Catalyst/catalyst support compositions having high reducibility and comprising a nanometric cerium oxide deposited onto a support substrate

Abstract

Catalyst/catalyst support compositions are characterized by a supported cerium oxide, deposited onto a silica, alumina, titanium or zirconium based support, including particles of said supported oxide deposited onto said support, individualized or in the form of aggregates, no greater than 500 nm in size and having, after 6 hours of calcination at a temperature of at least 800 C., a measured reducibility from 30 C. and 900 C. of at least 80%; such compositions are prepared by combining a colloidal dispersion of the supported oxide and a suspension of the support, drying the resulting mixture by atomization and drying the resulting product by calcination.

Claims

1. A catalyst/catalyst support composition in a powder form consisting essentially of: ceric oxide CeO.sub.2; and a support composed of silica, alumina, titanium oxide, zirconium oxide, or any combination thereof, the ceric oxide being deposited onto the support and being in the form of particles having a size of at most 50 nm, which are separate or in the form of aggregates, wherein the composition exhibits, after calcination at a temperature of at least 800 C. for 6 hours, a reducibility measured from 30 C. to 900 C. of at least 80%.

2. The catalyst/catalyst support composition as defined by claim 1, wherein said particles have a size of at most 10 nm.

3. The catalyst/catalyst support composition as defined by claim 1, exhibiting a reducibility of at least 85%.

4. The catalyst/catalyst support composition as defined by claim 1, wherein the support essentially consists of alumina or alumina stabilized by at least one element selected from among the rare earth metals, silicon and zirconium.

5. The catalyst/catalyst support composition as defined by claim 1, wherein the supported oxide is at most 30% by weight.

6. The catalyst/catalyst support composition as defined by claim 1, wherein the ceric oxide is at most 50% by weight of the composition.

7. The catalyst/catalyst support composition as defined by claim 1, wherein said particles, in a separate form, have a size of at most 5 nm.

8. A catalytic system which comprises a catalyst/catalyst support composition as defined by claim 1.

9. A catalyst/catalyst support composition in a powder form consisting essentially of: ceric oxide CeO.sub.2; and an alumina-based support made of alumina only or alumina stabilized by at least one element chosen from rare earth metals, silicon and zirconium, ceric oxide being deposited onto the alumina-based support and being in the form of particles having a size of at most 50 nm, which are separate or in the form of aggregates, wherein the composition exhibits, after calcination at a temperature of at least 800 C. for 6 hours, a reducibility measured from 30 C. to 900 C. of at least 80%, the alumina support of which additionally being devoid of the elements barium and strontium.

10. The catalyst/catalyst support composition as defined by claim 9, wherein the supported oxide is at most 30% by weight.

11. The catalyst/catalyst support composition as defined by claim 9, wherein said particles, separate or in the form of aggregates thereof, have a size of at most 10 nm.

12. The catalyst/catalyst support composition as defined by claim 9, wherein the ceric oxide is at most 50% by weight of the composition.

13. The catalyst/catalyst support composition as defined by claim 9, wherein said particles, in a separate form, have a size of at most 5 nm.

14. A process for the preparation of a catalyst/catalyst support composition as defined by claim 1, which comprises the following stages: (a) admixing a colloidal dispersion of the supported oxide and a suspension of the support; (b) drying the mixture thus formed by atomization; and (c) calcining the dried product thus obtained.

15. The process as defined by claim 14, wherein the colloidal dispersion of the supported oxide additionally comprises an amino acid.

16. A process for the preparation of a catalyst/catalyst support composition as defined by claim 1, which comprises the following stages: (a) providing a liquid mixture comprising a salt of cerium and a suspension of the support; (b) heating the mixture thus formed at a temperature of at least 100 C. to form a precipitate; (c) recovering the precipitate obtained from step (b) to form a recovered precipitate; and (d) calcining said recovered precipitate.

17. A process for the preparation of a catalyst/catalyst support composition as defined by claim 1, wherein the support comprises alumina stabilized by a stabilizing element selected from among the rare earth metals, barium and strontium, which comprises the following stages: (a) providing a liquid mixture comprising a colloidal dispersion of the supported oxide an aluminum compound and a compound of the stabilizing element; (b) mixing the mixture thus provided and a base, whereby a suspension comprising a precipitate is obtained; (c) drying the suspension thus obtained; and (d) calcining the dried product thus obtained.

18. A process for the preparation of a catalyst/catalyst support composition as defined by claim 1, wherein the support comprises alumina stabilized by a stabilizing element selected from among the rare earth metals, barium and strontium, which comprises the following stages: (a) providing a liquid mixture comprising a colloidal dispersion of the supported oxide and an aluminum compound; (b) mixing the mixture thus provided and a base, whereby a suspension comprising a precipitate is obtained; (c) adding a compound of a stabilizing element to the suspension thus obtained; (d) drying the suspension resulting from the preceding stage (c); and (e) calcining the dried product thus obtained.

19. The process as defined by claim 18, wherein the suspension resulting from stage (b) is subjected to a maturing before stage (c).

20. The process as defined by claim 18, wherein the suspension is dried by atomization.

21. The process as defined by claim 18, wherein step (c) additionally comprises separating the precipitate from the suspension, washing, and redispersing in water to form a second suspension.

22. The process as defined by claim 18, wherein the compound of the stabilizing element is a salt thereof.

23. A process for the catalytic treatment of exhaust gases emanating from an internal combustion engine comprising contacting the exhaust gases with the composition as defined by claim 14.

24. The process as defined by claim 23, said treatment including contacting the exhaust gases with a NOx trap.

Description

EXAMPLE 1

(1) This example relates to a composition based on cerium oxide on a silica support which is prepared by the second process described above.

(2) A mixture comprising 70% of SiO.sub.2 and 30% of CeO.sub.2 is prepared by mixing, in a beaker with stirring, 35 g of SiO.sub.2 powder (170 m.sup.2/g), dispersed in 500 ml of H.sub.2O at pH 0.5, with an acidic (pH 0.5) Ce(NO.sub.3).sub.4 solution comprising 15 g of CeO.sub.2 (CeO.sub.2 80 g/l). The combined mixture is transferred into an autoclave and brought to 150 C. for 2 hours while stirring at 300 revolutions/min.

(3) The cooled mixture is separated by filtration and washed with 2 l of water at pH 9. The cake obtained is dispersed in water at a concentration of oxide of 50 g/l (oxide) and then matured at 100 C. for 2 hours with stirring. After cooling, the suspension is separated by centrifuging. The cake is subsequently calcined under air at 800 C. for 6 h.

EXAMPLE 2

(4) This example relates to a composition based on a mixed oxide of cerium and of zirconium on a silica support which is prepared by the first process described above.

(5) A colloidal dispersion of particles of a mixed oxide of formula Ce.sub.0.5Zr.sub.0.5O.sub.2 is prepared beforehand.

(6) For this, 95.7 ml of a first solution, comprising 1.5 M/l of Ce(NO.sub.3).sub.4 and 0.5 M of HNO.sub.3, are mixed with 43.7 ml of a second solution comprising 3.3 M/l of ZrO(NO.sub.3).sub.2. The volume of the solution obtained is brought to 2300 ml by addition of water. The final pH is 1.9.

(7) 186 ml of 28% NH.sub.3 solution are added instantaneously. The pH increases to 10 and the formation of a precipitate is observed.

(8) The precipitate is filtered off and then washed with 2400 ml of deionized water. Washing is repeated 3 times in succession with an identical volume of washing solution. The pH of the final suspension is 7.5.

(9) The cake is resuspended in a solution comprising 20.2 g of a 68% nitric acid HNO.sub.3 solution (H.sup.+/(Ce+Zr)=0.75 in moles) and the volume is made up to 500 ml by addition of deionized water. The Ce+Zr concentration is equal to 0.29 mol/l. After stirring overnight, a colloidal dispersion with a size of 4 nm is obtained which is clear to the eye.

(10) Aminocaproic acid is added to this dispersion so as to obtain a final pH of 4.5 (98% 6-aminocaproic acid, Aldrich).

(11) 100 g of SiO.sub.2 powder (170 m.sup.2/g) are added to 550 ml of the colloidal dispersion of mixed Ce/Zr oxide in a beaker with stirring. The mixture is kept stirred for 15 minutes.

(12) This suspension is atomized at 110 C. (outlet temperature 110 C., inlet temperature 220 C.) with a flow rate of 1 l/h.

(13) The powder is calcined under air at 800 C. for 6 h.

EXAMPLE 3

(14) This example relates to a composition based on a mixed oxide of cerium and of zirconium on an alumina support which is prepared by the first process described above.

(15) A boehmite sol is prepared in a beaker equipped with a magnetic bar by dispersing, with stirring, 78.6 g of an AlOOH powder (Pural B21 alumina comprising 71.25% of Al.sub.2O.sub.2) in 700 ml of H.sub.2O brought to pH 2 using a 68% concentrated HNO.sub.3 solution. After a few hours, a gel is obtained at pH 4.

(16) At the same time, a colloidal dispersion of particles of mixed oxide of formula Ce.sub.0.5Zr.sub.0.5O.sub.2 is prepared as described in example 2. Aminocaproic acid is likewise added thereto, so as to obtain a final pH of 4.5.

(17) 2 liters of the dispersion of mixed oxide CeO.sub.2/ZrO.sub.2 are introduced with stirring at the rate of 50 ml/min into the 700 ml of the boehmite dispersion prepared above. The mixture is kept stirred for 15 minutes and then atomized (inlet temperature 245 C., outlet temperature 110 C.) at a flow rate of 1 liter/hour.

(18) The powder is calcined under air at 800 C. for 6 h.

(19) The characteristics of size of the supported oxide and of specific surface of the composition for the compositions prepared in examples 1 to 3 are given in the following table 1.

(20) TABLE-US-00001 TABLE 1 Sizes of the supported particles and specific surfaces Calcination 800 C./6 h Example X-ray size (nm) BETS (m.sup.2/g) 1 6 161 2 6 95 3 6 113

EXAMPLE 4

(21) In this example, the values for reducibility of the cerium, measured for the compositions described in the preceding examples, are given.

(22) The reducibility of the cerium is measured by temperature-programmed reduction in the following way. Use is made of a Micromeritics Autochem 2920 device with a quartz reactor and a 200 mg sample which has been calcined beforehand at 800 C. for 6 hours under air. The gas is hydrogen at 10% by volume in argon and with a flow rate of 25 ml/min. The temperature rise takes place from ambient temperature to 900 C. at the rate of 20 C./min. The signal is detected with a thermal conductivity detector. The temperature is measured in the sample using a thermocouple.

(23) The reducibility of the cerium is calculated from the hydrogen consumption, it being understood that mol of H.sub.2 consumed and measured by the method described above corresponds to 1 mol of reduced Ce(IV). The hydrogen consumption is calculated from the missing area of the hydrogen signal from the base line at 30 C. to the base line at 900 C. (respectively 600 C.) when the reducibility is measured between 30 C. and 900 C. (respectively 600 C.)

(24) The reducibility values are collated in table 2.

(25) TABLE-US-00002 TABLE 2 Reducibility of the cerium (%) Example 30 C.-600 C. 30 C.-900 C. 1 41 100 2 44 87 3 41 82

(26) The examples which follow relate to compositions for which the support is based on stabilized alumina.

(27) The starting materials used in these examples are as follows:

(28) Sasol SB1 boehmite comprising 78% by weight of oxide

(29) Barium nitrate comprising 99% by weight of oxide

(30) Lanthanum nitrate solution comprising 454 g/l of oxide

(31) Ceric nitrate solution comprising 256 g/l of oxide with a cerium(III) content of 0.11 mol/1 and a Ce(III)/total cerium atomic ratio of 0.06

(32) Zirconyl nitrate solution comprising 18.7% by weight of oxide

(33) 68 vol % nitric acid

(34) 28 vol % aqueous ammonia

EXAMPLE 5

(35) This example relates to a composition based on cerium oxide on a support made of alumina stabilized by lanthanum, the proportions by weight of which, expressed as oxide, are 75%, 20% and 5% respectively for the aluminum, the lanthanum and the cerium.

(36) 96 g of boehmite are added to an acidic solution, comprising 19 g of 5 mol/1 nitric acid and 300 ml of demineralized water, in a beaker equipped with a magnetic bar and a pH electrode. The pH of the solution is 1.9. 35 g of the cerium oxide sol described in example 4 of European patent No. 208 581 51 and then a mixture of 74 g of lanthanum nitrate in 50 ml of demineralized water are added to this mixture with stirring. This mixture is then gradually introduced with stirring into a vessel heel comprising 25 g of aqueous ammonia and 500 ml of demineralized water. The reaction mixture is subsequently subjected to maturing at 100 C. for one hour. The solid phase is separated from the supernatant by centrifugation. The solid phase is redispersed in demineralized water so that the total volume is 600 ml. The suspension is subsequently atomized on a device of Bchi Mini-Spray Dryer 190 type, inlet temperature=230 C., outlet temperature=115 C.

(37) The powder obtained is calcined under air at 600 C. for 2 h.

(38) The specific surfaces obtained after subsequent calcinations at different temperatures are shown below.

(39) 2 h 600 C.=166 m.sup.2/g

(40) 6 h 800 C.=120 m.sup.2/g

(41) 2 h 900 C.=112 m.sup.2/g

(42) 2 h 1000 C.=70 m.sup.2/g

EXAMPLE 6

(43) This example relates to a composition based on mixed oxide of cerium, of zirconium and of praseodymium comprising 25% by weight of oxides on a support formed of alumina stabilized by lanthanum in the overall proportions by weight, expressed as oxide, of 14%/6%/5%/15%/60% respectively for CeO.sub.2/ZrO.sub.2/Pr.sub.6O.sub.11/La.sub.2O.sub.3/Al.sub.2O.sub.3.

(44) 1) Preparation of a Colloidal Dispersion of CeO.sub.2ZrO.sub.2Pr.sub.6O.sub.11 Mixed Oxide

(45) A colloidal dispersion of particles of a CeO.sub.2ZrO.sub.2Pr.sub.6O.sub.11 mixed oxide is prepared beforehand according to the specific process described above.

(46) 320 g of cerium nitrate solution, 121.5 g of zirconyl nitrate solution and 70 g of praseodymium nitrate solution are introduced with stirring into a beaker comprising 151 ml of demineralized water. The solution thus obtained is gradually introduced with stirring into a vessel heel comprising 160 g of aqueous ammonia and 321 g of demineralized water. The solid phase is separated from the supernatant by filtration. The solid phase is washed with demineralized water or in a sintered glass filter. The solid phase is redispersed in a mixture of 45 g of nitric acid and 250 g of demineralized water. The solution obtained is then made up with demineralized water in order to achieve a total volume of 950 ml. The concentration of the solution is then 112 g/l of oxides and the pH is 1.0. This dispersion is heated at 80 C. for 1 h and a colloidal dispersion is obtained.

(47) 2) Deposition of the Colloidal Dispersion of CeO.sub.2ZrO.sub.2Pr.sub.6O.sub.11 Mixed Oxide on the Support Formed of Alumina Stabilized with Lanthanum

(48) A suspension of 38.5 g of boehmite and 8 ml of nitric acid (5M) is added to 125 g of the colloidal dispersion of CeO.sub.2ZrO.sub.2Pr.sub.6O.sub.11 mixed oxide prepared in 1). 28 g of lanthanum nitrate solution are subsequently added with stirring and the mixture is made up with demineralized water in order to obtain a total volume of 500 ml. This mixture is gradually introduced with stirring into a vessel heel comprising 22.5 g of aqueous ammonia and 200 ml of demineralized water. The final pH of the suspension is 9. The medium is subsequently heated at 95 C. for 1 h and is then cooled before being filtered in order to recover the solid phase, which is then washed with demineralized water on a sintered glass filter and redispersed in demineralized water (total volume of 500 ml) at a concentration of oxides of 100 g/l.

(49) The suspension is then atomized on a device of Bchi Mini-Spray Dryer 190 type, inlet temperature=225 C., outlet temperature=115 C.

(50) The powder obtained is calcined under air at 800 C. for 6 h.

(51) The specific surfaces obtained after subsequent calcinations at different temperatures are shown below.

(52) 6 h 800 C.=120 m.sup.2/g

(53) 2 h 900 C.=94 m.sup.2/g

(54) 2 h 1000 C.=71 m.sup.2/g

EXAMPLE 7

(55) This example relates to a composition based on mixed oxide of cerium, of zirconium and of praseodymium charged with 25% by weight of oxides on support made of alumina stabilized with barium in overall proportions by weight, expressed as oxide, of 14%/6%/5%/15%/60% respectively for CeO.sub.2/ZrO.sub.2/Pr.sub.6O.sub.11/BaO/Al.sub.2O.sub.3.

(56) A dispersion of 38.5 g of boehmite and 1.5 g of 15.2 mol/l concentrated nitric acid is added, with stirring, to 125 g of the colloidal dispersion of CeO.sub.2ZrO.sub.2Pr.sub.6O.sub.11 mixed oxide prepared in 1) of example 6. 515 ml of demineralized water are subsequently added with stirring in order to obtain a suspension comprising 50 g/l of oxides. This suspension is gradually introduced with stirring into a vessel heel comprising 21 g of aqueous ammonia and 176 ml of demineralized water. The final pH of the suspension is 9.3 at the end of the introduction. The medium is subsequently heated at 100 C. for 1 h and then it is cooled before being filtered in order to recover the solid phase, which is then washed with demineralized water on a sintered glass filter. The solid is subsequently redispersed in order to obtain a total volume of 500 ml in a solution of 160 ml of demineralized water and 13 g of barium nitrate.

(57) The suspension is then atomized on a device of Bchi Mini-Spray Dryer 190 type, inlet temperature=235 C., outlet temperature=110 C.

(58) The powder obtained is calcined under air at 600 C. for 2 h.

(59) The specific surfaces obtained after subsequent calcinations at different temperatures are shown below.

(60) 2 h 600 C.=142 m.sup.2/g

(61) 6 h 800 C.=112 m.sup.2/g

(62) 2 h 900 C.=93 m.sup.2/g

(63) 2 h 1000 C.=76 m.sup.2/g

EXAMPLE 8

(64) This example relates to a composition based on mixed oxide of cerium, of zirconium and of lanthanum comprising 25% by weight of oxides on a support formed of alumina stabilized with barium in overall proportions by weight, expressed as oxide, of 20.4/2.4/2.2/15/60% respectively for CeO.sub.2/ZrO.sub.2/La.sub.2O.sub.3/BaO/Al.sub.2O.sub.3.

(65) 1) Preparation of a Colloidal Dispersion of CeO.sub.2ZrO.sub.2La.sub.2O.sub.3 Mixed Oxide

(66) A colloidal dispersion of particles of a CeO.sub.2ZrO.sub.2La.sub.2O.sub.3 mixed oxide is first prepared according to the process described in example 6.

(67) 457 g of cerium nitrate solution, 51 g of zirconyl nitrate solution and 33.5 g of lanthanum nitrate solution are introduced with stirring into a beaker comprising 126 ml of demineralized water. The solution thus obtained is gradually introduced with stirring into a vessel heel comprising 165 g of aqueous ammonia and 316 g of demineralized water. At the end of the introduction, there is a pH of 9.2. The solid phase is separated from the supernatant by filtration. The solid phase is washed with demineralized water on a sintered glass filter. The solid phase is redispersed in a mixture of 50 g of nitric acid and 200 g of demineralized water. The solution obtained is then made up with demineralized water in order to achieve a total volume of 950 ml. The concentration of the solution is then 111.5 g/l of oxides and the pH is 0.9. This dispersion is heated at 80 C. for 1 h and a colloidal dispersion is obtained.

(68) 2) Deposition of the Colloidal Dispersion of CeO.sub.2ZrO.sub.2La.sub.2O.sub.3 Mixed Oxide on the Support Formed of Alumina Stabilized with Barium

(69) A suspension of 38.5 g of boehmite and 8.5 ml of nitric acid (5M) is added to 126 g of the colloidal dispersion of CeO.sub.2ZrO.sub.2La.sub.2O.sub.3 mixed oxide prepared in 1), followed by 535 ml of demineralized water. This mixture is gradually introduced with stirring into a vessel heel comprising 25 g of aqueous ammonia and 200 ml of demineralized water. The final pH of the suspension is 9.2. The medium is subsequently heated at 95 C. for 1 h and is then cooled before being filtered in order to recover the solid phase, which is then washed with demineralized water on a sintered glass filter. The solid is subsequently redispersed in a mixture of 13 g of barium nitrate and 160 ml of demineralized water. The mixture is subsequently made up with demineralized water in order to obtain a total volume of 500 ml for the suspension.

(70) The suspension is then atomized on a device of Bchi Mini-Spray Dryer 190 type, inlet temperature=225 C., outlet temperature=110 C.

(71) The powder obtained is calcined under air at 600 C. for 2 h.

(72) The specific surfaces obtained after subsequent calcinations at different temperatures are shown below.

(73) 2 h 600 C.=136 m.sup.2/g

(74) 6 h 800 C.=105 m.sup.2/g

(75) 2 h 900 C.=92 m.sup.2/g

(76) 2 h 1000 C.=76 m.sup.2/g

EXAMPLE 9

(77) This example relates to a composition based on mixed oxide of cerium, of lanthanum and of praseodymium comprising 25% by weight of oxides on a support made of alumina stabilized with lanthanum in the overall proportions by weight, expressed as oxide, of 19/18/3/60% respectively for CeO.sub.2/La.sub.2O.sub.3/Pr.sub.6O.sub.11/Al.sub.2O.sub.3.

(78) 1) Preparation of a Colloidal Dispersion of CeO.sub.2La.sub.2O.sub.3Pr.sub.6O.sub.11 Mixed Oxide

(79) A colloidal dispersion of particles of a mixed oxide of CeO.sub.2La.sub.2O.sub.3Pr.sub.6O.sub.11 mixed oxide is prepared beforehand.

(80) 426 g of cerium nitrate solution, 48 g of lanthanum nitrate solution and 38 g of praseodymium nitrate solution are introduced with stirring into a beaker comprising 147 ml of demineralized water. The solution thus obtained is gradually introduced with stirring into a vessel heel comprising 167 g of aqueous ammonia and 314 g of demineralized water. The solid phase is separated from the supernatant by filtration. The solid phase is washed with demineralized water on a sintered glass filter. The solid phase is redispersed in a mixture of 45 g of 68 vol % nitric acid and 300 g of demineralized water. The solution obtained is then made up with demineralized water in order to achieve a total volume of 1 l. The concentration of the solution is then 102 g/l of oxides and the pH is 1.0. This dispersion is heated at 80 C. for 1 h and a colloidal dispersion is obtained.

(81) 2) Deposition of the Colloidal Dispersion of CeO.sub.2La.sub.2O.sub.3Pr.sub.6O.sub.11 Mixed Oxide on the Support Made of Alumina Stabilized with Lanthanum

(82) A suspension of 38.5 g of boehmite and 1.7 ml of nitric acid (15.2M) is added to 137 g of the colloidal dispersion of CeO.sub.2La.sub.2O.sub.3Pr.sub.6O.sub.11 mixed oxide prepared in 1). The mixture is made up with demineralized water in order to obtain a total volume of 650 ml. The pH of the medium is 2.1. 28 g of lanthanum nitrate solution are then gradually introduced with stirring. The resulting mixture is gradually introduced with stirring into a vessel heel comprising 23 g of aqueous ammonia and 200 ml of demineralized water. The final pH of the suspension is 9.2. The medium is subsequently heated at 96 C. for 1 h and is then cooled to 50 C. before being filtered in order to recover the solid phase, which is then washed with demineralized water on a sintered glass filter and redispersed in 170 ml of demineralized water in order to obtain a concentration of oxides of 100 g/l.

(83) The suspension is then atomized on a device of Bchi Mini-Spray Dryer 190 type, inlet temperature=240 C., outlet temperature=115 C.

(84) The powder obtained is calcined under air at 800 C. for 6 h.

(85) The specific surfaces obtained after subsequent calcinations at different temperatures are shown below.

(86) 6 h 800 C.=102 m.sup.2/g

(87) 2 h 900 C.=83 m.sup.2/g

(88) 2 h 1000 C.=61 m.sup.2/g

EXAMPLE 10

(89) In this example, the values for reducibility of the compositions described in examples 5 to 9 are given.

(90) The reducibility of the compositions is measured by temperature-programmed reduction in the following way. Use is made of a Micromeritics Autochem 2920 device with a quartz reactor and a 200 mg sample which has been calcined beforehand at 800 C. for 6 hours under air. The gas is hydrogen at 10% by volume in argon and with a flow rate of 25 ml/min. The temperature rise takes place with ambient temperature to 900 C. at the rate of 20 C./min. The signal is detected with a thermal conductivity detector. The temperature is measured in the sample using a thermocouple.

(91) The reducibility of the cerium is calculated from the hydrogen consumption, it being understood that mol of H.sub.2 consumed and measured by the method described above corresponds to 1 mol of reduced Ce(IV). The hydrogen consumption is calculated from the missing area of the hydrogen signal from the base line at 30 C. to the base line at 900 C. when the reducibility is measured between 30 C. and 900 C.

(92) The reducibility values are collated in table 3.

(93) TABLE-US-00003 TABLE 3 Reducibility Reducibility of the of the cerium and Example cerium (%) praseodymium (%) 5 100 6 81 7 84 8 96 9 81

EXAMPLE 11

(94) Measurement of catalytic performance as storage capacity for NOx compounds in an oxidizing medium.

(95) This example illustrates the effectiveness of these materials used as precious metal support with regard to their storage capacity for nitrogen oxides NOx for catalytic compositions comprising 1% by weight of platinum prepared from the compositions of the preceding examples and in the following way.

(96) 5 g of compound according to one of the above examples are introduced into a beaker and then covered with water (50 ml) before the addition of a solution of tetraamine platinum hydroxide salt (3.125 ml at 16 g/1). After evaporation on a rotary evaporator, the catalytic composition thus obtained is dried in an oven at 120 C. for 2 h, then calcined at 500 C. under air for 2 h and aged at 700 C. under a 90% air/10% H.sub.2O mixture for 4 hours.

(97) The NOx storage capacity is measured under the following conditions:

(98) The catalytic composition as prepared above is introduced into a reactor and then pretreated at 300 C. for 12 hours under a gas stream with the following composition:
9% O.sub.2+10% H.sub.2O+2% SO.sub.2+79% N.sub.2 (corresponding to 1000 ppm of SO.sub.2).

(99) The reactor is isolated and then cooled to ambient temperature under an N.sub.2 stream. This catalytic composition, thus sulfated, is introduced into a fresh reactor and heated from a temperature of 150 C. to a temperature of 600 C. under a reducing gas stream with the following composition:
4.9% O.sub.2+10% CO+5% CO.sub.2+10% HC (2500 ppm of C.sub.3H.sub.6+2500 ppm of C.sub.3H.sub.8 in N.sub.2)+5% H.sub.2O+65.1% N.sub.2.

(100) The catalytic composition as prepared above was subsequently maintained under this reducing stream for 20 minutes at a temperature of 600 C. The purpose of this treatment is to simulate a sulfation/desulfation cycle.

(101) The reactor is subsequently isolated and then cooled under static conditions to ambient temperature. The reaction stream, with the composition: 10% O.sub.2+5% H.sub.2O+10% CO.sub.2+300 ppm NO in nitrogen, analyzed continuously by a Nicolet Magna 560 Fourier transform infrared (FT-IR) spectrometer, is introduced into the catalytic reactor, placed beforehand at the desired reaction temperature. After stabilization of the stream monitored by NO+NO.sub.2 analysis, the respective concentrations of NO and NO.sub.2 at the reactor outlet are continuously determined by the FTIR spectrometer.

(102) The integration of the NO and NO.sub.2 concentrations during the minute following the arrival of the reaction stream over the catalytic composition makes it possible to calculate the amount of trapped NOx compounds. The results are expressed by the amount by weight of trapped NOx compounds (%) at 1 minute, with respect to the total amount of NOx compounds fed during this minute.

(103) The measurements are subsequently carried out on other samples of catalytic compositions at the desired temperatures.

(104) The amounts of NOx compounds trapped at the temperatures of 200 C., 300 C., 350 C. and 400 C. are listed in table 4. The catalytic compositions of tests 1, 2 and 3 of this table correspond respectively to the products obtained after impregnation with platinum, according to the process described above, of the composition of example 5 (test 1), example 7 (test 2) and example 9 (test 3).

(105) TABLE-US-00004 TABLE 4 Trapped NOx compounds as % by weight Test 200 C. 250 C. 300 C. 400 C. Test 1 87 89 73 59 (composition example 5) Test 2 97 Not Not 92 (composition determined determined example 7) Test 3 75 Not Not 75 (composition determined determined example 9)

(106) It is seen, from the results of table 4, that the compositions of the invention exhibit a high. NOx storage efficiency in the temperature region between 200 C. and 400 C. They are very particularly effective in the region of low temperatures, at 300 C. or below, in particular at temperatures as low as 200 C. The catalytic compositions of the invention are therefore particularly effective over the entire temperature range and in particular at low temperatures, from 200 C. to 300 C. and more particularly still at 200 C. and 250 C.