Chemistry on single atoms:: key factors for the acetylene trimerization on MgO-supported Rh, Pd, and Ag atoms

Acetylene trimerization on Ag, Rh, and Pd atoms supported on the MgO (0 0 1) surface has been studied experimentally and theoretically. The electronic configuration of Ag renders these atoms inert both as free or supported atoms. The reactivity of Pd and Rh is strongly influenced by their adsorption and by their diffusion dynamics on the MgO surface. Pd atoms are turned into active catalysts only when adsorbed on oxygen vacancies (F centers). The low activation barriers computed for cyclotrimerization on free Rh atoms are substantially increased for Rh adsorbed on a MgO surface. Here, the reaction is only catalyzed when Rh is trapped on F centers. On steps, where Rh atoms are strongly bound, steric effects prevent the reaction to occur. (C) 2004 Elsevier B.V. All rights reserved.