<p>Carboxylic acids adsorption on
anatase TiO<sub>2</sub> 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 TiO<sub>2</sub> 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 <i>kT</i>. 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 TiO<sub>2</sub>
surfaces.</p><div>
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The Case of Formic Acid on Anatase TiO
2
(101): Where is the Acid Proton?
