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Cobalt-containing layered or zeolitic silicates as photocatalysts for hydrogen generation† - tefan Neat-u, Marta Puche, Vicente Forne S ´s and Hermenegildo Garcia*
Received 30th July 2014, Accepted 3rd October 2014 DOI: 10.1039/c4cc05931j www.rsc.org/chemcomm
Layered magadiite and zeolites Y containing framework Co or small CoO clusters in the pores have been synthesized and tested as photocatalysts for water splitting, in the absence and presence of methanol, upon UV or simulated sunlight irradiation; the best performing material was Co-magadiite.
There has been much interest shown in the photocatalytic production of solar fuels and particularly in hydrogen generation from water.1,2 In this field, most of the studies on hydrogen evolution are carried out using an excess of a sacrificial electron donor. Upon light absorption and generation of electrons and holes on the semiconductor, hydrogen generation by electron reduction of water has to take place at the same rate as hole consumption, since the overall reaction rate is controlled by the slower of the two processes.3 Therefore, in order to avoid kinetic limitations in the process of hydrogen generation, an excess of electron donors is generally used. While the presence of electron donors simplifies the study of hydrogen evolution, the overall water splitting, in where no donors are present, is more likely to be used for commercial application. In this context, it has been recently reported that cobalt oxide nanoparticles (NPs) are able to promote overall water splitting highly efficiently in the absence of electron donors using solar light.4 While titanium dioxide has been the most used as a photocatalyst for pollutant degradation in water and environmental remediation in general, there is an increasing interest for using TiO2 in any kind of process, including photocatalytic water splitting. TiO2 suffers, however, from the limitation of the lack of photocatalytic activity under visible light irradiation,5–7 and this limitation could be overcome by using cobalt oxide. In view of these precedents, it appears to be of interest to explore the photocatalytic activity of other cobalt-containing materials Instituto Univeristario de Tecnologia Quimica CSIC-UPV, Universidad Politecnica de Valencia, Av. De los Naranjos s/n, 46022, Valencia, Spain. E-mail: [email protected]; Fax: +34 96387 9444; Tel: +34 96387 7807 † Electronic supplementary information (ESI) available: Experimental details, photocatalyst characterization and photocatalytic reactions conditions. See DOI: 10.1039/c4cc05931j
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that either in the as-synthesized form or after adequate treatment have cobalt in a similar environment as cobalt oxide NPs. Two of these possibilities are to graft Co atoms on a silicate matrix or to incorporate small CoO clusters of a few number of atoms inside cavities of a porous host. Both approaches have been already applied for Ti-containing photocatalysts and have provided more efficient materials per Ti atom. In the present study, we have selected three different cobalt-containing silicates with different structures and evaluated their photocatalytic activity under two different conditions, either for the overall water splitting under UV irradiation in the absence of sacrificial electron donor or for hydrogen generation under simulated sunlight illumination in the presence of methanol. Specifically, we have selected a layered silicate with magadiite structure that has cobalt incorporated into the framework, replacing isomorphically Si atoms (Co-magadiite). The structure of magadiite is constituted by sheets of an array of Si4+ tetrahedra sharing the edges.8–10 The synthesis of Co-magadiite was carried out from SiO2 (Ludox AS-40) by hydrothermal treatment at 150 1C for 3 days in NaOH using trans-4-aminocyclohexanol as the structure directing agent as reported.11,12 A gel with the following molar composition was used: SiO2 : 0.2NaOH : 0.5trans4-aminocyclohexanol : 15H2O : 0.02Co(OAc)2. The XRD pattern of Co-magadiite is presented in Fig. 1. As can be observed, the Co-magadiite sample possesses an interlaminar distance of 1.55 nm and high crystallinity. Co-magadiite has been used as the source of Si and Co atoms in the synthesis of Faujasite Y framework containing cobalt.11,12 The rationale behind the use of magadiite as the source of Si in the synthesis of Faujasite Y is because it has been found that when the framework of Faujasite Y is being built during the hydrothermal synthesis, the use of magadiite as the Si source allows an easy co-incorporation of heteroatoms (Co atoms in this case) in the Faujasite Y framework, accompanying the introduction of tetrahedral Si atoms.13–15 The XRD characterization data show that isomorphic substitution of Si with Co atoms occur homogeneously, without causing structural changes of the silicate network, without introducing any diffraction peak due to the precipitated Co species or corresponding
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Fig. 1 Characterization data of Co-magadiite and [Co]Y materials: (A) DR-UV-Vis spectra of Co-magadiite and Co[Y]. The inset shows the region 500–700 nm of the spectrum were the characteristic d–d transition of Co2+ (Td) are present. (B) XRD patterns of the as-prepared Co-magadiite and Co[Y] (red) and their corresponding reference materials not containing cobalt species (black).
to the silicic acid. The incorporation of Co into the framework of the zeolite Y, as a structural defect, could introduce some lattice instability in the material. This aspect can be revealed in Fig. 1B where a strong intensity loss reflects the instability of the Co containing zeolite Y framework. Surface area and porosity measurements by isothermal nitrogen adsorption indicate that the Co[Y] sample has 380 m2 g 1 surface area and a pore volume of about 0.2 mL g 1. These values are common for this type of zeolite.16 The morphology of the particles was determined by electron microscopy that shows that magadiite is constituted by small NPs, while the average particle size of zeolite Y is about 100 nm and occasionally shows an octahedral shape (see ESI† for images). The loading of Co in [Co]Y was determined by ICP chemical analysis after dissolution of the zeolite Y framework, giving a value of about 2.3 wt%. The framework position of Co was determined by XRD and UV-Vis spectroscopy that shows a band at 220–250 nm corresponding to charge-transfer Co–O band in tetrahedral Co2+ in the framework accompanied by a weak band at 350 nm that is attributed to Co in trigonal coordination. In contrast to the case of Co-magadiite, [Co]Y also exhibits a set of bands between 500 and 650 nm that correspond to the d–d transitions in tetrahedral Co2+ ions. Fig. 1 shows characterization data of the sample employed in the present study. All characterization results described so far for the zeolite Y containing cobalt indicate that the Co occupies very likely lattice positions as reported in the literature.17,18 The X-ray photoelectron spectra of Co-magadiite and [Co]Y zeolite vacuum dried at 300 1C (10 6 mbar) are shown in Fig. 2A. In both cases, the spectra show a doublet (2p3/2 and 2p1/2) due to the spin–orbit coupling, the spacing between them being 16 eV, which is characteristic for the Co2+ species.19 Each of the doublet bands possess a satellite band with higher binding energy due to the atom relaxation process associated with a mechanism of
14644 | Chem. Commun., 2014, 50, 14643--14646
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Fig. 2 Surface characterization data of Co-magadiite and [Co]Y materials. (A) XP spectra of the photocatalysts in the Co 2p region. (B) Auger spectra of both photocatalysts.
ligand–metal electron transfer, such a behaviour being characteristic to the Co2+ ‘‘high spin’’ species.19 The binding energy of Co 2p3/2 in [Co]Y is 781.3 eV while in Co-magadiite it is much lower (780.6 eV), which is an indication that [Co]Y possess a higher dispersion of the Co2+ ions. In the case of the [Co]Y zeolite, the higher value of the binding energy may be due to different zeolite Magdelung constant. These results seem to exclude the presence of significant amounts of CoO inside the [Co]Y framework. To confirm the results obtained by XPS, a study using Auger spectroscopy has also been performed. The Auger spectra of cobalt, for both Co-magadiite and [Co]Y samples, are shown in Fig. 2B. In the case of the Co-magadiite material, the Auger peak maximum around 768 eV, accompanied by a shoulder at higher kinetic energies, is observed. The appearance of this shoulder suggests the presence of different cobalt species (small part of Co2+ might be extraframework cobalt). In the case of [Co]Y two peaks at 767 and 772 eV would also imply the existence of two types of Co species of a different nature. For instance, the parameter Auger value of 767 eV corresponds to low coordinated species of Co (Td), while the shoulder at 772 eV corresponds to the Co species having a more elevated coordination (pentagonal) due to possible coordination with water in the sample. Summarizing the results obtained from XPS and Auger, we can conclude that cobalt in the zeolite Y is in tetrahedral coordination and is highly dispersed in the inorganic matrix, which is comparable with its presence as part of the silicate framework in agreement with the results and conclusions obtained by other techniques. In addition, we prepared a third sample, [CoO]Y, to evaluate the photocatalytic activity. It is well known in zeolite science that heteroatoms occupying framework positions tend to migrate to extraframework locations by thermal or chemical stress.20
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Even framework aluminium can be removed to extraframework positions by this type of treatment.21 Therefore, the fact that framework cobalt is present in the as-synthesized Co[Y] material does not ensure that during the photochemical reaction a fraction of them migrates from the framework to the extraframework positions. To address this issue, we have also included in our photocatalytic activity test a third sample consisting in zeolite Y incorporating inside the pores small clusters of cobalt oxide ([CoO]Y). This sample was prepared by thermal treatment of [Co]Y. [CoO]Y was prepared starting from a [Co]Y zeolite that was submitted to calcination at 540 1C. This thermal treatment transforms framework Co atoms in a large proportion into extraframework CoOx clusters, similarly to what is known for Al3+ in aluminosilicates.22 This transformation is reflected in the UV-Vis spectrum of [CoO]Y where the triplet absorption characteristic of tetrahedral Co2+ can be seen with considerably higher intensity than for [Co]Y (see the ESI† for details). Chemical analysis after zeolite dissolution shows that the cobalt content of [CoO]Y is about 2.6 wt% and the small Co oxide clusters will be located in the supercages of the Faujasite. Preliminary EXAFS data on [Co]Y and [CoO]Y are in agreement with this change in the position and coordination of Co atoms in the structure. The purpose of the work is to determine if these samples exhibit also catalytic activity for overall water splitting in the absence of a sacrificial electron donor and correlate their activity with the structure of the sample. Two types of photocatalytic experiments were performed to evaluate the activity of the three samples. One of the photocatalytic tests was carried out without a sacrificial electron donor, and therefore the water has to become reduced to hydrogen and oxidized to form oxygen by quenching conduction band electrons and valence band holes, respectively. These experiments were carried out under UV-visible light irradiation to increase the efficiency of the process. The results obtained are summarized in Table 1. As can be seen there, the three samples exhibit photocatalytic activity for overall water splitting under UV-visible-light illumination, leading to the generation of H2 and O2 in the expected stoichiometric amounts (see Table 1). The most active sample was found to be Co-magadiite (1.434 mL of H2 per g of material). This higher photocatalytic activity could be due to a more appropriate energy level of electrons and holes,
Table 1 Hydrogen production rate of cobalt-containing catalysts under UV and simulated solar light irradiationa
Amount of H2 and O2 formed [mmol(g of Co) 1] Photocatalyst
UV
Solar
Co-magadiite [Co]Y [CoO]Y
127.56 (62.4)b 74.64 (36.0) 38.40 (18.0)
80.40 74.40 12.00
a
Reaction conditions: 25 mg of photocatalyst powder; 25 1C; 25 mL deionized water or H2O : CH3OH mixture (4 : 1 volumetric ratio) for the UV or solar simulation experiments, respectively; 125 or 1000 W, irradiation sources for the UV or solar simulation experiments; 12 h of irradiation time. b The numbers in brackets correspond to the amount of O2 formed.
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but it also indicates that highly dispersed isolated independent Co atoms embedded in the silicate matrix, as they are present in magadiite, can be active for water splitting. Although less active than Co-magadiite, this finding of photocatalytic activity of isolated Co atoms in the framework positions of the silicates was also observed for [Co]Y (0.963 mL of H2 per g of material). It should be noted that commercial CoO (Aldrich) does not exhibit any detectable photocatalytic activity under the same conditions (see ESI†), showing that the preparation procedure and particle size are crucial to observe the photocatalytic activity for overall water splitting reported in the literature.4 Another set of experiments was carried out using simulated solar light that irradiates a small percentage (about 4%) in the UVA region and a large proportion of the emitted photons correspond to wavelengths in the visible spectrum (about 48%).23 In this case, addition of methanol as sacrificial electron donor was used to increase the rate of hole consumption allowing the study of the water reduction process. The results obtained are also listed in Table 1. As it can also be seen there, under solar light illumination, Co-magadiite was the best performing photocatalyst (0.896 mL of H2 per g of material), and also again the three solids under study exhibit hydrogen generation activity. It is notable that Co-magadiite and [Co]Y containing isolated Co atoms in framework positions exhibit, also under simulated sunlight, notable photocatalytic activity that in this case is close for both materials. In these two types of photocatalytic experiments, the lowest catalytic activity was exhibited by the sample containing small clusters of CoO embedded within the pore system of zeolite Y. This seems to indicate that isolated Co ions have higher intrinsic photocatalytic activity than small Co NPs. In conclusion, in the present manuscript we have shown that Co ions grafted onto a silicate matrix, either in bidimensional silicic acids or having a zeolite structure are active centers for H2 generation in the presence and even in the absence of sacrificial electron donors. The photocatalytic data of small CoO clusters embedded into the voids of zeolite Y have lower photocatalytic activity than when the Co2+ ions are isolated and grafted into the silicate matrix. In a certain way, these data are analogous to those found for Ti-atoms grafted on silicates,24 except that in the case of Co2+ photocatalysis can be promoted by visible light. Considering the large variety of silicas and silicates, as well as the possibility to include other heteroatoms and particularly aluminium, our report opens the way to explore the photocatalytic activity of a wide range of Co and, in general, transition metal containing silicates as photocatalysts for hydrogen generation and water splitting. This work was supported by the Spanish Ministry of Economy and Competitiveness (Severo Ochea and CTQ-2012-32315) and the Marie Curie project PIEF-GA-2011-298740, and the Generalidad Valenciana (Prometeo 2012/2013).
Notes and references 1 X. B. Chen, S. H. Shen, L. J. Guo and S. S Mao, Chem. Rev., 2010, 110, 6503. 2 T. E. Mallouk, J. Phys. Chem. Lett., 2010, 1, 2738. 3 A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253.
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Communication 4 L. Liao, Q. Zhang, Z. Su, Z. Zhao, Y. Wang, Y. Li, X. Lu, D. Wei, G. Feng, Q. Yu, X. Cai, J. Zhao, Z. Ren, H. Fang, F. Robles-Hernandez, S. Baldelli and J. Bao, Nat. Nanotechnol., 2014, 9, 69. 5 R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269. 6 S. U. M. Khan, M. Al-Shahry and W. B. Ingler, Science, 2002, 297, 2243. 7 X. Chen, L. Liu, P. Y. Yu and S. S. Mao, Science, 2011, 331, 746. 8 W. Schiwieger, D. Heidemann and K.-H. Bergk, Rev. Chim. Miner., 1985, 22, 639. 9 J. M. Garces, S. C. Rocke, C. E. Crowder and D. L. Hasha, Clays Clay Miner., 1988, 36, 409. 10 T. J. Pinnavaia, I. D. Johnson and M. Lipsicas, J. Solid State Chem., 1986, 63, 118. 11 E. M. Barea, V. Fornes, A. Corma, P. Bourges, E. Guillon and V. F. Puntes, Chem. Commun., 2004, 1974–1975. 12 E. M. Barea, V. Fornes, A. Corma, P. Bourges and E. Guillon, Fr. Pat., 04/00842, 04/02326, 2004. 13 R. M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982. 14 C. S. Cundy and P. A. Cox, Chem. Rev., 2003, 103, 663.
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ChemComm 15 S. Shimizu, Y. Kiyozumi, K. Maeda, F. Mizukami, G. PalBorbely, R. M. Mihalyi and H. Beyer, Adv. Mater., 1996, 8, 759. 16 E. Nigro, F. Testa, R. Aiello, P. Lentz, A. Fonseca, A. Oszko, P. Fejes, A. Kukovecz, I. Kiricsi and J. B. Nagy, Stud. Surf. Sci. Catal., 2001, 140, 353. 17 A. A. Verberckmoes, M. G. Uytterhoeven and R. A. Schoonheydt, Zeolites, 1997, 19, 180. 18 A. A. Verberckmoes, B. M. Weckhuysen and R. A. Schoonheydt, Microporous Mesoporous Mater., 1998, 22, 165. 19 D. C. Frost, C. A. McDowell and I. S. Woolsey, Mol. Phys., 1974, 27, 1473. 20 B. M. Weckhuysen, R. R. Rao, J. A. Martens and R. A. Schoonheydt, Eur. J. Inorg. Chem., 1999, 565. 21 P. A. Wright and J. A. Conner, Microporous Framework Solids, The Royal Society of Chemistry, Cambridge, 2007. 22 Q. Tang, Q. Zhang, P. Wang, Y. Wang and H. Wan, Chem. Mater., 2004, 16, 1967. 23 Z. Sen, Solar Energy Fundamentals and Modeling Techniques, Springer Verlag, London, 2008. 24 M. Anpo, H. Yamashita, Y. Ichihashi, Y. Fujii and M. Honda, J. Phys. Chem. B, 1997, 101, 2632.
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Cobalt-containing layered or zeolitic silicates as photocatalysts for hydrogen generation† - tefan Neat-u, Marta Puche, Vicente Forne S ´s and Hermenegildo Garcia*
Received 30th July 2014, Accepted 3rd October 2014 DOI: 10.1039/c4cc05931j www.rsc.org/chemcomm
Layered magadiite and zeolites Y containing framework Co or small CoO clusters in the pores have been synthesized and tested as photocatalysts for water splitting, in the absence and presence of methanol, upon UV or simulated sunlight irradiation; the best performing material was Co-magadiite.
There has been much interest shown in the photocatalytic production of solar fuels and particularly in hydrogen generation from water.1,2 In this field, most of the studies on hydrogen evolution are carried out using an excess of a sacrificial electron donor. Upon light absorption and generation of electrons and holes on the semiconductor, hydrogen generation by electron reduction of water has to take place at the same rate as hole consumption, since the overall reaction rate is controlled by the slower of the two processes.3 Therefore, in order to avoid kinetic limitations in the process of hydrogen generation, an excess of electron donors is generally used. While the presence of electron donors simplifies the study of hydrogen evolution, the overall water splitting, in where no donors are present, is more likely to be used for commercial application. In this context, it has been recently reported that cobalt oxide nanoparticles (NPs) are able to promote overall water splitting highly efficiently in the absence of electron donors using solar light.4 While titanium dioxide has been the most used as a photocatalyst for pollutant degradation in water and environmental remediation in general, there is an increasing interest for using TiO2 in any kind of process, including photocatalytic water splitting. TiO2 suffers, however, from the limitation of the lack of photocatalytic activity under visible light irradiation,5–7 and this limitation could be overcome by using cobalt oxide. In view of these precedents, it appears to be of interest to explore the photocatalytic activity of other cobalt-containing materials Instituto Univeristario de Tecnologia Quimica CSIC-UPV, Universidad Politecnica de Valencia, Av. De los Naranjos s/n, 46022, Valencia, Spain. E-mail: [email protected]; Fax: +34 96387 9444; Tel: +34 96387 7807 † Electronic supplementary information (ESI) available: Experimental details, photocatalyst characterization and photocatalytic reactions conditions. See DOI: 10.1039/c4cc05931j
This journal is © The Royal Society of Chemistry 2014
that either in the as-synthesized form or after adequate treatment have cobalt in a similar environment as cobalt oxide NPs. Two of these possibilities are to graft Co atoms on a silicate matrix or to incorporate small CoO clusters of a few number of atoms inside cavities of a porous host. Both approaches have been already applied for Ti-containing photocatalysts and have provided more efficient materials per Ti atom. In the present study, we have selected three different cobalt-containing silicates with different structures and evaluated their photocatalytic activity under two different conditions, either for the overall water splitting under UV irradiation in the absence of sacrificial electron donor or for hydrogen generation under simulated sunlight illumination in the presence of methanol. Specifically, we have selected a layered silicate with magadiite structure that has cobalt incorporated into the framework, replacing isomorphically Si atoms (Co-magadiite). The structure of magadiite is constituted by sheets of an array of Si4+ tetrahedra sharing the edges.8–10 The synthesis of Co-magadiite was carried out from SiO2 (Ludox AS-40) by hydrothermal treatment at 150 1C for 3 days in NaOH using trans-4-aminocyclohexanol as the structure directing agent as reported.11,12 A gel with the following molar composition was used: SiO2 : 0.2NaOH : 0.5trans4-aminocyclohexanol : 15H2O : 0.02Co(OAc)2. The XRD pattern of Co-magadiite is presented in Fig. 1. As can be observed, the Co-magadiite sample possesses an interlaminar distance of 1.55 nm and high crystallinity. Co-magadiite has been used as the source of Si and Co atoms in the synthesis of Faujasite Y framework containing cobalt.11,12 The rationale behind the use of magadiite as the source of Si in the synthesis of Faujasite Y is because it has been found that when the framework of Faujasite Y is being built during the hydrothermal synthesis, the use of magadiite as the Si source allows an easy co-incorporation of heteroatoms (Co atoms in this case) in the Faujasite Y framework, accompanying the introduction of tetrahedral Si atoms.13–15 The XRD characterization data show that isomorphic substitution of Si with Co atoms occur homogeneously, without causing structural changes of the silicate network, without introducing any diffraction peak due to the precipitated Co species or corresponding
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Fig. 1 Characterization data of Co-magadiite and [Co]Y materials: (A) DR-UV-Vis spectra of Co-magadiite and Co[Y]. The inset shows the region 500–700 nm of the spectrum were the characteristic d–d transition of Co2+ (Td) are present. (B) XRD patterns of the as-prepared Co-magadiite and Co[Y] (red) and their corresponding reference materials not containing cobalt species (black).
to the silicic acid. The incorporation of Co into the framework of the zeolite Y, as a structural defect, could introduce some lattice instability in the material. This aspect can be revealed in Fig. 1B where a strong intensity loss reflects the instability of the Co containing zeolite Y framework. Surface area and porosity measurements by isothermal nitrogen adsorption indicate that the Co[Y] sample has 380 m2 g 1 surface area and a pore volume of about 0.2 mL g 1. These values are common for this type of zeolite.16 The morphology of the particles was determined by electron microscopy that shows that magadiite is constituted by small NPs, while the average particle size of zeolite Y is about 100 nm and occasionally shows an octahedral shape (see ESI† for images). The loading of Co in [Co]Y was determined by ICP chemical analysis after dissolution of the zeolite Y framework, giving a value of about 2.3 wt%. The framework position of Co was determined by XRD and UV-Vis spectroscopy that shows a band at 220–250 nm corresponding to charge-transfer Co–O band in tetrahedral Co2+ in the framework accompanied by a weak band at 350 nm that is attributed to Co in trigonal coordination. In contrast to the case of Co-magadiite, [Co]Y also exhibits a set of bands between 500 and 650 nm that correspond to the d–d transitions in tetrahedral Co2+ ions. Fig. 1 shows characterization data of the sample employed in the present study. All characterization results described so far for the zeolite Y containing cobalt indicate that the Co occupies very likely lattice positions as reported in the literature.17,18 The X-ray photoelectron spectra of Co-magadiite and [Co]Y zeolite vacuum dried at 300 1C (10 6 mbar) are shown in Fig. 2A. In both cases, the spectra show a doublet (2p3/2 and 2p1/2) due to the spin–orbit coupling, the spacing between them being 16 eV, which is characteristic for the Co2+ species.19 Each of the doublet bands possess a satellite band with higher binding energy due to the atom relaxation process associated with a mechanism of
14644 | Chem. Commun., 2014, 50, 14643--14646
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Fig. 2 Surface characterization data of Co-magadiite and [Co]Y materials. (A) XP spectra of the photocatalysts in the Co 2p region. (B) Auger spectra of both photocatalysts.
ligand–metal electron transfer, such a behaviour being characteristic to the Co2+ ‘‘high spin’’ species.19 The binding energy of Co 2p3/2 in [Co]Y is 781.3 eV while in Co-magadiite it is much lower (780.6 eV), which is an indication that [Co]Y possess a higher dispersion of the Co2+ ions. In the case of the [Co]Y zeolite, the higher value of the binding energy may be due to different zeolite Magdelung constant. These results seem to exclude the presence of significant amounts of CoO inside the [Co]Y framework. To confirm the results obtained by XPS, a study using Auger spectroscopy has also been performed. The Auger spectra of cobalt, for both Co-magadiite and [Co]Y samples, are shown in Fig. 2B. In the case of the Co-magadiite material, the Auger peak maximum around 768 eV, accompanied by a shoulder at higher kinetic energies, is observed. The appearance of this shoulder suggests the presence of different cobalt species (small part of Co2+ might be extraframework cobalt). In the case of [Co]Y two peaks at 767 and 772 eV would also imply the existence of two types of Co species of a different nature. For instance, the parameter Auger value of 767 eV corresponds to low coordinated species of Co (Td), while the shoulder at 772 eV corresponds to the Co species having a more elevated coordination (pentagonal) due to possible coordination with water in the sample. Summarizing the results obtained from XPS and Auger, we can conclude that cobalt in the zeolite Y is in tetrahedral coordination and is highly dispersed in the inorganic matrix, which is comparable with its presence as part of the silicate framework in agreement with the results and conclusions obtained by other techniques. In addition, we prepared a third sample, [CoO]Y, to evaluate the photocatalytic activity. It is well known in zeolite science that heteroatoms occupying framework positions tend to migrate to extraframework locations by thermal or chemical stress.20
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Even framework aluminium can be removed to extraframework positions by this type of treatment.21 Therefore, the fact that framework cobalt is present in the as-synthesized Co[Y] material does not ensure that during the photochemical reaction a fraction of them migrates from the framework to the extraframework positions. To address this issue, we have also included in our photocatalytic activity test a third sample consisting in zeolite Y incorporating inside the pores small clusters of cobalt oxide ([CoO]Y). This sample was prepared by thermal treatment of [Co]Y. [CoO]Y was prepared starting from a [Co]Y zeolite that was submitted to calcination at 540 1C. This thermal treatment transforms framework Co atoms in a large proportion into extraframework CoOx clusters, similarly to what is known for Al3+ in aluminosilicates.22 This transformation is reflected in the UV-Vis spectrum of [CoO]Y where the triplet absorption characteristic of tetrahedral Co2+ can be seen with considerably higher intensity than for [Co]Y (see the ESI† for details). Chemical analysis after zeolite dissolution shows that the cobalt content of [CoO]Y is about 2.6 wt% and the small Co oxide clusters will be located in the supercages of the Faujasite. Preliminary EXAFS data on [Co]Y and [CoO]Y are in agreement with this change in the position and coordination of Co atoms in the structure. The purpose of the work is to determine if these samples exhibit also catalytic activity for overall water splitting in the absence of a sacrificial electron donor and correlate their activity with the structure of the sample. Two types of photocatalytic experiments were performed to evaluate the activity of the three samples. One of the photocatalytic tests was carried out without a sacrificial electron donor, and therefore the water has to become reduced to hydrogen and oxidized to form oxygen by quenching conduction band electrons and valence band holes, respectively. These experiments were carried out under UV-visible light irradiation to increase the efficiency of the process. The results obtained are summarized in Table 1. As can be seen there, the three samples exhibit photocatalytic activity for overall water splitting under UV-visible-light illumination, leading to the generation of H2 and O2 in the expected stoichiometric amounts (see Table 1). The most active sample was found to be Co-magadiite (1.434 mL of H2 per g of material). This higher photocatalytic activity could be due to a more appropriate energy level of electrons and holes,
Table 1 Hydrogen production rate of cobalt-containing catalysts under UV and simulated solar light irradiationa
Amount of H2 and O2 formed [mmol(g of Co) 1] Photocatalyst
UV
Solar
Co-magadiite [Co]Y [CoO]Y
127.56 (62.4)b 74.64 (36.0) 38.40 (18.0)
80.40 74.40 12.00
a
Reaction conditions: 25 mg of photocatalyst powder; 25 1C; 25 mL deionized water or H2O : CH3OH mixture (4 : 1 volumetric ratio) for the UV or solar simulation experiments, respectively; 125 or 1000 W, irradiation sources for the UV or solar simulation experiments; 12 h of irradiation time. b The numbers in brackets correspond to the amount of O2 formed.
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but it also indicates that highly dispersed isolated independent Co atoms embedded in the silicate matrix, as they are present in magadiite, can be active for water splitting. Although less active than Co-magadiite, this finding of photocatalytic activity of isolated Co atoms in the framework positions of the silicates was also observed for [Co]Y (0.963 mL of H2 per g of material). It should be noted that commercial CoO (Aldrich) does not exhibit any detectable photocatalytic activity under the same conditions (see ESI†), showing that the preparation procedure and particle size are crucial to observe the photocatalytic activity for overall water splitting reported in the literature.4 Another set of experiments was carried out using simulated solar light that irradiates a small percentage (about 4%) in the UVA region and a large proportion of the emitted photons correspond to wavelengths in the visible spectrum (about 48%).23 In this case, addition of methanol as sacrificial electron donor was used to increase the rate of hole consumption allowing the study of the water reduction process. The results obtained are also listed in Table 1. As it can also be seen there, under solar light illumination, Co-magadiite was the best performing photocatalyst (0.896 mL of H2 per g of material), and also again the three solids under study exhibit hydrogen generation activity. It is notable that Co-magadiite and [Co]Y containing isolated Co atoms in framework positions exhibit, also under simulated sunlight, notable photocatalytic activity that in this case is close for both materials. In these two types of photocatalytic experiments, the lowest catalytic activity was exhibited by the sample containing small clusters of CoO embedded within the pore system of zeolite Y. This seems to indicate that isolated Co ions have higher intrinsic photocatalytic activity than small Co NPs. In conclusion, in the present manuscript we have shown that Co ions grafted onto a silicate matrix, either in bidimensional silicic acids or having a zeolite structure are active centers for H2 generation in the presence and even in the absence of sacrificial electron donors. The photocatalytic data of small CoO clusters embedded into the voids of zeolite Y have lower photocatalytic activity than when the Co2+ ions are isolated and grafted into the silicate matrix. In a certain way, these data are analogous to those found for Ti-atoms grafted on silicates,24 except that in the case of Co2+ photocatalysis can be promoted by visible light. Considering the large variety of silicas and silicates, as well as the possibility to include other heteroatoms and particularly aluminium, our report opens the way to explore the photocatalytic activity of a wide range of Co and, in general, transition metal containing silicates as photocatalysts for hydrogen generation and water splitting. This work was supported by the Spanish Ministry of Economy and Competitiveness (Severo Ochea and CTQ-2012-32315) and the Marie Curie project PIEF-GA-2011-298740, and the Generalidad Valenciana (Prometeo 2012/2013).
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