Vinyl halides adsorbed on TiO2 surface: FTIR spectroscopy studies and ab initio calculations

The adsorption at room temperature of vinyl fluoride and vinyl chloride on TiO2 was studied by Fourier-transform infrared spectroscopy. The surface acidity was preventively investigated through the adsorption at room temperature of carbon monoxide. Vibrational spectra of adsorbed vinyl halides were compared with those of the compounds in the gas-phase and the most interesting differences consist in a large shift of the C-H and C-F stretching vibrations and in a small displacement of the CC stretching mode. The obtained data led to a model of adsorbed vinyl halides on the TiO2 surface through the occurrence of an acid-base interaction between the halogen atom and the surface Lewis acid site (Ti4+), and an H-bonding between one hydrogen of the CH2 group and a surface basic site. A model where an oxygen ion (O2−) represents the surface basic site was considered and ab initio periodic calculations at DFT/B3LYP level were performed on the system CH2CHF/TiO2 using the rutile (110) surface. The optimization of the system gave rise to a positive binding energy and to an increase of the bond lengths C-F and C-H, involved in the H-bonding, in agreement with the results from the infrared data.