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Pd6O4+: an oxidation resistant yet highly catalytically active nano-oxide cluster.

The palladium oxide cluster Pd(6)O(4)(+) is formed as the sole product upon reaction of a bare palladium cluster Pd(6)(+) with molecular oxygen in an octopole ion trap under multicollision conditions. This oxide cluster is found to be resistant to further oxidation over a large temperature range, and further O(2) molecules merely physisorb on it at cryogenic temperatures. The particular stability of Pd(6)O(4)(+) is confirmed by the observation that the reaction of Pd(7)(+) with O(2) leads to fragmentation resulting in the formation of Pd(6)O(4)(+). However, in an oxygen-rich O(2)/CO mixture, Pd(6)O(4)(+) is identified as the catalytically active species that effectively facilitates the low-temperature oxidation of CO. Gas-phase reaction kinetics measurements in conjunction with first-principles calculations provide detailed molecular level insight into the nano-oxide cluster chemistry and are able to reveal the full catalytic combustion reaction cycle.