Consecutive Reduction of Five Carbon Dioxide Molecules by Gas-Phase Niobium Carbide Cluster Anions Nb₃C₄^-: Unusual Mechanism for Enhanced Reactivity by the Carbon Ligands

Studying the cleavage of the C═O bond during CO₂ activation at room temperature is highly significant for comprehending the CO₂ conversion processes. Herein, mass spectrometry experiments and density functional theory calculations indicate that the niobium carbide anions Nb₃C₄^- can continuously convert five CO₂ molecules to CO under thermal collision conditions, while the other clusters with less carbon ligands Nb₃C_1-3^- reduce fewer CO₂ molecules. Size-dependent reactivity of Nb₃C_1-4^- cluster anions toward CO₂ is observed. Interestingly, the carbon atoms in Nb₃C₄^- not only act as highly active adsorption sites for CO₂ but also serve as electron donors to reduce CO₂. The stored electrons are released through a carbon-carbon coupling process. Our findings on the role of carbon ligands in enhancing transition metal carbide reactivity can offer new insights for designing active sites on catalysts with both high activity and selectivity.