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.