Methane Adsorption on Graphitic Nanostructures: Every Molecule Counts

Abstract

Bundles of single-walled nanotubes are promising candidates for storage of hydrogen, methane, and other hydrogen-rich molecules, but experiments are hindered by nonuniformity of the tubes. We overcome the problem by investigating methane adsorption on aggregates of fullerenes containing up to six C(60); the systems feature adsorption sites similar to those of nanotube bundles. Four different types of adsorption sites are distinguished, namely, registered sites above the carbon hexagons and pentagons, groove sites between adjacent fullerenes, dimple sites between three adjacent fullerenes, and exterior sites. The nature and adsorption energies of the sites in C(60) aggregates are determined by density functional theory and molecular dynamics (MD) simulations. Excellent agreement between experiment and theory is obtained for the adsorption capacity in these sites.

Figure 1

Mass spectra of helium droplets doped with C₆₀ and CH₄. The most prominent ions have the stoichiometry [(C₆₀)_m(CH₄)_n]⁺, with m = 1–5 (a–e). The sections shown reveal the most significant abundance anomalies for each series.

Figure 2

Ion abundance of C₆₀–CH₄ complexes deduced from mass spectra. The main panel displays data for [(C₆₀)_m(CH₄)_n]⁺ versus n on a logarithmic scale for complexes containing up to six C₆₀; also shown is the abundance of [C₇₀(CH₄)_n]⁺. Statistically significant anomalies are labeled with the value of n. The abundance of [(C₆₀)₂(CH₄)_n]⁺ at around n = 56 is displayed in the inset.

Figure 3

Energy-resolved snapshots of C₆₀ dimer, trimer, and tetramer ions (a–c) with 80 adsorbed CH₄. Each complex is viewed from two different perspectives. CH₄ molecules are represented as spheres even though the energy dependence of their orientation is included in the calculations. The color of the molecules represents their energy, computed as a sum over all pairwise interactions with the fullerenes and all other CH₄. Strongly bound methanes (blue) reside in the “grooves”, which are marked by arrows for the dimer and trimer.

Figure 4

Structures and energetics extracted from MD simulations. For each panel, the histogram to the upper left (blue) represents the number of adsorbed CH₄ in [(C₆₀)_m(CH₄)₅₀₀]⁺ versus the distance from the center of the nearest fullerene; solid lines represent the accumulated sum ∑ of molecules. These histograms reveal the number of molecules in complete adsorbate layers. The histograms below (in red) reveal the number of molecules that are located in groove or dimple sites. The energy histograms (green) show the number of adsorbed molecules versus their energy.

Figure 5

The experimental ion abundance divided by a smooth function reveals another, rather weak anomaly. For the C₆₀ trimer and tetramer, the numbers (2 and 4, respectively) agree with the number of dimple sites.