Adsorption characteristics of 4-dimethylaminobenzoic acid on silver and titania: diffuse reflectance infrared Fourier transform spectroscopy study

We have investigated the adsorption behavior of 4-dimethylaminobenzoic acid (4-DMABA) on Ag and TiO2 by virtue of diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. 4-DMABA was found to chemisorb on silver as carboxylate, after deprotonation, with its two oxygen atoms bound symmetrically to the metal substrate. 4-DMABA was also very favorably adsorbed on TiO2 but with two different adsorption schemes. Most of the adsorbate was present as 4-DMABA(-) with its carboxylate group being held via a bidentate coordination to Ti4+ cations. The remaining adsorbate appeared to form a H-bond between the carbonyl group of 4-DMABA and the surface hydroxyl group of TiO2. Temperature dependent DRIFT spectra dictated that 4-DMABA molecules were surprisingly more strongly bonded to TiO2 than to Ag. This could be evidenced from the fact that the vibrational peaks of the adsorbed species were seen up to 600 K on TiO2, while they became substantially weakened around 450 K on Ag. From a cyclic thermal treatment, certain structural change appeared to occur around 400 and 500 K on Ag and TiO2,, respectively. (C) 2000 Elsevier Science B.V. All rights reserved.