Photocatalytic decomposition of L-phenylalanine over TiO2: Identification of intermediates and the mechanism of photodegradation

The purpose of this study was to get a better understanding on the key reaction steps of photocatalytic decomposition of L-phenylalanine. Therefore the reactions of L-phenylalanine occurring over UV excited TiO2 particles were investigated under anaerobic and aerobic conditions. Silver ions were used as electron traps and H2O2 molecules were applied as species reacting with conduction band electrons and valence band holes in argon-saturated aqueous suspensions. The photocatalytic performances were followed by measuring the concentration of L-phenylalanine, the formation of NH4+ and the total organic carbon (TOC) content of the reaction mixtures. In addition, UV-vis absorption and luminescence spectra and the pH of the aqueous phase of suspensions were also monitored. The initial rate of the NH4+ formation was found to be the same in the argon saturated (v(0,) (Ar) = (9.1 +/- 0.4) x 10(-7) M min(-1)) and in the air saturated (v(0,) (air) = (9.3 +/- 0.4) x 10(-7) M min(-1)) reaction mixtures, while significantly different values were obtained for the rate of disappearance of the phenylalanine (v(0,) (Ar) = 18.5 x 10(-7) M min(-1) and v(0,) (air) = 65 x 10(-7) M min(-1)) and for the rate of TOC decrease (v(0.Ar) = 17.5 x 10(-7) M min(-1) and v(0,) (air) = 142 x 10(-7) M min(-1)) in anaerobic and aerobic condition, respectively. Among the key intermediates of the photocatalytic decomposition the L-4-hydroxyphenylalanine (L-tyrosine) and the L-3,4-dihydroxyphenylalanine (L-DOPA) were identified by HPLC-MS method. Moreover, other hydroxyphenylalanines (3-hydroxyphenylalanine and 2-hydroxyphenylalanine) and dihydroxyphenylalanine isomers were also assigned by this technique. Ring opening reaction has been observed in conditions where oxygen containing reactive species, such as O2(-), HO2 and singlet oxygen form and can attack the dihydroxyphenylalanines. Mechanisms for the photocatalytic degradation of phenylalanine in anaerobic and aerobic conditions have been proposed. (C) 2010 Elsevier B.V. All rights reserved.