Highly selective visible-light-triggered CO₂ fixation to cyclic carbonates under mild conditions using TiO₂/multiwall carbon nanotubes (MWCNT) grafted with Pt or Pd nanoparticles

Irradiation of a series of aromatic epoxides and CO₂ in the presence of TiO₂, TiO₂/MWCNT, Pt//TiO₂/MWCNT and Pd/TiO₂/MWCNT photocatalysts was investigated with visible light-induced cycloaddition reactions for the production of cyclic carbonates with high activity and selectivity. The catalytic activity order of these catalysts was revealed as follows: Pd/TiO₂/MWCNT > Pt//TiO₂/MWCNT > TiO₂/MWCNT > TiO₂. The main features of this protocol are the transformation of CO₂ to cyclic carbonates at ambient temperature and under atmospheric pressure. The band gap energies for Pd/TiO₂/MWCNT and the Pt/TiO₂/MWCNT nanocomposites were estimated from the Kubelka-Munk plots to be ca. ∼2.39 and 2.33 eV, respectively, and they proved to be active catalysts in comparison with bare TiO₂ or TiO₂/MWCNT, which possess band gap values ca. ∼3.02 and 2.46 eV, respectively. A plausible mechanism explaining the behavior of CO₂ with the Pd/TiO₂/MWCNT nanocomposite was proposed. (authors)