WO2024145156A1 - A multi-layer article with platinum zeolite for improved selective oxidation of ammonia - Google Patents

Abstract

The present disclosure is directed to a multi-layered catalytic article (10) effective in selective oxidation of ammonia to nitrogen gas, the catalytic article comprising at least a top layer (11) comprising a selective catalytic reduction catalyst, a bottom layer (13) comprising an ammonia oxidation catalyst, and a substrate (15). The ammonia oxidation catalyst comprises platinum on a zeolite support. The present disclosure is also directed to a method of making the catalytic article, an emission treatment system for selectively oxidizing ammonia from an exhaust gas stream with the catalytic article, and a method for treating an exhaust stream containing ammonia with the emissions treatment system.

Description

A MULTI-LAYER ARTICLE WITH PLATINUM ZEOLITE

FOR IMPROVED SELECTIVE OXIDATION OF AMMONIA

[0001] The present disclosure relates to multi-layered catalytic articles that are effective in the selective oxidation of ammonia (NH3) to nitrogen gas. For example, the catalytic articles comprise a top layer that comprising an ammonia selective catalytic reduction (SCR) catalyst, a bottom layer comprising an ammonia oxidation (AMOx) catalyst, and a substrate, wherein the bottom layer AMOx catalyst comprises platinum on a zeolite support. In the present disclosure, the zeolite support of the AMOx catalyst may comprise a zeolite having a silica-to- alumina ratio in the range of about 5: 1 to about 100: 1 and/or the bottom layer may further comprise a catalytically-inert metal oxide. The present disclosure is also directed to methods of making catalytic articles, emission treatment systems for selectively oxidizing ammonia from an exhaust gas stream with catalytic articles, and methods for treating an exhaust stream containing ammonia.

[0002] Diesel engine exhaust is a heterogeneous mixture which contains not only gaseous emissions such as carbon monoxide (“CO”), unburned or partially burned hydrocarbons or oxygenates thereof (“HC”) and nitrogen oxides (“NOx”), but also condensed phase materials (liquids and solids) which constitute the so-called particulates or particulate matter. Often, catalyst compositions and substrates on which the compositions are disposed are provided in diesel engine exhaust systems to convert certain or all of these exhaust components to innocuous components. For example, diesel exhaust systems can contain one or more of a diesel oxidation catalyst, a soot filter and a catalyst for the abatement of NOx.

[0003] A proven NOx abatement technology applied to stationary sources with lean exhaust conditions is ammonia Selective Catalytic Reduction (SCR). In this process, NOx (such as nitrogen oxide and nitrogen dioxide) is catalytically reduced with an ammonia reductant in the presence of excess oxygen to form dinitrogen (N2, nitrogen gas) and steam over a catalyst, which is typically composed of base metals. This technology is capable of NOx reduction greater than 90%, and thus it represents one of the best approaches for achieving aggressive NOx abatement goals. SCR provides efficient conversions of NOx as long as the exhaust temperature is within the active temperature range of the catalyst.

[0004] Reduction of NOx species to N2 using ammonia is of interest for meeting NOx emission targets in lean burn engines. A consequence of using ammonia as a reductant is that under conditions of incomplete conversion or exhaust temperature upswings, the ammonia can slip from the exhaust of the vehicle. To avoid slippage, a sub-stoichiometric quantity of ammonia can be injected into the exhaust stream, but there will be decreased N0_x conversion. Alternatively, the ammonia can be overdosed into the system to increase N0_x conversion rate, but the exhaust then needs to be further treated to remove excess or slipped ammonia. Even at a sub-stoichiometric dosages, an increase in exhaust temperature may release ammonia stored on the NOx abatement catalyst, giving an ammonia slip. Conventional precious-metal based oxidation catalysts such as platinum supported on alumina can be very efficient at ammonia removal, but they produce considerable N2O and NOx as undesired side products instead of the desired N2 product.

[0005] Ammonia slip from the ammonia SCR catalyst presents a number of problems. The odor threshold for NH3 is 20 ppm in air. Eye and throat irritation are noticeable above 100 ppm, skin irritation occurs above 400 ppm, and the Immediately Dangerous to Life or Health (IDLH) is 500 ppm in air. NH3 is caustic, especially in its aqueous form. Condensation of NH3 and water in cooler regions of the exhaust line downstream of the exhaust catalysts will give a corrosive mixture.

[0006] Therefore, it is desirable to eliminate or substantially reduce the ammonia before it can pass into the tailpipe. An ammonia oxidation (AMOx) catalyst may be employed for this purpose, with the objective to convert the excess ammonia to N2. The ideal catalyst for ammonia oxidation will be able to convert ammonia at all temperatures where ammonia slip occurs in the vehicles driving cycle, and will produce minimal NOx by-products. This latter requirement is particularly critical since any production of NO or NO2 by the AMOx catalyst decreases the effective NOx conversion of the exhaust treatment system. The AMOx catalyst should also produce minimal N2O, which is a potent greenhouse gas.

[0007] These AMOx catalysts should be stable against the long term thermal, chemical, and physical stress of normal vehicle operation, which includes temperatures up to about 450° C for a typical diesel application. In addition, a vehicle exhaust system may operate for short periods at temperatures above 800° C, for example during the thermal regeneration of a particulate filter. It is important that an AMOx catalyst be stable to these acute thermal stressors as well. For this reason, accelerated aging conditions are identified that mimic the cumulative effects of these long-term and acute stressors on the catalyst activity. Such an aging condition involves exposure of the AMOx catalyst to temperatures of between 700 and 800° C for between 5 and 50 hrs in the presence of up to about 10% water vapor in air.

[0008] AMOx catalysts comprised of Pt supported on a metal oxide such as y-alumina are the most active NH3 oxidation catalysts known, exhibiting NH3 light-off temperatures below 250° C. They are highly effective for the removal of NHi from a gas stream under oxidizing conditions. However, the selectivity to N2 is not high enough to be applicable in a vehicle emission system. At 250° C, N2 selectivity is less than 50%, with the primary co-product of NH3 oxidation being N2O. As the temperature increases, N2 selectivity decreases. At 450° C, a supported Pt catalyst gives N2 selectivity less than 20%, with the majority of the products consisting of NO and NO2. Hence, there is a desire for ammonia oxidation catalysts with activity comparable to the supported Pt catalysts but with N2 selectivity greater than 60% across the temperature range from 250 to 450° C, which is the relevant temperature range for a diesel vehicle driving cycle.

[0009] The present disclosure provides an advanced multi-layered catalytic article for the selective oxidation of ammonia to nitrogen gas. The top layer of the catalytic article comprises an ammonia selective catalytic reduction (NH3-SCR) catalyst. The bottom layer comprises an ammonia oxidation (AMOx) catalyst and is positioned between the top layer and a substrate, the substrate having an inlet end, an outlet end, and an axial length. The catalytic article may have additional layers in between the top and bottom layer or between the bottom layer and the substrate. In particular, it was discovered that when the bottom layer AMOx catalyst comprises platinum on a zeolite support at a high coverage level instead of on the traditional aluminum oxide support and/or at a traditional coverage level, one could significantly lower the ammonia light-off temperature to meet key ammonia regulations or targets while maintaining a desired level of N2O selectivity. The light-off temperature is defined as the temperature at which 50% conversion efficiency is achieved. The improved performance was achieved by loading the platinum metal onto the zeolite support at various specific loading ranges and at various specific mass ratios, such as ranging from about 1 g/ft³ to about 10 g/ft³ for the loading range and from about 1 : 1000 to about 1 :20 for the mass ratio of Pt to zeolite support. The improved performance was also achieved using a variety of zeolite structures, including but not limited to those with framework type CHA and FAU.

[0010] These and other features, aspects, and advantages of the disclosure will be apparent from a reading of the following detailed description together with the accompanying drawings, which are briefly described below. Other aspect and advantages of the disclosed subject matter will become apparent from the following.

BRIEF DESCRIPTION OF THE FIGURES

[0011] To provide an understanding of aspects of the disclosure, reference is made to the appended drawings. The drawings are exemplary only and should not be construed as limiting the disclosure. The disclosure described herein is illustrated by way of example and not by way of limitation in the appended drawings. For simplicity and clarity of illustration, features illustrated in the drawings are not necessarily drawn to scale. For example, the dimensions of some features may be exaggerated relative to other features for clarity.

[0012] FIG. 1 shows a multi-layer structure 10 of an aspect of a catalytic article of the present disclosure with an SCR catalyst in the top layer 11, and an AMOx catalyst in the bottom layer 13 wherein the bottom layer is positioned between the top layer and a substrate 15.

[0013] FIG. 2 shows a line chart comparing ammonia conversion as a function of temperature for AMOx catalyst compositions of the present disclosure with platinum supported on a CHA support or on an aluminum oxide support, at high platinum coverage, to a standard reference Pt catalyst on an aluminum oxide support.

[0014] FIG. 3 shows a bar chart comparing Tso for AMOx catalyst compositions of the present disclosure with platinum supported on a CHA support or on an aluminum oxide support, at high platinum coverage, to a standard reference Pt catalyst on an aluminum oxide support.

[0015] FIG. 4 shows a bar chart comparing peak N2O yield for AMOx catalyst compositions of the present disclosure with platinum supported on a CHA support or on an aluminum oxide support, at high platinum coverage, to a standard reference Pt catalyst on an aluminum oxide support.

[0016] FIG. 5 shows a line chart comparing ammonia conversion as a function of temperature for AMOx catalyst compositions of the present disclosure with platinum supported on a CHA support, at different platinum coverage, to a standard reference Pt catalyst on an aluminum oxide support.

[0017] FIG. 6 shows a bar chart comparing T50 for AMOx catalyst compositions of the present disclosure with platinum supported on a CHA support, at different platinum coverage, to a standard reference Pt catalyst on an aluminum oxide support.

[0018] As used herein, “a” or “an” entity refers to one or more of that entity, e.g., “a support” refers to one or more supports or at least one support unless stated otherwise. As such, the terms “a” (or “an”), “one or more”, and “at least one” are used interchangeably herein.

[0019] As used herein, the term “about” means approximately, in the region of, roughly, or around. When the term “about” is used in conjunction with a numerical range, it modifies that range by extending the boundaries above and below the numerical values set forth. In general, the term “about” is used herein to modify a numerical value above and below the stated value by a variance of 5%. All numeric values are modified by the term “about whether or not explicitly indicated. Numeric values modified by the term “about” include the specific identified value. For example, “about 100” means a number ranging from 95 to 105, including 95, 100, and 105. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein.

[0020] As used herein, the term “catalyst” or “catalyst material” or catalytic material” refers to a material that promotes a reaction.

[0021] As used herein, the term “catalytic article” refers to an element that is used to promote a desired reaction. For example, a catalytic article may comprise a washcoat containing a catalyst species, e.g., a catalyst composition, on a substrate, e.g., a honeycomb substrate. [0022] As used herein, the terms “nitrogen oxides” or “NOx” refers to oxides of nitrogen.

Exemplary nitrogen oxides include oxide compounds, such as NO and NO2.

[0023] As used herein, “support” of a “catalyst” or “catalyst material” or “catalytic material” refers to material that receives the “catalyst” or “catalyst material” or “catalytic material” through precipitation, association, dispersion, impregnation, or other suitable means.

[0024] As used herein, the term “catalytically-inert metal oxide” refers to a metal oxide that is chemically stable and cannot be easily reduced or oxidized and is not active for the oxidation of ammonia at temperature less than 350°C, including, but not limited to, AI2O3, SiCh, ZrCh, TiCh, La2Os, Y2O3 or combinations thereof.

[0025] As used herein, the term “promoted” refers to use of a “promoter metal” that is intentionally added to a support, e.g., a zeolite, typically through ion exchange, as opposed to impurities inherent in the zeolite. The result is a metal that is supported on and/or in the support. In some aspects, promoter metals that can be used to prepare promoted zeolites of the disclosed compositions include, but are not limited to, iron (Fe) and copper (Cu). In some aspects, both copper and iron may be present as promoter metals. In some aspects, the zeolite is promoter free.

[0026] As used herein, the term “substantially free of promoter-ions” refers to a zeolite that does not include a “promoter metal.” In other words, no metal is intentionally added to the zeolite through ion exchange, as opposed to impurities inherent in the zeolite. In some aspects, promoter metals that are excluded from these zeolites of the disclosed catalyst compositions include, but are not limited to, metals from groups 3-12 of the periodic table, with the exception of Pt. In some aspects, promoter metals that are excluded from these zeolites include, but are not limited to, iron (Fe) and copper (Cu).

[0027] As used herein, the term “SCR catalyst” refers to an ammonia selective catalytic reduction catalyst that is effective to selectively catalyze a reduction of NOx to nitrogen gas in the presence of an ammonia reductant. As used herein, “ammonia reductant” as a reductant refers to ammonia in various forms, including anhydrous ammonia, aqueous ammonia, and ammonia precursors, such as urea, ammonium carbonate, ammonium carbamate, ammonium hydrogen carbonate, and ammonium formate. In some aspects, the ammonia reductant is urea. [0028] As used here, “substantially free” means “not intentionally added” and having only trace impurity and/or inadvertent amounts. For instance, it means less than 2 wt. % (weight %), less than 1.5 wt. %, less than 1.0 wt. %, less than 0.5 wt. %, less than 0.25 wt. %, less than 0.10 wt.%, or less than 0.05 wt. %, less than 0.01 wt. %, based on the weight of the indicated total composition.

[0029] As used herein, the term “substrate” refers to the monolithic object onto which a catalyst composition is placed, typically containing a plurality of channels extending from the inlet end to the outlet end and having a surface upon which a washcoat can be coated.

[0030] As used herein, the term “zeolite” refers to a specific example of a molecular sieve, including silicon and aluminum atoms. Zeolites are crystalline materials having rather uniform pore sizes which, depending upon the type of zeolite and the type and amount of cations included in the zeolite lattice, range from about 3 to 10 Angstroms (A) in diameter. Generally, a zeolite has an open 3 -dimensional framework structure composed of comer-sharing TO4 tetrahedra, where T is Al or Si, or optionally P. Cations that balance the charge of the anionic framework are loosely associated with the framework oxygens, and the remaining pore volume is filled with water molecules. The non-framework cations are generally exchangeable, and the water molecules removable.

[0031] The term “hydrogen-form zeolite” or “acidic zeolite” refers to a protonated zeolite (H-zeolite) or a zeolite which can be converted to an H-zeolite, such as NFL-zeolite. The term “H-Zeolite” refers to a zeolite having more than 90% exchangeable sites as protons (H+). [0032] As used herein, the terms “silica-to-alumina ratio” and “SAR” refer to the mole ratio of SiCh to AI2O3, calculated by the ratio mol SiCh/mol AI2O3.

Catalytic Article

[0033] In the present disclosure, a multi-layered catalytic article 10 is provided that is effective for the selective oxidation of ammonia to nitrogen gas. The catalytic article comprises a top layer 11, a bottom layer 13, and a substrate 15. The top layer 11 of the catalytic article comprises an ammonia selective catalytic reduction catalyst. The bottom layer 13 comprises an ammonia oxidation catalyst and is positioned between the top layer and a substrate 15, the substrate having an inlet end, an outlet end, and an axial length.

[0034] Top Laver [0035] The top layer 11 of the catalytic article comprises an ammonia selective catalytic reduction (SCR) catalyst. The SCR catalyst may comprise, or consist essentially of, a metal oxide-based SCR catalyst formulation, a molecular sieve-based SCR catalyst formulation, or mixture thereof.

[0036] In some aspects, the SCR catalyst of the present disclosure may comprise a metal ion- exchanged molecular sieve containing at least one metal component. In some aspects, the SCR catalyst is Cu-promoted zeolite or a Fe-promoted zeolite.

[0037] The molecular sieves of the SCR catalyst refer to support materials such as zeolites and other framework materials (e.g., isomorphously substituted materials), which may be in particulate form, and in combination with one or more promoter metals, used as catalysts. Molecular sieves are materials based on an extensive three-dimensional network containing generally tetrahedral type sites and having a substantially uniform pore distribution, with the average pore size being no larger than 15 Angstroms (A). The pore sizes are defined by the diameter of the largest diffusible sphere. According to some aspects, it will be appreciated that by defining the molecular sieves by their structure type, it is intended to include the structure type and any and all isotypic framework materials such as SAPO, ALPO, and MeAPO materials having the same structure type as the zeolite materials.

[0038] When the molecular sieve is a zeolite, the structure type of the zeolite may vary. In some aspects, the zeolite has a structure chosen from the framework type selected from AEI, AFT, AFX, AVL, BEA, CHA, DDR, EAB, EEI, ERI, FAU, FER, IFY, IRN, KFI, LEV, LTA, LIN, MOR, MER, MFI, MWF, NPT, PAU, RHO, RTE, RTH, SAS, SAT, SAV, SFW, TSC, UFI, and combinations thereof.

[0039] The metal component is intentionally added to the molecular sieves to enhance the catalytic activity compared to molecular sieves that do not have a metal intentionally added. These metals are often referred to as promoter metals and are added to the molecular sieve using ion-exchange processes or incipient wetness processes. Therefore, these ion-exchanged molecular sieves are often referred to as “metal -promoted” molecular sieves. In order to promote the SCR of nitrogen oxides, in some aspects, a suitable metal is exchanged into the molecular sieve component.

[0040] In some aspects, the molecular sieve may be subsequently ion-exchanged with one or more metals selected from alkali metals, alkaline earth metals, transition metals in Groups IIIB, IVB, VB, VIB VIIB, VIIIB, IB, and IIB, Group IIIA elements, Group IVA elements, lanthanides, actinides and a combination thereof. In some aspects, the molecular sieve of one or more embodiments may be subsequently ion-exchanged with one or more promoter metals such as copper (Cu), cobalt (Co), nickel (Ni), lanthanum (La), manganese (Mn), iron (Fe), vanadium (V), silver (Ag), and cerium (Ce), neodymium (Nd), praseodymium (Pr), titanium (Ti), chromium (Cr), zinc (Zn), tin (Sn), niobium (Nb), molybdenum (Mo), hafnium (Hf), yttrium (Y), and tungsten (W).

[0041] In some aspects, the metal component is a platinum group metal (PGM) selected from platinum (Pt), palladium (pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), osmium (Os), and combinations thereof. The terms platinum, palladium, rhodium, iridium, ruthenium, osmium and the like refer to the respective PGM compound, complex, or the like which, upon calcination or use of the catalyst, decomposes or otherwise converts to a catalytically active form, usually the metal or the metal oxide. For example, the metal component may be selected from platinum, palladium, rhodium, iridium, and combinations thereof. In some aspects the metal component is either platinum or palladium or combination of platinum and palladium. In some aspects, the metal component is not a PGM.

[0042] The concentration of metal component present in the metal ion-exchanged molecular sieve can vary but will typically be from about 0.1 wt. % to about 15 wt. % relative to the weight of the ion-exchanged molecular sieve, calculated as metal oxide. Likewise, the concentration of any optionally additional metal present in the metal ion-exchanged molecular sieve can also vary, but will typically be from about 0.1 wt. % to about 15 wt. % relative to the weight of the ion-exchanged molecular sieve, calculated as the metal oxide.

[0043] Alternatively, the SCR catalyst of the present disclosure may comprise a metal oxidebased SCR catalyst formulation. In some aspects, the metal oxide-based SCR catalyst formulation comprises oxides of vanadium (such as V2O5) or tungsten (WO3) or a mixture thereof supported on a refractory oxide. In some aspects, the SCR catalyst is a vanadia SCR. [0044] The refractory oxide may be selected from oxides of Al, V, W, Ti, Cu, Fe, Ni, Mn, Ce, La, Pr, Zn, Nb, Zr, Mo, Sn, Si, Ca, Y, and combinations thereof. Possible oxide supports include AI2O3, SiCh, SnCh, CeCh, ZrCh, MgO, La2Ch, CaO, Y2O3, TiCh, SiCh, FeOx, and MnOx. In some aspects, the oxide support is selected from TiCh, CeCh, ZrCh, AI2O3, and SiCh. [0045] When the refractory oxide is titania (e.g. TiCh), the concentration of vanadium oxide may range from about 0.5 to about 6 wt % and/or the concentration of tungsten oxide may range from about 5 to about 20 wt % (of the metal oxide based SCR formulation). When the refractory oxide is ceria (e.g. CeCh), the concentration of the vanadium oxide may range from about 0.1 to about 9 wt % and/or the concentration of tungsten oxide may range from about 0.1 to about 9 wt % (of the metal oxide based SCR formulation).

[0046] Bottom Laver [0047] The bottom layer 13 of the catalytic article comprises an ammonia oxidation (AMOx) catalyst comprising platinum on a zeolite support. The AMOx catalyst is effective in the oxidation of ammonia to N2, NO, NO2, or N2O.

[0048] In some aspects, the platinum is added to the zeolite support by a cation-exchange process of a platinum complex precursor. In some aspects, the platinum is added to the zeolite support by an incipient wetness impregnation process of a platinum complex precursor. Exemplary platinum precursors can be selected from tetraammineplatinum (II) salts, tetraammineplatinum (IV) salts, tetrachloroplatinum (II) salts, hexachloroplatinum (IV) salts, platinum hydroxides, platinum hydrates, platinum bis(acetylacetonate)s, and combinations thereof. In some embodiments, the platinum is derived from platinum nitrate, tetraamineplatinum nitrate, chloroplatinic acid, colloidal platinum nanoparticles, platinum amine complexes, or a combination thereof.

[0049] In some aspects, the zeolite support has a zeolite structure chosen from AEI, AFT, AFX, AVL, BEA, CHA, DDR, EAB, EEI, ERI, FAU, FER, IFY, IRN, KFI, LEV, LTA, LIN, MOR, MER, MFI, MWF, NPT, PAU, RHO, RTE, RTH, SAS, SAT, SAV, SFW, TSC, UFI, and combinations thereof. In some aspects, the zeolite support has a zeolite structure chosen from CHA, FAU, and combinations thereof. In some aspects, the zeolite support is an acidic zeolite. In some aspects, the acidic zeolite is a hydrogen-form zeolite.

[0050] In some aspects, the AMOx of the present disclosure has no metal other than the Pt intentionally added to the zeolite support. For example, palladium (Pd), rhodium (Rh), and gold (Au) may impair the effectiveness of the AMOx. In some aspects, the zeolite support to which the Pt is added is substantially free of promoter-ions. In some aspects, the zeolite support for the AMOx catalyst is substantially free of transition metals from groups 3-12 of the periodic table, with the exception of Pt. In some aspects, the zeolite support is substantially free of iron and copper. In some aspects, the zeolite support has less than about 0.10 % by weight of transition metals from groups 3-12 (excluding Pt), such as iron and/or copper. The zeolite support for the AMOx catalyst may have from about 0.01% to about 0.02% by weight of transition metals from groups 3-12 (excluding Pt), such as from about 0.01% to about 0.10% by weight of transition metals from groups 3-12 (excluding Pt).

[0051] Zeolites generally have a silica-to-alumina ratio of 2 or greater. For example, zeolites may have a silica-to-alumina ratio in the range of about 2: 1 to about 100: 1. In some aspects, the zeolite of the AMOx catalyst has a silica-to-alumina ratio in the range of about 5:1 to about 95: 1. In some aspects, the zeolite of the AMOx catalyst has a silica-to-alumina ratio in the range of about 10: 1 to about 50: 1. The zeolite of the AMOx catalyst may have a silica-to- alumina ratio greater than about 10: 1, greater than about 20: 1, greater than about 30: 1, greater than about 40: 1, or greater than about 50: 1. The zeolite of the AMOx catalyst may have a silica- to-alumina ratio less than about 100: 1, less than about 90: 1, less than about 80: 1, less than about 70: 1, or less than about 60: 1. In some aspects, the zeolite has a silica-to-alumina ratio in the range of about 20: 1 to about 90: 1, such as about 20: 1 to about 40: 1. The lower the silica- to-alumina ratio in a zeolite, the higher acid site density a zeolite would provide.

[0052] Additionally, acid sites are sites where a zeolite can store ammonia. A higher site acid density provided by a lower silica-to-alumina ratio would further increase the ammonia storage capability of the zeolite. Ammonia storage at 200°C of the zeolite of the AMOX catalyst with a silica-to-alumina ratio in the range of about 5: 1 to about 100: 1 of the present disclosure may be less than about 5 mmol NH3 per g of zeolite support. In some aspects, the zeolite of the AMOx catalyst has an ammonia storage in the range of about 0.1 mmol NH3 per g of zeolite support to about 10 mmol NH3 per g of zeolite support at 200 °C. In some aspects, the zeolite of the AMOx catalyst has an ammonia storage in the range of about 0.3 mmol NH3 per g of zeolite support to about 5 mmol NH3 per g of zeolite support at 200 °C. In some aspects, ammonia storage of the zeolite support of the AMOX catalyst may be greater than about 0.2 mmol NH3 per g of zeolite support, greater than about 0.4 mmol per g of zeolite support, greater than about 0.8 mmol per g of zeolite support, or greater than about 1.5 mmol per g of zeolite support. In some aspects, the zeolite support of the AMOX catalyst has an ammonia storage in the range of about 0.5 mmol per g of zeolite support to about 3 mmol per g of zeolite support. [0053] Furthermore, a lower silica-to-alumina ratio would typically lead to a less hydrothermally stable zeolite. Hydrothermal stability is an important consideration in the design of SCR and AMOx catalysts due to the high temperature of exhaust gases from a vehicle. Therefore, balancing the advantages and disadvantages of the silica-to-alumina ratio to obtain a zeolite support appropriate for an AMOx catalyst is not a straightforward task. [0054] In traditional AMOx catalysts, the loading of metal on the support is selected so that there is a low concentration of metal relative to the surface area of the support. In contrast, the AMOx catalyst of the present disclosure has a metal loading on the support such that there is high concentration of metal relative to the surface area of the support. While not being bound by any particular theory, it is believed that whereas a low concentration of metal relative to the surface area of the support yields the desired small metal particles, a high concentration of metal relative to the surface area of the support yields large metal particles with unexpectedly improved performance as a AMOx catalyst. This unexpected, improved performance is further highlighted by the generally accepted paradigm that catalysts with greater surface area to volume ratios (i.e., smaller particles) have enhanced catalytic activity compared to catalysts with lower surface area to volume ratios (i.e., larger particles).

[0055] In some aspects, the mass ratio of Pt to zeolite support for the AMOx catalyst ranges from about 1 : 1000 to about 1 :20. For example, the mass ratio of Pt to zeolite support for the AMOx catalyst ranges from about 1 :500 to about 1 :50. The lower end of the range can be about 1 :500, about 1 :600, about 1 :700, about 1 :800, about 1 :900, or about 1 : 1000. The upper end of the range can be about 1 :20, about 1 :30, about 1 :50, about 1 : 100, about 1:200, about 1:300, about 1 :400, or about 1 :500.

[0056] In some aspects, the platinum loading for the AMOx catalyst may be from about from about 1 g/ft³ to about 10 g/ft³ with respect to the total volume of the AMOx catalyst substrate. For example, the platinum loading for the AMOx catalyst may be from about from about 2 g/ft³ to about 8 g/ft³ with respect to the total volume of the AMOx catalyst substrate. The lower end of the range can be about 1 g/ft³, about 2 g/ft³, about 3 g/ft³, about 4 g/ft³, about 5 g/ft³, about 6 g/ft³, about 7 g/ft³, about 8 g/ft³, or about 9 g/ft³ with respect to the total volume of the AMOx catalyst substrate. The upper end of the range can be about 10 g/ft³, about 9 g/ft³, about 8 g/ft³, about 7 g/ft³, about 6 g/ft³, about 5 g/ft³, about 4 g/ft³, about 3 g/ft³, or about 2 g/ft³ with respect to the total volume of the AMOx catalyst substrate.

[0057] In some aspects, the zeolite support has a total surface area that is measured by N2 physisorption using the Brunauer-Emmet-Teller (BET) method. In some aspects the total surface area ranges from about 100 m²/g to about 1000 m²/g. In some aspects, the total surface are may be from 200 m²/g to about 800 m²/g. In other aspects, the total surface area may range from about 300 m²/g to about 600 m²/g. The lower end of the range can be about 100 m²/g, about 200 m²/g, or about 300 m²/g. The upper end of the range can be about 1000 m²/g, about 900 m²/g, about 800 m²/g, about 700 m²/g, or about 600 m²/g.

[0058] In some aspects, the density of platinum per unit total surface area of the zeolite support ranges from about 3 per 1000 nm² to about 1000 per 1000 nm². In some aspects, the density of platinum per unit total surface area of the zeolite support ranges from about 5 per 1000 nm² to about 1000 per 1000 nm². For example, the density may range from about 5 per 1000 nm² to about 500 per 1000 nm². The density may range from 10 per 1000 nm² to about 200 per nm². The lower end of the range can be about 3 per 1000 nm², about 6 per 1000 nm², about 15 per 1000 nm², or about 60 per 1000 nm². The upper end of the range can be about 1000 per 1000 nm², about 800 per 1000 nm², about 600 per 1000 nm², about 400 per 1000 nm², about 200 per 1000 nm², or about 100 per 1000 nm². [0059] In some aspects, the bottom layer further comprises a catalytically-inert metal oxide that is added after formation of the AMOx catalyst. Accordingly, the bottom layer may comprise, consist of, or consist essentially of the platinum supported on a zeolite support, as described herein, and optionally one or more catalytically-inert metal oxides. The catalytically- inert metal oxide may be present to reach a desired final platinum loading weight percentage for the bottom layer and/or a final solids concentration for the bottom layer composition to improve the application of the bottom layer composition to the support. In some aspects, the catalytically- inert metal oxide may be chosen from AI2O3, SiCh, ZrCh, TiCh, La2Ch, Y2O3, and combinations thereof. In some aspects, the catalytically-inert metal oxide may be chosen from AI2O3, SiCh, ZrCh, and combinations thereof.

[0060] Substrate

[0061] The catalytic article of the present disclosure comprises a substrate 15 upon which the top, the bottom, and any additional layers of the present disclosure disposed thereon. In some aspects, the layers are individually prepared and coated on a substrate. Useful substrates are 3- dimensional, having a length and a diameter and a volume similar to a cylinder. The shape does not necessarily have to conform to a cylinder. The length is an axial length defined by an inlet end and an outlet end.

[0062] According to some aspects, the substrate for the catalytic article may be constructed of any material typically used as a support for automotive catalysts and will typically comprise a metal or ceramic honeycomb structure. The substrate typically provides a plurality of wall surfaces upon which a wash coat composition, for example, is applied and adhered, thereby acting as a substrate for the catalyst composition.

[0063] Ceramic substrates may be made of any suitable refractory material, e.g., cordierite, cordierite-a-alumina, aluminum titanate, silicon titanate, silicon carbide, silicon nitride, zircon mullite, spodumene, alumina-silica-magnesia, zircon silicate, sillimanite, a magnesium silicate, zircon, petalite, a-alumina, an aluminosilicate and the like.

[0064] Substrates may also be metallic, comprising one or more metals or metal alloys. A metallic substrate may include any metallic substrate, such as those with openings or “punch- outs” in the channel walls. The metallic substrates may be employed in various shapes such as pellets, compressed metallic fibers, corrugated sheet or monolithic foam. Specific examples of metallic substrates include heat-resistant, base-metal alloys, especially those in which iron is a substantial or major component. Such alloys may contain one or more of nickel, chromium, and aluminum, and the total of these metals may advantageously comprise at least about 15 wt. % (weight percent) of the alloy, for instance, about 10 to about 25 wt. % chromium, about 1 to about 8 wt. % of aluminum, and from 0 to about 20 wt. % of nickel, in each case based on the weight of the substrate. Examples of metallic substrates include those having straight channels; those having protruding blades along the axial channels to disrupt gas flow and to open communication of gas flow between channels; and those having blades and also holes to enhance gas transport between channels allowing for radial gas transport throughout the monolith.

[0065] Any suitable substrate for the catalytic articles disclosed herein may be employed, such as a monolithic substrate of the type having fine, parallel gas flow passages extending there through from an inlet or an outlet face of the substrate such that passages are open to fluid flow there through (“flow-through substrate”). Another suitable substrate is of the type have a plurality of fine, substantially parallel gas flow passages extending along the longitudinal axis of the substrate where, typically, each passage is blocked at one end of the substrate body, with alternate passages blocked at opposite end-faces (“wall-flow filter”). Flow-through and wallflow substrates are also taught, for example, in International Application Publication No. WO 2016/070090 and U.S. Patent Application Publication No. 2012/0178380 Al, which are incorporated herein by reference in their entirety.

Preparation of Catalytic Article

[0066] In another aspect of the present disclosure, methods of making the catalytic article disclosed herein are provided. Generally, the catalytic article of the present disclosure is prepared by preparing a bottom layer composition, coating the substrate 15 with the bottom layer composition to form a bottom layer 13, preparing a top layer composition, and coating the top layer composition to form a top layer 11 above the bottom layer 13.

[0067] According to the present disclosure, the bottom layer (AMOx catalyst) of the catalytic article may be prepared by preparing a platinum composition, such as a platinum nitrate solution, comprising a desired concentration of platinum. The platinum composition may be loaded onto a zeolite support, such as via an incipient wetness impregnation. The platinum on zeolite support may then be mechanically mixed with a catalytically-inert metal oxide in order to reach a desired final platinum loading weight percentage and/or a final solids concentration for the bottom layer composition to be applied to the support. The bottom layer composition may be coated on the substrate and then subjected to calcination at a temperature ranging from about 450 to about 850 °C for at least one hour. The calcination temperature may range from about 450 to about 650 °C, about 450 to about 600 °C, about 450 to 500 °C, such as at 450 °C. [0068] According to the present disclosure, the top layer (SCR catalyst) of the catalyst article may be prepared by metal ion-exchanging a zeolite with a metal component, or by providing a metal component supported by an oxide support. The components may be combined to form the SCR catalyst by various methods. In some aspects, the components may be physically mixed, such as dry powder mixing. In some aspects, the components may be combined by dispersing the components in solvent, such as water, to form a slurry. In other aspects, the components may be co-milled, such as ball or wet milling. The resulting top layer composition may be applied to the support and then subjected to calcination at a temperature ranging from about 450 to about 850 °C for at least one hour. The calcination temperature may range from about 450 to about 650 °C, about 450 to about 600 °C, about 450 to 500 °C, such as at 450 °C. [0069] To simulate real world use scenarios, the composition of the top and/or bottom layer may be hydrothermally aged, such as at 750 °C for 20 hours with 10% steam in air prior to test for NOx conversion and N2O formation rates.

[0070] To produce a catalytic article of the present disclosure, a substrate is coated with the compositions of the top layer, bottom layer, and any additional layers, as disclosed herein. In some aspects, the catalytic article is a dual-layer coated substrate, with a bottom layer comprising the AMOx catalyst as disclosed herein and a top layer comprising the SCR catalyst as disclosed herein.

[0071] In some aspects, the bottom layer composition is coated directly on the substrate. In some aspects, the top layer composition is coated above the bottom layer. In some aspects, the top layer composition is coated directly on the bottom layer.

[0072] Various coating layers can be viewed as an undercoat, an overcoat, or an interlayer. An undercoat is a layer “under” a coating layer, an overcoat is a layer “over” a coating layer and an interlayer is a layer “between” two coating layers. The interlayer(s), undercoat(s) and overcoat(s) may contain one or more functional compositions or may be free of functional compositions

[0073] Accordingly, one or more “undercoats” may be present, so that at least a portion of the top and/or bottom layer is not in direct contact with the substrate (but rather, are in contact with the undercoat). One or more “overcoats” may also be present, so that at least a portion of the top and/or bottom layer is not directly exposed to a gaseous stream or atmosphere (but rather, are in contact with the overcoat). One or more interlayers may also be present, so that at least a portion of the top and bottom layers are not in direct contact with each other (but rather, are in contact with the interlayer). By way of example, a low temperature NOx adsorbent (LTNA) catalyst layer may be present as an interlayer between the bottom layer and top layer. [0074] The compositions of these layers may typically be applied in the form of one or more washcoats of the catalytic article. A washcoat is formed by preparing a slurry containing a specified solids content (e.g., about 10 to about 60% by weight) of catalyst in a liquid vehicle, which is then applied to a substrate using any washcoat technique known in the art and dried and calcined to provide a coating layer. If multiple coatings are applied, the substrate is dried and/or calcined after each washcoat is applied and/or after the number of desired multiple washcoats are applied.

[0075] After calcining, the catalyst loading obtained by the above described washcoat technique can be determined through calculation of the difference in coated and uncoated weights of the substrate. As will be apparent to those of skill in the art, the catalyst loading can be modified by altering the slurry rheology. In addition, the coating/drying/calcining process to generate a washcoat layer (coating layer) can be repeated as needed to build the coating to the desired loading level or thickness, meaning more than one washcoat may be applied for a given layer.

[0076] The present catalytic article comprises a top coating layer over a bottom coating layer. The SCR catalyst as disclosed herein is present in a top layer while the AMOx catalyst as disclosed herein is present in a bottom layer. Any one layer may extend the entire axial length of the substrate or only a portion of the axial length. For instance, a bottom layer may extend the entire axial length of the substrate and a top layer may also extend the entire axial length of the substrate over the bottom layer. Each of the top and bottom layers may extend from either the inlet end or the outlet end. The bottom and top layers may extend from the same or different substrate ends.

[0077] The catalytic article may advantageously be “zoned,” comprising zoned layers, that is, where the catalytic article contains varying compositions across the axial length of the substrate. This may also be described as “laterally zoned”. For example, either the top or bottom layer may extend from the inlet end towards the outlet end extending about 10%, about 20%, about 30%, about 40%, about 50%, about 60%, about 70%, about 80%, about 90%, or about 100% of the substrate length. The other layer may extend from the outlet end towards the inlet end extending about 10%, about 20%, about 30%, about 40%, about 50%, about 60%, about 70%, about 80%, about 90%, or 100% of the substrate length, wherein at least a portion of the top layer overlays the bottom layer.

[0078] This “overlay” zone may for example extend from about 5% to about 100% of the substrate length, for example about 5%, about 10%, about 20%, about 30%, about 40%, about 50%, about 60% about 70%, about 80%, or about 90% of the substrate length. [0079] In some aspects, the top and bottom layers may overlay only a portion of each other, providing a third “middle” zone. The middle zone may, for example, may extend from about 5% to about 80% of the substrate length, for example about 5%, about 10%, about 20%, about 30%, about 40%, about 50%, about 60% or about 70% of the substrate length. When a middle zone is present, either the top layer or the bottom layer may extend from the inlet end or the outlet end.

[0080] In some aspects, the bottom layer is disposed directly on the substrate, and the top layer is disposed on at least a portion of the bottom layer.

[0081] In some aspects, the catalytic article of the present disclosure has a platinum concentration, where at least 80%, such as 90%, 95%, or 99%, by weight of the platinum in the catalytic article is present in the AMOx catalyst of the bottom layer. In some aspects, 100% of the platinum in the catalytic article is present in the AMOx catalyst of the bottom layer.

Emissions Treatment Systems

[0082] In another aspect of the present disclosure, an emissions treatment system is provided for selectively oxidizing ammonia in the exhaust gas stream, such as of a diesel engine. The emissions treatment system comprises the catalytic article disclosed herein. The emissions treatment system may comprise additional components.

[0083] In some aspects, the emission treatment system may further comprise an ammonia reductant generator or injector. The ammonia reductant generator or injector may be in fluid communication and upstream of the catalytic article. Such a system may also comprise a pump, a reservoir, etc.

[0084] In some aspects, the emissions treatment system may further comprise a lean NOx trap (LNT) catalytic article comprising a substrate and a LNT catalyst composition. The LNT catalytic article would be in fluid communication with and downstream of the catalytic article. [0085] In some aspects, the emissions treatment system may further comprise a diesel oxidation catalyst (DOC) composition. The DOC composition maybe in a zone downstream from the catalytic article, in a third layer of the catalytic article, or part of the top or bottom layer of the catalytic article. In other aspects, the emissions treatment system may further comprise a low temperature NOx adsorbent (LTNA). The LTNA functionality may be part of a DOC composition separate from the catalytic article, part of the catalytic article, part of both the DOC composition and the catalytic article, or separate from the DOC composition and from the catalytic article.

[0086] In some aspects, the emissions treatment system may further comprise an H2-SCR catalyst composition. [0087] The relative placement of the various components present within the emissions treatment system can vary. In the present emissions treatment system, the exhaust gas stream is received into the catalytic article by entering the upstream end and exiting the downstream end. The inlet end of the substrate of the article is synonymous with the “upstream” end or “front” end. The outlet end is synonymous with the “downstream” end or “rear” end. The treatment system is, in general, downstream of and in fluid communication with an internal combustion engine, such as a diesel engine or a hydrogen internal combustion engine.

Method of Treating Exhaust Stream

[0088] Another aspect of the present invention is directed to a method of treating the exhaust gas stream containing ammonia. The method can include placing the catalytic article according to some aspects of the present disclosure downstream from an engine and flowing the engine exhaust gas stream through the catalytic article. In some aspects, the method further comprising placing additional components downstream from the engine as noted above with respect to the emission treatment system of the present disclosure. The catalytic articles are suitable for treatment of exhaust gas streams of an internal combustion engine, such as a diesel engine or a hydrogen internal combustion engine. The catalyst articles are also suitable for treatment of exhaust stream from stationary industrial processes, removal of noxious or toxic substances from indoor air or for catalysis in chemical reaction processes.

[0089] It will be readily apparent to one of ordinary skill in the relevant arts that suitable modifications and adaptations to the catalytic articles, emissions treatment systems, and methods for treating exhaust streams described herein can be made without departing from the scope of any embodiments or aspects thereof. The articles, systems, and methods provided are exemplary and are not intended to limit the scope of the claimed embodiments. All of the various embodiments, aspects, and options disclosed herein can be combined in all variations. The scope of the articles, systems, and methods described herein include all actual or potential combinations of embodiments, aspects, options, examples, and preferences herein. All patents and publications cited herein are incorporated by reference herein for the specific teachings thereof as noted, unless other specific statements of incorporation are specifically provided [0090] Before describing exemplary embodiments of the present disclosure, it is to be understood that the disclosure is not limited to the details of construction or process steps set forth in the following examples and is capable of other embodiments and of being practiced or being carried out in various ways.

[0091] Embodiments

[0092] Without limitation, some embodiments of the disclosure include: [0093] Embodiment 1. A catalytic article for the selective oxidation of ammonia to nitrogen gas, the catalytic article comprising a top layer and a bottom layer, the bottom layer positioned between the top layer and a substrate, the substrate having an inlet end, an outlet end, and an axial length; wherein the top layer comprises a selective catalytic reduction (SCR) catalyst, the bottom layer comprises an ammonia oxidation (AMOx) catalyst, and the AMOx catalyst comprises platinum (Pt) on a zeolite support, wherein the zeolite support comprises a zeolite having a silica-to-alumina ratio in the range of about 5: 1 to about 100: 1.

[0094] Embodiment 2. The catalytic article of Embodiment 1, wherein the mass ratio of Pt to zeolite support in the AMOx catalyst ranges from about 1 : 1000 to about 1 :20.

[0095] Embodiment 3. The catalytic article of Embodiment 1 or Embodiment 2, wherein the AMOx catalyst has a Pt loading ranging from about 1 g/ft3 to about 10 g/ft3 with respect to the total volume of the AMOx catalyst.

[0096] Embodiment 4. The catalytic article of any one of Embodiments 1 to 3, wherein the AMOx catalyst has a Pt density per unit surface area of the zeolite support of about 5 per 1000 nm2 to about 1000 per 1000 nm2.

[0097] Embodiment 5. The catalytic article of any one of Embodiments 1 to 4, wherein the zeolite support is a hydrogen-form zeolite.

[0098] Embodiment 6. The catalytic article of any one of Embodiments 1 to 5, wherein the zeolite support is substantially free of promoter-ions.

[0099] Embodiment 7. The catalytic article of any one of Embodiments 1 to 6, wherein the zeolite support comprises less than about 0.10% by weight of transition metals from the groups 3 - 12 of the periodic table, excluding Pt.

[0100] Embodiment 8. The catalytic article of any one of Embodiments 1 to 7, wherein the zeolite support has a zeolite structure chosen from the framework types CHA, FAU, BEA, MOR, FER, MFI, AEI, and combinations thereof.

[0101] Embodiment 9. The catalytic article of Embodiment 8, wherein the zeolite support has a zeolite structure chosen from the framework types CHA, FAU, and a combination thereof. [0102] Embodiment 10. The catalytic article of any one of Embodiments 1 to 9, wherein the zeolite support has a silica-to-alumina ratio in the range of about 5: 1 to about 95: 1.

[0103] Embodiment 11. The catalytic article of Embodiment 10, wherein the zeolite support has a silica-to-alumina ratio in the range of about 10: 1 to about 50: 1.

[0104] Embodiment 12. The catalytic article of any one of Embodiments 1 to 11, wherein the zeolite support has an ammonia storage in the range of about 0.1 to mmol NH3 per g of zeolite support to about 10 mmol NH3 per g at 200°C. [0105] Embodiment 13. The catalytic article of Embodiment 12, wherein the zeolite support has an ammonia storage in the range of about 0.3 mmol NH3 per g to about 5 mmol NH3 per g at 200°C.

[0106] Embodiment 14. The catalytic article of any one of Embodiments 1 to 13, wherein the bottom layer further comprises an additional catalytically-inert metal oxide.

[0107] Embodiment 15. The catalytic article of Embodiment 14, wherein the additional catalytically-inert metal oxide is chosen from A12O3, SiO2, ZrO2, TiO2, La2O3, Y2O3, and combinations thereof.

[0108] Embodiment 16. The catalytic article of any one of Embodiments 1 to 15, wherein at least 80% of the Pt in the catalytic article is located within the AMOx catalyst of the bottom layer.

[0109] Embodiment 17. The catalytic article of Embodiment 16, wherein 100% of the Pt in the catalytic article is located within the AMOx catalyst of the bottom layer.

[0110] Embodiment 18. The catalytic article of any one of Embodiments 1 to 17, wherein the substrate is a metallic, cordierite, or silicon carbide (SiC) substrate.

[OHl] Embodiment 19. The catalytic article of any one of Embodiments 1 to 18, wherein the substrate is a flow-through substrate or a wall-flow substrate.

[0112] Embodiment 20. The catalytic article of any one of Embodiments 1 to 19, wherein the SCR catalyst in the top layer is effective to selectively catalyze a reduction of nitrogen oxides (NOx) to nitrogen gas in the presence of an ammonia reductant, wherein the ammonia reductant comprises urea or ammonia.

[0113] Embodiment 21. The catalyst article of any one of Embodiments 1 to 20, wherein the SCR catalyst in the top layer comprises a Cu-exchanged zeolite or an Fe-exchanged zeolite. [0114] Embodiment 22. A method of making a catalytic article according to any one of Embodiments 1 to 21 comprising: preparing a bottom layer composition by impregnating the Pt onto the zeolite support, coating the substrate with the bottom layer composition to form the bottom layer, calcining the bottom layer composition at a temperature ranging from 450 °C to 850 °C for at least 1 hour, preparing a top layer composition comprising the SCR catalyst, and coating the top layer composition to form the top layer above the bottom layer, and calcining the top layer composition at a temperature ranging from 450 °C to 850 °C for at least 1 hour. [0115] Embodiment 23. The method of Embodiment 22, wherein preparing the bottom layer composition further comprises mechanically mixing the calcined Pt on zeolite support with an additional catalytically-inert metal oxide. [0116] Embodiment 24. An emission treatment system for selectively oxidizing ammonia from an exhaust gas stream, the system comprising the catalytic article according to any one of Embodiments 1 to 21 in fluid communication with the exhaust gas stream.

[0117] Embodiment 25. The emission treatment system of Embodiment 24, further comprising an ammonia injection port in fluid communication with the exhaust gas stream. [0118] Embodiment 26. A method for treating an exhaust stream containing ammonia, the method comprising passing the exhaust stream through the emissions treatment system of Embodiment 24 or Embodiment 25.

[0119] Embodiment 27. The method for treating an exhaust stream of Embodiment 26, wherein the exhaust stream is an internal combustion engine.

EXAMPLES

[0120] The following examples are intended to be illustrative and are not meant in any way to limit the scope of disclosure.

[0121] Example 1: Preparation of catalyst materials

[0122] AMOx catalysts were prepared by first impregnating a solution of platinum nitrate onto AI2O3, CHA zeolite (SAR = 29), or Y zeolite (SAR = 30). The impregnated powder was dried and calcined at 590°C for two hours to yield the Pt component of the AMOx catalyst. Pt components with different amounts of platinum, ranging from 0.6 wt% to 1.8 wt%, were prepared by adjusting the concentration of the platinum nitrate solution used in the impregnation step. Pt component materials containing more than 0.6 wt% Pt were then mechanically combined with an amount of a platinum-free catalytically-inert metal oxide so that the overall Pt loading of the powder was brought back to 0.6 wt%. The compositional details for each powder are described in Table 1. The resulting combined powders, all having overall Pt content = 0.6 wt%, were hydrothermally aged at 750 °C for 20 hours in air containing 10% steam.

Table 1. Description of catalyst powders

[0123] Example 2: Effect of the support on NHj activity

[0124] Each powder was pelletized, crushed, and sieved to a particle fraction between 250 pm to 500 pm. The reactor tube was charged with 33 mg of the sieved catalyst diluted with corundum to give a total bed volume of 1.0 cm³. The powder was exposed to a feed gas containing 200 ppm NH3, 6.5 vol.% H2O, 7 vol.% CO2, 10 vol.% O2, and a balance of N2. The total gas flow was 70 L/h. The system was allowed to equilibrate at each target temperature step until a steady-state temperature and gas composition in the outlet of the catalyst bed was achieved for 3 min. Data was collected and averaged for 30 sec before proceeding to the next temperature setpoint.

[0125] FIG. 2 shows ammonia conversion as a function of temperature for the powders C03, A09, A10, Al l, A26, and A27. Each of the data sets was fit to a sigmoid function where the inflection point corresponds to Tso, the temperature at which 50% of the NH3 is converted. A lower value for Tso indicates a more active catalyst for ammonia oxidation. FIG. 3 shows the Tso value for each of these catalyst formulations. The formulations A26 and A27, which contain 1.8 wt% Pt supported on either CHA zeolite or Y zeolite, show lower Tso than the formulations where Pt is supported on AI2O3. A26 and Al 1 both contain a mixture of CHA zeolite and AI2O3 but Pt is supported on CHA in A26 and on AI2O3 in Al 1. Since A26 exhibits Tso that is 20°C lower than Al l, we conclude that Pt supported on a CHA zeolite is more active for NH3 oxidation than Pt supported on AI2O3.

[0126] FIG. 4 shows the peak N2O yield for C03, A09, A10, Al l, A26, and A27. The chart shows there is minimal variability in peak N2O formation for this series of samples even though the Tso spans 40°C. In particular, the formulations A26 and A27 that have Pt supported on the zeolite show statistically similar peak N2O formation to the other samples having Pt supported on AI2O3. In this way we can achieve improvement in the NH3 conversion without penalty in formation of N2O. [0127] Example 3: Effect of Pt density on NH3 activity by Pt on zeolite support

[0128] FIG. 5 shows NH3 conversion as a function of temperature for the series of formulations C03, B01, B02, B03, B04, and A26, with a feed gas containing 200 ppm NH3, 10 vol.% O2, and a balance of argon. The key feature of this series is that as we proceed from B0 to A26, the density of platinum on the CHA zeolite support is increased while the total platinum content in the formulation is kept constant. All the formulations with Pt supported on CHA give a lower Tso value than the control samples C03 containing Pt supported on AI2O3. FIG. 6 shows Tso for each formulation, arranged with increasing coverage of platinum on the zeolite as one proceeds from left to right. The formulation B03, having 1.0 wt% Pt supported on CHA zeolite, gives the lowest Tso value. The formulations having 0.8 wt% Pt on CHA (B02) and 1.2 wt% Pt on CHA (B04) give somewhat higher values for Tso. This indicates that there is an optimum in Pt coverage on the CHA zeolite near 1.0 wt% platinum.

Claims

What Is Claimed Is:

1. A catalytic article for the selective oxidation of ammonia to nitrogen gas, the catalytic article comprising a top layer and a bottom layer, the bottom layer positioned between the top layer and a substrate, the substrate having an inlet end, an outlet end, and an axial length; wherein the top layer comprises a selective catalytic reduction (SCR) catalyst, the bottom layer comprises an ammonia oxidation (AMOx) catalyst, and the AMOx catalyst comprises platinum (Pt) on a zeolite support, wherein the zeolite support comprises a zeolite having a silica-to-alumina ratio in the range of about 5: 1 to about 100: 1.

2. The catalytic article of claim 1, wherein the mass ratio of Pt to zeolite support in the AMOx catalyst ranges from about 1 : 1000 to about 1 :20.

3. The catalytic article of claim 1 or claim 2, wherein the AMOx catalyst has a Pt loading ranging from about 1 g/ft³ to about 10 g/ft³ with respect to the total volume of the AMOx catalyst.

4. The catalytic article of any one of claims 1 to 3, wherein the AMOx catalyst has a Pt density per unit surface area of the zeolite support of about 5 per 1000 nm² to about 1000 per 1000 nm².

5. The catalytic article of any one of claims 1 to 4, wherein the zeolite support is a hydrogen-form zeolite.

6. The catalytic article of any one of claims 1 to 5, wherein the zeolite support is substantially free of promoter-ions.

7. The catalytic article of any one of claims 1 to 6, wherein the zeolite support comprises less than about 0.10% by weight of transition metals from the groups 3 - 12 of the periodic table, excluding Pt.

8. The catalytic article of any one of claims 1 to 7, wherein the zeolite support has a zeolite structure chosen from the framework types CHA, FAU, BEA, MOR, FER, MFI, AEI, and combinations thereof.

9. The catalytic article of any one of claims 1 to 8, wherein the zeolite support has a silica- to-alumina ratio in the range of about 5 : 1 to about 95: 1.

10. The catalytic article of any one of claims 1 to 9, wherein the zeolite support has an ammonia storage in the range of about 0.1 to mmol NH3 per g of zeolite support to about 10 mmol NH3 per g at 200°C.

11. The catalytic article of any one of claims 1 to 10, wherein the bottom layer further comprises an additional catalytically-inert metal oxide.

12. The catalytic article of any one of claims 1 to 11, wherein at least 80% of the Pt in the catalytic article is located within the AMOx catalyst of the bottom layer.

13. The catalytic article of any one of claims 1 to 12, wherein the substrate is a metallic, cordierite, or silicon carbide (SiC) substrate.

14. The catalytic article of any one of claims 1 to 13, wherein the substrate is a flow- through substrate or a wall-flow substrate.

15. The catalyst article of any one of claims 1 to 14, wherein the SCR catalyst in the top layer comprises a Cu-exchanged zeolite or an Fe-exchanged zeolite.

16. A method of making a catalytic article according to any one of claims 1 to 15 comprising: preparing a bottom layer composition by impregnating the Pt onto the zeolite support, coating the substrate with the bottom layer composition to form the bottom layer, calcining the bottom layer composition at a temperature ranging from 450 °C to 850 °C for at least 1 hour, preparing a top layer composition comprising the SCR catalyst, and coating the top layer composition to form the top layer above the bottom layer, and calcining the top layer composition at a temperature ranging from 450 °C to 850 °C for at least 1 hour.

17. The method of claim 16, wherein preparing the bottom layer composition further comprises mechanically mixing the calcined Pt on zeolite support with an additional catalytically-inert metal oxide.

18. An emission treatment system for selectively oxidizing ammonia from an exhaust gas stream, the system comprising the catalytic article according to any one of claims 1 to 15 in fluid communication with the exhaust gas stream.

19. The emission treatment system of claim 18, further comprising an ammonia injection port in fluid communication with the exhaust gas stream.

20. A method for treating an exhaust stream containing ammonia, the method comprising passing the exhaust stream through the emissions treatment system of claim 18 or claim 19.