IE911993A1

IE911993A1 - Material having a porous matrix containing a second¹component in dispersed form and process of manufacture

Info

Publication number: IE911993A1
Application number: IE199391A
Authority: IE
Original assignee: Rhone Poulenc Chimie
Priority date: 1990-06-13
Filing date: 1991-06-12
Publication date: 1991-12-18
Also published as: EP0461950A1; PT97922A; FI912821A0; KR920000380A; JPH04292480A; CA2044445A1; FR2663319A1; FI912821L; FR2663319B1

Abstract

Material comprising a porous matrix consisting of at least one oxide of a metallic element containing dispersed particles of oxide of at least one second metallic element and characterised in that the particles of the said second metallic element exhibit an increase in their average size of not more than 1000 % between the average size of these particles after a heat treatment at 350 DEG C for 4 hours and that of the particles after a heat treatment at 1100 DEG C for 4 hours. Process for the manufacture of this material by coprecipitation.

Description

The present invention relates to a material comprising a porous matrix comprising a second 5 component in dispersed form.

It relates more especially to a material comprising a porous matrix of inorganic oxide such as alumina, comprising an oxide of a second metallic element dispersed in the form of particles of thermally 10 stable average size.

Many materials have already been proposed, in particular in the heterogeneous catalysis of synthesis processes or in gas treatment processes, such as, for example, in the processes for treating exhaust gases 15 from internal combustion engines. These materials generally comprise a porous matrix in which are dispersed the catalytic components or additive components such as promoters, stabilisers or the like.

In order for the catalytic activity to be as 20 high as possible, the material must possess a high specific surface area, but also a very good dispersibility of the catalytic or additive components. In other words, the surface area of the catalytic or additive components must be as high as possible.

This surface area is a direct function of the average size of the particles of the said catalytic or additive components dispersed in the porous matrix.

The known materials possess a good dispersibility of the catalytic or additive components. However, when these materials are treated thermally at high temperature to obtain a stabilisation of these components, and in particular of their surface area, a large increase is observed in the size of the dispersed particles of catalytic or additive components. This increase, due to sintering of these particles together, directly decreases the catalytic activity of the material.

The object of the present invention is, in particular, to remedy these drawbacks by proposing a material composed of a porous matrix comprising, in dispersed form, particles of a second component, and in which the increase in the average size of these particles is small for thermal treatments at high temperature.

To this end, the invention proposes a material comprising a porous matrix consisting of at least one oxide of a metallic element, such as, for example, alumina, containing, in dispersion, particles of oxide of at least one second metallic element. This material is characterised in that the particles of oxide of the second metallic element display an increase in their average size equal to at most 1000% between their average size after a thermal treatment at 350’C for 4 hours and that of these particles after a thermal treatment at 1100*C for 4 hours.

£ Advantageously, this increase in average size is between 300% and 800%.

This small increase in the size of the particles may be explained, without this limiting the 5 scope of the invention, by a smaller degree of sintering of the oxides of the second element due, in particular, to the low mobility of the particles in the matrix and to trapping of these particles in the crystalline structure of the porous matrix.

According to an embodiment of the invention, the second metallic element does not yield a chemical compound or solid solution with the material forming the matrix. Thus, the dispersed particles consist exclusively of the oxide of the second element. is However, the second element can partially combine chemically with the matrix to yield either a complex salt or a solid solution, without thereby departing from the scope of the invention.

According to another feature of the invention, the weight concentration of the oxide of the second element in the material is between 5% and 70%, the weight concentration of the inorganic oxide forming the matrix being between 95% and 30%.

Advantageously, the concentration of the second component is between 7% and 55%.

Among inorganic oxides suitable for the invention as a compound for forming the porous matrix, alumina, silica, zirconium oxide, titanium oxide, a mixture of these, or the like, may be mentioned as examples .

Among these oxides, preferred oxides of the invention are alumina and silica.

As an oxide of the second metallic element suitable for the invention, elements of the lanthanide family, including yttrium, and more especially cerium, lanthanum, praseodymium, neodymium, alkaline earth metals such as barium, molybdenum, vanadium, or the like, may be mentioned as examples.

In the case where the oxide of the second element is cerium oxide and the matrix alumina, the average size of the cerium oxide particles is 5 nm at 350eC and 20 nm at llOO’C.

Advantageously, in this case, the weight concentration of cerium oxide is between 5% and 70%, and preferably between 7% and 50%.

The material of the invention is, in particular, suitable for the production of a catalyst, for example a catalyst for the treatment of exhaust gases from internal combustion engines. In effect, the very great thermal stability of the particles dispersed in the porous matrix enables a substantial catalytic » effect to be preserved even if the catalyst is subjected to a high temperature, which is common in this field of application.

Moreover, the good dispersibility of cerium 5 oxide, for example, permits an improved dispersibility of metallic elements such as the precious metals.

These improvements in dispersibility are, in particular, shown by measurement of the oxygen storage capacity in the case of cerium oxide.

The subject of the invention is also a process for the manufacture of the material according to the invention.

This process consists in coprecipitating the metallic element forming the matrix and the second metallic element in the form of an oxide precursor, in recovering the precipitated material and in calcining it after, where appropriate, a drying.

According to another feature of the invention, the precipitation of the materials is carried out in the form of a hydroxide, hydrate, oxide or carbonate, the difference between the pH values for precipitation of each of the components forming the material (the matrix component and the second component) under the conditions and the coprecipitation medium advantageously being less than 0.5 pH unit.

Advantageously, the process can comprise a step of crystalline development of the matrix. For example, this step can be a hydrothermal treatment of the coprecipitate.

The expression hydrothermal treatment is understood to mean a maintenance of the composition at specified temperature and pressure conditions for a crystalline phase to develop.

Thus, according to an embodiment of the invention, this hydrothermal treatment is carried out by a drying of the precipitate with continuous removal of the vapour emitted.

This continuous removal of the vapour emitted may be obtained by a drying under reduced pressure, or at atmospheric pressure with the vapour being swept by a stream of gas, for example a flow of air or inert gas (nitrogen) at the surface or through the precipitate arranged in an oven.

The temperature of drying of the precipitate is advantageously between 50eC and 150eC. The higher the pressure, the higher will be this temperature.

The pressure is, as a guide, less than 10000 Pa, and advantageously between 1000 and 10000 Pa.

The drying time is of the order of 1 hour to 100 hours.

According to a second embodiment, the hydrothermal treatment is carried out at a temperature of between 30eC and 100*C, either in the presence of water vapour at a pressure above atmospheric pressure, or with the product being suspended or made into a slurry in water. The duration of this treatment is advantageously less than 2 hours, and preferably of the order of a few minutes to 1 h 30 min. This product is then dried, either by one of the processes described above, or by a rapid drying process such as, for example, spray drying.

The product thus treated is then calcined at variable temperatures between 300eC and 1200eC.

According to a preferred embodiment of the invention, a process for the manufacture of the 10 material suitable for the invention consists in preparing a solution of one or more soluble compounds of the element forming the matrix, for example a salt, with one or more compounds of the second element or elements, for example salts of these compounds.

As soluble compounds of the element forming the matrix, the chlorides, nitrides and sulphates may be mentioned.

As soluble compounds of the second element, the nitrates, acetates and chlorides may be mentioned as examples.

In a first step, referred to as the precipitation step, a precipitating agent is mixed with the solution of soluble compounds. This precipitating agent comprises carbonate, oxalate and/or carboxylate ions and hydroxyl ions.

According to a preferred embodiment, the precipitating agent has a pH of between 3.5 and 9, and advantageously between 4 and 8.

This precipitation solution is obtained by adding an alkalinising agent, for example a base, to a solution of carbonate or hydrogen carbonate, for example.

As a suitable base, alkali metal or alkaline earth metal hydroxides and ammonia may be mentioned.

The latter is preferred since the ammonium anion may be readily removed.

According to another feature of the 10 invention, the precipitation solution contains (CO3)" and OH' ions in a ratio (CO3)"/OH* of between 0.25 and 4. According to another feature of the invention, and in particular when the second element is cerium, the latter is advantageously present in the form of cerium in the oxydation state 3+.

The concentrations of the element forming the matrix and the second element in the solution are not critical, and can vary within wide limits. However, advantageously, the total concentration of the element forming the matrix and the second element is between 0.1 mol/1 and 4 mol/1.

The concentration of the precipitating agent in the precipitation solution is not critical, and is limited, in particular, by the solubility coefficients of the compounds used.

The mixing of the precipitation solution with the solution containing the soluble compounds may be performed in any manner. g & Advantageously, this mixing is obtained by adding the precipitation solution to the solution of soluble compounds with stirring of the latter.

This mixing may be carried out in continuous 5 or discontinuous fashion, at a temperature which can vary within wide limits, this coprecipitation advantageously being carried out at a temperature between room temperature (15*C - 25’C) and 100’C, and preferably between 20 °C and 60*C.

Moreover, the quantity of solutions mixed, or the flowrates of the solutions and concentrations of the species in these solutions, are determined so that the quantity of precipitating agent relative to the species to be precipitated is at least equal to the stoichiometric quantity, and advantageously 5 to 10% greater than the latter.

According to a preferred embodiment of the invention, the coprecipitate is obtained by mixing a solution containing the soluble compounds with an ammonium carbonate solution whose pH has been adjusted by adding ammonia solution.

The coprecipitate obtained in the process of the invention comprises, in the case where the component forming the matrix is alumina, an alumina mainly in amorphous form.

The precipitate obtained is then filtered off. It can also be subjected to one or more washes. However, the latter are not mandatory.

This alumina will be crystallised in pseudoboehmite form, in particular during hydrothermal treatment of the coprecipitate, also referred to as the step of ageing of the precipitate, described above, before drying and calcination.

The structure of the compound is then determined, as well as the size of the particles of the oxide of the second element, for example by X-ray analysis and transmission electron microscopic analysis.

EXAMPLE 1 This experiment is carried out with a mixture of a solution containing 0.939 mol/1 of aluminium nitrate and 0.068 mol/1 of cerium nitrate and a solution containing 2.773 mol/1 of NHAHCO3 and 0.881 mol/1 of ammonia solution.

The flow rate of the two solutions is adjusted so as to obtain a ratio NH//NO3‘ in the region of 1.05.

The precipitation is carried out with stirring at a temperature of 25°C. The mixture has a pH varying between 5.6 and 6.3.

The precipitate is then washed with water.

The washing enables the NO3 and NH/ ions present in the filter cake to be removed. Thus, washing is advantageously continued as long as the weight concentration of nitrate and ammonium in the precipitate is less than 0.2%.

A The precipitate thus collected is subjected to an ageing by making the precipitate into a slurry in water (6% of precipitate) for 15 minutes at 100C.

The product is then spray dried at 150’C and 5 thereafter calcined at different temperatures.

The product obtained after calcination for 2 hours at 350°C possesses a pore volume of 1.6 cm3/g for pores having a diameter of between 10 nm and 100 nm (average diameter equal to 30 nm). This pore volume represents 85% of the pore volume of the pores of diameter smaller than 100 nm. The CeO2 particles have an average diameter, determined by X-rays, of 5 nm.

After calcination at 1100°C for 4 hours, the specific surface area is 7 0 m2/g and the total pore volume 0.9 cm3/g for pores of diameter in the region of 40 nm (diameter between 20 nm and 100 nm).

This pore volume represents 95% of the pore volume corresponding to pores of diameter smaller than 100 nm.

The material obtained comprises 20% of CeO2 in the Al2O3 matrix, in the form of CeO2 particles dispersed in the sheets formed by alumina in pseudoboehmite form. This structure is illustrated in Figure 1.

These cerium oxide particles have an average diameter of 19 nm after calcination at 1100"C for 4 hours, corresponding to an increase in size of 300%.

EXAMPLE 2 The example is repeated, but using a solution containing 0.300 mol/1 of cerium nitrate and 0.594 mol/1 of aluminium nitrate.

The precipitation is carried out under the conditions described in Example 1.

The product obtained, which contains 50% by weight of CeO2, possesses a specific surface area, after calcination at 350*C for 4 hours, equal to 160 m2/g. The size of the cerium oxide particles is 5 nm.

After calcination for 4 hours at 1100"C, the specific surface area is equal to 30 m2/g while the size of the cerium oxide particles is 25 nm. The increase in size of the CeO2 particles is hence 400%.

COMPARATIVE EXAMPLE 3 An alumina gel (marketed under the trade name VERSAL 250) is mixed with a hydrated cerium oxide. The product obtained is dried and calcined; it contains 20% by weight of CeO2.

The size of the cerium oxide particles after calcination at 350’C for 4 hours is 4 nm, but after calcination at 1100’C, this size is very much greater than 50 nm, which represents an increase in size markedly greater than 1000%.

Moreover, the specific surface area of this product is 50 m2/g at 1100*C.

This difference in size of the cerium oxide is particles is also demonstrated by measurement of the oxygen storage capacity of the material.

This oxygen storage capacity is determined according to the following test: a mass of material (0.3 g) is subjected to a flow of gas, helium at 2 bar and according to a flow rate of 10 1/h.

Into this carrier gas are injected, in the form of a pulse, either carbon monoxide (CO) at a concentration of 5%, or oxygen at a concentration of 2.5%.

The test is performed according to the following sequence: - temperature rise to 400°C under helium - dispatching of a series of pulses of oxygen - dispatching of a series of pulses of CO - dispatching of a further series of pulses of O2 - dispatching of alternating pulses of CO and O2.

To determine and assess the storage capacity, 20 the following are measured: - the percentage of CO converted during the 1st pulse of the series of pulses of CO - cumulative consumption of CO per gram of CeO2 after the series of pulses of CO - average consumption of CO per gram of CeO2 for each pulse of the series of alternate pulses of CO and 02.

Two materials were tested: material A according to the invention (Example 1) calcined at 850eC material B obtained by mixing an alumina gel (marketed under the trade name VERSAL 250) with a hydrated cerium oxide. The product obtained is dried and calcined at 850eC (Example 3).

The results are collated in the table below.

MATERIAL During th· 1st ssriss of pulses Cumulative consumption Alternated mlCO/8CeOj> C»Ox X CO converted with the 1st pulse mlCO/gCeO2 silCO/ jCeO^ CeOx A 23.4X 5.65 10.85 x:1.91 5.94 1.95 B 9.71 2.43 4.63 x:1.96 2.66 1.98 These results demonstrate clearly that the 20 quantity of oxygen stored by the cerium oxide, that is to say available for converting carbon monoxide to the dioxide, is greater with the material A according to the invention. This may be explained by the size of the CeO2 particles dispersed in the material A, which is smaller than the size of those dispersed in the material B.

A 14.1 EXAMPLE 4 The example 1 is repeated, but using a solution containing a titanous nitrate in the place of cerium nitrate solution.

The precipitation is carried out under the conditions described in Example 1.

Furthermore, a comparative example is made by impregnating an alumina gel (marketed under the trade name VERSAL 50) with a titanous nitric solution and calcining the impregnated material.

The both products are analysed by X. Ray analysis.

They contain particles of titane oxide. This titane oxide is cristallized under anatase and/or rutile forms, after calcining at 850°C. After calcining at 1200°C during 4 hours, titane oxide is only in rutile form.

The size of the titane oxide particles after calcination at 850°C for 6 hours is 16 nm in the product of the invention , 18 nm in the comparative example. After calcination at 1200 °C for 4 hours, the size of t ioane oxide particles is 100 nm in the invention while it is 1 40 nm in the comparative data.

Claims

1. Material comprising a porous matrix consisting of at least one oxide of a metallic element containing, in dispersion, particles of oxide of at 5 least one second metallic element, characterised in that the particles of the said second metallic element display an increase in their average size equal to at most 1000% between the average size of these particles after a thermal treatment at 350’C for 4 hours and that 10 of the particles after a thermal treatment at 1100°C for 4 hours.

2. Material according to claim 1, characterised in that the abovementioned increase is between 300% and 800%. 15

3. Material according to claim 1 or 2, characterised in that the second metallic element does not yield a compound with the metallic element of the matrix.

4. Material according to claim 1 or 2, 20 characterised in that the second metallic element combines partially with the metallic element of the matrix.

5. Material according to one of claims 1 to 4, characterised in that the weight concentration of 25 the oxide of the second element in the material is between 5% and 70%, the concentration of the inorganic oxide forming the matrix being between 95% and 30%.

6. Material according to claim 5, a characterised in that the concentration of the oxide of the second element is between 7% and 55%.

7. Material according to one of the preceding claims, characterised in that the inorganic 5 oxide forming the matrix is selected from the group comprising alumina, silica, zirconium oxide, titanium oxide or a mixture of these.

8. Material according to one of the preceding claims, characterised in that the oxide of 10 the second metallic element is an oxide of an element selected from the group comprising lanthanides, yttrium, molybdenum, vanadium and alkaline earth metals .

9. Material according to claim 8, 15 characterised in that the second metallic element is a lanthanide or a mixture of lanthanides, preferably cerium, lanthanum, praseodymium or neodymium.

10. Material according to claim 8, characterised in that the second element is an alkaline 20 earth metal, preferably barium.

11. Material according to one of the preceding claims, characterised in that the second metallic element is cerium, and in that the average size of the cerium oxide particles is between 15 and 25 35 nm after a thermal treatment at 1100’C for 4 hours and between 2.5 and 6 nm after a thermal treatment at 350 e C for 4 hours.

12. Material according to claim 11, a characterised in that the weight concentration of cerium oxide is between 5% and 70%, and advantageously between 7% and 50%.

13. Process for the manufacture of the 5 material according to one of the preceding claims, characterised in that it consists in coprecipitating the metallic element forming the matrix and the second metallic element in the form of an oxide precursor, in recovering the coprecipitated material and in calcining 10 it after, where appropriate, a drying.

14. Process according to claim 13, characterised in that it consists in carrying out a precipitation in the form of a hydroxide, hydrate or oxide, and in that the difference between the pH values 15. For precipitation of each of the components is less than 0.5 pH unit.

15. Process according to claim 13 or 14, characterised in that it comprises a step of hydrothermal treatment of the precipitate, before 20 calcination.

16. Process according to claim 15, characterised in that the hydrothermal treatment is carried out by a drying of the precipitate with continuous removal of the vapour emitted.

17. Process according to claim 16, characterised in that the drying is carried out under reduced pressure in order to remove the vapour emitted.

18. Process according to claim 16, Λ characterised in that the drying is carried out at atmospheric pressure with the vapour emitted being carried away by sweeping with a gas at the surface of the precipitate. 5

19. Process according to one of claims 16 to 18, characterised in that the drying temperature is between 50’C and 150°C.

20. Process according to claim 17, characterised in that the drying is carried out under a 10 pressure of between 1000 Pa and 10000 Pa.

21. Process according to claim 15, characterised in that the hydrothermal treatment is carried out under a pressure of water vapour, at a temperature of between 30°C and 100 °C for a period of 15 less than 2 hours.

22. Process according to claim 15, characterised in that the hydrothermal treatment is carried out by suspending the precipitate in water and maintaining the suspension at a temperature of between 20 30’C and 100’C for a period of less than 1 h 30 min.

23. Process according to one of claims 21 and 22, characterised in that the precipitate is dried at a temperature of between 80*C and 160*C.

24. Process according to one of claims 21 25 and 22, characterised in that the precipitate is dried according to the process described in one of claims 16 to 20.

25. Process according to one of claims 13 to * 24, characterised in that the precipitate is calcined at a temperature of between 300’C and 1200 °C.

26. Process for the manufacture of a composition according to one of claims 13 to 25, 5 characterised in that it consists in; - preparing a solution of soluble compounds of metallic elements forming the matrix and of soluble compounds of the second element, - mixing this solution with a precipitation composition 10 comprising carbonate, oxalate and/or carboxyl ions with hydroxyl ions, - separating the precipitate obtained, - subjecting the precipitate thus separated to a hydrothermal treatment, 15 - drying the product if appropriate, and - calcining it.

27. Process according to claim 26, characterised in that the precipitation composition is a mixture of ammonia solution and ammonium carbonate. 20

28. Process according to claim 26 or 27, characterised in that the soluble compounds of the element forming the matrix are selected from the group comprising the nitrates, sulphates and chlorides.

29. Process according to one of claims 26 to 25 27, characterised in that the soluble compounds of the second element are selected from the group comprising the nitrates and chlorides.

30. Process according to one of claims 26 to 27, characterised in that the precipitation composition is added in a stoichiometric quantity, and advantageously in 5 to 10% excess of the latter.

31. Process according to claim 30, 5 characterised in that the precipitation composition contains carbonate and hydroxyl ions in a ratio CO 3 “/OH' of between 0.25 and 4.

32. Process according to one of claims 26 to 31, characterised in that the precipitation is carried 10 out in discontinuous fashion by adding the precipitation composition to the solution of soluble compounds, or vice versa.

33. Process according to one of claims 26 to 31, characterised in that the precipitation is carried 15 out in continuous fashion.

34. Process according to one of claims 26 to 33, characterised in that the precipitation is carried out at a temperature of between 20 °C and 100°C.

35. Process according to one of claims 26 to 20 34, characterised in that the separation of the precipitate is carried out by filtration or centrifugation.

36. Process according to claim 35, characterised in that the separated precipitate is 25 washed.

37. A material according to claim 1, substantially as hereinbefore described and exemplified.

38. A process for the manufacture of a material according to claim 1, substantially as hereinbefore described and exemplified.

39. A material according to claim 1, whenever manufactured by a process claimed in a preceding claim.