JP2015147954A - Method and apparatus for producing mesoporous niobium oxide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mesoporous niobium oxide production method and a mesoporous niobium oxide production apparatus that can produce a mesoporous niobium oxide free of impurities with a low voltage and by a simple procedure, without using highly toxic reagent to human body, in an electrolytic synthesis by using a Nb electrode.SOLUTION: There is provided a mesoporous niobium oxide production method, including a Nb electrode oxidation step for oxidizing the Nb electrode by applying a voltage to the Nb electrode.

Description

  The present invention relates to a method for producing mesoporous niobium oxide and a production apparatus.

  The mesoporous material is characterized in that mesopores (pores of 2 nm to 50 nm) are uniformly and regularly arranged and have a large specific surface area. These materials typified by mesoporous silica are expected to be applied not only as a catalyst (utilization as a catalyst carrier) and an adsorbent, but also in the medical field as long as the material is highly biocompatible.

Nb ₂ O ₅ is a metal oxide similar to silica (SiO ₂ ) and exhibits high chemical stability compared to metal sulfides and the like, and thus can be used in the above fields. On the other hand, as a feature not found in SiO ₂ , the material itself can be used for light energy conversion. Nb ₂ O ₅ is an n-type semiconductor and can be used as a photoanode of a wet photovoltaic cell. In addition, Nb ₂ O ₅ has a Fermi level on the energetically lower side than TiO _2. Therefore, when used as a negative electrode substrate of a sensitized solar cell, Nb ₂ O ₅ is a commonly used TiO ₂ one. Compared with this, an improvement in open-circuit voltage can be expected. In fact, it has recently attracted particular attention as a negative electrode substrate for dye-sensitized solar cells, and there are many reports.

Reports on the synthesis of mesoporous niobium oxide (m-Nb ₂ O ₅ ) can be broadly classified into two types: bottom-up synthesis and electrolytic synthesis.
In the bottom-up synthesis, there is a problem that it is difficult to extend the mesopores perpendicularly to the substrate (see, for example, Non-Patent Document 1).
In the electrolytic synthesis, it is common to use a bias of 10 V to 60 V and a fluorine-based compound (for example, see Non-Patent Document 2). However, the fluorine-based compound has a problem that it is extremely harmful to the human body. Further, the conventional electrolytic synthesis has a problem that a high voltage needs to be applied.
Recently, it has been proposed to synthesize Nb ₂ O ₅ having macropores larger than 50 nm by applying a bias of 20 V at 200 ° C. in a glycerin solution of potassium phosphate in a Nb electrode (for example, Non-Patent Document 3). reference). However, since m-Nb ₂ O ₅ synthesized by the proposed technique contains impurities such as P (phosphorus), there is a problem that the performance is low when used as a negative electrode substrate of a dye-sensitized solar cell (for example, Non-Patent Document 4).

In normal electrolytic synthesis using an Nb electrode, Nb ₂ O ₅ exhibiting high chemical resistance is formed on the Nb electrode immediately after the bias is applied. As the synthesis time elapses, the Nb ₂ O ₅ formation rate and elution rate become constant, and finally m-Nb ₂ O ₅ is obtained. As such an eluent for Nb ₂ O ₅ , there has been little known so far other than harmful fluorine-based compounds.

  Therefore, in electrolytic synthesis using an Nb electrode, a mesoporous niobium oxide production method that can produce mesoporous niobium oxide containing no impurities at a low voltage and with an easy operation without using a reagent that is extremely toxic to the human body. In addition, the present situation is that provision of an apparatus for producing mesoporous niobium oxide is required.

Yang, P.A. et al. , Nature 1998, 396, 152. Wei, W. et al. Chem. Commun. 2012, 48, 4244. Lee, K.K. et al. Chem. Eur. J. et al. 2012, 18, 9521. Rani, R.D. A. et al. Chem. Commun. 2013, 49, 6349.

  An object of the present invention is to solve the above-described problems and achieve the following objects. That is, the present invention provides a mesoporous niobium oxide that can produce mesoporous niobium oxide containing no impurities at a low voltage and with an easy operation without using a reagent that is extremely toxic to the human body in an electrolytic synthesis using an Nb electrode. An object of the present invention is to provide a method for producing niobium and an apparatus for producing mesoporous niobium oxide.

Means for solving the problems are as follows.
The method for producing mesoporous niobium oxide of the present invention includes an Nb electrode oxidation step of applying a voltage to an Nb electrode as an anode in contact with an aqueous potassium hydroxide solution to oxidize the Nb electrode.

The apparatus for producing mesoporous niobium oxide of the present invention comprises:
An electrolytic cell;
An aqueous potassium hydroxide solution contained in the electrolytic cell;
An Nb electrode as an anode housed in the electrolytic cell and in contact with the aqueous potassium hydroxide solution;
It has the cathode accommodated in the said electrolytic vessel and in contact with the said potassium hydroxide aqueous solution, It is characterized by the above-mentioned.

  According to the present invention, the above-described problems can be solved, and in the electrolytic synthesis using the Nb electrode, no impurities are contained in a low voltage and an easy operation without using a reagent that is extremely toxic to the human body. A method for producing mesoporous niobium oxide capable of producing mesoporous niobium oxide and an apparatus for producing mesoporous niobium oxide can be provided.

FIG. 1A is a field emission scanning electron microscope (FE-SEM) image of an Nb plate before anodization. FIG. 1B is an FE-SEM image of the Nb plate after anodic oxidation (before firing). FIG. 1C is an FE-SEM image of the Nb plate after anodization (after firing). FIG. 2 is an XRD pattern. FIG. 3 is a Raman spectrum. FIG. 4 is a Raman spectrum. FIG. 5 is a graph showing IV characteristics of the dye-sensitized solar cell.

(Mesoporous niobium oxide production equipment)
The apparatus for producing mesoporous niobium oxide of the present invention includes at least an electrolytic cell, a potassium hydroxide aqueous solution, an Nb electrode, and a cathode, and further includes other members as necessary.

<Electrolytic cell>
There is no restriction | limiting in particular as a material of the said electrolytic vessel, According to the objective, it can select suitably, For example, a plastics, glass etc. are mentioned.
There is no restriction | limiting in particular as a shape, a magnitude | size, and a structure of the said electrolytic vessel, According to the objective, it can select suitably.

<Potassium hydroxide aqueous solution>
The aqueous potassium hydroxide solution is accommodated in the electrolytic cell.
There is no restriction | limiting in particular as a density | concentration of the potassium hydroxide in the said potassium hydroxide aqueous solution, Although it can select suitably according to the objective, 0.25 molL < ^-1 > ^-1 molL- ¹ is preferable, 0.5 molL < ^-1 > ^-1 molL ^-1 is more preferable. If the concentration is less than 0.25 mol L ⁻¹ , pores of around 100 nm may be partially generated, and if it exceeds ¹ mol L ⁻¹ , the generated mesoporous niobium oxide may peel from the Nb electrode. . It is advantageous in terms of structural uniformity that the concentration is within the more preferable range.

<Nb electrode (anode)>
The Nb electrode is used as an anode in the mesoporous niobium oxide production apparatus.
The Nb electrode is accommodated in the electrolytic cell and is in contact with the aqueous potassium hydroxide solution.

The material of the Nb electrode is Nb (niobium).
There is no restriction | limiting in particular as a shape, a magnitude | size, and a structure of the said Nb electrode, According to the objective, it can select suitably, For example, plate shape (Nb board) etc. are mentioned.

<Cathode>
The cathode is in contact with the potassium hydroxide aqueous solution in the electrolytic cell.
There is no restriction | limiting in particular as a material of the said cathode, Although it can select suitably according to the objective, Platinum (Pt electrode) is preferable from the point with a low hydrogen overvoltage.

<Other members>
There is no restriction | limiting in particular as said other member, According to the objective, it can select suitably, For example, a spacer etc. are mentioned.

<< Spacer >>
The spacer is disposed between the Nb electrode and the cathode.
There is no restriction | limiting in particular as a material of the said spacer, According to the objective, it can select suitably, For example, an insulating material etc. are mentioned. Examples of the insulating material include polytetrafluoroethylene.
By disposing the spacer between the Nb electrode and the cathode, the distance between the Nb electrode and the cathode can be easily maintained at a predetermined distance.

(Method for producing mesoporous niobium oxide)
The method for producing mesoporous niobium oxide of the present invention includes at least an Nb electrode oxidation step, preferably a heating step, and further includes other steps as necessary.

<Nb electrode oxidation process>
The Nb electrode oxidation step is not particularly limited as long as it is a step of applying a voltage to the Nb electrode to oxidize the Nb electrode, and can be appropriately selected according to the purpose. A step of oxidizing the Nb electrode by applying a voltage between the Nb electrode as the anode in contact with the potassium oxide aqueous solution and the cathode in contact with the potassium hydroxide aqueous solution in the electrolytic cell is preferable.
In the Nb electrode oxidation step, the Nb electrode is oxidized to produce mesoporous niobium oxide.
In the present invention, the mesoporous structure means a structure in which pores of about 2 nm to 50 nm are regularly arranged.

<< Nb electrode >>
The Nb electrode functions as an anode in the Nb electrode oxidation step.
The Nb electrode is in contact with an aqueous potassium hydroxide solution.

The material of the Nb electrode is Nb (niobium).
There is no restriction | limiting in particular as a shape, a magnitude | size, and a structure of the said Nb electrode, According to the objective, it can select suitably, For example, plate shape (Nb board) etc. are mentioned.

<< Aqueous potassium hydroxide >>
There is no restriction | limiting in particular as a density | concentration of the potassium hydroxide in the said potassium hydroxide aqueous solution, Although it can select suitably according to the objective, 0.25 molL < ^-1 > ^-1 molL- ¹ is preferable, 0.5 molL < ^-1 > ^-1 molL ^-1 is more preferable. If the concentration is less than 0.25 mol L ⁻¹ , pores of around 100 nm may be partially generated, and if it exceeds ¹ mol L ⁻¹ , the generated mesoporous niobium oxide may peel from the Nb electrode. . It is advantageous in terms of structural uniformity that the concentration is within the more preferable range.

<< Cathode >>
There is no restriction | limiting in particular as a material of the said cathode, Although it can select suitably according to the objective, Platinum (Pt electrode) is preferable from the point with a low hydrogen overvoltage.

<< Electrolysis tank >>
There is no restriction | limiting in particular as a material of the said electrolytic vessel, According to the objective, it can select suitably, For example, a plastics, glass etc. are mentioned.
There is no restriction | limiting in particular as a shape, a magnitude | size, and a structure of the said electrolytic vessel, According to the objective, it can select suitably.

<< Voltage application >>
There is no restriction | limiting in particular as said voltage, Although it can select suitably according to the objective, 1V-30V are preferable and 1V-5V are more preferable from the point which can oxidize said Nb electrode with a low voltage.

  There is no restriction | limiting in particular as the application time of the said voltage, According to the objective, it can select suitably, For example, 3 hours-6 hours etc. are mentioned.

<Heating process>
The heating step is not particularly limited as long as it is a step of heating the oxidized Nb electrode after the Nb electrode oxidation step, and can be appropriately selected according to the purpose.
The heating step is performed, for example, by removing the oxidized Nb electrode from the electrolytic cell.
By performing the heating step, an amorphous oxidized Nb electrode (mesoporous niobium oxide) can be crystallized.

  There is no restriction | limiting in particular as heating temperature in the said heating process, Although it can select suitably according to the objective, 400 to 500 degreeC is preferable and 400 to 450 degreeC is more preferable.

  There is no restriction | limiting in particular as heating time in the said heating process, Although it can select suitably according to the objective, 0.5 to 3 hours are preferable and 1 to 1.5 hours are more preferable.

The method and apparatus for producing mesoporous niobium oxide of the present invention does not require the use of harmful fluorine-based compounds, and uses the resources (potassium, water) that are abundant on the earth, and electrosynthesis of mesoporous niobium oxide. It can be performed.
The method and apparatus for producing mesoporous niobium oxide of the present invention can perform electrosynthesis of mesoporous niobium oxide at an applied voltage lower than that of conventional electrosynthesis of mesoporous niobium oxide.
The method and apparatus for producing mesoporous niobium oxide of the present invention can produce mesoporous niobium oxide containing no impurities.
In the present invention, K ₈ Nb ₆ O ₁₉ and hydrogen are generated during the electrolytic synthesis. K ₈ Nb ₆ O ₁₉ is a harmless and recyclable compound. Hydrogen is drawing attention as the next generation energy carrier. Therefore, in the electrolytic synthesis according to the present invention, the by-product is also valuable. K ₈ Nb ₆ O ₁₉ is, for example, a metal complex such as (NH ₄ ) ₃ [NbO (C ₂ O ₄ ) ₃ ] · H ₂ O (Bayot, DA et al., Inorg. Chem. 2006, 45, 4407.), H ₈ Nb ₆ O _{19 and} other hydrogen production photocatalysts (Kudo, A. et al., Bull. Chem. Soc. Jpn. 1992, 65, 1202.), Niobium-based oxide containing expected rare earth [La, K, □] ₂ Nb ₂ O _7-x (OH) ₂ (□ is a defect) (Nyman, M. et al., Chem. Mater. 2009, 21, 2201.) can be recycled.

  Examples of the present invention will be described below, but the present invention is not limited to the following examples.

The analyzers used in the following examples are as follows.
Field emission scanning electron microscope: JSM-6500F, manufactured by JEOL Ltd. Powder X-ray diffraction analyzer: MiniFlex600, manufactured by Rigaku Corporation Raman spectrum measuring device: LabRam HR, manufactured by Horiba Ltd. Krypton adsorption / desorption isotherm measuring device: BELSORP-max, manufactured by Nippon Bell Co., Ltd. SEM-EDS: photodiode SiP-N junction type semiconductor detector, manufactured by JEOL Ltd.

Example 1
<Production of _m-Nb 2 _{O 5>}
Mesoporous niobium oxide (m-Nb ₂ O ₅ ) was produced by electrolytic synthesis.
As electrodes, Nb plates (anode, 0.1 mm × 8 mm × 33.5 mm) and Pt plates (cathode, 0.1 mm × 10 mm × 20 mm) were used.
A plastic electrolytic cell (capacity 10 mL) was charged with 7 mL of a ¹ mol L ⁻¹ KOH aqueous solution and kept at room temperature (25 ° C.).
The interval between the Nb plate and the Pt plate was adjusted to 5 mm with a Teflon spacer (5 mm × 10 mm × 15 mm), and the Nb plate / the Teflon spacer / the Pt plate was immersed in the KOH aqueous solution. The KOH aqueous solution was stirred at 350 rpm, and a bias of 1 V (vs. Pt) was applied for 3 hours. After washing the electrode (anode) with water, the temperature was raised at 0.5 ° C. min ⁻¹ and firing at 450 ° C. for 1 hour to obtain m-Nb ₂ O ₅ .

<FE-SEM (field emission scanning electron microscope) observation>
From the FE-SEM image of the electrode obtained after anodic oxidation, it was confirmed that mesopores having a diameter of 5 nm to 10 nm that were not seen before anodic oxidation were uniformly formed (FIGS. 1A and 1B). Even if the obtained electrode was washed with water and fired at 450 ° C. for 1 hour (m-Nb ₂ O ₅ ), it was confirmed that the structure (mesopores) before firing was maintained (FIG. 1C).

<Powder X-ray diffraction>
Measurement of powder X-ray diffraction revealed that it was amorphous before firing, but after firing, showed a diffraction pattern attributed to orthohombic Nb ₂ O ₅ (PDF 30-873) (FIG. 2). . In FIG. 2, (a) is an XRD pattern of a sample after anodization (before firing) in this example, and (b) is after anodization in this example (after firing at 450 ° C. for 1 hour). It is an XRD pattern of a sample, and “Nb ₂ O ₅ ” is a diffraction pattern of Nb ₂ O ₅ (PDF 30-873).

<Raman spectrum>
When the sample before baking was identified from the Raman spectrum measurement result, it was found to be Nb ₂ O ₅ .nH ₂ O (FIG. 3). In FIG. 3, (a) is a Raman spectrum of the sample before anodization in this example, and (b) is Nb ₂ O ₅ .nH ₂ O synthesized by hydrolyzing K ₈ Nb ₆ O _19. It is a Raman spectrum of (comparative sample).

<Specific surface area>
The specific surface area of the anodized electrode (anode) was calculated from the krypton adsorption / desorption isotherm by the BET method. Assuming that the thickness of the mesoporous layer was 100 nm, the BET specific surface area was 892 m ² cm ⁻³ .

<M-Nb ₂ O ₅ formation mechanism>
Various studies were conducted in order to obtain knowledge about the formation mechanism of m-Nb ₂ O ₅ by anodic oxidation.
Anodization was performed for 24 hours, and the resulting aqueous solution was concentrated under reduced pressure. When a liquid phase product was confirmed by Raman spectroscopy, a peak attributed to water-soluble K ₈ Nb ₆ O ₁₉ was observed (FIG. 4). . In FIG. 4, (a) is a Raman spectrum of the liquid phase product after anodic oxidation, and (b) is K ₈ Nb ₆ O obtained by baking Nb ₂ O ₅ and KOH at 380 ° C. for 5 hours. ₁₉ is a Raman spectrum of (comparative sample).
Next, the vapor phase product was analyzed using a two-chamber electrolysis cell. In anodic oxidation, oxygen is necessary for the formation of m-Nb ₂ O ₅ , so the anode side (Nb plate) is vented, the cathode side (Pt plate) is replaced with an inert gas, and electrolysis is performed for 3 hours. The gas phase on the cathode side was sampled. As a result, it was confirmed by gas chromatography that only H ₂ was produced.
From the above results, it was found that the liquid phase product and the gas phase product were K ₈ Nb ₆ O ₁₉ and H ₂ , respectively.

When elemental analysis of m-Nb ₂ O ₅ was performed with SEM-EDS, the presence of K ⁺ was not detected, so it was confirmed that m-Nb ₂ O ₅ contained no impurities.

<Battery performance>
The performance evaluation of m-Nb ₂ O ₅ was performed.
The fired Nb plate (m-Nb ₂ O ₅ ) produced in Example 1 was immersed in an ethanol solution of Ru dye (N719) for 12 hours and washed with acetonitrile, and used as the negative electrode. The counter electrode is Pt sputtered FTO (Fourine-doped tin oxide), and the electrolyte is I ⁻ / I ₃ ⁻ acetonitrile solution [LiI (0.1 mol L ⁻¹ ), I ₂ (0.05 mol L ⁻¹ ), t-butyl]. Pyridine (0.5 molL ⁻¹ ) and dimethylpropylimidazolium iodide (0.6 molL ⁻¹ )] were used. Pseudo sunlight (AM1.5, 100 mWcm ⁻² ) was irradiated from the counter electrode side to evaluate battery performance. The results are shown in FIG. The battery performance was good.

Examples of the aspect of the present invention include the following.
<1> A method for producing mesoporous niobium oxide, comprising an Nb electrode oxidation step of applying a voltage to an Nb electrode as an anode in contact with an aqueous potassium hydroxide solution to oxidize the Nb electrode.
<2> The Nb electrode oxidation step applies a voltage between the Nb electrode serving as an anode in contact with the potassium hydroxide aqueous solution in the electrolytic cell and the cathode in contact with the potassium hydroxide aqueous solution in the electrolytic cell, The method for producing mesoporous niobium oxide according to <1>, which is a step of oxidizing a Nb electrode.
<3> The method for producing mesoporous niobium oxide according to <2>, wherein the cathode is a Pt electrode.
<4> The method for producing mesoporous niobium oxide according to any one of <1> to <3>, further including a heating step of heating the oxidized Nb electrode after the Nb electrode oxidation step.
<5> an electrolytic cell;
An aqueous potassium hydroxide solution contained in the electrolytic cell;
An Nb electrode as an anode housed in the electrolytic cell and in contact with the aqueous potassium hydroxide solution;
An apparatus for producing mesoporous niobium oxide, comprising a cathode housed in the electrolytic cell and in contact with the potassium hydroxide aqueous solution.
<6> The mesoporous niobium oxide production apparatus according to <5>, wherein the cathode is a Pt electrode.

Claims (6)

  A method for producing mesoporous niobium oxide, comprising an Nb electrode oxidation step of applying a voltage to an Nb electrode serving as an anode in contact with an aqueous potassium hydroxide solution to oxidize the Nb electrode.   In the Nb electrode oxidation step, a voltage is applied between the Nb electrode as an anode in contact with the potassium hydroxide aqueous solution in the electrolytic cell and the cathode in contact with the potassium hydroxide aqueous solution in the electrolytic cell, and the Nb electrode is The method for producing mesoporous niobium oxide according to claim 1, which is a step of oxidizing.   The method for producing mesoporous niobium oxide according to claim 2, wherein the cathode is a Pt electrode.   The method for producing mesoporous niobium oxide according to any one of claims 1 to 3, further comprising a heating step of heating the oxidized Nb electrode after the Nb electrode oxidation step. An electrolytic cell;
An aqueous potassium hydroxide solution contained in the electrolytic cell;
An Nb electrode as an anode housed in the electrolytic cell and in contact with the aqueous potassium hydroxide solution;
An apparatus for producing mesoporous niobium oxide, comprising a cathode housed in the electrolytic cell and in contact with the aqueous potassium hydroxide solution.   6. The apparatus for producing mesoporous niobium oxide according to claim 5, wherein the cathode is a Pt electrode.