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

The gas phase reactions of carbon monoxide with small mass-selected clusters of palladium, Pd-x(+) (x = 2-7), and their oxides, PdxO+ (x = 2-7) and PdxO2+ (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 PdxO(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.