JP2003024783A - Hydrogen generating catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydrogen generating catalyst which exhibits high hydrogen generation activity in a wide temperature range and is excellent in heat resistance. SOLUTION: The catalyst is composed of a carrier containing complex oxide powder comprising ceria and alumina, which are dispersed in a nanometer scale, and a noble metal supported on the carrier. The complex oxide powder has a high steam adsorption capacity, and the catalyst using the powder generates H2 by a steam reforming reaction. The noble metal supported on the carrier is reduced easily into a metallic state even when oxidized to recover its activity and has a large interaction with the carrier to control the particle growth of the metal. The steam reforming reaction is accelerated by the synergetic effect.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrogen generating catalyst, and more particularly, to a catalyst for efficiently generating hydrogen from a gas containing hydrocarbon, oxygen and steam by utilizing a steam reforming reaction.

[0002]

2. Description of the Related Art Hydrogen or a mixed gas of hydrogen and carbon monoxide is an important chemical raw material in many chemical industrial processes such as ammonia synthesis, methanol synthesis and oxo synthesis or in petroleum refining. Recently, the importance of hydrogen as a clean energy source such as a fuel cell has been increasing.

Further, also in purifying exhaust gas of an internal combustion engine,
Excellent ability to reduce NO _x, and hydrogen excellent ability to recover _the NO _x storage-reduction type catalyst desorbed sulfur oxides from _the NO _x storage material in which sulfur poisoning is now attracting attention.

As a method for producing hydrogen, a steam reforming reaction of hydrocarbon represented by the following formula is often used.

C _n H _m + nH ₂ O → nCO + (n + m / 2) H ₂ (−ΔH <0) Since this steam reforming reaction involves a large endotherm, it is necessary to supply necessary heat from the outside. . Therefore, in many cases, oxygen is added to the reaction gas and the progress of the steam reforming reaction is promoted by utilizing the reaction heat of the partial oxidation reaction or the oxidation reaction shown in the following formula.

C _n H _m + n / 2O ₂ → nCO + m / 2 H ₂ (−ΔH> 0) C _n H _m + (n + m / 4) O ₂ → nCO ₂ + m / 2 H ₂ (−ΔH> 0) ) In the steam reforming reaction, the CO shift reaction shown in the following equation simultaneously proceeds.

CO + H ₂ O → CO ₂ + m / 2H ₂ (−ΔH> 0) Various catalysts are used to promote the above reaction. For example, Japanese Patent Laid-Open No. 56-91844 discloses a hydrogen generating catalyst in which Rh is supported on zirconia. However, zirconia has a low heat resistance, and the specific surface area is reduced by heat during use, whereby the dispersibility of the supported Rh is reduced and the hydrogen generating ability is reduced.

Then, Japanese Patent Publication No. 6-4135 and Japanese Patent Laid-Open No. 3-8093
Japanese Patent Publication 7 discloses a hydrogen generation catalyst in which Rh is supported on a zirconia carrier partially stabilized by adding yttria or ceria. Further, JP-A-4-265156 discloses a hydrogen generation catalyst in which a noble metal is supported on ceria containing an alkali metal or an alkaline earth metal, and JP-A-11-226404.
The publication discloses a hydrogen generation catalyst in which Rh is supported on zirconia stabilized with an alkaline earth metal or a rare earth element.

[0009]

By the way, when the hydrogen generation catalyst is used in the exhaust gas of an internal combustion engine, or when it is used in a fuel reforming system of an internal reforming fuel cell mounted on an automobile or the like, the hydrogen generation catalyst is used. Will be used under various temperature conditions from low temperature to high temperature. Therefore, the hydrogen generation catalyst is required to be highly active in a low temperature range and have excellent heat resistance. However, no conventional hydrogen generation catalyst satisfies these two conditions.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a hydrogen generating catalyst that exhibits high hydrogen generating activity from a low temperature region to a high temperature region and is excellent in heat resistance.

[0011]

The hydrogen generation catalyst of the present invention for solving the above-mentioned problems is characterized in that it is a catalyst for generating hydrogen from a gas containing hydrocarbon, oxygen and water vapor, and ceria and alumina are both in the nm scale. And a noble metal supported on the carrier.

In the complex oxide powder contained in the hydrogen generation catalyst of the present invention, one particle without overlap using EDS of FE-STEM was subjected to a fine range analysis with a beam diameter of 5 nm. It is desirable that Ce and Al be detected at a composition ratio within ± 20% of the charged composition at 90% or more of the analysis points. Further, it is desirable that the composite oxide powder contains ceria in an amount of 50% by weight or more.

The precious metal preferably contains at least Rh.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The hydrogen generation catalyst of the present invention contains a composite oxide powder in which both ceria and alumina are dispersed in a carrier on a nm scale. This composite oxide powder has a high water vapor adsorption capacity. Further, the noble metal carried on this carrier is easily reduced to a metal state even if it is oxidized,
This restores the high activity of the precious metal. Furthermore, the interaction between the carrier and the noble metal is large, and the grain growth of the noble metal at high temperature is suppressed. It is considered that these synergistic effects promote the steam reforming reaction.

The state in which ceria and alumina are dispersed on the nm scale is obtained by performing a fine range analysis of one particle having no overlap using an EDS of FE-STEM with a beam diameter of 5 nm. It can be confirmed that Ce and Al are detected at a composition ratio within ± 20% of the charged composition at 90% or more of the points. FE-STEM is a Field Effect-Scanning
Abbreviation for Transmission Electron Microscopy, EDS
Is an abbreviation for Energy Dispersion Spectroscopy.

In this composite oxide powder, since ceria and alumina, which do not form a solid solution with each other, act as barriers to each other, sintering at high temperature is suppressed, and the pore volume of mesopores is kept high even after high temperature durability. can do. Mesopores are pores with a diameter of 2 to 50 nm in IUPAC, but may also be pores with a diameter of 1.5 to 100 nm due to the adsorption characteristics of molecules. The mesopores referred to here are the lower limit of 3.5 nm to 100 nm that can be measured in principle using a mercury porosimeter.
Means pores in the range.

In this composite oxide powder, the crystallite size of ceria calculated from the full width at half maximum of CeO ₂ (220) by X-ray diffraction was 5 to 5 after firing at 600 ° C. for 5 hours.
10-20nm, 1000 after firing at 10nm, 800 ℃ for 5 hours
It is desirable to have the characteristic that the thickness becomes 35 nm or less after firing at 5 ° C. for 5 hours. With such characteristics, sintering is reduced even after being exposed to high temperatures, and 600 ° C
The pore volume with a pore diameter of 3.5 to 100 nm is 0.07 cc / g or more after calcination for 5 hours, and the pore volume with a pore diameter of 3.5 to 100 nm is 0.04 cc / g after calcination for 5 hours at 800 ° C. It has a characteristic of g or more. This ensures a sufficient pore volume even after high temperature durability.

Furthermore, after firing at 600 ° C. for 5 hours, the pore volume with a pore diameter of 3.5 to 100 nm is 0.13 cc / g or more,
After calcination at 800 ° C. for 5 hours, the volume of pores having a diameter of 3.5 to 100 nm is more preferably 0.10 cc / g or more. The pore volume with a pore diameter of 3.5 to 100 nm is 0.19cc / g or more after calcination at 600 ° C for 5 hours, and the pore volume with a pore diameter of 3.5 to 100nm is 0.15cc after calcination at 800 ° C for 5 hours. cc / g
It is more desirable to have the above characteristics.

In the hydrogen generation catalyst of the present invention in which the carrier containing the composite oxide powder is loaded with the noble metal, the noble metal is loaded in the mesopores in a highly dispersed state, and the mesopores are used as the reaction field. Therefore, the activity is extremely high.
Furthermore, even after high-temperature durability, the mesopores, which are supporting sites for the noble metal, are sufficiently present, and the specific surface area is sufficiently large. Since the sintering of the oxide is suppressed, the grain growth of the noble metal is also suppressed, and the decrease in the activity after high temperature durability is greatly suppressed.

Ceria is preferably contained in the composite oxide powder in an amount of 50% by weight or more, and particularly preferably in an amount of 75% by weight or more. Therefore alumina is
Less than 50% by weight is preferred and less than 25% by weight is particularly desirable. When ceria is less than 75% by weight or less than 50%,
The hydrogen-producing ability in the low temperature range is reduced.

In order to produce this composite oxide powder, first, a precipitate of a ceria precursor and an alumina precursor or a compound of these precursors is precipitated from an aqueous solution in which a cerium compound and an aluminum compound are dissolved or a solution containing water. .

Salts are generally used as the cerium compound and the aluminum compound, and as the salts, sulfates, nitrates, chlorides, acetates and the like can be used. Water and alcohols can be used as the solvent for uniformly dissolving the salt. Further, for example, aluminum hydroxide, nitric acid, and water may be mixed and used as a raw material of aluminum nitrate.

The precipitation method is carried out by adjusting the pH mainly by adding ammonia water or the like, but the precursor of the more characteristic complex oxide can be obtained by various adjusting methods. For example, from an aqueous solution containing a cerium compound and an aluminum compound or a solution containing water, a method of precipitating these oxide precursors or compounds of these precursors at substantially the same time, or prior to the precipitation of the alumina precursor. There is a method of precipitating a ceria precursor (or vice versa).

Regarding the former method of precipitating almost simultaneously, a method of instantaneously adding ammonia water or the like and vigorous stirring, or a method of adjusting the pH at which the ceria precursor and the alumina precursor start to precipitate by adding hydrogen peroxide or the like. After that, there is a method of depositing a precipitate with aqueous ammonia.

Regarding the latter, the time required for neutralization with ammonia water or the like is set to be sufficiently long, preferably
A method of neutralizing for 10 minutes or more, or a pH at which a ceria precursor precipitates while monitoring the pH or a pH at which an alumina precursor precipitates, such as stepwise neutralization or maintaining such a pH. There is a method of adding a buffer solution.

In addition to ammonia water, an aqueous solution or alcohol solution in which ammonium carbonate, sodium hydroxide, potassium hydroxide, sodium carbonate, etc. are dissolved can be used. Ammonia and ammonium carbonate that volatilize during firing are particularly preferable. In addition, the pH of the alkaline solution is preferably 9 or more because it promotes the precipitation reaction of the precursor.

The precipitate thus obtained is calcined to obtain a composite oxide powder.

When the aging step is carried out, the heat of heating promotes the dissolution and reprecipitation and the growth of particles. This aging step is performed at room temperature or higher, preferably 100 to 2
It is desirable to carry out at 00 ° C, more preferably at 100 to 150 ° C. If the heating temperature is lower than 100 ° C, the aging-promoting effect is small and the aging time becomes long. Further, at a temperature higher than 200 ° C., the water vapor pressure becomes extremely high, so that a large-scale apparatus that can withstand high pressure is required, and the manufacturing cost becomes very high, which is not preferable. By firing the obtained precipitate, a complex oxide powder having a crystallite with a relatively high crystallinity and a large grain size is produced.

This firing step may be carried out in the atmosphere, and the temperature is preferably in the range of 300 to 900 ° C. Firing temperature is 3
When it is lower than 00 ° C, the stability as a carrier is substantially lost. In addition, firing at a temperature higher than 900 ° C causes a decrease in specific surface area, and is unnecessary from the viewpoint of usage as a carrier.

If the solution in which the precipitate is deposited is heated as it is to evaporate it to dryness, and further calcined, the aging step can be carried out during evaporation to dryness, but preferably at room temperature or higher.
It is better to keep it at 00 ° C or higher for aging.

The carrier referred to in the present invention may include the above-mentioned complex oxide powder, may be constituted only by the above-mentioned complex oxide powder, or may be constituted by mixing the above-mentioned complex oxide powder and the porous oxide powder. You can also do it. As the porous oxide, one kind or plural kinds of alumina, ceria, zirconia, titania, silica and the like can be used. When the carrier is a mixture of the composite oxide powder and the porous oxide powder, the content of the composite oxide powder is preferably 50% by weight or more. If the amount of the above complex oxide powder in the carrier is less than this, the hydrogen generating activity is reduced, which is not practical.

Noble metals supported on the carrier include Pt and R
It can be selected from h, Pd, Ir, etc., but preferably contains at least Rh. By supporting at least Rh, the hydrogen generation activity is particularly improved. The amount of the noble metal supported is preferably 0.1 to 10 g per liter of the carrier. If the supported amount is less than this, the hydrogen generating activity is low, and even if the supported amount is more than this, the hydrogen generating activity is saturated and grain growth between the noble metals may occur.

The hydrogen generating catalyst of the present invention is brought into contact with a gas containing at least hydrocarbon and steam to generate hydrogen with high activity from a low temperature region to a high temperature region by a steam reforming reaction. Further, it is preferable to contact with a gas containing oxygen. It is particularly preferable to use exhaust gas from a diesel engine as such gas. Exhaust gas from a diesel engine is rich in hydrocarbons, water vapor and oxygen, so that it is possible to generate hydrogen and purify exhaust gas from a diesel engine.

[0034]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples.

Example 1 Aluminum nitrate nonahydrate
0.2 mol (75.1 g) was mixed with 2000 ml of deionized water,
It was dissolved by stirring with a propeller stirrer for 5 minutes. Concentration there
304 g of 28 wt% cerium nitrate aqueous solution (0.5 in terms of CeO ₂₎
(Corresponding to mol) was mixed and stirred for 5 minutes. To the obtained mixed aqueous solution, 177 g of 25% ammonia water was added, and further
The solution was stirred for 10 minutes to obtain an aqueous solution containing a precipitate. This was subjected to an aging step of heat-treating at 120 ° C. for 2 hours under a pressure of 2 atm to ripen the precipitate.

Then, an aqueous solution containing the aged precipitate is added.
The composite oxide powder was prepared by heating at a heating rate of 100 ° C./hour, calcining at 400 ° C. for 5 hours, and further calcining at 600 ° C. for 5 hours. The obtained complex oxide powder contained about 89% by weight of Ce.
It is composed of O ₂ and about 11% by weight of Al ₂ O ₃ .

Using an EDS of FE-STEM, one particle of this composite oxide powder without overlap was subjected to elemental analysis with a beam diameter of 0.5 nm. The results are shown in Fig. 1. As the analysis conditions, "HD-2000" manufactured by Hitachi, Ltd. was used, and the measurement was performed at an acceleration voltage of 200 kV. This device is equipped with an EDX detector (Vantage EDX system manufactured by NCRAN), and elemental analysis can be performed with high sensitivity by the characteristic X-ray generated from the sample.

As can be seen from FIG. 1, even if a very small portion is analyzed with a beam diameter of 0.5 nm, the composition distribution of Ce and Al is ± with the theoretical atomic ratio (Ce: Al = 71: 29) as the center. It is clear that the concentration is within a narrow range, within 10%. If, for example, CeO ₂ and Al ₂ O ₃ exist as particles of 0.5 nm or more, a large number of 100% Ce or 100% Al should be detected by the above measurement.

This composite oxide powder was impregnated with a predetermined amount of an aqueous solution of rhodium nitrate having a predetermined concentration, and baked at 300 ° C. for 3 hours in the air to support 2% by weight of Rh. This was further molded into a pellet having a particle size of 0.5 to 1 mm by a conventional method to prepare a pellet catalyst.

Example 2 A pellet catalyst of Example 2 was prepared in the same manner as in Example 1 except that the amount of Rh supported on the composite oxide powder was 5% by weight.

Comparative Example 1 The pellet catalyst of Comparative Example 1 was prepared in the same manner as in Example 1 except that a commercially available γ-alumina powder (specific surface area 200 m ² / g) was used instead of the composite oxide powder. Prepared.

Comparative Example 2 A pellet catalyst of Comparative Example 2 was prepared in the same manner as in Example 1 except that a commercially available zirconia powder (specific surface area 100 m ² / g) was used instead of the composite oxide powder. .

Comparative Example 3 The same as Example 1 except that a commercially available yttria-containing stabilized zirconia powder (yttria content 5 mol%, specific surface area 85 m ² / g) was used in place of the composite oxide powder. Then, a pellet catalyst of Comparative Example 3 was prepared.

Comparative Example 4 A pellet catalyst of Comparative Example 4 was prepared in the same manner as in Example 1 except that a commercially available ceria powder (specific surface area of 75 m ² / g) was used instead of the composite oxide powder. .

Comparative Example 5 Comparative Example 5 was repeated except that a commercially available calcium-containing zirconia powder (calcium content 4 mol%, specific surface area 90 m 2 / g) was used in place of the composite oxide powder. The pellet catalyst of Example 5 was prepared.

Comparative Example 6 A commercially available calcium-containing zirconia powder (calcium content 4 mol%, specific surface area 90 m ² / g) was used in place of the composite oxide powder, and the amount of Rh supported was 5 wt%. A pellet catalyst of Comparative Example 6 was prepared in the same manner as in Example 1 except for the above.

<Test / Evaluation> A high temperature durability test was conducted on each of the above-mentioned catalysts by alternately flowing the lean gas shown in Table 1 for 4 minutes and the rich gas for 1 minute while heating the mixed gas at 700 ° C. for 5 hours. .

[0048]

[Table 1]

Then, after the high temperature durability test, each catalyst was fixed under normal pressure.
Loaded in a bed flow reactor, n-C₁₆H ₃₄(320ppm), O₂(0.2
5%), H₂O (10%), balance N₂Model exhaust gas consisting of 15
500 ℃ at a heating rate of 10 ℃ / minute while supplying at 00ml / minute
The temperature was raised to. Due to this model exhaust gas,
The steam reforming reaction, partial oxidation reaction, oxidation reaction and CO
The soft reaction progresses.

Therefore, assuming that the mass balance of C, H and O atoms before and after the reaction is the same, the formula (1) is derived from the following three formulas, and each of CO ₂ , CO and O ₂ measured during the temperature rise is calculated. The concentration of hydrogen produced was calculated by substituting the concentration into the equation (1). The results are shown in Figure 2.

C: 16 [nC ₁₆ H ₃₄ ] ₀ = 16 [nC ₁₆ H ₃₄ ] + [CO ₂ ] + [CO] H: 34 [nC ₁₆ H ₃₄ ] ₀ + 2 [H ₂ O] ₀ = 34 [nC ₁₆ H ₃₄ ] +2 [H ₂ O] +2 [H ₂ ] O: 2 [O ₂ ] ₀ + [H ₂ O] ₀ = 2 [O ₂ ] + [H ₂ O] + 2 [CO _{2] + [CO] [H} 2] (%) = (49/16) [CO 2] + (33/16) [CO] - 2 ([O 2] 0 - [O 2]) (1) ( Where [x] ₀ is the concentration of x before the reaction, and [x] is the x after the reaction.
It is understood from FIG. 2 that the catalyst of Example 1 exhibits higher hydrogen-producing ability from low temperature to high temperature as compared with the catalysts of Comparative Examples 1-5.

Next, Example 1, Comparative Example 1 and Comparative Examples 3 to
For the catalyst of No. 5, the Rh particle size before and after the high temperature durability test was measured by the CO pulse adsorption method. The results are shown in Fig. 3.

From FIG. 3, it is clear that the catalyst of Example 1 has a smaller Rh particle size after the high temperature durability test than the catalysts of the respective Comparative Examples, and the growth of Rh particles is suppressed.

On the other hand, Example 1, Comparative Example 1 and Comparative Examples 3 to
For catalyst No. 5, after firing at 500 ° C for 5 hours in the atmosphere, 250 ° C or 300 ° C for 10 minutes in a mixed gas consisting of C ₃ H ₆ (7000ppm), O ₂ (1%) and the balance N ₂ It was reduced. The oxidation state of Rh of each catalyst after the reduction treatment was measured by X-ray photoelectron spectroscopy (XPS). Rh was observed in a trivalent cation state and a zero valent metal state, and the result of measuring the ratio of Rh in the metal state is shown in FIG.

From FIG. 4, the catalyst of Example 1 has a higher ratio of Rh in the metal state than the catalyst of Comparative Example 1. Further, the result of FIG. 4 almost agrees with the result of FIG. That is, in the steam reforming reaction, Rh in the metal state is considered to be more effective than Rh in the cation state.

From the above, by using the composite oxide powder in which both ceria and alumina are dispersed on the nm scale as in Example 1, the grain growth of Rh in the high temperature durability test is suppressed, and Since the reduction of Rh to the metal state is promoted, it is clear that the hydrogen generation ability is excellent even after the high temperature durability test.

Next, the hydrogen production concentrations of the catalysts of Example 1, Example 2, Comparative Example 5 and Comparative Example 6 after the high temperature durability test are shown in FIG.
Shown in. From FIG. 5, the catalyst of Example 1 has a higher hydrogen-producing ability than the catalyst of Comparative Example 5, and the catalyst of Example 2 has a higher hydrogen-producing ability than the catalyst of Comparative Example 6. Therefore, it is understood that the effect of the composite oxide powder in which both ceria and alumina are dispersed on the nm scale is not affected by the amount of Rh supported.

Further, the catalyst of Comparative Example 6 shows almost the same hydrogen-producing ability as the catalyst of Comparative Example 5, whereas the catalyst of Example 2 has about twice the hydrogen-producing ability of the catalyst of Example 1. Is shown. That is, by using a composite oxide powder in which both ceria and alumina are dispersed on the nm scale as a carrier, Rh
The larger the supported amount of is, the higher the hydrogen production capacity is. It is considered that the effect was exhibited in Examples 1 and 2 because the number of active sites increased as the loading amount of Rh increased, but in the case of the catalysts of Comparative Examples 5 and 6, the loading amount was increased due to the grain growth of Rh. It is thought that the increase was offset.

[0059]

[Effects of the Invention] That is, according to the hydrogen generating catalyst of the present invention, hydrogen can be generated with high activity from a low temperature range to a high temperature range, and the activity can be maintained even after high temperature durability.

[Brief description of drawings]

FIG. 1 φ 0.5 of the composite oxide powder prepared in Example 1.
FIG. 7 is a distribution chart of atomic ratios of Al and Ce, showing the results of elemental analysis in the range of nm.

FIG. 2 is a graph showing the relationship between the incoming gas temperature and the hydrogen production concentration in the catalysts of Examples and Comparative Examples.

FIG. 3 is a graph showing the particle diameter of supported Rh in the catalysts of Examples and Comparative Examples.

FIG. 4 shows the metal state of the catalysts of Examples and Comparative Examples.
It is a graph which shows the ratio of Rh.

FIG. 5 is a graph showing the relationship between incoming gas temperature and hydrogen production concentration in the catalysts of Examples and Comparative Examples.

Continued front page    F-term (reference) 4G040 EA03 EA04 EA06 EA07 EC03                 4G069 AA03 BA01A BA01B BB02A                       BB06A BB06B BC16A BC16B                       BC43A BC43B BC69A BC71A                       BC71B BC72A BC74A BC75A                       CC17 EA02Y EB18Y EC06Y                       EC14Y EC15Y EC16Y EC27                       FC08                 5H027 AA02 BA01 DD09

Claims (5)

[Claims] 1. A catalyst for producing hydrogen from a gas containing hydrocarbon, oxygen and water vapor, the carrier comprising a complex oxide powder in which both ceria and alumina are dispersed on a nm scale, and the carrier supported on the carrier. A hydrogen generation catalyst characterized by comprising a noble metal. 2. The FE-STEM of the composite oxide powder
As a result of performing a minute range analysis of a single non-overlapping particle using an EDS with a beam diameter of 5 nm, 90 points at each analysis point were obtained.
% Or more of Ce and Al are detected at a composition ratio within ± 20% of the charged composition, The hydrogen generation catalyst according to claim 1. 3. The hydrogen generation catalyst according to claim 1, wherein the composite oxide powder contains ceria in an amount of 50% by weight or more. 4. The hydrogen generation catalyst according to claim 1, wherein the noble metal contains at least Rh. 5. The hydrogen generation catalyst according to claim 1, wherein the gas is exhaust gas from a diesel engine.