WO2019225715A1 - 水蒸気改質触媒 - Google Patents
水蒸気改質触媒 Download PDFInfo
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- WO2019225715A1 WO2019225715A1 PCT/JP2019/020543 JP2019020543W WO2019225715A1 WO 2019225715 A1 WO2019225715 A1 WO 2019225715A1 JP 2019020543 W JP2019020543 W JP 2019020543W WO 2019225715 A1 WO2019225715 A1 WO 2019225715A1
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
- B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
- B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
- B01J23/54—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
- B01J23/66—Silver or gold
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
- B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
- B01J21/02—Boron or aluminium; Oxides or hydroxides thereof
- B01J21/04—Alumina
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
- B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
- B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
- B01J23/40—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals of the platinum group metals
- B01J23/46—Ruthenium, rhodium, osmium or iridium
- B01J23/462—Ruthenium
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
- B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
- B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
- B01J23/48—Silver or gold
- B01J23/50—Silver
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
- B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
- B01J37/02—Impregnation, coating or precipitation
- B01J37/0201—Impregnation
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B3/00—Hydrogen; Gaseous mixtures containing hydrogen; Separation of hydrogen from mixtures containing it; Purification of hydrogen
- C01B3/02—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen
- C01B3/32—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air
- C01B3/34—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air by reaction of hydrocarbons with gasifying agents
- C01B3/38—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air by reaction of hydrocarbons with gasifying agents using catalysts
- C01B3/40—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air by reaction of hydrocarbons with gasifying agents using catalysts characterised by the catalyst
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
- C01B2203/02—Processes for making hydrogen or synthesis gas
- C01B2203/0205—Processes for making hydrogen or synthesis gas containing a reforming step
- C01B2203/0227—Processes for making hydrogen or synthesis gas containing a reforming step containing a catalytic reforming step
- C01B2203/0233—Processes for making hydrogen or synthesis gas containing a reforming step containing a catalytic reforming step the reforming step being a steam reforming step
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
- C01B2203/10—Catalysts for performing the hydrogen forming reactions
- C01B2203/1041—Composition of the catalyst
- C01B2203/1047—Group VIII metal catalysts
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
- C01B2203/10—Catalysts for performing the hydrogen forming reactions
- C01B2203/1041—Composition of the catalyst
- C01B2203/1047—Group VIII metal catalysts
- C01B2203/1064—Platinum group metal catalysts
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/30—Hydrogen technology
- Y02E60/50—Fuel cells
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P20/00—Technologies relating to chemical industry
- Y02P20/50—Improvements relating to the production of bulk chemicals
- Y02P20/52—Improvements relating to the production of bulk chemicals using catalysts, e.g. selective catalysts
Definitions
- the present invention relates to a steam reforming catalyst, and more particularly to a steam reforming catalyst for converting a hydrocarbon-based fuel into a mixed gas containing carbon monoxide and hydrogen by a steam reforming reaction.
- Hydrogen is used as a fuel for a fuel cell, for example, a polymer electrolyte fuel cell (PEFC).
- a fuel cell for example, a polymer electrolyte fuel cell (PEFC).
- PEFC polymer electrolyte fuel cell
- One of the industrial methods for producing hydrogen is a steam reforming method for hydrocarbon fuels such as natural gas and city gas.
- a steam reforming catalyst is used, and the hydrocarbon is converted into hydrogen-rich reformed gas by the reforming reaction with steam.
- a catalyst in which nickel or ruthenium is supported on an appropriate carrier made of a metal oxide such as alumina (aluminum oxide) is used.
- Patent Documents 1 and 2 a method using a catalyst in which platinum, rhodium, or both of them are supported on a steam reforming catalyst has been proposed.
- Patent Document 3 a method has been proposed in which ammonia in a hydrogen-rich gas obtained by reforming a raw fuel containing nitrogen or a mixture containing raw fuel and nitrogen is removed with an ammonia remover.
- an object of the present invention is to provide a steam reforming catalyst made of a relatively inexpensive ruthenium, in which ammonia production is suppressed.
- one embodiment of the present invention includes a hydrocarbon gas containing nitrogen gas, characterized by having a carrier and a metal catalyst made of an alloy containing ruthenium and silver supported on the carrier.
- the present invention relates to a steam reforming catalyst used for fuel reforming.
- the present inventors diligently studied to solve the above problems. As a result, when ruthenium was used alone as a metal catalyst in the steam reforming catalyst, it was found that an adsorption reaction of hydrogen and nitrogen occurred at the active site of the ruthenium and ammonia was generated through the reaction. . Therefore, as a result of further studies, the present inventors have further added silver to the ruthenium to constitute a metal catalyst made of an alloy containing ruthenium and silver, and supported the metal catalyst on a support. By forming a steam reforming catalyst, it is possible to suppress the dissociation of nitrogen at the active site (hereinafter also simply referred to as “active site”) that acts on the dissociative adsorption of nitrogen of ruthenium. It was found that it can be suppressed.
- active site the dissociation of nitrogen at the active site
- a steam reforming catalyst can be provided.
- Japanese Patent Application Laid-Open No. 2004-82034 discloses a steam reforming catalyst in which ruthenium, silver or the like is supported on a carrier, but the catalyst has a relatively high concentration of about several tens of ppm.
- a steam reforming catalyst used for reforming hydrocarbon compounds containing a sulfur component as an essential component which improves sulfur poisoning stability and provides a stable reaction system.
- the steam reforming catalyst according to the present invention is a catalyst used for reforming a hydrocarbon-based fuel containing nitrogen gas, and its use and operational effects are completely different.
- the steam reforming catalyst of the present invention is used for reforming a hydrocarbon-based fuel containing sulfur as an unavoidable impurity is not excluded.
- ⁇ -alumina can be used as the carrier.
- the catalytic activity of the metal catalyst that is, the steam reforming catalyst
- the catalytic activity can be maintained high, and the catalytic activity can be increased even when the steam reforming catalyst is disposed at a high temperature. Since it can maintain, durability can also be kept high.
- FIG. 1 is a STEM-EDX image of metal catalyst particles in one embodiment of the steam reforming catalyst of the present invention.
- the metal catalyst constituting the steam reforming catalyst needs to be made of an alloy of ruthenium (Ru) and silver (Ag).
- Silver has a higher electron affinity than ruthenium. Therefore, in the metal catalyst in this embodiment in which ruthenium and silver form an alloy, some of the ruthenium electrons move to silver, and the electron density of ruthenium is It is falling. As a result, dissociation of nitrogen (N 2 ) at the active site of ruthenium is suppressed, so that an active steam reforming catalyst can be provided in a state where ammonia production is suppressed.
- N 2 nitrogen
- ruthenium and silver form an alloy, that is, exist in a metal state, but may exist partially as ruthenium oxide or silver oxide within the range where the effects of the present invention are exhibited. Good.
- the metal catalyst made of an alloy containing ruthenium and silver may contain rhodium, nickel, cobalt, lanthanum, platinum and the like in addition to ruthenium and silver within the range where the effects of the present invention are exhibited.
- the steam reforming catalyst of the present embodiment may contain a metal catalyst that does not contain an alloy containing ruthenium and silver within the range where the effects of the present invention are exhibited.
- a metal catalyst consisting of ruthenium or only silver. You may contain the metal catalyst which consists of.
- ruthenium and silver form an alloy in the metal catalyst of the present embodiment is not particularly limited, and may be, for example, a solid solution, a eutectic, or an intermetallic compound.
- ruthenium and silver are close enough to exchange electrons, that is, if both ruthenium and silver are present in one metal catalyst particle. Good.
- the method for producing a metal catalyst made of an alloy containing ruthenium and silver is not particularly limited, for example, it can be obtained by the following method. That is, first, a precursor solution containing both ruthenium ions and silver ions is produced. As the precursor solution, for example, a mixed solution of a ruthenium nitrate aqueous solution and a silver nitrate aqueous solution can be used. Next, the precursor solution is impregnated into the support, dried, and then subjected to hydrogen reduction, whereby a steam reforming catalyst in which a metal catalyst made of an alloy containing ruthenium and silver is supported on the support can be obtained.
- a precursor solution containing both ruthenium ions and silver ions is produced.
- the precursor solution for example, a mixed solution of a ruthenium nitrate aqueous solution and a silver nitrate aqueous solution can be used.
- the precursor solution is impregnated into the support, dried, and then subjecte
- ruthenium and silver can be confirmed to form an alloy by various methods. For example, using STEM-EDX, it is confirmed that both ruthenium and silver are contained in one particle. be able to. Moreover, it can be confirmed that ruthenium and silver form an alloy also by observing the peak shift using XPS, for example. Specifically, for example, it can be confirmed by observing the spectrum of the Ru3d orbit as described in the column of Examples. When ruthenium and silver form an alloy, as described above, a part of the ruthenium electrons have moved to silver, and as a result, the position of the peak top of the Ru3d orbit shifts to the high energy side.
- ruthenium and silver form an alloy by confirming that the position of the peak top of the Ru3d orbit is shifted to the high energy side as compared with the case of a metal catalyst composed of a single ruthenium.
- the peak shift width is preferably 0.05 eV or more. In this case, it can be considered that an alloy is formed. Moreover, it is more preferable that it is 0.1 eV or more.
- the ratio of the silver content (atomic%) to the ruthenium content (atomic%) in the metal catalyst is preferably 0.01 to 0.2. Is more preferably from 0.01 to 0.1, and most preferably from 0.01 to 0.06. Thereby, dissociation of nitrogen at the active site of ruthenium can be effectively suppressed, and as a result, generation of ammonia can be effectively suppressed.
- (Ag / Ru) is obtained by STEM-EDX point analysis. Specifically, 10 metal catalyst particles in the steam reforming catalyst are randomly extracted, and the STEM-EDX points are respectively extracted. It is obtained by measuring the content of ruthenium and the content of silver by analysis to obtain (Ag / Ru) and taking the average.
- the amount of ruthenium in the metal catalyst is preferably 83.0 to 98.8 atomic%, the amount of silver is preferably 17.0 to 1.2 atomic%, and further, the amount of ruthenium is 90%. More preferably, the amount of silver is 9.9 to 98.8 atomic%, and the amount of silver is 9.1 to 1.2 atomic%. Also in this case, dissociation of nitrogen at the active site of ruthenium can be effectively suppressed, and as a result, generation of ammonia can be effectively suppressed.
- the material for the support of the steam reforming catalyst of the present embodiment is not particularly limited as long as it can support the metal catalyst described above, and oxidation of an alkali metal such as lithium, sodium, potassium, and cesium.
- Oxides of alkaline earth metals such as magnesium, calcium, strontium and barium, Group IIIA metal oxides such as scandium and yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium , Oxides of rare earth metals such as terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, etc., oxides of Group IVA metals such as titanium, zirconium, hafnium, oxides of aluminum, oxides of silicon, Unitary acids such as The like may be exemplified ones.
- aluminum oxide (alumina) is preferably used, and in particular, ⁇ -alumina can be preferably used. This is because when the above-mentioned metal catalyst is supported on a support made of ⁇ -alumina, the catalytic activity of the steam reforming catalyst can be maintained high, and even when the steam reforming catalyst is placed at a high temperature, This is because the catalytic activity can be kept high and the durability is excellent.
- a mixed oxide in which two or more of these oxides are mixed at an arbitrary ratio can also be used as a material for the support of the steam reforming catalyst of the present embodiment.
- alkaline earth metal oxides and aluminas such as magnesium-aluminum, calcium-aluminum, strontium-aluminum, barium-aluminum, and a combination of metals such as cerium-aluminum.
- a mixed oxide of lanthanoid metal oxide and alumina, a mixed oxide of zirconia and alumina, and the like are particularly preferably used.
- mixed oxides of oxides of alkaline earth metals such as magnesium, calcium and barium and oxides of lanthanoid metals such as cerium and alumina, or alkaline earth metals such as magnesium, calcium and barium.
- An oxide and a mixed oxide of zirconia and alumina are particularly preferably used.
- a carrier derived from a natural product such as zeolite can also be used.
- the form of the carrier or the steam reforming catalyst is not particularly limited.
- a carrier formed by tableting and pulverized to an appropriate range an extruded carrier, a powder or sphere, a ring, a tablet, a pellet
- a carrier formed into an appropriate shape such as a shape, a cylinder, or a flake shape can be used.
- molded in the honeycomb form may be used.
- the steam reforming catalyst of this embodiment is a steam reforming catalyst characterized by having the above-mentioned carrier and the above-mentioned metal catalyst.
- the steam reforming catalyst of the present embodiment can be produced, for example, by the method described above.
- the hydrocarbon fuel used for the steam reforming reaction by the steam reforming catalyst of the present embodiment includes nitrogen gas and an organic compound.
- the organic compound is preferably an organic compound having 1 to 40 carbon atoms, more preferably 1 to 30 carbon atoms.
- Specific examples of the organic compound include saturated aliphatic hydrocarbons, unsaturated aliphatic hydrocarbons, aromatic hydrocarbons, and the like.
- chain It can be used regardless of the shape or ring shape.
- Aromatic hydrocarbons can be used regardless of whether they are monocyclic or polycyclic. Such hydrocarbon compounds can contain substituents.
- either a chain or a ring can be used, and examples thereof include an alkyl group, a cycloalkyl group, an aryl group, an alkylaryl group, and an aralkyl group.
- these hydrocarbon compounds may be substituted with a substituent containing a hetero atom such as a hydroxy group, an alkoxy group, a hydroxycarbonyl group, an alkoxycarbonyl group, or a formyl group.
- saturated aliphatic hydrocarbons such as methane, ethane, propane, butane, pentane, hexane, dodecane, tridecane, tetradecane, pentadecane, hexadecane, heptadecane, octadecane, nonadecane, eicosane, ethylene, propylene, butene, pentene
- unsaturated aliphatic hydrocarbons such as hexene, cyclic hydrocarbons such as cyclopentane and cyclohexane
- aromatic hydrocarbons such as benzene, toluene, xylene and naphthalene.
- these mixtures can also be used conveniently,
- the material which can be obtained industrially cheaply such as natural gas, LPG, naphtha, gasoline, kerosene, light oil
- the hydrocarbon compound having a substituent containing a hetero atom include methanol, ethanol, propanol, butanol, dimethyl ether, phenol, anisole, acetaldehyde, acetic acid and the like.
- the hydrocarbon-based fuel may contain hydrogen, water, carbon dioxide, carbon monoxide, oxygen, and the like within a range that does not affect the steam reforming reaction.
- the catalyst of this embodiment can perform steam reforming in which ammonia generation is suppressed even when a hydrocarbon fuel containing a large amount of nitrogen is used.
- ammonia is used even when the content of nitrogen in the gas component excluding organic compounds in the steam and hydrocarbon fuel from all gas components used for steam reforming introduced into the catalyst is 50% by volume or more.
- the nitrogen content is 80 to 100% by volume, a dramatic ammonia suppression effect can be obtained compared to conventional catalysts.
- a silver nitrate crystal dissolved in deionized water and a ruthenium nitrate solution, the content of silver and ruthenium in the finally obtained steam reforming catalyst is the content (including support) described in the catalyst type column of Table 1
- the precursor solution was obtained by mixing so that the content of ruthenium or silver (wt%) in the steam reforming catalyst was obtained.
- the ⁇ -alumina pellet support was then impregnated with the precursor solution.
- the pellets were dried at 120 ° C. and then reduced with hydrogen to obtain steam reforming catalysts of Examples 1 to 3 (hereinafter also simply referred to as “catalyst”).
- the nitrogen content before being introduced into the catalyst is 99.995 vol%. .
- the conversion rate of methane gas was derived by the following equation from the concentrations (volume%) of carbon monoxide (CO), carbon dioxide (CO 2 ), and methane in the reactor outlet gas measured using a gas chromatograph.
- [CO] out represents the concentration of carbon monoxide (volume%)
- [CO 2 ] out represents the concentration of carbon dioxide (volume%)
- [CH 4 ] out represents the concentration of methane (volume%).
- Methane conversion rate ([CO] out + [CO 2 ] out) / ([CH 4 ] out + [CO] out + [CO 2 ] out)) ⁇ 100
- the amount of ammonia gas generated was measured using an indophenol blue absorptiometric method by absorbing ammonia with a boric acid solution at the reactor outlet. The measured values are shown in Table 1. “ND” in Table 1 means that the amount of NH 3 produced was less than the detection limit value.
- the content (atomic%) of ruthenium and silver and the ratio of the ruthenium and silver content (Ag / Ru) were determined from STEM-EDX point analysis.
- metal catalyst particles in the catalyst were extracted at random, and each of the Examples was performed 10 times, and the average of the obtained (Ag / Ru) was taken.
- the results are shown in Table 1.
- the analysis position was the center of the circumscribed circle of the metal catalyst particles, and the analysis target was particles in which both ruthenium and silver metal elements were detected at the analysis position.
- An example of a STEM-EDX image of the metal catalyst particles is shown in FIG. As shown in Table 1, it was confirmed that the metal catalyst particles contained both ruthenium and silver at any measurement point, that is, made of an alloy of ruthenium and silver.
- Example 1 A precursor aqueous solution containing a ruthenium nitrate solution was prepared in the same manner as in the Examples without using silver nitrate, and ⁇ -alumina pellet support was impregnated with the precursor solution. Next, the pellets were dried at 120 ° C. and then reduced with hydrogen to obtain a steam reforming catalyst in which a metal catalyst composed of ruthenium was supported on ⁇ -alumina. In addition, the measurement of the conversion rate of methane gas and the generation amount of ammonia gas was performed like the Example.
- a precursor aqueous solution containing silver nitrate crystals was prepared in the same manner as in the Examples, and an ⁇ -alumina pellet support was impregnated with the precursor solution. Next, the pellets were dried at 120 ° C. and then reduced with hydrogen to obtain a steam reforming catalyst in which a metal catalyst made of silver (Ag) was supported on ⁇ -alumina. In addition, the measurement of the conversion rate of methane gas and the generation amount of ammonia gas was performed like the Example.
- Example 1 The catalysts of Example 1 and Comparative Example 1 were pulverized with an agate mortar and subjected to XPS measurement under the following conditions to obtain a Ru3d orbital spectrum.
- the peak top position of Ru3d3 / 2 was 284.08 eV
- the peak top position of Ru3d5 / 2 was 279.88 eV.
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Abstract
Description
なお、本明細書において「~」とはその下限の値以上、その上限の値以下であることを意味する。
本実施形態において、水蒸気改質触媒を構成する金属触媒は、ルテニウム(Ru)と、銀(Ag)との合金からなることが必要である。銀はルテニウムと比較すると電子親和力が大きく、したがって、ルテニウムと銀とが合金を形成している本実施形態における金属触媒ではルテニウムの電子の一部が銀に移動しており、ルテニウムの電子密度が低下している。このことによりルテニウムの活性点における窒素(N2)の解離が抑制されるため、アンモニア生成を抑制した状態で、活性な水蒸気改質触媒を提供することができる。
すなわち、まずルテニウムイオンと銀イオンとの両方を含有する前駆体溶液を製造する。当該前駆体溶液としては、例えば硝酸ルテニウム水溶液と硝酸銀水溶液の混合溶液を用いることができる。次いで、当該前駆体溶液を担体に含浸させ、乾燥させたのち、水素還元することによって、担体にルテニウムと銀とを含む合金からなる金属触媒が担持された水蒸気改質触媒を得ることができる。
また、例えばXPSを用いてピークシフトを観察することによってもルテニウムと銀が合金を形成していることを確認することができる。具体的には、例えば実施例の欄に記載したようにRu3d軌道のスペクトルを観察することにより確認できる。ルテニウムと銀が合金を形成している場合、先述のとおりルテニウムの電子は一部が銀に移動しており、このことに起因してRu3d軌道のピークトップの位置は高エネルギー側にシフトする。したがって、単体のルテニウムからなる金属触媒の場合と比較してRu3d軌道のピークトップの位置が高エネルギー側にシフトしていることを確認することにより、ルテニウムと銀が合金を形成していることを確認できる。当該ピークシフト幅は0.05eV以上であることが好ましく、この場合合金が形成されていると考えることができる。また、0.1eV以上であることがより好ましい。
なお、(Ag/Ru)はSTEM-EDXの点分析により求められるものであり、具体的には、水蒸気改質触媒中の金属触媒粒子をランダムに10抽出して、各々についてSTEM-EDXの点分析によりルテニウムの含有量、銀の含有量を測定して(Ag/Ru)を求め、その平均をとることにより求められる。
本実施形態の水蒸気改質触媒の担体の材料としては上述した金属触媒を担持させることができるものであれば特に限定されるものではなく、リチウム、ナトリウム、カリウム、セシウムのようなアルカリ金属の酸化物、マグネシウム、カルシウム、ストロンチウム、バリウムのようなアルカリ土類金属の酸化物、スカンジウム、イットリウムのような周期律表第IIIA族金属の酸化物、ランタン、セリウム、プラセオジム、ネオジム、サマリウム、ユーロピウム、ガドリニウム、テルビウム、ジスプロシウム、ホルミウム、エルビウム、ツリウム、イッテルビウム、ルテチウムなどの希土類金属の酸化物、チタン、ジルコニウム、ハフニウムなどの周期律表第IVA族金属の酸化物、アルミニウムの酸化物、珪素の酸化物、などの単元系酸化物などを例示することができる。
本実施形態の水蒸気改質触媒は、先述の担体及び、先述の金属触媒を有することを特徴とする水蒸気改質触媒である。本実施形態の水蒸気改質触媒は例えば先述の方法で製造することができる。
本実施形態の水蒸気改質触媒によって水蒸気改質反応に供する炭化水素系燃料は、窒素ガス、及び有機化合物を含む。有機化合物は好ましくは炭素数1~40、より好ましくは炭素数1~30の有機化合物である。有機化合物としては具体的には、飽和脂肪族炭化水素、不飽和脂肪族炭化水素、芳香族炭化水素などを挙げることができ、また飽和脂肪族炭化水素、不飽和脂肪族炭化水素については、鎖状、環状を問わず使用できる。芳香族炭化水素についても単環、多環を問わず使用できる。このような炭化水素化合物類は置換基を含むことができる。置換基としては、鎖状、環状のどちらをも使用でき、例として、アルキル基、シクロアルキル基、アリール基、アルキルアリール基およびアラルキル基等を挙げることができる。また、これらの炭化水素化合物類はヒドロキシ基、アルコキシ基、ヒドロキシカルボニル基、アルコキシカルボニル基、ホルミル基などのヘテロ原子を含有する置換基により置換されていてもよい。
本実施形態の触媒は、窒素が多く含まれる炭化水素系燃料を用いた場合においてもアンモニア生成が抑制された水蒸気改質を行うことができる。例えば、触媒に導入される水蒸気改質に用いられる全気体成分から水蒸気及び炭化水素系燃料中の有機化合物を除いた気体成分中の窒素の含有量が、50体積%以上である場合においてもアンモニア抑制の効果が得られ、とりわけ窒素の含有量が80~100体積%である場合においても、従来の触媒と比較し飛躍的なアンモニア抑制の効果が得られる。
まず、脱イオン水に溶解した硝酸銀結晶と、硝酸ルテニウム溶液とを、最終的に得られる水蒸気改質触媒における銀及びルテニウムの含有量が表1の触媒種欄に記載の含有量(担体を含む水蒸気改質触媒中のルテニウムまたは銀の含有量(wt%))となるように混合し前駆体溶液を得た。次いで、α-アルミナペレット担体に対して前駆体溶液を含浸した。次いで、120℃にてペレットを乾燥後、水素還元し、実施例1~3の水蒸気改質触媒(以下、単に「触媒」ともいう)を得た。
また、窒素(N2)を25体積%含有するCH4ガスを原料とし、当該反応装置内に前記原料ガスと純水(H2O)を、SV=5,000h-1、S/C=2.7になるように調整して供給し、上記触媒を加熱する電気炉を750℃に保持して出口でのアンモニアガスの発生量を求めた。水蒸気改質に用いられる全気体成分のうち、水蒸気(H2O)及びメタン(CH4)を除く気体成分において、触媒に導入される前の窒素の含有量は、99.995体積%である。
メタン転化率=([CO]out+[CO2]out)/([CH4]out+[CO]out+[CO2]out))×100
なお、表1に示す通り、いずれの測定点においても金属触媒粒子がルテニウム及び銀の両方を含有すること、即ち、ルテニウムと銀との合金からなることが確認された。
硝酸銀を用いることなく、硝酸ルテニウム溶液を含む前駆体水溶液を実施例と同様にして調製し、α-アルミナペレット担体に対して前駆体溶液を含浸した。次いで、120℃にてペレットを乾燥後、水素還元しルテニウムからなる金属触媒をα-アルミナに担持させた水蒸気改質触媒を得た。なお、メタンガスの転化率及びアンモニアガスの発生量の測定は、実施例と同様にして行った。
硝酸ルテニウム溶液を用いることなく、硝酸銀結晶を含む前駆体水溶液を実施例と同様にして調製し、α-アルミナペレット担体に対して前駆体溶液を含浸した。次いで、120℃にてペレットを乾燥後、水素還元し銀(Ag)からなる金属触媒をα-アルミナに担持させた水蒸気改質触媒を得た。なお、メタンガスの転化率及びアンモニアガスの発生量の測定は、実施例と同様にして行った。
一方、ルテニウムのみからなる金属触媒を担持させて得た比較例1の水蒸気改質触媒はアンモニアガスの生成量が多かった。
また、銀のみからなる金属触媒を担持させて得た比較例2の水蒸気改質触媒はメタン転化率が低かった。
実施例1および比較例1の触媒をメノウ乳鉢で粉砕し、下記条件でXPS測定を行い、Ru3d軌道のスペクトルを得た。
ルテニウムのみからなる金属触媒のみを有する比較例1の水蒸気改質触媒では、Ru3d3/2のピークトップの位置は284.08eV、Ru3d5/2のピークトップの位置は279.88eVであった。
これに対し、実施例1の水蒸気改質触媒では、Ru3d3/2のピークトップの位置は284.28eV、Ru3d5/2のピークトップの位置は279.98eVであり、比較例1のピークトップの位置からRu3d3/2では0.2eV、Ru3d5/2では0.1eV高エネルギー側にシフトしていた。
このことからも、実施例1の水蒸気改質触媒における金属触媒ではルテニウムと銀とが合金を形成していることが確認された。
(XPS測定条件)
・分析装置:Thermo Fisher Scientific社製 K-Alpha+
・照射X線:単結晶分光AlKα線
・X線スポット系:400μm×800μm(楕円形)
・中和電子銃:使用
・ピーク位置補正:Al2pのピークトップ位置を74.0eVとして補正
また、実施例3の触媒についても同様に、XPS測定を行ったところ、実施例3の水蒸気改質触媒でも、比較例1のピークトップの位置から、Ru3d3/2では0.2eV、Ru3d5/2では0.1eV高エネルギー側にシフトしていた。
Claims (3)
- 担体及び、前記担体に担持させた、ルテニウムと銀とを含む合金からなる金属触媒を有することを特徴とする、窒素ガスを含む炭化水素系燃料の改質に用いられる水蒸気改質触媒。
- 前記ルテニウムの含有量(原子%)に対する前記銀の含有量(原子%)の比が、0.01~0.2であることを特徴とする、請求項1に記載の水蒸気改質触媒。
- 前記担体は、α-アルミナを含むことを特徴とする、請求項1又は2に記載の水蒸気改質触媒。
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