Carbon deposits originated from side organic reactions are known to strongly affect the performance of metal catalysts. Quite unexpectedly. the C atoms have been recently found to act favorably and to lead to the enhancement of the catalyst performance. In the present work we employ a density-functional method to uncover atomistic mechanisms of the very first step of modifying Pd nanoparticles by subsurface C. In the interior of Pd(111) facets C is most stable in octahedral subsurface sites; occupation of tetrahedral subsurface sites by adsorbed C atoms results in smaller stabilization. There, the surface-to-subsurface diffusion of C features distinctive activation barriers. However, near nanoparticle edges, where CHx decomposition precursors of C tend to be located, more mobile low-coordinated I'd atoms make the diffusion into tetrahedral subsurface sites almost non-activated. This peculiar "nano"-effect suggests that the initial low-temperature modification of Pd particles by atomic C is governed by a fast occupation of tetrahedral subsurface sites at edges, which therefore serve as a gate to the subsurface. (C) 2009 Elsevier Inc. All rights reserved.