JP3666588B2 - Method for producing hydrogen using multilayer thin film photocatalyst - Google Patents
Method for producing hydrogen using multilayer thin film photocatalyst Download PDFInfo
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- JP3666588B2 JP3666588B2 JP2001355461A JP2001355461A JP3666588B2 JP 3666588 B2 JP3666588 B2 JP 3666588B2 JP 2001355461 A JP2001355461 A JP 2001355461A JP 2001355461 A JP2001355461 A JP 2001355461A JP 3666588 B2 JP3666588 B2 JP 3666588B2
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- thin film
- hydrogen
- compound semiconductor
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- photocatalyst
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- 239000010409 thin film Substances 0.000 title claims description 83
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 title claims description 66
- 239000011941 photocatalyst Substances 0.000 title claims description 58
- 239000001257 hydrogen Substances 0.000 title claims description 56
- 229910052739 hydrogen Inorganic materials 0.000 title claims description 56
- 238000004519 manufacturing process Methods 0.000 title claims description 33
- 239000004065 semiconductor Substances 0.000 claims description 60
- 150000001875 compounds Chemical class 0.000 claims description 52
- RWSOTUBLDIXVET-UHFFFAOYSA-N Dihydrogen sulfide Chemical compound S RWSOTUBLDIXVET-UHFFFAOYSA-N 0.000 claims description 44
- 229910000037 hydrogen sulfide Inorganic materials 0.000 claims description 44
- 229910052980 cadmium sulfide Inorganic materials 0.000 claims description 37
- 229910052984 zinc sulfide Inorganic materials 0.000 claims description 37
- 238000004070 electrodeposition Methods 0.000 claims description 24
- 239000000463 material Substances 0.000 claims description 15
- 239000013078 crystal Substances 0.000 claims description 12
- 239000013626 chemical specie Substances 0.000 claims description 8
- 239000002994 raw material Substances 0.000 claims description 7
- 230000001678 irradiating effect Effects 0.000 claims description 4
- 239000005083 Zinc sulfide Substances 0.000 claims description 3
- DRDVZXDWVBGGMH-UHFFFAOYSA-N zinc;sulfide Chemical compound [S-2].[Zn+2] DRDVZXDWVBGGMH-UHFFFAOYSA-N 0.000 claims description 3
- WUPHOULIZUERAE-UHFFFAOYSA-N 3-(oxolan-2-yl)propanoic acid Chemical compound OC(=O)CCC1CCCO1 WUPHOULIZUERAE-UHFFFAOYSA-N 0.000 claims description 2
- 239000010410 layer Substances 0.000 description 62
- 238000000034 method Methods 0.000 description 52
- 239000000758 substrate Substances 0.000 description 37
- 239000000243 solution Substances 0.000 description 31
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Chemical compound [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 description 20
- 229910052717 sulfur Inorganic materials 0.000 description 19
- 239000011593 sulfur Substances 0.000 description 17
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 16
- 239000000126 substance Substances 0.000 description 16
- HEMHJVSKTPXQMS-UHFFFAOYSA-M Sodium hydroxide Chemical compound [OH-].[Na+] HEMHJVSKTPXQMS-UHFFFAOYSA-M 0.000 description 15
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- 238000006243 chemical reaction Methods 0.000 description 12
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- 239000012670 alkaline solution Substances 0.000 description 7
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- 238000000151 deposition Methods 0.000 description 7
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- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 6
- 239000010779 crude oil Substances 0.000 description 6
- 239000010408 film Substances 0.000 description 6
- 229910052719 titanium Inorganic materials 0.000 description 6
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- 238000002474 experimental method Methods 0.000 description 5
- 229910052757 nitrogen Inorganic materials 0.000 description 5
- 238000002360 preparation method Methods 0.000 description 5
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- 238000011084 recovery Methods 0.000 description 5
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 4
- LSNNMFCWUKXFEE-UHFFFAOYSA-N Sulfurous acid Chemical compound OS(O)=O LSNNMFCWUKXFEE-UHFFFAOYSA-N 0.000 description 4
- 238000002441 X-ray diffraction Methods 0.000 description 4
- 238000002149 energy-dispersive X-ray emission spectroscopy Methods 0.000 description 4
- 238000010438 heat treatment Methods 0.000 description 4
- -1 hydrogen sulfide Chemical class 0.000 description 4
- 150000002500 ions Chemical class 0.000 description 4
- 239000011734 sodium Substances 0.000 description 4
- 229910052979 sodium sulfide Inorganic materials 0.000 description 4
- GRVFOGOEDUUMBP-UHFFFAOYSA-N sodium sulfide (anhydrous) Chemical compound [Na+].[Na+].[S-2] GRVFOGOEDUUMBP-UHFFFAOYSA-N 0.000 description 4
- 229910021607 Silver chloride Inorganic materials 0.000 description 3
- 230000002776 aggregation Effects 0.000 description 3
- 238000004220 aggregation Methods 0.000 description 3
- 238000012790 confirmation Methods 0.000 description 3
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- 150000002431 hydrogen Chemical class 0.000 description 3
- 238000013032 photocatalytic reaction Methods 0.000 description 3
- 239000005077 polysulfide Substances 0.000 description 3
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- 150000008117 polysulfides Polymers 0.000 description 3
- 238000004064 recycling Methods 0.000 description 3
- HKZLPVFGJNLROG-UHFFFAOYSA-M silver monochloride Chemical compound [Cl-].[Ag+] HKZLPVFGJNLROG-UHFFFAOYSA-M 0.000 description 3
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 2
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 2
- RAHZWNYVWXNFOC-UHFFFAOYSA-N Sulphur dioxide Chemical compound O=S=O RAHZWNYVWXNFOC-UHFFFAOYSA-N 0.000 description 2
- 230000008901 benefit Effects 0.000 description 2
- 229910002092 carbon dioxide Inorganic materials 0.000 description 2
- 239000001569 carbon dioxide Substances 0.000 description 2
- 238000004140 cleaning Methods 0.000 description 2
- 238000007796 conventional method Methods 0.000 description 2
- 238000007323 disproportionation reaction Methods 0.000 description 2
- 238000010494 dissociation reaction Methods 0.000 description 2
- 230000005593 dissociations Effects 0.000 description 2
- 238000009713 electroplating Methods 0.000 description 2
- 238000000921 elemental analysis Methods 0.000 description 2
- 230000007613 environmental effect Effects 0.000 description 2
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- 239000011521 glass Substances 0.000 description 2
- XLYOFNOQVPJJNP-ZSJDYOACSA-N heavy water Substances [2H]O[2H] XLYOFNOQVPJJNP-ZSJDYOACSA-N 0.000 description 2
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 2
- 239000002245 particle Substances 0.000 description 2
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- 229910052950 sphalerite Inorganic materials 0.000 description 2
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- 229910052724 xenon Inorganic materials 0.000 description 2
- FHNFHKCVQCLJFQ-UHFFFAOYSA-N xenon atom Chemical compound [Xe] FHNFHKCVQCLJFQ-UHFFFAOYSA-N 0.000 description 2
- 229910052725 zinc Inorganic materials 0.000 description 2
- 239000011701 zinc Substances 0.000 description 2
- 229910000368 zinc sulfate Inorganic materials 0.000 description 2
- 239000011686 zinc sulphate Substances 0.000 description 2
- 241000894006 Bacteria Species 0.000 description 1
- YKYOUMDCQGMQQO-UHFFFAOYSA-L Cadmium chloride Inorganic materials Cl[Cd]Cl YKYOUMDCQGMQQO-UHFFFAOYSA-L 0.000 description 1
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- 239000004215 Carbon black (E152) Substances 0.000 description 1
- UGFAIRIUMAVXCW-UHFFFAOYSA-N Carbon monoxide Chemical compound [O+]#[C-] UGFAIRIUMAVXCW-UHFFFAOYSA-N 0.000 description 1
- 239000007832 Na2SO4 Substances 0.000 description 1
- 229910019891 RuCl3 Inorganic materials 0.000 description 1
- 238000003723 Smelting Methods 0.000 description 1
- PMZURENOXWZQFD-UHFFFAOYSA-L Sodium Sulfate Chemical compound [Na+].[Na+].[O-]S([O-])(=O)=O PMZURENOXWZQFD-UHFFFAOYSA-L 0.000 description 1
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 1
- DUDJJJCZFBPZKW-UHFFFAOYSA-N [Ru]=S Chemical compound [Ru]=S DUDJJJCZFBPZKW-UHFFFAOYSA-N 0.000 description 1
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- 239000000809 air pollutant Substances 0.000 description 1
- 231100001243 air pollutant Toxicity 0.000 description 1
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- 229910052793 cadmium Inorganic materials 0.000 description 1
- FRLJSGOEGLARCA-UHFFFAOYSA-N cadmium sulfide Chemical compound [S-2].[Cd+2] FRLJSGOEGLARCA-UHFFFAOYSA-N 0.000 description 1
- 229910052799 carbon Inorganic materials 0.000 description 1
- 229910002091 carbon monoxide Inorganic materials 0.000 description 1
- 239000003054 catalyst Substances 0.000 description 1
- 230000003197 catalytic effect Effects 0.000 description 1
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- 229930195733 hydrocarbon Natural products 0.000 description 1
- 150000002430 hydrocarbons Chemical class 0.000 description 1
- 238000005984 hydrogenation reaction Methods 0.000 description 1
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- 229910052938 sodium sulfate Inorganic materials 0.000 description 1
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Images
Classifications
-
- 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
- Y02E60/36—Hydrogen production from non-carbon containing sources, e.g. by water electrolysis
Description
【０００１】
【発明の属する技術分野】
本発明は、水素や硫黄などの化学物質を生成する化学工業分野、脱硫工程などで発生した硫化水素などを処理する化学工業分野、および悪臭物質や大気汚染物質を除去する環境保全分野などで利用可能な多層薄膜状光触媒の作製方法、およびその多層薄膜状光触媒を用いた水素の製造方法に関する。
【０００２】
【従来の技術】
光触媒技術の応用は、環境汚染物質や悪臭成分・雑菌などの分解など、様々な化学反応を促進する特性を利用した実用化が始まっている。その例としては、病院の手術室などで利用される抗菌タイル、空気清浄機やエア・コンディショナーのフィルタ、高速道路などの照明灯のガラスなどが挙げられる。これら光触媒の酸化促進能力を利用した実用化の一方で、水などに光触媒を作用させて水素を得ることや、炭酸ガスに作用させて炭素を固定還元することを目的とした研究も行われている。
【０００３】
一方、化石エネルギー資源の枯渇や地球温暖化による大気汚染など環境問題の観点から、クリーンで安全なエネルギーの獲得技術および環境汚染物質の浄化技術の確立が求められている。中でも光触媒の利用は有望であり、例えば、光触媒を石油精製や金属精練の脱硫工程に応用することが考えられる。
【０００４】
現在、一般的に行われている原油の脱硫工程を図４に示す。原油を蒸留する際に、重質ナフサを水素化精製して原油に含まれる硫黄分を全て硫化水素に変化させて回収している。
さらに、ここで発生した硫化水素は、図５に示すように、クラウス法と呼ばれるプロセスを経て、酸化して硫黄を回収している。このクラウス法は、硫化水素の３分の１を酸化して亜硫酸ガスとし、これと残りの硫化水素とを反応させて硫黄とするプロセスである。すなわち、化学式では以下のように表される。
【０００５】
【化１】
２Ｈ２Ｓ ＋ ３Ｏ２ → ２Ｈ２Ｏ ＋ ２ＳＯ２
４Ｈ２Ｓ ＋ ２ＳＯ２ → ４Ｈ２Ｏ＋ ６Ｓ
【０００６】
この硫黄回収工程は、亜硫酸ガスと硫化水素の反応だけでなく、加熱や凝集を繰り返すため、膨大なエネルギーを要している。また、亜硫酸ガスの管理にコストがかかるなどの問題を有している。
【０００７】
図４に示した原油の脱硫工程の重質ナフサの水素化生成には水素ガスが用いられるが、その水素ガス作製方法の１つは図６に示す方法である。
図６に示す水素ガス作製方法は、深冷水素精製法もしくは窒素洗浄法と呼ばれ、水素に富んだガスから水素を生成するもので、前述の炭化水素分解以外の工程から生成する粗ガスにも適用できる。
【０００８】
この作製方法では、原料ガスを圧縮し、水酸化ナトリウムで洗浄して炭酸ガス、硫化水素などをまず除去する。次に、熱交換器で低温の精製水素ガスによって冷却し、メタン及びＣ４以上の炭化水素ガスを液化除去する。ついで、窒素洗浄塔において塔底から塔頂へ上昇させて塔頂から下降する液化窒素によって洗浄することによって、一酸化炭素及び窒素が液状で塔底から排出され、精製分離された水素が塔頂から取り出される。
【０００９】
これらの水素作製方法では、主に硫化水素などの硫黄化合物による触媒の被毒を避けるための精製工程を必要とし、また加熱や凝集を繰り返すために膨大なエネルギーを要している。
【００１０】
【発明が解決しようとする課題】
上述したように、従来の硫化水素処理方法や水素製造方法では、加熱や凝集を繰り返すために膨大なエネルギーを必要としている。したがって、簡便に硫化水素から硫黄と水素を取り出すことができれば、硫化水素の処理と水素の製造とを同時に行うことができ、従来の硫化水素処理方法及び水素製造方法の問題を解決することができる。さらに、取り出した水素を脱硫工程に使用することで、有用なケミカル・リサイクルを実現することができる。
また、太陽光の波長のほぼ全域で作用する光触媒であれば、有効に太陽エネルギーを利用できる。
さらに、基材上に析出させ、基材上への固着性も改善された薄膜状の光触媒であれば、取り扱いや保管においても非常に便利なものとなる。
【００１１】
本発明は、上記従来技術の問題点に対処してなされたもので、硫化水素から水素を生成することができる化合物半導体による多層薄膜状光触媒の作製方法を提供することを目的とする。
また、本発明は、上記多層薄膜状光触媒を用いて硫化水素から水素を効率よく回収することができる水素の製造方法を提供することを目的とする。
【００１２】
【課題を解決するための手段】
即ち、本発明に係る多層薄膜状光触媒の作製方法は、化合物半導体原料となる化学種を含有してなる溶液に導電性基材を浸漬し、該基材表面に、第１の化合物半導体層および、該第１の化合物半導体層のバンドギャップと異なるバンドギャップを有する第２の化合物半導体層を電析により順次、或いは両半導体の混晶比を傾斜的に変化させて析出・固定化させることを特徴とする。ここで、混晶比を傾斜的に変化させるとは、例えばＺｎｘＣｄ１−ｘＳの混晶比Ｘを１から０まで連続的に変化させることをいう。そのようにして作製した膜は、片側はＺｎＳであり、徐々にＣｄＳが多く混ざった混晶となり、反対側はＣｄＳだけとなる薄膜である。これが即ち、混晶比を傾斜的に変化させて析出・固定化させた膜である。この作製方法は、電析法または電着法と呼ばれ、水溶液中の金属（錯）イオンを、カソード還元により導電性基材上に金属薄膜として析出させるいわゆる電気メッキの方法である。この方法は、電圧、溶液温度などによって析出を制御しやすく、基材上に析出物を薄膜状に析出させ、かつ固着性が良く、さらに保管や携帯性にも優れたものである。
【００１３】
ここで、該多層薄膜状光触媒の作製方法において、第２の化合物半導体層のバンドギャップが１．０〜２．５ｅＶであることが望ましい。さらに、該多層薄膜状光触媒の作製方法において、該化合物半導体はII−VI族化合物半導体或いは金属酸化物半導体であることが好ましい。またさらに、該多層薄膜状光触媒の作製方法は、該化合物半導体が、硫化亜鉛、硫化カドミウム、または硫化ルテニウムから選ばれることが望ましい。
【００１４】
本発明では、光触媒を基材上に多層かつ薄膜状に形成する。薄膜状に形成することによって、粒子状よりもハンドリング性に優れ、より広範囲な実用化が可能な光触媒とすることができる。また、薄膜状であるため少量で広い面積を覆う結果、触媒効果の大きい光触媒となりうる。
また、化合物半導体薄膜を多層状態に形成することによって、バンドの勾配が生じる結果、光照射で流れる電子が増大すると共に、太陽光をより多く吸収することができる。
【００１５】
また、本発明の薄膜状光触媒は、従来例に見られたような粒子状の光触媒のように溶液中に分散せず、電析によって基材上に固定化されるために、光の照射角を適切な角度に設定することで照射光を受けてその光エネルギーを変換する効率を向上させることができる。
【００１６】
ここで電析法について図２を用いて説明する。図２は、本発明による半導体化合物の多層薄膜状光触媒を作製する装置を示す模式図であり、電析法によって多層薄膜光触媒を製作する方法の１つを示している。符号を説明すると、１は多層薄膜状光触媒作製装置、２は対向電極、３は参照電極、４は導電性基材、５は溶液、６は磁気撹拌子、７は直流電源、８は陽極、そして９は陰極である。
前駆体となる化学種を含む溶液５に、導電性基材４、参照電極３および対向電極２を浸し、導電性基材４の表面に多層からなる化合物半導体層を析出・固定化させる。
【００１７】
この手法は図２に見るように複雑な装置を必要とせず、比較的低温条件での薄膜作製が可能である。また、溶液組成や印加する電圧を制御することにより所望の種類および膜厚を有する薄膜を得ることができ、基材に対する薄膜の電気化学的吸着という強い結合が期待できる。
ＺｎＳの薄膜作製は、化学種としてＮａ２Ｓ２Ｏ３を０．０１Ｍと、ＺｎＳＯ４を０．１Ｍ用い、参照電極としてＡｇ／ＡｇＣｌ電極を用い、そして陰極にチタンによる基材を用い７０℃において参照電極に対して−０．９Ｖで印加、そして対向電極（陽極）の電圧は参照電圧に固定した。対向電極には白金を用いた。
【００１８】
この電析法は、電気メッキと同じ原理で陽極に電圧を印加して基材上に選択的に析出を起こさせるので、他の薄膜析出法と比べて以下のような利点がある。
即ち、装置が簡単であること、蒸着法などに比べて、低温（室温近く）で析出させることができること、低コストで大きな面積に対しても同じように適用できること、および用途に適合した様々な基材に析出させることができることなどである。また化学的プロセスに比べて、半導体薄膜が導電性基材上に直接生成するため、基材への固着性に優れるという利点がある。
【００１９】
本発明の多層薄膜状光触媒において、基材としてはチタン、シリコン、ガラス、ニッケル、亜鉛、白金、樹脂などからなる基材が考えられる。このうち金属が望ましく、特に白金が望ましい。
しかし、上記光触媒の材料を有効に析出して固定化できるものであれば他の基材でも可能である。
【００２０】
薄膜の膜厚は、第１の化合物半導体層および第２の化合物半導体層を合わせたものが１００〜３００ｎｍであることが好ましい。
例えばＣｄＳ／ＺｎＳのように、基材上に異なる化合物半導体を多層状態で析出固定させるには、異なる材料を用いて上述した電析処理を順次、複数回行えばよい。
【００２１】
薄膜の構成成分を変化させるには、例えば、基材上に化合物半導体を析出させるための該化学種を含んだ溶媒およびその濃度、あるいは印加すべき電圧を変化させることによって可能である。これらを少しずつ変化させることにより両半導体の混晶比を傾斜的に変化させてもよい。
また、該溶媒の濃度、温度、印加する電圧および時間を変化させることによってその厚さを制御しうる。
【００２２】
請求項１に記載の多層薄膜状光触媒を用いた水素の製造方法は、化合物半導体原料となる化学種を含有してなる溶液に導電性基材を浸漬し、該基材表面に、第１の化合物半導体層および、該第１の化合物半導体層のバンドギャップと異なるバンドギャップを有する第２の化合物半導体層を電析により順次、或いは両半導体の混晶比を傾斜的に変化させて析出・固定化させた多層薄膜状光触媒を、硫化水素を含有する溶液中に浸漬し、光を照射することにより水素を製造することよりなり、
前記第１の化合物半導体および前記第２の化合物半導体は、硫化亜鉛および硫化カドミウムの組み合わせであることを特徴とする。
【００２３】
上記の如き多層薄膜状光触媒は、特に硫化水素を含有するアルカリ性溶液に浸漬することによって、高活性な光触媒の酸化還元作用により硫化水素から水素を製造することができる。ここで該アルカリ性溶液はそのｐＨが８〜１３であることが望ましい。
【００２４】
この多層薄膜状光触媒による水素の製造方法は、（１）アルカリ性の溶液に硫化水素を溶解する工程、（２）硫化水素の溶液に該多層薄膜状の光触媒を浸漬し、可視光・紫外線等の光を照射して水素ガスを回収する工程、（３）該水素ガス回収後の溶液から硫黄を回収し、硫黄回収後の溶液を硫化水素を溶解する溶液として（１）の工程に再循環する工程とで構成することができる。各工程の反応式は次式で表される。
【００２５】
【化２】
（１）Ｈ２Ｓ → Ｈ＋ ＋ ＨＳ−
（２）２ＨＳ− → Ｈ２ ＋ Ｓ２ ２−
（３）Ｓ２ ２− → Ｓ２− ＋ Ｓ
【００２６】
表１に示すように（後に詳述）、本発明による多層薄膜状光触媒を水溶液に浸漬して発生する水素発生効率は、単層式化合物半導体による光触媒を用いた場合に比較して、はるかに高い。その理由は、図３の（１）および（２）に示すように、多層式化合物半導体の場合は、基材と反対側の最表面の化合物半導体層（ＣｄＳ層）の電子１７（価電子帯にある電子）は光の照射によって励起し、ＣｄＳ層の伝導帯に入り（電子１８）、そしてバンドギャップの勾配に沿って移動してＺｎＳ層に入り、接している導電性基材１０に入り、その電子が水素イオンと結合しやすく、水素を発生させやすいためと考えられる。
【００２７】
ここでバンドギャップの勾配が生じる理由は、第１層および第２層のバンドギャップが異なるからである。
上記第２層は太陽光を良く吸収する物質で、例えばバンドギャップが１．０〜２．５ｅＶ、好ましくは１．５〜２．５ｅＶ、波長に換算すると９００〜６００ｎｍのものが好ましく、第１層目はそれと異なったバンドギャップを有して、好ましくは第２層よりもバンドギャップが大きいものであり、例えば３ｅＶである化合物半導体層である。
第１層と２層の化合物半導体層を入れ替えたとしても、同様にバンドギャップの違いに起因するバンドギャップの勾配が生じて電子の移動が行われやすくなると考えられるために、良好な光触媒機能を発揮する。
【００２８】
【発明の実施の形態】
以下、本発明にかかる化合物半導体による多層薄膜状光触媒の作製方法およびその多層薄膜状光触媒を用いた水素の作製方法の実施の形態を説明する。
第１の実施の形態
本発明にかかる多層薄膜状光触媒の作製方法は、化合物半導体の原料となる化学種を含む溶液中に基材を浸漬し、電析法により基材表面に化合物半導体を多層状態で析出・固定化させる。
【００２９】
電析法については前述の通りであり、電析法を用いてＺｎＳ層、ＣｄＳ層、ＺｎＳ層およびＣｄＳ層の多層薄膜（以下、ＺｎＳ／ＣｄＳ層と略す）、ＣｄＳ層およびＺｎＳ層の多層薄膜（以下、ＣｄＳ／ＺｎＳ層と略す）、およびＲｕＳ２層を作製した。ここでＺｎＳ／ＣｄＳ層などの表記は、左側が基材側であり、右側が最表面側である。
最初に、電析法によってＺｎＳ単層、およびＣｄＳ単層を作製して、その後、それぞれ異なった側の化合物半導体を続けて電析法により析出固着させて多層となした。
【００３０】
（１）ＺｎＳ層の作製
０．０１ＭのＮａ２Ｓ２Ｏ３と０．１ＭのＺｎＳＯ４を含む水溶液を作製し、温度を７０℃に保ち、Ａｇ／ＡｇＣｌ参照電極側から金属基材側に−０．９Ｖの直流電圧を印加した。対向電極は白金等がよく、金属基材としては白金、チタンなどがよい。
そこにおける反応式は以下の通りである。
【００３１】
【化３】
Ｓ２Ｏ３ ２−＋Ｚｎ２＋＋２ｅ→ＺｎＳ＋ＳＯ３ ２−
【００３２】
以上の手順に従った実験によって得られた基材上には、１時間当たり膜厚約１００ｎｍのＺｎＳ薄膜が電析により形成された。
この薄膜試料に対し、ＸＲＤ（Ｘ線回折）を用いて結晶相の同定を試みたが、回折ピークを生じるほど結晶性の良い薄膜の生成は確認できなかった。そこで次に、ＥＤＸ（エネルギー分散Ｘ線分析）によって、この薄膜の元素分析を行ったところＺｎとＳとが存在していることが確認され、この結果よりＺｎＳ薄膜が析出したことを確認した。また、ＳＥＭ顕微鏡によって調査したが、それによると析出した薄膜は基材の表面を固く覆っており、ほぼ球形分子の形をして多数が分散しながら丸く固まった形をなしていた。
【００３３】
（２）ＣｄＳ層の作製
０．０１ＭのＮａ２Ｓ２Ｏ３と０．１ＭのＣｄＣｌ２を含む水溶液を作製し、温度を７０℃に保ち、参照電極側から金属基材側に−０．７Ｖの直流電圧を印加した。対向電極は白金等がよく、基材としては白金、チタンなどがよい。
そこにおける反応式は以下の通りである。
【００３４】
【化４】
Ｓ２Ｏ３ ２−＋Ｃｄ２＋＋２ｅ→ＣｄＳ＋ＳＯ３ ２−
【００３５】
以上の手順に従った実験によって得られた基材上には１時間当たり膜厚約１００ｎｍのＣｄＳ薄膜が電析により形成された。
この薄膜試料に対し、ＸＲＤ（Ｘ線回折）を用いて結晶相の同定を試み、立方晶或いは六方晶を確認した。次に、ＥＤＸ（エネルギー分散Ｘ線分析）によって、この薄膜の元素分析を行ったところＣｄとＳとが存在していることが確認され、この結果よりＣｄＳ薄膜が析出したことを確認した。また、ＳＥＭ顕微鏡によって調査したが、それによると析出した薄膜は基材の表面を固く覆っており、ほぼ球形分子の形をして多数が分散しながら丸く固まった形をなしていた。
【００３６】
（３）ＺｎＳ／ＣｄＳ層の作製
上記ＺｎＳ層の作製をまず行い、続いてＺｎＳ層が析出した基材を上記ＣｄＳ層の作成装置の中に浸漬して、上述と同様の温度で、同様の電圧を印加して、該ＺｎＳ層の上からＣｄＳ層を固着させて、作製した。
そこにおける反応式は、上述の反応式が引き続いて起こるものである。
また、ＺｎＳ／ＣｄＳ層の析出の確認は、該各層の析出の確認方法で行った。
【００３７】
（４）ＣｄＳ／ＺｎＳ層の作製
上記ＣｄＳ層の作製をまず行い、続いて上記ＺｎＳ層の作成装置の中に該ＣｄＳ層の付着した基材を浸漬して、上記と同様の温度で、同様の電圧を印加して、該ＣｄＳ層の上からＺｎＳ層を固着させて、作製した。
そこにおける反応式は、上記の反応式が引き続いて起こるものである。
また、ＺｎＳ／ＣｄＳ層の析出の確認は、各層の析出の確認方法で行った。
【００３８】
（５）ＲｕＳ２層の作製
０．０４ＭのＮａ２Ｓ２Ｏ３、０．０４ＭのＮａ２ＳＯ４、および０．００８ＭのＲｕＣｌ３を含む水溶液を作製し、温度を２０℃に保ち、Ａｇ／ＡｇＣｌ参照電極側から金属基材側に−１．０Ｖの直流電圧を印加した。対向電極は白金等がよく、基材としては白金、チタンなどがよい。
そこにおける反応式は以下の通りである。
【００３９】
【化５】
２Ｓ２Ｏ３ ２−＋Ｒｕ３＋＋３ｅ−→ＲｕＳ２＋２ＳＯ３ ２−
【００４０】
以上の手順に従った実験によって得られた基材上には、１時間当たり膜厚約１００ｎｍのＲｕＳ２薄膜が電析により析出した。ＲｕＳ２薄膜の同定方法はＺｎＳ薄膜の場合と同様である。
【００４１】
第２の実施の形態（請求項１）
本発明にかかる多層薄膜状光触媒を用いた水素の製造方法は、化合物半導体原料となる化学種を含有してなる溶液に導電性基材を浸漬し、該基材表面に、第１の化合物半導体層および、該第１の化合物半導体層のバンドギャップと異なるバンドギャップを有する第２の化合物半導体層を電析により順次、或いは両半導体の混晶比を傾斜的に変化させて析出・固定化させた多層薄膜状光触媒を、硫化水素を含有する溶液中に浸漬し、光を照射することにより水素を製造することを特徴とする。
【００４２】
本実施の形態では、ＺｎＳ／ＣｄＳ層の化合物半導体薄膜を例とする水素の製造方法について説明する。
図７は、本発明にかかる多層薄膜状光触媒を使って水素を作製するのに用いられる水素製造装置を概略的に示す図である。この処理システムは、硫化水素を溶解する硫化水素溶解槽２１と、硫化水素を溶解した溶液から水素ガスを回収する光触媒反応槽２３と、水素ガス回収後の溶液から硫黄を回収し、硫黄回収後の溶液を硫化水素溶解槽２１へリサイクルする硫黄回収槽２５とで構成されている。
【００４３】
上記構成において、硫化水素溶解槽２１はアルカリ性の溶液を収容し、この中に硫化水素ガスが導入され、溶解される。アルカリ性溶液のｐＨは８〜１３が望ましい。
アルカリ性の溶液は、硫化水素の溶解・解離と反応の場の提供を行うだけで、それ自体の変化は起こらない。ここでは水酸化ナトリウム水溶液を例に挙げて説明するが、硫化水素を溶解・解離させるアルカリ性溶液であれば利用可能である。
【００４４】
硫化水素溶解槽２１内において、水酸化ナトリウム水溶液に硫化水素ガスが混入されると、次式に示すような中和反応が起こり、硫化水素ガスは水酸化ナトリウム水溶液に溶解し、硫化ナトリウム溶液となる。
【００４５】
【化６】
２ＮａＯＨ ＋ Ｈ２Ｓ → Ｎａ２Ｓ ＋ ２Ｈ２Ｏ
【００４６】
硫化ナトリウム溶液は光触媒反応槽２３に送られ、ここでは本発明にかかる多層薄膜状光触媒が浸漬され、光源２７からの白色光および／または紫外光が照射されると、次式に示すように、水素ガスとポリ硫化物イオンが生成し、水素ガスが回収される。
【００４７】
【化７】
２Ｓ２− → Ｓ２ ２− ＋ ２ｅ−
２Ｈ＋ ＋ ２ｅ− → Ｈ２↑
【００４８】
この溶液系にて硫化水素の溶解と水素発生を続けると、ポリ硫化物イオンはその溶解度を越えてしまい、ついには不均化反応（自己酸化還元反応）を起こして、次式に示すように、硫黄と硫化物イオンに変化する。
【００４９】
【化８】
Ｓ２ ２− → Ｓ２− ＋ Ｓ
【００５０】
硫黄回収槽２５では、上記不均化反応を利用してポリ硫化物イオンを硫黄として回収する。硫黄を回収された溶液は水酸化ナトリウム水溶液に戻るため、硫化水素を溶解するための溶液として再利用される。
上記の説明からも明らかなように、本実施の形態においては、多層薄膜状光触媒によって硫化水素を分解することで、クラウス法と同様に硫黄を回収することができ、しかも脱硫工程で消費される水素を製造することができる。さらに、この硫化水素処理方法および水素製造方法では、クラウス法のような加熱及び凝集など多くのエネルギーを必要とする操作を省くことができ、しかも硫化水素よりもさらに危険な亜硫酸ガスが関与しないという点で優れている。
【００５１】
（多層薄膜状光触媒による水素発生）
次に、上記試料について光触媒としての活性を調べるため、図８に示すような装置を用いて、光触媒による水素発生の実験を行った。この装置は、図８に示すように、石英ガラス製の光反応容器３１と、発生した水素ガスの定量を行う水素定量部分３３と、水素ガス発生によって装置内の圧力が上昇することを防ぐための溶液溜３５と、光線照射用の６００Ｗキセノンランプ（〜６００ｍＷｃｍ―２、図示省略）と、光線３７を集光するための集光レンズ３９と、光線３７を光触媒に照射するための反射鏡４１とで構成されている。操作は、初めに系全体を硫化ナトリウム水溶液で満たし、多層薄膜状光触媒を光反応容器３１の底に沈殿させ、ガス抜き栓４３を閉じた後、６００Ｗキセノンランプを容器３１の底部から点灯し、水素定量部分３３で一定照射時間ごとに水素発生量を測定するものである。ここで反射鏡４１は可視〜紫外光用（シグマ光機ＴＦＡＨ１００−１５−０．３）を、集光レンズは石英製のものを使用した。
【００５２】
上記の多層薄膜状光触媒による水溶液からの水素発生実験は、電析法によりチタン基材上に電析により作製した薄膜に、０．１ＭのＮａ２Ｓ水溶液１４０ｍｌ中で光照射したときの薄膜１ｃｍ２当たりの水素発生量を測定した。
試料としては表１に示した実施例１〜３、および比較例１〜３（ＺｎＳ）、比較例４〜６（ＣｄＳ）を用いた。
【００５３】
【表１】
表１において、浸漬時間とは、化合物半導体を電析法によって析出させ基材に固着させるために溶液に浸漬していた時間であり、２種類の場合は、それぞれ１＋１，１＋２などで時間単位で表した。電析法による各薄膜の析出条件は、前述したのと同じである。
水素発生率はフィルタを付けない状態（Ａ）およびフィルタ付き（Ｂ）に分け、フィルタを付けた場合は４００ｎｍ以下の光をカットし、４００ｎｍよりも長波長側の光のみ、即ち可視光側のみを用いた。
右端の水素発生率の減少割合（％）は、｛（Ａ−Ｂ）／Ａ｝x１００である。この数字が小さいことは、エネルギーの弱い光（可視光）であっても水素ガスを有効に発生させることを意味する。
【００５５】
比較例１〜６はいずれも単層のみからなる化合物半導体薄膜であり、その水素発生量は表１から分かるように、多層式の化合物半導体薄膜よりも、相当に低いといえる。一方、実施例１，２から分かるように、異なる化合物半導体からなる２層薄膜光触媒が、高い水素発生効率を示す。
【００５６】
図１は、前述の実施例１、比較例２および比較例５の薄膜を用いて光照射した時の光電流の波長依存性を示したものである。図中、ＡはＣｄＳ／ＺｎＳ（最表面）の２層薄膜（実施例１）、ＢはＣｄＳ単層薄膜（比較例５）、ＣはＺｎＳ単層薄膜（比較例２）の場合である。図から分かるように、ＣｄＳ／ＺｎＳ薄膜では単層薄膜の時と比べて全体的に光電流が顕著に増大しており、特に可視光側での光電流の増大が著しい。光電流が大きいということは、光照射による正孔と電子の分離が良好に行われていることを意味し、水素の発生効率が高いことを示唆するものである。
【００５７】
【発明の効果】
上述したように、本発明によれば、廉価で取り扱いに便利な高活性の化合物半導体からなる多層薄膜状光触媒を提供できる。本発明の多層薄膜状光触媒は、粒子状の光触媒に比べて、溶液中に分散せず、電析によって基材上に固定化されるために、光の照射角を適切な角度に設定することで照射光を受けてその光エネルギーを変換する効率を向上させることができる。
【００５８】
またこの多層薄膜状光触媒は、環境有害物質である硫化水素を原料として、簡単な工程で、亜硫酸ガスのような有害物質を発生することなく、工業全般に有用な硫黄と水素を効率よく安価に生成することができる。
【００５９】
さらに、この高活性光触媒を用いた水素の製造方法を原油などの脱硫工程に適用すれば、硫化水素の処理によって生じた水素を再度脱硫工程に使用することができ、極めて有用なケミカル・リサイクルが可能となる。
また、水素イオンから公害問題を引き起こすことなく水素ガスを製造することができる。
【図面の簡単な説明】
【図１】本発明によるＺｎＳおよびＣｄＳからなる２層式半導体化合物と、ＺｎＳ、ＣｄＳの単層からなる半導体化合物の光電気化学反応の波長依存性を示す図である。
【図２】本発明による半導体化合物の多層薄膜状光触媒を作製する装置を示す模式図である。
【図３】本発明による方法で、導電性基材に固着させた多層式半導体層の水素発生のメカニズムを表す模式図であり、（１）はその模式的断面図、（２）はそれぞれの半導体層に対応する電子のエネルギーバンドを表す模式図である。
【図４】一般的な原油の脱硫工程を概略的に示す図である。
【図５】硫化水素の処理方法の従来技術（クラウス法）を例示する図である。
【図６】水素作製方法の従来技術（深冷水素精製法もしくは窒素洗浄法）を例示する図である。
【図７】本発明の一実施の形態にかかる水素製造装置を概略的に示す図である。
【図８】本発明にかかる化合物半導体の多層薄膜状光触媒の活性を調べるための実験装置を示す図である。
【符号の説明】
１……多層薄膜状光触媒作製装置、２……対向電極、３……参照電極、４……導電性基材、５……溶液、６……磁気撹拌子、７……直流電源、８……陽極、９……陰極、１０……白金電極、１１……ＺｎＳ層、１２……ＣｄＳ層、１３……ＺｎＳ粒子、１４……ＣｄＳ粒子、１７……荷電子帯にある電子、１８……伝導帯に励起した電子、２１……硫化水素溶解槽、２３……光触媒反応槽、２５……硫黄回収槽、２７……光源、３１……光反応容器、３３……水素定量部分、３５……溶液溜、３７……光線、３９……集光レンズ、４１……反射鏡、４３……ガス抜き栓。[0001]
BACKGROUND OF THE INVENTION
The present invention is used in the chemical industry field for producing chemical substances such as hydrogen and sulfur, the chemical industry field for treating hydrogen sulfide generated in the desulfurization process, and the environmental conservation field for removing malodorous substances and air pollutants. The present invention relates to a method for producing a possible multilayer thin film photocatalyst and a method for producing hydrogen using the multilayer thin film photocatalyst.
[0002]
[Prior art]
The application of photocatalytic technology has begun to be put into practical use by utilizing characteristics that promote various chemical reactions, such as decomposition of environmental pollutants, malodorous components and bacteria. Examples include antibacterial tiles used in hospital operating rooms, filters for air purifiers and air conditioners, and glass for lighting on highways. While these photocatalysts have been put into practical use using the ability to promote oxidation, research aimed at obtaining hydrogen by acting a photocatalyst on water, etc., and fixing carbon by acting on carbon dioxide gas has also been conducted. Yes.
[0003]
On the other hand, from the viewpoint of environmental problems such as depletion of fossil energy resources and air pollution due to global warming, establishment of clean and safe energy acquisition technology and purification technology of environmental pollutants is required. Among them, the use of a photocatalyst is promising. For example, it can be considered that the photocatalyst is applied to a petroleum refining process or a metal smelting desulfurization process.
[0004]
FIG. 4 shows a crude oil desulfurization process that is generally performed at present. When crude oil is distilled, heavy naphtha is hydrorefined to convert all sulfur contained in the crude oil into hydrogen sulfide and recover it.
Further, as shown in FIG. 5, the hydrogen sulfide generated here is oxidized to recover sulfur through a process called Claus method. This Claus method is a process in which one third of hydrogen sulfide is oxidized to sulfurous acid gas, and this is reacted with the remaining hydrogen sulfide to form sulfur. That is, the chemical formula is expressed as follows.
[0005]
[Chemical 1]
2H2S + 3O2 → 2H2O + 2SO2
4H2S + 2SO2 → 4H2O + 6S
[0006]
This sulfur recovery process requires enormous energy because it repeats not only the reaction of sulfurous acid gas and hydrogen sulfide but also heating and aggregation. In addition, there is a problem such as costly management of sulfurous acid gas.
[0007]
Hydrogen gas is used for hydrogenation production of heavy naphtha in the crude oil desulfurization process shown in FIG. 4, and one of the hydrogen gas production methods is the method shown in FIG.
The hydrogen gas production method shown in FIG. 6 is called a deep cold hydrogen purification method or a nitrogen cleaning method, which generates hydrogen from a gas rich in hydrogen. Is also applicable.
[0008]
In this manufacturing method, the raw material gas is compressed and washed with sodium hydroxide to first remove carbon dioxide, hydrogen sulfide, and the like. Next, it cools with the low-temperature refinement | purification hydrogen gas with a heat exchanger, and liquefies and removes methane and C4 or more hydrocarbon gas. Next, in the nitrogen scrubber tower, the carbon monoxide and nitrogen are discharged from the tower bottom in a liquid state by being raised from the tower bottom to the tower top and washed with liquefied nitrogen descending from the tower top, and the purified and separated hydrogen is removed from the tower top. Taken from.
[0009]
These hydrogen production methods require a purification step for avoiding poisoning of the catalyst mainly by sulfur compounds such as hydrogen sulfide, and enormous energy is required to repeat heating and aggregation.
[0010]
[Problems to be solved by the invention]
As described above, the conventional hydrogen sulfide treatment method and hydrogen production method require enormous energy in order to repeat heating and aggregation. Therefore, if sulfur and hydrogen can be easily extracted from hydrogen sulfide, the treatment of hydrogen sulfide and the production of hydrogen can be performed at the same time, and the problems of conventional hydrogen sulfide treatment methods and hydrogen production methods can be solved. . Furthermore, useful chemical recycling can be realized by using the extracted hydrogen in the desulfurization process.
In addition, solar energy can be effectively used as long as it is a photocatalyst that operates over almost the entire wavelength of sunlight.
Furthermore, a thin film photocatalyst deposited on a substrate and improved in adhesion to the substrate is very convenient in handling and storage.
[0011]
The present invention has been made in response to the above problems of the prior art, and an object of the present invention is to provide a method for producing a multilayer thin film photocatalyst using a compound semiconductor capable of generating hydrogen from hydrogen sulfide.
It is another object of the present invention to provide a method for producing hydrogen that can efficiently recover hydrogen from hydrogen sulfide using the multilayer thin film photocatalyst.
[0012]
[Means for Solving the Problems]
That is,The present inventionThe method for producing a multilayer thin film photocatalyst according to the present invention includes immersing a conductive substrate in a solution containing a chemical species as a compound semiconductor raw material, the first compound semiconductor layer and the first compound on the surface of the substrate. The second compound semiconductor layer having a band gap different from the band gap of the compound semiconductor layer is deposited and fixed sequentially by electrodeposition or by changing the mixed crystal ratio of both semiconductors in a gradient manner. Here, changing the mixed crystal ratio in an inclined manner means changing the mixed crystal ratio X of ZnxCd1-xS continuously from 1 to 0, for example. The film thus prepared is a thin film in which one side is ZnS, gradually becomes a mixed crystal in which a large amount of CdS is mixed, and the other side is CdS alone. This is a film that is precipitated and fixed by changing the mixed crystal ratio in an inclined manner. This production method is called an electrodeposition method or an electrodeposition method, and is a so-called electroplating method in which metal (complex) ions in an aqueous solution are deposited as a metal thin film on a conductive substrate by cathodic reduction. This method makes it easy to control the deposition depending on the voltage, the solution temperature, etc., deposits the deposit on the substrate in a thin film, has good adhesion, and is excellent in storage and portability.
[0013]
Here, in the method for producing the multilayer thin film photocatalyst, the band gap of the second compound semiconductor layer is preferably 1.0 to 2.5 eV.Yes. TheFurthermore, in the method for producing the multilayer thin film photocatalyst, the compound semiconductor is preferably a II-VI group compound semiconductor or a metal oxide semiconductor.Yes. MaFurthermore, in the method for producing the multilayer thin film photocatalyst, the compound semiconductor is preferably selected from zinc sulfide, cadmium sulfide, or ruthenium sulfide.Yes.
[0014]
In this invention, a photocatalyst is formed in a multilayer and thin film form on a base material. By forming it in a thin film shape, it is possible to obtain a photocatalyst which is superior in handling property than a particulate shape and can be put into practical use in a wider range. Further, since it is a thin film, it covers a large area with a small amount, and as a result, it can be a photocatalyst having a large catalytic effect.
In addition, by forming the compound semiconductor thin film in a multilayer state, a band gradient is generated. As a result, electrons flowing by light irradiation increase and more sunlight can be absorbed.
[0015]
Further, the thin-film photocatalyst of the present invention is not dispersed in a solution like the particulate photocatalyst as seen in the prior art, but is immobilized on the substrate by electrodeposition, so that the light irradiation angle By setting the angle to an appropriate angle, the efficiency of receiving irradiation light and converting the light energy can be improved.
[0016]
Here, the electrodeposition method will be described with reference to FIG. FIG. 2 is a schematic view showing an apparatus for producing a multilayer thin film photocatalyst of a semiconductor compound according to the present invention, and shows one method for producing a multilayer thin film photocatalyst by an electrodeposition method. The
The
[0017]
This method does not require a complicated apparatus as shown in FIG. 2, and a thin film can be produced under relatively low temperature conditions. Further, a thin film having a desired type and film thickness can be obtained by controlling the solution composition and the applied voltage, and strong bonding such as electrochemical adsorption of the thin film to the substrate can be expected.
ZnS thin film is produced by using Na as a chemical species.2S2O30.01M and ZnSO40.1M, an Ag / AgCl electrode as a reference electrode, a titanium base material as a cathode, applied at -0.9 V to the reference electrode at 70 ° C., and the voltage of the counter electrode (anode) is a reference Fixed to voltage. Platinum was used for the counter electrode.
[0018]
This electrodeposition method has the following advantages compared to other thin film deposition methods because a voltage is applied to the anode by the same principle as electroplating to cause selective deposition on the substrate.
That is, the equipment is simple, it can be deposited at a low temperature (near room temperature) compared to the vapor deposition method, etc., it can be applied to a large area at the same cost, and there are various types suitable for use. For example, it can be deposited on a substrate. In addition, since the semiconductor thin film is directly formed on the conductive base material as compared with the chemical process, there is an advantage that the fixing property to the base material is excellent.
[0019]
In the multilayer thin film photocatalyst of the present invention, a substrate made of titanium, silicon, glass, nickel, zinc, platinum, resin, or the like can be considered as the substrate. Of these, metals are desirable, and platinum is particularly desirable.
However, other base materials can be used as long as the photocatalyst material can be effectively deposited and immobilized.
[0020]
The total thickness of the thin film is preferably 100 to 300 nm when the first compound semiconductor layer and the second compound semiconductor layer are combined.
For example, in order to deposit and fix different compound semiconductors in a multilayer state on a base material such as CdS / ZnS, the above-described electrodeposition treatment using different materials may be sequentially performed a plurality of times.
[0021]
The constituent of the thin film can be changed, for example, by changing the solvent containing the chemical species for precipitating the compound semiconductor on the substrate and its concentration, or the voltage to be applied. The mixed crystal ratio of both semiconductors may be changed in an inclined manner by changing them little by little.
The thickness can be controlled by changing the concentration, temperature, applied voltage and time of the solvent.
[0022]
In the method for producing hydrogen using the multilayer thin film photocatalyst according to
The first compound semiconductor and the second compound semiconductor are:Zinc sulfide and cadmium sulfideIt is the combination of these.
[0023]
The multilayer thin film photocatalyst as described above can produce hydrogen from hydrogen sulfide by the oxidation-reduction action of a highly active photocatalyst, particularly by immersing it in an alkaline solution containing hydrogen sulfide. Here, the alkaline solution preferably has a pH of 8 to 13.
[0024]
The method for producing hydrogen using the multilayer thin film photocatalyst is as follows: (1) a step of dissolving hydrogen sulfide in an alkaline solution; (2) immersing the multilayer thin film photocatalyst in a hydrogen sulfide solution; A step of recovering hydrogen gas by irradiating light, (3) recovering sulfur from the solution after recovering the hydrogen gas, and recycling the solution after recovering sulfur to the step (1) as a solution for dissolving hydrogen sulfide And a process. The reaction formula of each process is represented by the following formula.
[0025]
[Chemical 2]
(1) H2S → H+ + HS−
(2) 2HS− → H2 + S2 2-
(3) S2 2- → S2- + S
[0026]
As shown in Table 1 (detailed later), the hydrogen generation efficiency generated by immersing the multilayer thin film photocatalyst according to the present invention in an aqueous solution is much higher than that when a photocatalyst using a single layer compound semiconductor is used. high. The reason for this is that, as shown in FIGS. 3 (1) and (2), in the case of a multilayer compound semiconductor, the electrons 17 (valence band) of the outermost compound semiconductor layer (CdS layer) on the opposite side to the substrate are used. Are excited by light irradiation, enter the conduction band of the CdS layer (electrons 18), move along the band gap gradient, enter the ZnS layer, and enter the
[0027]
The reason why the band gap gradient occurs is that the band gaps of the first layer and the second layer are different.
The second layer is a substance that absorbs sunlight well. For example, the band gap is 1.0 to 2.5 eV, preferably 1.5 to 2.5 eV, and the wavelength is preferably 900 to 600 nm. The layer has a different band gap and preferably has a larger band gap than the second layer, for example, a compound semiconductor layer of 3 eV.
Even if the first compound semiconductor layer and the second compound semiconductor layer are exchanged, it is considered that the band gap gradient is similarly generated due to the difference in the band gap, and the electron transfer is easily performed. Demonstrate.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of a method for producing a multilayer thin film photocatalyst using a compound semiconductor according to the present invention and a method for producing hydrogen using the multilayer thin film photocatalyst will be described below.
First form of implementationstate
BookThe method for producing a multilayer thin film photocatalyst according to the invention involves immersing a substrate in a solution containing chemical species as a raw material for the compound semiconductor, and depositing and immobilizing the compound semiconductor on the substrate surface in a multilayer state by an electrodeposition method. .
[0029]
The electrodeposition method is as described above. Using the electrodeposition method, a multilayer thin film of ZnS layer, CdS layer, ZnS layer and CdS layer (hereinafter abbreviated as ZnS / CdS layer), a multilayer thin film of CdS layer and ZnS layer. (Hereinafter abbreviated as CdS / ZnS layer), and RuS2A layer was made. Here, in the notation such as ZnS / CdS layer, the left side is the substrate side, and the right side is the outermost surface side.
First, a ZnS single layer and a CdS single layer were prepared by an electrodeposition method, and then compound semiconductors on different sides were successively deposited and fixed by an electrodeposition method to form a multilayer.
[0030]
(1) Preparation of ZnS layer
0.01M Na2S2O3And 0.1M ZnSO4An aqueous solution containing was prepared, the temperature was kept at 70 ° C., and a DC voltage of −0.9 V was applied from the Ag / AgCl reference electrode side to the metal substrate side. The counter electrode is preferably platinum or the like, and the metal substrate is preferably platinum or titanium.
The reaction formula there is as follows.
[0031]
[Chemical 3]
S2O3 2-+ Zn2++ 2e → ZnS + SO3 2-
[0032]
A ZnS thin film having a film thickness of about 100 nm per hour was formed on the substrate obtained by the experiment according to the above procedure by electrodeposition.
For this thin film sample, an attempt was made to identify the crystal phase using XRD (X-ray diffraction), but it was not possible to confirm the formation of a thin film with good crystallinity to produce a diffraction peak. Then, when elemental analysis of this thin film was performed by EDX (energy dispersive X-ray analysis), it was confirmed that Zn and S were present, and from this result, it was confirmed that the ZnS thin film was deposited. Moreover, when investigated with the SEM microscope, according to it, the deposited thin film had covered the surface of the base material firmly, and it was the shape of the spherical molecule | numerator, and it was making it the shape which became round and hardened while many were disperse | distributing.
[0033]
(2) Preparation of CdS layer
0.01M Na2S2O3And 0.1M CdCl2The temperature was kept at 70 ° C., and a DC voltage of −0.7 V was applied from the reference electrode side to the metal substrate side. The counter electrode is preferably platinum or the like, and the substrate is preferably platinum or titanium.
The reaction formula there is as follows.
[0034]
[Formula 4]
S2O3 2-+ Cd2++ 2e → CdS + SO3 2-
[0035]
A CdS thin film having a film thickness of about 100 nm per hour was formed on the substrate obtained by the experiment according to the above procedure by electrodeposition.
An attempt was made to identify the crystal phase of this thin film sample using XRD (X-ray diffraction), and cubic or hexagonal crystals were confirmed. Next, when the thin film was subjected to elemental analysis by EDX (energy dispersive X-ray analysis), it was confirmed that Cd and S were present, and from this result, it was confirmed that the CdS thin film was deposited. Moreover, when investigated with the SEM microscope, according to it, the deposited thin film had covered the surface of the base material firmly, and it was the shape of the spherical molecule | numerator, and it was making it the shape which became round and hardened while many were disperse | distributing.
[0036]
(3) Preparation of ZnS / CdS layer
First, the ZnS layer is prepared, and then the base material on which the ZnS layer is deposited is immersed in the CdS layer forming apparatus, and the same voltage is applied at the same temperature as described above, thereby the ZnS layer. A CdS layer was fixed from above.
The reaction formula therein is the one in which the above-described reaction formula follows.
The confirmation of the deposition of the ZnS / CdS layer was performed by the method for confirming the deposition of each layer.
[0037]
(4) Preparation of CdS / ZnS layer
First, the CdS layer is produced, and then the substrate on which the CdS layer is adhered is immersed in the ZnS layer producing apparatus, and the same voltage is applied at the same temperature as described above, so that the CdS layer is produced. A ZnS layer was fixed from above the layer.
The reaction formula there is one that follows the above reaction formula.
Moreover, the confirmation of the precipitation of the ZnS / CdS layer was performed by the confirmation method of the precipitation of each layer.
[0038]
(5) RuS2Layer creation
0.04M Na2S2O30.04M Na2SO4, And 0.008M RuCl3A temperature of 20 ° C. was maintained, and a DC voltage of −1.0 V was applied from the Ag / AgCl reference electrode side to the metal substrate side. The counter electrode is preferably platinum or the like, and the substrate is preferably platinum or titanium.
The reaction formula there is as follows.
[0039]
[Chemical formula 5]
2S2O3 2-+ Ru3++ 3e−→ RuS2+ 2SO3 2-
[0040]
On the substrate obtained by the experiment according to the above procedure, RuS having a film thickness of about 100 nm per hour.2A thin film was deposited by electrodeposition. RuS2The method for identifying the thin film is the same as that for the ZnS thin film.
[0041]
Second embodiment (Claim 1)
In the method for producing hydrogen using the multilayer thin film photocatalyst according to the present invention, a conductive substrate is immersed in a solution containing a chemical species as a compound semiconductor raw material, and the first compound semiconductor is formed on the surface of the substrate. The second compound semiconductor layer having a band gap different from that of the first compound semiconductor layer is deposited and fixed sequentially by electrodeposition or by changing the mixed crystal ratio of both semiconductors in a gradient manner. The multilayer thin film photocatalyst is immersed in a solution containing hydrogen sulfide and irradiated with light to produce hydrogen.
[0042]
In this embodiment, a method for producing hydrogen will be described using a compound semiconductor thin film of a ZnS / CdS layer as an example.
FIG. 7 is a diagram schematically showing a hydrogen production apparatus used for producing hydrogen using the multilayer thin film photocatalyst according to the present invention. This treatment system includes a hydrogen
[0043]
In the above configuration, the hydrogen
Alkaline solutions only provide dissolution and dissociation and dissociation of hydrogen sulfide and a reaction field, and no change in itself occurs. Here, an aqueous sodium hydroxide solution will be described as an example, but any alkaline solution that dissolves and dissociates hydrogen sulfide can be used.
[0044]
In the hydrogen
[0045]
[Chemical 6]
2NaOH + H2S → Na2S + 2H2O
[0046]
The sodium sulfide solution is sent to the
[0047]
[Chemical 7]
2S2- → S2 2- + 2e−
2H+ + 2e− → H2↑
[0048]
If dissolution of hydrogen sulfide and generation of hydrogen continue in this solution system, polysulfide ions will exceed their solubility and eventually cause a disproportionation reaction (auto-redox reaction), as shown in the following equation: , Changes to sulfur and sulfide ions.
[0049]
[Chemical 8]
S2 2- → S2- + S
[0050]
In the
As is clear from the above description, in the present embodiment, sulfur can be recovered by decomposing hydrogen sulfide with a multilayer thin film photocatalyst as in the Claus method, and is consumed in the desulfurization process. Hydrogen can be produced. Furthermore, in this hydrogen sulfide treatment method and hydrogen production method, operations that require a lot of energy such as heating and coagulation as in the Claus method can be omitted, and sulfur dioxide gas that is more dangerous than hydrogen sulfide is not involved. Excellent in terms.
[0051]
(Hydrogen generation by multilayer thin film photocatalyst)
Next, in order to investigate the activity as a photocatalyst for the above sample, an experiment of hydrogen generation by a photocatalyst was performed using an apparatus as shown in FIG. As shown in FIG. 8, this apparatus prevents the pressure in the apparatus from rising due to the generation of hydrogen gas, the
[0052]
The hydrogen generation experiment from the aqueous solution using the multilayer thin film photocatalyst described above was performed on a thin film prepared by electrodeposition on a titanium substrate by an electrodeposition method.2Thin film 1cm when irradiated with light in 140ml of S aqueous solution2The amount of hydrogen generation per unit was measured.
As samples, Examples 1 to 3 shown in Table 1, Comparative Examples 1 to 3 (ZnS), and Comparative Examples 4 to 6 (CdS) were used.
[0053]
[Table 1]
In Table 1, the immersion time is the time during which the compound semiconductor was immersed in the solution in order to deposit it by electrodeposition and fix it to the substrate. In the case of two types, the time is 1 + 1, 1 + 2, etc. expressed. The deposition conditions for each thin film by the electrodeposition method are the same as described above.
Hydrogen generation rate is divided into the state without filter (A) and the case with filter (B). When the filter is attached, light of 400 nm or less is cut, and only light on the longer wavelength side than 400 nm, that is, only visible light side Was used.
The decrease rate (%) of the hydrogen generation rate at the right end is {(A−B) / A} × 100. When this number is small, it means that hydrogen gas is effectively generated even with light having low energy (visible light).
[0055]
Each of Comparative Examples 1 to 6 is a compound semiconductor thin film consisting of only a single layer, and as can be seen from Table 1, it can be said that the hydrogen generation amount is considerably lower than that of the multilayer compound semiconductor thin film. On the other hand, as can be seen from Examples 1 and 2, the two-layer thin film photocatalyst made of different compound semiconductors exhibits high hydrogen generation efficiency.
[0056]
FIG. 1 shows the wavelength dependence of the photocurrent when light is irradiated using the thin films of Example 1, Comparative Example 2, and Comparative Example 5 described above. In the figure, A is a CdS / ZnS (outermost surface) two-layer thin film (Example 1), B is a CdS single-layer thin film (Comparative Example 5), and C is a ZnS single-layer thin film (Comparative Example 2). As can be seen from the figure, in the CdS / ZnS thin film, the photocurrent is remarkably increased as compared with the single-layer thin film as a whole, and the increase in the photocurrent on the visible light side is particularly remarkable. A large photocurrent means that holes and electrons are well separated by light irradiation, and suggests that hydrogen generation efficiency is high.
[0057]
【The invention's effect】
As described above, according to the present invention, a multilayer thin film photocatalyst made of a highly active compound semiconductor that is inexpensive and convenient to handle can be provided. The multilayer thin-film photocatalyst of the present invention is not dispersed in the solution and is fixed on the substrate by electrodeposition as compared with the particulate photocatalyst, so that the light irradiation angle is set to an appropriate angle. Can improve the efficiency of receiving the irradiated light and converting its light energy.
[0058]
In addition, this multilayer thin film photocatalyst is made from hydrogen sulfide, which is an environmentally hazardous substance, as a raw material, and in a simple process, sulfur and hydrogen, which are useful throughout the industry, are efficiently and inexpensively produced without generating harmful substances such as sulfurous acid gas. Can be generated.
[0059]
Furthermore, if this hydrogen production method using a highly active photocatalyst is applied to a desulfurization process for crude oil or the like, the hydrogen generated by the treatment of hydrogen sulfide can be used again in the desulfurization process, and extremely useful chemical recycling can be achieved. It becomes possible.
Further, hydrogen gas can be produced from hydrogen ions without causing pollution problems.
[Brief description of the drawings]
FIG. 1 is a graph showing the wavelength dependence of the photoelectrochemical reaction of a two-layer semiconductor compound comprising ZnS and CdS according to the present invention and a semiconductor compound comprising a single layer of ZnS and CdS.
FIG. 2 is a schematic view showing an apparatus for producing a semiconductor compound multilayer thin film photocatalyst according to the present invention.
FIGS. 3A and 3B are schematic views showing a mechanism of hydrogen generation of a multilayer semiconductor layer fixed to a conductive substrate by the method according to the present invention, wherein FIG. 3A is a schematic cross-sectional view, and FIG. It is a schematic diagram showing the energy band of the electron corresponding to a semiconductor layer.
FIG. 4 is a diagram schematically showing a general crude oil desulfurization process.
FIG. 5 is a diagram illustrating a conventional technique (Klaus method) of a method for treating hydrogen sulfide.
FIG. 6 is a diagram illustrating a conventional technique (deep cold hydrogen purification method or nitrogen cleaning method) of a hydrogen production method.
FIG. 7 is a diagram schematically showing a hydrogen production apparatus according to an embodiment of the present invention.
FIG. 8 is a diagram showing an experimental apparatus for examining the activity of a compound semiconductor multilayer thin film photocatalyst according to the present invention.
[Explanation of symbols]
DESCRIPTION OF
Claims (1)
前記第１の化合物半導体および前記第２の化合物半導体は、硫化亜鉛および硫化カドミウムの組み合わせであることを特徴とする多層薄膜状光触媒を用いた水素の製造方法。A conductive base material is immersed in a solution containing a chemical species as a compound semiconductor raw material, and the first compound semiconductor layer and a band gap different from the band gap of the first compound semiconductor layer are formed on the surface of the base material. A multilayer thin film photocatalyst deposited and immobilized sequentially by electrodeposition or by changing the mixed crystal ratio of both semiconductors in an inclined manner is immersed in a solution containing hydrogen sulfide. Manufacturing hydrogen by irradiating light,
The method for producing hydrogen using a multilayer thin film photocatalyst, wherein the first compound semiconductor and the second compound semiconductor are a combination of zinc sulfide and cadmium sulfide .
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| US6967185B2 (en) * | 2004-01-28 | 2005-11-22 | De Nora Elettrodi S.P.A. | Synthesis of noble metal, sulphide catalysts in a sulfide ion-free aqueous environment |
| WO2010050226A1 (en) | 2008-10-30 | 2010-05-06 | パナソニック株式会社 | Photoelectrochemical cell and energy system using same |
| WO2011016244A1 (en) | 2009-08-05 | 2011-02-10 | パナソニック株式会社 | Photoelectrochemical cell and energy system using same |
| US8821700B2 (en) | 2009-11-10 | 2014-09-02 | Panasonic Corporation | Photoelectrochemical cell and energy system using same |
| CN102484303B (en) * | 2010-03-31 | 2015-12-09 | 松下知识产权经营株式会社 | The energy system of Optical Electro-Chemistry element and this element of use |
| CN104662204B (en) | 2012-10-31 | 2017-05-03 | 松下知识产权经营株式会社 | Photosemiconductor electrode, photoelectrochemical cell, and energy system |
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| CN102164653B (en) * | 2008-09-29 | 2015-05-27 | 开利公司 | Catalytic substrates and methods for creating catalytic coatings for indoor air quality applications |
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