JP2012250143A - Carbon monoxide methanation catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a CO methanation catalyst which has an activity to subject CO to methanation and then to eliminate in a wide temperature range from a low temperature while having a high reaction selectivity to the CO even if an introduced gas contains CO; and to provide a method for eliminating the CO in hydrogen using the catalyst.SOLUTION: The carbon monoxide methanation catalyst is constituted of: porous silica having 1-49 mass% of titanium; and ruthenium.

Description

  The present invention relates to a catalyst and a removal method for efficiently removing a trace amount of carbon monoxide contained in hydrogen.

  The hydrogen-rich fuel gas used for the electrochemical reaction in the fuel cell is usually a reformed gas obtained by chemically decomposing city gas or propane gas. These reformed gases contain about 1% carbon monoxide (CO) generated in the chemical decomposition step. A polymer electrolyte fuel cell using hydrogen as a fuel is provided with a platinum-based catalyst in the electrode portion. However, if the fuel gas contains CO, the platinum catalyst is poisoned, resulting in a problem that the cell efficiency is lowered.

In order to solve this problem, a method of selectively reducing CO in advance when supplying the reformed gas to the fuel cell is effective. At this time, it is said that the concentration of CO needs to be at least 100 ppm or less, desirably 10 ppm or less.
Selective oxidation catalysts are widely used to remove CO. Although the oxidation reaction of CO is a simple reaction as shown in Formula 1, since it is a reaction performed in a large excess of hydrogen, the oxidation reaction of hydrogen shown in Formula 2 can occur simultaneously. Since the consumption of hydrogen directly leads to a decrease in the power generation efficiency of the fuel cell, a high CO selectivity that selectively oxidizes CO in preference to hydrogen is required. However, there has been no carbon monoxide selective oxidation catalyst having sufficiently satisfactory selectivity. In addition, a device such as a blower for introducing oxygen has to be installed, resulting in a problem that the fuel reformer system becomes complicated and large.

Therefore, in recent years, a method has been devised in which CO is methanated and removed by reduction with hydrogen. The methanation reaction of CO is a reaction for generating methane and water from CO and hydrogen as shown in Equation 3, and is the reverse reaction of steam reforming. Since it is a CO removal system that does not require oxygen, it is possible to omit air supply machines. After supplying hydrogen containing methane to the hydrogen electrode of the fuel cell, it is recycled to the fuel reformer as off-gas, and a system that converts methane to hydrogen again by the steam reforming reaction is built to suppress the decrease in fuel efficiency. Can do.
As a CO methanation catalyst, a method in which ruthenium or nickel is supported on an inorganic carrier is known (see, for example, Patent Document 1). Don't be. In the reaction reformer, since the exhaust temperature of the water gas shift reaction in the previous stage introduced into the CO removal unit is about 200 ° C., it is necessary to separately install a heating device, which increases the cost of the fuel reformer. There was a problem. Therefore, a catalyst capable of removing CO at a low temperature of 200 ° C. or lower has been proposed (see, for example, Patent Document 2). However, the hydrogen gas that is shared by the water gas shift reaction contains about 20% of carbon dioxide (CO ₂ ) in addition to CO, and the side reaction CO ₂ methanation occurs as shown in Equation 4. In addition, hydrogen is consumed, which is not desirable, and there is a problem in that the reaction is runaway due to an exothermic reaction and the risk is high. In view of the above, a carbon monoxide methanation catalyst having the ability to achieve both CO removal efficiency and high reaction selectivity in a low reaction temperature range of 200 ° C. or lower is desired.

JP 2004-97859 A JP 2007-252990 A

The present invention has been made in view of the above circumstances, and its purpose is to have an activity capable of removing CO by methanation in a wide reaction temperature range from a low temperature, and even if CO ₂ is contained in the introduced gas. An object of the present invention is to provide a carbon monoxide methanation catalyst having high reaction selectivity with respect to CO, and to provide a method for removing CO in hydrogen using the catalyst.

As a result of intensive studies, the present inventors have found that the object of the present invention can be effectively achieved by supporting ruthenium in a particle form having a particle size of 1 to 5 nm on a porous silica containing titanium. The present invention has been completed.
That is, the present invention is as follows.
(1) A carbon monoxide methanation catalyst comprising a porous silica having a titanium content of 1 to 49% by mass and ruthenium.
(2) The porous silica has an average pore diameter of 1 to 10 nm, a specific surface area of 400 to 2000 m ² / g, and at least 1 in the position where the d interval of X-ray diffraction is larger than 2.0 nm. The carbon monoxide methanation catalyst according to (1), which has two peaks.
(3) The content of ruthenium in the catalyst is in the range of 0.5 to 20% by weight, and is supported in the form of particles having a particle diameter of 1 to 5 nm in the pores of the porous silica. The carbon monoxide methanation catalyst according to (1) or (2).
(4) The carbon monoxide methanation catalyst according to any one of (1) to (3) is contacted with a mixed gas containing hydrogen, carbon monoxide, and carbon dioxide to selectively methanate carbon monoxide. A carbon monoxide removal method characterized by the above.

According to the present invention, carbon monoxide methanation in which a reformed gas containing hydrogen gas as a main component and containing a small amount of CO is supported by ruthenium supported in the form of particles having a particle diameter of 1 to 5 nm on a porous silica containing titanium. By contacting with a catalyst, it has an activity capable of methanating and removing CO from a low temperature to a wide reaction temperature range, and has high reaction selectivity with respect to CO even if CO ₂ is contained in the introduced gas. In addition, stable performance can be imparted in the purification of hydrogen supplied to the fuel cell.

FIG. 1 is a diagram of the pore distribution of the silica porous body A. FIG. FIG. 2 is an X-ray diffraction pattern of the porous silica material A. FIG. 3 is a diagram showing the pore distribution of the porous silica A1. FIG. 4 is an X-ray diffraction pattern of the porous silica A1. FIG. 5 is a transmission electron micrograph of the carbon monoxide methanation catalyst A1. FIG. 6 is a transmission electron micrograph of the carbon monoxide methanation catalyst A1. FIG. 7 is a diagram showing the pore distribution of the porous silica A2. FIG. 8 is an X-ray diffraction diagram of the porous silica A2. FIG. 9 is a transmission electron micrograph of the carbon monoxide methanation catalyst A2. FIG. 10 is a transmission electron micrograph of the carbon monoxide methanation catalyst C. FIG. FIG. 11 is an X-ray diffraction pattern of a mixed powder of fine particle silica and titanium oxide.

Next, embodiments of the present invention will be described, but the technical scope of the present invention is not limited by these embodiments, and can be implemented in various forms without changing the gist of the invention. . Further, the technical scope of the present invention extends to an equivalent range.
The catalyst of the present invention is composed of a porous silica containing titanium and ruthenium.

  The porous silica in the present invention means a substance mainly composed of a silicon oxide having a porous structure. The porous silica preferably contains titanium from the viewpoint of improving methanation catalyst activity and CO reaction selectivity. The titanium content is preferably 1 to 49% by mass, more preferably 5 to 40% by mass, and most preferably 20 to 40% by mass. If the titanium content is low, the effect of improving the methanation catalyst activity and CO reaction selectivity will be small. If the content is high, the specific surface area and pore volume of the porous silica material will decrease, making it impossible to support ruthenium in a highly dispersed state. As a result, the methanation catalyst activity decreases. The titanium content was measured with an X-ray fluorescence analyzer.

Examples of a method for introducing titanium into the porous silica include a liquid phase method and a gas phase method. In the liquid phase method, titanium is introduced into the porous silica by dissolving a metal salt, metal alkoxide, etc. containing titanium in a solvent such as water, ethanol, benzene, etc., adding the porous silica to the solution, and mixing with stirring. Is done. In the vapor phase method, titanium alkoxide or the like that generates vapor or easily sublimates is used as a precursor, and titanium is introduced by bringing the vapor into contact with the porous silica.
As the titanium raw material, titanium alkoxides such as titanium isopropoxide and titanium methoxide, and titanium tetrachloride can be used.

  The average pore diameter of the porous silica in the present invention is in the range of 1 to 10 nm, preferably in the range of 1 to 5 nm. If the average pore diameter is less than 1 nm, gas diffusion is limited, and the reaction does not proceed efficiently. Further, those having an average pore diameter exceeding 10 nm are not preferable because it is difficult to control the particles to have a particle diameter of 1 to 5 nm when ruthenium is supported. The average pore diameter of the porous silica in the present invention was calculated by known nitrogen adsorption / desorption. That is, the average pore diameter was calculated by a known BJH method.

The specific surface area of the porous silica material 400~2000m ² / g or more, preferably in the range of 600~2000m ² / g. If the specific surface area is less than 400 m ² / g, ruthenium may not be supported in a highly dispersed state. In addition, those having a specific surface area greater than 2000 m ² / g are substantially difficult to produce. The specific surface area of the porous silica in the present invention was calculated by known nitrogen adsorption / desorption.

  Further, the porous silica has at least one peak at a position where the d interval is larger than 2.0 nm in the X-ray diffraction pattern. The X-ray diffraction peak means that a periodic structure having a d value corresponding to the peak angle is present in the sample. Therefore, having one or more peaks at a diffraction angle corresponding to a d value of 2 nm or more means that the pores are regularly arranged at intervals of 2 nm or more. The X-ray diffraction pattern of the porous silica in the present invention was measured by a fully automatic X-ray diffractometer.

  Although it does not specifically limit as a manufacturing method of the silica porous body in this invention, For example, it can manufacture as follows. First, an inorganic raw material and an organic raw material are mixed and reacted to form an organic matter-inorganic matter composite in which an inorganic matter skeleton is formed around the organic matter as a template. Next, a silica porous body is produced by removing organic substances from the obtained composite.

As the inorganic skeleton component, alkoxysilane such as tetramethoxysilane, tetraethoxysilane, and tetrapropoxysilane, sodium silicate, kanemite (NaHSi ₂ O ₅ .3H ₂ O), or silica can be used. These skeletal components form a silicate skeleton. These can be used alone or in admixture of two or more.

Although the organic raw material used as a casting_mold | template is not specifically limited, For example, surfactant is mentioned. The surfactant may be any of cationic, anionic, and nonionic, specifically, alkyltrimethylammonium (preferably alkyltrimethylammonium having an alkyl group having 8 to 18 carbon atoms). , Alkyl ammonium, dialkyl dimethyl ammonium, benzyl ammonium chloride, bromide, iodide or hydroxide, fatty acid salt, alkyl sulfonate, alkyl phosphate, polyethylene oxide nonionic surfactant, primary alkyl Examples include amines and triblock copolymer type polyalkylene oxides.
One of the above surfactants may be used alone, or two or more may be used in combination.

When mixing an inorganic raw material and an organic raw material, a suitable solvent can be used. Although it does not specifically limit as a solvent, For example, the mixture of water, an organic solvent, water, and an organic solvent etc. are mentioned.
The method of forming the complex of inorganic and organic is not particularly limited. For example, after dissolving the organic raw material in a solvent, adding the inorganic raw material and adjusting to a predetermined pH, the reaction mixture is then heated to a predetermined temperature. And a method in which the condensation polymerization reaction is carried out. The reaction temperature of the polycondensation reaction varies depending on the kind and concentration of the organic raw material and inorganic raw material to be used, but is usually 0 ° C to 100 ° C, preferably 35 ° C to 80 ° C.
The reaction time of the condensation polymerization reaction is usually 8 hours to 24 hours. In addition, the above condensation polymerization reaction may be performed either in a stationary state or in a stirring state, or may be performed in combination.

  A porous silica material can be obtained by removing the organic raw material from the composite obtained after the condensation polymerization reaction. Here, the removal of the organic substance from the complex of the organic substance and the inorganic substance can be performed by a method of baking at 400 to 800 ° C., a method of treating with a solvent such as water or alcohol, and the like.

  In the present invention, ruthenium is desirably supported in the form of particles having a particle diameter of 1 to 5 nm in the pores of the porous silica. When the particle diameter is smaller than 1 nm or larger than 5 nm, the methanation catalytic activity is lowered, which is not preferable. In addition, when supported outside the pores of the porous silica, metal particle growth proceeds to become coarse particles, which is not preferable because the methanation catalyst activity is lowered.

In the present invention, ruthenium chloride, ruthenium nitrate and the like can be used as the ruthenium raw material.
The method for supporting ruthenium in the pores of the porous silica material in the form of particles having a particle diameter of 1 to 5 nm is not particularly limited. Then dry. The drying conditions are not particularly limited, but are usually dried at 80 to 200 ° C. After drying, it can be reduced to obtain a carbon monoxide methanation catalyst.

As the reduction method, a method of treating with a reducing agent, heat, light or the like can be used. Which treatment method is used depends on the type of the metal raw material, but the conditions under which the metal raw material salt or complex salt decomposes to form metal particles are set. Moreover, since excessive treatment may increase the particle diameter due to sintering of the produced ruthenium particles, it is necessary to set appropriate conditions.
For example, when ruthenium chloride is used, hydrogen is used as a reducing agent and the treatment is performed at 200 ° C. or higher.

By contacting the carbon monoxide methanation catalyst of the present invention with hydrogen gas containing CO and CO ₂ , CO can be selectively converted to methane and removed. The contact method is not particularly limited, but a fixed bed method is desirable. The reaction temperature is preferably 100 to 400 ° C, more preferably 120 to 300 ° C, and most preferably 130 to 250 ° C. If the reaction temperature is too low, the CO conversion rate may not be sufficient, and if the reaction temperature is too high, the CO reaction selectivity may not be sufficient.
Next, the present invention will be described in more detail with reference to examples, but the technical scope of the present invention is not limited by the following examples.

Production Example 1 (Synthesis of porous silica)
50 g of water glass (No. 1 sodium silicate) (SiO ₂ / Na ₂ O = 2.00) was added to 1 L of ion-exchanged water containing 0.1 mol of docosyltrimethylammonium bromide and dissolved at 70 ° C. Further, 2N HCl was added to adjust the pH to 8.5, and the mixture was stirred at 70 ° C. for 3 hours. Thereafter, washing with water was repeated 5 times and dried at 40 ° C. The dried powder was heated in nitrogen gas at 450 ° C. for 3 hours and then fired in air at 550 ° C. for 6 hours to obtain porous silica A.

When the pore distribution of the obtained porous silica A was measured with a nitrogen adsorption device (Quadrasorb SI) manufactured by Quantachrome and determined by the BJH method, the average pore diameter was 4.2 nm (FIG. 1). Further, when the obtained porous silica was measured with an X-ray fluorescence analyzer, the titanium content was 0.05% by weight or less. Further, when the porous silica material A was analyzed with an X-ray diffractometer (RINT2000) manufactured by Rigaku Corporation, it had a peak at a position where the d interval of X-ray diffraction was 5.0 nm (FIG. 2). Furthermore, it was 970 m < ² > / g when the specific surface area of the silica porous body A was computed by the nitrogen adsorption / desorption method.

Production Example 2 (Introduction of titanium into the porous silica by the liquid phase method)
The process of impregnating 5 g of porous silica A obtained in Production Example 1 with 5 ml of n-isopropanol solution containing 2.5 g of titanium isopropoxide and drying at 120 ° C. for 24 hours was repeated twice to obtain porous silica containing titanium. Body A1 was obtained.
When the pore distribution of the obtained porous silica A1 was measured with a nitrogen adsorption device (Quadrasorb SI) manufactured by Quantachrome and determined by the BJH method, the average pore diameter was 3.7 nm (FIG. 3). Moreover, it was 33.0 weight% when the titanium content of obtained silica porous body A1 was measured with the X-ray fluorescence analyzer. Further, when the porous silica A1 was analyzed with an Rigaku X-ray diffractometer (RINT2000), it had a peak at a position where the d interval of X-ray diffraction was 4.9 nm (FIG. 4). Furthermore, it was 741 m < ² > / g when the specific surface area of the silica porous body A1 was computed by the nitrogen adsorption / desorption method.

Example 1 (Production of carbon monoxide methanation catalyst)
3 g of the porous silica A1 obtained in Production Example 2 was placed in a Schlenk tube, heated to 100 ° C., and vacuum degassed at 1 × 10 ⁻⁴ mmHg for 2 hours.
In a 50 ml eggplant-shaped flask, 0.18 g of ruthenium chloride n-hydrate (RuCl ₃ · nH ₂ O) and 4.8 ml of 6N hydrochloric acid were added and dissolved by heating at 60 ° C. to prepare an aqueous ruthenium chloride solution.

80 ml of water was added to the obtained aqueous ruthenium chloride solution, and further, porous silica A1 was added and stirred for 24 hours. Then, water was distilled off using an evaporator while heating to 70 ° C., and further dried in an oven at 110 ° C. for 24 hours.
Next, the obtained residue was calcined in air at 200 ° C. for 2 hours, and reduced in a dry hydrogen stream at 350 ° C. for 3 hours to obtain a carbon monoxide methanation catalyst A1 of the present invention. The amount of ruthenium supported by the carbon monoxide methanation catalyst A1 was 5% by mass as measured by a fully automatic X-ray fluorescence analyzer (XRF-1700WS). By observation with a transmission electron microscope, it was observed that ruthenium particles having a particle diameter of 2 to 3 nm were arranged at regular intervals along the pores (FIGS. 5 and 6).

Production Example 3
Titanium was introduced into the porous silica obtained in Production Example 1 by the CVD method using titanium chloride as a raw material to obtain porous silica A2 containing titanium. The obtained porous silica body A2 had an average pore diameter of 3.9 nm (FIG. 7). Moreover, it was 8.8 weight% when the titanium content of the obtained silica porous body A2 was measured with the X ray fluorescence analyzer. Further, when the porous silica A2 was analyzed with an X-ray diffractometer (RINT2000) manufactured by Rigaku Corporation, it had a peak at a position where the d interval of X-ray diffraction was 4.9 nm (FIG. 8). Furthermore, it was 835 m < ² > / g when the specific surface area of the silica porous body A2 was computed by the nitrogen adsorption / desorption method.

Example 2
A carbon monoxide methanation catalyst A2 was obtained in the same manner as in Example 1 except that the porous silica A2 was used instead of the porous silica A1. Observation of the transmission electron microscope showed that ruthenium particles having a particle diameter of 2 to 3 nm were arranged at regular intervals along the pores (FIG. 9).

Production Example 4
Porous silica B was obtained in the same manner as in Example 1 except that hexadecyltrimethylammonium bromide was used instead of docosyltrimethylammonium bromide. Subsequently, porous silica B1 was obtained in the same manner as in Production Example 2 except that porous silica B was used instead of porous silica A. The obtained porous silica body B1 had an average pore diameter of 2.3 nm. Moreover, it was 32.7 weight% when the titanium content was measured with the X ray fluorescence analyzer. Moreover, when it analyzed with the Rigaku X-ray-diffraction apparatus (RINT2000), it had a peak in the position where d space | interval of X-ray diffraction was 3.7 nm. Furthermore, it was 734 m < ² > / g when the specific surface area of the silica porous body B1 was computed by the nitrogen adsorption / desorption method.

Example 3
A carbon monoxide methanation catalyst B was obtained in the same manner as in Example 1 except that the porous silica B1 was used instead of the porous silica A1. By observation with a transmission electron microscope, it was observed that ruthenium particles having a particle diameter of 2 to 3 nm were arranged at regular intervals along the pores.

Comparative Example 1
A carbon monoxide methanation catalyst C was obtained in the same manner as in Example 1 except that the porous silica material not containing titanium obtained in Production Example 1 was used in place of the porous silica material A1. Observation of the transmission electron microscope showed that ruthenium particles having a particle diameter of 2 to 3 nm were arranged at regular intervals along the pores (FIG. 10).

Comparative Example 2
1.35 g of fine particle silica (Aerosil 300 (manufactured by Nippon Aerosil Co., Ltd.)) and 1.65 g of fine titanium oxide (manufactured by Wako Pure Chemical Industries) were mixed well to obtain a mixed powder of silica and titanium oxide. The titanium content of the mixed powder was measured by an X-ray fluorescence analyzer and found to be 32.7% by weight. Further, when this mixed powder was analyzed with an Rigaku X-ray diffractometer (RINT2000), there was no peak where the d-interval of X-ray diffraction was larger than 2 nm (FIG. 11). Furthermore, it was 162 m < ² > / g when the specific surface area of mixed powder was computed by the nitrogen adsorption / desorption method.

  A carbon monoxide methanation catalyst D was obtained in the same manner as in Example 1 except that the above mixed powder of silica and titanium oxide was used in place of the porous silica A1. It was observed that the aggregated ruthenium particles were supported on the outer surfaces of silica and titanium oxide by transmission electron microscope observation.

Test Example 1 (CO removal reaction using carbon monoxide methanation catalyst)
Put quartz wool into a SUS reaction tube with a diameter of 8 mm, mix and fill 1.5 g of glass beads and 0.1 g of carbon monoxide methanation catalyst, and install a heater on the outer periphery of the reaction tube to install carbon monoxide methane. A calibrator was created. The temperature of the catalyst layer was monitored by a thermocouple embedded in the catalyst layer, and the temperature of the catalyst layer was adjusted by the heater.

The catalyst layer was pretreated at 350 ° C. for 2 hours under a hydrogen stream before starting the reaction, and then activated by cooling to room temperature.
Thereafter, a simulated reaction gas (CO; 0.6%, CO ₂ ; 20.0%, hydrogen balance mixed gas) is introduced into the reaction tube at a gas flow rate of 3000 ml / (h · g _—catalyst ) using a mass flow meter. The reaction was carried out in the catalyst layer.

During the reaction, it was confirmed whether a predetermined flow rate was flowing with a soap film flow meter.
Table 1 shows the results obtained by conducting carbon monoxide methanation reaction while changing the catalyst layer temperature from 140 ° C to 230 ° C and measuring the gas composition at the outlet of the reaction tube by gas chromatography.
The CO selectivity was calculated by the following formula.

  Reaction conditions with CO concentration <10 ppm, selectivity> 50% are marked in bold.

In the carbon monoxide methanation catalysts C and D shown in Comparative Examples 1 and 2, CO removal activity is low, CO reaction selectivity is poor, CO ₂ methanation proceeds, and hydrogen is wasted. It was. On the other hand, the carbon monoxide methanation catalyst A1 shown in the examples can remove 100% of carbon monoxide at a temperature higher than the reaction temperature of 150 ° C., and has a high CO reaction selectivity of 50% or more up to 210 ° C. Could be maintained. Further, the carbon monoxide methanation catalyst A2 and the carbon monoxide methanation catalyst B shown in Examples 2 and 3 also showed performance close to this.

As described above, the carbon monoxide methanation catalyst of the present invention has an activity capable of methanating and removing CO in a wide reaction temperature range from a low temperature as compared with the conventional catalyst, and the introduced gas contains CO _2. Can also have high reaction selectivity with respect to CO, and can provide stable performance in hydrogen purification of fuel cells.

The carbon monoxide selective methanation catalyst of the present invention can remove CO by methanation in a wide reaction temperature range from low temperature, and has high reaction selectivity with respect to CO even if CO ₂ is contained in the introduced gas. It can have a stable performance in the purification of hydrogen supplied to the fuel cell, and contributes greatly to the industry.

Claims (4)

  A carbon monoxide methanation catalyst comprising a porous silica having a titanium content of 1 to 49% by mass and ruthenium. The porous silica has an average pore diameter of 1 to 10 nm, a specific surface area of 400 to 2000 m ² / g, and at least one peak at a position where the d-spacing of X-ray diffraction is larger than 2.0 nm. The carbon monoxide methanation catalyst according to claim 1.   The ruthenium content in the catalyst is in the range of 0.5 to 20 wt%, and is supported in the form of particles having a particle diameter of 1 to 5 nm in the pores of the porous silica. 2. The carbon monoxide methanation catalyst according to 2.   A carbon monoxide methanation catalyst according to any one of claims 1 to 3 is contacted with a mixed gas containing hydrogen, carbon monoxide, and carbon dioxide to selectively methanate carbon monoxide. Carbon oxide removal method.