C60Con complexes as hydrogen adsorbing materials

An active line of contemporary research is dedicated to the adsorption and storage of hydrogen on metal-doped carbon materials. Using density functional theory and van der Waals corrections we have studied molecular and dissociative adsorption of H₂ on neutral and cationic C₆₀Co_n complexes with n = 1–8. The Co atoms form compact clusters on the surface of the fullerene. Dissociative chemisorption of one H₂ molecule is more stable than molecular adsorption on neutral C₆₀Co_n, with the only exception of C₆₀Co. When C₆₀Co_n is ionized, the electronic charge deficit remains localized in the cobalt cluster. The molecular and dissociative adsorption features on cationic C₆₀Co_n⁺ and neutral C₆₀Co_n complexes are in general similar, but some differences can be highlighted. Molecular and dissociative H₂ adsorption on C₆₀Co₂⁺ are competitive; in fact, molecular adsorption is slightly more stable as a result of the localized electronic charge deficit on the Co dimer. Another difference is that the dissociative adsorption energies on some of the neutral C₆₀Co_n complexes are substantially larger than the dissociative adsorption energies on the corresponding C₆₀Co_n⁺ cationic complexes by amounts between 0.2 and 0.5 eV. Activation barriers for dissociation of the adsorbed H₂ molecule strongly depend on the charge state and cluster size. These barriers help to interpret the adsorption state (molecular or dissociated) of experimentally produced hydrogenated C₆₀Co_n⁺ complexes. Hydrogen saturation has been studied in two cases. C₆₀Co adsorbs three H₂ units in molecular form, and C₆₀Co_n adsorbs up to thirteen H₂ units, four dissociated and nine in molecular form.