WO2021142046A1

WO2021142046A1 - Acidic support for no x storage/release

Info

Publication number: WO2021142046A1
Application number: PCT/US2021/012390
Authority: WO
Inventors: Joseph A. Patchett
Original assignee: Basf Corporation
Priority date: 2020-01-10
Filing date: 2021-01-07
Publication date: 2021-07-15

Abstract

Disclosed herein are components capable of promoting a reaction between a NO_x and a hydrocarbon to form a nitrogen-substituted hydrocarbon, catalyst compositions comprising the same, articles comprising the same, and exhaust gas treatment system and methods of treating exhaust gases using any of the foregoing.

Description

ACIDIC SUPPORT FOR NOX STORAGE/RELEASE The present disclosure relates to catalysts and catalyst compositions for use in treating engine exhaust, methods for the preparation and use of such compositions, and catalyst articles and systems employing such compositions. Environmental regulations for emissions of internal combustion engines are becoming increasingly stringent throughout the world. Operation of a lean-burn engine, for example, a diesel engine, provides the user with excellent fuel economy due to its operation at high air/fuel ratios under fuel-lean conditions. However, diesel engines also emit exhaust gas emissions containing particulate matter, unburned hydrocarbons, carbon monoxide (CO), and nitrogen oxides (NOx), wherein NOx describes various chemical species of nitrogen oxides, including nitrogen monoxide and nitrogen dioxide, among others. NO_x are harmful components of atmospheric pollution. Various methods have been used for the treatment of NO_x-containing gas mixtures to decrease atmospheric pollution. An effective method to reduce NOx from the exhaust of lean-burn engines requires reaction of NO_x under lean burn engine operating conditions with a suitable reductant in the presence of a selective catalytic reduction (SCR) catalyst component. The SCR process typically uses as the reductant ammonia or a hydrocarbon in the presence of atmospheric oxygen, resulting in the formation predominantly of nitrogen and steam: 4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O (standard SCR reaction) 2NO2 + 4NH3 → 3N2 + 6H2O (slow SCR reaction) NO + NO₂ + NH₃ → 2N₂ + 3H₂O (fast SCR reaction) Current catalysts employed in the SCR process include molecular sieves, such as, e.g., zeolites, ion-exchanged with a catalytic metal, such as, e.g., iron or copper. Exemplary useful SCR catalyst component is able to effectively catalyze the reduction of the NO_x exhaust component at temperatures below about 600°C so that reduced NOx levels can be achieved even under conditions of low load, which typically are associated with lower exhaust temperatures. A major problem encountered in the treatment of automotive exhaust gas streams is the so-called “cold start” period, which is the time period at the beginning of the treatment process, when the exhaust gas stream and the exhaust gas treatment system are at low temperatures (i.e., below about 150ºC). At these low temperatures, exhaust gas treatment systems generally do not display sufficient catalytic activity for effectively treating hydrocarbons, nitrogen oxides (NOx), and/or carbon monoxide (CO) emissions. In general, catalytic components such as SCR catalyst components are very effective in converting NO_x to N2 at temperatures above about 200ºC but do not exhibit sufficient activities at lower temperature regions (less than about 200ºC), such as those found during cold-start or prolonged low-speed city driving. Therefore, catalytic components capable of capturing and storing such low-temperature NOx emissions, and being able to release them at higher temperatures (greater than about 200ºC) when catalytic components (e.g., SCR catalyst components) become effective, are in great demand. As a result, considerable efforts have been made to alleviate this problem. There are several ways to minimize NOx emissions during cold start periods. For instance, trapping systems have been developed which can store these exhaust gas emissions (i.e., hydrocarbon, CO, and NOx gases) at low temperatures and subsequently release them at higher temperatures, when the remaining catalytic components of the treatment system have attained sufficient catalytic activity. One such system is the Lean NOx Trap (LNT) catalyst, a well-known and commercially proven technology. Lean NOx trap (LNT) catalysts contain NOx adsorbent components that trap NOx under certain exhaust conditions. For example, the NO_x adsorbent components can comprise alkaline earth elements, e.g., alkaline earth metal oxides, such as, e.g., oxides of Mg, Ca, Sr, and/or Ba. Other LNT catalysts can contain rare earth metal oxides as NO_x adsorbent components, such as oxides of Ce, La, Pr, and/or Nd. LNT catalysts further contain a platinum group metal component, such as platinum dispersed on a refractory metal oxide (e.g., alumina) support for catalytic NOx oxidation and reduction. The LNT catalyst operates under cyclic lean (trapping mode) and rich (regeneration mode) exhaust conditions. Under lean conditions, the LNT catalyst traps and stores NOx as an inorganic nitrate (e.g., where the NOx adsorbent component is BaO or BaCO3, it is converted to Ba(NO3)2) upon reaction with (“trapping”) of NOx. The NOx adsorbent component then releases the trapped NOx and the platinum group metal component reduces the NOx to N2 under stoichiometric or transient rich engine operating conditions, or under lean engine operation with external fuel injected in the exhaust to induce rich conditions. NO to NO2 conversion is a prerequisite to efficient NOx trapping; however, the reaction rate is very slow when the temperature is below about 200ºC, which renders the traditional LNT catalyst unsuitable for trapping of cold-start NOx emission. Due to emission regulations becoming increasingly more stringent, it would be highly desirable to provide further materials for NOx adsorption to capture cold-start NOx emission. Employing a catalyst that is functional during low temperature operation (less than about 150°C) can help OEMs to meet the increasingly tighter emissions regulations (e.g., Euro-7 regulations). As more than 80% of cold-start NOx emission consists of NO, it is imperative that advanced NO_x adsorption materials have high efficiency for NO adsorption. Further, it can be highly desirable to provide materials that provide for NOx desorption at somewhat elevated temperatures (e.g., to ensure downstream catalysts such as selective catalytic reduction (SCR) catalysts are operational and can effectively treat the released NOx). The present disclosure provides a component capable of promoting NO_x conversion to a nitrogen-substituted hydrocarbon, where the component may be included within a catalyst composition. Components of this disclosure, as will be described in further detail herein, generally comprise a surface (e.g., an acidic surface) capable of promoting a reaction between a NO_x and hydrocarbons (both of which are commonly present in exhaust gas). The reaction product, in the form of a nitrogen-substituted hydrocarbon, can be readily adsorbed onto a hydrocarbon storage material (e.g., at low temperatures) and subsequently released therefrom (e.g., at high temperatures). The nitrogen-substituted hydrocarbon exhibits a higher desorption temperature than NO_x alone, rendering such catalyst compositions an effective means for NOx storage and release under various conditions. The disclosure further provides catalyst compositions, articles, systems, and methods involving such components. In one aspect is provided a catalyst composition comprising: a component comprising an acidic surface capable of promoting a reaction between a NO_x and a hydrocarbon to produce a nitrogen-substituted hydrocarbon; and a hydrocarbon storage material capable of storing the nitrogen-substituted hydrocarbon at a temperature below about 200°C. In some embodiments, the acidic surface comprises a Bronsted acid or a Lewis acid. In some embodiments, the acidic surface comprises a Bronsted acid. In some embodiments, the acidic surface comprises a Lewis acid. In some embodiments, the acidic surface comprises a platinum group metal. In some embodiments, the acidic surface comprises a sulfate surface. In some embodiments, the hydrocarbon storage material comprises a zeolite. The disclosure also provides a diesel oxidation catalytic article comprising a catalyst composition as disclosed herein. In some embodiments, the diesel oxidation catalytic article further comprises a platinum group metal on a refractory metal support. In some embodiments, the component comprising the acidic surface and the hydrocarbon storage material are mixed. The diesel oxidation catalytic article can, in some embodiments, comprise a first, upstream region and a second, downstream region, wherein: the first, upstream region comprises a component comprising the acidic surface; and the second, downstream region comprises a hydrocarbon storage material. In some embodiments, the first, upstream region contains a lower platinum group metal content than the second, downstream region. In a further aspect, the disclosure provides an exhaust gas treatment system for reducing NOx present in an exhaust gas stream, comprising: a diesel oxidation catalytic article as disclosed herein; and a conduit for fluid communication between the exhaust gas stream and the diesel oxidation catalytic article. In some embodiments, the exhaust gas treatment system further comprises a selective catalytic reduction (SCR) catalyst downstream of the diesel oxidation catalytic article. The disclosure additionally provides, in a still further aspect, a method for storing a NOx in an exhaust gas at a low temperature, comprising: contacting a NOx with a hydrocarbon under conditions effective to form a nitrogen-substituted hydrocarbon at a temperature below about 200°C. In some embodiments, the contacting further comprises contacting the NO_x, the hydrocarbon, or both the NO_x and the hydrocarbon with a component comprising an acidic surface capable of promoting reaction between the NOx and the hydrocarbon. The acidic surface of the component in the referenced method can vary. In some embodiments, the acidic surface comprises a Bronsted acid or a Lewis acid. In some embodiments, the acidic surface comprises a Bronsted acid. In some embodiments, the acidic surface comprises a Lewis acid. In some embodiments, the acidic surface comprises a platinum group metal. In some embodiments, the acidic surface comprises a sulfate surface. The method can, in some embodiments, further comprise trapping the nitrogen-substituted hydrocarbon at the temperature below about 200°C with a hydrocarbon storage material. The hydrocarbon storage material, in some embodiments, comprises a zeolite. In an additional aspect is provided a method for reducing a NOx in an exhaust gas at a low temperature, comprising: contacting a NO_x with a hydrocarbon at an acidic surface under conditions effective to form a nitrogen-substituted hydrocarbon at a temperature below about 200°C; trapping the nitrogen-substituted hydrocarbon at the temperature below about 200°C with a hydrocarbon storage material; and desorbing the nitrogen-substituted hydrocarbon at a temperature above about 200°C suitable for NOx reduction. The disclosure includes, without limitations, the following embodiments. Embodiment 1: A catalyst composition comprising: a component comprising an acidic surface capable of promoting a reaction between a NO_x and a hydrocarbon to produce a nitrogen-substituted hydrocarbon; and a hydrocarbon storage material capable of storing the nitrogen-substituted hydrocarbon at a temperature below about 200°C. Embodiment 2: The catalyst composition of Embodiment 1, wherein the acidic surface comprises a Bronsted acid or a Lewis acid. Embodiment 3: The catalyst composition of Embodiment 1, wherein the acidic surface comprises a platinum group metal. Embodiment 4: The catalyst composition of Embodiment 1, wherein the acidic surface comprises a sulfate surface. Embodiment 5: The catalyst composition of any one of Embodiments 1-4, wherein the hydrocarbon storage material comprises a zeolite. Embodiment 6: A diesel oxidation catalytic article comprising a catalyst composition of any one of Embodiments 1-5. Embodiment 7: The diesel oxidation catalytic article of Embodiment 6, further comprising a platinum group metal on a refractory metal support. Embodiment 8: The diesel oxidation catalytic article of Embodiments 6 or 7, wherein the component comprising the acidic surface and the hydrocarbon storage material are mixed. Embodiment 9: The diesel oxidation catalytic article of Embodiments 6 or 7, comprising a first, upstream region and a second, downstream region, wherein: the first, upstream region comprises the component comprising the acidic surface; and the second, downstream region comprises the hydrocarbon storage material. Embodiment 10: The diesel oxidation catalytic article of Embodiment 9, wherein the first, upstream region contains a lower platinum group metal content than the second, downstream region. Embodiment 11: An exhaust gas treatment system for reducing NOx present in an exhaust gas stream, comprising: the diesel oxidation catalytic article of any one of Embodiments 6-10; and a conduit for fluid communication between the exhaust gas stream and the diesel oxidation catalytic article. Embodiment 12: The exhaust gas treatment system of Embodiment 11, further comprising a selective catalytic reduction (SCR) catalyst downstream of the diesel oxidation catalytic article. Embodiment 13: A method for storing NO_x in an exhaust gas at a low temperature, comprising: contacting a NO_x with a hydrocarbon under conditions effective to form a nitrogen-substituted hydrocarbon at a temperature below about 200°C. Embodiment 14: The method of Embodiment 13, wherein the contacting further comprises contacting the NOx, the hydrocarbon, or both the NOx and the hydrocarbon with a component comprising an acidic surface capable of promoting reaction between the NO_x and the hydrocarbon. Embodiment 15: The method of Embodiment 14, wherein the acidic surface comprises a Bronsted acid or a Lewis acid. Embodiment 16: The method of Embodiment 14, wherein the acidic surface comprises a platinum group metal. Embodiment 17: The method of Embodiment 14, wherein the acidic surface comprises a sulfate surface. Embodiment 18: The method of any one of Embodiments 13-17, further comprising trapping the nitrogen-substituted hydrocarbon at the temperature below about 200°C with a hydrocarbon storage material. Embodiment 19: The method of Embodiment 18, wherein the hydrocarbon storage material comprises a zeolite. Embodiment 20: A method for reducing NOx in an exhaust gas at a low temperature, comprising: contacting a NOx with a hydrocarbon at an acidic surface under conditions effective to form a nitrogen-substituted hydrocarbon at a temperature below about 200°C; trapping the nitrogen-substituted hydrocarbon at the temperature below about 200°C with a hydrocarbon storage material; and desorbing the nitrogen-substituted hydrocarbon at a temperature above about 200°C that is suitable for NOx reduction. 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. The disclosure includes any combination of two, three, four, or more of the above-noted embodiments as well as combinations of any two, three, four, or more features or elements set forth in this disclosure, regardless of whether such features or elements are expressly combined in a specific embodiment description herein. This disclosure is intended to be read holistically such that any separable features or elements of the disclosed subject matter, in any of its various aspects and embodiments, should be viewed as intended to be combinable unless the context clearly dictates otherwise. Other aspects and advantages of the present disclosure will become apparent from the following. In order to provide an understanding of embodiments of the disclosure, reference is made to the appended drawings, which are not necessarily drawn to scale, and in which reference numerals refer to components of example embodiments of the disclosure. The drawings are provided by way of example only, and should not be construed as limiting the disclosure. FIG.1A depicts a perspective view of a honeycomb-type substrate which may comprise a catalyst composition in accordance with some exemplary embodiments of the present disclosure. FIG.1B depicts a partial cross-sectional view enlarged relative to FIG.1A and taken along a plane parallel to the end faces of the carrier of FIG.1A, which shows an enlarged view of a plurality of the gas flow passages shown in FIG.1A. FIG.2 depicts a cross-sectional view of an embodiment of an exemplary zoned catalytic article with partially overlapping layers. FIG.3 depicts a cross-sectional view of a different embodiment of an exemplary zoned catalytic article with partially overlapping layers. FIG 4 depicts a cross-sectional view of an embodiment of an exemplary zoned catalytic article with no overlapping layers. FIG.5 depicts a cross-sectional view of an embodiment of an exemplary layered catalytic article. FIG.6 depicts a cross-sectional view of an embodiment of a different exemplary layered catalytic article. FIG.7 depicts an embodiment of an engine treatment system. The present disclosure now will be described more fully hereinafter. Although the subject matter herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present disclosure. It will be apparent to those skilled in the art that various modifications and variations can be made to the method and apparatus of the present disclosure without departing from the spirit and scope of the disclosure. Thus, it is intended that the present disclosure include modifications and variations that are within the scope of the appended claims and their equivalents. It is to be understood that the disclosure is not limited to the details of construction or process steps set forth in the following description. The disclosure is capable of other embodiments and of being practiced or being carried out in various ways. Like numbers refer to like elements throughout. As used in this specification and the claims, the singular forms “a” or “an” entity refers to one or more of that entity, e.g., “a compound” refers to one or more compounds or at least one compound unless stated otherwise. As such, the terms “a” (or “an”), “one or more”, and “at least one” are used interchangeably herein. Catalyst Material The present disclosure provides a catalyst material comprising a component capable of promoting NO_x conversion to a nitrogen-substituted hydrocarbon. As used herein, the term “catalyst” refers to a material that promotes a chemical reaction. In some embodiments, the catalyst material further comprises an adsorbent for the storage and release of such nitrogen-substituted hydrocarbons (also referred to herein as N-hydrocarbons). As used herein, the terms “nitrogen oxides” or “NO_x” designate the oxides of nitrogen, such as NO, NO₂, or N2O. For example, “a NOx” refers to at least one oxide of nitrogen. The term “adsorbent” refers to a material that adsorbs and/or absorbs a desired substance (e.g., in this disclosure, nitrogen-substituted hydrocarbons and/or hydrocarbons). Adsorbents may adsorb and/or absorb (store) a substance at a certain temperature and desorb (release) the substance at a higher temperature. In the context of the disclosed catalyst materials, the hydrocarbon and nitrogen- substituted hydrocarbons can be adsorbed at a temperature below about 160°C or below about 200°C, and can be desorbed at a temperature above these values. As such, NOx can be effectively trapped (in the form of a nitrogen-substituted hydrocarbon), e.g., below temperatures at which a downstream selective catalytic reduction (SCR) catalyst is active, and can be released above such temperatures, such that it can be effectively treated by a downstream SCR catalyst at a suitable temperature 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 10%. The catalyst material according to the present disclosure is unique, e.g., in that it comprises a component that promotes the reaction of NOx present in an exhaust gas stream with hydrocarbons (which may also be present in the exhaust gas stream), e.g., to form one or more N-hydrocarbons. As used herein, “promotes,” in reference to a material, refers to a material that allows (at least in part) for the reaction to occur, enhances the extent of such reaction, or enhances the rate of such reaction. In some embodiments, a component capable of promoting NO_x conversion to a nitrogen-substituted hydrocarbon according to the present disclosure, is described herein as comprising an acidic surface. As used herein, an “acidic surface” refers to a surface of a material that can be characterized as capable of donating a proton (e.g., a “Bronsted acid” functionalized surface) or as capable of accepting an electron pair (e.g., a “Lewis acid” functionalized surface). Certain components capable of promoting NOx conversion to a nitrogen-substituted hydrocarbon can be described as comprising a “highly acidic” surface. The acidic surface can be characterized, in some embodiments, by multiple acid sites. Acidity can be determined by various methods. Acid strength is defined as the ability of a surface to convert an adsorbed neutral base into its conjugate acid. There are two main methods to determine the strength and amount of a solid acid: an amine titration method using indicators (e.g., n-butylamine titration) and a gaseous base adsorption method (ammonia and pyridine adsorption). These and other relevant methods of analyzing acidity of a surface are described, for example, in K. Tanabe et al., New Solid Acids and Bases: Their Catalytic Properties, Chapter 2, pp.5-11, which is incorporated herein by reference in its entirety. Suitable components capable of promoting NOx conversion to a nitrogen-substituted hydrocarbon include, but are not limited to, components comprising platinum group metals. Some components capable of promoting NOx conversion to a nitrogen-substituted hydrocarbon are materials comprising sulfate surfaces. In some embodiments, a component is incorporated within a catalyst composition, which inherently has a highly acidic surface. In other embodiments, it may be necessary to modify and/or treat a material to provide a highly acidic surface thereon. The compositions disclosed herein comprise, in addition to the component capable of promoting NOx conversion to a nitrogen-substituted hydrocarbon with acidic surface functionality, a component for the low temperature storage of hydrocarbons and nitrogen-substituted hydrocarbons. Such component can allow for the trapping of hydrocarbon present in the exhaust gas stream, as well as the N-hydrocarbons formed at the surface of the component capable of promoting NO_x conversion to a nitrogen-substituted hydrocarbon via reaction between hydrocarbons and NOx (as referenced herein above). This storage component can comprise any known hydrocarbon storage material, e.g., a micro-porous material, such as a molecular sieve, e.g., a zeolite or a zeolite-like material. As used herein, the term “molecular sieve” generally refers to framework materials such as zeolites and other framework materials (e.g., isomorphously substituted materials). Molecular sieves are materials based on an extensive three-dimensional network of oxygen ions containing generally tetrahedral type sites and having a substantially uniform pore distribution, with the average pore size being no larger than about 20 Å. The pore sizes are defined by the ring size. According to one or more embodiments, it will be appreciated that by defining the molecular sieves by their framework type, it is intended to include any and all zeolite or isotypic framework materials, such as, e.g., SAPO, AlPO, MeAPO, Ge-silicates, all-silica, and similar materials having the same framework type. Generally, molecular sieves are defined as aluminosilicates with open 3-dimensional framework structures composed of corner-sharing TO₄ tetrahedra, such as 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 are removable. In some embodiments, reference to an aluminosilicate zeolite framework type limits the material to molecular sieves that do not include phosphorus or other metals substituted in the framework. However, to be clear, as used herein, “aluminosilicate” excludes aluminophosphate materials such as SAPO, AlPO, MeAPSO, and MeAPO materials, and the broader term “zeolite” is intended to include aluminosilicates and aluminophosphates. As used herein, the term “aluminophosphate” refers to another specific example of a molecular sieve, including aluminum and phosphate atoms. In some embodiments, the molecular sieves are selected from aluminosilicate, borosilicate, gallosilicate, MeAPSO, and MeAPO compositions. These include, but are not limited to, SSZ-13, SSZ-62, natural chabazite, zeolite K-G, Linde D, Linde R, LZ-218, LZ- 235, LZ-236, ZK-14, SAPO-34, SAPO-44, SAPO-47, ZYT-6, CuSAPO-34, CuSAPO-44, Ti- SAPO-34, and CuSAPO-47. In some embodiments, the molecular sieves referred to herein comprise SiO₄/AlO₄ tetrahedra that are linked by common oxygen atoms to form a three-dimensional network. In other embodiments, the molecular sieves comprise SiO₄/AlO₄/PO₄ tetrahedra. Molecular sieves can be differentiated mainly according to the geometry of the voids which are formed by the rigid network of the SiO₄/AlO₄ or SiO₄/AlO₄/PO₄, tetrahedra. The entrances to the voids are formed from 6, 8, 10, or 12 ring atoms with respect to the atoms which form the entrance opening. In some embodiments, the molecular sieves comprise ring sizes of no larger than 12, including, e.g., ring sizes of 6, 8, 10, or 12. In some embodiments, the molecular sieve comprises an 8-ring small pore aluminosilicate zeolite. As used herein, the term “small pore” refers to pore openings which are smaller than about 5 Angstroms, for example, on the order of approximately 3.8 Angstroms. For example, the CHA structure is an example “8-ring” zeolite having 8-ring pore openings and double-six ring secondary building units and having a cage-like structure resulting from the connection of double six-ring building units by 4 rings. In some embodiments, the molecular sieve is a small-pore molecular sieve having a maximum ring size of eight tetrahedral atoms. 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 Angstroms to about 10 Angstroms in diameter. Aluminosilicate zeolites typically comprise framework structures including silica (Si) and alumina (Al), wherein the molar silica-to-alumina ratio (SAR) within the framework can vary over a wide range, but is generally 2 or greater. In some embodiments, the zeolite has an SAR in the range of about 2 to about 300, including about 5 to about 250, about 5 to about 200, about 5 to about 100, or about 5 to about 50. In some embodiments, the zeolite has an SAR in the range of about 10 to about 200, about 10 to about 100, about 10 to about 75, about 10 to about 60, and about 10 to about 50, about 15 to about 100, about 15 to about 75, about 15 to about 60, and about 15 to about 50, about 20 to about 100, about 20 to about 75, about 20 to about 60, or about 20 to about 50. In some embodiments, more than one zeolite is present such that the SAR of the first zeolite and the SAR of the second zeolite are the same. In some embodiments, more than one zeolite is present such that the SAR of the first zeolite and the SAR of the second zeolite are different Zeolites are generally identified by their framework topology. For example, the zeolite framework structure type is selected from framework types ABW, ACO, AEI, AEL, AEN, AET, AFG, AFI, AFN, AFO, AFR, AFS, AFT, AFX, AFY, AHT, ANA, APC, APD, AST, ASV, ATN, ATO, ATS, ATT, ATV, AVL, AWO, AWW, BCT, BEA, BEC, BIK, BOG, BPH, BRE, CAN, CAS, SCO, CFI, SGF, CGS, CHA, CHI, CLO, CON, CZP, DAC, DDR, DFO, DFT, DOH, DON, EAB, EDI, EEI, EMT, EON, EPI, ERI, ESV, ETR, EUO, FAU, FER, FRA, GIS, GIU, GME, GON, GOO, HEU, IFR, IFY, IHW, IRN, ISV, ITE, ITH, ITW, IWR, IWW, JBW, KFI, LAU, LEV, LIO, LIT, LOS, LOV, LTA, LTL, LTN, MAR, MAZ, MEI, MEL, MEP, MER, MFI, MFS, MON, MOR, MOZ, MSO, MTF, MTN, MTT, MTW, MWF, MWW, NAB, NAT, NES, NON, NPO, NPT, NSI, OBW, OFF, OSI, OSO, OWE, PAR, PAU, PHI, PON, RHO, RON, RRO, RSN, RTE, RTH, RUT, RWR, RWY, SAO, SAS, SAT, SAV, SBE, SBS, SBT, SFE, SFF, SFG, SFH, SFN, SFO, SFW, SGT, SOD, SOS, SSY, STF, STI, STT, TER, THO, TON, TSC, UEI, UFI, UOZ, USI, UTL, VET, VFI, VNI, VSV, WIE, WEN, YUG, ZON, and combinations thereof. In some embodiments, the zeolite framework is selected from AEI, AFT, AFV, AFX, AVL, CHA, DDR, EAB, EEI, ERI, IFY, IRN, KFI, LEV, LTA, LTN, MER, MWF, NPT, PAU, RHO, RTE, RTH, SAS, SAT, SAV, SFW, TSC, and UFI. The disclosed zeolites are generally prepared in the alkali metal form, wherein “alkali metal form,” as used herein, refers to zeolites having alkali metal ions residing within the zeolitic ion-exchange sites. Ion-exchange of zeolites in the alkali metal form with exchangeable cations, such as NH4⁺, H⁺, or alkaline earth metal ions, introduces exchangeable ions inside the zeolitic framework structure, changing the form of the zeolite. For example, ion-exchange of zeolites in the alkali metal form with NH4⁺ ions affords zeolites in the NH4⁺ form. Zeolites prepared for use in the catalyst compositions disclosed herein can be in the H⁺, NH₄ ⁺, alkali metal form, alkaline earth metal form, or combinations thereof. In some embodiments, the catalyst composition disclosed herein comprises at least one zeolite partially or completely in the H⁺ form. Zeolitic hydrocarbon storage materials can be natural or synthetic zeolites such as faujasite, chabazite, clinoptilolite, mordenite, silicalite, zeolite X, zeolite Y, ultrastable zeolite Y, ZSM-5 zeolite, offretite, or a beta zeolite. Certain zeolite adsorbent materials have a high silica-to-alumina ratio. The zeolites may have a SAR ratio of at least about 25:1, e.g., at least about 50:1, with example ranges of from about 25:1 to about 1000:1, about 50:1 to about 500:1, and about 25:1 to about 300:1. Non-limiting example zeolites include ZSM, Y, and beta zeolites. One adsorbent may comprises a beta zeolite of the type disclosed in U.S. Patent No.6,171,556, which is incorporated herein by reference in its entirety. In some embodiments of catalyst compositions disclosed herein, zeolites or other hydrocarbon storage components are used in an amount of about 0.05 g/in³ to about 1 g/in³. The disclosed component capable of promoting NOx conversion to a nitrogen-substituted hydrocarbon, in combination with the storage material, provides for low temperature trapping and storage of NOx within the catalyst composition in the form of a nitrogen-substituted hydrocarbon (e.g., at a temperature below about 160°C or at a temperature below about 200°C), and allows the nitrogen-substituted hydrocarbon to be released at a higher temperature (e.g., at temperatures at which traditional NO_x removal technologies, such as NH3-selective catalytic reduction (SCR), are active, e.g., at a temperature above about 160°C or a temperature above about 200°C. Such materials can exhibit high hydrocarbon and substituted hydrocarbon storage capacity within this temperature range. The disclosed combination of a component capable of promoting NOx conversion to a nitrogen-substituted hydrocarbon and hydrocarbon/nitrogen-substituted hydrocarbon storage material is in direct contrast to typical approaches. For example, the use of a component with an acidic surface (along with a trapping component) is in contrast to many known materials for NOx storage/release, which commonly have basic surfaces (e.g., BaO), and react with NOx to produce surface nitrates (e.g., BaNO3). In some embodiments, the catalyst composition disclosed herein exhibit higher NOx storage capacity per unit mass and/or store NOx over a broader temperature range than known compositions designed for such use. In some embodiments, a catalyst composition provided herein is in the form of a mixture (e.g., wherein the two components referenced herein above are provided within the same washcoat layer). However, in some embodiments, the catalyst composition can be provided in separated form, e.g., wherein the component capable of promoting NOx conversion to a nitrogen-substituted hydrocarbon and the hydrocarbon storage material are provided within separate washcoat layers, as will be described in further detail herein below. Other components of the disclosed catalyst composition can vary without departing from the disclosure, and include any catalytically active materials commonly employed in emission control systems of gasoline or diesel engines. Relevant components can depend, for example, on other catalyst compositions employed within a given exhaust gas treatment system and the like. In particular embodiments, the catalyst composition further comprises various components commonly employed within diesel oxidation catalysts. In some embodiments, the catalyst composition provides one or more components to oxidize the hydrocarbons and nitrogen-substituted hydrocarbons adsorbed by the hydrocarbon storage material when desorption occurs (at high temperature). In some embodiments, such functionality is included within the same catalyst composition; in other embodiments, it may be included in a separate catalyst composition and, in certain embodiments, on a different catalyst substrate. For example, in some embodiments, a catalyst composition can further comprise one or more catalytic metals impregnated or ion-exchanged in a porous support, with example supports including refractory metal oxides and molecular sieves. In some embodiments, the catalytic metal is selected from base metals, platinum group metals, oxides of base metals or platinum group metals, and combinations thereof. In some embodiments, the catalytic metal comprises platinum, palladium, or a mixture thereof. As used herein, “platinum group metal” refers to a platinum group metal or an oxide thereof, such as, e.g., platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), osmium (Os), iridium (Ir), and mixtures thereof. In some embodiments, the platinum group metal comprises a combination of platinum and palladium, such as in a weight ratio of about 1:10 to about 10:1. The concentrations of platinum group metal component (e.g., Pt, Pd, or a combination thereof) can vary, but may be from about 0.1 wt.% to about 10 wt.% relative to the weight of the porous support such as a refractory oxide support material (e.g., about 1 wt.% to about 6 wt.% relative to the refractory oxide support). As used herein, “base metal” refers to a transition metal or lanthanide (e.g., V, Mn, Fe, Co, Ni, Cu, Zn, Ag, Au, or Sn) or an oxide thereof that is catalytically active for oxidation of CO, NO, or hydrocarbon, or promotes another catalytic component to be more active for the oxidation of CO, NO, or hydrocarbon, and particularly includes copper, manganese, cobalt, iron, chromium, nickel, cerium, and combinations thereof. For ease of reference herein, concentrations of base metal or base metal oxide materials are reported in terms of elemental metal concentration rather than the oxide form. The total concentration of base metal in the base metal oxide component (e.g., copper, manganese, nickel, cobalt, iron, cerium, praseodymium, and combinations thereof) can vary, but may be from about 1 wt.% to about 50 wt.% relative to the weight of the porous support such as refractory oxide support material (e.g., about 10 wt.% to about 50 wt.% relative to the refractory oxide support). As used herein, “porous refractory oxide” refers to porous metal-containing oxide materials exhibiting chemical and physical stability at high temperatures (e.g., about 800°C), such as the temperatures associated with diesel engine exhaust. Example refractory oxides include, but are not limited to, alumina, silica, zirconia, titania, ceria, and physical mixtures or chemical combinations thereof, including atomically-doped combinations and including high surface area or activated compounds, such as activated alumina. Example combinations of metal oxides include, but are not limited to, alumina-zirconia, ceria-zirconia, alumina-ceria-zirconia, lanthana-alumina, lanthana-zirconia-alumina, baria-alumina, baria lanthana-alumina, baria lanthana-neodymia alumina, and alumina-ceria. Example aluminas include, but are not limited to, large pore boehmite, gamma-alumina, and delta/theta alumina. Example commercial aluminas include, but are not limited to, activated aluminas, such as high bulk density gamma-alumina, low or medium bulk density large pore gamma-alumina, and low bulk density large pore boehmite and gamma-alumina. High surface area refractory oxide supports, such as alumina support materials, also referred to as “gamma alumina” or “activated alumina” herein, may exhibit a BET surface area in excess of about 60 m²/g, e.g., up to about 200 m²/g or higher. Such activated alumina may be a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa, and theta alumina phases. As used herein, “BET surface area” has its usual meaning of referring to the Brunauer, Emmett, Teller method for determining surface area by N2 adsorption. In non-limiting examples, the active alumina has a specific surface area of about 60 m²/g to about 350 m²/g, such as, e.g., about 90 m²/g to about 250 m²/g. Catalytic Article In some embodiments, the disclosed catalytic composition may be disposed on a substrate. The substrate for the catalyst composition may be constructed of any material typically used for preparing automotive catalysts and may comprise a metal or ceramic honeycomb structure. The substrate may provide a plurality of wall surfaces upon which the catalyst composition is applied and adhered, thereby acting as a carrier for the catalyst composition. Example metallic substrates include, but are not limited to, heat resistant metals and metal alloys, such as, e.g., titanium and stainless steel as well as other alloys in which iron is a substantial or major component. Such alloys may contain one or more of nickel, chromium, and/or aluminum, and the total amount of these metals may comprise at least about 15 wt.% of the alloy, e.g., about 10 wt.% to about 25 wt.% of chromium, about 3 wt.% to about 8 wt.% of aluminum, and up to about 20 wt.% of nickel. The alloys may also contain small or trace amounts of one or more other metals, such as, e.g., manganese, copper, vanadium, titanium, and the like. The surface or the metal carriers may be oxidized at high temperatures, e.g., about 1000°C and higher, to form an oxide layer on the surface of the substrate, improving the corrosion resistance of the alloy and facilitating adhesion of the washcoat layer to the metal surface. Ceramic materials used to construct the substrate may include any suitable refractory material, e.g., cordierite, mullite, cordierite-α alumina, silicon nitride, zircon mullite, spodumene, alumina-silica magnesia, zircon silicate, sillimanite, magnesium silicates, zircon, petalite, α alumina, aluminosilicates, and the like. Any suitable substrate may be employed, such as a monolithic flow-through substrate having a plurality of fine, parallel gas flow passages extending from an inlet to an outlet face of the substrate such that passages are open to fluid flow. The passages, which are essentially straight paths from the inlet to the outlet, are defined by walls on which the catalytic material is coated as a washcoat so that the gases flowing through the passages contact the catalytic material. The flow passages of the monolithic substrate are thin-walled channels which can be of any suitable cross-sectional shape, such as trapezoidal, rectangular, square, sinusoidal, hexagonal, oval, circular, and the like. Such structures may contain from about 60 to about 1200 or more gas inlet openings (i.e., “cells”) per square inch of cross section (cpsi), more usually from about 300 cpsi to about 600 cpsi. The wall thickness of flow-through substrates can vary, with a typical range being between about 0.002 inches and about 0.1 inches. A representative commercially-available flow-through substrate is a cordierite substrate having about 400 cpsi and a wall thickness of about 6 mil, or about 600 cpsi and a wall thickness of about 4 mil. However, it will be understood that the disclosure is not limited to a particular substrate type, material, or geometry. In some embodiments, the substrate may be a wall-flow substrate, wherein each passage is blocked at one end of the substrate body with a non-porous plug, with alternate passages blocked at opposite end-faces. This requires that gas flow through the porous walls of the wall-flow substrate to reach the exit. Such monolithic substrates may contain up to about 700 or more cpsi, such as about 100 cpsi to about 400 cpsi or about 200 cpsi to about 300 cpsi. The cross-sectional shape of the cells can vary as described above. Wall-flow substrates may have a wall thickness between about 0.002 inches and about 0.1 inches. A representative commercially available wall-flow substrate is constructed from a porous cordierite, an example of which has about 200 cpsi and about 10 mil wall thickness or about 300 cpsi with about 8 mil wall thickness, and wall porosity between about 40% and about 70%. Other ceramic materials, such as, e.g., aluminum-titanate, silicon carbide, and silicon nitride, are also used as wall-flow filter substrates. However, it will be understood that the disclosure is not limited to a particular substrate type, material, or geometry. Note that where the substrate is a wall-flow substrate, the catalyst composition associated therewith (e.g., a catalyzed soot filter (CSF) composition) can permeate into the pore structure of the porous walls (i.e., partially or fully occluding the pore openings) in addition to being disposed on the surface of the walls. FIGs.1A and 1B illustrate an example substrate 2 in the form of a flow-through substrate coated with a washcoat composition as described herein. Referring to FIG.1A, example substrate 2 has a cylindrical shape and a cylindrical outer surface 4, an upstream end face 6 and a corresponding downstream end face 8, which is identical to end face 6. Example substrate 2 has a plurality of fine, parallel gas flow passages 10 formed therein. As seen in FIG.1B, flow passages 10 are formed by walls 12 and extend through carrier 2 from upstream end face 6 to downstream end face 8, the passages 10 being unobstructed so as to permit the flow of a fluid, e.g., a gas stream, longitudinally through carrier 2 via gas flow passages 10 thereof. As more easily seen in FIG.1B, walls 12 are so dimensioned and configured that gas flow passages 10 have a substantially regular polygonal shape. As shown, the catalyst composition can be applied in multiple, distinct layers if desired. In the illustrated embodiment, the catalyst composition comprises both a discrete bottom layer 14 adhered to the walls 12 of the carrier member and a second discrete top layer 16 coated over the bottom layer 14. The present disclosure can be practiced with one or more (e.g., 2, 3, or 4) catalyst layers and is not limited to the two-layer embodiment illustrated in FIG.1B. In describing the quantity of washcoat or catalytic metal components or other components of the composition, it is convenient to use units of weight of component per unit volume of catalyst substrate. Therefore, the units, grams per cubic inch (“g/in³”) and grams per cubic foot (“g/ft³”), are used herein to mean the weight of a component per volume of the substrate, including the volume of void spaces of the substrate. Other units of weight per volume, such as g/L, are also sometimes used. The total loading of the catalyst composition (including catalytic metal and support material) on the catalyst substrate, such as a monolithic flow-through substrate, can be from about 0.5 g/in³ to about 6 g/in³, such as from about 1 g/in³ to about 5 g/in³. Total loading of the platinum group metal or base metal component without support material can be in the range of about 5 g/ft³ to about 200 g/ft³ (e.g., about 10 g/ft³ to about 100 g/ft³). It is noted that these weights per unit volume are typically calculated by weighing the catalyst substrate before and after treatment with the catalyst washcoat composition, and since the treatment process involves drying and calcining the catalyst substrate at a high temperature, these weights represent an essentially solvent-free catalyst coating as essentially all of the water of the washcoat slurry has been removed. In some embodiments, it may be advantageous to separate certain components of catalyst compositions disclosed herein. For example, in some embodiments, the substrate is coated with a catalyst composition such that the inlet region of the catalyst is depleted in platinum group metal content and or has ratios of platinum group metals not designed for immediate (low temperature) oxidation. As such, in some embodiments, the disclosed catalytic article comprises an inlet zone with less oxidation activity than the rest of the catalytic article. In some embodiments, the component capable of promoting NO_x conversion to a nitrogen-substituted hydrocarbon is upstream of the hydrocarbon storage component. These components can be provided, in some embodiments, within separate washcoat layers coated on the substrate, e.g., in a so-called “zoned” configuration or a “layered” configuration. In some embodiments, the catalytic material on the substrate comprises multiple axial zones, wherein each zone has a different composition. For example, in some embodiments, the two separate washcoat compositions can be coated onto the substrate in an axially zoned configuration. In some embodiments, the same substrate can be coated once with a first washcoat composition and a second time with a second composition, wherein the first and second washcoat compositions are different. For example, in some embodiments, the two separate washcoat compositions may include two different components of the catalyst composition described herein. In one embodiment, the first catalyst composition may be coated first from the filter inlet end, and the second catalyst composition may be coated second from the filter outlet end. Non-limiting example zoned substrates coated with washcoat compositions such as the ones mentioned above, wherein the first washcoat composition (e.g., the first catalyst composition) is on the inlet end with washcoat coverage less than about 95% of the filter length and the second washcoat composition (e.g., the second catalyst composition) is on the outlet end with washcoat coverage less than about 95% of the filter length, are shown in FIGs.2-6. For example, referring to FIG.2, example substrate 22, having an inlet end 25, an outlet end 27, and an axial length extending between the inlet end 25 and outlet end 27 contains two separate washcoat zones. A first washcoat zone 24 and a second washcoat zone 26 are applied to the substrate 22. The first washcoat zone 24 extends from the inlet end 25 and comprises the first catalyst composition and a second washcoat zone 26 extends from the outlet end 27 and comprises the second catalyst composition. In some embodiments, the first washcoat zone 24 comprises the second catalyst composition and the second washcoat zone 26 comprises the first catalyst composition. In some embodiments, the first washcoat zone 24 comprises the first catalyst composition and the second washcoat zone 26 comprises the second catalyst composition. The first washcoat zone 24 of some embodiments extends from the front or inlet end 25 of the example substrate 22 through the range of about 5% to about 95%, from about 5% to about 75%, from about 5% to about 50%, or from about 10% to about 35% of the length of the substrate 22. The second washcoat zone 26 extends from the rear of outlet end 27 of the substrate from about 5% about 95%, from about 5% to about 75%, from about 5% to about 50%, or from about 10% to about 35% of the total axial length of the substrate 22. In the embodiment shown in FIG.2, the second washcoat zone 26 at least partially overlaps the first washcoat zone 24. In some embodiments, as seen in FIG.3, the first washcoat zone 24 extends from the inlet end 25 toward the outlet end 27. A second washcoat zone 26 is located adjacent and downstream from the first washcoat zone 24. The first washcoat zone 24 can at least partially overlap the second washcoat zone 26. In some embodiments, the first washcoat zone 24 comprises the first catalyst composition and the second washcoat zone 26 comprises the second catalyst composition. In some embodiments, the first washcoat zone 24 comprises the second catalyst composition and the second washcoat zone 26 comprises the first catalyst composition. The first washcoat zone 24 of some embodiments extends from the front or inlet end 25 of the substrate through the range of about 5% to about 95%, from about 5% to about 75%, from about 5% to about 50%, or from about 10% to about 35% of the length of the substrate 22. The second washcoat zone 26 extends from the rear of outlet end 27 of the substrate 22 from about 5% about 95%, from about 5% to about 75%, from about 5% to about 50%, or from about 10% to about 35% of the total axial length of the substrate 22. In some embodiments, referring to FIG.4, the same substrate can be coated with two types of washcoat slurries in two separate zones, wherein a first washcoat zone 24 includes a washcoat of a first catalyst composition and a second washcoat zone 26 includes a washcoat of a second catalyst composition are located side-by-side along the length of the substrate 22, with no overlap of the zones. In some embodiments, the first washcoat zone 24 comprises the second catalyst composition and the second washcoat zone 26 comprises the first catalyst composition. In some embodiments, the first washcoat zone 24 comprises the first catalyst composition and the second washcoat zone 26 comprises the second catalyst composition. The first washcoat zone 24 of some embodiments extends from the front or inlet end 25 of the substrate 22 through the range of about 5% to about 95%, from about 5% to about 75%, from about 5% to about 50%, or from about 10% to about 35% of the length of the substrate 22. The second washcoat layer 26 extends from the rear of outlet end 27 of the substrate 22 from about 5% to about 95%, from about 5% to about 75%, from about 5% to about 50%, or from about 10% to about 35% of the total axial length of the substrate 22. In some embodiments, as seen in FIG.5, a substrate 22 can be coated with a first washcoat zone 24 extending from the front or inlet end 25 of the substrate 22 to the rear or outlet end 27 of the substrate 22 and a second washcoat zone 26 that is coated over the first washcoat zone 24 proximate the front or inlet end 25 of the substrate 22 and extending across only a partial length of the substrate 22 (i.e., terminating before reaching the rear or outlet end 27 of the substrate 22). In some embodiments, the first washcoat zone 24 comprises the second catalyst composition and the second washcoat zone 26 comprises the first catalyst composition. In some embodiments, the first washcoat zone 24 comprises the first catalyst composition and the second washcoat zone 26 comprises the second catalyst composition. In some embodiments, the second washcoat zone 26 extends from the front of inlet end 25 of the substrate 22 from about 5% to about 95%, from about 5% to about 75%, from about 5% to about 50%, or from about 10% to about 35% of the total axial length of the substrate 22. The second washcoat layer 26 extends from the rear of outlet end 27 of the substrate 22 from about 5% to about 95%, from about 5% to about 75%, from about 5% to about 50%, or from about 10% to about 35% of the total axial length of the substrate 22. In some embodiments, as seen in FIG.6, a substrate 22 can be coated with a first washcoat zone 24 proximate the rear or outlet end 25 of the substrate 22 and extending only partially along the length of the substrate 22 (i.e., terminating before reaching the front or inlet end 25 of the substrate 22). The substrate 22 can be coated with a second washcoat zone 26. In some embodiments, the first washcoat zone 24 comprises the second catalyst composition and the second washcoat zone 26 comprises the first catalyst composition. In some embodiments, the first washcoat zone 24 comprises the first catalyst composition and the second washcoat zone 26 comprises the second catalyst composition. As seen in FIG.6, the second washcoat zone 26 extends from the front or inlet end 25 of the substrate 22 to the rear or outlet end 27 of the substrate 22 (and thus is coated completely over the first washcoat zone 26). In some embodiments, the first washcoat zone 24 extends from the rear of outlet end 27 of the substrate 22 from about 5% to about 95%, from about 5% to about 75%, from about 5% to about 50%, or from about 10% to about 35% of the total axial length of the substrate 22. Substrate Coating Process A catalyst composition described herein can be used in the form of a packed bed of powder, beads, or extruded granules. However, in some embodiments, the catalyst composition is coated on a substrate. The catalyst composition can be mixed with water (if in dried form) to form a slurry for purposes of coating a catalyst substrate. In addition to the catalyst particles, the slurry may optionally contain alumina as a binder, associative thickeners, and/or surfactants (including, e.g., anionic, cationic, non-ionic, or amphoteric surfactants). In some embodiments, the pH of the slurry can be adjusted, e.g., to an acidic pH of about 3 to about 5. When present, an alumina binder may be used in an amount of about 0.02 g/in³ to about 0.5 g/in³. The alumina binder can be, for example, boehmite, gamma-alumina, or delta/theta alumina. The slurry can be milled to enhance mixing of the particles and formation of a homogenous material. The milling can be accomplished in a ball mill, continuous mill, or other similar equipment, and the solids content of the slurry may be, e.g., about 20 wt.% to about 60 wt.%, more particularly about 30 wt.% to about 40 wt.%. In some embodiments, the post-milling slurry is characterized by a D90 particle size of about 10 microns to about 50 microns (e.g., about 10 microns to about 20 microns). The D90 is defined as the particle size at which about 90% of the particles have a finer particle size. The slurry can then be coated on the catalyst substrate using a washcoat technique known in the art. As used herein, the term “washcoat” has its usual meaning in the art of a thin, adherent coating of a material applied to a substrate, such as a honeycomb flow-through monolith substrate or a filter substrate which is sufficiently porous to permit the passage therethrough of the gas stream being treated. As used herein and as described in Heck, Ronald and Robert Farrauto, Catalytic Air Pollution Control, New York: Wiley-Interscience, 2002, pp.18-19, a washcoat layer includes a compositionally distinct layer of material disposed on the surface of a monolithic substrate or an underlying washcoat layer. A substrate can contain one or more washcoat layers, and each washcoat layer can have unique chemical catalytic functions. In some embodiments, the substrate is dipped one or more times in the slurry or otherwise coated with the slurry. Thereafter, the coated substrate is dried at an elevated temperature (e.g., about 100°C to about 150°C) for a period of time (e.g., about 1 hour to about 3 hours) and then calcined by heating, e.g., at about 400°C to about 600°C, for about 10 minutes to about 3 hours. Following drying and calcining, the final washcoat coating layer can be viewed as essentially solvent-free. After calcining, the catalyst loading 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 can be repeated as needed to build the coating to the desired loading level or thickness. The catalyst composition can be applied as a single layer or in multiple layers. A catalyst layer resulting from repeated washcoating of the same catalyst material to build up the loading level is typically viewed as a single layer of catalyst. In another embodiment, the catalyst composition is applied in multiple layers with each layer having a different composition. Additionally, the catalyst composition can be zone-coated (as referenced herein above), meaning a single substrate can be coated with different catalyst compositions in different areas along the gas effluent flow path. Emission Treatment System The present disclosure also provides an emission treatment system that incorporates a catalyst composition or an article described herein. The catalyst composition of the present disclosure may be used in an integrated emissions treatment system comprising one or more additional components for the treatment of gasoline or diesel exhaust gas emissions. As used herein, the terms “exhaust stream,” “engine exhaust stream,” “exhaust gas stream,” and the like refer to the engine effluent as well as to the effluent downstream of one or more other catalyst system components as described herein. One non-limiting example emission treatment system is illustrated in FIG. 7, which depicts a schematic representation of an emission treatment system 32. It is understood that this configuration is not intended to be limiting, but shows one possible arrangement of certain components for treatment of an exhaust gas stream. As shown, an exhaust gas stream containing gaseous pollutants and particulate matter is conveyed via exhaust pipe 36 from an engine 34 to a catalyst comprising the catalyst composition described herein, e.g., a diesel oxidation catalyst (DOC) 38. In the DOC 38, unburned gaseous and non-volatile hydrocarbons (i.e., the SOF) and carbon monoxide are largely combusted to form carbon dioxide and water. As described herein above, at least a portion of the NOx is trapped (and released at the appropriate temperature) in the form of a nitrogen-substituted hydrocarbon. Released hydrocarbons and/or nitrogen-substituted hydrocarbons may, in some embodiments, be oxidized within the DOC. The exhaust stream is next conveyed via exhaust pipe 40 to a catalyzed soot filter (CSF) 42, which traps particulate matter present within the exhaust gas stream. The CSF 42 is optionally catalyzed for passive or active soot regeneration. A CSF may comprise one or more platinum group metal components impregnated on a metal oxide support such as alumina, optionally further including an oxygen storage component (OSC), such as ceria, and may provide oxidation of both hydrocarbons and carbon monoxide. After removal of particulate matter, via CSF 42, the exhaust gas stream is conveyed via exhaust pipe 44 to a downstream selective catalytic reduction (SCR) component 16 for the further treatment and/or conversion of NOx. An SCR catalyst is adapted for catalytic reduction of nitrogen oxides with a reductant in the presence of an appropriate amount of oxygen. Reductants may be, for example, hydrocarbon, hydrogen, and/or ammonia. SCR catalysts may comprise a molecular sieve (e.g., a zeolite) ion-exchanged with a promoter metal such as copper or iron, with non-limiting example SCR catalysts including FeCHA and CuCHA. While the disclosure herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the disclosure set forth in the claims. Furthermore, various aspects of the disclosure may be used in other applications than those for which they were specifically described herein.

Claims

What is claimed is: 1. A catalyst composition comprising: a component comprising an acidic surface capable of promoting a reaction between a NOx and a hydrocarbon to produce a nitrogen-substituted hydrocarbon; and a hydrocarbon storage material capable of storing the nitrogen-substituted hydrocarbon at a temperature below about 200°C.

2. The catalyst composition of claim 1, wherein the acidic surface comprises a Bronsted acid or a Lewis acid.

3. The catalyst composition of claim 1, wherein the acidic surface comprises a platinum group metal.

4. The catalyst composition of claim 1, wherein the acidic surface comprises a sulfate surface.

5. The catalyst composition of any one of claims 1-4, wherein the hydrocarbon storage material comprises a zeolite.

6. A diesel oxidation catalytic article comprising a catalyst composition of any one of claims 1-5.

7. The diesel oxidation catalytic article of claim 6, further comprising a platinum group metal on a refractory metal support.

8. The diesel oxidation catalytic article of claim 6 or 7, wherein the component comprising the acidic surface and the hydrocarbon storage material are mixed.

9. The diesel oxidation catalytic article of claim 6 or 7, comprising a first, upstream region and a second, downstream region, wherein: the first, upstream region comprises the component comprising the acidic surface; and the second, downstream region comprises the hydrocarbon storage material.

10. The diesel oxidation catalytic article of claim 9, wherein the first, upstream region contains a lower platinum group metal content than the second, downstream region.

11. An exhaust gas treatment system for reducing NOx present in an exhaust gas stream, comprising: the diesel oxidation catalytic article of any one of claims 6-10; and a conduit for fluid communication between the exhaust gas stream and the diesel oxidation catalytic article.

12. The exhaust gas treatment system of claim 11, further comprising a selective catalytic reduction (SCR) catalyst downstream of the diesel oxidation catalytic article.

13. A method for storing NOx in an exhaust gas at a low temperature, comprising: contacting a NO_x with a hydrocarbon under conditions effective to form a nitrogen-substituted hydrocarbon at a temperature below about 200°C.

14. The method of claim 13, wherein the contacting further comprises contacting the NO_x, the hydrocarbon, or both the NO_x and the hydrocarbon with a component comprising an acidic surface capable of promoting reaction between the NOx and the hydrocarbon.

15. The method of claim 14, wherein the acidic surface comprises a Bronsted acid or a Lewis acid.

16. The method of claim 14, wherein the acidic surface comprises a platinum group metal.

17. The method of claim 14, wherein the acidic surface comprises a sulfate surface.

18. The method of any one of claims 13-17, further comprising trapping the nitrogen-substituted hydrocarbon at the temperature below about 200°C with a hydrocarbon storage material.

19. The method of claim 18, wherein the hydrocarbon storage material comprises a zeolite.

20. A method for reducing NOx in an exhaust gas at a low temperature, comprising: contacting a NOx with a hydrocarbon at an acidic surface under conditions effective to form a nitrogen-substituted hydrocarbon at a temperature below about 200°C; trapping the nitrogen-substituted hydrocarbon at the temperature below about 200°C with a hydrocarbon storage material; and desorbing the nitrogen-substituted hydrocarbon at a temperature above about 200°C that is suitable for NO_x reduction.