Rapid Energy Exchange between In Situ Formed Bromine Vacancies and CO₂ Molecules Enhances CO₂ Photoreduction

Photocatalytic reduction of CO₂ into fuels provides a prospective tactic for regulating the global carbon balance utilizing renewable solar energy. However, CO₂ molecules are difficult to activate and reduce due to the thermodynamic stability and chemical inertness. In this work, we develop a novel strategy to promote the adsorption and activation of CO₂ molecules via the rapid energy exchange between the photoinduced Br vacancies and CO₂ molecules. Combining in situ continuous wave-electron paramagnetic resonance (cw-EPR) and pulsed EPR technologies, we observe that the spin-spin relaxation time (T₂) of BiOBr is decreased by 198 ns during the CO₂ photoreduction reaction, which is further confirmed by the broadened EPR linewidth. This result reveals that there is an energy exchange interaction between in situ formed Br vacancies and CO₂ molecules, which promotes the formation of high-energy CO₂ molecules to facilitate the subsequent reduction reaction. In addition, theoretical calculations indicate that the bended CO₂ adsorption configuration on the surface of BiOBr with Br vacancies caused the decrease of the lowest unoccupied molecular orbital of the CO₂ molecule, which makes it easier for CO₂ molecules to acquire electrons and get activated. In situ diffuse reflectance infrared Fourier transform spectroscopy further shows that the activated CO₂ molecules are favorably converted to key intermediates of COOH*, resulting in a CO generation rate of 9.1 μmol g^-1 h^-1 and a selectivity of 100%. This study elucidates the underlying mechanism of CO₂ activation at active sites and deepens the understanding of CO₂ photoreduction reaction.