Preparation method of Pt-rare earth nano-alloy based on ionic liquid

Abstract

The invention discloses a method for preparing a Pt-rare earth nano-alloy by using an ionic liquid as a reaction medium, which comprises the following steps: reacting carbodiimide with lithium alkylide to generate amidino lithium salt in situ, and then reacting with a series of anhydrous rare earth chlorides to obtain a corresponding amidino rare earth complex. The preparation method comprises the following steps: preparing Pt nano-particles with different sizes by adopting an ethylene glycol or methanol reduction method, then dispersing or dissolving Pt and an amidino rare earth complex in ionic liquid, and removing an introduced molecular solvent; and heating to a certain temperature in an inert atmosphere for reaction, and after the reaction is finished, washing and separating to obtain a final product. The synthesis process is mild in condition and does not need high temperature and high vacuum. The Pt-rare earth nano alloy prepared by the method has potential application value in alkane dehydrogenation and fuel cell cathode oxygen reduction reaction.

Description

Technical Field

[0001] The invention relates to the synthesis of a Pt-rare earth alloy, in particular to a method for preparing a Pt-rare earth nano alloy based on an ionic liquid. Background Art

[0002] Pt alloy (Pt x M) catalysts are widely used in the field of catalysis, including fuel cell cathode oxygen reduction reaction and alkane dehydrogenation reaction. On the one hand, reducing the amount of Pt by designing Pt alloys is an important means to reduce the cost of catalysts; on the other hand, by changing the type and content of metal M, the electronic structure of Pt alloys can be optimized, further improving the reaction activity of the catalyst. However, the commonly used alloy catalysts formed by Pt and 3d transition metals (such as Fe / Co / Ni, etc.) are easy to dissolve out of the transition metal elements from the Pt alloy under actual working conditions, making it difficult to maintain long-term stability. Recent studies have shown that Pt-rare earth alloys can simultaneously achieve high catalytic activity and long-term stability of catalytic reactions, becoming one of the most popular alloy catalysts.

[0003] The bottleneck that prevents Pt-rare earth alloys from being widely used is that their synthesis is a huge challenge, mainly manifested in: (1) The standard reduction potentials of Pt and rare earth metal ions are very different, so it is difficult to control the simultaneous nucleation of the two metals; rare earth metals have a strong affinity for oxygen, so the preparation of Pt-rare earth alloys requires a strict anhydrous and oxygen-free environment. At present, the preparation of Pt-rare earth alloys mainly uses physical vapor condensation technology and high-temperature reduction method. Physical vapor condensation technology requires an ultra-high vacuum atmosphere and harsh preparation conditions, and is only suitable for small-scale laboratory preparation (Nat. Chem., 2014, 6, 732-738.). There are also reports on the preparation of Pt-rare earth nanoalloys by high-temperature reduction method. For example, Li et al. obtained a stable compound by embedding Pt and rare earth ions into a CN network, and then calcined the compound at 700°C in a 3.3% H2-Ar atmosphere to obtain a Pt-rare earth nanoalloy (J.Am.Chem.Soc., 2020, 142, 953-961.); Ryoo et al. constructed a mesoporous zeolite silanol nest, in which rare earth oxides existed as atomic metal compounds and formed nanoalloys with Pt under hydrogen heat treatment (700°C) (Nature 2020, 585, 221-224); CN114023976A used high-temperature molten salt as a medium (generally above 500°C) to prepare Pt-rare earth nanoalloy materials by molten salt electro-deoxidation. The high-temperature reduction method requires a high-temperature atmosphere or high-temperature molten salt, has high energy consumption, and particles are prone to aggregation.

[0004] Ionic liquids are a class of low-temperature molten salts (melting point < 100°C) composed of organic cations and organic or inorganic anions. Compared with traditional solvents, ionic liquids have many unique properties, such as low vapor pressure, good thermal and chemical stability, adjustable structure, good solubility for both inorganic and organic precursors, and wide electrochemical window. In recent years, researchers have synthesized more and more inorganic nanomaterials in ionic liquids, including metal elements and alloys, oxides, sulfides and porous materials (Adv. Mater., 2010, 22, 261-285; Chem. Eur. J., 2017, 23, 5391-5403.).

[0005] In the reported patents and literature, there is no report on the preparation of Pt-rare earth nano alloys using ionic liquids as reaction media. The present invention uses ionic liquids as reaction media and obtains nanoscale Pt-rare earth alloys by regulating the ionic liquid structure and reaction conditions. The synthesis process can be carried out below 220°C. Compared with physical vapor condensation technology and high-temperature reduction method, the reaction conditions are mild and the alloy size, morphology and chemical composition can be easily regulated. Summary of the invention

[0006] The purpose of the present invention is to provide a method for preparing a Pt-rare earth nanoalloy, which uses an ionic liquid as a reaction medium and directly prepares the Pt-rare earth nanoalloy by an ionothermal method. The synthesis process only needs to be carried out below 220°C, and the size, morphology and chemical composition of the obtained Pt-rare earth nanoalloy are controllable, and the synthesis conditions are mild.

[0007] The purpose of the present invention can be achieved by the following steps:

[0008] a. Select a suitable ionic liquid as the reaction medium. The ionic liquid containing halogenated anions imidazolium or quaternary phosphonium can be obtained by reacting imidazole or trialkylphosphine with halogenated alkane, and the ionic liquid containing non-halogenated anions imidazolium or quaternary phosphonium can be obtained by ion exchange between halogenated anions imidazolium or quaternary phosphonium salt and metal salt of target anion;

[0009] b. Use alcohol to reduce the platinum precursor to obtain 1-10nm Pt metal particles. Specifically: a certain amount of NaOH and polyvinyl pyrrolidone (PVP, K30) are dissolved in alcohol, and then mixed evenly with the alcohol solution of the platinum precursor, heated to 100-180°C under an inert atmosphere, maintained for 2-5h, and precipitated and dispersed using n-hexane and ethanol, and washed repeatedly several times to obtain Pt nanoparticles of different sizes;

[0010] c. Using diimine and alkyl lithium as raw materials, reacting to obtain amidine lithium salt, and using the in-situ generated amidine lithium salt to undergo replacement reaction with a rare earth metal salt to obtain a corresponding amidine rare earth complex;

[0011] d. Disperse the freshly prepared Pt nanoparticles in the ionic liquid, dissolve or disperse the amidine rare earth complex in the ionic liquid, mix them evenly, remove the molecular solvent introduced in the preparation process of Pt and the rare earth complex, react under an inert atmosphere of 120-220°C for 6-96h, and obtain the Pt-rare earth nanoalloy by centrifugation and washing.

[0012] The ionic liquid is one of 1-butyl-3-methylimidazolium bromide, 1-butyl-3-methylimidazolium bis trifluoromethanesulfonyl imide, 1-dodecyl-3-methylimidazolium bromide, 1-dodecyl-3-methylimidazolium bis trifluoromethanesulfonyl imide, tetrabutylphosphonium bromide, tetrabutylphosphonium bis trifluoromethanesulfonyl imide, tributyloctylphosphonium bromide, tributyldodecylphosphonium bromide, trihexyltetradecylphosphonium bromide, and trihexyltetradecylphosphonium bis trifluoromethanesulfonyl imide;

[0013] The alcohol is methanol or ethylene glycol, and the inert gas is one of nitrogen, argon or helium;

[0014] The platinum precursor is chloroplatinic acid, the rare earth metal salt is an anhydrous chloride of a rare earth metal, and the rare earth metal is any one of scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium;

[0015] According to the method of the present invention, the size, morphology and chemical composition of the Pt-rare earth nanoalloy can be adjusted by changing the type of ionic liquid, the type of rare earth metal salt, the size of Pt nanoparticles, the reaction temperature and the reaction time, and the crystal type, size, morphology and microstructure of the Pt-rare earth nanoalloy can be characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy.

[0016] Compared with the prior art, the present invention has the following characteristics:

[0017] (1) Compared with the methods for preparing Pt-rare earth nanoalloys reported in literature and patents, the method for preparing Pt-rare earth nanoalloys based on ionic liquid used in the present invention only needs to be carried out below 220°C, which greatly reduces the reaction temperature;

[0018] (2) Based on the designability of ionic liquids, a series of imidazole and quaternary phosphonium ionic liquids can be obtained by adjusting the anions and cations, providing a rich variety of reaction media for the preparation of Pt-rare earth nanoalloys;

[0019] (3) The size, morphology and chemical composition of Pt-rare earth nanoalloys can be controlled by adjusting the types of anions and cations in the ionic liquid, the structure of functional groups, and the temperature and time of the reaction. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1The nuclear magnetic resonance spectrum of the ionic liquid trihexyltetradecylphosphonium bistrifluoromethanesulfonimide salt synthesized by the present invention;

[0021] Figure 2 This is the X-ray diffraction pattern of the Pt-rare earth nano alloy synthesized by the present invention. DETAILED DESCRIPTION

[0022] The present invention will be further described below with reference to the accompanying drawings and examples.

[0023] Implementation Case 1

[0024] Synthesis of Pt-rare earth nanoalloy: 0.214 g polyvinyl pyrrolidone (PVP, MW≈40000) was dissolved in 5.0 mL ethylene glycol, 79 mg H2PtCl6·6H2O was dissolved in 5.0 mL ethylene glycol, and the two solutions were mixed evenly, heated to 160°C under Ar atmosphere, maintained for 3 h, precipitated and dispersed with n-hexane and ethanol, and washed repeatedly several times to obtain Pt nanoparticles with a particle size of 1.7 nm; 2.6 mL (17.3 mmol) N, N'-diisopropylcarbodiimide was dissolved in 80 mL THF and cooled to 0°C, and then 1.6 M methyl lithium (10.5 mL, 17.3 mmol) ether solution was slowly added dropwise. The mixture was warmed to room temperature and stirred under Ar atmosphere for 18 hours, cooled to 0°C, slowly added to a suspension of YCl3 (1.1 g, 5.7 mmol) in THF (60 mL), warmed to room temperature, stirred under Ar atmosphere for 36 hours, and the amidine yttrium complex was isolated.

[0025] The freshly prepared Pt nanoparticles were uniformly dispersed in 10 mL of ethanol by ultrasound, and the dispersion was added to 2.5 g of trihexyltetradecylphosphonium bis(trifluoromethanesulfonyl)imide salt, and the ethanol was removed under vacuum stirring at room temperature to obtain an ionic liquid dispersion A of Pt nanoparticles; 100 mg of yttrium amidinate complex was added to 2.5 g of trihexyltetradecylphosphonium bis(trifluoromethanesulfonyl)imide salt, and the dispersion or solution B was obtained by sufficient stirring under an inert atmosphere. Dispersion A was mixed with B, and continued to be stirred for 1 h, and then transferred to a reactor. The above steps were all carried out under inert conditions, reacted at 160°C for 24 h, and the final product was obtained after filtration, washing and drying.

[0026] Figure 1 is the nuclear magnetic resonance spectrum of the quaternary phosphonium ionic liquid trihexyltetradecylphosphonium bistrifluoromethanesulfonyl imide salt synthesized in Example 1, 1 H and 31 P NMR confirmed the structure of the ionic liquid; Figure 2The X-ray diffraction pattern of the obtained sample shows that compared with the commercial Pt / C catalyst, the diffraction peak of the sample is shifted to the right as a whole, which is consistent with the results reported in the literature (J. Mater. Chem. A, 2016, 4, 12232-12240.), proving that Pt x Successful synthesis of Y nanoalloy.

[0027] Implementation Case 2

[0028] Except that the crystallization time in Example 1 was changed to 30 hours, other conditions were the same as those in Example 1, and the obtained solid product was analyzed to be Pt-rare earth nano alloy;

[0029] Implementation Case 3

[0030] Except that the crystallization temperature of Example 1 was changed to 200° C., other conditions were the same as those of Example 1. The obtained solid product was analyzed to be Pt-rare earth nano alloy;

[0031] Implementation Case 4

[0032] Except that the ionic liquid in Example 1 was changed to trihexyltetradecylphosphonium chloride, other conditions were the same as those in Example 1, and the obtained solid product was analyzed to be Pt-rare earth nano alloy;

[0033] Implementation Case 5

[0034] Except that the rare earth metal precursor in Example 1 was changed to anhydrous lanthanum chloride, other conditions were the same as those in Example 1, and the obtained solid product was analyzed to be Pt-rare earth nano alloy.

[0035] The applicant declares that the above is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention are within the protection scope and disclosure scope of the present invention.

Claims

1. A method for preparing a Pt-rare earth nanoalloy based on ionic liquid, comprising the following steps: (1) using diimine and alkyl lithium as raw materials, reacting to obtain amidine lithium salt, and using the in-situ generated amidine lithium salt to undergo a replacement reaction with a rare earth metal ion to obtain a corresponding amidine rare earth complex; (2) Dispersing Pt nanoparticles uniformly in an ionic liquid to obtain a dispersion A; dissolving or dispersing an amidine rare earth complex in an ionic liquid to obtain a solution or dispersion B. Mixing A and B and stirring them uniformly; (3) transferring the mixed solution obtained in step (1) to a hydrothermal reactor and crystallizing at 120-220° C. for 6-96 hours; (4) The solid product obtained by crystallization is washed and dried at 80° C. for more than 12 h to obtain the final product.

2. The method for preparing a Pt-rare earth nanoalloy according to claim 1, wherein the ionic liquid is one of 1-butyl-3-methylimidazolium bromide, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-dodecyl-3-methylimidazolium bromide, 1-dodecyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, tetrabutylphosphonium bromide, tetrabutylphosphonium bis(trifluoromethanesulfonyl)imide, tributyloctylphosphonium bromide, tributyldodecylphosphonium bromide, trihexyltetradecylphosphonium bromide, and trihexyltetradecylphosphonium bis(trifluoromethanesulfonyl)imide.

3. According to the preparation method of Pt-rare earth nanoalloy as claimed in claim 1, the platinum precursor is chloroplatinic acid, the rare earth metal salt is anhydrous chloride of rare earth metal, and the rare earth metal is any one of scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.

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