Size-Dependent Reactivity of Co_n^- (n = 5-25) Cluster Anions toward Carbon Dioxide

A fundamental understanding of the reactivity evolution of nanosized clusters at an atomically precise level is pivotal to assemble desired materials with promising candidates. Benefiting from the tandem mass spectrometer coupled with a high-temperature ion-trap reactor, the reactions of mass-selected Co_n^- (n = 5-25) clusters with CO₂ were investigated and the increased reactivity of Co_20-25^- was newly discovered herein. This finding marks an important step to understand property evolution of subnanometer metal clusters (Co₂₅^-, ∼0.8 nm) atom-by-atom. The reasons behind the increased reactivity of Co_20-25^- were proposed by analyzing the reactions of smaller Co_6-8^- clusters that exhibit significantly different reactivity toward CO₂, in which a lower electron affinity of Co_n contributes to the capture of CO₂ while the flexibility of Co_n^- could play vital roles to stabilize reaction intermediates and suppress the barriers of O-CO rupture and CO desorption.