Size-dependent self-limiting oxidation of free palladium clusters.

Temperature-dependent gas phase ion trap experiments performed under multicollision conditions reveal a strongly size-dependent reactivity of Pd(x)(+) (x = 2-7) in the reaction with molecular oxygen. Yet, a particular stability and resistance to further oxidation is generally observed for reaction products with two oxygen molecules, Pd(x)O4(+). Complementary first-principles density functional theory simulations elucidate the details of the size-dependent bonding of oxygen to the small palladium clusters and are able to assign the pronounced occurrence of Pd(x)O4(+) complexes to a dissociatively chemisorbed bridging oxygen atomic structure which impedes the chemisorption of further oxygen molecules. The molecular physisorption of additional O2 is only observed at cryogenic temperatures. Additional experiments and simulations employing preoxidized clusters Pd(x)O(+) (x = 2-8) and Pd(x)O2(+) (x = 4-7) confirm the formation of the two different oxygen species.