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Aug 1, 2016 - Framework as an Efficient and Durable Catalyst for Promoting H2. Production .... The obtained samples were noted as Cu/MIL-101,. Ni/MIL-...
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Non-Noble-Metal Nanoparticle Supported on Metal−Organic Framework as an Efficient and Durable Catalyst for Promoting H2 Production from Ammonia Borane under Visible Light Irradiation Meicheng Wen,† Yiwen Cui,† Yasutaka Kuwahara,†,‡ Kohsuke Mori,†,‡,§ and Hiromi Yamashita*,†,‡ †

Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan Unit of Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Kyoto 606-8501, Japan § JST, PRESTO, 4-1-8 HonCho, Kawaguchi, Saitama 332-0012, Japan ‡

S Supporting Information *

ABSTRACT: In this work, we propose a straightforward method to enhance the catalytic activity of AB dehydrogenation by using non-noble-metal nanoparticle supported on chromium-based metal−organic framework (MIL-101). It was demonstrated to be effective for hydrogen generation from ammonia borane under assistance of visible light irradiation as a noble-metal-free catalyst. The catalytic activity of metal nanoparticles supported on MIL101 under visible light irradiation is remarkably higher than that without light irradiation. The TOFs of Cu/MIL-101, Co/MIL-101, and Ni/MIL-101 are 1693, 1571, and 3238 h−1, respectively. The enhanced activity of catalysts can be primarily attributed to the cooperative promoting effects from both non-noble-metal nanoparticles and photoactive metal−organic framework in activating the ammonia borane molecule and strong ability in the photocatalytic production of hydroxyl radicals, superoxide anions, and electron-rich nonnoble-metal nanoparticle. This work sheds light on the exploration of active non-noble metals supported on photoactive porous materials for achieving high catalytic activity of various redox reactions under visible light irradiation. KEYWORDS: metal−organic framework, non-noble-metal nanoparticle, H2 production, ammonia borane hydrolysis, visible-light-enhanced activity NH3BH3 + 2H 2O → 3H 2 + NH4 + + BO2−

1. INTRODUCTION With the increasing use of the limited fossil fuels and the world worldwide problem of air pollution and energy crises, the exploring of green and sustainable energy sources is highly desirable.1 Hydrogen has been considered a candidate for a renewable energy economy in consideration of its clean burning and high energy density, as well as an important reagent for the chemical industry.2 However, the efficient and safe storage, handling and distribution of hydrogen are still key issues for hydrogen economy in the future.3 An efficient way is to store hydrogen in the form of liquid or solid materials under ambient conditions, which can release hydrogen in situ at certain conditions at room temperature.4,5 Of all the available primary H2 storage materials, ammonia borane (AB) has received much attention as a promising hydrogen storage material due to its low molecular mass (30.87 g mol−1), remarkable theoretical hydrogen content (19.6 wt %), and high stability in solid state at ambient temperature.6 Those outstanding characteristics highly meet the environmental and socio-economic concerns. H2 released from AB can follow two different pathways, through pyrolysis under considerably high temperatures, yielding only 6.5 wt % hydrogen (1 equiv based on AB) or the hydrolysis of AB in the presence of suitable catalysts, producing 19.6 wt % hydrogen (3 equiv based on AB) under mild reaction conditions through eq 1.7 © XXXX American Chemical Society

(1)

To date, the hydrolysis of AB in homogeneous and heterogeneous catalytic systems have been well-reported. Different kinds of noble catalysts, such as Au, Ru, Pt, Rh, and Pd, have been widely applied for efficient H2 generation from AB hydrolysis under mild reaction conditions.8−10 Unfortunately, limited resources hinder large-scale energy application. To reduce the amount of noble metals, significant attention has been paid to synthesis of bimetallic nanoparticles for their potential application in catalysis owing to their synergic effect in tuning the electron distribution of alloy nanoparticles. It is wellknown that tuning the electronic density of metal-nanoparticles-based catalyst has emerged as an effective method to improve their catalytic performance. The electron redistribution makes the electron-rich catalytic surface of alloy nanoparticles, which favors the absorption of electrophilic partner compound as well as nucleophilic partner compounds inclined to absorb on positive catalytic surface of alloy nanoparticles. Up to now, several bimetallic nanoparticles (Pt−Ni, Ru−Co, and Au−Co) have been synthesized and shown significantly higher catalytic Received: April 11, 2016 Accepted: August 1, 2016

A

DOI: 10.1021/acsami.6b04169 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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impregnation method. For this purpose, 200 mg of NH2-MIL-101 was suspended in in 15 mL of deionized water and sonicated for 20 min. Then, it was mixed with a given amount of aqueous metal nitrate solution so as to have a target metal loading amount (2 wt %). The suspension was stirred vigorously for 4 h. The metal ions were reduced by injection of freshly prepared NaBH4 aqueous solution. The molar ratio between metal and NaBH4 is 1:10. The samples were collected by drying at 60 °C under vacuum to remove the solvent. Finally, the collected samples were keep under vacuum to prevent further oxidation by air. The obtained samples were noted as Cu/MIL-101, Ni/MIL-101, and Co/MIL-101, respectively. Cu/MIL-101 with different Cu loading amounts (0.5, 1.0, 1.5, 2.0, and 2.5 wt %) and Cu loaded on TiO2, ZrO2, ZrP, and Al2O3 (a target metal loading amount: 2 wt %) were also prepared with the method mentioned above. The metal nanoparticle loading amounts on the different supports were evaluated by inductively coupled plasma analysis. The results are summarized in Table S1. 2.3. AB Dehydrogenation. The catalytic performance of the prepared catalyst was investigated by H2 production from AB dehydrogenation. Briefly, 10 mg of catalyst was suspended in 5 mL of deionized water in a test tube (30 cm3) and bubbled with argon gas in the suspension for 30 min. Then, AB (40 μL, 0.5 M) was introduced into the mixture. Subsequently, the mixture was irradiated at wavelength λ > 420 nm from the side by using a 500 W Xe lamp. In the case of open system, a mixture containing 20 mg of catalyst and 5 mL of deionized water was placed into a reaction tube (30 cm3) with a reflux condenser and equipped with gas buret. After purging with argon gas, AB aqueous solution (0.2 M, 5 mL) was introduced into the reaction tube. For visible-light-assisted catalytic reaction, the mixture was irradiated at wavelength λ > 420 nm from the side using a 500 W Xe lamp. 2.4. Materials Characterizations. The crystallographic information on the catalysts were investigated by X-ray diffraction (Rigaku RINT2500 diffractometer with Cu Kα irradiation (λ= 1.5406 Å)). Nitrogen sorption analysis was performed at 77 K using BELSORPmax system (BEL Japan, Inc.). Samples were degassed under vacuum at 473 K for 24 h prior to data collection. TEM images were obtained on a Hitachi Hf-2000 FE-TEM (operated at 200 kV). UV−vis diffuse reflectance spectra were collected on a Shimadzu UV-2600 recording spectrophotometer. BaSO4 was used as background, and the absorption spectra were obtained using the Kubelka-munk function. Photoluminescence measurement was recorded on a fluorolog-3 spectrofluorometer (HORIBA) at room temperature.

activity in dehydrogenation of AB than their monometallic counterparts.11−13 Less expensive non-noble metals such as Fe, Co, Ni, and Cu have been developed for this purpose.14−17 However, it is still a challenge to achieve high catalytic activity based on those low-cost metals as a result of the magnetisminduced aggregation of metal particles, size, and easy oxidation while exposed to air.18 Alternatively, significant catalytic activity enhancement of AB decomposition can also be observed on plasmonic catalysts (Ag, MoO3−x, and Pd/MoO3)19−21 because the migration of photoexcited charges can promote the activation of the AB through the electron-rich NH3 moiety or electron-deficient BH3 moiety, as well as the active species generated during the photocatalytic reaction process further promoting the catalytic activity. Despite these achievements, the development of a high-performance, low-cost, and desired durability catalyst for AB dehydrogenation remains a great challenge. Metal−organic frameworks (MOF) are a promising class of porous materials and have been employed for dispersion and stabilization of metal nanoparticles.22 MOFs are composed of metal ions as connecting center and organic molecular as linker, which has high special surface area, uniform pore, and chemical diversity.23 Recently, different kinds of precious metal nanoparticles supported on MOFs were prepared for efficient H2 production from AB dehydrogenation.24 MOFs also show alluring optical properties because they can be modified by photoactive organic linkers or transitional metal ions.25 Up to now, many reactions have taken place on metal nanoparticles supported on MOFs.26 Upon light irradiation, the photoexcited charges are useful for promoting the redox reactions. However, very little attention has been paid to the utilization of their optical properties in enhancing chemical reaction.27 Therefore, the integration of non-noble-metal nanoparticle and photoreactive MOFs can allow efficient improvement in the performance of redox reaction under light irradiation. In this study, we propose a general way to enhance the catalytic activity of AB dehydrogenation by using non-noblemetal nanoparticle supported on a chromium-based MOF (MIL-101), which was synthesized by impregnation of metal precursors with MIL-101, followed by in situ reduction with NaBH4 prior to catalytic dehydrogenation of AB. The catalytic activity of metal nanoparticles supported on MIL-101 under visible light irradiation was remarkably higher than that without light irradiation. The possible mechanism of photocatalytically enhanced H2 generation from AB dehydrogenation catalyzed by non-noble nanoparticles supported on MOFs was discussed. It was suggested that superoxide anions and hydroxyl radicals, generated upon visible light irradiation, played vital roles in realizing high catalytic performance.

3. RESULTS AND DISCUSSION The crystallographic information on the catalysts was investigated by XRD and the results are shown in Figure 1.

2. EXPERIMENTAL SECTION 2.1. Preparation of NH2-MIL-101. Chromium-based aminefunctionalized MOF (NH2-MIL-101) was prepared using a published method.28 Typically, 0.76 g of chromic nitrate hydrate, 0.36 g of 2aminoterephthalic acid, and 0.2 g of sodium hydroxide were dissolved in 15 mL of deionized water. The mixture was stirred 5 min, and then transferred to Teflon-lined stainless-steel autoclaves, followed by heat treatment at 160 °C for 16 h and cooling to room temperature, after which the products were obtained through centrifugation. Then, the green precipitates were washed with methanol and water. Finally, the products were dried and activated by calcination at 200 °C for 6 h under vacuum. 2.2. Preparation of Metal-Nanoparticle-Loaded NH2-MIL101. In this study, the metal nanoparticles were prepared by

Figure 1. XRD pattern of MIL-101 and Cu/MIL-101. B

DOI: 10.1021/acsami.6b04169 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Three peaks were observed at around 3, 5, and 9° for pure MOF, which were in agreement with the reported simulated patterns,28,29 suggesting the successful fabrication of aminefunctionalized MIL-101. The broad Bragg reflections of samples indicated that the small particle size of MIL-101. The crystallinity of MIL-101 decreased after loading with Cu nanoparticles due to excess NaBH4 destroying the porosity of MIL-101. No new peaks appeared after loading with Cu nanoparticle, which can be attributed to the small Cu nanoparticle and low loading amount of Cu. Figure S2 displays the BET surface area plots of the catalysts before and after immobilization of Cu nanoparticles, and Table S2 shows the information of BET surface area (SBET) and micropore volume obtained from the analyses of N 2 adsorption−desorption by using Brunauer−Emmett−Teller (BET). MIL-101 exhibited a reversible type-I behavior, which is a typical feature of microporous materials. Loading of Cu nanoparticles decreased the surface area of MOFs, not only because the presence of excess NaBH4 partially collapsed the porosity of MIL-101 but also because the pore and internal cavities were blocked or occupied by Cu nanoparticles. The presence of Cu nanoparticle within MOFs was evidenced by the EDX analysis, where a signal of Cu can be observed in Figure S1. Inductively coupled plasma analysis (Table S1) further confirmed the presence of Cu within MOFs. Very smallsized Cu nanoparticles were loaded on MIL-101 (Figure S5). No significant agglomeration of Cu nanoparticles occurred on MIL-101, evidencing that Cu nanoparticles were welldistributed on MIL-101, probably because of the high surface area of MIL-101 and low loading weight of Cu. The diffuse-reflectance (UV−vis) spectra of MIL-101 and Cu/MIL-101 materials were recorded, and the results are displayed in Figure S3. Both MIL-101 and Cu/MIL-101 have an intensive response in the wavelength range of 200−300 nm, attributed to the π → π* transition of the organic linker, and the absorption peaks in the wavelength range of 300−450 nm are relative to the chromophore on the organic linker of MOF. The photoabsorption of MOFs at wavelength less than 450 nm are mainly derived from the organic linker of MOFs, as displayed in Figure S4. After the organic linker coordinated with inorganic cluster, the main absorption peak showed a slight blueshift. The bands in the low energy region between 550 to 700 nm are attributed to the d → d* transition of Cr3+ ions.23 In the case of sample Cu/MIL-101, no additional peak corresponding to the plasmon resonance of Cu was observed, further suggesting the small-sized Cu nanoparticle. Moreover, the presence of Cu nanoparticles on the surface of MIL-101 does not significantly affect the optical properties of MIL-101, indicating that it retains its MOF structure. The band gap energies of MIL-101 and Cu/MIL-101 calculated from a plot of (ahv)1/2 versus photo energy were 2.7 eV, demonstrating that Cu/MIL-101 is a visible-light-responsive catalyst. It is widely accepted that MIL-101 has acted as a promising support for applications in versatile catalytic reactions. However, very little attention has been paid to utilization of their optical properties to enhance the catalytic performance under light irradiation. In this study, the catalytic performance was estimated by H2 production from the hydrolysis of AB under visible light irradiation (λ > 420 nm). Figure 2 shows the H2 production from the hydrolysis of AB (40 μL, 0.5 M) catalyzed by Cu/MIL-101 and MIL-101 under argon atmosphere at ambient temperature with and without visible light irradiation. Our preliminary tests demonstrated that no H2

Figure 2. H2 generation from AB (40 μL, 0.5 M) dehydrogenation over MIL-101 and Cu/MIL-101 catalyst under visible light irradiation (λ > 420 nm) and dark conditions at ambient temperature (∼60 μmol H2 represents approximately 100% conversion of AB).

was analyzed by GC (Shimadzu GC14B gas chromatograph) in the absence of either catalyst or AB. Slight reaction occurred over sample without Cu nanoparticle loading. After immobilization of Cu nanoparticle, a considerable amount of H2 was generated, which suggests that the dehydrogenation of AB is catalyzed by Cu nanoparticle. The reaction rate of Cu/MIL-101 during the initial 5 min is slow but increases with increasing reaction time, which is probably due to the easily oxidized nature of copper preventing the catalytic activity of Cu/MIL101 during the initial 5 min. It is noteworthy that H2 production was significantly enhanced under visible light irradiation over Cu/MIL-101. Cu/MIL-101 could produce 53.2 μmol H2 in the initial 20 min, and it only took about 25 min to reach its maximum. In contrast, only 23.8 μmol H2 could be yielded under dark condition. The initial H 2 production rate over Cu/MIL-101 under visible light irradiation (2.66 μmol min−1) is almost two times higher than that under dark condition (1.19 μmol min−1). The enhanced catalytic activity of Cu/MIL-101 under visible light irradiation can be ascribed to the synergistic effect between Cu nanoparticles and MIL-101. The light emitted from a Xe lamp at λ > 420 nm contains some infrared light. The enhancement of catalytic activity under visible light irradiation might be attributed to the photothermal effect. The temperature of the suspension of Cu/MIL-101 was slightly heated up (about 30 °C) under light irradiation at wavelength of λ > 420 nm. To point out the contribution of infrared thermal heating, control experiments at 25 and 30 °C were performed with Cu/MIL-101 in the dark (Figure S6). The catalytic activity of H2 production from AB in the dark at 30 °C was a little higher than the production at 25 °C, much lower than that under visible light irradiation. These results demonstrated that the H2 production enhancement is primarily owing to the cooperation effect between MOF and Cu nanoparticles under visible light irradiation. Figure 3 displays the H2 production from the hydrolysis of AB catalyzed by Cu supported on different materials. The actual Cu contents loaded on different supports are summarized in Table S1. Among them, Cu/ZrO2 and Cu/ MIL-101 displayed considerable high activities under dark condition owing to the tiny and even distribution of Cu nanoparticles as shown in Figure S5. It is notable that Cu/MIL101 exhibited the highest catalytic activity and largest catalytic activity enhancement under visible light irradiation due to its unique optical property. No remarkable enhancement can be C

DOI: 10.1021/acsami.6b04169 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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photocatalytic reaction.30 Highly reactive hydroxyl radicals, which are continuously generated in a process of oxidation of hydroxide anions or water molecules by photogenerated hole, can initiate and promote the photocatalytic oxidative reaction.31 To clarify the effect of the hydroxyl radical in photocatlytic H2 generation from AB dehydrogenation over Cu/MIL-101 under visible light irradiation, 2-propanol (0.1 mL) was injected into the reaction mixture to act as hydroxyl radical scavenger after purging with argon for 30 min. The catalytic activity for AB dehydrogenation significantly decreased due to the decrease of the concentration of hydroxyl radical as shown in Figure 4,

Figure 3. H2 generation from AB (40 μL, 0.5 M) dehydrogenation on various catalysts under visible light irradiation (λ > 420 nm) and dark conditions at ambient temperature.

observed over Cu/Al2O3, Cu/ZrP, and Cu/ZrO2 under visible light irradiation under identical conditions. TiO2, the moststudied semiconductor, was selected in this study; however, Cu/TiO2 also showed no enhancement in the H2 production from AB hydrolysis under visible light irradiation due to its large band gap (3.2 eV for anatase). The visible-light-responsive semiconductor CeO2 was also used as support in this work. Slight enhancement was observed under visible light irradiation but was still lower than that of Cu/MIL-101, probably owing to the large Cu nanoparticle size of Cu/CeO2 (Figure S5) and inefficient photogenerated charge separation. From the abovementioned results, it is demonstrated that the large surface area and unique optical property of MOFs make them useful for the spatial loading of Cu nanoparticle for photocatalytic optimization of AB dehydrogenation under visible light irradiation. To the best of our knowledge, this is the first report of visible-light-induced hydrogen production enhancement from AB solution over non-noble-metal nanoparticles supported on MOFs. The effect of Cu loading amount was investigated. The optical property, crystallinity, and specific surface area of sample deceased with increase of Cu loading amount (Figure S7), owing to the increased NaBH4 amount. Figure S8 displayed the amount of H2 generated from AB dehydrogenation on different Cu loading amount samples after 30 min reaction under visible light irradiation and under the dark condition at ambient temperature. The optimum Cu loading amount for H2 generated from AB hydrolysis under visible light irradiation was found to be 2 wt %. The total amount of H2 evolved over 2 wt % Cu/MIL-101 was almost three times of that over 0.5 wt % Cu/MIL-101. With the increase of Cu loading amount from 0.5 to 2.0 wt %, the activity was enhanced under visible light irradiation and the dark condition due to the increase of active site number. However, a further increase of Cu loading amount to 2.5 wt % caused a decrease in activity under light irradiation, which can be ascribed to the aggregation of Cu nanoparticles evidenced by Figure S9. In general, the photocatalytic activity is highly dependent on the processes of electron/hole pair production, photogenerated electron/hole separation, interfacial charge migration, and surface reactions of the reagents diffused on the surface of the catalyst with these charge carriers. The efficient formation of radical intermediates upon light irradiation of photoactive materials in aqueous environment also greatly influences

Figure 4. Role of hydroxyl radical and superoxide anion on the photocatalytic performance of Cu/MIL-101 for H2 generation from AB (40 μL, 0.5 M) dehydrogenation under visible light irradiation (λ > 420 nm).

indicating that the formation of hydroxyl radical is an important factor to promote the H2 generation from AB dehydrogenation under visible light irradiation. In this work, terephthalic acid was used as a fluorescence probe to capture the photogenerated hydroxyl radical. Figure S10 shows the fluorescence signal intensity of a basic solution (NaOH, 10 mM) under different condition. As shown in Figure S10, no fluorescence peak was observed in the absence of Cu/MIL-101, and a low intensive fluorescence peak was observed at 426 nm in the absence of terephthalic acid. This fluorescence peak might be produced by the reaction between the organic linker of MOFs and photogenerated hydroxyl radical, suggesting the formation of hydroxyl radical upon visible light irradiation. The fluorescence peak was significantly enhanced after introducing terephthalic acid (5 mM) into the suspension of Cu/MIL-101, demonstrating that Cu/MIL-101 has ability to from the high concentration of hydroxyl radical upon visible light irradiation. Meanwhile, after introducing AB in the above suspension solution, a similar intensive fluorescence peak was observed accompanied by decrease of the catalytic activity for AB hydrolysis (Figure S11), further suggesting that the protogenerated hydroxyl radical is a principal contributor to achieve the high catalytic activity. Moreover, the intensity of fluorescence peak decreased by using the sample without Cu nanoparticle, indicating that Cu nanoparticle plays an important role in promoting the photogenerated charges separation, leading to the formation of hydroxyl radical with high concentration. The presence of superoxide anions formed by the reactions of dissolved oxygen molecular with photoexcited electron32 also plays a vital role in the heterogeneous photocatalytic reactions D

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mechanism (LCCT), then further to the metal nanoparticles (step 2). Subsequently, the electrons gathered on the surface of catalyst reacted with dissolved oxygen molecules to produce the superoxide anions. Meanwhile, the highly powerful oxidizing agents (hydroxyl radicals) were generated by the reaction between the hole in the HOMO state of linker with water molecules or hydroxide anions (step 3). AB is composed of BH3 dative bonded to NH3, in which B atoms carry negative H atoms whereas N atoms carry positive H atoms due to the electronegativity difference between N and B atoms.33 Additionally, the strength of B−N formed by sharing lone pair electrons between BH3 and NH3 group is weaker than ionic and covalent bonds. A plausible catalytic mechanism for AB hydrolysis over metallic nanoparticles under dark conditions was discussed based on the theoretical and experimental studies. H2 production from AB dehydrogenation involves adsorption of AB onto the metallic nanoparticle to form an activated complex species, followed by the dissociation of B−N by H2O molecules attacking to produce H2.34 As hydroxyl radicals and superoxide anions produced upon on visible light irradiation are important for enhancing the catalytic H2 production for AB dehydrogenation, we assume that not only the H2O molecule but also the photogenerated hydroxyl radicals and superoxide anions attack the Cu−AB complex species to dissociate the B−N bond (step 4). After dissociation of B−N band, the as-formed Cu−BH3 intermediate reacted with water to produce H2 along with the formation of a BO2− (step 5), and the as-formed NH3 reacts with H2O to generate NH4OH.35 In addition, we believe that the electron-rich Cu nanoparticle induced by photoactive MOFs also has positive effect on absorption of AB by the interaction between electrondeficient BH3 moiety of AB and Cu nanoparticle, forming an activated complex species,36 thus allowing the B−N bond to become susceptible to be attacked. On the basis of the above-discussed mechanism, we believed that this reaction mechanism also can apply to enhance the catalytic performances of Co and Ni nanoparticles supported MIL-101. UV−vis, XRD, and BET studies were used to characterize optical properties, microstructure, and specific surface area of Cu/MIL-101, Co/MIL-101, and Ni/MIL-101. As shown in Figure S12, similar crystallinity, specific surface

in the aqueous suspensions of photoactive catalysts. As a result, the initial reaction rate increased upon increased dissolved oxygen by purging oxygen gas in the mixture as shown in Figure 4, suggesting the positive effect of superoxide anions in enhancing the photocatalytic H 2 production from AB hydrolysis. Additionally, the efficient suppression of photogenerated electron and hole recombination can be promoted by continuous consumption of photoexcited electrons and holes, leading to high quantum efficiency for the photocatalytic reaction. A possible mechanism for visible-light-enhanced H 2 production from AB decomposition over Cu/MIL-101 is proposed in Scheme 1. After absorption of light by the linker Scheme 1. Possible Mechanism for Photocatalytic Enhancement of AB Dehydrogenation over Cu/MIL-101 under Visible Light Irradiation

of MOFs, an electron is excited from the HOMO state to LUMO state of linker, leaving a hole in the HOMO state of linker (step 1). The photoexcited electron can transfer to the cluster of MOFs through a linker-to-cluster charge-transfer

Figure 5. H2 production from AB (1 mmol AB/10 mL of H2O) dehydrogenation under visible light (red) irradiation (λ > 420 nm) and dark condition (black) at ambient temperature and the recycle study over (a) Cu/MIL-101, (b) Co/MIL-101, and (c) Ni/MIL-101 (experimental error within ±5%). E

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ACKNOWLEDGMENTS The present work was partially supported by Grants-in-Aid for Scientific Research (Nos. 26220911, 25289289, 26630409, and 26620194) from the Japan Society for the Promotion of Science (JSPS) and MEXT. We acknowledge Dr. Eiji Taguchi and Prof. H. Yasuda at the Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, for their assistance with the TEM measurements. Y.K., K.M., and H.Y. thank MEXT program “Elements Strategy Initiative to Form Core Research Center”.

area, and optical properties were observed for these three samples. For practical application, the catalytic performances of Cu/MIL-101, Co/MIL-101, and Ni/MIL-101 were examined in a gas buret system. As shown in Figure 5, all samples displayed significant enhancement of catalytic activities under visible light irradiation. The reaction rates of Cu/MIL-101, Co/ MIL-101, and Ni/MIL-101 reaching maximum under visible light irradiation were 3.76, 3.78, and 8.25 mL min−1, respectively. This corresponds to TOFs of 1693, 1571, and 3238 h−1 for Cu/MIL-101, Co/MIL-101, and Ni/MIL-101, respectively, which are comparable to or even higher than that of noble metal-nanoparticles-catalyzed AB hydrolysis.37−39 The stabilities of Cu/MIL-101, Co/MIL-101, and Ni/MIL-101 under visible light irradiation were also investigated. After each cycle, the generated H2 was removed by vacuumed and purging with argon gas. Subsequently, 1 mmol of AB was injected into the reaction vessel and followed by the same procedure. All samples exhibit remarkable durability and can be used three times with a slight decrease in activity. In the present study, the porous MOFs not only served as support for stabilizing the non-noble nanoparticles but also acted as semiconductors for visible light harvesting and producing active charges involved in enhancing the catalytic activity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b04169. XRD pattern; N2 adsorption−desorption isotherm; BET surface area and pore volume of samples of different samples; TEM images of different samples; influence of the loading amount of Cu on the H2 production from AB; fluorescence signal intensity of Cu/MIL-101 in terephthalic acid solution at different reaction condition under visible light irradiation for 30 min (PDF)

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REFERENCES

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4. CONCLUSIONS We have demonstrated for the first time that non-noble-metal nanoparticle supported on chromium based metal−organic framework can significantly enhance the catalytic activity of AB dehydrogenation under visible light irradiation, which serves as economic and efficient catalysts for H2 production in the hydrolysis of AB. Furthermore, those non-noble metals supported on MOFs also show high stability. The enhanced catalytic activity of non-noble-metal nanoparticle supported on MOFs can be primarily attributed to the efficient formation of photogenerated charges including hydroxyl radical, superoxide anion, and electron-rich non-noble-metal nanoparticles. The present work provides a new strategy for the development of highly active non-noble-metal supported on photoactive porous materials for achieving high catalytic activity of various redox reactions under visible light irradiation. We speculate that this method could be extended to other types of metal nanoparticles.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-(0)66879-7458. Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acsami.6b04169 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.6b04169 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX