Effects of hydrogen on the reactivity of O2 toward gold nanoparticles and surfaces

Density functional theory (DFT) was used to study the coadsorption of O-2 and H-2 or H on Au(111) and Au(100) and on free pyramidal clusters (Au-25 and Au-29) observed on titania- and ceria-based catalysts. For the adsorption of isolated O-2, no significant interaction was obtained between the flat metal surfaces and the oxygen molecule. Gold clusters (Au-14, Au-25, and Au-29) showed a higher reactivity, but this reactivity depended strongly on the type of uncoordinated sites exposed, ensemble effects, and the fluxionality of the metal nanoparticles. On the Au-14 and Au-29 clusters, a superoxo species and a peroxo moiety were formed, respectively. In contrast, no interaction between O-2 and Au-25 was seen because of the high coordination number of the exposed metal sites. In general, H-2 dissociates much easier on the gold clusters than O-2, but again Au-25 is less reactive than Au-29 or Au-14. The pyramidal Au-25 and Au-29 clusters illustrate the importance of geometry in the chemistry of gold nanoparticles: A smaller particle size does not necessarily imply a bigger chemical reactivity. With the presence of predissociated hydrogen on a gold system, the adsorption energy of oxygen is much higher in absolute value than for the O-2/Au systems, accompanied by the formation of a hydroperoxo species. The origin of the synergistic effect observed for the coadsorption of hydrogen and O-2 is the formation of O-H bonds that enhance the O <-> Au interactions, reducing (0.15-0.30 angstrom) the O-Au bond lengths. The reaction H(a) + OOH(a) - H2O2(a) was found to be highly exothermic on the Art clusters, An interesting case is when the oxygen orientation allows for simultaneous interaction with two H adatoms: The formation of hydrogen peroxide is readily achieved with an energy release of more than 30 kcal/mol. This reaction is only hindered by the need of a concerted approach of O-2 to the H adatoms.