JP6763009B2 - Oxygen generation catalyst - Google Patents

Description

The present invention relates to an oxygen generation catalyst, and more particularly to an oxygen generation catalyst that can be used in an oxygen electrode of a water electrolyzer or an anode of a fuel cell.

Ruthenium oxide, iridium oxide and the like are known as materials exhibiting oxygen production (OER) activity. A material exhibiting such OER activity is
(A) The catalyst on the oxygen electrode side of the water electrolyzer,
(B) When power generation is continued in a state where the fuel supply to some single cells in the fuel cell stack is interrupted, the oxidation of the carbon material generated at the anode of the single cell (fuel shortage cell) where the fuel supply is interrupted It is used as a catalyst for suppressing.

Among these, iridium oxide has higher durability of OER activity than other materials. Therefore, iridium oxide is often used as the oxygen generation catalyst. However, iridium oxide has low initial activity and high cost.
On the other hand, ruthenium oxide has a lower cost and higher initial activity than iridium oxide. However, ruthenium oxide has a problem that the durability of OER activity is low.

Therefore, various proposals have been made in order to solve this problem.
For example, in Patent Document 1,
(A) A hexachloroiridium acid (H ₂ IrCl ₆ ) solution is added to a suspension of TiO ₂ (BET> 300 m ² / g), and the suspension is heated to 70 ° C.
A method for producing an IrO ₂ / TiO ₂ catalyst is disclosed in which (b) the product is isolated by filtration and (c) the product is calcined at 400 ° C.

In the same document,
(A) By such a method, a catalyst in which iridium oxide (IrO ₂ ) particles are finely deposited on the inorganic oxide material (TiO ₂ ) can be obtained.
(B) When iridium oxide is deposited on an inorganic oxide material having a high specific surface area, the aggregation of iridium oxide is suppressed even after heat treatment, and the high specific surface area is maintained, whereas the particles are aggregated only with iridium oxide. Easy points and
(C) As a result, the IrO ₂ / TiO ₂ catalyst has a lower starting potential for oxygen evolution (ie, higher OER activity) than the IrO ₂ catalyst.
Is described.

Patent Document 1 describes that by supporting fine IrO ₂ particles on the surface of TiO ₂ , a decrease in OER activity (aggregation of IrO ₂ ) is suppressed.
However, the catalyst described in the same document is not configured to protect the catalyst surface, which is the starting point of catalyst deterioration, and does not solve the problem of durability. Moreover, since IrO ₂ is used as a catalyst, the cost is high.

Special Table 2007-514520

An object to be solved by the present invention is to provide an oxygen generation catalyst having an initial activity and durability equal to or higher than that of iridium oxide and at a lower cost than that of iridium oxide.

In order to solve the above problems, the oxygen generation catalyst according to the present invention is used.
It has a core and a shell that covers the surface of the core.
The core contains at least ruthenium oxide or metallic ruthenium on its surface.
The gist is that the shell contains titania or a composite oxide of titanium and ruthenium.

The oxygen generation catalyst according to the present invention exhibits initial activity and durability equal to or higher than those of conventional catalysts using iridium oxide. This is because the surface of the core containing ruthenium oxide or metallic ruthenium is covered with a shell containing titania or a composite oxide of titanium and ruthenium to protect the catalyst surface, which is the starting point of catalyst deterioration. Conceivable. Further, since the oxygen generation catalyst according to the present invention contains ruthenium oxide or metallic ruthenium as a main component, the cost is lower than that of a conventional catalyst containing iridium oxide as a main component.

3 is a TEM image of the oxygen generation catalyst obtained in Example 1 and Comparative Example 1. 8 is an EDX mapping of the oxygen generation catalyst obtained in Example 1. It is the initial water electrolysis activity of the oxygen generation catalyst obtained in Example 1 and Comparative Examples 1 to 3. It is the activity change in the potential cycle (0.07V⇔1.8V) of the oxygen generation catalyst obtained in Example 1 and Comparative Examples 1 to 3. FIG. 5 (A) shows changes in the IV characteristics of the oxygen generation catalyst obtained in Example 1. FIG. 5 (B) shows changes in the IV characteristics of the oxygen generation catalyst obtained in Comparative Example 1.

It is the activity change in the potential cycle (0.07V⇔1.8V) of the oxygen generation catalyst obtained in Examples 1-5 and Comparative Examples 1-2. It is the titania coverage dependence of the initial activity of the oxygen generation catalysts obtained in Examples 1 to 5 and Comparative Examples 1 and 2 and the activity after the durability test. It is the increase width of the potential when the potential cycle (0.07V⇔1.8V) is added to the oxygen generation catalysts obtained in Examples 1 to 5 and Comparative Examples 1 and 2. It is the titania coverage dependence of the increase width of the potential when the potential cycle (0.07V⇔1.8V) is applied to the oxygen generation catalysts obtained in Examples 1 to 5 and Comparative Example 1.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Oxygen generation catalyst]
The oxygen generation catalyst according to the present invention is
It has a core and a shell that covers the surface of the core.
The core contains at least ruthenium oxide or metallic ruthenium on its surface.
The shell contains titania or a composite oxide of titanium and ruthenium.

[1.1. core]
The core contains at least ruthenium oxide (RuO ₂ ) or metallic ruthenium (Ru) on its surface. Ruthenium oxide or metallic ruthenium may be contained only on the surface of the core or may be contained throughout the core. The surface of the core is preferably composed substantially only of ruthenium oxide or metallic ruthenium, but may contain unavoidable impurities. Since the central portion of the core does not contribute much to the oxygen generation reaction, it does not necessarily have to be composed of ruthenium oxide or metallic ruthenium, and may be composed of other materials.
The particle size of the core is not particularly limited. In general, the smaller the particle size of the core, the higher the effect can be obtained by adding a small amount. For that purpose, the average particle size of the core is preferably 1 μm or less. The average particle size of the core is preferably 500 nm or less, more preferably 200 nm or less.

[1.2. shell]
The shell contains titania (TiO ₂ ) or a composite oxide of titanium and ruthenium ((Ti, Ru) O ₂ ). As will be described later, the shell is formed by forming a titania precursor around the core and firing it. At this time, ruthenium oxide or metallic ruthenium on the core surface may react with the titania precursor to form a composite oxide. The shell is preferably composed substantially only of titania or a composite oxide, but may contain unavoidable impurities.
The shell covering the surface of the core is not particularly limited as long as it does not inhibit the OER activity of ruthenium oxide or metallic ruthenium and can improve the durability of ruthenium oxide or metallic ruthenium.

[1.3. Titania coverage]
"Titania coverage" refers to the number of layers of the titania atomic layer, assuming that all titanium contained in the shell is titania (TiO ₂ ) and that titania uniformly covers the surface of the core. Say. "1ML" represents one atomic layer titania. Specifically, "1ML" is defined as when the surface atomic number density of TiO ₂ (the total number of Ti and O per unit area) is 1.5 × 10 ¹⁵ cm ⁻² .

The smaller the titania coverage, the lower the durability of the oxygen generation catalyst. In order to obtain durability equal to or higher than that of iridium oxide, the titania coverage is preferably 0.05 ML or higher. The titania coverage is preferably 0.1 ML or more.
On the other hand, titania itself has no OER activity. Therefore, if the titania coverage becomes too large, the resistance to the diffusion of water to the core surface and the diffusion of oxygen from the core surface increases, so that the OER activity decreases. Therefore, the titania coverage is preferably 5.0 ML or less. The titania coverage is preferably 0.5 ML or less.

[1.4. Use]
The oxygen generation catalyst according to the present invention is
(A) A catalyst used for the oxygen electrode of a water electrolyzer,
(B) A catalyst added to the anode of a fuel cell (a catalyst for suppressing oxidation of carbon material generated at the anode of a fuel-deficient cell),
It can be used for such purposes.

[2. Manufacturing method of oxygen generation catalyst]
The oxygen generation catalyst according to the present invention can be produced by the so-called sol-gel method.

[2.1. Preparation of dispersion]
First, core particles containing ruthenium oxide or metallic ruthenium at least on the surface are dispersed in a solvent to obtain a dispersion liquid. The solvent is
(A) Core particles can be dispersed and
(B) A material capable of hydrolyzing and polycondensing a Ti source (alkoxide) so that the surface of the core particles is coated with the titania precursor.
It should be.
Examples of the solvent include alcohol, water, and a mixed solvent thereof.
The concentration of the core particles in the dispersion is not particularly limited as long as the core particles can be uniformly dispersed.

[2.2. Addition of Ti source]
Next, a Ti source is added to the dispersion. When a Ti source is added to the dispersion, hydrolysis and polycondensation of the Ti source proceed in the dispersion. As a result, precursor particles in which the surface of the core particles is coated with the titania precursor are obtained.
Examples of the Ti source include tetraisopropyl orthotitanium and tetrabutyl orthotitanium.
The optimum amount of Ti source to be added to the dispersion liquid is selected according to the desired composition.

[2.3. Heat treatment]
The precursor particles are recovered from the dispersion, dried, and then heat-treated. As a result, an oxygen generation catalyst in which the surface of the core particles is coated with a shell made of titanium or a composite oxide of titanium and ruthenium can be obtained.
The heat treatment is performed to dehydrate and crystallize the titania precursor in which OH groups remain. The heat treatment conditions are not particularly limited as long as the titania precursor can be dehydrated and crystallized. Usually, it is preferable to heat-treat in the air at 300 ° C. to 800 ° C. for about 0.5 to 3 hours.

[3. Action]
Ruthenium oxide and metallic ruthenium have a lower cost and higher initial activity than iridium oxide. However, ruthenium oxide and metallic ruthenium have low durability of OER activity.
On the other hand, the oxygen generation catalyst according to the present invention exhibits initial activity and durability equal to or higher than those of conventional catalysts using iridium oxide. This is because the surface of the core containing ruthenium oxide or metallic ruthenium is covered with a shell containing titania or a composite oxide of titanium and ruthenium to protect the catalyst surface, which is the starting point of catalyst deterioration. Conceivable. Further, since the oxygen generation catalyst according to the present invention contains ruthenium oxide or metallic ruthenium as a main component, the cost is lower than that of a conventional catalyst containing iridium oxide as a main component.

(Example 1, Comparative Examples 1 to 3)
[1. Preparation of sample]
[1.1. Example 1]
0.3 g of a commercially available ruthenium oxide catalyst was dispersed in 50 mL of a solvent (isopropanol 80%, water 20%). To this, 0.6 mL of tetraisopropyl orthotitanium (TTIP) was added, and the mixture was stirred at room temperature for 4 hours. After stirring, the dispersion was filtered to recover and dry the catalyst precursor. Further, the catalyst precursor was heat-treated at 400 ° C. for 1 hour in an air atmosphere to obtain an oxygen generation catalyst.
The amount of TTIP added in Example 1 corresponds to an amount in which five atomic layers of titania are formed on the surface of ruthenium oxide. Hereinafter, Example 1 will also be referred to as “5ML” using a unit called ML (Mono Layer).

[1.2. Comparative Examples 1 to 3]
A commercially available ruthenium oxide catalyst was heat-treated at 400 ° C. for 1 hour in an air atmosphere as it was without being modified with titania (Comparative Example 1).
Further, a commercially available ruthenium oxide catalyst that was not heat-treated (Comparative Example 2) and a commercially available iridium oxide catalyst (Comparative Example 3) were subjected to the test as they were.

[2. Test method]
[2.1. TEM observation and EDS mapping]
TEM observation and EDX mapping of the catalysts of Examples 1 and Comparative Examples 1 to 3 were performed.

[2.2. Activity and durability evaluation]
The catalysts of Example 1 and Comparative Examples 1 to 3 were applied to gold discs and dried. This was used as the working electrode for electrochemical measurement. The amount of catalyst supported was unified to 15 μg cm- ² . In Example 1, the "catalyst-supported amount" represents an amount excluding titania. The reference electrode was a reversible hydrogen electrode, the counter electrode was platinum, and the electrolytic solution was perchloric acid (0.1 M).
The measurement procedure is as follows.
(A) First, one cycle of potential scanning of 1.0 V⇔1.6 V was performed.
(B) Next, one cycle of potential scanning of 1.0 V⇔1.7 V was performed.
(C) Further, potential scanning of 0.07 V⇔1.8 V was performed for 20 to 50 cycles.

[3. result]
[3.1. TEM observation and EDS mapping]
FIG. 1 shows TEM images of the oxygen generation catalysts obtained in Example 1 and Comparative Example 1. Overall, the particles of Example 1 appear to be more rounded than those of Comparative Example 1. Further, when the TEM image of Example 1 is observed in detail, it seems that the ruthenium oxide particles (region surrounded by a broken line in FIG. 1) are covered with an amorphous substance (region surrounded by a broken line in FIG. 1). appear.

FIG. 2 shows the EDX mapping of the oxygen generation catalyst obtained in Example 1. The distributions of Ru and Ti almost overlap, and it is considered that the entire surface of the ruthenium oxide particles is covered with titania.

[3.2. Activity and durability evaluation]
[3.2.1. Initial activity]
FIG. 3 shows the initial water electrolysis activity of the oxygen generation catalysts obtained in Example 1 and Comparative Examples 1 to 3 (water electrolysis activity obtained in the measurement procedure (a)). The initial activity, untreated ruthenium oxide (RuO _2, Comparative Example 2)> the unmodified heat treatment ruthenium oxide (HT-RuO _2, Comparative Example 1)> titania modified ruthenium oxide (HT-TiO ₂ -RuO _2, Example 1) > IrO ₂ (Comparative Example 3). From FIG 3, HT-TiO ₂ -RuO ₂ is more active lower RuO ₂ and HT-RuO _2, than IrO ₂ it can be seen that a high activity.

[3.2.2. durability]
FIG. 4 shows changes in the activity of the oxygen generation catalysts obtained in Example 1 and Comparative Examples 1 to 3 during the potential cycle (0.07 V⇔1.8 V). In FIG. 4, “potential @ 0.5 mAμg ^-1 ” on the vertical axis represents the potential when the current density reaches 0.5 mA μg ^-1 when the potential is increased from 0.07 V. The smaller the "potential @ 0.5 mAμg ^-1 ", the higher the OER activity. Further, the values in the first cycle in FIG. 4 show that the current density is 0.5 mAμg ^-1 in the first cycle when the measurement procedure (c) is performed after the measurement procedures (a) and (b) are performed. Represents the potential when

In Figure 3, the activity of HT-RuO ₂ (Comparative Example 1) was greater than that of the HT-TiO ₂ -RuO ₂ (Example 1). However, the activity of Example 1 in the first cycle of FIG. 4 was almost the same as that of Comparative Example 1. This is because the deterioration has already progressed in the measurement procedure (a) + the measurement procedure (b), and the catalytic activities of both have become equivalent at the start of the measurement procedure (c). From FIG. 4, while HT-RuO ₂ (Comparative Example 1) and RuO ₂ (Comparative Example 2) have decreased activity (catalyst deteriorated) during the cycle, Example 1 has almost decreased activity. It turns out that there is no such thing.

FIG. 5 shows changes in the IV characteristics of the oxygen generation catalysts obtained in Example 1 (FIG. 5 (A)) and Comparative Example 1 (FIG. 5 (B)). From FIG. 5, it can be seen that when ruthenium oxide is modified with titania, the activity does not decrease even if a potential cycle is added. The initial activity is Comparative Example 1> Example 1, but the activities of both are reversed during the cycle. From the above, it was found that by modifying the surface of ruthenium oxide with titania, an OER catalyst having high activity and high durability can be obtained.

(Examples 2 to 5)
[1. Preparation of sample]
The amount of TTIP added was 0.06 mL (equivalent to 0.5 ML, Example 2), 0.03 mL (equivalent to 0.25 ML, Example 3), 0.012 mL (equivalent to 0.1 ML, Example 4), or 0. An oxygen generation catalyst was prepared in the same manner as in Example 1 except that it was 006 mL (equivalent to 0.05 ML, Example 5).

[2. Test method]
Activity and durability were evaluated in the same manner as in Example 1. The obtained current value was normalized by the weight of Ru.

[3. result]
[3.1. Activity change]
FIG. 6 shows the change in activity of the oxygen generation catalysts obtained in Examples 2 to 5 during the potential cycle (0.07 V⇔1.8 V). Note that FIG. 6 also shows the results of Example 1 and Comparative Examples 1 and 2. In FIG. 6, the vertical axis represents the potential reaching 0.5 mAμg ^-1, and the lower the vertical axis, the higher the OER activity.

In Comparative Example 1 (HT-RuO ₂ ) and Comparative Example 2 (commercially available RuO ₂ ), the activity decreased during the cycle, and deterioration of the catalyst was observed.
On the other hand, in Example 1 (5ML), the activity was hardly decreased. Further, in Examples 2 to 5 (0.5 to 0.05 ML) in which the coverage was reduced, deterioration was observed from 1 cycle to 20 cycles. However, there was almost no deterioration from 20 to 50 cycles, and the activity was higher than that of Comparative Example 1 even after 50 cycles.

[3.2. Coverage dependence of activity change]
FIG. 7 shows the titania coverage dependence of the initial activity of the oxygen generation catalysts obtained in Examples 2 to 5 and the activity after the durability test. Note that FIG. 7 also shows the results of Example 1 and Comparative Examples 1 and 2. In FIG. 7, the vertical axis represents the potential reaching 0.5 mAμg ^-1, and the lower the vertical axis, the higher the OER activity. Further, in FIG. 7, black circles represent the potential of the first cycle (initial activity), and white circles represent the potential of the 50th cycle (activity after the durability test). Further, in FIG. 7, the solid line represents the potential (initial activity) in the first cycle of Comparative Example 1 or 2, and the broken line represents the potential (activity after the durability test) in the 50th cycle.

From FIG. 7, it can be seen that the initial activity of Examples 1 to 5 is higher than that of Comparative Examples 1 and 2. The activity after the endurance test was similar. Furthermore, regarding the coverage dependence, both the initial activity and the activity after the durability test were maximized at 0.25 ML.

[3.3. Potential rise]
FIG. 8 shows the range of increase in potential when a potential cycle (0.07V⇔1.8V) is applied to the oxygen generation catalysts obtained in Examples 2 to 5. Note that FIG. 8 also shows the results of Example 1 and Comparative Examples 1 and 2.
Here, the "potential rise width" means the potential difference between the first cycle and the Nth cycle. FIG. 8 is a graph in which FIG. 6 is standardized as the increase width of the potential from the first cycle, and shows the degree of progress of deterioration from the first cycle. FIG. 8 shows that the deterioration progresses toward the upper side of the vertical axis.

From FIG. 8, it can be seen that the degree of deterioration of Examples 1 to 5 is smaller than that of Comparative Examples 1 and 2. Further, from FIGS. 7 and 8, it can be seen that Examples 1 to 5 have higher OER activity itself than Comparative Examples 1 and 2, and deterioration is less likely to proceed.

[3.4. Titania coverage dependence of potential increase range]
FIG. 9 shows the titania coverage dependence of the increase in potential when the potential cycle (0.07V⇔1.8V) is applied to the oxygen generation catalysts obtained in Examples 2 to 5. Note that FIG. 9 also shows the results of Example 1 and Comparative Example 1. In FIG. 9, black circles represent the potential difference between the first cycle and the 20th cycle, and white circles represent the potential difference between the first cycle and the 50th cycle. Further, FIG. 9 shows that the deterioration suppressing effect is greater toward the lower side of the vertical axis.

From FIG. 9, it can be seen that the larger the titania coverage, the greater the effect of suppressing deterioration. In 5ML, since there was no difference between the potential difference in the 20th cycle and the potential difference in the 50th cycle, both are displayed overlapping.

[3.5. Summary]
From the above, the effects of coating the surface of the ruthenium-based catalyst with titania can be summarized as follows.
(1) The OER activity after the endurance test of Examples 1 to 5 (after performing 50 times of 0.07V⇔1.8V cycle) is higher than that of Comparative Example 1 (see FIG. 6).
(2) Examples 1 to 5 have higher initial activity and activity after the durability test than Comparative Example 1. In addition, the initial activity and the activity after the durability test are both maximized at 0.25 ML (see FIG. 7).
(3) The degree of deterioration with respect to the number of cycles of Examples 1 to 5 is smaller than that of Comparative Examples 1 and 2 (see FIG. 8). Further, the degree of deterioration decreases as the coverage increases (see FIG. 9).

From the above, it has become possible to obtain an OER catalyst having high activity and durability by coating the surface of the ruthenium-based catalyst with titania of 0.05 ML or more and 5 ML or less as in the present invention. Durability increased as the titania coverage increased, but the absolute value of activity was highest near 0.25 ML. Durability and activity were higher than in Comparative Example 1 at all the coverage rates (0.05 ML or more and 5 ML or less) tested, but the optimum coverage rate is considered to be around 0.25 ML.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.

The oxygen generation catalyst according to the present invention can be used as a catalyst used for the oxygen electrode of a water electrolysis device, a carbon deterioration suppressing catalyst added to the anode of a fuel cell, and the like.

Claims (3)

It has a core and a shell that covers the surface of the core.
The core contains at least ruthenium oxide or metallic ruthenium on its surface.
The shell contains titania or a composite oxide of titanium and ruthenium .
The titania coverage is 0.05 ML or more and 5.0 ML or less . Oxygen generation catalyst.
However, the "titania coverage" refers to the titania atomic layer when it is assumed that all the titanium contained in the shell is the titania and the titania uniformly covers the surface of the core. The number of layers. The oxygen generation catalyst according to claim 1, which is used for the oxygen electrode of a water electrolyzer. The oxygen generation catalyst according to claim 1, which is added to the anode of a fuel cell.

Images