WO2013015261A1

WO2013015261A1 - Hydrogen combustion catalyst

Info

Publication number: WO2013015261A1
Application number: PCT/JP2012/068655
Authority: WO
Inventors: 久保　仁志; 淳一谷内; 宏史野口; 優輔大嶋
Original assignee: 田中貴金属工業株式会社
Priority date: 2011-07-28
Filing date: 2012-07-24
Publication date: 2013-01-31
Also published as: JP5123417B1; JP2013027811A

Abstract

This invention relates to a hydrogen combustion catalyst having a washcoat layer formed from zirconia and tungsten oxide, wherein the zirconia comprises monoclinic zirconia as the essential component and further includes at least one of cubic zirconia and tetragonal zirconia. This catalyst has sufficient catalytic activity even at low temperatures and is capable of maintaining this activity even with long-term use. This invention is a catalyst that is ideal for combustion treatment of hydrogen in exhaust gas from hydrogen production plants and hydrogen furnaces and the like.

Description

Hydrogen combustion catalyst

The present invention relates to a catalyst for burning hydrogen in an air-containing gas. In particular, the present invention provides a hydrogen combustion catalyst that can maintain catalytic activity for a long time even at a relatively low temperature.

Equipment that uses hydrogen, such as hydrogen production plants and hydrogen atmosphere furnaces, requires technology that removes used hydrogen contained in the exhaust gas and allows it to be disposed of in the environment as clean gas. For this reason, conventionally, a technique is known in which exhaust gas is passed through a catalyst layer, hydrogen is burned by a catalytic reaction to form water, and the generated moisture is removed and recovered (Patent Document 1).

JP 2008-139005 A

When burning the hydrogen in the exhaust gas, it is desirable that the catalytic action can be exhibited at the lowest possible temperature because hydrogen is explosive. In particular, in facilities that require enhanced safety, such as facilities that handle large amounts of hydrogen, such as high-purity hydrogen purification plants, it is essential to provide a hydrogen combustion catalyst that exhibits sufficient activity even at low temperatures.

As such a hydrogen combustion catalyst, a catalyst in which a catalyst metal such as platinum is supported on a pellet-like support made of a metal oxide such as silica or alumina has been proposed. However, even if these catalysts do not have sufficient catalytic activity at low temperatures, or have some activity at the beginning of use, their activity gradually decreases when used for about 3 days to 1 week. When the service period exceeds several months, it is difficult to maintain good catalytic activity.

Therefore, an object of the present invention is to provide a hydrogen combustion catalyst that has sufficient catalytic activity even at low temperatures and can maintain the activity for a long period of time.

In developing a catalyst that solves the above problems, the present inventors first conducted a screening test on the type of washcoat formed on the catalyst. In this screening test, the initial activity before the start of use was compared for a catalyst having a washcoat containing silica, alumina, or zirconia. As a result, it was found that having a washcoat containing zirconia or alumina provides a catalyst with relatively high initial activity even at low temperatures. Next, a catalyst using zirconia or alumina having a high initial activity was continuously used as a catalyst for one week or more, and a change in activity was observed. As a result, the activity of the catalyst using alumina significantly decreased in a short period of about 3 days. On the other hand, the catalyst using zirconia was found to be a catalyst capable of maintaining a certain degree of activity as compared with alumina, although a slight decrease in activity was observed.

The present inventors diligently studied a zirconia-containing catalyst that can maintain activity for a long period of time based on the above screening results. Further, the present inventors have found that a catalyst containing tungsten oxide together with zirconia as a washcoat and having a plurality of crystal structures as zirconia exhibits good catalytic performance for a long period of time. did.

That is, the present invention relates to a hydrogen combustion catalyst in which a catalyst metal is supported on a carrier having a washcoat layer. The washcoat layer is composed of zirconia and tungsten oxide, and the zirconia includes monoclinic zirconia as an essential component. Furthermore, the present invention relates to a hydrogen combustion catalyst containing at least one of cubic zirconia and tetragonal zirconia. The hydrogen combustion catalyst of the present invention has a high initial activity before starting the use of the catalyst, and the activity does not easily decrease even if the use is continued for a long time.

Zirconia has a tetragonal or cubic crystal structure under high temperature conditions, and a stable monoclinic crystal structure at room temperature. In addition to containing monoclinic zirconia that is stable at room temperature, the zirconia of the present invention is characterized by including a tetragonal or cubic crystal structure that usually exists only at high temperatures. Like the catalyst of the present invention, a catalyst containing two or more kinds of zirconia having a crystal structure and also containing tungsten oxide is a hydrogen combustion catalyst whose activity is unlikely to decrease for a long time.

The hydrogen combustion catalyst of the present invention has peaks at 2θ = 23.2 °, 23.6 °, 24.4 °, 28.3 °, 30.3 °, and 31.5 ° in X-ray diffraction. It is preferable. When having such an X-ray diffraction peak, the washcoat contains tungsten oxide and tends to be a catalyst containing zirconia having a plurality of crystal structures.

In X-ray diffraction, the appearance peaks at 2θ = 23.2 °, 23.6 °, and 24.4 ° are derived from tungsten oxide in the washcoat. Appearance peaks in the range of 2θ = 28.3 °, 30.3 °, 31.5 ° are derived from zirconia, and of these, peaks at 2θ = 28.3 °, 31.5 ° are in monoclinic zirconia. The peak at 2θ = 30.3 ° is derived from tetragonal or cubic zirconia. Thus, the zirconia contained in the catalyst of the present invention includes not only monoclinic crystals but also tetragonal crystals or cubic crystals.

As described above, the catalyst of the present invention contains tetragonal crystals or cubic crystals as zirconia, and further contains these tetragonal crystals or cubic zirconia within a specific range with respect to tungsten oxide or monoclinic zirconia. It is preferable. Specifically, in X-ray diffraction, the highest peak intensity (A) among peaks 2θ = 23.2 °, 23.6 °, 24.4 ° derived from tungsten oxide, tetragonal or cubic zirconia The peak intensity ratio of 2θ = 30.3 ° derived from (2) = 30.3 ° and the peak intensity ratio of 2θ = 28.3 ° of the peak derived from monoclinic zirconia (C) is shown below. It is preferable to be within the range.

That is, the ratio of the intensity (A) of the highest peak among 2θ = 23.2 °, 23.6 °, and 24.4 ° in X-ray diffraction to the peak intensity (B) of 2θ = 30.3 ° ( B / A) is preferably in the range of 0.05 to 1.0. B / A is more preferably 0.1 to 1.0. As described above, the peaks at 2θ = 23.2 °, 23.6 °, and 24.4 ° are derived from tungsten oxide, and the peak at 2θ = 30.3 ° is derived from tetragonal or cubic zirconia. The peak intensity ratio B / A indicates the abundance ratio of tetragonal or cubic zirconia to tungsten oxide. When the peak intensity ratio B / A is less than 0.05, the number of strong acid points on the catalyst is decreased, the activation rate of hydrogen as a reaction gas tends to be slow, and it is difficult to obtain a catalyst with high catalytic activity. On the other hand, if it exceeds 1.0, the dissociation and diffusion of oxygen on the tungsten oxide are slowed down, and it becomes difficult to activate oxygen, so that the hydrogen combustion activity by the catalyst tends to decrease.

The ratio (B / C) of the peak intensity (C) at 2θ = 28.3 ° and the peak intensity (B) at 2θ = 20.3 ° is in the range of 0.05 to 2.00. preferable. B / C is more preferably 0.10 to 1.0, and further preferably 0.10 to 0.80. Since the peak at 2θ = 28.3 ° is derived from monoclinic zirconia, the peak intensity ratio B / C is considered to indicate the abundance ratio of tetragonal or cubic zirconia to monoclinic zirconia. When the peak intensity ratio B / C is less than 0.05, the number of strong acid points on the catalyst is decreased, the activation rate of hydrogen as a reaction gas tends to be slow, and it is difficult to obtain a catalyst with high catalytic activity. On the other hand, if it exceeds 2.0, platinum, which is a catalyst metal particle, tends to cause grain growth from the preparation stage, the reaction area of the catalyst becomes small, the hydrogen combustion rate becomes slow, and the catalyst having high catalytic activity. It is hard to become.

As described above, as the peak intensity ratio of the X-ray diffraction in the present invention, the peak intensity obtained by subtracting the background is applied as the peak intensity used as the calculation standard. Specifically, the peak obtained by subtracting the background from the peak vertex intensity by subtracting the lowest intensity value within the range of 2θ from 25 to 35 ° from the 2θ value at which the target peak vertex is located. The strength can be determined.

In addition, the catalyst of the present invention has a total of two on both the low temperature side at a temperature of 150 to 250 ° C. and the high temperature side at a temperature of 550 to 700 ° C. in acidity measurement by ammonia temperature programmed desorption (NH ₃ -TPD) It is preferable that a peak appears. This acid point measurement shows the characteristics of the extreme surface that contributes to the reaction of the catalyst. If two kinds of zirconia are also present on the extreme surface of the catalyst in the acid point measurement, the catalyst activity decreases for a long time. It is because it becomes difficult to do. The two peaks in the acid point measurement indicate the presence of monoclinic crystals on the low temperature side and tetragonal or cubic zirconia on the high temperature side. On the other hand, in the case of a catalyst containing only monoclinic zirconia, a single ammonia desorption peak appears only on the low temperature side at a temperature of 150 to 250 ° C. when acid measurement is performed.

The hydrogen combustion catalyst of the present invention preferably contains 5 to 200 g / L of the washcoat layer described above, and particularly preferably 60 to 100 g / L. When the washcoat layer is less than 5 g / L, it is difficult to obtain an effect of maintaining the catalyst activity for a long time, and when it exceeds 200 g / L, gas diffusion to the catalyst metal is rate-determined and it is difficult to achieve high catalyst performance.

As the carrier in the hydrogen combustion catalyst of the present invention, a support having the above-described washcoat layer on a support having a shape such as foam metal, pellets, honeycomb, or net can be employed. The support can be made of an appropriate material such as ceramic, metal, and nonwoven fabric, and a metal honeycomb is particularly suitable.

As the catalyst metal supported on the support, a noble metal is preferable, and in particular, platinum, palladium or an alloy thereof is suitable for hydrogen combustion. The catalyst metal is in the form of metal particles formed by adsorbing a metal salt solution on a support and calcining or reducing, or colloidal (clustered) metal formed by adsorbing a previously prepared metal colloid solution on a support. Any state is acceptable. Depending on these conditions, the particle size of the catalytic metal is 10 to 200 nm. Further, the amount of catalyst metal supported (support rate) is not particularly limited, but is preferably 1 to 6 g / L based on the weight of the carrier. The physical properties of the catalyst according to the present invention are preferably those having a specific surface area of 15 to 100 m ² / g and an average pore diameter of 1 to 10 nm.

The hydrogen combustion catalyst of the present invention described above is a method for forming a washcoat layer in which a washcoat solution containing tungsten oxide and zirconia is impregnated with a carrier, and then drying and firing, and the carrier on which the washcoat layer has been formed is rapidly cooled. A step of impregnating or adsorbing a rapidly cooled carrier to a catalytic metal solution to support the catalytic metal, and a step of firing or reducing after the catalytic metal is supported, and the washcoat solution contains powder as zirconia. It can be produced by a production method using an aqueous solution of zirconia and a zirconium salt.

In the production method of the present invention, powdered zirconia and an aqueous solution of a zirconium salt such as zirconium nitrate are mixed and used as zirconia to be added to the washcoat solution. By mixing and using zirconia of different raw materials, that is, an aqueous solution of powdered zirconia and a zirconium salt, a catalyst containing zirconia having a plurality of crystal structures is easily obtained. Further, after impregnating the carrier with a washcoat solution, drying and baking, the carrier is rapidly cooled. By this rapid cooling, it becomes easy to become a catalyst containing tetragonal crystals or cubic crystals as zirconia. This is considered to be because zirconia, which has changed in phase from monoclinic to tetragonal or cubic by firing, tends to remain as tetragonal or cubic without returning to stable monoclinic. Hereinafter, the production method of the present invention will be described in detail.

Zirconium nitrate, acetate, sulfate, chloride, etc. can be used as the zirconium salt used in the washcoat solution. Specifically, zirconium acetate, zirconium chloride, zirconium sulfate, zirconium nitrate and the like are preferable, and zirconium nitrate and zirconium acetate are preferable. Zirconium nitrate becomes zirconia by the firing process. The powdery zirconia preferably has a purity of 98 wt% or more, and zirconium dioxide is preferred. The addition amount in the washcoat solution is preferably adjusted so that the amount of zirconia in the aqueous solution of zirconium salt is 10 to 70 wt% and the amount of powdered zirconia is 10 to 50 wt%.

The tungsten oxide is preferably in the form of powder and preferably has a purity of 95 wt% or more. A suitable type of tungsten oxide is tungsten (VI) WO ₃ . The crystal structure of tungsten oxide may be tetragonal, orthorhombic, monoclinic or triclinic. The content of tungsten oxide in the washcoat is preferably 5 to 90 wt%, particularly preferably 15 to 80 wt%. If the content of tungsten oxide is less than 5 wt%, it becomes difficult to become a highly active catalyst, and if it exceeds 90 wt%, it tends to be difficult to support the catalyst metal.

The washcoat layer can be formed by impregnating a carrier with a washcoat solution obtained by mixing the above zirconia and tungsten oxide, and then drying and baking. The drying condition of the washcoat layer is preferably 80 to 120 ° C., and the baking condition is preferably 500 to 700 ° C. and 1 to 2 hours. Thereafter, the carrier on which the washcoat layer has been formed is rapidly cooled. The rapid cooling is preferably performed at a cooling rate of 200 ° C./min to 50 ° C./min. This is because a catalyst containing tetragonal or cubic zirconia at an appropriate content is likely to be obtained.

The catalyst metal is supported by impregnating or adsorbing the support on which the washcoat layer has been formed in a catalyst metal solution and calcining at 250 to 700 ° C. The firing conditions for the catalytic metal layer are preferably 500 to 700 ° C. and 1 to 2 hours. You may reduce | restore on the conditions similar to baking as needed. As described above, in this production method, rapid cooling is performed after the formation of the washcoat layer. However, rapid cooling is preferably performed after the catalyst metal is supported and calcined (or reduced). This is because the catalyst is more likely to contain tetragonal or cubic zirconia.

The hydrogen combustion catalyst of the present invention has sufficient catalytic activity even at a low temperature, and can maintain its activity even after long-term use.

X-ray diffraction diagram of the hydrogen combustion catalyst in the embodiment (Example) X-ray diffraction pattern of the hydrogen combustion catalyst in the embodiment (comparative example) The acid point measurement result diagram of the hydrogen combustion catalyst in the embodiment Change of activity of hydrogen combustion catalyst with time in embodiment

Example 1
A hydrogen combustion catalyst containing powdered tungsten oxide and an aqueous solution of powdered zirconia and a zirconium salt was produced, and physical properties of the obtained catalyst were evaluated and a hydrogen combustion test was performed.

7 g of 99.5 wt% powdered tungsten oxide, 9.33 g of 99.5 wt% powdered zirconia, and 121.3 g of 47 wt% zirconium nitrate aqueous solution (25% zirconia solid content) were mixed well in a mortar. 100 g of water was added to give a proper viscosity to prepare a washcoat aqueous solution (powdered tungsten oxide: powdered zirconia: zirconia derived from zirconium salt = 15: 20: 65 (wt%)). A stainless honeycomb having a diameter of 25 mm, a length of 50 mm, and a capacity of 24.5 mL was immersed in this washcoat aqueous solution. Then, it dried at 120 degreeC for 1 hour, and baked in the 700 degreeC electric furnace for 2 hours. Through the above steps, a washcoat layer was adhered to the honeycomb at 80 g / L. The fired honeycomb was immediately removed from the furnace and rapidly cooled at a cooling rate of 150 ° C./min to obtain a hydrogen combustion catalyst. The rapid cooling after calcination is considered to cause martensitic transformation and easily become a catalyst containing not only monoclinic zirconia but also tetragonal or cubic zirconia .

A catalytic metal solution was prepared by diluting 1.23 g of 8% dinitrodiamine platinum aqueous solution with 100 g of water. The catalyst metal solution was impregnated with a honeycomb having a washcoat layer attached thereto, and dried at 120 ° C. for 1 hour. After drying, the catalyst metal was supported by firing for 2 hours in an electric furnace at 700 ° C. The fired honeycomb was immediately removed from the furnace and rapidly cooled at a cooling rate of 150 ° C./min to obtain a hydrogen combustion catalyst. When the physical properties of the obtained catalyst were measured, the surface area was 65 m ² / g, the average pore diameter was 2 nm, and the average particle diameter of platinum particles was 72 nm.

Example 2
In Example 1, a catalyst in which the ratio of powdered tungsten oxide, powdered zirconia and zirconium salt-derived zirconia in the washcoat was changed was produced. In adjusting the washcoat aqueous solution, 9.33 g of powdered tungsten oxide, 23.3 g of powdered zirconia, and 56 g of 47 wt% zirconium nitrate aqueous solution (zirconia solid content 25%) were mixed (powdered tungsten oxide: powdered zirconia: zirconium salt). The production conditions were the same as those of the catalyst of Example 1, except that the origin was zirconia = 20: 50: 30 (wt%). When the physical properties of the obtained catalyst were measured, the surface area was 87 m ² / g, the average pore diameter was 2 nm, and the average particle diameter of platinum particles was 45 nm.

Example 3
In Example 1, a catalyst in which the ratio of powdered tungsten oxide, powdered zirconia and zirconium salt-derived zirconia in the washcoat was changed was produced. In preparation of the washcoat solution, 37.3 g of powdered tungsten oxide, 4.7 g of powdered zirconia, and 23.4 g of 36.6 wt% zirconium acetate aqueous solution (zirconia solid content 20%) were mixed (powdered tungsten oxide: powder) The production conditions were the same as in the catalyst of Example 1, except that zirconia: zirconia derived from zirconium salt = 80: 10: 10 (wt%)). When the physical properties of the obtained catalyst were measured, the surface area was 46 m ² / g, the average pore diameter was 4 nm, and the average particle diameter of platinum particles was 109 nm.

Comparative Examples 1 and 2
A hydrogen combustion catalyst was produced in the same manner as in Example 1 except that alumina was used instead of zirconia in the preparation of the washcoat solution. In Comparative Example 1, only the zirconium nitrate aqueous solution was changed to an aluminum nitrate aqueous solution, and in Comparative Example 2, both the zirconium nitrate aqueous solution and the powdered zirconia were changed to the aluminum nitrate aqueous solution and the powdered alumina. As alumina, 595 g of 37.5 wt% aqueous aluminum nitrate solution (alumina solid content 5.1%) and 9.33 g of 99.5 wt% powdered alumina were used.

Comparative Example 3
Without adding powdered tungsten oxide, 16.3 g of powdered zirconia having a purity of 99.5% and only 121.3 g of a 47 wt% zirconium nitrate aqueous solution (25% zirconia solid content) were mixed to prepare a washcoat solution. Other conditions were the same as in Example 1.

Comparative Example 4
Without adding powdered zirconia, only 16.3 g of powdered tungsten oxide and 121.3 g of 47 wt% zirconium nitrate aqueous solution (zirconia solid content 25%) were mixed to prepare a washcoat solution. Other conditions were the same as in Example 1.

[Evaluation of the physical properties]
X-ray diffraction analysis was performed on the hydrogen combustion catalyst produced above (FIGS. 1 and 2). Further, acid point measurement was performed by ammonia adsorption temperature programmed desorption (TPD) (FIG. 3).

From the results of X-ray diffraction (Table 1, FIG. 1), the catalysts of Examples 1 to 3 containing an aqueous solution of powdered tungsten oxide, powdered zirconia and zirconium salt showed 2θ = 23.2 °, 23.6 °, It has a peak derived from 24.4 ° tungsten oxide and three peaks derived from monoclinic and tetragonal or cubic zirconia near 2θ = 28.3, 30.3, 31.5 °. It was.

On the other hand, the catalyst of Comparative Example 1 that does not contain an aqueous solution of zirconium salt (Table 1, FIG. 2) does not have a peak derived from tetragonal or cubic zirconia at 2θ = 30.3 °, and the powder The catalyst of Comparative Example 2 which does not contain an aqueous solution of zirconia and zirconium salt has no peaks derived from monoclinic and tetragonal or cubic zirconia at 2θ = 28.3, 30.3, 31.5 °. There wasn't. Further, the catalyst of Comparative Example 3 that does not contain powdered tungsten oxide does not have peaks derived from 2θ = 23.2 °, 23.6 °, and 24.4 ° tungsten oxide, and powdery zirconia is used. The catalyst of Comparative Example 4 which was not included did not have peaks derived from monoclinic zirconia at 2θ = 28.3 and 2θ = 31.5 °.

From the above results, it is possible to obtain a catalyst containing monoclinic zirconia by using powdered zirconia as the washcoat solution, and containing tetragonal or cubic zirconia by using an aqueous solution of zirconium salt. It turned out to be a catalyst.

In addition, as a result of acid point measurement (Table 1, FIG. 3), the catalysts of Examples 1, 3 and Comparative Example 3 have a peak derived from monoclinic zirconia at a temperature of about 180 ° C. And a peak derived from crystal zirconia. On the other hand, the catalysts of Comparative Examples 1 and 2 have only peaks derived from monoclinic zirconia around 150 ° C., and Comparative Example 4 has peaks derived from tetragonal or cubic zirconia around 700 ° C. It was only.

[Hydrogen combustion test]
The hydrogen combustion test was conducted as follows. Hydrogen and air mixed gas were introduced into the catalyst layer (catalyst volume 24.5 mL) filled with the catalyst obtained above, and the reduction rate of hydrogen in the mixed gas before and after passing through the catalyst layer was calculated as decomposition efficiency. The decomposition efficiency was calculated from the following formula, and the catalyst performance of each example was evaluated.
<(1- (Outlet concentration / Inlet concentration)) × 100> (%)

The hydrogen concentration in the mixed gas at the catalyst layer inlet and the catalyst layer outlet was measured using a gas chromatograph. In this embodiment, the gas chromatograph whose effective measurement upper limit of decomposition efficiency is 99.9% was used. The hydrogen concentration measurement test was performed under the following conditions.
・ Hydrogen concentration in mixed gas before passing through catalyst layer is 10000ppm
・ Catalyst layer inlet temperature 25 ℃
・ Catalyst layer catalyst volume 24.5mL
・ Mixed gas flow rate 2000mL / min

The hydrogen combustion catalyst was stored in air at room temperature and atmospheric pressure for 1 to 10 days, and the long-term change in catalyst activity over time was measured. Similarly, the catalytic activity after 4 months was also evaluated.

From the above results (Table 2, FIG. 4), the catalysts of Examples 1 to 3 were able to maintain high catalytic activity even after 4 months. On the other hand, the catalyst activities of Comparative Examples 1 to 4 decreased with the passage of the storage period.

The hydrogen combustion catalyst of the present invention is suitable for burning hydrogen in exhaust gas in a hydrogen production plant or a hydrogen atmosphere furnace.

Claims

In a hydrogen combustion catalyst in which a catalyst metal is supported on a carrier having a washcoat layer,
The washcoat layer consists of zirconia and tungsten oxide,
The zirconia contains monoclinic zirconia as an essential component, and further contains at least one of cubic zirconia and tetragonal zirconia.
X-ray diffraction has peaks at 2θ = 23.2 °, 23.6 °, 24.4 °, 28.3 °, 30.3 °, and 31.5 °,
The ratio (B / A) between the intensity (A) of the highest peak among 2θ = 23.2 °, 23.6 °, and 24.4 ° and the peak intensity (B) at 2θ = 30.3 ° is 0. The hydrogen combustion catalyst according to claim 1, wherein the hydrogen combustion catalyst is .05 to 1.0.
The ratio (B / C) of the peak intensity (C) at 2θ = 28.3 ° and the peak intensity (B) at 2θ = 20.3 ° is 0.05 to 2.00. Hydrogen combustion catalyst.
The hydrogen combustion catalyst according to any one of claims 1 to 3, which has peaks at temperatures of 150 to 250 ° C and 550 to 700 ° C in acidity measurement by an ammonia temperature-programmed desorption method.
A method for producing a hydrogen combustion catalyst according to any one of claims 1 to 4,
A step of forming a washcoat layer that is impregnated with a carrier in a washcoat solution containing tungsten oxide and zirconia, and then dried and fired;
A step of rapidly cooling the carrier on which the washcoat layer has been formed;
A step of impregnating or adsorbing a rapidly cooled carrier in a catalyst metal solution to support the catalyst metal;
A step of calcining or reducing after supporting the catalytic metal,
The method for producing a hydrogen combustion catalyst, wherein the washcoat solution uses powdered zirconia and an aqueous solution of zirconium salt as zirconia.