The application of iron oxide based photocatalysts in chromium photoredox chemistry

Abstract

Chromium(VI) is a reactive and highly toxic pollutant species which is present in certain industrial effluent streams as well as sea water and polluted ground water. Chromium(III), however, is relatively inert and less toxic. Conventional methods used to eliminate Cr(VI) from aqueous phase include its reduction to Cr(III) at acidic pH by reaction with strong reducing agents such as thiosulphate, FeSO4 and SO2, followed by precipitation as hydroxide in alkaline media. However, this procedure is not suitable for Cr(VI) elimination in dilute aqueous solutions. Thus, semiconductor photocatalysis has been studied as a possible alternative.
Photocatalysis is the process of using light to promote catalysis of reactions and normally involves the photoexcitation of a semiconductor catalyst. During photocatalysis, light of a wavelength corresponding to an energy greater than that of the band gap, Eg, of the material is incident on the catalyst, resulting in electrons being excited from the semiconductor valence band to its conduction band. These electrons can then reduce the Cr(VI) to Cr(III).
The semi-conductor used consists of nanoparticulate iron oxide embedded in a clay matrix. Clays are micro-crystalline layered minerals. This gives rise to interlayer spaces in which the iron oxide is ‘grown’. The clay structure restricts the growth of the iron oxide, resulting in nano-particulate sized semiconductor particles.
To monitor the changes in Cr(VI) concentration, a new, stable potentiometric method has been developed that involves the successful use of a gold electrode to measure Cr(VI) concentration, that has to date, not been reported elsewhere in literature. Results show that the nanocomposite does photo-reduce Cr(VI) to Cr(III) and does photo-oxidise ethanol. Modelling of the time dependence of the measured (photo-induced) potential has allowed for the extraction of key rate parameters for the Cr(VI) reduction process with a view to system optimisation.