A proof of the direct hole transfer in photocatalysis: The case ofmelamine

The photoinduced transformation of 2,4,6-triamino-1,3,5-triazine (melamine) was studied by using dif-ferent advanced oxidation technologies under a variety of experimental conditions. The systems involvinghomogeneous hydroxyl radicals, as generated by H2O2/hv, Fenton reagent, and sonocatalysis are inef-fective. However, melamine is degraded under photocatalytic conditions or by SO4•−(S2O82−/hv). Thetime evolution of long-living intermediates, such as 2,4-diamino-6-hydroxy-1,3,5-triazine (ammeline)and 2-amino-4,6-dihydroxy-1,3,5-triazine (ammelide), has been followed, being 2,4,6-trihydroxy-1,3,5-triazine (cyanuric acid) the final stable product. During both photocatalytic and S2O82−/hv experiments,in the early steps, a fairly stable intermediate evolving to ammelide is observed in a large extent. Thisintermediate was identified as 2,4-diamino-6-nitro-1,3,5-triazine. This indicates that the primary pho-tocatalytic event is the oxidation of the amino-group to nitro-group through several consecutive fastoxidation steps, and that a hydrolytic step leads to the release of nitrite in solution. To elucidate thenature of the oxidant species hole scavengers such as methanol and bromide ions were added to theirradiated TiO2. They completely stop the degradation, whereas chloride and fluoride ions decrease thedegradation rate. The study of the photocatalytic degradation rate of melamine at increasing concentrations using twodifferent commercial titanium dioxides, such as P25 and Merck TiO2, showed an intriguing behavior. Adrastic abatement of the melamine transformation rate was observed when coagulation of the P25 slurryoccurs due both to the pH change and melamine concentration effect that increase melamine adsorp-tion. In the presence of TiO2(Merck) the melamine initial degradation rates are significantly lower thanthose observed in the presence of P25 but are not depressed at larger concentrations. The experimentalevidences (e.g., absence of melamine adsorption onto TiO2surface at low concentrations or at acidic pHor due to the catalyst surface texture, and the lack of reactivity toward•OH free and bound) suggest thatthe effective photocatalytic mechanism is based on an outer sphere direct hole transfer to the melamine.Its formal potential lies in the range 1.9–2.3 V vs NHE. Then, the photodegradation of melamine is anefficient tool to evaluate the direct hole transfer ability of a photocatalyst.