Adjusting the Cu valent state in CeO2 nanorods loaded with Cu2O for controlling the electrochemical reduction of CO2 to methane

The challenge of converting carbon dioxide (CO₂) electroreduction into high value-added products is to obtain highly selective electrocatalysts. Research has shown that cuprous oxide (Cu₂O) effectively facilitates the activation of CO₂ and suppress the competing hydrogen evolution reaction. However, during the process of electrochemical reduction, the valence state of Cu₂O is prone to transformation. Herein, a series of Cu₂O–CeO₂-X catalysts (X represents different amounts of CeO₂ added) by applying Cu₂O onto Cerium oxide (CeO₂) nanorods were synthesized via the impregnation method and used as electrocatalysts. We found that the Faradaic efficiency of methane (FE-CH₄) for Cu₂O–CeO₂-2 reached 65.1 % at −1.6 V vs. RHE, exceeding many catalysts previously reported. From the Cu LMM Auger spectra, it was found that the valence state of Cu species in Cu₂O–CeO₂-2 is mainly Cu⁰ (accounting for 93.39 %), while in Cu₂O–CeO₂-3, the proportion of Cu⁰ is 56.34 %. This indicates that CeO₂ has a regulatory effect on the valence state of Cu species. Additionally, we observed the valence state of Cu in Cu₂O–CeO₂-2 after the CO₂ reduction reaction and found that the proportion of Cu⁰ changed little (94.97 %), which proves that CeO₂ plays a role in stabilizing the valence state of Cu. Research has demonstrated that Cu⁰ can adsorb the ∗CO intermediate on its surface, followed by a hydrogenation reaction to generate CH₄ (∗CO + H⁺ + e⁻ → ∗CHO). Copper-based catalysts in which Cu⁰ dominates among the Cu species are conducive to the conversion of CO₂ into CH₄. In situ ATR-SEIRAS spectra confirmed the presence of key intermediates such as ∗CHO and explored the formation pathway of CH₄ (CO₂ → ∗COOH → ∗CO → ∗CHO → ∗OCH₂ → CH₄). This study provides a new way to design efficient copper oxide-based catalysts for CO₂ electroreduction by adjusting and stabilizing the ratio of copper valence state reasonably.