Platinum group metal nanocrystal, and preparation method and application thereof

Abstract

The application provides a platinum group metal nanocrystal and a preparation method and application thereof, and the preparation method comprises the following steps: heating and reducing a solid-phase platinum group metal complex salt in a liquid-phase organic reducing agent, so that the solid-phase platinum group metal complex salt is gradually reacted and dissolved, and the platinum group metal nanocrystal is obtained. The platinum group metal nanocrystal, the preparation method and the application thereof are characterized in that the solubility of the solid-phase platinum group metal complex salt in the liquid-phase organic reducing agent is low, a solid-liquid reaction system is formed, and a chemical reduction method is used to make the platinum group metal precursor complex salt gradually react and dissolve, so that the controllable reduction of the platinum group metal and the easy control of the size and morphology are realized, and the platinum group metal nanocrystal with uniform size, narrow distribution and good stability is obtained.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of nanomaterials, and particularly to a platinum group metal nanocrystal and a preparation method and application thereof. BACKGROUND

[0002] The platinum group metal has excellent chemical stability and unique chemical activity, and plays an irreplaceable role in the field of catalysis. The nanometer-sized platinum group metal crystal can present tetrahedron, cube, octahedron, icosahedron, flake and the like, and exhibits physical and chemical properties related to the structure and size of the crystal. At present, the common method for platinum group metal nanocrystal is wet chemical synthesis, that is, under appropriate experimental conditions, the precursor is reduced to atoms in the solution phase, and then the crystal is grown. Typically, taking rhodium nanocrystal as an example, using polyhydric alcohol as a solvent, potassium hexachlororhodate can be reduced to the corresponding rhodium nanocrystal at a relatively high reaction temperature, but the size is generally less than 10 nm and almost cannot be adjusted; the solvothermal method can also be used to prepare rhodium nanocrystal, but the product has irregular morphology and wide size distribution. Although different synthesis methods can be used to prepare platinum group metal nanocrystals with various morphologies, the size cannot be changed by changing the reaction conditions, and the product morphology is affected.

[0003] In the conventional wet chemical synthesis method, the platinum group metal complex salt needs to be dissolved to form a solution first, and then the platinum group metal atom can be formed under the reaction conditions to participate in the growth of the crystal. For the platinum group metal, the standard reduction potential of most complex salts to atoms is low, and it is difficult to reduce under mild conditions, so it is difficult to controllably prepare platinum group metal nanocrystals; although the use of strong reducing agents can effectively produce platinum group metal atoms, the reduction rate cannot be adjusted and the metal bond energy is too high, so the growth of the crystal is also uncontrollable. At present, although some platinum group metal nanocrystals can be prepared using polyhydric alcohol as a solvent and a reducing agent, most complex salts have low solubility in polyhydric alcohol, the preparation efficiency is low, and it is difficult to synthesize a large amount at one time. SUMMARY

[0004] Based on the technical problems in the background art, the present application provides a platinum group metal nanocrystal and a preparation method and application thereof. By utilizing the low solubility of the solid-phase platinum group metal complex salt in the liquid-phase organic reducing agent, a solid-liquid reaction system is formed, and then a chemical reduction method is used to gradually react and dissolve the solid-phase platinum group metal complex salt, so that the controllable reduction of the platinum group metal and the easy control of the size and morphology are realized, and the platinum group metal nanocrystal with uniform size, narrow distribution and good stability is obtained.

[0005] The preparation method of the platinum group metal nanocrystal provided by the present application comprises: heating and reducing the solid-phase platinum group metal complex salt in the liquid-phase organic reducing agent to gradually react and dissolve the solid-phase platinum group metal complex salt, so as to obtain the platinum group metal nanocrystal.

[0006] Preferably, the solid platinum group metal complex salt is a chloroplatinum group metal acid salt;

[0007] Preferably, the chloroplatinum group metal acid salt has a general formula of A x MX y , A is an alkali metal cation or an ammonium cation, M is a platinum group metal element, X is a chloride ion, and x, y are integers greater than 0;

[0008] Preferably, the chloroplatinum group metal acid salt is at least one of potassium hexachlororhodate, ammonium hexachlororhodate, potassium hexachloroplatinate, sodium hexachloroiridate, sodium hexachloroosmate, potassium tetrachloropalladite, or ammonium hexachlororuthenate.

[0009] Preferably, the liquid phase organic reducing agent is a polyhydric alcohol;

[0010] Preferably, the polyhydric alcohol is at least one of ethylene glycol, oligomeric ethylene glycol, polymeric ethylene glycol, propylene glycol, oligomeric propylene glycol, polymeric propylene glycol, or butanediol.

[0011] Preferably, the molar ratio of the solid platinum group metal complex salt to the liquid phase organic reducing agent is 0.25-10:1000.

[0012] Preferably, the heating reduction specifically comprises: mixing the solid platinum group metal complex salt, the liquid phase organic reducing agent, and the organic protective agent, and then preheating the reaction, and then heating the reaction, so that the solid platinum group metal complex salt is gradually reacted and dissolved, i.e. the platinum group metal nanocrystal is obtained;

[0013] Preferably, after preheating the reaction of the mixture of the solid platinum group metal complex salt, the liquid phase organic reducing agent, and the organic protective agent, the method further comprises adding a solubility adjusting agent to continue preheating the reaction.

[0014] In the present application, by preheating the reaction, the dynamic dissolution process of the platinum group metal complex salt is adjusted without reducing the platinum group metal complex salt, and the concentration of the platinum group metal complex salt solution is controlled; by heating the reaction, the complex salt in the solution is reduced to atoms at a suitable temperature, and then crystals are formed.

[0015] Preferably, the organic protective agent is polyvinylpyrrolidone;

[0016] Preferably, the K value of the polyvinylpyrrolidone is 13-32;

[0017] Preferably, the molar ratio of the organic protective agent to the solid platinum group metal complex salt is 5-20:1.

[0018] Preferably, the solubility adjusting agent is used to change the solubility of the solid platinum group metal complex salt in the liquid phase organic reducing agent;

[0019] Preferably, the solubility regulator is an alkali metal salt or an ammonium salt;

[0020] Preferably, the alkali metal salt or the ammonium salt is at least one of cesium chloride, rubidium chloride, potassium nitrate, potassium sulfate, potassium chloride, potassium bromide, sodium bromide, ammonium bromide or ammonium chloride.

[0021] In the present application, the alkali metal salt or the ammonium salt regulates the solubility of the platinum group metal complex salt in the order of "sodium < potassium ~ ammonium < rubidium < cesium" for cations, and in the order of "chlorine < bromine < iodine" for anions.

[0022] Preferably, the molar ratio of the solubility regulator to the solid platinum group metal complex salt is 1-40:1.

[0023] Preferably, the preheating reaction temperature is 20-120℃, and the time is 0.5-24h; the heating reaction temperature is 130-200℃, and the time is 3-72h.

[0024] The present application also provides a platinum group metal nanocrystal prepared by the above preparation method.

[0025] The present application also provides a use of the above platinum group metal nanocrystal in catalyzing complete oxidation of low-concentration methane.

[0026] Compared with the prior art, the present application has the following advantages:

[0027] (1) The present application first proposes a dynamic dissolution reduction method, which takes advantage of the low solubility of platinum group metal complex salt precursors in polyols, breaks through the framework of traditional wet chemical synthesis method that requires the complex salt to be prepared into a solution before preparing platinum group metal nanocrystals, and realizes controllable reduction of platinum group metal complex salt and "one-pot" controllable preparation of platinum group metal nanocrystals through the design of "reduction while dissolution".

[0028] (2) The present application takes advantage of the complex salt particles suspended in polyols to provide reaction sites for the complex salt dissolved in polyols, promotes the formation of platinum group metal atoms, and further adjusts the number of suspended precursor complex salt particles by adding a solubility regulator, so as to ultimately achieve the purpose of adjusting the size of platinum group metal nanocrystals.

[0029] (3) The preparation method of the present application has clear principles, simple process and controllable products, and can be used for large-scale production. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 The figure is a schematic diagram of the principle of the preparation method of the present application;

[0031] Figure 2Transmission electron microscope image of Rh nanocube crystals prepared in Example 1 of the present application;

[0032] Figure 3 Transmission electron microscope image of Pt nanobrush crystals prepared in Example 5 of the present application;

[0033] Figure 4 Transmission electron microscope image of Rh nanocrystals prepared in Example 6 of the present application;

[0034] Figure 5 Transmission electron microscope image of Rh nanocube crystals prepared in Comparative Example 3 of the present application;

[0035] Figure 6 Transmission electron microscope image of Rh nanocube crystals prepared in Comparative Example 4 of the present application. DETAILED DESCRIPTION

[0036] Hereinafter, the technical solutions of the present application will be described in detail through specific examples, but it should be made clear that these examples are used for illustration, but not to be construed as limiting the scope of the present application.

[0037] Example 1

[0038] This example proposes a Rh nanocube crystal, and the specific preparation method is as follows: Figure 1

[0039] 50 mL of triethylene glycol was added to a 100 mL round-bottom flask, and then 0.5 mmol of potassium hexachlororhodate, 5 mmol of polyvinylpyrrolidone (K value of 23-27, molecular weight calculated according to monomer) was added, and preheating was carried out at 1500 r / min under stirring at 110°C for 3 h, and then the temperature was raised to 150°C, and the stirring heating was continued for 24 h. After the reaction was completed, the obtained product was added to 150 mL of acetone, and after being mixed thoroughly, centrifugation was carried out at 8000 r / min for 10 min, and after being washed once with water and acetone and centrifugation, the obtained precipitate was the Rh nanocube crystal, and the precipitate was dispersed in ethanol for storage.

[0040] Figure 2 The transmission electron microscope image of the Rh nanocube crystal with a size of 60.9±4.9 nm prepared in this example is shown in Figure 1, and it can be seen from the figure that the Rh nanocrystal prepared in this example is a cubic structure with a side length of 60.9±4.9 nm. Figure 2

[0041] Example 2

[0042] This example proposes a Rh nanocube crystal, and the specific preparation method is as follows:

[0043] ​​50mL triethylene glycol was added into a 100mL round bottom flask, then 0.5mmol potassium hexachlororhodate, 5mmol polyvinylpyrrolidone (K value 23-27, molecular weight calculated by monomer) were added, preheated at 110°C for 0.5h under stirring at 1500r / min, then 1mmol potassium bromide was added, preheated at 120°C for 1.5h under stirring, then heated at 140°C for 24h under stirring, after the reaction was completed, the product was added into 150mL acetone, mixed thoroughly, centrifuged at 8000r / min for 10min, washed once with water and acetone and then centrifuged, the obtained precipitate was the Rh nanometer cubic crystal, which was dispersed in ethanol for storage.

[0044] Figure 2 A transmission electron microscope image of the Rh nanometer cubic crystal with a size of 30.6nm prepared in this example is shown in Figure 1. Figure 2 It can be seen that the Rh nanocrystal prepared in this example is a cubic structure with a side length of 30.6±1.6nm.

[0045] Example 3

[0046] This example proposes a Rh nanometer cubic crystal, and the specific preparation method is as follows:

[0047] 50mL triethylene glycol was added into a 100mL round bottom flask, then 0.5mmol potassium hexachlororhodate, 5mmol polyvinylpyrrolidone (K value 29-32, molecular weight calculated by monomer) were added, preheated at 110°C for 0.5h under stirring at 1500r / min, then 1.67mmol potassium chloride and 4mmol potassium bromide were added, preheated for 2.5h under stirring, then heated at 150°C for 18h under stirring, after the reaction was completed, the product was added into 150mL acetone, mixed thoroughly, centrifuged at 8000r / min for 10min, washed once with water and acetone and then centrifuged, the obtained precipitate was the Rh nanometer cubic crystal, which was dispersed in ethanol for storage.

[0048] Figure 2 A transmission electron microscope image of the Rh nanometer cubic crystal with a size of 16.1±1.1nm prepared in this example is shown in Figure 3. Figure 2 It can be seen that the Rh nanocrystal prepared in this example is a cubic structure with a side length of 16.1±1.1nm.

[0049] Example 4

[0050] This example proposes a Rh nanometer cubic crystal, and the specific preparation method is as follows:

[0051] Into a 100 mL round bottom flask, 50 mL of triethylene glycol was added, followed by 0.5 mmol of potassium hexachlororhodate, 5 mmol of polyvinylpyrrolidone (K value of 29-32, molecular weight calculated based on monomer), and then the mixture was stirred at 110°C for 0.5 h at a rotation speed of 1500 r / min, followed by the addition of 13.3 mmol of potassium chloride and 8 mmol of potassium bromide, and then the mixture was stirred for 2.5 h, followed by the increase of the temperature to 150°C, and then the mixture was continuously stirred and heated for 48 h, and then the product was added to 150 mL of acetone, and then the mixture was centrifuged at a rotation speed of 8000 r / min for 10 min, and then the obtained precipitate was washed with water and acetone once and then centrifuged, and then the obtained Rh nanocubic crystals were dispersed in ethanol for storage.

[0052] Figure 2 A transmission electron microscope image of the Rh nanocubic crystals prepared in this example and having a size of 10.1±0.8 nm is shown in FIG. 1. Figure 2 As can be seen from FIG. 1, the Rh nanocrystals prepared in this example have a cubic structure with a side length of 10.1±0.8 nm.

[0053] Example 5

[0054] This example provides a Pt nanodendrite, and the specific preparation method is as follows:

[0055] Into a 100 mL round bottom flask, 50 mL of 1,3-propanediol was added, followed by 0.5 mmol of potassium hexachloroplatinate, 5 mmol of polyvinylpyrrolidone (K value of 29-32, molecular weight calculated based on monomer), and then the mixture was stirred at 110°C for 0.5 h at a rotation speed of 1500 r / min, followed by the addition of 3.5 mmol of potassium bromide, and then the mixture was stirred for 2.5 h, followed by the increase of the temperature to 130°C, and then the mixture was continuously stirred and heated for 24 h, and then the product was added to 150 mL of acetone, and then the mixture was centrifuged at a rotation speed of 8000 r / min for 10 min, and then the obtained precipitate was washed with water and acetone once and then centrifuged, and then the obtained Pt nanodendrites were dispersed in ethanol for storage.

[0056] Figure 3 A transmission electron microscope image of the Pt nanodendrites prepared in this example and having a size of 18.1±3.0 nm is shown in FIG. 4. Figure 3 As can be seen from FIG. 4, the Pt nanocrystals prepared in this example have a dendritic structure with a diameter of 18.1±3.0 nm.

[0057] Example 6

[0058] This example provides a Rh nanocrystal, and the specific preparation method is as follows:

[0059] Into a 100 mL round-bottom flask, 50 mL of triethylene glycol was added, followed by 0.5 mmol of sodium hexachlororhodate, 5 mmol of polyvinylpyrrolidone (K value of 29-32, molecular weight calculated by monomer), and then 2.5 mmol of sodium chloride or potassium chloride or rubidium chloride or cesium chloride was added, respectively, after preheating at 110°C for 0.5 h at a stirring speed of 1500 r / min. The preheating was continued for 2.5 h at 110°C, and then the temperature was raised to 160°C, and the stirring and heating were continued for 24 h. After the reaction was completed, the obtained product was added to 150 mL of acetone, mixed thoroughly, and then centrifuged at a speed of 8000 r / min for 10 min. After washing with water and acetone once and centrifuging, the obtained precipitate was the Rh nanocube crystal, which was dispersed in ethanol for storage.

[0060] Figure 4 The transmission electron microscope images of the Rh nanocrystals prepared by adding sodium chloride or potassium chloride or rubidium chloride or cesium chloride, respectively, for this example are shown in FIG. 1. Figure 4 As can be seen from FIG. 1, the Rh nanocrystals have different morphologies and sizes due to the different types of solubility adjustors. When cesium chloride is added, the sodium hexachlororhodate is almost insoluble in the liquid organic reducing agent, resulting in very little reaction degree and very low product yield, and thus the size of the nanocrystals is small. In general, as the solubility of the complex salt in the organic reducing agent decreases, the size of the Rh nanocrystals increases.

[0061] Comparative Example 1

[0062] This comparative example proposes a Rh nanocube crystal, and the specific preparation method is as shown below (which can refer to the literature “Kinetically Controlled Synthesis of Rhodium Nanocrystals with Different Shapes and a Comparison Study of Their Thermal and Catalytic Properties”, J. Am. Chem. Soc. 2021, 143, 6293-6302):

[0063] A solution containing 0.3 mmol of ascorbic acid, 0.9 mmol of potassium bromide and 1.2 mmol of polyvinylpyrrolidone (K value of 23-27, molecular weight calculated by monomer) in 13 mL of ethylene glycol was transferred to a three-necked flask, and was heated at 380 r / min for 1 h at 140 ℃, then a solution containing 0.12 mmol of sodium hexachlororhodate in 6 mL of ethylene glycol was added to the flask, the first 1.1 mL of ethylene glycol solution was added at a rate of 60 mL / h, and the remaining 4.9 mL of ethylene glycol solution was added at a rate of 4 mL / h, after 3 h of reaction, the solid product was collected by centrifugation, washed once with acetone and three times with a mixture of ethanol and acetone, to obtain Rh nanocubes with an average size of 4.5 nm.

[0064] Compared with the example, the solubility of potassium hexachlororhodate is much lower than that of sodium hexachlororhodate, so the potassium hexachlororhodate is in a solid solution state when added to triethylene glycol according to the amount added in the example, but the sodium hexachlororhodate is in a solution state when added to ethylene glycol according to the amount added in the example, but the size of the Rh nanocubes obtained by the former is much smaller than that in the example, and adjusting the amount added in the example cannot control the size of the Rh nanocubes obtained, and large-sized Rh nanocubes cannot be obtained.

[0065] Comparative Example 2

[0066] The present comparative example proposes a Rh nanocube, which is obtained by adding a precursor solution to the reaction system quantitatively through a syringe pump, so that small-sized Rh nanocubes can be grown in multiple steps, and the control of the crystal size is initially realized; the specific preparation method is as follows (which can be referred to the literature "Size-tunable rhodium nanostructures for wavelength-tunable ultraviolet plasmonics", Nanoscale Horiz., 2016, 1, 75-80):

[0067] 0.45 mmol of potassium bromide and 2 mL of ethylene glycol were added to a 20 mL flask, and heated at 160 ℃ for 40 min, then 0.045 mmol of rhodium (III) chloride hydrate and 0.225 mmol of polyvinylpyrrolidone were dissolved in 2 mL of ethylene glycol at room temperature, respectively, and the two solutions were simultaneously injected into the container at a rate of 1 mL / h through a double-channel syringe pump, after injection, the reaction mixture was kept at 160 ℃ for another 10 min, and then cooled to room temperature, to finally prepare Rh nanocubes with a size of 27 nm;

[0068] In a 20 mL glass container, 0.4 mL of the reaction mixture of Rh nanocube crystals with a size of 27 nm was mixed with 1.6 mL of ethylene glycol under stirring, the obtained reaction mixture was heated in an oil bath at 160°C for 40 min, 0.045 mmol of rhodium (III) chloride hydrate was dissolved in 2 mL of ethylene glycol, and 0.225 mmol of polyvinylpyrrolidone and 0.45 mmol of potassium bromide were co-dissolved in another 2 mL of ethylene glycol, the two solutions were simultaneously injected into the container at a rate of 1 mL / h by a syringe pump, after injection, the reaction mixture was kept at 160°C for another 10 min, and then cooled to room temperature. With this amount of Rh precursor, Rh nanocube crystals with a size of 59 nm can be prepared.

[0069] Compared with the examples, the former needs to be assisted by auxiliary equipment such as a syringe pump to obtain large-size Rh nanocube crystals, and also needs to be added to the reaction system by a syringe pump to drop the precursor solution in a quantitative manner, so that the small-size Rh nanocrystals are multi-step stacked and grown. Although the size control of the crystals is achieved to some extent, the complex preparation process increases the uncertainty of the crystal growth results, reduces the final yield of the product, and limits the scale-up synthesis of the size-controllable Rh nanocube crystals.

[0070] Comparative Example 3

[0071] The specific preparation method of the Rh nanocube crystals is as follows:

[0072] 50 mL of triethylene glycol was added to a 100 mL round-bottom flask, followed by the addition of 0.5 mmol of potassium hexachlororhodate, 5 mmol of polyvinylpyrrolidone (K value of 23-27, molecular weight calculated by monomer), and 5 mmol of sodium bromide. The mixture was preheated at 110°C with stirring at a speed of 1500 r / min until the complex salt was completely dissolved to form a clear solution. Then the temperature was raised to 150°C, and the heating was continued for 24 h with stirring. After the reaction was completed, the obtained product was added to 150 mL of acetone, and mixed thoroughly. The mixture was centrifuged at a speed of 8000 r / min for 10 min, and then washed with water and acetone once and centrifuged. The obtained precipitate was dispersed in ethanol for storage.

[0073] Comparative Example 3 and Example 1, the solubility of potassium hexachlororhodate in triethylene glycol is hardly reduced by sodium ions, while the solubility is sharply increased after the ligand exchange reaction between bromide ions and potassium hexachlororhodate. When the potassium hexachlororhodate is completely dissolved in triethylene glycol after the reaction, the gradual reaction and dissolution of the platinum group metal complex salt cannot be controlled during the entire reaction process, the complex salt is consumed in a short period of time, and a sustainable nanocrystal growth process is not formed, resulting in small-size Rh nanocube crystals.

[0074] Figure 5A transmission electron microscope image of Rh nanocubes with a size of 7.1±0.8 nm was prepared for the present comparative example, and the image is shown in Fig. 1. Figure 5 It can be seen that the Rh nanocrystals prepared in the present comparative example are cubic structures with a side length of 7.1±0.8 nm.

[0075] Comparative Example 4

[0076] The present comparative example provides a Rh nanocube, and the specific preparation method is as follows:

[0077] 50 mL of triethylene glycol was added to a 100 mL round-bottom flask, and then 0.5 mmol of potassium hexachlororhodate and 5 mmol of polyvinylpyrrolidone (K value of 23-27, molecular weight calculated based on monomer) were added. The mixture was heated to 150°C at a rotation speed of 1500 r / min, and the stirring was continued for 24 h. After the reaction was completed, the product was added to 150 mL of acetone, and the mixture was centrifuged at a rotation speed of 8000 r / min for 10 min. The precipitate was washed with water and acetone once and then centrifuged. The precipitate was dispersed in ethanol for storage.

[0078] Comparative Example 4 and Example 1, when not preheated, the entire reaction process cannot control the gradual reaction and dissolution of the platinum group metal complex salt, and a large number of small particles of the complex salt stably dispersed in the liquid organic reducing agent are not formed during the reaction process, resulting in a smaller size of only 16.7 nm.

[0079] Figure 6 A transmission electron microscope image of Rh nanocubes with a size of 16.7±2.2 nm was prepared for the present comparative example, and the image is shown in Fig. 1. Figure 6 It can be seen that the Rh nanocrystals prepared in the present comparative example are cubic structures with a side length of 16.7±2.2 nm.

[0080] Application Example

[0081] The present application provides an application of the above Rh nanocube in catalyzing complete oxidation of low-concentration methane, which specifically comprises:

[0082] The ethanol dispersion of the Rh nanocube prepared in Example 4 was mixed with 80-100 mesh quartz sand by the impregnation method, and the mixture was stirred under heating conditions until the ethanol was completely volatilized. The obtained solid mixture was calcined at 600°C overnight, and the catalyst was obtained after cooling. The content of metal Rh in the catalyst was about 3 wt%;

[0083] The reaction was carried out in a fixed bed quartz reactor (8 mm in diameter and 450 mm in length) under normal pressure at a feed flow rate of 100 mL / min, containing 0.2 vol% methane, 6 vol% oxygen and the rest nitrogen; 25 mg of the above catalyst was mixed with a certain amount of 80-100 mesh quartz sand to form a reaction bed layer with a height of about 3 cm and a volume of about 1.5 cm 3 The temperature of the catalytic bed was measured by a thermocouple and the feed and reaction products were analyzed on-line by a gas chromatograph.

[0084] The catalytic results show that the conversion of methane is 10% at about 350°C, 50% at about 460°C and 90% at about 550°C.

[0085] The above description is merely preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can make equivalent replacements or changes to the technical solutions and the inventive concept of the present application within the technical scope disclosed by the present application, which should be covered by the protection scope of the present application.

Claims

1. A method for preparing platinum group metal nanocrystals with uniform size, narrow distribution and good stability, characterized in that, The preparation method comprises the following steps: heating and reducing the solid-phase platinum group metal complex salt in the liquid-phase organic reducing agent, so that the solid-phase platinum group metal complex salt is gradually reacted and dissolved, and the platinum group metal nanocrystal is obtained; the solid-phase platinum group metal complex salt is at least one of potassium hexachlororhodate, ammonium hexachlororhodate, potassium hexachloroplatinate, sodium hexachloroiridate, sodium hexachloroosmate, potassium tetrachloropalladite or ammonium hexachlororuthenate; the liquid-phase organic reducing agent is at least one of ethylene glycol, oligomeric ethylene glycol, polyethylene glycol, propylene glycol, oligomeric propylene glycol, polypropylene glycol or butanediol; the molar ratio of the solid-phase platinum group metal complex salt to the liquid-phase organic reducing agent is 0.25-10:1000; the heating and reducing specifically comprises the following steps: mixing the solid-phase platinum group metal complex salt, the liquid-phase organic reducing agent and the organic protective agent, preheating the mixture, then heating the mixture, so that the solid-phase platinum group metal complex salt is gradually reacted and dissolved, and the platinum group metal nanocrystal is obtained; the preheating temperature is 20-120℃, and the preheating time is 0.5-24h; the heating temperature is 130-200℃, and the heating time is 3-72h; the organic protective agent is polyvinylpyrrolidone; the K value of the polyvinylpyrrolidone is 13-32; and the molar ratio of the organic protective agent to the solid-phase platinum group metal complex salt is 5-20:

1.

2. The method according to claim 1, wherein the platinum group metal nanocrystals have a narrow size distribution and good stability. After the preheating of the mixture of the solid-phase platinum group metal complex salt, the liquid-phase organic reducing agent and the organic protective agent, the method further comprises the following step: adding a solubility adjusting agent to the mixture and continuing the preheating.

3. The method according to claim 2, wherein the platinum group metal nanocrystals have a narrow size distribution and good stability. The solubility adjusting agent is used to change the solubility of the solid-phase platinum group metal complex salt in the liquid-phase organic reducing agent.

4. The preparation method of the platinum group metal nanocrystal with uniform size, narrow distribution and good stability according to claim 3, wherein the solubility adjusting agent is an alkali metal salt or an ammonium salt.

5. The preparation method of the platinum group metal nanocrystal with uniform size, narrow distribution and good stability according to claim 4, wherein the alkali metal salt or the ammonium salt is at least one of cesium chloride, rubidium chloride, potassium nitrate, potassium sulfate, potassium chloride, potassium bromide, sodium bromide, ammonium bromide or ammonium chloride.

6. The preparation method of the platinum group metal nanocrystal with uniform size, narrow distribution and good stability according to claim 2, wherein the molar ratio of the solubility adjusting agent to the solid-phase platinum group metal complex salt is 1-40:

1.

7. A platinum group metal nanocrystal prepared by the preparation method according to any one of claims 1-6.

8. The platinum group metal nanocrystal according to claim 7 in the application of catalyzing the complete oxidation of low-concentration methane.

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