than activities from experiments using only base metals even if both systems expressed surfaces of equivalent binding energies (see Fig. ). One possible explanation of this trend is that, although base metals may express thermodynamically active surfaces, these surfaces are less experimentally common than active surfaces on noble metals, either because particles are less likely to form or surfaces are deactivated. The latter prediction is supported by the experimental observation of oxides and sulfides on iron and nickle particles in a similar experimental system . Another compelling hypothesis is that 3d metals tend to have compact orbitals that cannot facilitate effective electron transfer . However, the activity of weak-binding noble metals Au and Ag are also known to be significantly enhanced in nanoparticle form, with explanations for this trend including surface strain and surface cleanliness . Alternatively, the active surfaces on noble metals may be less easily poisoned, aggregated, or passivated: these effects were not considered in this work. Differences in nanoparticle stability may also have a substantial effect on activity trends .