Strongly cluster size dependent reaction behavior of CO with $O2$ on free silver cluster anions

Reactions of free silver anions $Agn−$ $(n=1–13)$ with $O2,$ CO, and their mixtures are investigated in a temperature controlled radio frequency ion trap setup. Cluster anions $Agn−$ $(n=1–11)$ readily react with molecular oxygen to yield $AgnOm−$ $(m=2,$ 4, or 6) oxide products. In contrast, no reaction of the silver cluster anions with carbon monoxide is detected. However, if silver cluster anions are exposed to the mixture of $O2$ and CO, new reaction products and a pronounced, discontinuous size dependence in the reaction behavior is observed. In particular, coadsorption complexes $Agn(CO)O2−$ are detected for cluster sizes with $n=4$ and 6 and, the most striking observation, in the case of the larger odd atom number clusters $Ag7−,$ $Ag9−,$ and $Ag11−,$ the oxide product concentration decreases while a reappearance of the bare metal cluster signal is observed. This leads to the conclusion that carbon monoxide reacts with the activated oxygen on these silver clusters and indicates the prevalence of a catalytic reaction cycle.