A DFT investigation of CO oxidation over neutral and cationic gold clusters

Abstract

The interaction of CO and O2 with neutral and positively charged Au9 and Au13 clusters was studied using Density Functional Theory. The aim was the understanding of the elementary steps of the low temperature activity of supported gold nanoparticles towards carbon monoxide combustion, that is, the oxidation of CO to CO2 in presence of dioxygen molecules. The adsorption of a single CO molecule gives rise to a substantial electronic rearrangement on both neutral and cationic gold clusters. On the contrary, the adsorption of dioxygen produces an electron transfer from neutral gold clusters to the O2, while the interaction with cationic Au nanoparticles is simply electrostatic. Co-adsorption of CO and O2 on adjacent catalytic sites produces a synergic electronic rearrangement on neutral cluster, but carbon monoxide oxidation does not take place. It is only when the two reactants are forced to interact with the same Au catalytic site that a chemical reaction takes place, leading to the rupture of the O2 molecule on cationic Au13. On a neutral Au13 cluster, on the other hand, the formation of an adduct containing a weakly bound dioxygen and a not fully formed carbon dioxide molecule is observed. The adsorption of a second CO molecule gives rise on both neutral and cationic aggregates to the facile desorption of CO2. Detailed reaction paths and energy barriers are calculated for each CO oxidation process.