2-Ethanolamine on TiO₂ investigated by in situ infrared spectroscopy. Adsorption, photochemistry, and its interaction with CO ₂

In situ Fourier-transform transmission infrared spectroscopy has been employed to investigate the adsorption and photochemistry of 2-ethanolamine (HOCH₂CH₂NH₂) on TiO₂ as well as the interaction of CO₂ with 2-ethanolamine-modified TiO₂ surfaces. Intact HOCH₂CH₂NH₂ and the dissociative form of OCH₂CH₂NH₂ are found to be present on the surface with the saturation coverage of 2-ethanolamine at 35 °C, in comparison to the adsorption of CH₃CH₂CH ₂OH and CH₃CH₂CH₂NH₂ on TiO₂. CO₂ reacts with the -NH₂ basic centers of the surface 2-ethanolamine molecules, forming carbamate (-NHCOO^-) and ammonium (-NH₃⁺). Bicarbonate (HCO₃ ^-) is also formed due to the presence of residual water. The carbamate species of OCH₂CH₂NHCOO^- has a better thermal stability than HCO₃^-. In the presence of O ₂, as the TiO₂ surface with the saturation coverage of 2-ethanolamine is photoirradiated at 325 nm, the infrared studies suggest the formation of isocyanate (NCO), absorbed CO₂, OOCCH₂NH ₂, and carbonyl-containing species of formamide (HCONH₂) and formic acid (HCOOH). However, for the OCH₂CH₂NH ₂/TiO₂ surface, H₂O, formate (HCOO), carbonates, and a surface species carrying a C - N group are found, in addition to NCO. The photoproducts of NCO and absorbed CO₂ 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 TiO₂ in the presence of ¹⁶O₂ and ¹⁸O₂.