Method of treating powder made from cerium oxide using an ion beam

Abstract

A method of treating a powder (P) made from cerium oxide using an ion beam (F) in which: the powder is stirred once or a plurality of times; the ions of the ion beam are selected from the ions of the elements of the list consisting of helium (He), boron (B), carbon (C), nitrogen (N), oxygen (O), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe)the acceleration voltage of the ions of the beam is between 10 kV and 1000 kV; the treatment temperature of the powder (P) is less than or equal to Tf/3; the ion dose per mass unit of powder to be treated is chosen from a range of between 1016 ions/g and 1022 ions/cm2 so as to lower the reduction temperature of the powder made from cerium oxide (P).

Claims

1. A heterogeneous catalysis device, comprising: a region of transformation of a gas or of a liquid adapted to receive the gas or liquid for treatment, a powder based on cerium oxide disposed in the region and arranged to contact the gas or liquid for treatment, the powder based on cerium oxide having a cumulative dose of ions from prior ion bombardment treatment of between 10.sup.16 ions/gram of powder and 10.sup.22 ions/gram of powder, the ions being selected from ions of elements selected from the group consisting of helium (He), boron (B), carbon (C), nitrogen (N), oxygen (O), neon (Ne), argon (Ar), krypton (Kr) and xenon (Xe), wherein the powder based on cerium oxide comprises nano-holes having a greatest dimension of greater than or equal to 10 nm and a smallest dimension greater than equal to 1 nm.

2. The device of claim 1, wherein the device further comprises a honeycomb support, wherein one or more walls of the honeycomb support are coated with the powder based on cerium oxide.

3. The device of claim 2, wherein the honeycomb support is formed of alumina.

4. The device of claim 1, wherein the device is a catalytic converter.

5. The device of claim 1, wherein the powder based on cerium oxide is selected from the group consisting of ceria (CeO.sub.2) powder and a mixed cerium and zirconium oxide powder.

6. The device of claim 1, wherein the ions are selected from ions of elements selected from the group consisting of helium (He), boron (B), carbon (C), nitrogen (N), oxygen (O) and neon (Ne).

7. The device of claim 1, wherein the powder based on cerium oxide has a cumulative dose of ions from the prior ion bombardment treatment of between 10.sup.18 ions/gram of powder and 10.sup.20 ions/gram of powder.

8. The device of claim 1, wherein the powder has a grain size of about 10 nm to about 5 micrometers.

9. The device of claim 1, further comprising an oxidation catalyst disposed in the region.

10. The device of claim 9, wherein the oxidation catalyst is platinum.

11. The device of claim 1, wherein a working temperature of the powder based on cerium oxide is greater than or equal to 50 C. during heterogeneous catalysis.

12. The device of claim 11, wherein the working temperature of the powder based on cerium oxide is greater than or equal to 100 C. during heterogeneous catalysis.

13. The device of claim 5, wherein the mixed cerium and zirconium oxide powder is a ceria-zirconia (Ce.sub.0.7Zr.sub.0.3O.sub.2) powder.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other distinguishing features and advantages of the present invention will emerge in the description below of nonlimiting implementation of examples, in particular with reference to the appended drawings, in which:

(2) FIG. 1 illustrates the treatment by an ion beam of a powder (P) spread over a surface (S), the specific parameters of the treatment being specified as a thickness (e) of the powder to be treated and the distance (a) covered by the incident ions in the powder. A binomial powder mixing model was constructed by the inventors on the basis of this figure: in each mixing operation, a powder grain has a probability p=(a/e) of being within the region exposed to the beam and a probability q=(1(a/e)) of being outside the reach of the beam, with p+q=1;

(3) FIGS. 2.a to 2.b illustrate advantageous experimental yields (expressed in arbitrary units (AU)) obtained in converting hydrogen into water at different temperatures, with ceria (CeO.sub.2) and ceria-zirconia (Ce.sub.0.7Zr.sub.0.3O.sub.2) which are treated according to the process of the invention;

(4) FIG. 3 indicates, by extrapolation, the degree of treatment of a powder based on cerium oxide (on the ordinate, expressed as %) by a beam of nitrogen ions with a mean energy of 50 keV, as a function of the amount of powder spread over a surface area of 100 cm.sup.2 and of the total number of mixing operations N (on the abscissa), each being carried out before each treatment by an ion beam. This extrapolation is based on the powder treatment model set out in FIG. 1. The inventors estimate that a minimum degree of treatment equal to 10% is desirable on a powder and that it is preferable for this degree to be greater than 50%;

(5) FIG. 4 indicates, by extrapolation, the degree of treatment of a cerium oxide powder (on the ordinate, expressed as %) by a beam of nitrogen ions with a mean energy of 50 keV, for an amount of powder fixed at 0.15 mg, spread over 3 surfaces with respective areas equal to 50, 100 and 200 cm.sup.2, as a function of the total number of mixing operations N (on the abscissa), each being carried out before each treatment by the ion beam. This extrapolation is based on the powder treatment model set out in FIG. 1. The inventors estimate that a minimum degree of treatment equal to 10% is desirable on a powder and that it is preferable for this degree to be greater than 50%;

(6) FIG. 5 indicates, by extrapolation, the degree of treatment of a cerium oxide powder (on the ordinate, expressed as %), as a function of several types of ions (from the lightest, He, to the heaviest, Ar), with an energy of 50 keV, for an amount of powder fixed at 0.15 mg spread over a surface area fixed at 100 cm.sup.2, as a function of the total number of mixing operations N (on the abscissa), each being carried out before each treatment by the ion beam. This extrapolation is based on the powder treatment model set out in FIG. 1. The inventors estimate that a minimum degree of treatment equal to 10% is desirable on a powder and that it is preferable for this degree to be greater than 50%.

(7) For reasons of clarity, the dimensions of the different components represented in these figures are not necessarily in proportion with their true dimensions. The same numerical references in the different figures correspond to the same parts.

DETAILED DESCRIPTION

(8) According to examples of the implementation of the present invention, samples, ceria (CeO.sub.2) and also ceria-zirconia (Ce.sub.0.7Zr.sub.0.3O.sub.2) powders, were mixed and spread before each treatment over a surface made of aluminum and formed the subject of experimental studies for treatment with nitrogen ions emitted by an ECR source. The treatment consisted in repeating 16 times the same procedure comprising the following stages: firstly, mixing and uniform spreading of 150 mg of powder over a square surface of 100 cm.sup.2 with a fine brush, secondly, treatment of the spread powder with a beam with a diameter of 40 mm, moving at a rate of 80 mm/s, with an amplitude of movement of 6040 cm, 11 passes (corresponding to an advance of the beam of 30%), in order to achieve a dose of ions per unit of surface area equal to 510.sup.15 ions/cm.sup.2 at the end of each treatment; after having repeated the procedure 16 times, the total treatment dose accumulated by the 150 mg of powder spread over 100 cm.sup.2 is equal to 16510.sup.15100, i.e. 810.sup.18 ions, which amounts to a total treatment dose per unit of weight of powder equal to 510.sup.19 ions/gram of powder. The inventors estimate that the dose range in order to have a particularly effective treatment should preferably be between 10.sup.18 ions/gram and 10.sup.20 ions/gram of powder. The degree of treatment was evaluated at 80%, for a distance covered by the nitrogen ion at 50 keV in a ceria or ceria-zirconia powder evaluated at 0.2 microns and a thickness of ceria or ceria-zirconia powder, spread over 100 cm.sup.2, evaluated at 2 microns, i.e. 10 times the distance covered by the ion.

(9) The ion beam which was used to treat the preceding powders comprises N.sup.+ ions, the intensity of which is substantially equal to 0.58 mA, N.sup.2+ ions, the intensity of which is substantially equal to 0.32 mA, and finally N.sup.3+ ions, the intensity of which is substantially equal to 0.1 mA; the extraction and acceleration voltage of these ions is 35 kV; the N.sup.+ energy is 35 keV, the N.sup.2+ energy is 70 keV and the N.sup.3+ energy is 105 keV. It is estimated that the mean energy of these ions is approximately 50 keV.

(10) An experimental study in order to evaluate the impact of the treatment on the conversion of hydrogen into water as a function of the temperature was subsequently carried out on virgin and treated ceria and ceria-zirconia powders. This study consisted in injecting, into a cell, a stream of 5% hydrogen (H.sub.2) in a stream of argon (Ar), in a proportion of 25 cm.sup.3 min.sup.1, the temperature being varied between 30 and 800 C. according to a gradual rise in the temperature of 7.5 C./min. The degree of formation of water was measured as a function of the temperature at the surface or in the body of the virgin and treated powder.

(11) Such tests are representative of the reactions encountered in three-way catalytic converters, where cerium oxide compounds-buffers are added to alumina of a support in order to limit the impact of the variations in composition of the gases by storing oxygen when it is in excess in order to discharge it when it is in deficit.

(12) In order to validate the impact of the treatment by ionic bombardment of the cerium oxide powders for three-way catalytic converters, the inventors thus studied the reaction for the oxidation of hydrogen under the water form in the presence of cerium oxide, with or without contribution of platinum. This is because the reaction for the oxidation of hydrogen under the water form is very similar to and representative with regard to the chemical reactions of both types of oxidation reactions occurring in the three-way catalytic converter, namely that of carbon monoxide to give carbon dioxide and that of the nonincinerated gases with formation of water. Tables 1 and 2 and associated figures illustrate the improvements observed by the inventors and based on this reaction for the oxidation of hydrogen are obtained with cerium oxide powders treated according to the process of the invention. These observed improvements can be easily transposed, in terms of efficiency, to the oxidation reactions occurring in the three-way catalytic converter.

(13) Experimental results obtained for the reduction temperature at the surface and in the body for a virgin and treated ceria are recorded in table 1 and experimental results obtained for a virgin and treated ceria-zirconia are recorded in table 2.

(14) TABLE-US-00001 TABLE 1 Estimated doses Reduction Reduction (10.sup.17 ion/g of temperature at the temperature CeO.sub.2 sample powder) surface ( C.) in the body ( C.) Virgin 0 340 780 10 5 280 770

(15) TABLE-US-00002 TABLE 2 Estimated doses Reduction Reduction Ce.sub.0.7Zr.sub.0.3O.sub.2 (10.sup.17 ion/g temperature at temperature in sample of powder) the surface ( C.) the body ( C.) Virgin 0 358 561 10 5 270, 305, 370 408

(16) It is found, in table 1, that, for the treated ceria, the reduction temperature at the surface is very advantageously decreased, changing from 340 to 280 C., i.e. a decrease of 60 C.; the temperature in the body is decreased much less, changing only from 780 C. to 770 C.

(17) It is observed, in table 2, that, for the treated ceria-zirconia, the reduction temperature at the surface is very advantageously decreased at the surface, changing from 358 to 270, i.e. a decrease of 88 C.; it is decreased much more in the body, changing from 561 C. to 408 C., i.e. a decrease of 153 C.

(18) FIGS. 2.a and 2.b give the details of these results respectively for the ceria powder and the ceria-zirconia powder. The axis of the abscissae represents the axis of the temperatures on which movement takes place at the rate of 7.5 C./min and the axis of the ordinates represents the degree of production of water (expressed in arbitrary units). It is observed that, in FIG. 2.a, the curve (2) associated with the ceria powder treated according to the process of the invention very favorably breaks away from the curve (1) associated with the virgin ceria powder in the temperature region between 100 and 500 C.; the degree of production of water is twice as high there. Likewise, in FIG. 2.b, the curve (2) associated with the ceria-zirconia powder treated according to the process of the invention differs in a massive and high degree of production of water in a temperature range of less than approximately 200 C., with respect to the degree of the curve (1) associated with the virgin ceria-zirconia powder.

(19) The inventors were thus able to experimentally identify a particularly advantageous operating point of the process of the invention applied to powders based on cerium oxide, for a total dose per unit of weight of powder to be treated (510.sup.19 ion/g of powder) and a degree of treatment (80%). The inventors recommend broadening this operating point, preferably to a total dose range, as number of ions per unit by weight of powder to be treated, of 10.sup.18 and 10.sup.20 ion/g and to a range of degree of treatment of greater than 10%, preferably greater than 50%.

(20) The total dose per unit of weight of powder to be treated being fixed within the range recommended be; pw, the inventors have attempted, on the basis of a simple empirical model set out in FIG. 1, to understand and to estimate the degree of treatment of a powder as a function of the conditions employed by the process of the invention, such as the amount of powder, its spreading surface area or even the type and the energy of the ion of the beam. The treatment of a powder (P) spread over a substrate (S) according to an unchanging thickness (e), partially traversed by an ion beam (F), the reach of which into the powder delimits a region of travel with a maximum depth (a), is illustrated in FIG. 1. Below this region, the powder grains are not affected by the beam. This treatment model assumes that, at each mixing operation and before each treatment, a powder grain can be located in the region of travel of the incident ions or outside the same region. An increase in the region of travel of the incident ion, an increase in the mixing number or a reduction of the thickness of the powder to be treated, obtained by reduction in the amount of powder or by an increase in the spreading surface area, are so many factors which favor the probability of a powder grain being exposed at least once to the beam. The inventors have been able to quantify the impact of these factors on the degree of treatment; the associated trends are illustrated in FIGS. 3, 4 and 5.

(21) The impact of the spreading of a given amount of powder based on cerium oxide on the degree of treatment of the powder is represented in FIG. 3. The beam is composed of nitrogen ions with a mean energy of approximately 50 keV, covering a mean distance of 0.2 micron in the powder. For a spreading of 0.15 mg of cerium oxide powder spread over 100 cm.sup.2 and treated according to 16 mixing operations, the degree of treatment of the powder grains is approximately 80% (treatment conditions of the powders studied experimentally). It is observed that, in order to increase the efficiency of the mixing by 15%, changing from a degree of treatment of 80% to 95%, the mixing number can be multiplied by 2; otherwise it is possible to provide a more powerful mixing system. In order to achieve a minimum degree of treatment of 10%, the mixing number can be multiplied by 2 when the amount of powder to be treated is multiplied by 2.

(22) The impact of the spreading of a given amount of powder based on cerium oxide on the degree of treatment of the powder is represented in FIG. 4. The beam is composed of nitrogen ions with a mean energy of approximately 50 keV, covering a mean distance of 0.2 micron in the powder.

(23) Under the experimental conditions expressed above, 0.15 mg of powder spread and treated over 100 cm.sup.2 has a thickness of approximately 2 microns. The reach of the ions is approximately 0.2 micron. For 16 mixing operations, the inventors estimated that the powder had been treated with a degree of 80%. In order to increase the amount of powder to be treated by a factor of 2, the number of mixing operations can be increased by a factor of 2 in order to obtain an identical degree of treatment.

(24) In order to experimentally evaluate the impact of the surface area on the treatment of 150 mg of alumina powders with a total dose of 810.sup.17 nitrogen ions, i.e. 510.sup.19 ion/g of alumina powder, according to different spreading surface areas equal to 1200 cm.sup.2, 100 cm.sup.2 and 100 cm.sup.2, associated with a number of respective mixing operations equal to 2, 2 and 16, the inventors carried out an X-ray photoelectron spectroscopy (XPS analysis), also known as electron spectroscopy for chemical analysis (ESCA), of the powder. They were able to observe that the spectrum of the powder showed virtually no change in the first two cases, as if the powder grains had not been significantly exposed to the beam, unlike the third case, for which the electrostatic charges deposited by the beam on the powders made it impossible to carry out the XPS analysis. The inventors concluded that, in the first case, the powder thickness, estimated on average at 0.3 micron, which may be much less than 0.1 micron in places, appeared too low to be effectively treated by a beam, the distance covered of which in the powder is approximately 0.3 to 0.4 micron. The inventors recommend, preferably, spreading a given amount of powder over a surface area calculated in order for the thickness (e) to be at least equal to twice the mean distance covered by the ion in said powder. In the example of the second case, the mixing number was not sufficient; the inventors evaluated a degree of treatment of approximately 10% (based on the model of FIG. 1 applied to alumina powders) in order to obtain a significant signature by the XPS analysis. On this basis, the inventors recommend, preferably, a minimum mixing number which makes it possible to achieve a minimum degree of treatment at least equal to 10%. In the third case, the inventors observed that the powder grains were predominantly exposed to the beam in so far as they retain a persistent electrical charge, which makes the XPS analysis impossible. The inventors evaluated the degree of treatment at 60%, six times greater than that of the second case, estimated at 10%. On this basis, the inventors recommend, preferably, a minimum number of mixing operations which makes it possible to achieve, preferably, a degree of treatment of the powder grains at least equal to 50%.

(25) The impact of the choice of the type of ion on the degree of treatment, for 150 mg of powder based on cerium oxide spread over 100 cm.sup.2, is illustrated in FIG. 5. For any degree of treatment, it is observed that, for one and the same identical mixing number, helium makes possible a degree of treatment 60% greater than that of nitrogen, which has a degree of treatment 60% greater than that of argon. On this basis, the inventors recommend, preferably, the use first of light ions, such as helium (He), boron (B), carbon (C), nitrogen (N), oxygen (O) or neon (Ne).

(26) It should be remembered that the calculation of the degree of treatment of a powder based on the distance covered by an ion in a given metal oxide, a given incident ion with a given energy is based on the principles of the physics of the interactions of particles with matter. Methods and data which make it possible to carry out these calculations are disclosed in particular in the publications The Stopping and Range of Ions in Matter by J. F. Ziegler, volumes 2-6, Pergamon Press, 1977-1985, The Stopping and Range of Ions in Solids by J. F. Ziegler, J. P. Biersack and U. Littmark, Pergamon Press, New York, 1985 (new edition in 2009), and J. P. Biersack and L. Haggmark, Nucl. Instr. and Meth., vol. 174, 257, 1980.

(27) In addition, software has been developed and sold in order to facilitate or carry out such calculations, such as, for example, the software sold under the SRIM (The Stopping and Range of Ions in Matter) and TRIM (The Transport of Ions in Matter) names, developed in particular by James F. Ziegler.

(28) It is obvious that it is possible to combine the calculations and results set out so as to determine the desired number of mixing operations on a given spreading surface, in order to carry out the treatment of an amount of powder with a degree of treatment desired for the possible combinations, in particular of type of metal oxide powder, of type of ion for the treatment, of the energy of these ions.

(29) It is thus possible to carry out the choice of the dose of ions per gram of powder to be treated and of the number of mixing operations to be carried out on a spreading surface so as to treat the latter predictively.

(30) It should be noted that experimental observations on samples which have formed the subject of treatment by an ion beam can make it possible to confirm or adjust the range of dose of ions selected and the mixing number selected. Such observations can in particular be carried out by XPS, in order to refine the mixing, or by measurement of the temperature for reduction of the treated powder, in order to optimize the dose required.

(31) In order to extrapolate the preceding results to a system of vibrating mixing (bowl, plate, and the like) under a beam, the inventors recommend, for example, characterizing a system by depositing a layer of powder based on cerium oxide colored white on a layer of powder based on cerium oxide colored black and observing, as a function of the vibration time, the statistical mixing of the white and black grains at the surface of the powder as a function of the time with the aim of establishing a connection between the number of mixing operations and the vibration time of the device. At the time t0=0, 100% of the powder grains at the surface are white. When, at a characteristic time t1, 25% of the grains are white and 75% of the grains are black, the inventors estimate that 50% of the blended black and white powder grains have been exposed at least once to the beam. A correspondence between the mixing system with a vibrating device and the mixing system described above can thus be established, for which system it is possible to determine, with the binomial statistical model of FIG. 1, the mixing number necessary to treat, with a given degree of treatment, a powder, the amount of which is equal to the amount of white powder and black powder, the thickness of which is equal to the sum of the thicknesses of white powder and black powder and the spreading surface area of which is equal to that of the white powder, which is itself equal to that of the black powder. It is thus easy to proportionally link a mixing number to a duration of vibration of the device and to extrapolate these results in order to know, for example, the vibration time of the device in order to have a degree of treatment of the powder grains equal to 80%.

(32) The methodologies presented in the examples above allow a person skilled in the art to easily find the means for employing the process of the present invention via simple preliminary tests liable to specify the favorable treatment conditions in accordance with the ranges indicated.

(33) The invention is not limited to the implementational types exemplified and should be interpreted nonlimitingly and encompasses any equivalent embodiment. It should be noted that, while implementational examples with powders based on cerium oxide have been presented, the process according to the invention can be employed with a great many mixing systems and a great many metal oxide powders comprising cerium for the purpose of obtaining an increase in the porosity making possible better storage and better release of oxygen at the nanoscopic scale.