DFT study of PtH2 systems adsorbed on pristine and defective graphene

IGNACIO LÓPEZ CORRAL; RICARDO FACCIO; ALFREDO JUAN; MARCELO AVENA

Montevideo

Workshop; 5th Workshop on Novel Methods for Electronic Structure Calculations; 2013

Facultad de Química, Universidad de la República

We used density functional theory (DFT) to study adsorption of PtH2 systems onto pristine and defective graphene. The calculations were performed with VASP [1] and SIESTA [2] codes, using the generalized gradient approximation (GGA). The graphene surface was modeled by means of a 5 × 5 supercell system. We considered several adsorption sites on the carbon surface and both molecular as dissociated hydrogen-platinum coordination structures, including the so-called Kubas interaction [3]. Energetic, bonding and electronic analysis were addressed in all cases.Our results indicate that the most energetically favorable PtH2 system onto pristine graphene is a classic hydride without H-H bond (dH-H > 1.77 Å). The preferential hydrogen adsorption occurs when the Pt atom is located on the hollow site, above a carbon hexagon, with a binding energy of Eb(H2) = -2.57 eV. The Pt-H bond is elongated 2.0 % as a result of the interaction between the PtH2 hydride and the graphene sheet. Linear Pt-H-H dihydrogen complexes are much less stable on the pristine surface, with Eb(H2) = -0.62 eV, whereas supported Kubas-type systems are unstable and relax to classic hydrides. In contrast, a Kubas complex with a dH-H = 0.83 Å is the preferential carbon-supported PtH2 system when the Pt atom is adsorbed on a vacancy site, with Eb(H2) = -0.59 eV. The dissociative adsorption of the hydrogen molecule on defective graphene decorated with platinum leads to a Eb(H2) value almost 50% smaller than that corresponding to the Kubas complex.Our simulations suggest that the carbon vacancy decreases the catalytic effect of the supported Pt atom, favoring molecular hydrogen adsorption on the decoration. Thus, we could assume that the presence of defect in Pt-decorated carbon adsorbents play an very important role for hydrogen storage based on Kubas-type bonding.