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Hydrogen storage in Metal-Organic Frameworks

Metal-Organic Frameworks consist of inorganic subunits (secondary building units (SBU): transition materials, lanthanides, and oxygen.) connected to each other by organic ligands (carboxylates, imidazolates, etc.) thus forming ordered nanoporous crystalline hybrid networks (figure 1.12). The wide choice of metal centers and organic ligands allows the design of MOFs with adjustable crystalline properties and porosity. More than 60 000 MOFs have already been reported28. The most interesting MOFs for storage have very large surface areas and microporous volumes. Among the first hybrids studied for the storage of hydrogen, MOF-5 has a capacity of about 7 wt.% for a specific surface area of between 4000-5000 m2/g. Very recently, hybrids with a specific surface area up to 6000 m2/g have been synthesized. This is the case of NU-100 which contains micro and mesopores and can store about 9 wt.% of hydrogen at 77 K29.
Despite enormous research efforts towards the development of their specific surfaces, MOFs suffer from low binding energies with H2 molecules. For this reason, another part of the research effort is dedicated to increasing the adsorption energy. Among the various strategies envisaged, the insertion of metallic nanoparticles in the pores of MOFs is proposed to combine the adsorption properties of hydrogen at cryogenic temperature with the absorption properties of metals at room temperature. In fact, due to their ordered structure, the MOFs pores ensures the nanoconfinement and the monodispersion of the nanoparticles and, their stabilization against coalescence.
In this thesis project, one MOF was used as a confinement matrix for the nanoparticles. The MIL-101 ((Cr3O)X(H2O)2[O2C-C6H4-CO2]3, X = F-1, Cl-1, OH-1), discovered by F rey and co-workers in 2005 31, is a MOF with a very complex structure based on SBU of type μ3-oxo-centered trimers of octahedral Cr(III) connected to six other connectors through doubly deprotonated terephthalic acid linkers 31 . The resulting tetrahedron is assembled into the so-called “super tetrahedrons” leading to two types of mesoporous cages with internal free diameters of about 2.9 and 3.4 nm and pore windows of 1.2-1.6 nm (figure 1.13). The MIL-101(Cr) was selected as a host matrix in this study not only for its large specific surface area (SBET 3800 m2.g-1) and huge porosity (2.0 cm3.g-1) but also for its high thermal stability (up to 573 K) (Annex 1), and high chemical stability to water.

Nanoparticles/nanowires formation

Other top-down methods such as spark discharge generation allow the synthesis of free Mg or MgH2 particles with a size of 10-20 nm33. The reaction kinetics are very fast, and the desorption temperature is reduced compared to the bulk material. However, the synthesis is complex, and the quantities produced are limited. More original top-down methods (vapor deposition) have made it possible to develop one-dimensional nano-objects of Mg or MgH2 with a controlled morphology (nanowires with diameter <50 nm)34. Again, downsizing increases the reaction kinetics and decreases the desorption temperature compared to the bulk material. Nevertheless, the disadvantage of these methods is the instability of the nanostructure. Progressive recrystallization (increase in crystallite size with slowing of kinetics) is observed during absorption/desorption cycling and exposure at a relatively high temperature.

Nanoconfinement: monodispersion

In order to reduce the size of nanoparticles (< 10 nm) and to stabilize them against coalescence, a promising strategy is the confinement of particles within a porous inert matrix1. Many porous matrices are available such as carbons, MOFs, polymers, and oxides. Both top-down (melted infiltration under controlled atmosphere) and bottom-up methods (liquid impregnation of precursors and decomposition/reduction under controlled atmosphere) can be applied to synthesize nanoparticles of controlled size of metals/ alloys/hydrides confined in different host matrices. In addition, the control of the pore size of the matrices allows, in principle, to develop limited size nanoparticles.
For example, MgH2 nanoparticles confined in the pores of different carbons have been obtained with an extremely small size (1.3 nm)35. These nanoparticles show faster desorption kinetics than MgH2 bulk, which is also dependent on size: the smaller the size, the faster the kinetics. Another example that confirms the positive effects of nanoconfinement is the case of NaAlH4 complex hydride (mass capacity of 7.4 wt%)36. In the massive state, this hydride decomposes in two stages with the help of catalyst additives: NaAlH4 ↔ Na3AlH6 + Al + H2 ↔ NaH + Al + H2. (Equation 1.5). The same nanoconfined and additive hydride desorbs the hydrogen in a single reaction removing the intermediate step: NaAlH4 ↔ NaH + Al + H2. (Equation 1.6).
Thus, the kinetics are faster, the thermodynamics is modified, and the reversibility is improved.

Double Solvent Method (DSM)

In order to avoid metal nanoparticles aggregation on external surfaces of MOFs, the double solvent method was preferred. To remove the water molecules adsorbed on the surface and in the pores of MIL-101(Cr), a degassing step of the MOF at 493 K for 18 hours must be carried out before each synthesis and then handled without exposure to air. Then, the synthesis consists of the use of a mixture of a hydrophilic solvent (water), containing the metal precursor with a volume set equal or less than the pore volume of the adsorbent, and a hydrophobic excess solvent (pentane or hexane), playing an important role to suspend the adsorbent and facilitate the impregnation process. First, the freshly degassed 150 mg of MIL-101(Cr) is suspended in 30 mL of pentane in an ultrasonic bath for 20 minutes followed by 30 minutes of magnetic stirring. Then, a volume of the metal precursor solution, slightly less than the total pore volume of MIL-101(Cr), is added. The mixture was then stirred, in a beaker covered by a parafilm, under vigorous magnetic stirring (750 rpm) for 4 hours at room temperature. This step is very important because it will allow the good dispersion of the precursor solution within the MIL-101(Cr) pores. The total solvents evaporation is then carried out at room temperature by removing the parafilm and maintaining the magnetic stirring at 250 rpm. The recovered solid was dried at 343 K in an oven overnight. A schematic illustration of the synthesis method is represented in figure 2.1.

Scanning Electron Microscopy analyses

Scanning Electron Microscopy (SEM) is a classic technique used to observe the morphology of materials on a microscopic scale. The main use of this technique is to obtain images of the surface of the material. For this purpose, for conventional microscopes, the sample is placed in a chamber in which the vacuum is produced. In this work, scanning electron microscopy was only used to image the surface of the sample and, as such, qualitative or quantitative microanalyses were not performed using this technique. The average resolution of scanning electron microscopes is of the order of one nanometer. This last point shows that SEM images have the main utility of describing morphology rather than microstructure. A schematic illustration of a SEM is shown in figure 2.4.

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Transmission Electron Microscopy, Energy-dispersive X-Ray spectroscopy

Transmission Electron Microscopy (TEM) is used to characterize much finer samples and uses much higher voltages than those used for other microscopies such as scanning electron microscopy. The maximum acceleration voltages used in transmission electron microscopy range from 200 to 300 KV. TEM have the advantage of having resolutions much higher than those of SEM. The most recent models make it possible to distinguish the microstructure of the materials up to the scale of the atom and therefore have lower resolutions than the angstrom. The morphology of a TEM is shown in figure 2.5.
The image is then formed in the image space which is geometrically downstream of the sample. The radiation-matter interaction is identical to that occurring in a SEM. Nevertheless, the electrons passing through the sample can be either electrons transmitted or diffracted (elastic diffusion) or even diffused inelastically. The transmitted electrons are used for conventional TEM imaging in bright field. In this case, the areas with low electron density appear in the clear while the others are dark. The diffracted electrons are used to perform electron diffraction or dark field imaging during which one or more diffracted beams are selected. In this case, only the diffracting elements appear in the clear. This is dark field imaging. Finally, the inelastically scattered electrons make it possible to perform energy loss spectroscopy. The latter is a powerful tool allowing, in the case of a very small energy resolution, to identify the chemical nature of the elements. In addition, the X-ray photons emitted by de-excitation of the electronic cortege of the atoms of which a core electron has been ejected under the electronic impact give qualitative and quantitative chemical information of the target atom (EDS analysis: Energy Dispersive Spectroscopy). EDS analysis makes it possible to determine the composition and distribution of the chemical elements in the analyzed sample. Depending on the mode chosen, it is possible to carry out an elemental analysis at a precise point (on a volume of about 1 μm3) or on average on a surface, to form a concentration profile over a given distance or to carry out elementary mapping of a surface.

Palladium doping: Variable metal loading

Once the synthetic method optimized, four x-Pd@MIL-101(Cr) composites with the different metal loadings (x = 5, 10, 15 and 20 wt.%) were synthesized and characterized to study the effect of metal loading on the structural, nanostructural and textural properties. The hydrogen sorption properties will be then discussed, and the nanosize effect will be determined. It should be noted that the maximum metal loading has been imposed by the solubility limit of the precursor salt into water.

Table of contents :

1.1. Hydrogen aspects
1.1.1 Hydrogen: energy vector
1.1.2 Hydrogen production Fossil energy transformation Water electrolysis Biomass
1.1.3 Hydrogen transformation to energy Internal Combustion Engine Fuel Cell
1.2 Hydrogen storage
1.2.1 Gaseous storage Hydrogen storage by compression
1.2.2 Liquid storage Hydrogen storage by liquefaction
1.2.3 Solid storage Chemical storage Physical storage
1.3 Hydrogen storage in Metal-Organic Frameworks
1.4 Nanosizing effect
1.4.1 Nanostructuration by mechanical milling
1.4.2 Nanoparticles/nanowires formation
1.4.3 Nanoconfinement: monodispersion
2.1 Synthesis
2.1.1 MIL-101(Cr)
2.1.2 Nanoparticles confinement Liquid Impregnation (LI) Double Solvent Method (DSM)
2.2 Characterization
2.2.1 Physicochemical characterization Powder X-Ray Diffraction analyses Scanning Electron Microscopy analyses Transmission Electron Microscopy, Energy-dispersive X-Ray spectroscopy . Specific surface area & total pore volume X-Ray Absorption Spectroscopy Inductively Coupled Plasma Fourier Transform Infrared spectroscopy Differential Thermal Analysis coupled with Thermogravimetric Analysis .
2.2.2 Hydrogenation properties Thermo-Desorption Spectroscopy Pressure-Composition-Isotherm
3.1 Synthesis optimization
3.1.1 Impregnation and reduction method
3.1.2 Degassing effect
3.2 Palladium doping: Variable metal loading
3.2.1 X-Ray Diffraction
3.2.2 Fourier Transform Infrared Spectroscopy
3.2.3 Transmission Electron Microscopy
3.2.4 Scanning Electron Microscopy
3.2.5 Specific surface area & total pore volume
3.2.6 Inductively Coupled Plasma – Mass spectroscopy
3.3 Interaction with Hydrogen
3.3.1 Pressure-Composition-Isotherm at high pressure and low temperature (77 K)
3.3.2 Pressure-Composition-Isotherm at low pressure and low temperature (78 – 107 K)
3.3.3 Pressure-Composition-Isotherm at low pressure and room temperature
3.3.4 In-situ X-Ray Absorption Spectroscopy
3.3.5 Hydrogen desorption properties Hydrogen desorption for MIL-101(Cr) Hydrogen desorption for 10-Pd@MIL-101(Cr) composite Hydrogen diffusion Hydride formation Activation energy of desorption
3.4 Conclusion
3.5 Perspectives
4.1 Synthesis optimization by double solvent method
4.1.1 Reduction temperature effect
4.1.2 Reduction time effect
4.2 Rhodium doping: Variable metal loading
4.2.1 X-Ray Diffraction
4.2.2 Transmission Electron Microscopy
4.2.3 Specific surface area & total pore volume
4.3 Hydrogen sorption properties
4.3.1 Pressure-Composition-Isotherm at low temperature (78 – 107 K)
4.3.2 Pressure-Composition-Isotherm at low pressure and room temperature
4.3.3 Hydrogen desorption properties Hydrogen desorption for Rh@MIL-101(Cr) composite Air exposure effect Nanosize effect and the scaling law Cycling effect
4.4 Conclusion
5.1 Bimetallic nanoparticles doping
5.1.1 X-Ray Diffraction (XRD)
5.1.2 Transmission Electron Microscopy (TEM)
5.1.3 Energy-Dispersive X-ray spectroscopy (EDX)
5.1.4 Differential Thermal Analysis coupled with Thermogravimetric Analysis (DTA-TGA)
5.1.5 Specific surface area & total pore volume
5.2 Interaction with Hydrogen
5.2.1 Pressure-Composition-Isotherm at low pressure and room temperature
5.2.2 Hydrogen desorption properties
5.3 Conclusion


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