CO adsorption and oxygen activation on group 11 nanoparticles - a combined DFT and high level CCSD(T) study about size effects and activation processes

The focus of this study lies in the activation of molecular oxygen and reaction with CO within density functional theory (DFT) and high level CCSD(T) calculations. Therefore, we use M-13 and M-55 nanoparticles (NPs) and periodic M(321) surfaces as model systems and compare the catalytic activity of gold substrates to Ag and Cu based NP catalysts. In the first step, the adsorption energies of CO were compared for nanoparticles of different sizes for Au, Ag and Cu. The adsorption energies on M(321) and M-55 NPs (M = Au, Ag, Cu) are virtually identical. For smaller M-13 NPs the adsorption energies differ by approximate to 0.2 eV for Ag, approximate to 0.4 eV for Au, and approximate to 0.6 eV for Cu at the PBE level of theory. This can be explained by size effects, as the M-13 NPs show a more molecule-like character. Presumably, CO binds more strongly to these very small NPs at the PBE level of theory. However, a benchmark calculation in the framework of CCSD(T)-theory reveals an adsorption energy of CO on Au-13 of -0.88 eV, comparable to the adsorption energies calculated at the PBE level for Au-55 and Au(321). For Au-55, the adsorption energy calculated at the CCSD(T) level is -0.85 eV. This is in perfect agreement with the PBE result. In addition to adsorption energies, dissociation barriers have been calculated on M(321) surfaces. The dissociation energies of O-2 on coinage metal catalysts are high, so that direct CO-oxidation reactions with molecular oxygen should be the dominant reaction mechanism compared to the dissociation and reaction of CO and atomic oxygen at least for silver and copper catalysts.