C-C bond broken & repaired.
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C-C bond broken & repaired
Neutron Scattering Studies of Hydrogenation / Dehydrogenation of Carbon Nano-MaterialsAlexander I. Kolesnikov1*, Chun-K. Loong1, Vincent Leon1,2, Nicolas R. de Souza1, Alexander P. Moravsky3 and Raouf O. Loutfy31Argonne National Laboratory, Intense Pulsed Neutron Source, 9700 South Cass Avenue, Argonne, Illinois 60439, USA2CRMD-CNRS, Université d’Orléans, 1b rue de la Férollerie, 45071 Orléans, France3MER Corporation, 7960 South Kolb Road, Tucson, AZ 85706, USA
In the last decade there was a great interest in the investigation of interaction of hydrogen gas with different carbon nano-materials. Molecular hydrogen adsorbed in fullerenes, single wall carbon nanotubes (SWNT), carbon nano-fibers, etc. as well as chemically bound hydrogen were studied by different methods in order to get information on the properties of the adsorbed hydrogen and to develop an effective hydrogen storage material. The first publications (see e.g. ) on hydrogen storage in SWNT estimated a possible high hydrogen storage capacity for purified tubes at room temperature and ambient pressure. However further intense studies on a variety of nanocarbons showed variety of results that were often non-reproducible by independent measurements. Critical reviews [2-4] have identified the major factor underlying the discrepancy, besides sample impurity, to be the lack of fundamental understanding of the local configurations of the adsorbed hydrogen and its physical/electronic interactions with the nanostructure environment.
The most direct and complete information on hydrogen adsorption can be obtained by using inelastic neutron scattering (INS) method, because of very high neutron scattering cross-section of hydrogen (82.02 barns) compared to carbon (5.55 barns) atoms. Also, neutrons can easily penetrate different sample-container and environment walls that make possible to do INS experiments in situ, during charging/discharging the sample with hydrogen under high pressure at low or high temperatures. Up to now INS was mainly used for investigation of hydrogen uptake in pure SWNT [5-10]. In this work we present INS study of hydrogen adsorption/desorption behavior in a variety carbon nano-materials.
Materials and Methods
The high purity samples used in this study, SWNT of 14 Å diameter, double wall carbon nanotubes (DWNT) of about 25 Å external and 18 Å internal diameters, C60-fullerenes in SWNT of 15 Å diameter (C60-peapods), and single wall carbon nanohorns (SWNH) of rather broad size distribution (~50 Å), were produced by MER Corporation and characterized by TEM and neutron diffraction (ND).
The INS experiments were done on time-of-flight spectrometer of inverse geometry QENS (at IPNS, Argonne National Laboratory) which capable to simultaneously measure ND patterns and INS spectra in the energy transfer range 0-200 meV, covering wide range of neutron momentum transfer. Hydrogenation/dehydrogenation of carbon nano-materials were done in situ at temperatures 20-300 K and hydrogen gas pressures of 1-110 bar.
SWNT - 10 MPa @ 25 K, Physisorbed H2 molecules, no C-H bonds
Hydrogenated SWNT - 3 GPa quenched to 77 K, Molecular H2 & CHx complexes
Hydrogenated SWNT - 5 GPa annealed @ 374°C, Only CHx complexes remain
Hydrogenated C60 - 3 GPa quenched to 77 K, Molecular H2 & CHx complexes .
Results and Discussion
The obtained INS data for SWNT, in agreement with those by other workers [5-9], confirm the following observations: i) charged hydrogen at low temperatures (below ~40 K) behaves as molecular hydrogen, as evidenced by the observed para-ortho rotational transition at 14.7 meV nearly identical to that in bulk solid hydrogen, ii) at temperatures above ~50 K the para-ortho peak diminishes and a quasielastic component appears of increasing intensity with increasing temperature, indicating the release of hydrogen molecules from adsorption sites, and iii) after warming up to room temperature, about all of the hydrogen escapes, so that hydrogen cannot be stored at ambient conditions. Only a weak hydrogen signal (~5% of the original intensity) is seen when re-cooling the sample back to 10 K. Behavior of hydrogen in DWNT and C60-peapods are similar and do not favor these materials as a practical hydrogen storage medium.
A different behavior was observed for hydrogen in SWNH. We found that the widths of the para-ortho transition peak were more than twice of the corresponding ones in SWNT, DWNT and C60-peapods. Additionally, considerably stronger quasielastic components were observed at higher temperatures. More importantly, we found the recovery of the para-ortho peak at 10 K after warming up the SWNH sample to 290 K, indicating that hydrogen was retained in the SWNH sample after annealing. The next cycle of hydrogen adsorption/desorption in the same SWNH resulted in complete dehydrogenation of the sample at room temperature. The fresh SWNH sample contained many defects, as was seen in the TEM images taken from one of our samples at Argonne. In the nomenclature of (m,n)-SWNTs, the tube is in principle metallic if n-m is divisible by 3, otherwise it is semiconducting. The variance in the (n,m) combination in a nanohorn, required by the defect structure, will determine whether it is locally metallic or semiconducting. Such modified electronic states have been related to important stereochemical effects on hydrogen uptake. The first hydrogenation/dehydrogenation could cause the SWNH to reorganize in structure and to decrease defects, as was similarly seen in SWNT in .
High-resolution TEM images of SWNT (left), SWNH (right) and DWNT (bottom).
Formation of CHx complexes(?) .
Stretching vibrational band of C-H covalent bonds in SWNT is placed at lower energy than that of traditional covalent C-H bands such as in hydrocarbons and other materials.
ND patterns of SWNT and DWNT in the range of 1st Bragg peak from the (01) planes of the 2D hexagonal lattice of the nanotube bundles.
ND patterns of SWNT (curve 1), D2O in SWNT (2) and C60-peapods (3) measured at IPNS.