Microscopic characterization of the photocatalytic oxidation of oxalic acid adsorbed onto TiO 2 by FTIR-ATR Cecilia B. Mendive a,b,c, * , Detlef W. Bahnemann a , Miguel A. Blesa b,c,1 a Institut fu ¨r Technische Chemie, Universita ¨t Hannover, Callinstr. 3, D-30167 Hannover, Germany b Unidad de Actividad Quı ´mica, Centro Ato ´mico Constituyentes, Comisio ´n Nacional de Energı ´a Ato ´mica, Avenida General Paz 1499, 1650 San Martı ´n, Provincia de Buenos Aires, Argentina c Consejo Nacional de Investigaciones Cientı ´ficas y Te ´cnicas, Avenida Rivadavia 1917, 1033 Capital Federal, Argentina Available online 19 March 2005 Abstract Sequential FTIR-ATR spectra of an illuminated Degussa P-25 film deposited onto the ATR crystal were recorded to follow the photocatalytic oxidation of oxalic acid in acidic media. The results show that illumination ensues in the depletion of adsorbed oxalate from its equilibrium condition. The equilibrium spectral and thermodynamic data obtained previously from measurements in the dark were used to derive the time evolution, upon illumination, of the surface coverage of each of the three known surface complexes formed by oxalic acid adsorbed onto TiO 2 . The results demonstrate that the most stable species is also the most photo-labile one, and that the surface speciation is determined by a fast surface redistribution among the three species, without equilibration with the bulk solution. Thus, zero-order kinetics are observed at high degrees of coverage during the disappearance of the least stable species. At low concentrations, the disappearance of the most stable species is characterised by first-order kinetics. # 2005 Elsevier B.V. All rights reserved. Keywords: Surface phenomena; TiO 2 ; Oxalic acid; Photocatalytic degradation 1. Introduction Oxalic acid is a simple model compound of pollutants that contain more than one carboxylate group, and its behaviour during its heterogeneous photocatalytic oxidation should be representative for some of the main features of the whole class of organic acids. The interaction of oxalate with the photocatalyst surface in the dark has been thoroughly characterised by FTIR-ATR spectroscopy [1–4] and several papers have reported the kinetics of its photocatalytic degradation [5–10]. Oxalic acid is easily oxidised to carbon dioxide, thus minimising the problem of possible accumula- tion of intermediate products; its primary oxidation product, the radical anion CO 2  injects its electron into the conduction band and produces a second CO 2 molecule. Only if the pH is adequately high, carbonate may be retained at the photocatalyst’s surface. In this work, we report the results of a FTIR study of the changes that occur upon illumination of the surface of a thin TiO 2 layer in the presence of aqueous oxalic acid. Sequential spectra were obtained during a series of experiments that span a wide range of oxalic acid concentrations and light intensities. In a previous publication [11], we had shown that this technique was useful to characterise the formation of intermediates as well as the depletion caused by the photocatalytic oxidation of adsorbed catechol. Catechol forms only one surface complex, and thus the interpretation of the spectral data under illumination is straightforward. Oxalic acid, on the other hand, is known to form three different surface complexes and because of this the interpretation of the results is complicated and requires thorough mathematical analysis. In this work, we show that the use of the attenuated total reflection fourier trans- form infrared spectroscopy (ATR-FTIR) [12,13] technique www.elsevier.com/locate/cattod Catalysis Today 101 (2005) 237–244 * Corresponding author. Tel.: +49 511 762 5560. E-mail addresses: hansmann@iftc.uni-hannover.de (C.B. Mendive), bahnemann@iftc.uni-hannover.de (D.W. Bahnemann), miblesa@cnea.gov.ar (M.A. Blesa). 1 Tel.: 54 11 6772 7007. 0920-5861/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2005.03.016