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 Å) the O−Au bond lengths. The reaction H(a) + OOH(a) → H 2 O 2 (a) was found to be highly exothermic on the Au 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.