WO2018095882A1 - Iron containing catalyst - Google Patents

Iron containing catalyst Download PDF

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Publication number
WO2018095882A1
WO2018095882A1 PCT/EP2017/079844 EP2017079844W WO2018095882A1 WO 2018095882 A1 WO2018095882 A1 WO 2018095882A1 EP 2017079844 W EP2017079844 W EP 2017079844W WO 2018095882 A1 WO2018095882 A1 WO 2018095882A1
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Prior art keywords
platinum
zone
layer
catalyst
palladium
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PCT/EP2017/079844
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French (fr)
Inventor
Naohiro Kato
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Umicore Ag & Co. Kg
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Publication of WO2018095882A1 publication Critical patent/WO2018095882A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/02—Boron or aluminium; Oxides or hydroxides thereof
    • B01J21/04—Alumina
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • 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
    • B01D53/92—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
    • B01D53/944—Simultaneously removing carbon monoxide, hydrocarbons or carbon making use of oxidation catalysts
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/70—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper
    • B01J23/89—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with noble metals
    • B01J23/8906—Iron and noble metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/40—Catalysts, in general, characterised by their form or physical properties characterised by dimensions, e.g. grain size
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/50—Catalysts, in general, characterised by their form or physical properties characterised by their shape or configuration
    • B01J35/56—Foraminous structures having flow-through passages or channels, e.g. grids or three-dimensional monoliths
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/60—Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
    • B01J35/61—Surface area
    • B01J35/613—10-100 m2/g
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/60—Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
    • B01J35/61—Surface area
    • B01J35/615—100-500 m2/g
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/03—Precipitation; Co-precipitation
    • B01J37/038—Precipitation; Co-precipitation to form slurries or suspensions, e.g. a washcoat
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/10—Noble metals or compounds thereof
    • B01D2255/102—Platinum group metals
    • B01D2255/1021—Platinum
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/10—Noble metals or compounds thereof
    • B01D2255/102—Platinum group metals
    • B01D2255/1023—Palladium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/207—Transition metals
    • B01D2255/20738—Iron
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/90—Physical characteristics of catalysts
    • B01D2255/902—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
    • B01D2255/903—Multi-zoned catalysts
    • B01D2255/9032—Two zones
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2258/00—Sources of waste gases
    • B01D2258/01—Engine exhaust gases
    • B01D2258/012—Diesel engines and lean burn gasoline engines

Definitions

  • the present invention relates to the purification of exhaust gas of lean- burn internal combustion eng ines, in particular Diesel eng ines, of motor vehicles.
  • the exhaust gas of d iesel eng ines typical ly comprises carbon monoxide CO, hyd rocarbons HC and nitrogen oxides NO x , as wel l as a relatively high oxygen content of up to 15% by volume.
  • particulate emissions are present, these consisting predominantly of solid soot resid ue and possibly organic agg lomerates and orig inating from partially incomplete fuel combustion in the cylinder.
  • the carbon monoxide and hyd rocarbon pol lutant gases can be rendered harmless by oxidation over a suitable oxidation catalyst.
  • Diesel oxidation catalysts for oxidative removal of carbon monoxide and gaseous hydrocarbons have long been known in the prior art and have been described in a wide variety of different embodiments.
  • platinu m g rou p metals, in particu lar platinum and/or pal ladium are used as active components in these catalysts.
  • Such active components are usual ly provided on one or more support oxides, for example aluminum oxide or aluminum-sil icon mixed oxide.
  • Such oxides are freq uently stabil ized by the add ition of about 1 to about 10, in particular about 4 % by weig ht of lanthanum oxide .
  • SCR selective catalytic red uction
  • the NOx fraction of the exhaust gas predominantly comprises nitrogen monoxide NO, as wel l as NO2.
  • the NO2 part is reduced to NO as soon as the exhaust gas enters a platinum group metal containing catalyst using CO and HC as reductant.
  • Oxidation catalysts should ideally provide said NO2 to NOx ratio throughout the lifetime of the vehicle.
  • the activity of presently known oxidation catalysts decrease due to unavoidable thermal degradation processes. Consequently, they are designed to provide the appropriate NO2 to NOx ratio even at the end of the vehicle's lifetime with the consequence that the NO2 to NOx ratio is usually too high at the fresh state of the catalyst.
  • Oxidation catalysts containing iron are known already.
  • EP 0 714 692 Al discloses a titania powder having palladium and iron oxide (Fe203) deposited thereon. This powder is mixed with another refractory inorganic oxide and the thus obtained mixture is said to be capable of i.a. efficiently removing minute particulates entrained by the diesel engine exhaust gas
  • EP 2 000 202 Al discloses a catalyst for purification of exhaust gas comprising a noble metal supported on a metal-oxide support which support supports additional components which include iron.
  • the molar ratio (amount if iron/amount of noble metal) of an amount of the iron supported on the support to the amount of the noble metal ranges from 0.8 to 12 in terms of metals.
  • EP 2 147 720 Al discloses a three-way catalyst wherein a noble metal is supported on a metal oxide support which can comprise iron .
  • US2004/058810 pertains to a method for the catalytic conversion of carbon monoxide in a hyd rogen-containing gas mixture with water to form carbon d ioxide and hyd rogen by passing the gas mixture over a shift catalyst.
  • the shift catalyst may comprise platinum, pal ladium and iron .
  • US2015/158023 d iscloses a cold start catalyst which comprises a molecular sieve consisting essentially of a noble metal and a molecular sieve and a supported platinum group metal catalyst.
  • the document d iscloses a comparative catalyst which comprises a Fe/zeol ite-coated substrate.
  • US2011/158877 d iscloses a method for the catalytic decomposition of N 2 0 in an N2O- and NOx-containing gas in the presence of a catalyst, which catalyst contains a zeolite that has been loaded with a noble metal which can be platinum and/pal ladium and second metal which can be iron .
  • US2014/112849 d iscloses a process for the preparation of hig hly d ispersed transition metal on refractory oxides. Specifical ly, the document exemplifies a method for supporting plartinum and iron on alumina .
  • the inventors of the present invention surprising ly found that modification of oxidation catalysts comprising platin um and pallad ium by iron enhances NO2 formation after thermal deg radation .
  • the present invention accordingly relates to a catalyst comprising
  • first layer or zone which comprises platinum, pallad ium or
  • the catalyst comprises in the second layer or zone a base for pH adjustment.
  • bases can be organic bases but are more preferably inorganic bases, especially alkaline earth compounds like barium compounds. In case of barium compounds they are usually used in an amount of less than lOg/L when calculated as BaO of the support body.
  • the catalyst of the present invention is preferably free of NOx storage components.
  • the catalyst is free of tungsten, antimony, molybdenum, nickel, vanadium, manganese, bismuth, cobalt and zinc.
  • the catalyst comprises no other noble metals except platinum or platinum and palladium.
  • the catalyst doesn't comprise palladium as the only noble metal .
  • the weight ratio Pt : Pd in the second layer or zone is preferably 1 : 2 to 20 : 1, for example 1 : 1, 6 : 1, 10 : 1 or 12 : 1 or 20 : 1.
  • the amount of platinum or platinum and palladium in the second layer or zone is preferably 5g/cft to 180g/cft (0, 177 g/l to 6,357 g/l) based on the volume of the support body.
  • the carrier material of the second layer or zone which carries platinum or platinum and pal ladium, as well as iron is selected from the group consisting of aluminum oxide, aluminum-sil icon mixed oxides, aluminum oxide stabilized with lanthanum, composites or mixed oxides of the type Al 2 03-Zr02, Al 2 03-Zr0 2 -Ti0 2 or Al 2 03-Zr0 2 -Ti0 2 -Si02, sil icon dioxide and titanium d ioxide.
  • Preferred carrier materials are aluminum oxide, aluminum-sil icon mixed oxides and al uminum oxide stabil ized with lanthanum .
  • Al uminum oxide stabil ized with lanthanum usual ly comprises lanthanum in an amount of 1 to 10 wt. %, in particular 3 to 6 wt. %, calculated as La 2 03 und based on the total weight of the lanthanum stabil ized aluminum oxide.
  • the carrier materials of the second layer or zone usual ly have BET surfaces of 30 to 500 m 2 /g, in particular 100 to 250 m 2 /g (determined accord ing to DIN 66132) .
  • the second layer or zone catalyst usually contains iron in form of iron oxide Fe 2 03.
  • the first layer or zone comprises platin um, pal lad ium or platinum and pal ladium supported on a carrier material .
  • the carrier material of the first layer or zone is preferably one of the carrier materials d isclosed above.
  • the first layer or zone comprises one or more zeol ites, l ike for example beta zeol ite.
  • the support body comprises a first layer or zone which comprises platinum and pallad ium, supported on a carrier material comprising alumina, as wel l as a beta zeol ite, and a second layer or zone which comprises platinum or platinum and pal ladium, an iron compound and a carrier material characterized in that the ratio of the iron compound in g/l of the support body and calculated as Fe 2 03 and platinum or platinum and pallad ium in g/l of the support body and calculated as metal is 0.4 to 3.9.
  • the support body of the inventive catalyst is usually an inert ceramic or metal honeycomb body, in particular a flow-through or wall flow filter monolith. Ceramic honeycomb bodies are preferably cordierite, silicon carbide or aluminum titanate bodies whereas metal honeycomb bodies are preferably made of stainless steel.
  • the inventive catalyst can be manufactured by known methods, in particular by coating of the support body with a coating suspension (washcoat) which contains the catalytically active materials of the second layer or zone followed by coating with a coating suspension
  • washcoat which contains the catalytically active materials of the first layer or zone or vice versa.
  • the coating can be performed via conventional immersion, suction and pumping methods which are extensively described in literature and known to the person of skill in the art.
  • the coating is usually followed by calcination and optionally thermal reduction in an atmosphere which contains forming gas.
  • the catalytically active materials of the inventive catalyst can be coated over the complete length of the support body (in form of a layer) or only over a part of its length (in form of a zone).
  • the coating suspensions are manufactured by mixing the components or precursors thereof in an aqueous medium and subsequently milling it to the required particle size.
  • Platinum group metals are preferably introduced into the catalytic materials by impregnation, by sorption, by precipitation and by
  • platinum and palladium are introduced in the form of suitable water-soluble precursor compounds into a suspension containing the carrier material and fixed on the said carrier material a defined way by sorption and/or precipitation.
  • a suspension containing the carrier material into a solution of suitable water-soluble precursor compounds of platinum or platinum and palladium .
  • Iron can be introduced into the suspension in form of iron nitrate.
  • the catalyst of the present invention is suitable for the treatment of diesel exhaust gases, the treatment of the exhaust gas being carried out by passing the exhaust gas over the inventive catalyst.
  • the catalyst according to the invention provides the appropriate NO2 to NOx ratio for a downstream SCR catalyst constantly over the complete lifetime of a vehicle.
  • the catalyst of the present invention provides improved CO oxidation properties.
  • the present invention further relates to a method for providing a constant NO2 to NOx ratio in an exhaust gas which is appropriate for an SCR catalyst which forms part of an exhaust gas treatment system together with a catalyst as described above, characterized in that the exhaust gas is passed over the said catalyst before contacting the SCR catalyst.
  • the NO2 to NOx ratio in an exhaust gas which is appropriate for an SCR catalyst is preferably a ratio of NO2 to NOx of 0.3 to 0.7.
  • the said catalyst is part of the inventive exhaust gas treatment system together with an SCR catalyst.
  • the said catalyst is arranged in said exhaust gas treatment system upstream of the SCR catalyst.
  • the said catalyst and SCR catalyst are coated on different substrates.
  • the SCR catalysts may be coated on a flow through substrate or on a wall flow filter substrate.
  • Such substrates are of the type described above. Accordingly, they are preferably inert ceramic or metal honeycomb bodies of cordierite, silicon carbide, aluminum titanate or stainless steel .
  • Suitable SCR catalysts are based on mixed oxides or on zeol ites wherein the latter may be metal exchanged . Examples are in particular zeolites like ⁇ -zeol ite, ZSM-5 or small pore zeol ites like LEV, CHA and SAPO and ALPO materials which may be exchanged with Cu or Fe .
  • Preferred SCR catalysts are Cu- LEV, Cu-CHA and Cu-SAPO-34.
  • the inventive exhaust gas treatment system comprises a metering system for metering urea solution into the exhaust gas stream downstream of the oxidation catalyst and upstream of the SCR catalyst.
  • the urea dosed into the exhaust gas stream is hyd rolyzed to form ammon ia which reacts with NOx in the SCR reaction to from nitrogen and water.
  • the present invention further relates to a method for the oxidation of CO in an exhaust gas, characterized in that the exhaust gas is passed over the catalyst of the present invention .
  • Example 1
  • the catalyst obtained is hereinafter called C3.
  • the catalyst obtained is hereinafter called C4.
  • Example 1 was repeated with the exception that no iron was used .
  • the catalyst obtained is hereinafter called CCl .
  • Example 4 was repeated with the exception that no iron was used .
  • the catalyst obtained is hereinafter called CC2.
  • Catalysts CI, C2, C3 and CCl were aged and subsequently evaluated at a lab-reactor to determine the difference in N0 2 to NOx ratio at the catalyst outlet gas.
  • Catalysts C4 and CC2 were aged and subsequently evaluated at a lab- reactor to determine the difference in N0 2 to NOx ratio at the catalyst outlet gas. Aging
  • Drilled cores from the catalysts CI, C2, C3, C4, CC1 and CC2 were treated hydrothermally ( 10% H 2 0, 10% O2, N2 balance) in an oven at 800°C for a duration of 16 hours.
  • Each core was placed in a lab-reactor and the synthesized exhaust gas containing 12.5% O2, lOOOppm CO, 250ppm NO, 133ppm CsHe, 6.5% H2O, 6% CO2, balance N2 at a total flow of 2317L/h was passed through the core.
  • the temperature of the synthesized gas was increased from 75°C to 500°C at a rate of 15°C/min and during the temperature increase, NO2 to NOx ratio in the outlet gas was measured using conventional equipment.
  • the NO2/NOX ratio of the model gas after passing CI, C2 and CC1, respectively, is shown in Figure 1, while the NO2/NOX ratio of the model gas after passing C3 and CC2, respectively, is shown in Figure 2.
  • a washcoat containing 1.24 g/l of platinum, 0.62 g/l of palladium, 70 g/l of S1O2-AI2O3 and 27 g/l of beta zeolite was prepared and coated on a usual flow through substrate using a usual coating process.
  • step c) The washcoat obtained in step b) was coated over the catalyst obtained in step a) using a usual coating process.
  • the catalyst obtained is hereinafter called C5.
  • Example 5 was repeated with the exception that no iron was used in step b) .
  • the catalyst obtained is hereinafter called CC3.
  • the catalysts C5 and CC3 were treated hydrothermally ( 10% H 2 0, 10% 0 2 , N 2 balance) in an oven at 800°C for a duration of 16 hours.
  • a) 20 g/l of alumina stabilized with 4% by weight of La 2 03 in relation to the total mass of the mixed oxide was moistened with an aqueous solution containing 1.9 g/l of platinum in the form of tetraammine- platinum acetate and 0.32 g/l of palladium in the form of tetraammine- palladium acetate such that the pores of said alumina were filled, with the powder remaining free-flowing .
  • the moist powder was dried for a duration of 8 hours at 120°C and calcined for a duration of 4 hours at 300° C.
  • the obtained powder was suspended in water and iron nitrate in an amount of 3.1 g/l calculated as Fe 2 03 was dissolved into the aqueous suspension .
  • Base was added to the suspension to keep the pH value above 4.0 when necessary.
  • the suspension was milled to a mean particle size D99 below 5 microns.
  • D99 means, here, that the volume of the particles with particle sizes of below D99 adds up to 99% of the volume of all the particles.
  • a wall flow filter having a volume of 2.47L, a cell density of 300 cells per square inch, and a wall thickness of approximately 3.3mm was coated with the suspension above according to a) .
  • the suspension was pumped into the substrate from below until the inflow channels of the substrate were filled with suspension over their entire length . Then the slurry was pumped out again from below and then extracted, wherein the pump-out or extraction power was selected such that the
  • the resulting catalytically activated filter was calcined for a period of 4 hours at 350°C and then treated with forming gas for 2 hours at 500°C
  • the catalyst obtained is hereinafter called C6.
  • Example 6 was repeated with the exception that 6.2 g/l of iron
  • the catalyst obtained is hereinafter called C7.
  • Example 6 was repeated with the exception that no iron was used .
  • the catalyst obtained is hereinafter called CC4. Comparison of C6, C7 and CC4
  • Catalysts C6, C7 and CC4 were aged and subsequently evaluated at a lab-reactor to determine the temperatures required for 50% CO conversion (CO Tso) . Aging
  • Each core was placed in a lab-reactor and the synthesized exhaust gas containing 6% O2, 350ppm CO, 270ppm NO, 180ppm CsHe, 90ppm CsHs, 10% H2O, 10% CO2, balance N 2 at a total flow of 2000L/h was passed through the core.
  • the temperature of the synthesized gas was increased from 75°C to 500°C at a rate of 15°C/min and during the temperature increase, CO in the outlet gas was measured using conventional equipment.
  • the temperatures required for 50% CO conversion for C6, C7 and CC4, respectively, are shown in Table 1.

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Abstract

The present invention provides a catalyst comprising a support body, a first layer or zone which comprises platinum, palladium or platinum and palladium supported in a carrier material, and a second layer or zone which comprises platinum or platinum and palladium, an iron compound and a carrier material characterized in that the ratio of the iron compound in g/l of the support body and calculated as Fe2O3 and platinum or platinum and palladium in g/l of the support body and calculated as metal is 0.4 to 3.9 and wherein the second layer or zone is free of zeolites, ceria and alkali compounds.

Description

Iron contain ing catalyst
The present invention relates to the purification of exhaust gas of lean- burn internal combustion eng ines, in particular Diesel eng ines, of motor vehicles.
The exhaust gas of d iesel eng ines typical ly comprises carbon monoxide CO, hyd rocarbons HC and nitrogen oxides NOx, as wel l as a relatively high oxygen content of up to 15% by volume. In addition, particulate emissions are present, these consisting predominantly of solid soot resid ue and possibly organic agg lomerates and orig inating from partially incomplete fuel combustion in the cylinder. The carbon monoxide and hyd rocarbon pol lutant gases can be rendered harmless by oxidation over a suitable oxidation catalyst. Diesel oxidation catalysts for oxidative removal of carbon monoxide and gaseous hydrocarbons have long been known in the prior art and have been described in a wide variety of different embodiments. Usually, platinu m g rou p metals, in particu lar platinum and/or pal ladium are used as active components in these catalysts. Such active components are usual ly provided on one or more support oxides, for example aluminum oxide or aluminum-sil icon mixed oxide. Such oxides are freq uently stabil ized by the add ition of about 1 to about 10, in particular about 4 % by weig ht of lanthanum oxide .
NOx emissions are still a subject of environmental concern . One approach is to use the so-called selective catalytic red uction (SCR) to reduce NOx to nitrogen with ammonia, for example released from urea solution as reducing agent. SCR catalysts can be su pported on flow through substrates, as wel l as on filters like wall-flow filters (SDPF) .
The NOx fraction of the exhaust gas predominantly comprises nitrogen monoxide NO, as wel l as NO2. The NO2 part, however, is reduced to NO as soon as the exhaust gas enters a platinum group metal containing catalyst using CO and HC as reductant.
The reaction of NOx to nitrogen over an SCR catalyst is strongly affected by the ratio of NO2 in total NOx. In order to achieve said ratio, upstream oxidation catalysts are used to produce exhaust gas with the appropriate NO2 to NOx ratio. NO oxidation into NO2 happens efficiently only after the most part of CO and HC have been oxidized into CO2 and
Oxidation catalysts should ideally provide said NO2 to NOx ratio throughout the lifetime of the vehicle. The activity of presently known oxidation catalysts, however, decrease due to unavoidable thermal degradation processes. Consequently, they are designed to provide the appropriate NO2 to NOx ratio even at the end of the vehicle's lifetime with the consequence that the NO2 to NOx ratio is usually too high at the fresh state of the catalyst.
There is therefore a need for oxidation catalysts which provide the appropriate NO2 to NOx ratio constantly over the complete lifetime of the vehicle. It is in addition desirable that such oxidation catalysts provide improved CO oxidation as well.
Oxidation catalysts containing iron are known already. For example, EP 0 714 692 Al discloses a titania powder having palladium and iron oxide (Fe203) deposited thereon. This powder is mixed with another refractory inorganic oxide and the thus obtained mixture is said to be capable of i.a. efficiently removing minute particulates entrained by the diesel engine exhaust gas
EP 2 000 202 Al discloses a catalyst for purification of exhaust gas comprising a noble metal supported on a metal-oxide support which support supports additional components which include iron. The molar ratio (amount if iron/amount of noble metal) of an amount of the iron supported on the support to the amount of the noble metal ranges from 0.8 to 12 in terms of metals. EP 2 147 720 Al discloses a three-way catalyst wherein a noble metal is supported on a metal oxide support which can comprise iron .
US2004/058810 pertains to a method for the catalytic conversion of carbon monoxide in a hyd rogen-containing gas mixture with water to form carbon d ioxide and hyd rogen by passing the gas mixture over a shift catalyst. The shift catalyst may comprise platinum, pal ladium and iron .
US2015/158023 d iscloses a cold start catalyst which comprises a molecular sieve consisting essentially of a noble metal and a molecular sieve and a supported platinum group metal catalyst. The document d iscloses a comparative catalyst which comprises a Fe/zeol ite-coated substrate.
US2011/158877 d iscloses a method for the catalytic decomposition of N20 in an N2O- and NOx-containing gas in the presence of a catalyst, which catalyst contains a zeolite that has been loaded with a noble metal which can be platinum and/pal ladium and second metal which can be iron .
US2014/112849 d iscloses a process for the preparation of hig hly d ispersed transition metal on refractory oxides. Specifical ly, the document exemplifies a method for supporting plartinum and iron on alumina .
The inventors of the present invention surprising ly found that modification of oxidation catalysts comprising platin um and pallad ium by iron enhances NO2 formation after thermal deg radation .
The present invention accordingly relates to a catalyst comprising
- a support body,
- a first layer or zone which comprises platinum, pallad ium or
platinum and pallad ium supported in a carrier material , and
- a second layer or zone which comprises platinum or platin um and pal lad ium, an iron compound and a carrier material characterized in that the ratio of the iron compound in g/l of the support body and calculated as Fe203 and platinum or platinum and pal lad ium in g/l of the support body and calculated as metal is 0.4 to 3.9 and wherein the second layer or zone is free of zeolites, ceria and alkali compounds. In embodiments of the present invention in the second layer or zone the ratio of the iron compound in g/l of the support body and calculated as Fe203 and platinum or platinum and palladium in g/l of the support body and calculated as metal is 1 to 3. In embodiments of the present invention the catalyst comprises in the second layer or zone a base for pH adjustment. Such bases can be organic bases but are more preferably inorganic bases, especially alkaline earth compounds like barium compounds. In case of barium compounds they are usually used in an amount of less than lOg/L when calculated as BaO of the support body.
Besides that the catalyst of the present invention is preferably free of NOx storage components.
In other embodiments of the present invention the catalyst is free of tungsten, antimony, molybdenum, nickel, vanadium, manganese, bismuth, cobalt and zinc.
In further embodiments of the present invention the catalyst comprises no other noble metals except platinum or platinum and palladium. In particular the catalyst doesn't comprise palladium as the only noble metal .
In case the catalyst comprises platinum and palladium in the second layer or zone the weight ratio Pt : Pd in the second layer or zone is preferably 1 : 2 to 20 : 1, for example 1 : 1, 6 : 1, 10 : 1 or 12 : 1 or 20 : 1. The amount of platinum or platinum and palladium in the second layer or zone is preferably 5g/cft to 180g/cft (0, 177 g/l to 6,357 g/l) based on the volume of the support body.
In further embodiments of the present invention the carrier material of the second layer or zone which carries platinum or platinum and pal ladium, as well as iron is selected from the group consisting of aluminum oxide, aluminum-sil icon mixed oxides, aluminum oxide stabilized with lanthanum, composites or mixed oxides of the type Al203-Zr02, Al203-Zr02-Ti02 or Al203-Zr02-Ti02-Si02, sil icon dioxide and titanium d ioxide. Preferred carrier materials are aluminum oxide, aluminum-sil icon mixed oxides and al uminum oxide stabil ized with lanthanum .
Al uminum oxide stabil ized with lanthanum usual ly comprises lanthanum in an amount of 1 to 10 wt. %, in particular 3 to 6 wt. %, calculated as La203 und based on the total weight of the lanthanum stabil ized aluminum oxide.
The carrier materials of the second layer or zone usual ly have BET surfaces of 30 to 500 m2/g, in particular 100 to 250 m2/g (determined accord ing to DIN 66132) .
The second layer or zone catalyst usually contains iron in form of iron oxide Fe203.
The first layer or zone comprises platin um, pal lad ium or platinum and pal ladium supported on a carrier material . The carrier material of the first layer or zone is preferably one of the carrier materials d isclosed above.
In embodiments of the present invention, the first layer or zone comprises one or more zeol ites, l ike for example beta zeol ite.
In embodiments of the present invention the support body comprises a first layer or zone which comprises platinum and pallad ium, supported on a carrier material comprising alumina, as wel l as a beta zeol ite, and a second layer or zone which comprises platinum or platinum and pal ladium, an iron compound and a carrier material characterized in that the ratio of the iron compound in g/l of the support body and calculated as Fe203 and platinum or platinum and pallad ium in g/l of the support body and calculated as metal is 0.4 to 3.9. The support body of the inventive catalyst is usually an inert ceramic or metal honeycomb body, in particular a flow-through or wall flow filter monolith. Ceramic honeycomb bodies are preferably cordierite, silicon carbide or aluminum titanate bodies whereas metal honeycomb bodies are preferably made of stainless steel.
The inventive catalyst can be manufactured by known methods, in particular by coating of the support body with a coating suspension (washcoat) which contains the catalytically active materials of the second layer or zone followed by coating with a coating suspension
(washcoat) which contains the catalytically active materials of the first layer or zone or vice versa.
The coating can be performed via conventional immersion, suction and pumping methods which are extensively described in literature and known to the person of skill in the art. The coating is usually followed by calcination and optionally thermal reduction in an atmosphere which contains forming gas.
The catalytically active materials of the inventive catalyst can be coated over the complete length of the support body (in form of a layer) or only over a part of its length (in form of a zone).
The coating suspensions are manufactured by mixing the components or precursors thereof in an aqueous medium and subsequently milling it to the required particle size.
Platinum group metals are preferably introduced into the catalytic materials by impregnation, by sorption, by precipitation and by
"incipient wetness" methods known in the literature.
Preferably, platinum and palladium are introduced in the form of suitable water-soluble precursor compounds into a suspension containing the carrier material and fixed on the said carrier material a defined way by sorption and/or precipitation. Alternatively, it is possible introduce a suspension containing the carrier material into a solution of suitable water-soluble precursor compounds of platinum or platinum and palladium .
Iron can be introduced into the suspension in form of iron nitrate.
The catalyst of the present invention is suitable for the treatment of diesel exhaust gases, the treatment of the exhaust gas being carried out by passing the exhaust gas over the inventive catalyst. In particular, the catalyst according to the invention provides the appropriate NO2 to NOx ratio for a downstream SCR catalyst constantly over the complete lifetime of a vehicle. Also, the catalyst of the present invention provides improved CO oxidation properties.
Accordingly, the present invention further relates to a method for providing a constant NO2 to NOx ratio in an exhaust gas which is appropriate for an SCR catalyst which forms part of an exhaust gas treatment system together with a catalyst as described above, characterized in that the exhaust gas is passed over the said catalyst before contacting the SCR catalyst.
The NO2 to NOx ratio in an exhaust gas which is appropriate for an SCR catalyst is preferably a ratio of NO2 to NOx of 0.3 to 0.7.
The said catalyst is part of the inventive exhaust gas treatment system together with an SCR catalyst. When in use the said catalyst is arranged in said exhaust gas treatment system upstream of the SCR catalyst.
Preferably, the said catalyst and SCR catalyst are coated on different substrates.
The SCR catalysts may be coated on a flow through substrate or on a wall flow filter substrate. Such substrates are of the type described above. Accordingly, they are preferably inert ceramic or metal honeycomb bodies of cordierite, silicon carbide, aluminum titanate or stainless steel . Suitable SCR catalysts are based on mixed oxides or on zeol ites wherein the latter may be metal exchanged . Examples are in particular zeolites like β-zeol ite, ZSM-5 or small pore zeol ites like LEV, CHA and SAPO and ALPO materials which may be exchanged with Cu or Fe . Preferred SCR catalysts are Cu- LEV, Cu-CHA and Cu-SAPO-34.
In embodiments of the inventive exhaust gas treatment system it comprises a metering system for metering urea solution into the exhaust gas stream downstream of the oxidation catalyst and upstream of the SCR catalyst. The urea dosed into the exhaust gas stream is hyd rolyzed to form ammon ia which reacts with NOx in the SCR reaction to from nitrogen and water.
Metering systems which can be used according to the present invention are known to the skil led person and obtainable on the market place.
The present invention further relates to a method for the oxidation of CO in an exhaust gas, characterized in that the exhaust gas is passed over the catalyst of the present invention . Example 1
a) 2.0 g/l of platinum and 0.33 g/l of pallad ium in the form of their nitrates and iron nitrate in an amount of 1 g/l calculated as Fe203 were slowly added to the aq ueous suspension of 70 g/l of an al umina . Base was added if necessary to keep the pH above 4.0. Then the suspension was mil led to a particle size of D90 below 20 microns.
b) The washcoat obtained was coated on a usual flow throug h substrate using a usual coating process.
The catalyst obtained is hereinafter cal led CI . Example 2
Example 1 was repeated with the exception that 3g/l of iron (calculated
Figure imgf000010_0001
The catalyst obtained is hereinafter cal led C2. Example 3
Example 1 was repeated with the exception that 6g/l of iron (calculated
Figure imgf000011_0001
The catalyst obtained is hereinafter called C3.
Example 4
a) 2.0 g/l of platinum and 0.33 g/l of palladium in the form of their nitrates and iron nitrate in an amount of 9 g/l calculated as Fe203 were slowly added to the aqueous suspension of 105 g/l of an alumina .
b) The washcoat obtained was coated on a usual flow through substrate using a usual coating process.
The catalyst obtained is hereinafter called C4.
Comparison Example 1
Example 1 was repeated with the exception that no iron was used . The catalyst obtained is hereinafter called CCl .
Comparison Example 2
Example 4 was repeated with the exception that no iron was used . The catalyst obtained is hereinafter called CC2.
Comparison of CI, C2, C3 and CCl
Catalysts CI, C2, C3 and CCl were aged and subsequently evaluated at a lab-reactor to determine the difference in N02 to NOx ratio at the catalyst outlet gas.
Comparison of C4 and CC2
Catalysts C4 and CC2 were aged and subsequently evaluated at a lab- reactor to determine the difference in N02 to NOx ratio at the catalyst outlet gas. Aging
Drilled cores from the catalysts CI, C2, C3, C4, CC1 and CC2 were treated hydrothermally ( 10% H20, 10% O2, N2 balance) in an oven at 800°C for a duration of 16 hours.
Test condition at lab-reactor
Each core was placed in a lab-reactor and the synthesized exhaust gas containing 12.5% O2, lOOOppm CO, 250ppm NO, 133ppm CsHe, 6.5% H2O, 6% CO2, balance N2 at a total flow of 2317L/h was passed through the core. The temperature of the synthesized gas was increased from 75°C to 500°C at a rate of 15°C/min and during the temperature increase, NO2 to NOx ratio in the outlet gas was measured using conventional equipment. The NO2/NOX ratio of the model gas after passing CI, C2 and CC1, respectively, is shown in Figure 1, while the NO2/NOX ratio of the model gas after passing C3 and CC2, respectively, is shown in Figure 2.
In addition, in Figure 3 the relative increase in NO2/NOX ratio when the temperature of the synthesized gas has reached 250°C caused by the addition of iron is plotted against the ratio of the iron compound in g/l of the support body calculated as Fe2O3 and platinum or platinum and palladium calculated as metal in g/l of the support body.
Example 5
a) A washcoat containing 1.24 g/l of platinum, 0.62 g/l of palladium, 70 g/l of S1O2-AI2O3 and 27 g/l of beta zeolite was prepared and coated on a usual flow through substrate using a usual coating process.
b) 2.3 g/l of platinum and 0.38 g/l of palladium in the form of their nitrates and iron nitrate in an amount of 3.4 g/l calculated as Fe203 were slowly added to an aqueous suspension of 64 g/l of a Si02-Zr02- T1O2-AI2O3 and 16g/l of an alumina. Base was added if necessary to keep the pH above 4.0. Then the suspension was milled to obtain a washcoat.
c) The washcoat obtained in step b) was coated over the catalyst obtained in step a) using a usual coating process.
The catalyst obtained is hereinafter called C5.
Comparison Example 3
Example 5 was repeated with the exception that no iron was used in step b) .
The catalyst obtained is hereinafter called CC3.
Comparison of C5 and CC3
a) Aging
The catalysts C5 and CC3 were treated hydrothermally ( 10% H20, 10% 02, N2 balance) in an oven at 800°C for a duration of 16 hours.
b) The aged catalysts C5 and CC3 were evaluated on a 3.0L diesel engine under a dynamic test condition to determine the difference in NO2 to NOx ratio at the catalyst outlet gas. The range of the exhaust gas temperature in front of the catalysts during the test cycles was from about 25°C to about 370°C. NO2 to NOx ratio in the outlet gas was measured using conventional equipment.
The NO2/NOX ratio of exhaust gas after passing C5 and CC3 are shown in Figure 4
Example 6
a) 20 g/l of alumina stabilized with 4% by weight of La203 in relation to the total mass of the mixed oxide was moistened with an aqueous solution containing 1.9 g/l of platinum in the form of tetraammine- platinum acetate and 0.32 g/l of palladium in the form of tetraammine- palladium acetate such that the pores of said alumina were filled, with the powder remaining free-flowing . To fix the platinum and palladium, the moist powder was dried for a duration of 8 hours at 120°C and calcined for a duration of 4 hours at 300° C. The obtained powder was suspended in water and iron nitrate in an amount of 3.1 g/l calculated as Fe203 was dissolved into the aqueous suspension . Base was added to the suspension to keep the pH value above 4.0 when necessary. Then the suspension was milled to a mean particle size D99 below 5 microns. The designation D99 means, here, that the volume of the particles with particle sizes of below D99 adds up to 99% of the volume of all the particles.
b) A wall flow filter having a volume of 2.47L, a cell density of 300 cells per square inch, and a wall thickness of approximately 3.3mm was coated with the suspension above according to a) . The suspension was pumped into the substrate from below until the inflow channels of the substrate were filled with suspension over their entire length . Then the slurry was pumped out again from below and then extracted, wherein the pump-out or extraction power was selected such that the
proportion of solids contained in the suspension remained in the channel walls of the wall flow filter substrate in the desired quantity. The resulting catalytically activated filter was calcined for a period of 4 hours at 350°C and then treated with forming gas for 2 hours at 500°C The catalyst obtained is hereinafter called C6.
Example 7
Example 6 was repeated with the exception that 6.2 g/l of iron
(calculated as Fe203) were used .
The catalyst obtained is hereinafter called C7.
Comparison Example 4
Example 6 was repeated with the exception that no iron was used .
The catalyst obtained is hereinafter called CC4. Comparison of C6, C7 and CC4
Catalysts C6, C7 and CC4 were aged and subsequently evaluated at a lab-reactor to determine the temperatures required for 50% CO conversion (CO Tso) . Aging
Drilled cores from the catalysts C6, C7 and CC4 were treated
hydrothermally ( 10% H20, 10% 02, N2 balance) in an oven at 750°C for a duration of 16 hours.
Test condition at lab-reactor
Each core was placed in a lab-reactor and the synthesized exhaust gas containing 6% O2, 350ppm CO, 270ppm NO, 180ppm CsHe, 90ppm CsHs, 10% H2O, 10% CO2, balance N2 at a total flow of 2000L/h was passed through the core. The temperature of the synthesized gas was increased from 75°C to 500°C at a rate of 15°C/min and during the temperature increase, CO in the outlet gas was measured using conventional equipment. The temperatures required for 50% CO conversion for C6, C7 and CC4, respectively, are shown in Table 1.
Fe203/PGM [g/g] CO T50 [°C]
C6 1.40 144
C7 2.80 133
CC4 0 156

Claims

Patent Claims
1. Catalyst comprising
- a support body,
- a first layer or zone which comprises platinum, palladium or
platinum and palladium supported in a carrier material, and
- a second layer or zone which comprises platinum or platinum and palladium, an iron compound and a carrier material characterized in that the ratio of the iron compound in g/l of the support body and calculated as Fe2C>3 and platinum or platinum and palladium in g/l of the support body and calculated as metal is 0.4 to 3.9 and wherein the second layer or zone is free of zeolites, ceria and alkali compounds.
2. Catalyst according to claim 1, characterized in that the second layer or zone is free of other noble metals except platinum or platinum and palladium .
3. Catalyst according to claim 1 and/or 2, characterized in that in the second layer or zone the ratio of the iron compound in g/l of the support body and calculated as Fe203 and platinum or platinum and palladium in g/l of the support body and calculated as metal is 1 to 3.
4. Catalyst according to one or more of claims 1 to 3, characterized in that in the second layer or zone the weight ratio Pt : Pd is 1 : 2 to 20 : 1.
5. Catalyst according to one or more of claims 1 to 4, characterized in that in the second layer or zone the amount of platinum or platinum and palladium is 5 g/cft to 180 g/cft (0,177 g/l to 6,357 g/l) based on the volume of the support body.
6. Catalyst according to one or more of claims 1 to 5, characterized in that the carrier material in the second layer or zone is selected from the group consisting of aluminum oxide, aluminum-silicon mixed oxides, aluminum oxide stabilized with lanthanum, mixed oxides of the type ΑΙ2Ο3-ΖΓΟ2, ΑΙ2Ο3-ΖΓΟ2-Τ1Ο2 or Al203-Zr02-Ti02-Si02, silicon dioxide and titanium dioxide.
7. Catalyst according to one or more of claims 1 to 6, characterized in that the carrier material in the second layer or zone has a BET surface of 30 to 500 m2/g (determined according to DIN 66132).
8. Catalyst according to one or more of claims 1 to 7, characterized in that the first layer or zone comprises one or more zeolites.
9. Catalyst according to claim 8, characterized in that the first layer or zone comprises beta zeolite.
10. Method for providing a constant NO2 to NOx ratio in an exhaust gas which is appropriate for a SCR catalyst, characterized in that the exhaust gas is passed over the catalyst according to one or more of claims 1 to 9.
11. Method for the oxidation of CO contained in an exhaust gas, characterized in that the exhaust gas is passed over the catalyst according to one or more of claims 1 to 9.
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CN109821523A (en) * 2019-01-23 2019-05-31 内蒙古科技大学 A kind of preparation method and applications of rare-earth tailing base SCR catalyst

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CN109821522B (en) * 2019-01-23 2021-11-02 内蒙古科技大学 Method for preparing catalyst by deep processing of rare earth tailings and application thereof

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