pubs.acs.org/Langmuir © 2010 American Chemical Society
Influence of [Mo6Br8F6]2- Cluster Unit Inclusion within the Mesoporous Solid MIL-101 on Hydrogen Storage Performance )
Danil Dybtsev,†,‡,3 Christian Serre,*,† Barbara Schmitz,§ Barbara Panella,§ Michael Hirscher,§ Michel Latroche,^ Philip L. Llewellyn, Stephane Cordier,# Yann Molard,# Mohamed Haouas,† Francis Taulelle,† and Gerard Ferey†
)
† Institut Lavoisier de Versailles, UMR 8180, Universit e de Versailles St-Quentin en Yvelines, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France, ‡Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, Pr. Lavrentieva 3, Novosibirsk 630090, Russia, §Max-Planck-Institut f€ ur Metallforschung Heisenbergstrasse 3, 70569 Stuttgart, Germany, ^CMTR Team, Institut de Chimie et des Mat eriaux de Paris Est, CNRS, UMR 7182, 94320 Thiais Cedex, France, Laboratoire Chimie Provence, UMR 6264, Universit es Aix-Marseille I, II & III - CNRS, Centre de Saint J er^ ome, 13397 Marseille Cedex 20, France, and #Equipe CSM, UMR CNRS-UR1 6226 Sciences Chimiques de Rennes, Universit e Rennes I, Rennes, France. 3Present address: Graduate Institute of Advanced Materials Science, Pohang University of Science and Technology, San 31 Hyojadong, Pohang 790-784, Korea.
Received February 9, 2010. Revised Manuscript Received April 29, 2010 The inclusion of (TBA)2Mo6Br8F6 (TBA = tetrabutylammonium) containing [Mo6Br8F6]2- cluster units within the pores of the mesoporous chromium carboxylate MIL-101 (MIL stand for Materials from Institut Lavoisier) has been studied. X-ray powder diffraction, thermal analysis, elemental analysis, solid-state NMR, and infrared spectroscopy have evidenced the successful loading of the cluster. In a second step, the hydrogen sorption properties of the model cluster loaded metal organic framework (MOF) system have been analyzed and compared to those of the pure MOF sample, through a combination of adsorption isotherms (77 K, room temperature), thermal desorption spectroscopy, and calorimetry (calculated and experimental) in order to evaluate the hydrogen storage efficiency of the cluster loading.
Introduction Global warming as a consequence of the greenhouse effect urges our society to find suitable alternatives to fossil fuels. Hydrogen as an energy carrier has been proposed for transportation. However, many important technical issues still have to be sorted out such as its economically viable production and the discovery of efficient high capacity storage devices. So far, hydrogen can be stored as a compressed gas, in liquefied tanks, or adsorbed in solids. However, prior to applications, these materials should exhibit a high loading capacity, adequate reversibility, fast reactivity, and sustainability. Solid state storage of hydrogen can be performed through chemisorption, with a resulting high volumetric density at room temperature but usually with low weight capacity, strong thermal effects, and low kinetics or the need of high desorption temperatures which are not suitable for such applications.1-4 An alternative is to use adsorbents that can physisorb hydrogen. High loading capacities can be achieved with the required kinetics and regeneration conditions but with operating conditions at low temperature (liquid nitrogen temperature, i.e., 77 K). Carbon materials are among the best hydrogen sorbents5 with uptake up to 5 wt %5 at 87 K and 6 MPa, while zeolites exhibit lower capacities, up to 1.8 wt % at 77 K.6 Metal organic frameworks (MOFs) are an interesting class of porous solids with low framework (1) Schlapbach, L.; Z€uttel, A. Nature 2001, 414, 353. (2) Schlapbach, L. MRS Bull. 2002, 675. (3) Z€uttel, A. Mater. Today 2003, 24. (4) Sandrock, G. J. Alloys Compd. 1999, 293-295, 877. (5) Dillon, A. C.; Heben, M. J. Appl. Phys. 2001, A 72, 133 and references therein . (6) Langmi, H. W.; Walton, A.; Al-Mamouri, M. M.; Johnson, S. R.; Book, D.; Speight, J. D.; Edwards, P. P.; Gameson, I.; Anderson, P. A.; Harris, I. R. J. Alloys Compd. 2003, 356-357, 710. (7) Kitagawa, S.; Kitaura, R.; Noro, S.-I. Angew. Chem., Int. Ed. 2004, 43, 2334.
Langmuir 2010, 26(13), 11283–11290
density and high specific area, up to 5200 m2 3 g-1.7-9 They are built up by the association of inorganic subunits (clusters, chains, etc.) and polydendate organic linkers (carboxylates, phosphonates, imidazolates, etc.), which results in an unprecedented variety of structures with different compositions, structure, and pore systems. MOFs with very high surface area have been evaluated for hydrogen storage and exhibit large sorption capacities, up to 7.5 wt % at 77 K at high pressure (6-8 MPa), but with limited performances at room temperature (4.7 g of H2 per dm3 for MIL-101-Mo6Br8F6 against 2.1 g of H2 per dm3 for MIL-101. The evolution of the hydrogen sorption capacity has been evaluated as a function of the temperature (see Table 2). At low temperature (77 K), the weight capacity is about three times lower for the composite MIL-101-Mo6Br8F6 than for the MOF alone. This is in rather good agreement with the SSA reduction due to the mass uptake related to the cluster inclusion. However, at room temperature, the hydrogen uptake is preserved or slightly increased despite the significant mass difference. Therefore, bonding F-H interactions between the hydrogen molecules and the clusters cannot be ruled out at 298 K. To compare the volumetric hydrogen sorption capacity of MIL101 and MIL-101-Mo6Br8F6, the uptake per unit cell has been calculated. The hydrogen uptake of the sample nH and the sample mass ms together with the weight of anhydrous MIL-101 (Cr3FO(BDC)3) mMIL-101 and the weight of the metal cluster in the pores mM leads to the hydrogen uptake per unit cell . The calculated hydrogen uptake per unit cell for MIL-101 and MIL-101-Mo6Br8F6 is shown in Figure 4. In this comparison, the MIL-101-Mo6Br8F6 shows a steeper increase than MIL-101 and therefore better properties for low pressure gas storage. Isosteric heat of adsorption calculations, using the ClausiusClapeyron equation based on adsorption isotherms at different temperatures (for each material, five isotherms at temperatures between 77 and 127 K), were performed for the two samples Langmuir 2010, 26(13), 11283–11290
Dybtsev et al.
Article Table 2. Hydrogen Storage Performances of MIL-101 and MIL-101-Mo6Br8F6 at 77 and 298 K
hydrogen sorption in weight (%) capacity density of the porous solid (dried form) in g 3 cm-3 volumetric capacity (g 3 dm-3)
MIL-101 @ 77K, 8 MPa
MIL-101 @ 298 K, 8 MPa
MIL-101-Mo6Br8F6 @ 77K, 6 MPa
MIL-101-Mo6Br8F6 @ 298K, 8 MPa
6.1 0.49 30
0.4 0.49 1.85
1.9 1.2 23
0.4 1.2 4.7
Figure 4. Adsorption isotherms of MIL-101(Cr) (red squares) and MIL-101-Mo6Br8F6 (black triangles) at 77 K up to 2 MPa.
(Figure 5, left). In order not to overestimate the isosteric heat of adsorption calculated from the Clausius-Clapeyron equation, the heat of adsorption was additionally calculated by a virial type equation which yields very similar results and is shown for comparison in Figure 5 (left).35 Direct heat of adsorption measurements using an appropriate calorimeter device are shown in Figure 5 (right). A significant decrease in the calculated and measured isosteric heat of adsorption is denoted when the Mo6Br8F6 clusters are incorporated within the pores of MIL-101. The difference reaches about -2 kJ 3 mol-1 at very low coverage, and is close to -0.5 kJ 3 mol-1 when getting close to the full surface coverage. The calorimetric measurement allows a determination of the differential enthalpy of adsorption already at very low hydrogen pressures. Differences between the direct calorimetric results and the calculation of isosteric heats can be explained by the different methodologies,36 even whether the main conclusions are similar. In Figure 5, we can see two main energy regimes for the MIL-101 solid. The first, up to a relative coverage of around 0.1, corresponds to a sharp decrease in the enthalpies of adsorption from around -9 to -6 kJ 3 mol-1. As mentioned previously,16 this energy could seem to be due to the interaction of hydrogen with the Cr3þ unsaturated metal sites. The second energy regime above a relative coverage of 0.1 corresponds to energies of around -6 to -4 kJ 3 mol-1 and would correspond to surface coverage as indeed these energy values correspond to those observed for many other MOF or carbonaceous materials. The MIL-101-Mo6Br8F6 sample gives an adsorption energy profile with three regimes. The first regime observed with this material again seems to occur up to a relative filling of 0.1 (around 0.3 wt %); however, the initial energies observed here range from -13.5 to -8 kJ 3 mol-1. This clearly indicates the enhancement of the interaction of hydrogen due to this material. The initial energy of -13.5 comes close to the value recommended by Bhatia and Myers for a viable process for hydrogen storage.20 However, disappointingly, this value decreases too sharply for any practical use which would seem to indicate (35) Czepirski, L.; Jagiello J. Chem. Eng. Sci. 1989, 44(4), 797. (36) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption, surface area and porosity; Academic Press: London, 1999.
Langmuir 2010, 26(13), 11283–11290
only a quite small amount of high energy sites available to hydrogen on the cluster. A second regime is observed in the hydrogen interaction for coverage between 0.1 and 0.35 leading to enthalpy values between -8 and -6 kJ 3 mol-1 which may correspond to adsorption on or near the surface of the remaining complex. This would suggest a latent effect of the complex on the field gradient in the pore network which could be an idea for any other studies in this direction. Finally, the third energy regime, above a coverage of 0.35, mirrors the second regime on the unmodified MIL-101 (-6 to -4 kJ 3 mol-1) which would suggest coverage of the remaining surface of the material. Hydrogen thermal desorption spectroscopy (TDS) has been performed on both MIL-101-Mo6Br8F6 and MIL-101 solids to evaluate the consequences of the cluster loading in term of interactions with hydrogen molecules (Figure 6). Both desorption spectra (Figure 6) exhibit a maximum at very low temperature (23 K). Since it occurs at very low temperature and it is nonspecific to the sample, this can be assigned to the liquid hydrogen present in the system.35 Due to the overlap of this peak with the maxima of adsorbed hydrogen occurring at higher temperatures, no quantitative evaluation is thus possible. A qualitative comparison of the hydrogen desorption spectra of MIL-101 and MIL-101-Mo6Br8F6 however highlights a similar shape in the two desorption curves with maxima or shoulders corresponding to adsorbed hydrogen at 35, 50, and 72 K. However, for MIL-101, most of the hydrogen is released at temperatures below 40 K and only a smaller fraction is desorbed at higher temperatures, while for MIL-101-Mo6Br8F6 the major quantity of adsorbed H2 is released at temperatures higher than 40 K. Since the desorption temperature depends on the heat of adsorption, the hydrogen TDS spectra reflects well the stronger interaction of H2 with MIL-101-Mo6Br8F6 in agreement with the heat of adsorption especially at lower surface coverage (Figure 5).
Discussion Hydrogen sorption experiments were performed on the MIL101 and the MIL-101-Mo6Br8F6 in two pressure regions and are discussed in the following. Figure 2 shows excess hydrogen adsorption isotherms up to 8 MPa at 77 K and room temperature. The excess hydrogen adsorption in Figure 2 is given in wt % which is defined as mass of hydrogen adsorbed divided by the sum of the mass of the storage material and the stored hydrogen given in percent. For the 77 K isotherms, the typical IUPAC type I behavior with a steep initial increase followed by saturation at higher pressures is observed. The saturation value is about 6 wt % for MIL-101 and 2 wt % for MIL-101-Mo6Br8F6, which was expected due to the strong increase in density from 0.49 to 1.2 g 3 cm-3 upon loading of the cluster (Table 2). At room temperature, both materials show a linear increase of the hydrogen uptake up to 8 MPa where it reaches 0.4 wt %. As expected, the hydrogen storage capacity depends strongly on the pressure and temperature. Therefore, more precise measurements of gravimetric hydrogen sorption in the low pressure region up to 2 MPa are shown in Figure 3. Isotherms are measured with liquid nitrogen (77 K), with liquid argon (87 K), and with a heating system at temperatures above 87 K. For all temperatures, the hydrogen uptake is completely DOI: 10.1021/la100601a
11287
Article
Dybtsev et al.
Figure 5. (left) Isosteric heat of adsorption calculated from Clausius-Clapeyron equation at various temperatures for MIL-101 (triangles) and MIL-101-Mo6Br8F6 (squares). The gray small symbols represent an alternative approach to calculate the isosteric heat of adsorption with a virial type equation35 where the uncertainty is given by the light gray area. (right) Heat of adsorption for hydrogen in MIL-101 and MIL-101-Mo6Br8F6 from calorimetric measurements.
Figure 6. Thermal desorption spectroscopy of hydrogen in MIL-101(Cr) (left) and MIL-101-Mo6Br8F6 (right).
reversible and as an example for both materials one desorption isotherm is shown in Figure 3. By incorporation of the metal clusters, the gravimetric hydrogen storage capacity is reduced strongly. Due to the increase in crystal density from MIL-101 to MIL-101Mo6Br8F6, a comparison of the hydrogen uptake per volume behavior of MIL-101 and MIL-101-Mo6Br8F6 is more evocative and is given in Figure 4 for the uptake at 77 K up to 0.6 MPa. At pressures beyond 0.35 MPa, the volumetric hydrogen storage capacity was even increased by up to 20% by the inclusion of the metal clusters. At room temperature and 8 MPa, the hydrogen storage capacity of the MIL-101-Mo6Br8F6 is over twice that of MIL-101 (Table 2). This enhanced hydrogen uptake may be assigned to higher heat of adsorption for hydrogen in MIL-101-Mo6Br8F6. The isosteric heat of adsorption is evaluated from the isotherms in Figure 3 with a general form of the Clausius-Clapeyron equation. As isotherms were measured at five different temperatures, the isosteric heat of adsorption can be determined with high accuracy over a wide range of coverage (Figure 5). For MIL-101, the isosteric heat of adsorption is 5 kJ 3 mol-1 at low loading and decreases to 4 kJ 3 mol-1 at higher loading. By insertion of the [Mo6Br8F6]2- cluster units, the isosteric heat of adsorption is increased by approximately 1 kJ 3 mol-1 over the whole range of coverage from 0.02 to 0.8. The inclusion of well-defined [Mo6Br8F6]2- cluster units, containing ionic Mo-F bonds, in MIL-101 was performed to determine if very large pores could be used to incorporate molecular species with enhanced interactions with hydrogen, in order to foster 11288 DOI: 10.1021/la100601a
hydrogen adsorption at room temperature. Let us note that, to our knowledge, the [Mo6Xi8Fa6]2- unit constitutes with [(Nb6Fi6Xi6)(NCS)a6]4- (X=Br, I) and [Re4S4F12]4-, one of the rare examples of soluble nanosized fluorinated cluster units.37 This work was made with a fundamental approach in mind, knowing the fact that no real applications could be extracted because of its very large molar weight being further increased by the presence of tetrabutylammonium cations. Quantum calculations showed that apical linkers are very important in the stabilization of the whole [M6Li8L0 a6]n( cluster units.27 Recently, it was shown that apical fluorine enables a greater stabilization of bonding orbitals in Mo6Xi8Fa6 units and a higher HOMO-LUMO energy gap than apical chlorine or bromine and to a greater extent than iodine atoms.27b This effect is directly related to the very poor π-donor character of fluorine compared to other halogen ligands. The enhanced stabilization of the metal-fluorine bond is due to a higher electrostatic interaction component as already demonstrated for other metal halide complex series.38 Indeed, the Mo-Fa bond exhibits a strong ionic character compared to Mo-Cla, Mo-Bra, and Mo-Ia bonds. It explains that the increase in heat of adsorption observed upon loading of the cluster as well as an increase in the volumetric hydrogen sorption capacity at room (37) (a) Br€uckner, P.; Preetz, W.; Punjer, M. Z. Anorg. Allg. Chem. 1993, 619, 551. (b) Mironov, Y. V.; Yarovoi, S. S.; Naumov, D. Y.; Kuratieva, N. V.; Kozlova, S. G.; Simon, A.; Fedorov, V. E. Eur. J. Inorg. Chem. 2005, 12, 2476. (c) Naumov, N. G.; Cordier, S.; Perrin, C. Chem. Commun. 2008, 9, 1126. (38) Tilset, M.; Fjeldahl, I.; Hamon, J. R.; Hamon, P.; Toupet, L.; Saillard, J. Y.; Costuas, K.; Haynes, A. J. Am. Chem. Soc. 2001, 123(41), 9984.
Langmuir 2010, 26(13), 11283–11290
Dybtsev et al.
Article
temperature may be due to the presence of specific interactions between hydrogen and the terminal fluorine atoms present in the cluster and not to a simple reduction of the pore size. If the huge increase in density observed upon loading of the cluster makes the resulting sorption capacity not so attractive for practical applications, these results nevertheless pave the way for the inclusion of other lighter fluorinated clusters in mesoporous MOFs for further improvement of hydrogen sorption performance.
Experimental Section Synthesis of MIL-101(Cr). The crystalline MIL-101 product
was first hydrothermally synthesized at 220 C for 9 h, but the autoclave reactor after synthesis was slowly cooled down to room temperature for 3 h in order to obtain larger crystals of the unreacted terephthalic acid.22 The crystalline MIL-101 product in the solution was then doubly filtered off using two filters (glass and paper filters) with a pore size between 40 and 100 μm to remove the free acid. A solvothermal treatment of the product was sequentially performed using 1 g of MIL-101 per 50 mL of dimethylformamide (DMF) in a glass vial at 70 C during 3 h. The resulting solid was cooled down, filtered, and washed with 10 mL of DMF. The procedure was repeated twice. The solid was finally dried overnight at 150 C under air atmosphere. Preparation of (((n-C4H9)4)N)2Mo6Bri8Fa6. The substitution of apical bromine atoms in [Mo6Bri8Bra6]2- units by other halogen anions has been first reported by Preetz et al.32 For the present work, a slightly derived method has been developed in order to obtain [Mo6Bri8Fa6]2- units in high yield and higher quantities. The experimental process consists of a prior reaction in solution between [Mo6Bri8Bra6]2- units and AgBF4 to remove the six apical Br ligands that are subsequently replaced by fluorine ligands. Indeed, ((n-C4H9)4N)2Mo6Bri8Bra6 precursor was prepared from Cs2Mo6Br14 according to the experimental procedure reported in ref 34b. Afterward, (((n-C4H9)4)N)2Mo6Bri8Bra6 (3 g, 1.38 mmol) and AgBF4 (1.88 g, 9.64 mmol) were weighted in a dry box and introduced into a schlenck tube. Then 50 mL of a 50/50 acetone/ethanol mixture was introduced in the container. The solution was stirred overnight in the absence of light. After separation of the AgBr precipitate by filtration, 3.57 g of ((n-C4H9)4N)F 3 3H2O (13.8 mmol) was added to the filtrate. The mixture was stirred for 24 h in the absence of light at room temperature. A powder containing the experimental products ((n-C4H9)4N)2Mo6Bri8Fa6 and ((n-C4H9)4N)BF4, and the excess of AgBF4 was recovered after filtration and evaporation of the solvent. ((n-C4H9)4N)2Mo6Bri8Fa6 was then separated from the byproduct by three successive dissolutions/recrystallization at 0 C. For this last step, the powder was dissolved in 10 mL of CH2Cl2 and diethyl ether was added dropwise until the appearance of a small trouble at the top of the solution. Single X-ray diffraction studies of the selected single crystals revealed that the composition was (((n-C4H9)4)N)2Mo6Bri8Fa6-2CH2Cl2. Solvent molecules were eliminated after drying under vacuum at 60 C overnight to obtain pure (((n-C4H9)4)N)2Mo6Bri8Fa6 (Yield: 80%).
Incorporation of ((n-C4H9)4)N)2Mo6Bri8Fa6 in MIL-101(Cr). Yellowish-green MIL-101-Mo6Br8Br6 was made by stirring
the suspension of MIL-101 (500 mg), previously dried at 150 C, with (n-Bu4N)2[Mo6Br8F6] (500 mg) and hydrofluorhydric acid (0.1 mL of 50% aqueous solution, Prolabo 40% in water) in ethanol (5 mL, Aldrich 99%) at room temperature. After 3 h under stirring at room temperature, the solid was filtered, washed with small amount of EtOH (3 3 mL), and dried in oven at 100 C for 1 h. The final yield was 750 mg. Thermal Gravimetric Analysis. TGA was performed using a TA 2050 analyzer. Approximatively 10 mg of each sample was loaded in an aluminum sample holder from room temperature up to 500 C with a heating rate of 2 C/min and a mixture of oxygen (20%) and nitrogen (80%). Langmuir 2010, 26(13), 11283–11290
X-ray Powder Diffraction. X-ray diffraction patterns were collected on the solids before and after hydrogen sorption experiments on a D5000 (θ-2θ mode) Siemens diffractometer with λCu(KR1, KR2) = 1.54059, 1.54439 A˚. Nitrogen Sorption Experiments. BET surface areas were estimated using nitrogen adsorption experiments at 78 K and 1 bar using a Micromeritics Asap 2010 instrument. Samples were outgassed at 200 C overnight prior to the experiments. The nitrogen uptakes at saturation (P/P0 = 0.5) were close to 1100 and 220 mL 3 g-1 for MIL101b and MIL-101-cluster, respectively. The corresponding BET surface areas (calculated in the 0.05-0.2 P/P0 range) were 3400(60) and 690(10) m2 3 g-1, respectively. 19 F Solid-State NMR. 19F NMR solid-state NMR spectra were recorded at room temperature on a wide bore Bruker Avance spectrometer operating at a 19F resonance frequency of 470.5 MHz. A triple tuned (19F/1H/X) Bruker 2.5 mm MAS probe was used with 19F π/2 pulses of 1.6 μs. The spectra were acquired using a synchronized Hahn echo sequence with an echo delay of 33 μs and a magic-angle spinning speed of 30 kHz. The 19F NMR spectra were referenced with CFCl3 set to 0 ppm using as secondary standard NaF set to -221 ppm. Hydrogen Storage Measurements. High Pressure (8 MPa) Hydrogen Storage Measurements. Pressure-composition isotherm curves were determined using a volumetric device (Sievert’s method). About 0.7 g of powder was transferred into a tight stainless steel sample holder closed with a metal seal. The sample was then outgassed at 220 C during 16 h under primary vacuum for activation. At the end of the experiment, the sample holder was transferred into a glovebox under purified argon and weighted again to measure the mass loss from the heat treatment. All weight capacities refer here to the outgassed sample. Sample volume corrections were derived from density measurements obtained with a Helium Accupyc 1330 pycnometer on 100 mg of sample. The volumetric device with calibrated and gauged volumes was immersed in a temperature controlled water bath at 298 K, and high purity hydrogen (Alphagaz H22, 6N) was introduced step by step up to 6-8.6 MPa. The sample holder connected to the volumetric device was immersed in liquid nitrogen for measurements at 77 K or in the water bath for room temperature measurements (298 K). Pressure gas variations due to thermal equilibrium and hydrogen adsorption were measured using a calibrated pressure gauge. The equation of state for hydrogen was obtained from the program NIST12.39 2 MPa Hydrogen Sorption Experiments. Measurements were performed with an automated Sieverts apparatus PCTPro2000 (HyEnergy, CA), and they are described in detail elsewhere.40 A total of 98.48 mg of MIL-101 sample was evacuated for 30 min at 15 0 C. The volume of the sample was measured with helium at room temperature. For measuring the hydrogen adsorption isotherms, the sample was cooled with liquid nitrogen (77 K) and liquid argon (87 K) and with a cooling system for other temperatures. To compensate the cooled free volume, a blank sample with the same volume was measured and the data were subtracted from the MIL-101 adsorption data. A second measurement for each isotherm was done with only a few points, so the uncertainty which is added for every point is minimized. A total of 360.61 mg of MIL-101-Mo6Br8F6 was measured in the same way as described before. Microcalorimetry. Direct measurements of the differential enthalpies of adsorption of H2 at 77 K and up to 0.1 MPa were obtained with a Tian-Calvet type microcalorimeter coupled to a manometric apparatus built “in house”. A quasi-equilibrium volumetric procedure employs an extremely slow constant introduction of adsorptive gas, in the region of 2 cm3 3 h-1 for which it (39) Lemmon, E. W.; Peskin, A. P.; McLinden, M. O.; Friend, D. G. NIST12 Thermodynamic and Transport Properties of Pure Fluids, V5.0; NIST: Gaithersburg, MD, 2000. (40) Schmitz B.; M€uller, U.; Trukhan, N.; Schubert, M.; Ferey, G.; Hirscher, M. ChemPhysChem 9 (15), 2181.
DOI: 10.1021/la100601a
11289
Article was verified that the quasi-equilibrium conditions were fulfilled. This latter technique coupled with isothermal adsorption microcalorimetry allows direct access to a continuous measurement of the derived enthalpies of adsorption, -Δadsh, during the vertical (or near vertical) parts of the isotherm.41 Typical errors are of around (0.2 kJ 3 mol-1. Prior to each adsorption experiment, the samples were outgassed using sample controlled thermal analysis (SCTA).42 Around 50 mg of each sample was thus heated under a constant residual vacuum pressure of 10-2 mbar up to a final temperature of 200 C which was maintained until the residual pressure was less than 10-3 mbar. The H2 was obtained from Air Liquide (Alphagaz, France) and was 99.9999% purity. Thermal Desorption Spectroscopy. The home-built setup and the measuring technique for low temperature thermal desorption spectroscopy are described in detail elsewhere.43 For the measurements, the MIL-101 and MIL-101-Mo6Br8F6 samples (2.5-3.5 mg) were first heated at 473 K in vacuum (better 10-3 Pa) (41) Llewellyn, P. L.; Maurin, G. C. R. Chim. 2005, 8, 283. (42) Llewellyn, P.; Rouquerol, F.; Rouquerol, J. In Sample Controlled Thermal Analysis: Origin, Goals, Multiple Forms, Applications and Future; Toft Sorensen, O., Rouquerol, J., Eds.; Kluwer Academic Publishers: Dordrecht, 2003; p 135. (43) Panella, B.; Hirscher, M.; et al. Microporous Mesoporous Mater. 2007, 103, 230.
11290 DOI: 10.1021/la100601a
Dybtsev et al. to remove moisture and adsorbed gases. The sample were then loaded with 40 mbar of hydrogen (99.999% purity) at room temperature and cooled down to approximately 20 K with a liquid He flow cryostat. The nonadsorbed hydrogen was removed in vacuum (10-5 Pa) while the adsorbed hydrogen molecule stayed on the surface of the MIL sample due to the very low temperatures applied. The temperature of the sample was increased with a constant heating rate of 0.1 K 3 s-1 up to 370 K, and the desorbed hydrogen (mass m = 2) was recorded with a quadrupole mass spectrometer. In addition to molecular hydrogen, the masses of H2O (m=18) and of atomic hydrogen (m=1) were recorded. The amount of desorbed hydrogen can be quantified from the area under the desorption curve after careful calibration of the setup using hydrogen loaded palladium.
Acknowledgment. The authors are grateful to Dr. A. Vimont (LCS, Caen, France) for the help in interpretation of the IR experiments and INTAS for the funding (Grant 05-109-5007 for D.N.D.). Supporting Information Available: Additional figures. This material is available free of charge via the Internet at http:// pubs.acs.org.
Langmuir 2010, 26(13), 11283–11290
