Combined ATR-FTIR and DFT study of cyclohexanone adsorption on hydrated TiO2 anatase surfaces

The adsorption of cyclohexanone on different planes ((100), (101), and (001)) of anatase TiO2, with variable level of hydration, was evaluated by density functional theory (DFT) calculations. Surface hydration was found to affect the cyclohexanone adsorption enthalpy and the calculated infrared absorption frequencies of the preferred adsorbed configurations considerably. A good correlation was found between two experimentally observed absorption frequencies at 1694 and at 1681 cm–1 of cyclohexanone adsorbed on TiO2, determined by attenuated total reflection Fourier transform infrared (ATR-FTIR), and frequencies calculated for conformations of cyclohexanone interacting with the (101) surface with low and intermediate levels of hydration, respectively. The corresponding adsorption enthalpies of these adsorbed conformations amount to −23.5 and −37.0 kJ/mol, respectively. In addition, DFT calculations show that cyclohexanone physisorption on hydrated (100) yields an adsorption enthalpy of −89.4 kJ/mol. This might be a conformation contributing to irreversible adsorption of cyclohexanone known to occur experimentally after photocatalytic activation of TiO2. Other phenomena of relevance to irreversible adsorption are also briefly discussed