supercritical-nitrogen-processing-for-purification-reactive-porous

Supercritical Nitrogen Processing for the Purification of Reactive Porous Materials

Supercritical fluid extraction and drying methods are well established in numerous applications for the synthesis and processing of porous materials. ...

Empa, Swiss Federal Laboratories for Materials Science and Technology

Prolonged hydrogenation of C(60) molecules by reaction with H(2) at elevated temperature and pressure results in fragmentation and collapse of the fullerene cage structure. However, fragments can be preserved by immediate termination of dangling bonds by hydrogen. Here we demonstrate that not only fullerene fragments but also hydrogenated fragmented fullerenes (e.g., C(58)H(40) and C(59)H(40)) can be synthesized in bulk amount by high-temperature hydrogenation of C(60). We confirm successful synthesis of these species by matrix-assisted laser desorption ionization time-of-flight mass spectrometry and complete speciation of the resultant complex fullerene mixtures by high-resolution field desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry.

Synthesis of C59Hx and C58Hx fullerenes stabilized by hydrogen.

Complex solid hydrofulleride mixtures were synthesized by prolonged hydrogenation of C(60) at 120 bar hydrogen pressure, 673 K temperature, and different reaction periods. The high degree of hydrogenation was confirmed by infrared spectroscopy and X-ray diffraction. The identity of hydrogenation products was determined by high-resolution field desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry. Despite partial gas-phase fragmentation of hydrofullerene ions during mass analysis, the data suggest that the synthesized mixtures consist of mostly C(58-60)H(x) hydrofullerenes. Increasing the duration of hydrogenation results in synthesis of C(59)H(x) and C(58)H(x) as major products. Possible hydrofullerene fragmentation pathways during both material synthesis and mass spectrometric analysis are discussed. Gas-phase fragmentation in the mass spectrometer arises from hydrofullerene ions C(60)H(x)(+) with x > 40 and C(59)H(44)(+) with drastically decreased molecular stability relative to the known C(60)H(36).

Composition of hydrofullerene mixtures produced by C(60) reaction with hydrogen gas revealed by high-resolution mass spectrometry.

LiBH4 is a complex hydride and exhibits a high gravimetric hydrogen density of 18.5 wt %. Therefore it is a promising hydrogen storage material for mobile applications. The stability of LiBH4 was investigated by pcT (pressure, concentration, and temperature) measurements under constant hydrogen flows and extrapolated to equilibrium. According to the van 't Hoff equation the following thermodynamic parameters are determined for the desorption: enthalpy of reaction DeltarH = 74 kJ mol-1 H2 and entropy of reaction DeltarS = 115 J K-1 mol-1 H2. LiBH4 decomposes to LiH + B + 3/2H2 and can theoretically release 13.9 wt % hydrogen for this reaction. It is shown that the reaction can be reversed at a temperature of 600 degrees C and at a pressure of 155 bar. The formation of LiBH4 was confirmed by XRD (X-ray diffraction). In the rehydrided material 8.3 wt % hydrogen was desorbed in a TPD (temperature-programmed desorption) measurement compared to 10.9 wt % desorbed in the first dehydrogenation.

Stability and reversibility of LiBH4.

The determination of the equilibrium thermodynamic parameters of hydrogen storage materials from quasiequilibrium pressure data using the mass flow pressure concentration isotherm (pcI) method is presented. The method bases on the acquisition of pcI curves at different flow rates using a thermal mass flow controller to determine the amount of ad/desorbed hydrogen. These measurements provide a set of corresponding quasiequilibrium pressure functions from, which the true equilibrium pressure of the hydride is calculated by extrapolation to zero flow. The governing thermodynamic parameters can then be determined to characterize the material by the construction of a van't Hoff plot, extracting enthalpy of reaction DeltaH(r) and entropy of reaction DeltaS(r) from the equilibrium pressure p(eq) as a function of temperature. Naturally, true equilibrium can never be reached and therefore can only be approximated by measurement--a drawback that all experimental techniques share. This complication is alleviated by the flow-pcI approach at different flow rates. The compilation of the p(eq)(T) data from pcI-measurements can be performed by different methods, whereas the so called Sieverts apparatus is most commonly used. In this paper, we elaborate the differences and advantages of the mass flow-pcI over the Sieverts Apparatus and present measurements and results on LaNi(5) as a benchmark. Measurements at different flow rates are presented and equilibrium pressures at zero flow are achieved by extrapolation. The obtained results of DeltaH(d)=32.5 kJ mol(-1) H(2) and DeltaS(d)=115 J K(-1) mol(-1) H(2) (desorption process) perfectly match literature values, emphasizing the excellent quality of the measurements and the performance of this measurement apparatus.

Characterization of hydrogen storage materials by means of pressure concentration isotherms based on the mass flow method.

The hydrogen dynamics in solid and in liquid LiBH4 was studied by means of incoherent quasielastic neutron scattering. Rotational jump diffusion of the BH4- subunits on the picosecond scale was observed in solid LiBH4. The characteristic time constant is significantly shortened when the system transforms from the low-temperature phase to the high-temperature phase at 383 K. In the molten phase of LiBH4 above 553 K, translational diffusion of the BH4- units is found. The measured diffusion coefficients are in the 10(-5)cm2/s range at temperatures around 700 K, which is in the same order of magnitude as the self-diffusion of liquid lithium or the diffusion of ions in molten alkali halides. The temperature dependence of the diffusion coefficient shows an Arrhenius behavior, with an activation energy of Ea = 88 meV and a prefactor of D0 = 3.1 × 10(-4)cm2/s.

BH4− self-diffusion in liquid LiBH4.

The decomposition pathway is crucial for the applicability of LiBH(4) as a hydrogen storage material. We discuss and compare the different decomposition pathways of LiBH(4) according to the thermodynamic parameters and show the experimental ways to realize them. Two pathways, i.e. the direct decomposition into boron and the decomposition via Li(2)B(12)H(12), were realized under appropriate conditions, respectively. By applying a H(2) pressure of 50 bar at 873 K or 10 bar at 700 K, LiBH(4) is forced to decompose into Li(2)B(12)H(12). In a lower pressure range of 0.1 to 10 bar at 873 K and 800 K, the concurrence of both decomposition pathways is observed. Raman spectroscopy and (11)B MAS NMR measurements confirm the formation of an intermediate Li(2)B(12)H(12) phase (mostly Li(2)B(12)H(12) adducts, such as dimers or trimers) and amorphous boron.

Philippe Mauron

Hydrogen and Energy Laboratory

Empa, Swiss Federal Laboratories for Materials Science and Technology

Supercritical Nitrogen Processing for the Purification of Reactive Porous Materials