Infrared Spectroscopic Study of the Adsorption Forms of Cyanuric Acid and Cyanuric Chloride on TiO₂

Cyanuric acid is often found to be the end product in the hydrolysis of waste melamine and in the TiO₂-mediated photocatalytic decomposition of s-triazine-containing compounds used as herbicides or dyes. The photocatalytically recalcitrant nature of cyanuric acid on TiO₂ may be closely related to its adsorption properties, including the tautomeric forms present on the surfaces and their bonding structures, which remain to be determined. In this paper, we present the optimized adsorption structures of the four tautomeric isomers (triketo, diketo, monoketo, and triol) of cyanuric acid on a model rutile-TiO₂(110) surface and their vibrational absorptions. Experimentally, the adsorption structures of cyanuric acid and chloride on powdered TiO₂ are analyzed on the basis of the theoretically obtained, characteristic infrared information. Cyanuric acid on TiO₂ at 35 °C exists in triketo and hydroxylated forms, but the diketo becomes the predominant form on the surface at 250 °C, being bonded to a titanium site via one of its carbonyl groups and with a N-H···O hydrogen bonding interaction. Hydroxylation of cyanuric chloride occurs as it is adsorbed on TiO₂ at 35 °C. Upon being heated to 200 °C, the surface is mainly covered with the diketo form of cyanuric acid after the adsorption of cyanuric chloride.