Theoretical study of the reduction of uranium(VI) Aquo complexes on titania particles and by alcohols

To provide insights into the adsorption and photoreduction of uranium(VI) on TiO ₂, we have studied the structural and electronic properties of uranium(VI) aquo complexes adsorbed on stoichiometric and defected TiO ₂ surfaces and nanoparticles. Plane wave calculations with the pure PBE density functional and the PBE+U approach were used to study U ^VI complexes on a periodic rutile (110) slab. In addition, a nanoparticulate Ti ₃₈O ₇₆ cluster was used to simulate anatase nanoparticles. The electronic structures of the adsorbed U ^VI complexes indicate that the photoreduction process is a consequence of the photocatalytic properties of TiO ₂. The reduction of the adsorbed complexes can only occur if the energy of the incident photon exceeds the semiconductor band gap. The gap states induced by single or neighboring hydrogen atoms and oxygen vacancies at the rutile (110) surface cannot reduce adsorbed U ^VI complexes as the unoccupied 5-orbitals are found deeper in the conduction band. In the absence of a solid substrate, photoreduction proceeds by abstraction of a hydrogen atom from water or organic molecules present in solution. Photoreduction by chlorophenol results in lower product yield than reduction by aliphatic alcohols. This is because the triplet uranyl-chlorophenol complex is much more stable than similar complexes formed with methanol and ethanol. In the case of water, the hydroxyl photoproduct easily re-oxidizes the pentavalent species formed. In addition, it is easier for the triplet uranyl-water complex to decompose to the photoreactants.