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  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/46—Removing components of defined structure
  • B01D53/54—Nitrogen compounds
  • B01D53/56—Nitrogen oxides
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  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
  • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
  • B01D53/9404—Removing only nitrogen compounds
  • B01D53/9409—Nitrogen oxides
  • B01D53/9413—Processes characterised by a specific catalyst
  • B01D53/9422—Processes characterised by a specific catalyst for removing nitrogen oxides by NOx storage or reduction by cyclic switching between lean and rich exhaust gases (LNT, NSC, NSR)
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  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
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  • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
  • B01J37/0009—Use of binding agents; Moulding; Pressing; Powdering; Granulating; Addition of materials ameliorating the mechanical properties of the product catalyst
  • B01J37/0027—Powdering
  • B01J37/0036—Grinding
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  • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
  • B01J37/02—Impregnation, coating or precipitation
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• • B—PERFORMING OPERATIONS; TRANSPORTING
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  • B01D2255/2092—Aluminium
• • B—PERFORMING OPERATIONS; TRANSPORTING
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  • B01D—SEPARATION
  • B01D2255/00—Catalysts
  • B01D2255/90—Physical characteristics of catalysts
  • B01D2255/902—Multilayered catalyst
  • B01D2255/9022—Two layers
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2255/00—Catalysts
  • B01D2255/90—Physical characteristics of catalysts
  • B01D2255/91—NOx-storage component incorporated in the catalyst
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  • B01J23/40—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals of the platinum group metals
  • B01J23/46—Ruthenium, rhodium, osmium or iridium
  • B01J23/464—Rhodium
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
  • Y02A50/00—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE in human health protection, e.g. against extreme weather
  • Y02A50/20—Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters

Definitions

• the present invention relates to an NOx trap with a reduced Rh loading and to a method for the production of an NOx trap with a reduced Rh loading, as well as to a treatment system for an automobile exhaust gas stream and to a method for the treatment of automobile engine exhaust gas.
• TWC three-way conversion
• CO carbon monoxide
• HC hydrocarbon
• lean-burn engines Engines, especially gasoline-fueied engines, are being designed to operate under lean conditions as a fuel economy measure. Such engines are referred to as “lean-burn engines”. That is, the ratio of air to fuel in the combustion mixtures supplied to such engines is maintained considerably above the stoichiometric ratio so that the resulting exhaust gases are "lean", i.e., the exhaust gases are relatively high in oxygen content.
• lean-burn engines provide enhanced fuel economy, they have the disadvantage that conventional TWC catalysts are not effective for reducing NO x emissions from such engines because of excessive oxygen in the exhaust. Attempts to overcome this problem have included operating lean-burn engines with brief periods of fuel-rich operation (engines which operate in this fashion are sometimes referred to as “partial lean- burn engines”).
• the exhaust of such engines is treated with a catalyst/NO x sorbent (nitrogen oxide storage catalyst) which stores NO x during periods of lean (oxygen-rich) operation, and releases the stored NO x during the rich (fuel-rich) periods of operation.
• a catalyst/NO x sorbent nitrogen oxide storage catalyst
• the catalyst component of the catalyst NO x sorbent promotes the reduction of NO x to nitrogen by reaction of NO x (includ- ing NO x released from the NO x sorbent) with HC, CO and/or hydrogen present in the exhaust gas.
• WO 2008/067375 discloses a nitrogen oxide storage catalyst having a washcoat layer provided on a substrate, wherein said washcoat layer contains Rh and further elements which form a composition which are active in the abatement of nitrogen oxide by trapping and conversion thereof.
• a NOx storage catalyst design according to the present invention which involves lower loadings of platinum group met- als, in particular of Rh, affords improved performance with respect to the abatement of nitrogen oxide, especially during the cold-start period of the exhaust treatment process.
• the present invention relates to a nitrogen oxide storage catalyst comprising: a substrate;
• first washcoat layer disposed on the substrate, the first washcoat layer comprising metal oxide support particles and a nitrogen oxide storage material comprising at least one metal compound selected from the group consisting of alkaline earth metal compounds, alkali metal compounds, rare earth metal compounds, and mixtures thereof, at least a portion of said at least one metal compound being supported on the metal oxide support particles; and
• first washcoat layer contains substantially no Rh
• the second washcoat layer is disposed on 100 - x % of the surface of the first washcoat layer, x ranging from 20 to 80.
• any material may be used provided that it may support the washcoat layers of the nitrogen oxide storage catalyst and that it is resistant to the conditions which reign during the exhaust gas treatment process.
• Suitable substrates include any of those materials typically used for preparing catalysts, and will usually comprise a ceramic or metal honeycomb structure.
• the substrate according to the present invention may be of any conceivable shape, provided that it allows for the fluid contact with at least a portion of the washcoat layers present thereon.
• the substrate is a monolith, wherein more preferably the monolith is a flow-through monolith.
• the monolithic substrate preferably contains fine, parallel gas flow passages extending from an inlet to an outlet face of the substrate, such that passages are open to fluid flow.
• Such substrates are commonly referred to as honeycomb 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 washcoats are disposed, so that the gases flowing through the passages contact the catalytic material.
• the flow passages of the monolithic substrate are thin-wailed channels, which can be of any suitable cross-sectional shape and size such as trapezoidal, rectangular, square, sinusoidal, hexagonal, oval, or circular.
• Such structures may con- tain from 60 to 400 or more gas inlet openings (i.e., cells) per square inch of cross section.
• the nitrogen oxide storage catalyst comprises a monolith as the substrate, preferably a flow-through monolith, and more preferably a flow-through monolith having a honeycomb-structure.
• the substrate can be made from materials commonly known in the art.
• porous materials are preferably used as the substrate material, in particular ceramic and ceramic-like materials such as cordierite, ⁇ -alumina, an aluminosilicate, cor- dierite-alumina, silicon carbide, aluminum titanate, silicon nitride, zirconia, mullite, zircon, zircon mullite, zircon silicate, sillimanite, a magnesium silicate, petalite, spodu- mene, alumina-silica-magnesia and zirconium silicate, as well as porous refractory metals and oxides thereof.
• ceramic and ceramic-like materials such as cordierite, ⁇ -alumina, an aluminosilicate, cor- dierite-alumina, silicon carbide, aluminum titanate, silicon nitride, zirconia, mullite, zircon, zircon mullite, zircon silicate, sillimanite, a magnesium silicate, pet
• refractory metal refers to one or more metals selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, and Re.
• the substrate may also be formed of ceramic fiber composite materials.
• the substrate is preferably formed from cordierite, silicon carbide, and/or from aluminum titanate. In general, materials are preferred which are able to withstand the high temperatures to which a NOx storage catalyst is exposed to, in particular when used in the treatment of automotive exhaust gas.
• the substrates useful for the catalysts of embodiments of the present invention may also be metallic in nature and be composed of one or more metals or metal alloys.
• the metallic substrates may be employed in various shapes such as corrugated sheet or monolithic form.
• Suitable metallic supports include the heat resistant metals and metal alloys such as 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 advantageously comprise at least 15 wt.-% of the alloy, e.g., 10-25 wt.-% of chromium, 3-8 wt.-% of aluminum and up to 20 wt.-% of nickel.
• the alloys may also contain small or trace amounts of one or more other metals such as manganese, copper, vanadium, titanium and the (ike.
• the surface or the metal substrates may be oxidized at high temperatures, e.g., 1000°C and higher, to improve the resistance to corrosion of the alloys by forming an oxide fayer on the surfaces the substrates. Such high temperature-induced oxidation may enhance the subsequent adherence of the washcoat compositions to the sub- strate.
• any metal oxide support particles may be used in the first washcoat layer, provided that they can withstand the conditions encountered during the treatment of automotive exhaust gas, in particular with respect to the temperatures incurred by the NOx storage catalyst.
• the metal oxide support particles comprise at least one metal oxide selected from the group consisting of alumina, zirconia, zirconia- alumina, baria-alumina, lanthana-aiumina, lanthana-zirconia-alumina, and mixtures thereof. More preferably, the metal oxide particles comprise zirconia-alumina and/or lanthana-aiumina, even more preferably zirconia-alumina.
• the metal oxide support particles comprised in the first washcoat layer may be doped with one or more compounds.
• the metai oxide support particles, preferably alumina, comprised in the first washcoat layer are preferably doped with zirconia.
• the metal oxide support particles may be doped with any possible amount of zirconia, preferably with 0.5 to 40% zirconia, more preferably with from 1 to 30%, more preferably with from 5 to 25%, and even more preferably with from 10 to 20%.
• the particle size (d 90 ) of the metal oxide support particles comprised in the first washcoat layer preferably ranges from 5 to 20 pm, more preferably from 8 to 14 pm, more preferably from 9 to 13 ⁇ , and even more preferably from 10 to 12 pm.
• a particle size "d 90 " refers to the equivalent diameter where 90% of the number of particles per volume have a smaller diameter.
• the surface area of the metal oxide support particles may range from 50 to 350 m 2 /g, wherein preferably, the surface area ranges from 100 to 300 m 2 /g, more preferably from 130 to 270 m 2 /g, more preferably from 150 to 230 m 2 /g, and even more preferably from 170 to 190 m 2 /g.
• the surface area generally refers to the BET surface area, preferably to the BET surface area determined according to DIN 66135.
• said material comprises at least one element selected from the group consisting of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Ce, La, Pr, Nd, and mixtures the- reof, preferably at least one element selected from the group consisting of Mg, Ba, Ce, and mixtures thereof, more preferably, Ba and/or Ce.
• the nitrogen oxide storage material comprises a cerium compound and a barium compound, preferably ceria and barium carbonate, wherein the ceria : barium carbonate weight ratio preferably ranges from 1 : 1 to 1 : 20, more preferably of from 1 : 2 to 1 : 15, more preferably of from 1 :3 to 1 :10, and even more preferably of from 1 : 3.5 to 1 : 5.
• barium carbonate is preferably at least partially supported on particles comprising ceria, wherein the resulting particles preferably have a particle size (d 90 ) which ranges from 5 to 20 ⁇ , more preferably from 8 to 14 ⁇ , more preferably from 9 to 13 ⁇ , and even more preferably from 10 to 12 pm.
• the nitrogen oxide storage material may have any possible surface area, wherein according to a preferred embodiment, the surface area of the nitrogen oxide storage material ranges from 25 to 100 m 2 /g, more preferably from 30 to 80 m 2 /g, more preferably from 40 to 60 m 2 /g, and even more preferably from 45 to 55 m 2 /g.
• the first washcoat layer comprises at least one platinum group meta! selected from the group consisting of platinum, palladium, iridium, and mixtures thereof.
• the at least one platinum group metal is at least partially supported on the metal oxide support particles, wherein more preferably, different types of platinum group metals are supported on separate metal oxide support particles.
• the first washcoat layer comprises Pd and/or Pt, preferably Pd and Pt.
• the nitrogen oxide storage catalyst comprises Pd
• the total loading of Pd in the catalyst ranges from 5 to 25 g/ft 3 . More preferably, the total loading of Pd in the nitrogen oxide storage catalyst ranges from 8 to 20 g/ft 3 , more preferably from 10 to 17 g/ft 3 , more preferably from 1 1 to 16 g/ft 3 , more preferably from 12 to 15 g/ft 3 , and even more preferably from 13 to 14 g/ft 3 .
• the total loading of Pt in the catalyst is ranges from 5 to 200 g/ft 3 . More preferably, the total loading of Pt in the nitrogen oxide storage catalyst ranges from 20 to 150 g/ft 3 , more preferably from 40 to 120 g/ft 3 , more preferably from 50 to 100 g/ft 3 , more preferably from 60 to 80 g/ft 3 , and even more preferably from 65 to 75 g/ft 3 .
• the loading of the first washcoat layer in the nitrogen oxide storage catalyst may range from 0.5 to 20 g/in 3 , wherein the loading preferably ranges from 1 to 15 g/in 3 , more preferably from 3 to 10 g/in 3 , more preferably from 4 to 8 g/in 3 , more preferably from 5 to 7 g/in 3 , and even more preferably from 5.5 to 6.0 g/in 3 .
• the first washcoat layer of the nitrogen oxide storage catalyst comprises substantially no Rh.
• a material is preferably defined as not comprising a substantial amount of a specific element when it contains 0.001 wt.-% or less of said element, preferably 0.0005 wt.-% or less, more preferably 0.00001 wt-% or less, more preferably 0.000005 wt.-% or less, and even more preferably 0.000001 wt.-% or less thereof.
• said layer is disposed on 100-x% of the surface of the first washcoat layer, wherein x ranges from 20 to 80, x preferably ranging from 25 to 75, more preferably from 30 to 70, more preferably from 35 to 65, more preferably from 40 to 60, more preferably from 45 to 55, and even more preferably from 48 to 52.
• the washcoat layers are provided on the substrate in any conceivable fashion, wherein the first and second washcoat layers are preferably both disposed on the substrate starting from one end and/or side of the substrate body.
• said layers are provided on the substrate as continuous layers, wherein a continuous layer according to the present invention preferably refers to a layer which is uninterrupted along its entire length.
• the loading of Rh in the second washcoat layer may have any possible value, wherein the loading may range from 0.5 to 10 g/ft 3 .
• the loading of Rh in the second washcoat layer ranges from 1 to 8 g/ft 3 , more preferably from 2 to 6 g/ft 3 , more preferably from 2.5 to 5.5 g/ft 3 , more preferably from 3 to 5 g/ft 3 , and even more preferably from 3.5 to 4.5 g/ft 3 .
• said total loading may range from 0.25 to 5 g/ft 3 .
• the total loading of Rh ranges from 0.5 to 5 g/ft 3 , more preferably from 1 to 3 g/ft 3 , more preferably from 1.25 to 2.75 g/ft 3 , more preferably from 1.5 to 2.5 g/ft 3 , and even more preferably from 1.75 to 2.25 g/ft 3 .
• any possible loading may be chosen in principle.
• the loading of the second washcoat layer in the nitrogen oxide storage catalyst may range from 0.05 to 5 g/in 3 , preferably from 0.1 to 2 g/in 3 , more preferably from 0.2 to 1.5 g/in 3 , more preferably from 0.3 to 1 g/in 3 , more preferably from 0.4 to 0.6 g/in 3 , and even more preferably from 0.45 to 0.55 g/in 3 .
• the loading of the second washcoat layer in the portion of the catalyst containing the second washcoat layer is less than the loading of the first washcoat layer onto which it is disposed.
• the second washcoat layer comprises metal oxide support particles on which Rh is at least partially supported.
• any metal oxide particles may be used in the second washcoat layer, wherein metal oxide support particles are preferred which comprise at least one metal oxide selected from the group consisting of alumina, zirconia, zirconia-a!umina, baria- alumina, lanthana-alumina, lanthana-zirconia-alumina, and mixtures thereof, preferably wherein the metal oxide support particles comprise zirconia-alumina and/or lanthana- alumina, more preferably zirconia-alumina
• the metal oxide support particles comprised in the second washcoat layer may be doped with one or more com- pounds.
• the metal oxide support particles, preferably alumina, comprised in the second washcoat layer are preferably doped with zirconia.
• the metal oxide support particles of the second washcoat layer may be doped with any possible amount of zirconia, wherein the metal oxide support particles are preferably doped with 0.5 to 40% zirconia, more preferably with from 1 to 30%, more preferably with from 5 to 25%, and even more preferably with from 10 to 20% zirconia.
• the first and second washcoat layer of the nitrogen oxide storage catalyst comprise alumina doped with zirconia, wherein more preferably alumina comprised in the first washcoat layer is alumina doped with from 1 to 30 % zirconia, more preferably from 5 to 20 %, more preferably from 7 to 15 %, more preferably from 8 to 12 %, and even more preferably from zirconia 9 to 1 1 %, and alumina comprised in the second washcoat layer is alumina doped with from 1 to 50 % zirconia, more preferably from 5 to 40 %, more preferably from 10 to 30 %, more preferably from 15 to 25 %, more preferably from 18 to 22 %, and even more preferably from zirconia 19 to 21 %.
• the particle size (d 90 ) of the metal oxide support particles comprised in the second washcoat layer ranges from 5 to 20 pm, preferably from 8 to 14 ⁇ , more preferably from 9 to 13 pm, and even more preferably from 10 to 12 pm.
• a third washcoat layer is disposed onto at least a portion of the first washcoat layer onto which the second washcoat layer is not disposed, wherein said third washcoat layer com- prises Pd.
• the loading of Pd in the third washcoat layer may have any possible value, wherein the loading may range from 5 to 30 g/ft 3 .
• the loading of Pd in the third washcoat layer ranges from 10 to 25 g/ft 3 , more preferably from 13 to 21 g/ft 3 , more preferably from 15 to 19 g/ft 3 , more preferably from 16 to 18 g/ft 3 , and even more preferably from 16.5 to 17.5 g/ft 3 .
• the loading of the third washcoat layer in the nitrogen oxide storage catalyst may range from 0.05 to 5 g/in 3 , preferably from 0.1 to 2 g/in 3 , more preferably from 0.2 to 1.5 g/in 3 , more preferably from 0.3 to 1 g/in 3 , more preferably from 0.4 to 0.6 g/in 3 , and even more preferably from 0.45 to 0.55 g/in 3 .
• the loading of the third washcoat layer in the portion of the catalyst containing the third washcoat layer is less than the loading of the first washcoat layer onto which it is disposed.
• the nitrogen oxide storage catalyst comprises a third washcoat layer
• the third washcoat layer comprises substantially no Rh.
• Pd comprised in the third washcoat layer is at least partially supported on metal oxide particles.
• any metal oxide support particles may be used in the third washcoat layer, wherein metal oxide support particles are preferred which comprise at least one metal oxide selected from the group consisting of alumina, zirconia, zirconia-alumina, baria- alumina, lanthana-alumina, lanthana-zirconia-a!umina, and mixtures thereof, more preferably, wherein the metal oxide particles comprise zirconia-alumina and/or lanthana- alumina, even more preferably zirconia-alumina.
• the metal oxide support particles comprised in the third washcoat layer may be doped with one or more compounds.
• the metal oxide support particles, preferably alumina, comprised in the third washcoat layer are preferably doped with zirconia.
• the metal oxide support particles of the third washcoat layer may be doped with any possible amount of zirconia, wherein the metal oxide support particles are preferably doped with 1 to 50 % zirconia, more preferably from 5 to 40 %, more preferably from 10 to 30 %, more preferably from 15 to 25 %, more preferably from 18 to 22 %, and even more preferably from zirconia 19 to 21 %.
• the preferred substrate of the nitrogen oxide storage catalyst of the present invention which is a honeycomb substrate comprising a plurality of longitudinally extending passages formed by longitudinally extending walls bounding and defining said passages
• said honeycomb substrate comprises alternating inlet and outlet passages, said inlet passages having an open inlet end and a closed outlet end, and said outlet passages having a closed inlet end and an open outlet end, thus forming a wall-flow substrate.
• the loading of the washcoat layers thereon will depend on substrate properties such as porosity and wall thickness.
• the first washcoat layer onto 100 - x % of the surface of which the second washcoat layer is disposed is the first washcoat layer disposed on the walls of the inlet passages of the honeycomb substrate, wherein the second washcoat layer is preferably disposed on the first portion of said first washcoat layer extending from the inlet end of the honeycomb substrate (front section).
• said second washcoat layer is provided on the substrate as a continuous layer.
• the first washcoat layer onto which said third washcoat layer is disposed is the first washcoat layer disposed on the outlet passages of the honeycomb substrate, said third washcoat layer being disposed on at least a portion of said first washcoat layer of the outlet passages of the honey- comb substrate
• the third washcoat layer is disposed on 100 - x % of the surface of the first washcoat layer, x ranging from 20 to 80, preferably from 25 to 75, more preferably from 30 to 70, more preferably from 35 to 65, more preferably from 40 to 60, more preferably from 45 to 65, and even more preferably from 48 to 52. Furthermore, said third washcoat layer is preferably disposed on a portion of said first washcoat layer extending from the outlet end of the honeycomb substrate (rear section).
• the present invention is also directed to a treatment system for an automobile exhaust gas stream.
• the treatment system of the present invention comprises
• any conceivable combustion engine may be used in the treatment system of the present invention, wherein preferably a gasoline engine is used, and more preferably a direct injection gasoline engine.
• a catalytic device employing a three-way conversion (“TWC") catalyst may be used in conjunction with the nitrogen oxide storage catalyst of the present invention.
• TWC three-way conversion
• the TWC catalyst typically includes platinum, palladium and rhodium catalytic components dispersed on a high surface area refractory support and may also contain one or more base metal oxide catalytic components such as oxides of iron, manganese or nickel.
• Such catalysts can be stabilized against thermal degradation by expedients such as impregnating an activated alumina support with one or more rare earth metal oxides such as ceria.
• Such stabilized catalysts can sustain very high operating temperatures. For example, if a fuel cut technique is utilized, temperatures as high as 1050 °C may be sustained in the catalytic device.
• the catalytic device may be mounted close to the exhaust manifold of the engine.
• the TWC catalyst may warm up quickly and provide for efficient cold start emission control. Once the engine is warmed up, the TWC catalyst will remove HC, CO and NO x from the exhaust gas stream during stoichiometric or rich operation and HC and CO during lean operation.
• the nitrogen oxide storage catalyst is positioned downstream of the catalytic device where the exhaust gas temperature enables maximum NO x trap efficiency. During periods of lean engine operation, when NO x passes through the TWC catalyst, NO x is stored on the nitrogen oxide storage catalyst.
• the nitrogen oxide is then periodically desorbed and the NO x is reduced to nitrogen under periods of stoichiometric or rich engine operation.
• a catalytic device containing a TWC catalyst may be employed downstream of the nitrogen oxide storage catalyst of the invention. Such a catalytic device may serve to remove further amounts of HC and CO from the exhaust gas stream and, in particular, will provide for efficient reduction of the NO x to nitrogen under periods of stoichiometric or rich engine operation.
• the nitrogen oxide storage catalyst according to the present invention may be used in conjunction with a diesel oxidation catalyst (DOC), and a catalyzed soot filter (CSF), where the DOC and CSF are placed either before or after the nitrogen oxide storage catalyst.
• DOC diesel oxidation catalyst
• CSF catalyzed soot filter
• embodiments of the treatment system are preferred which, in addition to or alternatively to the three way catalyst, comprise a selective reduction catalyst pro- vided upstream or downstream from the nitrogen oxide storage catalyst, wherein embodiments are preferred with the selective reduction catalyst provided downstream from the nitrogen oxide storage catalyst.
• the treatment system of the present invention comprises both a three way catalyst and a selective reduction catalyst located downstream from the nitrogen oxide storage catalyst, wherein preferably, both the three way catalyst and the selective reduction catalysts are located on the same substrate.
• the nitrogen storage catalyst may be comprised on separate substrates with respect to the portions or sections of the catalyst which comprise the second washcoat layer, and those which do not comprise the second washcoat layer.
• the nitrogen storage catalyst comprises separate substrates with respect to the portions or sections of the catalyst which comprise the second washcoat layer, and those which comprise the third washcoat layer.
• the present invention also relates to a method for the treatment of automobile engine exhaust gas using the nitrogen oxide storage catalyst of the present invention. More specifically, the method of the present invention includes conducting an automobile engine exhaust gas over and/or through the nitrogen oxide storage catalyst, wherein the automobile engine exhaust gas is preferably conducted through the nitrogen oxide storage catalyst.
• the present invention also concerns a method for the treatment of automobile engine exhaust gas comprising: (i) providing a nitrogen oxide storage catalyst according to the present invention, and
• the automobile engine exhaust gas is from a gasoline engine, more preferably from a direct injection gasoline engine.
• the exhaust gas stream which is contacted with the nitrogen oxide storage catalyst of the present invention is alternately adjusted between lean and stoichiometric/rich operating conditions so as to provide alternating lean operating periods and stoichiometric/rich operating periods.
• the exhaust gas stream being treated may be selectively rendered lean or stoichiometric/rich either by adjusting the air-to-fuel ratio fed to the engine generating the exhaust or by periodically injecting a reductant into the gas stream upstream of the catalytic trap.
• the composition of the present invention is well suited to treat the exhaust of engines, including diesel engines, which continuously run lean.
• a suitable reductant such as fuel
• Partial lean-burn engines such as partial lean-burn gasoline engines, are designed with controls which cause them to operate lean with brief, intermittent rich or stoichiometric conditions.
• the automobile engine is operated periodically between lean and rich conditions.
• the present invention also relates to a method for its production.
• the present invention further relates to a method of producing a nitrogen oxide storage catalyst comprising the steps of:
• first washcoat layer comprising metal oxide support particles and at least one metal compound selected from the group consisting of alkaline earth metal compounds, alkali metal compounds, rare earth metal compounds, and mixtures thereof, at least a portion of said at least one metal compound being supported on the metal oxide support particles;
• x ranges from 20 to 80, preferably from 25 to 75, more preferably from 30 to 70, more preferably from 35 to 65, more preferably from 40 to 60, more preferably from 45 to 65, and even more preferably from 48 to 52;
• the washcoat layers may be provided on the substrate by any means commonly used in the art, wherein preferably the washcoat is applied to the substrate by a dip coating procedure, !n general, the preferred dip coating procedure may be conducted once to apply the washcoat layer in question, and may be repeated as many times as necessary for achieving the desired loading of said washcoat layer.
• the temperature and duration of said procedure are generally chosen such that the resulting dried product is essentially devoid of any solvent employed in the coating procedure.
• the temperature and duration are generally chosen such that a product is obtained which displays the chemical and physical transformations typical to the calcination process.
• the temperature at which the calcination procedure is conducted is comprised in the range of from 450 to 600 °C, more preferably of from 500 to 580 °C, and even more preferably of from 540 to 560 °C.
• the calcination procedure may be conducted under any suitable atmosphere, wherein the calcination is generally conducted under air.
• the metal oxide support particles comprise at least one metal oxide selected from the group consisting of alumina, zirconia, zirconia-alumina, baria-aiumina, lanthana-alumina, lanthana- zirconia-alumina, and mixtures thereof, wherein more preferably the metal oxide support particles comprise zirconia-alumina and/or lanthana-alumina, even more preferably zirconia-alumina.
• step (ii) of the inventive production method com- prises the steps of:
• step (f) providing a slurry comprising the milled particles obtained in step (e) and the composite materia! obtained in step (b) and milling the resulting mixture;
• step (g) coating the substrate with the slurry obtained in step (f).
• the slurry in step (f) contains substantially no Rh.
• step (a) the at least one metal compound in solution and the at least one metal compound present as particles do not contain the same metals, wherein more preferably the at least one metal compound in solution comprises Ba, and t e at least one metal compound in particle form comprises Ce, more preferably, wherein said particles comprise ceria.
• the temperature employed therein is preferably comprised in the range of from 500 to 800 °C, more preferably of from 600 to 750 °C, and even more preferably of from 680 to 720 °C.
• said procedure may be conducted in any known manner, provided that the platinum group metal respectively comprised therein may be effectively supported onto the metal oxide support particles. Preferably, this is achieved by an incipient wetness procedure.
• step (ii) comprises the steps of:
• step (b) drying and/or calcining the mixture to obtain a composite material; (f) providing a slurry comprising a solution comprising Pt and Pd, metal oxide support particles, and the composite materia! obtained in step (b) and milling the resulting mixture;
• step (g) coating the substrate with the slurry obtained in step (f ).
• said milling may be conducted in any known milling apparatus suited for the milling of particles as used in the present invention, wherein a milling apparatus is preferably employed which may grind the particles down to an average particle size (d 90 ) ranging from 5 to 20 pm, preferably from 8 to 14 m, more preferably from 9 to 13 pm, and even more preferably from 10 to 12 pm.
• d 90 average particle size
• a slurry is provided for the milling and the application of the components comprised in the respective washcoat layers onto the substrate.
• the slurry employed in the preferred embodiments of the inventive method may be provided according to any method known in the art using any suitable solvent, wherein aqueous solvents and in particular water, preferably as distilled water, are preferably used.
• step (iv) comprises the steps of:
• step (cc) coating the substrate with the slurry obtained in step (bb).
• steps (aa), (bb), and (cc) of said preferred embodiment it is further preferred that these respectively be conducted in the same fashion as steps (c) or (d), i.e. with respect to step (aa), (e), i.e. with respect to step (bb), and (g), i.e. with respect to step (cc).
• the inventive further comprises the steps of:
• steps (vi) and (vii) may either be conducted after step (v) or, alternatively, after step (iii) and prior to step (iv) of the inventive production method.
• step (vi) comprises the steps of:
• step (ff) coating the substrate with the slurry obtained in step (ee).
• steps (aa), (bb) and (cc) it is further preferred that steps (dd), (ee), and (ff) re- spectively be conducted in the same fashion as steps (c) or (d), with respect to step (dd), (e), with respect to step (ee), and (g) with respect to step (ff).
• the metal oxide support particles are impreg- nated by an incipient wetness procedure. Furthermore, it is prefered that in steps (g), (cc) and/or (ff) the coating is achieved by dip coating.
• the substrate is a honeycomb substrate, and the method comprises a further step of:
• step (viii) alternatively closing the inlet our outlet ends of the honeycomb substrate to form inlet passages having an open inlet end and a closed outlet end, and outlet passages having a closed inlet end and an open outlet end.
• the slurry is milled to a particle size (d 90 ) ranging from 5 to 20 pm, preferably from 8 to 14 pm, more preferably from 9 to 13 pm, and even more preferably from 10 to 12 ⁇ .
• the inventive method for the production of a nitrogen oxide storage catalyst is to be understood as comprising all of the features obviously necessary for obtaining the nitrogen oxide storage catalyst of the present invention, in particular with respect to the type and amount as well as with respect to the chemical and physical properties of the individual components comprised therein.
• the present invention relates to the products of the inventive production method per se, in particular with respect to the chemical and physical properties of a nitrogen oxide storage catalyst which is obtainable according to said method. Therefore, the present invention also relates to a nitrogen oxide catalyst obtainable according to the inventive method of producing a nitrogen oxide storage catalyst.
• Fig. 1 is a graph comparing the percent NO x conversion as a function of the temperature in °C for selected examples of the description, wherein the NOx tailpipe trigger was set to 100 ppm.
• Fig. 1 “ ⁇ ” represents the values obtained for Comparative Example
• "a” stands for the values obtained for Example 2
• “ A” stands for the values obtained for Example 3.
• Fig. 2 is a graph comparing the NO x storage in cycle 10 after 9 lean/rich cycles with a lean NOx trigger set to 100 ppm as a function of the temperature in °C for selected examples of the description, wherein the NOx tailpipe trigger was set to 100 ppm.
• Fig. 2 “ ⁇ ” stands for the values obtained for Comparative Example 1
• “ ⁇ ” stands for the values obtained for Example 2
• A stands for the values obtained for Example 3
• NOx Storage Cycle 10 / g stands for the
• Fig. 3 is a graph comparing the percent NO x conversion as a function of the temperature in °C for selected examples of the description, wherein the NOx tailpipe trigger was set to 100 ppm.
• A stands for the values obtained for
• Fig. 4 is a graph comparing the NO x storage in cycle 10 after 9 lean/rich cycles with a lean NOx trigger set to 100 ppm as a function of the temperature in °C for selected examples of the description, wherein the NOx taiipipe trigger was set to 100 ppm.
• A stands for the values obtained for Comparative Example 1
• B stands for the values obtained for Example 4
• NOx Storage Cycle 10 / g stands for the NOx Storage Efficiency in g/L.
• Fig. 5 is a graph comparing the percent NO* conversion as a function of the temperature in °C for selected examples of the description, wherein the NOx tailpipe trigger was set to 40 ppm.
• "a” stands for the values obtained for Comparative Example 1
• " ⁇ ” stands for the values obtained for Example 4.
• Fig. 6 is a graph comparing the NO x storage in cycle 10 after 9 lean/rich cycles with a lean NOx trigger set to 40 ppm as a function of the temperature in °C for selected examples of the description, wherein the NOx tailpipe trigger was set to 40 ppm.
• "a" stands for the values obtained for Comparative Example
• NOx Storage Cycle 10 / g stands for the NOx Storage in cycle 10 in g/L.
• BaC0 3 and Ce0 2 were intimately mixed and finely dispersed in a weight ratio of 1 :4.
• cerium oxide having a BET surface area of from 150 m 2 /g was mixed with a solution of barium acetate such that the BaC0 3 /Ce0 2 composite had a BaC0 3 content of from 25 wt.-%.
• the suspension of soluble barium acetate and Ce0 2 was then dried at a temperature of from 120 °C to obtain a solid mixture of barium acetate and ceria.
• the mixture was then heated at 700°C for 2 hours to form particles of ceria having barium carbonate supported on the ceria particles.
• the resulting BaC0 3 had an average crystallite size of from about 25 nm and the ceria had an average crystallite size of 10 nm.
• the BaC0 3 /Ce0 2 composite formed particles with an average size of from about 10 microns.
• the BET surface area of the particulate mixture is 50 m 2 /g.
• Pt and Rh are impregnated onto Al 2 0 3 by an incipient wetness procedure to yield 1.8 wt.-% Pt and 0.3 wt.-% Rh.
• Pd is impregnated separately onto alumina to a Pd loading of 1.4 wt.-%. in both cases, the alumina had a BET surface area of 200 m 2 /g and contained 10 wt.-% zirconia.
• a mixture of 1.65 g/in 3 of the Pt/Rh alumina and 0.4 g/in 3 Pd on alumina was prepared.
• a solution of zirconium acetate with a content of 0.2 g/in 3 was added, giving a slurry with a solid content of 45%. This slurry was milled with a ball mil! until a particle size of 12 micron (d 90 ) was obtained.
• Magnesium acetate was added to the slurry and stirred to dissolve, yielding 0.6 g/in 3 magnesium oxide.
• 3,4 g/in 3 of the Ba- C0 3 /Ce0 2 composite particles is added and the slurry is milled at pH 6.5-7 until a particle size of 11 micron (d 90 ) is obtained.
• a ceramic honeycomb substrate was coated with the slurry in a dip coating manner and then dried in a dryer and subsequently calcined in a furnace under air at 550 °C . The coating procedure was then repeated until a loading of 6.3 g/in 3 is achieved.
• the final nitrogen oxide storage catalyst displays a platinum loading of 72 g/ft 3 , a rhodium loading of 3.6 g/ft 3 , and a palladium loading of 14.4 g/ft 3 .
• Samples according to the present example were prepared with the addition of a second layer, as described below.
• Pt is impregnated onto Al 2 0 3 by an incipient wetness procedure to yield 1.8 wt.-% Pt.
• Pd is impregnated separately onto alumina to a Pd loading of 0.2 wt.-%.
• the alumina had a BET surface area of 200 m 2 /g and contained 10 wt.-% zirconia.
• a mixture of 1.5 g/in 3 of the Pt alumina and 0.4 g/in 3 Pd on alumina was prepared.
• a solution of zirconium acetate with a content of 0.15 g/in 3 was added, giving a slurry with a solid content of 42%. This slurry was milled with a ball mill until a particle size of 12 micron (dg 0 ) was obtained.
• Magnesium acetate was added to the slurry and stirred to dissolve, yielding 0.4 g/in 3 magnesium oxide.
• a precious metal is impregnated onto alumina with a BET surface area of 180 m 2 /g.
• the alumina is doped with 20 wt-% zirconia.
• Rhodium nitrate is impregnated onto the alumina to yield 0.5 wt.-% Rh.
• the alumina slurry is diluted to 35% solids with water.
• the pH is adjusted to 3.5 to 4 using tartaric acid.
• the slurry is then milled to about 12 micron (d 90 ) with a continuous mill. Subsequently, the pH is adjusted to 6.5 using MEA.
• the coated substrate is coated again with the slurry in a dip coating manner and then dried in a dryer.
• the substrate is then cal- cined in a furnace under air at 550 °C.
• the coating procedure yields the additional coat with a loading of 0.5 g/in 3 .
• the overall coating weight of the first and second washcoat layers in the final nitrogen oxide storage catalyst containing said layers is 6.3 g/in 3 .
• the final nitrogen oxide storage catalyst displays a platinum loading of 70 g/ft 3 , a rhodium loading of 4 g/ft 3 , and a palladium loading of 10 g/ft 3 .
• Samples according to the present example were prepared with the addition of a second layer (the "third washcoat layer” according to the invention), as described below.
• Pt is impregnated onto Al 2 0 3 by an incipient wetness procedure to yield 1.8 wt.-% Pt.
• Pd is impregnated separately onto alumina to a Pd loading of 0.2 wt.-%.
• the alumina had a BET surface area of 200 m 2 /g and contained 10 wt.-% zirconia.
• a mixture of 1.5 g/in 3 of the Pt alumina and 0.4 g/in 3 Pd on alumina was prepared.
• a solution of zirconium acetate with a content of 0.15 g/in 3 was added, giving a slurry with a solid content of 42%. This slurry was milled with a ball mill until a particle size of 12 micron (d 90 ) was obtained.
• Magnesium acetate was added to the slurry and stirred to dissolve, yielding 0.4 g/in 3 magnesium oxide.
• a precious metal is impregnated onto alumina with a BET surface area of 180 m 2 /g.
• the alumina is doped with 20 wt.-% of zirconia.
• Palladium nitrate is impregnated onto the alumina to yield 0.5 wt.-% Pd.
• the alumina slurry is diluted to 35% solids with water.
• the pH is adjusted to 3.5 to 4 using tartaric acid.
• the slurry is then milled to about 12 micron (d 90 ) with a continuous mill. Subsequently, the pH is adjusted to 6.5 using EA.
• the coated substrate is coated again with the slurry in a dip coating manner and then dried in a dryer.
• the substrate is then calcined in a furnace under air at 550 °C.
• the coating procedure yields the additional coat with a loading of 0.5 g/in 3 .
• the overall coating weight of the first and second layer in the final nitrogen oxide sto- rage catalyst is 6.3 g/in 3 .
• the final nitrogen oxide storage catalyst displays a platinum loading of 70 g/ft 3 and a palladium loading of 17 g/ft 3 .
• Examples 2 and 3 were respectively repeated, wherein the second layer coating according to Example 2 was only provided on 50% of the first layer coating, such that 50% of the front section of the honeycomb substrate relative to the longitudinally ex- tending passages was coated therewith, and the second layer coating (the "third wash- coat layer” according to the invention) according to Example 3 was provided on the remaining 50% of the first layer coating which had not been coated with a second layer coating according to Example 2, such that 50% of the rear section of the honeycomb substrate was coated therewith.
• the second layer coating according to Example 2 was only provided on 50% of the first layer coating, such that 50% of the front section of the honeycomb substrate relative to the longitudinally ex- tending passages was coated therewith
• the second layer coating (the "third wash- coat layer” according to the invention) according to Example 3 was provided on the remaining 50% of the first layer coating which had not been coated with a second layer coating according to Example 2, such that 50% of the rear section of the honeycomb substrate was coated therewith.
• the final nitrogen oxide storage catalyst displays a rhodium loading of 4 g/ft 3 in the front section of the catalyst containing the second layer coating according to Example 2, of 0 g/ft 3 in the section of the catalyst containing the second washcoat layer (the "third washcoat layer” according to the invention) according to Example 3, and a total loading of rhodium in the catalyst of 2 g/ft 3 .
• the palladium loading is of 10 g/ft 3 in the front section of the catalyst containing the second layer coating according to Example 2, of 17 g/ft 3 in the section of the catalyst containing the second washcoat layer (the "third washcoat layer” according to the invention) according to Example 3, and a total loading of palladium in the catalyst of 3.5 g/ft 3 .
• Example 2 and Comparative Example 1 (modification: no impregnation with Rh) were respectively repeated, wherein the layer coatings according to Example 2 were only provided on 50% of the honeycomb substrate, such that 50% of the front section of the honeycomb substrate relative to the longitudinally extending passages was coated according to Example 2, and the single layer coating according to the modified Comparative Example 1 was provided on the 50% of the honeycomb substrate which had not been coated with the layer coatings according to Example 2, such that 50% of the rear section of the honeycomb substrate was coated therewith.
• Catalytic traps were evaluated after aging for 25 hours at 800°C, as follows. An engine was set to an air/fuel ratio of 1 1 .6 for 2 minutes at the desired temperature to remove all stored NO x and oxygen from the catalyst. This mode represents rich engine operation. Subsequently, the engine was adjusted to an air/fuel ratio of 29.6 under constant NO x mass flow. This mode represents lean engine operation. During the whole test, the NO x concentration was measured before and after the NO x trap using a NO x analyzer.
• NO x NO x concentration (ppm)
• V volume flow (m 3 /h)
• V idea i ideal molar volume (I/mol) at STP
• the nitrogen oxide catalyst of the present invention displays an improved NO x storage and conversion efficiency compared to nitrogen oxide storage catalysts of the prior art exemplified by Comparative Example 1.
• the application of first and second wash- coat layers, as well as the application of first and third washcoat layers according to the present invention respectively leads to an improvement in both the NO x storage and conversion efficiencies of a nitrogen oxide storage catalyst, in particular at lower temperatures reflecting the "cold start" conditions typically encountered at the beginning of the automotive exhaust gas treatment process.
• a nitrogen oxide storage catalyst comprising a second washcoat layer comprising Rh (Example 2) or a third washcoat layer comprising Pd (Example 3) in addition to a first washcoat layer comprising a nitrogen oxide storage material, respectively leads to a clear improvement in the NO x conversion efficiency compared to an NO x storage catalyst according to Comparative Example 1 displaying a single washcoat layer. This improvement is particularly apparent at lower temperatures of the testing process, which reproduces the typical "cold start" environment in automotive exhaust gas treatment. This applies in particular with respect to Example 2, which comprises a topcoat containing Rh.
• the nitrogen oxide storage catalyst of Example 4 which combines the NO x storage catalyst design of Examples 2 and 3, and thus achieves a particularly reduced total loading of Rh, surprisingly displays a higher efficiency in both NO x conversion and NO x storage compared to the NO x storage catalyst of Comparative Example 1 which contains almost twice the total loading in Rh.
• said improved efficiency is particularly pronounced at lower testing temperatures reflecting the "cold start" environment in automotive exhaust gas treatment.
• a nitrogen oxide storage catalyst according to the present invention provides an improved catalyst performance in automotive exhaust gas treatment with respect to the abatement of nitrogen oxide, in particular with respect to the critical "cold start” start conditions, in combination with a con- siderabie reduction of the platinum group metais necessary for its performance, in particular with respect to the amount of Rh contained therein.

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