A bonding study of hydrogen dissociative adsorption on Pd decorated carbon nanotubes

IGNACIO LÓPEZ CORRAL; ESTEFANÍA GERMÁN; MARÍA ALICIA VOLPE; GRACIELA BRIZUELA

Tokyo

Simposio; 5th International Symposium on Surface Science and Nanotechnology; 2008

Surface Science Society Of Japan (SSSJ)

Hydrogen has a great potential as a clean fuel for automotive, but its practical application still requires the developing of materials that meets an adequate storage capacity. In recent years, nanostructured carbon materials and especially single-walled carbon nanotubes (SWCNTs) have been suggested as suitable media for hydrogen storage due to their high surface to volume ratios, even though the interaction between H2 molecule and bare carbon materials is very weak. The functionalization of SWCNTs by transition metal elements can promote their hydrogen storage capacity (1), often by means of the dissociation of adsorbed H2 and the later formation of strong bonds between hydrogen and metal atoms. Recently, it was observed that doping with Pd is a good alternative (2), but the theoretical studies involving this metal are still an open question. In this work we report a theoretical study on the H2 adsorption onto Pd decorated (10,0) zigzag SWCNT by semiempirical and DFT calculations, implemented respectively with YAeHMOP and VASP programs. First we investigated the preferential geometry for molecular and dissociative adsorption processes, using Pd1C190 clusters and considering specific adsorption sites (top, hollow and bridge). Then we study the atom-atom and orbital-orbital interactions by means of density of states (DOS), crystal orbital overlap population (COOP), and overlap population (OP). Results show a distinct preference for the bridge site, located between two neighboring C atoms, with the Pd atom situated to 1.84 Å respect to the carbon surface. The hydrogen-surface equilibrium distance is 4.25 and 2.86 Å for the molecular and dissociative adsorption, respectively, and in this ultimate case the H-Pd-H angle is 92º. The more visible changes in both adsorption processes involve a weakening of C-C bonds close to the bridge site and the formation of C-Pd and H-Pd bonds. However, the OP value for H-Pd bond in molecular state is only 1 % of the value in dissociative process, which is near to the OP value in atomic H adsorption. Therefore, we assume that the dissociation of adsorbed H2 is the preferential mechanism of interaction. We also found that H 1s, C 2pz and Pd 5s, 4px, 4pz and 4dxz orbitals play an important role in the C-Pd-H bonding.