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In Situ Neutron Powder Diffraction and X‑ray Photoelectron Spectroscopy Analyses on the Hydrogenation of MOF‑5 by Pt-Doped Multiwalled Carbon Nanotubes Heeju Lee,†,∥ Yong Nam Choi,†,* Sang Beom Choi,‡ Jung Hye Seo,§ Jaheon Kim,‡ In Hwa Cho,† Seunggi Gang,† and Cheol Ho Jeon§ †

Neutron Science Division, Korea Atomic Energy Research Institute, Daejeon 305-353, Korea Department of Chemistry, Soongsil University, Seoul 156-743, Korea § Division of Materials Science, Korea Basic Science Institute, Daejeon 305-333, Korea ∥ Research Institute for Basic Science, Sogang University, Seoul 121-742, Korea ‡

S Supporting Information *

ABSTRACT: Recent research on hydrogen storage in metal−organic frameworks focuses on how to achieve increased hydrogen binding energies by using doped metal, using unsaturated metal ions, or forming composites comprising Pt-doped carbon materials. In particular, noticeable progress using MOFs and Pt-doped carbons has been achieved to enhance the hydrogen storage capacity near room temperature. A three-component composite material, Pt-MWCNT-MOF5 (PMM5), which is a metal−organic framework (MOF-5) hybridized with Pt nanoparticles on multiwalled carbon nanotubes (MWCNT), stores 0.22 wt % hydrogen at 320 K and 30 bar, which is larger than 0.13 wt % at 300 K and 30 bar. Although the increased quantity is small, it is possible to detect the origin of uptake increase based on various analyses. In situ neutron diffraction experiments of deuterium-sorbed PMM5 with a temperature cycling process (D2 loading at 50 K → 4 K → 320 K, 2 h → 50 K → 4 K) result in a significant background increase owing to both chemisorbed deuterium atoms and a local deformation of the MOF-5 framework. Hydrogen loading in PMM5 induces significant binding energy shifts in C 1s and Zn 2p3/2 electrons in the X-ray photoelectron spectra, suggesting the chemical environment change in Zn4O(COO)6 coordination sphere in MOF-5. All accumulated experimental data support the fact that the hydrogen receptor is the oxygen atoms of benzene-1,4-dicarboxylates of MOF-5, which is facilitated by the embedded Pt-MWCNT.

1. INTRODUCTION

isolated unsaturated metal sites as strong binding sites that can be generated by removing coordinated solvent molecules through activation processes or the incorporation of those sites within organic linkers.6 MOF composites with carbon materials can be a versatile choice for improving or compensating MOF properties as gas storage materials. Compared to pristine MOFs, MOF/carbon composites can provide enhanced interactions with gas molecules, which is effective for storing or separating target adsorbates as shown in the composites of CNT and MOF-5,7,8 MIL-101,9 MIL-53-Cu,10 or HKUST-1 (or CuBTC)11 and activated carbon (AC) and MIL-101(Cr).12 The thermal conductivity of MOF-5, which is another important issue for rapid gas uptake and release in bulk, can be improved when it is hybridized with expanded natural graphite.13 Even the low moisture or water stability of MOF-5, which limits practical applications at a bulk scale, can be significantly improved in the

Metal−organic frameworks (MOFs) are considered as promising hydrogen storage materials owing to their large surface areas, tunable pore sizes, and functionality, which enables a large amount of hydrogen to be stored through physisorption under cryogenic conditions.1 A benchmark MOF, MOF-5 formulated as [Zn4O(BDC)3] (BDC = benzene-1,4dicarboxylate or terephthalate) with Zn4O(O2C)6 octahedral inorganic secondary building units, can be easily prepared with inexpensive compounds and has high BET surface areas ranging from 3100 to 3800 m2/g depending on its preparation and activation conditions2 and tunable pore environment when its BDC linkers are functionalized.3,4 MOF-5 can uptake up to 10.0 total wt % hydrogen at 77 K and 100 bar with a volumetric capacity of 66 g/L.2,5 However, the hydrogen storage amount is greatly reduced to ca. 1/10 at 298 K and 100 bar.2 This trend is observed for other types of MOFs which bind hydrogen through weak interactions.1 Thus, consensus has been made that storage materials must be able to bind hydrogen with pertinent energies at ambient temperature for mobile applications. A noticeable approach in this direction is to use © 2014 American Chemical Society

Received: December 6, 2013 Revised: January 28, 2014 Published: January 29, 2014 5691

dx.doi.org/10.1021/jp411955y | J. Phys. Chem. C 2014, 118, 5691−5699

The Journal of Physical Chemistry C

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form of MOF-5 composites with carbon nanotubes (CNT)14 or carbon-coats.15 The incorporation of Ni(II) and other ions in the inorganic secondary building units in MOF-516,17 or confinement of MOF-5 within mesoporous silica such as SBA15 are also alternative strategies in enhancing the hydrostability.18 Hybridization of pristine MOFs with inorganic solids has been considered as one of the breakthrough approaches for improving the hydrogen storage capability of MOFs near room temperature. Yang et al. reported a series of striking results that showed remarkably enhanced hydrogen storage capacities of the physical mixture of IRMOFs and Pt/AC (or with additional bridging carbon): for example, 0.4 wt % for MOF-5 and 3 wt % for MOF-5/Pt/AC/C at 298 K and 100 bar.19,20 The driving force was suggested as hydrogen spillover, which is believed to take place in inorganic systems that contain especially catalytic Pt nanoparticles. However, the identity of the hydrogen spillover in MOFs is still a controversy. Chuang et al. studied MOF-5/Pt/AC again and proposed a two-step mechanism of hydrogen uptake through hydrogen dissociation at high temperature followed by low temperature spillover on Pt/ AC.21 This observation is more or less in line with those of Yang and co-workers.19,20 However, opposite observation has been also reported that hydrogen spillover did not take place in the composites of MOF-5 and Pt/AC with or without bridging carbon.22,23 Regardless of the contradictory observations on the same composites, the hydrogen spillover approach is continuously being used to make room-temperature hydrogen storage materials based on MOFs. For example, Li and Wang concluded that the hydrogen spillover is noticeably dependent on the functional groups bonded to organic linkers and not on the type of metal ions as demonstrated in the Pt/AC composites with MIL-53(Al, Cr, or Fe), MIL-68(V), IRMOFs such as IRMOF-1 (or MOF-5), IRMOF-2, IRMOF-3, MTVMOF-5-AE, and MTV-MOF-5-AF.24 Moreover, MOF composites with Pd or Pt nanoparticles alone (without carbon support) were reported to exhibit enhanced hydrogen uptakes near room temperature mainly due to hydrogen spillover. MIL-100(Al) containing ca. 2.5 nm Pd particles prepared by the reduction of included PdCl42− showed excess hydrogen capacity of 0.35 wt % compared to 0.19 wt % of pristine MIL-101(Al) at 298 K and 40 bar.25 Pd/[SNU3](NO3)0.54, where ca. 3.0 nm Pd particles were produced through redox reactions involving organic linkers, also recorded 0.30 wt % (excess amount at 298 K and 95 bar) of hydrogen capacity, which is more than twice that of SNU-3.26 Pt/MOF177, where 2−5 nm Pt particles were prepared by hydrogenolysis of an infiltrated precursor, absorbed 0.5 wt % hydrogen after 2.5 wt % of the first uptake at 298 K and 144 bar.27 Chemical-vapor deposition and thermolysis of an organometallic precursor produced three Pt/IRMOF-8 composites, and one of them containing ca. 2.2 nm Pt particles achieved 0.85 wt % hydrogen capacity at 298 K and 100 bar.28 X-ray analyses (XRD, XPS, and XAES) on Pt-doped HKUST-1 (or CuBTC), MIL-53(Al), or ZIF-8 samples before and after H2 treatment suggested that the structural collapse of HKUST1 was due to the reduction of Cu(II) to Cu(0) by dissociated hydrogen atoms produced through spillover.29 Although there are many examples supporting the hydrogen capacity enhancement of MOFs mediated by catalytic metal nanoparticles alone or their composites with carbon, it seems that large hydrogen storage amounts are not guaranteed in

most cases. Most MOF composites with Pt/AC show marginal increases compared to their pristine MOFs at 298 K: MOF-5 (0.471 wt % dynamic) and MOF-5/Pt/AC (0.489 wt % dynamic),21 MIL-53(Al, Cr, or Fe) (0.15−0.18 wt %) and MIL53/Pt/ACs (0.35−0.41 wt %), IRMOFs (0.23−0.29 wt %) and IRMOF/Pt/ACs (0.38−0.76 wt %) at 73 bar.24 In addition, HKUST-1/Pt/AC exhibited less than 1 wt % hydrogen uptake (0.61 wt %) although the value is more than three-times that of HKUST-1 (0.17 wt % at 298 K and 20 bar).30 Interestingly, in this composite, the hydrogen receptor was identified as the oxygen atoms present on the benzene-1,3,5-tricarboxylate (BTC) ligands, not the aromatic carbon atoms based on IR spectroscopic analyses. This result was largely in line with the quantum mechanical calculations on the hydrogenation of benzene ring and carboxylate groups of BDC in MOF-5.31 However, other calculations suggested that hydrogenation of the carbon atoms in either BDC of MOF-5 or NDC (naphthalene-2,6-dicarboxylate) of IRMOF-8 is thermodynamically unfavorable.32 Despite the meaningful stride for hydrogen storage, a full understanding of those M/MOF (M = metal nanoparticles) or MOF/Pt/AC composites is still out of reach. A practical reason is that the composites are lacking homogeneity because solid components are just physically mixed. As it is practically impossible to achieve the same compositions and local structures in every particle of those hybrid materials, it is tremendously challenging to figure out the detail structures of the junction between MOF frameworks and M or Pt/AC. The same situation even occurs for Pt/AC itself, which makes the situation more complex due to the irregular and amorphous nature of AC. In this regard, MOF composites with Pt/CNT can be a better model system. Pt nanoparticles supported on the surface of CNT can dissociate hydrogen molecules and provide hydrogen atoms to MOF frameworks which are in proximity of Pt/CNT. In fact, a composite comprising of MOF5 and Pt/MWCNT (multiwalled CNT) was prepared, and was reported to absorb 1.25 wt % hydrogen at 298 K and 100 bar, a much enhanced value compared to 0.30 wt % for MOF-5 considered as a secondary hydrogen receptor.33 Here, we report a similar composite of MOF-5 and Pt/ MWCNT, and its hydrogen storage behavior has been investigated by in situ neutron powder diffraction (NPD) and X-ray photoelectron spectroscopy (XPS). In our previous report, we observed the liquid-like hydrogen stored in MOFs at 50 K by in situ NPD experiments.34 Now in this work, we applied this method for the characterization of the MOF-5/Pt/ MWCNT composite before and after hydrogen uptake at 320 K. Separately, XPS analyses were also carried out on the composite in order to identify hydrogen-receptors, which is critical for understanding Pt-assisted hydrogen storage in MOFs.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The samples in this work are Pt supported on MWCNT (termed PM), a composite of MOF-5 and MWCNT (termed MM5), and a composite of MOF-5 and PM (termed PMM5). PM,35 MM5,14 and PMM533 were respectively prepared according to the literature or modified methods as follows. MOF-5. A typical solvothermal reaction for MOF-5 was conducted by heating a reaction mixture of Zn(NO3)2·6H2O (670 mg) and 1,4-benzenedicarboxlylic acid (H2BDC, 125 mg) in N-methylpyrrolidone (NMP, 20 mL) sealed in a capped vial 5692



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at 105 °C for 1 day. After decanting the reaction solution, the remaining crystals in the vial were with neat N,Ndimethylformamide (DMF). DMF was decanted again, and the MOF-5 crystals were further washed with CH2Cl2 and stored in the solvent to replace DMF fully with more volatile CH2Cl2. Both MOF-5 and its composites were handled with caution not to expose them to moisture or ambient atmosphere directly. Bulk phase purity of the as-synthesized samples was confirmed by measurements of powder X-ray diffraction patterns and nitrogen adsorption isotherms. PM. MWCNT (510 mg) and 4,4-dipyridine (1.0 g) were dispersed in DMF (85 mL) for 6 h using an ultrasonicator. Into the reaction mixture was cautiously poured an NaOH ethylene glycol solution (340 mL) which contained both NaOH (3.4 g) and H2PtCl6·6H2O (51 mg), and the resulting mixture was stirred for 2 h at room temperature before reflux at 150 °C for 6 h under N2 atmosphere. The collected PM composite was rinsed with distilled water and ethanol, and dried at 80 °C for 12 h. Finally the dried sample was heated at 400 °C for 2 h to remove organic compounds. The Pt concentration determined by ICP-AES analyses was 12 200 ppm, which is converted to 1.22 wt %. MM5. Zn(NO3)2·6H2O (670 mg) and H2BDC (125 mg) were dissolved in NMP (20 mL). Into this solution was added MWCNT (5.0 mg), and it was dispersed by ultrasonication at ambient temperature for 1 h. When the reaction mixture sealed in a vial was heated at 105 °C for 1 day, MOF-5 crystals containing MWCNT were obtained. The washing procedure of the collected composites was similar to that applied to MOF-5 samples. Optical microscope images of MM5 particles indicated that MOF-5 domains were grown randomly with enclosing MWCNT (Figure 1a). The distribution of MWCNT in each MM5 particle was not homogeneous, and thus some MM5 particles had more MWCNT than others. PMM5. PMM5 was obtained by the same procedure used for the preparation of MM5 when PM (5.0 mg) was used instead of MWCNT. Zn(NO3)2·6H2O (670 mg) and H2BDC (125 mg) were dissolved in NMP (20 mL). Into this solution was added PM (5.0 mg) and dispersed by ultrasonication at ambient temperature for 1 h. When the reaction mixture sealed in a vial was heated at 105 °C for 1 day, MOF-5 crystals containing PM were obtained. The inclusion of Pt/MWCNT in MOF-5 crystals also happened irregularly, and thus some PMM5 particles looked black while others had a significant portion of transparent domain (Figure 1b). Manual separation of PMM5 particles with high Pt/MWCNT loading was not conducted for further analyses. Pt concentration was 153 ppm based on ICP-AES analyses, indicating that 1.24 wt % PM is embedded in PMM5. As the Zn concentration was 327 500 ppm in the same analysis, it is calculated that 6.2 × 10−4 Pt per one Zn4O unit is present in PMM5. 2.2. Physical Methods. Transmission electron microscopy (Cs-corrected STEM, JEOL JEM-ARM200F, 200 kV) and scanning electron microscopy (a dual beam focused ion beam equipment, Helios Nanolab, FEI) were employed for characterization of nano- and microscale morphologies. A Sivert-type apparatus equipped with a specially designed VTI-Cryostat (JANIS SHI-950) was used for hydrogen sorption measurements at 50−320 K. Samples were placed in a pressure cell made of a thin aluminum canister (thickness: 1 mm, volume: ca. 100 cc, maximum pressure: 40 bar). The temperatures and pressures of all spaces (paths, cylinders, and valves) were cautiously measured, and the temperature of the

Figure 1. Optical microscope images of (a) MM5 and (b) PMM5. The black domains contain MWCNT in (a) or Pt/MWCNT in (b).

metric tank (cylinder) was kept constant at 298 K using a circulated water chiller. Powder samples of PM (589 mg), MM5 (684 mg), and PMM5 (712 mg) were loaded into the aluminum canister during each experiment and were sealed with indium wires within a He-charged glovebox. Samples were degassed at room temperature by a turbo molecular pump (TPS-compact, Varian) until the gauge value was kept below 1 × 10−6 mTorr over 1 h. The volume of the sample container with a degassed sample was obtained accurately by measuring the equilibrium pressure after connecting to the He-gas charged (∼3 bar) metric tank, and the previous degassing process was then repeated again. The stabilization condition of the pressure at sample position was confirmed to be less than 20 μbar/s (1000−500A, Paroscientific). Both the accuracy and reliability of the measurements were confirmed using NaY and MOF-5 (Figure S1). In situ NPD patterns were measured by a high resolution powder diffractometer (HRPD) installed at the 30 MW “HANARO” reactor of the Korea Atomic Energy Research Institute (KAERI). A monochromatic neutron beam of λ = 1.835 Å reflected by a Ge(331) mosaic crystal monochromator was delivered to a sample, and the scattered neutrons were detected by a 32-channel 3He tube detector system. In situ measurements for MM5, PM5, and PMM5 were respectively conducted four times by changing the temperatures sequentially, as presented in Figure 2. The measurements of each sample were carried out as follows. The first NPD pattern of a sample was measured under a D2 atmosphere (12 bar) at 50 K, and the second pattern of the sample was measured at 4 K (case A). The temperature was raised to 320 K. After the sample was standing for 2 h under a D2 pressure (12 bar), the temperature was lowered to 50 K and the third pattern was 5693



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Figure 2. Schematic diagram shows the in situ NPD measurement conditions. After cooling an evacuated sample cell to 50 K, D2 gas is charged. The sample temperature is vied in a sequential order: 50 K → 4 K → 320 K → 50 K → 4 K. At the steps 1, 2, 4, and 5, neutron powder diffraction data are collected.

and mixed compounds). Phonon incoherence is produced due to a coupling of scattered neutrons with the emission or absorption of phonons, contributing to a base level of the background. All four incoherence types can be also grouped into elastic and inelastic scattering modes, as follows:

obtained. The sample was further cooled to 4 K and the fourth pattern was measured (case B). The two patterns (cases A and B) measured at the same temperature (4 K) were compared to find out any differences that might be aroused during the heat treatment at 320 K. As the temperature was changed upon loading the D2 to the sample, the NPD measurements started after confirming the system stabilization (ΔT: 0.5 K/min, ΔP: 20 μbar/s). High-resolution X-ray photoelectron spectroscopy (HRXPS) with monochromatic Al Kα X-ray radiation (1486.6 eV) operated at 120 W (AXIS Ultra DLD, Kratos Inc.) was used to investigate the compounds in this work. All samples were handled under an inert atmosphere of high purity helium gas (99.999%). To alleviate a charging problem on the samples, a sample holder was covered with a gold (Au) grid (G1000HSG3), which has 1000 lines/inch mesh and a 3.05 mm diameter. It was confirmed that each Au grid used to avoid the charging problem did not affect the XPS spectra by comparing the C1s spectra of the contaminated intrinsic carbon impurities in the Au grids (Figure S2). 2.3. Primer of Incoherent Neutron Scattering. Coherent neutron scattering produces an interference fringe, which corresponds to the Bragg peaks in the diffraction data. The incoherent scattering responsible for background signals results from scattering the length variance, ⟨b2⟩ − ⟨b⟩2, when a neutron is scattered by matter at a specific position. Scattering by mk of which interaction strength is bk, and has a probability f k (Σk f k = 1) gives binc2 = Σk f k bk2 − (Σk f k bk)2. This does not make any interference fringes between the scattered neutrons, and thus it merely increases the background monotonically if the Debye−Waller factor is neglected. Four origins are known to produce incoherence.36 Spin incoherence arises when a nucleus has a non trivial value of spin angular momentum (e.g., 1H, 2H = D, 23Na, 51V, 55Mn, etc.), and is particularly dominant when a material contains hydrogen (σinc(H) = 80.26 barn ≫ σinc(any other element)). Isotope incoherence is expected if an element has several isotopes with considerable abundances such as Cl, Ti, Ni, Cu, and Ag. Chemical incoherence can happen if a chemical site is occupied by multielements with intrinsic probabilities (e.g., metal alloys

σ ⎛ dσ ⎞el 2 ⎜ ⎟ = N inc = N ∑ ( bl 2 − bl̅ ) exp( −2Wl ) ⎝ dΩ ⎠inc 4π l

(1)

⎛ d2σ ⎞inel k′ 1 2 ( bl 2 − bl̅ ) exp( −2Wl ) ⎟ = ∑ ⎜ k l 4Ml ⎝ dΩdE ⎠inc ·∑ s

2Wl =

(k·els)2 ωs

2ns + 1 ± 1 δ(ω − ωs)

ℏ 2MlN

(κ·els ′)2 ωs ′

(2)

∑ s′

2ns ′ + 1 (3)

where exp(−2Wl) is the Debye-Waller factor, bl is the scattering length of lth atom, Ml is the mass of the atom at position l, κ = k′ - k is a momentum transfer, s stands for the mode of phonon, and els is the polarization vector for the atom at position l for mode s.

3. RESULTS AND DISCUSSION The inclusion of MWCNT or Pt/MWCNT (PM) in MOF-5 does not affect the long-range ordering of the MOF-5 framework, as seen in the indistinguishable neutron powder diffraction (NPD) patterns of the evacuated MOF-5, MM5, and PMM5 samples, respectively (Figure 3a). In the nitrogen gas adsorption experiment, both MM5 and PMM5 showed almost the same isotherms, while MOF-5 produced slightly greater nitrogen uptake (Figure S3a). Thus, the composites are suggested to have the same bulk properties as MOF-5 in terms of the framework structure and porosity. Under optical microscope observation or by the naked eye, the color of both MM5 and PMM5 is black, although some particles have transparent MOF-5 domains. This implies that the amount of 5694



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in PMM5. Further measurements were conducted for PMM5 at 320 K (Figure 3c). After the first cycle of sorption and desorption, the second isotherm data were collected without regeneration of the sample. That is, the sample cell was just evacuated at 320 K without additional heating. This second isotherm did not follow the first one, resulting in reduced hydrogen uptake. However, when the sample was activated at 423 K and for 12 h, it showed almost the same isotherm as the first one. This suggests that a pristine PMM5 sample saturated with chemically sorbed hydrogen does not receive further hydrogen if it is not activated at a higher temperature. The isotherm of the chemically sorbed PMM5, which produced the second isotherm, showed almost the same hydrogen storage capacity of those of MM5 or MOF-5, which works only under a physisorption process. The large difference between two kinds of samples is that only PMM5 contains Pt nanoparticles. Thus, the hydrogen uptake increase in PMM5 at 320 K can be attributed to the chemisorption process or hydrogenation in MOF-5 mediated by the catalytic Pt nanoparticles on MWCNT. The magnitude of increased hydrogen uptake is only ca. 0.1 wt %, which is far smaller than the enhancement observed for a composite of MOF-5 and Pt/MWCNT: a 0.95 wt % increase compared to that of a pristine MOF-5.33 However, the latter resulted from measurements at the condition of 298 K and 100 bar, and the composite has been known to have an additional hydrogenaccepting property. Although the absolute increased amount is small for PMM5, the increase tendency is in line with those observed for the composites of MOF and Pt/AC as mentioned in the Introduction. Besides the hydrogen storage amounts, the identification of hydrogen receptors is worth being pursued and characterized. First of all, it was necessary to find the Pt nanoparticles in PMM5, and thus electron microscope images of PM and PMM5 were obtained. In the TEM images of PM, it was clearly seen that ca. 2 nm Pt nanoparticles are doped on MWCNTs, the diameter of which ranges from ca. 10 to 20 nm (Figure 4a,c). The TEM images of PMM5 also showed that the Figure 3. (a) Neutron diffraction patterns for MOF5, MM5, and PMM5 measured at 50 K and vacuum. (b) Hydrogen gas adsorption isotherms of PMM5. (c) Hydrogen sorption isotherms measured at 320 K for MOF-5, MM5, and PMM5. In the case of PMM5, two additional measurements were processed with (crossed circles) and without (filled red circles) a sample regeneration treatment at 423 K in vacuum and for 12 h.

the embedded MWCNT and Pt in PMM5 is very small; in fact, 1.22 wt % of PM is present in PMM5 based on ICP-AES analyses, and the loading amount of Pt is also very small, 6.2 × 10−4 Pt per one Zn4O unit in MOF-5. Thus, it is understood that the measured BET surface areas of MOF-5 and PMM5 are almost the same with each other, that is, 3500 and 3400 m2/g, respectively. The hydrogen adsorption isotherms of PMM5 have been measured by changing the temperatures from 50 to 320 K. As expected, the adsorption amounts decreased quickly upon increasing the temperature up to 300 K (Figure S3b). However, the hydrogen uptake increased at 320 K although the absolute amount is small; at 30 bar, the hydrogen amounts were 0.13 wt % at 300 K and 0.22 wt % at 320 K (Figure 3b). As the increase in the hydrogen uptake amount does not happen for usual porous materials that store hydrogen through physisorption, the results can be attributed to the chemisorption of hydrogen

Figure 4. TEM images of (a) PM and (c) PMM5 are shown with their magnified images for the marked regions in (b) for PM and (d) PMM5, respectively. 5695



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Pt particles in PM were not detached during the solvothermal reaction for PMM5 (Figure 4b,d). It is also noticeable that the peripheral regions of the aggregated PM particles are contacting MOF-5 domains. In accordance with the optical microscopic images, PM is present in MOF-5. Additionally, we confirmed that Pt nanoparticles are present on the surfaces of MWCNT. These observations suggest that the Pt nanoparticles may absorb hydrogen molecules and dissociate them to produce activated hydrogen molecules or hydrogen atoms. These activated hydrogen species may react with the building block entities of MOF-5, which has been investigated in depth by in situ NPD and XPS experiments. As H atoms give rise to gigantic incoherent scattering, deuterium gas (D2) was used for the measurements of in situ NPD patterns for the analyses of PMM5. Both PM and MM5 were also subjected to the same NPD experiments; any changes in their NPD patterns may be attributed to the presence of Pt nanoparticles in PMM5. In a sealed cell, a sample was charged with a constant amount of D2 gas at 50 K and the subsequent measurement temperature was varied in a sequential order, as shown in Figure 2. The temperature decreased to 4 K before a temperature increase to 320 K. After holding a sample at 320 K for 2 h to facilitate any possible reactions of dissociated hydrogen with MOF-5 frameworks, the temperature decreased to 50 K and finally to 4 K in order to return to the first 4 K measurement condition. In situ NPD patterns were obtained twice at 4 and 50 K to monitor any changes before and after the sample holding at 320 K (Figure 5). Among the measured diffraction patterns at the 4 K conditions for PM, MM5, and PMM5, a significant variation in the background levels was observed only for PMM5 before and after 320 K (Figure 5c). This noticeable change indicates that the PMM5 sample must have absorbed deuterium at the 320 K condition. That is, the sample holding at 320 K induced a different state from that at lower temperature. The origin of background signals in NPD is attributed mostly to the incoherent scattering of a sample with a non-negligible contribution from local structural deformation.36 As a unit cell of MOF-5 contains 8 formula units, where the formula unit is Zn4O(BDC)3, there are many hydrogen atoms, that is, 96 H atoms per unit cell. As the most dominant contribution must be the “spin incoherence” from the hydrogen atoms in the BDC σinc(l) = 7707 barn, 99.97% from H), any linkers (∑cell l composition and structural changes by incorporation of D atoms in the MOF-5 framework will affect the background signals in the NPD patterns. A possible local deformation of the framework with an occurrence probability, f, will cause incoherent scattering: tot σinc

= 4π ∑ ( b − 2

Figure 5. Neutron powder diffraction patterns measured at 4 K before (black) and after (red) the temperature cycling process: (a) PM, (b) MM5, and (c) PMM5. The 320 K in parentheses denotes that the 4 K measurements were conducted after holding the samples at 320 K. No significant change was observed in PM and MM5, while the background of PMM5 shows a distinct increase after the temperature cycling process (∼12 bar for 2 h). int p σinc = nσinc(D) + 2f (1 − f )[32σcoh(Zn) + 64σcoh(C)

+ 114σcoh(O) + 96σcoh(H )]

whose maximum value is 77.1 barn for f = 0.5. However it is not sufficient to explain the increase of background level, ca. 10%, since the above calculation amounts to only 1% of the total spin incoherence, 7707 barns. Thus, it is necessary to introduce a more effective contribution to the incoherent scattering resulting from a local deformation, which is difficult to interpret with the current experimental data. Another possible contribution may be a change in lattice thermal motions. Random intrusion of D atoms into the framework may cause a lattice imperfection, i.e., defects. Owing to the creation of these defects, the number of phonon modes, ⟨2ns + 1⟩, may be increased. The exponent, −2Wl, in the Debye−Waller (DW) factor, exp(−2Wl), is proportional to the number of phonon modes. In general, the magnitude of 2Wl at

2 b ̅ )ltot

l

= 2f (1 − f ) ∑ 4πbl 2 l

= 2f (1 − f ) ∑ σcoh(l) l

(5)

(4)

where bl and σcoh(l) = 4πbl2 are the coherent scattering length and cross section of the lth atom in the unit cell, respectively. If all atoms in the distorted unit cell deviate from the original positions, the incoherence from the lattice distortion by the interposition of D atoms can be calculated as 5696



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Figure 6. XPS data of (a) PM, (b) PM:H2, (c) PMM5, and (d) PMM5:H2. Samples in panels b and d were exposed to H2 at 340 K and 10 bar for 2 h. The signals 2−5 in (a) are from the intrinsic defects within PM. After H2 exposure in panel b, some of the expoxide groups (3, C−O−C) changed into phenols (3′, C−OH) and sp2 carbons (2) by accepting H atoms, which results in a decrease of sp3 (relative) intensity and an increase of phenol signal (3′). The occurrence of a weak π−π* peak at 290.9 eV is a consequence of the splitting of a single π-band in aromatic rings into multiple πbands (π*) owing to the defect formation. The signals ii and 2, 3, and 5 in panel c are from the surface defects of PMM5 crystallites, which are exaggeratedly reflected on the spectra. Data (d) from the H2 exposed composite, PMM5:H2, show a big increase in ii and 2, and a newly emerging signal, 5 and 5′.

low temperature is very small (ca. 10−2), and thus the DW factor is not sensitive to the change in 2Wl. On the other hand, the incoherent inelastic scattering proportional to 2W l exp(−2Wl) ≈ 2Wl is more sensitive to the change in 2Wl. As a result, the reception of D atoms by MOF-5 is responsible for the abrupt increase in the NPD background signal of PMM5 after the thermal cycling process. It is noticeable that this partial and local distortion of the framework does not lead to the collapse of MOF-5 frameworks. Repeated measurements on different PMM5 samples produced the same results (Figure S4). While a possible structural change in MOF-5 is inferred by the background signal increase in the NPD pattern of PMM5, detailed information on the change in chemical entities are not able to be proposed by the NPD experiments alone. The incorporated D atoms must form chemical bonds with certain atoms in the MOF-5 frameworks, affecting the electronic energy levels of the bonded atoms and in turn reflected in the XPS spectra. Therefore, XPS analyses were conducted for PM and PMM5 before and after exposure to 10 bar H2 at 340 K for 6 h (Figure 6). As Pt nanoparticles are not present in MM5 samples, the catalytic dissociation of H2 is not expected. Indeed, their XPS spectra are the same before and after H2 exposure. As anticipated, PM and PMM5 produced apparent changes in the C 1s signals, whose origin is attributed to a hydrogenation of carbon atoms.

For a comparison, the XPS spectra of MOF-5, MWCNT, and MM5 were also investigated (Figures S5,6). A pristine MOF-5 produced the same signals as those in PMM5. In contrast, there are many peaks that do not belong to pure MWCNT. As is usually shown in defected single-walled or multiwalled carbon nanotubes, the MWCNT in this work seems to have some defects and nonaromatic carbon peaks corresponding to epoxide or carbonyl groups.37,38 The relative signal intensities of aromatic carbons (C sp2) and epoxide groups were increased, whereas those of C sp3 and carbonyl groups were decreased (Figures 6a,b and S7). This implies that MWCNT is also able to receive hydrogen in PM and PMM5. In the case of MM5, we could not observe any noticeable changes after exposure to 10 bar hydrogen atmosphere at 340 K (Figure S6). Compared to PM, exposure of PMM5 to H2 at 340 K for 6 h resulted in large changes in the C 1s and Zn 2p3/2 spectra (Figure 6c,d). In the C 1s spectra, a new broad signal (5 and 5′) at 290.3 eV is attributed to the formation of O∼C(5)−OH and HO−C(5′)−OH; the binding energy of the 1s electrons in the C(5) and C(5′) becomes greater than that for the C(4) because some electrons bound to the C(5) and C(5′) are transferred to the more electronegative −OH groups. In addition, the H atom attached to the C(3)O resulted in a signal increase at 286.2 eV, possibly owing to the C(2)−OH formation.39−41 In Figure 6d, a significant increase in the Zn(ii) peak at 1025.4 eV strongly supports that the oxygen atoms in BDC ligands are H atom receptors; the change in C 1s is 5697



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relatively weak, since the atomic sensitivity factor of C is 12.5 times smaller than that of Zn. The new chemical bonding between H and O atoms in BDCs will partially retrieve a distribution of the donated electrons for the Zn−O coordination bonds. Thus, the binding energy of the core electrons in Zn 2p3/2 orbitals would be shifted to higher values, Zn(i) → Zn(ii). Quantum mechanical calculations also predicted that the most favorable chemisorption site in MOF5 is the oxygen atoms in BDC.31,42 We believe that the dramatic changes in Zn 2p3/2 and C 1s spectra of PMM5 are decisive evidence of the H reception by oxygen atoms in the BDC ligands. Additional signals 2, 3, and 5 in PMM5 are ascribed to some oxygen-containing species present on the MOF-5 crystal surfaces (Figure 6c). On the crystalline surfaces, some BDCs are not able to be connected to the metal centers or vice versa. Therefore, water molecules or hydroxide ions are likely to interact with those terminal building units of MOF-5. Weak signals at the Zn(ii) position before H2 exposure, as shown in Figure 6c, may be originated from the surface defects of crystallites, like the cases in the C 1s spectra. A measurement retried on a different PMM5, which was exposed to H2 at 320 K and about 10 bar for 2 h, showed the same results as in Figure S8.

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Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Seung Hoon Choi, Dr. Dong Hyun Chung, and Dr. Daejin Kim of Insilicotech Co. Ltd. (Seongnam, Gyeonggido, Korea) for their valuable comments and Dr. Jouhahn Lee for experimental comments on the XPS experiment carried at Korea Basic Science Institute (Daejeon, Korea). TEM and SEM images were obtained at National NanoFab Center (Daejeon, Korea). Raw materials of MWCNT were gratefully donated by JEIO Co. Ltd. (Korea).

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REFERENCES

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4. CONCLUSIONS MOF-5 and nanocomposites, PM and PMM5, were synthesized, and their hydrogen storage properties were investigated by in situ NPD and XPS methods. Among them, only PMM5 shows prominent changes in its chemical status after the hydrogen gas exposure at warm temperatures (320 K). Those changes were reflected in the following experimental data: (i) a significant increase of the background level of the neutron diffraction pattern after the temperature cycling process, which implies the existence of chemisorbed hydrogen atoms, and (ii) consistent and systematic changes in the XPS spectra of Zn 2p and C 1s after H2 exposure to warm temperature, which suggest that the hydrogen receptor is the oxygen atom in the organic linkers (BDC) of MOF-5, which is similar to a composite, between HKUST-1 and Pt/AC.20 A simple hydrogenation by catalytic metal clusters might occur if the catalytic particles are sufficiently loaded within porous solids.27,28,43 However, in our PMM5, the doped Pt nanoparticles may not be responsible for the large increase of hydrogen uptake at 320 K, since the amount of Pt nanoparticles in the composite is very small. Therefore, it is concluded that the hydrogen uptake has been enhanced more than in a simple hydrogenation process. Some quantum mechanical calculations propose the possibility that the high migration energy of H atoms on a clean graphite-like carbon network can be reduced if enough oxygen surface groups are present on a graphite surface.44,45 Similar events may be applicable to the PM of PMM5, but the transfer mechanism of hydrogen into the organic linkers of MOF-5 cannot be described in detail with the current experimental data. However, this work supports the fact that the split hydrogen atoms are produced and transferred into the framework building blocks, that is, the BDC oxygen atoms.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental details on N2/H2 adsorption isotherm data, in situ neutron diffraction data, and XPS data. This 5698



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