Classifications

• • 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
• • 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
• • 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
• • 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
  • B01J23/002—Mixed oxides other than spinels, e.g. perovskite
• • 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
  • B01J23/16—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of arsenic, antimony, bismuth, vanadium, niobium, tantalum, polonium, chromium, molybdenum, tungsten, manganese, technetium or rhenium
  • B01J23/20—Vanadium, niobium or tantalum
• • 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
  • B01J37/031—Precipitation
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
  • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
  • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
  • F01N3/10—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust
  • F01N3/18—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust characterised by methods of operation; Control
  • F01N3/20—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust characterised by methods of operation; Control specially adapted for catalytic conversion ; Methods of operation or control of catalytic converters
• • 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
  • B01D2255/40—Mixed oxides
  • B01D2255/407—Zr-Ce mixed oxides
• • 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
  • B01D—SEPARATION
  • B01D2257/00—Components to be removed
  • B01D2257/40—Nitrogen compounds
  • B01D2257/404—Nitrogen oxides other than dinitrogen oxide
• • 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
  • B01J2523/00—Constitutive chemical elements of heterogeneous catalysts
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
  • F01N2570/00—Exhaust treating apparatus eliminating, absorbing or adsorbing specific elements or compounds
  • F01N2570/14—Nitrogen oxides
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
  • F01N2610/00—Adding substances to exhaust gases
  • F01N2610/02—Adding substances to exhaust gases the substance being ammonia or urea
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
  • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
  • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
  • F01N3/10—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust
  • F01N3/18—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust characterised by methods of operation; Control
  • F01N3/20—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust characterised by methods of operation; Control specially adapted for catalytic conversion ; Methods of operation or control of catalytic converters
  • F01N3/2066—Selective catalytic reduction [SCR]
• • 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 a process for treating a gas containing nitrogen oxides (NOx) using as catalyst a composition based on cerium oxide and niobium oxide.
• NOx nitrogen oxides
• One known method for this treatment is the SCR process in which NOx reduction is performed by ammonia or a precursor of ammonia such as urea.
• the SCR process allows an efficient treatment of the gases but nevertheless its effectiveness at low temperature remains to be improved.
• the catalytic systems currently used for the implementation of this process are often effective only for temperatures above 250 ° C. It would therefore be advantageous to have catalysts that can have significant activity at temperatures of 250 ° C or lower.
• the object of the invention is therefore to provide more efficient catalysts for SCR catalysis.
• the process of the invention is a process for treating a gas containing nitrogen oxides (NOx) in which a reduction reaction of NOx is carried out with a nitrogen reducing agent and is characterized in that
• the catalyst used in this reduction reaction is a catalytic system containing a cerium oxide-based composition which comprises niobium oxide with the following proportions by weight: - niobium oxide of 2 to 20%;
• rare earth is understood to mean the elements of the group consisting of yttrium and the elements of the Periodic Table with an atomic number inclusive of between 57 and 71.
• specific surface is meant the specific surface B.E.T. determined by nitrogen adsorption in accordance with ASTM D 3663-78 established from the BRUNAUER-EMMETT-TELLER method described in "The Journal of the American Society, 60, 309 (1938)".
• the calcinations mentioned in the description are calcinations under air unless otherwise indicated.
• the duration of calcination which is indicated for a temperature corresponds to the duration of the plateau at this temperature.
• composition of the catalytic system of the invention is characterized first of all by the nature and the proportions of its constituents.
• it is based on cerium and niobium, these elements being present in the composition generally in the form of oxides. These elements are also present in the specific proportions given above.
• the cerium oxide of the composition can be stabilized, by "stabilized” here means stabilization of the specific surface, by at least one rare earth other than cerium, in oxide form.
• This rare earth may be more particularly yttrium, neodymium, lanthanum or praseodymium.
• the stabilizing rare earth oxide content is generally at most 20%, preferably when the rare earth is lanthanum, more preferably at most 15% and preferably at most 10% by weight.
• the minimum stabilizing rare earth oxide content is that from which the stabilizing effect is felt and is generally at least 1%, more preferably at least 2%. This content is expressed as rare earth oxide relative to the mass of the stabilized rare earth cerium oxide-oxide complex.
• the cerium oxide can also be stabilized, always stabilizing in the sense of the specific surface, by an oxide chosen from silica, alumina and titanium oxide.
• the content of this stabilizing oxide can be at most 10% and more particularly at most 5%.
• the minimum content may be at least 1%. This content is expressed as stabilizing oxide relative to the weight of the stabilizing cerium oxide-oxide complex.
• the composition of the catalytic system of the invention comprises three constituent elements, again in the form of oxides, which are cerium, niobium and zirconium.
• niobium oxide from 2 to 20%
• the minimum proportion of zirconium oxide in the case of this second embodiment of the invention is preferably at least 10%, more preferably at least 15%.
• the maximum content of zirconium oxide may more particularly be at most 40% and even more particularly at most 30%.
• the composition of the catalytic system of the invention also contains at least one oxide of an element M chosen from the group comprising tungsten, molybdenum, iron, copper and silicon. , aluminum, manganese, titanium, vanadium and rare earths other than cerium, with the following proportions by mass:
• niobium oxide from 2 to 20%
• oxide of the element M up to 20%
• This element M can in particular act as a stabilizer of the surface of the mixed oxide of cerium and zirconium or improve the reducibility of the composition.
• This element M can in particular act as a stabilizer of the surface of the mixed oxide of cerium and zirconium or improve the reducibility of the composition.
• the maximum proportion of oxide of element M in the case of rare earths and tungsten may more particularly be at most 15% and even more particularly at most 10% by weight of oxide of element M ( rare earth and / or tungsten).
• the minimum content is at least 1%, plus especially at least 2%, the contents given above being expressed relative to the whole oxide of cerium oxide of zirconium oxide of the element M.
• the oxide content of the element M may more particularly be at most 10% and even more particularly at most 5%.
• the minimum content may be at least 1%. This content is expressed as the oxide of the element M with respect to the whole cerium oxide-zirconium oxide and oxide of the element M.
• the element M may be more particularly ryttrium, lanthanum, praseodymium and neodymium.
• the proportion of niobium oxide may be more particularly between 3% and 15% and even more particularly between 5% and 10%.
• the cerium content may be at least 65%, more particularly at least 70% and even more particularly at least 75% and that of niobium between 2 and 12% and more particularly between 2 and 10%.
• the compositions according to this variant have high acidity and reducibility.
• the cerium oxide content may be at least 60%, more particularly at least 65%, and that in zirconium oxide in a proportion by mass of at most 25%, more particularly between 15% and 25%.
• the proportion of niobium may even more particularly be less than 10% and for example between a minimum value which may be 2%, 4% or even 5% and a maximum value strictly less than 10%. 10% for example of at most 9% and more particularly at most 8% and even more particularly at most 7%.
• This niobium content is expressed in weight of niobium oxide relative to the mass of the entire composition.
• the compositions of the invention according to the first mode may comprise in addition to oxides of at least one metal M 'selected from the group comprising vanadium, copper, manganese, tungsten and iron in a proportion which may be between 1 and 10%, more particularly between 1% and 5%. %, more preferably between 1 and 3%, proportion expressed by weight of oxide of the metal relative to the entire composition.
• metal M 'selected from the group comprising vanadium, copper, manganese, tungsten and iron in a proportion which may be between 1 and 10%, more particularly between 1% and 5%. %, more preferably between 1 and 3%, proportion expressed by weight of oxide of the metal relative to the entire composition.
• compositions of the catalytic system of the invention finally have a sufficiently stable specific surface area, that is to say sufficiently high at high temperature, so that they are used in the field of catalysis.
• the compositions according to the first embodiment have a specific surface after calcination for 4 hours at 800 ° C. which is at least 15 m 2 / g.
• this surface under the same conditions, is generally at least 20 m 2 / g.
• the compositions of the catalytic system of the invention may have an area of up to about 55 m 2 / g still under the same calcination conditions.
• compositions of the catalytic system of the invention in the case where they contain a quantity of niobium of at least 10%, and according to an advantageous embodiment, may have a specific surface after calcination for 4 hours at 800 ° C. which is at least 35 m 2 / g, more particularly at least 40 m 2 / g.
• compositions of the catalytic system of the invention may have a surface after calcination at 900 ° C. for 4 hours which is at least 10 m 2 / g. Under the same calcination conditions they can have surface areas of up to about 30 m 2 / g.
• compositions of the catalyst system of the invention have high acidity which can be measured by a method of TPD analysis, which will be described later, and which is at least 5.10 "2, more preferably at least 6.10" 2 and even more particularly at least 7.10 -2 , this acidity being expressed in ml of ammonia (TPN: normal temperature and pressure) per m 2 (BET measurement) of product
• TPN normal temperature and pressure
• BET measurement m 2 of ammonia
• compositions of the catalytic system of the invention also have significant reducibility properties. These properties can be measured by the programmed temperature reduction measurement method (TPR) which will be described later.
• TPR programmed temperature reduction measurement method
• the compositions of the system Catalyst of the invention have a reducibility of at least 15, this reducibility being expressed in ml of hydrogen (TPN) per g of product.
• compositions may be in the form of a solid solution of the niobium oxides, the stabilizing element in the case of the first embodiment, zirconium and the element M or M 'in the cerium oxide for the other modes.
• zirconium we then observe in this case the presence of a single phase X-ray diffraction corresponding to the cubic phase of cerium oxide.
• the stability of this solid solution is such that its presence can be observed on compositions which may have been calcined up to 900 ° C., 4 hours.
• the invention also relates to the case where the compositions consist essentially of oxides of the abovementioned elements, cerium, niobium and, where appropriate, zirconium and element M or M '.
• consists essentially it is meant that the composition in question contains only the oxides of the abovementioned elements and that it contains no oxide of another functional element, that is to say capable of having a positive influence on the reducibility and / or the acidity and / or the stability of the composition.
• the composition may contain elements such as impurities which may notably come from its preparation process, for example raw materials or starting reagents used.
• compositions of the catalyst system of the invention may be prepared by the known impregnation process.
• a cerium oxide or a mixed oxide of cerium and zirconium prepared beforehand by a solution comprising a niobium compound, for example an oxalate or an oxalate of niobium and ammonium, is impregnated.
• a solution is used for the impregnation which contains a compound of this element M or M' in addition to the niobium compound.
• the element M or M ' may also be present in the starting cerium oxide which is impregnated.
• the dry impregnation consists in adding to the product to be impregnated a volume of an aqueous solution of the impregnant element which is equal to the pore volume of the solid to be impregnated.
• Cerium oxide or mixed oxide of cerium and zirconium must have specific surface properties which make it suitable for use in catalysis. Thus this surface must be stable, ie it must have a value sufficient for such use even at high temperature.
• Such oxides are well known.
• For the cerium oxides use may be made in particular of those described in patent applications EP 0153227, EP 0388567 and EP 0300852.
• compositions of the catalyst system of the invention may also be prepared by a second method which will be described below.
• This process comprises the following steps:
• the first step of this process involves a suspension of a niobium hydroxide.
• This suspension can be obtained by reacting a niobium salt, such as a chloride, with a base, such as ammonia, to obtain a niobium hydroxide precipitate.
• a niobium salt such as potassium or sodium niobate with an acid such as nitric acid to obtain a niobium hydroxide precipitate.
• This reaction can be done in a mixture of water and alcohol such as ethanol.
• the hydroxide thus obtained is washed by any known means and is then resuspended in water in the presence of a peptizing agent such as nitric acid.
• the second step (b1) of the process comprises mixing the suspension of niobium hydroxide with a solution of a cerium salt.
• This solution may also contain a zirconium salt and also the element M or M 'in the case of the preparation of a composition which further comprises a zirconium oxide or the oxide of this element M or M .
• These salts can be selected from nitrates, sulphates, acetates, chlorides, cerium-ammoniacal nitrate.
• zirconium salts By way of example of zirconium salts, mention may be made of zirconium sulphate, zirconyl nitrate or zirconyl chloride. Zirconyl nitrate is most commonly used.
• oxidizing agent for example hydrogen peroxide
• the different salts of the solution are present in the stoichiometric proportions necessary to obtain the desired final composition.
• the mixture formed from the niobium hydroxide suspension and the solution of the salts of the other elements is brought into contact with a basic compound.
• Hydroxide products can be used as base or basic compound. Mention may be made of alkali or alkaline earth hydroxides. It is also possible to use secondary, tertiary or quaternary amines. However, amines and ammonia may be preferred in that they reduce the risk of pollution by alkaline or alkaline earth cations. We can also mention urea.
• the basic compound may more particularly be used in the form of a solution.
• the reaction between the above mixture and the basic compound is preferably continuous in a reactor. This reaction is done by continuously introducing the mixture and the basic compound and continuously withdrawing also the product of the reaction.
• the precipitate which is obtained is separated from the reaction medium by any conventional solid-liquid separation technique such as, for example, filtration, decantation, spinning or centrifugation.
• This precipitate can be washed and then calcined at a temperature sufficient to form the oxides, for example at least 500 ° C.
• compositions of the catalytic system of the invention may be further prepared by a third method which comprises the following steps:
• a liquid mixture containing a cerium compound and, where appropriate, a zirconium compound and the M or M 'element is prepared for the preparation of the compositions containing the oxide zirconium and / or an oxide of the element M or M ';
• the cerium compound may be a cerium III or cerium compound
• the compounds are preferably soluble compounds such as salts. What has been said above for the salts of cerium, zirconium and the element M or M 'also applies here. It is the same for the nature of the basic compound.
• the different compounds of the starting mixture of the first step are present in the stoichiometric proportions necessary to obtain the desired final composition.
• the liquid medium of the first stage is usually water.
• the starting mixture of the first step can be indifferently obtained either from compounds initially in the solid state which will be introduced later in a water tank for example, or even directly from solutions of these compounds and then mixing, in any order, said solutions.
• the order of introduction of the reagents into the second step (b2) may be arbitrary, the basic compound may be introduced into the mixture or vice versa or the reagents may be introduced simultaneously into the reactor.
• the addition can be carried out all at once, gradually or continuously, and it is preferably carried out with stirring.
• This operation can be conducted at a temperature between room temperature (18-25 ° C) and the reflux temperature of the reaction medium, the latter can reach 120 ° C for example. It is preferably conducted at room temperature.
• the ripening is done by heating the middle.
• the temperature at which the medium is heated is at least 40 ° C, more preferably at least 60 ° C and even more preferably at least 100 ° C.
• the medium is thus maintained at a constant temperature for a period of time which is usually at least 30 minutes and more particularly at least 1 hour.
• the ripening can be carried out at atmospheric pressure or optionally at a higher pressure and at a temperature above 100 ° C. and in particular between 100 ° C. and 150 ° C.
• the next step (c2) of the process consists in mixing the suspension obtained at the end of the preceding step with a solution of a niobium salt.
• Niobium salt that may be mentioned niobium chloride, niobate potassium or sodium and especially here niobium oxalate and niobium oxalate and ammonium.
• This mixture is preferably at room temperature.
• steps of the process (d2) and (e2) consist in separating the solid from the suspension obtained in the preceding step, optionally washing this solid and then calcining it. These steps proceed in a manner identical to that described above for the second method.
• the third method may have a variant in which the compound of this element M or M' is not present in the stage ( a2).
• the compound of the element M or M ' is then added to step (c2) either before or after mixing with the niobium solution or at the same time.
• compositions of the catalyst system of the invention which are based on the oxides of cerium, niobium and zirconium and optionally an oxide of the element M may also be prepared by a fourth process which will be described below. below.
• This process comprises the following steps:
• the first step of the process consists in preparing a mixture in a liquid medium of a zirconium compound and a cerium compound and, where appropriate, of the element M.
• the various compounds of the mixture are present in the necessary stoichiometric proportions to obtain the desired final composition.
• the liquid medium is usually water.
• the compounds are preferably soluble compounds. This can be in particular salts of zirconium, cerium and element M as described above.
• the mixture can be indifferently obtained either from compounds initially in the solid state which will subsequently be introduced into a water tank for example, or even directly from solutions of these compounds and then mixed in any order of said solutions.
• the temperature at which this heat treatment, also called thermohydrolysis, is carried out is greater than 100 ° C. It can thus be between 100 ° C. and the critical temperature of the reaction medium, in particular between 100 and 350 ° C., preferably between 100 and 200 ° C.
• the heating operation can be carried out by introducing the liquid medium into a closed chamber (autoclave-type closed reactor), the necessary pressure then resulting only from the sole heating of the reaction medium (autogenous pressure).
• autogenous pressure the pressure in the closed reactor can vary between a value greater than 1 bar (10 5 Pa) and 165 bar (1 bar). , 65. 10 7 Pa), preferably between 5 Bar (5 ⁇ 10 5 Pa) and 165 Bar (1, 65. 10 7 Pa). It is of course also possible to exert an external pressure which is added to that subsequent to heating.
• the heating may be conducted either in air or in an atmosphere of inert gas, preferably nitrogen.
• the duration of the treatment is not critical, and can thus vary within wide limits, for example between 1 and 48 hours, preferably between 2 and 24 hours.
• the rise in temperature is carried out at a speed which is not critical, and it is thus possible to reach the reaction temperature set by heating the medium for example between 30 minutes and 4 hours, these values being given for all purposes. indicative fact.
• the reaction medium thus obtained is brought to a basic pH. This operation is performed by adding to the medium a base such as for example an ammonia solution.
• basic pH is meant a pH value greater than 7 and preferably greater than 8.
• the product as recovered can then be subjected to washes, which are then operated with water or optionally with a basic solution, for example an ammonia solution.
• the washing can be carried out by resuspension in water of the precipitate and maintenance of the suspension thus obtained at a temperature which can go up to 100 ° C.
• the washed product can optionally be dried, for example in an oven or by atomization, and this at a temperature which can vary between 80 and
• the process comprises a ripening (step c'3).
• the ripening is done under the same conditions as those described above for the third method.
• the ripening can also be carried out on a suspension obtained after putting the precipitate back into water.
• the pH of this suspension can be adjusted to a value greater than 7 and preferably greater than 8.
• the catalyst system used in the process of the invention contains a composition as described above, this composition being generally mixed with a material usually employed in the field of the catalyst system, that is to say a material chosen from among inert materials. thermally.
• This material may thus be chosen from alumina, titanium oxide, cerium oxide, zirconium oxide, silica, spinels, zeolites, silicates, crystalline silicoaluminium phosphates, calcium phosphates and crystalline aluminum.
• proportions between the composition and the inert material are those usually used in the technical field concerned herein and are well known to those skilled in the art.
• these proportions can be between 2% and 20% and more particularly between 2% and 10%, expressed as mass of inert material relative to the inert material and composition.
• the catalyst system used in the process of the invention may consist of the aforementioned mixture deposited on a substrate. More specifically, the mixture of the composition and the thermally inert material constitutes a coating (wash coat) with catalytic properties and this coating is deposited on a substrate of the type for example metal monolith, for example FerCralloy, or ceramic, for example in cordierite , of silicon carbide, of alumina titanate or of mullite.
• a coating with catalytic properties and this coating is deposited on a substrate of the type for example metal monolith, for example FerCralloy, or ceramic, for example in cordierite , of silicon carbide, of alumina titanate or of mullite.
• This coating is obtained by mixing the composition with the thermally inert material so as to form a suspension which can then be deposited on the substrate.
• the catalyst system used in the process of the invention may be based on the composition as described above, this being used in an extruded form. It can thus be in the form of a monolith having a honeycomb structure or in the form of a monolith of particle filter type (partially closed channels).
• the composition of the invention can be mixed with additives of known type to facilitate the extrusion and ensure the mechanical strength of the extruded.
• additives may be chosen in particular from silica, alumina, clays, silicates, titanium sulphate and ceramic fibers, especially in generally used proportions, ie up to about 30% by weight. compared to the entire composition.
• the invention also relates to a catalyst system which contains a zeolite in addition to the composition based on cerium and niobium oxides.
• the zeolite may be natural or synthetic and may be of aluminosilicate, aluminophosphate or silicoaluminophosphate type.
• a treated zeolite is preferably used to improve its hydrothermal stability.
• treatment of this type may be mentioned (i) the dealumination by steam treatment and acid extraction using an acid or a complexing agent (for example EDTA - ethylenediaminetetraacetic acid); by treatment with an acid and / or a complexing agent; by treatment with a gas stream of SiCl 4 ; (ii) cation exchange using polyvalent cations such as La; and (iii) the use of phosphorus-containing compounds.
• this zeolite may have an Si / Al atomic ratio of at least 10, more particularly at least 20.
• the zeolite comprises at least one other element chosen from the group comprising iron, copper or cerium.
• zeolite comprising at least one other element is meant a zeolite in the structure of which have been added by ion exchange, impregnation or isomorphous substitution one or more metals of the aforementioned type.
• the metal content may be between about 1% and about 5%, content expressed as the weight of metal element relative to the zeolite.
• aluminosilicate zeolites which may form part of the composition of the catalytic system of the invention are particularly suitable for those selected from the group comprising zeolites beta, gamma, ZSM 5 and ZSM 34.
• zeolites those of the type SAPO-17, SAPO-18, SAPO-34, SAPO-35, SAPO-39, SAPO-43 and SAPO-56.
• the mass percentage of zeolite relative to the total mass of the composition may vary from 10 to 70%, more preferably from 20 to 60% and even more preferably from 30 to 50%.
• this zeolite variant of the catalytic system it is possible to perform a simple physical mixture of the composition based on the cerium and niobium oxides and the zeolite.
• the gas treatment method of the invention is a SCR type process whose implementation is well known to those skilled in the art. It may be recalled that this process uses as reducing agent NOx a nitrogen reducing agent which may be ammonia, hydrazine or any suitable precursor of ammonia, such as ammonium carbonate, urea, ammonium carbamate, ammonium hydrogencarbonate, ammonium formate or organometallic compounds containing ammonia. Ammonia or urea may be more particularly chosen.
• a first reaction can be represented by equation (1)
• the method can be implemented for the treatment of a gas coming from an internal combustion engine (mobile or stationary), in particular from an engine of a motor vehicle, or gas coming from a gas turbine, from power stations operating on coal or fuel oil or any other industrial installation.
• an internal combustion engine mobile or stationary
• gas coming from a gas turbine from power stations operating on coal or fuel oil or any other industrial installation.
• the method is used for treating the exhaust gas of a lean-burn internal combustion engine or a diesel engine.
• the process can also be carried out using, in addition to the composition of the invention, another catalyst which is a catalyst for oxidation of the nitric oxide of the gas to nitrogen dioxide.
• another catalyst which is a catalyst for oxidation of the nitric oxide of the gas to nitrogen dioxide.
• the process is used in a system in which this oxidation catalyst is disposed upstream of the injection point of the nitrogen reductant in the exhaust gas.
• This oxidation catalyst may comprise at least one platinum group metal, such as platinum, palladium or rhodium, on a support of the alumina, ceria, zirconia or titanium oxide type, for example, the catalyst / support assembly. being included in a coating (washcoat) on a substrate of the monolithic type in particular.
• This example relates to the preparation of a composition comprising cerium oxide, zirconium oxide and niobium oxide in the following respective proportions by weight: 64% -26% -10%.
• a suspension of niobium hydroxide is first prepared by the following method.
• a solution of ammonia D is then prepared by introducing 1040 g of a solution (D1) of concentrated ammonia (29.8% of NH 3 ) in 6690 g of deionized water (D2).
• a solution E is prepared by mixing 4250 g of deionized water (E1), 1640 g of a solution (E2) of cerium (III) nitrate (30.32% CeO 2 ), 1065 g of a solution ( E3) of zirconium oxynitrate (20.04% ZrO2), 195 g of a solution (E4) of hydrogen peroxide (50.30% of H2O2), 1935 g of suspension C (4.08% by weight), Nb 2 Os). This solution E is stirred.
• the suspension is filtered, the solid product obtained is washed and calcined at 800 ° C. for 4 hours.
• compositions of these examples are prepared in the same manner as in Example 1. Solutions D and E are prepared with the same compounds but with different proportions.
• Example in the "Example” column for each example the numbers given below the example number correspond to the proportions their respective mass of cerium, zirconium and niobium oxides for the composition of the example in question;
• D 2 amount of deionized water used in the preparation of the ammonia solution D;
• This example relates to the preparation of a composition comprising cerium oxide, zirconium oxide and niobium oxide in the following respective proportions by weight: 72% -18% -10%.
• a solution of niobium oxalate (V) and ammonium is prepared by hot dissolving 192 g of niobium (V) oxalate and ammonium in 300 g of deionized water. This solution is maintained at 50 ° C. The concentration of this solution is 14.2% in Nb 2 Os. This solution is then introduced onto a powder of a mixed oxide of cerium and zirconium (mass composition CeO 2 / ZrO 2 80/20, specific surface after calcination at 800 ° C. 4 hours of 59 m 2 / g) up to saturation of the pore volume.
• the impregnated powder is then calcined at 800 ° C. (4 hour stage).
• compositions of these examples are prepared in the same manner as in Example 1. Solutions D and E are prepared with the same compounds but with different proportions.
• This example relates to the preparation of a composition comprising cerium oxide, zirconium oxide and niobium oxide in the following respective proportions by weight: 63% -27% -10%.
• a solution of nitrates of zirconium and cerium IV is prepared by mixing 264 g of deionized water, 238 g of cerium (IV) nitrate solution (252 g / L in CeO 2 ) and 97 grams of sodium hydroxide solution. zirconium oxynitrate (261 g / l ZrO 2 ). The concentration of this solution is 120 g / l of oxide.
• the suspension thus prepared is cured at 95 ° C. for 2 hours. The medium is then allowed to cool.
• a solution of niobium oxalate (V) is prepared by hot dissolving 44.8 g of niobium oxalate (V) in 130 g of deionized water. This solution is maintained at 50 ° C. The concentration of this solution is 3.82% in Nb 2 O 5 .
• the suspension is filtered and washed.
• the cake is then introduced into an oven and calcined at 800 ° C. (4 hour stage).
• This example concerns the preparation of a composition identical to that of Example 11.
• a solution of nitrates of zirconium and cerium IV is prepared by mixing 451 g of deionized water, 206 g of cerium nitrate solution (IV) (252 g / l CeO 2) and 75 g of zirconium oxynitrate solution (288 g / l in ⁇ 1 2). The concentration of this solution is 80 g / l of oxide.
• This solution of nitrates is introduced into an autoclave. The temperature is raised to 100 ° C. The medium is stirred at 100 ° C. for 1 hour. Let cool.
• the suspension is transferred to a stirred reactor of 1.5 liters.
• a solution of 6 mol / l of ammonia is introduced under stirring until a pH in the region of 9.5 is obtained.
• the suspension is cured at 95 ° C for 2 hours.
• the medium is then allowed to cool.
• a solution of niobium oxalate (V) is prepared by hot dissolving 39 g of niobium oxalate (V) in 13 g of deionized water. This solution is maintained at 50 ° C. The concentration of this solution is 3.84% The solution of niobium oxalate (V) is introduced in 20 minutes on the cooled suspension. The pH is then raised to pH 9 by adding an ammonia solution (32% NH 3 ).
• the suspension is filtered and washed.
• the cake is then introduced into an oven and calcined at 800 ° C. (4 hour stage).
• This example relates to the preparation of a composition comprising cerium oxide, zirconium oxide and niobium oxide in the following respective proportions by weight: 64% -27% -9%.
• This example relates to the preparation of a composition comprising cerium oxide, zirconium oxide and niobium oxide in the following respective proportions by weight: 19% -78% -3%.
• a solution of ammonia D is prepared as in Example 1 and with the same compounds but in the following proportions:
• a solution E is also prepared as in Example 1 and with the same compounds but in the following proportions:
• the acidity properties are measured by the TPD method which is described below.
• the probe molecule used to characterize acid sites in TPD is ammonia.
• the sample (100 mg) is heated to 500 ° C. under a stream of helium (30 ml / min) according to a rise in temperature of 20 ° C./min and is maintained at this temperature for 30 minutes in order to remove the vapor. water and avoid clogging the pores. Finally the sample is cooled to 100 ° C under a stream of helium at 10 ° C / min.
• the sample is then subjected to a flux (30 ml / min) of ammonia (5% vol of NH 3 in helium at 100 ° C. at atmospheric pressure for 30 minutes (until saturation). subjected for a minimum of 1 hour to a stream of helium (30 ml / min).
• TPD is conducted by raising the temperature by 10 ° C / min to 700 ° C.
• the concentration of the desorbed species that is to say ammonia
• the ammonia concentration during the desorption phase is deduced by calibrating the variation of the thermal conductivity of the gas flow measured at the outlet of the cell using a thermal conductivity detector (TCD).
• TCD thermal conductivity detector
• Table 3 the amounts of ammonia are expressed in ml (normal conditions of temperature and pressure) / m 2 (area at 800 ° C) of composition. The higher the amount of ammonia, the higher the surface acidity of the product.
• the reducibility properties are measured by performing a programmed temperature reduction (TPR) on a Micromeritics Autochem 2. This meter measures the hydrogen consumption of a composition as a function of temperature.
• TPR programmed temperature reduction
• the experimental protocol consists in weighing 200 mg of the sample in a previously tared container. The sample is then introduced into a quartz cell containing in the bottom of the quartz wool. The sample is finally covered with quartz wool and positioned in the oven of the measuring device.
• the temperature program is as follows:
• thermocouple placed in the quartz cell above the sample. Hydrogen consumption during the reduction phase is deduced by calibrating the variation of the thermal conductivity of the gas stream measured at the outlet of the cell using a thermal conductivity detector (TCD).
• TCD thermal conductivity detector
• the hydrogen consumption is measured between 30 ° C and 900 ° C. It is reported in Table 1 in ml (normal conditions of temperature and pressure) of H 2 per g of product. The higher this hydrogen consumption, the better the properties of reducibility of the product (redox properties). Table 3
• This example describes the catalytic properties of the compositions of the preceding examples in SCR catalysis. These properties are evaluated under the following conditions.
• compositions used are those directly derived from the syntheses described in the examples previous, that is to say compositions that have been calcined at 800 ° C 4 hours.
• compositions used are those of the preceding examples but after hydrothermal aging.
• This hydrothermal aging consists of continuously circulating a synthetic gas mixture of air containing 10% by volume of H 2 O in a reactor containing the composition. During the circulation of the gas, the temperature of the reactor is brought to 750 ° C. for 16 hours to overcome it.
• compositions are then evaluated in catalytic test.
• the composition (90 mg) is passed over a synthetic gaseous mixture (30 L / h) representative of the catalysis process (Table 4).
• the NOx conversion is monitored as a function of the temperature of the gas mixture.
• Example No. 14B is a comparative example with a composition based on vanadium oxide on a support based on titanium oxide and tungsten. The proportions are in mass.
• Example No. 14C is a comparative example with an aluminosilicate zeolite comprising iron. The proportions are in mass.
• Example No. 14D is a comparative example with an aluminosilicate zeolite comprising copper. The proportions are in mass. It appears from Table 5 that the products according to the invention are more efficient than the comparative products, especially after aging.
• This example illustrates the catalytic properties of the compositions according to the invention when they are used in a coating on a particle filter or else used in extruded form as described above.
• compositions used are compositions having undergone the hydrothermal treatment described above.
• compositions according to Examples 1, 14C and 14D are mixed in a mortar with a model soot (Carbon Black Cabot Eltex) in a mass proportion of 20% soot with 80% composition.
• model soot Carbon Black Cabot Eltex
• Thermogravimetric analysis is performed by circulating a flow of air (1 l / h) with a rise in ambient temperature at 900 ° C over 20 mg of the mixture prepared above.
• the mass loss of the sample is measured between 250 ° C and 900 ° C. It is considered that the loss of mass in this temperature range corresponds to the oxidation of the soot.
• the product of the invention makes it possible to reduce the ignition temperature by 90 ° C. and the light-off temperature by 70 ° C. relative to a combustion of soot without catalyst.
• the products of the comparative examples have no catalytic effect on the oxidation of soot.

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