Thermal CO₂ Adsorption and Activation on Copper Oxide Cluster Anions Cu_nO^- (n = 3-9)

Understanding the active sites of copper (Cu)-based catalysts toward CO₂ is a prerequisite for improving their rational design. The reactivity of copper oxide cluster anions Cu_nO^- (n = 3-9) and bare copper cluster anions Cu_n^- toward CO₂ has been investigated at room temperature by employing mass spectrometry combined with density functional theory (DFT) calculations. Only adsorption products are observed for the reaction of Cu_nO^- with CO₂. There is almost no observation for the reaction between bare Cu_n^- and CO₂ under our reaction conditions, and only a tiny adsorption of CO₂ on Cu₅^- cluster anions was observed. Our experimental results indicate that the doping of an O atom into bare copper cluster anions can boost the CO₂ adsorption capability at room temperature. Theoretical analysis indicates that the introduction of the O atom could change the charge distribution on bare copper cluster anions, making Lewis acid-base pairs Cu-O and Cu^δ+-Cu^δ-, which promotes the CO₂ adsorption with the formation of the O-C (Cu-C) and Cu-O bonds. The reaction rate constants exhibit size dependence yet remain on the same order of magnitude (∼10^-11 cm³·s^-1). More specifically, the reaction rates generally increase with larger cluster size. However, the n = 8 cluster exhibits a markedly lower reaction rate compared to both the n = 7 and n = 9 clusters, which could stem from its thermodynamically and kinetically less favorable reaction pathway. Our work provides molecular-level insights into the CO₂ adsorption and activation on anoxic copper catalyst surface by using mass spectrometry and DFT calculations.