Selective Photocatalytic Reduction of CO₂ to CH₄ Modulated by Chloride Modification on Bi₂WO₆ Nanosheets

Solar-driven photocatalytic CO₂ reduction into CH₄ with H₂O is considered to be a promising way to alleviate the energy crisis and greenhouse effect. However, current CO₂ photoreduction technologies tend to overlook the role of photooxidation half reaction as well as the effect of the protons produced by water oxidation on CH₄ generation, resulting in low CO₂ conversion efficiency and poor CH₄ selectivity. In the present study, a series of chloride-modified Bi₂WO₆ nanosheets were constructed in view of chloride-assisted photocatalytic water oxidation. The results show that the CH₄ yield of the synthesized sample can be enhanced up to about 10 times compared to that with no Cl^- modification. Besides, the selectivity of CH₄ can be regulated by the loading amount of chloride, varying from 51.29% for Bi₂WO₆ to 94.98% for the maximum. The increase of product yield is attributed to chloride modification, which not only changed the morphology of the catalyst, but also modified the pathway of water oxidation. Further studies on intermediate products and the density functional theory calculation confirm that the Cl^- ions on Bi₂WO₆ nanosheets not only promote H₂O oxidation, but also lower the energy barrier for intermediate *CHO generation, thus facilitating CH₄ production. The results gained herein may provide some illuminating insights into the design of a highly selective photocatalyst for efficient CO₂ reduction.