In heterogeneous catalysis, the routes whereby molecules come and go from the active sites can substantially affect their reactivity. Röttgen et al. have examined a case where direct and indirect adsorption processes compete: the oxidation of CO over Pd clusters supported on MgO films grown on a metal substrate. The Pd clusters (either Pd8 or Pd30) were mass-selected before deposition, and by changing their surface coverage, the authors could vary the ratio of incoming CO that adsorbed directly on the cluster versus that arriving via diffusion from the support. Data and modeling revealed that for the Pd8 clusters, the reaction probability was the same whether the CO arrived directly or by diffusion, whereas for the Pd30 clusters, the CO supplied by reverse spillover from the support was less reactive than that impinging directly. The results highlight the subtleties of structure-dependent activation energies. -- PDS
J. Am. Chem. Soc. 129, 10.1021/ja068437f (2007).
