The reaction of hydrogen gas with magnesium metal, which is important for hydrogen storage purposes, is enhanced significantly by the addition of catalysts such as Nb and V and by using nanostructured powders. In situ neutron diffraction on MgNb(0.05) and MgV(0.05) powders give a detailed insight on the magnesium and catalyst phases that exist during the various stages of hydrogen cycling. During the early stage of hydriding (and deuteriding), a MgH(1< x < 2) phase is observed, which does not occur in bulk MgH(2) and, thus, appears characteristic for the small particles. The abundant H vacancies will cause this phase to have a much larger hydrogen diffusion coefficient, partly explaining the enhanced kinetics of nanostructured magnesium. It is shown that under relevant experimental conditions, the niobium catalyst is present as NbH(1). Second, a hitherto unknown Mg-Nb perovskite phase could be identified that has to result from mechanical alloying of Nb and the MgO layer of the particles. Vanadium is not visible in the diffraction patterns, but electron micrographs show that the V particle size becomes very small, 2-20 nm. Nanostructuring and catalyzing the Mg enhance the adsorption speed that much that now temperature variations effectively limit the absorption speed and not, as for bulk, the slow kinetics through bulk MgH(2) layers.

Single-walled carbon nanotubes (SWNT) were reported to have record high hydrogen storage capacities at room temperature, indicating an interaction between hydrogen and carbon matrix that is stronger than known before. Here we present a study of the interaction of hydrogen with activated charcoal, carbon nanofibers, and SWNT that disproves these earlier reports. The hydrogen storage capacity of these materials correlates with the surface area of the material, the activated charcoal having the largest. The SWNT appear to have a relatively low accessible surface area due to bundling of the tubes; the hydrogen does not enter the voids between the tubes in the bundles. Pressure ± temperature curves were used to estimate the interaction potential, which was found to be 580 AE 60 K. Hydrogen gas was adsorbed in amounts up to 2 wt % only at low temperatures. Molecular rotations observed with neutron scattering indicate that molecular hydrogen is present, and no significant difference was found between the hydrogen molecules adsorbed in the different investigated materials. Results from density functional calculations show molecular hydrogen bonding to an aromatic CÀC bond that is present in the materials investigated. The claims of high storage capacities of SWNT related to their characteristic morphology are unjustified.

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...