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Hydrogen interaction with nanosized and ultradispersed metals

In heterogeneous catalysis, the design of atom-efficient catalysts, i.e., with improved performance (activity, selectivity, stability) and minimized amount of rare and expensive materials, remains a major challenge.26,27 The downscaling of metal particles to the ultra-small size range has become an important strategy for the design of new materials in this field.
As shown in Figure 1.6, the surface free energy increases with the decrease of metal particle size. The ratio of surface/volume atoms, i.e., the number of low-coordinated metal atoms, which often play as active sites, increases as the metal size decreases. Therefore, downsizing of metal particle catalysts increases the atom utilization and consequently, this might boost the catalytic performance. The metal catalyst in the ideal form of a series of single and discrete atoms may maximize the atom utilization to 100% and increase the specific catalytic activity. Furthermore, size reduction also affects the electronic properties of metals. For example, a continuous energy level is characteristic of bulk metal. The decrease of metal size (<2 nm) leads to a more discrete energy level distribution and a widening of the HOMO-LUMO gap in the case of clusters of several atoms. HOMO and LUMO are types of molecular orbitals, which stand for highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively. Single atoms show discrete electronic levels.

Hydrogen interaction with Metal Nanoparticles

Reducing the size of metal particles to several nanometres leads to a dramatic change in the physicochemical properties. For example: the melting point of Au nanoparticles was found to be much lower than the melting point of Au bulk owing to the large surface-to-volume ratio.34 Moreover, the thermal conductivity of silver nanoparticles was found smaller compared to metal bulk and it decreased with decreasing particle size.35 Therefore, the nanosize effect is also expected to change the hydrogen-metal interaction properties, e.g., the thermodynamics, the kinetics of reaction with hydrogen and eventually, showing trapping effects.
Pd is the only element in noble metals that absorbs hydrogen at ambient temperature and pressure forming an interstitial metallic hydride PdH0.7. For this reason, bulk Pd is the most studied element for hydrogen storage and is one of the best understood metal-hydrogen system. Therefore, nanosized Pd has become the model material to study the nanosize effect on metal–hydrogen interaction. Thus, the Pd-H system is used to discuss the size effect in this chapiter and this is also the object of our first experimental study in Chapter III.
The size effect is obviously observed in the Pressure-Composition Isotherms (PCI) in Figure 1.8. Overall, the hydrogen absorption capacity diminishes with decreasing the Pd size. The hydrogen solubility and solubility limit (αmax) in the α phase increase with decreasing the Pd size and the hydrogen solubility and solubility limit (βmin) in the 𝛽𝛽 phase decrease with reducing the Pd size. Consequently, the width of the plateau narrows and the critical temperature decreases with decreasing the Pd size. Furthermore, the slope of the pressure plateau becomes steeper with decreasing the particle size.

Hydrogen interaction with SACs

The downsizing of metal nanoparticles to metal single atoms causes dramatic changes in metal electronic properties and geometric structure. Consequently, the interaction between hydrogen and metals as nanoparticles and single atoms should have distinct behaviours. Unlike the hydrogen-bulk metal or hydrogen-metal nanoparticles systems, the hydrogen metal single atoms system has not been thoroughly studied so far. Szilágyi et al.90 reported a Pd SACs supported on a metal-organic framework [NH2-MIL-101(Cr)]. Using a combination of DFT calculations, in-situ Raman spectroscopy and temperature-desorption spectroscopy, Pd(H2) complexes (Figure 1.20) were proved to form on single Pd atoms. In addition, hydrogen was strongly bound in the Pd(H2) complex with an elongated H-H bond by over 15%. This elongation signified that there was an activation or destabilisation of the H–H bonds, which could have very important consequences in hydrogenation reactions.

Hydrogenation reactions with ultra-dispersed metal catalysts

As discussed previously, the hydrogenation is an important process in industries. There are many kinds of hydrogenation reactions: alkyne (𝐶𝐶≡𝐶𝐶), alkene (𝐶𝐶=𝐶𝐶), oxygennitrogen double bond (𝑂𝑂=𝑁𝑁), carbon-nitrogen double bond (𝐶𝐶=𝑁𝑁) and carbon-oxygen double bond (𝐶𝐶=𝑂𝑂) hydrogenations are common hydrogenation reactions, for example: hydrogenation of acetylene, hydrogenation of 1-3-butadiene, hydrogenation of CO2. The dominant catalysts for hydrogenation are metal nanoparticles owing to their high catalytically metal active sites originated from the high dispersion of nanoparticles on a support. However, it has been shown that the single atom catalysts are also reactive for this reaction, sometimes performing better in hydrogenation than the nanoparticles. For example, Hu et al.78 synthesized a Pd SACs supported on a graphitic carbon nitride. The Pd SACs catalyst was compared with a Pd nanoparticles sample embedded on the same support for the hydrogenation of styrene to ethylbenzene (Figure 1.22 a). Pd SACs catalyst was found to attain higher TOF value of 834 h-1 and yielded 98% conversion to ethylbenzene within 1.5h, while the TOF was 476 h-1 for Pd nanoparticles and the conversion was less than 55% in the same conditions. Wang et al.94 compared a Ru SACs on nitrogen-doped porous carbon with a Ru nanoparticles used in the hydrogenation of quinoline (Figure 1.22 b). The two catalysts showed 100% conversion of quinoline. However, the Ru SACs had a nearly 100% selectivity for the desired product to 1,2,3,4-tetrahydroquinolin (py-THQ) (2a), whereas the by-product 5,6,7,8-tetrahydroquinoline (21%) (bz-THQ, 2b) was found for Ru nanoparticles. Moreover, the Ru SACs catalyst was very stable, it did not deactivate (same activity and selectivity) after 5 cycles of reactions and its atomic dispersion remained unchanged. Shao et al.95 fabricated a stable atomically dispersed Ir catalyst supported on a porous organic polymer. The Ir SACs exhibited excellent catalytic activity during the liquid phase hydrogenation of CO2 to formate (Figure 1.22 c). In fact, the Ir SACs gave a turnover number (TON) as high as 6784 while the Ir nanocatalysts showed only several hundred TON value. The recycling tests indicated that there was no decrease in the catalytic activity after four uses.

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Metal single atom catalysts supported on carbon materials

The synthesis of metal single atom catalysts supported on carbon materials consists of 4 main steps as described in Figure 2.2. The sample 1 wt.% Ir SAC supported on AC was taken as an example:
(1) Impregnation: 200 mg of EDTA (ethylenediaminetetraacetic acid) was dispersed in 10 ml distilled water in a beaker under vigorous stirring, several drops of NH3·H2O were added to the solution to completely dissolve the EDTA. 3.8 mg of metal salt IrCl3·xH2O was then added to the solution. After the total dissolution of metal salt and the formation of a stable complex (the solution color is stable after about 6 h), 100 mg of the carbon host (AC or HSAG) was added.
(2) Evaporation: The solution was stirred (500 rpm) at room temperature for 2 h to completely disperse the carbon support and the metal complex. Afterwards, the solution was drying under stirring at 60 °C until the total evaporation of water (about 3 h). The recovered powder was dried at 70 °C in an oven overnight.
(3) Grinding: The dried solid was mixed with 500 mg of melamine and hand milled during 15 min.
(4) Calcination: The mixture was calcined at 800 °C under Ar flow (200 mL/min) for 2 h with a ramp of 3 °C/min. The 1 wt.% Ir SAC sample was obtained after calcination.

Transmission Electron Microscopy

Transmission electron microscopy (TEM) is an analytical tool to reveal sub-micrometer, internal fine structure in solids. It allows to obtain detailed micro-structural examination through high resolution and high magnification imaging because it produces images from a sample by illuminating the sample with a focused beam of high energy electrons. A schematic illustration of a TEM is shown in Figure 2.5.

Table of contents :

1.1 Hydrogen: generalities, and current uses
1.2 Hydrogen interaction with metals
1.3 Hydrogen interaction with nanosized and ultradispersed metals
1.3.1 Metal nanoparticles Synthesis of Metal nanoparticles Hydrogen interaction with Metal Nanoparticles
1.3.2 Single atom catalysts (SACs) Synthesis of SACs Hydrogen interaction with SACs
1.4 Hydrogenation reactions with ultra-dispersed metal catalysts
1.4.1 Hydrogenation of Butadiene
1.4.2 Hydrogenation of Levulinic Acid
1.5 References
2.1 Synthesis
2.1.1 Materials
2.1.2 Metal nanoparticles supported on carbon materials
2.1.3 Metal single atom catalysts supported on carbon materials
2.2 Characterization
2.2.1 Physicochemical characterization Powder X-ray Diffraction analyses Scanning Electron Microscopy Transmission Electron Microscopy Specific surface area & porosity Raman Spectroscopy X-Ray Photoelectron Spectroscopy X-Ray Absorption Spectroscopy Inductively Coupled Plasma Spectroscopy Thermogravimetric Analysis
2.2.2 Hydrogenation properties Pressure-Composition Isotherm Thermo-Desorption Spectroscopy
2.2.3 Catalysis Hydrogenation of butadiene Hydrogenation of levulinic acid
2.3 References
3.1 Synthesis of Palladium carbon materials
3.2 Pd nanoparticles supported on carbons
3.2.1 Physicochemical properties of materials
3.2.2 Nanosized Pd interaction with hydrogen Hydrogen sorption properties at room temperature Atomic Simulation Thermodynamic properties Desorption properties
3.3 Pd single atoms supported on carbons
3.3.1 Physicochemical properties of materials
3.3.2 Pd single atoms interaction with hydrogen
3.4 Conclusion
3.5 References
4.1 Synthesis of Iridium carbon materials
4.2 Physicochemical characterization
4.3 Formation process and stability of Ir-SAC sample
4.3.1 Formation process of Ir SAC sample
4.3.2 Stability of Ir-SAC sample
4.4 Catalytic test
4.4.1 Hydrogenation of butadiene
4.4.2 Hydrogenation of levulinic acid
4.5 Conclusion
4.6 Reference
5.1 Synthesis of single atom catalysts with various transition metals supported on activated carbon
5.2 Physicochemical characterization
5.2.1 Low metal loading SACs (< 5 wt.%)
5.2.2 High loading SACs (> 5 wt.%)
5.3 Catalytic test
5.3.1 Hydrogenation of butadiene
5.3.2 Hydrogenation of levulinic acid
5.4 Conclusion
5.5 Reference
General conclusion & perspectives


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