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

Abstract

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

Gaseous Photocatalytic Oxidation of Formic Acid over TiO2: A Comparison between the Charge Carrier Transfer and Light-Assisted Mars–van Krevelen Pathways

“…For example, Ngo et al attributed the photocatalysis of acetic acid over TiO 2 to hole mediation . Some studies were compared between thermocatalysis and photocatalysis of methanol, acetaldehyde, acetic acid, and formaldehyde and also did not evidence the light-assisted extraction of lattice oxygens in photocatalysis due to their different reaction pathways. − It has been also reported that the residual O 2 molecules contribute to high-vacuum CO photocatalysis under electron transfer mediation, rather than the light-assisted lattice oxygen extraction . Spectroscopic study has also shown that the light-assisted lattice oxygen extraction was unimportant in photocatalysis.…”

Simple and Scalable Synthesis of Vertically Aligned Anatase Nanowires for Enhanced Photoelectrochemical Performance

“…Deconvolution of the DTG peak between 100 and 525 °C yields two peaks, located at 260 and 310 °C for both CBD-A and CBD-N. These peaks correspond approximately to the expected maximum rate of weight loss for the dehydration of titanates , and the decomposition/deammonation of melamine, respectively. The additional weight loss in the first stage can, therefore, be attributed to the evolution of ammonia (during polycondensation), the decomposition of melamine and partial decomposition of poly s-triazine; while the second weight loss stage observed in both CBD-A and CBD-N can be attributed to the decomposition or desorption of poly s-triazine compounds.…”

“…It has been reported that cations, such as Na + , K + , or NH 4 + , present in solution act to stabilize the negatively charged titanate layers, thus preventing layer collapse and crystallization. , In the absence of a suitable cation, charge neutrality can only be maintained by layers collapsing through destabilization and recombination of TiO 6 octahedra . Several researchers have produced ammonia ions in situ by the acidic hydrolysis of urea, hexamethylenetetramine (HMT), and melamine in heated (>80 °C) solutions.…”

“…The radius of the triazine ring is similar to that of the ionic radius of the large Zr atoms, thus the incorporation of triazine compounds into the titanate layers may then result in the formation of a Srilankite-like phase upon calcination. Alternatively, Zhao et al noted that layered titanates that were either fully protonated or fully intercalated by NH 4 + would produce anatase TiO 2 upon calcination or hydrothermal treatment, whereas titanates partially intercalated by both NH 4 + and Na + would result in the formation of Brookite, which was a result of a lattice shear due to different deintercalation rates of NH 4 + and Na + . Similarly, melamine derivatives, which do not decompose until 320–650 °C, , intercalated into the titanate lattice may induce a lattice shear during dehydration and calcination, thus leading to an orthorhombic intermediate phase.…”

Titania-Carbon Nitride Interfaces in Gold-Catalyzed CO Oxidation

“…This is attributed to a molecular interaction between C 3 N 4 precursors and/or their gaseous decomposition products (NH 3 , HCN, cyanuric acid, and isocyanate) 50 and the hydrophilic, OH-terminated TiO 2 surface. 51,52 We speculate that the polycondensation process is then directed by the titania surface, 53 which drives/controls the morphology of the resulting composites, and that C 3 N 4 acts as a glue that makes the titania NT agglomerated structure more compact.…”