FR2763260A1 - Treatment of exhaust gases from diesel and lean burn IC engines to reduce emission of nitrogen oxide(s) - Google Patents

Abstract

Treatment of gases to reduce emissions of nitrogen oxides by the use of a catalyst based on at least one metal is claimed.The catalytic phase is obtained by irradiation of salt(s) of the metal(s) of the catalytic phase with an ionising or photoionising radiation source. Also claimed are the microaggregates obtained by the process and their use in the preparation of the catalytic system.

Description

GAS TREATMENT PROCESS FOR REDUCING EMISSIONS

  NITROGEN OXIDES USING A CATALYTIC PHASE CATALYST

  OBTAINED BY RADIOLYTIC OR PHOTOLYTIC REDUCTION AND

  SPECIFIC CATALYTIC COMPOSITIONS

RHONE-POULENC CHEMISTRY

  The present invention relates to a gas treatment process for the reduction of nitrogen oxide emissions using a catalytic phase catalyst obtained by radiolytic or photolytic reduction. It also relates to compositions

specific catalytics.

  It is known that the reduction of emissions of nitrogen oxides (NOx), exhaust gases from automobile engines in particular, is carried out using “three-way” catalysts which use the reducing gases present in the mixture stoichiometrically. . Any excess oxygen results in a sudden deterioration of

catalyst performance.

  However, certain engines such as diesel engines or gasoline engines operating in lean burn are fuel efficient but emit exhaust gases which permanently contain a large excess of oxygen of at least 5% for example. A standard three-way catalyst therefore has no effect on the NOx emissions of these engines. In addition, the limitation of NOx emissions is made imperative by the tightening of standards in automotive post combustion which

  now extend to this type of engine.

  There is therefore a real need for an effective catalyst for the treatment of gases containing NOx and, more particularly, for the treatment of exhaust gases.

  for the motors mentioned above.

  The object of the invention is therefore to find such a catalyst.

  For this purpose, the process according to the invention, for the treatment of gases for the reduction of emissions of nitrogen oxides, is characterized in that a catalyst is used comprising a catalytic phase based on at least one metal, this catalytic phase having been obtained by a process in which a metal salt or metal salts of said catalytic phase are irradiated with a radiation source

ionizing or photo-ionizing.

  The invention also relates to microaggregates in liquid medium or in solid medium capable of being obtained by the process which has just been described above for the

  preparation of the catalytic phase.

  These microaggregates are particularly suitable for the treatment of gases

  for the reduction of NOx emissions.

  Other characteristics, details and advantages of the invention will also appear

  more fully on reading the description which follows, as well as the various

  concrete but non-limiting examples intended to illustrate it.

  The essential characteristic of the process of the invention lies in the use of a specific catalyst. This catalyst or, more precisely, the catalytic phase thereof, are prepared by a process known as radiolytic or photolytic reduction. This

  process will be described more precisely below.

  It will be noted first of all that the process for preparing the catalytic phase can be implemented according to different embodiments. According to a first mode, the method is implemented essentially in a liquid medium. According to a second mode, the

  preparation of the catalytic phase involves a support.

  In the case of the first embodiment, the method comprises a first step in which a liquid medium is prepared which comprises a salt of the metal of the catalytic phase. It is entirely possible, in the context of the present invention, to use a catalyst whose catalytic phase can be based on several metals. In such a case, the aforementioned liquid medium will of course comprise several

  salts. Therefore, in the following text, when the description is made with reference to a

  single metal or a salt of a single metal, this description should be understood of course

  as being applicable to the case where several metals or several salts are

present.

  The salts may be chosen from the usual salts of the metals concerned. They can be inorganic or organic salts and, in particular, complexed metal ions. Examples of inorganic salts that may be mentioned include sulfates, nitrates, halides, in particular chlorides, and perchlorates. As organic salts, there may be mentioned carbonates, formates and acetates. The complexed metal ions can in particular be the ligands acetylacetonate, carbonyl, cyanide or EDTA. Preferably, salts are chosen which are soluble in the liquid medium which is prepared. We usually work with salt concentrations

  in the liquid medium between 10-5 and 10-1 mole. 1-1.

  The metals concerned are more particularly chosen from groups VIII, 1B, IIB, IIIB and IVB of the periodic table. The periodic classification of the elements to which reference is made is that published in the Supplement to the Bulletin of the French Chemical Society No. 1 (January 1966). Mention may more particularly be made of ruthenium, rhodium, nickel, palladium, iridium, platinum, silver, gold and tin. Among the combinations of these elements, it is possible to use more particularly in combination platinum and tin, platinum and palladium, platinum

and rhodium.

  To prepare the liquid medium, any solvent or any mixture of solvents is used.

  capable of dissolving the salts of the metals used.

  The solvents can in particular be polar solvents. Mention may be made, as polar solvents which are very suitable in the context of the present invention, of water, alcohols, acetonitrile, acetone, ammonia, tetrahydrofuran, ethylenediamine,

thioethers for example.

  In order to improve the efficiency of the radiolytic or photolytic reduction, the metal salt can be irradiated in the presence of one or more oxidizing radical scavengers. Thus, when the method is implemented in a liquid medium, this can advantageously comprise, in addition, one or more sensors of this type. The role of the sensor is to intercept the oxidizing radicals likely to form in particular in the liquid medium during irradiation. In fact, the irradiation or radiolysis or photolysis of the medium produces, in addition to solvated electrons, radicals, such as the OH 'radical in the case where the solvent is water, which can in turn oxidize part of the metal atoms formed and resulting from reduction by solvated electrons. This sensor can be chosen from primary or secondary alcohols. Mention may more particularly be made of isopropanol. For example, the addition of an RHOH alcohol which reacts rapidly with the OH radicals to give new reducing OH R radicals makes it possible to increase the yield of metal. Formates, sulfides and halides can also be suitable sensors. A suitably chosen salt of the metal can also act as a sensor. We can mention as

  for example an alkali formate or a formate of the metal which it is desired to reduce.

  The concentration of the oxidizing radical sensor is determined according to the quantity of metal to be produced. This concentration in the liquid medium can for example be between 10-3 and lmol.1'1. It will be noted that the oxidizing radical sensor can be used at the same time as a solvent or as one of the solvents of the abovementioned liquid medium. A mixture of water and isopropanol can thus be used to prepare this medium. The metal salt can also be irradiated in the presence of one or more surfactants. Thus, when the process is carried out in a liquid medium, this can also comprise one or more surfactants. The role of the surfactant is to limit the aggregation of incipient metal atoms and thus allow nanometric particles to be obtained. As an example of surfactants that can be used, there may be mentioned polyvinyl alcohol, polyacrylates, sodium dodecyl sulfate, sodium sulfonate, sodium polyvinyl sulfate, carbowax, polyacrylamide, poly-N-methylacrylamide. Polyvinyl alcohol and polyacrylates are preferred surfactants. The monomer concentration of the surfactant

  in the liquid medium is generally between 10-4 and 2 moles. ['1.

  One or more ligands can also be used to limit the aggregation of incipient metal atoms. As ligand which can be used, mention may be made of carbon monoxide (CO), triphenylphosphine, acetylacetonate, cyanide, EDTA. The ligand concentration can be identical to that given above for the surfactant. The ligand can either be part of the aforementioned salt of the metal or be added to the

liquid medium.

  It may be necessary to control the pH of the liquid medium. This pH should not be too high in order to avoid precipitation of a metal hydroxide by hydrolysis by the solvent. In addition, this pH should not be too low as this may affect the yield

  or the stability of metallic microaggregates.

  It can be noted that it is possible to control the size of the microaggregates which will be formed in the following irradiation step of the preparation process by varying the initial concentration of metal ions, the nature of the surfactant or of the ligand. , the ratio of metal ion concentration to agent concentration

surfactant or ligand and pH.

  The liquid medium thus prepared can be placed under vacuum or it can undergo degassing under nitrogen or argon for example. Note that in the particular case of use

  CO as a ligand, degassing can be done under CO.

  Irradiation is the second step in the phase preparation process

catalytic.

  This irradiation can be done by a source of ionizing radiation. As such a source, we can mention the sources of x-rays, y-rays or

  beams of accelerated particles, preferably accelerated electrons.

  One can also use a photo-ionizing radiation source. It can be

  a source of UV photons constituted for example by a lamp or a laser.

  It is possible to use a continuous source or a pulsed source.

  The irradiation is generally carried out with a dose sufficient to reduce all the metal ions. The irradiation is preferably carried out under inert gas or under vacuum. She

  can be performed at room temperature.

  At the end of the irradiation, a liquid medium is obtained which contains the metal or the

  metals in the form of microaggregates of metal atoms.

  As indicated above, the process for preparing the catalytic phase can

  be implemented using a support.

  The support can be of any type known in the field of catalysis. It can be more precisely chosen from alumina, silica, titanium oxide, zirconium oxide, lanthanide oxides, spinel type oxides, zeolites, silicates, crystalline silicoaluminum phosphates, crystalline aluminum phosphates or mixtures thereof. Among the lanthanide oxides, more may be mentioned

  especially cerium oxide.

  Several variants can be used in the context of the embodiment using a support. These variants include a first step in which a liquid medium is prepared in the manner described above. Consequently,

  the whole of the preceding description for this step also applies here.

  According to a first variant, the above-mentioned liquid medium is prepared. The irradiation is then carried out. In a following step, the liquid medium is brought into contact with the support. This contacting brings about the deposition on the support of the elements, in particular microaggregates, present in the liquid medium. We finally get the

  solid product obtained after contacting.

  This contacting can be done by introducing the support into the liquid medium and by stirring the mixture thus obtained. The solid phase is then separated from the mixture by any known means. The solid thus recovered is dried, this solid constitutes the

  catalyst comprising an active phase and a support.

  A second variant consists in preparing the liquid medium and additionally adding thereto the support in suspension. Then proceed as described for irradiation and then recover the solid phase from the medium obtained after irradiation. As before, the solid is dried. It should be noted in the context of this variant that the support can play the same role as the surfactant. This means that the liquid medium can be prepared with the oxidizing radical scavenger, the metal salt and the support and by adding no surfactant of the type described above or, optionally,

a smaller amount.

  A third variant consists in preparing a liquid medium as described above. In a second step, the support is impregnated with this liquid medium. The impregnated support is then dried. Finally, in a last step, we proceed to irradiation. It will be noted, for this variant, that the liquid medium which serves as impregnation solution must check the usual conditions to allow correct impregnation, in particular the pH conditions. In addition, when using an oxidizing radical sensor, this sensor can be present in the liquid medium so that it is then transferred to the support during impregnation with the element's salt. metallic. It can also be brought to the support during a separate impregnation step. In all cases for this variant, the sensor must be a non-volatile product. It is advantageous to use formates, sulfides and

halides.

  It will be noted for the variants of preparation of a supported catalyst that the catalyst obtained can be calcined before its use in catalysis. The purpose of such calcination is to remove any traces of sensor, ligand or agent.

  surfactant. This calcination is generally carried out in air, for example at 500 C.

  Another variant relates more particularly to the preparation of a catalyst whose active phase is based on at least two metals. It may be advantageous in this case to carry out irradiation at a high dose rate. In this case, the source can for example be a powerful source of X-rays, of y-rays or else a pulsed source of accelerated electrons. It can also be a powerful pulsed laser. By high dose rate is meant a minimum dose rate of at least 1 Gy.s- 1 and more particularly of at least 5Gy.s'1 for the sources of X-rays and y-rays; a minimum dose rate of at least 200Gy.s-1 and more particularly at least 1000Gy.s-1 for pulsed sources of accelerated electrons; a minimum dose rate of at least 0.1mJ.s'1 and more particularly at least lmJ.s'1 for lasers. Under such conditions, it is possible to obtain microaggregates bi or

  multimetallic at least partly in the form of alloys or solid solutions.

  The invention also relates to microaggregates of mono or multimetallic atoms

  likely to be obtained by the process which has been described above.

  More particularly, the invention relates, according to a first embodiment, microaggregates of platinum and tin in liquid medium which are characterized in that they are capable of being obtained by a process in which: - a liquid medium comprising platinum and tin salts and, optionally, an oxidizing radical scavenger, a surfactant or a ligand; - an irradiation of the medium prepared by a source of ionizing radiation is carried out

or photo-ionizing.

  According to another embodiment of the invention, the microaggregates of platinum and tin are in solid medium and they are characterized in that they are capable of being obtained by a process in which: - (a) one prepares a liquid medium comprising platinum and tin salts and, optionally, an oxidizing radical scavenger, a surfactant, a ligand or a support solid; - (b) an irradiation of the medium prepared by a source of ionizing or photo-ionizing radiation is carried out; (c) the liquid medium obtained after irradiation is brought into contact with a solid forming a support, if the latter has not been introduced in step (a); (d) the solid phase is separated and recovered from the media obtained following the steps

(goat).

  According to yet another embodiment of the invention, the microaggregates of platinum and tin are in solid medium and they are characterized in that they are capable of being obtained by a process in which: - a medium is prepared Itquide comprising platinum and tin salts and, optionally, an oxidizing radical scavenger, a surfactant, a ligand; - A support is impregnated with the aforementioned liquid medium; - the impregnated support is dried; - an irradiation of the support impregnated with a radiation source is carried out

ionizing or photo-ionizing.

  The invention also relates to microaggregates capable of being prepared by a

  process in which the irradiation was carried out at high dose rate.

  Since the microaggregates are defined above by their preparation process, everything that has been mentioned above concerning this process by radiolysis or photolysis therefore applies to the definition of these products. This is particularly the case with regard to

  relates to the supports usable in the preparation of microaggregates.

  Bi or multimetallic microaggregates and, in particular, those based on platinum and tin are characterized by the fact that they are at least partly in the form of alloys or solid solutions. The presence of an alloy or a solid solution can be demonstrated by uv-visible absorption spectroscopy of nanocolloids when the microaggregates are in liquid medium, by X or EXAFS diffraction or by microanalysis for microaggregates in solid medium, these measurements being made on

  samples obtained at different stages of irradiation, with increasing dose of irradiation.

  These microaggregates can also be characterized by the fact that they have a size of at most 25nm, more particularly at most 12nm and even more particularly at most 3nm. These sizes are measured by electron microscopy

with transmission.

  The invention applies to the treatment of gases which may comprise nitrogen oxides in combination optionally with carbon oxides and / or

  hydrocarbons, in particular for the reduction of nitrogen oxide emissions.

  The gases capable of being treated by the present invention are, for example, those originating from gas turbines, boilers of thermal power stations or even internal combustion engines. In the latter case, it may in particular be motors

  diesel or engines working in lean mixture.

  The invention thus applies to the treatment of gases which have a high oxygen content and which contain nitrogen oxides, with a view to reducing the emissions of these oxides. By gas having a high oxygen content is meant gases having an excess of oxygen relative to the quantity necessary for the stoichiometric combustion of fuels and, more specifically, gases having permanently an excess of oxygen relative to the stoichiometric value X = 1. The value; X is correlated to the air / fuel ratio in a manner known per se, in particular in the field of internal combustion engines. In other words, the invention applies to the treatment of gases from systems of the type described in the previous paragraph and operating continuously under conditions such as. is always strictly greater than 1. In the case of gases having a high oxygen content, the invention thus applies, on the one hand, to the treatment of engine gases operating in a lean burn mixture and which have a oxygen content (expressed by volume) generally between 2.5 and 5% and, on the other hand, the treatment of gases which have an even higher oxygen content, for example gases of engines of the diesel type, this that is to say at least 5% or more than%, more particularly at least 10%, this content can for example be

between 5 and 20%.

  The gases may contain hydrocarbons and, in such a case, one of the reactions

  that we seek to catalyze is the reaction HC (hydrocarbons) + NOx.

  The hydrocarbons which can be used as a reducing agent for the elimination of NOx are in particular the gases or the liquids of the families of saturated carbides, ethylenic carbides, acetylenic carbides, aromatic carbides and hydrocarbons from petroleum fractions such as for example methane , ethane, propane, butane, pentane, hexane, ethylene, propylene,

  Acetylene, butadiene, benzene, toluene, xylene, kerosene and gas oil.

  The gases can also contain, as reducing agent, organic compounds containing oxygen. These compounds can in particular be alcohols of the type, for example saturated alcohols such as methanol, ethanol or propanol; ethers such as methyl ether or ethyl ether; esters like acetate

methyl and ketones.

  The invention also applies to the treatment of gases not containing

  hydrocarbons or organic compounds as a reducing agent.

  The invention also relates to a catalytic system for the treatment of gases with a view to reducing the emissions of nitrogen oxides, gases which may be of the type mentioned above. This system is characterized in that it comprises a catalyst as obtained by the radiolysis or photolysis process described above or

  microaggregates in solid medium of the type also described above.

  In this system, the catalyst can be in various forms such as

  granules, balls, cylinders or honeycomb of variable dimensions.

  The catalysts according to the invention can also be used in catalytic systems comprising a coating (wash coat) incorporating these catalysts, the coating being placed on a substrate of the type, for example metallic monolith or

ceramic.

  In the case of application to exhaust gas treatment, the systems

  are mounted in a known manner and in the exhaust pipes of vehicles.

  Finally, the invention relates to the process for manufacturing the above-mentioned catalytic systems using a catalyst or microaggregates of the type described above.

  Concrete but non-limiting examples will now be given.

  Examples 1 to 4 relate to the preparation of the catalysts. In these examples, the products used are: Ultra pure water Isopropanol (Prolabo) Polyvinyl alcohol (PVA) (Molecular mass 86000, Aldrich) For irradiation, two sources are used: - a source of continuous radiolysis, with cobalt 60, manufactured by the company CIS Bio International and having an activity of 260TBq (or 7000Ci) with a maximum irradiation dose rate of 10kGy.h'1; the radiation field is panoramic all around the device and centered with respect to the sources; a pulsed radiolysis source constituted by an electron accelerator from the company CARIC with a power of 20kW, providing an energy of 10MeV. The pulse duration is 14ps at a frequency of 10 to 350Hz. Maximum dose rate

irradiation is 2.2kGy.s'1.

EXAMPLE 1

  An aqueous solution containing 2.5.10'2mole.1'1 of K2PtCI4, 2.5.10'2mole. 11 of PVA as a surfactant and 0.2 mol. 1.1 of isopropanol as an oxidizing radical scavenger is degassed under N2 and then irradiated by y radiation with a dose of 75kGy

and a dose rate of 10KGy.h-1.

  After irradiation, the solution is then brought into contact with stirring with 3 g of a cerium oxide (CeO 2) support for several hours. 1% by mass of metal (30 mg) is deposited. The supernatant is then separated from the solid product by centrifugation. The support with the impregnated metal is then dried in an oven at 60 ° C. overnight.

EXAMPLE 2

  An aqueous solution containing 2.5.10'3mole.1I1 of K2PtCI4, 2.5.10'2mole. 1I1 of PVA as a surfactant and 0.2mol. 1'1 of isopropanol as an oxidizing radical scavenger is degassed under N2 then irradiated by y radiation with a dose of 8kGy

and a dose rate of 10KGy.h-1.

  We then proceed to a support as in Example 1 but in

  using 3g of an alumina support (AI203).

EXAMPLE 3

  A 50/50 mixed water / isopropanol solution by volume containing 1.5. 10'3mole.l11 of K2PtCI4, 1.5.10-3mole.1-1 of SnCI4 and 10'2mole of PVA is degassed under N2 then irradiated by electron accelerator with a dose of 50kGy and a dose rate of 2.2Gy .s1 sufficient to completely reduce platinum and tin. The impregnation of an alumina support is then carried out as in Example 2.

EXAMPLE 4

  A 50/50 mixed water / isopropanol solution by volume containing 2.10'3mole. 1I1 of K2PtCI4, 10-3mole.1-1 of SnCI4 and 10'2mole of PVA is degassed under N2 then irradiated by electron accelerator with a dose of 50kGy and a dose rate of 2.2Gy.s'

  l Sufficient to completely reduce platinum and tin.

  The impregnation of a support in cerium oxide is then carried out as in

Example 1.

EXAMPLE 5

  This example relates to the use of the catalysts prepared in Examples 1 to

  4. These catalysts, once prepared, were calcined for 2 hours at 500 C.

  50 mg of the powdered catalyst are loaded into a quartz reactor. The powder

  used was previously granulated at 0.125 and 0.250mm.

  The reaction mixture at the inlet of the reactor has the following composition (by volume) for the tests carried out in the presence of a reducing agent (reduction) - NO = 300 vpm - C3H6 = 300 vpm - CO = 350 vpm

- 02 = 10%

- CO2 = 10%

- H2O = 10%

- N2 = qs 100%

The overall flow is 30 NI / h.

The VVH is 500,000 h-1.

  For the tests carried out in the absence of a reducing agent (decomposition), the CO and C3H6 flow rates are eliminated and replaced by an equivalent flow of nitrogen so

keep the same VVH.

  The NOx signal is given by an ECOPHYSICS NOx analyzer, based on the

principle of chemistry-luminescence.

  The catalytic activity is measured as a function of the temperature during a programmed temperature rise from 150 to 700 C at a rate of 15 C / min and from the following relationship: T (NOX) = 100 (NOx -NOx) / NOx with NOX signal of NOx at time t = 0 which corresponds to the signal of NOx obtained with the reaction mixture during the by-pass of the

  catalytic reactor and NOx is the NOx signal at time t. Because the catalysts can activate under the conditions of the test,

  the catalytic activity is given during the second consecutive passage under test under the same conditions. The highest T (NOx) values obtained below are given

and decaying.

Table 1

  Reduction tests Example 1 T (NO,) / Temperature 1st pass 2nd pass

1 22% / 380 C 26% / 320 C

2 26% / 355 C 48% / 250 C

3 28% / 360 C 42% / 275 C

4 18% / 300 C 23% / 260 C

Table 2

  Tests carried out in Decomposition Example 1 T (NO,) / Temperature 1st pass 2nd pass

1 20% / 405 C 22% / 380 C

2 31% / 365 C 36% / 300 C

3 27% / 365 C 31% / 365 C

4 18% / 405 C 23% / 355 C

  The comparison of decomposing examples 1 and 4 shows that at a lower content

  in platinum, a similar activity is obtained at a lower temperature.

Claims (20)

  1- A gas treatment process for the reduction of nitrogen oxide emissions, characterized in that a catalyst is used comprising a catalytic phase based on at least one metal, this catalytic phase having been obtained by a process in which a metal salt or metal salts of said catalytic phase are irradiated with   a source of ionizing or photo-ionizing radiation.   2- A method according to claim 1, characterized in that a catalyst is used comprising a catalytic phase based on at least one metal and a support for said phase, the catalyst having been obtained by a process in which: a liquid medium comprising a metal salt or metal salts of said catalytic phase; - An irradiation of the medium thus prepared is carried out by a source of ionizing or photo-ionizing radiation; - The liquid medium obtained after irradiation is brought into contact with said support;   - The solid product obtained is recovered after the contacting of the previous step.   3- A method according to claim 1, characterized in that a catalyst is used comprising a catalytic phase based on at least one metal and a support for said phase, the catalyst having been obtained by a process in which: a liquid medium comprising a metal salt or metal salts of said catalytic phase; - The support is impregnated with the liquid medium; - the impregnated support is dried; - an irradiation of the support impregnated with a radiation source is carried out ionizing or photo-ionizing.   4- A method according to claim 1, characterized in that a catalyst is used comprising a catalytic phase based on at least one metal and a support for said phase, the catalyst having been obtained by a process in which: a liquid medium comprising a metal salt or metal salts of said catalytic phase and the support; - An irradiation of the medium thus prepared is carried out by a source of ionizing or photo-ionizing radiation;   - The solid product is recovered from the medium obtained after the irradiation.   - Method according to one of the preceding claims, characterized in that the metal   belongs to groups VIII, IB, IIB, IIIB and IVB of the Periodic Table of the Elements.   6- Method according to one of claims 2 to 5, characterized in that the support is   chosen from alumina, silica, titanium oxide, zirconium oxide, lanthanide oxides, spinel type oxides, zeolites, silicates, phosphates   crystalline silicoaluminium, crystalline aluminum phosphates.   7- Method according to one of claims 2 to 5, characterized in that the liquid medium includes a polar solvent.   8- Method according to one of the preceding claims, characterized in that the source   of ionizing radiation is a source of x-rays, y-rays or even a   beam of accelerated particles, preferably accelerated electrons.   9- Method according to one of claims 1 to 7, characterized in that the source of   photo-ionizing radiation is a source of UV photons.   10- Method according to one of the preceding claims, characterized in that one uses   a catalyst comprising a catalytic phase based on at least two metals and which   was obtained by carrying out the above-mentioned irradiation from a source with a high dose rate.   11- Method according to one of the preceding claims, characterized in that the phase   catalytic is based on platinum and / or tin.   12- Method according to one of the preceding claims, characterized in that   irradiation takes place in the presence of at least one oxidizing radical sensor or on a   liquid medium comprising such a sensor.   13- Method according to one of the preceding claims, characterized in that   irradiation takes place in the presence of at least one surfactant or on a liquid medium including such an agent.   14- Method according to one of the preceding claims, characterized in that one treats   an exhaust gas from diesel engines or from engines operating in a lean mixture.   - Method according to one of the preceding claims, characterized in that the   oxygen content of the gases is at least 5% by volume.   16- Method according to one of the preceding claims, characterized in that one treats   a gas in the presence of a hydrocarbon or an organic compound containing oxygen. 17- Microaggregates of platinum and tin in liquid medium, characterized in that they are capable of being obtained by a process in which: - a liquid medium is prepared comprising salts of platinum and tin and, optionally a oxidative radical scavenger, surfactant or ligand; - an irradiation of the medium prepared by a source of ionizing radiation is carried out or photo-ionizing.   18- Microaggregates of platinum and tin in solid medium, characterized in that they are capable of being obtained by a process in which: (a) a liquid medium is prepared comprising salts of platinum and tin and, optionally, an oxidizing radical scavenger, a surfactant, a ligand or a solid forming a support; - (b) an irradiation of the medium prepared by a source of ionizing or photo-ionizing radiation is carried out; - (c) the liquid medium obtained after irradiation is brought into contact with a solid forming a support, if the latter has not been introduced in step (a); - (d) the solid phase is separated and recovered from the media obtained following the steps (goat).   19- Microaggregates of platinum and tin in solid medium, characterized in that they are capable of being obtained by a process in which: a liquid medium is prepared comprising salts of platinum and tin and, optionally, a oxidative radical scavenger, surfactant, ligand; - A support is impregnated with the aforementioned liquid medium; - the impregnated support is dried; - an irradiation of the support impregnated with a radiation source is carried out ionizing or photo-ionizing.   - Platinum and tin microaggregates according to one of claims 17 to 19,   characterized in that they are capable of being obtained by a process in which   the aforementioned irradiation is carried out by a source with a high dose rate.   21- Platinum and tin microaggregates, characterized in that they occur   in the form of an alloy or a solid solution.   22- Platinum and tin microaggregates, characterized in that they have a size of at most 25nm, more particularly at most 12nm and even more particularly at most 3nm.   23- Catalytic system for implementing the process according to one of the   Claims 1 to 16, characterized in that it comprises a catalyst as defined in   one of claims 1 to 16 or microaggregates as defined in one of claims 17 to 22.   24- A method of manufacturing a catalytic system according to claim 23, characterized in that one implements a catalyst as defined in one of   claims 1 to 16 or microaggregates as defined in one of claims 17 to 22.