- Email: [email protected]
TiO2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
A novel photo-thermochemical cycle of watersplitting for hydrogen production based on TiO2¡x/TiO2 Yanwei Zhang*, Jingche Chen, Chenyu Xu, Kewei Zhou, Zhihua Wang, Junhu Zhou, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
article info
abstract
Article history:
The traditional two-step thermochemical cycle of water-splitting that is based on metal-
Received 14 September 2015
oxide redox pairs requires a high reaction temperature. In this study, we introduced a
Received in revised form
photochemical reaction into this thermochemical cycle and established a novel photo-
3 December 2015
thermochemical cycle. In this new cycle, the process by which metal oxides are reduced
Accepted 4 December 2015
through concentrated solar energy is replaced with a photochemical reaction while water
Available online xxx
is still dissociated via a thermochemical reaction. Thus, photo-thermochemical cycles that combine these two reactions can be initiated at relatively low temperatures, unlike ther-
Keywords:
mochemical cycles operated at extremely high temperatures. In this research, TiO2 was
Thermochemical cycle
used as the catalyst, experiments were conducted to evaluate the feasibility of the pro-
Photo-thermochemical cycle
posed cycle, and a preliminary mechanism was developed for this cycle.
Water splitting
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Hydrogen production Oxygen vacancy
Introduction Environment-friendly hydrogen is an ideal alternative to fossil fuels. Developing an efficient hydrogen production system is thus an urgent issue. Compared with direct water-splitting at a high temperature, the thermochemical cycle that comprises multi-step reactions allows for a much lower temperature and minimizes the problem of H2 and O2 separation. The two-step thermochemical cycle based on metal oxide redox pairs is one of the most widely studied cycles. It consists of two reactions:
MxOy / MxOy1 þ 1/2O2 (g) (thermochemical reaction, >1500 C)
(1)
MxOy1 þ H2O / MxOy þ H2 (g) (thermochemical reaction, (2) 500e600 C) where MxOy and MxOy1 represent the metal oxide and its reduced state, respectively. The first step is the metal oxide reduction at temperatures usually above 1500 C. This step may use concentrated solar energy as the heat source. The second step is H2O dissociation through the reduced metal oxide or metal generated from the first step at relatively low temperatures, typically from 500 C to 600 C. Metal oxide is then recycled back to the first step. Thus, the net reaction is H2O splitting into H2 and O2, and separation of H2 and O2 is skillfully avoided.
* Corresponding author. Tel.: þ86 571 87953162; fax: þ86 571 87951616. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.ijhydene.2015.12.067 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang Y, et al., A novel photo-thermochemical cycle of water-splitting for hydrogen production based on TiO2x/TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.067
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
Fig. 1 e Schematic illustration of the photothermochemical cycle of water-splitting based on metal oxide.
Many possible materials have been investigated (mainly Zn/ZnO [1e3], FeO/Fe3O4 [4,5] and ferrites [6e8], Ce2O3/CeO2 [9e11] and ceria-based materials [12e15], SnO/SnO2 [16,17]), but temperatures above 1200 C are still required to acquire a reasonable amount of hydrogen. To reduce this temperature requirement, we introduced a photochemical reaction into this thermochemical cycle and established a novel photo-thermochemical cycle of watersplitting for hydrogen production (Fig. 1). In this new cycle, the process by which metal oxides are reduced through concentrated solar energy is replaced with a photochemical reaction while water is still dissociated via a thermochemical reaction. It can be expressed as follows (3,4):
MxOy / MxOy1 þ 1/2O2 (g) (photochemical reaction, room temperature) (3)
MxOy1 þ H2O / MxOy þ H2 (g) (thermochemical reaction, (4) 500e600 C)
(In this paper, MxOy represents TiO2, MxOy1 represents photo reduced TiO2) Thus, photo-thermochemical cycles that combine these two reactions can be initiated at relatively low temperatures, unlike thermochemical cycles operated at extremely high temperatures. Such reactions can also utilize the advantages of both solar luminous and thermal energies. TiO2 has been widely studied [18,19] as a chemically stable and excellent material in photocatalysis [20]. Oxygen vacancy on the TiO2 surface can be created by light irradiation as proved by many researchers. Wang [21] found that the TiO2 surface became super-hydrophilic after ultraviolet (UV) light illumination and that the XPS spectra proved the existence of photo-induced surface defects. Mezhennny [22] reported that line defects were formed on the (1 1 0)-(1 2) along the <0 0 1> direction by UV irradiation, whereas the (1 1 0)-(1 1) surface remained unchanged. The author even studied water adsorption on the (1 1 0)-(1 2) surfaces [23]. However, water dissociation for hydrogen production based on photo-induced oxygen vacancy has yet to be fully explored. TiO2 was used as the catalyst in the present study. Experiments were conducted to evaluate cycle feasibility, and a preliminary mechanism for the cycle was proposed.
Experimental Sample preparation 10 mL of tetrabutyl titanate was dissolved into 30 mL of absolute ethanol (solution A). 4 mL of glacial acetic acid and 5 mL of deionized water were mixed with 30 mL of absolute ethanol (solution B). Solution B was then added slowly to solution A under continuous stirring. The mixture was aged for 24 h at room temperature. The obtained gels were dried at 110 C, calcined at 700 C (heating rate ¼ 2 C/min), and cooled in the furnace.
Characterization The powder X-ray diffraction (XRD) pattern was acquired by a A) at 40 kV D/max 2550PC with Cu Ka radiation (Ka ¼ 1.540598
Fig. 2 e Experimental system for photo-thermochemical cycle of water-splitting. (a) Experimental system for photochemical reaction; (b) Experimental system for thermochemical reaction; 1: mass flowmeter; 2: two-way valve; 3: reactor; 4: Hg lamp; 5: TiO2 samples placed on a quartz glass; 6: water bubbler; 7: furnace; 8: gas chromatograph (TCD). Please cite this article in press as: Zhang Y, et al., A novel photo-thermochemical cycle of water-splitting for hydrogen production based on TiO2x/TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.067
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
Fig. 3 e X-ray diffraction pattern of TiO2.
3
Fig. 5 e Influence of the heating time on hydrogen production amount.
and 100 mA. The measurement was taken from 2q ¼ 10oe90 at intervals of 0.02 with 0.3 s of step time. The crystal size of the sample was calculated through the Debye e Scherrer equation. X-ray photoelectron spectroscopy (XPS) was carried out on Escalab 250Xi system using Mg Ka radiation under ultrahigh vacuum (UHV, 5 108Pa) and the spectra were calibrated by the C 1s peak at 284.6 eV. Electron paramagnetic resonance (EPR) measurements were performed on a Bruker A300 at 77 K using powdered TiO2 sample placed in a glass tube.
Reactivity test Fig. 2a shows the experimental system for photochemical reaction. 50 mg of TiO2 sample (weighted by analytical balance, Mettler Toledo, ME204E, d ¼ 0.1 mg) was dispersed on the bottom of the cylindrical reactor made up of stainless steel. Its inner cavity was 6.0 cm in diameter and 1 cm in height. A quartz window was covered on the top of the reactor during the photochemical reaction. Compressed argon (99.999%) with
Fig. 6 e Influence of the temperature on hydrogen production amount.
Fig. 4 e Influence of the irradiation time on hydrogen production amount.
Fig. 7 e Amount of hydrogen produced during five cycles.
Please cite this article in press as: Zhang Y, et al., A novel photo-thermochemical cycle of water-splitting for hydrogen production based on TiO2x/TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.067
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
Fig. 8 e The XPS O 1s spectra of the original sample, the sample after UV light irradiation and the sample after water dissociation.
a rate of 100 mL/min was passed through the reactor. The reactor was put under a 500 W Hg lamp (>254 nm) for 0e1 h. Fig. 2b shows the experimental system for thermochemical reaction. After the photochemical reaction, the quartz window on the top was replaced by a stainless steel lid. Argon was first passed through a water bubbler that was heated with a water bath at 70 C. The mixture was used to purge the reactor for 30 min before the valves on both sides were closed. A box-type furnace with a thermocouple (±2 C) inside was then used for heating. The H2 produced was measured by a gas chromatograph equipped with a thermal conductivity detector.
Results and discussions XRD analysis The XRD pattern (Fig. 3) shows that the TiO2 sample was a mixture of anatase and rutile. 2q values of 25.3 , 37.8 , 48.0 , and 62.7 corresponded to the (1 0 1), (0 0 4), (2 0 0), and (2 0 4) crystal planes of anatase. 2q values of 27.4 , 36.1 , 41.2 and 56.6 corresponded to the (1 1 0), (1 0 1), (1 1 1), and (2 2 0) crystal planes of rutile. Li [24] reported that the transformation temperature of
Please cite this article in press as: Zhang Y, et al., A novel photo-thermochemical cycle of water-splitting for hydrogen production based on TiO2x/TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.067
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
Fig. 9 e EPR spectra of the original sample, the sample after UV light irradiation and the sample after water dissociation. anatase-to-rutile is around 700 C, which is consistent with this result. The crystal size of TiO2 sample was 35.4 nm.
Activity tests Fig. 4 shows the amount hydrogen produced in six independent cycles with different irradiation time for photochemical reaction. During the thermochemical reaction, the TiO2 sample was heated at 600 C for 1 h. In this article, mL/g means the amount of hydrogen produced by per gram of TiO2. No Hydrogen was observed when there was no UV light irradiation for photochemical reaction. The amount of H2 produced increased with irradiation time in a 40-min period and remained unchanged with prolonged irradiation treatment. This result indicated that oxygen vacancy increases with the irradiation time; however, the total amount of oxygen vacancy that can be created was limited and was maximized at 40 min. Afterward, the oxygen vacancy concentration remained unchanged even if irradiation time was prolonged. Fig. 5 illustrates the amount hydrogen produced in six independent cycles with different heating time for thermochemical reaction. The irradiation time is 40 min for photochemical reaction, and the temperature is 600 C for thermochemical reaction. Overall, the amount of H2 produced increased with heating time. A small amount of hydrogen was
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produced in the first 10 min; this amount increased slightly by 20 min and rose sharply from 20 min to 30 min. This outcome may be attributed to the potentially lengthy heat transfer process occurring in the stainless steel reactor. As such, the TiO2 particles did not reach a certain temperature in the first 20 min, and the reaction rate was slow. Subsequently, this rate accelerated when the temperature was sufficiently high. As the reaction progressed, the surface oxygen vacancies decreased and the reaction was almost completed by 60 min. Fig. 6 depicts the influence of temperature on hydrogen production amount. The sample was first irradiated by UV light for 40 min and was then heated at each temperature for 60 min. No hydrogen was produced at room temperature. The amount of H2 generated increased with heating temperature; a small amount of H2 was produced at a temperature of 200 C, and this amount increased slightly when the temperature rose from 200 C to 400 C. This finding indicated that the reaction rate between oxygen vacancy and H2O is low at this temperature. When the temperature increased to 500 C and 600 C, the amount of H2 produced increased sharply, thus indicating that the reaction rate was significantly enhanced. Five successive photo-thermochemical cycles were repeated; the exposure time for the photochemical reaction was 40 min, and the heating time for the thermochemical reaction was 1 h at 600 C. Fig. 7 presents the amount of hydrogen produced. The TiO2 sample exhibited good cyclability. An average amount of 0.421 mL/g hydrogen was generated and the amount of hydrogen generated during cycles was stable. Besides, no other impurity gases were detected. The material sintering problem can hardly be avoided given the extremely high temperature in the two-step thermochemical cycle; this problem leads to a decline in hydrogen production. In the photo-thermochemical cycle, the maximum required temperature was considerably lowered; therefore, the sintering problem was avoided to a significant extent and material cyclability improved.
Comprehensive characterization The surface characterization of the samples was performed using XPS, the results of which are shown in Fig. 8. The O 1s peak was fitted to two peaks centered at 529.7 and 531.5 eV.
Fig. 10 e A preliminary mechanism for the photo-thermochemical cycle based on TiO2¡x/TiO2. Please cite this article in press as: Zhang Y, et al., A novel photo-thermochemical cycle of water-splitting for hydrogen production based on TiO2x/TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.067
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
The peak at 529.7 eV is ascribed to crystal lattice oxygen in TiO2, whereas the peak at 531.5 eV is ascribed to hydroxyl groups adsorbed on the surface. Physically adsorbed hydroxyl groups could not exist because the XPS system was utilized under the UHV condition; thus, the hydroxyl groups detected was associated with the surface defects, to which the hydroxyl groups were strongly bounded [25,26]. The hydroxyl groups clearly increased after exposure to the UV light for 40 min. This change equated to an increase in surface defects. The hydroxyl groups decreased after the thermal reaction of water splitting. Such decrease indicated that the surface defects were healed. The EPR spectra (Fig. 9) were used to identify the oxygen vacancy or Ti3þ in the sample. No observable EPR signals were detected for the original sample. Hence, oxygen vacancy and Ti3þ were not present in the sample. However, the sample resulted in a signal at g ¼ 2.003 after UV light irradiation for 40 min; this condition was ascribed to the oxygen vacancy [27,28]. No detectable signals of Ti3þ (g ¼ 1.94e1.98) was observed [29], because of the high instability of Ti3þ on the surface when exposed to air or water [30,31]. The signal peak disappeared after the thermochemical reaction. The EPR results were consistent with the XPS measurement.
Acknowledgements
Preliminary mechanism of photo-thermochemical cycle A preliminary mechanism for the cycle is established (Fig. 10) according to the characterization results and investigations of other researchers [21,22,32]. The TiO2 sample was photoexcited, and electronehole pairs were generated after the exposure of the TiO2 sample to UV light. The Ti4þ on the surface was reduced to Ti3þ by the electrons while the lattice oxygen (O2) was oxidized by the holes. These conditions led to the formation of oxygen vacancies on the surface. Water molecules were then absorbed on the oxygen vacancies and were easily decomposed when heating because the Ti3þ and oxygen vacancy were highly unstable. H2 was then generated, and oxygen vacancies were healed. This scenario can be expressed as follows (5e9): hv þ TiO2 / e þ hþ
was achieved with a photochemical reaction, while the water dissociation was still achieved with a thermochemical reaction. So, photo-thermochemical cycle can be operated at relatively low temperatures and it can utilize the advantages of both solar luminous and thermal energies. No hydrogen was detected without UV irradiation or heating, thereby indicating that the reaction was neither photocatalysis nor thermocatalysis. Rather, the reaction is a combined photo-thermochemical cycle. The influence of irradiation time, heating time, and temperature on hydrogen production was determined, and five successive photothermochemical cycles were repeated. An average amount of 0.421 mL/g H2 was produced during the five cycles, thus indicating that the photo-thermochemical cycle is feasible. The photo-thermochemical cycle mechanism was proposed based on the results of XPS and EPR. When the TiO2 sample was exposed to UV light, oxygen vacancy and Ti3þ were generated. This vacancy is highly unstable and can be easily healed under a water vapour atmosphere when heated. At the same time, H2 was produced.
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant no. 51276170).
Nomenclature UHV XRD XPS EPR UV Vo hþ e
ultrahigh vacuum X-ray diffraction X-ray photoelectron spectroscopy electron paramagnetic resonance ultraviolet oxygen vacancy hole electron
(5)
references e þ Ti4þ / Ti3þ
(6)
hþ þ O2 / O2 þ Vo
(7)
Vo þ H2O / Vo(H2O)
(8)
Ti3þ-Vo (H2O)-Ti3þ / Ti4þ-O-Ti4þ þ H2
(9)
Conclusions A novel photo-thermochemical cycle for hydrogen production was established. The reduction of metal oxides in this cycle
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Please cite this article in press as: Zhang Y, et al., A novel photo-thermochemical cycle of water-splitting for hydrogen production based on TiO2x/TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.067
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Please cite this article in press as: Zhang Y, et al., A novel photo-thermochemical cycle of water-splitting for hydrogen production based on TiO2x/TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.067
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
A novel photo-thermochemical cycle of watersplitting for hydrogen production based on TiO2¡x/TiO2 Yanwei Zhang*, Jingche Chen, Chenyu Xu, Kewei Zhou, Zhihua Wang, Junhu Zhou, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
article info
abstract
Article history:
The traditional two-step thermochemical cycle of water-splitting that is based on metal-
Received 14 September 2015
oxide redox pairs requires a high reaction temperature. In this study, we introduced a
Received in revised form
photochemical reaction into this thermochemical cycle and established a novel photo-
3 December 2015
thermochemical cycle. In this new cycle, the process by which metal oxides are reduced
Accepted 4 December 2015
through concentrated solar energy is replaced with a photochemical reaction while water
Available online xxx
is still dissociated via a thermochemical reaction. Thus, photo-thermochemical cycles that combine these two reactions can be initiated at relatively low temperatures, unlike ther-
Keywords:
mochemical cycles operated at extremely high temperatures. In this research, TiO2 was
Thermochemical cycle
used as the catalyst, experiments were conducted to evaluate the feasibility of the pro-
Photo-thermochemical cycle
posed cycle, and a preliminary mechanism was developed for this cycle.
Water splitting
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Hydrogen production Oxygen vacancy
Introduction Environment-friendly hydrogen is an ideal alternative to fossil fuels. Developing an efficient hydrogen production system is thus an urgent issue. Compared with direct water-splitting at a high temperature, the thermochemical cycle that comprises multi-step reactions allows for a much lower temperature and minimizes the problem of H2 and O2 separation. The two-step thermochemical cycle based on metal oxide redox pairs is one of the most widely studied cycles. It consists of two reactions:
MxOy / MxOy1 þ 1/2O2 (g) (thermochemical reaction, >1500 C)
(1)
MxOy1 þ H2O / MxOy þ H2 (g) (thermochemical reaction, (2) 500e600 C) where MxOy and MxOy1 represent the metal oxide and its reduced state, respectively. The first step is the metal oxide reduction at temperatures usually above 1500 C. This step may use concentrated solar energy as the heat source. The second step is H2O dissociation through the reduced metal oxide or metal generated from the first step at relatively low temperatures, typically from 500 C to 600 C. Metal oxide is then recycled back to the first step. Thus, the net reaction is H2O splitting into H2 and O2, and separation of H2 and O2 is skillfully avoided.
* Corresponding author. Tel.: þ86 571 87953162; fax: þ86 571 87951616. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.ijhydene.2015.12.067 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang Y, et al., A novel photo-thermochemical cycle of water-splitting for hydrogen production based on TiO2x/TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.067
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
Fig. 1 e Schematic illustration of the photothermochemical cycle of water-splitting based on metal oxide.
Many possible materials have been investigated (mainly Zn/ZnO [1e3], FeO/Fe3O4 [4,5] and ferrites [6e8], Ce2O3/CeO2 [9e11] and ceria-based materials [12e15], SnO/SnO2 [16,17]), but temperatures above 1200 C are still required to acquire a reasonable amount of hydrogen. To reduce this temperature requirement, we introduced a photochemical reaction into this thermochemical cycle and established a novel photo-thermochemical cycle of watersplitting for hydrogen production (Fig. 1). In this new cycle, the process by which metal oxides are reduced through concentrated solar energy is replaced with a photochemical reaction while water is still dissociated via a thermochemical reaction. It can be expressed as follows (3,4):
MxOy / MxOy1 þ 1/2O2 (g) (photochemical reaction, room temperature) (3)
MxOy1 þ H2O / MxOy þ H2 (g) (thermochemical reaction, (4) 500e600 C)
(In this paper, MxOy represents TiO2, MxOy1 represents photo reduced TiO2) Thus, photo-thermochemical cycles that combine these two reactions can be initiated at relatively low temperatures, unlike thermochemical cycles operated at extremely high temperatures. Such reactions can also utilize the advantages of both solar luminous and thermal energies. TiO2 has been widely studied [18,19] as a chemically stable and excellent material in photocatalysis [20]. Oxygen vacancy on the TiO2 surface can be created by light irradiation as proved by many researchers. Wang [21] found that the TiO2 surface became super-hydrophilic after ultraviolet (UV) light illumination and that the XPS spectra proved the existence of photo-induced surface defects. Mezhennny [22] reported that line defects were formed on the (1 1 0)-(1 2) along the <0 0 1> direction by UV irradiation, whereas the (1 1 0)-(1 1) surface remained unchanged. The author even studied water adsorption on the (1 1 0)-(1 2) surfaces [23]. However, water dissociation for hydrogen production based on photo-induced oxygen vacancy has yet to be fully explored. TiO2 was used as the catalyst in the present study. Experiments were conducted to evaluate cycle feasibility, and a preliminary mechanism for the cycle was proposed.
Experimental Sample preparation 10 mL of tetrabutyl titanate was dissolved into 30 mL of absolute ethanol (solution A). 4 mL of glacial acetic acid and 5 mL of deionized water were mixed with 30 mL of absolute ethanol (solution B). Solution B was then added slowly to solution A under continuous stirring. The mixture was aged for 24 h at room temperature. The obtained gels were dried at 110 C, calcined at 700 C (heating rate ¼ 2 C/min), and cooled in the furnace.
Characterization The powder X-ray diffraction (XRD) pattern was acquired by a A) at 40 kV D/max 2550PC with Cu Ka radiation (Ka ¼ 1.540598
Fig. 2 e Experimental system for photo-thermochemical cycle of water-splitting. (a) Experimental system for photochemical reaction; (b) Experimental system for thermochemical reaction; 1: mass flowmeter; 2: two-way valve; 3: reactor; 4: Hg lamp; 5: TiO2 samples placed on a quartz glass; 6: water bubbler; 7: furnace; 8: gas chromatograph (TCD). Please cite this article in press as: Zhang Y, et al., A novel photo-thermochemical cycle of water-splitting for hydrogen production based on TiO2x/TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.067
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Fig. 3 e X-ray diffraction pattern of TiO2.
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Fig. 5 e Influence of the heating time on hydrogen production amount.
and 100 mA. The measurement was taken from 2q ¼ 10oe90 at intervals of 0.02 with 0.3 s of step time. The crystal size of the sample was calculated through the Debye e Scherrer equation. X-ray photoelectron spectroscopy (XPS) was carried out on Escalab 250Xi system using Mg Ka radiation under ultrahigh vacuum (UHV, 5 108Pa) and the spectra were calibrated by the C 1s peak at 284.6 eV. Electron paramagnetic resonance (EPR) measurements were performed on a Bruker A300 at 77 K using powdered TiO2 sample placed in a glass tube.
Reactivity test Fig. 2a shows the experimental system for photochemical reaction. 50 mg of TiO2 sample (weighted by analytical balance, Mettler Toledo, ME204E, d ¼ 0.1 mg) was dispersed on the bottom of the cylindrical reactor made up of stainless steel. Its inner cavity was 6.0 cm in diameter and 1 cm in height. A quartz window was covered on the top of the reactor during the photochemical reaction. Compressed argon (99.999%) with
Fig. 6 e Influence of the temperature on hydrogen production amount.
Fig. 4 e Influence of the irradiation time on hydrogen production amount.
Fig. 7 e Amount of hydrogen produced during five cycles.
Please cite this article in press as: Zhang Y, et al., A novel photo-thermochemical cycle of water-splitting for hydrogen production based on TiO2x/TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.067
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Fig. 8 e The XPS O 1s spectra of the original sample, the sample after UV light irradiation and the sample after water dissociation.
a rate of 100 mL/min was passed through the reactor. The reactor was put under a 500 W Hg lamp (>254 nm) for 0e1 h. Fig. 2b shows the experimental system for thermochemical reaction. After the photochemical reaction, the quartz window on the top was replaced by a stainless steel lid. Argon was first passed through a water bubbler that was heated with a water bath at 70 C. The mixture was used to purge the reactor for 30 min before the valves on both sides were closed. A box-type furnace with a thermocouple (±2 C) inside was then used for heating. The H2 produced was measured by a gas chromatograph equipped with a thermal conductivity detector.
Results and discussions XRD analysis The XRD pattern (Fig. 3) shows that the TiO2 sample was a mixture of anatase and rutile. 2q values of 25.3 , 37.8 , 48.0 , and 62.7 corresponded to the (1 0 1), (0 0 4), (2 0 0), and (2 0 4) crystal planes of anatase. 2q values of 27.4 , 36.1 , 41.2 and 56.6 corresponded to the (1 1 0), (1 0 1), (1 1 1), and (2 2 0) crystal planes of rutile. Li [24] reported that the transformation temperature of
Please cite this article in press as: Zhang Y, et al., A novel photo-thermochemical cycle of water-splitting for hydrogen production based on TiO2x/TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.067
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Fig. 9 e EPR spectra of the original sample, the sample after UV light irradiation and the sample after water dissociation. anatase-to-rutile is around 700 C, which is consistent with this result. The crystal size of TiO2 sample was 35.4 nm.
Activity tests Fig. 4 shows the amount hydrogen produced in six independent cycles with different irradiation time for photochemical reaction. During the thermochemical reaction, the TiO2 sample was heated at 600 C for 1 h. In this article, mL/g means the amount of hydrogen produced by per gram of TiO2. No Hydrogen was observed when there was no UV light irradiation for photochemical reaction. The amount of H2 produced increased with irradiation time in a 40-min period and remained unchanged with prolonged irradiation treatment. This result indicated that oxygen vacancy increases with the irradiation time; however, the total amount of oxygen vacancy that can be created was limited and was maximized at 40 min. Afterward, the oxygen vacancy concentration remained unchanged even if irradiation time was prolonged. Fig. 5 illustrates the amount hydrogen produced in six independent cycles with different heating time for thermochemical reaction. The irradiation time is 40 min for photochemical reaction, and the temperature is 600 C for thermochemical reaction. Overall, the amount of H2 produced increased with heating time. A small amount of hydrogen was
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produced in the first 10 min; this amount increased slightly by 20 min and rose sharply from 20 min to 30 min. This outcome may be attributed to the potentially lengthy heat transfer process occurring in the stainless steel reactor. As such, the TiO2 particles did not reach a certain temperature in the first 20 min, and the reaction rate was slow. Subsequently, this rate accelerated when the temperature was sufficiently high. As the reaction progressed, the surface oxygen vacancies decreased and the reaction was almost completed by 60 min. Fig. 6 depicts the influence of temperature on hydrogen production amount. The sample was first irradiated by UV light for 40 min and was then heated at each temperature for 60 min. No hydrogen was produced at room temperature. The amount of H2 generated increased with heating temperature; a small amount of H2 was produced at a temperature of 200 C, and this amount increased slightly when the temperature rose from 200 C to 400 C. This finding indicated that the reaction rate between oxygen vacancy and H2O is low at this temperature. When the temperature increased to 500 C and 600 C, the amount of H2 produced increased sharply, thus indicating that the reaction rate was significantly enhanced. Five successive photo-thermochemical cycles were repeated; the exposure time for the photochemical reaction was 40 min, and the heating time for the thermochemical reaction was 1 h at 600 C. Fig. 7 presents the amount of hydrogen produced. The TiO2 sample exhibited good cyclability. An average amount of 0.421 mL/g hydrogen was generated and the amount of hydrogen generated during cycles was stable. Besides, no other impurity gases were detected. The material sintering problem can hardly be avoided given the extremely high temperature in the two-step thermochemical cycle; this problem leads to a decline in hydrogen production. In the photo-thermochemical cycle, the maximum required temperature was considerably lowered; therefore, the sintering problem was avoided to a significant extent and material cyclability improved.
Comprehensive characterization The surface characterization of the samples was performed using XPS, the results of which are shown in Fig. 8. The O 1s peak was fitted to two peaks centered at 529.7 and 531.5 eV.
Fig. 10 e A preliminary mechanism for the photo-thermochemical cycle based on TiO2¡x/TiO2. Please cite this article in press as: Zhang Y, et al., A novel photo-thermochemical cycle of water-splitting for hydrogen production based on TiO2x/TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.067
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The peak at 529.7 eV is ascribed to crystal lattice oxygen in TiO2, whereas the peak at 531.5 eV is ascribed to hydroxyl groups adsorbed on the surface. Physically adsorbed hydroxyl groups could not exist because the XPS system was utilized under the UHV condition; thus, the hydroxyl groups detected was associated with the surface defects, to which the hydroxyl groups were strongly bounded [25,26]. The hydroxyl groups clearly increased after exposure to the UV light for 40 min. This change equated to an increase in surface defects. The hydroxyl groups decreased after the thermal reaction of water splitting. Such decrease indicated that the surface defects were healed. The EPR spectra (Fig. 9) were used to identify the oxygen vacancy or Ti3þ in the sample. No observable EPR signals were detected for the original sample. Hence, oxygen vacancy and Ti3þ were not present in the sample. However, the sample resulted in a signal at g ¼ 2.003 after UV light irradiation for 40 min; this condition was ascribed to the oxygen vacancy [27,28]. No detectable signals of Ti3þ (g ¼ 1.94e1.98) was observed [29], because of the high instability of Ti3þ on the surface when exposed to air or water [30,31]. The signal peak disappeared after the thermochemical reaction. The EPR results were consistent with the XPS measurement.
Acknowledgements
Preliminary mechanism of photo-thermochemical cycle A preliminary mechanism for the cycle is established (Fig. 10) according to the characterization results and investigations of other researchers [21,22,32]. The TiO2 sample was photoexcited, and electronehole pairs were generated after the exposure of the TiO2 sample to UV light. The Ti4þ on the surface was reduced to Ti3þ by the electrons while the lattice oxygen (O2) was oxidized by the holes. These conditions led to the formation of oxygen vacancies on the surface. Water molecules were then absorbed on the oxygen vacancies and were easily decomposed when heating because the Ti3þ and oxygen vacancy were highly unstable. H2 was then generated, and oxygen vacancies were healed. This scenario can be expressed as follows (5e9): hv þ TiO2 / e þ hþ
was achieved with a photochemical reaction, while the water dissociation was still achieved with a thermochemical reaction. So, photo-thermochemical cycle can be operated at relatively low temperatures and it can utilize the advantages of both solar luminous and thermal energies. No hydrogen was detected without UV irradiation or heating, thereby indicating that the reaction was neither photocatalysis nor thermocatalysis. Rather, the reaction is a combined photo-thermochemical cycle. The influence of irradiation time, heating time, and temperature on hydrogen production was determined, and five successive photothermochemical cycles were repeated. An average amount of 0.421 mL/g H2 was produced during the five cycles, thus indicating that the photo-thermochemical cycle is feasible. The photo-thermochemical cycle mechanism was proposed based on the results of XPS and EPR. When the TiO2 sample was exposed to UV light, oxygen vacancy and Ti3þ were generated. This vacancy is highly unstable and can be easily healed under a water vapour atmosphere when heated. At the same time, H2 was produced.
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant no. 51276170).
Nomenclature UHV XRD XPS EPR UV Vo hþ e
ultrahigh vacuum X-ray diffraction X-ray photoelectron spectroscopy electron paramagnetic resonance ultraviolet oxygen vacancy hole electron
(5)
references e þ Ti4þ / Ti3þ
(6)
hþ þ O2 / O2 þ Vo
(7)
Vo þ H2O / Vo(H2O)
(8)
Ti3þ-Vo (H2O)-Ti3þ / Ti4þ-O-Ti4þ þ H2
(9)
Conclusions A novel photo-thermochemical cycle for hydrogen production was established. The reduction of metal oxides in this cycle
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Please cite this article in press as: Zhang Y, et al., A novel photo-thermochemical cycle of water-splitting for hydrogen production based on TiO2x/TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.067