Resolving Rotational Spectra of Hydrogen Adsorbed on a Single‐Walled Carbon Nanotube Substrate

Abstract

Some of the early results on hydrogen storage in single-walled carbon nanotubes (SWNTs) have not been reproduced, and at the moment it is generally believed that SWNTs store hydrogen in the molecular form in the same way as other graphitic materials with a high surface area.[1] Untreated nanotubes are bundled, which limits the accessible surface area drastically, but thermal and acidic treatments can increase the hydrogen storage capacity due to an increase in accessible surface area.[2] Furthermore, the adsorption energy of hydrogen on SWNTs and other nanostructured carbons are comparable.[3]Herein, we present a detailed description of the adsorption of hydrogen in SWNT materials.The energy E J of the rotational level J associated with the rotation of a hydrogen molecule is quantized: E J = J(J + 1)B, where B has the value 7.35 meV (59.3 cm À1 ).[4] The rotational spectrum of H 2 will be distorted when it is adsorbed and the triple degeneracy of the J = 1 level can be lifted, making the rotational levels split. We use the transition from J = 0 to J = 1, which, in a centrosymmetric environment, has two lines with relative intensities of 1:2. In general, a stronger interaction between the adsorbed molecule and the adsorbent leads to a more distorted spectrum, providing a sensitive and a local probe of the environment of an H 2 molecule. [3,5] Neutron scattering results of the region of interest (i.e., around the rotational peak) are presented in Figure 1. The insets (a) and (b) show the whole spectrum measured for the highest H 2 loading and the background run, respectively. The neutron spectra show a strong peak around 14.7 meV, the energy of the rotational transition. This peak indicates unambiguously that hydrogen is adsorbed in a molecular form. The rotational peak in the spectra is made up from more than one component. The additional structure of the rotational spectrum was not seen before due to a lack of resolution. [3,6] Initially, the peak shows a doublet. At higher loadings a broad peak adds up. The last spectrum shows these two contributions together with a sharper line on top.In Figure 2, fits using Gaussian lineshapes are given of each of the spectra with the separate contributions. Table 1 gives the parameters of the fits used. The spectrum with the lowest H 2 loading clearly shows two peaks (I and II) upon visual inspection with rough intensity ratios of 1:2. As described above, this is a spectrum of a single hydrogen site. However, the fit improved by adding a separate weak line in between the two others (peak III). This would correspond to a weakly populated second site where there is less anisotropy in the environment, that is, less distortion of the H 2 molecule. For a higher loading of hydrogen, the first two lines are always present, increasing in intensity for a filling of 85 mL at standard temperature and pressure (STP), but saturated for higher fillings.In the spectrum of 157 mL STP of adsorbed H 2 , the additional line III appears stronger and broadened. The highest loading Figure 1. Inela...

Inelastic neutron scattering of H2 adsorbed on boron substituted single walled carbon nanotubes

“…For both B-and C-SWNT samples, one observes very similar spectra that are at least doublets for loading level above the 0.5 H 2 :B. These spectra can be fitted to two Gaussian peaks and one obtains peak positions similar to those observed by others for as-prepared SWNTs [6][7][8], at 13.5 and 15.1 meV, but not by us for laser prepared materials [5]. Even though the peaks for the B-SWNT material seem to be very slightly broader than those from the C-SWNTs, there are no major differences in the spectra that could be interpreted as arising from a significant increasing in the hydrogen binding potential.…”

“…Subsequently data for as-prepared arc-produced materials were presented [6] and then reinterpreted and somewhat expanded [7], with a clear multi-peaked feature being rationalized in terms of the hydrogen experiencing a stronger rotational hindering potential in what is most likely the groove sites as compared to hydrogen on the exterior surfaces of the bundles. Similar multi-peak inelastic features were also seen for a carbon sample produced by decomposition of Fe(CO) 5 in a CO environment (HiPCo) [8].…”

“…Two recently proposed approaches that could increase the interaction of hydrogen with carbons are decorating the material with metal catalysts [2,3] and substituting boron for carbon [4]. Neutron scattering has previously been used to probe the local potential at a physisorption site via the rotational transition of molecular hydrogen [5][6][7][8]. Our initial study of hydrogen physisorbed on bundles of laser-produced, un-purified, nanotubes produced a spectrum that was considerably broadened from a nominally resolution limited rotational peak that was only slightly reduced from the free rotor valve of 14.7 meV [5].…”

Hydrogen adsorption in a copper ZSM5 zeoliteAn inelastic neutron scattering study

“…We found in a INS study of H 2 interacting with a cobalt aluminophosphates (CoALPOs) [9,10] that the interaction involved a high rotational barrier, implying a rather strong interaction of H 2 molecules with the CoALPO. The interaction with oxidic materials appears to be stronger than with carbon nanotubes, which have also been proposed for hydrogen storage and studied by INS [11,12]. Following our work on the CoALPOs, we now report a preliminary INS study of molecular hydrogen interacting with a copper ZSM5.…”

Neutron spectroscopy study of single-walled carbon nanotubes hydrogenated under high pressure

“…The INS technique has already been successfully applied for the investigation of low-pressure hydrogenated SWNT [4][5][6][7][8][9][10][11][12] and fullerenes [13], and also for the C 60 fullerenes hydrogenated under high pressure [14][15][16][17]. The present paper describes results of an INS investigation of SWNT and, for comparison, of C 60 hydrogenated at 3 GPa and T = 620 K. To analyze the nature of hydrogen bonding in these high-pressure compounds, the spectra were measured for the SWNT & H and C 60 & H samples in the quenched state and after partial removal of hydrogen by vacuum annealing.…”