阅读说明：本技术 纳米晶体复合体 (Nanocrystal composites ) 是由 都筑秀和 若江真理子 青木智 三好悟朗 阿部英树 于 2020-01-31 设计创作，主要内容包括：本发明的纳米晶体复合体(1)具备连结集合体(20)和负载于连结集合体(20)的纳米颗粒(30),上述连结集合体(20)是具有主表面(22)和端面(23)的2个以上的纳米晶体片(21)相互连结而成的。2个以上的纳米晶体片(21)分别为薄片状,2个以上的纳米晶体片(21)在主表面(22)间具有间隙G,间隙G配置成向连结集合体(20)的外侧开口。纳米颗粒(30)具有与2个以上的纳米晶体片(21)不同的金属元素,纳米颗粒(30)的视野面积相对于2个以上的纳米晶体片(21)的视野面积的比例为2％以上50％以下。(A nanocrystal composite (1) of the present invention comprises a connected assembly (20) and nanoparticles (30) supported by the connected assembly (20), wherein the connected assembly (20) is formed by connecting 2 or more nanocrystal sheets (21) having a main surface (22) and an end surface (23) to each other. The 2 or more nanocrystal sheets (21) are each in the form of a sheet, and the 2 or more nanocrystal sheets (21) have a gap G between the main surfaces (22), the gap G being disposed so as to open to the outside of the connection assembly (20). The nanoparticles (30) have a metal element different from the 2 or more nanocrystal sheets (21), and the ratio of the field area of the nanoparticles (30) to the field area of the 2 or more nanocrystal sheets (21) is 2% to 50%.)

1. A nanocrystal composite comprising a connected assembly and nanoparticles supported by the connected assembly, wherein the connected assembly is formed by connecting 2 or more nanocrystal sheets having a main surface and an end surface,

the more than 2 nano crystal plates are respectively in a sheet shape,

the 2 or more nanocrystal sheets have a gap between the major surfaces,

the gap is configured to be opened to the outside of the connection aggregate,

the nanoparticles have a metal element different from the 2 or more nanocrystal sheets,

the ratio of the field area of the nanoparticles to the field area of the 2 or more nanocrystal sheets is 2% to 50%.

2. The nanocrystal composite according to claim 1, wherein the nanoparticle has a particle diameter of 5nm or more and 100nm or less,

the nanoparticles are disposed on the major surface.

3. The nanocrystal composition of claim 1 or 2, wherein the 2 or more nanocrystal sheets are a first metal oxide and,

the nanoparticles are a second metal oxide different from the first metal oxide.

4. The nanocrystal composition of claim 3, wherein the first metal oxide is copper oxide.

5. The nanocrystal composite of claim 3 or 4, wherein the second metal oxide is a nanoparticle of cerium oxide or a mixture of cerium oxide and zirconium oxide.

6. The nanocrystal composition of any one of claims 1 to 5 for use in a redox catalytic reaction.

7. The nanocrystal composite according to any one of claims 1 to 5, which is used as a catalyst for exhaust gas purification.

Technical Field

The present invention relates to a nanocrystal composite, and more particularly, to a nanocrystal composite that can be repeatedly used while maintaining high catalytic activity in the purification of harmful gases such as NO and CO contained in automobile exhaust gas.

Background

In recent years, from the viewpoint of environmental problems, in order to reduce the toxicity of harmful gases such as CO and NO contained in automobile exhaust gas, these harmful gases can be efficiently converted into CO₂、N₂And the like, are attracting attention.

As such a catalyst, for example, a noble metal such as Pt, Pd, Rh, etc. is generally used. However, these noble metals are expensive, have limited resources, and have a problem of low throughput. Therefore, in order to improve the catalytic activity by a small amount, a method of making the surface area (surface area) on which a catalytic reaction occurs finer has been studied. That is, the diameter of the bulk metal catalyst is reduced from powder (powder) to fine crystals (micro crystals) and further to nanoparticles (nanoparticles) so that the surface area per unit amount (m) is reduced²/g) is increased, thereby enabling the catalyst to be improvedThe reaction amount is changed, and the catalytic activity is improved. As such a technique, a nanomaterial such as a nanosheet or nanoparticle made of Pt (platinum) has been reported (non-patent documents 1 to 4).

However, nanoparticles (primary particles) have a problem that they are easily aggregated with each other to form aggregated particles (secondary particles). When the nanoparticles are formed into aggregated particles, the surface area per unit amount becomes almost the same as that of the bulk metal catalyst, and the catalytic activity is also the same, so that the catalytic activity cannot be improved.

In order to solve the problem of the agglomerated particles, a medium has been studied in which nanoparticles composed of a noble metal such as Pt are dispersed in SiO₂And the like on the surface of the granular substrate. However, even if the nanoparticles are dispersed on the surface of the particulate matrix, the nanoparticles move and diffuse when used under high heat, and thus there is a problem that the nanoparticles are combined to form coarse particles. Since the combination of nanoparticles is coarsened, the surface area per unit amount becomes almost the same as that of the bulk, and the catalytic activity is also the same, so that the effect of improving the catalytic activity is not exhibited as in the case of the problem of aggregated particles.

In order to suppress a decrease in catalytic activity due to coarsening and coarsening of nanoparticles, a nano-single-crystal plate integrated catalyst (nanoflower) has been proposed in which nano-single-crystal plates having a specific surface of a specific single crystal as one surface are integrated so that the catalytic activity surfaces do not come into surface contact with each other between adjacent nano-single-crystal plates (patent document 1). Patent document 1 describes the following: by using the nano-single crystal plate integrated catalyst, even if the nanoparticles are combined, the catalytic active surfaces do not come into surface contact with each other, and a space (void portion) is secured in front of the catalytic active surfaces, so that the reduction of catalytic activity due to the coarsening of the combination of the nanoparticles can be suppressed, and the catalytic activity can be improved. Patent document 1 also describes the following: the catalytic reaction in the purification of the above-mentioned harmful gas can be efficiently performed by using a nano single crystal plate as CuO, which is a transition metal oxide, whose catalytic active surface is a (001) surface.

On the other hand, when a catalyst (nanoflower) integrated with a nano-single crystal plate is used for the purification of the harmful gas, it is desirable that the catalyst can be used while maintaining a high catalytic activity even when repeatedly exposed to a high temperature in order to efficiently perform a catalytic reaction.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2013-240756

Non-patent document

Non-patent document 1: joo, s.h.; choi, s.j.; oh, I; kwak, J; liu, Z; terasaki, o.; ryoo, r.; nature, 2001, 412, 169-

Non-patent document 2: wang, c.; daimon, h.; lee, y.; kim, j.; sun, s.; J.am.chem.Soc.2007, 129, 6974-6975

Non-patent document 3: wang, c.; daimon, h.; onodera, t.; koda, t.; sun, s.; angew.chem., int.ed.2008, 47, 3588-

Non-patent document 4: kijima, t.; nagatomo, y.; takemoto, h.; uota, m.; fujikawa, d.; sekiya, y.; kisshishita, t.; shimoda, m.; yoshimura, t.; kawasaki, h.; sakai, g.; adv.Funct.Mater.2009, 19, 1055-1058

Disclosure of Invention

Problems to be solved by the invention

The present invention has been made in view of the above circumstances, and an object thereof is to provide a nanocrystal composite that can maintain high catalytic activity well even when repeatedly exposed to high temperatures.

Means for solving the problems

As a result of intensive studies on the above problems, the present inventors have found that a nanocrystal composite comprising a connected aggregate of 2 or more nanocrystal sheets having a main surface and an end surface connected to each other and a nanoparticle having a metal element different from the 2 or more nanocrystal sheets, which is supported on the connected aggregate, and the nanoparticle supported on the connected aggregate can maintain high catalytic activity well even when the nanocrystal composite is repeatedly exposed to high temperatures, by appropriately controlling the amount of the nanoparticle supported on the connected aggregate.

That is, the gist of the present invention is configured as follows.

[1] A nanocrystal composite comprising a connected aggregate and nanoparticles supported by the connected aggregate, wherein the connected aggregate is formed by connecting 2 or more nanocrystal sheets each having a main surface and an end surface,

the above 2 or more nanocrystal sheets are respectively in the form of thin sheets,

the 2 or more nanocrystal sheets have a gap between the main surfaces,

the gap is configured to open to the outside of the connection aggregate,

the nanoparticles have a metal element different from the 2 or more nanocrystal sheets,

the ratio of the viewing area of the nanoparticles to the viewing area of the 2 or more nanocrystal sheets is 2% to 50%.

[2] The nanocrystal composite according to [1], wherein the nanoparticles have a particle diameter of 5nm to 100nm, and,

the nanoparticles are disposed on the major surface.

[3] The nanocrystal composite according to the above [1] or [2], wherein the 2 or more nanocrystal sheets are a first metal oxide, and,

the nanoparticles are a second metal oxide different from the first metal oxide.

[4] The nanocrystal composite according to [3], wherein the first metal oxide is copper oxide.

[5] The nanocrystal composite according to [3] or [4], wherein the second metal oxide is a cerium oxide nanoparticle or a nanoparticle of a mixture of cerium oxide and zirconium oxide.

[6] The nanocrystal composite according to any one of the above [1] to [5], which is used for redox catalytic reaction.

[7] The nanocrystal composite according to any one of the above [1] to [5], which is used as an exhaust gas purification catalyst.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a nanocrystal composite can be provided that can maintain high catalytic activity well even when repeatedly exposed to high temperatures.

Drawings

Fig. 1 is a schematic perspective view showing one embodiment of a nanocrystal composite of the present invention.

Fig. 2 (a) is an SEM image when the nanocrystal composite after the 1 st 600 ℃ catalyst evaluation in example 2 was observed at a magnification of 4000 times, and fig. 2 (b) is an SEM image when it was observed at a magnification of 70000 times.

Fig. 3 shows the results of X-ray crystal structure analysis of the nanocrystal composites after the 1 st 600 ℃ catalyst evaluation in example 2 and comparative example 2.

Fig. 4 is an SEM image of the nanocrystal composite after the 1 st evaluation of the 600 ℃ catalyst in example 3, observed at a magnification of 20000 times.

Fig. 5 is an SEM image of the nanocrystal composite after the 1 st evaluation of the 600 ℃ catalyst in comparative example 1, observed at a magnification of 20000 times.

Detailed Description

Hereinafter, a nanocrystal composite according to an embodiment of the present invention will be described with reference to the drawings.

< nanocrystal composite >

The nanocrystal composite of the present invention comprises a connected assembly and nanoparticles supported on the connected assembly, wherein the connected assembly is formed by connecting 2 or more nanocrystal sheets having a main surface and an end surface. The 2 or more nanocrystal sheets are each in the form of a sheet, and the 2 or more nanocrystal sheets have a gap between the main surfaces, and the gap is disposed so as to open to the outside of the joined assembly. The nanoparticles supported on the linked aggregate have a metal element different from that of the 2 or more nanocrystal sheets. In order to appropriately control the amount of nanoparticles supported on the connected aggregates, the ratio of the viewing area of the nanoparticles to the viewing area of 2 or more nanocrystal sheets is 2% to 50%.

Fig. 1 is a schematic perspective view showing an example of a nanocrystal composite according to an embodiment of the present invention. As shown in fig. 1, the nanocrystal composite 1 of the present invention has a connected assembly 20 in which 2 or more nanocrystal sheets 21 having a main surface 22 and an end surface 23 are connected to each other, and shows a flower shape. The connection state of the 2 or more nanocrystal sheets 21 is not particularly limited, and it is sufficient that the 2 or more nanocrystal sheets 21 are connected to form an aggregate.

The nanocrystal sheet 21 has a thin sheet-like shape in which the thickness of the end surface 23 is small relative to the size of the main surface 22. On the outer surface of the connection assembly 20, a gap G is formed between the main surfaces 22 of the adjacent 2 or more nanocrystal sheets 21, and the gap G is arranged so as to open to the outside of the connection assembly 20.

Specifically, the main surfaces of the nanocrystal sheet 21 mean surfaces having a large surface area in the outer surface of the sheet-like nanocrystal sheet 21, and are both surfaces defining the upper and lower edges forming end surfaces having a small surface area. For example, when nanocrystal composite 1 is used for a catalytic reaction, main surface 22 serves as a catalytically active surface exhibiting high catalytic activity. Therefore, the larger the surface area of the main surface 22, the more efficiently the catalytic reaction can be performed.

The minimum dimension of the main surface 22 of the nanocrystal sheet 21 is preferably 10nm or more and less than 1 μm, and the thickness t of the nanocrystal sheet 21 is preferably 1/10 or less of the minimum dimension of the main surface 22. This makes it possible to expand the area of the main surface 22 of the nanocrystal sheet 21 by about 10 times or more as compared with the area of the end surface 23, and to increase the catalytic activity per unit amount over that of nanoparticles. If the minimum size of the main surface 22 is 1 μm or more, it is difficult to connect the nanocrystal sheets 21 at high density, and if the minimum size is less than 10nm, there is a possibility that the gap G cannot be formed between the main surfaces 22 of the adjacent 2 or more nanocrystal sheets 21. In addition, the thickness t of the nanocrystal sheet 21 is preferably 1nm or more in order to suppress a decrease in rigidity of the nanocrystal sheet 21 in the thickness direction. The size of the main surface 22 of the nanocrystal sheet 21 is determined by measuring the nanocrystal sheet 21 separated from the connected assembly 20 as a single nanocrystal sheet so as not to impair the shape of the nanocrystal sheet 21. As a specific example of the measurement method, a rectangle Q of a minimum area circumscribing the main surface 22 of the nanocrystal sheet 21 is drawn, and the short side L1 and the long side L2 of the rectangle Q are measured as the minimum size and the maximum size of the nanocrystal sheet 21, respectively.

The nanocrystal sheet 21 is preferably a first metal oxide. Here, examples of the first metal oxide include oxides and composite oxides of noble metals, transition metals, or alloys thereof. Examples of the noble metal and its alloy include a metal composed of 1 component selected from the group consisting of palladium (Pd), rhodium (Rh), ruthenium (Ru), platinum (Pt), silver (Ag), and gold (Au), and an alloy containing 1 or more components selected from these. Examples of the transition metal and its alloy include a metal composed of 1 component selected from the group consisting of copper (Cu), nickel (Ni), cobalt (Co), and zinc (Zn), and an alloy containing 1 or more components selected from these.

In particular, the first metal oxide is preferably a metal oxide containing 1 or 2 or more metals selected from the group of transition metals. Such a metal oxide is present in a large amount on the earth as a metal resource, and is preferable from the viewpoint of suppressing the price because the cost is lower than that of a noble metal. Among them, preferred is a metal oxide containing 1 or 2 or more metals selected from the group of Cu, Ni, Co and Zn, and more preferred is at least copper. Examples of the metal oxide containing copper include copper oxide, Ni-Cu oxide, and Cu-Pd oxide, and among them, copper oxide (CuO) is preferable.

As shown in fig. 1, the nanocrystal composite 1 of the present invention has nanoparticles 30 supported on the connected aggregate 20, and the nanoparticles 30 are preferably supported in a dispersed manner. The nanoparticles 30 may be supported in the gap G or outside the gap G (for example, on the end surface 23 of the nanocrystal sheet 21) to hold the nanoparticles 30In view of the connection assembly 20, the load is preferably applied to the gap G. The nanoparticles 30 have a metal element different from 2 or more of the nanocrystal sheets 21, and preferably a second metal oxide different from the first metal oxide. Such a second metal oxide is, for example, preferably a metal oxide comprising cerium (Ce), more preferably nanoparticles of cerium oxide or a mixture of cerium oxide and zirconium oxide. The nanoparticles 30 are nanoparticles of cerium oxide or a mixture of cerium oxide and zirconium oxide (hereinafter, these are also collectively referred to as "CeO₂Nanoparticles "), since CeO₂The nanoparticles have oxygen-retaining properties, e.g. in the conversion of harmful gases such as CO, NO, etc. to CO₂、N₂When CeO is carried at a high temperature (for example, 600 ℃ C.) in a catalytic reaction with a non-harmful gas₂The nanocrystal composite 1 of nanoparticles is used as a catalyst, and then CeO₂The nanoparticles can temporarily hold oxygen atoms exfoliated from NO contained in the harmful gas, and can supply the oxygen atoms to the first metal oxide (for example, CuO) before the oxygen atoms are exfoliated from the first metal oxide. Namely, CeO₂The nanoparticles transfer oxygen atoms and exhibit a buffering action to mitigate the detachment of oxygen atoms from the first metal oxide. Therefore, the structure of the first metal oxide constituting the catalytically active surface of the nanocrystal sheet can be maintained, and thus the catalytically active surface can maintain high catalytic activity. As a result, the nanocrystal composite 1 can be used while maintaining high catalytic activity even when the catalytic reaction is repeated at high temperature. In the case where the nanocrystal composite 1 is used for a catalytic reaction, in order to effectively exert such a buffering action, the nanoparticles 30 are preferably disposed on the main surface 22 exhibiting high catalytic activity.

In order to support the nanoparticles 30 on the connected aggregate 20 well, it is necessary to appropriately control the amount of the nanoparticles 30. In the present invention, the ratio of the viewing area of the nanoparticles 30 (second metal oxide) to the viewing area of the 2 or more nanocrystal sheets 21 (first metal oxide) (viewing area ratio) is 2% to 50%, preferably 3% to 40%, and more preferably 4% to 30%. Here, the viewing field area of the 2 or more nanocrystal sheets 21 and the viewing field area of the nanoparticles 30 are the viewing field area observed by means capable of confirming the 2 or more nanocrystal sheets 21 and the nanoparticles 30, a microscope that realizes a magnification at which the shape can be visually confirmed, or the like, and for example, the viewing field area ratio can be calculated by performing element mapping of the 2 or more nanocrystal sheets 21 (first metal oxide) and the nanoparticles 30 (second metal oxide) using SEM-EDS (energy dispersive X-ray analysis). In this way, by controlling the ratio of the nanoparticles 30 to 2 or more nanocrystal sheets 21 within a specific range based on the ratio of the viewing area, the nanoparticles 30 can be favorably supported on the connected aggregate 20. If the ratio of the viewing area is less than 2%, the nanoparticles 30 supported on the connected aggregates 20 are too small, and the above-described buffer function cannot be properly performed. Therefore, when the above catalytic reaction is repeated at a high temperature, the structure of the catalytically active surface is destroyed, and the nanocrystal composite 1 cannot maintain a high catalytic activity. On the other hand, if the viewing area ratio exceeds 50%, the aggregation of the nanoparticles 30 becomes significant, and the nanoparticles 30 cannot be supported by the connected aggregate 20. In particular, if the ratio of the viewing area exceeds 80%, the aggregates of the nanoparticles 30 block the main surface 22, which is the catalytically active surface of the nanocrystal sheet 21, and the catalytic activity is significantly reduced, so that it is difficult to achieve the desired NO conversion even when the catalytic reaction is performed at a high temperature.

The nanoparticles 30 supported by the connected aggregate 20 are preferably secondary particles, and may be primary particles. The particle diameter (secondary particle diameter) of the nanoparticles 30 is not particularly limited as long as the secondary particles can be supported on the connected aggregate 20, and is preferably 5nm to 100nm, more preferably 20nm to 50 nm. If the particle diameter of the nanoparticles 30 is too large, the loading of the nanoparticles 30 on the connected assembly 20 and further in the gap G becomes difficult, and if the particle diameter of the nanoparticles 30 is too small, the retention of the nanoparticles 30 between the connected assembly 20, particularly the gap G, becomes difficult. The secondary particle size is not particularly limited, and can be measured, for example, by using an electron microscope (SEM: scanning electron microscope)And (4) measuring. In addition, the nanoparticles 30 are preferably dispersed in a ratio of 1 to 2 with respect to 20 nanocrystal sheets 21. With such a dispersed state, for example, in the case where the nanocrystal sheet is CuO, the CuO crystal is not decomposed into Cu₂O, Cu, the morphology of the nanocrystal sheet can be stably maintained.

The raw material of the first metal oxide and the raw material of the second metal oxide are not particularly limited, and include a step of dissolving these raw materials in a predetermined aqueous solution as described in the following method for producing a nanocrystal composite, and therefore these raw materials preferably include a hydrate of a metal constituting the first metal oxide and the second metal oxide, and more preferably a hydrate of a metal halide constituting the first metal oxide and the second metal oxide.

When the nanocrystal composite 1 of the present invention is used as a catalyst, the main surface 22 of the nanocrystal sheet 21 is a catalytically active surface, and therefore it is preferable that the main surface 22 of the nanocrystal sheet 21 has a specific crystal orientation.

In order to form the main surface 22 of the nanocrystal sheet 21 as a reductive active surface, the first metal oxide constituting the nanocrystal sheet 21 may be such that the surface of the metal atom exerting catalytic activity is oriented so as to be located on the main surface 22, and the main surface 22 is formed of a metal atom surface, and specifically, the ratio of the number of metal atoms in the metal atoms and oxygen atoms constituting the first metal oxide present on the main surface 22 is preferably 80% or more.

On the other hand, in order to configure the main surface 22 of the nanocrystal sheet 21 as an oxidative active surface, it is sufficient that the surface of oxygen atoms exerting catalytic activity in the first metal oxide configuring the nanocrystal sheet is oriented so as to be located on the main surface 22, and the main surface 22 is configured by the oxygen atom surface, and specifically, the number ratio of oxygen atoms in the metal atoms and oxygen atoms configuring the first metal oxide existing on the main surface 22 is preferably 80% or more.

By adjusting the number ratio of metal atoms or oxygen atoms in the metal oxide of the first type constituting the main surface 22 of the nanocrystal sheet 21 to the metal atoms or oxygen atoms according to the action of the active surface, the catalytic activity function of the main surface 22 can be improved, and a sufficient catalytic activity can be exhibited as the nanocrystal sheet 21 and, further, as the nanocrystal composite 1.

In addition, the reason why the main surface 22 of the nanocrystal sheet 21 is set to have a specific crystal orientation is because the crystal orientation existing in a large amount in the main surface 22 is different depending on the kind of the first metal oxide constituting the nanocrystal sheet 21. Therefore, although the crystal orientation of main surface 22 is not specifically described, for example, in the case where the first metal oxide is copper oxide (CuO), it is preferable that the main crystal orientation of the single crystal constituting the main surface, that is, the active surface is a (001) plane.

In addition, as a configuration in which main surface 22 is a metal atom plane, it is preferable that the crystal structure of the first metal oxide is a regular structure in which metal atom planes and oxygen atom planes are alternately laminated in a regular manner and the arrangement of atoms is regular, and the metal atom plane is located on main surface 22. Specifically, main surface 22 includes not only a structure composed of an aggregate of single crystals having the same orientation, but also a structure composed of an aggregate of single crystals having different crystal structures or different orientations, or an aggregate including grain boundaries, polycrystals, or the like, in which a metal atom plane is present on main surface 22.

The nanocrystal composite of the present invention can be used for various applications, and for example, can be suitably used as a catalyst for oxidation-reduction catalytic reaction, particularly for exhaust gas purification.

< method for producing nanocrystal composite >

Next, a method for producing the nanocrystal composite of the present invention will be described. The method for producing a nanocrystal composite according to the embodiment of the present invention includes a mixing step S1 and a temperature/pressure application step S2.

(mixing step S1)

The mixing process comprises the following steps: a hydrate of a compound containing a noble metal, a transition metal, or an alloy thereof (particularly, a hydrate of a metal halide) as a raw material of the first metal oxide; a hydrate of a compound containing Ce (particularly, a hydrate of Ce halide) as a raw material of the second metal oxide; and an organic compound having a carbonic acid diamide skeleton constituting a ligand of a metal complex as a precursor of the first metal oxide is dissolved in an aqueous solution (water). Examples of the hydrate of the metal halide include copper (II) chloride dihydrate, examples of the hydrate of the Ce halide include cerium (III) chloride heptahydrate, and examples of the organic compound having a carbonic acid diamide skeleton include urea. In the case of preparing a mixture of ceria and zirconia, examples of the raw material of the second metal oxide include cerium (III) chloride heptahydrate and zirconium oxychloride octahydrate.

It is preferable that the respective hydrates described above be mixed with urea after the organic solvent is added to the aqueous solution (water). As the organic solvent, ethylene glycol or the like can be used, and the organic solvent is preferably added so as to have a concentration of 50 mol% or less with respect to the aqueous solution. This can improve the dispersibility of the solute.

(hydrothermal Synthesis Process S2)

The hydrothermal synthesis step is as follows: the resulting mixed solution was subjected to a predetermined heat/pressure and left for a predetermined time. The mixed solution is preferably heated at 100 to 300 ℃. When the heating temperature is less than 100 ℃, the reaction of urea and metal halide cannot be completed, and when the heating temperature exceeds 300 ℃, the reaction vessel cannot bear the generated high vapor pressure. The heating time is preferably 10 hours or more. When the heating time is less than 10 hours, unreacted materials may remain. The predetermined pressure is preferably a pressure of 100 ℃ or higher of the vapor pressure (1 atm) of water. For applying a predetermined heat and pressure, for example, a method of heating and pressurizing using a pressure-resistant vessel can be mentioned.

Through the above steps, the nanocrystal composite 1 of the present invention in which the nanoparticles 30 are supported on the connected assembly 20 of the nanocrystal composite 1 can be produced simultaneously with the production of the nanocrystal composite.

While the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various changes can be made within the scope of the present invention including all the embodiments included in the concept of the present invention and claims.

Examples

Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited thereto.

Examples 1 to 4 and comparative examples 1 to 4

An aqueous solution was prepared by mixing 150ml of ethylene glycol and 150ml of water at room temperature and stirring for 1 hour. Next, cerium (III) chloride heptahydrate, copper (II) chloride dihydrate, and urea were added to the aqueous solution in predetermined addition amounts shown in table 1. The obtained solution was charged into a pressure-resistant vessel having an internal volume of 500ml, and the vessel was sealed under an air atmosphere. The pressure vessel was placed in a thermostatic bath, heated at 180 ℃ for 24 hours, and then cooled to room temperature. After 1 day at room temperature, the solution containing the precipitate was recovered from the vessel. After the precipitate in the solution was washed with methanol and pure water, it was dried at 70 ℃ for 10 hours in a vacuum environment to prepare a nanocrystal complex.

[ measurement and evaluation ]

The nanocrystal composites of the examples and comparative examples obtained as described above were used to perform the following measurements and characteristic evaluations. The measurement and evaluation conditions for each property are as follows. The results are shown in Table 1.

[1] Ratio of area of visual field

For the nanocrystal composites obtained in each of the examples and comparative examples, copper oxide (CuO) as a first metal oxide and cerium oxide nanoparticles (CeO) as a second metal oxide were carried out by EDS (energy dispersive X-ray spectrometer: SU-8020 manufactured by Hitachi High-Technologies, Inc.)₂Nanoparticles), determining CeO₂The ratio of the viewing area of the nanoparticles to that of CuO (viewing area ratio). Specifically, the observation magnification was set to 20000 times, and the element peak information detected by EDS was used to determine the distribution of each element of cerium (Ce), copper (Cu) and oxygen (O) in 1 visual field of 3 μm × 6 μmAnd (5) color discrimination. Next, two-dimensional image element mapping is performed to calculate the areas (viewing area) corresponding to the Ce element and the Cu element. The calculated area of the Ce element was divided by the area of the Cu element to obtain a ratio of the viewing area of the second metal oxide to the viewing area of the first metal oxide. In the above operation, mapping of 10 fields of view in total is randomly performed in fields of view at intervals of 10 μm or more, and the average value of the obtained ratios of the field of view areas is obtained as the field of view area ratio.

[2] Catalyst Performance

The catalyst performance was evaluated using an apparatus comprising a gas supply line, a reaction tube, and a gas sampling unit. The details are as follows.

First, 10 to 20mg of the nanocrystal composites obtained in each of examples and comparative examples were packed between glass filters as catalysts, and inserted into a reaction tube. Next, the reaction tube filled with the catalyst was set in the apparatus in the thermostatic bath. In order to eliminate the influence of moisture adhering to the surface of the sample, the sample was heated from room temperature to 200 ℃ for 30 minutes in a helium gas flow state, and then cooled to 100 ℃ for evaluation. Thereafter, the reaction gas (raw material gas) was switched to, and after holding for 15 minutes, the temperature was raised to 600 ℃ at 10 ℃/min, and the NO conversion was calculated from the collection/measurement of the reaction tube outlet gas at that time. Specifically, the NO conversion was calculated by the following formula (1).

NO conversion (%) ═ N₂(Outlet)/NO (raw gas) } × 100 … (1)

The raw material gas used was a mixed gas (helium equilibrium) of 1 vol% of carbon monoxide (CO) and 1 vol% of nitrogen monoxide (NO), and the flow rate was 20 mL/min. The catalyst performance was evaluated from the NO conversion by measuring the reaction tube outlet gas by GC-MASS. When the NO conversion was 50% or more, the steel sheet was evaluated as "good", and when the NO conversion was less than 50%, the steel sheet was evaluated as "bad". The NO conversion here means a value at which the catalyst is activated by heating based on the NO gas concentration at 100 ℃ switched to the reaction gas, and NO is reduced by reduction.

Further, the nanocrystal composites obtained in the examples and comparative examples could not maintain the structure and morphology of copper oxide when subjected to a catalytic reaction at 600 ℃, and there was a concern that the catalytic activity might deteriorate in the catalytic reaction after the 2 nd reaction. Therefore, after the 1 st catalytic reaction, the nanocrystal composites obtained in each example and comparative example were cooled to 100 ℃, and then heated again to 600 ℃, and the catalyst performance was evaluated again.

[3] Tissue observation

The observation of the structure of the nanocrystals was carried out using a scanning electron microscope (SEM: SU-8020 manufactured by Hitachi High-Technologies, Ltd.). Fig. 2 (a) is an SEM image when the nanocrystal composite after the 1 st 600 ℃ catalyst evaluation in example 2 was observed at a magnification of 4000 times, and fig. 2 (b) is an SEM image when it was observed at a magnification of 70000 times.

[4] Tissue stability in catalytic reactions

The structure analysis was performed by an X-ray diffraction apparatus ("D8 DISCOVER", Bruker AXS K.K. (now, manufactured by Bruker Japan K.K.) and whether or not the crystal structure of copper oxide could be maintained was confirmed by identifying the composition of the nanocrystal composites of each of the examples and comparative examples after the catalytic reaction by the X-ray crystal structure analysis.

Fig. 3 shows the results of X-ray crystal structure analysis of the nanocrystal composites after the 1 st 600 ℃ catalyst evaluation in example 2 and comparative example 2. Fig. 4 and 5 show the observation results of a nanocrystal complex in which the crystal structure can be maintained and a nanocrystal complex in which the crystal structure cannot be maintained and decomposed, respectively, by a scanning electron microscope. Fig. 4 is an SEM image when the nanocrystal composite after the 1 st 600 ℃ catalyst evaluation in example 3 is observed at a magnification of 20000 times, and fig. 5 is an SEM image when the nanocrystal composite after the 1 st 600 ℃ catalyst evaluation in comparative example 1 is observed at a magnification of 20000 times.

In the nanocrystal composites obtained in examples 1 to 4 shown in Table 1, the proportion of the visual field area was in the range of 2% to 50%, and CeO₂The nanoparticles are well dispersed. As shown in table 1, in examples 1 to 4, the NO conversion rates of the 1 st catalytic reaction and the 2 nd catalytic reaction were all 50% or more, and high catalytic activity was maintained. Therefore, the nanocrystal composites obtained in examples 1 to 4 were evaluated as CeO without covering the surface of the nanocrystal sheet made of CuO effective for the catalytic reaction₂The nanoparticles are well dispersed.

In examples 1 to 4, peaks of CuO were observed in the X-ray crystal structure analysis of the nanocrystal composites after the evaluation of the catalyst, while peaks of Cu and Cu were not observed₂O, and thus shows good tissue stability. As more specific results, in FIG. 3, peaks of CuO and CeO were observed in example 2₂Thus, it is found that CeO passes through₂The nanoparticles inhibit the decomposition of CuO. Further, as shown in FIG. 2, the nanocrystal composite, CeO, obtained after the evaluation of the 600 ℃ catalyst of the 1 st time in example 2 was observed₂The nanoparticles are also dispersed in the nanocrystal composite, and the surface of the nanocrystal sheet can be confirmed to be characteristic of the structure of the nanocrystal composite. Further, as shown in fig. 4, when the nanocrystal composite after the 1 st evaluation of the 600 ℃ catalyst in example 3 was observed, it was confirmed that the nanocrystal sheet having copper oxide (CuO) on the surface and the granular cerium oxide (CeO) were present₂Nanoparticles). From this, it was found that the crystal structure of copper oxide was maintained and the morphology was maintained in the nanocrystal composites obtained in examples 1 to 4.

On the other hand, in comparative examples 1 to 2 shown in table 1, although the NO conversion rate was 50% or more in the 1 st catalytic reaction, the NO conversion rate was less than 50% in the 2 nd catalytic reaction, and high catalytic activity could not be maintained. In addition, since no peak of CuO was observed in the X-ray crystal structure analysis after the evaluation of the catalyst, it was found that the surface of the nanocrystal sheet composed of CuO effective for the catalytic reaction was lost by the decomposition of CuO, and it was not possible to maintain a high catalytic activity. As more specific results, in fig. 3, no peak of CuO was observed in comparative example 2, and decomposition of CuO was confirmed. Furthermore, as shown in fig. 5, when the nanocrystal composite after the 1 st evaluation of the 600 ℃ catalyst in comparative example 1 was observed, the surface of the nanocrystal sheet characteristic to the structure of the nanocrystal composite could not be confirmed. From this, it was confirmed that in the nanocrystal composites obtained in comparative examples 1 to 2, decomposition of copper oxide occurred, the crystal structure of copper oxide could not be maintained, and the morphology was destroyed.

In comparative examples 3 to 4, the proportion of the visual field area was not in the range of 2% to 50%, and the NO conversion in the 1 st catalytic reaction was less than 50%, and high catalytic performance was not obtained. Therefore, in comparative examples 3 to 4, the performance of the catalyst after the 2 nd catalytic reaction was not evaluated.

From this, it is understood that the nanocrystal composites of the present invention described in examples 1 to 4 can maintain the morphology, crystal structure, and high catalytic activity of the catalytically active surface well even when repeatedly exposed to high temperatures. Therefore, it is found that the nanocrystal composite of the present invention is particularly useful for a catalytic reaction in purification of harmful gases contained in automobile exhaust gases.

Description of the symbols

1 nanocrystal complex

20 connected aggregate

21 nanocrystal sheet

22 major surface

23 end face

30 nanoparticles

G gap