Reactions of carbon monoxide with free palladium oxide clusters: strongly size dependent competition between adsorption and combustion

Abstract

The gas phase reactions of carbon monoxide with small mass-selected clusters of palladium, Pd_x⁺ (x = 2–7), and their oxides, Pd_xO⁺ (x = 2–7) and Pd_xO₂⁺ (x = 4–6), have been investigated in a radio frequency ion trap operated under multi-collision conditions. The bare palladium clusters were found to readily adsorb CO yielding a highly size dependent product pattern. Most interestingly, the reactions of the pre-oxidized palladium clusters with CO lead to very similar product distributions of Pd_x(CO)_z⁺ complexes as in the case of the corresponding pure Pd_x⁺ clusters. Consequently, it has been concluded that the investigated palladium oxide clusters efficiently oxidize CO under formation of the bare clusters, which further adsorb CO molecules yielding the previously observed Pd_x(CO)_z⁺ product complex distributions. This CO combustion reaction has been observed even at temperatures as low as 100 K. However, for Pd₂O⁺, Pd₆O⁺, Pd₆O₂⁺, and Pd₇O⁺ a competing reaction channel yielding palladium oxide carbonyls Pd_xO(CO)_z⁺ could be detected. The latter adsorption reaction may even hamper the CO combustion under certain reaction conditions and indicates enhanced activation barriers involved in the CO oxidation and/or the CO₂ elimination process on these clusters.