Structural transitions and stabilization of palladium nanoparticles upon hydrogenation

Using an explicit empirical potential and a combination of Monte Carlo and molecular dynamics simulations, we investigate the energetic, thermal and dynamical stability of hydrogenated palladium nanoclusters containing a few hundred atoms. At zero temperature, icosahedral clusters favour tetrahedral absorption sites, while cubic clusters preferentially absorb at octahedral sites. As a consequence, icosahedra can absorb a larger quantity of hydrogen than cubic clusters. However, even a moderate amount of absorbed hydrogen is found to favour cubic structures. There are two distinct effects of temperature. First, thermal desorption of hydrogen atoms can take place spontaneously even at low temperatures. Second, hydrogen rich cubic particles undergo a structural transition toward icosahedra before melting occurs. A simplified diagram is proposed to account for the observed behaviour. Finally, the coalescence between two clusters is studied dynamically for several clusters at various temperatures, both in the gas phase and for particles embedded in a simple model solvent. The heat produced from coalescence results in the desorption of hydrogen molecules. This leads to a significant cooling of the nanoparticle, thereby contributing to its stabilization.