Reduced maximum reducibility temperature zirconium oxide and cerium oxide based composition, method for the production and use thereof as a catalyst

Technical Field

[0001] The present invention relates to a composition based on zirconium oxide and cerium oxide with a low maximum reducibility temperature, to its method of preparation and to its use as a catalyst. In particular the present invention relates to compositions presenting a low reducibility temperature both of the surface and of the bulk composition material.

[0002] At the present time, catalysts called "multifunctional" catalysts are used for the treatment of exhaust gases from internal combustion engines (by automobile postcombustion catalysis). Multifunctional catalysts are understood to mean those capable of carrying out not only oxidation, in particular of carbon monoxide and the hydrocarbons present in exhaust gases, but also reduction, in particular of nitrogen oxides also present in these gases ("three-way" catalysts). Today, zirconium oxide and cerium oxide appear to be two particularly important and advantageous constituents for this type of catalyst. To be effective, these catalysts must have a high specific surface area even at high temperature.

[0003] Another quality required of these catalysts is reducibility. The term "reducibility" is understood to mean, here and for the rest of the description, the capacity of the catalyst to be reduced in a reducing atmosphere and to be reoxidized in an oxidizing atmosphere. This reducibility can be measured by the capacity to capture hydrogen. A well know measurement method is by temperature programmed reduction (TPR). It is due to the cerium in the case of compositions of the type of those of the invention, cerium having the property of being reduced or of being oxidized. This reducibility and, as a consequence, the effectiveness of the catalyst, are a maximum at a temperature that is currently rather high in the case of the known catalysts. This temperature is generally around 600° C. Background Art

[0004] For the purpose of lowering the reducibility temperature, compositions have been proposed, such as for example in US2006178261 A1 , that are based on zirconium oxide and cerium oxide comprising a zirconium oxide proportion of at least 50% by weight, having a maximum reducibility temperature of at most 500°C. However such compositions have a poor long-term stability.

[0005] For the purpose of increasing the stability of catalyst compositions, the addition of doping agents such as Y, La, Pr, or Nd has been proposed, however such doping leads to deteriorated reducibility.

[0006] WO2019238701 A1 discloses catalysts obtained by ion implantation of catalysts supported by aluminium oxide, cerium oxide, zirconium oxide, mixed cerium-zirconium oxide, titanium oxide or zeolite. The catalysts are active at a lower temperature than the untreated catalytic starting material. However only the reduction temperature of the surface species, for example Ce(IV) surface species, is lowered.

[0007] Now, there is a need for catalysts having an low reducibility temperature and therefore for catalysts having a higher performance, within lower temperature ranges and that present long-term stability.

[0008] The object of the invention is therefore the development of a stable catalyst having a low reducibility temperature. In particular the present invention relates to catalyst compositions presenting a low reducibility temperature both of the surface and of the bulk composition material.

[0009] Other features, details and advantages of the invention will become even more fully apparent on reading the following description, and from specific but nonlimiting examples intended to illustrate it.

[0010] The term "lanthanides" is understood to mean elements of the group formed by the elements of the Period Table with an atomic number lying between 57 and 71 included.

[0011] It should be pointed out that in the rest of the description, unless otherwise indicated, in the ranges of values given the limiting values are inclusive. Summary of Invention

[0012] The present invention concerns catalyst compositions based on zirconium oxide and cerium oxide comprising a zirconium oxide proportion of at least 30% by weight and a cerium oxide proportion of at least 25% by weight, further comprising at least 0.5% by weight, expressed as the oxide, of at least one dopant element, chosen from lanthanides other than cerium and from yttrium.

[0013] While not always detectable for lack of available detection methods, the compositions of the present invention are furthermore implanted with ions of nitrogen, argon or oxygen, resulting in the creation of free electrons in the composition’s lattice. The compositions of the present invention thus comprise ions of nitrogen, argon or oxygen.

[0014] The compositions of the invention, probably by way of the free electrons created in the composition’s lattice, have a reducibility temperature of Ce(IV) surface species of at most 400°C and a reducibility temperature of Ce (IV) bulk species of at most 550°C.

Detailed description of embodiments

[0015] The compositions of the invention are of the mixed oxide type, based on zirconium oxide and cerium oxide as main components. They may also include at least one dopant element chosen from lanthanides other than cerium and from Group III elements. In this case, the compositions may therefore be in particular ternary or quaternary compositions. The aforementioned at least one dopant element may more particularly be chosen from lanthanum, neodymium, praseodymium, gadolinium, yttrium and scandium. The compositions may more particularly comprise oxides of zirconium, cerium, yttrium and lanthanum or of zirconium, cerium, and yttrium.

[0016] The contents of the various constituents in the compositions of the invention may vary. These contents are expressed here, and for the rest of the description, as mass percentage of the constituent’s oxide (ZrO2, CeO2 and TR2O3, TR denoting the dopant, yttrium and/or a lanthanide other than cerium) relative to the overall composition. The zirconium content is at least 30%, and may particularly be at least 40%, even more particularly at least 45%, and even more particularly at least 47%. Furthermore, the zirconium content may be at most 65%, more particularly be at most 62% and even more particularly at most 55%. The cerium content is at least 25%, and may more particularly be at least 30% and even more particularly at least 35%. Furthermore, the cerium content may be at most 65%, more particularly at most 62% and even more particularly at most 55%. The content of a dopant is at least 0.5% by weight, expressed as the oxide. Furthermore the content of a dopant may be most 15% and it may be more particularly at most 10%, and it may be between 3% and 10%.

[0017] It should be noted that for the purpose of the present invention the proportions if the constituents of the compositions of the present invention do not take into account the implanted ions. Furthermore, The same proportions of zirconium oxide, cerium oxide, dopant oxide and precious metals, described for the ion implanted compositions of the present invention are also present in the starting material composition before implantation with ions. Therefore the proportions of the constituents of the starting material are not enumerated separately as the same proportions apply as for the implanted compositions of the present invention.

[0018] The compositions of the invention optionally further comprise precious metals. The nature of these metals and the techniques of incorporating them into these compositions are well known to those skilled in the art. The metals may be platinum, rhodium, palladium or indium, and they may especially be incorporated into the compositions by impregnation. Other methods are also known in the art for preparing metal comprising catalyst compositions, such as coprecipitation, impregnation-precipitation, sol-gel deposition, physical vapor deposition, chemical vapor deposition, atomic layer deposition, electroplating, underpotential deposition, cementation, transmetalation, and flame spray pyrolysis.

[0019] The inventors have surprisingly found that the ion implanted compositions of the present invention most surprisingly show a lowered reduction temperature of both surface and bulk Ce(IV), compared to the non-implanted starting material. Thus the process of the present invention allows for the manufacture catalysts that show high performance at low temperatures [0020] The compositions of the invention that do not comprise precious metals may be essentially characterized by the fact that they have a maximum reducibi lity temperature of surface Ce(IV) of at most 400° C. This maximum reducibility temperature may in particular be at most 385° C and even more particularly at most 370° C. Thus, this maximum temperature may be between 300° C and 400° C and especially between 330° C and 370° C.

[0021] Furthermore the compositions of the invention that do not comprise precious metals may be essentially characterized by the fact that they have a maximum reducibility temperature of bulk Ce(IV) of at most 550° C. This maximum reducibility temperature may in particular be at most 540° C and even more particularly at most 530° C. Thus, this maximum temperature may be between 500° C and 540° C and especially between 530° C and 540° C.

[0022] The compositions of the invention that do comprise precious metals may be essentially characterized by the fact that they have a maximum reducibility temperature of surface Ce(IV) of at most 140° C This maximum reducibility temperature may in particular be at most 120° C and even more particularly at most 110° C. Thus, this maximum temperature may be between 70° C and 140° C and especially between 80° C and 110° C.

[0023] Furthermore the compositions of the invention that do comprise precious metals may be essentially characterized by the fact that they have a maximum reducibility temperature of bulk Ce(IV) of at most 290° C. This maximum reducibility temperature may in particular be at most 280° C. and even more particularly at most 270° C. Thus, this maximum temperature may be between 220° C and 290° C and especially between 240° C and 270° C.

[0024] The reducibility of the compositions is determined by measuring their capacity to capture hydrogen as a function of temperature. This measurement is also used to determine a maximum reducibility temperature that corresponds to the temperature at which hydrogen capture is a maximum and in which, in other words, the reduction of cerium (IV) to cerium (III) is also at a maximum.

[0025] The compositions of the invention are characterized by the fact that they have free electrons trapped in the lattice, in particular due to the implantation of ions. Electron paramagnetic resonance spectroscopy (EPR) is used to determine the number free electrons trapped in the lattice. In order to quantify the number of free electrons that are not directly related to Ce ³⁺ -4f1 or O’ - 2p5, that are trapped in the lattice and responsible for the EPR signal at g- value of g = 2.0026, the following procedure was used: The first step consists in normalizing the EPR first-derivative spectrum to the sample mass (expressed in gram), the receiver gain and the number of accumulated scans then, the signal of interest is simulated using WinSimfonia software of the Broker company, considering a S = spin system with an isotropic g-value of 2.0026, and its position, that is its resonance magnetic field, its linewidth and its intensity are adjusted to perfectly fit the experimental data. The first- derivative simulated signal is then double-integrated to extract the value of the absorption curve area A, expressed in arbitrary units, that is finally compared to the one obtained for the EPR signal of a reference sample with a known concentration of electrons or spins in this case ‘Broker weak pitch’ reference sample with N _wp = 2.66x10 ¹³ spins/g.

[0026] Through ion implantation the compositions of the present invention may be provided with at least 3 x 10 ¹⁴ free electrons per gram in the lattice.

[0027] The preparation of the starting material for the compositions of the present invention is well known. Generally it comprises the following steps: a. a mixture comprising a zirconium compound, a cerium compound and, a compound of an aforementioned dopant element is formed; b. said mixture is brought into contact with a basic compound, by means of which a precipitate is obtained; c. said precipitate is separated and then calcined, forming a calcined precipitate; d. optionally, a precious metal is incorporated into the calcined precipitate; e. The starting material, generally obtained as a powder, prepared according to steps (a) to (d) are the implanted with ions of nitrogen, argon or oxygen.

[0028] The step (a) of the method therefore consists in preparing a mixture in a liquid medium of a zirconium compound, a cerium compound and optionally at least one compound of the additional aforementioned element.

[0029] The mixing is generally carried out in a liquid medium, which is preferably water.

[0030] The compounds are preferably soluble compounds. These may especially be zirconium, cerium, lanthanide, and yttrium salts. These compounds may be chosen from nitrates, sulphates, acetates, chlorides and ceric ammonium nitrates.

[0031] As examples, mention may thus be made of zirconium sulphate, zirconyl nitrate or zirconyl chloride. Most generally, zirconyl nitrate is used. Mention may also be especially be made of cerium (IV) salts such as, for example, nitrates or ceric ammonium nitrates, which are particularly suitable here. Ceric nitrate may be used. It is advantageous to use salts with a purity of at least 99.5% and more particularly at least 99.9%. An aqueous ceric nitrate solution may for example be obtained by the reaction of nitric acid on a hydrated ceric oxide prepared conventionally by reacting a solution of a cerous salt, for example cerous nitrate, with an ammonia solution in the presence of hydrogen peroxide. It is also possible in particular to use a ceric nitrate solution obtained by the method of electrolytic oxidation of a cerous nitrate solution, as described in the document FR-A-2 570 087, which constitutes here an advantageous raw material.

[0032] It should be noted here that the aqueous solutions of cerium salts and zirconyl salts may have a certain initial free acidity, which can be adjusted by the addition of a base or an acid. However, it is equally possible to employ an initial solution of cerium and zirconium salts having actually a certain free acidity as mentioned above and solutions that will have been neutralized beforehand to a greater or lesser extent. This neutralization may be carried out by the addition of a basic compound to the aforementioned mixture so as to limit this acidity. This basic compound may for example be an ammonia solution or a solution of alkali metal (sodium, potassium, etc.) hydroxides, but preferably an ammonia solution.

[0033] Finally, it should be noted that, when the starting mixture contains a cerium compound in which cerium is in the Ce(lll) form, it is preferable to employ, during the method, an oxidizing agent, for example hydrogen peroxide. This oxidizing agent may be used by being added to the reaction mixture during step (a) or during step (b), especially at the end of the latter step.

[0034] It is also possible to use a sol as starting compound of zirconium or cerium. The term "sol" denotes any system consisting of fine solid particles of colloidal dimensions, that is to say dimensions of between about 1 nm and about 500 nm, based on a zirconium or cerium compound, this compound generally being a zirconium or cerium oxide and/or hydrated oxide, in suspension in an aqueous liquid phase, said particles furthermore optionally being able to contain residual amounts of bonded or adsorbed ions, such as for example nitrate, acetate, chloride or ammonium ions. It should be noted that, in such a sol, the zirconium or cerium may be either completely in the form of colloids, or simultaneously in the form of ions and in the form of colloids.

[0035] It does not matter whether the mixture is obtained from compounds initially in the solid state, which will subsequently be introduced into an aqueous stock for example, or directly from solutions of these compounds, said solutions then being mixed in any order.

[0036] In step (b) of the method, said mixture is brought into contact with a basic compound. As base or basic compound, it is possible to use products of the hydroxide type. Mention may be made of alkali metal or alkaline-earth metal hydroxides. It is also possible to use secondary, tertiary or quaternary amines. However, amines and aqueous ammonia may be preferred in so far as they reduce the risks of contamination by alkali metal or alkaline-earth metal cations. Mention may also be made of urea. The basic compound is generally used in the form of an aqueous solution.

[0037] The way in which the mixture and the solution are brought into contact with each other, that is to say the order of introduction thereof, is not critical. However, this contacting may be carried out by introducing the mixture into the solution of the basic compound. This variant is preferable in order to obtain compositions in the form of solid solutions.

[0038] The contacting or the reaction between the mixture and the solution, especially the addition of the mixture into the solution of the basic compound, may be carried out in a single step, gradually or continuously, and it is preferably performed with stirring. It is preferably carried out at room temperature.

[0039] The product obtained may optionally be dried, by methods well known o the person skilled in the art., before the calcination step. Drying may be performed for example in an oven at a temperature of 100°C for more than 6 hours, for example up to 12 hours.

[0040] Step (c) of the process is a calcination step.

[0041] This calcination allows the crystallinity of the product obtained to be increased, and it may also be adjusted and/or chosen depending on the subsequent use temperature reserved for the composition according to the invention, taking into account the fact that the specific surface area of the product is lower the higher the calcination temperature employed.

[0042] According to a first method of implementation, the calcination takes place in an oxidizing atmosphere, for example in air. In this case, the calcination is generally carried out at a temperature of between 300 and 1000° C. for a time of generally at least 30 minutes. In an advantageous embodiment the calcination is carried out at a temperature of between 300 and 600°C, more advantageously between 400 and 500°C, even more advantageously between 450 and 550°C. In an advantageous embodiment the calcination is carried out for a duration of between 2 to 4 hours.

[0043] In optional step (d), a precious metal chosen from platinum, rhodium, palladium or iridium is incorporated into the composition, for example by impregnation or magnetron sputtering.

[0044] In the case of these uses in catalysis, the compositions of the invention may be employed in combination with precious metals. The nature of these metals and the techniques of incorporating them into these compositions are well known to those skilled in the art. For example, the metals may be platinum, rhodium, palladium or iridium, and they may especially be incorporated into the compositions by impregnation or magnetron sputtering.

[0045] The starting material of resulting of steps (a) to (d) is implanted with ions selected among the ions of nitrogen, argon, and oxygen. [0046] To the best of our knowledge, the ion implantation creates free electrons in the lattice of the material that improve the efficiency of the catalytic process and in particular reduce the reducibility temperatures of surface Ce(IV) and bulk Ce(IV) species. To the best of our knowledge, the number of free electrons in the lattice depends on the composition of the starting material, in particular of the relative amounts of cerium oxide, zirconium oxide and of the amount and type of dopant(s). It was in particular found that the defect creation efficiency, that is the amount of free electrons created in the lattice per implanted ion, depends on the composition of the starting material.

[0047] The present invention thus further concerns the use of implanted ions to reduce the reduce the reducibility temperatures of surface Ce(IV) and bulk Ce(IV) species of zirconium oxide and cerium oxide comprising a zirconium oxide proportion of at least 30% by weight and a cerium oxide proportion of at least 25% by weight, and at least 0.5% by weight, expressed as the oxide, of at least one dopant element, chosen from lanthanides other than cerium and from yttrium. The present invention in particular concerns the use of implanted ions, implanted according to the parameters mentioned in the present invention as mentioned herein. The present invention in particular also concerns the use of implanted ions in compositions having proportions of cerium, zirconium and dopant oxide as mentioned hereinabove. The present invention in particular also concerns the use of implanted ions to reduce the reducibility temperature of surface and bulk Ce(IV) species in the temperature ranges hereinbelow, both for compositions comprising precious metals and for compositions not comprising precious metals.

[0048] This ion implantation step comprises providing an ion beam, implanting the starting material with an ion beam dose per weight of starting material, comprised between 4.2 x 1 O ¹⁸ ions/g and 4 x 1 O ¹⁹ ions/g comprising monocharged or advantageously monocharged and multicharged ions with an energy of the monocharged ions in the ion beam from at least 10 keV to at most 100 keV;

[0049] Preferably, the ion beam is generated by a plasma filament ion beam source or an electron cyclotron resonance (ECR) plasma source, such as an ECR Plasma Immersion ion implantation (PHI) or preferably an ECR plasma confined with permanent magnets.

[0050] In some embodiments, the catalyst starting material is provided on a support and is mixed intermittently or continuously, so as to uniformly distribute the implanted ions in the catalyst starting material. Advantageously a carrier or support is used that provides continuous mixed during implantation for example a vibrating plate or bowl, a rotary bowl or a rotary drum. Preferably the carrier combines rotating and vibrating movements. It has been observed that the resulting implanted catalyst material is more homogeneously implanted when continuous mixing is provided, such as for example in a rotary bowl or drum. The catalyst starting material on the support shall advantageously form a layer of catalyst starting material having a thickness that is larger than the implantation depth of the ions in the catalyst starting material to avoid implanting ions in the support.

[0051] In some embodiments, at least part of the ions, preferably all ions, are derived from argon atoms, oxygen atoms and/or nitrogen atoms.

[0052] In some embodiments, at least part of the ions, preferably all ions, are derived from nitrogen atoms.

[0053] In some embodiments, at least 50% of the ions, preferably at least 75% of the ions, more preferably at least 90% of the ions, even more preferably at least 95% of the ions and most preferably 100% of the ions are derived from nitrogen atoms.

[0054] In some embodiments, at least part of the ions, preferably all ions, are derived from argon atoms.

[0055] In some embodiments, at least 50% of the ions, preferably at least 75% of the ions, more preferably at least 90% of the ions, even more preferably at least 95% of the ions and most preferably 100% of the ions are derived from argon atoms.

[0056] In some embodiments, the energy E of the monocharged ions in the ion beam is at least 10 keV, preferably at least 20 keV, more preferably at least 30 keV, even more preferably at least 40 keV and most preferably at least 50 keV.

[0057] In some embodiments, the energy E of the monocharged ions in the ion beam is at most 100 keV, preferably at most 90 keV, more preferably at most 80 keV, even more preferably at most 70 keV and most preferably at most 60 keV.

[0058] In some embodiments, the energy E of the monocharged ions in the ion beam is at least 10 keV to at most 100 keV, preferably at least 20 keV to at most 90 keV, more preferably at least 30 keV to at most 80 keV, even more preferably at least 40 keV to at most 70 keV and most preferably at least 50 keV to at most 60 keV.

[0059] In some embodiments, the ion beam comprises a mixture of differently charged ions, and therefore each differently charged ion may have a different energy. This is the result that the energy of the ions in the ion beam is the results of being accelerated by a voltage, preferably the extraction voltage. For example a nitrogen ion beam may comprise 58% N ⁺ ; 32%N ^2+; 9%N ³⁺ and 1 %N ⁺⁴ . When these ions are accelerated by a extraction voltage of 40 kV, the ion beam is made up of 58% of nitrogen ions with an energy of 40 keV, 32% of nitrogen ions with an energy of 80 keV, 9% of nitrogen ions with an energy of 120 keV and 1 % of nitrogen ions with an energy of 160 keV.

[0060] In some embodiments, the ion beam has an average charge (g _avr ) of at least 1 .00 to at most 5.00, preferably at least 1 .10 to at most 3.00, more preferably at least 1.20 to at most 2.00, even more preferably at least 1.30 to at most 1 .75 yet even more preferably at least 1 .40 to at most 1 .60 and most preferably at least 1 .50 to at most 1 .55. Herein, g _avr is the sum of all the charges in the ion beam divided by the number of ions in the ion beam.

[0061] In some embodiments, the monocharged ions in the ion beam have an average energy (E _avr ) of least 10 keV to at most 100 keV, preferably at least 20 keV to at most 90 keV, more preferably at least 30 keV to at most 80 keV, even more preferably at least 40 keV to at most 70 keV and most preferably at least 50 keV to at most 60 keV. Herein E _avr is the sum of all the energy values in the ion beam divided by the number of ions in the ion beam. Therefore, an ion beam with an g _avr of 1 .53 which is extracted by an extraction voltage of 40 kV has an E _avr of 61 .2 keV.

[0062] In some embodiments, the ions with the highest energy in the ion beam have an energy of at most 200 keV. In some embodiments, the ions with the lowest energy in the ion beam have an energy of at least 10 keV.

[0063] In some embodiments, the ion beam is generated by an ECR plasma confined with permanent magnets. Preferably the ion beam source comprises a mono- and multicharged ions plasma confined with permanent magnets which is generated by electron cyclotron resonance (ECR) using a high frequency, such as 2.45; 7.50 or 10.00 GHz. A monocharged ion is an ion bearing a single positive charge, a multicharged ion is an ion bearing more than one positive charge. The ion beam is then extracted to generate mono-multi-energies ions beam penetrating more deeply in the catalytic starting material. This kind of ion beam is more efficient to treat nanoparticles or catalytic material inside other material, such as support, or other catalytic material. Plasma filament ion beam sources and ECR Plasma Immersion ion implantation (PHI) sources generate molecular ions with lower charges states which have the drawbacks to be heavier with less energy, in others words to have reduced depth ranges to treat nanoparticles or catalyst.

[0064] In some embodiments, the ion beam dose is at least 10 ¹³ ions/cm ² , preferably at least 10 ¹⁴ ions/cm ² , even more preferably at least 10 ¹⁵ ions/cm ² at the point of contact with the catalyst starting material, where the catalyst starting material is considered to be forming an essentially flat surface

[0065] In some embodiments, the ion beam dose is at most 10 ¹⁸ ions/cm ² , preferably at most 10 ¹⁷ ions/cm ² , even more preferably at most 10 ¹⁶ ions/cm ² at the point of contact with the catalyst starting material, where the catalyst starting material is considered to be forming an essentially flat surface.

[0066] In some embodiments, the ion beam dose is at least 10 ¹³ ions/cm ² to at most 10 ¹⁸ ions/cm ² , preferably at least 10 ¹⁴ ions/cm ² to at most 10 ¹⁷ ions/cm ² , even more preferably at least 10 ¹⁵ ions/cm ² to at most 10 ¹⁶ ions/cm ² at the point of contact with the catalyst starting material, where the catalyst starting material is considered to be forming an essentially flat surface.

[0067] In some embodiments, the total ion beam dose is split into m separate doses, and wherein the catalytic starting material is mixed or stirred each time between the m different ion implantation treatments, preferably m is at least 4 to at most 64, more preferably at least 8 to at most 32, even more preferably at least 12 to at most 24 and most preferably at least 16 to at most 18. An amount of powder may be spread over a given area or surface and exposed to the ion beam m times to obtain a total ion dose. Each time, between the different doses, the powder may be mixed and may be spread again over the original area to allows to obtain a homogeneous treatment for the powder starting material. In some embodiments, m is at least equal to the ratio of the mean thickness of the powder spread over a given area and the mean free path of the ions inside the powder. The free path being the path ions travel inside the powder before they are stopped by the powder.

[0068] In some embodiments, the advancement step of the ion beam is at least 1 % to at most 50%, preferably at least 2% to at most 40%, more preferably at least 5% to at most 30%, even more preferably at least 7% to at most 20% and most preferably at least 10% to at most 15%. The ion beam may move in a series of round trips separated by a distance corresponding to a fraction of the ion beam diameter called advancement step. A step of 10% for a beam with a diameter of 22.5 mm, means that for each round trip a shift of 2.25 mm is performed. The advancement step may result in a high surface homogeneity of the treatment, preferably regardless the intensity distribution of the ion beam, which may be for instance be a Gaussian shape with more intensity at the centre and less intensity at the periphery.

[0069] In some embodiments, the method comprises n different implanting steps with n multiple doses Xi, preferably wherein each dose Xi is X/n, X being the total ion beam dose, i.e. the sum of the n doses . In some embodiments, the different implanting steps differ by at least one implantation parameter, e.g. different ions may be used in different steps. Preferably n is at most 3, more preferably n is at most 2, and most preferably n is 1 .

[0070] In some embodiments, the method comprises implanting the catalyst starting material with an ion beam that is performed at a pressure of at most 10’ ⁴ Torr, preferably at most 10’ ⁵ Torr, more preferably at most 10’ ⁶ Torr and most preferably at most 10’ ⁷ Torr.

[0071] In some embodiments, noble gas, such as Ar, Kr or Xe, is injected into the treatment chamber, preferably in lower vacuum levels, such as lower than 10’ ⁴ Torr, preferably lower than 10’ ⁵ Torr, more preferably lower than 10’ ⁶ Torr and most preferably lower than 10’ ⁷ Torr. These noble gasses at least partially supress the static electricity induced by the ion implantation of the catalytic material.

[0072] In some embodiments, the pressure in the treatment chamber is at least 3 x 10 ^-6 Torr, preferably at least 5x1 O’ ⁶ Torr more preferably at least 7 x 10’ ⁶ Torr, even more preferably at least 10x1 O’ ⁶ Torr and most preferably at least 20x1 O’ ⁶ Torr. These vacuum levels help to at least partially neutralize electrostatic barrier induced by the implanted ions. In some embodiments, the catalyst starting material is provided on a carrier or support comprising means for dissipating an static charges. For examples the support may comprise or consist of an electrically conducting material, such as a metal, and be electrically grounded.

[0073] In ion implantation on solid substrates, the ion implantation dose is usually expressed using the unit ions/cm ² . This dosage may be calculated using the following formula (units omitted): wherein D is the dosage [ions/cm ² ], I is the ion beam current [A], t is the implantation time [s], S is the surface area [cm ² ], q is the elementary charge 1.6x1 O’ ¹⁹ [Coulomb], This formula is easily adapted for mixtures of single charge and multicharge ions.

[0074] In some embodiments the ion dose is conveniently expressed using the unit ions/g. This dosage may be calculated using the following formula (units omitted): wherein, with the units in square brackets, D is the dosage [ions/cm ² ], I is the ion beam current [A], t is the implantation time [s], Q is the quantity of implanted catalyst starting material [g], q is the elementary charge 1.6x1 O’ ¹⁹ [Coulomb], This formula is easily adapted for mixtures of single charge and multicharge ions. [0075] When the catalyst starting material is evenly spread on a flat substrate, this dosage can be derived from the dosage expressed in ions/cm ² and the surface density o , in g/cm ² , of the evenly distributed catalyst starting material as follows:

[0076] The present invention further concerns compositions that are the result of any possible combination of the embodiments hereinabove.

[0077] The compositions of the invention, as described above or as obtained by the method mentioned above, are in the form of powders, but they may optionally be formed into granules, beads, cylinders or honeycombs of varying dimensions. These compositions may be applied to any support normally used in the catalysis field, that is to say, in particular, thermally inert supports. This support may be chosen from alumina, titanium oxide, cerium oxide, zirconium oxide, silica, spinels, zeolites, silicates, crystalline silicon aluminum phosphates and crystalline aluminum phosphates.

[0078] The compositions may also be used in catalytic systems. These catalytic systems may include a wash coat having catalytic properties and based on these compositions, on a substrate for example of the metal or ceramic monolith type. The wash coat may itself include a support of the type of those mentioned above. This wash coat is obtained by mixing the composition with the support so as to form a suspension that may then be deposited on the substrate.

[0079] These catalytic systems, and more particularly the compositions of the invention, may have very numerous applications. They are thus particularly well suited to, and therefore usable in, the catalysis of various reactions such as, for example, dehydration, hydrosulfurization, hydrodenitrification, desulfurization, hydrodesulfurization, dehydrohalogenation, reforming, steam reforming, cracking, hydrocracking, hydrogenation, dehydrogenation, isomerization, dismutation, oxychlorination and dehydrocyclization of hydrocarbons or other organic compounds, oxidation and/or reduction reactions, the Claus reaction, the treatment of internal combustion exhaust gases, demetalization, methanation, shift conversion, catalytic oxidation of soot emitted by internal combustion engines, such as diesel or petrol engines operating in lean mode. Finally, the catalytic systems and the compositions of the invention may be used as NOx traps.

[0080] Among the uses mentioned, the treatment of internal combustion engine exhaust gases (by automobile postcombustion catalysis) constitutes one particularly advantageous application.

[0081] Consequently, the invention also relates to the use of a composition or of a catalytic system such as those described above for the manufacture of a catalyst for automobile postcombustion.

[0082] The present invention in certain embodiments concerns the following items:

Item 1 . Composition based on zirconium oxide and cerium oxide comprising a zirconium oxide proportion of at least 30% by weight and a cerium oxide proportion of at least 25% by weight, characterized in that it further at least 0.5% by weight, expressed as the oxide, of at least one dopant element, chosen from lanthanides other than cerium and from yttrium, and in that it has a reducibility temperature of Ce(IV) surface species of at most 400°C and a reducibility temperature of Ce (IV) bulk species of at most 550°C.

Item 2. Composition according to Item 1 , characterized in that it comprises at least one dopant element chosen from lanthanum, neodymium, praseodymium, and yttrium.

Item 3. Composition according to any one of the preceding items, characterized in that it comprises a zirconium oxide proportion of at least 40% by weight, more particularly at least 45% by weight.

Item 4. Composition according to any one of the preceding items, characterized in that it has a maximum reducibility temperature of Ce(IV) surface species of at most 385°C, more particularly at most 370°C. Item 5. Composition according to any one of the preceding items, characterized in that it has a maximum reducibi lity temperature of Ce(IV) bulk species of at most 550°C, more particularly at most 530°C.

Item 6. Composition according to any one of the preceding items, characterized in that it comprises a cerium oxide proportion by weight of at most 65%, particularly at most 62% and even more particularly at most 55%.

Item 7. Composition according any one preceding item, characterized in that it comprises no precious metal.

Item 8. Composition according to any one of items 1 to 6, characterized in that it comprises a precious metal chosen from platinum, palladium and rhodium.

Item 9. Composition according to item 8, characterized in that it has a maximum reducibility temperature of Ce(IV) bulk species of at most 290°C, more particularly at most 280°C.

Item 10. Composition according to any one of items 8 or 9, characterized in that it has a maximum reducibility temperature of Ce(IV) surface species of at most 140°C, more particularly at most 120°C.

Item 11 . Composition according to any one of the preceding items, characterized in that it comprises a cerium oxide proportion by weight of at most 65%, particularly at most 62% and even more particularly at most 55%.

Item 12. Composition according to any one preceding item characterized in that it comprises at least 3 x 10 ¹⁴ free electrons per gram in the composition’s lattice.

Item 13. Composition according to any one preceding item characterized in that it comprises an ion dose per weight, comprised between 4.2 x 1O ¹⁸ ions/g and 4 x 1O ¹⁹ ions/g, in particular of ions of nitrogen. Item 14. Method of preparing a composition according to one of the preceding items, characterized in that it comprises the following steps: a. providing a starting material based on zirconium oxide and cerium oxide comprising a zirconium oxide proportion of at least 30% by weight, further comprising at least one dopant element chosen from lanthanides other than cerium and from yttrium and optionally further comprising a precious metal chosen from platinum, palladium and rhodium. b. providing an ion beam, c. implanting the starting material with an ion beam dose comprised between 4.2 x 10 ¹⁸ ions/g and 4 x 10 ¹⁹ ions/g comprising monocharged or monocharged and multicharged ions with an energy of the monocharged ions in the ion beam from at least 10 keV to at most 100 keV.

Item 15. Method according to item 14 wherein the ion beam is generated by a plasma source selected from a. a plasma filament ion beam source or b. an electron cyclotron resonance plasma source, selected among c. a ECR Plasma Immersion ion implanter or d. an electron cyclotron resonance plasma confined with permanent magnets.

Item 16. Method according to any one of items 14 to 15, wherein at least part of the ions or all ions, are derived from argon atoms, oxygen atoms and/or nitrogen atoms.

Item 17. Method according to any one of items 14 to 16, wherein the starting material comprises at least one dopant element chosen from lanthanum, neodymium, praseodymium, and yttrium. Item 18. Method according to any one of items 14 to 17, wherein the starting material comprises a zirconium oxide proportion of at least 40% by weight, more particularly at least 45% by weight.

Item 19. Method according to any one of items 14 to 18, wherein the starting material comprises a cerium oxide proportion by weight of at most 65%, particularly at most 62% and even more particularly at most 55%.

Item 20. Method according to any one of items 14 to 19, wherein the starting material comprises no precious metal.

Item 21 . Method according to any one of items 14 to 20, wherein the starting material comprises a precious metal chosen from platinum, palladium and rhodium.

Item 22. Method according to any one of items 14 to 21 , wherein the starting material comprises a cerium oxide proportion by weight of at most 65%, particularly at most 62% and even more particularly at most 55%.

Item 23. Catalytic system, characterized in that it comprises a composition according to one of Items 1 to 13.

Item 24. Method of treating the exhaust gases of internal combustion engines, characterized in that a catalytic system according to Item 23 or in that a composition according to one of Items 1 to 13 is used as catalyst.

Item 25. Composition according to any one of items 1 to 13 characterized in that it comprises implanted ions of nitrogen

[0083] Specific but nonlimiting examples will now be given.

[0084] The reducibility measurement is carried out by temperature programmed reduction (TPR) in the following manner. 40 mg catalytic test material is placed on a disk with a diameter of 1 mm. Optionally, the test material is submitted to a heating step in an Argon atmosphere wherein the test material is heated starting from room temperature to a temperature of 50°C at a heating rate of 20°C min ^-1 , at 50°C the testing material is equilibrated for 1 hour before it is cooled down to 40°C at a rate of 90°C min ^-1 . After this optional initial heating step, the test material on the disk is placed in an atmosphere consisting of 5%H2, the rest being Argon being paced over the test material at a flow rate of 50 cm ³ min ^-1 . The test material is equilibrated for 40 min at 20°C before it is heated to 800°C at a rate of 5°C min ^-1 . At 800°C the test material is, before it is cooled down at 5°C/min to a temperature of 20°C. The H2 concentration in the measurement cell is monitored to evaluate the test material’s efficiency.

[0085] TPR analysis of the tested materials show two peaks. The first peak at lower temperature corresponds to the reduction of surface species of Ce(IV) to Ce(lll). The second peak at higher temperature corresponds to the reduction of bulk species of Ce(IV) to Ce(lll).

[0086] The implantation step is performed in the following way. The starting material, from 0.5 to 16g, is placed in a microimplantor designed by the company Ionics, including an ECR (Electron Cyclotron Resonance) ion source powered by a 10 GHz and 50 W HF amplifier, and an ion extraction system of 10 kV (kiloVolt). The plasma of the ion source is confined by permanent magnets allowing the production of monocharged and multicharged ions). The starting material, particles agglomerated in powder form, are provided in a vibrating bowl, centered below the ion beam. The diameter of the powder at its surface is slightly larger than the diameter of the diameter of the ion beam. The pressure in the treatment chamber is kept at 10’ ⁵ mbar. The treatment is done with a mixture of mono and multicharged nitrogen ions, in the case of nitrogen ions for example 58% N ⁺ 32%, N ²⁺ , 9% N ³⁺ , and 1 % N ⁴⁺ , extracted by an extraction voltage of for example 35 kV. In the case of the nitrogen ion above the mean charge state is 1 .53 and the mean energy E _avr equals 53 keV at the extraction voltage above. The total dose can be implanted uniformly, without interruption, while the bowl was kept vibrating.

[0087] The starting material of the different examples are given in Table 1 [0088] Table 1

[0089] Comparative Example 1

[0090] 15g of Comparative Starting Material CeO2(40%)-ZrO2 (60%) was provided in a vibrating bowl centered below the ion beam. The diameter of the powder at its surface was slightly larger than the diameter of the diameter of the ion beam. The treatment was done with mono and multicharged nitrogen ions (58% N ⁺ 32% N ²⁺ , 9% N ³⁺ , 1 % N ⁴⁺ ) extracted by an extraction voltage of 35 kV, i.e. with a mean charge state of 1.53 and mean energy E _avr equal to 53 keV. The total dose of 4.49 x 10 ¹⁸ ions/g could be implanted without interruption while the bowl was kept vibrating.

[0091] The other examples were prepared in a similar way, only changing the implantation dosage. These parameters are summarized in Table 2 below.

[0092] Table 2

[0093] The results of the different materials above are summarized in Table 3 below. [0094] Table 3

[0095] The powders above were further characterized by EPR and showed the number N of free electrons per gram in the lattice shown in Table 4 below.

[0096] Table 4