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Photocatalytic Splitting of H2S to Produce Hydrogen by Gas-Solid Phase Reaction
CHINESE JOURNAL OF CATALYSIS Volume 29, Issue 4, April 2008 Online English edition of the Chinese language journal Cite this article as: Chin J Catal, 2008, 29(4): 313–315
SHORT COMMUNICATION
Photocatalytic Splitting of H2S to Produce Hydrogen by Gas-Solid Phase Reaction MA Guijun, YAN Hongjian, ZONG Xu, MA Baojun, JIANG Hongfu, WEN Fuyu, LI Can* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
Abstract: Hydrogen production by gas-solid phase photocatalytic splitting of H2S was investigated on five semiconductor photocatalysts including TiO2, CdS, ZnS, ZnO, and ZnIn2S4. ZnS shows the highest rate of hydrogen production under the identical conditions. Loading Ir on ZnS can effectively promote the hydrogen production. The addition of Cu2+ during the ZnS preparation significantly enhances the photocatalytic activity of the catalyst for hydrogen production. Key words: photocatalysis; hydrogen sulfide splitting; hydrogen; zinc sulfide
Hydrogen sulfide (H2S) is harmful to environment and virulent to human beings. It is abundant in natural gas and petroleum [1–3], and with the increasing consumption of crude oil, the amount of H2S released from petroleum refining has gradually increased. H2S is also contained in most natural gas wells. Up to 2002, the total quantity of sulfur contained in natural gas has reached 68 million tons in China [2]. Industrially, H2S is oxidized to sulfur (S) and water by the Claus process [3], where the hydrogen resource in H2S is oxidized to water. Therefore, there is a need to develop a clean and environmentally benign technology to recycle H2S gas. Photocatalytic splitting of H2S to produce hydrogen can release the hydrogen stored in H2S by utilizing solar energy. The photocatalytic splitting of H2S to produce H2 is usually carried out in aqueous solutions [4,5]. Very few researchers have reported on a gas-solid phase photocatalytic H2 production from H2S [6]. Here we report the photocatalytic production of H2 from H2S under anaerobic conditions by a gas-solid phase reaction. The reaction was performed in a self-made reactor. The activity of five commonly used photocatalysts, TiO2, CdS, ZnS, ZnO, and ZnIn2S4, was investigated for photocatalytic H2S decomposition. It was found that Cu-ZnS showed the highest rate of H2 production. TiO2 (Degussa P25), ZnO, and CdS were used as received. ZnS, Cu-ZnS, and ZnIn2S4 [4] were prepared by the hydrothermal method. Typically, a stoichiometric amount of
Zn(NO3)2·6H2O, Cu(NO3)2·3H2O, In(NO3)3·5H2O, and thiourea were added to a Teflon-lined stainless steel autoclave of 30 ml capacity. The autoclave was filled with double-distilled water up to 80% of its volume and then maintained at 200 °C for 20 h. The precipitate was filtered, washed with copious water, and dried at 80 °C in vacuum for 8 h. An appropriate amount of a noble metal (Ru, Rh, Pd, Pt, Ir, or Au) was loaded on the catalysts by in situ photochemical deposition [4,5]. Fig. 1 shows a schematic diagram of the external side and top views of the photocatalytic reactor. It is a closed fivechannel tabulated reactor using quartz glass as the shutter window and a 300 W Xe lamp illuminator. The single channel
Fig. 1. Schematic diagram of the external side (a) and top (b) views of the photocatalytic reactor.
Received date: 30 January 2008. * Corresponding author. Tel: +86-411-84379070; Fax: +86-411-84694447; E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (20503034, 20673112) and the National Basic Research Program of China (973 Program, 2003CB214504). Copyright © 2008, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
MA Guijun et al. / Chinese Journal of Catalysis, 2008, 29(4): 313–315
Table 1 Activity of different photocatalysts for hydrogen production by gas-solid phase photocatalytic splitting of H2S Photocatalyst Pt/TiO2
H2 production (ȝmol/h) 2.1
Pt/ZnO
1.6
Pt/CdS
< 0.5
Pt/ZnS
3.6
Pt/ZnIn2S4
1.9
Reaction conditions: catalyst, 0.40 g; Pt loading, 0.2%; reaction gas, 5% H2S in Ar; gas flow rate, (6 ± 0.5) ml/min; light source, 300-W Xe lamp.
volume was 7 cm ¯ 1 cm ¯ 0.8 cm. The particulate photocatalyst (0.40 g) was homogeneously dispersed in the reactor channels. The reactor was swept with Ar gas for 20 min to flush out air completely prior to each irradiation. A gas mixture of 5% H2S/Ar was flowed continuously through each of the reactor tunnel at a rate of (6 ± 0.5) ml/min. After reaction, H2S in the mixed gas was eliminated by purging in 2 mol/L NaOH and 0.5 mol/L CuSO4 solutions. The tail gas was gathered by draining the water. The amount of the H2 produced was analyzed using a gas chromatograph (TCD, Ar carrier, 5A molecular sieve). Table 1 lists the activity of the five catalysts for H2 production. CdS has been widely studied as a sulfide photocatalyst and shows a high H2 production rate in Na2S-Na2SO3 aqueous solution [5]. However, almost no H2 was detected with the CdS catalyst in the present gas-solid reaction. This result suggests that the photocatalytic performance of semiconductor catalysts under a gas-solid phase condition is quite different from that under a liquid-solid phase condition. Previous studies [7–12] revealed that in the presence of O2, H2S is oxidized to SO42í and H2O on a TiO2 surface in gas-solid phase photocatalytic reactions. Both photocatalytic H2 production activity and stability for gas-solid phase splitting of H2S were very low on TiO2 under the anaerobic reaction condition in our experiment, which is in agreement with the literature [8]. It can be seen from Table 1 that ZnS exhibited the highest H2 production activity. Therefore, further work was done on the modification of the ZnS photocatalyst. Loading a noble metal on the surface of a photocatalyst can significantly promote the photocatalytic hydrogen production activity [4,5,13,14]. This is because the loaded noble metal can capture the photo-generated electrons on the photocatalyst surface so that after excitation, the electrons are effectively separated from the holes, and the lifetime of the electrons is prolonged. This increases the proportion of electrons that participate in the photocatalytic reduction of protons to produce H2 [13,14]. On the other hand, the hydrogen overpotential of a noble metal is very low. Loading a noble metal on a semiconductor can lower the electrochemical reductive potential (E(H+/H2)), which favors photocatalytic H2 production [14,15].
Fig. 2. Average photocatalytic H2 production from H2S on ZnS loaded with noble metals. Reaction conditions are the same as in Table 1.
Fig. 3. Photocatalytic H2 production from H2S on ZnS, Ir/ZnS (0.2 wt% Ir), and Cu-ZnS (0.5 mol% Cu) catalysts. Reaction conditions are the same as in Table 1.
The rate of photocatalytic hydrogen production is highly dependent on the noble metal loaded on the photocatalyst. Therefore, Ru, Rh, Pd, Pt, Ir, and Au were loaded on ZnS by in situ photochemical deposition to evaluate the effect of the noble metal on the activity for H2 production. It can be seen from Fig. 2 that the loading of noble metals on ZnS enhanced the activity for gas-solid phase photocatalytic H2S splitting. Loading Ir on ZnS increased the rate of H2 production from 1.2 ȝmol/h to 4.5 ȝmol/h. Further modifications of the ZnS photocatalyst included the formation of a solid solution with other sulfides and doping with transitional metals. It is known that ZnS is a photocatalyst that is active only in the UV light region because of its large band gap of 3.2 eV. However, after doping with Cu2+, Ni2+, Pb2+, and Hg2+, ZnS shows H2 production under visible light irradiation [16–18]. We added an appropriate amount of these cations into the feedstock in the hydrothermal synthesis of ZnS and found that Cu-ZnS displayed a significantly enhanced activity for hydrogen production. XPS analysis showed that the copper species existed as Cu2+. Fig. 3 compares the photo-
MA Guijun et al. / Chinese Journal of Catalysis, 2008, 29(4): 313–315
catalytic H2 production over ZnS, Ir/ZnS (with 0.2 wt% Ir), and Cu-ZnS (with 0.5 mol% Cu) catalysts. It can be seen that the total amount of evolved H2 on Cu-ZnS is 80 ȝmol after 150 min, which is 20 times higher than that on ZnS. UV-Vis spectra showed that introducing Cu2+ into ZnS caused a shift of the absorption edge of ZnS from 400 nm to 450 nm. The higher activity of Cu-ZnS was possibly due to an extended light absorption range. This was supported by measuring the activity of ZnS and Cu-ZnS under visible light irradiation (Ȝ > 420 nm). Cu-ZnS showed a photocatalytic H2 production rate of 17 ȝmol/h, whereas no H2 was detected on ZnS. In summary, the activities of five catalysts (TiO2, CdS, ZnS, ZnO, and ZnIn2S4) were evaluated for gas-solid phase photocatalytic splitting of H2S in an anaerobic environment. The photoactivity of these catalysts is quite different in a gas-solid phase from that in a liquid-solid phase. ZnS showed the highest rate of H2 production, and its activity can be further improved by adding Cu2+ into the feedstock during the preparation of ZnS. Gas-solid phase photocatalytic splitting of H2S in an anaerobic environment can reclaim hydrogen energy from H2S by utilizing solar energy. Moreover, in the absence of oxygen, H2S can be selectively oxidized to elemental sulfur (S), which makes it possible to recycle S from H2S while producing H2.
[2] Yan T Zh, Gao L X, Miao M F. Chem Eng Oil Gas, 2002, 31(S1): 17 [3] Gargurevich I A. Ind Eng Chem Res, 2005, 44(20): 7706 [4] Lei Zh B, You W Sh, Liu M Y, Zhou G H, Takata T, Hara M, Domen K, Li C. Chem Commun, 2003, (17): 2142 [5] Bao N, Shen L, Takata T, Domen K, Gupta A, Yanagisawa K, Grimes C A. J Phys Chem C, 2007, 111(47): 17527 [6] Naman S A. Int J Hydrogen Energy, 1992, 17(7): 499 [7] Canela M C, Alberici R M, Jardim W F. J Photochem Photobiol A, 1998, 112(1): 73 [8] Guo J H, Jin Zh Sh, Zhang M, Huang Y, Yang J J, Zhang Zh J. Photogr Sci Photochem, 2004, 22(3): 211 [9] Kato S, Hirano Y, Iwata M, Sano T, Takeuhi K, Matsuzawa S. Appl Catal B, 2005, 57(2): 109 [10] Kataoka S, Lee E, Tejedor-Tejedor M I, Anderson M A. Appl Catal B, 2005, 61(1–2): 159 [11] Sopyan I. Sci Technol Adv Mater, 2007, 8(1–2): 33 [12] Portela R, Sánchez B, Coronado J M, Candal R, Suárez S. Catal Today, 2007, 129(1–2): 223 [13] Chen T, Feng Zh Ch, Wu G P, Shi J Y, Ma G J, Ying P L, Li C. J Phys Chem C, 2007, 111(22): 8005 [14] Wu Y. Catalysis Chemistry. Beijing: Science Press, 2000. 1345 [15] Fu X C, Shen W X, Yao T Y. Physical Chemistry. Beijing: Higher Education Press, 1990. 668
References
[16] Kudo A, Sekizawa M. Catal Lett, 1999, 58(4): 241 [17] Kudo A, Sekizawa M. Chem Commun, 2000, (15): 1371
[1] Wang J M. Petrol Process Petrochem, 1999, 30(5): 1
[18] Tsuji I, Kudo A. J Photochem Photobiol A, 2003, 156(1–3): 249
SHORT COMMUNICATION
Photocatalytic Splitting of H2S to Produce Hydrogen by Gas-Solid Phase Reaction MA Guijun, YAN Hongjian, ZONG Xu, MA Baojun, JIANG Hongfu, WEN Fuyu, LI Can* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
Abstract: Hydrogen production by gas-solid phase photocatalytic splitting of H2S was investigated on five semiconductor photocatalysts including TiO2, CdS, ZnS, ZnO, and ZnIn2S4. ZnS shows the highest rate of hydrogen production under the identical conditions. Loading Ir on ZnS can effectively promote the hydrogen production. The addition of Cu2+ during the ZnS preparation significantly enhances the photocatalytic activity of the catalyst for hydrogen production. Key words: photocatalysis; hydrogen sulfide splitting; hydrogen; zinc sulfide
Hydrogen sulfide (H2S) is harmful to environment and virulent to human beings. It is abundant in natural gas and petroleum [1–3], and with the increasing consumption of crude oil, the amount of H2S released from petroleum refining has gradually increased. H2S is also contained in most natural gas wells. Up to 2002, the total quantity of sulfur contained in natural gas has reached 68 million tons in China [2]. Industrially, H2S is oxidized to sulfur (S) and water by the Claus process [3], where the hydrogen resource in H2S is oxidized to water. Therefore, there is a need to develop a clean and environmentally benign technology to recycle H2S gas. Photocatalytic splitting of H2S to produce hydrogen can release the hydrogen stored in H2S by utilizing solar energy. The photocatalytic splitting of H2S to produce H2 is usually carried out in aqueous solutions [4,5]. Very few researchers have reported on a gas-solid phase photocatalytic H2 production from H2S [6]. Here we report the photocatalytic production of H2 from H2S under anaerobic conditions by a gas-solid phase reaction. The reaction was performed in a self-made reactor. The activity of five commonly used photocatalysts, TiO2, CdS, ZnS, ZnO, and ZnIn2S4, was investigated for photocatalytic H2S decomposition. It was found that Cu-ZnS showed the highest rate of H2 production. TiO2 (Degussa P25), ZnO, and CdS were used as received. ZnS, Cu-ZnS, and ZnIn2S4 [4] were prepared by the hydrothermal method. Typically, a stoichiometric amount of
Zn(NO3)2·6H2O, Cu(NO3)2·3H2O, In(NO3)3·5H2O, and thiourea were added to a Teflon-lined stainless steel autoclave of 30 ml capacity. The autoclave was filled with double-distilled water up to 80% of its volume and then maintained at 200 °C for 20 h. The precipitate was filtered, washed with copious water, and dried at 80 °C in vacuum for 8 h. An appropriate amount of a noble metal (Ru, Rh, Pd, Pt, Ir, or Au) was loaded on the catalysts by in situ photochemical deposition [4,5]. Fig. 1 shows a schematic diagram of the external side and top views of the photocatalytic reactor. It is a closed fivechannel tabulated reactor using quartz glass as the shutter window and a 300 W Xe lamp illuminator. The single channel
Fig. 1. Schematic diagram of the external side (a) and top (b) views of the photocatalytic reactor.
Received date: 30 January 2008. * Corresponding author. Tel: +86-411-84379070; Fax: +86-411-84694447; E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (20503034, 20673112) and the National Basic Research Program of China (973 Program, 2003CB214504). Copyright © 2008, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
MA Guijun et al. / Chinese Journal of Catalysis, 2008, 29(4): 313–315
Table 1 Activity of different photocatalysts for hydrogen production by gas-solid phase photocatalytic splitting of H2S Photocatalyst Pt/TiO2
H2 production (ȝmol/h) 2.1
Pt/ZnO
1.6
Pt/CdS
< 0.5
Pt/ZnS
3.6
Pt/ZnIn2S4
1.9
Reaction conditions: catalyst, 0.40 g; Pt loading, 0.2%; reaction gas, 5% H2S in Ar; gas flow rate, (6 ± 0.5) ml/min; light source, 300-W Xe lamp.
volume was 7 cm ¯ 1 cm ¯ 0.8 cm. The particulate photocatalyst (0.40 g) was homogeneously dispersed in the reactor channels. The reactor was swept with Ar gas for 20 min to flush out air completely prior to each irradiation. A gas mixture of 5% H2S/Ar was flowed continuously through each of the reactor tunnel at a rate of (6 ± 0.5) ml/min. After reaction, H2S in the mixed gas was eliminated by purging in 2 mol/L NaOH and 0.5 mol/L CuSO4 solutions. The tail gas was gathered by draining the water. The amount of the H2 produced was analyzed using a gas chromatograph (TCD, Ar carrier, 5A molecular sieve). Table 1 lists the activity of the five catalysts for H2 production. CdS has been widely studied as a sulfide photocatalyst and shows a high H2 production rate in Na2S-Na2SO3 aqueous solution [5]. However, almost no H2 was detected with the CdS catalyst in the present gas-solid reaction. This result suggests that the photocatalytic performance of semiconductor catalysts under a gas-solid phase condition is quite different from that under a liquid-solid phase condition. Previous studies [7–12] revealed that in the presence of O2, H2S is oxidized to SO42í and H2O on a TiO2 surface in gas-solid phase photocatalytic reactions. Both photocatalytic H2 production activity and stability for gas-solid phase splitting of H2S were very low on TiO2 under the anaerobic reaction condition in our experiment, which is in agreement with the literature [8]. It can be seen from Table 1 that ZnS exhibited the highest H2 production activity. Therefore, further work was done on the modification of the ZnS photocatalyst. Loading a noble metal on the surface of a photocatalyst can significantly promote the photocatalytic hydrogen production activity [4,5,13,14]. This is because the loaded noble metal can capture the photo-generated electrons on the photocatalyst surface so that after excitation, the electrons are effectively separated from the holes, and the lifetime of the electrons is prolonged. This increases the proportion of electrons that participate in the photocatalytic reduction of protons to produce H2 [13,14]. On the other hand, the hydrogen overpotential of a noble metal is very low. Loading a noble metal on a semiconductor can lower the electrochemical reductive potential (E(H+/H2)), which favors photocatalytic H2 production [14,15].
Fig. 2. Average photocatalytic H2 production from H2S on ZnS loaded with noble metals. Reaction conditions are the same as in Table 1.
Fig. 3. Photocatalytic H2 production from H2S on ZnS, Ir/ZnS (0.2 wt% Ir), and Cu-ZnS (0.5 mol% Cu) catalysts. Reaction conditions are the same as in Table 1.
The rate of photocatalytic hydrogen production is highly dependent on the noble metal loaded on the photocatalyst. Therefore, Ru, Rh, Pd, Pt, Ir, and Au were loaded on ZnS by in situ photochemical deposition to evaluate the effect of the noble metal on the activity for H2 production. It can be seen from Fig. 2 that the loading of noble metals on ZnS enhanced the activity for gas-solid phase photocatalytic H2S splitting. Loading Ir on ZnS increased the rate of H2 production from 1.2 ȝmol/h to 4.5 ȝmol/h. Further modifications of the ZnS photocatalyst included the formation of a solid solution with other sulfides and doping with transitional metals. It is known that ZnS is a photocatalyst that is active only in the UV light region because of its large band gap of 3.2 eV. However, after doping with Cu2+, Ni2+, Pb2+, and Hg2+, ZnS shows H2 production under visible light irradiation [16–18]. We added an appropriate amount of these cations into the feedstock in the hydrothermal synthesis of ZnS and found that Cu-ZnS displayed a significantly enhanced activity for hydrogen production. XPS analysis showed that the copper species existed as Cu2+. Fig. 3 compares the photo-
MA Guijun et al. / Chinese Journal of Catalysis, 2008, 29(4): 313–315
catalytic H2 production over ZnS, Ir/ZnS (with 0.2 wt% Ir), and Cu-ZnS (with 0.5 mol% Cu) catalysts. It can be seen that the total amount of evolved H2 on Cu-ZnS is 80 ȝmol after 150 min, which is 20 times higher than that on ZnS. UV-Vis spectra showed that introducing Cu2+ into ZnS caused a shift of the absorption edge of ZnS from 400 nm to 450 nm. The higher activity of Cu-ZnS was possibly due to an extended light absorption range. This was supported by measuring the activity of ZnS and Cu-ZnS under visible light irradiation (Ȝ > 420 nm). Cu-ZnS showed a photocatalytic H2 production rate of 17 ȝmol/h, whereas no H2 was detected on ZnS. In summary, the activities of five catalysts (TiO2, CdS, ZnS, ZnO, and ZnIn2S4) were evaluated for gas-solid phase photocatalytic splitting of H2S in an anaerobic environment. The photoactivity of these catalysts is quite different in a gas-solid phase from that in a liquid-solid phase. ZnS showed the highest rate of H2 production, and its activity can be further improved by adding Cu2+ into the feedstock during the preparation of ZnS. Gas-solid phase photocatalytic splitting of H2S in an anaerobic environment can reclaim hydrogen energy from H2S by utilizing solar energy. Moreover, in the absence of oxygen, H2S can be selectively oxidized to elemental sulfur (S), which makes it possible to recycle S from H2S while producing H2.
[2] Yan T Zh, Gao L X, Miao M F. Chem Eng Oil Gas, 2002, 31(S1): 17 [3] Gargurevich I A. Ind Eng Chem Res, 2005, 44(20): 7706 [4] Lei Zh B, You W Sh, Liu M Y, Zhou G H, Takata T, Hara M, Domen K, Li C. Chem Commun, 2003, (17): 2142 [5] Bao N, Shen L, Takata T, Domen K, Gupta A, Yanagisawa K, Grimes C A. J Phys Chem C, 2007, 111(47): 17527 [6] Naman S A. Int J Hydrogen Energy, 1992, 17(7): 499 [7] Canela M C, Alberici R M, Jardim W F. J Photochem Photobiol A, 1998, 112(1): 73 [8] Guo J H, Jin Zh Sh, Zhang M, Huang Y, Yang J J, Zhang Zh J. Photogr Sci Photochem, 2004, 22(3): 211 [9] Kato S, Hirano Y, Iwata M, Sano T, Takeuhi K, Matsuzawa S. Appl Catal B, 2005, 57(2): 109 [10] Kataoka S, Lee E, Tejedor-Tejedor M I, Anderson M A. Appl Catal B, 2005, 61(1–2): 159 [11] Sopyan I. Sci Technol Adv Mater, 2007, 8(1–2): 33 [12] Portela R, Sánchez B, Coronado J M, Candal R, Suárez S. Catal Today, 2007, 129(1–2): 223 [13] Chen T, Feng Zh Ch, Wu G P, Shi J Y, Ma G J, Ying P L, Li C. J Phys Chem C, 2007, 111(22): 8005 [14] Wu Y. Catalysis Chemistry. Beijing: Science Press, 2000. 1345 [15] Fu X C, Shen W X, Yao T Y. Physical Chemistry. Beijing: Higher Education Press, 1990. 668
References
[16] Kudo A, Sekizawa M. Catal Lett, 1999, 58(4): 241 [17] Kudo A, Sekizawa M. Chem Commun, 2000, (15): 1371
[1] Wang J M. Petrol Process Petrochem, 1999, 30(5): 1
[18] Tsuji I, Kudo A. J Photochem Photobiol A, 2003, 156(1–3): 249