Photocatalytic reactions of Cr(VI) reduction

M.I. LITTER

Conferencia; Conferencia; 2012

Our research group reported several studies on Cr(VI) RHP using TiO2, ZrO2, and Fe-TiO2 and Pt-TiO2 samples and gained insight about the processes taking place in the system. The standard reduction potentials of Cr(VI)/Cr(V), Cr(V)/Cr(IV) and Cr(IV)/Cr(III) couples are positive enough to be reduced by TiO2 conduction band electrons. Therefore, working with Cr(VI) in pure water TiO2 suspensions or in the presence of donor agents such as EDTA, oxalic acid, and citric acid, our team proposed for the first time that Cr(VI) RHP over TiO2 takes place by successive one electron transfer reducing steps ending in Cr(III), the stable final product. These assumptions were validated by EPR experiments in which Cr(V) species were detected. Our research group also reached to very important conclusions concerning the role of dissolved O2 in the Cr(VI) RHP, which was the object of controversy for many years. However, research of our team and some other investigations showed no particular effect of O2, confirmed by studies with platinized TiO2 and by spectroscopic evidences of the formation of a charge-transfer complex between Cr(VI) and TiO2, showing the existence of a strong interaction of the metal ion with the semiconductor surface. The behavior of Cr(VI) contrasts strongly with that of other metal ions such as Hg(II), whose reduction is greatly inhibited by O2. In this sense, Cr(VI) RHP is a unique system and the fact that its photocatalytic reduction can be made in air represents an important technological advantage. RHP of Cr(VI) has been reported to take place also under visible irradiation, in the presence of dyes such as alizarin red or hydroxoaluminiumtricarboxymonoamide phthalocyanine chelated to TiO2. The mechanism is different, involving excitation of the dye to a singlet state from which it can inject electrons to the TiO2 conduction band, leaving behind a radical cation that replaces holes or HO? as usual oxidant entities. However, the reducing power of the system remains intact, making possible Cr(VI) reduction. Cr(V) formation was confirmed by EPR measurements. The inhibition of formation of ROS in a photocatalytic reductive pathway by the fast trapping of electrons by Cr(VI) and the easier oxidability of 4-CP compared to AlTCPc protects the dye and avoids its photobleaching, making feasible Cr(VI) reduction by the use of solar radiation.