Microscopic Insights into the Catalytic Activity–Stability Trade-Off on Copper Nanoclusters for CO2 Hydrogenation to HCOOH

Lowly coordinated copper clusters are the most cost-effective benchmark catalysts for CO₂ hydrogenation, but there is a meticulous balance between catalytic activity and stability. Herein, density functional theory (DFT) calculations are implemented to examine the catalytic performance of Cu_n nanoclusters (n = 4, 8, 16, 32) in CO₂-to-HCOOH conversion. Facile activation of H₂ is observed with significant electron transfer from Cu_n to antibonding orbitals of H₂; conversely, the C-O bond of CO₂ is poorly activated due to a low degree of orbital overlap. During the reaction, structural fluxionality occurs on Cu₄ and Cu₈ because of the low stability; however, negligible deformation is observed on Cu₁₆ and Cu₃₂. In addition, Cu₁₆ achieves a good balance between the kinetics of each elementary reaction, which is, however, difficult to be maintained on Cu₄, Cu₈, and Cu₃₂. Therefore, Cu₁₆ satisfies the trade-off between activity and stability in CO₂-to-HCOOH conversion. Energy decomposition analysis clarifies that the activation barrier of the second hydrogenation originates from the energy of hydride desorption, the electronic repulsion energy due to hydroxyl group formation, as well as the energy for local Cu-O bond cleavage. The high energy demand on the second hydrogenation is mainly sourced from the last term. From the bottom up, this work provides microscopic insights into the catalytic activity-stability trade-off in CO₂ hydrogenation to HCOOH and would facilitate the rational design of advanced catalysts for the high-value utilization of CO₂ exhaust gas.