JP4620641B2 - Noble metal catalyst and method for producing the same - Google Patents
Noble metal catalyst and method for producing the same Download PDFInfo
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- 239000003054 catalyst Substances 0.000 title claims description 70
- 229910000510 noble metal Inorganic materials 0.000 title claims description 40
- 238000004519 manufacturing process Methods 0.000 title claims description 15
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Chemical compound [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 claims description 152
- 229910052697 platinum Inorganic materials 0.000 claims description 62
- 239000000495 cryogel Substances 0.000 claims description 41
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- CETPSERCERDGAM-UHFFFAOYSA-N ceric oxide Chemical compound O=[Ce]=O CETPSERCERDGAM-UHFFFAOYSA-N 0.000 claims description 25
- 229910000422 cerium(IV) oxide Inorganic materials 0.000 claims description 25
- 239000002253 acid Substances 0.000 claims description 13
- 229910052684 Cerium Inorganic materials 0.000 claims description 12
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- 238000005245 sintering Methods 0.000 claims description 7
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- HSJPMRKMPBAUAU-UHFFFAOYSA-N cerium(3+);trinitrate Chemical compound [Ce+3].[O-][N+]([O-])=O.[O-][N+]([O-])=O.[O-][N+]([O-])=O HSJPMRKMPBAUAU-UHFFFAOYSA-N 0.000 description 4
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- MYWQGROTKMBNKN-UHFFFAOYSA-N tributoxyalumane Chemical compound [Al+3].CCCC[O-].CCCC[O-].CCCC[O-] MYWQGROTKMBNKN-UHFFFAOYSA-N 0.000 description 1
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Description
本発明は、白金及びセリアを含む貴金属触媒とその製造方法に関する。 The present invention relates to a noble metal catalyst containing platinum and ceria and a method for producing the same.
近年、内燃機関、ボイラー等の排気ガス中の微粒子や有害物質は、環境への影響を考慮して排気ガス中から除去する必要性が高まりつつあり、各種排ガス浄化技術が提案されている。例えば、自動車の排気系には、酸化触媒、三元触媒、ＮＯＸ吸蔵還元型触媒等が配置され、主として貴金属の触媒作用によって排ガス中のＮＯＸ、ＨＣ、ＣＯ等の有害成分を浄化している。特に、酸化触媒としては、例えば、アルミナやシリカ等の担体に酸化活性の高い白金（Ｐｔ）を担持したものが知られている。 In recent years, there has been an increasing need to remove particulates and harmful substances in exhaust gas from internal combustion engines, boilers, and the like from the exhaust gas in consideration of environmental effects, and various exhaust gas purification technologies have been proposed. For example, the exhaust system of an automobile, an oxidation catalyst, three-way catalyst, NO X storage reduction catalyst and the like are arranged, to purify NO X in the exhaust gas, HC, harmful components such as CO mainly by the catalytic action of the noble metal Yes. In particular, as an oxidation catalyst, for example, a catalyst in which platinum (Pt) having high oxidation activity is supported on a carrier such as alumina or silica is known.
従来、多孔質セラミックス材料などの触媒担体に触媒金属を担持させるには、金属水溶液を用いた通常の含浸法による方法の他に、金属分散性を高めるために金属アルコキシドを用いたゾルゲル法が報告されている。従来のゾルゲル法によるアルミナ担持白金触媒の作製には、まずアルミニウムのアルコキシドをエステル系やジオル系のキレート剤で保護してからアルコール等の有機溶媒に溶解し、白金錯体塩と少量の水、触媒等と一緒に密閉容器に加え、場合によっては１週間ほど加温して徐々にアルミニウムの加水分解・ゲル化・ゲル熟成を進行させていた。 Conventionally, to support a catalytic metal on a catalyst carrier such as a porous ceramic material, a sol-gel method using a metal alkoxide to improve metal dispersibility has been reported in addition to a conventional impregnation method using a metal aqueous solution. Has been. In the preparation of an alumina-supported platinum catalyst by the conventional sol-gel method, an aluminum alkoxide is first protected with an ester-based or diol-based chelating agent and then dissolved in an organic solvent such as an alcohol, and then a platinum complex salt, a small amount of water, a catalyst. In some cases, it was heated for about a week, and the hydrolysis, gelation, and gel aging of aluminum were gradually advanced.
しかしながら、アルミニウムのアルコキシドは、加水分解速度が非常に高く、シリコンのそれに比べてその制御が難しいため、水分量や温度等を厳密に管理しながら、キレート剤で保護された有機溶媒中のアルミニウムアルコキシドを徐々に加水分解・重合させる、というデリケートな方法を採らざるを得なかった。 However, aluminum alkoxide has a very high hydrolysis rate and is difficult to control compared to that of silicon. Therefore, aluminum alkoxide in an organic solvent protected with a chelating agent while strictly controlling the amount of water and temperature is used. Therefore, the delicate method of gradually hydrolyzing and polymerizing the water must be taken.
しかも、上記アルコキシドを用いた従来のアルミナ系触媒の調製では、高価で危険な有機溶媒を使用し、且つ長時間掛けてゲル化を進行させる等、実用化に向けて多くの困難が立ちはだかっていた。更に、この方法で作製したゲルを凍結乾燥するためには、ゲル中の有機溶媒を水に置換する必要があり、その際、ゲルの破壊や白金金属イオンがゲルから流出してしまうという問題があった。 Moreover, in the preparation of the conventional alumina-based catalyst using the alkoxide, there are many difficulties for practical use, such as using an expensive and dangerous organic solvent and allowing the gelation to proceed for a long time. . Furthermore, in order to freeze-dry the gel produced by this method, it is necessary to replace the organic solvent in the gel with water, and in that case, there is a problem that the gel is broken or platinum metal ions flow out from the gel. there were.
そこで、上記問題を解決するため、本出願人は、先に、高表面積、高気孔率及び高耐熱性を有し、５００〜７００℃の雰囲気下で白金微粒子のシンタリングが抑制される貴金属触媒、及びその貴金属触媒を安全且つ低コストで製造することができる貴金属触媒の製造方法を提案した（特願２００６−４７９７４号）。
上記提案は、白金粒子の高い耐熱性とともに、高い触媒活性を有する点から優れたものであるが、更なる改良の要請がある。
Therefore, in order to solve the above problem, the present applicant has previously described a noble metal catalyst that has a high surface area, a high porosity, and a high heat resistance, and suppresses sintering of platinum fine particles in an atmosphere of 500 to 700 ° C. And a method for producing a noble metal catalyst that can be produced safely and at low cost (Japanese Patent Application No. 2006-47974).
The above proposal is excellent in terms of having high catalytic activity as well as high heat resistance of platinum particles, but there is a demand for further improvement.
本発明は、上述した従来技術の問題点に鑑みてなされたものであり、その目的とするところは、特願２００６−４７９７４号で提案した貴金属触媒について更に改良し、高い濃度の白金粒子を添加しても、アルミナクリオゲル（多孔質アルミナ構造体）の比表面積を大きく維持向上させるとともに、８００℃の雰囲気下でも白金粒子が高い耐熱性を有するような貴金属触媒とその製造方法を提供することにある。 The present invention has been made in view of the above-mentioned problems of the prior art. The object of the present invention is to further improve the noble metal catalyst proposed in Japanese Patent Application No. 2006-47974, and to add high concentration platinum particles. also, with greatly maintain and improve the specific surface area of the alumina cryogel (porous alumina structure), a process for its preparation and a noble metal catalyst such as a platinum particles has high heat resistance even in an atmosphere of 800 ° C. There is.
上述の目的を達成するため、本発明は、以下の貴金属触媒及びその製造方法を提供する
ものである。
In order to achieve the above object, the present invention provides the following noble metal catalyst and a method for producing the same.
［１］ 白金（Ｐｔ）とセリア（ＣｅＯ２）を含む、アルミナクリオゲルからなる貴金属触媒。 [1] A noble metal catalyst made of alumina cryogel containing platinum (Pt) and ceria (CeO 2 ).
［２］ 白金の含有量が０．５〜５質量％、セリアの含有量が１〜５０質量％である［１］に記載の貴金属触媒。 [2] The noble metal catalyst according to [1], wherein the platinum content is 0.5 to 5% by mass and the ceria content is 1 to 50% by mass.
［３］ ８００℃の雰囲気下で白金のシンタリングが、白金（Ｐｔ）を含み、セリア（ＣｅＯ ２ ）を含まないアルミナクリオゲルからなる貴金属触媒と比較して抑制される［１］又は［２］に記載の貴金属触媒。 [3] Platinum sintering is suppressed in an atmosphere of 800 ° C. as compared with a noble metal catalyst made of alumina cryogel containing platinum (Pt) and no ceria (CeO 2 ) [1] or [2 ] The noble metal catalyst as described in any one of.
［４］ 白金及びセリウムを含むベーマイトゾルを調製した後、前記べーマイトゾルをゲル化し、次いで凍結乾燥することにより、［１］に記載のアルミナクリオゲルからなる貴金属触媒を得る貴金属触媒の製造方法。 [4] A method for producing a noble metal catalyst comprising preparing a boehmite sol containing platinum and cerium, then gelling the boehmite sol, and then freeze-drying to obtain a noble metal catalyst comprising the alumina cryogel according to [1] .
［５］ 白金及びセリウムの添加前に、ベーマイトゾルに、酸をベーマイトに対して０．１７〜０．７０（酸／ベーマイトのモル比）の割合で添加する［４］に記載の貴金属触媒の製造方法。 [5] Before adding platinum and cerium, an acid is added to boehmite sol at a ratio of 0.17 to 0.70 (molar ratio of acid / boehmite) to boehmite. Production method.
本発明の貴金属触媒は、５質量％程度の高い濃度の白金粒子を添加した場合であっても、得られるアルミナクリオゲル（多孔質アルミナ構造体）の比表面積を大きくでき、しかも８００℃の高温雰囲気下でも白金粒子が高い耐熱性を有するという優れた効果を奏する。また、本発明の製造方法は、このような特性を有する貴金属触媒を効率的に製造することができる。 Noble metal catalyst of the present invention, even when added platinum particles of as high as 5% strength by weight, can increase the specific surface area of the resulting Alumina cryogel (porous alumina structure), addition of 800 ° C. Even under a high temperature atmosphere, the platinum particles have an excellent effect that they have high heat resistance. Further, the production method of the present invention can efficiently produce a noble metal catalyst having such characteristics.
以下、本発明の貴金属触媒及びその製造方法について詳細に説明するが、本発明は、これに限定されて解釈されるものではなく、本発明の範囲を逸脱しない限りにおいて、当業者の知識に基づいて、種々の変更、修正、改良を加え得るものである。 Hereinafter, the noble metal catalyst of the present invention and the method for producing the same will be described in detail. However, the present invention is not construed as being limited thereto, and is based on the knowledge of those skilled in the art without departing from the scope of the present invention. Various changes, modifications, and improvements can be added.
本発明は、特願２００６−４７９７４号で提案した貴金属触媒の改良である。
特願２００６−４７９７４号で提案した貴金属触媒は、白金微粒子のゲル中への埋没度が４５〜６０％（より好ましくは、５０〜５５％であり、且つ５００〜７００℃（より好ましくは、６００〜７００℃）の雰囲気下で白金微粒子のシンタリングが抑制されるとともに、高温耐熱性を有するものである。尚、本発明の貴金属触媒は、クリオゲルであることが好ましい。
The present invention is an improvement of the noble metal catalyst proposed in Japanese Patent Application No. 2006-47974.
The noble metal catalyst proposed in Japanese Patent Application No. 2006-47974 has an embedded degree of platinum fine particles in the gel of 45 to 60% (more preferably 50 to 55% and 500 to 700 ° C. (more preferably 600). Sintering of the platinum fine particles is suppressed under an atmosphere of ˜700 ° C. and has high temperature heat resistance, and the noble metal catalyst of the present invention is preferably a cryogel.
上記の貴金属触媒においては、ベーマイトゾルと塩化白金酸を出発原料としたゾルゲル工程とそれに引き続く凍結乾燥工程により得られたＰｔ／Ａｌ２Ｏ３クリオゲル触媒（Ｐｔ＝０．５〜５質量％）が、従来触媒に比較して白金粒子の高い耐熱性（５００〜７００℃）を有するものであった。これは約１ｎｍの白金微粒子が埋没度：４５〜６０％の割合でアルミナゲル中に埋め込まれ、この適度な埋め込みが白金微粒子の高い耐熱性を与えたものと考えられている。また、このクリオゲル触媒は、疑似大気中でのメタン酸化反応において高い触媒活性（すなわち低温でのより高い触媒活性）を示した。これは白金微粒子のアルミナクリオゲル担体上への高分散化により白金の表面露出率が向上し、またこの微粒子は上述のごとく耐熱性に優れ、高温焼成でも高い露出率を維持するため、と考えられた。 In the noble metal catalyst, a Pt / Al 2 O 3 cryogel catalyst (Pt = 0.5 to 5% by mass) obtained by a sol-gel process using boehmite sol and chloroplatinic acid as starting materials and a subsequent freeze-drying process is used. The platinum particles had higher heat resistance (500 to 700 ° C.) than the conventional catalysts. It is considered that about 1 nm of platinum fine particles are embedded in alumina gel at a ratio of burying degree: 45 to 60%, and this moderate embedding gives high heat resistance of the platinum fine particles. Moreover, this cryogel catalyst showed high catalytic activity (that is, higher catalytic activity at low temperature) in the methane oxidation reaction in a simulated atmosphere. This is because the platinum surface exposure rate is improved by high dispersion of platinum fine particles on the alumina cryogel support, and the fine particles have excellent heat resistance as described above, and maintain a high exposure rate even at high temperature firing. It was.
一方、本発明者が更に実験を進めたところ、添加する白金濃度が高くなると（〜５質量
％）、アルミナクリオゲル担体のＢＥＴ比表面積が小さくなることが判明した。担体の性能低下は長時間の触媒反応における触媒能の低下にもつながり、解決しなければならない重要な問題である。
On the other hand, when the present inventors further advanced the experiment, it was found that the BET specific surface area of the alumina cryogel support decreases as the platinum concentration to be added increases (˜5 mass%). The decrease in the performance of the support also leads to a decrease in the catalytic performance in a long-time catalytic reaction, which is an important problem to be solved.
また、クリオゲル触媒の白金粒子の耐熱性も、白金濃度が高くなると（〜５質量％）、８００℃の高温雰囲気では白金粒子のシンタリングが著しく、自動車排ガス触媒など、より高温での耐久性が求められるシステムへの適用が難しくなるという問題も発生する。 Also, the heat resistance of the platinum particles of the cryogel catalyst is such that when the platinum concentration increases (˜5 mass%), the sintering of the platinum particles is remarkable at a high temperature atmosphere of 800 ° C., and the durability at a higher temperature such as an automobile exhaust gas catalyst is high. There is also a problem that it is difficult to apply to the required system.
そこで、本発明者は、基本成分が特願２００６−４７９７４号で提案した貴金属触媒と同じく、Ｐｔ（活性金属種）とＡｌ２Ｏ３（担体）である白金−アルミナ系クリオゲル触媒に対し、第三成分を助触媒として導入することによって、上記問題の解決を目指した。 Therefore, the present inventor has compared the platinum-alumina cryogel catalyst whose basic components are Pt (active metal species) and Al 2 O 3 (support) as in the noble metal catalyst proposed in Japanese Patent Application No. 2006-47974. By introducing the three components as cocatalysts, we aimed to solve the above problems.
第三成分の選択にあたり、本発明者はいろいろな角度から検討した結果、セリウムに着目するに至り、セリウムを助触媒としてクリオゲルに導入することにより上記問題を解決できることを見出し、本発明を完成したものである。 In selecting the third component, the present inventors have studied from various angles. As a result, the inventors have focused on cerium, and found that the above problem can be solved by introducing cerium into the cryogel as a co-catalyst, thereby completing the present invention. Is.
本発明の貴金属触媒は、白金（Ｐｔ）とセリア（ＣｅＯ２）を含む、多孔質アルミナ構造体からなっている。ここで、白金の含有量は０．５〜８質量％が好ましく、１．０〜５質量％がより好ましい。また、本発明の貴金属触媒では、白金の含有量が３．０〜５質量％と高濃度になっても、多孔質アルミナ構造体の比表面積は低下せず、維持向上される。なお、白金の含有量が０．５質量％未満では触媒効果が低く、一方、白金の含有量が８質量％を超えると、多孔質アルミナ構造体の比表面積の低下や耐熱性の低下が生じ、好ましくない。 The noble metal catalyst of the present invention comprises a porous alumina structure containing platinum (Pt) and ceria (CeO 2 ). Here, the platinum content is preferably 0.5 to 8% by mass, and more preferably 1.0 to 5% by mass. Further, in the noble metal catalyst of the present invention, even when the platinum content is as high as 3.0 to 5% by mass, the specific surface area of the porous alumina structure is not lowered and maintained and improved. When the platinum content is less than 0.5% by mass, the catalytic effect is low. On the other hand, when the platinum content exceeds 8% by mass, the specific surface area and heat resistance of the porous alumina structure are reduced. Is not preferable.
セリアの含有量としては１〜５０質量％が好ましく、５〜４０質量％がより好ましい。セリアの含有量が１質量％未満の場合、セリア添加の効果が低く、一方、セリアの含有量が５０質量％を超えると、比表面積が低下して担体である多孔質アルミナ構造体の耐熱性が低下するという問題がある。 As content of ceria, 1-50 mass% is preferable, and 5-40 mass% is more preferable. When the content of ceria is less than 1% by mass, the effect of adding ceria is low. On the other hand, when the content of ceria exceeds 50% by mass, the specific surface area decreases and the heat resistance of the porous alumina structure as a carrier There is a problem that decreases.
本発明の貴金属触媒は、耐熱性に優れており、８００℃の雰囲気下で白金のシンタリングが、白金（Ｐｔ）を含み、セリア（ＣｅＯ ２ ）を含まないアルミナクリオゲルからなる貴金属触媒と比較して抑制されるという効果を奏するものである。 The noble metal catalyst of the present invention is excellent in heat resistance, and compared with a noble metal catalyst made of alumina cryogel containing platinum (Pt) and containing no ceria (CeO 2 ) in platinum sintering under an atmosphere of 800 ° C. in which an effect that is suppressed by.
次に、本発明に係る貴金属触媒は、白金及びセリウムを含むベーマイトゾルを調製した後、このべーマイトゾルをゲル化し、次いで凍結乾燥することにより、多孔質アルミナ構造体からなる貴金属触媒を得ることにより、製造できる。 Next, the noble metal catalyst according to the present invention is prepared by preparing a boehmite sol containing platinum and cerium, then gelling the boehmite sol and then freeze-drying to obtain a noble metal catalyst comprising a porous alumina structure. Can be manufactured.
上記製造方法において重要な技術的事項は、セリウムイオンがベーマイトゾルに含まれると、白金溶液を投入する際にゲル化速度が速まり、最終的に均一な混合ゲルが得られにくい、という点にある。 An important technical matter in the above production method is that when cerium ions are contained in the boehmite sol, the gelation speed increases when a platinum solution is added, and it is difficult to finally obtain a uniform mixed gel. is there.
そこで、セリウム導入の直前工程にあたるベーマイトゾル解こうプロセスにおいて、解こう目的に加える硝酸などの酸の添加量を増やして溶液をより酸性に傾けておくことで、白金溶液添加時の瞬時ゲル化を抑制した。 Therefore, in the boehmite sol peptization process, which is the process immediately before the introduction of cerium, increasing the amount of acid such as nitric acid added for the purpose of peptization and tilting the solution to make it more acidic allows instant gelation when adding a platinum solution. Suppressed.
すなわち、本発明の製造方法では、白金及びセリウムの添加前に、ベーマイトゾルに、酸をベーマイトに対して０．１７〜０．７０（酸／ベーマイトのモル比）の割合で添加することが好ましい。酸／ベーマイトのモル比は、０．３０〜０．６０がさらに好ましい。
なお、添加する酸としては特に限定されず、硝酸のほか、塩酸、硫酸などの無機酸を用いることができる。
That is, in the production method of the present invention, it is preferable to add acid to the boehmite sol at a ratio of 0.17 to 0.70 (molar ratio of acid / boehmite) to boehmite before adding platinum and cerium. . The acid / boehmite molar ratio is more preferably 0.30 to 0.60.
In addition, it does not specifically limit as an acid to add, In addition to nitric acid, inorganic acids, such as hydrochloric acid and a sulfuric acid, can be used.
次に、本発明の貴金属触媒の製造方法の一例として、主組成がアルミナ（Ａｌ２Ｏ３）で構成され、且つ白金（Ｐｔ）及びセリア（ＣｅＯ２）が分散された貴金属触媒（Ｐｔ／Ａｌ２Ｏ３クリオゲル）について説明する。上記Ｐｔ／Ａｌ２Ｏ３クリオゲルの製造方法は、大まかな工程として、（１）ゾル化工程、（２）白金錯体化工程、（３）ゲル化工程、（４）乾燥工程から構成される。 Next, as an example of the method for producing a noble metal catalyst of the present invention, a noble metal catalyst (Pt / Al) in which the main composition is composed of alumina (Al 2 O 3 ) and platinum (Pt) and ceria (CeO 2 ) are dispersed. 2 O 3 cryogel) will be described. The manufacturing method of the Pt / Al 2 O 3 cryogel is composed of (1) a sol process, (2) a platinum complex process, (3) a gel process, and (4) a drying process as rough processes.
（１）ゾル化工程
ゾル化工程は、アルミナ源であるＡＳＢ（Ａｌ（ｓｅｃ−ＢｕＯ）３）又はＡＩＰ（Ａｌ（ｉｓｏ−ＰｒＯ）３）を溶媒である水に投入し、アルコキシド加水分解後、ＨＮＯ３溶液（硝酸）を、硝酸／ベーマイトのモル比が０．１７〜０．７０となる割合で加えて、所定時間保持することにより、ゾルを邂逅、透明なベーマイトゾル（ＡｌＯＯＨ）を作製する。一方、硝酸セリウムをＣｅＯ２ベースで１〜５０質量％分取し、エチレングリコールを含む水溶液に溶解し、この溶液をベーマイトゾルに加え、攪拌する。
(1) Solation step In the solation step, ASB (Al (sec-BuO) 3 ) or AIP (Al (iso-PrO) 3 ), which is an alumina source, is added to water as a solvent, and after alkoxide hydrolysis, An HNO 3 solution (nitric acid) is added at a ratio such that the molar ratio of nitric acid / boehmite is 0.17 to 0.70, and is kept for a predetermined time, so that a transparent boehmite sol (AlOOH) is produced. . On the other hand, 1 to 50% by mass of cerium nitrate is collected based on CeO 2 and dissolved in an aqueous solution containing ethylene glycol. This solution is added to the boehmite sol and stirred.
（２）白金錯体化工程
白金錯体化工程は、ジカルボン酸系キレート剤を含む水溶液に、アンモニア水溶液を加えた溶液を調整後、白金酸（ＨＣＰＡ：ヘキサクロロ白金酸六水和物［Ｈ２（ＰｔＣｌ６）・６Ｈ２Ｏ］）から構成された白金ソースを投入し、沈殿物を生成させ、これを所定の温度で加熱・攪拌することにより、沈殿物が溶けて、均一な白金錯体溶液（白金ソース）を作製する。
(2) Platinum complexation step The platinum complexation step is a platinum acid (HCPA: hexachloroplatinic acid hexahydrate [H 2 (PtCl) after adjusting a solution obtained by adding an aqueous ammonia solution to an aqueous solution containing a dicarboxylic acid chelating agent. 6 ) · 6H 2 O]) is added to form a precipitate, which is heated and stirred at a predetermined temperature to dissolve the precipitate and form a uniform platinum complex solution (platinum). Source).
（３）ゲル化工程
ゲル化工程は、（２）で得られた白金ソースを投入後、尿素を加えて、所定時間保持した後、更に所定温度で所定時間保持することにより、ＨＣＰＡ／ベーマイトゲル（ＡｌＯＯＨ）を作製する。
(3) Gelation step In the gelation step, after adding the platinum source obtained in (2), after adding urea and holding it for a predetermined time, it is further held at a predetermined temperature for a predetermined time, whereby an HCPA / boehmite gel (AlOOH) is prepared.
（４）乾燥工程
乾燥工程は、通常乾燥によるキセロゲルを得る方法と、凍結乾燥によりクリオゲルを得る方法、とがある。後者は、ゲル化工程で得られたＨＣＰＡ／ベーマイトゲル（ＡｌＯＯＨ）を凍結し、凍結確認後、更にトラップ部冷却温度−８０℃以下の真空下で所定時間保持し、凍結乾燥する。ゲル化過程で形成した微細なネットワークを維持するために、従来法では、超臨界流体を用いた乾燥が行われてきたが、本発明では、凍結乾燥法を用いることにより、表面張力をキャンセルさせ、微細なネットワークを維持したままで乾燥ゲル（クリオゲル）を得ることができる。
(4) Drying process The drying process includes a method of obtaining xerogel by normal drying and a method of obtaining cryogel by freeze drying. In the latter case, the HCPA / boehmite gel (AlOOH) obtained in the gelation step is frozen, and after freezing is confirmed, the HCPA / boehmite gel (AlOOH) is further held for a predetermined time under a vacuum at a trap portion cooling temperature of −80 ° C. or lower and freeze-dried. In order to maintain the fine network formed in the gelation process, drying using a supercritical fluid has been performed in the conventional method, but in the present invention, surface tension is canceled by using the freeze drying method. A dry gel (cryogel) can be obtained while maintaining a fine network.
本発明を実施例に基づいて、更に詳細に説明するが、本発明はこれらの実施例に限られるものではない。 The present invention will be described in more detail based on examples, but the present invention is not limited to these examples.
（触媒合成例）
オイルバス８６℃中にて、アルミニウムトリブトキシド（Ａｌ（ｓｅｃ−ＢｕＯ）３）０．０２８６ｍｏｌを蒸留水２０ｍＬに加えて加水分解する。１Ｍ硝酸を５ｍＬ（硝酸／ベーマイトのモル比で０．１７）加えて８６℃で１時間オイルバス中にて攪拌することにより、濁ったゾルが解こうして半透明なベーマイトゾルが得られた。
(Example of catalyst synthesis)
In an oil bath 86 ° C., 0.0286 mol of aluminum tributoxide (Al (sec-BuO) 3 ) is added to 20 mL of distilled water for hydrolysis. By adding 5 mL of 1M nitric acid (nitric acid / boehmite molar ratio 0.17) and stirring in an oil bath at 86 ° C. for 1 hour, the cloudy sol was dissolved and a translucent boehmite sol was obtained.
硝酸セリウムをＣｅＯ２ベースで１〜５０質量％相当量を分取し、エチレングリコールを含む水溶液(セリウム：エチレングリコール＝１：１．１〜１．５（モル比)、２〜７ｍＬ）に溶解してオイルバス８６℃で３０分保持した。この溶液をベーマイトゾルに加えて攪拌を続けた。 1 to 50% by mass of cerium nitrate based on CeO 2 is collected and dissolved in an aqueous solution containing ethylene glycol (cerium: ethylene glycol = 1: 1.1 to 1.5 (molar ratio), 2 to 7 mL). The oil bath was kept at 86 ° C. for 30 minutes. This solution was added to the boehmite sol and stirring was continued.
一方で、０．０６８９Ｍのシュウ酸水溶液０．６ｍＬに、２０質量％のアンモニア水溶液０．１〜０．２ｍＬを加えた溶液を調製し、これに１．９９４質量％の塩化白金酸水溶液を０．３６〜１．８ｇ投入すると沈殿物が生成した。これを８６℃にて加熱攪拌すると沈殿物が解けて薄いレモンイエロー色の均一な溶液となった。この溶液を、セリウムを含んだベーマイトゾルに加えてしばらく攪拌を行った。その後、尿素０．３ｇを加えた後、スターラーを取り除きオイルバス８６℃で一晩放置し、得られたゲルを凍結乾燥した後、仮焼した。合成されたＰｔ／ＣｅＯ２−Ａｌ２Ｏ３クリオゲル触媒の白金含有量は５．０質量％で、セリアの含有量は１．０〜５０．０質量％の範囲内において、セリア含有量が１．０質量％（実施例１）、５．０質量％（実施例２）、１０．０質量％（実施例３）、２０．０質量％（実施例４）、３０．０質量％（実施例５）、４０．０質量％（実施例６）、５０．０質量％（実施例７）であった。 On the other hand, a solution obtained by adding 20 mass% ammonia aqueous solution 0.1-0.2 mL to 0.0689 M oxalic acid aqueous solution 0.6 mL was prepared, and 1.994 mass% chloroplatinic acid aqueous solution was added to this. When 36 to 1.8 g was added, a precipitate was formed. When this was heated and stirred at 86 ° C., the precipitates were dissolved and a light lemon-yellow uniform solution was obtained. This solution was added to boehmite sol containing cerium and stirred for a while. Thereafter, 0.3 g of urea was added, the stirrer was removed, and the mixture was left overnight in an oil bath at 86 ° C. The obtained gel was freeze-dried and calcined. The platinum content of the synthesized Pt / CeO 2 —Al 2 O 3 cryogel catalyst is 5.0 mass%, and the ceria content is 1.0 to 50.0 mass%, and the ceria content is 1 0.0% by mass (Example 1), 5.0% by mass (Example 2), 10.0% by mass (Example 3), 20.0% by mass (Example 4), 30.0% by mass (implementation) Example 5) 40.0 mass% (Example 6) and 50.0 mass% (Example 7).
また、比較例として、Ａｌ２Ｏ３クリオゲル（比較例１）、白金含有量が５．０質量％のＰｔ／Ａｌ２Ｏ３クリオゲル触媒（比較例２）、白金含有量が５．０質量％の含浸法によるＰｔ／Ａｌ２Ｏ３触媒（比較例３）を用いた。 Moreover, as a comparative example, Al 2 O 3 cryogel (Comparative Example 1), Pt / Al 2 O 3 cryogel catalyst having a platinum content of 5.0% by mass (Comparative Example 2), and platinum content of 5.0% by mass A Pt / Al 2 O 3 catalyst (Comparative Example 3) was used.
（１）ＢＥＴ比表面積の測定（図１）
図１は５質量％−Ｐｔ／Ａｌ２Ｏ３クリオゲルにＣｅＯ２を１〜３０質量％添加したときのＢＥＴ比表面積を示している。また、比較のためにＡｌ２Ｏ３クリオゲル（比較例１）、あるいはＣｅＯ２を添加しないＰｔ／Ａｌ２Ｏ３クリオゲル（比較例２）のＢＥＴ比表面積も示した。比較例１と比較例２を比べてわかるように、Ａｌ２Ｏ３クリオゲルに５質量％の白金を導入すると比表面積の低下が著しい。しかしながら、ＣｅＯ２を添加することにより比表面積の増大が認められ（実施例１〜５）、５．０質量％のＣｅＯ２添加（実施例２）では、白金を含まないＡｌ２Ｏ３クリオゲル（比較例１）よりも高い比表面積を有する結果となった。すなわち、第三成分としてセリアを導入することにより、白金の高濃度導入で通常見られるクリオゲル触媒の比表面積低下をくい止めることが可能となったことがわかる。
(1) Measurement of BET specific surface area (Fig. 1)
FIG. 1 shows the BET specific surface area when 1 to 30% by mass of CeO 2 is added to 5% by mass-Pt / Al 2 O 3 cryogel. For comparison, the BET specific surface area of Al 2 O 3 cryogel (Comparative Example 1) or Pt / Al 2 O 3 cryogel not added with CeO 2 (Comparative Example 2) is also shown. As can be seen by comparing Comparative Example 1 and Comparative Example 2, when 5 mass% platinum is introduced into the Al 2 O 3 cryogel, the specific surface area is significantly reduced. However, an increase in specific surface area was observed by adding CeO 2 (Examples 1 to 5), and when 5.0 mass% of CeO 2 was added (Example 2), Al 2 O 3 cryogel containing no platinum ( The result was that it had a higher specific surface area than Comparative Example 1). That is, it can be seen that by introducing ceria as the third component, it was possible to prevent the decrease in the specific surface area of the cryogel catalyst that is normally observed when introducing a high concentration of platinum.
なお、ＢＥＴ比表面積は、日本Ｂｅｌ（株）製のＢｅｌｓｏｒｐ Ｍｉｎｉ（比表面積・細孔分布測定装置）を用い、液体窒素温度でのＮ２吸着等温線から算出した。 The BET specific surface area was calculated from the N 2 adsorption isotherm at liquid nitrogen temperature using Belsorb Mini (specific surface area / pore distribution measuring device) manufactured by Bel Japan Co., Ltd.
（２）Ｘ線回折（ＸＲＤ）による白金の高温耐熱性の測定（図２）
図２は５質量％−Ｐｔ／Ａｌ２Ｏ３クリオゲルにＣｅＯ２を３０〜５０質量％添加したもの（実施例５〜７）を８００℃で焼成したときのＸＲＤスペクトルを示す。ＣｅＯ２を添加しないもの（比較例２）では、８００℃焼成で白金の鋭いピークが観察され、白金の焼結が進んでいることが示唆されたが、セリアを添加すること（実施例５〜７）によって高温下でも白金の焼結が抑制されたことがわかる。このように、セリアを助触媒として選択することにより、特願２００６−４７９７４号で提案した貴金属触媒（セリアなし）に比して、貴金属触媒の耐熱性を約１００℃向上させることが可能となった。
(2) Measurement of high-temperature heat resistance of platinum by X-ray diffraction (XRD) (Fig. 2)
FIG. 2 shows the XRD spectrum when 5 mass% -Pt / Al 2 O 3 cryogel added with 30-50 mass% of CeO 2 (Examples 5-7) was baked at 800 ° C. In the case where CeO 2 was not added (Comparative Example 2), a sharp peak of platinum was observed by firing at 800 ° C., suggesting that platinum was being sintered, but ceria was added (Examples 5 to 5). 7) that the sintering of platinum was suppressed even at high temperatures. Thus, by selecting ceria as a co-catalyst, the heat resistance of the noble metal catalyst can be improved by about 100 ° C. as compared with the noble metal catalyst proposed in Japanese Patent Application No. 2006-47974 (without ceria). It was.
（３）クリオゲル触媒を用いたメタンの酸化活性評価（図３）
図３は５質量％−Ｐｔ／Ａｌ２Ｏ３クリオゲルにＣｅＯ２を５〜５０質量％添加したもの（実施例２〜７）を触媒として、疑似大気雰囲気下でメタンの酸化活性を調べた結果を示す。比較例としてＣｅＯ２を添加しない５．０質量％−Ｐｔ／Ａｌ２Ｏ３クリオゲル触媒（比較例２）、あるいは市販アルミナを用いて含浸法により作製した５質量％−Ｐｔ／Ａｌ２Ｏ３触媒（比較例３）も同様に触媒活性を調べた。セリアを含まないＰｔ／Ａｌ２Ｏ３クリオゲル触媒（比較例２）は同じくセリアを含まない含浸法触媒（比較例３）より高い活性を示し、約１００℃ほど反応温度を低下させた。このＰｔ／Ａｌ２Ｏ３クリオゲル触媒にＣｅＯ２を添加することにより、触媒活性のさらなる増大が見られた（実施例２〜７）。最適量である約２０質量％のセリアを添加すること（実施例４）により、セリア
を含まないクリオゲル触媒（比較例２）と比較して約５０℃の触媒反応の低温化、すわなち触媒性能の向上が認められた。
(3) Evaluation of methane oxidation activity using cryogel catalyst (Fig. 3)
FIG. 3 shows the result of examining the oxidation activity of methane in a simulated atmospheric atmosphere using 5 mass% -Pt / Al 2 O 3 cryogel added with 5 to 50 mass% of CeO 2 (Examples 2 to 7) as a catalyst. Indicates. As a comparative example, a 5.0 mass% -Pt / Al 2 O 3 cryogel catalyst (Comparative Example 2) without adding CeO 2 or a 5 mass% -Pt / Al 2 O 3 catalyst prepared by impregnation using commercially available alumina. (Comparative Example 3) was also examined for catalytic activity. The Pt / Al 2 O 3 cryogel catalyst not containing ceria (Comparative Example 2) showed higher activity than the impregnation catalyst not containing ceria (Comparative Example 3), and the reaction temperature was lowered by about 100 ° C. By adding CeO 2 to this Pt / Al 2 O 3 cryogel catalyst, a further increase in catalytic activity was seen (Examples 2-7). By adding about 20% by mass of ceria which is the optimum amount (Example 4), the temperature of the catalytic reaction is lowered by about 50 ° C. compared to the cryogel catalyst not containing ceria (Comparative Example 2), that is, the catalyst. An improvement in performance was observed.
本発明の貴金属触媒及びその製造方法は、例えば、排ガス処理用の触媒の製造に好適に用いることができる。 The noble metal catalyst and the method for producing the same of the present invention can be suitably used for producing a catalyst for treating exhaust gas, for example.
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