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 CO2 activation 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 Ti18O22 and Bi4O7 cluster counterparts. DFT calculations combined with synchronous illumination X-ray photoelectron spectroscopy reveal that the Ti18O22 and Bi4O7 components form an S-scheme heterojunction, significantly increasing the reducing power of photogenerated electrons and spatial separation of photogenerated charges. While Ti18Bi4O29Bz26 has some catalytic activity in the cycloaddition reaction between CO2 and 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─CO2 reductive activation. This is not only the report on photocatalytic cycloaddition of CO2 using a Lewis acidic titanium-oxide cluster but also the example of the molecular S-scheme heterojunction to the best of our knowledge.