A twist about an atomistic explanation for the unusual particle size-dependent reactivity enhancement of supported gold clusters still exists in the surface science and catalysis communities. A molecular beam scattering technique allowed the active sites of a catalyst to be mapped. In doing so, copper and gold clusters, both supported on silica, were studied. Indeed, Au clusters showed quite unusual behavior, however, that would be consistent with the influence of defects rather than quantum size effects. (C) 2011 Elsevier B.V. All rights reserved.