First-principles theoretical study and scanning tunneling microscopic observation of dehydration process of formic acid on a TiO2(110) surface

We have studied the dehydration process of formic acid on a TiO2(110) surface by using first-principles theoretical calculations. Formic acid dissociatively adsorbs to form formate and hydroxyl. It turns out that simple decomposition processes of the formate on the stoichiometric surface are energetically unfavorable. The formation of H2O and O vacancies from two neighboring bridging hydroxyls is relatively easy and the activation barrier is calculated to be 114 kJ/mol. On the TiO2(110) surface with oxygen defect sites, formate adsorbs with one O at a defect site and with the other O on a five-fold Ti, forming a bridging configuration. Further decomposition of the formate occurs through a monodentate configuration with an activation barrier of 129 kJ/mol. We have also performed STM observation corresponding to the theoretical results. It was imaged that some formates were located along the oxygen row and at an intermediate position between the oxygen row and the Ti row at elevated temperatures at which reaction takes place, indicative of the interaction between oxygen vacancy and formate. The catalytic dehydration cycle is discussed based on these results.