JP6269912B2 - Catalyst for ethanol decomposition reaction - Google Patents

Description

  The present invention relates to a ferroelectric carrier catalyst having excellent low-temperature activity. More specifically, the present invention relates to a ferroelectric support catalyst that is excellent in low-temperature activity because the ferroelectric Curie point as a support has a low Curie point and polarization fluctuations occur at low temperatures.

Conventionally, a catalyst in which a metal is supported on a carrier is used as an oxidation-reduction catalyst in various reactions.
However, the use of ferroelectrics as carriers is not always widely known.
One of the present inventors discovered that lead deposited on the surface of lead titanate (Pb _x Sr _1-x TiO ₃ ), which is a ferroelectric material, is not oxidized and is in a metal state. Proposed as a catalyst. Moreover, lead titanate (Pb _x Sr _1-x TiO ₃ ) discloses that a catalyst prepared by a coprecipitation method is suitable. (Patent Document 1: JP 2009-209777 A, Patent Document 2: JP 2009-207978 A, Patent Document 3: JP 2009-207979 A)

However, when producing a ferroelectric by the coprecipitation method, the ferroelectric can be produced only by a combination of metals for which the coprecipitation method can be used. In addition, since the yield of the ferroelectric is small, the production efficiency is low. It was low and there was a problem in terms of economy.
Therefore, as a result of further investigation, a catalyst in which a metal is supported on the surface of a ferroelectric by firing a mixture of the powdered ferroelectric and the powdered metal while stirring is used. A catalyst in which the distance from the surface to the metal surface is 10 nm or less (that is, the metal particle diameter is 10 nm or less) is disclosed. (Patent Document 4: JP 2012-161751 A)

JP 2009-209777 A JP 2009-207978 A JP 2009-207979 A JP 2012-161751 A

The present inventors have further studied a result, the lower ferroelectric carrier Curie point (T _C), when a noble metal, even very reactive temperature close Curie point (T _C) is a case low, high The inventors have found that the catalyst activity is expressed and have completed the present invention.
An object of the present invention is to provide a catalyst having excellent low-temperature activity using a ferroelectric material carrying metal fine particles and / or metal oxide fine particles as a carrier.

Ferroelectric supported catalyst according to the present invention is a ferroelectric supported catalyst obtained by supporting a metal fine particle and / or metal oxide fine particles to ferroelectric particulate carrier, the Curie point of the ferroelectric (T _C ) Is in the range of 0 to 200 ° C., the average particle size of the ferroelectric fine particles is in the range of 10 to 200 nm, and the loading amount of the metal fine particles and / or metal oxide fine particles is the oxide of the ferroelectric carrier. It is characterized by being in the range of 1 to 30 parts by mass in terms of metal with respect to 100 parts by mass.

The ferroelectric material preferably has a band gap in the range of 3.2 to 4.0 eV.
The crystallite diameter of the ferroelectric fine particles is preferably in the range of 5 to 100 nm.

The ferroelectric is preferably a crystal having a perovskite crystal structure (oxygen octahedral structure) represented by ABO ₃ .
It is preferable that B is Ti or at least one element selected from Nb, Fe, Mn, Ta, Zr and La and Ti.
The A is preferably Ba or Ba and at least one element selected from Ca, Sr, Li, K, Na, Ag, La, and Tm.

The metal work function is preferably in the range of 4.5 to 6 eV.
The metal is preferably at least one selected from Pd, Rh, Au, Ru, Ni, Pt, Cu, Fe, and Ag.
The average particle diameter of the metal fine particles and / or metal oxide fine particles is preferably in the range of 1 to 20 nm.

The ferroelectric carrier catalyst according to the present invention is excellent in low-temperature activity and can be used for various catalytic reactions.

Ferroelectric Support Catalyst Hereinafter, the ferroelectric support catalyst according to the present invention will be described.
The ferroelectric carrier catalyst according to the present invention is a ferroelectric carrier catalyst in which metal fine particles and / or metal oxide fine particles are supported on a ferroelectric fine particle carrier.

A ferroelectric carrier ferroelectric is a substance in which electric dipoles are generally aligned without an external electric field, and the direction of the electric dipole can be changed by the electric field.
The ferroelectric used in the present invention is not particularly limited as long as it is a substance that causes polarization fluctuation described later, but is preferably a crystal having a perovskite crystal structure (oxygen octahedral structure) represented by ABO _3. .

It is preferable that B is Ti or at least one element selected from Nb, Fe, Mn, Ta, Zr and La and Ti. In the case of Ti and at least one element selected from Nb, Fe, Mn, Ta, Zr and La, the proportion of the Ti element is preferably 50% or more. When a perovskite type crystal structure consisting of elements Curie point (T _C) is low, it is possible to obtain an excellent catalyst activity at low temperature.

The A is preferably Ba or Ba and at least one element selected from Ca, Sr, Li, K, Na, Ag, La and Tm.
When the element A is any of these elements, the element B has a perovskite crystal structure in which the element B is mainly Ti or Ti and at least one element selected from Nb, Fe, Mn, Ta, Zr and La is blended. If a certain ferroelectric excellent perovskite type crystal crystallinity is obtained, the Curie point (T _C) is low, it is possible to obtain an excellent catalyst activity at low temperature.

The Curie point (T _C ) of the ferroelectric used in the present invention is preferably in the range of 0 to 200 ° C., more preferably 20 to 180 ° C. Here, the Curie point (Tc) means the temperature at which the ferroelectric material transitions to the paraelectric material.
When ferroelectric Curie point (T _C) is in the above range, the activity enhancing effect due to the polarization fluctuation in the vicinity of the Curie point, which will be described later (T _C), the reaction temperature is obtained an excellent catalyst activity even when low Can do.
It is difficult to obtain a ferroelectric having a Curie point (T _C ) of less than 0 ° C. When the Curie point (T _C ) exceeds 200 ° C., the polarization fluctuation is a temperature range in which the reaction temperature exceeds 200 ° C. However, as with the case of using a paraelectric material (non-ferroelectric material) such as alumina or titanium oxide that does not have polarization fluctuation at such a high temperature as a support, an improvement in activity depending on temperature is added, The activity improvement effect by polarization fluctuation is small.

This point will be described in further detail.
One of the inventors of the present application reports that polarization fluctuations appear in a ferroelectric material in Patent Document 4: Japanese Patent Application Laid-Open No. 2012-161751.
This “polarization fluctuation” is a change in the distribution of electric charge, resulting in a positive or negative charged micro-region, and an electric dipole moment between a pair of positively charged micro-regions and a negatively charged micro-region. Arise. This electric dipole moment is aligned in the direction of the electric field if an electric field is present outside, while it is in an arbitrary direction that is stable in terms of energy if there is no electric field outside. Here, the electric dipole moment in any direction is not always in a fixed direction, but the direction of the electric dipole moment is caused by fluctuations in the charge distribution in the ferroelectric due to thermal energy, etc. And the magnitude of the electric dipole moment is fluctuating.

This spatiotemporal fluctuation of the orientation of the electric dipole moment is also called “polarization fluctuation”, and this polarization fluctuation is a fluctuation (fluctuation) in picosecond units. It is considered that an electron-hole pair appears on the surface of the substrate in picosecond units, and catalytic activity is expressed by contacting the electron or hole with a reactant.
When the Curie point (T _C ) of the ferroelectric substance is low in the above range, it is understood that high activity is exhibited at low temperatures due to polarization fluctuations generated at low temperatures.
In the present invention, a known value is used for the Curie point (T _C ).

Next, the band gap of the ferroelectric used in the present invention is preferably in the range of 3.2 to 4.0 eV, and more preferably in the range of 3.5 to 4.0 eV.
When the band gap is in the above range, it means that the ferroelectric carrier has no or few oxygen vacancies or impurities (band gap is an index), and a highly active catalyst at low temperature can be obtained by polarization fluctuation. At this time, it is difficult to obtain one having a band gap exceeding 4.0 eV.

  In the present invention, the band gap is calculated from the absorption edge of the reflection spectrum by measuring the reflection spectrum using a device incorporating an integrating sphere unit (ISN-470) in the Shimadzu spectrophotometer V-550. did. Originally, it is desirable to measure a transmission spectrum, but the ferroelectric material used cannot transmit detection light because it is a fine particle powder. Therefore, a powder sample holder PSH-001 was used. A format was adopted in which the cells were evenly laid to a thickness of 1 to 2 mm in a 25 mm aperture cell inside the holder, and fixed by pressing with a quartz glass window, and the reflectance was obtained from the spectrum reflected by the incident light. The spectrum obtained by the spectrophotometer has the reflectance (% R) on the vertical axis and the wavelength (nm) on the horizontal axis. The absorption edge (wavelength region where the reflectance asymptotically approaches 0) appearing on the high energy side (low wavelength side) was confirmed, and 1240 ÷ absorption edge wavelength = band gap.

The ferroelectric used in the present invention uses fine particles. The average particle size of the ferroelectric fine particles is preferably in the range of 10 to 200 nm, more preferably 10 to 100 nm.
When the average particle size of the ferroelectric fine particles is less than 10 nm, the polarization fluctuation becomes extremely small and sufficient activity may not be obtained.
When the average particle size of the ferroelectric fine particles exceeds 200 nm, the average particle size of the supported fine particles tends to increase, although the amount varies depending on the amount of the supported metal fine particles or metal oxide fine particles, and the activity is insufficient. There is a case.

  The measurement method of the average particle size of such ferroelectric fine particles is as follows: scanning transmission electron microscope (STEM) (Hitachi, Ltd .: model S-5500) or field emission electron microscope (field emission TEM) (JEOL: JEM- 2100F), the particle diameter was measured for 50 particles, and the average value was obtained.

The crystallite diameter of the ferroelectric fine particles is preferably in the range of 5 to 100 nm, more preferably 7 to 50 nm.
When the crystallite diameter of the ferroelectric fine particles is less than 5 nm, it means that the crystallinity of the ferroelectric fine particles is low. For this reason, the metal fine particles supported on the surface of the ferroelectric fine particle carrier and / or Alternatively, the metal oxide fine particles may not be oscillated by the polarization fluctuation of the ferroelectric carrier and sufficient activity may not be obtained.
Ferroelectric fine particles having a crystallite diameter exceeding 100 nm are difficult to obtain, and even if they are obtained, the average particle size also increases, and the average particle size of the supported metal fine particles or metal oxide fine particles tends to increase. And the activity may be insufficient.

In the present invention, the crystallite diameter was determined by measuring the half width of the peak on the (110) plane near 2θ = 31.5 degrees with an X-ray diffractometer, and calculating by the following Scherrer equation.
D = Kλ / βcosθ
D: crystallite diameter (angstrom), K: Scherrer constant, λ: X-ray wavelength (1.7889 angstrom Cu lamp), β: half width (rad), θ: reflection angle.

Metal Fine Particles and / or Metal Oxide Fine Particles In the ferroelectric carrier catalyst of the present invention, the ferroelectric fine particles are used as a carrier, and metal fine particles and / or metal oxide fine particles are supported on the surface thereof.
As the metal, it is preferable to use a metal having a work function of 4.5 to 6.0 eV, more preferably 5.0 to 6.0 eV.
When a metal with a work function of 4.5 to 6 eV is supported on a ferroelectric material, it forms a Schottky junction, and the redox level of the gas to be generated and the bottom of the conduction band or valence band of the ferroelectric bridge on the staircase Is done. As a result, the potential potential of the metal fluctuates on the same time scale in the form oscillated by the polarization fluctuation of the ferroelectric carrier in the vicinity of the Curie point (T _C ), and the electrons in the metal fine particles or metal oxide fine particles change. It is thought that fluctuations occur and the exchange of electrons with reactants such as hydrocarbon gas diffused from the outside is promoted, and high activity is exhibited at low temperatures.

  On the other hand, in the case of a metal having a small work function, it is in ohmic contact with the Fermi level of the metal near the conduction band of the ferroelectric. In this case, the potential potential of the metal due to the polarization fluctuation of the ferroelectric does not sufficiently oscillate, and the activity tends to be insufficient. On the other hand, for large metals, Schottky junctions occur near the valence band of the ferroelectric. In this case, the p-type characteristics of the ferroelectric material are prominent, the potential potential fluctuation of the metal is limited, and the activity tends to be insufficient.

As the metal, at least one selected from Pd, Rh, Au, Ru, Ni, Pt, Cu, Fe, and Ag is used, and it is preferable that these metal fine particles and / or metal oxide fine particles are supported. .
These metal fine particles and / or metal oxide fine particles have a metal work function in the above range, and the potential potential of the metal due to the polarization fluctuation of the ferroelectric is sufficiently fluctuated, and it is considered that high activity is exhibited at a low temperature. .

The average particle diameter of the metal fine particles and / or metal oxide fine particles is preferably 1 to 20 nm, more preferably 2 to 15 nm.
It is difficult to obtain metal fine particles and / or metal oxide fine particles having an average particle size of less than 1 nm.
If the average particle diameter of the metal fine particles and / or metal oxide fine particles exceeds 20 nm, sufficient activity may not be obtained regardless of the type of metal and the supported amount described later.

The measurement method of the average particle diameter of such metal fine particles and metal oxide fine particles was calculated from the CO adsorption amount.
The amount of CO adsorption was measured using a catalyst analyzer (BEL-CAT, manufactured by Nippon Bell Co., Ltd.), and the pretreatment was measured with reference to “standardization of measurement method using reference catalyst” of the Reference Catalyst Committee of the Catalysis Society of Japan.
For the average particle of metal from CO adsorption, the average atomic weight and average atomic radius of the supported metal are calculated from the ratio of the supported metal, and the calculation formula described in the instruction manual for the catalyst analyzer (BEL-CAT) is used. Can be used to calculate.

Further, the supported amount of the metal fine particles and / or metal oxide fine particles is 1 to 30 parts by mass in terms of metal with respect to 100 parts by mass as the oxide (ABO ₃ ) of the ferroelectric carrier, and further 1 It is preferable that it exists in the range of 0.5-20 mass parts.
If the supported amount of metal fine particles and / or metal oxide fine particles is less than 1 part by mass in terms of metal, sufficient activity may not be obtained.
If the supported amount of the metal fine particles and / or metal oxide fine particles exceeds 30 parts by mass in terms of metal, the particle diameter of the metal fine particles and / or metal oxide fine particles becomes too large and sufficient activity is obtained. There may not be.

Method for Producing Ferroelectric Support Catalyst Such a method for producing a ferroelectric support catalyst according to the present invention is not particularly limited as long as the above-described catalyst is obtained. For example, the following impregnation / adsorption step, drying step And the manufacturing method which consists of a heat processing process is illustrated.

(Impregnation / adsorption process)
In advance, water is dripped onto a predetermined amount of the ferroelectric fine particle powder, and the amount of water dripped until it becomes almost paste-like (water absorption amount) is obtained.
Next, an aqueous metal salt solution having the same amount as the amount of dropped water is prepared, and the ferroelectric fine particle powder is mixed with the aqueous metal salt solution to absorb the aqueous metal salt solution.

(Drying process)
Next, the ferroelectric fine particle powder having absorbed the metal salt is dried.
The drying method is not particularly limited as long as moisture can be substantially removed, and a conventionally known method can be employed.
The drying temperature is generally 80 to 200 ° C, preferably 100 to 180 ° C.
The drying time varies depending on the drying temperature, but is generally 1 to 48 hours.

(Heat treatment process)
Next, heat treatment is performed in an oxidizing atmosphere such as air or oxygen, an inert gas atmosphere such as N ₂ , He, or Ar, or a reducing gas atmosphere such as H ₂ .
The selection of the atmosphere during the heat treatment can be appropriately selected depending on the type of reaction, the type of catalytic reaction, the type of ferroelectric, and the like.
A temperature at which the activity is sufficiently high is selected as the heat treatment temperature, but it is preferably in the range of 200 to 600 ° C., more preferably 250 to 550 ° C.

  Hereinafter, although an example explains, the present invention is not limited by these examples.

[Example 1]
Preparation of Ferroelectric Support Catalyst (1) 46 g of palladium nitrate aqueous solution (palladium nitrate concentration = 4.347% by mass) as ferroelectric fine particles BaTiO ₃ (manufactured by Kanto Denka Kogyo Co., Ltd .: average particle diameter = 50 nm, crystallite) (Diameter = 25.3 nm) was added with stirring, and then allowed to stand at room temperature for 3 hours.
Then, after drying at 150 ° C. for 24 hours, a ferroelectric carrier catalyst (1) was prepared by heat treatment at 400 ° C. for 3 hours in an air atmosphere.
With respect to the obtained ferroelectric carrier catalyst (1), the amount of metal fine particles supported is represented by a calculated value from the amount of raw material used, the average particle size was measured, and the results are shown in the table.

The ethanol-decomposed reaction powdered ferroelectric-supported catalyst (1) is pressed and pulverized and then granulated to prepare a granule sample with a particle size of 335-710 μm. Filled and subjected to reaction.
The reaction was carried out by introducing ethanol with a pulse size of 1 μL by the pulse method and changing the reaction temperature to 140, 170, 200, 230, 260 and 290 ° C.
The product gas was quantified using a gas chromatograph (manufactured by Shimadzu Corporation: GC-8A). At this time, thermal detector (Thermal Conductivity Detector, TCD) was used as a detector, and PEG 6000 Shimarite 10% TPA 60-80 mesh was used as a capillary column. Helium was used as the carrier gas, and the flow rate was 50 mL / min.
The decomposition rate is determined by the following equation, and the decomposition rate at each temperature is shown in the table.
Decomposition rate (%) = (Introduced ethanol amount−Unreacted ethanol amount) / Introduced ethanol amount × 100

[Example 2]
Preparation of Ferroelectric Support Catalyst (2) A ferroelectric support catalyst (2) was prepared in the same manner as in Example 1 except that 46 g of an aqueous palladium nitrate solution (palladium nitrate concentration = 10.868% by mass) was used.
With respect to the obtained ferroelectric carrier catalyst (2), the amount of metal fine particles supported and the average particle diameter were measured, and the results are shown in the table. Moreover, ethanol decomposition activity was evaluated and the results are shown in the table.

[Example 3]
Preparation of Ferroelectric Support Catalyst (3) In Example 1, except that 100 g of BaTiO ₃ (manufactured by Toda Kogyo Co., Ltd .: average particle size = 20 nm, crystallite size = 22.5 nm) was used as the ferroelectric fine particles. Similarly, a ferroelectric carrier catalyst (3) was prepared.
With respect to the obtained ferroelectric carrier catalyst (3), the amount of metal fine particles supported and the average particle diameter were measured, and the results are shown in the table. Moreover, ethanol decomposition activity was evaluated and the results are shown in the table.

[Example 4]
Preparation of Ferroelectric Support Catalyst (4) BaSrTiO ₃ was prepared as ferroelectric fine particles by the method in Patent Documents 1-3. The average particle diameter and crystallite diameter of the obtained BaSrTiO ₃ were measured, and the results are shown in the table.
Next, a ferroelectric carrier catalyst (4) was prepared in the same manner as in Example 1 except that 100 g of BaSrTiO ₃ particles were used.
With respect to the obtained ferroelectric carrier catalyst (4), the amount of metal fine particles supported and the average particle diameter were measured, and the results are shown in the table. Moreover, ethanol decomposition activity was evaluated and the results are shown in the table.

[Comparative Example 1]
Preparation of Ferroelectric Support Catalyst (R1) A ferroelectric support catalyst (R1) was prepared in the same manner as in Example 1 except that 46 g of an aqueous palladium nitrate solution (palladium nitrate concentration = 1.085% by mass) was used.
With respect to the obtained ferroelectric carrier catalyst (R1), the amount of metal fine particles supported and the average particle diameter were measured, and the results are shown in the table. Moreover, ethanol decomposition activity was evaluated and the results are shown in the table.

[Comparative Example 2]
Preparation of Ferroelectric Support Catalyst (R2) In Example 1, 100 g of BaTiO ₃ (manufactured by High Purity Science Laboratory Co., Ltd .: average particle size = 1000 nm, crystallite size = 36.4 nm) is used as the ferroelectric fine particles. A ferroelectric carrier catalyst (R2) was prepared in the same manner except that
With respect to the obtained ferroelectric carrier catalyst (R2), the amount of metal fine particles supported and the average particle diameter were measured, and the results are shown in the table. Moreover, ethanol decomposition activity was evaluated and the results are shown in the table.

[Comparative Example 3]
Preparation of non-ferroelectric (paraelectric) supported catalyst (R3) In Example 1, α-Al ₂ O ₃ (manufactured by Nippon Light Metal Co., Ltd .: particle size distribution 15) as non-ferroelectric (paraelectric) fine particles A non-ferroelectric (paraelectric) catalyst (R3) was prepared in the same manner except that 100 g) was used.
With respect to the obtained non-ferroelectric (paraelectric) supported catalyst (R3), the amount of metal fine particles supported and the average particle diameter were measured, and the results are shown in the table. Moreover, ethanol decomposition activity was evaluated and the results are shown in the table.

[Comparative Example 4]
Preparation of non-ferroelectric (paraelectric) carrier catalyst (R4) In Example 1, TiO ₂ (manufactured by JGC Catalysts & Chemicals, Inc .: HPW-60R, average particle diameter) as non-ferroelectric (paraelectric) fine particles = 60 nm, crystalline anatase) A non-ferroelectric (paraelectric) catalyst (R4) was prepared in the same manner except that 100 g was used.
With respect to the obtained non-ferroelectric (paraelectric) carrier catalyst (R4), the amount of metal fine particles supported and the average particle diameter were measured, and the results are shown in the table. Moreover, ethanol decomposition activity was evaluated and the results are shown in the table.

Claims (4)

A ferroelectric carrier catalyst comprising metal fine particles and / or metal oxide fine particles and a ferroelectric fine particle carrier, wherein the metal fine particles and / or metal oxide fine particles are supported on the ferroelectric fine particle carrier, The Curie point (T _C ) of the ferroelectric is in the range of 0 to 200 ° C., the average particle diameter of the ferroelectric fine particles is in the range of 10 to 200 nm, and the supported amount of metal fine particles and / or metal oxide fine particles Is in the range of 1 to 30 parts by mass in terms of metal with respect to 100 parts by mass as the oxide of the ferroelectric carrier, the ferroelectric is BaTiO ₃ or BaSrTiO ₃ , and the metal is Pd A catalyst for ethanol decomposition reaction comprising a ferroelectric carrier catalyst. The catalyst for ethanol decomposition reaction comprising the ferroelectric carrier catalyst according to claim 1, wherein the ferroelectric material has a band gap in the range of 3.2 to 4.0 eV. The catalyst for ethanol decomposition reaction comprising the ferroelectric carrier catalyst according to claim 1 or 2, wherein a crystallite diameter of the ferroelectric fine particles is in a range of 5 to 100 nm. The catalyst for ethanol decomposition reaction comprising the ferroelectric carrier catalyst according to any one of claims 1 to 3, wherein the metal has a work function in the range of 4.5 to 6 eV.