WO2004002611A1 - Doped ceria-containing zeolite-based nh3 scr catalyst - Google Patents

Doped ceria-containing zeolite-based nh3 scr catalyst Download PDF

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Publication number
WO2004002611A1
WO2004002611A1 PCT/GB2003/002663 GB0302663W WO2004002611A1 WO 2004002611 A1 WO2004002611 A1 WO 2004002611A1 GB 0302663 W GB0302663 W GB 0302663W WO 2004002611 A1 WO2004002611 A1 WO 2004002611A1
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Prior art keywords
catalyst
zeolite
ceria
dopant
catalyst according
Prior art date
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PCT/GB2003/002663
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French (fr)
Inventor
Valerie Marie Renee Houel
Timothy Ian Hyde
Isabel Zoe Jones
Raj Rao Rajaram
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Johnson Matthey Public Limited Company
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Priority to AU2003251128A priority Critical patent/AU2003251128A1/en
Publication of WO2004002611A1 publication Critical patent/WO2004002611A1/en

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    • 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
    • B01D53/9404—Removing only nitrogen compounds
    • B01D53/9409—Nitrogen oxides
    • B01D53/9413—Processes characterised by a specific catalyst
    • B01D53/9418—Processes characterised by a specific catalyst for removing nitrogen oxides by selective catalytic reduction [SCR] using a reducing agent in a lean exhaust gas
    • 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
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/86—Catalytic processes
    • B01D53/8621—Removing nitrogen compounds
    • B01D53/8625—Nitrogen oxides
    • B01D53/8628—Processes characterised by a specific catalyst
    • 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
    • B01J23/10—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of rare earths
    • 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
    • B01J29/00—Catalysts comprising molecular sieves
    • B01J29/04—Catalysts comprising molecular sieves having base-exchange properties, e.g. crystalline zeolites
    • B01J29/06—Crystalline aluminosilicate zeolites; Isomorphous compounds thereof
    • B01J29/40—Crystalline aluminosilicate zeolites; Isomorphous compounds thereof of the pentasil type, e.g. types ZSM-5, ZSM-8 or ZSM-11, as exemplified by patent documents US3702886, GB1334243 and US3709979, respectively
    • 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
    • B01J37/024—Multiple impregnation or coating
    • B01J37/0244—Coatings comprising several layers
    • 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
    • B01J37/024—Multiple impregnation or coating
    • B01J37/0246—Coatings comprising a zeolite
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2251/00—Reactants
    • B01D2251/20—Reductants
    • B01D2251/206—Ammonium compounds
    • B01D2251/2062—Ammonia
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/206—Rare earth metals
    • B01D2255/2065—Cerium
    • 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
    • B01D2255/20715—Zirconium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/50—Zeolites
    • 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
    • B01J2229/00—Aspects of molecular sieve catalysts not covered by B01J29/00
    • B01J2229/30—After treatment, characterised by the means used
    • B01J2229/42—Addition of matrix or binder particles
    • 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
    • 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
    • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00—Road transport of goods or passengers
    • Y02T10/10—Internal combustion engine [ICE] based vehicles
    • Y02T10/12—Improving ICE efficiencies

Definitions

  • the present invention relates to a catalyst for selectively reducing NO x with NH 3 , and in particular to a catalyst comprising an alumino-silicate zeolite having relatively higher selectivity at higher temperatures.
  • zeolite based catalysts are indicated for higher temperature applications (350-600°C or
  • zeolites tend to deactivate by a process called de-alumination whereby the Al 3+ in the SiO 2 -Al 2 O 3 framework migrates out of the structure. This leads to permanent deactivation and, in the extreme case, collapse of the crystalline structure.
  • Zeolite structures can be ion-exchanged by other cations while maintaining electrical neutrality, altering the chemistry of catalytically active sites.
  • a zeolite catalyst composition which comprises a metal (e.g., iron or copper) promoted zeolite, the zeolite being characterised by having a silica to alumina ratio of at least about 10 and a pore structure which is interconnected in all three crystallographic dimensions by pores having an average kinetic pore diameter of at least about 7 Angstroms.
  • WO-A-02/41991 (the entire contents of which are incorporated herein by reference) describes an improvement over the catalyst of US-A-4,961,917 in that the zeolite is a metal-promoted beta zeolite in which the zeolite is pre-treated so as to provide it with improved hydrothermal stability.
  • the stabilised beta zeolite is provided by incorporating into the zeolite structure no-framework aluminium oxide chains.
  • zeolite-based SCR catalyst such as ZSM-5, which desirably can contain from 1-10 wt% of metal e.g. copper, cerium, iron or platinum in order to improve low temperature SCR activity.
  • EP 0 299 294 describes a ferro-silicate zeolite for catalysing the selective reduction of nitrogen oxides from oxygen-containing exhaust gases with ammonia.
  • the invention provides a catalyst for catalysing the selective reduction of NO x with NH 3 , which catalyst comprising an alumino-silicate zeolite supporting doped ceria.
  • a catalyst for catalysing the selective reduction of NO x with NH 3 which catalyst comprising an alumino-silicate zeolite supporting doped ceria.
  • the ceria dopant can be any suitable element, but is typically a metal or metal oxide such as zirconium, lanthanum, alumina, yttrium, praseodymium, neodymium or any mixture of two or more thereof.
  • the ceria dopant is zirconium.
  • the ceria and the dopant can be present in an amount comprising from about 0.1 to about 20 wt% of the catalyst, such as from about 5 to about 15 wt%, and preferably about 10 wt%. It will be appreciated, however, that in certain embodiments of the invention high wt% of ceria and dopant together may promote undesirable non- selective oxidation of NH 3 .
  • the weight ratio of ceria to dopant can be from about 1:5 to about 5:1, such as about 1 :3 to about 3:1, preferably about 1:1.
  • the ceria and the or each dopant can be present in the catalyst as a mixture of each material, as a mixed oxide including ceria and at least one dopant or any mixture of two or more thereof.
  • At least some of the ceria and/or the dopant is present as an oxysulfur compound.
  • Methods for producing such oxysulfur compounds are known to the person skilled in the art and include contacting the zeolite with sulfuric acid, impregnation of the zeolite material with a sulfate-containing metal precursor e.g. Ce(SO 4 ) 2 (aq) (see US-A-4,780,445, the entire contents of which are incorporated herein by reference) or by lean hydrothermal sulfur ageing.
  • the catalyst can be any suitable alumino-silicate zeolite material for the purpose.
  • Typical zeolites which can be used, with advantage, in the present invention include ZSM-5, ZSM-8, ZSM-11, ZSM-12, ZSM-20, mordenite, gamma-zeolite, beta-zeolite, silicalite, X zeolite, Y zeolite, L zeolite, erionite, US Y zeolite or any mixture of two or more thereof.
  • the zeolite can be dealuminised by methods known in the art to increase the silica to alumina ratio if desired.
  • WO-A-02/41991 involves the pre-steaming of the zeolite material prior to the inclusion of the ceria and dopant.
  • the conditions can be 600 to 800°C, preferably 650-750°C for 0.25 to 8 hours, preferably 0.5 to 4 hours and most preferably 1 to 2 hours.
  • the invention provides a substrate comprising a catalyst according to the invention.
  • the substrate can be a monolith comprised of a refractory material such as a ceramic e.g. cordierite, or a metal such as FecralloyTM or stainless steel.
  • the arrangement of the monolith can be a honeycomb, a tube, a sheet, e.g. stainless steel plates arranged in parallel, or a mesh, particularly a wire mesh.
  • the zeolite can be used as a component in a substrate composition, e.g. including SiC, and extruded into pellets or formed into a monolith structure. Honeycomb cell densities vary, depending on their use.
  • the flow rate of the gas stream can depend on the catalyst volume. Typical flow rates in specific applications include about 3,000 to 5,000 1 hr "1 in high dust coal/petroleum-flred power plants having cell densities of from 9-11 cells per square inch (cpsi), 5,000 to 10,000 1 hr ' 1 in low dust coal/petroleum-fired power plants using 11-50 cpsi honeycomb monoliths and 20,000 to 40,000 1 hr "1 using 200 cpsi in gas turbines. Diesel engine applications can use 200-600 cpsi monoliths, e.g. a 400 cpsi configuration.
  • the invention provides a stationary power source comprising a catalyst according to the invention.
  • the stationary power source can be a coal or petroleum fired power plant or boiler, a gas turbine or a diesel engine.
  • the invention provides a mobile power source comprising a catalyst according to the invention.
  • the mobile power source can also be a diesel engine, and it can provide the propulsive force for a vehicle, such as a passenger vehicle, van or truck, for example.
  • the invention provides the use of a catalyst according to the present invention for reducing NO x in a gas stream using NH 3 .
  • the temperature of the gas stream in such use can be from about 350 to about 600°C, but is preferably in the range of from about 450 to about 550°C.
  • Such methods can include co-precipitation or wet impregnation of bulk zeolite with an aqueous solution including a mixture of ceria and the or each dopant salt, followed by drying and calcining the resulting material.
  • the zeolite support can be impregnated sequentially, first with an aqueous solution of the ceria salt or the or each dopant salt, followed by the drying and the calcining step, and the resulting material can then be impregnated with an aqueous solution of the or each dopant salt or the ceria salt.
  • the zeolite material is obtained, it is then made up into a washcoat and coated on the chosen substrate to obtain the desired loading.
  • a zeolite-containing washcoat can be prepared and coated on a suitable substrate, followed by a drying and calcining step.
  • the resulting coated substrate can then be wet impregnated with an aqueous solution including salts of the or each dopant and ceria, or a sequential application similar to the method outlined above.
  • Figure 1 is a graph showing NO x conversion against temperature for a fresh and aged catalyst comprising Ce-Zr/ZSM-5 according to the invention and a fresh and aged catalyst comprising Ce/ZSM-5 for the purposes of comparison.
  • a catalyst according to the invention was prepared by wet impregnation of cerium nitrate and zirconium nitrate on bulk ZSM-5. The resulting material was dried at 110°C for 24 hrs and calcined at 500°C for 2 hrs. The resulting catalyst material contained about 10wt% ceria and zirconia and the weight ratio of ceria to zirconia was approximately 1:1.
  • a comparative catalyst was prepared in a similar manner to include 5wt% ceria and no dopant.
  • a sample of each catalyst powder was subjected to lean hydrothermal ageing at 600°C in air/10% H 2 O for 337 hrs.
  • Fresh and aged powder samples of each catalyst were tested in a laboratory reactor in a synthetic gas mixture designed to mimic the composition of a diesel exhaust gas.
  • the synthetic gas composition used was: 200ppm NO, 200ppm NH 3 , 12% O 2 , 4.5% H 2 O, 4.5% CO 2 , 200ppm CO, lOOppm C 3 H 6 , 20ppm SO 2 , balance N 2 .
  • the reactor temperature was ramped at a rate of 5°C per minute and the NOx conversion downstream of catalyst sample was detected and plotted against temperature. The results are shown in Figure 1.
  • the fresh Ce-Zr/ZSM-5 catalyst according to the invention shows an increased selectivity for the selective reduction of NO x with NH 3 at temperatures above about 400°C compared with the Ce/ZSM-5 catalyst (without the dopant).
  • the improvement is more marked when the aged catalysts are compared.
  • the peak NO x conversion of the aged catalyst according to the invention is approximately 20% higher than the comparative catalyst and 20% greater NO x conversion at 550°C.
  • Light-off temperature the temperature at which a catalyst catalyses a reaction at 50% efficiency
  • the catalyst according to the invention catalyses peak, or within 10°C of peak, NO x conversion above 430°C in the important high temperature range of 400-600°C.

Abstract

A catalyst for catalysing the selective reduction of NOx with NH3 comprises an alumino-silicate zeolite supporting doped ceria. Suitable dopants include zirconium, lanthanum, alumina, yttrium, praseodymium, neodymium or any mixture of two or more thereof. The catalyst can be used for treating gas streams form stationary power sources such as coal or petroleum fired power plants or boilers, gas turbines or a diesel engines.

Description

ZEOLITE-BASED NEh SCR CATALYST
The present invention relates to a catalyst for selectively reducing NOx with NH3, and in particular to a catalyst comprising an alumino-silicate zeolite having relatively higher selectivity at higher temperatures.
It is known to remove oxides of nitrogen (NOx) pollutants from exhaust streams or other gaseous emission sources which contain excess oxygen by adding NH3 to the gas stream in a proportion based on the amount of NOx present and reacting the NH3 and NOx over a catalyst to form non-polluting N2 and H2O.
A wide variety of catalysts have been proposed for this purpose. Amongst these, zeolite based catalysts are indicated for higher temperature applications (350-600°C or
• above). The suitability of zeolite catalysts for SCR above about 450°C has been known since the 1970s, when the zeolite mordenite was identified as an active SCR catalyst. An advantage of zeolites is that when NOx is present, the catalyst does not oxidise ammonia to NOx according to the reaction 4NH3 + 5O2— 4NO + 6H2O. Therefore, unlike Pt- and V2O5-based catalysts, the selectivity of zeolite catalysts toward NOx conversion continually increases with temperature.
At exposure temperatures above about 600°C, in a high water content process stream, zeolites tend to deactivate by a process called de-alumination whereby the Al3+ in the SiO2-Al2O3 framework migrates out of the structure. This leads to permanent deactivation and, in the extreme case, collapse of the crystalline structure.
Zeolite structures can be ion-exchanged by other cations while maintaining electrical neutrality, altering the chemistry of catalytically active sites. See, for example, US-A-4,961,917 (the entire contents of which are incorporated herein by reference) which describes a zeolite catalyst composition which comprises a metal (e.g., iron or copper) promoted zeolite, the zeolite being characterised by having a silica to alumina ratio of at least about 10 and a pore structure which is interconnected in all three crystallographic dimensions by pores having an average kinetic pore diameter of at least about 7 Angstroms. Advantages of this material are described as high tolerance for sulfur poisoning, good activity for the SCR of NOx with NH3, good activity for the oxidation of NH3 with O2, and the retention of such good activities even under high temperature operations, e.g. 400°C, or higher, and hydrothermal conditions.
A problem with the zeolite catalyst composition described in US-A-4,961,917 is explained in WO-A-02/41991 wherein under harsh hydrothermal conditions, such as reduction of NOx from gas turbine exhaust at temperatures exceeding 500°C, the activity of the iron-promoted zeolite begins to decline. This decline in activity is believed to be due to destabilisation of the zeolite such as by dealumination and consequent reduction of metal-containing catalytic sites within the zeolite.
WO-A-02/41991 (the entire contents of which are incorporated herein by reference) describes an improvement over the catalyst of US-A-4,961,917 in that the zeolite is a metal-promoted beta zeolite in which the zeolite is pre-treated so as to provide it with improved hydrothermal stability. The stabilised beta zeolite is provided by incorporating into the zeolite structure no-framework aluminium oxide chains.
In our WO-A-99/55446 (the entire contents of which are incorporated herein by reference) we describe a zeolite-based SCR catalyst, such as ZSM-5, which desirably can contain from 1-10 wt% of metal e.g. copper, cerium, iron or platinum in order to improve low temperature SCR activity.
EP 0 299 294 describes a ferro-silicate zeolite for catalysing the selective reduction of nitrogen oxides from oxygen-containing exhaust gases with ammonia.
We have now devised a zeolite-based catalyst which has improved selectivity for high temperature NH3 SCR of NOx over prior art catalysts. Such improvement is retained following high temperature lean hydrothermal ageing of the catalyst.
According to one aspect, the invention provides a catalyst for catalysing the selective reduction of NOx with NH3, which catalyst comprising an alumino-silicate zeolite supporting doped ceria. An advantage of the catalysts of the present invention over prior art catalysts, such as iron beta zeolites, is that the activity of the iron beta zeolite catalysts is more significantly inhibited by the presence of hydrocarbon in the exhaust gas. We have also found that the catalysts of the present invention are particularly applicable when used to treat exhaust gases resulting from the combustion of low sulfur fuels such as MK1.
The ceria dopant can be any suitable element, but is typically a metal or metal oxide such as zirconium, lanthanum, alumina, yttrium, praseodymium, neodymium or any mixture of two or more thereof. Preferably, the ceria dopant is zirconium.
Together, the ceria and the dopant can be present in an amount comprising from about 0.1 to about 20 wt% of the catalyst, such as from about 5 to about 15 wt%, and preferably about 10 wt%. It will be appreciated, however, that in certain embodiments of the invention high wt% of ceria and dopant together may promote undesirable non- selective oxidation of NH3.
The weight ratio of ceria to dopant can be from about 1:5 to about 5:1, such as about 1 :3 to about 3:1, preferably about 1:1.
Without wishing to be bound by theory, we believe that the ceria and the or each dopant can be present in the catalyst as a mixture of each material, as a mixed oxide including ceria and at least one dopant or any mixture of two or more thereof.
In a preferred embodiment, at least some of the ceria and/or the dopant is present as an oxysulfur compound. Methods for producing such oxysulfur compounds are known to the person skilled in the art and include contacting the zeolite with sulfuric acid, impregnation of the zeolite material with a sulfate-containing metal precursor e.g. Ce(SO4)2 (aq) (see US-A-4,780,445, the entire contents of which are incorporated herein by reference) or by lean hydrothermal sulfur ageing.
The catalyst can be any suitable alumino-silicate zeolite material for the purpose. Typical zeolites which can be used, with advantage, in the present invention include ZSM-5, ZSM-8, ZSM-11, ZSM-12, ZSM-20, mordenite, gamma-zeolite, beta-zeolite, silicalite, X zeolite, Y zeolite, L zeolite, erionite, US Y zeolite or any mixture of two or more thereof. The zeolite can be dealuminised by methods known in the art to increase the silica to alumina ratio if desired. One such method is described in WO-A-02/41991 and involves the pre-steaming of the zeolite material prior to the inclusion of the ceria and dopant. The conditions can be 600 to 800°C, preferably 650-750°C for 0.25 to 8 hours, preferably 0.5 to 4 hours and most preferably 1 to 2 hours.
According to a further aspect, the invention provides a substrate comprising a catalyst according to the invention. Depending on the application, the substrate can be a monolith comprised of a refractory material such as a ceramic e.g. cordierite, or a metal such as Fecralloy™ or stainless steel. The arrangement of the monolith can be a honeycomb, a tube, a sheet, e.g. stainless steel plates arranged in parallel, or a mesh, particularly a wire mesh. Alternatively, the zeolite can be used as a component in a substrate composition, e.g. including SiC, and extruded into pellets or formed into a monolith structure. Honeycomb cell densities vary, depending on their use. The flow rate of the gas stream can depend on the catalyst volume. Typical flow rates in specific applications include about 3,000 to 5,000 1 hr"1 in high dust coal/petroleum-flred power plants having cell densities of from 9-11 cells per square inch (cpsi), 5,000 to 10,000 1 hr' 1 in low dust coal/petroleum-fired power plants using 11-50 cpsi honeycomb monoliths and 20,000 to 40,000 1 hr"1 using 200 cpsi in gas turbines. Diesel engine applications can use 200-600 cpsi monoliths, e.g. a 400 cpsi configuration.
According to a further aspect, the invention provides a stationary power source comprising a catalyst according to the invention. The stationary power source can be a coal or petroleum fired power plant or boiler, a gas turbine or a diesel engine.
According to a further aspect, the invention provides a mobile power source comprising a catalyst according to the invention. The mobile power source can also be a diesel engine, and it can provide the propulsive force for a vehicle, such as a passenger vehicle, van or truck, for example. According to a further aspect, the invention provides the use of a catalyst according to the present invention for reducing NOx in a gas stream using NH3. The temperature of the gas stream in such use can be from about 350 to about 600°C, but is preferably in the range of from about 450 to about 550°C.
Methods of manufacture of catalysts and catalyst coated substrates according to the invention are known to the person skilled in the art and no detailed explanation will be given here. Generally, such methods can include co-precipitation or wet impregnation of bulk zeolite with an aqueous solution including a mixture of ceria and the or each dopant salt, followed by drying and calcining the resulting material. In an alternative wet impregnation method, the zeolite support can be impregnated sequentially, first with an aqueous solution of the ceria salt or the or each dopant salt, followed by the drying and the calcining step, and the resulting material can then be impregnated with an aqueous solution of the or each dopant salt or the ceria salt. However the zeolite material is obtained, it is then made up into a washcoat and coated on the chosen substrate to obtain the desired loading.
Alternatively, a zeolite-containing washcoat can be prepared and coated on a suitable substrate, followed by a drying and calcining step. The resulting coated substrate can then be wet impregnated with an aqueous solution including salts of the or each dopant and ceria, or a sequential application similar to the method outlined above.
In order that the invention may be more fully understood, the following Example is provided by way of illustration only and with reference to the accompanying drawing, in which:
Figure 1 is a graph showing NOx conversion against temperature for a fresh and aged catalyst comprising Ce-Zr/ZSM-5 according to the invention and a fresh and aged catalyst comprising Ce/ZSM-5 for the purposes of comparison.
EXAMPLE
A catalyst according to the invention was prepared by wet impregnation of cerium nitrate and zirconium nitrate on bulk ZSM-5. The resulting material was dried at 110°C for 24 hrs and calcined at 500°C for 2 hrs. The resulting catalyst material contained about 10wt% ceria and zirconia and the weight ratio of ceria to zirconia was approximately 1:1.
A comparative catalyst was prepared in a similar manner to include 5wt% ceria and no dopant.
A sample of each catalyst powder was subjected to lean hydrothermal ageing at 600°C in air/10% H2O for 337 hrs.
Fresh and aged powder samples of each catalyst were tested in a laboratory reactor in a synthetic gas mixture designed to mimic the composition of a diesel exhaust gas. The synthetic gas composition used was: 200ppm NO, 200ppm NH3, 12% O2, 4.5% H2O, 4.5% CO2, 200ppm CO, lOOppm C3H6, 20ppm SO2, balance N2. The reactor temperature was ramped at a rate of 5°C per minute and the NOx conversion downstream of catalyst sample was detected and plotted against temperature. The results are shown in Figure 1.
As can be seen, the fresh Ce-Zr/ZSM-5 catalyst according to the invention shows an increased selectivity for the selective reduction of NOx with NH3 at temperatures above about 400°C compared with the Ce/ZSM-5 catalyst (without the dopant).
The improvement is more marked when the aged catalysts are compared. The peak NOx conversion of the aged catalyst according to the invention is approximately 20% higher than the comparative catalyst and 20% greater NOx conversion at 550°C. Light-off temperature (the temperature at which a catalyst catalyses a reaction at 50% efficiency) is shifted up 80°C for both catalysts following lean hydrothermal ageing, yet the catalyst according to the invention catalyses peak, or within 10°C of peak, NOx conversion above 430°C in the important high temperature range of 400-600°C. Thus the present invention provides considerable advantages over the prior art comparative catalyst.

Claims

CLAIMS:
1. A catalyst for catalysing the selective reduction of NOx with NH3, which catalyst comprising an alumino-silicate zeolite supporting doped ceria.
2. A catalyst according to claim 1, wherein the ceria dopant is zirconium, lanthanum, alumina, yttrium, praseodymium, neodymium or any mixture of two or more thereof.
3. A catalyst according to claim 1, wherein the ceria dopant is zirconium.
4. A catalyst according to claim 1, 2 or 3, wherein the ceria and dopant comprise from 0.1 to 20 wt% of the catalyst.
5. A catalyst according to claim 4, wherein the ceria and dopant comprise from 5 to 15 wt% of the catalyst.
6. A catalyst according to claim 4 or 5, wherein the ceria and dopant comprise 10 wt% of the catalyst.
7. A catalyst according to any of claims 1 or 6, wherein the weight ratio of ceria to dopant is 1:5 to 5:1.
8. A catalyst according to claim 7, wherein the weight ratio of ceria to dopant is 1 :3 to 3:1.
9. A catalyst according to claim 7 or 8, wherein the weight ratio of ceria to dopant is 1:1.
10. A catalyst according to any preceding claim, wherein at least some of the ceria and/or the dopant is present as an oxysulfur compound.
11. A catalyst according to any preceding claim, wherein the zeolite is ZSM-5, ZSM-8, ZSM-11, ZSM-12, ZSM-20, mordenite, gamma-zeolite, beta-zeolite, silicalite, X, zeolite, Y zeolite, L zeolite, erionite, USY zeolite or any mixture of two or more thereof.
12. A catalyst according to any of claims 1 to 10, wherein the zeolite is ZSM-5.
13. A catalyst according to claim 11 or 12, wherein the zeolite is dealuminised.
14. A substrate comprising a catalyst according to any preceding claim.
15. A stationary power source comprising a catalyst according to any of claims 1 to 13.
16. A stationary power source according to claim 15, wherein it is a coal or petroleum fired power plant or boiler, a gas turbine or a diesel engine.
17. A mobile power source comprising a catalyst according to any of claims 1 to 13.
18. A mobile power source according to claim 17, wherein it is diesel engine.
19. A vehicle including a mobile power source according to claim 17 or 18.
20. The use of a catalyst according to any of claims 1 to 13 for reducing NOx in a gas stream using NH3.
21. The use according to claim 20, wherein the temperature of the gas stream is from 350to 600°C.
22. The use according to claim 21, wherein the temperature of the gas stream is from 450 to 550°C.
23. The use according to claim 20, wherein the gas stream is from a stationary power source.
24. The use according to claim 23, wherein the stationary power source is a coal or petroleum fired power plant or boiler, a gas turbine or a diesel engine.
25. The use according to claim 20, wherein the gas stream is from a mobile power source.
26. The use according to claim 25, wherein the mobile power source is a diesel engine.
27. The use according to claim 25 or 26, wherein the mobile power source is part of a vehicle.
PCT/GB2003/002663 2002-06-28 2003-06-20 Doped ceria-containing zeolite-based nh3 scr catalyst WO2004002611A1 (en)

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