Quantum vs Thermal Effects in Formic Acid Adsorption on (101) TiO2 Anatase Surfaces

Carboxylic acids adsorption on anatase TiO₂ is a key process in circular economy and sustainability. Yet, in spite of several decades of investigations, its intimate working mechanisms still remain elusive. In particular, the behavior of the acid proton and its localization – either on the molecule or on the surface – are still open issues. By modeling the adsorption of formic acid on top of regular (101) anatase TiO₂ surfaces, we found that, in the 0 K limit, the acid proton is shared between a carboxylic oxygen and a surface oxygen. In this regime, the proton behavior is mainly governed by quantum delocalization effects in a single potential well. Nonetheless, as temperature is raised to room conditions, simulations evidenced a rapid “classical” shuttling of the proton due to the onset of a two-wells free energy profile separated by a free energy barrier of the order of kT. This picture, supported by the agreement between simulated and experimental IR spectra, shows that the titania surface acts like a protecting group for the carboxylic acid functionality. Such a conceptual insight might help rationalize the chemical processes of carboxylic species on TiO₂ surfaces.