Comparison of the Thermal and Photochemical Reaction Pathways of Melamine on TiO2

Fourier transform infrared spectroscopy was employed to study the thermal and photochemical reactions of melamine ((H2N)3(C3N3)) on TiO2. Also tested was the adsorption of urea, cyanamide (H2N–C≡N), dicyandiamide ((H2N)2C═N–C≡N), and cyanuric acid ((OH)3(C3N3)) for identifying possible reaction intermediates. It was found that the thermal decomposition of melamine starts with N–H bond scission, possibly forming intermediates such as (H2N)2(C3N3)NH– and (H2N)(C3N3)(NH)2–. Further loss of hydrogen atoms and ring-opening from these intermediates lead to the formation of −NCO (isocyanate) and −N3 (azide) on the surface. The TiO2-mediated photochemical reaction of melamine proceeds via a different mechanism, forming dicyandiamide. These thermal and photochemical reaction pathways of melamine on TiO2 are reported for the first time. They are different from previous studies showing the processes of polymerization, and substitution of NH2 by OH to form cyanuric acid and urea.