Atomic Eu Substitution in Cu 2 O Tailors C 1 and C 2+ Product Selectivity by Frustrated Deep Hydrogenation in Electrochemical CO 2 Reduction

The electrochemical CO₂ reduction reaction (eCO₂RR) is an important method to obtain high-value chemicals; however, selectively tailoring C₁ and C₂₊ products remains a challenge. Herein, we propose a novel and effective rare-earth Eu substitution strategy to tailor the selectivity of C₁ and C₂₊ products on Cu₂O sites by frustrated deep hydrogenation in the eCO₂RR. The incorporation of atomic Eu into Cu₂O can shift the dominant product from C₂₊ at low Eu content to CH₄ at high Eu content. For low Eu-doped Cu₂O (LD-Eu/Cu₂O), the total Faradaic efficiency (FE) of C₂₊ products reaches 79.39% with ethylene (C₂H₄) as the predominant product (FE: 49.27%) in the H-type cell; whereas high Eu-doped Cu₂O (HD-Eu/Cu₂O) promotes the formation of C₁ products, achieving a total FE of 50.25% with methane (CH₄) as the main product (FE: 47.21%). Eu substitution for tailoring the selectivity of C₁ and C₂₊ products on the Cu₂O site is also verified in the flow cell. Electrochemical in situ characterization and theoretical calculations suggest that low levels of Eu incorporation in Cu₂O weakens the π* antibonding interaction over the C═O bond, facilitating C-C coupling to lead the C₂₊ pathway via the frustrated deep hydrogenation of *CHO; whereas high Eu incorporation in Cu₂O strengthens the π* antibonding interaction, facilitating the deep hydrogenation of *CHO to CH₄ via the C₁ pathway. This work provides a new perspective on tailoring product selectivity by rare-earth-induced frustrated deep hydrogenation during the eCO₂RR.