DE19805259B4 - Oxygen-storing cerium complex oxide - Google Patents

Abstract

An oxygen-storing cerium complex oxide having the formula: Ce _{1- (1 + y)} Zr _x R _y O _2-z , where "R" represents a rare earth metal or rare earth metal other than Zer, where "z" represents the degree of oxygen deficiency determined by the valence of the rare earth metal contained, where 0.2≤x + y≤0.9, 0.1≤x ≤0.8 and 0.05≤y≤0.3, and wherein the cerium complex oxide carries a transition metal or transition metals selected from the group of transition metals reduced by the noble metals.

Description

BACKGROUND THE INVENTION

Field of the invention:

The present invention relates to an oxygen-storing cerium complex oxide which can be favorably used in a catalytic converter of a vehicle for efficiently cleaning exhaust gases of an internal combustion engine by removing nitrogen oxides (NO _x ), carbon monoxide (CO) and hydrocarbons (HC).

As is known, the exhaust gases of an automotive internal combustion engine inevitably contain harmful substances such as NO _x , CO, HC. Recently, in order to protect the environment, the restrictions on the exhaust gas have become more stringent.

on the other hand there is an increasing propensity to save fuel, an internal combustion engine in lean condition instead of stoichiometric To operate air-fuel mixture while a vehicle except of idling and acceleration phases running normally. For this reason, the Development of a catalytic converter, which is capable is the above harmful Substances not only in stoichiometric Condition but also in lean condition to remove efficiently, strong he wishes.

Most often, a so-called three-way catalyst is used to remove the above harmful substances. A three-way catalyst uses as an active substance a noble metal or metals, such as Pt, Pd and / or Rh, to reduce NO _x to N ₂ and to oxidize CO and HC to CO ₂ and H ₂ O. In this way, the catalytic converter functions both for oxidation and for reduction as a catalyst.

Various researches have been conducted to improve the operation of a three-way catalyst. One of the three-way catalysts that have followed such research uses cerium oxide (CeO ₂ ) with an ability to store oxygen; ie, the ability to store gaseous oxygen in the crystal structure and to deliver stored oxygen out of the crystal structure. Specifically, CeO _{2 is added} to a three-way catalyst to adjust the oxygen concentration of the gas phase such that in the oxygen-rich state, excess gas phase oxygen is intercalated in the CeO ₂ crystal structure to remove the catalyst in the reduction of NO _x to support N ₂ , and that in the CO and / or HC-rich state stored oxygen is released into the gas phase to assist the catalyst in the oxidation of CO and HC to CO ₂ and H ₂ O.

On the other hand, there is an increasing demand to shift the mounting position of the catalytic converter from below the floor panel to the exhaust manifold, which is close to the engine, whereby the catalyst can be warmed up quickly after starting the engine. However, the catalytic converter, when placed near the engine, may often be exposed to high temperatures of not lower than 900 ° C, which may cause growth of grain size of CeO ₂ due to sintering. As a result, the specific surface area of CeO ₂ and the oxygen storage capacity of CeO ₂ decrease. so that the desired improvement of the three-way catalyst can not be achieved.

In view of this fact, it has been proposed to add grain growth preventing elements such as Zr and La to CeO ₂ (see, for example, JP-A-61-262521 or JP-A-61-262522). However, it has been found that the added Zr or La crystallizes out (locally crystallized) on the grains of CeO ₂ , so that the inherent properties of CeO ₂ may be lost. Consequently, there are difficulties in using CeO ₂ at high temperatures.

From the post-published DE 196 44 276 A1 is a heat-resistant oxide expressed by the general formula Ce _{1- (1 + y)} Zr _x R _y O _2-z , wherein "R" represents a rare earth metal, "z" expresses an oxygen deficiency, "x" in the range of 0 , 15 to 0.70, "y" is in the range of 0.05 to 0.25, and (x + y) is in the range of 0.20 to 0.95.

EPIPHANY THE INVENTION

It is thus an object of the present invention, an oxygen-storing To provide Zer complex oxide which is capable of itself under difficult operating conditions, especially at high temperatures, to preserve a high oxygen storage capacity.

This object is achieved according to the features of claim 1. Accordingly, an oxygen-storing cerium complex oxide having the following formula is proposed: Ce _{1- (1 + y)} Zr _x R _y O _2-z , where "R" represents rare earth metal or rare earth metals other than Zer, where "z" represents the degree of oxygen deficiency determined by the valence of the rare earth metal contained, and wherein 0.2≤x + y≤0.9, 0.1≤x ≤0.8 and 0.05≤y≤0.3.

Preferably, at least a portion of the cerium complex oxide may be a solid solution. Specifically, a part of Ce in the crystal of CeO _{2 may be} replaced by Zr and the other rare earth metal in solid solution. The Zr in solid solution in the CeO ₂ crystal prevents mass transport of Ce (as a cation) and thus prevents grain growth (sintering) of CeO ₂ at high temperatures.

As As defined above, the content of Zr in the Zer complex oxide is defined as "x", between 0.1 and 0.8. A content of Zr below this content range is for the purpose of prevention grain growth of cerium complex oxide insufficient. A salary above this salary range to an excessive reduction Ce content and thereby unacceptable reduction the oxygen storage capacity of Zer complex oxide. To get a good result, "x" should preferably be in the range of 0.35 to 0.7 lie. It should be noted that the Zr incorporated into the cerium complex oxide will contain 1% to 2% hafnium (Hf), which is inevitable in Zr-ores is included.

The reason for the substitution of Ce in the CeO ₂ with the other rare earth metal in solid solution is that this substitution stabilizes the uniform fluorite structure of the cerium complex oxide at room temperature. The content of the other rare earth metal in the Zer complex oxide, defined as "y", is between 0.05 and 0.3. A content of the other rare earth metal under this range causes the difficulty of maintaining a stable and uniform fluorite structure because, due to the difference of the internal radii of Ce and Zr, the cerium complex oxide becomes a CeO ₂ region containing Zr and a ZrO ₂ region containing Ce splits. A content of the other rare earth metal over this range causes the excess rare earth metal to lead to by-products, while the contained Ce and Zr can not perform the corresponding functions due to the relative content reduction. In order to obtain good results, "y" should preferably be between 0.05 and 0.2.

The other rare earth metal "R" can, for example Y, Sc, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu include. These rare earth metals can be used individually or in combination be added. A preferred example is Y.

The Sum of "x" and "y", so "x + y", is chosen between 0.2 and 0.9. The content of Ce in the cerium complex oxide is represented by 1- (x + y) which is between 0.1 and 0.8. The oxygen storage capacity of Zer complex oxide is essentially determined by the fact that the Ce is oxidized or reduced becomes and its valence changes. Considering this fact, the zer-complex oxide is not able to one sufficient oxygen storage capacity as needed when the content of Ce is too low. on the other hand reduces the content of Zr and / or the other rare earth metal, if the content of Ce is too high, and these can not do their functions sufficiently fulfill. Consequently, the value of "x + y" should be between 0.2 and 0.9, preferably between 0.2 and 0.6.

The zer-complex oxide of the invention can be prepared by known methods. In a first example of a method for producing the zer-complex oxide, the following steps are performed. First, water is added to a carbonated powder and a slurry is provided. Next, an aqueous solution of a zirconium salt and a rare earth metal salt in predetermined stoichiometric ratio of the slurry is added. Finally, the slurry produced is heat treated after sufficient mixing and a desired cerium complex oxide generated.

The Cercarbonate powder can be any commercial, but it should preferably a possible large specific surface exhibit. For this purpose, preferably zerkarbonatpulver of not less than 0.1 μm Find use. 1 part by weight of water can be 10 to 50 parts by weight be given to such a Zerkarbonatpulvers to the above slurry to prepare.

The Zirconium salt may be, for example, zirconium oxychloride, zirconium oxynitrate, Zirconium oxysulfate or zirconium oxoacetate, of which zirconium oxynitrate is preferred. The rare earth metal can be, for example, sulphates, Nitrates, hydrochlorides, phosphates, acetates and oxalates, although a rare earth nitrate is preferred. In the provision above aqueous solution 1 part by weight of zirconium salt (selected from the above examples) and of the rare earth metal salt (selected from the above examples) in predetermined stoichiometric relationship dissolved in 0.1 to 10 parts by weight of water.

As described above, the provided in this way solution the slurry added and the resulting slurry after sufficient mixing heat treated. The heat treatment comprises a preceding step of vacuum drying in which the Slurry under Using a vacuum dryer is dried to a pre-dried To obtain a mixture, a step of heat drying in which the pre-dried mixture at 50 to 200 ° C for about 1 to 48 hours additionally dried and a baking step in which the additionally dried Mixture at about 350 to 1000 ° C, preferably at about 400 to 700 ° C, for about 1 to 12 hours, preferably for about 2 to 4 hours, baked. During the baking step should the baking conditions (baking temperature and baking time) depending on be chosen from the composition of the Zer complex oxide, so that at least part of the cerium complex oxide is in the form of a solid solution, around the thermal resistance of cerium complex oxide.

at a second example of a method of producing the cerium complex oxide will be the following Steps performed. First becomes a mixed saline solution prepared containing the corresponding salts of cerium, zirconium and a rare earth metal (except zer) in selected stoichiometry contains. Then is an alkaline aqueous solution or an organic acid the mixed solution added so that the corresponding salts of cerium, zirconium and rare earth metal coprecipitated become. After filtering, the resulting precipitate is heat treated to produce a desired precipitate Zer complex oxide to produce.

at the second method the salts, for example, sulfates, nitrates, hydrochlorides, phosphates, Acetates and oxalates include. The zirconium salts can, for example Zirconium oxychloride, zirconium oxynitrate, zirconium oxysulfate or Zirconium oxoacetate. According to the above examples of Zersalzen the rare earth metal salts sulfates, nitrates, hydrochlorides, phosphates, Acetates and oxalates include. The alkaline aqueous solution may be, for example, ammonia or aqueous ammonium carbonate. Furthermore, the organic acid, for example, oxalic acid or citric acid include.

The heat treatment in the second example, in the same way as in the first Example performed become.

at a third example of a method of producing the cerium complex oxide will be the following Steps performed. First becomes an alkoxide solution prepared, the zer, zirconium and a rare earth metal (except Zer) in chosen stoichiometry contains. Then, deionized water is added to the alkoxide solution to remove the cerium, the To precipitate or hydrolyze circminium and the rare earth metal. After filtering is the precipitation or the hydrolyzate is heat treated and the desired one Zer complex oxide produced.

The Alkoxides for producing the alkoxide solution can be used in the third example Methoxides, ethoxides, propoxides or butoxides before Zer, zirconium and the rare earth metal. Instead, ethylene oxide additives are also used of zer, zirconium and rare precious metals.

Also the heat treatment in the third example can be performed in the same way as in the first example.

The oxygen-storing cerium complex oxide of the present invention carries a transition metal or transition metals which can not be simultaneously assigned to the group of noble metals. A transition metal is an excellent catalyst for cleaning vehicle exhaust via redox reactions and can thus be advantageously combined with the oxygen-storing zer-complex oxide of the present invention to clean the vehicle exhaust even more efficient. Furthermore, the supported transition metal can also assist in the inclusion and release of the oxygen into and out of the cerium complex oxide.

The transition metal or the transition metals can or can e.g. Fe, Co, Ni and Cu, of which Fe is preferable.

The Transition metals described above can held on the Zer complex oxide by using known techniques become. For example, can first a solution which is a salt (e.g., 1 to 20% by weight) of a transition metal contains and the cerium complex oxide is then impregnated with this saline solution. The for Salts used for this purpose may for example, nitrates (e.g., palladium nitrate, rhodium nitrate, iron nitrate, Cobalt nitrate, nickel nitrate, copper nitrate), dinitrodiammine nitrates (e.g., dinitrodiammine palladium nitrate), amine nitrates (e.g., ammonium palladium (IV) nitrate) and chlorides (e.g., rhodium chloride). After this Impregnate and filtering, the zer-complex oxide is preferably heated up 50 to 200 ° C for about 1 to Dried for 48 hours and then at about 350 to 1000 ° C for about 1 to Baked for 12 hours.

alternative can the process to Application of the transition metal on the zer-complex oxide while the production of zer-complex oxide. In detail, will this is a solution which is a transition metal salt (which is identical to any of those described above) to which mixed saline solution or the alkoxide solution given if the latter solution precipitated so that the transition metal salt precipitates together with the other salts present. After filtration is the precipitation in the same manner as above for the preparation of the cerium complex oxide described a heat treatment subjected.

Other Features and advantages of the present invention will become apparent in the following Detailed description of preferred embodiments clearly.

LONG DESCRIPTION PREFERRED EMBODIMENTS

The preferred embodiments Present invention will be described below together with a comparative example described.

[1. embodiment]

In this embodiment, a cerium complex oxide having the composition Ce _0.6 Zr _0.3 Y _0.1 O _1.95 / 0.5 wt% Fe was prepared. Subsequently, the durability of the Zer complex oxide under severe operating conditions was evaluated by comparing the initial oxygen storage capacity of the Zer complex oxide with the oxygen storage capacity after aging after conducting a redox aging test at high temperatures. The results of the determination of durability are listed in attached Table 1.

It It should be noted that the Designation "/ 0,5 Wt% Fe "indicates that this Zer complex oxide carries 0.5% by weight of Fe, wherein the percentage based on non-Fe bearing cerium complex oxide is determined. Furthermore, in Table 1, the abbreviations "anf." "n. d. A." and "Ex." the Terms "initial," "after aging," and "comparative example." The numbering corresponds to the numbering of the embodiments.

(Preparation of Zer Complex Oxide)

The cerium complex oxide having the above composition was produced by the so-called alkoxide method. First, 38.4 g (0.102 mol) of cerium isopropoxide, 16.7 g (0.051 mol) of zirconium isopropoxide and 4.5 g (0.017 mol) of yttrium isopropoxide were dissolved in 200 ml of toluene to provide an alkoxide solution. Then, 600 ml of deionized water was gradually added to the alkoxide solution by dropping for about 10 minutes to cause hydrolysis of the alkoxides. Then, the resulting hydrolyzate (intermediate) was separated from the solution mixture by evaporation of the solvent and water. Thereafter, the intermediate was dried at 60 ° C for 24 hours under ventilation and then baked at 450 ° C for 3 hours in an electric oven to produce a cerium complex oxide powder having the composition Ce _0.6 Zr _0.3 Y _0.1 O _1.95 .

To apply the transition metal (Fe), 20 ml of deionized water was added to 20 g of the cerium complex oxide powder, and a slurry was produced. Further, 0.73 g of an iron (III) nitrate reagent (containing 13.8% by weight of iron) was dissolved in 20 ml of deionized water, and an iron nitrate solution containing 1% by weight of iron (III) was prepared. Then, the slurry was uniformly mixed with the iron nitrate solution and then dried by evaporating the H ₂ O. The thus-obtained powder was further dried at 60 ° C for 24 hours under ventilation and then baked at 450 ° C for 3 hours in an electric furnace to obtain a Fe-bearing cerium complex oxide having the composition Ce _0.6 Zr _0.3 Y _{0 , 1} O _1.95 / 0.5 wt .-% Fe produce.

(Determination of the initial Oxygen storage capacity, anf. OSC)

When Sample was 18 mg of the prepared as described above Fe bearing Zer complex oxide taken. The sample was initially in a 50% oxygen gas stream (the remainder was nitrogen) at 500 ° C for 15 minutes given. Subsequently The sample was subjected to a redox cycle, which was an alternating Gas flow from 20% hydrogen (the remainder was nitrogen) and 50% oxygen (the rest was nitrogen) at 500 ° C. This redox cycle was repeated three times while the weight of the sample constantly was measured by means of a thermobalance. The weight of the sample was shown in a diagram to the most stable redox cycle to find which as a basis for calculating a difference measured immediately after the oxidation and immediately served after reduction of measured sample weight. The calculated Weight difference, which for the oxygen weight stored and released by the sample representative was further used as the basis for calculating the number of moles used by oxygen per mole of the sample.

(Redox-aging test)

After determining the initial oxygen storage capacity, the sample was then subjected to a redox aging test in which the sample was exposed to gas conditions that simulated in simplified form the variation of current exhaust gases of a vehicle. Specifically, the sample was sequentially exposed to a purifying inert gas stream for 5 minutes, then to an oxidizing gas stream for 10 minutes, then back to the purifying inert gas stream for 5 minutes, and then to a reducing gas stream for 10 minutes, repeating this cycle four times over a total of 2 hours has been. The purifying inert gas stream contained 8% by volume of CO ₂ and 10% by volume of steam, with the remainder being N ₂ . The oxidizing gas stream contained 1% by volume of O ₂ , 8% by volume of CO ₂ and 10% by volume of steam, with the remainder being N ₂ . The reducing gas stream contained 1.5 vol.% CO, 0.5 vol.% H ₂ and 10 vol.% Water vapor with the remainder being N ₂ . Furthermore, the temperature of each gas stream was set equal to 1000 ° C.

(Determination of the oxygen storage capacity after the Aging, OSC n. D. A.)

To the implementation of the redox aging test was the oxygen storage capacity of the sample determined in the same way as in the determination of the initial Oxygen storage capacity.

(Determination of change the oxygen storage capacity)

In Table 1, the change of the oxygen storage capacity of the sample was calculated according to the following equation. Change (%) = 1 - (OSC nd A./anf. OSC) × 100.

As From the above equation, the value represents the change how much the initial oxygen capacity of the sample deteriorated after the redox aging test.

[2nd embodiment]

In this embodiment, a _{cerium complex oxide} of composition Ce _0.5 Zr _0.375 Y _0.125 O _1.9375 / 0.5 wt% Fe was prepared. Subsequently, in the same manner as in the first embodiment, the durability of the cerium complex oxide was determined under severe operating conditions. by first determining the initial oxygen storage capacity of the cerium complex oxide, then conducting the redox aging test, then determining the oxygen storage capacity of the zer-complex oxide after aging, and finally calculating the change in oxygen storage capacity. The results of the durability determination are shown in Table 1.

The cerium complex oxide used in the second embodiment was prepared in the same manner as that used in the first embodiment except that cerium isopropoxide, zirconium isopropoxide and yttrium isopropoxide in the above stoichiometric ratio other than the ratio in the first embodiment were mixed and the amount of ferric nitrate solution (containing 1% by weight of iron) to be added to the slurry of Ce _0.5 Zr _0.375 Y _0.12 O _1.9375 powder was suitably adjusted to the desired weight percentage of Fe on the To obtain Zer complex oxide.

[3rd embodiment]

In this embodiment, a _{cerium complex oxide} of composition Ce _0.4 Zr _0.45 Y _0.15 O _0.1925 / 0.4 wt% Co was prepared. Subsequently, in the same manner as in the first embodiment, the durability of the cerium complex oxide under severe operating conditions was determined by first determining the initial oxygen storage capacity of the cerium complex oxide, then the redox aging test, then the oxygen storage capacity of the zer-complex oxide after Aging determined and last the change in the oxygen storage capacity was calculated. The results of the durability determination are shown in Table 1.

The cerium complex oxide used in the third embodiment was prepared in the same manner as that used in the first embodiment except that cerium isopropoxide, zirconium isopropoxide and yttrium isopropoxide in the above stoichiometric ratio other than the ratio in the first embodiment were mixed a cobalt (II) nitrate solution (containing 1% by weight of cobalt) was used in place of the ferric nitrate solution and that the amount of the slurry of Ce _0.4 Zr _0.45 Y _0.15 O _1.925 powder was added Cobalt (II) nitrate solution was suitably adjusted to obtain the desired weight percent of Co on the Zer complex oxide.

[4th embodiment]

In this embodiment, a cerium complex oxide of composition Ce _0.2 Zr _0.6 Y _0.2 O _1.9 / 0.7 wt% Ni was prepared. Subsequently, in the same manner as in the first embodiment, the durability of the cerium complex oxide under severe operating conditions was determined by first determining the initial oxygen storage capacity of the cerium complex oxide, then the redox aging test, then the oxygen storage capacity of the zer-complex oxide after Aging determined and last the change in the oxygen storage capacity was calculated. The results of the durability determination are shown in Table 1.

The cerium complex oxide used in the fourth embodiment was prepared in the same manner as that used in the first embodiment except that cerium isopropoxide, zirconium isopropoxide and yttrium isopropoxide in the above stoichiometric ratio other than the ratio in the first embodiment were mixed a nickel nitrate solution (containing 1 wt% nickel) was used in place of the ferric nitrate solution, and that the amount of nickel nitrate solution to be added to the slurry of the Ce _0.2 Zr _0.6 Y _0.2 O _1.9 powder was suitable was adjusted to obtain the desired weight percent of Ni on the Zer complex oxide.

[5th embodiment]

In this embodiment, a cerium complex oxide of the composition of Ce _0.5 Zr _0.375 Y _0.125 O was _1.9375 / 1.0 provides wt .-% Cu Herge. Subsequently, in the same manner as in the first embodiment, the durability of the cerium complex oxide under severe operating conditions was determined by first determining the initial oxygen storage capacity of the cerium complex oxide, then the redox aging test, then the oxygen storage capacity of the zer-complex oxide after Aging determined and last the change in the oxygen storage capacity was calculated. The results of the durability determination are shown in Table 1.

The cerium complex oxide used in the fifth embodiment was prepared in the same manner as that used in the first embodiment except that cerium isopropoxide, zirconium isopropoxide and yttrium isopropoxide in the above stoichiometric ratio other than the ratio in the first embodiment were mixed to use a cupric nitrate solution (containing 1% by weight of copper) instead of the ferric nitrate solution, and that U.S. Pat the amount of _{cupric nitrate} solution to be added to the slurry of the Ce _0.5 Zr _0.375 Y _0.125 O _1.9375 powder was suitably adjusted to obtain the desired weight percentage of Cu on the Zer complex oxide.

[Comparative Example]

In this comparative example, a cerium oxide (CeO ₂ instead of a cerium complex oxide was prepared.) Subsequently, in the same manner as in the first embodiment, the resistance of the cerium oxide under severe operating conditions was determined by first determining the initial oxygen storage capacity of the cerium oxide, then the redox oxide. Aging test was carried out, then the oxygen storage capacity of the cerium oxide after aging was determined and finally the change in the oxygen storage capacity was calculated The results of the durability determination are shown in Table 1.

The Ceria used in the comparative example was in the same Prepared as the zer-complex oxide used in the first embodiment, except that only a Zerisopropoxidlösung used instead of an alkoxide solution was (because zirconium and yttrium were not included) and that the Fe applying process step was not performed.

[Result]

As from Table 1 shows the zer-complex oxide according to the embodiments a higher one initial oxygen storage (Initial OSC) as the comparative example, while there is less waste or a minor change between initial oxygen storage (Initial OSC) and Oxygen Storage Capacity After Aging (OSC n.d.A.) subject. This means that the Zer complex oxide according to the invention a higher oxygen storage even with repeated use at high temperatures can receive.

Of Further, those relating to the first and second embodiments are shown obtained results that the Fe-bearing Zer complex oxide a particularly high oxygen storage capacity both before and after the redox aging test, while only a small Waste in its oxygen storage capacity after the redox aging test subject.

• *Die Einheit der Sauerstoffspeicherkapazität (OSC) ist: (1/1000) × (mol O₂/mol Oxid)

As a result, it can be noted that the zer-complex oxide of the present invention can be favorably used in a three-way catalyst mounted in the vicinity of a vehicle engine, where very high operating temperatures are expected. Table 1

• * The unit of oxygen storage capacity (OSC) is: (1/1000) × (mol O ₂ / mol oxide)

Claims (6)

An oxygen-storing cerium complex oxide having the formula: Ce _{1- (1 + y)} Zr _x R _y O _2-z , where "R" represents a rare earth metal or rare earth metal other than Zer, where "z" represents the degree of oxygen deficiency determined by the valence of the rare earth metal contained, where 0.2≤x + y≤0.9, 0.1≤x ≤0.8 and 0.05≤y≤0.3, and wherein the cerium complex oxide carries a transition metal or transition metals selected from the group of transition metals reduced by the noble metals. Oxygen-storing cerium complex oxide according to claim 1, wherein in the formula, the relationships 0.4≤x + y≤0.8, 0.35≤x≤0.7 and 0.05≤y≤0.2 are satisfied. Oxygen-storing cerium complex oxide according to claim 1 or 2, where the rare earth metal is Y. Oxygen-storing cerium complex oxide after a the claims 1 to 3, wherein at least a part of the Zer complex oxide a solid solution is. Oxygen-storing cerium complex oxide after a the claims 1 to 4, wherein the transition metal is selected from a group consisting of Fe, Co, Ni and Cu. Oxygen-storing cerium complex oxide according to claim 5, in which the transition metal Fe is.