Abstract
Carboxylic
acid 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
both a 0 K and at finite temperatures, we found that, in the 0 K limit, the
acid proton is shared between an adsorbate 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, might help to shed further light on the chemical processes of carboxylic
species on TiO2 surfaces.
Supplementary materials
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supporting quantum vs thermal Xiv
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