The electrochemical reduction reaction of CO₂ (CO₂RR) facilitates the sustainable synthesis of fuels and chemicals. Grain-boundary-rich nanoparticles show superior CO₂RR performance, however, the nature of active sites remains elusive. Here, we utilized in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy to identify the active sites and achieve structure–activity correlations, by taking advantage of synthesizing colloidal nanoparticles with well-defined shapes. It indicates that grain boundaries selectively convert CO₂ to CO during the CO₂RR, and the terrace sites on Cu(100) facets are the active sites for the conversion of CO to C₂₊ products. Density functional theory calculations show that the optimal adsorption energies of CO₂ and CO on grain boundaries facilitate the conversion of CO₂ into CO, while the terrace sites on Cu(100) facets selectively convert CO to C₂₊ products by lowering the activation energy of C–C coupling process. This work provides experimental evidences for the rational design of highly selective catalysts to produce C₂₊ products via the CO₂RR.