  At the nanoscale, materials exhibit unique properties that differ greatly
from those of the bulk state. In the case of Ag$_x$Pt$_{1-x}$ nanoalloys, we
aimed to study the solid-liquid transition of nanoparticles of different sizes
and compositions. This system is particularly interesting since Pt has a high
melting point (2041 K compare to 1035 K for Ag) which could keep the
nanoparticle solid during different catalytic reactions at relatively high
temperatures, such as we need in the growth of nanotubes. We performed atomic
scale simulations using semi-empirical potential implemented in a Monte Carlo
code at constant temperature and chemical composition in canonical ensemble. We
observed that the melting temperature decreases with the size (pure systems and
alloys) and the composition. We show that the melting systematically passes
through an intermediate stage with a crystalline core (pure platinum or mixed
PtAg depending on the composition) and a pure silver liquid skin, which
strongly questions the idea of having a faceted solid particle in catalytic
reactions for carbon nanotubes synthesis.
