MIXED METAL OXIDE COMPOSITE FOR OXYGEN STORAGE

Abstract

The present invention relates to a composite oxide comprising ceria, praseodymia and alumina, wherein the cerium:praseodymium molar ratio of the composite oxide is 84:16 or less, as well as to a method of preparing the composite oxide and to its use, in particular in a method of treating an exhaust gas stream.

Claims

1. A composite oxide, comprising: ceria, praseodymia, and alumina, wherein the cerium:praseodymium molar ratio of the composite oxide is 84:16 or less.

2. The composite oxide according to claim 1, wherein the content of aluminum is in the range of from 0.2 to 70 mol.-% based on 100 mol.-% of the total moles of cerium, praseodymium and aluminum in the composite oxide.

3. The composite oxide according to claim 1, wherein the alumina is dispersed in the solid solution of ceria and praseodymia.

4. The composite oxide according to claim 1, wherein the composite oxide displays a BET surface area determined according to DIN-ISO 9277 in the range of from 15 to 300 m.sup.2/g after aging at 950° C. for 12 h in air containing 10 vol -% of steam.

5. The composite oxide according to claim 1 which further comprises one or more catalytic metals.

6. The composite oxide according to claim 1 wherein the composite oxide is comprised in a catalyst system for exhaust gas treatment.

7. A method of preparing a composite oxide comprising ceria, praseodymia, and alumina, comprising: (a) mixing one or more precursor compounds of ceria, one or more precursor compounds of praseodymia, optionally one or more precursor compounds of zirconia and/or optionally one or more precursor compounds of one or more rare earth oxides other than ceria and praseodymia, one or more precursor compounds of alumina, and one or more basic compounds in a solvent system for obtaining a suspension; (b) optionally heating the suspension obtained in step (a); (c) optionally adding one or more surfactant compounds to the suspension obtained in step (a) or (b); (d) separating the solids from the suspension obtained in step (b) or (c); (e) optionally washing the solids obtained in step (d); (f) optionally drying the solids obtained in step (d) or (e); (g) optionally calcining the solids obtained in step (d), (e), or (f); wherein the cerium:praseodymium molar ratio of the suspension obtained in step (a) is 84:16 or less.

8. The method according to claim 7, wherein the content of aluminum in the suspension obtained in (a) is in the range of from 0.2 to 70 mol.-% based on 100 mol.-% of the total moles of cerium, praseodymium and aluminum in the suspension.

9. The method according to claim 7, wherein the one or more precursor compounds of alumina are selected from the group consisting of colloidal alumina, colloidal aluminum oxide hydroxides, colloidal aluminum hydroxides, and combinations of two or more thereof.

10. The method according to claim 7, wherein the optional heating in step (b) is carried out at a temperature in the range of from 80 to 250° C.

11. The method according to claim 7, wherein the optional heating in step (b) is carried out under autogenous pressure.

12. The method according to claim 7, wherein the method further comprises (h) impregnating the solids obtained in step (d), (e), (f), or (g) with one or more catalytic metals.

13. A composite oxide obtained and/or obtainable by a process according to claim 7.

14. A process of treating an exhaust gas stream, comprising: (1) providing an exhaust gas stream; (2) contacting the exhaust gas stream of (1) with a catalyst comprising a composite oxide comprising ceria, praseodymia, and alumina according to claim 1.

15. A catalyst, catalyst support, or catalyst component, comprising: the composite oxide according to claim 1.

Description

DESCRIPTION OF THE FIGURES

[0144] FIG. 1 is a graphical representation of the results from lambda-sweep catalyst testing in Example 13 performed on the samples from Examples 1-7 and Comparative Examples 8-11 as contained in Table 4 displayed as a bar chart. In the Figure, the values displayed in the abscissa “X” stand for the average conversion in % of NO (top chart), HC (middle chart), and CO (bottom chart) as obtained for the samples from the respective examples and comparative examples as obtained in the fresh state (light grey bar on the left), after hydrothermal aging for 5 h (grey bar in the middle), and after hydrothermal aging for 20 h (dark grey bar on the right).

[0145] FIGS. 2 and 3 respectively display an image of a “fresh” (i.e. after having been subject to calcination at 600° C.) ceria-praseodymia-alumina composite mixed oxide according to the present invention, as obtained from transmission electron microscopy (TEM). In the images, the ceria-praseodymia-alumina composite mixed oxide product and unreacted pradeodymium oxide are respectively designated.

[0146] FIGS. 4 and 5 respectively display an image of a hydrothermally aged ceria-praseodymia-alumina composite mixed oxide according to the present invention as obtained from transmission electron microscopy (TEM), wherein the sample has been subject to hydrothermal aging at 1000° C. for 5 hours in air and 10 vol. % of steam. In the images, the ceria-praseodymia-alumina composite mixed oxide product as well as praseodymium aluminum oxide side-product are respectively designated.

EXAMPLES

[0147] Lambda-Sweep Testing

[0148] For aging, powder samples were placed as shallow bed in high temperature resistant ceramic crucibles and heated in a muffle furnace. Aging was carried out under a flow of air and 10% steam controlled by a water pump. The temperature was ramped up to a desired value (1000° C.) and remained at the desired temperature for a desired amount of time (5 h or 20 h) before the heating was switched off.

[0149] For determining the catalytic activity of the new as well as reference samples, all samples were impregnated with a solution of palladium nitrate for a target loading of 0.5 wt.-% of Pd based on 100 wt.-% of the composite oxide, mixed with 3 wt % boehmite dispersion as binder, dried, and subsequently calcined at 550° C. The resulting cake is crushed and sieved, a size fraction of 250-500 μm is used for testing fresh and after oven aging (1000° C., 5 h or 20 h, 10% steam/air). Tests were performed in a 48-fold parallel testing unit. 100 mg of the respective samples were diluted to a volume of 1 mL using corundum of the same particle size fraction and placed in a reactor.

[0150] To assess catalytic performance of the materials in a three way catalytic converter, the response of the samples to a modification of the air to fuel ratio was tested in a A-sweep test at different temperatures. Powder samples prepared as described above were exposed to a gas feed with oscillating composition (1 s lean, 1 s rich) at a GHSV of 70000 h.sup.−1 with a defined average A value (ratio of actual and stoichiometric air/fuel ratio). The composition of the gas stream under rich and lean conditions is described in Table 1 below.

TABLE-US-00001 TABLE 1 lean and rich gas compositions for lambda-sweep testing Lean Rich CO [vol.-%] 0.7 2.33 H2 [vol.-%] 0.22 0.77 O2 [vol.-%] 1.8 ± Δ 0.7 ± Δ HC (Propylene:Propane 2:1) [ppmv C.sub.1] 3000 3000 NO [ppmv] 1500 1500 CO2 [vol.-%] 14 14 H2O [vol.-%] 10 10

[0151] At stationary temperatures (250, 300, 350, 450° C.), the steady state conversion of CO, NO and HC was measured at 5 discrete average λ values of 1.02, 1.01, 1.00, 0.99, 0.98, adjusted by modifying the amount of oxygen (parameter Δ in the Table 1) without disturbing the amplitude of the rapid oscillations. This simulates to some extent a range of load points of a gasoline engine and probes for good oxygen storage capacity as well as platinum group metal (PGM) activity. For sample ranking, an average conversion over the λ window 1.02-0.98 was calculated for each temperature.

[0152] X-ray Diffraction

[0153] For X-Ray diffraction (XRD), data were collected on a Bruker AXS D8 C2 Discover. Cu K.sub.α radiation was used in the data collection. The beam was narrowed and monochromatized using a graphite monochromator and a pinhole collimator (0.5 mm). Generator settings of 40 kV and 40 mA were used. Samples were gently ground in a mortar with a pestle and then packed in a round mount. The data collection from the round mount covered a 20 range from 16° to 53.5° using a step scan with a step size of 0.02° and a count time of 600s per step. GADDS Analytical X-Ray Diffraction Software was used for all steps of the data analysis. The phases present in each sample were identified by search and match of the data available from Inorganic Crystal Structure Database (ICSD).

[0154] Nitrogen Adsorption Measurements

[0155] N.sub.2-Adsorption/desorption measurements were carried out on a Micromeritics TriStar II.

[0156] Samples were degassed for 30 minutes at 150° C. under a flow of dry nitrogen on a Micromeritics SmartPrep degasser.

Example 1

Preparation of a Ceria-Praseodymia-Alumina Composite Mixed Oxide

[0157] This example describes the preparation of a composite oxide of cerium, praseodymium and aluminum in the respective molar metal proportions of 50%, 40%, 10%. In a beaker 0.05 mol Ce, applied as (NH.sub.4).sub.2Ce(NO.sub.3).sub.6, were dissolved in 150 ml deionized water (DI-water) under stirring (Solution A). A second solution (Solution B) was prepared by dissolving 0.04 mol Pr, applied as Pr(NO.sub.3).sub.3×6 H.sub.2O, and 0.01 mol Al, applied as Al(NO.sub.3).sub.3×9H.sub.2O in 50 ml DI-water. Solutions A and B were stirred until all of the applied solids have been dissolved. A precipitation vessel was prepared by diluting NH.sub.3, applied as concentrated ammonia solution (25%), with DI-water. The total volume of the mixture was 200 ml at the end. The mixture of concentrated ammonia in DI-water was found to have a pH value of 12. Solution A and B were added consecutively and drop wise into the precipitation vessel using a flow rate of 10m1/min under constant stirring of the resulting mixture. During the precipitation process the pH value was not allowed to drop below 9. This was controlled by constantly adding additional ammonia solution (25%). The suspension was stirred for 15 minutes before being transferred into an autoclave (50% fill quantity) and stirred for 2h at 150° C. The suspension was allowed to cool to room temperature overnight, before 0.022 mol of lauric acid (LA) (0.22 mol LA per mol of Ce, Pr, and Al employed) was added. The mixture was stirred until total dilution of the lauric acid was achieved. The suspension was filtered with a blue ribbon filter thereafter and washed with ammonia solution (25%) until the filter cake was free of NO.sub.3.sup.− ions. The filter cake was dried at 40° C. and subsequently calcined at 600° C. for 4 h using a muffle furnace.

Example 2

Preparation of a Ceria-Praseodymia-Alumina Composite Mixed Oxide

[0158] This example describes the preparation of a composite oxide of cerium, praseodymium and aluminum in the respective molar metal proportions of 50%, 45%, 5%. The starting materials used in this preparation included 0.05 mol of Ce applied as (NH.sub.4).sub.2Ce(NO.sub.3).sub.6 (Solution A), and for solution B 0.045 mol Pr, applied as Pr(NO.sub.3).sub.3×6 H.sub.2O, and 0.005 mol Al, applied as Al(NO.sub.3).sub.3×9H.sub.2O. The procedure described in Example 1 was followed.

[0159] Example 3: Preparation of a ceria-praseodymia-lanthana-alumina composite mixed oxide

[0160] This example describes the preparation of a composite oxide of cerium, praseodymium, aluminum and lanthanum in the respective molar metal proportions of 45%, 45%, 5%, 5%. The starting materials used in this preparation included 0.045 mol of Ce applied as (NH.sub.4).sub.2Ce(NO.sub.3).sub.6 (Solution A), and for solution B 0.045 mol Pr, applied as Pr(NO.sub.3).sub.3×6 H.sub.2O, 0.005 mol Al applied as Al(NO.sub.3).sub.3×9H.sub.2O and 0.005 mol La applied as La(NO.sub.3).sub.3×H.sub.2O. The procedure described in Example 1 was followed, wherein lanthanum was added as a part of Solution B.

Example 4

Preparation of a Ceria-Praseodymia-Yttria-Alumina Composite Mixed Oxide

[0161] This example describes the preparation of a composite oxide of cerium, praseodymium, aluminum and yttrium in the respective molar metal proportions of 45%, 45%, 5%, 5%. The starting materials used in this preparation included 0.045 mol of Ce applied as (NH.sub.4).sub.2Ce(NO.sub.3).sub.6 (Solution A), and for solution B 0.045 mol Pr, applied as Pr(NO.sub.3).sub.3×6 H.sub.2O, 0.005 mol Al applied as Al(NO.sub.3).sub.3×9H.sub.2O and 0.005 mol Y applied as Y(NO.sub.3).sub.3×6 H.sub.2O. The procedure described in Example 1 was followed, wherein yttrium was added as a part of Solution B.

Example 5

Preparation of a Ceria-Praseodymia-Neodymia-Alumina Composite Mixed Oxide

[0162] This example describes the preparation of a composite oxide of cerium, praseodymium, aluminum and neodymium in the respective molar metal proportions of 45%, 45%, 5%, 5%. The starting materials used in this preparation included 0.045 mol of Ce applied as (NH.sub.4).sub.2Ce(NO.sub.3).sub.6 (Solution A), and for solution B 0.045 mol Pr, applied as Pr(NO.sub.3).sub.3×6 H.sub.2O, 0.005 mol Al applied as Al(NO.sub.3).sub.3×9H.sub.2O and 0.005 mol Nd applied as Nd(NO.sub.3).sub.3×6 H.sub.2O. The procedure described in Example 1 was followed, wherein neodymium was added as a part of Solution B.

Example 6

Preparation of a Ceria-Praseodymia-Lanthana-Yttria-Alumina Composite Mixed Oxide

[0163] This example describes the preparation of a composite oxide of cerium, praseodymium, aluminum, lanthanum and yttrium in the respective molar metal proportions of 45%, 40%, 5%, 5%, 5%. The starting materials used in this preparation included 0.045 mol of Ce applied as (NH.sub.4).sub.2Ce(NO.sub.3).sub.6 (Solution A), and for solution B 0.040 mol Pr, applied as Pr(NO.sub.3).sub.3×6 H.sub.2O, 0.005 mol Al applied as Al(NO.sub.3).sub.3×9H.sub.2O, 0.005 mol La applied as La(NO.sub.3).sub.3×H.sub.2O and 0.005 mol Y applied as Y(NO.sub.3).sub.3'6 H.sub.2O. The procedure described in Example 1 was followed, wherein yttrium and lanthanum were added as a part of Solution B.

Example 7

Preparation of a Ceria-Praseodymia Composite Mixed Oxide

[0164] This example describes the preparation of a composite oxide of cerium, praseodymium and aluminum in the respective molar metal proportions of 50%, 40%, 10%. In a beaker 0.05 mol Ce, applied as (NH.sub.4).sub.2Ce(NO.sub.3).sub.6, were dissolved in 150 ml deionized water (DI-water) under stirring (Solution A). Solution B was prepared by dissolving 0.04 mol Pr, applied as Pr(NO.sub.3).sub.3×6 H.sub.2O in 50 ml DI-water. Solutions A and B were stirred until all of the applied solids have been dissolved. A precipitation vessel was prepared by diluting NH.sub.3, applied as concentrated ammonia solution (25%), with DI-water. The total volume of the mixture was 400 ml at the end. The mixture of concentrated ammonia in DI-water was found to have a pH value of 12. Under constant stirring 0.01 mol aluminum was added, using a colloidal aqueous suspension of alumina (particle size ˜200 nm) as aluminum source. Solution A and B were added consecutively and drop wise into the suspension in the precipitation vessel using a flow rate of 10 ml/min under constant stirring of the mixture. During the precipitation process the pH value was not allowed to drop below 9. This was controlled by constantly adding of additional ammonia solution (25)%. The suspension was stirred for 15 minutes before being transferred into an autoclave (50% fill quantity) and stirred for 2 h at 150° C. The suspension was allowed to cool to room temperature overnight, before 0.022 mol of lauric acid (LA) (0.22 mol LA per mol of Ce, Pr, and Al employed) was added. The mixture was stirred until total dilution of the lauric acid was achieved. The suspension was filtered with a blue ribbon filter thereafter and washed with ammonia solution (25%) until the filter cake was free of NO3.sup.− ions. The filter cake was dried at 40° C. and subsequently calcined at 600° C. for 4 h using a muffle furnace.

Comparative Example 8

Preparation of a Ceria

[0165] This example describes the preparation of cerium oxide. The starting material used in this preparation included 0.1 mol of Ce applied as (NH.sub.4).sub.2Ce(NO.sub.3).sub.6. The procedure described in Example 1 was followed. No solution B was prepared.

Comparative Example 9

Preparation of a Ceria-Praseodymia Mixed Oxide

[0166] This example describes the preparation of a composite oxide of cerium and praseodymium, in the respective molar metal proportions of 50%, 50%. The starting materials used in this preparation included 0.05 mol of Ce applied as (NH.sub.4).sub.2Ce(NO.sub.3).sub.6 and 0.05 mol Pr, applied as Pr(NO.sub.3).sub.3×6 H.sub.2O. The procedure described in Example 1 was followed. No aluminum was added to Solution B.

Comparative Example 10

Preparation of a Ceria-Praseodymia Mixed Oxide

[0167] This example describes the preparation of a composite oxide of cerium and praseodymium in the respective molar metal proportions of 50%, 50%. In a beaker 0.05 mol Ce, applied as (NH.sub.4).sub.2Ce(NO.sub.3).sub.6 and 0.05 mol Pr, applied as Pr(NO.sub.3).sub.3×6 H.sub.2O, were dissolved in 300 ml deionized water (DI-water) under stirring (Solution A). The further procedure described in Example 1 was followed. No solution B was prepared.

Comparative Example 11

Preparation of a Ceria-Zirconia Mixed Oxide

[0168] This example describes the preparation of a composite oxide of cerium and zirconium in the respective molar metal proportions of 50%, 50%. In a beaker 0.05 mol Ce, applied as (NH.sub.4).sub.2Ce(NO.sub.3).sub.6 and 0.05 mol Zr, applied as ZrO(NO.sub.3).sub.2×H.sub.2O (Zr content was determined gravimetrically prior to use), were dissolved in 300 ml deionized water (DI-water) under stirring to form Solution A. The further procedure described in Example 1 was followed. No solution B was prepared.

[0169] The compositions of Examples 1-7 and Comparative Examples 8-11 are summarized in Table 2. The numbers represent molar contents (in %) of respective composite oxide constituents normalized to 100%.

TABLE-US-00002 TABLE 2 Composition of samples from Examples 1-7 and Comparative Examples 8-11. Composition, mol. % Sample CeO.sub.2 ZrO.sub.2 LaO.sub.1,5 YO.sub.1,5 NdO.sub.1,5 PrO.sub.1,83 AlO.sub.1,5 EXAMPLE 50 — — — — 40 10 1 EXAMPLE 50 — — — — 45 5 2 EXAMPLE 45 — 5 — — 45 5 3 EXAMPLE 45 — — 5 — 45 5 4 EXAMPLE 45 — — — 5 45 5 5 EXAMPLE 45 — 5 5 — 40 5 6 EXAMPLE 50 — — — — 40 10 7 COMP. EX. 100 — — — — — — 8 COMP. EX. 50 — — — — 50 — 9 COMP. EX. 50 — — — — 50 — 10 COMP. EX. 50 50 — — — — — 11

Example 12

Surface Area Ddetermination (BET)

[0170] Table 3 provides data on the BET surface area determined by the standard N.sub.2-adsorption/desorption method. The samples were analyzed fresh, meaning after calcination at 600° C., as well as after being aged at 1000° C. for 5 hours in air and 10 vol. of steam. The data (rounded to full numbers) are discussed in the following. Examples 1-7 exhibit a surface area equal or higher than 80 m.sup.2/g before and a surface area equal or higher 10 m.sup.2/g after aging. Comparative examples 8 to 10 have surface areas below 73 m.sup.2/g fresh and below 10 m.sup.2/g after aging. Comparative example 11 has a surface area of 62 m.sup.2/g before and 29 m.sup.2/g after aging. Thus, it has surprisingly been found that the relatively large surface area of the fresh samples according to the present invention is contributed by the content of alumina in the formulation. After aging, the samples containing alumina quite unexpectedly still show higher surface areas than samples prepared from Ce and Pr or Ce only (Examples 8 to 10). However the surface area after aging is lower than those measured for the comparative sample 11 made from Ce and Zr. The data reveal that the addition Al of to the formulation results in notably higher surface areas in the fresh state and to an overall higher thermal stability compared to samples prepared from Ce and Pr only.

TABLE-US-00003 TABLE 3 BET surface area of the samples from Examples 1-7 and Comparative Examples 8-11 fresh and after hydrothermal aging BET Surface Area, m.sup.2/g Sample Fresh 1000° C., 5 hrs.sup.a EXAMPLE 1 88 11 EXAMPLE 2 80 10 EXAMPLE 3 90 10 EXAMPLE 4 81 10 EXAMPLE 5 84 10 EXAMPLE 6 91 11 EXAMPLE 7 106 11 COMP. EX. 8 54 3 COMP. EX. 9 51 8 COMP. EX. 10 72 2 COMP. EX. 11 62 29 .sup.aHydrothermal aging conditions: 1000° C. for 5 hours in air and 10 vol. % of steam.

Example 13

Lambda-Sweep Catalyst Testing

[0171] Table 4 shows catalytic data obtained from lambda-sweep testing in the catalytic experiment as described further above. A graphical representation of the result displayed in Table 4 is provided in FIG. 1. Thus, the A-sweep data at 300° C. reveals equivalent fresh performance relative to the comparative examples. However, after aging at 1000° C., examples 1-7 surprisingly show significantly superior conversions since they are less affected by hydrothermal aging, i.e. the comparative examples loose a large fraction of the fresh activity while the examples 1-7 show slower deterioration.

TABLE-US-00004 TABLE 4 Results from lambda-sweep catalyst testing performed on the samples from Examples 1-7 and Comparative Examples 8-11. Average conversion [%] in a λ-window 0.98-1.02 at 300° C. Aged (1000° C.) Aged (1000° C.) Fresh H.sub.2O/air 5 hrs H.sub.2O/air 20 hrs Sample X-CO X-HC X-NO X-CO X-HC X-NO X-CO X-HC X-NO EXAMPLE 1 90.8 83.8 72.9 94.1 72.5 67.4 90.2 68.9 56.1 EXAMPLE 2 97.6 90.9 74.3 92.1 78.1 59.2 86.3 68.7 33.2 EXAMPLE 3 94.6 88.5 73.3 90.3 79.3 59.5 73.2 58.4 25.4 EXAMPLE 4 96.6 89.6 73.4 91.5 81.2 62.9 92.6 75.1 46.2 EXAMPLE 5 97.7 90.4 71.0 93.0 76.8 52.2 92.3 75.9 44.9 EXAMPLE 6 97.0 90.2 72.7 92.0 82.6 61.3 89.2 71.1 38.9 EXAMPLE 7 97.4 88.9 81.7 90.8 79.3 69.7 92.8 79.7 70.8 COMP. EX. 8 90.3 76.5 81.9 27.7 10.6 8.7 — — — COMP. EX. 9 89.8 81.7 66.6 74.2 74.6 40.2 62.2 59.0 27.8 COMP. EX. 10 91.1 74.8 52.2 50.0 32.4 13.1 22.6 5.4 7.7 COMP. EX. 11 90.6 87.5 55.2 50.0 49.6 34.2 49.3 47.8 27.6

[0172] These results are particularly unexpected relative to the performance of the ceria-zirconia mixed oxide catalyst according to comparative example which, as observed in the determination of the BET surface area (cf. Table 3), appeared to display a greater resistance to hydrothermal aging not only with respect to the other comparative examples devoid of zirconia, but also with respect to the inventive examples. Accordingly, it has quite unexpectedly been found that despite the better stabilization of the ceria-containing oxygen storage material with the aid of zirconia as practiced in the art, the inventive composite materials containing praseodymia in addition to alumina display superior results in the conversion of CO, HC, and NO in exhaust gas not only in a fresh state, but quite surprisingly clearly outperform such oxygen storage materials according to the art after prolonged periods of aging, as evidenced by the results from the lamda-sweep catalyst testing results displayed in Table 4.

[0173] Thus it has quite surprisingly been found that the specific catalyst composites of the present invention containing a ceria-paraseodymia mixed oxide in addition to alumina displays superior catalytic results in the treatment of automotive exhaust gas compared to oxygen storage materials according to the art.