure includes a catalyst support and a palladium catalyst layer electrostatically adsorbed to an outer surface of the catalyst support. The catalyst support includes an alumina substrate, a first ceria layer disposed on and extending substantially continuously above the alumina substrate, and a second colloidal ceria layer formed directly on the first ceria layer above the alumina substrate. The palladium catalyst layer, comprising an atomic dispersion of palladium ions, is electrostatically adsorbed to the outer surface of the catalyst support. The atomic dispersion of palladium ions can be electrostatically adsorbed to the outer surface of the catalyst support at the locations of surface defect sites in the second colloidal ceria layer.

[0026] The first ceria layer may have a thickness greater than or equal to about 30 m 2 / g to less than or equal to approximately 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.

[0027] The second colloidal ceria layer may have a thickness greater than or equal to about 50 m 2 / g to less than or equal to approximately 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.

[0028] The BET surface area of the second colloidal ceria layer may be greater than the BET surface area of the first ceria layer.

[0029] The first ceria layer and the second colloidal ceria layer can together comprise greater than or equal to about 5% to less than or equal to about 40% by weight of the layered catalyst structure.

[0030] The palladium catalyst layer may constitute greater than or equal to about 0.1% to less than or equal to about 5% by weight of the layered catalyst structure.

[0031] The present invention discloses the following technical solutions:

[0032] Scheme 1. A method for preparing a layered catalyst structure for purifying an exhaust gas stream, the method comprising:

[0033] (a) applying a palladium-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 on 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 palladium-containing precursor solution comprises a positively charged palladium complex in an aqueous medium and has a pH greater than a point of zero charge of the second colloidal ceria layer; and

[0034] (b) Heating the catalyst support and the palladium-containing precursor solution to evaporate the aqueous medium, decompose the positively charged palladium complex, and form an atomic dispersion of palladium ions on the outer surface of the catalyst support.

[0035] Option 2. The method of Option 1, wherein the aqueous medium comprises an aqueous ammonium hydroxide solution.

[0036] Option 3. The method of Option 1, wherein the palladium-containing precursor solution has a pH of greater than or equal to about 11 to less than or equal to about 12.

[0037] Scheme 4. The method of Scheme 1, wherein the positively charged palladium complex comprises a tetraamminepalladium(II) complex.

[0038] Option 5. The method of Option 1, wherein, when the palladium-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 the positively charged palladium complex is electrostatically adsorbed onto the outer surface of the catalyst support.

[0039] Option 6. The method of Option 5, wherein the positively charged palladium complex is electrostatically adsorbed onto the outer surface of the catalyst support at the location of surface defect sites in the second colloidal ceria layer.

[0040] Option 7. The method of Option 1, further comprising:

[0041] dissolving or dispersing a palladium salt in an aqueous ammonium hydroxide solution to form a palladium-containing precursor solution of step (a),

[0042] The palladium salt comprises at least one of palladium nitrate, tetraamminepalladium dinitrate, tetraamminepalladium dichloride, tetraamminepalladium acetate, and tetraamminepalladium sulfate, or a combination thereof.

[0043] Option 8. The method of Option 1, wherein the catalyst support is impregnated with the palladium-containing precursor solution in step (a) by a wet impregnation technique or an incipient wetness technique.

[0044] Item 9. The method of Item 1, wherein the catalyst support and the palladium-containing precursor solution are heated in step (b) at a temperature of greater than or equal to about 350°C to less than or equal to about 800°C.

[0045] Scheme 10. The method of Scheme 1, wherein heating the catalyst support and the palladium-containing precursor solution in step (b) releases nitrogen, nitrogen oxides, ammonia, and / or water gas or vapor.

[0046] Scheme 11. A method for preparing a layered catalyst structure for purifying an exhaust gas stream, the method comprising the following steps in sequence:

[0047] (a) applying a cerium-containing precursor solution to the outer surface of an alumina substrate;

[0048] (b) heating the alumina substrate and the cerium-containing precursor solution at a first temperature to form a first ceria layer on the outer surface of the alumina substrate, the first ceria layer extending substantially continuously on the outer surface of the alumina substrate;

[0049] (c) applying a colloidal suspension containing ceria to the alumina substrate above the first ceria layer;

[0050] (d) heating the alumina substrate, the first ceria layer, and the ceria-containing colloidal suspension at a second temperature to deposit colloidal ceria particles directly on the first ceria layer and form a catalyst support comprising the alumina substrate, the first ceria layer, and a second colloidal ceria layer overlying the first ceria layer on the alumina substrate;

[0051] (e) applying a palladium-containing precursor solution to the outer surface of the catalyst support, the palladium-containing precursor solution comprising a positively charged palladium complex in an aqueous medium and having a pH greater than the point of zero charge of the second colloidal ceria layer; and

[0052] (f) heating the catalyst support and the palladium-containing precursor solution at a third temperature to evaporate the aqueous medium, decompose the positively charged palladium complex, and form an atomic dispersion of palladium ions on the outer surface of the catalyst support.

[0053] Option 12. The method of Option 11, wherein the cerium-containing precursor solution of step (a) comprises a cerium salt in an aqueous medium, and wherein the aluminum oxide substrate and the cerium-containing aqueous precursor solution are heated in step (b) at a first temperature of greater than or equal to about 550° C. to less than or equal to about 1050° C. in an oxygen-containing ambient.

[0054] Option 13. The method of Option 11, wherein the ceria-containing colloidal suspension of step (c) comprises a plurality of colloidal ceria particles suspended in an aqueous medium, wherein the colloidal ceria particles have a D50 diameter of greater than or equal to about 5 nm to less than or equal to about 20 nm, and wherein the alumina substrate, the first ceria layer, and the ceria-containing colloidal suspension are heated in step (d) at a second temperature of greater than or equal to about 350° C. to less than or equal to about 800° C. in an oxygen-containing ambient.

[0055] Item 14. The method of Item 11, wherein in step (e), the aqueous medium comprises an aqueous ammonium hydroxide solution, and wherein the palladium-containing precursor solution has a pH of greater than or equal to about 11 to less than or equal to about 12.

[0056] Scheme 15. The method of Scheme 11, wherein the positively charged palladium complex comprises a tetraamminepalladium(II) complex.

[0057] Scheme 16. The method of Scheme 11, wherein when the palladium-containing precursor solution is applied to the outer surface of the catalyst support in step (e), a net negative charge is provided to the outer surface of the catalyst support, and the positively charged palladium complex is electrostatically adsorbed onto the outer surface of the catalyst support.

[0058] 17. The method of 11, wherein the catalyst support and the palladium-containing precursor solution are heated in step (f) at a third temperature of about 350° C. or greater and about 800° C. or less in an oxygen-containing environment, and wherein heating the catalyst support and the palladium-containing precursor solution in step (f) releases nitrogen, nitrogen oxides, ammonia, and / or water gas or vapor.

[0059] 18. A layered catalyst structure for purifying an exhaust gas stream, the layered catalyst structure comprising:

[0060] 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

[0061] A palladium catalyst layer comprises an atomic dispersion of palladium ions electrostatically adsorbed to the outer surface of the catalyst support at the locations of surface defect sites in the second colloidal ceria layer.

[0062] 19. The layered catalyst structure of 18, wherein the first ceria layer has a thickness greater than or equal to about 30 m 2 / g to less than or equal to approximately 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, wherein the second colloidal ceria layer has a pore volume of greater than or equal to about 50 m 2 / g to less than or equal to approximately 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, and wherein the BET surface area of the second colloidal ceria layer is greater than the BET surface area of the first ceria layer.

[0063] Item 20. The layered catalyst structure of item 18, wherein the first ceria layer and the second colloidal ceria layer together comprise greater than or equal to about 5% to less than or equal to about 40% by weight of the layered catalyst structure, and wherein the palladium catalyst layer comprises greater than or equal to about 0.1% to less than or equal to about 5% by weight of the layered catalyst structure.

[0064] Further areas of applicability will become apparent from the description provided herein.The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

[0066] Figure 1 is a schematic depiction of the steps in a method for preparing a layered catalyst structure, wherein an Al2O3 substrate is impregnated with a Ce-containing precursor solution.

[0067] Figure 2 After the Al2O3 substrate and the Ce-containing precursor solution have been subjected to a first heat treatment to form a first CeO2 layer on the surface of the Al2O3 substrate Figure 1 Schematic depiction of an Al2O3 substrate.

[0068] Figure 3 yes Figure 2 Schematic depiction of an Al2O3 substrate and a first CeO2 layer, where the first CeO2 layer is impregnated with a Ce-containing colloidal suspension.

[0069] Figure 4 is included Figure 3 Al2O3 substrate and the first CeO2 layer and by Figure 3 Schematic depiction of a catalyst support with 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.

[0070] Figure 5 yes Figure 4 Schematic depiction of a catalyst support, wherein the second colloidal CeO2 layer of the catalyst support is impregnated with a Pd-containing precursor solution.

[0071] Figure 6 yes Figure 5 A magnified view of the surface of the second colloidal CeO2 layer depicts the negatively charged –O - The groups (shown as circles) interact with the positively charged [Pd(NH3)4] 2+ Electrostatic interactions between complexes (shown as squares □).

[0072] Figure 7 is included Figure 5 The catalyst support and Figure 5 Schematic depiction of a layered catalyst structure in which a catalyst support and a Pd-containing precursor solution are subjected to a third heat treatment to form a Pd catalyst layer directly on the surface of the second colloidal CeO2 layer of the catalyst support.

[0073] Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. DETAILED DESCRIPTION

[0074] The exemplary embodiments are provided to make this disclosure thorough and fully convey its scope to those skilled in the art. Many specific details, such as examples of specific compositions, components, devices, and methods, are set forth to provide a full understanding of the embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be used, and the exemplary embodiments may be embodied in many different forms, and none of them should be construed as limiting the scope of the present disclosure. In some exemplary embodiments, well-known methods, well-known device structures, and well-known technologies are not described in detail.

[0075] The terms used herein are only for the purpose of describing exemplary embodiments and are not intended to be limiting. Unless the context clearly indicates otherwise, the singular forms "a", "an" and "the" used herein are intended to also include plural forms. The terms "comprising", "including" and "having" are inclusive and therefore specify the presence of specified features, elements, compositions, steps, integers, operations and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or combinations thereof. Although the open term "comprising" should be understood as a non-limiting term for describing and claiming the various embodiments described herein, in some aspects, the term alternative is understood to be a more restrictive and constrained term, such as "consisting of" or "substantially consisting of". Therefore, for any given embodiment that lists compositions, materials, components, elements, features, integers, operations and / or process steps, the present disclosure also clearly includes an embodiment consisting of or substantially consisting of these listed compositions, materials, components, elements, features, integers, operations and / or process steps. 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 that materially affect the basic and novel characteristics, but may include in the embodiment any compositions, materials, components, elements, features, integers, operations and / or process steps that do not materially affect the basic and novel characteristics.

[0076] Any method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the order discussed or illustrated, unless expressly specified as an order of performance. It is also to be understood that additional or alternative steps may be used unless otherwise indicated.

[0077] When a component, element, or layer is referred to as being "on," "engaged," "connected," or "coupled" to another element, element, or layer, it may be directly on, directly engaged, connected, or coupled to the other component, element, or layer, or there may be intervening elements or layers. Conversely, when an element is referred to as being "directly on," "directly engaged," "directly connected," or "directly coupled" to another element or layer, there may be no intervening elements or layers. Other words used to describe the relationship between elements should be interpreted in a similar manner (e.g., "between" vs. "directly between," "adjacent" vs. "directly adjacent," etc.). As used herein, the term "and / or" includes a combination of one or more of the related enumeration items.

[0078] Although the terms first, second, third, etc. may be used in this article to describe various steps, elements, components, regions, layers and / or sections, unless otherwise indicated, 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, terms such as "first", "second" and other ordinal terms do not imply order or sequence when used in this article. Therefore, the first step, element, component, region, layer or section discussed below can be referred to as the second step, element, component, region, layer or section without departing from the teachings of the exemplary embodiments.

[0079] For ease of description, spatially or temporally relative terms, such as "front," "back," "inside," "outside," "lower," "below," "lower," "upper," and the like, may be used herein to describe the relationship of one element or feature to one or more other elements or features as shown in the drawings. Spatially or temporally relative terms may be intended to encompass different orientations of the device or system in use or operation in addition to the orientation depicted in the drawings.

[0080] Throughout this disclosure, numerical values represent approximate measurements or range limits and include slight deviations from the given values and embodiments that have approximately the listed values, as well as embodiments that have exactly the listed values. Except for the examples provided at the end of the detailed description, all numerical values for parameters (such as amounts or conditions) in this specification (including the appended claims) should be understood to be modified by the term "approximately" in all cases, regardless of whether "approximately" actually appears before the numerical value. "Approximately" means that the specified numerical value allows for a certain slight imprecision (close to the accuracy of the value; approximately or reasonably close to the value; almost). If the imprecision provided by "approximately" is not understood in this ordinary sense in the art, "approximately" as used herein at least refers to the variation that may be caused by the common methods of measuring and using such parameters. For example, "approximately" may include less than or equal to 5%, optionally less than or equal to 4%, optionally less than or equal to 3%, optionally less than or equal to 2%, optionally less than or equal to 1%, optionally less than or equal to 0.5%, and in some aspects, optionally less than or equal to 0.1%.

[0081] Furthermore, disclosure of ranges includes disclosure of all values within the entire range and further subdivided ranges, including endpoints and subranges given within those ranges.

[0082] As used herein, unless otherwise indicated, the terms "composition" and "material" are used interchangeably to refer generally to a substance containing at least a preferred chemical constituent, element or compound, but may also contain additional elements, compounds or substances, including trace amounts of impurities.

[0083] Example embodiments will now be described more fully with reference to the accompanying drawings.

[0084] The present disclosure relates to a layered catalyst structure containing palladium (Pd), ceria (CeO2), and aluminum oxide (Al2O3) for purifying exhaust gas streams from combustion processes, and to a method for preparing the Pd / CeO2 / Al2O3 layered catalyst structure. The layered catalyst structure includes a catalyst support comprising an Al2O3 substrate, a first CeO2 layer deposited on the Al2O3 substrate, and a second colloidal CeO2 layer comprising a plurality of colloidal CeO2 particles deposited on the Al2O3 substrate above the first CeO2 layer. An atomic dispersion of palladium (Pd) is formed as a catalyst layer on the exposed surfaces of the high-surface-area colloidal CeO2 particles using a strong electrostatic adsorption technique, which can achieve a relatively high Pd atomic loading on the catalyst support. Furthermore, the strong electrostatic adsorption technique promotes the deposition of discrete Pd atoms on the catalyst support and inhibits the formation and / or aggregation of relatively large Pd particles on the catalyst support, which can help maximize the number of active Pd catalyst sites on the catalyst support without increasing the Pd loading.

[0085] In practice, the disclosed Pd / CeO2 / Al2O3 layered catalyst structure can help catalyze the conversion of unburned hydrocarbons (HC), carbon monoxide (CO), nitric oxide (NO) and nitrogen dioxide in an exhaust gas stream from a combustion process into carbon dioxide (CO2), nitrogen (N2) and water (H2O) before the exhaust gas stream is discharged into the surrounding environment.

[0086] Now refer to Figure 1 , preparing layered catalyst structure 10 ( Figure 7 ) may include preparing a catalyst support 12 ( Figure 4 ) and wherein a palladium (Pd) catalyst layer 14 ( Figure 7 ) to form a second step of the layered catalyst structure 10. In the first step, an aluminum oxide (Al2O3) substrate 18 defining an outer surface 20 may be provided. The Al2O3 substrate 18 is porous and provides the layered catalyst structure 10 with excellent thermal and mechanical stability. The Al2O3 substrate 18 may comprise an Al2O3 powder including a plurality of Al2O3 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 Al2O3 particles in the Al2O3 powder may 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 Al2O3 substrate 18 may have a particle size of about 80 m 2 / g BET surface area and approximately 0.6 mL / g pore volume.

[0087] like Figure 1 As shown in FIG, the Al2O3 substrate 18 may 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 Al2O3 substrate 18. The Ce-containing precursor solution 22 may include a cerium salt dissolved or dispersed in an aqueous medium. The cerium salt may include a cerium cation (Ce 3+ ) and nitrate (NO3 – ), sulfate (SO4 2– ), carbonate (CO3 2 − ), citrate, halide ions (such as F – or Cl – ), alkoxide, phenoxide, acetate, benzoate, oxalate, acetylacetonate and / or carboxylate counterions (anions). The aqueous medium may comprise water (H2O). The amount of cerium in the Ce-containing precursor solution 22 may be selected to achieve a target cerium loading on the Al2O3 substrate 18. In various aspects, the amount of cerium in the Ce-containing precursor solution 22 may comprise from 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 may comprise cerium in an amount that constitutes about 10% by weight of the Ce-containing precursor solution 22. The Al2O3 substrate 18 may be impregnated with the Ce-containing precursor solution 22 using a dry impregnation or incipient wetness technique, wherein the volume of the Ce-containing precursor solution 22 applied to the Al2O3 substrate 18 is substantially equal to the calculated pore volume of the Al2O3 substrate 18.

[0088] Now refer to Figure 2 , after applying the Ce-containing precursor solution 22 to the outer surface 20 of the Al2O3 substrate 18, the Al2O3 substrate 18 and the Ce-containing precursor solution 22 are subjected to a first heat treatment to remove the aqueous medium and anions of the cerium salt (e.g., by evaporation) and deposit a first CeO2 layer 24 on the outer surface 20 of the Al2O3 substrate 18. The first heat treatment may include heating the Al2O3 substrate 18 and the Ce-containing precursor solution 22 at a temperature of greater than or equal to about 550°C to less than or equal to about 1050°C in an oxygen (O2)-containing environment (e.g., air) for a duration of 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 Al2O3 substrate 18. In various aspects, the first thermal treatment may include heating the Al 2 O 3 substrate 18 and the Ce-containing precursor solution 22 in an O 2 -containing ambient at a temperature of approximately 800° C. for a duration of approximately 2 hours to form a first CeO 2 layer 24 on the outer surface 20 of the Al 2 O 3 substrate 18 .

[0089] The first CeO2 layer 24 is porous and extends substantially continuously on the outer surface 20 of the Al2O3 substrate 18. The first CeO2 layer 24 may have a thickness greater than or equal to about 30 m 2 / g to less than or equal to approximately 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. In various aspects, the impregnation and heat treatment steps can be repeated to achieve a target CeO 2 loading on the Al 2 O 3 substrate 18.

[0090] Now refer to Figure 3 After forming the first CeO2 layer 24, the first CeO2 layer 24 is impregnated with a cerium (Ce)-containing colloidal suspension 26, for example by applying the Ce-containing colloidal suspension 26 directly onto the first CeO2 layer 24 over the outer surface 20 of the Al2O3 substrate 18. The Ce-containing colloidal suspension 26 may comprise a plurality of colloidal CeO2 particles suspended in an aqueous medium. The colloidal CeO2 particles may consist essentially of CeO2 and have a D50 diameter of greater than or equal to approximately 5 nanometers to less than or equal to approximately 20 nanometers, or greater than or equal to approximately 10 nanometers to less than or equal to approximately 15 nanometers. The aqueous medium may comprise water (H2O) and, optionally, an acid, such as acetic acid, which may help maintain the colloidal CeO2 particles suspended in the aqueous medium of the Ce-containing colloidal suspension 26. The amount of colloidal CeO2 particles in the Ce-containing colloidal suspension 26 may be selected to achieve a target colloidal CeO2 particle loading. In various aspects, the colloidal CeO2 particles 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 various aspects, the first CeO2 layer 24 can be impregnated with the Ce-containing colloidal suspension 26 using a dry impregnation or incipient wetness impregnation technique, wherein the volume of the Ce-containing colloidal suspension 26 applied to the first CeO2 layer 24 is substantially equal to the calculated pore volume of the first CeO2 layer 24.

[0091] Now refer to Figure 4The Al2O3 substrate 18, the first CeO2 layer 24, and the Ce-containing colloidal suspension 26 are subjected to a second heat treatment to form a second colloidal CeO2 layer 28 directly on the first CeO2 layer 24. The Al2O3 substrate 18, the first CeO2 layer 24, and the second colloidal CeO2 layer 28 together constitute the catalyst support 12, and the second colloidal CeO2 layer 28 defines the outer surface 32 of the catalyst support 12. During the second heat treatment, the Al2O3 substrate 18, the first CeO2 layer 24, and the Ce-containing colloidal suspension 26 are heated to remove the aqueous medium (e.g., by evaporation) and deposit the second colloidal CeO2 layer 28 directly on the first CeO2 layer 24 above the Al2O3 substrate 18. The second thermal treatment may include heating the Al2O3 substrate 18, the first CeO2 layer 24, and the Ce-containing colloidal suspension 26 in an oxygen-containing O2 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 various aspects, the second thermal treatment may include heating the Al2O3 substrate 18, the first CeO2 layer 24, and the Ce-containing colloidal suspension 26 in the O2-containing environment at a temperature of approximately 550°C for a duration of approximately 2 hours to form the second colloidal CeO2 layer 28 on the Al2O3 substrate 18.

[0092] The second colloidal CeO2 layer 28 is porous and can be chemically and mechanically bonded to the first CeO2 layer 24, for example, via lattice matching. The second colloidal CeO2 layer 28 can consist essentially of CeO2 and can have a thickness of approximately 50 m 2 / g to approximately 180 m 2 / g and a pore volume of about 0.2 mL / g to about 1.5 mL / g. In various aspects, the impregnation and heat treatment steps can be repeated one or more times to achieve a target colloidal CeO 2 particle loading on the Al 2 O 3 substrate 18 .

[0093] The first CeO2 layer 24 can help the second colloidal CeO2 layer 28 adhere to the surface 20 of the Al2O3 substrate 18 and can extend substantially continuously above the surface 20 of the Al2O3 substrate 18. The second colloidal CeO2 layer 28 can be formed on the Al2O3 substrate 18 above the first CeO2 layer 24 and can cause the catalyst support 12 to have a relatively high number of surface defect sites. Defect sites on the outer surface 32 of the catalyst support 12 can occur, for example, at the locations of oxygen vacancies, interstitial atoms, lattice substitutions, dislocations, grain boundaries, or impurities in the second colloidal CeO2 layer 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 CeO2 layer 28 compared to the first CeO2 layer 24 can cause the second colloidal CeO2 layer 28 to have a relatively high surface defect density compared to the first CeO2 layer 24. Furthermore, it is believed that the most chemically reactive sites on the metal oxide surface occur at the locations of the surface defect sites. Thus, forming the second colloidal CeO 2 layer 28 above the first CeO 2 layer 24 on the Al 2 O 3 substrate 18 may increase the surface defect density of the catalyst support 12 and may provide a layered catalyst structure 10 having improved catalytic activity.

[0094] Now refer to Figure 5 After forming the second colloidal CeO2 layer 28, a palladium (Pd)-containing precursor solution 30 is applied to the outer surface 32 of the catalyst support 12, so that the Pd-containing precursor solution 30 at least partially impregnates the catalyst support 12 by infiltrating its pores. Figure 5As shown, a Pd-containing precursor solution 30 can be applied directly to a surface 32 of the catalyst support 12 such that the Pd-containing precursor solution 30 wets the surface of the second colloidal CeO2 layer 28 and infiltrates its pores. The Pd-containing precursor solution 30 can include a palladium salt dissolved or dispersed in an aqueous medium. The palladium salt can include palladium(II) nitrate, Pd(NO3)2; palladium(II) tetraammine dinitrate, [Pd(NH3)4](NO3)2; palladium(II) tetraammine dichloride, [Pd(NH3)4](Cl)2; palladium(II) tetraammine acetate, [Pd(NH3)4](CH3COO)2; and / or palladium(II) tetraammine sulfate, [Pd(NH3)4]SO4. The aqueous medium can include an aqueous ammonium hydroxide (NH4OH) solution, comprising an ammonium hydroxide compound dissolved or dispersed in water (H2O). The amount of palladium in the Pd-containing precursor solution 30 can be selected so as to provide a target Pd loading for the layered catalyst structure 10. The Pd-containing precursor solution 30 can be prepared by dissolving or dispersing the palladium salt in an aqueous solution of NH4OH. In various aspects, the palladium salt can be provided in the form of a hydrate complex, such as palladium (II) nitrate dihydrate (Pd(NO3)2·2H2O). The aqueous NH4OH solution can comprise 56.6% by weight of NH4OH and the remainder water (equivalent to 28 wt% NH3 in H2O). Without intending to be bound by theory, it is believed that the dissolution of the palladium salt in the aqueous NH4OH solution can result in the formation of a palladium salt in the Pd-containing precursor solution 30 that is covered by NO3 - 、Cl - 、CH3COO - and / or SO4 2- Anion-balanced positively charged [Pd(NH3)4] 2+ complex.

[0095] The Pd-containing precursor solution 30 is formulated to facilitate the separation of Pd using a strong electrostatic adsorption technique. 2+ ions on the outer surface 32 of the catalyst support 12. More specifically, the Pd-containing precursor solution 30 is formulated to promote the [Pd(NH3)4] 2+ Strong electrostatic interaction between the complex and the outer surface 32 of the catalyst support 12. To achieve this, the pH of the Pd-containing precursor solution 30 is controlled or adjusted so that the pH of the Pd-containing precursor solution 30 is greater than the zero charge point (PZC) of the second colloidal CeO2 layer 28 of the catalyst support 12. Figure 6As shown, when the Pd-containing precursor solution 30 is applied to the surface 32 of the catalyst support 12, the functional groups on the surface of the second colloidal CeO2 layer 28 are deprotonated and converted to negatively charged species 34 (shown as circles ○), resulting in a net negative charge on the surface 32 of the catalyst support 12. In various aspects, the functional groups on the surface 32 of the catalyst support 12 can include hydroxyl groups (-OH), and the hydroxyl functional groups can be deprotonated and converted to negatively charged -OH on the surface 32 of the catalyst support 12. - species. Without intending to be bound by theory, it is believed that the positively charged [Pd(NH3)4] 2+ The complex 36 (shown as a square) will be Coulombically attracted to the negatively charged species 34 on the surface of the second colloidal CeO2 layer 28 and electrostatically adsorbed to 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 the deprotonation of the functional groups and the formation of the negatively charged species 34 can be favored at the location of the surface defect sites in the second colloidal CeO2 layer 28.

[0096] In various aspects, the PZC of the second colloidal CeO2 layer 28 of the catalyst support 12 can be from about 6.7 to about 8.6 at a temperature of about 25°C. Therefore, in order to promote the [Pd(NH3)4] 2+ The strong electrostatic interaction between the complex and the surface 32 of the catalyst support 12 can control or adjust the pH of the Pd-containing precursor solution 30 so that the Pd-containing precursor solution 30 has a pH greater than or equal to about 11 to less than or equal to about 12 at a temperature of about 25°C.

[0097] In various aspects, the catalyst support 12 can be impregnated with the Pd-containing precursor solution 30 using a wet impregnation technique, a dry impregnation technique, or an incipient wetness technique. If a wet impregnation technique is used, the volume of the Pd-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 technique or an incipient wetness technique is used, the volume of the Pd-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.

[0098] Now refer to Figure 7 The catalyst support 12 and the Pd-containing precursor solution 30 are subjected to a third heat treatment to deposit the Pd catalyst layer 14 directly on the surface 32 of the catalyst support 12 (i.e., directly on the surface of the second colloidal CeO2 layer 28) and form the layered catalyst structure 10. During the third heat treatment, the catalyst support 12 and the Pd-containing precursor solution 30 can be heated to remove the aqueous medium and the anions of the palladium salt, decomposing the [Pd(NH3)4] 2+The complex 36 is formed, reaction byproducts are removed from the catalyst support 12 (e.g., by evaporation), and the Pd catalyst layer 14 is deposited directly on the surface 32 of the catalyst support 12. Chemical compounds that can be released from the catalyst support 12 and the Pd-containing precursor solution 30 in the form of gases or vapors during the third thermal treatment can include nitrogen (N2), nitrogen oxides (e.g., N2O, NO2, and / or NO), ammonia (NH3), and / or H2O.

[0099] The third thermal treatment may include heating the catalyst support 12 and the Pd-containing precursor solution 30 in an oxygen-containing O2 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 five hours. In various aspects, the third thermal treatment may include heating the catalyst support 12 and the Pd-containing precursor solution 30 in the O2-containing environment at a temperature of approximately 500°C for a duration of approximately two hours to form the Pd catalyst layer 14 directly on the surface 32 of the catalyst support 12.

[0100] The Pd catalyst layer 14 may include Pd deposited directly on the surface 32 of the catalyst support 12 (ie, on the surface of the second colloidal CeO 2 layer 28 ). 2+ ions and optionally a plurality of sub-nanometer sized Pd particles. Without intending to be bound by theory, it is believed that [Pd(NH3)4] 2+ Coulombic forces between the complex and the negatively charged species 34 on the catalyst support 12 can contribute to the [Pd(NH3)4] 2+ The complex 36 is anchored to the surface 32 of the catalyst support 12 at specific locations of the negatively charged species 34, such as specific locations of surface defect sites in the second colloidal CeO2 layer 28. Furthermore, during the third heat treatment, these strong electrostatic interactions can suppress the [Pd(NH3)4] 2+ The migration of the complex 36 on the surface 32 of the catalyst support 12 can promote the separation of the Pd 2+ ions onto the catalyst support 12, rather than forming and / or agglomerating relatively large Pd and / or PdO particles. In various aspects, when present, the sub-nanometer-sized Pd particles in the Pd catalyst layer 14 can have a D50 particle size of less than or equal to about one (1) nanometer.

[0101] The Pd catalyst layer 14 may consist essentially of palladium and may constitute greater than or equal to about 0.1% to less than or equal to about 5.0% by weight of the layered catalyst structure 10. For example, the Pd catalyst layer 14 may constitute greater than or equal to about 0.8% to less than or equal to about 3.5% by weight of the layered catalyst structure 10.

[0102] The second colloidal CeO2 layer 28 and the first CeO2 layer 24 may consist essentially of CeO2, and the second colloidal CeO2 layer 28 and the first CeO2 layer 24 may together 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 CeO2 layer 28 and the first CeO2 layer 24 may constitute about 30% by weight of the layered catalyst structure 10.

[0103] The layered catalyst structures 10 can be substantially spherical particles having a diameter of greater than or equal to 200 microns to less than or equal to 500 microns. A plurality of layered catalyst structures 10 can be positioned in the path of an exhaust gas stream from a combustion process to help catalyze the conversion of unburned hydrocarbons (HC), carbon monoxide (CO), nitric oxide (NO), and nitrogen dioxide in the exhaust gas stream to carbon dioxide (CO2), nitrogen (N2), and water (H2O). The plurality of layered catalyst structures 10 can be positioned in the path of the exhaust gas stream, for example, by filling a pipe or passageway with a fill volume of the layered catalyst structures 10 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 layered catalyst structures 10 in the path of the exhaust gas stream, the layered catalyst structures 10 can be screened so that, in practice, the layered catalyst structures 10 exhibit a D50 particle size of about 245 microns to about 450 microns.

[0104] The HC, CO, and NO conversion efficiency of the layered catalyst structure 10 can be evaluated by exposing a certain volume of the layered catalyst structure 10 to a simulated exhaust gas stream containing CO, NO, C3H6, and C3H8. The simulated exhaust gas 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 certain temperature range. The term "T50" refers to the temperature at which the layered catalyst structure 10 achieves 50% conversion efficiency. The layered catalyst structure 10 can have a T50 for CO oxidation of approximately 220°C, a T50 for NO reduction of approximately 273°C, a T50 for C3H6 oxidation of approximately 270°C, and a T50 for C3H8 oxidation of approximately 390°C.

[0105] Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) can be used to confirm and / or validate the isolated Pd 2+ ions on the surface of the second colloidal CeO2 layer 28 of the layered catalyst structure 10. A DRIFTS spectrum of CO adsorption collected at 25°C on a volume of the layered catalyst structure 10 exposed to a 1000 ppm CO feed can be depicted at 2141 cm -1 and 2097 cm -1The DRIFTS spectrum of NO adsorption collected on a volume of the layered catalyst structure 10 exposed to a feed of 1000 ppm NO at 25 °C can be depicted at 1836 cm -1 The presence of these bands indicates that the isolated Pd is adsorbed on the surface of the second colloidal CeO2 layer 28 of the layered catalyst structure 10. 2+ Carbonyl and nitrosyl groups on ions.

[0106] The above description of the embodiments is provided for the purpose of illustration and description. It is not intended to be exhaustive or to limit the present disclosure. Elements or features of a particular embodiment are generally not limited to that embodiment, but are, where applicable, interchangeable and applicable to a selected embodiment, even if not explicitly shown or described. They may also be varied in many ways. Such variations should not be considered a departure from the present disclosure, and all such modifications are intended to be included within the scope of the present disclosure.

Claims

1. A method for preparing a layered catalyst structure for purifying an exhaust gas stream, the method comprising: (a) applying a palladium-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 continuously on 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 palladium-containing precursor solution comprises a positively charged palladium complex in an aqueous medium and has a pH greater than a point of zero charge of the second colloidal ceria layer; and (b) heating the catalyst support and the palladium-containing precursor solution to evaporate the aqueous medium, decompose the positively charged palladium complex, and form an atomic dispersion of palladium ions on the outer surface of the catalyst support, The aqueous medium comprises an aqueous ammonium hydroxide solution, and the palladium-containing precursor solution has a pH of greater than or equal to 11 and less than or equal to 12.

2. The method of claim 1, wherein the positively charged palladium complex comprises a tetraamminepalladium(II) complex.

3. The method according to claim 1, wherein When the palladium-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 the positively charged palladium complex is electrostatically adsorbed onto the outer surface of the catalyst support.

4. The method of claim 3, wherein the positively charged palladium complex is electrostatically adsorbed onto the outer surface of the catalyst support at the locations of surface defect sites in the second colloidal ceria layer.

5. The method of claim 1, further comprising: dissolving or dispersing a palladium salt in an aqueous ammonium hydroxide solution to form a palladium-containing precursor solution of step (a), The palladium salt comprises at least one of palladium nitrate, tetraamminepalladium dinitrate, tetraamminepalladium dichloride, tetraamminepalladium acetate, and tetraamminepalladium sulfate, or a combination thereof.

6. The method of claim 1, wherein the catalyst support is impregnated with the palladium-containing precursor solution in step (a) using a wet impregnation technique or an incipient wetness technique.

7. The method of claim 1, wherein the catalyst support and the palladium-containing precursor solution are heated in step (b) at a temperature of greater than or equal to 350°C to less than or equal to 800°C.

8. The method of claim 1, wherein heating the catalyst support and the palladium-containing precursor solution in step (b) releases nitrogen, nitrogen oxides, ammonia and / or water gas or vapor.

9. A method for preparing a layered catalyst structure for purifying an exhaust gas stream, the method comprising the following steps in sequence: (a) applying a cerium-containing precursor solution to the outer surface of an alumina substrate; (b) heating the alumina substrate and the cerium-containing precursor solution at a first temperature to form a first ceria layer on the outer surface of the alumina substrate, the first ceria layer continuously extending on the outer surface of the alumina substrate; (c) applying a colloidal suspension containing ceria to the alumina substrate above the first ceria layer; (d) heating the alumina substrate, the first ceria layer, and the ceria-containing colloidal suspension at a second temperature to deposit colloidal ceria particles directly on the first ceria layer and form a catalyst support comprising the alumina substrate, the first ceria layer, and a second colloidal ceria layer overlying the first ceria layer on the alumina substrate; (e) applying a palladium-containing precursor solution to the outer surface of the catalyst support, the palladium-containing precursor solution comprising a positively charged palladium complex in an aqueous medium and having a pH greater than the point of zero charge of the second colloidal ceria layer; and (f) heating the catalyst support and the palladium-containing precursor solution at a third temperature to evaporate the aqueous medium, decompose the positively charged palladium complex, and form an atomic dispersion of palladium ions on the outer surface of the catalyst support, The aqueous medium comprises an aqueous ammonium hydroxide solution, and the palladium-containing precursor solution has a pH of greater than or equal to 11 and less than or equal to 12.

10. The method of claim 9, wherein the cerium-containing precursor solution of step (a) comprises a cerium salt in an aqueous medium, and wherein the aluminum oxide substrate and the cerium-containing aqueous precursor solution are heated in step (b) at a first temperature of greater than or equal to 550° C. to less than or equal to 1050° C. in an oxygen-containing environment.

11. The method of claim 9, wherein the ceria-containing colloidal suspension of step (c) comprises a plurality of colloidal ceria particles suspended in an aqueous medium, wherein the colloidal ceria particles have a D50 diameter of greater than or equal to 5 nm to less than or equal to 20 nm, and wherein the alumina substrate, the first ceria layer, and the ceria-containing colloidal suspension are heated in step (d) at a second temperature of greater than or equal to 350° C. to less than or equal to 800° C. in an oxygen-containing environment.

12. The method of claim 9, wherein the positively charged palladium complex comprises a tetraamminepalladium(II) complex.

13. The method of claim 9, wherein: When the palladium-containing precursor solution is applied to the outer surface of the catalyst support in step (e), a net negative charge is provided to the outer surface of the catalyst support, and the positively charged palladium complex is electrostatically adsorbed onto the outer surface of the catalyst support.

14. The method of claim 9, wherein the catalyst support and the palladium-containing precursor solution are heated in step (f) at a third temperature of greater than or equal to 350° C. to less than or equal to 800° C. in an oxygen-containing environment, and wherein heating the catalyst support and the palladium-containing precursor solution in step (f) releases gas or vapor of nitrogen, nitrogen oxides, ammonia and / or water.

15. A layered catalyst structure for purifying an exhaust gas stream, the layered catalyst structure comprising: a catalyst support comprising an alumina substrate, a first ceria layer disposed on and extending continuously over the alumina substrate, and a second colloidal ceria layer formed on the first ceria layer directly over the alumina substrate; and A palladium catalyst layer comprising an atomic dispersion of palladium ions electrostatically adsorbed to the outer surface of a catalyst support at the locations of surface defect sites in a second colloidal ceria layer, wherein the raw materials for preparing the palladium catalyst comprise an aqueous medium and a palladium-containing precursor solution, wherein the aqueous medium includes an aqueous ammonium hydroxide solution, and the palladium-containing precursor solution has a pH greater than or equal to 11 and less than or equal to 12.

16. The layered catalyst structure of claim 15, wherein the first ceria layer has a thickness greater than or equal to 30 m 2 / g to less than or equal to 150 m 2 / g and a pore volume greater than or equal to 0.2 mL / g to less than or equal to 1.5 mL / g, wherein the second colloidal ceria layer has a pore volume greater than or equal to 50 m 2 / g to less than or equal to 180 m 2 / g and a pore volume of greater than or equal to 0.2 mL / g to less than or equal to 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.

17. The layered catalyst structure according to claim 15, wherein The first ceria layer and the second colloidal ceria layer together constitute greater than or equal to 5% to less than or equal to 40% by weight of the layered catalyst structure, and wherein the palladium catalyst layer constitutes greater than or equal to 0.1% to less than or equal to 5% by weight of the layered catalyst structure.

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