Analogy between homogeneous and heterogeneous catalysis by subnanometer metal clusters: Ethylene oxidation on Ag trimers supported on MgO(100)

An analogy between homogeneous catalysis and heterogeneous catalysis by very small, subnanometer-size metal clusters (or heterogeneous ultrananocatalysis) is proposed. We start from the idea that the catalytically active species in heterogeneous ultrananocatalysis are ligand/cluster/support complexes generated in situ under reaction conditions in the high coverage regime, and make a parallel between such species and the metal complexes of homogeneous catalysis. To this aim, we use first-principles simulations and focus on a specific example selecting as catalytic complex a Ag-3(CO3)/MgO(1 0 0) compound which has been suggested to form during the CO oxidation reaction catalyzed by silver trimer (Ag3) deposited on the regular MgO(1 0 0) surface. We investigate whether such a complex can be used as a catalyst in a different reaction: the partial oxidation of ethylene to ethylene oxide (or EtOx reaction). The EtOx catalytic cycle over Ag-3(CO3)/MgO(1 0 0) is determined, and it is found that it presents an overall reaction energy barrier of 0.9 eV. A detailed Molecular Orbital (MO), charge and fragment analysis is then conducted on the species involved in the catalytic cycle, highlighting the similarities of the given catalytic chemistry with homogeneous analogues. The proposed analogy between homogeneous and heterogeneous catalytic aggregates seems promising to widen the spectrum of heterogeneous catalyst design and realize synergistic investigations with accumulated knowledge on homogeneous systems. (C) 2014 Elsevier B.V. All rights reserved.