Unveiling the Nanoconfinement Effect in CO₂ Electroreduction to CH₄ over Mesoporous Cu-CeO₂ Nanospheres

Nanoconfinement provides a promising strategy to promote the electrochemical CO₂ reduction reaction (CO₂RR) owing to enhanced reactant enrichment and collision. However, the nanoconfinement influence on the CH₄ selectivity from the CO₂RR with related regulation mechanism is unclear. Herein, a series of mesoporous CeO₂ loaded Cu catalysts with controllable pore size (1.3-5.5 nm) are designed to modulate the CO₂RR selectivity to CH₄. It is found that decreasing the pore size can apparently enhance the CO₂RR performance while inhibiting the HER activity. Moreover, a volcano-type relationship between the CH₄ selectivity and the pore diameter is observed among these catalysts, while Cu-mCeO₂-3.0 (pore diameter of 3.0 nm) shows the highest CH₄ Faradaic efficiency (66.1 ± 2.9%). The in situ experiments and DFT calculations illustrate that a smaller pore size with stronger confinement over Cu-mCeO₂-x can promote the adsorption and transformation of reactants (*CO, *CHO, etc.) for CH₄ production, but too narrow confined space (1.3 nm) will contribute to much higher intermediate coverage and promote their collision for C-C coupling to C₂₊ products instead, thus reducing the CH₄ selectivity. This work provides designing insights into metal/oxide catalysts with controllable pore size to study the nanoconfinement effect on the CO₂RR-to-CH₄ activity, which can be extended to other oxide-based catalytic reactions.