In Situ/Operando Insights into the Selectivity of CH₄/C₂H₄ in CO₂ Electroreduction by Fine‐Tuning the Composition of Cu/SiO₂ Catalysts

Copper-silica-based catalysts have drawn much attention for the remarkable product selectivity in the electrochemical CO₂ reduction reaction, particularly toward CH₄ and C₂H₄. However, systematic studies exploring the underlying reasons for the selectivity differences are lacking. Herein, Cu/SiO₂ catalysts with different Cu/Si ratio are controllably synthesized by fine-tuning the precursor ratio, enabling selective CO₂ electroreduction to CH₄/C₂H₄. Specifically, at a current density of 200 mA cm^-2, Cu/SiO₂-10 including majority of CuSiO₃ facilitates the electroreduction of CO₂ to CH₄ with a Faradaic selectivity ratio of CH₄/C₂H₄ (9.3/1), whereas Cu/SiO₂-50 primarily consisting of CuO exhibits a Faradaic selectivity ratio of C₂H₄/CH₄ (16.7/1). X-ray photoelectron spectroscopy and in situ Raman spectroscopic characterization reveal that CuSiO₃ in the catalysts remains stable and no valve state change occurs during the reaction, while CuO in the catalysts is reduced in situ to Cu⁰/Cu⁺ during the reaction. In situ infrared spectroscopic and temperature-programmed desorption of CO characterization further reveal that CuSiO₃ has a stronger protonation capacity and promotes the direct protonation of adsorbed *CO species to CH₄, while Cu⁰/Cu⁺ is more conducive to the CC coupling between the intermediate species *CHO with adsorbed *CO species to form C₂H₄ due to the stronger CO adsorption capacity and higher coverage.