JP2011089143A - Method for producing mono-component system and bi-component system cubic type metal nanoparticle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing mono-component system and bi-component system cubic type metal nanoparticles, which enables control of the deposition part of a shell metal and the amount thereof with respect to a core-shell bicomponent system metal nanoparticles in which the core is made of platinum or palladium. <P>SOLUTION: A specific adsorption auxiliary and a water-soluble dibasic acid salt are mixed in a solution of an ionic compound of platinum or palladium, and the mixture is deposited under reduction condition. For example, a core metal is platinum, a shell metal is silver, and, by controlling a hydrogen reduction time, a reduction temperature, an Ag/Pt mol ratio and a dibasic aid salt/Pt mol ratio, the part at which Ag is deposited on the surfaces of cubic platinum nanoparticles and the amount thereof are controlled. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for producing metal nanoparticles used in catalysts, electronic devices and the like.

Platinum is used as an exhaust gas purification for automobiles and as a fuel cell catalyst. However, since it is rare and expensive, improvement in catalytic performance per weight has been an issue.
The platinum nanoparticles usually have a cubic octahedral structure, and the catalytic activity differs greatly between the (111) plane and the (100) plane exposed on the surface. For example, the activity of the electrode catalyst of the oxygen electrode of the fuel cell is (100) Since the surface is said to be higher than the (111) surface, a reduction in the amount of platinum used can be expected by using cubic platinum nanoparticles whose entire surface is a (100) surface.
The same is expected for palladium nanoparticles.

  The binary core / shell nanoparticles were obtained by self-organizing rhodium, palladium, and platinum nanoparticles having a particle diameter of 2 nm onto the surface of spherical silver nanoparticles having a particle diameter of 10 nm or more. Nanoparticles are described in, for example, Non-Patent Document 1, but there is no report regarding the production of cubic binary nanoparticles having higher catalytic activity.

In Non-Patent Document 2, binary metal nanoparticles are obtained by precipitating Pd particles on cubic platinum nanoparticles by a liquid phase method.
Specifically, after adding long-chain alkylammonium ions to an aqueous chloroplatinic acid (K ₂ PtCl ₄ ) solution to obtain cubic platinum nanoparticles by sodium borohydride reduction, while maintaining the solution at pH 9, K ₂ PdCl ₄ and long-chain alkylammonium ions are added, and ascorbic acid reduction is performed to grow Pd on the cubic platinum nanoparticles.
When the obtained nanoparticles are used in the formic acid oxidation reaction, poisoning resistance is improved because Pd suppresses the production of carbon monoxide, which is a side reaction.
However, the nanoparticles obtained by this method are obtained by growing cubic Pd nanoparticles on cubic platinum nanoparticles, and do not control the precipitation site of Pd.

In Non-patent Document 3, binary metal nanoparticles whose shape is controlled by epitaxially growing Pd and Ag on octahedral gold nanoparticles are obtained.
In this method, a dilute solution of nanoparticles obtained by reducing sodium borohydride with chloroauric acid (HAuCl ₄ ) using a long-chain alkylammonium ion as a protective agent is used as a seed nucleus, and the seed nucleus is slowly added in the presence of HAuCl _4. By growing, octahedral gold nanoparticles are obtained.
Then, cubic type Au-Pd core-shell metal nanoparticles or Au-Ag core-shell metal nanoparticles are prepared by adding H ₂ PdCl ₄ or AgNO ₃ .
Although the shape of the nanoparticles obtained by this method is controlled, the entire surface is covered with the second component (here, Pd, Ag), and the precipitation site of the second component is not controlled.

Non-Patent Document 4 prepares carbon-supported Au—Co alloy nanoparticles by mixing gold chloride (AuCl ₃ ) and cobalt chloride (CoCl ₂ ) into a carbon support and performing thermal decomposition under a hydrogen atmosphere and high temperature.
A platinum monolayer film is formed by underpotential deposition on Au phase-separated to the particle surface by high-temperature treatment.
However, the produced carbon-supported nanoparticles have many preparation steps, and the shape of the obtained nanoparticles is a cubic octahedron, and the shape of the nanoparticles themselves is not controlled.
Further, platinum as a shell metal covers the entire particle surface.

Non-Patent Document 5 reports a method for producing metal nanoparticles having rhodium as a core and platinum as a shell by ethylene glycol reduction of metal ions.
It is prepared by adding a PVP (polyvinylpyrrolidone) -protected rhodium nanoparticle solution prepared in advance to an ethylene glycol solution of platinum chloride (PtCl ₂ ) and heating and stirring in a nitrogen atmosphere.
During heating, a temperature increase from 80 ° C. to 130 ° C. is necessary, and the temperature increase rate needs to be controlled depending on the size of Rh used as the core.
However, the shape of the nanoparticles obtained by this method is a cubic octahedron, the shape of the nanoparticles themselves is not controlled, and the shell metal platinum covers the entire particle surface.
Moreover, since PVP which is a polymer is used as a protective agent, there is a concern about a decrease in catalytic activity.

Spontaneous Formation of Core / Shell Bimetallic Nanoparticles: A Calorimetric Study, J. Phys. Chem. B, 109 (2005), 16326-16331 Localized Pd overgrowth on Cubic Pt Nanocrystals for Enhanced Electrocatalytic Oxidation of Formic Acid, J. Am. Chem. Soc. 130 (2008), 5406-5407 Epitaxial Growth of Heterogeneous Metal Nanocrystals: From Gold Nano-octahedra to Palladium and Silver Nanocubes, J. Am. Chem. Soc. 130 (2008), 6949-6951 Platinum Monolayer on Nonnoble Metal-Noble Metal Core-Shell Nanoparticle for O2 Reduction, J. Phys. Chem. B, 109 (2005), 22701-22704 Rh-Pt Bimetallic Catalysts: Synthesis, Characterization, and Catalysis of Core-Shell, Alloy, and Monometallic Nanoparticles, J. Am. Chem. Soc. 130 (2008), 17479-17486

  The present invention can control the size of cubic metal nanoparticles by using a novel protective agent, and can control the deposition site and amount of shell metal in the core-shell binary metal nanoparticles and An object of the present invention is to provide a method for producing binary cubic metal nanoparticles.

According to the present invention, a unitary cubic metal nanoparticle production method comprises mixing a specific adsorption aid and a water-soluble dibasic acid salt in a solution of an ionic compound of platinum or palladium, under reducing conditions. It is made to precipitate.
Here, the specific adsorbent is a chemical species that suppresses the growth of the (100) plane of particles that are generally intended to have a cubic octahedral structure and acts to grow the (111) plane. Ions and bromine ions are preferred, and examples of the addition method include sodium salts and potassium salts.
The dibasic acid salt is not particularly limited as long as it is a water-soluble compound that acts as a protective agent for metal nanoparticles and has two functional groups per molecule, but a typical example is a salt of a dicarboxylic acid.
Specifically, salts of alkylene dicarboxylic acids such as malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid and sebacic acid, and meta and para aromatics such as isophthalic acid and terephthalic acid An example is a salt of a dicarboxylic acid.
Particularly preferred are succinic acid salts.
This type of protective agent has a low molecular weight compared to conventional sodium polyacrylate, and can be easily removed from the surface of the metal nanoparticles to obtain an active surface.
In addition, the size and uniformity of the size of the cubic metal nanoparticles are facilitated.

As a second object, the core-shell metal nanoparticles, which are binary cubic metal nanoparticles according to the present invention, are obtained by using cubic metal nanoparticles in the production method according to claim 1. Precipitating as a core metal, and then adding any one of Ag, Au, Pd and Ru as a shell metal under reducing conditions to deposit the shell metal site-specifically on the core metal. Features.
When such a production method is adopted, for example, the core metal is platinum and the shell metal is silver, and the cubic reduction is achieved by controlling the hydrogen reduction time, reduction temperature, Ag / Pt mol ratio, and dibasic acid salt / Pt mol ratio. The site | part and quantity which Ag precipitates on the surface of a platinum nanoparticle can be controlled.
Here, for example, when a protective agent such as disodium succinate is used as the dibasic acid salt, it is presumed that one functional group is adsorbed on the surface of the core particle and the other functional group acts to take in the shell metal ion. Therefore, it seems that the deposition site | part to the core particle of shell metal was able to be controlled.
Moreover, as a result of evaluating the lattice spacing by X-ray diffraction and HRTEM, it is clear that Ag deposited on the surface of the cubic platinum nanoparticles exists in an AgI state and changes to Ag under electron beam irradiation or under vacuum conditions. Became.

In the present invention, compared to conventional sodium polyacrylate, by using a low molecular dibasic acid salt as a protective agent, cubic type metal nanoparticles can be produced and the ion concentration of platinum or palladium is specific. By adjusting the amount of the adsorbing aid, the metal nanoparticles can be made smaller and uniform in size.
In addition, when shell metal ions are added after the precipitation of the cubic core metal particles, the use of dibasic acid salt as a protective agent makes it possible to control the deposit site and amount of the shell metal.
The generated cubic nanoparticles have a van der Waals interaction with a flat surface larger than that of the usually obtained cubic octahedral particles, so that the particles can be easily and temporarily arranged.

The precipitation transition of Ag is shown. The TEM observation result of the cubic type platinum nanoparticle controlled to about 10 nm in Example 1 is shown. The TEM observation result of the cubic type platinum nanoparticle controlled to about 5 nm in Example 2 is shown. The TEM observation result of the cubic type platinum nanoparticle controlled to about 2.5 nm in Example 3 is shown. The TEM observation result which changed ratio of Ag / Pt in Example 6 is shown. The XRD spectrum of a binary metal nanoparticle is shown. The HRTEM result of a binary metal nanoparticle is shown.

  Hereinafter, the method for producing cubic metal nanoparticles according to the present invention will be described in detail with respect to a unitary platinum nanoparticle, but it can also be applied to a unitary palladium nanoparticle, and the core-shell metal nanoparticle may be a core metal. It is also possible to adopt platinum or palladium as the shell metal and use Ag, Au, Ru, or Pd as the shell metal (excluding Pd as the shell when the core is palladium).

When depositing Ag on the platinum core particles, control the hydrogen reduction time and temperature, the amount of Ag precursor added, the Ag / Pt mol ratio, and the succinic acid / Pt mol ratio, thereby controlling the Ag precipitation site and amount. can do.
Although specific conditions also change depending on the combination of the mutual conditions, the following can be said generally when other conditions are standard values (median values).
The precipitation amount of Ag can be controlled at a hydrogen reduction time in the range of 20 to 40 minutes, and when it exceeds 40 minutes, the thickness of Ag covering the entire platinum nanoparticles increases.
The hydrogen reduction temperature is preferably controlled in the range of 20 to 80 ° C.
The Ag precursor to be added is preferably controlled in the range of 0.1 to 5 in terms of Ag / Pt mol ratio, and is preferably controlled in the range of 1 to 4 in terms of succinic acid / Pt mol ratio.
These relationships are summarized in FIG.
Ag begins to precipitate at the corners of the cubic platinum nanoparticles due to the action of succinic acid, and when Ag grows, the edges become connected, and then precipitates on the surface, increasing the thickness.

Example 1
Sodium succinate-protected cubic platinum nanoparticles control (Preparation of disodium succinate-protected cubic platinum nanoparticles with a platinum ion concentration of 1.0 mM (particle size 10 nm))
Pt: disodium succinate: NaI = 1: 2: 10 (molar ratio)
(1) 5 ml of K ₂ PtCl ₄ solution (20 mM), 0.15 g of NaI powder, 4 ml of sodium succinate (50 mM) and 91 ml of pure water were placed in an insulator-type flask.
(2) The temperature of the oil bath was set to 40 ° C. and stirred.
(3) Bubbling with Ar gas for 30 minutes.
(4) Bubbling with H ₂ gas for 20 minutes.
(5) The mixture was stirred for 5 to 15 hours in an H ₂ gas atmosphere.
In order to obtain a solution in which particles are dispersed, stirring is preferably stopped in about 5 hours.
The TEM grid was prepared by placing one drop of the solution on the grid with a Pasteur and air drying overnight.
The TEM observation was performed at a magnification of 100,000 to 400,000, the platinum nanoparticle diameter and shape were measured, and the average particle diameter and the proportion of cubic platinum nanoparticles were calculated.
The results are shown in Table 1.
The results of Example 1 and Examples 2 to 5 described later are summarized in Table 1.
When the particles were taken out for measurement, the following operation was performed.
When the particles had already precipitated, it was filtered using filter paper and washed several times with pure water.
If dispersed in a solution, the solution is concentrated with an evaporator, then filtered using filter paper, washed several times with pure water, or placed in a centrifuge tube and centrifuged (6,000 rpm, 5-10 minutes) and washed several times with pure water.
The TEM photograph is shown in FIG.

(Example 2)
(Preparation of disodium succinate protected cubic platinum nanoparticles with a platinum ion concentration of 0.5 mM (particle size 5 nm))
Pt: disodium succinate: NaI = 1: 2: 10 (molar ratio)
(1) An insulator-type flask was charged with 2.5 ml of a K ₂ PtCl ₄ solution (20 mM), 0.075 g of NaI powder, 2 ml of sodium succinate (50 mM), and 95.5 ml of pure water.
(2) The temperature of the oil bath was set to 60 ° C. and stirred.
(3) Bubbling with Ar gas for 30 minutes.
(4) Bubbling with H ₂ gas for 5 minutes.
(5) H ₂ gas was released.
(6) The mixture was stirred at room temperature for 5 minutes to 15 hours.
The color of the solution is slightly brown compared to 10 nm cubic platinum nanoparticles.
A TEM photograph at this time is shown in FIG.
When the stirring time was 15 hours, the cubic particles were rounded.

(Example 3)
(Preparation of disodium succinate-protected cubic platinum nanoparticles with a platinum ion concentration of 0.25 mM (particle size 2.5 nm))
Pt: disodium succinate: NaI = 1: 2: 10 (molar ratio)
(1) 1.25 ml of K ₂ PtCl ₄ solution (20 mM), 0.0375 g of NaI powder, 1 ml of sodium succinate (50 mM), and 97.8 ml of pure water were placed in an insulator-type flask.
(2) The temperature of the oil bath was set to 60 ° C. and stirred.
(3) Bubbling with Ar gas for 30 minutes.
(4) Bubbling with H ₂ gas for 5 minutes.
(5) The mixture was stirred at room temperature for 5 minutes to 15 hours.
A TEM photograph at this time is shown in FIG.
At a temperature of 60 ° C., 2.5 nm nanoparticles were obtained with a selectivity of 52%.

(Examples 4 and 5)
In the operation (1) of Example 1, 4 ml of 50 mM aqueous solution of adipic acid or azelaic acid was added instead of succinic acid and stirred.
As shown in Table 1, the average particle size of the obtained cubic platinum nanoparticles is 10.2 nm for succinic acid, 14.8 nm for adipic acid, and 16.3 nm for azelaic acid, which increases the alkyl chain length of the dicarboxylic acid. As the particle size increased.

(Example 6)
Compounding of disodium succinate protected cubic platinum nanoparticles with silver Pt: disodium succinate: NaI = 1: 2: 10 (molar ratio)
(1) A three-necked flask was charged with 5 ml of a K ₂ PtCl ₄ solution (20 mM), 0.15 g of NaI powder, 4 ml of sodium succinate (50 mM), and 86 ml of water.
(2) The temperature of the oil bath was set to 40 ° C. and stirred.
(3) Bubbling with Ar gas for 30 minutes.
(4) Bubbling with H ₂ gas for 20 minutes.
(5) A predetermined amount of AgNO ₃ (20 mM) was added by syringe.
(6) The mixture was stirred under an H ₂ gas atmosphere.

(Ag amount change)
The amount of the AgNO ₃ aqueous solution added in (5) was changed, that is, the Ag / Pt ratio was changed.
Ag / Pt = 1/10, 1/5, 1, 2, and 4 are conditions in which 0.5 ml, 1 ml, 5 ml, 10 ml, and 20 ml of an AgNO ₃ aqueous solution were added in the operation (5), respectively.
It was confirmed from the TEM image and the EDX spectrum that the greater the amount of Ag, the greater the amount of Ag covering the cubic platinum nanoparticles.
TEM results are shown in FIG. 5 and EDX results are shown in Table 2.
First, it is considered that Ag precipitates at the corners of cubic platinum nanoparticles and gradually covers the whole.
From the results of the EDX spectrum of Table 2, it can be considered that iodine is easily attached to Ag or AgI is precipitated because the amount of iodine is increased as the amount of precipitated Ag is increased.

(Examples 7 and 8)
(Change in amount of protective agent)
Preparation was performed by changing to disodium succinate / Pt = 1, 4 in the procedure (1) of Example 6.
Table 3 summarizes the EDX results of Examples 6 to 8 and Examples 9 and 10 described below.
As the amount of the protective agent increased, the amount of Ag deposited on the cubic platinum nanoparticles increased.
Under the condition of disodium succinate / Pt = 4, Ag covered the entire cubic platinum nanoparticles and was rounded, and the shape of the original cubic platinum nanoparticles could not be confirmed.
It can be seen that disodium succinate is closely related to the precipitation of Ag on the cubic platinum nanoparticles.

(Examples 9 and 10)
(Temperature change)
Experimental Example 6 After operation (5) (after adding Ag), the temperature was changed to 25 and 80 ° C.
Under the condition of 25 ° C., nanoparticles in which Ag was deposited in a site-specific manner were also seen from the TEM image, but nanoparticles containing only Ag were also seen.
From the results of EDX shown in Table 3, a large amount of Ag is contained at 25 ° C.
On the other hand, at 80 ° C., the amount of Ag deposited on the cubic platinum nanoparticles is large and aggregation proceeds.
At low temperatures, the succinic acid on the cubic platinum is stabilized, preventing the precipitation of Ag on the platinum, and the succinic acid becomes more unstable by raising the temperature. At 40 ° C. in Example 6, the succinic acid density is low. It is thought that it was deposited from the corner and deposited so as to cover the entire nanoparticle at 80 ° C.
40 ° C. is the optimum condition for site-specific precipitation of Ag.

Next, the results of XRD measurement and HRTEM measurement of Ag deposited on the corners of the cubic platinum nanoparticles are shown in FIGS.
From the XRD spectrum, in addition to the peak derived from Pt metal, a peak corresponding to β-AgI was observed.
The interstitial distance of the Pt-Ag cubic binary metal nanoparticles estimated by HRTEM is 0.195 nm at the center of the cube and 0.202 nm at the corner, and these distances are respectively Pt (200) of bulk metal. And Ag (200).
Therefore, although Ag seed | precipitates as AgI on the cubic type Pt nanoparticle, it is guessed that Ag ion was reduce | restored to metal Ag by electron beam irradiation for TEM observation.

  The one-dimensional or binary cubic metal nanoparticles obtained by the production method according to the present invention can be miniaturized to a particle size of 2.5 to 3 nm, and have a high catalytic activity because of the uniform cubic shape. In addition, it is expected to be used for electronic devices because it is easy to parallel.

Claims (6)

In a solution of an ionic compound of platinum or palladium,
A specific adsorption aid,
A method for producing cubic metal nanoparticles, comprising mixing water-soluble dibasic acid salts and precipitating them under reducing conditions.   The method for producing cubic metal nanoparticles according to claim 1, wherein the specific adsorption aid is a sodium or potassium salt of iodine or bromine.   The method for producing cubic metal nanoparticles according to claim 1 or 2, wherein the water-soluble dibasic acid salt is a salt of succinic acid.   Cubic metal nanoparticles are deposited as a core metal by the production method according to any one of claims 1 to 3, and then ions of any one of Ag, Au, Pd and Ru different from the core metal under reducing conditions A method for producing core / shell metal nanoparticles, wherein the shell metal is site-specifically deposited on the core metal by adding a shell metal as a shell metal. The core metal is platinum and the shell metal is silver,
By controlling the hydrogen reduction time, the reduction temperature, the Ag / Pt mol ratio, and the dibasic acid salt / Pt mol ratio, the site and amount of Ag deposited on the surface of the cubic platinum nanoparticles are controlled. The manufacturing method of the core-shell metal nanoparticle of Claim 4.   Cubic platinum or palladium single or binary core / shell metal nanoparticles obtained by the production method according to claim 1.