S-Scheme Bi-oxide/Ti-oxide Molecular Hybrid for Photocatalytic Cycloaddition of Carbon Dioxide to Epoxides

The Lewis acidic sites and reducing power of a photocatalyst are critical for its performance in CO₂activation for cycloadditions. In this study, we designed and synthesized a Ti₁₈Bi₄O₂₉Bz₂₆(Bz = benzoate) cluster molecule that contains Lewis acidic sites on the surface and combines Ti₁₈O₂₂and Bi₄O₇cluster counterparts. DFT calculations combined with synchronous illumination X-ray photoelectron spectroscopy reveal that the Ti₁₈O₂₂and Bi₄O₇components form an S-scheme heterojunction, significantly increasing the reducing power of photogenerated electrons and spatial separation of photogenerated charges. While Ti₁₈Bi₄O₂₉Bz₂₆has some catalytic activity in the cycloaddition reaction between CO₂and epoxides at room temperature, light irradiation significantly increases both the conversion rate and the selectivity of the cyclocarbonate product. Mechanistic studies show that both electrons and holes contribute to the improved performance when exposed to light, and that the increased reducing power overcomes the cycloaddition reaction's limiting step CO₂reductive activation. This is not only the report on photocatalytic cycloaddition of CO₂using a Lewis acidic titanium-oxide cluster but also the example of the molecular S-scheme heterojunction to the best of our knowledge.