Photocatalytic transformation of 4-chloro-2-methylphenoxyacetic acid (MCPA) on several kinds of TiO2

The photocatalytic transformation of MCPA on TiO2 leads to 4-chloro-2-methylphenol (P3) as the major intermediate. Some minor intermediates were also identified, namely methylhydroquinone (P1), 5-chloro-2-hydroxy-3-methylphenylacetic acid (P2), 4-chloro-2-methylanisole (P5) and 4-chloro-2-methylphenylformate (P4). The rate of transformation is higher with Degussa P25 than with Millennium PC50, PC100, PC105 and PC500 used as slurries (1 g l−1). There is no apparent relationship between the rate of transformation and the surface area of the catalyst. The immobilization of TiO2 on glass fibres significantly reduces the reaction rate, but it has the important advantage of eliminating the problem of filtration. Except with PC100 and PC500 kinetics obey a first-order law. The photocatalytic transformation is probably due to two different processes involving hydroxyl radicals and positive holes.