First-principles insights into CO₂ reduction and competing HER on transition metal-loaded Zr₁₂O₁₂ nanocages

The catalytic reduction of CO₂ into value-added chemicals using renewable energy is a promising strategy for effective CO₂ management, but it often competes with the hydrogen evolution reaction (HER). Herein, we present a systematic first-principles investigation of Zr₁₂O₁₂ and M@Zr₁₂O₁₂ ( M = Fe, Co, Ni, Cu, Zn) nanocages to elucidate their potential as photocatalysts for the CO₂ reduction reaction (CO₂RR). We reveal that late 3 d transition metals with partially filled d orbitals (Fe, Co, and Ni) are stably anchored on the Zr₁₂O₁₂ nanocages, whereas Cu and Zn atoms are not stabilized due to their fully filled d orbitals. The pristine zinc oxide nanocage exhibits a band gap of 2.07 eV and can convert CO₂ to CO under a low potential of 0.71 eV. Loading with Fe, Co, or Ni further narrows the band gap and enhances CO₂ activation. However, CO desorption from Fe@Zr₁₂O₁₂, Co@Zr₁₂O₁₂, and Ni@Zr₁₂O₁₂ is hindered by large energy barriers. Interestingly, Ni@Zr₁₂O₁₂ facilitates spontaneous CO₂ reduction to OCCO, a key intermediate for C₂ products, though its strong H adsorption may block active sites. In contrast, the non-doped cage, and the corresponding Fe@Zr₁₂O₁₂, and Co@Zr₁₂O₁₂ can form OCCO under low external energies, with Fe- and Co-loaded nanocages showing higher CO₂ selectivity over HER. These findings provide atomistic level insights for engineering single-atom Zr-oxide photocatalysts for selective CO₂ reduction to C₂ products.