DE3835183A1 - Rhodium-free three-way catalyst - Google Patents

Abstract

The invention relates to a three-way catalyst composed of active aluminium oxide, palladium, platinum and cerium dioxide, the catalyst, when Al2O3 is present as free-flowing support, containing 5-20% by weight of CeO2 and, when Al2O3 is present as coating on a honeycomb-like inert support, containing 25-50% by weight of CeO2.

Description

The invention relates to a catalyst with a on Transition row aluminum oxide applied active Phase of 0.03-3 wt .-% palladium and platinum with a Weight ratio between palladium and platinum of 0.1: 1 to 10: 1 and a content of ceria, the quantities by weight of precious metal, ceria and Alumina supplement to 100%, obtained by Impregnating the optionally grid-stabilized Support material with an aqueous solution of a salt of palladium and platinum, drying and annealing Temperatures above 250 ° C, optionally in one Hydrogen-containing gas stream.

Due to the recently increased sharply Rhodium prices and supply uncertainty emerged for the preparation of catalysts for the purification of Exhaust gases from internal combustion engines the need Catalyst compositions to develop, which under Renouncing rhodium an equivalent conversion of in contained in the exhaust gases of internal combustion engines Pollutants CO, hydrocarbons and one for the Practical application sufficient sales of Have nitric oxides.

It has been found that in catalysts, both Palladium as well as platinum, while retaining the usual amounts of precious metals, a replacement of rhodium is possible, provided that at the same time the invention, high amount of CeO₂ present in the carrier material.

At the same time containing platinum and palladium Catalysts are used to reduce Although pollutant emissions have long been known, however the effect of such catalysts was only in it, CO and hydrocarbons by oxidation too remove. Only through the use of the invention However, high amounts of CeO₂ in the carrier material were Pt / Pd catalysts obtained in addition to the Have the ability to reduce nitrogen oxides. These Ability was so far only the very expensive rhodium and attributed to the barely available Iridium, which is why all Three-way catalysts for the simultaneous conversion of CO, Hydrocarbons and nitrogen oxides contain rhodium had to and contained in praxi too.

The invention accordingly provides a catalyst with one on transitional alumina applied active phase of 0.03-3 wt .-% palladium and platinum with a weight ratio between Palladium and platinum from 0.1: 1 to 10: 1 and a Content of ceria, wherein the amount by weight of Precious metal, ceria and alumina at 100% supplement, obtained by impregnating the optionally lattice-stabilized support material with an aqueous Solution of a salt of palladium and platinum, drying and annealing at temperatures above 250 ° C, optionally in a hydrogen-containing Gas flow.  

The catalyst is characterized in that it is at Presence of alumina in bulk forms 5-20, preferably 11-20% by weight of ceria and when present of the aluminum oxide as a coating on a honeycomb inert carrier of ceramic or metal 25-50% by weight Contains ceria, wherein the alumina before the Impregnate with the solution of palladium and Platinum salt with an aqueous solution of cerium salt is impregnated or in the presence of the aluminum oxide on a honeycomb inert carrier as well Cerium compound in the form of solid to the alumina is admixed and that the obtained Catalyst precursor then each in air at 300-950 ° C. preferably 600-700 ° C, is annealed.

The above teaching of the invention differentiates the Amounts of ceria for the first time depending on the Application form of the aluminum oxide as a coating (washcoat) on an inert monolithic or honeycomb Carrier or as shaped bulk material (balls, Extrusions, tablets or the like), as was found, that both species because of different Diffusion ratios of bulk solids and washcoats are different to dope.

As alumina transition series all crystal modifications of Al₂O₃ (individually or in a mixture) with the exception of α -Al₂O₃ into consideration, wherein the BET specific surface area between 40 and 250 m² / g may be.

The bulk density of molded bulk material from the provided active, d. H. catalysis-promoting Aluminum oxide is on average 500 kg / m³. That through Impregnate with aqueous cerium salt solutions, drying and Calcined introduced ceria permeates the Alumina molding substantially uniform.

Per unit volume of the catalyst the same Oxygen storage capacity by ceria as in aluminum oxide coated monoliths or honeycombs reach, in which the Al₂O₃ content in the range of 100 kg / m³ catalyst volume, the cerium content must to a correspondingly lower concentration be adjusted those of the monolith catalyst.

Surprisingly, it turns out that in the Triple combination Pd / Pt / CeO₂ the function of Precious metal component when used commonly Precious metal quantities to approximately the same level as at usual, platinum, rhodium and ceria-containing Formulations can be brought, provided the inventively provided increased amounts of ceria come into use. For the precious metal components come the common starting materials in the form water-soluble salts for use.

In the catalysts of the invention for the purpose of increasing activity, high temperature resistance, the so-called. Lean stability in exhaust gas compositions of λ <1 and the fatigue life in operation, up to 20 wt .-% of the amount of alumina by zirconium dioxide, lanthanum oxide La₂O₃ neodymium oxide Nd₂O₃, praseodymium Pr₆O₁₁, nickel oxide NiO be replaced as a single substance or in admixture.

Further, using or co-using nickel oxide NiO results in an increase in hydrocarbon conversion in the rich exhaust region and a substantial reduction in the undesirable emission of hydrogen sulfide occurring in rich operation (ie at λ <1).

For the introduction of the important modification component Ceria CeO₂ in the necessary high Concentrations are in addition to cerium nitrate, ammonium cerium nitrate, Cerium oxalate, cerium chloride, cerium carbonate, cerium oxide or Cerium hydroxide and other cerium salts especially the Cerium (III) acetate suitable. It can be in the form of aqueous Impregnating solutions for the production of Bulk catalysts and monolithic or Honeycomb catalysts are used. In the production but the latter species is also the Possibility of all the mentioned compounds in the form of Add solids to the alumina.

A proven measure, in particular for stabilization the specific surface of the active alumina during continuous operation of the catalyst, is the Lattice of alumina by alkaline earth metal oxide, Silica, zirconia or by oxides of Pre-stabilize rare earths. According to a variant the invention is used with advantage.

In the context of the invention also proves a Measure to separate the two precious metals from each other as conducive to the preservation of specific intrinsic effects of each metal.  

Therefore, there is an advantageous embodiment of Invention in that when depositing the aluminum oxide as a coating on an inert honeycomb carrier the Ceria and optionally the other components containing alumina by means of an aqueous Suspension in two layers on the inert support is applied, wherein the first layer with aqueous Platinum salt solution impregnated, dried and optionally between tempered and the second Layer impregnated with aqueous palladium salt solution and is dried and wherein the obtained Catalyst precursor then, optionally in one Hydrogen-containing gas stream, is annealed.

Another subject concerns the use of the featured catalyst for simultaneous conversion of carbon monoxide, hydrocarbons and nitric oxide the exhaust gases of internal combustion engines.

The invention will be described below by embodiments further explained.

example 1

A honeycomb of cordierite 62 cells / cm², 102 mm in diameter and 152 mm in length by dipping in a 35% aqueous suspension containing γ -Al₂O₃ (140 m² / g), cerium (III) acetate and zirconyl acetate and in the these substances - calculated as oxides - in the ratio Al₂O₃: CeO₂: ZrO₂ = 58: 39: 3 templates, coated.

Excess suspension was removed by blowing and the coated monolith after drying 120 ° C for 2 h at 600 ° C annealed, leaving the acetates CeO₂ and ZrO₂ formed. The applied coating sat down from 128 g of Al₂O₃, 86 g of CeO₂ and 7 g of ZrO₂ together. The thus coated honeycomb body was then with an aqueous solution containing 0.59 g of Pd in Form of Pd (NO₃) ₂ and 1.18 g of Pt in the form of H₂PtCl₆ contained, occupied by impregnation.

After drying the precious metal impregnated Monoliths were subjected to a 4-hour reduction in Forming gas (N₂: H₂ = 95: 5) at 550 ° C.

Example 2

It was prepared according to Example 1 with a catalyst the only difference being that now 0.88 g Pd and 0.88 g Pt were applied.

Example 3 (comparative example)

A honeycomb body was as described in Example 1, with provided an oxide coating. Subsequently, instead of of Pd and Pt but under the same production conditions 1.47 g of Pt in the form of H₂PtCl₆ and 0.29 g of Rh in the form of RhCl₃ applied by impregnation.  

Example 4 (Comparative Example)

A ceramic honeycomb body according to Example 1 was coated with a 30% aqueous suspension containing CeO₂, introduced as Ce (NO₃) ₃, and γ -Al₂O₃ (140 m² / g) in a ratio of 5:95. After tempering 152 g Al₂O₃ and 8 g of CeO₂ were present on the monolith. The remaining production parameters corresponded to Example 1.

Example 5

From those prepared according to Examples 1-4 Catalysts became cylindrical parallel to the cells Test specimen drilled with 38 mm diameter, in one Multi-chamber test reactor installed and in the exhaust stream of a Ottomotors in terms of their function as Three-way catalytic converter checked.

The test engine was a 4-cylinder injection engine with 1781 cm³ displacement, equipped with a K-JETRONIC the company Bosch.

In order to evaluate the low-temperature activity of the catalysts, that temperature was determined at which 50% of the carbon monoxide of the hydrocarbons and of the nitrogen oxides contained in the exhaust gas stream is converted at λ = 0.995.

In addition, the catalytic activity was measured at 450 ° C in a dynamic test at a sweep frequency of 1 Hz and a λ sweep width of 0.034.

The space velocity was 64 000 h ^-1 . Before the catalyst, the exhaust gas composition varied as follows

CO 2.4-1.4 vol.% HC 450-350 ppm NO _x 2500-2000 ppm O₂ 1.0% by volume CO₂ 13-14 vol.%

To determine the creep behavior, the Catalysts for 100 hours on the engine at exhaust gas temperatures operated between 450 and 850 ° C.

The results of these investigations with the catalysts of the invention are together with those of the comparative catalysts in Table 1 contain.

As the measurements show, the invention Pt / Pd catalysts according to Examples 1 and 2 the Pt / Rh catalyst of Comparative Example 3 in the particularly important dynamic conversion in the fresh condition and equal after 100 h engine aging only in lightning are both in Fresh state as well as after aging disadvantages the Pt / Ph three-way catalyst available. These disadvantages are however - especially with regard to the very good ones Results in the dynamic conversion - not so serious, than that of an application inventive Pt / Pd catalysts as Three way catalysts would be in practice contrary.  

Comparative Example 4 corresponds in the Catalyst formulation of a commercial Pt / Pd oxidation catalyst with low CeO₂ content and differs from the invention Three-way catalyst by its purpose and the strongly changed CeO₂ concentration in the carrier material.

The catalytic activity of such a commercially available oxidation catalyst in the three-way catalyst test used here, especially with respect to the NO _x conversion, is significantly lower than that of the high-titer Pt / Pd catalysts according to the invention according to Examples 1 and 2. Because of this result, those intended expressly for oxidation purposes Pt / Pd catalysts with low CeO₂ content in practice for use as Dreiwegkatalysatoren not suitable, while for this application, the catalysts of the invention according to Examples 1 and 2 have a sufficient catalytic activity.

The following examples 6-8 are intended to show that the even a Pt / Pd three-way catalysts according to the invention higher catalytic activity than commercial ones Pt / Rh three-way catalysts, as described, for. B. in the German Patent 29 07 106 are described, respectively.  

Example 6

A ceramic monolith 62 cells / cm², 102 mm in diameter and 152 mm in length was covered by immersion with a suspension containing γ -Al₂O₃ (150 m² / g), cerium acetate and zirconyl nitrate in the ratio of the oxides Al₂O₃: CeO₂: ZrO₂ = 65: 28: 7 contained.

After blowing out excess suspension, the coated honeycomb body dried at 120 ° C and 1 h activated at 900 ° C.

The coating amount was 145 g Al₂O₃, 62 g of CeO₂ and 15.5 g of ZrO₂. On this provided with carrier material Monoliths were then from aqueous solution 0.69 g Pd in the form of PdCl₂ and 1.39 g of Pt in the form of H₂PtCl₆ applied by impregnation. Following the Drying of the impregnated molding at 150 ° C. followed by a two-hour reduction 500 ° C in a stream of hydrogen.

Example 7 (comparative example)

He differed from it but by the composition of the support material (139 g Al₂O₃, 10 g of CeO₂, 12 g of ZrO₂ and 6 g Fe₂O₃), which consists of an aqueous suspension of γ -Al₂O₃ (150 m² / g), cerium acetate, zirconyl acetate and iron oxide Fe₂O₃ was applied and characterized in that now 1.47 g Pt in the form of H₂PtCl₆ and 0.29 g Rh in the form of Rh (NO₃) ₃ were impregnated.

Example 8

The catalysts prepared according to Examples 6 and 7 were tested successively in the exhaust gas stream of a gasoline engine with regard to the effect as a three-way catalyst. The test conditions were similar to those described in Example 6, except that a λ sweep width of 0.068 and a space velocity of 73,000 h ^{-1 were used} to determine the dynamic conversion. Therefore, the following exhaust gas composition was present:

CO 3.3-2.2 vol.% HC 510-420 ppm NO _x 1500-2100 ppm O₂ 1.6% by volume CO₂ 12-13 vol.%

The pollutant conversions of the catalysts were in the fresh state, and 24 h tempering in air at Measured 950 ° C, see. see Table 2.

In the fresh state, the inventive Pt / Pd catalyst over the Pt / Rh comparative catalyst in the dynamic test comparably high Conversion rates. He has in the lightning behavior regarding CO and HC advantages (lower temperatures at the 50% conversion), but disadvantages regarding Nitrogen oxides.  

Subsequently, the same catalysts were annealed for 24 hours at 950 ° C in air to perform a test of catalyst stability at times lean engine operation ( λ <1), as is common in modern three-way concepts, and simultaneously present high temperatures.

The high ceria content Pt / Pd catalyst of the present invention then has much higher conversion rates in the dynamic test and significantly better light-off performance than the comparative example. Its 50% conversion of CO, HC and NO _x are values <450 ° C outside the range of conventional measurements and were therefore no longer detected.

Example 8

A cylindrical honeycomb made of cordierite with 102 mm Diameter, 76 mm length and a cell density of 62 Cells / cm² is dipped in a 30% aqueous Suspension, the alumina (140 m² / g), cerium acetate and Contains zirconium acetate coated.

The excess suspension is made by blowing with Compressed air removed and the coated monolith at Dried 120 ° C. This coating process will if necessary repeated to the desired Apply coating amount. Subsequently, the coated monolith annealed at 600 ° C for 45 minutes, wherein cerium and zirconium acetate to the respective Decompose oxides. The amount and type of applied Oxides are given in Table 3.  

The thus coated monolith is treated with an aqueous Solution of PdCl₂ and H₂PtCl₆ impregnated, the Pd and Pt in the ratio 1: 2 contains. The angry Precious metal amount is 1.1 g per catalyst.

To the drying of the noble metal-impregnated monolith at 150 ° C, a two-hour reduction in Forming gas (N₂: H₂ = 95: 5) at 550 ° C at.

Example 10

A catalyst according to Example 9 was prepared with the only difference being that the ratio Pd: Pt now 3: 1 was.

Example 11

Catalyst prepared according to Example 9, with the Difference that instead of cerium acetate solid CeO₂ (obtained by thermal decomposition of cericarbonate in air 500 ° C) was used.

Example 12

Catalyst prepared according to Example 9, wherein the Coating suspension contains nickel oxide.

Example 13

Catalyst prepared according to Example 9, wherein the Coating suspension Cerium acetate in high concentration and rare earth acetates (La: Nd: Pr = 61: 21: 8) contains.  

Example 14

Catalyst prepared according to Example 11 with the single differences that the catalyst no ZrO₂ contained and not reduced.

Example 15

A catalyst with a layered structure and Dimensions, coating and precious metal content as in Example 9 is prepared as follows:

In a first production cycle 2/3 of the applied total coating amount. The Coated monolith is dried, 45 min. At 600 ° C. annealed in air and then with a H₂PtCl₆ solution coated, dried and tempered at 500 ° C in air.

In the second production cycle, the Pd-containing Monolith with the missing third of the coating provided, dried and annealed at 600 ° C for 45 min. Then it is impregnated with Pd (NO₃) ₂ solution, dried and in forming gas (5% hydrogen in Nitrogen) at 550 ° C for 2 hours.

Example 16

On 1 dm³ of a spherical support of γ -Al₂O₃ (particle diameter 2-4 mm, tamped density 560 g / dm³, specific surface area 105 m² / g, pore volume 0.85 cm³ / g) containing 2 wt .-% La₂O₃ / Nd₂O₃ ( La: Nd = 2: 1) is pre-stabilized, 47 g of CeO₂ are applied by impregnation.

The impregnation takes place in two steps Pour over with an aqueous solution of cerium acetate. To Each impregnation step is followed by drying at 120.degree and a one-hour annealing at 500 ° C.

Subsequently, 1.2 g of precious metal in the form of a aqueous solution of Pd (NO₃) ₂ and H₂PtCl₆ applied, wherein Pd and Pt in the weight ratio 2: 5 are present. After drying at 120 ° C and a heat treatment Air at 450 ° C, the catalyst for 1 hour at 550 ° C. with forming gas (N₂: H₂ = 95: 5) reduced.

Example 17

On 1 dm³ of a spherical support of γ -Al₂O₃ (particle diameter 2-4 mm, tamped density 430 g / dm³, specific surface area 108 m² / g, pore volume 1.08 cm³ / g), are twice impregnated with the corresponding acetates 80 g of CeO₂ and 10 g ZrO₂ applied. Drying and tempering conditions corresponded to Example 16.

For subsequent precious metal impregnation were Pd (NO₃) ₂ and H₂PtCl₆ used. The precious metal concentration was 1.0 g / dm 3 catalyst, the Weight ratio Pd: Pt = 1: 1. After drying at 120 ° C, the catalyst at 650 ° C with Forming gas (N₂: H₂ = 95: 5) reduced.  

Example 18

The catalysts of Examples 9-17 were subjected to a performance test with a synthetic exhaust gas mixture in the fresh state and after a 12-hour thermal aging at 800 ° C in air. For this purpose, cylindrical specimens with a diameter of 25 mm and a length of 75 mm were drilled out of the monolithic catalysts and measured in a test reactor at a space velocity of 50,000 h ^-1 . Bulk quantities of the bulk catalysts were tested.

Prüfgaszusammensetzung CO₂ 14 vol.% O₂ 0.75 ± 0.75 vol.% CO 1 vol.% ± 1 vol.% H₂ 0.33% by volume C₃H₆ / C₃H₈ (2/1) 0.05% by volume NO 0.1% by volume H₂O 10 vol.% N₂ rest

The dynamic test was carried out at a frequency of 1 Hz at 400 ° C. The light-off behavior was measured at λ = 0.995 for NO and at λ = 1.01 for CO and hydrocarbons in each case at a heating rate of 30 ° K / min.

The results of the test of catalytic activity are summarized in Table 4.  

Table 1

Catalytic activity of the catalysts according to Examples 1-4

Table 2

Activity comparison of the catalysts according to Example 6 and 7

Table 3

Composition of the catalysts of Examples 9-15

Table 4

Starting behavior and pollutant conversions in the dynamic test for the catalysts according to Example 9-17

Claims (6)

1. Catalyst with an active phase applied to alumina of the transition series of 0.03-3 wt .-% palladium and platinum with a weight ratio between palladium and platinum of 0.1: 1 to 10: 1 and a content of ceria, wherein the Weight amounts of noble metal, ceria and alumina to 100%, obtained by impregnating the optionally lattice-stabilized support material with an aqueous solution of a salt of palladium and platinum, drying and annealing at temperatures above 250 ° C, optionally in a hydrogen-containing gas stream, characterized in that, in the presence of the alumina in bulk form, the catalyst contains 5-20% by weight of ceria and, in the presence of the alumina coating on a honeycomb ceramic or metal inert support, 25-50% by weight of ceria, the alumina preceding the Impregnate with the solution of the palladium and platinum salt with an aqueous solution is impregnated by Cersalz or mixed in the presence of the alumina on a honeycomb inert carrier as a cerium compound in the form of solid to the alumina and that the catalyst precursor is then in each case in air at 300-950 ° C, preferably 600-700 ° C, annealed , 2. Catalyst according to claim 1, characterized, that up to 20 wt .-% of the amount of alumina Zirconium dioxide, lanthanum oxide La₂O₃, neodymium oxide Nd₂O₃, Praseodymium oxide Pr₆O₁₁, nickel oxide NiO as Single substance or in mixture is replaced. 3. Catalyst according to claim 1 or 2, characterized, that the ceria is incorporated as cerium (III) acetate. 4. Catalyst according to claims 1 to 3, characterized, that the lattice of the alumina through Alkaline earth metal oxide, silica, zirconia or stabilized by rare earth oxides. 5. Catalyst according to claims 1 to 4, characterized, that upon deposition of the aluminum oxide as a coating on an inert honeycomb carrier, the ceria and optionally the other components containing alumina by means of an aqueous Suspension in two layers on the inert support is applied, wherein the first layer with aqueous platinum salt solution, dried and optionally between tempered and the second layer with aqueous palladium salt solution impregnated and dried and wherein the catalyst precursor obtained then, if appropriate in a hydrogen-containing gas stream, annealed becomes.   6. Use of the catalyst according to claims 1 to 4 for the simultaneous conversion of Carbon monoxide, hydrocarbons and nitric oxide the exhaust gases of internal combustion engines.