Rare earth phosphate alumina composite material for emission control catalyst and preparation method thereof

Abstract

The present invention relates to a composition for a catalyst system in an emission control system, comprising a transition alumina-based material and a rare earth phosphate, and to a method for preparing the same.

Description

Technical Field

[0001] The present invention relates to a composition for a catalyst system for use in an emission control system, the composition comprising a transition alumina-based material and a rare earth phosphate, and to a method for preparing the same. Background Art

[0002] In the prior art, among emission control catalysts, three-way catalyst systems (hereinafter referred to as "TWC") include precious metals such as platinum (Pt), palladium (Pd) or rhodium (Rh), carriers such as alumina, ceria, zirconia, ceria-zirconia composite oxides, and catalyst supports made of ceramic or metal materials. In these TWC systems, rhodium plays a very important role as an active component, mainly used to convert harmful NO into x Converted to N2. It is well known that rhodium supported on alumina tends to be deactivated under lean conditions by an irreversible reaction with Al2O3, forming catalytically inactive rhodium aluminate. Therefore, there is a need to enhance the thermal stability of supported rhodium. Thermal stability (or thermostability) refers to the ability of a substance to resist irreversible changes in its chemical and / or physical structure, for example, by resisting one or more of decomposition, sintering, chemical changes, or phase transitions at relatively high temperatures.

[0003] The prior art has offered various solutions, such as using ZrO2 as a rhodium support material. However, stricter regulations and more challenging thermal stress requirements have necessitated improved supports for rhodium-based washcoats. An improvement was proposed by 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 support materials for rhodium in TWCs. However, a problem with using these bulk rare earth phosphates as supports is that the rhodium does not disperse well on the supports.

[0004] Therefore, there is a need to improve these systems. Summary of the Invention

[0005] According to one aspect of the present invention, there is provided a composition for use as a support in a catalyst system, comprising:

[0006] i) transition alumina based materials; and

[0007] ii) Rare earth phosphates characterized in that, after calcination at 1000°C for 3 hours, their crystallite size is less than 50 nm, preferably less than 15 nm, and most preferably less than 10 nm.

[0008] Transition aluminas refer to all aluminas in the transition state between boehmite and alpha alumina.

[0009] According to another aspect of the present invention, there is provided a method for preparing the above-mentioned composition for use as a support in a catalyst system, the method comprising:

[0010] i) providing a transition alumina rare earth oxide material, wherein the transition alumina rare earth oxide material is prepared by a method comprising the following steps:

[0011] a) preparing a suspension comprising a transition alumina precursor;

[0012] b) preparing an aqueous solution containing a rare earth salt (rare earth salt solution);

[0013] c) combining the suspension with a rare earth salt solution to form an alumina rare earth salt mixture;

[0014] d) drying the alumina rare earth salt mixture to form a dried alumina rare earth salt mixture; and

[0015] e) calcining the dried alumina rare earth oxide salt mixture to form a transition alumina rare earth oxide material;

[0016] ii) impregnating the transition alumina rare earth oxide material with an aqueous solution comprising phosphate ions to form an impregnated transition alumina rare earth oxide material; and

[0017] iii) calcining the impregnated transition alumina rare earth oxide material. DETAILED DESCRIPTION

[0018] The composition includes a transition alumina-based material and a rare earth phosphate.

[0019] The transition alumina-based material includes transition alumina, and most preferably transition alumina derived from boehmite, transition alumina derived from silica-alumina, transition alumina derived from doped alumina, or transition alumina derived from a mixture thereof. Doped alumina refers to alumina doped with alkaline earth metal oxides, ZrO2, rare earth oxides, or TiO2, each in the range of 0.1 wt.% to 20 wt.%, preferably 1 wt.% to 5 wt.%, or alumina doped with a mixture thereof. The transition alumina-based material is preferably a transition alumina derived from boehmite.

[0020] Transition aluminas refer to all aluminas in the transition state between boehmite and alpha alumina.

[0021] According to one embodiment of the present invention, the composition comprises at least 50 wt.% of a transition alumina-based material, preferably at least 60 wt.% of a transition alumina-based material, and most preferably 70 wt.% of a transition alumina-based material.

[0022] Transition alumina-based materials can have 50m 2 / g to 300m 2 / g of specific surface area, and 0.1ml / g to 1.5ml / g of pore volume.

[0023] The specific surface areas reported herein were measured by BET using nitrogen according to DIN-ISO 9277. The pore volume and pore size distribution were measured using the method of Barrett, Joyner, and Halenda (BJH) at 77 K using nitrogen adsorption.

[0024] The rare earth phosphate is preferably LaPO 4 , YPO 4 or NdPO 4 , and more preferably LaPO 4 .

[0025] According to one embodiment of the present invention, the composition comprises 3 to 50 wt.% of rare earth phosphate, preferably 10 to 30 wt.% of rare earth phosphate, more preferably 5 to 30 wt.% of rare earth phosphate, each calculated as the corresponding rare earth oxide.

[0026] The rare earth phosphate preferably has a crystallite size of less than 15 nm after calcination at 1000°C for 3 hours, and most preferably a crystallite size of less than 10 nm after calcination at 1000°C for 3 hours.

[0027] The crystallite size is determined by the Scherrer method using the (21-2) reflection in the X-ray powder diffraction pattern of the composition after heat treatment at 1000°C for 3 hours in air (index based on the monoclinic monazite structure, ICSD reference code: 98-007-9747) in the case of La and Nd phosphates, and by the Scherrer method using the (020) reflection in the X-ray powder diffraction pattern of the composition after heat treatment at 1000°C for 3 hours in air (index based on the tetragonal zircon structure, ICSD reference code: 98-002-4514) in the case of Y phosphate.

[0028] The rare earth phosphate is preferably uniformly dispersed in the matrix of the transition alumina-based material. Without being bound by theory, applicants believe that the uniform dispersion of small rare earth phosphate crystals enables the transition alumina matrix to act as a diffusion barrier, thereby resulting in the beneficial properties of the composite material. Homogeneity is measured by scanning electron microscopy (SEM) cross-sectional imaging, optionally in conjunction with EDX elemental mapping, which reveals the domain size of the transition alumina-based material and the rare earth phosphate.

[0029] The compositions of the present invention preferably have a 2 / g, preferably 70m 2 / g to 150m 2 The BET specific surface area is 2000 nm and the pore volume is 0.2 ml / g to 1.2 ml / g, preferably 0.3 ml / g to 1.0 ml / g. These measurements are performed as described above.

[0030] According to a second aspect of the present invention, there is provided a method for preparing a composition for use as a support in a catalyst system, the method comprising:

[0031] i) providing an alumina rare earth oxide material, wherein the transition alumina rare earth oxide material is prepared by a method comprising the following steps:

[0032] a) preparing a suspension comprising a transition alumina precursor;

[0033] b) preparing an aqueous solution containing a rare earth salt;

[0034] c) combining the suspension with a rare earth salt solution to form an alumina rare earth salt mixture;

[0035] d) drying the alumina rare earth salt mixture to form a dried alumina rare earth salt mixture; and

[0036] e) calcining the dried alumina rare earth oxide salt mixture to form a transition alumina rare earth oxide material;

[0037] ii) impregnating an alumina rare earth oxide material with an aqueous solution containing phosphate ions to form an impregnated alumina rare earth oxide material; and

[0038] iii) calcining the impregnated alumina rare earth oxide material.

[0039] The phosphate ions may comprise or in other words may be derived from phosphoric acid, orthophosphate, hydrogenphosphate, dihydrogenphosphate, diphosphate, triphosphate, metaphosphate, pentaphosphate, polyphosphate, or mixtures thereof.

[0040] The transition alumina precursor preferably comprises an alumina hydrate of the general formula Al(OH)3 and / or AlOOH*xH2O, or a mixture thereof. The alumina precursor is preferably boehmite. The alumina precursor may further comprise silica, titania, a water-soluble salt of an alkaline earth metal, zirconium, or a mixture thereof. The alumina precursor preferably comprises at least 50 wt.% of an alumina hydrate of the general formula Al(OH)3 and AlOOH*xH2O, or a mixture thereof.

[0041] The suspension comprising the transition alumina precursor is preferably a boehmite suspension prepared by hydrolysis of an aluminum alkoxide.

[0042] The suspension comprising the transition alumina precursor preferably comprises the alumina precursor and water in a ratio of at least 2:98 to 20:80. The suspension may also comprise a pH adjusting additive such as a carboxylic acid or ammonia, preferably a monocarboxylic acid such as acetic acid.

[0043] The rare earth salt is a water-soluble salt, preferably a rare earth acetate, more preferably lanthanum (La) acetate, yttrium (Y) acetate, neodymium (Nd) acetate or a mixture thereof, more preferably lanthanum (La) acetate.

[0044] The rare earth salt solution preferably comprises a rare earth salt and water corresponding to a rare earth oxide content of 2 to 20 wt.% in the solution.The suspension may further comprise a pH adjusting additive, such as a carboxylic acid or ammonia.

[0045] The mixed alumina rare earth salts preferably contain at least 50 wt.% of a transition alumina precursor, preferably at least 60 wt.% of a transition alumina precursor, and most preferably 70 wt.% of a transition alumina precursor.

[0046] The alumina rare earth salt mixture preferably contains 3 to 50 wt.% of rare earth salt, preferably 10 to 30 wt.% of rare earth salt, more preferably 5 to 30 wt.% of rare earth salt, calculated as the corresponding rare earth oxide.

[0047] The alumina rare earth salt mixture is preferably spray dried.

[0048] The dried alumina rare earth salt mixture is then preferably calcined at a temperature of 450 to 1200°C for a period of 0.5 to 5 hours to form a transitional alumina rare earth oxide material.

[0049] The impregnation of the transition alumina rare earth oxide material can be carried out by any impregnation method known in the art. Preferably, the well-known incipient wetness impregnation method is used, preferably by filling 80-100% of the pore volume of the alumina rare earth oxide material with an aqueous solution containing phosphate ions, preferably phosphoric acid. Preferably, the solution is used for impregnation in an amount that provides a molar ratio of phosphoric acid to rare earth element of 10 to 100 mol%. For example, if the rare earth element is La and if the ratio is 100 mol%, then there are equimolar amounts of La and PO4 3- , and only LaPO4 exists on the transition alumina. If this ratio is 10 mol%, only 10 mol% of LaPO4 and 90 mol% of La in the form of oxide exist on the transition alumina.

[0050] The impregnated transition alumina rare earth oxide material is then preferably calcined at a temperature of 600 to 1100° C., preferably 900 to 1100° C. and most preferably 1000° C. Calcination may be carried out for a period of 0.5 to 5 hours, preferably 3 hours.

[0051] The thermal stability of the composition of the present invention (according to claim 1 and obtained by the method of claim 10) can be determined by calcining the composition at 1200°C for 3 hours. After calcining the composition at 1200°C for 3 hours, the composition is preferably characterized by one or more of the following:

[0052] The composition has a BET specific surface area of at least 40 m 2 / g, preferably 45m 2 / g to 90m 2 / g;

[0053] - the composition has a pore volume of 0.3 ml / g to 1.0 ml / g, preferably 0.4 ml / g to 0.9 ml / g,

[0054] The BET surface area and pore volume measurements were performed as described above.

[0055] The invention will now be described with reference to non-limiting examples and the accompanying drawings, in which:

[0056] Figure 1 is a powder XRD of the composition obtained in Example 1 compared with Comparative Example 1 showing the difference in crystallinity of LaPO4;

[0057] Figure 2 (material of the present invention) is the SEM of Example 1 and shows uniform white spots; and

[0058] Figure 3is a SEM of Comparative Example 1 and shows white spots in a gray matrix, which means that there are RE-rich regions (white regions) and Al-rich regions (gray regions).

[0059] Example:

[0060] The crystallite size was determined by the Scherrer method as described above.

[0061] Surface area was measured by BET and pore volume was measured by N2 adsorption as described above.

[0062] experiment

[0063] Example 1

[0064] A transition alumina rare earth oxide material made of 20 wt. % lanthanum oxide having a specific surface area (BET) of 130 m 2 / g and a transition alumina with a pore volume of 0.97 ml / g. The product was dried at 120°C and finally calcined at 1000°C for 3 hours.

[0065] Example 2

[0066] A transition alumina rare earth oxide material made of 20 wt.% yttrium oxide having a specific surface area (BET) of 134 m 2 / g and a transition alumina with a pore volume of 0.98 ml / g. The product was dried at 120°C and finally calcined at 1000°C for 3 hours.

[0067] Example 3

[0068] A transition alumina rare earth oxide material made of 20 wt. % neodymium oxide having a specific surface area (BET) of 130 m 2 / g and a transition alumina with a pore volume of 0.97 ml / g. The product was dried at 120°C and finally calcined at 1000°C for 3 hours.

[0069] Example 4

[0070] A transition alumina rare earth oxide material made of 15 wt. % lanthanum oxide was impregnated with an aqueous solution of phosphoric acid (9.7 wt. % H3PO4), the material comprising a specific surface area (BET) of 145 m 2 / g and a transition alumina with a pore volume of 0.94 ml / g. The product was dried at 120°C and finally calcined at 1000°C for 3 hours.

[0071] Comparative Example 1

[0072] LaPO4 was prepared according to Example 3 of EP 2754489 A1:

[0073] Phosphoric acid solution was added to the lanthanum nitrate solution in an amount to give a 1:1 molar ratio of La to P. The pH was adjusted to 8 by adding ammonia solution. The precipitate was isolated by filtration and finally calcined at 900° C. for 5 hours.

[0074] The obtained LaPO4 was combined with alumina. The LaPO4 powder and the specific surface area (BET) were 151m 2 / 4 wt.% lanthanum-doped alumina with a pore volume of 1.02 ml / g was slurried and wet-milled. The suspension was then spray-dried and calcined at 1000°C for 3 hours to obtain a comparative composite material.

[0075] Comparative Example 2

[0076] According to Example 6 of GB 1431868, CePO4 was prepared by evaporating (NH4)2Ce(NO3)6 on alumina using commercially available PURALOX TH100 / 150 and heating to 400°C overnight. Phosphoric acid was then added to the cooled mixture. After calcination at 1000°C for 3 hours, the comparative product was tested and no CePO4 was found; only AlPO4 was found. The specific surface area (BET) of the comparative product was 45 m 2 / g.

[0077] The results are included in Table 1 below:

[0078] Table 1:

[0079]

[0080] *As is

[0081] **After additional calcination at 1200°C for 3 h (for thermal stability)

[0082] Figure 1 The XRD patterns of the materials obtained in Example 1 and Comparative Example 1 are shown. The difference in crystallinity of LaPO4 is clearly shown by the width of the diffraction lines. Table 1 lists the values extracted by the Scherrer method.

[0083] also, Figure 2 and 3 The SEM cross-sectional view in Example 1 ( Figure 2 ) and Comparative Example 1 ( Figure 3). Note that the difference in homogeneity between the two samples is evident by the well-defined white spots (corresponding to LaPO4-rich areas), indicating a lower homogeneity.

Claims

1. A method for preparing a composition for use as a support in a catalyst system in an emission control system, the method comprising: i) providing a transition alumina rare earth oxide material, wherein the transition alumina rare earth oxide material is prepared by a method comprising the following steps: a) preparing a suspension comprising a transition alumina precursor; b) preparing an aqueous solution comprising a rare earth salt, wherein the transition alumina precursor comprises an alumina hydrate of the general formula Al(OH)3 or AlOOH*xH2O or a mixture thereof; c) combining the suspension with a rare earth salt solution to form an alumina rare earth salt mixture; d) drying the alumina-rare earth salt mixture to form a dried alumina-rare earth salt mixture; e) calcining the dried alumina rare earth salt mixture to form a transition alumina rare earth oxide material; ii) impregnating the transition alumina rare earth oxide material with an aqueous solution containing phosphate ions to form an impregnated alumina rare earth oxide material; and iii) calcining the impregnated transition alumina rare earth oxide material, wherein the composition comprises: A. Transitional alumina-based materials; and B. A rare earth phosphate characterized in that it has a crystallite size of less than 50 nm after calcination at 1000° C. for 3 hours, the rare earth phosphate comprising LaPO 4 , YPO 4 , NdPO 4 or a mixture thereof.

2. The method according to claim 1, wherein the rare earth salt is lanthanum acetate, yttrium acetate, neodymium acetate or a mixture thereof.

3. The method of claim 1 or claim 2, wherein the impregnated transition alumina rare earth oxide material is calcined at a temperature of 600°C to 1100°C for 0.5 to 5 hours. The method according to claim 1 , wherein the rare earth salt is lanthanum acetate.

5. The method of claim 1 or claim 2, wherein the impregnated transition alumina rare earth oxide material is calcined at a temperature of 1000°C for 3 hours.

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