Theoretical Study of the Hydrogen Absorption Mechanism into a Palladium Nanocube Coated with a Metal-Organic Framework

We analyzed theoretically the hydrogen absorption properties and kinetics into Pd nanocubes coated with copper(II) 1,3,5-benzenetricarboxylate (HKUST-1), which is a type of metal-organic framework, using density functional theory. We prepared an interface model consisting of the Pd(100) surface and Cu-edged HKUST-1 structure and calculated the hydrogen adsorption and absorption energies in a Pd nanocube model. To discuss the kinetics of the hydrogen absorption, we also evaluated the hydrogen diffusion barrier near the interface. Compared with bare Pd, the hydrogen diffusion barrier from the surface to the subsurface decreased. We found that the adsorption of HKUST-1 on the Pd nanocube leads to chemical and steric effects for the diffusion rate increase of hydrogen. As a chemical effect, hydrogen adsorption was destabilized by the change of electronic structure of the Pd surface because of the atomic charge displacement between the Pd and Cu atoms in HKUST-1. As a steric effect, a new hydrogen diffusion path from the unique Pd5Cu octahedral site was created.