EVIDENCE FOR A SURFACE DUAL HOLE - RADICAL MECHANISM IN THE TIO2 PHOTOCATALYTIC OXIDATION OF 2,4-DICHLOROPHENAXYACETIC ACID

The photocatalytic degradation of 2,4-dichlorophenoxyacetic acid (2,4- D) in UV-illuminated aqueous TiO2 suspension was studied at pH 1-12. A t pH similar to 3, the initial step is chiefly direct hole (h(+)) oxid ation, while below and (especially) above pH 3, it shifts progressivel y to a hydroxyl radical (HO.)-like reaction following rate-limiting h( +) oxidation of surface hydroxyls. The hole pathway gives products exp ected from one-electron oxidation of the carboxyl group-near stoichiom etric yield of (CO2)-C-14 from [carboxy-C-14]-2,4-D and high yields of 2,4-dichlorophenol (DCP), 2,4-dichlorophenol formate (DCPF), and form aldehyde (HCHO)-and is little affected by 0.1 M methanol or tert-butan ol. The weak competition by alcohols is proof of a surface reaction si nce the alcohols scavenge free HO.. The radical pathway results in low yields of (CO2)-C-14, DCP, DCPF, and HCHO, indicating that attack shi fts to the aromatic rings and is strongly inhibited by the alcohols. S olvent kinetic and product D isotope effects at pH 2 and pH 12 are con sistent with the dual mechanism. Shift to the radical mechanism at low pH may result from a lower oxidation potential and/or coordinating ab ility of the R-CO2H, while the shift at high pH is due to enhanced oxi dation potential of the increasingly charged surface, charge repulsion of carboxylate, and/or OH- competition with carboxylate for coordinat ion sites. At pH similar to 3, judging from scavenger effects, the rad ical mechanism predominates for DCP transformation, whereas the hole m echanism predominates for carboxyl-bearing byproducts formed during la te stages of mineralization.