JP2013046883A - Pt-HIGH DISPERSION SUPPORTED CATALYST AND MANUFACTURING METHOD OF THE SAME - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Pt-high dispersion supported catalyst in which a Pt particle is supported to a carrier surface of TiC or the like in a minute and uniform manner, and to provide a manufacturing method of the same.SOLUTION: The support (wherein, C is excluded) that has electron conductivity is retained in a container, the steam of a Pt complex is introduced into the container, and the carrier surface is made to adsorb the Pt complex. Next, an inert gas is flowed in the container, and the remaining Pt complex is exhausted. A reducer gas including hydrogen is introduced in the container, and the Pt complex is reduced. An inert gas is flowed in the container, and the remaining reducer gas is exhausted. An adsorption gas including CO is introduced in the container, and CO is made to be adsorbed to the Pt particle surface generated by the reduction of the Pt complex. In addition, an inert gas is flowed in the container, and the remaining adsorption gas is exhausted.

Description

  The present invention relates to a Pt highly dispersed supported catalyst and a method for producing the same.

  As an electrode catalyst used in a fuel cell, a catalyst (Pt-supported carbon (Pt / C)) in which Pt or a Pt alloy having a particle diameter of several nanometers is supported on carbon powder having a high specific surface area is generally used. The Pt / C catalyst has a problem that the utilization rate of expensive Pt is low because only atoms near the surface of the Pt particles function as a catalyst. For this reason, several techniques for limiting the use of Pt to a few atomic layers from the particle surface and substituting the inside of the particle with another material have been proposed.

For example, Non-Patent Document 1 discloses an electrode (GC / TiC / Pt) in which TiC particles having a particle diameter of about 40 nm are supported on the glassy carbon surface and Pt is supported on the TiC surface by an electrodeposition method.
In FIG. 1 of the same document, an SEM image 1000 times larger than GC / TiC / Pt is described. In this SEM image, aggregated Pt particles can be confirmed.
FIG. 2 of the same document describes a GC / TiC / Pt cyclic voltammogram. The electrochemical surface area (ECSA) per Pt weight determined from the amount of Pt electrodeposition and the amount of hydrogen adsorbed in the cyclic voltammogram in the figure is 3.7 m ² / g.

  As described in Non-Patent Document 1, when an electrodeposition method is used, Pt particles can be supported on the TiC surface. However, in the electrodeposition method, the size of the Pt particles varies and considerably large Pt particles are generated. When Pt particles are large, the specific surface area is small, and the area that can function as a catalyst is small, so the catalyst per Pt weight is low. If Pt can be supported more finely and uniformly on the surface of TiC particles, the catalytic activity per Pt weight is considered to be improved.

"Titaniumu carbide nanoparticles supported Pt catalysts for methanol electrooxidation in acidic media" Y. Ou, et al., J. Power Sources, 2010, 195, 1365

  The problem to be solved by the present invention is to provide a highly dispersed Pt supported catalyst in which Pt particles are finely and uniformly supported on the surface of a support such as TiC and a method for producing the same.

In order to solve the above problems, a method for producing a Pt highly dispersed supported catalyst according to the present invention comprises:
A Pt complex adsorption step of holding a carrier having electron conductivity (excluding C) in a container, introducing a vapor of Pt complex into the container, and adsorbing the Pt complex on the surface of the carrier;
A first purge step of flowing an inert gas into the vessel and discharging the remaining Pt complex;
Introducing a reducing gas containing hydrogen into the vessel to reduce the Pt complex;
A second purge step of flowing an inert gas into the vessel and discharging the remaining reducing gas;
A CO adsorption step of introducing an adsorption gas containing CO into the container and adsorbing CO on the surface of the Pt particles generated by the reduction of the Pt complex;
A third purge step of flowing an inert gas into the container and discharging the remaining adsorbed gas.
The Pt highly dispersed supported catalyst according to the present invention is obtained by the method according to the present invention.

  When the Pt complex is adsorbed on the surface of the carrier and the Pt complex is reduced, Pt particles are deposited on the surface of the carrier. Next, when the precipitated Pt particles are exposed to a CO atmosphere, CO is adsorbed on the surface of the Pt particles. The CO adsorbed on the surface of the Pt particles has an effect of preventing the newly formed Pt from being deposited on the already deposited Pt particles by reduction of the Pt complex. Therefore, when the adsorption of the Pt complex, the reduction of the Pt complex, and the CO adsorption are repeated a plurality of times, the Pt particles can be finely and uniformly deposited on the surface of the carrier.

It is process drawing of the manufacturing method of the Pt high dispersion carrying catalyst which concerns on this invention. It is a TEM image of the Pt / TiC catalyst obtained by the method according to the present invention. 2 is a particle size distribution of Pt particles (with CO adsorption) and Pt particles (without CO adsorption) determined from a TEM image. It is a cyclic voltammogram of Pt / TiC (with CO adsorption), Pt / TiC (without CO adsorption), TiC and Pt / C. It is a figure which shows the relationship between the process temperature of ALD, and CO adsorption amount.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Pt high dispersion supported catalyst]
The Pt highly dispersed supported catalyst according to the present invention comprises a Pt particle supported on a support surface in a highly dispersed state. Such a Pt highly dispersed supported catalyst is obtained by the method described later.

[1.1. Carrier]
In the present invention, a material having electron conductivity (excluding C) is used for the carrier. An example of such a carrier is TiC.
The particle size of the carrier is not particularly limited and can be arbitrarily selected according to the purpose. In general, the smaller the particle size of the support, the higher the activity per weight.

[1.2. Pt particles]
Pt particles are supported on the surface of the carrier. If a method described later is used, Pt can be supported on the carrier surface with high dispersion.
The degree of dispersion of Pt can be expressed by the CO adsorption amount in CO pulse measurement or the electrochemical surface area (ECSA) obtained from the hydrogen adsorption wave area in CV measurement. The CO adsorption amount and ECSA are correlated with each other, and the higher the value, the finer the Pt particles.
In the case of using the method described later, when the manufacturing conditions are optimized,
(1) CO adsorption amount in CO pulse measurement is 30 mL / g or more, and / or
(2) A catalyst having an ECSA of 75 m ² / g or more obtained from the hydrogen adsorption wave area during CV measurement is obtained.

[2. Method for producing Pt highly dispersed supported catalyst]
FIG. 1 shows a process chart of a method for producing a Pt highly dispersed supported catalyst according to the present invention. In FIG. 1, the method for producing a Pt highly dispersed supported catalyst according to the present invention includes a Pt complex adsorption step, a first purge step, a Pt complex reduction step, a second purge step, a CO adsorption step, and a third purge. Process.

[2.1. Pt complex adsorption process]
In the Pt complex adsorption step, as shown in FIG. 1A, a carrier having electron conductivity (except for C) is held in a container (not shown), and the vapor of the Pt complex is introduced into the container. And the Pt complex is adsorbed on the surface of the carrier.
In the present invention, a material having electron conductivity (excluding C) is used for the carrier. Since the details of the carrier are as described above, the description thereof is omitted.

“Pt complex” refers to a compound in which a ligand is bonded to a Pt ion. In the present invention, the Pt complex used as a raw material needs to generate a predetermined vapor pressure without being decomposed at the process temperature. The Pt complex used as the raw material preferably has a vapor pressure of 100 Pa or higher at 90% of the decomposition temperature T (K).
Examples of the Pt complex that satisfies such conditions include methylcyclopentadienyltrimethylplatinum, cyclopentadienyltrimethylplatinum, ethylcyclopentadienyltrimethylplatinum, and tetrakis (trifluorophosphine) platinum.

  When the Pt complex is sealed in a vapor generation container maintained at a predetermined temperature (for example, 25 to 80 ° C.) and a carrier gas (inert gas) is introduced into the vapor generation container, the Pt complex vapor is converted into The contained carrier gas is obtained. The Pt complex vapor + carrier gas thus obtained is introduced into a container holding the carrier.

When Pt complex vapor + carrier gas is introduced into a container holding a carrier, a single molecular layer of Pt complex is adsorbed on the surface of the carrier as shown in FIG. At this time, the amount of adsorption on the surface of the carrier changes according to the temperature (adsorption temperature) in the container holding the carrier.
In general, when the adsorption temperature is too low, the Pt complex aggregates and does not adsorb uniformly on the surface of the carrier, and Pt is deposited as coarse particles. Therefore, the adsorption temperature is preferably 100 ° C. or higher.
On the other hand, if the adsorption temperature is too high, the Pt complex may be thermally decomposed. Therefore, the adsorption temperature is preferably 200 ° C. or lower.

  The time (adsorption time) for introducing the Pt complex vapor + carrier gas into the container holding the carrier may be a time during which the single molecular layer Pt complex can be adsorbed on the carrier surface. Although depending on the adsorption temperature, the adsorption time is usually about 10 to 40 minutes.

[2.2. First purge step]
As shown in FIG. 1B, the first purge step is a step of flowing an inert gas into the container and discharging the remaining Pt complex.
When the Pt complex vapor described below is reduced with the Pt complex vapor remaining in the container, the Pt complex vapor floating in the container may become Pt and be deposited on the surface of the carrier. Therefore, in order to carry Pt in a highly dispersed manner on the surface of the carrier, it is necessary to discharge excess Pt complex vapor from the container before the reduction of the Pt complex.

Excess Pt complex vapor is discharged by flowing an inert gas (for example, N ₂ gas, rare gas such as Ar) through the container. At this time, the temperature in the container is not particularly limited as long as it is a temperature at which excess Pt complex vapor can be discharged. However, if the temperature in the container greatly fluctuates before and after purging, the Pt complex adsorbed on the support surface may be desorbed. Also, unnecessary temperature fluctuations complicate temperature control. Therefore, in the first purge step, the temperature in the container is preferably maintained at the same temperature as the Pt complex adsorption step.
The flow rate of the inert gas, the purge time, etc. are not particularly limited as long as the excess Pt complex vapor is discharged out of the container.

[2.3. Pt complex reduction process]
As shown in FIG. 1C, the Pt complex reduction step is a step of reducing the Pt complex by introducing a reducing gas containing hydrogen into the container.
A gas containing hydrogen is used as the reducing gas. The reducing gas may contain a gas other than hydrogen. The gas other than hydrogen may be a gas that does not hinder the reduction of the Pt complex (for example, an inert gas). In order to efficiently reduce the Pt complex, the hydrogen concentration in the reducing gas is preferably as high as possible, and 100% (pure hydrogen) is particularly preferable.

The reduction temperature of the Pt complex may be a temperature at which the Pt complex can be reduced by a reducing gas. For example, when hydrogen gas having a concentration of 100% is used as the reducing gas, the reduction of the Pt complex proceeds at 25 ° C. or higher. Further, when the Pt complex is reduced, Pt aggregation occurs, so the reduction temperature affects the size of the Pt particles.
In general, the higher the reduction temperature, the faster the reaction rate of the reduction reaction. In order to complete the treatment in a short time, the reduction temperature is preferably 100 ° C. or higher.
On the other hand, if the reduction temperature is too high, the particles become coarse. Therefore, the reduction temperature is preferably 200 ° C. or lower.

The flow rate of the reducing gas, the reduction time, and the like are not particularly limited as long as the Pt complex is completely reduced. Although depending on the reduction temperature and the concentration of the reducing gas, the reduction time is usually about 10 to 30 minutes.
When the Pt complex is reduced under predetermined conditions, one atomic layer of Pt is ideally formed on the surface of the carrier as shown in FIG. In practice, the precipitated Pt often agglomerates into fine Pt particles.

[2.4. Second purge step]
As shown in FIG. 1D, the second purge step is a step of flowing an inert gas into the container and discharging the remaining reducing gas.
If the process proceeds to the next step with the reducing gas remaining in the container, the reducing gas remaining in the container may react with the newly introduced gas in the container. For example, in the case where the introduction of the Pt complex vapor and the reduction of the Pt complex are repeated several times, if the Pt complex vapor is newly introduced with the reducing gas remaining in the container, before the adsorption of the Pt complex to the surface of the carrier occurs. Reduction of the Pt complex vapor occurs. Therefore, in order to carry Pt on the carrier surface in a highly dispersed manner, it is necessary to discharge excess reducing gas from the container before CO adsorption.

Excess reducing gas is discharged by flowing an inert gas into the container. At this time, the temperature in the container is not particularly limited as long as it can discharge excess reducing gas. However, if the temperature in the container fluctuates greatly before and after purging, the temperature control becomes complicated. Therefore, it is preferable to maintain the temperature in the container at the same temperature as in the Pt complex reduction step.
The flow rate of the inert gas, the purge time, and the like are not particularly limited as long as excessive reducing gas is discharged to the outside of the container.

[2.5. CO adsorption process]
As shown in FIG. 1 (e), the CO adsorption process is a process in which an adsorption gas containing CO is introduced into the container and CO is adsorbed on the surface of Pt particles generated by reduction of the Pt complex.
When the carrier on which Pt is deposited is exposed to a CO atmosphere, CO is adsorbed on the Pt surface. The CO adsorbed on the Pt surface has an effect of preventing further new Pt from being deposited on the Pt deposited on the support surface.

A gas containing CO is used as the adsorption gas. The adsorbed gas may contain a gas other than CO. The gas other than CO may be any gas that does not interfere with the adsorption of CO onto the Pt surface (for example, an inert gas).
The CO concentration contained in the adsorbed gas can be arbitrarily selected according to the purpose. In general, if the CO concentration is too low, efficient adsorption of CO becomes difficult. Therefore, the CO concentration is preferably 100 ppm or more.

When the adsorption gas is introduced into the container holding the carrier, CO is adsorbed on the Pt surface as shown in FIG. At this time, the amount of CO adsorbed on the Pt surface changes according to the temperature (adsorption temperature) in the container holding the carrier.
In general, the higher the adsorption temperature, the more difficult it is for CO to be adsorbed on the Pt surface. Therefore, the adsorption temperature is preferably 200 ° C. or lower.
The time (adsorption time) for introducing the adsorbed gas into the container holding the carrier may be a time during which CO can be adsorbed on the Pt surface. Although depending on the adsorption temperature, the adsorption time is usually about 1 to 3 hours.

[2.6. Third purge step]
As shown in FIG. 1 (f), the third purge step is a step of flowing an inert gas into the container and discharging the remaining adsorbed gas.
When the process proceeds to the next step with the adsorbed gas remaining in the container, the CO remaining in the container reacts with the newly introduced Pt complex, and before the adsorption of the Pt complex to the surface of the carrier occurs, the Pt complex May be reduced. Therefore, in order to carry Pt in a highly dispersed manner on the carrier surface, it is necessary to discharge excess adsorbed gas from the container before proceeding to the next step.

Excess adsorption gas is discharged by flowing an inert gas into the container. At this time, the temperature in the container is not particularly limited as long as it is a temperature at which excess adsorbed gas can be discharged. However, if the temperature in the container greatly fluctuates before and after purging, desorption of CO adsorbed on the Pt surface may occur. Also, unnecessary temperature fluctuations complicate temperature control. Therefore, the temperature in the container is preferably maintained at the same temperature as the CO adsorption step.
The flow rate of the inert gas, the purge time, and the like are not particularly limited as long as the excess adsorbed gas is discharged outside the container.

[2.7. Number of process iterations]
Each step described above may be performed at least once. However, there is a limit to the amount of Pt supported on the carrier surface in one treatment. Therefore, in order to carry a predetermined amount of Pt on the surface of the carrier, it is preferable to repeat the above-described steps a plurality of times.
Generally, the amount of Pt supported increases as the number of process repetitions increases. However, repeating more than necessary has no real benefit. The optimum number of repetitions is selected according to the particle size of the carrier, the concentration of the gas used in each step, and the like.

When the series of processes described above is repeated a plurality of times, the last CO adsorption step and the third purge step can be omitted.
Further, when the last CO adsorption step and purge step are performed, CO is adsorbed on the Pt surface of the obtained catalyst. In general, CO adsorbed on the surface of Pt causes poisoning of Pt. However, when the obtained catalyst is used for the cathode of a fuel cell, CO is oxidized with oxygen, so there is no risk that CO will deteriorate the catalyst performance. Further, even when the catalyst is used for the anode, the deterioration of the catalyst performance can be suppressed by performing the oxidation treatment before use.

[2.8. Process temperature]
Each step described above is not necessarily performed at the same temperature, and the temperature may be set independently for each step. However, if the temperature is changed for each process, the adsorbed gas is desorbed and the temperature control becomes complicated. Accordingly, the Pt complex adsorption step, the first purge step, the Pt complex reduction step, the second purge step, the CO adsorption step, and the third purge step are each performed at a temperature of 100 ° C. or higher and 200 ° C. or lower. Is preferred.
In order to avoid unintentional desorption of adsorbed gas and complicated temperature control, these steps are preferably performed at a constant temperature T. In this case, T is preferably 100 ° C. or higher and 200 ° C. or lower.

[3. Action of Pt highly dispersed supported catalyst and production method thereof]
An atomic layer deposition (ALD) method is known as a method for forming a thin film in atomic layer units.
For example, when two types of reactants are used as raw materials, the production of a thin film using the ALD method is as follows:
(A) a step of adsorbing the first reactant on the substrate surface;
(B) exhausting the first reactant;
(C) reacting the first reactant adsorbed on the substrate surface with the second reactant, and
(D) The step of exhausting the second reactant is performed as one cycle, and these steps are repeated a plurality of times. The film thickness is controlled by controlling the number of cycles.

  When such an ALD method is applied to support Pt on the support surface, Pt can be supported on the support surface with relatively high dispersion. However, if the conventional ALD method is applied to Pt support as it is, aggregation of Pt atoms deposited on the surface of the carrier occurs and Pt particles become coarse.

  On the other hand, when the Pt complex is adsorbed on the support surface and the Pt complex is reduced, Pt particles are deposited on the support surface. Next, when the precipitated Pt particles are exposed to a CO atmosphere, CO is adsorbed on the surface of the Pt particles. The CO adsorbed on the surface of the Pt particles has an effect of preventing the newly formed Pt from being deposited on the already deposited Pt particles by reduction of the Pt complex. Therefore, when the adsorption of the Pt complex, the reduction of the Pt complex, and the CO adsorption are repeated a plurality of times, the Pt particles can be finely and uniformly deposited on the surface of the carrier.

(Example 1, Comparative Examples 1-3)
[1. Preparation of sample]
[1.1. Example 1]
TiC particles carrying Pt (Pt / TiC) were produced by repeating the following steps (1) to (6) four times. The container temperature was in the range of 100 to 200 ° C.
(1) Ar gas containing vapor of Pt complex (methylcyclopentadienyltrimethylplatinum) was introduced into a container containing TiC particles, and the Pt complex was adsorbed on the TiC particles. The gas flow rate was 0.3 L / min and the adsorption time was 35 min.
(2) The inside of the container was purged with Ar gas. The gas flow rate was 1 L / min, and the purge time was 5 min.
(3) H ₂ gas was introduced into the container to reduce the Pt complex. The gas flow rate was 0.2 L / min, and the reduction time was 30 min.
(4) The inside of the container was purged with Ar gas. The gas flow rate was 1 L / min, and the purge time was 5 min.
(5) N ₂ balanced CO gas (CO concentration: 100 ppm) was introduced into the container, and CO was adsorbed on the deposited Pt. The gas flow rate was 0.5 L / min, and the adsorption time was 3 h.
(6) The inside of the container was purged with Ar gas. The gas flow rate was 1 L / min, and the purge time was 5 min.

[1.2. Comparative Examples 1 to 3]
Pt / TiC was produced in the same manner as in Example 1 except that CO adsorption and subsequent Ar purge were not performed (Comparative Example 1).
Furthermore, as a comparison, TiC (Comparative Example 2) and commercially available Pt-supported carbon (Pt / C) (Comparative Example 3) were also used for the test.

[2. Test method]
[2.1. TEM observation]
The obtained particles were observed by TEM.
[2.2. Particle size distribution]
100 Pt particles were randomly selected from the TEM image, and the particle size was measured.
[2.3. CO adsorption amount]
The CO adsorption amount was determined by CO pulse measurement.
[2.4. ECSA]
For each particle, a cyclic voltammogram (CV) was measured under the conditions of potential width: 0.05 to 1.0 V and potential sweep rate: 100 mV / s. The electrochemical surface area (ECSA) was determined from the hydrogen adsorption wave area during CV measurement.

[3. result]
[3.1. TEM observation]
FIG. 2 shows a TEM image of the Pt / TiC catalyst obtained in Example 1. FIG. 2 shows that Pt particles of about 2 nm are supported on the TiC surface with high dispersion.
[3.2. Particle size distribution]
FIG. 3 shows the particle size distribution of Pt particles of the Pt / TiC catalyst measured from the TEM image. FIG. 3 shows that the Pt / TiC catalyst (Example 1) subjected to CO adsorption has fewer Pt particles of 2.6 nm or more than the Pt / TiC catalyst (Comparative Example 1) not subjected to CO adsorption. Recognize. This is presumably because the growth of Pt particles was suppressed by the CO adsorption process.

[3.3. CO adsorption amount]
In the case of Pt / TiC obtained in Example 1, the CO adsorption amount determined by CO pulse measurement was 32 mL / g. This was 1.8 nm in terms of Pt particle size.
[3.4. ECSA]
FIG. 4 shows cyclic voltammograms of various particles. Table 1 shows ECSA obtained from the hydrogen adsorption wave at CV.
The ECSA of Pt / TiC obtained in Example 1 was 87.9 m ² / g, which was about 20% larger than the conventional Pt / C (Comparative Example 3).

(Example 2)
[1. Preparation of sample]
Pt / TiC was prepared according to the same procedure as in Example 1. However, the process temperature (container temperature) was constant (room temperature, 100 ° C., 150 ° C., or 200 ° C.).
[2. Test method]
The CO adsorption amount was determined by CO pulse measurement.
[3. result]
FIG. 5 shows the relationship between the ALD process temperature and the CO adsorption amount. At room temperature (25 ° C.), the CO adsorption amount was 10 mL / g or less. On the other hand, when the process temperature was 100 to 200 ° C., the CO adsorption amount was 30 mL / g or more. This is probably because, in the range of 100 to 200 ° C., the Pt complex is uniformly adsorbed on the surface of the carrier, whereas the Pt complex is not uniformly adsorbed on the surface of the carrier and aggregates at room temperature.

  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 scope of the present invention.

  The Pt highly dispersed supported catalyst according to the present invention can be used as an electrode catalyst for a fuel cell.

Claims (6)

A Pt complex adsorption step of holding a carrier having electron conductivity (excluding C) in a container, introducing a vapor of Pt complex into the container, and adsorbing the Pt complex on the surface of the carrier;
A first purge step of flowing an inert gas into the vessel and discharging the remaining Pt complex;
Introducing a reducing gas containing hydrogen into the vessel to reduce the Pt complex;
A second purge step of flowing an inert gas into the vessel and discharging the remaining reducing gas;
A CO adsorption step of introducing an adsorption gas containing CO into the container and adsorbing CO on the surface of the Pt particles generated by the reduction of the Pt complex;
And a third purge step of flowing an inert gas into the vessel and discharging the remaining adsorbed gas. 2. The high Pt dispersion support according to claim 1, wherein the Pt complex adsorption step, the first purge step, the Pt complex reduction step, the second purge step, the CO adsorption step, and the third purge step are repeated a plurality of times. A method for producing a catalyst.
However, the last CO adsorption step and the third purge step can be omitted.   The Pt complex adsorption step, the first purge step, the Pt complex reduction step, the second purge step, the CO adsorption step, and the third purge step are performed at a temperature of 100 ° C to 200 ° C. 3. A process for producing a Pt highly dispersed supported catalyst according to 1 or 2.   The method for producing a Pt highly dispersed supported catalyst according to any one of claims 1 to 3, wherein the support is TiC.   A Pt highly dispersed supported catalyst obtained by the method according to any one of claims 1 to 4. CO adsorption amount in CO pulse measurement is 30 mL / g or more, and / or
6. The Pt highly dispersed supported catalyst according to claim 5, wherein ECSA obtained from a hydrogen adsorption wave area at the time of CV measurement is 75 m ² / g or more.

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