Visible-Light-Promoted Photocatalytic Hydrogen Production by

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Visible-Light-Promoted Photocatalytic Hydrogen Production by Using an Amino-Functionalized Ti(IV) Metal-Organic Framework Yu Horiuchi, Takashi Toyao, Masakazu Saito, Katsunori Mochizuki, Masatoshi Iwata, Hideyuki Higashimura, Masakazu Anpo, and Masaya Matsuoka J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp3046005 • Publication Date (Web): 07 Sep 2012 Downloaded from http://pubs.acs.org on September 20, 2012

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The Journal of Physical Chemistry

Visible-Light-Promoted Photocatalytic Hydrogen Production by Using an Amino-Functionalized Ti(IV) Metal–Organic Framework Yu Horiuchi,† Takashi Toyao,† Masakazu Saito,† Katsunori Mochizuki,‡ Masatoshi Iwata,‡ Hideyuki Higashimura,‡ Masakazu Anpo,† and Masaya Matsuoka†,* †

Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture

University, 1-1, Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan ‡

Advanced Materials Research Laboratory Sumitomo-Chemical Co., Ltd., 6, Kitahara, Tsukuba,

Ibaraki 300-3294, Japan

KEYWORDS: Metal–organic framework, Visible-light-responsive photocatalyst, Hydrogen production, ESR measurement

1 ACS Paragon Plus Environment


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ABSTRACT The present article describes the hydrogen production from an aqueous medium over aminofunctionalized Ti(IV) metal–organic framework (Ti-MOF-NH2) under visible-light irradiation. Ti-MOF-NH2, which employs 2-amino-benzenedicarboxylic acid as an organic linker, has been synthesized by a facile solvothermal method. Pt nanoparticles as co-catalysts are then deposited onto Ti-MOF-NH2 via a photodeposition process (Pt/Ti-MOF-NH2). The XRD and N2 adsorption measurements reveal the successful formation of a MOF framework structure and remaining its structure after deposition of Pt nanoparticles. The observable visible-light absorption up to around 500 nm can be seen in the DRUV−vis spectrum of Ti-MOF-NH2, which is associated with the chromophore in the organic linker. Ti-MOF-NH2 and Pt/Ti-MOF-NH2 exhibit efficient photocatalytic activities for hydrogen production from an aqueous solution containing triethanolamine as a sacrificial electron donor under visible-light irradiation. The longest wavelength available for the reaction is 500 nm. The results obtained from wavelengthdependent photocatalytic tests and photocurrent measurements as well as in-situ ESR measurements demonstrate that the reaction proceeds through the light absorption by their organic linker and the following electron transfer to the catalytically active titanium-oxo cluster.

1. INTRODUCTION Photocatalytic hydrogen production from water splitting is of considerable interest owing to its potential application to clean and renewable energy production. Earlier efforts have demonstrated that semiconductor-type photocatalysts can be used to promote this process. However, these types of photocatalysts are typically active only under UV light irradiation


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conditions, which accounts for only 3–5% of the solar spectrum. Therefore, the development of catalysts for the water splitting reaction that respond to visible light and, thus, can effectively utilize sun light, is extremely important. To date, several types of hydrogen production photocatalysts, which operate under visible light irradiation conditions, have been developed. Included in this group are heteroatom-doped,1-5 non-oxide type,6-10 dye-sensitized,11-14 plasmonic,15 and Z-scheme type photocatalysts.16-17 Moreover, in the field of metal complex catalysts, visible-light-driven hydrogen production from water has been realized by employing a multicomponent system consisting of photosensitizers, redox mediators, and water reduction catalysts. Although all of these systems have beneficial features, the design of alternative photocatalysts with reduced complexity is a significant goal. Great attention in the area of photocatalytic hydrogen production has been given to inorganic–organic hybrid materials. This interest is driven by the potential benefit of integrating the functions of inorganic and organic components within a single composite.18-22 Metal–organic frameworks (MOFs), also referred to as porous coordination polymers (PCPs), are a new class of hybrid porous materials that possess three-dimensional crystalline frameworks comprised of metal-oxo clusters and organic linkers.23-24 Owing to their inherent structural characteristics, such as large surface areas and well-ordered porous structures, these materials serve as adsorbents,25-28 catalyst supports,29-32 and catalysts33-37 as well as agents for separations38-40 and drug delivery.41-42 Additionally, the topology and surface functionality of MOFs can be readily tuned by varying the constituent metals and bridging organic linkers. At the current time, a major interest in MOFs focuses on the simultaneous utilization of the unique characteristics of the metal-oxo clusters and organic linkers.



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In the context of their application to visible-light-promoted photocatalytic hydrogen production, MOFs possess great flexibility in terms of framework design. As a result, it should be possible to integrate a photosensitizer and a water reduction catalyst within a single MOF system. In independent studies, Garcia et al.43 and Majima et al.44 observed that electron transfer takes place from the photo-excited organic linker to the metal-oxo cluster within MOF-5. This process, termed linker-to-cluster charge-transfer (LCCT), occurs in photocatalytic reactions and photoluminescence in which MOF-5 participates. Garcia et al. also described a method for photocatalytic hydrogen production, using a Zrbased MOF and irradiation with 370 nm light.45 In considering the features of the hydrogen production system via a LCCT mechanism, we realized that the conduction band (CB) edge position of titanium-oxo cluster would be suitable for efficient charge transfer from the excited state of the organic linker owing to the fact that the CB potential of a titanium-oxo cluster is more positive than that of the zirconium counterpart. Hence, Ti-based MOFs expected to be superior photocatalysts for the hydrogen production reaction. Recently, Li et al. firstly reported the successful tuning of the absorption of a Ti-based MOF by using a visible-light-active organic linker, 2-amino-benzenedicarboxylic acid, and its application to a CO2 reduction photocatalyst operating under visible light.46 Furthermore, we have attained the visible-light-promoted photocatalytic hydrogen production by using Ti-based MOFs on the basis of the same LCCT mechanism47-48 and reported the part of the results in JP patent.49 Herein we prepared an amino-functionalized Ti(IV) MOF (Ti-MOF-NH2) that contains 2amino-benzenedicarboxylic acid (H2BDC-NH2; C8H7NO4), which is utilized in place of the conventional organic linker of 1,4-benzenedicarboxylic acid (H2BDC; C8H6O4) in Ti-MOF (MIL-125), as the light absorbing component. Ti-MOF-NH2 was then probed as a visible-light-


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responsive material for photocatalytic hydrogen production reaction. The action spectrum of TiMOF-NH2 along with the results of in-situ ESR experiments demonstrate that hydrogen production takes place efficiently through the LCCT mechanism illustrated in Figure 1. The observations made in this investigation are described and discussed below.

Figure 1. A schematic illustration of photocatalytic hydrogen production reaction over Ptsupported Ti-MOF-NH2 on the basis of the LCCT mechanism.

2. EXPERIMENTAL 2.1. Materials. Tetrapropyl orthotitanate (TPOT; Ti(OC3H7)4) was purchased from Tokyo Kasei Kogyo Co., Ltd. 2-Amino-benzenedicarboxylic acid (H2BDC-NH2; C8H7NO4), 1,4benzenedicarboxylic acid (H2BDC; C8H6O4), N’N-dimetylformanide (DMF; (CH3)2NCHO), and methanol (CH3OH) were purchased from Nacalai Tesque Inc. Hydrogen hexachloroplatinate(IV) hexahydrate (H2PtCl6•6H2O) was purchased from Kishida Chemicals Co., Ltd. 2.2. Synthesis of MOF photocatalysts.

Ti-MOF-NH2 was synthesized by using a

solvothermal method that is based on one in the preparation of Ti-MOF reported by Dan-Hardi et



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al.50 A mixture of TPOT (2.4 mL), H2BDC-NH2 (2.2 g), DMF (36 mL), and methanol (4.0 mL) was subjected to solvothermal conditions in a Teflon-lined stainless steel autoclave for 48 h at 423 K under autogenous pressure. The formed precipitate was separated by filtration, washed repeatedly with DMF and dried at room temperature overnight. Finally the obtained powder sample was dried under vacuum for 1 h to remove the residual H2BDC-NH2. Pt nanoparticles as co-catalysts were deposited on Ti-MOF-NH2 by employing a photodeposition method. Typically, a suspension containing Ti-MOF-NH2 (0.40 g), 0.039 м H2PtCl6 methanol solution (0.55 mL), DMF (20 mL), and methanol (20 mL) was irradiated with UV light from a 100 W high-pressure Hg lamp at 5 mW cm-2 for 3 h with stirring. The obtained precipitate was separated by centrifugation, washed repeatedly with DMF and dried in air at room temperature overnight. Finally the obtained powder was dried under vacuum for 1 h, yielding Pt/Ti-MOF-NH2. For comparison purposes, conventional Ti-MOF and Pt-deposited Ti-MOF (Pt/Ti-MOF) were also prepared by using H2BDC. 2.3. Characterization. Standard θ–2θ X-ray diffraction (XRD) data were recorded on a Shimadzu X-ray diffractmeter XRD-6100 using Cu Kα radiation (λ = 1.5406 Å). Specific surface areas were estimated from the amount of N2 adsorption at 77 K using the BET (BrunauerEmmett-Teller) equilibrium equation. Diffuse reflectance UV–vis (DRUV–vis) spectra were obtained with a Shimadzu UV–vis recording spectrophotometer 2200A. Electron spin resonance (ESR) spectra were recorded with a JEOL JES-RE-2X at 77 K. Prior to the measurements, the sample immersed in an aqueous solution containing 0.01 м triethanolamine (TEOA; C6H15NO3) (2 mL) was added an in-situ cell, evacuated at 77 K to remove dissolved oxygen, and then irradiated with visible light having a wavelength of λ > 420 nm from a Xe lamp (500 W; San-Ei Electric Inc. XEF-501S) with an appropriate cut-off filter for 3 h at room temperature.


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2.4. Photoelectrochemical measurements.

The photoelectrochemical test system was

composed of an HZ3000 potentiostat (Hokuto Denko Corp.), a Xe lamp (the same lamp used in ESR measurements) with an appropriate cut-off filter, and a homemade three-electrode cell with Pt mesh as the counter electrode, saturated calomel electrode (SCE) as the reference electrode, and Ti-MOF-NH2 as the working electrode. As the electrolyte solution, 0.25 м K2SO4 aqueous solution containing 0.01 м TEOA was used. For the preparation of the Ti-MOF-NH2 electrode, a suspension made of Ti-MOF-NH2 and methanol was deposited onto indium tin oxide coated polyethylene naphthalate (ITO-PEN) film by a doctor blade coating method and then dried at 373 K for 1 h. The spectral response of the Ti-MOF-NH2 electrode was measured by monitoring the photocurrent signal produced by a light irradiation through an appropriate cut-off filter under a constant potential of 0.5 V vs. SCE. The area irradiated was set at 1.0 cm2. 2.5. Photocatalytic hydrogen production reaction. The photocatalyst (10 mg) and 0.01 м aqueous TEOA solution (2 mL) were added to a pyrex reaction vessel connected to vacuum line. The resulting mixture was evacuated at 77 K to remove dissolved oxygen. Subsequently, the sample was irradiated from the side with a Xe lamp (the same lamp used in ESR measurements) through an appropriate cut-off filter with stirring at room temperature. After the reaction, the resulting gas was analyzed by using a GC with a Shimadzu GC-12A with a thermal conductivity detector equipped with MS-5A column.

3. RESULTS AND DISCUSSION 3.1 Characterization of MOF photocatalysts. In Figure 2 are displayed XRD patterns for Ti-MOF-NH2 and Ti-MOF, before and after the photodeposition of Pt nanoparticles. As can be



seen by viewing these images, Ti-MOF-NH2 exhibits the same diffraction pattern as Ti-MOF, indicating that a MIL-125 structure is produced when H2BDC-NH2 is utilized as an organic linker.50 Additionally, no traces of characteristic peaks corresponding to bulk titanium dioxide phases, such as anatase and rutile, are observed in the diffraction pattern of Ti-MOF-NH2. This result indicates that the framework of Ti-MOF-NH2 consists of small titanium oxide clusters in the absence of bulky aggregated titanium oxide species. After photodeposition of Pt nanoparticles, the diffraction patterns of Ti-MOF-NH2 and Ti-MOF remained nearly unchanged, suggesting that the photodeposition process does not influence the structures of these materials.

2500 cps (d) Pt/Ti-MOF-NH2 (c) Ti-MOF-NH2

Intensity

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(b) Pt/Ti-MOF

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Figure 2. XRD patterns of (a,b) Ti-MOF and (c,d) Ti-MOF-NH2 before (a,c) and after (b,d) photodeposition of Pt nanoparticles.

To evaluate the pore structures and specific surface areas of the materials, N2 adsorption measurements were performed before and after photodeposition of Pt nanoparticles. The specific surface areas of Ti-MOF-NH2 and Ti-MOF were determined to be 1101 and 1202 m2 g-1, respectively, by using BET method based calculations on N2 adsorption isotherm data. These


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findings show that Ti-MOF-NH2 and Ti-MOF have large specific surface areas associated with their microporous structures. Although the specific surface areas of these materials decrease slightly after Pt nanoparticle photodeposition owing to a pore blockage, Ti-MOF-NH2 and TiMOF maintain their porous structures and high specific surface areas of 910 and 946 m2 g-1, respectively. In Figure 3 are displayed DRUV–vis spectra of Ti-MOF-NH2 and Ti-MOF, together with those of H2BDC-NH2 and H2BDC as reference samples (inset). The spectrum of Ti-MOF shows absorption bands below 350 nm, while an observable absorption band is seen in the spectrum of Ti-MOF-NH2 up to around 500 nm that is associated with the chromophore in BDC-NH2 units. The results demonstrate that Ti-MOF-NH2, containing BDC-NH2 units as the organic linker, is a visible-light absorbing MOF.

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Figure 3. DRUV–vis spectra of Ti-MOF (dotted line) and Ti-MOF-NH2 (solid line). Inset shows the DRUV–vis spectra of H2BDC (dotted line) and H2BDC-NH2 (solid line).



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3.2. Photocatalytic hydrogen production. In an investigation exploring the potential photocatalytic activity of Pt/Ti-MOF-NH2, hydrogen production from an aqueous solution containing this catalyst and 0.01 м TEOA as a sacrificial electron donor was monitored while being subjected to visible light irradiation at room temperature. Optical filters were used to modulate the wavelength of the broad-band visible light source. The time course of photocatalytic hydrogen production under visible light irradiation (λ > 420 nm) over Pt/Ti-MOFNH2 for 9 h, with intermittent evacuation and exposure to atmospheric conditions every 3 h, is displayed in Figure 4. Continuous hydrogen production occurs from the beginning of the irradiation period, and the total evolution of hydrogen after 9 h irradiation reaches 33 µmol. These results clearly show that Pt/Ti-MOF-NH2 acts as an efficient visible-light-responsive, hydrogen production photocatalyst and that it does not lose its photocatalytic activity over at least three cycles. After the reaction Pt/Ti-MOF-NH2 possesses slightly lower BET specific surface area of 742 m2 g-1 compared to that of the fresh sample (910 m2 g-1). Moreover, the intensities of the diffraction peaks corresponding to the MIL-125 structure decrease marginally, as displayed in Figure S1. The inspection of these results demonstrates that the porous network of Pt/Ti-MOF-NH2 is partly deteriorated, which, however, has only a small impact on photocatalytic activity. In addition, as shown in Figure S2, when the reaction was performed over Pt/Ti-MOF-NH2 without a 3-hour evacuation interval, the amount of hydrogen evolved increases steadily for 6 h periods. After 6 h irradiation, however, the evolution rate is slightly decreased. It can be considered that the decrease of the concentration of sacrificial electron donor and/or the partial deterioration of MOF framework structure during the reaction is responsible for the slightly-decreased activity.


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Figure 4. The time course of photocatalytic hydrogen production under visible-light irradiation (λ > 420 nm) over Pt/Ti-MOF-NH2 for a total of 9 h with intermittent evacuation and exposure to atmospheric conditions every 3 h.

In contrast, Pt/Ti-MOF displays no photocatalytic activity when employed under the same reaction conditions. This finding indicates that visible-light absorption characteristics associated with the BDC-NH2 moiety are indispensable for promoting the photocatalytic hydrogen production reaction under visible-light irradiation. Incidentally, Ti-MOF-NH2, not containing deposited Pt nanoparticles, exhibits a slightly lower photocatalytic activity than does Pt/Ti-MOFNH2 (Figure S3), suggesting that efficient charge separation caused by the presence of Pt nanoparticles as a co-catalyst plays a role in enhancing the efficiency of the hydrogen production reaction. Meanwhile, when other sacrificial electron donors, such as TEA (triethylamine), EDTA (ethylenediaminetetraacetic acid), and methanol, are employed, Pt/Ti-MOF-NH2 does not promote the reaction. This may be due to the weak oxidation power of the organic linker of BDC-NH2.



The dependence of the hydrogen evolution rate on the wavelength of incident light employed in the Pt/Ti-MOF-NH2 photocatalytic system is displayed in Figure 5A. The hydrogen evolution rate is observed to depend strongly on wavelength in a manner that correlates with absorption intensities in the visible spectrum of Ti-MOF-NH2 (Figure 5A(g)). The similar results were obtained by photoelectrochemical measurements. As shown in Figure 5B, the wavelengthdependent photocurrents observed for the Ti-MOF-NH2 electrode in an electrolyte containing TEOA under visible-light irradiation (at a constant potential of 0.5 V vs. SCE) demonstrated that

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(e)

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Figure 5. The dependence of (A) the hydrogen evolution rate for Pt/Ti-MOF-NH2 and (B) the photocurrent for the Ti-MOF-NH2 electrode on the wavelength of incident light (a–f and a’–f’), and DRUV–vis spectrum of Ti-MOF-NH2 (g,g’). Wavelength region of irradiated light: (a,a’) 380–420 nm, (b,b’) 420–450 nm, (c,c’) 450–480 nm, (d,d’) 480–500 nm, (e,e’) 500–550 nm, and (f,f’) above 550 nm. Wavelength region of irradiated light was controlled by using six different cut-off filters (λ > 380, 420, 450, 480, 500, and 550 nm). The photocurrents were recorded in 0.25 м K2SO4 aqueous solution containing 0.01 м TEOA under visible-light irradiation at a constant potential of 0.5 V vs. SCE.


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the trend of photocurrent response shows good parallel relationship with the absorption spectrum. These findings indicate that the organic linker of BDC-NH2 absorbs the incident visible light and that its excited state transfers electrons to the CB of titanium-oxo clusters in the manner expected for operation of LCCT mechanism. On the other hand, irradiation of photocatalytic system with UV-light (λ > 300 nm) from a 500 W Hg lamp (Ushio Inc.), Pt/Ti-MOF-NH2 leads to a much higher hydrogen evolution rate (11.7 µmol h-1) as a consequence of direct excitation of titaniumoxo clusters. 3.3. Clarification of the reaction mechanism. The results of in-situ ESR measurements after visible-light irradiation also confirmed that LCCT mechanism is followed in the photocatalytic process. For this measurement, Ti-MOF-NH2 was immersed in a solution of 0.01 м aqueous TEOA, and the suspension was evacuated at 77 K. This was followed by irradiation with visible light (λ > 420 nm) for 3 h at room temperature. After irradiation, the ESR spectrum was recorded at 77 K. As show in Figure 6, the ESR spectrum after visible-light irradiation contains signals which have parameters that are characteristic of paramagnetic Ti3+ centers in a distorted rhombic oxygen ligand ﬁeld (gx = 1.980, gy = 1.953, and gz = 1.889).50-51 In contrast, no signals assignable to Ti3+ species are observed in the ESR spectrum of Ti-MOF after visible-light irradiation (Figure S4). These results suggest that visible light promotes transfer of photogenerated electrons from the excited BDC-NH2 group to the CB of titanium-oxo cluster, resulting in the formation of Ti3+ species. After exposure to air, the signals in the Ti-MOF-NH2 spectrum corresponding to Ti3+ species disappear, indicating the reoxidation of generated Ti3+ species to Ti4+ species.



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gx =1.980 before irradiation after irradiation

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Figure 6. ESR spectra observed at 77 K for Ti-MOF-NH2, immersed in aqueous 0.01 м TEOA solution, before (dotted line) and after (solid line) visible-light irradiation (λ > 420 nm). The suspension was degassed under vacuum at 77 K and irradiated with visible light for 3 h at room temperature, followed by spectrum acquisition at 77 K.

To gain further evidence for the operation of LCCT mechanism, Pt/Zr-MOF-NH2, a Ptsupported UiO-66 type MOF comprised of BDC-NH2 units and zirconium-oxo clusters whose CB potential is more negative than that of titanium-oxo cluster, was prepared. The successful formation of the MOF framework structure was confirmed by XRD measurements (Figure S5). Prolonged λ > 420 nm irradiation (24 h) of the Pt/Zr-MOF-NH2 system did not lead to production of hydrogen under our experimental conditions, in spite of the fact that Pt/Zr-MOFNH2 has excellent visible light absorption characteristics (Figure S6). In view of the fact that Pt/Zr-MOF-NH2 can promote the hydrogen production reaction when irradiated with UV light, it appears that electron transfer from the excited organic linker to the zirconium-oxo cluster is


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inefficient compared to that occurring in Pt/Ti-MOF-NH2 and Ti-MOF-NH2 because the zirconium-oxo cluster has a highly negative CB edge. Additionally, in-situ ESR measurements before and after visible-light irradiation of Zr-MOF-NH2 system provided informative results. In particular, the absence of characteristic signals in the spectrum assignable to Zr3+ species after visible-light irradiation demonstrates that electron transfer does not occur in this system. The results summarized above clearly demonstrate that the photocatalytic hydrogen production reactions using Pt/Ti-MOF-NH2 and Ti-MOF-NH2 proceed through a pathway involving light absorption by the BDC-NH2 chromophore and subsequent efficient electron transfer to the titanium-oxo cluster.

4. CONCLUSTION In the effort described above, we have prepared and studied a new visible-light-responsive metal–organic framework (MOF) photocatalyst containing a 2-amino-benzenedicarboxylic acid organic linker. The amino-functionalized Ti(IV) MOF (Ti-MOF-NH2) was found to efficiently photocatalyze the hydrogen production reaction in an aqueous solution containing TEOA as a sacrificial electron donor under visible-light irradiation conditions. In this process, the organic linker absorbs visible light and its excited state transfers electrons to the CB of the photocatalytically active titanium-oxo cluster. To the best of our knowledge, this is the first example of photocatalytic hydrogen production system employing MOF materials and visiblelight irradiation at wavelengths up to 500 nm. The observations made in this investigation should offer new insight into the design and manipulation of specifically functioning MOFs that have



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applications in the fields of photo-functionalized materials, such as photocatalysts and luminescent materials.


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ASSOCIATED CONTENT Supporting Information. Figure S1. XRD patterns of Pt/Ti-MOF-NH2 before and after photocatalytic hydrogen production. Figure S2. Photocatalytic hydrogen production under visible-light irradiation (λ > 420 nm) for 9 h over Pt/Ti-MOF-NH2. Figure S3. Photocatalytic hydrogen production under visible-light irradiation (λ > 420 nm) for 3 h over Pt/Ti-MOF-NH2 and Ti-MOF-NH2. Figure S4. ESR spectra observed at 77 K for Ti-MOF immersed in aqueous 0.01 м TEOA solution before and after visible-light irradiation (λ > 420 nm). Figure S5. XRD patterns of Zr-MOF-NH2 and Pt/Zr-MOF-NH2. Figure S6. DRUV–vis spectra of Zr-MOF-NH2 and Zr-MOF. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS The present work is supported by a Grant-in-Aid for Scientific Research (KAKENHI) from Ministry of Education, Culture, Sports, Science and Technology of Japan (No. 21550192).



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