JP7726526B2 - Novel porous metal oxide - Google Patents

Description

The present invention relates to a novel porous metal oxide with high oxygen generation catalytic activity. More specifically, it relates to a novel porous metal oxide with high oxygen generation catalytic activity, which is obtained by calcining ionic crystals.

Hydrogen production by water electrolysis, which converts electricity generated from renewable energy sources into hydrogen, is attracting attention as a technology that can contribute to the realization of a decarbonized society. Water electrolysis involves a hydrogen evolution reaction at the cathode and an oxygen evolution reaction at the anode. However, the high overvoltage of the anode oxygen evolution reaction is a particular challenge. Platinum group metals such as iridium (Ir) and ruthenium (Ru) and their oxides are considered promising oxygen evolution catalysts with low overvoltage and high activity. These catalysts have been further enhanced in activity by microparticulating them (Non-Patent Documents 1 and 2), making them porous (Non-Patent Document 3), and alloying them (Non-Patent Document 4). It is also known that highly active oxygen evolution catalysts can be produced using synthesis methods similar to those used for Adams catalysts or methods based on these methods (Non-Patent Documents 5 and 6, Patent Document 1). However, both production methods require complex multi-step procedures and require strict control of production conditions, such as pH adjustment and the amount of reducing agent.

Meanwhile, the present inventors have proposed a method for recovering platinum group metals as precipitates from hydrochloric acid solutions using a primary amine compound as a recovery agent. They have reported that rhodium (Rh), which is known to be extremely difficult to actively recover among platinum group metals, can also be recovered as a precipitate using a primary amine compound. The Rh recovered as a precipitate has been found to be an ionic crystal consisting of a Rh(III) chloride complex ion ([RhCl ₆ ] ^3- ) and a protonated amine compound (Non-Patent Documents 7 to 9, Patent Documents 2 and 3).

Japanese Patent Application Laid-Open No. 2020-132465 WO2017/170444 JP 2019-163502 A

ACS Catal. , 2017, 7, 5983-5986. Int. J. Hydrog. Energy, 2020, 45, 33491-33499. Nanoscale, 2017, 9, 9291-9298. ACS Nano, 2019, 13, 13225-13234. Int. J. Hydrog. Energy, 2018, 43, 19460-19467. Catalysis, 2019, 9, 318. ACS Omega, 2019, 4, 1868-1873. ACS Omega, 2019, 4, 14613-14620. Metals, 2020, 10, 324.

The object of the present invention is to provide a new porous metal oxide that has high oxygen generation catalytic activity while being easily produced.

In order to achieve the above object, the present inventors have conducted various studies and found that a porous metal oxide having high oxygen evolution catalytic activity can be easily produced by calcining an ionic crystal composed of a platinum group metal chloride complex ion and a protonated primary amine compound, and have arrived at the present invention.
1. A porous metal oxide obtained by firing an ionic crystal consisting of a platinum group metal chloride complex ion and a protonated primary amine compound.
2. The porous metal oxide of 1 above, wherein the firing temperature is 360°C to 600°C.
3. The porous metal oxide according to 1 or 2 above, wherein the chloride complex ions of platinum group metals are [IrCl ₆ ] ³⁻ and/or [RuCl ₆ ] ³⁻ .
4. The porous metal oxide according to any one of 1 to 3 above, wherein the primary amine compound is an aromatic primary amine compound and/or an aliphatic primary amine compound.
5. The porous metal oxide according to any one of 1 to 4 above, which has an electrochemically active surface area of 90 to 700 ^{m 2} /g when measured in a 0.5 mol/L sulfuric acid solution.
6. An oxygen generating catalyst which is the porous metal oxide according to any one of 1 to 5 above.
7. An oxygen generating catalyst precursor, which is an ionic crystal consisting of a platinum group metal chloride complex ion and a protonated primary amine compound.
8. (1) A step of mixing a primary amine compound with a hydrochloric acid solution containing a platinum group metal;
(2) A method for producing porous metal oxide, which includes a step of calcining the resulting precipitate.

The present invention provides a new porous metal oxide that has high oxygen generation catalytic activity and is easily manufactured.

The structure of an ionic crystal consisting of iridium chloride complex ions ([IrCl ₆ ] ^3- ) and protonated p-phenylenediamine (PPDA) is shown. This is the result of X-ray structural analysis. The principle of producing porous iridium oxide by calcination is shown schematically. An example of the flow of manufacturing porous iridium oxide is shown below. The current density-potential curve when examining the oxygen generating overvoltage is shown. The oxygen generating overvoltage is the value obtained by subtracting the theoretical potential for oxygen generation, 1.23 V, from the voltage (horizontal axis) when the current density (vertical axis) reaches ¹⁰ mA/cm². The durability of electrodes made from each material is shown ((A) porous iridium oxide, (B) porous ruthenium oxide). If the change in potential over time is constant, the electrode has high durability.

The present invention will be described in detail below.
1. Chloride complex ions of platinum group metals There are six types of platinum group metals: ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt). The chloride complex ions of platinum group metals used in the present invention may be any of these chloride complex ions, but chloride complex ions of iridium (III), ruthenium (III), or both are more preferred, and chloride complex ions of iridium (III) are particularly preferred.
In terms of catalytic activity as a metal oxide, iridium oxide and ruthenium oxide are superior to other metal oxides. Furthermore, when comparing iridium oxide and ruthenium oxide, ruthenium oxide has a superior catalytic activity, but iridium oxide is significantly superior in terms of durability. From the viewpoint of the balance between catalytic activity and durability, it is particularly preferable to use an iridium (III) chloride complex ion.

2. Primary Amine Compound The primary amine compound used in the present invention is not particularly limited, but is preferably an aromatic primary amine compound and/or an aliphatic primary amine.
The aromatic ring of the aromatic primary amine compound may be a heterocycle such as a nitrogen-containing heterocycle.
The aliphatic primary amine is preferably a linear alkyl monoamine, and more preferably a linear alkyl monoamine having 6 to 18 carbon atoms. A linear alkyl monoamine having 6 to 18 carbon atoms allows selective recovery of platinum group metals (see Patent Document 2), making it possible to produce a catalyst from low-grade raw materials and facilitating the reuse of the catalyst.
Also, different primary amine compounds may be used in combination.
Examples of the aromatic primary amine compound include the following.
(I) Aromatic primary monoamine compounds such as aniline, 4-butylaniline, and 4-phenoxyaniline; (II) Aromatic primary diamine compounds such as p-phenylenediamine (PPDA), m-phenylenediamine, 2,6-diaminopyridine, and 4,4'-oxydianiline;
(III) Aromatic primary triamine compounds such as 1,3,5-benzenetriamine, melamine, and 2,4,6-triaminopyrimidine,
(IV) Benzylamine compounds having two or more amines bonded thereto, such as m-xylylenediamine (MXDA) and p-xylylenediamine. Examples of aliphatic primary amine compounds include the following.
(V) Linear alkyl monoamines having 6 to 18 carbon atoms, such as n-nonylamine. In protonated primary amine compounds, the amino group is often protonated, but if the heterocycle contains a nitrogen atom, the nitrogen atom may also be protonated. For example, in the case of aniline, the amino group is protonated, and in the case of melamine, one or two nitrogen atoms in the heterocycle are protonated.

3. Ionic Crystals The ionic crystals formed in the present invention can be used as catalyst precursors that become porous metal oxides for oxygen generating catalysts by calcination.
The ionic crystals consist of platinum group metal chloride complex ions and protonated primary amine compounds, but since these two are the main components, they may also contain other mixtures as long as they do not affect the formation of the ionic crystals.
Ionic crystals are formed by ionic bonding between negatively charged platinum group metal chloride complex ions and positively charged protonated primary amine compounds. For example, an ionic crystal of iridium chloride complex ions ([IrCl ₆ ] ^3- ) and protonated 4-butylaniline has a structure in which the basic unit is an ion pair in which six protonated 4-butylaniline anions and three chloride anions surround [IrCl ₆ ] ^3- .
For example, an ionic crystal of [IrCl ₆ ] ^3- and protonated p-phenylenediamine (PPDA) is constructed by combining [IrCl ₆ ] ^3- , protonated PPDA (PPDA-2H ⁺ ), chloride anions, and water in a ratio of [IrCl ₆ ] ^3- : PPDA-2H ⁺ : Cl ^- : H ₂ O = 1:2:1:2 (Figure 1).
There is no particular limit to the particle size of the ionic crystals, and they may be a mixture of various particle sizes, but for ease of handling, crystals in the range of 1 μm to 5 mm are more preferable. The particle size is calculated from the pore size of the filter through which they pass.
Furthermore, the ionic crystal may be either a single crystal or a polycrystal, and there is no limit to the size of the crystallite (or crystal if it is a single crystal). Larger crystals are not necessarily preferable.

4. Porous Metal Oxide Porous metal oxide can be produced by firing ionic crystals. For example, firing an ionic crystal consisting of iridium chloride complex ions ([IrCl ₆ ] ^3- ) and p-phenylenediamine (PPDA) forms pores, allowing the production of porous iridium oxide (Figure 2, SEM image at the bottom right of Figure 3).
Porous ruthenium oxide can be produced from ionic crystals containing ruthenium (III). Furthermore, porous alloy metal oxide (iridium oxide/ruthenium oxide) can be obtained from ionic crystals containing both iridium (III) and ruthenium (III). It is also possible to combine metals to form alloys.
The main components of iridium oxide and ruthenium oxide in the present invention are IrO ₂ and RuO ₂ , but they may also contain oxides with different oxidation numbers, such as Ir ₂ O ₃ and Ru ₂ O ₃ .
In the present invention, calcination removes the primary amine compound that forms ionic crystals, forming voids (pores), resulting in a porous metal oxide. Since the electrochemically active surface area (ECSA), which is the area of the portion that actually functions as a catalyst, is large as described below, it can be said to have a dense porous structure.
There is no method that can uniquely define this porous structure, and it is impossible and impractical to identify porous metal oxides from their structure. Although the BET specific surface area is a tentative index of porosity, it is needless to say that the BET specific surface area cannot uniquely define the porous structure.

5. Production of Porous Metal Oxides Using Ionic Crystals Ionic crystals can be produced by methods that have already been reported (Non-Patent Documents 7 to 9, Patent Documents 2 to 4). Ir(III) and Ru(III) exist in hydrochloric acid solutions as chloride complex ions ([IrCl ₆ ] ^3- and [RuCl ₆ ] ^3- ) similar to Rh(III), and are therefore thought to form ionic crystals with primary amine compounds, similar to Rh, and can be produced by methods that have already been reported.
Specifically, for example, a platinum group metal (e.g., Ir(III))-containing ionic crystals can be produced by mixing a primary amine compound with a platinum group metal dissolved in hydrochloric acid, and then leaving the mixture to stand, shaking, or stirring. The resulting precipitate can be collected to obtain the ionic crystals. "3. Ionic Crystals"
As described above, it is not necessary to grow crystals, so it is sufficient to mix the ingredients to precipitate ionic crystals, and the operation may be any method such as standing, shaking, stirring, or a combination of these. In this way, the ionic crystals of the present invention can be easily produced without complex conditions.
Next, the recovered ionic crystals are calcined. The calcination temperature is more preferably 360 to 600°C, even more preferably 380 to 480°C, and particularly preferably 380 to 400°C. Calcination is more preferably carried out in an air atmosphere by heating to the target temperature at a temperature increase rate of more preferably 10 to 50°C/min, even more preferably 15 to 25°C/min, and maintaining the target temperature for more preferably 10 to 60 minutes, even more preferably 10 to 20 minutes.
As described above, according to the method for producing porous metal oxide of the present invention, porous metal oxide can be produced simply by using ionic crystals as a catalyst precursor and calcining it.
Furthermore, while conventional methods could only use high-purity starting materials (such as chloroiridic acid or iridium complexes), the present invention obtains ionic crystals using the same principle as the platinum group metal selective recovery method (Non-Patent Documents 7-9, Patent Documents 2 and 3). Therefore, even if the iridium or ruthenium starting materials contain other metal elements, ionic crystals containing only iridium or ruthenium can be selectively obtained, making it possible to produce catalysts from low-grade starting materials. Furthermore, used catalysts can be regenerated by leaching them with hydrochloric acid. By using primary amine compounds such as 4-butylaniline, p-phenylenediamine, melamine, and 2,6-diaminopyridine, ionic crystals containing only iridium or ruthenium can be obtained, even if other platinum group metal elements are mixed in.

6. Uses of Porous Metal Oxide The porous metal oxide of the present invention has catalytic activity for oxygen generation and can be used, for example, together with a conductive support such as carbon fiber as the anode of a water electrolysis device.

7. Catalytic Activity of Porous Metal Oxide When an oxygen generating catalyst is cast onto a glassy carbon electrode and further coated with Nafion (trade name) to prepare an electrode, the catalytic activity in a 0.5 M aqueous sulfuric acid solution is preferably as follows:
Regarding the performance as a catalyst, for example, when cyclic voltammetry is performed using the electrode prepared above as a working electrode, at a potential range of 1.2 to 1.4 V and a sweep rate of 5 to 80 mV/s, and the electrochemically active surface area (ECSA) is calculated from the increase in the electric double layer with respect to the sweep rate, assuming the specific capacity of iridium oxide and ruthenium oxide to be 35 μF/cm ² , an ECSA of 90 to 700 ^{m 2} /g is more preferable, 200 to 700 ^{m 2} /g is even more preferable, and 500 to 700 m ² /g is particularly preferable.
Furthermore, when linear sweep voltammetry is performed from a potential of 1.1 V to 1.9 V at a sweep rate of 10 mV/s, and the difference between the potential at which a current density of 10 mA/ ^cm2 is reached and 1.23 V, which is the theoretical potential for oxygen generation, is calculated as the oxygen generating overvoltage, the oxygen generating overvoltage is more preferably 240 to 350 mV, even more preferably 240 to 320 mV, and particularly preferably 240 to 285 mV.
When the working electrode is rotated at 1600 rpm, the current density is fixed at 10 mA/ ^cm2 , and the change in potential over time is measured to evaluate durability, it is more preferable that the durability be 12 hours or more for iridium oxide, and 5 hours or more for ruthenium oxide.
Surprisingly, despite the extremely simple manufacturing method of the present invention, the porous iridium oxide manufactured by this method exhibits excellent oxygen evolution catalytic activity with an oxygen evolution overpotential of 0.34 V or less (Table 1).
Furthermore, "Ir(III)-PPDA 380°C" has an oxygen evolution overpotential of 270 mV (Table 1), which shows almost the highest oxygen evolution catalytic activity of any iridium oxide catalyst.
Furthermore _, the durability is surprisingly longer than that of commercially available products, and it is particularly surprising that the durability of ruthenium oxide, which has low stability, has improved to more than 5 hours.

Example 1 Production of Porous Iridium Oxide 1. Production of Porous Iridium Oxide p-phenylenediamine (PPDA) hydrochloride was added to a 5 M hydrochloric acid solution containing iridium (Ir) so that the ratio of Ir:PPDA was 1:8, and the mixture was allowed to stand for one week.
The resulting precipitate was collected by filtration. This precipitate (ionic crystals in which Ir(III) and p-phenylenediamine (PPDA) were ionically bonded) was heated to 380°C at a rate of 20°C per minute in an air atmosphere and baked for 15 minutes to obtain porous iridium oxide (Figure 3).

Example 2: Production of porous iridium oxide by varying the heating temperature during firing Porous iridium oxide was obtained in the same manner as in Example 1, except that the heating temperature was changed in the range of 360 to 580°C (see the firing temperature for "Ir(III)-PPDA" in Table 1).

Example 3 Production of Porous Iridium Oxide Using Different Aromatic Primary Amine Compounds Porous iridium oxide was obtained in the same manner as in Example 1, except that the hydrochlorides of 4-butylaniline (BuA), melamine, or m-xylylenediamine (MXDA) were used (Table 1, "Ir(III)-BuA" to "Ir(III)-MXDA").

Example 4 Production of Porous Iridium Oxide Using an Aliphatic Primary Amine Compound Porous iridium oxide was obtained in the same manner as in Example 1, except that n-nonylamine (NoA) hydrochloride was used (Table 1 "Ir(III)-NoA").

Example 5 Production of Porous Ruthenium Oxide Porous ruthenium oxide was obtained in the same manner as in Example 1, except that a 5 M hydrochloric acid solution containing ruthenium (Ru) was used (Table 1 "Porous Ruthenium Oxide").

Example 6 Production of Porous Iridium Oxide/Ruthenium Oxide p-phenylenediamine (PPDA) hydrochloride was added to a 5 M hydrochloric acid solution containing iridium Ir and ruthenium Ru so that the Ir:Ru ratio in the ionic crystal was 1:1. Except for this, porous iridium oxide/ruthenium oxide was obtained in the same manner as in Example 1 (Table 1 "Porous Iridium Oxide/Ruthenium Oxide").

Example 7 Catalyst Evaluation 1. Preparation of Working Electrode The porous iridium oxide prepared in the previous example was dispersed in a solvent consisting of a 1:2 volumetric mixture of water and 2-propanol, and the dispersion was cast onto a glassy carbon electrode with an electrode area of 0.196 ^cm² , resulting in 0.128 mg/ ^cm² of iridium oxide being placed on the electrode. After drying, 5 µL of a 0.25 wt% Nafion ethanol solution was cast onto the electrode, coating it with Nafion. Glassy carbon electrodes carrying porous ruthenium oxide or porous iridium oxide/ruthenium oxide were also prepared using a similar method.
For comparison, electrodes were also prepared in the same manner using commercially available iridium oxide (Aldrich, Product No. 206237) and ruthenium oxide (Aldrich, Product No. 238058).
2. Electrochemical Measurements Electrochemical measurements were carried out using the glassy carbon electrode prepared as described above as the working electrode, a platinum wire as the counter electrode, and a silver-silver chloride electrode as the reference electrode, with a 0.5 mol/L aqueous sulfuric acid solution (saturated with nitrogen gas) as the electrolyte. All potentials are shown converted to the reversible hydrogen electrode (RHE) standard. Commercially available iridium oxide and ruthenium oxide were used as controls.
2-1 Evaluation of Electrochemically Active Surface Area Cyclic voltammetry was performed at a potential range of 1.2 to 1.4 V, with the sweep rate varying from 5 to 80 mV/s. The electrochemically active surface area (ECSA) was calculated from the increase in the electric double layer relative to the sweep rate, assuming that the specific capacity of iridium oxide and ruthenium oxide was 35 μF/ ^cm² .
2-2 Evaluation of oxygen generating overvoltage Linear sweep voltammetry was performed from a potential of 1.1 V to 1.9 V at a sweep rate of 10 mV/s, and the difference between the potential at which a current density of 10 mA/ ^cm2 was reached and the theoretical potential for oxygen generation, 1.23 V, was taken as the oxygen generating overvoltage.
2-3 Evaluation of durability The working electrode was rotated at 1600 rpm, the current density was fixed at 10 mA/ ^cm2 , and the change in potential with time was measured.
2-4. Measurement results The porous iridium oxide produced from ionic crystals consisting of Ir(III) and p-phenylenediamine (PPDA) (Table 1, "Porous iridium oxide Ir(III)-PPDA") had a maximum ECSA of 536 m ² /g when the calcination temperature was 380°C, and the ECSA decreased when calcined at lower or higher temperatures. Regardless of the calcination temperature, the porous iridium oxide produced from ionic crystals consisting of Ir(III) and PPDA (Table 1, "Porous iridium oxide Ir(III)-PPDA") showed a larger ECSA than commercially available iridium oxide ("Iridium oxide (commercially available)").
Porous iridium oxide produced from ionic crystals consisting of Ir(III) and PPDA (porous iridium oxide "Ir(III)-PPDA" in Table 1) showed the lowest oxygen evolving overvoltage of 270 mV when the calcination temperature was 380°C. The oxygen evolving overvoltage correlates with the magnitude of the ECSA, and the larger the ECSA, the lower the overvoltage tended to be. Regardless of the calcination temperature, porous iridium oxide produced from ionic crystals consisting of Ir(III) and PPDA (porous iridium oxide "Ir(III)-PPDA" in Table 1) showed a lower overvoltage (oxygen evolving overvoltage can be determined from Figure 4) than commercially available iridium oxide ("Iridium oxide (commercially available)" in Table 1), demonstrating its superiority as an oxygen evolving catalyst.
Porous iridium oxides prepared from ionic crystals consisting of various amine compounds and Ir(III) (Table 1, "Ir(III)-BuA,""Ir(III)-Mermamine,""Ir(III)-MXDA," and "Ir(III)-NoA") all exhibited larger ECSA and lower oxygen evolution overpotential than commercially available iridium oxide (Table 1, "Iridium oxide (commercial product)"). This indicates that the performance as an oxygen evolution catalyst is not significantly affected by the type of amine compound.
Porous ruthenium oxide produced from ionic crystals composed of Ru(III) and PPDA (porous ruthenium oxide of "Ru(III)-PPDA" in Table 1) exhibited a lower oxygen evolving overvoltage than porous iridium oxide produced in the same manner (porous iridium oxide of "Ir(III)-PPDA, 380°C" in Table 1). Ru is known to exhibit higher oxygen evolving catalytic activity than Ir, and this result reflects this. Furthermore, porous ruthenium oxide exhibited a larger ECSA and lower oxygen evolving overvoltage than commercially available ruthenium oxide (ruthenium oxide (commercial product) in Table 1), indicating that, like iridium oxide, the oxygen evolving catalytic activity was improved by making the ionic crystals porous through calcination.
The porous oxide (iridium oxide/ruthenium oxide) of an alloy containing Ir and Ru in a molar ratio of 1:1 (Table 1, porous iridium oxide/ruthenium oxide in "Ir(III)/Ru(III)-PPDA") exhibited an oxygen evolution overvoltage intermediate between that of porous iridium oxide (Table 1, porous iridium oxide in "Ir(III)-PPDA, 380°C") and porous ruthenium oxide. This indicates that the oxygen evolution catalytic activity can be changed by controlling the molar ratio of Ir and Ru.
The durability test results showed that the potential of commercially available iridium oxide rose significantly in about 1.5 hours, but the potential of porous iridium oxide did not change significantly even after a long measurement period of 12 hours (Figure 5(A)). Furthermore, the potential of commercially available ruthenium oxide rose significantly immediately, but the potential of porous ruthenium oxide did not change significantly over a long measurement period of 5 hours (Figure 5(B)). This indicates that increasing porosity improves not only the oxygen evolving overpotential but also the durability of the catalyst.

This invention makes it possible to produce an oxygen generating catalyst with high catalytic activity using a simple method. This catalyst can be used as an electrode for water electrolysis, making it useful in industries involved in the process of converting electricity produced from natural energy into hydrogen through the electrolysis of water and storing it, particularly in industries related to water electrolysis equipment.

Claims (8)

A porous metal oxide formed by calcining ionic crystals consisting of platinum group metal chloride complex ions and protonated primary amine compounds. The porous metal oxide of claim 1, wherein the firing temperature is 360°C to 600°C. 3. The porous metal oxide of claim 1, wherein the chloride complex ions of platinum group metals are [IrCl ₆ ] ^3- and/or [RuCl ₆ ] ^3- . The porous metal oxide of any one of claims 1 to 3, wherein the primary amine compound is an aromatic primary amine compound and/or an aliphatic primary amine compound. 5. The porous metal oxide according to claim 1, which has an electrochemically active surface area of 90 to 700 ^{m 2} /g when measured in a 0.5 mol/L sulfuric acid solution. An oxygen generating catalyst comprising the porous metal oxide of any one of claims 1 to 5. An oxygen evolution catalyst precursor, an ionic crystal consisting of a platinum group metal chloride complex ion and a protonated primary amine compound. (1) mixing a primary amine compound with a hydrochloric acid solution containing a platinum group metal;
(2) A method for producing porous metal oxide, which includes a step of calcining the resulting precipitate.