Hydrogenation on metal surfaces; why are nanoparticles more active than single crystals?

Under low-pressure conditions, hydrogenation of alkenes, such as 2-pentene and ethene, is shown to occur on supported palladium nanoparticles, whereas single-crystal palladium surfaces are inactive. This finding is rationalized on the basis of the accessibility of weakly bound subsurface hydrogen (see picture), which is enhanced on particles of nanometer dimensions