Bonding in hydrogen-platinum Kubas complexes on defective graphene layers

IGNACIO LÓPEZ CORRAL; RICARDO FACCIO; ALFREDO JUAN

Matsue

Simposio; 7th International Symposium on Surface Science; 2014

Surface Science Society Of Japan (SSSJ)

Carbon nanomaterials decorated with transition metal atoms are promising systems for hydrogen storage at ambient conditions. The enhanced hydrogen storage capacity has been ascribed to different mechanisms: physisorption, hydride formation, spillover and Kubas bonding. Recent STEM observations and modeling1) suggest that the Kubas-type bonding can occur in parallel with the other hydrogen storage mechanisms. Moreover, its contribution to the excess uptake may be bigger than previously thought for carbon materials decorated with highly dispersed metal nanoparticles. In this work we performed Density Functional Theory (DFT) calculations in order to study several PtH2 systems adsorbed on defective graphene, including Kubas-type complexes. The GGA-PBE exchange-correlation functional2) was implemented in VASP3) and SIESTA4) codes. Graphene surfaces were modeled using a 5x5 supercell. The bonding evolution during the adsorption process was addressed in all cases. We found that the most stable PtH2 system on defective graphene corresponds to the Kubas-type complex, with the H-H distance elongated to 0.83 Å. However, this Kubas complex is unstable on pristine graphene and a hydride is obtained on all adsorption sites, according previous studies5). We considered several initial configuration for the Kubas complex on defective graphene, but at the end all of them conducted to final geometry where the H2 is orientated almost perpendicular to the outermost C-Pt bond. This particular orientation can be understood from the density of states in which there are important contributions from hydrogen states. Therefore, we selected the eigenvalues for that energy regions and we plotted the charge density associated to the individual eigenvalues at gamma-point. The H2 molecule also interacts with one carbon atom from graphene through s-H and  p-C states. In this way, the strong Pt-graphene interaction reduces the reactivity of the free metal atom, and at the same time graphene provides an interaction with the H2 molecule for the formation of the Kubas-type complex. According to these results, single atom vacancies passivated with individual Pt atoms could provide an interesting system for hydrogen storage applications.