Mechanisms of Direct and TiO2‐Photocatalysed UV Degradation of Phenylurea Herbicides

Abstract

Phenylurea herbicides undergo low-yield (ϕ_PI .⁺) are highly acidic (−1.5K_a.). Pulse radiolysis experiments allowed limitation of the reduction potential of phenylurea^.+ within 2.22 V versus the normal hydrogen electrode (NHE).⁺/phenylurea)λ=193 nm) photodegradation of phenylureas correspond to a photo-Fries rearrangement. One-electron reduction with e yields the corresponding radical anions (phenylurea^.−), for which 4.3K_a show that in photocatalysis the generation of phenylurea^.− and O₂^.− on the surface of the photocatalyst may be competitive. High reactivity toward e is predicted from linear free-energy relationships (LFER) for phenylureas bearing electron-withdrawing groups. Reaction with HO^. takes place mainly via addition to the aromatic ring and/or H^. abstraction from a saturated carbon atom (98 %), rather than one-electron oxidation (2 %). High reactivity toward oxidation by HO^. is predicted from LFER for phenylureas bearing electron-donating groups. Adsorption studies for TiO₂ in its polymorphic forms of rutile and anatase, as well as with the commercial mixture Degussa P-25, show photocatalysis is independent of the specific area of the catalyst. A variety of compounds are generated during the photocatalytic degradation of Diuron, while only two hydroxychloro derivatives are observed upon prolonged direct 365 nm irradiation. The photocatalytic degradation proceeds mainly by oxidation of the Me group of the side chain, hydroxylation of the aromatic ring, and dechlorination. The photoproducts of photocatalytic degradation differ from one polymorphic form of TiO₂ to another.