Hexavalent chromium, Cr(VI), a carcinogenic pollutant made famous by the movie Erin Brockovich, can be reduced to nontoxic trivalent chromium using photocatalysts such as TiO₂. Nanoparticles of TiO₂, irradiated by sunlight or an artificial UV source, undergo bandgap excitation, forming electron-hole pairs. The excited electrons are available for redox reactions with species near the particle surfaces.

Marta I. Litter, Universidad Nacional de General San Martín, Argentina, and colleagues studied the dyamics and kinetics of this electron transfer process. They used TiO₂ nanoparticles in formic acid, which acted as an electron donor/hole scavenger.

Using a stopped-flow technique, they determined the rate of formation of a TiO₂-Cr(VI) surface complex, which they confirmed to be a dichromate species. However, kinetic analysis indicates that the formation of this complex is not a requirement for electron transfer from TiO₂ to Cr(VI). The team also observed that Cr(VI) reacts so rapidly with available electrons that the presence of O₂ does not affect the photocatalytic reaction.

• Application of the stopped flow technique to the TiO₂-heterogeneous photocatalysis of hexavalent chromium in aqueous suspensions. Comparison with O₂ and H₂O₂ as electron acceptors,
  Jorge Martín Meichtry, Ralf Dillert, Detlef Bahnemann, Marta Irene Litter,
  Langmuir 2015.
  DOI: 10.1021/acs.langmuir.5b00574