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Cite this: DOI: 10.1039/c4cc02994a Received 23rd April 2014, Accepted 12th June 2014
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Amine-functionalized MIL-101(Cr) with imbedded platinum nanoparticles as a durable photocatalyst for hydrogen production from water† Meicheng Wen,a Kohsuke Mori,ab Takashi Kamegawaab and Hiromi Yamashita*ab
DOI: 10.1039/c4cc02994a www.rsc.org/chemcomm
The incorporation of Pt nanoparticles into a highly stable and porous amine-functionalized MIL-101(Cr) was performed for construction of a visible light driven H2 evolution system with high activity and strong stability.
Energy shortage and environmental pollution are becoming more and more urgent due to continuous consumption of limited fossil fuels, which need to be resolved without any delay. Considerable efforts have been invested in photocatalytic H2 production from water and purification of wastewater.1 Hydrogen is highly flammable and an abundant chemical substance on earth. Many approaches to improving the hydrogen production by the decomposition of hydrogen carriers have been investigated.2 Among those technologies, the conversion of solar radiation into chemical energy for water splitting has been emerging as a most promising method. Numerous approaches have been devoted to the development of visible-light-driven photocatalysts, for instance, N-TiO2,3 metallic doped TiO24 and other visible light-responsive semiconductors (CdS, CuInZnS).5 Increasing efforts have been made to achieve molecular-based H2 evolution systems composed of a photosensitizer, a sacrificial reagent, and a non-semiconducting hydrogen generating catalyst.6 The use of a photosensitizer, such as organic dyes and organometallic complexes, is considered as an effective strategy for absorption of visible light, which acts as chlorophyll to collect sunlight energy and then transport it to a photocatalyst causing reduction of protons to H2, while the oxidized photosensitizer returns to its ground state by accepting an electron from the sacrificial reagent. The overall reaction is considered as a visible-light-driven reduction of protons by a a
Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan. E-mail: [email protected]; Fax: +81-6-6879-7457; Tel: +81-6-6879-7457 b Unit of Elements Strategy Initiative for Catalysts & Batteries Kyoto University, ESICB, Kyoto University, Japan † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cc02994a
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sacrificial reagent to molecular H2 through electron relay. For example, [Ru(bpy)3]2+ (bpy = 2,2-bipyridine) has been utilized as a photosensitizer, and noxious methyl viologen (MV2+) was used as an electron relay to produce H2 under visible light.7 In order to develop a recyclable photocatalyst and removal noxious methyl viologen, photosensitizer [Pt(tpy)Cl]Cl complexes were attached to mesoporous silica or intercalated into a layered niobate for H2 production under visible light irradiation.8 More recently, a photocatalyst (Pt) and a photosensitizer (rose bengal) were immobilized on layered double hydroxide for producing H2.9 Despite these achievements, the thermal instability of metal complexes or photobleaching of photosensitizers limits their practical application. Therefore, the development of a visiblelight-driven H2 production system with high stability and activity is still a great challenge. Metal–organic frameworks (MOFs) are an attractive class of porous organic–inorganic crystalline materials and have been employed in photocatalysis, selective gas sorption, membranes and electrical conductivity.10 Most of them are prepared using inorganic metal ions as connecting centers and organic moieties as linkers. They offer significant new chemical diversity because they can be modified by functional groups.11 However, there is limited attention focused on their utilization in photocatalysis. For instance, Ti-based and Zr-based MOFs can be applicable to CO2 reduction to form the formate anion, oxidation of organic compounds and H2 production from water.12 Subsequently, Pt NPs were imbedded into the cavities of two phosphorescent UiO MOFs built from Ir–phosphor-derived linear dicarboxylate linkers. The photoactive MOFs serve as highly efficient photocatalysts for hydrogen generation.13 More recently, Ott and co-workers described the successful incorporation of a molecular diiron catalyst into MOFs and significant enhancement in photochemical hydrogen production.14 These achievements demonstrate MOFs to be a potential new class of photocatalysts for application in versatile photocatalytic reaction. However, compared with the conventional photocatalysts of metal oxides, the photocatalytic properties of MOFs have remained unexplored.
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It is well known that amine-functionalized MIL-101(Cr) acts as a promising material for applications ranging from gas capture to catalytic reaction.15 Presently, there are no reports examining the role of NH2-MIL-101(Cr) in photocatalytic H2 evolution from dye solution. Herein, we present a new type of H2 evolution system, in which Pt NPs were loaded on NH2-MIL101(Cr) to act as a catalyst, Rhodamine B to act as chlorophyll to collect sunlight energy, and TEOA to act as a sacrificial reagent. Pt/NH2-MIL-101(Cr) can be reused at least 5 times without any loss of catalytic activity, proposing an economical method to reuse the photocatalyst. Moreover, no significant change was observed in the concentration and the main peak position of RhB in UV-vis spectra after five consecutive cycles, indicating extremely high chemical stability. NH2-MIL-101(Cr) was prepared using a method published by Chen10c with a small modification using chromic nitrate hydrate, 2-aminoterephthalic acid and sodium hydroxide. The synthesized NH2-MIL-101(Cr) (0.2 g) was suspended in H2O (20 mL) and sonicated for 20 min until it became highly dispersed. Then 3.98 mL (Pt: 1.5 wt%) of aqueous H2PtCl4 solution (3.861 mM) was added and stirred at room temperature for 8 hours. After the reaction, the products were extracted by centrifugation and washed with water. Finally, the products were dried under vacuum followed by 1 h of H2 reduction at 473 K. Pt/NH2-MIL-101(Cr) with a different level of Pt loading (0.5, 1.0, 1.5, 2.0, 3.0 wt%) was prepared. 1.5 wt% Pt loaded SiO2 was also prepared by a similar method. Fig. S1 (ESI†) shows the XRD patterns of the fresh and Pt-imbedded NH2-MIL-101 materials. The broad Bragg reflections of samples were observed, indicating the small particle size of NH2-MIL-101. The presence of Pt NPs has no significant influence on homogeneity and crystallinity in the XRD pattern, suggesting that the integrity and crystallinity were preserved. The absence of the characteristic Pt peak was due to the low Pt loadings. As shown in Fig. S2 (ESI†), the completely reversible isotherm exhibited Type-I behavior. The Brunauer– Emmett–Teller (BET) surface area (SBET) and pore volume (Vp) calculated from N2 adsorption–desorption were 1436 m2 g 1 and 0.9855 cm3 g 1 and 1121 m2 g 1 and 0.8374 cm3 g 1 for original NH2-MIL-101(Cr) and 1.5 wt% Pt NP embedded NH2-MIL-101(Cr). A decrease in the BET surface area for Pt/NH2-MIL-101 indicates that the internal cavities are occupied or blocked by Pt NPs formed within the pores of the material. MIL-101(Cr) has been widely used as a support in the domains of catalysis due to several unprecedented features such as mesoporous cages, microporous windows, giant cell volume, huge surface area, functional groups and numerous unsaturated Cr(III) sites.15 In this study, Pt NPs have successfully encapsulated onto the surface of amine functionalized MIL-101(Cr) by ionic interaction of the positively charged surface ammonium group with anionic PtCl62 , followed by the reduction with H2. Fig. 1 presents a TEM image of Pt/NH2MIL-101(Cr). The average particle size of NH2-MIL-101(Cr) was around 50 nm. The Pt NPs with the average size of 3.75 0.5 nm can be clearly observed. No significant agglomeration of Pt NPs occurred in the Pt/NH2-MIL-101(Cr) composites after H2
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Fig. 1
TEM images of 1.5 wt% Pt NP imbedded NH2-MIL-101(Cr).
Fig. 2 UV-vis spectra of (a) NH2-MIL-101, (b) 1.5 wt% Pt/NH2-MIL-101, and (c) RhB aqueous solution.
treatment, indicating that Pt NPs were highly distributed on NH2-MIL-101(Cr). Both NH2-MIL-101 and 1.5 wt% Pt/NH2-MIL-101 have a clear optical response in the visible light region as shown in Fig. 2. The bands in the high energy region can be assigned to the p-p* transition of the linker and in the region at around 600 nm can be assigned to d-d transition bands of Cr3+ (5d) ions.16 The UV-vis spectrum of RhB in TEOA, and H2O solution exhibits intense absorption bands in the low energy region (450 nm o l o 600 nm). The results demonstrate that our H2 evolution system containing RhB solution and Pt/NH2-MIL-101 is a visible-light-absorbing system. As shown in Fig. 3, the photocatalytic H2 production was performed under irradiation of visible light. A sample (10 mg) was suspended in the H2O (4 mL) and TEOA (1 mL) mixture containing 1 mM RhB. Our preliminary tests demonstrated that no significant reaction was observed in the absence of either light irradiation or a photocatalyst, suggesting that the H2 production was mainly driven by photocatalysis. No appreciable H2 production was detected without a RhB molecule. This indicates that the irradiated light is absorbed mainly by the RhB photosensitizer. It is noteworthy that the pure NH2-MIL-101(Cr) exhibited photocatalytic activity in the
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Fig. 3 Photocatalytic H2 production over (a) 1.5 wt% Pt/NH2-MIL-101(Cr), (b) NH2-MIL-101(Cr). The inset shows photoluminescence spectra of RhB solution (a) without catalyst, (b) with NH2-MIL-101(Cr), and (c) with 1.5 wt% Pt/NH2-MIL-101(Cr).
presence of RhB and irradiation of visible light even in the absence of Pt NPs, which indicates that numerous electrophilic unsaturated Cr(III) sites that appeared in the MOFs act as reactive sites to receive photoelectrons leading to the reduction of protons to H2, resulting in reduction of the self-quenching rate of RhB.17 To the best of our knowledge, there are no reports examining photocatalytic H2 production by using pure NH2-MIL-101(Cr) as a catalyst. Anatase TiO2, the most-studied semiconductor with remarkable activity for photocatalytic H2 production and mesoporous material SiO2, was selected in this system. Unlike NH2-MIL-101(Cr) with unsaturated metal sites, no significant reaction was observed by using SiO2 as a catalyst in our system and very low activity can be observed over TiO2 with the anatase phase due to the photo-excited electron injected from RhB to the conduction band of TiO2. Platinum nanoparticles have presented superior physical and chemical properties in producing H2.18 After loading with Pt nanoparticles, the Pt/NH2-MIL-101(Cr) composite showed much higher photocatalytic activity than pure NH2-MIL-101(Cr), Pt/TiO2, and Pt/SiO2 under the identical conditions (Fig. S3, ESI†). The enhanced photocatalytic activity of the Pt–NH2-MIL-101(Cr) composite might be attributed to the synergistic effect between Pt NPs and NH2-MIL-101(Cr). The inset in Fig. 3 shows the photoluminescence spectra of the RhB photosensitizer containing TEOA 20 vol% aqueous solution. The intensity slightly decreased by the addition of NH2-MIL-101, while a significant decrease was observed in the presence of Pt/NH2-MIL101(Cr). These results suggest that the occurrence of excited electron injection from RhB to NH2-MIL-101(Cr) and the Pt nanoparticles enhance the electron transfer. Fig. 4 shows the total amount of H2 evolved from the system over different Pt amounts after 6 h of visible light irradiation. The optimum Pt amount for the photoreaction in our system was found to be around 1.5 wt% Pt. The total amount of H2 evolved from the system using 0.5 wt% Pt/NH2-MIL-101(Cr) was 28.6 mmol. This corresponds to a turnover number (TON) of 110 molH2 molcat (Pt loading 0.5 wt%). The TON for the 3 wt% Pt loading sample decreased to 16.5 molH2 molcat. With the increase of Pt loading from 0.50 to 1.5 wt%, the activity enhanced owing to the increased number of active sites. A further increase of Pt
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Fig. 4 Influence of the loading amount of Pt on the H2 production.
loading to 3.0 wt% caused a decrease in activity, which can be attributed to the gathering of Pt into large nanoparticles evidenced by Fig. S4 (ESI†). Furthermore, the effects of the RhB concentration on H2 production were investigated. The optimum RhB concentration for the photocatalytic H2 production in this system was found to be around 1 mM as shown in Fig. S5 (ESI†). The efficient utilization of visible light has been a great concern for the conversion of solar light to chemical energy. Therefore, the development of a visible-light-driven H2 production system has been studied intensively using various approaches. Photocatalytic H2 production from our system was investigated under Xe lamp (500 W) irradiation using colored filters with different cutoff wavelengths. The increase in the amount of H2 formed on Pt/NH2-MIL-101 was directly proportional to light intensity as shown in Fig. S6 (ESI†). The highest activity was observed under UV-Visible irradiation. Moreover, this photocatalytic system can produce H2 even under irradiation at wavelengths longer than 500 nm, indicating the efficient utilization of visible light. For economic application, the stability of photocatalysts has always been a concern, it is important to investigate the stability and reusability of Pt/NH2-MIL-101 in photocatalytic H2 evolution. At the end of each run, the reaction vessel was bubbled with argon gas for 30 min to remove the produced H2. Subsequently, the same procedure was applied to the following cycle. Fig. S7a (ESI†) demonstrates that this system showed excellent durability and could be used repetitively 5 times with a slight decrease in activity under visible light irradiation. It is interesting to note that the concentration of RhB shows no significant decrease after the 1st cycle (Fig. S7b, ESI†). Compared with the initial concentration, the decreased concentration of RhB after the 1st cycle was mainly attributed to the absorbed RhB on Pt–NH2-MIL-101. After five consecutive cycles, almost the same concentration of RhB with a maximum absorption at 554 nm accompanied by the shoulder peak at 520 nm was monitored. Unlike the heavy halogen-substituted xanthene dye, which undergoes the photobleaching through reductive quenching during light irradiation;16 the main absorption peak of RhB in our system did not shown any shift or decrease in intensity after five consecutive cycles, demonstrating the high chemical stability.
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or self-quenching of the photosensitizer, which may offer more opportunities for photocatalytic applications. The present work was supported by Strategic China–Japan Research Cooperative Program 2012 from JST. A part of this work was also performed under a management of ‘Elements Strategy Initiative for Catalysts & Batteries (ESICB)’ supported by MEXT.
Notes and references
Scheme 1 A plausible mechanism for visible light induced photocatalytic H2 over Pt/NH2-MIL-101 from RhB solution.
The possible mechanism for H2 production from our system is illustrated in Scheme 1. After the absorption of visible light by the RhB molecule, an excited photoelectron is formed from the LUMO state of the RhB molecule. The generated photo-excited electron transfer occurs not only to the Pt NPs (path 1) but also to the unsaturated Cr(III) sites (path 2). The injected electrons to the unsaturated Cr(III) sites are occasionally transferred to the Pt NP sites (path 3). Such an electron transfer pathway can be evidenced by the significant decrease of emission intensity of RhB in the presence of NH2-MIL-101(Cr) with and without Pt. Moreover, the incident visible light absorption by the organic linker of NH2-MIL-101(Cr) produces the photo-excited electrons,19 which firstly transfer to chromium-oxo through a linker-to-cluster charge-transfer mechanism (LCCT)13 and then to the Pt NPs (path 4). Finally, the proton which gathered on the surface of NH2-MIL-101(Cr) by hydrogen bonding are transferred to the Pt NPs or unsaturated Cr(III) sites in NH2-MIL-101(Cr) by water,20 and react with photoelectrons to produce H2 (paths 5 and 6). In this case, the porous MOFs not only served as electric conductors to promote the electron transfer in facilitating the separation of photoelectrons from RhB, but also act as photo-electron generators to enhance the activity of H2 production. Meanwhile, the oxidized RhB molecule then comes back to its ground state by accepting an electron from TEOA. In summary, this work demonstrated a novel method for the construction of a H2 evolution system. This system exhibited high activity and strong durability in visible light induced photocatalytic H2 production owing to cooperative promoting effects from both the Pt imbedded MOF catalysts and the dye solution in collecting sunlight energy, transporting photoexcited electrons, protecting the photosensitizer, reducing the self-quenching of RhB, enhancing light harvest, and formation of well dispersed catalyst nanoparticles on the MOFs. It supplied a platform to design other photosensitizer/MOF based catalyst systems with strong stability against either destroying
Chem. Commun.
1 (a) T. Kamegawa, S. Matsuura, H. Seto and H. Yamashita, Angew. Chem., Int. Ed., 2013, 52, 916; (b) H. X. Li, Z. F. Bian, J. Zhu, D. Q. Zhang, G. S. Li, Y. N. Huo, H. Li and Y. F. Lu, J. Am. Chem. Soc., 2007, 129, 8406; (c) Q. J. Xiang, J. G. Yu and M. Jaroniec, J. Am. Chem. Soc., 2012, 134, 6575; (d) J. G. Yu, J. Zhang and S. W. Liu, J. Phys. Chem. C, 2010, 114, 13642. 2 (a) M. Martis, K. Mori, K. Fujiwara, W. S. Ahn and H. Yamashita, J. Phys. Chem. C, 2013, 117, 22805; (b) K. Fuku, R. Hayashi, S. Takakura, T. Kamegawa, K. Mori and H. Yamashita, Angew. Chem., Int. Ed., 2013, 52, 7446; (c) K. Mori, M. Dojo and H. Yamashita, ACS Catal., 2013, 3, 1114. 3 Y. N. Huo, Z. F. Bian, X. Y. Zhang, Y. Jin, J. Zhu and H. X. Li, J. Phys. Chem. C, 2012, 116, 18873–18877. 4 K. Fuku, T. Kamegawa, K. Mori and H. Yamashita, Chem. – Asian J., 2012, 7, 1366. 5 (a) W. Y. Chen, T. H. Yu, K. J. Chao and S. Y. Lu, Int. J. Hydrogen Energy, 2013, 38, 9665; (b) X. S. Tang, Q. L. Tay, Z. Chen, Y. Chen, G. K. L. Goh and J. M. Xue, J. Mater. Chem. A, 2013, 1, 6359. 6 E. Borgarello, J. Kiwi, J. K. Ezio, E. Pelizzetti, M. Visca and M. Gratzel, J. Am. Chem. Soc., 1981, 103, 6324. 7 A. J. Esswein and N. D. Nocera, Chem. Rev., 2007, 107, 4022. 8 (a) K. Mori, S. Ogawa, M. Martis and H. Yamashita, J. Phys. Chem. C, 2012, 116, 18873; (b) K. Mori, K. Watanabe, K. Fuku and H. Yamashita, Chem. – Eur. J., 2012, 18, 415. 9 J. D. Hong, Y. B. Wang, J. S. Pan, Z. Y. Zhong and R. Xu, Nanoscale, 2011, 3, 4655. 10 (a) K. M. Laurier, F. Vermooratele, R. Ameloot, D. De Vos, J. Hofkens and M. Roeffaers, J. Am. Chem. Soc., 2013, 135, 14488; (b) Y. Horiuchi, T. Toyao, M. Saito, K. Mochizuki, M. Iwata, H. Higashimura, M. Anpo and M. Matsuoka, J. Phys. Chem. C, 2012, 116, 20848; (c) Y. C. Lin, C. L. Kong and L. Chen, RSC Adv., 2012, 2, 6417; (d) M. Sadakiyo, H. Okawa, A. Shigenatsu, M. Ohba, T. Yamada and H. Kitagawa, J. Am. Chem. Soc., 2012, 134, 5472; (e) N. Stock and S. Biswas, Chem. Rev., 2012, 112, 933. ´rey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, 11 G. Fe ´ and I. Margiolaki, Science, 2005, 309, 2040. S. Surble 12 (a) C. Wang, E. K. deKrafft and W. B. Lin, J. Am. Chem. Soc., 2012, 134, 7211; (b) Y. H. Fu, D. R. Sun, Y. J. Chen, R. K. Huang, Z. X. Ding, X. Z. Fu and Z. H. Li, Angew. Chem., Int. Ed., 2012, 51, 3364. 13 Y. Horiuchi, T. Toyao, M. Takeuchi, M. Mastuoka and M. Anpo, Phys. Chem. Chem. Phys., 2013, 15, 13243. 14 S. Pullen, H. H. Fei, A. Orthaber, S. M. Cohen and S. Ott, J. Am. Chem. Soc., 2013, 135, 16997. ´rey and J. S. Chang, Adv. 15 D. Y. Hong, Y. K. Hwang, C. Serre, G. Fe Funct. Mater., 2009, 19, 1537. 16 A. Modrow, D. Zargarani, R. Herges and N. Stock, Dalton Trans., 2012, 41, 8690. 17 J. Pan, G. Liu, G. Q. Lu and H. M. Cheng, Angew. Chem., Int. Ed., 2011, 50, 2133. 18 M. Z. Zhu, Y. T. Lu, Y. K. Du, X. M. Wang and P. Yang, Int. J. Hydrogen Energy, 2011, 36, 4298. 19 J. He, Z. Yan, J. Q. Wang, J. Xie, L. Jiang, Y. M. Shi, F. G. Yuan, F. Yu and Y. J. Sun, Chem. Commun., 2013, 49, 6761. 20 B. Wu, X. C. Lin, L. Ge, L. Wu and T. W. Xu, Chem. Commun., 2013, 49, 143.
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Cite this: DOI: 10.1039/c4cc02994a Received 23rd April 2014, Accepted 12th June 2014
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Amine-functionalized MIL-101(Cr) with imbedded platinum nanoparticles as a durable photocatalyst for hydrogen production from water† Meicheng Wen,a Kohsuke Mori,ab Takashi Kamegawaab and Hiromi Yamashita*ab
DOI: 10.1039/c4cc02994a www.rsc.org/chemcomm
The incorporation of Pt nanoparticles into a highly stable and porous amine-functionalized MIL-101(Cr) was performed for construction of a visible light driven H2 evolution system with high activity and strong stability.
Energy shortage and environmental pollution are becoming more and more urgent due to continuous consumption of limited fossil fuels, which need to be resolved without any delay. Considerable efforts have been invested in photocatalytic H2 production from water and purification of wastewater.1 Hydrogen is highly flammable and an abundant chemical substance on earth. Many approaches to improving the hydrogen production by the decomposition of hydrogen carriers have been investigated.2 Among those technologies, the conversion of solar radiation into chemical energy for water splitting has been emerging as a most promising method. Numerous approaches have been devoted to the development of visible-light-driven photocatalysts, for instance, N-TiO2,3 metallic doped TiO24 and other visible light-responsive semiconductors (CdS, CuInZnS).5 Increasing efforts have been made to achieve molecular-based H2 evolution systems composed of a photosensitizer, a sacrificial reagent, and a non-semiconducting hydrogen generating catalyst.6 The use of a photosensitizer, such as organic dyes and organometallic complexes, is considered as an effective strategy for absorption of visible light, which acts as chlorophyll to collect sunlight energy and then transport it to a photocatalyst causing reduction of protons to H2, while the oxidized photosensitizer returns to its ground state by accepting an electron from the sacrificial reagent. The overall reaction is considered as a visible-light-driven reduction of protons by a a
Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan. E-mail: [email protected]; Fax: +81-6-6879-7457; Tel: +81-6-6879-7457 b Unit of Elements Strategy Initiative for Catalysts & Batteries Kyoto University, ESICB, Kyoto University, Japan † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cc02994a
This journal is © The Royal Society of Chemistry 2014
sacrificial reagent to molecular H2 through electron relay. For example, [Ru(bpy)3]2+ (bpy = 2,2-bipyridine) has been utilized as a photosensitizer, and noxious methyl viologen (MV2+) was used as an electron relay to produce H2 under visible light.7 In order to develop a recyclable photocatalyst and removal noxious methyl viologen, photosensitizer [Pt(tpy)Cl]Cl complexes were attached to mesoporous silica or intercalated into a layered niobate for H2 production under visible light irradiation.8 More recently, a photocatalyst (Pt) and a photosensitizer (rose bengal) were immobilized on layered double hydroxide for producing H2.9 Despite these achievements, the thermal instability of metal complexes or photobleaching of photosensitizers limits their practical application. Therefore, the development of a visiblelight-driven H2 production system with high stability and activity is still a great challenge. Metal–organic frameworks (MOFs) are an attractive class of porous organic–inorganic crystalline materials and have been employed in photocatalysis, selective gas sorption, membranes and electrical conductivity.10 Most of them are prepared using inorganic metal ions as connecting centers and organic moieties as linkers. They offer significant new chemical diversity because they can be modified by functional groups.11 However, there is limited attention focused on their utilization in photocatalysis. For instance, Ti-based and Zr-based MOFs can be applicable to CO2 reduction to form the formate anion, oxidation of organic compounds and H2 production from water.12 Subsequently, Pt NPs were imbedded into the cavities of two phosphorescent UiO MOFs built from Ir–phosphor-derived linear dicarboxylate linkers. The photoactive MOFs serve as highly efficient photocatalysts for hydrogen generation.13 More recently, Ott and co-workers described the successful incorporation of a molecular diiron catalyst into MOFs and significant enhancement in photochemical hydrogen production.14 These achievements demonstrate MOFs to be a potential new class of photocatalysts for application in versatile photocatalytic reaction. However, compared with the conventional photocatalysts of metal oxides, the photocatalytic properties of MOFs have remained unexplored.
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It is well known that amine-functionalized MIL-101(Cr) acts as a promising material for applications ranging from gas capture to catalytic reaction.15 Presently, there are no reports examining the role of NH2-MIL-101(Cr) in photocatalytic H2 evolution from dye solution. Herein, we present a new type of H2 evolution system, in which Pt NPs were loaded on NH2-MIL101(Cr) to act as a catalyst, Rhodamine B to act as chlorophyll to collect sunlight energy, and TEOA to act as a sacrificial reagent. Pt/NH2-MIL-101(Cr) can be reused at least 5 times without any loss of catalytic activity, proposing an economical method to reuse the photocatalyst. Moreover, no significant change was observed in the concentration and the main peak position of RhB in UV-vis spectra after five consecutive cycles, indicating extremely high chemical stability. NH2-MIL-101(Cr) was prepared using a method published by Chen10c with a small modification using chromic nitrate hydrate, 2-aminoterephthalic acid and sodium hydroxide. The synthesized NH2-MIL-101(Cr) (0.2 g) was suspended in H2O (20 mL) and sonicated for 20 min until it became highly dispersed. Then 3.98 mL (Pt: 1.5 wt%) of aqueous H2PtCl4 solution (3.861 mM) was added and stirred at room temperature for 8 hours. After the reaction, the products were extracted by centrifugation and washed with water. Finally, the products were dried under vacuum followed by 1 h of H2 reduction at 473 K. Pt/NH2-MIL-101(Cr) with a different level of Pt loading (0.5, 1.0, 1.5, 2.0, 3.0 wt%) was prepared. 1.5 wt% Pt loaded SiO2 was also prepared by a similar method. Fig. S1 (ESI†) shows the XRD patterns of the fresh and Pt-imbedded NH2-MIL-101 materials. The broad Bragg reflections of samples were observed, indicating the small particle size of NH2-MIL-101. The presence of Pt NPs has no significant influence on homogeneity and crystallinity in the XRD pattern, suggesting that the integrity and crystallinity were preserved. The absence of the characteristic Pt peak was due to the low Pt loadings. As shown in Fig. S2 (ESI†), the completely reversible isotherm exhibited Type-I behavior. The Brunauer– Emmett–Teller (BET) surface area (SBET) and pore volume (Vp) calculated from N2 adsorption–desorption were 1436 m2 g 1 and 0.9855 cm3 g 1 and 1121 m2 g 1 and 0.8374 cm3 g 1 for original NH2-MIL-101(Cr) and 1.5 wt% Pt NP embedded NH2-MIL-101(Cr). A decrease in the BET surface area for Pt/NH2-MIL-101 indicates that the internal cavities are occupied or blocked by Pt NPs formed within the pores of the material. MIL-101(Cr) has been widely used as a support in the domains of catalysis due to several unprecedented features such as mesoporous cages, microporous windows, giant cell volume, huge surface area, functional groups and numerous unsaturated Cr(III) sites.15 In this study, Pt NPs have successfully encapsulated onto the surface of amine functionalized MIL-101(Cr) by ionic interaction of the positively charged surface ammonium group with anionic PtCl62 , followed by the reduction with H2. Fig. 1 presents a TEM image of Pt/NH2MIL-101(Cr). The average particle size of NH2-MIL-101(Cr) was around 50 nm. The Pt NPs with the average size of 3.75 0.5 nm can be clearly observed. No significant agglomeration of Pt NPs occurred in the Pt/NH2-MIL-101(Cr) composites after H2
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Fig. 1
TEM images of 1.5 wt% Pt NP imbedded NH2-MIL-101(Cr).
Fig. 2 UV-vis spectra of (a) NH2-MIL-101, (b) 1.5 wt% Pt/NH2-MIL-101, and (c) RhB aqueous solution.
treatment, indicating that Pt NPs were highly distributed on NH2-MIL-101(Cr). Both NH2-MIL-101 and 1.5 wt% Pt/NH2-MIL-101 have a clear optical response in the visible light region as shown in Fig. 2. The bands in the high energy region can be assigned to the p-p* transition of the linker and in the region at around 600 nm can be assigned to d-d transition bands of Cr3+ (5d) ions.16 The UV-vis spectrum of RhB in TEOA, and H2O solution exhibits intense absorption bands in the low energy region (450 nm o l o 600 nm). The results demonstrate that our H2 evolution system containing RhB solution and Pt/NH2-MIL-101 is a visible-light-absorbing system. As shown in Fig. 3, the photocatalytic H2 production was performed under irradiation of visible light. A sample (10 mg) was suspended in the H2O (4 mL) and TEOA (1 mL) mixture containing 1 mM RhB. Our preliminary tests demonstrated that no significant reaction was observed in the absence of either light irradiation or a photocatalyst, suggesting that the H2 production was mainly driven by photocatalysis. No appreciable H2 production was detected without a RhB molecule. This indicates that the irradiated light is absorbed mainly by the RhB photosensitizer. It is noteworthy that the pure NH2-MIL-101(Cr) exhibited photocatalytic activity in the
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Fig. 3 Photocatalytic H2 production over (a) 1.5 wt% Pt/NH2-MIL-101(Cr), (b) NH2-MIL-101(Cr). The inset shows photoluminescence spectra of RhB solution (a) without catalyst, (b) with NH2-MIL-101(Cr), and (c) with 1.5 wt% Pt/NH2-MIL-101(Cr).
presence of RhB and irradiation of visible light even in the absence of Pt NPs, which indicates that numerous electrophilic unsaturated Cr(III) sites that appeared in the MOFs act as reactive sites to receive photoelectrons leading to the reduction of protons to H2, resulting in reduction of the self-quenching rate of RhB.17 To the best of our knowledge, there are no reports examining photocatalytic H2 production by using pure NH2-MIL-101(Cr) as a catalyst. Anatase TiO2, the most-studied semiconductor with remarkable activity for photocatalytic H2 production and mesoporous material SiO2, was selected in this system. Unlike NH2-MIL-101(Cr) with unsaturated metal sites, no significant reaction was observed by using SiO2 as a catalyst in our system and very low activity can be observed over TiO2 with the anatase phase due to the photo-excited electron injected from RhB to the conduction band of TiO2. Platinum nanoparticles have presented superior physical and chemical properties in producing H2.18 After loading with Pt nanoparticles, the Pt/NH2-MIL-101(Cr) composite showed much higher photocatalytic activity than pure NH2-MIL-101(Cr), Pt/TiO2, and Pt/SiO2 under the identical conditions (Fig. S3, ESI†). The enhanced photocatalytic activity of the Pt–NH2-MIL-101(Cr) composite might be attributed to the synergistic effect between Pt NPs and NH2-MIL-101(Cr). The inset in Fig. 3 shows the photoluminescence spectra of the RhB photosensitizer containing TEOA 20 vol% aqueous solution. The intensity slightly decreased by the addition of NH2-MIL-101, while a significant decrease was observed in the presence of Pt/NH2-MIL101(Cr). These results suggest that the occurrence of excited electron injection from RhB to NH2-MIL-101(Cr) and the Pt nanoparticles enhance the electron transfer. Fig. 4 shows the total amount of H2 evolved from the system over different Pt amounts after 6 h of visible light irradiation. The optimum Pt amount for the photoreaction in our system was found to be around 1.5 wt% Pt. The total amount of H2 evolved from the system using 0.5 wt% Pt/NH2-MIL-101(Cr) was 28.6 mmol. This corresponds to a turnover number (TON) of 110 molH2 molcat (Pt loading 0.5 wt%). The TON for the 3 wt% Pt loading sample decreased to 16.5 molH2 molcat. With the increase of Pt loading from 0.50 to 1.5 wt%, the activity enhanced owing to the increased number of active sites. A further increase of Pt
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Fig. 4 Influence of the loading amount of Pt on the H2 production.
loading to 3.0 wt% caused a decrease in activity, which can be attributed to the gathering of Pt into large nanoparticles evidenced by Fig. S4 (ESI†). Furthermore, the effects of the RhB concentration on H2 production were investigated. The optimum RhB concentration for the photocatalytic H2 production in this system was found to be around 1 mM as shown in Fig. S5 (ESI†). The efficient utilization of visible light has been a great concern for the conversion of solar light to chemical energy. Therefore, the development of a visible-light-driven H2 production system has been studied intensively using various approaches. Photocatalytic H2 production from our system was investigated under Xe lamp (500 W) irradiation using colored filters with different cutoff wavelengths. The increase in the amount of H2 formed on Pt/NH2-MIL-101 was directly proportional to light intensity as shown in Fig. S6 (ESI†). The highest activity was observed under UV-Visible irradiation. Moreover, this photocatalytic system can produce H2 even under irradiation at wavelengths longer than 500 nm, indicating the efficient utilization of visible light. For economic application, the stability of photocatalysts has always been a concern, it is important to investigate the stability and reusability of Pt/NH2-MIL-101 in photocatalytic H2 evolution. At the end of each run, the reaction vessel was bubbled with argon gas for 30 min to remove the produced H2. Subsequently, the same procedure was applied to the following cycle. Fig. S7a (ESI†) demonstrates that this system showed excellent durability and could be used repetitively 5 times with a slight decrease in activity under visible light irradiation. It is interesting to note that the concentration of RhB shows no significant decrease after the 1st cycle (Fig. S7b, ESI†). Compared with the initial concentration, the decreased concentration of RhB after the 1st cycle was mainly attributed to the absorbed RhB on Pt–NH2-MIL-101. After five consecutive cycles, almost the same concentration of RhB with a maximum absorption at 554 nm accompanied by the shoulder peak at 520 nm was monitored. Unlike the heavy halogen-substituted xanthene dye, which undergoes the photobleaching through reductive quenching during light irradiation;16 the main absorption peak of RhB in our system did not shown any shift or decrease in intensity after five consecutive cycles, demonstrating the high chemical stability.
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or self-quenching of the photosensitizer, which may offer more opportunities for photocatalytic applications. The present work was supported by Strategic China–Japan Research Cooperative Program 2012 from JST. A part of this work was also performed under a management of ‘Elements Strategy Initiative for Catalysts & Batteries (ESICB)’ supported by MEXT.
Notes and references
Scheme 1 A plausible mechanism for visible light induced photocatalytic H2 over Pt/NH2-MIL-101 from RhB solution.
The possible mechanism for H2 production from our system is illustrated in Scheme 1. After the absorption of visible light by the RhB molecule, an excited photoelectron is formed from the LUMO state of the RhB molecule. The generated photo-excited electron transfer occurs not only to the Pt NPs (path 1) but also to the unsaturated Cr(III) sites (path 2). The injected electrons to the unsaturated Cr(III) sites are occasionally transferred to the Pt NP sites (path 3). Such an electron transfer pathway can be evidenced by the significant decrease of emission intensity of RhB in the presence of NH2-MIL-101(Cr) with and without Pt. Moreover, the incident visible light absorption by the organic linker of NH2-MIL-101(Cr) produces the photo-excited electrons,19 which firstly transfer to chromium-oxo through a linker-to-cluster charge-transfer mechanism (LCCT)13 and then to the Pt NPs (path 4). Finally, the proton which gathered on the surface of NH2-MIL-101(Cr) by hydrogen bonding are transferred to the Pt NPs or unsaturated Cr(III) sites in NH2-MIL-101(Cr) by water,20 and react with photoelectrons to produce H2 (paths 5 and 6). In this case, the porous MOFs not only served as electric conductors to promote the electron transfer in facilitating the separation of photoelectrons from RhB, but also act as photo-electron generators to enhance the activity of H2 production. Meanwhile, the oxidized RhB molecule then comes back to its ground state by accepting an electron from TEOA. In summary, this work demonstrated a novel method for the construction of a H2 evolution system. This system exhibited high activity and strong durability in visible light induced photocatalytic H2 production owing to cooperative promoting effects from both the Pt imbedded MOF catalysts and the dye solution in collecting sunlight energy, transporting photoexcited electrons, protecting the photosensitizer, reducing the self-quenching of RhB, enhancing light harvest, and formation of well dispersed catalyst nanoparticles on the MOFs. It supplied a platform to design other photosensitizer/MOF based catalyst systems with strong stability against either destroying
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