Volume 14 Issue 1 - May 21, 2010 PDF
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Chemistry of Glycolic Acid (HOCH2COOH) on Titanium Dioxide
Department of Chemistry, College of Sciences, National Cheng Kung University
 
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Air pollution is one of the major concerns in environmental protection. Organic compounds in aerosols distributed in the atmosphere contain a substantial amount of carboxylic acid molecules. It has been reported that 3.6% of the organic content is composed of glycolic acid [1]. TiO2 is a chemically stable semiconductor and used as a photocatalyst for degradation of organic compounds. As TiO2 absorbs photons with energy higher than its bandgap, electrons in the valence band can be promoted into the conduction band, generating electron-hole pairs which are the reaction initiation species for the organic molecules adsorbed on TiO2. This article presents the results of the fundamental interaction between glycolic acid molecule and TiO2, including adsorption, thermal evolution and photodecomposition of glycolic acid on TiO2.

Figure 1. Temperature-dependent infrared spectra of HOCH2COOH on TiO2.
The TiO2 powder used for this study was supported on a tungsten fine grid and mounted inside the infrared cell for in-situ Fourier-transform transmission infrared spectroscopy to investigate the adsorption and chemistry of glycolic acid introduced into the cell and adsorbed on the TiO2 surface. The infrared spectra obtained after adsorption of HOCH2COOH on TiO2 at 35 ˚C, followed by surface heating up to 300 ˚C, are presented first and shown in Figure 1. In the 35 ˚C spectrum, several characteristic bands reveal the dissociative adsorption forms of glycolic acid on TiO2. Figure 2 (a) and (b) show the adsorption structures with the associated infrared peaks. Increasing the surface temperature to 150 ˚C causes the enhanced absorptions at 1420, 1563 and 1756 cm-1 which are also observed in the adsorption of glyoxylic acid (HCOCOOH) on TiO2 and are attributed to the structure of Figure 2(c). This set of peaks almost vanishes at 275 ˚C, but with the major peaks appearing at 1360, 1386, 1563, 2836, 2895, 2933 and 2954 cm-1 which are ascribed to CH3O and HCOO (Figure 2(d) and (e)) [2,3].
Figure 2. Surface species generated after adsorption of glycolic acid on TiO2 at different temperatures.

Fig 3. Infrared spectra taken before and after the indicated photoirradiation times for glycolic acid adsorbed on TiO2.
Glycolic acid molecules on TiO2 are also subjected to photodecomposition at 325 nm in the presence of O2. The infrared result is shown in Figure 3. Photoirradiation induces increased intensities at 1329, 1356, 1438, 1540, 1575, 2362, 2886 and 2955 cm-1 which are attributed to HCOO, CO2 and CO3. Figure 4 displays the reaction pathway.

Efficient degradation of organic pollutants by a photocatalyst, such as TiO2, relies on fundamental understanding of the interaction of organic molecules with the catalyst. Here, we report an interesting outcome for glycolic acid, with –COH and –COOH functional groups, on TiO2.
Figure 4. Photodecomposition pathway of glycolic acid on TiO2.

References
  1. S.R. Souza, P.C. Vasconcellos, L.R.F. Carvalho, Atoms. Environ. 33 (1999) 2563.
  2. W.-C. Wu, C.-C. Chuang, J.-L. Lin, J. Phys. Chem. B 104 (2000) 8719.
  3. G. Busca, J. Lamotte, J.-C. Lavalley, V. Lorenzelli, J. Am. Chem. Soc. 109 (1987) 5197.
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