RU2797344C2 - Composite material based on rare-earth metal phosphates and aluminium oxide for application in catalysts for exhaust gas emission control and method for its production - Google Patents

Abstract

FIELD: compositions; catalytic systems; exhaust gas emission control systems.

SUBSTANCE: invention relates to a composition for use in a catalytic system in exhaust gas emission control systems containing a material based on transitional alumina and a rare earth phosphate, and to a method for producing the same. The composition for use as a carrier in a catalytic system in exhaust gas emission control systems contains: i) a transitional aluminium oxide material and ii) a rare earth phosphate that has a crystallite size of less than 50 nm after the composition is tempered at 1000°C for 3 hours. The invention also relates to a process for preparing a composition for use as a carrier in a catalytic system.

EFFECT: improvement of carriers for porous coatings based on rhodium, resistant to thermal stresses.

13 cl, 3 dwg, 2 tbl, 6 ex

Description

FIELD OF TECHNOLOGY

The present invention relates to a composition for use in a catalytic system in exhaust gas emission control systems containing a transition alumina material and a rare earth phosphate, and to a method for producing the same.

PRIOR ART

Prior art catalysts to control exhaust emissions used three-way catalyst (TWC; from English: three-way catalyst) systems containing a noble metal such as platinum (Pt), palladium (Pd) or rhodium (Rh), carrier a catalyst such as alumina, cerium oxide, zirconia, cerium-zirconium complex oxide; and a catalyst support made of ceramic or metallic materials. The rhodium in these TWC systems acts as a very important active ingredient, primarily for converting harmful nitrogen oxides (NOx) into N ₂ . It is well known that alumina-supported rhodium is typically deactivated under lean fuel conditions by irreversibly reacting with Al ₂ O ₃ to form catalytically inactive rhodium aluminate. Therefore, there is a need to improve the thermal stability of supported rhodium. Thermal stability (or thermal stability) is understood as the ability of a substance to resist an irreversible change in its chemical and/or physical structure at a relatively high temperature, for example, the ability not to undergo decomposition, sintering, chemical change or phase transition.

Various solutions have been proposed in the prior art, such as the use of ZrO ₂ as a carrier material for rhodium. However, due to tightening regulations and more stringent requirements for thermal stress resistance, there is a need for improved carriers for porous rhodium-based coatings. An improvement was proposed in M. Machida et al.: "Tuning the Electron Density of Rh Supported on Metal Phosphates for Three-Way Catalysis", J. Phys. Chem. C 2015, 119, 11653-11661, which describes the use of rare earth phosphates as a carrier material for rhodium in TWC systems. However, a problem with using these bulk rare earth phosphates as carriers is that the rhodium does not distribute well over the carrier.

Therefore, there is a need to improve such systems.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a composition for use as a carrier in a catalyst system is provided, comprising:

i) transition alumina material; And

ii) a rare earth phosphate, wherein the rare earth phosphate is characterized in that it has a crystallite size of less than 50 nm, preferably less than 15 nm, and most preferably less than 10 nm, after calcination at 1000° C. for 3 hours.

In the context of the present invention, transition alumina is understood to mean all aluminas in the transition state between boehmite and alpha alumina.

According to a further aspect of the present invention, there is provided a process for preparing said composition for use as a carrier in a catalyst system; way includes:

i) obtaining a material based on transition alumina and oxide of a rare earth element, and the material based on transition alumina and oxide of a rare earth element is obtained by a method comprising the following steps:

a) obtaining a suspension containing a precursor (precursor) transition alumina;

b) obtaining an aqueous solution containing a rare earth salt (rare earth salt solution);

c) combining the slurry with the rare earth salt solution to form a mixture of alumina and rare earth salt;

d) drying the mixture of alumina and rare earth salt to obtain a dried mixture of alumina and rare earth salt; And

e) calcining the dried mixture of alumina and rare earth salt to form a transition alumina and rare earth oxide material;

ii) impregnating (impregnating) the transition alumina and rare earth oxide material with an aqueous solution containing phosphate ions to obtain an impregnated (impregnated) transition alumina and rare earth oxide material; And

iii) calcining the impregnated material based on transitional alumina and rare earth oxide.

INFORMATION CONFIRMING THE POSSIBILITY OF IMPLEMENTING THE INVENTION

The composition contains a material based on transitional alumina and rare earth phosphate.

The transition alumina material comprises a transition alumina, and most preferably a transition alumina derived from (derived from) boehmite, a transition alumina derived from aluminosilicate, a transition alumina derived from doped alumina, or derived from mixtures thereof. Under the doped alumina in the context of the present invention understand alumina doped with oxides of alkaline earth metals, ZrO ₂ oxides of rare earth elements or TiO ₂ , the content of each of which lies in the range from 0.1 wt. % up to 20 wt. %, preferably from 1 wt. % up to 5 wt. %, or mixtures thereof. The transition alumina material is preferably a transition alumina derived from boehmite.

Under the transition alumina in the context of the present invention is meant any alumina that is in a transitional state between boehmite and alpha alumina.

According to an embodiment of the present invention, the composition contains at least 50 wt. % material based on transition alumina, preferably at least 60 wt. % material based on transition alumina, and most preferably 70 wt. % material based on transition alumina.

The transition alumina material may have a specific surface area of 50 m ² /g to 300 m ² /g and a pore volume of 0.1 ml/g to 1.5 ml/g.

The specific surface area reported in this publication was measured by the Brunauer, Emmett and Teller (BET) method using nitrogen according to DIN-ISO 9277. Pore volume and pore size distribution were measured using the method proposed by Barrett, Joyner, Halenda (BJH; from English: Barrett, Joyner and Halenda), using nitrogen adsorption at 77 K.

The rare earth phosphate is preferably LaPO ₄ , YPO ₄ or NdPO ₄ , and more preferably it is LaPO ₄ .

According to an embodiment of the present invention, the composition contains from 3 wt. % up to 50 wt. % rare earth phosphate, preferably from 10 wt. % up to 30 wt. % rare earth phosphate, more preferably from 5 wt. % up to 30 wt. % phosphate of the rare earth element, each time in terms of the corresponding oxide of the rare earth element.

The rare earth phosphate preferably has a crystallite size that is less than 15 nm after calcination at 1000°C for 3 hours, and most preferably it has a crystallite size that is less than 10 nm after calcination at 1000°C for 3 hours.

The crystallite size was determined by the Scherrer method using (21-2) reflection (indexing based on monoclinic monazite structure, reference code ICSD: 98-007-9747) in the diffraction pattern of the composition obtained by powder X-ray diffraction after heat treatment at 1000 °C for 3 hours in air, in the case of La and Nd phosphates and the Scherrer method using (020) reflection (indexing based on the tetragonal structure of zirconia, reference code ICSD: 98-002-4514) in the diffraction pattern of the composition obtained by X-ray powder diffraction after heat treatment at 1000°C for 3 hours in air, in the case of Y phosphate.

The rare earth phosphate is preferably homogeneously dispersed in the matrix of the transition alumina material. Without wishing to be bound by a particular theory, Applicant believes that a homogeneous dispersion of fine crystals of rare earth phosphate in a transition alumina matrix acts as a diffusion barrier leading to advantageous composite properties. Homogeneity is measured by cross-sectional imaging using a scanning electron microscope (SEM; from English: scanning electron microscope), optionally combined with element mapping by means of energy dispersive X-ray spectroscopy (EDX; from English: energy dispersive X-ray spectroscopy), revealing dimensions material domains based on transition alumina and rare earth phosphate.

The composition of the present invention preferably has a BET specific surface area of at least 50 m ² /g, preferably 70 m ² /g to 150 m ² /g, and a pore volume of 0.2 ml/g to 1.2 ml/g, preferably from 0.3 ml/g to 1.0 ml/g. These measurements were made as described above.

According to a second aspect of the present invention, there is provided a process for preparing a composition for use as a carrier in a catalyst system; this method includes:

i) obtaining a material based on alumina and oxide of a rare earth element, and the material based on alumina and oxide of a rare earth element is obtained by a method comprising the following steps:

a) obtaining a suspension containing a transitional alumina precursor;

b) obtaining an aqueous solution containing a salt of a rare earth element;

c) combining the slurry with the rare earth salt solution to form a mixture of alumina and rare earth salt;

d) drying the mixture of alumina and rare earth salt to obtain a dried mixture of alumina and rare earth salt; And

e) calcining the dried mixture of alumina and rare earth salt to obtain a material based on transition alumina and rare earth oxide;

ii) impregnating the transition alumina/rare earth oxide material with an aqueous solution containing phosphate ions to obtain an impregnated transition alumina/rare earth oxide material; And

iii) calcining the impregnated material based on transitional alumina and rare earth oxide.

Phosphate ions may include or, in other words, may be derived from phosphoric acid, orthophosphates, hydrogen phosphates, dihydrogen phosphates, diphosphates, triphosphates, metaphosphates, pentaphosphates, polyphosphates, or mixtures thereof.

The transition alumina precursor preferably comprises alumina hydrates of the general formulas Al(OH) ₃ and/or AlOOHxH ₂ O or mixtures thereof. Preferably, the alumina precursor is boehmite. The alumina precursor may further comprise silica, titanium dioxide, water-soluble alkaline earth metal salts, zirconium, or mixtures thereof. Preferably, the alumina precursor contains at least 50 wt. % aluminum oxide hydrates of general formulas Al(OH) ₃ and/or AlOOH⋅xH ₂ O or mixtures thereof.

The slurry containing the transition alumina precursor is preferably a boehmite slurry obtained by hydrolysis of aluminum alkoxide.

The slurry containing the transition alumina precursor preferably contains the alumina precursor and at least water in a ratio of 2:98 to 20:80. The suspension may additionally contain pH modifying auxiliaries, for example carboxylic acid or ammonia, preferably monocarboxylic acids such as acetic acid.

The rare earth salt is a water soluble salt, preferably rare earth acetate, more preferably La acetate, Y acetate, Nd acetate or mixtures thereof, and more preferably La acetate.

The salt solution of the rare earth element preferably contains a salt of the rare earth element and water in an amount corresponding to the content of the oxide of the rare earth element in the solution from 2 wt. % up to 20 wt. %. The suspension may additionally contain pH-modifying auxiliaries, such as carboxylic acid or ammonia.

The mixture of alumina and rare earth salt preferably contains at least 50 wt. % transition alumina precursor, preferably at least 60 wt. % transition alumina precursor, and more preferably at least 70 wt. % transition alumina precursor.

The mixture of alumina and a salt of a rare earth element preferably contains from 3 wt. % up to 50 wt. % salt of a rare earth element, preferably from 10 wt. % up to 30 wt. % salt of a rare earth element, more preferably from 5 wt. % up to 30 wt. % salt of the rare earth element, in terms of the corresponding oxide of the rare earth element.

The mixture of alumina and rare earth salt is preferably spray dried.

The dried mixture of alumina and rare earth salt is then calcined, preferably at 450° C. to 1200° C., for a period of 0.5 hour to 5 hours to form a transition alumina and rare earth oxide material.

The impregnation of the transition alumina and rare earth oxide material can be performed by any impregnation method known in the art. Preferably, a well-known water capacity impregnation method is used, preferably filling 80% to 100% of the pore volume of the transition alumina-rare earth oxide material with an aqueous solution containing phosphate ions, preferably containing phosphoric acid. Preferably, a solution is used for impregnation in an amount that provides a molar ratio of phosphoric acid to rare earth element, ranging from 10 mol. % up to 100 mol. %. For example, if the rare earth element is La, and if the ratio is 100 mol. %, then this is an equimolar amount of La and PO ₄ ^3- , and only LaPO ₄ is present in the transition alumina. If the ratio is 10 mol. %, then only 10 mol. % LaPO ₄ and 90 mol. % La is in the oxide form.

The impregnated transition alumina and rare earth oxide material is then calcined, preferably at 600°C to 1100°C, preferably 900°C to 1100°C, and most preferably at 1000°C. The calcination can be carried out for a period of time from 0.5 hour to 5 hours, preferably for 3 hours.

The thermal stability of the composition of the present invention (corresponding to claim 1 of the claims and obtained by the method according to claim 10) can be determined by calcining the composition for 3 hours at 1200°C. After the composition has been calcined for 3 hours at 1200°C, the composition is typically characterized by one or more of the following features:

- the composition has a specific surface area, determined by the BET method, equal to at least 40 m ² /g, preferably from 45 m ² /g to 90 m ² /g;

- the composition has a pore volume from 0.3 ml/g to 1.0 ml/g, preferably from 0.4 ml/g to 0.9 ml/g,

moreover, measurements of specific surface area by the BET method and pore volume were performed as described above.

BRIEF DESCRIPTION OF GRAPHICS

Hereinafter, the present invention will be described on the basis of non-limiting examples of its implementation and drawings, in which:

Fig. 1 is an X-ray powder diffraction pattern of the composition obtained in Example 1 compared to Comparative Example 1, which shows the difference in the crystal structure of LaPO ₄ ;

Fig. 2 (inventive material) is a scanning electron micrograph (SEM) of Example 1 and shows homogeneous white spots; And

Fig. 3 is a scanning electron micrograph (SEM) of Comparative Example 1 and shows white spots in a gray matrix, indicating the presence of RE-rich domains (white area) and Al-rich domains (gray area).

DESCRIPTION OF EXAMPLES OF CARRYING OUT THE INVENTION

The crystallite size was determined by the Scherrer method as described above.

Surface area is measured by the BET method and pore volume is measured using N ₂ adsorption as described above.

Experiments

Example 1

A material based on transitional alumina and rare earth oxide containing 20 wt. % lanthanum oxide and transition alumina, which had a specific surface area (BET) equal to 130 m ² /g, and a pore volume equal to 0.97 ml/g, were impregnated with an aqueous solution of phosphoric acid (14.4 wt.% H ₃ RO ₄ ). The product was dried at 120°C and then calcined at 1000°C for 3 hours.

Example 2

A material based on transitional alumina and rare earth oxide containing 20 wt. % yttrium oxide and transition alumina, which had a specific surface area (BET) of 134 m ² /g and a pore volume of 0.98 ml/g, were impregnated with an aqueous solution of phosphoric acid (13.2 wt.% H ₃ RO ₄ ). The product was dried at 120°C and then calcined at 1000°C for 3 hours.

Example 3

A material based on transitional alumina and rare earth oxide containing 20 wt. % neodymium oxide and transition alumina, which had a specific surface area (BET) of 130 m ² /g and a pore volume of 0.97 ml/g, were impregnated with an aqueous solution of phosphoric acid (14.4 wt.% H ₃ RO ₄ ). The product was dried at 120°C and then calcined at 1000°C for 3 hours.

Example 4

A material based on transitional alumina and rare earth oxide, containing 15 wt. % lanthanum oxide and transition alumina, which had a specific surface area (BET) equal to 145 m ² /g, and a pore volume equal to 0.94 ml/g, were impregnated with an aqueous solution of phosphoric acid (9.7 wt.% H ₃ RO ₄ ). The product was dried at 120°C and then calcined at 1000°C for 3 hours.

Comparative Example 1

LaPO ₄ received according to Example 3 from the publication EP 2754489 A1.

The phosphoric acid solution was added to the lanthanum nitrate solution in an amount to provide a 1:1 molar ratio of La and P. The pH was adjusted to 8 by adding ammonia solution. The precipitate that formed was separated by filtration and then calcined at 900° C. for 5 hours.

The resulting LaPO ₄ was combined with alumina. Powder LaPO ₄ and 4 wt. % lanthanum doped alumina having a specific surface area (BET) of 151 m ² /g and a pore volume of 1.02 ml/g was slurried and wet milled. Then the suspension was dried by spray drying and calcined at 1000°C for 3 hours to obtain a comparative composite material.

Comparative Example 2

CePO ₄ was prepared according to Example 6 of GB 1431868 by sputtering (NH ₄ ) ₂ Ce(NO ₃ ) ₆ onto alumina using the commercially available PURLOX TH100/150 product and heated at 400° C. overnight. Phosphoric acid was then added to the cooled mixture. After calcination for 3 hours at 1000° C., the comparative product was tested and no CePO ₄ was found; found only AlPO ₄ . The specific surface area (BET) of the comparative product was 45 m ² /g.

The results are presented in Table 1 below.

Fig. 1 shows the XRD pattern of the materials obtained in Example 1 and Comparative Example 1. The difference in the crystal state of LaPO ₄ is clearly seen in the width of the diffraction lines. The values obtained by the Scherrer method are shown in Table 1.

In addition, the cross-sectional images obtained by SEM shown in FIG. 2 and FIG. 3 show the materials obtained in Example 1 (FIG. 2) and Comparative Example 1 (FIG. 3). Of note is the difference in homogeneity of the 2 samples, manifested by the appearance of distinct white spots corresponding to zones enriched in LaPO ₄ , which indicate lower homogeneity.

Claims (23)

1. Composition for use as a carrier in a catalytic system in exhaust gas emission control systems, containing: i) transition alumina material and ii) a rare earth phosphate, wherein the rare earth phosphate has a crystallite size of less than 50 nm after the composition is calcined at 1000° C. for 3 hours. 2. The composition according to claim. 1, characterized in that the material based on transitional alumina contains a transitional alumina derived from boehmite, aluminosilicate, doped alumina, or mixtures thereof. 3. Composition according to claim. 1 or 2, characterized in that it contains at least 50 wt.%, preferably at least 70 wt.% material based on transition alumina. 4. Composition according to claim 1, characterized in that the rare earth phosphate contains LaPO ₄ , YPO ₄ , NdPO ₄ or mixtures thereof. 5. The composition according to any one of paragraphs. 1-4, characterized in that it contains from 3 wt.% to 50 wt.% rare earth phosphate, preferably from 10 wt.% to 30 wt.% rare earth phosphate. 6. Composition according to any one of paragraphs. 1-4, characterized in that the rare earth phosphate has a crystallite size of less than 15 nm after calcining the composition at 1000°C for 3 hours. 7. The composition according to any one of paragraphs. 1-4, characterized in that the rare earth phosphate has a crystallite size of less than 10 nm after calcining the composition at 1000°C for 3 hours. 8. The composition according to any one of paragraphs. 1-4, characterized in that the specific surface area determined by the BET method is at least 50 m ² /g, preferably from 70 m ² /g to 150 m ² /g. 9. The composition according to any one of paragraphs. 1-4, characterized in that the pore volume is from 0.2 ml/g to 1.2 ml/g, preferably from 0.3 ml/g to 1.0 ml/g. 10. A method of obtaining a composition for use as a carrier in a catalytic system, which includes: i) obtaining a material based on transition alumina and oxide of a rare earth element, and the material based on transition alumina and oxide of a rare earth element is obtained by a method comprising the following steps: a) obtaining a suspension containing a transitional alumina precursor; b) obtaining an aqueous solution containing a salt of a rare earth element, and the transition alumina precursor includes alumina hydrates of General formulas Al(OH) ₃ or AlOOH.xH ₂ O or mixtures thereof; c) combining the slurry with the rare earth salt solution to form a mixture of alumina and rare earth salt; d) drying the mixture of alumina and rare earth salt to obtain a dried mixture of alumina and rare earth salt; e) calcining the dried mixture of alumina and rare earth salt to obtain a material based on transition alumina and rare earth oxide; ii) impregnating the transition alumina/rare earth oxide material with an aqueous solution containing phosphate ions to obtain an impregnated transition alumina/rare earth oxide material; And iii) calcining the impregnated material based on transitional alumina and rare earth oxide. 11. The method of claim 10, wherein the rare earth salt is a rare earth acetate, preferably La acetate, Y acetate, Nd acetate, or mixtures thereof, and more preferably it is La acetate. 12. The method according to claim 10, characterized in that the impregnated material based on transition alumina and rare earth oxide is calcined at a temperature of from 600°C to 1100°C for a period of time from 0.5 hour to 5 hours, preferably at temperature equal to 1000°C for 3 hours. 13. The method according to any one of paragraphs. 10-12, characterized in that the composition further differs in features according to any one of paragraphs. 1-9.

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