Continuous preparation method of Pd-based bimetallic nanoflowers

Abstract

The continuous preparation method comprises the following steps that a, a raw material A and a raw material B are introduced into a first micro-mixer to be mixed, a first mixed solution is obtained, the raw material A is an aqueous solution of palladium salt, the raw material B is an aqueous solution of other transition metal salts except the palladium salt, and a second mixed solution is obtained; the raw material A and the raw material B both contain a cationic surfactant; b, introducing the first mixed solution and a raw material C into a second micro-mixer, and mixing to obtain a second mixed solution, wherein the raw material C is an aqueous solution of a reducing agent; and c, the second mixed solution is introduced into a microreactor for an oxidation-reduction reaction, and the Pd-based bimetallic nanoflower is obtained. According to the preparation method, rapid and uniform mixing of the reaction raw materials is achieved through the micro-fluidic technology, and the prepared Pd-based bimetallic nanoflower has a unique hierarchical structure, a high specific surface area and uniform particle size and morphology distribution.

Description

Technical Field

[0001] The present invention belongs to the field of nanomaterials, and relates to a continuous preparation method of Pd-based bimetallic nanoflowers, specifically a continuous preparation method of Pd-based bimetallic nanoflowers based on microfluidic technology. Background Art

[0002] Bimetallic nanomaterials have received extensive attention in recent years due to their unique physical and chemical properties, surface catalytic activity, and good electrochemical performance. Especially Pd (palladium)-based bimetallic nanomaterials have been widely studied due to their superior performance in the fields of catalysis, energy conversion, and energy storage.

[0003] Citation Document 1 discloses a preparation method of controllable bimetallic alloy nanoparticles, which uses n-butyllithium as a strong reducing agent, and oleylamine and trioctylphosphine as protecting agents to simultaneously reduce acetylacetonate salts, chloride salts, or acetate salts of Pt / Pd-M (where M is Ni, Fe, Co, Mn, Pd, Zn, Cu, Mo, etc.) to form uniform and size-controllable Pt / Pd-M alloy nanoparticles.

[0004] Citation Document 2 discloses a preparation method of Bi-Pd bimetallic nanocrystals, which includes dissolving Bi salts and Pd salts in oleylamine, and through heating and reduction reactions, and then through centrifugal separation, drying and other steps, Bi-Pd bimetallic nanocrystals with specific morphology and size are prepared.

[0005] The nanoflower structure is a unique nanostructure, and its layered or petal-like morphology endows it with a rich pore structure and a high specific surface area. This structural feature significantly enhances the catalytic performance of the material. Nanoflower-shaped alloy materials can not only provide more active sites, but also optimize the transfer path of reactants and improve the catalytic reaction rate. In the fields of energy catalysis, sensors, and electrochemistry, the nanoflower structure exhibits excellent performance.

[0006] Citation Document 3 discloses a preparation method of a three-dimensional flower-shaped supported bimetallic copper-nickel nanocatalyst, which in-situ grows a layered copper-nickel-aluminum-containing bimetallic hydroxide on the surface of amorphous alumina microspheres, and then calcines and reduces at high temperature to finally obtain a highly dispersed flower-shaped bimetallic CuNi nanocatalyst.

[0007] Citation Document 4 discloses a preparation method of a palladium-copper bimetallic nanoflower-like peroxide, which mixes sodium tetrachloropalladate, tungsten hexacarbonyl, copper dichloride dihydrate, dimethylformamide, and glacial acetic acid, and through steps such as ultrasonic treatment, oil bath heating, centrifugal washing, etc., palladium-copper bimetallic nanoflowers are prepared.

[0008] As an emerging synthesis method, microfluidic technology provides innovative technical means for the preparation of nanomaterials by achieving precise fluid control and enhanced mass transfer and mixing within a microreactor. Compared with traditional batch synthesis, microreactors, with their unique flow characteristics and high surface area-to-volume ratio, can significantly improve the mixing efficiency, shorten the reaction time, and reduce energy consumption. In the process of nanomaterial preparation, microfluidic technology has advantages such as efficient mixing and heat and mass transfer, precise control of reaction parameters, scalability, repeatability, and environmental friendliness.

[0009] Citation 5 discloses a microfluidic preparation technology for size-controllable monodisperse gold nanoparticles, including a microreaction system constructed based on this gold particle preparation method, the acquisition and transfer of the gold nanoparticle reaction precursor solution, the continuous controllable preparation of gold nanoparticles, and the obtaining of a toluene solution of uniformly monodisperse gold nanoparticles.

[0010] Citation documents:

[0011] Citation 1: CN103192086A

[0012] Citation 2: CN113664215B

[0013] Citation 3: CN106955709A

[0014] Citation 4: CN111992732A

[0015] Citation 5: CN108555309A Summary of the Invention

[0016] Problems to be Solved by the Invention

[0017] In existing research, for example, in the above-mentioned documents (Citation 3 and Citation 4), the preparation of bimetallic nanoflowers usually adopts traditional batch synthesis processes, which have problems such as difficult precise control of reaction conditions, non-uniform particle size and morphology of products, poor repeatability between batches, and low synthesis efficiency, and cannot meet the requirements of high-efficiency and large-scale continuous preparation.

[0018] As an advanced synthesis method, microfluidic technology has been preliminarily applied in the preparation of metal nanomaterials, especially showing certain advantages in the synthesis of single-metal nanoparticles (such as cited reference 5). However, due to the complex hierarchical structure and composition distribution of bimetallic nanoflowers, the formation process of their morphology and alloyed or core-shell structure poses higher requirements for the selection of reaction systems and also stricter control requirements for reaction conditions (such as rapid mixing and uniform concentration / temperature fields). There are currently no reports on the preparation of such complex structures through microreactors. In addition, traditional methods usually use toxic reducing agents (such as oleylamine) or high-energy-consuming synthesis processes, which not only increase the environmental burden but also raise production costs. Therefore, developing an efficient, green, and controllable method for preparing Pd-based bimetallic nanoflowers based on microfluidic technology has important scientific value and practical significance.

[0019] Solutions for Solving the Problems

[0020] After research, the present inventors have proposed a continuous preparation method for Pd-based bimetallic nanoflowers based on microfluidic technology. This method utilizes a microfluidic synthesis platform to achieve efficient control of reaction conditions by selecting metal precursors and reasonably adjusting process parameters, thereby ensuring that the products have uniform size and morphology.

[0021] Specifically, the present invention solves the technical problems of the present invention through the following solutions.

[0022] [1]. A continuous preparation method for Pd-based bimetallic nanoflowers, which comprises the following steps:

[0023] a. Feeding raw material A and raw material B into a first micromixer for mixing to obtain a first mixed solution, wherein raw material A is an aqueous solution of palladium salt, raw material B is an aqueous solution of other transition metal salts except palladium salt, and both raw material A and raw material B contain cationic surfactants;

[0024] b. Feeding the first mixed solution and raw material C into a second micromixer for mixing to obtain a second mixed solution, wherein raw material C is an aqueous solution of a reducing agent;

[0025] c. Feeding the second mixed solution into a microreactor for redox reaction to obtain Pd-based bimetallic nanoflowers.

[0026] [2]. The preparation method according to [1], wherein,

[0027] The second mixed solution substantially has a homogeneous dispersion system, and the mixing time in step b is 0.1 to 10 s.

[0028] [3]. The preparation method according to [1] or [2], wherein,

[0029] The palladium salt is selected from one or more of sodium tetrachloropalladate and potassium tetrachloropalladate;

[0030] The other transition metal salts except the palladium salt are selected from one or more of copper chloride, silver nitrate, chloroauric acid, and potassium chloroplatinate;

[0031] The cationic surfactant includes a cationic surfactant of the quaternary ammonium salt type.

[0032] [4]. The preparation method according to any one of [1]-[3], wherein,

[0033] The concentration of the palladium salt in the raw material A is 0.001 - 0.01 mol / L;

[0034] The concentration of the other transition metal salts except the palladium salt in the raw material B is 0.001 - 0.01 mol / L;

[0035] The concentration of the cationic surfactant in the raw material A and the raw material B is independently 0.001 - 0.05 mol / L.

[0036] [5]. The preparation method according to any one of [1]-[4], wherein,

[0037] The molar ratio of the palladium salt in the raw material A to the other transition metal salts except the palladium salt in the raw material B is (1 - 10):1.

[0038] [6]. The preparation method according to any one of [1]-[5], wherein,

[0039] The reducing agent is selected from one or more of ascorbic acid and citric acid.

[0040] [7]. The preparation method according to any one of [1]-[6], wherein,

[0041] The concentration of the reducing agent in the raw material C is 0.1 - 0.5 mol / L.

[0042] [8]. The preparation method according to any one of [1]-[7], wherein,

[0043] The first micro mixer and the second micro mixer are independently a microchannel mixer, a membrane dispersion mixer, or a micro sieve pore mixer;

[0044] The micro reactor is a microchannel reactor or a coiled tube micro reactor.

[0045] [9]. The preparation method according to any one of [1]-[8], wherein,

[0046] The temperature in the first micro-mixer is 10 - 50 °C, and the mixing time is 0.1 - 10 s;

[0047] The temperature in the second micro-mixer is 10 - 50 °C;

[0048] The temperature in the micro-reactor is 10 - 60 °C, and the residence time is 0.1 - 20 min.

[0049]

[10] . The Pd-based bimetallic nanoflowers prepared by the preparation method according to any one of [1] - [9], wherein the particle size of the Pd-based bimetallic nanoflowers is below 500 nm.

[0050] Effects of the Invention

[0051] The present invention provides a continuous preparation method of Pd-based bimetallic nanoflowers based on microfluidic technology, which not only significantly improves the morphology and size uniformity of the product, but also improves the reaction efficiency and batch-to-batch repeatability. This method uses green and environmentally friendly raw materials and processes, showing good potential for industrial application. Further, the method of the present invention has strong versatility and can be extended to the continuous and controllable synthesis of other bimetallic and even multi-metallic nanomaterials, having broad reference significance and application value.

[0052] Specifically, the present invention has achieved the following beneficial technical effects:

[0053] 1. The present invention realizes the rapid and uniform mixing of reaction raw materials through microfluidic technology. The prepared Pd-based bimetallic nanoflowers have a unique hierarchical structure and high specific surface area, and the particle size and morphology distribution are uniform; compared with the traditional batch synthesis method, the preparation method of the present invention effectively avoids the problem of product polydispersity caused by insufficient mixing.

[0054] 2. Based on the enhanced mass transfer and heat transfer performance of the micro-reactor, the present invention significantly improves the reaction efficiency, shortens the reaction time, and provides a practical solution for the synthesis of nanomaterials on an industrial scale through a continuous production mode.

[0055] 3. The present invention uses green reducing agents such as ascorbic acid to replace traditional toxic chemicals, and at the same time utilizes the high-efficiency mass transfer characteristics of the micro-reactor to reduce energy consumption, embodying the concept of green chemistry and providing new ideas for the preparation of environmentally friendly nanomaterials.

[0056] 4. Based on the high-precision regulation characteristics of microfluidic technology, the present invention is not only applicable to the preparation of Pd-based bimetallic nanoflowers, but also can be extended to the continuous and controllable synthesis of other bimetallic and even multi-metallic nanomaterials by adjusting the types of metal precursors and process parameters, further enhancing the scope of application and practical application value of this method. Description of the Drawings

[0057] Figure 1 Schematic process flow diagram of a specific embodiment of the preparation method of the present invention;

[0058] Figure 2 TEM images of the PdCu bimetallic nanoflowers prepared in Examples 1 - 3;

[0059] Figure 3 TEM images of the PdCu bimetallic nanoparticles without flower-like branches prepared in Comparative Example 4;

[0060] Figure 4 HRTEM and EDS images of the PdCu bimetallic nanoflowers prepared in Example 3;

[0061] Figure 5 XRD patterns of the PdCu bimetallic nanoflowers prepared in Examples 1 - 3;

[0062] Figure 6 XPS patterns of the PdCu bimetallic nanoflowers prepared in Examples 1 - 3;

[0063] Figure 7 HAADF-STEM and EDS images of the Pd-based bimetallic nanoflowers prepared in Examples 4 - 6;

[0064] Figure 8 XRD patterns of the Pd-based bimetallic nanoflowers prepared in Examples 4 - 6. Detailed implementation manners

[0065] The following will detail various exemplary embodiments, features, and aspects of the present invention. The special word "exemplary" here means "serving as an example, embodiment, or illustration". Any embodiment described as "exemplary" here does not necessarily have to be interpreted as superior to or better than other embodiments.

[0066] In addition, to better illustrate the present invention, numerous specific details are given in the following detailed implementation manners. Those skilled in the art should understand that the present invention can still be implemented without certain specific details. In other instances, methods, means, equipment, and steps well-known to those skilled in the art are not described in detail to highlight the gist of the present invention.

[0067] Unless otherwise stated, the units used in this specification are all international standard units, and the numerical values and numerical ranges appearing in the present invention should be understood to include the inevitable systematic errors in industrial production.

[0068] In this specification, the meaning expressed by using "can" includes both the meaning of performing a certain process and not performing a certain process.

[0069] In this specification, the "some specific / preferred embodiments", "some other specific / preferred embodiments", "embodiments", etc. mentioned refer to the specific elements (e.g., features, structures, properties, and / or characteristics) related to such embodiments, which are included in at least one of the embodiments described herein, and may or may not exist in other embodiments. Additionally, it should be understood that the elements may be combined in various embodiments in any suitable manner.

[0070] In this specification, the numerical range expressed as "numerical value A to numerical value B" refers to the range that includes the endpoint numerical values A and B.

[0071] In this specification, the numerical range expressed as "above" or "below" refers to the numerical range that includes this number.

[0072] In this specification, the use of "optional" or "optional / optionally" means that certain substances, components, implementation steps, applied conditions, and other factors are used or not used.

[0073] In this specification, unless otherwise specified, the "normal temperature" or "room temperature" generally refers to the temperature at 23 ± 2 °C.

[0074] In this specification, the unit names used are all international standard unit names, and unless otherwise stated, the "%" used represents the weight or mass percentage content.

[0075] In this specification, "comprising", "having", "including", or "containing" may refer to inclusive or open-ended, and do not exclude additional, unrecited elements or method steps. At the same time, "comprising", "having", "including", or "containing" may also represent a closed type, excluding additional, unrecited elements or method steps.

[0076] In this specification, the "substantially" or "essentially" means that the error is less than 1%, or less than 0.8%, or less than 0.6% compared with the relevant perfect standard or theoretical standard. Additionally, when "all" or "all of" is mentioned in this specification, its meaning also refers to "substantially" or "essentially" "all" or "all of".

[0077] In this specification, unless otherwise stated, the "many", "multiple", "a plurality of", etc. in which "many" represents a numerical value of 2 or more.

[0078] In this specification, the transition metals include five elements: palladium (Pd), copper (Cu), gold (Au), silver (Ag), and platinum (Pt).

[0079] In this specification, transition metal salts include their various hydrates. For example, copper chloride includes anhydrous copper chloride, copper chloride dihydrate, etc.

[0080] The object of the present invention is to provide a continuous preparation method of Pd-based bimetallic nanoflowers, which comprises the following steps:

[0081] a. Feeding raw material A and raw material B into a first micromixer for mixing to obtain a first mixed solution. The raw material A is an aqueous solution of a palladium salt, and the raw material B is an aqueous solution of other transition metal salts except the palladium salt. Both raw material A and raw material B contain a cationic surfactant;

[0082] b. Feeding the first mixed solution and raw material C into a second micromixer for mixing to obtain a second mixed solution. The raw material C is an aqueous solution of a reducing agent;

[0083] c. Feeding the second mixed solution into a microreactor for redox reaction to obtain Pd-based bimetallic nanoflowers.

[0084] The following will respectively describe in detail each step in the preparation method of the present invention.

[0085] Step a

[0086] In the present invention, feeding raw material A and raw material B into a first micromixer for mixing can enable the reaction raw materials for producing Pd-based bimetallic nanoflowers to perform liquid-liquid mixing and / or liquid-liquid dispersion in the micromixer. Since the micromixer can increase the contact specific surface area between raw material A containing an aqueous solution of a palladium salt and raw material B containing an aqueous solution of other transition metal salts except the palladium salt, the mixing and / or dispersion among the raw materials before the reaction become more uniform, which is beneficial to the subsequent reaction.

[0087] In the present invention, the raw material A is an aqueous solution of a palladium salt and contains a cationic surfactant. The present invention has no particular limitation on the palladium salt used as long as it is water-soluble. In some specific embodiments, the palladium salt is selected from one or more of sodium tetrachloropalladate, potassium tetrachloropalladate, or their hydrates.

[0088] In the present invention, the raw material B is an aqueous solution of other transition metal salts except the palladium salt and contains a cationic surfactant. In some specific embodiments, the other transition metal salts except the palladium salt are selected from one or more of copper chloride, silver nitrate, chloroauric acid, potassium chloroplatinate, or their hydrates.

[0089] In the present invention, the palladium salt and other transition metal salts other than the palladium salt are reasonably selected so that the first mixed solution formed after mixing the raw material A and the raw material B is substantially in a solution state, which is conducive to the sufficient mixing of the palladium salt and other transition metal salts other than the palladium salt. In some specific embodiments, the first mixed solution substantially has a homogeneous dispersion system.

[0090] In some specific embodiments, the concentration of the palladium salt in the raw material A is 0.001 - 0.01 mol / L, preferably 0.001 - 0.005 mol / L, more preferably 0.002 - 0.005 mol / L. For example, it can be 0.001 mol / L, 0.002 mol / L, 0.0025 mol / L, 0.003 mol / L, 0.004 mol / L, 0.005 mol / L, 0.006 mol / L, 0.007 mol / L, 0.008 mol / L, 0.009 mol / L, 0.01 mol / L, etc.

[0091] In some specific embodiments, the concentration of other transition metal salts other than the palladium salt in the raw material B is 0.001 - 0.01 mol / L, preferably 0.001 - 0.005 mol / L, more preferably 0.002 - 0.005 mol / L. For example, it can be 0.001 mol / L, 0.002 mol / L, 0.0025 mol / L, 0.003 mol / L, 0.004 mol / L, 0.005 mol / L, 0.006 mol / L, 0.007 mol / L, 0.008 mol / L, 0.009 mol / L, 0.01 mol / L, etc.

[0092] It should be noted that the upper limit of the above concentration range is general. For specific palladium salts and other transition metal salts other than the palladium salt, the upper limit of the above concentration should also be below their solubility.

[0093] In some specific embodiments, the molar ratio of the palladium salt in the raw material A to other transition metal salts other than the palladium salt in the raw material B is (1 - 10):1, preferably (1 - 5):1, more preferably (1 - 3):1. For example, it can be 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, etc. By adjusting the ratio of the palladium salt to other transition metal salts other than the palladium salt within the above range, it can be ensured that the generated Pd-based bimetallic nanoparticles do not agglomerate and self-assemble into a flower-like structure.

[0094] In the present invention, the cationic surfactants in raw material A and raw material B are adsorbed on the surface of metal ions through electrostatic interaction, preventing the premature reduction or aggregation of metal ions. They serve as both stabilizers during the preparation process of metal precursors and structure-directing agents for the formation of Pd-based bimetallic nanoflowers. Specifically, the cationic surfactant first combines with palladium ions (present in the form of negatively charged PdCl 4 2- in raw material A) through electrostatic interaction, and then the palladium ions are reduced to Pd clusters under the action of a reducing agent. The Pd clusters can catalyze the reduction of other transition metal ions except palladium ions in raw material B, thereby obtaining Pd-based bimetallic nanoflowers. In this process, on the one hand, the cationic surfactant prevents the aggregation of nanoparticles through the steric hindrance effect of its long carbon chain, and on the other hand, it also acts as a structure-directing agent for the formation of Pd-based bimetallic nanoflowers.

[0095] In some specific embodiments, the cationic surfactant includes cationic surfactants of quaternary ammonium salts, such as one or more of cetyltrimethylammonium bromide (CTAB), cetyltrimethylammonium chloride (CTAC), octadecyltrimethylammonium bromide (OTAB), and octadecyltrimethylammonium chloride (OTAC).

[0096] In some specific embodiments, the concentrations of the cationic surfactants in raw material A and raw material B are independently 0.001 - 0.05 mol / L, preferably 0.001 - 0.02 mol / L, more preferably 0.002 - 0.01 mol / L, and can be, for example, 0.001 mol / L, 0.003 mol / L, 0.005 mol / L, 0.01 mol / L, 0.015 mol / L, 0.02 mol / L, 0.025 mol / L, 0.03 mol / L, 0.035 mol / L, 0.04 mol / L, 0.045 mol / L, 0.05 mol / L, etc.

[0097] In some specific embodiments, a pump (such as a peristaltic pump, syringe pump, diaphragm pump, etc.) is used to introduce raw material A and raw material B into a first micro-mixer.

[0098] In some specific embodiments, raw material A and raw material B are introduced into the first micro-mixer at flow rates in the range of 0.01 - 1 mL / min, preferably 0.1 - 0.5 mL / min.

[0099] In some specific embodiments, by adjusting the flow rate ratio of the two feed liquids in the first micro-mixer, that is, the flow rate ratio of raw material A to raw material B, the molar ratio of the palladium salt contained in raw material A to other transition metal salts except the palladium salt contained in raw material B is (1 - 10):1.

[0100] Furthermore, the flow rate ratio of raw material A to raw material B can be adjusted by regulating the flow rates of raw material A and raw material B introduced into the first micro-mixer. In some specific embodiments, the volume flow rate ratio of raw material A to raw material B can be (0.5 - 5):1.

[0101] The present invention places no particular limitation on the first micro-mixer used in step a, and it can be any micro-mixer known in the art suitable for liquid-liquid mixing, such as a micro-channel mixer, a membrane dispersion mixer, or a micro-sieve pore mixer, etc. In a preferred embodiment, the first micro-mixer is a micro-channel mixer.

[0102] In some specific embodiments, the channel diameter of the first micro-mixer is 0.1 - 2 mm, preferably 0.2 - 0.8 mm, and can be, for example, 0.2 mm, 0.25 mm, 0.5 mm, etc.

[0103] In some specific embodiments, the temperature in the first micro-mixer is 10 - 50 °C, preferably 20 - 40 °C, and can be, for example, 10 °C, 15 °C, 20 °C, 25 °C, 30 °C, 35 °C, 40 °C, 45 °C, 50 °C, etc.; by controlling the temperature in the micro-mixer within the above range, blockage of the reaction pipeline can be prevented and the mixing efficiency can be improved.

[0104] In some specific embodiments, the mixing time of raw material A and raw material B in the first micro-mixer is 0.1 - 10 s, preferably 0.2 - 5 s, and can be, for example, 1 s, 2 s, 3 s, 4 s, 5 s, 6 s, 7 s, 8 s, 9 s, etc.; by controlling the mixing time within the range of 0.1 - 10 s, both sufficient mixing and production efficiency can be taken into account. Among them, the "mixing time" refers to the average residence time of the materials in the micro-mixer.

[0105] Step b

[0106] The present invention feeds the first mixed liquid and raw material C into a second micro-mixer for mixing to obtain a second mixed liquid. By carrying out liquid-liquid dispersion of the first mixed liquid and raw material C in the second micro-mixer, the first mixed liquid and raw material C can be mixed sufficiently. In the present invention, when the first mixed liquid containing the metal precursor solution and raw material C containing the reducing agent solution are pre-mixed in the second micro-mixer before the redox reaction occurs, due to the very small internal size of the second micro-mixer, the specific surface area when the two contact is very large and the shearing effect is very strong, which can enable the metal precursor solution and the reducing agent solution to be well dispersed immediately after mixing in the second micro-mixer. That is, the uniform dispersion of the reaction precursors provides uniform reaction conditions for the subsequent reduction of metal ions and the nucleation and growth of metal nanoparticles.

[0107] Specifically, a pump (such as a peristaltic pump, syringe pump, diaphragm pump, etc.) can be used to introduce raw material C into the second micromixer. Additionally, the flow rate ratio of the first mixed solution to raw material C can be adjusted by regulating the flow rates of the first mixed solution and raw material C introduced into the second micromixer. In some specific embodiments, the volume flow rate ratio of the first mixed solution to raw material C is (0.5 - 5):1, for example, it can be 0.8:1, 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, etc.

[0108] The present invention places no particular limitation on the second micromixer used in step b, and it can be any micromixer known in the art suitable for liquid-liquid mixing, such as a microchannel mixer, membrane dispersion mixer, or micro sieve pore mixer, etc. In a preferred embodiment, the second micromixer is a microchannel mixer.

[0109] In some specific embodiments, the channel diameter of the second micromixer is 0.1 - 2 mm, preferably 0.2 - 0.8 mm, for example, it can be 0.2 mm, 0.25 mm, 0.5 mm, etc.

[0110] In some specific embodiments, the temperature in the second micromixer is 10 - 50 °C, preferably 20 - 40 °C, for example, it can be 10 °C, 15 °C, 20 °C, 25 °C, 30 °C, 35 °C, 40 °C, 45 °C, etc.; by controlling the temperature in the second micromixer to 10 - 50 °C, uniform mixing of the metal precursor solution and the reducing agent solution can be achieved. The mixing time in the second micromixer is 0.1 - 10 s, preferably 0.2 - 5 s, for example, it can be 0.5 s, 1 s, 3 s, 5 s, 7 s, 9 s, etc.; by controlling the mixing time to 0.1 - 10 s, both good dispersion and production efficiency can be taken into account.

[0111] It should be noted that in the second micromixer, the main process is the dispersion of the metal precursor solution and the reducing agent solution, but at the same time, a certain amount of redox reaction will also occur between the metal precursor and the reducing agent. Therefore, a certain amount of metal atoms or nanoclusters will also be generated after mixing in the second micromixer.

[0112] In some specific embodiments, the reducing agent is selected from one or more of ascorbic acid and citric acid.

[0113] In some specific embodiments, the concentration of the reducing agent in raw material C is 0.1 - 0.5 mol / L, preferably 0.2 - 0.5 mol / L, for example, it can be 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, etc.

[0114] In some specific embodiments, the raw material C is introduced into the second micro-mixer at a flow rate in the range of 0.01 - 1 mL / min, preferably 0.1 - 0.5 mL / min.

[0115] In some specific embodiments, the second mixed solution substantially has a homogeneous dispersion system.

[0116] Step c

[0117] In the present invention, the second mixed solution is introduced into a micro-reactor for a redox reaction to obtain Pd-based bimetallic nanoflowers.

[0118] In some specific embodiments, in a microchannel reactor, a redox reaction occurs between the metal precursor solution and the reducing agent in the second mixed solution, nucleation and growth form Pd-based bimetallic nanoflowers, and the formed nanoflowers are dispersed in water in the form of a colloid under the stabilizing action of a cationic surfactant.

[0119] In the present invention, the reducing agent reduces metal ions in the metal precursor solution to metal atoms, for example, reducing palladium ions (present in the form of PdCl 4 2- and other transition metal ions (such as Cu2+) to metal atoms (Pd, Cu, etc.).

[0120] In the present invention, the cationic surfactant regulates the reduction of metal ions and the growth of nanoparticles through electrostatic interaction and steric hindrance effects, and guides the self-assembly of nanoparticles to form a hierarchical or petal-shaped nanoflower structure.

[0121] The present invention places no particular limitation on the micro-reactor used in step c, which can be any micro-reactor known in the art suitable for liquid-liquid mixing, such as a microchannel reactor or a coiled tube micro-reactor, etc. In a preferred embodiment, the micro-reactor is a microchannel reactor. Among them, the channel diameter of the micro-reactor is 0.8 - 5 mm, preferably 0.8 - 2 mm, and can be, for example, 0.8 mm, 1 mm, 1.5 mm, etc.; the length is 1 - 20 m, preferably 3 - 10 m.

[0122] In the present invention, there is no particular limitation on the material of the micro-reactor, and it can be selected from one or more of polytetrafluoroethylene, perfluoroethylenepropylene, and soluble polytetrafluoroethylene, for example.

[0123] In some specific embodiments, the reaction temperature in the microreactor is 10 to 60 °C, preferably 20 to 50 °C, and can be, for example, 10 °C, 15 °C, 20 °C, 25 °C, 30 °C, 35 °C, 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, etc. By controlling the reaction temperature within the above range, it is possible to ensure that the metal precursor solution reacts fully with the reducing agent while maintaining a stable nucleation environment. If the reaction temperature is too low, the reaction rate will be too slow, which is not conducive to initial nucleation; while if the reaction temperature is too high, the reaction will be too fast, making it difficult to form the desired pore structure, and then leading to agglomeration of the nanoflowers. Therefore, selecting an appropriate reaction temperature range is crucial for obtaining an ideal nanoflower structure.

[0124] In some specific embodiments, the residence time in the microreactor is 0.1 to 20 min, preferably 3 to 10 min, and can be, for example, 0.5 min, 1 min, 3 min, 5 min, 7 min, 9 min, 10 min, 13 min, 15 min, 17 min, 19 min, etc. By controlling the residence time within the above range, it is possible to ensure the uniform nucleation and growth of metal atoms, form a stable nanoflower structure, and avoid adverse phenomena such as incomplete reaction or insufficient nucleation caused by too short residence time.

[0125] Other Steps

[0126] The preparation method of the present invention also optionally includes one or more steps selected from setting up the reaction device, post-treatment and purification of the product.

[0127] In some specific embodiments, the preparation method of the present invention further includes a step of setting up the reaction device before step a. Specifically, in this step, the various components of the reaction device are connected so that the reaction materials can flow through the reaction device. The various components of the reaction device include, but are not limited to, the pumps, micromixers, microreactors, etc. described above.

[0128] In some specific embodiments, the preparation method of the present invention further includes a step of post-treatment and purification of the obtained Pd-based bimetallic nanoflowers after step c. The post-treatment and purification steps include one or more selected from centrifugation, water washing, alcohol washing, etc.

[0129] In some specific embodiments, after step c, the reaction solution containing Pd-based bimetallic nanoflowers is centrifuged. The centrifugation speed can be 5000 - 10000 r / min, and the centrifugation time can be 5 - 10 min. One or more of deionized water, ethanol, or isopropanol are used to wash the centrifuged Pd-based bimetallic nanoflowers multiple times. The water wash and / or alcohol wash can be carried out separately one or more times, and after each single water wash or alcohol wash, separation is carried out by centrifugation and / or filtration to remove unreacted substances and surfactants, ultimately obtaining high-purity Pd-based bimetallic nanoflowers.

[0130] The present invention also relates to Pd-based bimetallic nanoflowers prepared by the preparation method according to the present invention. Through the implementation of the above preparation method, the Pd-based bimetallic nanoflowers prepared by the present invention have a high specific surface area and abundant active sites.

[0131] For the Pd-based bimetallic nanoflowers prepared by the present invention, their particle size is below 500 nm, preferably below 400 nm, more preferably below 300 nm, and further preferably 10 - 200 nm. The particle size is obtained by statistically analyzing the particle size of the nanoflower particles in the TEM image using Nano Measurer software.

[0132] In the Pd-based bimetallic nanoflowers prepared by the preparation method of the present invention, the dispersion degree of the nanoflower particles is high. In the present invention, a high dispersion degree means that the Pd-based bimetallic nanoflowers are dispersed in the form of single particles and are relatively evenly dispersed, without large-area aggregation and no agglomeration between particles. The highly uniform dispersion structure of the nanoflower particles can be fully demonstrated by the morphological structure characterized in the high-resolution transmission electron microscope (HRTEM) and transmission electron microscope (TEM) images.

[0133] Examples

[0134] The embodiments of the present invention will be described in detail below in conjunction with examples. However, those skilled in the art will understand that the following examples are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. For those not specified in the examples, the conventional conditions or the conditions recommended by the manufacturer are used. For the reagents or instruments not specified for the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0135] The parameters of the microreaction equipment used in the following examples are as follows:

[0136] The first micro mixer is a T-shaped microchannel mixer with a channel diameter of 0.5 mm.

[0137] The second micro mixer is a T-shaped microchannel mixer with a channel diameter of 0.5 mm.

[0138] The microreactor is a microchannel reactor made of polytetrafluoroethylene, with a channel diameter of 0.8 mm and a length of 5 m.

[0139] Example 1:

[0140] A continuous preparation method of PdCu bimetallic nanoflowers is as Figure 1 shown, and the specific steps are as follows:

[0141] Take an aqueous solution of sodium tetrachloropalladate (concentration 0.0025 mol / L) as raw material A, an aqueous solution of anhydrous copper chloride (concentration 0.0025 mol / L) as raw material B, and an aqueous solution of ascorbic acid (concentration 0.3 mol / L) as raw material C. Cetyltrimethylammonium chloride (CTAC, concentration 0.005 mol / L) is added as a surfactant to both raw material A and raw material B. Feed raw material A through a peristaltic pump (flow rate 0.3 mL / min) and raw material B through a syringe pump (flow rate 0.1 mL / min) into the first micromixer (mixing temperature 30 °C, mixing time 1 s) for mixing. Among them, the total flow rate of raw material A and raw material B is 0.4 mL / min, and the flow rate ratio is 3:1, to obtain the first mixed solution. Feed raw material C through a syringe pump (flow rate 0.1 mL / min) and the first mixed solution into the second micromixer (mixing temperature 30 °C, mixing time 1 s) for mixing to obtain the second mixed solution. The second mixed solution enters the microchannel reactor connected to the outlet of the second micromixer (reaction temperature 30 °C, residence time 5 min) for redox reaction, and then centrifugation is carried out (centrifugation speed 10000 rpm, centrifugation time 10 min), and about three water washes and three alcohol washes (ethanol) are carried out to obtain the PdCu bimetallic nanomaterial (denoted as Pd3Cu1), which is dispersed and stored in ethanol or water.

[0142] Example 2:

[0143] Compared with Example 1, the difference is that: the total flow rate of raw material A and raw material B is 0.4 mL / min, and the flow rate ratio of raw material A and raw material B is 2:1.

[0144] The PdCu bimetallic nanomaterial prepared in this example is denoted as Pd2Cu1, which is dispersed and stored in ethanol or water.

[0145] Example 3:

[0146] Compared with Example 1, the difference is that: the total flow rate of raw material A and raw material B is 0.4 mL / min, and the flow rate ratio of raw material A and raw material B is 1:1.

[0147] The PdCu bimetallic nanomaterial prepared in this example is denoted as Pd1Cu1, which is dispersed and stored in ethanol or water.

[0148] Example 4:

[0149] A continuous preparation method of PdAu bimetallic nanoflowers is as Figure 1 shown, and the specific steps are as follows:

[0150] Take sodium tetrachloropalladate aqueous solution (concentration 0.0025 mol / L) as raw material A, chloroauric acid aqueous solution (concentration 0.0025 mol / L) as raw material B, and ascorbic acid aqueous solution (concentration 0.3 mol / L) as raw material C. Cetyltrimethylammonium chloride (CTAC, concentration 0.005 mol / L) is added as a surfactant in both raw materials A and B. Feed raw material A through a peristaltic pump (flow rate 0.2 mL / min) and raw material B through a syringe pump (flow rate 0.2 mL / min) into a first micromixer (mixing temperature 30 °C, mixing time 1 s) for mixing. Among them, the total flow rate of raw materials A and B is 0.4 mL / min, and the flow rate ratio is 1:1, obtaining a first mixed solution. Feed raw material C through a syringe pump (flow rate 0.1 mL / min) and the first mixed solution into a second micromixer (mixing temperature 30 °C, mixing time 1 s) for mixing to obtain a second mixed solution. The second mixed solution enters a microchannel reactor connected to the outlet of the second micromixer (reaction temperature 30 °C, residence time 5 min) for redox reaction, and then centrifugation is carried out (centrifugation speed 10000 rpm, centrifugation time 10 min), and about three water washes and three alcohol washes (ethanol) are carried out to obtain PdAu bimetallic nanomaterials (denoted as PdAu), which are dispersed and stored in ethanol or water.

[0151] Example 5:

[0152] A continuous preparation method of PdAg bimetallic nanoflowers is as Figure 1 shown, and the specific steps are as follows:

[0153] Sodium tetrachloropalladate aqueous solution (concentration 0.0025 mol / L) was used as raw material A, silver nitrate aqueous solution (concentration 0.0025 mol / L) was used as raw material B, and ascorbic acid aqueous solution (concentration 0.3 mol / L) was used as raw material C. Cetyltrimethylammonium chloride (CTAC, concentration 0.005 mol / L) was added as a surfactant to both raw material A and raw material B. Raw material A was delivered to the first micromixer (mixing temperature 30 °C, mixing time 1 s) through a peristaltic pump (flow rate 0.2 mL / min), and raw material B was delivered through a syringe pump (flow rate 0.2 mL / min) for mixing. Among them, the total flow rate of raw material A and raw material B was 0.4 mL / min, and the flow rate ratio was 1:1, obtaining the first mixed solution. Raw material C was delivered to the second micromixer (mixing temperature 30 °C, mixing time 1 s) through a syringe pump (flow rate 0.1 mL / min) and the first mixed solution for mixing to obtain the second mixed solution. The second mixed solution entered a microchannel reactor connected to the outlet of the second micromixer (reaction temperature 30 °C, residence time 5 min) for redox reaction, and then centrifugation was carried out (centrifugation speed 10000 rpm, centrifugation time 10 min), and about three water washes and three alcohol washes (ethanol) were carried out to obtain PdAg bimetallic nanomaterials (denoted as PdAg), which were dispersed and stored in ethanol or water.

[0154] Example 6:

[0155] A continuous preparation method of PdPt bimetallic nanoflowers is as Figure 1 shown, and the specific steps are as follows:

[0156] Sodium tetrachloropalladate aqueous solution (concentration 0.0025 mol / L) was used as raw material A, potassium chloroplatinate aqueous solution (concentration 0.0025 mol / L) was used as raw material B, and ascorbic acid aqueous solution (concentration 0.3 mol / L) was used as raw material C. Cetyltrimethylammonium chloride (CTAC, concentration 0.005 mol / L) was added as a surfactant to both raw material A and raw material B. Raw material A was delivered to the first micromixer (mixing temperature 30 °C, mixing time 1 s) through a peristaltic pump (flow rate 0.2 mL / min), and raw material B was delivered through a syringe pump (flow rate 0.2 mL / min). Among them, the total flow rate of raw material A and raw material B was 0.4 mL / min, and the flow rate ratio was 1:1, obtaining the first mixed solution. Raw material C was delivered to the second micromixer (mixing temperature 30 °C, mixing time 1 s) through a syringe pump (flow rate 0.1 mL / min) and the first mixed solution for mixing to obtain the second mixed solution. The second mixed solution entered a microchannel reactor connected to the outlet of the second micromixer (reaction temperature 30 °C, residence time 5 min) for redox reaction, and then centrifugation was carried out (centrifugation speed 10000 rpm, centrifugation time 10 min), and about three water washes and three alcohol washes (ethanol) were carried out to obtain PdPt bimetallic nanomaterials (denoted as PdPt), which were dispersed and stored in ethanol or water.

[0157] Comparative Example 1:

[0158] Compared with Example 1, the difference is that the flow rate of raw material A is 0.4 mL / min, and the flow rate of raw material B is 0 mL / min.

[0159] The nanomaterials prepared in this comparative example are Pd nanoparticles.

[0160] Comparative Example 2:

[0161] Compared with Example 1, the difference is that the flow rate of raw material A is 0 mL / min, and the flow rate of raw material B is 0.4 mL / min.

[0162] No nanoparticles were prepared in this comparative example.

[0163] Comparative Example 3:

[0164] With the same flow rate as in Example 1, raw materials A, B, and C were simultaneously injected into a microchannel reactor (reaction temperature 30 °C, residence time 5 min) for redox reaction, and then centrifugation was carried out (centrifugation speed 10000 rpm, centrifugation time 10 min), and about three water washes and three alcohol washes (ethanol) were carried out.

[0165] The PdCu bimetallic nanomaterials prepared in this comparative example have extremely uneven particle size distribution and self-nucleated Pd nanoparticles appear.

[0166] Comparative Example 4:

[0167] Compared with Example 3, the difference lies in that neither raw material A nor raw material B contains a cationic surfactant.

[0168] Characterization and Analysis

[0169] The obtained nanomaterials were characterized by means such as transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma spectroscopy (ICP-OES), etc. to determine the composition, structure, morphology, etc. of the materials.

[0170] The preparation process of the samples for TEM and HRTEM characterization was as follows: The bimetallic nanomaterials prepared in each example and comparative example were dispersed in ethanol, and 1 to 3 drops of the dispersion were dropped onto an ultrathin carbon film and dried moderately.

[0171] 1. Characterization by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive spectroscopy (EDS)

[0172] The PdCu bimetallic nanomaterials prepared in Examples 1 - 3 and Comparative Example 4 were characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive spectroscopy (EDS).

[0173] The transmission electron microscopy (TEM) images were taken using a JEOL HT7700 transmission electron microscope, and image acquisition was achieved using an acceleration voltage of 120 kV.

[0174] The high-resolution transmission electron microscopy (HRTEM) images and energy-dispersive spectroscopy (EDS) images were taken using a JEOL high-resolution field emission transmission electron microscope. Ultra-high-resolution image acquisition was achieved using a field emission transmission electron microscope with 200 kV. At the same time, the composition at the nanoscale was collected using the STEM attachment combined with energy spectroscopy.

[0175] From Figure 2As can be seen from the transmission electron microscopy (TEM) images shown, the PdCu bimetallic nanomaterials with different ratios all exhibit a flower-like hierarchical structure, while the nanomaterials obtained without adding Cu (i.e., Comparative Example 1) are uneven Pd nanoparticles. In addition, as the ratio of the Cu precursor increases, the size of the nanoparticles becomes more uniform and shows narrower branches. It should be noted that when the ratio of the Cu precursor exceeds that of the Pd precursor, the Cu content in the finally prepared nanomaterials no longer increases and the morphology no longer changes. In addition, nanoparticles cannot be prepared when only the Cu precursor is present (i.e., Comparative Example 2).

[0176] Figure 3 Figure 4 is the transmission electron microscopy (TEM) image of the PdCu bimetallic nanomaterial (Pd1Cu1) prepared in Comparative Example 4. As can be seen from Figure 3 it, the nanomaterials obtained without adding a cationic surfactant are PdCu bimetallic nanoparticles without flower-like branches.

[0177] Figure 4 A,[[]] Figure 4 B are respectively the high-resolution transmission electron microscopy (HRTEM) image and the energy-dispersive spectroscopy (EDS) image of the PdCu bimetallic nanomaterial (Pd1Cu1) prepared in Example 3. As can be seen from Figure 4 A, a clear hierarchical flower-like structure is observed, Figure 4 and B shows that Pd and Cu elements are uniformly distributed on the nanoflowers, proving the uniformity of the composition.

[0178] 2. Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis

[0179] Inductively coupled plasma optical emission spectrometry (ICP-OES) was used to analyze the actual metal ratio by an Agilent 5800 inductively coupled plasma optical emission spectrometer (ICP-OES). The ICP-OES test results of the PdCu bimetallic nanomaterials prepared in Examples 1-3 are shown in Table 1.

[0180] Table 1. ICP-OES test results

[0181] Examples Pd (mole percentage%) Cu (mole percentage%) Example 1 76.92 23.08 Example 2 69.00 31.00 Example 3 55.23 44.77

[0182] As can be seen from Table 1, the PdCu ratios in the PdCu bimetallic nanomaterials prepared in Examples 1-3 are basically consistent with the metal precursor feeding ratios.

[0183] 3. X-ray diffraction (XRD) characterization

[0184] X-ray diffraction (XRD) was performed using a Rigaku D / max-2550 X-ray diffractometer. The test conditions were continuous scanning, and the angular range was 35-75°.

[0185] The X-ray diffraction (XRD) patterns of the PdCu bimetallic nanomaterials prepared in Examples 1-3 are as Figure 5 shown. It can be Figure 5 seen that except for Pd3Cu1, both Pd2Cu1 and Pd1Cu1 form a homogeneous alloy structure without phase separation. As the Cu content increases, the diffraction peaks gradually shift to red, and Pd3Cu1 shows diffraction peak splitting, indicating that part of Pd nucleates by itself to form Pd nanoparticles.

[0186] 4. X-ray photoelectron spectroscopy (XPS) characterization

[0187] X-ray photoelectron spectroscopy (XPS) was measured using an X-ray photoelectron spectrometer of the Axis Supra model from Shimadzu Corporation of Japan. This system is equipped with an Al Kα X-ray source (1486.6 eV), the measurement scanning power is 200 W, and the core-level spectrum power is 300 W.

[0188] The X-ray photoelectron spectroscopy (XPS) patterns of the 3d orbit of palladium element and the 2p orbit of copper element in the PdCu bimetallic nanomaterials prepared in Examples 1-3 are as Figure 6 shown. It can be Figure 6 seen that as the copper content in the PdCu bimetallic nanomaterials increases, the 3d peak of palladium shifts to a lower binding energy, while the 2p peak of copper shifts to a higher binding energy, which proves that there is an electronic interaction between copper and palladium during the alloying process of PdCu bimetals.

[0189] 5. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) combined with energy-dispersive spectroscopy (EDS) characterization

[0190] The morphology and composition of the Pd-based bimetallic nanomaterials obtained in Examples 4-6 were characterized by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) combined with energy-dispersive spectroscopy (EDS).

[0191] The HAADF-STEM and EDS patterns of the Pd-based bimetallic nanomaterials prepared in Examples 4-6 are as Figure 7 shown. It can be Figure 7 seen that except for the PdAu bimetallic nanomaterials, both the PdAg bimetallic nanomaterials and the PdPt bimetallic nanomaterials are homogeneous alloy structures. The reason why the PdAu bimetallic nanomaterials form a gold core-palladium shell structure is that the standard electrode potential of AuCl 4 - is much higher than that of PdCl 4 2- .

[0192] Furthermore, the XRD patterns of the Pd-based bimetallic nanomaterials prepared in Examples 4-6 are as Figure 8 shown. It is further Figure 8 proved that the PdAu bimetallic nanomaterial has a unique core-shell structure, and both the PdAg bimetallic nanomaterial and the PdPt bimetallic nanomaterial are homogeneous alloy structures.

[0193] 6. Particle size analysis

[0194] The particle size is obtained by statistically analyzing the sizes of the nanoparticles in the TEM images using the Nano Measurer software, and the results are shown in Table 2.

[0195] Table 2. Results of particle size analysis

[0196] Examples Nanomaterials Particle Size (nm) Example 1 Pd3Cu1 14 Example 2 Pd2Cu1 26 Example 3 Pd1Cu1 24 Example 4 PdAu 200 Example 5 PdAg 60 Example 6 PdPt 200 Comparative Example 1 Pd 36 Comparative Example 4 Pd1Cu1 30

[0197] Industrial Applicability

[0198] The preparation method of the present invention is simple, efficient, and has good generality, and can be widely used for the large-scale preparation of bimetallic nanoflowers in industry, especially having important application prospects in the field of catalysts.

Claims

1. A continuous preparation method of Pd-based bimetallic nanoflowers, characterized in that: The steps include: a. The raw material A and the raw material B are passed into a first micromixer to mix to obtain a first mixed solution, wherein the raw material A is an aqueous solution of a palladium salt, the raw material B is an aqueous solution of a transition metal salt other than a palladium salt, and both the raw material A and the raw material B contain a cationic surfactant; b. The first mixed solution and the raw material C are passed into a second micro-mixer to mix to obtain a second mixed solution, wherein the raw material C is an aqueous solution of a reducing agent; c. Passing the second mixed solution into a microreactor for redox reaction to obtain Pd-based bimetallic nanoflowers.

2. The preparation method according to claim 1, characterized in that: The second mixed liquid is substantially a homogeneous dispersion system, and the mixing time in step b is 0.1 to 10 seconds.

3. The preparation method according to claim 1 or 2, characterized in that: The palladium salt is selected from one or more of sodium tetrachloropalladate and potassium tetrachloropalladate; The other transition metal salts except the palladium salt are selected from one or more of cupric chloride, silver nitrate, chloroauric acid, and potassium chloroplatinate; The cationic surfactants include quaternary ammonium salt-type cationic surfactants.

4. The preparation method according to any one of claims 1 to 3, characterized in that: The concentration of palladium salt in the raw material A is 0.001-0.01 mol / L; The concentration of other transition metal salts in the raw material B except palladium salt is 0.001-0.01 mol / L; The concentrations of the cationic surfactant in the raw material A and the raw material B are independently 0.001 to 0.05 mol / L.

5. The preparation method according to any one of claims 1 to 4, characterized in that: The molar ratio of the palladium salt in the raw material A to the other transition metal salts except the palladium salt in the raw material B is (1-10):

1.

6. The preparation method according to any one of claims 1 to 5, characterized in that: The reducing agent is selected from one or more of ascorbic acid and citric acid.

7. The preparation method according to any one of claims 1 to 6, characterized in that: The concentration of the reducing agent in the raw material C is 0.1-0.5 mol / L.

8. The preparation method according to any one of claims 1 to 7, characterized in that: The first micromixer and the second micromixer are each independently a microchannel mixer, a membrane dispersion mixer or a micromesh mixer; The microreactor is a microchannel reactor or a coil-type microreactor.

9. The preparation method according to any one of claims 1 to 8, characterized in that: The temperature in the first micro mixer is 10 to 50° C., and the mixing time is 0.1 to 10 seconds; The temperature in the second micro mixer is 10 to 50° C.; The temperature in the microreactor is 10-60° C., and the residence time is 0.1-20 min.

10. The Pd-based bimetallic nanoflower prepared by the preparation method according to any one of claims 1 to 9, characterized in that: The particle size of the Pd-based bimetallic nanoflower is less than 500 nm.

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