WO2011064655A2

WO2011064655A2 - Particulate filter, catalytic compositions useful for regenerating said filter and processes for their preparation

Info

Publication number: WO2011064655A2
Application number: PCT/IB2010/003039
Authority: WO
Inventors: Marco Arimondi; Luca Castellani; Evgeny Kopnin; Agusti Sin Xicola
Original assignee: Pirelli & C. S.P.A.
Priority date: 2009-11-27
Filing date: 2010-11-26
Publication date: 2011-06-03
Also published as: WO2011064655A3

Abstract

A particulate filter (1) is described comprising a filtering body (5) formed by a porous substrate and provided with a plurality of gas channels (6', 6") defined by gas- permeable walls (7) of said substrate, said gas-permeable walls (7) being coated with a catalytic composition comprising: a) a metallic phase having the formula: Ag_1-xPd_x, wherein x is a number comprised between 0 and 0.2; and b) a cerium oxide doped with at least one rare earth group metal having the formula: Ce_1-yR_yO_2-Y/2, wherein R is a rare earth group metal selected from Gd, Sm, Eu, La, and mixtures thereof and y is a' number comprised between 0.1 and 0.5. Catalytically active compositions useful for regenerating a filter (1) of this type, the use of these compositions as a catalytic active coating of a filtering element of an exhaust gas emitted by an internal combustion engine (4), as well as processes for preparing the filter (1) and compositions are also described.

Description

PARTICULATE- FILTER, CATALYTIC COMPOSITIONS USEFUL FOR REGENERATING SAID FILTER AND PROCESSES FOR THEIR PREPARATION

DESCRIPTION Background of the invention

The present invention refers to a particulate filter, to catalytic compositions useful for regenerating this type of filter, to the use of these compositions as a catalytic active coating of a filtering element of an exhaust gas emitted by an internal combustion engine, as well as to processes for preparing the filter and the aforementioned compositions.

Within the framework of the description and in the subsequent claims, the term "particulate filter" is used to indicate a filtering element of suitable configuration adapted to hold soot particles present in the exhaust gases of an internal combustion engine, in particular diesel engines. Generally, filters of this type are known in the art with the acronym DPF (Diesel Particulate Filter).

Within the framework of the description and in the subsequent claims, the term "catalytically active" is used to indicate the ability of the compositions described herein or of a coating layer including the same, to promote the burning reaction of soot particles accumulated in the filter above a critical temperature indicated hereinafter by the term ignition temperature or Tj.

Related art

As is known, the automotive industry is currently strongly committed - also due to the pressure of ever more stricter regulations - to find effective solutions to the problem of removing particles of unburnt material present in the exhaust gases of internal combustion engines, in particular diesel engines. These particles generally consist of a particulate material (known with the acronym PM) in form of particles of carbonaceous material, generally referred to in the field by the term soot, being extremely small in size and, as such, responsible for the greatly undesired pollution of the so-called "fine particulate" kind and source of considerable environmental and public health problems. In order to remove the soot particles from the exhaust gases, it was proposed in the art to mount on the exhaust conduits of the engine filtering systems comprising a particulate filter capable of holding the particles which are then burnt at high temperature by means of an operation known with the term of filter regeneration.

This operation, however, cannot be easily carried out essentially because the oxidation of the soot particles accumulated in the filter can occur solely upon reaching a critical temperature, or ignition temperature Tj, which cannot be easily achieved by the exhaust gases of the engine, especially during the use of the vehicle at a low speed for example during urban circulation.

In order to overcome in some way this drawback and thus promoting a more efficient regeneration of the filter, the so-called "active" regeneration methods were proposed, for example by providing additional heat to the filter from an external source or, on alternatively, by reducing the ignition temperature of the soot particles so as to obtain their combustion in an easier way.

Thus, for example, the particulate filter can be provided with heating devices periodically activated for heating the exhaust gases upstream of the filter or, as proposed by the vehicle manufacturer PSA Peugeot Citroen, by providing the vehicle with a special tank wherein a cerium oxide precursor (ceria) is stored which precursor is mixed with the fuel and which has the double characteristic of promoting the formation of agglomerates of particles without a chemical bond (more easily "withheld" by the filter) and lowering the ignition temperature of the soot particles. Then, the filter is periodically "cleaned" by burning the captured agglomerates: this operation is carried out by means of a fuel post-injection thus making the exhaust gases hotter, which exhaust gases heat in turn the agglomerates in the filter burning them.

In order to promote a more efficient regeneration of the filter, the so-called "passive" regeneration methods were also proposed, which are essentially based on the action of catalytic compositions which are suitably supported and distributed inside the filter, in particular on the walls of the gas channels formed in the filter, which catalytic compositions allow to suitably reduce the ignition temperature of the soot particles to values which are more easily achieved by the exhaust gases when the vehicle is travelling. Within the context of particulate filters which may be regenerated by means of the so- called "passive" methods, document EP 1 356 864 discloses filters including catalytic compositions without platinum group metals (known in the art with the acronym PGM - Platinum Group Metals) comprising cerium oxide stabilized by silver and/or cobalt, which catalytic compositions would facilitate the oxidation of soot particles during the regeneration of a particulate filter.

In an embodiment, EP 1 356 864 discloses a particulate filter comprising a filtering body formed by a porous substrate made of cordierite and provided with a plurality of gas channels defined by gas-permeable walls of the substrate on which a base layer including cerium oxide stabilised with a combination of Zr, Sm and Y and a catalytic layer (washcoat) applied on the base layer and comprising Ag and cerium oxide stabilized with a combination of the same metals Zr, Sm and Y of the rare earth group, are formed in succession.

In connection with particulate filters which may be regenerated by means of the so- called "passive" methods, on the other hand, document EP 1 820 561 discloses a particulate filter comprising a filtering body formed by a porous substrate (for example made of cordierite) and provided with a plurality of gas channels defined by gas- permeable walls of the substrate coated by a catalytic layer containing alumina and a composite oxide containing Ce as the main component and a rare earth group metal different from Ce or an alkaline-earth metal and Pt loaded on the alumina and on the composite oxide.

The rare earth group metal is preferably selected from Sm and Gd, while the alkaline- earth metal is preferably selected from Mg, Ca, Sr and Ba.

Summary of the invention In connection with particulate filters which may be regenerated by means of the so- called "active" methods, the Applicant observed that these filters require additional apparatuses for heating the filter or additional apparatuses for dosing ceria, hence substantially complicating both installation and control leading to an undesired cost increase. In particular, the Applicant observed that the particulate filters based on the dosage of ceria not only require the installation of a special tank of this component which should be periodically filled and of systems for regulating the correct dosage of ceria, but they also have an operation which can be jeopardised by the clogging of the porous substrate due to the formation of agglomerates of particles promoted by ceria. In connection with the particulate filters which may be regenerated by means of the so- called "passive" methods, the Applicant has experimentally found that the reduction of the ignition temperature of soot particles which may be achieved by the catalytic compositions including Ag or Pt disclosed by the aforementioned documents EP 1 356 864 and EP 1 820 561 is still insufficient to ensure a suitable regeneration of the filter especially during the use of the vehicle at a low speed for example during urban circulation.

The Applicant also observed, in connection with the particulate filters disclosed by document EP 1 820 561, that the catalytic compositions based on Pt supported on alumina and Ce oxide show a further series of drawbacks and precisely that of catalysing entirely undesirable reactions such as oxidation of S0₂ to S0₃ with the formation of sulphated ashes and particles (sulphates), that of being susceptible to poisoning by sulphur compounds (invariably present in diesel which is the fuel used in diesel engines) and that of having a considerably high cost which makes their use considerably unappealing from the economic point of view.

The Applicant has surprisingly discovered that it is possible to achieve considerable improvements in terms of efficiency and easiness of the regenerating operations of the "passive" type in a particulate filter by providing in the filter a catalytic composition forming a catalytically active layer and comprising a metallic phase based on metallic Ag and a cerium oxide doped with some specific metals of the rare earths group present in the catalytic composition in a specific range of molar % with respect to cerium.

More particularly, the Applicant experimentally observed that it is possible to achieve a considerable reduction of the ignition temperature of the soot particles withheld by the particulate filter with respect both to the filters disclosed by document EP 1 356 864, and to the filters disclosed by document EP 1 820 561 by means of the aforementioned combination of Ag with a suitably doped cerium oxide and, this, without having the disadvantages related to the use of Pt (undesired reactions, possible poisoning and cost).

More particularly according to a first aspect, the present invention relates to a particulate filter, comprising a filtering body formed by a porous substrate and provided with a plurality of gas channels defined by gas-permeable walls of said substrate, said gas- permeable walls being coated with a catalytic composition comprising: a) a metallic phase having the formula: Agi_-xPd_x, wherein x is a number comprised between 0 and 0.2; and b) a cerium oxide doped with at least one rare earth group metal having the formula: Cei_-yRy02_-y/₂, wherein R is a rare earth group metal selected from Gd, Sm, Eu, La, and mixtures thereof and y is a number comprised between 0.1 and 0.5.

Within the framework of the present description and in the subsequent claims, the term "metallic Ag" is used to indicate that silver is present in the catalytic composition in the zero oxidation state thereof and not in ionic form when the catalytic composition is subjected to analysis by X-ray diffraction (XRD).

Within the framework of the present description and in the subsequent claims, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being preceded in all instances by the term "about". Also, all ranges of numerical entities include all the possible combinations of the maximum and minimum numerical values and all the possible intermediate ranges therein, in addition to those specifically indicated herein. The Applicant experimentally observed that the efficiency of the regeneration operations of the filter can be considerably increased due to a substantial reduction of the ignition temperature of the soot particles withheld by the filter with respect to that which may be achieved using the filters of the prior art mentioned above, by coating the gas-permeable walls of the substrate which forms the filtering body with a catalytic composition formed by a first component constituted by a metallic phase comprising metallic Ag and by a second component constituted by a ceramic oxide (cermet) formed by a cerium oxide doped with at least one rare earth group metal selected from Gd, Sm, Eu, La, and mixtures thereof, wherein the molar % of the doping metal R of the rare earth group in cerium oxide is comprised between 10% and 50%. As a matter of fact, the Applicant experimentally observed that if the molar percentage of the aforementioned doping metals of the rare earth group in the doped cerium oxide is lower than 10% the catalytic activity is worse than that which may be achieved by the aforementioned catalytic composition comprising Ag and cerium oxide stabilized with a combination of the same metals Zr, Sm and Y of the rare earth group disclosed by document EP 1 356 864, while if the molar percentage of the aforementioned doping metals of the rare earth group is greater than 50% there is no improvement of the catalytic activity which instead tends to worsen again along with the increase of the molar percentage of the doping metal.

In this connection, the Applicant deems, without however wishing to be bound by any interpretative theory, that with the aforementioned specific selection of the doping metal between Gd, Sm, Eu and La and with the aforementioned specific selection of the molar % of such a metal within the doped cerium oxide, the crystalline lattice of cerium oxide is altered so as to create an oxygen ion vacancy which facilitates the transport of material within the lattice in the specific catalytic conditions which occur during the filter regenerating operations. In particular, the Applicant deems that when the molar % of the aforementioned doping metal of the rare earth group in the cerium oxide is comprised between 10% and 50% the crystalline lattice thus modified of the doped cerium oxide facilitates the transport of activated ionic species of the atmospheric oxygen which are thus capable of reaching the soot particles much more easily causing their combustion.

The Applicant also deems, without however wishing to be bound by any interpretative theory, that below or above such limits of molar % of the doping metal there is either an insufficient alteration of the crystalline lattice or, alternatively, a segregation of an oxide of the doping metal which oxide lacks per se any catalytic activity and thus negatively affects - at the same weight - the performance of the catalytic composition in the specific catalytic conditions which occur during the filter regenerating operations.

In connection with the technical effects described above, the Applicant observed that the use of Gd, Sm, Eu and La as a doping metal of the cerium oxide, that is, of metals having an atomic radius similar to that of Ce but having a lower oxidation state, achieves the two-fold advantageous technical effect of creating the aforementioned oxygen ion vacancy and the ensuing ionic mobility without however altering in a substantial way the structure of the crystalline lattice of the cerium oxide.

Advantageously, the technical effects observed with the particulate filter of the invention are achieved by using non-toxic and relatively inexpensive elements with the ensuing advantages in terms of low environmental impact and low cost.

According to the invention, the Applicant also experimentally observed that an optional addition of Pd in the metallic phase of the aforementioned catalytic composition, in a molar % not exceeding 20 mol % of the total moles of the metallic phase based on metallic Ag, advantageously allows both to further reduce the ignition temperature of the soot particles during the filter regenerating operations and to improve the thermal stability of Ag slowing its sintering and maintaining its catalytic efficiency.

According to the invention, furthermore, the Applicant experimentally observed that the addition in the catalytic composition of a third component comprising an oxide MO_z of an alkaline or alkaline-earth metal M, wherein M is a metal selected from K, Ba, Sr, Ca, Mg and mixtures thereof and z is 0.5 or 1 depending upon the metal valence, allows to achieve - in an entirely unexpected way - a further considerable reduction of the ignition temperature of the soot particles during the filter regenerating operations.

More particularly, the Applicant experimentally observed that this additional lowering of the ignition temperature of the soot particles is particularly significant when the catalytic composition includes the aforementioned oxide MO_z of an alkaline or alkaline- earth metal M in combination with a cerium oxide doped with Gd.

According to a second aspect thereof, the present invention relates to a post-treatment system for eliminating or minimising the emission of soot particles present in an exhaust gas emitted by an internal combustion engine comprising at least one particulate filter as described herein.

Advantageously, the post-treatment system of the invention allows to increase the filter regeneration efficiency and, along therewith, to increase the service life and the efficiency of the post-treatment system.

According to a third aspect thereof, the present invention relates to a process for eliminating or minimising the emission of soot particles present in an exhaust gas emitted by an internal combustion engine comprising: a) introducing a stream of said exhaust gas into a filter for removing said particles comprising a filtering body provided with a plurality of gas channels defined by gas-permeable walls of a porous substrate forming said body, said gas-permeable walls being coated with a catalytic composition as described herein; b) passing said gas stream through said gas-permeable walls so as to remove the soot particles from the gas stream and hold the soot particles in contact with said catalytic composition; c) burning the soot particles bringing said catalytic composition to a temperature equal to or greater than 250°C.

Advantageously, the process for treating the exhaust gases according to the invention allows for an efficient removal of the soot particles by combustion already starting from a temperature of 250°C, compatible with a use of a vehicle even in urban circulation and at which the catalytic composition is already capable of accelerating the kinetics of the combustion reaction of the particles.

According to a fourth aspect thereof, the present invention relates to a catalytic composition for burning soot particles present in an exhaust gas emitted by an internal combustion engine, comprising: a) a metallic phase having the formula: Agi_-xPd_x, wherein x is a number comprised between 0 and 0.2; and b) a cerium oxide doped with at least one rare earth group metal having the formula: Ce_1-yR_y0_2-y _2i wherein R is a rare earth group metal selected from Gd, Sm, Eu, La, and mixtures thereof and y is a number comprised between 0.1 and 0.5.

Advantageously, the catalytic composition of the invention allows to achieve the technical effects described above and has the additional preferred advantageous features which shall be clearer hereinafter.

According to a fifth aspect thereof, the present invention relates the use of a catalytic composition as described herein as a catalytically active coating of a filtering element of an exhaust gas emitted by an internal combustion engine.

According to a sixth aspect thereof, the present invention relates a process for the preparation of a particulate filter, comprising: a) providing a filtering body formed by a porous substrate and provided with a plurality of gas channels defined by gas-permeable walls of said substrate; b) providing an aqueous solution or suspension comprising: bl) at least one precursor compound of metallic Ag; b2) at least one precursor compound of cerium oxide doped with at least one rare earth group metal R selected from Gd, Sm, Eu, La, and mixtures thereof; b3) at least one ethylenically unsaturated water-soluble monomer including an ester group; b4) at least one cross-linking water-soluble monomer including at least two ethylenically unsaturated ester groups; b5) at least one radical polymerisation initiator; and optionally b6) at least one precursor compound of an oxide of an alkaline or alkaline-earth metal selected from K, Ba, Sr, Ca, Mg, and mixtures thereof; c) impregnating the porous substrate of the filtering body with said aqueous solution or suspension; d) polymerising said water-soluble monomers b3) and b4) so as to obtain, on the gas- permeable walls of the porous substrate, a layer of a gel formed by a hydrophilic polymer incorporating said at least one precursor compound of the metallic Ag, at least one precursor compound of said doped cerium oxide and optionally said at least one precursor compound of said oxide of an alkaline or alkaline-earth metal; e) thermally treating the gel thus obtained at a temperature comprised between 450 and 600°C for a time comprised between 2 and 10 h so as to coat the gas-permeable walls of the porous substrate with a catalytic composition in form of solid particles, comprising metallic Ag, cerium oxide doped with at least one rare earth group metal and optionally an oxide of an alkaline or alkaline-earth metal.

Within the framework of the present description and in the following claims, the term "aqueous solution or suspension" is used to indicate a solution or suspension wherein the used solvent is water or a water mixture and at least one water-soluble solvent. For the purposes of the invention, the water-soluble solvents which may be used can be selected from alcohols, glycols, tetrahydrofuran and dioxane.

Advantageously, the process for preparing the particulate filter according to the invention allows to deposit the catalytic composition directly on the porous substrate of the filtering body without the need to provide any base layer capable of preventing interactions between the catalytic composition and the substrate, as required, for example, by document EP 1 356 864, and/or allows to increase the adhesion of the catalytic composition to the substrate.

This feature is particularly advantageous, since it not only reduces the amount of reagents required for forming a catalytically active layer on the gas-permeable walls of the substrate, but also simplifies the manufacturing operations to the benefit of the filter production costs.

According to a preferred embodiment of the invention, an amount of catalytic composition comprised between 2 and 50 g per litre of filter and, more preferably, comprised between 5 and 30 g per litre of filter, is deposited on the filter.

According to a seventh aspect thereof, the present invention relates a process for preparing a catalytic composition comprising metallic Ag, a cerium oxide doped with at least one rare earth group metal R selected from Gd, Sm, Eu, La, and mixtures thereof and optionally an oxide of an alkaline or alkaline-earth metal M selected from , Ba, Sr, Ca, Mg, and mixtures thereof, wherein the process comprises: a) providing an aqueous solution or suspension comprising: al) at least one precursor compound of metallic Ag; a2) at least one precursor compound of cerium oxide doped with at least one rare earth group metal R selected from Gd, Sm, Eu, La, and mixtures thereof; a3) at least one ethylenically unsaturated water-soluble monomer including an ester group; a4) at least one cross-linking water-soluble monomer including at least two ethylenically unsaturated ester groups; a5) at least one radical polymerisation initiator; and optionally a6) at least one precursor compound of an oxide of an alkaline or alkaline-earth metal selected from , Ba, Sr, Ca, Mg, and mixtures thereof; b) polymerising said water-soluble monomers a3) and a4) so as to obtain a gel formed by a hydrophilic polymer incorporating said at least one precursor compound of metallic Ag, at least one precursor compound of said doped cerium oxide and optionally said at least one precursor compound of said oxide of an alkaline or alkaline-earth metal; c) thermally treating the gel thus obtained at a temperature comprised between 450 and 600°C for a time comprised between 2 and 10 h so as to obtain said catalytic composition in form of solid particles, comprising metallic Ag, cerium oxide doped with at least one rare earth group metal and optionally an oxide of an alkaline or alkaline-earth metal.

Advantageously, the aforementioned preparation processes allow to obtain a catalysed particulate filter and, respectively, a catalytic composition more reproducible with respect to the sintering processes of the prior art, using non-toxic substances having a low environmental impact and to obtain at the same time a catalytic composition having a high surface area to the benefit of the catalytic efficiency of the composition.

In this concern, the Applicant deems, without however wishing to be bound by any interpretative theory, that these advantageous technical effects may be ascribed to the specific so-called sol-gel synthesis technique of the catalytic composition which allows to operate with non-toxic substances having a low environmental impact and, above all, to carry out the synthesis of the final catalytic composition by means of a gel thermal treatment decidedly milder than those required by the conventional sintering treatments. The present invention in at least one of the aforementioned aspects may have at least one of the following preferred features.

In a preferred embodiment of the invention, the molar % of the doping metal R of the rare earth group in cerium oxide is comprised between 20% and 40% on the total of the doped cerium oxide, that is, the subscript y in the aforementioned formula (I) is a number comprised between 0.2 and 0.4.

The Applicant experimentally observed that in this way it is advantageously possible to achieve a considerable reduction of the ignition temperature of the soot particles in the filter regenerating operations. In a preferred embodiment, the molar ratio between the metallic phase based on metallic Ag and the doped cerium oxide Cei-_yRy0_{2-y 2} in the catalytic composition is comprised between 0.25 (1 :4) and 4 (4: 1), more preferably, between 0.5 (1 :2) and 2 (2:1) and, still more preferably, it is equal to 1 (1 :1).

Also in this case, the Applicant experimentally observed that in this way it is possible to achieve a considerable reduction of the ignition temperature of the soot particles in the filter regenerating operations.

Within the framework of the preferred embodiment of the invention in which the catalytic composition also includes Pd in the metallic phase based on metallic Ag, the molar % of Pd on the total moles of the metallic phase is comprised between 5% and 10%, that is, x in the aforementioned formula Agi_-xPd_x is a number comprised between 0.05 and 0.1.

As a matter of fact, the Applicant experimentally observed that in this way it is possible to achieve a considerable reduction of the ignition temperature of the soot particles during the filter regenerating operations achieving at the same time an improvement of the thermal stability of the metallic Ag during the filter regenerating operations.

Within the framework of the aforementioned preferred embodiment of the invention illustrated above in which the catalytic composition further also includes an oxide MO_z of an alkaline or alkaline-earth metal selected from K, Ba, Sr, Ca, Mg, and mixtures thereof, the molar ratio between the oxide MO_z and the metallic phase is comprised between 0.001 and 1, more preferably, between 0.01 and 0.5 and, still more preferably, between 0.05 and 0.2.

As a matter of fact, the Applicant experimentally observed that in this way it is possible to achieve a further considerable reduction of the ignition temperature of the soot particles in the filter regenerating operations, in particular when the doping metal R of the cerium oxide is Gd.

In a preferred embodiment, the molar ratio between the metallic phase, the doped cerium oxide and the oxide MO_z of~an alkaline or alkaline-earth metal is comprised between 1 : 1 :0.05 and 1 : 1 :0.5 and, more preferably, between 1 : 1 :0.05 and 1 : 1 :0.2.

As a matter of fact, the Applicant experimentally observed that in this way it is possible to achieve a considerable reduction of the ignition temperature of the soot particles in the filter regenerating operations. In a preferred embodiment of the invention, the doping metal R of the cerium oxide is Gd preferably present in a molar % comprised between 20% and 40% with respect to the total moles of the doped cerium oxide, that is, the subscript y in the formula Cei- _yGd_y0_{2-y 2} is comprised between 0.2 and 0.4.

Within the framework of this preferred embodiment in which the doping metal R of the cerium oxide is Gd, the Applicant observed that the addition of Pd in the Ag-based metallic component in a molar % comprised between 5% and 10% on the total moles of the Ag-based metallic component (i.e. the subscript x in the formula Agi-_xPd_x is comprised between 0.05 and 0.1) is particularly advantageous in terms of reduction of the ignition temperature of the soot particles in the filter regenerating operations and in terms of thermal stabilisation of the metallic Ag.

Still within the framework of this preferred embodiment wherein the doping metal R of the cerium oxide is Gd, the Applicant further observed that the addition of an oxide MO_z of an alkaline-earth metal M selected from Ba, Sr, Ca or Mg so that the molar ratio between the metallic phase based on metallic Ag, the doped cerium oxide Cei-yGd_y0_2-y/₂ and the oxide MO_z of the alkaline or alkaline-earth metal is comprised between 1 : 1 :0.05 and 1 :1 :0.5, is particularly advantageous in terms of reduction of the ignition temperature of the soot particles in the filter regenerating operations.

In a preferred alternative embodiment of the invention, the doping metal R of the cerium oxide is Sm, Eu or La and it is preferably present in a molar % comprised between 20% and 30% with respect to the total moles of the doped cerium oxide, that is, the subscript y in the formula Cei_-yR_y0_2-y/2 is comprised between 0.2 and 0.3.

Within the framework of this preferred embodiment in which the doping metal R of the cerium oxide is Sm, Eu or La, the Applicant further observed that the addition of an oxide MO_z of an alkaline-earth metal selected from Ba, Sr, Ca or Mg so that the molar ratio between the metallic phase based on metallic Ag, the doped cerium oxide Cei- _yGd_y0_2-y/2 and the oxide MO_z of the alkaline-earth metal is comprised between 1 :1 :0.05 and 1 : 1 :0.1, is particularly advantageous in terms of reduction of the ignition temperature of the soot particles in the filter regenerating operations.

Within the framework of the preferred embodiments of the particulate filter and of the catalytic composition of the invention, the Applicant observed that particularly efficient preferred compositions in terms of stability and reduction of the ignition temperature of the soot particles in the filter regenerating operations are the following (the values - in moles - of each component of the composition are indicated in brackets): Ag/Ceo.₆Gdo.₄0_{I 8} (1 : 1); Ago.9Pdo.i/Ceo.8Gdo.20i.₉(l :l); Ag/Ceo.₈Gdo.₂0_{1 9}/BaO (1 : 1 :0.05); Ag/Ceo.gGd^Oi ₉ BaO (1 :1 :0.1); Ag/Ce_{0 8}Gdo_.20, ₉/SrO (1 : 1 :0.1); Ag/Ceo.sGdo_.20] ₉/CaO (1 : 1 :0.1); Ag/Ce₀.₈Sm_{0 2}0, ₉ (1 : 1); Ag/Ce₀.₈Sm₀.₂Oi.₉/BaO (1 : 1 :0.05); Ag_{0 9}Pdo i/Ce₀._gSmo ₂0,.₉/BaO (1 : 1 :0.05); Ag/Ceo.₈Euo.₂0_{1 9} (1 :1); Ago.₉Pdo_.i/Ceo.₈Euo_.20i._9/BaO (1 :1 :0.05); Ag/Ceo.₈Euo_.2Oi.₉/BaO (1 : 1 :0.05); Ag/Ce_0.8Gdo.₂0,.₉/CaO (1 : 1 :0.05).

In a preferred embodiment, the filter according to the invention finds a particularly effective application in a post-treatment system adapted to eliminate or rninimise the emission of soot particles present in an exhaust gas of a diesel engine which is currently widely used both in motor vehicles and in other applications such as electric power units, diesel boilers and, generally, systems for reducing combustion fumes.

Within the framework of the aforementioned process for eliminating or minimising the emission of soot particles present in an exhaust gas emitted by an internal combustion engine, it is preferable and advantageous to burn the soot particles by bringing the catalytic composition to a temperature comprised between 250 and 500°C, more preferably, to a temperature comprised between 300 and 400°C.

According to the invention, it is advantageously possible to effectively remove the soot particles by combusting the same at a temperature range that is definitely lower - at the same operating conditions - than that required to achieve a similar effect within reasonable times using the particulate filters and catalytic compositions of the prior art.

The Applicant also experimentally observed that at the same temperature and combustion conditions, the catalytic compositions and the catalysed filters of the invention allow to carry out the combustion of the soot particles within a time period definitely lower than that which may be achieved with the catalytic compositions and filters of the prior art.

In a preferred embodiment of the aforementioned process for preparing the particulate filter and of the catalytic composition according to the invention, the aforementioned at least one precursor compound of the metallic Ag, at least one precursor compound of the doped cerium oxide and optionally at least one precursor compound of the oxide of an alkaline-earth metal are selected from the salts soluble in water.

Preferably, the aforementioned at least one precursor compound of the metallic Ag is AgN0₃.

Preferably, the aforementioned at least one precursor compound of said doped cerium oxide is a mixture of salts including at least one cerium salt selected from Ce(N0₃)₃, Ce(CH₃COO)₃, Ce(N0₂)₃, optionally hydrated, and mixtures thereof, and at least one salt of said rare earth group metal R selected from R(N0₃)₃, R(CH₃COO)₃, R(N0₂)₃, optionally hydrated, and mixtures thereof.

Within the framework of this preferred embodiment, said at least one cerium salt and said at least one salt of the rare earth group metal R are present in said mixture of salts in a molar ratio comprised between 4 (4: 1) and 1.5 (3:2).

In a preferred embodiment, the aforementioned at least one precursor compound of the metallic Ag and said at least one precursor compound of the doped cerium oxide are present in the aqueous solution or suspension in a molar ratio comprised between 0.25 (1 :4) and 4 (4:1).

Preferably, the aforementioned at least one precursor compound of said oxide of an alkaline or alkaline-earth metal is selected from M(N0₃)_2> M(CH₃COO)₂, M(N0₂)₃, optionally hydrated, and mixtures thereof.

In a preferred embodiment, the aforementioned at least one precursor compound of the metallic Ag, at least one precursor compound of the doped cerium oxide and at least one precursor compound of the oxide of an alkaline-earth metal M are present in the aqueous solution or suspension in a molar ratio comprised between 1 :1:0.05 and 1 :1 : 0.5.

In a preferred embodiment, in the aforementioned process for preparing the particulate filter and the catalytic composition according to the invention the aqueous solution or suspension further comprises at least one precursor compound of metallic Pd. Preferably, said at least one precursor compound of metallic Pd is selected from Pd(CH₃COO)₂, Pd(N0₃)₂ x 2H₂0, PdO x H₂0, and mixtures thereof.

Within the framework of this preferred embodiment, the aforementioned at least one precursor compound of the metallic Ag and at least one precursor compound of metallic Pd are present in the aqueous solution or suspension in a molar ratio comprised between 9:1 and 4:1.

In a preferred embodiment, in the aforementioned processes for preparing the particulate filter and the catalytic composition according to the invention, the aforementioned at least one ethylenically unsaturated water-soluble monomer including an ester group has the following structural formula:

wherein R is hydrogen, (Q-C alkyl, aryl or aryl(C₁-C4)alkyl; Ri is a Ci-C₈ hydrocarbon group containing at least one polar group selected from -COOH, -NH_2; -NHR', -N(R')₂, -OH, -OR' -S0₃H, -SH, wherein R' is a (C₁-C₆)alkyl group; and R₂ is hydrogen, methyl, ethyl, propyl or phenyl.

In a particularly preferred embodiment, R' is a (Ci-C4)alkyl group.

For the purposes of the invention, the aforementioned at least one ethylenically unsaturated water-soluble monomer including an ester group is an acrylic monomer preferably selected from 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2- hydroxyethyl methacrylate (HEMA), 2-hydroxypropyl methacrylate, 2-hydroxyethyl phenyl acrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, butanediol monoacrylate, 2-(2-ethoxy)ethyl acrylate, dimethylaminoethyl acrylate, and dimemylaminoethyl methacrylate.

In a preferred embodiment, in the aforementioned processes for preparing the particulate filter and the catalytic composition according to the invention, the aforementioned at least one cross-linking water-soluble monomer is selected from diacrylates and triacrylates wherein the acrylate groups are bonded to alkoxylate moieties or to linear polyoxyalkylene units. For the purposes of the invention, the aforementioned at least one cross-linking water- soluble monomer is a water-soluble monomer preferably selected from polyethylene glycol dimethacrylate (PEGDMA), polyethylene glycol diacrylate or trimethylolpropane-triacrylate ethoxylate. In a preferred embodiment, in the aforementioned processes for preparing the particulate filter and the catalytic composition according to the invention the aforementioned at least one radical polymerisation initiator is an azo-compound, a peroxide or a persalt.

For the purposes of the invention, the aforementioned at least one radical polymerisation initiator is preferably selected from α,α'-azoisobutyronitrile (AIBN), tetramethylene- e&ylenediamine, hydrogen peroxide, benzoyl peroxide, dicumyl peroxide, ammonium, sodium or potassium persulfate, and mixtures thereof.

In a preferred embodiment, in the aforementioned processes for preparing the particulate filter and the catalytic composition according to the invention, the aforementioned aqueous solution or suspension further comprises at least one non-precipitating chelating agent selected from citric acid, maleic acid, EDTA, and mixtures thereof.

In a preferred embodiment of the processes for preparing the particulate filter and the catalytic composition according to the invention, the polymerisation of the water-soluble monomers is carried out by thermally treating said aqueous solution or suspension at a temperature comprised between 70 and 100°C. Preferably, the aforementioned thermal polymerisation of the water-soluble monomers is carried out for a time comprised between 5 and 30 min.

As described above, in the processes for preparing the particulate filter and the catalytic composition according to the invention, the thermal treatment of the gel formed by the hydrophilic polymer is carried out at a temperature comprised between 450 and 600°C for a time comprised between 2 and 10 h.

In this way, it is advantageously possible to prepare a particulate filter and, respectively a catalytic composition, by means of a thermal treatment considerably milder than the sintering treatments of the prior art with a reduction of energy consumption, obtaining at the same time a filter and a catalytic composition having considerably higher performances than those of the filters and of the catalytic compositions of the prior art.

In a preferred embodiment of the process for preparing the particulate filter according to the invention, the impregnation of the porous substrate of the filtering body with the aqueous solution or suspension including the precursors of the various components of the catalytic composition and the water-soluble monomers precursors of the hydrophilic polymer, is carried out by dipping the filtering body in this aqueous solution or suspension for a time which may be easily determined by a man skilled in the art and suitable to achieve a satisfactory impregnation of the porous substrate and in particular of the internal zones of the filtering body crossed by the exhaust gas stream.

Preferably, the impregnation of the porous substrate of the filtering body is carried out by dipping the filtering body in the aqueous solution or suspension including the precursors of the various components of the catalytic composition and the water-soluble monomers precursors of the hydrophilic polymer for a time comprised between a few seconds and 5 minutes.

In this way, it is advantageously possible to carry out a substantially complete impregnation of the filtering body thus allowing a subsequent uniform coating of the gas-permeable walls of the porous substrate by the final catalytic composition obtained after thermal treatment .

Brief description of the drawings

Additional features and advantages of the invention will become more clearly apparent by the following description of some preferred embodiments thereof, given hereinbelow by way of illustration and not of limitation, with reference to the attached drawings. In such drawings: figure 1 is a simplified diagram of a post-treatment system for eliminating or minimising the emission of soot particles present in an exhaust gas emitted by an internal combustion engine comprising at least one particulate filter according to the invention; - figure 2 is a perspective schematic view, in partial cross-section and in partial exploded view and in enlarged scale, of a particulate filter according to the invention; figure 3 is a cross-sectional view in enlarged scale illustrating one of the gas permeable walls of a filtering body of the particulate filter of figure 2; figure 4 is a graph illustrating the X-ray diffraction data of a preferred catalytic composition according to the invention; figure 5 is a graph illustrating the C0₂ emission data in an analysis wherein two preferred catalytic compositions according to the invention and some comparative catalytic compositions are subjected to temperature-programmed oxidation conditions; figure 6 is a graph illustrating the data regarding the soot speed of combustion as a function of temperature in a thermal gravimetric analysis of two preferred catalytic compositions according to the invention and of a comparative catalytic composition according to the prior art.

Detailed description of the currently preferred embodiments

With reference to figure 1, a particulate filter according to a preferred embodiment of the invention positioned in an exhaust conduit 2 of a post-treatment system 3 for eliminating or rriinimising the emission of soot particles present in an exhaust gas emitted by an internal combustion engine 4, for example a diesel engine, is generally indicated at 1. The post-treatment system 3 is mounted on a vehicle, preferably a car.

As schematically illustrated in figure 2, the filter 1 comprises a filtering body 5, for example of the monolithic type or of the assembled portions type, preferably substantially cylindrical, formed by a porous substrate made of a material having a suitable thermal-mechanical resistance at temperatures exceeding 700°C and a suitable corrosion resistance.

For the purposes of the invention, the material of the porous substrate can be of the ceramic or metal type; preferably, such a material is recrystallised silicon carbide and preferably superficially oxidised; alternatively, the material of the porous substrate can be selected from Si-SiC, S13N4, cordierite, Al₂Ti0₅, ceramic foam, metal wool or metal mesh.

The filtering body 5 of the filter 1 has a substantially honeycomb structure comprising a plurality of gas channels 6', 6", adjacent and parallel to each other, defined by gas- permeable walls 7 of said substrate.

More particularly, the filtering body 5 has a structure such that a first half of the gas channels 6' is closed, for example by respective plugs 8, preferably essentially made of the same material of the substrate, at an end 6b downstream with respect to the flowing direction of the exhaust gases according to a substantially chequered configuration so as to form as many inlet passages 6' of the exhaust gases, while a second half of the gas channels 6" is closed, for example by respective plugs 9, also preferably made of the same material of the substrate, at an end 6a upstream with respect to the flowing direction of the exhaust gases according to a substantially chequered configuration complementary to the preceding one, so as to form as many outlet passages 6" of the exhaust gases.

In this way and as better illustrated by the arrows in figure 2, a path of the exhaust gases is created within the filtering body 5 which allows for the gas entry into the inlet gas channels 6' open at the upstream end 6a thereof, a crossing of the gas-permeable walls 7, the entry into the outlet gas channels 6" open at the downstream end 6b thereof and thus the exit from the filter 1.

More particularly and as illustrated in figure 3, each gas-permeable wall 7 comprises micro-pores (channels for the gas passage) 10 which put the inlet passages 6' of the exhaust gases in communication with the adjacent outlet passages 6" of the exhaust gases so that the exhaust gases flow through the micro-pores 10 as is better shown by the arrows in such a figure. The soot particles are thus trapped and they are essentially deposited on the surfaces of the gas-permeable walls 7 of the inlet passages 6' and on the walls of the micro-pores 10. The gas-permeable walls 7 and the micro-pores 10 are coated with at least one layer 1 1 of a catalytic composition according to the invention to promote the combustion of the trapped soot particles and, thus, to promote the passive regeneration of the filter 1 already at temperatures compatible with the use of a vehicle even in urban circulation, for example 250-300°C. The results of a series of experimental tests carried out by the Applicant with the aim of proving the improved performance of the particulate filter 1 and of the catalytic composition according to the present invention shall be provided in the following examples, merely provided by way of illustration and not of limitation.

According to the examples indicated below, a series of catalytic compositions according to the invention were synthesized by means of a sol-gel preparation process illustrated above, as well as some comparative catalytic compositions prepared according to respective preparation processes indicated hereinafter.

In the examples that follow, the reagents that were used had a purity of at least 99% unless otherwise indicated. The activity of the synthesised catalytic compositions in the oxidation of soot particles was thus evaluated in atmospheric conditions and using atmospheric air by means of Thermal Gravimetric Analysis (TGA) at a variable temperature. The performance of some of the catalytic compositions according to the invention was further evaluated, with respect to some comparative catalytic compositions by means of analysis of the gases emitted in temperature-programmed oxidation conditions, (Temperature Programmed Oxidation - Evolved Gas Analysis; TPO-EGA) at a variable temperature.

The performance of a catalytic composition according to the invention was further evaluated in terms of oxidation speed of the soot particles at a constant temperature,, with respect to a comparative catalytic composition by means of a Thermal Gravimetric Analysis (TGA) at a constant temperature. EXAMPLE 1

(Comparative)

Preparation of Ceo_.sGdo ₂Οι_.9

A first comparative catalytic composition was prepared by means of a sol-gel synthesis process as follows. 0.4680 g Ce( 0₃)₃ x 6 H₂0 and 0.1220 g Gd(N0₃)₃ x 6 H₂0 were added to 20 ml of H₂0 while stirring; the resulting solution was heated to 50°C so as to obtain a saline concentration of 0.135 mol/1. About 3 g of citric acid were added to achieve a better homogenisation. 20 ml of 2-hydroxyethyl methacrylate (HEMA), 10 ml of poly (ethylene glycol) dimethacrylate (PEGDMA) having an average molecular weight of 750 and about 50 mg of AIBN (radical polymerisation initiator) were subsequently added. The solution was heated to 80°C up to a gel formation. The resulting gel was subjected to thermal treatment in atmospheric air at 500°C for 5 h and the resulting powder having the composition Ceo_.8Gdo_.2Oi_.9 was mixed in a mortar.

EXAMPLES 2-9

(Comparative)

Eight comparative catalytic compositions were prepared by means of a sol-gel synthesis process entirely analogous to that of the previous example 1 (except for the amount of reagents and proportions thereof, which can be easily determined by means of routine stoichiometric calculations by a man skilled in the art as a function of the desired molar ratios in the final catalytic composition) so as to obtain as many catalytic compositions having the following formulae (the molar ratios between the components of the composition are indicated in brackets): Example 2 Ce0₂

Example 3 Ag/Ce0₂ (l :l)

Example 4 Ago_.9Pto. l/Ceo.gGdo^Oi <

Example 5 Ago_.5Pto.₅/Ceo.8Gdo.₂01 <

Example 6 Pt/Ce_{0 8}Gdo ₂O_{l 9} (l :l)

Example 7 Ag/Ceo.8Ndo ₂0_{) 9} (l :l)

Example 8 Ag/Ce_{0 8}Yo.₂0,_.9(l :l)

Example 9 Ag/Ce_0.95Gdo_.050 \ ₉₇₅ ( 1 : 1 )

In connection with comparative example n. 2, the catalytic composition having the formula Ce0₂ was prepared following the same preparation methods of example 1 except for the fact that the solution of the reagents contained no salt R(N0₃)₃ x 6 H₂0 of the rare earth group metal R. In connection with comparative example n. 3, the catalytic composition having the formula Ag/Ce0₂ was prepared following the same preparation methods of example 1 except for the fact that the solution of the reagents contained AgN0₃ as the Ag precursor compound and contained no salt R(N0₃)₃ x 6 H₂0 of the rare earth group metal R.

In connection with comparative example nos. 4 and 5, the catalytic compositions having the formula

(1 :1) and Ag_{0 5}Pto.₅/Ceo ₈Gdo.₂Oi ₉ (1 :1) were instead prepared following the same preparation methods of example 1 using AgN0₃ as the Ag precursor compound and hydrogen hexachloroplatinate (TV) monohydrate (H₂PtCl₆ x H₂0 - Sigma-Aldrich, purity of 99.9 %) as water-soluble salt adapted to provide the desired Pt amount. In connection with comparative example n. 6, the catalytic composition having the formula Pt/Ce_0.8Gdo₂0i ₉ (1 :1) was instead prepared following the same preparation methods of example 1 using hydrogen hexachloroplatinate (TV) monohydrate (Η₂Ρί0₆ x H₂0 - Sigma-Aldrich, purity of 99.9 %) as water-soluble salt adapted to provide the desired Pt amount. In connection with comparative example nos. 7, 8 and 9, the catalytic compositions having the formula Ag/Ceo.8Nd .₂Oi.9 (1 :1), Ag/Ceo Yo.₂Oi.9 (1 :1) and Ag/Ceo.9₅Gd ₀₅01.975 (1 :1) were instead prepared following the same preparation methods of example 1 using AgN0₃ as the Ag precursor compound.

EXAMPLE 10

(Comparative)

Preparation of Ag/Ceo.₈4Zro.₀8₅Smo.₀6₅Yo.oiOi.962₅ (1:1.065)

A comparative catalytic composition comprising Ag and cerium oxide stabilised with Zr, Sm and Y according to EP 1 356 864 was prepared by means of the following preparation process (catalytic composition of the filter CPF-15 disclosed in such reference). 68.50 g AgN0₃, 155 g Ce(N0₃)₃ x 6 H₂0, 8.5 g ZrO(N0₃)₂ x 6H₂0, 12.8 g Sm(N0₃)₃ x 6 H₂0 and 3.0 g Y(N0₃)₃ x 6H₂0 were added to 325 ml of distilled H₂0 while stirring. After drying at room temperature overnight, the sample was dried at 105°C for 10 h and calcined at 600°C for 4 h. The resulting powder was thus mixed in a mortar.

A comparative catalytic composition having the formula Ag/Ceo.₈₄Zro.o85Smo.o65Yo.oiOi.9625 (1:1.065) was obtained.

EXAMPLE 11

(Invention)

Preparation of Ag/Ceo_.8Gdo_.2Oi.9 (1 :1)

A first catalytic composition according to the invention was prepared by means of a sol- gel synthesis process as follows.

0.4680 g Ce(N0₃)₃ x 6 H₂0, 0.1220 g Gd(N0₃)₃ x 6 H₂0, 0.2295 g of AgN0₃ and 3.0 g of citric acid were added to 20 ml of H₂0 while stirring; the resulting solution was heated to 50°C so as to obtain a saline concentration of 0.135 mol/1. 20 ml of 2- hydroxyethyl methacrylate (HEMA), 10 ml of poly(ethylene glycol) dimethacrylate (PEGDMA) having an average molecular weight of 750 and about 50 mg of AIBN (radical polymerisation initiator) were subsequently added. The solution was heated to 80°C up to a gel formation. The resulting gel was subjected to a thermal treatment in atmospheric air at 500°C for 5 h and the resulting powder was mixed in a mortar.

The X-ray diffraction analysis, the results of which are visible in the graph of figure 4, confirmed the fluorite-like structure of cerium oxide doped with Gd and the metal nature at zero oxidation state of the Ag. EXAMPLE 12

(Invention)

(1:1)

A second catalytic composition according to the invention was prepared by means of a sol-gel synthesis process as follows.

0.4680 g Ce(N0₃)₃ x 6 H₂0, 0.1220 g Gd(N0₃)₃ x 6 H₂0, 0.2066 g of AgN0₃, 0.1516 g of Pd(CH₃COO)_2j preliminarily dissolved in about 10 ml of a solution of HN0₃ at 63% by weight, and 3.0 g of citric acid were added to 20 ml of H₂0 while stirring. The resulting solution was heated to 50°C so as to obtain a saline concentration of 0.135 mol/1. 20 ml of 2-hydroxyethyl methacrylate (HEMA), 10 ml of poly(ethylene glycol) dimethacrylate (PEGDMA) having an average molecular weight of 750 and about 50 mg of AJBN (radical polymerisation initiator) were subsequently added. The solution was heated to 80°C up to a gel formation. The resulting gel was subjected to thermal treatment in atmospheric air at 500°C for 5 h and the resulting powder was mixed in a mortar.

EXAMPLE 13

(Invention)

(1 :1 :0.1)

A further catalytic composition according to the invention was prepared by means of a sol-gel synthesis process as follows.

0.4095 g Ce(N0₃)₃ x 6 H₂0, 0.1 193 g Gd(N0₃)₃ x 6 H₂0, 0.2295 g of AgN0₃ , 0.0354 g of Ba(N0₃)₂ and 3.0 g of citric acid were added to 20 ml of H₂0 while stirring; the resulting solution was heated to 50°C so as to obtain a saline concentration of 0.135 mol 1. 20 ml of 2-hydroxyethyl methacrylate (HEMA), 10 ml of poly(ethylene glycol) dimethacrylate (PEGDMA) having an average molecular weight of 750 and about 50 mg of ΑΓΒΝ (radical polymerisation initiator) were subsequently added. The solution was heated to 80°C up to a gel formation. The resulting gel was subjected to thermal treatment in atmospheric air at 500°C for 5 h and the resulting powder was mixed in a mortar. EXAMPLES 14-41

(Invention)

Additional catalytic compositions according to the invention were prepared by means of a sol-gel synthesis process analogous to those of the examples reported above (except for the amount of reagents and proportions thereof, which can be easily determined by means of routine stoichiometric calculations by a man skilled in the art as a function of the desired molar ratios in the final catalytic composition) so as to obtain as many catalytic compositions having the following formulae (the molar ratios between the components of the composition are indicated in brackets):

Example 14 Ag/Ce₀.₈Gdo.₂0,.₉(2:l)

Example 15 Ag/Ce₀₈Gdo.₂Oi.₉(l:2)

Example 16 Ag/Ceo.₆Gdo.₄Oi₈(l:l)

Example 17 Ag/Ce₀₅Gd₀.₅O, ₇5(l:l)

Example 18

Example 19 Ago ePdo !/Ceo gGdozO_Li/BaO (1:1:0.05)

Example 20 Ag/Ce.₇₅Gdo₂Oi ₉BaO (1:1:0.05)

Example 21 Ag/Ceo.sGdo.20! ₉/BaO (1:1:0.5)

Example 22 Ag/Ce.₆Gdo ₄Oi ₉/BaO (1:1:0.1)

Example 23 Ag/Ce₀₈Gdo.₂Oi ₉/SrO (1:1:0.1)

Example 24 Ag/Ce₀.₈Gdo.₂Oi.₉/CaO (1:1:0.1)

Example 25 Ag/Ce₀₈Smo.₂Oi.₉(l:l)

Example 26 Ag/Ceo.₆Smo_.40i.₈(l:l)

Example 27 Ag/Ce₀.₈Smo.₂Oi ₉/BaO (1:1:0.05)

Example 28 Ag/Ceo.gSmo ₂Oi ₉/BaO (1:1:0.1)

Example 29 Ag₀₉Pdo i/Ceo.gSmo Oi.g/BaO (1:1 :0.05)

Example 30 Ag/Ce₀₈Lao.₂Oi.₉(l:l)

Example 31 Ag/Ceo.₈Euo.₂O_l9(l:l);

Example 32 Ag/Ce₀₈Euo.Oi.₉/BaO (1 : 1 :0.1 ); Example 33 Ag_0.9Pdo i/Ceo_.8Euo_.20,_.9/BaO (1 :1 :0.05);

Example 34 Ag/Ce₀₈Euo ₂Oi_.9/BaO (1:1 :0.05);

Example 35 Ag/Ceo sGdo sO! VCaO (1 :1:0.05);

Example 36 Ag/Ce₀.₈Gdo_.20_{1 9}/CaO (1 :1 :0.2);

Example 37 Ag/Ce_{0 8}Gdo_.2Oi.₉/MgO (1 :1:0.1);

Example 38 Ag/Ce₀.₈Smo_.2Oi.₉/CaO (1 :1 :0.1);

Example 39 Ag/Ceo.₈Smo.20_{1 9}/MgO (1 :1:0.1);

Example 40 Ag/Ce₀.₈Gdo.₂0_{1 9}/K0₀.5 (1 :1 :0.1);

Example 41 Ago_.9Pdo.i/Ce_{0 8}Smo_.20_{1 9}/KOo_.5 (1 : 1 :0.1).

EXAMPLE 42

(Invention)

(Preparation of a catalysed particulate filter with Ag/Ceo.₈Gdo ₂0| 9/BaO ( 1 : 1 :0.1 ))

A catalysed particulate filter according to the invention was prepared by means of the following preparation process.

49.49 g Ce(N0₃)₃ x 6 H₂0, 12.59 g Gd(N0₃)₃ x 6 H₂0, 24.27 g of AgN0₃ and 3.73 g of Ba(N0₃)₂ were dissolved in 80 ml of H₂0 while stirring and heating to 50°C. The total saline concentration in the solution thus obtained was equal to about 3.75 M (Ce:Gd:Ag:Ba molar ratio equivalent to 1.43:0.36:1.79:0.18). 80 ml of 2-hydroxyethyl methacrylate (HEMA), 40 ml of poly(ethylene glycol) dimethacrylate (PEGDMA) having an average molecular weight of 750 and about 100 mg of AIBN (radical polymerisation initiator) were subsequently added.

A laboratory scale cylindrical filter 1 made of reciystallised silicone carbide having the structural features described above, a diameter of 15 mm and a length of 80 mm, was impregnated by means of the aforementioned solution by dipping the filter in the solution for 2 min.

The filter thus impregnated was then heated in an oven at 80°C for 2 h up to gel formation and then subjected to thermal treatment in atmospheric air at 500°C for 5 h. After the thermal treatment, the filter was cleaned by means of a compressed air stream and weighed to determine the total amount of catalytic composition loaded on the filter which turned out to be equal to about 25.9 g 1 of filter.

EXAMPLE 43

(Evaluation of the catalytic activity by TGA analysis at a variable temperature)

The catalytic compositions (comparative and according to the invention) prepared as described above were subjected to thermogravimetric analysis to verify their catalytic efficiency. More particularly, these analyses were carried out according to the following experimental protocol. 10 mg of catalytic composition and 90 mg of artificial soot particles (Printex® U - Degussa, primary average diameter of the particles: 25 nm) were intimately mixed in a mortar according to a 1 :9 weight ratio.

The thermogravimetric tests were carried out using a portion of the previously prepared catalyst + soot mixture (about 6-8 mg), with a heating speed of 10°C/min using a Mettler-Toledo TGA850 apparatus and supplying a 60 cm³/min air flow rate into the apparatus.

The results of the TGA tests are expressed as ignition temperature (Tj) of the soot particles. This temperature corresponds to the first peak of the second derivative of the TGA loss curve which in turn corresponds to the temperature at which the combustion reaction of the soot in air is triggered.

The subsequent Table 1 shows the data related to the measurements carried out both on the catalytic compositions according to the invention and on the comparative catalytic compositions.

Table 1 also contains, by way of comparison, the data detected only with the soot particles (Printex® U) without any catalytic composition and the data detected by mixing 90 mg of soot particles (Printex® U) with 10 mg of Pt catalyst (Pt black from Alfa Aesar GmbH - surface area of 27 m7g).

EXAMPLE 44

(Evaluation of the catalytic activity of the catalytic compositions by TPO-EGA analysis)

The comparative catalytic compositions of examples 1 and 3 and the inventive catalytic compositions according to examples 1 1 and 13 prepared as described above, were subjected to an analysis of the gases emitted in programmed oxidation conditions (TPO- EGA) to verify their catalytic efficiency. More particularly, these analyses were carried out according to the following experimental protocol.

1.4 mg of catalytic composition and 12.6 mg of artificial soot particles (Printex® U - Degussa, primary average diameter of the particles: 25 ran) were intimately mixed in a mortar according to a 1 :9 weight ratio.

The analysis of the emitted gases was carried out by positioning the aforementioned mixture in an air stream having a flow rate of 10 1/hr, heating the mixture at a heating speed of 10°C/min and analysing the emitted gases by means of a gas chromatograph using a Micro-GC Chrompack CP-2002P apparatus.

Both the C0₂ and CO peaks, respectively indicating a complete or incomplete combustion of the soot particles, were measured in the emitted gases during the experimentation.

The C0₂ peak corresponds to the maximum speed of the combustion reaction observed in the experimental conditions hence the corresponding peak temperature can in turn be related to the catalytic efficiency of the specific material analysed.

The subsequent Table 2 shows the data regarding the measurements carried out by using two catalytic compositions according to the invention and two comparative catalytic compositions. Also in this case, Table 2 contains by way of comparison the data detected only with the soot particles (Printex® U) without any catalytic composition and the data detected by mixing 12.6 mg of soot particles (Printex® U) with 1.4 mg of Pt catalyst (Pt black from Alfa Aesar GmbH - surface area of 27 m7g).

The results of the tests carried out are also graphically represented in figure 5 for a more prompt visual analysis.

EXAMPLE 45

(Evaluation of the catalytic activity by TGA analysis at a constant temperature)

The inventive catalytic compositions according to the preceding examples n. 13 [Ag/Ceo_.gGdojC VBaO (1:1:0.1)] and n. 24 [Ag/Ceo_.gGdo_.iC CaO (1:1:0.1)] and the comparative catalytic composition according to the prior art example n. 10 [Ag/Ceo g^ro.ossSmo.oesYo.oiOi 9625 (1 :1.065)] prepared as described above were subjected to a thermogravirnetric analysis to verify their catalytic efficiency. More particularly, these analyses were conducted according to the following experimental protocol.

40 mg of catalytic composition and 60 mg of artificial soot particles (Printex® U - Degussa, primary average diameter of the particles: 25 nm) were intimately mixed in a mortar according to a 2:3 weight ratio.

The thermogravirnetric tests were carried out in air using a portion of the previously prepared catalyst + soot mixture (about 6-8 mg), maintaining the sample at a predetermined and constant temperature (isothermal method) using a Mettler-Toledo TGA850 apparatus and by supplying a flow rate of 60 cm /min into the apparatus.

The following Table 3 shows the results of the isothermal TGA tests carried out at nominal temperatures of 275°C, 300°C, 325°C and 350°C (with actual temperature values indicated in the Table), while figure 6 shows the curves representing the most significant parameter for each of the aforementioned catalytic compositions, i.e. the oxidation speed of the soot particles that is the amount of soot consumed per unit of time.

*** * ***

By comparing the experimental data reported in the aforementioned Tables 1-3 and in figures 5 and 6, it is evident that the particulate filter and the catalytic compositions of the present invention constituted by a metallic phase comprising metallic Ag and optionally Pd and by a ceramic oxide (cermet) formed by a cerium oxide doped with at least one rare earth group metal selected from Gd, Sm, Eu and La present in a specific molar %, allow to considerably improve the efficiency of the filter regenerating operations with respect to what may be achieved using the filters and the catalytic compositions of the prior art.

As a matter of fact, both a drastic reduction of the ignition temperature Ti of the soot particles and a considerable increase of the oxidation speed of such particles was experimentally observed in the same experimental conditions and, this, along with the additional advantages of using inexpensive reagents having a low environmental impact. In particular, the Applicant observed considerable improvements by using in the catalytic composition a cerium oxide doped with Gd at amounts comprised between 20 and the 40 mol % on the total of the doped cerium oxide and a cerium oxide doped with an amount of Sm, Eu or La comprised between 20 and 30 mol % on the total of the doped cerium oxide.

The Applicant also observed that the addition of an oxide of an alkaline or alkaline-earth metal suitably selected from K, Ba, Sr, Ca and Mg and with a suitable selection of the molar ratio thereof with respect to the metallic phase in the catalytic composition triggers quite unexpectedly the advantageous effect of an additional marked reduction of the ignition temperature Tj of the soot particles and an additional considerable increase of the oxidation speed of such particles.

This effect was particularly marked in combination with doped cerium oxide including an amount of Gd comprised between the 20 and 40 mol % or an amount of Sm, Eu or La comprised between 20 and 30 mol % on the total of the doped cerium oxide.

Lastly, the analysis of the emitted gases in programmed oxidation conditions (TPO- EGA) enabled to experimentally observe that the catalytic compositions and thus the possible particulate filter thus coated of the present invention allow to achieve a complete combustion of the soot particles shown by the substantial absence of CO in the emitted gases during the thermal conditions typical of filter regenerating operations.

Clearly, those skilled in the art may introduce variants and modifications to the above described invention, in order to satisfy specific and contingent requirements, variants and modifications which fall anyhow within the scope of protection as is defined by the following- claims.

TABLE 1

Example n° Composition of the catalyst Ti (°C)

31 (inv) Ag/Ceo.₈Euo.₂0, .9(l :l) 402

32 (inv) Ag/Ce_{0 8}Euo₂Oi ₉/BaO (1:1 :0.1) 419

33 (inv) Ago_.9Pdo_.i/Ceo.₈Euo.₂0,.₉ BaO (1:1 :0.05) 403

34 (inv), Ag/Ce_{0 8}Euo ₂θ_{! 9}/BaO (1:1 :0.05). 403

35 (inv) Ag/Ceo_.8Gdo_.20,_.9/CaO (1 :1:0.05); 395

36 (inv) Ag/Ce_{0 8}Gdo_.2Oi_.9/CaO (1 :1 :0.2); 413

37 (inv) Ag/Ceo_.8Gdo_.20,.9/MgO (1 :1 :0.1); 410

38 (inv) Ag/Ceo_.8Smo_.2Oi_.9/CaO (1 :1 :0.1); 406

39 (inv) Ag/Ceo_.8Smo_.2Oi_.9 MgO (1:1:0.1); 418

40 (inv) Ag/Ceo_.8Gdo ₂Oi_.9/K0_{0 5} (1:1 :0.1); 455

41 (inv) Ago.9Pdo.i/Ce₀.₈Sm₀.₂Oi.₉/ Oo.5 (1:1 :0.1). 440

(The molar ratios between the components of the catalytic composition are indicated in brackets)

Ti = ignition temperature of the particles

TABLE 2

(Evaluation of the catalytic activity by TPO-EGA analysis)

n.d. = not detectable TABLE 3

(Evaluation of the catalytic activity by TGA analysis at constant temperature)

Example n° 13 (inv)

T (°C) Speed (%/min) complete combustion t (h)

272 0.075 22.2

296 0.22 7.6

322.5 0.73 2.3

349 1.5 1.1

Example n° 24 (inv)

T (°C) Speed (% min^" ') complete combustion t (h)

271 ¹¹ 0.080 20.8

297 0.285 5.8

324 0.95 1.8

350 2.90 0.6

Example n° 10 (comp)

T (°C) Speed (%/min) complete combustion t (h)

271 0.027 61.7

296 0.085 19.6

322 0.25 6.7

348 0.69 2.4

Claims

1. Particulate filter (1), comprising a filtering body (5) formed by a porous substrate and provided with a plurality of gas channels (6', 6") defined by gas- permeable walls (7) of said substrate, said gas-permeable walls (7) being coated with a catalytic composition comprising: a) a metallic phase having the formula: Agi_-xPd_x, wherein x is a number comprised between 0 and 0.2; and b) a cerium oxide doped with at least one rare earth group metal having the formula: Cei_-y y0₂-y ₂, wherein R is a rare earth group metal selected from Gd, Sm, Eu, La, and mixtures thereof and y is a number comprised between 0.1 and 0.5.

2. Filter (1) according to claim 1, wherein the molar ratio between the metallic phase and the doped cerium oxide in the catalytic composition is comprised between 0.25 and 4.

3. Filter (1) according to claim 1, wherein the catalytic composition further comprises: c) an oxide of an alkaline or alkaline-earth metal M having the formula: MO_z, wherein M is a metal selected from K, Ba, Sr, Ca, Mg, and mixtures thereof and z is 0.5 or 1.

4. Filter (1) according to claim 3, wherein the molar ratio between the oxide MO_z of an alkaline or alkaline-earth metal and the metallic phase is comprised between 0.001 and 1.

5. Filter (1) according to claim 3, wherein the molar ratio between the metallic phase, the doped cerium oxide and the oxide MO_z of an alkaline or alkaline-earth metal is comprised between 1 : 1 :0.05 and 1 : 1 :0.5.

6. Filter (1) according to claim 1, wherein R is Gd and y is comprised between 0.2 and 0.4.

7. Filter (1) according to claim 3 and 6, wherein M is Ba, Sr, Ca or Mg and wherein the molar ratio between the metallic phase, the doped cerium oxide and the oxide MO_z of an alkaline or alkaline-earth metal is comprised between 1 : 1 :0.05 and 1 : 1 :0.5.

8. Filter (1) according to claim 1, wherein R is Sm and y is comprised between 0.2 and 0.3.

9. Filter (1) according to claim 3 and 8, wherein M is Ba, Sr, Ca or Mg and wherein the molar ratio between the metallic phase, the doped cerium oxide and the oxide MO_z of an alkaline or alkaline-earth metal is comprised between 1 : 1 :0.05 and 1 : 1 :0.1.

10. Filter (1) according to claim 1, wherein said catalytic composition is selected from Ag/Ceo,6Gdo_j4Oi,8 (1 :1);

(1 :1 :0.05); Ag/Ce₀,₈Gdo,₂Oi,₉/BaO (1 :1 :0.1); Ag/Ceo.sGd^O^/SrO (1 :1 :0.1); Ag/Ceo_,8Gdo^O,yCaO (1 :1 :0.1); Ag/Ceo_,8Smo,₂0,_;9 (1 :1); Ag/Ce₀,8Sm₀₎₂O_l>9/BaO (1:1 :0.05); Ago_>9Pdo,i/Ceo_,8Smo,₂0,_j9 BaO (1:1:0,05); Ag/Ce_0,8Euo_,2Oi,₉ (1:1); Ago^Pdo/Ceo.sEuo^yBaO (1 :1 :0.05); Ag/Ce₀,₈Euo^O,_>9/BaO (1 :1 :0.05); Ag/Ce_0,8Gdo,20,yCaO (1 :1 :0.05).

11. Post-treatment system (3) for eliminating or minimising the emission of soot particles present in an exhaust gas emitted by an internal combustion engine (4) comprising at least one particulate filter (1) according to any one of claims 1-10.

12. Post-treatment system (3) according to claim 11, wherein said internal combustion engine is a diesel engine.

13. Process for eliminating or rninimising the emission of soot particles present in an exhaust gas emitted by an internal combustion engine (4) comprising: a) introducing a stream of said exhaust gas into a filter (1) for removing said particles comprising a filtering body (5) provided with a plurality of gas channels (6', 6") defined by gas-permeable walls (7) of a porous substrate forming said body, said gas-permeable walls (7) being coated with a catalytic composition in accordance with any one of claims 1-10; b) passing said gas stream through said gas-permeable walls (7) so as to remove the soot particles from the gas stream and hold the soot particles in contact with said catalytic composition; c) burning the soot particles by bringing said catalytic composition to a temperature equal to or greater than 250°C.

14. Catalytic composition for burning soot particles present in an exhaust gas emitted by an internal combustion engine (4) comprising: a) a metallic phase having the formula: Agi_-xPd_x, wherein x is a number comprised between 0 and 0.2; and b) a cerium oxide doped with at least one rare earth group metal having the formula: Cei_-yR_y0₂._y/2, wherein R is a rare earth group metal selected from Gd, Sm, Eu, La, and mixtures thereof and y is a number comprised between 0.1 and 0.5.

15. Use of a catalytic composition according to claim 14 as a catalytically active coating of a filtering element of an exhaust gas emitted by an internal combustion engine (4).

16. Process for the preparation of a particulate filter (1 ), comprising: a) providing a filtering body (5) formed by a porous substrate and provided with a plurality of gas channels (6', 6") defined by gas-permeable walls (7) of said substrate; b) providing an aqueous solution or suspension comprising: bl) at least one precursor compound of metallic Ag; b2) at least one precursor compound of cerium oxide doped with at least one rare earth group metal R selected from Gd, Sm, Eu, La, and mixtures thereof; b3) at least one ethylenically unsaturated water-soluble monomer including an ester group; b4) at least one cross-linking water-soluble monomer including at least two ethylenically unsaturated ester groups; b5) at least one radical polymerisation initiator; and optionally b6) at least one precursor compound of an oxide of an alkaline or alkaline-earth metal selected from K, Ba, Sr, Ca, Mg, and mixtures thereof; c) impregnating the porous substrate of the filtering body (5) with said aqueous solution or suspension; d) polymerizing said water-soluble monomers b3) and ) so as to obtain, on the gas- permeable walls (7) of the porous substrate, a layer of a gel formed by a hydrophilic polymer incorporating said at least one precursor compound of metallic Ag, at least one precursor compound of said doped cerium oxide and optionally said at least one precursor compound of said oxide of an alkaline or alkaline-earth metal; e) thermally treating the gel thus obtained at a temperature comprised between 450 and 600°C for a time comprised between 2 and 10 h so as to coat the gas-permeable walls

(7) of the porous substrate with a catalytic composition in form of solid particles, comprising metallic Ag, cerium oxide doped with at least one rare earth group metal and optionally an oxide of an alkaline or alkaline-earth metal.

17. Process for preparing a catalytic composition comprising metallic Ag, a cerium oxide doped with at least one rare earth group metal R selected from Gd, Sm, Eu, La, and mixtures thereof and optionally an oxide of an alkaline or alkaline-earth metal selected from K, Ba, Sr, Ca, g, and mixtures thereof, wherein the process comprises: a) providing an aqueous solution or suspension comprising: al) at least one precursor compound of metallic Ag; a2) at least one precursor compound of cerium oxide doped with at least one rare earth group metal R selected from Gd, Sm, Eu, La, and mixtures thereof; a3) at least one ethylenically unsaturated water-soluble monomer including an ester group; a4) at least one cross-linking water-soluble monomer including at least two ethylenically unsaturated ester groups; a5) at least one radical polymerisation initiator; and optionally a6) at least one precursor compound of an oxide of an alkaline or alkaline-earth metal selected from K, Ba, Sr, Ca, Mg, and mixtures thereof; b) polymerising said water-soluble monomers a3) and a4) so as to obtain a gel formed by a hydrophilic polymer incorporating said at least one precursor compound of metallic Ag, at least one precursor compound of said doped cerium oxide and optionally said at least one precursor compound of said oxide of an alkaline or alkaline-earth metal; c) thermally treating the gel thus obtained at a temperature comprised between 450 and 600°C for a time comprised between 2 and 10 h so as to obtain said catalytic composition in form of solid particles, comprising metallic Ag, cerium oxide doped with at least one rare earth group metal and optionally an oxide of an alkaline or alkaline-earth metal.

18. Process according to claim 16 or 17, wherein said aqueous solution or suspension further comprises at least one precursor compound of metallic Pd.

19. Process according to claim 16 or 17, wherein said at least one ethylenically unsaturated water-soluble monomer including an ester group has the following structural formula:

wherein R is hydrogen, (Ci-C₄)alkyl, aryl or aryl(Ci-C4)alkyl; Ri is a C]-Cg hydrocarbon group containing at least one polar group selected from -COOH, -NH₂, -NHR', -N(R')₂, -OH, -OR' -SO3H, -SH, wherein R' is a (Ci-C₆)alkyl group; and R₂ is hydrogen, methyl, ethyl, propyl or phenyl.

20. Process according to claim 16 or 17, wherein said at least one cross-linking water-soluble monomer is selected from diacrylates and triacrylates wherein the acrylate groups are linked to alkoxylate moieties or to linear polyoxyalkylene units.

21. Process according to claim 16 or 17, wherein said at least one radical polymerisation initiator is an azo-compound, a peroxide or a persalt.

22. Process according to claim 16 or 17, wherein said polymerisation of the water- soluble monomers is carried out by thermally treating said aqueous solution or suspension at a temperature comprised between 70 and 100°C.