The case of formic acid on (101) anatase TiO2: where is the acid proton?

Carboxylic-acid adsorption on anatase TiO2 is a relevant process in many technological applications. Yet, despite several decades of investigations, the acid-proton localization—either on the molecule or on the surface—is still an open issue. By modeling the adsorption of formic acid on top of anatase(101) surfaces, we highlight the formation of a short strong hydrogen bond. In the 0 K limit, the acid-proton behavior is ruled by quantum delocalization effects in a single potential well, while at ambient conditions, the proton undergoes a rapid classical shuttling in a shallow two-well free-energy profile. This picture, supported by agreement with available experiments, shows that the anatase surface acts like a protecting group for the carboxylic acid functionality. Such a new conceptual insight might help rationalize chemical processes involving carboxylic acids on oxide surfaces.