Visualizing the dynamic evolution of light-sensitive Cu1+/Cu2+ sites during photocatalytic CO2 reduction with an advanced in situ EPR spectroscopy

Elucidation of the dynamic evolution of active sites is still a challenge in investigating the catalytic mechanism mainly due to the difficulty in accurately detecting the transient structural changes of active sites under operating conditions. Here, we develop an advanced in situ electron paramagnetic resonance (EPR) spectroscopy, which could sensitively monitor and visualize the dynamic evolution of paramagnetic active sites during photoreduction CO₂. In situ results reveal that the photoactivated Cu¹⁺ sites from CuO nanoclusters/TiO₂ serve as the authentic active sites in the reaction and exhibit self-regenerative capability. The CO₂ molecules can acquire electrons and get activated by the photoactivated Cu¹⁺, leading to the transition of Cu¹⁺ sites into Cu²⁺ sites. Subsequently, the Cu²⁺ sites expedite the generation of hydrogen protons through antiferromagnetic coupling with hydroxyl radicals, thereby promoting the production of the final product CH₄ via a multi proton-coupled electron transfer (PCET) process. This work reveals and visualizes the dynamic evolution of Cu-based active sites during photocatalytic reactions by combined in situ characterizations, providing new perspectives on the mechanistic understanding of paramagnetic active sites under operation.