Visualizing alkali metal aggregation-induced coordination in CO₂ activation on copper

Alkali metals are widely recognized as promotors in CO₂ activation and conversion. However, how the alkali metals activate CO₂ molecules and stabilize the reaction intermediates remains controversial due to the lack of atomic-scale characterization. Here, using scanning tunneling microscopy and non-contact atomic force microscopy, we directly visualize the coordination structure of alkali metal cations (K⁺ and Cs⁺) and CO₂ reaction intermediates on copper surfaces. At the initial step, we find the aggregation of alkali ions into trimers to facilitate the activation of CO₂. Subsequently, the activated CO₂^δ- undergoes C-C coupling to form oxalate on Cu(100), that is coordinated with four alkali ions. Density functional theory calculations reveal the cooperative role of alkali trimers in stabilizing key intermediates, overcoming Coulomb repulsion, and significantly lowering the reaction barrier for CO₂ conversion. Higher CO₂ pressure promotes the production of two-dimensional ordered alkali carbonate films. Our findings provide valuable insights for designing efficient catalysts for carbon capture and utilization.