Copper-based catalysts enable C-C coupling to produce multicarbon products via carbon dioxide reduction reaction (CO2RR). Recently, C2 products have been successfully catalyzed; however, the selectivity and activity of higher-carbon products are still deficient. In this work, unveiled by density functional theory, we find that the diversification of the protonation sites of the various C2 and C1 intermediates during CO2RR limited the C3 catalytic cycles. The oxygen-deficient Cu2O(110) surface is found to promote the coupling of a specific C2 intermediate (H2CCO) and CO at Cu1+ and Cu0 mixed-valence regions with a proper activation energy of H2CCO··CO coupling around 1.00 eV and a thermodynamically favorable transition. The Bader charge analysis of H2CCO reveals that the C2 end of H2CCO possesses a high positive charge, suggesting the C2-C1 coupling as a nucleophilic addition reaction process in the mixed-valence boundary region. This material-related insight offers an implication for designing CO2RR electrocatalysts, producing high-carbon products.