FTIR study of adsorption and photodegradation of L-α-alanine on TiO2 powder

An infrared study of the adsorption of L-α-alanine (L-Ala) on TiO2 powder in aqueous suspension was carried out. When the amino acid is in the zwitterionic form and the TiO2 surface is positively charged, experimental results indicated that L-Ala molecules are adsorbed on titania through the carboxylated group, which gives rise to a bridged structure with the positive centers (protonated hydroxyls) of the TiO2 surface. Adsorbed amino acid was photodegradated in model conditions by exposing the sample to UV light through the quartz walls of the IR cell. The amine function was found to be preferentially removed from the adsorbed L-Ala molecules during the initial period of UV irradiation, and then the complete mineralization of the rest of the molecular structure occurred. This surface process agrees with the results of the analysis of the species that diffuse from the surface of the photocatalyst into the bulk of the liquid medium in the actual photodegradation carried out with TiO2 suspended in aqueous solutions of L-Ala.