(575q) Infrared Spectroscopy and DFT Calculation Studies of the Effect of Gas Phase Water on Formic Acid Adsorption on TiO2 Anatase (101) Surface
Photocatalytic oxidation is a valuable tool for the cleaning of air streams with low concentrations of volatile organic compounds. Gas phase water has been found to dramatically increase the rate of photooxidation of some organics on TiO2. By examining the adsorption of these organics in the presence of gas phase water, a better understanding of this reaction mechanism can be developed. Formic acid was selected because it is a small molecule, and showed a large increase of rate with gas phase water. However, other organics of interest, including acetic acid and tetrahydrofuran, have also been studied.
The adsorption of organics on the dry and hydrated TiO2 anatase (101) surface was examined by multiple techniques such as infrared spectroscopy and density functional theory (DFT) calculations. Infrared spectroscopy revealed changes in the distribution of adsorption states when gas phase water was present; several of the changes suggest an increase in the relative concentration of adsorbed formate. DFT calculations also showed the dissociation of an oxygen-hydrogen to form a solvated formate ion in the presence of water. In this contribution, we report on comparisons between experimental and model studies of the effect of water on adsorbate structure and binding affinity for several organics.
For IR experiments, formic acid was adsorbed onto TiO2 that had been pressed into a tungsten grid. Water was then added and spectra taken for various pressures. The system was then evacuated, and another spectra taken. DFT calculations were performed using Vienna Ab-initio Simulation Package, VASP.
Infrared spectroscopy experiments with formic acid and d-formic acid showed two peaks in the CH stretching region, one assigned to formic acid and one to formate. As gas phase water was fed to the system, the intensity of the formate signal grew as the formic acid signal decreased. Changes were also seen in the O-H region. The C-O bend peak at 1190 disappeared when gas phase water was introduced into the system and did not return when the system was evacuated. This was also seen in DFT frequency calculations. The C=O carbonyl stretch at 1650 was also reduced with the presence of water. DFT results also showed the appearance of a formate ion in the presence of water.