Associative desorption of hydrogen at edges and facets on Pt nano-particles (NPs) was studied using density functional theory. The goal was to identify catalytically active sites on Pt NPs for the hydrogen evolution reaction. Since NPs used in catalysis typically contain over a thousand atoms, calculations of whole particles are too demanding and the adsorption sites were instead modeled by periodic face centered cubic slabs representing an array of edges between two (111) micro-facets or edges between (111) and (100) micro-facets. The width of the facets in the periodic representations was systematically increased to reach converged results for binding and activation energy. For maximum hydrogen coverage, edges between (111) micro-facets were found to be several orders of magnitude more active than edges between (100) and (111) micro-facets or flat terraces. Unlike the missing row Pt(110)-(2 × 1) surface, which has sometimes been used as a simple model for edges between (111) micro-facets, the converged edge model does not show the recently reported reentrant behavior in desorption mechanism (Gudmundsdóttir et al., Phys Rev Lett 108:156101, 2012).