Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he Hydrogen absorption in Ni and Pd: A study based on atomistic calculations E.A. Crespo a , M. Ruda b,Ã , S. Ramos de Debiaggi c a Dpto. de Fı ´sica, Fac. de Ingenierı ´a, Universidad Nacional del Comahue, Buenos Aires 1400, 8300 Neuque ´n, Argentina b Centro Ato ´mico Bariloche, 8400 Bariloche, Argentina, y Centro Regional Universitario Bariloche, Universidad Nacional del Comahue, Bariloche, Argentina c Dpto. de Fı ´sica, Fac. de Ingenierı ´a, Universidad Nacional del Comahue - CONICET, Buenos Aires 1400, 8300, Neuque ´n, Argentina article info Article history: Received 25 August 2007 Received in revised form 4 December 2007 Accepted 4 December 2007 Available online 19 February 2008 Keywords: Hydrogen in metals Nanoparticles Pd Ni Simulation Monte Carlo abstract 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. & 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. 1. Introduction Pressure–composition isotherms are a fundamental tool to investigate the interaction of hydrogen with metals. These curves characterize the phase transition from a low concen- tration solid solution of H in the metal matrix to a high H concentration hydride phase. For both Pd and Ni, the metals we studied here, this transition does not involve a structural phase transformation but only a change in the lattice parameter of the metallic host. The hydride phase can be thought of as an expanded fcc metallic lattice with H occupying all the octahedral sites [1,2]. In the solid solution to hydride transition Ni changes its lattice parameter from 0.352 to 0.372 nm (6%), while for Pd the lattice parameter expands from 0.388 to 0.402 nm (4%) [3]. Palladium absorbs hydrogen easily and therefore the Pd–H system has been extensively studied [3]. Nickel requires large pressures for absorbing hydrogen and forming the NiH hydride and there are large hysteresis effects in the two experimental absorption–desorption isotherms available in the literature at 298 and 338 K [3]. Both systems (Pd–H and Ni–H) cover a wide range of the behaviour of fcc Me–H systems. From the theoretical point of view physically accurate Me–H interatomic models are ARTICLE IN PRESS 0360-3199/$ - see front matter & 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.12.010 Ã Corresponding author. Tel.: +54 2944 445278. E-mail addresses: ecrespo@uncoma.edu.ar (E.A. Crespo), ruda@cab.cnea.gov.ar (M. Ruda), ramos@uncoma.edu.ar (S. Ramos de Debiaggi). INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 33 (2008) 3561– 3565