We report the synthesis of a highly active, supported nanostructured metal nanoparticle catalyst with an ultra-thin porous shell and gaps between the metal nanoparticles and the shell for size-selective reactions. The size-selectivity of the catalysts could be realized through the porous shell. The gaps were able to reduce catalytic activity loss due to the contact areas between the shell and the catalytic sites. Evaluations of the activity and selectivity of the catalysts were made by catalytic hydrogenation of n-hexene versus cis-cyclooctene. Further verification of the high catalytic activity of the nanostructured catalysts was by oxidation of carbon monoxide.