Hydrogen absorption in Ni and Pd: A study based on atomistic calculations

We implemented a model at the atomistic level to simulate metal–hydrogen (Me–H) pressure–composition equilibrium isotherms and we applied it to Ni and Pd, both at the bulk and in nano-sized particles. We used potentials of the embedded atom type (EAM) to describe the interaction between atoms [Daw MS, Baskes MI. Embedded-atom method: derivation and application to impurities, surfaces, and other defects in metals. Phys Rev B 1984; 29(12): 6443–53; Angelo JE, Moody NR, Baskes MI. Trapping of hydrogen in nickel. Model Simul Mater Sci Eng 1995;3:289–307] and we performed Monte Carlo simulations to calculate the isotherms. For the bulk the simulations are sensitive to the interatomic potentials chosen. The tested potentials for Pd–H were not adequate since they did not exhibit the characteristic plateaux observed experimentally. The Ni–H potentials predicted a clear transition but at lower hydrogen pressures.For Ni and Pd nano-sized clusters our simulations predict enhanced hydrogen solubilities and vanishing plateaux when compared to the bulk. For both types of nanoparticles H atoms were segregated to the surface, but in Ni the effect was stronger.