2-Ethanolamine on TiO₂ Investigated by in Situ Infrared Spectroscopy. Adsorption, Photochemistry, and Its Interaction with CO₂

In situ Fourier-transform transmission infrared spectroscopy has been employed to investigate the adsorption and photochemistry of 2-ethanolamine (HOCH 2 CH 2 NH 2 ) on TiO 2 as well as the interaction of CO 2 with 2-ethanolamine-modified TiO 2 surfaces. Intact HOCH 2 CH 2 NH 2 and the dissociative form of OCH 2 CH 2 NH 2 are found to be present on the surface with the saturation coverage of 2-ethanolamine at 35 °C, in comparison to the adsorption of CH 3 CH 2 CH 2 OH and CH 3 CH 2 CH 2 NH 2 on TiO 2 . CO 2 reacts with the −NH 2 basic centers of the surface 2-ethanolamine molecules, forming carbamate (−NHCOO − ) and ammonium (−NH 3 + ). Bicarbonate (HCO 3 − ) is also formed due to the presence of residual water. The carbamate species of OCH 2 CH 2 NHCOO − has a better thermal stability than HCO 3 − . In the presence of O 2, as the TiO 2 surface with the saturation coverage of 2-ethanolamine is photoirradiated at 325 nm, the infrared studies suggest the formation of isocyanate (NCO), absorbed CO 2, OOCCH 2 NH 2, and carbonyl-containing species of formamide (HCONH 2 ) and formic acid (HCOOH). However, for the OCH 2 CH 2 NH 2 /TiO 2 surface, H 2 O, formate (HCOO), carbonates, and a surface species carrying a C═N group are found, in addition to NCO. The photoproducts of NCO and absorbed CO 2 reveal the C−C bond dissociation pathway of the surface 2-ethanolamine under illumination. Photoreaction mechanisms involving hole capture at different reaction centers and formation of organoperoxide or tetraoxide species have been proposed, on the basis of the products and intermediates identified in the 2-ethanolamine photocatalytic degradation on TiO 2 in the presence of 16 O 2 and 18 O 2 .