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  • 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
  • B01D53/46—Removing components of defined structure
  • B01D53/54—Nitrogen compounds
  • B01D53/56—Nitrogen oxides
• • 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
  • 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
  • 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
• • 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
  • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
  • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
  • B01J23/54—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
  • B01J23/56—Platinum group metals
  • B01J23/63—Platinum group metals with rare earths or actinides
• • 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
  • B01J23/76—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
  • B01J23/83—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36 with rare earths or actinides
• • 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
  • 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
• • 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
• • 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
  • B01D2255/20—Metals or compounds thereof
  • B01D2255/206—Rare earth metals
  • B01D2255/2061—Yttrium
• • 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
  • B01D2255/20707—Titanium
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
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  • B01D2255/40—Mixed oxides
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2255/00—Catalysts
  • B01D2255/90—Physical characteristics of catalysts
  • B01D2255/91—NOx-storage component incorporated in the catalyst
• • 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2258/00—Sources of waste gases
  • B01D2258/02—Other waste gases
  • B01D2258/0241—Other waste gases from glass manufacture plants

Definitions

• the present invention relates to the field of purification of a gaseous pollutant gas essentially of NO x type. More particularly, the invention relates to catalytic gas treatment systems, especially at the exhaust outlet of a diesel engine or gasoline, allowing the elimination of said polluting species, by reduction of said NO x type species.
• a conventional three-way catalyst allows the combined treatment of NO x , CO and HC pollutants and their conversion into neutral and chemically harmless gases such as N 2 , CO 2 and H 2 0.
• a very good system efficiency is not achieved only by a continuous adjustment of the richness of the air-fuel mixture. It is thus known that the slightest deviation from the stoichiometry of the mixture causes a large increase in pollutant emissions.
• NO x trap NO x trap
• the major disadvantage of such a system is however that the reduction of NOx can be achieved at the cost of overconsumption of fuel.
• the desorption of NOx entrapped on the catalyst and their catalytic reduction in nitrogen gas N 2 can only be obtained in the presence, at the level of the reduction catalyst, of a sufficient quantity of the species reducing agents in the form of hydrocarbons or carbon monoxide CO or gaseous hydrogen 3 ⁇ 4.
• Hydrogen gas can itself be obtained by a catalytic reaction between HC hydrocarbons and water vapor or between CO and water vapor.
• the object of the present invention is to provide a simple and autonomous catalytic system, allowing the direct elimination of pollutants of the NOx type, in particular NO 2 , contained in a polluted gas to be treated, said system allowing the conversion of said NO x in 2 without the use of an additional material adsorbing NO x very selectively, that is to say more than 80% or more than 90 mol% of the converted NOx are converted into N 2 , said system being able to operate regardless of the global atmosphere of the gases to be treated: oxidizing, neutral or reducing, in particular oxidizing and under low temperature conditions, especially below 500 ° C.
• the present invention relates to the use for the reduction of the NO x , in particular NO 2 , oxidizing polluting species contained in a gas to be purified, of a catalytic system comprising or consisting of an oxide to the molar formulation:
• M is an element chosen from: Gd, Y, Se, Sm, La, Pr, Nd, Er, Tb,
• N is an element having several valence degrees selected from: Ti, V, Cr, Mn, Fe, Co, Ni, Cu,
• x is greater than 0.05.
• the electronic resistivity of the ionic and electronically conductive oxide is less than 20 ohms. cm at 400 ° C and its ionic conductivity is between 1 and 10 ⁇ 4 Siemens / cm at 400 ° C.
• the oxide ion and electronic conductor is of the formula CEI y _ z 02- x M y, and wherein y is between about 0.1 and about 0.3.
• the ionic and electronically conductive oxide corresponds to the formula Cei- y O- x Gd y , and wherein y is from about 0.1 to about 0.3.
• z is greater than 0 and preferably between 0.01 and 0.2.
• the ionic and electronic conducting oxide has the formula CEI y _ x 02- z Y y Ti z wherein y is from about 0.1 to about 0.3 and wherein z is between 0.01 and 0.1.
• x is between 0.1 and 0.4, preferably between
• the oxide comprises oxygen vacancies and part of the Ce 4+ cations of the crystal lattice is converted into Ce 3+ cations.
• the proportion of Ce 3+ ions is preferably between 25 and 75%, in particular between 40 and 70%.
• proportion of the cations Ce 3+ we mean the proportion of trivalent cations as calculated according to the Ce 3+ / Ce 4+ ratio. The method of determining such a ratio by magnetic susceptibility measurements is well known and for example described in the publication "Catalysis Today, vol. 54, pp. 93-100 (1999).
• the ionic and electronic conductive oxide has a specific surface area of between 0.5 and 100 m 2 / g, preferably between 1 and 30 m 2 / g.
• the catalytic system as previously described does not require additional metals, especially precious metals, of the type known to date to promote the conversion or reduction of NO x to N 2 , especially those based on the Rh, Cu, Ni type.
• selective catalyst for the oxidation of NO to NO 2 it is understood a catalyst allowing the conversion of more than 50 mol% of NO to NO 2 preferably more than 75% or even more than 80% or even more than 85 mol% of NO to NO 2 .
• the catalytic system according to the invention allowing a extremely selective and efficient reduction of nitrogen dioxide NO 2 to N 2 , but a significantly lower activity of the conversion of nitrogen monoxide NO / N 2 O to N 2 .
• an oxidation catalyst for example comprising platinum.
• Pt or other metals such as Pd, Ag, Fe, Co, or Au, in particular by the known impregnation techniques, CEI formulation oxide y - z 0 2 -x M y N z.
• the catalytic system used according to the invention can be implemented according to various possible modes, according to any technique known in the art.
• Powder bed structures may comprise oxide powder constituted by or incorporating a system used according to the invention.
• Inorganic structures, filtering or not, formed of porous walls of inorganic material for example a ceramic material or foam-type structures are also likely to be constituted by or support cerium oxide corresponding to the molar formulation previously described.
• the porous inorganic structure is impregnated with an aqueous solution comprising particles of cerium oxide corresponding to the preceding molar formulation or its precursors.
• the inorganic material constituting out or part of the inorganic structure is chosen from the following list, alone or in combination: metal or sintered metal, cordierite, aluminum titanate, alumina, mullite, silicon nitride, silicon carbide .
• Electronically conductive inorganic material structures such as silicon carbide or metals for regenerating the catalytic system by polarization.
• the catalytic system as described above may advantageously be used in a structure for the depollution and / or filtration of a gas charged with gaseous pollutants and possibly solid particles, in particular an exhaust gas from a motor vehicle, consisting of a porous inorganic material on which said catalytic system is deposited.
• the present invention is thus particularly applicable in the structures used for the purification and filtration of an exhaust gas of a diesel engine.
• Such structures generally referred to as particle filters, comprise at least one, and preferably a plurality, of monolithic honeycomb blocks.
• the block or blocks comprising a set of adjacent ducts or channels of axes parallel to each other separated by walls porous, closed by plugs at one or the other of their ends to define inlet ducts opening on a gas intake face and outlet ducts opening on a discharge face of gas, so that the gas passes through the porous walls.
• Examples of such assembled or unassembled structures are for example described in EP 0816065, EP 1142619, EP1306358 or EP 1591430.
• the catalytic system makes it possible to convert the NO x oxidizing polluting species even if the atmosphere of the gases is generally oxidizing, that is to say in particular in the case of the treatment of the exhaust gases resulting from a poor initial mixture of air / fuel,
• the catalytic system according to the invention is autonomous, that is to say it allows the continuous conversion of NO x in 2 without the assistance of an additional compound allowing the prior storage NO x before treatment.
• the present invention further relates to the use of a catalytic system as previously described for the depollution of a gas from a glass furnace.
• the invention and its advantages will be better understood on reading the nonlimiting examples which follow:
• a precursor of the oxide is synthesized by a sol-gel method, by mixing the reagents given below, in equivalent proportions to obtain in the initial mixture, in moles: 0.2 part of Y, 0.75 part of This, 0.05 part of Ti.
• yttrium acetate tetrahydrate Y (CH 3 COO) 3 , 4H 2 O (99-102%, Alfa Aesar),
• Titanium isopropoxide Ti (OC 3 H 5 ) 4 (99.9%, Alfa Aesar).
• precursor salts are dissolved in ultrapure distilled water.
• a salt solution and an organic solution are prepared.
• the precursor salts in proportions desired in order to obtain the compound Ceo, 75YO, 2T10, 05O2, as described above, are dissolved in ultrapure distilled water at ambient temperature and with stirring.
• PEG 2000 creates a three-dimensional periodic polymeric network in which salts can be incorporated. Drying and one evaporation of the solvent are 1 to a rotary evaporator under reduced pressure (P a ⁇ tm 1100 mbar) and 65 ° C.
• the removal step of the organic compounds is carried out by heating in air at 600 ° C for 30 minutes.
• the climb ramp is 50 ° C / h.
• the previous gel is placed in an alumina crucible to be certain to eliminate all organic residues.
• the oxide thus recovered, of composition Ceo, 75Y0, 2T10, 05O2 has a yellow color after manual grinding with the mortar. Its specific surface is measured by a conventional surface analysis according to the BET method. This method of measuring specific surface area by inert gas adsorption has been developed by S. Brunauer, PHEmmet and J. Teller is well known to those skilled in the art. Its specific surface is about 57 m 2 / g.
• Oxygen deficit rate x of the oxide was evaluated according to the invention according to conventional techniques, by reduction in programmed temperature (often referred to as temperature-programmed reduction or TPR).
• the samples are heated under an atmosphere comprising 3 ⁇ 4 (3 ⁇ 4 / He mixture with 1 mol% 3 ⁇ 4 1.8 L / h, 10 ° C / min) up to 900 ° C.
• hydrogen consumption is directly correlated with the parameter x.
• This step of reduction at high temperature in a hydrogen atmosphere according to the invention makes it possible to eliminate part of the oxygens not only at the surface but also in the entire volume of the oxide, with the creation of oxygen vacancies throughout the network. lens.
• a step of creating oxygen vacancies could be accompanied by a partial conversion of a part of the Ce 4+ cations of the Ce 3+ cation network.
• a first catalytic system according to the invention is thus obtained.
• Example 1 was reproduced in the same manner except that the last reduction step, that is to say the removal of part of the oxygen from the crystalline lattice of the oxide n has not been done.
• the last reduction step that is to say the removal of part of the oxygen from the crystalline lattice of the oxide n has not been done.
• an oxide with the general formulation Ceo, 75YO, 2T10, 05O2 of surface area 57 m 2 / g this time with no oxygen vacancy.
• the oxide synthesized according to the example is the oxide synthesized according to the example.
• composition Ceo, 75Y0, 2T10, 05O2, with a specific surface area of 57 m 2 / g is preheated at 1250 ° C. for 8 hours in air before the reduction step under pure hydrogen.
• the specific surface area measured after treatment at 1250 ° C is decreased to 2 m 2 / g.
• the oxide is then subjected to the same treatment in a hydrogen atmosphere and at a high temperature as described in Example 1 above, for a time sufficient to obtain a very oxygen-deficient oxide.
• the experimental protocol for obtaining is identical to that already described for Example 2 except that the last step of removing part of the oxygens from the crystal lattice of the oxide is not carried out.
• an oxide of general formulation Ceo, 75Yo, 2Ti0, O502 with no oxygen vacancy is obtained.
• the powder is impregnated with a platinum precursor in the form of platinum dinitrite diamine, (N3 ⁇ 4) 2 Pt (NO 2 ) 2 (Alfa Aesar, 1.7% by weight of Pt), then poured into a flask and mixed with a solution platinum precursor and ultrapure water. The flask is immersed in a water bath and heated to a temperature of 70 ° C, with stirring for 70 minutes.
• the flask is removed from the water bath and cooled to room temperature. The flask is then re-plunged and gently heated under primary vacuum to 60 ° C. After the solvent has completely evaporated after about 2 hours at 60 ° C., the powder is oven-dried at 110 ° C. for 3 hours and then ground. The catalytic system thus obtained is then calcined in air at 500 ° C. for one hour. After impregnation, the oxide of general formulation Ceo, 8Gdo, 202 comprises about 0.3% of Pt relative to the total mass of the oxide and precious metal powder system.
• the oxide is then subjected to treatment in an atmosphere of pure hydrogen and at a high temperature (greater than or equal to 600 ° C.) as described in Example 1 above, for a time sufficient to obtain a oxide very deficient in oxygen.
• a high temperature greater than or equal to 600 ° C.
• Example 3 was reproduced in the same way, except that the last step of severe reduction for the removal of a part of the oxygens from the lattice of the oxide by reduction was not made.
• an oxide of general formulation Ceo 8Gdo 202 comprising platinum, but not lacunar with oxygen, is obtained.
• a fourth catalytic system is prepared consisting of an oxide deficient in oxygen identical to that described in Example 3 but not impregnated with platinum.
• the experimental protocol of obtaining is identical to that already described for example 3 except the step of impregnation of the oxide with Pt.
• the end is thus obtained according to this example a catalyst system without Pt consisting of an oxide of the general formulation Ce ODMS, 2 0i, 7.
• Example 4 was reproduced in the same manner, except that the last severe reduction step for the removal of part of the oxygen from the crystal lattice of the oxide by reduction did not occur. been carried out.
• an oxide of general formulation Ceo, 8GdO, 202 with no oxygen vacancy and without Pt is obtained.
• the samples of the examples of the invention and previous comparative, finely ground in the form of a powder are placed in a U-shaped fixed-bed quartz reactor. More precisely, about 500 mg of the catalytic system are introduced into the reactor. , equipped with a porous quartz frit supporting the powder.
• the volume of the catalyst is of the order of 0.3-0.4 cm 3 .
• the catalyst is heated under a current of He (5 L / h) at 250 ° C and at atmospheric pressure. Once the temperature of 250 ° C. has been stabilized, a reaction mixture composed of a 02 / He mixture is introduced onto the catalyst at a total flow rate of 10 L / h and a hourly space velocity of between 25,000 and 35,000 h -1 .
• the gaseous mixture to be cleaned up initially comprises 500 ppm of O2. It is representative of a highly oxidizing atmosphere, in the sense for example of an exhaust gas from a diesel engine operating in lean mixture or a gas from a burner of a glass furnace.
• On-line gas analysis is performed by an infra-red analyzer for NO and 2O, ultraviolet for NO2, and by a microchromatograph equipped with two analysis modules and two catharometric detectors, for oxygen analysis and nitrogen.
• the evolution of the concentrations of NO, NO 2 and N 2 and N 2 O in the gases to be treated is monitored as a function of time as soon as the NO 2 / He mixture is introduced onto the catalyst for approximately 2 hours.
• An overall molar percentage of conversion of NOx to N2 by the catalyst system is then calculated over this period, as reported in Tables 1 and 2.
• Example 1 shows that the catalytic systems according to the invention can convert NO x to 2 even if they have very small specific surface areas, while maintaining good selectivity.
• the selectivity of the systems according to the invention is very high. This was determined by measuring the percentage of product 2 (in umol) relative to the amount of NO x converted (N 2 + N 2 0) by the catalyst system during the duration of the test (6500 seconds). In this case, the system according to the invention show no presence of NO x converted 2 0 because the selectivity is 100%.
• Example 3 The comparison between Example 3 and Example 4 indicates that the presence of a precious metal is not necessary for the catalytic reaction.
• the presence of platinum on oxygen deficient oxide can even under certain conditions lead to a decrease in the NOx conversion rate, which appears extremely surprising, especially when the gas to be treated contains a large majority of nitrogen dioxide.
• other tests carried out by the applicant have shown that the presence of noble metal catalyst such as Pt, however, may be advantageous when the gas to be treated comprises this time significant amounts of nitric oxide.
• Example 5 shows that the NO x conversion effect can be directly correlated with the oxygen deficit state of the crystal lattice, the conversion rate of NO x decreasing with the value of the rate x of oxygen vacancies in the crystal lattice of the oxide.
• a significant conversion of pollutants is observed at a low temperature, that is to say at 250 ° C, while the usual catalysts are active at much higher temperatures .
• the current three-way catalysts, especially zeolites are mainly active between 700 and 1000 ° C.
• the proportion of the cations in the oxide is measured by the Ce 3+ / Ce 4+ ratio, obtained by means of magnetic susceptibility measurements (also called “Magnetic Balance Measurement” or “magnetic balance”). known in the field.
• the proportion of Ce 3+ ion is of the order of 40 to 60%.
• the conductivities of the oxide can be measured by conventional impedance techniques, for example as described in the publication "Acta Materialia, vol. 56, pp. 4658-4672 (2008).
• the ionic and electronic conductivities of the samples according to the examples according to the invention are much lower than those of the comparative examples, and they are the only ones to be in accordance with the subject of the present invention: electronic resistivity less than 20 ohms. cm at 400 ° C and ionic conductivity of between 1 and 10 ⁇ 4 Siemens / cm at 400 ° C.

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