Ammonia slip catalyst

Abstract

The present invention relates to an ammonia slip catalyst article comprising a substrate, a first catalyst coating, and a second catalyst coating. The first catalyst coating comprises an ammonia oxidation catalyst. The second catalyst coating comprises a selective catalytic reduction catalyst and is formed by calcining a second catalyst washcoat comprising polysaccharide particles.

Description

[0001] AMMONIA SLIP CATALYST

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to an ammonia slip catalyst for treating combustion exhaust gas, and particularly for reducing ammonia slip associated with a selective catalytic reduction process.

[0004] BACKGROUND OF THE INVENTION

[0005] Combustion of fuels such as hydrocarbons produces engine exhaust or flue gas that contains, in large part, relatively benign nitrogen (N2), water vapor (H2O), and carbon dioxide (CO2). But the exhaust gas may also contains, in relatively small part, noxious and / or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, and particulate matter (mostly soot). To mitigate the environmental impact of exhaust gas released into the atmosphere, it is desirable to eliminate or reduce the amount of undesirable components, preferably by a process that, in turn, does not generate other noxious or toxic substances.

[0006] NOx, which includes nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O), is a burdensome component to remove from exhaust gas generated by lean burn engines. The reduction of NOx to N2 is particularly problematic in lean burn exhaust gas because the exhaust gas contains enough oxygen to favor oxidative reactions instead of reduction. Notwithstanding, NOx can be reduced by a process commonly known as Selective Catalytic Reduction (SCR). An SCR process involves the conversion of NOx, in the presence of a catalyst and with the aid of a nitrogenous reducing agent, such as ammonia, into elemental nitrogen (N2) and water. In an SCR process, a gaseous reductant such as ammonia is added to an exhaust gas stream prior to contacting the exhaust gas with the SCR catalyst. The reductant is absorbed onto the SCR catalyst and the NOx reduction reaction takes place as the gases pass through or over the SCR catalyst.

[0007] Most SCR processes utilize a stoichiometric excess of ammonia in order to maximize the conversion of NOx. Unreacted ammonia that passes through the SCR process (also referred to as “ammonia slip”) is undesirable, because the slipped ammonia gas can react with other combustion species and / or negatively impact the atmosphere if released. To reduce ammonia slip, exhaust treatment systems can include an ammonia oxidation catalyst (AMOX) downstream of the SCR catalyst article. Catalysts for oxidizing excess ammonia in an exhaust gas are known. For example, U.S. Pat. No. 7,393,511 describes an ammonia oxidation catalyst containing a precious metal, such as platinum, palladium, rhodium, or gold on a support of titania, alumina, silica, zirconia, etc.

[0008] Dhillon, P. S., et al. recently reported a dual-layer ammonia slip catalyst containing macropores in the SCR catalyst layer produced by using sacrificial agents such as poly(tert- butyl acrylate) and yeast. See Dhillon, P. S. et al., “Enhanced transport in washcoated monoliths: Application to selective lean NOx reduction and ammonia oxidation”, Chemical Engineering Journal, volume 377,119734 (1 December 2019).

[0009] It is an object of the present invention to provide an ammonia slip catalyst article with improved catalytic performance.

[0010] SUMMARY OF THE INVENTION

[0011] According to a first aspect there is provided an ammonia slip catalyst (ASC) article comprising:

[0012] (a) a substrate;

[0013] (b) a first catalyst coating; and

[0014] (c) a second catalyst coating; wherein the first catalyst coating comprises an ammonia oxidation catalyst; wherein the second catalyst coating comprises an SCR catalyst; and wherein the second catalyst coating is formed by calcining an SCR catalyst washcoat comprising polysaccharide particles.

[0015] According to another aspect there is provided a method of preparing an ammonia slip catalyst (ASC) article, the method comprising:

[0016] (a) applying an ammonia oxidation catalyst slurry on a substrate;

[0017] (b) applying an SCR catalyst slurry comprising polysaccharide particles on the substrate;

[0018] (c) drying and / or calcining a coated substrate from steps (a) and (b).

[0019] According to a further aspect there is provided an exhaust gas treatment system comprising a selective catalytic reduction (SCR) article and the ammonia slip catalyst (ASC) article as described herein.

[0020] According to a further aspect there is provided a fuel combustion and exhaust gas treatment system comprising an engine and the exhaust-gas treatment system as described herein.

[0021] According to a further aspect there is provided a vehicle comprising the fuel combustion and the exhaust gas treatment system as described herein. According to a further aspect there is provided a method for the treatment of an exhaust gas, the method comprising passing an exhaust gas through the exhaust gas treatment system as described herein.

[0022] BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The accompanying drawings, described below, illustrate exemplary embodiments and are not to be considered limiting of the scope of the invention. The figures are not necessarily to scale, and certain features and certain view of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness.

[0024] FIG. 1 shows NH3 conversion of Catalyst A and Catalyst B.

[0025] FIG. 2 shows NOx remake during NH3 conversion of Catalyst A and Catalyst B.

[0026] FIG. 3 shows NH3 conversion of Catalyst C and Catalyst D.

[0027] FIG. 4 shows NOx remake during NH3 conversion of Catalyst C and Catalyst D.

[0028] FIG. 5 shows NH3 conversion of Catalyst E and Catalyst F.

[0029] FIG. 6 shows NOx remake during NH3 conversion of Catalyst E and Catalyst F.

[0030] FIG. 7 shows NH3 conversion of Catalyst G, Catalyst H and Catalyst I.

[0031] FIG. 8 shows NH3 and NO concentration of Catalyst G, Catalyst H and Catalyst I.

[0032] DETAILED DESCRIPTION OF THE INVENTION

[0033] The present disclosure will now be described further. In the following passages different aspects / embodiments of the disclosure 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. It is intended that the features disclosed in relation to the product may be combined with those disclosed in relation to the method and vice versa.

[0034] According to a first aspect there is provided an ammonia slip catalyst (ASC) article comprising:

[0035] (a) a substrate;

[0036] (b) a first catalyst coating; and

[0037] (c) a second catalyst coating, wherein the first catalyst coating comprises an ammonia oxidation catalyst; wherein the second catalyst coating comprises an SCR catalyst; and wherein the second catalyst coating is formed by calcining a second catalyst washcoat comprising polysaccharide particles.

[0038] The ammonia slip catalyst (ASC) article comprises a substrate, which may be a flow- through substrate or a filter substrate. In some embodiments, the ASC article comprises a flow-through substrate. In some embodiments, the ASC article comprises a filter substrate.

[0039] The substrate can be a ceramic substrate or a metallic substrate. The ceramic substrate may be made of any suitable refractory material, e.g., alumina, silica, titania, ceria, zirconia, magnesia, zeolites, silicon nitride, silicon carbide, zirconium silicates, magnesium silicates, aluminosilicates, metallo aluminosilicates (such as cordierite and spudomene), or a mixture or mixed oxide of any two or more thereof.

[0040] The metallic substrates may be made of any suitable metal, and in particular heat- resistant metals and metal alloys such as titanium and stainless steel as well as ferritic alloys containing iron, nickel, chromium, and / or aluminum in addition to other trace metals.

[0041] The flow-through substrate is preferably a flow-through monolith having a honeycomb structure with many small, parallel thin-walled channels running axially through the substrate and extending throughout from an inlet or an outlet of the substrate. The channel crosssection of the substrate may be any shape, but is preferably square, sinusoidal, triangular, rectangular, hexagonal, trapezoidal, circular, or oval. The flow-through substrate may have porous channel walls which allows catalyst coatings to penetrate into the substrate walls. For certain applications, the flow-through monolith substrate has a cell density of about 600 to 800 cells per square inch, and / or an average internal wall thickness of about 0.18-0.35 mm, or about 0.20-0.25 mm. For certain other applications, the flow-through monolith substrate has a low cell density of about 150-600 cells per square inch, or about 200-400 cells per square inch.

[0042] The filter substrate is preferably a wall-flow monolith filter substrate. The channels of a wall-flow filter are alternately blocked, which allow the exhaust gas stream to enter a channel from the inlet, then flow through the channel walls, and exit the filter from a different channel leading to the outlet. Particulates in the exhaust gas stream are thus trapped in the filter.

[0043] The ammonia slip catalyst article of the present invention comprises a first catalyst coating comprising an ammonia oxidation catalyst. An ammonia oxidation catalyst may be formulated to oxidize excess ammonia and prevent it from being released to the atmosphere. The ammonia contained in an exhaust gas stream is reacted with oxygen over the ammonia oxidation catalyst to form N2.

[0044] According to some embodiments, the ammonia oxidation catalyst may include a supported platinum group metal which is effective to remove ammonia from the exhaust gas stream. In some embodiments, the platinum group metal includes platinum, palladium, ruthenium, rhodium, iridium, osmium, or combinations thereof. In some embodiments, the platinum metal component comprises platinum and / or palladium. In some embodiments, the platinum group metal comprises platinum. In some embodiments, the platinum group metal comprises palladium.

[0045] According to some embodiments, the platinum group metal is supported on a refractory inorganic oxide. Examples of suitable refractory inorganic oxides include, but are not limited to, alumina, silica, titania, ceria, and zirconia, as well as physical mixtures, chemical combinations and / or atomically-doped combinations thereof. In some embodiments, the refractory inorganic oxide may contain a mixed oxide such as silica-alumina, amorphous or crystalline aluminosilicates, alumina-zirconia, alumina-lanthana, alumina-chromia, alumina- baria, alumina-ceria, and the like. An exemplary refractory inorganic oxide comprises y- alumina having a specific surface area of about 50 to about 300 m2 / g.

[0046] In some embodiments, the first catalyst coating comprises platinum and alumina.

[0047] In some embodiments, the ammonia oxidation catalyst may comprise a zeolitic or non- zeolitic molecular sieve, which may have any one of the framework structures as described in the SCR catalyst section herein. In some embodiments, a platinum group metal may be supported by the molecular sieve, being distributed on the external surface or in the channels, cavities, or cages of the molecular sieve. In some embodiments, a molecular sieve may be physically mixed with an oxide-supported platinum group metal.

[0048] In some embodiments, the first catalyst coating may include an ammonia oxidation catalyst blended with an SCR catalyst. The SCR catalyst may consist of any one of the SCR catalysts described herein. In some embodiments, the first catalyst coating consists of a physical mixture of an oxide-supported platinum group metal and an SCR catalyst. In some embodiments, a platinum group metal may be distributed on the external surface or in the channels, cavities, or cages of the SCR catalyst.

[0049] The first catalyst coating may contain about 0.03 to 2.5 weight percent (wt%), or about 0.1 to 0.6 wt% platinum group metal based on the weight of the refractory inorganic oxide.

[0050] The amount of the platinum group metal on the substrate (commonly referred to as “platinum group metal loading”) can be about 0.5 to 20 g / ft3. The platinum group metal loading is the amount of platinum group metal in a unit volume of the substrate.

[0051] The washcoat loading of the first catalyst coating is generally greater than 0.10 g / in3, greater than 0.20 g / in3, or greater than 0.30 g / in3, e.g., 0.10 to 1.0 g / in3or 0.20 to 0.8 g / in3. Washcoat loading is the amount of washcoat after calcination per unit volume of the substrate.

[0052] The first catalyst coating can be formed on the substrate by a washcoating procedure with an ammonia oxidation catalyst slurry comprising the supported platinum group metal described herein. The ammonia oxidation catalyst slurry may comprise platinum group metal in a concentration of 0.015 to 0.7 g / L. The ammonia oxidation catalyst slurry may have the refractory inorganic oxide in an amount of 6 to 75, or 20 to 65 g / L. The ammonia oxidation catalyst slurry may be applied to the substrate by known methods. There are many suitable ways to apply the ammonia oxidation catalyst slurry to the substrate. For example, coating of the substrate can be performed by immersing the substrate vertically in the slurry such that the desired coating length is achieved. The substrate can be left in the slurry for a sufficient period of time to allow the desired amount of the catalyst slurry to deposit on the substrate. The substrate is removed from the slurry, and excess slurry is removed from the substrate first by allowing it to drain from the substrate, then by blowing on the slurry on the substrate with compressed air (against the direction of slurry penetration) or applying a vacuum.

[0053] Another method for coating the substrate can be found in US6599570B1. The method comprises the steps of (a) locating a containment means on top of a monolithic substrate; (b) dosing a predetermined quantity of a catalyst slurry into the containment means; and (c) applying vacuum to draw the catalyst slurry into at least a portion of the monolithic substrate.

[0054] The coated substrate may be dried, for example, at a temperature of about 110 to 300°C, and / or calcined at a temperature of about 300 to 550°C.

[0055] The ASC article comprises a second catalyst coating comprising an SCR catalyst.

[0056] The SCR catalyst generally comprises an oxide of a base metal, a molecular sieve, a metal-exchanged molecular sieve, or a mixture thereof. The base metal can be selected from the group consisting of cerium (Ce), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), tungsten (W), vanadium (V), and mixtures thereof. SCR catalysts consisting of vanadium supported on a refractory inorganic oxide such as alumina, silica, zirconia, titania, ceria and combinations thereof are well known and widely used commercially in mobile applications. Typical compositions are described in US4,010,238 and US4,085,193, of which the entire contents are incorporated herein by reference.

[0057] The SCR catalyst may comprise vanadium and antimony. Antimony may be present in an amount such that molar ratio of antimony to vanadium is greater than 0.5, or from 0.6 to 0.9. Antimony may be present as Sb20s. By way of example, the SCR catalyst may comprise vanadium in an amount of 2-6 wt%, or 3-5 wt% on a V2O5 basis, and antimony in an amount of 3-8 wt% on an Sb20s basis, relative to the total weight of the SCR catalyst. The form of vanadium and antimony present in the catalyst article after calcination is not necessarily V2O5 and Sb2O5.

[0058] In addition to vanadium and antimony, the SCR catalyst may additionally comprise cerium. The molar ratio of cerium to vanadium is generally greater than 3: 10. The SCR catalyst may comprise cerium in an amount of 1-10 wt% or 2-5 wt% on a CeC>2 basis. The form of Ce present in the catalyst is not necessarily CeC>2. Employing cerium and vanadium in such proportions reduces vanadium volatilisation without reducing the activity of the SCR catalyst. The reduced volatilisation permits a greater loading of vanadium, which is desirable for improving the efficiency of the SCR catalyst. By way of example, the SCR catalyst may comprise 2-6 wt% V2O5, 2-6 wt% CeC>2, and 3-8 wt% Sb2Os relative to the total weight of the SCR catalyst.

[0059] The SCR catalyst can comprise a molecular sieve or a metal-exchanged molecular sieve. As is used herein "molecular sieve" is understood to mean a metastable material containing tiny pores of a precise and uniform size that may be used as an adsorbent for gases or liquids. The molecular sieve can be a zeolitic molecular sieve, a non-zeolitic molecular sieve, or a mixture thereof.

[0060] A zeolitic molecular sieve is a microporous aluminosilicate having any one of the framework structures listed in the Database of Zeolite Structures published by the International Zeolite Association (IZA). The framework structures include, but are not limited to those of the CHA, BEA, FAU, LTA, MFI, AEI, and MOR types. Non-limiting examples of zeolites having these structures include chabazite, faujasite, zeolite Y, ultrastable zeolite Y, beta zeolite, mordenite, silicalite, zeolite X, and ZSM-5. Aluminosilicate zeolites can have a silica-to- alumina molar ratio (SAR, defined as SiO2 / AhO3) from 5 to 100, from 10 to 80, or from about 10 to 30.

[0061] As used herein, the term “non-zeolitic molecular sieve” refers to corner sharing tetrahedral frameworks where at least a portion of the tetrahedral sites are occupied by an element other than silicon or aluminum. Specific non-limiting examples of non-zeolitic molecular sieves include silicoaluminophosphates such as SAPO-34, SAPO-37 and SAPO 44. The silicoaluminophosphates can have framework structures that contain framework elements that are found in zeolites, such as BEA, CHA, FAU, LTA, MFI, MOR and other types described below.

[0062] The SCR catalyst can comprise a small-pore, a medium-pore ora large-pore molecular sieve, or combinations thereof.

[0063] The SCR catalyst can comprise a small-pore molecular sieve selected from the group consisting of aluminosilicate molecular sieves, metal-substituted aluminosilicate molecular sieves, aluminophosphate (AIPO) molecular sieves, metal-substituted aluminophosphate (MeAIPO) molecular sieves, silico-aluminophosphate (SAPO) molecular sieves, and metal substituted silico-aluminophosphate (MeAPSO) molecular sieves, and mixtures thereof. The SCR catalyst can comprise a small-pore molecular sieve selected from the group of Framework Types consisting of AGO, AEI, AEN, AFN, AFT, AFX, ANA, APC, APD, ATT, CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI, GIS, GOO, IHW, ITE, ITW, LEV, KFI, LTA, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SIV, THO, TSC, UEI, UFI, VNI, YUG, and ZON, and mixtures and / or intergrowths thereof. The small pore molecular sieve may be selected from the group of Framework Types consisting of AEI, AFX, CHA, DDR, ERI, ITE, KFI, LTA, LEV, and SFW. The SCR catalyst can comprise a medium-pore molecular sieve selected from the group of Framework Types consisting of AEL, AFO, AHT, BOF, BOZ, CGF, CGS, CHI, DAC, EUO, FER, HEU, IMF, ITH, ITR, JRY, JSR, JST, LAU, LOV, MEL, MFI, MFS, MRE, MTT, MVY, MWW, NAB, NAT, NES, OBW, PAR, PCR, PON, PUN, RRO, RSN, SFF, SFG, STF, STI, STT, STW, -SVR, SZR, TER, TON, TUN, UOS, VSV, WEI, and WEN, and mixtures and / or intergrowths thereof. The medium pore molecular sieve may be selected from the group of Framework Types consisting of FER, MFI, and STT.

[0064] The SCR catalyst can comprise a large-pore molecular sieve selected from the group of Framework Types consisting of AFI, AFR, AFS, AFY, ASV, ATO, ATS, BEA, BEC, BOG, BPH, BSV, CAN, CON, CZP, DFO, EMT, EON, EZT, FAU, GME, GON, IFR, ISV, ITG, IWR, IWS, IWV, IWW, JSR, LTF, LTL, MAZ, MEI, MOR, MOZ, MSE, MTW, NPO, OFF, OKO, OSI, RON, RWY, SAF, SAO, SBE, SBS, SBT, SEW, SFE, SFO, SFS, SFV, SOF, SOS, STO, SSF, SSY, USI, UWY, and VET, and mixtures and / or intergrowths thereof. The large pore molecular sieve can be selected from the group of Framework Types consisting of BEA, MOR and OFF.

[0065] A metal exchanged molecular sieve can have at least one metal from one of the groups VB, VI B, VII B, VII IB, IB, or I IB of the periodic table deposited onto extra-framework sites on the external surface or within the channels, cavities, or cages of the molecular sieves. Metals may be in one of several forms, including, but not limited to, zerovalent metal atoms or clusters, isolated cations, mononuclear or polynuclear oxycations, or as extended metal oxides. The metals can be iron, copper, and mixtures or combinations thereof.

[0066] A metal exchanged molecular sieve can contain in the range of from about 0.10 wt% to about 10 wt% of a group VB, VI B, VI IB, VI I IB, IB, or 11 B metal located on extra framework sites on the external surface or within the channels, cavities, or cages of the molecular sieve. The metal exchanged molecular sieve can be a copper (Cu) supported small pore molecular sieve having from 0.1 to 20.0 wt%, 1 wt% to 6 wt%, or from 1.8 wt% to 4.2 wt% copper relative to the total weight of the metal exchanged molecular sieve.

[0067] The metal exchanged molecular sieve can be an iron (Fe) supported small pore molecular sieve having from 0.1 to 20.0 wt%, 1 wt% to 6 wt%, or from 1.8 wt% to 4.2 wt% iron relative to the total weight of the metal exchanged molecular sieve.

[0068] The second catalyst coating is formed by calcining a second catalyst washcoat comprising polysaccharide particles.

[0069] The second catalyst washcoat can be formed on the substrate by a washcoating procedure with an SCR catalyst slurry comprising an SCR catalyst and polysaccharide particles.

[0070] The SCR catalyst slurry comprise polysaccharide particles as pore formers. A polysaccharide is a polyacetal having oxygen bridges linking the monosaccharide monomers. Suitable polysaccharides may have a degree of polymerization of greater than 10, or greater than 30. Degree of polymerization is the number of monomeric units in a polymer or oligomer.

[0071] In some embodiments, the polysaccharide is cellulose or agar.

[0072] In some embodiments, the polysaccharide is cellulose. Cellulose is an organic compound with the formula (CeH OsJn and consisting of a linear chain of D-glucose units. Degree of polymerization, n, of suitable celluloses is generally greater than 100, greater than 500, or greater than 1000.

[0073] The suitable cellulose particles may be spheres, beads, granules, or may have other irregular shapes.

[0074] Suitable cellulose as a pore former in the SCR catalyst slurry may have a mean particle size, D50 by volume (Dv50), of from 0.05 to 100 pm, from 0.1 to 80 pm, from 0.5 to 50 pm, or from 1 to 20 pm.

[0075] The Dv50, Dv10, Dv90 are parameters to evaluate the particle size of a powder material. For example, if the Dv50 of a sample is 5 pm by volume distribution, it means that the volume of particles less than 5 pm accounts for 50% of the total sample volume, and likewise, the volume of particles greater than 5 pm is also 50%. Similarly, if the Dv90 of a sample is 5 pm by volume distribution, it means that the volume of particles less than 5 pm accounts for 90% of the total sample volume, and likewise, the volume of particles greater than 5 pm is 10%. If the Dv10 of a sample is 5 pm by volume distribution, it means that the volume of particles less than 5 pm accounts for 10% of the total sample volume, and likewise, the volume of particles greater than 5 pm is 90%.

[0076] One example of a suitable cellulose is microcrystalline, ultra-fine cellulose powder having an Dv50 of about 10 pm.

[0077] In some embodiments, dry agar powder can be used as the pore former. Agar generally consists of a mixture of two polysaccharides: agarose and agaropectin, with agarose making up about 70% of the mixture, while agaropectin makes about 30% of it. Agarose is a linear polymer, made up of repeating units of agarobiose, a disaccharide made up of D- galactose and 3,6-anhydro-L-galactopyranose. Agaropectin is a heterogeneous mixture of smaller molecules that occur in lesser amount, and is made up of alternating units of D- galactose and L-galactose heavily modified with acidic side-groups, such as sulfate, glucuronate, and pyruvate.

[0078] It is known that agar exhibits hysteresis because when mixed with water, it solidifies and forms a gel at about 32-42°C, which is called the gel point, and melts at 85°C, which is the melting point. Hysteresis of agar occurs due to the difference between the gel point and melting point temperatures.

[0079] Swollen agar hydrogel beads can be prepared by mixing dry agar powder with water, under vigorous stirring, using a heating step (ramp to 90°C followed by a hold lasting for a minimum of 20 minutes) to ensure its complete melting and dissolution. A cooling step to room temperature is then carried out under vigorous stirring, enabling only a partial networking of the fully hydrated polymer coils, and so the formation of macrogel beads rather than a homogenous gel block. The macrogel suspension is subject to high shear mixing, to enable further particle reduction to a suitable range for pore former applications. A Silverson high shear mixer can be used to achieve the desired particle size distribution, stirring the sample at 3000-4000 rpm for a time period of, for example, from 1 to 5 hours. Generally, the polymer strands in the macrogel beads obtained with this process hold onto water and the gel particles contain greater than 99 wt% water.

[0080] In some embodiments, swollen agar hydrogel beads are used as pore former in the SCR catalyst slurry.

[0081] In some embodiments, swollen agar hydrogel beads have a Dv50 of from 10 to 800 pm, from 20 to 600 pm, 50 to 400 pm, or 60 to 200 pm, when the particle size analysis is carried out using laser diffraction on a 0.8 wt% macrogel sample.

[0082] The SCR catalyst slurry is generally coated to the substrate by the washcoating procedure described herein.

[0083] The second catalyst coating is formed by calcining the second catalyst washcoat, for example, at a temperature of from 300 to 500°C. Prior to calcination, it may be dried at a temperature of about 110 to 300°C.

[0084] The ASC articles of the present invention may be prepared with various configurations. In some embodiments, the coatings are arranged such that the exhaust gas contacts the second catalyst coating before contacting the first catalyst coating.

[0085] In a first configuration, an ASC article can comprise a first catalyst coating comprising an ammonia oxidation catalyst and a second catalyst coating comprising an SCR catalyst, wherein the second catalyst coating is located in a layer over the first catalyst coating and the second catalyst coating covers all of the first catalyst coating.

[0086] In a second configuration, an ASC article can comprise a first catalyst coating comprising an ammonia oxidation catalyst and a second catalyst coating comprising an SCR catalyst, where the first catalyst coating extends from the outlet end toward the inlet end, covering less than a full length of the substrate, and the second catalyst coating extends the entire length of the substrate, completely overlapping the first catalyst coating.

[0087] In a third configuration, an ASC article can comprise a first catalyst coating comprising an ammonia oxidation catalyst and a second catalyst coating comprising an SCR catalyst, where the first catalyst coating extends from the outlet end toward the inlet end, covering less than a full length of the substrate, and the second catalyst coating extends from the inlet end towards the outlet end, partially overlapping the first catalyst coating. The second catalyst coating can overlap the first catalyst coating by an amount from about 10% to about 95%, or about 50% to about 95%.

[0088] In a fourth configuration, an ASC article can comprise a first catalyst coating comprising an ammonia oxidation catalyst and a second catalyst coating comprising an SCR catalyst, where the first catalyst coating extends from the outlet end toward the inlet end, covering less than a full length of the substrate, and the second catalyst coating extends from the inlet end towards the outlet end, without overlapping the first catalyst coating. There may be a space between the first catalyst coating and the second catalyst coating, the first catalyst coating and the second catalyst coating may meet but not overlap, or there may be a slight and insubstantial overlap of the first and second catalyst coating.

[0089] In a fifth configuration, an ASC article can comprise a first catalyst coating comprising an ammonia oxidation catalyst and a second catalyst coating comprising an SCR catalyst, where the first catalyst coating extends from the inlet end toward the outlet end, covering less than a full length of the substrate, and the second catalyst coating extends the entire length of the substrate, completely overlapping the first catalyst coating.

[0090] In a sixth configuration, an ASC article can comprise a first catalyst coating comprising an ammonia oxidation catalyst and a second catalyst coating comprising an SCR catalyst, where the first catalyst coating extends from the inlet end toward the outlet end, covering less than a full length of the substrate, and the second catalyst coating extends from the outlet end toward the inlet end, covering less than the full length of the substrate, and partially overlapping the first catalyst coating. The second catalyst coating can overlap the first catalyst coating by an amount from about 10% to about 95%, or about 50% to about 95%.

[0091] According to another aspect there is provided a method of preparing an ammonia slip catalyst (ASC) article, the method comprising:

[0092] (a) applying an ammonia oxidation catalyst slurry on a substrate;

[0093] (b) applying an SCR catalyst slurry on the substrate;

[0094] (c) drying and / or calcining a coated substrate from steps (a) and (b).

[0095] According to a further aspect there is provided an exhaust gas treatment system comprising a selective catalytic reduction (SCR) article and ammonia slip catalyst (ASC) article as described herein. The SCR article generally comprises a flow-through substrate as described herein and a SCR catalyst as described herein

[0096] According to a further aspect there is provided a fuel combustion and the exhaust gas treatment system comprising an engine and the exhaust-gas treatment system as described herein.

[0097] According to a further aspect there is provided a vehicle comprising the fuel combustion and exhaust gas treatment system as described herein. According to a further aspect there is provided a method for the treatment of an exhaust gas, the method comprising passing an exhaust gas through the exhaust gas treatment system as described herein.

[0098] EXAMPLE 1 : Catalyst A

[0099] A flow-through honeycomb core (4.66 inches by 3.0 inches, 400 cpsi, 4 mil wall thickness) was coated with an oxidation catalyst slurry containing platinum nitrate, alumina, and a bare molecular sieve, to form a bottom layer. It was dried at 100°C and calcined at 500°C. The oxidation catalyst layer washcoat loading was 1 .0 g / i n3.

[0100] A SCR catalyst slurry was prepared by mixing a copper ion-exchanged aluminosilicate zeolite CHA 1 having a particle size Dv50 of 3.5 pm and copper content relative to the weight of CHA of 4 wt%, alumina (particle crystallite size of 6 nm), cellulose rheology modifier, and a cellulose pore-former having a Dv50 of about 10 pm. The amount of pore former is 12.5 wt% with respect to the total weight of the copper ion-exchanged aluminosilicate zeolite CHA and the binder. The SCR catalyst slurry was coated over the oxidation catalyst layer so that the bottom layer is completely covered. It was dried at 100°C calcined at 500°C. Catalyst A obtained has a Pt loading of 3 g / ft3and an SCR catalyst washcoat loading of 2.4 g / in3.

[0101] COMPARATIVE EXAMPLE 2: Catalyst B

[0102] Catalyst B was prepared by following the same procedure for Catalyst A, except that the pore-former was not present in the SCR catalyst slurry.

[0103] EXAMPLE S: Catalyst C

[0104] A flow-through honeycomb core (4.66 inches by 3.0 inches, 400 cpsi, 4 mil wall thickness) was coated with an oxidation catalyst slurry containing platinum nitrate, alumina, and a bare molecular sieve to form a bottom layer. It was dried at 100°C and calcined at 500°C. The oxidation catalyst layer washcoat loading was 1.0 g / in3.

[0105] A SCR catalyst slurry was prepared by mixing a copper ion-exchanged aluminosilicate zeolite CHA 2 having a Dv50 of 3.5 pm and a copper content relative to the weight of CHA of 3.3 wt%, an alumina binder (particle crystallite size of 6 nm), cellulose rheology modifier, and a cellulose pore-former having a Dv50 of about 10 pm. The amount of pore-former is 10 wt% with respect to the total weight of the copper ion-exchanged aluminosilicate zeolite CHA and alumina. The SCR catalyst slurry was coated over the oxidation catalyst layer so that the bottom layer is completely covered. It was dried at 100°C calcined at 500°C. Catalyst C obtained has a Pt loading of 5 g / ft3and an SCR catalyst washcoat loading of 3.0 g / in3.

[0106] COMPARATIVE EXAMPLE 4: Catalyst D

[0107] Catalyst D was prepared by following the same procedure for Catalyst C, except that the pore-former was not present in the SCR catalyst slurry.

[0108] EXAMPLE S: Catalyst E

[0109] A flow-through honeycomb core (4.66 inches by 3.0 inches, 400 cpsi, 4 mil wall thickness) was coated with an oxidation catalyst slurry containing platinum nitrate, alumina support, and a bare molecular sieve to form a bottom layer. It was dried at 100°C and calcined at 500°C. The oxidation catalyst layer washcoat loading was 1 .0 g / in3.

[0110] A SCR catalyst slurry was prepared by mixing a copper ion-exchanged aluminosilicate zeolite CHA having particle size Dv50 of 3.5 pm and copper content relative to the weight of CHA of 3.3 wt%, an alumina binder (particle crystallite size of 6 nm), a cellulose rheology modifier, and a cellulose pore-former having a Dv50 of about 10 pm. The amount of pore former is 9 wt% with respect to the total weight of the copper ion-exchanged aluminosilicate zeolite CHA and alumina. The SCR catalyst slurry was coated over the oxidation catalyst layer so that the bottom layer is completely covered. It was dried at 100°C calcined at 500°C. Catalyst E obtained has a Pt loading of 3 g / ft3and an SCR catalyst washcoat loading of 2.2 g / in3.

[0111] COMPARATIVE EXAMPLE 6: Catalyst F

[0112] Catalyst F was prepared by following the same procedure for Catalyst E, except that the pore-former was not present in the SCR catalyst slurry.

[0113] EXAMPLE 7: Catalyst G

[0114] Swollen agar hydrogel beads preparation: Agar powder was suspended in water, under heated stirring at 90°C, to ensure its complete melting and dissolution. This was followed by a cooling step and high shear mixing to form macrogel beads. Particle size analysis carried out using laser diffraction on a 0.8 wt% macrogel sample shows Dv10 = 40 pm, Dv50 = 90 pm, Dv90 = 180 pm.

[0115] A flow-through honeycomb core (5.66 inches by 6.0 inches, 400 cpsi, 4 mil wall thickness) was coated with an oxidation catalyst slurry containing platinum nitrate and alumina support. It was dried at 100° C and calcined at 500°C. The oxidation catalyst layer washcoat loading was 0.35 g / in3.

[0116] A SCR catalyst slurry was prepared by mixing a copper ion-exchanged aluminosilicate AEI having a Dv90 of 8.3 pm and copper content relative to the weight of AEI of 4 wt%, alumina having a particle crystallite size of 6 nm, a cellulose rheology modifier, and the swollen agar hydrogel beads (the amount of the dry agar is 20 wt% with respect to the total weight of the copper ion-exchanged aluminosilicate zeolite AEI and alumina). The SCR catalyst slurry was coated over the oxidation catalyst layer so that the bottom layer is completely covered. It was dried at 100°C calcined at 500°C. Catalyst G obtained has a Pt loading of 1 g / ft3and an SCR catalyst washcoat loading of 2.4 g / in3.

[0117] EXAMPLE 8: Catalyst H

[0118] A flow-through honeycomb core (4.66 inches by 3.0 inches, 400 cpsi, 4 mil wall thickness) was coated with an oxidation catalyst slurry containing platinum nitrate and alumina support, to form a bottom layer. It was dried at 100°C and calcined at 500°C. The oxidation catalyst layer washcoat loading was 0.35 g / in3.

[0119] A SCR catalyst slurry was prepared by mixing a copper ion-exchanged aluminosilicate AEI having particle size Dv90 of 8.3 pm and a copper content relative to the weight of AEI of 4 wt%, alumina (particle crystallite size of 6 nm), and dry agar beads (Dv10 = 6 pm, Dv50 = 20 pm , Dv90 = 50 pm; the amount of the dry agar is 20 wt% with respect to the total weight of the copper ion-exchanged aluminosilicate zeolite AEI and the binder). The SCR catalyst slurry was coated over the oxidation catalyst layer so that the bottom layer is completely covered. It was dried at 100°C calcined at 500°C. Catalyst H obtained has a Pt loading of 1 g / ft3and an SCR catalyst washcoat loading of 2.4 g / in3.

[0120] COMPARATIVE EXAMPLE 9: Catalyst I

[0121] Catalyst I was prepared by following the same procedure for Catalyst G, except that the pore-former was not present in the SCR catalyst slurry.

[0122] EXAMPLE 10: Catalyst Performance Test

[0123] Cores (1”x3”) were cut from Catalyst A and Catalyst B and were aged at 650°C for 100 h in an atmosphere of 10% water in air before being tested for NH3 conversion and N2 selectivity. Two test conditions were probed - the first a temperature ramp in 500 ppm NH3 at 180k SV (350ppm CO, 5% H2O, 8% O2 and N2 balance); the second a series of steady state points at increasing temperatures in 75 ppm NH3 at 120k (10% O2, 5% water and N2 balance).

[0124] Catalyst A and Catalyst B exhibit similar NH3 conversion activity FIG. 1 Catalyst A results in a lower NOx remake than Catalyst B, as shown in FIG. 2 and Table!

[0125] Table 1 . Summary of catalytic activity and NOx remake of Catalyst A and Catalyst B

[0126] EXAMPLE 11 : Catalyst Performance Test

[0127] Cores (1”x3”) were cut from Catalyst C and Catalyst D and were hydrothermally aged at 650°C for 50 h in an atmosphere of 10% water in air before being tested for NH3 conversion and N2 selectivity using a temperature ramp in 500 ppm NH3 at 180k SV (350 ppm CO, 5% H2O, 8% O2 and N2 balance). Catalyst C shows improved NH3 conversion over catalyst D, as shown in FIG. 3. Catalyst C results in a lower NOx remake than Catalyst D as shown in Table 2 and FIG. 4.

[0128] Table 2. Summary of catalytic activity and NOx remake of Catalyst C and Catalyst D

[0129] EXAMPLE 12: Catalyst Performance Test Cores (1”x3”) were cut from Catalyst E and Catalyst F and were hydrothermally aged at 750°C for 16 h in an atmosphere of 10% water in air before being tested for NH3 conversion and N2 selectivity using a temperature ramp in 500 ppm NH3 at 180k SV (350 ppm CO, 5% H2O, 8% O2 and N2 balance). Catalyst E gave the same NH3 conversion as catalyst F as shown in FIG. 5. Catalyst E gave a lower NOx remake than Catalyst F as shown in FIG. 6 and in Table 3.

[0130] Table 3. Summary of catalytic activity and NOx remake of catalysts E and F

[0131] EXAMPLE 13: Catalyst Performance Test

[0132] Cores (1”x1”) were cut from Catalyst G, Catalyst H and Catalyst I and hydrothermally aged at 800°C for 16 h in an atmosphere of 10% water in air before being tested for NH3 conversion and N2 selectivity using a temperature ramp in 500 ppm NH3 at 215k SV (5% H2O, 14% O2 and N2 balance). The NH3 conversion of Catalyst G and Catalyst H are compared with that of Catalyst I in FIG. 7. The results indicate that Catalyst G and Catalyst H have superior NH3 conversion over Catalyst I. FIG. 8 shows Catalyst H makes the same level of NOx remake as Catalyst I despite improved conversion so has a lower NOx remake selectivity than Catalyst I. In the case of catalyst G, the improved conversion is resulting in increased NOx remake despite presence of pore former, as shown in Table 4.

[0133] Table 4. Summary of catalytic activity and NOx remake of catalysts G, H and I

Claims

Claims:1 . An ammonia slip catalyst article comprising:(a) a substrate;(b) a first catalyst coating; and(c) a second catalyst coating; wherein the first catalyst coating comprises an ammonia oxidation catalyst; wherein the second catalyst coating comprises an SCR catalyst; and wherein the second catalyst coating is formed by calcining a second catalyst washcoat comprising polysaccharide particles.

2. The ammonia slip catalyst article of claim 1 , wherein the first catalyst coating comprises platinum and alumina.

3. The ammonia slip catalyst article of claim 1 , wherein the polysaccharide is cellulose or agar.

4. The ammonia slip catalyst article of claim 1 , wherein the polysaccharide is cellulose.

5. The ammonia slip catalyst article of claim 4, wherein the cellulose has a Dv50 of from 0.05 to 100 pm.

6. The ammonia slip catalyst article of claim 4, wherein the cellulose has a Dv50 of from 0.5 to 50 pm.

7. The ammonia slip catalyst article of claim 4, wherein the cellulose has a Dv50 of from 1 to 20 pm.

8. The ammonia slip catalyst article of claim 1 , wherein the polysaccharide is agar.

9. The ammonia slip catalyst article of claim 1 , wherein the polysaccharide particles are swollen agar hydrogel beads.

10. The ammonia slip catalyst article of claim 9, wherein the swollen agar hydrogel beads have a Dv50 of from 10 to 800 pm, and wherein the Dv50 is determined using laser diffraction on a 0.8 wt% macrogel sample.11 . The ammonia slip catalyst article of claim 9, wherein the swollen agar hydrogel beads have a Dv50 of from 50 to 400 pm, and wherein the Dv50 is determined using laser diffraction on a 0.8 wt% macrogel sample.

12. The ammonia slip catalyst article of claim 9, wherein the swollen agar hydrogel beads have a Dv50 of from 60 to 200 pm, and wherein the Dv50 is determined using laser diffraction on a 0.8 wt% macrogel sample.

13. The ammonia slip catalyst article of any of claims 1-12, wherein the substrate is a flow- through substrate, and wherein the second catalyst coating is located in a layer over the first catalyst coating and the second catalyst coating covers all of the first catalyst coating.

14. A method of preparing an ammonia slip catalyst article, the method comprising:(a) applying an ammonia oxidation catalyst slurry on a substrate;(b) applying an SCR catalyst slurry on the substrate;(c) drying and / or calcining a coated substrate obtained from steps (a) and (b).

15. An exhaust gas treatment system comprising a selective catalytic reduction (SCR) article and the ammonia slip catalyst (ASC) article of any of claims 1-13.

16. A vehicle comprising the fuel combustion and exhaust gas treatment system of claim 15.

17. A method for the treatment of an exhaust gas, the method comprising passing an exhaust gas through the exhaust gas treatment system of claim 15.

Images