JP2005034682A - CO conversion catalyst and method for producing the same - Google Patents

Abstract

In a fuel cell system, a catalyst suitable for reducing carbon monoxide from a hydrogen-containing gas used in a polymer electrolyte fuel cell, having excellent durability such as heat resistance and oxidation resistance, and A CO shift catalyst capable of performing a CO shift reaction with high efficiency and a method for producing the same are provided.
A CO conversion catalyst characterized in that Cu as an active metal is supported on a support made of alumina or boehmite, and an active metal Cu impregnated and supported on a support containing alumina or boehmite. A process for producing a CO conversion catalyst characterized by the above.
[Selection] Figure 1

Description

【００１３】
一方、燃料電池システムの運転方法のうち、ＤＳＳ運転とは、通常一日一回起動停止する運転方法を指すものであるが、特に一日一回起動停止に限定されるものではなく、頻繁に起動停止する運転または適宜起動停止する方法も含まれ、本発明のＣＯ変成触媒はかかる運転方法でも長期間使用に適している。
【００１４】
次に、本発明のＣＯ変成触媒について詳細に説明する。
本発明のＣＯ変成触媒は、アルミナ（Ａｌ_２Ｏ_３）またはベーマイト（ＡｌＯ（ＯＨ））からなる担体に、活性金属としてＣｕが担持されている。アルミナまたはベーマイトは水を吸着できる担体であり、活性金属であるＣｕと組み合わせることにより、ＣＯを主にＣＯ_２およびＨ_２に変換する。アルミナの種類は限定されず、α−，β−，γ−等のいずれの形状のものも任意に用いることができるが、中でもγ型のアルミナは特に好ましい。担体となるアルミナまたはベーマイトの含有量は、通常５０重量％以上、好ましくは６０重量％以上９０重量％以下、特に好ましくは７０重量％以上８０重量％以下である。この担体は高比表面積を有し、通常１００ｍ^２／ｇ以上５００ｍ^２／ｇ以下、好ましくは１５０ｍ^２／ｇ以上４００ｍ^２／ｇ以下の比表面積を有する。
【００１５】
活性金属である銅（Ｃｕ）は、アルミナ等の担体の表面付近に分散して担持されている。ＣＯシフト反応を効率的に進行させるには、水を吸着するアルミナ等の担体とＣＯを引きつけるＣｕによる活性点が多く存在する状態がよい。本発明で用いられるアルミナ等は高比表面積を有する多孔質の担体であるため、表面にＣｕを分散して担持させることにより、多くの活性点を形成させることができる。これにより、担体担持型の触媒であってもＣＯシフト反応を高効率に進行させることができる。
Ｃｕ成分であるＣｕ（還元前はＣｕＯ）の含有量は、通常４０重量％以下、好ましくは３５重量％以下５重量％以上、より好ましくは３０重量％以下１０重量％以上、特に好ましくは３０重量％以下２０重量％以上である。このようなＣｕ成分が、例えば粒子状単体として多く存在すると酸素によって容易に酸化反応を起こしてしまうが、本発明のように担体上に担持されている性状では、酸素の流入があっても容易には酸化反応を生じず、活性金属としての触媒活性を維持できる。
【００１６】
本発明では、前記担体として、アルミナ単独の場合の他、アルミナに他の酸化物を添加した担体を用いることも可能であり、Ａｌ_２Ｏ_３・ＺｒＯ_２、Ａｌ_２Ｏ_３・ＳｉＯ_２又はＡｌ_２Ｏ_３・ＴｉＯ_２などの複合酸化物が好適に用いられる。これら複合酸化物からなる担体も、アルミナ同様に高比表面積を有するものであり、いずれの担体成分を用いた場合にも高効率なＣＯシフト反応が可能であり、耐酸素性等の耐久性も有する。なお、上記複合酸化物とアルミナまたはベーマイトとを混合して、担体を構成させることも可能である。
また、前記活性金属としては、Ｃｕ単独の場合の他、Ｃｕと他の金属成分Ｍｅ、具体的にはＺｎ、Ｃｏ、Ｎｉ、Ｆｅ又はＣｒを含有することもできる。ＣｕとＭｅとの量比は特に限定されるものではなく任意に設定可能であるが、例えばＣｕ：Ｍｅ＝１：０．１〜１の範囲で用いることができる。
【００１７】
次に、本発明のＣＯ変成触媒の製造方法について説明する。
本発明の製法では、アルミナまたはベーマイトを含む担体に、活性金属であるＣｕを含浸法により担持する。具体的には、例えば担体に、Ｃｕ水溶液を滴下して攪拌しながら均一にしたＣｕ溶液のＣｕを含浸担持する。次いで、Ｃｕ水溶液をＣｕ換算で一定量以上加えた後、１００℃以上１２０℃以下にて蒸発乾固させて水分を蒸発させる。蒸発後、３００℃以上８００℃以下で焼成して、本発明の触媒が得られる。また、Ｃｕ以外にも活性金属成分を含む場合には、Ｃｕと、Ｚｎ、Ｃｏ、Ｎｉ、Ｆｅ、又は、Ｃｒのいずれかの金属を含む各混合水溶液を加えて、含浸担持する。含浸担持後に得られた触媒は、ＣＯ変成反応を行う前に、触媒の活性化処理として水素含有ガスによる水素還元処理を行う。
【００１８】
次いで、本発明のＣＯ変成触媒を燃料電池システムに適用した場合の実施の形態について説明する。
本発明により得られるＣｕ含有触媒を、固体高分子型燃料電池（ＰＥＦＣ）システムで用いる場合には、図１に示すような形態が考えられる。改質触媒を有する改質器１を用いて水素を製造する方法であり、水素製造においてはメタン、プロパン等が原料として用いられる。
図１で都市ガス（主にメタン）又はＬＰＧ（主にプロパン）を原料とする場合には、先ず硫黄分を除去する。次いで、約７００℃付近にて改質器１の改質触媒によって、下記（２）式又は（３）式の水蒸気改質反応を生じさせて水素含有ガスを得る。
【００１９】
【化２】

【００２０】
得られた水素含有ガスは約１０％程度のＣＯ成分を含み、このＣＯは燃料電池の働きを阻害する被毒物質として作用する。そこで、後段に設けられる本発明のＣＯ変成触媒２において、通常２５０℃以下、好ましくは１５０℃以上２３０℃以下にてＣＯシフト反応を生じさせて、ＣＯを二酸化炭素に変換する（上記（１）式）。
ＣＯ変成触媒２を経たガスからは一酸化炭素が通常３０００ｐｐｍ以上４０００ｐｐｍ以下にまで減少、除去されているが、燃料電池本体４に導入する原料ガスは、通常２０ｐｐｍ以下好ましくは１０ｐｐｍ以下のＣＯ濃度であることが必要である。ＣＯ除去触媒３をＣＯ変性触媒２の後流に設けることにより、更なるＣＯ除去を行う。ＣＯ除去装置３では、下記（４）式もしくは（５）式によってＣＯ低減を行う。
【００２１】
【化３】

【００２２】
このようにＣＯ濃度が低下した水素含有ガスは、燃料電池４に送られて、アノード電極側での電極反応に利用される。燃料電池４では、アノード電極にてアノード電極触媒により、水素がＨ_２から２Ｈ^＋と２ｅ⁻となり、Ｈ^＋が電解質に拡散し、ｅ⁻は電極間を繋ぐ線を移動する。一方、カソード電極においてカソード電極触媒により、Ｈ^＋とｅ⁻と酸素からＨ_２Ｏが生じる。これらの反応を合わせて電池反応が構成され、起電力を得ることができる。
【００２３】
以下、実施例により本発明をより詳細に説明するが、本発明はこれら実施例によって何ら制限されるものでない。
【００２４】
【実施例】
実施例１（触媒の調製）
〔ＣＯ変成触媒１の調製〕
硝酸アルミニウム（Ａｌ（ＮＯ_３）_３・９Ｈ_２Ｏ：Ｍｗ３７５．１３）３７．５ｇを水１Ｌに添加して、６０℃加熱したのち溶解させて、硝酸アルミニウム水溶液を得た。この溶液に、アンモニア水をｐＨ＝７になるまで添加して、水酸化アルミニウム沈殿物を生成させた。該沈殿物を１時間熟成した後、水洗濾過し、乾燥させてから、５５０℃、５時間焼成して、γ−Ａｌ_２Ｏ_３（担体１）を得た。
得られた担体１（１０ｇ）に、硝酸銅（Ｃｕ（ＮＯ_３）_２・６Ｈ_２Ｏ）水溶液を滴下して攪拌しながら、均一にＣｕ溶液を含浸担持した。Ｃｕ水溶液をＣｕ換算で１ｇ滴下した後、攪拌しながら１１０℃にて蒸発乾固させ、２４時間で水分を蒸発させた。蒸発後に得られたサンプルを、５００℃、５時間空気中で焼成して、触媒１（ＣｕＯ／γ−Ａｌ_２Ｏ_３）を得た。
【００２５】
〔ＣＯ変成触媒２〜５の調製〕
上記触媒１の調製法において、硝酸アルミニウム水溶液に塩化チタニウム塩酸溶液、シリカゾル、又は、オキシ硝酸ジルコニウムを加えて、ＡｌとＴｉ、ＡｌとＳｉ、およびＡｌとＺｒの混合水溶液（各々原子比で８０：２０の組成）をそれぞれ得た。これらの水溶液に対して、触媒１と同様な方法にてアンモニアを添加して、沈殿物を得て、乾燥焼成を行い、複合酸化物Ａｌ_２Ｏ_３・ＴｉＯ_２、Ａｌ_２Ｏ_３・ＳｉＯ_２、およびＡｌ_２Ｏ_３・ＺｒＯ_２ の担体を得た。これらの担体に対して、触媒１と同様な方法によりＣｕＯを担持して、触媒２（ＣｕＯ／Ａｌ_２Ｏ_３・ＴｉＯ_２）、触媒３（ＣｕＯ／Ａｌ_２Ｏ_３・ＳｉＯ_２）、および触媒４（ＣｕＯ／Ａｌ_２Ｏ_３・ＺｒＯ_２）を得た。
【００２６】
〔ＣＯ変性触媒５の調製〕
上記触媒１の調製法において、上記担体１の代わりに市販のベーマイト（ＡｌＯＯＨ）を担体に用いた以外は触媒１と同様な方法により、ＣｕＯを担持して触媒５（ＣｕＯ／ＡｌＯＯＨ）を得た。但し、焼成時には、ＡｌＯＯＨはＡｌ_２Ｏ_３に相変化する。
【００２７】
〔ＣＯ変性触媒６〜７の調製〕
上記触媒１の調製法において、得られた担体１（γ−Ａｌ_２Ｏ_３）１０ｇ上に、硝酸銅をＣｕ換算でそれぞれ２ｇ（触媒６）あるいは３ｇ（触媒７）担持した後、触媒１と同様な方法にてＣｕＯ／γ−Ａｌ_２Ｏ_３触媒を調製した。本触媒を触媒６、触媒７とする。
【００２８】
〔ＣＯ変性触媒８〜１２の調製〕
上記触媒１の調製法において、得られた担体１（γ−Ａｌ_２Ｏ_３）１０ｇ上に硝酸銅をＣｕ換算で１ｇと、硝酸亜鉛、硝酸コバルト、硝酸ニッケル、硝酸鉄、又は、硝酸クロムを、Ｚｎ換算で０．３ｇ、Ｃｏ換算で０．３ｇ、Ｎｉ換算で０．３ｇ、Ｆｅ換算で０．３ｇ、Ｃｒ換算で０．３ｇの各混合水溶液を滴下して、触媒１と同様に含浸担持して、触媒８（ＣｕＯ・ＺｎＯ／γ−Ａｌ_２Ｏ_３）、触媒９（ＣｕＯ・ＣｏＯ／γ−Ａｌ_２Ｏ_３）、触媒１０（ＣｕＯ・ＮｉＯ／γ−Ａｌ_２Ｏ_３）、触媒１１（ＣｕＯ・ＦｅＯ／γ−Ａｌ_２Ｏ_３）、および、触媒１２（ＣｕＯ・Ｃｒ_２Ｏ_３／γ−Ａｌ_２Ｏ_３）を得た。
【００２９】
〔比較触媒１の調製〕
比較触媒として、従来の沈殿法で調製した触媒を以下に記す。
硝酸銅（Ｃｕ（ＮＯ_３）_３・６Ｈ_２Ｏ）と硝酸亜鉛（Ｚｎ（ＮＯ_３）_２・６Ｈ_２Ｏ）と硝酸アルミニウム（Ａｌ（ＮＯ_３）_３・９Ｈ_２Ｏ）が原子比でＣｕ：Ｚｎ：Ａｌ＝１００：１００：２０の比率になるように混合水溶液を調製した。この混合水溶液に炭酸ナトリウムをｐＨ＝７になるまで滴下して複合水酸化物沈殿を得た。この複合水酸化物沈殿物を水洗、濾過を行ったのち、１１０℃で乾燥を行い、３００℃で５時間焼成を行い、比較触媒１を得た。
【００３０】
〔触媒の物性評価〕
上記触媒１〜１２及び比較触媒１の物性評価として、一点Ｎ_２吸着法で測定した比表面積測定を実施した。表１には、触媒リストとともに比表面積測定結果を示す。
【００３１】
実施例２（触媒の評価・試験）
〔触媒活性評価〕
触媒１〜１２および比較触媒１を用いて、以下の試験条件にてＣＯ変性反応試験を行った。触媒は１ｍｍに加圧整粒した後、固定床流通式マイクロリアクタに５ｃｃ充填した。触媒の活性化処理として、１％Ｈ_２／Ｎ_２バランスのガスを２００℃、ＧＨＳＶ１０００ｈ^−１の条件で触媒に供給して、活性金属のＣｕを還元して金属状態にする。触媒出口の水素濃度をモニターしながら、入口水素濃度と同様の濃度となった時点で還元処理を終了した。
所定温度に加熱したあと、反応ガスを供給して、ＣＯの減少量をＣＯ計（ＮＤ−ＩＲ法）によりモニターした。
（活性評価条件）ガス組成：ＣＯ ０．３％、ＣＯ_２ ２０％、Ｈ_２Ｏ ３０％、Ｈ_２ 残４９．７％
接触時間（ＧＨＳＶ）：３０００ｈ^−１ 、温度１７０℃、２００℃
表１に、初期の活性評価としてＣＯ転化率を記す。なお、ＣＯ転化率は以下の式にて定義する。
ＣＯ転化率＝（（入口ＣＯ濃度―出口ＣＯ濃度）／入口ＣＯ濃度）×１００
【００３２】
〔強制劣化試験〕
触媒の強制劣化試験として、上記活性評価後において、２００℃で空気をＧＨＳＶ１０００ｈ^−１の状態で１０分間触媒に供給した。この際、触媒の発熱が認められるが、最高発熱温度を測定した。さらに、空気暴露後に上記活性評価条件（１７０℃）において、活性評価を行い、触媒の劣化挙動を観察した。
表１に、空気暴露による最高発熱温度、及び強制劣化後の１７０℃におけるＣＯ転化率を記す。
【００３３】
【表１】

【００３４】
上記結果より、本発明の触媒１〜１２は初期のＣＯシフト活性は比較触媒１と遜色ない性能を示すことが分かり、さらに、比較触媒１に比べて空気暴露による発熱上昇が少なく、これに伴う活性低下も小さいことが明らかになった。この理由は、本発明の触媒は高比表面積担体に活性金属のＣｕが高分散担持されているために、酸素漏洩時においても、酸素とＣｕの反応が緩やかであるため、温度上昇が抑制されるためである。
また、本発明の触媒は上記物性を有するため、高温時のシンタリングが抑制できるため耐熱性に優れ、さらに水分存在化での水蒸気酸化を抑制することから耐水性にも優れている。以上のことから、本発明の触媒は既存の沈殿系触媒より耐酸化性、耐熱性、耐水性に強い長寿命触媒となることが分かった。
【００３５】
【発明の効果】
本発明の触媒は、システムが高温になる際にも触媒劣化しない耐熱性を有し、且つ、減圧によるシステム中への酸素混入においても容易には発熱反応が進行しない耐酸素性を有している。また、触媒成分の中でも、一般に吸水性が少ないアルミナを担体として用いるので、シンタリングの問題が生ぜず耐水性にも優れている。一方で、本来のＣＯシフト反応においても低温活性に優れており、高効率にＣＯシフトを行い、ＣＯ濃度を大幅低減させることができる。
したがって本発明によれば、燃料電池システムの温度条件や運転条件に依存せず、触媒劣化による性能低下を起こさないＣＯ変成触媒を提供できる。本発明のＣＯ変成触媒を用いれば、特にＤＳＳ運転等において触媒が劣化する現象を効果的に防止でき、触媒を長期間使用できる。
さらに、従来のＣｕとＺｎの複合酸化物を調製する際の複雑な工程を要する共沈法と異なり、高比表面積を有する担体を用いた含浸法によって簡易な工程によって製造できる。
【図面の簡単な説明】
【図１】燃料電池システムの概略構成を模式的に示すブロック図である。
【符号の説明】
１ 改質触媒
２ ＣＯ変成触媒
３ ＣＯ除去触媒
４ 燃料電池
１０ 燃料電池システム[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a CO shift catalyst suitable for reducing carbon monoxide from a hydrogen-containing gas used in a polymer electrolyte fuel cell in a fuel cell system, and a method for producing the same.
[0002]
[Prior art]
The polymer electrolyte fuel cell (PEFC) is expected to be applied as a power source in a wide range of fields such as an automotive power source and a distributed power source because it has low pollution and higher thermal efficiency. In the fuel cell system, as shown in FIG. 1, the reforming catalyst of the reformer 1, the CO shift catalyst (CO shift catalyst) 2, and the CO removal catalyst 3 are arranged in series in the preceding stage of the fuel cell 4. Thus, CO removal is performed together with hydrogen production. For the operation of a series of these reformers, in a domestic fuel cell, DSS (Daily Start-up and Shut-down) operation that stops operation at night is more economical and convenient than continuous operation. Are better.
However, in the DSS operation, the combustible gas removal method and the start-up method at the time of stop are complicated, and there is a problem that the deterioration of the catalyst becomes remarkable. In particular, as the CO conversion catalyst 2, a Cu-ZnO-based composite oxide catalyst is generally used, but this catalyst has drawbacks that are poor in heat resistance, water resistance and oxygen resistance, and is due to steam oxidation at the time of stopping power generation and reduced pressure. There were problems of heat generation due to inhalation of air (oxygen) into the system and deterioration due to high temperature exposure at startup.
From the viewpoint of efficiently performing CO shift by the conventional Cu—ZnO-based composite oxide catalyst, this is formed as a mixture in which Cu and ZnO are mixed as fine particles, and the particulate Cu is highly concentrated. It was because it contained. In general, it is formed as a mixture of fine particles using a precipitation method, and the contact area between the particles of Cu and ZnO is increased to advance the CO shift reaction. However, in the properties of such a Cu—ZnO-based composite oxide catalyst, especially when it comes into contact with oxygen by inhaling air, Cu is immediately oxidized to cause an exothermic reaction, which is a major cause of catalyst deterioration. It was.
[0003]
Due to the above problems, when a Cu—ZnO-based composite oxide catalyst is used as a CO conversion catalyst in the past, in order to cope with catalyst deterioration in DSS operation, careful attention is paid to the operation surface, and expensive control equipment is installed. It was installed to protect the catalyst. However, this is a high cost process and it has been difficult to realize an economical DSS process.
On the other hand, the conventional catalyst is a catalyst system mainly intended for a chemical plant that operates continuously, and is suitable for performing a steady and constant operation. However, a catalyst system used for a hydrogen production apparatus having a large load fluctuation such as a fuel cell. Is not necessarily suitable (see, for example, Patent Document 1 and Patent Document 2).
[0004]
[Patent Document 1]
Japanese Patent Publication No. Hei 4-75058 [Patent Document 2]
Japanese Patent Publication No. 5-83304
[Problems to be solved by the invention]
In view of the above problems, the present inventors do not depend on temperature conditions or operating conditions in order to enable a simple DSS operation method, do not cause performance degradation due to deterioration of the catalyst over time, and are resistant to heat and acid. In order to develop a CO conversion catalyst that has excellent durability such as chemical conversion and can also effectively perform the original CO shift, the inventors have intensively studied.
As a result, the present inventors have found that the above problem can be solved by a catalyst in which Cu, which is an active metal, is supported on an Al-based carrier that is a basic oxide and can activate and adsorb water. I found it. Compared with the conventional coprecipitation method of Cu and Zn, a relatively inexpensive base metal catalyst is prepared by an impregnation method in which an active metal Cu is supported on a support mainly composed of Al. It has been found that a catalyst excellent in durability such as durability can be obtained. The present invention has been completed from such a viewpoint.
[0006]
[Means for Solving the Problems]
That is, the present invention provides a CO conversion catalyst characterized in that Cu is supported as an active metal on a support made of alumina (Al ₂ O ₃ ) or boehmite (AlO (OH)). Alumina or boehmite is a carrier capable of adsorbing water, and in combination with Cu, which is an active metal, converts CO mainly into CO ₂ and H ₂ . The content of alumina or boehmite as a support is usually 50% by weight or more, preferably 60% by weight or more and 90% by weight or less, particularly preferably 70% by weight or more and 80% by weight or less, and Cu (CuO before reduction treatment) The content of is usually 40% by weight or less. The carrier is a carrier having a high specific surface area, and usually has a specific surface area of 100 m ² / g or more and 500 m ² / g or less, preferably 150 m ² / g or more and 400 m ² / g or less.
[0007]
The carrier includes at least one selected from the group consisting of a carrier obtained by adding another oxide to alumina, Al ₂ O ₃ .ZrO ₂ , Al ₂ O ₃ .SiO ₂ and Al ₂ O ₃ .TiO ₂ . An alumina complex oxide can also be used. In addition to Cu alone, the active metal may contain Cu and at least one metal selected from the group consisting of Zn, Co, Ni, Fe and Cr.
Furthermore, the present invention provides a method for producing a CO conversion catalyst characterized by impregnating and supporting Cu as an active metal on a support made of alumina or boehmite, followed by drying and firing. After the calcination, a hydrogen reduction treatment with a hydrogen-containing gas is usually performed as a catalyst activation treatment.
[0008]
The catalyst of the present invention is excellent in heat resistance, has a low degree of heat generation even in the presence of oxygen, and is excellent in oxygen resistance, so that it can withstand long-term use of a fuel cell system involving DSS operation and the like. Have. Further, among the catalyst components, alumina having a low water absorption is generally used as a carrier, so that there is no problem of sintering and water resistance is excellent. Furthermore, unlike the conventional coprecipitation method that requires a complicated process when preparing a complex oxide of Cu and Zn, it can be manufactured by a simple process by an impregnation method using a carrier having a high specific surface area.
[0009]
In the present invention, the catalyst is effectively prevented from deteriorating by supporting the active metal Cu on the support. Specifically, it is necessary to support Cu on the surface of the carrier having a high surface area, and if there are too many Cu components, heat deterioration or deterioration due to oxygen may occur, which is not preferable. In the catalyst, Cu is usually supported by 40% by weight or less, preferably 35% by weight or less and 5% by weight or more, more preferably 30% by weight or less and 10% by weight or more. The primary particle size of the Cu component after being supported is usually 5 nm or more and 100 nm or less, preferably 10 nm or more and 50 nm or less, and is dispersed and supported on the surface of the carrier.
The catalyst of the present invention can effectively prevent deterioration of the catalyst even under conditions where oxygen is mixed in or under high temperature conditions. Since the reaction temperature on the LTS side in the fuel cell system is about 200 ° C. and the heat-resistant temperature of the catalyst is about 700 ° C., it can be sufficiently applied during the operation of the system or when the operation is repeatedly stopped. Degradation can be avoided effectively.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
First, an outline of a fuel cell system in which the CO conversion catalyst of the present invention is used will be described. FIG. 1 schematically shows a system 10 for introducing a raw material gas into a fuel cell.
The polymer electrolyte fuel cell (PEFC) is low in pollution and has high efficiency, so that it can be applied as a power source in a wide range of fields such as an automotive power source and a distributed power source. In order to supply hydrogen, which is a fuel, to the fuel cell system, for example, a method by hydrogen production using the reformer 1 can be mentioned. In such hydrogen production, hydrocarbon fuels (city gas, methane, provan, kerosene, dimethyl ether, etc.) are used as raw materials. Gases such as methane and propane are raw materials that are sufficiently popular as fuel gases, and these gases are reformed into hydrogen H ₂ in the reformer 1. At this time, carbon monoxide CO and carbon dioxide CO ₂ are simultaneously generated.
A platinum catalyst is mainly used for the electrode of the polymer electrolyte fuel cell 1, but since this catalyst is easily poisoned by carbon monoxide, CO is removed from the fuel gas mainly containing hydrogen as much as possible, It is necessary to reduce the CO concentration. Therefore, as shown in FIG. 1, in the downstream of the reformer 1, the CO conversion catalyst 2 is first used to reduce the CO concentration in the gas. Next, the CO concentration in the gas sent to the fuel cell main body 4 is reduced to 10 ppm or less by using a CO removal catalyst 3 such as a CO selective oxidation catalyst provided in the subsequent stage.
[0011]
The CO shift catalyst 2 reduces the CO concentration in the hydrogen-containing gas produced by the reformer 1 to about 3000 ppm at the catalyst outlet. There are two types of CO conversion catalyst 2, an HTS catalyst that undergoes a shift reaction at a high temperature of about 450 ° C. and an LTS catalyst that undergoes a shift reaction at a low temperature of about 200 ° C. The CO conversion catalyst of the present invention usually has a temperature of 250 ° C. or less. The reaction is performed and is suitable as an LTS catalyst. The CO shift catalyst 2 performs a shift reaction for generating carbon dioxide and hydrogen from CO and water vapor in the gas according to the following equation (1).
[0012]
[Chemical 1]

[0013]
On the other hand, among the operation methods of the fuel cell system, the DSS operation usually refers to an operation method of starting and stopping once a day, but is not particularly limited to starting and stopping once a day. An operation for starting and stopping or a method for appropriately starting and stopping is also included, and the CO conversion catalyst of the present invention is suitable for long-term use even in such an operation method.
[0014]
Next, the CO conversion catalyst of the present invention will be described in detail.
In the CO conversion catalyst of the present invention, Cu is supported as an active metal on a support made of alumina (Al ₂ O ₃ ) or boehmite (AlO (OH)). Alumina or boehmite is a carrier capable of adsorbing water, and in combination with Cu, which is an active metal, converts CO mainly into CO ₂ and H ₂ . The type of alumina is not limited, and any shape such as α-, β-, and γ- can be used. Of these, γ-type alumina is particularly preferable. The content of alumina or boehmite as a carrier is usually 50% by weight or more, preferably 60% by weight or more and 90% by weight or less, and particularly preferably 70% by weight or more and 80% by weight or less. This carrier has a high specific surface area, and usually has a specific surface area of 100 m ² / g or more and 500 m ² / g or less, preferably 150 m ² / g or more and 400 m ² / g or less.
[0015]
Copper (Cu), which is an active metal, is dispersed and supported near the surface of a carrier such as alumina. In order for the CO shift reaction to proceed efficiently, a state where there are many active sites due to a carrier such as alumina that adsorbs water and Cu that attracts CO is preferable. Since alumina or the like used in the present invention is a porous carrier having a high specific surface area, many active sites can be formed by dispersing and supporting Cu on the surface. Thereby, even if it is a support | carrier support-type catalyst, CO shift reaction can be advanced with high efficiency.
The content of Cu as a Cu component (CuO before reduction) is usually 40% by weight or less, preferably 35% by weight or less, 5% by weight or more, more preferably 30% by weight or less, 10% by weight or more, particularly preferably 30% by weight. % Or less and 20% by weight or more. If such a Cu component is present in a large amount, for example, as a particulate simple substance, an oxidation reaction is easily caused by oxygen. However, in the properties supported on the carrier as in the present invention, it is easy even if oxygen flows in. Does not cause an oxidation reaction and can maintain catalytic activity as an active metal.
[0016]
In the present invention, as the support, in addition to the case of alumina alone, it is also possible to use a support obtained by adding other oxides to alumina. Al ₂ O ₃ .ZrO ₂ , Al ₂ O ₃ .SiO ₂ or Al A composite oxide such as ₂ O ₃ .TiO ₂ is preferably used. Supports made of these composite oxides also have a high specific surface area like alumina, and can use any of the support components to perform a highly efficient CO shift reaction and have durability such as oxygen resistance. . In addition, it is also possible to mix the composite oxide and alumina or boehmite to form a carrier.
In addition to Cu alone, the active metal may contain Cu and another metal component Me, specifically Zn, Co, Ni, Fe, or Cr. The amount ratio of Cu and Me is not particularly limited and can be arbitrarily set. For example, Cu: Me = 1: 0.1 to 1 can be used.
[0017]
Next, the manufacturing method of the CO conversion catalyst of this invention is demonstrated.
In the production method of the present invention, Cu, which is an active metal, is supported on a support containing alumina or boehmite by an impregnation method. Specifically, for example, a Cu aqueous solution is impregnated and supported on a carrier while the Cu aqueous solution is dropped and stirred. Next, after adding a certain amount of Cu aqueous solution in terms of Cu, water is evaporated by evaporating to dryness at 100 ° C. or higher and 120 ° C. or lower. After evaporation, the catalyst of the present invention is obtained by calcination at 300 ° C. or higher and 800 ° C. or lower. When an active metal component is included in addition to Cu, each mixed aqueous solution containing Cu and any metal of Zn, Co, Ni, Fe, or Cr is added and impregnated and supported. The catalyst obtained after impregnation is subjected to a hydrogen reduction treatment with a hydrogen-containing gas as a catalyst activation treatment before the CO shift reaction.
[0018]
Next, an embodiment when the CO conversion catalyst of the present invention is applied to a fuel cell system will be described.
When the Cu-containing catalyst obtained by the present invention is used in a polymer electrolyte fuel cell (PEFC) system, a form as shown in FIG. 1 can be considered. In this method, hydrogen is produced using the reformer 1 having a reforming catalyst. In the hydrogen production, methane, propane or the like is used as a raw material.
In FIG. 1, when city gas (mainly methane) or LPG (mainly propane) is used as a raw material, the sulfur content is first removed. Next, a steam reforming reaction of the following formula (2) or (3) is caused by the reforming catalyst of the reformer 1 at around about 700 ° C. to obtain a hydrogen-containing gas.
[0019]
[Chemical 2]

[0020]
The obtained hydrogen-containing gas contains about 10% of CO component, and this CO acts as a poisoning substance that inhibits the function of the fuel cell. Therefore, in the CO conversion catalyst 2 of the present invention provided in the latter stage, a CO shift reaction is usually caused at 250 ° C. or lower, preferably 150 ° C. or higher and 230 ° C. or lower to convert CO into carbon dioxide ((1) above) formula).
Carbon monoxide is usually reduced to 3000 ppm or more and 4000 ppm or less from the gas that has passed through the CO conversion catalyst 2, but the raw material gas introduced into the fuel cell body 4 is usually 20 ppm or less, preferably 10 ppm or less. It is necessary to be. Further CO removal is performed by providing the CO removal catalyst 3 in the downstream of the CO modification catalyst 2. In the CO removing device 3, CO is reduced by the following equation (4) or (5).
[0021]
[Chemical 3]

[0022]
The hydrogen-containing gas having a reduced CO concentration is sent to the fuel cell 4 and used for the electrode reaction on the anode electrode side. In the fuel cell 4, hydrogen is changed from H ₂ to 2H ⁺ and 2e ⁻ by the anode electrode catalyst at the anode electrode, H ⁺ diffuses into the electrolyte, and e ⁻ moves along a line connecting the electrodes. On the other hand, H ₂ O is produced from H ⁺ , e ⁻ and oxygen by the cathode electrode catalyst in the cathode electrode. A battery reaction is constituted by combining these reactions, and an electromotive force can be obtained.
[0023]
EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not restrict | limited at all by these Examples.
[0024]
【Example】
Example 1 (Preparation of catalyst)
[Preparation of CO shift catalyst 1]
Aluminum nitrate _{(Al (NO 3) 3 ·} 9H 2 O: Mw375.13) to 37.5g was added to water 1L, dissolved after heating 60 ° C., to obtain a aqueous solution of aluminum nitrate. To this solution, aqueous ammonia was added until pH = 7 to form an aluminum hydroxide precipitate. The precipitate was aged for 1 hour, washed with water, filtered, dried and then calcined at 550 ° C. for 5 hours to obtain γ-Al ₂ O ₃ (carrier 1).
The resulting support 1 (10 g) was impregnated and supported uniformly with a Cu solution while dropping and stirring a copper nitrate (Cu (NO ₃ ) ₂ .6H ₂ O) aqueous solution. After dropping 1 g of Cu aqueous solution in terms of Cu, it was evaporated to dryness at 110 ° C. with stirring, and water was evaporated in 24 hours. The sample obtained after evaporation was calcined in air at 500 ° C. for 5 hours to obtain catalyst 1 (CuO / γ-Al ₂ O ₃ ).
[0025]
[Preparation of CO shift catalysts 2-5]
In the preparation method of the catalyst 1, a titanium chloride hydrochloric acid solution, silica sol, or zirconium oxynitrate is added to an aluminum nitrate aqueous solution, and a mixed aqueous solution of Al and Ti, Al and Si, and Al and Zr (each in an atomic ratio of 80: 20 compositions) were obtained respectively. Ammonia is added to these aqueous solutions in the same manner as for catalyst 1, a precipitate is obtained, dried and fired, and composite oxides Al ₂ O ₃ .TiO ₂ and Al ₂ O ₃ .SiO _{2 are obtained.} And a support of Al ₂ O ₃ .ZrO ₂ was obtained. On these supports, CuO is supported by the same method as that for catalyst 1, catalyst 2 (CuO / Al ₂ O ₃ · TiO ₂ ), catalyst 3 (CuO / Al ₂ O ₃ · SiO ₂ ), and catalyst 4 (CuO / Al ₂ O ₃ .ZrO ₂ ) was obtained.
[0026]
[Preparation of CO-modified catalyst 5]
In the preparation method of the catalyst 1, a catalyst 5 (CuO / AlOOH) was obtained by supporting CuO by the same method as the catalyst 1 except that commercially available boehmite (AlOOH) was used instead of the support 1 as the support. . However, during firing, AlOOH changes to Al ₂ O ₃ .
[0027]
[Preparation of CO-modified catalysts 6-7]
In the preparation method of the catalyst 1, after 2 g (catalyst 6) or 3 g (catalyst 7) of copper nitrate was supported on 10 g of the obtained carrier 1 (γ-Al ₂ O ₃ ) in terms of Cu, A CuO / γ-Al ₂ O ₃ catalyst was prepared in the same manner. This catalyst is referred to as catalyst 6 and catalyst 7.
[0028]
[Preparation of CO-modified catalysts 8-12]
In the preparation method of the catalyst 1, 1 g of copper nitrate is converted into Cu on 10 g of the obtained support 1 (γ-Al ₂ O ₃ ), and zinc nitrate, cobalt nitrate, nickel nitrate, iron nitrate, or chromium nitrate is added. Then, 0.3 g in terms of Zn, 0.3 g in terms of Co, 0.3 g in terms of Ni, 0.3 g in terms of Fe, and 0.3 g in terms of Cr are dropped and impregnated in the same manner as Catalyst 1 Supported catalyst 8 (CuO.ZnO / γ-Al ₂ O ₃ ), catalyst 9 (CuO.CoO / γ-Al ₂ O ₃ ), catalyst 10 (CuO.NiO / γ-Al ₂ O ₃ ), catalyst 11 (CuO.FeO / γ-Al ₂ O ₃ ) and catalyst 12 (CuO.Cr ₂ O ₃ / γ-Al ₂ O ₃ ) were obtained.
[0029]
[Preparation of Comparative Catalyst 1]
As a comparative catalyst, a catalyst prepared by a conventional precipitation method is described below.
Copper nitrate (Cu (NO ₃ ) ₃ · 6H ₂ O), zinc nitrate (Zn (NO ₃ ) ₂ · 6H ₂ O) and aluminum nitrate (Al (NO ₃ ) ₃ · 9H ₂ O) are Cu: A mixed aqueous solution was prepared so as to have a ratio of Zn: Al = 100: 100: 20. Sodium carbonate was added dropwise to the mixed aqueous solution until pH = 7 to obtain a composite hydroxide precipitate. The composite hydroxide precipitate was washed with water, filtered, dried at 110 ° C., and calcined at 300 ° C. for 5 hours to obtain Comparative Catalyst 1.
[0030]
[Evaluation of catalyst properties]
As physical property evaluation of the catalysts 1 to 12 and the comparative catalyst 1, specific surface area measurement measured by a one-point N ₂ adsorption method was performed. Table 1 shows the specific surface area measurement results together with the catalyst list.
[0031]
Example 2 (Evaluation and test of catalyst)
[Evaluation of catalyst activity]
Using the catalysts 1 to 12 and the comparative catalyst 1, a CO modification reaction test was performed under the following test conditions. The catalyst was pressure sized to 1 mm, and then 5 cc was packed into a fixed bed flow type microreactor. As a catalyst activation treatment, a 1% H ₂ / N ₂ balance gas is supplied to the catalyst under the conditions of 200 ° C. and GHSV 1000 h ⁻¹ to reduce the active metal Cu to a metallic state. While the hydrogen concentration at the catalyst outlet was monitored, the reduction treatment was terminated when the concentration became similar to the inlet hydrogen concentration.
After heating to a predetermined temperature, a reaction gas was supplied and the amount of CO decrease was monitored by a CO meter (ND-IR method).
(Activity evaluation conditions) Gas composition: CO 0.3%, CO ₂ 20%, H ₂ O 30%, H ₂ remaining 49.7%
Contact time (GHSV): 3000 h ⁻¹ , temperature 170 ° C., 200 ° C.
Table 1 shows the CO conversion rate as an initial activity evaluation. The CO conversion rate is defined by the following formula.
CO conversion rate = ((inlet CO concentration−outlet CO concentration) / inlet CO concentration) × 100
[0032]
[Forced deterioration test]
As a forced deterioration test of the catalyst, after the activity evaluation, air was supplied to the catalyst at 200 ° C. for 10 minutes in a state of GHSV1000h- ¹ . At this time, although heat generation of the catalyst was observed, the maximum heat generation temperature was measured. Furthermore, after air exposure, the activity was evaluated under the above-described activity evaluation condition (170 ° C.), and the deterioration behavior of the catalyst was observed.
Table 1 shows the maximum exothermic temperature due to air exposure and the CO conversion at 170 ° C. after forced deterioration.
[0033]
[Table 1]

[0034]
From the above results, it can be seen that the catalysts 1 to 12 of the present invention show an initial CO shift activity comparable to that of the comparative catalyst 1, and further, there is less increase in heat generation due to air exposure compared to the comparative catalyst 1, which is accompanied by this. It was revealed that the decrease in activity was small. The reason for this is that the catalyst of the present invention has a high specific surface area carrier in which Cu, an active metal, is highly dispersed and supported, so even during oxygen leakage, the reaction between oxygen and Cu is slow, so that the temperature rise is suppressed. Because.
In addition, since the catalyst of the present invention has the above-described physical properties, it is excellent in heat resistance because sintering can be suppressed at high temperatures, and also in water resistance because it suppresses steam oxidation in the presence of moisture. From the above, it has been found that the catalyst of the present invention is a long-life catalyst that is stronger in oxidation resistance, heat resistance, and water resistance than existing precipitation catalysts.
[0035]
【The invention's effect】
The catalyst of the present invention has heat resistance that does not cause catalyst deterioration even when the system becomes high temperature, and has oxygen resistance that does not easily cause an exothermic reaction even when oxygen is mixed into the system due to reduced pressure. . Further, among the catalyst components, alumina having a low water absorption is generally used as a carrier, so that there is no problem of sintering and water resistance is excellent. On the other hand, the original CO shift reaction is also excellent in low-temperature activity, can perform CO shift with high efficiency, and can greatly reduce the CO concentration.
Therefore, according to the present invention, it is possible to provide a CO shift catalyst that does not depend on the temperature condition or operating condition of the fuel cell system and does not cause a decrease in performance due to catalyst deterioration. If the CO conversion catalyst of the present invention is used, it is possible to effectively prevent the deterioration of the catalyst, particularly in DSS operation, and the catalyst can be used for a long time.
Furthermore, unlike the conventional coprecipitation method that requires a complicated process when preparing a complex oxide of Cu and Zn, it can be manufactured by a simple process by an impregnation method using a carrier having a high specific surface area.
[Brief description of the drawings]
FIG. 1 is a block diagram schematically showing a schematic configuration of a fuel cell system.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Reforming catalyst 2 CO conversion catalyst 3 CO removal catalyst 4 Fuel cell 10 Fuel cell system

Claims (7)

A CO conversion catalyst characterized in that Cu is supported as an active metal on a support made of alumina or boehmite. 2. The CO conversion catalyst according to claim 1, wherein the content of alumina or boehmite serving as the carrier is 50% by weight or more and the content of Cu is 40% by weight or less. The CO conversion catalyst according to claim 1 or 2, wherein the support has a specific surface area of 100 m ² / g or more and 500 m ² / g or less. The carrier is a carrier obtained by adding another oxide to alumina, and at least one selected from the group consisting of Al ₂ O ₃ .ZrO ₂ , Al ₂ O ₃ .SiO ₂ and Al ₂ O ₃ .TiO _2. The CO conversion catalyst according to any one of claims 1 to 3, wherein the catalyst is an alumina-based composite oxide. The active metal contains Cu and at least one metal selected from the group consisting of Zn, Co, Ni, Fe and Cr. The CO conversion catalyst according to Item. A method for producing a CO conversion catalyst, comprising impregnating and supporting Cu as an active metal on a support made of alumina or boehmite, followed by drying and firing. The method for producing a CO conversion catalyst according to claim 6, wherein after the calcination, a reduction treatment with a hydrogen-containing gas is performed as an activation treatment of the catalyst.

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