Rapid Energy Exchange between In Situ Formed Bromine Vacancies and CO 2 Molecules Enhances CO 2 Photoreduction

Abstract

Photocatalytic reduction of CO 2 into fuels provides a prospective tactic for regulating the global carbon balance utilizing renewable solar energy. However, CO 2 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 CO 2 molecules via the rapid energy exchange between the photoinduced Br vacancies and CO 2 molecules. Combining in situ continuous wave-electron paramagnetic resonance (cw-EPR) and pulsed EPR technologies, we observe that the spin–spin relaxation time (T 2 ) of BiOBr is decreased by 198 ns during the CO 2 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 CO 2 molecules, which promotes the formation of high-energy CO 2 molecules to facilitate the subsequent reduction reaction. In addition, theoretical calculations indicate that the bended CO 2 adsorption configuration on the surface of BiOBr with Br vacancies caused the decrease of the lowest unoccupied molecular orbital of the CO 2 molecule, which makes it easier for CO 2 molecules to acquire electrons and get activated. In situ diffuse reflectance infrared Fourier transform spectroscopy further shows that the activated CO 2 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 CO 2 activation at active sites and deepens the understanding of CO 2 photoreduction reaction.