Exhaust gas treatment system

Abstract

The present invention provides an exhaust gas treatment system comprising an intake for receiving an exhaust gas from an internal combustion engine and a catalyst system, which catalyst system comprises: a substrate having an LNT zone extending from an inlet end and a TWC zone extending from an outlet end; or a first upstream substrate comprising an LNT zone and a second downstream substrate comprising a TWC zone. Also provided is a combustion and exhaust system and a method of its use; and a method for treating methane in an exhaust gas.

Description

[0001] Exhaust Gas Treatment System

[0002] The present invention relates to an exhaust gas treatment system, in particular for the treatment of an exhaust gas from the combustion of an alternative fuel, such as natural gas, and to a combustion and exhaust system and a method for its use. In particular, the exhaust gas treatment system is especially suited for ensuring low levels of NOx and CO emissions from such combustion whilst also achieving good methane conversion.

[0003] Alternative fuels, such as natural gas, hydrogen, ammonia, methanol, propane or blends of one or more thereof, are of increasing interest as alternative fuels for vehicles and stationary engines that traditionally use gasoline and diesel fuels. Natural gas is composed mainly of methane (typically 70-90%) with variable proportions of other hydrocarbons such as ethane, propane and butane (up to 20% in some deposits) and other gases. It can be commercially produced from oil or natural gas fields and is widely used as a combustion energy source for power generation, industrial co-generation and domestic heating. It can also be used as a vehicle fuel.

[0004] Natural gas can be used as transportation fuel in the form of compressed natural gas (CNG) and liquefied natural gas (LNG). CNG is carried in tanks pressurised to 3600 psi (-248 bar) and has an energy density around 35% of gasoline per unit volume. LNG has an energy density 2.5 times that of CNG and is mostly used for heavy-duty vehicles. It is cooled to liquid form at -162°C and as a result the volume is reduced 600 fold meaning LNG is easier to transport than CNG. Bio-LNG could be an alternative to natural (fossil) gas, being produced from biogas, derived by anaerobic digestion from organic matter such as landfill waste or manure.

[0005] Natural gas has a number of environmental benefits: it is a cleaner burning fuel typically containing few impurities, it contains higher energy (Btu) per carbon than traditional hydrocarbon fuels resulting in low carbon dioxide emissions (25% less greenhouse gas emissions), and it has lower emissions of PM and NOx compared to diesel and gasoline. Biogas could reduce such emissions further.

[0006] Further drivers for the adoption of natural gas include high abundance and lower cost compared to other fossil fuels. As a result there has been development of natural gas combustion engines. These are typically operated in one of two main operating modes: stoichiometric conditions (A = 1) and lean burn conditions (A > 1.3). The challenge for natural gas combustion engines is found in abating methane emissions since this molecule requires a very high temperature to activate the C-H bond leading to its conversion to CO2. In addition, a different type of catalyst is needed to reduce NOx to ultralow NOx levels. Currently only oversize LNTs or urea-SCR system can achieve those levels.

[0007] Accordingly, there is a desire for the provision of an improved system for natural gas combustion and exhaust gas treatment, particularly to handle the CH4, NOx and CO emissions observed. It is an object of the present invention to address this problem, tackle the disadvantages associated with the prior art, or at least provide a commercially useful alternative thereto.

[0008] According to a first aspect there is provided an exhaust gas treatment system comprising an intake for receiving an exhaust gas from an internal combustion engine and a catalyst system, which catalyst system comprises either: a substrate having an LNT zone extending from an inlet end and a TWC zone extending from an outlet end; or a first upstream substrate comprising an LNT zone and a second downstream substrate comprising a TWC zone.

[0009] In one embodiment of the first aspect, the catalyst system of the exhaust gas treatment system comprises a substrate having an LNT zone extending from an inlet end and a TWC zone extending from an outlet end. This single substrate embodiment is advantageous because it allows for the provision of a less complex system architecture. In an alternative embodiment of the first aspect, the catalyst system of the exhaust gas treatment system comprises a first upstream substrate comprising an LNT zone, and a second downstream flow-through substrate comprising a TWC zone. This separate substrate embodiment is advantageous because it facilitates the use of off-the-shelf components taken from other applications, particularly since suitable LNT and TWC components are already known and commercialised. Additionally, separate substrates are advantageous as they can be configured to fit into a volume-constrained system through interconnected piping in cases where a single longer zoned catalyst system would not fit.

[0010] In the following passages different aspects / embodiments are defined in more detail. Each aspect / embodiment so defined may be combined with any other aspect / embodiment or aspects / embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

[0011] In the following reference is made to the “LNT” or the “TWC”. For the avoidance of doubt, these terms refer equally to the LNT zone and the TWC zone, respectively, of the catalyst system.

[0012] The present inventors were seeking a system suitable for abating the emissions from a CNG fuelled engine. They considered the known urea-fed SCR systems and found that urea injection was not desirable for small CNG applications as it adds complexity to an otherwise relatively simple unit. They therefore set out to devise a system that was optimally suited for this engine-emission-type.

[0013] The inventors have found that providing a specific zoned catalyst system helps to mitigate the emissions. In particular, the zoned catalyst system has an LNT-type catalyst at the front and a TWC-type catalyst at the rear. The inventors particularly noted that using this system the overall conversion reaches levels not achievable by a single LNT or TWC sized the same as the full system. Furthermore, there was higher NOx conversion and higher CO conversion.

[0014] Unlike a normal CNG engine, the use of the present invention benefits from the use of a lean / rich cycle more commonly employed for diesel engines. The three-way catalyst (TWC) is placed / zoned downstream of a Lean NOx Trap (LNT) under lean / rich cyclic conditions typical of LNTs. The LNT converts / stores most of the emissions during the lean cycle. During the rich cycle most of the stored NOx is reduced, however NOx, CO and CH4 slips through the LNT. The LNT produces an exotherm that heats up the downstream TWC by around 100°C. The hotter TWC oxidizes the CH4 and CO with the stored O2.

[0015] In addition the LNT-out NOx and CO spikes are minimized, reaching ultra-low NOx levels at the tailpipe, not achievable by an LNT sized the same as the full system and CO conversion of 99.9%. The overall conversion reaches levels not reachable by a single LNT or TWC sized the same as the full system.

[0016] Surprisingly, the combination of the two products that typically work on different types of cycles (LNT under long lean / short rich cycles, TWC under perturbations around stoichiometric conditions) serves to create a synergetic boost on emission control. These benefits are particularly observed for a CNG-engine as described herein. In these applications a single substrate combining both functionalities may be employed. Alternatively, a combination of a first substrate having LNT functionality and a second substrate having TWC functionality may be employed for simplicity of the system architecture and hence lower costs.

[0017] The present invention relates to an exhaust gas treatment system, preferably for the treatment of an exhaust gas from the combustion of natural gas or a blend of natural gas an another fuel (such as hydrogen).

[0018] The exhaust gas treatment system is suitable for the treatment of an exhaust from engines powered by alternative fuels, for example, natural gas, hydrogen, ammonia, methanol, propane or a blend of one or more thereof.

[0019] In particular, the exhaust gas treatment system is suitable for the treatment of an exhaust from an internal combustion engine fuelled by natural gas or a blend of natural gas with one or more other alternative fuels, for example, a blend of natural gas with hydrogen. That is, the exhaust gas treatment system is suitable for the catalytic treatment of exhaust gases from a natural-gas combustion engine in order to convert or transform components of the gases before they are emitted to the atmosphere in order to meet emissions regulations. When natural gas is combusted it will produce both carbon dioxide and water, but the exhaust gas also contains an amount of additional methane (and other short chain hydrocarbons) that needs to be catalytically removed before the exhaust is emitted to the atmosphere. The combustion also produces NOx and CO gases which need to be treated.

[0020] The LNT and TWC zones may be disposed on a single substrate or on separate substrates. The substrate(s) may be any of those materials typically used for preparing catalysts, and will typically comprise a ceramic or metal honeycomb structure.

[0021] Any suitable substrate may be employed, such as a monolithic substrate of the type having fine, parallel gas flow passages extending therethrough from an inlet or an outlet face of the substrate, such that passages are open to fluid flow therethrough (referred to herein as flow- through substrates). The passages, which are essentially straight paths from their fluid inlet to their fluid 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.

[0022] Alternatively, the monolithic substrate may be in the form of a wall-flow filter. The flow passages of the monolithic substrate are thin-walled channels, which can be of any suitable cross-sectional shape and size such as trapezoidal, rectangular, square, sinusoidal, hexagonal, oval, circular, etc.

[0023] Such monolithic substrates may contain up to about 900 or more flow passages (or “cells”) per square inch of cross section, although far fewer may be used. For example, the substrate may have from about 7 to 600, more usually from about 100 to 400, cells per square inch (“cpsi”). The cells can have cross sections that are rectangular, square, circular, oval, triangular, hexagonal, or are of other polygonal shapes. The ceramic substrate(s) may be made of any suitable refractory material, e.g., cordierite, cordierite-alumina, silicon nitride, or silicon carbide, or the substrates may be composed of one or more metals or metal alloys.

[0024] The catalyst article may comprise a flow-through substrate or two flow-through substrates arranged in series.

[0025] Where the LNT and TWC are disposed on a single substrate, the substrate has an LNT zone extending from an inlet end. The inlet end is the end which, in use, receives the untreated gases. Where the LNT and TWC are disposed on separate substrates, the LNT is positioned upstream with respect to normal flow of exhaust gas through the catalyst system in use. In the present invention, this means that the gases to be treated reach the LNT before they reach the TWC.

[0026] LNTs are a well-known component in diesel exhaust gas treatment systems. Examples of suitable LNT compositions and configurations are described in WO2013093597, WO2018042181 and WO2018178686, the contents of which are incorporated herein by reference.

[0027] LNTs are devices that adsorb NOx under lean exhaust conditions, release the adsorbed NOx under rich conditions, and reduce the released NOx to form N2. More specifically, the NOx trap performs three functions. First, nitric oxide reacts with oxygen to produce NO2 in the presence of the oxidation catalyst. Second, the NO2 is adsorbed by the NOXadsorbent in the form of an inorganic nitrate (for example, BaO or BaCCh is converted to Ba(NOs)2 on the NOx adsorbent). Lastly, when the engine runs under rich conditions, the stored inorganic nitrates decompose to form NO or NO2 which are then reduced to form N2 by reaction with carbon monoxide, hydrogen and / or hydrocarbons in the presence of the reduction catalyst. Typically, the nitrogen oxides are converted to nitrogen, carbon dioxide and water in the presence of heat, carbon monoxide and hydrocarbons in the exhaust stream. Accordingly, the key components of an LNT are a NOx adsorbent for the storage of NOx and an oxidation / reduction catalyst. Preferred LNTs comprise Pt, Rh and Ba. Such LNTs may be Pd-free.

[0028] Suitable NOXadsorbent component is typically an alkaline earth metal (such as Ba, Ca, Sr, and Mg), an alkali metal (such as K, Na, Li, and Cs), a rare earth metal (such as La, Y, Pr, and Nd), or combinations thereof. These metals are typically found in the form of oxides. The alkali or alkali earth metal is preferably barium. Preferably the barium, where present, is present in an amount of 400 to 1200gft3, more preferably 600 to 1000gft'3and most preferably 700 to 900gft-3.

[0029] The oxidation catalyst is typically one or more noble metals, preferably platinum and / or palladium, and / or rhodium. Preferably, platinum is included to perform the oxidation function and rhodium is included to perform the reduction function. The lean NOXtrap catalyst preferably comprises 10 to 200 g / ft3, more preferably 50 to 150 g / ft3, most preferably 100 to 130 g / ft3. Preferably the LNT comprises the Pt and Rh in a ratio of from 5:1 to 1 :5, more preferably 4:1 to 1 :1.

[0030] The oxidation / reduction catalyst and the NOXadsorbent are typically loaded on a support material such as an inorganic oxide for use in the exhaust system. Suitable supports include ceria and / or alumina optionally doped with one or more further elements, such as rare earth metals. Preferred supports preferably have a surface area in the range 10 to 1500 m2 / g, pore volumes in the range 0.1 to 4 mL / g, and pore diameters from about 10 to 1000 Angstroms. High surface area inorganic oxides having a surface area greater than 80 m2 / g are particularly preferred, e.g. high surface area ceria or alumina. The LNT may be free from zirconia.

[0031] The LNT zone is preferably formed as one or more washcoat layers on the substrate. Many designs of LNT are known which include two or more, or three or more, layers. It may be desirable, for example, to isolate the Rh from the Pt. The washcoat layers can be formed using conventional techniques, which may involve dipping a substrate in a suitable washcoat composition. Where the LNT and TWC zones are disposed on a single substrate, since the LNT extends from the inlet end, washcoating will typically involve dipping an inlet end of the substrate in suitable washcoat compositions. After dipping the coated substrate will be dried and calcined before use. Where the LNT and TWC zones are disposed on a single substrate, preferably the LNT zone extends from 20 to 80%, preferably 60 to 80% of an axial length of the substrate. Where the LNT and TWC zones are disposed on separate substrates with equivalent cell density, preferably the upstream substrate has a greater volume compared to the downstream substrate. The provision of the LNT preferably as the dominant component of the substrate ensures that the LNT traps the exhaust components and the TWC acts to handle slipped components during the rich cycle.

[0032] The LNT zone preferably comprises a catalyst composition comprising: i) Pt and Rh in a total amount of from 25 to 200gft’3, preferably in a ratio of Pt to Rh of from 4:1 to 1 :1 ; ii) Ba in a total amount of from 700 to 900gft-3; iii) alumina and / or ceria, preferably both.

[0033] Where the LNT and TWC are disposed on a single substrate, the substrate has a TWC zone extending from an outlet end. The outlet end is the end which, in use, releases the treated gases. Where the LNT and TWC are disposed on separate substrates, the TWC is positioned downstream with respect to normal flow of exhaust gas through the catalyst system in use. In the present invention, this means that the gases to be treated reach the LNT before they reach the TWC.

[0034] TWCs are a well-known component in gasoline exhaust gas treatment systems. TWCs perform three main functions: (1) oxidation of CO; (2) oxidation of unburned hydrocarbons; and (3) NOx reduction to N2. Examples of suitable TWC compositions and configurations are described in WO2021165697 and EP3699410, the contents of which are incorporated herein by reference.

[0035] Three-way catalysts (TWCs) are intended to catalyse three simultaneous reactions: (i) oxidation of carbon monoxide to carbon dioxide, (ii) oxidation of unburned hydrocarbons to carbon dioxide and water; and (iii) reduction of nitrogen oxides to nitrogen and oxygen. The active components in a typical TWC comprise one or more of platinum, palladium and rhodium, supported on a high surface area oxide, and an oxygen storage component. TWCs generally combine a supported PGM component (typically Pd is preferred) with an oxygen storage component (OSC), such as a mixed oxide of ceria and zirconia. An OSC is an entity that has multi-valence state and can actively react with oxidants such as oxygen or nitrous oxides under oxidative conditions, or reacts with reductants such as carbon monoxide (CO) or hydrogen under reducing conditions. Suitable oxygen storage components include cerium-containing species, for example, ceria or cerium-containing mixed oxides. Praseodymia can also be included as an OSC. Preferably, the OSC comprises or consists of one or more mixed oxides. The OSC can be ceria or a mixed oxide comprising cerium. The OSC may comprise a mixed oxide of cerium and zirconium; a mixed oxide of cerium, zirconium, and neodymium; a mixed oxide of praseodymium and zirconium; a mixed oxide of cerium, zirconium and praseodymium; or a mixed oxide of praseodymium, cerium, lanthanum, yttrium, zirconium and neodymium. Preferably the OSC is selected from the group consisting of cerium oxide, a ceria-zirconia mixed oxide, and an alumina-ceria- zirconia mixed oxide.

[0036] A loading of the OSC is preferably at least 2.2g / in3. Preferably the loading of the OSC is from 2.2 to 4g / in3, preferably from about 2.4 to about 2.6g / in3and most preferably about 2.5g / in3.

[0037] Preferably the TWC comprises one or more platinum group metals (PGM) selected from Pd, Pt and Rh on a support (preferably Pd and Rh and preferably no Pt). The palladium component and the rhodium component may have a weight ratio of from 200:1 to 1 :200. Preferably, the palladium component and the rhodium component have a weight ratio of from 100:1 to 1 :100. More preferably, the palladium component and the rhodium component have a weight ratio of from 50:1 to 1 :50. Most preferably, the palladium component and the rhodium component may have a weight ratio of from 15: 1 to 1 : 15.

[0038] The support for the PGMs may be independently selected from the group consisting of alumina, silica-alumina, alumino-silicates, alumina-zirconia, alumina-ceria, and alumina- lanthanum. Suitable supports are well known in the art. Preferably the high surface area support has a surface area of at least 70 m2 / g, for example at least 80m2 / g, at least 150 m2 / g or at least 200 m2 / g.

[0039] The TWC zone is preferably formed as one or more washcoat layers on the substrate. Many designs of TWC are known which include two or more, or three or more, layers. It may be desirable, for example, to isolate the Rh from the Pd. The washcoat layers can be formed using conventional techniques, which may involve dipping a substrate in a suitable washcoat composition. Where the LNT and TWC zones are disposed on a single substrate, since the TWC extends from the outlet end, these will typically involve dipping an outlet end of the substrate in suitable washcoat compositions. After dipping the coated substrate will be dried and calcined before use.

[0040] Where the LNT and TWC zones are disposed on a single substrate, preferably the TWC zone extends from 20 to 80%, preferably 20 to 40% of an axial length of the substrate. In the preferred embodiment where the TWC is the minor component of the catalyst article, the TWC serves to address the slip from the LNT.

[0041] Preferably the TWC zone comprises Pd, Rh and an Oxygen storage component (OSC).

[0042] Preferably the OSC in the TWC does not contain doped or pure ceria. Preferably the OSC comprises a mixed oxide of ceria and zirconia, preferably wherein the zirconia represents at least 10wt% of the mixed oxide.

[0043] The TWC may comprise one or more alkaline earth metals and, preferably the alkaline earth metals comprise barium. Preferably the TWC comprises the alkaline earth metal in an amount of up to 400 g / ft3. Preferably the TWC comprises less alkaline earth metal than the LNT. Preferably the TWC contains at most 50wt% of the alkaline earth metal of the LNT, preferably at most 40wt%.

[0044] The TWC zone preferably comprises a catalyst composition comprising: i) Pd and Rh in a total amount of from 25 to 200gft3, preferably in a ratio of Pd to Rh of greater than 5:1 , preferably greater than 10:1; ii) Ba in a total amount of less than 400gft-3, preferably 100 to 400 gft3; iii) a zirconia-containing mixed oxide; and iii) alumina.

[0045] Where the LNT and TWC zones are disposed on a single substrate, preferably the LNT zone and the TWC zone together substantially cover the substrate. That is, it is not desirable for there to be any regions of uncoated substrate between the two zones. Indeed, in some embodiments there may be small amounts of overlap to avoid this potential uncoated region.

[0046] Where the LNT and TWC zones are disposed on separate substrates, the upstream and downstream substrates are preferably arranged adjacent with minimal gap therebetween. Preferably the LNT zone covers the whole of the upstream substrate and preferably the TWC zone covers the whole of the downstream substrate (i.e. as a washcoat on the channels of each substrate). Preferably a volume ratio of the upstream substrate to the downstream substrate is from 20:80 to 80:20, more preferably from 60:40 to 80:20 such that there is a greater volume of the LNT.

[0047] According to a further aspect there is provided a combustion and exhaust system comprising, an internal combustion engine and an exhaust gas treatment system as described above (i.e. either a single article or two separate flow-through substrates) arranged to receive and treat an exhaust gas from the combustion engine. Preferably the internal combustion engine is a natural gas combustion engine. The natural gas combustion engine may be fuelled by natural gas or a blend of natural gas with a renewable fuel (e.g. hydrogen).

[0048] According to a further aspect there is provided a method for the combustion of a fuel comprising natural gas in the combustion and exhaust system described herein, the method comprising: operating the combustion engine under lean conditions whereby NOx is stored in the LNT zone; and intermittently operating the combustion engine under rich conditions, whereby NOx is released from the LNT zone and remediated in the TWC zone.

[0049] According to a further aspect there is provided a method of treating an exhaust gas comprising methane, which method comprises contacting the exhaust gas with a catalyst system comprising: a substrate having an LNT zone extending from an inlet end and a TWC zone extending from an outlet end; or a first upstream substrate comprising an LNT zone and a second downstream substrate comprising a TWC zone.

[0050] Washcoating techniques will now be described further. The washcoating is preferably performed by first slurrying finely divided particles of the catalysts in an appropriate solvent, preferably water, to form a slurry. The slurry preferably contains between 5 to 70 weight percent solids, more preferably between 10 to 50 weight percent. Preferably, the particles are milled or subject to another comminution process in order to ensure that substantially all of the solid particles have a particle size of less than 20 microns in an average diameter, prior to forming the slurry. Additional components, such as stabilizers or promoters may also be incorporated in the slurry as a mixture of water soluble or water-dispersible compounds or complexes.

[0051] The substrate(s) may then be coated one or more times with the slurry such that there will be deposited on the substrate the desired loading of the catalysts.

[0052] After the substrate has been coated with the catalyst composition slurry, the coated substrate is preferably dried and then calcined by heating at an elevated temperature to form the catalyst. Preferably, the calcination occurs at 400 to 600°C for approximately 1 to 8 hours.

[0053] Figures

[0054] The invention will now be described further in relation to the following non-limiting examples, in which:

[0055] Figure 1 shows predicted CO Conversion modelled under 9 representative engine conditions. In this Figure the columns for each condition start (i.e. on the left) with the paired TWC and end (on the right) with the paired LNT, with the inventive pairing being the middle column.

[0056] Figure 2 shows predicted CO Conversion modelled under 9 representative engine conditions. In this Figure the columns for each condition start with a single LNT and end with the paired LNT, with the inventive pairing (LNT / TWC) being the middle column.

[0057] Figure 3 shows predicted NOXConversion modelled under 9 representative engine conditions. In this Figure the columns for each condition start with the paired TWC and end with the paired LNT, with the inventive pairing being the middle column.

[0058] Figure 4 shows predicted NOXConversion modelled under 9 representative engine conditions. In this Figure the columns for each condition start with a single TWC and end with the paired TWC, with the inventive pairing being the middle column.

[0059] Figure 5 shows predicted CH4 oxidation modelled under 9 representative engine conditions. In this Figure the columns for each condition start with paired LNTs, followed by the inventive pairing, and end with a single TWC. Figure 6 shows a schematic of a suitable exhaust treatment system 1. The system comprises an CNG engine 5 and an exhaust treatment system 10. The system 10 starts with an intake 15 for receiving exhaust gases from the engine 5 and exhausting treatment gas from an exhaust pipe 20 to the atmosphere. The system 10 comprises an article 25 as described herein having an upstream LNT zone 26 and a downstream TWC zone 27. Downstream of the article 25 is a particulate filter 30, preferably a wall-flow filter.

[0060] Examples

[0061] The invention will now be disclosed further in relation to the following non-limiting examples.

[0062] A series of three representative fuelling conditions - lean, stoichiometric, and rich, and three operating temperatures (inlets ~430°C, 510°C, and 580°C, designated as [e.g. Rich] “1”, “2”, “3” respectively) were modelled and offer a total matrix of nine test conditions to compare catalysts against the catalyst combination disclosed herein. For simplicity, the modelling compares an LNT / TWC combination article against a pair of LNT articles and a pair of TWC articles having the equivalent volume (and, as appropriate, compositions).

[0063] As can be seen from Figure 1 , under fuel rich conditions, a paired LNT fails to eradicate CO emissions, with over 50% of CO passing through unconverted. Similarly, in Figure 2, under lean, and to a less extent even stoichiometric conditions, a paired TWC of equivalent volume fails to mitigate NOx emissions to requisite levels. The LNT+TWC catalyst combination in series however maintains full conversion across all 9 conditions for both pollutants. That is, overall conversion reaches levels not achievable by a single LNT or TWC sized the same as the full system.

[0064] As compared to either a single TWC or a paired TWC of equivalent volume, Figure 3 shows the LNT+TWC configuration yields higher NOXconversion under stoichiometric and lean fuel conditions.

[0065] As compared to either a single LNT or a paired LNT of equivalent volume, Figure 4 shows the LNT+TWC configuration yields higher CO conversion under rich fuel conditions.

[0066] Figure 5 serves to highlight the necessity of a TWC in the system. As compared to a single TWC of equivalent volume, the LNT+TWC configuration achieves equivalent or close to equivalent methane conversion. In contrast, even paired LNTs fail to mitigate CH4 slip under all conditions, with only partial conversion under rich engine conditions.

[0067] As used herein, the singular form of “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. The use of the term “comprising” is intended to be interpreted as including such features but not excluding other features and is also intended to include the option of the features necessarily being limited to those described. In other words, the term also includes the limitations of “consisting essentially of” (intended to mean that specific further components can be present provided they do not materially affect the essential characteristic of the described feature) and “consisting of” (intended to mean that no other feature may be included such that if the components were expressed as percentages by their proportions, these would add up to 100%, whilst accounting for any unavoidable impurities), unless the context clearly dictates otherwise.

[0068] It will be understood that, although the terms "first", "second", etc. may be used herein to describe various elements, layers and / or portions, the elements, layers and / or portions should not be limited by these terms. These terms are only used to distinguish one element, layer or portion from another, or a further, element, layer or portion. It will be understood that the term “on” is intended to mean “directly on” such that there are no intervening layers between one material being said to be “on” another material. Spatially relative terms, such as “under”, "below", "beneath", "lower", “over”, "above", "upper" and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s). It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if a device as described herein is turned over, elements described as "under” or “below" other elements or features would then be oriented “over” or "above" the other elements or features. Thus, the example term "under" can encompass both an orientation of over and under. The device may be otherwise oriented and the spatially relative descriptors used herein interpreted accordingly.

[0069] The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.

Claims

Claims:1 . An exhaust gas treatment system comprising an intake for receiving an exhaust gas from an internal combustion engine and a catalyst system, which catalyst system comprises: a substrate having an LNT zone extending from an inlet end and a TWC zone extending from an outlet end; or a first upstream substrate comprising an LNT zone and a second downstream substrate comprising a TWC zone.

2. The exhaust gas treatment system according to claim 1 , wherein the LNT zone comprises a catalyst composition comprising Pt, Rh and Ba.

3. The exhaust gas treatment system according to claim 1 or claim 2, wherein the LNT zone comprises a catalyst composition comprising: i) Pt and Rh in a total amount of from 25 to 200gft’3, preferably in a ratio of Pt to Rh of from 4:1 to 1 :1 ; ii) Ba in a total amount of from 700 to 900gft-3; iii) alumina and / or ceria, preferably both.

4. The exhaust gas treatment system according to any preceding claim, wherein the TWC zone comprises a catalyst composition comprising Pd, Rh and an OSC.

5. The exhaust gas treatment system according to claim 4, wherein the TWC zone comprises a catalyst composition comprising: i) Pd and Rh in a total amount of from 25 to 200gft3, preferably in a ratio of Pd to Rh of greater than 5:1 , preferably greater than 10:1 ; ii) Ba in a total amount of less than 400gft-3, preferably 100 to 400 gft3; iii) a zirconia-containing mixed oxide; and iii) alumina.

6. The exhaust gas treatment system according to any preceding claim, wherein the TWC zone comprises a first total amount of alkaline earth metal and the LNT comprises a second total amount of alkaline earth metal, and wherein the first total amount of alkaline earth metal is at most 50wt% the second total amount of alkaline earth metal, preferably at most 40wt%.

7. The exhaust gas treatment system according to any preceding claim which comprises a substrate having an LNT zone extending from an inlet end and a TWC zone extending from an outlet end, wherein the substrate is a flow-through substrate.

8. The exhaust gas treatment system according to claim 7, wherein the LNT zone extends from 20 to 80%, preferably 60 to 80% of an axial length of the substrate; and / or wherein the TWC zone extends from 20 to 80%, preferably 20 to 40% of an axial length of the substrate; and / or wherein the LNT zone and the TWC zone together substantially cover the substrate.

9. The exhaust gas treatment system according to any of claims 1 to 6, which comprises a first upstream substrate comprising an LNT zone and a second downstream substrate comprising a TWC zone, wherein the first upstream substrate is a flow-through substrate and / or the second downstream substrate is a flow-through substrate.

10. The exhaust gas treatment system according to claim 9, wherein the upstream substrate has a larger volume than the downstream substrate.

11. A combustion and exhaust system comprising, an internal combustion engine and an exhaust gas treatment system according to any preceding claim arranged to receive and treat an exhaust gas from the combustion engine.

12. A combustion and exhaust system as claimed in claim 11, wherein the internal combustion engine is a natural gas combustion engine.

13. A method for the combustion of a fuel comprising natural gas in the system according to claim 11, the method comprising: operating the combustion engine under lean conditions whereby NOx is stored in the LNT zone; and intermittently operating the combustion engine under rich conditions, whereby NOx is released from the LNT zone and remediated in the TWC zone.

14. A method for treating methane in an exhaust gas, which method comprises contacting the exhaust gas with a catalyst system comprising: a substrate having an LNT zone extending from an inlet end and a TWC zone extending from an outlet end; ora first upstream substrate comprising an LNT zone and a second downstream flow- through substrate comprising a TWC zone.

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