JP2007239053A - Method for producing alloy nanoparticle and hydrogen storage alloy - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a new production method where alloy nanoparticles in which palladium and metal atoms other than that are solid-soluted to form a single kind of crystalline lattices even without applying a hydrogen absorption/discharge cycle, and to provide a palladium/rhodium (Pd/Rh) alloy in which phase separation is not caused even if the same cycle is applied. <P>SOLUTION: A solution comprising a Pd salt, a metal salt as an Rh salt or a gold (Au) salt, an organic high polymer, and polyhydric alcohol is heated, so as to simultaneously reduce Pd ions included in the Pd salt and metal ions included in the metal salt. Then, the alloy nanoparticles in which Pd atoms and metal atoms are solid-soluted to form a single kind of crystal lattices is obtained. Further, as the alloy which can be produced by this method, the hydrogen storage alloy composed of the alloy nanoparticles comprising Pd and Rh, and in which the Pd and Rh form a single kind of crystal lattices, and the single kind of crystal lattices are retained even after the application of a hydrogen absorption/discharge cycle is provided. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing alloy nanoparticles, and further relates to a new hydrogen storage alloy that can be produced by this method.

  Hydrogen has a low environmental impact because the product after combustion is water, and is expected as a future main fuel. Storage of hydrogen requires a hydrogen storage alloy that reversibly absorbs / releases hydrogen. Palladium (Pd) has a hydrogen storage property, and it is known that the hydrogen storage amount increases by alloying with rhodium (Rh).

  The inventor previously proposed a new method for producing alloy nanoparticles (Patent Document 1). This production method changes the nanoparticle structure from the core-shell type to the solid solution type by applying a hydrogen absorption / release cycle to the nanoparticles having a core-shell type structure obtained by the alcohol reduction method. is there. The nanoparticles having a core-shell structure are formed by first forming metal nanoparticles as cores (Pd nanoparticles in the examples of Patent Document 1) by an alcohol reduction method, and then forming a shell on the surface of the metal nanoparticles. It is produced by depositing metal (in this example, platinum (Pt) shell).

  Preparation of core-shell type nanoparticles by the alcohol reduction method has been tried for various metals (Patent Document 2; in particular, [0003] [0004]). According to Patent Document 2, when palladium chloride and platinum chloride are simultaneously reduced by an alcohol reduction method, nanoparticles having a core made of Pt and a shell made of Pd are formed ([0003]). In addition, nanoparticles having a core made of gold (Au) and a shell made of Pt can be obtained by simultaneous reduction by an alcohol reduction method ([0004]). Since polyvinylpyrrolidone (PVP) used in the alcohol reduction method has a stronger interaction with platinum than gold, even if Au and Pt are reduced simultaneously, Au is reduced before Pt, and Au core / Pt shell A structure is generated.

There is also a report on simultaneous reduction of Pd salt and Rh salt by the alcohol reduction method (Non-patent Document 1). Referring to the analysis result of EXAFS of the nanoparticles obtained in Non-Patent Document 1, the coordination number of Pd is larger than the coordination number of Rh (Table II-V). This indicates that Pd forms a core and Rh forms a shell.
JP 2005-272970 A JP 9-225317 A Harada et al., “Structural Analysis of Polymer Protected Palladium / Rhodium Bimetallic Clusters Using Exhausts”, The Journal of Physical Chemistry, American Chemical Society, 1993, 97 Volume 41, 10741-1049

  The nanoparticles reported above are expected to be used as hydrogen storage alloys, catalysts and the like. However, as disclosed in Patent Document 1, in order to obtain solid solution-type nanoparticles having a single crystal lattice, it was necessary to apply a hydrogen absorption / release cycle a plurality of times. According to Patent Document 1, solid solution type nanoparticles have better hydrogen storage properties than core / shell type nanoparticles. Therefore, considering the use as a hydrogen storage alloy, it is desirable that the nanoparticles are of a solid solution type from the stage of production.

  Even in applications other than hydrogen storage alloys, it is complicated to apply a hydrogen absorption / release cycle to core / shell type nanoparticles in order to achieve conversion to a solid solution type, resulting in an increase in manufacturing costs.

  In addition, as described above, the Pd / Rh alloy has a larger hydrogen storage capacity than Pd, but there is a problem that the alloy shows a tendency of phase separation due to the hydrogen absorption / release cycle. When phase separation occurs, the effect of increasing the amount of hydrogen occlusion by alloying with Rh is lost. The Pd / Rh alloy also has a problem that the hydrogen storage pressure increases due to alloying with Rh.

  The object of the present invention is to obtain alloy nanoparticles in which Pd atoms and metal atoms other than Pd are in solid solution to form a single kind of crystal lattice without applying a hydrogen absorption / release cycle. It is to provide a new manufacturing method. Another object of the present invention is to provide a hydrogen storage alloy comprising a Pd / Rh alloy that does not phase separate even when a hydrogen absorption / release cycle is applied.

  The present invention includes a Pd ion contained in a Pd salt and the above metal salt by heating a solution containing a Pd salt, a metal salt that is an Rh salt or an Au salt, an organic polymer, and a polyhydric alcohol. A method for producing alloy nanoparticles is provided in which metal nanoparticles are simultaneously reduced to obtain alloy nanoparticles in which a Pd atom and a metal atom derived from the metal ion are in solid solution to form a single type of crystal lattice.

Another aspect of the present invention is an alloy nanoparticle containing Pd and Rh. In this alloy nanoparticle, Pd and Rh form a single kind of crystal lattice, and a hydrogen absorption / release cycle is formed in the alloy nanoparticle. The present invention provides a hydrogen storage alloy in which a single type of crystal lattice is maintained even after application of. Specifically, a hydrogen absorption / release cycle for determining whether or not a single type of crystal lattice is maintained is specifically a treatment under the conditions described later (2 hours in a hydrogen atmosphere at 573 K and 20 atm (2 MPa)). And then evacuating to 7 × 10 ⁻⁴ Pa).

  According to the present invention, alloy nanoparticles having a single type of crystal lattice (in other words, completely solid solution type) can be obtained without applying a hydrogen absorption / release cycle. Polyhydric alcohols are considered to have a stronger reducing action than monohydric alcohols typified by ethanol. When this strong reduction action is applied to the simultaneous reduction of palladium ions and specific metal ions, alloy nanoparticles in a completely solid solution type can be obtained without applying a hydrogen absorption / release cycle.

  Further, according to the present invention, it is possible to provide a hydrogen storage alloy composed of a Pd / Rh alloy that maintains the effect of alloying without phase separation even after applying a hydrogen absorption / release cycle.

  Even if a strong interaction of polyhydric alcohol is applied, a metal having a strong tendency to separate from Pd, such as iridium (Ir), cannot form solid solution type alloy nanoparticles with Pd. The metals such as Rh described above have a high compatibility with Pd to some extent, and it is considered that solid solution type alloy nanoparticles are formed when the strong reducing power of polyhydric alcohol is applied.

  The alloy nanoparticles according to the present invention are produced by a so-called alcohol reduction method. As conventionally known, the alcohol reduction method is a method for producing nanoparticles in which metal ions are reduced in a solution containing alcohol in the presence of an organic polymer.

  As the organic polymer, a water-soluble polymer is preferable, and specifically, a polymer having a cyclic amide structure such as polyvinylpyrrolidone (PVP) is preferable. For example, polyvinyl alcohol, polyvinyl ether, polyacrylate, polyacrylonitrile, or the like may be used.

  Alcohol acts as a reducing agent in solution. In the present invention, a polyhydric alcohol typified by ethylene glycol is used. Polyhydric alcohols can exhibit a higher reducing action than monohydric alcohols. Examples of the polyhydric alcohol include ethylene glycol, propylene glycol, glycerin and the like.

  The temperature at which the solution is heated to reduce the metal ions is 90 ° C or higher, for example, 90 ° C to 198 ° C, more preferably 90 ° C to 150 ° C. In order to obtain alloy nanoparticles that are completely solid solution by simultaneous reduction, it is desirable to reduce metal ions in as short a time as possible. The time for heating at the above temperature is, for example, within 1.5 hours, preferably within 1 hour. This is because heating in a short time tends to provide alloy nanoparticles that are completely in a solid solution type.

  When the amount of the organic polymer in the solution is relatively increased, the particle size of the deposited nanoparticles becomes small, and this can be used to control the particle size of the nanoparticles. By adjusting the concentration of the metal salt to be added, it is possible to control the amount of the deposited metal and thus the particle size of the nanoparticles. The uniformity of the composition of the obtained particles is also high. Thus, the alcohol reduction method is excellent in controllability such as particle size, and is suitable as a method for producing nanoparticles.

  The particle diameter of the alloy nanoparticles according to the present invention is not particularly limited, but is preferably 100 nm or less, such as 0.5 nm to 100 nm, more preferably 1 nm to 100 nm, particularly 2 nm to 50 nm, especially 2 nm to 20 nm, preferably 10 nm or less. There may be.

  The alloy nanoparticles obtained by the present invention are preferably used in a state of being coated with an organic polymer, particularly when the particle size is small. This state is effective for preventing oxidation of fine particles. The organic polymer used as the protective agent is not particularly limited, and various polymers used in the alcohol reduction method may be used as they are.

  The hydrogen storage alloy according to the present invention may be obtained by using an Rh salt as a metal salt in the production method of the present invention. In this hydrogen storage alloy, Pd and Rh are in solid solution at the atomic level before application of the hydrogen absorption / release cycle.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the following examples are only examples of the embodiments of the present invention, as described above in this section, and do not limit the present invention.

(Synthesis of Pd / Rh alloy nanoparticles)
0.111 g (1 mmol) of PVP was added to 20 ml of ethylene glycol and dissolved by stirring with a magnetic stirrer to prepare a PVP ethylene glycol solution. Meanwhile, 0.0133 g of palladium nitrate (II) n hydrate (containing 39.9% by weight of water) (0.05 mmol) and 0.0132 g of rhodium (III) chloride trihydrate were dissolved in 2 ml of water. Thus, a Pd—Rh mixed aqueous solution was obtained. The Pd—Rh mixed aqueous solution was added to the PVP ethylene glycol solution, and heating was started. When the temperature reached 95 ° C., the mixture was stirred for 1 hour to obtain a dark brown solution.

  After this solution returned to room temperature, it was transferred to a centrifuge tube, diethyl ether and acetone were added (sample solution: diethyl ether: acetone = 5: 4: 5, volume ratio), and this solution was centrifuged. Thereafter, the supernatant was discarded, and the resulting black precipitate was dissolved in 5 ml of ethanol. Further diethyl ether was added and the cloudy solution was centrifuged again. The supernatant was discarded again, 5 ml of acetone and 15 ml of diethyl ether were added, and the mixture was further centrifuged. The precipitate thus obtained was washed with acetone three times, vacuumed and dried to obtain Pd / Rh alloy nanoparticles.

(Observation using transmission electron microscope (TEM))
Pd / Rh alloy nanoparticles were observed by TEM. The TEM observation was performed with an acceleration voltage of 200 kV and a magnification of 100,000 times. For the average particle diameter, about 300 particles were selected from an arbitrary area in the TEM photograph, and the diameter (particle diameter) was measured. A TEM photograph is shown in FIG. 1, and the measurement results of the particle diameter are shown in FIG. From the statistical results shown in FIG. 2, the average particle size and standard deviation were calculated. The average particle diameter of the Pd / Rh alloy nanoparticles was 4.0 nm, and the standard deviation was 0.9 nm.

(Analysis by powder X-ray diffraction)
The Pd / Rh alloy nanoparticles obtained above were encapsulated in a 0.5 mmφ glass capillary. The measurement was performed using synchrotron radiation having a wavelength of 0.068812 nm. FIG. 3 shows the measurement results for Pd / Rh alloy nanoparticles together with Pd nanoparticles and Rh nanoparticles. From this result, it was confirmed that the Pd / Rh alloy nanoparticles had a single face-centered cubic (fcc) lattice. Pd nanoparticles and Rh nanoparticles were also obtained by the same alcohol reduction method as described above.

Furthermore, in order to observe the structural change accompanying the hydrogen absorption / release cycle, the Pd / Rh alloy nanoparticles obtained above were held at 573 K in a 2 MPa hydrogen atmosphere for 2 hours, and then 7 × 10 ⁻⁴ Pa. And the above cycle was applied once. The X-ray diffraction patterns before and after application of the hydrogen absorption / release cycle are shown in FIG. As shown in FIG. 4, after application of the hydrogen absorption / release cycle, the Pd / Rh alloy nanoparticles showed no phase separation tendency and the peaks that appeared in the X-ray diffraction pattern were rather sharp.

  For comparison, a hydrogen absorption / release cycle was applied once to the bulk Pd / Rh alloy under the same conditions as described above. X-ray diffraction patterns before and after application of this cycle are shown in FIG. In the bulk alloy, it can be seen that the peak was split by hydrogen treatment (for example, the peak indicated by the arrow in FIG. 5). This peak splitting indicates that Pd and Rh are phase separated.

  As described above, in the bulk alloy, the alloy tends to phase-separate by application of the hydrogen release / absorption cycle, but in the alloy nanoparticles, it was confirmed that the effect of Rh addition was not lost even by application of the same cycle. .

(Confirmation of hydrogen storage capacity)
A PCT (Hydrogen Pressure-Composition-Isotherms) curve was measured for the Pd / Rh alloy nanoparticles obtained above. For the measurement, a PCT automatic characteristic measuring device (manufactured by Suzuki Shokan) was used. The results are shown in FIG. The measurement temperature was 303K.

  In the case of a bulk Pd / Rh alloy foil produced by a melt quenching method or the like, it is known that the hydrogen occlusion pressure significantly increases with the addition of Rh. However, as shown in FIG. 6, in the alloy nanoparticles, a plateau region appears at a hydrogen pressure of 0.1 MPa (760 Torr) or less. As shown in FIG. 6, since the Pd / Rh alloy nanoparticles are still in the plateau region even at a hydrogen pressure of 1 MPa, it is considered that the hydrogen storage amount is larger under high pressure.

  According to the production method of the present invention, nanoparticles in which a plurality of types of metal atoms are alloyed at the atomic level can be obtained without applying a hydrogen absorption / release cycle. The present invention has high utility value in the fields of conductive pastes, catalysts, and hydrogen storage alloys. Furthermore, the present invention has great utility in the field of hydrogen storage alloys as providing a Pd / Rh alloy that does not show a tendency to phase separate even when applied with a hydrogen absorption / release cycle.

It is a TEM photograph of the Pd / Rh alloy nanoparticle produced in the Example. It is a figure which shows the particle size distribution of the Pd / Rh alloy nanoparticle produced in the Example. It is a figure which shows the X-ray-diffraction pattern of the Pd / Rh alloy nanoparticle, Pd nanoparticle, and Rh nanoparticle which were measured in the Example. It is a figure which shows the X-ray-diffraction pattern before and behind application of the hydrogen absorption / release cycle of the Pd / Rh alloy nanoparticle measured in the Example. It is a figure which shows the X-ray-diffraction pattern before and behind application of the hydrogen absorption / release cycle of a Pd / Rh bulk alloy measured in the Example. It is a figure which shows the PCT curve (hydrogen atmosphere pressure: 0.1 MPa, 1 MPa) of the Pd / Rh alloy nanoparticle measured in the Example together with the PCT curve of the Pd bulk.

Claims (6)

  A solution containing a palladium salt, a metal salt that is a rhodium salt or a gold salt, an organic polymer, and a polyhydric alcohol is heated, and palladium ions contained in the palladium salt and metal ions contained in the metal salt The alloy nanoparticle manufacturing method which obtains the alloy nanoparticle which contains a palladium atom and the metal atom derived from the said metal ion, and has a single kind of crystal lattice by reducing simultaneously.   The method for producing alloy nanoparticles according to claim 1, wherein the polyhydric alcohol is ethylene glycol.   The method for producing alloy nanoparticles according to claim 1, wherein the organic polymer is polyvinylpyrrolidone.   The method for producing alloy nanoparticles according to claim 1, wherein the metal salt is a rhodium salt.   The alloy nanoparticles comprising palladium and rhodium, wherein the palladium and rhodium form a single kind of crystal lattice in the alloy nanoparticles, and the single kind is also applied after applying a hydrogen absorption / release cycle to the alloy nanoparticles. A hydrogen storage alloy that maintains the crystal lattice.   Obtained by heating a solution containing a palladium salt, a rhodium salt, an organic polymer, and a polyhydric alcohol, and simultaneously reducing palladium ions contained in the palladium salt and rhodium ions contained in the rhodium salt. The hydrogen storage alloy according to claim 5.