EP2379204A1

EP2379204A1 - Purification structure including a polarised electrochemical catalysis system

Info

Publication number: EP2379204A1
Application number: EP09804298A
Authority: EP
Inventors: Philippe Vernoux; Abdelkader Hadjar; Agnès PRINCIVALLE; Christian Guizard
Original assignee: Centre National de la Recherche Scientifique CNRS; Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Current assignee: Centre National de la Recherche Scientifique CNRS; Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Priority date: 2008-12-17
Filing date: 2009-12-16
Publication date: 2011-10-26
Also published as: FR2939696B1; WO2010076508A1; FR2939696A1

Abstract

The invention relates to a catalytic filter for purifying a polluted gas, including a honeycomb structure, in which at least one porous electronically conductive material forms the walls of said structure, and an electrochemical system for treating said gas, including a layer of an ionically conductive and electronically insulating material D, the ionic conductor D including or being made of a zirconium oxide partially substituted by a trivalent M + or divalent cation M'²⁺, a catalyst A suitable for converting NO_x to NO₂and a catalyst B suitable for reducing NO_x to N₂and a means for applying a voltage or a current between an electrode W connected to the catalyst A and an auxiliary electrode CE connected to the catalyst B, such that said layer of zirconium oxide can be retained in a reduced state by applying said voltage or said current.

Description

PURIFICATION STRUCTURE INCORPORATING A POLARIZED ELECTROCHEMICAL CATALYSIS SYSTEM

The present invention relates to the field of structures for purifying a gas charged with gaseous pollutants essentially of NO _x type. More particularly, the invention relates to honeycomb structures, in particular used to treat the exhaust gas of a gasoline or preferably diesel engine, and incorporating an electrochemical catalyst system.

The techniques and problems related to the purification of polluted gases, especially at the exit of the exhaust lines of petrol or diesel motor vehicles, are well known in the art. A conventional three-way catalyst allows the joint treatment of pollutants NO _x , CO and HC and their conversion into neutral and chemically harmless gases such as N ₂ , CO ₂ and H ₂ O. A very good efficiency of the system 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 said mixture causes a large increase in pollutant emissions. To solve this problem, it has been proposed to incorporate into the catalyst materials temporarily allowing to fix the NO _x (often called in the NO _x trap) when the mixture is poor (that is to say under stoichiometric). The major disadvantage of such a system is however that the reduction of NOx can be achieved at the cost of overconsumption of fuel, during a subsequent phase of operation in rich mixture. The desorption of the NOx trapped on the catalyst and their catalytic reduction in nitrogen gas N ₂ can not only in the presence, at the level of the reduction catalyst, of a sufficient quantity of the reducing species, in the form of hydrocarbons or carbon monoxide CO or hydrogen H ₂ , the hydrogen being able to be -Even obtained by a catalytic reaction between HC hydrocarbons and water vapor or between CO and water vapor. The NOx adsorbing materials are generally alkali metal or alkaline earth metal oxides, in particular barium oxide, a material currently known to be the most effective in this field. These solutions, however, pose a problem of chemical stability because the NOx traps are poisoned by sulfur oxides (SOx) contained in the exhaust gas including diesel engine. Moreover, the precursors of barium being difficult to disperse on the support material, it is necessary to provide significant amounts. This leads to reducing the accessibility of the noble metals of the catalytic system by a "covering" effect by barium oxide. A possible but too expensive solution would be to increase the contribution of noble metals in the system. In addition, these barium-based NOx trap systems pose hygiene and environmental problems, barium being listed as heavy metal.

At present, no simple catalytic system is known, which allows a substantial conversion of NO _x to N ₂ in a poor atmosphere, that is to say in the presence of an excess of oxygen. One of the aims of the present invention is to overcome such a deficiency, in particular by allowing the conversion of a substantial amount of NOx even when the air / fuel ratio of the exhaust gas is poor. According to one possible approach, US Pat. No. 6,878,354 describes a combination of catalysts for oxidation of HC and CO and reduction of NO _x electrochemically. Such systems appear advantageous because they allow an electrochemical reaction between two reduction catalysts A and oxidation B linked together by both an electronic conductor C and an ionic conductor D. According to this publication, such a system makes it possible in particular to to increase the catalytic conversion of polluting species, especially in a lean burn engine operation.

According to the authors, the presence of such a system comprising an ionic conductor and an electronic conductor simultaneously makes it possible to oxidize the reducing species of the HC, CO, soot or H _{2 type} and the reduction of the NO _x type oxidizing species according to the principle general illustrated by Figure 1 of this patent.

However, the effectiveness of such a system appears limited because its proper implementation requires close contact between the four elements constituting the electrochemical system. Thus, in the embodiments described in US Pat. No. 6,878,354, two catalysts A and B are deposited in the form of particles on the monolith, one for the reduction of NOx and the other for the oxidation of HC hydrocarbons. , CO. The efficiency of such a system then strongly depends on the deposition conditions of the catalysts A and B and the electronic conductors C and ion D. Indeed, the properties obtained are highly dependent on the dispersion of the different phases corresponding to the different constituents on the medium used, a connection being necessary between these four elements for the proper functioning of the electrochemical system. In addition, the efficiency of the conversion of the polluting species can also be substantially limited by the intrinsic characteristics of the materials used as the ionic and electronic conductor. More precisely, since the electrochemical system consists of small grains randomly arranged with respect to each other, its efficiency is necessarily limited, on the one hand, by the connections between the grains and, on the other hand, by the small quantity of conductors (electrons). and / or ions) available for the proper functioning of the electrochemical catalysis system.

The object of the present invention is to provide a solution for solving the problems described above. In particular, one of the aims of the present invention is to provide a structure for the purification of a polluted gas, in particular a structure for filtering an exhaust gas resulting from a gasoline or preferably diesel engine loaded with pollutants. gaseous and solid particles, able to work effectively regardless of the richness of the air / fuel mixture. In another aspect, the invention relates to novel electrochemical systems, having an NO _x storage ability comparable to systems using the presence of alkali or alkaline earth metals of the barium type.

According to another aspect, the invention relates to a NOx storage / reduction device of the particle filter type making it possible to limit the operating frequencies in rich mixture necessary for the destocking and the reduction of NO _x , or even to to overcome and thus avoid overconsumption in fuel, characterizing the electrochemical systems already described. More specifically, the invention relates in its most general form to a device for the purification of a polluted gas, for example an exhaust gas from a diesel engine or gasoline, comprising: - one and preferably a plurality monolithic honeycomb blocks, typically interconnected by a joint cement and constituting a particulate filter, said block or blocks comprising a set of adjacent channels of axes parallel to each other separated by porous walls, closed by plugs at one or the other of their ends to define inlet ducts opening on a gas inlet face and outlet ducts opening on a gas evacuation face, in such a way the gas passes through the porous walls in which at least one porous electronic conductive material forms the walls of said structure, an electrochemical system for treating said gas comprising: a layer of a material D ionic conductor by the oxygen ions and electronic insulator, the ionic conductor D comprising or consisting of a zirconium oxide partially substituted by a trivalent cation M ³⁺ or divalent M ' ²⁺ , a catalyst A for converting NOx to NO2, to contact of the ion conductor D and electrically connected to an electrode W and, an N ₂ NOx reduction catalyst B, in contact with the ion conductor D and electrically connected to a counter-electrode CE, the catalyst B being in electrical contact with the material electronic conductor forming the honeycomb walls and said ionic conductor layer D being disposed between the catalyst A and the catalyst B, means for applying a voltage or a current between said electrode W and said counter-electrode CE, such that said zirconium oxide layer can be maintained in a reduced state by the application of said voltage or electric current.

According to an advantageous embodiment of implementation of the invention, the plugs obstructing the output channels on the front face of the filter and plugs obstructing the input channels on the rear face of the filter are ionic conductors, and are electrically interconnected to constitute respectively the reference electrode W and the counter-electrode CE. For example, the electronic conductive plugs may consist of SiC doped with aluminum.

Catalyst A is preferably chosen to also be suitable for the oxidation of soot.

Catalyst B is preferably chosen to also be suitable for the oxidation of polluting species of the HC, CO or H ₂ hydrocarbon type.

The term ion conductor comprising or consisting of a partially substituted zirconium oxide with a cation valence lower than that of zirconium.

Typically, the ion conductor according to the invention can be formulated by an equation:

(Zrθ2- _X) i- _y (M ₂ θ3_ _x) _y, M being a cation of valency 3 or

(ZrO ₂ -x) Y _'(Me _IM-x) _y', wherein M 'is a cation of valency 2. In a preferred embodiment, x is less than 0.5. Preferably, x is less than 0.1, and very preferably, x is less than 0.05. Advantageously, x is greater than 0.005, or even greater than 0.01. In the preceding formula, it is of course always strictly greater than 0 and strictly less than 2. y is generally less than or equal to 0.5. Preferably, y is less than or equal to 0.25 and very preferably y is less than or equal to 0.1. In the preceding formula, y 'is of course strictly greater than 0 and strictly less than 1. y' is generally less than or equal to 0.6. Preferably, y 'is less than or equal to 0.3 and most preferably y' is less than or equal to 0.15. M is for example selected from the group consisting of Y ³⁺ , Sc ³⁺ or rare earths.

M 'is for example selected from the group consisting of Ca ²⁺ and Sr ²⁺ .

According to one particular aspect of the invention, the ionic conductor D can also play a role of support material for the catalytic system and, for this purpose, preferably has a large specific surface area even after a calcination of 800 and 1000 ^° C. In particular the ionic conductor advantageously has a specific surface area of at least 5 m ² / g, preferably at least 10 m ² / g, or even at least 50 m ² / g.

According to the invention, the ionic conductor based on zirconia is an ionic conductor by oxide ions O ^{2 ~} , the conduction being obtained by substitution of certain zirconium atoms with lower valence transition metal cations as previously described.

The trivalent or divalent substitution cations may be chosen according to the invention, in the group consisting of Y ³⁺ , Sc ³⁺ , Ca ²⁺ , Sr ²⁺ or rare earths. Good understood, this list is in no way limiting and other substitution cations, divalent or trivalent, may be considered without departing from the scope of the present invention. The ionic conductivity obtained by such a substitution is typically between 1 and 10 ^{~ 4} S / cm in the temperature range of 150 to 800 ^° C.

For the purposes of the present description, the porous inorganic material has an open porosity, conventionally measured by mercury porosimetry, greater than 10%, preferably greater than 20%, or even greater than 30%. Too little porosity of the material constituting the filtering walls leads to a too high pressure drop. Too high a porosity of the material constituting the filtering walls leads to insufficient filtration efficiency.

According to one possible mode, the porous inorganic material is based on silicon carbide SiC, preferably recrystallized at a temperature of between 2100 and 2400 ^° C. In particular, the inorganic material may be based on doped SiC, for example with aluminum or nitrogen, and such that its electronic resistivity is preferably lower than 20 Ohm. cm at 400 ⁰ C, more preferably less than 15 Ohm. cm at 400 ⁰ C and more preferably less than 10 Ohm. cm at 400 ^° C. By the expression "SiC-based" is meant in the sense of the present description that the material consists of at least 25% by weight, preferably at least 45% by weight and so very preferred at least 70% by weight of SiC. Catalyst A most often comprises at least one precious metal selected from Pt and / or Pd and / or Rh and / or Ag and / or Au and / or transition metals, especially Cu, Fe, Ni, Co, and transition metal oxides such as Mn ₂ θ3, C0 ₃ O ₄ . Catalyst A used for the reaction of For example, the reduction is selected from catalysts well known in the art for their activity and preferably their selectivity to NOx reduction reactions. They can in particular be chosen from rare earths which can help to fix or trap NOx deposited in admixture with an active ingredient including precious metals or transition metals according to the known technique.

The catalyst B most often comprises at least one precious metal selected from Pt and / or Pd and / or Rh and / or Ag and / or Au and / or transition metals, in particular Cu, Fe, Ni, Co and / or or the oxides of the transition metals. Without departing from the scope of the invention, surprisingly, the catalyst B can advantageously be chosen from catalysts otherwise well known in the art for their activity and preferably their selectivity vis-à-vis the oxidation reactions of hydrocarbons . In particular, the reforming and steam reforming catalysts used in the field of petrochemistry and refining can be used according to the invention as catalyst B. Those deposited in admixture with an active ingredient including precious metals Pt, Pd, Rh, or transition metals, especially Cu, Fe, Ni, Co, can be effectively used according to the invention.

According to the invention, the device as just described can be implemented according to the various methods that will now be exposed. The following operating modes are representative of certain preferred modes of the invention, but it is understood that other modes are also possible, without departing from the scope of the invention.

According to a first possible mode of implementation of the device, the ionic conductor D is reduced beforehand by electrochemical activation and it imposes periodically a potential or a negative current between the electrodes W and CE, to maintain in the reduced state the ionic conductor at least at the interface between the layer D and the catalyst A during the phases of operation of the engine in which the structure honeycomb is fed by a gas mixture low in fuel.

For example, according to this first embodiment, the ionic conductor based on zirconia is reduced beforehand by electrochemical activation. This electrochemical activation consists of a negative electric polarization of high amplitude, that is to say of at least -2.2

V, or at least -4V, between the reference electrode W

(Also called working electrode W), for example connected to catalyst A and the counter-electrode CE, for example connected to catalyst B. Preferably this polarization is carried out at a temperature of at least 200 ^° C., and more preferably at least 400 ^° C. for at least a few minutes and at most for one hour.

Such a polarization has the effect of locally reducing the ionic conductor at least at the interface between the layer D and the catalyst A, that is to say, to bring it into a deficit state (i.e. under stoichiometric) in oxygen. For the purpose of the present description, the negative potential imposed between the electrodes W and CE corresponds to the minimum value, deduced from the ohmic drop of the system, to electrochemically reduce the ionic conductor layer D, at least on the electrode side W.

The duration of the polarization is deliberately short (for example from 1 to 60 minutes) in order to reduce the layer D to a small thickness, preferably a few microns, for example from 1 to 100 microns, compatible with a surface adsorption mechanism . The inventors have observed that this polarization makes it possible to significantly and unexpectedly increase the storage capacity of the NO _x of the catalyst pair A / ionic conductor D. According to this first mode, the polarization of the layer D is not continuous but is triggered periodically during or immediately after an engine operating phase in which the honeycomb structure is fed with a fuel-rich mixture (air / fuel stoichiometry less than 1). By applying such a polarization, it can thus be ensured that the oxygen sub-stoichiometry of the layer D is maintained and regenerated at each passage in a rich medium. In another aspect, in the event of significant loss of catalytic activity detected by a downstream NO _x sensor, for example more than 30%, an electrochemical activation of the ion conductor D resets the performance of the pollution control system.

According to a second embodiment, alternative or complementary to the first, the polarization is this time applied continuously, but is reversed periodically. This operating mode, which can be activated unlike the previous one without the need for a fuel-rich phase, allows during a first phase to increase (unexpectedly) the NO _x storage capacity by the catalyst pair. A / ionic conductor D. During a second phase, by polarity inversion

(resulting from the application of a current or a positive voltage between W and CE), the reducing catalyst torque

A / ionic conductor D is regenerated, i.e.

NO _x previously stored during the previous phase by the catalyst pair A / ionic conductor D are released in the form of NO _{x in} particular NO 2 and / or NO and / or N ₂ O. The NO _x , thus in sufficient concentration, can then be reduced essentially by means of electrochemical activation on catalyst B.

According to this mode of operation, one alternates:

negative polarization phases, in order to maintain in a reduced state (that is to say in a sub-stoichiometric oxygen surface state) the ionic conduction layer D in contact with the catalyst A, said phase of polarization leading surprisingly to the storage of NOx, and

phases of positive electrical polarization between W and CE, during which the nitrogen oxides are converted into nitrogen gas.

According to a third embodiment according to the invention, alternative to or complementary to the previous ones, alternating the two regeneration paths of the reduction catalyst A, as described previously in connection with respectively the first and second embodiments above. According to this mode, the decomposition of the nitrates is alternately obtained during certain phases by the introduction of a rich fuel mixture and in other phases by a positive electric polarization between W and CE, according to the operating modes previously described.

In this third embodiment, the reduced state of the layer D (oxygen sub-stoichiometry) can be preserved and maintained by the operating phases during which the structure is fed with a fuel-rich mixture. According to one possible embodiment applying to all the modes previously described, the electrochemical activation of the layer D by application of a negative voltage or current between W and CE can be triggered in the event of a significant loss, for example of at least 30%, of the conversion to NO _x by the device.

Of course, other modes of operation of the device can be deduced without difficulty from the foregoing. It is particularly understood that all these modes, even if they are not explicitly described in the present description, should be considered as an integral part.

The present invention is particularly applicable in structures used for the purification and filtration of an exhaust gas of a diesel engine. Such structures, generally referred to as particle filters, include one and preferably a plurality of monolithic honeycomb blocks. Unlike the purification devices described above, in such filters, the block or blocks comprising a set of adjacent ducts or channels of axes parallel to each other separated by porous walls, closed by plugs to one or the other. other of their ends to define inlet ducts opening on a gas inlet face and outlet ducts opening on a gas evacuation face, so that the gas passes through the porous walls. Examples of such assembled or unassembled structures are for example described in publications EP 0816065, EP 1142619, EP1306358 or EP 1591430. In such filter structures, the gases are forced through the walls. The work carried out by the Applicant has shown that the use of an electrochemical catalyst system as described above surprisingly allows on the one hand a very good conversion of the polluting species without substantially increasing the pressure drop generated by the introduction of the filter on the exhaust line.

Such a system can also contribute to improving the regeneration efficiency of the filter by promoting a higher oxidation rate of the soot.

In a preferred embodiment of the invention, which is however not limiting, the filter body according to the invention is thus essentially constituted of a porous silicon dioxide matrix, optionally doped, which constitutes the walls of a filtering structure. Honeycomb. In a first step, the catalyst B is deposited for example by impregnation in the porosity of the walls of the filter body. According to the invention, the catalyst B is deposited in proportions and conditions which preferably allow the particles constituting it to be percolated between them, in the form of a film or an electronically conductive network. Without departing from the scope of the invention the particles may be deposited on a catalyst support consisting of an electronically conductive material.

In a second step, the ionically conductive material D is then deposited on the surface of the impregnated SiC walls and covered by the catalyst B in a layer whose thickness is preferably adjusted to remain permeable to the exhaust gases passing through the wall of the filter and to lead to a percolating ion driver network. In a third step, the second reducing catalyst A is then deposited on the layer of the ionic conductive material, for example according to conventional impregnation techniques, such that the layer of material D electrically isolates the two catalysts. According to the invention, just like catalyst B, catalyst A is deposited in proportions and conditions permitting the percolation of the particles constituting it, in the form of a film or an electronically conductive network.

A complete implementation of the method for obtaining a filter according to the invention is illustrated in the following description, in connection with the appended figures among which:

Figure 1 is a schematic representation in section of a wall piece of a catalytic filter obtained according to the invention, incorporating in its porosity a catalyst B (step 1). Figure 2 is a schematic sectional representation of a wall piece of a catalytic filter obtained according to the invention, incorporating in its porosity catalyst B covered by a layer of ionic conductive material D (step 2). FIG. 3 is a diagrammatic representation in section of a wall piece of a catalytic filter obtained according to the invention, incorporating in its porosity catalyst B covered by a layer of an ionic conductive material D, said material D separating catalysts A and B (step 3).

Figure 4 is a schematic representation in section of a portion of a catalytic filter obtained according to the invention, wherein the inlet channels are blocked by plugs. FIG. 5 shows an elevational view of the inlet 5a and outlet 5b faces of the filtration system according to the invention.

Figure 6 illustrates the experimental device used in the examples to demonstrate the effectiveness of a device according to the present invention.

In the embodiment shown in FIG. 1, a longitudinal sectional view of the parallel walls 1 of a conventional honeycomb particle filter has been schematized from the front face 5 of the gas inlet. example as described in applications EP 816065 or EP 1142619.

The constituent material of the walls 1 of the filter consists of an electronically conductive material, for example SiC doped with aluminum Al, whose open porosity is typically close to 45%. The filter alternately comprises input channels 2 and output channels 3, depending on the position of the plugs. In the honeycomb structure not yet clogged as shown in FIG. 1, the catalyst B is deposited by a sol-gel process and according to well-known methods, in the form of nanometric particles 4 of Rh metal. particles are thus dispersed in the porosity of the electron-conducting SiC, inside the walls 1 of the channels 2, 3. The impregnation can be carried out only in the lower part of the filter, for example on the first third of the length of the channels from the front face 5 of the gas inlet. Of course, the impregnation can, without departing from the scope of the invention, relate to a more or less important part of the walls 1, in particular to limit the losses due to the presence of the catalyst and fully retain the structure its role filtration. As shown in FIG. 1, the impregnation with the catalytic solution is carried out under conditions permitting the diffusion of the catalytic solution throughout the thickness of the walls 1. However, it would not be possible to depart from the invention if only the more external walls were impregnated. In general, the impregnated thickness of the walls 1 is at least of the order of 500 nm. After this first impregnation with the catalyst B, the outlet channels 3, on the side of the front face 5 of the honeycomb structure of the uncapped filter, are masked with a resin or a wax 6 that can be eliminated by a simple heating at a temperature typically less than or equal to 300 ⁰ C, so as not to deteriorate the catalyst layer already present, according to masking techniques well known to those skilled in the art. The structure is then dried.

As illustrated in FIG. 2, a sol-gel deposition of the ionic conductor D is then performed.

According to an exemplary embodiment, the initial sol-gel solution used for the deposition is obtained by mixing the nitrates of Yttrium Y (NO ₃ ) (H ₂ O) ₆ and Zirconium ZrO (NO ₃ ) ₂ (H ₂ O) ₆ in respective proportions resulting in an amount of 8 mol% of Y ₂ θ 3 in the yttria zirconia layer 7 finally obtained after heating. According to the invention, the deposited layer may protrude beyond the portion 4 of the walls impregnated by the catalyst B. The porosity of this ionic conducting layer 7 is chosen to be sufficiently low to ensure good ionic conduction. In addition, the thickness of the ionic conducting layer 7 is also sufficiently small and porous to allow it to pass through the exhaust gases during operation. The resin masks 6 are then removed and the layer 7 of ionic conductor is matured, by a thermal treatment under air at 300 ^° C. The treatment makes it possible to clear the outlet channels 3. It may be necessary to finely grind the surface of these channels to finish removing the resin residues. After removal of the mask, the filter is preferably again impregnated in the sol-gel solution of the ion conductor D so as to cover only the lower end 8 of the channels, for example to a height of about 1 mm.

The output channels 3 are then masked again at the front face 5 of the filter, with the same resin 6 as previously described.

A solution of catalyst A is prepared from Pt salts. According to well-known techniques, the solution is matured in the form of a sol gel and then the catalyst filter A is impregnated with this solution by simple immersion as previously under form of a film or layer 9 disposed on the surface or in the porosity of the ionic conductive layer 7. According to one possible mode, a deposition of a SiC layer can be carried out before the deposition of Pt, in order to ensure the electrical percolation and the good dispersion of the Pt. The resin masks 6 are then removed by a thermal treatment under air at 300 ^° C. The treatment makes it possible to clear the outlet channels 3. It may be necessary to finely grind the surface of these channels to finish removing resin residues.

According to conventional techniques, the input channels are then masked with a resin or a suitable paper, according to techniques well known in the art. On the front face 5 of the filter, the outlet channels 3 of the filter, which are not masked by the resin, are then closed off by electronic conductive plugs 11, as shown in FIG. 4. The plugs 11 are conventionally obtained by the application, on the side of the front face 5 and at the end of the outlet channels 3, of a SiC-based slip doped with aluminum to increase the electronic conduction, followed by a heat treatment of solidification at 500 ^° C.

According to the same method already used for the inlet plugs, on the rear face 10 of the filter, the inlet channels are then closed by plugs 12 which are also electronically conductive. The method thus used then leads to a filter illustrated schematically in FIG. 4.

FIG. 5 shows an elevational view of the inlet 5a and outlet 5b faces of a filtration system according to the invention produced from the filter shown in FIG. 4.

As illustrated in FIG. 5 and previously described, the plugs 11 obstructing the output channels 3 on the input face (FIG. 5a) and obstructing the input channels 2 on the output face (FIG. 5b) are consisting of conductive SiC. On the inlet face, all plugs 11 are interconnected by metal wires 13 to constitute the reference electrode W

(Figure 5a). The reference electrode W is thus directly in contact with the catalyst A as shown in FIG. 4.

The catalyst B is placed on the surface and / or in the walls of the filter in the form of particles 4. The catalyst particles B are thus in contact with the electron-conductive SiC walls of the monolith but electronically isolated from the catalysts A by the D-layer. solid electrolyte.

All the plugs 12 of the output face of the filter (FIG. 5b) are interconnected by metal wires 14 to form the counter-electrode CE. The counter CE electrode is thus obtained by the electronic contact of all layers or catalyst films B.

In operation, the application of a negative potential difference U between W and CE has the effect of causing the migration of part of the O ^{2 ~} ions contained in the ionic conductive layer D. According to an astonishing aspect never described, such an application promotes, within the filtration structure, the storage and concentration of NO _x at the interface between the layer D and the catalyst A, as well as their reduction in a subsequent phase.

In operation, the combustion gases from the engine enter the filter structure via the inlet channels 2 and are in direct contact with the layer 9 of the catalyst A. The gases then pass through the porous walls 1 and then come into direct contact with the catalyst. the layer 4 of the catalyst B. The gases, substantially free of their pollutants, are then discharged through the outlet channels 3.

The invention is however not limited to the embodiment as previously described but must be understood as referring to any catalyzed filtering structure comprising at least a catalyst A and a catalyst B and polarized according to the above principles. The filtration structures in accordance with the invention comprise inlet channels and adjacent outlet channels delimited by porous walls whose ends are alternately plugged on one side and then on the other, according to conventional particle filter technology. for internal combustion engine. According to an essential characteristic of the invention, the catalyzed filter is polarized by the application of an electric current or a voltage typically from an electric generator, preferably such that the current density applied between electrode W and the counter electrode CE is greater than 0.01 mA / cm ² , preferably greater than 0.05 mA / cm ² and typically between 0.01 and 10 mA / cm ² . The surface taken into account is the sum of the specific surfaces of the deposit A and the deposit B. The invention also relates to an exhaust line incorporating the combustion gas depollution device of a diesel engine preferably equipped as described above. . The device may comprise in particular a 0 to 10 Volts absolute voltage setting system and an associated control system according to the engine speed to other data provided by sensors, in particular one or more NO _x sensors.

The device according to the invention may also comprise additional means allowing the implementation of an onboard control of the catalytic activity of the catalyst. According to a possible embodiment, the control can consist simply of the stopping of the polarization and the measurement of the remaining potential difference between the electrode W and the counterelectrode CE. From reference values, typically established on the basis of model cells, corresponding to the potential difference measured without voltage, it is possible to estimate in a simple and fast way the efficiency of the catalyst and to apply electrochemical activation if necessary to generate the reduction of catalyst A and thus promote the catalytic efficiency of the system. This onboard control of the catalytic efficiency is complementary to that of the NOx sensor. According to another possible embodiment, the measurement of the voltage difference potential can induce or participate among other parameters (pressure drop, weight of the filter, etc.) in the choice of the threshold of tripping or stopping of a regeneration phase.

The electrochemical system according to the present invention is further adapted not only to the reduction of NO _x but also allows, jointly, at the level of catalysts A or B and depending on their respective electric polarization, the oxidation of soot (solid particles) present. in the walls of the particulate filter, or the oxidation of the gaseous reducing species contained in the exhaust gas such as unburnt hydrocarbons and / or carbon monoxide and / or HC + H ₂ O type steam reforming reactions - ^ 3/2 H ₂ + CO and / or reactions of gases with water of CO + H ₂ O -> H ₂ + CO _{2 type} .

The electrochemical system according to the present invention has many advantages:

the electrons necessary for the NO _x reduction reaction, or even the anions required for the combined oxidation reaction of HC and / or CO, are advantageously directly supplied by the support, which makes it possible to ensure the operation of the electrochemical system with a quantity of conductive species that is almost constant and not limited by the size of the solid electrolyte or electrolytes,

the operation of the electrochemical catalysis system is improved since all the catalyst particles A and B are active: the electrochemical system is active regardless of the relative disposition of the particles of the catalysts A and B in the porosity of the material constituting the walls,

- the use of additional material for the proper functioning of the system and the storage of NOx, Barium type is no longer necessary, the storage function being provided by the electrochemical system itself, in particular by the ionic conductor of the type according to the invention,

the electrons and anions necessary for the electrochemical de-stocking of the NOx (before their reduction) are provided electrochemically and by the polarization of the system, probably according to a reaction of the type:

ZrO (NO ₃ ) ₂ + O ^{2 "} → ZrO ₂ + 2 NO ₂ + O ₂ + 2 e ^" As never described in the literature in this field, the tests carried out by the applicant have also shown that the application of a negative bias according to the invention, that is to say the application of a negative potential difference between the reference electrode and the counter electrode (resulting for example by the contribution of electrons at the of the reference electrode) allowed not only the effective implementation of the electrochemical system according to the invention but also had the effect of significantly increasing the storage capacity of NO _x even in the absence of alkali or alkaline metals. in addition to and in the vicinity of the reduction catalyst A, and this even during the operating phase of the lean mixture type, that is to say in the presence of substantial amounts of oxygen. Such a property allows, within the catalyzed filter according to the invention, a continuous electrochemical reduction of the NO _x , whether during the operating phases of the engine in a rich mixture, corresponding according to the invention to a NO _x destocking phase, or also surprisingly during the operating phases of the engine lean mixture, a sensitive portion of the NOx being directly captured and stored by the catalyst A, in contact with the ion conductor D maintained according to the invention, in a reduced state, before being reduced to the catalyst B.

The tests carried out by the applicant have also shown that an operation of the device which is the subject of the present invention, by successive cycles of the negative bias / positive bias type, also makes it possible on the one hand to efficiently store NOx and to concentrate them efficiently during a desorption phase to promote their transformation into nitrogen gas N ₂ . Such an operation advantageously makes it possible to limit or even get rid of the operating phases in a rich mixture and consequently to avoid overconsumption of fuel.

These advantages are particularly illustrated by the following exemplary embodiments:

EXAMPLE 1

An electrochemical catalysis system 100 was carried out on the basis of a dense yttria-stabilized zirconia tube 101 (ZrO 2) o, 92 (Y 2 O 3) 0.08, 42 cm long and 3 mm thick. The tube constitutes the ionic conductor D (O ^{2 ~} ion conduction). As shown in FIG. 6, two platinum films 102, 103 are deposited from a platinum lacquer respectively on the two outer and inner faces of the tube, over a distance d of 6 cm.

The tube is calcined at 500 ^° C. for 1 hour in air after each deposition step.

The two films of Pt 102, 103 thus constitute respectively the reference electrode and the counter- electrode for the inner face and are connected to means 104 for applying a bias voltage. The reference electrode 102 is exposed on the outside face to the reaction mixture consisting of a gas comprising nitrogen monoxide and oxygen as described in Table 1 while the electrode 103 is kept in air.

EXAMPLE 2 The same type of system is realized as above but the yttrie zirconia is reduced by a so-called "electrochemical pre-activation" step by means of a negative polarization of -4 V, ohmic drop included, for 1 hour at 400.degree. ⁰ C by applying a potential difference between the reference electrode and the counter-electrode for the inner face.

On each system of Example 1 and Example 2 several cycles were then carried out to obtain an average value representative of the storage capacity in a mixture rich in oxygen.

Between each series of measurements, the surface of the catalyst Pt is regenerated by a scanning under a current of a neutral gas (H _e ) so as to achieve the destocking of all the nitrogen oxides, without reducing agent and consequently without catalytic reduction of NOx by a reducing gas is possible. In this case, only the electrochemical reduction can occur. Experimental measurement data are provided in Table 1.

Table 1: Experimental data

The experimental results are reported in Table 2:

Table 2: Experimental results

The experimental results reported in Table 2 show that the storage capacity of the system of Example 1, in which a non-prereduced zirconia is used and no NOx trap of alkali or alkaline earth metal type, is very weak or non-existent. . The system according to Example 2 with pre-reduced zirconia shows a significant increase in the storage capacity of the nitrogen oxides of the catalytic system despite the presence of a large amount of oxygen in the gas to be treated and a specific surface area of YSZ exposed to extremely low gas (less than 1 m ² / g), which had never been observed until then. We also observe with example 2 a significant conversion by lane electrochemical NOx in N ₂ during the destocking phase.

Example 3: The system described in Example 2 was subjected to polarization cycling.

In a first step, the reference electrode and the counter-electrode are connected to a generator 104 and subjected to a bias voltage of -4 volts for 5 minutes.

In a second step, the system is subjected to a positive bias of +4 V volts for about 30 seconds.

The adsorbed NOx content in a first cycle was first measured, and so on until a tenth negative and positive polarization cycle.

Experimental measurement data are provided in Table 3.

Table 3: Experimental data

The experimental results of the tests (polarized or unpolarized device) are reported in Table 4.

Table 4: Experimental results

The experimental results reported in Table 4 by comparison with Example 1 (see Table 2) show an increase in the nitrogen storage capacity of the catalytic system when the system is polarized despite the presence of a significant amount of oxygen in the gas to be treated.

As has never been described, it is observed that this storage capacity for nitrogen oxides is maintained even after a significant number of cycles, which demonstrates the effectiveness of the positive polarization for regenerating the catalyst, that is to say say electrochemically break down the nitrates stored on the ionic conductor.

EXAMPLE 4

An electrochemical catalysis system 100 was made on the basis of porous tubes made of zirconia 105 stabilized with yttrium (ZrO 2) o, 92 (Y 2 O 3) 0.08, of length 42 cm and thickness 3 mm, placed in a reactor. quartz 106. The tube constitutes the ionic conductor D (conduction by O ^{2 ~} ion). As shown in FIG. 7, a film 102 of platinum and a film 103 of Rh are deposited from platinum and rhodium lacquers on the two inner and outer faces of the tube respectively, over a distance d of 6 cm. The tube is calcined at 500 ^° C. for 1 hour in air after each deposition step.

The two films 102 and 103 respectively of Pt and Rh respectively constitute the working electrode and the counter electrode and are connected to means for applying a bias voltage 104. In contrast to the preceding examples, the tube porous with these two electrodes is exposed to the same reaction mixture on the inner and outer faces of the tube.

EXAMPLE 5

The same type of system is realized as previously in Example 4, but the yttrie zirconia is this time reduced by a so-called "electrochemical pre-activation" step by means of a negative bias of -4 V, including ohmic drop, during 1 hour at 400 ^° C. by applying a potential difference between the working electrode and the counter-electrode for the internal face.

On each system of Example 4 and Example 5 several cycles were then carried out to obtain an average value representative of the storage capacity in a mixture rich in oxygen.

Between each series of measurements, the surface of the catalyst Pt and the catalyst Rh is regenerated by scanning under a current of a neutral gas (H _e ) so as to achieve the destocking of all the nitrogen oxides, without reducing agent and therefore, no catalytic reduction of NOx by a reducing gas is possible. In this case of f igure, only the electrochemical reduction can occur.

Experimental measurement data are provided in Table 5.

Test conditions

Preparation Porous tube (40%) of YSZ (ionic conductor) sample

(Pt / YSZ tube) Composition of the gas to be purified: NO / O ₂

Content:

Storage phase (= oxidizing): NO (1200 ppm) O ₂ : 0.7%, 2% and 6% (O ₂ pressure) Destocking phase: He

Gas flow rate: 4.3 1 / h Temperature: 400 ⁰ C

Location of measurement sensors: thermocouple (Temperature) placed at 3 mm (approximately) from the surface of the catalyst, gas analysis is carried out at the reactor outlet by IR + μGC analyzers.

Polarization Measure 1: Measure 2: Electrical None -Polarization Voltage = -4V [Oxide ion migration from the reference electrode to the electrolyte (D layer)]

- Limit current (max) induced in the storage phase: -50 μA / cm ² (for 2% and 6% of O ₂ ) and -30 μA / cm ² (for 0.7% O ₂ )

-Nature of conductors: ionic conductor by 0 ^{2 ~} (YSZ)

Table 5: Experimental data

The experimental results are reported in Table 6.

Table 6: Experimental results

The experimental results reported in Table 6 show that the storage capacity of the system of Example 4, in which a non-prereduced zirconia is used and no NOx trap of alkali or alkaline earth metal type, is very weak or non-existent. . The system according to Example 5 with pre-reduced zirconia shows a significant increase in the storage capacity of the nitrogen oxides of the catalytic system despite the presence of a large amount of oxygen in the gas to be treated and a specific surface area of YSZ exposed to extremely low gas (less than 5 m ² / g), which had never been observed until then. We also observe with Example 5 a significant electrochemical conversion of NOx to N2 during the destocking phase.

Example 6

The system described in Example 5 was subjected to polarization cycling.

First, the working electrode and the counterelectrode are connected to a generator 104 and subjected to a bias voltage of -4 volts for 5 minutes. In a second step, the system is subjected to a positive polarization of + 4 V volts for about 30 seconds. The adsorbed NOx content in a first cycle was first measured, and so on until a tenth negative and positive polarization cycle. Experimental measurement data are provided in Table 7.

Table 7: Experimental data

The experimental results of the tests (polarized or unpolarized device) are reported in Table 8.

Table 8: Experimental results

The experimental results reported in Table 8 by comparison with Example 4 of Table 6 show an increase in the nitrogen storage capacity of the catalytic system when the system is polarized despite the presence of a large amount of oxygen in the system. gas to be treated.

We observe that this storage capacity of nitrogen oxides is maintained even after a significant number of cycles which demonstrates the effectiveness of the positive polarization to regenerate the catalyst, that is to say, electrochemically decompose the nitrates stored on the ionic conductor.

Claims

An apparatus for purifying a polluted gas, for example an exhaust gas from a diesel engine or gasoline, comprising: a. one and preferably a plurality of monolithic honeycomb blocks constituting a particle filter, said block or blocks comprising a set of adjacent channels (2, 3) with mutually parallel axes separated by porous walls (1), plugged by plugs (11, 12) at one or other of their ends (5, 10) to delimit inlet ducts (2) opening on an inlet face (5) of the gases and outlet ducts (3) opening on a discharge face (10) of the gases, such that the gas passes through the porous walls (1) in which at least one porous electronic conductive material forms the walls of said structure , b. an electrochemical system for treating said gas comprising: a layer (7) of an ionic conductor material D by oxygen ions and an electronic insulator, the ionic conductor D comprising or consisting of a zirconium oxide partially substituted with a trivalent M ³ cation ⁺ or divalent M ' ²⁺ , a catalyst A (9) for converting NOx to NO2, in contact with the ion conductor D and electrically connected to a W electrode and - a B (4) reduction catalyst NOx in N ₂ in contact with the ion conductor D and electrically connected to a counter-electrode CE, the catalyst B being in electrical contact with the electronic conductive material forming the walls (1) of the honeycomb and said layer (7) of ion conductor D being disposed between the catalyst A and the catalyst B, c. means for applying a voltage or a current between said electrode W and said counter-electrode CE, such that said zirconium oxide layer can be maintained in a reduced state by the application of said voltage or said electric current.

2. Device according to claim 1, wherein the plugs (11) obstructing the outlet channels (3) on the front face (5) of the filter and the plugs (12) obstructing the inlet channels (2) on the face back (10) of the filter are ionic conductors, and are electrically connected to each other to constitute respectively the reference electrode W and the counter-electrode CE.

3. Device according to claim 2, wherein the electronic conductive plugs consist of SiC doped with aluminum.

4. Device according to one of the preceding claims, wherein the catalyst A is chosen to also be suitable for the oxidation of soot.

5. Device according to one of the preceding claims, wherein the catalyst B is chosen to also be suitable for the oxidation of polluting species HC, CO or H ₂ hydrocarbon type.

6. Device according to one of the preceding claims, wherein the ionic conductor responds to the formulation (Zrθ2- _X) i- _y (M ₂ θ3_ _x) _y wherein M is a cation of valency 3 preferably selected from the group consisting of Y ^3+, Sc ^3+, or rare earths and being strictly greater than 0 and strictly less than 2.

7. Device according to claim 6, wherein y is less than or equal to 0.5, preferably wherein y is less than or equal to 0.25 and very preferably in which y is less than or equal to 0.1.

8. Device according to one of claims 1 to 5, wherein the ionic conductor corresponds to the formulation (ZrO ₂ - _x ) iy _' (M'Oi- _x ) y _' , M 'being a cation of valence 2 preferably selected from the group consisting of Ca ²⁺ and Sr ²⁺ and y 'being strictly greater than 0 and strictly less than 2.

9. Device according to claim 8, wherein y 'is less than or equal to 0.6, preferably wherein y' is less than or equal to 0.3 and very preferably wherein y 'is less than or equal to 0 15.

10. Device according to one of claims 6 to 9, wherein x is less than 0.5, preferably wherein x is less than 0.1, and wherein x is greater than 0.005, preferably wherein x is greater than 0.01.

11. Device according to one of the preceding claims, wherein the ionic conductive material D has a specific surface area of at least 5 m ² / g.

12. Device according to one of the preceding claims, wherein the porous inorganic material is based on silicon carbide SiC, optionally doped, for example by aluminum or nitrogen, so that its electronic resistivity is less than 20 Ohm. cm at 400 ^° C., preferably less than 15 ohm. cm at 400 ⁰ C and very preferably less than 10 Ohm. cm at 400 ° C.

13. Device according to one of the preceding claims, wherein the catalyst A comprises at least one precious metal selected from Pt and / or Pd and / or Rh and / or Ag and / or Au and / or transition metals, including Cu, Fe, Ni, Co, and transition metal oxides such as Mn ₂ θ 3, CO ₃ O 4 and / or in which the catalyst B comprises at least one precious metal selected from Pt and / or Pd and / or Rh and / or Ag and / or Au and / or transition metals, especially Cu, Fe, Ni, Co and / or transition metal oxides.

14. Method of implementation of a device according to one of the preceding claims, wherein the ionic conductor D is reduced beforehand by electrochemical activation and wherein periodically imposes a potential or a negative current between the electrodes W and CE. to keep the ion conductor in the reduced state at least at the interface between the layer D and the catalyst A during the operating phases of the engine in which the honeycomb structure is fed with a gas mixture rich in fuel .

15. Method of implementation of a device according to one of claims 1 to 13 wherein the polarization is applied continuously and reversed periodically and in which is alternated therefore: negative polarization phases, to maintain in a reduced state the ionic conduction layer D in contact with the catalyst A, said polarization phase leading to the storage of NOx, and positive electric polarization phases between W and CE, during which the nitrogen oxides are converted to nitrogen gas.

16. Method of implementation of a device according to one of claims 1 to 13 wherein it implements alternately one or other of the methods described in claims 14 and 15.